EARYH 7 DAY Samoa mm“! :36 Lithology and Origin of Middle Ordovician Caleareous Mudmound at Meiklejohn Peak, Southern Nevada ’ GEOLOGICAL SURVEY PROFESSIONAL PAPER 871 Lithology and Origin of Middle Ordovician Calcareous Mudmound at Meiklejohn Peak, Southern Nevada By REUBEN JAMES ROSS, JR., VALDAR JAANUSSON, and IRVING FRIEDMAN GEOLOGICAL SURVEY PROFESSIONAL PAPER 871 A description of the various carbonate sediments constituting the Stromatactis-ricli mound core and the basal “zebra limestone, ” isotopic analyses of contrasting components, and review of possible complex origins of components UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTONII975 fin: .». s gag—.3 “t' if. .3“ ‘ ,_ I}; m V 3 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Ross, Reuben James, 1918— Lithology and origin of Middle Ordovician calcareous mudmound at Meiklejohn Peak, southern Nevada. (Geological Survey Professional Paper 871) Supt. of Docs. no.: I 19.16:871 Bibliography: p. 1. Rocks, Carbonate. 2. Geology, StratigraphinOrdovician. 3. Geology—Nevada—Meiklejohn Peak. 1. Jaanusson, Valdar. Ill Friedman, Irving, 1920— 111. Title. IV. Series: United States Geological Survey Professional Paper 871. QE71.15.C3R67 551.7'31 74—28131 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, DC. 20402 Stock Number 024—001—02690—2 CONTENTS Abstract ________________________________ Introduction Purpose of report ________________________ Acknowledgments General description and geologic setting of the mudmound Lithology _______________________________ Laminated limestone (zebra limestone), by Reuben James Ross, Jr _______________ Geologic relations _____________________ Calcilutite layers Radiaxial fibrous calcite layers _____________ Trace-element analysis __________________ Insoluble residue Underlying limestone Massive core facies of the carbonate mudmound, by Valdar Jaanusson __________________ Macroscopic constituents Stromatactis —————————————————————— Macroscopic skeletal particles ____________ Fine-grained limestone ________________ Microscopic constituents Main microscopic constituents ____________ Sparry calcite _____________________ 12 15 15 20 20 20 22 23 24 25 26 Lithology—Continued Massive core facies of the carbonate mudmound, by Valdar Jaanusson—Continued Microscopic constituents—Continued Composition of skeletal sand Comparison with selected other Paleozoic carbonate mounds Isotopic interpretations, by Irving Friedman Origin of the mudmound ______________________ Origin of the zebra limestone, by Reuben James Ross, Jr. Geochemical evidence Physical evidence _____________________ Zebra limestone above base of mound ________ Pelletoid calcilutite ____________________ Aggrading neomorphism: Other views ________ Evidence of rupture structures _____________ Rapid submarine lithification ______________ Derivation from algal mats _______________ Effect of organic substances _______________ Theory of lifting by crystal growth ___________ Conclusions Origin of the core of the mudmound, by Valdar Jaanusson References cited ___________________________ ILLUSTRATIONS FIGURE Page 1. Map of Nevada _______________________________________________________ 2 2. Photograph showing carbonate mudmound from the north _____________________________ 3 3. Photograph showing Meiklejohn Peak and the carbonate mudmound from the west ______________ 4 4. Diagram showing approximate positions of collections on mudmound _______________________ 5 5-34. Photographs: 5. Cyclically laminated zebra limestone ___________________________________ 8 6. Zebra limestone from flank of mudmound; polished section ______________________ 9 7. Cyclic zebra limestone; large thin section _________________________________ 10 8. One cycle of zebra limestone; thin section ________________________________ 10 9. Pelletoid calcilutite within radiaxial fibrous calcite ___________________________ 10 10. Disrupted layers of zebra limestone; thin sections ____________________________ 11 11. Cyclic succession of three calcilutites, one partly converted to radiaxial calcite; thin section ___ 12 12. Pelletoid calcilutite appears crossbedded between disrupted laminae; polished surface ______ 12 13. Calcilutite caa being converted to radiaxial fibrous calcite; thin section _______________ 13 14. Radiaxial fibrous calcite and distribution of insoluble minerals in calcilutites; polished sections - 14 15. Zebra limestone, three cycles; thin section ________________________________ 15 16. Pelletoid crossbeds grading to radiaxial calcite; collodion peel and polished section ________ 16 17. Radiaxial fibrous calcite associated with pelletoid calcilutite; collodion peel ____________ 16 18. Incomplete conversion of calcilutite caa to radiaxial calcite; polished sections ____________ 17 19. Detail of “cabbagehead”; thin section ___________________________________ 18 20_ Zebra limestone associated with “cabbageheads” ____________________________ 18 21. Nautiloid chambers filled with para-axial calcite; shells surrounded by calcilutite and radiaxial fibrous calcite ______________________________________________ 19 22. Poorly laminated limestone below mudmound; polished section ____________________ 20 III Page 33 33 34 35 38 39 40 40 41 41 42 43 43 45 46 IV CONTENTS Page FIGURE 23. Poorly laminated limestone below mudmound; polished and thin sections ______________ 21 24. Coarse calcarenite below mudmound ___________________________________ 22 25. Pelletoid texture, poorly laminated limestone; vertical and horizontal thin sections ________ 23 26. Sponge roots; polished section _______________________________________ 24 27. Calcite-filled tubes in calcilutite; thin section ______________________________ 24 28. “Microstromatactis” in mound core; thin section ____________________________ 25 29. Echinoderm plate partly converted to radiaxial calcite; thin section _________________ 25 30. Fossils relict in radiaxial calcite; thin section ______________________________ 26 31. Ostracodes recrystallized into radiaxial calcite; thin section ______________________ 26 32. Two generations of limestone in mound core; thin section _______________________ 26 33. Faulted “microstromatactis”; thin section ________________________________ 27 34. Spiculaelike skeletal grains in mound core; thin sections ________________________ 29 35. Frequency composition diagram for ac13 ________________________________________ 33 36. Frequency composition diagram for 6018 ________________________________________ 34 37—42. Photographs: 37. Radiaxial calcite in contact with calcilutite caa that is partly converted to pelletoid calcilutite; polished sections _____________________________________________ 36 38. Relationship of radiaxial calcite, pelletoid and equigranular ca; between ca! layers; collodion peel 37 39. Radiaxial fibrous calcite grading into pelletoid caa; collodion peel __________________ 37 40. Cyclic calcilutites of zebra limestone; polished section _________________________ 38 41. Cyclic calcilutites and radiaxial fibrous calcite; polished section ___________________ 39 42. Filaments of Sphaerocodium; thin section ________________________________ 42 TABLES TABLE 1. #0353 Page Relative proportions of microscopic constituents of mudmound __________________________ . Relative proportions of organic components of skeletal sand ____________________________ . Isotopic analysis of limestone beds covering the mudmound ____________________________ . Isotopic analysis of zebra limestone and core of mudmound ____________________________ LITHOLOGY AND ORIGIN OF MIDDLE ORDOVICIAN CALCAREOUS MUDMOUND AT MEIKLEJOHN PEAK, SOUTHERN NEVADA By REUBEN JAMES Ross, JR, VALDAR JAANUSSONI, and IRVING FRIEDMAN ABSTRACT The large dome—shaped (300 In wide by 80 m high) calcareous mud- mound at Meiklejohn Peak is the largest and most accessible of three such Ordovician mudmounds in southern Nevada. It has been studied in hopes of deciphering the origin of zebra limestone which forms its base and of Stromatactis which is abundant in the main, virtually un- stratified, core of the mound. The mudmound is completely enclosed within the Middle Ordovician Antelope Valley Limestone; it is un- derlain by calcarenite and poorly laminated limestone of the Paiute Ridge Member. Silty limestone of the Ranger Mountains Member in— tertongues with the lower third of the mound and abuts against and completely covers its higher parts. Cyclic laminated zebra limestone that forms the lower part of the mudmound is characterized by repeated sequences of three, or two, distinctive calcilutites. Radiaxial fibrous calcite separates some cycles; in these the third calcilutite tends to be missing. The origin of zebra limestone may be variously attributed to formation of submarine hardgrounds, to parallel shear cracks, to displacive precipitation, or to the precipitation of radiaxial fibrous calcite in parallel cavities of in- credible extent. Some radiaxial calcite may have filled cavities but most resulted from recrystallization or neomorphic aggradation of other preexisting metastable carbonate. It seems likely that origin of the zebra limestone was linked to algal mats in light of geologic and geochemical evidence. The core of the mound is composed of 40—60 percent calcareous mud, 2060 percent sparry calcite, and 4—11 percent skeletal sand. Stromatactis is a prominent constituent of the sparry calcite. Regardless of the origin of Stromatactis as such, the radiaxial fibrous calcite crystals are the result of diagenetic change. The calcareous mud of the mound comprises at least two and possibly three generations. Within skeletal sand, echinoderms predominate; spicules, possibly of sponges, are second in abundance. Ostracodes outnumber trilobites. Neither bryozoans nor any other organism seems to have been present in numbers adequate to bind the mound sediments together. Stromatactis seems to represent cavity fillings. The radiaxial mosaic of Stromatactis was fully developed before the mound had ceased to grow. This indicates an early lithification of the mound. Examination of isotopes of oxygen and carbon indicates that calcilutites and radiaxial fibrous calcite formed in the same seawater. No evidence for fresh vadose water is present. ‘Naturhistoriska Riksmuseet, Stockholm, Sweden. INTRODUCTION Three Middle Ordovician carbonate mudmounds are known in southern Nevada (localities shown in fig. 1, this report; Ross and Cornwall, 1961; Ross, 1972, fig. 1). These mounds resemble somewhat younger Ordovician features in the Siljan district, central Sweden (Thorslund, 1936; Thorslund and J aanusson, 1960, p. 24— 35), and Carboniferous “knoll reefs” of northwest and west—central Ireland (Schwarzacher, 1961; Lees, 1964). Development of sparry calcite in the main part of the biggest of the Nevada mudmounds may have some rela- tion to structures called Stromatactis, well known in middle and upper Paleozoic mudmounds in Western Europe (Black, 1952; Bathurst, 1959; Schwarzacher, 1961; Lees, 1964; Jaanusson, 1975). In the basal part of the Nevada mound there is enigmatic, thinly in- terlayered calcilutite and sparry calcite which together resembles “sheet spar” described by Lees (1964, p. 523- 524) and “zebra limestone” described by Fischer (1964, p. 115, figs. 15, 17, and 18). At least three generations of sparry calcite are present. In the early phase of development there were two generations, one producing Stromatactis and the other producing the laminar sparry calcite near the base. Limestone boulders, some of prodigious size, are known in southern Quebec (Mystic conglomerate) (Cooper, 1956, p. 14, 15, 31) and at Lower Head, western Newfoundland (Whittington, 1963, p. 7). Although no Stromatactis has been reported in them, some of these boulders are composed of limestone seemingly very similar to that in the main body of the Nevada mud- mounds and the fossils obtained from them are so similar that their temporal correlation seems obvious (Cooper, 1956, p. 15, 31; Whittington, 1963; Ross, 1967, 1970, 1972). Fossils from beds surrounding these boulders in Newfoundland corroborate the correlation with beds covering the Nevada mudmounds. 2 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA A ON CITY 0 \ 380A \ \\ \ Nevada \ Test Site Meiklejohn Peakx XAysees ‘\ Peak \ \ Laso 36°33 Vegas 0 \ Q \ ‘C M 0 100 200 MILES Q | 1 l l \ a I ' | | D 0 100 200 KILOMETRES FIGURE 1. — Map of Nevada showing location of Nevada Test Site. Coevally formed Middle Ordovician mudmounds are known at Meiklejohn Peak and near Aysees Peak. The largest and most readily accessible Nevada mud- mound is on the west face of Meiklejohn Peak, Bare Mountain quadrangle, 6 miles east of the town of Beatty, Nev. (fig. 2; Ross, 1972, fig. 2). Cephalopods from this mound have been studied by R. H. Flower; a stratigraphic resumé of the fossils of the mound and covering beds, as well as descriptions of brachiopods and trilobites, was presented by Ross (1967, 1972). Krause (1972) studied the inarticulate brachiopods of the mudmound and the covering beds; he concluded that infaunal lingulides preferred the covering beds, whereas abundant acrotretids probably were at- tached to algae on the mound. The numerous complete bivalved acrotretids indicate that the upper part of the mound was deposited in relatively quiet water. A Master’s thesis by Elizabeth Stilphen Yancey (1971), mentioned this locality. PURPOSE OF REPORT It is our purpose to describe the calcareous mudmound at Meiklejohn Peak in more detail than has been attempted previously (Ross, 1972), to compare it with similar structures in Europe with which Jaanusson has the greater familiarity, and to describe and speculate about the origins of various kinds of sparry calcite and about origins of the mound itself. The origin of the carbonate mudmound at Meiklejohn Peak has been enigmatic ever since the mound was first reported (Ross and Cornwall, 1961). Though it con- stitutes only a small part of the total volume, the origin of the basal zebra limestone facies has been particularly puzzling. Most colleagues knowledgeable in the study of car- bonates with whom we have discussed possible origins have urged us to consider the high proportion of radiax- ial fibrous calcite as the filling of former cavities. But all those who have accompanied us to the mound have agreed that, geologically and mechanically, cavities amounting to 60 percent of the volume of the basal facies are unreasonable and that the spar must have preferen- tially replaced some other material. It is our belief that the kinds of deformation shown by the laminated zebra limestone must have taken place prior to lithification and that many of these reflect earlier topography over which consecutive parallel laminae have been draped. In the main body of the mound Stromatactis accounts for about 20 percent of the volume. Some of it may have filled cavities but at least part replaced preexisting car- bonate mud. ACKNOWLEDGMENTS In examining the mudmound in the field, we had the assistance of N. F. Sohl in 1969, W. T. Dean and Ellis Yochelson in 1970, and A. J. Rowell and F. F. Krause in 1972, each of whom gave important suggestions about field observations and their interpretation. In particular, Rowell and Krause pointed out that mound sedimenta- tion, whether along the base or above interfingerings of flank lithology high on the sides, is initiated by the laminated facies. Our conversations with J. L. Wray, Richard Rezak, L. A. Hardie, R. N. Ginsburg, P. R. Rose, and M. J. Brady have been concerned mainly with possible origins of the mudmound and expectable effect of algae on sedimentation. We are indebted to R. E. Wilcox for his petrographic examination of minerals in the insoluble residues from the laminated facies of the mudmound and to B. F. Leonard III and W. N. Sharp for X—ray diffraction identification of the same residues. Leonard and D. M. Pinckney have discussed replace- ment fabrics in carbonate rocks with us; 0. B. Raup, R. J. Hite, and W. C. Culbertson called our attention to similarities in lamination of certain saline deposits and the basal facies of the mudmound. A. T. Myers made a spectrographic analysis of the laminated limestone on which R. C. Surdam of the University of Wyoming ran numerous X—ray microprobe traverses. Thin and polished sections were prepared by M. E. Johnson and L. /Ranger Mountains Member Aysees Member above mound INTRODUCTION FIGURE 2. — The carbonate mudmound on the west side of Meiklejohn Peak as seen from the north. Stratigraphic units indicated are the Lower Ordovician Nine- mile Formation and the Paiute Ridge, Ranger Mountains, and Aysees Members (Byers and others, 1961) of the Antelope Valley Limestone. LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA .smE 3m chmv S cm «Eu 933 E Sod 8 8m 32? E vcsoficsz .Q .muv «Enemmza E m»: mmum awuoem .695 2: 3 £8 :wnéco =E 553m 3:: Sea 5% ma 95955": 2: was xwwm ~50meme l .m "mm—36E INTRODUCTION Aweinsmv canoEwSE Bmcofimu 2: E maosoozoo =mm£ @3033 98 $383 2335: Mo 28338 BNECBEQAN I .v HEDGE mmmn. “Eamm oo memla oo Rmmoomfl OO hmmmn. Oo mvmwo OU wmmmo v.8; £95262 6 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA A. Wilson. R. E. Miller made a biochemical analysis of the laminated facies and established the presence of fatty acid and lipids. At the 1971 annual meeting of the Geological Society of America (Washington, DC.) and at the 1972 annual meeting of the American Association of Petroleum Geologists (Denver, Colo.), we discussed most features of the mound with many geologists, among them M. J. Brady, John Harper, James Lee Wilson, Graham Davies, P. N. Playford, Perry Roehl, H. Zankl, Richard Rezak, Robert Dunham, P. W. Choquette, and R. W. Beales. Jaanusson and Ross were joined in an examination of the mound on August 2—3, 1972, by Nils Spjeldnaes, C. P. Hughes, R. A. Fortey, J. L. Henry, and L. R. M. Cocks. P. E. Cloud, Jr., joined Ross in a visit to the mound on April 14, 1973. GENERAL DESCRIPTION AND GEOLOGIC SETTING OF THE MUDMOUND As shown by Cornwall and Kleinhampl (1961) the geologic setting of the carbonate mudmound at Meikle- john Peak is complex. The mound and surrounding strata are tilted tectonically with dips of 45° toward the east-northeast (fig. 2; Ross, 1972, fig. 4). The underlying calcarenitic limestone north of the mound has been dis- placed by a steep normal fault (Ross, 1972, fig. 3). The entire Ordovician section has overridden Devonian dolomite on a thrust fault. All three known Ordovician carbonate mounds in southern Nevada lie above limestone of the Paiute Ridge Member and within the Ranger Mountains Member (Byers and others, 1961), both constituting the lower member of the Antelope Valley Limestone (Ross, 1967, pl. 11). Immediately covering each mound is thin- bedded, silty, nodular limestone. As shown by Ross (1972) the mounds lie within the Orthidiella zone; cor- relations by McKee, Norford, and Ross (1972) indicate that this zone is equivalent to the graptolite zones of Isograptus caduceus and Paraglossograptus etheridgei. The exact geometry of the carbonate mudmound is not known. In fact, we do not know what part of the mound is exposed to view, how much may be buried by covering sediments, or how much may have been eroded away. The exposed section suggests that the bottom is nearly flat, and that the mound is thickest in its middle and thinnest at the sides. It is about 300 m (1,000 ft) across its greatest visible dimension and 80 m (270 ft) high (figs. 3, 4). The laminated zebra limestone at the base of the mound is underlain by interbedded coarse calcarenite, dark—gray, silty, nodular limestone, and crudely laminated, partly stylolitic limestone; this underlying unit is about 9 m (30 ft) thick and can be followed ap- proximately 0.9 km (0.6 mi) northward above the Ninemile Formation. This unit is equivalent to the Paiute Ridge Member of the Antelope Valley Limestone on the Nevada Test Site. Although another mound is present at the same stratigraphic level east of the Nevada Test Site (Ross and Cornwall, 1961), its exposure is much smaller and it seems to lack the basal zebra limestone facies. We do not know whether that exposure represents a lateral tip of a large mound or the center of a small mound; the laminated facies may be present but hidden from view. The mound at Meiklejohn Peak shows evidence of several generations of fracturing, the details of which have not been explored. Clearly a system of cavities opened after formation of radiaxial fibrous calcite in the core and in the basal laminated beds. In many places cavities are filled with brown-weathering material. The cavities resemble distal parts of postdepositional fissures which are known in most other carbonate mounds. Such fissures are particularly conspicuous in the mounds of the Siljan district, Sweden, because they are filled by black graptolite shale. Within the core of the Meiklejohn Peak mound one can find several patches of intrafor- mational conglomeratic material; it is also common in similar carbonate mounds elsewhere. The mound core is composed mainly of Stromatactis- bearing calcilutite which shows only slight evidence of bedding. Some channels or pockets are filled with shells of cephalopods, brachiopods (particularly Idiostrophia), trilobites, and gastropods. In the upper part of the core Stromatactis constitutes about 20 per cent of the volume of the rock. The entire basal part of the core is laminated limestone composed of couplets of calcilutite (micrite) and radiaxial fibrous calcite. Each couplet is 6—10 mm thick. Following the example of Fischer (1964, p. 115), we call this basal facies zebra limestone. The aggregate thickness of the facies is highly variable, in some places being 20 cm and in others 9 m. Covering the mound are thin beds of siliceously silty, highly fossiliferous, partly nodular, dark-gray limestone, whose silty parts weather grayish orange. Here and there in the lower third of the Ranger Mountains Member this lithology forms tongues into the mass of the mudmound. Two probable channels in the mound seem to have been filled by it. But the covering beds, particularly in the up- per part of the mound, butt against the sloping sides of the mound and at the top cover it. Lithology of the part of the Ranger Mountains Member that forms the mound cover (Orthidiella zone) was studied by Jaanusson in thin sections of a series of samples collected at the northern end of the mound. Jaanusson found that the covering limestone is preponderantly a sparitic calcarenite with the former voids between sand grains partially or completely filled with sparry calcite. Most of the limestone is a pelletal, LITHOLOGY 7 sparitic calcarenite in which the majority of sand grains are cryptocrystalline pellets, mostly 0.08 to 0.2 mm in diameter. During deposition, the pellets were obviously indurated. The pelletal sand includes varying amounts of skeletal sand grains which dominate in some thin sec- tions and form skeletal sparry calcarenite. The para—ax- ial sparry calcite has to a large extent assimilated adjoin- ing parts of skeletal grains making it difficult to deter- mine the original grain boundaries. Therefore, it was im- possible to apply modal analysis for quantitative deter- mination of original sand constituents. In some places, the matrix is micritic, the interstices between sand grains being filled with carbonate mud, but such rock (micritic calcarenite) seems to be quan- titatively less important than sparitic calcarenite. Radiaxial calcite was not observed in any of the thin sec- tions studied. The irregular, argillaceous intercalations between limestone beds abound in quartz silt grains, mostly 0.02 to 0.06 mm in diameter. Jaanusson found that the limestone covering the mound is very different from that of the mound core. He concluded that a considerable winnowing of the sedi- ment occurred and that deposition was in an environ- ment with a much higher water energy than that during which the mound core was formed. LITHOLOGY Each lithologic facies of the mudmound was examined in thin section. Because its origin is highly controversial, the zebra limestone was subjected to other analyses. A semiquantitative spectrographic analysis was run by A. T. Myers. The laminae of calcilutite and radiaxial spar were analyzed for the isotopes of oxygen and carbon by Friedman. Insoluble residue was examined and iden- tified by R. E. Wilcox and B. F. Leonard III. A sample was subjected to X—ray fluorescence and microprobe analysis by Ronald C. Surdam of the University of Wyoming. Representative thin sections of samples from the core of the mudmound were examined in detail by V. J aanus- son. LAMINATED LIMESTONE (ZEBRA LIMESTONE) By REUBEN JAMES Ross, JR. GEOLOGIC RELATIONS At the north end of the carbonate mound, about 30 feet (9 m) of the zebra limestone is strikingly exposed (fig. 5; Ross, 1972, fig. 6). Some individual laminar couplets may extend laterally for tens of feet (more than 3 in) without break, but others are of much shorter ex- tent. Lateral terminations of couplets are sedimentary rather than tectonic and account for variations in thickness of the laminated facies along the bottom of the mound. Although the laminated limestone forms the base of the mound core, it also occurs in restricted areas higher along the edges of the core. Wherever core mud has been deposited over channels or tongues of dark, silty limestone of the covering lithology the initial phase of mound deposition is laminated. An example was called to our attention by A. J. Rowell and F. F. Krause (Oct. 6, 1971) about 80 feet (25 m) above the base and 200 feet (61.5 m) from the south end of the mound (USGS colln. D2334 CO; fig. 6). In one place near the middle of the base where the laminated rock totals about 35 cm (1.2 ft) in thickness, two flat segments are connected without interruption by a quasi—flexure as if a very thick carpet were draped over a single step, the riser of which is also about 35 cm (1.2 ft) high. No fracture or tectonic displacement accounts for the flexure. Close to this place, about 3 m (10 ft) above the bottom of the mound, Professor Nils Spjeldnaes (Aug. 3, 1972) noted that the zebra limestone facies was interrupted by a cavity which itself was filled with shells of about 30 nautiloid cephalopods and three generations of car- bonate mud and sparry calcite. The visible, lens—shaped cross section of the cavity was almost 1 foot (30 cm) in height and 2 feet (60 cm) in width. Cephalopods were not confined to the youngest part of the cavity although the majority were “nested” therein. This disparity indicates that the laminated zebra facies had already formed early in the depositional and diagenetic regime because the cephalopods are the same as those found in the zebra limestone about 60 m (200 ft) farther south (R. H. Flower, written commun., 1972). CALCILUTITE LAYERS The laminated limestone exemplified by USGS colln. D2325 CO is composed mostly of alternating layers of calcilutite and of radiaxial fibrous calcite (fig. 7). Any attempt to categorize the kinds of calcilutite present or its relations with the radiaxial mosaic runs into complex- ities that defy simple interpretations. Any description of this rock seemingly involves speculation about its origins. In the simplest form, each layer of calcilutite consists of two parts (fig. 8): 1. A lower fossiliferous calcilutite (cal), the upper and lower surfaces of which are highly irregular. Fossil material is contributed by ostracoda, trilobita, and echinodermata. Attitudes of fossils parallel layers of calcilutite (fig 9) except where cal is flexed or dis- rupted; there, fossils may rest at angles highly canted relative to the attitude of the laminae (fig. 10). 2. An upper pelletoid calcilutite (C32) in which the diameter of the pelletlike particles is about 0.05 mm. This material seems to fill depressions in the upper surface of the fossiliferous calcilutite. In 8 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 4 . FIGURE 5. — Cyclically laminated zebra limestone. This lithology is exposed through a thickness of 20 cm to 9 in (1—30 ft) along the base of the mound. many cases all depressions are filled and a flat sur- face results (figs. 7, 8). In other places, a third, more homogeneous, unfos- siliferous, equigranular calcilutite (caa) is present above the pelletoid mud (caz) and seems to hold the position of the radiaxial fibrous spar (fig. 11). This third calcilutite (033) locally appears crossbedded with pelletoid material (fig. 12) or intergrading with pelletoid calcilutite. Its usual position is above the pelletoid material (C32) and below the next higher fossiliferous calcilutite (cal) (figs. 11, 13). The zebra limestone is remarkable for its constant LITHOLOGY 9 M FIGURE 6. — Polished section of zebra limestone from south side of mound high on flank. All three calcilutites (ca‘, cag, and can) are de- veloped. Disruption and deformation took place while C82 and 033 were saturated with water and hence semifluid. cal was more coherent, but not lithified. Radiaxial fibrous calcite (rax) formed only along bottom contact of cal. Section illustrated in figure 40 cut along indic- ated line. USGS colln. D2334 CO. cyclicity whether at the bottom of the mound (figs. 7, 14) or above intertongued sediments higher on the sides of the mound (figs. 6, 11). I estimate that at the north end there are 900 cycles present within a thickness of 9 m. It should further be noted that when present the radiaxial fibrous calcite discussed below invariably oc- curs at the bottom contact of calcilutite cal further emphasizing the cyclic nature of the rock. From the zebra limestone Jaanusson made a modal analysis of four separate layers of calcilutite ca; in thin section after visiting the mound in 1972. (For method see Jaanusson, 1972.) He found that the mean values of the main constituents are (1) matrix, 95 percent (range, 9297 percent); (2) para-axial calcite, 2 percent (range, 0.1v3 percent); and (3) skeletal sand, 3 percent (range, 1—6 percent). Compared with the bulk of the limestone in the massive mound core (mean values for matrix, 56 per- cent; para-axial calcite, 21 percent; radiaxial calcite, 16 percent; and skeletal sand, 7 percent), the absence of radiaxial calcite and the low values of para-axial calcite in calcilutite cal of the zebra limestone are particularly noteworthy. Qualitative examination of peels and thin sections—too thick for a reliable modal analysis—of other cal layers suggests that these data are fairly representative. The average amount of skeletal sand is less in 10 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA FIGURE 8. — Cyclic laminae. Depressions in upper surface of fossiliferous calcilutite (cal) are filled or covered by pelletoid calcilutite (ca-2). Space along median seam in radiaxial fibrous calcite (rax) opened during preparation of thin section. Lower tier (tr) of radiaxial calcite is well defined by micritic peloids along its top. USGS colln. D2325 CO. “if? FIGURE 7. ~ Large section of laminated zebra limestone, showing FIGURE 9. — Pelletoid material within radiaxial fibrous calcite (rax) cyclic nature. Calcilutite ca1 is fossiliferous; cag is pelletoid and fills is cut by median seam. Seam may be a secondary feature. Pelletoid hollows in top of ca. Radiaxial fibrous calcite (rax) in every example material would not have been deposited unsupported in middle is immediately below calcilutite cal. In the bottom of rax, a thin tier and upper half of cavity but could be relict calcilutite. partly con- of clear radiaxial calcite (tr) is common. Compare with figure 8. verted by aggrading neomorphism. F, fossils. Thin section. USGS USGS colln. D2325 CO. colln. D2325 CO. LITHOLOGY 11 FIGURE 10. — Disrupted layers of zebra limestone contrast with those shown in figure 7, which are less than 60 cm distant. Orientation of fossils in disrupted layers of calcilutite ca; might be interpreted to indicate that calcilutite cal was divided by vertical crevices. USGS colln. D2325 CO. A, Thick section illustrating fluid appearance of calcilutite (cax) between torn layers of ca. Orientation of large fossil fragment (F) in center of section agrees with that of small fossils in B and C. B, Small part of section near top ofA. Cross sections of fossils (F) are oriented at high angles to enclosing layers of calcilutite cai, therein contrasting with fossils in figure 9. Radiaxial calcite (rax and tr) might be lining of partly filled cavity, but would imply a different orientation than fossils. Radiaxial calcite may also have resulted from neomorphic conversion of preexisting aragonitic calcilutite. C, Small part of section near top of A and close to B. Note cross section of inarticulate brachiopod (ia) projecting downward into calcilutite (can) here made pelletoid. If boundaries of calcilutite cal were frac- ture or shear surface, the fossil could not have survived. Despite at- titude of seeming crossbeds of ca, radiaxial calcite (tax) is developed preferentially immediately beneath each layer of ca. FIGURE 11.— Thin section showing cyclic nature of laminae and selective aggrading neomorphism along contact between bottom of fossiliferous calcilulite (cal) and top of equigranular calcitutite (cag). Note that both cal and C83 are being converted to radiaxial calcite (rax) along this contact in both cycles. USGS colln. D2324 CO, from same sample as shown in figure 6. calcilutite cal than in the core but the ranges overlap. The difference is statistically significant at P=0.01 level. This does not necessarily mean much, on account of the small area of cal layers analyzed. Nevertheless, it does indicate that the carbonate mud which formed 031 layers was finer grained than most of the carbonate mud in the core. Owing to the small amount of skeletal sand present in the measured areas of ca layers, composition of the skeletal sand was calculated for all four analyzed areas together. The data, in percent, are as follows: Percent Echinodermata ______________ Trilobita __________________ 14 Ostracoda _________________ 10 Brachiopoda ________________ 3 Mollusca __________________ 1 Indeterminable ______________ 20 Trilobites are more common than ostracodes, whereas in the core, the reverse is true; no “spiculae” were found. LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA FIGURE 12. — In contorted or disrupted parts of zebra limestone cyclic succession of calcilutites may be disturbed. Here layers of calcilutite cal are separated by seemingly crossbedded interlayer- ing of pelletoid and equigranular calcilutite(ca3),Radiaxialflbrous calcite (rax) is immediately below cal. In some cycles a tier (tr) of radiaxial calcite appears to underlie caa and is analogous to tr in figure 8. Polished surface. USGS colln. D2325 CO. Owing to the small area of ca layers measured, the low content of skeletal sand present, and the relatively high amount of small indeterminable grains, these data may not be representative for cal rock as a whole. But the analysis does suggest that the rock forming calcilutite cal was somewhat finer grained than most calcilutite in the massive core; its small content of drusy calcite in- dicates a diagenetic history different from that of the core. Jaanusson found that the pelletoid calcilutite (C82) was too recrystallized (with many indistinct grain boundaries) to yield reliable data by modal analysis. RADIAXIAL FIBROUS CALCITE LAYERS The coarse calcite in the zebra limestone exhibits the essential characteristics of Bathurst’s (1959, p. 511—512; 1971, p. 426-427) radiaxial fibrous mosaic. In the simplest form, each radiaxial fibrous calcite layer is divided symmetrically into upper and lower halves by a median seam to which coarse crystals are roughly perpendicular. The seam is remarkably smooth despite the large side of the adjoining crystals (figs. 14, 15). Zon- ing in the coarse radiaxial calcite is disposed sym- metrically above and below relative to the seam (fig. 14A); the zoning is not necessarily parallel to the top or bottom contact of the spar. LITHOLOGY 13 lain- im;-w€v;m’s J FIGURE 13. — Thin section cut normal to surface shown in figure 6 and parallel to surface shown in figure 40. Two cycles of fossiliferous calcilutite (cal) are shown. Pelletoid calcilutite (ca?) and very fine equigranular calcilutite (033) complete the sedimentary cycle. Ag- grading neomorphism has resulted in partial conversion of caa and bottom of overlying cal to radiaxial calcite (rax). Calcilutite above C33 is not fragmental; if it were, it should also be spread to left on “floor” over (282. USGS colln. D2334 CO. In many places the radiaxial calcite layer is somewhat more complex. Below the thick symmetrical calcite layer is one, very rarely more than one, thin tier of radiaxial calcite. This thin tier can be distinguished in plane- transmitted light with assistance of the pelletoid material incorporated in its bottom and in the bottom of the overlying radiaxial calcite (figs. 8, 14A, 14B). In polarized light, whether reflected or transmitted, the boundary between the upper thick symmetrical radiax— ial calcite and the lower thin tier of radiaxial calcite is distinct only where peloids intervene; laterally the dis- tinction between the two may disappear (fig. 148). The tier is remarkably like the radiaxial calcite that occurs beneath seemingly crossbedded calcilutite in disrupted parts of the zebra limestone (figs. 103, 16, 17). The crystals of calcite in the thick, symmetrical calcite tend to be largest close to the median seam and smallest along contacts with calcilutite (figs. 14C, 18). Large crystals exhibit undulate extinction and in- tergrown boundaries (figs. 14B, 14C). Relict extinction suggests that many large crystals have grown at the ex- pense of their neighbors, be it by “cannibalism,” aggrading neomorphism, or some other means. Pelletoid material and other bits of calcilutite are common within the radiaxial fibrous calcite. This material may be along the bottom of the thin lower radiaxial calcite tier or along the bottom of the lower half of the symmetrical thick radiaxial calcite (fig. 8). Although the pelletoid and calcilutitic material is usually below the median seam of the thick radiaxial calcite (fig. 15), it appears to be above or across the seam in some instances (fig. 9). At the south end of the outcrop and less than 3 m (10 ft) above the base of the mudmound the zebra limestone overlies a tongue of dark-gray, nodular limestone of the covering facies (Ranger Mountains Member) and takes two forms. The first of the forms occurs immediately above the nodular limestone as irregular masses not more than 20 cm across and 10 cm high composed of very thin discon- tinuous laminae of calcilutite cal and radiaxial sparry calcite (fig. 19). There is a suggestion that these laminar bodies grew as small heads on the upper surface of the nodular limestone. Laminae range from 0.25 to 2.0 mm in thickness. The small finely laminated masses are reminiscent of a cross section through a head of cabbage. The thickness of radiaxial calcite between layers of calcilutite ca, is about equal to the thickness of the layers of ca. Although space taken up by radiaxial calcite may once have been occupied by calcilutite, it seems equally likely here that radiaxial calcite has filled spaces between thin crusts composed of calcilutite cal. Covering not only the nodular limestone but also the more finely laminated “cabbageheads” is the zebra limestone in a second, more conventional form. Here (fig. 20), it is composed of alternations of calcilutite cal and pelletoid calcilutite C32; in some couplets C32 may have been deposited in two generations. Along the con- tact between (332 and the next succeeding layer of ca, radiaxial calcite has formed irregularly. Laterally, to the north, the layers of pelletoid calcilutite C32 are taken over by radiaxial fibrous calcite. The field evidence suggests that the pelletoid calcilutite has been selectively replaced by or recrystallized into radiaxial calcite. Just as elsewhere along the base of the mudmound, the lateral extent of the zebra limestone facies, the thinness and irregularity of calcilutite cm, and the even spacing of laminae argue against the mechanical possibility that the radiaxial calcite filled cavities. Chambers of nautiloids are filled with both para-axial and radiaxial calcite (fig. 21). Most contain para-axial 14 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA LITHOLOGY 15 calcite; para—axial calcite occurs only as filling of nautiloid chambers or as filling of fractures that cut all other lithologies. Radiaxial fibrous calcite surrounds all shells except where they are still imbedded in calcilutite; one nautiloid chamber shown slightly left of center of figure 21 is mostly filled with radiaxial calcite but also contains calcilutite. Only the para-axial mosaic is cer- tainly a cavity filling. The radiaxial calcite may have resulted from the conversion of calcilutite surrounding and in some places partially filling shells. TRACE-ELEMENT ANALYSIS A semiquantitative six-step spectrographic analysis was run by A. T. Myers on both the calcilutite and the radiaxial fibrous calcite in order to determine whether a significant chemical difference existed between them. The results (Myers, written commun., Dec. 1, 1971) were as follows: USGS colln. D2325 C0 Calcilutite Radiaxial calcite Fe (percent) _________ .01 .003 Mg _______________ 0.3 0.3 Ca _______________ >10 >10 Ti _______________ 0.01 Detected Si _______________ 2.0 .02 Al _______________ 0.15 0.01 Mn (ppm) ___________ 200 70 Ba ______________ 70 50 Cr ______________ 10 5 Cu _______________ 15 5 Mo _______________ 10 Not detected Pb _______________ 20 15 Sr _______________ 500 500 V ________________ 20 20 FIGURE 14 (facing page). — Polished sections of zebra limestone. USGS colln. D2325 CO. A, Ordinary light; X 6. Two complete cycles of laminated sediment. Only part of zoning in radiaxial fibrous calcite (rax) is symmetrical about median seam. Lowest zonal band is lower tier of calcite (tr) as in figure 8. Calcilutites ca; and cm are difficult to differentiate under ordinary light. B, Polarized light; part of view shown in A; X 9. Within radiaxial fibrous calcite (rax) crystal faces are intergrown; shadowy relicts of small crystals are visible within larger crystals; the smaller, younger crystals tend to be at the top and bottom margins. Median seam (ms) is relatively smooth. Quartz, K—feldspar, and goethite stand in relief and are most abun- dant in fossiliferous calcilutite (cal), less abundant in pelletoid calcilutite (032), and rare in radiaxial calcite. Lower tier of calcite (tr) is clear at middle but fades to right, where peloids have already been recrystallized to spar. C, Polarized light; small area of lower half of middle radiaxial fibrous calcite (rax) in B; X 40. Peloids (P) are relict, not having been recrystallized; therefore, the top of the lower calcite tier (tr) is distinct, and this tier may be comparable to tiers beneath C83 in figure 16B. Pelletoid calcilutite (C82) under high magnification and polarized light appears crystalline intermediate between coarse radiaxial calcite and more finely crystalline ca). D, Polarized light; part of View shown in B and C. X 40. Quartz and K‘ feldspar stand in relief; they are most abundant in calcilutite cal, less abundant in ca, and very sparse in radiaxial calcite (tr and rax). FIGURE 15. — Thin section of three cycles in zebra limestone. Bright points within fossiliferous calcilutite (cal) are crystals of quartz and K-feldspar. Relict peloids and small bodies of calcilutite in radiaxial calcite (rax) may be comparable to those above calcilutite caa in figure 13. USGS colln. D2325 CO. The calcilutite is consistently richer in trace elements than the radiaxial calcite, but the amounts concerned are not particularly unusual. The higher percentage of silicon in the calcilutite can be credited to the presence of quartz and feldspar. INSOLUBLE RESIDUE Photographs taken in polarized light of a polished sur- face of a sample from USGS collection D2325 CO (fig. 14D) indicate the presence of material that is harder than calcite, particularly in the fossiliferous calcilutite (cal). Microscope examination of thin sections suggests that particles of quartz and an opaque mineral are the more resistant minerals seen in relief on the polished sur- face. Part of one sample from collection D2325 CO dissolved in dilute hydrochloric acid yielded 1.3 percent by weight insoluble residue. A second sample yielded 3.33 percent by weight insoluble residue. The particles of residue are almost entirely in the silt sizes; some particles are as large as very fine sand. By volume about 70 percent of 16 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA FIGURE 16. (above and upper right) — Layers of fossiliferous calcilutite (cal) separated partly by radiaxial calcite (rax) and partly by seemingly crossbedded pelletoid calcilutite (caa), which also grades into radiaxial calcite. Pockets of calcilutite ca; are overlain by a thin layer of radiaxial calcite (tr) just as in undisturbed parts of the zebra limestone. (See, for example, fig. 8.) Lower tier (tr) of radiaxial calcite may be analogous to that in figures 8, 13, 14A, and 14 C. USGS colln. D2325 CO. A, Collodion peel. B, Polished surface. these particles are quartz, slightly fewer than 25 percent are potassic-feldspar, and about 7 percent are opaque. According to R. E. Wilcox (written commun., Feb. 25, 1972), “Most of the particles of quartz and feldspar are made up of nuclei, probably detrital, about which have grown mantles of authigenic material to provide crude to well-developed euhedral crystal forms.” According to B. F. Leonard III (written commun., Jan. 17, 1972), most of the opaque mineral is goethite. Assuming that the typical distribution of these insolu- ble minerals is shown on the polished section (figs. 143, 14D), we estimate that 70 percent are in the fossiliferous calcilutite (cal) and 25—30 percent are in the overlying pelletoid material (caz). A trace (1—5 percent) is present in the radiaxial calcite, which makes up more than half the total rock. FIGURE 17. — Radiaxial fibrous calcite (rax) associated with seemingly pelletoid calcilutite between layers of calcilutite cal. Radiaxial calcite on right would not be discontinuous if it had lined a cavity prior to filling by calcilutite. Radiaxial calcite appears to have grown along more permeable sedimentary contacts. Pelletoid sediment may have provided permeable channel. Or pelletoid texture may be structure grumeleuse, resulting from aggrading neomorphism. As shown in figure 16, depressions above cal may be filled by pelletoid calcilutite (082); much of space between layers of ca] is filled by typically equigranular 083. Collodion peel. USGS colln. D2325 CO. LITHOLOGY 17 In laminated sediments in Shark Bay, western Australia, quartz constitutes 1 percent of the sediment in the intertidal zone and 2—20 percent in the supratidal zone (Davies, 1970, table 1, p. 177). The small content of quartz in the laminated sediments at Meiklejohn Peak is therefore not unusual. Concerning Shark Bay, Davies (1970, p. 186) further noted that “pyrite or a related iron sulphide” is present in laminated sediments from the intertidal zone and that “gypsum is absent.” Both these conditions are also met by the laminated facies from Meiklejohn Peak. After an X—ray microprobe analysis of a sample of this Ordovi- cian zebra limestone, R. C. Surdam (oral commun., Apr. 5, 1972) concluded that there is no difference in iron, calcium, or magnesium content between the calcilutite layers and the radiaxial fibrous calcite, and that the chemical suite present does not indicate deposition in a hypersaline or supratidal environment. FIGURE 18. (left and above) —— Polished sections showing radiaxial fibrous calcite (rax) divided by nearly flat median seam (ms). Seam above calcilutite C83 is jagged; terminations of crystals below seam in C33 also seem jagged. Top of calcilutite caa partly converted to radiaxial calcite by aggrading neomorphism below seam. If conver- sion of C33 continued, the tier of calcite (tr) below caa would corres- pond to lower tier (tr) in figures 8, 14B, and 14C. Polarized light. USGS colln. D2334 CO. A, ~>< 20. A part of the sample shown in figure 41, this surface parallels and is 7 mm distant from that in the samples illustrated on figures 13 and 40. B, Enlarged view of part of A, X 39. A sample (USGS colln. D2343 CO) from the un- laminated upper part of the mudmound, 28 m (86 ft) above the base yielded a residue which is 2.16 percent by weight of the original sample. X-ray analysis by W. N. Sharp indicated that quartz and mica, probably illite, are the prominent constituents of this residue, and that some hematite is present. UNDERLYING LIMESTONE Northward from the mudmound the underlying calcarenite is exposed in a deep gully beyond which it is displaced westward by a fault. This calcarenite was in- itially considered to be a detrital apron derived from the mound; however, its continuity north and south beneath the zebra limestone of the mound shows it to be a part of the limestone 9 m (30 ft) thick that lies stratigraphically below the mound and that extends over a wide area as the topmost part of the Paiute Ridge Member of the Antelope Valley Limestone. In the lower third of the 9- metre-thick limestone a crudely laminated interval (figs. 22, 23) resembles the zebra limestone but lacks the predominance of radiaxial calcite and the striking cyclic nature. 18 FIGURE 19. — Thin section from small, irregular “cabbagehead” mass composed of calcilutite cal and radiaxial fibrous calcite (rax). About 10 feet (3 m) above base of mound at south end of outcrop. USGS colln. D2454a CO. In the calcarenite (fig. 24) recrystallization has obliterated the boundaries of skeletal grains making planimetric measurements of the original composition of the rock impossible. Some echinodermal material is pre— sent and ostracodes appear to be abundant. A collection (USGS colln. D2390 CO) was made at the bottom of the crudely laminated facies at a point where coarse sparry calcite is a minor constituent in the hope that the crude laminae might be analogous to the calcilutite layers of the overlying zebra limestone. A greater proportion of the rock is composed of fos- siliferous calcilutite (figs. 22, 23). Layers, possibly of pel- letoid origin (figs. 238, 25), about 3 mm thick are in- terbedded and seem to be equivalents of the radiaxial calcite layers in the overlying zebra limestone. Spicules are present in clumps within the fossiliferous calcilutite (figs. 26, 238). No such spicular clumps have been found in the zebra limestone. The poorly laminated limestone lacks the uniform cyclicity of the zebra limestone. The fossiliferous calcilutite cal forms rather irregular layers 5—30 mm thick. Some of these layers have been bioturbated and LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA locally the material has invaded lower layers in what ap— pear to have been liquid protrusions from above; in other places there is as much evidence for upward flow of sedi- ment as for downward flow. In still other places the sedi- ment that appears to have been fluid in one spot has clearly been fractured a few millimetres away. Where fossils are abundant a considerable amount of pelletoid calcilutite is protected by “umbrellas” of larger shells. Within the fossiliferous calcilutite cal are numerous scattered dark-gray shapes in which spicules are promi- nent (figs. 238, 26). Associated with the spicules is a fine anastomosing network of clear sparry calcite. After ex- amining samples of the spicules and network, Robert Finks reported (oral c0mmun., Nov. 9, 1972) that the straight spicules are probably root tufts of a lithistid sponge for which there is no other immediate evidence. He also thought that the anastomosing network of calcite was too irregular and devoid of overall geometry and spicular shape to belong to any sponge now known in the lower Paleozoic. In his opinion, this fine network might best be explained as the matrix in an original pel- FIGURE 20. — Thin section from zebra limestone facies adjacent to “cabbagehead” mass shown in figure 19. Laminae composed of alter- nating calcilutite ca; and pelletoid calcilutite (€82). Radiaxial calcite (rax) is incipient below cal. Laterally layer C82 becomes radiaxial fibrous calcite. At bottom of section note scour—and—fill(?) in cag. USGS colln. D2454b CO. LITHOLOGY 19 FIGURE 21. — A small part of a vertical section of a pocket or “nest” and partly with radiaxial calcite. Presence of calcilutite may have filled with shells of nautiloid cephalopods. All the space surrounding been essential to formation of radiaxial fibrous calcite. Para-axial shells not now occupied by calcilutite (ca) is radiaxial fibrous calcite calcite (pa) occurs outside chambers only in fractures which cut all (rax). Almost all chambers are filled with blocky para-axial calcite other lithologies. Collodion peel, photographed by transmitted light mosaic (me). A few chambers are filled with radiaxial calcite (rax). with one polarizer. USGS colln. D2389 CO. One chamber slightly left of center is filled partly with calcilutite letoid calcilutite (Beales, 1958, pl. 1, figs. 1—3); Ross here texture found in the thin (2—5 mm thick) layers that are considers this interpretation less likely for the network interstratified with the fossiliferous calcilutite (figs. 23A, associated with spicules than for the somewhat similar 25A, 258). Both textures are shown in figure 238. 20 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA FIGURE 22. — Polished section of poorly laminated limestone below mudmound. Dark-gray shapes contain spicular roots (sr) of sponges. Thin layers are pelletoid. Some thin layers and sparry calcite are fill- ings of internal cavities. USGS colln. D2390 CO. In texture the thin interlayer shown in figure 27 somewhat resembles the late Paleozoic hydrozoan Palaeoaplysina described by Davies (1971, fig. 4 B, E). Dr. Davies kindly examined the sample illustrated and expressed the opinion (written commun., Nov. 3, 1972) that it was more readily interpreted as a pelletoid calcilutite with sparry matrix; however, he also sug- gested that the texture might be interpreted as a clastic biomicrite with algal(?) borings. Some of the thin layers in this crudely laminated facies contain sparry calcite and may have a different origin. These tend to be discontinuous and to cut across the fossiliferous calcilutite cal. Many, but not all, appear to have been open fractures in the calcilutite, some cut- ting steeply across beds and some running nearly paral- lel. These presumed fractures are partly filled with geopetal sediment much of which was pelletoid. Most of the calcite bodies are entirely sparry para-axial mosaic; a few have radiaxial calcite enveloping the para-axial mosaic. The spar seems to have been formed in small discontinuous cavities. However, surrounding many of these calcite bodies is a halo of goethite at the contact with the calcilutite; such halos are common in replace- ment textures and it is difficult to explain the concentra- tion of goethite within floor, walls, and ceiling of a cavity. MASSIVE CORE FACIES OF THE CARBONATE MUDMOUND By VALDAR J AANUSSON MACROSCOPIC CONSTITUENTS In the massive limestone forming the core of the mound, three main macroscopic constituents (here defined as structures larger than about 0.5 cm) can be distinguished: (1)sparry calcite bodies (Stromatactis), (2) skeletal particles, and (3) the rest of the rock, macroscopically mostly of fine-grained appearance. STROMA TA CTIS The sparry calcite bodies are elongated in cross section and distributed throughout the core facies with the long axis approximately parallel to the former depositional surface. Their thickness rarely exceeds 2 cm. The floor in most examples is fairly even, whereas the roof varies from even to digitate. Examination of these structures is made difficult by the sparsity of natural plane surfaces that are perpendicular to the depositional surface. Only a few such surfaces of sufficent size were found in the outcrop area of the mound core. Point counting in the field on three surfaces, each approximately 0.25 m2 and all located in the uppermost part of the mound, showed that the macroscopic sparry calcite bodies form about 15, 17, and 18 percent of the volume of the rock, respec- tively. Examination of thin sections revealed that the sparry calcite bodies are composed of radial calcite (Bathurst, 1959) — that is, radiaxial fibrous calcite with undulose extinction, irregular, often highly digitate in- tercrystal boundaries, and numerous subgrains. The mode of occurrence, general shape, and microstructure of the sparry calcite bodies resemble those in what is generally known as Stromatactis, and the same term is used here for the structures from the LITHOLOGY 21 core of the Meiklejohn Peak mound. The Stromatactis in the core differs from the sparry calcite layers in the zebra limestone by the much more irregular shape of the in- dividual bodies, by their seemingly irregular distribu- tion, and by their smaller size. The maximum observed length of a cross section of Stromatactis in the core of the mound is 24 cm; most are considerably shorter. The microstructure of Stromatactis and the sparry calcite layers in the zebra limestone, on the other hand, is very similar. The radiaxial crystals or mosaic of crystals from FIGURE 23 (left and above). — Polished and thin sections of sample of poorly laminated facies. USGS colln. D2390 CO. A, Polished section showing fossiliferous calcilutite (ca) separated by thin layers per- forated by anastomosing tubes (tu). Tubes now filled with calcite mosaic, which may be matrix around pellets. Large dark-gray mass was produced by sponge and contains spicules (sp) of Pyritonema(?) and meshwork (M) of uncertain origin shown enlarged in B and C. B, Brachiopod(?) attached to long spicule (sp) paralleling plane of sec- tion appears as round spot in dark spicular mass in A. Discontinuous meshwork (M) below spicule may be sponge or calcite matrix around pellets of calcilutite. Abundant small white circles near top of photomicrograph are cross sections of spicules. Texture at bottom may be sparry calcite matrix around pellets or structure grumeleuse. Thin section. C, Thin section parallels B but does not cut brachiopod or large spicule. Large white circles are cross sections of Pyritonema(?) spicules. Peculiar discontinuous meshwork in center of view also appears below brachiopod in B. the floor and roof normally meet in a distinct seam (fig. 28). None of the Stromatactis examined in the core of the mound exhibited a central, “residual” cavity or a well- defined central filling with para-axial (Bathurst, 1964) calcite. The absence of such structures may depend on the relatively small dimensions of Stromatactis in the Meiklejohn Peak mound or on their sparsity. The origin of Stromatactis has been, and still is, very much disputed and numerous different explanations have been proposed. Comparison of Stromatactis from the core of the Meiklejohn Peak mound with similar 22 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA . 1 FIGURE 24. — Coarse calcarenite below mudmound, about 60 m (200 ft) from the south end of outcrop. USGS colln. D2332 CO. structures from various other Paleozoic carbonate mounds suggests that the main factor in the origin is common to all. The material available for comparison includes Stromatactis from the Devonian of the Dinant Basin in Belgium (including samples from the quarries around Philippeville, the type area for Stromatactis Du- pont, 1881), from the Ordovician of Sweden, and from the Carboniferous of England and Ireland. The material from the Meiklejohn Peak mound alone is too limited for forming a satisfactory basis of an extensive discussion of the whole problem of Stromatactis. Some observations presented here are deemed to be pertinent for under- standing the origin of the structures. Several observations indicate that, irrespective of what the origin of Stromatactis may be, the shape and size of the radiaxial crystals, and possibly the whole radiaxial mosaic as such, is not original but a result of diagenetic processes. The radiaxial mosaic has demonstrably incorporated skeletal particles (Black, 1952; Orme and Brown, 1963; Jaanusson, 1975) and grown at their expense. The material from the core of the Meiklejohn Peak mound shows numerous examples of this phenomenon (figs. 29, 30). A particularly interesting example (fig. 31) shows carapaces of ostracodes that oc- cur in various stages of incorporation into radiaxial mosaic within a Stromatactis. Some of the carapaces are situated at the median seam of the radiaxial mosaic. The radiaxial mosaic has probably also assimilated fine- grained matrix, although this cannot be proved. A mosaic which is very similar to the radiaxial mosaic can be shown to result from recrystallization (increase in the size of crystals) of finely fibrous normal calcite mosaic and, possibly, when aragonite is transformed into calcite in situ in closed space. Probably one of these processes was operative in the formation of radiaxial mosaic on Stromatactis. Subsequently, the mosaic grew by incorporating adjoining grains or extending into voids. The core of the Meiklejohn Peak mound contains numerous narrow veins filled with calcite. Some of the fissures probably formed early during the diagenetic history of the mound. The normal vein filling of these fis- sures is para-axial sparry calcite but where narrow veins cut a Stromatactis the calcite in the vein filling is generally radiaxial and in optical continuity with the crystals in adjoining radiaxial mosaic (fig. 15). This phenomenon has been observed in almost all carbonate mounds with Stromatactis from which material has been available. Orme and Brown (1963) suggested that this proves the mosaic of Stromatactis to postdate the forma- tion of fissures, but in my opinion, a more plausible ex- planation is precipitation in a fissure with the crystal structure controlled by that of the wall. There are exam- ples in the core of the Meiklejohn Peak mound where a vein cuts an echinoderm grain and the filling of the vein at that place is calcite in optical continuity with both halves of the echinoderm grain. Thus the present radiaxial mosaic of Stromatactis is most likely secondary. Determining the primary struc— ture is difficult. This type of mosaic occurs in what demonstrably have been cavities in sediment (for exam- ple, in closed shells of brachiopods), but it is also known to replace original organic structures (stromatoporid coenosteums and stromatolitelike organic structures in the Upper Ordovician carbonate mounds of Sweden). The presence of a distinct median seam might indicate filling of a cavity, because such seams have not been observed to be associated with recrystallization into a radiaxial mosaic. MACROSCOPIC SKELETAL PARTICLES Macroscopic skeletal particles are scarce in most of the core of the Meiklejohn Peak mound. The point- counted surfaces mentioned previously (p. 20) did not include any skeletal particle of macroscopic size. Macrofossils tend in the core facies to be assembled into pockets or channellike structures but their importance relative to the total volume of the core of the mound is small. LITHOLOGY 23 .;x FIGURE 25. — Pelletoid(?) texture in thin light-gray layer similar to that below dark spicular mass in figure 23A and to that at bottom of figure 23B. Thin sections of poorly laminated facies below mudmound. USGS colln. D2390 CO. A, Vertical section. B, Horizontal section. FINE-GRAINED LIMESTONE In some of the samples several generations of fine- grained limestone can be distinguished; other samples are more homogeneous and contain possibly only one generation of micrite. The various generations differ mainly by microstructure and contact relations. Dif- ferences in color are so slight that recognizing the genera- tions in the field is difficult, unless the rock surface is etched or stained, or both. The seemingly earliest generation of the fine—grained limestone (Li 1) is the commonest and resembles layer cal in the zebra limestone. It normally consists of cryp- tocrystalline micrite with varying amounts of skeletal grains (fig. 32). The skeletal grains of sand size are mostly more abundant than in cal, and so the sediment is coarser grained than the limestone of the cal layers. What probably is the next generation (Li 2) has a more varied composition. Micrite similar to that of Li I is common, but the limestone also contains layers or patches of peloid limestone similar to that in 032 of the zebra limestone recrystallized portions, and spar, radiaxial as well as para-axial (fig. 32). Where typically developed, the boundaries between Li I and Li 2 are well defined (fig. 32). The complex dis- tribution pattern of both generations suggests a system of burrows or borings in Li 1 filled with Li 2. With the material at hand, it is difficult to prove whether burrow- ing or boring was responsible for the pattern—whether the cavities had been formed in soft sediment or in a rock. Some observations, such as shells protruding from Li 1 into Li 2, suggest burrows, but the evidence is in- conclusive. ' The succession of the generations Li I and Li 2 is not always clear, and further studies may reveal a more com- plicated pattern of relations. In the available samples it is difficult to see any clear spatial relation between the two generations of fine— grained limestone and individual bodies of Stromatactis, comparable to that between the layers of spar and C32 layers of calcilutite in the zebra limestone or between Stromatactis and layers considered as internal sediment described from English and Irish Carboniferous car- bonate mounds (Bathurst, 1959; Schwarzacher, I961; Lees, 1964). In this respect further studies are necessary. The third generation of fine-grained limestone (Li 3) is relatively rare but distinctive. It mostly consists of a 24 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA FIGURE 26. ! Polished section showing one of the dark masses illustrated in figure 22 (sr) where large spicules (sp) are par- ticularly abundant and probably constituted roots of a sponge. USGS colln. D2390 CO. relatively homogeneous, fine-grained microspar with few skeletal grains. It generally occurs as a layer or fissure- filling at a steep angle or perpendicular to the former depositional surface. The boundaries against Li I and Li 2 are sharp and accentuated by a thin layer of sparry, para-axial calcite, 0.07—0.15 mm thick. The relation of Li 3 to Stromatactis is illustrated in figure 33. There a small Stromatactis is faulted into four pieces, only three of which are visible in the photomicrograph (the fourth piece was just outside the right field of View). The almost straight margin of the Stromatactis to the right is formed by the fault which separates the third and fourth pieces. It cuts through a normally developed radiaxial mosaic in the same way as the first and second faults do. Faulting at the right formed a fissure, 2.6 to 2.9 mm wide, which is now filled with the limestone of the generation Li 3. The faulting also caused a vertical displacement of about 2.5 mm. This example shows conclusively that Li 3 postdates the formation of Stromatactis and that the general struc- tural pattern of the radiaxial mosaic must have been fully developed before the deposition of Li 3. The sedi- ment above and below the Stromatactis may still have been somewhat friable when faulting took place, as judged from the smooth curvature of the margin of the fault at some places (fig. 33). At other places, par- ticularly where the wall is coated with spar, the wall of the fissure is straight. In parts of the mound the core is transected by macroscopically distinguishable, locally irregular fis- sures of varying width filled with brown-weathering limestone. This limestone obviously is still later than Li 3. MICROSCOPIC CONSTITUENTS The microscopic composition of the limestone in the core of the Meiklejohn Peak mound was studied by modal analysis in a series of thin sections. The samples from which thin sections were prepared were inten- tionally taken from the macroscopically fine-grained parts of the core. Thus, the quantitative data apply only to this macroscopic component, comprising 80—85 per- FIGURE 27. — Vertical section across thin layer of calcilutite perforated by irregular calcite-filled tubes, similar to those shown in figure 25. These may be sparry matrix around indistinct pellets or algal bor- ings in calcilutite. USGS colln. D2390 CO. LITHOLOGY cent of the volume of the core. The general objectives of the study of microscopic carbonate constituents, as well as the methods, were reviewed and discussed by Jaanus— son (1972). The series of samples for constituent analysis was col- lected by Ross in 1970 along a section through the middle part of the core of the mound (samples D2341 CO to D2349 CO; fig. 4). In two thin sections (from D2341 CO and D2346 CO) recrystallization has nearly obliterated original grain boundaries; these thin sections were not measured. One thin section, from the sample D2349 CO, showed two clearly different rock types (calcilutite and calcarenite) separated by a sharp boundary; the areas of both types were measured separately. Patches of rock within almost all thin sections showed various degrees of aggrading neomorphism, and measurements involved more subjective judgments than is usual for such modal analysis. Still, qualitative study of peels from additional samples collected by Jaanusson in 1972 seems to in- dicate that the quantitative data are reasonably representative for the fine-grained limestone of the mound core. Owing to difficulties in distinguishing the generations Li I and Li 2 in most thin sections, these limestones are not discussed separately. The limestone of the genera- tion Li 3 was not included in the measured area. R «I , '.', ‘. i ' -\ .' , FIGURE 28. — Thin section of a “microstromatactis” (ms) from the core of the mound. The surrounding limestone is either first(Li1)or se- cond (Li 2) generation, except to the left, where a third generation of limestone (Li 3) is in contact with the radiaxial calcite. USGS colln. D2345 CO. Photograph by U. Samuelson of Naturhistoriska Riksmuseet. Lithologic sequence explained on p. 23. “mam. ‘* ' is. ' 9 ‘ ., ,. a . FIGURE 29. — Plate of an otherwise unidentified echinoderm partly converted to radiaxial fibrous calcite (rax). Thin section. USGS colln. D2345 CO. MAIN MICROSCOPIC CONSTITUENTS In the core of the Meiklejohn Peak carbonate mound, the main microscopic constituents (table 1) of the limestone are (1) sparry calcite, (2) skeletal sand, and (3) matrix. Sparry calcite was considered as a separate con- stituent only when a well-defined mosaic of calcite crystals was 0.1 mm long or larger. Skeletal sand is defined as those skeletal particles 0.1 mm long or larger in thin section (Jaanusson, 1952, 1972). The matrix — the rest of the rock — consists of material of various origin, such as skeletal grains smaller than 0.1 mm, car- bonate mud, terrigenous mud, and probably at least 40 percent calcium carbonate cement. Much of the matrix is recrystallized into an ultramicroscopic mosaic of calcite crystals whose original constituents are no longer recognizable. Some thin sections contain dark, cryptocrystalline, pelletlike spots, almost completely surrounded by spar. In places they seem to represent indurated pellets, in others, it is difficult to prove whether they are depositional features (represent pelletlike sedimentary 26 grains) or are phenomena developed during recrystal- lization (structure grumeleuse). Many of the spots are shorter than 0.1 mm and have indistinct boundaries. In the thin section D2343 CO, the peloids 0.1 mm long or larger comprise 8 percent of the measured area; in all other thin sections, they are much rarer or are absent. The peloids are tabulated as matrix. Modal analyses show that the fine-grained parts of the rock consist, in average, of 37 percent sparry calcite, 7 percent skeletal grains, and 56 percent matrix. SPARRY CALCITE About 44 percent of the microscopic sparry calcite is radiaxial and the rest is para-axial. The microscopic bodies of radiaxial sparry calcite (fig. 28; Ross, 1972, fig. 7) resemble Stromatactis in microstructure and general shape. For convenience they are termed here FIGURE 30. — Thin section from USGS colln. D2342 CO showing fossil ostracodes (0s) and pieces of calcilutite (ca) relict in radiaxial fibrous calcite (rax). LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA “ :. ,t .. ‘, -" FIGURE 31. — Thin section of a “microstromatactis” with in- corporated ostracode carapaces (arrows) in various degrees of recrystallization into radiaxial calcite. USGS colln. D2344 CO. Photograph by U. Samuelson, Naturhistoriska Riksmuseet. i FIGURE 32. — Thin section from the core of the mound showing distribution of two generations of limestone (Li I and Li 2). The large sparry calcite mosaic in Li 2 is radiaxial, as is the spar inside a partially closed brachiopod shell (br) which is mostly filled with micrite. USGS colln. D2343 CO. Photograph by U. Samuelson, Naturhistoriska Riksmuseet. LI'I‘HOLOGY 27 “microstromatactis.” There are all gradations in size from microstromatactis (with a thickness less than 1 mm) to Stromatactis of macroscopic dimensions. Other microscopic mosaics of radiaxial calcite have less regular outlines and lack a clear median seam. The nature of such mosaics is not always clear. Some represent peripheral cuts of microstromatactis; others suggest for- mation by aggrading neomorphism. Where former original cavities can be demonstrated, such as within closed carapaces of ostracodes and closed shells of brachiopods, the cement is mostly radiaxial calcite. Some such intragranular cavities contain a central filling of para-axial calcite which is later than the radiaxial calcite. FIGURE 33 (right). — Thin section of a “microstromatactis” which is faulted into four pieces (the fourth piece is to the right just outside the lower right corner of the photograph). The fissure at the right is filled with limestone of the generation Li 3. USGS colln. D2343 CO. Photograph by U. Samuelson, Naturhistoriska Riksmuseet. TABLE 1. — Relative proportions of microscopic constituents for six collections taken stratigraphically through center of the carbonate mudmound at Meiklejohn Peak [Positions of collections indicated in fig. 4. Collection D2349 CO from nodular limestone immediately covering mound] Sparry calcite USGS - Para-axial Skeletal Meters Ft, Colln. No. Matrix ':' RadIaXIal sand D2349 .- 80— 02348 I 02347 70— 60_-200 I D2345 J I 50— . 02344 _ p 40-4 30——100 I D2343 - 20— I 02342 . - 10- —— f 0 0 $333: 0 20 4o 60 80 o 20 40 60 0 20 40 PERCENT 28 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA TABLE 2. — Relative proportions of organic components of the skeletal sand fraction shown in table 1, for six collections taken stratigraphically through center of the carbonate mound [Positions of collections indicated in figure 4. Collection D2349 CO from nodular limestone immediately covering mound] USGS Spiculae— like Trilobita Bryozoa Meters Ft. Colin. No. Echinodermata rods Ostracoda Brachiopoda Mollusca Nuia Algae Indeterminate 02349 “ _ . I. i L b 80- D2348 I D2347 I L 70— 60_— 200 I 02345 fi * I I F 50— I 02344 _ - I I I I — 40— 30-—100 I 02343 — - I h I I _ 20— I D2342 — F I 1 10— 0—— 0 0 20 40 60 0 20 40 0 20 0 10 0 10 0 10 0 10 0 20 0 10 0 20 PERCENT Para-axial calcite frequently forms relatively coarse mosaics similar in size and shape to microstromatactis except that the outline tends to be much more irregular and a median seam is seldom present. Such para-axial mosaics may belong to a generation different from that of radiaxial calcite but this could not be proved. Where both types of sparry calcite mosaics are in contact, the boundary between the mosaics is generally transitional. However, this may be a phenomenon of the same nature as the development of radiaxial calcite in those parts of narrow veins that cut a radiaxial mosaic; the structure of the crystals acting as seeds may have become propagated beyond the original boundary of a mosaic. Radiaxial and para-axial sparry calcites tend fre- quently to be concentrated in different patches — in dif— ferent samples or in different parts of a thin section. This affects, then, not only the relatively large mosaics but also intragranular and intergranular cement. Near the top of the mound para-axial sparry calcite predominates, whereas in the lower and middle parts of the mound radiaxial calcite predominates. The implica- tion of these differences in distribution is not clear. Skeletons which were originally aragonitic (here regarded as part of the skeletal sand) have mostly been replaced by a para-axial sparry calcite even in areas where radiaxial calcite dominates. This might suggest that the transformation of aragonite to calcite was later than the formation of intragranular radiaxial spar. However, there are exceptions and further studies are needed. Much of the microscopic para-axial sparry calcite forms irregular patches of relatively fine-grained mosaic which obviously formed by aggrading neomorphism, as defined by Bathurst (1971, p. 481). COMPOSITION OF SKELETAL SAND In the thin sections studied the amount of skeletal sand varies between 4 and 11 percent (table 1), and thus the rock can be characterized as a calcilutite. Calcarenitic areas do occur, particularly in pockets abounding in skeletal remains, but they are quan- titatively unimportant. The sediment was mud- supported. Within the skeletal sand (table 2) echinoderms LI'I‘HOLOGY 29 FIGURE 34. — Spiculaelike skeletal grains showing rays composed of para-axial sparry calcite. USGS colln. D2343 CO. Photographs by U. Samuelson, Naturhistoriska Riksmuseet. A, Three of four rays shown; X 35. B, Four rays; the ray to the left merges into radiaxial sparry calcite; X 100. predominate by volume. Second in volume are straight spiculaelike rods. Some show four short rays (fig. 34B) and thereby resemble pentact megascleres 0f sponges, whereas others have four relatively long rays (fig. 34A). In thin sections the “spiculae” mostly occur as circular to elliptical cross sections of rods. Some may be Pyritonema, which is similar in microstructure and general dimensions. The original composition of the skeleton was probably aragonite because the “spiculae” are now a relatively coarse para-axial mosaic of sparry calcite. This often hampers recognition of the spiculaelike skeletal grains; some spiculae either have not been distinguished from sparry calcite or have been confused with other groups which originally had aragonitic skeleton, such as mollusks or an enigmatic tubelike microfossil with a very thin wall. Many tubes are filled with sparry calcite and then the wall is difficult to recognize. Aragonite as a skeletal mineral is, to the best of our knowledge, not reported among sponges and the attribution of the spiculaelike skeletal grains is at present uncertain. By volume ostracodes predominate over trilobites. In this respect composition of skeletal constituents in the core of the Meiklejohn Peak mound differs from that of many other Ordovician limestones. A component in the skeletal sand of the Meiklejohn Peak mound, not known from European Ordovician limestones, is the enigmatic Nuia, widespread in Lower and Middle Ordovician limestones of North America (Toomey and Klement, 1966; Johnson, 1966; Toomey, 1967). Originally the skeleton of this possible alga probably was calcite. COMPARISON WITH SELECTED OTHER PALEOZOIC CARBONATE MOUNDS In several respects the Meiklejohn Peak mound resem- bles certain other Paleozoic carbonate mounds, such as the Ordovician Kullsberg and Boda Limestones of the Siljan district of Sweden and the Carboniferous “reef knolls” of the British Isles and Belgium. In all these mounds, as well as in several others, Stromatactis is an important macroscopic constituent. In the Kullsberg Limestone Stromatactis forms about half the volume of the rock and in northwestern European Carboniferous 30 mounds the importance of Stromatactis seems to be closely comparable. The Meiklejohn Peak mound differs in its smaller content (15—20 percent) of the sparry calcite bodies. In all these carbonate mounds microscopic sparry calcite is abundant, radiaxial as well as para-axial. In the Kullsberg Limestone it forms, in average, about a third of the volume of what macroscopically is fine- grained limestone (J aanusson, 1975). Microscopic sparry calcite is abundant also in thin sections examined from the northwestern European Carboniferous mounds. In the Meiklejohn Peak mound the macroscopically fine- grained limestone of the core abounds in sparry calcite, averaging about a third of the volume of the rock. A part of the sparry calcite may have been precipitated as ce- ment but a part is demonstrably formed by aggrading neomorphism, that is, by recrystallization of sedimen- tary particles and carbonate cement into sparry calcite mosaic after aragonite had either dissolved or been transformed into calcite. The carbonate mounds in Sweden, Belgium, and the British Isles consist of limestones of high purity (97—99 percent Ca003; in the core of the Meiklejohn Peak mound the amount of in- soluble residue is about 2—3 percent). A contributing fac- tor to the extensive recrystallization of such limestones may be the low content of terrigenous material, par- ticularly clay minerals (Zankl, 1969) and other im- purities which can hinder crystal growth. The skeletal constituents of the Meiklejohn Peak mound (table 2) differ markedly from those of the Or- dovician mounds of the Siljan district, Sweden; (J aanus- son, 1975), the Devonian Upper Koneprusy mounds in Bohemia (Jaanusson, 1975), and the Carboniferous “reef knolls” of the British Isles and Belgium. In all those car- bonate mounds fenestrate bryozoans (phylloporinids in the Ordovician mounds and fenestellids in the Devonian and Carboniferous mounds) are abundant and com- monly are the predominant component of the skeletal sand. Abundance of fenestrate bryozoans has been reported also from several other carbonate mounds (Pray, 1958; Cotter, 1965; and others). The abundant fronds of fenestrate bryozoans formed grain-supported sediment; not all the extensive intergranular voids became filled with sediment; some voids persisted and sparry calcite cement was subsequently precipitated therein. The fenestrate bryozoans may also have acted as sediment traps and contributed to the growth of the car- bonate mounds. In the Meiklejohn Peak mound bryozoans are very rare and no fenestrate form has been found. Bryozoans are much more common in the upper member of the Antelope Valley Limestone well above the mound but do not form an important constituent of the skeletal sand therein. Many Paleozoic carbonate mounds show evidence of LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA an early lithification. That a mound was lithified prior to deposition of the covering beds can be demonstrated in numerous examples. During final settling of a mound, broad dilatational crevices were formed in the then- lithified rock and subsequently filled with sediment from above. (See Isberg, 1917, 1918, and others for the Ordovi- cian carbonate mounds of the Siljan district, Sweden; Chlupaé, 1955, for Devonian Upper Koneprusy “reefs” of Bohemia; Philcox, 1963, for Carboniferous “reef knolls” of Ireland; Pray, 1965, for “banks” of Waulsortion type in the Sacramento Mountains, New Mexico; Cotter, 1965, for Mississippian “banks” of central Montana.) In the Ordovician carbonate mounds of the Siljan district, Sweden, it can be shown that not only was the mound lithified before formation of the crevices but the main features of the radiaxial mosaic in Stromatactis were already fully developed (Jaanusson, 1975). This proves that Stromatactis formed before development of the crevices. This relation sets an upper limit for the time of formation of the radiaxial mosaic in Stromatactis and the lithification of the mound. The Meiklejohn Peak mound apparently had no major postdepositional crevices comparable to those in other mounds. However, fissures are known which were ob- viously formed in a lithified sediment before the growth of the mound had ceased. The main features of the radiaxial mosaic of Stromatactis can be shown to have been developed when the mound was still growing. This agrees with the suggestions by Lecompte (1936, 1937), Schwarzacher (1961), and Lees (1964) as to the very early formation of the spar in Stromatactis. ISOTOPIC INTERPRETATIONS By IRVING FRIEDMAN The contrasting components of the zebra limestone, the beds covering the mudmound, and cement from breccias and fractures that cut both the mound and covering beds were analyzed for the isotopes C13 and 018 (tables 3, 4). All isotopic data are given in per mil. The (SC13 values are in respect to PDB (Peedee belemnite), and the 50la is given in respect to SMOW (standard mean ocean water). The isotopic data are given in per mil and are precise to 10.1 percent. The (SC13 of the samples is plotted on a frequency com- position diagram, figure 35. The 6C‘3 compositions of the calcilutite and coarse radiaxial and para-axial calcite are very similar and equal to the 6C13 of modern marine car- bonate. The small spread in (SC13 values indicates that they formed from a well-mixed carbon reservoir and that carbon from organic matter was an unimportant compo- nent. Organic carbon has a 6G” of —8 to —26 per mil, compared to present-day ocean water bicarbonate of -2 per mil. The covering beds, by contrast, show great variability in 6C”, and about half the samples have a ISOTOPIC INTERPRETATIONS 31 5C13 that is much lighter than any of the calcilutite or coarse calcite samples. This enrichment in C12 and the variability of 5C13 can be accounted for if the covering beds formed in an environment in which 002 from organic matter formed an important but varying part of the C02 reservoir, in addition to the normal atmosphere— derived ocean water bicarbonate. Keith and Parker (1965) found that the (5C13 of mollusk shells growing in marginal marine environments was dependent upon the accessibility of continental (land plant-derived) carbon, and that the 6C13 values of the samples approached the marine values as the sampling sites progressed from estuarine to marginal bays. The samples from marginal bays also show a variable 5C”, which Keith and Parker (1965, p. 127) attribute to “variable effects due to locally-produced C02 from decompositon of organic detritus, both continental and marine (Landegren, 1954) and from respiration of aquatic plants. Development of a local C13 deficiency by aquatic plants in waters with restricted circulation is consistent with observations by Wickman (1952).” That the covering beds contained organic matter is shown by the rich fauna (trilobites and particularly brachiopods). The decomposition of the soft parts probably contributed light C02 to the pore water. Restricted circulation resulted in some of this 002 being incorporated into the shells of organisms living in this environment. In the mudmound the similarity of 5C13 between the calcilutite and the coarse calcite indicates either that both formed from the same well-mixed, large carbon reservoir, or that one formed from this reservoir and the other formed by recrystallization of the first-formed material without a significant admixture of carbon from decomposing organic matter. The cause of this recrystallization can be found in the greater solubility of the fine particles as compared to the coarse spar crystals. The large surface area of the calcilutite particles results in greater surface energy and therefore less stability for those grains than for the large calcite crystals. The process of solution of the finest grains and growth of the coarse calcite would have been aided by small temperature oscillations, which would have occurred before deep burial of the mound. The process probably almost stopped when burial was deep enough to damp out diurnal and seasonal temperature fluctuations, and this may account for the persistence of some calcilutite. Possibly, the recrystallization was facilitated because some of the original calcilutite was not calcite but a form of calcium carbonate (aragonite or vaterite) which is thermodynamically less stable than calcite. The isotopic data for the cement of the dolomite fault breccia show that this material is similar in 6018 values to the calcilutite and spar, and probably formed in Or— dovician time. If it formed from fresh water, a lower 5018 value would be expected, and if it formed from seawater in later times, its 6018 value should be heavier. Figure 36 is a frequency 6018 diagram. The samples from covering beds and calcilutite have similar ranges of composition, from+20.3 to +225. The coarse calcite covers a slightly larger range, from +20.3 to +232, with a clustering of values between +21.8 and +232. The radiaxial fibrous calcite therefore has about the same 6018 value as do covering beds and calcilutite, but ap- pears to be slightly enriched by about 0.5 per mil in 018. One explanation for the rather small spread of 5018 values is that the carbonate formed under marine condi- tions with little influence from fresh water. Another ex- planation is that all the samples have been altered and their (5018 content modified. However, if alteration had occurred with water of different 6018 than the original seawater, we would expect either that the fine submicron calcilutite would have exchanged more than the very coarse millimetre-sized radiaxial calcite grains, or that they both would have exchanged completely. This latter case would have resulted in a very uniform 6018 content rather than the range of 5018 values found. The first case would have resulted in the calcilutite having much lighter 6018 values than the coarse calcite. The similarity TABLE 3. —- Isotopic analysis of beds of Antelope Valley Limestone around mudmound, shown in fig. 4 [All values are given in per mil] Sample No. 50” Description 5C” 3376-30_-_ Middle, thick-bedded Aysees Member about 50 ft above highest point of mudmound. D2335 CO. Dolomitized __ 29___ Dolomite fracture filling in D2335 CO 47 _ _ _ Covering beds of Ranger Moun- tains Member abutting bioherm, approx. 200 ft above base of mound. D2340 CO. Calcilutite fraction _____ 48 _ _ _ _______ do 13 _ _ _ Covering beds of Ranger Moun- tains Member abutting bioherm, about 195 ft above base of mound. D1970 CO —1.0 +21.4 (20.6) —1.6 +19.2 +20.6 +206 +21.8 49 _ _ _ Covering beds of Ranger Moun- tains Member, about 5 ft below D1994 CO, about 108 ft above base of mound D2326 CO. Dark-gray fine-grained limestone 50 ___ Covering beds of Ranger Moun- tains Member, about 5 ft below D1994 CO, about 108 ft above base of mound D2326 CO. Grayish-orange- weathering calcarenite _ _ _ —l.4 +215 —1.2 +21.5 32 ___ Covering beds, close to mound on south side, about 150 ft above base. Worm tubes. 40 ft south of D2334 CO. (71/30) —0.5 +21.7 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA TABLE 4. — Isotopic analysis of limestone from main core of mudmound and from zebra limestone l“Spar" refers to radiaxial fibrous calcite in this table. Samples are listed stratigraphically from top to bottom. All values are given in per mil] Sparry calcite Calcilutite Sample No. Description 6C“ 150‘3 (SC‘3 150‘3 3376—52 _____ Nodular limestone caé) ing mound. USGS colln. D2338 8. _____ _ _ .. _ _ _ +1.1 +21.8 52 _____ Para axial calcite in “birdseye” D2338 _________________ -0 9 +225 .. _ - _ _ _ 53 _____ TolgmostC outcrop of mudmound (71/2). _____________ ___ ___ -0.5 +22.1 51 _____ Topmost outcropp of mudmound (71/2). D2348 CO __________ - _ _ - _. _ —0.3 +19.6 32 _____ Cephalopod, chamber filling, 170 ft above base of mound (71/27). D2331 CO. Inner part ___________ -0.5 +21.4 _ _. - _ _ _ 32 _____ Cephalopod, chamber filling 170 ft above base of mound (71/27). D2331 CO. Outer part ___________ +0.3 +21.5 _ _ _ _ _ ._ 35 _____ Lens of covering bed litholo , 30 ft south of D2331 CO. D2333g50 _ __ _ _ _ +0 1 +22 3 27 _____ Laminated limestone, 143 ft above base of mound. D2344 CO _____ 0.0 +21.7 +0.4 +21.6 28 _____ Random sample, zebra limestone, less than 40 ft above base of mound. Spar ________________ —0.2 +21.5 _ .. _ _ _ _ Calcilutite, fossiliferous _____ _ - .. _ _ _. -0.2 +21.3 ________________ -0.1 +20.5 ___ ___ Calcilutite, fossiliferous _____ _ _ _ _ _ _ —0.2 +20.4 _________ do ________ ___ ___ —0.2 +20.6 Calcite vein, crosscutting _ _ _ _ — 1.8 +13.6 _ _ _ _ _ _ 43 _____ Calcilutite, fossiliferous1 _ - _ _ _ _ —0.6 +21.3 42 _____ Spar, in upper dark zone1 _______ —0.4 +21.9 .. _ _ _ _ ._ 36 _____ Spar, below median seam1 _______ —0.1 +21.8 - _ _ _ _ _ 37 _____ Spar, lower dark zone 1 ________ —0.3 +21.8 - _ _ _ - _ _____ Calcilutite, fossiliferous1 _ _. _ _ _ _ _ _ _ - _ _ _ —0.5 +21.0 44 _____ Sparl __________________ 0.0 +22.4 _ _ _ _ _ _ 45 _____ Calcilutite, fossiliferous1 _______ _ _ _ _ _ _ -—0.1 +20.9 46 _____ Sparl __________________ —0.2 +21.4 ___ ___ 41 _____ Calcilutite, fossiliferous1 _______ _ _ _ _ _ .. -0.5 +20.9 40 _____ Spar, two dark zones above seaml _ _ —0.3 +22.0 _ _ _ _ _ _ 39 _____ Spar, above median seam‘ ______ —0.1 —-22.4 _ _ _ - _ _ 18 _____ Spar and calcilutite lamina2 _____ —0.1 +22.4 — 1.0 +20.4 +0.1 +22.4 —1.0 +20.4 0.0 +22.7 —0.4 +20.3 +0.3 +22.6 0.0 +20.7 24 _____ Spar, upper half2 ____________ +0.1 +22.4 ._ _ _ _ _ _ 23 _____ Spar, lower half2 ____________ —0.1 +22.4 _ _ ._ _ _ _ 22 _____ Calcilutite, fossiliferrous2 _______ - _ _ .. _ _ —0.2 +207 21 _____ Spar, upper half‘ ____________ +0.2 +22.4 _ _ - _ _ _ 20 _____ Spar, lower half? ____________ +0.1 +22.1 _. _ _ _ _ _ 19 _____ Calcilutite, fossiliferousz _______ _ _ _ _ _ _ —0.4 +20.9 3379— 8 _____ Calcilutite, fine, light gray, wackestone3 _____________ _ .. _ _ _ _ +0.2 +223 9 _____ Spar, filling gastropod shell3 _ _ _ _ 0.0 +22.2 - .. - _ _ _ 10 _____ Spar, lining channel (?)3 ______ +0.2 +23.2 _ _ _ .. _ _ 7 _____ Calcilutite, fossiliferous adjacent to spar above seam4 _________ _ _ _ _ _. _ +0.4 +21.3 6 _____ Spar, outer zone, above seam‘ ._ _ _ +0.1 +22.4 _ _ _ _ _ _ 5 _____ Spar, at center, above seam‘ _ _ _ ._ +0.2 +20. 3 _ _ - _ _ - 1 _____ Spar, at center, below seam‘ _ _ _ _ —-0.1 +22. 0 _ _ _ _ _ _ 2 _____ Spar, outer zone, below seam‘ ._ _ _. +0.3 +22. 2 _ _ _ _. - 3 _____ Calcilutite, pelletoid‘ ________ _ _ _ _ _ _ -—0. 1 +21.1 4 _____ Calcilutite, fossiliferous“ _______ _ _ _ _ _ _ —0. 1 +21.3 3376-54 _____ Calcarenite, 15 ft below base of mound near center. D2330 CO _______ _ .. _ _ _ _ +0.3 +222 3376-55 _____ Para-axial calcite in fracture or seam cutting calcarenite _________ —0.7 +20.4 _ _ _ _ _ _ 3379-19 _____ Black calcilutite, fine1 ________ _ _ _ _ _ _ —0.7 +20.5 20 _____ Black calcilutite, coarse1 ______ _ _ _ - _ _ +0.4 +20.6 21 _____ Brown calcilutite1 __________ _ _ - _ _ _ —0.1 +20.5 22 _____ Black pelletoid calcilutite1 _____ - _ _ _ _ _ — 0.2 + 22,1 23 _____ Spar1 __________________ -1.1 +21.0 ___ ___ 24 _____ Brown calcilutitel __________ _. _ _ _ _ _ 0.0 +20.6 25 _____ Spar1 __________________ +0.2 +22.8 _ _ _ _ _ _ 'Zebra limestone, approx. 60 ft above base of mound. D2325 CO. This sample taken from a huge block, which lies askew to sur- rounding rock. A. J. Rowell (oral commun., 1971) proposes that this block may have tumbled from a part of the mound long since removed by erosion, perhaps in Ordovician time. This possibility needs consideration. ’Random samples, zebra limestone above base of mound at south end. "Channel fill, near base of mound. D2324 CO. ‘Below basal bed of " ", poorly ‘ ' ’ " ‘ (no ‘ ' ). D2327 CO. ORIGIN OF THE MUDMOUND 33 Dolomitized{ olornitized __ l i Covering beds T i Calcilutite O : \l NUMBER OF SAMPLES 0) | 01 | Radiaxial calcite l llll 0 | I -4 —3 —2 l — 1 l 0 +1 5cm FIGURE 35. — Frequency composition diagram for 60” in samples of calcilutite and radiaxial calcite from zebra limestone, from mound core, and from limestone of covering beds. in 6018 values between the two precludes any important amount of recrystallization in fresh (light 6018) water. The isotopic evidence from 6C13 and 6018 is consistent with the spar’s having formed by recrystallization of the fine-grained calcilutite. Further, this recrystallization must have taken place in water of the same isotopic com— position as that in which the calcilutite originally formed and at a temperature, in general, the same as or 5°—10°C cooler than the temperature of formation of the fine lime mud. From evidence already presented we have concluded that later alteration of the calcite by fresh water was un- important, and that the calcite (or aragonite) has the same 6018 content as it originally had. Shell material forming in the present marine environment has a 6015 value of +30 to +32 (Epstein and others, 1951). Forming carbonate of +21.5 per mil from water of 0 per mil (present-day ocean) requires a temperature of about 65°C. An alternate explanation for the +21.5 per mil is that the entire Ordovician ocean—or the part of it which formed the mudmound—had a 6018 value of —-10 per mil. This hypothesis is consistent with the 018 data of Perry and Tan (1972) on marine chert. Perry and Tan postulate a change in the oceans of 15 per mil from early Precambrian to Holocene. Lowenstam (1961) presented evidence for the relative constancy in 013 content of the oceans from late Mississippian to Holocene. Our data suggest that most of the 018 change in the oceans since early Precambrian took place in the Ordovician- Mississippian interval. ORIGIN OF THE MUDMOUND ORIGIN OF THE ZEBRA LIMESTONE By REUBEN JAMES Ross, JR. Because the laminated zebra limestone forms the base of much of the mudmound one must attempt to under- stand its origin in order to understand the reasons for the mound’s existence. Grossly, the striking feature of the laminated facies is the seeming alternation of calcilutite and radiaxial fibrous calcite (figs. 5, 7). The formation of similar calcite layers elsewhere has had widely differing interpretations (Bathurst, 1959, p. 511-512; Schwar- zacher, 1961, p. 1494—1495; Lees, 1964, p. 518, 523—524; Fischer, 1964, p. 114—116, figs. 15, 16F, 17, 18; Ross, 1972, p. 6—8; Kendall and Tucker, 1973; Jaanusson, 1974). In the search for a modern analog of this varvelike sediment, we may fail if we look for a modern zebra limestone instead of an alternation of two kinds of calcilutites, one of which under conditions extant in the Ordovician ocean might have changed into radiaxial calcite selectively. 34 The origin of the zebra limestone is complex. No sim- ple explanation satisfies all the evidence, much of which is contradictory. Any theory of the origin of this striking rock must at least take into account: 1. The great lateral extent and the aggregate thickness of the zebra limestone couplets. 2. The cyclic nature of the laminated rock, involving a repetitive stratigraphic sequence of three calcilutites, designated cal, caz, and cas. 3. The determination of whether radiaxial fibrous calcite is a cavity filling or a product of neomorphic recrystallization. 4. The position of radiaxial fibrous calcite when present always immediately below calcilutite cal. 5. The disposition of geopetal sediment, particularly peloids, relative to radiaxial fibrous calcite. 6. Disposition and orientation of fossils within calcilutite cal, sometimes parallel to and sometimes normal to the stratification. 7. Occurrence of calcilutite C83 (possibly also C32) as seemingly crossbedded “internal sediment” where the layers of calcilutite cal are disturbed or dis— rupted. 8. The drape of zebra limestone without attenuation over a “step and riser” in preexisting topography and similar physical relationships. 9. The results of isotopic and other analyses. LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA In attempting to explain the origin of the zebra limestone, we have considered the above evidence although not necessarily in the order listed. GEOCHEMICAL EVIDENCE Various analyses were made not only of the laminated limestone but also of the more massive part of the mound core. We found (1) that all but a trace of insolu- ble component is in calcilutites cal and C32 rather than the radiaxial calcite mosaic, (2) that trace elements are somewhat more abundant in the calcilutite than in the radiaxial calcite, and (3) that X-ray microprobe analysis showed no significant difference between the calcilutite and radiaxial calcite in regard to Fe, Mg, and Ca. Ac- cording to R. C. Surdam, there is no geochemical evidence for a hypersaline or supratidal environment for either component. The isotopic data suggest that the radiaxial calcite was deposited in higher salinities or lower temperature than the calcilutite in the lowest part of the zebra limestone, but both were deposited under the same con— ditions higher in the mound. This evidence is important in eliminating the possibility that origin of the radiaxial calcite is allied to percolation of meteoric water as Dunham (1969, p. 160—163) proposed for the Permian Townsend Mound in New Mexico. It further requires that genesis and diagenesis of both radiaxial calcite and Dolomitized Dolomitized Covering beds l | Calcilutite NUMBER OF SAMPLES Radiaxial calcite 1* | l | 0 l I 20.0 19.0 21.0 | I 22.0 23 .0 24.0 8018 FIGURE 36.— Frequency composition diagram for 6018 in samples from calcilutite and radiaxial spar of zebra limestone, from mound core, and from limestone of covering beds. ORIGIN OF THE MUDMOUND 35 calcilutite have been affected by essentially the same factors. The analytical evidence therefore favors a totally marine origin for the zebra limestone. PHYSICAL EVIDENCE Most geologists knowledgeable in carbonate sedimen- tology who have examined hand specimens and thin sec- tions of the zebra limestone from Meiklejohn Peak have insisted that the radiaxial fibrous calcite was the primary filling of former cavities. Such an interpretation would agree with the original inference of Bathurst (1959, p. 511; 1971, p. 426—427) concerning the habit of radiaxial fibrous calcite. However, it has been my belief, based on field evidence, that the radiaxial fibrous calcite must have replaced some other substance. The distinction between the two interpretations could be significant in regard to the depth of water in which the zebra limestone was deposited. Therefore the evidence is here reviewed. More than half the total volume of the zebra limestone is composed of radiaxial fibrous calcite (fig. 14). As shown in figure 5 (and Ross, 1972, fig. 6) the laminae may extend several metres (tens of feet) laterally and are present through a stratigraphic thickness of 1—10 m (3—30 ft). It seems mechanically impossible to maintain more than half the volume of the rock as simultaneously open galleries in which radiaxial fibrous calcite even- tually grew. Even if such open galleries could have existed simultaneously the stratigraphic evidence calls for deposition of calcilutite C32 in depressions above every layer of cm, followed by growth of a thin tier of radiaxial fibrous calcite in virtually every gallery. Water level in each gallery would have been very shallow; if it had filled the galleries the fibrous calcite would have grown from the ceiling as well as from the floor. Above each tier peloids of micrite would have been scattered; only then could the galleries have been filled with water to their ceilings so that radiaxial fibrous calcite could grow downward as well as upward. The circumstances required to control such water levels and sedimentation within a stack of galleries would try the ingenuity of a wizard, let alone a hydrologic engineer. That the radiaxial fibrous calcite now occupies space which was previously simultaneously open galleries is here considered impossible. The pos- sibility that such galleries could have been maintained open even one at a time is unlikely but conceivable. The steplike flexure of the zebra limestone mentioned previously would have been structurally beyond belief if composed of parallel shells of calcilutite cal separated by parallel layers of water of thickness equal to that of the calcilutite. Some substance must have intervened between layers of calcilutite C31 to maintain spacing between them. To the left of the hammer in figure 5 there appears to be a lump of calcilutite that has depressed the sur- rounding laminae. Actually this may be the filling of some sort of depression in the laminated sediment but that makes little difference. What is important is the fact that the seeming lump is completely surrounded by radiaxial calcite. A similar lump was isolated from a more accessible part of the outcrop and sectioned. It was found to be suspended in radiaxial calcite in three dimensions. It is unlikely that a lump of calcilutite could be suspended so that it was completely surrounded by a cavity and that the cavity was subsequently filled by radiaxial calcite or acicular aragonite. I therefore conclude that radiaxial fibrous calcite must have replaced some other substance which may have been mineral, or vegetable, or partly both. Typical cavities filled with alternating generations of sparry calcite lining and geopetal sediments are il- lustrated by Sander (1951, p. 183, fig. 28; p. 184, fig. 29) in limestones of the Austrian Alps. Coarse calcite lines not only the floor but also the walls and ceiling of the cavities. We know of no way to explain precipitation of sparry or radiaxial fibrous calcite at selected discon- nected spots along the floor, walls, or ceiling of a totally submerged cavity. Nor can we explain deposition of calcite on the walls or ceiling without deposition on the floor of a partially submerged cavity. Yet both circum- stances are exemplified at Meiklejohn Peak. The large block of laminated limestone from which the section shown in figure 7 was taken was variously cut and polished to reveal that the radiaxial fibrous calcite pinches laterally (figs. 16B, 37) and yet the fossiliferous calcilutite layers cal maintain their same vertical spac- ing. Where the sparry calcite is lacking, the space is filled by a layer of very fine, mostly equigranular calcilutite, much of which seemingly exhibits very regular foreset bedding. This very fine calcilutite is an original layer of sediment in a cyclic sequence (figs. 7, 11). As shown in figures 16A, 17, 37A, 37B, and 38, parts of the very fine calcilutite (caa) and the bottom of the fossiliferous calcilutite (cal) seem to be partly converted into radiaxial fibrous calcite. In discussing Irish Waulsortian mounds, Lees (1964, p. 523—524) attributed the origin of “sheet spars” to the fill- ing of extensive nearly horizontal cracks caused by shear failure, but he met with difficulties in explaining how roofs of such extensive cavities could be supported. Indeed, he questioned how such cavities could even open in wet calcareous mud and he appealed to localization of layered organic matter to hold the cavities open. Commenting on supposed cavities in which the sparry calcite of Stromatactis grew, Bathurst (1959, p. 514) stated, “the labyrinthine cavity system, supported now by such a small volume of intervening siltstone, could never have existed in an empty state all at once; it would LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA 36 ORIGIN OF THE MUDMOUND 37 have collapsed.” After thoroughly discussing possible ex- planations, Bathurst (1959, p. 520) concluded that “for the maintenance of cavity roofs and steep slopes a sedi- ment binder was necessary” and like Lees he considered the possibility “that the cavities are, at least in part, molds of a buried organism which decayed.” The position in which we now find radiaxial fibrous calcite in many examples is occupied laterally by calcilutite cag (figs. 10B, 12, 16, 39, 40). Much of this calcilutite appears to be crossbedded. In some instances the radiaxial calcite appears to be interbedded within the crossbeds (figs. 10B, 16, 37A, 38, 39). One might be tempted to attribute these relationships to alternating calcite lining of the walls of a cavity and internal sedimentation, like that clearly illustrated by Bathurst (1971, fig. 311). The superficial resemblance to Bathurst’ s example requires discussion. In Bathurst’ s illustrated example, an unfilled residual cavity still exists and the calcite crystals are terminated with “dogteeth” pointing into the cavity. Equally impor- tant, every layer (or lining) of calcite buried on the floor and wall has a continuous counterpart on the ceiling. As Bathurst’s photograph clearly shows, terminations of calcite crystals are preserved even where buried. In contrast, no examples of similar-looking calcite from the zebra limestone at Meiklejohn Peak had pointed terminations (figs. 17, 37A, 37B, 38); we have found no open cavities. There are more layers of radiax- ial calcite on conjectural floors and walls than on ceilings (figs. 10B, 16). There may be a layer of radiaxial calcite on a supposed ceiling but not on the corresponding floor, and vice versa (figs. 17, 39). Calcite layers within and paralleling crossbeds are associated with and grade into pelletoid calcilutite; some such layers terminate without completely following the contact between crossbeds (fig. 17). Evidently these examples from the Nevada Ordovi- cian differ significantly from Bathurst’s (1971, fig. 311) and the radiaxial calcite may be the result of neomorphic processes (Folk, 1965, p. 23; Bathurst, 1971, p. 481—503) rather than the lining of a cavity. We do not suggest that the mudmound of Meiklejohn Peak is or was devoid of cavities that became filled by spar. On the contrary, we can demonstrate the presence, in the upper part of the mound, of “birdseye” and similar structures which are conceded to be cavity fill- ings. But we insist that, within the zebra limestone, FIGURE 37. (facing page) — Polished section showing detailed views of the same small part of sample illustrated in figure 168. USGS colln. D2325 CO. A, The inner surfaces of the coarse radiaxial calcite (rax) lack crystal terminations; such terminations would have been ex- pected if calcite lined the inside of a cavity prior to filling by calcilutite €83; photographed in polarized light. B, Equigranular calcilutite (caa) appears to have been partly converted to pelletoid calcilutite (C82) as first step toward conversion to radiaxial fibrous calcite (rax); photographed in ordinary light. FIGURE 38. — Coarse calcite along contact at bottom of calcilutite cal is discontinuous and could not have formed lining of a submerged cavity prior to filling by seemingly crossbedded calcilutite (cas). Pel- letoid layers, fractures, and contacts may have provided permeable channels along which aggrading neomorphism produced radiaxial calcite (rax). Note lack of crystal terminations on upper side of lower calcite layer (tr). USGS colln. D2325 C0. Collodion peel. FIGURE 39. — Coarse calcite (tr) at bottom of crossbedded calcilutite can is discontinuous; it resembles lower tier (tr) in radiaxial calcite (rax) of figure 14B. Radiaxial calcite (rax) above crossbeds is con- tinuous with coarse calcite in pelletoid layers, but is not continuous with lower calcite (tr). Therefore, rax, tr, and coarse calcite within pelletoid layers did not form the lining of a cavity prior to deposition of crossbedded calcilutite caa. Median seam (ms) is comparable to same feature in figures 7, 8, 9, 14B, and 15, and has no relation to middle of a preexisting cavity. Presence of calcilutite caz filling depressions in the upper surface of cal indicates that calcilutite cm was essentially horizontal prior to deposition of crossbedded calcilutite cag. Radiaxial fibrous calcite may have resulted from neomorphic conversion of calcilutite caa selectively along pelletoid layers. Collodion peel. USGS colln. D2325 CO. 38 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA cavity filling played a minor role unless such filling took place layer by layer accompanied by almost immediate lithification of preceding layers. ZEBRA LIMESTONE ABOVE BASE OF MOUND The previous discussion applies to the zebra limestone at the base of the mudmound. That occurrence is not unique. High on the south side of the mound a tongue of covering rock extends into the mound. As noted previously, the mound lithology is re-initiated above this tongue by deposition of zebra limestone. As shown in figures 6 and 40, the laminae are generally less regular and more deformed than in the basal part of the mound. Nonetheless the same four lithologic elements are pres- ent, listed stratigraphically: 4. Radiaxial fibrous calcite (rax). Particularly along the contact below cal and above 032. 3. Very fine equigranular calcilutite (033) deposited above the pelletoid calcilutite. Locally crossbed- ded. Overlain by another fossiliferous calcilutite ca. 2. Pelletoid calcilutite (C82), filling depressions and other irregularities in the top of the fossiliferous calcilutite cal. Perhaps somewhat plastic. 1. Fossiliferous calcilutite (cal), in places much dis- rupted. Most of the insoluble residue—authigenic quartz and potassium feldspar—concentrated in this unit. The block shown in figure 6 was cut at right angles along the line indicated and polished; the resulting sur- face is shown in figure 40. The fourth and fifth laminae from the bottom in figure 40 are of particular interest, because seeming conversion of the calcilutite to radiaxial fibrous mosaic is incomplete; it can be seen in greater detail in figure 13. In the thin section (fig. 13) two layers of fossiliferous calcilutite cal are shown. The lower is overlain by evenly bedded pelletoid calcilutite (caz), and that in turn is overlain by very fine equigranular calcilutite (C33). However, unit caa has been changed partly to radiaxial calcite, as has the bottom of the overlying fossiliferous calcilutite cal in the next cycle. N0 neomorphism has af- fected the pelletoid layer. Neomorphism seems to have taken place extensively along the upper contact of the equigranular calcilutite ca3 and above the median seam of the radiaxial calcite (rax). Above the unconverted calcilutite C83 is considerable fragmental material, which might be interpreted as fragments loosened from the ceiling and fallen to rest on the floor of a cavity. Such an interpretation is untenable here; it fails to explain the lack of similar material all along the supposed floor to the left in this view. Calcilutite C33 appears to be vir- tually in place and only partly converted into radiaxial fibrous calcite. -- ~ Mi FIGURE 40. — Cyclic calcilutites cal, 032, and caa in polished surface normal to surface shown in figure 6. In fourth and fifth cycles from bottom, calcilutite C83 is incompletely replaced by radiaxial fibrous calcite (rax). Thin section illustrated in figure 13 was cut from this surface. USGS colln. D2334 CO. ORIGIN OF THE MUDMOUND 39 Figure 41, another polished section, shows lithologic relations 7 mm from the surface shown in figure 40. The same features are present except that in figure 41 the two patches of coarse calcite (tr) in the lower part of the very fine equigranular calcilutite have coalesced. Two enlarged views in polarized light of this polished mirror image are instructive. Figure 18A shows the full thickness of equigranular calcilutite ca3 completely con- verted to radiaxial calcite (rax) on the right and divided into upper and lower halves by the smooth median seam. To the left the median seam is above the unconverted calcilutite; in this sector the crystals of spar are large above the seam and only very small below. This sector magnified still further is shown in figure 18B. Figure 18B presents an informative puzzle. If we as- sume that the radiaxial calcite is filling a cavity, the me- FIGURE 41. — Two patches of the same radiaxial calcite as shown in figure 13 and figure 40 have coalesced below calcilutite caa in a dis- tance of 7 mm. Polished surface, facing that shown in figure 40 at a distance of 7 mm. USGS colln. D2334 CO. dian seam should be the surface along which crystals growing from floor and ceiling met. As discussed previously, we may also assume that dogtooth calcite crystals face the inside of a cavity. The radiaxial calcite above 083 seems to have such teeth pointing downward into the calcilutite; we therefore might suppose that calcilutite ca3 filled a cavity of which the radiaxial calcite immediately above formed the ceiling. But this same calcite, we have already noted, should have been growing upward to close a cavity at the median seam. How radiaxial calcite crystals could have remained suspended in space to grow both upward and downward to form the floor of one cavity and the ceiling of another is difficult to comprehend. Therefore, I suggest that the equigrains of calcilutite cas began to aggrade to form increasingly larger crystals upward toward the median seam. This was no cavity floor. The median seam is jagged here; it would have become smooth when crystals above and below grew to the same size. Calcilutite ca3 was being selectively con- verted to radiaxial fibrous calcite. PELLETOID CALCILUTITE Beales (1958, p. 1867—1871) reviewed the occurrence of “Bahaman type” limestone characterized by its pel- letoid texture and he favored a nonskeletal aggregation of finely crystalline aragonite or calcite in agitated and possibly supersaturated water to form such sediments. Whether the pelletoid material within the Meiklejohn Peak zebra limestone was derived from fecal pellets or from mechanically aggregated microcrystalline car- bonate or is a result of structure grumeleuse (Cayeux, 1935, p. 271—272) is uncertain, but a case could be made for each of these origins. The correct interpretation bears on the origin of radiaxial fibrous calcite. In figure 39, two layers of calcilutite cm are separated by what appears to be crossbedded pelletoid calcilutite and homogeneous calcilutite. A tier of calcite (tr) separates the bottom of the crossbedded calcilutite from cal but pinches out to the left; scattered small depres- sions in the top of cal are filled with another calcilutite (C82) beneath the tier of calcite. Above the crossbedded calcilutite is a small body of radiaxial fibrous calcite which grades downward into two of the crossbeds; these two crossbeds are pelletoid, but they are so impregnated with radiaxial calcite that there is no clear boundary between the calcite and the pelletoid calcilutite. The radiaxial calcite at the bottom is not the same as that on top; if it were, both would grade into the cross- beds. The lower one clearly does not. Therefore, there was no submerged cavity between the two layers cal at the time the lower radiaxial calcite was formed or it would have formed continuously and simultaneously along the floor and on the ceiling. If the pelletoid layers were originally pelletoid their 4O LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA high permeability may have permitted easy flow of supersaturated water, in the same manner as contacts between the layer. Increased permeability may have stimulated the formation of radiaxial fibrous calcite in selected parts of the sediment. On the other hand, some unknown factor, perhaps in— creased permeability along sedimentary contacts, may have enhance the conversion of homogeneous calcilutite to grumeleuse calcilutite as an intermediate step in the change to radiaxial fibrous calcite. In figure 17, radiaxial fibrous calcite is somehow in- timately related to pelletoid material. And in figure 38 there is a suggestion that pieces of calcilutite con- siderably larger than pellet-size are in the process of be- ing surrounded by or perhaps converted to calcite. Two even more detailed views, figures 103 and 10C, also could be interpreted to show that radiaxial calcite is formed with the assistance of or at the expense of pel- letoid material. These two photomicrographs show parts of the thick section shown in figure 10A. Shinn (1969, p. 133, figs. 21, 22) observed that the pel— letoid texture of a modern carbonate mud may be accen- tuated by inward replacement or recrystallization of the periphery of each original peloid. This observation sug- gests to us that a similar mechanism could modify any calcilutite which had been divided into pellet-sized par- ticles or aggregates. AGGRADING NEOMORPHISM: OTHER VIEWS As early as 1904 Cullis concluded that aragonitic mud and fossil shells had been converted to calcite spar at Funafuti atoll, Ellice Islands; this evidence has been reviewed by Bathurst (1971, p. 350—355). Bathurst (1971, p. 489—490) also recorded the discovery that aragonite shells may be replaced by fibrous sparry aragonite. Kendall and Tucker (1973) carefully reviewed the evidence on the formation of radiaxial fibrous calcite and concluded that it is a product of replacement of acicular metastable carbonate and that replacement took place by a process of solution-precipitation and by “migration of a fluid film through the acicular host.” According to this theory the acicular carbonate filled voids prior to replacement. Ross, Friedman, and J aanusson (1971) considered that radiaxial fibrous calcite could be a product of selective recrystallization of carbonate mud in a totally marine environment. The process they envisaged was similar to that of Kendall and Tucker but was independent of a preexisting acicular cement. EVIDENCE OF RUPTURE STRUCTURES Although the spacing of laminae is remarkably uni- form there are evidences of minor deformation, rupture of layers, and flexures irregularly throughout the facies (fig. 5). To examine an example of such disruption the block of limestone shown in figure 10 was cut serially, sectioned, polished, and etched for collodion peels. As already noted, there are striking differences from as well as strong similarities to, the section shown in figure 7. (The two sections occurred within 60 cm of each other.) One section of the disrupted rock (fig. 10C) is par- ticularly enigmatic. In most of the subparallel laminae of the zebra limestone, fragments of fossils which characterize the calcilutite cal are oriented randomly or roughly parallel to the laminations (figs. 9, 11) as might be expected. However, where the the calcilutite cal is deformed into flexures and(or) ruptured, the layering in calcilutites C32 and C33 appears crossbedded and the fossil fragments in calcilutite layers cal may be oriented at angles as high as 90° to the lamination (figs. 103, 100, 12). It seems patently impossible to devise a mechanism by which these laminae could have been deposited on edge as the orientation of fossils seems to require. Strangely, these discordant orientations are not limited to the small areas of deformed laminae and are independent of the seeming crossbedding of ca2 and 033. Moreover, a shell of an inarticulate brachiopod still partly enclosed in calcilutite (fig. 100) projects into a combination of radiaxial fibrous calcite and pelletoid crossbeds. How could this fossil have survived any mechanical separation of layers of calcilutite cal? We may suppose that the layers of calcilutite cal were all part of a single body, that it was cut by subparallel tensional fractures which resulted in cavities, and that these cavities were filled by semifluid, partly pelletoid calcilutite. But if there were any component of shear, as suggested by Lees (1964, p. 523—524), the fossil should have been broken. Indeed, one wonders how the fossil could have avoided breaking unless the intervening pel- letoid material acted truly as a fluid of low viscosity. On the other hand we may be dealing with cyclically deposited sedimentary layers one of which (caz and(or) ca3) has been partly and preferentially replaced by or recrystallized into radiaxial fibrous calcite in each cycle. Originally, this layer may have behaved much like a fluid because of high water content, while calcilutite cal was more cohesive and coherent because of its organic content. Philip N. Playford kindly examined examples of the zebra limestone and pointed out (oral commun., April 1972) that the accordance in attitude of the supposed crossbeds (figs. 10C, 12) might indicate that spaces between many layers of calcilutite ca] were open at the same time, canted at a steep angle to the horizontal, and filled by a succession of nearly horizontal pelletoid and equigranular calcilutite layers; he further suggested that key layers found in a succession of spaces could confirm this interpretation. Key horizons cannot be traced precisely from one set of “crossbeds” to the next and ORIGIN OF THE MUDMOUND 41 there is no simple sedimentary explanation for the three divergent contradictory orientations of (1) layers of calcilutite cal, (2) positions of fossils within yet normal to layers of ca, and (3) crossbedded calcilutite ca3. We have considered the possibility that layers of calcilutite cal might have become indurated and buckled in a manner similar to that deduced from Or- dovician limestone in the Baltic area by Lindstrom (1963, p. 252—256) or similar to the cemented layers forming today in the Persian Gulf (Shinn, 1969, p. 112—119, 122, 128). Buckling of such layers involves shear and we have already discounted any explanation involv ing shear. Although we do not know how the sediment was deposited, we propose that the calcilutites 082 and ca3 might have behaved thixotropically because of high water content. Perhaps hydraulic pressure contributed locally to the rupture of calcilutite cm and more broadly to its support. Distortion and rupturing of layers, as i1- lustrated in figures 6, 10, and 40, may have occurred in such a fluid environment, wherein unconsolidated thix- otropic calcareous muds (car and (38.3) flowed through ruptures in more competent cal to mix with other uncon- solidated mud in younger or older cycles. This theory eliminates the necessity for mechanically impossible open galleries equaling 60 percent of the final volume of the rock. It provides for local disruption of some layers and the undisturbed nature of others. It ac- counts for flowage of internal sediment, permitting as much upward as downward movement. Accordingly, each layer of fossiliferous calcilutite cal may be considered as the more competent layer in a repeated cycle and may represent a time of slow sedimentation and of partial winnowing of the calcilutite to concentrate fossil remains. Layers of pelletoid and homogeneous calcilutite (082 and caa) may have been deposited over a much shorter time and in more tur- bulent water. The unconsolidated and saturated mud (C32 and caa) could have provided the permeable avenues wherein aggradational neomorphism eventually con- verted the calcilutite to radiaxial fibrous calcite. This conversion may have taken place selectively along cer— tain sedimentary contacts. Whether pelletoid muds con- tributed to permeability and easier access of CaC03- bearing water or were the result of neomorphic produc- tion of structure grumeleuse is uncertain. In either process, rapid crystallization, probably of aragonite, within the saturated, unconsolidated carbonate mud was necessary to preserve the cyclic nature of the zebra limestone. RAPID SUBMARINE LITHIFICATION Alternatively, one may speculate that rapid cementa- tion or aggrading neomorphism contributed to the production of successive hard, lithified sedimentary crusts a few millimetres thick and that each crust ex- panded laterally as it lithified, according to Shinn’s (1969, p. 128, fig. 17) observations in the Persian Gulf. As a result, each new crust may have had the strength necessary to hold itself off the underlying crust with a few points of contact. In the resulting slim cavities calcite or aragonite may have crystallized from saturated seawater, or pelletoid and fine homogeneous carbonate mud may have been introduced from the overlying sea bottom. DERIVATION FROM ALGAL MATS In describing Alpine Triassic zebra limestone, very similar to the Ordovician laminated limestone from Meiklejohn Peak, Fischer (1964, p. 127) noted their close resemblance to modern algal mat deposition. Such mat deposits come most readily to mind when we seek a modern laminated analog. Direct evidence of algae within the carbonate mound is scarce. Algae are present but those few that are preserved could hardly have acted as the binder for the entire mound. Sphaerocodium (fig. 42) is present in col— lections D2349 CO, D2348 CO, D2343 CO (fig. 4) as scat- tered filaments. Nowhere does it form the closely packed structures reported in Devonian reefs of Australia (Wray, 1967, p. 35-40) and Canada (Wray and Playford, 1970, p. 547-552, pl. 2, figs. 1-4). The problematic micro-organism Nuia is found in the calcarenite parts below and in the mound in collections D2341 CO (echinodermal lime packstone), D2345 CO, D2346 CO, D2347 CO, D2348 CO, and D2349 CO (fig. 4). This form was reported by Toomey (1970, p. 1325, fig. 12a) and by Toomey and Klement (1966) in Lower Or- dovician carbonate mounds of the El Paso Limestone, west Texas. At Meiklejohn Peak Nuia is a minor con— stituent and seems to occur in fragmental deposits within the mound. In fact Nuia could not have been significant in binding the mound together. Rowell and Krause (1972, 1973) inferred that plants, presumably algae, were present on the mound, because two genera of inarticulate brachiopods were adapted for attachment to “cylindrical objects.” Although we have no direct evidence that algae were abundant enough to form extensive mats, we must not completely exclude the possible former existence of green or blue-green algae, all remains of which have dis- appeared. G. M. Friedman and others (1973) showed that car- bonate laminites may have been deposited within algal mats in the geologic past, and they suggested a mechanism for the formation of calcite cement in layers within the mats themselves. Their observations support the belief that algae not only may trap and hold sedimentary particles but also may be directly responsi- ble for the “precipitation of carbonate laminites which 42 preserve the morphology of the mats even after the organic matter has become degraded and has disap- peared” (Friedman and others, 1973, p. 552). Such a process, if cyclic, could result in formation of zebra limestone one lamina at a time without requiring any voids. Furthermore, irregularities in the bottom con- figuration would be duplicated by the mats; damage to mats would result in depressed features like those shown in figure 5. Local upward disruption of laminae could have resulted in structures shown in figures 6 and 10. The trapping of small fossils on edge (fig. IOC) within an algal mat is conceivable. Without such a mat it is nearly impossible to explain why such fossils are not parallel to the stratification. Otte and Parks (1963, p. 394) concluded that radiaxial fibrous calcite forming Stromatactis-like masses in Up- per Pennsylvanian and Lower Permian limestone in New Mexico had taken the place of a firm-bodied organism capable of supporting mud layers but incapable of secreting a calcareous skeleton. Some modern blue-green algae have the consistency of tough rubber. Layers of such a rubbery organic material might have supported FIGURE 42. — Filaments of Sphaerocodium found in main core of mudmound, particularly at top. Thin section. USGS colln. D2348 CO. LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA layers of calcilutite and later been replaced by sparry aragonite. Perhaps, one need no longer speculate on this sort of origin for the radiaxial calcite, inasmuch as G. M. Friedman and coauthors (1973) have observed a reasonable analog. The zebra limestone might have formed on a tidal flat but Hardie and Ginsburg (1971) found that cyclic laminae on Bahaman tidal flats are formed during periods of major storms, not as the result of diurnal tides. They further noted that persistent winds, over 30 km/hr, “are needed to suspend pelleted lime mud * * * and flood the tidal flats with sediment-charged seawater. Deposition from these floodwaters produces either a thin mud lamina (made by the flypaper-like trapping action of algal mats) or a couplet consisting of a basal mud layer overlain by a sorted layer of fine sand peloids.” Like the couplets observed by Hardie and Ginsburg on Andros Island, Bahama Islands, the laminae at Meikle- john Peak are composed of a basal mud (cal) (rich in os- tracodes and other fossils) and a layer of well-sorted pel- letoid mud (032). But a third layer of calcilutite is pres- ent in the cycle; where laminae lie flat, this third calcilutite (C33) is equigranular, but where laminae are disrupted or deformed, calcilutite ca3 appears pelletoid and crossbedded (fig. 38, 39). In Shark Bay, Western Australia, well-laminated sedi- ments are, according to Davies (1970, p. 183—186), only found with smooth algal mats and they occur in two en- vironments—in the outer intertidal zone and in the landward parts along the floors of tidal channels. Neither dolomite nor gypsum is present in sediments in the intertidal zone, but both are present, in varying amounts, in the supratidal zone (Davies, 1970, p. 177, table 1). The laminated rock at Meiklejohn Peak lacks any evi- dent dolomite, magnesian calcite, or gypsum. Therefore, deposition in the intertidal, rather than the supratidal, zone is favored. The small “cabbagehead” masses mentioned previously (p. 13) (fig. 19) may have resulted from deformed growth of algal mat over preexisting topographic irregularities (compare with Davies, 1970, figs. 12 B, C). Similarly, deformation of calcilutite ca; as shown in figure 10 may have been caused by breaching of such a “cabbagehead,” by upward movement of fluid mud or of gas (Davies, 1970, p. 191—192), or by bioturba- tion. EFFECT OF ORGANIC SUBSTANCES Recently, Meyers and Quinn (1971, p. 992) showed that fatty acids and other lipids are absorbed by calcite in seawater and may provide a coating that prevents the solution of calcite in seawater which is undersaturated for calcite. The coating may also prevent the precipita- ORIGIN OF THE MUDMOUND 43 tion of calcite in supersaturated seawater. It therefore seems to us possible that the presence or absence of such organic substances within some of the components of the laminae at Meiklejohn Peak could have influenced the formation of sparry calcite, although identifying that in— fluence now may be virtually impossible. The important influence of blue-green algae, other organisms, and organically derived chemicals in the for- mation of modern laminated carbonate sediments in the Gulf of Aqaba has been demonstrated by Friedman and others (1973, p. 553-556). These same authors (p. 552, figs. 9, 14) showed not only that the algal mat encloses great amounts of particulate carbonate but also that the laminites may consist of alternating layers of “cryp- tocrystalline high magnesian calcite and fibrous aragonite.” Such alternations are suggestive of the cyclic calcilutites and radiaxial fibrous calcite of the zebra limestone. Bathurst (1967) called attention to a “subtidal gelatinous mat” which acts to stabilize the calcareous sands of the Great Bahamas Bank. This mat is widespread. The mat seems to be partly algal but it is in- habited by and is the food source for such a multitude of minute animals that its precise origin is a mystery. Ac- cording to Bathurst, such a mat could leave no direct evidence in the geologic record, for it disappears as soon as buried. And yet if this mat were suddenly buried without disruption by very fine sediment, we believe an organic film would surely be trapped. Zangerl (1971) reviewed the effect of decaying animal matter in the formation of concretions, involving the rapid crystallization of calcium or other carbonates. There should be little doubt that organic substances may exert influences on the deposition of carbonate sedi- ments, influences so complex and so little understood that current conventional thinking about the origin of limestone is inadequate. THEORY OF LIFTING BY CRYSTAL GROWTH Several of our colleagues suggested that the sparry and radiaxial calcite may have grown in place, “shouldering” its way along the contact between couplets; they proposed that the growth of the calcite, like the growth of gypsum and trona in the Pennsylvanian Paradox Member of the Hermosa Formation and in the Eocene Green River Formation (Deardorff and Mannion, 1971, p. 35; O. B. Raup, R. J. Hite, and W. C. Culbertson, oral commun., 1972) might have lifted the overlying layers as it grew—the displacive precipitation theory of Folk (1965, p. 24). In many of the Ordovician laminae there are pelletoid bodies within the lower layers of the radiax- ial fibrous spar (fig. 8); these bodies might be compared with fragments of the underlying shale which have been lifted during crystal growth into the coarsely crystalline gypsum or trona of the Pennsylvanian and Eocene deposits. However, at Meiklejohn Peak where the radiaxial fibrous calcite layer is incompletely formed (fig. 7), the calcilutite couplets obviously were not mechanically disturbed by formation of the calcite. In such places the radiaxial calcite appears to have grown at the selective expense of one or more of the calcilutites. CONCLUSIONS The original sediment of the zebra limestone was probably an alternation of partly or wholly cemented fos- siliferous calcilutite with unconsolidated saturated car- bonate mud. Much of the latter was pelleted. Whether pelleted muds were produced as fecal pellets or whether they were in some way related to algal mats is unknown. No direct evidence for such algal origin was found. In either circumstance pelleted texture would have produced high permeability, thereby aiding the diffusion of seawater saturated with CaC03. The depth of water is uncertain. If the formation of the laminae was related to algal mats one might expect very shallow, even supratidal, conditions to have been possi- ble. Isotopic analysis indicates that all components of the mound were formed under the same conditions, precluding vadose water as an agent of diagenesis and making unlikely a supratidal environment for formation of the radiaxial fibrous calcite in the zebra limestone. Inasmuch as the main core of the carbonate mound rests upon the zebra limestone, growth of the mound may have started in depths not exceeding 30 m. Shinn (1969, p. 110, 112—117) showed that submarine cementation can take place in depths of 30 m in the Persian Gulf although examples in which he found mul- tiple crusts of relatively close vertical spacing were at depths of less than 3 m. He also found that sub- marine cementation took place in areas of low sedimen- tary accumulation such as the windward side of offshore highs. Presumably the essential factors of low sediment accumulation, saturated seawater, and permeable sedi- ment could be found at considerable depths. We do not know what, if any, influence algae, sponges, or other organisms had in protecting the depositional surface, in binding the sediment, or in controlling crystallization of CaC03. Although Meiklejohn Peak must have been scores of miles from the shore, it surely was not subjected to wave action sufficiently destructive to tear apart the laminated zebra limestone. The fact that cephalopod shells are most common in the laminated facies, particularly in pockets, suggests that their empty shells, weighted down by cameral deposits and waterlogged, were swashed into shallow depressions and cemented into the surrounding rock. Ginsburg and James (1973, p. 24) have discovered off the coast of British Honduras that lithification of limestone has taken place at depths not less than 20 m in completely submarine conditions; one 44 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA specimen was found to be less than 2,500 years old. They have found that the cement is aragonite and high- magnesium calcite, both of which are unstable minerals. The cement, according to Ginsburg and others (1973, p. 781), “almost obliterates many primary depositional fabrics.” If such is the case in the Neogene off British Honduras we can speculate that the inversion of the ce- ments to calcite will unavoidably result in further recrystallization and obliteration of original sedimentary fabric. Ginsburg and James’ discovery of cavities filled or lined with “dense, radiating fibrous growths of aragonite” provides a lithology which could readily in- vert to the radiaxial fibrous calcite of the zebra limestone and the Stromatactis-like bodies of the main mudmound at Meiklejohn Peak. Despite our lack of positive evidence for algal mats, the observations of G. M. Friedman and others (1973) suggest that the origin of the zebra limestone should be attributed to mats. Most fossils in calcilutite cal are oriented parallel to lamin‘ations where layers are un- disturbed but the shells lie at angles as high as 90° to laminations, particularly where layers are disturbed; each lamina was deposited initially as a layer draped over various irregular surfaces where fossils settled and were trapped in the cohesive “flypaper” of algal fila- ments. By no analogy other than to algal mats can we as easily explain the great lateral extent of laminae in the zebra limestone, the complete isolation of lumps of calcilutite, and the steplike flexure of laminae. Seasonal climatic fluctuations—whether producing storms, changes in water chemistry, or alternations of algal growth—could have accounted for the varvelike cyclic nature of the calcilutites cal, cm, and caa. The radiaxial fibrous calcite may have been a product of selective recrystallization of aragonite needles within the algal mat, or of micritic metastable carbonate within or below the mat, or both. One of the calcilutites, possibly cal, may represent the micritic material within the algal mat itself and therefore may appear to have been the most competent layer in the cycle. Locally, cal was deformed just as we might expect an algal mat to be where ruptured by bioturbation, wave action, or the upward pressure of gas or contained fluid. Locally, there must have been a briefly fluid, possibly thixotropic, pelletoid calcilutite C32 and calcilutite cag between layers of ca; whether the resulting carbonate slurry filled cavities from above or was deposited as a layer in a repeated cycle and was locally squeezed hydraulically into flow structures cannot be proved. The zebra limestone seems to indicate that intertidal or extremely shallow infratidal deposition initiated the building of the carbonate mudmound and Meiklejohn Peak. However, the depth to which algal mats may ex- tend depends largely on the abundance and voracity of browsing marine fauna. The occurrence of minor amounts of zebra limestone above tongues of cover beds, approximately 25 m above the bottom of the mound, proves that for the first third of its growth the mound was only slightly above the sur— rounding bottom. We must therefore conclude that the covering silty limestone of the Orthidiella zone was likewise deposited in shallow water in the vicinity of present-day Meiklejohn Peak. Although some radiaxial fibrous calcite may occupy former cavities it may also have resulted from recrystal- lization and neomorphic aggradation of other metastable carbonate. Whether triggered by organic substances, by the instability of aragonite, or by greater permeability, this neomorphism was highly selective. Further, it must have taken place very early in the diagenetic process, perhaps as rapidly as the algae decomposed. These conclusions free us from the necessity of postulating the prior existence of cement-filled cavities wherever radiaxial fibrous calcite is encountered. We need not pursue an ontological speculation on the critical size of a cavity. If, as Kendall and Tucker (1973) believe, radiaxial fibrous calcite is derived by recrystallization from acicular aragonite, why should it not be derived even more readily from micritic aragonite or high-magnesian calcite? If such a fine-grained aragonite were converted by aggrading neomorphism to coarse radiaxial spar the oldest crystals — those that increased most by incor- porating their neighbors — should be the largest (figs. 140, 18B). Bathurst (1958, p. 15, 20; 1959, p. 509) showed that grain (crystal) size of calcite increases with growth away from the wall of the cavity; that is, the older crystals are the smaller. I do not dispute this finding, but I question the belief that direction of in- crease in grain size always indicates the center of a former cavity. The direction of neomorphic conversion of micrite would be from coarse (old) to fine (young) crystals, a very different conclusion involving no cavity at all. Neomorphic conversion can be demonstrated at Meiklejohn Peak; the conversion of an echinodermal plate to radiaxial calcite is shown in figure 29. Calcite and fossils left relict in radiaxial fibrous calcite are il- lustrated in figures 30 and 31. I suggest that the median seam, which is virtually ubi- quitous in such calcite, does not represent the surface along which crystals grown from opposing ceiling and floor have met. The seam may be entirely related to recrystallization, being some sort of surface of equal pressure between two tiers of large aggrading crystals. ORIGIN OF THE MUDMOUND 45 ORIGIN OF THE CORE OF THE MUDMOUND By VALDAR JAANUSSON The Meiklejohn Peak mound had no skeletal frame. Constituents of the skeletal sand do not indicate the presence of organisms that could have acted as impor- tant sediment traps. Sponges are a possible exception, provided that the spiculaelike skeletal particles belong to this group. Nevertheless, the “spiculae” do not seem sufficiently numerous, and macroscopic skeletons of sponges are very rare. The prolific occurrence of sponges (and Pyritonema) in the so-called sponge beds of the Ikes Canyon section, Toquima Range, Nevada, is not as- sociated with a mound. Whatever organisms did control the growth of the mound must have lacked preservable hard parts. M. J. Brady (oral commun., Nov. 1971; written com- mun., April 3, 1972) found that low banks of lime mud in lagoons of Yucatan are not dependent on any framework organisms. The buildup of mud is related to hydrologic conditions — to changing currents, to bottom configura- tion, to wind direction, and to tidal currents. As soon as the mudbanks “became prominences on the lagoon floor” they seemingly grow, because they disrupted sediment-laden currents. Understanding the original nature of the carbonate mound requires interpretation of how Stromatactis formed. These structures may occupy 50 percent or more of the volume of a carbonate mound. Even at the Meikle- john Peak mound they form 15—20 percent of the volume. Bathurst (1959) suggested that Stromatactis represents cavity-fillings — a proposal accepted by most subse- quent writers who investigated such structures. This suggestion implies that during growth or diagenetic history of a mound a considerable volume was occupied by cavities of macroscopic size. That Stromatactis in fact is cement precipitated into cavities can be proved in many instances but this does not necessarily mean that all Stromatactis-like calcite bodies have the same origin. Such calcite bodies can originate through recrystalliza- tion although bodies so formed lack a median seam. Moreover, radiaxial mosaic forming Stromatactis can demonstrably grow at the expense of included sedimen- tary grains and surrounding sediment. The quantitative importance of the latter process, though not always evi- dent, seems to be small in comparison with the volume of a Stromatactis. The evidence from the Meiklejohn Peak mound is not conclusive but if the median seam of radiaxial mosaic does indicate cavity-filling, then most of the Stromatactis in the mound has been formed as ce- ment precipitated into cavities. Interpretation of the growth of the mound depends on that of the diagenesis in the early generations of limestone. If in the Meiklejohn Peak mound the generation Li 1 (fig. 32) was bored by organisms—if it was lithified prior to the deposition of Li 2 —lithification of parts of the mound took place early enough for forma- tion of hardgrounds. In that case, the mound probably represented a lithoherm (as defined by Neumann and others, 1972) and not a mound of carbonate mud. In fact, much of what now looks like a lithified carbonate mud would possibly not have been soft sediment at all but a rock largely formed by calcium carbonate micrite precipitated from the seawater. Recent limestone, formed largely by precipitation of high-magnesian calcite, has been described from the Mediterranean (Alexandersson, 1969). Some of the rock from the lithoherms of the Straits of Florida (Neumann and others, 1972) is closely similar to the mound at Meikle- john Peak in appearance (my observations on the material exhibited by A. C. Neumann at the AAPG meeting in Denver, April 1972) as well as composition. This type of rock is termed here “thalassic limestone.” Particles of sediment were trapped and encrusting organisms were incorporated therein, but it formed mainly by precipitation of calcium carbonate from seawater. The growth of thalassic limestone tends to be irregular. Cavities of various shape and size can be enclosed within the rock and filled by trapped sediment or drusy cement, or both. The organisms associated with modern thalassic limestones are mainly those that bore into the rock or attach themselves upon the surface of the rock, that is, boring endofauna and sedentary epifauna. Such rock tends to be intensely bored by various organisms (Alexandersson, 1969). If the Meiklejohn Peak mound and similar carbonate mounds were once lithoherms, how the large volume of cavities could be formed in the mound and could exist without collapsing would be easily explained. The early formation of spar in Stromatactis would also be easy to understand. More difficult to explain would be the locally distinct lineation parallel to the depositional sur- face of the former cavities within the mound. During field examination of the Meiklejohn Peak mound we considered the possibility that the mound represented a lithoherm. However, no structures could be recognized that would indicate the former presence of hardgrounds. No bored surfaces were found, or even any unmistakable traces of boring activities. The presence of several generations of limestone was discovered later, during laboratory examination, in some samples. Skeletal remains of encrusting organisms are rare in the macrofauna of the mound. However, on Ordovician hardgrounds (discontinuity surfaces) an encrusting epifauna is generally notoriously poorly represented (J aanusson, 1961). It is also important to note that when the growth of a Stromatactis carbonate mound had 46 LITHOLOGY AND ORIGIN, MUDMOUND AT MEIKLEJOHN PEAK, NEVADA ceased, the mound was demonstrably lithified and formed a hardground before covering beds were deposited. This cannot be proved with respect to the Meiklejohn Peak mound, but it is true for numerous other Paleozoic carbonate mounds. At least in the Or- dovician carbonate mounds of Sweden, the mounds’ up— per surfaces show no evidence of borings; environmental conditions obviously were not suitable for boring organisms. An epifauna encrusting the surface has not been observed. If the Meiklejohn Peak mound was not a lithoherm, it presumably was soft carbonate mud which lithified early but not at the interface between sediment and seawater. If the mound had emerged before deposition of the cover- ing beds, the cavities could have formed as a result of differential shrinkage within the mound during subaerial exposure. Emersion and shrinkage could have been fol- lowed by internal sedimentation, lithification, and cementation in the cavities (Dunham, 1969). However, contact relations between the mound and the covering beds do not suggest a phase of emersion between these two periods of deposition. Moreover, the data on (3018 in- dicate that the spar and calcilutite have the same isotopic composition and that precipitation of the spar from fresh water is very unlikely (p. 31). It is difficult to find a suitable actuogeological model for the Meiklejohn Peak mound if formation of the ex— tensive cavities and early lithification took place within the carbonate mud in constantly submarine conditions, as postulated by many writers for similar carbonate mounds (Bathurst, 1959; Schwarzacher, 1961; Lees, 1964; Heckel, 1972, and others). The origin of the cavities has been variously explained: space left after decay of soft organisms (Bathurst, 1959; Lees, 1964), dif- ferential settling and shear failure (Schwarzacher, 1961), solution (Textoris and Carozzi, 1964), burrows made by animals (Shinn, 1968), and differential compaction (Heckel, 1972). Some of the explanations (solution, bur- rows) are very unlikely for the Meiklejohn Peak mound and for other mounds herein mentioned. There is no positive or negative evidence for the former existence of abundant soft organisms whose decay could have left the cavities after the enclosing carbonate mud had become firm (see also Heckel, 1972). Shear failure alone cannot be an important factor in forming the cavities, because it implies that the mound was originally much larger. Dif- ferential settling and differential compaction, or a com- bination of both, are possible agencies in formation of cavities within a soft sediment but our knowledge is scanty about what really happens when a mound con- sisting of carbonate mud settles and compacts. Small cavities of the general shape of microstromatactis have been experimentally produced by settling Holocene car- bonate mud (Cloud, 1960, 1962). However, whether the same or a similar process can produce macroscopic cavities aggregating as much as 50 percent of the total volume of a carbonate mud is not known. Differential settlement of a mound has demonstrably caused transverse fissures and crevices but formation of a con- siderable volume of horizontal cavities is not readily ex- plained from the standpoint of soil mechanics. Thus, if the Meiklejohn Peak mound was once a mound of soft carbonate mud continuously in a submarine environ- ment, there is much in the diagenetic history of the mound that we do not understand. The depth of water during formation of the Meikle- john Peak mound is difficult to determine. The presence of calcareous algae suggests deposition within the general limits of the photic zone. The bedded limestone covering the mound abounds in what probably have been indurated pellets. In modern seas such pellets are not known to be formed below the upper photic zone, and they may indicate similar shallow-water conditions during the deposition of the covering beds. Regardless of whether the Meiklejohn Peak mound was or was not a lithoherm, the calcarenitic covering beds give the impression of having been deposited in an environment with a higher average water energy level than that which prevailed during deposition of the soft carbonate mud of the mound. The significance of this difference is not clear; in carbonate sediments estimating the water energy level from the grain size distribution is usually difficult mainly because of the trapping and binding ac- tion of plants (Ginsburg and Lowenstam, 1958; Baars, 1963; Lynts, 1966; Scoffin, 1970) and the local occur- rence of gelatinous mats on the surface of the sediment (Bathurst, 1967). The aspects related to depth of deposi- tion can be discussed with greater confidence when the carbonate lithology of contemporaneous beds has been studied in a regional scale. REFERENCES CITED Alexandersson, Torbjiirn, 1969, Recent littoral and sublittoral high-Mg calcite lithification in the Mediterranean: Sedimentology, v. 12, p. 47—61. Baars, D. L., 1963, Petrology of carbonate rocks, in Bass, R. D., and Sharps, S. L., eds, Shelf carbonates of the Paradox basin, a sym- posium—Four Corners Geol. Soc., 4th Field Conf., 1963: Durango, Colo., Petroleum Inf., p. 101—129. Bathurst, R. G. C., 1958, Diagenetic fabrics in some British Dinantian limestones: Liverpool and Manchester Geol. Jour., v. 2, no. 1, p. 11—36, 2 text figs, 1 pl. 1959, The cavernous structure of some Mississippian Stromatactis reefs in Lancashire, England: Jour. Geology, v. 67, no. 5, p. 506-521. 1964, The replacement of aragonite by calcite in the molluscan shell wall, in Imbrie, John, and Newell, Norman (eds.), Approaches to paleoecology: New York, John Wiley and Sons, Inc., p. 357—376. 1967, Subtidal gelatinous mat, sand stabilizer and food, Great Bahama Bank: Jour. 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E., 1970, Some occurrences of Devonian reef-building algae in Alberta: Canadian Petroleum Geology Bull., v. 18, no. 4, p. 544—555, pls. 1—2. Yancey, E. S., 1971, Early Middle Ordovician marine benthic com- munities in southern Nevada and California: Univ. California (Berkeley) M.A. thesis, 98 p., illus. Zankl, H., 1969, Structural and textural evidence of early lithification in fine-grained carbonate rocks: Sedimentology, v. 12, p. 241—256. Zangerl, Rainer, 1971, On the geologic significance of perfectly preserved fossils, in Extraordinary fossils—~Pt. I: North Am. Paleont. Convention, Chicago 1969, Proc., v. 2, p. 1207—1222. fiUS. GOVERNMENT PRINTING OFFICE: l975—677-3l3/63 . '3 v: ‘ ‘~ I. V f -r ‘ i Fa wig; ,l ,. ,3 , {-92:31 I w 15%;: Geology of the Golden Quadrangle, Colorado GEOLOGICAL SURVEY PROFESSIONAL PAPER 872 MAY 28 1978: U.S.S.D. GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO FRONTISPIECE.—Physlographic setting of the Golden quadrangle as viewed northward from Lookout Mountain. The hummocky surface of Table Mountains. The escarpment between the Southern Rocky Mountains and the Great Plains physiographic provinces forms the Group. The Rocky Flats Alluvium (Qrf) forms the gently eastward sloping surface beyond the Tertiary intrusives. Arrows indicate the an old landslide (Is) lies between Golden and North Table Mountain. Other landslides are present on the steep flanks of North and South east (right) face of Mount Zion. The sinuous ridge in the left middle ground is a hogback formed by resistant sandstone in the Dakota approximate directions of the west and south edges of the quadrangle. Geology of the: Golden Quadrangle, Colorado By RICHARD VAN HORN GEOLOGICAL SURVEY PROFESSIONAL PAPER 872 Geology of part of the Denver urban area, Colorado, with emphasis on deposits of Pleistocene age and their economic potential, engineering characteristics, and environmental implications UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1976 UNITED STATES DEPARTMENT OF THE INTERIOR THOMAS S. KLEPPE, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Van Horn, Richard, 1920— Geology of the Golden quadrangle, Colorado. (Geological Survey Professional Paper 872) Bibliography: p. Includes index. Supt. of Docs. no.: I l9.l6:872 1. Geology—Colorado—Jefferson Co. I. Title. II. Title: Golden quadrangle, Colorado. III. Series: United States Geological Survey Professional Paper 872. QE92.J4V36 557.88'84 75—619065 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024-001-02794—1 CONTENTS Abstract ........................................................................... Introduction .............. Acknowledgments. Previous work ........... Physiography ................................................................................ Stratigraphy .................................................................................. Precambrian rocks .......................................................... Metamorphosed sedimentary and volcanic(?) rocks ...... Mica schist unit ................................................ Hornblende gneiss unit ........................................... Interlayered hornblende gneiss, amphibolite, and biotite-quartz-plagioclase gneiss ........... Garnetiferous schist ......................................... Mica schist ........................................................ Layered calc—silicate gneiss and associated rock ............................................................... Microcline-quartz-plagioclase-biotite gneiss unit .. Interlayered gneiss ............................................ Garnetiferous quartz-biotite schist . Igneous rocks .................................................................. Pegmatite ....................................... Hornblende-biotite lamprophyre ............................ Paleozoic rocks ....................................................................... Pennsylvanian and Permian Fountain Formation.. Permian ......................... Lyons Sandstone ..................................................... Paleozoic and Mesozoic(?) rocks ............................................ Permian and Triassic(?) ................................................. Lykins Formation ................................................... Mesozoic rocks ...................... Jurassic ................ . ......... Ralston Creek Formation. Morrison Formation ................................................ Cretaceous ....................................................................... Dakota Group undifferentiated Benton Shale .......... Niobrara Formation ..................... Fort Hays Limestone Member ......................... Smoky Hill Shale Member .............................. Pierre Shale ............................... Fox Hills Sandstone.. Laramie Formation Arapahoe Formation ............................................... Cretaceous and Tertiary ................................................. Denver Formation ................................................... Laramie, Arapahoe, and Denver boundary problems .............................................................. Tertiary igneous rocks ........................................................... Monzonite ....................................................................... Latite ............................................................................... Quaternary deposits ............................................................... Alluvium .............................................. Longitudinal stream and terrace profiles .................... . 1: N mmu‘mmoaoaoa—‘H'fi <11wa ooooocoooooooooom 43 47 48 50 53 55 56 Stratigraphy—Continued Quaternary deposits—Continued Pleistocene(?) deposits .................................................... Pre-Rocky Flats alluvium ....................................... Pleistocene deposits ........................................................ Nebraskan or Aftonian ............................................ Rocky Flats Alluvium... Kansan or Yarmouth ........ Verdos Alluvium .............................................. Illinoian or Sangamon ............................................ Slocum Alluvium... Bull Lake and Pinedale Louviers A11uvium.... Broadway Alluvium ......................................... Holocene deposits ........................................................... Pre-Piney Creek alluvium(?) Piney Creek Alluvium ...... Post-Piney Creek alluvium ..................................... Pleistocene and Holocene deposits ....................................... Alluvial fan deposits ............ Transported mantle.. Colluvium ............. Loess ............................................................................... Landslides ....................................................................... Artificial fill .................................................................... Pleistocene and Holocene soils ............................................. Structural geology ............................ Folds ............. Faults ........... Geologic history ............................................................................ Economic geology ......................................................................... Coal .............. Oil ............. Uranium Gold ....................................................................................... Clay ........................................................................................ Silica sand ........ Dimension stone. Limestone ........ Crushed rock .......................................................................... Sand and gravel ..................................................................... Engineering geology ........... Foundation conditions Swelling clay .................................................................. Differential settlement .................................................... Landslides ................................................... Workability ............................................................ Septic systems ..... Cut-slope stability ..... Earthquake hazard ................................................................. Historical geologic events at Golden, C010,, and vicinity ........... References cited ................................................................... VII Page 57 57 58 58 58 62 62 67 67 69 69 72 75 75 76 76 79 79 80 82 84 87 91 92 95 95 101 103 103 103 103 103 104 104 104 104 105 105 105 105 105 105 106 106 107 107 107 108 109 113 VI II Page TABLE 1. Formations of the Golden quadrangle, Colorado ............................................................................................................................ 5 2. Thickness of beds between fossil zones of the Pierre Shale ............................................................................... 21 3. Fossils found in the Golden quadrangle, Colorado, from the Pierre Shale arranged according to zone .................................. In pocket 4. Semiquantitative X-ray mineralogic determinations of samples from Upper Cretaceous formations in northeastern Colorado ......................................................................................................................................................................................... 26 5. Lithology of fragments larger than 0.074 mm in washed samples of the Denver and Arapahoe Formations ............................... 38 6. Chemical analyses of monzonite, latite, and tuffaceous rocks of the Denver Formation ................................................................ 47 7. Modal analyses of Tertiary intrusive and extrusive rocks in the vicinity of the Golden quadrangle... 48 8. Pebble counts from alluvial deposits in the Golden and adjacent quadrangles . . ....... 56 9. Minerals in samples of volcanic ash from near Golden ....................................... 65 10. Complete and partial rock analyses of volcanic ash samples from near Golden ............................................................................ 65 11. Quantitative spectrographic analyses of glass shards from volcanic ash samples near Golden, Colo., and Orleans, Nebr .......... 66 12. X-ray analyses of soils developed on alluvium in the Golden quadrangle ..................................................................................... 94 13. Ratio of mica to montmorillonite of soil samples from the Golden quadrangle ........................................................................... 95 14. Number of soil samples from the Golden quadrangle in which the amount of mica was respectively greater or less than the amount of montmorillonite ..................................................................................................................................................... 95 MEASURED SECTIONS OF BEDROCK AND SURFICIAL DEPOSITS Page G25—A. Fort Hays Limestone Member of the Niobrara Formation .................................................................................................................. 19 B. Smoky Hill Shale Member of the Niobrara Formation ................................................. 20 C. Pierre Shale, Ralston Reservoir and Ralston Creek ..... 25 G24—A. Pierre Shale, in prospect trench ............................................................................................................................ 25 G25—D. Fox Hills Sandstone, south side of Ralston Creek ........................................................................................... 31 G24—B. Fox Hills Sandstone, in prospect trench ............................................................................................................................................... 31 C. Laramie Formation, in prospect trenches ............................................................................................................................................ 34 D. Arapahoe Formation ................................... 37 G23. Denver Formation .................... 4O G143. Soil on Rocky Flats Alluvium. 60 GF5. Verdos Alluvium .................................................................................................................................................................................... 62 G101. Soil on Verdos Alluvium ....................................................................................................................................................................... 65 G72. Eroded soil on Slocum Alluvium... 69 G109. Soil on Broadway Alluvium ....... 75 G55. Soil on transported mantle depos1t... 82 G62. Soil on 10655 deposit .............................................................................................................................................................................. 85 G94. Landslide in Golden ............................................................................................................................................................................... 88 ILLUSTRATIONS Page FRONTISPIECE. Photograph showing the physiographic setting of the Golden quadrangle. PLATE 1. Profiles of major streams in the Golden quadrangle ................................................................................................................ In pocket FIGURE 1. Index map showing location of the Golden quadrangle and other quadrangles in the Denver area 2 2. Shaded relief map of the Golden quadrangle showing principal landforms ............................................................................ 4 3. Map showing localities of measured sections and samples obtained for lithologic and materials-tests data of the Golden quadrangle mentioned in this report ......................................................................................................................... 7 4. Diagram showing correlation of formations mostly of Paleozoic age as used in the vicinity of Golden by various authors. 9 5. Photograph of ripple marks in the Lykins Formation at Red Rocks Park ............................................................................... l2 6. Diagram showing correlation of formations mostly of Mesozoic and Tertiary age as used in the Golden area by various authors ...................................................................................................................................................................................... l4 7. Columnar section of the Dakota Group ............................ 16 8. Photograph of the hogback of the Dakota Group north of Tucker Gulch... 17 9. Diagram showing lithologic subdivisions of the Pierre Shale near Golden ............................................................................. 22 10. Diagram of lithologic sections of the Fox Hills Sandstone near Golden .................................................................................. 29 11. Photograph showing the Fox Hills Sandstone exposed in the Denver and Rio Grande Western Railroad cut at Plastic sidin ........................................................................................................................................................................................ 30 12. Map shgowing locations of abandoned coal mines and the approximate area of mined-out coal in the Golden quad- rangle ........................................................................................................................................................................................ 33 CONTENTS TABLES FIGURE CONTENTS IX 13. Correlation chart of the Laramie, Arapahoe, and Denver Formations as used in the vicinity of Golden by various Page authors ................................................................................... 43 14. Correlation chart of the Denver Formation as used in the vicmity of Golden by various authors .......................................... 45 15-17. Photographs: 15. Latite lava flows and the underlying Tertiary and Cretaceous Denver Formation exposed at Castle Rock .............. 46 16. Contact between Pierre Shale and monzonite in the water outlet tunnel on the east side of Ralston dike .......... 50 17. Air oblique view of well-developed columnar structure in latite of lava flows 2 and 3 on the north side of South Table Mountain ............................................................................................................................................... 51 18. Sketch of section showing sedimentary rocks between the two youngest lava flows ................................................................ 52 19. Map showing contours drawn on the base of the middle latite lava flow and its relation to Ralston dike ............................ 54 20. Chart showing correlation of Quaternary formations as used in the vicinity of Golden by various authors and showing the time of formation and relative development of old soils .................................................................................................. 55 21. Photograph of alluvial terraces in the valley of Ralston Creek ................................................................................................. 57 22. Cumulative curve showing the size distribution of sample R46 of pre-Rocky Flats alluvium ................................................. 58 23. Cumulative curves showing the size distribution of samples of Rocky Flats Alluvium ......... 59 24. Profile of the crestline of the hogback of the Dakota Group .................................................................... 60 25. Cumulative curve showing the size distribution of a sample of Verdos Alluvium from locality G101 ................................... 63 26. Photographs of crudely stratified deposit of mixed Verdos Alluvium, old alluvial fan, and rhyolitic volcanic ash, and closeup of the volcanic-ash-bearing silt bed ........................................................................................................................... 54 27. Cumulative curve showing the size distribution of a sample of Slocum Alluvium from locality G82... 68 28. Photograph of normally sorted Louviers Alluvium deposited by Clear Creek .............................................. 70 29—31. Cumulative curves showing the size distribution of samples: 29. Louviers Alluvium deposited by Clear Creek ............................................................................................................... 71 30. Louviers Alluvium deposited by Ralston Creek ........................................................................................................... 71 31. Soil horizons developed on Louviers Alluvium ........................................................................ 73 32. Photograph of soil developed on Broadway Alluvium northwest of 50th Avenue and Indiana Street .................................... 75 33—37. Cumulative curves showing the size distribution of samples: 33. Soil horizons developed on Broadway Alluvium at locality GlO9 .............................................................................. 76 34. Piney Creek Alluvium from locality 842+00 ................................................................................................................. 77 35. Post-Piney Creek alluvium ............................................................................................................................................ 78 36. Old alluvial fan deposits ................................................................................................................................................ 80 37. Transported mantle ................................................................................................................................ 81 38. Diagrammatic maps and profiles showing the development of transported mantle surfaces by steam erosion .. 83 39. Cumulative curves showing the size distribution of samples of colluvium ...................................................... 34 40. Cumulative curves showing the size distribution of samples of loess ....................................................................................... 85 41—44. Photographs: 41. Loess with a moderately well developed soil unconformably overlying the Cca soil horizon developed on Louviers Alluvium .................................................................................................................................................. 86 42. Old landslide showing a disoriented mass of Denver Formation overriding Verdos Alluvium. 88 43. Large composite landslide with a typically hummocky surface ................................................................................... 89 44. Active slump landslide ..................................................................... 90 45. Cumulative curves showing the size distribution of samples of artificial fill ........................................................................... 91 46. Photograph of poorly compacted fill in abandoned clay pits of the Laramie Formation ........................................................ 92 47. Diagram showing soil-profile horizon terminology used in this report ......................... 93 4-8. Graph showing percent of clay minerals in various soils in the Golden quadrangle ................. 96 49. Map showing structure contours on the top of the Fox Hills Sandstone in the Golden quadrangle. ..................... 98 50. Sketch map of the fault slice in the Golden fault zone at Tucker Gulch in the Golden quadrangle ...................................... 100 51—53. Photographs: 51. Part of the Golden fault zone in the north bank of Tucker Gulch ............................................................................. 101 52. Structural distress in an apartment building west of the Colorado School of Mines ....... 106 53. Hummocky topography on an ancient landslide reactivated by highway construction ............................................. 107 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO By RICHARD VAN HORN ABSTRACT The Golden quadrangle, Jefferson County, Colo., lies at the western edge of the rapidly developing Denver metropolitan area. As in many other rapidly developing urban areas, the rock types and geologic setting are not everywhere entirely suitable for certain types of construction. Rec- ognition of geologic factors is essential to sound planning for efficient land use in this area of variable foundation conditions and dwindling availability of construction materials. The quadrangle lies athwart the boundary between the Front Range of the southern Rocky Mountains and the Colorado Piedmont of the Great Plains. No geologic map accompanies this report; it has, instead, been published as U.S. Geological Survey Miscellaneous Geologic Investiga- tions Map I—761—A (Van Horn, 1972). Precambrian metamorphosed sedi— mentary and volcanic rocks form the mountains of the Front Range. They include a mica schist unit; hornblende gneiss unit and four sub- units; a microcline—quartz—plagioclase—biotite gneiss unit; and inter- layered gneiss unit. These rocks have been folded, faulted, and intruded by pegmatite and hornblende-biotite lamprophyre dikes. The Precam- brian rocks are overlain unconformably by Paleozoic sedimentary rocks of the Fountain Formation. Paleozoic, Mesozoic, and Cenozoic sedimen- tary rocks overlying the Fountain are, from oldest to youngest, the Lyons Sandstone, Lykins Formation, Ralston Creek Formation, Morrison Formation, Dakota Group, Benton Shale, Niobrara Formation, Pierre Shale, Fox Hills Sandstone, Laramie Formation, Arapahoe Formation, and Denver Formation. These rocks have been folded, faulted, and in- truded by monzonite dikes. Lava flows of latite occur within the Denver Formation. Unconformably overlying the bedrock are superficial deposits of Pleistocene and Holocene age which include: pre-Rocky Flats alluvium, Rocky Flats Alluvium, Verdos Alluvium and a contained rhyolitic vol- canic ash possibly equivalent to the Pearlette Ash Member of the Sappa Formation, Slocum Alluvium, Louviers Alluvium, Broadway Alluvium, Piney Creek Alluvium, post-Piney Creek alluvium, alluvial fan depo- sits, transported mantle deposits, colluvium, loess, landslide deposits, and artificial fill. Near, and west of, the foothills of the Front Range of the Rocky Moun- tains are steeply dipping metamorphic, sedimentary, and igneous rocks. East of the foothills the sedimentary rocks dip gently eastward into the Denver basin. The Precambrian rocks record probable volcanic and marine sedimentary events and were deformed at least three times. The Paleozoic and Mesozoic rocks record two invasions of shallow seas into the area. The youngest Mesozoic and Tertiary rocks indicate volcanic activity in the foothills and in the Front Range. The landscape of today is the result of recurrent episodes of erosion and alluviation during Quater- nary time. At least eight periods of alluviation resulted in some changes of major valleys and several instances of stream piracy. The alluvial deposits had soils formed on them and were eroded during intervals be- tween alluviations. Strongly developed soils formed prior to deposition of the Louviers Alluvium, and less strongly developed soils formed after deposition of the Louviers. Deposits of alluvial fans, transported mantle, colluvium, and landslide debris have been forming continuously throughout Quaternary time. In 1968 there were five active landslides on North and South Table Mountains. Sources for large amounts of sand and gravel, rock for crushing, coal, common clay, and shale suitable for expanded aggregate are present in the area. Foundation problems may result from potentially expansive parts of the colluvium, transported mantle, and the Ralston Creek, Morrison, Benton, Pierre, Fox Hills, Laramie, Arapahoe, and Denver Formations. Differential settlement is a potential cause of foundation failure in deposits of loess, artificial fill, and landslide debris. The surficial materials, and most of the shale, the mudstone, and the Denver Formation (except the latite), probably can be excavated with power equipment. The remainder of the rock units locally will require blasting to excavate. The alluvial deposits provide the best drainage for disposal of effluent from small septic systems, but care should be taken to see that the systems are not discharging noxious wastes into the ground water. The cut-slope stability of the geologic units is affected by attitude of the bedding and joints, rock type, and physiographic setting. Some of the geologically significant events that occurred during 26 of the years be— tween 1878 and 1914 are tabulated. INTRODUCTION Knowledge of the composition, physical properties, and geometry of the materials that underlie the land surface is a necessary adjunct to proper land-use planning in urban areas. This report delineates the rock and surficial depo- sits, describes their structural and topographic settings, and provides information pertaining to their suitability for various construction and other economic uses. Rec- ognition of these geologic factors is essential to sound planning for efficient land use in this area of variable foundation conditions and dwindling availability of con- struction materials. The Golden quadrangle, Jefferson County, Colo., is one of several quadrangles in the rapidly developing Denver metropolitan area that have been, or are being, studied by the U.S. Geological Survey (fig. 1). Fieldwork started in the Golden quadrangle during the summer of 1952 and was carried on intermittently until the winter of 1962. The geology was mapped on aerial photographs at a scale of 1217,500 and transferred to a topographic base map by means of an ER—‘55 projector. The geologic map has been published separately as Miscellaneous Geologic Investi- gations Map I—761—A (Van Horn, 1972), which should be consulted for better understanding of this report. Features pertaining to bedrock are shown in red on the geologic GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO 105°22'30" , . u o o ' , ,, o , 15 730 105 104 52 30 40 00 \FBoulder ' \ O \ 0 an <9 0Q? 06 0% 4w" V0 (be 0‘ 06 <0 9Q] C10 0 v I l 52 30 Standley \ Lake v. 0 e \\ 4‘? 4° vq‘ 6 («,5 \ V & <2? 0" \ Q) \ ,— .—r 1. _ _.'_|. _ L. Mag/3,- Creek 4Q? ". e e 45: Indian Gulch v- _ Mt ZI nX M \930 L k {Mt /\ Lakewood Gulch n 95$ 4" X °° °” 90% DENVER Q‘ Q’ \ _ 0 ° <2~ - in; Q \é‘ All], \ Q~ weir Gulch <0 ‘ 6 «,4 60 \MO @OGréen " | Q Mtn I— I—1 “é— " I" ' 960‘ Oak \ Red Rocks Park LJ __.—. l— - Morrison L I": _I \ a L__ ( _> S. 37, 30.. \\O V 4E O Marstor} \( Reservozr 4 % '7 <3? Q Q?‘ 30' - OLyons raa of his A '99:” ' oLimon COLORADO oPueblo 39°22'30" 0 5 10 MILES : I u] I: l 1‘ l I [ I 0 5 10 KILOMETRES PHYSIOGRAPHY 3 map (Van Horn, 1972), whereas those pertaining to the surficial deposits are shown in black. The red locality numbers shown on the geologic map are keyed to Pierre Shale fossil localities in table 3. Localities from which samples were obtained for lithologic and materials tests are shown in figure 3. Color designations in this report follow the Munsell color identification system (Goddard and others, 1948; Munsell Color Co., Inc., 1954). The rocks older than the Pierre Shale are given cursory treatment in this report as they were described in considerable detail in a publication on the adjoining Ralston Buttes quad— rangle (Sheridan and others, 1967). Data gathered since a bedrock geology map was issued (Van Horn, 1957b) have required changes in the mapping of faults in the northwestern part of the quadrangle and revision of the contact between the Laramie and Arapa- hoe Formations in the northern part. The delineation and terminology of the Precambrian map units also have been changed to accord with the subdivisions mapped in the Ralston Buttes quadrangle (Sheridan and others, 1967). ACKNOWLEDGMENTS Cretaceous fossils were identified by W. A. Cobban, unless otherwise noted. These determinations were essen- tial in delineating the complicated structural patterns in the Pierre Shale in the northern part of the quadrangle. The Los Angeles abrasion test and some of the other materials tests mentioned in the report were made by personnel of the Colorado Department of Highways. Clay and other minerals in the Pierre, Fox Hills, and Laramie Formations were identified principally by A. J. Gude III using X-ray diffraction techniques on the fraction smaller than 74 microns. One suite of 10 samples from the Denver and Rio Grande Western Railroad (previously known as the Denver and Salt Lake Railway) cut at Plastic siding was identified by F. A. Hildebrand using information obtained from X-ray diffraction on particles of less than one-half micron, microscopic examination of thin sections, the electron microscope, and differential thermal analysis of the fraction smaller than 2 microns. By the use of X-ray diffraction techniques Paul Blackmon identified the clay and other minerals found in soils developed on 4 FIGURE 1.—Location of the Golden quadrangle and other quadrangles in the Denver area for which geologic maps have recently been pub- lished by the U.S. Geological Survey. Eldorado Springs (Wells, 1967); Louisville (Spencer, 1961; Malde, 1955); Ralston Buttes (Sheridan and others, 1967); Golden (this report; Van Horn, 1957b, 1972); Den- ver area (Hunt, 1954); Morrison (Smith, 1964; Gable, 1968; Scott, 1972); Indian Hills (Scott, 1961; Bryant and others, 1973); Littleton (Scott, 1962); Platte Canyon (Peterson, 1964); Kassler (Scott, 1963a, b); Evergreen (Sheridan and others, 1972); Parker (Maberry and Lindvall, 1972); Arvada (Lindvall, 1972). Quaternary deposits. G. A. Macdonald, H. A. Powers, and R. E. Wilcox were most helpful in the interpretation of some of the volcanic features found at Ralston dike and at North Table Mountain. The cooperation of Mrs. Pansy Parshall Hook, of the Golden Historical Museum, and Miss Patricia Van Horn enabled me to compile the data contained in the summary of historical geologic events (p. 108). All the landowners were gracious in allowing me access to their property. PREVIOUS WORK Although the Golden area had been settled by 1850 and coal mines were operating in 1861 or 1862, the first general geologic study of the area was that by Hayden (1869). More detailed work by members of Hayden’s survey was reported in 1874 and the first colored geologic map of the area was published in 1877 (Hayden, 1874, 1877). The next, and perhaps the most important, geologic study of this area was published in 1896 (Emmons and others, 1896). Subsequent publications deal mainly with particular stratigraphic or structural problems in the area east of the Front Range. Important contributions to the present understanding of structure and stratigraphy in the area have been made by Leo Lesquereux, Arthur Lakes, H. B. Patton, G. L. Cannon, N. M. Fenneman, G. B. Richard- son, W. T. Lee, Victor Ziegler, ]. Harlan Johnson, F. H. Knowlton, R. L. Heaton, F. M. Van Tuyl, T. S. Lovering, W. A. Waldschmidt, R. W. Brown, and L. W. LeRoy. Many of the publications are concerned with the contro- versial sequence of beds above the Fox Hills Sand- stone—the “Laramie problem”—and with the details of the Golden fault and associated faults. These problems are briefly discussed in this report at the beginning of the sections on Cretaceous and Tertiary stratigraphy and on structure. PHYSIOGRAPHY The Golden quadrangle is principally in the Colorado Piedmont section of the Great Plains physiographic province but includes a small part of the Front Range of the Southern Rocky Mountains province (frontispiece and fig. 2). The dividing line between these two provinces is the steep escarpment that marks the east edge of the hard Precambrian rocks of the mountains. East of the escarp- ment, hogbacks and valleys formed on the upturned sedi- mentary rocks of Paleozoic and Mesozoic age make up the foothills. East of the foothills most of the area is terraced and slopes eastward. This general eastward slope is broken only by the lava-capped mesas of North and South Table Mountains, which rise steeply BOO—1,000 feet above Clear Creek, and by Ralston dike, which forms a steep-sided GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO 105°15' 39°52'3o~ 1 2'30" R. 70 W 10' R.69W 50’ 4730" 39°45' ‘ O-~O 1 $ ('3 MILES I l 1 l 3 KILOMETRES FIGURE 2.-——The principal landforms of the Golden quadrangle are strikingly displayed by this shaded—relief map. Alluvial terraces in the eastern part of the quadrangle rise westward and abut sharp-crested, northwest—trending hogbacks and the foot of the Rocky Mountain Front Range. This pattern is interrupted near Golden by two lava-capped mesas. From US. Geological Survey Golden 7‘xé-minute quadrangle (shaded relief), scale 124,000, 1965. 105'07’30' PRECAMBRIAN ROCKS 5 ridge southeast of Ralston Reservoir. Most of the bedrock is covered by terrace alluvium, and on the steeper slopes, by colluvium. Landslides form small but distinctive patches of hummocky terrain on many of these steep slopes. STRATIGRAPHY The stratigraphic nomenclature used in this report has evolved over a period of almost 100 years. During this time some stratigraphic terms have been used in many differ- ent ways, formations have been redefined and renamed, and the age designation of some rock units has been changed. Table 1 shows the geologic units in the Golden quadrangle and indicates which of them were not mapped separately. The measured section G25 (fig. 3) of bedrock included in this report is part of an almost continuous exposure along Ralston Reservoir and Ralston Creek. The section was measured when the water in the reservoir was at a low stage, and a tape-and-compass survey was made between control stations located by planetable. The individual sections for the Fountain through Benton were included in another report (Van Horn, in Sheridan and others, 1967) and are not repeated here. The numbering system of beds used in that report is continued here. PRECAMBRIAN ROCKS The nomenclature of the Precambrian rock units used in the earlier map of the Golden quadrangle (Van Horn, 1957b) has been discarded in favor of the nomenclature of the more precisely named rock units in the adjoining Ralston Buttes quadrangle (Sheridan and others, 1967). These new units were traced eastward into the Golden quadrangle from the edge of the Ralston Buttes quad- rangle by photogeologic methods and comparison with the earlier mapping. As the rocks have been described in detail by Sheridan, Maxwell, and Albee (1967), they are discussed only briefly here. The Precambrian rocks are de- scribed in order of their appearance from north to south because their relative ages are unknown. METAMORPHOSED SEDIMENTARY AND VOLCANICO) ROCKS Metamorphic rocks, mostly covered by colluvium, occupy an area of about 1.5 square miles in the southwest- ern part of the quadrangle. These resistant rocks form the easternmost slopes of the Front Range and, although out- crops are common, individual units are generally diffi- cult to follow for any distance. These rocks were orig- inally deposited as sediments (Lovering and Goddard, 1950, p. 20) and perhaps in part as volcanics (Sheridan and others, 1967, p. 5), and were later metamorphosed. The strong easterly foliation, which dips from 58° N. to 56° 5., is a reflection of original sedimentary bedding. The rocks have been folded and faulted and have been intruded by pegmatite and hornblende-biotite lamprophyre dikes. The Precambrian rocks are overlain with angular uncon- formity by sedimentary rocks of the Fountain Formation. MICA SCI-HST UNIT A narrow band of mica schist, north of Cressmans Gulch, is principally muscovite, biotite, and quartz. Alter- nating layers of different amounts of these materials give the rock a light- to dark-gray banding. It commonly weathers brown and at places the mica gives it a silvery sheen. HORNEBLENDE GNEISS UNIT The horneblende gneiss unit consists of four subunits: (l) interlayered hornblende gneiss, amphibolite, and biotite-quartz-plagioclase gneiss; (2) garnetiferous schist; (3) mica schist; and (4) layered calc-silicate gneiss and asso- ciated rock. INTERLAYERED HORNBLENDE GNEISS, AMPHIBOLITE, AND BIOTITE-QUARTZ-PLAGIOCLASE GNEISS The interlayered hornblende gneiss, amphibolite, and biotite-quartz-plagioclase gneiss extends from Golden Gate Canyon northward to a point about halfway to Cress- mans Gulch. The unit is predominantly a gray biotite gneiss composed of biotite, plagioclase, and quartz. This gneiss is interlayered with dark-gray to black amphibolite, hornblende gneiss, and biotite gneiss. The amphibolite is principally hornblende and plagioclase. The hornblende and biotite gneisses are principally plagioclase, quartz, and hornblende or biotite. Several of these dark-colored layers, 3—10 feet thick, crop out on the north side of Golden Gate Canyon. Near the north border of this unit a 10-foot- thick layer of light-gray quartzite forms a bold west-trend- ing outcrop, interrupted by a pegmatite dike. The quartzite is composed of subround quartz grains and small amounts of biotite. GARNETIFEROUS SCHIST A layer of garnetiferous schist, 50—80 feet thick, is present in Golden Gate Canyon. It is principally quartz and feldspar, but contains garnet and biotite as accessory minerals. The rock is light to medium gray with black flecks of biotite and flattened purple garnet crystals as much as one-half inch in diameter. The garnetiferous schist is lenticular and is enclosed in the layered calc-sili- cate gneiss and associated rock. This layer was previously mapped as garnet gneiss (Van Horn, 1957b). MICA SCHIST The mica schist south of Cressmans Gulch is poorly exposed in the Golden quadrangle. It is light to medium gray and in the Golden quadrangle is principally quartz, muscovite, and biotite. The mica schist forms a thick layer overlain and underlain by the layered calc-silicate gneiss and associated rock. LAYERED CALC-SILICATE GNEISS AND ASSOCIATED ROCK The layered calc-silicate gneiss and associated rock occurs on both sides of Cressmans Gulch. It is various GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO TABLE l.—Formati0ns of the Golden quadrangle, Colorado Era System Series Age Rock unit Artificial fill Holocene Post-Piney Creek alluvium Pinev Creek Alluvium Order Landslides Holocene doeS Colluvium and . nOt Transported mantle Pleistocene show Voung age Alluvial fan re|a~ Old E tions Loess N Quaternary _ t o Pinedale Broadway Alluvmm : 8 Bull Lake Louviers Alluvium Pleistocene Sangamon or Slocum Alluvium IIImouan rm Ya outh or Verdos Alluvium Bearlette Ash Member equivalentl Kansan Af onian or t Rocky Flats Alluvium Neb raskan Plaistocenfl?) Pre-Rockv Flats alluvium ‘Tertiary Paleocene Denver Formation and associated Tertiary latite and rnonzonite Arapahoe Formation Laramie Formation Fox Hills Sandstone Pierre Shale Smoky HiII Shale Member Niobrara Formation Upper Fort Hays Limestone Member Cretaceous .g Carlile Shale equivalent' 3 Bridge Creek Shale Member equivalent:l v, Benton Greenhorn a) Limestone Hartland Shale Member equivalent1 2 Shale e uivalenl:l q Lincoln Limestone Member equivalentl Graneros Shale equivalent‘ Lower Dakota South Platte Formationl GFDUP Lytle Formation' _ Morrison Formation Jurassm Upper Ralston Creek Formation Triassic(?) Strain Shale Memberl 2 Glennon Limestone Memberl 2 Q Lykins Formation Bergen Shale Memberl 2 g Permian Falcon Limestone Member' 2 8 Harriman Shale Member‘ 2 at? Lyons Sandstone Farms |- - ~ vanign Fountain Formation Hornblende biotite lamprophyre Order Pegmatite may lnterlayered . . . t not _ Garnetlferous quartz-biotlte schlst gneisses re» Microcline- uartz- la ioclase-biotite neiss unit flect q P 9 9 age Layered calc-silicate gneiss and associated Precambrian '°°k Mica schist Hornblende gneiss Garnetiferous schist unit lnterlayered hornblende gneiss, amphibolite and biotite-quartz-plagioclase gneiss Mica schist unit 'Not mapped separately. 1 Of LeRoy (1946). PRECAMBRIAN ROCKS 105°15' 1230~ R. 70 w. 10' R. 69 w. 105'07'30' 39°52'3o~ _ . J , ‘ . V , 1 2 ° | l | | 3 KILOMETRES O——O _‘_ N— FIGURE 3.—Loealities of measured sections and samples obtained for lithologic and materials-tests data of the Golden quadrangle mentioned in this report. Base from US. Geological Survey Golden 35-minute quadrangle (shaded relief), scale 1224,000, 1965. 8 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO shades of gray, apparently well laminated, and is composed principally of epidote, quartz, hornblende, and plagioclase. MICROCLINE-QUARTZ-PLAGIOCLASE-BIOTITE GNEISS UNIT Microcline-quartz-plagioc1ase-biotite gneiss, or briefly, microcline gneiss, occupies most of the area between Golden Gate and Clear Creek Canyons. Midway between the two canyons are thin layers of amphibolite and dikes of hornblende-biotite lamprophyre. Pegmatite dikes have intruded the rock near the east end of the outcrop. The main body of the rock is a pink to gray, fine- to medium-grained foliated gneiss. Small amounts of biotite give a crudely layered and black speckled appearance. The rock is composed principally of quartz, microcline, and plagioclase with smaller amounts of biotite. However, in the first canyon north of Clear Creek (Magpie Gulch), the rock is interlayered with dark-gray amphibolite that contains plagioclase, quartz, and hornblende in about equal amounts, and a small amount of biotite. INTERLAYERED GNEISS The interlayered gneiss extends from the southern quadrangle boundary part way up the north valley wall of Clear Creek to about the working face of the large abandoned quarry. The north margin is marked by a bed of garnetiferous quartz-biotite schist, a subunit of the interlayered gneiss. The interlayered gneiss is dominantly a gray fine- grained plagioclase-quartz-biotite gneiss; at places the rock is pink because of potassic feldspar. Interlayered with the gneiss are thin beds of dark-gray amphibolite. The rock is hard and breaks into sharp angular fragments when fresh. Fresh rock from the abandoned quarry in sec. 32 shows minor alteration along fractures and of the feldspar; a sample examined by J. Berman (Hickey, 1950, p. 10) was predominantly quartz, potassic feldspar, and oligoclase, with smaller amounts of micas, hornblende, magnetite, apatite, zircon, and sphene. GARNETIFEROUS QUARTZ-BIOTITE scmsr A layer of garnetiferous quartz-biotite schist, 15-25 feet thick, marks the north contact of the interlayered gneiss in Clear Creek Canyon. The garnetiferous quartz-biotite schist is predominantly quartz, biotite, and garnet. The rock is mottled medium gray, pale pink, and dark brown, and the garnet crystals give the weathered surface a rough, bumpy appearance. The garnet crystals, which range from 0.1 to 1 inch in diameter, are larger and more abundant in the western part of the outcrop area than in the eastern part. They are fractured, have inclusions of quartz and mica, and generally are altered. Small blebs of fresh- looking garnet are deep ruby red. This layer was previously mapped as garnet gneiss (Van Horn, 1957b). IGNEOUS ROCKS Igneous rocks consisting of pegmatite and hornblende- biotite lamprophyre intrude the Precambrian rock units at several places, but do not intrude the younger sedimen- tary rocks. They are probably of Precambrian age. PEGMATITE Three small pegmatite dikes are shown on the accom- panying geologic map. They are coarsely crystalline and consist principally of quartz, potassic feldspar, and small amounts of mica. The dikes cut across the foliation of the enclosing metamorphic rocks. The largest dike is 1,300 feet long and 100 feet wide. HORNBLENDE-BIOTITE LAMPROPHYRE Two east-trending hornblende-biotite lamprophyre dikes, previously mapped as biotite syenite (Van Horn, 1957b), are located between Clear Creek and Golden Gate Canyon. They are dark gray flecked with pink, and are composed mainly of biotite, hornblende, and potassic feldspar. The biotite forms irregularly shaped black phenocrysts as much as 5 mm across, and the pink to white feldspar and dark-green hornblende phenocrysts are gen- erally smaller. Although these dikes are plainly visible on aerial photographs, in the field they are poorly exposed but are marked by a weathered zone that is less resistant than the enclosing rock. At the only exposure found, the dike is 2 feet thick. Both dikes terminate abruptly at the contact of the Precambrian rocks and the Fountain Forma- tlon. PALEOZOIC ROCKS Rocks of Paleozoic age include the Fountain Forma- tion, Lyons Sandstone, and the lower part of the Lykins Formation. These rocks are sparsely fossiliferous and orig- inally all were included in a red-bed sequence assigned to the Trias (Hayden, 1869). Later, Emmons, Cross, and Eldridge (1896) assigned them to the Wyoming Forma- tion. The Wyoming Formation has been abandoned. The rocks formerly assigned to it in the Denver area were sub- divided into the Fountain, Lyons, and Lykins by Fenne- man (1905). (See fig. 4.) PENNSYLVANIAN AND PERMIAN FOUNTAIN FORMATION In the Golden quadrangle the Fountain Formation consists of about 800 feet of conglomerate, sandstone, and mudstone. North of Tucker Gulch it occupies a valley between the mountains to the west and a low hogback of Lyons Sandstone to the east; south of Tucker Gulch the upper half is cut out by the Golden fault and the lower half is overlain by thick deposits of surficial material. The Fountain Formation rests on an eroded surface cut on folded and faulted Precambrian rocks, thus marking a pro- nounced angular unconformity. Sparse pebbles of lime- PALEOZOIC ROCKS UTHOLOGY HAYDEN EMMONS AND FENNEMAN LEROY ““3 PAPER (1869) OTHERS (1896) (1905) (1946) UNIT AGE E 9 0: Strain Strain 2 Shale Shale E Member Member1 _) l— , C C C c ,9 .9 .9 E, E Crinkled E Glennon E Glennon E E Sandstone 5 Limestone 5 Limestone L L'- Member L'- Member “- Member‘ ‘3. 3 8 3 Da 5‘; S; Bergen .3 Bergen _l .1 Shale _l Shale Member Member1 8 Falcon Falcon g Limestone Limestone E Member Member1 . 0 Harriman Harriman “'35 "" Shale Shale E’ Member Member' E Cl.) 2 > c 3 E 9 , z gg Lyons Sandstone Lyons Formation Lyons Sandstone <_( L C E 03 (I u.) m _7 y e .6 E E, g E ‘5 z a v E E S 2 E o 0 Z O U) Ll. LL < ._ c c c > °§ '5 a E 4 o s g g 5 "J o o 0 2 u. u. u. 2 Lu 0. PRECAMBRIAN PRECAMBRIAN ‘Of LeRoy (1946), Shale or mudstone Limestone Conglomerate Sandstone FIGURE 4.—Correlation of formations mostly of Paleozoic age as used in the vicinity of Golden by various authors. stone in the Fountain suggest that Mississipian and older Paleozoic rocks were once present nearby. These older rocks evidently were completely eroded from the Golden area before deposition of the Fountain began in the Middle Pennsylvanian. The Fountain is composed of pink to reddish- orange, coarse- to fine-grained, crossbedded, arkosic conglomeratic sandstone and conglomerate interbedded with lenticular, dark-reddish-brown, micaceous, silty, indurated mudstone and pinkish-gray, fine-grained, crossbedded, quartzose sandstone. The base of the forma~ tion is commonly marked by a sedimentary breccia containing many angular cobbles and small boulders. Within a few feet this breccia gives way to the normal conglomeratic sandstone. Conglomeratic sandstone is generally predominant in the formation, except for the lower 150 feet where mudstone is dominant. Well-de- veloped mudcracks were seen in the mudstone in one place west of the quadrangle. In the upper 30 feet there are several lenticular quartzose sandstone beds similar to 10 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO sandstone in the overlying Lyons Sandstone. The F oun- tain generally weathers to smoothly rounded outcrops. The coarse fraction of the formation, which includes cobbles as much as 7 inches in diameter, is composed of quartz and pink feldspar and minor amounts of schist, gneiss, quartzite, granite, and limestone. The finer frac- tion is principally quartz and feldspar, much of which appears to be pink microcline. The grains are subangular to subround and moderately to strongly cemented in a clayey matrix that has much red iron oxide, probably hematite, and silica. Thin light-gray streaks, spots, and zones are common; south of Clear Creek the light-gray color is as prevalent as the pink. The color change, which most commonly is roughly parallel to the bedding and is very abrupt, in most places occurs in the sandstone and conglomerate beds, generally where they are in contact with the mud- stone. There is no apparent change in texture or degree of cementation of the sandstone across the color boundary. At most places the light gray is adjacent to a dark-red mud- stone, but at some places the mudstone also contains light- gray horizons. X-ray fluorescence analyses of four samples indicate there is less elemental iron in the light-gray beds than in adjacent red beds (Hubert, 1960, p. 59). The Fountain overlies the Precambrian rocks with pro- nounced angular unconformity; the base of the Fountain cuts sharply across the structural and lithologic trends in the Precambrian. Although the contact is poorly exposed at most places, the float indicates that the eroded surface of the Precambrian, on which the Fountain was deposited, is relatively planar. At a few places outside the Golden quad- rangle the rocks underlying the Fountain are deeply weathered (Wahlstrom, 1948; Hubert, 1960, p. 46). AGE AND CORRELATION No fossils have been found in the Fountain in or near this area, hence the Pennsylvanian and Permian age designation depends on correlation of units outside the area believed to be equivalent to the Fountain. The first fossils noted in this formation, dated as Early Pennsylvanian by David White, were found in the Glen Eyrie Shale Member of the Fountain Formation (Finlay, 1907, p. 588—589) about 80 miles south of Golden. The Ingleside Formation, defined from outcrops about 80 miles north of Golden (Butters, 1913, p. 68), is now regarded as equivalent to the upper part of the Fountain Formation, and contains fusulinids of Permian age (Heaton, 1933, p. 119; Maughan and Wilson, 1960, p. 42). Thus, the Fountain appears to be Pennsylvanian in the lower part and Permian in the upper part. This concept had been intimated earlier by Maher (1946, p. 1760) on the basis Of correlation of widely spaced well logs. ORIGIN The Fountain is of terrestrial origin. The conglom- eratic beds at many places occupy channels that cut into underlying mudstone or conglomeratic beds. Cross- bedding is well developed in the conglomeratic beds. The mudstone beds are massive, generally less than 20 feet thick, and appear to be lenticular. The mudstones rep- resent flood-plain or lake deposits cut by streams that deposited the coarse material in the eroded channels. The terrestrial sediments of the Fountain interfinger eastward with marine Pennsylvania and Permian deposits (Maher and Collins, 1952). The source of the arkosic material in the Fountain was a Precambrian highland of Pennsylvanian-Permian time—the ancestral Rocky Mountains—west of Golden. The highland had been mostly stripped of any earlier Paleozoic rocks and had been deeply weathered before the Fountain was deposited on a gently eastward-sloping surface of low relief. PERMIAN LYONS SANDSTONE The Lyons Sandstone, about 150 feet thick, is light-gray to grayish-orange, fine- to medium-grained, crossbedded, quartzose sandstone but includes some conglomerate, silt— stone and mudstone. The light-gray color is predominant in this area and for many years the Lyons was called the “creamy sandstone” and was thought to be a separate unit from the grayish-orange Lyons found farther north. The Lyons Sandstone forms a low hogback north of Tucker Gulch, about midway between the Front Range and the high hogback formed by the Dakota Group. South of Tucker Gulch it has been cut out by the Golden fault. The sandstone, which makes up the greatest part of the Lyons, is composed principally of fine- to medium-grained quartz particles, strongly cemented by silica. Most of the grains are subround and many have a slightly to moderately frosted surface. A small amount of light-gray argillaceous material generally is present. The cross- bedding, though present at most outcrops, is not well. displayed in this area. Thompson (1949) described the crossbedding as well as many other unusual features of the Lyons. A bed of light-gray conglomerate as much as 100 feet thick marks the top of the formation north of Cressmans Gulch. The pebbles and cobbles (as much as 4 inches in diameter) are predominantly quartz and chert but include some sandstone. Between Cressmans Gulch and Tucker Gulch this conglomerate is missing, and mudstone of the Lykins Formation rests directly on sandstone of the Lyons; but south of Golden, conglomerate is again present at the top of the Lyons (LeRoy, 1946, p. 28). A similar conglom- erate is interbedded with sandstone in the lower 40 feet of the Lyons in Cressmans Gulch. This lower unit contains sandstone concretions as much as 2 feet in diameter, and it fills channels cut into the underlying Fountain Forma- tion. Thin beds of siltstone are present at a few places PALEOZOIC AND MESOZOIC(?) ROCKS 11 throughout the Lyons. Some of them fill channels cut into underlying sandstone beds. A thin reddish-brown mud- stone bed that occurs in the Ralston Buttes quadrangle (Van Horn, in Sheridan and others, 1967, p. 41) was not found in the Golden quadrangle, but could be present in the poorly exposed upper 40 feet of the formation. Although the contact with the underlying Fountain Formation is lithologically abrupt and unconformable, the unconformity is possibly no more significant than the numerous unconformities represented by stream-channel- deposits cutting into mudstone beds within the Fountain. The presence of several lenticular Lyons-like quartzose sandstone beds in the upper 30 feet of the Fountain prob- ably indicates that the sea was encroaching on the area, and these sandstone beds may represent an interfingering of the Lyons and Fountain. Although part of the upper Fountain has undoubtedly been removed by the trans- gressing sea, this boundary is, perhaps, best described as being transitional. AGE AND CORRELATION No fossils, other than a few reptile tracks, have been found in the Lyons. The Permian age was assigned by Gil- more (in Lee, 1927, p. 12) on the basis of these tracks. This age is corroborated by the stratigraphic position of the Lyons between two formations of established Permian age—the Ingleside and the Lykins. ORIGIN The origin of the Lyons has been the subject Of much conjecture. Thompson (1949, p. 67-71) proposed a beach- deposit origin because of features similar to those he observed on a modern beach near Balboa, Calif. Certain aspects of the crossbedded sandstones have been construed to indicate eolian deposition (Tieje, 1923, p. 202), and the conglomerates are probably fluvial. The Lyons was prob- ably deposited at or near the strandline of a transgressing sea, and many of the complex terrestrial and marine features of a seacoast are present in it. PALEOZOIC AND MESOZOICO) ROCKS PERMIAN AND TRIASSIC(?) LYKINS FORMATION The Lykins Formation, as used in this report, follows the original definition of Fenneman (1905, p. 24) as partly amended by LeRoy (1946, p. 31). It consists principally of mudstone and smaller amOunts of limestone and is about 450 feet thick. The Lykins crops out north of Tucker Gulch in the western half of the valley between the low hogback formed by the Lyons and the high hogback formed by the Dakota. South Of Tucker Gulch the Lykins Formation is cut out by the Golden fault, except at the small knoll along the west side Of sec. 21,T.3 S., R. 70 W., where a thin slice of it is exposed between faults in the Golden fault zone. Limestone float occurs at places be- tween this knoll and Clear Creek. This float seems to be de- rived from some of the limestone beds in the Lykins, and probably marks the trace of the Golden fault. The Lykins Formation is principally a grayish-red mudstone but includes some limestone and a few thin beds of very fine grained sandstone. LeRoy (1946, p. 30—47) has named and described five members of this formation. In ascending order they are the Harriman Shale Member, 55 feet thick; Falcon Limestone Member, 3 feet thick; Bergen Shale Member, 30 feet thick; Glennon Limestone Member, 14 feet thick; and Strain Shale Member, 350 feet thick. The two limestone members are in the lower fourth of the formation and a third limestone (unnamed), about a foot thick, is 13 feet above the base. LIMESTONE BEDS The unnamed limestone bed 13 feet above the base of the Lykins is very light to medium gray. It contains many small vugs and weathers to a sugary texture. The Falcon Member and the lower 4 feet of the Glennon Member are very light gray, hard, finely crystalline to dense, dolomitic limestone; the upper 10 feet Of the Glennon Member is a reddish-brown, tough, silty limestone. Vugs in the lime- stones contain calcite crystals. Although both limestones have the characteristic irregular wavy bedding of the “crinkled” limestone or sandstone of Fenneman (1905, p. 25), the bedding is mostly obscured because the alternate bands are nearly the same color. A small outcrop of the Falcon Limestone Member along the extension of Ala- meda Parkway in Red Rocks Park, 6 miles south of Golden, appears to have poorly developed ripple marks (fig. 5). They are similar to current ripple marks modified by wave action (Schrock, 1948, p. 95). The wavy bedding is emphasized by alternating reddish-brown and pale-red beds. The bulk of the rock ispale red, but the bedding is marked by a thin reddish-brown layer. Individual waves have a length of about 1 inch and an amplitude of 0.2 inch. Many of the individual laminae are subparallel to adja- cent laminae but at places several laminae may converge to form a thin band in which individual laminae cannot be distinguished. At other places adjacent subparallel laminae may diverge and then converge, to form small lentils. There are as many as 100 of these laminae per inch. Where weathered, the upper surfaces of the laminae have a distinctive dimpled appearance. Individual dimples are roughly circular, about an inch in diameter, and 0.2 inch deep. Adjacent areas contain small domes of the same size. The domes are generally connected to other domes by low saddles. The overall appearance of these features is similar to interference ripple marks. At some places there are domelike structures as much as 3 feet in diameter with amplitudes of about 0.5 foot. Individual laminae in the domes have the characteristic wavy bedding, although the amplitude of each wave is very small and the laminae usually are parallel. l2 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO FIGURE 5.—Ripple marks in the Lykins Formation exposed along the extension of Alameda Parkway in Red Rocks Park. Several disassociated segments of the dayscladacean alga Mizzia minuta Johnson and Dorr (USGS accession No. EG-55—12D [No. 55—RR—5 of Van Horn, in Sheridan and others, 1967, p. 42]) were found and identified by Richard Rezak (written commun., Jan. 27, 1956). The collections came from the upper part of the Glennon exposed along Ralston Creek just west of the Golden quadrangle. According to Rezak the genus M izzia has been found only in Permian rocks of the world, and in North America it has been recognized only in rocks of Guadalupian age. Rezak also stated that the large domelike structures in the Glennon appear to be due to blue-green algae. MUDSTONE BEDS Mudstone of the Harriman, Bergen, and Strain Members is grayish red, tough, and moderately cal- careous. The basal 2 feet of the Harriman is very sandy and moderately calcareous but within a few feet grades upward into the normal very fine grained mudstone. The mud- stone is composed of subangular to subround silt-sized grains in a matrix of clay-sized material. The reddish- brown clay matrix, which presents avery uniform appear- ance to the unaided eye, contains many thin anasto- mosing channels of denser color when viewed in thin section. These zones are probably the result of a secondary concentration of iron oxide. At places there are mottles as much as an inch in diameter of light-bluish-gray mud- stone, and at one place the small channelways of iron oxide terminate abruptly against these mottled spots. The mottled spots probably are also secondary and were formed after the channelways. The Strain Member is slightly coarser grained than the other members and contains some 0.1- to l-inch-thick beds of light-gray, very fine grained, silty sandstone interbedded with the mud- stone. The poorly displayed bedding in the mudstone gen- erally consists of uniformly sized and roughly parallel layers of thick to thin beds. At several places, however, the bedding is very lenticular; each lentil is about an inch long and 0.3 inch thick, and overlaps adjacent lentils. The lenticular structures, which are figured by Van Horn (in Sheridan and others, 1967, p. 43), do not form a well- defined zone. They are similar to the structures described by Moore and Scruton (1957, p. 2733-2735) which were found in the part of the ”regular layers” that occurred in water 6—360 feet deep near the Mississippi Delta. The contact of the Lykins with the underlying Lyons Sandstone is lithologically abrupt. The discontinuous conglomerate at the top of the Lyons could be construed to result from an erosional period intervening between the two formations. The merging of the Lyons and Lykins in northern Colorado described by Butters (1913, p. 70) indi- cates that any unconformity found at the contact has only local significance and that no long period of time inter- vened between the deposition of these formations. This merging relationship in northern Colorado has been reaf- firmed in terms of currently accepted nomenclature by Maughan and Wilson (1960, p. 39-40). They showed the Lyons interfingering with the lower part of the Satanka Shale and stated that the upper part of the Satanka is equivalent to the lower part of the Lykins. AGE AND CORRELATION The Permian age of the lower part of the Lykins (below the Strain Shale Member) is established by the occurrence of the algae M izzia in the Glennon Member. The Permian age has been postulated for many years, most recently by Broin (1958) and by Oriel and Craig (1960, p. 45), who have correlated the “crinkled” sandstone or limestone of Fenne- man (Glennon Limestone Member of LeRoy) with the Forelle Limestone of Permian age. Broin also correlated two carbonate-evaporite members of the Lykins that over- lie the Forelle in northern Colorado with the Ervay Tongue of the Phosphoria Formation (Permian) and the Little Medicine Tongue of the Dinwoody Formation (Triassic). A dolomitic limestone bed that is about 90 feet above the Glennon Limestone has been correlated with the Ervay Tongue of the Park City Formation by M. R. Mudge (in McKee and others, 1959). Mudge placed the contact between rocks of Permian and Triassic age at the top of the dolomitic limestone. Even though no limestone or dolo- mite beds were recognized above the Glennon in the vicinity of Golden, probably the upper part of the Strain is Triassic. MESOZOIC ROCKS 13 ORIGIN The algae and the ripple marks(?) indicate a shallow water origin of the limestone beds in the Lykins. According to Richard Rezak (written commun., Jan. 27, 1956), “the association of fragments of Mizzz'a with stromatolitic sediments [the large domelike structures] indicates a shallow-water, nearshore, marine envi- ronment, possibly intertidal. The crinkly limestone of the Lykins (Glennon Limestone Member), especially the upper 8 inches of the topmost unit at Ralston Reservoir, very closely resembles the intertidal algal deposits of south Florida and the Bahamas.” The crinkled structure of limestones in the Lykins has been ascribed to tectonic stress (Fenneman, 1905, p. 26) and to deformation of the limestone before it had consolidated into a rock (LeRoy, 1946, p. 37). It seems to me, however, that most of these features are a fortuitous preservation of ripple marks and algal growths. MESOZOIC ROCKS Rocks of the Mesozoic age include the upper part of the Lykins Formation (described in the previous section), Ralston Creek Formation, Morrison Formation, Dakota Group, Benton Shale, Niobrara Formation, Pierre Shale, Fox Hills Sandstone, Laramie Formation, Arapahoe Formation, and lower part of the Denver Formation. The upper part of the Lykins Formation is probably Triassic in age, the Ralston Creek and Morrison are of Jurassic age, and the rest are of Cretaceous age. (See fig. 6.) The Benton, Niobrara, and Pierre are of marine origin; the Fox Hills is transitional between marine and terrestrial; the rest are ter- restrial. JURASSIC RALSTON CREEK FORMATION The Ralston Creek Formation was recognized and de- scribed as the Ralston Formation by LeRoy (1946, p. 47—57) and renamed by Van Horn (1957a, p. 756). The Ralston Creek is exposed at a few places near the western base of the high hogback formed by the Dakota Group. It is about 100 feet thick. The Ralston Creek is composed of varicolored clay- stone, limestone, calcareous siltstone, and chalcedony. Light-gray to grayish-red siltstone beds are predominant in the lower third of the formation; these massive beds, 1—15 feet thick, are generally calcareous but a few appear to be siliceous. The rest of the formation is principally clay- stone but includes several thin limestone and siltstone beds. The claystone is grayish red, grayish orange, dusky red, pale green, or light gray; it is tough, silty, and generally calcareous, and occurs in beds 1—14 feet thick. The limestone is generally light gray, but at places is grayish red or grayish orange. When weathered it has the rough grainy appearance of an indurated siltstone. At places disseminated nodules or thin layers of chal- cedony form a small but important part of the formation. It is moderate red, white, and light and dark gray, and occurs as discrete layers as much as 0.4 foot thick or as small disseminated nodules in siltstone or limestone, in the middle and upper part of the Ralston Creek. The relation of the Ralston Creek to the underlying Lykins Formation is not clear. At most places in the Golden and adjoining Ralston Buttes quadrangles, the two formations appear to be conformable and there is no evidence of any extended interval of erosion or nondepo- sition between the two. Just west of the Golden quad- rangle at Ralston Creek, however, a 5-fOOt-thick bed of sandstone at the base of the Ralston Creek Formation may be an erosional remnant of the Entrada Sandstone (Van Horn, in Sheridan and others, 1967, p. 44). This sand- stone was not found elsewhere in the Ralston Buttes or the Golden quadrangles. It is not possible, at present, to definitely correlate this bed with the Entrada or to deter- mine how much Entrada may have originally been present. Possibly the sandstone bed is merely a stream- channel deposit and has no regional significance. AGE AND CORRELATION The Jurassic age of the Ralston Creek is shown by fossils collected near the Golden quadrangle. Fossil collections from the Ralston Creek Formation of the Kassler quad- rangle, 20 miles south of Golden, include “fresh-water gastropods identified by J. B. Reeside as Gymulus vetemus Meek and Hayden and Lymnaea morrisonensis Yen, and algae identified by R. E. Peck as Echinochara spinosa Peck of Kimmeridgian age” (Scott, 1963b, p. 92). Unio-like pelecypods identified by T. C. Yen (Imlay, 1952, p. 961) which were collected from just west of the Golden quad- rangle may compare with Vetulonaia faberz' Holt, a species found in the Morrison Formation. From this same locality LeRoy (1946, p. 51, 55) had previously collected and iden- tified the algae Aclistochara, which he believed indicated a freshwater or possibly brackish-water environment. The zone containing chalcedony has been widely used in sub- surface correlations in the Denver basin, and Ogden (1954) has pointed out the occurrence of chalcedony at about this same horizon at several places in the Rocky Mountain area. The chalcedony beds have also been used as a paleo- tectonic marker to separate the Kimmeridgian from the Oxfordian by McKee and others (1956), who placed the base of the overlying Morrison Formation at the top of chalcedony beds. The base of the Morrison in the present report, however, is placed at the base of a sandstone about 25 feet above the chalcedony beds. ORIGIN Coarsening of the upper part of the Lykins Formation probably indicates a withdrawal of the sea. Fossils in the succeeding Ralston Creek show that some of it was depos- l4 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO EXPLANATION UTHOLOGY HAYDEN EMMONS AND FENNEMAN BROWN LEROY REICHERT THIS PAPER “ (71869) OTHERS (1896) (1905) (1943) (1946) (1954) UN” AGE Denver .5 Golden Member Denver Tertiar Lava flow Denver Formation 3 Pleasant View Formation Denver V . E Member l — — — Formation ‘5 Formation u. Hornblende— S d 5 augite phase an y, shaly E srltstone or, 8 :rapa'l” o t mudstone Coal Arapahoe é FIint-chert rma '0" Arapahoe strata Formation A Arapahoe I: phase Formation Laramie 3 v Conglomer- m Formation t M be & Sandstone a 9 em ' Laramie Laramie Laramie Laramie Laramie , Formation Formation Formation Formation Formation Volcalnc debt“ Fox Hills Fox Hills Fox Hills Fox Hills Fox Hills FOX Hills Formation Formation Sandstone beds Formation Formation Conglomerate Fort Pierre Pierre Pierre Shale Pierre Fort Pierre he"? Formation Format'on Shale Late Formation E] Group Cretaceous Coal Lat 6 amount 'L_'LJ' 'L_* g g g o geologic — —‘— —r— —r— '23 . ~,—, . 'fi _ section not _.__ _L__ T E Aplshapa E Aplshapa E Smoky Hill Shown _ ; _l_ _ Niobrara Niobrara Niobrara “5- Shale E Shale LE Shale Member —'— Division Formation Formation a w in r a» a 5 a e a Very calcar — E 3 2 eous Shale Timpas Timpas Fort Hays Limestone Limestone Limestone Member li:l:i:l Carlile Shale equivalent Limestone 8::th Fort Benton _ Benton Benton Benton Ehreenhorn A A A A Benton Formation NOt d'SCUSSEd Formation Formation Shale lmestone Group equwalent Chalcedony G’anefos —‘_— Shale equivalent a. 3 South E , \— ar Dakota Dakota Dakota F 05km," F Dak°$a o Platte Cre‘aceyous Group Formation Formation ormatlon ormatlon '3 Formation ' X (B D Lytle Formation Morrison FMorrison FMorrison Morrison Formation . ormatlon ormation Formation . Morrison . Jurassw Formation Jurassw / Ralston Ralston Ralston Creek / Formation Formation Formation Upper Trias Division Lykins Triassic( 7) WV°"""9 Formation . l Formation Lyklns Lykins Lyklns Formation Formation Formation —-——- Permian FIGURE 6.—Correlati0n of formations mostly of Mesozoic and Tertiary age as used in the Golden area by various authors. MESOZOIC ROCKS 15 ited in fresh or brackish water, and gypsum beds found only 10 miles south of Golden (LeRoy, 1946, p. 53) indicate it was deposited near sea level. It is possible that the marine conditions passed gradually into the terres- trial and that the Lykins and the Ralston Creek were not separated by a long period of deposition and erosion. MORRISON FORMATION The Morrison Formation, about 250 feet thick, consists of varicolored mudstone, limestone, siltstone, and sand- stone. North of Tucker Gulch the Morrison crops out on the west slope of the high hogback formed by the Dakota Group; south of Tucker Gulch it crops out only in the small knoll along the west side of sec. 21, T. 3 S., R. 70 W., as part of a thin slice in the Golden fault zone. South of this place is has been cut out by the Golden fault. The Mor- rison Formation is principally greenish-gray, dusky-red, and dark-gray claystone and mudstone. Montmorillonite, illite, and kaolinite are present throughout the formation (Keller, 1953). A basal sandstone, 10—40 feet thick, is very light gray and contains limonite dots and concretions as large as one-half inch in diameter. It may be thin bedded, crossbedded, or massive. The sandstone cuts out beds in the underlying Ralston Creek Formation at places. Above the basal sandstone the lower part of the Morrison is principally dusky-red mudstone and thin beds of very light gray, fine- grained, calcareous sandstone. The middle part of the Morrison is poorly exposed but appears to consist mostly of greenish-gray to dusky-red claystone and of some beds of medium-gray dense, brittle limestone. In the upper part of the Morrison, thin beds of light- to dark- gray sandstone are interbedded with thick beds of claystone. A few thin beds of fine-grained sandstone near the top contain tiny white blebs of interstitial clay and a small amount of limonite nodules and black mineral grains. The quartz grains are mostly subround and have a frosted to matte surface. These sandstones break down when soaked in water and a moderate quantity of clay is visible. The basal sandstone of the Morrison Formation partly lies in channels cut into the Ralston Creek Formation. This disconformable relation, which is well exposed on the south side of Ralston Reservoir, is probably of minor extent inasmuch as the chalcedony beds of the Ralston Creek Formation are not known to be cut out by erosion nor is the top of the Ralston Creek known to be signifi- cantly higher than the chalcedony beds in this area. AGE AND ORIGIN N0 fossils were identified from the Morrison Formation in the Golden quadrangle. Algae, freshwater mollusks, dinosaurs, and plant remains have been reported from nearby areas, and most geologists now agree that the Mor- rison is of Late Jurassic age and of terrestrial origin as established earlier by Baker, Dane, and Reeside (1936, p. 55, 58, 63) and Yen (1952, p. 34, 35). It was probably depo- sited on a surface of low relief that contained sluggish streams and many lakes and swamps. CRETACEOUS Rocks of Cretaceous age, which make up the bulk of the sedimentary rocks in the Golden quadrangle, consist of the Dakota Group, Benton Shale, Niobrara Formation, Pierre Shale, Fox Hills Sandstone, Laramie Formation, Arapahoe Formation, and the lower part of the Denver Formation. The total thickness of the Cretaceous rocks is about 10,000 feet, and much of this is contained in one formation—the Pierre Shale. DAKOTA GROUP UNDIFFERENTIATED The Dakota Group along the eastern front of the Rocky Mountains in Colorado has been studied intensively by K. M. Waagé. The results of his investigations have been published in several papers, three of which contain strati- graphic information from the vicinity of Golden (Waagé, 1955, 1959, 1961). The Dakota Group of the present report is virtually the same as that of Waagé. The two formations of the Dakota (South Platte above and Lytle below) and their several members were not mapped separately by me in the Golden quadrangle, although they are shown on my measured section along Ralston Creek in the adjacent Ralston Buttes quadrangle (Van Horn, in Sheridan and others, 1967, p. 46—47) and in figure 7 of the present report. The Dakota Group consists predominantly of sand- stone, siltstone, and claystone, but near the base it contains some conglomerate and conglomeratic sandstone. The thickness of the Dakota is generally about 300 feet in this area, but it varies considerably along the strike. South of Tucker Gulch the Dakota has been cut out by the Golden fault; north of Tucker Gulch it forms a high, steeply sloping hogback (fig. 8). Between Tucker Gulch and the hogback is a thin slice of the upper part of the Dakota with faults on both sides. From Tucker Gulch to Van Bibber Creek the east side of the hogback is marked by a long sinuous scar caused by the surface caving into an abandoned clay mine. Rocks of the Dakota Group in most places are noncal- careous, hard, and resistant to erosion, and in many places they contain fragments of fossil plants. The rocks are mostly light-gray, fine- to medium-grained sandstone that is commonly crossbedded and ripple marked at places, light- to medium-gray siltstone, and dark-gray claystone. The sandstone beds at the base of the South Platte and Lytle Formations are locally conglomeratic; the pebbles are predominantly light-gray chert. These beds generally 16 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO Benton Shale First sandstone of Waagé (1955) Van Bibber Shale Member- first shale of Waage’ (1955) Kassler Sandstone Member—second sandstone of Waage (1955) Second shale, third sandstone, and third shale of Waagé (1955) Plainview Sandstone Member— fourth sandstone of Waagé (1955) __/——‘——,—\ Fourth shale of Waage (1955) Basalconglomeratic sandstone (Waage (1955) EXPLANATION .5923"; :2 ,;..°.av Conglomeratic ‘ '4" " sandstone .. Sandstone Siltstone S a E C‘aymne E LE xxxxxxxMarkerbed E E o. g . 5 e FEET METRES 3 (D O 0 o ”3 10 5 g 20 3 m 30 10 Ct 40 50 15 C .9 ‘6 E 0 LL 2 ;. .J Morrison Formation FIGURE 7.—The Dakota Group showing the lithology and the nomenclature used in the text. appear massive although they are locally crossbedded. At many places they have yellowish-brown ferruginous spots as large as one-quarter inch in diameter. Sandstone and siltstone beds in the upper part of the Dakota are cross- bedded and ripple marked at many places. Ripple marks are particularly well exposed along the sinuous scar on the bed forming the footwall of the clay mine between Tucker Gulch and Van Bibber Creek. The top of the Dakota is composed of a thick, light-gray, medium- to fine-grained, quartzose sandstone. Prominent jointing is also well exposed in the clay mine. The joints are spaced from 6 inches to 12 feet apart, are nearly vertical and perpendicular to bedding, and form planes of weakness along which blocks of sandstone tend to break. Where exposed in clay mines a few hundred feet below the ground surface, the joints appear to be tight even though some are slightly offset. The joints are most closely spaced where the Dakota is near the Golden fault just north of Tucker Gulch; they are more widely spaced to the north where the fault is farther east. Several small faults, virtually parallel to the joints, are also exposed in the old clay workings. Claystone beds, though generally covered, form a large part of the Dakota. Those in the South Platte Formation are generally dark gray and weather into small, roughly cubical blocks. The Claystone in the Lytle Formation is commonly greenish gray but is otherwise similar in appearance to the Claystone above. Clay-mineral deter- minations reported by Keller (1953, p. 93) and Wage? (1961, p. 12 and 23) indicate that kaolinite is probably the predominant clay mineral in the Dakota of the Golden area, although montmorillonite and illite are also present. MESOZOIC ROCKS 17 FIGURE 8.—Southwestward view of the hogback of the Dakota Group north of Tucker Gulch. The nearly accordant ridges of the Pre- cambrian metamorphic rocks of the Front Range, in the back- ground, result from late Tertiary erosion. The white scars on the hogback of the Dakota Group, in the middle distance, are the result of mining of refractory clayrock in the Dakota. A similar scar can be seen on the small knoll of Laramie Formation, just to right of the most distant ranchhouse, which resulted from open- The second shale, which is near the middle of the South Platte Formation, is a mixture of kaolinite and illite (Waage: 1961, p. 33). The relation of the Dakota Group to the underlying Morrison is not clear in this area: both formations consist of lithologically similar sandstone and mudstone near the contact, the principal difference being the interstitial blebs of white clay in the sandstone of the Morrison. Beds of sandstone in the Dakota are more abundant and slightly thicker than in the Morrison, and they do not contain interstitial clay. No unconformity was seen between the formations, although Waage’(1955, p. 23) indicated that the contact is marked by an unconformity both north and south of the Golden quadrangle. AGE No fossils have been reported from the Lytle Forma- pit mining of refractory clayrock. The high grass-covered hill shown near the center of the right side of the picture is a monzonite intrusive of Paleocene age. The dark-gray gullied scar on its left side is the result of mining Pierre Shale for use as expanded aggre- gate. The mottled dark- and light-gray artificial fill shown at the center of the left side of the picture is barnyard manure. Photo- graph by H. E. Malde, U.S. Geological Survey, taken June, 1, 1971. tion, but Waagé (1961, p. 10) indicated that it is probably equivalent to the part of the Cloverly Formation of Wyoming that contains Early Cretaceous nonmarine fossils. Very few fossils have been reported from the South Platte Formation in this area. Tracks of Ignotomis mc- connellz' Mehl (a bird), Walteria jeffersonensis Mehl (a web-footed quadruped), and (?)Anomoepus sp. (probably a bipedal dinosaur) from the Dakota between Tucker Gulch and Van Bibber Creek have been reported by Mehl (1931, p. 441—452). Fossils found in the Denver area are principally land plants but also include dinosaur tracks, fish scales, and some poorly preserved casts of bivalves from the uppermost sandstone (Johnson, 1931, p. 358—360). Marine fossils, chiefly Inoceramus coman- cheanus Cragin (Early Cretaceous), have been reported in the Golden area by Waagé (1961, p. 78) from his second shale near the middle of the South Platte Formation. 18 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO ORIGIN The Lytle is similar to the Morrison Formation and is undoubtedly of terrestrial origin. Most of the fossils in the South Platte Formation indicate a terrestrial origin except for the marine pelecypod Inocemmus comancheanus (Waage: 1961, p. 41). Utilizing these meager paleonto- logic and lithologic data, Waage’(1961, p. 93—96) proposed a deltaic origin for the South Platte Formation in the vicinity of Golden. The marine fossils in the second shale of Waage'(1955) indicate a transgression of the sea across the delta, but deltaic conditions resumed with the advent of the overlying Kassler Sandstone Member and con- tinued for the remainder of the Dakota deposition. BENTON SHALE The Benton Shale, about 500 feet thick, is principally shale, but contains thin beds of bentonite, siltstone, and limestone. North of Tucker Gulch it is exposed in a few places east of the hogback formed by the Dakota Group, but south of the gulch it is cut out by the Golden fault. One outcrop in the north bank of Tucker Gulch (see fig. 51) shows Benton sandwiched between the Dakota (not visible in fig. 51) to the east and Fountain to the west. The outcrop is narrower, perhaps as a result of faulting, south of Van Bibber Creek than it is north of the creek. The mapped distribution of the shale south of Van Bibber Creek is based on a single, small, poorly exposed outcrop of limestone that crops out about 1 mile north of Tucker Gulch. The limestone is believed to be part of the Fort Hays Limestone Member of the Niobrara Formation. Possibly the southeast-trending fault that crosses the Dakota at Van Bibber Creek trends more southerly and is a strike fault cutting out part of the Benton. If so, this indicates that the fault is vertical or dips eastward and that the rocks east of the fault have been moved upward rela- tive to those west of the fault. An excellent exposure along the bank of Ralston Reser- voir, in the adjoining Ralston Buttes quadrangle, the paleontologic and lithologic equivalents of the widely used subdivisions of the Benton Group—in ascending order, the Graneros Shale, Greenhorn Limestone, and Carlile Shale—were recognized. Within the limits of the Greenhorn Limestone the equivalents of the Lincoln Limestone Member, Hartland Shale Member, and Bridge Creek Limestone Member were also recognized. The Benton Shale is poorly exposed in the Golden quad- rangle, so these equivalents are only discussed in general- ized form in this report. A more detailed discussion and a measured section at Ralston Reservoir are given in an earlier report (Van Horn, in Sheridan and others, 1967, p. 48—49). . GRANEROS SHALE EQUIVALENT The lower 180 feet of the Benton—the Graneros Shale equivalent—is principally dark-gray, noncalcareous, clayey shale which weathers into a multitude of flat flakes. The shale beds are generally about 3 feet thick but are as much as 40 feet thick at places. The shale is interbedded with 0.1- to l-foot-thick beds of blocky-weathering silt- stone. The siltstone is calcareous at places; it is dark yellowish brown in the upper part and dark to medium gray in the lower part. X-ray examination of a shale sample from the Graneros (unit 119 of measured section of Van Horn, in Sheridan and others, 1967, p. 49) of the Ralston Buttes quadrangle showed major amounts of quartz, minor amounts of kaolinite, mica, and chlorite, and traces of feldspar (table 4). GREENHORN LIMESTONE EQUIVALENT The middle part of the Benton—the Greenhorn Lime- stone equivalent—is about 260 feet thick. Like the lower part, the middle is principally noncalcareous, fissile, clayey shale, but unlike the lower part it is black to light gray. The beds are generally about 3 feet thick but are much thicker at places. Very distinctive beds of very light bentonite, generally less than 1 foot thick, are interbedded with the shale in the lower 140 feet. At most places the ben- tonite beds have been stained pale yellowish orange. No clay mineral tests were made of the bentonite, but according to J. L. Stout (LeRoy and Schieltz, 1958, p. 2459) it is probably montmorillonite. A few thin medium- to yellowish-gray limestone beds are present in this part of the section. The upper 120 feet of the Greenhorn equiv- alent is principally black to light-gray, fissile shale. Throughout the upper 120 feet some very thin hard cal- careous bands or lenses are present, and near the base are two limestone beds. CARLILE SHALE EQUIVALENT The upper 70 feet of the Benton Shale—the Carlile Shale equivalent—is principally medium-dark—gray noncal- careous siltstone, but includes a few thin beds of fine- grained sandstone. Mineral determinations were made of two samples that were not precisely located within the Benton. X-ray analysis of a sample from the fault zone in Tucker Gulch showed major amounts of kaolinite, minor amounts of mica and chlorite, and traces of quartz, feldspar, and dolomite(?). The coarse fraction of this sample, examined with a binocular microscope, was almost entirely sand- and silt-sized aggregates of clay particles. It also contained less than 1 percent mica flakes, discus-shaped gypsum particles that were finely striated, and rounded to sub- angular, clear to frosted quartz grains. A sample of Benton from the vicinity of Golden is reported to contain 40 percent illite and 10 percent kaolinite (Mielenz and others, 1951, footnote 13, p. 323). It is interesting to note that none of the three samples of shale from the Benton shown in table 4 contains montmorillonite. At Ralston Creek the ripple-marked upper surface of the top part of the Dakota Group is overlain by minutely crossbedded siltstone, about 12 feet thick, of the Benton MESOZOIC ROCKS 19 Shale. These transitional siltstone beds pass laterally into the typical black shale of the Benton which overlies the Dakota in most of the Golden quadrangle. AGE AND ORIGIN No fossils were found in the Benton of the Golden quadrangle. Marine fossils were reported by Van Horn (in Sheridan and others, 1967, p. 42) from the Greenhorn Limestone equivalent. According to W. A. Cobban (written commun., Oct. 27, 1954) the fossils indicateaLate Cretaceous age (Cenomanian and Turonian) for this part of the Benton. The entire formation is undoubtedly of marine origin. NIOBRARA FORMATION The Niobrara Formation, about 350 feet thick, is composed of the Fort Hays Limestone Member and the overlying Smoky Hill Shale Member. Both members contain abundant Foraminifera which were noted but not identified. The measured section of locality G25A—D included in this report is a continuation of the previously published measured sections of Paleozoic and Mesozoic rocks in the adjoining Ralston Buttes quadrangle (Van Horn, in Sheriden and others, 1967). Bed 180 (G25—A) of the present report immediately overlies bed 179 of the earlier report along the northwest shore of Ralston Reser- voir. The earliest study of the Niobrara in the Golden area used a threefold division of the formation (Emmons and others, 1896, p. 66—69). The lower division is equivalent to the Fort Hays Limestone Member, and the middle and upper divisions are equivalent to the Smoky Hill Shale Member. Later workers, using terminology applied to the N iobrara in southeastern Colorado, called the lower lime- stone division the Timpas Limestone and the upper two divisions the Apishapa Shale. The Timpas, at its type locality, includes more than the lower division limestone, so the term was misapplied in the Denver area. The Timpas and Apishapa have been abandoned everywhere in favor of Fort Hays and Smoky Hill, respectively (Scott and Cobban, 1964). FORT HAYS LIMESTONE MEMBER The Fort Hays Limestone Member, about 28 feet thick, is 78 percent limestone and 22 percent shale. It occurs north of Tucker Gulch and east of the hogback of the Dakota Group, where it generally forms a small bench between the Benton Shale and the Smoky Hill Shale Member. South of Van Bibber Creek only one outcrop of limestone was seen and that was a poorly exposed outcrop located about 1 mile north of Tucker Gulch. North of Van Bibber Creek there are several good exposures in abandoned limestone quarries. The Fort Hays is principally medium- to yellowish- gray, hard, dense limestone. Individual limestone beds are as much as 2 feet thick but are generally about 1 foot thick. They are interbedded with medium-dark-gray, calcareous shale beds that are about 03 foot thick. Fragments of Pseudopema and Inoceramus are abundant at some places in the member, and are particularly noticeable on weathered surfaces. Large fish teeth are also present and are most abundant in the shaly limestone bed at the base. No evidence of an unconformity between the N iobrara Formation and the underlying Benton Shale was noted in the Golden quadrangle, although there is certainly an abrupt lithologic boundary between the siltstone of the Benton and the overlying limestone of the Niobrara. An unconformity, or at least a diastem, between these units in eastern Colorado was reported by Johnson (1930a). AGE AND ORIGIN The fossil Inoceramus deformis Meek (fossil loc. l; USGS Mesozoic loc. D17) was collected from near the middle of the member a short distance north of measured section G25—A. This fossil, according to W. A. Cobban (written commun., Oct. 27, 1954), marks the lower part of the Niobrara Formation and equivalent rocks over the entire western interior region, and is of Late Cretaceous age. The Fort Hays is of marine origin. SECTION G25—A.—Fort Hays Limestone Member of the Niobrara For- mation [Measured on the northwest side of Ralston Reservoir, Golden quadrangle; location shown in fig. 3. Tr., Trace] Thickness Niobrara Formation: (I!) Smoky Hill Shale Member, unit 231. Fort Hays Limestone Member: 230. Limestone, medium- to yellowish-gray, dense; abundant fragments of Inoceramus and Pseu- dopema are exposed on weathered surface ....... 0.5 229. Shale, medium-dark-gray, calcareous ................... 2.0 228. Limestone, medium- to yellowish-gray, dense ..... .4 227. Shale, medium-dark-gray, calcareous ................... 4 226. Limestone, medium- to yellowish-gray, dense ..... 3 225. Shale, medium-dark-gray, calcareous ................... .5 224. Limestone, medium- to yellowish-gray, dense ..... 3 223. Shale, medium-dark-gray, calcareous ................... 2 222. Limestone, medium- to yellowish-gray dense; attitude N. 1° W., 57° E ................. 1.0 221. Shale, medium-dark-gray, calcareous .2 220. Limestone, medium- to yellowish-gray, dense ..... 1.6 219. Shale, medium-dark-gray, calcareous ................... .1 218. Limestone, medium- to yellowish-gray, dense ..... .3 217. Shale, medium-dark-gray, calcareous ................... .4 216. Limestone, medium- to yellowish-gray, dense. Los Angeles Abrasion loss is 24.4 percent ........ 2.1 215. Shale, medium—dark-gray, calcareous ................... .1 214. Limestone, medium- to yellowish-gray, dense ..... .9 213. Shale, medium-dark-gray, calcareous ................... Tr. 212. Limestone, medium- to yellowish-gray, dense ..... .7 211. Shale, medium-dark-gray, calcareous ................... .1 210. Limestone, medium- to yellowish-gray, dense ..... .4 209. Shale, medium-dark—gray, calcareous ................... .1 208. Limestone, medium- to yellowish-gray, dense ..... .1 207. Shale, medium-dark-gray, calcareous ................... .1 206. Limestone, medium- to yellowish-gray, dense ..... 1.5 205. Shale, medium-dark-gray, calcareous ................... .l 20 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO Niobrara Formation—Continued Thl‘hm’“ Fort Hays Limestone Member—Continued (It) 204. Limestone, medium- to yellowish-gray, dense ..... 1.0 203. Shale, medium-dark-gray, calcareous ................... .2 202. Limestone, medium- to yellowish-gray, dense ..... .7 201. Shale, medium-dark-gray, calcareous ................... .3 200. Limestone, medium- to yellowish-gray, dense ..... .3 199. Shale, medium-dark-gray, calcareous ................... Tr. 198. Limestone, medium- to yellowish-gray, dense; some stylolitic structure ..................................... 1.0 197. Shale, medium-dark-gray, calcareous ............. .. .2 196. Limestone, medium- to yellowish-gray, dense ..... 1.1 195. Shale, medium-dark-gray, calcareous ................... .1 194. Limestone, medium- to yellowish-gray, dense; some ferruginous concretions, as much as ‘A by 1% in ............................................. 1.3 193. Shale, medium-dark-gray, calcareous ................... .1 192. Limestone, medium- to yellowish-gray, dense ..... 2.2 191. Shale, medium-dark-gray, calcareous ................... Tr. 190. Limestone, medium- to yellowish-gray, dense ..... .8 189. Shale, medium-dark-gray, calcareous; contains thin limestone lentils as much as 4 ft long ...... .2 188. Limestone, medium- to yellowish-gray, dense; contains some stylolitic structures ..................... 1.3 187. Shale, medium-dark-gray, calcareous ................... .1 186. Limestone, medium- to yellowish-gray, dense; contains ferruginous concretions as much as ‘5 by 2 in ............................................................. .9 185. Shale, medium-dark-gray, calcareous ....... .1 184. Limestone, medium- to dark-gray, dense. .2 183. Shale, medium-dark-gray, calcareous ....... .5 182. Limestone, yellowish-gray, shaly .......................... .4 181. Shale, medium—dark-gray, calcareous ................... .2 180. Limestone, yellowish-gray, shaly; numerous fish teeth .................................................................... .5 Note: The Fort Hays is very fossiliferous and contains numerous microfossils as well as macrofossils. Inoceramusdeformis is diagnostic of the Fort Hays and is abundant throughout this outcrop. Total Fort Hays Limestone Member ............................. 28.1 Total N iobrara Formation ............................................. 347.1 Benton Shale, unit 179 of Van Horn (in Sheridan and others, 1967, p. 48). SMOKY HILL SHALE MEMBER The Smoky Hill Shale Member of the Niobrara Forma- tion, about 320 feet thick, is composed of shale, chalk, and bentonite. It is present east of the bench formed by the Fort Hays north of Tucker Gulch but is poorly exposed at most places. The Smoky Hill is principally composed of light- to yellowish-gray calcareous fissile shale. Illite and mont- morillonite are the predominant clay materials in the shale beds of the upper part of the member (LeRoy and Schieltz, 1958). Two persistent thin-bedded chalk beds are present in the Smoky Hill—a 25-foot-thick light-gray chalk bed 150 feet above the base, and a 5-foot-thick medium-bluish-gray to yellowish-gray chalk bed 260 feet above the base. Thin layers of bentonite and selenite are present at some places. The clay minerals of three bentonite beds in the upper part of the Smoky Hill were determined by Schieltz (LeRoy and Schieltz, 1958); the upper two contain over 70 percent montmorillonite, whereas the lower bed (average of the three samples shown by Schieltz on p. 2456 and 2458) contains about 50 percent kaolinite and 20 percent montmorillonite. The top of the Smoky Hill is placed at the top of the thick sequence of yellowish~gray-weathering highly calcareous shales. Beds overlying the Smoky Hill are very weakly calcareous or noncalcareous shales that weather to a medium gray or light brown. The contact between the Smoky Hill and the underlying Fort Hays is conformable. AGE AND ORIGIN The Late Cretaceous age of the Smoky Hill is determined by its stratigraphic position between the Fort Hays and the Pierre Shale, both of Late Cretaceous age. Marine fossils were found at two places in the upper chalk. Fossil locality 2 of the geologic map (Van Horn, 1972) (unit 235 of measured section G25—B) contained large, smooth Baculites sp. (USGS Mesozoic loc. D18) that is encrusted with 10 individuals of the very rare barnacle Stramentum haworthz' (Williston). J. B. Reeside, Jr. (written commun., Sept. 14, 1954), who identified the barnacle, stated that all the specimens previsously recorded came from the Smoky Hill Chalk Member of Kansas. Fossil locality 3 (USGS Mesozoic 10c. D641) contained both of these species as well as Pseudopema congesta (Conrad). The fossils all indicate a marine origin. SECTION G25—B.—Smoky HillShale Member ofthe Niobrara Formation [Measured on the northwest side of Ralston Reservoir, Golden quadrangle; location shown in fig. 3] Thickness Pierre Shale, unit 237. (It) Niobrara Formation: Smoky Hill Shale Member: 236. Shale, dusky-yellow, calcareous ............................ 55 235. Chalk, medium-bluish-gray to grayish-yellow; l- to 6-in. beds, forms persistent outcrop in vi- cinity of Ralston Reservoir. Fossil collection D—18 (map locality 2) contained a portion of a large straight cephalopod. Attitude N. 5° W., 75° W .................................................................. 5 234. Shale, light-gray, grayish-yellow, light-greenish- gray, and light- and moderate-brown, cal- careous. Contains thin bentonite and selenite layers .................................................................. 12 233. Shale, grayish-yellow, calcareous .......................... 70 232. Chalk, light-gray; 1- to 4-in. beds ..... 25 231. Shale, grayish-yellow, calcareous ...... 152 Total Smoky Hill Shale Member .................. 319 Fort Hays Limestone Member, unit 230. PIERRE SHALE The Pierre Shale is predominantly shale but includes some relatively thick siltstone and silty sandstone beds. The Pierre crops out in the western part of the quadrangle from the southern to the northern boundaries. It has been MESOZOIC ROCKS folded, faulted, and intruded by mafic monzonite. As a result of these structural complications the outcrop width ranges from a few hundred feet in the southern part of the quadrangle to several thousand feet in the central and northern parts. The contact between the yellowish-gray Smoky Hill Shale Member and the overlying olive-gray Pierre Shale is well exposed along the northwest shore of Ralston Reser- voir. Here the Smoky Hill is strongly calcareous whereas the Pierre is weakly calcareous or noncalcareous. Except for the marked lithologic change, the contact appears con- formable. A hiatus between the two formations, however, has been indicated by Reeside (1957, table 1 and fig. 16) and by LeRoy and Schieltz (1958, p. 2463). This hiatus is indi- cated by the sharp change in color and lithology between the two formations in the Golden quadrangle. FOSSlL ZONES Most of the structures shown in the Pierre are based on the mapping of fossil zones—stratigraphic zones ranging in thickness from a few feet to several hundred feet, throughout which a particular fossil or commonly asso- ciated fossils are found. These zones do not represent exact stratigraphic horizons as single beds of volcanic ash do; instead, the particular fossil, or commonly associated fossils, are found throughout a stratigraphic zone ranging in thickness from a few feet to several hundred feet. They are, however, mutually exclusive in that any one 21 diagnostic fauna does not occur in underlying or over- lying zones, with the exception of the zone of Inoceramus typicus, which has been found to extend into the under- lying zones of Baculites eliasi at places outside the Golden quadrangle. For reasons explained in the description of the Inocemmus typicus zone, the zone of I. typicus, rather than the more conventionally used zone of Baculites baculus, is shown on the geologic map (Van Horn, 1972). On that map the zone lines are drawn on the stratigraphi- cally highest occurrence of the diagnostic fauna; thus they approximate an exact stratigraphic position and can be used for structural and stratigraphic control. Table 2 shows the names, thicknesses, and cumulative height of zones above the base of the Pierre. The thickness of beds between the fossil zones and distances above the base of the Pierre Shale given in the following paragraphs are not exact dimensions, but are reasonable approximations based on measured sections and on computed sections. The computed sections were determined by plotting attitudes of beds and locations of fossil zones on the geologic map (Van Horn, 1972). These data were used to compute true thicknesses of beds between fossil zones using the distances measured from the map. There is no place in the quadrangle where a single section undisturbed by faulting, or containing clearly delineated fossil zones, can be used to determine the thickness of all the zones. Therefore, the composite thickness of each zone (see table 2) was determined by comparison with thickness TABLE 2.—Thz'clmess (in feet) of beds between fossils zones of the Pierre Shale [Leaders (...) indicate no data] Locality ................................................................. Denver and Rio North parts Ralston Creek North parts Composite Grande Western secs. 28 and 29, and reservoir secs. 8, 9, and 10, Railroad‘ T. 2 5., R. 70 W.‘ at locality GZS-C T. 3 S., R. 70 W. Thickness to .......................................................... Underlying Base of Underlying Base of Underlying Base of Underlying Base of Underlying Base of zone Pierre zone Pierre zone Pierre zone Pierre zone Pierre Top of Pierre Shale .......................... 1,010 7,580 860 6,750 2625 24,645 1,025 5,010 1,000 7,250 Baculites clinolobatus. 320 6,570 470 5,890 2610 24,020 350 6,250 grandis ................. 360 6,250 2510 25,420 ........................ 360 5,900 Inoceramus typicus. 1,450 5,890 700 4,910 ........................ 1,450 5,540 Baculites eliasi ......... 470 4,440 600 4,210 350 3,410 550 4,090 reesidei 40 3,970 380 3,610 2500 23,060 40 3,540 compressus . . .. ................ 300 3,930 ................................................ 340 3,500 Didymoceras eheyennense ................. 260 ........................ 260 3,160 Exiteloceras jenneyi .......... 1,100 ........................ 5200 2,900 Didymoeems steuensom'. ............ 400 2,560 400 2,700 nebmscense ............................................. 400 2,160 400 2,300 Baculites scotti .................................. 200 1,760 200 1,900 n. sp. ............. 250 1,560 250 1,700 gregoryensis .. 200 1,310 200 1,450 gilberti ........... 580 1,110 3600 1,250 asperiformis .. ........................ 100 650 macleami ...... 130 530 175 550 oblusus ....................................................................................... 400 400 375 375 Base of Pierre Shale .......................... 0 0 0 0 0 0 0 0 0 0 ILower part from Van Horn (in Sheridan and others, 1967). 2A fault is present between this and the next underlying zone; therefore, the thickness may be abnormal. 3Poor control. 22 of other zones or, where one or more zonal boundaries could not be effectively located, with thickness of several zones. As an example, the thickness between the Baculites macleami zone and the B. obtusus zone was determined in the following way. At one place the thickness is 130 feet and at another it is 150 feet (table 2). At these places the B. obtusus zone is 400 and 350 feet, respectively, stratigraphi- cally above the base of the Pierre (an average of 375 ft). At two other places the thickness between the same two baculite zones could not be determined directly, but the thickness from the B. maclearm' zone to the base of the Pierre was 700 and 450 feet. At these two places the average thickness between B. obtusus and the base of the Pierre (375 ft) subtracted from 700 and 450 gives an approximate thickness of 325 and 75 feet of beds between B. macleami and B. oblusus zones. When these are added to the two direct measurements of 130 and 150 feet and the sum is divided by four the average thickness between the two fossil zones is 170 feet, which is rounded to 175 feet in table 2. In this fashion the thickness of beds between each fossil zone was determined; in some places the possible effect of structure was considered and in other places thickness determined from isolated exposures not shown in table 2 also tempered my judgment. Thus, the thicknesses shown in the composite columns of table 2 are interpretations based on measurements given in the other columns of the table and on the geologic map (Van Horn, 1972). This blend of information gives an approximate thickness of 7,250 feet of Pierre Shale. This thickness is in close agree— ment with that reported in the well log of the S. D. Johnson 1 Farmers Highline Canal and Reservoir Co. well (American Stratigraphic Co., 1956), which, after correction for a 2° dip, gives a thickness of 7,240 feet for the Pierre. The fossil localities and zone lines are shown on the geologic map (Van Horn, 1972), but the fossils are listed in table 3 of the present report. Four poorly defined major lithologic units can be recognized in the Pierre. In ascending order these are the lower shale unit (about 1,200 ft thick), lower sandstone unit (700 ft), upper shale unit (4,300 ft), and upper transi- tion unit (1,000 ft). These arecomparable to the lithologic units described by Scott and Cobban (1965), except that in the Golden quadrangle the boundary between the two middle units is placed 1,500 feet nearer the base of the section, at the top of the Hygiene Sandstone Member. The relation of various lithologic subdivisions of the Pierre Shale is shown in figure 9. LOWER SHALE UNIT The lower 1,200 feet of the Pierre is olive-gray, clayey, noncalcareous to slightly calcareous fissile shale. A few thin beds of bentonite are interbedded with the shale, and a few small selenite crystals are present in the lower 100 feet. GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO NORTHEAST OF BOULDER (SCOTT AND COBBAN .1965) FEET ABOVE BASE Fox Hills Sandstone 8,000~ 5 'g \\ GOLDEN OUADRANGLE ‘ g \ (THIS REPORT) .1 \ E \ FEET — E \\ ABOVE _ 9 Fox Hills Sandstone BASE 7,000 — é — 7,000 3 Upper transition ~ — \\ unit — \ _ \ _ \ ‘ Mobridge Member 6,000 — equivalent — 6,000 _ E _ _. 3 h 2 m z 5 _ u. _ 6 _ D. a 5,000 — 3 — 5,000 Upper shale ‘ unit ' 4.000 ~— Richard 85 Mbr ~ 4000 _ Larimer Ss Mbr \ _ Rocky Ridge 85 Mbr \\ _ \ \ ~ \ 7 .t' Larimer Sandstone ' g Member ’ 3,000 — g Terry Sandstone i— 3,000 _ E, Member Upper F _ 8 shale _ g unit _ o _ _l _ \ _ \ 2,000 — Hygiene Sandstone \\ — 2,000 _ Member _ Lower _ \\ sandstone - \ unit ' \ 1,000 _ w — 1,000 E _ 5 l— _ E Lower shale _ 0 unit _ .J _ 0 — — 0 Niobrara Formation Niobrara Formation FIGURE 9.—Lithologic subdivisions of the Pierre Shale near Golden. Illite is the dominant clay mineral in the shale of this part of the Pierre (LeRoy and Schieltz, 1958, p. 2457). Mont- morillonite, which is dominant in the thin bentonite beds, is also present in small amounts in the shale. Baculites obtusus Meek was found at one locality (see MESOZOIC ROCKS table 3, D832) about 400 feet above the base of the Pierre. At this place the fossil is in light-olive-gray to dark-reddish- brown, finely crystalline, silty, calcareous concretions. Baculites macleami Landes is present at two localities (see table 3) 500—600 feet above the base of the Pierre; at a locality in the Ralston Buttes quadrangle, it is present about 700 feet above the base. The fossil occurs in hard cal- careous concretions and is associated with cone-in-cone structure. A single, isolated fossil locality (table 3) yielded Baculites asperiformis Meek at a stratigraphic horizon about 600 feet above the base of the Pierre. The fossil is younger than B. macleami according to W. A. Cobban (oral commun.). Baculites gilberti Cobban is found near the top of the lower part of the Pierre, about 900—1200 feet above the base. Some of these fossils weather out of shale and appear phosphatic, whereas others are associated with hard calcareous concretions. Two thin monzonite dikes intrude the shale on the west side of the Ralston Reservoir in the Baculites gilberti zone. These dikes could be traced for only a short distance, and the outcrop is entirely below the high-water level of the reservoir. LOWER SANDSTONE UNIT The lower sandstone unit, about 700 feet thick, is principally siltstone, silty shale, and sandstone. The upper and lower boundaries are poorly defined. The unit overlies the lower shale unit and is much more arenaceous than the rest of the Pierre. It is partially exposed on the northwest side of Ralston dike, along the north edge of Ralston Reservoir, and west of Rocky siding on the Denver and Rio Grande Western Railroad. At these places the unit is interbedded silty shale and sandy siltstone, and near the top it contains a 50-foot-thick silty sandstone. The lower sandstone unit is equivalent to the Hygiene Sandstone Member of Fenneman (1905, p. 31) which, according to Scott and Cobban (1959, p. 126), contains Baculites gregoryensis Cobban and Baculites scotti Cobban. The lowest fossils indicative of the zone of Bacu- lites gregoryensis Cobban are about 1,300 feet above the base of the Pierre. These commonly occur in medium- gray, finely crystalline calcareous concretions, which weather light gray to moderate yellowish brown and contain sparse grains of green glauconite. The next highest fossil zone, the Baculz'tes n.sp. zone, is in a light-olive-gray silty to sandy shale containing some moderate-yellowish-brown-weathering thin sandstone beds and ironstone concretions. This is overlain by the Baculz'tes scotti zone, the top of which is about 1,900 feet above the base of the Pierre. This zone is principally silt- stone and sandstone, and the fossils weather from sandy beds or are in concretions. The sandy beds are light-gray fine-grained sandstone or sandy siltstone that weather 23 moderate reddish brown andcontain small amounts of green glauconite and unidentified rounded black blebs. The concretions are finely crystalline, calcareous, and generally sandy; they are dark gray but weather moderate yellowish orange. The upper 50 feet is marked by massive to indistinctly and minutely crossbedded light-gray sand— stone that locally is mottled dark gray and contains small particles of carbonaceous material at places. UPPER SHALE UNIT The upper shale unit of the Pierre Shale includes the fossil zones from Didymoceras nebmscense to Baculites clinolobatus. This unit, about 4,300 feet thick, is prin- cipally dark- to olive-gray clayey to sandy shale but contains some thin siltstone and sandstone beds. In the Golden quadrangle the lower 1,500 feet of the upper shale unit includes the upper part of the lower sandstone unit of Scott and Cobban (1965). (See fig. 9.) This part of the Pierre, which is very sandy north of Boulder, is principally shale in the Golden quadrangle. This change in lithology probably represents a shaling up of the sandy interval and possibly is related to the distance from the shoreline. Some of the sandstone beds, such as the Larimer Sandstone Member in the zone of Baculites reesidei, may extend rela— tively unchanged between Golden and Boulder. The fossil zones cut across this change in lithology, so that zones that are in the lower sandstone unit north of Boulder are in the upper shale unit near Golden. In the lower part of this unit the zones of Didymocems nebrascense (Meek and Hayden), D. stevensoni (Whitfield), Exilelocems jennyi (Whitfield), and D. cheyennese (Meek and Hayden) are respectively about 2,300, 2,700, 2,900, and 3,160 feet above the base of the Pierre. The fossils are generally in medium- to dark-gray, hard, finely crystalline, calcareous concretions. The concretions weather to light gray and in places have a grainy appearance. The lower three fossil zones consist principally of noncalcareous shale but they also contain a few thin siltstone beds. The predominant clay mineral in the lower part of this unit is montmorillonite. The zone of Didymocems cheyennense is principally shale or sandy shale, some of which is calcareous. The fossils commonly are in hard, finely crystalline to dense, calcareous concretions. The concretions in the lower part are medium gray to dark reddish brown and contain minor amounts of unidentified small black blebs. Concretions in the upper part are light brown to olive gray and weather to a yellowish or moderate yellowish brown. They contain small amounts of sand—sized grains of quartz, greenish- gray glauconite, and unidentified black blebs. The blebs are larger and more abundant than in other zones. The Baculites compressus zone, 3,500 feet above the base of the Pierre, and the Baculites cuneatus zone are poorly 24 marked zones in which the fossils mainly weather out of shale, but also are present in concretions. The Baculites cuneatus zone is overlain by the Baculites reesidei zone, about 3,540 feet above the base of the Pierre. The B. reesidez' zone is principally noncalcareous shale but, adjacent to Ralston Creek, it contains at least one 24-foot-thick bed of very fine grained silty sandstone (unit 246 of the measured section G25-C at Ralston Creek and fossil locality 44, D26). The sandstone was seen only for a few hundred feet north and east of Ralston dike; it is cut out to the north by a fault and to the south by Ralston dike. The sandstone is light gray (weathering dark yellowish orange), hard, and greatly fractured. It is composed principally of quartz grains but contains moderate amounts of an unidentified dark mineral and mica. It is possibly equivalent to the sandstone at 4,780 feet below the surface (3,710 ft above the base of the Pierre) in the S. D. Johnson 1 Farmers Highline Canal and Reservoir Co. well (American Stratigraphic Co., 1956). The fossils are in sandstone and siltstone and in concretions weathering out of shale. The siltstone is light gray to dark yellowish orange and generally sandy. The concretions are light to dark gray, silty and calcareous. According to Scott and Cobban (1959, p. 128), Baculites reesidei occurs in the Larimer Sandstone Member of the Pierre. The succeeding 2,700 feet is principally clayey to silty, fissile shale but contains some thin beds of siltstone and sandstone. At places the shale is calcareous and contains hard calcareous concretions. The zone of Baculites eliasz' Cobban, about 4,090 feet above the base of the Pierre, is marked by moderate-yellowish-brown to dark—gray cal- careous concretions. They are dense to finely crystalline and have a sugary texture on weathered surfaces. The Baculites baculus zone is not shown as such on the geo- logic map (Van Horn, 1972), although it is shown in table 3, because no positively identified Baculites baculus was found in the Golden quadrangle. Instead a zone of Inoceramus typicus is shown, the top of which is approximately equivalent to the Baculites baculus zone of others (Scott, 1962, 1963b; Smith, 1964). Fossils in this zone generally are in yellowish-brown-weathering thin shaly limestone. Several outcrops of the limestone are present near the zone line of Inoceramus typicus in the salient of Pierre Shale southeast of Ralston dike. The upper part of the upper shale unit of the Pierre Shale contains the large Baculites grandis Hall and Meek. This fossil is present about 5,900 feet above the base of the Pierre. It was found weathering out of shale and also in hard, dense, cal- careous concretions. The zone of Baculites clinolobatus, as shown on the geo- logic map (Van Horn, 1972), forms the least satisfactory zone as a stratigraphic horizon marker. This zone could not be drawn so as to remain a relatively constant distance from the underlying Baculites grandis zone and the over- lying Fox Hills Sandstone. This lack of uniformity is GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO probably due to a combination of a thick, sparsely fossili- ferous zone and faulting. Fossils indicative of the 3am- lites clinolobatus zone are present in the narrow band of Pierre shown in the southern part of the quadrangle. A silty sandstone at fossil locality D28l (south of Clear Creek) contains both Baculites clinolobatus Elias and Tenuipteria fibrosa (Meek and Hayden). The T. fibrosa, which is found only in this zone, was also found near the east end of the salient of Pierre southeast of Ralston dike. Baculites clinolobatus Elias is the guide fossil to the Mobridge Member of South Dakota of the Pierre which is equivalent to the Baculites clinolobatus zone of this report. UPPER TRANSITION UNIT The upper 1,000 feet of the Pierre is principally silty shale but contains many siltstone and silty sandstone beds. These coarser grained beds generally form thin interbeds between thicker shale beds but at places are as much as 10 feet thick. Mineral analyses by A. J. Gude III (see table 4) of eight samples from shale and siltstone beds in the upper part of the Pierre showed that both rock types contained, among other minerals, montmorillonite, kaolinite, and small amounts of illite and dolomite. An analysis by F. A. Hildebrand of a ninth sample (siltstone) showed mont- morillonite, kaolinite, and small amounts of a mica-type mineral. Montmorillonite is the predominant clay mineral. At Ralston Creek Sphenodiscus sp. (fossil locality D271, unit 248 of the measured section G25—C at Ralston Creek) was found in a silty shale in the upper 50 feetof the Pierre. According to W. A. Cobban (written commun., Oct. 27, 1954), this form is not known to exist in rocks older than the Mobridge Member of the Pierre Shale of South Dakota, although it is found in the Fox Hills Sandstone. Species of Sphenodicus are also present in the Fox Hills Sandstone of other areas. AGE The fossils collected from the Pierre Shale were identi- fied by W. A. Cobban (written commun.), who stated that they are of Late Cretaceous age (Campanian and Maestri- chtian). ORIGIN The fossils found in the Pierre Shale of the Golden quadrangle (table 3) all indicate a marine origin for this unit. The many siltstone and sandstone beds, some of which contain small amounts of finely divided plant remains, probably indicate a relatively shallow sea with land areas not too far distant. IThe fossil-collection localities table in Van Horn (1972) should be corrected to show that Map. No. 79 is USGS Mesozoic Locality D—28, 80 is D—27, and 81 is D—835. MESOZOIC ROCKS 25 SECTION G25—C.—Pierre Shale [Measured on both sides of Ralston Reservoir and Ralston Creek, Golden quadrangle. Line of section shown in fig. 3] Fox Hills Sandstone, unit 249. Thickness Pierre Shale: (1‘) 248. Shale, medium-gray, silty; fossil locality D27, map locality 80, Sphenodiscus sp. 30 ft below top, light- brown stains common. Contains numerous 0.1- to 0.2-ft-thick soft siltstone beds ................................... 40 247. 1,525 246. Covered; sporadic outcrops of shale ............................. Sandstone, yellowish-gray, very fine grained, silty, hard, greatly fractured. Outcrops on east side of Ralston dike. Contains fossil collection D26, map locality 44 Inocemmus cf. 1. vanuxemi Meek and Hayden which appears to be indicative of the Baculites reesidei zone ............................................... 25 245. Covered, occasional outcrops of medium-gray shale. Contains some hard lenticular concretions. Fossil collections D25 and D734 (map localities 25 and 24) in zone of Didymocems stevensoni 110 ft below the top. Base of this unit is marked by greatly frac- tured and faulted zone on strike with Ralston dike ............................................................................ Covered, probably shale. Shale adjacent to monzonite of Ralston dike is baked. The monzonite has a 3-in.-wide chilled border ........................................... Sandstone, light-gray mottled dark-gray, massive, silty, hard, and fine-grained. Contains carbonace- ous material, probably plant fragments. Attitude N. 5° W., 59° E. Outcrops on west side of Ralston dike. Contains fossil collection D3450 (map locality 21) in zone of Baculitcs scotti ................................... 50 Shale, medium-gray to light-olive—gray, clayey, Con- tains hard lenticular yellow-weathering, sandy con- cretions on south side of reservoir. Attitude N. 5° W., 39° E., near base. On north side of Ralston Reservoir this unit has attitude of N. 4° W., 64° E .......................................................................... 75 Siltstone, yellowish-gray, sandy. Weathers into rounded to hackly (angular, very irregular shape) %- to 2-in. blocks. Contains a few calcareous con» cretions and fossil collection D776 (map locality 17) in the zone of Baculites n. sp. aff. gregoryensis Cobban ...................................................................... 25 Siltstone, poorly exposed, light-olive-gray to yel- lowish-gray, sandy, soft. At places weathers to hackly shaped ‘A- to 2-in. blocks ............................... Shale, olive-gray, clayey, very fissile. Contains phos- phatic baculite casts and hard lenticular concretions in upper 100 ft. Contains two intersecting monzo- nite dikes in upper part. The shale contains some thin bentonitelike layers in the lower 500 ft. Fish jaw and oysters 10 ft above base. Fossil collections D20, D22, and D841 (map localities 8, 10, and 9) from the zone of Baculites gilberti in the upper 200 ft .......................................................................... 238. Shale with cone-in-cone structure. Attitude of bedding is N. 2° E., 72° E. Fossil collection D842 (map local- ity 5) from the zone of Baculites macleami ............. l 237. Shale, olive—gray, clayey, very fissile; selenite in lower 100 ft .......................................................................... 630 244. 430 243. 242. 241. 240. 270 239. 740 485 Total Pierre Shale ..................................................... 4,296 Niobrara Formation, unit 236. SECTION G24—A.—Pierre Shale [Measured in a prospect trench at locality G24 (fig. 3) in the SW‘A sec. 28, T. 3 5., R. 70 W. It is continuous with a section of the Fox Hills Sandstone measured at the same locality] Fox Hills Sandstone, unit 24. Thickness Pierre Shale: (f!) 23. Shale, light-olive-gray (5Y 5/2), clayey; ......................... 0.5 22. Sandstone, grayish-orange (lOYR 7/ 4), fine-grained ..... .5 21. Shale, light-olive-gray (5Y 5/2), clayey .......................... .7 20. Sandstone, grayish-orange (lOYR 7/4), fine-grained ..... .7 19. Shale, light-olive-gray (5Y 5/2), clayey .......................... 1.2 18. Sandstone, grayish-orange (lOYR 7/4), fine-grained ..... .4 l7. Shale, light-olive-gray (5Y 5/2), clayey .......................... 2.3 16. Sandstone, grayish-orange (lOYR 7/4), fine-grained ..... 1.4 15. Shale, light-olive-gray (5Y 5/2), clayey .......................... 1.0 14. Sandstone, grayish-orange (lOYR 7/4), fine—grained ..... 1.0 13. Shale, light-olive-gray (5Y 5/2), clayey .......................... .7 l2. Covered .............................................. 9.7 11. Shale, light-olive-gray (5Y 5/2), clayey .......................... 5.0 10. Sandstone, grayish-orange (lOYR 7/4), fine-grained ..... 3.9 9. Shale, light-olive-gray (5Y 5/2), clayey .......................... 2.2 8. Sandstone, grayish-orange (lOYR 7/4), fine-grained ..... 2.1 7. Shale, light-olive-gray (5Y 5/2), clayey .......................... 1.3 6. Sandstone, grayish-orange (lOYR 7/4), fine-grained ..... 1.9, 5. Shale, light-olive-gray (5Y 5/2), clayey .......................... 1.2 4. Sandstone, grayish-orange (lOYR 7/4), fine—grained ..... .7 3. Shale, light-olive-gray (5Y 5/2), clayey .......................... 69.0 2. Siltstone, light-gray (N7) to light-brownish-gray (5YR 6/1), thin—bedded. Attitude N. 26° W., 65° W ............ 25.4 1. Shale, light-olive-gray (5Y 5/2), clayey; base covered 7.1 Total Pierre Shale measured ................................... 139.9 FOX HILLS SANDSTONE The Fox Hills Sandstone is present on the west side of the low discontinuous hogback of the Laramie Forma- tion. It forms a narrow, north—trending band that extends across the Golden quadrangle. In the northern part of the area it is principally sandstone, about 60 feet thick, and becomes more shaly southward. In the vicinity of Golden, the Fox Hills is about 100 feet thick and contains more shale than sandstone. The beds dip steeply eastward in the northern part of the area; south of Ralston Creek, how- ever, they generally are overturned and dip steeply west- ward. Foraminifera, present in a few shale beds, were the only fossils found in the Fox Hills. The delineation of the contact between the Fox Hills and adjacent formations is rather arbitrary in the south- ern part of the Golden quadrangle. The shale in the Fox Hills is indistinguishable from shale in the Pierre. Shale in both formations, however, is fissile and generally grayish olive, whereas the clay or claystone in the over- lying Laramie Formation is blocky and of a different color. Sandstone in the Fox Hills is generally grayish orange, silty, and fine grained. It is moderately to poorly cemented except at a few places where well-cemented zones form hard rounded concretions as much as a foot across. A more rounded outcrop, finer grain size, browner color, and a somewhat dirty look distinguish sandstone of the Fox 26 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO TABLE 4,—Semiquantitative X—ray mineralogic determinations of samples from Upper Cretaceous formations in northeastern Colorado [Identified by A. J. Gude 111 except for loc. MS. The amounts indicated are relative only to other minerals in the same sample and should not be compared to other samples. For those determinations in which the minerals are reported in order of relative abundance, the most abundant are shown on the table by the number 1 and the less abundant minerals are shown by successively larger numbers. the letter designations, from most to least abundant, are: VA, very abundant; A, abundant: Mj, major amount; P, present; Mn, minor amount; TL, trace; ?, questionably pre- sent Lithology: co., conglomerate; ss, sandstone; Sl, siltstone; c1, claystone; sh. shale] E .3 . o' 34‘ g E E z E 5 E 2 E 2 t :7: Formation E E E g E g E g E ,4: 8. 'g g 2' . .E g '3 _§ ‘23 g g .3 g .2 i g Remarks :2 <2 2 Q E E o E a 0' <3 :3: A _ 5 G23 70 Tr. ............... VAl cl 2; E G2; 18 Tr. ...A. ..... VA2 58 - .. G . .......... 2 . 3 .2 (.33 3 ii. ..... 3A ..... “Br 2:} Duplicate samples- § ,§ 023 1 .......... VA Tr. W St 8, E 1.22 ..... Tr. ..... A Tr. Mn1 55 E 19. L1 ..... Tr. ..... A VA ..... ss 18 33 204,714 .......... VA ..... A co = 3 EC] 3 207,203 ..... Tr. Mj Tr. sz ss 2 '2 EC] 7 207,204 ..... Mn Mj ..... Mn1 55 E E ECI 8 207,205 .......... Mj .......... a ‘3 r2 ECH 10 207,206 .......... Mj ..... Mj? ct ECG 12 207,207 .......... Mj Mj ..... ss ECG 14 207,208 ..... Mj Mj .......... a ECF 16 207,209 .......... Mj .......... a ,5 ECE 19 207,210 .......... Mj .......... ct % ECD 21 207.211 ..... Mn Mi .......... a 9 ECD 24 207,212 .......... M] ..... Mn a 3 ECC 26a 207,213 .......... Mj .......... a “g“ ECC 26b 207,214 .......... Mj ..... Mn a g 18 30 204,715 .......... VA .......... a E 18 29 204,716 .......... VA ..... Mn co .2 18 28 204,717 .......... VA .......... sh g 18 26 204,718 .......... VA .......... sh .g 18 25 204.719 .......... VA ..... A 55 = § 18 22 204,720 .......... VA ..... Mn 55 g g 18 20 204,721 .......... VA ..... A sh g g 18 18 204,722 .......... VA .......... sh ,g 2 18 15 204,723 .......... VA ..... A 55 ,2 18 13 204,724 .......... VA ..... P 55 g 18 10 204,725 .......... VA ..... Mn 55 3 DSL 11 254,377 ..... Mn .......... Mj5 Tr ..... Mn .......... a . . . DSL 12 254,378 Mn M1 ........................................ a Eganfrgifffi‘i‘éfifpéfie DSL 13 254,379 ..... M] .......... Mn5 Tr. ..... Mn .......... a weathered an tolower DSL 14 254.380 ..... Mj ............... Tr. ..... Mn .......... a lessweathged' an ' DSL6 35 IWX387J ..... 1 ......................... 2 .......... 55 p ' g DSL" 39 wa3871 ..... 1 2 .................... 3 .......... a & DSLG 42 1wx387H 2 1 3 .................... 4 .......... ss 3 DSL" 43 1wx387G 1 2 3 .................... 4 .......... a . 3 DSL 43 207,350 Mj Mj .................... Mj ..... Mj a} D“Pllca‘e samples- DSL6 44 1wx387F 2 1 3 .................... 4 .......... ss DSL6 441wx387E 2 1 3 . ............... 4 .......... 55} Do DSL 44 207,349 ..... Mj ......................... Mj ..... Mj ss ' G25 279 219,979 Mn Mn ..... Tr. .......... Tr. A Tr. Tr.1 ct G25 278 219,980 P Mn ..... Tr. ............... A ..... A2 55 025 277 219,981 P Mn ..... Tr. ............... A ..... P2 55 E DSLG 1 1wx387D 1 2 3 .................... 4 .......... 55} D0 «=2 a“ DSL 1 207,348 ..... Mn .................... Mn Mn Mn Mn 55 ' g :3; ECB 29 207,215 Mn Mn ......................... Mj ..... Mj2 ss 2 :2 ECA 31 207,216 ..... Mn ......................... Mj ..... Mj2 55 6‘5 18 8 204,726 1 3 ..... 2 ......................... Mn sh g DSL6 c1wx387c 1 3 2 ............... 4 .......... ct E DSL C 207,347 Mj Mn .................... Mn Mn Mn Mn ctI Do. x : DSL6 2-1wx387B 2 3 4 .................... 5 1 ..... 55} Do 12 2 DSL 2 207,346 Mn Mj .................... P Mj Mj Mj ss ' 6; ECA 33 207,217 Mn Mn ......................... Mj ..... Mj2 ss 3 ECA 34 207,218 Mj Tr. .................... Mn M] Mn ..... sh ECA 35 207,219 .................... Mn7 ..... Mn M] M] Mn2 55 See footnote at end of table, p. 27. MESOZOIC ROCKS 27 TABLE 4.—Semiquantitative X—ray mineralogic determinations of samples from Upper Cretaceous formations in northeastern Colorado—Continued '8 9 73 5 ‘E .5 3 g g . i E E 3 .. .- 0 Formation E; 3- f2: 8 E g E E 5 2 a :30 Remarks n r: . .g a e .3 ,2 ~ g 2 ta 3 e s- 3 53 as s s s a S s 3 5 5 .2 a DSL" A IWX387A l ............... 5 3 ..... 55} Do DSL A 207,345 Mj .......... P M1 M1 ..... ss ECA 36a 207,220 ............... Mn M3 ..... Mn2 ct ECA 36b 207,221 .......... M] Mn M] ..... Mnl ct g 18 6a 204,727 1 .......... Tr. .......... P sh = 18 6b 204,728 1 Mn7 ..... Tr. .......... Mn sh E 18 3 204,729 1 Mn7 ..... Tr. .......... Mn sh g 18 2 204,730 1 ......................... Mn sh 2 g 18 1 204,731 1 ..... M... Tr M. ....... TP sh E >- NVH 1 254,367 Mj ..... n ..... n ..... r. ct - . 2 E NVH 2 254,368 Mj ..... Tr ..... Mn ..... Tr a £3333?le $33;ng ,5 D NVH 3 254,369 Mj ..... Mn ..... Mn Tr. Tr ct weathered En "310w” m NVH 4 254,370 Mj ..... Mn Tr Mn Tr. Tr a less weathged’ an ' NVH 5 254,371 Mj ..... Mn > Mn ..... Tr ct P ~ NVH 6 254,372 Mj ..... Mn ? Mn ? Tr ct g g MVH 7 254,373 Mj Mn ............... Tr. ..... Mn .......... sh S 8 g MVH 8 254,374 Mj Tr. ........................................ sh Do ig 2 MVH 9 254,375 Mj Tr. ............... .> ..... .> .......... sh ' 3 s MVH 10 254,376 Mj Tr. ............... Tr. ..... Tr. ..... 2 sh G25 119 263,782 ..... Mn .......... Mn7 Mn ..... Mj ..... Tr. sh 5 2 U ..... 263,781 ..... Mj .......... Mn7 Mn P Tr. ..... Tr. sh g 3 MS ..... 289,481 ..... Mn ..... Mj .............................. sh ‘Polash feldspar. 5Interlayered montmorillonite-illite. 2Plagioclase. ‘Identiiied by F. A. Hildebrand and others using thin section, differential thermal, electron 31nterlayered montmorillonite-kaolinite. microscopy, and X-ray diffraction techniques. ‘Metahalloysite. 7Chlorite. SAMPLE LOCALITIES AND REMARKS G23. NWVNE‘A sec. 21, T. 3., R. 70 W. Golden quadrangle L22. NE'ANE‘A sec. 7, T. 3 S., R. 69 W. Golden quadrangle. L1. SE‘ANEl/r sec. 33, T. 3 S., R. 70 W, Golden quadrangle 18. NW'ASW'A sec. 20, T. 7 S. R. 68 W., Kassler quadrangle ................................................. Based on section in Scott (1963b, p. 105). EC]. NE‘ANEl/r sec. 22, T. 10 S., R. 60W ,Elbert County ........ Based on Dane and Pierce (1936). ECI. SE‘ASW‘A sec 19, T. 7 S. ., R. 60 W., Elbert County.. Do. ECH. SE‘ASW‘A sec. 2, T. 7 S. ., R. 60W ., Elbert County.... Do. ECG. NW‘ANW‘A sec. 19, T. 9 S. ., R.58 W, Elbert County.. Do. ECF. Center NW‘A sec. 2, T. 8 S. ., R.60 W, Elbert County Do. ECE. SW‘ASWV sec. '7, T. 9 S. ., R. 5l8w ,Elbert County ...... Do. ECD. SE‘ASE‘A sec 6, T. 9 S. ., R. 58 W. Elbert County... Do. ECC. SW‘ANW‘A sec. 5, T. 8 S. ., R 57 W., Elbert County ........... . Do. DSL. Center 8% sec. 21, T. 2 S., R. 70W, Golden quadrangle ................................................. Denver and Rio Grande Western Railroad cut. The Pierre sample is from 5 it below the base of the Fox Hills. Based on section by LeRoy (1946, p 89, 93). G25. Center sec. 5, T. 3 S., R. 70 W., Ralston Buttes quadrangle ............................................ To see. 33, .2 S., R. 70 W., Golden quad- ' rangle. ECB. NE‘ANW‘A sec. 18, T. 8 S., R. 57 W., Elbert County ......................................................... Based on Dane and Pierce (1936). ECA. NW‘ANE‘A sec. 1, T. 7 S. R. 58 W. Elbert County ................. Do. NVH. NW‘ANW‘A sec. 4, T. 2 S. R. 70 W. Louisville quadrangle ......... Collected by H. E. Malde and R. Van Horn. MVH. SW‘ANWV sec. 29, T. l S. ., R. 70 W. Eldorado Springs quadrangle" Do. U. NW‘ASW'A sec. 21, T. 3 S., R. 70 W. Golden quadrangle. . MS. ............................................................................................................................................ Near Golden. From Mielenz, Greene, and Schieltz (1951), collector unknown. Hills from sandstone in the Laramie Formation. The dominantly fresh- or brackish-water deposits above. In sandstone beds in the underlying Pierre are similar to 'view of this environment indefinite boundaries are, sandstone in the Fox Hills, but most are thinner and are perhaps, expectable. interbedded with thicker shale beds. As now defined (Lov- The clay mineralogy of the Fox Hills just south of the ering and others, 1932) the Fox Hills is a transition phase Golden quadrangle was examined by Gude (1950). He between predominantly marine shale below and pre- determined that eight out of nine samples contained more 28 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO montmorillonite than kaolinite. Montmorillonite also exceeds kaolinite in three samples of the Fox Hills from the Denver and Rio Grande Western Railroad cut at Plastic siding in the north part of the Golden quadrangle (see table 4); all three samples also contained dolomite. X— ray analysis by Gude of five samples from the Fox Hills collected by Van Horn in the Kassler quadrangle 24 miles south of Golden also showed montmorillonite is more abundant than kaolinite, and four of these also contained trace amounts of dolomite. The relative amounts of mont- morillonite and kaolinite were not distinctive in five samples from the east side of the Denver basin in the vicin- ity of Limon, Colo., about 90 miles southeast of Golden. There the amounts of montmorillonite and kaolinite are the same in three samples, montmorillonite exceeds kao- linite in one sample, and kaolinite exceeds montmoril- lonite in one sample. (See table 4, localities ECA and ECB.) However, two of the samples did contain dolomite. LOCAL CORRELATION Shale and sandstone are complexly intertongued in the Fox Hills as indicated by mineralogic, sedimentologic, and paleontologic evidence. In the north part of the Golden quadrangle the Fox Hills comprises two sand- stone units separated by a thin clay. The upper part of the lower sandstone unit intertongues with shale beds to the south, and in the southern part of the Golden quadrangle and in the Morrison quadrangle a thick shale unit is present between the two sandstone units. (See fig. 10.) In the northern part of the area the Fox Hills is com- posed of two sandstone beds, separated by a thin seam of clay. (See fig. 11.) The lower sandstone, which is grayish orange, is about 30 feet thick. It is very fine grained and very silty. Many poorly defined bedding planes indicate it is thin bedded. The upper sandstone, a massive bed about 35 feet thick, is very pale orange. It is fine grained and silty. Many very fine grained, rounded, dark-gray mineral grains give the sandstone a speckled appearance. No Foraminifera or other fossils were found in the clay seam or either sandstone. Mineral analysis by F. A. Hildebrand shows that both sandstones contain, in probable order of abundance, montmorillonite, kaolinite, and small amounts of a mica-type mineral; the clay seam contains montmorillonite, small amounts of a mica type mineral, and kaolinite. Analyses by A. J. Gude III of samples from the same localities gave similar results except that they showed no mica-type minerals. Both analysts reported the presence of calcite in the lower sandstone. The Fox Hills does not appear to change significantly from the railroad cut south to Leyden Creek although it is poorly exposed. The Fox Hills does not crop out between Leyden and Ralston Creeks. At Ralston Creek the Fox Hills has changed in litho- logy. There it is about 70 feet thick but a few thin shale beds occur in the lower part of the formation. The lower 13 feet is mainly sandstone but the succeeding 17 feet is sand- stone in beds 1—3 feet thick separated by shale beds gen- erally less than 1 foot thick. The sandstone is grayish orange, silty, and very fine grained. The shale is medium gray and generally is fissile. The upper 40 feet of the Fox Hills at Ralston Creek is sandstone in two beds (units 277 and 278 of measured section G25—D at Ralston Creek). The lower sandstone bed is grayish orange, silty, and fine grained. It is composed principally of angular, clear to milky, quartz grains but includes as much as 15 percent black minerals and mica. A single small shell fragment and a probable fish scale were found in a washed sample. The coarse fraction of washed samples consists mainly of aggregates of silt and very fine grained sand. The upper sandstone bed is similar to the lower except it is light gray and is not as silty. X-ray analyses indicate that both sand- stone beds contain more montmorillonite than kaolinite. No dolomite, however, was present. The lower 30 feet of the Fox Hills at Ralston Creek is correlated with the lower sandstone at the Denver and Rio Grande Western Railroad cut. The upper 40 feet of the Fox Hills at Ralston Creek is correlated with the upper sand- stone of the Fox Hills at the railroad cut. The log of the S. D. Johnson 1 well (fig. 3), located east of Hyatt Lake and4 miles southeast of the Fox Hills exposure at Ralston Creek, shows 165 feet of Fox Hills which consists, in ascending order, of 90 feet of fine-grained gray sandstone with some interbedded shale in the lower part, 10 feet of gray shale, and 65 feet of medium-grained white sand- stone (American Stratigraphic Co., 1956). Only the lower 90 feet of the Fox Hills shown in this log is equivalent to the Fox Hills as mapped in the Golden quadrangle. The Fox Hills is exposed in a prospect trench near the northwest corner of Golden. (See measured section G24—B.) Here it consists of 10 feet of sandstone (units 24—26) overlain by 62.7 feet of shale (unit 27), and capped by 25 feet of alternating sandstone and shale (units 28-33). The sandstone beds are mainly grayish orange and fine grained. Samples of some beds examined with a micro- scope contain many silt-sized grains. They are composed principally of quartz grains but contain small amounts of mica and unidentified dark minerals. The shale beds are light olive gray, fissile, and clayey. Foraminifera resem- bling Haplophragmoides and Robulus are present in the middle and lower parts of the 62.7-foot shale bed. The stratigraphic correlations are less clear near Golden than at Ralston Creek. The lower contact of the Fox Hills is placed at the base of a relatively thick sandstone. This sandstone is underlain by 40 feet of interbedded shale and sandstone in '/2- to 5-foot-thick beds. Because the shale pre- MESOZOIC ROCKS A 29 1. West Alameda Parkway east of Morrison (LeRoy, 1946. p. 88,sec. 1); SW34 sec. 24, T. 4 S., R. 70 W. 2. South of Golden (LeRoy, 1946, p. 88, sec. 2, and p. 2 97 below bed 79 of sec. 4, as amended by Gude, 1950);NWV4 sec. 3, T. 4 S., R. 20 W. EXPLANATION 3. Prospect trench near northwest corner of Golden, measured section, locality G24; SW'A sec. 28, T. 3 S., R. 70 W. SOUTH 4. Ralston Creek, measured section, locality G25; 1 SWV4 sec. 33, T. 2 S., R. 70 W. 5. Denver and Rio Grande Railroad cut at Plastic siding locality DSL (LeRoy, 1946, p. 89, sec. 3); 3 SW% sec. 21, T. 2 S., R. 70 w. Interbedded shale and sandstone — a . Siltstone 33 F0 ram/e F " H’lls so’marm 4 h 1 and” '7 NORTH 5 W 0178 Silty shale _ Clayey shale \ Limes U—27 \ \ \ tone \\ \‘e§: U478 Unit no. of ‘—\—\: _______ \ \ measured \ \ \ ‘ * section ~===:\:_‘ FEET METRES _______ 0 0 10 U—24 1 Pierre Shale 20 3O 10 40 o 1 MILE o 1 KILOMETRE FIGURE 10,—Lithologic sections of the Fox Hills Sandstone near Golden. dominates and most of the sandstone beds are less than 2 feet thick, these beds are assigned to the Pierre Shale. The lower sandstone beds assigned to the Fox Hills and the overlying 62.7-foot-thick shale are probably equivalent to the lower 30 feet of the Fox Hills at Ralston Creek. The al- ternating sandstone and shale sequence in the upper part of the prospect trench is tentatively correlated with the upper 40 feet of sandstone at Ralston Creek. (See fig. 10.) The upper contact of the Fox Hills at the prospect trench is placed at the base of a medium-grained light-gray sandstone. This sandstone looks cleaner than underlying sandstone because of its lighter color and coarser texture; it is similar in appearance to the sandstone beds higher in the section. In addition, the beds principally composed of clay—sized material that underlie this sandstone are fissile, whereas those that overlie the sandstone are blocky. South of Golden, LeRoy (1946, p. 86, 88) indicated that the upper part of the Fox Hills is mainly shale and the lower part is a sandstone 10—30 feet thick. Thin sandstone beds are present at a few places in the upper part of the shale. (See section 1, fig. 10.) Several sandstone beds are present in the upper part of the formation just south of Golden (Gude, 1950, p. 170241705). These beds are included in the Laramie Formation by LeRoy (1946, p. 97, units 80—88), although, east of Morrison, he included sandstone and an underlying thick shale in the F ox Hills. The sandstone in the lower part of the Fox Hills south of Golden, indicated by LeRoy, is probably equivalent to the sandstone in the lower part of the Fox Hills in the pros- pect trench. The upper shale bed of LeRoy probably corre- lates with the 62.7-foot-thick shale bed in the middle part of the formation in the prospect trench. The sandstone in 30 \ FIGURE ll.—Southward view of the Fox Hills Sandstone exposed in the Denver and Rio Grande Western Railroad cut at Plastic siding, near the north end of the Golden quadrangle. The lower sandstone is to the right of sample C, which is the clay seam separating the two sandstone beds of the Fox Hills. The letters show the units the upper part of the formation at the prospect trench is probably equivalent to the additional sandstone and shale beds that Gude (1950) included in the Fox Hills just south of Golden. These relations lead me to believe that there is an intertonguing of shale and sandstone beds in the Fox Hills of the southern part of the Golden quadrangle; the shale in the middle pinches out or grades into sandstone to the north, and the upper sandstone grades into shale and sandstone to the south. AGE The Late Cretaceous age of the Fox Hills is established by its stratigraphic position. The Fox Hills overlies Pierre Shale and underlies Laramie Formation, which are both of Late Cretaceous age. GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO it ale sampled at locality DSL reported in table 4. The serial numbers of the samples are: A (207,345 and IWX387A); B (207,346 and IWX387B); C (207,347 and IWX387C); D (207,348 and IWX387D); E (207,349 and IWX387E); F (IWX387F); G (207,350 and IWX387G); H (IWX387H). ORIGIN The Foraminifera in shale of the Fox Hills near Golden indicate a marine origin. LeRoy (1946, p. 90) reported several types of Foraminifera from the shale, and Moody (1947, p. 1461) reported several macrofossils of marine origin also from the shale. In addition, Moody (p. 1461) reported that sandstone of the Fox Hills underlying the shale contains Baculites sp. aff. asper Morton and Pteria nebmscana Evans and Shumard. This meager marine fauna, plus the lithology of these beds, probably indicates that the sandstone is of marine origin. The presence of finely broken plant remains in the sandstone beds (see measured section at Ralston Creek, and Moody, 1947, p. 1461) indicates a nearshore environment. MESOZOIC ROCKS 31 SECTION G25—D.—Fox Hills Sandstone [Measured on the south side of Ralston Creek, Golden quadrangle. Line of section shown Laramie Formation, unit 279. Fox Hills Sandstone: 278. 277. 276. 275. 274. 273. 272. 271. 270. 269. 268. 267. 266. 265. 264. 263. 262. 261. 260. 259. 258. 257. 256. 255. 254. 253. 252. 251. 250. in fig. 3] Sandstone, very light gray, fine—grained, soft; pre- dominantly quartz with some dark minerals; occa- sional layers with many plant fragments. Differs from underlying sandstone in color and appears slightly more massive. X-ray laboratory sample No. 219,980, table 4, taken 2 ft above base ...................... Sandstone, grayish-orange, massive fine-grained; pre- dominantly quartz, with some dark minerals. X- ray laboratory sample No. 219,981 table 4, taken 2 ft below top ............................................................ Shale, medium-gray, clayey.... Sandstone, grayish-orange, massive, very fine grained. Predominantly quartz with some dark minerals ...... Shale, medium-gray, clayey .......................................... Sandstone, grayish-orange, massive, very fine grained; contains rare plant fragments; predominantly quartz with some dark minerals ............................... Shale, medium-gray, silty ............. . Sandstone, grayish-orange, massive, very fine grained; predominantly quartz with some dark minerals ...... Shale, medium-gray, clayey .......................................... Sandstone, grayish-orange, massive, very fine grained; predominantly quartz with some dark minerals ...... Shale, medium-gray, clayey .......................................... Sandstone, grayish-orange, thin-bedded, very fine grained, silty; predominantly quartz with some dark minerals ............................................................ Shale, medium-gray, clayey .......................................... Sandstone, grayish-orange, thin-bedded, very fine grained, silty; predominantly quartz with some dark minerals ............................................................ Shale, medium-gray, clayey .......................................... Sandstone, grayish-orange, massive, very fine grained; predominantly quartz with some dark minerals ...... Siltstone, moderate—brown, sandy; thin layers of plant fragments ......................................................... Sandstone, grayish-orange, massive, very fine grained; predominantly quartz with some dark minerals ...... Shale, medium-gray, clayey .......................................... Sandstone, grayish-orange, massive, very fine grained; predominantly quartz with some dark minerals. Attitude N. 6° W., 80° W. ......................................... Siltstone, moderate-brown, sandy; thin layers of plant fragments ................................................................... Sandstone, grayish-orange, thin-bedded, very fine grained, silty; some layers of plant fragments ......... Siltstone, moderate-brown, sandy; thin layers of plant fragments ......................................................... Sandstone, grayish-orange, thin-bedded, very fine grained, silty .............................................................. Shale, medium-gray, clayey .......................................... Sandstone, grayish-orange, thin-bedded, very fine grained, silty .............................................................. Shale, medium-gray, clayey ........ .. Sandstone, grayish-orange, thin-bedded, very fin grained, silty .............................................................. Shale, medium-gray, clayey .......................................... Th irkness (fl) 25.4 Fox Hills Sandstone—Continued ”will“; 249. Sandstone, grayish-orange to moderate-yellow- ish-brown, silty; with some 0.1—in.—diameter ferruginous concretions. Lower 1 ft contains thin beds of medium-gray shale and grayish-orange sandstone ................................................................... 3.0 Total Fox Hills Sandstone .................................... 67.8 Pierre Shale, unit 248. SECTION G24—B.—-Fox Hills Sandstone [Measured in one of a series of prospect trenches at locality G24 (fig. 3) in the SW‘A sec. 28, T. 3 S., R. 70 W. It is continuous with a section of the Pierre Shale and 1h? Laramie Formation measured at this same locality] Laramie Formation, unit 34. Thickness Fox Hills Sandstone: (ll) 33. Shale, light-olive—gray (5Y 5/2), clayey; and four thin interbedded clayey siltstone beds near the middle ..... 4.4 32. Sandstone, grayish-orange (IOYR 7/4), fine-grained; and a few thin interbedded light-olive-gray shale beds .............................................................................. 3.5 31. Shale, light-olive-gray (5Y 5/2), clayey; and a few thin interbedded grayish-orange siltstone beds .................. 4.8 30. Sandstone, grayish-orange (IOYR 7/4), fine-grained; and two thin shale beds near the base ....................... 2.5 29. Shale, light-olive-gray (5Y 5/2), clayey, and a few thin siltstone beds near the top .......................................... 2.8 28. Sandstone, light~gray (N7), medium-grained; moder- ately stained grayish orange by limonite ................... 5.4 27. Shale, light-olive-gray (5Y 5/2), clayey. Contains a few small clusters of gypsum. Several species of fossil Foraminifera are present in the lower half ................ 62.7 26. Sandstone, grayish-orange (IOYR 7/4 ), fine-grained... 7.8 25. Shale, light-olive-gray (5Y 5/2), clayey .......................... .8 24. Sandstone, grayish-orange (IOYR 7/4), fine-grained. . 1.7 Total Fox Hills Sandstone ...................................... 96.4 LARAMIE FORMATION The Laramie Formation, as used in this report, follows the definition by Emmons, Cross, and Eldridge (1896, p. 72—77). The lower contact is at the top of grayish-orange, fine-grained sandstone or fissile shale of the Fox Hills Sandstone, and the upper contact is at the base of the con- glomerate of the Arapahoe Formation. Where the con- glomerate is absent or obscured, as it is in most of the northern part of the area, the upper contact is very indefi- nite. The formation in general appears to thicken east- ward, from about 600 feet at Golden to nearly 1,000 feet east of Hyatt Lake. Between Golden and Ralston Creek the Laramie forms several isolated knolls, and just north of Leyden Creek it forms a sharp hogback nearly a mile long and about 100 feet high; at other places it has been smoothly truncated and does not form distinctive ridges. Between Upper Long Lake and Van Bibber Creek the outcrop area of the forma- tion is displaced almost a mile eastward by the fault south- east of Ralston dike. ” 32 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO Beds in the Laramie are lenticular and, although gen- eralized zones have been traced for many miles, indi- vidual beds within these zones probably pinch out within a few miles. The lower part contains about equal amounts of sandstone and claystone whereas Claystone is predomi- nant in the upper part. In the Golden quadrangle, coal beds are present in the lower part. The sandstone is light gray and medium grained. It is principally quartz but contains small amounts of dark minerals. Large concretionlike masses of sandstone occur at a few places. Normally the beds are moderately well cemented, although at places they may be either hard and quartzitic or only moderately cemented. The sandstone beds are massive to thin bedded, and at a few places they are crossbedded. They are generally noncalcareous. The sand grains are generally subround and relatively uniform in grain size within a single sample. The matrix usually appears crystalline and does not constitute a large propor- tion of the rock. At places very little cement is present and as a result the rock has a porous appearance. At other places some white clay is present with the cement. A few thin fine-grained sandstone and siltstone beds are in the lower part, but both are more common in the upper part. Claystone in the Laramie is generally medium to dark gray although at places it has a brownish or reddish tinge. Claystone is predominant in the upper part of the Laramie; well logs in the eastern part of the quadrangle show only minor amounts of sandstone in the upper part. Within the Golden quadrangle the ratio of Claystone to sandstone increases to the east—where the Laramie is much thicker—and appears to increase slightly to the north. The Claystone in the upper part also appears to be more arenaceous than the Claystone to the west. Investigation of the clay minerals of the Laramie by Gude (1950) suggested that the Laramie could be sub- divided into upper and lower. parts in which kaolinite is predominant, and a middle part that is predominantly montmorillonite. Additional clay mineral investigation of Claystone and sandstone beds in the Laramie (see table 4) only partly confirms such a subdivision. In the Golden quadrangle the lower 100 feet of the Laramie exposed along the Denver and Rio Grande Western Railroad cut was sampled for clay minerals. In 10 of 12 samples analyzed from six beds, kaolinite is predominant. In one sample montmorillonite is predominant, and in another kaolinite and montmorillonite are approximately equal in quantity. A mica-type mineral is also present in five of the samples. In a presumed basal Laramie sample from Ralston Creek, kaolinite and montmorillonite are present in about equal quantities, although montmorillonite is predominant in the underlying Fox Hills. Dolomite is present in only one sample of the Laramie in the Golden area but is present in several samples of the Fox Hills and Pierre. Samples from 24 miles south of Golden, in the Kassler quadrangle, show a twofold division of the Laramie in which the upper part is predominantly montmorillonite and the lower part is predominantly kaolinite. Farther east on the east side of the Denver basin, near Limon, Colo., the clay minerals throughout the Laramie appear to be pre- dominantly montmorillonite. Coal beds, as much as 14 feet thick, are present in the lower 200 feet of the Laramie. No fresh coal was seen during fieldwork for this report, but Eldridge reported (Emmons and others, 1896, p. 317—387) that it is black, lustrous, and hard. It contains small amounts of pyrite. The many abandoned coal mines give an insight into the geologic structure of the area and are therefore dis- cussed in some detail. The lower 200 feet of the Laramie contains from one to six known coal beds, ranging in thickness from 1 to 14 feet. Individual coal beds have not been correlated from mine to mine, but within each mine individual beds have been traced sufficiently to indicate that the western edge of the Laramie mostly dips nearly vertically for as much as 700 feet below the ground surface and then bends sharply eastward to become nearly hori- zontal. North of the Denver and Rio Grande Western Rail- road the dip at the western edge of the outcrop becomes less steep. Information on the coal mines is from various sources including the field notebooks of Whitman Cross and George H. Eldridge, the report by Emmons, Cross, and Eldridge (1896, p. 333—338), and maps on file with the Colorado State Bureau of Mines. The location of the mines and the approximate position of the mined-out coal beds are shown in figure 12. Most of the mines were de- veloped from vertical shafts sunk into the Fox Hills. Drifts were then driven eastward at various levels to intersect the coal in the Laramie. The portal of the southernmost mine in the quadrangle, the White Ash mine, was on the south side of Clear Creek, just northwest of the Laramie out- crops shown on the geologic map. Here, the coal beds dip steeply west (overturned) from the surface downward for about 650 feet. From this point to the lowest workings, another 80 feet down, the coal is essentially vertical. In the Loveland and New Loveland mines, about 2,000 feet north on the same bed, the coal is essentially vertical between 300 and 500 feet below the surface. At the New (Little) White Ash mine, one-half mile north of Clear Creek, the coal dips 75°-80° W. (overturned) to a depth of 317 feet. The coal in the next three mines to the north dips steeply west. In the Ralston Springs mine, about one-quarter mile south of Van Bibber Creek, the coal dips west for 175 feet below the surface. South of Ralston Creek in the Tindall mine (sometimes called Tindale mine) the coal dips west for 500 feet below the surface, although at the 500-foot level the beds are nearly vertical. The nearly horizontal beds in the workings of the new Leyden mine are 1,500 feet east and 2,000 feet north of and 650 feet lower than the coal at the MESOZOIC ROCKS 105°1 5’ 39'52'30' ‘ . 1 .230“ R. 70 W. 10' R. 69 W. 47'30' M? EXPLANATION CE Inclined shaft El Vertical shaft )— Portal of adit ® Area of mined-out coal Area of mined-out near- vertical coal bed 1 |2 1'3 MILES l 1 2 3 KILOMETRES O——O 105°07'30' FIGURE 12.—Abandoned coal mines and the approximate area of mined-out coal in the Golden quadrangle. Base from U.S. Geological Survey Golden ”4-minute quadrangle (shaded relief), scale 124,000, 1965. 33 34 bottom of the Tindall shaft. Assuming the two coal beds are close to the same stratigraphic position, the radius of bending to change from vertical to horizontal is probably relatively short. A similar situation is indicated at Leyden Creek where drill~hole data (Gude and McKeown, 1952) indicated that the coal at the outcrop is dipping steeply eastward at about 5,600 feet altitude, whereas nearly hori- zontal beds in the new Leyden mine were worked 1,500 feet east of the outcrop at an altitude of 4,962 feet. The coal at the Rocky Flats mine in Barbara Gulch dips about 75° E. at the surface. The coal in the Capitol (Cap Rock) mine, 1,000 feet north of Rocky Flats Lake, (not in fig. 12), dips 45° E. at the surface. At several places gently dipping beds of younger formations are present a few thousand feet east of nearly vertical outcrops of Laramie. The sharp bend in the formation, indicated by the change in dip, may partly account for the difference in thickness of the Laramie at the outcrop, where it is about 600 feet thick, and in the subsurface in the eastern part of the quadrangle, where it is nearly 1,000 feet thick. The great stress that must have accompanied this bending pos- sibly caused plastic flowage of the claystone with a conse- quent thinning. The difference in thickness could also be due to more erosion in the west, to original thickening of deposits toward the center of the basin, or to a combina- tion of all three factors. The contact between the Laramie and the underlying Fox Hills varies from an abrupt lithologic change to a gradual change from fissile shale and grayish-orange sandstone in the Fox Hills to blocky claystone and light- gray sandstone in the Laramie. At different places Laramie rocks at the contact may be either sandstone or claystone on either sandstone or shale. There was undoubtedly some erosion as the sea retreated from the area and as rivers flowed out across the vacated sea bottom. Any such erosion was probably minor, however, and the boundary is prob- ably best described as transitional rather than uncon- formable. AGE No fossils from the Laramie were collected during the present investigation although many leaf fragments were seen. Horn cores of the dinosaur Triceratops have been reported by Brown (1943, p. 69); Ostrea glabra were found in the lower part, and Unio sp.? at higher horizons by Eldridge (Emmons and others, 1896, p. 77). A compre- hensive report on the Laramie flora of the Denver basin by Knowlton (1922) lists over 100 species of fossil plants. The fossils indicate a Late Cretaceous age. ORIGIN The coal beds in the lower part of the Laramie were formed in swamps in an area of low relief and sluggish drainage. According to Knowlton (1922, p. 99) the plants grew in a warm humid climate. GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO SECTION G24—C.—Laramie Formation [Measured in a series of prospect trenches at locality G24 in the SW sec. 28, T. 3 5.. R. 70 W. (fig. 3). [t is continuous with a section of the Fox Hills Sandstone and the lower part of the Arapahoe Formation measured at this same locality] Thickness Laramie Formation: (ft) 107. Claystone, medium- to light-gray (N5 to 7), silty ....... 13.7 106. Sandstone, light-gray (N7), fine—grained, silty, limo- mite-stained ................................................................ 1.4 105. Claystone, medium-gray (N5), silty, slightly cal- careous ....................................................................... 22.9 104. Sandstone, grayish-orange (lOYR 7/4), fine-grained, silty, limonite stained ............................................... .3 103. Claystone, medium-gray (N5), silty, slightly cal- careous ........................................... 3. 7 102. Covered ......................................... 23. 9 101. Claystone, medium-gray (N5), silty ................ 4.8 100. Sandstone, light-gray (N7), very fine grained, s11t 4.9 99. Covered ......................................................................... 43.0 98. Claystone, medium-gray (N5), silty; streaked by very light gray calcium carbonate .................................... 11.0 97. Covered ......................................................................... 28.1 96. Claystone, medium- to light-gray (N5 to 7), silty. Atti- tude N. 13° W., 85° W ............................................... 3.7 95. Claystone, medium- to light-gray (N5 to 7), silty; greatly stained by very light gray calcium carbonate and grayish—orange limonite streaks. Unit contains organic matter and is slightly coaly at the base ....... 30.2 94. Sandstone, light-gray (N 7), fine-grained, silty; moder- ately stained by calcium carbonate and limonite ..... 2.1 93. Claystone, medium-gray, silty; greatly stained by cal- cium carbonate and limonite .................................... 8.4 92. Siltstone, light- to medium-gray (N7 to 5); greatly stained by calcium carbonate .................................... 14.4 91. Claystone, medium-gray (N5), silty; greatly stained by calcium carbonate and limonite ............................... 4.9 90. Sandstone, grayish-orange (lOYR 7/4), fine-grained silty ............................................................................ 7.0 89. Claystone, medium—gray (N5); greatly stained by calcium carbonate and limonite ............................... 2.3 88. Siltstone, light- gray (N7), sandy; moderately stained by calcium carbonate ................................................ 2.0 87. Claystone, medium gray (N5), silty; moderately stained by calcium carbonate and limonite ............. 2.1 86. Claystone, medium-gray (N5); and interbedded light- gray (N7) siltstone ..................................................... 3.8 85. Siltstone, light~gray (N7), thin, wavey-bedded; and minor amounts of interbedded medium-gray (N5) silty claystone ............................................................ 28.8 84. Covered ......................................................................... 175.5 83. Sandstone, light-gray (N7), fine-grained, silty; moder- ately streaked by calcium carbonate ......................... 10.6 82. Claystone, medium-gray (N5); moderately stained by calcium carbonate and limonite ............................... 6.6 81. Sandstone, grayish—orange (lOYR 7/4), medium— grained; moderately stained by limonite. Contains a minor amount of dark-gray mineral grains .......... 1.5 80. Claystone, medium-gray (N5). Contains a l-ft and a 0.5-ft-thick grayish-orange limonite stained zone near the top and a l-ft-thick light-brown bed near the middle ................................................................. - 6.3 79. Siltstone, medium-gray (N5) ........................................ .4 78. Claystone, medium-gray (N5), silty ............................. 1.8 77. Siltstone, medium-gray (N5). Lower half is stained grayish orange by limonite and contains plant fragments ................................................................... 2.0 MESOZOIC ROCKS Laramie Formation—Continued ”’57:” 76. Claystone, medium-gray (N5), silty ............................. 3.2 75. Sandstone, light-gray (N7) to grayish-orange (lOYR 7/4), fine-grained; and interbedded light-gray siltstone and medium-gray silty Claystone ............... 9.2 74. Sandstone, light-gray (N7), medium-grained; moder- ately stained by limonite. Attitude N. 20° W., 89° W ......................................................................... 23.7 73. Claystone, medium-gray (N5), silty; upper 0.6 ft greatly stained by limonite. Contains a few 0.1-ft- diameter limonite concretions .................................. 6.9 72. Claystone, dark-gray (N3). Contains a moderate amount of organic material including plant frag- ments ......................................................................... 10.6 71. Claystone, medium-gray (N5); and thin beds of fine- grained, grayish-orange sandstone ........................... 1.8 70. Claystone, medium~gray (N5); slightly stained by limonite ..................................................................... 2.9 69. Sandstone, light-gray (N7), medium-grained, even- bedded. Attitude N. 23° W., 78° W ........................... 4.7 68. Claystone, dark-gray (N3). Contains abundant plant fragments ................................................................... 2.4 67. Sandstone, light-gray (N7), medium-grained; greatly stained grayish orange by limonite. Contains abun- dant plant fragments in lower 2 ft ........................... 5.2 66. Claystone, medium-gray (N5); moderately stained grayish orange by limonite. Contains moderate amount of plant fragments ....................................... 5.1 65. Siltstone, light-gray (N7), clayey; moderately stained by limonite ................................................................ 5.1 64. Claystone, medium-gray (N5), silty. Contains several zones of abundant plant fragments, and two thin coal beds .................................................................... 10.3 63. Sandstone, grayish-orange (lOYR 7/4), medium- grained ....................................................................... 1.6 62. Claystone, light-gray (N7) ............................................ 9.8 61. Sandstone, light-gray (N7), medium-grained; contains minor amounts of dark-gray grains. Lower part is greatly stained dark yellowish brown and has con- cretions 3—5 ft in diameter and small wormlike con- cretions. Attitude N. 25° W., 73° W .......................... 10.5 60. Claystone, medium-gray (N5) ...................................... 9.1 59. Sandstone, light-gray (N7), medium-grained .............. 10.1 58. Claystone and thin coal bed ......................................... 2.0 57. Sandstone; bottom surface stained reddish brown and contains plant fragments ................................... 5.5 56. Claystone, medium-gray (N5) ...................................... 1.6 55. Sandstone, light-gray (N7), medium-grained. Con- tains a few plant fragments and a 0.5-ft-thick medium-gray Claystone bed ...................................... 3.0 54. Claystone, medium-gray (N5) ...................................... 4.9 53. Sandstone, light-gray (N7), medium-grained; lower 2 ft stained grayish orange by limonite. Contains minor amount of dark mineral grains ..................... 26.4 52. Sandstone, light-gray (N7), fine-grained ...................... 8.7 51. Claystone, light-gray (N7) ............................................ 3.7 50. Siltstone, light-gray (N7) .............................................. 1.9 49. Sandstone, light-gray (N7), medium-grained, greatly stained grayish orange by limonite .......................... 7.6 48. Covered ......................................................................... 13.7 47. Claystone, medium-gray (N5), silty ............................. 1.2 46. Sandstone, light-gray (N7), medium-grained, mottled grayish orange. Contains a few 0.1-ft-diameter limonite concretions ................................................. 1.7 35 Laramie Formation—Continued ”'37” 45. Claystone, medium-gray (N5), silty. Attitude N. 25° W., 65° W .................................................................. 3.0 44. Sandstone, very light gray (N8), medium-grained ....... 5.4 43. Claystone, light-gray (N7), silty ................................... 1.3 42. Sandstone, very light gray (N8).. 2.6 41. Claystone, light-gray (N7) ...... 2.6 40. Sandstone, light-gray (N7), medium— to coarse— grained, greatly stained grayish orange by limonite. Attitude N. 26° W., 79° W ........................................ 27.1 39. Claystone, medium-gray (N5), mottled reddish brown ........................................................................ .9 38. Sandstone, grayish-orange (lOYR 7/4), fine-grained... .3 37. Claystone, medium-gray (N5), mottled reddish brown ........................................................................ .6 36. Sandstone, grayish-orange (lOYR 7/4), fine-grained... .3 35. Claystone, medium-gray (N5), mottled reddish brown ........................................................................ .3 34. Sandstone, light-gray (N7), medium—grained ..... 4.2 Total Laramie Formation ..................................... 733.2 ARAPAHOE FORMATION The Arapahoe Formation, like the underlying Laramie, is principally composed of discontinuous beds of sand» stone and Claystone. In the Golden quadrangle the base is marked by a poorly exposed or discontinuous conglom- erate and the top by the appearance of the andesitic debris in the Denver Formation. The Arapahoe Formation underlies most of the northeastern three-fourths of the quadrangle, where it forms small isolated exposures. The Arapahoe, as used in this report, follows the definition of Eldridge (1889, p. 97—99), for reasons explained on page 43. The Arapahoe is 400—500 feet thick. North of Ralston Creek the lower contact of the Arapa- hoe Formation was seen at only two places in the field. The location of the contact is based on three criteria: outcrops of conglomerate in the roadcut for State High- way 93 on the north side of Ralston Creek and in the NEIANE‘A sec. 7, T. 2 S., R. 69 W. (Louisville quadrangle); electric well logs 500 feet south of the NM cor. sec. 29, T. 2 S., R. 69 W. (log interpreted by George H. Chase of the U.S. Geol. Survey) and another 800 feet east of Hyatt Lake; and interpolation of subsurface contours drawn on the base of the Laramie Formation (fig. 49). Analysis of these data indicated that the Laramie-Arapahoe contact should be near the surface in the eastern parts of the valleys of Woman and Leyden Creeks. Some corroboration for locating the contact here was obtained in that the contact is just below a hard conglomerate bed in the center of sec. 23, T. 2 S., R. 70 W. and a conglomeratic sandstone 1,000 feet south of the NE. cor. sec. 31, T. 2 8., R. 69 W. In addition, no conglomerate was found stratigraphically lower than the contact shown on the map, although con- glomerate beds are present above the contact at several places. These data have required a revision of the location 36 of the lower contact from that shown previously (Van Horn, 1957b). The Arapahoe Formation is poorly exposed in this area. The most complete exposure is on the campus of the Colorado School of Mines in Golden: the basal com- glomerate is exposed in the abandoned claypits south of the athletic field, progressively younger conglomerate and sandstone beds are discontinuously exposed adjacent to the curving road just east of the claypits, and gray sandy claystone and thin sandstone are exposed in an artificial cut 500 feet due east of the claypit symbol. Andesitic—de- bris-bearing beds of the Denver Formation were tempo- rarily exposed in the excavation for a new gymnasium (not shown on the map of Van Horn, 1972) at the north corner of 14th and Maple Streets (on the map, 700 ft east of the claypit symbol). The basal conglomerate of the Arapahoe is also expOSed in two prospect trenches west of Golden and north of Clear Creek. (See measured section at locality G24—D.) The basal conglomerate forms a low mound at a few places between Golden and Van Bibber Creek. The only other outcrop of unequivocal basal Arapahoe con- glomerate is on the west side of a deep highway cut east of Ralston Reservoir on the north side of Ralston Creek. Here the basal conglomerate is a weakly cemented pebbly sand- stone about 15 feet thick. Isolated exposures of the Arapahoe north and northeast of North Table Mountain are principally of sandstone and conglomeratic sandstone, although many temporary exposures and a few natural cuts indicate that the formation is principally a sandy or silty claystone. Of nearly 100 sandstone outcrops seen in this area, roughly 50 percent are noncalcareous sandstone, 15 percent are cal- careous sandstone, and 35 percent are conglomeratic sand— stone. Most of the conglomeratic sandstone occur in out- crops presumably near the base of the Arapahoe but a few are in the middle or upper part. Several outcrops of a coarse conglomeratic sandstone are present near the junc- tion of Van Bibber Creek and McIntyre Street. At several places calcareous sandstones are 100—200 feet higher than the presumed base of the formation, and only a few are in the upper part. Noncalcareous sandstone is present throughout the formation. The sandstone in the Arapahoe ranges from light gray to moderate yellowish brown; most beds have a brownish or yellowish tinge. The sand grains are generally sub- angular to subround and consist principally of quartz although small amounts of dark-gray chert are present. The grain size within a single sample generally varies widely, and the different beds range from predominantly very fine grained to conglomeratic. The matrix is gen- erally clay or silt and at many places it forms as much as 50 percent of the rock. At some places very little matrix is present, resulting in a very porous rock. Pebbles in the GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO conglomerate and conglomeratic sandstones were derived from both metamorphic and sedimentary rocks. Quartz, feldspar, and chert are most common—the feldspar appears altered and 'silicified at many places—but some metamorphic rock and sandstone generally are present. The beds are generally massive but at some places are thin bedded or crossbedded. The claystone is light to dark gray or brown and gen- erally silty or sandy. Black carbonaceous fragments of fossil plants are present locally. The coarse fraction of the claystone is predominantly quartz. At places thin beds of siltstone are interbedded with the claystone. The presence of thin conglomerate layers just below the main Arapahoe conglomerate at Golden led Brown (1943, p. 68) to believe that the contact of the Arapahoe with the underlying Laramie was erosional, but transitional. For this, and other reasons Brown believed that the erosion did not require a great lapse of time. AGE The only fossils from the Arapahoe that were identified during this investigation consisted of several thin, round, calcareous, waferlike freshwater sponges, 1—3 inches in diameter, which were found in a claystone on the north side of the railroad cut in the SW‘ASW‘A sec. 23, T. 2 S., R. 70 W. The fossils were identified as Spongillidae Gray, 1867, by Richard Rezak of the US. Geological Survey (written commun., Jan. 4, 1957). According to Rezak this family ranges in age from Jurassic to Holocene. Very few other fossils have been reported from the Arapahoe. Four species of dinosaurs from the Arapahoe were reported in Emmons, Cross, and Eldridge (1896, p. 227), and the presence of dinosaurs from the Arapahoe near Golden was reported by Brown (1943, p. 70). The dinosaurs are refer- able to the Cretaceous. No flora from undoubted Arapa- hoe beds have been identified. Knowlton (1922, p. 102—104) stated that only one of two previously believed Arapahoe floral collections was possibly from the Arapahoe. The beds from which this possible Arapahoe flora came have recently been mapped as Dawson Arkose by Scott (1963b). The distinction between Arapahoe and Dawson is, perhaps, a fine point inasmuch as the two formations interfinger, and the collection was probably made from a horizon equivalent to the Arapahoe. Brown (1943) pointed out that the flora and meager fauna of the Laramie, Arapa- hoe, and lower Denver Formations are similar, and they are of Cretaceous age. ORIGIN The fossil dinosaur, sponges, and leaf fragments and the conglomerates indicate the terrestrial origin of the forma- tion. MESOZOIC ROCKS SECI'ION G24—D.——Arapahoe Formation (part) [Measured in a series of prospect trenches at locality G24 in the SW'A sec. 28. T. 3 5., R. 70 W. It is continuous with a section of the Fox Hills Sandstone and Laramie Formation measured at this same locality]. Th irknesx Ara ahoe Form tion: p 3 (ft) 112. Covered. lll. Claystone, medium-gray (N5), silty; mottled by limonite stains ........................................................... Sandstone, light-gray (N7), very fine grained, silty; mottled by limonite stains. Contains discontinuous streaks and lenses of claystone .................................. Claystone, medium-gray (N5), silty. Becomes lighter in color and sandier toward base. Grades into underlying sandstone ................................................ Sandstone, grayish-orange (lOYR 7/4), medium- grained, conglomeratic, friable. Pebbles are pre- dominantly gray chert and minor amounts of red chert, pink and gray granite, and sandstone. Atti- tude N. 10° W., 90° ................................................... 20.6 86.3 44.5 110. 11.3 109. 9.9 108. Total Arapahoe Formation measured ................... Laramie Formation, unit 107. CRETACEOUS AND TERTIARY Although the Cretaceous—Tertiary boundary is shown on the map (Van Horn, 1972) in the lower part of the Denver Formation, the two parts of the formation are not discussed separately. Both parts are similar in lithology, and probably are not separable where paleontologic de- terminations are lacking. At the west, south, and east sides of North Table Mountain the Cretaceous part of the Denver is 257, 260, and 200 feet thick, respectively, and the Tertiary part, below the base of the capping lava flows, is 380, 500, and 380 feet thick, respectively. DENVER FORMATION The distinguishing characteristic of the Denver F orma- tion in the Golden quadrangle is the presence of volcanic debris. The base is “determined by the first appearance of eruptive material among the particles derived from the crystalline or older sedimentary rocks” (Emmons and others, 1896, p. 160). The Denver Formation is present in the southern part of the Golden quadrangle. It crops out at many places in Golden, on North and South Table Moun- tains, and at a few places south of Clear Creek in the east- ern part of the quadrangle. Three lava flows occur in the upper part of the formation. The part of the formation that presumably extended above the highest lava flow has been removed by erosion in this area, and less than 800 feet of the formation remains. The Denver is composed of light-gray to brown tuffaceous silty claystone, tuffaceous arkose, and andesitic conglomerate. The base is marked by the first appearance of volcanic material, composed of angular fragments and euhedral crystals of hornblende, augite, white feldspar (probably andesine), and magnetite. Rounded quartz 37 grains are present in the lower 200 feet of the formation but above this are rarely present. The amount of angular vol- canic debris increases rapidly in progressively younger beds; also, within a few hundred feet of the base and con- tinuing to the highest exposures, there are many beds that contain rounded pebbles and cobbles of andesite (Cross, in Emmons and others, 1896, p. 315). The claystone and siltstone beds in the Denver are simi- lar in gross aspect to those in the Arapahoe. At some places a careful inspection with a hand lens, however, will reveal the presence of short, stubby, dark-colored prismatic crystals (hornblende and augite) typical of the Denver. The residue of a washed sample of the Denver will invariably contain angular t0 euhedral grains of dark minerals and clear to milky plagioclase feldspar. Mont- morillonite-type day with excessive swelling capabilities occurs in the Denver Formation in the Denver area (Judd and others, 1954, p. 5, 15, 16). In the Golden quadrangle the Denver Formation, as shown by tests, also has exces- sive swelling capabilities. Montmorillonite is the pre- dominant clay material in the three samples determined by Gude (1950, table3). Tuffaceous arkose, as used in this report, consists pre- dominantly of sand-sized feldspar of volcanic origin, although part of the material may have been reworked by streams or wind. The tuffaceous arkose is nearly quartz- free, except in the lower 200 feet of the Denver which may contain as much as 50 percent quartz grains. The tuffaceous arkose is light gray to light brown and contains characteristic angular broken fragments and euhedral crystals of dark minerals and white to milky plagioclase feldspar. Quartz, if present, occurs as sub- round to subangular grains in sharp contrast to the feld- spar and dark minerals. The particles larger than 0.074- mm diameter from washed samples of the tuffaceous arkose show a great variability in mineral content when examined under a binocular microscope. (See table 5.) In general, more than one-half the particles are fresh, angular, clear to milky plagioclase. About 5 percent are dark, angular, prismatic fragments and euhedral crystals of hornblende and augite, but the amount ranges from a trace to 37 percent. The mafic minerals or the plagioclase may be present in about equal quantities, or either may be greatly predominant over the other. Generally, there is less than 1 percent small euhedral magnetite octahedrons, many of which have rounded corners. Less than 1 percent biotite is also present in most samples. At places crypto- crystalline altered glass was seen adhering to crystals of plagioclase and to the augite or hornblende. The tuffa- ceous arkose beds are generally massive although at a few places they are crossbedded. The lack of rounding of the crystals and the bedding indicate that fluviatile trans- portation was at a minimum. 38 TABLE 5.—Lithology (in percent) of fragments larger than 0.074 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO mm in washed samples of the Denver and Arapahoe Formations [.,.,, absent; Tr., Trace; Ka. Arapahoe Formation of Cretaceous age; Kdv, Denver Formation (lower part) 0‘ Cretaceous age; TdV. Denver Formation (UPP‘?r part) Of Tertiary 38‘“ a. above the contact; b, below the contact. 55, sandstone; st, siltstone; ta, tuffaceous arkose]. Locality No .................................................... Section Ravine‘ U.S. Highway 62 -‘ 025 ‘st Sample No ..... 1.39 L38 L37 L8 L35 L34 L7 L6 U—l U—2 U—10 U—11 U—67 U—2 U—6 U—12 U—13 U-14 Formation ------ K8 Kdv Kdv K8 Kdv Kdv Kdv Kdv Ka Kdv Tdv Tdv Tdv Kdv Tdv Tdv Tdv Tdv Stratigraphic distance (in feet) from Ka—KDV coma“ ..... 1,50 350 375 1,25 375 21275 3350 a400 b2 32 a303 a318 a593 al 77 a270 a378 3403 3468 Rock Type .................................... ss ta ta ss ta ta ta ta st 51 st ta ta ta ta ta ta ta Aggregates of silt and clay 56 10 80 50 85 40 70 Rounded pink feldspar ................ 55 25 Tr. Tr. Rounded quartz ........................... 60 20 30 40 Tr. 3 15 20 47 5 Tr. Tr. Tr. Rounded white feldspar ............... l 5 90 Unclassified rounded grains ........ l 6 Rounded black minerals .............. 1 Tr. Tr. White cementing material ........... 35 30 35 30 5 55 Rounded andesite(?) ..................... 5 58 Tr. Angular white feldspar ................ 35 30 60 30 30 10 50 5 20 40 95 60 80 Angular hornblende or augite ..... 5 5 40 2 5 2 5 9 2 5 2 5 39 19 Angular magnetite ............... 1 2 10 l .5 1 1 Tr. Tr. Tr. l Biotite ................ 1 l 5 Tr. l 3 Tr. Tr. Tr. Tr. Tr. Tr. l Tr. ‘NEKNWlt sec. 25, T. 4 S., R. 70 W., Morrison quadrangle. 2SI‘Z‘tSEl‘ sec. 3 and SWltSWlé sec. 2, T. 4 5., R. 70 W., Morrison quadrangle. Grain mounts and thin sections of several samples of the Denver were examined microscopically. The white to milky subangular feldspar is probably andesine, some of which appears to be altering to clay. The subangular to euhedral mafic minerals are principally black to light- green hornblende, black to reddish-brown basaltic horn- blende, and black to dark-green augite. Hornblende appears to predominate in the lower part of the Denver, but augite occurs in progressively greater amounts toward the upper part where the two are about equal. Biotite is generally present in small amounts. At places the tuffaceous arkose is composed principally of cryptocrystalline grains of lathlike minerals in a cryptocrystalline groundmass. At other places the rock is a crystal tuff principally composed of unattached subhedral to euhedral crystals in a cryptocrystalline groundmass. The cryptocrystalline material is generally altered but at a few places appears fresh. A sample of the tuffaceous arkose of unit 40, of the measured section at locality G23, is a lithic tuff (Williams and others, 1958, p. 151, fig. 48c). A thin section of this sample contains cryptocrystalline altered glass, plagioclase, and an opaque mineral, prob- ably magnetite. About 20 percent of the thin section is composed of rounded to angular sand-sized grains, each consisting of many minute plagioclase laths. Each grain forms a faint but distinct unit, being slightly darker than the surrounding matrix under optimum light conditions. These grains may have been expelled from a volcanic vent as discrete glassy globules. Another 20 percent of the thin section is composed of randomly oriented discrete laths of plagioclase and black blebs of the opaque mineral. The aNW‘ANI-l‘t sec. 21, T. 3 S., R. 70 W., Golden quadrangle. ‘Center N'ré sec. 27, T. 3 S., R. 70 W., Golden quadrangle. remaining 60 percent of the thin section is a cryptocrystal- line altered glass matrix. The matrix is a very finely com- minuted dust that probably settled out of the atmosphere with the discrete grains. The conglomerate beds range from a normal con- glomerate, composed principally of pebbles in a sandy matrix, to a rock consisting of scattered pebbles and boulders in a clayey matrix. The pebbles consist almost entirely of a porphyritic volcanic rock that ranges from nearly aphanitic to coarsely porphyritic. The many petro- graphic determinations made by Cross (Emmons and others, 1896, p. 315) indicate that the pebbles are prin— cipally andesites of many different kinds. A few pebbles of metamorphic rock were found in these conglomerates in the Golden quadrangle. The euhedral crystals and angular crystal fragments of plagioclase and hornblende or augite, typical of the Denver, are almost everywhere visible in the matrix. The normal conglomerate ranges from cemented pebbles in a sand or silt matrix to pebbly tuffaceous arkose. The pebbles generally are 1—2 inches in diameter, but a boulder about 2 feet across is present in a bed underlying the lowest lava flow on North Table Mountain. The con- glomerate beds are generally massive, but at a few places are crossbedded. The conglomerate looks very much like the Quaternary alluvium but is darker in color and finer grained. Conglomerates of this type were, no doubt, deposited as alluvium by streams. The pebbly mudstone is very different in appearance. The pebbles, and locally cobbles and small boulders, are sparsely scattered throughout a clayey or silty matrix. MESOZOIC ROCKS 39 Generally there is a crude gradation in size, the coarser material being in the lower part of the beds. Locally petri- fied logs stand erect in the beds and apparently are in their position of growth. The enclosing material is poorly sorted and massive. At a few places the pebbly mudstone beds are underlain by thin ashy-appearing deposits. The pebbly mudstone beds fit the criteria outlined by Mullineaux and Crandell (1962) for volcanic mudflows. The probable origin of these mudstone beds was first called to my attention by G. E. Lewis and verified by D. R. Crandell, both of the U.S. Geological Survey, from expo- sures on Green Mountain a few miles south of Golden. The highest beds in the Denver Formation in the Golden quadrangle underlie the lava flows of North and South Table Mountains. Any Denver or other Tertiary deposits that may have originally overlain these flows have been removed by erosion. The nature of the contact of the Denver with the under- lying Arapahoe Formation is not clearly established. There is a change in lithology that is probably grada- tional within 100 feet. According to Brown (1943, p. 84), the fauna and flora of the Laramie continue into the Arapahoe and the lower part of the Denver. In the Golden quadrangle the contact is exposed only in a small cut near the base of the west side of North Table Mountain (west of the 67° dip symbol on the map; Van Horn, 1972) in the NW‘ANE‘A sec. 21, T. 3 S., R. 70 W. Here a yellowish-gray sandy siltstone of the Arapahoe is overlain, with no evidence of unconformity, by a pale-yellowish-brown silt- stone of the Denver. (See measured section G23.) In an Arapahoe hand specimen a few very fine rounded quartz grains are visible, whereas in a Denver specimen a few small, black subhedral crystals are visible. Samples of each were washed through a No. 200 U.S. Standard Sieve and examined with a binocular microscope. (See table 5, samples U—l and U—2, locality G23.) The Arapahoe residue contained about 80 percent aggregates of silt and very fine sand particles; 20 percent white to clear, subangular to rounded, predominantly unfrosted quartz; and traces of mica and a black mineral. The sample was leached with hydrochloric acid to free more of the grains, and a few particles of white feldspar and subround, elon- gate, grains of a black mineral were noted. The residue from the Denver sample contained 50 percent aggregates of silt- and clay-sized particles; 47 percent white to clear, subangular to rounded quartz; 2 percent green and red- dish-brown broken subhedral to euhedral crystals; 0.5 per- cent black euhedral magnetite octahedra (the corners are slightly rounded); and a trace of mica. When the sample was leached with hydrochloric acid about 3 percent of a weathered white feldspar was seen. The black mineral in the Arapahoe appears to be abraded, whereas the fresh angular surfaces and crystal faces of the green and red- dish-brown minerals in the Denver have undergone little wear. X-ray examinations of the clay-sized fraction of beds on both sides of this contact show no significant dif- ferences (table 4). This inconspicuous contact gives little hint of the drastic change in mineralogy and mode of deposition that followed the accumulation of the Arapa- hoe Formation. The contact between the Arapahoe and Denver probably is transitional. AGE The tuffaceous beds of the Denver Formation provided excellent conditions for the preservation of fossils, which reveal that the lower part of the Denver is Cretaceous in age and the upper part Paleocene. Over 200 species of fossil plants were listed by Johnson (1931, p. 370—374) as having been reported from the Denver. He also listed five species of dinosaurs, two of turtles, one of fish, one of crocodile, several mammalian teeth and bones, and five freshwater species of invertebrates. A bone fragment was found in dark-brown tuffaceous arkose in the irrigation ditch in the SW‘ASW‘A sec. 28, T. 3 S., R. 69 W., Arvada quadrangle, at an altitude of 5,520 feet. The fragment was tentatively identified as “dino- saurian” by G. E. Lewis of the U.S. Geological Survey (written commun., April 29, 1954). Thus, this tuffaceous arkose and underlying rocks are tentatively of Cretaceous age. A few fossils, mostly leaves, were collected in the Golden quadrangle. Several fossil plant localities were reported earlier (Van Horn, 1957b); since then two additional 10- calities have been found—one on the north side and one on the west side of North Table Mountain. The northern lo- cality, at an altitude of 6,250 feet, is in the NW‘ASW‘A sec. 15, T. 3 S., R. 70 W. The western locality (unit 18, measured section G23), at an altitude of 6,160 feet, is along the road shown ascending the west side of North Table Mountain. Both localities contain Allantodiopsis erosa (Lesquereux) Knowlton and Maxon and the northern locality also contains Ficus planicostata Lesquereux. The fossils were identified by R. W. Brown of the U.S. Geo- logical Survey (written commun., Sept. 28, 1953), who indicated that they were of Paleocene age. The Cretaceous—Tertiary boundary occurs in the lower part of the Denver Formation. Its location is based princi- pally on paleontologic data supplemented by a color change of questionable value. The Cretaceous—Tertiary boundary does not form a satisfactory formational boundary in the Golden quadrangle, and the line shown on the geologic map (Van Horn, 1972) should be used with discretion. The best exposure of the boundary is at the section measured by Brown (1943, p. 74) at the southeast corner of South Table Mountain just south of the Golden quadrangle. Here the boundary is at the base of a thick light-colored zone overlying a thick brown zone. On the north side of South Table Mountain and on North Table Mountain a light-colored zone associated with the boun- dary is split by a 10- to 20-foot-thick light-brown zone, and 40 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO the boundary is put at the base of the lower light-colored zone. (See fig. 15.) Unfortunately, these beds cannot be traced continuously, owing to poor exposures. ORIGIN The Denver Formation seems to have been deposrted on a gently sloping surface of low relief and, as indicated by the abundant flora, in a climate that was wetter and warmer than the present-day climate. Brown (1962, p. 96) stated that the Rocky Mountains and Great Plains were in a general area of warm temperate climate with a medium amount of precipitation. A study of pollen and spores from beds of the southeast corner of South Table Mountain indicates a vegetation dominated by sub- tropical elements but also containing temperate elements (E. B. Leopold, written commun., Oct. 22, 1957). The vitric and crystal tuff beds are predominantly pyro- clastics and show little evidence of reworking by streams. The rounded andesite pebbles in many of the conglom— erate beds, however, show that west of the Golden quad- rangle streams were actively eroding and abrading their beds. The volcanic mudflows indicate that the source of the andesitic debris was relatively near the west edge of the present deposit. The general lack of material other than volcanic, as first noted by Cross (1889), indicates that the older beds to the west were mainly covered by volcanic deposits. As shown by the fact that beds of equivalent age to the south, the upper part of the Dawson Arkose, have little andesitic debris, the volcanic cover was probably of local extent in a topographically higher area west of Golden and Morrison. No potential source dikes for the volcanic cover have yet been found in this area. Possibly, however, the Precambrian rocks to the west are intruded by andesitic dikes similar to the one in the Kassler quad- rangle reported by Scott (1963b, p. 106—107). SECTION GZ3.—Denver Formation [Measured at locality G23 along the road to the top of North Table Mountain in the NBA sec. 21. T. 3 5., R. 70 W.] Denver Formation: _ Thickntn (fl) 83. Latite of lava flow 3, dark gray (N3); weathers yel- lowish gray (5Y 7/2); dense. Contains phenocrysts of black augite and clear to white feldspar ................ (1) 82. Siltstone, greatly baked, sandy; contains a few pebbles and cobbles of latite and altered andesite(?). It has been burned various shades of red and gray. The upper contact is slightly baked and slopes S. 8°. The lower contact is also baked ................................. 5.5 81. Siltstone, baked, sandy; contains a few scattered pebbles and cobbles of rounded latite, some of which are greatly altered. This appears to be a large mass of Denver Formation included in unit 80. (See fig. 18.) See footnote on p. 41. Denver Formation—Continued Thitknm 80. Pebbles, cobbles, and boulders of angular latite (similar 0’) to unit 83) in a matrix of vesicular volcanic froth. It contains many pockets and thin skewed bands of baked Denver Formation, and includes unit 81 79. Tuffaceous arkose, light-gray (N7), fine-grained; con- tains very light gray feldspar and a dark mineral, probably augite. Appears to have been cut by erosion associated with the overlying unit ............................. 4.0 78. Claystone, moderate-reddish-brown (10R 4/6) (dry), silty; contains a few pebbles and cobbles of greatly altered andesite. Most of the outcrop has a strongly baked appearance. At places it is greatly fractured and slickensided .......................................................... 77. Conglomerate, very pale orange (lOYR 8/2) to dark- yellowish-brown (lOYR 4/2); composed of cobbles and boulders of latite with many augite crystals in the matrix. A medium-grained tuffaceous arkose is at the top in places. Forms hard, rounded outcrop... 8.5 76. Claystone, grayish-black (N2), pebbly; baked appear- ance. Lies at base of a channel cut into the latite and may be a mudflow ............................................... .6 22.1 12.0 75. Latite of lava flow 2, dark-gray (N3); weathers yel- lowish gray (5Y 7/2); dense. Contains phenocrysts of black augite and clear to white feldspar. Augite crystals not as prominent as flow 1. A channel cut into unit 75 is filled by units 76 to 82 ....................... 153.5 74. Conglomerate, moderate-yellowish-brown (lOYR 5/4) (dry); contains pebbles and boulders of latite. The matrix contains many large, loose augite crystals similar to those in the ledge south of Castle Rock on South Table Mountain. The top 2-12 in. is baked black. Forms hard, rounded outcrop .......................... 73. Tuffaceous arkose, very light gray (N8) (dry), fine- grained; contains some coarse augite and andesite grains. Forms hard, cliffy outcrop ............................. 1.4 72. Tuffaceous arkose, pale-yellowish-brown (lOYR 6/2) to light-gray (N7), coarse-grained, conglomeratic; contains augite or hornblende and latite or andesite grains. Pebbles are strongly altered latite .................. 4.6 71. Tuffaceous arkose, very light gray (N8) to light-gray (N7), medium-grained, conglomeratic. Contains interbedded claystone. Contains fossil wood. Forms hard, cliffy outcrops .................................................... 7.1 70. Claystone and siltstone, very light gray (N8) to grayish- orange (lOYR 7/4); in l- to 6-in. beds. Forms hard, cliffy outcrop. Locality of fossil fern leaf (Eg—53—18) determined by R. W. Brown to be Paleocene in age (table 4, locality G23, unit 70) .................................... 7.0 69. Siltstone, dark-yellowish-brown (lOYR 4/2) (dry), sandy. Contains many subround pebbles of latite or andesite. Sand grains are very light gray feldspar and 10.3 augite or hornblende. Forms hard, cliffy outcrops.... 4.2 68. Claystone, pale-yellowish-brown (lOYR 6/2) (dry), silty. Forms hackly slope ...................................................... 9.2 67. Tuffaceous arkose, moderate—yellowish-brown (lOYR 5/4) to dark—yellowish-orange (lOYR 6/6); stained a dark-yellowish-brown (lOYR 4/2) at places; silty. Contains very light gray feldspar, andesite, augite or hornblende, and minor amounts of pale-pink feldspar and bentonite(?). Forms hard, cliffy outcrops (table 5, locality G23, unit 67) .................................... 5.3 66. Siltstone, light-gray (N7) (dry), sandy. Sand grains are principally quartz, very light gray feldspar, augite or hornblende. Cliff former ........................................ 1.7 See footnote on p. 41. Denver Formation—Continued MESOZOIC ROCKS Thickness 65. Tuffaceous arkose, pale-yellowish-brown (lOYR 6/2) W 64. 63. 62. 61. 59. 58. 57. 56. 55. 54. 53. (dry), silty, crossbedded; conglomeratic at places. Channels into unit 64. Pebbles are greatly altered andesite which at a few places appeared to be vesicu— lar. Forms hard, rounded outcrop ............................. Tuffaceous arkose, very light gray (N8) to light-gray (N7), silty, very fine grained. Broken by nearly verti- cal, north-trending fault with l-ft displacement. Cliff former ......................................................................... Tuffaceous arkose, yellowish-gray (5Y 8/1) (dry), stained grayish-orange (lOYR 7/4) (dry), fine- to coarse-grained; contains quartz and very light gray feldspar with minor amounts of dark-gray to black minerals. Forms hard cliffy outcrops. Attitude N. 50° 12., 2° S ..................... Siltstone, dark-yellowish-brown (lOYR 4/2) (damp) to yellowish-gray (5Y 8/1) (dry), sandy. Forms slope of hackly fragments ........................................................ Tuffaceous arkose, very light gray (N8) to light-gray medium-grained, conglomeratic, massive; contains quartz, very light gray feldspar, augite or hornblende, and minor biotite. Contains many pebbles and cobbles of altered latite which do not appear to have the large phenocrysts of latite flow 1. Forms hard, rounded outcrop ........................................................ Siltstone, brownish-gray (5YR 4/1) (dry), sandy .......... Sandstone, very light gray (N8), massive, medium- grained. Contains quartz, biotite, augite or horn- blende, and minor amounts of pale-pink and very light gray feldspar. The top is displaced 8 in. by a fault with attitude N. 3° W., 70° E ......................... Sandstone, very light gray (N8) (dry), silty, fine-grained; contains quartz, biotite, augite or hornblende, and minor amounts of pale—pink and very light gray feldspar ....................................................................... Covered .......................................................................... Sandstone, very light gray (N8) (dry), silty, fine-grained; contains quartz, biotite, augite or hornblende, and minor amounts of pale-pink and very light gray feldspar; bentonitelike clay stringers skewed to bedding. It is massive to wavy bedded and near the base it contains a few latite cobbles. Outcrops form small cliffs, rounded knobs, andv thinly covered slopes .......................................................................... Latite of lava flow 1, dark-gray (N3); weathers yellow- ish gray (5Y 7/2); dense. Contains phenocrysts of black augite and clear to white feldspar. Augite crystals are very prominent—more so than in any other latite flow in Table Mountain or monzonite in Ralston dike. The middle part is covered by slope wash. The top 4 ft is pale yellowish brown (lOYR 6/2) to pale brown (5YR 5/2) and vesicular... Conglomerate, dark-yellowish-orange (lOYR 6/6), cobbly; cemented with silt and clay. Some of the pebbles and cobbles are similar to the underlying tuffaceous arkose and others to the overlying latite, but are well rounded. Most are so weathered that they were unidentifiable other than being igneous. The largest boulder seen was 2 ft long. This bed is the basal Denver Formation of Reichert (1954) Tuffaceous arkose, very pale orange (lOYR 8/2) (dry), coarse- to fine-grained; contains quartz, very light gray and pink feldspar, and black minerals ............. 8.3 4.6 1.7 2.3 2.3 13.3 5.3 5.0 11.3 158.3 5.8 5.6 'Not part of Denver Formation and thickness not included in total thickness of Denver. Denver Formation—Continued 52. 51. 50. 49. 48. 47. 46. 45. 44. 43. 42. 41. 40. 39. 38. 37. 36. Siltstone, pale-brown (5YR 5/2) (damp) to grayish- orange-pink (5YR 7/2) (dry). Forms hard sloping outcrop ....................................................................... Tuffaceous arkose, light-gray (N7) (damp), very light gray (N8) (dry), fine-grained. Forms hard, rounded outcrop ....................................................................... Siltstone, medium-light-gray (N6) (damp) to light- gray (N7) (dry); massive. Forms hard, hackly frag- mented sloping outcrop ............................................. Tuffaceous arkose, light-gray (N7) (damp) to very light gray (N8) (dry), fine-grained, massive. Contains pink and very light gray feldspar and minor amounts of a black mineral. Stained moderate greenish yellow (lOY 7/ 4) at places. Forms hard, rounded to sloping outcrop. Massive ........................................................ Siltstone, medium-light-gray (N6) (clamp) to light- gray (N7) (dry); massive. Forms hard, hackly frag- mented sloping outcrop ............................................. Tuffaceous arkose, light-gray (N7 ) (clamp) to very light gray (N8) (dry); fine-grained. Forms hard, rounded outcrop ....................................................................... Siltstone, medium—light-gray (N6) (damp) to light— gray (N7) (dry), massive. Forms hard, hackly frag- mented sloping outcrop .............................................. Tuffaceous arkose, light-gray (N7) (damp) to very light gray (N8) (dry), fine-grained, massive. Contains pink and very light gray feldspar and minor amounts of a black mineral. Stained moderate greenish yellow (lOY 7/4) at places and has a one-half-in. dark- reddish-brown (10R 3/4) band parallel to bedding at middle. Forms hard rounded to sloping out‘ crop ............................................................................. Siltstone, light-gray (N7) (damp) to very light gray (N8) (dry) ............................................................................. Tuffaceous arkose, yellowish-gray (5Y 8/1), conglom- eratic; contains quartz and andesite pebbles. Sand- sized fraction contains pink and very light gray feldspar and minor amounts of black minerals ......... Siltstone, light-gray (N7) (damp) to very light gray (N8) (dry). Two 6- to 8-in.-thick grayish-orange-pink (5YR 7/2) siltstone beds are near the base. Forms hard, sloping outcrop ................................................. Siltstone, pale-brown (5YR 5/2) (damp) to grayish- orange-pink (5YR 7/2) (dry). Forms hard sloping outcrop ........................................................................ Tuffaceous arkose, yellowish-gray (5Y 8/1); conglom- eratic contains quartz and andesite pebbles. Sand- sized fraction contains pink and very light gray feldspar and minor amounts of black minerals. Clay binder is white but mostly altered to yellowish gray. Massive with occasional crossbedding. Forms, hard, rounded outcrop ......................................................... Tuffaceous arkose, light-gray (N7); lithology similar to unit 33. The basal half is medium grained to con- glomeratic. The unit is cut out at south end of the outcrop by a channel fill of the overlying unit ......... Siltstone, medium-light-gray (N6) (damp) to light- gray (N7) (dry), massive. Forms hard, hackly frag- mented sloping outcrop .............................................. Tuffaceous arkose, light-gray (N7); lithology similar to unit 33. The basal half is medium grained to con- glomeratic ................................................................... Siltstone, medium-light-gray (N6) (damp) to light- gray (N7) (dry), massive. Forms hard, hackly frag- mented sloping outcrop .............................................. 4] Thickness (f!) 2.3 3.3 1.5 3.3 1.0 11.3 2.6 4.3 3.5 3.5 1.3 2.7 4.3 42 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO Denver Formation—Continued Thicknm 35. Tuffaceous arkose, light—gray (N7); lithology similar (It) to unit 33. The basal half is massive, medium- grained to conglomeratic sandstone. The upper half is crossbedded ‘in thin beds, fine-grained sandstone. Forms hard, rounded outcrop .................................... 7.8 34. Claystone, pale-brown (5YR 5/2) (damp) to grayish- orange-pink (5YR 7/2) (dry), silty. Forms hard, rounded outcrop ......................................................... 3.7 33. Tuffaceous arkose, light-gray (N7) (damp) to very light gray (N8) (dry), massive. Principally quartz and very light gray feldspar and minor amounts of pale-pink feldspar and a black mineral. Forms hard, rounded outcrop ........................................................................ 3.8 32. Claystone, very light gray (N8) (dry) and light-gray (damp) ......................................................................... 1.9 31. Tuffaceous arkose, very light gray (N8) ........................ 1.2 30. Tuffaceous arkose, light-gray (N7), silty ....................... 2.1 29. Claystone, grayish-orange-pink (5YR 7/2) .................... 1.0 28. Tuffaceous arkose, very light gray (N8); stained moder- ate yellow (5Y 7/6) at places ........................... 3 27. Claystone, very light gray (N8)... .3 26. Siltstone, very light gray (N8) ........................................ .6 25. Tuffaceous arkose, very light gray (N8) ........................ 8 24. Conglomerate, very light gray (N8) to light-gray (N7); composed chiefly of andesite pebbles of very light gray (N8) feldspar, augite or hornblende, and bio- tite. Sand size has these minerals plus quartz. Forms hard, rounded outcrop ................................................ 4.7 23. Tuffaceous arkose, light-gray (N7) to very light gray (N8), fine- to medium-grained, even-bedded to massive. Contains very light gray and pale-pink feldspar, quartz, biotite, and augite or hornblende. Conglomeratic at places. The pebbles are mainly andesite but two 6-in. cobbles were of Precambrian orgin. One was biotite granite gneiss and the other was composed of very light gray feldspar and biotite. Forms hard, rounded outcrop .................................... 7.2 22. Conglomerate, light-olive-gray (5Y 5/2), silty, massive. Pebbles are light-gray andesite. The sand-size frac— tion contains quartz and biotite and minor amounts of very light gray feldspar and hornblende or augite. Forms hard, rounded outcrop .................................... 1.2 21. Tuffaceous arkose, dusky-yellow (5Y 6/4), silty, cross- bedded; lithologically similar to unit 20. Has 1- to 2-in-wide zones of calcium carbonate mixed with opal which crosscut bedding planes. Forms hard, rounded outcrop. Attitude N. 4° W., 8° E .................. 5.3 20. Tuffaceous arkose, light-olive-gray (5Y 5/2), silty; composition similar to unit 19 but also contains some andesitic material. Conglomeratic at places with strongly weathered light and very light gray andesitic pebbles. Forms hard, rounded outcrop ....... 2.7 19. Siltstone, olive-gray (5Y 3/2), sandy. Sand grains are biotite and very light gray feldspar with minor quartz and hornblende or augite ................................ 1.1 18. Tuffaceous arkose, very light gray (N8), medium- grained; contains quartz, very light gray feldspar, black augite or hornblende, and very minor biotite. Conglomeratic at places. Forms hard, rounded out- crop. Fossil leaf fragments are present locally. (Table 4, locality G23, unit 18.) ............................................. 5.6 17. Claystone, dark-gray, contains phenocrysts of un- identified black mineral .............................................. .5 16. Tuffaceous arkose, very light gray (N8) to light-gray (N7), medium-grained; crossbedded at places; con- tains quartz, pink feldspar, unidentified black Denver Formation—Continued 16. Tuffaceous arkose unit—Continued 15. 14. 13. 10. mineral and some very light gray feldspar and ande- sitic material at base; grades up into light-olive—gray (5Y 6/1) silty sandstone, Entire section below top of this unit has weathered into thinly covered steep slope ............................................................................ Siltstone, pale-brown (5YR 6/2), sandy ......................... Tuffaceous arkose, very light gray (N8) to light-gray (N7), mediumgrained; crossbedded at places. Con- tains quartz, pink feldspar, unidentified black min- eral, and some very light gray feldspar and andesitic material. A fault (attitude of N. 1° 12., 84° W.) dis— places the top of the unit 18 in. Forms 3 hard, rounded outcrop ......................................................... Siltstone, pale-brown (5YR 5/2), clayey; contains a few quartz sand grains and thin seams of moderate- yellow (5Y 7/6) and light-olive and brown (5Y 5/6) bentonitic-appearing clay with a greasy luster .......... Tuffaceous arkose, very light gray (N8) to light-gray (N7), medium-grained; crossbedded at places with quartz, pink feldspar, and unidentified black mineral. Contains lenses of conglomerate and clayey silt. Conglomerate pebbles are badly weathered to very light gray (N8), pale red purple (5RP 6/2), grayish red purple (5RP 4/2), pale pink (5RP 8/2), and light gray (N 7). They appear to be composed of very light gray and pale-pink feldspar with a black mineral which is either hornblende or augite. Attitude N. 40° E., 12° E. ............................................................... . Tuffaceous arkose, very light gray (N8) to light-gray (N7), medium-grained; crossbedded at places, con- taining light—gray feldspar, pink feldspar, and un- identified black mineral. Hard, rounded outcrop but breaks into 2-in. blocks. (Table 5, locality G23, unit 11.) ....................................................................... Siltstone, light-olive-gray (5Y 5/2); sandy with limon- ite staining near top. Attitude N. 5° E., 20° E. (Table 5, locality G23, unit 10.) ............................................ Covered ........................................... . Tuffaceous arkose, yellowish-gray (5Y 7/2), medium- grained; contains light-gray feldspar, andesite, and minor pink feldspar. Evenly and thinly bedded. Atti- tude N. 5° W., 29° E. .................................................. Siltstone, brownish-gray (5YR 4/1), sandy with a few noncalcareous white spots, massive. Fault N. 12° W., 84° W., near top of bed with minor displacement and l in. of gouge ....................................................... Siltstone, dusky-yellow (5Y 6/ 4), sandy, massive .......... Siltstone, light-olive-gray (5Y 6/1), massive. The base of this bed is about the base of the Tertiary part of the Denver Formation ............................................ Tuffaceous arkose, moderate-olive-brown (5Y 4/4), fine-grained, massive, silty; contains light-gray feldspar, andesite, and pink feldspar. Attitude N. 2° 12., 26° E. ................................................................. Covered ......................... Siltstone, pale-yellowish-brown (lOYR 6/2); contains a few black subhedral crystals. Attitude N. 4° W., 67° E. (Tables 4 and 5, locality G23, unit 2.) ............ Total Denver Formation ......................................... Arapahoe Formation: l. Siltstone, yellowish-gray (51/ 7/2). (Tables 4 and 5, locality G23, unit 1.) ................................................... Total Arapahoe Formation measured .................... Thickness (It) 3.2 1.1 29.6 15.0 3.3 25.0 5.3 4.6 3.0 4.9 5.0 250.0 2.0 634.0 2.0 2.0 MESOZOIC ROCKS LARAMIE, ARAPAHOE, AND DENVER BOUNDARY PROBLEMS The Laramie, Arapahoe, and Denver Formations are not readily distinguishable at many places; therefore, the boundaries between them have been shown differently by various workers. The major changes of the stratigraphic position of the boundaries and the different nomen- clature are shown in figure 13. After examining the various proposed contacts I decided that the original formational contacts which were established by Emmons, Cross, and Eldridge (1896), and which were used in reports by Van Horn (1957b, 1972) and Smith (1964), are workable at most places, and therefore, their usage is followed in the present report. 43 Dissatisfaction with the designation of Laramie, Arapa- hoe, and Denver has been expressed by several geologists who have studied these formations in the Denver area (Johnson, 1930b, p. 14; 1931, p. 368; Brown, 1943, p. 77; LeRoy, 1946, p. 101; Reichert, 1954, p. 17; 1956, p. 110). Johnson believed that the Arapahoe and Denver as orig- inally defined should be combined into one formation. Brown believed that the Laramie should be expanded to include the Arapahoe and the Cretaceous part of the Denver; the basal conglomerate of the Arapahoe was desig- nated as the Arapahoe Conglomerate Member of the Laramie Formation, and the name Denver was restricted to the beds above the Cretaceous—Tertiary boundary. LeRoy, in partial agreement with Johnson, believed that the Arapahoe and Denver should be combined into one NORTH GREEN MOUNTAIN- GREEN MOUNTAIN— TABLE SOUTH TABLE MOUNTAIN NORTH AND SOUTH TABLE MOUNTAINS MOUNTA'N EMMONS AND BROWN (1943) LEROY (19461 RE'CHERT THIS PAPER OTHERS (1896) (1954) Lava flow 2 Lava flow —— . .. “\ Lava flow DIVISlOn 50 _ AIL) _ \ \ \ ,_ \ Lava flow 2 3 @ E 170 lgolden 60 \ \ Lava flow 2 ‘° a) ember "E 380 0 Pleasant fl View 100 h m _ qumber E g . ___________ Tertiary __________ Tv1 , , », a: 280 8 E B Cretaceous D J! u: > Division T o C 580 v1 E_ _____________ 8 B W 'U B a: .. S 290 ~93 a o c S 8 260 1’ I a: 8 .93 a: m g r; g g 500 U '2 2 D. L / / : .D / / c 5 — ‘ . ——— -\ g o . \ 5 I 700 \ a. w w \ s 2 E \ 5 § 8 Upper 9 3 __ _ _ _ E 4: u, . . ‘ 400—600 3 E S g dlvmon ~ (8 450 8 500 200 E E ‘ o < 5 3’. .5 ,— ... m E . // E ‘5. E - O // / /Arapahoe O 22 100 E q, Con Iom- 0‘ {‘3 Eu " Cdnglom- Lower 50—100 / ergte S “’ S g erate __~ division i~___fi LMimbi____ 0 _*~ 0 m .2 ,2 .2 'g E ' E g E 5 Es V‘ , ta . (v .I _l .1 " ‘4’th M W M W FEET METRES 0 50 100 150 200 O 50 FIGURE 13.—Corre1ation chart of the Laramie, Arapahoe, and Denver Formations as used in the vicinity of Golden by various authors. Thicknesses, in feet, are indicated by figures on right sides of columns. TV] is oldest lava flow. 44 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO unit; he also named and defined two members in the Ter- tiary part of the combined Arapahoe-Denver. Reichert rec- ognized the Arapahoe but placed the upper boundary at the base of a basalt—andesite conglomerate bed in the Cre- taceous part of the Denver. The boundary between the Laramie and Arapahoe Formations is retained in this report as originally defined because of the persistent nature of the lithologic change and the conglomerate at the base of the Arapahoe. All the authors cited placed a contact at this stratigraphic posi- tion although Brown considered it as only a boundary between members. The subtle lithologic differences between the rocks on each side of this boundary seem to justify retaining the formations as originally defined. The conflicting views on the Arapahoe and Denver F or- mations are more difficult to resolve. Johnson wanted to combine the two formations because he believed they were defined because of a misconception as to the age of the Denver (Johnson, 1930b, p. 14). He also pointed out a similarity of contained fossils and origin of the forma- tions. In the original definition, Cross (1889, p. 122) stated, “The independence of the [Denver] formation was first es- tablished by the character of the materials in its sedi- ments.” This is certainly a valid reason for establishing a formation. LeRoy, like Johnson, also combined the Arapahoe and Denver in spite of recognizing a distinctive difference in lithology—the contact between his flint- chert and hornblende-augite phases is the contact between the Arapahoe and Denver Formations. In recognizing this significant lithologic change LeRoy (1946, p. 103) stated, ”the mineralogic break between these two phases on the west slope of Green Mountain is of considerable magni- tude and may be placed within several feet.” In spite of this, LeRoy stated (1946, p. 101 ), “From lithic evidence the Arapahoe—Denver boundary as originally placed by Cross and Eldridge is very indefinite.” LeRoy presumably con- cluded that the two formations should be combined because he saw the contact at only one place, on the west slope of Green Mountain. Although the Denver—Arapahoe contact is rarely ex- posed, there are several places where abrupt changes from normal sedimentary to volcanic mineralogy take place. (See table 5.) A break of considerable magnitude is exposed on the southwest side of Green Mountain in the SE‘ASW‘A sec. 24, T. 4 S., R. 70 W., Morrison quadrangle. This is probably the Section Ravine of Emmons, Cross, and Eld- ridge (1896, p. 160—161). Here the strata change is marked by the abrupt appearance of volcanic debris within a few feet. The abrupt appearance of volcanic debris is also seen in exposures on the west side of US. Highway 6 in the SE‘ASE‘A sec. 3, T. 4 S., R. 70 W., Morrison quadrangle, where the highway changes direction from N. 40° W. to N. 80° W. A washed sample of the Arapahoe from just west of the curve contained about 55 percent weathered pink feld- spar, 40 percent subangular to round quartz, 5 percent sub- angular to subround white feldspar, and a trace of magne- tite and mica (table 5, sample L8). The next exposure to the southeast, part way up the hill on the west side of the road, is of Denver‘ Formation. A washed sample from this exposure contained 60 percent of white angular to sub- angular feldspar with many crystal or cleavage faces; 5 percent of black and light-green, angular to euhedral crys- tals of hornblende or augite; 1 percent of magnetite in very small irregular black blebs; and a trace of reddish-brown biotite crystals (table 5, sample L35). Other beds on the east side of the highway are similar to the last described bed. Two other examples of abrupt appearance of volcanic debris have been described in the sections of this report on the Arapahoe and the Denver Formations. Although the upper part of the Arapahoe and the lower part of the Denver are exposed together at very few places, the dif- ference in lithology is enough to establish the validity of separate formational status for both of them. The contact shown on the map (Van Horn, 1972) may not be exact at all places but is well within the accuracy of generally ac- cepted geologic map standards. The problem of the Denver—Arapahoe contacts that Brown (1943), LeRoy (1946), and Reichert (1954) placed within the Denver Formation as originally described is somewhat more complex. (See fig. 14.) It should first be pointed out that the lithology of beds above and below these proposed contacts is the same. The beds range from claystone to conglomerate and are composed dominantly of volcanic debris. Neither Brown, Reichert, nor LeRoy questioned this. Brown established his boundary (the Cre- taceous-Tertiary boundary) first, and in this area it was based on a single, well-described exposure on South Table Mountain. LeRoy, though not accepting Brown’s boun- dary as a formational boundary, did accept it as the lower boundary for a member of the Denver that could be recog- nized only on North and South Table Mountains (LeRoy, 1946, p. 103, 104). By thus defining the Pleasant View Member of the Denver, LeRoy virtually recognized Brown’s Cretaceous-Tertiary boundary as a mappable contact around North and South Table Mountains. Reichert did not accept Brown’s boundary as a mappable contact but did relate his own Arapahoe—Denver contact to Brown’s boundary by using the base of the highest dino- saur-bearing bed (a conglomeratic sandstone) of Brown’s South Table Mountain exposure for the Arapa- hoe—Denver contact. Reichert stated that this bed is the lowest, thickest, and most prominent basalt-andesite con- glomerate in the area and that it can be traced from Golden to Colorado Springs (Reichert, 1954, p. 21). Scott (1962 , 1963b) did not mention this bed in either the Littleton or Kassler quadrangle although he did mention that a tongue of Denver in the Littleton quadrangle has andesitic sand- stone. During a field conference in 1953, Brown pointed out two places on the north side of South Table Mountain where he had found Paleocene fossils. During this confer- ence we located the Cretaceous—Tertiary boundary on the FIGURE 14,—Correlation chart of the Denver Formation as used in the vicinity of Golden by various authors. (1) West side North Table Mountain (G23), NE‘A sec. 21, T. 3 S., R. 70 W.; (2) north- west corner South Table Mountain, 811% sec. 27, T. 3 S., R. 70 MESOZOIC ROCKS FEET METRES 0-H O 50— 100— 150v - — 50 200 —J W.; (3) northeast corner South Table Mountain, NE‘A sec. 26, T. south side of North Table Mountain. Using these data and some later collections, I mapped the Cretaceous—Tertiary boundary at the base of a thick sequence of light-colored beds in accordance with Brown’s criteria (1943, p. 74). The three boundaries on the Table Mountains estab- 45 EXPLANATION 63 Lava Conglomerate Tuffaceous arkose Siltstone Claystone Covered, — probable lithology shown on right Fossil leaves Fossil vertebrate Number refers to unit no. of measured section G23 3 S.,‘ R. 70 W.; (4) southeast corner South Table Mountain, NW‘A sec. 31, T. 3 3., R. 70 W. (Morrison quadrangle). The sections are from Emmons, Cross, and Eldridge (1896), Brown (1943), LeRoy (1946), and Reichert (1954). lished by LeRoy, by Reichert, and by Brown and me were not in accordance. At the southeast corner of South Table Mountain, where the relations are clear, the contacts es- tablished by LeRoy and Brown coincide and are about 70 feet above the contact established by Reichert (fig. 14, sec. 46 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO 4—, y "I" III" III" "I". e a , ’ ~ FIGURE l5.-—-Latite lava flows (Tv2 and Tv3) and the underlying Ter- tiary and Cretaceous Denver Formation exposed at Castle Rock on the east side Of Golden. (See fig. 14.) The Cretaceous—Tertiary boundary (B) is at the base of the light-colored beds near the bottom of the slope. The Denver—Arapahoe boundary established by Reichert (R) (1954) is farther up the slope at the top of the light- colored beds. Outcrops of andesitic conglomerate (A) occur at several 4). On the northeast corner of South Table Mountain the Cretaceous—Tertiary boundary established by Brown and me is about 150 feet lower than the presumably same boun- dary established by LeRoy (fig. 14, sec. 3). Reichert’s contact is about midway between these two boundaries; it has moved from 70 feet below the Cretaceous—Tertiary boundary to 70 feet above it in a diStance of about 2 miles. LeRoy’s boundary, which supposedly coincides with the Cretaceous—Tertiary boundary, has moved to a position 150 feet above it. ‘ ‘ , On the northwest side of South Table Mountain, below :3- places. The one at the upper right is truncated by flow 2 a mile south of the pictured area. The vesicular zone between the two lava flows is near the base of the cliff. The light~colored vertical scar on Castle Rock (F) marks the site of the 1958 rockfall, debris of which can be seen on the slope to the right of the scar. Photograph by H. E. Malde, US. Geological Survey. Castle Rock, the position of LeRoy’s boundary could not be established. Reichert, however, showed his boundary to be at 5,850 feet altitude which is at the base of a dark- colored sequence of beds and overlies a light-colored se- qdence. (See figs. 14 and 15, sec. 2.) I believe the Creta- ceous—Tertiaryrboundary to be at the base of this light- colored sequence at 5,820 feet altitude. Thus Reichert’s boundary is here 30 feet above the Cretaceous-Tertiary boundary. A similar relation prevails on North Table Mountain where Brown and I established the Cretaceous—Tertiary TERTIARY IGNEOUS ROCKS boundary near the southwest corner at about 150 feet below the base of the lowest lava flow. The position of LeRoy’s boundary is not clear on North Table Mountain but he stated that sandstones in the lower third of his Pleasant View Member are contemporaneous with the lowest lava flow. Reichert was more definite and placed his boundary at the base of a conglomerate 10 feet below the base of the lowest lava flow, on the west side of North Table Moun- tain (fig. 14, section 1). Reichert’s contact here, at about the same position as LeRoy’s boundary, is approximately 140 feet above the Cretaceous—Tertiary boundary. After recognizing these relationships I walked com- pletely around both North and South Table Mountains. The results, as might be expected, were far from satis- factory because of the thick colluvial deposits and many landslides. Neither LeRoy’s, Brown’s, nor Reichert’s units could be traced. The conglomerate beds are not persistent and are present at many different positions. Brown’s contact is not satisfactory because there are several differ- ent horizons where light-colored beds overlie dark-colored beds. Without distinctive fossils I could not pick the proper horizon. At this point I decided that the boun- daries described by Emmons, Cross, and Eldridge (1896), which are based on distinctive mineralogic or composi- tional differences, were the most practical formation boundaries. Although the Cretaceous—Tertiary boundary is also shown, it is essentially a paleontologic boundary and is only approximately located. TABLE 6.—Chemieal analyses (in percent) of monzonite, 47 TERTIARY IGNEOUS ROCKS Lava flows interbedded with the Tertiary part of the Denver Formation, and irregular intrusive bodies and dikes that intrude the Pierre Shale, constitute the Tertiary igneous rocks in the Golden quadrangle. Sills of similar material intrude the Fort Hays Limestone Member of the Niobrara Formation in the adjoining Ralston Buttes quadrangle (Van Horn, in Sheridan and others, 1967). These rocks, formerly classed as diorite and basalt, were determined to be mafic monzonite and its extrusive equiv- alent, mafic latite, by W. T. Pecora of the US. Geological Survey (written commun., May 11, 1955). Although Wald- schmidt (1939) and Cross (in Emmons and others, 1896) both recognized that potassic feldspar is present in these rocks, they did not realize that it exceeded the amount of plagioclase feldspar. The large amount of potassic feld- spar is shown by the high potassium oxide content of the rock (table 6) and by the thin sections analyzed (table 7). Both intrusive and extrusive rocks are composed prin- cipally of potassic feldspar (probably sanidine), plagio- clase (andesine-labradorite), and augite. Olivine, mag- netite, biotite, and apatite are present in smaller amounts. (See table 7.) The potassic feldspar occurs as large, poorly defined, colorless grains containing inclusions of plagio- clase and other minerals. Cross indicated that potassic feldspar also occurs in the groundmass (Emmons and others, 1896, p. 303). The plagioclase forms sharply latite, and tuffaceous rocks of, and interbedded with, the Denver Formation [Samples were analyzed by P. L. D. Elmore, K. E., White, and S. D. Botts by methods similar to those described by Shapiro and Brannock (1956)] Locality No (fig. 3) ...... 031 (330 032 (:22 G7 062 G65 G66 G60 Lab. No ....................... 149006 149007 149008 149010 149012 149013 149014 149015 149016 3102 .................... 49,2 53.2 54.1 54.0 52.4 60.3 54.2 56.1 50.6 Ale:;.. 15 16.4 16.8 17 15.2 17.6 15.2 18.8 20.6 Fe203 .. 6.9 5.1 4.6 4.1 4.4 4.4 8.6 5.2 6.8 F60 ..... 3.5 3.8 3.8 4.2 4.5 .31 2 .68 1.8 MgO... 6.2 3.6 3.2 3.2 6.8 .75 2.3 1.1 1.2 CaO .................... 6.5 6.7 6.4 6.2 7 1 5.4 5.2 2.1 NazO ............. 2.3 3.2 3.2 3.5 2.7 1.7 2.3 3.1 3.1 K230 ..... 3.6 4.6 4.7 4.7 3.3 9.1 4 2 4.2 T102. 1 .86 .82 .82 .82 1 .98 .54 .96 P205.. .78 .58 .54 .56 .45 .06 .57 .24 .32 MnO .28 .16 .16 .16 .15 .02 .12 .11 .08 H20. 4.7 2 1.6 1,3 2.2 3.7 3.9 6.5 9 C02 ..................... .08 .20 .08 .05 .08 .08 <.05 <.05 .06 Sum ......... 100 100 100 100 100 100 100 100 101 SAMPLE DESCRIPTION AND LOCALITY Golden quadrangle, T. 3 S., R. 70 W. Latite from middle of lowest flow in the SW‘ANW'A sec. 14. Latite from middle of intermediate flow in the SW‘ASW'A sec. 14. Latite from lower part of highest flow in the SE'ANE'A sec. 21. Monzonite from south of Ralston dike in the NW‘ASEK sec. 9. Monzonite from Ralston dike in the NE‘ANE‘A sec. 5. G31. G30. 032. G22. G7 . Morrison quadrangle, SW‘ANW‘A sec. 31, T. 3 S., R. 69 W. Tuffaceous claystone from Cretaceous part of Denver Formation. Tuffaceous sandstone from Cretaceous part of Denver Formation. Tuffaceous sandstone from Tertiary part of Denver Formation. Tuffaceous sandstone from Tertiary part of Denver Formation. 062. 065 . G66. G68. 48 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO TABLE 7.—Modal analyses, in volume percent, of Teitiary intrusive and extrusive rocks in the Golden quadrangle and vicinity [The Tvl samples are from North Table Mounwin and the Tv2 and Tv3 samples are from North and South Table Mountains. The Leyden sill locality is in the NW‘ASW‘A sec. 29, T. 2 8., R. 70 W., Ralston Buttes quadrangle. The Green Mountain locality is in the SW‘ANE'A sec. 12, T. 4 S., R. 70 W., Morrison quadrangle. and according to Smith (1964) is equivalent to TVS. Analyst: Barrie H. Bieler. About 2,000 points counted per thin section] Location Number Potassic Plagioclase Augite Olivine Biotite Magnetite Apatite of samples feldspar feldspar Leyden sill.... 1 46.9 32.9 11.8 1.9 2.4 4.0 0.1 Ralston dike ........... 6 35.8 28.1 23 4.1 1.9 6.0 1.2 Tvl .. 5 34.8 30.7 22.5 6.8 .l 4.8 .3 TV2 .. 4 37.4 32.5 18.1 3 .2 7.4 1.2 TV3 ............... 6 38.2 36.4 15.8 2.7 .5 5.2 .8 Green Mountain. 1 38.7 33.1 19.7 3.4 .4 4.5 .2 defined colorless grains both as phenocrysts and in the inicrocrystalline groundmass. The augite forms large sharply defined to ragged, pale-brown to pale-green phenocrysts that contain inclusions of magnetite, apatite, biotite, and plagioclase. The augite also occurs as small somewhat irregular grains. Olivine is present as small or large, rather irregular, yellowish-brown to dark-green phenocrysts, and generally is greatly altered. Magnetite forms black irregularly rounded to nearly square small to medium size grains. Apatite is in short stubby crystals or as rounded to nearly square, small, gray grains. The biotite phenocrysts are strongly pleochroic from light yellowish brown to dark orange and are generally small and ragged. Much of the rock shows signs of slight alteration but not enough to significantly affect its physical properties. According to Schlocker (1947), alteration products in these rocks are composed of clay minerals of the mom. morillonite—nontronite group. MONZONITE The monzonite intrusives are west of, or associated with, the Golden fault. In the Golden quadrangle the mon- zonite intrudes only the Pierre Shale and is not known to intrude any younger rock in the vicinity. Ralston dike, the largest of the intrusives, forms the large hill southeast of Ralston Reservoir. Two thin intersecting dikes northwest of Ralston dike are visible on the northwest side of Ralston Reservoir only when the water is at a low stage. They can be traced only a short distance and neither seem to be connected to Ralston dike at the surface. Southeast of Ralston dike are 10 irregularly shaped intrusive bodies that range in shape from thin tabular to roughly circular. In hand specimen the monzonite intrusives are very dark gray rocks composed of short stubby phenocrysts in an aphanitic groundmass. The phenocrysts, of glassy feld- spar and very dark green to almost black augite, generally are only a few millimetres long. At one place on Ralston dike, however, a single plagioclase crystal 3 cm long and an augite crystal 2 cm long were seen. The groundmass weathers to a medium gray or moderate brown from which the almost black augite grains stand out in sharp contrast. Rounded knobs, as much as 1 foot in diameter, due to spheroidal weathering are present at several places. A few small xenoliths were found enclosed by the mon- zonite. On Ralston dike one was of metamorphic rock and another was of sandstone. A quartzite xenolith was found near the center of the largest intrusive southeast of Ralston dike. The metamorphic rock and the quartzite un- doubtedly came from the Precambrian basement but the sandstone could not be identified as to source. None of the xenoliths have been appreciably metamorphosed by the monzonite. Ralston dike, about 7,500 feet long and 2,000 feet wide, is canoe shaped, with a hollowed-out center occupied by Upper Long Lake. The west side is about 400 feet above the lake but the east side is only 20—50 feet above the lake. The lake occupies an undrained basin that has been increased in volume by two small artificial dams on the east side. Eldridge (in Emmons and others, 1896, p. 282) reported the presence of outcrops of Pierre Shale on the west side of the basin and concluded that the entire basin area was of shale. Whitman Cross (unpub. notes) also examined the basin; he did not find any shale, but con- cluded that shale probably underlies the basin because of the depression. Later, when the lake was at a low level, the area was investigated by Waldschmidt (1939, p. 11), who was not able to find any shale. No evidence of shale was found during my investigation of the basin. From out- crops now available the basin walls appear to be princi- pally brown colluvium containing abundant monzonite fragments. In the lower part of the basin the colluvium is overlain by 1—2 feet of pale-brown loess. Outcrops of mon- zonite extend down to the shore of Upper Long Lake at a few places. The absence of gravel around the margin of the lake indicates that the part of the basin lower than the adjoining Rocky Flats Alluvium has been formed since Nebraskan time; the part lower than the base of the dams, the original undrained depression, must have been exca- vated by wind erosion—the depth of the erosion is not known. TERTIARY IGNEOUS ROCKS 49 The joint system developed in Ralston dike was not studied; there are, however, three sets of conspicuous joints. The most persistent set strikes nearly east and dips a few degrees to either side of vertical. A second strikes ap- proximately north and dips 20°—70° E. A third set, which is generally less well developed than the others, strikes from a few degrees west of north to northeast and dips 18°—60° W. A tabular platy structure is developed at several places; at the north end of the dike the plates are parallel to the first joint set described, but in the abandoned quarry on the northwest side they are parallel to the third joint set. At two places a vertical pseudocolumnar structure is faintly developed. No obvious evidence of multiple intrusion was seen at Ralston dike. The rock, however, is not entirely uniform. Several samples taken from the water tunnels driven into the dike and from the quarries show minor differences in the size of phenocrysts and the quantity of some of the minerals. These differences could be due to multiple intru- sions of slightly different character or to differentiation within a single intrusion. The contact of Ralston dike with the enclosing Pierre Shale is discordant and ranges from gently undulatory to sharply irregular. The contact is clearly exposed in the inlet and outlet tunnels of Upper Long Lake and at a few places in surface outcrops. On the west side of Ralston dike the contact dips 50°-60° E., about 10° gentler than the shale. In the abandoned quarry on the northwest side the contact is very irregular with large jagged embayments of monzonite into the Pierre. Only 500 feet north of the quarry, the contact is very smooth and regular. Here, how- ever, the Pierre is brecciated for 6 inches and the mon: zonite for 1 foot on either side of the contact. The shale within 2 inches of the contact appears baked and is ab- normally hard, brittle, and fractured for several feet to the west. Both the brecciated and normal monzonite are slick- ensided. In the tunnel through the west side of the dike the contact is gently undulating; it strikes N. 22° E. and dips 61° E., whereas the Pierre strikes N. 3° W. and dips 69° E. Here, a strongly developed east-striking vertical joint system in the monzonite is also present, though less strongly developed, in the shale. Several small faults with thin gouge zones are present in the shale. On the east side of Ralston dike the contact between the monzonite and Pierre Shale dips about 45° W., whereas the shale dips from vertical to 70° W. (overturned). At the abandoned quarry on the northeast side, the contact is gently rolling and the shale is bleached and hardened for many feet east of the contact. Both the monzonite and shale are cut by slickensided faults of small displacement. The outlet tunnel under Upper Long Lake contains an in- teresting exposure of the contact. (See fig. 16.) Here the contact, though very irregular, dips about 45° W. The monzonite apparently follows fractures in the shale, and the contact consists of many short, nearly straight seg- ments. Some of these segments intersect at angles ap- proaching 90°, such as the one at the top of figure 16 above the point of the pick. Thin fingers of shale extend into the monzonite shown in the upper right of the figure, and a small finger of monzonite shown just above the pick is entirely surrounded by shale. The monzonite is strongly brecciated for 6 feet west of the contact. The shale, though strongly jointed, is not obviously brecciated. Slickensided surfaces with many different orientations are abundant in both shale and monzonite. The baking of the shale on both sides of Ralston dike, the transection of shale stratification by the monzonite, the monzonite embayments, and interfingering into the shale show the intrusive nature of Ralston dike. The commonly shared slickensides and joint pattern record tectonic move- ment of both shale and monzonite at some time after solidification of the monzonite. Similar features, though not so well displayed, indicate that the irregular-shaped monzonite intrusives southeast of Ralston dike under- went a similar sequence of events. The monzonite bodies that intrude the Pierre Shale southeast of Ralston dike are texturally and mineralogi- cally similar to Ralston dike. The western three irregular- shaped bodies are roughly alined along a major fault. A short thin tabular dike is present just west of the southern- most body. One large and five small intrusives are east of the fault. A small body of Pierre Shale (not shown in Van Horn, 1972) is apparently enclosed by monzonite near the southwest corner of the largest of these intrusives. The monzonite of this large intrusive is coarser grained than most of the other intrusives. The large number and close grouping of these intrusives indicate to me the possibility that they are cupolas and all merge into a single large mass within a few hundred feet of the ground surface. (See Van Horn, 1972, cross section B—B’.) The monzonite dikes all have similar composition and probably are of nearly the same age. Some of the intrusives seem to be related to a late stage of movement on the Golden fault. Displaced fossil zones in the Pierre Shale indicate that a major fault strikes into the north end of Ralston dike where a strongly brecciated zone of Pierre is exposed in a roadcut just north of the dike and just east of the Ralston Reservoir dam. A similar major fault is indicated by displaced fossil zones southeast of Ralston dike along the line of smaller intrusives. These faults line up with the axis of the shallow basin occupied by Upper Long Lake. If a major displacement had affected Ralston dike it would have resulted in obvious offset of the east and west parts because of the converging dips of the contacts. On the other hand, minor tectonic movement of the dike is recorded by the slickensided surfaces and the joints that cross the monzonite contact and by the zone of weakness indicated by the presence of the Upper Long 50 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO FIGURE 16.—Contact between Pierre Shale (P) and monzonite (M) in by L. M. Gard, U.S. Lake basin. Thus, Ralston dike seems to have been in- truded during the late stages of movement of the Golden fault. LATITE North and east of Golden, latitic lava flows are inter- bedded with the rocks of the Tertiary part of the Denver Formation. These lavas are divisible into three separate flows, which Waldschmidt (1939, p. 16) called basalt flows 1 (oldest), 2, and 3 (youngest). All three flows are similar in composition to the Tertiary intrusives just described and are classified as latite in this report. The vesicular top of the two oldest flows are still preserved at most places, but the top of the youngest flow is everywhere eroded. Flow 1, although similar to the other flows, contains no- ticeably larger and more abundant phenocrysts of augite the water outlet tunnel on the east side of Ralston dike. Photograph Geological Survey. than the others. It is texturally similar to the coarse mon- zonite of the largest intrusive southeast of Ralston dike. The lava was not extruded as a broad blanket but was confined to several tongues, which probably occupied shallow depressions on the ground surface. These tongues are about 60 feet thick in their central parts but thin laterally. The lower 3—5 feet of the large flow on the west side of North Table Mountain consists of an angular, vesicular clinkery mass typical of aa. The flow apparently caused little disturbance to the underlying conglomerate bed. The massive lava above the base shows no flow struc- ture except for a nearly vertical tubular opening, prob- ably caused by rising gas. The vesicular top, which is as much as 20 feet thick in better exposures, consists of many small to large circular openings, some of which are elon- gate upward. Many of these openings are filled with TERTIARY IGNEOUS ROCKS 51 FIGURE l7.——Well-developed columnar structure in latite of lava flows 2 and 3 on the north side of South Table Mountain. The vesicular zone between the flows is near the base of the cliffs. Debris from fallen columns litters the colluvial and landslide slopes of the base of the cliff. Two active landslides are formed on the flanks of older weathered zeolites. These flows are underlain by as much as 200 feet of the Paleocene part of the Denver Formation, and are overlain by 120 feet (west) to 200 feet (east) of the Paleocene part of the Denver, which is in turn overlain by flow 2. Flow 2 apparently thickens eastward and northward. It is about 45 feet thick on South Table Mountain but on North Table Mountain it is 70 feet thick at the southwest corner, 100 feet thick at the southeast corner, and 125 feet thick at the northeast corner. The upper part has been removed by erosion on the south part of South Table Mountain. Hand—level elevations of 10 points at the base of slides that reach to the base of the lava. The base of Castle Rock and part of Golden are visible at the extreme upper right of the picture. Photograph by R. B. Colton and J. H. Hartshorn, US. Geological Survey. the flow show that the northern part slopes southeast at about 150 feet per mile, but the slope is much less near the east end of South Table Mountain (fig. 19). Part of this slope is probably due to uplift of the western part. The basal few feet of flow 2 may be either massive and dense or moderately vesicular latite. Above this the flow is hard dense latite. At most places a well-developed colum- nar structure is visible in the latite. (See fig. 17.) The columns are 3—20 feet across, but most are about 12 feet. Locally a poorly developed horizontal sheeting intersects the columnar structure and causes the latite to break into thin tabular plates. At many places the columns are inter- 52 sected by randomly oriented, moderately dipping joints. Where these joints dip toward the face of clifflike out- crops they form planes of weakness leading to rockfalls. The top 20—40 feet of flow 2 is vesicular, some of the vesi- cles beng several feet long. At many places these vesicles are filled with zeolites, most of which are weathered. Many large and beautiful crystal clusters of fresh zeolites have been collected, in years past, from this zone in several now- abandoned quarries on both North and South Table Mountains. The zeolites have been well described by several authors (Cross and Hillebrand, 1882, p. 452, 458; Emmons and others, 1896, p. 292—298; Waldschmidt, 1939, p. 31—38). Waldschmidt included an excellent summary of the existing literature on zeolites in this area. The Denver Formation between flows 1 and 2 thickens eastward on North Table ountain. On South Table Mountain from Castle Roc southeastward, beds of the Denver underlying flow 2 are cut out. The prominent outcrop below the latite cliff just visible at the right side of figure 15 is a Denver Formation conglomerate containing augite crystals. The conglomerate is 14 feet below the latite and was traced southeastward into the Morrison quad- rangle, where it converges with the latite. At a few places as much as 1 foot of the Denver Formation underlying flow 2 has a black, burned appearance. This is presumably due to GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO combustion of vegetation on the ground surface that was overwhelmed by the hot lava flow. The upper vesicular zone is the little-eroded original top of flow 2. In most exposures flow 3 lies directly on the vesi- cular top of flow 2. A single outcrop, however, on the quarry road on the west side of North Table Mountain (measured section G23) gives evidence of deposition of sediments between flows 2 and 3 (fig. 18). At this locality a stream eroded a steep-walled channel about 50 feet into flow 2. Sediments deposited in this channel include con- glomerates, sandstone, and possibly volcanic mudflows. The earliest sediment (unit 76, fig. 18) may have been a vol- canic mudflow—possibly hot. It consists of a black baked- appearing pebbly claystone 2—8 inches thick that follows the convolutions of the underlying latite surface. The peb- bly claystone is overlain by about 8 feet of a stream-depo- sited bouldery conglomerate (unit 77) containing many loose augite crystals. The conglomerate is overlain by a probable volcanic mudflow (unit 78) about 12 feet thick, composed of sparse andesitic pebbles and cobbles in a silty claystone matrix. At places the mudflow has been altered to shades of yellow and red by later steam explosions or hot-water solutions. This mudflow rises high on the north side of the channel where it abuts against the smooth surface of the eroded latite of flow 2. Near the middle of the SOUTH FEET METRES O , O 10 20 3O 10 4O EXPLANATION Red or black baked or burned area Sand SSSSS __ $22"; #23:} Rounded pebbles Clay . °- ° » -. '° to boulders Silt «u u. . r v‘ u .4. v v-" .. a1} - u - uh . s u'A. ‘ 4.. Angular pebbles to boulders in vesicular frothy-looking lava O 10 20 30 40 FEET O 10 METRES Lava-flow Slope wash Contact — Dashed where approximately located FIGURE l8.——Section exposed along the road to the quarry at the west side of North Table Mountain, showing sedimentary rocks between the two youngest lava flows (Tv3 and Tv2). The numbered units are keyed to the numbers in measured section G23 of the Denver Formation. QUATERNARY DEPOSITS 53 channel a small patch of tuffaceous arkose (unit 79) over- lies the mudflow. Overlying and to the south of the arkose is a breccia of pebble- to boulder-sized angular pieces of latite and Denver Formation in a matrix of vesicular vol- canic frothy-looking rock (unit 80). This frothy-looking rock is exposed for about 22 feet vertically and for about 50 feet west of the tuffaceous arkose where it appears to contain a large mass of Denver Formation (unit 81). The frothy-looking rock is present above and south of unit 81 where it underlies a conglomeratic sandy siltstone (unit 82), which at the south end of the exposure is overlain by massive latite of flow 3 (unit 83). The frothy deposit is probably the result of a leading tongue of flow 3 moving into the stream channel and encountering water-satu- rated sediments. The water vaporized suddenly, causing the molten latite to practically explode and also bringing about the oxidation and burned appearance of nearby sediments. After the water had all vaporized, the massive latite of flow 3 was able to fill and overflow the old channel in a normal manner. Flow 3 is the thickest of the latite flows. The top has everywhere been eroded off, so the total thickness is not known. On the west side of North Table Mountain, how- ever, 172 feet of the flow still remains. The remaining thickness at other places on North Table Mountain is con- siderably less and is generally 50—90 feet. On South Table Mountain flow 3 is 50—60 feet thick on the north side, but has been removed by erosion on the south side. It is a hard dense latite similar to the other Tertiary igneous rocks. No vesicular zone is now present in the upper part. The well- developed columnar structure is similar to that described for flow 2 and, where joints dip toward vertical faces, flow 3 is also subject to rockfalls. The similarity of the Tertiary extrusive and intrusive igneous rocks indicates that all were emplaced at about the same time. If it were otherwise, magmatic differentiation in the source chamber would probably have produced dif- ferent kinds of rock. Some authors have indicated that the series of conical hills near the west side of North Table Mountain represent the original vent from which the lava flowed. The abundant tabular plates at this place are prob- ably the result of flow structure, but they also can be seen on higher parts of the flow at the southwest and the north- east corners of North Table Mountain. Dikelike features at the conical hills appear to be case-hardened joints rather than dikes. The similarity in texture of the intrusive and extrusive rock probably indicates that the intrusive rock was relatively close to the then-existing surface. This is further borne out by the inverted cone shape of Ralston dike which indicates it may have been expanding as it approached the earth’s surface. In addition, the projected contours of the base of flow2 pass about 500 feet above the high part of Ralston dike. (See fig. 19.) The trend of these contours of the base of flow 2 pass about 500 feet above the high part of Ralston dike. (See fig. 19.) The trend of these contours is at 90° to a line drawn from the center of Ralston dike to the east part of North Table Mountain. Thus, the source of the lava flows was probably one or more of the monzonite intrusives northwest of North Table Mountain. QUATERNARY DEPOSITS The Quaternary deposits form the most important known economic resource in the area and are the most widespread material of concern to the construction industry. Deposits of Quaternary age in the Golden quad- rangle include alluvium, colluvium, loess, transported mantle, artificial fill, and landslides. The age assigned to the deposits has a fivefold basis: topographic position, contained volcanic ash, relict soils, stratigraphic posi- tion, and contained fossils. The topographic position of the various terrace deposits is the principal means of iden- tifying relative ages of the terraces; within any one valley the higher terraces are the oldest. Correlation between valleys and with glaciations was possible because a vol- canic ash similar to and considered to be of equivalent age to the Pearlette Ash Member of the Sappa Formation (Condra and others, 1950, p. 22) is incorporated in the terrace deposits in the valleys of Clear and Ralston Creeks. (See fig. 20.) Degree of development of relict soils was used extensively to correlate the Louviers Alluvium and younger terrace deposits between valleys but was used very cautiously for the older deposits. The use of soils as strati- graphic markers is more completely explained in the section on soils on page 93. The superposition of depo- sits was used at a few places for determining relative age of adjoining deposits. Fossil vertebrates and invertebrates at a very few places served to corroborate in a general way the correlation of deposits with glacial stages arrived at by other means. Other, more intangible, criteria which were useful for distinguishing the younger from older deposits were color, grain size, and degree of coherence. Most of the locality numbers mentioned in the text are plotted on the preliminary surficial geologic map of Van Horn (1968) and all are shown in figure 3. Deposits less than 3 feet thick are not shown on the geologic map. The Unified Soils Classification2 (US. Bureau of Reclamation, 1960, p. 379—400) is used at several places in the following text to describe the various surficial depo- sits. This classification has value in predicting the physical properties and some possible construction uses of surficial deposits (Van Horn, 1968b). 2The letter symbols of the Unified Soil Classification indicate the kind of material contained in the class. The first letter indicates the grain size and inorganic or organic character of the material: G is gravel, Sis sand, M is silt (nonplastic fines), C is clay (plastic fines), O is organic silt or clay, and Pt is peat and other highly organic material. The second letter indicates some other property of the material: W is well graded (engineering sense), P is poorly graded (engineering sense), M is silty, C is clayey, L is low liquid limit, and H is high liquid limit. Soils that fall on or near the boundary between two classes are designated by the combined and hyphenated symbols of the two classes, such as CL—ML. 54 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO 105°15’ ' ~ 39'52‘30' , A » ~ - , [14230, R 70 w. 10' R. 69 w. “ EXPLANATION 105°07'30” 05317 CONTROL POINT — Showing altitude, in feet ———- CONTOURS DRAWN ON BASE OF MIDDLE LA- —6500— TITE FLOW (Tv2) — Dashed where projected. Contour interval 50 feet 4730' L? 1| % 1? MILES I l O | 1 2 3 KILOMETRES FIGURE 19.—Coumours drawn on the base of the middle lame lava flow (Tv2), showing its relation to Ralston dike. Base from US. Geological Survey Golden 7%-minute quadrangle (shaded relief), scale 124,000, 1965. QUATERNARY DEPOSITS 55 ALLUVIUM The sand and gravel of the alluvial deposits have certain similarities that are common to all such deposits and thus need not be repeated. Alluvial deposits are lenticular in cross section and elongate in longitudinal section. Each deposit is thickest near the center of the valley and thins toward the valley sides. In the older deposits, the central part (near the center line of the present valley) has been removed by erosion to leave terraces at various heights above the modern streams. The deposits are generally thin at the mountain front but thicken rapidly as they pass into the foothills; they seem to maintain a roughly consistent thickness downstream, but some, particularly those along the smaller streams, become slightly thinner downstream. The sand and gravel in the alluvial deposits are generally coarser near the mountain front, where boul- ders commonly occur, but become finer eastward. At a few places, however, large boulders are present several miles east of the mountains. The larger streams generally contain the coarsest material; the youngest deposits—those of Pinedale and Holocene age—are generally finer grained than older deposits at similar distances from the mountain front. The lithology of the gravel in the stream deposits reflects the lithologic types of bedrock found upstream from the deposit. The gravel in all stream deposits is almost entirely derived from the Precambrian rocks in the mountain area, although the Tertiary monzonite and latite make up a small part of deposits that are down- stream from the Table Mountains and the Ralston dike. Pebble counts of flood-plain deposits west of the moun- tain front (see table 8) show that deposits of several streams are readily distinguishable. Clear Creek and Mount Vernon Creek (south of the quadrangle) both contain many recognizable and distinctive pebbles and cobbles of very light granitic rock not found in any of the other streams. Gravel from Coal Creek (north of the quad- rangle), which contributed the deposit on Rocky Flats, contains a distinctively high proportion of quartzite from Coal Creek. The term “quartzite from Coal Creek” here refers to distinctive bluish-gray quartzite pebbles and cobbles derived from Precambrian rocks named the Coal Creek Quartzite by Boos and Boos (1934, p. 306; 1957, p. 2612) and referred to as the quartzite-schist sequence (a localized lithologic facies of the Idaho Springs Forma- tion) by Wells, Sheridan, and Albee (1964, p. 02). This VAN HORN,( IN VAN AGE HUNT (1954) MALDE (1955) SCOTT(1962, 1963a) Sgiafiégéhia’wf 241093” THIS PAPER Protohistoric Artificial fill PostPine Artificial fill . ‘ VL— and historic Post-Piney Creek Fog-Pm” Creek P°“’P'"e}’ Creek Creek Post-PinevCreek alluvium alluvium alluVIum alluvuum alluvium alluvium g a, w w W-M > w w w T/ a I: ,_ 8 Piney Creek Piney Creek Piney Creek T"; Piney Creek Piney Piney Creek E To: Alluvium Alluvium Alluvium 8’ .3 Alluvium Afire§k Alluvium E I m i UVIUm ‘o S Alluvium and cobble E M-S 3’ Pre Piney Creek Pre Kiney W >9 E . . P . gravel on rock- E .2 3 Eolian sand alluvium Creek '§£§§¥5%§5k % 3 cut benches 3 8 g __ colluvium recognized 5, 3 L; M I Alluvium in E E M M g Pre-Pinev M W W 5 g 8 8 Gravel lower part of 8 g Gravel Alluvial Broadway Creek Broadway Broadway Broadway : -‘ E fill Lysngfig: “4 r... fl” Al'W'Um aIIuvIum Alluvium Alluvium Alluvium 4‘; 3 7 l | / ’ M = E 52 All ‘5' I I : b M-S Younger loess LoNtll—vsiers gainers Lngif/iers g m _, uv-a grave Co ble gravel Louviers Alluvium ' ' ' 0 “1.6L ."1 AlluVIum Alluwum Alluvtum I s l l 3 s 'a s s ' s l ' I§ c I in; Part of gravelly 3 Id 1 l o m E E, I l8; phase of undif— o g 0 er Ioess I (J. i" g g 5 g “L 1% % ferentlated up- 5 U Slocum Slocum I | “I‘ 2 _ _ . . u. , .. E E -= 1 lg 3 land deposns in .5 E Slocum Alluvium Alluvuum Alluvium l I l E '12 in — l l: a: low topographic g g ' l t l E 3 .1. if, position ._.- 13 l “I'll. I8 . . . '7. a: l .. ":5 c Gravel capping hill- I "I S E E S S S "I l : I; g 5 E tops west of South 4 | I '7; g Verdos Verdos | i l lg 5 Q Platte River valley .J. l l Terrace gravel Lu ; Verdos Alluvium Alluvium Alluvium I "I". +5 >_ A l cx- I L: I. I l- l | (I '8 A 5| : | 1 I: g l l | S l: S S S I | l -' §_ § “I i : Gravel on Rocky | : R k Fl 1 R k Fl : “I ‘1' Fl §°é Ii. :I 27.2.15 2:3...1“ w: :i < Z l I l of upland gravel | I | I | l a. I l | | , | “I IAIN 9 E : ..:. l I : Pre-Rocky Pre-Rocky .1. l hl I 3 g | | t.:. Old gravel l l Flats Flats : | I l E 8 "I I | i l alluvium alluvium I a: : “i' FIGURE 20.—Correlation of Quaternary formations as used in the vicinity of Golden by various authors, showing the time of formation and relative development of old soils. Not all age equivalents are those assigned by the original author. The symbol / / / S / / / indicates time of formation of a soil and the degree of development: S, strong; M, moderate; W, weak. A indicates rhyolitic volcanic ash. 56 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO TABLE 8.-—-Pebblc counts from alluvial deposits in the Golden and adjacent quadrangle: [The figures are in percent of particles larger than one-quarter inch. About 150 pebbles were counted for each sample. Within each drainage basin the samples farther to the right are farther east ofvthe Front Range. ..... not present. Qr, Rocky Flats Alluvium; Qpr, pre~Rocky Flats alluvium; Qpp, post-Piney Creek alluvium; Qb, Broadway Alluvium; Qv, Verdos Alluvium; Q10, Louviers Alluvium; Qof, old alluvial fan deposit; Qs, Slocum Alluvium; re, transported mantle] Drainage basin ........................................ Coal Creek Ralston Creek Van Bibber Creek TUCKCI’ Clear 093k Mount Gulch Vernon Creek Field locality ,,,,, 6143 R46 R40 R28 (3101 GlllC G114 R41 R39 0109 R42 R43 095 G99 G48 G82 085 R44 Formation symbol ................................... Qr Qpr Qpp Qb Qv Q10 re Qpp QV Qb Qpp Qpp Qof Q01 Q10 Q5 re QPP Pegmatite ................................ 29 41 17 7 16 37 ll 39 17 16 11 27 9 5 15 23 Granitic ......... 5 15 6 7 29 39 5 29 20 25 32 45 21 52 49 48 Granodiorite.. 6 6 1 1 2 2 3 16 42 4 11 Quartzite .............. 9 29 6 4 7 23 33 3 3 8 7 15 3 Quartzite‘ ............ 80 27 9 6 Gneiss and schist 10 27 24 52 52 27 26 45 12 52 51 56 27 15 21 8 26 Sedimentary ............. 5 73 6 2 4 1 .. 8 l Latite or monzonite l 5 1 1 Syenite .................... 4 Unidentified ............................ 3 4 9 7 ll 3 3 2 12 10 Total ................................ 100 100 101 100 100 101 100 100 100 101 98 102 99 99 99 100 100 100 IQuartzite from Coal Creek. SAMPLE LOCALITIES 0143. NE‘ASW‘A sec. 21, T. 2 S., R. 70 W., Golden Gll4. NW‘ANE'A sec. 6, T. 3 S., R. 69 W., Golden 095. NWMSE‘A sec. 28, T. 5 5.. R. 70 W., Golden quadrangle. quadrangle quadrangle. R46. NE‘ANW‘A sec. 29, T. 2 5., R. 70 W., Ralston R41. NE‘ASEB‘ sec. 7, T. 5 S., R. 70 W., Ralston Buttes 099. NWMNW‘A sec. 34, T. 3 5., R. 70 W., Golden Buttes quadrangle. quadrangle. quadrangle. R40? NW'4$E‘A sec. 31, T. 2 S.. R. 70 W., Ralston R39. NEléNWlé sec. 8, T. 3 5., R. 70 W., Ralston G48. SW%SW% sec. 27, T. 3 S., R. 70 W., Golden Buttes quadrangle. Buttes quadrangle. quadrangle. R28. SE%SE‘A sec. 6. T. 8 5., R. 70 w.. Ralston Buttes 0109. Nrusm sec. 13, T. 3 s., R. 70 W., Golden 682. SWKNW'A sec. 24, T. 3 s., R. 70 w., Golden quadrangle. quadrangle. quadrangle. GlOl. NWKSEK sec. 33, T. 2 S., R. 70 W., Golden R42. NW'ASE‘A sec. 20, T. 3 S., R. 70 W., Golden G85. NW‘lNEM sec. 16, T. 3 S., R. 69 W., Arvada quadrangle. quadrangle. quadrangle. _ Glll. NWlfiNE‘A sec. 11, T. 3 S., R. 70 W., Golden R43, NEKNElé sec. 32, T, 3 S., R. 70 W., Golden R44. NW‘ASE‘A sec. 17, T. 4 5., R. 70 W., Morrison quadrangle. quadrangle. quadrangle. unit crops out west and northwest of the Golden quad- rangle in Coal Creek valley and on the north side of Ralston Creek valley. Gravel from Ralston Creek has a mixture of quartzite from Coal Creek and other less dis- tinctive metamorphic rocks. Deposits of Van Bibber Creek and Tucker Gulch are mostly metamorphic gneiss and schist with no readily distinctive rock type, which is in itself a distinctive characteristic. Streams such as Woman Creek (which heads in Rocky Flats Alluvium from Coal Creek) and Leyden Creek (which heads in Rocky Flats and Verdos Alluvium from Coal and Ralston Creeks) assume the lithologies of these alluviums. Alluvial deposits in each valley become younger with decreasing height above the valley floor. At most places it appears that each alluvium was deposited on a stream-cut bedrock floor, only to be cut through and partially removed during the next erosion cycle. The remnants at the sides of the valley were left as terraces. (See fig. 21.) The new bedrock floor was then covered by a younger allu- ’vium. This process was repeated several times during the Pleistocene. However, it is possible that alluvium under- lying the flood plains of some creeks was deposited in late Pleistocene time and that subsequent erosion has not yet cut down to bedrock. LONGITUDINAL STREAM AND TERRACE PROFILES Longitudinal stream and terrace profiles (pl. 1) were prepared for Clear, Van Bibber, Ralston, and Leyden Creeks, and Tucker Gulch. Each stream valley was divided in to three to five fairly straight sections and the profile laid out along the approximate center of each section. The stream and terrace elevation at right angles to this line were then projected to the centerline. No allowance was made for the valleyward slope of some of the terraces or for slight deviation in direction of some of the Pleistocene valleys from the modern valley. The latter is the reason for the anomalous difference in altitude of the two differently oriented projected slopes of the Verdos Alluvium of Ralston Creek between the first and second bends from the west. Here the upper terrace surface was projected along lines greatly skewed from the earlier direction of stream- flow. The small segment of Van Bibber Creek that joined Clear Creek near the present locality of Mount Olivet Cemetery during Pleistocene time is plotted separately. The profiles show reasonably well the slope and con- tinuity of the terraces, the relation of younger to older ter- races, and the approximate amount of erosion that occurred between periods of deposition. The profiles show that the terrace segments along any particular valley fall QUATERNARY DEPOSITS 57 FIGURE 21.—Alluvial terraces in the valley of Ralston Creek. From oldest to youngest: Rocky Flats Alluvium (Qrf), Verdos Alluvium (in foreground) (Qv), Slocum Alluvium (Qs). Louviers Alluvium (Qlo), Broadway Alluvium (Qb), and Piney Creek Alluvium (Qp). along the same generally eastward-sloping plane and, therefore, provide supplemental evidence for correlation of the segments. The profiles also show the way in which the transported mantle and colluvium cut across, and are generally steeper than, the main terraces. The stream and terrace profiles generally are concave up. Clear and Van Bibber Creeks, however, show slightly bimodal profiles. The junction of the two concave profiles of Clear Creek is near the easternmost bend of the profile (pl. 1F). The apparently slower rate of incision of the creek indicated by this junction may be due to the creek's having to remove recurring landslide debris shed from the Table Mountains. Van Bibber Creek shows a more distinctly bimodal character; the junction of the two concave seg- ments is just west of the intersection with profile E near Ulysses Street (pl. 1C, E). A smoothly concave profile, without bimodal char- acter, results if the profile west of Ulysses Street is joined to the profile from Ulysses Street to Mount Olivet Cemetery (pl. 1E). It thus seems that Van Bibber Creek originally .0), . 79' '5. '0 «c, (a /.J 5 North Table Mountain , ‘ South Table QUE) The hummocky ridge extending northeast (to the left) from North Table Mountain is a large landslide. The paved road in the lower left has now been destroyed by a landslide. View is southeast from the Verdos surface east of State Highway 93. flowed directly into Clear Creek in the vicinity of the ceme- tery. The bimodal character of the present course prob- ably indicates that the lower part of the Van Bibber Creek was captured by a more vigorously eroding stream tribu- tary to Ralston Creek during or shortly after the depo- sition of the Broadway Alluvium in Pinedale time. PLEISTOCENEG) DEPOSITS PRE-ROCKY FLATS ALLUVIUM One small deposit of pre-Rocky Flats alluvium south of Rocky siding in the northwest part‘of the quadrangle is probably the correlative of the old gravel described by Malde (1955, p. 223). The deposit slopes east-northeast about 250 feet per mile and is 250 feet higher than Leyden Creek. The ground surface is littered with cobbles and small boulders. The deposit is a moderate-reddish-brown, coarse, poorly sorted, bouldery sand and gravel in a clayey sand matrix. The common large size of the boulders is about 3 feet in diameter; the largest boulder seen was 6 feet in dia- 58 meter. The coarse material is predominantly sandstone and some conglomerate (73 percent), and quartzite from Coal Creek (27 percent). Not present in the pebble count sample but found elsewhere were small amounts of grani- tic rocks, some of which are rotten, and metamorphic rocks. About 50 percent of the deposit is clayey sand. (See fig. 22.) Much of the clay is in silt- and sand-sized aggre- gates. The thickness of the deposit ranges from 6 to 15 feet. The contact with the underlying Pierre Shale is gently rolling. Until some distinctive fossils are found, the age of the pre-Rocky Flats alluvium must remain in doubt, although I believe the age is probably early Nebraskan. This belief is based on the topographic position of this alluvium, which is about 1,000 feet below the late Tertiary surface in the mountains to the west (Van Horn, in Sheridan and others, 1967, p. 51) but only 50 feet higher than the nearby Rocky Flats Alluvium. PLEISTOCENE DEPOSITS NEBRASKAN OR AFTONIAN ROCKY FLATS ALLUVIUM Deposits of Rocky Flats Alluvium derived from Coal, Ralston, and Clear Creeks are present in the area. The deposit from Coal Creek is in the northwestern part of the quadrangle although Coal Creek itself is north of the quadrangle. This deposit has the shape of a broad gently rounded and eastward-sloping alluvial fan. The deposit GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO from Ralston Creek, on the south side of Ralston dike, forms a terracelike eastward-sloping plane. The deposit from Clear Creek, not shown on the map, is a small ir- regular outcrop high on the south valley wall. COAL CREEK The Rocky Flats Alluvium derived from Coal Creek, the most extensive of the three Rocky Flat deposits, is in the northwest part of the quadrangle. The upper surface is very even and slightly concave up (pl. 1A). The easterly slope is about 120 feet per mile along the west edge of the quadrangle and 80 feet per mile near the eastern part of the deposit. The upper surface is 220—300 feet above Leyden Creek. The alluvium on Rocky Flats is generally about 40 feet thick but ranges from 0 to at least 50 feet. In the western part of the area the relatively smooth upper surface is broken only by a few shallow longi- tudinal drains that carry water only during the infrequent heavy rains. The eastern part of the deposit is deeply and intricately dissected by intermittent streams that have cut through the alluvium into the underlying bedrock. The ground surface at Rocky Flats is littered with cobbles and pebbles of quartzite. At some places small hillocks of fine- grained material, similar to mima or soil mounds, rise above the general level of the surface. The mounds are about 25 feet in diameter and 1—2 feet high. No mounds are present on the Rocky Flats Alluvium south of Rocky Flats. The character of the cut surface on bedrock underlying SIZE OF PARTICLES, IN MILLIMETRES 0.01 0.1 1.0 10.0 100.0 70‘ 60* 40- / 30* / 20— / 10* / PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN Unified 5°" d _ assificatior‘ / #_ _L , i .77, 4W4 CLAY SILT SAND GRANULES PEBBLES COBBLES FIGURE 22.—Cumulative curve showing the size distribution of sample R46 of pre-Rocky Flats alluvium. Sample from the NW% sec. 29, T. 2 S., R. 70 W. (Ralston Buttes quadrangle). QUATERNARY DEPOSITS the alluvium is not very well known. The lower part of the Laramie Formation appears to form a nearly continuous north-trending ridge transverse to the east-sloping Rocky Flats surface. Outcrops, shallow prospect pits, and clay mines show that the top of the ridge is at or just below the top of the alluvium at most places where it crosses Rocky Flats. Geologic mapping, well logs, and geophysical surveys show that the alluvium has its normal thickness of about 40 feet within a few hundred feet both east and west of the ridge. This indicates that the erosion surface under the Rocky Flats Alluvium is composed of two concave-up sections, separated by the north-trending ridge of Laramie Formation. Near the junction of State Highways 72 and 93 the allu- vium is a poorly sorted coarse sand and gravel, whereas 2 miles eastward it is a pebbly coarse sand. The size dis- tributions of several samples are shown in figure 23. The pebbles and cobbles are subround to subangular. Boul- ders as much as 2 feet in diameter are present at many places in the Rocky Flats Alluvium. The largest boulder seen in the area is near the base of the deposit, about 500 feet north of Plastic siding. The exposed part of the boulder is 15 by 21 feet, attesting to the power of the mudflow, or possibly a stream that moved it at least 2 miles. The coarse fraction of the alluvium is predom- inantly quartzite from Coal Creek, but it also contains small amounts of sedimentary, metamorphic, and grani- tic rocks (table 8). The upper 3 feet commonly contains a 59 clayey sand and gravel, silty sand, or the clayey B horizon of a pre-Bull Lake soil. Some of this material is undoubtedly younger than Aftonian inasmuch as it over- lies a pre-Bull Lake soil and bears a post-Pinedale soil. The amount of this material, however, is small and gen- erally less than 3 feet thick, so it was not mapped. RALSTON CREEK A small deposit of Rocky Flats Alluvium transects the south end of Ralston dike. Remnants of this deposit extend east of State Highway 93. The top of the deposit forms an inclined plane that slopes eastward about 140 feet per mile. At its east end the deposit is about 350 feet higher than Ralston Creek. The west end of the deposit heads just east of a wind-gap cut in the Dakota Group. The deposit is about half a mile south of, and 250 feet higher than, the present valley of Ralston Creek through the Dakota. During Rocky Flats time the Rocky Flats Alluvium extended through and west of the wind gap. This deposit of Rocky Flats Alluvium has been truncated by a younger northeast-sloping deposit of Verdos Alluvium derived from Van Bibber Creek (Van Horn, in Sheridan and others, 1967). The Rocky Flats Alluvium from Ralston Creek, about 30 feet thick, consists of coarse sand and gravel con- taining boulders as large as 2 feet in diameter. At places the deposit contains appreciable clay. The rocks are prin- cipally metamorphic and consist of schist, gneiss, and a SIZE OF PARTICLES, IN MILLIMETRES 0.00 0.01 0.1 1.0 10.0 100.0 100 90— 80- 70— 60- 50— 40— 30— 20“ / PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN / O > CLAY SILT VT] SAND GRANULES PEBBLES COBBLES FIGURE 23.—Cumulative curves showing the size distribution of samples of Rocky Flats Alluvium. Samples 575+00 from SW‘ANW‘A sec. 21, 0171 from NW‘ASW‘A sec. 23, and G170 from SW‘ANW‘A sec. 21, all T. 2 S., R. 70 W. 60 GEOLO Y OF THE GOLDEN QUADRANGLE, COLORADO bluish-gray quartzite like the quartzit from Coal Creek. The pebbles and cobbles are subrou d to subangular. Southwest of Ralston dike the upper p rt of the alluvium is moderately cemented by calciu carbonate; the cemented zone is overlain by 1.5 feet of fine-grained allu- vium. The calcium carbonate may epresent the Cca horizon of a pre-Bull Lake soil. Th overlying fine- grained alluvium is probably the res lt of sheetwash reworking the older alluvium. The calcium carbonate in the soil was not The Rocky Flats Alluvium south of Ralston dike was probably derived from Ralston Creek. The bluish-gray quartzite from Coal Creek crops out no farther south than the north side of the valley of Ralston Creek in the Ralston Buttes quadrangle and, no doubt, served as the source of the bluish-gray quartzite in the alluvium. Therefore, the alluvium must have come, at least in part, from Ralston Creek. The profile drawn along the crestline of the hog- back formed on’ the Dakota Group (fig. 24) in the Ralston Buttes quadrangle indicates that during Nebraskan time the low point in the valley may have been farther south than at present. Upstream from the point of crossing the Dakota hogback, the valley of Ralston Creek has changed very little from its ' position in Nebraskan time. The presence of the broad shoulder and the Rocky Flats Allu- vium on the south side of Ralston dike and the absence of both on the north side of the dike lead me to suspect that Ralston Creek was confined to the area south of Ralston dike during Nebraskan time. It probably joined Clear Creek at some place east of North Table Mountain. CLEAR CREEK A small deposit of bouldery and cobbly sand and gravel, too small to be shown on the map, is exposed on the steep northeast flank of Mount Zion just south of Clear Creek at the southwest corner of the Golden quadrangle at an altitude of about 6,050 feet, about 350 feet above Clear NORTH 7000’ Present profile 6500’ 6000’ 5500’ Crestline of Dakota Hogback Creek. The deposit consists principally of subround cobbles and small boulders that contain the very light gray granitic rock typical of alluvium along Clear Creek. It is overlain by 7 feet of colluvium, the lower 3 feet of which is strongly impregnated by calcium carbonate that rep- resents a pre-Bull Lake Cca soil horizon. The upper 4 feet has no soil and is colluvium of Holbcene age. The sand and gravel deposit is probably a remnant of a once much larger deposit of Rocky Flats Alluvium. SotL At a few places the remnant of a once thick, strongly de- veloped soil is exposed along the margins of or in cuts into the Rocky Flats Alluvium. This soil is similar to and cor- relative with the pre-Bull Lake soil of Hunt (1954, p. 126) and Malde (1955, p. 247). An excellent exposure of the soil on Rocky Flats, which was measured on the east bank of the South Boulder Diversion Canal between State Highways 72 and 93, is shown in the following section G143. Malde (p. 250) also indicated a low-lime facies of this soil west of the center of sec. 15, T. 2 S., R. 70 W., but this facies is not apparent in the Golden quadrangle, prob- ably because of poor exposures. SECTION Gl43.—Soil on Rocky Flats Alluvium [Measured on east bank of South Boulder Diversion Canal, between State Highways 72 and 93, NE‘ASW‘A sec. 2|. T. 2 S., R. 70 W.l 2] Thickness (fl) 1. Spoil from canal ................................................................ 4.0 2. Clayey sand, dark-brown (7.5YR 3/2), nonclacareous, poorly sorted.3 Contains some pebbles and cobbles. Gradational into unit 3 ................................................. .2 3. Clayey sand, dark-brown (7.5YR 3/2), noncalcareous, medium subangular blocky, pH6.9.5 Contains 'A-in. and smaller blebs of reddish-brown clay similar to 4. Contains some pebbles and cobbles. Gradational into unit 4 .............................................................................. .6 See footnotes at end of stratigraphic section. SOUTH 7000’ Wind gap graded to Roeky Flats ‘—\‘_\Alluvium 6500’ Ralston Creek 6000’ O——O 5500' 1 MILE I l . 1 KILOMETRE . FIGURE 24.—Profile of the crestline of the hogback formed by the Dakota Group in the vicinity of Ralston Creek in the Ralston Buttes quadrangle, just west of the Golden quadrangle. The dashed line is the inferred crestline profile during Nebraskan and Aftonian time. QUATERNARY DEPOSITS 4. Sandy silty clay, reddish-brown (2.5YR 3/2), noncal— Thickness careous, angular blocky to prismatic, pH 6.5.5 Contains (ft) some pebbles and cobbles. Grades into unit 5 .............. 5. Sandy silty clay, yellowish-red (5YR 4/6), noncalcareous, angular blocky to prismatic, pH 6.5.3 Contains some pebbles and cobbles. Sharp to gradational uneven con- tact with unit 6 .............................................................. 6. Silty to cobbly sand and gravel, pinkish-white (7.5YR 8/2), strongly calcareous, moderately to strongly cemented caliche. Maximum size is 2 ft and common large size is 10 in. 3 The coarse fraction is about 80 percent quartzite from Coal Creek, 10 percent meta- morphic, 5 percent granitic, and 5 percent sedimentary (table 8). Base not exposed ............................................ 2.2 1.5 3.0 ‘Mechanical analyses of the units in this exposure shown in Van Horn (1968) do not include pebbles larger than 1 in. ”he U numbers in Van Horn (I968) correspond to the next highest unit number in this measured section, thus U] of Van Horn (1968) corresponds to unit 2 of the measured section, and U2 to unit 3, and so forth. 35cc table 12, fig. 48, and Van Horn (1968, sample 6143). At places as much as 6 feet of unit 6 is exposed; north- ward, toward the flume, the heavy caliche gradually changes to a poorly defined thin layer of calcium car- bonate stringers and blebs under the thick clay layers. Pebbles and cobbles of bluish-gray quartzite from Coal Creek are present throughout this section but are much more common toward the base. A change in the clay minerals is indicated by the mica-to-montmorillonite ratio which decreases abruptly between units 3 and 4 (table 12). Unit 2 of the section is probably an A horizon of H010- cene age; similar horizons are present at the top of all deposits in the area except for the post-Piney Creek deposits. Unit 3 is a B horizon, probably of post-Pinedale age, that has developed on an older B horizon or on a now~ unrecognizable Pinedale deposit. A similar horizon is de- scribed in the section of the Louviers Alluvium along Clear Creek. The small reddish—brown blebs of clay in this horizon probably were in the process of being changed to dark brown when the soil-forming period ended. The abrupt change in the mica-to-montmorillinite ratio from about 6/1 to 1/4 may also indicate that a long hiatus existed between the formation of units 3 and 4. The differ- ence in the soils is attributed to the fact that different kinds of soil develop under different climatic and biologic conditions. Units 4 and 5 are the B1 and B2 horizons of a pre-Bull Lake soil, and unit 6 is the Cca horizon. AGE AND CORRELATION The Nebraskan or Aftonian age of the Rocky Flats Allu- vium is inferred from the volcanic ash that is similar to, and correlated with, the Pearlette Ash Member of the Sappa Formation of Kansan age which is contained in the next lowest alluvium about a mile south of Rocky Flats. ORIGIN The alluvium on Rocky Flats has the shape of a gently eastward-sloping alluvial fan with its apex at the mouth of 61 Coal Creek Canyon. Prior to Kansan time it was undoubt- edly much more extensive than it is at present. It probably extended eastward to the South Platte River and may have joined with other similar deposits to the north and south in much the same way that the modern flood plains of adjacent valleys join. The thickness of the deposits and the possible presence of bedrock interfluves indicate that most of the Rocky Flats Alluvium in the Golden area is the result of alluviation, although the lower few feet may be the result of pedimentation. Pediment gravels which are associated with lateral planation are generally only a few feet thick (Denny, 1965, p. 21, 58). Although lateral plana- tion was no doubt an important factor in shaping the bed- rock surface underlying the Rocky Flats Alluvium, it seems rather unlikely that a stream with the small drainage area that Coal Creek has would be able to plane the bedrock effectively while distributing a surficial load of 40 feet of coarse alluvium. Pediment gravel originating from Coal Creek, a small stream, probably would be present only in the lower few feet of the existing deposit. The remainder of the deposit, then, is the result of allu- viation. Two major inferences about the origin of the Rocky Flats Alluvium, mostly based on indirect evidence, are possible. First, the Rocky Flats Alluvium is principally a product of processes other than pedimentation in the Golden quadrangle, as discussed above. Second, it is very likely that interfluves of Cretaceous and Paleocene bed- rock separated most segments of the valleys of Coal, Ralston, and Clear Creeks within the limits of the Golden quadrangle; so the deposits were probably confined within the limits of separate valleys. The Rocky Flats Alluvium south of Ralston dike appears to have been deposited in a relatively narrow valley between Ralston dike on the north and the large ir- regular intrusive on the south. The presenceof a bedrock interfluve between Ralston and Clear Creek's is partly sug- gested by the absence of Van Bibber Creek from this area until after Kansan time. During Kansan and earlier time Van Bibber Creek joined Ralston Creek near Ralston Reservoir (Van Horn, in Sheridan and others, 1967, p. 57). For the lower segment of Van Bibber Creek to have cap- tured the upper segment south of Ralston Reservoir it must have been cutting easily eroded material, such as the soft Cretaceous and Paleocene deposits. The hard sand- stones of the Laramie Formation and‘Dakota Group are broken by faults, which would locally permit the rela- tively easy removal of those otherwise resistant rocks. The lack of any deposits or bedrock benches west of North Table Mountain or north of Ralston dike at the proper level for Rocky Flats Alluvium may also indicate former bedrock interfluves in these areas. If these former bedrock interfluves of this old narrow valley were present they were formed principally of soft Cretaceous and Paleocene rocks that have since been 62 almost completely eroded away, and the topography has been reversed. The old alluvium now forms an interfluve extending a short distance east of the mountains. In this context the alluvium seems to be a terrace deposit. The Rocky Flats Alluvium and the bedrock interfluves were eroded prior to the deposition of the next younger Verdos Alluvium. KANSAN OR YARMOUTH VERDOS ALLUVIUM The Verdos Alluvium, which contains a rhyolitic vol- canic ash member probably equivalent to the Pearlette Volcanic Ash Member of the Sappa Formation of Kansan age, is present at several places in the quadrangle. The largest deposit of Verdos Alluvium is north of Ralston Creek between Leyden and Ralston Reservoir. The sev- eral isolated hills north of Leyden Creek and west of Leyden are capped by Verdos Alluvium derived from Ralston Creek, as are several ridges extending east of Rocky Flats. The long east- trending ridge north of Leyden Lake is capped partly by Verdos Alluvium and partly by transported mantle derived from the Verdos Alluvium. The bedrock knoll west of Hyatt Lake probably was once part of the surface on which the Verdos was deposited. The higher knolls on the ridge south of Van Bibber Creek are capped by transported mantle deposits probably derived from Verdos Alluvium. Several isolated deposits of Verdos Alluvium are on the north side of Clear Creek at Golden. Volcanic ash was found in the Ralston and Clear Creek drainages. RALSTON CREEK Adjacent to Ralston Creek the upper surface of the Verdos Alluvium is 170—200 feet above the creek, and about 170 feet below the upper surface of the Rocky Flats Allu- vium. The Verdos slopes 80—70 feet per mile slightly north of east, in the general direction of Standley Lake. The physiographic relations indicate that the Verdos at Leyden Junction and east of Rocky Flats was deposited by an an- cestral northeast-flowing Ralston Creek. The character of the bedrock surface beneath the Verdos is poorly known. The general eastward slope of the surface is interrupted by the north-trending ridge of the lower part of the Laramie Formation, which rises to or near the upper surface of the Verdos at several places. The alluvium re- sumes its normal thickness to the east and to the west of this transverse ridge of the Laramie, thus giving the lower surface of the Verdos a bimodal character. An apparent northward slope of the lower surface is exposed in a cut for State Highway 93, made about 1960, north of Ralston Creek. Additional evidence for a northerly component of slope of the lower surface is the southward thinning of the deposit between Ralston Reservoir and Tucker Lake (Van Horn, 1972). The northerly slope indicates that the orig- inal southern limit of the deposit probably was not very far south of the present southern limit. GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO This poorly sorted alluvium ranges in thickness from 0 to at least 40 feet and averages about 30 feet. It ranges from a cobbly sand and gravel to a pebbly sand. The size dis- tribution of a sample from locality G101 on the east side of State Highway 93 near the middle part of the deposit is shown in figure 25. The Verdos is extremely variable in size gradation within short distances but in general is finer grained toward the east. The coarse fraction is principally of granitic rock, gneiss, and pegmatite but includes quartzite, schist, granodiorite, sandstone, and conglom- erate. North of Ralston Reservoir the alluvium is com- plexly mixed with old alluvial fan deposits. Here it con- tains appreciably greater amounts of silt and sedimentary rock fragments. Several species of Foraminifera reworked from the Cretaceous bedrock are present at places in the deposit. Rock fragments below a depth of 3 feet generally are sound; above this, however, fragments commonly are rotten, and appreciable interstitial clay is present. At places the upper 2—5 feet has a poorly to strongly cemented caliche layer. In the large borrow pit north of Ralston Reservoir the caliche is in several layers 1—2 feet thick separated by l- to 2-foot- thick layers of silty to clayey sand or gravel and sand. At other places in the pit there is only one caliche layer. A measured section along the south side of the borrow pit follows. SECTION GF5.—Verdos Alluvium [Measured along the south side of borrow pit north of Ralston Reservoir, SEléNE'A sec. 52, T. 2 S., R. 70 W.) Thickness l. Clayey to cobbly sand, red (10R 4/6). At places upper 0‘) 1 ft has thin calcium carbonate stringers ...................... 4.0 2. Silty to cobbly coarse sand and few small boulders, white (lOYR 8/2) at top to very pale brown (lOYR 7/4) at base. Upper 3 ft is well cemented with calcium car- bonate, but lower 4 ft is only moderately to weakly cemented ......................................................................... 3. Silty to pebbly coarse sand, very pale brown (lOYR 8/ 2); some narrow stringers of calcium carbonate. Contains fossil gastropods Oreohelix sp. indet., Pupilla muscorem (Linné), Vallom'a gracilicosta Reinhardt, and Zonitoides arboreus (Say) ....................................... 4. Silty coarse sand, light-reddish-brown (5YR 6/ 4). Persist- ent zone of calcium carbonate concretions as much as 4 in. in diameter at top. Base covered ........................... 7.0 2.5 3.0 Total thickness measured ........................................... 16.5 The fossils in unit 3 were identified by A. B. Leonard of the Kansas Geological Survey (written commun., Jan. 15, 1953). Other collections from this pit identified by Leonard include Pupilla blandi Morse and Deroceras laeve (Miiller). The faunule is terrestrial and indicates a woodland—plains border area. The Oreohelix suggests montane conditions, which is not surprising in view of the 6,100-foot altitude and the nearness to the Front Range. Deposits in the measured section probably include a soil profile. Unit 1 may have been a B horizon that was ex- tensively reworked by streams that incorporated much additional material into a new deposit. If so, unit 1 is QUATERNARY DEPOSITS 63 younger than Yarmouth. The thin calcium carbonate zone in the upper part of this unit is probably the result of a post-Sangamon soil-forming period. The lower units seem to be an undisturbed pre—Bull Lake Cca horizon, impregnating sand and gravel of Kansan or Yarmouth age. At the west end of the pit is a small lenticular deposit of silt containing about 10 percent rhyolitic volcanic ash shards (tables 10 and 11, locality 631). (See fig. 26.) The lens is about 30 feet long and 1—1 .5 feet thick. It is very light gray where dry and grayish orange where damp. The lens is principally composed of silt-sized, rounded, and frosted quartz grains and aggregates of clay particles but includes small amounts of mica and dark mineral grains. The volcanic ash shards are clear to pitted, curved frag- ments that are mostly pieces of bubble junctures but include a few nearly complete bubbles. A few slender dark- brown crystals of chevkinite attached to the volcanic glass shards were identified by H. A. Powers of the U.S. Geo- logical Survey (oral commun.). Analyses of specially cleaned glass shards (table 11) show marked similarity to analyses of the Pearlette Ash Member at the type locality of the Sappa Formation in Nebraska (Miller and others, 1964, p. 26). About 10 feet of Verdos Alluvium mixed with an old alluvial fan deposit overlies the volcanic ash. The top of the mixed deposit is marked by as much as 2 feet of very light gray poorly cemented caliche (fig. 26). The gently undulating top of the caliche, probably the roots of a pre- Bull Lake Cca soil horizon, is unconformably overlain by 1—2 feet of younger colluvium. The several isolated deposits of Verdos Alluvium north of Leyden Creek are eroded remnants of the large deposit north of Ralston dike. These isolated deposits are similar to the main deposit but probably contain more quartzite derived from reworking of the adjacent Rocky Flats Allu- vium. The Verdos Alluvium on the long ridge extending east from Leyden Junction is a continuation of the Verdos Alluvium north of Ralston dike and undoubtedly was originally continuous with it. Much of the quartzite in the alluvium on the ridge probably was derived from reworking the Rocky Flats Alluvium. Volcanic ash has been reported from this gravel (George Chase, U.S. Geol. Survey, oral commun.) but was not seen during the present study. The eastern part of this ridge is covered by as much as 5 feet of loess but it thins westward, and although present at the west end it is not mapped there. A slightly lower terracelike surface north of the ridge is underlain by a transported mantle deposit probably derived from the Verdos Alluvium. CLEAR CREEK North of Clear Creek at Golden, three isolated deposits of Verdos Alluvium contain thin deposits of volcanic ash probably equivalent to the Pearlette Ash Member of the SIZE OF PARTICLES, IN MILLIMETRES 0.01 0.1 1.0 10.0 100.0 70* 60~ 50— 40— 30— 20— 10— PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN 0 (3? y/ - 9 ‘ % T CLAY SILT SAND # fi__JL W M' GRANULES PEBBLES COBBLES FIGURE 25.—Cumulative curve showing the size distribution of a sample of Verdos Alluvium from locality 6101. Sample is from the NW‘ASE‘A sec. 33, T. 2 S., R. 70 W. 64 FIGURE 26.—Crudely stratified deposit of mixed Verdos Alluvium and old alluvial fan in the NW‘ANE‘A sec. 32, T. 2 S., R. 70 W. (locality G31). A, Far view. The lenticular silt bed (S) contains about 10 percent rhyolitic volcanic ash probably equivalent to the Pearlette Ash Member of the Sappa Formation. The thin light- colored caliche bed just below the top of the excavation is probably GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO the basal part of a pre-Bull lake Cca soil horizon. Unconformably overlying the caliche is colluvium (C) of Holocene age. The low grassy slope behind the cow is a spoil bank from the South Boulder Diversion Canal. B, Closeup of the volcanic-ash-bearing silt bed (S) and part of the mixed Verdos Alluvium and old alluvial fan deposit. the east boundary of sec. 28, T. 3 S., R. 70 W. (locality 694). (See fig. 42.) These four deposits do not now form a flat surface but are undoubtedly erosional remnants of a former terrace deposit that sloped about 60 feet per mile eastward between the Table Mountains. The deposit in the Sappa Formation. The westernmost deposit, which is not shown on the geologic map, is exposed in the western- most prospect trench in the NE‘ASW‘A sec. 28, T. 3 S., R. 70 W. (locality G24). A fourth deposit, also not shown on the map, has been overridden by a landslide that lies athwart QUATERNARY DEPOSITS 65 terrace was at least 40 feet thick. No deposit of Verdos Allu- vium attributable to Clear Creek was found south of the creek. The nearly circular deposit of Verdos capping the hill in northeast Golden (Graveyard hill) contains a 2-foot-thick deposit of rhyolitic volcanic ash in the lower part. The Verdos overlying the ash consists of about 40 feet of poorly sorted interbedded sand and cobbly sand and gravel. The coarse material contains a large proportion of light- to medium-gray granitic rock derived from the Silver Plume Granite in the headwaters of Clear Creek. The Verdos exposed in the prospect trench is of similar lithology, as are the other two deposits. In the prospect trench more than 3 feet of cobbly alluvium is overlain by rhyolitic vol- canic ash, which in turn is overlain by colluvium. The de- posit overridden by the landslide contains many cobbles and boulders of latite in addition to the granitic rock. The volcanic ash and topographic position relative to other nearby alluvial deposits indicate that these four isolated deposits are Verdos Alluvium. VOLCANIC AsH MEMBER Volcanic ash in the Verdos Alluvium is a distinctive marker and forms a basis for correlating deposits in the Golden quadrangle with deposits elsewhere in the Denver area (Scott, 1962, 1963a; Hunt, 1954). The mineralogy and chemical composition of this ash strongly indicate that it is equivalent to the Pearlette Ash Member of the Sappa Formation and therefore is of late Kansan age. The ash deposits range from light gray to white and have a fluffy or powdery texture. The tiny, broken shards are visible with a hand lens and create a myriad of bril- liant bluish-white sparkles when viewed in strong light. When examined through a microscope the shards are seen to be mainly curved plates of clear to cloudy glass. Many of the plates are strongly pitted and the pits filled with a white clay mineral. (These fillings are removed by ultra- sonic cleaning before the ash is analyzed chemically.) Many broken bubble junctions with jagged edges and a few unbroken bubbles are present in the ash. The chem- ical composition of ultrasonically cleaned shards (table 10, sample E1818) is similar to that of a rhyolite. Tiny crystals of brown chevkinite and green ferroaugite, some with particles of glass still attached to them, are an integral part of the ash. E. J. Young, of the U.S. Geological Survey, identified these minerals in samples from the two west- ernmost deposits at Golden (table 9), and H. A. Powers, also of the U.S. Geological Survey, identified chevkinite in the sample from north of Ralston Reservoir (oral communs.). A possible extension of the Pearlette Ash Member into Colorado and other Western States was indicated by Powers, Young, and Barnett (1958). Powers (1961, p. B261—B263; fig. 111.1) subsequently proposed that the Pearlette Ash had 0.12—0. 15 percent chlorine and 0.12-0.17 percent fluorine and that it differed from several other TABLE 9.—Minerals in samples of volcanic ash from near Golden, Colo. [Mineral identification by E. J. Young, U.S. Geological Survey. X, present; absent; ?, identification not certain] Locality No Serial No G24 2633 l 6 G95 280636 Chevkinite ............................................................... Green ferroaugite .................................................... Magnetite ............ Hematite ............................................ Ilmenite ............................................. Quartz and feldspar ...... Blue-green hornblende. Green hornblende ......... Red-brown hornblende ........................................... Brown hornblende .................................................. Biotite ...................... Zircon ...................................................................... Tourmaline ............................................................. Sillimanite ..... Fayalite ........................................................ Garnet ......................................................... Hypersthene .. Apatite ............................................ Sphene ............................................ Monazite .................................................................. ><><>< >< XE ><><><><><>< ><><><><><><§ ><§ 'V'V.“'."><><><><><§ SAMPLE LOCALITIES G24. NEltswlA sec. 28, T. 3 S., R. 70 W., Golden quadrangle. G93. Center sec. 28, T. 3 S., R. 70 W., Golden quadrangle. TABLE 10.—Complete and partial rock analyses (in percent )of volcanic ash samples from near Golden, Colorado Locality No. . lG24 E1818 2 G93 G 2860 ’ 051 E2073 ’ M555 E2072 72.91 11.86 .58 .83 less 0 ................. .07 Total ....... 'Analyst: Paula Monlalto. 2Analysts: E. L. Munson and V. C. Smith. ’Analyst: D. F. Powers. SAMPLE LOCALITIES . NE'ASW‘A sec. 28, T. 3 5., R. 70 W., Golden quadrangle. ‘ . Center sec. 28, T. 3 8., R. 70 W., Golden quadrangle. GSI. NW'ANEM sec. 32, T. 2 S., R. 70 W., Golden quadrangle. M533. SE'A sec. 3, T. 4 5., R. 70 W., Morrison quadrangle. silicic ash deposits in these constituents. The volcanic ash deposits in the Golden area fall within the limits assigned to the Pearlette (table 10). The partial composition of the 66 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO Pearlette Ash Member at the type locality of the Sappa Formation, and at several other places in Kansas and Ne- braska, was published by Miller, Van Horn, Dobrovolny, and Buck (1964, p. 26—28). The volcanic ash from the Golden area (table 1 1) is chemically similar to the ash from the other areas. Of the 24 elements determined in most of the samples from near Golden, 18 are within the limits shown for the Kansas and Nebraska samples. The other six elements are all present in higher percentages in one or more of the Golden area samples than in the Kansas and Nebraska samples. The small and inconstant differences, however, do not appear to be significant: boron, beryl- lium, and manganese are high in samples from localities G31 and MS33; cobalt and strontium were high in locality G24; fluorine is high in samples from localities G24 and 031, but still within the limits shown by Powers (1961); zirconium is high in all four of the samples from the Golden area. The ash at locality 024 is the type 0 Pearlette and is about 0.6 million years old, according to G. A. Izett of the US. Geological Survey (oral commun., July 1975). SOIL The original soil developed on the Verdos Alluvium has been largely eroded at most exposures I examined. At some places only a caliche layer, which probably is the Cca horizon of a pre-Bull Lake soil, is present. This layer ranges in thickness from 0 to 7 feet but is absent at many places. In the deep cut for State Highway 93 north of Ralston Creek, a thick reddish-brown clayey gravel over- lies a thick moderately cemented caliche zone, as shown in the following measured section. SECTION GlOl.—Soz’l on Verdos Alluvium [Measured in the east bank of highway cut in the NWASE'A sec. 33, T. 2 5., R. 70 W.] Thickness (I!) l. Spoil and old road fill ....................................................... 2.2 2. Clayey sand and gravel, dark-reddish-brown (2.5YR 3/4), noncalcareous; no soil structure. The pebbles and cobbles are stained dark reddish brown and many are rotten. The matrix is damp sandy clay. Grades into unit 31 ............................................................................. .9 3. Clayey sand and gravel, dark-red (2.5YR 3/6) mottled dark-reddish-brown (2.5YR 3/4), noncalcareous, no soil structure. Similar to unit 2 except for color; grades into unit 41 ..................................................................... 1.4 4. Cobbly sand and gravel, clayey at top to silty at base, pinkish-gray (5YR 7/2), strongly calcareous. Calcium See footnote at end of stratigraphic section. TABLE 11.—Quantitative spectrographic analyses (272 percent by weight) of glass shards from volcanic ash samples near Golden, Colo., and Orleans, Nebr. [Samples weredisaggregated and the glass shards were cleaned with an ultrasonic transducer; shards were separated from contaminants by use of an electromagnet; and analyses were made by generally accepted spectrographic methods. Reported results have an overall accuracy of 1:15 percent, except that they are less accurate near limits of detection] Locality No .............................. 1G3) ‘024 2 093 1 M533 Type section of the Sappa Formation’ Serial No .................................. E2073 E1818 02860 E2072 02861 02862 02868 02864 Element‘ (5) (5) < 0.00005 (5) < 0.00005 < 0.00005 < 0.00005 < 0.00005 0.003 0.002 < .001 0.003 < .001 < .001 < .001 < .001 .016 .021 .032 .016 .017 .018 .016 .016 .0017 .0010 .0007 .0013 .0009 .0006 .0009 0006 .0001 .0002 < .0001 < .0001 < .0001 < .0001 < .0001 < .0001 .0001 .0001 < .0001 .0001 < .0001 < .0001 < .0001 < .0001 .0006 .0005 .0004 .0008 .0004 .0004 .0003 .0002 1.2 (5) 1.1 1.3 .93 .84 .92 .91 .0026 .0026 .0024 .0027 .0023 .0022 .0023 .0022 .011 .012 .013 .014 .014 .011 .010 .010 .034 (6) .026 .032 .023 .022 .024 .020 .0006 .0006 .0005 .0005 .0003 .0005 .0005 .0004 .008 .007 .007 .008 .007 .005 .005 .005 < .0004 < .0003 < .0002 < .0004 < .0002 < .0002 < .0002 < .0002 .005 .004 .004 .005 .004 .004 .005 .004 < .0003 .0003 < .0005 < .0003 < .0005 < .0005 < .0005 < .0005 .0011 .0011 .001 .0009 .001 .001 .001 .001 .0014 .0084 < .001 .0020 <.001 < .001 < .001 < .001 .077 (6) .087 .084 .082 .076 .074 .078 < .0003 .0003 < .0005 < .0003 < .0005 < .0005 < .0005 < .0005 .011 .012 .009 .013 .010 .008 .009 .008 .0010 .0011 .0010 .0011 .0010 .0010 .0010 .0010 .030 .029 .030 .036 .027 .021 .020 .021 IAnalys‘s: p. R. Barnett, N. M. Conklin, and J. C Hamilton. ‘Also looked for in all samples but not detected: As, Au, Bi, Cd, Ge, 1n, Pt, Sb, Ta, Th, T1, U, W. and Zn. 5Also looked for but not detected. “Not determined. SAMPLE LOCALITIES 2Analyst: P. R. Barnett. 3Ana1yses from Miller and others (1964, p. 26, 27). 051. NWMNEM sec. 32, T. 2 5., R. 70 W., Golden quadrangle. M533. SE“ sec. 3, T. 4 S., R. 70 W., Morrison quadrangle. G24. NEKSWK sec. 28, T. 3 5., R. 70 W., Golden quadrangle. 02861.64. SEKNE“ sec. 11. T. 2 N., R. 20 W., Stamford Quadrangle, Nebraska. 093. Center sec. 28. T. 3 S., R. 70 W., Golden quadrangle. QUATERNARY DEPOSITS 67 Thickness ( f l) Unit 4.—Continued carbonate forms a moderately to weakly cemented zone at the top but decreases to slender stringers and small blebs toward base. Some pebbles and cobbles have a thick rind of hard caliche. Contains a few rotten schist pebblesl .......................................................................... 5. Silty to cobbly sand and gravel and interlensing silty sand, reddish-yellow (5YR 6/6). Pebbles and cobbles are principally metamorphic and granitic rocks similar to post-Piney Creek alluvium of Ralston Creek.l (See table 8.) The largest rock seen is 2 ft in diameter and the common large size is 6 in. (See sample 010] of Van Horn (1968) for mechanical analysis.) Seeps are present at the base of the thickest part at the bedrock contact. Unconformably overlies the Arapahoe and Laramie Formations ...................................................... 2.2 15 to 30 1See table 12. Units 2 and 3 of the measured section are B horizons of a pre-Bull Lake soil. No soil structure was found but the large number of pebbles and cobbles and the high moisture content would probably tend to obscure any structure. Well-developed clay skins coat the pebbles and cobbles. The Cca horizon (unit 4) is thin for a pre-Bull Lake soil, but Cca horizons, because of their variability owing to local conditions, are not dependable criteria for use in soil stratigraphy. The A horizon and upper part of the B horizon have been removed by erosion. At many places along the excavation for the aqueduct north of Ralston Reservoir the Cca horizon is overlain by 1—3 feet of moderate—reddish-brown silty to cobbly sand and gravel. The contact between the two is very sharp and uneven. This seems to indicate that the upper part of the Verdos Alluvium has been reworked and that some of the B horizon of the original soil has been incorporated into the younger deposit overlying the caliche. AGE AND CORRELATION The age of the Verdos Alluvium is believed to be Kansan and Yarmouth chiefly because of the contained volcanic ash that is correlated with the Pearlette Ash Member of late Kansan age. The ash and underlying deposits of Verdos Alluvium are Kansan. The alluvium overlying the ash as well as the eroded soil profile may be of late Kansan or early Yarmouth age. The soil represented by the reddish- brown B horizon or the eroded Cca horizon probably de- veloped during Yarmouth time. The correlation of the volcanic ash in the Golden quad- rangle with the Pearlette Ash Member of the Sappa Forma- tion forms the main basis for the ages assigned to the terrace deposits in the Golden quadrangle. This correla- tion is corroborated by the topographic position of the terrace and the soil: the next younger terrace deposit also has a pre-Bull Lake soil, but the next one younger than that does not. Hence, the Verdos is the next to the youngest pre-Bull Lake deposit and is therefore consistent with a Kansan and Yarmouth age assignment. ILLINOIAN OR SANGAMON SLOCUM ALLUVIUM The most continuous deposit of Slocum Alluvium is on the south side of Ralston Creek, but smaller deposits are adjacent to Clear Creek and Standley Lake. The Slocum borders valleys in the area and has a definite terrace aspect. It is the oldest terrace deposit that at places is overlain by a younger terrace deposit within the limits of the Golden quadrangle. It is as much as 40 feet thick. RALs'rON CREEK The upper surface of the Slocum south of Ralston Creek slopes southeastward about 100 feet per mile. The Slocum is about 200 feet lower than the Rocky Flats Alluvium east of Ralston dike and about 100 feet lower than the Verdos Alluvium north of Ralston Creek. A low ridge at the south side of the surface south of Ralston Creek is thinly veneered with gravel and separates the Ralston Creek drainage from the Van Bibber Creek drainage. The Slocum Alluvium adjacent to Ralston Creek is a poorly sorted, silty to cobbly sand and gravel that is 10—30 feet thick. The proportion of silt and clay increases out- ward from the center of the valley as the thickness of the deposit decreases. At places the upper 3 feet contains some rotten cobbles, mostly of schist. Northeast of North Table Mountain the Slocum deposit is crossed obliquely by the present course of Van Bibber I Creek. On the south side of the creek it continues south- eastward to the vicinity of West 54th Avenue and McIntyre Street where it forms a terrace just a few feet above the next youngest terrace of Louviers Alluvium. Prior to deposi- tion of the Louviers the Slocum probably continued southeastward and merged with the Slocum from Clear Creek. It may still be present under the Louviers, which has, however, obscured the relation between the deposits in this area. The Slocum south of Van Bibber Creek prob- ably was deposited in part by Van Bibber Creek. The deposit is lower than the adjoining large deposit of Slocum to the northwest (pl. 10). This is probably a result of erosion of the deposit south of Van Bibber Creek, although possibly the latter deposit has been miscor- related and should be included in the Louviers Alluvium. CLEAR CREEK The Slocum Alluvium adjacent to Clear Creek com- prises five terrace remnants that are located on both sides of the creek. The upper surfaces slope eastward 55 feet per mile, are about 150 feet below the Rocky Flats Alluvium, 70 feet below the Verdos, and 75 feet above Clear Creek. The alluvium is a cobbly sand and gravel that contains very little silt or clay. It is poorly to normally sorted as de- termined by the sorting coefficient of Trask (1932, p. 72). The deposit contains pebbles and cobbles of Silver Plume Granite that distinguish alluvium of Clear Creek. Except for a few small remnants, the Slocum has been eroded from 68 the valley of Clear Creek. The deposits at Golden gen- erally are less than 10 feet thick (see measured soil section G72) and probably represent the thin edge of the deposit near the valley walls. The base of the deposit is exposed at the west end of a gravel pit south of Fairmount School, in the NWA sec. 24, T. 3 S., R. 70 W. At the west end of the pit the contact with the underlying Denver Formation is about 10 feet higher than the top of the nearby Louviers Alluvium of Bull Lake age. The bedrock surface slopes eastward and in the east- ern part of the gravel pit is not exposed even though the pit has been excavated almost to the same level as the top of the Louviers Alluvium. This thickness of gravel indicates that at least part of the valley in which Louviers was de- posited had been eroded by Illinoian time. One mile east of the pit the upper surface of the Slocum Alluvium slopes under, and is buried by, the Louviers Alluvium. This superposition of alluvium is the oldest example found in the Golden quadrangle where a younger alluvium physically overlies an older alluvium rather than being in a valley incised below the base of the older deposit. The Slocum at the pit near Fairmount School is 20—35 feet thick and, besides the typical Clear Creek clasts, it con- tains pebble- to boulder-sized fragments of latite. The silt- sized fraction contains small amounts of augite, hom- blende, magnetite, and white feldspar derived from the Denver Formation. The largest boulder seen was 4 feet in maximum dimension. The common large size is 0.6 foot. GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO Figure 27 shows the size distribution of a sample from locality G82 of this deposit. Rotten cobbles are present throughout the deposit but increase in number toward the top where they make up about 10 percent of the deposit. They account for 20 percent of the pre-Bull Lake soil that is present at a few places on the deposit. STANDLEY LAKE The Slocum north of Standley Lake is a silty to cobbly gravel that is more than 15 feet thick. The cobbles are prin- cipally quartzite from Coal Creek. The deposit is capped by a calcium carbonate zone 0—7 feet thick that may be the Cca horizon of a pre-Bull Lake soil. At places it forms a strongly cemented caliche as much as 3 feet thick. SOIL The soil seen on the Slocum Alluvium is everywhere partly eroded and generally only a Cca horizon 1—7 feet thick is present. Even the Cca horizon is completely eroded at many places. The calcium carbonate of the Cca horizon may either form a strong hard cement or be loosely dis- seminated throughout the zone. At a few places as much as 6 inches of moderate-reddish-brown clayey B soil horizon overlies the Cca. An eroded soil on the Slocum (described in the following measured section) was exposed at locality G72 in the excavation for the metallurgical building of the Colorado School of Mines northwest of the corner of 15th and Arapahoe Streets in Golden. SIZE OF PARTICLES, IN MILLIMETRES 0.00 0.01 100 0.1 1.0 10.0 100.0 (D O | 70— 60" 50- 30— 20— 10* PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN 0 ed soil cIassifica ion GP Unifi CLAY SILT SAND GRANULES PEBBLES COBBLES FIGURE 27,—Cumulative curve showing the size distribution of a sample of Slocum Alluvium from locality G82, SW‘ANW‘A sec. 24, T. 3 S., R. 70 W. QUATERNARY DEPOSITS SEcrION G72.—Eroded soil on Slocum Alluvium {Measured in the excavation for the Colorado School of Mines metallurgical building in NW‘ANW‘A sec. 34, T. 3 S., R. 70 W.] Thickness (I!) l. Clayey silt, grayish-black. Probably artificial fill ............. 0.5 2. Clayey to cobbly sand, moderate-reddish-brown (10R 4/ 6). Many light-gray rounded granite cobbles, some rotten. Forms discontinuous layer ............................................ 0—15 3. Silty to cobbly sand and gravel, very light gray, zone of calcium carbonate accumulation. Forms discontinuous layer ................................................................................ 0—1.5 4. Cobbly sand and gravel, yellowish—gray (5Y 8/1). Com- mon large size of rounded cobbles is 8 in. Many pebbles and cobbles of light-gray granite .................................. 3.5—5 5. Claystone of Denver Formation ........................................ 6.0 Unit 2 of this section is probably the B or Bca horizon of a pre-Bull Lake soil. No soil structure was noted and pos- sibly it is merely a reworked B horizon. The Cca horizon, unit 3, is very thin, which may also indicate that the over- lying unit is a reworked deposit. The rounded cobbles of light-gray granite which distinguish this as an alluvium deposited by Clear Creek are in sharp contrast to the angular pebbles and cobbles of gneiss and schist in the adjoining old alluvial fan. A roadcut south of Ralston Creek and 1% miles east of Ralston dike exposed 2 feet of reddish-brown pebbly to clayey silt overlying Slocum Alluvium. The silt has well- developed prismatic structure with clay skins on the prism faces. Clay skins have also formed around the pebbles. The silt overlies 1—2 feet of strongly cemented pebbly to cobbly caliche, which overlies a cobbly sand and gravel. The exposure is poor but appears to be of a strongly developed pre-Bull Lake soil on Slocum Alluvium. AGE AND CORRELATION The Illinoian or Sangamon age of the Slocum Alluvium is established by the volcanic ash in the next highest (older) alluvium, by the strongly developed pre-BuIl Lake soil on the deposit, and by the fact that the overlying allu- vium is capped by a well-developed soil of probable post- Bull Lake age. ORIGIN East of North Table Mountain the lower surface of the Slocum slopes eastward and passes below the upper surface of the next younger Louviers Alluvium. Expo- sures and well logs are not good enough to permit tracing the contact between the two alluviums for any distance. However, part of the alluvium under the Louviers terrace almost certainly is Slocum, and part of the thick alluvium under the flood plain of Clear Creek possibly is Slocum. If the latter could be proved it would indicate deep erosion by Illinoian time and extensive alluviation during Illinoian time. No evidence to support such an extensive fill terrace has yet been found in the deep gravel pits in the flood plain. 69 BULL LAKE AND PINEDALE The deposits of Bull Lake and Pinedale age were formerly shown as being of Wisconsin age in the Denver area. Because of nomenclature changes it has become more appropriate to refer to their age as Bull Lake and Pine- dale, even though this represents no change in their chronologic placement. The former usage and the usage of the present report are as follows: Usage of the Former usage present report Recent ..................................................... Holocene Post-Wisconsin ..... post-Pinedale Late Wisconsin ..... Pinedale Mid-Wisconsin ........................ post-Bull Lake Early Wisconsin .......................... Bull Lake Pre-Wisconsin ........................................ pre-Bull Lake The Louviers Alluvium of Bull Lake age and the Broadway Alluvium of Pinedale age are both present in this area. This twofold division of the Bull Lake and Pine- dale was not recognized by Hunt (1954) or Malde (1955), although they did recognize more than one deposit of Bull Lake and Pinedale age. They both used the soil developed on the Pinedale deposits to delineate the Bull Lake and Pinedale deposits at most places. Locally they correlated the strong soil developed on the Louviers Alluvium with a pre-Bull Lake soil and, thus, included part of the Louviers in their pre-Bull Lake deposits. The soil of post-Bull Lake age described by Malde (1955, p. 252) and by Hunt (1954, p. 109) appears to be the same as the. late Pinedale and superimposed early Holocene soils of the present report and of my earlier report (Van Horn, 1967). This soil appears to be the same as the early Holo- cene soil developed on the pre-Piney Creek alluvium exposed only along small tributary streams described by Scott (1962, 1963a). I correlate all alluvial deposits that bear this soil and are younger than Louviers with the Broadway Alluvium (Van Horn, 1967). LOUVIERS ALLUVIUM The Louviers Alluvium, of Bull Lake age, forms broad, well-defined terraces in the southern and eastern parts of the quadrangle. It is present in the valleys of all major streams except Leyden Creek. The upper surface slopes eastward 50—100 feet per mile, and in most places is about 40 feet above stream level. The deposit is generally a coarse cobbly sand and gravel that is normally to poorly sorted. There generally is less than 10 percent silt and clay in the deposit, except for the soil horizon, which contains more than 10 percent. The largest deposits are adjacent to Clear and Ralston Creeks. CLEAR CREEK In the valley of Clear Creek the Louviers is present in the vicinity of 8th Street in north Golden and also under the main business district of Golden near 13th and Washing- ton Streets. Here the pebbles and cobbles are subround to 70 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO round and are predominantly of granitic rock typical of Clear Creek. The deposit is normally sorted. Boulders as large as 1 foot across are present, but the common large size is 6 inches (fig. 28). The size distribution of a sample from this locality, G48, is shown in figure 29. Where the gravel is at the surface in the excavation a few feet west of the area shown in figure 28 the upper 2 feet is stained brown by iron oxide and appears strongly weathered, but no soil struc- ture was seen in the excavation. A coarse cobbly sand and gravel that underlies the brewery just east of Kenneys Creek is probably Louviers; the upper part, however, has been so extensively reworked during construction of the brewery that it has been mapped as artificial fill. The lower part of the young alluvial fan deposit shown at the mouth of Tucker Gulch in north Golden was prob- ably deposited at the same time as the Louviers Alluvium in the vicinity of 8th Street. The upper part of the fan is much younger, and has been covered by recent floods. Farther east, the Louviers exposed in the valley across the creek from the Golden Sewage Disposal plant (NEl/4 FIGURE 28,—Normally sorted Louviers Alluvium deposited by Clear Creek at locality G48 is shown at the lower left of the photograph. Behind the pick, unconfonnably overlying the Louviers, is dark- brown silty sand of the Broadway Alluvium deposited by Kenneys Creek. The Broadway is inconspicuously overlain by an artificial fill of very dark brown sandy silt above the pick head. The upper 2—6 inches of the exposure is spoil from the excavation. The eroded surface of the Louviers trends N. 30° W. The exposure was in an excavation along the south side of 13th Street, between Jackson and Ford Streets, in Golden. The pick handle is 1.5 feet long; the view is to the northwest. sec. 27, T. 3 8., R. 70 W.) is coarse cobbly sand and gravel. The owner of a well drilled in this deposit reported 47 feet of sand and gravel. The well bottomed in clay, pre- sumably Denver Formation. East of the Table Mountains the Louviers is marked by broad terraces of cobbly sand and gravel on both sides of Clear Creek. On the north side, near the corner of West 44th Avenue and McIntyre Street, the deposit is more than 20 feet thick. It is probably much thinner east of Mount Olivet Cemetery. It is about 10 feet thick on the south side of Clear Creek except near West 32d Avenue and Youngfield Street, where locally it is more than 20 feet thick, and at places is almost entirely composed of sand. At many places near West 32d Avenue and Youngfield Street the Louviers is overlain by a thin silt layer that locally may be as much as 20 feet thick. This may be loess although it is crudely bedded and appears to grade into the colluvium on the flanks of South Table Mountain. At a few places the top of the gravel under- lying the silt is marked by a moderate to strong accumu- lation of calcium carbonate, probably a Cca soil horizon. GRAVEL PITS IN CLEAR CREEK FLoon PLAIN East of North Table Mountain, exposures in several sand and gravel pits and prospect pits in the flood plain of Clear Creek show as much as 60 feet of cobbly sand and gravel, overlain in most places by 3—4 feet of fine- to coarse-grained alluvium. The cobbly sand and gravel is massive to poorly bedded and contains lenses of pebbly sand. A few boulders of granite and latite were as much as 5 feet in diameter. At places small amounts of gold are extracted as a byproduct of the gravel production. No indications of major unconformities were seen in the sand and gravel. Locally the upper few feet of the deposit is iron stained and contains partially rotten cobbles of granite, but the staining and rotting both decrease with depth. No zones of calcium carbonate accumulation were seen. No evidence for the age of the cobbly sand and gravel was found, but it is presumed to be at least as old as the Louviers Alluvium and is shown as Louviers(?) on the geologic map (Van Horn, 1972). The overlying 3—4 feet of alluvium is mapped as post~Piney Creek alluvium. RALsTON CREEK The other major deposit of Louviers Alluvium is on the south side of Ralston Creek. The thickness of this deposit is not known but it is probably less than 20 feet at most places. This deposit of coarse cobbly sand and gravel (fig. 30) has a very indefinite east boundary. The main body probably occupied the position of the present valley of Ralston Creek north of the bedrock ridge in secs. 5 and 6, T. 2 S., R. 69 W. A smaller body may once have existed south of this ridge. Both, however, have been completely removed by erosion. The two deposits of transported mantle shown on the north side of Ralston Creek near Ralston Church (sec. 31, QUATERNARY DEPOSITS 71 SIZE OF PARTICLES, IN MILLIMETRES 0.00 0.01 0.1 1.0 10.0 100.0 100 80- 50- 30 ' \" Ex 0 SS‘Ifi : \ . . 'I c\3 nIIIEd 591/ 20— /‘ PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN CLAY SILT SAND GRANULES PEBBLES COBBLES FIGURE 29,—Cumulative curves showing the size distribution of samples of Louviers Alluvium deposited by Clear Creek. Sample G48 is from the SW‘ASW‘A sec. 27, T. 3 S., R. 70 W.; G162, from the NE‘ASW‘A sec. 19, T. 3 S., R. 69 W.; and 690, from the NW‘ANE‘ASW'A sec. 29, T. 3 S., R. 69 W. SIZE OF PARTICLES, IN MILLIMETRES 0.00 0.01 0.1 1.0 1. 1 . 2100 00 000 E 9 U, 90— — LLI E (I) 80— z /7 < E 70— /— [I E _, 60— — < 2 W 50~ _ (I) UJ .J 9 40— _ I- E n. 30— 5-31. _ LL -\ Owls/A o / E; zoe — < ’2 w 10— — O 35 G111c—Unified,soil classification GP a— 0 l I A CLAY SILT SAND GRANULES PEBBLES COBBLES FIGURE 30.—-Cumulative curves showing the size distribution of samples of Louviers Alluvium deposited by Ralston Creek.Sample GlllC is from the NW‘ANE'A sec. 11 and 0169 from the NE‘ASE‘A sec. 1, T. 3 S., R. 70 W. 72 T. 2 S., R. 69 W.) are graded to about the same level as the Louviers. These deposits may be about the same age as the Louviers but they have more silt- and clay-sized material and grade imperceptibly upward to a level higher than the adjacent Louviers. STANDLEY LAKE Louviers Alluvium in the large area near the northeast corner of the quadrangle is probably less than 10 feet thick at most places. This coarse cobbly sand and gravel is prin- cipally composed of quartzite reworked from the Rocky Flats Alluvium. The small patch of Louviers(?) shown northwest of Upper Twin Lake marks an early course of the stream now occupying the valley to the north. The valley was probably cut during the early erosional stages of Bull Lake time and abandoned before the main body of alluvium was deposited. VAN BIBBER CREEK A small deposit of Louviers near the corner of 54th Avenue and McIntyre Street was deposited by Van Bibber Creek as shown by the fact that it contains no light-gray granite. The lack of any deposits of Louviers in the present valley of Van Bibber Creek east of McIntyre Street prob- ably indicates that Van Bibber Creek was flowing south of its present course and joined Clear Creek in the vicinity of Mount Olivet Cemetery throughout Bull Lake time. The Louviers was not recognized in the area north of North Table Mountain although it may be present under a thin cover of younger alluvium. The old fan deposit east of the Dakota hogback may be equivalent to the Louviers. It is not mapped as Louviers because of the coarse angular nature of the material in the deposit, and because of the fanlike shape. SOIL Like most Pleistocene soils in the area, the soil on the Louviers has been mostly removed by erosion. The thickest soil seen on the Louviers is in a sand and gravel prospect, locality G111, in the Ralston Creek drainage (fig. 31, samples GlllA, GlllB; fig. 30, sample G111C). Here the B horizon is 1.5 feet thick. The top 0.5 foot is dark reddish-brown, silty sand and gravel which overlies 1 foot of dark-brown silty and pebbly sand. Samples for clay mineralogy determinations (fig. 48, table 12) showed that the upper 0.5 foot contained 20 percent clay-sized material and the lower 1 foot contained 10 percent. The Cca horizon in the prospect pit consists Of 0—2.5 feet of sand and gravel moderately cemented by very light gray calcium carbonate. The B horizon of the post-Bull Lake soil of the Golden area is characterized by a well-developed prismatic struc- ture. Illuviated clay has formed coatings on the prism faces and around pebbles and cobbles. The horizon is generally noncalcareous. It is mostly dark brown but may have a red- GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO dish cast in the upper part. It was recognized only on allu- vial deposits, where it is as much as 1.5 feet thick. The calcium carbonate in the Cca horizon is so variable in amount and thickness that it is not useful for recognizing this soil. AGE AND CORRELATION The Bull Lake age of the Louviers Alluvium is established by the topographic position of its terrace and by the relative degree of development of the soil formed on it. The Louviers forms the second terrace below the vol- canic-ash-bearing Verdos Alluvium of Kansan or Yarmouth age. The soil is not as strongly developed as the pre-Bull Lake soils, but is more strongly developed than the soil on the next youngest terrace deposit, the Broad- way Alluvium. Thus, the soilon the Louviers fits into the soil-stratigraphic succession outlined by Morrison and Frye (1965). (See p. 93.) BROADWAY ALLUVIUM The Broadway Alluvium, of Pinedale age, is generally a fine-grained alluvium and consists of silty to pebbly sand. At a few places in the western part of the quadrangle it is a coarse cobbly sand and gravel. Examples of the coarse phase are exposed adjacent to Clear Creek east of North Table Mountain and along Ralston Creek east of State Highway 93. The deposits of Broadway Alluvium are probably less than 10 feet thick at most places. The major deposits were formed by Ralston and Van Bibber Creeks. Smaller deposits are present west of Standley Lake and adjacent to Clear Creek. CLEAR CREEK Only two small areas of Broadway Alluvium were found along Clear Creek. The only exposure of the deposit seen in Golden had no soil developed on it but showed the stratigraphic relation of the Broadway Alluvium over- lying and cutting into the underlying Louviers Alluvium (fig. 28). The Broadway Alluvium shown in figure 28 was probably derived from Kenneys Creek and is a fine-grained facies at the edge of the deposit. This terrace of Broadway Alluvium is 5—10 feet above Clear Creek and 10—15 feet below the top of the Louviers Alluvium. The other deposit of Broadway Alluvium, east of North Table Mountain, is about 10 feet above Clear Creek and 40 feet below the top of the Louviers Alluvium. Here the Broadway is a poorly sorted cobbly sand and gravel with a moderately developed soil on it. The A horizon of the soil consists of 0.2 feet of a very dark grayish brown humic, friable, silty coarse sand. This is underlain by 1.5 feet of dark-brown B horizon that is noncalcareous and has thin clay skins on the faces of the fine subangular blocky soil peds (individual natural soil aggregates). The B horizon is a cohesive, plastic clayey coarse sand that grades into the overlying A horizon and the underlying cobbly gravel. No zone of calcium carbonate accumulation (Cca horizon) QUATERNARY DEPOSITS 73 was found. Pebbles and cobbles of the light-gray granitic rock typical of Clear Creek deposits predominate, but no bedding characteristics were visible in the limited expo- sure. The coarse cobbly alluvium seems to extend entirely down the terrace scarp to the level of the modern flood plain of Clear Creek. VAN BIBBER CREEK The Broadway Alluvium adjacent to Van Bibber Creek grades rapidly eastward from a coarse cobbly sand and gravel to a fine sand (fig. 32, sample locality G109, unit 7). The greatest thickness of the al luvium seen was 6 feet. The alluvium probably is no more than 15 feet thick in the western part of the quadrangle and may be less than half that thick in the eastern part. Along the western part of Van Bibber Creek the Broad- way Alluvium is a coarse cobbly sand and gravel with sub- angular particles. It forms a terrace about 20 feet above the modern stream. North of North Table Mountain this terrace is only about 5-10 feet above stream level and is at about the same level as the adjacent Piney Creek Allu- vium—at places 1—2 feet of Piney Creek overlies the Broadway. During the early part, if not all, of Pinedale time Van Bibber Creek flowed southeast from the north- east corner of North Table Mountain toward Mount Olivet Cemetery. The alluvium deposited in this gently sloping valley segment is a silty to pebbly sand. It was deposited in a broad shallow valley that was cut a few feet in to the Louviers Alluvium. At places deep cuts reveal that the Broadway is underlain by a coarse cobbly sand and gravel that has a strong calcium carbonate zone. When the Broadway Alluvium was deposited by Van Bibber Creek, Clear Creek must have been depositing its gravel at a lower level. Van Bibber Creek probably joined Clear Creek southeast of Mount Olivet Cemetery. The channel Van Bibber Creek cut through the Louviers Allu- vium also must have been southeast of Mount Olivet Cemetery. Erosion has removed any direct evidence of this old channel as well as the eastern end of the Broadway Alluvium of Van Bibber Creek. This segment of Van Bibber Creek was abandoned in the latter part of Pine- dale or early Holocene time when Van Bibber Creek was. captured by a tributary of Ralston Creek at about the point where Ulysses Street crosses the creek. Only one small deposit of Broadway Alluvium was found in the postcapture segment. It could not be deter- mined if this was deposited before or after the capture. I suspect, but can find no evidence to prove, that the large area of Piney Creek Alluvium near the east border of the quadrangle in the present valley of the creek is underlain by Broadway Alluvium. SIZE OF PARTICLES, IN MILLIMETRES 0.00 0.01 0.1 1.0 100.0 100 80 '- 70 _ 60'- 50 - 40 — 30_/ 20— / d 5011 cIaSS' G111A—Unifie l ___________ 4___ | I 10—/ PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN l J L J CLAY SI LT SAND H—A H—J GRANULES PEBBLES COBBLES FIGURE ESL—Cumulative curves showing the size distribution of samples from soil horizons developed on Louviers Alluvium. Sample 0162 is from the NE‘ASW‘A sec. 19, T. 3 S., R. 69 W.; unit 1 is from the A horizon, unit 2 the B1 horizon, and unit 3 the B2 horizon. Samples 6111A and GlllB are from the NWANE'A sec. 11, T. 3 S., R. 70 W.; 0111A is from the B1 horizon and 01118 the B2 horizon. 74 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO RALSTON CREEK The Broadway Alluvium along Ralston Creek is a coarse cobbly sand and grave] in the western part of the quadrangle. Here it is about 20 feet above the Piney Creek Alluvium and nearly 30 feet above Ralston Creek. To the east the Broadway becomes much finer grained and its terrace converges with the terraces of the younger Piney Creek Alluvium until the Broadway is only a foot or so higher than the Piney Creek and is covered by a veneer of younger alluvium. Local residents report that prior to construction of the Ralston Reservoir darn the bigger floods would submerge areas above the level of the Broadway terrace. These exceptional floods, and similar ones that would presumably have occurred in Piney Creek time, would account for the veneer of younger alluvium overlying the Broadway Alluvium. In the eastern part of the quadrangle the lower part of the Broadway is a light- yellowish-brown silty sand. This is overlain by an upper part of dark-gray slightly calcareous sandy clay as much as 3 feet thick. The soil developed on the sandy clay has 0.2 foot of very dark gray friable, humic sandy silt. This is underlain by a B horizon consisting of 0.6 foot of very dark gray noncalcareous sandy clay that has clay skins coating the medium to fine angular blocky peds. LEYDEN CREEK A small terrace deposit of moderate-yellowish-brown sandy silt at the town of Leyden is the only Broadway Allu- vium found along Leyden Creek. The soil developed on the deposit is similar to that on other deposits of Broadway Alluvium. Near the crossing of Alkire Street and Leyden Creek a soillike horizon on a terrace deposit superficially resembles soils developed on Broadway Alluvium but lacks the usual soil structure found in the B horizon. It was therefore included with the Piney Creek Alluvium. The Broadway Alluvium found along Leyden Creek is the oldest alluvium in the valley directly attributable to Leyden Creek. The narrow, unterraced, V-shaped valley indicates that most of the cutting in this valley is not very old and probably occurred between Illinoian and Pine- dale time. STANDLEY LAKE The Broadway Alluvium west of Standley Lake is principally yellowish-brown sandy silt or sand. A cobbly sand and gravel unit is present at a few places but appears to be lenticular. The coarse fraction is mainly quartzite from Coal Creek. The deposit forms a terrace about 20 feet above the modern streams and 30 feet below the Louviers Alluvium. The B horizon of the soil on the Broadway is as much as 0.5 foot thicker in this area than elsewhere. Where the B horizon is thick, the Cca horizon is thin or absent. According to Joseph B. Brown of the U.S. Department of Agriculture (oral commun.) the unusual thickness of the B horizon may be due to the lack of calcareous material in the parent material, which is principally quartzite. Because the downward-percolating water is not clogged with calcium carbonate the B horizon of a soil tends to develop more rapidly. SOIL The soil generally found on the Broadway Alluvium is a moderately well developed pedocal, which I believe was formed mainly during two separate soil-forming in- tervals (Van Horn, 1967). The earlier interval followed the deposition of the Broadway Alluvium, forming the post- Broadway Alluvium soil, and the later interval followed the deposition of lower Holocene terrestrial deposits and formed the early Holocene soil. Soils which probably are equivalent to either the post-Broadway or early Holocene soil have been described previously (Van Horn, in Sheridan and others, 1967, p. 56, 57). A similar soil has developed on a fine-grained alluvium in the SW. cor. NW‘ANW‘A sec. 24, T. 2 S., R. 70 W., in the Golden quad- rangle. Here the soil consists of 2 feet of dark-grayish- brown to black clayey to sandy silt that contains thread- like streaks of very light gray calcium carbonate. It is over- lain by 3 feet of colluvium. The relation of this soil and the alluvium to other alluviums is not known. I believe it is either the post-Broadway or the early Holocene soil. Where the upper surface of the Broadway Alluvium has been continuously exposed since the end of deposition, as it has at most places in the Golden quadrangle, it is capped by a dark-gray to dark-grayish-brown, humic, massive to thin platy A horizon about 0.5 foot thick. This overlies a 0.5- to l-foot—thick B horizon, generally with a sharp contact. The B horizon is dark brown, prismatic, noncal- careous, and moderately plastic. The prism faces contain faint to moderately well developed clay skins. Clay skins also are present around pebbles found in this horizon. At places the lower part of the B horizon is slightly to mod- erately calcareous and contains light-gray calcium car- bonate in thin stringers and as coatings along prism and joint faces. The B horizon grades into the underlying Cca horizon which is 0.5—3 feet thick. The Cca horizon does not have a cemented caliche zone such as may occur in the older soils. The calcium carbonate is generally present as coatings along joint faces and on the underside of pebbles or as tiny spots disseminated throughout the deposit. The clay mineralogy, size gradation, and other materials tests results are shown in figures 33 and 48, and in table 12, and in Van Horn (1968). The following section, locality G109, showing a moderately well developed soil on Broadway Alluvium from Van Bibber Creek, was exposed in an excavation for a greenhouse northwest of the corner of 50th Avenue and Indiana Street in the NE‘ASE‘A sec. 13, T. 3 S., R. 70 W. (figs. 32 and 33). QUATERNARY DEPOSITS SECTION G109—Soil on Broadway Alluvium [Measured in an excavation at the northwest corner of 50th Avenue and Indiana Street, Golden, NEltSElt sec. 13, T. 3 s., R. 70 W.] Thickness (It) 1. Spoil. Clayey to pebbly silt. Base nearly level but has minor undulations ......................................................... 2. A horizon and plowed layer. Dark-grayish-brown (2.5Y 4/2), massive to faintly thin, platy, silty sand and a few pebbles. The unit has fair dilatancy, slight plastic— ity, and low dry strength. Base forms sharp level con— tact with unit 3 just below top of ruler in figure 32‘... .5 3. B horizon. Dark—yellowish-brown (10YR 4/4) mottled dark-brown (lOYR 4/3), noncalcareous clayey sandy silt and a few pebbles. The ped faces of the poorly to moderately developed, fine, angular, blocky to prismatic structure contains a moderate accumulation of clay skins in the upper part. The unit has low dila- tancy, moderate plasticity, and high dry strength. Grades into unit 41 ........................................................ .3 4. Bca horizon. Light-olive-gray (5Y 6/2), ranging from dark-yellowish-brown at top to light-gray at base, massive, slightly to moderately calcareous, clayey to silty sand. The unit has fair dilatancy, moderate plas— ticity, and moderate dry strength. The contact with unit 5 is gradational ...................................................... .3 5. Clca horizon. Light-gray (5Y 6/2), massive, strongly cal- careous, silty sand and some pebbles. It contains abundant calcium carbonate mostly disseminated throughout the deposit but concentrated at a few places into thin white streaks and as rinds on the bottoms of a few pebbles. The unit has fair dilatancy, moder- ate plasticity, and low dry strength. It grades into the underlying unitl ............................................................. .4 6. C2ca horizon. Yellowish-brown (lOYR 6/8), moderately to strongly calcareous, massive silty sand and a few pebbles. The unit has good dilatancy, slight plasticity, and low dry strength. It grades into the underlying unit ................................................................................. .5 7. C3 horizon, alluvium. Dark-yellowish-brown (lOYR 4/4), massive, silty, fine- to medium-grained sand and a few pebbles. The alluvium has good dilatancy, slight plasticity, and low dry strength. The base is not exposed1 .......................................................................... .5 0.5 1See table 12 and figures 33 and 48. AGE AND CORRELATION The Broadway Alluvium is considered to be of Pinedale age. The moderately well developed soil formed on the deposit is similar in degree of development to the early Holocene soil of Scott (1962, 1963a), which he said had developed on the pre-Piney Creek alluvium of the tribu- tary valleys in the Kassler and Littleton quadrangles, Colorado. New exposures in these quadrangles resulting from the Plum Creek floods of 1965 have provided evidence that terrace deposits Scott had correlated with the pre-Piney Creek alluvium are probably Broadway Allu- vium and that the moderately well developed soil was formed only on the Broadway Alluvium and not on younger deposits (Van Horn, 1967). The deposit also occupies a stratigraphic position similar to that of the upper part of the deposit which caps the Broadway terrace 75 FIGURE 32.—Soil developed on Broadway Alluvium at locality G109 northwest of 50th Avenue and Indiana Street, Golden, NE‘ASE‘A sec. 13, T. 3 S., R. 70 W. The material above the top of the 20-inch ruler is the A horizon. and which was considered by Hunt (1954, p. 104) to be Pinedale in age. HOLOCENE DEPOSITS PRE-PINEY CREEK ALLUVIUM(?) Deposits of pre-Piney Creek alluvium were not mapped or definitely recognized in the Golden quadrangle although they are present in the adjoining Ralston Buttes quadrangle. At two places west of Standley Lake a black, massive silty clay, about 2 feet thick, underlies Piney Creek Allu- vium. The contact is conformable at both places. In the SWANW‘A sec. 24, T. 2 S., R. 70 W., cobbly sand and gravel underlying the black silty clay yielded a metacarpal bone identified by G. E. Lewis of the U.S. Geological Survey as Bison cf. B. bison (Linnaeus). It is probably of post- Mankato (post-middle Pinedale) age. The black bed, although its origin is not clear, may represent a humified surface horizon formed during a period of slope stability between the pre-Piney Creek alluvium (Scott, 1962, p. 30) and deposition of the Piney Creek Alluvium. If so, the cobbly sand and gravel underlying the black silty clay is probably pre-Piney Creek alluvium. A similar bed was reported (Van Horn, in Sheridan and others, 1967, p. 56—57) in the Ralston Buttes quadrangle. These beds may be equivalent to a very dark gray humic soil horizon, about 2 feet thick, that caps a terrace deposit about 20 feet above stream level near the Kassler quadrangle (Van Horn, 1967). 76 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO SIZE OF PARTICLES, IN MILLIMETRES O O 0.01 100.0 a 8.0 LO 0 I PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN O CLAY SILT SAND H.) GRANULES PEBBLES COBBLES FIGURE 33.—Cumulative curves showing the size distribution of samples from soil horizons developed on Broadway Alluvium at locality G109, NE’ASE‘A sec. 13, T. 3 S., R. 70 W. Units 2, 3, 5, and 7 are described in the accompanying measured section at locality G109. PINEY CREEK ALLUVIUM The Piney Creek Alluvium generally is crudely to mod- erately well-bedded, dark-gray to dark-brown sandy silt or silty sand. The upper part usually contains much humic material. A very weak azonal soil is developed on the Piney Creek at several places. In the western part of the quad- rangle lenses of gravel and sand locally are present in the lower part of the deposit. In the eastern part of the quad- rangle the deposit contains more clay, and tends to stand in nearly vertical banks where gullied. The size distribu- tion curve of a sample of Piney Creek Alluvium adjacent to Leyden Creek is shown in figure 34. The material is a silty clay of low plasticity. The Piney Creek was not recognized in the valley of Clear Creek, although it is present in all the other stream valleys. In the western part of the area the Piney Creek forms a terrace 6—10 feet above the present streams and below the terrace composed of Broadway Alluvium. Farther east- ward along most streams, the two terrace surfaces merge and in the eastern parts of Van Bibber and Leyden Creeks the Piney Creek Alluvium appears to overlie the upper terrace surface of the Broadway Alluvium. In these places the Piney Creek is probably very thin, perhaps only 3—4 feet thick. In the valley of Ralston Creek the Broadway forms a terrace a few feet above the Piney Creek, but the Broadway has a thin layer of younger alluvium over it. This younger alluvium probably represents flood deposits of Piney Creek and possibly post-Piney Creek age. In the vicinity of Indiana Street, probably as a result of a post- Piney Creek flood, Ralston Creek left the valley it had followed in Piney Creek time and cut a short stretch of new channel into the Broadway Alluvium, in which it is now flowing. SOIL The soil developed on the Piney Creek is a very weak azonal soil. At most places the A horizon consists of 0.2—0.6 foot of noncalcareous, slightly platy dark-grayish-brown sandy silt. At a few places this is underlain by as much as 1 foot of slightly calcareous sandy silt that contains some widely disseminated pinhead-size spots and vertically oriented threadlike streaks of a white salt—probably calcium carbonate. No zone of 'clay accumulation was seen. AGE AND CORRELATION The alluvium is correlated with the Piney Creek because of the thin azonal soil developed on the alluvium and because the younger deposits bear no soil. The age is post- Mankato (post-middle Pinedale) as shown by the bison found in the pre-Piney Creek alluvium(?) underlying the Piney Creek west of Standley Lake, and from Holocene fossils found in the Piney Creek in the adjoining Ralston Buttes quadrangle (Van Horn, in Sheridan and others, 1967, p. 54). POST-PINEY CREEK ALLUVIUM Post-Piney Creek alluvium is present in the beds of most of the streams in the area but is very thin. It is QUATERNARY DEPOSITS 77 SIZE OF PARTICLES, IN MILLIMETRES O O 0.01 0.1 1.0 10.0 100.0 .. 8.0 1.0 O l 80— 70— 60'- 50- 30— 20" 10— PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN // o > CLAY SILT W4 WA SAND GRANULES PEBBLES COBBLES FIGURE 34.—Cumulative curve showing the size distribution of a sample of Piney Creek Alluvium from locality 842+00 (Van Horn, 1968) adjacent to Leyden Creek in the NE'ANE‘A sec. 36, T. 2 S., R. 70 W. mapped only along Clear Creek and Tucker Gulch in the southern part of the quadrangle where the deposits are thickest. The size distribution curves of samples collected near the mountain front from the major streams in the vicinity of the Golden quadrangle are shown in figure 35. No soil or soil structure was found on the post-Piney Creek alluvium. Exposures are rare inasmuch as the present streams have cut into this deposit at very few places. CLEAR CREEK The post-Piney Creek alluvium in the western part of the quadrangle is a coarse cobbly sand and gravel. West of the mountain front boulders are common in the stream- bed. Mechanical analysis of a gravel bar in the stream in this area shows that the material is poorly sorted and contains less than 1 percent silt and clay (fig. 35). Within the city limits of Golden the flood plain has been greatly restricted by artificial fill dumped on the alluvium. East of a small dam near the east end of North Table Mountain, Clear Creek has made a shallow cut into the post-Piney Creek alluvium. At the base of the cut is 2—4 feet of cobbly sand and gravel that may be an older alluvium. This is overlain by 1—5 feet of silty sand with no obvious stratification. The silty sand is overlain by 0.2 foot of concrete that appears to have been dumped as waste material from construction of the nearby dam. The concrete is overlain by 0.4 foot of silty sand similar to the material under the concrete. The top of the upper silty sand is level with the adjacent terrain and this sand is assumed to represent postdam flood deposits. No soil or evidence of weathering was found in any of the beds. East of North Table Mountain exposures in several sand and gravel pits and gravel prospect pits show 3—4 feet of fine to coarse alluvium overlying a thick cobbly sand and gravel. The cobbly sand and gravel is presumed to be Louviers Alluvium. The overlying fine to coarse allu- vium ranges from black or dark-brownish-gray clayey to sandy silt, near the edge of the valley, to a brown silty cobbly sand and gravel, near the middle of the valley. No soil was found on this material. Near the east boundary of the quadrangle are several 1- to 2-foot-high, northeast- trending terracelike scarps. The dark-brown, pebbly to silty sand exposed on the surfaces adjacent to these scarps has no soil developed on it. TUCKER GULCH The only other significant deposit of post-Piney Creek alluvium is in Tucker Gulch. Here the alluvium is coarse cobbly to bouldery sand and gravel (fig. 35). Many of the fragments are subangular. The not-uncommon historic torrential floods have deposited an unknown thickness of this material in the steeply sloping narrow valley down- stream from the Front Range. At a few places upstream from the mountain front the post-Piney Creek has been entirely removed by erosion and the stream has cut down to bedrock (Van Horn, in Sheridan and others, 1967, p. 56, pl. 1). Near the mouth of Golden Gate Canyon a cut in this alluvium shows 5 feet of cobbly sand and gravel. 78 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO SIZE OF PARTICLES, IN MILLIMETRES 0.00 0.01 0.1 1.0 10.0 100.0 E 100 / O / E, 90— z — LIJ '3 U3 80— — Z R44—Unified 501 <( E 70— — u: 5 _, 60- — < E "’ 50— _ U) LU d ” _ o _ i: 40 /0 EC 03’ s 3. / LL ,1. z /: R40—Unified soil classificatior GW 0 6A’ . (\C’) 5 20 _ R43—Unified soil classification GW // WW _ < 2” \659 i— /’ r,o'\ c E 10— I m _ e ,0» LIJ 9“ n. 0 A H.) CLAY SILT SAND GRANULES PEBBLES COBBLES FIGURE 35.—Cumulative curves showing the size distribution of samples of post-Piney Creek alluvium. Sample R40 is from Ralston Creek, NW‘ASE‘A sec. 31, T. 2 S., R. 70 W. (Ralston Buttes quadrangle); R41 from Van Bibber Creek in the NE‘ASE‘A sec. 7, T. 3 S., R. 70 W. (Ralston Buttes quadrangle); R42 from Tucker OTHER CREEKS The post-Piney Creek alluvium is poorly exposed in the other creeks of the Golden quadrangle. Where exposed it generally is gray to brown crudely bedded sand and sandy silt that contains pebbles and cobbles at a few places. At most places the alluvium is in the bottoms of valleys cut into the Piney Creek Alluvium, although at a few places there is a veneer of overbank flood deposits of post-Piney Creek alluvium overlying the terrace of Piney Creek Allu- vium. West of Ralston Reservoir the post-Piney Creek alluvium in Ralston Creek is coarse sand and gravel. (See fig. 35.) A stream cut in the NW‘ANW‘A sec. 25, T. 2 S., R. 70 W. reveals 2 feet of crudely bedded post-Piney Creek allu- vium unconformably overlying an older fine-grained allu— vium that is presumed to be Piney Creek. A bone found 0.4 foot below the top of the post-Piney Creek alluvium was identified by G. E. Lewis of the US. Geological Survey as probably 303 tauras, the domestic cow. This would indicate that the upper part of the section was deposited after 1850. Additional evidence of the age of post-Piney Creek allu- vium was found on the south bank of Van Bibber Creek where it crosses the hogback of the Dakota Group. Here a scattering of chips and broken points showed the presence of Indian occupation of this bank that is about 6 feet above Gulch in the NW‘ASE‘A sec. 20, T. 3 S., R. 70 W. (Golden quad- rangle); R43 from Clear Creek in the NE‘ANE‘A sec. 32, T. 3 S., R. 70 W. (Golden quadrangle); R44 from Mount Vernon Creek in the NW‘ASEl/i sec. 17, T. 4 S., R. 70 W. (Morrison quadrangle). the creek bottom. A prominent dark-gray silty sand layer about 2 feet below the top of the bank contained charcoal. A ISO-pound sample from this layer yielded 10.2 grams of charcoal, which was dated at 1,0501200 years before present (Rubin and Alexander, 1960, p 156, sample W—616). The coarse fraction of the same sample contained a few dozen rodent pellets, some of which appeared charred, about 10 pounds of angular to subangular rock fragments as much as one-half inch in diameter, several bone frag- ments, a few flakes of chert and one of obsidian, and one side-notched point that is 2.05 cm long and 1.1 cm wide just forward of the notch. Mr. Arnold Withers of the University of Denver identified the point as a side-notched point possibly of a Late Woodland culture. (The point is in the Denver University collection under accession No. K—4—l 1.) The deposit was originally believed to be Piney Creek Alluvium but the radiocarbon date, the side- notched point, and the lack of a soil on the deposit all indicate that the deposit is post-Piney Creek. Because of its small areal extent and thinness, however, the deposit is included in the Piney Creek on the geologic map. In 1966 the site was studied by Nelson (1969), who concluded that there are three separate cultural layers at the site that range in age from 800 to 2,140 i145 years be- fore present. PLEISTOCENE AND HOLOCENE DEPOSITS The foregoing bits of information seem to indicate that post-Piney Creek alluviation started prior to the year 200 BC. and that a period of erosion started some time after this. A second, and perhaps equally plausible, interpre- tation is that this area has been undergoing erosion since the end of Piney Creek alluviation and that the thin, rela- tively minor amounts of post-Piney Creek alluvium found in this area are only the overbank flood deposits of the present streams. Evidence supporting this interpretation was seen south of Denver where the 1965 flood of Plum Creek deposited as much as 1 foot of nearly continuous sand on the thin humic A horizon of the terrace 8 feet above the flood plain, and a thin discontinuous layer of sand on the thick humic A horizon of the terrace 25 feet above stream level. A list of 19 floods that occurred in the vicinity of the Golden quadrangle during 26 of the years between 1878 and 1914—an average of almost one every year—has been made in this report. (See p. 108.) PLEISTOCENE AND HOLOCENE DEPOSITS Several different kinds of deposits of Pleistocene, Holo- cene, or undifferentiated Pleistocene and Holocene age shown on geologic map (Van Horn, 1972) consist of alluvial fan deposits, transported mantle, colluvium, loess, artificial fill, and landslides. Some of these deposits grade into each other and into the various alluvial deposits described earlier; others have sharp boundaries. Only broad age distinctions have been made within these deposits, although finer subdivisions are possible at some places. ALLUVIAL FAN DEPOSITS Alluvial fan deposits are a heterogeneous mixture of particles ranging from boulders to clay. In plan view the deposits generally have the shape of a partially opened fan, with the narrow, or handle, end pointing upstream and the wide, lobate end downstream. They are stream deposits brought about by a sudden lessening of the carrying power of a stream. This is generally caused by a flattening of the gradient of a stream or by a stream in a narrow valley issuing onto a wide flat plain, or both. The lithology of the deposits reflects the bedrock in the valley from which they originate. Two ages of alluvial fans are distinguished on the geologic map (Van Horn, 1972); geomorphic relations of deposits was the main criterion used in establishing the relative age, and the degree of soil development was used to a much lesser degree. The old alluvial fans are of Bull Lake age or older, whereas the young alluvial fans are of Pine- dale age or younger. OLD ALLUVIAL FAN DEPOSITS The old alluvial fans that debouch from the mountains are poorly sorted bouldery sand and gravel near the moun- tain front but rapidly grade to finer grained sand and 79 gravel away from the mountains. Beds of clayey to sandy silt are present near the terminal end of some deposits. The old fan in the south part of Golden was deposited on a very irregular surface. Near Washington Avenue beds of brown sandy silt as much as 5 feet thick overlie clean sand. (See sample G99, fig. 36.) Other exposures in this same area show subangular silty to cobbly sand and gravel, containing boulders as much as 2.5 feet in diameter, resting on the Denver Formation. At some places many of the cobbles and small boulders in the upper part of the deposit are rotten. Although no well-developed soil was found on the deposit, a calcium carbonate cemented zone as much as 3 feet thick locally is present in the top part. At a few places this zone has been eroded and younger allu- vium or colluvium has been deposited in channels cut in the old fan. Other old fan deposits are similar but exposures are gen- erally poor. An excellent exposure of the terminal end of a fan is exposed in the gravel pits just north of the Louviers Alluvium along 8th Street in Golden (NW‘ASE‘A sec. 28, T. 3 S., R. 70 W.). Here the fan deposit has been truncated by erosion before deposition of the Louviers. The upper surface of the fan is at the same height as the adjacent Slocum Alluvium and is probably of the same age. The old fan deposit is composed of material from the small canyon north of Clear Creek and contains none of the light- colored granite so typical of Clear Creek. The deposit is poorly sorted cobbly sand and gravel (G95, fig. 36) that is horizontally and evenly, though somewhat crudely, bedded. The bedding gives the impression of a terrace alluvium constructed by Clear Creek but the shape and lithology indicate alluvial fan deposition. At this place the deposit probably resulted from a mixing of the two pro- cesses. A few hundred feet uphill from the north edge of this fan a thin deposit of cobbly sand and gravel from Clear Creek is overlain by Pearlette-like volcanic ash. This material no doubt was once part of an extensive alluvial fill of Verdos Alluvium that was mostly eroded away before the old fan alluvium was deposited. A small remnant of a still older fan is preserved on the small knoll one-half mile north of Clear Creek and just east of the Front Range. The older fan probably was graded to the level of the Verdos Alluvium. Both fan deposits are as much as 20 feet thick. The large old alluvial fan near Van Bibber Creek north- west of North Table Mountain may be intermediate between a fan and a terrace deposit. This fan deposit is poorly exposed, but it appears to be composed of silty to bouldery sand and gravel, coarser at the west end. The soil that formed on it has been removed by erosion except for a strong accumulation of calcium carbonate. This deposit probably resulted when Van Bibber Creek left the rela- tively confined valley west of the Dakota Group and debouched into the wide valley east of the Dakota. It is tentatively correlated with the Louviers Alluvium because 80 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO SIZE OF PARTICLES, IN MILLIMETRES 0.00 0.01 0.1 _| O O 1.0 '10.0 (D O 80 '70 60— 50— 40 30 2O 10 PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN J CLAY SILT SAND GRANULES PEBBLES COBBLES FIGURE 36.—Cumulative curves showing the size distribution of samples of old alluvial fan deposits. Sample G99 is from the NW‘ANW'A sec. 34, and sample G95 is from the NW‘ASE‘A sec. 28, T. 3 S., R. 70 W. it is about the same height above stream level as the Louviers terrace to the west and because the oldest allu- vium that truncates the fan is probably the Broadway Allu- vium. The old fans north of Ralston Reservoir appear to merge and interfinger with the Verdos Alluvium. At a few places the old fans are cut by small channels filled with younger material. The old fan alluvium contains much silt and calcium carbonate where exposed east of the South Boul- der Diversion Canal (fig. 26). YOUNG ALLUVIAL FAN DEPOSITS Included as young alluvial fan deposits are all the fan deposits that are younger than Bull Lake. The fans west of the longitude of Golden are a silty to cobbly sand and gravel, whereas the fans east of Golden appear to have more silt- and clay-sized material and fewer cobbles. The upper few feet of most young fans is clayey silt grading downward into a coarser material. At places the bottoms of cobbles and pebbles in the upper part of the coarser material have a thin calcium carbonate rind, probably part of the Cca horizon of a soil. No well-preserved recogniz- able soil was found on any of the young fan deposits. Their age was determined by their relation to terrace deposits they truncate or overlie. Most of these young fans prob- ably were formed before deposition of the Piney Creek Alluvium, but sediment is still being deposited on many of them. The young fan formed at the mouth of Tucker Gulch near its junction with Clear Creek is intermediate between an alluvial fan and a terrace deposit. It has the shape of an alluvial fan and is composed of poorly sorted sand and gravel that contains many subangular cobbles and boul- ders. The material is principally from Tucker Gulch although some is from Clear Creek. It resembles a terrace deposit in that the upper surface is gently sloping and bulges only slightly above the upper surface of the Louviers Alluvium deposited by Clear Creek. The fan deposit probably resulted from the loss of carrying power where Tucker Creek encountered the flatter gradient of Clear Creek. Several historic floods have added material to the upper part of the deposit. The lower part of the deposit probably is equivalent to the old alluvial fan deposit. TRANSPORTED MANTLE Transported mantle, as the name implies, is surficial material that has been moved and deposited by a com- bination of processes. It seems to be partly alluvium and partly colluvium. The deposits consist of a mixture of boulder- to clay-sized material that underlies planar surfaces that slope more steeply than stream terraces (pl. 1). It ranges from a clayey to cobbly sand and gravel to pebbly sandy silt. At many places the deposits become coarser grained downslope. The upper few feet of the transported mantle commonly has abundant calcium car- bonate and may contain some reddish-brown to brown PLEISTOCENE AND HOLOCEN E DEPOSITS SIZE OF PARTICLES, IN MILLIMETRES 0.00 100 90 80 70 60 50 40 30 20 / / PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN CLAY SILT FIGURE 37,—Cumulative curves showing the size distribution of samples of transported mantle. Sample 705+00 is from the NE‘ASW‘A sec. 23, T. 2 S., R. 70 W.; 775+00 from SW‘ANE‘A sec. 25, T. 2 5., R. 70 W.; 850+00 from SE‘ANE‘A sec. 36, T. 2 S., clay. At a few places the lower part is made up of clean cobbly sand and gravel. The gravel clasts are well rounded and predominantly of Precambrian rocks derived from the Front Range. In general it is coarser and less plastic than colluvium and finer and more plastic than alluvium (fig. 37). The transported mantle is 0—15 feet thick. It is equiv- alent to similar deposits mapped as residuum by Hunt ”(1954, p. 101) and Van Horn (1968) and as undif- ferentiated upland deposits by Malde (1955, p. 233). Scott (1962, 1963a) has included similar deposits in the Little- ton and Kassler quadrangles with the next lowest terrace or pediment alluvium. Wells (1967) may have mapped some of these deposits as colluvium. Transported mantle deposits generally head at an iso- lated hill, although some form slopes between alluvial terraces or between a terrace and the adjacent flood plain. The downstream ends of transported mantle deposits gen- erally tend to merge with topographically lower (younger) alluvial terraces. But some of the transported mantle surfaces, if projected upstream from the headward end, will rise above much older terrace surfaces. The trans- ported mantle on the ridge north of Leyden is an example (p1. 1A): it starts below an obvious break in slope at the eastern end of the Rocky Flats Alluvium and maintains an even slope eastward to the narrow remnant of the Verdos Alluvium east of Indiana Street (State Highway 72). This v ’RI—A v ILv" SAND‘ GRANULES PEBBLES COBBLES R. 70 W.; 980+00 from NW‘ANW‘A sec. 8, T. 3 5., R. 69 W.; G114 from NW‘ANE‘A sec. 6, T. 3 S., R. 69 W.; G157 from NW‘ASW‘A sec. 9, T. 3 S., R. 69 W.; 085 from NW‘ANE‘A sec. 16, T. 3 5., R. 69 W. (Arvada quadrangle). segment of transported mantle probably was formed and deposited mainly by Ralston Creek during an erosional period between deposition of the Rocky Flats and Verdos Alluviums. Later, after deposition of the Verdos Alluvium by Ralston Creek, another deposit of transported mantle lying east of Indiana Street and north of the narrow ridge of Verdos, was formed on a surface cut by Ralston Creek. The two transported mantle deposits merge and the boundary between them is indistinguishable. Thus, this seemingly continuous deposit of transported mantle is actually two deposits of different ages. Similar deposits of transported mantle southeast of Leyden are graded to the Louviers Alluvium and are probably pre-Bull Lake in age (pl. 1A). The large deposit of transported mantle south of Van Bibber Creek near the east boundary of the quadrangle has been truncated by the Louviers Alluvium deposited by Clear Creek (pl. 1C). It may be pre-Slocum in age. The uphill part, forming the present divide between Clear and Van Bibber Creeks, is very close to the altitude expectable for the Verdos Allu- vium from which the transported mantle probably was derived. Sample 685 (fig. 37) was collected from this deposit just east of the quadrangle. The large body of transported mantle in secs. 7, 8, and 9 just north of Van Bibber Creek at the east boundary of the quadrangle appears to be graded to the level of the Piney 82 Creek Alluvium at the low end of the deposit, and to a level above the Louviers Alluvium from Ralston Creek at the high end (pl. 13). Exposures 30 feet above the Piney Creek terrace of Van Bibber Creek show 1 foot of strongly calcium-carbonate—impregnated silty to clayey sand, over- lain abruptly by 0.6 foot of clayey material that has pris- matic structure and poorly developed clay skins. Farther uphill a similar sequence shows 2.5 feet of dark-brown, slightly plastic, clayey to pebbly coarse sand (locality G157, fig. 37) overlying a strongly calcium-carbonate-im- pregnated silty sand and gravel. The upper part of the coarse sand has some clay skins developed around the pebbles, and the lower part has sparse %—1 inch spots of calcium carbonate. The underlying strongly calcium-car- bonate-impregnated zone in both of these exposures is probably the Cca horizon of a soil developed on trans- ported mantle. The soil is the same age as, or older than, the soil developed on the Louviers Alluvium. The over- lying material is probably colluvium bearing a soil equiv- alent to the soil formed on the Broadway Alluvium. Both deposits probably extend under the Piney Creek Allu- vium in Van Bibber Creek. If so, the eastern part of Van Bibber Creek valley must have been excavated to about its present depth before Pinedale time at the latest. This deep— ening may have been related to the erosion that removed the Louviers Alluvium of Ralston Creek east of Hyatt Lake. Transported mantle deposits are overlain by a thin cover of colluvium or, at a few places, by loess. They are generally underlain by bedrock. The following section describes the soil horizons at locality G55. SEcrION G55.—Soil on transported mantle deposit [Measured on east side of a roadcut in the SWléNWK sec. 29, T. 2 S., R. 69 W] Thickness 1. Al horizon. Very dark brown, weakly calcareous, very (/2) finely prismatic, sandy silt; friable, contains many roots ................................................................................ 0.2 2. A3 horizon. Very dark gray brown, noncalcareous, very finely prismatic, sandy silt; friable, contains many roots ................................................................................ .4 3. B horizon. Dark-brown, noncalcareous, medium angular blocky, sandy to silty clay. Clay skins are present on vertical faces. Grades into unit 4 ................................... 4. Cca horizon. Dark'brown, moderately calcareous, mas- sive, sandy silt. Contains lenses and streaks of light- gray calcium carbonate. The material on which this soil formed is probably loess. This unit truncates the next two underlying units ............................................. .9 5. IIBb horizon. Yellowish-red, moderately calcareous to noncalcareous, coarse angular blocky, clayey sand. Clay skins are present on vertical faces. Moderately calcareous in areas where thin streaks of calcium car- bonate extend downward from the overlying unit, non- calcareous where these streaks are not present. Grades 1.2 into underlying unit ...................................................... 1.3 6. IlClca horizon. Pinkish-white, strongly calcareous, massive, cobbly to sandy silt ......................................... 1.3 7. IIC2ca horizon. Grayish-brown, strongly calcareous, massive, cobbly sand and gravel. Base covered ............. 2.0 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO Units 1—4 of this section probably are loess and they extend across a bedrock ridge. Units 6 and 7, consisting of transported mantle, lie in a channel cut into the bedrock. After the transported mantle was deposited a soil formed on it. This soil was truncated and loess was deposited on the truncated surface. A later soil formed on the loess. The older soil is probably pre-Bull Lake in age and the younger soil is probably equivalent to the soil formed on the Broad- way Alluvium. Unit 5 is a B horizon formed on the trans- ported mantle. AGE The soil formed on transported mantle is mostly eroded. Generally only the remnants of a Cca horizon are found although at some places the reddish-brown or brown clay in the upper part may represent a Bca soil horizon. This soil is probably no younger than the soil formed on the Louviers Alluvium. No younger soils were found on trans- ported mantle. The transported mantle deposits were formed at dif- ferent times prior to Pinedale time. The relation of some deposits to the terrace gravels both upslope and down- slope indicates the age of the deposit. ORIGIN The origin of the transported mantle is complex but streams and slopewash seem to be the two major processes involved. I visualize one way the deposits may have formed as follows. The broad flood plains of the Pleistocene streams (fig. 38A) were eroded between major periods of alluviation. This erosion formed steeply sloping valley sides adjacent to the newly eroded valleys (fig. 383). As the flood plains were gradually incised they were left as terraces. Downstream the meanders of the stream were wider, perhaps controlled by gently dipping bedrock, and by erosion-planed slopes that were not as steep as the terrace scarps but were steeper than the old flood plain. The resulting surface sloped less steeply than the sides of the incised valley upstream, but more steeply than the original terrace (or old flood plain) surface. The streams, though dominantly eroding, left a thin deposit of allu- vium, mostly reworked from older terraces and collu- vium, on the eroded bedrock surfaces. Colluvium that accumulated on the slopes was reworked by rill wash and by the streams. Because the transported mantle deposit was thin and the slopes were moderately steep, some fine- grained material from the underlying bedrock was also incorporated into the deposit. COLLUVIUM Colluvium consists of material that has been moved down steep slopes by creep and sheet wash and, at a few places, a few minor alluvial deposits formed by short, steep-gradient streams that flow only during and shortly after rainstorms. The colluvial deposits grade into, and interfinger with, alluvial terrace deposits and transported PLEISTOCENE AND HOLOCENEDEPost 83 Old terrace Old terrace Old terrace llKllllllllKl\l (“Will l\\l\ll\\l[l\\l\l\ \\\\\\\\\\\\\\\ \\ \\ \ \ \\\\\\\\\\\\\\\ lll/llllllllll‘ I\I\I |\\ll\l\\\\\|\l\ \I)\I\\lllll))/)ll Flood plain New terrace New terrace 8,, \ (UK (M \l\kl\ll ((\t((l(ll 9s Wlllllxn \ \\\\ \\\\ ” llwlllll l)l\ l l’\)l(\i l\ll ll )/ l)\\> Floodplain )U H)“ (fl! /(///)/ //_/ New terrace A4 New terrace : ’“J .Transported , A A’ B B’ C C 7-? Imantlef, r‘// "l// ) Hill/1') I/l) / Ill/l I ) I’//// / ////,/ //////////l/ // W/I//// / /////l /)////// //l////)//// l///////,’// // l/l/ (W Will/HM “HM (/lr/HHI/l “HUM/(HKU/I Old terrace Old terrace Old terrace A Flood plain A' B Newterrace Flood plain B’ C Newterrace Transported Flood W FIGURE 38.—The development of transported mantle surfaces by s tream erosion: A, the original flood plain; B, and intermediate stage after incision of the flood plain and formation of terrace remnants, a narrow upstream valley, and a wide downstream valley; C, the transported-mantle surface. mantle. The material composing the colluvium is derived from both bedrock and surficial deposits. Colluvium is mostly a massive to crudely bedded clayey to sandy silt but locally either the sand or the clay can predominate. At places a predominantly silty upper part grades downward into a predominantly sandy lower part. Colluvial deposits on the flanks of the mountains and hogbacks locally contain boulders (fig. 51). Slopes below alluvial terraces may have an appreciable percentage of rounded cobbles and pebbles. The size distribution curves for several samples of colluvium are shown in figure 39. Colluvial deposits generally overlie very irregularly sloping bedrock surfaces. At many places the bedrock projects upward nearly through the colluvium. The thickness of the colluvium ranges from a few inches to 20 feet; on gentle slopes it is generally less than 10 feet. Large changes in thickness can be found within a few hundred feet; thus, predictions about the depth of colluvium are unreliable, and probably some colluvium that is shown on the map is less than 3 feet thick. The deposition and history of colluvium and land- slides, particularly on the Table Mountains, are in- timately related. All the landslides mapped have moved some colluvium, and some of the older landslides may have been covered by deposits of younger colluvium. Where the topography indicates a former landslide, the deposit has been mapped as a landslide regardless of the amount of postlandslide colluvial cover. Such a colluvial deposit was temporarily exposed in a pipeline trench that traversed the upper part of an old landslide on the north side of South Table Mountain in sec. 36, T. 3 S., R. 70 W. The trench, about 6 feet deep, is 100 feet west of the power- line shown on the map (Van Horn, 1972). The deposit exposed in the trench from the Welch Ditch to the base of the lava flow capping the mountain consists of moderate- yellowish-brown clayey silt of colluvial origin. The uphill end of the deposit has many subangular cobbles and boul- ders of latite which decrease in number toward the down- hill end, where there are only a few. About a third of the way up the trench is a one-quarter-inch-thick reddish- brown nearly horizontal layer with several discontinuous nearly horizontal fractures; the contact with the overlying material is a smooth slickensided surface, possibly a slip plane. This surface may be the bottom of the landslide, in which case a similar (but steeply dipping) slip plane would be expected in the upper part of the trench, at the uphill end of the landslide. No such slip plane was found in the upper part of the trench, but it may have been covered by postlandslide colluvium to a depth greater than the depth of the trench. At many places the Piney Creek Alluvium merges head- ward into a colluvial slope. The western part of Leyden 84 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO SIZE OF PARTICLES, IN MILLIMETRES 0.00 100.0 _‘ O O (D O I so— 50— 4o— \8 \ y \/ 30 — \6 20— PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN —/ Y CLAY SILT SAND GRANULES PEBBLES COBBLES FIGURE 39.——Cumulative curves showing the size distribution of samples of colluvium. Sample 82l+00 is from the SE‘ASE'A sec. 25, 830+00 from NE‘ANE‘A sec. 36, and 860+00 from NE‘ASE‘A sec. 36, all from T. 2 S., R. 70 W.; sample 890+00 is from NE‘ANE‘A sec. 1, T. 3 S., R. 70 W.; sample 960+00 is from NW‘ANE‘A sec. 7; 1020+00 from SE‘ASE‘A sec. 5, G161 from NE‘ASW‘A sec. 7, all from T. 3 S., R 69 W. ' Gulch has many such examples, indicating colluvial activity during alluviation. Soils that form on colluvial deposits .are in general poorly developed. They tend to be destroyed even as they are formed by the slow downward movement and by deposition of material in the upper part of the deposit. No well-developed soil was found on the colluvial deposits although at several places there are weathered zones be- lieved to be comparable with the soil formed on the Broad- way Alluvium. Locally thick accumulations of calcium carbonate indicate that parts of the deposits are probably pre-Bull Lake in age. LOESS Loess—wind-deposited silt and sandy silt—forms a widespread, but generally thin, layer over much of the quadrangle. At places it overlies alluvial deposits ranging in age from Nebraskan to Bull Lake. Loess shown on the geologic map is at least 3 feet thick. Thinner loess occurs west to the mountain front, but probably no farther. It is as much as 2% feet thick on the transported mantle east of Rocky Flats (see measured section G55, p. 82) and 1% feet thick on the Verdos Alluvium east of Ralston Reservoir. The loess that overlies the Verdos Alluvium in the NW‘ANW‘A sec. 33, T. 2 S., R. 69 W. is typical of most loess in the area. It is a pale-brown inorganic sandy silt. Bedding is not visible except at a few places near the base of ' the deposit. Vertical jointing is present at most exposures but is not always obvious. At many places a few pebbles are widely scattered through the deposits. The loess generally is less than 10 feet thick. At locality G173 (fig. 40), where the loess is 3 feet thick, the bottom 2 feet was sampled for laboratory tests. Possibly some alluviated clay from the soil, equivalent to the soil formed on the Broadway Alluvium, was included in the sample. The grain size distribution of both this sample and a sample from modified loess on the side of the valley (10c. G172, fig. 40) is similar to that of sandy loess in central Nebraska and to that of some of the colluvium in the Golden quadrangle. The thickest loess is in the northeast part of the quad- rangle in the small valley in the 5% sec. 33, T. 2 S., R. 69 W. Here the loess probably was partly reworked into modified loess. The material is a pale-brown inorganic sandy silt to silty sand. Crude horizontal bedding is visible on a few wind-etched surfaces on the outcrop. No vertical jointing is apparent. The deposit is coarser, better sorted, and much thicker than the loess overlying the Verdos Alluvium. In the railroad cut just east of the quadrangle it was more than 25 feet thick (sample G172, fig. 40). This material was probably reworked by slope wash during deposition and is, therefore, a modified loess; the modification by slope wash probably accounts for the crude bedding. The coarseness indicates that it must have been derived from a PLEISTOCENE AND HOLOCENE DEPOSITS 85 SIZE OF PARTICLES, IN MILLIMETFIES 0.00 0.01 0.1 1.0 10.0 100.0 _| O 0 £0 0 I 80- 70‘— / ,/ 60—: 40— 10/ ’/ PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN ./ O CLAY SILT SAND GRANULES PEBBLES COBBLES FIGURE 40.—Cumulative curves showing the size distribution of samples of loess. Sample G172 is from the SE‘ASW‘A sec. 33 (Arvada quad- rangle), G178 from NW‘ANW‘A sec. 33, and G144 from NE‘ANW‘A sec. 33 (Arvada quadrangle), T. 2 S., R. 69 W. nearby source, possibly from alluvium along Leyden and Ralston Creeks. The youngest alluvium on which loess was definitely recognized is the Louviers. A loesslike deposit, too small to be shown on the map, is included in the colluvium south of, and slightly higher than, the small patch of Broadway Alluvium one-half mile northwest of Upper Twin Lake. The loesslike deposit is of sandy silt and is 1.5 feet thick. SOIL The soil developed on the loess is comparable to the soil formed on the Broadway Alluvium. It consists of the ubiquitous 0.5 foot of A horizon, about 1 foot of dark- brown, noncalcareous, blocky clayey B horizon, and as much as 1.5 feet of calcareous Cca horizon with the calcium carbonate present as small pods and streaks along joints (measured section G55, units 1—4, p. 82). At places where the loess is thin the Cca horizon extends down into the top of underlying older deposits or soil horizons. No deposits were found overlying the loess. Where loess overlies Louviers Alluvium, in some places separate soils seem to be developed on the silt and on the alluvium. At most such places the silt has a soil similar to that on the Broadway Alluvium with a moderately cal- careous Cca horizon. This soil is underlain by a pebbly to cobbly gravel with a strong caliche zone in the top. The change is abrupt and at places is marked by a layer of cobbles or by channels cut into the caliche and filled with uncemented material. Such a change occurs at the locality (G62) photographed in figure 41 and is described in the following measured section. SEcrION G62.—Soil on loess deposit [Measured in a south-trending gully (shown in fig. 41) between the stream and the un- improved road in the SE‘ASEM sec. 13, T. 2 S., R. 70 W.] Thickness (I!) l. A horizon; dark-grayish-brown (lOYR 4/2), noncalcar- eous, thin platy, sandy silt. Grades into unit 2 ............ 2. B horizon; dark-brown (lOYR 3/3), noncalcareous, med- ium subangular blocky, clayey to sandy silt. Grades into unit 3 ...................................................................... .8 3. Cca horizon; yellowish-brown (lOYR 5/4), moderately calcareous at top to strongly calcareous at base, mas- sive, sandy silt. Calcium carbonate is in scattered lenses, 2—5 mm long, at top but becomes disseminated toward base. Abrupt unconformable contact with unit 4 ......... 1.3 4. IIClca horizon; pinkish-white (5YR 8/2), very strongly calcareous, massive, silty to cobbly sand and gravel. Calcium carbonate forms moderately cemented caliche at places. The pebbles and cobbles generally have a l-mm-thick rind of calcium carbonate. Below and to right of the pick shown in figure 41 a narrow vertical walled channel, filled with cobbly sand and gravel, is cut into the upper part. The pebbles and cobbles in the channel are coated with calcium carbonate but are 0.3 not cemented .................................................................. LB 5. IIC2ca horizon; yellowish-brown (lOYR 5/4), calcareous massive, silty sand. Calcium carbonate forms numerous hard nodules, 2—5 mm in diameter Base covered .......... 1.0 86 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO FIGURE 4l.—L0ess with a moderately well developed soil unconformably overlying the Cca soil horizon developed on Louviers Alluvium in the SE‘ASE‘A sec. 13, T. 2 S., R. 70 W. The numbers refer to the unit numbers on the accompanying measured section at locality G62. This sequence of deposits probably contains two sep- arate soil profiles. Unit 1 is a humic A horizon of H010— cene age. Units 2 and 3 are the B and Cca horizons Of a soil similar to that formed on the Broadway Alluvium. They are developed on silt that is probably loess. There is a somewhat greater than normal amount of calcium car- bonate in unit 3 for the soil developed on the Broadway Alluvium, but not excessively so. This unit is only 1.3 feet thick, the upper part of which is low in calcium car- bonate. The concentration of calcium carbonate may be due tO deposition in a thinner than usual zone and also because there was more free calcium carbonate available in the parent material of the loess which consisted partly of the underlying older soil. The undulating top of unit 4 and sand- and gravel-filled channels cut into it represent an erosional unconformity. Malde (1955, fig. 54 and p. 236) indicated that units 4 and 5 are pre-Bull Lake (pre— Wisconsin) soils, but I interpret them as the Cca horizon of a moderate or strong soil of indeterminate age; judging from their topographic position I believe them to be the post-Bull Lake soil developed on Louviers Alluvium. AGE AND CORRELATION A Pinedale age for the loess in the Golden quadrangle is indicated by its position overlying strong calcium car- bonate accumulations on terraces no younger than Louviers Alluvium, and by the soil developed on the loess, which is comparable to the soil on the Broadway Allu- vium. Although some of the loess may be older than Pine- dale no strongly developed soil was found on or within the loess. The loess is probably correlative with the eolian silt and sand Of Malde (1955, p. 240), the eolian deposits of Hunt (1954, p. 108), and the younger loess of Scott (1962, 1963a), although all three believed the loess to be of Bull Lake age. The soils on the loess described by Malde and by PLEISTOCENE AND HOLOCENE DEPOSITS 87 Hunt are similar to the soil on the loess in the Golden quadrangle. LANDSLIDES Landslides are most common on the flanks of North and South Table Mountains but also are present on many other steep slopes in the quadrangle. Bedrock formations involved in landsliding are Pierre Shale, Laramie F orma- tion, Arapahoe Formation, Denver Formation, and rocks of Precambrian age. Colluvium is the principal surficial deposit affected by landslides. although the Verdos Allu- vium is overridden by one slide; flood-plain alluvium and presumably the underlying Louviers(?) Alluvium are reported to have been involved in another (Van Horn, 1954, p. 15). Most landslides in this area are of the slump type in which the upper surface of the sliding block rotates backward relative to the direction of movement. At places the rotation is not obvious and the slides appear to be slow debris slides. Rockfall is another common landslide type, but no rockfall deposits have been mapped. A small in- cipient block-glide landslide is forming in Precambrian rocks west of Golden. The summary of historical geologic events (p. 108) is an annotated list of landslides, floods, and other geologic phenomena of the Golden area that have been reported in the Golden Globe newspaper for 26 of the years between 1878 and 1914. These records show that 25 landslides, seven of them rockfalls and 15 other types of slides, had been reported. It can safely be assumed that there were other landslides that were not reported, because none of the five landslides that were active during the first half of 1968 were reported in the local newspapers. If only half the active landslides have been reported in the Golden Globe, it would indicate that there are at least two active land- slides per year. The landslides shown on the geologic map (Van Horn, 1972) are differentiated only by the quality of the evidence for movement, as follows: (1) All slides for which I have seen compelling evidence of movement, including plainly visible bounding faults and scarps, dislocation of struc- tures, and younger material overridden by older material are shown with a triangular pattern. (2) The large masses that have been inferred, by interpretation of their topog- raphy, to be landslides, are shown without a pattern. The evidence for mapping the landslides shown in the Golden quadrangle (Van Horn, 1972) consists of: dis- cordant, short, terracelike landforms that lack terrace gravels; active slides reported by residents of the area and in old newspaper accounts; and the presence of one outcrop showing a slide plane under an old slide. The topography of most slides consists of an abrupt flattening of a normally steep slope at the head of the landslide. Where erosion has not completely obliterated it, a nearly vertical scarp is visible just above the landslide. At many places the head of the slide has a slight slope backward toward the main hillside, caused by rotation of the land- slide block. On large slides or at a common junction of several smaller slides a small undrained depression exists at the head. Below the head, the body of the slide is a steeply sloping, hummocky, irregular surface. This surface was probably caused by what originally were minor scarps, or perhaps separate smaller slides. Many of the large slide areas appear to be a composite of many small, but now indistinct slides (fig. 21). Acursory inspec- tion of the sides of North and South Table Mountains gives the impression of a series of terracelike levels. Close examination of these levels, however, reveals that they are at many different altitudes and that they slope toward the mountainside at many places, and no rounded stream sand and gravel nor isolated pebbles were found on them. The toes of old slides are generally obscure. The toes of active slides behave in various ways. At places the toes form nearly vertically rising transverse ridges as much as 10 feet above adjoining undisturbed surfaces. (See fig. 44.) The material exposed in the scarps formed by the rising toe rapidly ravels to form flatter slopes, which may be covered by vegetation within a few years if no new movement takes place. At one place a transverse ridge of this type is reported to have formed in the modern channel of Clear Creek (Van Horn, 1954, p. 15). The creek was dammed for a few hours but quickly eroded the obstruction. At other places the toes merely slide over downhill slopes. At some places the toes move into artificial excavations and commonly are removed as the slides slowly advance. One such place is along the Farmers Highline Canal north of the sewage disposal plant (sec. 27, T. 3 S., R. 70 W.). Here the water in the ditch is able to carry away the fine material, but cobbles and boulders of latite accumulate in the ditch and must periodically be removed. According to Mr. Stott of the ditch company (oral commun.) pilings from the railroad 100 feet uphill have been found in the uphill bank of the ditch. Mr. Stott stated that in most years evidence of movement of the uphill bank of the canal was noticeable during the spring and summer. The downhill bank appears to be stable. When examined in April 1961, the railroad tracks, normally straight, had bulged about 3 feet downhill and had vertical waves of long wavelength and small amplitude. Telephone poles above the canal leaned at various angles, some uphill and others down- hill. The highway above the railroad, originally con- structed with an even grade, had several large sags. The only place that an unequivocal slide plane was found exposed in an old slide was in a residential section in the northeast part of Golden (fig. 42), described in the following measured section. 88 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO FIGURE 42.—An old landslide at Plateau Parkway in Golden in the SE‘ANE‘A sec. 28, T. 3 S., R. 70 W., showing a disoriented mass of Denver Formation (unit 2) that has overridden Verdos Alluvium (unit 6). The Denver Formation has moved southward along the slide plane (unit 3) from the flank of North Table Mountain which is to the left of the area pictured. The numbers refer to unit numbers in the accompanying measured section at locality 694. View toward the east. SECTION G94.—Landslide in Golden [Measured behind a house in the SE‘ANE'A sec. 28, T. 3 S., R. 70 W.] Thickness l. Colluvium; clay to boulders as much as 3 ft in maximum W dimension ....................................................................... 3.0 2. Sandstone and claystone; bedding greatly fractured and dis- rupted (but appears to dip steeply north) ..................... 10 3. Silty clay, dark-brown, slickensided; strikes N. 70° E., dips 12° N. Striations plunge 8° N. 30° E ............................. .1 4. Silty sand light-olive-gray; appears to be a mixture of debris from the Denver Formation and alluvium of Clear Creek. A prominent light-gray, 0.5-ft-thick cal- cium~carbonate-impregnated layer is in upper foot ..... 1-25 5. Clayey sand, moderate-ye]lowish-brown; similar to unit 4 .............................................................................. 3 6. Cobbly sand and gravel; cobbles predominantly rounded, Of Precambrian origin, and typical of alluvium from Clear Creek. Also contains many subangular cobbles and boulders of latite. The deposit partially cemented by light-gray calcium carbonate. Top of this unit is about level with base of the volcanic ash in the Verdos Alluvium ‘/4 mile to the east. Base not exposed ............. 6 Unit 1 is colluvium that is mixed with debris from old rockfalls. Unit 2 is a badly fractured mass of Denver Formation that has slid over the underlying Pleistocene deposits on the slickensided slide plane (unit 3). Units 4 and 5 are probably prelandslide colluvium. The calcium carbonate in unit 4 does not represent a soil zone. No bedding was seen in units 4 and 5 and they do not appear to have been particularly affected by the overriding land- slide. Unit 6 is a stream-deposited sand and gravel that is almost certainly a remnant of the Verdos Alluvium. The calcium-carbonate-cemented layer in unit 6 is probably the eroded Cca horizon of a pre-Bull Lake soil. The land- slide occurred after the formation of the pre-Bull Lake soil and before the accumulation of as much as 3 feet of collu- vium of probable Holocene age (unit 1). No indisputable evidence of present activity of the landslide was found. The telephone pole and the fence visible in the picture are not out Of line, and no surface cracks had opened in the excavation or uphill from the toe. Some cracks occurring in nearby buildings may be due to expansive clay in the Denver Formation or to settling that is normal in new structures. The area uphill from this landslide is a gently sloping terracelike plane. It is similar in shape and general aspect to the other old landforms on the flanks of North and South Table Mountains that have been mapped as landslides. The largest landslide in the area, at the northeast corner of North Table mountain (figs. 21 and 43), covers about three-quarters of a square mile. A hand-dug cistern 12 feet PLEISTOCENE AND HOLOCENE DEPOSITS 89 FIGURE 43.—A large composite landslide with a typically hummocky surface at the northeast corner of North Table Mountain. No historic movement has been reported on this landslide, but a horse tooth found in the landslide deposit may indicate some movement deep located in the upper part of this landslide showed 10 feet of hard silty clay overlying claystone of the Denver Formation. A tooth was collected by the owner of the prop- erty in the cistern from near the base of the silty clay. The tooth was identified as an upper molar of Equus sp. by the late Mrs. Jean Hough of the U.S. Geological Survey. Mrs. Hough (written commun., Nov. 25, 1953) stated that the tooth was not fossilized and was probably of Holocene age. This would mean that the horse died some time after AD. 1500 when the horse was beginning to be reestablished on this continent by Spanish importation. The burial of the tooth almost certainly would not be due to historic ac- cumulation of 10 feet of colluvium; therefore a more rea- sonable explanation for the burial of the tooth is that it had been washed into a deep crack, probably resulting from landslide movement, which has now healed and is no longer visible. There is no other evidence indicating move- ment of this landslide in historic time. Landslides are forming at present, and two landslides on South Table Mountain became active in 1967 and were still moving in 1968. One of these, in the SW‘ANW‘A sec. 25, T. 3 S., R. 70 W., is between the Welch and Agricultural Ditches. (See fig. 44.) Prior to the formation of this land- slide in 1967 the ground had not shown signs of move- ment. There is no indication of any disturbance occurring within the past few years that might have triggered move- ment of this landslide. The slope, therefore, must have been in a potential state of imbalance for many years, and the pressure generated by the mass of the slide finally overcame the coefficient of friction of the material in the since AD. 1500 Lava flows 2 (TV2) and 3 (Tv3) cap North Table Mountain and lava flow 1 (Tv1) fills an old channel in the Denver Formation part way down the slope. Photograph, toward the southeast, by H. E. Malde, US. Geological Survey. slide, perhaps aided by a gradually increased water content. The other slide, in the SWl/i of the same section, is in the lower end of an ancient landslide. This ancient landslide became activated shortly after material had been excavated from the toe of the slide. During 1968 three older slides in the NBA sec. 27, T. 3 S., R. 70 W., were also active; two on South Table Mountain involved a highway (fig. 17), and one on North Table Mountain was at the former junction of State Highway 58 and Easley Road, about 500 feet north of the Golden Sewage Disposal plant. A new landslide adjoining an old landslide occurred on the north side of a cut for the relocated State Highway 58 in the NW‘ANE'A sec. 27, T. 3 S., R. 70 W. on North Table Mountain. I first saw this landslide in December 1969 shortly before the highway was completed. The landslide involved the upper part of the cut and slowly moved toward the highway. There was no apparent rotation of the mass and it appeared to be a slow debris slide. Several additional units slid downhill before the entire new land- slide was removed by excavation in about March 1970. Rockfalls, though not frequent, occur with great rapidity. Seven of the slides reported in the newspapers (p. 108) probably were rockfalls. Two small rockfalls from South Table Mountain were investigated shortly after their occurrence. They both came from the west side of the latite flows capping Castle Rock (fig. 15). The first, of about 400 cubic yards, happened at 5:20 am. on a foggy Sunday morning, March 23, 1958. The roar, like a blast of dynamite, awakened a few light sleepers and startled a passing newspaper delivery boy. The temperature was 90 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO FIGURE 44.—An active slump landslide in the SW‘ANW‘A sec. 25, T. 3 S., R. 70 W. The head of the slump has dropped down and the toe is bulging up indicating rotational movement. The Welch Ditch is just above the nearly vertical main scarp of the landslide. The toe of the landslide has bulged upward about 10 feet. The close to freezing and the relative humidity was 100 percent. When the area was examined 2 days later it was determined that a single 100-foot-high column of latite, about 15 by 8 feet wide at the base and 18 by 5 feet wide at the top, had collapsed. A 6-foot—deep pile of freshly broken rock of many sizes formed a fan-shaped deposit which extended about 100 feet out from the original base of the column. This pile Of debris contained many precariously perched boulders. Extending about 100 feet horizontally south- west from the debris was a zone of pulverized earth that was liberally sprinkled with freshly broken flat rock frag- ments as much as 6 inches across. A few trails, consisting of gouges and furrows, extended from the zone of pulverized earth to the bottom of the steep slope about 500 feet from the column. At the base of the steep slope were rectangu- lar blocks of freshly broken rock about 12 inches across. One rectangular block 4 by 3.5 by 2.5 feet moved a distance most recent movement is indicated by the raw dirt scarp at the base Of the toe. The landslide started in 1967; the photograph was taken in 197] by H. E. Malde, US. Geological Survey. Note the columnar jointing in the latite lava flow capping South Table Mountain, and the numerous boulders Of latite on the slope. of 900 feet before sliding to a stop in a driveway about 50 feet from a house. The tracks of this block were plainly visible for several hundred feet upslope. The last big bound Of this boulder was 24 feet long and occurred not more than 75 feet from its final resting point. The freshly broken rock had an angular blocky appearance and is marked by many very light yellowish gray percussion points. When examined 4 years later these percussion points were still visible but were medium gray and the rock no longer had the original freshly broken appearance. A second but smaller rockfall, in February 1962, consisted of about 32 cubic yards of latite that fell from the recess formed by the fall of March 1958. The rock followed the same course as the earlier fall. One block about 3 by 3 by 2 feet rolled and bounded about 500 feet out from the base of the latite cap. Concentrations of similar blocky boulders around the PLEISTOCENE AND HOLOCENE DEPOSITS 9l SIZE OF PARTICLES, IN MILLIMETRES 0.00 0.01 0.1 1.0 100.0 100 90— 60— 50- 40— 30 — W’é PERCENTAGE OF PARTICLES SMALLER THAN SIZE SHOWN a“: .c\ CLAY SILT SAND Afl—J H4 GRANULES PEBBLES COBBLES FIGURE 45.—Cumulative curves showing the size distribution of two samples of artificial fill. Sample 830+00 is from the NE‘ANE'A sec. 36 and sample 870+00 is from the SE’ASEV; sec. 36, T. 2 S., R. 70 W. flanks of North and South Table Mountains indicate that rockfalls have been numerous. The strongly jointed vertical columns are poorly supported at many of the high vertical faces around the mountains. Frost action during each winter tends to open the joints a small amount, and to push the columns outward. Colluvial action tends to move material downward away from the base and to leave the columns unsupported. When the column reaches a position of imbalance, it falls and cascades down the slope with great rapidity. Many of the boulders visible on the flanks of North and South Table Mountains (fig. 44) undoubtedly are the result of rockfalls. The third type of movement, block glide, was found only near the SE. cor. sec. 29, T. 3 S., R. 70 W. This slide is uphill from a steep quarry face and was probably initiated by the quarrying. The quarry, started in 1948 or 1949 at the site of an older quarry, was excavated rapidly into the mountain. By September 1951 a single large crack out- lining the shape and maximum size of the slide had de- veloped several hundred feet uphill from the quarry face. By 1953 the main crack had branched at the east end and several small transverse cracks had developed between the quarry and the main crack. During July 1953 the surface of the slide was 4 feet lower than the adjacent undisturbed surface and was separated from it by a crack 10 feet deep and 6 feet wide. In 1959 the crack was nearly filled with debris but the head of the slide was about 10 feet below the undisturbed colluvium of the crown. No further change has been noted in this slide.3 The slide involves both Pre- cambrian bedrock and surficial colluvium. The slide plane was not located although it probably is present in the steep quarry face and in the cutbank above US. High- way 6. Rotational movement of the slide is not apparent; rather, the slide mass seems to have very slowly glided down a fractured surface or joint plane. ARTIFICIAL FILL Artificial fill ranges from well-compacted layers of silt and gravel in structures like Ralston Reservoir dam to un- compacted, uncontrolled gully-fill dumps of tree stumps, grass clippings, boulders, and miscellaneous dirt and trash like the fill east of the County Hall of Justice in Golden (sec. 34, T. 3 S., R. 70 W.). Artificial fill ranges in thickness from 150 feet at Ralston Reservoir dam to a few inches around building sites. Only areally large fills more than 3 feet thick were mapped (Van Horn, 1972). The well- compacted fills include the many highway and railroad fills, dams, and fills around large buildings. Most of these fills are of selected material, principally poorly sorted sand containing small amounts of pebbles, silt, and clay. The size distribution of two samples that were collected from highway fills is shown in figure 45. The sample from locality 870+00 contained fragments of “red dog,” an oxidized clinker commonly found on coal mine dumps. ’ln june 1973, as this manuscript was being prepared, new cracks could be seen above the original main scarp and a longitudinal crack had opened on the east side downslope from the original main scarp. 92 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO FIGURE 46.—Poorly compacted fill in abandoned clay pits of the Laramie Formation west of the Colorado School of Mines campus, sec. 33, T. 3 S., R. 70 W. Apartment buildings behind the fill are founded on the sandstone ribs that form the walls of the clay pits. Photograph, toward the southeast, by H. E. Malde, US. Geological Survey. Parts of the highway and railroad fill are cobbly sand and gravel. Partly compacted fills include controlled city and county dumps that are burned and then compacted with bulldozers and industrial dumps such as the one extending west of Golden on the north side of Clear Creek. The city and county dumps are a chaotic mixture of organic and inorganic debris that is generally covered by a few feet of locally derived gravel and silt. The industrial dump principally consists of locally derived earth materials from excavations, but it also may contain broken pottery and porcelain. Poorly compacted fills, which gen- erally occur in gullies used as dumps, are similar to the partly compacted fills except that in the partly compacted fills some compaction is brought about by trucks crossing the fill to dump at the edge. At irregular intervals the material is pushed over the edge by bulldozers. The debris filling the 01d clay pits west of the Colorado School of Mines is a poorly compacted fill (fig. 46). The apartment buildings behind the visible fill were constructed over a well-compacted fill in the same clay pits. The load-bearing members of these buildings were placed on the sandstone ribs that separated the old clay pits. In general this procedure has been satisfactory but a few of the apartments pictured at the left of figure 46 have shown signs of structural distress. At other places the well- compacted fill has settled, forming sags in the otherwise smooth topography. PLEISTOCENE AND HOLOCENE SOILS One important criterion used to establish the relative age of the various deposits is the degree of soil development on the deposit. In this report soils or soil profiles are commonly described in three parts (master horizons) which are called the A, B, and C horizons (US. Dept. of Agriculture, 1962). The A horizon is usually dark PLEISTOCENE AND HOLOCENE SOILS 93 or medium gray. It contains organic and mineral matter, but has been depleted in clay and calcium carbonate. The B horizon underlies the A horizon and is a zone of deposi- tion of clay derived from the A horizon. Calcium car- bonate has been partly leached from the B horizon. The C horizon underlies the B horizon (or the A horizon where the B horizon is missing) and is a zone in which some chemical weathering has taken place. The Cca horizon, which is in the upper part of the C horizon, is a zone in which calcium carbonate has been deposited. A number following the master horizon letter indicates that two or more phases of this horizon have been recognized. (See fig. 47.) A Roman numeral (II) preceding the master horizon letter indicates that a lithologic discontinuity overlies the units so designated. Any of these horizons may be absent from a soil profile because of erosion or lack of sufficient soil development. The upper layers have been eroded from many of the older soil exposures. The B and Cca horizons have not yet developed in the youngest soil. Dark-colored horizon with a high content of A A1 organic matter mixed with mineral matter. Bl Transitional to B, but more like B than A. May be absent. Maximum accumulation of silicate clay minerals B B2 or of iron and organic matter; maximum development of blocky or prismatic structure, or both. r____ / Bca Horizons Bca and Cca are layers of accumulated \ Cca caICIum carbonate found in some sorls. Lv~_fi Mineral horizon or layer, excluding bedrock, C C1 that has been relatively little affected by pedogenic process. The number indicates C2 the relative vertical position in the C horizon. FIGURE 47.—Soil-profile horizon terminology used in this report. Modified from US. Department of Agriculture (1951, p. 173; 1962). The term “degree of soil development” is a general expression for summarizing the differences in thickness of the soil profile, and in thickness and other characteristics of individual horizons (Richmond, 1962, p. 25). It is a convenient method of comparing soils of different ages that have developed on similar parent materials. For instance, a well-developed soil may be thicker or have more clay accumulation in the B horizon than a poorly developed soil. The extensive sequence of terraces, in which each suc- cessive terrace is lower and younger than the adjoining terrace, simplified the comparison of the extent of soil de- velopment on the various deposits. The deposits in the flood plain of the existing streams have no soil developed on them. The lowest terrace deposit (Piney Creek Al- luvium) has a thin A horizon underlain at a few places by small blebs of calcium carbonate. The next higher terrace deposit (Broadway Alluvium) has a thin but complete soil profile consisting of a thin A horizon, nearly 1 foot of brown B horizon, and a Cca horizon 1.5 feet thick. The next older terrace deposit (Louviers Alluvium) has a soil which is generally eroded but which has as much as 1.5 feet of reddish-brown B horizon overlying as much as 3 feet of Cca horizon. At places this soil is indistinguishable from the older soils. The pre-Bull Lake alluvial deposits have thick, well- developed soil profiles that have been eroded and were not differentiated. They have reddish- to yellowish-brown B horizons as much as 4 feet thick and Cca horizons as much as 6 feet thick. James Thorp (US. Dept. of Agriculture, oral commun.) indicated that the soil on the Rocky Flats Alluvium might be distinguished from other pre-Bull Lake soils by its very red B horizon. The analysis of soil developed on Rocky Flats Alluvium at locality G143 (table 12) shows a much greater amount of montmorillonite than the soil developed on Verdos Alluvium and thus tends to corroborate ThQrp’s observation. The soils in the Golden area are in a stratigraphic sequence similar to the sequence outlined by Morrison and Frye (1965). They indicated that three very strongly developed soils of pre-Bull Lake age are followed by a strongly developed soil of post-Bull Lake age which is suc- ceeded by as many as three less well-developed soils. The differences in degree of soil development were first established along Clear and Ralston Creeks where there are readily distinguishable terrace sequences. These soils were then correlated with soils in adjoining areas where the terrace sequence is not so clear. At a few places younger soils were found stratigraphically overlying older soils. It was found that the B horizon forms the most distinctive and consistently equally developed horizon in any par- ticular soil. The Cca horizon varies greatly in thickness and amount of accumulation of the calcium carbonate in short distances. It is, no doubt, influenced by the amount of limestone or other calcareous rocks in the parent ma- terial. There is no way to tell how much of the calcium carbonate may be due to evaporation at the top of some past water table, or how much calcium carbonate has been carried away in solution and did not precipitate. In spite of these difficulties the Cca horizon still provides an indica- tion of the degree of soil development on a deposit and, if used with caution, provides a useful tool in correlating deposits. In addition to the field criteria just discussed for differ- entiating soils, X—ray analyses of 30 samples from seven soil profiles of different ages show differences in kind and amount of clays (table 12). These differences persist in . not present] ped on alluvium in the Golden quadrangle [Analyst Paul Blackmon. Tr., trace: TABLE 12.—X-my analyses of soils develo Estimated parts in 10 (uornex; pazrs -pues sql ur 5! auou Surumssz) aldmes [2101 U; earn: maxed uornu} pans-ms ur alduuzs [mm ur 231m waned [1011321] pazrs-Aep ui 9[dures [€101 ur 12er moored (uornm; pazrs-pues sq] u! s! auou Bugumsse) aldmes [2101 u; atyuomroluluour moored uogner; pans-ms ur aldwes 12101 ur auuomrounuour maxed uornm; pausing u! ardures [m0] ur alruommmzuom auaoiad Aep lll3319d setup ariuonpounuoui a) 931m ’0 oneu .01 saw." QUHOIHIOUHUOIU 01 earn! JO one)! suuomroun Hour 01 earur 30 open Aep wound [2101 9113123 mdsplad zuenb 911110qu arruomromruom-eorw aznmruua A alruuoex alruomiounuow mm ‘oN aldums pue Aztlem auoz HOS Broadway Alluvium wgwwvwwvnq9 —N®5‘Ql\al\'—‘V‘©N __ __ Qwvwvanngw m.—..—1~——.——<fiq~—u—. vwm®99®®9wv wow—nowmcovn— _ M 55“!”99‘99995‘1‘9. mom~mm~~m~ _ ._.._ -—< m. ‘9“.“1 co coco -— ca ”.lfilquqqmflf—I“? coon—tauw—u—m—t .— .—1 ~— Tr Tr fl. Tr 2 2+ fl: 1+ 1+ 1+ <1 ++ _.._. 1+ 2+ 2+ 4+ m m E :1 :1 O O O N N U U .—.m MN —<.\l ) or less (<) than the amount of montmorillonite Age of deposit on which soil formed ........................................ Pinedale Bull Lake Pre—Bull Lake A Mica > montmorillonite ..... Mlca < montmorillonite ..... O0 B Mica > montmorillonite ..... Mica < montmorillonite ..... Soil horizon [\D 00 0 wk C Mica > montmorillonite ..... Mica < montmorillonite ..... 1 C2 Mica > montmorillonite ..... Mlca < montmorillonite ..... ”‘0 ON Orb OH HO N30 bah—l Both the similarities and the differences may be explained by climatic differences and relative grain size of the mica and montmorillonite. The montmorillonite particles are generally smaller than the mica particles and because of this are probably easier to move downward (illuviate) than the mica. This would increase the pro- portion of montmorillonite in the lower part of the soil. Within any given time span similar climatic conditions within the small area represented by the Golden quad- rangle should lead to similar rates of illuviation in deposits of similar texture, thus giving similar mica to montmorillonite ratios in deposits of the same age. Dif- ferent climatic conditions during successive periods of time (or different length of the soil-forming period) should cause different amounts of illuviation, hence dif- ferent mica-to-montmorillonite ratios in B horizons of deposits of different ages. STRUCTURAL GEOLOGY The dominant structural features of the area are the east- dipping limb of the Front Range anticline and the west- dipping Golden fault, both of which are attributed to the Laramide revolution. The concept of the Front Range anticline has long been accepted by geologists working in the Rocky Mountains. The east limb of the anticline is indicated by the steeply dipping to overturned beds of Paleozoic and Mesozoic age in the western part of the Golden quadrangle. The west limb of this anticline is so far removed from the Golden quadrangle that the east limb of the anticline is herein considered a monocline. FOLDS Folds in the area consist of the Front Range monocline and several smaller flexures (fig. 49). The monocline of the Front Range is the dominant fold of the Golden quad- rangle. This steeply east-dipping to overturned structure trends slightly west of north along the west edge of the quadrangle. East of the Golden fault the Upper Creta- ceous rocks dip vertically downward for hundreds of feet, then abruptly bend eastward to dip gently toward the center of the Denver basin. This abrupt change is shown by the change in dips at the surface on the west side of North Table Mountain. Another large but less conspicuous fold is present at Golden where the rocks east of the Golden fault on the crenulated limb of the Front Range monocline form a broad overturned to vertically plunging syncline normal to the monocline. One limb of the syncline trends north and the other southeast. It is best outlined by the vertical to overturned beds of the Fox Hills and Laramie Formations; the axis is where the vertically plunging syncline symbol is shown in figure 49 at the contact of the two formations. Gentle west dips of a few degrees on the east flanks of 96 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO #1 60 _ EXPLANATION _ _ 60 Values are in percent * All other clay-size minerals _ 3.0) Montmorillonite 50 — - 50 M'ca 24.0 -— 336 4° ‘ ’7 7 ~ 40 21 .1__ l— ‘ _ Z '2 8 LIJ 35 21.5 g __‘ LU “- 30 — 20.0 205 _ 30 CL 20 - 20 10 _ I 10 A B C1ca C3 A B1 32 A B C1caCZ G109 G115 G113 Soils on Broadway Alluvium G111 B1 82 C1 C2 Soils on Louviers Alluvium A B1 B2 Cca G162 B1B2C1caC2 A 81131 llCca G101 G143*1132 Soils on pre-Bull Lake alluvium FIGURE 48.—-Percentage of clay minerals in the clay-sized fraction of various soils in the Golden quadrangle, Colorado. ‘1 the A and B horizons are probably developed on a thin Pinedale deposit. The localities are given in table 12. North and South Table Mountains indicate the presence of a broad, shallow syncline under these mountains. The dips are corroborated by subsurface data on the top of the Fox Hills that show a broad gently southeast-plunging anticline that enters the quadrangle north of Leyden (Spencer, 1961) and trends east of the Table Mountains (Stewart, 1955, p. 30). The north end of the west limb of the anticline and the north end of the syncline that trends southward to the Table Mountains are well displayed by structure contours interpolated from altitudes of the coal horizon at various places in the new Leyden Coal mine taken from the files of the Colorado State Bureau of Mines. (See figs. 12 and 49.) FAULTS Most of the faults in the Golden quadrangle are related to, or continuations of, faults in the Precambrian tenane to the west. The largest fault in the quadrangle, the Golden fault, probably is a southern extension of the Livingstone fault from the Ralston Buttes quadrangle STRUCTURAL GEOLOGY (Van Horn, in Sheridan and others, 1967, p. 72). The total amount of displacement along the Golden fault zone is not known but is probably very large. Just north of Golden, at the point of apparent maximum stratigraphic dis- placement, about 8,800 feet of stratigraphic section has been cut out by the fault. The stratigraphic displacement decreases rapidly to the north, but it is interesting to note that, depending on the attitude of faults and bedding, major movement on the fault could have led to very little apparent stratigraphic displacement. The Golden fault has been the subject of controversy concerning not only its angle of dip, but also its location and even its existence. In my opinion, the Golden fault is a moderately to steeply west-dipping reverse fault of large displacement. The marked reduction of the geologic section at Golden was noted by A. R. Marvine (in Hayden, 1874, p. 136), who mentioned the possibility of this being the result of faulting or unconformity. He seemed to prefer the unconformity concept inasmuch as no fault is shown on his map (p. 147) or on the atlas map (Hayden, 1877, pl. 12). A similar conclusion was reached by Lakes (1889), although in his section at Clear Creek (pl. 3, p. 58) he strongly supported the possibility of faulting. Eldridge strongly favored an unconformity as the cause of the thin section at Golden (1889; Emmons and others, 1896). The assault on the unconformity concept probably began with Patton’s (1905) description of some remarkable faulting in the Dakota Group south of Golden. Work by Richardson (1912, p. 429) several miles south of Golden caused him to question the unconformity concept at Golden, and he sug- gested that the reduction in section might be due to faulting. Lee (1915, p. 32) also expressed doubt about the proposed unconformity at Golden. The presence of a fault was convincingly argued by Zeigler (1917) to explain the omission of beds at Golden. He believed that the fault was a steep-angle reverse fault and he showed the fault passing west of Ralston dike, but concurred with Eldridge (Emmons and others, 1896, pl. 10) in locating two trans- verse faults southeast of Ralston dike. Johnson (1925, 1930b) and Van Tuyl and McLaren (1932) agreed with Zeigler. Waldschmidt (1939) was the first to suggest that the fault southeast of Ralston dike was a single fault rather than two transverse faults, and that in the vicinity of Ralston dike it was a low-angle thrust fault. He also believed that there was a fault west of Ralston dike but could find no evidence for it. The low-angle thrust fault concept was later applied to the Golden fault throughout the Golden-Morrison area by Stewart (1952, p. 966). Van Horn (1957b) reverted to the steep-angle reverse fault. The Golden fault was described by Boos and Boos (1957, p. 2640) as a thrust-fault belt consisting of underthrusts. Osterwald (1961, p. 226) and Harms (1961, p. 413) both concluded that the Golden fault is a high-angle reverse fault. Berg (1962, p. 707) concluded that faults in the Golden zone dip 35°—50° SW. 97 The principal evidence for the steep angle of the fault is found at three places. The first, and southernmost, place is about 9 miles south of Golden, where evidence for the steep angle of the fault was shown in detail by Smith (1964, section B—B’). By comparing the logs of wells with the outcrop of the Golden fault, Smith showed that the fault zone dips 50°—60° to the southwest, and that the southwest side has moved up relative to the northeast side. The second place is the south bank of Clear Creek where the fault contact between the Pierre Shale and the overriding Lyons and Fountain is plainly visible from the north side of the creek. Here the apparent dip is more than 45° W. The third place is east of the mouth of Golden Gate Can- yon where several boreholes were drilled. D. F. Tobin (oral commun.), who examined the cores from the bore- holes, believed that the fault dips in excess of 60‘? In addi- tion the displacement of the fossil horizons in the Pierre Shale north of Van Bibber Creek shown by Van Horn (1972) is generally compatible with a west-dipping reverse fault. The beds on the west override and cut out the beds on the east. The salient of Pierre Shale extending eastward near Ralston dike, however, is apparently anomalous. The only place the Golden fault is known to be asso- ciated with a long looping salient eastward is east of the intrusives southeast of Ralston dike. Because of this unique situation the salient seems to be related to the intrusives. Another relationship brought out by the mapping (Van Horn, 1972) is that the number and con- centration of the intrusives suggest that they are all part of a single large intrusive lying near the surface. The location of fossil zones in the Pierre Shale suggests that there are two other faults in this area. The westernmost is indicated by the less-than-norrnal thickness of beds between the Didymoceras cheyennense zone and the Baculites eliasi zone. The other fault is shown by the repetition of the Baculites grandis zone. Fossils typical of the older Inoceramus typicus zone are present between the two. Geologic cross section B—B’ (Van Horn, 1972) is drawn through this area. The attitudes of the faults and beds shown in the cross section are based on meager surface exposures. These relations suggest that the long looping salient of Cretaceous rock is essentially a small upright spoon—shaped overthrust plate that formed during a late stage of activity along the Golden fault. Other theories that were considered to account for the salient required that the salient be part of a syncline, a recumbent anticline, a low-angled Golden fault, two transverse faults, or a land- slide. These theories were rejected because they did not fit the observed relations of the beds, the faults, or the intru- sives. A possible interpretation of the sequence of events leading to the formation of the spoon-shaped overthrust plate is that the block west of the main Golden fault moved upward and eastward after the beds had been folded into a nearly vertical attitude. The intrusion then moved up this 98 \ 105°15' 39°52’30“ 1 _ GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO 1230' R. 70 W 10' R. 69 W. 105'07'30' 47'30’ 2 | l l 2 3| MILES I 3 KILOMETRES FIGURE 49.——Structure contours on the top of the Fox Hills Sandsrone in the Golden quadrangle. Map explanation is at top of facing page. Base from US. Geological Survey Golden 7%-minute quadrangle (shaded relief), scale 124,000, 1965. STRUCTURAL GEOLOGY 99 EXPLANATION -—4600— STRUCTURE CONTOUR DRAWN ON TOP OF THE FOX HILLS SANDSTONE — Dashed where ap- proximately located. Contour interval 100 feet FAULT — Approximately located ANTICLINE — Showing crestline and direction of plunge. Dashed where approximately located SYNCLINE — Showing troughline and direction of plunge. Dashed where approximately located *— VERTICALLY PLUNGING SYNCLINE 0 WELL USED FOR CONTROL fault zone. As the intrusive approached the surface, pres- sure generated by the heat, chemical reactions, and the mass of the intrusive exerted strong forces against the adjoining sedimentary rock. This force ruptured and pushed a thin slice of sedimentary rock eastward along a spoon-shaped fault surface. The bottom of the spoon is probably opposite the main body of the intrusive. The large degree of overturning in the overthrust slice may be due to drag or to a preintrusive overturned fold. The west- ern part of the salient was faulted up relative to the eastern part during a late stage of the formation of the salient, thus repeating the Baculites grandis zone. All movement stopped when the pressure caused by the intrusive was relieved by the magma reaching the surface. Later erosion left the slice exposed as a long eastward-looping salient of older rocks overlying younger rocks. Thick slices of sedimentary rock that have been caught in the Golden fault have been reported from localities southeast of Morrison (Berg, 1962), at Tucker Gulch (Van Tuyl and McLaren, 1932), and northwest of the Golden quadrangle in the Livingstone breccia reef (Levering and Goddard, 1950). Similar slices are exposed in the south valley wall of Clear Creek just west of the Pierre Shale and in Tucker Gulch just east of the Front Range. These slices, though not unique, do reveal something of the mechanics of movement of the fault blocks. The fault exposed in the north bank of Tucker Gulch is of interest because of the peculiar position of the Benton Shale sandwiched between two older formations (figs. 50 and 51). A bedding plane in the Fountain Formation 20 feet west of the fault strikes N. 5° W. and dips 51° E. The fault, which has a 2-inch-wide gouge zone, strikes N. 20° E. and dips 82° W. A 3-inch-wide bentonite seam in the Benton, 12 feet east of the fault, strikes N. 20° E. and dips 88° E. The Benton is exposed for 61 feet east of the fault and appears to maintain the same attitude. The bentonite beds indicate this may be in the lower part of the Greenhorn equivalent. East of the Benton is 30 feet of colluvium which is followed by beds in the Dakota Group that strike N. 30° W. and dip 51° SW. Exposures in an old prospect shaft nearby indicate that the beds are in the upper part of the Dakota. Dakota outcrops continue another 144 feet to the east where they strike N. 50° W. and dip 70° SW. A small outcrop of Pierre Shale is present 90 feet east of the Dakota exposure; the intervening fault and bedrock are covered by colluvium. The Morrison Formation crops out 400 feet south of the Dakota along the south side of a farm access road on the south side of Tucker Gulch. The atti- tude of the Morrison is similar to that of the Dakota. Float from the Lykins Formation is present on the south side of the knoll south of the Morrison Formation. These rela- tions suggest that the beds are overturned. The location of some of the faults in the Pierre Shale north of Ralston dike is uncertain. Most of the area is covered by surficial deposits but possible discrepancies in the fossil zones of the Pierre Shale suggest that faults asso- ciated with the Golden and Livingstone faults are present in the area. Several thousand feet of left-lateral dis- placement along the Livingstone fault is indicated by the offset of the basal Pierre contact between the Ralston Buttes and Eldorado Springs quadrangles (Wells, 1967; Van Horn, in Sheridan and others, 1967). In the Golden quadrangle the most northwesterly fault is part of this system, although here the offset is not so clear cut. South of the junction of this fault with the west-trending fault one- half mile north of Ralston Reservoir the stratigraphic interval between the Baculites scotti and the Didymocems cheyennense zones appears to be shorter than normal. This shortening probably indicates deletion of section by reverse faulting. (See Van Horn, 1972, geologic section A—A’.) On the next fault to the east, intervals between the fossil zones suggest that nearly 1,000 feet of section has been cut out by reverse faulting. The northward extension of this eastern fault is believed to fade out into bedding- plane faults of unknown displacement and extent. South- ward, near Ralston Creek, the faults north of Ralston dike join with the Golden fault and the small overthrust fault that forms the salient southeast of Ralston dike. The junc- tion is covered by surficial deposits and individual dis- placements are not known. An alternative interpretation based on the same data by Scott and Cobban (1965) shows the Golden fault terminating 2 miles north of Ralston dike and shows no connection with the Livingstone fault. The southeast-trending fault adjacent to Van Bibber Creek is the extension of a fault mapped in the Ralston Buttes quadrangle, where it appears to offset the Lyons Sandstone. The offset shown in the Dakota Group (Van Horn, 1972) is less certain and probably is smaller than the offset shown for the Lyons. The small faults shown in the Dakota are undoubtedly related to this fault. The Benton Shale appears to be thinner south of the fault than it is to the north. The fault very probably bends to the south and 100 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO EXPLANATION Kp Pierre Shale Kb Benton Shale Kd Dakota Group Jm Morrison Formation 'EPIk Lykins Formation Ply Lyons Sandstone PIPf Fountain Formation pCr Precambrian rocks Contact 82 —1——— FAULT — Showing dip. Dashed where approximately located STRIKE AND DIP 0F BEDS fi— Inclined 5/ + Vertical —q- Overturned 5/ 0 500 1000 FEET O 100 200 300 METRES FIGURE 50.—-Sketch map of the fault slice in the Golden fault zone at Tucker Gulch in the Golden quadrangle. becomes a west-dipping, reverse, strike fault that joins the Golden fault to the south. The several small faults in the Dakota north of Tucker Gulch, and the east-trending fault in sec. 21 north of the kilns and east of the Golden fault are all related to the Golden fault. Along the Golden fault zone there are possibly similar faults that are obscured by the surficial deposits. The south—dipping fault shown passing through the quarry north of Clear Creek in the Precambrian rocks is also a reverse fault as is shown by the offset of the Precam- brian-Fountain contact south of the creek’s mouth (Van Horn, 1972). An east-dipping reverse fault in sec. 26, T. 3 S., R. 70 W., on the north flank of South Table Mountain, affected lava flows 2 and 3 and the underlying sedimentary rock of the Denver Formation. It has a throw of 60 feet and a heave of 40 feet; the west side moved down and to the east relative to the east side. The fault could not be traced more than 100 yards. A graben of Yarmouth(?) or younger age, with 5 feet of displacement, in a trench in the NE‘ASW‘A sec. 28, T. 3 S., R. 70 W., was described by Scott (1970, p. Cl5—C16). It involves the Fox Hills Sandstone, Verdos Alluvium, and colluvium; it is nearly parallel to the strike of the Fox Hills. The relationship of the small downdropped block on the downdropped side of the Golden fault to the Golden fault is not clear. It should be noted, however, that the graben is near the locality of the New (Little) White Ash mine which was reported to have surface subsidence in 1903. (p. 109) GEOLOGIC HISTORY 101 FIGURE 51.—Part of the Golden fault zone in the north bank of Tucker Gulch. The pick is on the Fountain Formation and the hat is on Benton Shale; both formations are dipping east. The fault zone, about 2 inches wide, is marked by the west-dipping white streak just above and to the left of the hat. The bouldery deposit overlying the irregularly eroded bedrock surface is colluvium. GEOLOGIC HISTORY Rocks exposed in the Golden quadrangle record events that occurred at times ranging from the Precambrian to the present. The oldest rocks in the area record the accumulation of sedimentary and possibly volcanic rocks in Precambrian time. According to Sheridan, Maxwell, and Albee (1967, p. 73) these rocks were metamorphosed, intruded, and subjected to at least three periods of de- formation during Precambrian time. From some time in the later Precambrian until Penn- sylvanian time, erosion dominated the area and created a major unconformity that is marked by the contact between the Precambrian rocks and the terrestrial Fountain Forma- tion. Little is known of events during the time interval rep- resented by this unconformity. Limestone pebbles in the Fountain Formation suggest that sedimentary rocks may have been deposited in nearby areas, but if so they were completely removed by erosion before the Fountain Formation was deposited. Streams flowing over the eroded surface of the folded and faulted Precambrian rocks deposited the sediments of the Fountain Formation on lowlands that bordered a sea to the east. Following depo- sition of the Fountain, the sea encroached on this land area and the Lyons Sandstone was deposited near the shore- line. Later, as the Perrnian sea encroached farther on the land, the red mudstones and algal limestones of the Lykins Formation were deposited in shallow water; the lime- stones may have formed in intertidal zones. Marine condi- tions probably persisted into Early Triassic time, and then the sea withdrew. Some erosion probably took place during later Triassic and(or) Early Jurassic time. Sedimentation began again in Late Jurassic time with the deposition of the Ralston Creek Formation (including a basal 5-ft-thick bed of sandstone possibly equivalent to the Entrada Sandstone) and the succeeding Morrison Formation. These fine-grained terrestrial sediments were deposited by sluggish rivers on a low flat plain and in lakes. and swamps during Late Jurassic time. Terrestrial conditions persisted into the Early Cre- - taceous, when the sea began to readvance on the land. The rocks of the Dakota Group were deposited at the margin of this sea, and with continued encroachment of the sea the succeeding Benton, Niobrara, and Pierre Formations were deposited in deeper waters. According to Lovering and Goddard (1950, p. 58) the uplift of the present Front Range probably began about middle Pierre time, al- though the Denver basin had been subsiding and filling for some time prior to the uplift. The uplift of the Front Range marked the beginning of the Laramide orogeny, 102 which culminated in early Tertiary time. As a result of this uplift the Paleozoic and Mesozoic formations were bent upward along the mountain front, and movements along the Golden and associated faults occurred in several stages. Volcanism taking place in the rising mountains of the Front Range during Late Cretaceous and early Tertiary time caused the flood of volcanic tuff, mudflows, and tuffaceous terrestrial deposits of the Denver Formation. Ralston dike and associated intrusives were emplaced during Paleocene time, after most of the Laramide fault- ing had taken place. The lava flows of North and South Table Mountains probably came from a volcanic vent a short distance above the present crest of Ralston dike. All traces of Tertiary sedimentary rocks younger than the Denver Formation have been eroded from the Golden quadrangle, but erosion surfaces cut during the latter part of the Tertiary are still partially preserved in the moun- tains to the west (Van Horn, in Sheridan and others, 1967). These younger erosion surfaces, of probable Pliocene age, are about 2,000 feet above the present stream level and 1,500 feet above the lower Pleistocene alluvium (Malde and Van Horn, 1965, p. 42). Thus the erosion between late Tertiary and early Pleistocene time greatly exceeded erosion during the Pleistocene and Holocene. Quaternary history in the Golden quadrangle is marked by recurrent episodes of erosion, alluviation, and soil formation. During each of the three major pre-Bull Lake glaciations the major streams eroded broad terracelike surfaces on the bedrock. These surfaces were then allu- viated by the streams. There is some evidence pointing to the possibility that during early Pleistocene time the stream valleys were separated by interfluves that extended east of the foothills. Clear, Tucker, Ralston, and Coal Creeks all had well-established drainages in about their present location by Nebraskan time, although Ralston Creek occupied a valley on the south side of Ralston dike. During Nebraskan time the Rocky Flats Alluvium, and probably the pre-Rocky Flats alluvium, were deposited on broad, gently sloping surfaces cut by the individual streams. A soil of Aftonian(?) age was then developed on these deposits. The deposits of Nebraskan age were eroded and the Verdos Alluvium of Kansan age was deposited in valleys cut below the Rocky Flats Alluvium. During Kansan time a rhyolitic volcanic ash, probably equivalent to the Pearlette Ash Member of the Sappa Formation, fell on the area and is locally preserved in the Verdos Alluvium. By Kansan time Ralston Creek was flowing northeastward north of Ralston dike, and across the areas now occupied by Leyden Creek and by Standley Lake. Van Bibber Creek had established its present valley west of Ralston dike by Kansan time but flowed northward to join Ralston Creek west of Ralston dike. A soil of Yarmouth(?) age was de- veloped on the Verdos Alluvium. GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO The streams again cut into the older deposits and the Slocum Alluvium of Illinoian age was deposited in the new valleys. The area draining into Standley Lake was established as a basin separate from the rest of the Golden quadrangle between Kansan and Illinoian times. Slocum Alluvium was deposited in this basin by a stream orig- inating on Rocky Flats. The basin has been isolated from the rest of the Golden quadrangle since Illinoian time, and at present drains northeastward into the South Platte River. By Illinoian time Ralston Creek had established its present valley east of Ralston dike but may have joined Clear Creek near Mount Olivet Cemetery. Van Bibber Creek established its present valley south of Ralston dike by Illinoian time and probably joined Ralston Creek just north of North Table Mountain. A soil of Sangamon(?) age developed on the Slocum Alluvium. The Louviers Alluvium was deposited during Bull Lake time in a valley cut into the Slocum. At about this time Ralston Creek shifted its eastern valley about 2 miles northward and joined Clear Creek about 4 miles east of Mount Olivet Cemetery. Van Bibber Creek, however, stayed in the old valley and joined Clear Creek in the vicinity of Mount Olivet Cemetery. The post-Bull Lake soil, a moderately to strongly developed soil, was formed on the Louviers and correlative deposits in the interval between Bull Lake and Pinedale times. The Louviers Alluvium was then partly eroded and the Broadway Alluvium of Pinedale age was deposited in the newly formed valley. At this time silt blown from the flood plains of streams formed a thin deposit of loess on the older deposits. Van Bibber Creek was captured from the Clear Creek drainage by a tributary of Ralston Creek toward the end of, or shortly after, Pinedale time, at a point northeast of North Table Mountain. At about this same time Van Bibber Creek captured part of the Tucker Gulch drainage in the mountains to the west (Van Horn, in Sheridan and others, 1967, p. 57). A weak soil, the post-Broadway Alluvium soil, prob- ably formed on the Broadway Alluvium, loess, and other deposits of Pinedale age. Narrow valleys were then eroded into the Broadway Alluvium. It is possible that deposits of pre-Piney Creek alluvium or colluvium were deposited in these valleys, but they have not been recognized in the Golden quadrangle. I believe that a weak soil developed after the pre—Piney Creek deposits formed. This weak soil, where added to an exposed previously developed post- Broadway Alluvium soil, resulted in a moderately well developed soil (Van Horn, 1967 ). Piney Creek Alluvium, of Holocene age, was deposited in valleys cut into the Broadway Alluvium. A weak soil developed on the Piney Creek. Valleys have been cut into the Piney Creek, and at places post—Piney Creek alluvium is being deposited on the flood plains of the stream. Leyden Creek, a relatively young stream, was estab- ECONOMIC GEOLOGY lished after Kansan time and prior to Pinedale time. At present its valley heads one-half mile from the point where Coal Creek issues from the foothills in the Eldorado Springs quadrangle. In this area Leyden Creek has incised its valley lower than, and has a steeper gradient than, Coal Creek. These relations indicate that the capture of Coal Creek by Leyden Creek, although not imminent, is possible. Deposits of alluvial fans, transported mantle, collu- vium, and landslide debris probably have been forming continuously throughout Quaternary time. At most places their age was not determined. In 1968, as this report was being written, there were at least five active landslides on the flanks of South and North Table Mountains. ECONOMIC GEOLOGY Economic deposits that have been developed in the Golden quadrangle are sand, gravel, limestone, riprap, crushed rock, dimension stone, clay, coal, gold, and feld- spar (for ceramic glaze). Potential economic deposits are silica sand, uranium, oil, and gas. There are no known economic metallic ore deposits in the Golden quadrangle although some gold is produced as a byproduct from the mining of sand and gravel. In 1966 only common clay and sand and gravel were produced in the quadrangle. COAL Subbituminous coal occurs in several lenticular bodies in the lower part of the Laramie Formation. No coal has been mined from the Golden quadrangle since 1950. The earliest recorded production was in 1873, but some mines were producing 20 years earlier. All mines except the new Leyden mine are in the area of steeply dipping outcrops. The mines near the outcrop were generally worked from vertical shafts sunk about 100 feet west of the outcrop. In most places the coal dipped steeply west (overturned) so a few hundred feet below the surface the coal was within 50 feet of the shaft. In the lower parts of the deeper mines the coal was steeply east dipping. The new Leyden mine, now used for natural gas storage, is in the gently east-dipping strata in the eastern part of the area. The coal generally is in two beds which are separated by a distance of 10—20 feet: (1) The westernmost (lower) bed, 2—8 feet thick, and (2) the main coal bed, 8—14 feet thick. From the surface downward for about 100 feet the coal is generally fractured and lusterless. Below about 100 feet, however, it is black, lustrous, and hard, and it slacks readily on exposure. The coal has a relatively low ash content. The grade of the coal is better in the steeply dipping rocks than in the gently dipping rocks. Samples from the Leyden No. 3 mine (new Leyden of the present report) show 20 percent moisture, 55 percent fixed carbon, 7 percent ash, 0.7 percent sulfur, and 9,500 Btu (Fieldner and others, 1937). 103 An estimated 10 million tons of coal has been mined from 13 mines in the quadrangle. Assuming an average thickness of 6 feet of minable coal, 250 million tons of coal still lies within 1,000 feet of the surface. OIL Oil seeps occur in and adjacent to the Golden quad- rangle but commercial production of oil has not yet been attained. Oil seeps have been reported from Halfmile Gulch (which joins Tucker Gulch from the northwest at the west boundary of the quadrangle) and from the east- ernmost tunnel on US. Highway 6 in Clear Creek Canyon. Both of these occurrences are in Precambrian rocks just west of the Golden quadrangle. Oil seeps have also been reported from a clay mine in the Dakota Group north of Tucker Gulch and from a water well south of the amphitheater in Red Rocks Park (Morrison quadrangle). Oil possibly is trapped in a zone under the Golden fault, or in small anticlines such as the one east of the Table Mountains. This structure was tested by a well east of Hyatt Lake in 1955. Although the well was drilled into the Lyons Sandstone, no commercial oil shows were found. Possible producing units include the Lyons Sandstone, Dakota Group, the equivalent of the Carlile Shale in the Benton Shale, Fort Hays Limestone Member of the Niobrara Formation, and sandstone or siltstone beds in the Pierre Shale. URANIUM A potential economic deposit of uranium occurs in Tucker Gulch where a 6-inch-thick zone of high radio- activity occurs in the Benton Shale just east of the Foun- tain—Benton fault contact. Radiometric and radiochem- ical analyses of this material show 0.08 percent equivalent uranium but 0.019 percent uranium. The material that gives the equivalent uranium reading is composed of radioactive disintegration products of uranium, indi- cating a possibility that commercial deposits of uranium exist at depth. A uranium prospect in the Laramie Formation at Leyden Creek in the SEl/zSW‘A sec 28, T. 2 S., R. 70 W. (Gude and McKeown, 1952) has been abandoned. Several prospects have been driven on faults in the interlayered gneiss in Indian Gulch and one on the south side of Clear Creek. The prospect south of Clear Creek is on a ridge, where a brecciated fault dips 70° S. and gives a scintil- lometer count of several times the background. GOLD Gold was first mined in the Golden quadrangle in 1859 from placer deposits at the site of the town of Arapahoe Bar on the flood plain of Clear Creek near the SW. cor. sec. 24, T. 3 S., R. 70 W. (Henderson, 1926). Placer deposits were subsequently worked along Clear Creek but no large 104 discoveries were made in Jefferson County. From 1858 to 1962, 16,558 fine ounces of gold worth $553,084 was pro- duced in Jefferson County (Prommel and Hopkins, 1964). Much of this came from Clear Creek but part of it came from west of the Golden quadrangle. Since World War II a small, but unrevealed, amount of gold has been recovered in the quadrangle as a byproduct of some of the gravel- mining operations in the flood plain of Clear Creek. The gold is fine grained but is coarser in the lower and upper thirds of the gravel deposits (W. H. Slensker, oral commun.). CLAY Clay has been mined from the Fountain, Lykins, Mor- rison, Benton, Pierre, Laramie, and Arapahoe Formations, and Dakota Group in or near the Golden quadrangle. The principal productive beds are in the Dakota, Laramie, and Benton. Refractory-grade clay occurs in the South Platte Formation of the Dakota Group, and isolated deposits of refractory clay also occur in the Laramie (Waagé, 1952, p. 378, 385). Most of the readily accessible refractory clay has been mined from the area. Substantial reserves of clay are present north of Van Bibber Creek, but they may have a high iron content (Waagé, 1961, p. 88, 89). Most common clay produced in ,the area is from the Laramie Formation. The best grades are generally in the lower beds, although most clay beds in the formation are usable. Common clay of the Laramie has been mined in the following areas: Golden, half a mile south of Van Bibber Creek, Leyden Creek to the Denver and Rio Grande Western Railroad, and south of Rocky Flats Lake. Exten- sive deposits of clay of unknown quality are present in the intervening areas and in the northeastern part of the quad- rangle. Clay from the upper part of the Pierre Shale has been treated and expanded to form a lightweight aggregate. In the Golden quadrangle two pits adjacent to monzonite intrusions produced shale for a short time. In 1964 an area 2 miles north of the quadrangle produced at a rate of a few hundred thousand tons a year (Bush, 1964, p. 200), and this locality was still producing in 1967. Clay from the Benton Shale has a tendency to bloat and is generally blended with other clays. Waagé (1952, p. 388) stated that the Benton might be a source of bloating clay suitable for manufacturing lightweight concrete blocks. Benton Shale has been produced from a small pit west of North Table Mountain and from a pit north of Van Bibber Creek. SILICA SAND No quarries developed for silica sand have been noted in the Golden quadrangle, but Argall (1949, p. 355, 357) stated that silica sand has been quarried from the Dakota Group north of Golden and at several other places in Colo- rado. The Dakota in the Golden quadrangle is similar to these deposits and is a potential source of silica sand. GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO DIMENSION STONE Although parts of the Lyons and Lykins Formations and latites of Tertiary age have been used for dimension stone, no quarries in the Lyons or Lykins have been oper- ated in the Golden quadrangle. The Lyons Sandstone exposed in the Golden quadrangle does not have the highly prized pink color of the stone quarried at Lyons, Colo. The crinkled limestone of the Lykins has recently been used for a decorative stone finish on fireplaces in some of the new homes in the Denver area, but it is not used as a structural stone nor is it used on the outside of buildings. Latite from quarries on North and South Table Moun- tains has been used as cobblestone and building stone. It made an excellent cobblestone, but this use is outmoded; latite also has had some use as a building stone, but it does not have a pleasing aspect, owing to the somber color. Sources of lichen-covered field rock (moss rock) for facing include the outcrop areas of rocks of Precambrian age, and the Fountain, Lyons, Morrison, and Laramie Formations, and Dakota Group. Most of these will not break into predictable shapes and are brown or gray. LIMESTONE Limestone beds in the Lykins and Niobrara Forma- tions were extensively quarried for mortar and smelter flux many years ago, but limestone no longer is used as a source for this material in the Golden quadrangle. The use of limestone for nonchemical purposes is discussed in the following section. CRUSHED ROCK Several quarries have extracted rock from the Precam- brian interlayered gneiss and the Tertiary igneous rock exposed in the Golden quadrangle. Both of these materials have been used for concrete aggregate and riprap. Lime- stone suitable for use as crushed rock is also present in the area. Interlayered gneiss from a quarry in Clear Creek Canyon, half a mile west of Golden, has been used as concrete aggregate and riprap. The material is fine to coarse grained, harsh (produces very angular fragments when crushed), nonreactive, and sound (Hickey, 1950, p. 10). In addition to concrete aggregate and riprap, it is suit- able for railroad ballast, highway base course, and other uses where hard, durable, angular, crushed rock is desired. Latite has been quarried from North and South Table Mountains, and monzonite from Ralston dike, for use as riprap, concrete aggregate, and road metal. Both rocks are hard, tough, durable, and harsh. The only limestone quarried in this area in recent years came from a small quarry in the upper part of the “crinkled limestone” (Glennon Limestone Member of the Lykins of LeRoy) in the NBA sec. 17, T. 3 S., R. 70 W. This material is used to surface private driveways. It has a pleasing reddish color and should wear well but would ENGINEERING GEOLOGY probably wear excessively if used on heavy duty roads and highways. A sample of this material from Ralston Reservoir showed 44.0 percent loss in the Los Angeles abrasion test. A sample from the 4-foot-thick dolomitic limestone that underlies this bed showed a 44.7 percent loss in the Los Angeles abrasion test. The Fort Hays Limestone Member of the Niobrara Formation contains a considerable amount of inter- bedded shale. A sample of this limestone from Ralston Reservoir showed only 24.4 percent loss in the Los Angeles abrasion test; another sample from the Pueblo area (100 miles south of Golden) gave a loss of 24.7 percent. It is hard, durable, harsh, and tough; it would probably be suitable for mineral aggregate in highway construction. The light color of the Fort Hays would be particularly de- sirable for the wearing or armor course of asphaltic concrete. Probably neither the Fort Hays nor the Glennon would be suitable for concrete aggregate, although, to my knowledge, no tests for this use have been made. SAND AND GRAVEL Sources of sand and gravel include Rocky Flats, Verdos, Slocum, Louviers, and Broadway Alluviums, and trans- ported mantle and alluvial-fan deposits. About 250 million cubic yards of sand and gravel suitable for concrete and mineral aggregate is present in the quadrangle, but urban development is rapidly encroaching on the most de- sirable deposits and probably only a small part of these deposits will ever be mined. About 7 million cubic yards had been mined in 1964. The alluvial deposits adjacent to and underlying Clear Creek and the Louviers and Broadway Alluviums adjacent to Ralston Creek are the best sources of concrete aggre- gate. The material ranges from silt to cobble size and appears to be sound. The lithology of the deposits is shown in table 8. The material has been used locally for both concrete and bituminous aggregate and for many other purposes. Most of the other deposits contain enough silt and clay to make them less desirable for concrete aggre- gate, although they generally are suitable for use as mineral aggregate. The alluvial fans and other gravelly deposits near the mountain front generally contain a large proportion of boulders. None of the deposits are known to have a harmful amount of deleterious minerals, although bituminous binder might strip from the quartzite in the deposits from Coal Creek. Small amounts of placer gold are extracted as a byproduct from some of the sand and gravel operations on Clear Creek. Where available the size distribution of the material is shown separately in the present report in the geologic de- scription of each unit. Materials test data consisting of mechanical analyses, coefficient of sorting, Atterburg limits, pH, swell capacity, specific gravity, and soil clas- sification (both Unified and AASHO) of 57 samples from the quadrangle have previously been published (Van Horn, 1968). 105 ENGINEERING GEOLOGY An economic and physical relationship exists between man and the different geologic units because of the way in which he uses and builds on them. In general the geologic units have a definite range of physical properties, and their gross reaction to outside forces acting on them is to some extent predictable. The physical character of geologic units has an effect on, among other things, foundation conditions, workability, suitability of septic systems, cut— slope stability, and earthquake hazard. Construction materials were discussed in a previous section of this report. FOUNDATION CONDITIONS Metamorphic rock, igneous rock, and sandstone (except in the Denver Formation) generally provide good to excel- lent foundation conditions. The alluvial terrace deposits provide good foundations, but at places high water tables may exist at basement level. Several geologic units provide poor foundation conditions in the quadrangle; these are parts of the Ralston Creek, Morrison, Benton, Pierre, Fox Hills, Laramie, Arapahoe, and Denver Formations, and colluvium, transported mantle, loess, artificial fill, and landslide deposits. In all these units except the last three, swelling clays cause the potentially poor foundation conditions in the area. The chief source of potential failure of foundations in the last three units is differential settlement, and in the event of landslides the entire struc- ture may move laterally. Recognition of these potential hazards can lead to proper foundation design prior to con- struction. SWELLING CLAY Some clay minerals, principally montmorillonite, undergo expansion and contraction related to wetting and drying. Structures built on deposits containing such minerals can be damaged by the alternate swelling and shrinking of the clay. In the Golden quadrangle the Denver Formation contains abundant montmorillonite (table 4), and structures built on the Denver have sus- tained damage to a greater degree than structures built on other geologic units. This greater amount of damage may be more apparent than real, however, because more than half the structures in the area that are built on potentially expansive formations are built on the Denver. Some damage has also been sustained by structures built on clay and shale beds in the Laramie Formation and the Pierre Shale. Expansive clays may also be found in clay and shale beds in the Arapahoe, Fox Hills, Benton, Morrison, and Ralston Creek Formations, all of which are known to con- tain montmorillonite. The thick shale and mudstone beds in the Lykins and Fountain Formations are not believed to be expansive but probably should be tested prior to con- struction of any structure on them. The presence of montmorillonite does not auto- 106 matically indicate poor foundation conditions. Ralston Reservoir dam and a tunnel into Upper Long Lake are both in Pierre Shale, which contains abundant mont- morillonite. Neither structure has shown any indication of distress caused by swelling clays. The Smoky Hill Shale Member of the Niobrara Formation contains some thin bentonite beds which probably are mainly montmoril- lonite. Because the beds are thin and widely spaced they probably will have little effect on foundation conditions. Colluvium, transported mantle, loess, and soils locally contain montmorillonite. Materials tested show that some samples of these deposits show high swell and plasticity indexes (Van Horn, 1968), both of which indicate poten- tially hazardous swelling properties. Holtz and Gibbs (1954) pointed out that materials having a high plasticity index generally are more expansive than those having a low plasticity index. Further testing is advisable on any material that has a plasticity index of 20 or more before constructing any buildings on it. DIFFERENTIAL SETTLEMENT Loess, when saturated with water, tends to settle under heavy loads. The amount of settling frequently is not the same at all places, and consequently may cause unusual stress in the structures. Artificial fill, where poorly compacted, may also settle differentially under heavy loads; it also hides older more stable deposits so that parts of a building founded on fill may inadvertently be partly or entirely supported by the older deposit. (See fig. 52.) This is particularly possible along the outcrop of the Laramie and Arapahoe Formations where abandoned clay pits that are separated by vertical walls of sandstone may be used for dumps. Differential settlement is possible wherever a structure is fOunded on two different geologic units. A potential hazard in the area is the surface expression of cave-ins of old mine workings. N 0 such cave-ins were seen in the area but the possibility is present, particularly along the outcrop of the Laramie Formation. These cave-ins tend to become obscure as time passes, and examples of cave-ins reported in the Golden Globe November 29, 1902, and March 14, 1903 (see p. 109) are no longer visible because the surface openings are now filled with debris. LANDSLIDES Landslides provide the least stable, and perhaps the least predictable, foundations. The mapping of landslides in a rapidly growing urban area presents delicate economic problems. The property involved in landslides is gen- erally on the higher slopes and is esteemed as view prop- erty. At my present state of knowledge I am unable to predict when, or if, any such property located on a land- slide, whether old or recent, will move downslope. Consultant engineers can design buildings that will with- stand such movement but the cost may be prohibitive, and GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO FIGURE 52.—Structural distress in an apartment building west of the Colorado School of Mines that was constructed over a well- compacted artificial fill. The load-bearing members of most of the apartment buildings in this area were reportedly founded on sandstone ribs that separated abandoned clay pits (now filled) as much as 50 feet deep, or on caissons to sandstone. The building pictured, however, was reportedly placed on footings 3 feet deep founded on thick artificial fill overlying a clay pit. Photograph by M. M. Lemke, Colorado School of Mines. perhaps unnecessary. The incidence of active slides is not high at present, but when man tampers with the natural slopes by excavating the toe of the slope, or adding abnormal amounts of water from septic tanks and lawn watering, the incidence may increase. Besides, the home- owner who finds himself in possession of part of the “Heartbreak Hills” (Johns, 1958) with his home being torn asunder won’t care about the incidence rate of land— slides. The principal areas of unstable slopes are the sides of North and South Table Mountains underlain by the Denver Formation. (See p. 108.) Numerous old landslides and debris from rockfalls can be seen in this area, and several areas on the north side of South Table Mountain ENGINEERING GEOLOGY have moved in recent years (Van Horn, 1954). These land- slides appear to have formed both by rotational move- ment along a curved slide plane and by debris sliding down a planar slide plane. Several similar, but generally smaller, landslides occur in the lower 1,000 feet of the Pierre Shale and several good examples can be seen on the west side of Ralston Reservoir. The reactivation of an old landslide in 1959 in the basal part of the Arapahoe Forma- tion just north of Ralston Creek forced the relocation of State Highway 93 (fig. 53). Several small slides are in the upper part of the Laramie Formation on the south side of Leyden Creek. Landslides and rockfalls also occur in the Precambrian rocks. WORKABILITY The surficial materials and most of the shale, mud- stone, and rocks of the Denver Formation (except the latite) can probably be excavated with power equipment. The other rock units locally will require blasting to excavate. The alluvial deposits, being relatively free of clay, will be easiest to work with power equipment, and colluvium and landslide deposits will probably provide relatively difficult working conditions. On Rocky Flats the allu- vial-fan deposits and the Rocky Flats Alluvium contain some very large boulders that will require special han- dling. The Rocky Flats Alluvium locally contains abun- dant clay and, as a result, it is more difficult to work with power equipment than are other alluviums. 1 07 SEPTIC SYSTEMS Sewage disposal in much of the quadrangle is by small septic systems. The alluvial deposits provide the best drainage for disposing of the effluent from these systems. Care should be taken to see that the systems are in good condition and are not discharging noxious wastes into the ground water. The many shallow wells in the alluvium could be contaminated by malfunctioning septic systems. Fine-grained deposits such as colluvium, loess, the Piney Creek Alluvium, parts of the Broadway Alluvium, and transported mantle are less desirable but can be used for septic systems. They do not drain as well as the allu- viums. It would be injudicious to allow such systems to discharge into landslide deposits because additional water would only increase the danger of movement. The bedrock formations are not suitable for disposal of septic-system effluent. CUT-SLOPE STABILITY The cut-slope stability of the geologic units in the quad- rangle differs greatly from one unit to another. In areas of low dip the sandstone beds are stable in nearly vertical cuts. In areas of steeply dipping beds or joints, cuts in any of the bedrock units should be designed for the attitude of the bedding and joints at the particular area, because bedding and joints together may outline blocks of rock that can slide downslope if an artificial cut removes their natural support. The hogback ridges and metamorphic FIGURE 53.—Hummocky topography on an ancient landslide (dotted line) reactivated by highway construction in 1959 on State High- way 93, sec. 33, T. 2 S., R. 70 W. Three generations of high- ways are visible. The oldest (arrows) forms a faint trace that ascends the slope without a U-turn. It was constructed prior to 1895 on the ancient landslide and had not been broken by land- slides until the 1959 reactivation. The highway with the U-turn was constructed prior to 1942 and was widened and rebuilt in 1959. Movement on the landslide started (dashed lines) before con- struction was completed, and the highway was relocated. The re- newed movement also disrupted the trace of the oldest highway. Photograph, toward the northeast, by H. E. Malde, U.S. Geo- logical Survey. 108 terrane of the mountains are particularly hazardous areas in this respect. Shale and claystone beds are generally weak, and design of cutoslopes should account for strength of the particular beds as well as the attitude of the joints and bedding. Cut-slopes in clay and shale can be unstable in any setting. The steepness of cut-slopes in the surficial deposits should not exceed the minimum requirements of the Colorado Industrial Commission. Water saturation, which may be found in these units, provides hazardous excavating conditions and calls for extra caution. Cuts into landslide areas should only be made after a thorough slope-stability analysis. EARTHQUAKE HAZARD There is no evidence for movement along any of the faults in the Golden area during Holocene time, which encompasses roughly the last 10,000 years. Several of the earthquakes centered around the Denver area since 1962 have been felt in the quadrangle, but, to my knowledge, no structural damage in the Golden area has been caused by these tremors. In the event of strong earthquakes affecting the Golden area, landslides and colluvial deposits would be the least stable material, and bedrock the most stable. HISTORICAL GEOLOGIC EVENTS AT GOLDEN, COLO., AND VICINITY This is a summary of historical events with geologic implications that have taken place at and near Golden as recorded in old issues of the Golden Globe that are now kept in the Pioneer Museum at Golden, Colo. The news- papers examined were published during the years 1878—87, 1895 to mid-1903, 1906—11, and 1913—14, a total of 26% years. The first item is not, strictly speaking, part of this record, but this early notice of a rockfall inspired'my search for old records of landslides. The record is not complete but the dates covered will give some idea of the frequency of the events. Landslides, rockfalls, and floods account for most of the events recorded herein. During the reading of these old papers it became evident that not every slide nor all movements of a particular slide were reported. Judging from inferences read in the old papers and from present-day newspaper standards probably 50 percent or less of the active landslides and rockfalls were reported. June 22, 1871. An estimated 1,000 tons of rock fell from the south face of North Table Mountain with a roar like an earthquake [re— ported in the Golden Transcript newspaper column “90 Years Ago Today” for June 22, 1961.] July 5, 1879. Hailstones as much as 7 inches in diameter caused much damage in Golden. GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO March 12, 1881. A landslide occurred near the Valley Smelter on the south side of North Table Mountain. The material slid down a wet bedrock surface. Deep cracks formed near the Church Ditch, and a small hill rose beneath the railroad tracks. On March 19, 1881, the Golden Globe reported that as many as 40 men had been employed in the previous week to keep the track straight, but that it was still bulged considerably. August 10, 1881. A cloudburst in Clear Creek Canyon 1% miles east of Beaver Brook caused 22 “landslides" in 5 miles downstream. The railroad line was broken at several places, and debris as much as 5 feet deep was deposited on the track. [These 22 “landslides” may not have been true landslides and are not included in the count of landslides] June 10, 1882. Floods down Kinney Run [now called Kenneys Creek] and Tucker Gulch washed out the railroad beds. July 3, 1882. Flood in Tucker Gulch caused moderate damage. July 21, 1883. A landslide on the south side of North Table Mountain near the Valley Smelter caused damage to the Church Ditch and the railroad. Subsequent movement was reported on July 28 and August 4 of 1883. August 11, 1883. Small flood on Tucker Gulch washed out roads. July 14, 1887. A cloudburst caused floods in Tucker Gulch, Dry [now called Van Bibber], and Ralston Creeks. At Glencoe [now covered by waters of Ralston Reservoir] Ralston Creek was 150 feet wide and swept away the Post Office. May 16, 1896. Another landslide occurred on the Gulf Railroad oppo- site the Carpenter place [on the south side of North Table Mountain]. July 25, 1896. Heavy rain the previous week washed out the Gulf Railroad tracks at Chimney Gulch [southwest of the campground, and across Clear Creek, at Golden]. July 24 and 25, 1896. Widespread floods on these dates were reported in the papers issued on August 1 and 8, 1896, and mentioned in the paper of May 21, 1898. Floods occurred in Mount Vermon Can- yon, Cub Creek, Clear Creek, Tucker Gulch, Crismans [now called Cressmans] Gulch, and Ralston Creek. Great damage was sustained in Golden, and by the bridges and railroad tracks up Clear Creek Canyon. Six deaths resulted, three in Golden and three in Mount Vernon Canyon. August 15, 1896. In Clear Creek Canyon two men, who were working on the railroad, were injured by a falling rock and three others were injured by being buried in a landslide. August 22, 1896. A rockslide in Clear Creek Canyon 3 miles west of Golden near Guy Gulch delayed the train, and 100 feet of track was washed out near Beaver Brook. September 5, 1896. Two men, while working on the railroad, were injured by dirt and rock caving on them. No location is given. September 12, 1896. A small flood occurred on Tucker Gulch. September 19, 1896. A man showed the editor a bottle of crude petro- leum and water he had recovered from a crevice in some rocks near Golden Gate Canyon. [This started Golden’s first oil boom and within a month a well was being drilled in the vicinity of the brick- yard north of town—subsequent issues of the newspaper indicate that it was a dry hole.] June 5, 1897. Another landslide on the Agricultural Ditch just west of Rees Easely’s [South Table Mountain]. July 10, 1897. The railroad is building a 350-foot bridge at the slide 1% miles below Golden. August 7, 1897 . A small flood occurred in Tucker Gulch. April 8, 1899. A rockslide in Clear Creek Canyon derailed a Colorado and Southern locomotive. July 22, 1899. A flood occurred on Ralston Creek. August 12, 1899. A rockslide at the mouth of Golden Gate Canyon damaged the road. May 5, 1900. The landslide below the old smelter moved again. REFERENCES CITED May 26, 1900. A great mass of rock broke off the face of Castle Rock with a report like a cannon. November 8, 1902. The ground north of Clear Creek and below the smelter is moving the Colorado and Southern Railroad tracks. November 29, 1902. A man almost stepped into a cave-in of an old coal cavern southwest of Golden. March 14, 1903. A yawning chasm that should be fenced has formed at the little [new] White Ash Coal mine [called the New (Little) White Ash in the present report] north of Golden. April 4, 1903. A flow of 40,000 gallons of water per day from an old coal mine was intercepted by the tunnel through the Laramie For- mation for the Welch Ditch southwest of Golden. September 16, 1907. A 12- by 12- by 12-foot boulder fell from the Tram- way Quarry [SE‘A sec. 22, T. 3 S., R. 70 W.] on the south side of North Table Mountain and lodged in the Church Ditch just above a house. April 20, 1907. A man was buried alive in a ditch being dug for a retaining wall on the landslide below the smelter. May 4, 1907. Cracks opened in the ground below the Tramway Quarry [SE‘A sec. 22, T. 3 S., R. 70 W.] on the south side of North Table Mountain and the ground seemed to be moving. A huge mass of rock ploughed down the mountain below the quarry. June 15, 1907. Ground near 14th and Ford Streets in Golden caved into Kinney Run [Kenneys Creek]. July 13, 1907. A flood on Clear Creek was caused by a cloudburst near Black Hawk. Water was as much as 5 feet deep on 11th Street and was running 2 feet over the top of the Ford Street bridge. The Washington Street bridge was moved off its abutments. There was much damage to the railroad west of Golden and flood debris was found 20 feet above the bed of Clear Creek. Damage amounted to about $50,000 [reported in the Oct. 20, 1907, issue]] July 10, 1909. A cloudburst west of Golden caused Clear Creek to rise 5 feet in 20 minutes. Lawns and irrigation ditches were covered with sand and boulders. July 31, 1909. A flood on Tucker Gulch washed out all bridges between Golden Gate Canyon and Clear Creek except the Colorado and Southern Railroad bridge which was completely covered with water. Flood waters reached to Washington Ave. May 17, 1913. An immense boulder, accompanied by many small rocks, rolled from the top of North Table Mountain into the Church Ditch narrowly missing a rural mailman. It had rained heavily the pre- ceding night. August 16, 1913. A flood on Kinney Run [Kenneys Creek] was caused by a cloudburst [estimated 4 inches of rain in ‘xé hour]. About $10,000 damage was sustained by homes, businesses, and bridges on Ford Street. September 6, 1913. Small floods occurred in several gulches in Golden. The railroad tracks west of Golden were covered by tons of rock and sand at Magpie Gulch [north side of Clear Creek, SW sec. 28, T. 3 S., R. 70 W.] . March 14, 1914. A large deposit of bitumen—soaked sandstone was found near Turkey Creek south of Morrison. April 4, 1914. A man was seriously injured at the rock quarry on North Table Mountain when pieces of basalt [latite] fell from the cliff without warning. May 2, 1914. At 4:00 a.m. Tuesday morning a few thousand tons of rock slumped several yards and covered up the access road at the Tramway Quarry [SE‘A sec. 22. T. 3 S., R. 70 W.] on North Table Mountain. Heavy snow and rain have softened the “clay and ash." The ditch company and the railroad experienced much trouble in previous years when great sections of hillside moved slowly down toward Clear Creek. July 18, 1914. On Saturday morning the biggest slide ever reported here took place just east of the Frank Owen's Ranch on South Table 109 Mountain. Large crevices opened just below the Welch [Golden Canal] Ditch. A great mass of earth several hundred feet across moved several feet down the mountain. Some ground moved as a unit but some was badly cracked. The Agricultural Ditch was destroyed and the level ground at the base of the mountain was lifted into a ridge 15 feet high. REFERENCES CITED American Stratigraphic Co., 1956, S. D. Johnson Farmers Highline Canal and Reservoir Co., well no. 1: Well log no. 760, sec. 7, T. 3 S., R. 69 W., Jefferson County, Colo. Argall, G. 0., Jr., 1949, Industrial minerals of Colorado: Colorado School Mines Quart, v. 44, no. 2, 477 p. Baker, A. A., Dane, C. H., and Reeside, J. 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A Page A horizon, soil, defined ..... r. 92 Broadway Alluvium ..72, 74, 75 Piney Creek ......... 76 Rocky Flats Alluvium ........................... 61 A1 horizon, soil, transported mantle deposit.. 82 A3 horizon, soil, transported mantle deposit.. 82 Abstract ....... l Aclistochara. 13 Aflonian deposits. 53 Age, Cretaceous..... . 15 Holocene .................. . 61, 74 post-Piney Creek. 70 Jurassic. 13 Pennsylvanian . 8 Permian .......... 8, 10, 11 Pleistocene ..... 57 Aftonian.. . 58 Bull Lake . 69, 72 Kansan 62 Nebraskan.. 58 Pinedale ..... 69, 86 pre-Bull Lake. 69 Yarmouth 62 Precambrian.“ 5 Quaternary .. 53 Tertiary....... . 37, 47 Triassic ..... 11 Ages, redefined 69 Agricultural Ditch, landslides 89 Algae. Aclislocham 13 dayscladacean. 12 Echinorham spinosa ......................................... 13 A" ‘ "r ' emm 39 Alluvial fan deposits. 79 Alluvium, described. 55 Louviers 69 post-Piney Creek 70, 76 pre-Piney Creek . 75 pre-Rocky F1ats.. 57 Rocky Flats..... 58 Slocum ........ 67 soil horizons 93 Verdos ...... 62 workability” 107 X-ray analysis . 95 A ,t sp. l7 Anticline, Front Range ............................................ 95 Arapahoe Bar, gold.... 103 Arapahoe Formation.. 35 age... 86 boundary 43 clay production 104 fossil plants ................... 36 foundation conditions .. 105 d section 37 Artifical fill 91 clay pits 92 differential settlement 106 B B horizon, soil, Broadway Alluvium..t......,..,,......72. 74, 75 defined 93 Louviers Alluvium .. 72 Rocky Flats Alluvium .. . 59, 61 INDEX [Page numbers of major references are in italics] B horizon, soil—Continued Page Slocum Alluvium ............ .. 68, 69 transported mantle deposit. 82 Verdos Alluvium 62, 67 Baculites asp” 30 asperi/ormis . 23 baculus 21 c r. 23 23 21 23 24 gregoryensis ...................................................... 23 m ' m' 22 obfusus 22 "J ' 23 Stem... 23 n. sp. .. 23 sp,........... 20 Barbara Gulch, coal 34 Barnacle, Smoky Hill Shale Member“ 20 Bca horizon, soil, Broadway Alluvium 75 Slocum Alluvium 69 Benton Shale. 18 age... 19 clay production 104 faulting ............ 99 foundation conditions .. 105 uranium ..................... 105 Bentonite, Greenhorn Limestone equivalent. 18 Smoky Hill Shale Member...........,....,.. 20 Biotite syenite. Se: Dikes, hornblende-biotite lamprophyre. Bird tracks, Dakota Group... 17 Bison B. bison .................... 75 Boundary. Cretaceous-Tertiary .. 37 Laramie, Arapahoe, and Denver Formation.... 43 Broadway Alluvium, age 75 correlation.. 69 gravel ...... 105 Leyden Creek 74 Mount Olivet Cemetery. 75 Ralston Creek 74 sand...... 105 soil 74, 93 soil, measured section. 75 Standley Lake ...... 74 Van Bibber Creek. 73 Bull Lake, post-, soil .. 72 pre-, glaciations... 102 soil ..63, 66, 68, 69 Bull Lake age, alluvial fan deposits 79 Bull Lake deposits... . 69, 72 Bull Lake time, alluvtum deposition. 102 C C horizon, defined 93 Cap Rock mine ..... 34 Capitol mine ................ 34 Carlile Shale equivalent. 18 Castle Rock.... 52 rockfall.... 89 Cca horizon, soil, alluvial fan deposit 80 Broadway Alluvium .......................................... 74 defined 93 Cca horizon, soil—Continued Page Ines: 85 Louviers Alluvium ..... t, 70, 72 Rocky Flats Alluvium .. 60, 61 Slocum Alluvium . 68, 69 transported mantle deposit .. 82 Verdos Alluvium.. ..63, 66, 67 Cephalopods, Fox Hills Sandstone 30 Pierre Shale .............................. 21 Smoky Hill Shale Member................ 20 Chalcedony beds, Ralston Creek Formation . 13 Clay 104 swelling ............. 105 Clay pits, artifiaal fill. 92 Clear Creek, alluvial fan deposns .. 79 gravel ..................................... 105 Golden fault zone.... 99 Louviers Alluvium .. . 69 post-Piney Creek alluvium ............................... 77 profile 56 pl. 1 Rocky Flats Alluvium 50 sand ................... 105 Slocum Alluvium 67 transported mantle 81 Verdos Alluvium.. 63 uranium ...................... 103 Clear Creek Canyon, garnet .. 8 interlayered gneiss 8 microcline gneiss unit 8 oil ................................ 103 Premmbrian rocks, crushed ............ 104 Coal 32 Barbara Gulch .. 34 production .. 103 red dog. 91 Goal Creek, Rocky Flats Alluvium. . 58 Goal Creek Quartzite 55, 58, 59, 60, 68, 74 Colluyium, foundation conditions. 106 graben 100 Leyden Gulch 83 Piney Creek Alluvium. 83 South Table Mountain 83 Welch Ditch.,,,,............ 85 Columnar jointing, rockfalls. 91 Concretions, Morrison Formation 15 Pierre Shale ............................ . 23 Cow 78 Creamy sandstone. 10 Cretaceous rocks. 15 Cretaceous-Tertiary boundary 37, 39, 43 Cressmans Gulch, mica schist unit. 5 hornblende gneiss unit Crinkled limestone.,............ D Dakota Group. 15 age ......... l7 clay production. 104 faulting...r....... 99 origin ....... 18 silica sand 104 Dakota hogback, Louviers Alluvium ......... 72 Rocky Flats Alluvium, Ralston Creek 60 113 114 Dakota hogback—Continued Page Van Bibber Creek, post-Piney Creek alluvium .. 78 Dayscladacean alga 12 Denver and Rio Grande Western Railroad cut, Fox Hills Sandstone. 28 Laramie Fomtation 32 Denver Formation 37 age......... 39 boundary 43 faulting . 100 foundation conditions . .. 105 1and<1idP 88 d wtinn 40 origin ......... 40 39 107 62 D ", eras aha, 23 ' "up 23 ‘ 23 Bikes, hornblende-biotite lamprophyre .................. B Dimension stone ...... 104 Dinosaur fragments . 39 Dinosaur horn, Laramie Formation . 34 Dinosaur tracks, Dakota Group. l7 Dumps 92 E, F Earthquake hazards ..... 108 Echinocham spinosa... 13 Economic geology.... 103 Engineering geology 105 Entrada Sandstone 13 Equus sp.. 89 Exitelocnasm jmnyt. 23 Fairmount School, Slocum Alluvium. . 68 Farmers Highline Canal, landslide ..... . 87 Fault: 96 evidenc‘e, fossil zones ............. 97. 99 Ralston dike. 99 F icur planicostala 39 Fish teeth, Niobrara Formation 19 Floods, alluvium, Ralston Creek. 76 listed 108 Plum Creek, historic. 75 Plum Creek deposits. 79 Ralston Creek, historic .. 74 Tucker Gulch, historic 0, 77, 8O Flows... 50 Foramiriifera, Fox Hills Sandstone 25, 28 Fort Hays Limestone Member 19 crushed rock ........... 105 ‘. wtinn 19 Fossil localities, described ..... 1 Fossil zones 21 fault evidence. 97, 99 Fossil plants, Aclistochur 13 Allanlodiopst'r 2705a 39 dayscladacean ........ 12 Echinochara spinosa 13 Ficus planicostata. 39 petrified logs... 39 Fossils, Baculites asper 30 Bamliles aspnilorm 23 baculus....... 21 clinolobalus 23 compressus 23 cumulus” 23 elimi. 21 gilbnt 23 grandu . 24 gregorymst's 23 maclearni 22 abturus 22 insider 23 um“. 23 n. sp. 23 sp... 20 Fossils—Continued harnarlp Bison B. bison Bos Mum: ..... Derocetas law: ................. Didymacerar cheyznnenre nebmscense rteuznsoni. F ' ’ jennyx' fish teeth Fountain Formation. Gymulus velmus..... Haplophragmoider ............................ Irl r '- deformis typicur... Lymnam monisonensis Miuia minuta .............. Oreohelix sp. indet Ostreu glabru......... Pseudopema... cangesta .. Ptert'a nebrascena Pupilla Mandi... Sphmodirnzs sp Spongillidac ...... "‘ ‘ haworlhi Tmuipteria fibrosa ..... tracks, Dakota Group. Triceratops .............. Unio ..... sp... Valltmia gracilicorla... 7 ' '4 arbor Foundation conditions Fountain Formation ...... age .................. clay production. faulting ...... fossils .......... foundation conditions origin Fox Hills Sandstone age. foundation conditions graben .. local correlation measured sections origin" Freshwater mollusks, Morrison Formation Front Range anucline Garneros Shale equivalent....................................... Gas tnrave Gastropods, freshwater, Jurassic Verdos Alluvium. Geomorphology. See Phystography. Geologic history, summarized ................................. Glauconite, Pierre Shale..... Gold, Louviers Alluvium production Golden fault evidence. Golden Gate Canyon, hornblende gneiss unit microcline gneiss unit....... post-Piney Creek alluvium Graben, Fox Hills Sandstone... Graveyard hill, Verdos Alluvium Greenhorn Limestone equivalent faulting Gyzaulus vetemus Halfmile Gulch, 011 all. J ‘rr Heartbreak oHills" Page 20 75 78 62 23 23 23 105 103 103 28 GEOLOGY OF THE GOLDEN QUADRANGLE, COLORADO Historical geology ...................... History, geological, summarized recent Holocene age, alluvium deposition ........................ Holocene deposits Holocene soils ..... Horse, modern . Hyatt Lake, oil. Verdos Alluvium ............................................... Igneous rocks. crushed ........... latite, columnar jointing dimension stone. flow 1 ............. flow 2. flow 3 monzonite Tertiary Ignolomir mcconnelli . Illinoian deposits ...... alluvium deposition 111ite, Carlile Shale equivalent. Morrison Formation Indian artifacts ................ Indiana Street, Ralston Creek, Broadway Alluvium ............................................ Inut (hm L tic/omits typicus.. lntrustves J,K Johnson 1 well, S. D Jurassic rocks ........... Kansan deposits .......... Kansan time, alluvium deposition Kaolinite, Carlile Shale equivalent Dakota Group ...... IJramie Formation Morrison Formation. Smoky Hill Shale Membe .. Kenneys Creek, Broadway Alluvrum Louviers Alluvium . Landslides .. active .. .. Table Mountains Agricultural Ditch ..... fossil: foundation conditions...................................... “\\\‘\\\ - Op \\\.\. 06 'X — - 7%\. x\ 00° 5600’ _ ‘ \\\\ \_ 7 _ Ore (south of Leyden Lake) Ore (northeast Of _ 5600, l Ralston Creek \ KN Ore (south of Fremont School) Fremont School) 5500’ 7 l R rQ,\ ~\ - -\ — 5500’ ' OD Ralston CreW'WN , 5400' 5400 B. RALSTON CREEK I C C WEST EAST ,_ 7 6200’ 6200 0'0 Z w \‘ '\-/\ f Orf \ - 0 LL \X .\>\ z > , 6100'* ‘-\\ g 8 I a 6100 '\,\_\ 0' Ob Oyf \>\'\.L‘\ I 6000’7 ‘**i.-;\~\.\ ~ 7 6000' Op ’\"\4‘\‘\‘:\ l0 ‘T\es:»\mf 0% 5900’7 \\‘\\\S\\\\\.~\\.\ Z 3 L“l 5 7 5900' %§-\ 0 ”—- Engu Van Bibber Creek \\\\\\‘\\\\,.\ E 8 0|]: glo , 5800’7 \\\\ 0“ CL (I I- 012 75800 -77ze\ m Mg 9e 'w—K- , ~\\.\.\ 05 IL“ _ , 5700 7 §$~\Olm 2 Lu 5700 l '\ \l: \‘\ Q _ —l | i"‘~77;:\77.'° 0“ DE 5600'7 l l \-\\‘$- E 2 0’6 _ Ore 7 5600’ l | '\.\. on o. N -\. 5500’ 7 l | \’ w “ 5500' | l Van Bibber Creek \\\\\\..\409 | l \'\ '\\\\.\ , l l \' \“‘" > 5400' 5400 C. VAN BIBBER CREEK D z 5 0’ NORTHWEST g E SOUTHEAST r . O V ' 6600 oflghLOEEbjnch LuLal E 6600 6500’ \\\\\77\\\\ 76500' G)lVliddlerock \\\\7\¥_\ bench \\\\\O , Saddle 6427 , ' _ 6400 6400 North Table 0 Mountain Low rock , 6300' bench * 6300 E 6200' 7 \ ’ 620° NOR THWEST g]: F’ \\\ Ulysses Street 919 2 Lu J \ , l— _ SOUTHEAST '7 \ 76100 , l Ob Olo :' 6100 \\ 2 m 5700 ‘~_/_ ______ 3'3: 2 Lot. 5700' \\\ E El ~——l-‘=\--l1~%§ Os (of Ralston Creek) g E O ‘ ~\. 2 o 7 6000' “‘~‘-4¥7N'° 60 s. ' ‘—-777 ' 00 §/\E\\{Ob _ g E 5600 l Ob7 —————— \ Ol.0\' Os (of Clear Creek) 5600 Precambrian rocks / ~:’\\ \\\\ i- -\. MOW” Olivet Cemetery , éw'M/y/y ;\.\ Op \\ 10V (base) 0V , 5900: l Ob \ \—»\.\OL 5900 T I nil/C? are/<22?» } '-\ \‘ 3 Concealed M 5500' l ' CH0 (Of Clear creek) 5500’ ‘ . , 7x . re \-‘\. . QCO , ////////. / \ \/ Landslide (OI) u_| , l Opp (of Clear Creek) 5800’7 ,. /; // ./ . .44 / . -‘ ‘ 5| 7 5800 , I , Po/st—Precambrran rocks/y . , ,, , . 4Os El 5400 5400 fiéfi¢V{/// , , ,. \ - Qvf gl . E. LATE PLEISTOCENE COURSE OF VAN BIBBER CREEK 5700’ 7 iflW// ///'/// //// Tucker Creek \‘4\\°-l ’ 5700 Stream bed and Piney Creek Alluvium (Op) not shown . . / \l l . 5600' ' l i 5600 l | 5500’7 l ’ 5500 l 5400, 5400' D. TUCKER CREEK F F, McIntyre Street . EAST ‘ Y WEST Washington Avenue Lava flow 2 (Tertiary) E 5 l Oungfield Street 6200' 6200 — a E Bl‘D T Z o O|Z ‘33 E L fl 2 (T ‘ ) 3l9 ava ow ertiar )— , 6100’7 ‘5 V 8'8 76100 ‘2 \ Olw D‘UJ 6000'- Elli l 7 6000’ , Qrf LLlLu a O I l 5 gill— l ’ 5900’ ~ 3 CV ~. lg | — 5900 <( / Ov (base) lw Z uJ LL g LI_]J : "K {V Denver 5800’ _ 3 El E g g ‘ l Tertiary-Cretaceous boundary ' Tertiary~Cretaceous boundary _ 5800' E E 6 g E | 5‘40: '\‘ Clear Creek 0 O 0 w. 5700’~ \T' ‘\ \kfi — 5700' Precambrian \ I\‘\.TH\TH.T T_l—‘ ______________ . Qslldeposited by Ralston Creek) . rocks ‘ “DenverrFm fix\ _______________________ ' T“ l“-- ' , 5600 7 ‘ “ (pair . l \\\ QNMOIO ________ . Os .. ‘ 5600 c' | . . ‘: . '\ ; .‘N lede ‘t db v B'bb . E g E ( retaceou a? E2”? 3:3 Cretaceous} l \ §‘\\ \Q7‘_Z‘___p_o:lj V an I erCreek) g u. . R“~ . 5500" '5 ”:6 EE .9 65% l : \'\ \, ‘x_ 7 5500 H m H fi' . g E % xg fig an, } Clear Creek \ \‘\. "'\ LE 5': 5 85 as 95 ' I \.\g\ 5400’ V V " " < V l \‘ 5400’ F. CLEAR CREEK 1 0' 1050760 39°52’30Hl . \R‘Ml‘u Fl [3 L ‘4 EXPLA NATION '30“ ’ \ . HOLOCENE DEPOSITS RELICT SURFACE OF A SURFICIAL Leyden JunctTon l . ‘ ‘ DEPOSIT 7 Dashed where Inferred P —P , ' . . . . ' Qpp OSt iney Creek alluvrum Dot 1ndicates pomt at Wthh eleva— Qp piney Creek Alluvium tion was measured. “(base)” indi- cates that the profile was drawn HOLOCENE AND PLEISTOCENE DEPOSITS on the base of the deposit 50 .«W ...... Q1 Landslide . —-— North side of creek Qco Colluv1um T. 2 S T. 3 s. g Qre Transported mantle ¥~fi South side of creek % -- Order does ny Young alluvial fan not show 1 - L n PLEISTOCENE DEPOSITS age re a BEDROCK CONTACT Poorly 77.7 tIOHS exposed or concealed i, Qof Old alluvial fan gr” Qb Broadway Alluvium FAULT — Poorly exposed or con- 3 . _ _ cealed Qlo Louv1ers Alluv1um S . . . 0 .fi Q Slocum Alluvmm -<—>- ARROWS — Indlcate the d1fferent Qv Verdos Alluvium position on the profile of a point projected from the stream due to Orf Rocky Flats Alluvium a change in the stream direction 4730” D S IN THE GOLDEN Q U ADRANGLE, COLORADO, AND THEIR RELATION TO THE NEARBY GEO LOGIC UNITS 39°45 mi Ow—O l 2 3 KILOMETRES Index map showing locations of longitudinal stream profiles A-F and geologic section G. Base from US Geological Survey Golden 71/2-minute quadrangle (shaded relief), scale l:24,000, 1965. SCALE 1:24 000 1 1/2 0 1 MILE }___| l'———l i : . i L : : 1 5 o 1 KILOMETRE l—l l—l i—Li i——-i i——i th—él VERTICAL EXAGGERATION X8 fiUj. GOVERNMENT PRINTING OFFICE: 1976—677-340/64 . 7. g '. NV ' - ' ‘ (v :nM-twmwi-‘V'p’n TABLE 3.—Fors1'ls found in the Golden quadrangle. Colorado, from the Pierre Shale arranged according to zoml fossils were identified by W.A. Cobban (written commun., 1954, 1955, 1956) and were collected by Richard Van Horn, W. A. Cobban, G. R. Scott, and Antonio Segovia, all with the US. Geological Survey] Batuhm Baculue: aspejlormir gregoryem‘ir Bamliter, n. sp. Buulim Matti Didymoeerax nebramme Didyrrmeerar rtwmtmi E xitelaeera: Didymocem theyenneme Buuli It: tompresrus Bamlim' cantata: Buuliter "main Baruliter baculur Baeulim grandix Batu/im [linolobalur jennzyi USGS Mesozoic locality no ............................................................ 0843 D3446 D20 D837 D776 D829 D830 D3450 0640 024 D591 D734 D740 D703 D1208 D704 D706 D596 D737 D644 D642 D40 D590 D736 D473 D26 D476 D3448 D78 D143 D638 01206 D733 Dl205 D27 D25 Pclecypods Inoceramus s aztrbaiol’janemi: Aliev :ulztmpreme: Meek an eyeloider Wegner :arlzate Warren tenuilimatur Hall and Meek com/e114; Hall and Meek ., barabini Morton lllllllllll {Intermix Meek and Hayden .. 'uarmxemi Meek and Hayden. balehii Meek and Hayden oblmgor White tagenrzs Owen nebraseemir Ow .. mat/earni Douglas rubcirculari: M eek_ typimx (Whitfield) Tmuipteria fibrota (Meek and Hayden) .............................. Phelopleria an 11><1 1 111111 ans and Shumard) . sub/wit Whitfield ...... Ortrea sp. inmata Meek and Hayden ,,,,,,,,,,,,,,, 111111><11 11><111><11 111111111 1111111 n. sp (Gryphaeortrea) n, sp. Nueularza sp ................................ bimleala (Meek and Hayden). 1 5 o’eridmtalir (Morton). 111111 111111111 111111 C ymbop ora sp ........ n, sp. canonemir (Meek) ............. grad/1’1 (Meek and Hayden) 111111111 1 Nueula planimarginata Meek and Hayden tamellata Meek and Hayden ............. " sp . Anomia sp. Limopxi: sp Them? cireu 1111 , .1 sp,,. Gastropods Anixomyon alveolar (Meek and Hayden) .. borealtt (Morton) ....................... patelliformit (Meek and Hayden) A porrhazt met/t1 Whitfiel d Piertothzlu: sp ............... Drepanothilu: evami Cossman. Bellifumt? sp. . Cephalopods Batuliter sp obtum: matlearm Lan atperiformit Meek g1lbem Cobban gregoryenm Cobban n. sp. aff. gregorymm Cobban 110111 Cobban ...... [rielemayi William rugorut Cobban (late form). mmprerru: Say . remdei Elias ..... undalm Ste henson. jensem' Cob an. elim' Cobban..,, [laviformit Stephenson ., batulu: Meek and Ha den gran/111 Hall and Meek. elirzolobatux Elias ......... 1111 1111111 11 1111x11 1111111111111111><11 1111111111111111><11 1111111111111111><11 111111111111 1 1><1 111111 111 11111 P J L "1.". .1 n. sp Trachyreaphiler spinngtr (Schliiter). Anapaehydmu: sp Didymoeeras sp nebrarcenre (Meek and Hayden). Itewmom (Whitfield) ............... theyermerm’ (Meek and Hayden) Haploxcaphzle: sp ........... brew: (Meek) ............. quadrangularir (Meek and Hayden) 7101105145 (Owen) .......... Eutrephoeerar sp ......... mortar/(me (Meek). Solenocera: sp. mortom (Meek and Ha den). Exiteloeerarjermeyi (Whitfie d). Oxybelocerar sp ............... Plaeenneeras meek1 Boehm SJ ‘ Sp 11111111 11111 111111 111111 11111111 11111111 11111111111111 1111111 11111111 11111111111><11 1 111111 111111 1111 11111111111111><1111 11><11‘v 111111111 111111111 111111111 111111111 1111 1111111111><11 1111 11111111111 111111111 111111111 111111111 11111 1111 111 11><1 111111111111111111>< 1111111111111><11111 1111 111111 1111 1111 11111111 11111><1 111 111111 11111111111Q21111 11><11 1111111 111><111 1111 1 111 11>< 1111>< 1111111 111111111 1111111 111111T-:><111111111 11111111 1111111 111111 11111111 11111 11111111 1111 111111 11111111 a1111111111111111 1 1 1 11111111 ><1111111 ><11 x11111111 ><1111 11111111111111111 111111111><11 11111111><11111111 1 1111 11111111111111111 1111111111111>< 1 1111 1><11 11111111111111111 1111 1111111><11111111>< 1111111><11111111>< 11111K1Q111111111 15.2111 1111><11 11111 1111111 1111111 1111 11 1 11 1 1 1 11 1 111111111 11 1111 1 1><111|1 111111 1 1111 ><11111 1111 1111 11 11111 111 11 1111 111111 11111 111111 1111 1111111 1 ><1 11><11111 11111111 1111 1111111 1 111111 111 1 111111111 111111111 111x33; 1111111><1 1 111111111 1 111111111 111111111 1111 11111 1111 1 11 111 1111 11111 111 11 1 1111 11><11 '1111>< 1 1111111 111111111 1 11111 111111111 1111><1 111 121 111111 11111><111 111111 111111 111111 111111111 111111111 1111111 111111111 111111111 1><1111111 1><11 111111111 111111111 1 111111111 u :2 1111 11111 1 111111 1111111 111111 1111111 1111><11 1 1|1><1Q1 1111111 1111111 1111111 1111111 1111111 1111111 1111111 1111111 1 1111111 111111 11111 1111111 1111111 111111 1111111 1111111 1111111 1111111 11111 111111 1111111 11><11><| 1111 1 1 11111><1 111111 1 1 1 1 111111 11 11111111 11111111>< 1111111 1 111111 11111 1111111 1111111 11111 1111111 111111 1111 1 111111111 1111 1111111111 1111 1111111 1 111><11111111111 1111111 11111 1 1 1111111><1 1 111 1111 111><111111 111111111111 1111111111111 1 11 1111111 1111 11111111><111111 ‘11111111><1111 11111><111 111><><11111 1111><11 1111><11111111111111 1111><1 1 1><1111 11111 1111 11111>< 1><11 511111111 1><1 111111 111111111111 111111 111111 1111><1111 111x1111111 1111><1111 1111‘v1 1111><11 111111 1111 11><><111 11 1111 11111 11111111 ‘Additional specimens not included in the table are the calcareous worm tube Serpula eretaeea (Conrad) from map locality D737; the coral Trothocyathur? sp. from map localities D40, D590, and D594; the ochinoid Hemiaxter sp, from locality D24; the brachiopod Lingula mbspatulata Hall and Meek from D590 and D594; scaphopods Dentalium graeile Meek and Hayden from D24, Dentalium sp. from D704; aptychus, the aperture cover of a baculite, is from D704; fragments of a small crab are from D736; fish scales and teeth are from D19 and 0831. «aims. GOVERNMENT PRINTING OFFICE: 1976—677»340/64 / . 0575/ ”:7 DAY“ Cambrian and Ordovician Rocks of Southern Arizona $.23; and New Mexico and Westernmost Texas GEOLOGICAL SURVEY PROFESSIONAL PAPER 873 ”.mm.” m DOCUMWTS DLPAR = .J‘IEM ..... Nov 6 1975 usmav , ”may j‘rfl;::13€*~ffi __v- U335), Cambrian and Ordovician Rocks of Southern Arizona and New Mexico and Westernmost Texas By PHILIP T. HAYES assisted by GEORGE C. CONE GEOLOGICAL SURVEY PROFESSIONAL PAPER 873 A stratigraphic and petrologic study of lower Paleozoic rocks in the Mexican Highland physiographic section as a framework for evaluating their oil and gas possibilities UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Hayes, Philip Thayer, 1923- Cambrian and Ordovician rocks of southern Arizona and New Mexico and westernmost Texas. (Geological Survey Professional Paper 873) Bibliography: p. Includes index. Supt. of Docs. no.: 119.16:873 1. Geology, Stratigraphic—Ordovician. 2. Geology, Stratigraphic-Cambrian. 3. Geology—Southwest, New. 1. Cone, George C., joint author. 11. Title. 111. Series: United States Geological Survey Professional Paper 873. QE660.H39 551.7'2’0979 75-619221 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024—001—02718—6 CONTENTS Page Page Abstract ......................................................................................... l Rock-stratigraphic units—Continued Introduction ............................ . ..................................................... 1 Middle Ordovician unconformity ......................................... 51 Purpose of study. 1 Middle and Upper Ordovician rock units 52 Regional setting. 2 Montoya Group ..................... 52 Physiography ...... 2 Second Value Dolomite. 53 Structural geology ................................................ 3 Aleman Formation ......... 57 Rocks .............................................................................. 4 Cutter Dolomite ...................................................... 61 Nature of Cambrian and Ordovician outcrops and Rocks directly overlying Cambrian and Ordovician strata.. 63 subsurface data ............................................................ 4 Silurian rocks ................................................................. 63 Previous work ......... 5 Devonian rocks ............................................................ 65 Present work 6 Post-Devonian rocks.. .................................................. 65 Initial office work.. 6 Time—stratigraphic units ............................... 66 Fleldwork ........................................................................ 6 Laboratory work ............................................................. 7 Cambrian ------------------- 66 Compilation ................................................................... 8 Middle Cambrian ~~ 56 Acknowledgments .................................................................. 8 Dresbachian age “““ -- 67 Rock—stratigraphic units... 8 Franconian and Trempealeauan ages 69 Precambrian rocks ................. 8 Ordovician ............................................................................. 70 Character and distribution _____________ 8 Early and middle Canadian ........................................... 70 Relief at the top of the Precambrian. 9 Early late Canadian ........................................................ 72 Cambrian and Lower Ordovician rock units _ 9 Late late Canadian ......................................................... 73 Western part of study region .......................................... 10 Early Middle Ordovician.... 73 Bolsa Quartzite ........................................................ 10 Late Middle OTdOViCian ----- 74 Abrigo Formation ............................ 14 Early Late Ordovician ----- 75 Lower member .................................... 14 Late Late Ordovician ..................................................... 75 Middle member ......... 13 Oil and gas possibilities ............................................................... 76 Upper sandy member ..... 23 Possible source beds.... 76 Copper Queen Member.. 25 Possible reservoir rocks.. 77 El Paso Limestone .................................................. 26 Possible trap surfaces ................................................. 78 Easm PM of study region ------------------------------------------- 27 Control points ............................................................................... 79 81155 Sandstone """""" 27 Selected measured sections ............................................................ 79 El Paso Group ................... 35 . Previous nomenclature .............. 35 Pasotex section......... “““““ 79 Nomenclature used in this report .................... 35 Garden Canyon section '1 """ 81 Hitt Canyon Formation ................................... 35 McKelligon Canyon section 82 McKelligon Limestone ..................................... 39 Brandenburg Mqumain section. 82 Padre Formation iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii 42 Nantac le sectlon ............................................................... 83 Central part of study region ....................... 45 F0551] lists ...................................................................................... 84 Coronado Sandstone ............................. 45 References cited ............................................................................. 88 El Paso Limestone .................................................. 47 Index ............................................................................................. 93 ILLUSTRATIONS Page PLATE 1. Selected graphic sections of Cambrian and Lower Ordovician rocks in southern Arizona and New Mexico and western Texas ................................................................................................................................................................. In pocket FIGURE 1. Index map of the report region ........................................................................................................................................................ 2 2. Keysort card used in data compilation .................................................................. 5 3. Map showing rocks at top of Precambrian ............................................................................................. 7 4. Map showing preserved thickness of rocks of Sauk sequence... 12 5. Diagram of stratigraphic nomenclature and correlations ........................................................... 15 6. Map showing distribution areas of major stratigraphic units ........................................................................................................ 16 III CONTENTS IV Page FIGURE 7. Photomicrographs of Bolsa Quartzite.: ............................................................................................................................................. 17 8. Photomicrographs of lower member of Abrigo Formation ............................................................................................................. 19 9. Photographs showing outcrops of middle member of Abrigo Formation ............................... 20 10. Photomicrographs of middle member of Abrigo Formation .................................................................. 21 11. Photomicrographs of upper sandy member of Abrigo Formation ......................................................... 23 12. Photograph showing cross-laminated sandstone in upper sandy member of Abrigo Formation. .......................................... 24 13. Photomicrographs of Copper Queen Member of Abrigo Formation ..................................................................................... 26 14. Lithofacies map of Bliss Sandstone ............................................................................ 28 15. Photomicrographs of Bliss Sandstone .................. 30 16. Photographs showing outcrops of Bliss Sandstone ...... 33 17. Diagram showing subdivisions of the El Paso Limestone (or Group) as used by various workers ............................................... 34 18. Photographs showing outcrops of Hitt Canyon Formation ................................................................. 37 19. Photomicrographs of Hitt Canyon Formation ....................................... 38 20. Photomicrographs of McKelligon Limestone ......................... 41 21. Photographs showing outcrops of McKelligon Limestone.... ....................................................................................... 42 22. Photomicrographs of Padre Formation ......................................................................................................................... 44 23. Photographs showing outcrops of Padre Formation ........................................................................................................... 45 24. Photomicrographs of Coronado Sandstone ........................ 47 25. Photomicrographs of El Paso Limestone ............................................ 49 26. Photographs showing outcrops of El Paso Limestone ................................................................ . ........................... 50 27-30. Maps showing: 27. Distribution of rocks of Sauk sequence before end of Cretaceous time ............................................................................. 51 28. Rocks overlying Sauk sequence ............................................................. 52 29. Distribution and preserved thickness of Montoya Group ................................................................................................... 54 30. Distribution, thickness, and facies of Second Value Dolomite ........................................................................................... 56 31. Photomicrographs of Cable Canyon Sandstone Member of Second Value Dolomite ............. 58 32. Photographs showing outcrops of Upham Dolomite Member of Second Value Dolomite... 59 33. Photomicrographs of Upham Dolomite Member of Second Value Dolomite ............................ 50 34. Photograph of sandy dolomite of Cable Canyon Sandstone Member of Second Value Dolomite ........................................ 61 35. Photographs of cherty dolomite of Aleman Formation ......................................................................................................... 62 36. Map showing distribution and thickness of Aleman Formation ............................................................................................... -. ..... 63 37. Photomicrographs of Aleman Formation ......................................................................................................................................... 64 38. Photograph showing silicified colonial coral in Aleman Formation ...................... 66 39. Map showing distribution and thickness of Cutter Dolomite ............................................. 57 40. Photomicrographs of Cutter Dolomite ............................................... 58 41. Photographs showing outcrops of Cutter Dolomite .............................................................................................................. 69 42-51. Maps showing: 42. Distribution and thickness of Fusselman Dolomite ............................................................................................................ 69 43. Distribution, thickness, and facies of Middle Cambrian rocks ................... 70 44. Distribution, thickness, and facies of rocks of Dresbachian age ....................................... 71 45. Distribution, thickness, and facies of rocks of Franconian and Trempealeauan ages ....................................................... 72 46. Distribution, thickness, and facies of rocks of early and middle Canadian age ................................................................ 73 47. Distribution and thickness of rocks of early late Canadian age ......................................................................................... 774 48. Distribution and thickness of rocks of late late Canadian age ........................................................................................... 75 49. Relative favorability of Cambrian rocks as source beds for petroleum ......................................... 76 50. Relative favorability of Lower Ordovician rocks as source beds for petroleum ........................... 77 51. Rock units that underlie dark shale-bearing rocks of Devonian age ................................................................................. 78 TABLE Page TABLE 1. Data on control points used in compilation of thickness and facies maps ...................................................................................... 96 CAMBRIAN AND ORDOVICIAN ROCKS OF SOUTHERN ARIZONA AND NEW MEXICO AND WESTERNMOST TEXAS By PHILIP T. HAYES, assisted by GEORGE C. CONE ABSTRACT The area of study is defined by the Mexican—United States border on the south, roughly by lat 34° N. on the north, and roughly by long 105° and 112° W. on the east and west, respectively. This area is chiefly in the Mexican Highland section of the Basin and Range province but includes parts of adjoining physiographic subdivisions. The study, which was designed to aid in the evaluation of the oil and gas possibilities of the region, involved much library research, the exam- ination of 76 outcrop localities, the laboratory study of about 750 out- crop samplés, and the examination of cuttings from a few oil explora- tion holes. Cambrian and Ordovician rocks in the report region are included in two main depositional sequences that are separated by a regional uncon- formity: Cambrian and Lower Ordovician rocks of the Sauk sequence at the base and a Middle and Upper Ordovician sequence assigned to the Montoya Group above. The rocks of the Sauk sequence result from deposition in and on the margins of a shallow shelf sea that transgressed across the region from the west and southwest. As a result, the base of the sequence is of probable early Middle Cambrian age at the west edge of the region and of early Early Ordovician age at the east edge. Post-Early Ordovician regional emergence and consequent erosion at the top of the Sauk sequence was greater in the west and north than in the east and south, with the result that rocks just beneath the upper unconformity are locally as old as late Middle Cambrian in the west and as young as late Early Ordovician in the southeast. The rocks of the Sauk sequence are locally more than 1,500 feet thick at several localities near the south edge of the region, and they thin to a wedge edge at the north. In most of southern Arizona the rocks of the Sauk sequence are included in the Bolsa Quartzite of Middle Cambrian age and the over- lying Abrigo Formation. The Abrigo is divided into a lower member of Middle Cambrian age and three members of Late Cambrian age—the middle, upper sandy, and Copper Queen Members. All the members of the Abrigo are increasingly sandy toward the north and east. Progressing east-northeastward the B0153 Quartzite thins to a deposi- tional wedge edge by onlap, and the lower three members of the Abrigo Formation grade into the Coronado Sandstone of easternmost Arizona and westernmost New Mexico. In that area, rocks equivalent to the Copper Queen Member of the Abrigo are assigned to an informal lower member of the El Paso Limestone which overlies the Coronado Sand- stone. Above the somewhat sandy lower member of the El Paso in that area is a nonsandy upper member of Early Ordovician age. Farther east- ward the Coronado Sandstone thins by onlap, and the upper part of the Coronado and lower member of the El Paso Limestone grade into the Bliss Sandstone. In most of southern New Mexico and western Texas the rocks of the Sauk sequence are assigned to the Bliss Sandstone and the overlying El Paso Group which is made up, in ascending order, of the Hitt Canyon Formation, McKelligon Limestone, and Padre Formation. Both the base and the top of the Bliss are younger eastward. Whereas the Bliss is probably entirely of Late Cambrian age in far western New Mexico, it is entirely of Early Ordovician age in the eastern part of the report region. All formations of the El Paso Group are made up dominantly of carbo- nate rocks which become increasingly sandy eastward. The Montoya Group of Middle and Late Ordovician age discon- formably overlies Lower Ordovician rocks in much of western Texas and southern New Mexico but only barely extends into Arizona; the rocks of the group are limited by erosional wedge edges on the north and west. The maximum thickness of the group is slightly more than 500 feet. Throughout the region the group is divided in ascending order into the Second Value Dolomite, the Aleman Formation, and the Cutter Dolo- mite. In much of the region the Second Value Dolomite is divided into the Cable Canyon Sandstone Member and the overlying Upham Dolo- mite Member, but the Cable Canyon is missing in some large areas. Our stratigraphic and petrologic studies indicate that, although the Cambrian and Ordovician rocks of the region cannot be regarded as having a high potential as oil- or gas-bearing rocks, possibilities for petroleum reservoirs do exist. The Abrigo Formation could contain petroleum reservoirs in southernmost Arizona, and rocks of the El Paso and Montoya Groups could contain reservoirs wherever they occur. Perhaps the most favorable exploration target would be porous rock at the top of the El Paso Group in areas where it is unconformably overlain by shale-bearing Devonian beds. INTRODUCTION PURPOSE OF STUDY As the nation’s reserves of oil and gas diminish and the demand for petroleum products increases, the search for new reserves must extend into areas that have heretofore been considered to have only marginal potential. It is the purpose of this study to evaluate the possibilities for oil and gas production from lower Paleozoic rocks in one of these marginal areas, the Mexican Highland section of southern Arizona and New Mexico and westernmost Texas, and to provide sufficient data on the rocks for others to make their own evaluations or to stimulate further study. In addition, knowledge of the paleotectonic framework established in early Paleozoic time should provide a firmer basis to begin similar studies on younger rocks in the region. The area studied (fig. 1) is bounded on the south by the Mexican border, on the east roughly by the 105th meri- 2 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS dian west of Greenwich, on the west roughly by the 112th meridian, and on the north roughly by the 34th parallel north. These boundaries were chosen because lower Paleozoic rocks east of the study area are relatively well known from drill-hole data; lower Paleozoic rocks are absent over most of the area immediately north of the study area; and outcrops and drill-hole penetrations of Paleozoic rocks west of the study area are extremely sparse. REGIONAL SETTING PHYSIOGRAPHY Most of the area covered by this report is a part of the Mexican Highland section of the Basin and Range physio- graphic province, but the area also includes a part of the Sonoran Desert section on the west, the Sacramento section on the east, and the southern margin of the C010- rado Plateaus province on the north (fig. 1). The Mexican 112° 111° 110° 109° ! - r‘""“1 ! / l I MARICOPA i I C A T R O N I '154\ I l ~-~———-J \ .° ' i 162\\ ————— ~L —- \ 15 ) ' "T \\ .0 f, '150 I __ \ \ /“ “grim I GREENLEE I 33 ° Ma \\ \ / I 'm K oMorenoi I O \ v I P I N A L \ \ \ .102 .76 l \ Brandanbprg \ «1% G R A N T Mountam G R A H A M ZlN i \A 8': o o I , R EV; o 2-831LATEHA 10:7 a; 1 E ° Silver City V1??? MT§ ’40:; 77 [re a I ‘ a area ”W‘F‘ 110‘“ ’5 1% l Lac 27" i‘Z“*---— (d o WATERMAN,80 .150 7,; 0 -—-—-—~—-__.._._____ 1).; . . MOUNTAINS ‘5‘ ’7’ I c c; I . 0 Z I 7 \’ A ( OTucson i 272. 0 O! HIDALGO I A z LITTLE 0 NM ‘553 m DRAGOON 4408660 .59 l I .74 .75 04,458 I; 23 to 32° v MOUNTAINS )4 e4 0. Z [I ’II/ s o c < l 10 6‘ 4‘; .58 z I. I e C O C H I S E 0679 I -I z 67 8:) % rag-5’1 E 8 98 A § ‘7 '7 ’S« In 2' - I $" 259 (23371 gowé [76,67 gt: ________‘_____. § cf» 1 ’L‘”.64 I 5 \\\ SANTA CRUZ J '70 a $16? ' 5 25: \ 4 «L m 394 BIG 24 \\ ”é a Garden «9 066.51 HATCHET v 00" Canyon MU LE 63 53 I MTS \L 1, 00 83 10$.66 MT5 9 ‘Q l 1‘7 ¢' ’\v°o 6.50 Bisbee, $00 \ / ’ / s9 \h""'7fpj———~—L-—7®L_._____~QNITED_S_”_FATES__:7 _ _ __ __ MEXICO 31° .Hc o 50 100 MILES I IT II 1 II | I o 50 100 KILOMETRES I f I FIGURE l.—Index map of the report region, showing county names, boundaries of physiographic subdivisions (dashed where approximate), province; IIB, Mexican Highland section of Basin and Range province; IIC, INTRODUCTION Highland section is characterized by generally north trending upfaulted mountain ranges separated by broad intermontane basins which have been filled to varying degrees by alluvial debris from the ranges. The Sonoran Desert section is similar but is lower in overall altitude, the ranges are narrower, and the intermontane basins broader in proportion. The Sacramento section is characterized by broad moderately dissected ranges underlain by generally 108° 107° 3 gently eastward-dipping rocks. The part of the Colorado Plateaus province included in the report area is dissected plateau country in which Paleozoic rocks are covered by Cenozoic volcanic rocks. STRUCTURAL GEOLOGY Local geologic structure in the Basin and Range pro- vince becomes increasingly complex from east to west 106° 105° 2 i 00 i i \ AN . 35 ,-~B 1\ ‘ ms 3345-135‘; LINCOLN ‘ I 111 _L FRA ”m_ 87%;; '28 i : F“ -__..._ m z . 37 ' i ' . (g T ”1......— 1 .0 F || 2 o 1 | I ———--~—-L MUD SPRINGS 01° I ,Mu I 7 MOUNTAINS~J3 ‘ H" SIERRA I; k n “a 3'14 117 ge/i 4 ”fl..-” ‘-—‘—~\ \ >15 ° /$.11 r OTERO '0 \ .43 ‘C' // ' 113 CHAVES H .26 .33 R :16 / s'ACRAMENTo l .1“ | .91 \ 19 _—‘.,.____..._._J .27 i MTS 1 l 1140 .021 . km 0 l‘ >" 22 :12 1 i . H g 3 GRANT “fizecooxs “'4 3.18 y .108 HEQWMWWW \fizi ___- RANGE R0 )> ”10 .\ 20 BLEDO 42 E / 2 . IIB IMOUNTAIN' U, / \ i s / 1 \V I .1 9 I DONA ANA ‘ // 94. i VICTORIO I / 120 i §M0l.JNTAINS FLomoA 4 i / 5 L 90 39. MTS I / Turner 1 State LUNA Capitol ; . L IN a .40 9°” 1 nigfimfikms'm ~_ NE¥%{%IQOmwu—M "74 32“ CEDAR r"".2f4" '— E MOUNTAIN I l ’88 '121 129 i; [I RANGE 123 o _..___‘ 122 \ I McKelligon Canyons/2.: 93 HUEEZO : \ F’”"’““"""‘"‘”‘” ____.._W.._. 45 ELPASO 6 MN \fl 1 U) l \ .56 m | \ \ Pasotex m j \ i \ HUDSPETH a, m \ 'rfi‘so “1 I \\ I \\\ If; . a I \~1 -1\35\\ > ___i K \\ Van Horn ngAYLOR \\area a MTS.\ \\ \\ r 42 / \ \szsz, \ g, 4§BEACH \\4)’z i MTN 6’ 7 31° 4’6 0‘9» i *1;\ v7 1 OO\& i T‘\ \\ // E i I and localities (control points, table I) referred to in the text. I, Colorado Plateaus province; IIA, Sonoran Desert section of Basin and Range Sacramento section of Basin and Range province; III, Great Plains province. 4 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS across the report area. In the Sacramento section the rocks for the most part are gently dipping and are interrupted only by gentle folds and high-angle normal faults of moderate displacement. Normal faults of larger displace- ment, more steeply dipping beds, and minor thrust faults are characteristic of the eastern part of the Mexican High- land section, but most individual ranges (and presumably the intervening basins) are relatively simple structurally as compared with those farther west. For the most part, local structure is complex in the western part of the Mexican Highland section, where there were repeated tectonic events throughout most of the Mesozoic and Cenozoic Eras. Structure in the Sonoran Desert section is not well understood but is complex also. Structure of the rocks underlying the thick volcanics of the southern margin of the Colorado Plateaus is unknown but may by simpler than that to the south. Some workers (Muehlberger, 1965; Poole and others, 1967) have proposed that west-northwest- or northwest- trending right-lateral fault zones of up to 200 miles of displacement cross all or parts of the present report region in the Basin and Range province. Certainly there are important major structural lineations in the region that have such trends, such as the Texas lineament of western Texas (Albritton and Smith, 1965), but compelling evidence for large-scale wrench faulting has not been presented. Indeed, the distribution of Precambrian rocks in the region (fig. 3) and the isopach and facies maps prepared during this investigation seem to refute the existence of wrench faults with displacements of more than a few tens of miles at the most. ROCKS Rocks at the top of the crystalline basement are dominantly plutonic rocks of granitic to dioritic compo- sition throughout the report region; but crystalline schists and gneisses are dominant over an extensive area in south- eastern Arizona, and similar metamorphic rocks including quartzites are locally present throughout the region. Sedimentary rocks with minor associated basalt that have been invaded by abundant diabase dikes and sills are present between the crystalline basement and the Paleo— zoic under an extensive area of Arizona. Similar Precam- brian sedimentary rocks with locally associated rhyolite intervene between the basement and the Paleozoic in some areas in western Texas and eastern New Mexico. In most instances these Precambrian layered rocks display only slight angularity with the overlying Paleozoic. The Paleozoic Era throughout the report region is re- presented by sedimentary rocks, dominantly carbonates, that were deposited in or on the margins of shallow shelf seas or, in the Pennsylvanian and Permian, in intracra- tonic basins. Mesozoic rocks are very irregularly distributed in the region. Triassic and Jurassic rocks are missing over most of the region; however, there are some Triassic sedimen- tary rocks toward the north edge of the region in New Mexico, local but in places thick sequences of volcanic and epiclastic rocks of both Triassic and Jurassic age in south- eastern Arizona, a few plutons of both Triassic and Juras- sic age in the same area, and Jurassic carbonate rocks of marine origin along the Mexican border in western Texas. Lower Cretaceous strata of marine origin are widely dis- tributed in western Texas and, with increasing propor- tions of nonmarine sediment, extend across southernmost western New Mexico and into southeastern Arizona. Upper Cretaceous strata partly of marine origin once covered most of the northern part of the region and are locally preserved. Nonmarine Upper Cretaceous beds are locally present in southeastern Arizona and extreme southwestern New Mexico. Volcanic rocks of latest Creta- ceous age are present at many localities throughout the Arizona part of the region and in southwestern New Mexico. Plutonic rocks of latest Cretaceous age are fairly widely distributed in southeastern Arizona. The character and distribution of Cenozoic rocks in the region are extremely variable. Cenozoic sedimentary rocks are entirely of continental origin and consist of debris shed from rising mountains; they are most abundant in the in termontane areas of the Mexican Highland and Sonoran Desert sections of the Basin and Range province. A wide diversity of Cenozoic volcanic rocks occurs throughout the region but are most abundant in the Colorado Plateaus province and most diverse in western parts of the Basin and Range. Both hypabyssal and plutonic igneous rocks of Cenozoic age are irregularly distributed throughout the region. NATURE OF CAMBRIAN AND ORDOVICIAN OUTCROPS AND SUBSURFACE DATA Cambrian and Ordovician rocks are at depth under much of the Sacramento section of the Basin and Range province, except where removed by later Paleozoic ero- sion; outcrops are limited to a few areas near the west and south margins of the section. Considerable information on the rocks, however, is available from exploratory drill- hole data. Outcrops of Cambrian and Ordovician rocks occur in a large percentage of the mountain ranges of the Mexican Highland section, but drill-hole data for those rocks are sparse in the section, especially toward the west. In the eastern part of the section, outcrops are more extensive, are relatively free of internal structure, and are relatively free of hydrothermal alteration. Conversely, toward the western part of the section the outcropping rocks occur in shorter outcrop bands, are in many places severely faulted, and generally show the effects of mild to severe silicifica- tion, epidotization, and dolomitization. In the Sonoran Desert section the rocks are even more INTRODUCTION o ,. 2 2 g a 2 'o g- n g ‘5 9 Age assignment g g .2 r___/C ———‘fi OE JIHGFEDCBA . * I. I 7 4 2 I 2 t 7 A 2 I s z I 2 I From lIteratureu’ mmmmmmmm :5 u 23 22 m :0 ID to I7 16 15 u Is I: n In 9 a 7 p , s 4 s z I r :u ., sec. NO .............................. FM .............................. THICKNESS ................................. g Fusse'mar‘ “E” CA/MG ......................................... I“ M°m°ya o 3 w UNIT .................................. MBR. ................................ POHOSITY .................................. Simpson E a; ,t . E- El Paso i} m 9 DESCRIPTION. X—LAM. . E. g E ' X—LAM. as. BlIss g g :4 2 ~ @Ab'igc’ as t I2 q a» Bolsa \- ‘3 Tu 51 In U _ m 3 E Longfellow 2 - a Coronado III . b E Precambrlan e V N V 7 I L28 L27 Lzamm L23 L22 L2I Lzom LIa L17 LIe LI5 LU man LII muunuuumnu tn I N .s I: a: v l- h In 3: N C a D a .o E r~ 2 _ c < _ .t' C III E N D o l— v ,t ~ll] 8 ‘_ v K55 0713 | L r. 3- .2. ans EN T 0. III S c a— 9 g E E“ t % E m " b . x. 3 Unlts O In EN .5 E 2 a Silurlan 3 I. Ordovician "g EN Cambrian ; a- Precambrian g Eu Upper § ... ' In a. MIddle > an Lower ‘0 Lnnuc Visited section-' [Elm 59 V8 59 95 LE 95 6E OIEHBZWNEHEHMUHW fitZiLllZ'L'IZVLIIZiLllzvtlvalZ'leiLIR mwOI-l—-unOmOC>OOrDOmmmIOUOO111C) OUIajaOm‘OODU°3"O°W=3’mmOIomz‘mma‘ moon-~o«\°€1:nm<03—:=Hm3;-033H‘E.agc 2,03<3_o§a-'*omg;m°maflg N°3uaa~<:m n 7,2-.~ EXPLANATIO N Surface locality or drithole samples exam Surface locality reported in literature Drill—hole data Other locality referred to in text Isopach — Showing thickness, in feet l l —300-— / Hembrillo Agua Chiquita Canyon A ined in this study sequence of Sloss, Krumbein, and Dapples (1949) and 81055 (1963). l4 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS careful comparison of detailed graphic sections on which all diagnostic fossils of the Abrigo are n ted indicates that the top of the Bolsa in easternmost area may be younger than to the west. (See pl. 1.) ABRIGO FORMATION The Abrigo Limestone was named by Ransome (1904) for exposures in the Mule Mountains, Cochise County, Ariz., and the type section was remeasured and divided into four members by Hayes and Landis (1965) at Mount Martin (fig. 4 and 10c. 65, fig. 1). Called Abrigo Forma- tion everywhere but at the type locality, the Abrigo crops out at many places in the western area as shown in figure 6 but does not occur farther east. The four members of the type section were extended over all the eastern part of the western area by Hayes (1972) and called, in ascending order, lower member, middle member, upper sandy mem- ber, and Copper Queen Member. In this report the informal members are extended, with reservations, to the western part of the western area and are described separ- ately on the following pages. LOWER MEMBER GENERAL DESCRIPTION The lower member of the Abrigo Formation is a mod- erately nonresistant unit several hundred feet thick that in outcrop generally forms a slope between the much more resistant underlying and overlying units. Lithologically, the member varies markedly from one area to another but, in general, is made up dominantly of fine—grained terri— genous clastic rocks. DISTRIBUTION AND STRATIGRAPHIC LIMITS The lower member of the Abrigo Formation is present wherever Cambrian rocks occur in the western area as shown in figure 6. In most areas it conformably overlies the Bolsa Quartzite and is conformably overlain by the middle member of the Abrigo. Locally, in the northern and western parts of the western area where the Bolsa is absent over old Precambrian hills, the lower member un- conformably overlies Precambrian sedimentary rocks. PETROLOGY The lithologic characteristics of the lower member Of the Abrigo Formation change almost completely from the southern part of the region to the northern part. In the Mule, Huachuca, and Patagonia Mountains at the south (locs. 65, 66, and 83) the member is made up almost entirely of roughly equal proportions of micaceous silty shale and thin-bedded limestone, but a few thin beds of quartzite and brown-weathering dolomite are generally near the base. In the Santa Catalina and Galiuro Moun- tains and Superior area (locs. 77, 102, and 154) the mem- ber is made up almost entirely of siltstone and fine- grained sandstone. In the intervening area the member is intermediate in composition but in an irregular manner. Toward the east in the intervening area abundant shales extend farther north than they do to the west, whereas abundant carbonates extend farther north in the western part of the intervening area. The following comments on the petrology of the lower member are based on the out- crop examination of the unit at 18 localities and on the examination of 80 thin sections. Limestones were studied most carefully and shales least. Shale in the lower member is in general fissile. It is commonly micaceous and usually silty and slightly cal— careous. It is mostly medium gray to olive gray where fresh but weathers to a yellowish brown. In mineralized and slightly metamorphosed areas, such as the Tombstone area (10c. 70), shale in the member has been altered to hornfels. Limestone in the lower member of the Abrigo Forma- tion rarely occurs in units more than a few feet thick and is generally thinly or very thinly bedded; it is commonly irregularly laminated as well. Most is medium gray on fresh surfaces and light gray or yellowish gray on weathered surfaces. Yellowish-gray-weathering lime- stone is typically fairly silty, whereas light-gray-weather- ing limestone is mostly free Of terrigenous silt. Silty and relatively silt-free limestone are commonly irregularly interlaminated. Variably silty and (or) argillaceous lami- nated lime mudstone is the most common limestone type in the member (fig. 8C). Some lime mudstones appear in thin section to have been peloid, and others show lithi- clasts that may have been desiccation chips. Lithiclast lime wackestones to packstones made up of chips of lime mudstone in lime mudstone matrix (fig. 8A, B) are also common, as are algal grainstones (fig. 8D). F ossil-bearing lime mudstones and skeletal wackestones are present but not common. Both glauconite (fig. 8A) and hematite (fig. 8D) are common as impurities in lower member limestones. Dolomite in the lower member is most abundant in the facies intermediate between the southern shale and lime- stone facies and the northern siltstone and sandstone facies. Where present it is medium gray and generally weathers to a dark yellowish brown. It is commonly silty or sandy and is generally thinly laminated. Some dolomite in the Slate and Vekol Mountains (locs. 81 and 82) is sandy dolomite chip conglomerate which has chips of laminated hematitic dolomitic sandstone in a matrix of sandy fine- grained dolomite. The coarse siltstones and fine-grained sandstones that make up most Of the member in the north and part of it elsewhere are in general yellowish to reddish gray on fresh surfaces and weather to a wide variety of colors ranging from yellowish gray to dark reddish brown. It is thinly t0 thickly bedded, commonly laminated, locally cross-lami- nated, commonly friable, and mostly only weakly resis- tant. Siltstone exceeds sandstone in abundance at most ROCK—STRATIGRAPHIC UNITS l5 AGE WESTERN AREA CENTRAL AREA EASTERN AREA Late Late Ordovucian Cutter Dolomite Early Late Ordovician Aleman Formation 7 l Montoya Group Late Middle Second Value Dolomite Ordovician Early Middle Ordovician ORDOVlClAN Late late 1“ Canadian T Early late Canadian Middle and early Canadian Early Ordovician El Paso Late Late _/ Limestone guess ,/~ Cambrian / (Trempealeauan J/Jr/ Queen > r/“Fr and Franconian Member Stages“) 0° / mo———~—~——§ f / t Early Late )/ Q0 sandy ”J“ Cambrian A ‘80 member (Dresbachian / 79—————A-—— Coronado Stage“) / Sandstone if A] Middle member fJ Lower member CAMBRlAN Midd le Cambrian Bolsa Quartzite Early Cambrian lUpham Dolomite Member 2 Cable Canyon Sandstone Member. 3Unnamed rocks of early Middle Ordovician age. 4of Howell and others (1944) FIGURE 5.—Stratigraphic nomenclature and correlations used in this report for Cambrian and Ordovician rock units. The locations of the western, central, and eastern areas are shown in figure 6. The letters on the right edge of the left column show correlations with Ordovician fossil zones of Utah (Hintze and others, 1969). 16 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS R11? #7, 1i1° 110° , 109° 7 108° 197°”, 1C‘)6° 195° ,7 l , — ' 1 fl \\\ V‘\ Area where CambriaT’and Ordovician rocks are absentii /,\ /fi._.i if \ x . , ma » . /,/,-:L_-_-1.74_4 \ I, I \ ___ _. ’— c ___ “““ J’ \\ A’M", n f L l \ M l 1 33°- \V L \ 7L /! \\ i'”_’fl 4‘ CENTRAL I\R‘EA . ,// [J \ 1 47 Area of'loccurrencb of _ R _',/ ' \\L / i I \ , \ T—T I ‘__7_!VESTERN AREA Coronado Sandstonel |_- ,_k_._._._! 1 IL\'/ 1 A ‘‘‘‘‘‘‘‘‘‘‘‘ —4 t L_ ____ l ,——-——-V rea of occurrence of -—l__ h. l I EASTERN AREA 1 l Balsa Ouartzne and i \ ‘ ' Area of occurrence of i ' Abrigo Formation I <8\ ! i BlisslSandstone ' i f “L H 1 ___L__ «marge-He H l 1 l _ _____ 4 SEE, \ ‘i _____ (BEEPLSIAIEHL. ' '1 l \\ . , z \ : MEXICO ”a, . x \\ _l | \ | \\ i i \‘L ' \ g \ '1 i \ ‘‘‘‘‘ ‘i ————— \ l \‘ K 1 Western limit of El Paso Limestone Western limit of Montoya Group \\ i 31w x l g 0 50 100 MILES \\ .1 \ I 0 so 100 KlLOMETRES “\ - l t i 1 l I ,, l , C) , FIGURE 6.—Distribution areas of major stratigraphic units discussed in text and shown in figure 5. localities (fig. 8E); most of the sandstones present are fine to very fine grained, but some are medium grained. Grain- size sorting is mostly fairly good, but sorting ranges from very good to very poor. The grains are mostly quartz; but feldspar grains are fairly common, and grains of glauco— nite, calcite, dolomite, and chert also occur. Most of the siltstones and sandstones are cemented with quartz, cal- cite, or dolomite, but clay minerals and iron oxides are commonly present in the matrix. SEDIMENTARY STRUCTURES The most notable sedimentary structures observed in limestones and dolomites of the lower member of the Abrigo Formation are mud cracks and intraformational chip conglomerates. Burrowed lime mudstone was noted at a few localities. Tracks and trails are abundant and conspicuous on the bedding surfaces of many siltstone and fine-grained sand- stone beds, and Scolithus tubes as much as 1 cm wide and 25 cm long are commonly noted in sandstone beds. Many sandstone beds show conspicuous small- and medium-scale cross-laminations. At Nugget Canyon (10c. 77) in southeastern Pinal County most of the cross- laminae are inclined toward the southwest. Slump structures were noted in siltstone beds at a few localities. POROSITY Porosities in carbonate rocks of the lower member of the Abrigo Formation are very low, but some sandstones and siltstones have moderate porosity. The effective porosities of two limestone samples checked were 1.4 and 2.1 per— cent, and the effective porosities of two silty dolomites checked were 0 and 1.1 percent. Six siltstone and sand- stone samples were checked for total porosity, and the resultant porosities ranged from 7.3 to 13.9 percent and averaged 9.5 percent. The effective porosities of six sand- stone and siltstone samples were also checked, and the resultant porosities ranged from 1.3 to 7.0 percent and averaged 4.5 percent. THICKNESS The lower member of the Abrigo Formation ranges in thickness from 238 feet at Johnson Peak (10c. 75) in north- western Cochise County to 665 feet at Waterman Moun- tains (10c. 80) in northern Pima County; it is 252 feet thick in the type section of the Abrigo at Mount Martin (10c. 65). The great thickness at locality 80 is probably due to west- ward thickening at the expense of the underlying Bolsa Quartzite. The lower member is less than 480 feet thick at all other localities. FossrLs AND AGE On the basis of fossils, chiefly trilobites, found by various workers in Cochise and Pima Counties, Ariz., at or near the northern Swisshelm Mountains, Mount Martin, French Joe Canyon, Tombstone, Johnson Peak, and the Waterman Mountains (locs. 64, 65, 69, 70, 75, and 80), the age of the lower member of the Abrigo Formation is Middle Cambrian and probably mostly late Middle Cam- brian. Fossils from the northern Swisshelm Mountains and French Joe Canyon (at or near locs. 64 and 69) have ROCK-STRATIGRAPHIC UNITS FIGURE 7.—Ph0tomicrographs of Bolsa Quartzite. Bar in A is 1 mm; all views are same scale. A, Poorly sorted arkosic quartzite from near base of formation in the northern Swisshelm Mountains (10c. 64). Such rock is abundant in the lower part of the Bolsa wherever it overlies a crystalline terrane. Besides abundant feldspar (F), mostly microperthite, the section contains rock fragments (R), magnetite, and considerable sericite in the matrix. Note the intergranular weld- ing of quartz grains especially in the upper part of the picture. Crossed nicols. B, Poorly sorted granule-bearing quartzite from 299 feet above base of formation at Garden Canyon (loc. 66). Large frag— ment of quartzite at left was derived from the underlying Precam- brian. Like most quartzite from this high in the Bolsa, this rock is virtually devoid of feldspar. This quartzite is less perfectly welded than most in the Bolsa, and there is some sericite between many quartz grains. Crossed nicols. C, Very well sorted siliceous fine- grained orthoquartzite from upper part of formation at Nugget Can- yon (loc. 77). Rock such as this is characteristic of the upper part of the Bolsa wherever it occurs. The original grain boundaries are nearly all lost. Crossed nicols. D, Poorly sorted quartzose sandstone cemented with hematite from upper part of formation at Mount Martin (10c. 65). This rock type is present at nearly all Bolsa localities but makes up a very minor part of the sequence. Such rock is much more prevalent in the Bliss Sandstone. Plain light. 18 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS been reported on by A. R. Palmer (in Gilluly, 1956), and more collections from those localities were made during this study and reported on by M. E. Taylor of the US. Geo- logical Survey (written commun., Aug. 16, 1971, and Jan. 24, 1973). Fossils from or near Mount Martin (10c. 65) in the Mule Mountains have been reported on ‘by Stoyanow (1936) and by A. R. Palmer (in Gilluly, 1956). Fossils from the Tombstone area and Johnson Peak (at or near locs. 70 and 75) have been reported on by A. R. Palmer (in Gilluly, 1956). Collections were made from the lower member during this study in the Waterman Mountains (10c. 80) and contain several genera of trilobites indicative of a late Middle Cambrian age, according to M. E. Taylor (written commun., Aug. 16, 1971, and Jan. 24, 1973). Fossils col- lected from the member during this study are given under “Fossil Lists” near the end of the report. The age of the lower member at northern localities where no diagnostic fossils have been found is presumed to be Middle Cambrian on the basis of stratigraphic position and presumed equivalence to the lower member of southern localities. UPPER CONTACT The contact of the lower member of the Abrigo with the overlying middle member is gradational but at most localities can be placed rather precisely. In the southern- most localities, where considerable shale is present in the lower member, the contact is placed at the top of the high- est shale or siltstone bed beneath the 100 feet or more of limestone that makes up the middle member. Farther north, where the lower member consists largely of silt- stone and fine-grained sandstone and the upper member contains resistant sandstone or quartzite at the base or is made up entirely of such rock, the contact is placed where siltstone or weakly resistant fine-grained sandstone gives way abruptly upward to relatively resistant coarser grained sandstone or quartzite. MIDDLE MEMBER GENERAL DEsCRIPrION The middle member of the Abrigo Formation, like the lower member, changes completely in lithologic character from south to north but can be recognized by its strati- graphic position and its contrast to the underlying and overlying units. In the south the member is made up almost entirely of distinctively ribbed limestone or dolo- mitized limestone. In the northernmost localities it is made up almost entirely of resistant sandstone. In inter- vening areas it is resistant sandstone at the base and chiefly dolomite in the upper part. The middle member nearly everywhere contrasts fairly sharply with the relatively nonresistant fine-grained clastic rocks of the underlying member and with the sandy dolomites and dolomitic sand- stones of the overlying member. Wherever completely pre- served, the member is more than 100 feet thick and is no- where much more than 300 feet thick. DISTRIBUTION AND STRATIGRAPHIC LIMITS The middle member of the Abrigo Formation is con- fined to the western area as outlined in figure 6. It is pres- ent throughout that area except where removed or not deposited at the extreme north, where removed by Mesozoic erosion in much of the central part of the area, and where locally removed by pre-Late Devonian erosion in the west. The middle member of the Abrigo wherever seen over- lies the lower member with conformable contact. It is conformably overlain by the upper sandy member of the Abrigo in most areas except in the far west (Slate and Vekol Mountains, locs. 81 and 82), where it is overlain discon- forrnably by the Martin Formation of Devonian age. The middle member grades laterally northeastward into the Coronado Sandstone of the central area. PETROLOGY The middle member of the Abrigo Formation consists almost entirely of distinctively ribbed limestone or dolomitized limestone in and west of the Mule Mountains (10c. 65). Not far to the north and east of the Mule Moun- tains, in the Swisshelm and Little Dragoon Mountains (locs. 64 and 75), limestone is predominant, but a few thin sandstone beds are present. Farther north and west, in southern Pinal County (locs. 77, 81, and 82), sandstone is dominant in the lower part of the member, and carbonate, in that area chiefly dolomite, is dominant in the upper part. Still farther north, at and near Brandenburg Moun- tain (loc. 102), the member consists entirely of sandstone. The following descriptions of the rocks of the middle FIGURE 8.—Photomicrographs of lower member of Abrigo Formation. Bar in A is 1 mm; all views are same scale. A, Coarse lithiclast lime packstone from 105 feet above base of member at French Joe Canyon (10c. 69). Parts of two large rounded lithiclasts (upper right and bottom) and all of one rounded lithiclast (upper left) of slightly silty lime mudstone are visible. Matrix (in upper middle) is sparrite containing quartz silt (white spots) and glauconite (dark circular spots). Plain light. B, Coarse lithiclast lime grainstone from 322 feet above base of member at Garden Canyon (10c. 66). Parts of three lithiclasts made up of slightly silty lime mudstone are visible. Matrix material is silty and argillaceous lime mudstone. Many of the thin limestone beds in the lower member in the southern part of the region are made up of such rock. Plain light. C, Laminated lime mudstone (below) and silty argillaceous lime mudstone (above) from 300 feet above base of member at French Joe Canyon (loc. 69). This thinly laminated rock is very similar to that of the lithiclasts in B. Such rock is more common in the middle member of the Abrigo. Plain light. D, Ferruginous grainstone from 204 feet above base of member at Garden Canyon (10c. 66). The spheroidal grains are largely made of iron oxide but many contain sparry calcite (S). They may be altered grains of glauconite, completely replaced ooids, or largely replaced algal structures (GirvanellaP). Such rock occurs sparingly in the lower member at several localities. Crossed nicols. E, Well-sorted porous slightly ferruginous quartz siltstone from Brandenburg Mountain (10c. 102). Most dark spots are pore spaces but some are grains of iron oxide. Most of the lower member is made up of such rock near the transition of the Abrigo into the Coronado sandstone. Crossed nicols. ROCK-STRATIGRAPHIC UNITS 20 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS member are based on outcrop examination of the member at 17 localities and on the examination of 26 thin sections. Most of the limestone in the middle member either is distinctively ribbed with layers of medium-light-gray- weathering limestone and slightly more resistant light- brown-weathering silty limestone (fig. 9A) (both medium gray on fresh fracture) or is irregularly laminated in a dis- tinctive manner (fig. QB). Much of the limestone is lime mudstone or vaguely peloid lime mudstone that may or may not be fossil bearing. Also common are skeletal lime wackestones (fig. 10A), algal(?) and oolitic lime pack- stones (fig. 103), and lithiclast lime wackestones that con- tain large disk-shaped clasts of lime mudstone in lime mudstone or sparry calcite matrices (fig. 10C). At Ameri- can Peak (loc. 83) and some other areas the limestones are partially or completely dolomitized but retain their dis- tinctive ribbing or laminations. Dolomite greatly dominates limestone in the intermediate facies between the southern limestone facies and the northern sandstone facies. In this intermediate area the dolomites are nearly all thinly laminated and some are cross-laminated. They are commonly brownish gray on fresh surfaces and yellowish brown on weathered surfaces. Most of the dolomites are either sandy or silty and commonly occur as interlaminated sandy and very fine to medium-grained dolomites. Beds of edgewise chip con- glomerate containing chips of laminated sandy or non- sandy dolomite or of dolomitic sandstone in a dolomite matrix are conspicuous in the member in the inter- mediate facies. The sandstone that makes up the middle member at the north is light to medium gray on fresh surfaces and weathers to a banded gray or yellowish brown. It is fairly well sorted fine- to medium-grained orthoquartzite cemented by quartz. Most beds are fairly resistant but some are friable. The more resistant beds commonly have ”case- hardened” weathered surfaces but are less well cemented away from the weathered surfaces. Many beds are cross- laminated. Sandstones that occur to the south in the intermediate facies are largely concentrated at the base of the member where they have been included in a local Southern Belle Member (Creasey, 1967a). The sandstones of the inter- FIGURE 9.—Outcrops of middle member of Abrigo Formation. A, Silty and nonsilty limestone about 180 feet above base of member at French Joe Canyon (loc. 69). Pencil gives scale. “Ribbed” limestone like this is characteristic of the middle member in southern and western Cochise, eastern Pima, and Santa Cruz Counties, Ariz. (fig. 1). B Thinly layered slightly silty limestone with irregular very silty laminae in Pedregosa Mountains (10c. 61). Note similarity of this outcrop to that of Hitt Canyon Formation shown in Figure 18A. C, Cross-laminated slightly glauconitic calcareous sandstone in Vekol Mountains (10c. 82). Rock such as this is tentatively assigned to the middle member, but the member has this lithology only in the Slate and Vekol Mountains (locs. 81 and 82). ROCK-STRATIGRAPHIC UNITS FIGURE lO.—Photomicrographs of middle member of Abrigo Forma- tion. Bar in A is 1 mm; all views are same scale. A, Partly dolomitized slightly silty skeletal lime wackestone from 85 feet above base of member in northern Swisshelm Mountains (10c. 64). Such rock is abundant in the middle member in all southern localities. Plain light. B, Partly dolomitized silty oolite lime packstone from 150 feet above base of member at Garden Canyon (10c. 66). The oolites are largely dolomitized, whereas the silty micrite matrix is not. Oolitic rock such as this occurs commonly only in the Huachuca and Patagonia Mountains (locs. 66 and 83). Plain light. C, Lithiclast lime grainstone from base of member at Mount Martin (loc. 65). Clasts are slightly argillaceous lime mudstone, and matrix is mostly sparry calcite. (Compare fig. BB.) Such rock is common in the middle member in southern localities. Plain light. D, Poorly sorted siliceous ortho- quartzite from lower part of member at Nugget Canyon (loc. 77). Quartzite such as this is not present in the member at southern localities but is conspicuous at most northern and western localities. Crossed nicols. 22 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS mediate area tend to be less well sorted than to the north (fig. 10D). In the western part of the intermediate area the sandstones commonly have calcareous or dolomitic cement and weather dark red to moderate brown; some of them are glauconitic and many are hematitic. SEDIMENTARY STRUCTURES Limestone or dolomite chip conglomerates or edgewise conglomerates occur in carbonate beds in the middle member of the Abrigo at all localities except at the far north, where the member is made up entirely of sand- stone. Burrowed lime mudstone is occasionally seen in the member. Sandstone beds of the northern and intermediate areas commonly are conspicuously cross-laminated. Most cross- laminations are small- to medium-scale planar type, but some trough cross—laminations are present in western sec- tions (fig. 9C). Dolomite beds of the intermediate area also commonly display small-scale cross-laminations. Tracks and trails may be noted on the top surfaces of sandstone beds in the northern and intermediate facies, but they are not abundant. Ponosn'v The limestones of the middle member of the Abrigo Formation mostly seem to be very low in porosity, as indi- cated by thin-section examination. Pores formerly pre- sent in the unit are now largely cement filled, as seen in outcrop samples. Cement-filled or cement-reduced moldic and microfracture porosity are the mOSt commonly noted types. One exceptional sample from Johnson Peak in the Little Dragoon Mountains (10c. 75) which showed con- spicuous fenestral porosity in thin section was checked for effective porosity and found to have 7.1 percent. Dolomite from the intermediate facies is also seen to be very low in porosity. One sample tested from the Vekol Mountains (loc. 82) showed 1.9 percent effective porosity. Some of the well-sorted and imperfectly cemented sand- stone frorri Brandenburg Mountain (10c. 102) shows con- siderable porosity in thin section. Two samples checked for total porosity showed 6.0 percent and 13.6 percent. The more poorly sorted sandstones from the inter- mediate area seem to be lower in porosity than the northern sandstones. One sample checked had 4.6 percent total porosity, and another had only 1.5 percent effective porosity. THICKNESS Where the middle member of the Abrigo Formation is completely preserved and has not been thinned by post- Cambrian erosion, it ranges in thickness from 139 feet in the northern Swisshelm Mountains (10c. 64) to 306 feet at Garden Canyon in the Huachuca Mountains (10c. 66); it is 201 feet thick at the type section of the Abrigo at Mount Martin (10c. 65). Despite the large differences in thickness there seems to be no regionally consistent pattern of thick- ening and thinning. FossrLs AND AGE The middle member of the Abrigo Formation is mostly of Dresbachian (of Howell and others, 1944) or early Late Cambrian age on the basis of fossil—chiefly trilo- bite—collections made at or near the northern Swisshelm Mountains, Mount Martin, French Joe Canyon, Tomb- stone, Dragoon Mountains, Johnny Lyon Hills, Johnson Peak, and the Slate Mountains (locs. 64, 65, 69, 70, 71, 74, 75, and 81). At Mount Martin (10c. 65) in the Mule Mountains the basal few feet of the member is apparently of latest Middle Cambrian age (Hayes and Landis, 1965), and the same is true in the northern Swisshelm Mountains (10c. 64), where fossils possibly assignable to the Bolaspidel la zone were collected 8 feet above the base of the member (M. E. Taylor, written commun., Jan. 24, 1973). Fossils collected during this study within 18 feet above the base of the member in the Slate Mountains (10c. 81) (see “Fossil Lists”), however, are definitely of early Late Cambrian age (M. E. Taylor, written commun., Jan. 24, 1973). On the basis of fossils from Cochise County reported by A. R. Palmer (in Gilluly, 1956) from at or near localities 64, 65, 69, 70, 71, and 75; by Cooper and Silver (1964) from locality 74; and by us at localities 64, 65, and 69 (see “Fossil Lists”), forms that are representative of the Cedaria zone of the Dresbachian Stage are seen to be characteristic of most of the middle member. The younger Dresbachian fossils are found near the top at localities 65 and 74. The age of the middle member of northern areas where diagnostic fossils have not been found is assumed to be very nearly the same as that of southern areas because of conformable stratigraphic position beneath the upper sandy member. UPPER CONTAcr The contact between the middle member and the over- lying upper sandy member is conformable but generally fairly sharp. In the south, where the middle member is made up of gray ribbed limestone, the contact is marked by a rather abrupt change to reddish-brown-weathering sandy dolomite and dolomitic sandstone of the upper sandy member. In the north, where the middle member is made up of gray- to pale-yellowish-brown-weathering siliceous sandstone, the contact with the upper member is also easily recognized by a fairly abrupt upward change to brown-weathering dolomitic sandstone and sandy dolomite. Only in the area of the intermediate facies of the middle member where brown-weathering sandy dolomites and subordinate dolomitic sandstones make up much of the upper part of the middle member is the contact subtle. Indeed, Creasey (1967a) reasonably included the upper part of the middle member and the overlying beds in his Peppersauce Member in the Santa Catalina Mountains (10c. 77). However, I believe that a contact within the Peppersauce at that locality can be recognized at a horizon where there is an abrupt upward change to conspicuously ROCK-STRATIGRAPHIC UNITS 23 sandier and darker brown-weathering beds. This is very nearly at the same horizon as the top of the middle mem- ber to the north and south. On the basis of thickness and lithology in the Slate and Vekol Mountains (locs. 81 and 82) in the intermediate area, the upper sandy member seems to be missing, and the middle member is discon- formably overlain by the Martin Formation of Devonian age. UPPER SANDY MEMBER GENERAL DESCRIPTION The upper sandy member of the Abrigo Formation, 100-180 feet thick where completely preserved, is made up entirely of brown-weathering dolomitic sandstone and brownish-gray-weathering sandy dolomite. The member is very persistent in lithology throughout its area of occur- rence but contains more dolomite toward the south and west and more sandstone toward the north and east. DISTRIBUTION AND STRATIGRAPHIC LIMITS The upper sandy member is confined to the western area as shown in figure 6. It is missing because of post-Cam— brian erosion, however, over much of the western part of that area and over some of the central part. The upper sandy member of the Abrigo conformably overlies the middle member and, where completely preserved, is conformably overlain by the Copper Queen Member. In most western sections the Copper Queen is missing because of erosion, however, and the upper sandy member of the Abrigo is disconformably overlain by the Martin Formation of Devonian age. The upper sandy member grades eastward into the upper part of the Coronado Sandstone of the central area (fig.5). PETROLOGY The upper sandy member of the Abrigo Formation shows very little diversity of lithology vertically or regionally. In southern sections, however, there tends to be an alternation of sandy dolomite and dolomitic sand- stone, with dolomite being slightly dominant, whereas in the northernmost localities the unit is made up almost entirely of dolomitic sandstone. A few southern localities (Mount Martin, French Joe Canyon, and American Peak, fig. 4 and locs. 65, 69, and 83, fig. 1) contain some sandy or silty limestone. The following descriptions of the lithology are based on outcrop examination of the upper sandy member at 11 localities and on the examination of 20 thin sections. Sandstones in the member are well-sorted very fine to medium-grained orthoquartzites with dolomite cement (fig. 11); some are glauconitic and a few are slightly hema- titic. They are olive gray to pinkish gray on fresh surfaces FIGURE ll.—Photomicrographs of upper sandy member of Abrigo For- mation. Bar on A is 1 mm; views are same scale. A, Well-sorted fine-grained laminated dolomitic sandstone from lower part of mem- ber in northern Swisshelm Mountains (loc. 64). By point count this section contains 79 percent quartz, 20 percent dolomite grains, and 1 percent feldspar. This and B are the most abundant rock types in the member at most localities. Crossed nicols. B, Medium-grained glauconitic, hematitic, dolomitic quartz sandstone from Branden- burg Mountain (loc. 102). Quartz (Q) is most abundant, but rounded glauconite (G) grains are common, as is hematite (black) and dolomite (D) cement. Plain light. 24 and weather moderate brown or dark reddish brown. Most are cross-laminated (fig. 12) and are moderately resistant. Dolomites in the upper sandy member are evenly very fine to medium grained lithiclast dolomite grainstones (dolarenites). They range from very slightly quartzose to extremely quartzose, and some are glauconitic. They are mostly olive gray on fresh surfaces and weather to light brownish gray. Many dolomite beds are cross-laminated and are moderately resistant. The few limestones seen at Mount Martin, French Joe Canyon, and American Peak (locs. 65, 69, and 83) are very fine to medium grained lithiclast lime grainstones (cal- carenites). Most contain quartz silt or sand and a few are glauconitic. Otherwise they are similar to the dolomites. SEDIMENTARY STRUCTURES Cross-laminations are by far the most notable sedimen- tary structures of the upper sandy member (fig. 12). The cross-laminations are small to medium scale and are of the planar type. They occur throughout the member and throughout the region. Dolomite edgewise conglomerates that are made up of clasts of dolomite in a dolomite matrix may be found in most Cochise, Santa Cruz, and eastern Pima County sections. POROSI'I'Y Porosities in the upper sandy member of the Abrigo F or- mation are higher than in most of the rock units described in this report. The sandstones of the northern part of the region tend to be most porous, and dolomites of the southern part, least porous. Intergranular porosity is by far the most common type in sandstones, and both inter- granular and fracture porosities are found in dolomites. Two sandstones checked from Nugget Canyon (10c. 77) had 13.3 and 14.2 percent total porosity; the one with 14.2 percent total porosity had 120 percent effective porosity. Two dolomites checked for total porosity had 2.1 and 8.8 percent. The effective porosities of three dolomites checked were 2.8, 4.2, and 6.7. THICKNESS Where overlain by the Copper Queen Member, and thus completely preserved, the upper sandy member of the Abrigo Formation ranges in thickness from 101 feet at Nugget Canyon in the Santa Catalina Mountains (10c. 77) to 180 feet in the northern Swisshelm Mountains (10c. 64); it is 166 feet thick at the type section of the Abrigo at Mount Martin in the Mule Mountains (10c. 65). Fossns AND AGE Fossils are not abundant in the upper sandy member of the Abrigo Formation, but trilobites have been reported from the unit in western Cochise County from at or near Mount Martin, Tombstone, Dragoon Mountains, and Johnson Peak (locs. 65, 70, 71, and 75) by A. R. Palmer (in Gilluly, 1956) and from the Johnny Lyon Hills (10c. 74) by Cooper and Silver (1964). In addition we collected trilo- CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS ~ww- FIGURE 12.—Torrentially cross-laminated glauconitic and dolomitic sandstone in upper sandy member of Abrigo Formation at Branden— burg Mountain (loc. 102). Figure 113 is a thin-section photomicro- graph of rock from this outcrop. Such rock is common in the mem- ber at all localities where the member is present. Note strong simi- larity of this outcrop to that of Bliss Sandstone in figure 16A. bites from what appears to be the member from Picacho de Calera (loc. 160) in Pima County. (See “Fossil Lists”) All the collections reported that are without doubt from the member are assignable to the Crepicephalus or A phelaspis zones of the Dresbachian Stage of the Late Cambrian. Although some localities have not yielded fossils, on the basis of the uniformity of lithology and fauna over con- siderable distances the upper sandy member is assumed to be of closely similar age throughout the region. UPPER CONTACT The upper sandy member is conformably overlain-by the Copper Queen Member, or, where the Copper Queen has been removed by pre-Devonian erosion, it is discon- formably overlain by the Martin Formation of Devonian age. The contact between the upper sandy member and the Copper Queen, though conformable, is marked by a generally abrupt change from sandstone or decidedly sandy dolomite to nonsandy or only slightly sandy dolo- mite or limestone. Where the Martin Formation overlies the upper sandy member, the precise position of the con- tact, though disconformable, is not everywhere immedi- ately evident. In places where a thin conglomeratic zone is at the base of the Martin there is no difficulty in determin- ing the contact, but there are many localities in which slightly sandy brown-weathering cross-laminated dolo- mite at the base of the Martin directly overlies sandy brown-weathering cross—laminated dolomite at the top of the upper sandy member of the Abrigo. If diagnostic fos- sils cannot be found, the Martin dolomites can most easily be recognized by the vague low-angle cross-laminations as ROCK-STRATIGRAPHIC UNITS 25 contrasted to the higher angle planar cross-laminations of the upper sandy member. In addition, the Martin is dominantly yellowish brown or brownish gray, whereas the upper sandy member is mostly moderate brown. Where the contact is well exposed it can be seen to be knife sharp along a surface of a few inches of relief. COPPER QUEEN MEMBER NAME AND TYPE AREA The Copper Queen Limestone was named by Stoyanow (1936) for exposures in the Mule Mountain at Mount Mar- tin (fig. 4 and 10c. 65, fig. 1 ) as a separate formation but was considered by Hayes and Landis (1965) as the Copper Queen Limestone Member of the Abrigo Limestone in the same area. The member designation has since been extended to other localities in the western area (fig. 6) as the Copper Queen Member of the Abrigo Formation (Hayes, 1972). GENERAL DESCRIPTION At its type area the Copper Queen Member consists dominantly of thinly bedded laminated slightly sandy to silty limestone, and in all other areas it consists mostly of laminated slightly sandy dolomite. DISTRIBUTION AND STRATIGRAPHIC LIMITS The Copper Queen Member of the Abrigo Formation as currently defined is limited to the western area, as shown in figure 6. It is missing because of post-Cambrian erosion over most of the area but is preserved at a few localities in the eastern part. The Copper Queen Member conformably overlies the upper sandy member of the Abrigo. At the extreme east in the Pedregosa and Swisshelm Mountains (as at locs. 61 and 64, fig. 1) it is conformably overlain by the El Paso Lime- stone, but farther west it is disconformably overlain by the Martin Formation of Devonian age. The Copper Queen Member grades laterally eastward into an informal lower member of the El Paso Limestone in the central area (fig. 5). PETROLOGY The Copper Queen Member is made up almost entirely of fairly resistant medium-gray to pinkish-gray limestone at and near the type area and of light-brownish-gray dolo— mite at other localities. A few thin beds of dolomitic sand- stone are present at most places. The following descrip- tions of the rocks in the unit are based on the outcrop examination at five widely separated localities and on the examination of six thin sections. The limestone at the type area is mostly laminated slightly sandy fine- to medium-grained lithiclast lime grainstone (calcarenite) and fossil-bearing lithiclast lime grainstone, but some interlaminated lime mudstone is present, as is oolite lime grainstone and packstone (fig. 13A). The dolomite that makes up the member at most locali- ties is either lithiclast dolomite grainstone (dolarenite) (fig. 138) or dolomitized fine- to medium-grained lithi- clast lime grainstone; most is slightly sandy or silty and some is thinly laminated. The sandstone in the Copper Queen Member is well- sorted and brown-weathering generally cross-laminated dolomitic sandstone like thatwhich makes up much Of the underlying upper sandy member. SEDIMENTARY SrnucrUREs Notable sedimentary structures are virtually absent in the Copper Queen Member except for very minor dolo- mite intraformational conglomerate at Brandenburg Mountain (loc. 102) and for cross-laminations in most of the thin sandstone beds. POROSITY The carbonate rocks of the Copper Queen Member appear on the outcrop and in thin section to be very low in porosity, although some dolarenites have considerable cement-filled intergranular porosity. One dolomite sample checked by the kerosene displacement method had 2.0 percent total porosity. The very thin sandstone beds present at most localities may have porosities comparable to those of the sand- stones Of the upper sandy member but are probably much too thin to be effective reservoir rocks. THICKNESS Only two sections were examined in which the Copper Queen Member is conformably overlain by the El Paso Limestone and thus completely preserved from post-Cam- brian erosion. In those sections, both in southeastern Cochise County, the Copper Queen is 120 feet thick (Pedregosa Mountains, 10c. 61) and 148 feet thick (northern Swisshelm Mountains, 10c. 64). At other locali- ties the Copper Queen ranges in thickness from a few feet to slightly less than 100 feet. Fossns AND AGE Large numbers of trilobites and a few brachiopods have been collected from the type area of the Copper Queen Member at Mount Martin (10c. 65), where the unit is dis- conformably overlain by Devonian rocks (A. R. Palmer, in Gilluly, 1956; Hayes and Landis, 1965; this report, in “Fossil Lists”). These fossils are representative of zones of Franconian age (of Howell and Others, 1944). The highest collection, from 16 feet below the top of the member, indi- cates an age very near the Franconian and Trempeal- eauan boundary (M. E. Taylor, written commun., Jan. 24, 1973). Species of the brachiopod Billingsella have been recovered from the lower part of the complete Copper Queen sequence in the northern Swisshelm Mountains (10c. 64) (Epis and Gilbert, 1957) and from the thin remaining part of the member at Brandenburg Mountain (near loc. 102) (Krieger, 1968e). Billingsella was also found with Ptychaspis-zone trilobites at the type area and is indicative of a Franconian age. It seems rather certain that 26 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS the lower part of the Copper Queen is of Franconian age, and it is assumed that the upper barren part in the more complete sections may be of Trempealeauan (of Howell and others, 1944) or latest Cambrian age. UPPER CONTACT At the extreme east the Copper Queen Member is conformably overlain by the El Paso Limestone with an indefinite contact. The contact used by Epis and Gilbert (1957) was placed at an upward change from dolomite to limestone, but I (Hayes, 1972) chose a contact 55—75 feet lower at a change from sandy but noncherty dolomite below to nonsandy but somewhat cherty dolomite above. To the west the Copper Queen Member is disconform- ably overlain by the Martin Formation of Devonian age. The basal Martin beds in most places where they overlie the Copper Queen consist of cross-laminated sandy or silty dolomite. Not far above the base of the Martin in most sec- tions is a distinctive bed of nearly white orthoquartzite. EL PASO LIMESTONE NAME AND TYPE LOCALITY The El Paso Limestone was named by Richardson (1904, 1908) for exposures in the Franklin Mountains, El Paso County, Tex., near the Scenic Drive section (fig. 4 FIGURE 13.—Photomicrographs of Copper Queen Member Of Abrigo Formation. Bar in A is 1 mm; views are same scale. A, Largely silicified oolite lime packstone from Nugget Canyon (10c. 77). White is chert, black is hematitic dust, and gray is chiefly dolomite. At most localities rock in this member is dolomitized and has no relict structure. In the Mule Mountains (10c. 65), where undolomitized, and 10c. 45, fig. 1). At the type locality and throughout western Texas and much of southern New Mexico, the El Paso is now considered a group, but in westernmost New Mexico and in southern Arizona, its formational rank is retained. (See fig. 17 and attendant text discussion.) GENERAL DESCRIPTION The El Paso Limestone of the western area (fig. 6) con- sists almost entirely of thinly bedded limestone or dolo- mite that is slightly cherty at a few horizons. DISTRIBUTION AND STRATIGRAPHIC LIMITS In the western area (fig. 6) the El Paso Limestone is limited to a few exposures at and near the Pedregosa Mountains, Leslie Pass, and the northern Swisshelm Mountains (locs. 61, 63, and 64). It conformably overlies the Copper Queen Member of the Abrigo Formation and is disconformably overlain by strata of late Middle Devonian age. PETROLOGY The El Paso Limestone was given cursory examination at three localities in the western area. It does not differ ap- preciably from the upper part of the El Paso of the central area which is described later in this report. Most of the limestones are fine- to coarse-grained lithiclast or skele- tal-lithiclast lime packstones to wackestones; some con- the member consists largely of lithiclast lime grainstone or fossil- bearing lithiclast lime grainstone. Plain light. B, Well-sorted quartz- ose lithiclast dolomite grainstone (dolarenite) from northern Swiss- helm Mountains (loc. 64). This rock type is characteristic of the member at eastern localities. Plain light. ROCK-STRATIGRAPHIC UNITS tain quartz silt. The lower 50-80 feet is dolomitized at most localities, and locally all of the El Paso is dolomite. Chert occurs in nodules and lenses. SEDIMENTARY STRUCTURES Sedimentary structures noted in the El Paso in the western area include minor small-scale cross-laminations in slightly silty dolomite at one locality, probable algal- mat dolomite in the lower part at most localities, and some beds of burrowed limestone at all localities. POROSITY No porosity measurements were made of El Paso Lime- stone from the western area; presumably it differs little in porosity from the El Paso of the central area described later in this report. On the outcrop, porosities apparently are very low. THICKNESS The thickest known section of El Paso in the western area is at Leslie Pass (10c. 63) in the Swisshelm Moun- tains, where Epis and Gilbert (1957) measured 435 feet of the formation. The El Paso thins abruptly westward to a wedge edge and is absent west of long 109°45’ W. FOSSILS AND AGE No guide fossils have been found in the basal 120 feet or more of the El Paso Limestone in the western area. Cephalopods reported by Epis and Gilbert (1957) from higher beds indicate an age for those beds no Older than Early Ordovician zone D of Utah (Hintze and others, 1969) (fig. 5). Lithologic correlation with El Paso sections in the central area suggests, however, that the highest El Paso beds of the western area cannot be much younger than zone D and certainly are not as young as zone G. The barren basal beds of the El Paso may contain equivalents of one or all of zones A, B, and C and could conceivably contain beds of latest Cambrian age. The highest fossils known in the underlying Copper Queen Member of the Abrigo Formation are of Franconian age. Until further information is available, it is arbitrarily assumed that the uppermost beds of the Abrigo are of Trempealeauan age (latest Cambrian) and that the Cambrian and Ordovician boundary is near the arbitrary contact between the Abrigo and El Paso in the western area. UPPER CONTACT The contact of the El Paso with the overlying Devonian beds is disconformable and easily recognized; the rela- tively pure carbonate rocks of the El Paso contrast sharply with the soft-weathering siltstones and silty carbonate rocks of the Devonian. EASTERN PART OF STUDY REGION BLISS SANDSTONE NAME AND TYPE LOCALITY The Bliss Sandstone was named by Richardson (1904) for exposures in the Franklin Mountains, Tex. (between 27 locs. 44 and 45, fig. 1); a formal type section was notdesig- nated. A section in McKelligon Canyon (fig. 14) (10c. 92) near the type locality is described and is designated the principal reference section in this report. (See description under “Selected Measured Sections”) GENERAL DESCRIPTION In its type locality and at all localities to the east of the longitude of the type locality, the Bliss is made up almost exclusively of sandstone. To the northwest and west of the type locality, significant amounts of sandy limestone 0r dolomite and (or) siltstone or shale are present at most localities. The Bliss ranges in thickness from 0 to about 375 feet, the thickest sections being toward the south. DISTRIBUTION AND STRATIGRAPHIC LIMITS The Bliss Sandstone is confined to the eastern area as outlined in figure 14. The name Bliss is now applied to the basal Paleozoic clastic formation throughout that area. Gordon and Graton (1907) used the term Shandon Quartz- ite for the unit in part of southwestern New Mexico, but it was never widely adopted. Darton (1917b) and Paige ( 1916) abandoned Shandon and extended Bliss into that area. Flower (1958) suggested that the term Shandon be revived and that the Bliss be only a part of the Shandon, but the term Bliss is well established, and such a division seems unnecessary, inappropriate, and undesirable. The Bliss unconformably overlies Precambrian rocks wherever it occurs and is conformably overlain by the El Paso Group, except very locally at the extreme north (Mockingbird Gap and Fra Cristobal Range, locs. 34 and 37), where it is disconformably overlain by Pennsylva- nian rocks. PETROLOGY In the type locality and in most areas east of long 107° W., the Bliss Sandstone is made up mostly of sandstone, generally more than 90 percent (fig. 14). The subordinate Iithologies in these eastern areas include thin interbeds of siltstone or shale and, rarely, a thin bed or two of gener- ally sandy limestone or dolomite. West of long 107° W., interbedded carbonate rock occurs in the formation in ap- preciable amounts at most localities and is the dominant lithology in some places. Siltstone and Shale are impor- tant constituents of the formation at several localities northwest Of the type locality, most notably at Eaton Ranch in the San Mateo Mountains and at Amphitheater Canyon in the Fra Cristobal Range (locs. 35 and 36, fig. 1) near the north wedge edge of the Bliss. Hematite is an important cementing agent of the Bliss in most areas, par- ticularly west of long 106° W., and thin beds of oolitic hematite are interbedded with sandstone at many locali- ties, especially toward the northwest. Glauconite-rich beds are common at all localities except at the extreme south- east (Beach Mountain, loc. 48) and southwest (Mescal Canyon, 10c. 24) of the area of Bliss distribution in the report area. The presence of rather abundant hematite and 28 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS 1112° 111° 110° 109° \V\ \ A 1' l « M" ‘ i \\\ \/ A P A C HE I a R co A\\ GILA {1 i ; CATRON l M A I P \\ \ Area where Cambrial‘r and Qrdoynfién r cks are absent I r _. “1 xxx» . 1 / __l__0___._.‘ —— I ;-,____—— r -———~«-_._._x //’( ’ GREENLEE ' 123 J ' A535 l #)Coolidge Dam Nantac Rim . PIN A L I I 33 ...... o I WESTERN AREA ’ l Area of occurrence of I \ I . lmperia Balsa Quartznte and Mountain CENTR AREA AbrIgo Formation Area ofoccurrence of l Coronado Sandstone I . \ o “ ° ' 23‘1 :00 o GRAHAM ( 21E ' Removed by M~-~_ ( O1m (Mesozoic erosion! 1. __.___;~__-- E12 -——~—~ g (\ «’5 La. 5 225 ‘1 z ’ L . ___.._...__..._I._—1 .' l l f i o Dos Cabezas i ° ‘ 370 388 . .116 A Ahep I 396 . 32° ._ P I M A PBC ass PI’eagher « l . Mountain ' l J 610] ‘ l COCHISE Portal l o , r—w-m..c._.__ _ . i \\\ j 1 A44o \\ [———~ SANTA CRUZ l \\L 454 l A499 l i A344 \ \h—w —-—-—- —— __..lL __UNITE_Q__STATES MEXICO ‘~ _' “ EXPLANATION ! 31° . A370 Thickness, in feet — Measured K or verified during present study 3 '2 (I) 50 100 MILES (7 l I I II I II I | 1 i O 50 100 KILOMETRES l l l i 1 glauconite in the Bliss gives a dark appearance to the formation as a whole, and, especially in western and northern localities, outcrops of the Bliss are conspicu- ously darker than overlying and underlying units. The weathered colors of individual beds range from blackish red and dark greenish gray to pinkish gray and yellowish gray, but most beds are in the dark-reddish-brown to pale- brown range. Freshly fractured rock is generally some- what paler, and most is in the pale-red to pinkish-gray range. FIGURE l4.—Lithofacies of Bliss Sandstone and some thicknesses The following statements on the texture and composition of sandstone and siltstone from the Bliss are based on field examination at 25 localities and on the examination of 84 thin sections, of which 70 were point counted. An additional 18 thin sections of carbonate rock from the Bliss were examined. Nearly all sandstones in the Bliss are grain-supported arenites; most are orthoquartzites (fig. 15H) or feldspath- ic sandstones (fig. 15A), and a few, especially near the base, are arkoses (fig. 15E). Glauconite grains are present in ROCK-STRATIGRAPHIC UNITS 29 108° 107° 1Q6° 105° i Area where CambriLn and Ordovician rocks are absent ol 1 ‘ CATRON Eaton Ranch SOCORRO . — l I ., MOCkIngblfd GW\ L I N C O L N Fra Cristobal / [.0 Flange , I ‘13 __..‘ Sierra Oscura l *(southl _ ‘ 1- ———-~ -1 . r \ - l \\ o i" '— -’ — \\ CHAVES/‘433" J __,.A \ ' /’l O T E R O \0 o \ | / l C \ // . . Agua Chiquita 1 10 CanyonA o l \ / I.EDDY .— —-—-—-—'—‘ .1 . ' l o I ‘ RN AREA ‘ 105 V' - ccurrence of a I Sandstone L EEWMEQm———— —--32° —'- TEXAS - l u _ . I E L P A S O . l i 373 H U D s P E T H l Z Pasotex \ O m l a I ' m l ' °° ._l \1 . . D \0 U More than 25% siltstone and (or) shale (I?) . i \ {’0 o E 4’ I ‘ z N» ‘93 : 10—25% siltstone and (or) shale {’0 \12’6‘ Beach Mt" E 0 fl ...... . 310 l l ’ More than 10% hemafite \\\ I | ‘- / i 5—10% hematite \\ Q/ i I l W of Bolsa Quartzite, Coronado Sandstone, and Bliss Sandstone. Bliss sandstones in small amounts at most localities, but such grains are known to occur in abundance only in two discrete areas: (1) a large area in south-central New Mexico and extreme western Texas bounded roughly by lines con- necting Molinas Canyon, south Sierra Oscura, and Scenic Drive (locs. 16, 28, and 45); and (2) a smaller area in Grant County, N. Mex., including Wemey Hill, Lone Moun- tain, and San Lorenzo (locs. 20, 21, and 38). Lewis (1962) discussed the occurrence of glauconite in the Bliss near Lone Mountain (loc. 21). All observed sandstones of the Bliss are cemented by quartz (fig. 15A, H), carbonate (fig. 15D), or hematite (fig. 15C) or by a combination of these (fig. 15F, G). Quartz cement is the most abundant, but at nearly all localities either carbonate cement or hematite cement or both occur in significant amounts. No variation in the distribution of carbonate cement seems to be regionally consistent, but the decided tendency is for the cement to be more common in the upper part of the formation. On the other hand, hematite cement seems to be common only in north- 30 FIGURE 15. (Above and facing page).—Photomicrographs of Bliss Sand- stone. Bar in A is 1 mm; all views are same scale. A, Poorly sorted very coarse grained sandstone from base of formation at Beach Moun— tain (10c. 48). This sandstone, which rests on Precambrian sedi- mentary rocks, contains large lithic fragments (R) of siltstone and quartzite as well as some feldspar (F). Cement is quartz, and original boundaries of quartz grains are largely obscured. Crossed nicols. B, Very sandy fine-grained dolomite from about 60 feet above base of formation at Capitol Dome (10c. 39). Rock as dolomitic as this is interbedded with sandstone in most western localities The dolomite appears to be slightly metamorphosed. Plain light. C, Extremely CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS hematitic sandstone from 2 feet above base of formation at Rhodes Canyon (loc. 10). Here hematite (black) occurs both as cement and as replacement(?) of quartz (white) grains. Similar hematitic sand- stone occurs in both Cambrian and Ordovician parts of the Bliss in most northern localities. Plain light. D, Very dolomitic sandstone from lower part of rock assigned by Zeller (1965) to the Bliss Sand- stone at Mescal Canyon (loc. 24). This rock is not overlain by less dolomitic sandstone and might more properly be assigned to the overlying Hitt Canyon Formation. Plain light, E, Poorly sorted coarse arkose from base of formation at Capitol Dome (10c. 39). ROCK-STRATIGRAPHIC UNITS Feldspar (F) is nearly as abundant as quartz (Q). Matrix is largely quartz and feldspar silt but includes considerable sericite (S). Arkose such as this is common where the Bliss rests on a surface of moder- ately high relief carved in Precambrian granite. Crossed nicols. F. Calcareous and hematitic laminated sandstone from 30 feet above base of formation at Werney Hill (10c. 20). Here the hematite (black) apparently has replaced some of the sparry calcite cement around quartz grains. This type of sandstone is very common in the Bliss. Crossed nicols. G, Hematitic siliceous sandstone from 14 feet above base of formation at Cable Canyon (10c. 15). Here the chief cement- ing material is quartz, but hematite forms rings around most grains and also occurs as large hematitic ooids. Such rock occurs most commonly in more northern localities of the Bliss. Plain light. H, Porous well-sorted medium-grained siliceous sandstone from about 160 feet above base of formation at Werney Hill (10c. 20). This quartz- ose sandstone is cemented with quartz in optical continuity with grains, but most original grain boundaries are evident. Some of the black areas are porous voids, and others are quartz at extinction. Such sandstone is generally present in the Bliss but is not pre- dominant in most areas. Crossed nicols. 32 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS western areas as shown in figure 14, where it may occur in any part of the formation from the base to near the top. Where various cements occur together, quartz cement generally replaces carbonate cement and hematite cement generally replaces both quartz and carbonate cement. In addition to hematite cement in many of the sand- stones of the Bliss, hematite ooids are commonly present in many northern and western localities (fig. 15G) and greatly predominate over quartz grains in some beds. Five selected samples that appeared to be particularly high in hematite were examined for total iron content by the atomic-absorption method by Violet Merritt of the US. Geological Survey. A sample from a poorly exposed bed about 25 feet above the base of the Bliss at Lone Mountain (10c. 21) in Grant County showed 41 percent iron as FezOs, and a sample from a 4-foot-thick bed 12 feet above the base of the Bliss at the Winston section in northeastern Sierra County showed 37.5 percent iron as Fe203. Kelley (1949) discussed the potential of some of these oolitic hematites of the Bliss as iron-ore deposits and described the brief mining of such ore in the Caballo Mountains, N. Mex. (near Cable Canyon, 10c. 15). Sandstone at the base of the Bliss is locally conglomeratic and is nearly everywhere very coarse and poorly sorted (fig. 15A, E). Near the top of the Bliss most sandstones are fine grained and fairly well sorted. The areas where siltstone and shale are moderately abundant in the Bliss Sandstone are generally coincident with areas where hematite, glauconite, and carbonate are also moderately abundant. (See fig. 14.) Although none of the fine-grained rocks were examined in the laboratory, field examination indicated that most of the fine silt- stones and the shales are calcitic, hematitic, or glauconitic, or all three. Limestone and dolomite beds included in the Bliss Sandstone are for the most part, similar to those in the lower part of the overlying El Paso Limestone and are not described in detail here. Nearly all are quartzose (fig. 153), many are glauconitic, and some are slightly hematitic. Most of the limestones are fine-grained lithiclast lime grainstones to packstones, some are coarse lithiclast lime wackestones, and a few are oolite packstones. Some of the dolomites appear to be true dolarenites, but most are prob— ably dolomitized lithiclast limestones. Figure 14 and plate 1 together roughly show the lateral and vertical distribu- tion of carbonate beds in the Bliss. SEDIMENTARY STRUCTURES Small- and medium-scale planar cross-laminations (fig. 16A) are the most conspicuous and most abundant sedi- mentary structures seen in the Bliss Sandstone. Cross— laminations can be seen in all parts of the sequence at all localities, although most of the sandstone beds in the formation are not cross-laminated at most places. Seeland (1968) measured the attitudes of inclination of numerous sets of cross-laminated beds in the Bliss for at least six localities within the present report area. A majority of those measured at two western localities (San Lorenzo and Capitol Dome, locs. 38 and 39) dip southwestward and agree rather closely with most sets he measured to the west in the Bolsa Quartzite and Coronado Sandstone. Sets that were measured farther east are less consistent from one place to another—Slightly east of north at Agua Chiquita Canyon and near the Pasotex section (locs. 1 and 56), east- southeast near South Ridge and McKelligon Canyon (locs. 14 and 92), and southwest at Beach Mountain (10c. 48). Other sedimentary structures seen at many places in sandstone of the Bliss are fucoidal tracks and trails on bed- ding surfaces and Scolithus tubes. Intraformational chip conglomerates were noted in dolomite or dolomitic sandstone beds at several localities f' . l6B . ( 1g ) POROSITY Porosities of sandstones from the Bliss vary but, in general, are higher than those of the more siliceous Bolsa Quartzite. The average effective porosity of four samples tested with the mercury porosimeter is 3.4 percent; the average total porosity of eight other samples tested by the kerosene-displacement method is 7.6 percent; and the aver- age porosity of five additional samples checked by point counting is 4.8 percent. The total porosity of two carbonate samples tested from the Bliss is 3.5 percent and 4.7 percent. Most porosity observed in thin sections of carbonates from the Bliss is microfracture porosity; some carbonates showed vuggy porosity, and one lithiclast lime grainstone had intrapar- ticle porosity. THICKNESS The thickness of the Bliss Sandstone ranges from depositional and erosional wedge edges to as much as 373 feet at the Pasotex section (10c. 56) in the Hueco Moun- tains, Tex. Erosional wedge edges occur along the north boundary of the Bliss distribution area and around a large Mesozoic highland in western New Mexico (fig. 14). In general, the thickest complete sections are in the southern part of the region, but even there, thicknesses change radi— cally within a few miles owing to relief on the underlying surface. In the Franklin Mountains of western Texas the Bliss is locally absent because of nondeposition (Kottlow- ski and others, 1969; Harbour, 1972) between McKelligon and Hitt Canyons (locs. 92 and 44), where the formation is 267 and 210 feet thick, respectively (fig. 14). FOSSILS AND AGE Guide fossils are sparse in the Bliss Sandstone. Late Cambrian fossils have been found at some localities, and Early Ordovician forms have been found at others. Flower (1969) has discussed the problems relating to the age of the Bliss on a regional basis. AS a whole, the fauna] evidence available, together with lithologic correlations, indicates that the Bliss becomes younger from west to east. VRQQK-STRATIGRAPHIC UNITS 33 FIGURE 16.—Outcrops of Bliss Sandstone at Lone Mountain (loc. 21). A, Torrentially cross-laminated sandstone 112 feet above base of Bliss Sandstone. Such sandstone is common in the Bliss at most localities. B, Intraformational conglomerate about 150 feet above base of Bliss Sandstone. Penny gives scale. Here sandy dolomite clasts are in a At locality 20 (fig. 1) near Werney Hill in Grant County, one of the westernmost exposures of the Bliss, we found brachiopods about 30 feet above the base of the Bliss, which Reuben J. Ross, Jr. (written commun., May 11, 1970) identified as Eorthis sp. of Late Cambrian (Trempealeauan) age. Ballman (1960) had previously reported the Late Cambrian trilobite Camaraspis sp. from near the base of the Bliss at the same locality. The eastem- most locality in which Cambrian fossils have been found in the Bliss is San Diego, or Tonuco, Mountain in Dona .Ana County, about 12 miles north-northwest of Robledoi Mountain (10c. 42), where trilobites of Late Cambrian (Franconian) age were reported to occur by Flower (1953). East of the longitude of San Diego (Tonuco) Mountain only Ordovician fossils have been reported from the Bliss. There seems to be little doubt that the top of the Bliss is younger in the east than toward the west. According to Flower (1969) the upper Bliss of the Van Horn area in Texas (near Beach Mountain, 10c. 48) contains impressions of cephalopods that indicate an age as young as that of beds 100 feet above the Bliss in areas to the west. King (1965) concluded that the entire Bliss of the Van Horn area is of Ordovician age. Earliest Ordovician fossils are known from the upper part of the Bliss at least as far west as the Mud Springs Mountains of New Mexico (10c. 13) (Flower, 1953). All the fauna] evidence available used in conjunction with lithologic correlations suggests that in the western part of the region the lower part of the Bliss is Late Cambrian (Franconian) in age and that the upper part is of dolomite matrix. This part of the Bliss at this locality is probably of earliest Ordovician age and thus somewhat younger than similar rock in the El Paso Limestone at Preacher Mountain (loc. 23). (See fig. 26A.) Early Ordovician age. The evidence further indicates that at the easternmost localities the entire Bliss is of Ordovician age and that the top is younger than to the west. Exactly where the base of the formation becomes Ordovician eastward is unknown, but our interpretation is shown on plate 1 and in figure 46. CONTACTS The basal contact of the Bliss Sandstone with underlying Precambrian rocks is everywhere uncon- formable and sharp. In most places the Bliss overlies crystalline rocks, but even where it overlies younger Pre- cambrian sedimentary rocks there is no problem in recognizing the unconformable contact because the basal Bliss contains pebbles of the underlying rocks (King, 1965). The upper contact of the Bliss with the overlying Hitt Canyon Formation of the El Paso Group varies somewhat in nature from west to east. At most western localities the contact is intertonguing. At such places, as exemplified at Lone Mountain (10c. 21) in Grant County (pl. 1), where limestones lithologically similar to those of the basal Hitt Canyon Formation occur below sandstones like those of the Bliss, the contact is usually chosen at the top of the highest conspicuous sandstone. At most eastern localities the contact is gradational but fairly sharp—sandstone at the top of the Bliss gives way rather abruptly to sandy lime- stone or dolomite at the base of the Hitt Canyon. At most Texas localities the contact is very abrupt and has been interpreted as a disconformity (Richardson, 1904; Cloud and Barnes, 1946). As noted by King (1965). if the contact at 34 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS CABALLO WEST TEXAS AND FRANKL'N MOUNTA'NS MOUNTAINS NEW MEXICO Cloud 233:; Kelley and nd Lucia Harbour and This Barnes Leah/1MB (1969) (1972) Silver report (1946) (1969) (1952) Unit C FloridaFMmountains : Ranger Upper .9 Peak limestone E . Scenic Fm zone _ Unlt . ,o B Drlve u. 2 b Frn E o 3 Cindy Upper sandy Low3r sandy Fm zone member lSection missing) Unit 323 McKeIIieon McKelligon ,M‘dd” McKeIIigon Canyon Frn "mesmne Limestone Fm zone Unit Bl Bat Cave Chamizal Middle sandy Frn Upper sandy Jose Fm Fm zone member Victorio g Hills "g Fm E Hag Hill IL°W°' 5 Middle Unit Fm llmestone u. member A zone c o >. : an Cooks 0 .. Fm 5' I Sierrite —————— Limestone Bowen Lower Lower Sierrite nd Limestone Fm Szaoney sandy member FIGURE 17.— Subdivisions of the El Paso Limestone (or Group) as used by various workers in the region, showing their probable relations to one another. Flower (1964) and LeMone (1969), in some areas other than the Franklin Mountains, recognized the Big Hatchet Forma- tion between the Sierrite and Cooks and, in ascending order, the Mud Spring Mountain and Snake Hills Formations between the Jose and McKelligon Canyon. Where the nomenclatures of Flower (1964) and LeMone (1969) do not coincide, that of LeMone is used— for example, Flower used the terms Victorio, McKelligon, and Florida for the Victorio Hills, McKelligon Canyon, and Florida Mountains. ROCK-STRATIGRAPHIC UNITS 35 the Texas localities is indeed disconformable, the hiatus represented by the disconformity must be slight. EL PASO GROUP PREVIOUS NOMENCLATURE The El Paso Limestone was named by Richardson (1904) for exposures near Scenic Drive (fig. 4 and 10c. 45, fig. 1) in the Franklin Mountains of Texas. He later restricted the El Paso by removing the upper part as the Montoya Limestone (Richardson, 1908). Gordon and Graton (1907) used the term Mimbres Limestone for the unit in part of southwestern New Mexico; that term was abandoned and was replaced there with the name El Paso by Paige (1916) and Darton (1917b), and since then the name has been extended throughout much of the present report region. Kelley and Silver (1952), working in the Mud Springs and Caballo Mountains (between locs. 13 and 16), proposed elevating the El Paso to a group composed of two formations, the Sierrite Limestone and the overlying Bat Cave Formation (fig. 17). With varying degrees of success, other workers have attempted to extend these sub- divisions to nearby areas in New Mexico. Flower (1964) and LeMone (1969) also considered the El Paso as a group and proposed as many as 10 formations in the group (fig. 17). The lowest of these was called the Sierrite Limestone although Flower (1964) recognized that his Sierrite represented a shorter time interval of deposition and included a more restricted lithology than the Sierrite of Kelley and Silver (1952). Lucia (1969) also regarded the El Paso of the type locality as a group, but he divided it into six formations. He accepted Flower’s McKelligon Formation in the middle of the El Paso but proposed five new formational names to replace most of those used by Flower (1964) and LeMone (1969), as is shown in figure 17. Harbour (1972) maintained a formational status for the El Paso but divided it into six informal subdivisions that coincide almost exactly with the formations of Lucia (1969). All the various subdivisions of the El Paso referred to above, as well as the informal subdivisions used by Cloud and Barnes (1946) in western Texas, are recognizable and useful in the local areas in which they were proposed; and some of the contacts can be recognized with a reasonable degree of certainty over much of the region of the El Paso distribution. NOMENCLATURE USED IN THIS REPORT The El Paso is here considered a group throughout the eastern area as outlined in figure 6; west of there it is considered a formation. Three formations of the El Paso Group are recognizable, mappable, and correlatable with a reasonably high degree of certainy throughout the report region (fig. 17). The lowest of these, herein named the Hitt Canyon Formation, coincides with unit A of Cloud and Barnes ( 1946) and includes as many as seven formations of Flower (1964) and LeMone (1969), three formations of Lucia (1969), and three informal zones of Harbour (1972). The McKelligon Limestone in the middle of the group is an adoption of the McKelligon Formation of Flower (1964); it coincides with units B1 and Bza of Cloud and Barnes (1946) and with the middle limestone zone of Harbour (1972). The upper formation is the herein-named Padre Formation, which includes units sz and C of Cloud and Barnes ( 1946), two formations of Flower (1964) that coincide with Cloud and Barnes’ units, two formations of Lucia (1969), and two informal zones of Harbour (1972) that coincide with Lucia’s formations. HI'IT CANYON FORMATION NAME AND TYPE SECTION The Hitt Canyon Formation, here named, comprises the lower 531 feet of the El Paso Limestone (here elevated to El Paso Group) as described by Harbour (1972) in his stratigraphic section 5 (Hitt Canyon, fig. 4 and 10c. 44, fig. 1, of this report) measured on a spur on the east side of the Franklin Mountains, El Paso County, Tex., about half a mile south of the drainage course of Hitt Canyon. This section (units 5-13 as described by Harbour) is designated as the type section. As thus defined, the Hitt Canyon Formation comprises the lower sandy, lower limestone, and middle sandy zones of the El Paso as used by Harbour (1972) and is equivalent to unit A of the El Paso as used by Cloud and Barnes (1946) at Beach Mountain (loc. 48 of this report). The section of unit A at Beach Mountain is here designated as the principal reference section of the Hitt Canyon. In so naming the Hitt Canyon, we recognize that as defined it includes an interval in the Franklin Mountains that was divided into four formations by Flower ( 1964) and that was divided into three formations by Lucia (1969). All of those formations as defined are so thin, have mutual contacts that are so subtle, and generally occur on such steep slopes that they are not practically mappable units at scales most commonly adopted for mapping in the region. All the units are recognizable, however, in the Franklin Mountains; and many can be recognized with some degree of confidence in some other areas and, thus, would be suit- able members of the Hitt Canyon. Unfortunately, none of the names used by Lucia (1969) was taken from properly named geographic features. The Sierrite Limestone of Flower (1964) is not acceptable as a member name because it is a redefinition of the Sierrite of Kelley and Silver (1952). The Jose of Flower (1964) cannot be used because it was not named for a properly recognized geographic feature. Flower’s (1964) Cooks and Victorio might be acceptable as local member names, but we believe that the contact between them, being a contact of dolomite and limestone, is not of stratigraphic significance. 36 GENERAL DESCRIPTION The Hitt Canyon Formation is a generally thinly layered carbonate unit which is notably quartzose in the basal part and which is commonly quartzose and locally oolitic in the uppermost part. It ranges in thickness from slightly more than 200 feet to nearly 600 feet. DISTRIBUTION AND STRATIGRAPHIC LIMIrs The Hitt Canyon Formation as presently defined is restricted to the eastern area as outlined in figure 6. Its equivalents extend farther to the west but, for the present, are assigned to the El Paso Limestone. The Hitt Canyon nearly everywhere conformably overlies the Bliss Sandstone except very locally where the Bliss is absent owing to nondeposition and the Hitt Canyon unconformably overlies Precambrian rocks. In the southern part of the region the Hitt Canyon is con- formably overlain by the McKelligon Limestone of the El Paso Group. To the north it is disconformably overlain by the Montoya Group of Middle to Late Ordovician age or by younger rocks. PETROLOGY The Hitt Canyon Formation is made up almost entirely of rather thinly layered carbonate rocks that in most localities are conspicuously quartzose in the basal 30-100 feet and somewhat quartzose in the top 50-125 feet. The carbonate rocks are dominantly limestone in the vicinity of the Caballo and Hueco Mountains (locs. 13, 15, and 56), dominantly dolomite to the north and east (locs. 1, 7, 9, 18, 27, 28, 35, and 48), and variable in other areas. The following descriptions of the lithologic character of the Hitt Canyon are based on the outcrop examination of the formation at 24 localities and on the examination of 112 thin sections. Carbonate beds in the lower part of the Hitt Canyon are in general made up of alternating very quartzose laminae and generally thicker slightly quartzose laminae; the relatively quartzose laminae usually weather to pale brown and stand out in relief on the outcrop (fig. 18A). The intervening less quartzose laminae are mainly fine- grained lime grainstones or packstones made up of variable ratios of algal(?) and other skeletal grains and lithiclasts (fig. 19A); fossil-bearing lime mudstones are relatively sparse. The amount of quartz sand in the lower part of the formation increases from west to east, and at Beach Mountain (10c. 48) a considerable amount of sand- stone is included. Above the basal quartzose part of the formation, limestone fabrics are more diverse but tend to reflect less agitated depositional environments. Skeletal lime wacke- stone (fig. 198) and fossil-bearing lime mudstone dominate, but fossil-bearing oolite lime packstones and lithiclast lime packstones are also common and become increasingly abundant upward. Some lime mudstones and wackestones display an intricate network of burrows filled with slightly silty lime mudstone (fig. 188). Coarse lithi- CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS clast lime wackestones occur both in continuous beds (fig. 18C ) and, at some localities, in “channels” cut in massive fossil-bearing lime mudstone. Although, in general, the middle part of the Hitt Canyon is free of quartz sand and has a low quartz silt content, it does become somewhat quartzose eastward (as at Agua Chiquita Canyon and Beach Mountain, locs. 1 and 48). The upper part of the Hitt Canyon, like the lower part, is quartzose at most localities and, even where it is not, can generally be separated from the middle part by the increasing abundance of medium-gray-weathering beds of lithiclast lime packstone to grainstone, oolite packstone to grainstone (fig. 19C, D), and, in some localities, coarse- grained dolarenite. As with the lower and middle parts of the Hitt Canyon, the upper part is particularly quartzose toward the east and, at Agua Chiquita Canyon and Beach Mountain (locs. l and 48), contains beds of dolomitic quartz sandstone. Secondary chert is irregularly distributed in the Hitt Canyon Formation at most localities. It generally occurs as sparingly distributed irregular nodules of dark- to light- gray porcelaneous chert and sparsely occurs as nearly complete replacement of beds as much as 2 feet thick; it is most abundant in the middle part of the formation. SEDIMENTARY STRUCTURES A moderate diversity of sedimentary structures indicative of deposition in shallow subtidal and low inter- tidal environments occurs in the Hitt Canyon Formation. Much more work than was expended on this project would have been necessary to have evolved a detailed environ- mental analysis. The comments below are sufficient for a general analysis and may be sufficient to stimulate further research by sedimentary petrologists. lntraformational conglomerates (fig. 18C) can be seen in any part of the formation but seem to be more common in the basal and uppermost parts. Most contain small chips, but some have chips of algal-mat dolomite as much as 2 inches across. Algal-mat dolomites from which the chips in some of the conglomerates were derived are present in the formation, particularly toward the base, but are not abundant. Many of the intraformational chip con- glomerates may be indicative of subaerial desiccation, but other desiccation features, such as mud cracks and bird’s- eye structures, are sparse. _ Oncolites and digitate algal stromatolites are commonly seen, especially in the middle part of the formation. Also commonly seen in the middle part of the formation, especially at Mud Springs Mountains, Cable- Canyon, San Lorenzo, Capitol Dome, and the Pasotex section (locs. 13, 15, 38, 39, and 56), are elongate mounds of fossil—bearing lime mudstone that were channeled and filled with coarse skeletal-lithiclast lime packstone to wackestone; these are similar to carbonate mounds in the overlying McKelligon Limestone described by Toomey (1970). ROCK-STRATIGRAPHIC UNITS 37 Some beds, especially in the middle part of the Hitt Canyon, were intricately burrowed (fig. 183). Burrowed beds as much as 3 feet thick are typically very extensive, and one was used by Harbour (1972) as a marker horizon throughout a large part of the Franklin Mountains. Surface tracks and trails, though less easily seen than burrowed rock, are also common in the middle part of the Hitt Canyon. The sandstones, sandy dolarenites, and lime grain- stones in the lower and upper parts of the Hitt Canyon commonly display small-scale low-angle cross- laminations that are indicative of oscillating currents. Poxosrnr Limited studies indicate that the greatest porosities in the Hitt Canyon Formation are in the upper part of the formation and that, in general, porosities in the formation are higher in western Texas than in New Mexico. Because of the effects of near-surface weathering, porosity measure- ments of rocks from the outcrop probably do not accurately represent the porosities that the same rocks might have if they were deeply buried. Nevertheless, we determined the effective porosities of 13 outcrop samples by the mercury—porosimeter method and the total porosities of an additional 28 samples by the kerosene- displacement method; most of those samples and others were checked in thin section for porosity types. The highest limestone porosities we measured are in oolite grainstones and coarse lithiclast lime packstones from the upper half of the formation. The highest effective porosity measured for these rock types was 6.1 percent (at Mud Springs Mountains, 10c. 13), and the lowest total porosity determined was 2.6 percent. Most of the porosity in these rocks is interparticle porosity, but some is intra- particle porosity. Medium- and coarse-grained dolomites from the upper half of the formation vary greatly in porosity, but some show total porosities from 4.0 percent to 5.4 percent. The highest measured porosity is a combination of inter- particle and fracture porosity. Except for a few rocks with moldic, vuggy, or fracture porosity, most lime mudstones or wackestones, fine- FIGURE 18.—Outcrops of Hitt Canyon Formation. A, Thinly layered silty finely oolitic limestone and fossil lime wackestone 70 feet above base of formation at type section, Hitt Canyon (10c. 44). A thin- section photomicrograph of similar rock from a few feet above this outcrop is shown in figure 19A. Much of the lower part of the forma- tion has this general appearance. B, Burrowed limestone 247 feet above base of formation at Hitt Canyon (10c. 44). Pencil gives scale. Slightly silicified silty burrow fillings stand out in relief. This is the “rusty-weathering band” of Harbour (1972, fig. 4). Similarly burrowed limestones are found in the formation at many localities. C, Intraformational conglomerate from a few feet below top of for— mation of Lone Mountain (loc. 21). Pencil gives scale. In this outcrop dolomite fragments are in a limestone matrix. Rock of similar struc- ture is present in the formation at most localities. A thin-section photo- micrograph of generally similar rock from the El Paso Limestone is shown in figure 25C. 38 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS grained algal(?) lime grainstones, and fine-grained dolomites in the Hitt Canyon have porosities of less than 3 percent. Solution breccias formed beneath the uncon- formity at the top of the El Paso Group extend downward into the Hitt Canyon Formation at Molinas Canyon (10c. 16) in the Caballo Mountains, and perhaps elsewhere. As noted under the discussion of porosity of the McKelligon Limestone, such breccias could be very porous. THICKNESS The Hitt Canyon Formation ranges in thickness from an erosional wedge edge at the north and around a large Mesozoic highland in western New Mexico to 568 feet at Mescal Canyon (10c. 24) in the Big Hatchet Mountains; at Hitt Canyon, its type section (10c. 44), it is 531 feet thick. Where the formation is conformably overlain by the McKelligon Limestone it thins both northward and east- ROCK-STRATIGRAPHIC UNITS 39 FlGURE 19.——Photomicrographs of Hitt Canyon Formation. Bar in A is 1 mm; all views are same scale. A, Laminated silty glauconitic algal lithiclast lime grainstone from 80 feet above base of formation at type section, Hitt Canyon (10c. 44). This fine-grained but “high- energy" rock type is very common in the lower sandy part of the Hitt Canyon. Silt-sized glauconite grains are not visible in this view but are present in the thin section. Rounded grains of calcite (D) (probably echinodermal) are present but are not as abundant as algal(?) fragments (A) (the small circular alga is Nuia). Quartz (white) silt is nearly always present in small quantities in the lower part of the Hitt Canyon. Plain light. B, Silty skeletal lime wackestone from 150 feet above base of formation at Werney Hill (10c. 20). This rock type with or without quartz silt is the dominant rock type in the middle part of the Hitt Canyon at most localities. Plain light. C, Partly dolomitized coarse fossil-bearing lithiclast lime packstone from 300 feet above base of formation at Capitol Dome (10c. 39). Dolomite in large rhombs has nearly replaced the micrite in the lithiclasts. Dolomite has replaced much of the matrix on the left side. Probable fossil fragments (F) are a minor constituent, as are scattered grains of quartz silt (white). Calcarenite such as this is common in the upper part of the Hitt Canyon at most localities. Plain light. D, Sandy oolite lime packstone from about 380 feet above base of formation in Mud Springs Mountains (10c. 13). Micrite matrix is completely dolomitized. Quartz silt and sand in matrix shows as white. This rock type is interbedded with lithiclast lime packstone and is present near the top of the formation at nearly all but easternmost and westernmost localities. Plain light. ward, and its thinnest known uneroded section is at Agua Chiquita Canyon (loc. l) in the Sacramento Mountains, where it is 207 feet thick. FossrLs AND AGE Although the Hitt Canyon Formation is rarely con- spicuously fossiliferous, it has yielded fossils from different horizons at many localities, and its age is thus reasonably well known. The base of the formation is younger from west to east and probably from south to north, whereas the top of the formation is apparently older from west to east. The following comments on the age of the Hitt Canyon are largely based on published faunal information and regional stratigraphic correlations and partly on additional fossil data from collections made during this study. The lower 100 feet or so of the Hitt Canyon Formation has yielded brachiopods and trilobites from at least five localities (Cooks Range, Lone Mountain, Mescal Canyon, San Lorenzo, and Hitt Canyon, locs. 17, 21, 24, 38, and 44) that indicate correlation with the Early Ordovidian Symphysurz’na zone B of Utah (Hintze and others, 1969) (fig. 5). The fossils from locality 17 in the Cooks Range were reported by Jicha (1954), those from locality 44 in the Franklin Mountains were reported by Harbour (1972), and those from localities 21, 24, and 38 in Grant and Hidalgo Counties were collected during the present study (see “Fossil Lists”). To the east at Beach Mountain (10c. 48), from within 11 feetof the base of the formation, Cloud and Barnes (1946) reported fossils that indicate equivalence with the slightly younger zone C of Utah. The top of the Hitt Canyon Formation appears to be as old as zone E of Utah at Beach Mountain (10c. 48), where Cloud and Barnes (1946) collected brachiopods similar to Diaphelasma within 9 feet of the top, and definite Diaphelasma 133 feet below the top. Westward, the top of the Hitt Canyon is as young as zone C of Utah. Zone G trilobites were collected from the upper part of the Hitt Canyon at Mescal Canyon, San Lorenzo, and Capitol Dome (locs. 24, 38, and 39) during this study (see “Fossil Lists”), and at Mescal Canyon they were found 195 feet below the top. Fossils indicative of zone G were also collected from near the base of the overlying McKelligon Limestone at Mescal Canyon. Fossils indicative of zone D of Utah have been found in the Hitt Canyon Formation at many localities and are most commonly found in the middle part of the forma- tion. Fossils indicative of the Utah zones C and E seem to be missing west of Beach Mountain (10c. 48), and zone F seems to be missing at all localities. I believe that zone E is generally present as a barren zone, that the absence of zone F in the region may be due to a regional depositional hiatus, and that zone C may be represented at some localities by barren rocks and may be truly absent at others. CONTACTS The Hitt Canyon Formation nearly everywhere over- lies the Bliss Sandstone with a gradational but generally rather abrupt contact that is described in the text section on the Bliss. Locally, in the Franklin Mountains of western Texas, the Hitt Canyon unconformably overlies Precambrian rocks and is conglomeratic in the lower part, as has been described by Kottlowski, LeMone, and Foster (1969). In the southern part of the region the Hitt Canyon is conformably overlain by the McKelligon Limestone. In eastern localities the contact is marked by an abrupt change from relatively nonresistant sandy and silty car- bonate rocks at the top of the Hitt Canyon to relatively resistant and thick-bedded carbonate rocks at the base of the McKelligon. In western localities, where the upper part of the Hitt Canyon is less sandy and silty and the basal McKelligon is less thickly bedded, the contact is more subtle but is chosen at a horizon where bedding thickness rather abruptly increases. In the northern part of the region, where the McKelligon Limestone is absent, the Hitt Canyon is disconformably overlain by rocks of the Montoya Group or younger strata, and the contact is easily recognizable. MCKELLIGON LIMESTONE NAME AND TYPE SECTION The McKelligon Limestone was named the McKelligon Formation by Flower (1964)1 for McKelligon Canyon in the southern part of the Franklin Mountains in Texas (fig. 4 and Ice. 92, fig. 1). It was assigned a type section by ‘Sometimes called “McKelligon Canyon Formation" by Flower and other authors. 40 LeMone (1969), who modified its name to McKelligon Canyon Formation; the type section is approximately along the line of a section measured by Cloud and Barnes (1946) in the Franklin Mountains. Its base is about 2,000 feet southeast of Comanche Peak as shown on the El Paso 7V2-minute topographic sheet. As adopted for use in this report as a part of the El Paso Group, the McKelligon Limestone at its type section comprises Cloud and Barnes’ units B1 and B23 and apparently includes a few feet of beds at the base that LeMone (1969) did not include. GENERAL DESCRIPTION The McKelligon Limestone is made up almost entirely of limestone and (or) dolomite that is relatively free of quartz sand, thick bedded, and resistant as compared with the underlying Hitt Canyon and overlying Padre Forma- tions. It is 350—700 feet thick at most localities where its top is preserved but thins to less than 200 feet at one western locality. DISTRIBUTION AND STRATIGRAPHIC LIMITS The McKelligon Limestone is restricted to the eastern area as outlined in figure 6. It is missing, probably owing to erosion, at northern localities in that area but is present at all southern localities. The McKelligon everywhere overlies the Hitt Canyon Formation with conformable contact. At a few southern localities it is conformably overlain by the Padre Formation, but in most of the area it is disconformably overlain by rocks of the Montoya Group of Middle and Late Ordovician age. PETROLOGY The McKelligon Limestone is made up almost entirely of thickly bedded carbonate rocks that at most localities contain scattered irregular nodules of chert. The carbonate rocks are dominantly limestone in much of the central part of the McKelligon distribution area (locs. l3, 15, 17,39, 44, 45, and 56), domina’ntly dolomite to the east (locs. l, 11, 18, 27, and 48) and in one western locality (loc. 21), and variably mixed in most of the remainder of the region. The following descriptions of the lithologic character of the McKelligon are based primarily on the outcrop examina— tion of the formation at 14 localities and on the examination of 28 thin sections. Most of the McKelligon is characterized by two general carbonate rock types: (1) skeletal lime wackestones (fig. 20C) that may grade to skeletal lime packstones or to sparsely fossiliferous lime mudstones and algal bound— stones; and (2) lithiclast or skeletal-lithiclast lime pack- stones (fig. 20D) that grade to wackestones (fig. 208) 0r grainstones. The first general rock type is generally light gray and commonly occurs in broad moundlike bodies or thick beds and is most abundant in the lower part of the formation. The second broad type is generally medium gray and occurs commonly in sharp contact with rock of the first group (figs. 20A, 21A) and locally as channel fill in CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS rock of the first group (fig. 213); it is the dominant rock type in the upper part of the formation. As described by Klement and Toomey (1967), much of the lime mudstone may have been formed by the destruction of skeletal grains by the blue-green alga Gimanella. SEDIMENTARY STRUCTURES The most abundant and conspicuOus sedimentary structures in the McKelligon Limestone are channeled carbonate mounds such as those described in detail by Toomey (1970) in the lower part of the formation near the Scenic Drive section (10c. 45) in the Franklin Mountains. Such mounds were observed in the formation at all localities visited except Beach Mountain (10c. 48), but they are inconspicuous in completely dolomitized sections and are sparse at several localities. The mounds are made up of the first general rock type described above, and the channels cut in the mounds are made up of the second general rock type described above. Nearly all mounds contain digitate algal stromatolites and well-preserved specimens of the sponges Archaeoscyphia and Calathz'um. Toomey (1970) believed that the mounds grew in a shallow subtidal environment and that the channels were cut sub- aerially in an intertidal environment. Other sedimentary structures common in the McKelligon are burrowed limestones and small mounds built up by Pulchrilamz'na (Toomey and Ham, 1967) or stromatolites (fig. 21). Possible algal-mat dolomite and mud cracks were observed in the upper part of the forma- tion at Agua Chiquita Canyon (10c. 1). POROSITY Observations on the outcrop, thin-section examina- tions, and limited laboratory data all indicate that porosities in the McKelligon Limestone are low through- out the report area but seem to be highest in the eastern part. Of nine measurements of the total porosity by the kerosene-displacement method, the two highest were 2.5 percent in dolomites from Agua Chiquita Canyon and Beach Mountain (locs. l and 48) in the east. The effective porosities of only two rocks from Little San Nicholas Canyon and San Andres Canyon (locs. l8 and 27) were measured by the mercury porosimeter and found to be 1.4 and 0.7 percent, respectively. The only porosity observed in rocks of the McKelligon on the outcrop was vuggy porosity in dolomites from localities 1 and 48. The chief type of porosity observed in thin sections was cement- filled or cement-reduced microfracture porosity. As noted by Lucia (1971), solution-collapse features are locally present beneath the unconformity at the top of the El Paso Group and in places extend well into the McKelligon Formation. The porosity characteristics of the solution breccias were not appraised during this study, but Gibson (1965) noted “a zone of porosity and permeability up to several hundred feet in thickness”"immediately below the erosion surface” on top of Lower Ordovician rocks in the subsurface of western Texas. ROCK-STRATIGRAPHIC UNITS FIGURE 20.—Photomicrographs of McKelligon Limestone. Bar in A is 1 mm; all views are same scale. A, Fossil-bearing argillaceous lime mudstone sharply overlain by fossil»bearing lithiclast lime wackestone from 220 feet above base of formation at Capitol Dome (Inc. 39). Lithidasts (dark) in upper part appear to have been derived from underlying argillaceous lime mudstone. Both of these rock types are abundant in the McKelligon Limestone. Plain light. B, Completely (lolomitiled (‘oarse lithiclast lime wackestone from ll 1 feet above base of formation at San Andres Canyon (loc. 27). Except 4l for grain size this rock probably once looked much like the upper part of A. Relict structures like this show up in many dolomites in otherwise relatively unaltered rocks. C, Skeletal lime wackestone from 8 feet above base of formation in Cooks Range (10c. 17). This is a common rock type of the McKelligon. Plain light. D, Skeletal- oolitiC-lithiclast lime packstone from 320 feet above base of forma- tion at Hitt Canyon (loc. 44). Much of the upper McKelligon from this area is made up of this rock type. Plain light. 42 THICKNESS The McKelligon Limestone in the southern part of the region, where it is conformably overlain by the Padre Formation and is thus completely preserved, ranges in thickness from 166 to 690 feet. It is thickest at its type section at Scenic Drive (loc. 45) and thinnest about 110 miles to the west at Mescal Canyon (10c. 24). It thins depositionally northward as well as westward and is only 329 feet thick at San Andres Canyon (10c. 27), which is about 65 miles north of the type section. Farther to the north and west the McKelligon thins to an erosional wedge edge. FossrLs AND AGE The McKelligon Limestone is not rich in guide fossils but has yielded some fossils, notably cephalopod siphuncles, from most localities. Flower (1969) indicated that the McKelligon is an age equivalent of the Jefferson City Formation of Missouri, and, indeed, the McKelligon has yielded Jefferson City fauna] equivalents from the Scenic Drive section in the Franklin Mountains (10c. 45) on the east (Cloud and Barnes, 1946) to Mescal Canyon in the Big Hatchet Mountains (10c. 24) on the west (Zeller, 1965). Because the underlying Hitt Canyon Formation contains beds as young as zone G of Utah (Hintze and others, 1969) in its western exposures and because the over- lying Padre Formation is as old as zone G or H at its base, the entire McKelligon throughout the region is probably of zone C age (fig. 5). However, because the top of the underlying Hitt Canyon appears to be younger westward, the zone G equivalents at the base of the McKelligon are probably older in the east than in the west. UPPER CONTRACT The McKelligon is overlain conformably by the Padre Formation in the southern part of the region. The contact is marked at most localities by the abrupt upward change to notably sandy or silty dolomites or dolomitic sand- stones at the base of the Padre. At Capitol Dome in the Florida Mountains (10c. 39), where the Padre is not con- spicuously silty or sandy at the base, the contact is marked by an abrupt change from relatively thick bedded lime- stone in the McKelligon to relatively thin bedded poorly exposed possibly argillaceous limestone at the base of the Padre. PADRE FORMATION NAME AND TYPE SECTION The Padre Formation, the highest formation in the El Paso Group, is here named for Padre Canyon in the Hueco Mountains, Hudspeth County, Tex., shown on the Borrego 15-minute topographic quadrangle. Its type section (Pasotex, fig. 4 and loc. 56, fig. 1) at lat 30°41 ’20” N. and long 105°54’20” W. is on a ridge crest above a small unnamed canyon 2.8 miles due east of Padre Canyon and is described under “Selected Measured Sections” in this CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS FIGURE 2l.—Outcrops of McKelligon Limestone at Hitt Canyon (10c. 44) in the northern Franklin Mountains. A, Interlayered lithiclast limestone, skeletal lime packstone, and some burrowed limestone 420 feet above base of formation (10c. 44). The diverse layers indicate alternating subenvironments of deposition. Figure 20D is a thin- section photomicrograph of rock like that in the darker bed beneath the pencil. B, Scour channel of limestone conglomerate in thinly layered skeletal lime wackestone high in formation. Such channels are commonly associated with carbonate mounds in the McKelligon. Thin layering beneath pencil may be Pulchrilamina or may be of stromatolitic origin. report. The Padre Formation in the Scenic Drive section of the Franklin Mountains (10c. 45) coincides with Cloud and Barnes’ (1946) units B2b and C of the El Paso measured at that locality, and that section may be regarded as the principal reference section. As defined, the Padre Formation includes the Scenic ROCK-STRATIGRAPHIC UNITS 43 Drive and Florida Mountains Formations of LeMone (1969). The latter is so thin and so local that it is un- mappable at standard mapping scales. The name Scenic Drive with minor modification in definition might have been used for the Padre except that the American strati— graphic code discourages the naming of units after ephemeral artificial features. The Padre also includes the Cindy and Ranger Peak Formations of Lucia (1969). Although distinguishing the two units is possible at some localities, it is not at others. Lucia’s units would make useful local members except that the name Cindy was apparently not taken from a recognized geographic feature. GENERAL DESCRIPTION The Padre Formation is made up of limestones and dolomites that are thinner bedded and less resistant than the underlying McKelligon Limestone and that are notably quartzose at the base at most localities; litho- logically it is more similar to the Hitt Canyon Formation than to the intervening McKelligon Limestone. The Padre ranges in thickness from a wedge edge to nearly 400 feet. DISTRIBUTION AND STRATIGRAPHIC LIMITS The Padre Formation is limited by definition to the eastern area as outlined in figure 6 and by erosion to the southern edge of that area. It everywhere overlies the McKelligon Limestone with sharp but conformable contact. In most of the region it is overlain disconformably by rocks of the Montoya Group of Middle and Late Ordovician age; locally, near the extreme southeast corner of the report region it is discon- formably overlain by beds of Middle(?) Ordovician age (King, 1965) that have been equated with the Simpson Group of Oklahoma by Jones (1953). PETROLOGY Most of the lithologies in the Padre Formation are very similar to lithologies found in the lower formations of the El Paso Group. The basal part of the formation is generally characterized by sandy or silty dolomite or dolomitic sandstone that grades upward into carbonate rocks that are much like those found in the middle part of the Hitt Canyon Formation. Some of these higher carbonate beds at most localities are extremely cherty, and at Capitol Dome in the Florida Mountains (10c. 39) chert makes up about one-third of a 73-foot-thick interval. The following comments on the lithologic character of the Padre Formation are based on the outcrop examination of the formation at eight localities and on the examination of 12 thin sections. Except at Capitol Dome (loc. 39), the basal 15—80 feet of the Padre Formation consists of quartzose saccharoidal dolomite (figs. 22A and 23A) or dolomitized lithiclast lime grainstone (fig. 228). At most localities the quartzose dolomite grades upward into silty and generally thinly laminated dolomite (fig. 238). Above these are a variety of fossil-bearing lime mudstones (fig. 22C) to skeletal lime wackestones that are commonly interbedded with coarse lithiclast lime packstones or grainstones (fig. 22D) much like some of those in the McKelligon Limestone. Oolite lime grainstones (fig. 22E) like those in the upper part of the Hitt Canyon Formation are also present. In the Florida and Franklin Mountains (locs. 39, 44, and 45) these upper carbonates are mostly limestones, whereas farther east at Beach Mountain and the Pasotex section (locs. 48 and 56) they are largely dolomitized. SEDIMENTARY STRUCTURES The basal sandy dolomites of the Padre Formation commonly display conspicuous small-scale cross- laminations that may represent beach deposition (fig. 23A). Most of the very thin laminations in the dolomites above the basal sandy zone are interpreted to represent tidal-flat algal-mat structure. This interpretation is strengthened by the association of thin interbeds of dolomite chip con- glomerate, which are especially notable in the Pasotex sec- tion in the Hueco Mountains (10c. 56). The intraclasts in these conglomerates are interpreted to be desiccation chips of algal-mat dolomite. Burrowed limestones are sparse in the upper part of the formation. POROSITY The highest porosities in the Padre Formation seem to be in some of the very sandy basal beds. Interparticle and vuggy porosity in basal sandy dolomite from the Pasotex section in the Hueco Mountains (10c. 56) showed a total porosity of 6.7 percent when checked by the kerosene-dis- placement method and an effective porosity of 3.5 percent as checked with the mercury porosimeter. Other samples of basal sandy dolomite showed effective porosities ranging from 0.7 to 2.6 percent. The carbonate beds higher in the formation seem mostly to be very low in porosity. One thin section of rock from Mescal Canyon in the Big Hatchet Mountains (10c. 24) revealed some microfracture porosity, and other thin sections showed cement-filled fracture, moldic, and channel porosity. Only two limestone samples were checked for total porosity, and each had less than 1 percent. Like the McKelligon Limestone, the Padre Formation locally contains solution breccias beneath the uncon- formity at its top which could be zones of high porosity and permeability. THICKNESS The total original thickness of the Padre Formation is not preserved in the region inasmuch as the top contact is everywhere disconformable. The thickest section seen is its type section in the Hueco Mountains (Pasotex, 10c. 56), where the formation is 382 feet thick. As shown in figure FIGURE 22.—Photomicrographs of Padre Formation. Bar in A is 1 mm; all views are same scale. A, Very quartzose dolomite from near base of formation at Hitt Canyon (10c. 44). This rock is very similar to some basal rock of the El Paso Group and was probably deposited in a similar environment. Plain light. B, Slightly silty lithiclast lime grainstone from low in formation at Beach Mountain (10(:. 48). This is another “high-energy” rock from low in the Padre, but it is a type found only at this eastern locality. Plain light. C, Lime mudstone from 127 feet above base of formation at Capitol Dome (10c. 39). Rock like this is not dominant but is common well above the base of the Padre. The white flecks are probably minute skeletal CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS grains. Plain light. D, Skeletal lime wackestone overlain by coarse lithiclast lime grainstone from about 125 feet above base of forma- tion at Hitt Canyon (loc. 44). The large rounded clasts in the upper part were derived from rock very similar to that below. Similar rocks are found in the McKelligon Limestone. (Compare fig. 20A.) Plain light. E, Skeletal-oolite lime grainstone from about 200 feet above base of formation at Hitt Canyon (loc. 44). This laminated rock from high in the Padre Formation is similar to rock low in the Hitt Canyon Formation (fig. 19A) and probably was deposited in a similar environment. Fossil fragments in the picture are con— spicuous; very small oolites are not. Plain light. ROCK-STRATIGRAPHIC UNITS 45 48, the formation is more than 200 feet thick at several extreme southern localities but thins to a wedge edge not far to the north and west. FOSSILS AND Aer-2 The Padre Formation is not richly fossiliferous, but it does contain abundant trilobites and brachiopods in some beds at a few localities, and it has sparse cephalopod siphuncles and gastropods. The Scenic Drive and Florida Formations Of Flower (1969), which are included here in the Padre, are the age equivalents of Lower Ordovician zones H through J and possible zone K of Utah (Hintze and others, 1969) (fig. 5). Fossils collected during the present study tend to substantiate that age assignment. Species of the brachiopods Hesperonomia and Diparelasma collected from the lower part Of the formation at Capitol Dome in the Florida Mountains and at Mescal Canyon in the Big Hatchet Mountains (locs. 39 and 24) suggest correlation with Utah zones G or H according to Reuben J. Ross, Jr., of the US. Geological Survey. Trilobite and brachiopods collected from the top of the formation at Capitol Dome (10c. 39) are suggestive of zone J to Ross, as are collections from near the top of the formation at Hitt Canyon in the Franklin Mountains (10c. 44) that were reported by Harbour (1972). UPPER CONTACT The upper contact of the Padre is everywhere discon- formable. Throughout most of the region the Padre is overlain with sharp contact by rocks of the Montoya Group of late Middle and Late Ordovician age; but in a small area in the Baylor Mountains (around loc. 47) at the extreme east edge of the report area, pre-Montoya rocks of Middle Ordovician age disconformably overlie the Padre Formation (King, 1965). CENTRAL PART OF STUDY REGION CORONADO SAN DSTONE NAME AND TYPE LOCALITY The Coronado Quartzite was named by Lindgren (1905) for exposures in the Clifton-Morenci area (near 10c. 84, fig. 1). The name was used only in the area of the type locality until recently when I (Hayes, 1972) extended it to include rocks formerly improperly assigned to the Bolsa Quartz- ite. Because most of the formation in most of the area of its extent is not truly quartzite, the name Coronado Sand- stone is preferable for regional use. GENERAL DESCRIPTION The Coronado Sandstone consists dominantly of moderately dark weathering sandstone that is variably cemented by quartz and carbonate. In most of its area of occurrence it ranges in thickness from about 200 feet to about 600 feet. DISTRIBUTION AND STRATIGRAPHIC LIMITS The Coronado Sandstone occurs only in the central area as outlined in figure 14. Until recently (Hayes, 1972), the name was used only in the vicinity of the type locality. The Coronado unconformably overlies Precambrian rocks throughout its area of occurrence, and it is con- formably overlain by the El Paso Limestone except in some northern areas where the Coronado is discon- formably overlain by Devonian or Mississippian rocks. The Coronado is a lateral facies equivalent of the lower three members of the Abrigo Formation and, locally, of the upper part of the Bolsa Quartzite of the western area; the upper part of the Coronado is a lateral equivalent of the Bliss Sandstone of the eastern area. (See fig. 5 and pl. 1.) PETROLOGY Virtually all the Coronado is made up of sandstone in the northern part of its distribution area, and at least 70 percent of the formation is sandstone in southern locali- FIGURE 23.—Outcr0ps of Padre Formation at Hitt Canyon (10c. 44) in the northern Franklin Mountains. A, Cross~laminated sandy dolo- mite about 50 feet above base of formation. Penny gives scale. B, Slightly silty laminated dolomite about 100 feet above base of for- mation. 46 ties. Most southern sections contain some siltstone and (or) shale, and at Dos Cabezas (10c. 60), near the westward transition of the Coronado into the Abrigo Formation, there is some sandy dolomite in the Coronado. Outcrop colors of the Coronado are mostly reddish brown to pale brown but range from pale red to olive brown. On fresh fracture much of the rock is pinkish gray but ranges to dusky red and dark greenish gray. Sandstones in the Coronado range from arkose to ortho- quartzite; in Dapples’ (1972) classification, most are quartz- or dolomite—cemented arenites but a few are sub- wackes. As with the Bolsa Quartzite and Bliss Sandstone, grain size decreases upward. The following descriptions of sandstones from the Coronado are based on outcrop examination of the formation at seven localities and on the examination of 37 thin sections, of which 29 were point counted. Quartz is the most abundant cementing material in sandstones from the Coronado (fig. 24A, B), but dolomite cement is not uncommon in the southern part of the Coronado distribution area, especially in the middle and upper parts of the formation. Hematite cement occurs in a few beds. Feldspar content of Coronado sandstones decreases upward. True arkoses are present at the base of the section at many localities and make up much of the lower two- thirds of the formation on Nantac Rim (loc. 150). Feldspar rarely makes up more than 5 percent of the rock from the upper one-third of the Coronado and averages less than 2 percent. Rock fragments are present in small amounts in many samples (fig. 248) but do not constitute even 2 percent of any sandstone examined. Glauconite grains are present in many samples and abundant in a few, particularly from southern outcrops (fig. 24C). On the whole, glauconite is much more abundant in the Coronado than in the Bolsa Quartzite but not as abundant as in the Bliss Sandstone. Grains of opaque minerals, most commonly hematite, are present in most samples and abundant in some (fig. 24C). As with glauconite, hematite is more abundant in the Coronado than in the Bolsa and less abundant than in the Bliss. Sandstones containing clay or sericite in the matrix seem to be more common in the Coronado than in either the Bolsa Quartzite or the Bliss Sandstone and these minerals occur in some beds throughout the distribution area of the Coronado. Several samples examined can be classified as subwackes rather than as arenites. It is suspected, however, that much of the fine matrix in the subwackes results from alteration of feldspars and ferromagnesian minerals. Siltstones and silty shales from southern localities of the Coronado are mostly dark greenish gray to reddish brown. Most seem to be sericitic or chloritic; some are limonitic or glauconitic; a few are calcareous. The few beds of dolomite that occur in some Coronado sections are all silty to sandy and irregularly micrograined CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS to coarse grained. They are medium gray to grayish red on fresh fracture and weather to yellowish brown. SEDIMENTARY STRUCTURES Small- to medium-scale planar cross-laminations are by far the most conspicuous sedimentary structure seen in the Coronado but certainly do not occur in all beds. Seeland (1968) measured numerous attitudes of cross-laminations in the Coronado at Apache Pass and Morenci (locs. 59 and 84), and I noted many on Nantac Rim (ICC. 150). The majority of those at Apache Pass are inclined to the south- west, whereas those at Morenci and Nantac Rim dip mostly a little to the north of west. Tracks and trails on bedding planes and Scolithus tubes are present in a few beds and seem to be more common in northern exposures than in southern ones. POROSITY Most samples from the Coronado Sandstone are very low in porosity. Of 26 samples point counted for porosity, 18 showed less than 1 percent, 4 showed between 1 percent and 2 percent, 2 showed about 3 percent each, and 2 showed 11 percent and 15 percent (fig. 248). Only two samples were checked by the mercury porosimeter; these showed 1.4 percent and 3.5 percent porosity. The two fairly porous samples were from near the top of the formation at Morenci and Nantac Rim (locs. 84 and 150), near the northern known limits of occurrence of the Coronado. THICKNESS The Coronado Sandstone ranges in thickness from 370 feet to 610 feet in the sections where it is conformably over- lain by the El Paso Limestone; there is no known place where the formation is absent because of nondeposition over local relief on the Precambrian, as happens with both the Bolsa Quartzite and the Bliss Sandstone. The Coronado is less than 200 feet thick at Imperial Mountain and Coolidge Dam (locs. 76 and 151), where its top was eroded before Late Devonian or Early Mississippian time, and presumably the formation comes to an erosional wedge edge not far to the north (fig. 14). Fossns AND AGE N0 fossils closely diagnostic of age have been found in the Coronado Sandstone. However, because of the con- formable relations of the Coronado with overlying rocks of known Late Cambrian (Franconian) age, at least the upper part of the Coronado is probably no older than early Late Cambrian (Dresbachian) age. This conformity and the distinct lithologic resemblances of the Coronado with sandy facies of the Abrigo Formation indicate that the thick sections of Coronado range in age from Middle Cambrian through early Late Cambrian and that the thinner sections may be entirely of early Late Cambrian age (pl. 1, fig. 5). CONTACTS The Coronado Sandstone unconformably overlies Pre- ROCK-STRATIGRAPHIC UNITS 47 FIGURE 24.—Photomicrographs of Coronado Sandstone. Bar in A is 1 mm; all views are same scale. A, Poorly sorted coarse arkosic sand- stone from basal part of formation near Portal (10c. 57). Feldspar (F) is fairly abundant in the basal Coronado, as it is in the basal Bolsa Quartzite. This rock, which is probably an age equivalent of the lower member of the Abrigo of western localities, is less per- fectly cemented with quartz (qc) than is most Bolsa. Dusty sericite (S) and hematite (black) is in matrix. Original boundaries of quartz grains show clearly. (Compare fig. 7B) Crossed nicols. B, Well-sorted sandstone from near top of formation near Morenci (10c. 84). Most grains are quartz but fragments of schist (finely spotted grains) are common. Cement is quartz in optical continuity with grains. Much of the black in the section is open pore space, but some is quartz at extinction. Crossed nicols. C, Laminated glauconitic hematitic fine-grained sandstone from 412 feet above base of formation near Portal (10c. 57). This rock is very glauconitic (medium gray) and hematitic (black) and is probably equivalent to the upper sandy member of the Abrigo Formation. (Compare fig. 11.) Plain light. cambrian rocks at all localities. At most localities the underlying rocks are crystalline rocks, but at and near Imperial Mountain and Coolidge Dam (locs. 76 and 151) the Coronado overlies Precambrian sandstone or quartzite with nearly parallel contact of very low relief. In these two areas it is possible to mislocate the basal contact. Simons (1964), who assigned the beds here called Coronado to the Bolsa Quartzite near Imperial Mountain (10c. 76), conceded that he may have included Precambrian beds in his Bolsa, and I believe that he did. Iam not certain thatI properly placed the contact at Coolidge Darn (10c. 151); Krieger (1961) has discussed the nature of the Pre- cambrian-Cambrian contact in this area. The contact of the Coronado Sandstone with the over- lying El Paso Limestone is conformable but in most places can be loeated with little equivocation as sandstone of the Coronado gives way rather abruptly to sandy dolomite at the base of the El Paso. EL PASO LIMESTONE NAME AND TYPE LOCALITY The name and type locality of the El Paso Limestone were discussed earlier in this report in the section on the El Paso Group in the eastern part of the study area. 48 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS GENERAL DESCRIPTION In the central area the El Paso Limestone is divided into lower and upper members. The lower member consists mostly of sandy or silty dolomite that has interbeds of dolomitic sandstone in the upper part, and the upper member is made up of relatively pure carbonate beds that locally contain some rather cherty intervals. DISTRIBUTION AND STRATIGRAPHIC LIMITS The El Paso Limestone crops out in several extended outcrop bands in the southern part of the central area (fig. 6) and occurs sporadically in the northern part east of long 110° W. Wherever the El Paso occurs in the central area, it conformably overlies the Coronado Sandstone. It is dis- conformably overlain at most localities by Devonian rocks and at Morenci (fig. 4 and 10c. 84, fig. 1) by upper Middle Ordovician rock assigned to the Second Value Dolomite. PETROLOGY The lower member of the El Paso Limestone in the central area is made up almost entirely of dolomite, quartzose dolomite, and dolomitic sandstone. The upper member is largely dolomite at most localities but contains considerable limestone at Preacher Mountain (10c. 23) and at Apache Pass (10c. 59). The following descriptions of these rocks are based on outcrop examination of the formation at eight localities in the central area and on the examination of 61 thin sections. Nearly all the dolomite in the lower member appears to be dolarenite (fig. 25A). There is minor intraformational dolomite conglomerate at most southern localities and, in some instances, dolomitized calcerenite. Most of the dolomite in the member is at least slightly sandy or silty, and some of it is very sandy (fig. 25A, B). Most is faintly to distinctly grain-size laminated. Most of the sandy dolomites from northern localities (locs. 84, 150, and 162) are glauconitic; in these, the glauconite grains are rounded particles of the same general grain size as the accompanying quartz sand. Sandstone in the lower member is generally well sorted fine- to coarse-grained orthoquartzite with dolomite cement. Some is glauconitic. Carbonates in the upper member are more varied than those in the lower member and include lime mudstones to grainstones and dolomite boundstones. Lithiclast lime packstones and grainstones (fig. 25C), some fossil bearing, may be most abundant and are similar to the dolarenites of the lower member except that they are rarely sandy. Lime mudstone, fossil-bearing lime mudstone, and skeletal lime wackestone (fig. 25C) are also found in the member. The lime mudstones are commonly slightly silty and argillaceous (fig. 25D). Algal-mat dolomites (dolomite boundstones) are present in most sections. The lime- stones have been largely dolomitized in most areas, but considerable undolomitized limestone remains at Preacher Mountain and Apache Pass (locs. 23 and 59). SEDIMENTARY STRUCTURES Many of the dolarenites and dolomitic sandstone beds in the lower member display conspicuous small— and medium-scale planar cross-laminations. Some cross- laminated dolomite occurs in the upper member near Morenci (10c. 84). Intraformational chip conglomerates are present in the lower member at most localities and in the upper member at a few localities (fig. 26A). Mud cracks were observed in the upper member near Morenci and to the northwest (locs. 84, 150, and 162). Algal-mat dolomite is present in the upper member at several localities, and at Morenci (10c. 84) it displays bird’s- eye structures (fig. 26B). Burrowed limestone or dolomite is present in the upper member at nearly all localities and in the lower member near Portal (10c. 57). Tracks and trails were observed on bedding planes in the lower member at Nantac Rim and Barlow Pass (locs. 150 and 162). _ POROSITY Some dolomitic sandstone and sandy dolomite from the lower member of the El Paso Limestone has moderate porosity, but most outcrop samples of the formation in the central area are low in porosity. Eight samples from the lower member were checked for effective porosity with the mercury porosimeter, and the porosities determined ranged from 0.5 to 6.4 percent and averaged 3. 1 percent; the highest porosities were from Morenci and Nantac Rim (locs. 84 and 150), and the lowest were from Preacher Mountain and Portal (locs. 23 and 57). The rocks with the greatest porosities had dominantly between-particle porosity but some vuggy and fracture porosity. Six samples of carbonate rocks from the upper member were checked by the mercury porosimeter and showed effective porosities from 0.7 to 3.8 percent and averaged 1.9 percent. A wide variety of porosity types, largely cement reduced or cement filled, was noted in these rocks. Between-particle, fracture, and vuggy porosities are most common, but moldic, fenestral, and burrow porosities were also noted. THICKNESS In the central area the thickness of the El Paso Lime- stone ranges from a measured maximum of 887 feet in the Portal section (loc. 57) to an erosional wedge edge on the west and presumably on the north. Even the maximum thickness does not represent an original thickness inasmuch as the top of the El Paso is disconformable throughout the area. The thickness of the lower member, which is conformable at both base and top, ranges from 86 feet at Preacher Mountain (10c. 23) to 181 feet at Portal (10c. 57). ROCK-STRATIGRAPHIC UNITS 49 FIGURE 25.-—-Photomicrographs of El Paso Limestone. Bar in A is 1 mm; all views are same scale. A, Well-sorted quartzose dolarenite from lower part of formation near Dos Cabezas (10c. 60). Note simi- larity to figure 138, which is of a very nearly coeval rock. Plain light. B, Metamorphosed sandy dolomite from lower part of for- mation at Apache Pass (10c. 59). Before metamorphic recrystalliza- tion of the dolomite this rock probably looked much like that in A or in figure 138. (Compare fig. 33D.) Plain light. C, Large clast of silty skeletal lime wackestone (right) in recrystallized matrix from 220 feet above base of formation near Dos Cabezas (loc. 60). Clast at lower left is rounded calcite crystal of echinodermal origin. A few quartz silt grains show as white. Intraformational limestone conglomerates (very coarse lithiclast lime wackestones to grain stones) like this are found in the lower part of the El Paso and in the Hitt Canyon Formation at most localities, especially toward the south. (See fig. 18C.) Plain light. D, Laminated silty and argilla- ceous lime mudstone from 454 feet above base of formation at Apache Pass (loc. 59). Rock such as this is common in the upper member of the El Paso Limestone in Arizona and is present but uncommon in the Hitt Canyon Formation. Plain light. 50 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS FIGURE 26.—Outcrops of El Paso Limestone. A, Intraformational con- glomerate 80 feet above base of formation at Preacher Mountain (10c. 23). Flat dolomite clasts are in a sandy dolomite matrix. This part of the El Paso at this locality is probably of latest Cambrian FOSSILS AND AGE The El Paso Limestone of the central area includes beds of Late Cambrian and Early Ordovician age. The lower member has yielded only Cambrian fossils, and the upper member has yielded only Ordovician fossils; presumably the systemic boundary lies near the contact between the lower and upper members. Lindgren (1905) collected fossils from 20 feet above the base of rocks now assigned to the El Paso near Morenci (10c. 84) that Charles D. Walcott thought were probably uppermost Cambrian. Sabins (1957) reported Billingsella sp., which is indicative of a Franconian (Late Cambrian) age, from the basal beds of the El Paso on Blue Mountain (near 10c. 58). We found brachiopods identified by M. E. Taylor of the US. Geological Survey (written commun., Aug. 16, 1971) as Billingsella cf. B. coloradoensis from 52 feet above the base of the lower member on Nantac Rim (loc. 150). From 98 feet above the base (and 83 feet below the top) of the lower member near Portal (10c. 57) we collected the brachiopod Plectotrophia sp., identified by Reuben J. Ross, Jr., of the US. Geological Survey (written commun., Oct. 9, 1970), and possible Matthevia, as determined by Ellis L. Yochelson of the US. Geological Survey (written commun., Dec. 6, 1971). These forms indicate a Late Cambrian age. Unfortunately, age- diagnostic fossils have not been found in the upper part of the lower member. The upper part of the El Paso of the central area has yielded cephalopods and sparse gastropods of Early Ordovician age from Preacher Mountain (10c. 23) (Gillerman, 1958), Apache Pass (10c. 59) (Sabins, 1957), age. Similar intraformational conglomerates can be found in various parts of the Abrigo Formation at various localities and in the Bliss Sandstone. (See fig. 163) B, Bird’s-eye structures in dolomite about 200 feet above base of formation near Morenci (10c. 84). Dos Cabezas (10c. 60) (Gilbert, 1875; Darton, 1925), and Morenci (10c. 84) (Lindgren, 1905). Gastropods collected by Gillerman (1958) from near Preacher Mountain (10c. 23) were believed by R. H. Flower (cited in Gillerman, 1958) to represent his first endoceroid zone, which, according to Flower (1969), is equivalent to Early Ordovican zone D of Utah (Hintze and others, 1969). We collected generically unidentified endoceroid siphuncles from 102 to 117 feet above the base of the upper member of the El Paso at Preacher Mountain (10c. 23), and I presume that these represent Flower’s first endoceroid zone. According to Flower (cited in Sabins, 1957) a nautiloid siphuncle from about 150 feet above the base of the El Paso (very low in the upper member) at Apache Pass (10c. 59) probably represents the lower of the two cephalopod zones of the Gorman Formation of Texas and thus also would probably represent zone D of Utah. The cephalopods from Dos Cabezas (loc. 60) were thought by Flower (cited in Sabins, 1957) to be of middle Canadian age. The fossils from Morenci (10c. 84) were regarded by Walcott (quoted in Lindgren, 1905) to be of Early Ordovician age. On the basis of the fossil evidence from the central area and lithologic correlation with fossil-bearing beds in the eastern and western areas, the lower member of the El Paso Limestone in the central area is assumed to be of Franconian and Trempealeauan (late Late Cambrian) age. It is, thus, equivalent to the Copper Queen Member of the Abrigo Formation to the west and to the lower part of the Bliss Sandstone to the east (pl. 1). The upper member of the El Paso in the central area is assumed to be of Early Ordovician age and may contain beds at the top of the thickest sections as young as Early Ordovician zone G of ROCK-STRATIGRAPHIC UNITS 51 Utah. It is, thus, equivalent to the upper part of the Bliss Sandstone, all or most of the Hitt Canyon Formation, and possibly, at its thickest sections, the basal part of the McKelligon Limestone of the eastern area (fig. 5). UPPER CONTACT The upper contact of the El Paso is disconformable throughout the central area. At Morenci (10c. 84) the El Paso is overlain by a thin remnant of the Second Value Dolomite, and at that locality both the El Paso and the Second Value were included in the Longfellow Lime- stone until the Longfellow was abandoned by Hayes (1972). Elsewhere the El Paso is sharply overlain by shaly strata of Middle or Late Devonian age. MIDDLE ORDOVICIAN UNCONFORMITY Throughout the report region the rocks of the Sauk sequence of Sloss, Krumbein, and Dapples (1949) and Sloss (1963) are separated from the overlying Middle Ordovician or younger rocks by a disconformity. In general, the amount of geologic time represented by the disconformity increases from east to west and from south to north. This is due both to a general westward and north- ward increase in age of the top of the Sauk sequence (fig. 27; pl. 1) and to a general but irregular westward and northward slight decrease in age of the rocks immediately overlying the Sauk sequence (fig. 28). In Culberson County, Tex., at and near locality 47 (fig. 1) in the southeast, pre-Montoya Group rocks that have been correlated with the Middle Ordovician Simpson Group of Oklahoma overlie the Padre Formation of late late Canadian age, and the disconformity represents only latest Early Ordovician and early Middle Ordovician time. In most of westernmost Texas and southern New Mexico the Montoya Group overlies the El Paso Group; the El Paso becomes increasingly older at the top northward and westward, so at places such as Morenci, Winston, and Eaton Ranch (locs. 84, 41, and 35) the disconformity represents much of Early Ordovician and most of Middle Ordovician time. Still farther west and north, beyond the erosional edge of the Montoya Group, the pre-Montoya and one or more post-Montoya disconformities have coalesced, and so in much of southern Arizona Devonian rocks overlie Sauk sequence rocks that become older at the top westward. Thus, in western Pima and Final Counties (locs. 80—82), the disconformity at the top of the Sauk sequence represents all of Late Cambrian, Ordovician, Silurian, and Early and Middle Devonian time. In areas where later erosion has removed the rocks of the Sauk sequence the effects of the Middle Ordovician uncon- formity are, of course, lost. Thus, in the Diablo region of western Texas, Permian rocks overlie the Precambrian; on the Burro uplift of western New Mexico and in a large area in southern Arizona, Cretaceous rocks overlie the Pre- cambrian; and along the entire north edge of the region, Devonian to Pennsylvanian rocks overlie the Pre- cambrian (figs. 27, 28). 33“ Older\ Precam‘brian fl . \ 2 r 21' Early Late-Cambrian I ‘a ' Morenci’ F~,/‘ 32" l ! ARIZONA 1/ NEW MEXICO _E;IW‘ Late Cambrian l . Early and Middle Canadian Older Precambrian ‘ ~. a; ___J-—— _____l \ . | Early late Canadian UNITED STATES MEXICO 0 50 100 MILES O 50 100 KILOMETRES i l 1 1 FIGURE 27.—Generalized map showing distribution of rocks of the Sauk sequence before the end of Cretaceous time and the age of rocks at the top of the sequence. Ages of pre-Sauk rocks are shown where rocks of the Sauk sequence are missing. 110” CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS 108° 107° 109° [—7 Devonian ——__J ‘ l l Eaton Ranch 1‘"— Pennsylvanian ___._] _____ Winston 33° Vekol Mts _ \Slate Mts 1. R __‘t__a ________.__-L\___ _1‘I_ I 15’ C (I) To .2 Waterman _____ _ Ordovician 3, “Mrs l m I ' Montoya Group 0. 1 1 :5 1 L 9 32°? ‘ . 8 . ,____|__- ‘2 [ Devonian 2E '— 1 o 1 o lid “ \ 8. ,_______{ E 2 \ E \\ I , a: 3 “x (7) \\\ _J I < {=1 \ \\L 1 Z \\ E ‘x— _____ .i_-___11‘£I_TED STATES “3’ MEXICO % 3 1° 0 so 100 MILES 41 50 100 KILOMETRES ~\\ I \1 I 1 1 FIGURE 28.—Genera1ized map showing rocks MIDDLE AND UPPER ORDOVICIAN ROCK UNITS MONTOYA GROUP The Montoya Limestone was named by Richardson (1908) for exposures near the Scenic Drive (fig. 29 and 10c. 45, fig. 1) in the Franklin Mountains, Tex. Richardson (1914) extended the name eastward to the Baylor Mountains and Beach Mountain (locs. 47, 52, and 50), and Darton (1917a) extended the name throughout much of southern New Mexico. Entwistle (1944) subdivided the Montoya at Boston Hill in the Silver City area, New Mexico (10c. 91), into three members that were previously described as informal members by Darton (1917a); Entwistle’s Montoya consisted in ascending order of the Second Value, Par Value, and Raven Members. Kelley and Silver (1952) recognized the same subdivisions in the Caballo and Mud Springs Mountains (locs. 13—16) of New Mexico and elevated the Montoya to group rank. They believed that Entwistle’s type sections were inadequate because of faulting and mineralization2 and renamed the Par Value and Raven Members the Aleman and Cutter Formations, respectively. The Second Value Member was not used, and rocks formerly assigned to it were divided into the Cable Canyon Sandstone and overlying Upham Dolomite. Pray (1953), working in the Sacramento Moun- tains, N. Mex. (around locs. 1—3), used the term Montoya Formation for the lower part of the Montoya of other workers and applied the name Valmont Dolomite to the beds referred to the Raven Member by Entwistle (1944) and 2I do not agree In my examination of Entwistle's (1944) Boston Hill section (loc. 91) I had no dilliculty in identifying units across the minor Iaults. overlying the Sauk sequence and older rocks. the Cutter Dolomite by Kelley and Silver (1952). Although Entwistle’s (1944) names had priority, the subdivisions of Kelley and Silver (1952) have since been used either as formational names in the Montoya Group or as member names in the Montoya Dolomite at most localities where the Montoya occurs in the report region. Pratt (1967), in his mapping near Silver City (around Lone Mountain, 10c. 22), considered the Montoya as a group but found that he was unable to map the Cable Canyon separately from the Upham at his 1224,000 mapping scale. He considered the Cable Canyon Sandstone and Upham Dolomite as members of the Second Value Dolomite, which was over- lain by the Aleman Formation and Cutter Dolomite. In his discussions of regional stratigraphy, Flower (1965, 1969) considered the Montoya as a group comprising in ascending order the “Second Value Formation, Par Value- Aleman Formation, and the Raven-Cutter-Valmont Formation." Although I believe that Entwistle’s (1944) type section of his subdivisions of the Montoya is adequate and that his older names unfortunately were neglected in favor of newer names, the units of Kelley and Silver (1952) are obviously rather firmly established. In this report, following the example of Pratt (1967), the Montoya is considered as a group made up in ascending order of the Second Value Dolomite, the Aleman Formation, and the Cutter Dolomite. In most areas, where a basal sandstone unit is recognizable in the Second Value, the Second Value is further divided into the Cable Canyon Sandstone Member and the overlying Upham Dolomite Member (fig. 5). ROCK-STRATIGRAPHIC UNITS 53 The Montoya Group as a whole is restricted mostly to southern New Mexico and western Texas but extends a few miles into Arizona at one locality. The thickness of the group exceeds 500 feet locally but thins to erosional wedge edges to the north and west and locally elsewhere (fig. 29). SECOND VALUE DOLOMITE NAME AND TYPE LOCALITY The Second Value Dolomite was named by Entwistle (1944) as a member of the Montoya Dolomite for exposures at Boston Hill (fig. 30 and 10c. 91, fig. 1) near Silver City, N. Mex. Pratt (1967) recognized the Second Value as a formation made up of the Cable Canyon Sandstone Member at the base and the Upham Dolomite Member at the top. The Cable Canyon and Upham were named as formations by Kelley and Silver (1952) for exposures in Cable Canyon (10c. 15) in the Caballo Mountains, N. Mex. The terminology used by Pratt ( 1967) is used here except in areas where the Cable Canyon is missing and the Upham Dolomite Member is synonymous with the Second Value Dolomite. GENERAL DESCRIPTION The Second Value Dolomite, 35-145 feet thick, in general occurs as a conspicuous dark-weathering cliff. The Cable Canyon Sandstone Member is present at the base at a majority of localities and ranges in thickness from less than 1 foot to slightly more than half the thickness of the entire formation. It is made up of poorly sorted medium-grained dolomitic sandstone that may be either sharply or gradationally overlain by the Upham Dolomite Member. The Upham is dolomite at most localities but limestone at a few. It is somewhat variable in detail but is generally fine grained and is everywhere either apparently unbedded or indistinctly bedded. DISTRIBUTION AND STRATIGRAPHIC LIMITS As shown in figure 30 the Second Value Dolomite is present in most of western Texas and southern New Mexico but extends into only a very small part of eastern- most southern Arizona. Rocks characteristic of the Upham Dolomite Member occur over the entire Second Value distribution area, but the Cable Canyon Sandstone Member is absent from the south-central part of the region between Bishop Cap, Capitol Dome, and Long Canyon (locs. 4, 39, and 46), in the north-central part of the region around Johnson Park Canyon, Capitol Peak, and the south Sierra Oscura (locs. 7, 9, and 28), and at the extreme west edge of the Second Value distribution area near Morenci (10c. 84). The Second Value Dolomite disconformably overlies progressively older rocks from southeast to northwest. Along the southeastern edge of the region it overlies Middle Ordovician rocks that have been correlated with the Simpson Group of Oklahoma (fig. 28). Over most of the region, however, it overlies Lower Ordovician forma- tions of the El Paso Group that become progressively older northwestward, as shown in fugures 27 and 28. The Second Value is overlain with generally abrupt but apparently conformable contact by the Aleman Forma- tion throughout all but the extreme north and west edges of its distribution area. Near those edges, where post- Montoya erosion removed the Aleman but not all of the Second Value, the Second Value is unconformably over- lain by Devonian or younger rocks. PETROLOGY CABLE CANYON SANDSTONE MEMBER The Cable Canyon Sandstone Member is made up almost entirely of poorly sorted dolomitic orthoquartzite that is light to medium gray on fresh fracture but is moderate brown to dark brownish gray on weathered surfaces. At some localities, particularly toward the west, very sandy detrital dolomite is present or dominant. At a few localities the sandstones have dominantly siliceous rather than dolomitic cement. The following comments on the petrology of the member are based on outcrop examination of the member at 25 localities and on the examination of 23 thin sections, of which 21 were point counted. Grain-Size sorting in the sandstones seems to be poor to very poor (fig. 31A, B) in most areas, but on the basis of sieve analyses of sands from Lead Canyon in the Sacra— mento Mountains (loc. 3) and from Jose (loc. 6) in the Cooks Range, Howe (1959) reported an average Trask co- efficient of sorting of 1.67, which would indicate well— sorted sand. Grains in most samples range in size from coarse silt or very fine sand to coarse sand or granules. The coarser grains are generally well rounded. The sand grains are dominantly quartz, but rock fragments of chert, siltstone, mudStone, or dolomite are locally common (fig. 31A,.C). Feldspar grains and grains of opaque minerals are commonly present in trace amounts but are never abundant. Fine- tomedium-grained dolomite cements most sand- stone of the Cable Canyon and generally makes up one- fourth to one-half Of the rock. Calcite cement is occasionally present. Near Lake Valley (10c. 19), the Cable Canyon has been pervasively silicified, and quartz cement has patchily replaced about half of the dolomite cement. Virtually all the cement is quartz in an elongate northern area in western Socorro and Sierra Counties including Eaton Ranch, Winston, and South Percha Creek (locs. 35, 41, and 43) (fig. 313). Dolomite included in the Cable Canyon is typically fine to medium grained and is very sandy. At some western localities it is Slightly silty and fine grained and contains abundant irregular pockets or connected borings of dolomitic sandstone (fig. 31D). Flower (1961) mentioned ”saccharoidal” sandstones at the base of the Cable Canyon at a few localities that he believed are unlike the remainder of the unit. He proposed that they are remnants of an older unit which he correlated with the Harding Sandstone of Colorado. Although we 54 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS visited those localities, we were not convinced of the distinction. UPHAM DOLOMITE MEMBER The Upham Dolomite Member is nearly everywhere made up of medium- to dark-gray massively bedded resistant fine to very finely crystalline dolomite that generally weathers to olive gray or brownish gray but also to a distinctive mottled yellow gray and olive gray (fig. 32A). It commonly displays a very rough weathered sur- face (fig. 32B). At a few localities the dolomite is slightly calcareous, and at a few others much or all of the member is made up of limestone. The basal few feet is commonly slightly sandy. Small quantities of nodular chert occur locally in the member. The following descriptions of the rock are based on the outcrop examination of the member at 33 localities and on the examination of 47 thin sections. Most thin sections of dolomite from the Upham show only irregularly microcrystalline to finely crystalline dolomite and display no relict limestone fabric (fig. 333). A few, however, display relict images of fossil-bearing lime mudstone, skeletal lime wackestone to packstone (fig. 33A), or fossil-bearing lithiclast lime wackestone to pack- stone; the skeletal lime wackestone to packstone is probably the most abundant. Thin sections of pelmatozoan(?) limestone from the Cooks Range and Morenci (locs. 17 and 84) (fig. 33E) showed little that was not evident in hand specimen. Some dolomites from mineralized areas display a fibrous fabric (fig. 33D) that may be characteristic of hydrothermal dolomite. These offer no clue as to their original depositional fabric. Finely crystalline dolomites from throughout the region commonly display vug fillings of coarsely crystalline dolomite (fig. 33C). On the outcrop these vug fillings appear as conspicuous white spots on a dark background. Quartz sand grains in dolomites from the lower part of the Upham are similar to those in the Cable Canyon Sandstone Member but are in general finer. SEDIMENTARY STRUCTURES Notable sedimentary structures are rare in the Second Value Dolomite. Indistinct cross-laminations were observed in the Cable Canyon Sandstone Member at a few localities but seem to be absent from most areas. Irregular burrow fillings and pockets of dolomitic sandstone in sandy dolomite are present in the upper part of the Cable Canyon at several localities in the western part of the region (fig. 34). Faint indications of thin beds of intra- formational conglomerate were noted in the Upham Dolomite Member at two localities. POROSITY The poor grain-size sorting of the Cable Canyon Sand- stone Member allows for very little intergranular porosity in the unit, but many samples show considerable cement- reduced or cement-filled vuggy, channel, or fracture porosity. The total porosities of seven samples were checked by the kerosene-displacement method; these ranged from 3.2 to 7.6 percent and averaged 4.8 percent. The sample that showed 7.6 percent total porosity had an effective porosity of 6.4 percent, as checked by the mercury porosimeter, which indicates that most of the pore spaces were effectively interconnected. However, three other effective porosity determinations showed only 1.6, 1.9, and 2.8 percent. 110° 7 f [—H‘f r ‘ I \ /’{ i 33°" . D 32°77 C A <2 3 ZN o o ‘ 0 £21 3 «1&1 [Z ‘ I l .________ __UNITEQSTATESJ _~_ MEXICO 314M ROCK-STRATIGRAPHIC UNITS Vuggy porosity was observed in a large proportion of thin sections of samples from the Upham Dolomite Mem- ber, but most of the vugs are cement reduced or cement filled. Moldic porosity is also common in the member, and microfracture porosity is not unusual. The total porosi- ties of 11 samples checked ranged from 0 to 8.8 percent and averaged 3.0 percent. The effective porosities of 12 samples checked ranged from 0.7 to 5.1 percent and averaged 2.0 percent. Two samples were checked by both methods and 108° 197° 55 showed 5.1 and 8.8 percent total porosity but only 1.1 and 2.2 percent effective porosity, respectively. These large dif- ferences are explained by the nature of the porosity observed in the thin sections of the two samples—moldic in one and vuggy in the other. THICKNESS The Second Value Dolomite ranges from a maximum measured thickness of 144 feet at Point of Mountains in Culberson County, Tex. (10c. 50), on the east to erosional 105° AMud Springs Mts + South Ridge 0 ACable Canyon Boston Hill EXPLANATION + Surface data reported in literature 0 Drill— hole data —500— Isopach — Showing thickness, in feet O—r—O A Surface locality or drill-hole samples examined in this study _‘ ll l l l i"'“"”-. Agua Chiquita l Baylor Ni” . . 31° i 100 KILOMETRES FIGURE 29,—Distribution and preserved thickness of Montoya Group. 56 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS wedge edges on the north and west and around some post- Ordovician high areas (fig. 30); it is 80 feet thick at its type locality at Boston Hill (10c. 91). Its minimum measured thickness is 35 feet at San Lorenzo (10c. 38), where the formation is conformably overlain by the Alemen Forma- tion and thus is not thinned by erosion. Although the Second Value does vary irregularly in thickness, it is generally thickest toward the southeast and thinnest toward the northwest. The thickness of the Cable Canyon Sandstone Member is highly irregular in the region. It is absent, owing to non- deposition, or only inches thick in the triangular area in south—central New Mexico and extreme western Texas defined by Bishop Cap, Capitol Dome, and Long Canyon (locs. 4, 39, and 46) and at Johnson Park Canyon, Capitol Peak, and south Sierra Oscura (locs. 7, 9, and 28) in cen- tral New Mexico. At its type locality at Cable Canyon (10c. 15) in the Caballo Mountains it has a reported thickness of 35 feet (Kelley and Silver, 1952) and is nearly as thick at several other nearby localities. The greatest known thick- ness is 55 feet at Point of Mountains (10c. 50) in the Sierra Diablo, but only about 20 miles away in the southern Baylor Mountains (10c. 52) the thickness is only 2 feet. The thickness of the Upham Dolomite Member where completely preserved ranges from 16 feet at San Lorenzo (10c. 38) to 115 feet at Rhodes Canyon (10c. 10) in the San Andres Mountains. The Upham is reported to be 78 feet thick at its type section at Cable Canyon (10c. 15) (Kelley and Silver, 1952). FOSSILS AND AGE Marine invertebrate fossils occur in the Second Value Dolomite at nearly all localities. Corals are most conspic— uous but brachiopods, mollusks, and pelmatozoan remains are common; trilobites and bryozoans are sparse. No forms that had not previously been found in the forma- tion were collected during this study. As discussed by Howe (1959), Hill (1959), and Flower (1961, 1969), differ- ent faunal elements suggest ages ranging from Black River (late Middle Ordovician) to Red River (Late Ordovician). For the present it seems safest to say that the Second Value is roughly of late Middle Ordovician (Trenton) age. It may also be of early Late Ordovician age and correlative with the Bighorn Dolomite of Wyoming. CONTACTS The base of the Second Value Dolomite lies discon- formably on earlier Ordovician rocks throughout the report region, and the contact is everywhere abrupt. The Second Value is conformably overlain by the Aleman Formation, but the contact in most places is rather abrupt inasmuch as the massive chert—free carbonates of the Second Value generally give way rather abruptly to more distinctly bedded finer grained and very cherty carbonates 0f the Aleman. In this study the contact was selected at the lowest appearance of abundant chert regardless of the nature of the carbonates. The contact between the Cable Canyon Sandstone Member and the Upham Dolomite Member apparently is conformable at all localities. Generally, however, the con- tact can be picked within very narrow limits because the dolomitic sandstone or conspicuously sandy dolomite at the top of the Cable Canyon gives way rather abruptly to only slightly sandy dolomite at the base of the Upham. At most localities a distinct bedding plane separates the two members. 110° 109° T ”—w 1 1 I If. g! l APACHE / I . GILA P , I ;r\; \c// , f C A T R O N g i l GREENLEE i Thinnedgflitojfl_%f? 930 /L///—‘—/ % .. . .Morenci/ I \\ \‘ \) ”xxx-us, i , . 1 32" I ‘ i . 0/ i f ‘Lake |’ i \\ .\\\lfl/loonl_‘main Valley\ \ Chiquita l \ \ Cgoks —--7’———i I| Canyon L \ \\ Range +Jose / O T E R O +— /’ xl i \\ \\__a’¢o i l \ \ «0’ N A A N A i \\\L~U N A ’,)’ D 0 l A -— —-— 1 — ___ Capltol a’ 1°—-~“ ‘ Dome I L-*‘ \ i Hitt / A ' . Canyon 32° i be” I l \ —'7.0 [I l ___. x x . . l z” UNITED STATES As°em° 0"“ . Lo 2 m _.__....___._ __._.___W.... . n I’M” MEXICO \AL P A S 0 303mg,“ x 1;,— ——’T \ i I ’03 l \ m l \ 7° . m . Mountain A l EXPLANATION \\ O i \ z + Surface data reported in literature 0 Drill-hole data —100— Isopach — Showing thickness, in feet ——10-— Facies line —~ Showing percentage of quartz silt and sand in formation 0 50 | 100 KILOMETRES FIGURE 30.—Distribution, thickness, and facies of Second Value Dolomite. 58 FIGURE 31.—-Photomicrographs of Cable Canyon Sandstone Member of Second Value Dolomite Bar in A is 1 mm; all views are same scale. A, Coarsegrained dolomitic sandstone from 6 feet above base of member at Lone Mountain (10c. 22) Most grains are quartz, but near base of picture is large dark fragment of silty mudstone whose source is unknown. Cement is dolomite, some of which has replaced margins of quartz grains. Crossed nicols. B, Poorly sorted ortho- quartzite from member on South Percha Creek (loci 43). Sandstone such as this, without dolomite cement, is unusual in the Cable Can- yon and may result from replacement of dolomite by quartz in this CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS sample. Crossed nicols. C, Dolomitic sandstone from 4 feet above base of member at Point of Mountains (loc. 50). Grains are mostly quartz and cement is mostly dolomite, but large area of dolomite at base is part of a large detrital fragment of dolomite—not an un- common feature of the Cable Canyon. Crossed nicols. D, Bottom edge of dolomitic sandstone fill in burrowed silty dolomite from 3 feet above base of member at Boston Hill (loc. 91). Sand—filled burrows and pockets in dolomite are common in the Cable Canyon at several western localities. Crossed nicols. ROCK-STRATIGRAPHIC UNITS 59 FIGURE 32.—Outcrops of Upham Dolomite Member of Second Value Dolomite. A, Mottled dolomite near base of member near Scenic Drive section in southern Franklin Mountains (loc. 45). The origin of this mottling is unclear, but such mottling is common in the Upham in and near extreme western Texas and is seen sporadically bution area. The Aleman is conformably overlain by the Gutter Dolomite except near the margins of its distribu- tion area, where it is unconformably overlain by post— Ordovician rocks. PETROLOGY The Aleman Formation is made up dominantly of generally thinly laminated microcrystalline cherty dolo- mite at most sections, but considerable limestone is present at several southern localities extending from the Florida Mountains to the Sierra Diablo (locs. 39, 44, 46, and 50), and the carbonate is all limestone in the Cooks Range (10c. 17). The carbonate rocks are mostly distinctly bedded and average medium gray on fresh fracture and light medium gray to light olive gray on weathered surfaces. Nearly white to very dark gray chert that occurs in extensive horizontal lentils as much as several inches thick and in elongate nodules makes up 10—45 percent of the formation, and it probably averages 25 percent over the region. A thin bed of sandstone was seen near Winston in the Sierra Cuchillo (10c. 41). The following comments on the lithology of the Aleman are based on outcrop examina- tion of the formation at 27 localities and on the examina- tion of 43 thin sections. Because the Aleman is largely or completely dolomi- tized at most localities, relatively few thin sections of lime- stone were examined, but many of the dolomites display a rather distinct relict texture. Most of the carbonate is (or was) lime mudstone or fossil-bearing lime mudstone; much is thinly laminated and on occasion shows oncolite structures (fig. 37F). Skeletal lime wackestone (fig. 37A) and skeletal lime packstone are sparsely represented. elsewhere. Penny gives scale. B, Very rough weathering calcareous dolomite about 40 feet above base of member at Hitt Canyon in northern Franklin Mountains (10c. 44). Such rough weathered sur- faces are characteristic of the Upham at many localities. Pencil gives scale. Siliceous spicules can be seen in many lime mudstones whether or not they have been dolomitized (fig. 37D). Most dolomite in the formation is microcrystalline (fig. 378), but some laminae or irregular areas are very finely crystalline to medium-crystalline dolomite (fig. 37E). Many of the chert bands and nodules in the Aleman are sharply bounded (fig. 373), but others have gradational boundaries with the dolomite (fig. 378, E). In addition, most carbonate in the formation contains considerable silica in the form of siliceous spicules (fig. 37D), inter- granular chert (fig. 373, F), chert veinlets, and incipient chert nodules (fig. 37D). In most thin sections the forma- tion of chert nodules or lentils postdated dolomitization, but in some thin sections later dolomite can be seen as veins in chert (fig. 37E). Much of the chert in the forma- tion may have been derived from the abundant siliceous spicules in the carbonate. SEDIMENTARY STRUCTURES Megascopic sedimentary structures that might be of use in interpreting depositional environments are not common in the Aleman Formation. The thinly laminated dolomites common to much of the formation might be interpreted as algal-mat dolomites, but, other than scat- tered oncolites (fig. 37F), sedimentary structures that are usually associated with supertidal and intertidal algal mats are lacking. Evidence for aerated shallow marine waters occurs in the Franklin Mountains area (Hitt Can- yon and Sugarloaf, locs. 44 and 93), where colonial coral colonies in apparent growth position are found at one horizon (fig. 38). 6O CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS POROSITY Porosities in the Aleman Formation are low. The total porosity of five samples was checked by the kerosene-dis- placement method; the porosities ranged from 0.7 to 2.7 percent and averaged 1.5 percent. The effective porosity of 10 different samples was checked by the mercury porosi- meter; the porosities ranged from 0.5 to 4.4 percent and averaged 1.5 percent. In thin section the most commonly observed types of pore space were microfractures, minute vugs, and fOSSil molds. Possible fenestral porosity was noted in one sample. THICKNESS The Aleman Formation has a maximum measured thickness of 238 feet in the northern Baylor Mountains (loc. 47) and thins to an erosional wedge edge on the west and north and around some post-Ordovician high areas ROCK-STRATIGRAPHIC UNITS 61 FIGURE 33.—Photomicrographs of Upham Dolomite Member of Second Value Dolomite. Bar in A is 1 mm; all views are same scale. A, Completely dolomitized skeletal lime wackestone from about 100 feet above base of member in Rhodes Canyon (10c. 10). Few dolo- mitized samples of the Upham show such distinct relict structure. Here the lime mud matrix has recrystallized into a matte of fibrous dolomite, whereas the fossil fragments are mostly replaced by more coarsely crystalline dolomite. Plain light. B, Very finely crystalline dolomite from near base of member in Agua Chiquita Canyon (loc. 1). Many Upham samples look like this in thin section and offer little clue as to the original sedimentary structure. It may have been lime mudstone. Plain light. C, Coarsely crystalline dolomite filling in vug in very finely crystalline dolomite from 95 feet above base of Second Value Dolomite at Bishop Cap (10c. 4). Such vugs are common in the Upham but are, as in this instance, generally filled. Even where only partially filled, the pore spaces, though abundant in places, do not appear to be interconnected. Plain light. D, Finely crystalline fibrous dolomite from about 20 feet above base of member at Boston Hill (10c. 91). This dolomite, which is from a mineralized area, appears to have been metamorphosed and may once have looked like A or B. There is virtually no suggestion of the original sedimentary fabric. (Compare fig. 253.) Crossed nicols. E, Coarsely recrystallized pelmatozoan(?) limestone from 7 feet above base of member in Cooks Range (10c. 17). This is from one of the very few localities where the Upham is limestone. Crossed nicols. (fig. 36). Its minimum measured thickness in areas where conformably overlain by the Cutter Dolomite and thus presumably completely preserved is 47 feet at Mescal Can- yon in the Big Hatchet Mountains (10c. 24). In its typical locality at Cable Canyon (10c. 15) in the Caballo Moun- tains the Aleman has a reported thickness of 107 feet (Kelley and Silver, 1952). In general, the formation is less than 100 feet thick in the western part of the region and more than 150 feet thick in much of the central and south- eastern part of the region (fig. 36). FOSSILS AND AGE The Aleman Formation is not conspicuously fossil- iferous, but at most localities there are a few abundantly fossiliferous beds and other scattered fossils. Brachiopods and corals are most common, but bryozoans, mollusks, trilobites, conodonts, and other forms have been found. Previously unreported bryozoans and conodonts col- lected during this study are listed under “Fossil Lists." Because of difficulties of correlating the fossil zones of the Aleman with Ordovician zones of the central United States, the precise age of the Aleman is still in doubt. How- ever, both Howe (1959) and Flower (1969) agreed that it belongs to the Late Ordovician. Howe (1959) believed that there is no faunal hiatus between the Aleman and the underlying Second Value Dolomite, but Flower (1969) believed that part of the lowest Upper Ordovician is missing between the formations. Because the two forma- tions seem to be lithologically gradational, I believe that no real hiatus exists and that the Second Value Dolomite of late Middle Ordovician age is conformably overlain by the Aleman of Late Ordovician age. UPPER CONTAcr The contact of the Aleman Formation with the over- lying Cutter Dolomite is gradational and arbitrary. At most localities it can be picked fairly closely as the hori- zon at which very cherty beds of the Aleman give way upward to virtually chert-free beds Of otherwise similar lithology in the Cutter. Because the boundary is arbitrary, different workers might choose the contact differently at some localities. A general decrease in thickness of the Ale- man westward and a concomitant increase in thickness of the Cutter westward might indicate that the contact is generally in progressively older beds westward. CUTTER DOLOMITE NAME AND TYPE LOCALITY The Cutter Dolomite was named by Kelley and Silver (1952) for exposures in the Caballo Mountains, N. Mex., at Cable Canyon (fig. 39 and 10c. 15, fig. 1). GENERAL DESCRIPTION The Cutter Dolomite, which has a maximum thick- ness of 300 feet but is generally less than 200 feet thick, is made up almost entirely of yellowish-gray-weathering medium- to light-gray microcrystalline dolomite that typically crops out in steep slopes. DISTRIBUTION AND STRATIGRAPHIC LIMITS The Cutter Dolomite underlies much of westernmost Texas and southern New Mexico, except in two large and many small post-Ordovician high areas, but it does not extend into Arizona (fig. 39). It conformably overlies the Aleman Formation and is disconformably or uncon- formably overlain by post-Ordovician rocks. PETROLOGY The Cutter Dolomite consists almost entirely of obscurely to distinctly bedded medium- to light-gray FIGURE 34.—Sandy dolomite with burrow fillings of dolomitic sand- stone 12 feet above base of Cable Canyon Sandstone Member of Second Value Dolomite at Lone Mountain (10c. 22). Rock such as this is present at several localities. (See photomicrograph of Similar rock in fig. 310.) 62 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS FIGURE 35,—Cherty fine-grained dolomite, 26—32 feet above base of Ale- man Formation at Hitt Canyon in the Franklin Mountains (loc. 44). Pencil gives scale. Several habits of chert occurrence are shown microcrystalline dolomite that in general weathers to a pale yellowish gray; in some areas similarly colored lime- stone is present in the formation. The basal few feet of the formation is commonly marked by an argillaceous or slightly silty interval. Some nodular chert is present at most localities. The following comments on the petrology of the Cutter are based on the outcrop examination of the formation at 21 localities and on the examination of 40 thin sections. Although the Cutter is now largely dolomite, thin sec- tions commonly display indistinct to conspicuous relict textures of the original limestone. Most of the rock in the formation seems to have been lime mudstone or fossil- bearing lime mudstone (fig. 403), but beds of dolomitized skeletal wackestone to packstone are also present (figs. 40A, 41A). More than half of the dolomitized lime mud- stone is conspicuously laminated (figs. 40C, 418). Most of the dolomites are largely microcrystalline, but laminae or patches of very fine to medium-crystalline dolomite are commonly present (fig. 40B, C). The two limestone samples examined show scattered dolomite rhombs or dolomitized patches (fig. 40A). A few samples of dolomite from the Cutter contain siliceous spicules or chert replacement of fossil debris, but silica in any form is much scarcer in the Cutter than in the underlying Aleman Formation, where carbonates are otherwise very similar to those in the Cutter. Minor argillaceous material and traces of quartz silt were noted in a sample from near the base of the Cutter at Long Canyon (10c. 46) in the Hueco Mountains. Specks of probable carbonaceous or bituminous material were seen in several samples. in A and another in B. The bedded chert under the pencil in A and the elongate lenticular chert in B are most characteristic of the Aleman at most localities. SEDIMENTARY STRUCTURES Sedimentary structures useful for paleoenvironmental interpretation seem to be virtually nonexistent in the Cutter Dolomite. The common very thin laminations might be interpreted to be of algal-mat origin, but no features were noted that are suggestive of subaerial desic— cation of tidal-flat algal mats. The only evidence of cur- rent action noted was in a thin lenticular bed of cross-lami- nated saccharoidal dolomite 40 feet above the base of the Cutter at Lone Mountain (10c. 22). POROSITY Most of the Cutter Dolomite is very low in effective porosity, but some of it has moderate total porosity in the form of small vugs and fossil molds. Only three samples were checked for total porosity by the kerosene-displace- ment method, and these had 2.1, 3.6, and 4.2 percent porosity. Several other samples viewed in thin section had visible porosities estimated at 5-10 percent. The effective porosities of nine samples checked by the mercury porosi- meter ranged from 0.2 to 2.3 percent and averaged only 0.8 percent. The sample whose measured total porosity was 4.2 percent showed an effective porosity of only 0.3 per- cent. This disparity is not surprising, considering that vuggy and moldic porosity is not interconnected. Although microfractures were noted in many samples in thin section, these were nearly all filled by cement. THICKNESS Because the Cutter Dolomite has a disconformable or unconformable upper contact throughout the report region, its total original thickness is probably nowhere preserved. Its greatest thicknesses are near the west edge of ROCK-STRATIGRAPHIC UNITS 63 its distribution area, but it thins suddenly to an erosional wedge edge farther westward as well as northward. The maximum measured thickness is 293 feet at Mescal Can- yon in the Big Hatchet Mountains (10c. 24), and the reported (Kelley and Silver, 1952) thickness at the Cutter type locality at Cable Canyon (10c. 15) in the Caballo Mountains is 129 feet. FOSSILS AND AGE Fossils are sparse to absent in much of the Cutter Dolo- mite, but at nearly all localities there are a few beds rich in corals or brachiopods. Mollusks and conodonts may be recovered from some beds. As summarized by Howe (1959) and Flower (1961, 1969), the Cutter is of Richmond (Late Ordovician) Age. Howe (1959) believed that deposition was continuous from Aleman into Cutter time, but Flower (1969) suggested that a minor erosion interval separated deposition of the two formations over most of the region. On the basis of lithology and the seeming westward increase in age of the contact between the formations, I am strongly inclined to agree with Howe (1959) that deposi— tion was virtually continuous. UPPER CONTACT The Cutter is disconformably overlain by the Fussel- man Dolomite of Silurian age over much of the region, but near the north and west erosional edges and along some post-Ordovician high areas, rocks of Devonian, Pennsyl- vanian, Permian, or Cretaceous age may unconformably overlie the Cutter. Distinguishing the Cutter from post— Silurian rocks is no problem, but the dolomite-dolomite contact between the Cutter and the Fusselman, though 1 1 2° 1 1 0° 111° 109° knife sharp, can seem subtle to those unfamiliar with it. With a little experience, however, one can readily detect the slight change in color and texture between the two dolomites. The Fusselman weathers to a brownish gray or olive gray or to a yellowish gray that is darker than that of the Cutter. It is nearly everywhere more coarsely grained than the Cutter and is almost everywhere more massively bedded. ROCKS DIRECTLY OVERLYING CAMBRIAN AND ORDOVICIAN STRATA Silurian strata overlie Ordovician rocks over much of southern New Mexico and western Texas, Devonian rocks overlie Cambrian or Ordovician rocks over most of southern Arizona and part of southern New Mexico, and various rocks younger than Devonian overlie the Cam- brian or Ordovician in small local areas in various parts of the region. Because the character of overlying rocks can be of major significance in evaluating the oil and gas potential of the underlying rocks, brief descriptions of the rocks that overlie the Cambrian or Ordovician are presented. SILURIAN ROCKS Silurian strata assigned to the Fusselman Dolomite disconformably overlie the Middle and Upper Ordovician Montoya Group over much of southern New Mexico and western Texas (fig. 42). The formation is many hundreds of feet thick in part of southern New Mexico and in western Texas but thins to an erosional wedge edge to the west and north. 108° 32° . Mescal I LLonQ Canyon Canyon J EXPLANATION \ < 5| \ F‘—‘_“—l El \\\ J l {24' \ i—_ l < \- _______ J_»_L_I_NITED STATES MEXICO 31°— so 100 MILES 50 100 KILOMETRES l l A Surface locality or drill-hole samples examined in this study\\\ + Surface data reported in literature \ ° Drill—hole data “159—- lsopach — Showing thickness, in feet FIGURE 36.—Distribution and thickness of Aleman Formation. 64 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS FIGURE 37 (facing page and above).—Photomicr0graphs of Aleman For- mation. Bar in A is 1 mm; all views are same scale. A, Dolomitized skeletal wackestone from about 75 feet above base of formation in San Andres Canyon (10c. 27). Rock such as this is common but not predominant in the Aleman. Plain light. B, Chert-banded micro- (rystalline dolomite from about 25 feet above base of formation at Capitol Dome (loc. 39). The lower very thin chert lamina is fairly sharply bounded, whereas the upper one has diffused boundaries. Flecks of silica (white) occur in the dolomite, and dolomite flecks remain in the chert. The dolomite may be dolomitized lime mud- stone. Plain light. C, Chert and microcrystalline dolomite from 4 feet above base of formation at Bishop Cap (loc. 4). This View shows a zone more than 1 mm thick of incompletely silicified dolomite between chert at top and slightly siliceous dolomite on bottom. Boundaries between chert and dolomite are diffuse like this in most thin-section views although on the outcrop or in hand specimen the boundaries appear to be very sharp. Plain light. D, Siliceous mirrocrystalline dolomite with chert veinlet from 5 feet above base of formation at Hitt Canyon (loc. 44). The dolomite contains numer- ous siliceous sponge spicules. These may be the source of the silica ROCK-STRATIGRAPHIC UNITS 65 in the chert veinlet and in the diffused area of “incipient” chert at upper left. Plain light. E, Chert with veinlets of microcrystalline dolomite from near top of formation at Long Canyon (10c. 46). Here the process of silicification appears to have been reversed ~— chert at left is veined with microcrystalline dolomite (dark areas). Plain light. F, Partly silicified section of part of a dolomitized onco— lite from top of formation at Capitol Peak (Ice. 9). The microcrystal- line dolomite in the curved laminae of this oncolite are about one- half replaced by granular chert. Crossed nicols. The Fusselman consists almost entirely of coarsely microcrystalline to medium-crystalline massively bedded dolomite throughout most of its distribution area; lime- stone occurs very locally. Most of the dolomite is medium gray on fresh fracture and weathers to yellowish gray or light olive gray. Large irregular chert nodules are abun- dant in some areas and absent from others. Dolomitiza- tion of the Fusselman is generally so complete that original limestone fabrics are lost, but faint relict textures in a few of 14 thin sections cut from rock collected from the basal part of the formation suggest that much of the rock may have ranged from lime mudstone to skeletal lime wackestone. The determined total porosities of six samples of F ussel- man Dolomite ranged from 0.7 to 4.8 percent and averaged 3.0 percent. One sample that had 4.3 percent total porosity proved to have an effective porosity of only 0.7 percent, indicating that the pore spaces were not effectively inter- connected. The effective porosities of three other samples were 1.4, 2.2, and 2.6 percent. The low effective porosities were probably due to the vuggy nature of the porosity noted in most thin sections. The Fusselman is nearly everywhere overlain by dark shaly Devonian strata except locally near the Plymouth 1 Federal well (ICC. 108), Cable Canyon (10c. 15), Baylor Mountains (10c. 52), and Victorio Mountains (10c. 90); at these four localities the Devonian was removed and the Fusselman is overlain, respectively, by Mississippian, Pennsylvanian, Permian, and Cretaceous rocks. DEVONIAN ROCKS Devonian strata up to a few hundred feet thick discon- formably overlie Cambrian, Ordovician, or Silurian strata over much of the report region, as indicated in figure 51. Dark-gray marine shales and siltstones assigned to several widespread or local stratigraphic units make up a signifi- cant part of the Devonian in western Texas, New Mexico, and easternmost Arizona; but farther west in Arizona, carbonates of shallow marine origin assigned to the Martin Formation are the dominant lithology of the Devonian (Poole and others, 1967). The Devonian rocks, particularly those to the east, offer excellent potential as petroleum source beds. POST-DEVONIAN ROCKS Pennsylvanian strata of marine origin made up of fine to coarse terrigenous clastics and carbonates directly over— lie Ordovician rocks in a wedge-shaped area near the north edge of the Ordovician distribution area in central New Mexico. These Pennsylvanian rocks, which contain pos- sible petroleum source beds, have been described by Kottlowski (1960b) and Bachman (1968). Permian strata of marine origin overlie Ordovician rocks in the subsurface of eastern New Mexico and western Texas along the margins of areas uplifted in Pennsyl- vanian time, but unless already penetrated by drill holes the precise locations of such areas is speculative. A hole 66 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS FIGURE 38.—Silicified colonial coral 131 feet above base of Aleman Formation at Hitt Canyon in the Franklin Mountains (10c. 44). Pen- cil points up. These colonies are most commonly found in the for- mation in extreme western Texas and nearby areas. (Turner 1 State) drilled at locality 105 (fig. 1) penetrated Permian strata that directly overlie Lower Ordovician rocks. The nature of the Permian rocks in the region was reviewed by McKee, Oriel, and others (1967). Cretaceous strata overlie Ordovician rocks at Capitol Dome (10c. 39, fig. 1) and probably overlie Cambrian and (or) Ordovician rocks elsewhere in the subsurface along the margins of areas in western New Mexico and Arizona that were uplifted in Mesozoic time. The nature and distri- bution of the Cretaceous rocks of the region, which toward the north might offer some potential as petroleum reser- voir rocks, were summarized by Hayes (1970). TIME-STRATIGRAPHIC UNITS CAMBRIAN MIDDLE CAMBRIAN DEFINITION AND DISTRIBUTION Middle Cambrian rocks as interpreted in this report include the Bolsa Quartzite and overlying lower member of the Abrigo Formation in the western part of the region and the lower part of the Coronado Sandstone in the central part of the region (fig. 5); Middle Cambrian rocks are absent, probably owing to nondeposition, in the eastern part of the region. Middle Cambrian rocks are thus present in most of southern Arizona except in a broad area where they were presumably removed by erosion during Cretaceous time (fig. 43). THICKNESS Middle Cambrian rocks of the region, which uncon- formably overlie Precambrian rocks on a surface of vari— able relief, thin irregularly northeastward from 800 or 1,000 feet in the southwestern part of the region to a depositional wedge edge in the vicinity of the Ari- zona—New Mexico State line (fig. 43). In the Caborca area of Sonora, about 110 miles southwest of American Peak (10c. 83), apparently near the margins of the early Paleozoic Cordilleran geosyncline (Poole and Hayes, 1971), Middle Cambrian rocks are more than 2,600 feet thick (Cooper and Arellano, 1952). LITHOLOGY In the southwestern part of the region, the basal part of the Middle Cambrian is represented by the dominant sand- stones of the Bolsa Quartzite, and the upper part, by the shales and limestones of the lower member of the Abrigo Formation. Northward and eastward from there the basal part of the Bolsa laps out irregularly on a Precambrian paleoslope; the basal part of the lower member of the Abrigo apparently grades into sandstones at the top of the Bolsa; and the upper part of the lower member of the Abrigo changes irregularly in facies from dominant shales and limestones to siltstones and fine-grained sandstones. Still farther eastward the siltstones and fine-grained sand- stones of the lower member of the Abrigo grade into coarser sandstones of the basal part of the Coronado Sand- stone. As a result, in the southwesternmost part of the region the Middle Cambrian rocks are only about one-half sandstone and about two-thirds total terrigenous clastic rocks, whereas near the eastern edge of its occurrence the Middle Cambrian is mostly sandstone (fig. 43). Con- comitant with the northeastward lapping out of the lower beds and coarsening of the upper beds, there is a north— eastward increase in feldspar content of sandstones at given horizons and an apparent increase in glauconite in the same direction. CONDITIONS OF DEPOSITION The thickness variations, gross lithofacies trends, and broad regional considerations all rather strongly suggest that the Middle Cambrian strata of the report region were deposited on a surface of modest relief near the margins of a shelf sea that transgressed northeastward perhaps 120 miles during the roughly 20 million years of Middle Cam- brian time. Whereas reasonable confidence can be maintained for the broad concept of sedimentation near the margins of a TIME-STRATIGRAPHIC UNITS 67 northeastward-transgressing sea, less certainty can be claimed for interpretations of the subenvironments in which various Middle Cambrian lithotypes were deposited. Generally, the sandstone of the Bolsa Quartzite and basal Coronado Sandstone can be called transgressive sandstone, but whether the sands were deposited in a high- energy beach environment as interpreted by Lochman- Balk (1971) or on offshore bars affected by strong ebbtide currents as interpreted by Seeland (1968) is unsettled. Con- sidering the regional stratigraphy, lithologies, and sedi- mentary structures as a whole, I tend to favor the beach- sand interpretation. The rare scattered small phosphatic brachiopods as well as the occasional glauconite could have been washed landward onto a beach and are no proof of a subtidal marine depositional site; also, the surface tracks and trails and the Scolithus tubes common in the Bolsa, especially in the upper part, are common enough in modern beach sands. I also concur with Lochman-Balk (1971) that both the sandy and the shaly facies of the lower member of the Abrigo Formation were probably deposited in an intertidal environment. The mud cracks and mud- chip conglomerates (fig. 88) common in the member are evidence of probable periods of subaerial desiccation. Paleoenvironmental interpretations by Kepper (1972), based on detailed studies in Nevada and Utah, suggest that the westward transition from intertidal to subtidal condi- tions in Middle Cambrian time may have been in eastern Nevada. DRESBACHIAN AGE DEFINITION AND DISTRIBUTION As interpreted in this report, rocks of Dresbachian age 7112 ,, 1711“ 7 110° _ 109° T A l I r f”. | ‘ If r"—~—:\ r\i—L\// . i \ /’/~ I _____ J, \ Li I \ - 1 I _____ \ / T I U i \ I \\ ,6 I o, E _ . I \ ' I ,. retaceous .—______‘ i g :0 ' ergsvon l T ——————— —i I of“ . II~ ___________ L I 332 . l s \- r ----- a. I ‘\ —J I <2 \\\i_\ I Z - ——————— J_-_HI£I§2§IAI§8____ _____ MEXICO 31°" 0 50 100 MILES _ - Drillehole data 0 50 100 KILOMETRES 41', A Surface locality or drill \ \ a, \ l S \ 1 m ‘ \ / \ / EXPLANATION ‘ Surface locality or drill-hole samples examined in this study‘ + Surface data reported in literature 7.300" Isopach — Showing thickness, in feet —50—— Facies line — Showing percentage of terrigenous clastics in Dresbachian rocks L O 50 100 MILES O 50 100 KILOMETRES FIGURE 44.—Distribution, thickness, and facies of rocks of Dresbachian age. Rocks interpreted to be of early and middle Canadian age in the report region consist of the El Paso Limestone as it is used in the western area (figs. 5, 6), most of the upper member of the El Paso Limestone of the central area, and part or all of the Bliss Sandstone and all of the Hitt Can- yon Formation of the eastern area. Strata of early and middle Canadian age may once have extended across the entire report region but are now missing in most of Arizona, along the entire north edge of the region, and locally elsewhere owing to several episodes of post-Ordovician erosion (fig. 46). THICKNESS In areas where strata of early and middle Canadian age are conformably overlain by beds of late Canadian age and thus have not been thinned by erosion, they range in thick- ness from about 260 feet at Lone Mountain near Silver City (fig. 46 and loc. 21, fig. 1) to about 685 feet at Hitt Canyon in the Franklin Mountains (loc. 44). They seem to thin fairly regularly to the west, north, and east of extreme western Texas (fig. 46). Farther to the north and west they are thinned by erosion on top and eventually thin to a wedge edge. LITHOLOGY Near the west edge of the distribution area of strata of early and middle Canadian age the strata are made up almost entirely of relatively pure limestone and (or) dolomite, and the estimated content of terrigenous clastics is less than 5 percent. Eastward from there, as the Bliss Sandstone at the base makes up an increasing proportion of the interval and as the carbonates become sandier, content of quartz silt and sand increases to about 50 percent at Beach Mountain (10c. 48) near the southeast corner of the report region (fig. 46). Some shale and silt- stone is present in the Ordovician part of the Bliss Sand- stone, particularly toward the north. A highly generalized summary of the diversities of composition, texture, and sedimentary structures found in the lower and middle Canadian rocks follows. In general, the Bliss Sandstone, which lies at the base of the sequence in all but the western part of the region and whose top becomes younger eastward, can be described as consisting dominantly of quartz- or carbonate-cemented grain- supported quartz arenites that commonly display planar cross—laminations; hematite cement and grains of hematite ooids are locally common, especially toward the north and west. In general, the carbonates of early and middle Canadian age that occur just above the basal sand- stone in the central part of the region are dominantly quartzose algal and lithiclast lime packstones to grainstones that are commonly cross-laminated and that in places contain beds interpreted to be algal-matdolomite and beds of limestone or dolomite chip conglomerate. Above these beds in the central part of the region, non- sandy fossil-bearing lime mudstones and skeletal lime wackestones are dominant; locally some of the lime mud- stones contain channels of skeletal-lithiclast lime pack- stone. Burrows, tracks, and trails are common in these rocks and oncolites and digitate algal stromatolites are locally found. Carbonates in the upper part of the lower and middle Canadian sequence in the central part of the region tend to be quartzose and are made up largely of dolarenite, lithiclast lime packstone and oolite pack- stone, all of which may be cross-laminated. Limestone or dolomite chip conglomerates are also locally present in these upper beds. In the western part of the region the carbonates are basically similar to those just described in the central part but are not sandy in either the lower or the upper parts. In the southeastern part of the region the carbonates are generally similar to those in the lower and upper parts of the sequence in the central part of the region; nonsandy carbonates are relatively scarce even in the middle part of the sequence. CONDITIONS OF DEPOSITION Strata of early and middle Canadian age in the region were deposited near the margins and in shallow waters of the sea that had transgressed northeastward and eastward across most of the region during Middle and Late Cambrian time. During earliest Canadian time the sea migrated farther eastward and covered the entire region. The Bliss Sandstone at the base of the lower and middle Canadian sequence is inferred to have been deposited 72 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS primarily in a beach environment. Most of the overlying carbonates in the region are interpreted to have been deposited in a limy intertidal environment, but some carbonates in the middle part of the Hitt Canyon Formation in all but the easternmost part of the region and some of the carbonates in the El Paso Limestone in extreme western New Mexico and eastern Arizona probably were deposited in a shallow subtidal environ- ment. Apparently very slight changes in sea level caused repeated alternations from intertidal to shallow subtidal conditions over much of the region during much of early and middle Canadian time. The isopachs (fig. 46) of lower and middle Canadian rocks show that, although transgression took place primarily eastward, there was a pronounced depositional thinning northward from the western tip of Texas. Although the present northern limit of lower and middle Canadian rocks is a post-Early Ordovician erosional edge, a depositional edge probably existed within about 100 miles to the north of the present erosional edge. EARLY LATE CANADIAN DEFINITION AND DISTRIBUTION As used in this report, early late Canadian time is the time during which the McKelligon Limestone was deposited. It corresponds approximately to the time of deposition of the Jefferson City Dolomite of Missouri, to the Jeffersonian Stage of Flower (1964) of New Mexico, or to the time of deposition of most of the strata that contain Early Ordovician fossil zone G of Utah (Hintze and others, 1969). In the report region rocks of early late Canadian age include the McKelligon Limestone of the El Paso Group in most of southern New Mexico and western Texas and the upper part of the upper member of the El Paso Lime- stone in southwesternmost New Mexico and south- easternmost Arizona (fig. 5). THICKNESS Rocks of early late Canadian age in the report region are thickest at the Scenic Drive section (10c. 45, fig. 1) in the Franklin Mountains in extreme western Texas, where they are 715 feet thick. They thin depositionally from there to the west, north, and northeast, and farther in those directions they are further thinned by erosion on top until they reach an erosional wedge edge (fig. 47). The thinnest completely preserved section of lower upper Canadian rocks, where they are believed to be conformably overlain by upper upper Canadian rocks, is 329 feet at San Andres Canyon (10c. 27), about 67 miles north of the Scenic Drive section. LITHOLOGY Strata of early late Canadian age in the region consist almost exclusively of limestone and dolomite that is locally cherty. Details of the lithologic character and sedimentary structures of these strata are given under the descriptions of the McKelligon Limestone in the eastern part of the study region and the upper member of the El Paso Limestone in the central part. In general, the strata are similar throughout the report region, but sedimentary structures and limestone textures interpreted to be indica- tive of deposition in subtidal waters are more abundant in 112° 111° 110° 109° 108° 107° 106° 105° I I l r r-— I l l l l‘ i l l I ‘ l t \ / pf v . ! -___ . ‘‘‘‘ 7.-__. l 33“~ r-..— . . l; l— + o ‘ U a A ll 1 -_ L___.. _____ .1 ° | Area where base onlaos Precambrian i . | 32._ .. 11 'L‘ . NEW MEXIC‘IO . . ‘ 6. . umnwm. l... J 0‘ 06 l . Amp; 1 |_ 5—; XAS 1’ e——————I" * t I §°eiw ! ' "25132.81 __ ' \\ ' I 6,, l ‘ 2E 9%“ng F MEXICO \ \\\ -i l . SE ’Ié'. ‘ «,0 Pennsylvanian \\L . I A l ' E 3 ; erosional‘ \ __I_ _L < m / . edge l — —————————— __ z '. . EXPLANATION \ l- .+ 1 31°— A Surface locality or drill-hole samples examined in this study 1+. 0 50 100 MILES + Surface data reported in literature \ ‘l 50 100 KILOMETRES ' D’m'hme data DepositionaH?) \\ ' I 1 ~20()« Isopach — Showing thickness, in feet ‘ edge \Wé/ FIGURE 45.—Distribution, thickness, and facies of rocks of Franconian and Trempealeauan ages. TIME-STRATIGRAPHIC UNITS 73 112° 111° 110° 109° 108° | \V‘ l l r //——_ 1 l \\ P“ /' f-—-— JJ . \J 7) edge I l . \ ,/ W ——___. \\ . l ‘ + _ 7 330‘ 4- \\ «/ ’i— . . . i V a I + 0’9 Thlnned by erosuon on top 1‘ Lone l" * * ' ° 4? l __ . __ -— . . i éritetaceous QO(__ "“‘————4—-———»_—J $0.559" _j:~ . F“‘F-——f ‘\ ‘30‘ l x l “2 ~~‘ ‘20. + 0 \~ 32° ii '3 ‘ ’3‘ v + , 3' z _|____ 1) + 9; I . o ,100 ’65 Hitt o o 0’ / Canyo __{' ‘ * E. I <1 5:. a" Z ‘ r—————— o , z ' ‘ \(l‘lle'E J A ‘3 Q g A:/ -'-\\ E\p\ —J l _ . p: 3 ’l \ I 6DYIO‘KKTJE‘s‘ ‘ ' ‘ ‘ :5? i <§ ’ “ 0 _____ _i________co J’ EXPLANATION . Surface locality or drill-hole samples examined in this study 310, t Surface data reported in literature ' Drill-hole data 0 50 100 Ml ES . ‘- 70500—301950h — SSlLowing thicknetss, in feet I .1 d - --- on our —— owmg percen age 0 quar 2 SI t an 0 5° 10° K'LOMETRES‘ sand in lower and middle Canadian rocks L l l | l l FIGURE 46.—Distribution, thickness, and facies of rocks of early and middle Canadian age. East of dashed-line A—A’ the entire Bliss Sandstone is presumed to be of this age. the south-central part of the area and in the lower part of the sequence. Conversely, sedimentary structures that are interpreted to be indicative of deposition in an intertidal environment greatly dominate in the vicinity of Beach Mountain and Agua Chiquita Canyon (locs. 48 and l) to the southeast and northeast and in some extreme western localities. CONDITIONS OF DEPOSITION All the rocks of early late Canadian age in the report region are interpreted to have been deposited in intertidal or shallow subtidal environments of a sea that had trans- gressed across the region during Cambrian and earlier Ordovician time. Limy subtidal conditions probably pre- dominated in the south-central. part of the region in earli- est late Canadian time, but as time progressed the sea apparently began a slow intermittent southward regres- sion, and limy intertidal conditions become of increasing importance. Intertidal conditions were probably predom- inant throughout early late Canadian time in areas to the west, north, and east of the south-central part of the region. During this entire time the margins of the sea probably were near the north edge of the report region not far beyond the present erosional wedge edge of the McKel- ligon Limestone and equivalents (fig. 47). Lithofacies and thickness trends indicate that deeper marine waters prob- ably lay to the south of the report region. LATE LATE CANADIAN DEFINITION AND DISTRIBUTION Late late Canadian time as used here refers to the time of deposition of the Padre Formation of the El Paso Group (fig. 5). It thus corresponds closely to the Cassinnian Stage of Flower (1964) and to the time of deposition of the strata that contain Early Ordovician fossil zones H through J of Utah (Hintze and others, 1969). Rocks of late late Canadian age (the Padre Formation) in the report region are restricted to a part of southern New Mexico and western Texas as shown in figure 48. Their distribution is limited to the west, north, and east by a Middle Ordovician erosional edge. THICKNESS AND LITHOLOGY The thickness and lithology of the Padre Formation, and thus of upper upper Canadian rocks, were described earlier in this report under a discussion of the eastern part of the study region. CONDITIONS OF DEPOSITION Strata of late late Canadian age in the report region were deposited on or near the margins of the same sea in which lower and middle Canadian rocks were deposited. The crossbedded sandy saccharoidal dolomites at the base of the Padre are interpreted to represent beach deposition fol- lowing a slight regression of the sea during late middle or early late Canadian time. This regression was followed by slight northward transgression and a return to the type of limy intertidal environment that apparently dominated middle Canadian time. EARLY MIDDLE 0RDOVICIAN DEFINITION AND DISTRIBUTION As used here, early Middle Ordovician time was all the 74 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS time between the end of deposition of the Padre Forma- tion of the El Paso Group and the beginning of depos- ition of the Second Value Dolomite of the Montoya Group. Rocks of early Middle Ordovician age occur only in the extreme southeast corner of the report region in and near the Baylor Mountains (10c. 47, fig. 1). The rocks are unnamed but have been correlated by Jones (1953) with the Simpson Group of Oklahoma. DESCRIPTION The unnamed rocks of early Middle Ordovician age in and near the Baylor Mountains (10c. 47) were not studied in detail during this investigation but have been described by King (1965). The strata are not more than 80 feet thick. According to King (1965), The strata include two layers of medium-grainded brown calcareous sandstone, between which are beds of shaly or silty limestone and marl, some of which are green. Sedimentary analyses of the sandstone indicate a textural resemblance to the St. Peter Sandstone of the northern interior region (Howe, 1959, p. 2289—2291) "‘ * "‘ GEOLOGIC HISTORY On the basis of the character and distribution of the rocks of early Middle Ordovician age at the southeast edge of the report region, of the age and nature of Lower Ordo- vician rocks beneath the early Middle Ordovician uncon- formity over most of the region, and of the character of upper Middle Ordovician rocks overlying the unconfor- mity, reasonable speculations on early Middle Ordo- vician history can be made. The entire region was elevated slightly above sea level at about the beginning of early Middle Ordovician time, and the resulting land surface sloped gently southward to southeastward. The Lower Ordovician rocks on that surface were then eroded slightly, with erosion being somewhat greater in the north and west than in the south and east. The products of ero- sion were carried southeastward to a sea which was trans- gressing toward the report area from the southeast and which reached the southeast corner of the region to leave marginal marine sediments there. Probable minor local upwarping in parts of south- western New Mexico and possibly in western Texas dur- ing early Middle Ordovician time allowed the local ero- sional exposure of Precambrian rocks. Evidence for this is not direct but is deduced from the nature of rocks in the lower part of the Montoya Group. LATE MIDDLE ORDOVICIAN DEFINITION As used herein, late Middle Ordovician time coincides with the time of deposition of the Second Value Dolomite of the Montoya Group (fig. 5). DISTRIBUTION, THICKNESS, AND LITHOLOGY The distribution, thickness, and lithology of rocks of late Middle Ordovician age are described in detail under the description of the Second Value Dolomite and are graphically summarized in figure 30. CONDITIONS OF DEPOSITION The lithologic characteristics, sedimentary structures. and fossil types found in the Second Value Dolomite in the region all suggest that these strata of late Middle Ordo- vician age were deposited in a rather agitated subtidal ma- 112" 111° 110° 109° 108° 107° 106“ 'i \f I ‘[ T r—— l t l I l l 1 ‘ \ \ l ' ’ I _._l | _ \ l‘L‘V/ [ ‘ 13.x: 1 r r c__ ,N . ___L___ _______ cal. . I \ / f + . . _____ 7 l ————— J \ < i ‘ l i \ l ‘ a ‘ \ /"l . 33 ‘t/ i K\ . , \ I ‘1 I \ —.— ‘ __ l Cretaceous erosional edge ___1 _________ —4 ( __ 1‘ ___________ L l l , ‘ 32°— 1 Pre—Middle Devonian erosional edge e c ,.-_ —- l ‘29: L ' ,______.___f . g: \:Scenic \\ I ’ E 3 / , Drive \\ Pal I < m \ \\L\ _]I_ ‘ z / \ \_____ UNITED STATES [ _ _______________ _______ "J MEXICO EXPLANATION A Surface locality or drill—hole samples 31° ’ examined in this study 0 50 100 MILES + Surface data reported in literature 0 50 100 KILOMETRES ' DUN—how data I l l ~400-~ lsopach — Showing thickness, in feet FIGURE 47.—Distribution and thickness of rocks of early late Canadian age. TIME-STRATIGRAPHIC UNITS 75 rine environment. The thickness and distribution pattern of the Second Value Dolomite and broader regional con- siderations suggest that the sea in which the Second Value sediments were deposited transgressed rapidly across the region from the south or southeast. Inasmuch as the pres- ent distributional limits of the Second Value seem every- where to be erosional wedge edges, it is assumed that the margins of the sea were somewhat farther west and north than the present limits of the Second Value. Knowledge of the distribution, thickness variations, and nature of the Cable Canyon Sandstone Member at the base of the Second Value Dolomite and observations of the basal contact of the Second Value and the rocks beneath it allow the generalization that the land area that was inun- dated in late Middle Ordovician time was a nearly feature— less plain interrupted by broad slightly elevated areas on which Precambrian rocks may have been exposed. One of these broad slightly elevated terrains was located in the vicinity of the present boundary of Grant and Sierra Counties in southwestern New Mexico (figs. 1, 30). There the Second Value Dolomite is relatively thin and its terri- genous sand content is particularly high. The nature of the sand suggests that much of it was derived from Pre- cambrian rocks rather than from older Paleozoic rocks. A smaller high area is indicated near the southwestern part of Luna County, N. Mex. (figs. 1, 30), and sand percen- tages, but not thicknesses, suggest the existence of a Pre- cambrian source area near the boundary between Hudspeth and Culberson Counties, Tex. (figs, 1, 30). EARLY LATE ORDOVICIAN DEFINITION Early Late Ordovician as used in this report is the time of deposition of the Aleman Formation of the Montoya Group (fig. 5). As so defined, its beginning and ending are imprecise inasmuch as both the base and the top of the Ale- man are conformable and probably vary somewhat in age across the region. DISTRIBUTION, THICKNESS, AND LITHOLOGY The distribution, thickness, and lithology of rocks of early Late Ordovician age are described in detail under the description of the Aleman Formation, and the distribu- tion and regional variations in thickness are shown graph- ically in figure 36. CONDITIONS OF DEPOSITION The lithology, fossil types, and sedimentary structures of the Aleman Formation considered together suggest deposition in well-aerated but quiet, warm, shallow, sub- tidal marine waters. The lithologic change across the con- formable contact between the Aleman and the underlying Second Value Dolomite is interpreted to represent pri- marily a decrease in agitation of the waters at about the beginning of Late Ordovician time, possibly due to a slight deepening of the waters and a greater distance to the shoreline that must have existed to the north and west of the present erosional wedge edges of the Aleman (fig. 36). LATE LATE ORDOVICIAN DEFINITION Late Late Ordovician time as used here is synonymous 112° 111° 110° 109° 108° 107° 106° 105° l \\ l I r /r—-‘— l l 1 1 ll l l \/\ | ' g | __I l _ H‘¥// i ‘ t i I" ~—‘ r\2 I ___J_ __________ ' l -r _+_‘_| I \ 1/ l i— f ‘ ————— _] I ————— l \ l l ‘ ‘ T ”‘1 ‘ \ I l _ t .f,_ 33°- \ ,/ T l I r l \ 4 _.: , \ . I l ‘1 ' ‘ l \ — i ' —- l ‘2 L l ——————————— —J ( -—'—'—'1. ye ___________ L ' 1 l l I, \ a? t 32 i O 211 L NEW_M_E_X_I_QQ_ 1—— 0 7-1" " EXAS I ‘21 g r 246 ‘ _____ ' o m K .335 l \ l— “l E 5 ‘-\ 382 ~\ l 01 3 \ | o \\ _ , . a E, 1 \L\ | z \ I x _______ _L _____ U_N_I_TED STATES EXPLANATION \- MEXICO A Surface locality or drill-hole samples examined in this \ 31°, study where interval is absent \\ o 50 100 MILES + Surface locality reported in literature where interval is absent\\ , . . \ I 50 100 KILOMETRES 0 Drill hole where interval is absent \\ ’ ‘70 Surface locality examined in this study — Showing ‘\ \ thickness of interval, in feet \\ \ *246 Surface locality reported in literature — Showing thickness \\ of interval, in feet FIGURE 48.—Distribution and thickness of rocks of late late Canadian age. 76 with the time of deposition of the Cutter Dolomite of the Montoya Group (fig. 5). Because the basal contact of the Cutter with the Aleman Formation is conformable and may be older toward the west, the beginning of late Late Ordovician time as used here is very loosely defined. DISTRIBUTION, THICKNESS, AND LITHOLOGY The distribution, thickness, and lithology of rocks of late Late Ordovician age in the region are described in detail under the description of the Cutter Dolomite. Figure 39 shows the distribution and approximate preserved thickness graphically. CONDITIONS OF DEPOSITION On the basis of the conformable nature of the contact between the Cutter Dolomite and underlying Aleman Formation, and on the basis of the lithologic similarity of the two formations, except in abundance of chert, it is assumed that the depositional environment in the region changed very little from early Late Ordovician to late Late Ordovician time. The region during late Late Ordovician time is thus believed to have been the site of a warm, shallow, quiet, but aerated, sea that had transgressed northward or northwestward across the region in late Middle Ordovician time. The much greater abundance of chert in the Aleman Formation as compared with the Cutter Dolomite may be due to some possibly subtle change in diagenetic conditions rather than in the condi- tions of deposition from early to late Late Ordovician time. OIL AND GAS POSSIBILITIES The following general comments on the oil and gas possibilities of Cambrian and Ordovician rocks in the report region are not intended to be a thorough evalua- tion of the subject; oil and gas explorationists must, of necessity, make their own evaluations. POSSIBLE SOURCE BEDS Marine shales, which presumably offer the maximum potential for petroleum generation, are sparse in the Cam— brian and Ordovician rocks of the region except in a part of the distributionarea of the lower member of the Abrigo Formation. Marine carbonates, however, can serve as petroleum source beds and are widely distributed in the Cambrian and Ordovician sequences of the region in the Abrigo Formation and in the El Paso and Montoya Groups. Sandstones such as those in the Bolsa Quartzite, the Coronado and Bliss Sandstones, and the Cable Canyon Sandstone Member of the Second Value Dolomite prob- ably have very low potential as source beds. Sandy carbo— nates, such as are found in various parts of the Cambrian and Ordovician sequences throughout the region, pre- sumably are intermediate between carbonates and sand- stones as potential source rocks. Accepting the conclusion of Cordell (1972) that significant oil generation and flush migration can take place only in rocks that have been buried to depths of at least several thousand feet, I think CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS 112° 111° 110° 109° l v‘\ l f PkPACHF/"‘I l MARICOPA \ GILA r—Lv/f CATRON 'r—‘—-— _/_,r\) l a O i \ ,/ l -l <2 —__ . \ L . z zx \ I LL} 0‘fl 33., \ /‘T i E E: _____ FINAL (/1 i GRAHAM K\ o «a: ‘ i A \fi Z GRANT . \\ “‘1 e "1 s L _____ S. "I I ! 32" ll L LGO 31° 50 100 MILES 50 100 KlLOMETRES I I l FIGURE 49,—Relative favorability of Cambrian rocks in the region as source beds for petroleum. G, area in which potential is good; F, area in which potential is fair; P, area in which potential is poor; A, area in which Cambrian rocks are either missing or lacking in significant potential. that the Cambrian and Ordovician rocks of the report region have all been buried deeply enough by later Paleozoic and Mesozoic rocks to have been capable of generating significant amounts of petroleum. However, none of the units was very deeply buried before being elevated and subjected to subaerial erosion, and, as sug- gested by Gibson (1965), oil that was indigenous may have escaped during the period of erosion and reduced rock pressure. On the basis of the above facts and assumptions, subjective judgments can be made as to which rocks in which areas offer the greatest potential as petroleum source beds. Cambrian rocks in the region seem to offer the greatest potential as source beds in southwestern Cochise County, Ariz., where the lower member of the Abrigo Formation contains a relatively large percentage of marine shale and where the middle member and Copper Queen Member of the Abrigo are fairly pure marine carbonate (fig. 49). The potential of Cambrian rocks presumably decreases to the west, north, and east of southwestern Cochise County owing either to increased sandiness of the rocks or to absence by erosion of favorable beds or both. Cambrian rocks offer virtually no potential as source beds in western Texas, New Mexico, and Arizona north of lat 33° N. Probably the best potential petroleum source beds among Lower Ordovician rocks in the region are the fairly pure carbonates of the McKelligon Limestone in extreme 011. AND GAS POSSIBILITIES 77 western Texas and south-central New Mexico. Moder— ately pure carbonates also occur in the Hitt Canyon and Padre Formations and in the El Paso Limestone of the New Mexico-Arizona border area, but these formations also contain sandy and silty carbonates that presumably are less favorable. Figure 50 is a subjectively drawn map outlining areas of varying degrees of petroleum source-bed potential in Lower Ordovician rocks in the region based primarily on the total thickness of moderately pure carbo- nates in the sequence. ‘ The Upper Ordovician carbonates preserved in the Montoya Group are presumed to be potential petroleum source rocks. The greatest potential would seem to be in areas where the Montoya Group is thickest (fig. 29) and where the smallest proportion of the group is made up of terrigenous sandstone (fig. 30). The thickest sequence and the smallest proportion of terrigenous sandstone coincide in southwestern Otero County, N. Mex., and bordering areas. POSSIBLE RESERVOIR ROCKS In order for a rock unit to serve as a petroleum reservoir, at least four conditions must be met: (1) the potential reser- voir must have direct avenue to petroleum source rock or be its own source rock; (2) it must have sufficient porosity to hold significant quantities of oil or gas; (3) it must be sufficiently permeable to yield oil to a well; and (4) it must be confined by a trapping surface which will prevent any contained oil or gas from escaping naturally. In evalua- ting the reservoir possibilities of Cambrian and Ordovi- cian rock units in the report region, emphasis in this sec- tion is placed on rocks that lie above potential source rocks (as subjectively appraised in the preceding section) on the assumption that petroleum migration is generally upward. (The possibility of downward migration from post-Ordovician rocks is briefly reviewed in the section on trap surfaces.) Accurate appraisal of the porosity characteristics of all the Cambrian and Ordovician rocks that underlie the region cannot be made on the basis of porosity determinations on fewer than 200 weathered rock samples collected at the surface, but some generalizations can be made. No permeability tests were made, but with the knowledge that permeability is directly related to effec- tive porosity and pore size, some generalizations can also be made on relative perImeabilities. The subject of reser- voir trap surfaces is discussed briefly in the next section. The Bolsa Quartzite, Coronado Sandstone, and Bliss Sandstone can all be presumed to have a very low probability as reservoir rocks. In addition to the fact that most rock in all three formations is low in porosity, none of the three formations overlies potential source beds. As noted in the preceding section, Cambrian rocks offer a negligible potential as petroleum source beds in Arizona north of lat 33° N. Regardless of porosity characteristics, therefore, the Abrigo Formation of northern areas can be virtually eliminated as containing petroleum reservoirs. Toward the south, however, the lower and middle mem- bers and the Copper Queen Member of the Abrigo do offer some possibility as petroleum source beds. Rocks in the lower member of the Abrigo in the south appear to be very low in porosity, but at least some rock in the middle mem- 112° 111° 110° 109° 108° 107° 106° 105° . _ l r——— l i i V\ ,1 KPACHE/ SOCORRO MARICOPA \ ’ ...:s , '—-—___J\ GILA r\) CATRON , _ I \ l/‘f ’ ' -‘—~J \\ L; 33°E \\ /_/i PlNAL VI i iGRAHAM A R I Z O N Ai _ _______________ —4 '1‘ _______ g A ..... 32o? PIMA I l | . r ----- a, \ ‘\\ I_‘J - I \.L SANTA CRUZ . \\_9NITE21§_TAT_ES ' MEXICO 31°?- 50 100 MILES 50 100 KILOMETRES I l I l J_ 1 FIGURE 50.——Relative favorability of Lower Ordovician rocks in the region as source beds for petroleum. G, area in which potential is good; F, area in which potential is fair; P, area in which potential is poor; A, area in which Lower Ordovician rocks are absent. 78 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS ber, the upper sandy member, and the Copper Queen Member has moderate porosity. The three upper members of the Abrigo of southern areas, therefore, all have at least a modest chance of containing petroleum reservoirs if suit- able trap conditions exist. Because the rocks of the El Paso Group and the El Paso Limestone have at least some potential as source beds wherever they occur, they must be considered also for their possibility as reservoir rocks. Although local porosity may be present in any of these rocks, our studies suggest that the highest effective porosities are found in various rock types in the upper part of the Hitt Canyon Formation and in the basal part of the Padre Formation. The latter overlies the greatest thickness of potential petroleum source rocks so, even though it is more restricted in areal occurrence (com— pare figs. 46, 48), it is here regarded as the most favorable stratigraphic zone for petroleum reservoirs in Lower Ordovician rocks of the region. Of course, other reservoir possibilities exist in the weathered and brecciated zone beneath the unconformity at the top of the El Paso Group regardless of the stratigraphic position of the unconfor- mity within the group. The Montoya Group, like the El Paso Group and equivalents, contains potential petroleum source beds wherever it occurs in the region. However, our studies do not indicate the existence of any widespread permeable horizons in the Montoya Group. The Upham Member of the Second Value Dolomite is locally relatively high in vuggy or moldic porosity, but permeabilities are probably low. In general, the probability of the Montoya Group containing productive oil or gas reservoir rocks is moderate at best. Perhaps the greatest possibility in the group is the weathered and fractured rock just beneath the unconformity at its top. POSSIBLE TRAP SURFACES A detailed appraisal of possible trap surfaces within the Cambrian and Ordovician rocks of the report region is beyond the scope of this report. Suffice it to say that al- though impervious shales, which form the best trap rocks, are virtually nonexistent in the sequence, relatively imper- meable carbonate beds occur that might serve to trap oil present in some of the more permeable beds. For example, the relatively permeable upper part of the Hitt Canyon Formation is overlain in much of south-central New Mexico and western Texas by the relatively impermeable McKelligon Limestone, and the relatively permeable beds of the basal part of the Padre Formation are overlain by much less permeable beds higher in the formation. With regard to trap surfaces, it may be particularly sig- nificant that most commercial occurrences of oil in Ordo- vician rocks in easternmost New Mexico and western Texas east of the present report region are just beneath un— conformities (Gibson, 1965). In many of these reservoirs, shales correlated with the Simpson Group of Oklahoma directly overlie reservoirs at the top of rocks correlated with the El Paso Group; in others, Devonian shales over- lie reservoirs in either Lower or Upper Ordovician rocks. Jones and Smith (1965) believed that most oil in these -112.“ 111° 110° 109° 108° 107° 106° 105° FT \\ . l Tl IXPACH ' .~ l l l‘ ‘ l MARICOPA \ '° 1' SOCORRO r-—‘ LINCOLN l {._~__ GILA ' Devonian absent . \ ____ _____ 1' \ 1"] “t \ . 33° \\ ,/ T ‘74-’— FINAL V A RI 2 o N A 02:22:" _ _______________ a Little or no shale in Devonlian l 32” PIMA i 1‘ l COCHISE : . _____ ' : n \\ F —i ' .' :9 \\ [__J l . . l cc \\LSANTA CRUZ . ; 1‘ m \\ ________ I_J_NiTED ST ‘3 1 a l 191157166" -. “UDSPET” \ 2 \ 31° \\ i o 50 :00 MILES \\ II \ 1 50 100 KILOMETRES l l FIGURE 51.—Generali7ed map showing rock units that underlie dark shale-bearing rocks of Devonian age. SELECTED MEASURED SECTIONS 79 reservoirs migrated downward from younger rocks. Rocks correlated with the Simpson Group are not present in the report region west of the easternmost edge, but Devonian shale-bearing rocks overlie Cambrian, Ordovician, or Silurian rocks over a large part of the region. Figure 51 is a generalized map showing the rock units that uncon- formably underlie Devonian shale-bearing rocks in the New Mexico and Arizona parts of the region; note that the Devonian sequence contains little or no shale in much of Arizona. The Devonian shales, where present, besides being possible source rocks for downward migrating oil, might form an excellent trap surface for oil accumulated in porous weathered zones beneath the unconformity. On the bases of known oil occurrences east of the region, porosity determinations made during this study, and observations on the distribution of solution breccias beneath unconformities in the report region, specula- tions can be made that the most favorable areas for oil accumulations beneath Devonian shales in the report region would be where such shales unconformably over- lie Lower Ordovician rocks—that is, in northeastern Cochise County or southeastern Graham County, Ariz. CONTROL POINTS A total of 105 control points was used in preparing the various isopach and facies maps appearing in this report; of these, 88 are surface localities and 17 are drill holes. We visited 68 of the surface localities during our fieldwork and examined samples from 3 of the drill holes in the laboratory. Table 1 (end of report) gives information on each control point, including location, principal references in the literature, the nature of our work (if any) at each locality, and a subjective appraisal of the quality and accessibility of each visited surface locality; it also gives the stratigraphic and time divisions for which each control point was used. SELECTED MEASURED SECTIONS Five important stratigraphic sections that we measured and described in the field and for which there are no ade- quate descriptions in previously published reports are re- produced on the following pages. The descriptions of 10 other sections, most of which have previously been reported, have been placed on open file (Hayes, 1975) because our measurements or descriptions differ markedly from those already published and because the sections are not shown graphically on plate 1. PASOTEX SECTION (10c. 56) [Section of Bliss Sandstone, Hilt Canyon Formation, McKelligon Limestone, and Padre Formation measured in offset segments starting at about lat 31°40’45” N. and long 105°54’35” W. in the Hueco Mountains, Hudspeth County. Tex. (fig, 1). The Hill Canyon was measured about 2,000 feet to the east of the Bliss, the McKelligon was measured about 1,500 feet to the northwest of the Bliss, and the Padre Formation was measured about 2,500 feet northeast of the Bliss. This section includes lhe type section of the Padre Formation. The geology of the area that includes the S(‘(|l()n was mapped by King, King, and Knight (1945)] Thickness (189‘) Second Value Dolomite. Padre Formation: 71. Covered. Probably dolomite like unit 69 .............................. 15 70. Dolomite, very fine grained, silty, yellowish-orange; in ledge ............................................................................... 2 69. Dolomite, very fine grained, pale-yellowish—gray to light-gray; thinly bedded; poorly exposed ........................ ll 68. Dolomite, very fine grained, silty, yellowish-orange ............ l 67. Dolomite, like unit 69 ........................................................... 27 66. Limestone, light-gray; interlaminated, skeletal and lithiclast packstone; thinly bedded; poorly exposed ......... l 65. Dolomite, like unit 69 ........................................................... 3 64. Dolomite, like unit 68 ........................................................... l 63. Dolomite, like unit 69 ............................................ 36 62. Limestone, like unit 66 ......................................................... 20 61. Dolomite, fine-grained, pale-yellowish-gray; thinly laminated in part; mostly thinly bedded; contains scattered very irregular nodules of reddish-brown- weathering chert; contains some intraformational conglomerate; top of ledgy slope ...................................... 80 60. Dolomite, fine- to medium-grained, yellowish-brown- weathering; thinly laminated; mostly in beds 6-12 in. thick; contains a lenticular chert bed as much as 6 in. thick at 33 it above base; grades upward into unit 61 ........................................................ 103 59. Dolomite, sandy and silty, yellowish-brown; very poorly exposed; in slope .................................................. 4 58. Dolomite, fine- to medium-grained; thinly laminated; in beds 6-12 in. thick; contains some intraforma— tional conglomerate 10 ft above base .............................. 13 57. Dolomite, like unit 59 ................................ 56. Dolomite, fine- to medium-grained, yellowish—brown- weathering; thinly laminated; in beds 6-12 in. thick; 1 ft of intraformational conglomerate begins 38 ft above base ..................................................................... 48 55. Dolomite, medium-grained, sandy, yellowish-brown- weathering; thickly bedded; top of unit forms a conspicuous ledge ............................................................ 14 Total thickness of Padre Formation ........................ 382 McKelligon Limestone: 54. Dolomite, fine-grained, light-olive-gray, yellowish- gray-weathering; in beds 6-12 in. thick ........................... 21 53. Dolomite, slightly silty, yellowish-brown; very poorly exposed; in slope .................................................. l2 Dolomite, thinly laminated ................................................. 25 51. Alternating ledges of crudely laminated light-gray detrital limestone that contains scattered lenses and nodules of chert and slopes of poorly exposed nodu- lar limestone; there are thin beds of brownish-gray- weathering dolomite at 17 and 122 ft above base ........... 166 50. Dolomite, yellowish-brown-weathering; laminated ........... 2 49. Limestone, light-gray, detrital; crudely laminated; thickly bedded; contains some burrowed limestone; in steep slope; alternates with slopes of poorly exposed nodular limestone ........................................ 102 48. Limestone, like unit 49 but lacks slopes of nodular limestone; contains some Calathz'um sp. at base ............ 47 47. Limestone, light-gray, detrital; crudely laminated; contains scattered endoceroid cephalopod siphuncles in middle part; in steep slope ........................................ 100 i 46. Limestone, light-medium-gray and medium-gray; irregularly laminated in part; contains mounds of 80 CAMBRIAN AND 0RDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS Thickness (In?) liglll- medium-gray skeletal lime mudstont' ('hztn- neled by medium-gray skeletal-lithochast lime wackestone to packstone; contains several beds that contain up to 5 percent reddish-brown-weathering chert nodules; in steep slope ......................................... 110 1 Total thickness of McKelligon Limestone ............. 585 1 Hitt Canyon Formation: 45. Limestone, partly sandy medium-gray and medium- dark-gray, thinly bedded; consists of alternating medium-gray skeletal lime wat'kestone and medium- (lurk-gray sandy lithiclast limt' packstone; abun- dant gastropods in basal bed ........................................... 88 44. Limestone, light-medium-gray and medium-gray; has some laminae and burrow fillings of light-brown- weathering silty limestone; contains widely scattered chert nodules and cephalopod siphuncles; skeletal lime wackestone is dominant, but lithiclast lime packstone, stromatolitic limestone and minor limestone intraformational conglomerate are present.... 110 43. Limestone, medium-light-gray; skeletal lime wacke- stone type; contains a few laminae of medium-gray lithiclast lime packstone; contains a few 1- to 2-in.- thick lenses of chert and scattered nodules of chert; contains cephalopod siphuncles 5 ft above base ............. 28 42. Limestone, medium-light-gray; skeletal lime wacke- stone type; contains some silty brown-weathering limestone as laminae and burrow fillings, some stromatolitic limestone, and some limestone intra- formational conglomerate ............................................... 37 41. Limestone, medium-lightgray and medium-gray, massive; dominantly fossil-bearing lime mudstone with digitate algal structures that contains channel fillings of lithiclast lime packstone ................................. 8 40. Limestone; interlaminated medium-light-gray lime- stone and abundant dark-brown-weathering very silty limestone that gives unit an overall dark banded appearance; contains some limestone intra- formational conglomerate and some fucoidal mark- ings; grades upward into unit 41 .................................... 38 39. Limestone, medium-light-gray; thinly laminated; contains considerable limestone intraformational con- glomerate and abundant fucoidal markings ................... 30 38. Limestone, medium-light-gray; thinly interlaminated sandy and nonsandy limestone whose sandy laminae stand out in relief and decrease upward; contains a few thin beds of limestone intraformational conglomerate .................................................................... 30 37. Sandstone, very dolomitic, slightly hematitic and glauconitic, fine-grained, well-sorted, grayish- orange-weathering; thinly laminated; contains abundant gastropods in top 1 in ..................................... 2 36. Limestone, like unit 38 ....................................................... 10 35. Dolomite, very sandy, light-brown to grayish-orange» weathering; thinly laminated; in l- to 2-in.-thick beds at base, massive above; forms basal part of cliff ................................................................................... 26 34. Conglomerate; pebbles and cobbles of sandstone like unit 33 in a matrix of gray-weathering medium— grained glauconitic, hematitic, dolomitic sand- stone; unconformity(?) at base ......................................... N) Total thickness of Hitt Canyon Formation ............. 409 Bliss Sandstone: 33. Sandstone, grayish-orange-weathering, medium~ 32. 31. 30. 29. 28. 27. 26. 25. 24. 23. 22. 21. 20. 15. l4. l3. Thickness (feel) grained, subangular, very glauconitic, slightly hematitic, dolomitic; thinly laminated; thinly bedded .............................................................................. 3 Sandstone, like unit 33, but crudely cross-laminated in beds 1-5 ft thick; forms ledge ...................................... 30 Covered. May be shaly thin-bedded dolomitic sandstone .......................................................................... 11 Sandstone, moderate-ye]lowish-brown- to grayish- orange-weathering, medium-grained, subangular, very glauconitic, slightly hematitic, slightly dolo- mitic; crudely cross-laminated; in beds 1—3 ft thick; forms ledge ....................................................................... 8 Sandstone, light-brown-weathering, medium- to coarse-grained, dolomitic; cross—laminated; mostly in thin beds; contains poorly exposed shaly partings ................................................. Dolomite, moderate-yellowish-brown-weathering, medium-grained; contains very sandy laminae in lower one-third, brown shaly sandstone in middle one—third, and edgewise conglomerate in upper one~third ........................................................................... 2 Sandstone, grayish~red, medium-grained, hematitic; very glauconitic near top; cross-laminated; thinly bedded; in slope; mostly covered in middle part ............ l7 Sandstone, grayish-red, medium-grained, hematitic, cross-laminated; in 6- to 24-in.-thick beds; in a conspicuous ledge ............................................................ 6 Sandstone, pale-red to yellowish-gray, medium- to coarse-grained; hematitic streaks increase upward; crudely laminated; displays fucoidal markings; in thick beds; alternates with subordinate similar but glauconitic sandstone in thin beds that are poorly exposed ................................................................. 59 Sandstone, pale-yellowish-gray, medium- to coarse- grained, with rounded grains; laminated; contains fucoidal markings; in rounded ledge ............... 2 Sandstone, like unit 24, but very friable and poorly exposed; in slope .............................................................. 10 Sandstone, pinkish-gray, reddish—brown-weathering, medium- to coarse-grained, subangular to sub- rounded; cross«laminated in part; in indistinct thick beds; in conspicuous rounded ledge ................................ 2O Sandstone, light-pinkish-gray, medium- to coarse- grained, subangular to subrounded; cross-laminated in part; in indistinct thick beds; grades into unit 22; in humpy slope ................................................................ 41 Sandstone, light-pinkish-gray, medium- to coarse- grained, subangular to subrounded, friable; massively bedded; iron-oxide-stained upper contact; in slope ............................................................................. 19 Sandstone, pale-red, coarse-grained, with fairly well sorted subrounded grains; cross-laminated; in beds 1-2 ft thick; fairly resistant .............................................. 4 Sandstone, pale-red, medium- to coarse-grained, with fairly well sorted subrounded grains; cross-laminated; in beds 3-12 in. thick ....................................................... 13 Mostly covered. A few inches of pale-red coarse-grained sandstone in middle ......................................................... 6 Sandstone, pale-red, coarse-grained, with fairly well sorted subrounded grains; cross-laminated; in rounded ledge ....................................... 3 Covered .................................................... 2 Sandstone, like unit 16 ........................................................ 5 Interbedded sandstone and sandy shale; sandstone is SELECTED MEASURED SECTIONS 8] Thickness (feel) coarse grained, friable, thinly bedded and displays fucoidal markings on the tops of some beds .................. 8 12. Covered ................................................................................ 5 ll. Sandstone, dar’kyellowish-gray, medium-brown- weathering, medium to very coarse grained, with poorly sorted subroundetl grains; cross—laminated; resistant ................ 2 10. Covered ................. 3 9. Sandstone, like unit 11 ........................................................ 6 8. Sandstone, brownish-gray, moderate-brown-weathering, coarse-grained, with poorly sorted subangular grains; arkosic, slightly glauconitic, slightly hematitic; thinly bedded; poorly exposed. ............................ 10 7. Sandstone, like unit 11.. 2 Sandstone, like unit 8 ....... 16 5. Sandstone, dark-yellowish-gray, moderate-brown- weathering, medium to very coarse grained, with poorly sorted subrounded grains; cross-laminated: two beds in resistant ledge..... 3 4. Sandstone, like unit 8 .................... l4 3. Sandstone, like unit 8, but well exposed... 4 2. Sandstone, like unit 8 .......................................................... ll 1. Sandstone, brownish-gray, moderate-brown-weathering, coarse-grained, with scattered granules and poorly sorted subangular grains; arkosic, slightly glauconitic, slightly hematitic; laminated; 4-ft-thick bed at base and 2-ft-thick beds above; in resistant ledge: unconformable at base .......................................... 10 Total thickness of Bliss Sandstone ........................... 373 Precambrian granite. GARDEN CANYON SECTION (Ice. 66) [Section of Bolsa Quartzite and Abrigo Formation measured in sec. 5 (projected), T. 23 5.. R. 20 E., and on ridge south of Garden Canyon in the Huachuca Mountains, Cochise County, Ariz. (fig. 1). The geology of the area that includes the section was mapped by Hayes and Raup (1968)] Thickness rise!) Martin Formation. Abrigo Formation: Upper sandy member: 43. Dolomite, very sandy, very poorly exposed; uncon- formable at, top ........................................ Total thickness of upper sandy member... Middle member: 42. Limestone, medium-dark-gray, medium-light-gray- weathering; contains laminae of yellowish-brown- weathering silty limestone that stands out in relief; in 1- to l2-in.‘thick beds except 180-210 ft above base, where it is nearly massive, and top 30 ft, where it is platy bedded; moderately resistant; mostly inter- laminated lime mudstone, algal(?) lime packstone, and skeletal lime wackestone ..................................... 41. Claystone, calcareous, platy, very poorly exposed ........ 40. Limestone, medium-dark-gray, medium-light-gray- weathering, microcrystalline; contains a few layers of limestone edgewise conglomerate; in beds 3-12 in. thick: in cliff .............................................................. 29 Total thickness of middle member .......................... 306 .. 270 Lower member: 39. Limestone, platy-bedded, argillaceous, and calcareous shale with crinkly laminae; poorly exposed ................... 11 38. 37. 36. 34. 32. 31. 30. 29. 28. 27. 26. 24. 23. 22. 21. 20. 15. 14. 13. 12. 11. 10. Thitltnesx (fee!) Covered. Float of yellow-brown-weathering fissile shale ................................................................................. 24 Like unit 39 ................................................. 47 Limestone, medium-dark-gray, medium-light-gray- weathering, microcrystalline; in x/4-in.thick laminae separated by l—mm-thick laminae of yellowish- brown-weathering silty limestone that stands out in relief... ...................................................... l3 Limestone, medium-dark-gray, medium-light-gray- weathering, microcrystalline; in 1A-in.-thick laminae chert .................................................................................. 3 Limestone, medium—dark-gray, medium-light-gray- weathering, microcrystalline; in 3- to 12-in.-thick beds that are separated by claystone partings ........ . 23 Claystone, pale-yellowish-brown-weathering; contains minor thin beds of light-gray-weathering medium- gray limestone .................................................................. 13 Limestone, like unit 36, but with thin argillaceous streaks ............................................................................... 10 Like unit 33 ......................................................................... 15 i Limestone, medium-gray, light-gray-weathering, fine-grained; contains crinkly laminae of silty limestone about ‘A~in. thick ............................................. 2 1 Like unit 33 .............. Like unit 30 .............. Like unit 33 ......................................................................... 28 Limestone, medium-gray, light-gray-weathering, fine- grained; contains irregular crinkly laminae ‘/4-in. thick of silty limestone; in beds 3.12 in. thick; about 40 percent of unit is interbeds of less resistant platy shale in 2- to 4-ft-thick intervals ..................................... 27 Shale, partly silty, medium-gray, brownvweathering, fissile; poorly exposed ...................................................... 4% Limestone; algal(?) packstone type; contains crinkly laminae; in 3- to 12»in.-thick beds ........... 3 Shale, like unit 25 ............................................................... 6 Limestone; alga1(?) packstone type; contains minor partings of shale; in 3- to l2-in.-thick beds .................... 6 Shale, partly silty, medium-gray, brown—weathering, fissile; contains very minor interbedded limestone ......... 8% Limestone, glauconitic(?), laminated, in beds 3-12 in. thick ................................................................................. 3 Shale, calcareous, partly silty, medium-gray, brown- weathering, platy; poorly exposed .................................. 12 Limestone, medium-gray; contains irregular crinkly laminae up to % in. thick of brown-weathering silty limestone; contains interbeds of calcareous shale ........... 35 Shale, calcareous, partly silty, medium-gray, brown- weathering; contains very minor interbedded lime- stone; imperfectly exposed ............................................... 6 Limestone, medium-gray; contains irregular crinkly laminae up to '/z-in. thick of brown-weathering silty limestone; in beds 3-18 in. thick; contains partings of brown-weathering medium-gray silty shale ............... 18 Like unit 17 ......................................................................... 75 Limestone, like unit 16, but without shale partings .......... 3 Shale, platy, and minor interbedded dolomite; poorly exposed ............................................................................ l4 Dolomite, very fine grained, medium-gray, dark— yellowish-brown-weathering; laminated ......................... 4 Shale, platy; poorly exposed ................................. 4 Dolomite, like unit 12 ........................................... 6 Sandstone, dolomitic, dark-yellowish-brown- weathering, fine-grained; faintly laminated; in 82 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS Thickness (I!!!) 1-in.-thick beds ................................................................. 6 8. Sandstone, siliceous, fine-grained, cross-laminated; resistant ............................................................................ 8 7. Covered ........ 2 6. Dolomite, slighily silty, very fine grained, medium- gray, dark- -yellowisli- brown- -weathering; in 1- to 4-in.-thick beds .................................. Total thickness of lower member ............................. 466 Total thickness of Abrigo Formation ...................... 775 Bolsa Quartzite: 5. Shale, platy, siliceous; contains about 10 percent interbedded siliceous sandstone like unit 4; in slope ................................................................................. 26 4. Sandstone, fine-grained, siliceous; in 6- to l2-in.-thick beds; interbedded with considerable claystone; in steep ledgy slope .......................................................... 64 3. Like unit 5 ........................................................................... 43 2. Sandstone, fine-grained, siliceous, pale-red- to pale- reddish-purple-weathering; strongly cross-laminated; contains thin interbeds of brown-weathering siliceous shale ...................................................... l. Sandstone, siliceous, pale-red- to pale-reddish-purple- weathering; largely strongly cross-laminated; grain size decreases upward from very coarse grained and granular at base to fine grained at top; mostly in beds 2-10 ft thick; contains occasional partings of shaly sandstone in upper 275 ft; entire unit in cliff; unconformable at base ............................................. Total thickness of Bolsa Quartzite .................. Precambrian granite. McKELLIGON CANYON SECTION (10c. 92) [Principal reference section of Bliss Sandstone measured at about lat 3|°50|A N. and long 106°29‘A’ W. in McKelligon Canyon in Franklin Mountains El Paso County Tex (fig 1) The geology of the area that includes the section was mapped by Richardson (1909)] Thickness (feet) Hitt Canyon Formation. Bliss Sandstone: 8. Sandstone, fine- to coarse-grained, largely hematitic, partly glauconitic, pale-brown, pale-brown- to grayish-red~weathering; grains are subrounded and poorly sorted; in beds 6 in. -2 ft thick; top is covered .............................................................................. 30 7. Sandstone, fine- grained, pale- brown; in beds less than 1 ft thick; imperfectly exposed ........................................ 35 6. Sandstone, medium- to fine-grained, hematitic, slightly glauconitic, pale-brown, grayish-red-weathering; cross-laminated in part; some beds contain Scolithus tubes; mostly in beds less than 1 ft thick; imperfectly exposed ......................................................... 44 5. Sandstone, coarse-grained to slightly granular, slightly glauconitic, grayish-orange—pink, pale» brown-weathering; in beds 2—6 in. thick; imperfectly exposed ............................................................................. 15 4. Sandstone, fine-t0 coarse-grained, slightly feldspathic, very pale red, pale-brown-weathering, cross- laminated ......................................................................... 44 3. Sandstone, medium- to fine'grained, pale-yellowish- brown, mostly in beds 3—12 in. thick; poorly exposed; Thickness (feel) may have shale partings; in slope .................................. 29 2. Sandstone, medium- to coarse-grained, slightly felds- pathic, pale-yellowish-brown, light-brown— weathering; grains are fairly well sorted and sub- rounded; in beds 2-6 in. thick with shale partings between beds .................................................................... 61 1. Sandstone, like unit 2, but poorly exposed; uncon- formable at base ............................................................... 9 Total thickness of Bliss Sandstone ........................... 267 Precambrian granite. BRANDENBURG MOUNTAIN SECTION (loc. 102) [Section of Bolsa Quartlite and Abrigo Formation measured on ridge in NMSW'A sec. 7. T. 6 W., R. 17 E., on a spur on the west side of Brandenburg Mountain in the northern Galiuto Mountains, Pinal County, Ariz. (fig. 1). The geology of the area that includes the section was mapped by Krieger (196811)] Thickness (feet) Martin Formation. Abrigo Formation: Copper Queen Member: 17. Dolomite,fine- to medium-grained, partly glauconitic; contains sandy and silty laminae; contains some intraformational chip conglomerate; poorly exposed .................................................................. . Total thickness of Copper Queen Member .............. 25 Upper sandy member: 16. Sandstone, coarse-grained, dolomitic, medium»brown- weathering; cross-laminated in 2- to 6-in. sets; thickly bedded; fairly resistant ......................................... 91 Total thickness of upper sandy member .................. 91 Middle member: 15. Sandstone, medium-grained, pale-yellowish«brown- weathering; cross-laminated in part; contains some fucoidal markings; fairly nonresistant except for hardening of weathered surfaces of some beds ................................................................................... 41 14. Sandstone, fine- to medium-grained, feldspathic, siliceous, banded very light and medium gray; in ledgy slope in which about one-third of sandstone is in ledges made resistant by hardening of weathered surfaces and two- thirds is unhardened pale- -yellowish- -brown- weathering sandstone in slopes ........................................................................... 23 13. Sandstone, like unit 14 but only about two- thirds is in ledges and one-third in slopes ................................ 143 12. Sandstone, like unit 14 ........................ Total thickness of middle member .......................... 245 Lower member: 11. Sandstone, fine-grained, silty, pale-red-weathering, friable; poorly exposed .................................................... 16 10. Sandstone, fine-grained, grayish-red-weathering; in beds of variable thickness; weakly resistant ................ 39 9. Siltstone, argillaceous, very slightly sandy, moderate-yellow-weatheririg; nonresistant, very poorly exposed; has thin interbeds of fine to very fine grained yellowish-brown- to reddish- brown-weathering sandstone ........................................... 70 8. Siltstone, like unit 9 but without sandstone interbeds ........................................................................... 87 7. Sandstone, fine-grained, slightly feldspathic, SELECTED MEASURED SECTIONS 83 Thickness (In!) light-gray- to light-brown-weathering, laminated; contains abundant fucoidal markings; mostly friable; mostly thinly bedded ........................................... 30 Total thickness of lower member ............................. 242 Total thickness of Abrigo Formation ...................... 603 Bolsa Quartzite: 6. Sandstone, fine to slightly coarse grained, siliceous, very light gray to light-yellowish-gray, cross- laminated; in 6- to 24-in.-thick beds; very resistant; grades abruptly into unit 7 .............................................. 24 5. Sandstone, medium-grained, poorly sorted, banded very light gray, reddish—brown, and grayish-orange- pink; cross-laminated in beds 3-36 in. thick; less resistant than units 4 and 6 ............................................. 43 4. Sandstone, very fine to very coarse grained, siliceous; weathers yellowish-gray but has some reddish-gray Streaks; cross-laminated; in l- to 3-ft-thick beds; resistant ............................................................................ 36 3. Sandstone, coarse grained to granular, grayish- orange-pink-weathering; very thinly bedded; poorly exposed ............................................................................... 6 2. Siltstone, sandy, hematitic, argillaceous, dark-reddish- brown; poorly exposed in part .......................................... 16 1. Conglomerate; composed of pebbles and cobbles of diabase; unconformable at base ....................................... Total thickness of Bolsa Quartzite ........................... 127 M Precambrian diabase. NANTAC RIM SECTION (loc. 150) [Section of Coronado Sandstone and El Paso Limestone measured at about lat 33°]? N. and long 109°403i’ W. east of Cold Spring Trail on escarpment of Nantac Rim, Graham County,_Ariz. (fig. 1). The geology of the area including the section was mapped by Bromfield, Eaton, Peterson, and Ratte (1972)] Thickness (feet) Morenci Shale. El Paso Limestone: Upper member: 21. Dolomite, grayish-red, yellowish-gray-weathering, fine- to medium-grained; irregularly laminated; contains many layers of intraformational chip conglomerate; contains abundant fucoidal markings on bedding planes; thinly bedded; contains about 1 ft of sandstone like unit 19 at 83 ft above base; poorly exposed, especially near top; position of upper contact approximate .......................................... 140 i 20. Dolomite, much like unit 21, but somewhat better exposed ........................................................................... 28 Total thickness of upper member .................... 168 t Lower member: 19. Dolomite, very sandy, very glauconitic, fine— grained, dark-reddish-brown-weathering, laminated; thinly bedded; imperfectly exposed ............. 6 18. Dolomite, partly sandy, glauconitic; laminated in part; contains many fucoidal markings; imperfectly exposed ........................................................................... 13 17. Dolomite, very sandy, very glauconitic, fine- to medium-grained; cross-laminated in part; contains 4-in.-thick layer of dolomite chip conglomerate .......... 2 16. Dolomite, silty, slightly glauconitic, fine- to medium-grained, grayish-red, yellowish-gray- weathering; crudely laminated in part; contains Thickness (I!!!) abundant fucoidal markings 15 ft above base; in 3- to l2—in.-thick beds; poorly exposed in part ............... 25 15. Dolomite, very silty, glauconitic, very fine grained, grayish-red, reddish-brown-weathering; contains abundant fucoidal markings; thinly bedded ................ l4. Dolomite, silty, slightly glauconitic, fine- to medium-grained, grayish-red, yellowish-gray— weathering; crudely laminated in part; contains some mud cracks; in 3- to l2-in.-thick beds; poorly exposed near top; fossil colln. USGS 7036-CO from 26 ft above base ....................................................... 49 13. Limestone, dark-gray, crystalline; thinly bedded; poorly exposed ................................................. l2. Covered ............ Total thickness of lower member .................... Total thickness of El Paso Limestone ..................... 298 t Coronado Sandstone: 11. Sandstone, medium-grained to Slightly granular, glauconitic, reddish-brown-weathering, laminated; poorly exposed; on dip Slope .......................................... 22 10. Sandstone, siliceous, medium-grained to locally gritty, pale-yellowiSh-gray to very light gray, cross-laminated; imperfectly exposed .............................. 100 9. Sandstone, arkosic, coarse-grained and gritty; cross-laminated in thin sets; resistant ............................. l3 8. Sandstone, arkosic, medium-grained; cross-laminated in part; thinly bedded; poorly exposed ........................... 13 7. Sandstone, very arkosic, coarse-grained and gritty, yellowish-brown-weathering; cross-laminated in 2— to 4-in.-thick sets; resistant .......................................... 25 6. Conglomerate, granule, arkosic, orange-brown- weathering; indistinctly bedded in 1- to 4-in.-thick beds; in knobby ledge ...................................................... 9 5. Sandstone, arkosic, siliceous, coarse-grained and gritty, reddish-brown to brownish-red; weathers to a banded reddish brown and yellowish brown; cross-laminated in part; contains some fucoidal markings; in beds up to 3 ft thick; resistant ............................................................................ 30 4. Sandstone, arkosic, siliceous, medium-grained to partly gritty, reddish-brown to brownish-red; weathers to a banded reddish brown and yellowish brown; cross-laminated in part; contains some fucoidal markings in beds 1-6 in. thick; resistant; in ledges .................... ................................. 3. Sandstone, very arkosic, coarse-grained, reddish-brown- to yellowish-brown-weathering; contains some layers of granule conglomerate; cross-laminated; in beds 3-18 in. thick; fairly resistant; top 3 ft covered ............................................................................. 46 2. Sandstone, much like unit 3, but mostly covered ............... 38 l. Conglomerate, granule, very arkosic; contains some pebbly layers; in beds 2 in. to 2 ft thick that are very indistinct except in top 12 ft; a few beds are faintly cross-laminated; contains minor inter- bedded coarse-grained arkosic sandstone in top 12 ft; mostly nonresistant; in slope interrupted by a few ledges; poorly exposed in part in basal 68 ft; unconformable at base ................................................ 193 47 Total thickness of Coronado Sandstone .................. 536 Precambrian granodiorite. 84 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS FOSSIL LISTS Fossils that were collected from Cambrian and Ordovician rocks during the present investigation and that have been retained in the collections of the US. Geological Survey are listed on the following pages; fossils that were discarded after identification are not listed. Localities are shown in figure 1. Abrigo Formation, lower member: Locality 64 (northern Swisshelm Mountains): Collection 7308-CO—collected from 249 feet above base of member and identified by M. E. Taylor (written commun., Jan. 24, 1973). Trilobite: Eldoradia sp. Other: Inarticulate brachiopod, gen. and sp. indet. Locality 69 (French Joe Canyon): Collection 7032-CO—collected from 165 feet above base of member and identified by M. E. Taylor (written com- mun., Aug. 16, 1971). Trilobite: PModocia sp. A. Other: cf. Chancelloria sp. Hyolithid, undet. Problematicum, undet. Locality 80 (Waterman Mountains): Collection 7285-CO—c011ected from 588 feet above base of member and identified by M. E. Taylor (written commun., jan. 24, 1973). Trilobite: Olenoides sp. Other: Inarticulate brachiopod, gen. and sp. indet. Collection 7035-CO—collected from 595 feet above base of member and identified by M. E. Taylor (written commun., Aug. 16, 1971). Trilobites: Alokistocare sp. Baltagnostus or Kormagnostus sp. cf. Elmthia sp. PModocia sp. indet. Ptychopariid, gen. and sp. indet. Collection 7286-CO—collected from 600 feet above base of member and identified by M. E. Taylor (written commun., Jan. 24, 1973). Trilobites: Olenaides sp. cf. Modocia sp. Bolaspidid, gen. and sp. indet. Abrigo Formation, middle member: Locality 64 (northern Swisshelm Mountains): Collection 7309~CO—c011ected from 2 feet above base of mem- ber and identified by M. E. Taylor (written commun., Jan. 24, 1973). Trilobites: cf. Bolaspidella sp. cf. Modocia sp. Ptychopariid, gen. and sp. undet. Collection 6979-CO—c011ected from 91 feet above base of member and identified by M. E. Taylor (written commun., Sept. 14, 1970). Trilobites: Cedarina aff. C. obtusans Duncan cf. Kormagnostus sp. Meteomspis sp. Undet. trilobite fragments Collection D2089—collected from 92 feet above base of mem- ber and identified by A. J. Rowell (written commun., Oct. 19, 1970). Brachiopods: Lingulella sp. Curticia sp. Locality 65 (Mount Martin): Collection 7291-CO—c011ected from above base of member) and identified by M. E. Taylor (written commun., Jan. 24, 1973) Trilobites: Arapahoia sp. cf. Bynumia sp. A PCedan'a sp. Coosia-like pygidium Kormagnostus sp. cf. Meteoraspis sp. Granulose ptychopariid, gen. and sp. indet. Other: Hyolithid, gen. and sp. indet. Collection 7292-CO—Collected from 133 feet above base of member and identified by M. E. Taylor (written com- mun., Jan. 24, 1973). Trilobites: Arapahoia sp. PMenomonia sp. ”Modocia” sp. Cedariid, gen. and sp. indet. Other: Inarticulate brachiopod, gen. and sp. indet. Collection 7293-CO—collected from 148 feet above base of member and identified by M. E. Taylor (written commun., jan. 24, 1973). Trilobites: Arapahoia sp. Bynumia sp. A Cedaria sp. Kormagnostus sp. Ptychopariid, gen. and sp. undet. Collection 7294—CO—collected from 164 feet above base of member and identified by M. E. Taylor (written commun., Jan. 24, 1973). Trilobites: Arapahoia spp. cf. Meteomspis spp. Locality 69 (French Joe Canyon): Collection 7033-CO—c011ected from 10 feet above base of member and identified by M. E. Taylor (written commun., Aug. 16, 1971). Trilobites: cf. Blountia sp. PBrassicephalus sp. PCedaria sp. Kormagnostus sp. Brachiopod: cf. Dicellomus sp. Collection 7034-CO—collected from 118 feet above base of member and identified by M. E. Taylor (written commun., Aug. 16, 1971). Trilobites: FOSSIL LISTS 85 PAnkoura sp. Arapahoia sp. Crepicephalid, indet. Kingstonia cf. K. montanensz's Lochman Kormagnostus sp. cf. Meteoraspis sp. Other: Oboloid brachiopod, indet. Echinodermal debris Locality 81 (Slate Mountains): Collection 7289-CO—collected from 18 feet above base of member and identified by M. E. Taylor (written commun., Jan. 24, 1973). Trilobites: cf. Kormagnostus sp. Cedariid, gen. and sp. indet. Other: Obolinoid brachiopod, gen. and sp. indet. Burrows Hyolithid, gen. and sp. indet. Collection 7290—CO—collected from 34 feet above base of member and identified by M. E. Taylor (written commun., Jan. 24, 1973). Trilobite: Ptychopariid, gen. and sp. indet. Other: Inarticulate brachiopod, gen. and sp. indet. Sponge spicule Collection 7287-CO—collected from 135 feet above base of member and identified by M. E. Taylor (written commun., Jan. 24, 1973). Trilobite: Arapahoia sp. Other: Obolinoid brachiopod, gen. and sp. indet. Burrows Locality 160 (Picacho de Calera): Collection 7281-CO—collected from about 265 feet above base of member and identified by M. E. Taylor (written com- mun., Jan. 24, 1973). Trilobites: Crepicephaliid, gen. and sp. indet. cf. Holcacephalus sp. ?Coosiid, gen, and sp. indet. Other: Linguloid brachiopod, gen. and sp. indet. Collection 7282-CO—collected from about 275 feet above base of member and identified by M. E. Taylor (written com- mun., Jan. 24, 1973). Trilobites: PModocia sp. Crepicephaliid, gen. and sp. indet. Granulose ptychopariid, gen. and sp. indet. Abrigo Formation, upper sandy member: Locality 160 (Picacho de Calera): Collections 7283-CO and 7284-CO—collected from about 10 feet above base of member and identified by M. E. Taylor (written commun., jan. 24, 1973). Trilobite: Aphelaspis sp. Abrigo Formation, Copper Queen Member: Locality 65 (Mount Martin): Collection 7295-CO—collected from 30 feet above base of member and identified by M. E. Taylor (written commun., Jan. 24, 1973). Trilobites: Drumaspis cf. D. maxwelli Resser Idahoia sp. Agnostid, gen. and sp. undet. Brachiopods: Billingsella sp. Inarticulate brachiopod, gen. and sp. indet. Other: Pelmatozoan debris Collection 7396-CO———collected from 35 feet above base of member and identified by M. E. Taylor (written commun., Jan. 24, 1973). Trilobites: Drumaspis cf. D. walcotti Resser Idahoia sp. Maladia sp. Ptychaspis cf. P. miniscaensis (Owen) Agnostid, gen. and sp. undet. Dikelocephalid, gen. and sp. undet. Undet. trilobite, Lecanopyge-like Brachiopods: Acrotretid, gen. and sp. undet. Linguloid, gen. and sp. indet. Other: Pelmatozoan columnals Sponge spicules (monaxon) Problematical tubular fossils, undet. Collection 7298-CO—collected from 78 feet above base of member and identified by M. E. Taylor (written commun., Jan. 24, 1973). Trilobites: Briscoia sp. PProsaukia sp. Rasettia sp. Catillicephalid, gen. and sp. undet. Ptychopariid, gen. and sp. undet. Other: Inarticulate brachiopod, gen. and sp. undet. Hyloithid, gen. and sp. indet. (??Kygmaeoceras sp.) Gastmpod, gen. and sp. undet. (high-spired form). Bliss Sandstone: Locality 20 (Werney Hill): Collection D2187—CO—collected from 30 feet above base of formation and identified by R. J. Ross, Jr. (written com- mun., May 11, 1970). Brachiopod: Eorthis sp. El Paso Limestone, lower member: Locality 57 (Portal): Collection D2247-CO—collected from 98 feet above base of member. Brachiopod identified by R. J. Ross, Jr. (written commun., Oct. 9, 1970), and mollusk by E. L. Yochelson (written commun., Dec. 6, 1971). Brachiopod: Plectotrophia sp. Mollusk: Matthevia? sp. Locality 150 (Nantac Rim): Collection 7036-CO—collected from 52 feet above base of member from unit 14 (see description of Nantac Rim mea- sured section) and identified by M. E. Taylor (written com- mun., Aug. 16, 1971). Brachiopod: Billingsella cf. B. coloradoensis (Shumard) 86 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS Other: Hyolithid, undet. Hitt Canyon Formation: Locality 1 (Agua Chiquita Canyon): Collection] D2274-CO—collected from 73 feet above base of formation and identified by L. A. Wilson (written commun., Apr. 22, 1971). Conodonts: Drepanodus suberectus (Branson and Mehl) Oz'xtodus forceps Lindstrom O. parallelus Pander Drepanodus sp. Oneotodus variabilis Lindstrom Scolopodus sp. Paltodus sp. Collection 6987-CO—collected from 195 feet above base of formation and identified by J. W. Huddle and J. J. Kohut (written commun., Sept. 28, 1970). Conodonts: Drepanodus parallelus (Branson and Mehl) D. sculponea Lindstrom D. suberectus (Branson and Mehl) D. sp. Oistodus sp. Paltodus sp. Scolopodus quadruplicatus Branson and Mehl Locality 21 (West Lone Mountain): Collection D2256-CO—collected from 122 feet above base of formation. Brachiopod and trilobites identified by R. J. Ross, Jr. (written commun., Jan. 13, 1971), and conodonts by L. A. Wilson (written commun., Mar. 17, 1971). Brachiopod: Nanorthis sp. Trilobites: Hystricurus sp. Indeterminate scraps Conodonts: Distacodus? simplex Furnish Drepanodus homocuruatus Lindstrom D. subereclus (Branson and Mehl) Scolopodus cumutiformz's (Branson and Mehl) S. linealus (Furnish) Scandodus furnishi Lindstrom S. pipa Lindstrom Oistodus lanceolatus Pander 0. sp. Paltodus variabilis Furnish Collection D2257-CO—collected from 190 feet above base of formation. Mollusks identified by E. L. Yochelson (written commun., Jan. 28, 1971), and conodonts by L. A. Wilson (written commun., Mar. 1, 1971). Mollusks: Indet. genus like Ophileta or Lecanospira Indet. bellerophontacean Conodonts: Oislodus sp. Acontiodus sp. Scolopodus cornutiformz's Branson and Mehl S. quadruplicatus Branson and Mehl S. sp. Paltodus sp. Locality 24 (Mescal Canyon): Collection l)2204—CO—collected from 73 feet above base of formation and identified by R. J. Ross, Jr. (written com- mun., July 22, 1970). Trilobites: Kainella sp. Leiastegium sp. Collection D2205-CO—c011ected from 117 feet above base of formation and identified by L. A. Wilson (written commun., Dec. 2, 1970). Conodonts: Oistodus inaequalis Pander O. parallelus Pander O. abundans Branson and Mehl Oneotodus sp. Drepanodus homocurvatus Lindstrom D. lineatus Furnish Acodus oneotensis Furnish Paltodus cf. P. inconstans Lindstrom Collection D2209-CO—c011ected from 375 feet above base of formation. Trilobites and brachiopod identified by R. J. Ross, Jr. (written commun., July 22, 1970, and Dec. 2, 1970), echinoderm material by James Sprinkle (written commun., Dec. 2, 1970), and conodonts by L. A. Wilson (written com- mun., Dec. 2, 1970). Trilobites: Asaphellus cf. A. riojanm Harrington and Leanza Hystricurinid Brachiopod: Small camerellid(?), fragmentary Echinoderm material: One rliombiferan cystoid(?) plate with rhombs Two large fold plates of unknown origin Several different types of possible crinoid radial plates One disklike columnal Conodonts: Drepanodus homocumatus Lindstrom Oistodus inaequalis Pander Drepanodus suberectus (Branson and Mehl) Locality 38 (San Lorenzo): Collection D2190-CO—collected from about 65 feet above base of formation and identified by L. A. Wilson (written com- mun., July 22, 1970). Conodonts: Acontiodus staufferi Furnish A. sp. Drepanodus sp. Oistodus parallelus Pander Scolopodus sp. Cordylodus sp. Collection D2191-CO—collected from about 80 feet above base of formation. Trilobite identified by R. J. Ross, Jr. (written commun., July 22, 1970), and conodonts by J. W. Huddle (written commun., Sept. 25, 1970). Trilobite: Bellefontia sp. Conodonts: Acanthodus cf. A. uncinatus Furnish A. sp. Acontiodus aff. A. propinquus Furnish A. aff. A. staufferi Furnish A. aff. A. sulcatus Furnish Cordylodus sp. Drepanodus parallelus Branson and Mehl D. suberectus (Branson and Mehl) Oistodus sp. Paltodus sp. Scolopodus cf. S. cornutiformz's Branson and Mehl S. sp. FOSSIL LISTS Collection D2192-CO—collected from about 250 feet above base of formation. Trilobites identified by R. J. Ross, Jr. (written commun., July 22, 1970), conodonts by L. A. Wilson (written commun., Dec. 2, 1970), and echinoderm material by James Sprinkle (written commun., Dec. 2, 1970). Trilobites: Leiostegium sp. (probably new) Aulacoparia? huygenae of Flower (probably same as Asaphellus riojanus of Ross, 1970) Hystricurid, unidentified Bathyurid pygidium Conodonts: Drepanodus suberectus (Branson and Mehl) D. homocuruatus Lindstrom D. sp. Oistodus forceps Lindstrom Echinoderm material: Two highly silicified cylindrical stem segments(?) Two possible oblong columnals(?) showing a possible “twist” in the orientation of the two faces like col- umnals in the Carboniferous Platycrinus - crinoid? Seventeen calyx plates with exospire-type respiratory folds developed as external ridges radiating to the plate sides; the two types—one where whole plate folds into ridges and troughs and the other where the plate is thick and has slits covered over externally by a thin calcite sheet—both resemble plates found in the crinoid Palaeocrinus, in some paracrinoids, and in the eocrinoid(?) Macrocystella Locality 39 (Capitol Dome): Collection D2193-CO—collected from about 300 feet above base of formation. Trilobites and brachiopod identified by R. J. Ross, Jr. (written commun., July 22, 1970, and Dec. 2, 1970), and conodonts by J. W. Huddle (written commun., Sept. 25, 1970). Trilobites: Asaphellus cf. A. n'ojanus (See Ross, 1970, pl. 13.) Leiostegium cf. L. tyboensis Ross Hystricurid (See Ross, 1970, pl. 10, figs. 32-34.) Brachiopod: Nanorthis? sp. Conodonts: Drepanodus suberectus (Branson and Mehl) Oistodus sp. Scandodus sp. McKelligon Limestone: Locality 27 (San Andres Canyon): Collection D2248-CO—collected from 87 feet above base of formation and identified by J. K. Rigby (written commun., June 3, 1970). ReceptaculitidP: Calathium fittom' Billings Locality 39 (Capitol Dome): Collection D2232-CO—collected from about 220 feet above base of formation. Conodonts identified by L. A. Wilson (written commun., Nov. 27, 1970), and other fossils by R. J. Ross, Jr. (written commun., Sept. 24, 1970, and Nov. 27, 1970). Brachiopod: Nanorthis cf. N. hamburgensis (Walcott) Conodonts: Scolopodus triplicatus Ethington and Clark S. cf. 5. alums Bradshaw Drepanodus homocuruatus Lindstrom Ulrichodina sp. 87 Oistodus forceps Lindstrom Other: Gastropods, indet. Sponges Padre Formation: Locality 24 (Mescal Canyon): Collection D2210-CO—collected from 94 feet above base of formation. Brachiopods identified by R. J. Ross, Jr., and conodonts by L. A. Wilson (written commun., Dec. 2, 1970). Brachiopods: Diparelasma sp. Hesperonomia sp. Conodonts: Drepanodus homocuruatus Lindstrom Scolopodus n. sp. of Lindstrom, 1954 S. gracilis Ethington and Clark S. variabilis Ethington and Clark 5. sp. Oistodus longiramis Lindstrom 0. inaequalis Pander 0. sp. Panderodus sp. Collection D22ll-CO—collected from 100 feet above base of formation and identified by L. A. Wilson (written commun., Dec. 2, 1970). Conodonts: Drepanodus homocumatus Lindstrom D. arcuatus Pander Scandodus sp. Paltodus sp. Locality 39 (Capitol Dome): Collection D2194-CO—collected from 27 feet above base of formation. Trilobites and brachiopod identified by R. J. Ross, Jr. (written commun., July 22, 1970, and Dec. 2, 1970), and conodonts by L. A. Wilson (written commun., Dec. 2, 1970). Trilobites: Ptyocephalus sp. Peltabellia? sp. Brachiopod: Diparelasma sp. Conodonts: Gothodus sp. Drepanodus homocurvatus Lindstrom Scolopodus quadruplicatus Branson and Mehl Scolopodus sp. Falodus prodentalus Graves and Ellison Oistodus sp. Acodus sp. Collection D2195-CO—collected from 42 feet above base of formation. Brachiopod identified by R. J. Ross, Jr., and conodonts by L. A. Wilson (written commun., Dec. 2, 1970). Brachiopod: Diparelasma sp. Conodonts: Distodus lanceolatus Pander O. parallelus Pander O. aff. O. forceps Lindstrom Scandodus pipa Lindstrom S. sp. Panderodus sp. Drepanodus arcuatus Lindstrom D. homocumatus Lindstrom Acontiodus sp. Scolopodus filosus Ethington and Clark 88 Collection D2196-CO—collected from 90 feet above base of formation and identified by R. J. Ross, Jr. (written com- mun., July 22, 1970). Trilobite; Pseudocybele sp. (indet. but more like P. lemurei Hintze than like P. nasuta Ross) Collection D2197-CO—collected from about 200 feet above base of formation and identified by R. J. Ross, Jr. (written commun., July 22, 1970). Brachiopods: Diparelasma? sp. Hesperonomia? sp. Syntrophid, perhaps Syntrophopsis Trilobites: Indet. asaphids Aleman Formation: Locality 16 (Molinas Canyon): Collection D2234-CO—collected from 7 feet above base of for- mation and identified by R. J. Ross, Jr. (written commun., Sept. 24, 1970). Brachiopod: Zygospira sp. Collection 6988-(IO—c011ected from 7 feet above base of for- mation and identified by J. W. Huddle and J. J. Kohut (written commun., Sept. 28, 1970). Conodonts: Phragmodus undatus Branson and Mehl Oulodus oregonia Branson, Mehl, and Branson Locality 93 (Sugarloaf): Collection 6908—collected from 145 feet above base of for- mation and identified by O. L. Karklins (written commun., June 28, 1970). Bryozoans: Cyphotrypa? sp. Calloporella? sp. Hallopom? sp. Cutter Dolomite: Locality 24 (Mescal Canyon): Collection D2212-CO—collected from 75 feet above base of formation. Brachiopods identified by R. J. Ross, Jr., and conodonts by L. A. Wilson (written commun., Dec. 2, 1970). Brachiopods: Paucicmra? sp. Fragments of large orthids Conodonts: Belodina ornatus (Branson and Mehl) B. wykoffensis (Stauffer) B. grandis (Stauffer) Oistodus inclinatus Branson and Mehl Trichonodella sp. Panderodus intermedius (Branson, Mehl, and Branson) P. sp. Diepanodus sp. D. homocuwatus Lindstrom Cyrtoniodus sp. Collection 6986-CO—collected from 288 feet above base of formation and identified by J. W. Huddle and J. J. Kohut (written commun., Sept. 28, 1970). Conodonts: Belodina profunda (Branson and Mehl) Drepanodus subarectus Branson and Mehl Ozarkodina tennis Branson and Mehl Oislodus sp. Panderodus gracilis (Branson and Mehl) Plectodina furcata? (Hinde) N. Genis? CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS Locality 39 (Capitol Dome): Collection D2233-CO—collected from 9 feet above base of for- mation. Brachiopods identified by R. J. Ross, Jr., and cone- donts by L. A. Wilson (written commun., Sept. 24, 1970). Brachiopods: Paucicrum sp. Hebertella sp. ( Rhynchonellis undet. Conodonts: Trichonodella sp. Ligonodina sp. Ozarkodina sp. Locality 93 (Sugarloaf): Collection 6989-CO—collected from 49 feet above base of for mation and identified by J. W. Huddle and J. J. Kohut (written commun., Sept. 28, 1970). Conodonts: Belodina profunda (Branson and Mehl) Drepanodus subereclus Branson and Mehl Ozarkodina polita (Hinde) Panderodus sp. REFERENCES CITED Albritton, C. C., Jr., and Smith, J. 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Southern Belle Member ............... 20, 68 upper sandy member .................... 23 Agua Chiquita Canyon, fossils ,,,, 40, 86 petrology ........................ 36 Aleman Formation of the Montoya Group .52, 57, 75 American Peak ............................ 23 petrology ................... 20 Amphitheater Canyon, petrology , . , . . 27 Apache Pass, fossils ........................ 50 petrology ............................. 48 Aphelaspis zone .............. . . . 24 Archaeoscyphia ............................ 40 B Basin and Range province, structural geology ............... 3 Bat Cave Formation of the El Paso Group. . . 35 Beach Mountain, petrology ........... 27, 36, 43, 71 Big Hatchet Mountains ooooooooooooooooooooo 9 fossils .................................. 45 Bighorn Dolomite ..................... 56 Billingsella ............................. 25 coloradoensis ....................... . , 50 sp ....................................... 50 Bliss Sandstone ....................... 27, 45, 50, 69 measured stratigraphic section ........... 80, 82 petroleum potential ..................... 76, 77 Bolaspidellu zone ........................... 22 Bolsa Quartzite of the Abrigo Formation . , , , 10, 66 measured stratigraphic section .......... 82, 83 petroleum potential ..................... 76, 77 Brachiopods ................ 25, 39, 45, 50, 56, 61, 63 Brandenburg Mountain, petrology ........... 18, 25 Brandenburg Mountain section .............. 82 Bryozoans .................................. 56, 61 C Caballo Mountains, mining ................. 32 petrology ............................... 36 Cable Canyon, petroleum potential .......... 76 Cable Canyon Sandstone ................... 52 Cable Canyon Sandstone Member of the Second Value Dolomite .......... 53 Calathium .................................. 40 Camaraspis sp, ............................. 33 Cambrian timestratigraphic units .......... 66 Canadian age, time-stratigraphic unit ....... 70 Capitol Dome, fossils. . i ............ 45, 87, 88 petrology ............................... 43 Cassinnian Stage, time-stratigraphic unit . . i 73 Cedaria zone ............................... 22 Cephalopods ....................... 27, 33, 42, 45, 50 Colorado Plateaus, physiography ............ 2 structural geology ....................... 4 INDEX [Page numbers of major references are in italic] Page Conodonts .................................. 61, 63 Control points ........... 79 Cooks Range, petrology . . . . 59 Copper Queen Member of the Abrigo Formation ............. 25, 50, 69 measured stratigraphic section ,,,,,,,,,, 82 petroleum potential .......... . i , 77 Corals .................................... 5,6 61,63 Coronado Quartzite ......................... 45 Coronado Sandstone ........... 23, 45, 68, 69 measured stratigraphic section ,,,,,,,,,, 83 petroleum potential .............. 76, 77 Crepicephalus zone ,,,,,,,,,,,,,,,,,,,,,,,,, 24 Cretaceous rocks ............................ 66 Cutter Dolomite of the Montoya Group , . 52, 61 , 76 Cutter Formation ........................... 52 D Demingian Stage ........................... 70 Devonian rocks, Martin Formation .......... 65 Diaphelasma ............................... 39 Diparelasma ................................ 45 Dos Cabezas, fossils ........................ 50 Dresbachian age, time—stratigraphic unit . . , . 67 E Early Canadian age, time-stratigraphic unit ............................. 70 Early late Canadian age, timestratigraphic unit ............................. 72 Early Late Ordovician age, timestrati- graphic unit .................... 75 Early Middle Ordovician age, time-strati- graphic unit .................... 75 El Paso Group ............ 35 McKelligon Limestone 72 petroleum potential ..................... 76 El Paso Limestone ,,,,,,,,,,,,,,,,,,,, 25, 26, 47, 69 measured stratigraphic section ,,,,,,,,,, 83 petroleum potential ..................... 77 Eorthis sp ................................... 33 F Faulting .................................... 4 Florida Mountains .............. . , 9, 43 fossils .................................. 45 Florida Mountains Formation versus Padre Formation ................ 43 Folding ............................... 4 Fossils listed ,,,,,,,,,,,,,,,,,,,,,,,,,, 84 Fra Cristobal Range, petrology .......... i 27 Franconian age, timestratigraphic unit ..... 69 Franklin Mountains ........................ 9 fossils .................................. 45 petrology ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 43 French Joe Canyon, fossils .................. 84 petrology ............................... 23 Fucoids ..................... 11, 16, 32, 37, 48, 67, 71 Fusselman Dolomite ........................ 63 G Galiuro Mountains, petrology ............... 14 Garden Canyon section ..................... 81 Page Gas and oil potential ........................ 76 Gasconadian Stage ......................... 70 Gastropods ................................. 45, 50 Giruanella ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 40 H Harding Sandstone ......................... 53 Hesperanomia .............................. 45 Hitt Canyon, fossils ......................... 45 Hitt Canyon Formation of the El Paso Group .................. 35, 51 measured stratigraphic section . . 80 petroleum potential ............... 77 Huachuca Mountains ................. 11 petrology ............................... 14 Hueco Mountains, petrology ................. 36 J, L Jefferson City Dolomite ..................... 72 Jefferson City Formation ................... 42 Jeffersonian Stage, time-stratigraphic unit. ............................. 72 Johnny Lyon Hills, petrology ............... 11 Late late Canadian age, time-strati- graphic unit .................... 73 Late Late Ordovician age, timestrati- graphic unit ,,,,,,,,,,,,,,,,,,,, 75 Late Middle Ordovician age, time-strati- graphic units ................... 74 Little Dragoon Mountains, petrology ........ 18 Longfellow Limestone ....................... 51 M McKelligon Canyon section ................. 82 McKelligon Limestone, El Paso Group ..... 39, 51, 72 measured stratigraphic section .......... 79 petroleum potential ,,,,,,,,,,,,,,,,,,,,, 76 Martin Formation ........................... 24, 65 Mattheuia .................................. 5O Measured stratigraphic sections ............. 79 Mescal Canyon, fossils ................ 45, 86,87, 88 petrology ............................... Mexican Highland, physiography ,,,,,,,,,,, 22 structural geology ,,,,,,,,,,,,,,,,,,,,,,, 4 Middle Cambrian, time-stratigraphic units ............................ 66 Middle Canadian age, time-stratigraphic units 70 Mimbres Limestone, El Paso Group ,,,,,,,,, 35 Molinas Canyon, fossils ..................... 88 Mollusks ................................. 56, 61, 63 Montoya Formation, , i , ,,,,,,,,,,,,,,,,, 52 Montoya Group ...................... 52 Aleman Formation ...................... 75 petroleum potential ..................... 76, 78 Second Value Dolomite ............ 51, 53, 74, 78 Montoya Limestone of the El Paso Group i i i 35 Montoya Limestone of the Montoya Group . . 52 Morenci, fossils ............................. 50 Mount Martin, fossils ........ . i 25, 84, 85 petrology ................ , . . , 23 Mule Mountains, petrology ,,,,,,,,,,,,,,,,,, 14, 18 93 94 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS Page N, 0 Nantac Rim, fossils ....................... 85 petrology ............................. 46 Nantac Rim section . , . .................... 83 Oil and gas potential ...................... 76 Ordovician time-stratigraphic units ......... 73 Ordovician unconformity ................... 51 P Padre Formation of the El Paso Group ...... 42 measured stratigraphic section .......... 79 petroleum potential ..................... 77 Par Value Member of the Montoya Limestone ...................... 52 Pasotex section ............................. 79 Patagonia Mountains, petrology ............. 14 Pelmatozoa ................................ 56 Peloncillo Mountains ................. 9 Pennsylvanian rocks .................. 65 Permian rocks ........................ 65 Petroleum potential ....................... 65, 66, 76 Physiography ............................... 2 Picacho de Calera, fossils ......... . 85 Plectotrophia sp ............................. 50 Preacher Mountain, fossils .................. 50 petrology ..................... 48 Ptychaspis zone ................... . . 25 Pulchrilamina .............................. 40 Page R, S Raven Member of the Montoya Limestone . . . 52 Sacramento section, physiography .......... 2 structural geology .................. 4 San Andres Canyon, fossils . r 87 San Lorenzo, fossils ............ 86 San Mateo Mountains, petrology ‘ . . 27 Santa Catalina Mountains, petrology . 14 Sauk sequence .............................. 9 Scenic Drive Formation versus Padre Formation ...................... 42 Scolithus tubes ..................... 11, 16, 32, 46, 67 Second Value Dolomite of the Montoya Group ........................ 51, 53, 74 petroleum potential ..................... 78 Second Value Member of the Montoya Limestone ...................... 52 Shandon Quartzite .......................... 27 Silurian rocks, Fusselman Dolomite 63 Sierra Cuchillo ..................... r . , . 59 Sierrite Limestone of the El Paso Group ..... 35 Slate Mountains, fossils rrrrrrrrrrrrrr r r 85 petrology .................. 14 Sonoran Desert, physiography r 2 structural geology .............. 4 Southern Belle Member of the Abrigo Formation ...................... 20, 68 Stratigraphic sections, measured ............ 79 Structural geology .......................... 3 Page Sugarloaf, fossils ........................... 88 Superior area, petrology. . . 14 Swisshelm Mountains, fossils ............... 84 petrology ............................... 18 Symphysurina zone ......................... 39 T, U, V, W ’I‘imestratigraphic units ................... 66 Tombstone area, petrology .................. 14 Trempealeauan age, time-stratigraphic unit ............................. 69 Trilobites ............ 16, 22, 24, 25, 33, 39, 45, 56, 61 Upharn Dolomite ............................ 52 Upham Dolomite Member of the Second Value Dolomite ................. 54 petroleum potential ..................... 77 Unconformity, Middle Ordovician ........... 51 Valmont Dolomite .......................... 52 Vekol Mountains ............................ 9 petrology ............................... 11, 14 Waterman Mountains, fossils ............... 84 Wemey Hill, fossils .................. 85 West Lone Mountain, fossils .......... 86 TABLE 1 96 Locality: Coordinates: (ZAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS TABLE 1.—Data on control points used in compilation of thickness and fairs maps than by geographic coordinates. Stratigraphic units: Dolomite Member; 7c, Aleman Formation. Ages: A, Middle Cambrian; B, Dresbachian (of Howell and others, 1944); Number followed by w indicates drill hole. Drill—hole locations are given by land coordinates rather l, Bolsa Quartzite; 2, Abrigo Formation; 2c, upper sandy member; 2d, Copper Queen Member; 3, Coronado Sandstone; 4, Bliss Sandstone; 5, El Paso Group (or Limestone); 6, corrulatives of Simpson Group; 7, Montoya Group; 7a, Cable Canyon Sandstone Member; 7b, Upham C, Franconian and Trempealeauan (of Howell and others, 1944); D, early and middle Canadian; E, early late Canadian; F, late late Canadian; G, early Middle Ordovician; H, late Middle Ordovician; 1, early Late Ordovician; J,, late Late Ordovician. Land status: References: Remarks: G, accessible local, State, or federal government lands; R, restricted government lands; 1, Indian lands; P, private property. NMBMMR, New Mexico Bureau of Mines and Mineral Resources; PBSL, Permian Basin Sample Library. Spld., section sampled during this investigation; temeas., section remeasured and described during this investigation; meas., section measured for first time during this investigation; spls. ex., drill—hole samples examined; mod. th., previous descriptions used but thickness modified by measurenents made during this investigation; illus., Sauk sequence rocks of this locality are graphically illus— trated on pl. 1; desc., written description of section is given in text; 0F, written description of section is in an open—file report (Hayes, 1975); mod. strat., locality visited and original strati— Quality or accessibility: E, excellent; G, good; F, fair; P, poor. graphic assignment changed during this investigation. U >\ to u 2 a z 2 >‘ . m ‘H U a & Locality name County Lat N. Long W. H m Ages 5‘ E g References Remarks fig gig ABC DEF A-F GHIJ H-J "a g E 3 a a a 3 NEW MEXICO l Agua Chiquita Canyon —————— 32°41'20" 105°52'30" 4,5 ——- XX— X -——— — G F G Spld. 2 Alamo Canyon-~ 32°51' 105°51'45" 7 —————— — —XXX X G P G 00. 3 Lead Canyon- 32°48'45" 105°56' 7 ------ — -XXX X - — - 4 Bishop Cap—— 32°11'15' 106°33'30" 7 —————— - —XXX X E E R Remeas. 5 Webb Gap ------------------ 32°05' 106°32'45" 7 ------ - -XXX X - - - 6 Jose 32°34'30" 107°44'20" 7 ------ — -XXX X - - - -—--d0--- 7 Johnson Park Canyon ——————— Socorro———— 33°29' 106°27'40" 4,5,7 -—— X-- X —XXX X G G R Bachman (1965, 1968)—— Spld. 8 Fairview Mountain ————————— Sierra 33°28‘ 106°30'45" 4,5,7 —-- X—— X —X—— X — - — Bachman (1965)— 9 Capitol Peak ————— -—-do— 33°25'15" 106°25'30” 4,5,7 -—— X—- X -XX— X G F R —-——do ------ —— Remeas., illus. 10 Rhodes Canyon— ———do- 33°16'30" 106°37'15" 4,5,7 -—- X—— X —XXX X G G R Kottlowski and others Spld. 11 Hembrillo Canyon ------ Dona Ana——- 32°56'30" 106°36'30" 4,5,7 ——— XX- X —XXX X - — — 12 Ash Canyon ------ - _--do ------ 32°37'30" 106°32' 4,5,7 --- XXX X —XXX X — - — l3 Mud Springs Mountains- Sierra ————— 33°09'15" 107°18'15" 4,5,7 -—X XX- X -XXX X G F G Kelley and Silver Remeas., illus. (1952). 14 South Ridge -------- 33°01'30" 107°14' 4,5,7 --X -—- X —XXX X - - — —-—-do-- 15 Cable Canyon ——————— 32°55'45" 107°l4' 4,5,7 ——X XX- X —XXX X E G G -——-do -------- Remeas. part, illus. 16 Molinas Canyon ——————— 32°47'45" 107°13'40” 4,5,7 -—X ——— - —XXX X F P G ——--do ———————————————— Remeas. part. 17 Cooks Range (north)-— 32°35'15" 107°44'20" 4,5,7 —-X XX— X —XXX X F F P Jicha (1954) —————————— Remeas., 0F. 18 Little San Nicholas Dona Ana-—- 32°34'15" 106°30'40" 4,5,7a, ——- XXX X -XX— — E G R Bachman and Myers Remeas. part, Canyon. 7b, 7c (1969). illus. 19 Lake Valley—— Sierra ————— 32°44'30" 107°35‘15" 7 —————— - —XXX X F G P Jicha (1954)—— — Remeas. 20 Werney Hil]-- 32°26'45" 108°15'20” 4 --X --- — —--- - F F P Bellman (1960)- - Do. 21 Lone Mountain 32°42'45" 108°12' 4,5 --X XX— X ———— - E G P Pratt (1967)-——— Spld. 22 Lone Mountain 32°42' 108°ll' 7 —————— — -XXX X E G P ———-do ---------- Do. 23 Preacher Mountain- 32“07' 108°59' 3,5 -XX XX- X ——-- — F G G Gillerman (1958) Do. 24 Mescal Canyon 31°40' 108°23'15" 4,5,7 ——X XXX X -XXX X G G G Zeller (1965)——— Remeas., illus. 25 Ram Gorge--—- 31°4l' 108°23'15" 4 --X -—— - —-—- - - - - ---—do ---------------- 26 Bear Mountain ———————— 32°49'45" 108°21'45" 7 ------ — —XXX X G G G Pratt and Jones (1961) Spld. 27 San Andres Canyon ————————— 32°44'30" 106"33'4S" 4,5,7 ——— XXX X —XXX X E G R Bachman and Myers Remeas.,OF. (1963). 28 Sierra Oscura (south) ————— 33°32' 106°19'45" 4,5,7 ——— X—— X —XX- X R Bachman (1968)—- Remeas., illus. 29 Sierra Oscura (middle— 33°33' l06°20'15" 4,5,7 --- X-- X —X-- X — - - ————do- south). 30 Sierra Oscura (middle)—--- 33°35' 106°21' 4,5 --- X-- X ---- - - - - -—--d0- 31 Sierra Oscura (middle- 33°36'30" 106°21'40" 4,5 --— X—— X ———— — — - — --——do— north). 32 Sierra Oscura (north) ----- 33°38' 106°22'15" 4,5 --- X—— X ——-- - - - - ----d0 ---------------- 33 Mockingbird Gap (south) 33°34'20" 106°25'45" 4,5 --— X-- X - - — - -—-—do- 34 Mockingbird Gap (north) - 33°35'30" 106°25'45" 4 —-— X-- X - - - — ----do- 35 Eaton Ranch ——————————————— 33°37'15" 107°16' 4,5,7 ~-X X—— X F G G Kelley and Furlow Remeas., illus. (1965). 36 Amphitheater Canyon ——————— Sierra ————— 33°22'30" 107“06‘45" 4,5 ——— X-- X ---- — F P P Kelley and Silver Remeas., illus. (1952). 37 Fra Cristobal Range —-—do ------ 33°24'45" 107°06'45" 4 ——— X-— X --—— - - - - -—-—do ---------------- (north). 38 San Lorenzo ——————————————— Grant —————— 32°48'15" 107°S7' 4,5,7a, —-X XX— X —X—— — F E P Jones and others Remeas. part. 7b (1967). 39 Capitol Dome ———————— Luna ——————— 32°08'40" 107°39' 4,5,7 ——X XXX X —XXX X G G G Lochman—Balk (l958)--- Remeas., illus. 40 Cedar Mountain Range ———do —————— 32°02' 108°09'15" 7a,7b, —————— — —XX- — F F G Bromfield and Wrucke Meas. 7c (1961). 41 Winston ——————————————————— Sierra ————— 33°21' 107°36'20" 4,5,7 ——X X—~ X —XXX X G F C Johns (1955) —————————— Mostly remcas., OF. 42 Robledo Mountain—-—— Dona Ana——— 32°27'15" 106°55' 7 —————— — —XXX X F F G Kottlowski (1960a)—~—- Remeas. part 43 South Percha Creek—- Sierra ————— 32°53'45" 107°43'10" 4,5,7 ——X X—— X -XXX X P U G Kuellner (1954) ——————— Spld. DATA ON CONTROL POINTS 97 TABLE lr—Data on tontrol points used in. compilation of thickness and facies maps—Continued (J >\ f: '2 32' an 3‘ Q 3 3 5,, m u .o “3 7‘; “:1. Locality name County Lat N. Long w. .503 Ages 3‘ '3 6", References Remarks V - "-4 (1} § E 13 ABC DEF A—F GHIJ H—J j; 3 g u :l u N m 0’ < J TEXAS 44 Hitt Canyon --------------- El Paso---- 31°59' 106°30' 4,5,7 —-X XXX X ~XXX X E F P Pray (1958), Harbour Spld. (1972). 55 Scenic Drive--- —--do ------ 31°47' 106°29' 5 --- -XX — —-—- - G E G Cloud and Barnes Do (1946); others. 46 Long Canyon --------------- Hudspoth—-- 31°46'30" 105°59' 7 ------ — —XXX X G P Howe (1959) ----------- Do 47 Baylor Mountains (north)-- Culberson—- 31°14'30" 104°45'15" 6,7 —————— - XXX- X - - - King (1965) ----------- 48 Beach Mountain ------------ ——-do ------ 31°09' 104°51' 4,5,7 --- XXX X —XXX X G E P Cloud and Barnes 00‘ (1946); King (1965). 50 Point of Mountains -------- -—-do —————— 31°28'20" 104°51'45" 7 ------ — —XX- X G F P King (1965) Do. 52 Baylor Mountains (south)-- ---do —————— 31°11'40" 104°A6' 6,7 ------ — XXX— X G G P ——-—-do ----- Mod. th. 56 Pasotex ------------------- Hudspeth--- 31°41' 105°5h' 4,5 -—X XXX X ---- - F G P This report—-- Meas. ARIZONA 57 Portal -------------------- Cochise-——- 31°56'30" 109°09' 3,5 XXX XX— X --—— - F P Sabins (1957) --------- Meas. 3, remeas. unit 5, illus. 58 Blue Mountain ------------- -—-do —————— 32°06'15" 109°12' 3,5 --- XX- - F G G —- Remeas. 59 Apache Pass ————————— -——do ------ 32°10' 109°26'45" 3,5 XXX X-- - F G G - Do 60 Dos Cabezas ------ 32°11'20" 109°39' 3,5 XXX X—- - G G P - Spld 61 Pedregosa Mountains ---do ------ 31°33'45" 109°24'45" 1,2,5 ——X —~— — P P G — - -- Remeas 63 Leslie Pass ——————————————— -—-do —————— 31°34'30" 109°30'30" 2c,2d, ——X X-- - -——- — P G P Epis and Gilbert Spld 5 (1957). 64 Northern Swisshelm -——do —————— 31°43' 109°33'30" 1,2,5 XXX X—- X -——— — G P P Epis and Gilbert Do Mountains. (1957); Hayes (1972). 65 Mount Martin -------------- ---do —————— 31°26'15" 109°56'15" 1,2 XXX ——— X —-—— - F P P Hayes and Landis Do (1965). 66 Garden Canyon ------------- ——-do ------ 31°27'45" 110°20'15" 1,2 XX— -—- X ---— - F F R This report ------------ Meas., desc. 67 Northern Whetstone Pima ------- 31°52'45" 110°27'15" 2 —X- ——— - ———— - F P G Tyrrell (1957) ————————— Spld. Mountains. 69 French Joe Canyon—-- Cochise-—-- 31°47'45" 110°23'20" 1,2 XX- ——- X —-—- - G F G Creasey (1967b)-— - Remeas. 70 Tombstone ------- - --—do —————— 31°40'15" 110°04'40" 1,2 XX- -—- X ---- - F F G Gilluly (1956) — DO 71 Dragoon Mountains - ---do- 31°59'45" 109°56'15" 1,2 XX- ~—— X -——- - P P G --—-do ----------------- Spld. 72 Rattlesnake Ridge-—— ---do ------ 32°12'20" 110°12' 2 -XX --— - -——- - — - - Cooper and Silver (1964). 74 Johnny Lyon Hills——— ---do ------ 32°06'40" 110°13'15" 2 -XX --— — ———— — - — — ----do-—— - 75 Johnson Peak -------- —--do ------ 32°06'45" 110°06' 1,2 XXX --- ——-- - E P ----do-— Remeas. part, illus. 76 Imperial Mountain-—— Graham ----- 32°56'30" 110°20'15" 3 X ----- X — P P P Simons (1964) —————————— Remeas., 0F. 77 NUBSEE Canyon—--- 32°31'40" 110°43'15" 1,2 XXX ——- X — E F G Creasey (l967a)-—-— Remeas., illus. 80 Waterman Mountains—- 32°21'15" 111°28'20" 1,2 X ----- X ——-— — P G G McClymonds (1959a) ----- Remeas. part., illus. 81 Slate Mountains ----------- —--do ------ 32°35'15" 111°5/4' 1,2 XX- —-— X --—-— — E E I McClymonds (1959b) ————— Remeas. part, 0F. 82 Vekol Mountains—- -—-do ------ 32°35'15" 112°06'45" 1,2 X ----- X --—- — G P I —-~-do ————————————————— Kemeas., illus. 83 American Peak-—-- Santa Cruz- 31°25'45" 110°42'30" 1,2 XX— ——- X —--- — P E G F. S. Simons (written Spld. commun. , 1973). 8/4 Morenci ------------------- Greenlee—-— 33°04'30" 109°18' 3,5,7 —XX X—- X -X—— X G E P Lindgren (1905) ———————— Remeas. part, illus. TEXAS 88 Anthonys Nose--—— El Paso---— 31°57' 106°29'45" 4,5,7 --X XXX X -XXX X - - - Harbour (1972) --------- NEW MEXICO 89 North Anthonys Nose ——————— Dona Ana--— 32"02'30" 106°31'30" 7 ------ — -XXX X - - — Harbour (1972) --------- 90 Victorio Mountains -------- Luna ------- 32°11' 108°05' 7 —————— — -XXX X P G Kottlowski, in Griswold Spld. (1961). 91 Boston Hill --------------- Grant ------ 32°45'h5" 108°17'30" 7 ------ - -XXX X F G P Entwistle (1944) ------- Do. TEXAS 92 McKelligon Canyon --------- El Paso---- 31°50'30" 106°29'30" 4 --X ——- - -——- — E E G Meas., desc. 93 Sugarloaf ----------------- —--do —————— 31°49'55" 106°28' 7 ------ — -XXX X F G G Meas. , 0F. 98 CAMBRIAN AND ORDOVICIAN ROCKS OF ARIZONA, NEW MEXICO, AND TEXAS TABLE l.—Data on control points used in compilation of thickness and facies maps—Continued A .3 3 -—< a .,. v, 3 . m a 3 I: :0 Locality name County Lat N. Long w. 1:0 :1: Ages >\ a 3 References Remarks ‘3: 3 3 ABC DEF A-F GHIJ H-J :3. g "3 ° 2 5 .4 w 'u .4 ,_, g U G "’ o 53 3 NEW MEXICO 94W Humble Oil and Refining Eddy ------- (sec. 23, T. 23 8., 4,5,6, --— XX- X XXXX - - — NMBMMR sample library—— Spls. ex. Huapache 2. R. 22 E.) 7. 98 Northern Animas Mountains- Hidalgo—-—- 31°S3' 108°43'30" 4,5,7 ------ X —--- P F G J. M. Soule (written Spld. commun., 1973). ARIZONA 102 Brandenburg Mountain —————— Pinal ------ 32°55'20" 110°39' 1,2 XXX —-— X —--- G F I This report ------------ Meas., desc. NEW MEXICO 105W Turner State 1 ———————————— Otero —————— (sec. 36, T. 25 5., 4,5 --- XX— X ———— — — - NMBMMR sample library-- Spls. ex. R. 16 E.) 108W Plymouth Federal 1 -------- ---do ------ (sec. 15, T. 20 5., 7 ------ — --—- — - - PBSL log --------------- R 9 E ) 110“ Standard Oil of Texas —--do ------ (sec. 18, T. 21 5., 4,5,7 —-- X-— X —XXX - — ~ NMBMl'fll sample library—— Spls. ex. Scarp (Blaize) 1. R. 18 E.). 111" Magnolia Petroleum Co. Chaves ----- (sec. 31, T. 17 5., 4,5,7 —————— X --—— - — — Roswell Geol. Soc. Black Hills 1. R. 21 E.). (1952). 112w Standard Oil of Texas Eddy ------- (sec. 16, T. 21 S , 4,5,7 ------ X —--- — - — Roswell Geol. Soc. 1 State E6584. R. 22 E ) (1957). 113” Southern Prod. Co. et a1., 0tero —————— (sec. 5, T 17 8., 4,5,7 ------ X ——-- — - - Roswell Geol.,_Soc. Cloudcroft 1 (Swank). R. 12 E.). (unpub.). 114W Southern Prod. Co. Eddy ------- (sec. 24, T. 18 5., 4,5,7 ——- --— X --—— - — - --—-do-—- - Elliott l. R. 23 E.). 116w Sun Oil Co. Victorio 1--—- Sierra ----- (sec. 25, T. 10 5., 4,5,7 ------ X ——-- - - - Kottlowski (1963) ------ R. l W.). 117W Sunray-Midcontinent Oil ---do ------ (sec. 23, T. 15 S , 7 —————— — --—- — - - —---do ----------------- Co. Federal l—M. R. 2 W ) 119W Turner Everett 1 —————————— Otero —————— (sec. 34, T. 22 S , 7 ------ — ---- — - - ----do ----------------- R. 10 E.). 120W Turner Evans 1--—— ---do —————— (sec. 22, T. 24 S., 7 ------ _ --—- — - - —---do ----------------- R. 12 E.). TEXAS 121w Magnolia Oil Co. l-39881—- Hudspeth——— (sec. 36, Blk. 70 7 ------ - ---— - — - Kottlowski (1963) ------ Univ. Lands). 122w California Co. University- -——do ------ (sec. 24, Elk. E, 4,5,7 ------ X ---— - - - ----do ----------------- Theissen 1" Univ. Lands). 123" Jones Sorley 1 ------------ El Paso——-- (sec. 17, Blk. 5, 4,5 —————— X -——— - — — —---do ----------------- PSL Survey). 129w General Crude Merrill and Hudspeth—-— (sec. 8, Blk. 69, 4,5 —————— X -—-- - — - PBSL 10g ——————————————— Voyles 1. T-2). 135w Gulf Oil Corp. I-J --—do-—---- (see. 14, Blk. 19, 4,5,7 ------ X -——— - - - USGS records ——————————— Burner-State "B." PSL Survey). ARIZONA 150 Nantac Rim ---------------- Graham ————— 33°13' 109°40'45" 3,5 XXX X—- X --—- G F 1 Spld.,desc. 151 Final ------ 33°10'30" 110°31'20" 3 X-- —-— X ---— G E I This report-- Meas. 154 Superior ------------------ —--do ------ 33°18'30" 111°06' 2 XX- ——- X —--- F G P Peterson (1969)-——- _- Spld., mod. strat. 158 Eagle Peak ———————————————— Cochise-——- 32°09'45" 110°25'15" 1,2 —XX —-- - ---— P P i} F. W. Plut (written Spld. commun., 1973). 160 Picacho de Calera --------- Pima ------- 32°21'20" 111°11'45" 1,2 —-- --— ---- P G P Bryant (l952)--— Do. 162 Barlow Pass ——————————————— Graham ————— 33°16'30" 109"45'30" 4 ——X ——- ——-— F F I This report-—--- Meas. fiUS. GOVERNMENT PRINTING OFFICE: 1975—677-340/15 v .5? hr _. UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY PROFESSIONAL PAPER 873 PLATE 1 82 VEKOL MOUNTAINS 80 WATERMAN 7 MOUNTAINS7 W It I11 - L. | r- 4—. Abrigo Formation DRESBACHIANI AGE Al MIDDLE CAMBRIAN 33"I— 32°- 31°— \ _ \k 0 0 50 50 100 K \ LI 15 / 11FI I \1 “TI 21 \ ‘ I I I LI J—i FI‘ I _ 18 B l I— ' I I 60 75 23 I I 39 DI I ,4' I NEW ME _1_ I_ l 44 TEXAS 57l — E 64 I _— _— I I 56 \ 65 24 I \ \\ 100 MILES INDEX MAP SHOWING LOCATIONS ILOMEIRES OF MEASURED SECTIONS I I I | l 39 CAPITOL DOME W l I I F l I I I I I I I I Padre Formation I I 1 B I l I I F 18 LATE LATE CANADIAIRII :2: I II LITTLE EARLY LATE CANADI I SAN NICHOLAS 21 I CANYON LONE MOUNTAIN I I ' /' ME LATE CANADIAN AGE (WEST) I I I I +Padre Formation I F / EARLY LATE , . 7 fCANADIAN AGE / I 7 McKelIigon Limestone g' I / I 7 9 I 15 o I / CABLE 13 / I g I CANYON MUD SPRINGS / / n. I // M0 .. II UNTAINS 7 I“ I / MCKelIigon Limestone M ' / EARLY LATE CANADIAN AGE II I I I ? L/T- ? MIDDLE AND EAR LY CANADIAN AGE I—IUI I I 84 jg; I I MORENCI I I I F / I71 I I I I ..I. I l I 9 EARLY LATE CANADIAN AGE I I 150 7*” ‘i' I I 1‘ <3 MIDDLE AND EARLY CANADIAN AGE 1‘ I I T . . 1 o I I NANTAC 7 I I I 7 , ‘. e I f I HIM / .I . . . 7 /~ - I: ' 17 I I I I l I I: HItt Canyon Formatlon / f - -/ LL, I I I I / l I g , , I f E 7 I ' I / 8 7 1 I B, m / f _ 1 1 A MIDDLE AND EARLY I E 7 I f > A E / HIttCanvon Formamn I I I AMPHIIIEIEATER 35 77 CANADIAN AGE 7 W” 7 g 7 {4. ? ORDOVICIAN ? ...I. 7 MIDDLE AND EARL 44 , IT: CANYON EATON UAINIEEN 102 TREMPEALEAUAN‘ 35—; cl” 3 CAMBRIAN a.) -/'II'A" TREMPEALEAUANI ANY CANADIAN AGE HITT I I I _ .. ....“ BRANDENBURG AND 7 E , 7 :5 . '. DFRANcoNIANI AGES ? CANYON {.1 I .-.- - MOUNTAIN FRANCON'AN‘ 7“,; F I -/- ~- g ;.;.-_’.-_ 7 -.I .I _‘ Copper Queen I r Member AGES ? ? .' I. 7 $142K f '—'I} i I. .~~ DRESBACHIAN1 AGE _‘I' ' _‘ .2 l 7.. . . U er sand ‘ I' member - - E A I I Bliss Sandstone Q '- W V : . L AGE R . . . / ARI-Y CANADIAN C In: . . . 39 _ .. MIDDLE AND E A 1'. ' : ' CAPITOL ./ W '. '1‘ 3 ~ - DOME ormat‘IO“ _,‘ TREMPEALEAUAN‘ AND FRANCONIANl AGES 9 MI: Pad‘eF I /' ‘ Middle member I8 II‘IA L B . . g é;g,‘° I 47‘ ./. . 5 U, W I I . s ’54 IS I I '5 474—0 T 4/94,, 475 LITTLE “T CHIANI AGE - U l L 044, g DRESBA QAMBRIAN g 1 I I l 47‘5C 40,4 SAN NICHOLAS Le MIDDLE a I I 4/1/40, ’I/ 495 I CANYON —7-81 __.7 O I F I 4/I/ . -: - . I 1 4% ' f :2 .2 . I 24 I - II I f HEMBRILLO Lower member MESCAL I I / CANYON CANYON 1e LATEE CARP l W LP‘ U“ I F McKelligon Limestone / . ‘ F EARL I I '- 'I I S / I . O I I o I 2 all“ I a I . Qua I E I f [LI I ”I I RHOIIIJES f I 90“" I EARLY LATE CANADIAN AGE I I EARLY LATE CANADIAN AGE // CANYON ,7. T“? MIDDLE AND EARLY CANADIAN AGE +1.. 2,112 MIDDLE AND EARLY CANADIAN AGE .‘ . I l I I l ./. ' _'/_'I' 0/ / ‘]"I_ III F './' / / 9 f.__ 23 I l I .7. /. CAPITOL 28 +4 - - 7 PREACHER 1' ,1 L‘Ir' 7 , , PEAK SIERRA +2.. MOUNTAIN .I1.I I I1 I OSCURA / .2,‘ i F 7 , I T . 7 I 7 (SOUTH) +. I.. / Hitt Canyon Formation I I i F 7 l / I , _ / I J I 7 . . ...+ 1 I I I I I I f / z .1” m / .._I_ I 'I'I1.'f _ . _ TI f /‘ E _{_.,. 7 H I .‘ZI‘I F 43) 8 / / «I- ~ /- 1 . . j 7 —-I — . 8 I II_ 5. I) 0683 III 7:, TI'I / IE] I ..— / I CABEZAS 1" _ ”7" l 4-7!- I I .14.... l j. / . .— . . . f _ :6:- / / . . . A 75 . .,. MIDDLE A\ID EARLY CANADIAN AGE —- » 7 7 ORDOVICIAN 77 ? . . . _ . NUGGET . . .. TREMPEALEAUANl AND ' 7- .. . '. .‘ CAMBRIAN JOHNSON .,. .. FRANCONIANI AGES '~ 7 -/- - CANYON PEAK :27.- 7 7 7‘ 9’.» .. . . . . , . _.— I :,:1.- Copper Queen Member M $733,; ,7.-. F 77:" TREMPEALEAUAN AND 56 ' '7 .' 1’.‘ . 27‘ ' I FRANCONIAN‘ AGES PASOTEX E; .' ,1 TREMPEALEAIJANl AND FRANCONIAN‘ AGES . ./. 7 7 - TREMPEALEAUAN M 2+2“:- DRESBACHIANl AGE § ' 1' '\ AND —' ‘ 48 ..-.4. 2 - ~ f I-‘-1-;‘;' Upper sandy member ' ‘I‘ .- ' FRANCONIANI E '/'_l BEACH :-1+: I I AGES 1 ’ MOUNTAIN . . . . 7 ' / ‘.'—‘-.' I1 I f . I .7\ . 7 44 +1.1 '-,;I I I I' ' I j 5 '1 'DRESBACI-IIAN1 AGE ”III 7 1 _;/' I. I CANYON :7 l f . . - . f . C Middle member . g j I I 7 Padre Formation .g I I-.' I9 ' g —/ f g I g 3 3 — f 3 y_;_.__?w$ 7 _- '-, I —/ 8 M MIDDLE CAMBRIAN g -‘T. . _// / 7 _...— to ' __ . 5 g : : ../. '/ — < 3 Z . I / LATE LATE CANADIAN AGE ' . f . _ EAR . . 1 . . / / LY LATE CANADIAN AGE ~/- ; - EXPLANATION I ~ - Lower member I I I 1 Rock symbols I I I I. ' Conglomerate I I I .'- : : . I Sandstone I g . I I 9 f Siltstone I I (3 McKelligon Limestone O I I as; f I 1 1—1 I D’ I I 57 E Limestone J I I I PORTAL I I @ Dolomite J l I I1 I l 1 l f I I I II f _ 7L- Modifying symbols I r . l EARLY LATE CANADIAN AGE I:I s d 1321' '_I.1I: MIDDLE AND EARLY CANADIAN AGE _.. any if: 34-1.- F :_ Silty I I II—‘I‘I .... METRES FEET I i I / 0 ~_ 0 El Shaly l l I I Hitt Canyon Formation _. I 64 1 I , TI . . ,1 I NORTHERN —‘ Glauconitic I I F ‘1"I—‘I’ _.. SWISSHELM 1 1 l .1‘. -"—.' g ./_ I—ioo '1‘ I I 1 1 —./.-.- MOUNTAINS g / SI Iceous I I f III—II '- T‘I’: E —. . I I I -I . I. I I l j / Calcarous I {4. It 1- g 7— — I I I I I CI.“ / Dolomitic I | IIZ'.'I.' I4 I E I I1 1 I I -/— — 300 --- 7-". _ l I l 100 _ F Fossils noted in text I f BI‘55 Sandstone I I T f f Diagnostic fossils observed in field f / I— 400 or reported in literature 65 ORDOVICIAN MOUNT CAMBRIAN —- Formation boundary MARTIN 5 _' MIDDLE AND EARLY I F Copper Queen Member 00 —— Member boundary ORDOVICIAN ?_ ::_? CANADIAN AGE II F CAMBRIAN TREMPEALEAUANl AND GAEIISJEN I I f 7 —7+— Approximate time-stratigraphic boundary FRANCONIANI AGES D CANYON T::, I A TE: AII ' ' . 33 .. 1 Upper sandy member 200 NO ages are shown In capital latters .I TI” ‘Of Howell and others (1944). AMERICAN I III I . PEAK I l I ‘i' . .- +I.. --I-- :r.. I . . . III 11—? 4—: F IIFI +1.. I F I I F m I I _L f Middle member I II 8 ,7. ---l— I_I.1 I I; _. I _I l 1 TE: 4—1”I II- I F III-’7‘- 1 I44 C LI- 23+} "’ ' _3: 7 DRESBACHIAN _I_I_ g AGE I___ %: '9: :—:3: MIDDLE .—:—:—. g CAMBRIAN ———?:: f T; 5 I 1 I LI" 5 1 1 _ 5 I I 1 u. "T— - . 0 . FI“ I I47 8, 'I'Il— L; 1I1I " ‘: ‘I_II Lower member .1.. I __"__ _Q I _.|.. . 1 L I—I—I < 7 ' I I I ' 4_.. ___. _ 1 —_—:- . I I —_—_- . . 1 - _ _ I I . I I I :-:—: . 4:: 2-:- ‘ ~ 112° 111° 110° 109 108° 107° 106° 105° 7—1—— I I l I -4 T \,\ I I < B I I B’ I I ‘ I l \ .I"\ Z R I 35 CT :—:—: I“ 7 / 8 Q .J. 28 r r\ l H T T 6 _ :—:—: ' ..-g \ 150' fi': . 36 L- ‘ I 'Z J SELECTED GRAPHIC SECTIONS OF CAMBRIAN AND LOWER ORDOVICIAN ROCKS IN SOUTHERN ARIZONA AND NEW MEXICO AND WESTERN TEXAS filnterior—Geologieal Survey, Reston, Va.—1975 fiUS. GOVERNMENT PRINTING OFFICE: I975—677-340/l5 ‘ m. “and.” North American Species of Tempskya and Their Stratigraphic Significance By SIDNEY R. ASH and CHARLES B. READ With a section on STRATIGRAPHY AND AGE OF THE TEMPSKYA-BEARING ROCKS OF SOUTHERN HIDALGO COUNTY, NEW MEXICO By ROBERT A. ZELLER, JR. GEOLOGICAL SURVEY PROFESSIONAL A description of two new species of the Early Cretaceous tree fern Tempskya and a discussion of the characters and distribution of the other North American species UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Ash, Sidney R 1928— North American species of Tempskya and their stratigraphic significance. (Geological Survey professional paper ; 874) Bibliography: p. Includes index. Supt. of Docs. no.: I 19.16:874 1. Tempskya. 2. Paleobotany—Cretaceous. 3. Paleobotany—New Mexico—Hidalgo Co. 4. Paleobotany— North America. 5. Geology, Stratigraphic—Cretaceous. I. Read, Charles Brian, 1907— joint author. II. Title. III. Series: United States. Geological Survey. Professional paper ; 874. QE965.A83 561’.’73 75—619201 For sale by the Superintendent of Documents, US. Government Printing Office Washington DC 20402 Stock Number 024—001-0732-1 CONTENTS Page Page Abstract ________________________________________ 1 Stratigraphy and age of the Tempskya-bearing rocks Introduction _____________________________________ 1 of southern Hidalgo County, New Mexico, by Acknowledgments _________________________________ 2 Robert A. Zeller, Jr _____________________________ 16 Hlstory of investigations of Tempslcya ______________ 2 Introduction _________________________________ 16 Occurrences of Tempskya in the Unlted States _______ 6 Cretaceous strati h 17 In-place specimens ___________________________ 6 grap y """""""""""" Rocky Mountain geosyncline _______________ 6 Late Albian biota in the Mojado Formation _____ 18 Blackleaf Formation ___________________ 6 Biota in the type Moj ado Formation in Colorado Group (undifferentiated) _____ 6 Mojado Pass ___________________________ 20 Wayan Formation """""""""""""" 9 Biota in the Moj ado Formation of the Aspen Shale ----------------------- 3 Animas Mountains ______________________ 21 gfgwpgfiksgajfmgtfi‘::::::: 1‘0 Conclusions regarding the age of the biota__ 22 Cedar Mountain Formation ____________ 11 SyStematic descriptions ——————————————————————————— 22 Burro Canyon Formation _____________ 12 Tempskya reesidei Ash and Read, n. sp _________ 25 Dakota Sandstone ———————————————————— 12 Tempskya zelleri Ash and Read, n. sp ___________ 28 Mex1can geosyncllne ------------------- 13 Comparisons of the species of Tempskya _____________ 31 Mojado Formation ____________________ 13 S . 33 Atlantic Coastal Plain ____________________ 13 ynOPS‘S —————————————————————————————————— Patapsco Formation ___________________ 13 Selected references _______________________________ 36 Reworked specimens ___________________________ 14 Index ___________________________________________ 41 ILLUSTRATIONS [Plates follow indexl PLATES 1—4. Tempskya reesidez‘ Ash and Read, n. sp. 5—7. Tempskya zelleri Ash and Read, n. sp. 8. Tempskya knowltom‘ Seward. 9. Tempskya minor Read and Brown. 10. Tempskya grandis Read and Brown. 11. Tempskya wyomingensis Arnold. 12. Tempskya wesselli Arnold. 13. Tempskya superba Arnold. Page FIGURE 1. Reconstruction of Tempskya rossica by Kidston and Gwynne-Vaughn ________________________________ 4 2. Reconstruction of Tempskya by Andrews and Kern _________________________________________________ 5 3. Index map of the United States showing general location of occurrences of Tempskya spp ____________ 7 4. Correlation chart of parts of the Cretaceous system in the United States showing rock sequences where Tempskya spp. have been reported ___________________________________________________________ 8 5. Index and topographic map of part of southwestern New Mexico showing known exposures of the Mojado Formation, Tempskya localities, and the type sections of the several Cretaceous formations mentioned in this report _______________________________________________________________________________ 16 6. Stratigraphic sections of the Mojado Formation ___________________________________________________ 19 7—11. Drawings: 7. Cross section of the false trunk of Tempskya reesidei n. sp _________________________________ 27 8. Reconstruction of the basal part of the false trunk of Tempskya reesidei n. sp ________________ 28 9. Cross sections of the false trunk of Tempskya reesidei n. Sp ______________________________ 29 10. Cross section of the false trunk of Tcmpskya zcllcri n. sp __________________________________ 30 11. Diagrammatic sketches of the well-known species of Tempskya ____________________________ 34 III NORTH AMERICAN SPECIES OF TEMPSKYA AND THEIR STRATIGRAPHIC SIGNIFICANCE By SIDNEY R. ASH and CHARLES B. READ ABSTRACT Two new species of the fossil tree fern Tempskya—T. reesidei and T. zelleri—are described in this report. Both are from the Lower Cretaceous Mojado Formation of Hidalgo County, N. Mex., and are associated with marine invertebrates that date them quite adequately. T. reesidei is somewhat smaller than, but reminiscent of, T. grandis, whereas T. zellem' appears to be related to the group that is characterized by T. knowltom‘. The report also contains a history of the investigations of Tempskya and a resumé of the 43 reported occurrences of Tempskya in the United States. In-place occurrence of the genus Tempskya in the United States now total 34; all but one are from west of the Mississippi River, and most of them are in strata in the Rocky Mountain area. In addition, rocks of Late Cretaceous or Cenozoic age, particularly, contain much reworked Tempskya material, once again in the Western United States. In-place occurrences of the genus Tempskya seem to be main- ly in rocks of latest Early Cretaceous (Albian) age, as indicated by the associated invertebrate fossils. The age of a few of these localities, however, is still questioned by some observers. INTRODUCTION In 1955, Robert A. Zeller, J r., of the State Bureau of Mines and Mineral Resources Division of the New Mexico Institute of Mining and Technology, showed the junior author of the present report a peculiar specimen of obviously organic origin in an effort to obtain an approximate generic identification. The specimen was a poorly preserved cast in fine-grained sandstone. Read expressed the opinion that the specimen might be a fragment of the enigmatic fern genus Tempskya rather than a sponge which it superficially resembled. Later in 1955, the junior author accompanied Carle H. Dane, of the US Geological Survey, and Zeller to the locality in the southern part of the Big Hatchet Mountain area, Hidalgo County, N. Mex., where the fossil had been obtained. On that trip, many well-preserved speci— ments of Tempskya were collected. At first (Zeller and Read, 1956), these specimens were thought to be examples of Tempskya minor Read and Brown. Additional study, however, now indicates that the species actually differs from all others, and it is described as Tempskya zellem’ n. sp. in this report. During the 1955 field trip, Zeller pointed out that the specimens of Tempskya, were associated with many marine invertebrates. A few collections of the invertebrates were made and sent to the late John B. Reeside, J r., for study. Later in 1955, and from then until his death in July 1958, there was active cor- respondence between the authors of this report and Reeside concerning the nature of the occurrence and the age of the containing strata. In the spring of 1956, Reeside visited the localities in Hidalgo County and made additional collections of the invertebrates. As indicated elsewhere in this report, he concluded, on the basis of the marine fossils, that the contain- ing rocks are Early Cretaceous (Albian) in age. Continued correspondence between Reeside and Read led to speculation that all the American occur- rences of the genus Tempskya may be in Albian strata. Unfortunately, before Dr. Reeside’s death, it was impossible for the junior author of this report to go into the details of the many occurrences. How- ever, early in 1959, a detailed study of the new species from the Big Hatchet Mountains began, together with the assembling of data on American occurrences in general. Later in 1959, Robert A. Zeller obtained additional specimens of Tempskya in similar Cretaceous marine strata on the eastern flanks of the Animas Moun- tains, Hidalgo County, N. Mex. These specimens were determined to belong to a second new species, here to be named and described as Tempskya reesidei. In 1960, the authors of this report Visited the locality with Zeller and collected a considerable quantity of material, including additional pieces of the false trunk of the holotype and of the paratype as shown in figure 8. It was then decided to reevaluate the various species of Tempskya that are known in the United 1 2 NORTH AMERICAN SPECIES OF TEMPSKYA States, both as regards their morphologic charac- teristics and also the stratigraphic occurrences. The principal product of the study is this report, al- though a preliminary paper assessing the strati- graphic value of Tempskya in the western United States has been published (Read and Ash, 1961b). Early during this investigation it was recognized that the association of marine invertebrates with Tempskya in southwestern New Mexico was espe- cially significant. As a result, during the mid-1960’s, Zeller prepared a section for this report in which he discussed the Lower Cretaceous stratigraphy of the area and described the occurrence of invertebrates with the fern in some detail. Since then, it has become necessary to make a few changes in the sec- tion in order to bring it up to date. Unfortunately, Zeller’s unexpected death early in 1970 prevented him from seeing these changes. They, however, are so small that they do not modify the section in any essentials, and it is still very close to the version originally submitted by Zeller. ACKNOWLEDGMENTS The writers are indebted to Professor Chester A. Arnold, University of Michigan, for the loan of type slides and the gift of type material of Tempskya wesselii, T. wyomingensis, and T. superba, which have been used in this report. We are also indebted to Sergius H. Mamay and Arthur D. Watt, US Geological Survey, for the loan of thin sections and type material of T. grandis, T. knowltom‘, and T. minor, and also for locality data regarding occur- rences of Tempskya. E. Guerry Newton, U.S. Geo- logical Survey, has checked the bibliography used in this report. John D. Strobell, US. Geological Survey, made a special trip to the Carrizo'Mountains, Ariz. and N. Mex., to help the junior author collect from a locality earlier discovered by Strobell. Thanks are here expressed for the interest and encouragement given by the late Carle H. Dane, US. Geological Survey, in connection with the study. The late Esther R. Applin, US. Geological Survey, supplied helpful information on the vertical range of C7'ib7’atina texana, one of the critically diagnostic Foraminifera used in stratigraphic correlation of the Lower Cretaceous rocks in southwestern New Mexico. Mr. Takio Suski, University of California, Los Angeles, identified some of the earlier inverte- brate collections that were made by Zeller. The staff of paleontologists of the US. Geological Survey and the US. National Museum (Natural History) have made identifications of most of the collections of invertebrate fossils, and their contributions are acknowledged with thanks. They include Norman F. Sohl, who studied the gastropods; Ruth Todd, who examined the Foraminifera; the late John B. Ree- side, J r., who reviewed the entire fauna; and W. A. Cobban, who worked on the marine fauna from the Animas Mountains. Dr. Eugene Callaghan, former director of New Mexico Bureau of Mines and Mineral Resources, supported and personally encouraged the earlier efforts of Zeller in connection with the stratigraphic study of the Cretaceous rocks that led to the discov- ery of Tempskya in southwestern New Mexico. Mr. Allen M. Alper of the Corning Glass Company worked with Zeller in the mapping of the Mojado Formation in the Animas Mountains, a project that led to the discovery of one of the new species here described. The assistance of Mr. W. R. West, Carolin-a Bio- logical Supply Company, who supplied us with in- formation on the new occurrence of Tempskya in North Carolina, is appreciated. Finally, the writers wish to acknowledge particu- larly the aid and encouragement of W. W. Rubey and the late John B. Reeside, Jr. Rubey, who for many years has conducted stratigraphic and struc- tural studies in western Wyoming and adjacent parts of Idaho, is responsible for many of the discoveries of Tempskya localities and has provided much thoughtful and helpful information regarding the stratigraphic occurrences. Reeside encouraged Brown and Read during their earlier investigations of Tempskya and was both extremely interested in and actively concerned with the occurrences of Tempskya in southwestern New Mexico. HISTORY OF INVESTIGATIONS OF TEMPSKYA In 1937, the junior author and one of his asso- ciates published a historical statement regarding the genus Temps/cya. The report (Read and Brown, 1937) has been out of print for some time. In conse- quence, a summary is needed of the historical ac- count earlier published. The first written record of Tempskya is an ac- count of Endogem‘tes erosa (Stokes and Webb, 1824) in a report on the plant material collected by Mantell in Tilgate forest. The plant material was believed to have affinities with palms, hence the name. Cotta (1832) described similar material under the name Porosus marginatus and suggested that it might be part of a large fern stem. Mantell (1833) agreed with this opinion after reexamining material he himself had collected some 10 years before. HISTORY OF INVESTIGATIONS 3 Fitten (1836) described similar material collected near Hastings. He returned apparently to the opin- ion of Stokes and Webb (1824) and referred the specimens to Endogem’tes erosa. Unger (1845) reexamined the material under question, as well as new collections, and expressed his opinion that Endogenites erosa was simply a mode of preservation of Protoptem’s. Protopteris was the generic name used in the early days of paleo— botany for certain fern stems that were either petri- fied or preserved as casts or molds. In 1845, Corda published his observations on a large series of petrifications of various types and established the genus Tempskya in honor of a con- temporary naturalist, Tempsky. The type material was four specimens from localities in Bohemia and adjacent regions. The generic description (Corda, 1845, p. 81) is as follows: Truncus * * * Rachis rotundata, plicata vel alata; cortice crassiuscula, fasciculis vasorum ternatis, majori clauso vel lunulato et supra incurvo, minoribus oppositis lunulatis. Radices minutae numerosissimae; fasciculo vasorum centrali unlco. Corda’s interpretation was that Tempskya was a member of the Phthoroptem‘des. The material was believed to be silicified masses of branched petioles, sheathed by a thick mat of roots. The species T. pulchra, T. macrocaula, T. microrrhiza, and T. schimpem' were described. The preservation of the material that Corda studied appears to be rather poor. In consequence, although the account is his- torically quite important, few data of morphological value resulted from the investigation. In 1871, Schenk suggested that Tempskya is a complete stem of marattiaceous affinities, the vascu- lar bundles being sheathed in a ground mass of parenchyma and sclerenchyma. Feistmantel in 1872 suggested that Tempskya is not a valid genus but rather a type of preservation of certain kinds of fern stems. This opinion im- pressed a number of investigators, and several papers were published in support of the opinion. Velenovsky (1888) supported the theory and pro- duced additional corroborative evidence. Seward, in his catalog of the Wealden flora pub- lished in 1894, included a very interesting and val- uable account of the literature on Tempskya. Feist- mantel’s ideas were seriously questioned, although not completely discredited. Seward’s conclusions (1894, p. 158) are as follows: In Tempskya schimpe'ri we have masses of branched diarch fern roots associated with petiole axes, which occasionally afford evidence of branching; probably some forms of Tempskya and Protoptem's are very closely related, if not identical plants; but, so far as English specimens are concerned, there is an absence of any direct proof of such organic connection between the two fossils, as Feistmantel and Velenovsky have previous- ly suggested. The true nature of Tempskya was first tentatively hinted in 1897 by Stenzel, who offered three possible morphological explanations: 1. Lateral organs of a tree fern growing downward and encased in downward-growing roots. (This hypothesis is similar to that suggested by Corda, although not identical with it.) 2. Independent stems climbing upward between roots. 3. Upward-growing and branching fern stems en- cased in their own downward-growing roots. Stenzel appears to have preferred the third ex- planation, although there is no final commitment. This explanation is the one accepted by all later investigators. Detailed morphological studies of well-preserved Tempskya material were carried out for the first time in 1911 by Kidston and Gwynne-Vaughan. These investigators described T. Tossica from the Karanganda River basin in Russia. Although the material is very well preserved, its geologic age is uncertain; the material came from a Tertiary con- glomerate but, in the opinion of all investigators, has been reworked from older strata. These investi- gators concluded that the siliceous masses that had caused so much speculation in the past were false trunks, or dichotomously branching systems of stems sheathed in a mass of adventitious roots. Kidston and Gwynne-Vaughan speculated on the growth habit of T. Tossica and suggested that it may have stood erect and produced a crown of leaves or fronds. Although they were unable to establish close aflini- ties, they suggested that Tempskya belongs in some family of the Leptosporangiatae. The first American material referred to Tempskya was described by Berry (1911b) from the Patapsco Formation of the Atlantic Coastal Plain. The ma- terial is poorly preserved, and although the validity of the generic identification is unquestioned, Berry’s account of T. whitei does not contribute greatly to morphological knowledge. In 1915, Dr. Marie Stopes presented a summary of investigations of Tempskya in the catalog of Lower Greensand plants of Great Britain. In addition, she redescribed T. erosa which, as stated above, was the first species to be noted. Stopes included a recon- struction of T. rossz'ca which was provided by Kid- ston and Gwynne-Vaughn. The drawing shows an erect false trunk with a terminal cluster of small fronds similar to those borne by modern tree ferns and cycads. (See fig. 1.) 4 NORTH AMERICAN SPECIES OF TEMPSKYA ..._., FIGURE 1.—Restoration of Tempskya rossica by Kidston and Gwynne-Vaughn. The figure is based mainly on the specimen of T. rossica they described in 1911 from the Karaganda River basin of Russia. It was first published by Stopes (1915, text-fig. 5, p. 15) and is reproduced here with the permission of the Trustees of the British Museum (Natural History). Professor A. C. Seward of Cambridge University described the first well-preserved specimen of Tempskya from the United States in 1924. The specimen was obtained by A. C. Silberling in 1908 from the Cretaceous rocks in the valley of the Musselshell River in central Montana. Because of the nature of the preservation, the specimen was at first interpreted as a caudal spine of the dinosaur Stegosaurus. (See Seward 1924, p. 485.) F. H. ‘Knowlton recognized the true nature of the material, but, being busy on other investigations, suggested that it would be appropriate for Seward to study the specimen. Tempskya knowltom‘ is a very well pre- served specimen, and the study in general corrob- orates that of Kidston and Gwynne-Vaughn on T. rossica. There is one significant difference between the two species, however. In T. knowltom‘, the entire false stem is dorsiventral by reason of the orienta- tion of the individual true stems; this arrangement caused Seward (1924, p. 505) to describe T. knowl— tom’ as “a root-encircled bundle of stems, obconical and tapering, lying obliquely in the soil, a few of the stem branches bearing crowded fronds near the ground level.” Seward suggested that Tempskya may be a member of the Schizaeaceae. This conclu- sion was arrived at partly on the basis of stelar sim- ilarities and in part on the characteristics of spores and sporangia that were found not only in T. knowltom, but also in some of the English material. During the period 1930—35, W. W. Rubey and sev— eral of his associates were quite active in carrying out a mapping program in eastern Idaho and adja- cent parts of Wyoming. Many specimens of Temp- skya were collected from the Wayan Formation of Idaho and the Aspen Shale of Wyoming. These units, which are in part correlative, are classified as Cre— taceous in age. The abundant well—preserved material collected by this group was of sufficient interest to Read and Brown that they decided to collaborate in an account. The report was published in 1937 and was an attempt to discuss all the Tempskya material known at that time in the United States. Two new species, T. grandis and T. minor, were described, and additional observations were published on T. knowl- tom’ and T. whitei. Two groups or subgenera of Tempskya. were rec- ognized on the basis of radial symmetry of the false trunks in one group as opposed to the dorsiventral nature of the false trunks in the other group. After the characteristics of the various species of Temp- skya had been reviewed and compared with those of modern ferns, it was concluded that a new fam- ily, the Tempskyaceae, should be provisionally estab- lished inasmuch as within the Leptosporangiatae reasonable comparisons may be made between the species of Tempslcya and the Schizaeaceae, the Lox- somaceae, and the Gleicheniaceae. Read and Brown (1937) speculated on the prob- able growth habit of the species of Tempskya and suggested that in the case of the group represented by T. grandis, all the facts, including the radial sym- metry of the false trunks, supported the idea that these ferns had a habit similar to that of low tree ferns. However, they noted that the plants repre- sented by the dorsiventral false trunk types typified by T. knowltom' might have had a liana- or vinelike growth habit. An attempt was made to discuss the stratigraphic significance of the American temp- HISTORY OF INVESTIGATIONS 5 skyas. Read and Brown concluded that in the West- ern United States, the genus Tempskya ranges in age from Turonian to Senonian. However, in the Eastern United States, T. whitei occurs in Albian rocks and hence is older than the western occurrences. These conclusions were, of course, based to a large extent on the opinions then held by invertebrate paleontolo- gists regarding the age of the containing rocks. Material was reported from Wyoming, Idaho, Mon- tana, Oregon, Nevada, Utah, and Maryland. In 1939, Read published a summary of his views on the growth habit of Tempskya and on the possible course of development of the habit. He restated the opinion that on the basis of the morphology of the false trunks there are two groups of Tempslcya, one of which is radially symmetrical and the other dorsi— ventral. The dorsiventral false trunk types were believed to have been lianalike and also to have been the more primitive group. He speculated that the radial forms were developed from these climbing types as a result of the assumption of a free upright habit. This change may have been quite accidental, or possibly the result of an increase in rigidity of the composite organ because of-increase in size of the true stems. Another American who has studied Tempskya is Chester A. Arnold. In 1945, he described two new species, T. wesselii from Montana and Oregon and T. wyomingensis from Wyoming. The concept of two groups or subgenera of Tempskya proposed by Read and Brown (1937) was accepted by Arnold, with certain reservations, and he placed his new species in the radially symmetrical group. Later, Arnold (1958) described a new species of Tempskya from Nebraska and named it T. superba. This species is similar to T. grandis internally, but the stems of T. superba are larger than those of T. grandis. This new species also was assigned to the radially symmetrical group. In 1947, Andrews and Kern published a very ex- cellent discussion of large collections of Tempskya material made by Thomas, Manion, and Andrews. Their report contributes very little to the morpho- logical details described by Kidston and Gwynne- Vaughn, Seward, and Read and Brown, but the paper has a very stimulating discussion of false trunk morphology. These investigators suggested that in all probability all the false trunks referred to Tempskya were originally radially symmetrical and that accidents of preservation are responsible for the alleged dorsiventral types. Their reconstruction of a radially symmetrical Tempskya shows a treelike fern in which the upper two-thirds of the false trunk is characterized by irregular clusters of small fronds (see fig. 2). Andrews and Kern made com- parisons between Tempskya and Hemitelz’a smithz‘i, H. crenulata, Todea barbara, and Dicksom'a fibrosa. In addition, they discussed the late Paleozoic zygop- FIGURE 2.—Reconstruction of Tempskya by Andrews and Kern (1947). The figure is based mainly on specimens of T. wesselii Arnold (1945) from the Wayan Formation of south- eastern Idaho. Reprinted with the kind per- mission of H. N. Andrews. 6 NORTH AMERICAN SPECIES OF TEMPSKYA terid tree fern Clepsydropsis austmlis and pointed out analogies with Tempslcya. In 1961, the authors of the present report re- appraised (Read and Ash, 1961a, b) the strati- graphic distribution of Tempskya in the western part of the United States in the light of modern data. We showed that specimens of the genus often occur in or adjacent to marine units containing in- vertebrates of latest Early Cretaceous (Albian) age. We concluded, therefore, that the age of Tempskya is more restricted than formerly thought (Read and Brown, 1937) and that the genus is probably of latest Early Cretaceous (Albian) age in the West- ern United States. In 1968, a new Tempskya. was described from southern England by Chandler. The single small specimen on which her study is based resembles T. grandis, but because of the limited amount of ma- terial available, it was not referred to a species. The age of the specimen is questionable, as it was found on the surface of the beach at the famous upper Eocene London Clay plant locality near Sheppey in Kent. The new Tempskya is silicified, whereas plant remains in the London Clay are usually pyritized or preserved in some other manner. Thus, it seems unlikely that the new Tempskya was originally de- posited in the London Clay. Chandler suggested (1968, p. 179) that the most likely source of the specimen is the lower Eocene Woolwich Beds, which are exposed at nearby Herne Bay and which contain silicified dicotyledonous wood. Another likely source for the specimen is the Lower Cretaceous Lower Greensand, which is exposed in nearby areas and which yielded T. erosa, the first species of the genus to be recognized. Several nicely preserved specimens of Tempskya have been reported from gravel deposits in Harnett County, NC, by West (1968, 1970) of the Carolina Biological Supply Company. The fossils have not yet been described, but pictures supplied by Mr. West show that they do not represent any of the species considered in this report. OCCURRENCES OF TEMPSKYA IN THE UNITED STATES Specimens of Tempskya have been reported from about 43 localities in the United States (fig. 3). Most of the localities are in the Rocky Mountain geosyncline region of the western interior of the United States. Two are in the Mexican geosyncline area in the southwestern United States, and two are in the Eastern United States. Thirty—four of these localities contain specimens that are in place. These in-place specimens in the Rocky Mountain geosyn- cline area are considered first, by formations from north to south. Those in the Mexican geosyncline region and the one in the Atlantic Coastal Plain in the Eastern United States are then briefly dis- cussed. Specimens that clearly have been reworked into Tertiary or Quaternary formations from older rocks are dealt with last. Locality numbers used in the text are keyed to the index map (fig. 3), and in-place specimens are keyed to the correlation chart (fig. 4). IN-PLACE SPECIMENS ROCKY MOUNTAIN GEOSYNCLINE BLACKLEAF FORMATION The holotype of Tempskya, wesselii was collected from the “bad lands” northwest of Great Falls, Mont., at locality 1 (fig. 4) by Louis Wessel (Arnold, 1945, p. 26). The collector thought that the specimen came from the Kootenai Formation of Early Cre- taceous age. It may have come, however, from the overlying Blackleaf Formation, which is also ex- posed in the area and which is known to contain Tempskya a few miles south of the “bad lands” at locality 2. Specimens of T. knowltom’ have been col— lected from the Vaughn Member of the Blackleaf Formation (of the Colorado Group) west of Great Falls at locality 2, as reported by Cobban (1951, p. 2180). These specimen-s, which were identified by R. W. Brown, occur in the so-called red speck zone. An additional specimen of Tempskya sp. was col- lected by Fergus Mitchell in 1947 near locality 3. It probably was also derived from the “red speck zone” or adjacent strata. The Bootlegger Member of the Blackleaf, which overlies the Vaughn Member, contains the ammonite Neogastroplites, which is considered to be of late Albian Age (Reeside and Cobban, 1960, p. 17, 60; Cobban and others, 1959, p. 2792). Thus, the speci- mens of Tempskya in the Blackleaf are of late Early Cretaceous (Albian) age or slightly older. COLORADO GROUP (UNDIFFERENTIATED) The first well-preserved example of the genus Tempskya to be described from the United States was obtained by A. C. Silberling in 1908 in Montana at locality 4. The fossil was eventually sent to Pro- fessor A. C. Seward who published (1924) an ac- count of the specimen, naming it T. knowltom‘. At that time it was thought that the specimen probably came from the Kootenai Formation of Early Cre- taceous age. Later, when Read and Brown (1937 ) redescribed T. knowltom’, they showed that the speci- men undoubtedly came from what is now called the shale and sandstone member of the Colorado Shale. OCCURRENCES 7 117° 113° 109° 105° 101° T‘T_r+—_L——~l-——IE‘LN§E____L-T_l—————\—-—"1 UNITED STATES l WASHINGT I I ON I \ NORTH DAKOTA . ’I' L 8 1 X 198024 BMW; 2, 3 IGreat Falls 7‘2 —— \ I \ l \ | W \) Helena . MONTANA . l \‘ 6 4'5 stoma “/‘e’r' F—— —— __”- l e OREGON (A ““0 o AA I 45 ° ‘ ‘\ 413% § I sOUTH DAKOTA A Bakgro I \ _ _ —— — - , _ - T . 25(R) w 3th 7(R) IDAHO ’ ’ A 0 Buffalo m f ‘1 my 0 “x 24(R) 3; 26 oh — I °a 28(R) _ _ / —— ,/— o Chadron NEBRASKA 41°_ 0 Denver | \ NEVADA fl COLORADO \ KANSAS |>(35 Durango L 0 fl, .— 37°‘ \ __ 6‘___ __ _.___—’ *‘ 0KLAHOMA’ 29 36 L___/—- NEW MEXICO ‘ | l ‘ \ l \ L35 Vegas ARIZONA \ o 39(R)x o/r38(R) X «P § § 8 O 50 100 MILES 0 50 ISOKWLOMEYRES 3 33° 0: I40 —— L15!“ J l lCl)O 290 390 MILES ‘I’ T I 100 200 300 KILOMETRES FIGURE 3.—Genera1 location of occurrences (X) of Tempskya spp. in the United States. Numbers refer to localities mentioned in text and figure 4; (R) indicates that remains of this plant have been reworked from Cretaceous rocks into younger units. 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Distribution—This species occurs in the Mojado Formation on the east flank of the Animas Moun- tains, Hidalgo County, N. Mex., at USGS fossil plant locality 9703. Tempskya zelleri Ash and Read, n. sp. Plates 5—7 ; figure 10 1956. Tempskya minor Read and Brown: Zeller and Read, p. 1804. Diagnosis—False trunks small to medium, as much as 17 cm in diameter, radially symmetrical; individual stems usually 3—8 mm in diameter, dis- tinctly angular in cross section; internodes very long, usually showing one, rarelytwo or three, leaf traces in a transverse section; cortex three—layered, outer layer narrow, composed of parenchyma, middle layer wide, composed of sclerenchyma, inner layer Wide, composed of parenchyma; outer endodermis present; outer pericycle and phloem thin; xylem exarch with little interspersed parenchyma; inner phloem, inner pericycle, and endodermis present; pith consisting of thin outer layer of parenchyma and a central area of sclerenchyma. Free petioles rare. Adventitious roots usually vertically oriented. Description—The largest specimen of T. zellem’ that has been collected is about 17 cm by 11 cm in diameter; it is illustrated diagrammatically in figure 10. Thirty-two stems are present and are oriented so that the false‘ trunk has a crude radial symmetry. The stems are all in a central zone in the false trunk, on the exterior of which is a rindlike area made up entirely of adventitious roots. The stems are all vertical or nearly vertical in their orientation in the false trunk, are small, and are equally dichotomous. The low number of leaf traces associated with each stem suggests relatively long internodes, longer than in T. reeside'i Ash and Read. FIGURE 8.—Reconstruction of the basal part of the false trunk of the paratype of Tempskya reesidei n. sp. Fragments of the specimen that were recovered are shown unpatterned and the missing parts are patterned. The position of the cross sections in figures 9A and B are indicated by the straight lines at a and b, respectively. Places where stems of woody dicotyledonous plants adhered to the exterior of this false trunk of T. reesidei when it was alive are indi- cated by d. Solid lines show the present extent of this foreign vegetative material, and the dashed lines delineate the impressions they made on the surface of the false trunk by other parts of these same woody stems. USNM 167547. SYSTEMATIC DESCRIPTIONS 29 €=> @@ @ (9% @ W @ @3 © @3 k? IQ K3>® 2 CENTJMETRES L‘_l B FIGURE 9.——Cross sections of the false trunk of the paratype of Tempskya reesidei shown in figure 8; USNM 167547. A, Section cut through about the middle of the specimen (at a in fig. 8) , showing the comparatively large num— ber and orientation of the stems and the remains of three foreign vegetative bodies (d). Compare the number of stems in this section with the three in the section cut through the basal part of the fossil shown in figure QB. Ad- ventitious roots are not shown in the figure although they are abundant in the section. B, Section cut through the basal part of the specimen (at b in fig. 8). Note the rarity of stems in this section. The bulk of the false trunk at this level is composed of adventitious roots, The most characteristic feature of all specimens of this species that have been examined is the angu- lar nature of both the steles and the stems when viewed in cross section. It was at first thought that this angularity might be due to crushing of the plant material during the processes of sedimentary load- ing and postdepositional deformation of the contain- ing rocks in the Big Hatchet Mountains. However, as is indicated by the diagram (fig. 10), the largest specimen is nearly equidimensional in cross section, and there is no apparent relationship between the but simplification they are not shown. small amount of flattening that may be inferred and the orientation of the stems. In addition, the tissues of the stem, as Will be shown at a later point, are not crushed or deformed any more than in other species of Tempskya that do not show the angular characteristics that seem to mark this form. In con- sequence, the writers are forced to assume that the angularity is characteristic of the species, just as it is a characteristic of some genera and species of modern ferns. The general characteristics of the stems are illus- 30 NORTH AMERICAN SPECIES OF TEMPSKYA 2 CENTIMETRES J FIGURE 10.—Cross section of the false trunk of the holotype of Tempskya zelleri n. sp. showing the distribution and crude radial orientation of the stems. In this specimen they are concentrated more or less near the middle of the false trunk and generally are transversely sectioned, contrasting in both of these features with T. reesidei n. sp. Compare the diagram with the cross sections of T. reesidei n. sp. (figs. 7 and 9A). Another significant fea— ture of the species that can be seen in the figure is the angularity of the stems which contrasts with the rounded stems of T. rees’idei n. sp. and all the other known species of the genus. Adventitious roots are present throughout the section, but for simplification they are shown as small circles in a part of the illustration. USNM 167543. trated on plates 5 and 6. They are dorsiventral and give off leaf traces in two ranks. Only rarely is more than one leaf trace seen in a single transverse section associated with a single stem. (See plates 5 and 6.) In consequence, it may be assumed that the inter- nodes are somewhat longer than in T. reesidei. From the point of their inception the leaf traces are seen to be oblique in comparison with the orientation of the stem, so it may be assumed that they pass out from the parent stem at rather low angle to the stem. In the stems examined, the epidermis is rarely preserved. The outer cortex appears to be a zone about one-fifth the width of the entire cortex. It is poorly preserved and parenchymatous. The middle cortex consists of sclerenchyma, which judging from the sizes of the lumens, may be stone cells, and is at least three times the thickness of the outer cortex. It is usually dark brown, and the lumens appear to COMPARISONS OF THE SPECIES 31 be nearly closed. Bordering the middle cortex is the inner cortex which is at least twice the Width of the outer cortex and on the average slightly narrower than the middle cortex. The inner cortex is thinner walled than the middle cortex and appears to be parenchyma. The endodermis is fairly well defined in this species, and internal to it is a very thin zone of pericycle and phloem. These tissues are all illus- trated on plate 7. The xylem is exarch and, like Tempskya minor, the protoxylem appears to be a series of clusters rather than a continuous ring. One especially prominent cluster of protoxylem is seen on plate 7. It will be noted on this plate that adjacent to the prominent protoxylem, large metaxylem cells extend to the interface of the xylem and phloem. The xylem is from four to as many as eight cells wide. It is characterized by an abundance of small-celled xylem and parenchyma interspersed with the scalari- form tracheids. A narrow zone of inner phloem and pericycle within the xylem ring is bounded by the inner endodermis. The pith is characterized by a thin outer ring of parenchyma and a central area of sclerenchyma. Leaf traces cut at low levels are shown on plate 6, figure 3 (upper left), and others cut at higher levels are on plate 6, figure 2. The leaf traces depart at a very low angle from the xylem ring and from the stem in general. Thus, on plate 6, figure 3, the xylem ring of the stem is in cross section except for the part that is bulging into a leaf trace. In this pro- trubence the tracheids are slightly oblique. At a still higher level (pl. 6, fig. 3; pl. 7), the same obliqueness may be noted. The leaf traces are inverted U shaped with invaginated or recurved ends. The xylem is two or three cells wide except at the terminations, where knoblike bulges cause it to be as much as six cells wide. The xylem is completely surrounded by phloem, a thin zone of pericycle, and the endodermis. The inner cortex within the inverted U of the stele is a very thin zone of parenchyma, and the middle cortex is a thick zone of sclerenchyma. The cortex on the exterior of the inverted U of the stele consists of a much thicker inner parenchymatous zone and a somewhat thinner sclerenchym'atous middle cortex. Phyllopodia are rarely seen in specimens attrib— uted to this species. The roots are diarch and exarch, have only a few tracheids, are sheathed by a thin zone of phloem and pericycle, and the stele is bounded by a very well defined endodermis (pl. 7). The cortex appears to be almost entirely sclerenchymatous. No evidence of an outer lacunar cortex was observed in the roots of this species. The long axes of the adventitious roots of T. zelleri appear to be oriented parallel to the long axes of the stem in most cases. Exceptions, of course, occur at the point of departure of the adventitious roots from the stem where the long axes are at nearly right angles. The orderly orientation of the adventitious roots in this species contrasts strongly with the random orientation of the roots, with the resultant tangle that appears to be so characteristic of T. reesidei. Remarks—The species is named for Robert A. Zeller, J r., who collected many of the specimens on which this species and T. reesidei n. sp. are based and who was an authority on the geology of south- western New Mexico before his untimely death in 1970. Material.—Holotype: USNM 167543. Paratypes: USNM 167541, 167542, 167544. Distribution—T. zellem‘ occurs in the Mojado Formation south of the Big Hatchet Mountains, Hidalgo County, N. Mex., at USGS fossil plant local- ity 9702. COMPARISONS OF THE SPECIES OF TEMPSKYA At present, nine species of Tempskya have been described from North America. These are Tempskya grandis Read and Brown, T. knowltoni Seward, T. minor Read and Brown, T. reesidei Ash and Read, T. superba Arnold, T. wesselii Arnold, T. whitei Berry, T. wyomingensis Arnold, and T. zelleri Ash and Read. In addition, several species are known in Europe, and two of these, T. rossica Kidston and Gwynne-Vaughan and the new Tempskya from Eng- land are based on very well preserved material. The following discussion is restricted to the well-pre- served species in North America and to T. rossica. The several European species that are based on poorly preserved material, and T. whitei Berry, which is also known only from poor specimens, are mentioned only incidentally. False trunks—The several species of the genus Tempskya are tentatively classified into two groups according to the symmetry of the false trunks, one being characterized by dorsiventral false trunks and the other by radially symmetrical false trunks. T. knowltoni and T. minor continue to be the only known species that have dorsiventral false trunks. The radially symmetrical group now includes T. grandis, T. rossica, T. snperba, T. wesselii, T. wyom- ingensis, and the two new species described here, T. reesidei and T. zelleri. Stems.—It was suggested by Read and Brown 32 NORTH AMERICAN SPECIES OF TEMPSKYA (1937) and again by Arnold (1945) that variations in dispositions of tissues in the cortex of the stems must have systematic value. The writers agree with the earlier findings and place emphasis on the corti- cal characteristics. Two groups of Tempskya may be recognized on the basis of the disposition of tissues in the cortex, the first characterized by a simple cortex consisting of inner parenchymatous, middle sclerenchymatous, and outer parenchymatous zones. the outer paren- chymatous zone, in some cases, being reduced to only a few cells in thickness. The second group is charac- terized by an inner mixed zone of parenchyma and sclerenchyma, a middle sclerenchymatous, and an outer parenchymatous zone. Parenthetically, the usage of the terms inner, middle, and outer zones of the cortex does not mean that these are homologous tissues (Arnold, 1945). As here used, the terms simply refer to position with respect to the epi- dermis and to the stele. The group characterized by the simple three- layered type of cortex includes T. zelleri, T. knowl- toni, T. minor, and T. rossica. Idealized diagrams of the stems of these species are shown in figure 11, and anatomical details of T. zelleri are illustrated on plates 5—7. Details of T. knowltoni are shown on plate 8 and those of T. minor on plate 9. In these four species, the cortex is characterized by an inner zone of parenchyma, a middle zone of sclerenchyma, and an outer zone of parenchyma. The relative widths of these zones probably are fairly constant, but the exact widths are probably functions of rela- tive sizes of the stems. In the second group, characterized by mixed tis- sues in the inner cortex, several architectural pat- terns as viewed in cross sections appear to be of specific value Diagrams of the stems of T. reesidei, T. grandis, T. wyomingensis, T. wesselii, and T. snperba are also shown in figure 11. Anatomical de- tails of T. reesidei are illustrated on plates 1—4, Whereas those of T. grandis are shown on plate 10, those of T. wyomingensis, on plate 11, those of T. wesselli, on plate 12, and those of T. superba, on plate 13. In Tempskya grandis and T. snperba an irregular and discontinuous band of sclerenchyma divides the inner and dominantly parenchymatous cortex into three bands, as is indicated in the diagrams. In T. reesidei, T. wesselii, and T. wyomingensis the inner cortex is characterized by inner and outer zones of sclerenchyma separated by a continuous or discon- tinuous band of parenchyma. According to Arnold (1945), in T. wesselii and T. wyomingensis these bands of sclerenchyma are actually stone cells. In T. reesidei the middle belt of parenchyma i-s discon- tinuous by reason of radial bands of sclerenchyma that connect the inner and outer continuous zones of sclerenchyma. In T. wesselii and T. wyomingensis, the middle zone of parenchyma is continuous, ac- cording to Arnold (1945). Epidermal emergences that appear to be simple hairs are characteristic of at least two species, Tempskya zelleri and T. minor. Because the epider- mis is rarely preserved in most stems, such struc- tures may have occurred in other species. Except for differences in diameters, the steles of all species are fairly similar in their characteristics, although there are some slight differences. For in- stance, the protoxylem in most species appears to form a continuous or nearly continuous band on the exterior of the xylem cylinder, whereas in T. zelleri and T. minor it is discontinuous and occurs as dis- crete strands. Earlier it was thought by one of the writers that variations in amounts of xylem parenchyma might be of value in separating some of the species (Read and Brown, 1937). Arnold (1945) discussed this problem and was inclined to question the value of this criterion. After reviewing the matter on the basis of new material in hand as well as the material earlier described, the writers are now inclined to agree with Arnold. The xylem is rarely more than 9—10 cells wide and is usually less. The morphological details of the leaf traces and phyllopodia are in general similar, except for the envelopment of the epidermis. At present, except for the variations in cortex, the writers are unable to recognize any differences in these organs in the various known species. One observation of possible value has been made, however, regarding the presence or absence of pre- served phyllopodia. In most species these organs are rarely noted, and in some, as T. knowltoni, they have not been reported, whereas, in T. reesidei they are quite abundant. The pith in all known species of Tempskya con- sists of two zones. The outer zone is parenchymatous or a mixture of parenchyma and sclerenchyma. In general, it is similar to, but thinner than, the inner cortex of the stem. The inner or central zone of the pith in all species is sclerenchyma and similar to the sclerenchyma of the middle cortex, although somewhat thicker walled. All investigators of the morphologic characteris- tics of the genus Tempslcya have formally or in- formally used stem diameter as a basis for specific SYNOPSIS 33 separation. However, as Arnold (1958) has pointed out, the use of diameter as a character trait always raises a question whether exceedingly large speci- mens that are otherwise similar to smaller stems may simply be indicative of very robust individuals. Thus, T. superba Arnold appears to be identical in all respects with T. grandis Read and Brown, but the stem diameter in T. superba is approximately twice that of T. grandis. Nevertheless, in the ab- sence of data to the contrary, the writers are of the opinion that stem diameters are a valid criterion for distinguishing species. Earlier, Read and Brown (1937) expressed the opinion that stem diameter and length of internodes might correlate directly. At that time, Tempskya grandis and T. rossz'ca were the only two large- stemmed species known (>4 mm in diameter), and both of them are characterized by relatively short internodes. Similarly, T. knowltom' and T. minor were the only small-stemmed (<4 mm in diameter) types known, and both are characterized by rela- tively long internodes. T. wesselii is in stem size somewhat larger in general than T. knowltom’ and Txminor, but as indicated elsewhere, it is character- ized by very long internodes comparable in length to those of T. zelleri n. sp. T. wyomz’ngensis is intermediate in average diam- eter between T. grandis and T. superba, but it has short internodes. T.‘ reesidei n. ’sp. has small stems and relatively long internodes, whereas T. zelleri n. sp., which also has small stems, has very long internodes. It is thus apparent that there is no cor- relation between stem diameter and length of inter- nodes (Arnold, 1945). Roots.-—The roots of all known species of Temp- slcya appear to be similar, if not identical. In most species the outer cortex is unknown. However, in T. minor and T. wesselii, several observers (Read and Brown, 1937; Arnold, 1945; Andrews and Kern, 1947) have noted an outer lacunar cortex (pl. 9, fig. 3) as well as a well-defined epidermis bearing simple root hairs. We believe that these observations have been made on exceedingly well-preserved speci- mens and that the outer lacunar zone in the cortex is probably characteristic of all species. In general, the (adventitious roots of all species parallel or subparallel the long axes of the stems. However, in one species, T. reesidei, a relatively high percentage of roots clearly do not parallel the courses of the stems (pl. 3, fig. 1). The importance of this characteristic in the specific concept of T. reesidei can only be evaluated after material from many localities has become available. SYN OPSIS In an earlier report on Tempskya, (Read and Brown, 1937) an effort was made to summarize the distinctions of the better known species. This group— ing of salient characteristics seems to have been helpful to some of the investigators who have worked on Tempskya in more recent years. Accord- ingly, a new synopsis has been prepared which in- cludes the better known species of the genus. I. Individual stems of false trunk medium to large (5—15 mm) in cross-sectional diameter. The inner parenchymatous layer of the cortex containing continuous and (or) dis- continuous bands of sclerenchyma. Much parenchyma in the xylem ring. False trunks have radial symmetry. A. Inner parenchymatous layer of cortex and ex- terior of pith contains a single discontinuous and irregular band of sclerenchyma. Internodes short, permitting much overlapping (3—5) of leaf bases. 1. Individual stems, 10—15 mm in cross- section diameter _________________ ________________ Tempskya superba 2. Individual stems medium, 4—6 mm in cross-section diameter ___________ « -_ ________________ Tempskya grandis B. Inner parenchymatous layer of cortex contains two bands of sclerenchyma separated by a con- tinuous or discontinuous band of parenchyma. The outer band of sclerenchyma is continuous, and the inner band may be either continuous or discontinuous. Exterior of pith does not con- tain a band of sclerenchyma. Internodes long to very long, permitting very slight to slight overlapping (1-3) of leaf bases. 1. The two bands of sclerenchyma in the inner parenchymatous layer of cor- tex are connected locally by strands of sclerenchyma, giving an impres- sion of “islands” of parenchyma sur- rounded by sclerenchyma. Internodes are long, permitting slight overlap- ping (2—3) of leaf bases __________ ________________ Tempskya reesidei 2. The two bands of sclerenchyma in the inner parenchymatous layer of cor- tex are completely separated from each other by a continuous band of parenchyma. a. Internodes very long, permit- ting only very slight over- lapping (1—2) of leaf bases. Individual stems 4.0—5.0 mm in cross-sectional diameter__ ________ Tempskya wesselli b. Internodes medium in length, permitting slight overlap- ping (2—3) of leaf bases. In- dividual stems 6.0—8.0 mm in cross-sectional diameter__ _ _ _ Tempskya wyomingensis 34 NORTH AMERICAN SPECIES OF TEMPSKYA FIGURE 11.—Diagrammatic sketches of the stems of the well-known species of Tempskya. Cortical scleren- chyma and similar tissues in the pith are shown as black; xylem of the stele is shown by the dark stipple pattern; the parenchyma is shown by a light stipple pattern; and phloem and pericycle are shown by clear bands. All X 10 except I which is X 8. A, Tempskya zellem'. Adapted from slides of the type specimen. B, T. reesidei,Adapted from slides of the type specimen. C, T. minor. Adapted from Read and Brown, 1937, pl. 36, fig. 3, and slides of the type E specimen. D, T. yrandis. Adapted from Read and Brown, 1937, pl. 33, fig. 4, and slides of the type specimen. E, T. wyomingensis. Adapted from Arnold, 1945, pl. 10, fig. 1, and slides of the type specimen. F, T. rossica. Adopted from Kidston and Gwynne-Vaug- han, 1911, pl. 2, fig. 10. G, T. knowltom’. Adapted from Read and Brown, 1937, pl. 32, fig. 1, and slides of the type specimen. H, T. wesselii. Adapted from Arnold, 1945, pl. 8, fig. 1, and slides of the type specimen. 1, T. superba. Adapted from Arnold, 1958, pl. 2, fig. 2, and slides of the type specimen. SYNOPSIS FIGURE 11.—— ( Continued.) 35 36 NORTH AMERICAN SPECIES OF TEMPSKYA 11. Individual stems of false trunk small to medium (2.5—8.0 mm) in cross-sectional diameter. The inner parenchyma- tous layer of the cortex does not contain continuous or discontinuous layers of sclerenchyma. Little parenchyma in the xylem ring. False trunks have either dorsiventral or radial symmetry. A. Individual stems and steles angular in cross sec- tion, approximately 3—8 mm in diameter. Inter- nodes very long, permitting only very slight overlapping (1—2) of leaf bases _____________ ___________________________ Tempskya zelleri B. Individual stems and steles round to subround in cross section. Internodes long, permitting slight overlapping (2—3) of leaf bases. 1. Petioles common in false trunks; stems approximately 2.0—3.5 mm in diame- ter. False trunks are dorsiventral--- __________________ Tempskya minor 2. Petioles rare in false trunks. Stems are approximately 2.5—7.0 mm in diame- ter. False trunks are dorsiventral or radially symmetrical. a. False trunk dorsiventral. Xylem exarch. Xylem ring containing little if any par- enchyma. Stems are 2.5—3.5 mm in diameter __________ _______ Tempskya knowltoni b. False trunk radially symmet- rical. Xylem exarch or pos- sibly slightly immersed in some specimens. Xylem ring containing much paren- chyma. Stems are 6.0—7.0 mm in diameter __________ _________ Tempskya rossica SELECTED REFERENCES Publications that are cited in this paper and other publica- tions in which Tempskya is mentioned or described are in- cluded here. Items marked with an asterisk (*) contain descriptions and discussions of the genus Tempskya and its several species. Anderson, J. L., 1948, Cretaceous and Tertiary subsurface geology: Maryland Dept. Geology, Mines and Water Re- sources Bull. 2, p. 1—1‘13, app. p. 385—441. Andrews, H. N., Jr., 1943, Notes on the genus Tempslcya: Am. Midland Naturalist, v. 2.9, no. 1, p. 133—136. 1947, Ancient plants and the world they lived in: Ithaca, N.Y., Comstock Publishing Co., 279 p. 1948, Fossil tree ferns of Idaho: Archaeology, v. 1, no. 4, p. 190—195, 8 figs. 1961, Studies in paleobotany, with a chapter on paly- nology by Charles J. Felix: New York, John Wiley and Sons, 487 p. * 1970, Filicophyta Incertae Sedis, in Andrews, H. N., Jr., and others, Traité de Paléobotanique; time IV, fascicule 1, Filieophyta: Paris, Masson et Cie, p. 457—491. [In French] *Andrews, H. N., and Kern, E. M., 1947, The Idaho Tempskyas and associated fossil plants: Missouri Bot. Garden Annals, v. 34. no. 2, p. 119—183, app. p. 185—186, pls. 15—27, 8 text-figs. *Arnold, C. A., 1945, Silicified plant remains from the Meso- zoic and Tertiary of western North America. I, Ferns: Michigan Acad. Sci., Arts and Letters Papers, 1944, v. 30, p. 3—34. * 1958, A new Tempskya: Michigan Univ. Mus. Paleon- tology Contr., v. 14, no. 8, p. 133-142, 3 pls., 1 fig. Berry, E. W., 1911a, Correlation of the Potomac formations: Maryland Geol. Survey, Lower Cretaceous [Volume], p. 153—172. * 1911b, Systematic paleontology of the Lower Creta— ceous deposits of Maryland; Plantae; Pteridophyta: Mary— land Geol. Survey, Lower Cretaceous [Volume], p. 214- 312, pls. 12-41. 1929, The Coastal Plain deposits: Maryland Geol. 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B., Jr., 1944, Map showing thickness and general character of the Cretaceous deposits in the Western Interior of the United States: U.S. Geol. Survey Oil and Gas Inv. Map OM—10. Reeside, J. B., Jr., and Cobban, W. A., 1960, Studies of the Mowry Shale (Cretaceous) and contemporary formations in the United States and Canada: U.S. Geol. Survey Prof. Paper 355, 126 p., 58 pls., 80 figs. Reeside, J. B., Jr., and Weymouth, A. A., 1931, Mollusks from the Aspen Shale (Cretaceous) of southwestern Wyoming: U.S. Natl. Mus. Proc., v. 78, art. 17, 24 p., 4 pls. Richards, H. G., Groot, J. J., and Germeroth, R. M., 1957, Cretaceous and Tertiary geology of New Jersey, Delaware and Maryland, Field Trip no. 6, in Geol. Soc. America, Guidebook for field trips, Atlantic City Meeting, 1957: p. 183—216, 7 figs. Rubey, W. W., and Callaghan, Eugene, 1936, Magnesite and brucite, in Hewett, D. F., and others, Mineral resources of the region around Boulder Dam: U.S. Geol. Survey Bull. 871, p. 113—143. SELECTED REFERENCES 39 Sahni, B., 1928, On Clepsydropsis austrah‘s, a zygopt/erid tree- fern with a Tempskya-like false stem, from the Carbonifer- ous rocks of Australia: Royal Soc., London, Philos. Trans. ser. B, v. 217, p. 1—37, pls. 1—6. Sample, C. H., 1932, Cribratina, a new genus of Foraminifera from the Comanchean of Texas: Am. Midland Naturalist, v. 13, no. 5, p. 319—323, pl. 30. *Schenk, August, 1871, Beitrage zur Flora der Vorwelt; IV, Die fossile Flora der nordwestdeutchsen Wealdenforma- tion: Palaeontographica, v. 19, p. 203—262, pls. 42, 43. Schimper, W. P., 1869—74, Traite de paléontologie vegetale, ou la flore du monde primitif daus ses rapports avec los formations géologiques et la Flore du Monde actual: Paris, 3 v. and atlas, 110 pls. (See v. 1, p. 697, 698; v. 3, p. 530.) 1880, Palaeophytologie, in Zittel, K. A., Handbuch der Palaeontologi: Munich and Leipzig, R. Oldenbourg, Abt. 2, Lieferung 2, p. 153-232. (See pl. 2, p. 151.) Schultz, A. R., 1914, Geology and geography of a portion of Lincoln County, Wyoming: U.S. Geol. Survey Bull. 543, 141 p., 11 pls. *Seward, A. 0., 1894-95, Catalogue of the Mesozoic plants in the Department of Geology, British Museum (Natural History). The Wealden flora: London, British Mus. (Nat. Hist), 2 v., 31 pls. (See v. 1, p. 148-159.) 1923, The use of the microscope in palaeobotanical re- search: Jour. Royal Micros. Soc., 1923, p. 299—302, pl. 11. (Not seen.) * 1924, On a new species of Tempskya from Montana, Tempskya knowltoni, sp. nov.: Ann. Botany, v. 38, p. 485—507, figs. 1—3, pls. 16, 17. Shawe, D. R., Simmons, G. G., and Archbold, N. L'., 1968, Stratigraphy of Slick Rock district and vicinity, San Miguel and Dolores Counties, Colorado: U.S. Geol. Sur- vey Prof. Paper 576—A, 108 p. Simmons, G. C., 1957, Contact of Burro Canyon Formation with Dakota Sandstone, Slick Rock District, Colorado, and correlation of Burro Canyon Formation: Am. Assoc. Petroleum Geologists Bull., v. 41, no. 11, p. 2519—2529, 3 figs. Skolnick, Herbert, 1958a, Lower Cretaceous Foraminifera of the Black Hills area: Jour. Paleontology, v. 3.2, no. 2, p. 275—285, pls. 36—38, 1 fig. 1958b, Stratigraphy of some Lower Cretaceous rocks of Black Hills area: Am. Assoc. Petroleum Geologists Bull., v. 42, no. 4, p. 787—815, 18 figs. Spangler, W. B., and Peterson, J. J ., 1950, Geology of Atlantic Coastal Plain in New Jersey, Delaware, Maryland, and Virginia: Am. Assoc. Petroleum Geologists Bull., v. 34, no. 1, p. 1—99, 1 pl., 24 figs. *Stenzel, K. G. W., 1897, Verkieselte Fame von Kamenz in Sachsen: K. Mineralog.-Geol. und Praehist. Mus. Dresden Mitt, no. 13, p. 1—24, pls. 1—3. (See p. 3.) Sterzel, J. T., 1893, Die Flora des Rothliegenden im Plauem schen Grunde bei Dresden: K. Sachsische Gesell. Wiss., Math-Phys. Klasse Abh., v. 19, 172 p. (See p. 129, pl. 12, figs. 8, 9.) (Not seen.) [Stokes, Charles, and Webb, P. B.], 1824, Descriptions of some fossil vegetables of the Tilgate Forest in Sussex: Geol. Soc. London Trans, 2d ser., v. 1, p. 423—426, pls. 45—47. (No authors names given.) Stokes, W. L., 1944, Morrison Formation and related deposits in and adjacent to the Colorado Plateau: Geol. Soc. America Bull., v. 55, no. 8, p. 951—992, 5 pls., 5 figs. 1952, Lower Cretaceous in Colorado Plateau: Am. Assoc. Petroleum Geologists Bull., v. 36, no. 9, p. 1766— 1776, 3 figs. 1955, Non-marine Late Jurassic and Early Cretaceous formation, in Wyoming Geol. Assoc., Guidebook, 10th Ann. Field Conf., 1955, Green River Basin: p. 80—84. *Stopes, M. C., 1915, Catalogue of the Mesozoic plants in the British Museum (Natural History). The Cretaceous flora, Part 2, Lower Greensand (Aptian) plants of Bri- tain: London, British Museum (Nat. History), 360 p., 32 pls. (See p. 13, 19—21, text-figs. 2—5.) Strobell, J. D., Jr., 1956, Geology of the Carrizo Mountains area in northeastern Arizona and northwestern New Mexico: U.S. Geol. Survey Oil and Gas Inv. Map 0M— 160, 2 sheets. Tidewell, W. D., and Hebbert, Naomi, 1972, Tempskya from the Cedar Mountain Formation near Moab, Utah [abs]: Geol. Soc. America Abs. with programs, v. 4, no. 6, p. 417—418. Unger, Franz, 1845, Synopsis plantarum fossilium: Leipzig, 330 p. (See p. 107). 1841—47, Chloris protogaea [in 10 parts]: Leipzig, 149 p., 50 pls. (See p. 52). Not seen. 1850, Genera et species plantarum fossilium: Vienna, 627 p. (See p. 201.) Veatch, A. C., 1907, Geography and geology of a portion of southwestern Wyoming, with special reference to coal and oil: U.S. Geol. Survey Prof. Paper 56, 178 p., 26 pls. *Velenovsky, Josef, 1888, Die Farne der Bfihmischen Kreide- formation: K. Bohmiche Gesell. Wissen. Abh., Folge 7, v. 2, no. 8, 32 p., 6 pls. (See p. 23.) Vokes, H. E., 1948, Cretaceous Mollusca from depths of 4875 to 4885 feet in the Maryland E-sso well: Maryland Dept. Geology, Mines and Water Resources Bull. 2, p. 126— 151, pls. 3—4. 1957, Geography and geology of Maryland: Maryland Dept. Geology, Mines and Water Resources Bull. 19, 243 p., 28 pls., 32 figs. West, W. R., 1968, Petrified wood in North Carolina: Caro- lina Tips, v. 31, no. 9, p. 1—2, 2 figs. 1970, Tempskya in North Carolina: Lapidary Jour., v. 23, no. 11, p. 1552—1556, figs. 1—9. Wieland, G. R., 1922, [Progress of cycadophyte investigation], in Carnegie Inst. Washington Year B00k No. 20, p. 452— 457. Yen, T.-C., 1952, Age of the Bear River Formation, Wyoming: Geol. Soc. America Bull., v. 63, no. 8, p. 757—764. 1954, [Discussion: Age of the Bear River Formation]: Am. Assoc. Petroleum Geologists Bull., v. 38, no. 11, p. 2412—2413. Young, R. G., 1960, Dakota group of Colorado Plateau: Am. Assoc. Petroleum Geologists Bull., v. 44, no. 2, p. 156—194, 20 figs. Zeller, R. A., Jr., 1958a, Reconnaissance geologic map of Dog Mountains quadrangle: New Mexico Bur. Mines and Mineral Resources, Geol. Map 8, scale 1:62,500. 40 NORTH AMERICAN SPECIES OF TEMPSKYA 1958b, The geology of the Big Hatchet Peak quad— rangle, Hidalgo County, New Mexico: Univ. Ca1if., Los Angeles, Unpub. Ph.D. dissert., 260 p., 7 p1s., 1 fig. 1958c, Preliminary composite stratigraphic section, Big Hatchet Peak quadrangle, Hidalgo County, south- western New Mexico, in Roswell, Geol. Soc. Guidebook, 11th Field Conf., The Hatchet Mountains and the Cooks Range-Florida Mountain areas, Grant, Hid-algo and Luna Counties, southwestern New Mexico May 14—16, 1958: p. 10. 1965, Stratigraphy of the Big Hatchet Mountains area, New Mexico: New Mexico Bur. Mines and Mineral Re- sources, Mem. 16, 128 p., 6 pls., 18 figs. Zeller, R. A., Jr., and Alper, A. M., 1965, Geology of the Walnut Wells quadrangle, Hidalgo County, New Mexico: New Mexico Bur. Mines and Mineral Resources Bull. 84, 105 p., 2 pls., 13 figs. Zeller, R. A., Jr., and Read, C. B., 1956, Occurrence of Tempslcya minor in strata of Albian age in southwestern New Mexico [abs.]: Geol. Soc. America Bull., v. 67, no. 12, pt. 2, p. 1804. INDEX [Italic page numbers indicate descriptions and major references] Page A Acknowledgments _____________________ 2 Aclistocham mundula -_ ________ 11 Actemellu sp _______ - 20 Adventitious roots ____________________ 23 Age of Tempskya-bearing rocks, New Mexico ___________________ 16 Alamo Hueco Mountains (N. Mex.) ---- 17 Albian Age ___________ 1, 5, 6, 9, 10, 11, 12, 13, 14, 17, 18, 20, 21, 22 Ammonite _____________________________ 20 Anatomical organization ______________ 23 Anchu‘ra mudgeana ___________________ 21 Animas Mountains (N. Mex.) ______ 1, 17, 18, 21, 22, 28 Anemia, sp ___________________________ 21 Aptian Age ____________________ 12, 13, 14, 1'7 Aptyacella (Nerinaides) n. sp _________ 20 Area 51) ______________________________ 20 Arundel Formation ___________________ 14 Aspen Shale (Idaho) -_ _____ 9, 11 Aspen Shale (Wyo.) _____________ 4, 9, 14, 15 Atlantic Coastal Plain area specimens -_ 18 austmlie, Clepsydropsis ............... 6 B barbare, Todeu _______________________ 5 Baseline Sandstone ___________________ 11 basiniformis, Corbulu _________________ . 22 Bear River Formation (Wyo.) --- 9, 10, 11, 15 Big Hatchet Mountains (N. Mex.) __________ 1, 17, 18, 31 Biota, age ____________________________ 22 Black Hills (S. Dak.) __ _ 14 Blackleaf Formation _- - 6 Brachydontes sp _____ - 20 Breviarca sp _________________________ 20 Burro Canyon Formation (Utah) _____ 12 C Cadulus sp ___________________________ 21, 22 Cemptonectea inconspicuus ____________ 21 Cardium kansasense ___________________ 21 Carrizo Mountains - 2 Cassiope sp ________ 20 cau’rinus, Melampus ___________________ 11 Cedar Mountain Formation (Utah) ___ 11 Cenomanian Age ________________ 10, 13, 14, 22 Cephalopod ________________ 21, 22 Cerithium pecoense ___ _ - 20 Chadron Formation (Nebr.) - 14 Chara stantoni ________________________ 11 Charophytes __________________________ 11, 12 Chinle Formation - -- 14 Cinulia __________________ -- 21 Classificatory viewpoints - -- 3,4 Clavator harriai _______________________ 11 Clepsydropsia uustralis ________________ 6 Cloverly Formation (Wyo. and Mont) _- -- 11, 12, 15 Coal beds ____________________________ 10, 12 Page Colorado Group (undifferentiated) (Mont) __________________ 6 Comparisons of species of Tempskya ___ 31 Conglomerate _________________________ 17, 22 Coniobasis ortmanni -- 11 Co’rbula basiniformis ___ 22 sp _______________________ 21 Cortex, morphological details __________ 32 Cowboy Spring Formation ____________ 17,22 crenulata, H emitelia _________ _ 5 Cretaceous stratigraphy --_ _______ 17 Cribmtina teacnma, ________ -- 2, 18, 21, 22 Crossbedding, fluvial type _____________ 13 Cycad fragments _____________________ 15 Cycadeoidea ___________________________ 14 Cymbopho'ra, sp _______________________ 21 Cyprimeria gigantea ___________________ 21 D Dakota Sandstone, Arizona ___________ 14 Colorado New Mexxco -- Utah _______ Dentulium sp _________________________ 21, 22 Depositional environment, continental--- 18 nearshore ------------------------- 20 Devils River Limestone (Georgetown)-- 20 Dicksonia fibrasa --------------------- 5 Dicotyledonae ------------------------- 13 Dinosaur remains __________________ 11, 14, 15 Diplodon n. sp ----------------------- 11 Dorsiventral growth --------- 23, 24, 25, 30, 31 douglassi, Protelliptio _- 12 Drepanochilus kiowana ---------------- 22 E (Echinobathm‘a) n. sp., Pyrazus ------ 20 Edwards Formation (Tex.) ------ - 20 emoryi, Trigom'u __ - 22 endlichi, Lioplaac __ - 11 Endogem'tes erosa. --------------------- 2, 3 Engonoceras serpentinum ----------- 20, 21, 22 Epidermal hairs ---------------------- 25 erase, Endogem'tes -------------------- 2, 3 Tempskya 3. 6 Eupo‘ra aneatue ----------------------- 12 European faunal sequence ------------ 16 Exogym sp --------------------------- 22 F False trunks symmetry terminal parts farri, Lampsilis ______________________ 12 fibrosw, Dicksonia --_- - 5 Fish teeth and bones -- - 11 Fluid transportation __ _______ 24 Foraminifera ------------------- 10, 13, 21, 22 Fredericksburg age ------------------- 17, 20 Fresh water, mollusks - - 10 ostracods -------------------- 11 Frontier Formation (Wyo.) ----- 9, 10, 11, 15 Page G Gannett Group (Wyo. and Mont.) __-- 11, 12 Gastroliths --------------------------- 15 Gastropods 11, 13, 20, 21, 22 geminata, Nerinea -------------------- 20 gigantea. Cyprimeria. ------------------ 21 glabra, Ne’rinea geminata ------------- 20 Gleicheniaceae ------------------------ 4 Glen Rose age -------------------- 17 grandis, Tempskya. _____ 2, 4, 5, 6, 9, 27, ‘31, 32, 33, 34; pl. 10 Graneros Shale (S. Dak.) ____________ 14 Growth habit _____________________ 3, 4, 5, 24 Gryphaea ----------------------------- 12 H hamili, Um'o ------------------------- 11 harrisi, Cla’uator --------------------- 11 Helix sp ----------------------------- 11 Hell-to-Finish Formation 17 Hemitelia crenulata ------------------- 5 smithii ___________________________ 5 History of investigations of Tempskya" 2 I inconspicuus, Campttmectes ___________ 21 In-place specimens ------------------- 6 Introduction -------------------------- 1 K kunsasense, Cardium ----------------- 21 kamasensis. Turritella ---------------- 22 Key to species -- ---------------------- 38 kiowana, Drepanochilus _______________ 22 knawltoni, Tempskya ___- 2, 4, 6, 12, 24, 25, 31, 32, 33, 34, 36; pl. 8 Kootenai Formation ---------------- 6, 11, 12 L La Sal Mountains (Utah) ____________ 12 Lampailia farm' -. Leaf emergence Leaf traces, abundance --------------- 25 morphological details ------------- 23 Leptosporangiatae -------------------- 3, 4 Lima sp ----------------------------- 21 Liaplax endlichi ----------------------- 11 Little Hatchet Mountains (N. Mex.) --------------- 17 London Clay (Great Britain) --------- 6 Lopha quadriplicata. ------------------ 21, 22 Lower Greensand (Great Britain) ---- 3,6 Loxsomaceae ------------------------- 4 Lunatia ______________________________ 21 Lymnaea (Pleurolimnaea) n. sp ------ 11 M McElmo Formation ---- macrocaula, Tempskua ---------------- 3 41 42 Page marginatus, Porosus __________________ 2 Marine aspect, sandstone _____________ 13 Marine invertebrate faunas ___- 1, 9, 13, 14, 16, 17, 18, 20, 22 Mascal age ___________________________ 15 M elampus caurinus 11 Mesaverde Group _ 15 Metacyris ____________________________ 11 Mexican geosyncline area specimens __ 1.9 micrmrhiza, Tempskya ________________ 3 Microtaenia paucifolia ________________ 11 minor, Tempskya ______ 1, 2, 4, 9, 10, 11, 15, 31, 32, 33, 34, 36; pl. 9 Moenkopi Sandstone (Ariz.) __________ 14 Mojado Formation (N. Mex.) __ 2, 18, 17, 28, 31 biota, Animas Mountains _________ 21 Mojado Pass _________________ 20 montanensis, Viviparus _ 11 Morrison Formation ____________ 11, 12, 13, 15 Mowry Shale _______ mudgeana, Anchura ___________________ 21 mundula, Aclistocha'm ________________ 11 Musculiopsis sp _______________________ 11 N Neithea teammz _______________________ 21 NeOCOmian Age _______________________ 14 Neogastroplites ______________________ 6, 9, 10 Nerinea geminata ___- __ 20 geminata. glabm _ _______ 20 shumlensis _____________ 20 sp _______________________________ 20 (Nerinoides) n. sp., Aptyxella. ________ 20 N erita sp ______________________ _ _ 20 (Theilostyla) sp __ __ 20 Newcastle Sandstone - __ 10 Nucula sp ____________________________ 21 O Occurrences of Tempslcya in the United States ___________________ 6 Oligocene age _________________________ 14 ortmanni, Coniobasis ____________ 11 Ostrea perverse _________________ - 20, 21 sp _________________ - 20 Overton Fanglomerate ________________ 10, 11 P Paleocene age ________________________ 15 Patapsco Formation (Md.) _ 3,18 paucifolia, Microtaem'a _______ 11 pawpawensis, Polychasmim ___________ 21 pecoense, Cerithium ___________________ 20 Pyrazus __________________________ 20 Pelecypods __________________ 11, 12, 20, 21, 22 perverse, Ostrea. _ ___________ 20,21 Petrified wood -__ ___________________ 14 Photosynthesis _ ___________________ 24 Phthoropterides _______________________ 3 Physa uaitata _________________________ 11 Pinyon Conglomerate ___ __ 15 Planorbis pmecursoris ____________ 11 Pleistocene age _______________________ 14 INDEX Page (Pleurolimnaea) n. sp., Lymnaea _____ 11 Plicatula sp __________________________ 20, 21 Polychasmma. pawpawensis ___________ 21 Porosus marginatus __________ 2 Postdepositional deformation __________ 29 Potomac Group _______________________ 13, 14 praecursoris, Planorbis ________________ 11 Protelliptio douglassi __________________ 12 Protocardiu texuna ____ _ 20, 21, 22 Protopteris _________ ___- 3 Pseudonerinea sp __ -- 20 pulchra, Tempskya ____________________ 3 Purgatoire Formation (N. Mex.) ______ 20 Pyrazus (Echinobathyra) sp __________ 20 pecoense _________________________ 20 Q quadriplicata, Lopha. __________________ 21, 22 Quaternary age _______________________ 14, 15 R Raritan Formation ____________________ 13 Reconstructions of growth habit ______ 24 reesidei, Tempskya _- 1, 13, 22, 24,25, 28, 29, 30, 31, 32, 33, 34; pls. 1—4; figs. 7—9 Reworked specimens ___________________ 14 Rocky Mountain geosyncline area specimens ________________ 6 rosaica, Tempskya, __ 3, 4, 31, 32, 33, 34, 36; pl. 8 S Scaphopods ___________________________ 21, 22 schimperi, Tempskya 3 Schizaeaceae ________ 4 Sedimentary loading, preservation ___- 29 Selected references ____________________ .96 Senonian Age ________________________ 5 seriatimgmnulata, Turritella ___ 20, 21 serpentinum, Enganoce’ras __ __ 20, 21, 22 Serpula sp ________________ __ 21 shumlensis, Nerinea ___________________ 20 Sierra Rica ___________________________ 17 Skull Creek Shale _ 10 smithii, Hemitelia _ 5 Soil, fossil ________ 18 stamttmi, Chara. _______________________ 11 Stems, characteristics _________________ 23, 29 diameter ____________ dichotomy ___________ functional distance _ Stone cells ___________________________ Stratigraphy of the Tempakya-bearing Rocks, New Mexico ______ 16 superba, Tempskyauu 2, 5, 31, 32, 33, 34; pl. 13 Synopsis _____________________________ 3.? Systematic descriptions _______________ 22 T Tellina sp ____________________________ 20, 21 Tempskya. __ _____ 1 22 erosa _ _ _ _ _______________ 3, 6 grandis ____________ 2. 4, 5, 6. 9. 27. 31. 32. 33 34; pl. 10 Page knowltom‘ ___________ 2, 4, 6, 12, 24, 25, 31, 32, 33, 34, 36; pl. 8 macroeaula, _______________________ 3 micron-him __ ________________ 3 minor _______________ 1, 2, 4, 9, 10, 11, 15, 31, 32, 33, 34, 36; pl. 9 pulchm ___________________________ 3 range 11 reesidei ,___ 1, 13, 22, 24, 25, 28, 29, '30, 31, 32, 33, 34; pls. 1—4; figs. 7—9 Tossica, _____________ 3, 4, 31, 32, 38, 34, 36 schimpen‘ _________________________ 3 superba _________ 2, 5, 31, 32, 33, 34; p]. 13 wesselii _____________ 2, 5, 6, 24, 27, 31, 32, 33, 34; pl. 12 whitei _____________________ 3, 4, 5, 13, 31 wyomingensis _____________ 2, 5, 15, 31, 32, 33, 34; pl. 11 zelleri ______ 1, 13, 18, 20, 22, 24, 28, 30, 31, 32, 33, 34, 36; pls. 5—7; fig. 10 sp ____________________ 6, 9, 10, 12, 14, 15 Tempskyaceae ________________________ 4 tewuna, Crib'ratina. ______________ 2, 18, 21, 22 Neithea 21 Protoca/rdiu ____________________ 20, 21, 22 Texas Coastal Plain faunal sequence ___ 16 (Theilostyla) sp., Nerita ______________ 20 Thermopolis Shale (Wyo.) _________ 10, 14, 15 Todea barbara _________________ 5 Trigom‘a emo'rm‘ ________________ 22 Trinity age ___- 11 Turonian Age ___ _______________ 5 Turn'tellu kansasensis ________________ 22 seriatimgranulata _________________ 20, 21 U U-Bar Formation ____________________ 17 Unconformitia _______________________ 17, 22 Unio hamili _____ 11 vetustus ___________________ 11 sp ________________________ 11 usitata, Physu ________________________ 11 V vetustus, Univ ________________________ 11 Viviparus montanensis ________________ 11 W Washita age ____________________ 18, 20, 21, 22 Wayan Formation (Idaho) - 4, 9 Wealden flora (England) _____________ 14 wesselii, Tempskya. ______ 2, 5, 6, 24, 27, 31, 32, 33, 34; pls. 10, 12 whitei, Tempskya. ______________ 3, 4, 5, 13, 31 Willow Tank Formation (Nev.) _______ 10 Woodbine Formation (Tex.) ___ 20 Woolwich Beds (Great Britain) 6 Worm ________________________________ 21 wyomingensis, Tempskya ______ 2, 5, 15, 31, 82, 33, 34; pl. 11 Y, Z Yoldia sp _____________________________ 21 zelleri, Tempskya ______ 1, 13, 18, 20, 22, 24, 28, 30, 31, 32, 33, 34, 36; pls. 5—7; fig. 10 sfirUS. GOVERNMENT PRINTING OFFICE: 1975 0—211—319/37 PLATES l- 13 Contact photographs of the plates in this report are available, at cost, from US. Geological Survey Library, Federal Center, Denver, Colorado 80225 PLATE 1 FIGURES 1,2. Tempskya reesidei Ash and Read, n. sp. (p. 25). From USGS fossil plant locality 9703. 1. Cross section of the false trunk of the holotype showing the radial distribution of stems. The section is also shown in figure 7. Note that the stems are cut transversely in the central part of the false trunk whereas they are cut rather obliquely near the exterior. USNM 167546f, X 3/4. Two stems and associated leaf traces in the central part of the false trunk. A petiole has just separated from the left-hand stem, and a leaf trace is in an advanced stage of separating from the same stem, as indicated by the protuberance that has formed on the stem. A leaf trace that is nearly free is as- sociated with the right-hand stem. The pronounced thinning of the stele in the same stem indicates that another leaf trace is in the process of forming. The “islands” of parenchyma in the inner cortex show as light irregularly shaped areas just outside of the steles. A few adventitious roots that are randomly oriented are noticeable in the picture. Slide USNM 1675469., X 5. GEOLOGICAL SURVEY PROFESSIONAL PAPER 874 PLATE 1 TEMPSKYA REESIDEI ASH AND READ, N. SP. PLATE 2 Tempskya reesidei Ash and Read, n. sp. (p. 25). From USGS fossil plant locality 9703. Cross section a short distance away from the center of the false trunk. Here the stems are somewhat obliquely oriented, and the petioles are at a more oblique angle. Slide USNM 167546b, X 5. GEOLOGICAL SURVEY PROFESSIONAL PAPER 874 PLATE 2 FIGURES 1—3. PLATE 3 Tempslcya recsidei Ash and Read, n. sp. (p. 25). From USGS fossil plant locality 9703. 1. Cross section of two stems which are in the proce5s of dividing. A leaf trace has started to separate from the left-hand stem, whereas a free petiole has just formed above the right-hand stem, and a leaf trace has begun to form on the same stem. The “islands” of parenchyma in the inner cortex that are typi- cal of this species are clearly visible in some places. Note, especially in the upper right-hand corner, that the long axes of some of the roots extend horizontally or at random, rather than vertically as they do typically in T. zellem’ and other species of Tempskya. Slide USNM 167546c, X 10. . A stem that has just begun to divide into two stems. The process of dividing has not progressed as far in this example as in the stem shown in figure 1, so the stem is only slightly bilobed. A leaf trace has started to form from the right-hand lobe and a free petiole is just above the narrowed part of the stem. It probably arose from this same stem at a lower level. In most of this view, the long axes of the adventitious roots are oriented vertically, although a few can be seen that are randomly oriented. Slide USNM 167546d, X 10. . The basal part of the false trunk of the paratype. This is a view of the side opposite that shown in figure 8. Compare with the picture of the holotype of T. knowltom' given by Seward (1924, pl. 16, fig. 1). USNM 167547, about X 1/3. GEOLOGICAL SURVEY PROFESSIONAL PAPER 874 PLATE 3 TEMPSKYA REESIDEI ASH AND READ, N. SP. PLATE 4 Tempskya recsidei Ash and Read, n. sp. (p. 25). From USGS fossil plant locality 9703. A stem and two associated leaf traces. The leaf trace in the upper left is nearly free, whereas the other is in a much less advanced stage of departure, and the leaf gap in the stele is beginning to close. Note that the stelar cells in the stem are cut transversely, whereas those in the leaf traces are cut on oblique angle. The “islands” of parenchyma surrounded by sclerenchyma in the inner cortex of the stem are clearly visible in this section. e, approximate position of the epidermal area; ph, phloem. Slide USNM 167546e, X 25. GEOLOGICAL SURVEY PROFESSIONAL PAPER 874 PLATE 4 TEMPSKYA REESIDEI ASH AND READ, N. SP. PLATE 5 ‘Tempskya zelleri Ash and Read, n. sp. (p. 28). From USGS fossil plant locality 9702. General view of part of the false trunk of the holotype showing the distribution and orientation of the stems. Note that only one departing leaf trace is associated with most stems. Two exceptions are near the figure margin in the upper left. Each of these contains one leaf trace that is in an advanced stage of de- parture and another that is in an earlier stage. This feature indicates that the internodes are fairly long in comparison with those of T. grandis. Also note the fact that there are no free petioles in this section, a characteristic of the species. Slide USNM 167543b. X 5. GEOLOGICAL SURVEY PROFESSIONAL PAPER 874 PLATE 5 i1}; ,F-gyi‘ 4'. H\\\I/\l\\Y-f h.“ '9...‘:' ¢. M?‘ q .‘n. W( TEMPS ’IA ZELLERI ASH AND READ, N. SP. PLATE 6 FIGURES 1—3. Tempskya zelleri Ash and Read, n. sp. (p. 28). Slide USNM 16754313., x 10. From USGS fossil plant locality 9702. 1. Cross section of a stem in which the xylem of the leaf trace is free and the leaf gap in stem has closed but the cortex of the stem and leaf trace is still united. Note the typical knoblike bulges at the ends of the xylem in the leaf traces in this and the other figures on the plate. Practically all the adventitious roots in the figures are cut transversely, indicating that they are vertically oriented, as is usual in this species. 2. Another stem in the false trunk. The xylem of one leaf trace is now free, and a second one is in the early stages of departure to the left. 3. Six stems with leaf traces in various stages of departure. A stem that is in the process of bifurcating is in the lower left of the figure. GEOLOGICAL SURVEY PROFESSIONAL PAPER 874 PLATE 6 “"3; ‘1 ‘ ,3} Mg "‘-. ’2“ ..r_ ‘ '." . .- ’11,. - TEMPst/A ZEiLERI ASH AND READ, N. SP. o,: "7' PLATE 7 Tempskya zelleri Ash and Read; n. sp. (p. 28). From USGS fossil plant locality 9702. Transverse section of a stem containing two departing leaf traces. One leaf trace at the upper left is in an advanced stage of departure, although the associated leaf gap in the stele is still present. A second leaf trace which has just begun to form is at the upper right. A leaf gap has formed at the left side of the trace, whereas the xylem of the trace and stele are still united at the right. The knoblike bulges at the ends of the xylem in the left-hand leaf trace are well developed. Angularity of the stem is particularly noticeable in this view. A few of the adventitious roots are randomly oriented, although most are vertically oriented as is characteristic of this species. Slide USNM 167543a, X 25. GEOLOGICAL SURVEY PROFESSIONAL PAPER 874 PLATE 7 I ‘ TEMPSKYA ZELLERI ASH AND READ, N. SP. FIGURES 1—3. PLATE 8 Tempskya knowltom’ Seward, 1924 (p. 4). 1. 2, 3. General view of part of the false trunk showing several pairs of stems imbedded in a groundmass of adventitious roots. Note the absence of free petioles in the section. They are characteristically rare in the false trunk of this species as they are in T. zellem‘ n. sp. and T. rossica. The dense, thick sclerotic cortex that is typical of T. knowltom' is evident in this figure and the others on the plate. The occurrence of just one or two leaf bases with each stem indicates fairly long internodes and contrasts with T. grandis where there are often four or more leaf bases with a single stem. Slide USNM 39266 (section V), X 3. Cross section of a pair of stems that are in the process of dividing. Note that there is one departing leaf trace with each stem and that both have a protuberance marking the beginning of a second leaf trace. The excellent preservation of most tissues in this fossil is well illustrated in figure 3. Both from slide USNM 39266 (secticn V); 2, X 10; 3, X 25. PROFESSIONAL PAPER 874 PLATE 8 GEOLOGICAL SURVEY TEMPSKYA KNO WLTONI SEWARD FIGURES 1—3. PLATE 9 Tempskya minor Read and Brown, 1937 (p. 4). 1. Part of the false trunk showing the general distribution of the small stems typical of this species im- bedded in a groundmass of adventitious roots. The occurrence of only one or two departing leaf traces with each stem suggests that the internodels are fairly long and are comparable in length with those in T. rossica, T. knowltom‘, and T. wyomingensis. Slide USNM 39260c, X 5. Two stems in which the xylem cylinder is well preserved. Slide USNM 39‘254a, X 10. 3. No stems that are in the process of dividing. Apparently dividing of the stem began at a slightly lower level. A departing leaf trace is present at the upper left of the right-hand stem. The protuberance on the side of each stele marks the beginning of a leaf trace. Slide USNM 39260c, X 25. 5° GEOLOGICAL SURVEY PROFESSIONAL PAPER 874 PLATE 9 . y ~ 4 ® - - . .. ~'l\. ‘ .v _ TEMPSKYA MINOR READ AND BROWN PLATE 10 FIGURm 1—4. Tempskya grandis Read and Brown, 1937 (p. 4). 1. Transverse section of a stem with three leaf traces in various stages of departure and two leaf-trace protuberances. At least one of the traces is free or practically so. Note the comparatively large num- ber of departing leaf traces and leaf-trace protuberances associated with each stem on this plate. This demonstrates that the internodes are fairly short in T. grandis, contrasting strongly with T. wesselii and T. zellem’ which have long internodes and usually only one leaf trace and possibly one leaf-trace pro- tuberance associated with each stem. Slide USNM 39267p, X 3. 2. A stem with several departing leaf traces and leaf-trace protuberances. Slide USNM 39164b, X 5. 3. Part of the side of a stem showing the three-layered cortex. The discontinuous and irregular band of scler- enchyma in the inner cortex near the stele is exceptionally well preserved in this specimen, although a similar band of sclerotic tissue in the pith is not as clear. Slide USNM 39267i, X 25. 4. Cross section of a stem which has at least six departing leaf traces. The one on the left edge of the pho- tograph has just become free of the stem, whereas near the center of the photograph the stele of another has just separated from the stele of the stem. Near the bottom of the picture an adventitious root is in- serted on the stem. Slide USNM 39267j, X 25. PROFESSIONAL PAPER 874 PLATE 10 GEOLOGICAL SURVEY TEMPSKYA GRANDIS READ AND BROWN PLATE 1 1 FIGURES 1—4 Tempskya wyomingensis Arnold, 1944 (p. 5). All figures are from the holotype (No. 23400) in the Museum of Paleontology, University of Michigan. 1, 2 Two general views of part of the false trunk showing several stems with departing leaf traces and two free petioles surrounded by a groundmass of adventitious roots which are mainly vertically oriented. Note that two or more departing leaf bases are attached to each stem in these and the other figures on the plate, indicating that the nodes are fairly long. x 5. 3. A rather small stem from which three leaf traces are in the process of departing. This same stem is V shown in figure 1. X 10. 4. A somewhat larger stem with twc departing leaf traces and one free petiole that evidently just separated from the stem. The outer continuous band of sclerenchyma is clearly visible, but the inner discontinuous band is not as clear. The two protuberances above the stele mark the beginning of a leaf trace. X 10. PROFESSIONAL PAPER 874 PLATE 11 GEOLOGICAL SURVEY TEMPSK YA WYOMINGENSIS ARNOLD PLATE 12 FIGURES 1—3. Tempskya wesselli Arnold, 1944 (p. 5). All figures are from slides made from the holotype (23399) in the Museum of Paleontology, University of Michigan. 1. A stem from which a leaf trace is in the process of separating. The two continuous bands of sclerenchyma separated by a complete band of parenchyma are clearly visible surrounding the stems and the leaf traces in all figures on the plate. In the example shown in this figure the leaf gap has closed, the bands of scleren- chyma are still complete, and there is a gap in the parenchymatous layer where the inner band of the leaf trace is still fused with the inner band of the stem. Most of the adventitious roots in this figure are cut transversely. An exception is the root that is cut more or less tangentially at the lower left. X 25. General view of a part of the false trunk showing the distribution and remains of four stems in a ground- mass of adventitious roots. The stem near the left side of the photograph has been penetrated by several roots. The stem near the center is shown in figure 1. X 5. A stem from which a leaf trace is in the process of separating. In this example the separation has not pro- ceeded quite as far as in figure 1 and a leaf gap is still present. An adventitious root which probably arose from the stem is at the lower right imbedded within the band of parenchyma. The root is cut trans- versely and probably is vertically oriented. A second adventitious root that is in the process of separating from the stem is at the lower left. It is cut tangentially and apparently is horizontally oriented. The xylem of the root is not connected with the xylem of the stem, but the sclerenchyma bands of the stem cortex are connected with the sclerenchymatous cortex of the root. X 10. GEOLOGICAL SURVEY PROFESSIONAL PAPER 874 PLATE 12 TEMPSKYA WESSELLI ARNOLD PLATE 13 Tempskya superba Arnold, 1958 (p. 5). A stem with three leaf bases in various stages of departure. Short internodes in this species are responsible for this noteworthy feature of T. superba. The remains of sev- eral adventitious roots are visible including one that is in the process of separating from the right side of the stem. Although this fossil is not very well preserved, the thick band of parenchyma in the inner cortex and the discontinuous, irregular band of sclerenchyma that characteristically surrounds the vascular cylinder in this species are visible in a few places, X 10. The figure is from a. slide (USNM 1675453) made from the holotype (34561) in the Museum of Paleontology, University of Michigan. GEOLOGICAL SURVEY PROFESSIONAL PAPER 874 PLATE 13 \‘fl’ TEMPSK YA S UPERBA ARNOLD 63$ EARTH SCIENCES HBRARY 7 DAY Deformations Associated With Relaxation of Residual Stresses in a Sample of Barre Granite From Vermont 1 GEOLOGICAL SURVEY PROFESSIONAL mcuMENTS DEPARIMW APR 15 “375 LlBRRRY UNIVERSITY OF “LEON t ”A“ 7 1975 PAPER 875 Deformations Associated with Relaxation of Residual Stresses in a Sample of Barre Granite From Vermont By T. o. NICHOLS, JR. GEOLOGICAL SURVEY PROFESSIONAL PAPER 875 A study of measurable-mobilized strains that occur when a granitic rock sample, containing stored elastic deformational energy, is physically dissected in a systematic manner UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1975 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Nichols, T. C. Deformations associated with relaxation of residual stresses in a sample of Barre Granite from Vermont. (Geological Survey Professional Paper 875) Based on the author’s thesis study (M.S.), Texas A&M University. Bibliography: p. Supt. of Docs. no.2 119.16:875 1. Rock deformation. 2. Granite—Vermont. I. Title. II. Series: United States Geological Survey Professional Paper 875. QE604.N52 S22'.3 74—26896 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 ~ Price $1.25 (paper cover) Stock Number 024—001-02601 CONTENTS Page Glossary of uncommon terms used in text ___________ IV Experimental results — Continued Abstract _______________________________ 1 Phase III—Continued Introduction ____________________________ 1 Other significant petrofabric features ________ Acknowledgments _________________________ 1 Discussion _____________________________ Previous work ___________________________ 2 Strain magnitudes ______________________ Recognition of residual stress ________________ 2 Behavior of the block — Coring of top surface ______ Investigations to determine the nature of residual stress 2 Behavior of the top surface ______________ The Barre Granite ______________________ 2 Behavior of side-2 and bottom surfaces _______ Experimental procedure _____________________ 3 Behavior of the block — Coring of bottom surface ___ Phase I _____________________________ 3 Behavior of block — Coring of side-2 surface ______ Instrumentation _____________________ 3 Behavior of block — Total experiment __________ Spurious signals associated with the experimental Behavior of block —- Summary _______________ technique _______________________ 3 Comparison of relieved strains with X-ray prestress deter- Succession of coring and collection of data _____ 4 minations __________________________ Phase II ____________________________ 5 Relieved residual stress field and comparison with in situ Observation of surface fabric _____________ 5 determinations _______________________ Phase III ____________________________ 5 Relieved residual stresses _______________ Petrofabric examination ________________ 5 Relieved residual stresses compared with in situ Experimental results _______________________ 6 stress field _______________________ Phase I _____________________________ 6 Deformation mechanisms _______________ Presentation of strain data _______________ 6 Relation of fabric anisotropy to residual stress fields Phase II ____________________________ 19 Speculation and possible future work ________ Surface fabric changes _________________ 19 Conclusions _____________________________ Phase III ____________________________ 19 References cited __________________________ Anisotropy of petrofabric elements __________ 19 ILLUSTRATIONS FIGURE 1. Drawing of block of Barre Granite, showing rift, grain, and hardway planes, and top, bottom, and side—2 faces positions of the 45°-strain rosettes ________________________________________________ 2—5. Photographs showing: 2. Top after annuli have been broken out ____________________________________________ 3. Bottom after successively coring with 5.1-, 10.1-, and 15.0-cm core bites ________________________ 4. Bottom showing fractured interior annulus _________________________________________ 5. Side 2 after coring with 10.1-cm core bit ___________________________________________ 6—13. Diagrams showing vectors of principal strain changes caused by each coring at numbered gage locations on top, bottom, and side-2 surfaces: . After top 5.1-cm coring _____________________________________________________ . After top 10.1-cm coring _____________________________________________________ . After top 15.0-cm coring _____________________________________________________ . After bottom 5.1-, 10.1-, and 15.0-cm coring _________________________________________ - 10. After side-2 10.1-cm coring ___________________________________________________ 11. Total net strain changes after 10.1—cm coring on side 2 __________________________________ 12. Over quartz and feldspar grains after the 10.1-cm and 15.0-cm corings on the top surface ______________ 13. Over quartz and feldspar grains on the top surface after the 5.1- and 10.1—cm corings on the bottom face _____ 14. Reflected-light photographs of surfaces dyed with organic fluorescent dye before and after coring __________ 15. Photomicrographs of Barre Granite showing fractures parallel to rift plane and parallel to lift plane ________ 16. Diagram showing a cylindrical cantilever loaded at the base _______________________________ 17. Schematic diagram showing changes of stored energy in a residual energy model resulting from elastic and permanent deformations of the model elements ____________________________________ 18. Exaggerated illustration of deformations, following the top 15.0-cm coring, of the inner and outer annuli _____ 19. Diagram showing inferred in situ stress vectors as calculated from deformations of the block of Barre Granite __ “3QO HI Page 19 19 19 20 21 22 23 23 23 23 25 26 26 26 28 29 31 31 32 Page 0‘01ka 10 12 14 16 17 18 19 20 21 23 24 29 29 IV TABLE CONTENTS TABLES 1. Strain differences, Ace and A B e, at gages 1 through 34 ________________________________________ 2. Strain differences, A6 and AB 6, at gages F1, F2, and Q ______________________________________ 3. Magnitude of A062 and respective thicknesses of annuli after 15.0-cm overcoring on the top surface. The A052 and annuli thicknesses are shown for each gage location on the annuli surfaces ____________________________ 4. Stress changes determined on the top central core and on the outer annulus after top 15.0-cm coring __________ GLOSSARY OF UNCOMMON TERMS USED IN TEXT Definitions used here are consistent with those of quoted authors or of others considered to be experts in the field. Locking domainzA volume and shape in rock that can contain in equilibrium a particular system of internally balanced forces (Varnes and Lee, 1972). Overcore: To make a cut with a core barrel around an instrumented volume of rock, separating that volume from the remaining rock mass. Prestrain: Strain that results from prestress. The term alludes to locked-in, potentially recoverable strains that reflect rock dis- tortions related to the history of internal or external loads. Prestress: The term used in this text is synonymous with residual stress. The term alludes to that part of the previously applied internal or external forces which is stored as residual stress. Residual strain: Locked-in, potentially recoverable elastic distortions, satisfying internal equilibrium conditions with no exter- nal loads or temperature gradients (Friedman, 1972). Residual stress: A stress system satisfying internal equilibrium, with no external loads or temperature gradients (McClintock and Argon, 1966). Strain relief or stress relief: The mobilization of stored elastic energy (stress or strain) by creating physical changes within an existing force field. 6 8 22 27 DEFORMATIONS ASSOCIATED WITH RELAXATION OF RESIDUAL STRESSES IN A SAMPLE OF BARRE GRANITE FROM VERMONT BY T. C. NICHOLS, JR. ABSTRACT A cube of Barre Granite (approximately 22 cm on a side), free of boundary loads, was sequentially and concentrically drilled out with 5.1-, 10.1-, and 15.0-cm core bits on the top and bottom sides and with a 10.1-cm core bit on a third side. The top and bottom sides are parallel to the grain or lift plane, which is horizontal in the quarry, and the third side is parallel to the rift plane, which is vertical and strikes N. 30° E. Strains resulting from overcoring were recorded on these sides with thirty-six 45°-rosette resistance strain gages. The strain changes observed upon coring seem to have been derived from a complexly changing internal residual stress field. Initially, as the top surface was overcored, the resulting strain changes were mostly compressional; that is, the block contracted. Strain changes became more extensional as the bottom of the block was overcored. Finally, there were large net strain changes that were radially compressional and tangentially extensional, relative to each set of concentric over- cores. The largest strain changes were observed adjacent to freshly cut sur- faces, yet significant changes were monitored over the entire surface of the block after each overcoring. Cumulative strain changes monitored on opposing gages symmetrically located across a freshly cut surface were, much of the time, similar in sense, magnitude, and direction. The residual stress field is related to the changing geometry of the body, the induced free surfaces, and seemingly to the degree of homogeneity, orientations, and magnitudes of the stresses when they were frozen in. Cumulative strain changes at individual gage locations were large — as much as 330 X 10’6 (extension) and —275 X 10*6 (com- pression). The deformation generally appears to be elastic. At certain gages, however, there were occasional large anomalous strain changes that are interpreted to result from permanent deformations, such as in- tergranular movements or intragranular gliding. These mechanisms may also be partly responsible for a pronounced creep event that was observed after the top 15.0-cm coring was completed; the whole block, which had initially contracted, expanded for 48 hours after the coring event, until the net strain change was nearly zero. The extensional deformations typically occurring across a pilot borehole as a result of stress-relief overcoring techniques can be caused by at least two possible stress conditions an externally applied com- pressional stress field or the mobilized tensile component of the inter- nal or residual stress field. When externally applied compressive forces are cut by overcoring, the innner and outer walls of the newly created annulus will expand. An internally balanced stress field may be par- tially relieved upon cutting of the outer cylindrical surface of the newly created annulus. If the tensile energy is relieved locally, then com- pressive strains are mobilized, causing the annulus to become thinner. The interior wall thus expands, and the exterior wall contracts. A deformation-measuring device placed in the interior hole measures ex- tension of the interior wall in both stress conditions considered. INTRODUCTION With any deformation of a natural rock mass, whether manmade or natural, there is an accompanying distur- bance of the preexisting quasistatic elastic stress field. The stresses, whether residual, gravitational, or present- day tectonic, are at least momentarily unbalanced, and energy is expended in some type of work or heat transfer. A better understanding of how released residual-elastic- strain energy is expressed at free boundaries is needed. Systematic quantitative knowledge of this phenomenon would be of great practical value for the design of mines, quarries, and engineering structures. Also, this knowledge may well be of value to earthquake predic- tion. I believe that the source of a significant quantity of the total energy released has been residual strain (stress). If the release of these stresses can be predicted quantitatively in terms of a stress and (or) strain tensor, the information will be valuable in design and contruc- tion practices. In the present research, a systematic investigation of residual-strain relief was conducted to determine the magnitudes and distribution of the resulting strain changes as the geometry of a block of rock was modified and to discover the relation of these strains to fabric elements. “Prestress, residual stress, prestrain, residual strain,” and other uncommon terms used in this text are defined in the glossary. This report is based on a thesis study conducted at Texas A & M University in partial fulfillment of the re- quirements for the Master of Science degree. ” {‘ H (( ,, “ ACKNOWLEDGMENTS I am pleased to acknowledge the active support, criticism, thoughtful comments, and many ideas provided by Melvin Friedman, John W. Handin, David W. Stearns, David J. Varnes, Fitzhugh T. Lee, and John F. Abel, Jr. Thomas A. Bur, US. Bureau of Mines, supplied the block of Barre Granite upon which the experiment was 1 2 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT performed. The US. Geological Survey and the Ad- vanced Research Projects Agency provided much of the instrumentation and apparatus necessary to perform the experiment. PREVIOUS WORK RECOGNITION OF RESIDUAL STRESS The existence of residual stress in rocks and rock masses has been recognized for a long time — at least 180 years (Varnes, 1969). Much thought has been given recently to this subject (Lowry, 1959; Pincus, 1964; Friedman, 1967, 1971, 1972; Denkhaus, 1967; Voight, 1967; Emery, 1968; Nichols and others, 1969; Price, 1969; Hoskins and Daniels, 1970). Several of these authors have made measurements of residual strains in laboratory experiments. Price (1969, p. 7) observed siltstone specimens that expanded in creep under uniax- ial compressive loads of 640 bars and greater, and he at- tributed this expansion to the release of residual stresses. Friedman (1967) used X-ray techniques to determine the existence of residual elastic strains within quartzose rocks. Also, Friedman and Logan (1970) related similarly determined data to the orientations of fractures induced in quartzose rock specimens under controlled boundary loads. Friedman (1971, 1972) has summarized informa- tion about residual strains, primarily from his X-ray studies. INVESTIGATIONS TO DETERMINE THE NATURE OF RESIDUAL STRESS Very few investigations have been undertaken to determine the extent of the relief of residual stresses within rock masses and the manner in which the released energy is expressed at free rock boundaries. Emery (1964) used photoelastic transducers to demonstrate the existence of elastic strain energy in sandstones, foliated quartzites, granites, and pebble conglomerates. He concluded that these rocks contain substantial amounts of potential elastic strain energy that in some cases is recoverable and that has pronounced measurable anisotropy. Pincus (1964) also used photoelastic transducers, and he was able to demonstrate that 5.4-cm(NX)-diameter rock disks, relieved by small-diameter (0.4-cm) holes, showed relief patterns around the holes. Sharp (1969) made residual-stress determinations on core samples taken from the Red Mountain mid-Tertiary intrusive complex, west of Denver, Colo. The determined secondary principal stresses were attributed to stored energy related to the forcible emplacement of a mid- Tertiary stock. Also, Sharp alluded to the relief of strain as a possible cause of failure for future mine shafts in the area. Varnes (1969) designed and built a physical model consisting of individual units representing grains in a rock. The model behavior simulates that of stress relief within a rock. The grain units capable of interacting viscously and elastically with elastic and frictional restraints. Varnes stated that experiments are in progress to detect effects of size and geometry of the model. E. G. Bombolakis (in Friedman and Logan, 1970, p. 387) attempted to determine the size effects on relieving residual stresses by overcoring. Starting with 12.5-cm cores of granite near Chelmsford, he successively over- cored 45°-rosette resistance strain gages with 7 .5-, 5.4-, and 3.8-cm core bits, and after each overcoring he measured still further strain relief. Hoskins and Daniels (1970) determined directions and relative magnitudes of residual strains in six different specimens by undercoring with different sized core bits. J. F. Abel, Jr., and T. C. Nichols, Jr., performed similar experiments in US. Geological Survey laboratories on Precambrian metasedimentary and granitic rocks from Colorado and on Paleozoic granite from North Carolina. Friedman (1971) conducted experiments in which he measured residual strains in the grains of naturally deformed sandstones. Using X-ray techniques, Fried— man compared the “before” and “after” state of residual strain in individual sandstone grains that were relieved by etching with hydrochloric and hydrofluoric acids. Similarly, he compared the “before” and “after” state of residual strain in individual sand grains of laboratory- compacted and welded specimens, freed merely by etching with water. Gallagher (1971) concluded from photoelastic model studies that residual elastic strains detected in sandstones by X-ray method are average values that in- clude the strains within the grains and the cementing matrix over the volume scanned. THE BARRE GRANITE Barre Granite was chosen for this experiment because much work had already been done on its mineralogy and on the nature of its elastic anisotropy. White (1946) recognized that much nonuniform energy is relieved in quarry operations at Barre, Vt.; Hooker and Johnson (1969) measured the in situ stress field in the Smith granite quarry and at nearby outcrops close to Barre, and they made laboratory measurements on cores from which values of the anisotropic elastic moduli were calculated. Douglass and Voight (1969) found microfractures and fluid inclusions (healed microfractures) to be strongly oriented parallel to the rift plane, which is vertical and strikes about N. 80° E. They found a smaller concentra- tion of microfractures in the grain direction parallel to the horizontal lift plane, but where the quartz optic axes are strongly oriented parallel to the rift plane, they related these features to the laboratory-determined com- pliance ellipsoid. EXPERIMENTAL PROCEDURE 3 Willard and McWilliams (1969) found a similar strong concentration of fractures parallel to the plane of easiest tensile fracture, which is most likely the rift plane. Bur, Hjelmstad, and Thill (1969) measured velocities and relative sonic amplitudes within the Barre Granite that show well-developed orthotropic symmetry. The plane containing the axes of highest and intermediate velocity and relative amplitude coincides with the rift plane, wheras the plane containing the axes of highest and intermediate attenuation coincides with the hardway plane (M. Friedman, oral commun, 1971). Melvin Friedman (oral commun., 1971) used X-ray- diffraction techniques to determine the state of prestress (residual stress). The chip of granite used for that deter- mination was taken from the block used in the experi- ment for the present report. Nur and Simmons (1969) and Nur (1971) showed that acoustic birefringence is induced by stress in the Barre Granite and that the velocity anisotropy is dependent on both crack distribution and orientation of applied load. The mineral composition of the Barre Granite, as determined by Chayes (1952), is as follows: Mineral Percent Quartz _____________________________ 25 Potash feldspar ________________________ 20 Plagioclase feldspar _____________________ 35 Biotite _____________________________ 9 Muscovite ___________________________ 9 Accessories __________________________ 1 The block used here has a similar modal composition and has an average grain size of about 2.0 mm. EXPERIMENTAL PROCEDURE A block of Barre Granite from the Wetmore and Morris quarry near Barre, Vt., was supplied for this ex- periment by T. R. Bur, Geophysicist, Twin Cities Min- ing Research Center, U.S. Bureau of Mines, Minneapolis, Minn. The block, nearly cubic and 22 cm on a side, was taken from a larger master block ex- cavated at the quarry level 67 m below the ground sur- face, and it is oriented so that the sides are parallel to the rift, hardway, and grain direction (fig. 1). The top and bottom of the block are parallel to the grain or lift direc- tion, sides 1 and 3 are parallel to the hardway plane, and sides 2 and 4 are parallel to the rift plane. The rift plane in this quarry strikes N. 30° E. (T. R. Bur, oral com- mun., 1971), and the grain is approximately horizontal. The experiment was conducted in three phases, as follows: Phase I. The determination of principal strain changes that occur on the surface of the block as a result of the systematic creation of new surfaces by core drilling. Phase II. The observation of surface fabric changes that result from the creation of new surfaces. Phase III. A cursory petrofabric examination to cor- roborate known anisotropy of fabric elements and to determine possible changes in fabric caused by the creation of new surfaces. PHASE I INSTRUMENTATION Initially the block was polished on three surfaces: the top, bottom, and side 2 (fig. 1). Then 45°-rosette resistance strain gages were mounted on the polished surfaces and numbered as in the configuration shown in figure 1B. The rosettes marked F1, F2, and Q are those placed on individual feldspar and quartz grains. The grid elements of these three rosettes have a gage length of 0.76 mm. The rest of the rosettes have grid elements with gage lengths of 3.18 mm, and they cover several crystals or grains at each location. The gage circuit con— sisted of a half bridge with the sensing gage in the active arm and a compensating gage, mounted on a wafer of Barre Granite, in the compensating arm. Strain readings were made with a Budd SR4 Strain Indicator. The temperature during readings never varied more than 0.3°C and, during the experiment, ranged from 21.0° to 230°C. Changes of strain were calculated from readings that were taken within 0.3°C of each other. SPURIOUS SIGNALS ASSOCIATED WITH THE EXPERIMENTAL TECHNIQUE Before the strain-relief experiment was started, three initial checks were made to determine the approximate deviations of strain readings associated with the ex- perimental procedures themselves. 1. Because the block was to be stress relieved by creating new surfaces with a diamond-drill bit, it was desirable to determine the effect of drilling water. Therefore, one of the faces was flooded and allowed to dry in a 12-hour period. By randomly comparing before and after readings of individual gages in three rosettes (a total of seven gages), it was found that the average deviation was only 7 X 10—6. 2. Before and after each drilling operation, wire leads had to be unsoldered and soldered. Before and after readings on four gages unsoldered and resoldered had an average deviation of only 3 X 10—6. 3. Each of these checks included changes caused by switching the circuits through a low-resistance wafer switch and changes due to drift of the strain indicator. The combined switch and strain in- dicator deviation was 2 X 10‘6 per degree. Thus the probable “accidental error” after any single drilling operation is probably no greater than 10 X 10’6 if readings are taken with 0.3°C of each other. DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT SIDE I EXPLANATION A I N30 S ' h ' 0‘ >11 traln gage S owmg 5| l ¢ reference number TOP 0‘ 6‘84 //V 9' Strain gage covering ! >0 discrete quartz grain SIDE 3 I f _ F F Strain gages covering i fll A Z discrete feldspar grains 5 | N ;______ N / / ' HARDWAY M I l / 507-7044 #0 22 CM / A SIDE 1 SIDE 1 TOP w v m LL! 0. LLI 21 9 >>F <8 9 < E 30 m 1 F2 <9 «I: <22 m < A 31& 29 <10 <23 b 1 2 3 4 5 6 7 14 15 16 17 18 19 20 27 28 /|\/l\\V/I\/I\/I\/I\ \V\V\V<\I/\I/\V 3% < /1\ >11 <24 47 >12 <25 33 <34 N N .— LLI LU <13 uQJ <26 0 1:1 TOP ‘7) BOTTOM(V|EWED FROM ABOVE) 6 SIDE 2 <7: SIDE 3 SIDE 3 BOTTOM B FIGURE 1. — Block of Barre Granite used in this experiment. A, Block showing rift, grain, and hardway planes. B, Top, bottom, and side-2 faces of block showing positions of the 45°~strain rosettes. SUCCESSION OF CORING AND COLLECTION OF DATA After the initial checks were made, the strain gages were read once a day for 3 days, and during this time the block was stable within 15 X 10‘6. After the 5.1- and 10.1-cm cuts, the strain changes ceased after the first 24 hours. After the 15.0-cm coring there was a creep event at all gages over 48 hours. When the block had stabilized, the cores created by drilling were broken out (fig. 2), and slight further strain relief occurred. The block was then turned over and drilled successively on the bottom face in the same manner as before (fig. 3). The depth of the cuts, however, was only 9.5 cm, and the cuts did not penetrate to the hole already drilled from the top side. Thus, each core was still attached to a small thickness of rock. Strain changes recorded after each of the bottom cuts largely ceased within 12 hours. There were, however, four grid elements in separate rosettes that showed small creep after the 5.1-cm cut. These circuits were checked for bad FIGURE 2. — Top of block after annuli have been broken out. Core is 10.1 cm in diameter. solder joints, worn leads, or bad switch connections, but none were found. EXPERIMENTAL PROCEDURE 5 FIGURE 3. — Bottom of block after successively coring with 5.1-, 10.1- and 15.0-cm core bits. FIGURE 4. — Bottom of block showing fractured interior annulus (arrow). An attempt was made to break the cores free from the rock, as had been done at the top. However, the interior annulus was chipped at the lip (fig. 4), leaving two small rock fragments free of the annulus with the strain rosette still intact. It was decided then to leave the other an- nulus intact until the experiment was finished. This phase of the experiment was completed when the block was again rotated and side 2 (fig. 5) was cut with a 10.1-cm bit to a depth of nearly 13 cm, thus nearly free- ing the core on the bottom. The strain changes that ac- curred after this cut also ceased within 12 hours. FIGURE 5. — Side 2 after coring with 10.1-cm core bit. PHASE II OBSERVATION OF SURFACE FABRIC Changes in the rock fabric that occurred as a result of the coring were looked for —— specifically, the develop- ment of microfractures on the top surface of the block. In order to emphasize the fabric elements, especially microfractures and grain boundaries, several staining techniques were attempted. The most successful was a penetrant organic dye that emphasized both the microfractures and the grain boundaries (Gardner and Pincus, 1968). The part of the top surface stained for observation covered approximately one-half of the northeast quadrant and extended from the outer edge of the block to within 5 cm of the center. For the purpose of comparing fabric elements after strain relief, large-scale photographs were made at identical locations before and after the block was drilled (fig. 6). The location seen in figures 6A and 6B is about midway between gages 6 and 9, and the location seen in figures 6C and 6D is about midway between gages 7 and 8. PHASE III PETROFABRIC EXAMINATION Extensive petrofabric analysis of the Barre Granite has been done by Douglass and Voight (1969) and Willard and McWilliams (1969). Nonetheless, two sets of three mutually orthogonal thin sections from the block used in this experiment were examined to corroborate the anisotropy of petrofabric elements. Also, these sec- tions were examined to discover changes in the fabric produced by overcoring. The thin-section locations were chosen in an area of (1) high strain relief directly under gage 6 (fig. 1), and (2) moderate strain relief 5 cm below gage 4 (fig. 1). 6 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT EXPERIMENTAL RESULTS PHASE I PRESENTATION OF STRAIN DATA Strain data were recorded at all locations after each coring of the block. Changes of strain were used to calculate the maximum and minimum (secondary) prin- cipal strain changes by the rectangular-rosette method (Frocht, 1941, p. 37). The maximum principal strain changes (A61) were regarded as most extensional, and the least principal strain changes (A62) as least extensional; extensional strain was taken as positive and com- pressional strain as negative. The principal strain changes so calculated are changes in response to the cut- ting of new surfaces. The residual strains are those, with reference to an original undisturbed state of zero strain, that occurred when the stored energy was locked in; therefore, they are opposite in sign to the relieved strains measured experimentally. It is important to note that data on strain changes are mostly those determined from either of two zero readings; namely, (1) the initial reading prior to any drilling of the block, and (2) the stable reading after completely coring the top but prior to any coring on the bottom. Strain changes (Ac) relating to these readings will be denoted A06 and A36, respectively. Any other zero points will be specified in the text. The strain differences A6 at all gages shown in tables 1 and 2 were calculated from stable readings 24 hours after each coring. The stable 48-hour reading after the 15.1- cm coring on the top surface was made because the 24- hour reading was not stable. Arms a, b, and c designate the individual gages on each rosette, arm a being the most clockwise and arm c the most counterclockwise. In- dividual gage orientations are shown in figure 1. TABLE 1. — Strain differences, A06 and A 3 e, at gages 1 through 34 [The strain differences at all gages were calculated from stable readings 24 hours after each coring, except as noted. Arms a, b, and c designate individual gages on each rosette, arm a being the most clockwise and arm 6 the most counterclockwise Individual gage orientations are shown in fig. 1. Leaders (_ _ _1) indicate no data available] Ame (X 10*) based on stable zero reading prior to 5.1»cm coring on top face 13¢ (X 10 5) based on stable zero reading prior to 5‘1-cm coring on bottom face Gage Arm After After After After After After After After N0. 5.1-tcm lOl—cm 15,0l-cm 15:0-cm 5.1-Icm 10.1:cm 15.0:cm 10.1:cm coring coring coring cormgon coring coring coring coring on top on top on top top face on on on on side face face face (48 hrs‘) bottom bottom bottom face face face face 1 a #10 #13 3 14 _____ ____ ____ -___ b #3 #16 39 57 _____ ____ ____ ____ c #1 #18 1 15 ____ ____ ____ ____ 2 a #5 #45 #63 #51 ____ ____ ____ ____ b 7 54 84 91 ____ ____ ____ ____ c 3 #116 #109 #94 _____ ___._ ____ ____ 3 a #57 #102 #93 #58 ____ ____ ____ _____ b 20 #40 #25 #9 ____ ____ ____ ____ c #6 #111 #98 #65 ____ ____ ____ ____ 4 a #76 #98 #82 #51 ___- ____ ____ __.__ b #55 #60 #49 #23 ____ ____ ____ ____ c #52 #62 #55 #32 _____ ____ ____ _____ 5 a #51 #125 #132 #105 _____ ____ ____ ____ b 2 #43 #36 #7 ____ ____ ____ ____ c #21 #183 #176 #154 _____ ____ ____ ____ 6 a #3 #51 #69 #54 ____ ____ ____ ____ b #17 #21 #19 11 ____ ____ ____ ____ c #12 #38 #141 #118 ____ ____ ____ ____ 7 a 6 #3 #2 19 34 28 27 16 b #5 #7 20 40 19 9 27 6 c 10 14 22 40 30 18 26 16 8 a #27 #17 #21 22 34 39 _ __ _ __ __ b #15 #14 15 41 63 0 ____ ____ c 12 45 48 139 28 16 _ _ _ _ _ _ _ _ 9 a #10 #17 #14 8 ____ ____ ____ ____ b #2 6 6 10 ____ ____ __-_ ____ c #8 #23 #113 #98 ____ ____ __.._ .____ 10 a #65 #84 #66 #53 ____ ____ ____ ____ b 40 #12 1 11 ____ ____ ____ ____ c #11 #152 #134 #137 ____ ____ ____ ____ 11 a #58 #106 #94 #77 ____ ____ ____ ____ b 19 #53 #43 #23 ____ ____ ____ ____ c #27 #73 #58 #42 ____ ____ ____ ____ 12 a #16 #174 #167 #142 ____ ___._ ____ ____ b #14 30 44 72 ____ ____ _.___ ____ c 0 #23 #29 #11 ____ ____ ____ ____ 13 a #8 #16 #9 16 24 13 27 10 EXPERIMENTAL RESULTS TABLE 1. — Strain differences, Age and A B 6, at gages 1 through 34 — Continued A“: (X 10 6) based on stable zero reading prior to 5,1-cm coring on top face Age (X 10 5) based on stable zero reading prior to 5.1-cm coring on bottom face Gage Arm After After After After After After After After No. 5,1:cm 10.1:cm 1510:cm 15.}0-cm 5.1-crn 10.1:cm 15.0:cm 10.1:cm coring coring coring coring On coring cormg coring coring on top on top on top top face on on on on side face face face (48 hrs‘) bottom bottom bottom face face face face b —6 —6 24 47 26 16 26 13 c 0 6 4 36 20 —17 26 7 14 a —11 —29 4 17 9 15 6 25 b ~18 —32 —10 0 —2 11 48 22 c —5 —20 —12 0 15 27 28 28 15 (1 ~19 ~35 ~9 0 4 ~14 ~28 41 b ~16 ~35 ~6 1 5 39 ~6 30 0 ~15 ~31 ~7 5 —4 10 ~61 32 16 (1 ~18 ~32 —7 1 ~39 4 7 46 b ~20 ~47 ~18 ~1 41 17 20 0 c ~21 ~38 ~8 ~6 ~10 ~33 6 55 17 (1 ~13 ~30 ~12 ~2 ~22 ~1 18 ___- b ~17 ~28 ~12 6 ~28 6 29 ____ 0 ~17 ~37 ~3 10 ~29 ~1 32 ____ 18 (1 ~23 ~43 ~27 ~11 ~40 ~52 3 32 b ~24 ~46 ~25 ~8 30 ~26 9 32 c ~25 ~45 ~20 ~7 ~5 2 ~2 69 19 (1 ~25 ~41 ~18 ~1 6 3 ~67 32 b ~25 ~47 ~20 ~6 8 39 ~36 29 0 ~24 ~46 ~25 ~14 4 ~32 ~21 28 20 a ~19 ~33 ~14 0 11 10 22 24 b ~22 ~39 ~15 ~4 7 11 43 30 c ~18 ~39 ~20 ~11 20 16 ~7 34 21 a ~9 ~20 14 5 10 24 18 30 b ~5 ~12 22 14 25 40 85 35 c 3 ~5 20 13 28 32 26 30 22 (1 ~13 ~16 37 62 11 8 ~20 29 b ~15 ~33 ~1 ~16 7 ~54 ~15 33 c ~7 ~18 49 66 35 103 ~8 65 23 a ~17 ~33 ~26 ~21 ~28 ~4 7 8 (1 ~14 ~36 ~22 ~32 22 ~22 11 13 0 ~20 ~31 ~1 6 2 ~39 7 15 24 a ~13 ~35 ~3 3 4 ~35 17 ~30 b ~16 ~34 ~7 ~7 38 8 10 26 c ~18 ~41 ~19 ~13 ~18 7 7 47 25 a ~14 ~24 ~2 8 12 17 ~26 ___- b ~19 ~37 ~13 ~4 19 45 ~30 ____ 0 ~14 ~30 ~26 ~11 4 ~37 ~52 __-_ 26 (1 ~16 ~28 ~14 ~5 5 13 33 27 b ~8 ~22 0 8 16 38 76 29 c ~8 ~20 ~1 6 14 29 23 29 27 a ~23 ~34 ~15 ~1 15 11 14 ~1 b ~17 ~14 12 26 9 7 30 9 c ~19 ~28 ~11 3 10 9 8 12 28 a ~13 ~20 ~2 9 9 9 2 2 b ~6 ~3 24 41 16 13 6 46 c ~8 ~8 9 25 11 2 2 20 29 u ___- ___- ___- ____ ___- ___- ____ ~6 b ____ ____ ____ ____ ___- ___- ____ ~48 c ____ ____ ____ ___- ____ ____ ___ ~32 30 a ~2 ~6 12 26 6 17 13 12 b ~4 ~9 12 22 13 33 14 42 c ~13 ~24 ~6 4 8 10 15 24 31 a ____ ____ ____ ___- ___- ___- ____ 29 b ___- ____ ___- ____ ___- ___- ___- 10 c ____ ____ ____ ____ ___- -___ ___- 26 32 a ~10 ~19 2 12 14 18 0 17 b ~22 ~33 ~7 7 6 4 8 28 c ~17 ~26 2 9 19 6 13 16 33 a ____ ____ ___- ____ ___- ___- ____ 16 b ____ __-_ ____ ____ ____ __-_ ___- 4 c ____ ___- ____ ___ ____ ___- ___- 16 34 (1 ~33 ~53 ~37 ~24 8 1 12 12 b ~27 ~41 ~17 ~5 12 17 31 25 c ~26 ~44 ~38 ~18 5 8 10 12 ‘The 48-hour reading was made because the 24-hour reading was not stable. DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT TABLE 2. — Strain differences, As and A Be, at gages F1, F 2, and Q [The strain differences at all gages were calculated from stable readings 24 hours after each coring, except as noted. Arms 0, b, and c designate individual gages on each rosette, arm a being the most clockwise and arm 0 the most counterclockwise. Individual gage orientations are shown in fig. 1. Leaders (_-,_) indicate no data available] As (X 10’“) based on a stable zero reading prior to the 10.1-cm coring on top face A86 (X 10‘5) based on a stable zero reading prior to the 5.1-cm coring on bottom face Gage Arm After the After the After the After the 5.1-cm After the No. 10:1-cm 15:0-cm 15.0-cm coring on bottom 10.1-cm coring on coring on coring on face coring on top face top face top face bottom (48 hrs‘) (41 days) face F1 ________ a — 25 —46 2 62 23 131 b — 17 42 76 19 24 78 c — 4 25 53 86 43 148 F2 ________ a —51 —75 —49 ____ ____ ____ b —80 —149 —106 ____ ____ ____ c —55 -83 —34 ____ ____ ____ Q _________ a —41 16 55 37 19 36 b — 32 18 62 0 15 64 c — 37 23 55 34 1 21 'The 48-hour reading was made because the 24-hour reading was not stable. Figures 6-13 show vectors representing the computed principal strain changes at each gage location on the top, bottom, and side-2 surfaces after each coring operation. At locations where strains are not shown, the strain values were very small or the gages were destroyed by drilling. Also plotted in figures 6 and 7D are vector representations of the prestrain contained within quartz grains as determined by X-ray diffraction techniques from chips taken from the same block. Prestrain has an opposite sense to strain caused by overcoring. Figures 6, 7, and 8 show the principal strain changes Am and A062 that occur on the top, bottom, and side-2 surfaces after each of the 5.1-, 10.1-, and 15.0—cm corings of the top surface. Figures 9, 10, and 11 show strain changes on top, bottom, and side-2 surfaces after similar corings of the bottom surface and after the 10.1-cm cor- ing of side 2. Figures 11, 12, and 13 show the principal strain changes that occur at the individual feldspar- and quartz-grain rosettes. Figure 12 shows these changes after the 10.1- and 15.0—cm corings of the top surface, The initial readings for these changes were taken after the 5.1-cm coring. Figure 13 shows these changes after the 5.1- and 10.1-cm corings of the bottom surface. The pertinent strain-change data are presented in the following succession: 1. Age on the top surface after the 5.1-cm coring on top (fig. 6).After the 5.1-cm coring, the only significant strain changes on the block occurred in the vicinity of the freshly cut surface (fig. 6). The minor principal strain changes, A062, are all compressional and of large magnitudes (up to —120 X 10’6), whereas A061 are either extensional or compressional but very small in magnitude (less than 40 X 10’s). The orientation of the A052 and A051 axes on the outside of the circular cut is nearly radial and tangential to the cut, respectively, whereas the similar axes at the center of the core (gage 4) do not seem to closely aline with those on the out- side. 2. Age on the top, bottom, and side 2 after the 10.1-cm coring on top (fig 7). After the 10.1-cm coring on top, the cumulative strain changes Age on all surfaces are significant. The larger A05, however, are on the top surface near the newly created 10.1-cm circular cut (fig. 7B). The Aoe2 are all compressional, with magnitudes as large as —250 X 10‘s; and the A051 are either compressional or extensional, with magnitudes of as much as 60 X 10‘s. The larger A062 and A061 axes, both exterior and interior to the 10.1-cm diameter cut, tend to be radial and tangential, with noticeable rota- tion at gages 11 and 12. Within the zone just outside the cut, at gages 6 and 9, the A062 are much less com- pressive than those at gages 2 and 12, and, conversely, at gages 5 and 10 on the newly formed annulus across the cut from 6 and 9, and A062 are more compressive than at similar gages 3 and 11 diametrically opposite, across the cut from 2 and 12. The unequal strain behavior at these gages is even more clearly demonstrated in figure 7A, which shows the principal strain changes that occurred between the 5.1-cm and the 10.1-cm coring. The A062 and A061 on the center core changed only slightly (fig. 7A). The A06 on the bottom and side-2 surfaces (figs. 7C, 7D) are nearly all compressive and have magnitudes commonly as large as —65 X 10*. On the bottom surface, the A062 and A061 axes aline very well with those of similar positions on the top sur- face. The A062 and A061, nearly all compressive on the bot- tom surface, are larger toward the center, thus reflec- ting the radial disturbance on top. The A06 on side 2, all compressive, are small near the top of the block but become larger toward the bot- EXPERIMENTAL RESULTS “8 5‘1—CM CORING 31% “was: Prestrain at gage 4 as determined by Xeray diffraCtometry $12 N LU ‘13 o a EXPLANATION 10 Strain gage reference number 0 100 H Extensional strainechange vector l_|_.l VGC‘OV magnitude (X 10-“) H Compressional strainrchange vector A coring at surface FIGURE 6. — Vectors of princi a1 strain changes A061 and A062 on the to surface caused by top 5.1-cm corin (Chan es that p p g g occurred between the time of the initial reading, prior to coring of the top surface, and the top 5.1-cm coring). tom. With one exception, the A052 are nearly vertical, cm cut, the largest Age are still on the top surface (fig. and the A061 are nearly horizontal. 8B), and all the A062 are compressive, with magnitudes 3. Age on the top, bottom, and side 2 after the 15.0-cm as large as —275 X 10‘s. The Aoel are both compressive coring on top (fig. 8). Twenty-four hours after the 15.0- and extensional, with magnitudes as large as 90 X 10 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT SIDE 1 8 SIDE 1 21 v v 3 Lu TOP In BOTTOM (VIEWED 9 9 FROM ABOVE) w m 9} *22 /'—\ / / +23\\/10.1—CM CORING 10.1—CM CORING // 5‘1_CM ”\(CORING / \ 14 15 I/ 16 {/ 17 \ 18 \ 19 20 \ / \ Jrza / \\ / \_// N H425 N 3 “9’ 5 (/1 SIDE 3 $26 SIDE 3 A C SIDE 1 TOP I I I I v 8 LU TOP :3 SIDE 2 I I 5.1—CM I I 9 D I I CORING—I I m a 10.1—CM 9I I I 3’30 I I/CORING | I I I 10.1—CM CORING I I I I R; «Z 3‘32 |__.__.I__ 27__I____I '38 35“ N w .— 9 Prestrain at gage 4 as E ”3 determined by X»ray (7, SIDE 3 diffractometry BOTTOM B EXPLANATION D 0 28 Strain—gage reference number 100Vector magnitude (X 10‘6) <—> Extensional strainichange vector A Coring at surface H compressional strain-change vector /’_\ coring projected to surface FIGURE 7. — Vectors of principal strain changes after top 10.1-cm coring. A, Top surface. Principal strain changes A61 and A62 (changes occurring in the time between the stable reading (5.1-cm coring) prior to the 10.1-cm coring of top sur- face. B, Top surface. Strain changes A051 and A062 (changes after the initial reading made prior to coring top surface) after coring. C, Bottom surface. Strain changes A061 and A052 after coring. D, Side-2 surface. Strain changes A061 and A052 after coring. 10‘s. The orientations of the A062 and A061 axes are all gage 4 on the center core and at all gages on the inner nearly radial and tangential, except at gage 13. At annulus, the Ace have remained nearly the same. EXPERIMENTAL RESULTS 11 Small extensional strain changes created by the 15.0- cm coring (fig. 8A) are not readily detected on the A06 of the inner annulus at gages 3, 11, 5, and 10. On the outer annulus, A062 increases are largest at gages 6 and 9, which previously showed the smallest A062 increase after the 10.1-cm coring. Exterior to the cut, the Age remain small in contrast to the large A06 found exter- nal to previous cuts. On the bottom and side 2 (figs. 8C, 8D), the A06 were still mostly compressional but much smaller in magnitude than after the 10.1-cm coring, thus in- dicating that the exterior of the block was extending instead of contracting, as observed previously. The orientations of the A06 axes remain approximately the same as they were following the 10.1-cm coring. There is one strain response on the bottom surface at gage 22 that appears to be anomalous. The A062 and A061 are both extensional; A061 is nearly 90 X 10*. Following the 15.0-cm coring, the gages on the top, bottom, and side 2 showed continued creep for 48 hours. Figure 8E shows the strains that occurred on the top side of the block during the second 24 hours after the 15.0—cm coring. All the strains are exten- sional, with the principal strain axes approximately radial and tangential. The radial strain at gage 8 is ex- ceptionally large, about 110 X 10‘5. Compare figures 8C and 8D, which show the A06 on side 2 and on the bottom 24 hours after drilling, with figures 8F and 80, which show the same quantities approximately 48 hours after, when all creep had ceased. The A06 orien- tations remained approximately the same in all cases, but extensional strain changes, with one exception, reduced the compressional A06 to very small values, approaching the initial strain state prior to disturbing the block. During the observed-creep episode, a con- tinued anomalous extensional-strain response was recorded at gage 22 that had been observed to have anomalous extensional A06 after the 12-hour reading. The annuli and center core were broken out of the block, leaving on the top surface only the strain-gage rosettes 1, 8, 7, 13, Q, and F on the periphery. The block was then turned over, and coring was started in the same sequence on the bottom surface as on the top. The strain changes A36 were then referenced to a stable zero reading just prior to the 5.1-cm coring of the bottom surface. . A36 on the bottom, side 2, and top surfaces after the 5.1-, 10.1-, and 15.0-cm corings on the bottom (fig. 9). The strain changes ABE on the bottom side (figs. 9A, B, E) following the 5.1-, 10.1-, and 15.0-cm corings are in many ways similar to those that occurred on the top surface as it was cored. The strain changes A362 and A351 on the bottom center core are both compressive and of similar magnitude to those that were recorded on the central core of the top surface. The orientations, however, are rotated nearly 90°; the A352 and A361 axes nearly coincide with A051 and A052 axes, respectively. Like those on top, the largest strains on the bottom occurred nearest to the fresh cuts; the orientations of the A362 and A351 axes are radial and tangential, respectively. The AB 62 in the peripheral zone are mostly compressive and of magnitudes as large as —145 X 10‘6 (fig. 9C, gage 25); the A361 are nearly all extensional and of magnitudes as large as 145 X 10‘6 (fig. 9C, gage 26). The compressive A362 induced by the cuts on the bottom surface are much less com- pressive than the A062 on the top surface, and the A361 are much more extensional than are the A061. The AB 61 on the peripheral surface external to the 15.0-cm out are especially large, indicating that the exterior rind of the block is expanding. The A362 and A361 on side 2 and at the remaining two good gages (7 and 13) on top (figs. 9E, D, respec- tively) corroborate the expansion of the exterior rind. The A362 are mostly extensional or slightly compressional, whereas the A361 are all extensional, and the orientations have remained nearly the same as they were after coring the top surface —— that is, ver- tical and horizontal on side 2, radial and tangential on top. Maximum magnitudes of extensional strain are 90 X 10‘6 (fig. 9D, gage 7), and maximum magnitudes of compressional strain are —15 X 10‘6 (fig. 9D, gage 30). The smallest Age on side 2 are in the middle (gages 27, 28, and 32), nearest to the thin interior rock plate still connecting the annuli t0 the main rock mass. After the bottom was completely cored, the center core was broken out, and an attempt was made to break the annuli away from the thin rock plate at the bottom of the cut. The annuli did not break; instead, three small chips were broken away from the central annulus, resulting in the strain changes shown in figure 9F. At the gage locations (16, 23, and 24) on the freed chips and near the new break, there are large strain changes, both extensional and compressional, from 200 X 10‘6 to —150 X 10‘6. Smaller changes oc— cur on the still-intact protion of the annulus at gage 18. Also, a small change occurs on the outer annulus adjacent to one of the fractured chips at gage 22. . ABE on the bottom, side-2, and top surfaces after the 10.1-cm coring on side 2 (fig. 10). The block was then rotated, and side 2 was cored with the 10.1-cm core bit to a depth of 12.0 cm, partially relieving the thin rock plate still holding the bottom annuli. Resulting strain changes ABE, referenced to the zero reading established prior to coring the bottom, are shown in figures IOB, C, and D. On the bottom surface the orientations of the A352 and A361 axes remained approximately the same, but in most places the A362 12 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT S‘DE‘ +8 SIDE 1 $21 V TOP V BOTTOM (VIEWED ”J “J FROM ABOVE)/ \\ 3, 3 /// 22 \\\ / / \\ \\ / / A423 \ \ / / /_\ \ \ 14 I 15 / 16 / 17 \ 18 \ 19 I 20 f I ¢ I k I\ \ I f /I l I I \ / / \ \ \\ /'/ / / \ \ *2“ // / \\ \\‘_// // \ x25 / \ / N \\ // Lu ‘T 9 ,26 0’ SIDE 3 C SIDE 1 X8 TOP I I I I I i v ‘0 SIDE 2 I I I I I 3 3 ' | l | I | 5 15.0-CM CORING a I I I 3’? I I | I I I I01 @213th 5.17CM . —CM | I rCORING—JILORINQ: I I I I I I I I I I I I 1 :14: 3: LEZI _.I__I__ I21_J__J_ +2234 N {.34 _ ”é a m :7) SIDE 3 BOTTOM D FIGURE 8. — Vectors of principal strain changes after top 15.0-cm coring. A, Top surface. Principal strain changes A61 and changes A051 and A062 (changes since the initial reading made prior to coring top surface) 24 hours after coring. C, hours after coring. E, Top surface. Principal strain changes A61 and A62 (changes since the 24 hour reading) 48 hours surface. Strain changes A061 and Age: 48 hours after coring. axes decreased in compression, and the A351 axes increased in extension. Similarly, on side 2 the orientations of the Age remained nearly identical, but the magnitudes, es- pecially of the A361, changed somewhat. Radial A352 changed very little; either they became slightly more compressive or slightly more extensional, Whereas the tangential A361 have all increased in extension to magnitudes of nearly 100 X 10’6 (gage 30). On the cen- tral core at gage 27 the compressional A362 and the EXPERIMENTAL RESULTS 13 SIDE’I v u.I D a EXPLANATION 28 Strain—gage reference number 1. 9 Extensional strain-change vector H Compressional strainichange vector 0 100 I I..__|_I Vector magnltude (X 10“”) Coring at surface —— Coring projected to surface SIDE1 TOP I I I I I I V BOTTOM (VIEWED ‘21 m I I I I I I I3 FROM ABOVE/,— \\ ”9J SIIDEZ I I I I I _ co ”’ // 22 \\ | I I 43° I am / /*\ \ I I I I10.1—CMI I / \ \ I I ICORING / / ,«23 \ I I / / \ I I I5.1—CMCORING I W. IIIIII / / \ I I L: I 1;; I 3‘; I/ I: I. 1:3 I 13 I 2: L_..32_L_I_ +27_i_4_3‘2§. I I \ / / I \ \ \\ / / / \ \ v / / \ \ x24 / / \ \\\_/// / \ / \\\ \25 /// 3 is“ I. 0’ c7: [26 SIDE 3 BOTTOM G A52 (changes since the stable reading made prior to the coring) 24 hours after the 15.0-cm coring. B, Top surface. Strain Bottom surface. Strain changes A051 and A062 24 hours after coring. D, Side-2 surface. Strain changes A061 and A052 24 after the same coring (creep event). F, Bottom surface. Strain changes A061 and A062 48 hours after coring. G, Side-2 extensional A 361 axes are, respectively, vertical and horizontal. On the top surface the orientations of A362 and A361 have rotated slightly, but they still remain nearly radial and tangential to the original 15.0-cm core hole. Both A362 and A361 have become largely extensional, with magnitudes as large as 145 X 10‘6 (gage 7). To demonstrate more clearly the geometric effect of the freshly cut surfaces, three additional gages (29, 31, and 33) were placed radially on side 2 (along with 14 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT SIDE 1 v ‘ 21 Lu BOTTOM (VIEWED BOTTOM (V‘EWED 9 FROM ABOVE) /,__\ FROM ABOVE) (/1 \\ // $22 \ / \ / //P\\ \ / / 2123 \\ \ / / \ / \ / / \ \\ . 16 13 \ 19 20 104 [ 1‘5 (74/4 ) __ I - l\ \ 5.1—Ch7/CORING \ \\ / / \ \ 3:24 // / \ \ / // \ _ \\ a 25 // \\ // N \_/ w 9 - 26 (n SIDE 3 A SIDE 1 21 V LU BOTTOM (VIEWED 9 FROM ABOVE) U) / / \ / \ / / \ / \ 15 19 \ 14 20 \ / \ CORING1O.1—CM(/ZORING \ / \ / \ / \ 25 / \ / N \\ _/// W Lu _ E E “26 5 w m SIDE 3 SIDE 3 B D FIGURE 9. (above and facing page). — Vectors of principal strain coring. D, Top surface. Strain changes A B 61 and A B 62 after bottom changes caused by bottom coring. A, Bottom surface. Strain changes A361 and AB 62 (changes after stable reading made prior to coring bottom surface) after bottom 5.1-cm coring. B, Bottom surface. Strain changes A361 and A362 after bottom 10.1-cm coring. C, Bottom surface. Strain changes A B 61 and A B 62 after bottom 15.0-cm gages 28, 30, 32, and 34) prior to the 10.1-cm coring. Figure 10A shows the principal strain changes that oc- curred at these seven gage locations from immediately before to 12 hours after the 10.1-cm cut. External to 15.0-cm coring. E, Side-2 surface. Strain changes A361 and AB 62 after bottom 15.0-cm coring. F, Bottom surface. Strain changes A51 and A52 caused by chipping interior annulus. Strain changes deter- mined from equilibrium state prior to chipping. the cut (gages 28 through 34), the minimum principal strain changes are entirely radial, and the maximum principal strain changes are tangential. The magnitudes and senses of strain changes very con- EXPERIMENTAL RESULTS 15 EXPLANATION 32 Strain—gage reference number <—> Extensional strain»change vector H Compressional strain-change vector 0 100 l_.__1_I Vect0r magnitude (X 10'°) Coring at surface ———— Coring projected to surface SIDE 2l ' m | I Lu TOP I 9 I "’ I 30 I I r I | | 15.0—CM CORING I I '32 27 28 I "j: —: —:_":_:—:: — t: I I I I I I I | Dam I I I I I I CORING\I I I I I I 34 I10.1—CMI I I H CORING I ‘_ I I I 15.0—CM I I I I ICORING “é (/7 I I I I I BITTOM I n I I l I E SIDE 1 V LIJ BOTTOM (VIEWED 9 FROM ABOVE) U) 15.0—CM CORING CHIP 4.0—CM DEEP N LU 9 SIDEus1 F siderably, but the orientations remain consistent with the geometry of the cut. The maximum principal strain changes are all extensional and range from 10 X 10‘6 to 60 X 10‘6, whereas the minimum principal strain changes are generally small extension or com- pression (less than 10 X 10*), except for one com- pressional value of —55 X 10‘s. 6. Total strain changes A66 = A06 + A36 on the bottom, side—2, and top surfaces after all drilling was com- pleted (fig. 11). The 10.1-cm coring on side 2 com- pleted the drilling program, after which final calculations of the principal strain changes A662 and A661 were made for the total duration of the experi- ment — that is, strain changes were determined from the time of the initial reading prior to the first over- core on top to the final stable reading after the 15.0- cm coring on side 2. Figure 11 shows these total prin- cipal strain changes at the locations where the strain gages were still recording. On the bottom surface (fig. 11B), the total principal strain changes A662 and Abel on the cored annuli are radically compressional and tangentially extensional, with one exception. At gage 22 on the outer annulus, toward side 1, the A662 (—50 X 10*) is compressional and tangential, and the A661 (360 X 10‘s) is exten- sional and radial. At gage locations (4, 20, 21, and 26) external to the annuli, the A66 are mostly extensional and tangential, indicating circumferential expansion of the outside rind. The radial strains at these locations are very small. The total compressional strain magnitudes are as much as —115 X 10‘6 (gage 15), and the extensional strain magnitudes were as much as 370 X 10‘6 (gage 22). On side 2 (fig. 11C,) the A662 external to the 10.1-cm cut are radial and mostly compressional. The A662 at gage 32 is extensional. The corresponding A661 are all extensional and tangential. Magnitudes of the com- pressional A662 are as much as —80 X 10‘6 (gage 34), and magnitudes of the extensional A661 are as much as 105 X 10‘6 (gage 30). On the center core, A662 is com- pressional and vertical, and Abel is extensional and horizontal. On the top surface (fig. 11A) at the remaining locations external to the cut, the total A66 are entirely extensional, nearly radial and tangential, and with magnitudes as large as 130 X 10‘6 (gage 7). . A6 on the quartz and feldspar grains after the 10.1 and 15.0-cm corings on the top and after the 5.1-cm coring on the bottom (figs. 12, 13). Figure 12 shows the prin- cipal strain changes that took place on the individual feldspar (F1,F2) and quartz (Q) grains after the 10.1- and 15.0-cm corings on top and, finally, 48 hours after the 15.0-cm coring on top. These changes refer back to an initial reading taken after the 5.1-cm but before the 10.1-cm coring. 16 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT after the stable reading prior to the side-2 10.1-cm coring. B, Top surface. Strain changes A3 61 and A 3 62 (changes after stable reading prior to coring bottom surface). C, Bottom surface. Strain changes A 361 and A362. D, Side-2 surface. Strain changes A361 and A362. TOP I I SIDE 1 m I SIDE 2 | " BOTTOM (VIEWED I: I I 3 FROM ABOVE) w I/15.0—CM COBING “' | w I / l fi10.1—CM_ I X 10.1—CM CORING I CORING | | I I I 20 L$:::::*_:——_—A{J I14 X T I 7 | I I | | I | | I I | I I I I | | ____ I I 10.17CM “’ I I l 15.0—CM CORING N I COBING I I u: uOJ L15 O_C'\Il L—5.1—CM | I I 9 (7) IcOBING I COR'NG‘_I I I "’ ._,i, | | | | BOTTOM SIDE 3 A C SIDE 1 TOP I I q TOP 0') I I m Lu SIDE 2 g 9 I 30 I a) ”I <——l——> I _.__._ I‘15.OACM CORING 10.1—CM I I CORING I I I 10.1—CM CORING I | | I32 28_I I— - —l | I I I I ' —a—«———— I I I I I I I I‘c‘éslzflé H31. I I I N .— 3 I I Law I | I I: «+13 a lag-as I MA I I «7» SIDE 3 I I I I I BOTTOM I | I B EXPLANATION D 0 100 13 Strain—gage reference number I Vector magnitude (X 10"”) 4—D Extensional strain-change vector Coring at surface >—< Compressional strain-change vector —-— — Coring projected to surface FIGURE 10. — Vectors of principal strain changes caused by side-2 10.1-cm coring. A, Side-2 surface. Strain changes Ael and A62 EXPERIMENTAL RESULTS 17 SIDE 1 TOP SIDE 4 CORING EXPLANATION 27 Strain— gage reference number H Extensional strain~change vector H Compressional strain-change vector 0 100 L.__l_i Vector magnitude (X 10'6) Coring at surface ~——‘ Coring projected to surface SIDE 1 v BOTTOM (VIEWED TOP g FROM ABOVE) m SIDE 2 (—1). LL! 9 30 U) 15.0—CM CORING l—__ 10.1—CM CORING 10.1~CM CORING i i i : I : | l l | l l I 14 H» 20 A 15.0—CM 32 28 X comwgli‘ltfi‘ ‘+’ at: l I comma: i I N | F 5 i 3 «i, m l eagles/IO ‘7’ SIDE 3 l I j B TTOM i FIGURE 11. — Vectors of total net strain changes Abel and A’oez (total strain change for experiment) after 10.1-cm coring on side 2. A, Top surface. B, Bottom surface. C, Side-2 surface. 18 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT FIGURE 13. —— Vectors of principal strain changes A361 and A362 D (strain changes after stable reading made prior to coring bottom) over quartz (Q) and feldspar (F1) grains on top surface. A, 24 hours after the 5.1-cm coring on the bottom face. B, 41 days after the 5.1- cm coring on the bottom face. C, 24 hours after the 10.1-cm on the bottom face. |DE1 SIDE4U) k0 TOP Figure 13 shows the strain changes on the same gages which took place during the 5.1- and 10.1-cm corings on the bottom. The initial reading was just prior to the bottom 5.1-cm coring. Following the 10.1-cm corings on the top (fig. 12A), the principal strain changes in individual quartz and feldspar grains are all compressional but are not ob- viously radial or tangential. The magnitudes (as much as —80 X 10‘6) tend to get smaller with distance out- ward from the fresh cut. At the 12-hour reading following the 15.0-cm coring (fig. 123), the quartz and feldspar locations external to the cut have A6 are radial, and the A61 are tangen- tial. The A62 at gage F1 becomes much larger in com- pression (—55 X 10‘6), whereas the A62 at gage Q and the Ae1 at both gages Q and F1 become more exten- sional (as large as 50 ><10‘6). During the same interval, the A6 at gage F2 on the outer annulus does not change much in orientation, but the A62 becomes larger in compression (—150 X1045), and the A61 is smaller in compression (—15 X 10*). After the 15.0-cm coring, both the quartz and the feldspar gages recorded significant creep. Figure 12C shows the strain changes after creep had ceased at 48 hours, indicating slight to moderate extensional changes in all directions and some rotation of the A6 orientations on the quartz grain. Twenty-four hours after the 5.1—cm coring on the bottom, thequartz and feldspar grains showed in- creased extensional strains (fig. 13A). The strain changes, A 362 and A 361, with respect to the bottom zero readings are both extensional; the A 361 at gage F1 is large (131 X1045). The orientations of the A362 and A61 axes, however, are not tangential or radial. A creep event occurred at gages Q and F1 for 41 days after the 5.1-cm bottom overcore. Figure 133 shows 10.1—CM CORING EXPLANATION the A 36 of the quartz and feldspar grains (gages Q and H Extensional strain—change vector F1) after 41 days, and the creep had ceased. Both the ’ ‘ C°mp'e“‘°”a' St'ain’Change mm“ feldspar and the quartz grains show compressional 0 100 - - - 1 Vector magnitude (x 101,) creep changes, w1th rotatlons of orlentatlons of A62 and A Coring at surface A61- After the 10.1-cm coring on the bottom, the FIGURE 12. —— Vectors of principal strain changes on quartz (Q) and strain changes, A851 and A362, at the quartz and feldspar F1, F2) grains on the top surface after 10.1-cm and 15.0-cm feldspar grains are again highly extensional With ’ coring on the top surface but references to the stable reading after . ,6 . the 5.1-cm coring. A, Changes measured after the 10.1—cm coring. B, magnltUdeS as large as 181 X 10 (flg' 13C)‘ The Changes measured 24 hours after the 15.0-cm coring. C, Changes orientations 0f A851 and A352 are nearly the same as measured 48 hours after the 15.0-cm coring. they were after the 5.1-cm corlng on the bottom. SlDE 4 SIDE 4 SIDE 4 SIDE’I TOP 15.0—CM CORING SIDE1 TOP #1) 15.0—CM COFHNG SIDE1 TOP 15.0—CM CORING C EXPLANATION H Extensional strainechange vector 0 100 Vector magnitude (X 10'5) Coring at surface DISCUSSION 19 PHASE II SURFACE FABRIC CHANGES Figures 14A and C show enlarged photographs of sur- face areas that were decorated with organic dyes prior to coring the block. The area in figure 14A is 14.5 cm from the center and midway between gages 6 and 9 (a location of high strain response), and the area in figure 14C is 16.9 cm from the center and midway between gages 7 and 8 (low strain response). Figures 143 and D show the same areas after the coring on the top surface was finished. upon comparing fabric elements at both locations before and after coring, one finds no obvious changes except im- mediately adjacent to the fresh surface seen in figure 148. The development of a zone of microfracturing about 1 mm thick can be inferred by the higher intensity of refracted light. This zone of microfracturing was cor- roborated by thin-section examination. Elsewhere, the microfractures seen in these photographs can be iden- tified in nearly every case both before and after coring. In a few cases the fractures may appear to be wider from one photograph to another, but this difference may be caused by either crack displacement or difference in the intensity of reflected light. PHASE III ANISOTROPY OF PETROFABRIC ELEMENTS Figure 15 shows photomicrographs of microfractures in the lift and rift planes. Open and healed fractures, seen as linear elements on the lift plane, are parallel to the rift plane (fig. 15A), and similar fractures, seen as linear elements in the rift plane, are, in turn, parallel to the lift plane (fig. 158). These fracture sets have very strong preferred orientations parallel to both planes; the most dense fracture set is parallel to the lift plane. OTHER SIGNIFICANT PETROFABRIC FEATURES Other fabric features observed are the large numbers of quartz grains with undulatory extinction and clusters of small quartz and feldspar grains along larger grain boundaries that indicate some degree of recrystalliza- tion. Examination of two sets of orthogonal thin sections, one taken from a highly strained area (figs. 14A, B) and the other from a moderately strained area (figs 14C, D), reveals no changes of fracture spacing or orientations that would indicate any incipient fracture development upon overcoring and strain relief. DISCUSSION STRAIN MAGNITUDES The strain data obtained by successively creating new axially symmetrical surfaces within a 22—cm cube of Barre Granite suggest that rock masses can store a significant amount of residual stress that is capable of being unlocked or mobilized over large areas in a com- plex manner. The block released principal strain 20 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT FIGURE 14. — Reflected-light photographs of surfaces dyed with organic fluorescent dye before and after coring. Areas A and A’ are identical, as are areas B and 8’. Scale divisions, 1 mm. A, Location on outer annulus between gages 9 and 6; before coring. B, Same loca- tion as A, after coring. A large number of new fractures occur within changes as much as —275 X 10‘6 (compression) and 330 X 10‘6 (extension) on newly cut surfaces. BEHAVIOR OF THE BLOCK — CORING OF TOP SURFACE As the top of the block was cored successively out- ward, the strain changes adjacent to the fresh surfaces became highly compressional in radial directions but became only slightly compressional or slightly exten- sional in circumferential directions. The principal strain changes within 2 cm of both sides of the circular cuts all 1‘s? v «i the first 2 mm from the cut. Otherwise no new fractures can be iden- tified. C, Location 2.5 cm radially exterior to the location in A; before coring. D, Same location as in C, after coring. No new frac- tures can be identified. had nearly the same sense, orientation, and magnitude; strain changes on the outer skin (rind) were much less compressional. Forty-eight hours after the top 15.0-cm coring, these strains had returned nearly to the precoring strain conditions. On the bottom and side 2, all strain changes were moderately compressional after the 10.1-cm coring, but they had recovered to nearly zero 48 hours after the 15.0- cm coring. The largest strain changes (near the bottom of side 2, gage 32) agree very well with adjacent strain DISCUSSION 2]. FIGURE 15. — Photomicrographs of Barre Granite. A, Fractures (arrow) parallel to rift plane; plane of photomicrograph is parallel to lift plane. B, Fractures parallel to lift plane; plane of photomicrograph is parallel to rift plane. changes in the same direction on the bottom surface (gage 20), whereas smaller strain changes toward the top of side 2 (gage 30) agree very well with adjacent strain changes in the same direction on the top surface (gage 7). Thus, as coring proceeded, the block deformed in a manner related primarily to the geometry of the newly created surfaces, the initial shape of the block, and the release of available residual stress. BEHAVIOR OF THE TOP SURFACE After the initial 5.1-cm drilling, the central core deformed elliptically, as would be expected of a cylinder cut from a mass having a prestress that was non- hydrostatic. The circular cut allowed equal degrees of freedom for deformations in all directions normal to the circular axis, so that the strains should reflect the state of prestress. After the 10.1- and 15.0-cm corings, the large com- pressive radial strains within the annuli indicate that each annulus became thinner. The tangential strains were small. The inferred net circumferential deforma- tion on the middle circumference of the innner annulus, based on tangential strains, is compressional, whereas the net circumferential deformation on the middle cir- cumference of the outer annulus is extensional. using the average radial and tangential strains on each annulus at the middle circumference, one can calculate the radial displacements from the formulas for radial and tangen- tial infinitesimal strain, from McClintock and Argon (1966, p. 60). 6U Err : _ , 67' and 1/ 6U" + U 6 — r— — ’ 96 60 r where 6 = average radial strain, 699 = average tangential strain, Ur = radial displacement, U, = tangential displacement, and r = mean radius. 22 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT If the annuli remain circular during deformation, then U9 = constant, and aU. _0 Thus, 69 (Jr : r699. For the inner annulus, at gages 3, 5, 10, and 11, the average 609 =—24 X 1076, Ur = 3.80 (—24 X 1076) = —91 X 10’6 cm. For the outer annulus, gages 2, 6, 9, and 12, the average 39 X 10‘6, 6.35 (39 X 10‘6) = 245 X 10‘6 cm. 699: Ur S0 II Thus, the middle circumferences of both annuli have been radially deformed, the inner annulus by —91 X 10‘6 cm, and the outer annulus by 245 X 10"3 cm. These deformations are based on the assumption that the an- nuli remain circular as they deform, which they ob- viously do not. The intent is merely to give the reader some feel for how on the average, the annuli surfaces are being displaced relative to the center of the top surface. The strains, created by relief of the prestress, must be contained within the body of the annuli, where the least constraint is radial and the maximum constraint of freedom is circumferential. Thus, upon relieving the residual stress field, the geometry of the new surfaces control the release of stress such that the radial strain components are the greatest, and the circumferential strain components are the least. The nonhydrostatic aspect of the residual stress field is suggested by the varied magnitudes of strain changes both in radial and circumferential directions, and also by the unequal prin- cipal strain changes on the central core. Also, the varied magnitudes in part may be related to the thickness of the annuli. In table 3 the annulus thickness and magnitude of A062 are given at each gage site. In general, where the annuli are thinnest the magnitudes are largest, but there are exceptions that cast some doubt on this relationship. When the total thicknesses of the annuli are summed in the direction of gages 9 and 12 and then in the direction of gages 2 and 6, the sums are, respectively, 8.265 cm and 8.313 cm. Yet the average A062 for these same directions are, respectively, —181 X 10‘6 and —224 X 10*, suggesting that the total thickness of both annuli may not be as significant as other factors (such as anisotropy of the internally stored energy) in controlling strain magnitudes. On the top surface the strain changes never became highly compressional at gages 1, 7, 8, and 13 on the outer edge (rind). After the 10.1- and 15.0-cm corings were TABLE 3.—Magnitude 0f A062 and respective thicknesses of annuli after 15.0-cm overcoring on the top surface. The A052 and annuli thicknesses are shown for each gage location on the annuli surfaces. Annulus A06 24 hours Gage No.l after 15.0-cm Inner or Thickness overcoring outer (cm) (X 10’s) 12 Outer __________ 1.912 —255 5 Inner __________ 1.925 — 274 2 Outer __________ 1 .933 — 257 10 Inner __________ 1 .966 — 206 9 Outer __________ 2.046 — 149 6 _ _ do __________ 2.060 — 198 11 Inner __________ 2.341 —113 3 _ _ do __________ 2.395 - 166 1Listed in order of thickness of annulus. made, the rind experienced very small radial and cir- cumferential strains. The average circumferential strain from gages 1, 7, 8, and 13, 699 = 28 X 10‘“, was used to calculate the radial displacement of the rind, U, = 249 X 10‘6 cm. Thus, the rind increased in radius about the same as the outer annulus. BEHAVIOR OF SIDE-2 AND BOTTOM SURFACES The side-2 and bottom faces can be regarded as part of the external rind relative to the coring of the top surface. The strains on these faces, especially those on the lower half of the block that attained their greatest magnitude after the 10.1-cm coring, are smaller and much more un- iform than those on the top face. The bottom face in- dicates nearly equal strain changes at most of the locations monitored, and the axes of the principal strain changes reflect the orientations of the axes of these prin- cipal strain changes on the top surface. The strains seen on the bottom outer rind apparently reflect an interior stress state created by the coring procedure. If the strains caused by cutting a uniform residual stress field are uniformly distributed down the cylindrical annulus segment to the base of the cut, one can imagine a cylin- drical cantilever type of loading (fig. 16) applied through these segments to the lower half of the rock. The still- intact lower rind responds with diminished but uniform strains that reflect the cantilever loading. On the other hand, the rind of the upper half of the block is attached to the lower rock mass only at the base of the 10.1-cm cut. The very small strain changes on the upper rind are attributed to the combination of relieved stresses adja- cent to the newly cut surface and the cantilever stresses distributed through the attachment zone. After the 15.0-cm coring and associated creep event, the surfaces on the lower half of the rock recovered ex- tensionally to nearly the precoring condition of strain. The upper exterior rind, made thinner by coring, recovered to the initial state and then became very slightly extensional. Thus, it appears that the combined DISCUSSION FIGURE 16. — A cylindrical cantilever oriented vertically upon a block. The moment caused by gravity loading is equal to zero. Applied forces (F) on boundary are distributed through the base onto the block as shown by arrows DF. interaction of the internal cantilever stresses and the relieved residual stresses produces a net strain change of nearly zero on the side-2 and bottom surfaces, both being uncut surfaces on the external rind. BEHAVIOR OF THE BLOCK — CORING OF BOTTOM SURFACE When the block was turned over and cored on the bot- tom, the responses were similar —— that is, the central 5.1-cm core had all compressive strains having magnitudes similar to those of the central core on the top. The orientations of the principal strain axes were rotated about 70°. The annuli had principal strains oriented radially and tangentially; the radial strains were also compressive but of lesser magnitudes than those on top. However, the tangential strains were nearly all extensional, and they became very large, especially on the exterior rind, as the coring progressed outward. Side 2 became nearly all extensional horizontally, with practically no vertical change, and the exterior rind on the top was highly extensional. Total strains in the quartz and feldspar grains on the top exterior rind were entirely extensional after the 5.1-cm and 10.1-cm cor- ings, and the A86 were oriented neither radially nor tangentially, except after the 41-day creep event (fig. 8). During this creep event, the A35 became more compressional, and their axes rotated to radial and 23 tangential positions. The creep event noted on these gages was not observed at most of the other gages. BEHAVIOR OF BLOCK — CORING 0F SIDE-2 SURFACE After the final 10.1-cm coring on side 2, further exten- sion was observed at nearly all locations on the block, with the exception of slight increases of radial compres- sion around the freshly cut surface. Thus, it seemed that the interior restraints or balances had been partly destroyed or damaged so that the block expanded. BEHAVIOR OF BLOCK —— TOTAL EXPERIMENT The net strains for the entire experiment on the bot- tom annuli (fig. 113) are radially compressive and tangentially extensional. On the same surface, the ex- terior rind has nearly zero radial strains (gages 14, 21, 26) but has large extensional tangential strains. On side 2 (fig 110), the radial strains (with one exception) are compressional, and the tangential strains are all exten- sional. The two remaining locations (gages 7, 13) on the top surface (fig. 11A) In general, the final net strains seem to indicate that the central core, both annuli, and the remaining parts of the block are radially shortened and tangentially expanded. The radial compression decreases outward, whereas the tangential extension in- creases outward. BEHAVIOR OF BLOCK — SUMMARY The foregoing discussion shows that stress relief and the resulting strains manifested at free surfaces of the block are very complex. The residual state of stress within the body changes in a complex manner as new boundaries (free surfaces) are introduced. The initial strain changes, as the block of granite was cored on the top surface, indicate that the mobilized part of the residual stress field was tensile in sense — that is, all the relieved strains had been compressive. Yet, as more data were gathered from further coring of the block, many gages expanded, implying that certain com- pressive elements of the residual state were relieved. For example, the data recorded before and after the final 10.1-cm coring on side 2 indicate that a nearly totally compresssional component of the residual stress field was relieved. The net strain changes for the entire ex- periment, however, were radial compressions and tangential expansions. Because the force field within the block had to be in- ternally balanced, the question arises as to why all the initial relieved strains had the same sense. One has to in- quire: What are the stress-storage and release mechanisms that allow such deformation to occur? Ac- cording to McClintock and Argon (1966, p. 434), residual stresses occur on a microscopic scale in metals as a result of anisotropic plastic behavior, dislocations, and in- clusions; they occur on a macroscopic scale as a result of 24 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT plastic flow or creep and volume changes caused by metallurgical or cchemical processes. Jaeger and Cook (1969, p. 362) alluded to viscoelastic effects to account for residual stresses in rocks. Varnes (1969, p. 417) wrote of frictional, time-dependent, and elastic restraints as attributes of internally balanced stress fields. Bjerrum (1967, p. 45) concluded that diagenetic bonds that welded clay particles together while under maximum consolidation load account for locked-in strain energy of clay deposits. Seemingly, from these statements, the storage of elastic strain energy requires a permanent or time-dependent change of rock structure that fixes or locks-in energy imparted by internal or external force fields. The locking elements can be thought of as passive restraints preventing the active elastic energy from es- caping. At any point in the rock, however, there must be a balance of forces to maintain static equilibrium. Thus, the passive restraint elements must provide sufficient resistive forces to balance the active forces that result from stored elastic energy. If the passive restraints are destroyed or if they decay with time, the active elastic energy is mobilized, and strains result. An analogy of this can be seen in Gallagher’s (1971, p. 92) model, in which he applied a biaxial compressive load to an aggregate of photoelastic disks and simultaneously cemented the aggregate. Upon removing the external loads, he found reduced compressive elastic strains stored in the disks, locked in or balanced by the cement. In Gallagher’s experiment, the cement was elastic and did not allow a viscous relief of the stored energy. However, if the cement were viscous, the stored strain energy contained in the disks would, in time, be totally relieved. Inasmuch as the cemented model was in a compressed state, the strains relieved within the disks by the viscous relaxation of the cement would be exten- sional. Relieved strains of compressive sense would, however, occur locally as the cement matrix relaxed. From this example it is inferred that the internal forces resulting from the stored energy must be essentially balanced at any particular time becuase there are no ex- ternally applied forces; yet, as the cement relaxes with time, the same stored energy will create measurable ex- tensional strains at the boundaries of the model. In figure 17A a schematic approach similar to that presented by Varnes (1969, p. 419) demonstrates how energy release, caused by permanent and elastic defor— mations, may generate net strains of only one sense. Figure 17A shows the initial equilibrium condition of a hypothetical residual stress model after external restraints and (or) loads have been removed. The restraining and pushing elements represent, respec- tively, interior tensional and compressional forces that must be equal at equilibrium. The separation of units is an arbitrary linear distance between elements of the model. Changes in this distance thus represent strains. Tension Compressionl _ 4— A Separation of units (It) Initial equilibrium position Expanding elements AH Tension , . . nts 2 training eleme \Res Separation of ““lzf l | units (H+A,u) 1 compressonl _/ Expanding elements —_ B Intermediate equilibrium position Tension _ Restraining elements Separation of units (n+Au’) Final equilibrium position Compression —— Expanding elements C FIGURE 17. ._ Changes of stored energy in a residual energy model resulting from elastic and permanent deformations of the model elements. Areas 1, 1’, and 1” show work done. Areas 2 and 2’ show stored tensile energy in the restraining elements, whereas areas 3 and 3' show stored compressional energy in the pushing elements. A, Initial equilibrium condition with external restraints removed. B, Partial release of energy after some deformation of restraining elements. C, Complete release of energy after deformation of restraining elements. Area 1 represents the work done in attaining equilibrium, area 2 shows the stored tensile energy, and area 3 shows the stored compressional energy. Figure DISCUSSION 25 173 shows the changes that occur when elastic and per- manent deformations of the elements are introduced into the model. Assume that the permanent deformations cause a change of modulus for both the restraining (passive) and pushing (active) elements. In the model shown here, the moduli of these elements have been successively softened by decreasing, in equal in- crements, the angle of slope shown for the elements, thereby allowing the equilibrium position to migrate. The amount of remaining stored energy shown by the areas of triangles 2’ and 3’ has decreased, allowing the units to separate by an amount Au. Finally, as shown in figure 17C, permanent deforma- tion has completely relaxed the restraining bonds, and no more energy is stored in the system, and a net exten- sional strain is inferred by the increased separation AM’ of the units. Therefore, the release of stored energy by means of permanent and elastic deformations is ac- complished by extensional strain. Compressiona] strains on this model are very local, being in the vicinity of the bonds being destroyed by permanent deformation and, thus, are not seen in the illustrated net strain. The same model may be used for the granite block by inverting the compression and tension axes in figure 17, thereby allowing the release of stored energy by com- pressional, rather than extensional, strains. The horizon- tal axis, instead of being “separation of units,” becomes “contraction of units.” In this model, stored energy when relieved causes compressive strains. As the block was cored on the bottom and side 2, further relieving the internal stress field, extensional strains became predominant. Up to this time the mobilized component of the residual energy largely produced compressional strains. Thus, the internal stress state seemingly was modified so that extensional strains instead of compressional strains became mobilized. The new internal balance of forces can be ex- plained by the superposition of the unrelieved part of the residual force field and newly freed forces that were generated by mobilized residual strains. These forces, just freed from locally balanced fields, must find new restraints within the body in order to become balanced again. The new restraints are defined by the body material and are modified by the body’s geometry. Thus, as the new forces are distributed over the body, each point in the body will have a new equilibrium condition. The geometric controls provided by the new boundaries help in part to determine the nature (direction and magnitude) of the superposed forces, because the boun— daries must completely contain these forces. A force field controlled by geometry will generally have uniform gradients of both tension and compression in order to maintain equilibrium. Accordingly, both compressional and tensile strain components can be expected upon relieving the field. In the granite block, possibly such a superposed geometric stress field, created by the initial overcoring, was being relieved as additional coring proceeded; therefore, both tensional and compressional strains were observed. In all rock types the balanced internal force field at a point is probably some combination of a boundary- induced geometrically controlled force field superposed upon a local intergranular or intragranular force field. Each of these fields, in itself, balanced, but at any point in the rock mass, the combined fields are balanced. COMPARISON OF RELIEVED STRAINS WITH X-RAY PRESTRESS DETERMINATIONS The X-ray technique described by Friedman (1967) was used to measure the three-dimensional state of prestress or residual elastic strain in the quartz grains of the granite block. The 19 X 16 X 6 mm chip of granite used for the determination was taken from the top, directly under gage 4, less than 0.1 mm below the sur- face. If residual strains are locked in the rock, ideally they should be equual in magnitude, but opposite in sign, to the strains obtained by completely removing the restraints. Because the relieved strains are a function of the geometry of the block and core surfaces and of the chang- ing residual stress as the block was successively cored, the only valid comparison is that of the strain changes at gage 4 in the center of the top surface with the X-ray prestress determined at this location. This prestress, determined by Friedman (in Handin, 1972), is shown by vectors in the lower right part of figures 6 and 7D. The principal strain changes at gage 4 are compatible to the X-ray-determined prestress at the same location, in both sense and direction, but have diminished magnitudes. The following evidence illustrates the point that the residual stresses changed as the geometry of the block and cored surfaces changed. The X-ray-determined prestress in another granite chip of the same master granite block from which the block for this experiment was taken (61 = 130 X 10‘6 horizontal and bearing N. 40° W., e 2 = —65 X 10‘6 and vertical, (3 =—250 X 10‘6 horizontal and bearing N. 50° E.), is very different from the prestress determined at gage 4. The variation of residual stress is further corroborated by the general lack of agreement between the X-ray data at gage 4 and the strain changes that occurred over the rest of the block. Thus, the prestress within the granite may change con- siderably at any location as a result of changing restraints and body geometry. The behavior of the block of Barre Granite, however, implies that the small X-ray chip at gage 4 should be greatly relieved as compared with the original block of rock. this is also demonstrated by the fact that the strain changes, induced in shall chips as they are fractured away from an already partly relieved annulus, are very 26 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT large (fig. 9C), yet, these chips are not as small as those used for X-ray analyses. The relieved strains at gage 4 superposed upon the residual stresses remaining in the cip must represent the original prestress; hence, the relieved strains should be compatible in sense and orien— tation to the prestress in the chip. The prestress as measured with X-rays may represent a diminished surface prestress that reflects the principal axes of the residual stress in the larger rock mass, or it may reflect a modification thereof. Moreover, differences can arise between the states of stress measured by X- rays and strain-relief techniques that are attributed to heterogeneous residual strains which, when viewed on the scale of the quartz crystal, give rise to local variations in measurement. Interestingly, the X-ray—determined prestress direc- tions and relative magnitudes on the chip from the master granite block are in good agreement with the principal axes of the ultrasonic velocity and attentuation determined by Bur, Hjelmstad, and Thill (1969), whereas the X-ray—determined prestress and strain-relief data on the experimental block are not in good agree- ment with the ultrasonic data. At this point one wonders how small or how large the stress-locking domain is, if in fact there is more than one locking domain, and if a locking-domain gradient exists. The term “stress-locking domain” as used here has been defined by Varnes and Lee (1972, p. 2865) as “that volume and shape within a particular rock that can con- tain a balanced force system of a certain maximum in- tensity without application of exterior loads.” Friedman (1968) experimentally demonstrated that residual strains in individual quartz grains within quartzose sandstones relax virtually to zero when etched by hydrofluoric acid, thus implying that the stress-locking domain must be larger than an individual grain. On a much larger scale, one can imagine large locking domains controlled by the original geometry and the geometries of progressive solidification of a granitic pluton. Between these two extremes it is quite probable that there is a locking-domain gradient which may be in- terrupted by local discontinuities and tectonic structural elements that in themselves define discrete locking domains. RELIEVED RESIDUAL STRESS FIELD AND COMPARISON WITH IN SITU DETERMINATIONS RELIEVED RESIDUAL STRESSES Because the stresses relieved are complexly dis- tributed, there was no attempt made to compute a stress ellipsoid representing the preexisting stress state within the undisturbed block. The behavior of the block ade- quately demonstrates that the state of residual stress is controlled by the prestress condition and by geometry of the overcores and the block. It is questionable that elastic compliances measured in the laboratory are equal to the in situ compliances because unavoidable perma- nent structural damage occurs as a result of coring the laboratory samples. However, to get some idea of the magnitudes of these stresses, the principal strain changes shown on the central top core (gage 4) and the outer annulus (gages 2, 6, 9, 12, fig. SB), after the top 15.0-cm coring, and on bottom central core (gage 17) after the bottom 5.1—cm coring (fig. 9A), were converted to average principal stress changes (table 4) on the assumption that the rock is elastically isotropic. The same sign convention is used for stress as was used for strain. The stress-change ellipses for the central cores, by changing signs, represent the relieved part of the residual stress field, as presented in table 4. No attempt is made to correct for the strong elastic anisotropy that has been demonstrated by several in- vestigators (Douglass and Voight, 1969; Hooker and Johnson, 1969; Bur and others, 1969). The Young’s modulus (Et) used to calculate these values is an average of the maximum and minimum tangent moduli deter- mined by Douglass and Voight (1969) on a similar block of Barre Granite from the 40-meter level in the Smith quarry. RELIEVED RESIDUAL STRESSES COMPARED WITH IN SITU STRESS FIELD In table 3 it was pointed out that relieved residual stresses can be sizable. To compare the values of the strains and the resulting calculated stresses observed in this experiment with in situ stress measurements, the strains are first reduced to deformations and, thence, to stresses by calculations similar to those of the US. Bureau of Mines (Merrill and Peterson, 1961; Hooker and Johnson, 1969). The assumptions used herein for the conversion of strains to deformation are tenuous; none- theless, despite the lack of corroborating deformation data, they are useful to demonstrate in the following calculations that strains derived from freed residual stresses may be significant components of deformations measured by in situ overcoring techniques. A standard method for determining the in situ state of stress is analogous to determining the theoretical state of stress in an externally loaded elastic isotropic plate from the measured deformations of a pilot hole within the plate. The method assumes the condition of either plane strain or plane stress and also assumes the rock mass to be elastic, isotropic, and infinite in extent. If the rock mass behaves anisotropically, measurements can be made to account for this anisotropy (Hooker and John- son, 1969). However, for the purpose of the following calculations, the Barre Granite is here regarded as isotropic. The method of calculating in situ stresses for a three- axis borehole-deformation gage requires that defor- DISCUSSION 27 TABLE 4. — Stress changes determined on the top central core and on the outer annulus after top 15. 0-cm coring Equivalent relieved Principal Principal residual strain stress principal Segment changes changes stresses Bearing (X 10"") (bars) (bars) Am A051 A002 Aoai (77f mr 027’ or Outer annulus — 218 39 — 76 - 6 6 76 Tangential Radial to (average). to annulus. annulus. Top central —85 —53 —36 —27 27 36 N. 3° E. N. 87° W. core. Bottom central —85 —35 —30 —22 22 30 N. 78° E. N, 12° W. core. mations be measured across three different diameters of a pilot hole as the stress field around the hole is relieved by concentric overcoring. Thus, any applied forces are removed, and the borehole-deformation meter records the elastic deformation (change in diameter) of the original hole in three directions, generally 120° apart. The borehole-deformation data are used to determine the change in the stress field that occurs upon relief of the applied forces. In that the calculated stress changes are inferred to be equal and opposite to the applied stress field, one merely inverts the sense of the calculated stresses and implies that these data represent the true in situ stress field. This approach works well if the forces acting on the borehole are, in fact, applied at a distance and are not internal (residual). In the block of Barre Granite, there are no applied boundary forces, and the deformations are generated merely by cutting away parts of the block to relax all or part of the residual stresses. The central borehole is the first to be cut, and, as can be inferred by the radial com- pression and tangential extension seen in figure 6, the hole expands upon cutting. Then, as the inner and outer annuli are cut, the hole continues to become larger, yet the rock mass is becoming smaller, seemingly as the result of relieving internally stored tensile energy. However, for the central hole to become larger as a result of cutting boundary forces, these forces would have to be compressional. Thus, an identical deformation of the central hole can be obtained from two quite different states of loading — one consisting of external com- pressive forces, and the other consisting of excess ten— sional energy stored internally. Unfortunately, the deformations across the central hole could not be measured during the experiment; therefore, they must be determined on the basis of measured strains surrounding the hole. Also, because the strain-gage rosettes were placed diametrically across the hole in only two orthogonal directions, deformations can be determined only in these directions. The deformation in a third direction was calculated from knowledge of the ratio of in situ stresses, as determined by Hooker and Johnson (1969) in the field. To determine the diametral deformations of the inner core hole, the inner and outer annuli are considered to be a composite hypothetical annulus, about 5.1 cm thick, to approximate the geometry of an in situ stress determina- tion. The strains of the two annuli are reasonably uni- form in magnitude and direction, and, thus, they are superposed by averaging along the same radial direction. The strains so determined are assumed to be the average values of the single 5.1-cm annulus. From the strains recorded as the 5.1-cm borehole was initially cut, it can be seen that there was significant deformation adjacent to the borehole and outward to within 25.4 mm of the wall. Further strains that occurred by making the 15.0-cm drill cut then relieved the rest of the annulus. The radial strains were all compressive, and calculated deformations indicate that the borehole wall moved out. The median circumferences of both annuli, comprising the hypothetical annulus, were displaced with respect to their initial positions occupied before the 5.1-cm overcoring. As illustrated in figure 18, the median circumference of the inner annulus was determined by circular deformation to be displaced inwardly 91 X 10'6 cm, and the median diameter of the outer annulus was displaced outwardly 245 X 10‘6 cm. If the relative dis- placements of both annuli are taken into account, a summed neutral circumference of zero radial defor- mation for the composite hypothetical annulus can be located outward at slightly greater than three-fourths of the way between the inner and outer surfaces of the outer annulus. This is 5.5 mm from the outer surface, or 45.3 mm from the inner borehole boundary. If the strains A06 are assumed to be nearly uniform in the radial direction, the inner radial borehole deformation (U R ) in any direc- tion on the annulus created by a 15.0-cm core bit, can be calculated from the neutral circumference by summing the average strains over the estimated distances of defor- mation, UR = 45.3 X “ER annulus”— (25.4 “eR borehole + 72”), where “6R annulus” is the average radial strain of the annulus “e R borehole” is the average radial strain adjacent to the borehole, and the + 72 X 10‘6 is the average radial circular deformation that occurred prior to cutting the outer annulus. The diametral deformation is expressed as U d = 2 U R . The following diametral deformations U1, oriented N. 30° 13., and Us, oriented 90° to U1, were calculated for the top surface. 28 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT U1: 1,142 ><10‘6 cm and U3 = 1,710 X 10‘6 cm, exten- sional deformations. Using equations for the 45° rosettes (Merrill and Peterson, 1961), and using the ratio of the in situ prin- cipal stresses already determined by Hooker and John- son (1969) at the Smith quarry, one can approximate values of stress relief in the granite. E S+T=— (U1+U3), 2d d U=E[(S+T)+2(S—T)cos20], where S and T = major and minor secondary in situ principal stresses, respectively; d = diameter of borehole = 5.1 cm; = Young’s modulus = 3.41 X 105 bars; and = counterclockwise angle from S to U. Thus, S + T = —95 bars. Hooker and Johnson’s data (1969) show that the ratio S/T is 0.51 for the state of stress (isotropic assumption) at the 100-m level in the Smith quarry. Assuming that this ratio is nearly the same for my block taken at the 67 - m level of the Wetmore and Morris quarry, S = 0.51T, and T + 0.51T = —95 bars; and in figure 19 the in situ stress or inferred exterior load is then: T = —63 bars Bearing of T = N. 6° W. S = —32 bars Bearing of S = N. 84° E. The “isotropic” values of S and T determined by Hooker and Johnson (1969) at the surface (1) and at the 100-m level (2) are CDPH (1) T = —32 bars S = —8 bars Bearing ofT = N. 27° E. (2) T = —188 bars 8 = —96 bars Bearing ofT = N. 14° W. By comparing all these values one can readily see that the estimated deformations caused by relieving residual stresses may be significant components of the total measured in situ deformations. Further, as demonstrated here, the residual stress orientations and magnitudes are similar enough to the in situ deter- minations that the in situ stress field could be entirely residual. DEFORMATION MECHANISMS It has been suggested that fractures are produced upon relieving crystalline rocks from high in situ stress con- centrations (Hooker and Duvall, 1966, p. 14). The per- manent structural damage caused by such opening of fractures, according to Norman (1970, p. 21), can occur within cores as they are removed from the outcrop. Nor- FIGURE 18. — Exaggerated illustration of deformations, following the [> top 15.0-cm coring, of the inner and outer annuli. The deformations are relative to the initial annuli positions prior to the 5.1-cm coring. Solid lines show the initial positions; dashed lines show the final positions. Distances between dashed and solid lines represent defor- mations. man demonstrated that the microfractures in crystalline rocks near Atlanta, Ga., are preferentially oriented nor- mal to the axis of maximum strain relief, and he at- tributed much of the nonlinear deformation in this direc- tion to the opening of fractures. However, the block of Barre Granite, as it was being relieved by coring, showed very little evidence of new fracturing. During the initial corings the minor principal strain changes were all radially compressive, and the magnitudes appeared to change uniformly away from the cut. It is difficult to imagine fractures opening in such a strain field. If fracture development were a domi- nant mechanism, one would expect a very erratic behavior under the strain gages wherever fractures oc- curred. No such behavior was observed until the strain changes started to become extensional. Figure 8C and F and 9A, B, and E show anomalous strain changes that developed at one location (gage 22) as extensional strain changes become more dominant. Figure 90 shows strain changes at gage locations on the interior annulus that have obviously been freed by bounding fractures (fig.4). These changes are very large, and they appear to be somewhat random in orientation, which implies localized stress changes that are probably caused by fracture development or grain-boundary movement. The photographs of dyed areas before and after drill- ing (fig. 14) show little or no evidence of new fracture development except immediately adjacent to the newly cut surface. Figures 14A and B show a location on the outer annulus at which some of the largest strains oc- curred. Even though existing microfractures are plainly visible, a comparison of the before and after photographs shows no new fracturing except within 2 mm from the new surface. Similarly, the photographs of a location nearer the edge of the block that experienced much less strain show no new microfractures (figs. 140, D). The general lack of fracture development upon stress relief is consistent with the conclusion of Nur and Sim- mons (1970) that microfractures are not introduced dur- ing drilling unless the stress field is very high. It is suggested, therefore, that the deformations observed in the Barre Granite black were largely elastic. The perma- nent structural damage, of the kind alluded to by Nor- man (1970), probably occurred in lesser amounts and most likely was accomplished by slip along grain boun- daries or possibly even by intragranular-creep mechanisms. DISCUSSION 29 1.27 cm 1.27 cm Final median diameter 6.35 cm > Center Initial median diameter \%\\<§8 >< 10'5cm \ 245 X 10'6 cm 28 X 10‘5 cm Outer annulus 1.27 cm 1.27 cm Center Initial median diameter Q\150 x 10‘6 cm K91 X 10—6 cm %325 X 10'6 cm Inner annulus N o 64 BARS 3 ;L_I_L__I ‘7, SIDE 3 FIGURE 19. — Inferred in situ stress vector components calculated from deformations of the block of Barre Granite. S is maximum prin- cipal stress (least compressive). T is minimum principal stress (most compressive). Dashed line shows exaggerated deformation ellipse. RELATION OF FABRIC ANISOTROPY TO RESIDUAL STRESS FIELDS There are strong preferred orientations of fabric elements in the Barre Granite that have been deter— mined by previous investigators. Douglass and Voight (1969) have found microfractures and fluid inclusions to be strongly oriented parallel to the rift direction or the plane of easiest parting, which in the Barre Granite is vertical and strikes about N. 30° E. They have found a small concentration of microfractures parallel to the horizontal grain or lift. According to Douglass and Voight, microfractures and fluid inclusions are the fabric elements most influential to the mechanical behavior of the granite. Directions normal to microfractures and fluid-inclusion planes are parallel to the direction of maximum compliance and the direction of minimum tensile strength. In addition, the quartz optic axes are strongly oriented nearly parallel to the rift plane. Willard and McWilliams (1969) found a similar strong concentration of microfractures parallel to the plane of easiest tensile fracture (the rift plane). Other manifestations of the anisotropy are the measured sound velocities and relative amplitudes within the Barre Granite (Bur and others, 1969), which Show well-developed orthotropic symmetry. The plane defined by the maximum and intermediate axes of the velocity and relative amplitude fields nearly coincide with the rift plane (M. Friedman, oral commun., 1971). 30 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT Cursory examinations of three mutually perpendicular thin sections taken from my block reveal that the densest microfracture sets are parallel to the rift and lift plane (fig. 15). There, the fractures in the lift plane are apparently better developed than those in the rift plane. These results do not agree with those of Douglass and Voight (1969), who found the fracturing in the rift plane and the least in the plane of lift. Thus, either there are local variations of fracture spacings or one must enter- tain the possibility that the block is misoriented. In that the orientations have been rechecked and found to be correct, it is assumed that there are localized variations in the fracture development. Although the fabric elements definitely are related to deformational behaviors observed in the laboratory and in quarry operations, a similar relation to either the in situ stress field or the residual stress field is not as clear. The average bearings for the minimum and maximum principal stresses determined in situ by Hooker and Johnson (1969) are, respectively, N. 14° E. and S. 76° E. However, at the two sites they tested, the orientations corrected for anisotropy are variable; for example, 02 ranges from N. 8° W. to N. 32° E. The inferred minimum principal stress determined by the method of Hooker and Johnson on my block, corrected for anisotropy, has a bearing of approximately N. 1° W. The minimum prin- cipal relieved residual stresses as determined by calcula- tion of a stress ellipse on the top and bottom central cores (table 3) have bearings of N. 3° E. and N. 78° E., respectively. Therefore, stresses calculated by relieving the in situ stress field and (or) the residual stress field do not seem to relate very well to the velocity or fabric data, which coincide with the rift-plane strike of N. 30° E., or to the hardway plane (plane of maximum tensile strength). The perpendicular alinement of most microfractures to the minimum principal stress, as observed by Norman (1970) in the Georgia rocks, does not hold here, so it does not seem reasonable that stress- relief microfracturing appreciably influences the measurement of residual stresses. In that no fabric elements seem to aline with the prin- cipal directions of stress relief, it is difficult to relate residual stresses to any strong fabric anisotropy. The reasons for this lack of correlation are not at all clear from this study, but there are some indications. Some of the fabric elements seen during microscopic examination suggest possible mechanisms of locking stresses in. The granite has clearly undergone deformation sometime during its history. The quartz grains show large amounts of undulatory extinction, and the feldspar and quartz grains are highly fractured. Also, the presence of small grains that have formed along larger feldspar- and quartz-grain boundaries is evidence of syntectonic recrystallization. Dislocation motions probably occurred within the con- stituent minerals, as indicated at least for quartz by the undulatory extinction, to produce self locally balanced stress fields. After the geologic deformation attenuated, the rock fabric contained large amounts of elastic strain energy locked in or keyed in by dislocations, grain- boundary restraints, and external restraints on the rock mass as a whole. The cutting of new surfaces removed the external restraint and caused permanent defor- mations, such as new fractures, grain-boundary slip, and renewed dislocation activity that allowed the relief of local restraints. Deformation then continued until the internal forces again became balanced by the restraints. McClintock and Argon (1966, p. 420) said that, in metals, “residual stresses arise from dislocation pile ups, kink boundaries, deformation-induced tilt boundaries, deformation twins and other diffusionless shear transfor- mations.” In such a manner, anisotropic elastic or viscoelastic strain energy could be stored within a rock mass without any apparent relationship to fabric anisotropy. Carter (1969) calculated the stress required for specific edge-dislocation spacings genetically associated with quartz deformation lamellae. A spacing of 315 angstrom units, or 3 X 105 edge dislocations per centimeter, can account for the observed change of birefringence across the lamellae of 0.002. The differential stress necessary for a stress—optical effect of this magnitude is about 5.4 X 103 bars (aux 103 bars and 022 = 0.8 X 103 bars). Thus, owing to permanent deformation, large stress differences can be stored within the grains of the rock. The stored stress associated with the deformation lamellae can be relieved if the dislocations migrate such as to reduce their density along the lamellae. It is not known whether the dislocations move at room temperature upon relaxa- tion of constraints about a given grain. Clearly, lamellae do exist in freed, isolated quartz grains. In the Barre Granite the relieved stresses are 1 or 2 orders of magnitude less than the values calculated by Carter (1969). However, deformation lamellae are not conspicuous in the granite, and the only evidence of dis- location lineups is the undulatory extinction. Nonetheless, sufficient locked-in energy can probably be associated with dislocations alone to account for the residual stress in the Barre Granite, provided that even a fraction of that energy is recovered. The actual mechanisms of storing residual stresses cannot be determined from microscopic petrofabric studies, simply because the specimens become relieved when they are prepared for observation. With existing techniques it is impossible to look at the “before” (natural) state. In light of Carter’s (1969) calculations, however, an in- direct approach for estimating residual stresses can be CONCLUSIONS 31 used; for example, the approach can be used to statistically compare the relative development of inten- sity of deformation lamellae or undulatory extinction to the magnitude of relieved residual stresses. SPECULATION AND POSSIBLE FUTURE WORK The relieved strains measured in this experiment in- dicate that there are large amounts of potential strain energy stored in rock masses which are capable of being mobilized when new surfaces are created. To know the geometry and size of these stored-energy regimes and what the mechanisms are that store and relieve them would be of great practical value. The mobilized energy alluded to undoubtedly contributes to the deformational behavior of rock masses in mining and quarrying operations, as well as in darn excavations. Significant quantities of energy stored as residual strain conceivably may be relieved in large-scale fracturing of rock masses, as in earthquakes. Therefore, the relieved energy may contribute to overall behavior of earthquake events, es- pecially the aftershocks. As has already been suggested by others (Varnes, 1969; Durrance, 1969), the release of strain energy is probably a significant factor in mechanical-weathering processes. Certainly, the relieved strains measured in this experiment would indicate that relieved strains can either aid or hinder mechanical—weathering processes. For example, if the residual-energy state is such that the resulting relieved strains are extensional, the mechanical weathering would be abetted; whereas, if the residual— energy state is such that the relieved strains are com- pressive, the mechanical weathering would be hindered. Future work attempting to define the mechanisms that store residual stress in rocks and to determine how these same mechanisms relieve stress will be important. Determining how large the stress-locking regimes can be and how much available energy may be present within any regime are also important. A definition of the geometry of these regimes may give clues as to the genesis of the residual stress. CONCLUSIONS The conclusions to be drawn from the release of residual stresses by drilling concentric slots of successively larger diameter in a block of Barre Granite are as follows: 1. The relieved strains are large, but because of the changes with successive cuts, the strains are not compatible with the calculation of a unique residual-stress tensor for the block. These strains show that significant quantities of stored strain energy can be mobilized. After the top of the block had been cored, the relieved compressive strains indicated that initially there was an excess of tensile residual energy in the block with a relieved tensile maximum-principal- stress component of at least 6 bars. As the coring was continued on the bottom and on side 2, the relieved strains became more exten- sional, indicating that the residual stress had become more compressional. Finally, the net strains indicated that the relieved part of the residual stress field was both tensile and compressional, with the minimum prin- cipal (compressive) stress being circumferential. 2. Contractions of cores and annuli cannot be caused by the opening of fractures; thus, the deformations produced by stress relief are not accompanied by appreciable new microfracturing. 3. The strains resulting from stress relief are not limited to short distances from the fresh cuts; rather, away from the cuts they diminish uniformly over the entire surface in a pattern related to the geometry of the cuts and the initial shape of the block. The largest strains are adjacent to the new surfaces. 4. Identical deformations measured across a borehole, as a result of stress-relief overcoring techniques, can be produced by two different balanced—stress systems. Mobilization of tensile residual stress energy can produce the same deformation as exter- nally applied compressional forces. 5. Although the anisotropic deformations of the rock are definitely related to the fabric anisotropy, the residual stress is not clearly related to these fabric elements. In fact, the internally balanced stress field seems to change as the geometry of the body changes, yet it maintains an inherent orientation, as can be seen by the elliptical deformation of the annuli. The measured residual stress field is related at least to the geometry of the body and to the degree of homogeniety, orientations, and magnitudes of the originally frozen-in stresses. 6. The X-ray determined prestress was compatible in sense and direction with the strain changes at the same location (gage 4). The magnitudes of the strain changes, however, were smaller than those of the X-ray-determined prestress. Elsewhere on the block, the strain changes were compatible at some locations with only the direction of the X-ray- determined prestress at gage 4. The X-ray prestress that is determined in a small chip may represent the direction and sense of the prestress at the same point in the block prior to the dissection of the chip. However, the magnitudes will have been diminished by at least the magnitudes of the relieved strains. 32 DEFORMATIONS, RELAXATION OF RESIDUAL STRESSES, BARRE GRANITE FROM VERMONT REFERENCES CITED Bjerrum, Laurits, 1967, Progressive failure in slopes of overcon- solidated plastic clay and clay shale: Am. Soc. Civil Engineers Proc., v. 93, paper 5456, Jour. Soil Mechanics and Found. Div., v. 93, no. SM5, pt. 1, p. 1-49. Bur, T. R., Hjelmstad, K. E., and Thill, R. E., 1969, An ultrasonic method for determining the attenuation symmetry of materials: U.S Bur. Mines Rept. Inv. 7335, 8 p. Carter, N. L., 1969, Flow of rock-forming crystals and aggregates, in Riecker, R. E., ed., Rock mechanics seminar, v. 2: Bedford, Mass, Air Force Cambridge Research Lab., p. 509-594. Chayes, Felix, 1952, The finer-grained calcalkaline granites of New England: Jour. Geology, v. 60, no. 3, p. 207-254. Denkhaus, H., 1967, Report on theme 4, residual stresses in rock masses: Intemat. Soc. Rock Mechanics Cong, 1st, Lisbon, Por— tugal, Proc., v. 3, p. 312-319. Douglass, P. M., and Voight, Barry, 1969, Anisotropy of granites — A reflection of microscopic fabric: Geotechnique, v. 19, no. 3, p. 376- 398. Durrance, E. M., 1969, Release of strain energy as a mechanism for the mechanical weathering of granular rock material: Geol. Mag, v. 106, no. 5, p. 496-497. Emery, C. L., 1964, Strain energy in rocks, in State of stress in the earth's crust — Internat. Conf., Santa Monica, Calif., 1963, Proc.: New York, Am. Elsevier Publishingg Co., p. 234-279. 1968, Strain in rocks, in Engineering geology and soils engineer- ing symposium, 6th Ann., Boise, Idaho, 1968, Proc.: Boise, Idaho Dept. Highways, p. 355-364. Friedman, M., 1967, Measurement of the state of residual elastic strain in quartzose rocks by means of X-ray diffractometry: Norelco Reporter, v. 14, no. 1, p. 7-9. 1968, X-ray analysis of residual elastic strain in quartzose rocks, in Gray, K. E., Basic and applied rock mechanics: Baltimore, Port City Press, Inc., p. 573-597. __1971, Residual elastic strains in rock, in Studies in rock fracture: U.S. Army Corps Engineers Tech. Rept. 5, Contract DACA 73-68- 0-0004, 55 p. __._1972, Residual elastic strains in rocks: Tectonophysics, v. 15, no. 4, p. 297-330. Friedman, M., and Logan, J. M., 1970, The influence of residual elastic strain on the orientations of experimental fractures in three quart- zose sandstones: Jour. Geophys. Research, V. 75, no. 2, p. 387-405. Frocht, M. M., 1941, Photoelasticity, v. 1: New York, John Wiley & Sons, Inc., 411 p. Gallagher, J. J ., Jr., 1971, Photomechanical model studies relating to fracture and residual elastic strain in granular aggregates, in Studies in rock fracture: U.S. Army Corps Engineers Tech. Rept. 3, Contract DACA 73-68-C-0004, 127 p. Gardner, R. D., and Pincus, H. J., 1968, Fluorescent dye penetrants applied to rock fractures: Internat. Jour. Rock Mechanics and Mining Sci., v. 5, no. 2, p. 155-158. Handin, John, 1972, Studies in rock fracture, Task 1: U.S. Army Corps Engineers Quart. Tech. Rept. 17, Contract DACA 73-68-C-0004, p. 2-5. Hooker, V. E., and Duvall, W. 1., 1966, Stress in rock outcrops near Atlanta, Georgia: U.S. Bur. Mines Rept. Inv. 6860, 18 p. Hooker, V. E., and Johnson, C. F., 1969, Near-surface horizontal stresses, including the effects of rock anisotropy: U.S. Bur. Mines Rept. Inv. 7224, 29 p. Hoskins, E. R., and Daniels, P. A., 1970, Measurements of residual strain in rocks: Am. Geophys. Union Trans, v. 51, no. 11, p. 826. Jaeger, J. C., and Cook, N. G. W., 1969, Fundamentals of rock mechanics: London, Methuen & Co., Ltd., 513 p. Lowry, W. D., 1959, Expansion domes and shear cones in Mount Airy granite [N.C.]: Mineral Industries Jour., v. 6, no. 4, p. 1—6. McClintock, F. A., and Argon, A. W., 1966, Mechanical behavior of materials: Reading, Mass, Addison Wesley Publishing Co., 770 p. Merrill, R. H., and Peterson, J. R., 1961, Deformation of a borehole in rock: U.S. Bur. Mines Rept. Inv. 5881, 32 p. Nichols, T. 0., Jr., Lee, F. T., and Abel, J. F., Jr., 1969, Some in- fluence of geology and mining upon the three-dimensional stress field in a metamorphic rock mass: Assoc. Eng. Geologists Bu11., v. 6, no. 2, p. 131-143. Norman, C. E., 1970, Geometric relationships between geologic struc- ture and ground stresses near Atlanta, Georgia: U.S. Bur. Mines Rept. Inv. 7365, 24 p. Nur, Amos, 1971, Effects of stress on velocity anisotropy in rocks with cracks: Jour. Geophys. Research, v. 76, no. 8, p. 2022-2034. Nur, Amos, and Simmons, Gene, 1969, Stress-induced velocity anisotropy in rock — An experimental study: Jour. Geophys. Research, v. 74, no. 27, p. 6667-6674. 1970, The origin of small cracks in igneous rocks: Internat. J our. Rock Mechanics and Mining Sci., v. 7, no. 3, p. 307-314. Pincus, H. J., 1964, Discussion of “Strain energy in rocks,” by C. L. Emery (1964), in State of stress in the Earth’s crust ——- Internat. Conf., Santa Monica, Calif., 1963, Proc.: New York, Am. Elsevier Publishing Co., p. 269-279. Price, N. J ., 1969, Laws of rock behavior in the earth’s crust, in Somer- ton, W. H., Rock mechanics theory and practice: Baltimore, Port City Press, Inc., p. 3-23. Sharp, J. E., 1969, Engineering geology studies of the Henderson no. 1 shaft, Henderson mine, Climax Molybdenum Co., Empire, Colorado: Denver, Colo., Adm. rept. to Climax Molybdenum Co., 63 p. Varnes, D. J ., 1969, Model for simulation of residual stress in rock, in Somerton, W. H., Rock mechanics theory and practice: Baltimore, Port City Press, Inc., p. 415-426. Varnes, D. J., and Lee, F. T., 1972, Hypothesis of mobilization of residual stress in rock: Geol. Soc. America Bu11., v. 83, no. 9, p. 2863-2866. Voight, Barry, 1967, Interpretation of in situ stress measurements: Internat. Soc. Rock Mechanics Cong., lst, Lisbon, Portugal, 1966, Proc., V. 3, p. 332-348. White, W. S., 1946, Rock-bursts in the granite quarries at Barre, Ver- mont: U.S. Geol. Survey Circ. 13, 15 p. Willard, R. J., and McWilliams, J. R., 1969, Microstructural tech- niques in the study of physical properties of rock: Internat. Jour. Rock Mechanics and Mining Sci., v. 6, no. 1, p. 1-12. if! U.S. GOVERNMENT PRINTING OFFICE: 1975—677-308/30 7 DAY 1:, 1m SCIENCES LIBRARY, :56 Cauldron Subsidence of Oligocene Age [7?ka At Mount Lewis, Northern Shoshone Range, Nevada GEOLOGICAL SURVEY PROFESSIONAL PAPER 876 a, . LIBRARY '. ”JIM/5R9, ,“r lepAl ”CITE!” ; Aug 27 ms [1.5.5.0. Cauldron Subsidence of Oligocene Age At Mount Lewis, Northern Shoshone Range, Nevada By CHESTER T. WRUCKE and MILES L. SILBERMAN GEOLOGICAL SURVEY PROFESSIONAL PAPER 876 UNITED STATES GOVERNMENT PRINTING OFFICE,WASHINGTON: 1975 UNITED STATES DEPARTMENT OF THE INTERIOR STANLEY K. HATHAWAY, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Wrucke, Chester T. 1927— Cauldron subsidence of Oligocene age at Mount Lewis, northern Shoshone Range, Nevada (Geological Survey Professional Paper 876) Bibliography: p. 19-20. Supt. of Docs. No.: 119.16:876 1. Volcanism-Nevada—Shoshone Mountains. 2. Subsidences(Earth movements)—Shoshone Mountains. 3. Geology, Stratigraphic—Oligocene. I. Silberman, Miles L., joint author. 11. Title. III. Series: United States. Geological Survey. Professional Paper 876. QE461.W89 557.3'08s[551.2’1] 75—619173 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, DC. 20402 Stock Number 024—001—02693—1 CONTENTS Page Page Abstract __________________________________________________ 1 Extrusive and sedimentary rocks ___________________________ 15 Introduction ______________________________________________ 1 Deposits on Mount Lewis and in Indian Creek Valley ____ 15 Geologic setting __________________________________________ 3 Caetano Tuff __________________________________________ 15 Boundary fault ____________________________________________ 3 Depositional sequence __________________________________ 16 Internal structure ________________________________________ 10 History of volcanism ______________________________________ 16 Intrusive rocks ____________________________________________ 11 Caetano Tuff and the Mount Lewis cauldron ________________ 17 Pipes and plugs ______________________________________ 12 Comparison of the Mount Lewis and other cauldrons ________ 18 Intrusive quartz latite breccia of Gilluly and Gates ______ 12 References cited __________________________________________ 19 Bikes ________________________________________________ 13 Intrusive sequence ____________________________________ 14 ILLUSTRATIONS Page FIGURE 1. Generalized geologic map of part of north-central Nevada showing location of the Mount Lewis cauldron, selected lithologic units, faults, and aeromagnetic contours _______________________________________________________________________ 2 2. Geologic map of the Mount Lewis area showing the ring fault _______________________________________________________ 4 3. Map of' the Mount Lewis area showing Tertiary igneous rocks, selected faults, and the location of chemically analyzed samples and samples dated by K—Ar methods ____________________________________________________________________________ 6 4. Geologic maps and sections of the southern margin of the Mount Lewis area ___________________________________________ 8 5. Map showing attitudes of thrust faults in and around the Mount Lewis cauldron _______________________________________ 10 6. Chart showing sequence and duration of events at Mount Lewis and vicinity and differentiation index as a function of age of igneous rocks _________________________________________________________________________________________________ 17 7. Histogram of K-Ar and fission-track ages of the Caetano Tuff _________________________________________________________ 18 TABLES Page TABLE 1. Chemical analyses, norms, and semiquantitative spectrographic analyses of Tertiary igneous rocks, Mount Lewis area and vicinity, Nevada _______________________________________________________________________________________________ 13 2. Summary of K-Ar ages of igneous rocks from Mount Lewis and vicinity _______________________________________________ 14 3. Analytical data for new K-Ar ages of igneous rocks from Mount Lewis _______________________________________________ 15 III CAULDRON SUBSIDENCE OF OLIGOCENE AGE AT MOUNT LEWIS, NORTHERN SHOSHONE RANGE, NEVADA By CHES'I‘ER T WRL'CKF. and MILES L. SILBERMAN ABSTRACT In the Shoshone Range of north-central Nevada, a ring fault that dips vertically to 65° inward outlines a deeply eroded cauldron 16 km in diameter. Thrust faults of Early Mississippian and early Mesozoic age that cut Paleozoic and Mesozoic strata in the cauldron have been deformed by the subsidence and dip inward, steeply near the ring fault, more gently in the interior, where they form a concentric pat- tern around a centrally located cluster of plugs and breccia pipes. The plugs and breccia pipes are the largest intrusive bodies in the subsided block. Plugs of quartz monzonite, rhyolite porphyry, and pumiceous vitrophyre occupy central positions in each of three breccia pipes; a dacite plug and two rhyolite plugs occur several kilometers from the pipes. Other intrusive bodies include a subhorizontal mass of quartz latite breccia at the summit of Mount Lewis, many dikes in the western part of the cauldron, and an intrusive breccia in the ring fault. The intrusive deposits, some of which have been studied in detail by Gilluly and Gates, contribute only a few percent of the exposed rocks in the cauldron. Extrusive rocks in the vicinity of Mount Lewis are preserved mainly within the cauldron. One sequence of extrusive and interlayered vol- caniclastic rocks crops out at the summit, on a few spurs that extend out from the summit, and in lowland east of the mountain. This sequence is composed of altered tuffs, dacite lava flows and agglomer- ate, rhyolite welded tuffs, conglomerate, and sandstone. There is another sequence at the north margin of the cauldron and in an extensive volcano-tectonic depression centered 25 km south of Mount Lewis. It consists of andesite lava flows and rhyolite welded ash-flow tuff of the Caetano Tuff. K-Ar dates and field relations provide a basis for determining only the general chronology of igneous events related to cauldron subsi- dence at Mount Lewis. Collapse occurred after the ring fault cut a 35.1-m.y.-old granodiorite pluton and before the fault was invaded by 33.2-m.y.-old intrusion breccia. Age determinations of 34.4 m.y. from the dacite plug and 34.7 and 33.2 m.y. from the quartz latite intrusion breccia record volcanism interpreted as occurring before collapse. Subsidence is thought to have resulted from eruption of some of the 33—31-m.y.-old Caetano Tufffrom the Mount Lewis cauldron. Uplift of volcanic and sedimentary rocks in the center of the cauldron 850 m relative to deposits near the ring fault, and the dips away from the summit in patches of these rocks on spurs flanking the mountain may result from mild resurgent doming. Volcanic activity in the northern Shoshone Range during an igne- ous cycle that existed from 38 to 31 m.y. ago began with intrusions of small granodiorite and quartz monzonite plutons, continued with emplacement of plugs and presubsidence tuffs at Mount Lewis, and ended with eruption of the Caetano Tuff from the volcano-tectonic depression and the Mount Lewis cauldron. The cauldron thus formed differs from many cauldrons in the western United States and from igneous ring complexes in Great Britain, New England, Nigeria, and Norway by exposing only a small percentage of intrusive rocks, prob— ably because the subjacent pluton at Mount Lewis did not rise to high levels during resurgent doming as in most ring complexes and caul- drons. INTRODUCTION One of the most prominent structural features of the Shoshone Range in north-central Nevada (fig. 1) is a ring fault 16 km in diameter (fig. 2). On the deeply incised flanks of Mount Lewis, the highest peak at the northern end of the range, this fault bounds Paleozoic and Mesozoic stratified rocks that foundered during vol- canic collapse. Within the subsided mass, breccia pipes, plugs, and dikes, together with remnants of tuffs and volcaniclastic deposits, record an episode of Oligocene volcanism. The collapse structure, here named the Mount Lewis cauldron, is one of the oldest Tertiary volcanic centers in Nevada. It is relatively deeply eroded, as the greater part of the rocks exposed lie well below the base of the original volcanic edifice. Our interest in the subsidence structure was aroused when we noticed on the geologic map of the northern Shoshone Range by Gilluly and Gates (1965), a fault pattern strikingly concentric about the volcanic center at Mount Lewis and markedly different from the struc- tural grain of the surrounding area. In order to deter- mine the significance of the fault pattern, Wrucke spent several weeks examining structural, stratigraphic, and igneous features in and around the subsided mass and made a geologic sketch map (fig. 4) ofa key area at the cauldron margin. Silberman obtained K-AR dates (ta- bles 2, 3) that helped establish the sequence of igneous and structural events. Together we visited many localities and collected samples for isotopic dating. Al- though this paper documents critical episodes in the history of volcanism at Mount Lewis, we recognize that the area is structurally far too complex to be fully un- derstood from reconnaissance examinations. Our in- terpretations have benefited greatly from progress made in the understanding of cauldrons and calderas since Gilluly and Gates completed fieldwork in the Shoshone Range in 1959. CAULDRON SUBSIDENCE OF OLIGOCENE AGE, MOUNT LEWIS, NORTHERN SHOSHONE RANGE, NEVADA 1 17°00' 116°30' l .\ ' ~ 'ATTLE s MOUNT .1 IN / ,\\ // % >t % S? Area of 4" figures 2. <8 40°30' — 40°00' 0 5 10 MILES i—1_LT—J 0 5 10 KILDMETRES Oligocene I EXPLANATION Qg Gravel Caetano Tuff Tbxv Breccia in breccia pipes ng-' Granodiorite Phanerozoic rock, undivided Contact Fault Q9 .1900 Magnetic contours C amour interval 500 gammas; 50 gammas at Mann! Lewis QUATERNARY TERTIARY FIGURE 1 .—Generalized geologic map of part of north-central Nevada showing location of the Mount Lewis cauldron, selected lithologic units, faults (after Gilluly and Gates, 1965; Roberts and others, 1967; Stewart and McKee, unpub. data; and this report), and aeromagnetic contours (after Philbin and others, 1963). BOUNDARY FAULT 3' The principal results of our study include recognition of the ring fault and the character of the rocks emplaced in it, structures developed in the downdropped pile, timing of subsidence relative to igneous events elsewhere in the range, and the effects of a preexisting pluton on the collapsed mass. Features critical to the history of subsidence have been found in structural and stratigraphic relations in Paleozoic rocks at the south- -west margin of the cauldron. Our study has benefited from enlightening discus- sions of cauldron subsidence with R. A. Bailey, E. H. McKee, and D. C. Noble, the last two in the field and in the office; and we are grateful to T. E. Mullens and J. H. Stewart for sharing with us observations on the stratig— raphy of Paleozoic strata southwest of Mount Lewis. GEOLOGIC SETTING The Shoshone Range is a northeast-trending, eastward-tilted fault block in the northwestern part of the Great Basin. In this region, the oldest formations exposed consist of miogeosynclinal rocks of Cambrian to Mississippian age in the lower plate of the Roberts Mountains thrust and eugeosynclinal strata of about the same age range in the upper plate of the thrust. Thrusting during the Antler orogeny, which culmi- nated in Early Mississippian time, juxtaposed these plates. Upper Paleozoic rocks are sparse in the region, as are Mesozoic rocks, remnants of which rest on thrust faults of Mesozoic age. Cenozoic volcanic rocks cover large areas of some ranges. In the vicinity of the Mount Lewis cauldron, bedrock consists principally of siliceous sedimentary rocks of Ordovician, Silurian, and Devonian age in numerous thin thrust slices above the Roberts Mountains thrust (fig. 2). Carbonaceous strata of Cambrian to Silurian age below the thrust crop out in a window immediately south of the ring fault. It is significant that all but one remnant of upper Paleozoic rock and the only scraps of Mesozoic strata in the northern 60 km of the range occur within the cauldron, some at relatively low altitudes. Overlying the older rocks are lava flows, welded tuffs, water-laid tuffs, and sedimentary rocks of Oligocene age. Some of these rocks are scattered, unnamed de- posits in the cauldron, but most rocks of this age in the northern Shoshone Range belong to the Caetano Tuff, a composite sheet of welded ash flows that crops out ex— tensively 20—35 km south of Mount Lewis and in adja- cent ranges (fig. 1). The relation of the Caetano Tuff and the unnamed deposits to the Mount Lewis cauldron is discussed in a later section. The Mount Lewis cauldron lies along a narrow zone of steep faults, small plutons, and numerous mineral de- posits that trend northwest across the range. Many of the steep faults are cut by plutons of Oligocene age. About 35 km southeast of Mount Lewis, this zone merges with a strikingly linear north-northwest— trending zone that extends from the Simpson Park Mountains at least to the Sheep Creek Range (fig. 1) and consists of strong magnetic anomalies (Philbin and others, 1963; Robinson, 1970) that follow a swarm of diabase dikes of Miocene age and a system of faults as young as Pliocene. The northwest-trending zone that crosses the Shoshone Range and that part of the north- northwest-trending zone southeast to the Cortez Moun- tains form much of the Battle Mountain-Eureka min- eral belt of Roberts (1966). These belts seem to be deep- seated features that have been the locus of considerable igneous activity; the zone crossing the Shoshone Range may have controlled the ascent of magma in the Mount Lewis area. Although no belt of comparable length con- taining high-angle faults is known to intersect the northwest-trending zone in the vicinity of Mount Lewis, the numerous north-trending dikes west and southwest of the summit (fig. 2) may follow structures that were important in localizing the cauldron. Only the southeastern half of the cauldron is exposed. The northwestern half is downfaulted and buried be— neath a thick cover of valley fill west of the steep Basin and Range faults that border the Shoshone Range. BOUNDARY FAULT The fault that encircles the subsided mass at Mount Lewis traces a smooth curve for 29 km around the south and east flanks of the mountain (fig. 2). It is a typical ring fault in that it bounds a nearly perfect semicircular area that forms the exposed southeastern half of a circu- lar subsidence structure. The fault is covered by al— luvium around the western perimeter of the cauldron. Along the southern margin, the fault has been invaded by dikes for 5.6 km of its extent; at the east edge of the cauldron, the fault is interrupted by a pluton. Elsewhere it is a well-defined single fault that dips 90°—65° inward. The ring fault, composed of several segments, was mapped by Gilluly and Gates (1965) as separate faults of different origins. The Trout Creek fault they inter— preted to be a tear fault in the upper plate of the Roberts Mountains thrust along the south margin of the caul- dron. The Hilltop and Bateman faults were considered by them to be two thrusts in the upper plate along the east margin of the cauldron (fig. 3). A critical element of their interpretation is that these faults originated by processes unrelated to volcanism. The tear-fault concept was devised by Gilluly (1960; 4 CAULDRON SUBSIDENCE OF OLIGOCENE AGE, MOUNT LEWIS, NORTHERN SHOSHONE RANGE, NEVADA ill“. Ml" ‘ 116°50' I Qal 40°25' — 0 1 2 MILES l——-r—Lr—‘ 0 I 2 KILOMETRES FIGURE 2.—Geologic map of the Mount Lewis area, showing the ring fault. Modified from Gilluly and Gates (1965). Location shown in figure 1. see also Gilluly and Gates, 1965, p. 98— 100) to explain However, the Ordovician rocks have been found by us to the development of folds and an overturn in the Roberts underlie as well as overlie the Cambrian strata on the Mountains thrust near the mouth of Trout Creek, 8 km northwest—trending ridge north of the ring fault, as il- south-southwest of Mount Lewis. They assumed that in lustrated in figure 43. Specifically, the contact of the the Trout Creek area the Roberts Mountains thrust southeast-trending prong of the Cambrian rocks of that separated rocks of Cambrian age in the lower plate from ridge was found to be the basal contact of those rocks rocks of Ordovician age in the upper plate (fig. 4A). dipping west slightly steeper than the slope of the ridge Roberts Mountains thrust Oligocene 4L Upper plate Lower plate EXPLANATION Qal Alluvium Tv T“ \ Tv, extrusive igneous rocks and sedimentary rocks Ti, intrusive igneous rocks and intrusive breccia in dikes Tbx, intrusive breccia of breccia pipes my lllll Caetano Tuff and associated andesite \I \ t / [ngk / l / Granodiorite TIPIP Sedimentary rocks Slaven Chert Chert, quartzite, siltstone, and greenstone Shwin Formation Limestone, dolomite, shale, and quartzite Contact Steep fault, dashed where inferred M Paleozoic thrust fault _A_A_.A._A._ Mesozoic thrust fault —l— Syncline TERTIARY QUATERNARY PENNSYL— VANIAN, PER— NIAN AND ORDOVICIAN DEVO— SILURIAN CAM- BRIAN CAMBRIAN, ORDOVICIAN, MIAN, AND TRIASSIC AND SILURIAN BOUNDARY FAULT 5 crest that they cap; that is, the contact forms a some- what rounded V that points upridge. Because the Cambrian rocks rest on upper-plate Ordovician strata, they must belong in the upper plate: they form a thrust slice torn from the lower plate during thrust faulting and interleaved with wedges of upper plate formations. A limestone sliver of Silurian Roberts Mountains For— mation, a short distance southeast of the Cambrian rocks and immediately north of the Trout Creek fault, has a similar origin, for it, too, is of lower-plate lithology but is now interleaved with upper—plate rocks. The Roberts Mountains thrust is not exposed north of the ring fault. Slices of rocks originally in the lower plate, but now incorporated Within upper-plate rocks of the Roberts Mountains thrust, occur adjacent to the Gold Acres window (Wrucke and Armbrustmacher, 1969) on the east flank of the range. Since the Cambrian and adjacent rocks north of the Trout Creek fault are part of the upper plate, the structural relations between them need not be explained by folding of the thrust or by movements related to tear faulting, so the Trout Creek fault may be interpreted as a normal rather than a tear fault. As discussed below, we interpret this normal fault to be a segment of the ring fault bounding the Mount Lewis cauldron. The structural break originally mapped as a tear fault is shown by Gilluly and Gates (1965) as beginning 8 km east of the range front and extending eastward (fig. 3). We believe, however, that the Trout Creek fault extends to the west edge of the range, as shown in figures 2 and 4B. Near the mountain front, the bound- ary fault dips steeply northeast into the cauldron, separating stratigraphic units of different structural blocks. Our geologic sketch map (fig. 4B) of the area near the mountain front differs from the geologic map of Gilluly and Gates (reproduced as our figure 4A) in the iden- tification of some of the formations and in the existence of thrust faults as well as in the extent of the Trout Creek fault. In particular, rocks labeled Hanson Creek Formation or the Roberts Mountains Formation in the lower plate of the Roberts Mountains thrust south of Trout Creek in figure 4A are identified as quartzite, shale-quartzite-limestone, and limestone in figure 43. We believe that the Roberts Mountains Formation does not crop out adjacent to the Trout Creek fault in this part of the area, although it crops out farther south as shown in figure 4B. The limestone that we mapped with rocks designated Cambrian(?) and OrdoVician(?) re- sembles the Bullwhacker Member of the Windfall For- mation at Eureka, Nev. (Nolan and others, 1956; T. E. Mullens, oral commun., 1972), but the identity of the quartzite and the shale-quartzite-limestone units re- mains uncertain. The shale-quartzite-limestone unit is CAULDRON SUBSIDENCE 0F OLIGOCENE AGE, MOUNT LEWIS, NORTHERN SHOSHONE RANGE, NEVADA Tqm ‘ TS' h V- ‘ § 40°25' Tbx \Horse ‘Canyon \\ pipe V652" 5 M1038 \ a” 'V 6 _- :T°.° 116°50' . '. Ii .. .‘ j ~ ........... l’ ______________ <9 4464’ 44/ Msz Tq pl a ,9 6x ? Q a, 2 MILES 0 1 2 KILOMETRES Geology modified from Gilluly and Gates (1965) FIGURE 3,—Map of the Mount Lewis area showing Tertiary igneous rocks, selected faults, and the location of chemically analyzed samples and samples dated by K-Ar methods. Location shown in figure 1. mostly dark-gray to black, sandy, phyllitic shale and dark-brownish-gray phyllitic sandstone with subordi- nate light-gray quartzite and thin-bedded dark-blue- gray limestone. The geology of the lower plate of the Roberts Mountains thrust south of the Trout Creek fault is more complex than the geology of the upper plate immediately north of that fault and will not be understood until studied in detail. Other critical relations that involve the Trout Creek fault at the south edge of the cauldron can be observed at the small granodiorite body 31/2 km east of the mountain front (figs. 2, 3). Although the contact at the north edge of this granodiorite is not exposed, the pluton crops out boldly as far north as the projection of the ring fault, suggesting that the granitic mass ends there. Further- more, the northern 30 m of the body is crossed by BOUNDARY FAULT 7 EXPLANATION Z . ' . - ’ _ < -‘.:Qg.-'-i :l E . . -. . LL] Gravel : ' a - e o ‘ D are}; 0 , VTca , Caetano Tuff Tc, rhyolite tufl' Tca, andesite Igneous rocks and breccia qu, quartz porphyry in dikes, rhyolite por- u phyry in Pipe Canyon pipe, and intrusive >4 E breccia in Trout Creek fault g 8 _ Tqm, quartz monzonite _ ; .20 Tpv, pumiceous vitruphyre m O qu, quartz latite intrusion breccia E TV, tufls, lava flows. agglomerate, and minor sedimentary rocks Td, intrusive dacite Tbx, breccia in breccia pipes T Sedimentary rocks '''''''' _ Granodiorite D Z < 2 U MZPZ "‘ 8 _ 8 O Sedimentary and igneous ‘ 8 fl rocks .4 2 < l: Contact Fault _—A—A— Thrust fault Suwteeth on upper plate +2 Locality of chemically analyzed specimen Number is specimen number of table I x Wl421 K-Ar sample locality Number is specimen number of table 2 numerous fractures that parallel the fault, dip 75° N., and produce a strong sheeted appearance in the granodiorite. These fractures probably formed during development of the ring fault. If this is true, this pluton, dated at 35.1 my (table 2, sample M103B), predates the ring fault. The most problematic feature along the ring fault is the granodiorite intrusion at the east margin of the cauldron (figs. 2, 3). As mapped by Gilluly and Gates (1965, pl. 1), this pluton seems to cut across the Bate- man segment of the ring fault as though it were younger. Indeed, we have found no evidence that the Bateman fault crosses the pluton. The isotopic dates discussed below, however, show that the granodiorite predates the volcanism at Mount Lewis; rocks in the cauldron must, therefore, have subsided around the plu- ton. The contact of the pluton and host rocks is so poorly exposed that it is not possible to determine if it is brec- ciated. Gilluly and Gates (1965) show that strata around the granitic mass tend to follow the outline of the body as though they were draped around it. This area warrants further examination, but we believe that the interpretation presented here is correct. A modification of the draping theory was suggested to us by Roy A. Bailey (written commun., 197 3) of the US. Geological Survey: Because of the rigidity of the granodiorite, the ring fault may have split into two faults around the pluton—the Hilltop fault and the Bateman fault (fig. 3). The Hilltop fault formed as the Trout Creek fault turned northward and, with decreas- ing displacement, passed west of the pluton and died out 8 km northeast of Mount Lewis. Segments of the Hilltop fault appear to be nearly vertical, suggesting that it may be a normal fault rather than a reverse fault. Along the steep, west-dipping Bateman fault to the east, dis- placement decreased southward and died out at the north edge of the granodiorite pluton. The combined effect of decreasing displacement in opposite directions on these faults produced an unbroken arcuate ramp-like structure leading from the caldera rim to the floor—a feature that Bailey states (written commun., 1973) is not uncommon in calderas. Evidence that the Bateman fault forms a segment of the ring fault is that the Bateman fault dips steeply west and follows the arcuate trend of the Trout Creek fault and the southern part of the Hilltop fault. Jux- taposition of stratigraphic units of different structural masses on opposite sides of the fault could be explained as easily by normal as by reverse movements. Evidence favoring the ring-fault hypothesis include the curved trace of the Trout Creek, Hilltop, and Bate- man fault segments roughly concentric about the intru- sive centers at Mount Lewis and the difference in stratigraphy and the contrasting styles of the structure on opposite sides of the ring fault. Inside the cauldron, thrust sheets dip toward the interior and strike approx- imately parallel to the ring fault; outside the cauldron, thrusts are subhorizontal. In summary, the ring fracture can be traced as a single fault around the exposed part of the cauldron CAULDRON SUBSIDENCE OF OLIGOCENE AGE, MOUNT LEWIS, NORTHERN SHOSHONE RANGE, NEVADA EXPLANATION UPPER PLATE OF LOWER PLATE OF ROBERTS MOUNTAINS THRUST ROBERTS MOUNTAINS THRUST : - - 1 ' ‘ } QUATERNARY "ll - SILURIAN Sedimentary rocks Roberts Mountains Formation - Quartz porphyry TERTIARY Valmy Formation Hanson CIek Formation _ ORDOVICI AN \'- I \ 7 fl Granodiorite Eureka Quartzite Shwin Formation _ C AMBRI AN Eldorado Dolomite ~ Contact % F ult §® a CV ,, 116°57' .A-A-A-A-A— ’ Thrust fault Sawteeth on upper plate 40°22'30" E ; ' . TROUT CREEK FAULT Oh Pv Base from U-S- Geological Survey, 1215340 Geology generalized slightly from Unedited manuscript, Mount Lewis, 1950 Gilluly and Gates (1965) A A ' ROBERTS MOUNTAINS THRUST\Q5 7W \ ------- ' 7000' 0 v. '/2 MILE I : l . : . I ' I——r—J—I——4 i E 5 “ 0 v. V: KILOMETRE 6000' ' ‘ ' 5000' comaun INTERVAL 200 FEET (61 METRES) Section drawn by C. T. Wrucke using geology from Gilluly and Gates (1965) FIGURE 4.—Geologic maps and sections of the southern margin of the Mount Lewis area. A, as mapped BOUNDARY FAULT EXPLANATION UPPER PLATE LOWER PLATE OF ROBERTS OF ROBERTS MOUNTAINS THRUST MOUNTAINS THRUST ]» QUATERNARY "ll - . , _ SILURIAN’ Roberts Mountains Roberts Mountains Formation Formation J = \ I \\ , ~ .‘ — . , 9 0" ~ ORDOVICIAN Quartz porphyry Intruswe breccna TERTIARY Valmy Formation V : 3 A Granodiorite E Limestone .2 § _‘ ' ORDOVICIAN(?) i g ' " j — AND 3 «5 Shaleaquanzue, and CAMBRIAN(?) {if = limestone g .. . .. V; Quartzite — Shwin Formation _ C AMBRI AN % st . 115°57' Eldorado Dolomite ~ Contact Fault _A_A_A._A_A_ Thrust fault 40°22‘30" Sawteeth on upper plate to. stam- . amaze: 9. 0-5.. “’0 02v». 7 - I was: ‘oooooo >~ mo: 9.9, T ’ 3%.». A , \ Tib D ¢ 'fil ~ ’ ROB RT M0 TAINS in?“ a ‘ HRUST ‘ 5 N P‘ "3b Base from US. Geological Survey, 1:15,840 o Reconnaissance geology by C. T. Wrucke, 1972 Unedited manuscript, Mount Lewis, 1950 A : A ' 7W 0 v. v; MILE T-T_’—L~‘l—_—J 0 VI ‘/2 KILOMETRE “If CONTOUR INTERVAL 200 FEET (61 METRES) by Gilluly and Gates (1965, pl. 1). B, as sketched for this report. Location shown in figure 1. 10 CAULDRON SUBSIDENCE OF OLIGOCENE AGE, MOUNT LEWIS, NORTHERN SHOSHONE RANGE, NEVADA 116°50' l ?\ iv 30\ 2\ 7,\ 56<\ 23/ \9]\,\7 25\\ [k 0 /u. ”l [2/ 32“37‘\\ \[2 I9 27\ \3: +2, X 321 “77/ 13“ a/-\ ”7/25 4/1 20\ 272/ Mi 3 ms“ 4. ,f \3“ mx 40°25' — ER /2I \25 / 37\ 50\ 7 r A. >9 t” 26 17/ ”O 25/ X l7 27/ ” \{5 ”’3‘; Mount Lewis 3"“ / ., \” l” r/ ,2 27 \>/9 5 l1” 9/ [27/ i / \z 72 Il‘l 20 30/ \ ,2 / x \3 \62 3 / l5 4 \4;\ at; 25/ {-4 Iqs 25 / 19 \a “X 3" l” 5/ \2' 4”- ,./ 27\ /,2 7>\ f” l’” \56 ’°\ \’5 \20 3/ /25 /«3/0 357/ 29/ ’7, \‘Z i Inc 1 l \(25 0 1 2 3 M|LES 0 1 2 3 K|LOMETRES FIGURE 5.——Attitudes of thrust faults in and around the Mount Lewis cauldron. Location shown in figure 1. except for the segment occupied by the granodiorite on the east side. It dips vertically to 65° inward and sepa- rates rocks of such differing structural styles that the internal structure of the subsided mass must offer addi- tional clues to its origin. INTERNAL STRUCTURE Thrust faults provide the best reference surfaces for study of the internal structure of the subsided mass because they are the prevolcanic features most likely to have been subhorizontal before subsidence. In contrast, bedding or other planar structures within thrust sheets commonly are highly contorted as a result of deforma- tion during thrusting. This is especially true of chert and argillite strata; quartzite beds generally retain the attitude of overlying and underlying thrusts. Most INTRUSIVE ROCKS 1 1 thrusts in the Shoshone Range are warped and some have been folded sharply; most thrust faults outside the cauldron are inclined at angles less than 30°. The ap- proximate attitudes of thrusts within the subsided mass have been calculated using the three-point method and the relation of the trace of the faults across the land surface as shown on Gilluly and Gates’ map (1965, pl. 1). The results, illustrated in figure 5, show clearly that the thrusts in the downdropped mass tend to parallel the boundary fault and to dip inward toward the breccia pipes. Thrusts tend to dip more steeply near the boundary fault than in the central part of the downdropped pile. Within the outer kilometer of the cauldron, thrusts have an average dip of 38°; in the remainder of the cauldron interior and in the area of figure 5 outside the cauldron, the average dip of the thrusts is 24°. This basining effect strongly suggests that the boundary fault dips generally inward and that the internal mass collapsed into increasingly more restricted space downward, uptilting the peripheral layers into gener- ally steep attitudes along the cauldron margin. Direct or indirect evidence of an inward-dipping boundary fault has been found at many other cauldrons, including the Paresis igneous complex of South Africa (Siedner, 1965), Ossipee cauldron, New Hampshire (Kingsley, 1931), Oslo region (Oftedahl, 1953), and Glen Coe, Scot- land (Taubeneck, 1967; Reynolds, 1956). Some thrust faults in the subsided mass diverge from the general inward dip. At the west edge of the range, a few thrusts have probably been affected by drag along Basin and Range faults. In the western part of the southeast quadrant of the cauldron, the attitudes shown in figure 5 define a north-northeast-trending synform ‘that probably formed during thrusting prior to vol- canism. The few aberrant attitudes detract only slightly from the general symmetry of the inward dips. Attitudes of thrust faults outside the cauldron show little conformity to the boundary fault. They record a vague northerly strike common to rocks in the upper plate of the Roberts Mountains thrust in the northern Shoshone Range. Evidence that the rocks dip outward from the boundary fault as a result of tumescence from rising magma was not found. Considering the variation in strike and dip of thrust faults outside the cauldron, the degree to which the attitudes parallel the boundary fault within the subsided mass is all the more remarkable. The structural complexities at the southwest margin of the cauldron suggest that the downdropped block subsided at least 950 m in that area. This conclusion is based on the observation that the Roberts Mountains thrust crosses the high ridge south of Trout Creek fault but is not exposed along Trout Creek, which lies in the first deep canyon north of the fault. However, the Cam- brian and Silurian formations that interleave with typ- ical upper plate Ordovician rocks just inside the caul- dron form a sequence that probably lies only a short distance above the Roberts Mountains thrust, for the same Cambrian and Silurian rocks are here interpreted as occurring respectively immediately above or im- mediately below the thrust south of the boundary fault (figs. 2, 43). Therefore, the amount of subsidence may not have been much more than 950 m. In the northeast- ern part of the cauldron, the thrust contact between the Ordovician rocks and the underlying Devonian chert is not exposed, but rocks outside the ring fault are Devo- nian; consequently, the subsidence in that part of the cauldron must have been greater than the 335 m maximum topographic relief in the Ordovician rocks, as measured in a section normal to the ring fault. Mesozoic rocks exposed within the cauldron, some at relatively low altitudes, are preserved as a result of the downdrop- ping; rocks of this age do not crop out for many kilometres outside the structure. The absence of a Mesozoic reference plane outside the fault prevents an estimate of subsidence of these rocks. The presence of a simple ring fault rather than a complicated zone of boundary faults and the relatively unbroken pattern of thrusts in the subsided mass suggest that the cauldron subsided more or less as a single intact block. Resurgent doming at Mount Lewis is suggested by late Paleozoic and early Mesozoic rocks and by Tertiary volcanic rocks that in general are topographically high at the summit of Mount Lewis and dip toward topo- graphically lower areas nearer the ring fault. The dif- ference in elevation between the late Paleozoic and Mesozoic rocks at the summit and rocks of the same age 5.6 km northwest is about 850 In, between the volcanic rocks on Mount Lewis and the tuffs in Indian Creek Valley 4 km east about 800 In. These values are not much less than the 3,500—5,000 foot (1,065—1,520 m) probable range of structural relief that Smith and Bailey (1968) report for resurgent domes. Positive evi- dence of an igneous mass that might have caused the doming suggested here has not been found, but the pattern of magnetic contours shown on figure 1 is suggestive of a buried pluton. An aeromagnetic map by Philbin, Meuschke, and McCaslin (1963) shows that in the northern Shoshone Range there is a strong associa- tion between positive magnetic anomalies and known granitic plutons (see Wrucke and others, 1968, fig. 1). Possibly the subjacent magma chamber beneath Mount Lewis is relatively deep. INTRUSIVE ROCKS Within the block here interpreted to be downfaulted by volcanic subsidence, Gilluly and Gates (1965) map- 12 ped three breccia pipes, many small bodies of quartz monzonite, a dacite plug, three rhyolite plugs, a tabular mass of intrusive breccia, and innumerable dikes (fig. 3). Most of the intrusive centers are slightly elliptical in plan; the largest is a pipe 1.3x 2 km, but no single body dominates as a major intrusive center. Paleozoic and Mesozoic country rock separate the larger intrusive bodies, although a narrow neck of intrusive breccia con- nects two of the pipes. The pipes and plugs are clustered in an area about 7 km across slightly southwest of the center of the cauldron. Together the intrusive rocks, exclusive of the tabular mass of breccia at the summit, constitute only about 3 percent of the area exposed within the cauldron. A positive magnetic anomaly (Philbin and others, 1963) that may be related to a larger subjacent intrusive body lies only a short dis- tance west of the pipes (fig. 1). PIPES AND PLUGS The breccia pipes, named the Horse Canyon, Pipe Canyon, and Rocky Canyon breccia pipe (fig. 3) by Gil- luly and Gates (1965, p. 66— 73), contain two varieties of breccia, foundered blocks of sedimentary rocks, and plugs of Tertiary igneous rocks. Around the margin of each pipe are discontinuous patches of coarse breccia composed of angular fragments ranging in size from microscopic chips to blocks as much as 100 feet long of wall rock and sparse pieces of Tertiary porphyries. This breccia is interpreted as having formed by slumping and landsliding of the sides of the pipes. Intruded into the coarse breccia as dikes and veinlets and as masses oc- cupying much of the inner part of the Horse Canyon pipe and the southern and eastern parts of the Pipe Canyon pipe is fine breccia composed of angular to rounded fragments ranging in size from microscopic to about 3 cm across. One-half to two-thirds of this breccia is a thorough mixture of Paleozoic rock types, the remain- der, clasts of Tertiary igneous rocks and chips of quartz, plagioclase, biotite, and hornblende crystals. Frag- ments of pumice, tuff, and glass occur in the fine breccia of the Pipe Canyon pipe; the matrix is a dense siliceous, isotropic to devitrified glassy mass containing mineral fragments and rock flour. As interpreted by Gates (1959) and Gilluly and Gates (1965), this breccia formed by volatiles rising from a cupola of magma, penetrated the overlying rock and brecciated it, causing turbulence that resulted in abrasion and mixing of the fragments. Steam generated from ground water percolating into the fragmented rock may have aided the process of brec- ciation; the brecciation could have occurred in several pulses followed by periods of rock bursting and collapse stoping. Magma intruding the rock debris fragmented as it congealed, adding crystal fragments and pieces of igneous material to the breccia. A conglomerate consist- CAULDRON SUBSIDENCE OF OLIGOCENE AGE, MOUNT LEWIS, NORTHERN SHOSHONE RANGE, NEVADA ing chiefly of pebbles of Paleozoic sedimentary rocks and Tertiary volcanic rocks occurs in the Horse Canyon pipe and probably formed close to the vent from fine breccia reworked by running water before subsiding into the pipe. The pipes, as described by Gates (1959) and Gilluly and Gates (1965), contain plugs intrusive into the breccias—a plug of quartz monzonite porphyry in the Horse Canyon pipe, a rhyolite porphyry in the Pipe Canyon pipe, and a plug of pumiceous vitrophyre in the Rocky Canyon pipe. The pumiceous vitrophyre is a ve- sicular, locally fragmental rock composed of altered and devitrified glass that Gilluly and Gates (1965) interpret as having been a viscous gas-charged magma which on reaching the surface may have formed a welded tuff. Lenses of block agglomerate and several varieties of tuff incorporated in the vitrophyre record eruptions older than the vitrophyre. There is no firm evidence that the quartz monzonite of the Horse Canyon pipe breached the surface. However, the abundance of shards and fragments of collapsed pumice in the fine breccia of the Pipe Canyon pipe suggest that explosive eruptions may have occurred at the vent, although this is not spe- cifically mentioned by Gilluly and Gates. All the plugs described here are altered, and none of them has been chemically analyzed. A dacite plug and a rhyolite plug occur on the east slope of Mount Lewis, outside the breccia pipes (fig. 5). As described by Gilluly and Gates (1965), both plugs have a zone of intrusive breccia at the contact, and both are flow banded. A specimen of the dacite collected for this study consists of plagioclase, biotite, and hornblende phenocrysts in a purplish-gray aphanitic matrix. In thin section the dacite is seen to be devitrified glass containing numerous small grains and pools of late quartz. According to the chemical analysis (table 1, column 1) this rock is a dacite. Gilluly and Gates (1965) propose that the dacite and rhyolite plugs may have fed explosive eruptions and could have been the roots of domes. INTRUSIVE QUARTZ LATITE BRECCIA OF GILLULY AND GATES The volcanic rock at the summit of Mount Lewis (fig. 3) is described by Gilluly and Gates (1965) as a subhori- zontal mass of intrusive quartz latite breccia emplaced along the unconformity between Paleozoic rocks and an overlying sequence of Tertiary extrusive and sedimen- tary rocks. The breccia, which shows evidence of having been subjected to great turbulence during deposition, is described as being composed of fragments of quartz la- tite porphyry, glass, Paleozoic rocks, and of broken crys- tals of plagioclase, hornblende, biotite, and quartz. The rock fragments commonly are 1—10 mm in diameter, but INTRUSIVE ROCKS 13 TABLE 1.—Chemical analyses, norms, and semiquantitative spectro- graphic analyses of Tertiary igneous rocks, Mount Lewis area and vicinity, Nevada [Methods used are those described in Shapiro and Brannock (1962). Supplemented by atomic absorption, Analysts: P. Elmore, L. Artis, G. Chloe, H. Smith, J. Kelsey, and J. Glenn. Semiquantitative spectrographic analysis. Results are to be identified with geometric brackets whose boundaries are 12. 0.08, 0.56, 0.38, 0.26, 0.18, 0.12 . . . ., but are reported arbitrarily as midpoints of those brackets, 1, 0.7, 0.5, 0.3, 0.2, 0.15, 01 ,,,,, The precision of a reported value is approximately plus or minus 1 bracket at 68 percent or 2 brackets at 95 percent confidence. Analysts: C. Heropoulos and R. Mays] Specimen No. 1 2 3 4 5 Chemical analyses (weight percent) 67.6 63.6 66.2 68.3 70.2 14.1 16.7 16.5 15.0 15.3 2.1 3.1 3.6 2.3 1.2 1.2 .44 .08 .72 1.3 .88 .89 1.0 1.0 .59 3.2 2.9 1.4 2.0 1.2 2.8 4.1 4.6 3.6 3.2 3.3 4.1 4.2 3.1 4.8 1.6 1.2 1.2 1.5 1.2 .85 1.2 .45 .87 .16 .41 .50 .48 .44 .37 .15 .15 .18 .12 .16 .05 .04 .05 .04 .02 1.7 1.0 <.05 1.0 .32 Sum ,,,,,,,,, 100 100 100 100 100 C.I.P.W. norms 35.60 19.69 19.40 32.79 30.71 19.51 24.25 24.83 18.32 28.36 23.71 34.72 38.95 30.47 27.07 4.15 7.09 5.77 2.82 2.88 2.19 2.22 2.49 2.49 1.47 _,,, ,,,, ,,,, ,_,, .82 284 .10 ,_,_ 1.18 1.74 .14 3.03 3.60 1.49 _,, 78 .95 .28 .84 .70 ,,,, ,,,, .34 ,,, ,,,. .36 .36 .43 .28 .38 3.87 2.28 ,_. _ 2.27 .73 4.40 2.93 2.27 4.69 3.78 Semiquantitative spectrographic analyses (in parts per million) 1,500 5 1,000 2,000 1,500 1, ,,, ,,, , ., 5 3 5 , 5 2 10 _ , 3 1 3 15 50 , 50 ,,, 300 200 500 10 ,_,, 10 ,, ,_,_ 2 , 20 15 20 15 7 5 5 700 300 1,000 200 50 15 30 15 15 10 15 10 150 100 100 100 Description of analyzed samples; locations shown in figure 3. 1. Biotite dacite from plug, contains late quartz. 2. Biotite dacite from lava flow in volcanic rocks (unit Tv of Gilluly and Gates, 1965, pl. 1 J on Mount Lewis. 3. Biotite rhyodacite from lava flow in volcanic rocks (unit Tv ofGilluly and Gates, 1965, pl. 1) on Mount Lewis. 4. Quartz latite from quartz latite intrusion breccia (unit Tg of Gilluly and Gates, 1965, pl. 1) on Mount Lewis. 5. Rhyolite porphyry, devitrified from dike in intrusive breccia of ring fault. a few rounded clasts of granitic rock resembling the granodiorite at Granite Mountain reach cobble size. The fragmental material is set in a groundmass of glass shards, dustlike opaque minerals, and fine-grained iso- tropic to faintly birefringent grains. According to Gilluly and Gates (1965), this groundmass resembles the mat- rix of the fine breccia in the Horse Canyon and Pipe Canyon pipes. One sample of the breccia that was chem- ically analyzed (table 1, column 4) is a quartz latite. The. geologic map of Gilluly and Gates (1965) shows that the intrusive breccia is at least 240 m thick. Although Gil- luly and Gates (1965) present detailed discussions of the intrusive origin of the unit, we have found that the unit contains rocks with abundant flattened pumice lapilli; and these rocks closely resemble welded ash flows. Moreover, the presence of granitic cobbles and the great thickness of the unit lead us to speculate that the unit might have been formed of rocks deposited subaerially; subsequently these rocks may have been jumbled and thickened during mass sliding. DIKES The dikes are the youngest intrusive rocks in the cauldron. Many of them form a north-trending swarm that extends from south of the boundary fault into the subsided block and across the Horse Canyon and Pipe Canyon pipes (fig. 3). According to Gates (1959) and Gilluly and Gates (1965), the dikes range in composition from andesite to rhyolite; many are quartz latite and quartz monzonite. Because many of the dikes are al- tered, Gilluly and Gates (1965) found that it was not always practicable to distinguish the various kinds. They designated many of them quartz porphyry. Most of the dikes shown in figures 3 and 4, regardless of compo- sition, are labeled quartz porphyry. Our observations show that the eastern dike along the Trout Creek fault is a quartz porphyry and that the western dike and at least the western 500 m of the middle dike, beginning at sample locality W1241 (fig. 3), are intrusive breccia locally intruded by rhyolite porphyry. The western dike also extends north into the cauldron. These breccia dikes are 03—35 m wide; those in the Trout Creek fault dip vertically to 65° toward the center of the cauldron. The intrusive breccia of the dikes contains 15—25 per- cent unsorted angular mineral grains and 1—25 percent angular rock fragments set in a dense, siliceous, nearly isotropic groundmass. One sample contains about 25 percent shards and collapsed pumice fragments. Most mineral grains are quartz, plagioclase, and biotite, but sanidine occurs locally; all range in size from a few millimeters to pieces barely discernible through a hand lens. Rock fragments consits of chert, argillite, and por- phyries, and their lengths are as great as several centimetres. The most distinctive feature of the intrusive breccia is a layering parallel to the walls of the dikes. This feature is well developed in the western dike in the Trout Creek fault and it occurs locally in the middle dike. The layers are light to very dark gray, sharply defined, and com- monly 2—30 mm wide. In detail, contacts between layers are irregular; the layers interfinger at all scales from 14 CAULDRON SUBSIDENCE OF OLIGOCENE AGE, MOUNT LEWIS, NORTHERN SHOSHONE RANGE, NEVADA TABLE 2.—Summary of K-Ar ages of igneous rocks from Mount Lewis and vicinity Specimen . . Age1 Average age2 No. Location Mineral (m.y ) (m.y.) Reference Rock Type W1416 , __,. , W _ West-central sec. 14, T. 30 Biotite 32.1:10 32.4:05 This work. Rhyolite welded ashflow tuff from , R. 45 E., 40°28’12” Sanidine 32.8:10 the Caetano Tun" within N., 116°52'19” W. cauldron. W1421 .7 ,. _ . SW cor, sec. 21, T. 29 N., Biotite 33.6110 332:0.6 _______ do 777777 Intrusivebrecciainringfractureat R. 45 E., 40°21’52” N., Sanidine 32,8¢1.0 south margin of cauldron, 116°55’34" W. 141 . , M. . , H , ,,, SW‘ANW‘A sec. 7, T. 29 N., Biotite 34.4:0-7 34.&0.7 McKee and others, Dacite plug, southeast flank of R. 46 E., 116°50’49” W,, 1971, p. 40 Mount Lewis. 40°23’57" N. 139 W “H... _. n , NE‘ANW‘A sec, 12, T. 29 N., ,,,,,, do ,,,,,, 34.7:14 34.7:14 ,,,,,,, do ,,,,,, Quartz latite intrusion breccia of R. 45 E., 40°24’14” N., Gilluly and Gates (1965) from 116°51’42" W. summit of Mount Lewis. 140 ,,,,,,,,, . NE%NW% sec. 12, T. 29 N., ,,,,,,, do ,,,,,,, 332:0.7 332:0.7 _______ do. "W , Do. R. 45 E., 40°24’09” N., 116°51’43” W, M103B , _ NW, , NW‘A sec. 28, T. 29 N., ,,,,,,, do, _ , _ 35110.7 35.1:07 Silberman and McKee, Biotite-granodiorite stock, R. 45 E., 40°21’39" N., 1971, p. 24. precauldron intrusive from south 116°55'25” W. mar in of cauldron, MB8 W, ,,,,,_ North-central sec. 3, Hornblende 381:0.8 38.1108 ,,,,, do 7". . Hornb ende granodiorite, T. 29, N., R. 46 E. precauldron intrusive from east 40°24’55” N., 116°46’ 56” W. margin of cauldron. (incorrectly located in the published reference). Granite Sec. 13 and parts of 11, Biotite 38.0 37 0+0 6 McKee and Silberman, Biotite, hornblende,granodiorite of Mountain 12, 14, and 24, T, 29 N., Homblende 36.7 ‘ 7 ' 1970, p, 2324, no. Granite Mountain, 14 km east of p]uton.. W . M, R. 46 E. Biotite 37.0 Average of four mineral 5.6, table 4. Mount Lewis. Hornblende 36.0 ages on two samples. Tenabo Southern arts of secs. 8 Biotite 37-4 37 6+0 4 McKee and Silberman, Biotite,hornblende, ranodiorite of pluton..,,,,, H, and 9, . 28 N., R. 47 E, Homblende 38.2 ' ’ ' 1970, . 2324, no. Tenabo pluton. 18 in southeast Biotite 337.2 Average of three mineral 2_3, ta le 4, of Mount Lewis ages from two samples. Caetano Regional distribution in Various minerals 33.5 Range 34.1—30.65 J. H, Steward and Rhyolite welded tuff. Tufi‘. N, ,, fl , Shoshone, Toiyabe Ranges, and mineral 30.6 (33.6—30.6) for E. H, McKee Fish Creek Mountains, pairs from 11 31.2 K-Ar ages alone. (unpub. data) and Battle Mountain. separate samples. ' 31}: Average 32.5. 31.2 33,5 32.3 32,7 ‘330 A34.1 32.0 IPlus-minus represents estimated analytical uncertainty at one standard deviation (68 percent confidence level) and is based on statistical analysis ofa large number of replicate potassium and duplicate argon analyses (Silberman, 1971), 2Plus-minus represents standard deviation calculated from ages ofdifferent minerals and from one or more samples. the widest layers down to the thinnest ones, many of which taper gradually though irregularly to a fine point. The dark layers are intrusive breccia. Most of the light-gray layers are rhyolite porphyry composed of quartz, plagioclase, sanidine, and biotite phenocrysts set in a matrix of devitrified glass, but some are bleached breccia. A chemical analysis of the rhyolite porphyry is given in table 1, column 5. Evidently the rhyolite porphyry invaded the rising intrusive breccia and was itself brecciated. Volatiles streaming through the layers promoted devitrification of the rhyolite and altered some of the breccia. Most of the breccia and invading rhyolite moved by flowage parallel to the dike walls to produce the well-defined layers. INTRUSIVE SEQUENCE The order in which the various intrusive bodies were emplaced in the cauldron can be determined only in part from intrusive relations. Following and in part contem- poraneously with development of the pipe breccias, the plugs were intruded into the pipes. None of the plugs are in contact with other pipes. The dacite plug, however, probably formed before the pumiceous Vitrophyre, for the vitrophyre engulfed dacite agglomerate thought to have come from the dacite plug (Gilluly and Gates, “Incorrectly reported as 38.2 my. by Silberman and McKee (1971). aFission track ages. slncludes fission track ages of note 4 above. 31965). The dacite plug is likely of presubsidence origin, as the dacite agglomerate is thought to have been emplaced before cauldron collapse. The intrusive quartz latite breccia of Gilluly and Gates contains fragments of the dacite plug and is therefore younger than the plug despite map relations that suggest the converse as dis- cussed by Gilluly and Gates (1965), but the age of the breccia relative to the other intrusive rocks and to sub- sidence is uncertain. The rhyolite porphyry plug is probably a relatively late intrusive rock at Mount Lewis because it closely resembles the rhyolite dikes. Clearly most of the dikes are younger than the plugs. The dikes show no evidence of having been faulted, therefore, they probably formed after the cauldron col- lapsed. Because the breccia and rhyolite dikes in the Trout Creek fault occur in a structural feature inter- preted to have resulted from subsidence, they must have been emplaced during or after subsidence. A few dikes of andesite cut dikes of quartz latite and rhyodacite por- phyry and seem to be the youngest intrusives in the Mount Lewis cauldron. The sequence of intrusive events cannot be de- ciphered entirely by the isotopic ages. With the excep- tion of the breccia in the ring fault (tables 2, 3, sample W1421) and the dacite plug on the southeast spur of EXTRUSIVE AND SEDIMENTARY ROCKS ' TABLE 3,—Analytical data for new K—Ar ages of igneous rocks from Mount Lewzs [Potassium analyses were done by flame hotometer usin lithium metaborate fusion, on lithium serving as an internal standar . Analyst: Lois Echlocker. Argon analyses were made by standard isotope dilution procedures (Dalrymple and Lanphere. 1969). A Nair-type, 60°, 6-inch mass 5 ectrometer operated in the static mode was used for the mass analysis. Analyst: M. L. Silberman] 40 Ar40d Arrad Sample No. Mineral K20 ’3 40 ($85) mol/g Armtal ' ' (percent) W—1416 ______ Biotite ,,,,,,, 8.40 ‘10 '8‘3‘Jgo ______ 8.37 4.009X 10 80.5 32.1:10 ni ine 11.17 '10 ._,_Do _______ 11420 5.459x 10 90.1 32.8:10 w—1421 ,,,,,, Biotite ______ 8.64 -10 S‘“Eo ______ 8.54 4.303X 10 88.9 33.6210 ani ine 12.47 -10 "D0 777777 12.45 6.093X10 76.1 328:1.0 Constantsusedzxe=0585x 10 ‘loyr _ 1, )‘B = 4.72 X 10 '10yr'1, K40/Ktotal: 1.22 X 10 ‘4 g/g. Mount Lewis (table 2, sample 141), the intrusive igne- ous rocks are hydrothermally altered and do not yield samples suitable for K-Ar age determination. EXTRUSIVE AND SEDIMENTARY ROCKS In the Vicinity of Mount Lewis, remnants of sedimen- tary deposits, tuffs, and lava flows cap the summit, crown high ridges, and make up exposures in low areas near the edge of the subsided mass (fig. 2). Most of these deposits occur within the ring fault. The original areal extent of the deposits is not known, but some of the tuffs, as discussed below, probably blanketed large areas. DEPOSITS ON MOUNT LEWIS AND IN INDIAN CREEK VALLEY The oldest layered rocks of Tertiary age in the Mount Lewis cauldron are conglomerate and impure coal that crop out in Indian Creek Valley east of Mount Lewis. These rocks, mapped as Tertiary sedimentary rocks by Gilluly and Gates (1965, pl. 1), are included in the unit designated Tertiary extrusive igneous rocks and sedimentary rocks in figure 2 but are shown separately in figure 3. An important feature of the conglomerate, which consists of well-rounded pebbles and cobbles chiefly of Paleozoic sedimentary rocks, is that it con- tains clasts of granodiorite derived from the stock at Granite Mountain, 9 km east of Mount Lewis. These granitic clasts indicate that the streams depositing them drained areas outside the cauldron; furthermore, the streams must have existed before eruptions at Mount Lewis because the conglomerate lacks volcanic debris. Other sedimentary rocks in the unit mapped as Ter- tiary sedimentary rocks by Gilluly and Gates (1965, pl. 1) are younger than the conglomerate in Indian Creek Valley. They include reworked breccia in the Horse Canyon pipe and water-deposited tuff in the Rocky Canyon pipe. Sedimentary rocks that crop out west of .15 the mouth of Pipe Canyon are not described by Gilluly and Gates (1965). According to Gilluly and Gates (1965), dacite block agglomerate, tuff, silts‘ ne, mudstone, and conglomer- ate overlie the conglomerate and coal in Indian Creek Valley. These rocks crop out in several patches that differ in stratigraphy from place to place because of lensing and pinching out of layers. Gilluly and Gates (1965, p. 61) state that the block agglomerate is the dominant Tertiary rock in Indian Creek Valley. This observation may be correct, but we have found that the largest remnant of Tertiary rock in the valley consists principally of massive, poorly consolidated, altered tuffs, and conglomerates containing clasts of plagioclase-rich latite and rhyodacite porphyries. We estimate the sequence of extrusive and sedimentary rocks above the basal coal and conglomerate to be 100— 150 m thick. The volcanic rocks lying on the intrusive breccia at the summit of Mount Lewis, according to Gilluly and Gates (1965), consist of tuffaceous lake beds, rhyolite tuff, mud-flow breccia, arkose, and dacite block agglom- erate. We have found, in addition, rhyolite welded tuff, a rhyodacite lava flow (table 1, column 3), and a dacite lava flow (table 1, column 2) on this unit. On the geologic map of Gilluly and Gates, the remnant of this unit exposed on Mount Lewis is shown to be about 50 m thick. The tuffs and sedimentary rocks on Mount Lewis and in Indian Creek Valley may be approximately contem- poraneous. Fresh rocks in both areas are similar in composition, and the altered rocks of these areas are similar in appearance. Moreover, Gilluly and Gates (1965) report dacite block agglomerate in both areas. CAETVANO TUFF Welded rhyolite tuff and andesite flows here inter- preted to be part of the Caetano Tuff crop out at the north edge of the cauldron, in a few small areas south- west of the cauldron, and in patches along the mountain front northwest of Mount Lewis (figs. 2, 3). The tuff in the northern part of the cauldron is chiefly medium gray, locally bleached light gray to white, and struc- tureless to weakly layered. It is composed of about 30 percent subhedral and broken crystals of quartz, sanidine, plagioclase, and biotite in a devitrified matrix of shards and trains of glassy clasts that tend to be wrapped around phenocrysts. A chemical analysis (Gil- luly and Gates, 1965, p. 85, column 13) shows that the rock is rhyolite and that it resembles the Caetano Tuff of the type locality, Caetano Ranch, 42 km southeast (Gil- luly and Masursky, 1965, p. 77). As mapped by Gilluly and Gates (1965), the tuff at the north end of the caul- dron is at least 180 m thick. It appears to overlie, rather than underlie, the andesite, which is about 60 m thick. 16 DEPOSITIONAL SEQUENCE The relative ages of the layered rocks in the cauldron, inferred from a few isotopic dates and from the intrusive order of dikes and plugs, indicate that the tuffs in Indian Creek Valley and those at the summit of Mount Lewis formed before the Caetano Tuff. An isotopic age of 34.4 m.y. for the dacite plug, assumed to be the source of the dacite agglomerate, indicates that these rocks, and pre- sumably the extrusive rocks at Mount Lewis, formed only a short time before deposition of the 32.4«m.y.-old Caetano Tuff exposed in the cauldron (table 2). The relation of the volcanic rocks to cauldron subsidence at Mount Lewis is discussed in the next section. HISTORY OF VOLCANISM Field relations supplemented by K-Ar dates (tables 2, 3) provide at present only a sketchy account of vol- canism in the Mount Lewis cauldron. The isotopic dates cannot be used to establish a detailed sequence of events for several reasons. Many of the rocks, particularly the intrusives but also some of the extrusives, are too al- tered hydrothermally to provide suitable samples for K—Ar dating. Moreover, the uncertainty of the K-Ar dating method as applied to rocks of this age is of the order of 3 percent for a single age determination (McKee and Silberman, 1970). This uncertainty for rocks of Oligocene age is approximately 1 m.y., a time interval that is long compared with the duration of cauldron— forming events such as ash-flow eruption and cauldron subsidence (Smith and Bailey, 1968). Finally, data available are not sufficient for a detailed account of the relation of all volcanic rocks in the region to events that occurred at Mount Lewis. Nevertheless, K-Ar ages pro- vide an overall time framework for the igneous and structural events at Mount Lewis and nearby areas in the Shoshone Range. Volcanism and subsidence of the Mount Lewis caul- dron were preceded by an episode of granitic intrusive activity along the northwest—trending mineral belt that crosses the range in the vicinity of the cauldron. This activity, which occurred from about 38 to 35 my. ago (latest Eocene—early Oligocene), resulted in emplace- ment of five small plutons that range in composition from granodiorite to quartz monzonite, all medium to coarse grained. These magmatic bodies probably did not breach the surface, as extrusive rocks of this age are unknown in the region. The oldest of these plutons is the 38.1 my. (table 2, sample MB8) body that interrupts the ring fault at the east edge of the cauldron (figs. 2, 3), the one around which strata in the cauldron appear to have been draped during collapse. The youngest of these plu- tons, which lies adjacent to the cauldron at the south margin, has been cut by the ring fault. The clasts of granodiorite from Granite Mountain in the basal con- CAULDRON SUBSIDENCE OF OLIGOCENE AGE, MOUNT LEWIS, NORTHERN SHOSHONE RANGE, NEVADA glomerate of Indian Creek Valley indicate an episode of deep erosion before volcanism at Mount Lewis. Volcanism in the Mount Lewis area began about 35 my. ago as recorded by K—Ar dating (table 2). The oldest ages obtained from volcanic rocks are 34.4 m.y. from the dacite plug on the east spur of the mountain and 34.7 m.y. from a sample of the quartz latite intrusion breccia of Gilluly and Gates (1965). The 34.7—m.y. date has a large analytical uncertainty due to high atmospheric contamination and is therefore less reliable than a 33.2-my. date from another sample of the intrusion breccia. The quartz latite intrusion breccia may be younger than the sequence of volcanic and sedimentary rocks correlated with the dacite plug as Gilluly and Gates (1965) imply. The breccia and the volcanic and sedimentary rocks are interpreted as forming before subsidence because they are older than the regionally extensive Caetano Tuff. The tuff is thought to have initiated the volcanic collapse. The youngest dated rock within the subsided block is a 32.4-m.y.-old rhyolite ash flow from the Caetano Tuff near the north edge of the cauldron. Rhyolite intrusive breccia from the Trout Creek fault was dated at 33.2 m.y. This breccia and associated rhyolite in this fault could have fed some of the eruptions that represent parts of the Caetano Tuff. Volcanic collapse at Mount Lewis began after emplacement of the 35.1-m.y. granodiorite that is faulted at the south margin of the cauldron and before the rhyolite breccia and associated dike rock invaded the ring fault about 33.2 m.y. ago. Subsidence could have been contemporaneous with or earlier than emplacement of the breccia and dikes in the ring fault. Presumably the pumiceous vitrophyre of the Rocky Canyon pipe, the rhyolite plug of the Pipe Canyon pipe, and the late rhyolite dikes within the cauldron could have occupied conduits that fed eruptions during or after collapse, but none of these rocks has been dated radiometrically. The andesite dikes could have breached the surface during or following subsidence to feed lava flows associated with rhyolite of the Caetano Tuff at the north edge of the cauldron. A chart, figure 6, represents graphically our proposed sequence of struc— tural and igneous events in the Mount Lewis area. Meager evidence of resurgent doming in the Mount Lewis cauldron includes the 850-m displacement of the extrusive and sedimentary rocks at the summit relative to the sequence in Indian Creek Valley; the tilting away from the summit of small patches of similar rocks on the north and northeast spurs of the mountain; and the dip of late Paleozoic and early Mesozoic rocks close to the summit of Mount Lewis toward topographically low areas near the ring fault. In a general way, volcanic rocks in the center of the cauldron are structurally high, those near the ring fault are low. CAETANO TUFF AND THE MOUNT LEWIS CAULDRON 17 Approximate limits on time of cauldron subsidence Range of K-Ar ages obtained on the Caetano Tuff Range of K-Ar ages obtained on precauldron granodiorites m _. Caetano Tuff — / Quartz Iatite intrusive breccia Mount Lewis — I Y\ 4| I \J\ I _ Qua latite intrusive breccia, Mount Lewis E5 80 — E g _ Granodiorite, south margin of cauldron E g h Granodiorite, 18 km southeast of Mount Lewis g _ E LL u. _ a 70 Granodiorite, 9 km east of Mount Lewis — Granodiorite, east margin of cauIdron 60 _ é .: ,_ B . . E 2 g Intruswe brecCIa, Trout Creek faut E z , . g E E Caetano Tuff, north end oL/caul ron Dacrte FIUQ. MOUM Lewts E E D / 21 U ' | I 1 | I | I D so 31 32 33 34 35 36 a7 38 39 MILLIONS OF YEARS BEFORE PRESENT FIGURE 6.—Sequence and duration of events at Mount Lewis and vicinity and differentiation index as a function of age of igneous rocks. CAETANO TUFF AND THE MOUNT LEWIS CAULDRON The Caetano Tuff was named by Gilluly and Masursky (1965, p. 73—78) for a thick sequence of welded ash flows and associated water-laid tuffs, sandstones, and conglomerates in the northern Toiyabe Range. Masursky (1960) writes that the formation lies mainly in an east-west-trending volcano-tectonic de- pression that Burke and McKee (1973) report to be 8—16 km wide and 115 km long. The depression extends from the northern Toiyabe Range across the Shoshone Range south of the Mount Lewis cauldron. Isolated areas of Caetano Tuff occur east and northwest of the cauldron (fig. 1). J. H. Steward and E. H. McKee (unpub. data) suggest that the tuff erupted from the volcano-tectonic depression and point out that the formation is at least 2,400 m thick in the eastern part of the depression and less than 150 m thick in most outliers near Mount Lewis. E. H. McKee (oral commun., 1973) believes, moreover, that the source lies in the part of the depres- sion that crosses the Shoshone Range 18—35 km south of Mount Lewis because the tuff in that area is thicker than elsewhere, has not developed horizontal to sub- horizontal layering typical of thinner outliers, is perva- sively altered, contains large zones of breccia, and in- cludes many more xenoliths than elsewhere. We suggest, however, that at least part of the Caetano Tuff was erupted from Mount Lewis. K-Ar ages shown by the histogram, figure 7, suggest that the Caetano Tuff is early Oligocene in age. These ages were obtained from stratigraphically uncorrelated samples collected at widely scattered localities, mainly in the volcano-tectonic depression. An age of 32.4 m.y. from a sample obtained at the north edge of the Mount Lewis cauldron lies within the range of ages reported for the Caetano Tuff. The histogram (figure 7) suggests that tuffs identified as part of the Caetano Tuff were emplaced during an interval perhaps as long as 3.5 m.y. Although there is no way of knowing if the Caetano Tuff erupted more or less continuously as the data suggest, the 3.5-my. interval seems long for eruptions from one volcanic center. Smith and Bailey (1968) indicate that a major ash flow may be emplaced during a period of 10 years or less. We 18 CAULDRON SUBSIDENCE OF OLIGOCENE AGE, MOUNT LEWIS, NORTHERN SHOSHONE RANGE, NEVADA Summary of ages of the Caetano Tuff: 34 E] K-Ar age D Fission-track age l 30 NUMBER OF DATES Bah BID? .1 my. 33.6 33.5 33.5 33.0 32.7 32.3 32.0 31.5 31.2 31.2 30.6 Average age = 32.4 m.y. Standard deviation = 1- 1.1 my. (3.5 percent) 35 MILLIONS OF YEARS BEFORE PRESENT FIGURE 7.—Histogram of K-Ar and fission-track ages of the Caetano Tuff (After J. H. Stewart and E. H. McKee, unpub. data, 1972), grouped in 0.2-m.y. intervals. suggest that the Caetano Tuff is a complex unit that may have several different source areas. Eruption of at least some of the Caetano Tuff from the Mount Lewis cauldron is suggested by the overlap of the K-Ar ages with the limits of cauldron subsidence at Mount Lewis (fig. 6); the distribution of the formation east, south, and northwest of the mountain; and the relatively structureless altered deposit of the tuff at the north edge of the cauldron. Further work is needed to determine the location of eruptive vents of the Caetano Tuff, but it is the only tuff \of regional extent and ap— propriate age that could have caused subsidence at the Mount Lewis cauldron. Chemical and age data suggest that eruption of the Caetano Tuff was the final event in a single cycle of igneous activity in the northern Shoshone Range. The cycle began with emplacement of small granodiorite plutons, mainly east and southeast of Mount Lewis, and continued after an interval of erosion and dissection of these early granitic rocks with intrusion of dacite and quartz monzonite plugs and contemporaneous eruption of dacite lavas and quartz latite breccias and tuffs in the Mount Lewis area. This magmatic cycle ended with emplacement of andesite lava flows and rhyolite ash flows of the Caetano Tuff. The chemical progression of the rocks thus formed is summarized in a chart, figure 6, that shows a plot of the differentiation index (Thornton and Tuttle, 1960) of analyzed rocks as a function of their age. The evolution of igneous rocks in the northern Shoshone Range is typical of the general history of igne- ous activity in central Nevada, where Tertiary igneous activity began abruptly about 38 my ago with eruption of andesitic to dacitic lavas from numerous centers scat— tered over the region. These lavas were accompanied by or preceded by emplacement of small granodiorite to quartz monzonite plutons at shallow crustal levels (McKee and Silberman, 1970; McKee and others, 1971). About 33 my ago, local andesitic and dacitic volcanism was followed by eruption of widespread quartz latitic to rhyolitic ash flows that blanketed large parts of the region. Small amounts of andesite occur with these silicic rocks (Burke and McKee, 1973). Ash-flow vol- canism continued for 10—12 m.y. (McKee and Silber- man, 1970; McKee and others, 1971), characterized by differentiation of more primitive intermediate magmas (McKee and Silberman, 1970; Noble, 1972). In the northern Shoshone Range, the andesite flows and rhyo- lite tuffs of the Caetano Tuff and the quartz latite brec- cias and tuffs of Mount Lewis represent the later period of generally silicic volcanism. COMPARISON OF THE MOUNT LEWIS AND OTHER CAULDRONS The Mount Lewis cauldron differs from most other cauldrons of Cenozoic age in the western United States in being deeply eroded below the original volcanic pile; it differs from many ring complexes of the world in exposing only a small percentage of intrusive rocks. Within the cauldron, the base of the layered volcanic rocks is exposed at a high elevation on the summit of Mount Lewis; below the summit, the cauldron is ex- posed by erosion to depths as great as 1,100 In. No vestige of the original Oligocene topography remains. In contrast, cauldrons such as the Valles (Smith and others, 1961; Smith and Bailey, 1968), of Pleistocene age in New Mexico, the Timber Mountain (Christiansen and others, 1965; Carr and Quinlivan, 1968; Byers and others, 1968), of Miocene age in Nevada, and the Creede (Steven and Ratté, 1965; Lipman and others, 1970), of Oligocene age in Colorado, retain original topographic elements of their central resurgent domes, the sur- REFERENCES CITED rounding moats, and the caldera walls. The Phanerozoic and Precambrian host rocks are exposed at the San Juan-Silverton cauldron complex of Oligocene age in Colorado (Burbank and Luedke, 1969; Luedke and Bur- bank, 1968), but detailed maps of the area have not been published. Because of the deep level of exposure at Mount Lewis, it seems reasonable to compare the Mount Lewis cauldron with igneous ring structures, as they, too, are eroded below the original topographic features. In contrast to the 3 percent of igneous rocks that crop out in the exposed part of the Mount Lewis cauldron, igneous ring complexes commonly consist of 30—100 percent intrusive rocks. This is true of ring structures in New Hampshire (Kingsley, 1931; Billings, 1956), Nigeria (Jacobson and others, 1958; Turner, 1963, 1968), Norway (Oftedahl, 1953), and Queensland (Branch, 1966). Ring complexes in these areas contain a central pluton as well as ring intrusions. Most of the ring com— plexes preserve downdropped volcanic rocks and only small amounts of prevolcanic host rocks. Magma reached high structural levels at all these ring complexes. Smith and Bailey (1968) have inter- preted the central plutons in some of the ring complexes of Nigeria, Norway, and New Hampshire as subvolcanic analogs of resurgent domes. Considering this interpre- tation and the high percentage of subvolcanic host rocks enclosed by the ring fault at Mount Lewis, we conclude that any resurgence at Mount Lewis was small, possibly because the subjacent pluton did not rise to high levels as in most ring complexes or possibly because it was much smaller than the plutons causing resurgence in the other complexes discussed. REFERENCES CITED Billings, M. P., 1956, The geology of New Hampshire, pt. II, Bedrock geology: Concord, New Hampshire Planning and Development Comm., 203 p. Branch, C. D., 1966, Volcanic cauldrons, ring complexes, and as- sociated granites of the Georgetown inlier, Queensland: Aus- tralia Bur. Mineral Resources, Geology and Geophysics Bull. 76, 158 p. Burbank, W. S., and Luedke, R. G., 1969, Geology and ore deposits of the Eureka and adjoining districts, San Juan Mountains, Col- orado: U.S. Geol. Survey Prof. Paper 535, 73 p. Burke, D. B., and McKee, E. 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S., 1956, The strati- graphic section in the vicinity of Eureka, Nevada: US. Geol. Survey Prof. Paper 276, 77 p. Oftedahl, Christofi'er, 1953, Studies on the igneous rock complex of the Oslo region, XIII, The cauldrons: Norske Vidensk.-Akad. Oslo Skr., Mat.-Naturv. Kl., no. 3, 108 p. Philbin, P. W., Meuschke, J. L., and McCaslin, W. E., 1963, Aeromagnetic map of the Roberts Mountains area, central Ne- vada: U.S. Geol. Survey open—file map, scale 1:125,000. Reynolds, D. L., 1956, Calderas and ring complexes: Nederl. Geol.- Mijnb. Gen., Verh., Geol. Ser., d1 16, p. 355—379. , Roberts, R. J., 1966, Metallogenic provinces and mineral belts in Nevada: Nevada Bur. Mines Rept. 13, pt. A, p. 47—72. Roberts, R. J ., Montgomery, K. M., and Lehner, R. E., 1967, Geology and mineral resources of Eureka County, Nevada: Nevada Bur. Mines Bull. 64, 152 p. Robinson, E. S., 1970, Relations between geologic structure and aeromagnetic anomalies in central Nevada: Geol. Soc. America Bull., v. 81, no. 7, p. 2045—2060. Shapiro, Leonard, and Brannock, W. W., 1962, Rapid analysis of silicate, carbonate, and phosphate rocks: U.S. Geol. Survey Bull. 1144—A, p. A1—A56. Siedner, Gerard, 1965, Structure and evolution of the Paresis igneous complex, South West Africa: Geol. Soc. South Africa Trans, v. 68, p. 178—202. Silberman, M. L., and McKee, E. H., 1971, K-Ar ages of granitic plutons in north-central Nevada: Isochron/West, no. 71—1, p. 15— 32. 20 Smith, R. L., and Bailey, R. A., 1968, Resurgent cauldrons, in Coats, R. R., Hay, R. L., and Anderson, C. A., eds., Studies in volcanology—A memoir in honor of Howel Williams: Geol. Soc. America Mem 116, p. 613—663. Smith, R. L., Bailey, R. A., and Ross, C. S., 1961, Structural evolution of the Valles caldera, New Mexico, and its bearing on the emplacement of ring dikes: U.S. Geol. Survey Prof. Paper 424-D, p. D145—D149. Steven, T. A., and Ratté, J. C., 1965, Geology and structural control of ore deposition in the Creede district, San Juan Mountains, Col- orado: U.S. Geol. Survey Prof. Paper 487, 90 p. Taubeneck, W. H., 1967, Notes on the Glen Coe cauldron subsidence, Argyllshire, Scotland: Geol. Soc. America Bull., v. 78, no. 11, p. 1295— 1316. Thornton, C. P., and Tuttle, O. F., 1960, Chemistry of igneous rocks, CAULDRON SUBSIDENCE OF OLIGOCENE AGE, MOUNT LEWIS, NORTHERN SHOSHONE RANGE, NEVADA Pt. 1, Differentiation index: Am. Jour. Sci., v. 258, no. 9, p. 664— 684. Turner, D. C., 1963, Ring structures in the Sara-Fier Younger Granite complex, northern Nigeria: Geol. Soc. London Quart. Jour., v. 119, pt. 3, no. 475, p. 345—366. 1968, Volcanic and intrusive structures in the Kila-Warji ring-complex, northern Nigeria: Geol. Soc. London Quart. Jour., v. 124, pt. 1, no. 493, p. 81—89. Wrucke, C. T., and Armbrustmacher, T. J ., 1969, Structural controls of the gold deposit at the open-pit mine, Gold Acres, Lander County, Nevada [abs]: Geol. Soc. America, Abstracts with Pro- grams, V. 1, no. 3, p. 75. Wrucke, C. T., Armbrustmacher, T. J ., and Hessin, T. D., 1968, Dis- tribution of gold, silver, and other metals near Gold Acres and Tenabo, Lander County, Nevada: U.S. Geol. Survey Circ. 589, 19 p. l? U.S. GOVERNMENT PRINTING OFFICE: 1975-0-689-910/85 7 DAY THE BLACK HILLS—RAPID CITY FLOOD ”"“ or JUNE 9—10, 1972: p A DESCRIPTION OF THE STORM AND FLOOD E75, Report prepared jointly by the 0.8. Geological Survey w. 7 and the National Oceanic and Atmospheric Administration ".5. DEPARTMENT OF THE INTERIOR o ".8. DEPARTMENT OF COMMERCE ,v..____ ___,‘.~., mg ,;:..,.W . , .. ,._..- " 4L” ; ;j L: U;:’faif§{.ii§‘i 3' L ‘ ,JUL 21:37:; ‘ :.r..“\“ir3. {I yyvtll-l‘J'yfll'TV (,th CAI EQEVHA : p. . ‘r" .,._. ‘lei‘f .‘r I ‘ THE BLACK HILLS—RAPID CITY FLOOD OF JUNE 9—10, 1972: A DESCRIPTION OF THE STORM AND FLOOD By FRANCIS K. SCHWARZ, LAWRENCE A. HUGHES, and E. MARSHALL HANSEN of the National Weather Service, and M. S. PETERSEN and DONOVAN B. KELLY of the US. Geological Survey GEOLOGICAL SURVEY PROFESSIONAL PAPER 877 Report prepared jointly by the US. Geological Survey and the National Oceanic and Atmospheric Administration UNITED STATES GOVERNMENT PRINTING OFFIGE, WASHINGTON: 1975 UNITED STATES DEPARTMENT OF THE INTERIOR UNITED STATES DEPARTMENT OF COMMERCE ROGERS C. B. MORTON, Secretary FREDERICK B. DENT, Secretary GEOLOGICAL SURVEY NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION V. E. McKelvey, Director Robert M. White, Administrator Library of Congress Cataloging in Publication Data Main entry under title: The Black Hills-Rapid City flood of June 9-10, 1972. (Geological Survey professional paper; 877) Bibliography: p. Includes index. Supt. of Docs. no.: I 19.161877 1. Rapid City, S. D.—Flood, 1972. 2. Floods—Black Hills, S. D. and Wyo. I. Schwarz, Francis K. II. United States. Geological Survey. III. United States. National Weather Service. IV. Series: United States. Geologi- cal Survey. Professional paper; 877. GB1225.SSB52 551.4’8 74-32079 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 FOREWORD The U.S. Geological Survey and the National Weather Service have a long history of cooperation in monitoring and describing the Nation’s water cycle—the movement of water as atmospheric moisture, as precipitation, as runoff, as streamflow, as ground water, and finally, through evaporation, its return to the atmosphere to begin the cycle over again. The cooperative effort has been a natural dovetailing of technical talent and responsibility: the National Weather Service as the Federal agency responsible for moni- toring and predicting atmospheric moisture and precipitation, for forecasting river flow, and for issuing warnings of destructive weather events; and the U.S. Geological Survey as the primary agency for monitoring the quantity and quality of the earthbound water resources. This report represents another step in the growth of our cooperative efforts. In some ways, this closer working arrangement has been spurred by five major flood disasters that have struck the Nation in the last 5 years. In August 1969, the remnants of Hurricane Camille caused flooding of the James River and other streams in central Virginia that left 152 people dead or missing. In February 1972, the failure of a coal-waste dam sent a flood w-ave down the Buffalo Creek valley of West Virginia, leaving 118 people dead or missing. On June 9, 1972, extremely heavy rains over the eastern Black Hills of South Dakota produced record-breaking floods on Rapid Creek and other streams, leaving 237 dead and 8 missing. Beginning on June 18, 1972, the remains of Hurricane Agnes pro- duced floods in the eastern United States from Virginia to New York that killed 117 people in what has been called the worst natural disaster in American history. Most re— cently, the spring 1973 floods on the Mississippi River produced a record 89 days of floodflow at Vicksburg, Miss, and 78 days at St. Louis, Mo.; inundated more than 11 million acres of land; and damaged over 30,000 homes. These disasters have underlined the need to know more about and respect the force and flow of floodwater and have given impetus to further cooperation between the U.S. Geological Survey and the National Weather Service to combine their respective studies and information about flood events into single, unified reports. Hopefully, this documen- tation of the Black Hills—Rapid City flood will aid the understanding of such flood disasters and will help improve human preparedness for coping with future floods of a similar catastrophic magnitude. Joseph S. Cragwall, ,J r. George P. Cressman Chief Hydrologist Director U.S. Geological Survey National Weather Service III Foreword _ _ _ _ Abstract _____ CONTENTS ——————————————————————————————————— III The flood ________________________________________ ___________________________________ 1 Rapid Creek __________________________-______ The Black Hills—Rapid City flood: An extreme storm, Boxelder Creek ______________________________ Battle Creek _________________________________ a flood, and human tragedy _____________________ 1 S co 11d kn 1 d m nt _______________________ 1 Grace Coolidge Creek _________________________ pe a ac ow e g e 5 Spring Creek ________________________________ Conversion factors ________-_____________________; 3 Elk Creek basin ______________________________ Meteorologic setting ______________________________ 3 Bear Butte Creek ____________________________ Precipitation averages and extremes ____________ 3 Cheyenne River ______________________________ Storm-centered maximums ____________________ 4 Flood volumes _______________________________ The role of orography in producing extraordinary Relative magnitude __________________________ rainfalls ___________________________________ 5 The building storm _______________________________ 5 The destruction __________________________________ Upper-air conditions _________________________ 5 The victims _________________________________ Surface weather features ______________________ 10 The damage _________________________________ Moisture considerations _______________________ 13 The Witnesses ________________________________ Radar indications ____________________________ 18 The rain ________________________________________ 19 Selected references ______________________________ Variation of rainfall with time ________________ 20 Index ___________________________________________ ILLUSTRATIONS FIGURE 1. Map showing the relation of the flood area (June 9—10, 1972) to South Dakota and the United States 2. Graph showing probable maximum precipitation amounts for Rapid City and vicinity, June 9—10 _-_ 3—9. Maps showing— 3. Upper-air configuration prior to storm, 500-mb analysis _________________________________ 4. Upper-air configuration prior to storm, 850-mb analysis _________________________________ 5. Upper-air soundings for Rapid City ____________________________________________________ 6. Surface analysis for period 0600—1500 MDT, June 9 ____________________________________ 7. Surface analysis for period 1800 MDT, June 9, to 0300 MDT, June 10 _________________ 8. Hourly surface winds observed at Rapid City Weather Service Office, June 9—10 ___________ 9. Composite 1, OOO-mb dewpoints (° F) ______________________________________________________ 10. Graph showing 1 ,0-00 mb dewpoints (° F) at the Rapid City Weather Service Office ________________ 11. Satellite photographs showing cloud cover on June 9 ____________________________________________ 12. Photograph showing radar image at 2230 MDT, June 9 (0430Z, June 10), during the period of the in- tense rainfall near Rapid City __________________________ 1 __________________________________ 13. Map showing composite radar echoes, 1800 MDT, June 9, to 0100 MDT, June 10 (OOOOZ to 0700Z, June 10) ______________________________________________________________________________________ 14. Map showing National Weather Service station locations and 1,000-foot elevation contours _________ 15. Total storm isohyetal map for June 9—10 ________________________________________________________ 16. Graph indicating mass rainfall curves for selected stations that had rainfall in excess of 4.0 inches 17. Map showing stream-gaging stations and miscellaneous measurement sites in the Rapid City area ____ 18. Graph indicating rise, peak, and decline of Rapid Creek at Canyon Lake Reservoir gaging station, June 9—10, 1972 __________________________________________________________________________ 19. Photograph of Canyon Lake Reservoir after the collapse of the dam ____________________________ V Page 21 21 34 315 35 36 37 37 37 38 39' 39‘ 40 41 45 Page 11 14 18 19 19 20 12 22 23 24 31 32 33 34 VI FIGURES 20—26. TABLE CONTENTS Graphs indicating— 20. 21. 22. 23. 24. 25. 26. Peak discharge at Rapid City, June 9, 1972, and peak discharge at Farmingdale, June 10, 1972 Progression of flood crest down Rapid Creek ___________________________________________ Peak discharge of Elk Creek at Elm Springs, June 11 __________________________________ Peak discharge at the gaging station 12.5 miles northeast of Sturgis, on Bear Butte Creek, June 10 --------------------‘""""r ___________________________________________ Peak discharge of the Cheyenne River at Wasta, June 11 _______________________________ Comparison of the Black Hills floods with maximum floods previously determined in the United States ___________________________________________________________________________ Comparison of the Black Hills flood peaks with maximum floods previously determined in South Dakota 27. Photographs showing homes which have been searched and condemned ___________________________ 28. Photographs showing Chapel Lane ending at the entrance to Canyon Lake and cars stacked by the force of the floodwater _____________________________________________________________________ Photograph showing survivors of the Rapid City flood standing in line for fresh water _______________ 29. 99199.”? TABLES Storms exceeding 10 inches in 6 hours over the northwestern Great Plains _______________________ Wind direction and speed at a point 40 nautical miles northeast of Rapid City_ S. Dak., on June 9—10, 1972 Total rainfall, June 9—10, at miscellaneous gaging stations in the Black Hills __________________ Daily precipitation in Black Hills and vicinity __________________________________________________ Hourly precipitation in Black Hills and vicinity _______________________________________________ Flood stages and discharges in the Rapid City area during the June 1972 flood and during previous maximum floods __________________________________________________________________________ Damage estimates for the Black Hills flood _____________________________________________________ Page 35, 36 36 37 37 40 41 42 43 45 Page 4 13 25‘ 29 30 38 41 THE BLACK HILLS-RAPID CITY FLOOD OF JUNE 9-10, 1972: A DESCRIPTION OF THE STORM AND FLOOD By FRANCIS K. SCHWARZ, LAWRENCE A. HUGHES, and E. MARSHALL HANSEN, of the National Weather Service, and M. S. PETERSEN and DONOVAN B. KELLY, of the US. Geological Survey ABSTRACT On June 9, 1972, an almost stationary group of thunder- storms formed over the eastern Black Hills of South Dakota near Rapid City and produced record amounts of rainfall and flood discharges. Nearly 15 inches of rain fell in about 6 hours near Nemo, S. Dak., and more than 10 inches of rain fell over a 60—square-mile area. The resulting floods were the highest ever recorded in South Dakota. At least 18 of the 27 streams where peak flows were computed experienced flows that exceeded the 50-year flood. At least 237 people died in the Black Hills flood and 8 people were still listed as missing 6 months after the flood. Another 3,057 people were injured, and total damage is esti- mated to have exceeded $160 million. As documented by radar and satellite images and by radio- sonde observations, as well as by much other weather data, the unusual and excessive rainfall was partly the result of a strong, low-level, easterly airflow that forced moist air upslope over the Black Hills. This sustained orographic effect helped the air to rise, cool, become very unstable, and release its moisture in repeating thunderstorms. Another important contributing factor was the unusually light winds at higher levels. Those light winds did not disperse the moist air or move the thunderstorms along to prevent the extreme con- centration of rainfall. Instead, these conditions allowed heavy rainfall to remain almost stationary along the eastern slopes of the Black Hills. Rainfall data from 24 observing stations and from over 200 miscellaneous sites show that all reported rains greater than 4 inches fell on the eastern slopes of the Black Hills. At least 6 inches of rain fell over a 300-square-mile area that lies primarily between 4,000 and 5,000 feet elevation. Almost all the flood peaks occurred between 2230 MDT on June 9 and 0100 MDT on June 10, 1972, in a flood belt about 40 miles long and 20 miles wide along the eastern slopes of the Black Hills. This belt extended from Sturgis, S. Dak., on the north to Hermosa, S. Dak., on the south, with Rapid City near the center. To document the flood, peak discharge de- terminations were made at 49 sites. Records show that about 13,000 acre-feet of water flowed through Rapid City during the 2 days of flooding. At one point during the night of June 9, the floodwaters rose about 3.5 feet in 15 minutes. Coming ofl" the slopes of the Black Hills, the flood peak traveled the 22 miles between Deer Creek and Rapid City in about 3.5 hours. THE BLACK HILLS—RAPID CITY FLOOD: AN EXTREME STORM, A FLOOD, AND HUMAN TRAGEDY June 9 was the first day of one of the early week- ends of the 1972 summer season in Rapid City. Along with the area’s 50,000 residents, the first of the year’s expected 6 million visitors to the Black Hills were in Rapid City (fig. 1) looking forward to the start of the “Dakota Days” celebration. The day ended in tragedy: 237 dead, 8 missing, 3,057 injured, 1,335 homes and 5,000 automobiles destroyed, and total damage in excess of $160 million in the Black Hills area. This report is primarily designed to describe and reconstruct the unusual physical events surrounding this disaster: the unusual weather conditions that produced nearly 15 inches of rain in 6 hours at one location and more than 10 inches of rain over some 60 square miles and the resulting extreme flood. Although the physical facts of the storm and result- ing flood are the primary subject of this report, any description of the Black Hills—Rapid City flood of 1972 would be incomplete without a chapter on the human tragedy that resulted. SCOPE AND ACKNOWLEDGMENTS The large amount of information needed to docu- ment the Black Hills flood required the cooperation and effort of many individuals and government agencies. Their aid and the data they supplied are gratefully acknowledged. At the heart of this report is a core of basic data: precipitation measurements from some 240 loca- tions; flood-discharge measurements at 49 sites; and flood profiles surveyed along 19 miles of Rapid Creek. Added to this are bits of vital information from many sources: radar images of the storm from Na- 1 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 103° \ RAPID CITY / 44" \ I 0 10 / 20 30 MILES 3O 4O KILOMETERS \‘~. I T“‘\-‘_ / (‘_ \-~~\___ I KN ------ \_"- \. . .> I ”OHM“ NORTH DAKDYA ‘ .' (\ . |‘ = L \MINNESOWA'. ——‘_~\ \_ __ ————— - / . ‘“’~F ‘ ~ ______ < a? i l = : 50““ anom : \-. WISCONSH‘ 1 \. :l .1 L ........... k, --------- «m. i kw,“ ‘l \,. ‘- NEBRASKA K r) ‘. “““ 2 M“ P \. A; L _____________ Sin—i /_ ,Lmes \‘ND‘AN \ : ‘ W \s i §"\._ I,‘ (wESTM/v ‘ L , "‘~ l lN \: \r.’ /" Jr r: “G i, VlRffin“,K ““5” ‘ MlSSOuil ;\ =_1~_J~J - .- .\ ngucw OKLAHOMA TEXAS 0 300 600 MILES o 400 800 KILOMETERS \ FIGURE 1.—Location of flood area. Rapid City and the flood area lie along the eastern slopes of the Black Hills in west- central South Dakota. METEOROLOGIC SETTING 3 tional Weather Service and US. Air Force stations; satellite images of the continental and regional weather patterns; readings from radiosonde obser- vations; the weather and flood observations of eye- witnesses; a detailed engineering survey with special computations; and a comparison with previous floods and storms. The effort and skill of our coworkers, including the Geological Survey field hydrologists stationed at Rapid City and those detailed to Rapid City from Colorado, Illinois, Kansas, Missouri, Ne- braska, North Dakota, and South Dakota, and the National Weather Service meteorologists and ob- servers on duty in Rapid City and at headquarters in Kansas City and Washington, DC, made possible the collection and interpretation of the extensive meteorological and hydrological data related to this unusual storm and the resulting flood. We also acknowledge the direct assistance of the US. Bureau of Reclamation and the National Forest Service who furnished much data as well as men to assist with the fieldwork. The US. Army Corps of Engineers and the National Guard also greatly aided our efforts in the field. Finally, we thank the resi- dents of the Black Hills for their assistance in docu— menting this natural disaster. CONVERSION FACTORS English units have been used in much of this re- port. The following factors may be used to convert the English units published herein to the Interna tional System of Units (SI). Multiply English units By To obtain SI units inches (in.) 25.4 millimeters (mm) .0254 meters (m) feet (ft) .3048 meters (m) miles (mi) 1.609 kilometers (km) knots (kts) 1.853 kilometers per hour (km/hr) .5148 meters per second (m/s) square miles (mi2) 2.590 square kilometers (kmz) cubic feet (ft3) 28.32 cubic decimeters (dnf) .02832 cubic meters (m3) cfs—day [(fts/s)-day] 2447 . cubic meters (m3) 2.447X10“ cubic hectometers (hma) acre-feet (acre-ft) 1233 CUbiC meters (m3) 1.233><10‘3 cubic hectometers (hma) 1.233 X10 '6 cubic kilometers . (kma) cubic feet per second 28.32 liters per second (fta/s) (US) 28.32 cubic decimeters per second (dms/s) .02832 cubic meters per second (ma/s) METEOROLOGIC SETTING PRECIPITATION AVERAGES AND EXTREMES Precipitation in the Black Hills averages 24 inches a year, considerably greater than the 16 inches in the surrounding plains (US. Dept. of Commerce, 1968). The higher precipitation is accounted for by the greater frequency, longer duration, and some- times greater intensity of storms along the terrain slopes, as compared to the frequency, duration, and intensity of storms over the relatively smooth terrain of the adjoining plains. The slopes provide the slight triggering or lift of nearly saturated air that is sufl‘i— cient to begin showers. Directly west of Rapid City the mean terrain slope is approximately 100 feet per mile, while farther to the south, west of Hermosa, the mean slope increases to 200 feet per mile. Monthly precipitation maps show that the 24 inches of precipitation is fairly well distributed throughout the year (US. Dept. of Commerce, 1968). Maximum 1-day rainfalls of record tend to occur more frequently and also tend to be higher in the Black Hills than in the neighboring plains. From about 1913-63, the average of maximum 1-day rain- falls of record for 12 stations in the Black Hills (elevations over 4,000 ft) was 4.4 inches, compared with 3.6 inches for 9 South Dakota stations (eleva- tions of 2,000 to 4,000 ft) located in the area sur- rounding the Black Hills (Jennings, 1952, unpub. summary tabulations National Weather Service). The maximum 1-day rainfall from both groups of stations is the same, 7.1 inches. Hydrologic conditions in the Black Hills area of South Dakota were not abnormal immediately prior to the events of June 9-10, 1972. Scattered showers occurred throughout the area on several days during the early part of June, but generally the quantity of rain that fell was small (0.2 in. or less). Pactola Dam, however, had higher rainfall amounts reported on June 2, 3, and 5, with 0.81 inches, 0.61 inches, and 2.49 inches, respectively, recorded for those days. These amounts of rainfall saturated the soil, and although there were only small scattered showers from June 5 to the date of the flood, the soil was probably still wet from the previous rains. These conditions would increase the amount of run- off and quicken the time of concentration for the flood of June 9. Some judgment on the rarity of the reported value of 15 inches in about 6 hours on June 9—10, 1972, can be obtained by a comparison of the greatest 6- hour amounts that occurred during a nearly uniform period of record at several stations. Published sum- maries on the greatest 6-hour rainfall depths are available for 38 recording rain-gage stations in South Dakota for an average of 12 years of record. The greatest 6-hour amount of the three recorders in the Black Hills is 4.36 inches recorded at Spearfish (the average of the maximum amounts for the three 4 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 stations is 2.85 inches). The greatest amount at the 35 stations outside the Black Hills is 4.52 inches recorded at Howard (average of the maximum amounts for the 35 stations is 3.01 inches). Because the Black Hills area is represented by fewer stations, there is a much higher probability of recording an extreme amount outside the Black Hills than in them. Even so, it can still be inferred from the data that the Black Hills are not particularly more likely to have heavy short-duration rains than the sur- rounding area. Detailed maps of rainfall frequency for the Black Hills of South Dakota have been prepared in an unpublished study by the National Weather Service. In addition to using rainfall-frequency values com- puted for stations with precipitation observations, the relations between rainfall-frequency values and meteorologic, climatologic, and topographic factors were used to interpolate between stations. The 100- year 6-hour precipitation-frequency map shows an elongate 3.6-inch rainfall-depth center between the 4,000— and 5,000-foot contours on the eastern slopes of the Black Hills; this depth decreases to 3.4 inches over the plains to the east of the 3,000-foot contour. North and south of the Black Hills values decrease to less than 3 inches. STORM-CENTERED MAXIMUMS Although useful in comparing long-term precipita- tion rates, the 6—hour precipitation figures recorded at rain-gaging stations only rarely record the maxi- mum amount of precipitation produced during a storm. With few exceptions, recorder stations are not located at the center of peak rainfall. The maxi- mum reported rainfall in the Black Hills flood, the storm-center maximum of 15 inches of rain in just over 6 hours measured near Nemo, S. Dak., in a “bucket survey” conducted following the storm, should only be compared with other similar storm- centered values measured on the northwestern Great Plains. Some 132 such storms have been analyzed in the northwestern Great Plains States of North Dakota, South Dakota, Kansas, Montana, Wyoming, and Colorado (US. Army Corps of Engineers, 1945). Of these, 10 storms had point rainfall values of 10 inches or more in 6 hours and two exceeded the 15-inch, 6-hour precipitation of the Black Hills storm (table 1). Outstanding for northwestern Great Plains is the Cherry Creek, 0010., storm of May 30—31, 1935, with two station 6—hour values of 24 inches of precipita- tion. These amounts were measured approximately 150 miles apart, one at an elevation of 3,500 feet, the other at about 6,000 feet. One other storm, centered TABLE 1.—Storms exceeding 10 inches in 6 hours over 10- sqnare-rm'le areas in the northwestern Great Plains Maximum Average . Location Date _dept.h 5323:? (““1”) (inches) Near Burlington, Colo _______ May 30—31, 1935 20.6 24.0 Near Stanton, Nebr ___________ June 11, 1944 13.4 15.5 Near Nemo, S. Dak __________ June 9—10, 1972 ..-- 15.0 Near Neosho Falls, Kans _____ Sept 12, 1926 13.4 13.6 Grant Township, Nebr ________ June 3‘4, 1940 13.0 (1) Greeley, Nebr _________________ June 5, 1896 12.0 (1) Near Colorado Springs, Colo __ June 17, 1965 11.5 12.8 Near Greeley, Nebr ___________ Aug. 12, 1966 11.4 11.8 Springbrook, Mont __________ June 19, 1921 10.5 (1) Near Pueblo, Colo ____________ June 3, 1921 10.4 (1) Sharon Springs, Kans _________ May 30, 1938 10.0 (1) 1Insufficient data to distinguish between maximum station value and average depth over 10 square mils. near Stanton, Nebr., on June 11, 1944, also exceeded the Black Hills storm with a reported 15.5 inches of precipitation. Probable maximum precipitation (PMP) is defined as the maximum precipitation amount that would result from an optimum combination of meteorological factors. For a given location and time of the year, it represents the estimated greatest depth of precipitation meteorologically possible for a given duration. Its derivation involves the ju- dicious selection of a set of synoptic conditions rele- vant to the area and season under consideration which satisfies the maximization requirement. The probable maximum precipitation for Rapid City and vicinity for June 9—10, shown in figure 2, was in- terpolated between the May and June maps shown in Hydrometeorological Report No. 33 (Riedel and others, 1956). For purpose of comparison, the actual rainfall amount of 12.8 inches recorded by Rapid City gage 5NW for a duration of 12 hours (fig. 16) constitutes about 65 percent of the probable maxi- N # .1 N O H O! 12 PROBABLE MAXIMUM PRECIPITATION, IN INCHES DURATION. IN HOURS FIGURE 2.—Probable maximum precipitation amounts for Rapid City and vicinity, June 9-10. THE BUILDING STORM 5 mum precipitation for a 10-square-mile area. The highest storm rainfall amount of 15 inches in just over 6 hours was reported near Nemo from a “bucket survey.” Using a duration of 6 hours and an area coverage of 10 square miles, this highest observed rainfall near Nemo would constitute about 90 per- cent of the probable maximum precipitation. These comparisons provide some insight With regard to the severity of the storm. Heavy rains have occurred previously in the Black Hills but were not measured. Notable is the storm of June 1907. Although rain measurements were not available for the Black Hills, 7.1 inches was caught in 24 hours on June 12-13 at Fort Meade (elev 3,600 ft) near the northern foothills. Hermosa, south of Rapid City, measured 1.80 inches in 2 days. A record 24-hour rainfall of 7 .1 inches at Custer (elev 5,300 ft) on April 17, 1920, was apparently caused by a local intense thunderstorm. Of the 14 stations then reporting in the Black Hills, the second highest daily rain was 2.10 inches. On July 13, 1962, heavy local rains occurred in the Black Hills. Ten rainfall sta- tions in and near the Black Hills averaged about 1 inch in a day (13th or 14th) ranging from a low of 0.15 to a high of 1.64 inches. THE ROLE OF OROGRAPHY IN PRODUCING EXTRAORDINARY RAINFALLS The exact role of orography is difficult to deter- mine in situations where extraordinary rains from persisting, or repeating, thunderstorms occur. The maximum areal storm rainfall amount (depth-area- duration data) has been determined for nearly 700 storms in the eastern two-thirds of the United States, including more than 50 storms that produced rainfalls of 10 inches or more in 6 hours or less. The vast majority of these extraordinary rains occurred over terrain which could not give significant vertical uplift. In those cases where orography does appear to play a role, the terrain usually acts only as a mechanism for “fixing” or holding in place the per- sisting or reoccurring massive and efficient rain— producing thunderstorms. The Smethport, Pa., storm of July 17-18, 1942, which produced more than 30 inches of rainfall in an estimated 41/; hours, occurred Where ground slopes were not a significant factor in producing uplift currents. Likewise, the Camille-generated massive repeating thunderstorms of August 19—20, 1969, in Virginia, produced a 20-inch rainfall center in a basically nonorographic region, (Schwarz, 1970) although the first impressions were that the terrain played a substantial role. The Camille storm again demonstrated that the thunderstorm mecha— nism itself, fueled by the latent heat of condensa- tion, was of overriding importance. Closer to the setting of the Black Hills downpour, the Cherry Creek, Colo., storm of May 30—31, 1935, produced two relatively com-parable rainfall centers of 24 inches in about 6 hours. One of the centers was lo- cated where terrain-produced vertical air currents of consequence were not possible. These cases of extraordinary rains strikingly em- phasize the fact that the thunderstorm mechanism itself, unaided by orographic lifting, is capable of producing the largest observed rains of record for durations of 6 hours or less. In the Black Hills storm of June 9-10, 1972, terrain played a somewhat greater role than in the extraordinary storms just discussed, especially in increasing the rainfall vol- ume and concentrating the greatest rainfall amounts in an elongate pattern parallel to the mean terrain contours. A more definite answer to the importance of terrain effects must await more detailed study of this unusual storm in particular, and, more gener- ally, of the flood-producing, massive efl‘icient thun- derstorm phenomenon itself. THE BUILDING STORM Extraordinary events do not have ordinary ex- planations. The rainfall that began in the Black Hills on the afternoon of June 9, 1972, fits such a cate- gory. Although it is difficult to explain the exceed- ingly complex systems that cause extraordinary rains, the following section provides a discussion of atmospheric phenomena considered important in the production of this unusual rainfall. Ongoing and future studies Will have to thoroughly investigate the mesoscale features before such extraordinary rain- fall events can ever be explained fully. In brief, the unusual and excessive rainfall was partly the result of a strong low-level easterly air— flow that forced moist air upslope over the hills. This sustained orographic effect helped the air to rise, cool, become very unstable, and release its moisture in repeating thunderstorms. Another important con- tributing factor was the unusually light Winds at higher atmospheric levels. These light Winds did not disperse the moist air or move the thunderstorms along to prevent the extreme concentration of rain- fall. Instead, these conditions allowed heavy rainfall to remain almost stationary along the eastern slopes of the Black Hills. UPPER-AIR CONDITIONS The upper-air configuration prior to the storm shows a prominent, fairly stationary, long-wave ridge over the Great Plains with the 500-mb (milli- bar) ridge line just to the east of Rapid City (fig. 3). There was also a very weak smaller scale trough oriented northwest-southeast through southwest 6 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 APPROXIMATE SCALE 0 100 200 300 400 500 MILES l I l l | I I | I I I l | 0 100 200 300 400 500 600 KILOMETERS E X P LA N AT] 0 N — 13 552 1 Pressure-surface height contour Observation station Shows height of 500-mb pressure surface in tens of meters. Upper number is air temperature, in degrees Celsius. Lower number Contour interval 60 meters. Datum is mean sea level is depression of dewpoint temperature below air temperature, in de- grees Celsius. Shaft indicates wind direction. Barbs on shaft indicate wind speed, in knots. Lona barb :10 knots; short barb :5 knots. M indicates missing data/or that element — ——— — -25" —- — — —-—- Line of equal air temperature H Interval 5 degrees Celsius 8 Geographic center of maximum height FIGURE 3.——Upper-air configuration prior to storm. Left chart: 500-mb analysis, 0600 MDT THE BUILDING STORM 50° 40' 35° APPROXIMATE SCALE 100 200 300 400 500 M | l ES l J I I l | | | | | | 100 200 300 400 500 600 KILOMETERS O—r—O E X P LA N AT I O N ‘7‘— 1 5x 582 22 . Pressure-surface height contour Observatlon 31381110“ Shows height of 500-mb pressure surface in tens of meters. Upper number is air temperature, in degrees Celsius. Lower number Contour interval 60meters. is depression of dewpoint temperature below air temperature, in de- grees Celsius. Shaft indicates wind direction. Barbs on shaft indwate wind speed, in knots. Long barb :10 knots; short barb :5 knots Datum is mean sea level —————— _20° ——————— Line of equal air temperature H Interval 5 degrees Celsius 8 Geographic center of maximum height (1200Z), June 9, 1972. Right chart: 500-mb analysis, 1800 MDT, June 9, 1972 (OOOOZ, June 10). THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 APPROXIMATE SCALE 100 200 I J I I i O‘r—O 400 500 MILES l J I 100 200 300 400 500 600 KILOMETERS EXPLANATION 19K. 14 Observation station Upper number is air temperature, in degrees Celsius. Lower number is depression of dewpoint temperature below air temperature, in de- grees Celsius. Shaft indicates wind direction. Barbs on shaft indicate wind speed, in knots. Long barb :10 knots; short barb :5 knots. LV beside the station indicates light and variable winds. Speed less than 8 knots.M indicates data are missing either because of an instru- ment malfunction or because the constant pressure surface is below the terrain elevation for the station L2 Geographic center of minimum height I41 Pressure-surface height contour Shows height of 850-mb pressure surface in tens of meters. Contour interval .90 meters. Datum is mean sea level ————————— 20° ————-———— Line of equal air temperature Interval 5 degrees Celsius FIGURE 4.——Upper-air configuration prior to storm. Left chart: 850-mb analysis, 0600 MDT THE BUILDING STORM 9 50' ' 45‘ . \2 40° . APPROXIMATE SCALE 0 100 200 300 400 500 MILES |_ | J I l l I l I I | | l 0 100 200 300 400 500 600 KILOMETERS EXPLANATION A. la 14 Observation station Ge - - - . , o a he center of minimum hel ht Upper number is air temperature, in degrees Celsius. Lower number gr p g is depression of dewpoint temperature below air temperature, in de- grees Celsius. Shaft indicates wind direction. Barbs on shqfl indicate 147 wind speed, in knots. Lona barb :10 knots; short barb :5 knots. M indicates data are missing either because of an instrument malfunc- p _ ‘ ur tion or because the constant pressure surface is below the terrain ele - ressure surface height conm vation for the station Shows height of 850-mb pressure surface in tens of meters. Contour interval .90 meters. Datum is mean sea level I @I ————————— 20° — ———————— Line of equal air temperature Geographic center of maximum height Interval 5 degrees Celsius (12002), June 9, 1972. Right chart: 850-mb analysis, 1800 MDT, June 9, 1972 (OOOOZ, June 10). 10 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 Wyoming. By 1800 MDT (mountain daylight time), on June 9 (OOOOZ (Greenwich mean time), June 10), the trough had moved into northeast Wyoming. An important characteristic of the upper-air conditions was the prevalence of light winds aloft through the Dakotas and westward that indicated the lack of a steering current. This allowed the massive thunder- storms to stay in approximately the same area for hours. The 850-mb charts for June 9 at 0600 MDT (1200Z) and for June 9 at 1800 MDT (OOOOZ, June 10) show both a large Canadian high-pressure sys- tem centered well north of the North Dakota border and a weak low-pressure system centered near the Colorado-Wyoming border (fig. 4). These two broad- scale features moved southward during the next 12 hours and provided a flow of air from a general southeasterly direction over South Dakota. The 850—mb map for 0600 MDT does not reflect low-level moisture in Nebraska, just south of South Dakota. Temperature dewpoint depressions of 22°C and 18°C at 0600 MDT (1200Z) give 850—mb dew- points of 2°C and 4°C, respectively, at North Platte, Nebr., and Omaha, Nebr. Just to the north, however, at Huron, S. Dak., the moisture at 850-mb was quite high, with a reported 19°C temperature and 4°C temperature dewpoint depression, giving a dewpoint of 15°C. In contrast, over Rapid City at 0600 MDT (1200Z), measurements showed the dewpoint to be ~15°C (fig. 5). A cyclonic curvature of the flow resulted from a combination of a southwesterly flow over Nebraska and a southeasterly flow over South Dakota. This cyclonic curvature at the 850-mb level probably resulted in a low-level convergence that helped to transport vertically the high surface mois- ture (discussed later). By 1800 MDT on June 9 (OOOOZ, June 10), a significant increase in moisture at 850-mb is noted over Rapid City where the dew- point has risen 32 degrees to an unusually high value of +17°C. The main feature of the upper-level charts appears to have been the prevalence of only light winds. The broad-scale features of the upper-air charts con- tained no significantly strong indicators that would suggest an exceptionally heavy rainfall. SURFACE WEATHER FEATURES Prior to the South Dakota rain, a large high- pressure . system in Canada was pushing slowly southward. Early on June 9th, the leading edge of this colder air mass stretched west-southwestward across southern Lake Michigan, and westward into South Dakota, from a weak Low near northeastern New York State. The main feature of the low-level flow over South Dakota was its easterly direction throughout most of the 9th. Thus, it was given an upslope motion by both the large-scale slope of the Great Plains and the more pronounced local terrain of the Black Hills. “To the south of the leading edge of the cooler air mass, the prevailing weather systems were quite weak. Warm, moderately moist air was characteris- tically present on a synoptic scale (a concentrated tongue of high moisture is discussed later). On the National Meteorological Center surface charts for June 9, the surface front was positioned close to, but generally a little north of, the Rapid City area. Fig— ures 6A—D show the surface maps for the period of 0600—1500 MDT (1200Z—2100Z) , June 9, prior to the rains. A weak low-pressure system existed early on the 9th in eastern Colorado and western Nebraska. This low-pressure center remained weak and rather diffuse, and it appeared to move a little southward during the day. The weakness of the systems to the south of the leading edge of the cooler air produced weak pressure gradients and light winds on a synop- tic scale. Although a surface front was nearby, and upslope flow from the east prevailed, it must be kept in mind that both of these surface synoptic-scale features occur many times without producing heavy, much less exceptional, rainfall. Thus, neither the broad- scale upper-air features, nor the broad-scale surface features indicated the storm would bring unusually heavy rainfall. Figures 7A—D show the 3-hour surface maps dur- ing the period of heavy rainfall over the Black Hills. The winds, cloud cover, temperature, dewpoint, and significant weather characters have been included to clarify the variation that occurred with both time and space. Of interest is the zone of thunderstorms in the vicinity of the front and the area of shower activity that extended to the south of Rapid City along the eastern Colorado border. A strong influx of a rather narrow band of high moisture, related to mes-oscale effects that increased the wind, appears to have been an important low- level feature contributing to this unusual deluge. To investigate this feature, a study was made of the development of surface pressure gradients directed upslope in the Black Hills using some of the surface geostrophic winds at grid points over the contiguous United States routinely computed by the National Weather Service in Kansas City with techniques developed by Sangster (1960). The Winds at the nearest grid point to Rapid City, 40 nautical miles northeast, are shown in table 2. Since the airflow pattern was almost stationary THE BUILDING STORM 11 400 500 as 8 PRESSURE, IN MILLIBARS \1 E3 800 — Dewpoint temperature EXPLANATION ————————€329o-——————— Line of constant potential temperature, in degrees Kelvin —~—eum——— ~ Line of constan? wet-bulb potential temperature, in degrees Kelvin Air or dewpoint temperature 0600 MDT, June 9, 1972 Air or dewpoint temperature 1800 MDT, June 9,1972 Dewpoint \ emperature \\\ \* 900 — 1000 — Rapid City 1030 l l l l l —40° —30° — 20° — 10° 0° 10° 20° 30° 40° TEMPERATURE, IN DEGREES CELSIUS FIGURE 5.—Upper-air soundings for Rapid City. and had only slight curvature, gradient winds would have differed little from these values. The pro- nounced afternoon wind maximum caused strong terrain-induced lifting because of the nearly north- south orientation of both the general terrain con- tours and the sharp rises of the Black Hills. Ob- served hourly surface winds at the Rapid City Weather Service Office for the storm period (fig. 8) may be compared with the computed surface geo- strophic winds (table 2). Hourly weather reports at Rapid City highlight the repetitive nature of the thunderstorm activity. Thunderstorms began at the airport station a little before 2000 MDT. Thunderstorms of various inten- sities were reported continuously for all observa- tions until just before the 0200 MDT June 10 obser- 12 THE BLACK HILLS-RAPID CITY FLOOD, JUNE 9—10, 1972 45° I 40' *“ l A —0600 MDT (I2002) 110° 105’ 100° 95° 90° I I 1\ I I ,,— —— ... I au— 40 j \ [Li—’lrfifl/” \ I i I . \ C —1200 MDT (18002) D—1500 MDT (2IOOZ) APPROXIMATE SCALE 0 100 200 300 400 MILES 0 100 200 300 400 500 KILOMETERS EXPLANATION 1008 I2 Line of equal atmospheric pressure at sea level Interval 4 millibars Geographic center of low-pressure system —_.—_.___ —v—v— __-—_—v— Warm front Cold front Stationary front FIGURE 6.—Surface analysis. A, 0600 MDT (1200Z), June 9, 1972. B, 0900 MDT (1500Z), June 9, 1972. C, 1200 MDT (1800Z), June 9, 1972. D, 1500 MDT (2100Z), June 9, 1972. THE BUILDING STORM TABLE 2.—Surface geostrophic wind direction and speed for a point 40 nautical miles northeast of Rapid City S. Dak. on June 9—10, 1972 Time (MDT) Direction/speed M 0600 110°/15 0900 100°/22 1200 120°/29 1500 130°/38 1800 130°/45 2100 130°/31 0000 130°/18 vation. Such a continuation of thundershower activ— ity is typical of extreme rains that produce flooding over drainage areas as large as several hundred square miles. It appears that the large-scale weather effects were weak but oriented to augment any mesoscale factors favorable for upward motions or for the triggering of thunderstorms. Because most of the precipitation fell in a north-to—south band along the first major upslopes west of Rapid City, the local terrain appears to have been effective in at least triggering thunderstorms in an environment of pre- vailing low-level easterly winds that brought in large amounts of moisture. MOISTURE CONSIDERATIONS An analysis of the surface dewpoints of the air mass feeding into the storm area from the east showed they were quite close to the maximum 12- hour persisting dewpoint values for the region and season (US. Department of Commerce, 1968, p. 59). Where differences in elevations prevail, an accurate portrayal of surface moisture is not possible unless the dewpoints are reduced to a common level. Re- ducing surface dewpoints to 1,000—mb has proved a useful procedure for storm-moisture analysis. An accepted technique of adjusting surface dewpoints to the 1,000-mb level is to assume the variation with elevation was equivalent to the adiabatic tempera- ture lapse rate for saturated air. Such an adjustment was applied to dewpoints on the 3-hour surface maps from 0900 MDT (1500Z), through 1800 MDT (OOOOZ, June 10), June 9 to produce a map of com- posite 1,000-mb dewpoints for the period (fig. 9). A narrow band of 1,000-mb dewpoints, close to max- imum values for the sea-s'on and area, provided a con- centrated supply of low-level moisture that was car- ried into the heavy rain area by the low-level easterly winds. Figure 10 shows the hourly trend in 1,000- mb dewpoints on June 9, 1972, for Rapid City and demonstrates how the dewpoints slowly approached a maximum persisting value of 725° F before the heavy rain. Upper-air soundings for Rapid City (fig. 5) show a dramatic change in moisture and stability through 13 depth between the morning and the evening of the 9th. Previous studies of extreme thunderstorm rain have shown that when there is high-moisture influx at low levels, the presence of a mechanism (or mechanisms) to produce or enhance low-level con- vergence of this air will result in thunderstorms that quickly transport the low-level moisture through deep layers. ' Additional insight into the Rapid City storm is derived from a mesoscale study using satellite pic- tures and Sangster’s surface charts. Because the Winds were southerly from about the 850-mb level up to the 500-mb level and because the air was quite dry in this layer southward of Rapid City (see fig. 3), an increase in mean moisture over the Black Hills would be difficult to explain on the basis of mid-level advection. However, a feature that could have contributed materially to the low-level wind maximum (table 2) and somewhat to the increased mid-level moisture was associated with a mesoscale cloud mass first noticed near the northeastern tip of Colorado in a satellite photograph for 0836 MDT (1436Z) on the 9th. This cloud mass had moved to near Rapid City by 1454 MDT (2054Z) (figs. 11A— D). The hourly weather reports show a cloud base in this area to be at about 7,000 feet, with some asso- ciated rain starting in the mid-afternoon. The sur- face geostrophic vorticity charts (not shown) show the relation between the vorticity field and this cloud mass. The charts indicate a vorticity maximum in eastern Colorado at 0000 MDT June 9 (0600Z, June 9) that splits in two. The northern maximum moved with the cloud mass seen in the satellite photographs to create another vorticity maximum near Rapid City at 1800 MDT on the 9th (0000Z, June 10), the time of strongest upslope wind and shortly after the start of the heavy rains. The movement of the vorticity system caused the pressure at Rapid City to hold steady during the day, with the pressure to the east rising as the high-pres- sure center pushed south. These conditions materi- ally increased the low-level wind and the upslope motion in later afternoon. The mid—level increase in moisture made the uplift more effective in causing saturation. Shortly after 1800 MDT on the 9th, (0000Z, June 10), the mesosystem, then approxi- mately over the Black Hills, impinged on the almost- stationary front and moved southeastward along it. The front then moved southward as a cold front, ending the rains in the Black Hills. Thus, the mesosystem seen in both the satellite and special surface charts may have been the pri- mary cause of the unusual intensity of the weather, providing initial mid-level moisture, the strong THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 ' *“‘“"67“-~-“~'+ ‘ 70: i 75 '1 (7'. japus it] ‘ I 66 0'4? 300 400 MILES l l | | l 400 500 KILOMETERS o-o 'g’- ‘8" FIGURE 7.—(F‘or explanation see facing page.) THE BUILDING STORM 15 EXPLANATION 70%,. WW 54 Observation station Upper number is air temperature, in degrees Fahrenheit. Lower number is dewpoint temperature, in degrees Fahrenheit. Shafl indicates wind direction. Barbs on shafl indicate wind speed, in knots. Long barb :10 knots; short barb :5 knots. A circle around the station indicates the wind is calm PRESENT WEATHER (ww) 4 .] L‘ht' "b1, thdh d . 1g nmg V131 e no un er ear Rain (NOT freezmg and NOT falling as showers) during ) ( past hour, but NOT at time of observation 0 Precipitation within sight, reaching the ground, but distant Q from station . . _ t Showers of ram during past hour, but NOT at time of ( .) observation . . . . . . O Prec1p1tatlon w1thiglil§lgzrreachmg the ground, near to Intermittent rain (NOT freezing), slight at time of u at station observation R 0. Thunder heard, but no precipitation at the station Continuous rain (NOT freezing), Slight at time of observation 0 R] v Thunderstorm (with or without precipitation) during Slight rain shower(s) past hour, but NOT at time of observation . K Slight or moderate thunderstorm without hail, but with rain SKY COVER 0 CD 0 clear 40 percent 70 or 80 percent G) O O 10 percent 50 percent 90 percent 0 ED 0 20 or 30 percent 60 percent 100 percent H Geographic center of high—pressure system +_A_. _'—V_ Warm front Cold front Stationary front 1012 Line of equal atmospheric pressure at sea level Interval 4 millibars FIGURE 7.—Surface analysis. A, 1800, MDT June 9, 1972 (0000Z, June 10). B, 2100 MDT, June 9, 1972 (0300Z, June 10). (For C and D parts see following pages.) 16 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 95° -Jgfl _ / . z ax?“ 0 100 200 400 M|LES I I I I J r I I I I I 0 100 200 300 400 500 KILOMETERS D FIGURE 7,—Surface analysis—Continued. THE BUILDING STORM 17 EXPLANATION 70¢; ww 54 Observation station Upper number is air temperature, in degrees Fahrenheit. Lower number is dewpoint temperature, in degrees Fahrenheit. Shafi indicates wind direction. Barbs on shaft indicate wind speed, in knots. Lona barb :10 knots; short barb :5 knots. A circle around the station indicates the wind is calm PRESENT WEATHER (ww) <. o] L' ht ' ' ‘bl , th (1 h d _ 1g nmg VISI e no un er ear Rain (NOT freezmg and NOT falling as showers) during )4 past hour, but NOT at time of observation Precipitation within sight, reaching the ground, but distant {7] from station . Showers of rain during past hour, but NOT at time of ( . ) observation Precipitation within sight, reaching the ground, near to Intermittent rain (NOT freezing), slight at time of but NOT at station observation R u Thunder heard, but no precipitation at the station Continuous rain (NOT freeqins‘), slight at time Of observation R 0 V Thunderstorm (with or without precipitation) during Slight rain shower(s) past hour, but NOT at time of observation . K Slight or moderate thunderstorm without hail, but with rain SKY COVER 0 Q 0 clear 40 percent 70 or 80 percent (D 0 O 10 percent 50 percent 90 percent 0 9 O 20 or 30 percent 60 percent 100 percent H Geographic center of high-pressure system _‘—A_ _V—V_ Warm front . Cold front Stationary front 1012 Line of equal atmospheric pressure at sea level Interval :5 millibars FIGURE 7,—Surface analysis—Continued. C, 0000 MDT (0600Z), June 10, 1972. D, 0300 MDT (0900Z), June 10, 1972. 18 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 24 l l I I I I I I I r I _ _‘\ _\ __ _K\ 20 _ /E Note: Wind direction is determined at 36 _ 9 compass points. Number and length W of barbs denote wind speed to the near- _ _\ A est 5 knots (1/2 barb) _ *W a) 16 —‘ — '5 k z x — __ E d 12 —— —- Lu E —\ (I) _ _ D .Z. /’ f 3 8 — —— _ __\ _. 4 _ B _ O l I I I I I I I I I I I I I l I 13 14 15 16 17 18 19 20 21 22 23 24 1 2 3 4 5 (MDT) JUNE 9 JUNE 10 FIGURE 8.—Hourly surface winds observed at Rapid City Weather Service Office, June 9—10, 1972. Winds (aided by the advancing cold front), and the strong terrain-induced vertical motion that contrib- utes to such unusual rains. Without the mesosystem, the rains probably would not have been devastating. RADAR INDICATIONS The radar at Ellsworth Air Force Base was opera- tive for only part of the period of intense rains on the evening of the 9th. Figure 12 shows the PPI scope radar image at 2230 MDT (0430Z, June 10), during the period of intense rain near Rapid City and Pactola Dam. The echo just to the right of the center of the radar scope is just east of the region of largest rainfall amounts for the storm. The other PPI photograph available from the Ellsworth radar for 1930 MDT (0130Z, June 10), (not shown), shows a large area of precipitation extending north- northwest to south-southeast just west of Rapid City. Although nearly 250 miles away, the National Weather Service radar at Huron, S. Dak., showed echoes of the large persisting thunderstorm tops over the Black Hills during the period of heavy rain- fall. At this distance, these echoes came from clouds that had built higher than 40,000 feet and were con- siderably taller than any other echoes observed from Huron. A composite of these echoes (fig. 13) during the period 1800 MDT, June 9 to 0100 MDT, June 10 (0000—0700Z, June 10) indicates the concentration of high-level echoes over the eastern slopes of the Black Hills and suggests a line of echoes from thun- derstorms to the southeast that also appears on the satellite photographs. The observation of these echoes at extreme range in South Dakota is similar to the observation of distant radar echoes of pro- longed thunderstorm-produced rain in Virginia asso- ciated with the remnants of Hurricane Camille, August 19—29, 1969. Echoes of Camille thunder- storms in Virginia were persistently detected some 200 miles away by the Pittsburgh, Pa., radar (Schwarz, 1970). An hour-by-hour comparison shows close agree- ment between echo positions and areas of high pre- cipitation rates. Because of the extreme distance between the Huron radar station and the Black Hills storm, some error in position, on the order of 15 miles, is caused by atmospheric refraction. If echo positions were adjusted 15 miles to the west, six out THE RAIN NORTH DAKOTA \ “I NEBRASKA § ..__......—...— ..__—......._...- APPROXIMATE SCALE 1C.” 2r | 300 KILOM ETERS 3(l)O MILES O—r—O I 200 EXPLANATION 60 Line of equal dewpoint temperature Interval 2 degrees Fahrenheit. Based on composite of 1000-?mill1bar dewpmlnts FIGURE 9.—Composite 1,000-mb dewpoints (°F). 19 of seven precipitation rates greater than 1.0 inch per hour would be covered by a Huron-detected echo. Thus, as in the Camille case, radar echoes at large distances may perform a useful function in recog- nizing potential for extreme rains, even though un- certainties exist as to the exact locations and amounts. THE RAIN Rainfall data from 24 Weather Service observing stations, with either recording or nonrecording rain gages (fig. 14) , were supplemented by over 200 rain- fall reports from other agencies and from surveys after the flood. These data were used to produce an isohyetal map for the storm (fig. 15). According to the available rainfall data for the Black Hills storm (tables 3, 4, and 5), a maximum storm value of 15 inches of rain fell near Nemo, S. Dak., about 16 miles northwest of Rapid City. All rainfalls greater than 4 inches occurred on the east slopes of the Black Hills. The area of approxi- mately 300 square miles that received a minimum of 6 inches of rainfall lies primarily between 4,000 and 5,500 feet elevation (fig. 15). Numerous areas that received 10 or more inches of rain are scattered within this area. The largest 10-inch center, covering approximately 39 square miles, is about 15 miles west-northwest of Rapid City. The second largest center, of approximately 19 square miles, is about 15 miles southwest of Rapid City. There does not appear to be a simple or direct relation between maximum rainfall centers and ter- rain features at these locations except for the slight indication that east-facing valleys may have con- tributed to some forced convergence of the prevail- ing low-level winds. 'é‘ 3" I | I I I I I I I I D .— < '_ _ z w: “2-5170 — 3‘15 On. A. 60— — 53 a: _ _ “3 3°w— — :35 s. _ _ g I I I I I I I I I I I I I I I I I I I I I I I JUNE 9 l3 14 15 16 17 18 19 HOUR (MDT) 20212223241 JUNE 10 FIGURE 10.—1,000-mb dewpoints at the Rapid City Weather Service Office (°F) . 20 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 FIGURE 11.—Cloud cover. A, 0836 MDT (1436Z), June 9, 1972. B, 1024 MDT (1624Z), June 9, 1972. C, 1239 MDT (1839Z), June 9, 1972. D, 1454 MDT (2054Z), June 9, 1972. VARIATION OF RAINFALL WITH TIME Mass curves of rainfall for the six recording gages and one nonrecording rain gage where storm rainfall exceeded 4 inches show the variation of rainfall with time (fig. 16). The curves of figure 16 and the other recording gage data (table 5) indicate that almost all the rains at individual stations occurred within a 12—hour period. Even more striking, these mass curves show that 85—96 percent of the total rainfall occurred within 6 hours. Because the rain did not begin or end simultaneously at each station, however, the duration of the storm for the area encompassed by the 4-inch precipitation line was about 16 hours. Although the recorder-gage data show that the onset of rainfall progressed from north to south, a more detailed description of storm development can be obtained from visual observations made by a number of individuals in the Rapid City area. Photo- graphs of radar echoes from Ellsworth AFB and the South Dakota School of Mines lend support to the visual descriptions. The earliest signs of convective activity began about 1330 MDT, (1930Z), in Wyo- ming, approximately 60 miles west-northwest of Rapid City. Additional cells developed during the next 3 to 4 hours along the eastern edge of the Black THE FLOOD 21 FIGURE 12.—Radar image at 2230 MDT, June 9 (0430Z, June 10), during the period of intense rainfall near Rapid City. The echo just to the right of the center of the radar scope is, slightly east of the region that received the most rainfall during the storm. Hills about 25 miles north of Rapid City and began building farther south. By 1800 MDT (OOOOZ, June 10), a nearly solid line of cells extended from the northern edge of the Black Hills to just west-north- west of Rapid City. Stations A2 NFS and A5 NFS, roughly 25 miles northwest of Rapid City, were the first of the re- corder stations to receive rainfall, beginning about 1500 MDT (2100Z) , June 9. These stations operated by the South Dakota School of Mines are two of seven gages in close proximity that recorded between 8.1 and 11.82 inches of rainfall. Rainfall at Plot 67 and Arboretum (two of three gages located at site NFS in figure 14) began abOut 1600 MDT (2200Z), but totaled only 5.51 and 5.14 inches, respectively. According to the meteorologist-in-charge at the National Weather Service Office at Rapid City, almost simultaneously with the development of con-— vection cells north of Rapid City, a line of thunder- storms developed to the southeast at about 1500 MDT (2100Z) and moved toward the west-north- west with subsequent cells forming on the trailing end of the line. These cells increased markedly in intensity during the next hour or two, and by 1800 MDT (0000Z, June 10), the line of thunderstorms had become almost solid. Rain began falling at Pactola Dam at about 1700 MDT (2300Z) as the leading edge of a line of thunderstorms reached there from the southeast. Between 1800 (0000Z) and 1930 MDT (0130Z, June 10) the gap filled in between the line of thun- derstorms south of Rapid City and the initial thunderstorm development northwest of Rapid City to form a continuous line of thunderstorms covering the eastern slopes of the Black Hills. This system appeared to drift slowly eastward after about 2200 MDT (0400Z, June 10), although light showers per- sisted to the west for several hours. In the vicinity of Rapid City the heaviest rainfall continued until about midnight (fig. 16). At the Rapid City airport, most of the rain fell between 2230 MDT on the 9th (0430Z, June 10), and 0100 MDT (0700Z) on the 10th; little rain fell after 0400 MDT (1000Z) on June 10. THE FLOOD Because the Black Hills floods of June 1972 were caused by a concentrated group of thunderstorms, most of the rain fell in a short period of time. Con- sequently, many of the flood peaks occurred at ap- proximately the same time, between 2230 MDT on June 9 and 0100 MDT on June 10. At least 18 of the 27 streams where peak flows were computed experi- enced flows that exceeded the 50-year flood (a flood that is equaled or exceeded on the long-term average of only once every 50 years) as defined by Patterson (1966). The extreme flooding was largely confined to a flood belt about 40 miles long and 20 miles wide along the eastern slopes of the Black Hills, from Sturgis on the north to Hermosa on the south and with Rapid City near the center. Most of the streams draining this flood belt continue to flow their sep- arate ways eastward until they reach the Cheyenne River about 30 miles east of the Black Hills. Peak discharges for this flood were determined at 49 sites in the flood-affected area. Table 6 compares these data to the maximum stages and discharges experienced at the same sites in previous floods. The sites are numbered and listed in downstream order and are shown in figure 17. RAPID CREEK The flood in the Rapid Creek basin is significant because of the severe damage done in the Rapid City area. The stream flows through the center of Rapid City, where most of the flood plain has been devel- oped for homes and businesses. A few other streams in the vicinity had flooding comparable to that of Rapid Creek, but did not flood densely populated 22 46° 104° THE BLACK HILLS-RAPID CITY FLOOD, JUNE 9—10, 1972 100° 99° __—_. —__ __§_ —‘ 45° 44° SOUTH Huron o DAKOTA BLACK HILLS echoes 43° __‘__ APPROXIMATE SCALE 20 40 l l I I 20 4O 6O O—-—O ALBERS EQUAL AREA PROJECTION STANDARD PARALLELS AT 29%° AND 455/5 60 80 100 MILES l l I l 80 100 KILOMETERS FIGURE 13.—Composite radar echoes, 1800 MDT, June 9, to 0100 MDT, June 10 (0000—0700Z, June 10), 1972. areas. Consequently, they received less public atten- tion. The total drainage area of the Rapid Creek basin upstream from the Canyon Lake Reservoir is 371 square miles; however, only about 51 square miles, the area downstream from the gage on Rapid Creek below Pactola Reservoir, contributed to the flood runoff. Prior to the storm on June 9, Pactola Reser- voir was receiving an inflow of fro-m 60 to 70 cfs (cubic feet per second) and was discharging 74 cfs. On June 9 at 1600 MDT, inflow to Pactola Reservoir began to increase, but discharge from the reservoir was held steady at 74 cfs. Inflow to Pactola Reser- voir continued to increase until approximately 2000 MDT, when it reached a peak of about 2,200 cfs. Re- leases from the reservoir were maintained at 74 cfs until midnight, when outflow from the reservoir was cut off completely, and no water was released until 2000 MDT on June 13, when a discharge of 105 cfs began. Therefore, it can be seen that because of Pactola Reservoir the upper 320 square miles of the Rapid Creek basin played no significant role in the Rapid City flooding. The heavy rainfall over the Rapid Creek basin was concentrated over streams that enter Rapid Creek in the 9 miles between Pactola Reservoir and the Rapid Creek gage above Canyon Lake Reservoir. The principal streams entering Rapid Creek in this reach are Deer Creek, tributary from the north, and Prairie Creek and Victoria Creek, tributary from the south. Peak discharge of Deer Creek was deter— mined at the Deer Creek campground to be 3,530 cfs from a drainage area of 4.28 sq mi, a unit runoflf of 825 cfs per sq mi. The peak discharge of Victoria Creek at Victoria Lake dam was determined to be 6,860 cfs from a drainage area of 6.71 sq mi, a unit discharge of 1,020 cfs per sq mi. The peak on Prairie Creek could not be determined but was estimated to be about 5,500 cfs. Rapid Creek peaked at 31,200 cfs at the gage THE FLOOD 104°00' 103'45’ 30’ 103°15’ 44°30' 44°3o' .0 + + __ _ ______ SPEARFISH "I I \ _Black H71; area a: iaft shan relative to South Dakota RAPID CITY 0 O + 44-00' .RAPID CITY WSOAP 44-00' I. IAS O OH ERMOSA 00 a CUSTER MT COOLIDGE O EXPLANATION O Nonrecording gage O Recorder gage WIND CAVE NM 43°30'f- + + 4330’ 104°00' 103°45' 30' BUgKSI-OO 103"15' A000 ®H0T SPRINGS .ORAL o 5 10 15 20 25 MILES I I I I I I I I I I I I . O 5 10 15 20 25 30 35 KILOMETERS FIGURE 14.——Station locations and 1,000-foot elevation contours 24 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 103°45’ 103'30’ 103°15' 103°00’ 102‘45’ 102‘30’ 44° 30’ 44' 15’ 44° 00! 43° _ 45’ 43' 30' 2 % EXPLANATION ta —_ 2 _— Isohyets of total storm rainfall Interval 2 inclws. Areas of largest total storm rainfall, more than 10 inches, are shaded 9% 4% o——o I 20 15 I l 20 MILES l I 25 KILOM ETERS FIGURE 15,—Total storm isohyetal map for June 9—10, 1972. Rainfall occurred within a 16-hour period. Data based on climatologic network and supplemented by about 200 unofficial precipitation reports. THE FLOOD 25 TABLE 3.—Total rainfall, June 9-10, at miscellaneous gaging stations in the Black Hills [The following data have been collected through the cooperative efforts of the National Weather Service, Bureau of Reclamation, Forest Service, Corps of Engineers, South Dakota School of Mines, Black Hills Water Conservancy District, and the help of the general public. Most of the rain fell dur- ing a. period flrom 6:30 pan. June 9 to 12:30 a.m. June 10 (MDT). Additional data on intensity from recording gages will appear in the publication under hourly precipitation data] County 131?:— Range Section 3:55:33. 333w Type 1 Remarks 2 SN 6E 29 SW 14 2.02 A 8055 SN 3E 31 NE 14 2.21 A 8001 38 4E 18 SE 14 3.0 B David Renshaw 3 in. Water Coffee Can 38 4E 24 NW 4.5 A Roy Brummelt R-l, Custer 48 7E 1 2.6 B 3N Fairburn ZS SE 8 NW ‘24 4.25 B 4N Hermosa ZS 8E 18 SE 14 5.0 B 2N Hermosa 3S 4E 23 SE 3.9 A Custer, S. Dak. 38 6E 27 NW 14 3.05 A 8032 3S 4E 23 NW 14 3.75 A 8031 48 SE 1 CNTR 2.20 A 8107 38 7E 24 NE 1%; 2.75 A 8036 3S 7E 16 NE 14 3.53 A 8033 3S 9E 9 NE 14 2.05 A 8037 6S 9E 17 NW 111. 2.75 A 8088 48 7E 28 NW 3.0 A Tim Conrad R—25, 2W Hwy 79, French Creek, Fairburn 2S 8E 32 C 3.1 B Hermosa RC 2.76 A Beth Kukuk R—70, 1022 Quincy, Rapid City Custer ______________ 3S 4E 23 NE 3.5 B Custer Custer ______________ 2S 8E 32 NE 3.0 A Wayne Warren R—81, Hermosa Custer ______________ 48 4E 5 NW 22 B Jim Carson NFS Custer ______________ 58 4E 13 SW 225 B Pringle Custer ______________ 58 4E 33 SE 2.1 B 4 SW Pringle Hwy 89 Custer ______________ 48 3E 26 SE 1.25 B 9 SW Custer Custer ______________ 68 9E 14 SW 2.3 B 38 3E 32 SW 1-0 B 10 W Custer Hwy 16 3s 9E 3 SE 1.75 B 4S 7E 6 NW 3.2 B 38 4E 26 NE 4-6 B Evaporation 3S 5E 21 W 124 3.8 B 3S 4E 1 SW 4.5 B 4S 4E 15 SE 3.7 B 5S 8E 14 SW 14 3.5 B 3S 11E 9 SE 14 1.5 B 33 9E 9 N 17/2 2.08 A 4S 10E 3 NW 14 2.6 B 4S 10E 13 E 1/2 1.7 B 2S 10E 29 W 1/2 2.33 B 2S 10E 30 SW ”A. 2.75 B 6S 9E 13 NW 14 2.6 B 68 9E 24 SW 14 2.25 B 68 9E 19 E 1/2 3.75 B 58 8E 33 SE 14 3.25 B GS 7E 12 NW 14 2.0 B GS 8E 33 NE 1/4 2.75 B ES 9E 34 NW IA. 2.65 B 58 SE 2 NW 14 4.0 B Estimated GS 6E 3 SE 14 3.0 B 6S 6E 9 N 1/2 2.8 B 6S 6E 14 SE 14 3.2 B 6S 5E 25 2 1.6 B GS 6E 10 SE 14 2.8 B ' 4S 7E 27 NE 1A 5.0 B French Creek R—79 68 6E 22 3.25 B 3 W Buffalo Gap GS 6E 24 NE 3.1 A Weather Bureau-Buffalo Gap 38 6E 8 NE 4.23 A Black Hills Playhouse 2S 5E 30 C 4.48 A 8103 3S 7E 24 NE 3.13 A 8034 4S 7E 11 W 3.5 B 35 6E 5 SE 14 4.67 B 38 6E 22 SW 14 3.21 A 38 8E 7 NW 14 3.35 A 48 3E 4 NW 14 1.0 A 4S 6E 26 SW 14 3.2 A 4S 7E 12 CE 14 2.6 A 58 9E 32 SE 1/4 3.5 A GS 4E 31 NW 14 .75 B GS 7E 29 CS 1%; 3.1 A See footnotes at end of table. 26 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9-10, 1972 TABLE 3.—Total rainfall, June 9-10, at miscellaneous gaging stations in the Black Hills—Continued Total County 2:221" Range Section 3311:3911: rainfall Typel Remarks 9 Fall River ___________ 78 6E 2 NE 14 3.0 B Fall River ___________ 78 8E 16 SE 14 1.5 B Fall River ___________ 78 5E 31 NW 1A, 2.7 B Fall River ___________ 78 5E 8 SW 14 1.7 B . Fall River ___________ 7S 5E 20 .75 B 4W of Hot Springs Fall River ___________ 7S 4E 19 C .75 A Fall River ___________ 7S 5E 30 E 1/2 2.5 A Fall River ___________ 78 7E 33 CE 1/2 2.5 A Fall River ___________ 7S 8E 8 SW 14 1.7 A 7S 8E 31 NW 1%; 2.70 B Fall River ___________ 8S 6E 4 SE 14 .90 A Lawrence ____________ 5N 4E 18 SE 8.0 B Garbage Can Lawrence ____________ 3N 4E 18 SE 3.0 B David Renshaw Lawrence ____________ 5N 4E 18 SW ‘4 3.75 A 8004 Lawrence ____________ 7N 3E 26 NW 14 2.85 A 8002 Lawrence ____________ 6N 2E 24 NW 14 1.85 A 8097 Lawrence ____________ 3N 1E 31 NE 14 .50 A 8109 Lawrence ____________ 4N 3E 25 SE 14 2.99 A 8013 Lawrence ____________ 3N 3E 24 SE 14 6.65 A 8105 Lawrence ____________ 6N 4E 10 CNTR 4.36 A 8003 Lawrence ____________ 4N 3E 17 SE 1/4. 1.60 A 8012 Lawrence ____________ 3N 5E 16 SW 14 8.15 A 8102 . Lawrence ____________ 2N 4E 4 5.94 A 3 Rfiiirg Gages 5.90 to 5.94 m. — 7 Lawrence ____________ 3N 5E 16 SW 14 10.00 B Gaigagoe Can 13 in. Reduced to .9 in. Lawrence ____________ 3N 5E 16 SW 14 10.00 A NW Nemo 6 pm. to 2 am. . Lawrence ____________ 3N 5E 27 NE 15.0 __ Woodrow Weathers R—71 (Ven- fied) Nemo Job Corps Lawrence ____________ 6N 4E 1 NE 5.0 B MS. Fred Swesey, Wash Tub, Whitewood Lawrence ____________ 2N 5E 6 NE 10.55 A Trout Haven Lawrence ____________ 2N 4E 4 NW 5.51 R Plot 67 Lawrence ____________ 2N 4E 4 NW 5.14 R, Arb. Lawrence ____________ 6N 4E 5 NE 3.0 A Alvin Jensen, Whitewood Lawrence ____________ 6N 4E 33 NE 5.5 A Frank Willson, Whitewood Lawrence ____________ 4N 2E 22 NW 2.08 A Jim Carson NFS Lawrence ____________ 5N 2E 23 5.0 B Jim Carson NFS (Deadwood) Lawrence ____________ 3N 5E 27 SE 7.5 B Jim Carson NFS Lawrence ____________ 3N 4E 26 S 1/2 12.7 B Lawrence ____________ 4N 4E 33 SW 14 4.9 B Lawrence ____________ 3N 4E 6 SW 1A 4.7 B Lawrence ____________ 3N 4E 18 NE 1/4 6.5 B Est—6.5 to 7.0 Lawrence ____________ 3N 5E 31 NW 10.55 A Pilot Knob Lawrence ____________ 2N 5E 2 SE 8.0 B South Canyon, Rapid City Lawrence ____________ 2N 5E 7 SW 10.0 B Jim Carson NFS Lawrence ____________ 2N 4E 12 SE 14 12.0 B Lawrence ____________ 2N 7E 5 SE 14 3.06 A 8068 Lawrence ____________ 2N 5E 34 NE 12.0 A Ms. Joseph Gairtner, N of Pactola R—2 N of Trout Haven, W of Nemo Rd. Meade ______________ 3N 6E 15 SE 4.25 B Walker—5 gallon Bucket Straight Sides—Rt A Box 96 Piedmont Meade ______________ 7N 6E 35 C 3.75 B Roy Costell Meade ______________ 5N 5E 25 SW 3.8 A Blackhills Nat Cemetary Meade ______________ 3N 6E 9 SE 5.75 A Jack McLean, Piedmont Meade ______________ 2N 7E 8 NW 3.9 A John Kane, Black Hawk Meade ______________ 4N 6E 32 SE 1/4 5.88 A 8008 Meade ______________ 5N 9E 34 SW 14 1.65 A 8073 Meade ______________ 4N 7E 18 SE 14 2.45 A 8071 Meade ______________ 6N 6E 12 SE 14 3.80 A 8082 Meade ______________ 7N 7E 13 NE 1A. 2.01 A 8084 Meade ______________ 3N 9E 3 SE 1/4 1.75 A 8064 Meade ______________ 6N 6E 33 NW 14 4.25 A 8080 Meade ______________ 5N 7E 24 NW 14 2.09 A 8075 Meade ______________ 5N 5E 15 SE 1/4 5.70 A 8006 Meade ______________ 4N 6E 4 NE 14 4.85 A 8078 Meade ______________ 5N 7E 1 SW 14 2.88 A 8076 Meade ______________ 4N 6E 18 SE 1/4. 6.68 A 8007 Meade ______________ 6N 10E 21 NE IA 2.75 A 8036 Meade ______________ 2N 10E 19 SE 14 2.32 A 8049 Meade ______________ 7N 6E 30 SW 1/ 2.82 A 8083 Meade ______________ 6N 5E 13 NW 14 2.87 A 8081 Norris Roverce Meade ______________ 5N 7E 6 SE 1/4 2.41 A 8079 Meade ______________ 5N 9E 19 NE IA 2.00 A 8074 See footnom at end of table. THE FLOOD 27 TABLE 3.—Total rainfall, June 9—10, at miscellaneous gaging stations in the Black Hills—Continued County £11?:- Range Section 2:31-1:11. Egg“ Type 1 Remarks 3 Meade ______________ 4N 5E 3 11.82 A 7 Rain Gages 8.1 to 11.82 R—56 Meade ______________ 3N 6E 2 sw 4.0 A John Hauer R—89, 1 N 1 W, Piedmont Meade ______________ 3N 6E 22 SE 3 3 A John Hauer R—90, 2E of I—90 on Elk Creek Rd. Meade ______________ 5N GE 5 NW 4.5 B Chas R086 (Sturgis) Meade ______________ 6N 7E 10 C 4.25 A Vernon Bachard B—2 Meade ______________ 7N 6E 35 C 3.0 B Don Costell . Meade ______________ 7N 6E 17 C 2.75 B Kenneth Holst (Sturgis) Meade ______________ 6N 5E 12 C 3.3 A Frank chka (Sturgls) Meade ______________ 6N 5E 30 C 4.85 A E. H. Fallett, Whitewood _ Meade ______________ 3N 6E 15 SE 4.5 B Walker, Rt A, Box 96, 1 S Pied- mont Meade ______________ 3N 6E 19 C 7.0 B Don Nelson, Bucket Overflow, Rt 8, Box 103A, Piedmont, By Stage-Barn Canyon R—43 Meade ______________ 2N 7E 5 SE 14 3.06 A 8068 Meade ______________ 3N 10E 5 SE 1 65 A 8063 Meade ______________ 6N 10E 21 NW 1 15 A 8062 Meade ______________ 5N 7E 1 SW 2.68 A 8076 Meade ______________ 2N SE 7 10 55 A Meade ______________ 3N 5E 27 SW 1A. 7.5 A Meade ______________ 5N 3E 23 SE14 5.0 A Pennington __________ 2N 7E 14 N 4.0 B Art E. Fland, 4109 W. Chicago, Rapid City Pennington __________ 1N 7E 7 SW 3.8 B Emil Magnusson R—28, 2305 Arrow Drive, Rapid City Pennington __________ 1N 7E 7 SW 3.5 A Robert Biegler R—30, 319 E. Tallent, Rapid City Pennington __________ IS 6E 2 SE 5.5 B H813) Gross, 2230 Alamo, Rapid ity Pennington __________ 1S 5E 80 SE 5.14 A Gene Barker R—83, Hill City Pennington __________ 1N SE 7 NE 3.20 B Ken Elliott, St. Andrew Lincoln, Rapid City Pennington __________ IS 4E 30 SW 5.2 B Jim Carson NFS Pennington __________ 18 4E 25 SW 4.85 B Jim Carson NFS Pennington __________ IS 5E 20 NW 6.0 B Jim Carson NFS Pennington __________ 1N 2E 26 NE 1.2 B Jim Carson NFS Pennington __________ 1S 5E 32 NE 1%; 8.9 B Bucket Overflow Pennington ___________ IS 5E 32 SW% 12.5 B Bucket Overflow Pennington __________ 2S 5E 2 N 1%; 8.25 B Bucket Overflow Pennington __________ 1N 7E 21 N 1%.» 7.0 B Bucket Overflow Pennington __________ 1N 7E 33 NW 14 6.25 B Bucket Overflow Pennington __________ 1N 7E 29 SE 14 7.5 B Bucket Overflow Pennington __________ 1N 6E 36 NW 1%; 5.6 B Pennington __________ IS 6E 2 SW 14 11.75 B Pennington __________ 1S GE 9 NW $4 8.75 B Bucket Overflow Pennington __________ 18 5E 36 NE 14 12.6 B Pennington __________ IS 5E 22 S 1/2 10.0 B Pennington __________ IS 5E 19 NW 14 4.52 A Pennington __________ 1S 4E 14 NE $4 7.0 B Bucket Overflow Pennington __________ ZS 5E 2 SW 1/1 8.2 B Pennington __________ IS 5E 21 CE 8.0 B Pennington __________ 1S 5E 22 SW 1A 8.0 B Pennington __________ IN 5E 34 C 10.6 B Pennington __________ 1N GE 8 NW 1A. 8.0 B Pennington __________ 2N 7E 31 NW 14 4.5 B Pennington __________ 1N 6E 12 SE 14 6.0 B Pennington __________ 2N 5E 28 C 11.64 A Pennington __________ IS GE 8 NW 1A. 14.01 A Gage Overflowed 8024 Pennington __________ IN 3E 6 C 1.89 A 8014 Pennington __________ 1N 2E 26 SW 14 .50 A 8101 Pennington __________ 2S 3E 24 NE 14 2.12 A 8099 Pennington __________ 1N SE 6 SW 14 3.00 A Pennington __________ IS 4E 14 NE 2.9 A Julius Hardi R—72, Box 9, Hill City Pennington __________ 1N 7E 1 NE 5.5 B M. S. Hausen R—75, Wastebasket 6.25 mi 9th & St. Patrick, . Rapid City Pennington __________ 1S 6E 34 C 12.0 B Janice Grow R—27, Garbage Can Pennington __________ 1S 6E 34 C 10.51 A Sheridan Lake Pennington __________ 2N 5E 16 SW 10.0 B Pennington __________ 2S 6E 8 NE 7.0 B Owner of Rushmore View Motel Keystone See footnotes at end of table. 28 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 TABLE 3.—Total rainfall, June 9—10, at miscellaneous gaging stations in the Black Hills—Continued County 2g?- Range Section 2233:: Eggm Type 1 Remarks 3 Pennington __________ IS 7E 3 SW 5.25 A Ms. Michael Kloster Key Rt. Box 178 Man Across From Reptile Gardens R—92 ‘ Pennington __________ 2N 9E 17 C 2.5 A Herbert Vadney R—46, 9272 Lin- coln Drive EAFB Pennington __________ 28 6E 1 SW 5.0 A Arthllllr Haub B—3, 4 S Rocker- V1 e Pennington __________ IS 6E 24 SW 5.0 B Joe Dunn—Can, 1 S Rockerville .4 Pennington __________ IS 6E 28 NW 7.0 B Dean J ohnson—31B, Coffee Can Overflow, 3W Rockerville Pennington __________ 1S 7E 3 NE 34 11.0 A Albert Frame R—52, 30 Gallon Garbage Can 131/2 in., Sprmg Creek Camp Ground Pennington __________ 2N 5E 16 SW 11.0 B _ Pennington __________ 2S 6E 8 NE 12.0 B Art Iones—Case of 10 1n. Pop Bottles, Keystone Pennington __________ 2N 5E 31 SE 6.0 A Greunwald, Silver City Pennington __________ 2N 6E 35 SW 14 11.75 A Container Overtopped, R—74 Straight Sided Flower Urn, Larry Wright, Rt 1, Box 27, 5S Wild Irishman Gulch Pennington __________ 2S 5E 4 NE 9.5 B Z Olson B—5, Box 295, Hill City 3 SE Hill City _ _ Pennington __________ 1s 5E 23 NW 9.0 B Neil Hodges B—6, 4 NE Hill Clty at Three Forks Pennington __________ IS 5E 32 NE 9.0 B A. W. McCamly B—7, B0); 387, Hill City, 1.8 S Hill City Pennington __________ IS 6E 28 NE 14 12.0 A Dean Johnson R—91, 2—5 Gallon Buckets Tepee Gulch Hwy 16 Cos-Deer Huts Pennington __________ 2N 7E 32 NE 1%; 13.0 A Container Overtopped R—31 Glass Tube 71/1» Rain Gage Ted Plisterer, Rt 1, Box 34A, 5W RC on Nemo Rd Pennington __________ 2N 6E 31 SE 13.5 B Jim Eade—5 Gallon Side Pail R—44, Johnson Siding Pennington __________ 1N 5E 2 NW 7.0 A Kathleen Mack R—55, 115 Frank- lin, Rapid City Pennington __________ 1S 8E 20 N 1/2 2.5 B 5 SE Spring Creek Pennington __________ 1S 8E 22 S 1/2 3.85 B 2E Spring Creek, Hwy 79 Pennington __________ 2S 6E 18 NW 11.0 B Glen Tally R—68, Straight Sided Bucket, Grizzly Canyon Key- stone, Bucket Overtopped Pennington __________ 2S 5E 31 SW 6.5 A HaII-lrl’l Iéruse R—67, Box 514, i ity Pennington __________ 2S 4E 13 NE 4.5 B Container Overtopped B—8, Her- bert Trumpeter, 48 Hill City Pennington __________ 1N 4E 31 SE 2.2 A John McVey Pennington __________ 1S 6E 12 SE 5.5 A Larrlyi Jacobson B—9, 1 N Rocker- v1 e Pennington __________ 2N 7E 28 SE ‘4 4.70 B 8019 Pennington __ IS 7E 28 NW 14 6.60 B 8023 Pennington __________ 2S 4E 9 NW 14 2.74 B 8089 Pennington __________ 1N 7E 10 NE 14 3.51 B 8100 Maralee Dennis Pennington __________ 1S 5E 12 SE 14 14.00 B Round Bottom Boat R—42 Pennington __________ 1S 8E 28 NE 2.50 A G. Z. Olson, Box 295, Hill City, 3 SE Hill City Pennington __________ 28 5E 6 SW 14 9.0 B A .W. McCamly, Box 387, Hill City, 28 Hill City Pennington __________ 1N 5E 10 NE 14 7.16 AR Cloey Simpson, Keystone Box 155, S End Pactola Dam Pennington __________ 2N 5E 31 SE 6.0 B Greunwald, Silver City Pennington __________ 2N 3E 11 NE 1.7 B Darrell Kenrison, Rockford Pennington __________ 2N 5E 34 NE 12.0 A Joseph Gairtner R—2, 9 in. Rain Gage, N of Pactola Pennington __________ 1S 4E 16 NE 7.2 B Newton Fork Pennington __________ 1N 6E 9 NW 9.5 B Gayle Jorgenson. Big Piney Subdivision near Rimrock Club, Hisega Turnofi' Pennington __________ IS 6E 26 SE 5.4 B 28 Rockerville Pennington __________ IS SE 5 10.00 B Bucket Overflow R—7 Pennington __________ 1S 5E 24 NE 14 11.5 B Ralph Rossknecht, Traile Pk- . Calumet Pt on Sheridan Lake Pennington __________ 25 6E 32 NW 3.6 A Dr. R. A. Kovarik, R—53, 2.5 ' SW Rapid City Penmngton __________ 1S 8E 28 NE 3.0 A Wayne Warren See footnotu at end of table. THE FLOOD 29 TABLE 3.——Total rainfall, June 9—10, at miscellaneous gaging stations in the Black Hills—Continued Town- Quarter Total County ship Range Section section rainfall Type 1 Remarks 3 Pennington __________ 2S 8E 19 NE 4.0 R IAS 9001 Pennington __________ 2N 3E 23 SE 1.72 R Rochford Pennington __________ 1N 9E 17 C 2.32 R Rapid City Pennington __________ 1N 5E 11 C 7.07 R Pactola Dam - Pennington __________ 2N 6E 32 SW 12.8 A Observer took times Pennington __________ 1N 10E 24 SE 1A. 1.7 A Pennington __________ 1N 7E 21 NE 1A 4.0 A Pennington __________ 1N 6E 36 NW 14 5.5 B Pennington __________ 1N GE 9 CN 1/2 6.5 A Pennington __________ 1N GE 9 CN 1/2 7.9 B Pennington __________ IS 6E 8 NW 14 6.6 B Pennington __________ IS 6E 9 NW 1A, 9.75 B Pennington __________ 1S 8E 15 SE 14 3.85 B Pennington __________ IS 8E 20 NW 14 3.50 B Pennington __________ 15 9E 12 NE 14 1.50 A Pennington __________ ls 10E 26 SE 14 1.72 A Pennington __________ 1S 11E 26 NW 1A. 1.68 B Pennington __________ ZS 9E 6 C 2.5 B Pennington __________ ZS 12E 11 CE 1/2 1.5 B Shannon ____________ 41N 47W 4 NW 1/4 3.5 B Shannon ____________ 41N 47W 4 NW 1A 2.0 B 1 The type A refers to rain gage. Type B refers to bucket. aUnder remarks, all 4 digit numbers refer to School of Mines. Rain gages and numbers of the type R—55 are bucket survey reference numbers. TABLE 4,—Daily precipitation (inches), Black Hills and vicinity [T= less than measurable amount; MSG: missing data] Time . Location Of June 8mm“ Lat Long “55;,“ 8 9 10 11 (MDT) Buskala Ranch 103°49’ 7 p.m. T 0.08 1.58 ___ Custer ...................................... 103°46’ 7 a,m. ___ __- 3.45 ___ Deadwood ________ 103°44’ 7 p.m. 0.05 30 1.73 ___ Deerfield 4 NW 103°54’ 7p.m. 20 ___ .60 ___ Farmingdale 4 N _ 44°02’ 102°54’ 6 p.m. ___ ___ 1.68 ___ Fort Meade ___- 44°24’ 103°28' 5 p.m. ___ 45 4.38 ___ Hermosa 1 W _ 43°50’ 103°13’ 7a.m. ___ -._ 2.81 T Hill City ____ 43°56' 103°34’ 9a.m. .02 ___ 5.14 ___ Hot Springs ___ ______ 43°26’ 103°28’ 7p.m. ___ ___ 1.31 ___ Lead 1 SE _-_-_ _____________________________ 44°21’ 103°46’ 7p.m. 03 17 1.65 T Lead 6 SSW _______________________________ 44°17’ 103°48’ 7p.m. T .40 1.04 ___ Mount Coolidge _____________________________ 43°45’ 103°29’ 6 p.m. ___ .05 MSG ___ Mount Rushmore National Memorial Park --- 43°53’ 103°27’ 6 p.m. ___ ___ 6.30 0.02 Pactola Dam _______________________________ 44°04’ 103°29’ 8 a.m. ___ ___ 7.16 ___ Rapid City _________________________________ 44°04’ 103°16’ 6 p.m. ___ 3.60 ___ Rapid City, WSO AP ______________________ 44°03' 103°04’ 12 p.m. --_ 2.08 .24 _.._ Spearfish ___________________________________ 44°21’ 103°53’ 8 a.m. --_ T 1.68 .32 above Canyon Lake Reservoir, with all the flow being derived from about 51 sq mi of drainage area down- stream from Pactola Reservoir. This gives a unit discharge of 612 cfs per sq mi for the peak. The gage on Rapid Creek above Canyon Lake Res- ervoir was inundated by floodwater but remained intact, allowing part of the record to be salvaged. The recorder tape showed a stage of 1.65 feet, then a gradual rise in water surface beginning at 1900 MDT and continuing until 2000 MDT, when the water began a sharp but even rise until 2100 MDT, when it leveled off for 15 minutes (fig. 18). The creek had risen approximately 3 feet in 2 hours. From 2115 MDT until 2315 MDT, the creek rose another 12 feet, beginning with a rapid rise of about 3.5 feet in 15 minutes. The flood peaked at a stage of 15.77 feet, 7.69 feet higher than the previous flood of record. The gates on the spillway at Canyon Lake Reser- voir were opened to lower the lake level but were soon clogged by large amounts of floating debris. Several men were assigned to keep the debris mov- ing through the spillway, but the task became im- possible when a boat dock from the lakeshore broke loose and lodged in the spillway. After this, several boats were caught in the spillway and, along with the boat dock, blocked the exit to the lake. Shortly thereafter, water started to flow over the left end of the 500-feet-long earthfill dam. The water level of the lake continued to rise and soon overtopped the entire dam. Although the main break in the dam probably washed out fairly rapidly, it was reported that water flowed over the top of the dam for ap~ proximately 45 minutes before the dam gave way. Most reports give the time when the dam broke as 2245 MDT, which coincides fairly well with the 30 TABLE 5.—Hourly precipitation THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 (inches) in Black Hills and vicinity Station Location June 9 (MDT) Lat Long 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 0000 Edgemont 25 NNW .............. 43°39’ 103°56’ ___- ___- ___- _--_ 0.03 0.01 ___- _-_- 0.23 0.06 0.04 Oral _____________________________ 43°24’ 103°16’ ____ ---_ -__- __..- ___- ___- ___- -..-_ 2.35 1.13 .07 Pactola Dam ............... ___ 44°04' 103°29’ _-_- ___- 0.01 0.04 .25 1.37 0.86 1.37 1.83 .57 .30 Rapid City, WSO AP __________ 44°03’ 103°04’ -___ ___- ___- -__- ___- ___- ___- -..-_ ___- .61 1.16 Rochford _________________________ 44°07’ 103°43’ ____ __-_ .05 .03 .05 .33 .12 .15 .17 .26 .20 Spearfish 1 W ___________________ 44°29’ 103°53’ ___- ___- ____ ____ _-__ .08 .02 .11 .14 .25 .31 Wind Cave ....................... 43°33' 103°29’ ___- ___- .02 .02 -,--- ___- ___- ___- 1 99 .30 12 Station Location June 10 (MDT) Lat Long 0100 0200 0300 0400 0500 0600 0700 0900 1000 Edgemont 25 NNW ............. 43°39’ 103°56’ 0.04 0.04 0.02 ___- ___- ___- ___- ___- ___- Oral ____________________________ 43°24’ 103°16’ .03 .01 .01 ___- ___- ___- ___- ___- ___- Pactola Dam ____________________ 44°04’ 103°29’ .26 .14 .02 0.01 ___- ___- 0.02 0.04 0.01 Rapid City. WSO AP ____________ 44°03’ 103°04’ .31 .13 .09 .02 --__ ___- _-__ ___- ___- Rochford ________________________ 4°07 103°43’ .34 .03 .02 .02 ___- ___- ___- ___- ___- Spearfish 1 W ___________________ 44°29’ 103°53’ .31 .26 .02 ___- ___- ___- ___- _--_ ___- Wind Cave _____________________ 43°33’ 103°29’ .04 ___- .01 ...__ ___. ___- ___- ___- ___- . Location June 10 (MDT) Stan” Lat Long 1500 1700 1800 1900 2000 Edgemont 25 NNW _____________________ 43°39’ 103°56’ ___- ___- ___- _-__ ___- Oral __________________________________ 43°24’ 103°16’ ___- _-__ ___- ___- .._-_ Pactola Dam _________________________ lid-“04’ 103°29’ --.- ___- ___- _-__ _--_ Rapid City, WSO AP ___ 44°03’ 103°04’ ___- --_- ___- ___- ___- Rochford _________ _-_ 44°07’ 103°43’ ___- ___- ___- ---- ___- Spearfish 1 W ___- _________ ___ 44°29’ 103°53' ___- 0.01 0.01 0.14 0.06 Wind Cave 43°33 103°29’ ___- ___- ___- ___- ___- Notes: 1. No precipitation on June 8 or 11 at these stations. 2. Other usually unpublished hourly data shown as mass curves, fig. 16. timing of the flood crest flowing out of Cleghorn Canyon and entering Rapid Creek immediately up- stream from Canyon Lake. (See fig. 19.) The effect of the failure of the dam at Canyon Lake Reservoir on the subsequent flood wave and on the total damage is difl‘icult to assess. Lacking actual recorded measurements of the flood wave, sev- eral indirect measurements have been computed for Rapid Creek. Each of these indirect measurements is based on a reconstruction of the water-surface profiles and on the shape and roughness of the chan- nel, but each is subject to some error. The extreme nature of the flood and the associated floating debris (houses, trees, cars, and trailers) may affect the relation between measured high-water marks and actual discharge. By adding together the 31,200 cfs peak discharge measured on Rapid Creek above Can- yon Lake Reservoir and the 12,600 cfs peak dis- charge that poured from Cleghorn Canyon into Rapid Creek, some 43,000 cfs, or 86 percent, of the peak discharge of 50,000 cfs determined to have been carried by Rapid Creek through Rapid City at midnight on June 9 (fig. 20A) can be accounted for. On the basis of computations of discharge, because of the relatively small size of Canyon Lake Reser- voir, and because the dam did not give way abruptly, the failure of the dam does not appear to have been a major contributor to the peak discharge of Rapid Creek at Rapid City. The vast amount of water coming down Rapid Creek and the several streams contributing to the flow of Rapid Creek far over- shadowed the amount of water in the small reser- v01r. After moving through the city, the flood peak on Rapid Creek passed into the Great Plains, where it spread out over a wide area; much of the water was stored in the wider flood plain. By the time the flood peak reached the gaging station at Farmingdale, the the peak discharge had attenuated to 7,320 cfs. This gives a unit discharge of 26 cfs per sq mi for the 283 sq mi drainage below Pactola Dam (fig. 203). This peak discharge was only about one-seventh the peak discharge measured in Rapid City but was still a peak of record at that site. The flood peak traveled the 65 river miles from Deer Creek near Pactola Reservoir to Farmingdale in about 17 hours, or an average speed of 3.8 miles per hour (fig. 21). Com- ing off the slopes of the Black Hills, the flood peak had traveled at about twice that speed, covering the approximately 22 river miles between Deer Creek and Rapid City in about 31/; hours. After the flood, high-water marks were surveyed along the banks of Rapid Creek through Rapid City. From this survey and the resulting flood profiles, a map was prepared for an earlier report showing areas of the city inundated by floodwaters (Larimer, 1973). BOXELDER CREEK The headwaters of Boxelder Creek are located in an area where up to 12 inches of rain fell during the night; as a result, this creek experienced very THE FLOOD 31 14 I I 12 — — 11.2 in., A5 NFS (Sturgis 5 SSW) 1o — _ 8.57 in.,A2 NFS _ _ (Sturgis 5 SSW) / / m 8 - / — / 5 / E / E _ _ I“ l: Lu 6 _ _ 0 5.51 in., Plot 57 (Rgrygrd 4 NE) _ // //”—- _ / ,x’ //I/” I 4 _ _ H // / l I _ 5.14 in., Arb; (Rochford 4 NE)— 2 — _ I o / I 1 l 1200 1500 1800 2100 0000 0300 0600 JUNE 9 JUNE 10 l | l I 12.8 in., Rapid City 5 NW (5 observations) .v’ ..._..._’-- _ 7.07 in., 6427 Pactola Darn _,_./ I | | I l 1 500 18 2 1 00 0000 0300 0600 0900 JUNE 9 JUNE 10 FIGURE 16.—Mass rainfall curves for selected stations where rainfall exceeded 4.0 inches. high flooding. Although flooding was extreme, total damage was fairly low compared to that along Rapid Creek, because of the area’s low level of develop- ment. Significant damage was done to one of the bridges on Interstate 90 between Rapid City and Sturgis as a result of stream scour; damage was also done to other roads and ranches. Peak discharge of Boxelder Creek was computed at several locations. At the gaging station on Box- elder Creek near Nemo, the discharge was deter- mined to be 30,100 cfs from a drainage area of 96 ’ sq mi, or a unit discharge of 314 cfs per sq mi. The '; gaging station was so completely destroyed that no i remnants of the gaging station could be found. 32 J: h o .___T.!. “fir—r THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 z _; - _ - — — — L32: 0 10 20 30 MILES i 1 l | I I I l o 10 20 30 4o KILOMETERS EX PLANATIO N . O . . A . . I . Gagmg station Partlal record station Miscellaneous Site FIGURE 17.——Stream-gaging stations and miscellaneous measurement sites in the Rapid City area. THE FLOOD 33 15 | I l I I I I I I I I I I I I Peak 15.77 ft i \ 2315 hr l \ 14 - \ — \\ \\ 12 — \ - \\ 10 I— \ — \ I; \ I“ \ u. z \ _ 8 — \ — Id (5 \\ < F \ (I) \ 6 — \\ _ \ \ \ \\\\ 4 ~ EXPLANATION — Recorded gage height ——— Estimated gage height Stage 0.00:3,40739 ft above mean sea level 2 — _ o l l I l l I I L l I I I I I 0000 0600 1200 1800 0000 0600 1200 1800 0000 June 9 June 10 TIME, IN HOURS FIGURE 18.—At the gaging station above Canyon Lake Reservoir, Rapid Creek began rising slowly at 1900 MDT on June 9. By 2000 MDT, the stream started rising faster, finally peaking some 14 feet above the 1900 MDT level at a discharge of 31,200 cfs by 2315 MDT. The North Fork, Middle Fork, and South Fork of Boxelder Creek all cross Highway 385 about 4 to 6 miles upstream from Nemo, and all three of these sites were noted to have had very little flow during the storm. From Nemo Road, 7 miles northwest of Nemo, there was practically no flow into Reausau Lake, which is on a tributary to Boxelder Creek. Rainfall, and therefore runoff, was not unusually heavy upstream from this point. This moderate rain- fall in effect eliminated approximately 45 sq mi of contributing drainage area above the gage on Boxelder Creek near Nemo, raising the unit dis- charge at Nemo to about 590 cfs per sq mi for the contributing area. A discharge of 1,180 cfs was determined on Boxel— der Creek at Benchmark, near Nemo. Total drainage area above this point is 37.2 sq mi; however, only about half of the drainage area contributed to this runoff because rainfall was light on the headwaters. Another indirect measurement was made at a stream-gaging station on Estes Creek near Nemo, a tributary to Boxelder Creek. The gage was washed out and destroyed, and the hydrograph record was lost. This creek had a peak discharge of 6,620 cfs 34 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 FIGURE 19.—View of Canyon Lake Reservoir following the collapse of the dam. Jackson Boulevard is visible at the top. (American Red Cross photograph.) from a drainage area of 6.14 sq mi, a unit discharge of 1,080 cfs per sq mi. A third measurement was made on the main stem of Boxelder Creek at the point where the creek leaves the mountains and flows onto the plains. At this point, a peak discharge was computed to be 51,600 cfs from a drainage area of 117 sq mi, or a unit discharge of 442 cfs per sq mi. BATTLE CREEK Battle Creek and its tributaries were in the area of some of the most intense rainfall; extreme flood- ing was found all along the stream. Eight people were killed in the small town of Keystone, built on the banks of Battle Creek in a little canyon near Mount Rushmore National Memorial. Much of the town was washed away. Peak discharges were determined at six points in the basin; on a small tributary to Battle Creek; on Battle Creek upstream from Keystone; on Grizzly Bear Creek near its junction with Battle Creek, at Keystone; at the gaging station downstream from Keystone; at the canyon mouth where Battle Creek empties into the plains; and on the plains near Hermosa. The measurement on the small tributary to Battle Creek was made specifically to get infor- mation on discharge in small drainage areas. This measurement was made at a crest-stage gage 2.8 miles northwest of Keystone, with a drainage area of 0.88 square miles. Discharge at the peak was 1,330 cfs. This represents a unit discharge of 1,510 cfs per sq mi. Battle Creek upstream from Keystone had a peak discharge of 10,800 cfs, and Grizzly Bear Creek peaked at 6,230 cfs. Unit discharges were 794 and 676 cfs per sq mi, respectively. The U.S. Geological Survey has maintained a gage on Battle Creek downstream and 4.5 miles southeast of Keystone for the past 11 years. The peak dis— charge at this site was 26,200 cfs from a drainage area of 66 sq mi, giving a unit runoff of 400 cfs per sq mi. The discharge of Battle Creek at the canyon mouth was 44,100 cfs from a drainage area of 110 THE FLOOD 35 50 I N O | H O I ' I | 1200 0000 1200 June 9 June 10 TIME, IN HOURS A 8 l I l O 0000 0000 DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECOND \1 l l 1200 0000 1200 June 10 June 11 TIME. IN HOURS B 0000 FIGURE 20.—Peak discharges on Rapid Creek. A, At midnight on June 9, a peak discharge of about 50,000 cfs was carried through Rapid City by Rapid Creek. By contrast, the average discharge for June 9 was 1,050 cfs, and for June 10, 5,600 cfs. B, Around 1330 MDT on June 10, the flood peak reached the gaging station on Rapid Creek near Farmingdale. Peak discharge at this point was 7,320 cfs. sq mi. This represents a unit discharge of 400 cfs per sq mi. One more gage is located on Battle Creek near Hermosa, 18 miles south of Rapid City on the plains. Here the peak discharge was 21,400 cfs from a drainage area of 178 sq mi. The reduction in peak discharge between the canyon mouth and Hermosa demonstrates how quickly discharge attenuated after reaching the plains area. GRACE COOLIDGE CREEK Grace Coolidge Creek basin is located to the south of the heavy rainfall area and discharges into Battle Creek upstream from Hermosa. Grace Coolidge Creek did not experience heavy flooding; the peak discharge at the gaging station near Custer was only 709 cfs from a drainage area of 25.3 sq mi. SPRING CREEK In the upper Spring Creek basin there was very little runoff in streams west of Hill City. The peak discharge of Newton Fork, 3.9 miles northwest of Hill City, was only 21 cfs. There was, however, high runoff from tributaries in the lower Spring Creek basin. Because of the variability Within the basin, the discharge was determined in several places so that the origin and amount of water could be documented. High runoff appeared between Hill City and the Sheridan Lake area. Measurements were made on Palmer Creek near Hill City and on Spring Creek and Horse Creek near their entries into Sheridan Lake to determine the runoff from the headwaters area. Palmer Creek, 3 miles east of Hill City, peaked at 4,370 cfs from a drainage area of 8.24 sq mi, a unit discharge of 530 cfs per sq mi. At Sheridan Lake, Where the drainage is 58.0 sq mi, Spring Creek had a peak discharge of 5,630 cfs, a unit dis- charge of 97 cfs per sq mi. Horse Creek, another major tributary to Sheridan Lake, produced a peak discharge of 1,830 cfs, or a unit discharge of 181 cfs per sq mi.- Sheridan Lake stored some water for a short time and slightly reduced the flood peaks downstream. Rockerville Gulch near Rockerville is a small tribu- tary to Spring Creek and is located near the center of heavy rainfall. The discharge obtained from a drainage area of 1.79 sq mi was 1,560 cfs, or a unit discharge of 870 cfs per sq mi. The flow of Spring Creek was also determined where it crosses Highway 328. Representing the runoff from a drainage area of 88.9 sq mi, the peak 36 0000 I I I I I I THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9-10, 1972 1600 0800 I TIME AND DATE OF FLOOD CREST I I T I I I I I I I 3; 1600— h a 2 _ In “I 3 g" m 3 = 5 a .. 2 :1 no ,9 7-1 a g“ 3 I: .E g x In 0 no 'I §§ 1% a a: s e 9 g g g °8°°—gu s2 a: :3 s a B - a? 08 I? S ': S S S > g 5 “3,7, L: to o .9 z > "a 2 o E Sn 8% u .2 .2 .9 5 s a .2 a D-U OH 5 I > w u U) > o u. , II 0000 . I I I I I | I I I I I I I I I I I 92 88 84 80 76 72 68 64 60 56 52 48 44 40 36 32 28 24 20 16 DISTANCE UPSTREAM FROM RIVER MOUTH, IN MILES FIGURE 21.—Timing of flood peak down Rapid Creek. discharge was 14,900 cfs, or a unit discharge of 168 cfs per sq mi. At the point where the creek crosses Highway 16, Spring Creek had a peak discharge of 21,800 cfs from a drainage area of 103 sq mi, or a unit dis- charge of 212 cfs per sq mi. At the gaging station near Hermosa, Spring Creek had spread out into the plains area. Here the stream has a drainage area of 199 sq mi and the discharge had attenuated to 13,400 cfs, a unit discharge of 67 cfs per sq mi. ELK CREEK BASIN The Elk Creek basin, draining eastward from the Black Hills, is one of the larger basins in the flood area. The basin did not receive the heaviest rainfall and consequently did not have as much runofl" as some of the streams farther south. Flooding in this basin was significant, however, damaging creek beds, roads, bridges, and a few ranches. Because only a few people inhabit the basin, property damage was relatively slight. At the point where it leaves the hills, Elk Creek produced a peak discharge of 11,600 cfs. This is a unit discharge of 256 cfs per sq mi. About 15 miles upstream from this site, Elk Creek crosses Highway 385 and was noted to have had practically no flow at the time of the peak. By the time Elk Creek reached Elm Springs, some 40 miles east on the plains, the peak discharge had attenuated to 1,880 cfs (fig. 22). zooo , I T 1800 1600 1400 1200 - Daily discharge June 11:385 cfs June 12:1,370 cfs June 13:515 cfs 1000 800 600 400 200 DISCHARGE. IN CUBIC FEET PER SECOND I 0000 I 1200 June 12 TIME. IN HOURS 1200 June 13 0 0000 1200 0000 Ju ne 1 1 FIGURE 22.-—The peak discharge of Elk Creek at Elm Springs, some 40 miles east of the Black Hills, was 1,880 cfs on June 11. THE FLOOD BEAR BUTTE CREEK The Bear Butte Creek basin, a tributary to the Belle Forche River, was on the northern fringe of the storm area and not in the area of most intense rainfall. A discharge of 19,000 cfs was computed for Bear Butte Creek at a discontinued gaging-station site near Galena, 4 miles southwest of Sturgis. This represents a unit discharge of 398 cfs per sq mi from the 47.6 sq mi basin. A peak discharge of 19,500 cfs was measured at a miscellaneous site just upstream from Sturgis. This represents a unit discharge of 368 cfs per sq mi from 53.0 sq mi. At the Bear Butte gage site downstream from and 12.5 miles northeast of Sturgis, the discharge was only 7,220 cfs (fig. 23). This latter site includes the flow of Deadman Gulch. Deadman Gulch, tributary to Bear Butte Creek at Sturgis, carried considerable runoff—enough to over— flow its banks in the city and to discharge water down many of the streets of Sturgis. Basements were flooded in a few houses. A discharge of 4,740 cfs was measured in Deadman Gulch just upstream from Sturgis, giving a unit discharge of 797 cfs per sq mi from 5.95 sq mi. The fill under the bridge of 8 I I I I I I I I I I I O E 7 _ Peak=11.32 ft 7,220 cfs _ In (I) E m 6 I_ _ ’E u: L 9 5— — m :a 0 Daily discharge ‘5 4 _ June 10:2,940 cfs <0 June 11:806 cfs _ D Z 5‘: D 3 — O I ’— E 2 _ LII 0 n: < 5 1— ‘L’ D O I I I I I I I I l I I 0000 0800 1600 0000 0800 1 600 0000 June 10 TIME, IN HOURS June 11 FIGURE 23.—At the gaging station 12.5 miles northeast of Sturgis, the peak discharge of Bear Butte Creek had atten- uated to 7 ,220 cfs about equal to a 40-year flood. 37 the interstate highway scoured badly but traffic was not interrupted and the bridge structure was not damaged. Elsewhere in Sturgis, traffic was delayed for a few hours in places, but no major damage was done. In the same vicinity, Vanocker Creek had significant runoff but was not as high as Bear Butte Creek. CHEYENNE RIVER All the flooding covered in this report lies within the Cheyenne River basin, and all discharge from the flood, except for losses through evaporation and seepage, eventually ended up in the Cheyenne River. The water was stored in vast overflow areas as it reached the plains, however, and the peaks were diminished considerably. As a result, the peak dis— charge of the Cheyenne River at. the gaging station near Wasta, S. Dak., 43 miles east of Rapid City, was only 11,800 cfs (fig. 24). Furthermore, the peak was broad, hardly pushing the Cheyenne River out of the main channel banks at the peak stage. FLOOD VOLUMES The floods in the Rapid City area struck quickly and viciously but did not last long. Flood peaks were sharp. The stage and discharge of Rapid Creek at Rapid City rose quickly, and the water overtopped 212 I I I I I I I I I I I Peak=8.19 ft 11,800 cfs ... ._. I I.‘ O I Daily discharge June 10:1,390 cfs June 11:9,240 cfs June 12:5,780 cfs 0 I I I I I I I I I I I 1600 0000 0800 1600 0000 0800 1600 June 10 June 11 June 12 DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECO D cl TIME, IN HOURS FIGURE 24.—By the time it reached Wasta, the Cheyenne River had received peak discharges from most of the streams in the flood area. Yet, because of attenuation, the peak discharge on the Cheyenne was only 11,800 cfs, about equal to the 2-year flood. 38 its banks at about 2215 to 2230 MDT on the 9th. By 0400 MDT to 0500 MDT the next morning, the water was back within the banks of the stream. Because of the short duration of the peak, the total volume of water in the flood was relatively small. For exam- ple, total flow in the Rapid Creek above Canyon Lake was a little more than 9,000 acre-feet during the 2-day period June 9, 10. Total flow for Rapid Creek at Rapid City was a little more than 13,000 acre-feet during the same 2-day period, and Boxelder Creek near Nemo flowed a little more than 19,000 acre-feet. As a basis for comparison, Pactola Reser- voir was about half full and contained some 60,000 acre-feet of water at the time of the flood—about four times the total floodflow of Rapid Creek at Rapid City. After the floodwater flowed downstream from Rapid City, it spread out over a broad flood plain. Because the flood event came and left quite rapidly, ground-water levels were not raised significantly. THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9-10, 1972 Instead, the water was temporarily stored in the flood-plain alluvium. Thus the major effect was to sustain discharges at a higher than normal level. For example, the flow of Rapid Creek at Rapid City 1 week after the flood was about 500 percent higher than the flow 1 week before the flood; 2 weeks after the flood, the flow was still about 400 percent higher. RELATIVE MAGNITUDE Flood stages and discharges for the June floods are summarized for the Rapid City area in table 6. The table also shows previous peaks of record for selected sites. Although the 1972 floods generally ex- ceeded previous peaks of record in the stricken area, floods of greater relative magnitudes have been ex- perienced in the United States (fig. 25). Figure 26 shows the comparison between the 1972 Black Hills floods and previous maximum floods experienced in South Dakota. TABLE 6.—Flood stages and discharges in the Rapid City area, during the June 1972 flood and during previous maximum floods [The first column under “Maximum floods previously known” shows the period of known floods before June 1972. This period may be longer than the actual period of record, because records of historical floods may have been obtained. The last column contains the recurrence interval in years of the June 1972 peak discharge] Maximum floods previously known Maximum June 9—10, 1972 N Stat' St d 1 Drainage G Discharge G Discharge 0 ion ream an p ace Prior to a e ~ _ age ‘ - No. of determination (52113;) June 1972 height 039:? 13:53: Hour height CF62? 1:33;; Period Year (ft) per interval 1 Day (MDT) (ft) per interval 1 second (yrs) second (yrs) 1 4025 Beaver Creek near Buffalo Gap (4.5 130 1937—72 1938 16.46 11,700 >100 10 0100 7.09 829 4 mi upstream from mouth). 1927 18.0 _____ 2 4026 Cheyenne River near Buffalo Gap 9,810 1968—72 1971 11.44 17,600 10 10 0700 7.24 2,670 <2 (12 mi east of Buffalo Gap). 3 _--_ Battle Creek at Keystone _________ 13.6 ____ ___- ___ 9 2130 ___- 10,800 >100 4 ___- Grizzly Bear Creek near Keystone 9.22 ___- ___- ___ 9 2130 ——-— 5.230 >100 (tributary to Battle Creek). 5 4038 Battle Creek tributary near Keystone .88 1956—72 1967 5.41 16 ___ 9 2100 18.8 1,330 >100 (2.8 mi northwest of Keystone). 6 4040 Battle Creek near Keystone (15 mi 66 1945—47 1965 3.71 718 4.5 9 2200 14.5 26,200 >100 southwest of Rapid City). 1961—72 7 ___- Battle Creek at canyon mouth near 110 _______ __-_ ___- ..... _-- 9 2245 ---- 44,100 >100 Hermosa (2.2 mi southwest of Hermosa). 8 4050 Grace Coolidge Greek near Custer 25.3 1945—47 1947 4.50 206 2.0 10 0245 3.64 709 7 (11.5 mi east of Custer). 1967—72 9 4060 Battle Creek at Hermosa (0.8 mi 178 1949—72 1952 14.00 2,950 12 9 2300 17.72 21,400 >100 south of Hermosa). 10 4068 Newton Fork near Hill City (3.9 mi 28.25 1969-72 1971 3.62 25 2 9 2145 3.51 21 2 northwest of Hill City). 11 4069 Palmer Creek at Hill City (3 mi 8.24 1956—72 1962 7.55 2350 >100 9 ___- 17.06 4,370 >100 east of Hill City). 12 ___- Spring Creek at entrance to Sheridan 58.0 _______ ___- ___- ..... ___ 9 2400 ———- 5.630 >100 a e. 13 4069.5 Hirse Creek at entrance to Sheridan 10.1 ___- ___- _____ ___ 9 2400 6-05 1:830 >100 ake. 14 .0. Spring Creek below Sheridan Lake 88.9 ____ ____ _____ ___ 10 0033 ___- 14,900 >100 at Highway 328. 15 ___- Rockerville Gulch at Rockerville ___- 1.79 -___ ____ _____ ___ 9 2300 ___- 1.660 >100 16 ___. Spring Creek at Highway 16 near 103 ___- ___- _____ ___ 10 0200 ---- 21.800 >100 léoclgerville (8 mi south of Rapid ity . 17 4085 Spring Creek near Hermosa (17 mi 199 1949—72 1967 5.49 772 6.2 10 0300 13.12 13,400 >100 south of Rapid City). 18 4088.5 Silver Creek near Rochford (1.1 mi 26.20 1969—72 1969 4.17 13 2 10 0645 3.49 23.3 <2 east of Rochford). 19 4089 Heeley Creek near Hill City (13.5 mi 4.86 1969—72 1969 ___- 15 2 10 0700 2.96 1 <2 northwest of Hill City). 20 4090 Castle Creek above Deerfield Reser- 83 1948—72 1952 5.81 1,120 45 10 0430 2.14 19 <2 voir near Hill City (30 mi west of Rapid City). 21 4100 Castle Creek below Deerfield Dam (28 96 1946—72 1952 3.87 200 10 ___- nu . 18 <2 mi west of Rapid City). (daily) (daily) 22 4105 Rapid Creek above Pactola Reservoir 292 1953—72 1965 10.44 2,060 20 10 0905 5.74 228 <2 at Silver City (17 mi west of Rapid City). 23 4115 Rapid C'reek below Pactola Dam (13 31 1332—373 1952 46.74 2.170 ___ 9 2030 8-52 378 --- mi west of Rapid City). — 24 ___- Rapid Creek at Highway 40 (3 mi 8.35 ....... ___- ___- ..... -—- 9 2245 —-~- 5350 >100 west from Rapid City). See footnotes at end of table. THE DESTRUCTION 39 TABLE 6,—Flood'stages and discharges in the Rapid City, South Dakota, area during the June 1972 flood and during previous maximum floods—Continued Maximum floods previously knowu Maximum June 9—10, 1972 N Stat' St d 1 Drainage P’ to G Discmrge G Discharge o. 1011 ream an p ace nor age _ a e - _ No. of determination are“. June 1972 height Cf‘n’” Rem" Hour heigght 0“” Rec“ (sq m1) . eet rence feet renoe Penod Year (ft per intervall Day (MDT) (ft) per intervall second (YI‘S) second (yrs) 25 __-- Deer Creek at Deer Creek camp- 4.28 ....... .-.- -___ ..... .-_ 9 2030 -___ 8,530 >100 ground (8 mi west of Rapid City). 26 ____ Victoria Creek at Victoria Dam near 6.71 ....... -.-.. _--- _____ --- 9 2100 ---- 6,860 >100 Rapid City. 27 4125 Rapid Creek above Canyon Lake 3 52 1946—72 1952 8.08 2,600 24 9 2315 15.77 31,200 >100 near Rapid City. 28 -___ Cleghorn Canyon at Rapid City (ones 6.95 1962 ____ 2,920 >100 9 2200 ___- 12,600 >100 half mi from mouth). 29 __-_ Lime Creek at 36th and West Main 2.51 ____ ____ _____ __- 9 2300 ____ 481 >100 in RapidC ity. 30 4140 Rapid Creek at Rapid City ........ 391 13403-06 1962 13.27 3,300 36 9 2400 15.45 50,000 >100 1 —72 1920 ..... 31 4215 Rapid Creek near Farmingdale (20 3283 1946—72 1947 8.4 2,640 12 10 1330 11.85 7,320 >100 mi southeast of Rapid City). 32 4217.5 Rapid Creek tributary near Farm- 1.51 1970—72 1970 3.87 35 210 10 __._ _-_- 1 <2 ingdale (3.8 mi southeast of Farm- ingdale). 33 4223.95 Boxelder Creek at Benchmark near 37.2 _______ _--- ____ ..... ___ 9 2100 ___- 1,180 7 Nemo (31/2 mi northwest of Nemo). 34 4224 Estes Creek near Nemo (1.6 mi 6.14 1969—72 1970 5.41 (5) -__ 9 2200 (0) 6,620 >100 southeast of Nemo). 35 4225 Boxelder Creek near Nemo (12 mi 96 1945—47 1946 45.75 1,180 4 9 2230 20.4 30,100 >100 northwest of Rapid City). 1966—72 1911 14 ..... 36 ____ Boxelder Creek at Nemo road near 117 ....... -_..- ....-_ _____ ..-_ 10 0030 ____ 51,600 >100 Rapid City (8 mi northwest of Rapid City). 37 4232.5 Boxelder Creek tributary at New .14 1970—72 ____ ____ 1 ___ 10 _-_- ____ 3 <2 Underwood (0.1 mi west of New Underwood). 38 4235 Cheyenne River near Waste (43 mi 12,800 1914—15 1932 12.28 46,300 20 11 1330 8.19 11,800 2 east of Rapid City). 1928—32 1915 13.5 _____ 1934—72 1920 17 ..... 39 _--_ Elk Creek at canyon mouth near 45.5 _______ .--- --_- ..... -.... 9 2400 ____ 11,600 >100 Piedmont (4 mi northwest of Pied— mont). 40 -___ Stagebarn Canyon near Piedmont (2 16.8 ....... ---- -.-— ..... ..-- 10 1245 ____ 4,100 >100 mi southeast of Piedmont). 41 4255 Elk Creek near Elm Springs (38 mi 540 1949—72 1952 410.61 8,540 12 11 1100 8.70 1,880 2 northeast of Rapid City). 1962 11.0 _____ 1920 4 17 ..... 42 4370 Belle Fourche River near Sturgis (20 5,870 1945—72 1946 13.86 17,900 8 10 1700 10.56 7,060 <2 mi northeast of Sturgis).1962 14.32 _____ 43 4371 Boulder Creek near Deadwood (3.5 1.69 1956—72 1962 8.46 210 15 9 2215 5.59 59 <2 mi east of Deadwood). 44 4372 Bear Butte Creek near Galena (dis- 47.6 1965—69 1965 861 4,950 >100 9 11.78 19,000 >100 continued gaging station, 4 mi southwest of Sturgis). 45 _--_ Begr Butte Creek at Interstate 90 at 53.0 ....... --_- -_-- ..... --- 9 -.-- 19,500 >100 turgis. 46 --__ Deéadman Gulch at Interstate 90 at 5.95 _______ __-- _..__ _____ --_ 9 ____ 4,740 >100 turgis. 47 4375 Bear Butte Creek near Sturgis (28 192 1945—72 1962 412 45 12,700 >100 10 0330 11.32 7,220 40 mi north of Rapid City). 48 4380 Belle Fourche River near Elm 7,210 1928—32 1964 15.90 45,100 45 11 0600 8.62 9,810 3 Springs (4.3 mi northwest of Elm 1934—72 1927 21.8 _____ Springs). 1933 20 _____ 49 4385 Cheyenne River near Plainview (10 21,600 1950—72 1957 e--- 41,700 (7) 11 2230 9.44 14,600 <2 mi southeast of Plainview). 1965 11.68 _____ 1920 17.5 _____ 1927 14 _____ 1From Patterson (1966) and Hardison (1973). 2 Approximate. 3 Contributing area downstream from Pactola Dam. ‘Datum then in use. 5 Discharge not determined. 9 Datum destroyed by flood. 7 Regulated. THE DESTRUCTION Just as we have tried to describe the storm and flood in terms of physical facts and numbers, we can try to add another dimension to the flood by describ- ing the death and destruction inflicted in the Rapid City area the night of June 9, 1972. To the scientific facts that describe the magnitude and rarity of this natural disaster, we can add the human effect of the flood: the number of deaths and the destruction, the words of the witnesses, and photographs of the aftermath. THE VICTIMS According to the American Red Cross, 237 people died in the Black Hills floods. Another 3,057 people were injured, including 118 who needed hospital care. Perhaps more startling, 6 months after the flood, eight people were still listed as missing. Based on incomplete newspaper descriptions, the dead ranged in age from 3 months to 94 years. About 25 percent of the Victims were over 60 years old, 15 percent were under 13, and 30 percent were under 30. Some 10 percent of those who died in the flood 40 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 100,000 C 5 a ° ° / m 10,000 — / ° _ a: / ii: / 0 . o // . 0 L1 0 , / Lu y / “- o . o /. o E: o / // a / O . // z :— / o /// g o o / . < o / E, o . EXPLANATION m D 1000 — —— Maximum known floods in U.S. by 1965 _‘ a: (Matthai, 1969) Lu 9- —- Maximum known floods In U.S. by 1950 (Hoyt and Langbien, 1955) ___.__.— Maximum known flood in U.S. by 1890 (Creator, Justin, and Hinds, 1950) 0 Maximum flood—Black Hills 1972 0 Maximum flood-other parts of the United States 0.] 1.0 10 100 1000 CONTRIBUTING DRAINAGE AREA, IN SQUARE MILES FIGURE 25.—Comparison of the Black Hills floods with maximum floods previously determined in the United States. were tourists from 13 States and from France en- joying the Wild beauty of the Black Hills. THE DAMAGE Total damage in the Black Hills flood is estimated (written c-ommun., U.S. Army Corps of Engineers) to exceed $160 million (table 7). The National Flood Insurance Association reported that 29 flood insur- ance policies were in effect in the area, covering less than $300,000 in damage, or about one-half of 1 percent of the damage to homes and businesses. The Red Cross counted an overall total of 770 permanent homes and 565 mobile homes destroyed in the flood; an additional 2,035 permanent homes and 785 mobile homes suffered major damage (fig. 27). The Soil Conservation Service estimates 5,000 automobiles were destroyed in the flood, and the Fed— eral Highway Administration reports that highway damage was severe. Within the Federally aided high— way system, major repairs were required at about 160 locations, including the replacement of washed- out bridges and the removal of accumulations of debris. (See fig. 28.) THE DESTRUCTION 41 1,000,000 I I T EXPLANATION 0 Previous peaks in South Dakota 2 O 1972 Black Hills flood peak 0 0 I.” w 5 100,000P ‘1 n. o t: E o 0 g D 2 . o O 8 O O 0 g 0 o o o o o 0 0° 0 E. ' o o o m o o o o c: 0 o z: . ° 0 8 ° 0 ‘6 o < . . 0 O 5 10,000 — — ‘2 o o 0 0° 9 o t ' 0 ° 0 o x . O o O o 0 5 ° ° 6. o o o 0 o o o o o . o o . 0 1000 I I ° I I (11 1.0 10 100 1000 10,000 DRAINAGE AREA, IN SQUARE MILES FIGURE 26.——Comparison of the Black Hills flood peaks with maximum floods previously determined in South Dakota. TABLE 7.——Damage estimates for the Black Hills flood [Furnished by the U.S. Army Corps of Engineers] Urban damages: Residential damage: Rapid City ___________ $ 35,095,000 Boxelder ______________ 1,158,000 Sturgis _______________ 1,247,000 Keystone _____________ 137,000 Commercial and industrial damage: Rapid City ____________ $ 30,881,000 Boxelder ______________ 75,000 Sturgis _______________ 213,000 Keystone ______________ 1,499 ,000 Utilities ______________________________ Associated economic losses ______________ Total urban damages _________________ Rural damages ____________________________ Transportation damages ___________________ Secondary economic losses: Estimated loss in tourist income ______________;___ $ 30,000,000 Estimated interest costs for reconstruction losses ______ 6,3 64,000 Total secondary economic losses _______ Estimated total losses ________________ $ 37,637,000 $ 32,668,000 $ 10,338,000 6,341,600 $ 86,984,600 $ 6,217,800 $ 35,380,400 $ 36,364,000 $164,946,800 THE WITNESSES Hundreds of stories of horror and devastation were related by witnesses who survived the floods. Most of the following quotes were first reported by the “Rapid City Journal” as that paper recorded eye- witness accounts of the destruction of the commu- nity. Some of the eyewitnesses had been warned by friends and officials of the flood danger and had escaped to higher ground. Many others heard the warnings but did not heed them: When we heard the warning, we thought he (the Mayor) was kidding. We just sat there, and pretty soon this big bunch of water came down the creek. We ran next door and suddenly the water was up to my neck. The top of a house came float- ing by and we grabbed on to that. A little way downstream we got ofl" and climbed on the roof of a neighbor’s house. We stayed there until the flood began to fall on Saturday. Some were swept away, trying to struggle to- safety; others were more fortunate: The people were crying and screaming for help. And you could just see the people floating around, just before your eyes. Some going under . . . some floating in a car or in a house . . . it’s just terrible. A car floated down the main street where we live, so we tried to make it on foot toward a hill and couldn’t make it. My husband almost floated away, but 42 THE BLACK HILLS—RAPID CITY FLOOD, JUNE 9—10, 1972 FIGURE 27.——One of 770 permanent homes and one of 565 mobile homes destroyed in the flood. The homes have been searched (S) for victims and condemned (X). (American Red CrOSs photographs.) THE DESTRUCTION 43 FIGURE 28.—The roads, the bridges, and the cars they served suffered much of the damage caused by the Black Hills floods. Chapel Lane, now bridgeless, ends suddenly at the entrance to Canyon Lake. The cars were stacked by the force of the water as it pushed through Rapid City. (“Rapid City J ournal” photograhs.) (Continued on following page.) the children grabbed him. We went back to the house and the water came up to our necks. We just stood there until the water came down to our waists and we were rescued. Many of the victims, campers in the Black Hills area near the streams, were taken by surprise by the sudden increase in discharge and were swept away in their sleep or before they could reach high ground: A little before 11 p.m., Tom heard water coming in the cabin. He woke us all up. We couldn’t open the cabin door to get out because of water outside. I kicked out a window and right then a car smashed into it. We all grabbed a mattress in the one room in the cabin and floated in the water—it was four or five feet deep—and the cabin started floating downstream. It went at least a mile and then one wall of the cabin broke away from the rest of it. I’d given myself up for dead, I thought this was it. And for three of the six companions of this boy, it was. Many people were found the next morning hang- ing onto trees, poles, wires, or anything else that they could grab: I looked out the Window and I saw him in the flashes of lightning. He was caught in a tree over there. He was a brave youngster, only about 10. I kept hollering at him to hold on and climb higher. He was saved several hours later. Miraculous and sad tales were told of how invalids were saved or lost. One 71-year-old lady kept her invalid granddaughter alive by holding her on a mattress that floated on the water in the room: When the water reached my chest, I thought “if it goes any higher, this will be it.” I was so afraid she would tip and roll off, so I stood right beside the mattress, balancing *** it was all so slimy. Vicki simply couldn’t do it (climb up on the roof). Besides, I didn’t think taking her up on the roof in the dark and the pouring rain would be any safer for her. This is but one of many heroic deeds performed this unforgettable night. Many stories were yet to be told, and just as many would never be told (fig. 29). 44 THE BLACK HILLS-RAPID CITY FLOOD, JUNE 9—10, 1972 Figure 28,—Continued SELECTED REFERENCES 45 FIGURE 29.—Ironically, one of the major problems that faced the survivors of the Rapid City flood was a lack of adequate drinking water. Truly a case of “water, water everywhere, but not a drop to drink.” (American Red Cross photograph.) SELECTED REFERENCES Creager, W. P., Justin, J. D., and Hinds, Julian, 1950, Engi- neering for dams: New York, John Wiley and Sons, Inc., v. 1, p. 101—127. Hardison, C. H., 1973, Estimation of 100-year flood magni- tudes at ungaged sites: U.S. Geol. Survey open-file report, 10 p. Hoyt, W. G., and Langbein, W. B., 1955, Floods: Princeton ‘ Univ. Press, 469 p. Jennings, A. H., 1952, Maximum 24-hour precipitation in the United States: U.S. Weather Bur. Tech. Paper 16, 284 p. Larimer, O. J., 1973, Flood of June 9—10, 1972, at Rapid City, South Dakota: U.S. Geol. Survey Hydrol. Atlas HA—511. Matthai, H. F., 1969, Floods of June 1965 in South Platte River basin, Colorado: U.S. Geol. Survey Water—Supply Paper 1850—B, 64 p. Patterson, J. L., 1966, Magnitude and frequency of floods in the United States; Part 6—A, Missouri River Basin above Sioux City, Iowa: U.S. Geol. Survey Water-Supply Paper 1679, 471 p. Riedel, J. T., Appleby, J. F., and Schloemer, R. W., 1956, Seasonal variation of the probable maximum precipita- tion east of the 105th meridian for areas from 10 to 1000 square miles and durations of 6, 12, 24, and 48 hours: Hydrometeorological Report No. 33, 58 p. Sangster, W. E., 1960, A method of representing the horizon- tal pressure force without reduction of station pressures to sea level: Jour. Meteorology, v. 17, no. 4, p. 166—176. Schwarz, F. K., 1970, The unprecedented rains in Virginia associated with the remnants of Hurricane Camille: Monthly Weather Rev., v. 98, no. 11, p. 851—859. U.S. Army Corps of Engineers, 1945, Storm rainfall in the United States: U.S. Army Corps of Engineers [rept.]. U.S. Department of Commerce, 1968, Climatic atlas of the United States, U.S. Govt. Printing Office, 80 p. U.S. Geological Survey, 1970, National atlas of the United States: Washington, D.C., U.S. Govt. Printing Office, 417 p. U.S. National Weather Service, 1972, Precipitation frequency atlas of western United States: Weather Bur. Tech. Paper 16, 284 p. Page Areal extent of flooding _______________ 21 Areas of moderate rainfall ________ 33, 35, 36 Atmospheric conditions, surface weather features __________________ 10 upper air _________________________ 5 Battle Creek _________________________ .94 Battle Creek near Hermosa - 35 Bear Butte Creek .77 Belle Forche River 37 Bibliography __________________________ 45 Boxelder Creek _______________________ 30 Boxelder Creek near Nemo __ _ 31 Bucket surveys of rainfall ___________ 4, 5 Camille storm (Virginia) of August 19—20, 1969 _______________ 5, 18 Canyon Lake Reservoir _______________ 22, 29 collapse of dam, effect on Rapid Creek flood wave _________ 30 Cherry Creek, 0010., storm of May 30—31, 1935 ______________ 4, 5 Cheyenne River .97 Cleghorn Canyon 30 Comparisons with previous floods in area _____________________ 29, 8.? Conversion factors for units of measure- ment ..................... 3 Damage ______________________________ 40 Deadman Gulch _______________________ 37 Deer Creek ___________________________ 22 Dewpoints. adjustment to 1,000-mb level 13 Drainage area, Bear Butte Creek _____ 37 below Pactola Dam _______________ 30 Boxelder Creek ________________ 31, 33, 34 Deadman Gulch __________________ 37 Deer Creek ______ _ 22 Estes Creek ______________________ 3 Grace Coolidge Creek _____________ 35 Palmer Creek ____________________ 35 Rapid Creek ______________________ 22 Rockerville Gulch _____________ __ 35 small areas, data on discharge --_- 34 Spring Creek ..................... 35 Victoria Creek ____________________ 22 850-mb upper-air analysis ____________ 10 Elk Creek basin -- -_ -_ ._ __ 86 Elm Springs ___________ __ 36 Estes Creek near Nemo ______________ 33 Eyewitness accounts __________________ 41 Farmingdale __________________________ 30 500-mb upper-air analysis _____________ 5 Flood-frequency relations _____________ 38 Flood peaks, timing __________________ 21 See Peak discharges. Flood profiles, distance surveyed ______ 1,30 Flood volumes ________________________ 87 INDEX [Italic page numbers indicate major references] Page Gage height, Rapid Creek _____________ 29, 39 Galena _______________________________ 37 Geostrophic winds ___- _ 10, 11, 13 Grace Coolidge Creek - -- 35 Grizzly Bear Creek --_- -- 34 Ground-water levels __________________ 38 Hermosa __________________________ 34, 35, 36 Hill City ___ __ 35 Horse Creek _- 35 Howard ______________________________ 4 Hydrologic conditions immediately prior to the flood event ________ 3 Isohyetal map of storm _______________ 19 Keystone _____________________________ 34 Maximum areal storm rainfall amounts (depth-area—duration data) 5, 19 storm center values ____________ 4, 15, 19 Mesosystem, importance of in causing rains _____________________ 18 Meteorologic setting ___________________ 3 Middle Fork, North Fork, South Fork, Boxelder Creek, discharge” 33 Monthly precipitation, distribution of yearly average ____________ 3 Mount Rushmore National Memorial ___ 34 Newton Fork _________________________ 35 1,000-mb dewpoints on June 9, hourly trend _____________________ 13 Orographic effects, role in storm__ 3, 5, 11, 13 Pactola Reservoir, rainfall in area im- mediately prior to date of flood _____________________ 3 role as flood-control measure in Rapid City flooding ______ 22 Palmer Creek _________________________ 35 Peak discharges, attenuation of __ 30, 35,36 Battle Creek (six sites) ________ 34 Bear Butte Creek __ __ ___- 37 Boxelder Creek --_ - 31, 33, 34 Cheyenne River __________________ 37 Cleghorn Canyon _________________ 30 Deadman Gulch _____ -- 37 Deer Creek _- -_ 22 Elk Creek _____________ __ 36 Estes Creek near Nemo ___________ 33 Farmingdale ______________________ 30 Grace Coolidge Creek - -- 35 Horse Creek ___________ _- 35 indirect measurements - __ 30 measurement number _____________ 1 Palmer Creek _____________________ 35 Prairie Creek _____________________ 22 * U.S. GOVERNMENT PRINTING OFFICE: 1975 0—555—475/74 Peak discharges—Continued previous peaks of record - Rapid Creek ______________ speed ___________________________ Rockerville Gulch Spring Creek Victoria Creek ____________________ Prairie Creek Precipitation averages and extremes--- Precipitation measurements, number -- Previous storms, June 1907 ___________ April 17, 1920 ____________________ July 13, 1962 Probable maximum precipitation ______ Radar images of storm activity _______ Radiosonde observations Rainfallofrequency data ............... Rapid Creek ___________ _- Reausau Lake Recurrence intervals __ Rockerville Gulch Satellite images of continental and regional weather patterns-. Severity of the storm, percent of pvrob- able maximum precipitation Sheridan Lake ________________________ 6-hour 100-yr rainfall-frequency maps-- 6-hr precipitation depths, for period of record (12 yr) for storm of June 9—10, 1972 _____ Smethport, Pa., storm of July 17—18, 1942 ______________________ Soil, saturation state, flood concentration time Spearfish Spring Creek ________________________ Stanton, Nebr, storm of June 11, 1944-- Storm-moisture analysis ______________ Sturgis _______________________________ Surface front, positioning of June 9-- The building storm ___________________ The destruction The flood _____________________________ Thunderstorm activity ________________ early convective cell development--- latent heat of condensation ________ 24-hr precipitation depths ____________ Vanocker Creek Variation of rainfall with time - Victims Victotria Creek Victoria Lake dam __ Vorticity systems --------------------- Yearly precipitation, averages for Black Hills and surrounding plains 47 13, 70A! P RTH 5r ,NCES LIBRARY, \ 9g? . , P} Evaluatlon of Ground-Water Degradatlon "f ? Resulting from Waste Disposal to Alluvium near Barstow, California GEOLOGICAL SURVEY PROFESSIONAL PAPER 878 Prepared in cooperation with the U .S . Marine Corps and the California Water Resources Control Board 31%" . ...__ Fiwcuwmwrs DEPARTMENT] OCT 1 5 1975 ‘ ‘ LIUKAR ’ liflkirwrciumm f 1‘ Ana 2 7 1975 U.S.S.D. Evaluation of Ground-Water Degradation Resulting from Waste Disposal to Alluvium near Barstow, California By JERRY L. HUGHES GEOLOGICAL SURVEY PROFESSIONAL PAPER 878 Prepared in cooperation with the US. Marine Corps and the California Water Resources Control Board UNITED STATES GOVERNMENT PRINTING OFFICE,WASHINGTON:1975 UNITED STATES DEPARTMENT OF THE INTERIOR STANLEY K. HATHAWAY, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Hughes, J. L. Evaluation of ground-water degradation resulting from waste disposal to alluvium near Barstow, California. (Geological Survey Professional Paper 878) Bibliography: p. 33 Supt. of Docs. No.: I 19.16z878 1. Water, UndergroundeollutionVBarstow region, Calif. 1. United States. Marine Corps. II. California. State Water Resources Control Board. 111. Title: Evaluation ofground-water degradation... IV. Series: United States. Geological Survey. Professional Paper 878. TD224.C3H83 363.6’1 75—619167 For sale by the Superintendent of Documents, US Government Printing Office Washington, DC. 20402 Stock Number 024-001—02694—5 CONTENTS Page Page Abstract __________________________________________________ 1 Sources of ground-water degradation—Continued Introduction ________________________________________________ 1 Industrial and domestic waste—Continued Geographic setting ______________________________________ 3 US. Marine Corps Supply Center, Nebo ______________ 14 Well-numbering system ________________________________ 3 Mining and milling operations ______________________ 14 Water-quality definitions and standards __________________ 4 Irrigation return ________________________________________ 14 Methods of investigation ________________________________ 5 Distribution and evaluation of chemical substances affecting Field methods ______________________________________ 5 ground-water degradation ____________________________ 15 Analytical methods ________________________________ 6 ' Dissolved solids ________________________________________ 2O Geohydrology ______________________________________________ 7 Detergents (as MBAS) __________________________________ 22 Ground-water geology __________________________________ 7 Dissolved organic carbon ________________________________ 26 Ground-water hydrology ________________________________ 7 Nitrogen ______________________________________________ 26 Sources of ground-water degradation ________________________ 9 Chloride. ______________________________________________ 29 Natural ________________________________________________ 9 Effect of pumpage on water quality at the USMC Supply Center 29 Industrial and domestic waste __________________________ 9 Rate of ground-water movement ____________________________ 32 City of Barstow ____________________________________ 9 Summary __________________________________________________ 32 Atchison, Topeka and Santa Fe Railway ______________ 10 Selected references __________________________________________ 33 ILLUSTRATIONS Page FIGURE 1. Index maps showing location of project area ______________________________________________________________________ 2 2. Graphs showing comparison of streamflow in Mojave River at Barstow with water levels in well 9N/1E—18E1 ____________ 4 3. Map showing geology, waste~disposal sites, wells used to sample ground-water quality, and water-level contours for 1945 and spring 1972 ____________________________________________________________________________________________ 8 4. Schematic diagram showing water use in project area ______________________________________________________________ 11 5. Graphs showing change in concentration of selected chemical constituents in wells 9N/1W—9H5,—9H7, and —10J3 ________ 21 6. Sections showing vertical distribution of dissolved solids __________________________________________________________ 23 7. Map showing areal distribution of DOC (dissolved organic carbon) and detergents (as MBAS) _________________________ 24 8-11. Sections showing vertical distribution of: 8. Detergents (as MBAS) ____________________________________________________________________________________ 25 9. Dissolved organic carbon __________________________________________________________________________________ 27 10. Nitrogen ________________________________________________________________________________________________ 28 11. Chloride ________________________________________________________________________________________________ 30 12. Graphs showing fluctuation of dissolved solids in well 9N/1W—13E1 (Nebo 4) and water-supply demands at the Marine Corps Supply Center (N ebo) __________________________________________________________________________________ 31 13. Map showing area affected by a 96-hour aquifer test at US. Marine Corps Supply Center ____________________________ 32 TABLES Page TABLE 1. Drinking—water standards ________________________________________________________________________________________ 5 2. Water budget __________________________________________________________________________________________________ 10 3. Chemical analyses of the domestic wastes of the city of Barstow ____________________________________________________ 12 4. Chemical analyses of the industrial wastes of the Atchison, Topeka and Santa Fe Railway __________________________ 16 5. Chemical analyses of the domestic and industrial wastes of the US. Marine Corps Supply Center (Nebo) __________________ 18 6. Chemical analyses of mining wastes ______________________________________________________________________________ 20 7. Comparison of water quality in shallow and deep test wells at USMC Supply Center golf course ______________________ 22 8. Data on USMC Supply Center wells (Nebo) ______________________________________________________________________ 31 In IV CONTENTS CONVERSION FACTORS Factors for converting English units to the International System of Units (SI) are given below to four significant figures. However, in the text the metric equivalents are shown only to the number of significant figures consis- tent with the values for the English units. English Multiply by Metric (SI) acre 4.047 X 10‘1 ha (hectare) acre-ft (acre-foot) 1.233 X 10'3 hm3 (cubic hectometre) acre-ft (acre-foot) 1.233 X 103 m3 (cubic metre) ft (foot) 3.048 X 101 cm (centimetre) ft (foot) 3.048 X 10—1 m (metre) ft/mi (feet per mile) 1.890 X 10—1 m/km (metre per kilometre) gal (gallon) 3.785 X 10—3 m3 (cubic metre) gal/min (gallon per minute) 5.451 m3/d (cubic metre per day) in. (inch) 2.540 cm (centimetre) Mgal/d (million gallons per day) 3.785 X 103 m3/d (cubic metre per day) mi (mile) 1.609 km (kilometre) mi2 (square mile) 2.590 km2 (square kilometre) EVALUATION OF GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL TO ALLUVIUM NEAR BARSTOW, CALIFORNIA By JERRY L. HUGHES ABSTRACT Part of the alluvial aquifer along the Mojave River near Barstow, Calif, has been subjected to pollution from percolation of industrial wastes and municipal sewage for nearly 60 years. Effluent discharges have contained high concentrations of detergents, nitrogen, chromium, oil and grease, phosphates, phenols, and other organic and inorganic chemical substances. The poor quality ground water resulting from the discharge of these wastes has forced abandonment of several domestic wells because of taste, odor, and foaming and is endangering a well field serving the U.S. Marine Corps Supply Center at Nebo. The nature and occurrence of the degraded ground water, which is moving in very permeable river-channel deposits at an estimated rate of 1.0—1.5 ft (30—45 cm) per day, is described and outlined both areally and vertically. The concentration of dissolved solids, detergents, dissolved organic carbon, total nitrogen, and chloride were studied in three dimensions. The distribution of chemical constituents in the ground water indicates that a plume of degraded water, produced by percolation from abandoned waste-disposal facilities near Barstow, is moving near the base of the aquifer. Since 1910 this degraded plume has moved downgradient about 4 mi (6.4 km). A more recent overlying plume of degraded water occurs near the downstream edge of the deeper plume. This overlying plume is produced by percolation from sewage-treatment facilities installed in 1968. Concentrations of detergents in ground water beneath this waste—disposal facility reflect the current use of readily biodegradable linear alkylate sulfonate type detergents, in contrast to the nonbiodegradable alkyl benzene sulfonate types in the deeper plume. The concentration and distribution of nitrogen and chloride in ground water in the vicinity of the US. Marine Corps golf course suggest that the gradual increase in dissolved solids in the Marine Corps wells is in part due to the use of treated sewage effluent on the golf course. Areal and vertical mapping of the degraded water indicates that the water supply at the Marine Corps Supply Center will also be affected by the degraded water in the river-channel deposits if no corrective measures are taken. INTRODUCTION In many arid regions ground water in alluvial aquifers is commonly the only reliable source for man’s water requirements. These aquifers have also consti- tuted a convenient “out of sight—out of mind” medium for disposal of municipal and industrial wastes. These practices may effectively meet immediate water-supply and disposal requirements but later they can produce a legacy of ground—water quality problems that must be faced not only by future generations, but, in some cases, by the same entities and individuals responsible for the degradation. Ground-water degradation in the alluvial aquifer near Barstow is endangering the well field that is the source of water for the USMC (US. Marine Corps) Supply Center at Nebo. Water managers in the area have been aware that a ground-water quality problem existed, but they had no adequate data for implement- ing effective management practices designed to al- leviate the problem. The chemical nature, the horizon- tal and vertical extent of the degraded plume, and the dynamic characteristics of the hydrologic system must be understood before proper corrective measures can be taken. In June 1971 the US. Geological Survey began an investigation of ground-water degradation near Barstow, Calif. The project area is in the lower and middle Mojave River basin, San Bernardino County, about 95 mi (155 km) northeast of Los Angeles. The general study area is about 200 mi2 (520 km2) and includes the reach of the Mojave River from above Barstow to below the USMC Supply Center at Nebo (fig. 1). The Mojave River at Barstow is dry except during periods of flooding. The flooding is caused by heavy precipitation on the mountainous areas 50 mi (80 km) to the southwest. Precipitation at Barstow averages about 5 in. (13 cm) per year and produces negligible ground-water recharge. Wells perforated in younger and older alluvial deposits of Holocene and Pleistocene age are the only dependable source of water for the area’s main water users (the city of Barstow and the USMC Supply Center at Nebo). The water supply for the city of Barstow is derived from wells in the younger alluvium north and west of the city where the chemical quality of ground water is generally good. The USMC supply wells are in the younger alluvial aquifer about 4 mi (6.4 km) southeast of Barstow. ’ 2 GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL TO ALLUVIUM, BARSTOW, CALIF. 124° 123° 122° 42114 n W 42° 410 o 40" 31319” R-ZW- 117°oo'R.1w. R.1E. ‘ s E E g 15 m T. 10 N. a a _ E PrOJect area\ ‘0 ‘ __ 34°55] Y O ermo @ . . . ———’ / - ~ 8.1153/ W T. 9 N. Marine Corps Supply Center/ @ (Nebo area) 1 o 2 4 6 MILES / o 2 4 6 KILOMETRES / SACRAMEN’ SSE—,L {V 1 a 2.3 rfi San FrancnscoL 37° 100 MILES O 100 KILOMETRES FIGURE 1.—Location of project area. Since the early 1950’s, the chemical quality of ground water along the Mojave River for about 4 mi (6.4 km) southeast of Barstow has deteriorated significantly. The first recorded complaint of objectionable taste and odor was in 1952. Additional complaints about the quality of the ground water resulted in abandoning several wells. Chemical analyses of water samples from these wells indicated that the ground water in this area had degraded. A series of investigations by the California Department of Public Works (1952), California De- partment of Public Health and California Department of Water Resources (1960), and California Department of Public Health (1966, 1970) indicated that the degradation had resulted from local municipal and industrial waste disposal. The areas of degraded ground water were delineated on the basis of taste, odor, and the presence of detergents. A comparison of the degraded areas in each of the above reports indicated that the degraded ground water was moving downgradient toward the well field at the USMC Supply Center. Should the degraded water appreciably affect the quality of the water pumped at the USMC Supply Center, it would be necessary to treat or obtain water from other sources at considerable cost. The purpose of this study was to identify and describe the areas of degraded ground water and to evaluate the INTRODUCTION 3 potential effect on the quality of water pumped from the well field at the USMC Supply Center. This objective required an investigation of the aquifer system and identification of the chemical nature of the ground water with particular emphasis on the areal and vertical distribution of the degraded water and its rate and direction of movement. The scope of the study included: (1) Collecting hydrologic, geologic, and chemical data from published and unpublished sources; (2) augering sufficient test wells that, when combined with existing irrigation and domestic wells, would provide a network of sampling points encompassing the areas of degraded ground water; (3) collecting and analyzing water samples from selected wells; (4) establishing a monthly program for ground-water quality sampling and water-level mea- surement for selected wells; and (5) performing an aquifer-evaluation test at the USMC Supply Center. Some of the hydrologic interpretation used in this report was provided by S. G. Robson, who concurrently investigated the feasibility of utilizing digital-modeling techniques for studying ground-water quality (Robson, 1974). The model simulates the behavior of dissolved- solids concentration in ground water. The Barstow area, in large part, was selected for the model study because of the abundant hydrologic and chemical data generated by this study. These data are tabulated in a separate report entitled “Selected Data on Wells in the Barstow Area, Mojave River Basin, California” (Hughes and Patridge, 1973). The author thanks the many persons who have kindly given time, information, and guidance during the study and in the preparation of this report. The following agencies and firms provided data and made this report possible: Atchison, Topeka and Santa Fe Railway Co.; Betz Laboratories; Brown and Caldwell, Consulting Engineers; California Department of Public Health; California Department of Water Resources; City of Barstow; Mojave Water Agency; Nalco Chemical Co.; Neste, Brudin, and Stone, Consulting Engineers; San Bernardino County Flood Control District; Southern California Edison 00.; Southern California Water 00.; US. Marine Corps, Nebo Annex; and US. Navy, San Diego. Particular thanks are given to D. L. Patridge, Geological Survey, for his assistance in collecting and organizing the large quantities of data necessary for this report. Other persons in the Geological Survey who contributed time and knowledge in interpretation of data include S. G. Robson, C. 0. Morgan, G. A. Miller, G. G. Ehrlich, and L. A. Eccles. GEOGRAPHIC SETTING The Mojave River is the main source of recharge to local aquifers. The river, which traverses the project area from west to east (fig. 1), originates in the San Bernardino Mountains about 50 mi (80 km) southwest of Barstow and empties into playa lakes about 60 mi (96 km) east of Barstow (Miller, 1969). Tributary inflow to the Mojave River is very small in the reach downstream from the San Bernardino Mountains, and the streambed is dry near Barstow except during infrequent periods of heavy runoff. The climate in the lower and middle Mojave River basin is arid; average annual precipitation is less than 5 in. (13 cm) (data from US. National Weather Service, 1972). The primary source of ground-water recharge to the Mojave River aquifer system is precipitation on and subsequent runoff from the San Bernardino Mountains. Annual precipitation in the San Bernardino Mountains is greater than in any other area of southern California, sometimes exceeding 75 in. (190 cm). During 1969 rainfall at Barstow was 1.0 in. (2.5 cm) in January and 2.2 in. (5.6 cm) in February, yet rainfall in the San Bernardino Mountains was sufficient to produce streamflow at Barstow in excess of 120,000 acre-ft (150 hm3) (Hardt, 1969, p. 5). Excluding the wet year of 1969, records of flow in the Mojave River at Barstow indicate a comparatively dry period since 1948 (fig. 2). During 1932—48, floodflows in the Mojave River provided large quantities of water for ground-water recharge. The frost-free growing season in the Barstow area is about 250 days. Temperatures below freezing often occur between November and March. Midafternoon temperatures during July and August are frequently above 100°F (38°C). In arid and semiarid regions, the quantity of water that evaporates from soil and that is transpired by plants generally is less than the potential evapotranspi- ration because water is not usually readily available. Thornthwaite (1948) devised a method for computing evapotranspiration based on mean monthly tempera- tures and latitude of the area. Hardt (1971, p. 7) compared evapotranspiration computed by the Thornthwaite method with mean monthly precipitatidn for the Mojave River basin and found that precipitation exceeds potential evapotranspiration only during 3 months of the year—December, January, and February. The potential evapotransporation was estimated to be 35 in. (89 cm) per year or about seven times greater than mean annual precipitation. WELL-NUMBERING SYSTEM Wells are numbered according to their location in the rectangular system for subdivision of public land. As shown by the diagram, that part of the number preceding the slash, as in 9N/1W—9B1, indicates the township (T. 9 N.); the number following the slash GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL TO ALLUVIUM, BARSTOW, CALIF. 15° I I I I I I I I I 3' 2‘5 0g”, olt‘ 27,0: I- - Em mg 9: $0100 mg m< ’— I— ,_u. <0 <0 g Lu 3") OI oD d9 _I2 m u.< D 23 50 E0 < mo 0:; 1:1: I“; 5—!- ”’z 0 I I M I l 2°°° I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 61° \Land surface - 605 mid 1980 - [fig 5 E 0.5 mi (0.8 km) below Nebo—USMC Supply Center 5 5 El -' altitude 1996.8fl (609.6 m) _. < 93% — 600 Egg (m (E 3> 1960 30 u.O u. m 0% 0“ _ U) ém 595 8% ,_uJ 3 F“— \~‘\ \\ EE .1; ~~\\ \\ i2 < 1940 \A z \ — 590 \_‘ 1920 l I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 585 1931 1995 1940 1945 1950 1955 1960 1965 1970 1972 YEAR FIGURE 2,—Comparison of streamflow in Mojave River at Barstow with water levels in well 9N/1E—18E1. indicates the range (R. 1 W.); the number following the dash indicates the section (sec. 9); the letter following the section number indicates the 40-acre (16.2-ha) subdivision according to the lettered diagram. The final digit is a serial number for wells in each 40-acre (16.2-ha) subdivision. The area covered by this report lies east and west of the San Bernardino meridian and north of the San Bernardino base line. WATER-QUALITY DEFINITIONS AND STANDARDS Terms commonly used to identify conditions of water quality include degradation, pollution, and contamina- tion. Because these qualitative terms are sometimes considered synonymous, definitions used in this report will generally follow those given in the 1969 Porter- Cologne Water Quality Control Act of the State of California. Briefly, they are: degradation—a general term used to identify water quality that has been impaired for use but not necessarily to a degree that prohibits its beneficial use; pollution—a condition of water quality resulting from use that limits its further beneficial use; contamination—water that is generally considered a hazard to public health because of its content of poisonous constituents or disease-causing organisms. To evaluate the quality of water in the Barstow area, R.2W. R.1W. H.115. R.2E. H.3E. T. 12 N. (Z) 5; 3": - E T. 11 N. 1: — 53 E 2 5 T. 10 N. w T.9N \ 6 5 4 l a l 2 1 — T.8N. 7 8 T‘°~~ 9N/1W—-SB1 18 17 o/ D C B A T.7N 19 20 3‘ 30 29 2\ E F G H 31 32 33\ 9 M L K J N P Q R INTRODUCTION TABLE l—Drinking water standards [Modified from American Water Works Association (1971, p. 20—32); constituents in milligrams per litre] Chemical constituents U.S. Public Health Service 1962 World Health Organization European, 1961 World Health Organization, International 1963 Recommended limitl limit‘ Tolerance Recommended limitl Tolerance limitl limitl Recommended Acceptable Tolerance limitl limit‘ Alkyl benzene sulfonate (ABS) ________________________ Ammonia ..... Arsenic (As) Barium (Ba) Boron2 (BL. Cadmium (Cd) Calcium (Ca) ______________________ Carbon chloroform extract (CCE) __ Carbon dioxide, free (C02) ________ Chloride (Cl) ________________________________________ Chromium, hexavalent (Cr ‘ s) ________________________ Copper (Cu) __________________ Cyanide (CN) _________ ._ _ Dissolved solids (D8)-“ ___ Fluoride (F) ___________ ___ Hydro en ion concentration (pH) Iron( e) ______________________ _ Lead (Pb) _______________ _._ Magnesium (Mg) ____________________________________ Magnesium + sodium sulfate __________________________ Manganese (Mn) ______________________ Nitrogen (N ), Nitrite + Nitrate2 ...... Oxygen, dissolved (01) _____________________ Phenolic compounds (as phenol)” _____ Selenium (Se) Silver (A ) Sulfate ( ) Zinc (Zn) ..... 0.5 ‘Recommended limit: USPHS: Concentrations which should not be exceeded where more suitable water su plies are available WH , European: Concentrations above which may give rise to esthetic and other troublesome problems. WHO, International: Concentrations which are generally satisfactory to the con- sumer. Acce table limit: 0, International: Concentrations above which the potability of the water would be “markedly" impaired. Tolerance limit: USPHS: Concentrations above which shall constitute grounds for rejection of the su pl . WHS, uropean: Concentrations above which are likely to give rise to actual danger to health. results of chemical analyses of waste effluent and ground water were compared with the generally accepted standards of water quality. The suitability of water for industrial, domestic, and irrigation use depends to a large extent on the type and total concentration of ions and compounds in solution. All widely recognized standards for drinking water specify that water used for drinking should be clear, colorless, odorless, pleasant to taste, and free from toxic com- pounds and pathogenic organisms. Probably the most widely used criteria in the United States for determin- ing suitability of water for drinking are from the U.S. Public Health Service (1962), summarized in table 1. Drinking-water standards established by World Health Organization, European (1961) and World Health Organization, International (1963) are also summa- rized for comparison (Am. Water Works Assoc, 1971). The standards summarized in table 1 are not to be confused with those established by the California Water Resources Control Board, Lahontan Region, for waste disposal to alluvium in the Barstow area. These discharge requirements are briefly discussed in this WHO, International: Concentrations above which may give rise to actual danger to ea t . 2 From U.S. Public Health Service Pub. 1880, 1969. 3 Concentrations in excess of 0.2 mg/l indicate need for additional analyses to determine the causative agent. ‘ Recommended limit is 0.05 mg/l for water entering the distribution system; 3.0 after 16-hour contact with new pipes. . -" Dependent on annual average maximum daily air temperature over not less than a 5-year period. 5 Where fluridation is practiced, minimum recommended limits are also specified. " Range, minimum to maximum limits, " In larger installations where removal ofiron is economic, water entering the distribution system should not contain more than 0.1 mg/l. 9 Upper limit should be 0.1 mg/l; 0.3 permitted after 16-hour contact with lead pipes. in Not more than 30 mg/l if the sulfate content equals or exceeds 250 mg/l. ‘1 Minimum concentration. report in the section on “Sources of Ground-Water Degradation.” METHODS OF INVESTIGATION FIELD METHODS During the latter part of 1971 and the first 6 months of 1972, more than 50 test wells were augered in the river-channel deposits of the Mojave River. Most of these wells were located in areas of historic and current waste disposal and in areas where the ground water was reported to be degraded. A few wells were augered outside the above areas in order to better define the limits of the degraded ground water. Several well points, each at a different depth, were installed at most drill sites. Depths of the test wells ranged from 25 to 135 ft (7.6 to 41 m) below land surface. All test wells completed for this project were cased with 2-in. (5.08 cm) I.D. (inside diameter) PVC (polyvinyl chloride) pipe with a screened well point attached at the bottom. Well points were 2 ft (0.6 m) 6 GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL TO ALLUVIUM, BARSTOW, CALIF. long, 1% in. (3.175 cm) in diameter, and number 18 slot (0.18-in. or 0.46—cm opening). A 5-ft (1.5-m) length of 2-in. (5.08—cm) I.D. galvanized pipe was fitted to the top of each test well in order to minimize damage from vandalism and floods. Several precautions were taken to avoid contamina- tion of the test well during augering and development. The use of oil on bolts, auger flights, and other drill equipment was kept to a minimum. Wells were developed by lowering a weighted, flexible, 3Ipin. (1.90-cm) PVC pipe into the test well and airlifting water from the well with compressed air. Development usually required an hour or more, generally at a rate of about 5 gal/min (25 m3/d). All test wells augered were allowed to equilibrate with the ground water a minimum of 60 days before sampling. The sampling procedure involved the lowering of an air line to a place just above the well point where pumping by airlift began and was continued until discharge from the well was clear of sediment. The air line was then slowly raised to a level where discharge from the well was reduced to a near minimum. At this level the airlift compressor was operated for a period sufficient to insure that the discharge was free of sediment and that any water that might have been aerated by the deeper pumping had been removed. After washing all sampling apparatus, the air line was removed from the well and a flow-through, or self- washing, thief sampler of 750-m1 (millilitres) capacity was lowered to the top of the well point and a water sample taken. This thief sampler is a spring-loaded, teflon-sealed, brass cylinder open at both ends until triggered by a weighted messenger at the desired level of sampling. Chemical analyses of ground-water samples collected directly from the airlift discharge differed from those taken with the thief sampler. These differences were attributed to the aeration associated with the airlift process which caused oxidation, precipitation, and accelerated aerobic degradation. Values of the chemical parameters compared were generally lower for the airlift method: field pH values were higher (hydrogen ion concentration lower); calcium, magnesium, chromium, and iron were lower; ammonia, phosphate, dissolved organic carbon, and particularly the MBAS (methyl blue active substances) values were lower, especially in the samples thought to contain the newer biodegradable LAS (linear alkylate sulfonate) detergents. ANALYTICAL METHODS During this investigation about 150 water samples from wells were analyzed. Analysis included the major anions and cations plus arsenic, phosphate (P04), am- monia (NH3 as N), chemical oxygen demand (COD), oil and grease, detergents (as MBAS), hexavalent chromium (Cr+6), and dissolved organic carbon (DOC). Copper (Cu), mercury (Hg), and chromium (Cr+3) were analyzed in samples from selected wells. During the course of sampling, several laboratories were selected for a cross-check of sample analyses. The results of this check indicated that laboratory results were generally satisfactory except for analysis of some of the nonconservative biodegradable constituents. The geographically closest laboratory was selected to analyze these biodegradable constituents to reduce the time period for handling; this laboratory also produced the most consistent results. Most of the‘ common constituents (major anions and cations) were analyzed by the Geological Survey central laboratory, Salt Lake City, Utah. The procedures used for filtering and preserving samples complied with analytical techniques that are described by the Ameri- can Public Health Association (1971). Water samples to be analyzed for the nonconservative, biodegradable chemical compounds were delivered from the field to the State of California Public Health Laboratory in Los Angeles within 48 hours. The presence of hexavalent chromium, a readily reduced ion, was determined in the field by adding 2.5 ml of diphenylcarbozide to a 50-ml aliquot of sample. The mixture was allowed to stand for 5 minutes, and a pink to red coloration indicated the presence of hexavalent chromium. Samples containing hexavalent chromium were sent to the California Public Health Laboratory ,for quantitative analysis. One of the chemical constituents determined in all samples was the synthetic surfactant constituent in detergents. The method for determination of detergents is by MBAS. This method does not distinguish among the several types of surfactants that may exist in water (American Public Health Association, 1971). DOC (dissolved organic carbon) was a most useful parameter in locating areas affected by industrial- and domestic-waste disposal. DOC indicates the presence of water-soluble organic compounds. Water samples to be analyzed for DOC were stored at low temperature (4°C) after filtration through a silver-membrane filter. Both the low temperature and the trace quantity of silver that were dissolved in the sample from the silver- membrane filter inhibited bacterial activity. Effects on analytical results from time delays between field sampling and laboratory analysis were less critical for DOC than for other nonconservative compounds be- cause of the inhibited bacteria. All DOC samples were analyzed at the Geological Survey organic geochemis- try laboratory in Denver, Colo. GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL TO ALLUVIUM, BARSTOW, CALIF. 7 GEOHYDROLOGY GROUND-WATER GEOLOGY The ground-water geology of the project area has been described in previous reports by Thompson (1929); California Department of Water Resources (1967); Bader, Page, and Dutcher (1958); Page and Moyle (1960); Page, Moyle, and Dutcher (1960); Dyer, Bader, Giessner, and others (1963); Miller (1969); and others. In general, the surface geology of the highland areas (fig. 3) is characterized by outcrops of consolidated rocks of Tertiary and Quaternary age, which yield very little water—less than 100 gal/min (550 m3/d)—to wells. The lowland areas are characterized by unconsolidated alluvial deposits consisting of alluvium and fan deposits of Pleistocene age and alluvium and fan deposits of Holocene age. River-channel deposits are a part of the younger alluvium (Holocene) and compose much of the aquifer system studied in detail. Data from more than 50 test wells augered during this study (fig. 3) indicate that the river-channel deposits are highly permeable and consist of moderately well-sorted, subrounded to subangular, fine to coarse sand and gravel. Wells in these deposits typically yield from several hundred to more than 1,000 gal/min (5,500 m3/d). Clay is uncommon in the river-channel deposits and where present occurs in thin (0.5—1.5 ft or 0.15—0.46 In) lenses. The hydraulic character of the younger alluvial deposits seems to approach that of a homogene— ous and isotropic medium. Although stratification in sand layers may retard the vertical movement of ground water, well logs and water-level data collected during this project suggest that wide variations in vertical permeability are not of major consequence. Test wells were drilled to depths of as much as 150 ft (45 In); the contact with the underlying older alluvial deposits was not identified during drilling but is probably not much greater than 150 ft (45 m) below land surface. The older alluvial (Holocene—Pleistocene?) deposits are poorly sorted and consist of clay, silt, sand, gravel, and boulders. They are locally cemented and generally yield small to moderate quantities of water to wells. Water in this aquifer is typically high in dissolved solids, boron, and fluoride (Hughes and Patridge, 1973). In the descriptions shown on most commercial well logs, the geologic contact between the younger river-channel deposits and the older, less permeable alluvium is not well defined. Many of the high-yielding wells in the project area probably derive water from a combination of river-channel deposits, alluvial-fan deposits, and older alluvial deposits. The lithologic log of well 9N/1W—14B3 (Koehler, 1969) suggests that intertongu- ing of river-channel deposits and alluvial-fan deposits occurs to depths of as much as 150 ft (45 111). During a 1969 aquifer test on well 9N/1W—14B3, a deep-well flowmeter survey indicated that most of the yield was from the upper 150 ft (45 m) of these intertonguing deposits (well is perforated from 109 to 312 ft or 33 to 95 m). The Waterman fault trends northwestward across the Mojave River along the east side of the USMC Supply Center (fig. 3) where its trace is buried by unfaulted surficial sediments. The upper limit of faulted or fault-affected river-channel deposits is not known. Differences in water levels across the fault of 45 ft (14 m) in 1965 (Miller, 1969) and 30 ft (9 m) in 1971 (Hughes and Patridge, 1973) have been observed. GROUND-WATER HYDROLOGY The alluvial aquifers near Barstow receive recharge as underflow from the west and south and from floodflow in the river. The principal direction of ground-water movement is eastward. Contours of ground—water levels (1972) indicate a gradient from west to east of 10—15 ft/mi (1.9—2.8 m/km) in the reach upstream from the Waterman fault, and a somewhat steeper gradient downstream from the fault (fig. 3). Long-term hydro- graphs indicate that water levels in wells in the younger alluvial aquifer reflect the intermittent surface flow in the Mojave River. Steady declines in the ground-water level in some areas exceed 40 ft (12 m) during dry periods when no surface flow occurs and may be followed by as much as 50 ft (15 m) of recovery during a year with ample floodflow in the Mojave River. Figure 2 illus- trates the response of water levels in well 9N/1E—18E1 to fioodflow in the Mojave River. This well is 88 ft (27 m) deep, is located adjacent to the river, and is reportedly perforated entirely in river-channel deposits. Ground-water development in the Barstow area has resulted in a complex local system of recharge and discharge. The effect of this development has been to lower the water table (fig. 3), thereby reducing the volume of ground water in storage and the rate of ground-water flow eastward. The water budget for the Barstow area can be divided into the following categories: (1) Recharge by un- derflow, surface water, sewage effluent, and irrigation return; and (2) discharge by underflow and pumpage (table 2). Recharge by underflow is the subsurface inflow from the aquifers west of Barstow and from the much less permeable aquifer southeast of Barstow. Variations in this quantity of recharge represent changes in inflow due to changes in saturated thickness of the aquifers west of Barstow. The aquifer system south of Barstow is undeveloped, and available data suggest that the system has undergone very little change in head. Recharge from effluent occurs where there is deep 8 GROUND‘WATER DEGRADATION RESULTING FROM WASTE DISPOSAL TO ALLUVIUM, BARSTOW, CALIF. 117000, R.1W. R-l ~11°55’ ‘1 T. 10 N. , \ 34°55 ' (“xx/ T.9N.§ 34°52’ v~ Base from U.S. Geological Survey Geology modified after G.A. Miller, 1969 Barstow and Daggett 1:62 500, 1956 Roads as of 1972 1 5/2 0 1 M'LE E 1 ‘5 o 1 KILOMETRE EEE CONTOUR INTERVAL 40 FEET DATUM IS MEAN SEA LEVEL FIGURE 3.—Geology, waste-disposal sites, wells used to sample ground-water quality, and water-level contours for 1945 and spring 1972. percolation of sewage-effluent water. This recharge large variations in this quantity shown in table 2 are originates from two main sources in the Barstow area: due to correspondingly large variations in the saturated (1) The city and railroad sewage-treatment facilities thickness of the aquifer downstream from the fault. (sites A, B, and C, fig. 3), and (2) the USMC The term “pumpage” as used in table 2 represents the sewage-treatment facilities (site D, fig. 3). net consumptive use of water extracted from the Discharge by underflow from the area occurs along aquifer. When the pumping well is in the area of use, the the Mojave River northeast of the Waterman fault. The quantity of extracted water that percolates and returns Holocene Pleisto- cene f—A‘x SOURCES OF GROUND-WATER DEGRADATION 9 EXPLANATION an Younger alluvial deposits QUATER - NARY Older alluvial deposits Consolidated rocks, undifferentiated } TERTIARY AND QUATERNARY Contact Fault Dotted where concealed. U, upthrown side; D, downthrown side 1980+ Water—level contour, 1972 Shows altitude ofwater level. Contour in- terval 10ft (3 m). Datum is mean sea level. Arrow shows direction of move- ment ----- 1980----- Water-level contour, 1945 Based on sparse measurements. Contour interval 10 ft (3 m). Datum is mean sea level. WASTE-DISPOSAL SITES A City of Barstow (abandoned, 1953) A.T. and S.F. Railway (abandoned, 1968) B City of Barstow (abandoned, 1968) C City of Barstow A.T. and S.F. Railway D USMC golf course E USMC Supply Center 031 Test well .63 Irrigation-domestic well @82 USMC-City of Barstow supply well to the aquifer was subtracted from the total to calculate the pumpage. Evaluation of the volumes of recharge and discharge (table 2) indicates that for the 26-year period (1946—71), the volume of ground water in storage was reduced by an estimated 13,000 acre-ft (16 hm3). The beneficial effects on ground water in storage by surface-water recharge during large floods are illustrated by omitting the 1969 flood recharge from the change in storage computations; under these conditions the compared decline in storage was approximately 32,000 acre-ft (40 hm3) in 26 years. SOURCES OF GROUND-WATER DEGRADATION Figure 4 is a diagram of the system or cycle of water use in the project area. Some ground water recharged to the basin along the Mojave River is pumped upgradient from Barstow. After use by the city and other agencies, some of this water is returned to the ground-water system by percolation from the city’s waste-disposal facilities. Water for irrigation in the area follows a similar pattern: that is, it is pumped, used, and some is returned to the ground-water body, except that the excess water available for percolation does not contain some of the chemical constituents that are added during domestic and industrial use. Water is used at the USMC Supply Center for a combination of industrial, domestic, and irrigation purposes similar to that upstream, only on a smaller scale. Sources of impairment of ground-water quality in the Barstow area include: (1) Natural; (2) domestic- and industrial-waste disposal; and (3) irrigation return. Much of the historic information in the following discussion of sources of ground-water degradation was taken from a report by the California Department of Public Health and California Department of Water Resources (1960, p. 18—32). NATURAL Ground-water inflow from areas south of the river is generally high in dissolved solids. For modeling purposes, S. G. Robson (1974; Hughes and Robson, 1973) assumed inflow from the south to contain an average of about 1,000 mg/l (milligrams per litre) dissolved solids. Because the rate and volume of inflow associated with this source are comparatively low (table 2), the influence on the ground-water quality in the basin is small. INDUSTRIAL AND DOMESTIC WASTE CITY OF BARSTOW The city of Barstow has operated three sewage- treatment plants since 1938. Each plant has been located adjacent to or constructed on the river-channel 10 GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL TO ALLUVIUM, BARSTOW, CALIF. TABLE 2.—Water budget, in acre-feet [After Hughes and Robson, 1973] Recharge Discharge Change in Period Duration Underflow from Effluent storage (yr) Surface Irrigation Underflow Pumpage pefilod water , return to east \West South Orgiylr'gfi USMC 1946—51 ________________ 6 6,600 720 2,100 3,300 2,460 540 —3,540 —22,440 — 10,260 1952 __________________ 1 1,100 120 7,370 640 410 320 —630 —3,140 6,190 1953—57 ________________ 5 5,500 600 0 3,750 2,300 3,750 - 1,800 —24,300 — 10,200 1958 __________________ 1 1,100 120 9,900 860 610 1,200 —490 —7,080 6,220 1959—68 ________________ 10 8,000 1,200 2,600 12,200 3,500 113,600 —4,200 -55,600 —18,700 1969 __________________ 1 800 120 20,280 1,690 380 11,230 —-700 -4,680 19,120 1970—71 ________________ 2 1,600 240 0 3,700 960 12,840 —840 — 13,960 —5,460 Period total ______ 26 24,700 3,120 42,250 26,140 10,620 23,480 —12,200 —131,200 —13,090 1Includes 150 acre-feet per year irrigation return from USMC golf course. deposits of the Mojave River, and the city has used this medium for disposal of waste effluents. Effluent from the 1938 plant (site A, fig. 3) was discharged directly to the Mojave River bed. That facility reportedly handled only domestic wastes, with the exception of synthetic detergents and soaps from laundries after 1945. In 1953 a new sewage-treatment plant (site B, fig. 3) was constructed about half a mile east and downstream from site A. This plant provided increased capacity and additional stages of treatment for handling the city’s growing volume of sewage. The design capacity of the plant was 2.25 Mgal/d (1.8 X 1010 m3/d) for primary treatment and 1.0 Mgal/d (7.9 X 109 m3/d) for secondary treatment. In 1965 the average daily flow was 1.2 Mgal/d (9.4 X 109 m3/d); therefore 0.2 Mgal/d(1.6 X 109 m3/d) did not receive secondary treatment. Disposal of effluent from this facility was by direct percolation and evaporation from oxidation ponds. Some treated effluent was diverted for irrigation of alfalfa (fig. 4). The city of Barstow’s effluent reportedly consisted only of treated domestic wastes with the possible exception of synthetic detergents and soaps from laundries. Prior to 1953 the Atchison, Topeka and Santa Fe (AT. and SF.) Railway treated and disposed of its domestic and industrial wastes, but with construction of the larger city plant, the company restricted its treatment to industrial wastes and discharged domestic wastes to the city sewer system. The constant increase in volume of waste effluent associated with a growth of population and industry required a larger sewage-treatment facility, and in late 1968 a new treatment facility was built by the city of Barstow on a site a short distance upstream from the USMC Supply Center (site C fig. 3). At present (1972), the new facility provides primary treatment, with mechanical aeration in one pond and six additional oxidation ponds. The plant was modified to provide complete secondary treatment and became operational in March 1973 (Robert Beach, oral commun., city of Barstow, 1972). Chemical analyses of waste from the city of Barstow’s facilities (table 3) indicate that the effluent has been high in detergents (as MBAS), with concentrations ranging from 0.48 to 16 mg/l. The average has been well above the limits set for discharge at this facility by the California Water Quality Control Board, Lahontan Re- gion. The Board’s maximum limits for detergents (as MBAS) in the effluent that percolates to the ground- water basin has been, at different times, 0.5 mg/l and 1.0 mg/l. The concentration of phenols in the Barstow effluent has been as high as 0.15 mg/l. The Board’s maximum limit for phenols is 0.002 mg/l. The concen- tration of dissolved solids, chloride, and sodium has been marginal compared with their respective limits of 900, 120, and 160 mg/l. ATCHISON, TOPEKA AND SANTA FE RAILWAY Available records of industrial waste discharge by the AT. and SF. Railway suggest that discharge began about 1910 with the construction of a drain system from the shop and yards to the Mojave River (site A, fig. 3). This waste effluent included fuel oils and solvents. In 1915 an oil trap was constructed so that the fuel oils could be separated from the water and reused. This facility was reportedly small and required retention time for separation of the oil, suggesting that during periods of peak loads, spills and direct discharge to the river probably occurred. With the exception of an attempt to absorb some of the oils passing the traps, this form of treatment did not change until 1952. Much of the need to modify industrial—waste handling by the railroad was caused by the conversion from steam- powered to diesel-powered locomotives, which began in 1942 and was completed in 1952. With the advent of diesel engines came a need for coolants with corrosion- preventative compounds for use in radiators. An additive containing hexavalent chromium was used beginning about 1948. Until 1952 it was common practice to drain the radiator coolants into the waste-disposal system which discharges to the Mojave SOURCES OF GROUND-WATER DEGRADATION 1 1 UPSTREAM (in) l Municipal and industrial use <— Lateral ground-water inflow Evapotranspiration Lateral ground-water inflow—— Local domestic pumpage Railroad use, treatment, and disposal Evapotranspiration \ \ \ \\ \ \ - \ City \ disposal Irrigation \ Abandoned return \ 1953 \ \ . \ Evaporation E Evapotranspiration \ i— 9 City a) disposal in: LIJ Abandoned l-_I- 1968 8 Irrigation < Abandoned 1964 City Evaporation Evapotranspiration disposal Evaporation Lateral ground-water inflow <—— Lateral ground-water inflow Evapotranspiration Lateral ground-water inflow Municipal USMC and Industrial industrial and use domestic disposal Evaporation < (out) DOWNSTREAM FIGURE 4.—Water use in project area. River (California Department of Public Health and facility produced an effluent containing emulsified oil, California Department of Water Resources, 1960, p. 29). ‘ grease, synthetic detergents, and a disinfectant with a A laundry was constructed in 1949 by the railroad to high concentration of phenolic compounds. wash grease- and oil-saturated rags and clothing. This During 1954 and 1958, two dikes were constructed by 12 GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL T0 ALLUVIUM, BARSTOW, CALIF. TABLE 3.—Chemical analyses of the [Constituents in milligram per litre, except total iron, boron, and arsenic in micrograms per litre; Source and site locations (see fig. 3) 3 as Q Water temperature (°C) Silica (SiQ) Bicarbonate (1100;) Total iron (Fe) Magnesium (Mg) Carbonate (COa) Sulfate (804) Chloride (Cl) Potassium (K) bldest city facilit (site ). Santa Fe domestic treatment (site B). Old city facilit (site ). Irrigation. Storage ponds (located about 9% mile downstream from site B). Present facility: Pond 1 (site C). Pond 2 (site C). Pond 3 (site C). Pond 4 (site C). Pond 5 (site C). Pond 6 (site C). 10-19—48 ________ 5-13-52 ________ NICO H .p H Ho M q GI 00 q q on o; ..I r-n O on H I l I I W r—t N 03 03 I I I I H CD on O O W a: W {D 0! g o I—I D-l 0’: on )—‘ H g N] q H H as a: 5 g (D <0 .3 OOOOOOOMOO D—l U‘ N ..I W In I: I: I: I: I: 131 I: I: I: I: I: 145 :1: I: I: 727 20 171 15 '45s 156 146 _ 150 _-,- 133 183 ____ on- 230 I: I: I: I: 165 ____ 39 12 165 15 184 0 134 139 50 58 20 164 14 205 0 159 163 _ .-_ 96 14 200 12 403 ———_ 240 186 ____ _.__ ____ 165 ____ ____ ———— 190 110 Z: I: 230 210 286 I: III I: '2'66 III '2'56 '2'i5 197 __1_ 64 17 195 4 342 ———— 210 176 ____ ____ 220 ____ .1__ ____ ____ .___ -__. ———— -—_— 225 ____ __,_ ____ 0,- ____ ____ ———— _»—— 220 .-__ .___ ____ ____ ___- ____ -—»- ———_ 227 1 Southern pond. 2 Northern pond. 5 Biofilter. domestic wastes of the city of Barstow water temperature in degrees Celsius; specific conductance in microth at 25°C; and pH] SOURCES OF GROUND-WATER DEGRADATION 13 Ammonia (NHo) Phosphate (P04) Boron (B) Specific conductance pH Oil and grease Detergents (MBAS) Arsenic (AS) Total chromium (Cr) Hexavalent chromium (Cr ‘ ‘) Copper (Cu) i g Fluoride (F) ...F.F.H H OQQAmNQmAN b-‘D-l ii-n .P qow UI I D I I 1100 1.0 1.6 h a Nitrate (N05) 83 no... pJHwHOHp-H 1280 600 l ,000 1,100 1366 00 500 800 1,100 900 300 766 100 §§ Dissolved solids l ,000 997 820 554 624 «aw on ma §§§§ §§ §§§§§§§§ HH HHHHHHHH HHH 1,400 1I'i66 1,700 1.400 95” 0-10: 914.03 “3°03 ,.. “flfiflflflflflfiflfl whmummmwmmo#mwm . I I .43.“: T‘ N.“ mam-i H was . 51.4.4 owns] '73 w s 9 SS sss O 0 uh huh @NM 1.0 CI SPF-”9°? SHOmO bub man ... n m w mm 990m ' Go I I I I 2228 .05 .00 .00 14 the railroad in the river (site A, fig. 3) to hold treated effluent. The diked areas provided for physical separa- tion of oil and sludge but did not control the percolation of synthetic detergents, chromium, emulsified oils, and phenolic compounds. The dikes were constructed of river-channel sand, which offered little resistance to floods. Both dikes were destroyed by floods in April 1958 but were reconstructed during the summer of 1958. From 1959 to 1968, the railroad installed facilities for flocculation, surface stripping, and oxidation of wastes. It also changed from the use of ABS to LAS (nonbiode- gradable to biodegradable) synthetic detergents and abandoned its laundry and caustic cleaning vats at the diesel shop, thereby eliminating two sources of phenolic compounds from the waste effluent. In 1968 the railroad modified the treatment of wastes to comply with the effluent standards established by the California Water Resources Control Board, Lahontan Region (resolution 66—18, 1966) and by the city of Barstow. Most railroad waste subsequently was ex- ported to the new city waste-treatment facility (site C, fig. 3) by means of the city sewer system. The chemical quality of waste from the facilities of the AT. and SF. Railway (table 4) has varied depending on the source, rate and volume of discharge, and sampling site. These variations are indicated in part by pH values, which range from very acidic, 3.9, to very basic, 12.4. Comparison of concentrations in wastes for detergents (0.35—64 mg/l), phenols (0.000—1.0 mg/l), oil and grease (0.04—4,000 mg/l), chlorides (34—496 mg/l), boron (0.06—46 mg/l), sodium (SO—1,200 mg/l), dissolved solids (31 1—2,700 mg/l), total chromium (0.00—4.80), and hexavalent chromium (0.000—11 mg/l), with require- ments of the California Water Resources Control Board, Lahontan Region (resolution 66— 18, 1966), indicates that at times concentrations far above published standards have been available for percolation to the ground-water system. US. MARINE CORPS SUPPLY CENTER, NEBO Facilities for treatment and disposal of industrial and domestic waste at the USMC Supply Center were built in 1942 (site E, fig. 3) and were subsequently modified and expanded in 1952 and 1957. The present system provides primary treatment and some degree of secondary treatment. Disposal of treated effluent is by evaporation or by direct percolation to the Mojave River. Until late 1972 treated effluent was also used as irrigation water for a local golf course (site D, fig. 3). This method of disposal has since been discontinued. Some concentrated industrial-waste products are trans- ferred to the USMC supply annex at Yermo (fig. 1) for disposal. According to a report on the waste-treatment GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL TO ALLUVIUM, BARSTOW, CALIF. facilities at the USMC Supply Center (Brown and Caldwell, 1970), the plant is inadequate and difficult to operate, resulting in occasional spills and odors. Consideration is now being given to reconstructing and updating the facilities, or transferring all effluent to the city of Barstow for treatment and disposal. Chemical analyses of waste discharged by the USMC Supply Center (table 5) indicate that the concentrations of oil and grease and phenols in the waste effluent have been as high as 79 and 0.06 mg/l. As a result of influent industrial wastes, pH values in the effluent have ranged from 7.2 to 10.0. MINING AND MILLING OPERATIONS Two mining and milling operations were active in the Barstow area until 1954—the PCMMC (Pacific Coast Milling and Mining Co.) and a tungsten mill. The PCMMC disposed of its industrial wastes in an area adjacent to a complex of wells now operated as the city’s water supply (located about half a mile upstream from site A, not shown in fig. 3). Sparse records do not indicate the volumes of waste discharged, but they do indicate that the waste was high in iron, aluminum, copper, lead, calcium, magnesium, and sodium. The concentrations of dissolved solids in waste effluent were 2,300 mg/l and 3,200 mg/l in March and August 1952 (table 6). Disposal of wash water from the tungsten mill was at the same location as the waste—disposal facility of the AT. and SF. Railway and the city’s first disposal site A (fig. 3). Available chemical analyses indicate that for those substances analyzed (table 6), the waste water was apparently similar to the local ground water. The volume of waste was estimated to be 25—50 acre-ft (30,000—60,000 m3) per year. IRRIGATION RETURN The high rate of evapotranspiration in the Barstow area requires that large quantities of water be applied to crops. Alfalfa, the major crop in this area, transpires about 3 ft (0.9 m) of water per year (Meyer and Horn, 1955, p. 197) when about 5—7 ft (1.5—2.1 m) ofwater per year is applied for irrigation. Allowing for consumptive use, leaching of minerals from the soils, and the solution of fertilizers, the concentration of dissolved solids in irrigation-return water could be at least doubled. Assuming that the applied irrigation water in the northeastern part ofsecs. 4 and 10, T. 9 N., R. 1 W. (fig. 3) contained 350—450 mg/l of dissolved solids, the irrigation-return water could contain more than 700 mg/l of dissolved solids. The annual volume of irrigation-return water (table 2) has been estimated to be (1) nearly equal to annual underflow into the Barstow area from the west, (2) DISTRIBUTION AND EVALUATION OF CHEMICAL SUBSTANCES AFFECTING GROUND-WATER DEGRADATION greater than the average annual volume of domestic and industrial waste that percolated to the river from the city treatment facilities prior to 1952, and (3) only slightly less than the average annual volume associated with the facilities operated'from 1962 to 1968. Until 1964, effluent from the city of Barstow treatment facility (site B, fig. 3) was used for irrigation of alfalfa at a site about half a mile downstream. That part of the effluent not required for irrigation was discharged by the city of Barstow to the Mojave River (California Department of Public Health and California Department of Water Resources, 1960, p. 24—26). This source of ground-water recharge is included with effluent-recharge data in table 2. Chemical analyses of water in storage ponds where effluent was retained prior to use for irrigation indicate that the water was acceptable for irrigating alfalfa. Chemical analyses for other than the major constituents were not available. Effluent from the waste-water treatment facilities at the USMC Supply Center (site E, fig. 3) was used for irrigating a golf course (site D, fig. 3) between 1959 and 1972. The volume of effluent used annually varied only slightly and totaled about 4,000 acre-ft (5 hm3) or approximately 300 acre-ft (0.4 hm3) per year for 14 years. Most of the treated water was applied during the summer. Chemical analyses of the treated water indicated a concentration of dissolved solids of about 1,000 mg/l (table 5). The concentrations of most chemical constituents were within the generally ac- cepted limits for irrigation. Because of the high consumptive use and the presence of sandy soils at the golf course, about half of the applied water returned to the aquifer, therefore, the concentration of dissolved solids in the return water is estimated to be 2,000 mg/l. Chemical analyses of the water from the USMC Supply Center oxidation pond (mixed effluent, table 5), which has been used for golf-course irrigation, indicate that the concentrations of most chemical constituents are generally within the acceptable limits recom- mended for percolation by the California Water Resources Control Board, Lahontan Region (resolution 66—18, 1966). The concentration of phenols, oil and grease, fluoride, and detergents was occasionally above those limits. DISTRIBUTION AND EVALUATION OF CHEMICAL SUBSTANCES AFFECTING GROUND-WATER DEGRADATION Percolation of waste effluents from industrial and municipal sources, plus irrigation return, has seriously affected the quality of the ground water in the younger alluvial aquifer east of Barstow. The extent of the degraded ground water has been identified both areally and vertically by the concentration and distribution of 15 dissolved solids, detergents (as MBAS), dissolved organic carbon, total nitrogen, and chloride. The vertical sections (see figs. 6, 8—1 1) that are used to illustrate the distribution of selected chemical sub- stances in ground water are based on chemical analyses of samples collected from March 27 to April 7, 1972. The patterns of distribution of the chemical constituents were drawn on the basis of chemical analyses of water from wells which are either on or near the trace of the vertical sections and reflect hydrologic stresses result- ing from recharge and discharge in the aquifer. Two planes of projection are used (see figs. 6, 8—11) to illustrate the vertical distribution of selected chemical substances in ground water—the longitudinal section which is more or less along the flow system, and the cross sections which are at right angles to the longitudinal section. The trace of the longitudinal section A—A’ remains unchanged for each of the chemical substances discussed. The cross sections are shown at three different locations (B—B’, C—C', and D—D’) to better illustrate conditions related to varia- tions in types and methods of waste disposal. The patterns of distribution of chemical substances suggest a certain uniformity of occurrence and flow within the hydrologic system. However, monthly sampling of three test wells, 9N/1W—9H5, —9H7, and —10J3, indicates that the distribution of chemical constituents in the ground water is changing with time, particularly in the deeper zone. Such changes in ground-water quality with time are to be expected because of the movement of fluids in the aquifer, and because the quality and the quantity of wastes discharged to the ground-water system have varied in time. Figure 5 illustrates the changes in selected chemical constituents in the ground water in these wells with time; the changes are based on the samples of water and resultant analyses. Maps showing the areal distribution of water quality in an aquifer system are often misleading because they generally do not show variations in chemical concentra— tions with depth. In this report, maps showing the areal distribution of various constituents are used only to illustrate, for discussion purposes, those areas degraded by a selected minimum concentration of a chemical substance. Some similarities in the distribution of general chemical quality are apparent in each of the sections in figures 6, 8—11. Along section A-A’ data indicate a general increase in constituent concentration with depth upstream from well 9N/1W—9B1. This increase has resulted from large volumes of floodwater contain- ing low dissolved solids that has moved downward from the stream channel into the aquifer and depressed the poorer quality water that had previously percolated to 16 GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL T0 ALLUVIUM, BARSTOW, CALIF. TABLE 4.—Chemical analyses of the industrial wastes [Constituents in milligrams per litre, except total iron, boron, and arsenic in micrograms per litre; Source and site loca- tions (see fig 3) Water temperature (°C) Silica (SiOr) Total iron (Fe) Calcium (Ca) Magnesium (Mg) Sodium (Na) Potassium (K) Bicarbonate (HCOa) Carbonate (COa) Sulfate ($0.) Chloride (Cl) Fluoride (F) Ditch. She and ya:- (site A). (site A). Percolation pond. Main shop treated effluent (site A). Ditch. Main shop and skim pit overflow (site A). l4 ____ __-_ 88 III: 366 10 ll 12 13 133 98 80 294 230 87 41:3 ‘o.7 .4 of the Atchison, Topeka and Santa Fe Railway water temperature in degrees Celsius; specific conductance in micromhos at 25°C; and pH] Nitrate (Na) DISTRIBUTION AND EVALUATION OF CHEMICAL SUBSTANCES AFFECTING GROUND-WATERlDEGRADATION Ammonia (NH‘) Phosphate (P04) Boron (B) Dissolved solids Specific conductance pH Detergents (MBAS) Arsenic (A 3) Total chromium 0'} (Cr '6 Hmvalent chromium_ 17 Capper (Cu) 0 1 1:200 13200 1,200 1560 1,500 1,100 1:200 1:206 1:600 I .900 90.00 can 1 l l H wsssws on HU'IWQQNI 1 0.6 8.9 9.8 9.2 8.3 9.7 8.2 8.9 9.4 8.8 9.6 8.8 9.4 8.8 9.5 7.1 .6 . 1 13’ “951599.495” NIOUIQONNv 1 ,_. 995°“: can-u». , coon m‘o’o. v H 3 MN) HD—I 9. WM? 'o'cm .:°=°=' l . o . A H 1 0. 1 6 4. 2. 1. 2. whopwhhem 19.3 18 GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL TO ALLUVIUM, BARSTOW, CALIF. TABLE 4.—Chemical analyses of the industrial wastes of g, A .8 g - g - 3 a ‘51 {a E0 ,- 3: 6 is» L A e a V a 8 g 9 ~ g a ”it E8 3 s 9 ‘5 E E 2 % g z E V .5; '0 .. 3V "" a — E a: E a g E % E E a a s t = s 2 5° ‘~ - .3 a s 2° a (3;: c: 3 iii a 5 2 g a? a: o m o E? Ditch. Water 10—19—48 77777777 ”.7 118 50 15 23 723 .u- 0 91 64 57 5 softener and ,_._ 8 0 1 0 106 We, 0 70 52 32 0 laundry , .7 - 31 400 240 1 1 ,200 9 _ , , A 157 63 68 2 (site A). 38 36 0 15 1 215 61 ,___ 222 54 70 6 "ii :I j: "11 21" ’iéé 133” ”’6 71765 ’iéi 5'7" "8- Dischargeto .1” ____ ”A ____ , , ,.. ,_,_ WW H” ”W ,,__ ,_ , city sewer ._._ .0- .1»- __,. , ,,,, W. A" "V H . _ .w. (siteC). 7—» ,,__ ———» .... 1.. 470 W.— 272 ,1" ”A 234 41 1At entrance to drain (ditch). TABLE 5.—Chemical analyses of the domestic and industrial [Constituents in milligrams per litre, except total iron, boron, and arsenic in micrograms per litre; cl: A 3’5 1 3 A no 33°” 2 E :2 m “‘ "J A 3 V ,.. V A r: A c e a v s d 3 8 3 E g E 5 5 E ‘9 § 3 m3 V ‘F‘ E a E "' .8 3 E E m a -— H = a I- L. E : 3 0 g 0 En - fi 3 s... o o o N a”: 0 Ti a 3 o .2 3 2 3 :3 ~ Q a: [- o E a. an a: o a. Sewage plant 10—19—48 40 500 41 13 137 ._._ 248 100 68 ,___ effluent 5—23—51 _ 12 ____ 42 11 293 1,“ 327 95 83 -,__ (site E). 1—10—60 1.1, ___, H" _1__ 236 __1_ ,__, _._ ”H 5.7 4—14—61 ,,__ ,,__ ___, ,- ,1” ___ 1-1, ,,,, 8— 3—61 ,,__ 1," ___, 300 _1,_ m, ____ ____ 31—27—681 24 0 220 ,,._ 128 164 1.45 6—19—68‘ 32 220 1,,_ 255 184 1.38 9-2&68‘ 22 100 e". 280 _ 350 196 1.23 1— 8—69 28 0 24 207 327 76 174 .68 3—20—69 30 5 15 224 235 250 176 1.52 7—15—69 26 .0 33 249 165 290 200 .93 9-30-69 34 .0 18 214 295 225 192 1.07 6— 3—70 ____ ____ __1_ "w ___- ”W ____ ,___ _,__ 9—21—70 (hr) 1030 -,,, ___, __,_ 210 __,_ 232 224 ”A- 1400 -_,- ____ ____ 186 ____ 220 180 1 13 1815 _-__ a“ ____ 178 ,___ 228 180 1.18 2215 _,__ ,1" ____ 178 -___ 218 184 1.13 9-22-70 (hr) 0215 _-_- ___A ,1“ 182 ”h ____ 228 180 1.13 061 _--- __-1 _-__ 170 ."1 -___ 228 180 1.18 9-22/23—702". _,__ _,_, _,1_ 184 0,, q" 226 188 1.15 9-23-70 ____ ____ ____ 186 __,- _,__ 200 180 1.13 10—28—70 ,___ ___- ____ 248 -M- ”W 280 220 .88 12— 2—70 ,___ -__1 _-_, 200 1--- v“ 230 230 .70 12—23—70 --__ -9, ____ 135 "H -_,_ 240 230 .84 3—31—71 _,__ __,, 1__, 310 ,,,_ ____ 262 220 1.0 4—27—71 __-_ __1_ .0- 258 .,__ ____ 330 224 1.14 5—26—71 ___1 ",1 ___, 252 236 1.2 6—15—71 .0- 286 ____ 277 240 1.11 7-28-71 ____ __._ 230 ____ 270 226 .9 9-30-71 ____ -_-_ 197 ____ 220 224 1.3 Averaged 8—9/10—70 ____________ “A, ,_,_ .___ ____ 101 .0. q" 118 117 96 industrial wastes. Averaged 8—9/10—70 ____________ .1" ,1,_ ,___ ____ 161.7 11,. ____ 205 160 1.05 domestic wastes. GOlf course 8—17—61 ____________ 35 50 55.5 20.5 180 11.3 "1. 203 194 1.08 sprinklers (site D). ‘Quarterly summary. the river-channel deposits from abandoned upstream waste-discharge sites A and B (fig. 3). Distribution of chemical substances in ground water beneath the present city of Barstow treatment facility (site C, fig. 3) reflects the quality, quantity, and timing of sewage percolation. Estimates by the California Department of Public Health (1970, p. 7) of the rate of waste percolation from the city oxidation ponds indicate that percolation from pond 2 is approximately 10 times that of the other ponds. Pond 2 has a greater percolation rate than the other ponds because when the other ponds are either full, drying, or under repair, six lagoons adjacent to pond 2 are used for percolation of the wastes. These lagoons, located between the ponds and the center of the river (fig. 3), are constructed of river sand and are capable of percolating large volumes of waste effluent. The two irregularities in the shallow plume of degraded water that are shown in figures 6, 8—11 reflect and are DISTRIBUTION AND EVALUATION OF CHEMICAL SUBSTANCES AFFECTING GROUND-WATER DEGRADATION 19 the Atchison, Topeka and Santa Fe Railway—Continued E Q A A E - - ,, g g 2 g - 5: g g :1 2 2 - ,g 5 a. 6 a v a E 3 - n e .. + 3 E g {3 E g 8 5‘0 *3 ‘5, E 5 5 9 3 a g - 2 e 3 “a 2’» -: .. s “ ~ g g 8 5 § 1 . a .2 a E g :E Z— § :8 Q m a. if. 0 G <2 8 11': O 8 1,250 2,400 7,000 11.6 .. _ 2 .__- _-__ 60 311 631 103 .... .-.- ... .-.- .... ...- ---- 1 .-._ .... 1,000 2,700 13,000 12.4 ,7 _ , _ ___ 7 0 .-__ 0 0 -___ 2,400 696 1,400 11.1 ’02" 37 0 0 "‘5; I: ‘600 366 "0225 10.6 ’10 e7 7’ '1: I: f: 0"" .I: ..-. ———— 9.7 ____ _-__ 2,300 3.9 100 42.7 32 .___ 30 .000 ..__ ___. ___. 3.4 ___, .-.. 2,400 4-3 061 _... 64 .._. 24 .000 ..__ 05 .__. ___- 200 1,590 2,300 ~-- 28 ._.. 1 8 ._.. .... .050 .... 2At end of drain (ditch). wastes of the U .S. Marine Corps Supply Center (Nebo) water temperature in degrees Celsius; specific conductance in micromhos at 25°C; and pH] E 8 A 2 .2 A 5 2 9 a .. 5 .8 ‘5 m g 8 - 6 e = 3 § E -- fi 2 z 3 - 3 5 ° 3 E E . 9 V on CG '3 ° Sb 1: E 3 Q, 9 3 .1: v > g -- 'u g}, u a ,,, a D- : ‘—' ,_ g E I- ~ ; a ‘3 .3 g 2 2 m E .3 ‘3 3 :5 a Z a. an cu m a. a. o o :2 m :3 .__- 1,400 506 820 72 ___. _._. ___. _,_ _.__ 710 569 1,000 7 5 0 04 79 .... 0.0 ___- 1,600 _-__ 1,400 ..._ .0. .1“ 2 .. ._._ :2 1,400 I: I: I: I: I: 530 I: I: I: 1.8 900 780 _... 7.5 .018 ___. .18 __.- .__, ___. .0 30 840 _. . 9.2 .00 ..._ .06 .._. ..__ ___. 2.7 850 830 ___. 9.4 <.06 .___ .13 __-. .__. ... 23.9 2,500 940 __._ 8.1 .018 ..._ .52 ..._ .... 0.1 5.2 1,200 710 ___. 9.4 ___. .___ .06 .-._ ___. .15 4.0 1,730 790 ___. 9.0 00 ._.. .03 .... .___ .0 .0 820 780 ___- 7.3 ..__ ___. .0 ..__ .-.- .0 ___- ...- ___- 1,700 7.6 .003 41.3 .80 .012 0.000 ___. ..__ 1,280 990 1,500 9.6 016 ._._ .68 01 00 ._._ ,___ 1,120 960 1,500 9.3 032 __.. .48 01 00 ___- ,___ 1,420 900 1,400 9.0 028 -___ .52 00 00 ___- ____ 1,180 860 1,300 8.8 048 .-.- .40 01 00 ..__ .._. 1,320 860 1,300 8.6 .028 .-__ .22 01 .00 .___ ._.. 1,460 830 ,300 8.7 .013 _.__ .42 00 .00 ..._ ___. 1,300 900 1,400 9.0 .032 11.5 .45 01 .00 .... ___. 1,120 960 ___. 8.6 .032 11.5 .44 .._. .00 ._.. ___- 830 1,200 .-.- 9.3 .003 3 .26 .___ .01 ..__ 1,600 990 .... 8.5 .001 6.3 .56 .00 1,000 1,100 .... 7.4 .003 6.4 .48 .02 700 990 .... 10.0 .002 14.5 .32 .00 ___- ___- 1,140 -..- 9.2 .009 _..- .76 .02 ___. ___. 2,300 1,160 .... 9.8 .006 3.6 62 .... .00 --__ _,_, 1,800 1,230 .... 8.8 .001 8.2 44 ___. .01 .._. -_-. ___. 1,550 1,150 ..-. 8.6 .005 _-.- 16 ___. .00 ___. ._._ ____ 900 1,040 1,500 . 9.0 .036 .2 027 .... .00 .___ ___. 11.3 40 .-.. .-.- 8.5 .... .... 1.33 ..-_ .__. 1... ___ 13.07 1,190 ...- ..._ ___- ____ .___ .36 __._ -._- -__- ___. 11.0 2,000 1,000 1,500 .,.. _,__ _,__ .3 _,_1 ___1 _-._ INumerical average. attributed to this irregular distribution of recharge. The first irregularity is located beneath pond 2 and results from waste percolation from the lagoons; the other is near ponds 6 and 7. Both irregularities are elongated downstream (section A—A ’) and toward the center of the river (figs. 6, 8, 9, sectionsB—B’ and C—C ’) in response to the local system of ground-water flow. The hydrologic effects on the aquifer system caused by the city’s waste discharge are somewhat similar to the effects caused by flood recharge. The discharge and concomitant recharge to the aquifer have depressed or displaced underlying ground water of different quality. SectionA—A ’ (figs. 6, 8—11) suggests that a body of better quality ground water has been isolated between the deeper, older degraded ground water and the younger plume produced by present waste percolation. The quality of ground water in the shallow plume becomes better with depth. Chemical analyses of ground-water TABLE 6.—Chemical analyses of mining wastes [Constituents in milligrams per litre, except total iron, boron, and arsenic in micrograms per litre; water temperature in degrees Celsius; specific conductance in micromhoe at 25°C; and pH] GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL TO ALLUVIUM, BARSTOW, CALIF. (no) Jaddoo 5 i i 5 5 5: samples from shallow wells adjacent to the present """ ' oxidation ponds are very similar to the analyses of waste effluent in each of the ponds. Recharge to ground water from irrigation of the USMC golf course with treated effluent has produced an underlying body of degraded ground water that reflects both the quality and quantity of the effluent used. The vertical distribution of water quality beneath the golf course is similar to that described beneath the city’s treatment facility (figs. 6, 8—11), except for the absence of the deeper, older plume of poorer quality water. The quality of ground water beneath the golf course is much poorer at a depth of 50 ft (15 m) than at a depth of 100 ft (31 m) below land surface. The only apparent source for this poor-quality ground water is the irrigation-return water that had been concentrated by domestic and -° : - i industrial use,by evapotranspiration, and by solution of fertilizers prior to and during infiltration. Chemical and hydrologic data indicate that the distribution of water quality in the vicinity of the golf course is influenced by the Waterman fault and by ('os) mums amass § . extensive pumping at the USMC Supply Center. Because the Waterman fault tends to retard ground- water flow, drawdown of the water table caused by pumping at the USMC Supply Center is exaggerated and tends to reverse the natural ground-water gradient. This reversal in gradient has resulted in ground-water flow from areas of poor-quality water near the golf 1 : : ’ course and the river-channel deposits toward the USMC ‘ ‘ ‘ ' Supply Center wells (figs. 10 and 11, section D—D’). .\ (an) ammos : 5 2 7 6 7 72 Hd 0155:4145 aoumanpuoo ogioads g 8 § § § 8 ,000 svnos peAlossio § 469 526 469 2 300 3 200 o (H) “0108 g H“ 550 450 430 1 550 2,6 1 0 (vow mum 2*”83 170 (d) aPl-mnlzl (10) GPFIONO g “'3 8 {'3 {I 330 (comaqeuoqmg ocooo ca (90011) awuoqmvia was 298 7 5 38 ()1) umisssgod 4 125 127 98 640 1 000 DISSOLVED SOLIDS (3w) ummaufiaw "‘ N 2 18 13 6 Dissolved solids are a measure of the amount of dissolved mineral matter in water and may be (1) determined by totaling all anions and cations analyzed, (2) determined by weighing the dry residue after evaporation, or (3) approximated from specific conduc- tance. In the Barstow area the concentration of dissolved solids in ground water ranges from less than 500 to more than 2,000 mg/l (Hughes and Patridge, 1973). Available data suggest that the concentrations of dissolved solids in floodflows that recharge the aquifer are about 150 mg/l (Miller, 1969, p. 19). Very few wells have been drilled south of the river in the older, more consolidated deposits; therefore, chemical data are sparse on ground-water inflow from these areas. Analyses of ground-water samples from wells 9N/1W—15Q1 and —15Q2 (fig. 3) and wells 9N/1W—27D1 and 9N/1E —19J1—5 (not shown in fig. 3) indicate that dissolved (no) wmolao 33338 48 130 9 ll (as) mm 191% 8 I 1 l l 3:666 3 8 16 r ‘O!S) Boms " (3°) ammxaduxaq 13mm "is 3—27—52 __1__ __ 8—11—52 to <- 3130 é :5 r-( (s ‘39 996) suoueooI ans pus aomos Tungsten mill ore-wash water (Site A) ‘Not shown in fig. 3; located about 9% mile upstream from site A. Pacific Coast Mining & Milling Co. sump.l solids in ground water south of the river are generally high and vary directly with well depth. The concentra- tions of dissolved solids in water from wells 9N/1E—19J1 DISTRIBUTION AND EVALUATION OF CHEMICAL SUBSTANCES AFFECTING GROUND-WATER DEGRADATION 21 I I I I OCt. Aug. Sept Juw June 1 972 May ApL Man A—A Well 10J3 Feb. J I. I I I I I I EHLI'I Had SWVHQI-I-IIW NI INEQOHLIN SV N N '- EIVHJJN 'I' ELIHJJN :IO NOIIVHINEONOO 3H1” Had SWVHBITIIW NI ‘3CHUO‘IHO :IO NOIlVHlNEONOO 001 1971 100 I I I I I | EXPLANATION 0—0 We" 9H7 Oct. Aug. Sept Juw I I | o——0 WeII 9H5 June 1972 May Apr. FIGURE 5.—Change in concentration of selected chemical constituents in Wells 9N/1W—9H5, —9H7, and —10J 3. Man Feb. A‘ 0/ Oct. 1971 I | I VP 0) N V— O 1300 1100 — 900 _ 800 I I § 8 gum Had swvusmlw NI .— 9 3V9“ do NO'lVHlNEONOO aum Had swvuen'nw NI 'SGI‘IOS CIEATOSSICI :IO NOILVHLNSONOO 22 and—19J5, which are 250 and 660 ft (76 and 201 m) deep, range from 800 mg/l in the shallow well to 2,300 mg/l in the deep well. Water from other wells south of the river (9N/1W—15Q1, ~15Q2, and —27D1) contains less than 800 mg/l of dissolved solids. Robson (1974; Hughes and Robson, 1973) estimated that ground water entering the area from the south contained an average of 1,000 mg/l of dissolved solids. The effect of this inflow on the quality of water in the river-channel deposits is probably minor compared with the quantity and quality of municipal- and industrial-waste disposal to ground water. Volume of inflow from the south has been estimated to be 120 acre-ft (0.15 hm3) per year (table 2). Inflow from the consolidated rocks to the north is probably negligible. Available data suggest that the greatest impact on quality of ground water in the project area has resulted from the large volumes of ground water used for domestic, industrial, and agricultural purposes. Water used for these purposes and returned to ground water has increased in dissolved solids by (1) addition of solid or liquid concentrates, such as detergents, fertilizers, rust inhibitors, oil and grease, and a variety of organic matter; and (2) evapotranspiration associated with agriculture and with exposure in waste-treatment facilities. The concentrations of dissolved solids in industrial and municipal wastes have averaged more than 1,000 mg/l, and some effluent from the railroad facilities has exceeded 2,500 mg/l of dissolved solids (tables 3, 4, 5, and 6). The distribution of dissolved solids in ground water in the Barstow area is illustrated in sections A—A’ and B—B’ (fig. 6). The concentration of dissolved solids in water from well 9N/1W—10J4 is very similar to waste water in the city’s oxidation ponds. During 1970, dissolved solids in these ponds averaged more than 950 mg/l (table 3). In 1972 dissolved solids in ground water from well 9N/1W—10J4 were approximately 1,000 mg/l (Hughes and Patridge, 1973). This well is about 300 ft (91 m) downstream from the ponds. The adverse effects on ground water caused by using treated and reclaimed sewage on the golf course are indicated by water-quality data from wells 9N/1W— 1 1P1, —11Q1, and—11R1 (table 7) and the distribution of nitrogen and chloride in section D—D’ (figs. 10 and 11). Wells 9N/1W—11P1, —11Q1, and —11R1 are approxi- mately 50 ft (15.2 m) deep, and their ground water contains a very high concentration of chloride, sulfate, and dissolved solids compared with adjacent deeper wells 9N/1W—11P2, —11Q2, and—11R2, which are about 100 ft (30.5 m) in depth. The distribution of chloride and nitrogen in section D—D’ appears to be influenced by pumping at the USMC Supply Center and by the barrier effects of the Waterman fault. GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL TO ALLUVIUM, BARSTOW, CALIF. TABLE 7,—Comparison of water quality in shallow and deep test wells at USMC Supply Center golf course Constituents, in milligrams per litre Depth Well No. (ft) . Dfifihfd Calcium Magnesium Sodium Sulfate Chloride 9N/1W—11K5 ,,,,,,,,, 56 413 64 11 62 100 40 11K6 ,,,,,,,, 43 666 68 14 140 190 31 11K7 ,,,,,,,, 102 382 54 9.4 61 7s 41 11M1 ,,,,,,,, 51 504 74 14 74 110 35 um ........ 131 459 69 11 72 94 61 11P1 ......... 52 726 35 7.1 210 130 120 11132 115 500 73 14 73 100 76 11Q1 ........ 53 1,520 190 35 300 370 420 11622 ........ 105 439 67 12 64 88 54 11R1 ........ 52 1,160 140 27 220 280 310 11R2 ........ 102 339 53 9.6 64 77 46 DETERGENTS (AS MBAS) Methyl blue active substances (MBAS) refers to a colormetric test used to measure the apparent concen- tration of synthetic detergents in water. This test does not distinguish between the nonbiodegradable ABS (alkyl benzene sulfonate) and the biodegradable LAS forms. Synthetic detergents were first used in the United States about 1946, and now they constitute the principal washing compounds for industrial and domes- tic purposes. These compounds contribute to water- quality problems when they percolate to ground water, usually as part of the effluent from industrial and domestic waste sources. Problems associated with treatment and disposal of water that contains deter- gents were partly solved in 1964 with the development of the biodegradable LAS and with restrictions placed on the use of the nonbiodegradable ABS. Experiments conducted on the biodegradation poten- tial of LAS and ABS (Halvorson and Ishaque, 1969, p. 571—576) indicated that under proper aerobic con- ditions, LAS would biodegrade by 97.5 percent in 120 hours at 25°C while ABS remained unaffected. Under similar aerobic conditions, the rate of biodegradation of LAS at a temperature of 10°C decreased by a factor of 2.5, and at 2:0.1°C no biodegradation of LAS was observed over a period of 12 days. Similar experiments for anaerobic conditions showed that neither LAS nor ABS was appreciably biodegraded at 25°C. The results of these experiments strongly suggest that the concen- tration of ABS detergents in domestic and industrial wastes in the project area during 1946—65 biodegraded very little prior to percolation into the river-channel deposits. Since 1965, the amount of biodegradation of detergents was probably largely dependent on water temperature in the treatment plant. Winter tempera- ture in Barstow averages 10°C: with nighttime temperature falling below 2°C (data from National Weather Service, 1972). During summer months, temperatures probably are sufficiently high to permit maximum biodegradation. Synthetic detergents are not indigenous to ground DISTRIBUTION AND EVALUATION OF CHEMICAL SUBSTANCES AFFECTING GROUND-WATER DEGRADATION 23 N Barstow waste-disposal ponds F USMC g 0? (projected) golf course‘i ‘? '- '2. E E 1' a ,1, :r <2- 2— A Z 3 g El» 3' e g E e 9 E :3 2080's _ " _ F—I'fii— . _ 7 7 7 7 z , 2, 2st E sees: a _630 °’ ‘0' °’ 2 _ 2 2 2 2 2' _ Water table , - w 2 3-, a, a, a, ,_ . °‘ g —620 2000' — =- ~e1o \ \ _ ‘ ,5. ) —600 3:0} | vol, U, l 590 2 1920— an, . — Eh. Older aIIuvium <5oo EJII': 5 —580 _ I ’K\?\\ \ I-|_J I 2 00\_ I \\?\ ~ 93 35.33% «EN 3380 028MB 838mg GOD we :ossflbmmw ~m2¢395 30653 90 >=QIUEmD Fm© =03 osmoEouéouaHEH E. :03 30% E0 <9: «6 :2: 530.3 .8 8 Race nagging—So Am< 041 209 500— — (0.1 892:! 0: 40°F- — 300 I I I I I I I I I I I I I I I I I I I 9° I I I I I I I I I I I I I I I I I I I _ ‘2 70—- 9 _ a! o 50— I5 ‘é so— 2 .2. ui 40_ _ 2 n. 2 E 30..— 5 .— E 5 20— 10 I I I I I I I I I I I I I I 1 I I 5‘” I I I I I I I I I I I I I I I I I I I 3% x” "-3.1 932' 400— _ no Eu. go 2'9 300— _ :9 200 I I I I I I I I I I I I I I I I I I I 1953119541195511956I1957I195811959I1960I1961I19621196311964I1965I1966]1967I1968I1969I1970I1971I1972 FIGURE 12.—F1uctuation of dissolved solids in well 9N/1W—13E1 (Nebo 4) and water-supply demands at the Marine Corps Supply Center' (Nebo). yielding well, was abandoned in 1968 and was replaced in 1969 with a new, high-yielding well, N ebo 6 (table 8). Another well, Nebo 3, was reperforated and modified in 1972 to increase production. An aquifer test was conducted at the USMC Supply Center in March 1972 to define the approximate area TABLE 8.-—Data on USMC Supply Center wells (Nebo) Depth to Well number Pum - ——“-— Date Def?" Pegga- water (if below raging USMC USGS drilled ( ) land surface - . (fl) datum)‘ (cal/mm) Nebo 1 9N/1w—14Bl _-__ 1942 192 $0421 51.29 .--_ ,V.. 1 1—1 1 .--- ____ 2 14A2 ____ 1942’ 408 107—407 61.13 668 3 1432 _ __ - 19473 208 37—280 60.20 386 4 13E] __,_ 1954 348 48-348 61.10 1,039 5 13E2 ____ 1961 450 65-440 57.40 585 6 1433 ___, 1969 336 109-312 52.35 770 IMeasured March 29—April 1972. 2Date ori inaIly drilled; reconstructed 1958 “Replac pump, cleaned, dreperforated,‘JuIy1972. influenced by pumping and thereby to identify the possible source or sources affecting the quality of water at the base. USMC supply wells 9N/1W—13E1, —13E2, —14A2, and —14B3 (table 8), which were pumped at full capacity for 96 hours, discharged a total of about 11 million gallons (4.2 X 104 m3). Response of ground- water levels to the discharge was measured in the four pumping wells and in 17 nonpumping observation wells. The combined discharge from the pumping wells is comparable to peak water-supply demands at the USMC Supply Center during the summer (fig. 12). Test results were used to delineate an area of drawdown equal to or greater than 0.25 ft (0.08 m) as shown in figure 13. Initially, most of the water-level decline was in the immediate area of the pumping wells. As pumping continued, water-level declines extended 32 R.1W. E 7%., J2.“ “to," I” ,...«-’1f'”"' ‘. “an“. C F C§DN1 orps Supply (Nebo area) " Base from US. Geological Survey Daggett 1:62 500, 1956 Roads as of 1972 0 V2 1 MILE 0 .5 1 KILOMETRE EXPLANATION Concealed fault Area affected by drawdown in well Drawdown equal to or greater than 0.25 ft (0.08 m). Queried where doubtful ©A2 USMC supply well 0N1 Observation well FIGURE 13.—- Area affected by a 96-hour aquifer test at US. Marine Corps Supply Center. Waste-disposal sites (C, D, E) as identified in figure 3. outward. After the drawdown cone was subjected to the barrier effect to ground-water flow caused by the Waterman fault, the area affected by the pumping extended northwestward along the fault at a greater rate than on the downstream side. The shape of the cone of depression south and east of the well field could not be observed because of the lack of wells in those areas. Although the aquifer along the Mojave River north and west of the USMC Supply Center’s well field is capable of supplying large quantities of water, water in this area is also potentially the most detrimental to the quality of water pumped at the USMC base. Percolation from the present waste-disposal facilities for the city of Barstow, a plume of older, degraded water in the river-channel deposits, and the USMC Supply Center’s golf course are in this direction. Therefore, unless some changes are made to alter ground-water flow paths, the water pumped at the USMC Supply Center can be GROUND-WATER DEGRADATION RESULTING FROM WASTE DISPOSAL TO ALLUVIUM, BARSTOW, CALIF. expected in future years to increase in dissolved solids, chloride, and those compounds indicated by DOC. Because of the proximity of the USMC golf course, it seems that degraded ground water from this source would first reach the USMC supply wells. As stated earlier, the city of Barstow sewage— treatment facility is currently (1972) completing mod- ification of its plant to provide full secondary treatment of domestic and industrial wastes. This reconstruction is designed to improve the quality of effluent for percolation. The USMC Supply Center stopped the use of treated effluent on the golf course in October 1972. The use of fresh water on the golf course will probably not improve the quality of water pumped at the USMC Supply Center immediately, although with time the general quality of ground water in the vicinity of the golf course will probably improve. The combined effects of percolating effluent of better quality by the city of Barstow and by the USMC Supply Center will be to improve the quality of water in the shallow ground- water zones. RATE OF GROUND-WATER MOVEMENT The average velocity of ground water in the river- channel deposits has been estimated by interpretation of both hydrologic and chemical data. Ground-water velocities estimated from hydrologic data indicate a flow rate of approximately 1.0 ft (0.3 111) per day. The hydrologic data used in this estimate were generated by a digital model of the ground-water system in the project area (S. G. Robson, written commun., 1972). The velocity of ground-water flow was also estimated by comparing the present downstream location of the degraded plumes with the first records of waste disposal at specific sites in the alluvial deposits. The nose of the plume outlined by DOC (fig. 7) is approximately 4.5 mi (7.2 km) below the point where the railroad reportedly first began to discharge wastes. Assuming about 60 years has elapsed since that discharge began, a downgradient maximum velocity of about 1.0 ft (0.3 m) per day would be required to achieve the present downstream location of the DOC plume. A similar computation using detergents (as MBAS) indicated that a downgradient maximum velocity of approximately 1.5 ft (0.46 m) per day would be required to account for the downstream location of the plume (fig. 7). This estimate assumes that synthetic detergents were first used in the Barstow area in 1945 and that the city of Barstow’s facility at site B (fig. 3) was the point of waste discharge. SUMMARY The degraded ground water that has resulted from municipal and industrial waste discharge in the Barstow area has been delineated both areally and SELECTED REFERENCES vertically for the period March—April 1972. The quality of water in the river-channel deposits is part of a dynamic system and is constantly changing in response to changes in quality and quantity of recharge and location of waste water and irrigation-return water. The distribution of chemical quality in the river- channel deposits indicates that an old plume of degraded water is moving near the base of the alluvial aquifer. Since 1910 this degraded plume has moved downgradient about 4.5 mi (7.2 km). A more recent overlying plume of degraded water occurs near the downstream edge of the deeper plume. This plume has been produced by percolation from sewage-treatment facilities installed in 1968. Interpretation of hydrologic and chemical data indicates that the rate of movement in the deeper zone of degraded ground water is 1.0—1.5 ft (03—046 m) per day. Providing that the present (1972) conditions of recharge and discharge continue, it can be expected that the water supply at the USMC Supply Center will significantly reflect the poor quality ground water in the river-channel deposits within the next few years, even though the quality of the ground water in the shallow zones beneath the golf course may improve. Distribution of nitrogen and chloride in the ground water suggests that the gradual increase in dissolved- solids concentrations in the USMC Supply Center’s wells has resulted in part from the use of treated sewage effluent on the USMC golf course. SELECTED REFERENCES American Public Health Association, 1971, Standard methods for examination of water and wastewater [13th ed.]: 874 p. American Water Works Association, 1971, Water quality and treatment [3d ed.]: McGraw-Hill Publishing Co., 654 p. Bader, J. S., Page, R. W., and Dutcher, L. C., 1958, Data on wells in the upper Mojave Valley area, San Bemardino County, California: U.S. Geol. Survey open-file report, 238 p. Brown, K. W., and Caldwell, D. H., 1970, Domestic and industrial waste study, Marine Corps Supply Center, Barstow, California: Consulting rept. to U.S. Marine Corps, 66 p., app. A—G. California Department of Public Health, 1966, Barstow ground-water study: Rept. to Lahontan Regional Water Quality Control Board, 12 p. and 7 tables. 1970, Barstow ground-water study: Rept. to California Re- gional Water Quality Control Board, Lahontan Region, 14 p. California Department of Public Health and California Department of Water Resources, 1960, Ground-water quality studies in Mojave River valley in vicinity of Barstow, San Bemardino County: Rept. to Lahontan Regional Water Quality Control Board, 60 p. and app. California Department of Public Works, Division of Water Resources, 1952, Investigation of Mojave River, Barstow to Yermo: Rept. to Lahontan Regional Water Quality Control Board, 40 p. California Department of Water Resources, 1967, Mojave River R U.S. GOVERNMENT PRINTING OFFICE: 1975-0-689-910/86 33 ground-water basins investigations: California Dept. Water Resources, Bull. 84, 151 p. California Water Resources Control Board, 1963, Water quality criteria, edited by J. E. McKee and H. W. Wolf [2d ed.]: Pub. 3—A, 548 p. (repr. 1971). 1969, The Porter-Cologne Water Quality Control Act: Rept. to Regional Water Quality Control Boards, 44 p. Dyer, H. B., Bader, J. S., Giessner, F. W., and others, 1963: Data on wells and springs in the lower Mojave Valley area, San Bemardino County, California: California Dept. Water Re- sources Bull. 91—10, 212 p. Halvorson, Harvest, and Ishaque, M., 1969, Microbiology of domestic wastes. III. Metabolism of LAS-type detergents by bacteria from sewage lagoon: Canadian Jour. Microbiology 15, p. 571—576. Hardt, W. F., 1969, Mojave River basin ground-water recharge with particular reference to the California floods of January and February 1969; U.S. Geol. Survey open-file report, 13 p. 1971, Hydrologic analysis of Mojave River basin, California, using electric analog model: U.S. Geol. Survey open-file report, 84 p. Hughes, J. L., and Patridge, D. L., 1973, Selected data on wells in the Barstow area, Mojave River basin, California: U.S. Geol. Survey open-file report, 102 p. Hughes, J. L., and Robson, S. G., 1973, Effects of waste percolation on ground water in alluvium near Barstow, California, in Under- ground waste management and artificial recharge— Interna- tional Symposium: Am. Assoc. Petroleum Geologists, U.S. Geol. Survey, and Internat. Assoc. Hydrol. Sci, v. 1, p. 91—129. Koehler, J. H., 1969, Water resources at the Marine Corps Supply Center, Barstow, California for the 1968 fiscal year: U.S. Geol. Survey open-file report, 16 p. Malcolm, R. L., and Leenheer, J. A., 1972, Organics Task Group: U.S. Geol. Survey Research Note 112, 3 p. Meyer, C. B., and Horn, W. L., 1955, Water utilization and requirements of California: California Water Resources Control Board, Bull. 2, v. 1, 227 p. Miller, G. A., 1969, Water resources of the Marine Corps Supply Center area, Barstow, California: U.S. Geol. Survey open-file report, 51 p. Page, R. W., and Moyle, W. R., J r., 1960, Data on wells in the eastern part of the middle Mojave Valley area, San Bernardino Co., California: California Dept. Water Resources Bull. 91—3, 223 p. Page, R. W., Moyle, W. R., Jr., and Dutcher, L. C., 1960, Data on wells in the west part of the middle Mojave Valley area, San Bernardino County, California: California Dept. Water Re- sources Bull. 91—1, 126 p. Robson, S. G., 1974, Feasibility of digital water-quality modeling illustrated by application at Barstow, California: U.S. Geol. Survey Water—Resources Inv. 46—73, 66 p. Thompson, D. G., 1929, The Mohave Desert Region, California, a geographic, geologic, and hydrologic reconnaissance: U.S. Geol. Survey Water-Supply Paper 578, 759 p. Thomthwaite, C. W., 1948, An approach toward a rational classifica- tion of climate: Geographic Review, v. 38, no. 1, p. 55—94. U.S. National Weather Service, 1972, Climatological data, Washington: U.S. Weather Bur. mo. rept. U.S. Public Health Service, 1962, Drinking water standards, 1962: Pub. 956, 61 p. 1969, Observations of Continental European solid waste management practices: Pub. 1880, 46 p. 7 DAY .< m ‘6}- W57 ‘Tx 4, r 43 Silurian-Devonian Peleeypods and Paleozoic Stratigraphy of Subsurface Rocks in Florida and Georgia m and Related Silurian Pelecypods ., From Bolivia and Turkey GEOLOGICAL SURVEY PROFESSIONAL PAPER 879 DOCUMENTS DEMRTMENT‘ MAR l 4 1976 LIBRARY , ”fir/5523577 OF CAIJFORHM WI Silurian-Devonian Pelecypods and Paleozoic Stratigraphy of Subsurface Rocks in Florida and Georgia and Related Silurian Peleeypods From Bolivia and Turkey By JOHN POJETA, JR., JIRI Kifii, and JEAN M. BERDAN GEOLOGICAL SURVEY PROFESSIONAL PAPER 879 General geologic setting of the Florida and Georgia subsurface Paleozoic; systematics and the biostratigraphic, paleoecologic, and paleobiogeographic significance of the pelecypods UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Pojeta, John. Silurian-Devonian pelecypods and Paleozoic stratigraphy of subsurface rocks in Florida and Georgia and related Silurian pelecypods from Bolivia and Turkey. (Geological Survey professional paper 879) Bibliography: p. Supt. of Docs. no.: I 19.162879 1. Lamellibranchiata, Fossil. 2. Paleontology—Silurian. 3. Paleontology—Devonian”; Geology, Stratigraphic —Pa1eozoic. 5. Geology—Florida. 6. Geology—Georgia. I. Krii, Jiri, joint author. 11. Berdan, Jean Milton, joint author. 111. Title. IV. Series: United States. Geological Survey. Professional paper 879. QE811.P65 564'.11'09758 74—16046 For sale by the Superintendent of Documents, US. Government Printing Office Washington DC. 20402 Stock Number 024-001-02731-3 CONTENTS Page Abstract ___________________________________________ 1 Introduction ________________________________________ 1 Acknowledgments ___________________________________ 2 General geologic setting of subsurface Paleozoic rocks in Florida and Georgia ______________________________ 2 History Of drilling ____________________________ 2 General setting _______________________________ 4 Lower Ordovician sandstones __________________ 4 Middle or upper Ordovician clastic rocks ________ 5 Silurian and Devonian shales __________________ 6 Middle (?) Devonian clastic rocks ______________ 7 Igneous rocks ________________________________ 9 Structural relationships ________________________ 10 Summary ____________________________________ 10 Page Systematic paleontology ______________________________ 10 Florida and Georgia wells ___________________ ,___ 11 Chandler well ____________________________ 11 Tillis well ________________________________ 12 Cone well ________________________________ 14 Ragland well _____________________________ 17 Bolivia collection ______________________________ 17 Biostratigraphy _____________________________________ 18 Paleoecology and environment of deposition ____________ 2O Paleobiogeography __________________________________ 24 References cited _____________________________________ 27 Index ______________________________________________ 31 ILLUSTRATIONS [Plates follow index] PLATE 1. Cheiopteria, Dualina, Pterochaenia, Actinoptem'a, Leptodesma, Joachymia, and Mytilarca. 2. Lunulacardium, Panenka, Actinopteria, and Butom'cella. 3. Eoschizodus?, Numl'ites, Arism'gia, Tentaculites, Actinopteria, Pleurodapis, and Ptem'nopecten?. 4. Plectonotus (Tritonophon), pholadomyacean, Prothyn‘s, Modiomo'rpha?, Dawsonocems, and Parakionoceras. 5.- Palaeoneilo, Deceptfix, Nuculites, Mytila'rca, Actinoptem‘a, and Dualina. Page FIGURES 1—4. Index maps showing: 1. Subsurface distribution of Paleozoic rocks and structure contours on the top of Paleozoic rocks in peninsular Florida and adjacent parts of Georgia and Alabama and location of wells discussed in this report _________________________________________________________________________ 3 2. The four wells which have yielded pelecypods in Florida and Georgia __________________________ 11 3. Silurian pelecypod locality in Turkey ______________________________________________________ 16 4. Silurian pelecypod locality in Bolivia ______________________________________________________ 17 5. Chart showing occurrences of pelecypod taxa in Bohemia and the Cone well of Florida ______________ 20 6. Diagrams showing percentages of various pelecypod ecologic types for the Tillis and Gone wells and the collection from Bolivia ____________________________________________________________________ 22 7. Mercator projection of the position of Late Silurian-Early Devonian landmasses and waterways and dis- tributions of various genera which occur in the Gone and Tillis wells ____________________________ 25 8. South-polar projection of the position of Late Silurian—Early Devonian landmasses and waterways and distributions of various genera which occur in the Cone and Tillis wells ______________________ 26 TABLES Page TABLE 1. Summary of the inferred life habits, by number of species, of the pelecypods from the Ragland, T-illis, and Cone wells and from Bolivia _____________________________________________________________ 22 2. Percentage distribution of the life habits of the pelecypods from the Tillis and Cone wells and from Bolivia ___________________________________________________________________________________ 22 III SILURIAN-DEVONIAN PELECYPODS AND PALEOZOIC STRATIGRAPHY OF SUBSURFACE ROCKS IN FLORIDA AND GEORGIA AND RELATED SILURIAN PELECYPODS FROM BOLIVIA AND TURKEY By JOHN POJETA,JR.,JkaK1\,{f2/, and JEAN M. BERDAN ABSTRACT The subsurface sedimentary Paleozoic rocks beneath north- ern Florida and adjacent parts of Georgia and Alabama comprise a sequence of quartzitic sandstones and micaceous shales, dark-gray shales, and red and gray siltstones ranging in age from Early Ordovician to Middle Devonian. The Silurian-Devonian pelecypod faunas from four wells (three of which, the Ragland, Cone, and Tillis wells, are in Florida, and one of which, the Chandler, is in Georgia) are described and illustrated. Also described are Silurian pelecypods from one locality in Bolivia and one in Turkey. Biostratigraphically, the faunas from the American wells range in age from Wenlockian or Ludlovian (Silurian) to Middle Devonian; the Bolivian specimens are probably Lud- lovian (Late Silurian); and the Turkish specimens are probably Wenlockian or Ludlovian (Silurian). Paleoeco- logically, the strata in the American wells represent shallow- water normal marine environments, and all pelecypods known from them belong to one of three life-habit groups—byssally attached, burrowing, or reclining. The Bolivian and Turkish pelecypods likewise belong only to these three life—habit groups. Analysis of the geographic distribution of the Florida Paleozoic pelecypod genera shows that they are closest to the forms found in central Bohemia and Poland; elements of this fauna also occur in Nova Scotia, North Africa, and South America. INTRODUCTION This paper is based upon material from four cores from Florida and Georgia and from two small col- lections, one each from Bolivia and Turkey (figs. 2—4). All six localities have yielded Silurian-Devon- ian pelecypods, and four of them show affinities to the Late Silurian—Early Devonian pelecypod fauna of Bohemia and Poland. The four American cores are from the following wells: (1) Mont Warren et al. A. C. Chandler No. 1, Early County, Ga.; (2) Coastal Petroleum Co. J. B. and J. T. Ragland No. 1, Levy County, Fla.; (3) Sun Oil Co. J. H. Tillis No. 1, Suwannee County, Fla; and (4) Humble Oil and Refining Co. J. P. Cone No. 1, Columbia County, Fla. The Chandler well reached a depth of 7,320 ft; Paleozoic rocks were first encountered at a depth of 6,600 ft. Palmer (1970) described some pelecypods from this well, but none of the mollusk fauna from the other wells has been described previously. The Ragland well reached a depth of 5,850 ft; Paleozoic rocks were first encountered at a depth of 5,792 ft. The Tillis well reached a depth of 3,572 ft, and the first Paleozoic rocks were recorded at a depth of 3,480—3,500 ft. The Cone well penetrated to a depth of 4,444 ft, and the top of the Paleozoic was reached at 3,482 ft. The taxonomic part of this paper is divided geo- graphically into sections on the southeastern United States, Bolivia, and Turkey, and comparisons are made with Bohemia and Poland where the Silurian and Devonian rocks contain similar or identical faunal elements. Each of the American wells is suffi- ciently different from the others that its fauna is described separately. The faunas are compared with each other and with those of central Europe. The pelecypod fauna from the Chandler well is probably Middle Devonian in age, that of the Tillis well is Early Devonian, that of the Cone well is Late Silurian (Pridolian), and the fossils from the Rag- land well are Silurian in age (Wenlockian-Lud- lovian). The material from Turkey and Bolivia is Silurian in age (Wenlockian-Ludlovian). Although the pelecypods are taxonomically varied, all belong to one of three life-habit groups: byssally attached, burrowing, or reclining. The abundance of byssate and reclining forms suggests that the en- closing sediments were laid down in relatively shal- low water. The abundance of paleotaxodonts in some of the collections also suggests this. 2 SUBSURFACE PALEOZOIC IN FLORIDA AND GEORGIA We have plotted the distributions of the pelecypod genera on a reconstruction of the positions of Late Silurian—Early Devonian landmasses and have indi- cated the probable oceanic surface current patterns. The distribution suggests that the similarity between the faunas of Bohemia and Florida may have been due to the transport of pelecypod larvae by a warm current from central Europe down the east coast of North America. Responsibility for the general geologic setting section is that of Berdan; Pojeta and Kr'iz are re- sponsible for the rest of the paper. ACKNOWLEDGMENTS We would like to thank the following persons for their help and cooperation: Necdet Ozgiil, of the Turkish Geological Survey, for collecting the speci- mens from Turkey and W. T. Dean, of the Geologi- cal Survey of Canada, for bringing the Turkish ma- terial to our attention; Mario Suérez—Riglos brought the Bolivian specimens to our attention; E. L. Yochelson and John S. Peel and the late Edwin Kirk identified the gastropods and crinoids respectively; Bedrich Bouéek, R. H. Flower, P. M. Kier, and G. A. Cooper examined the tentaculites, cephalopods, crinoids, and brachiopods respectively; graptolites were identified by W. B. N. Berry; J. M. Schopf identified the acid-resistant microfossils, and frag- ments of plant megafossils were identified by F. M. Hueber. K. V. W. Palmer loaned us the specimens she de- scribed from the Chandler well; these specimens are stored at the Paleontological Research Institution (PRI). The Turkish specimens are the property of the Maden Tetkik ve Arama (MTA). All other figured material is deposited in the United States National Museum (USNM). Preparation of the section on the general geologic setting of the subsurface Paleozoic rocks in Florida and adjacent parts of Georgia and Alabama would not have been possible without the advice and en- couragement of P. L. Applin and the late E. R. Applin, who initiated and coordinated much of the work on the Paleozoic of this area. Unattributed re- marks in this section on the ages and lithologies of Paleozoic rocks are based on the examination by Berdan of cores and samples provided by numerous oil companies and by the Florida Geological Survey. Special thanks are due the following: the late Her- man Gunter and the late R. 0. Vernon, both Direc- tors of the Florida Geological Survey; the late D. J. Munroe and the late Louise Jordan, of the Sun Oil Co., who made available the cores of the Chandler, Tillis, and Ragland wells; A. C. Raasch, Jr., and E. T. Caldwell, of the Humble Oil and Re- fining Co., who made available the cores of the Cone well. GENERAL GEOLOGIC SETTING OF SUBSURFACE PALEOZOIC ROCKS IN FLORIDA AND GEORGIA HISTORY OF DRILLING The first intimations that Paleozoic rocks underlay the Mesozoic section in Florida and adjacent parts of Georgia and Alabama came in the late twenties (Gunter, 1928) and thirties (Campbell, 1939). At that time no fossils had been found; the first wells were drilled by cable tools, and no cores were avail- able, so that Gunter (1928, p. 1108) interpreted the highly micaceous cuttings from Ocala Oil Corp. Clark-Ray—Johnson well No. 1 (fig. 1, well 8, sec. 10, T. 16 S., R. 20 E., Marion County, Fla.) as meta- morphic rock and compared them with the meta- morphosed Paleozoic formations farther north beneath the Cretaceous. In 1939 (completed Jan. 1940) the St. Mary’s River Oil Corp. Hilliard Tur- pentine Co. No. 1 (fig. 1, well 22, sec. 19, T. 4 N., R. 24 E., Nassau County, Fla.) penetrated 168 ft of dark-gray, non-calcareous, slightly micaceous shale with interbedded siltstone and quartzite. This was also a cable-tool well, and Campbell (1939, p. 1713) suggested that the sequence might represent the Chattanooga Shale. It was not until the period from 1943 through 1950, when many wells were drilled with rotary rigs and cored (fig. 1) that enough fossils were found to provide a tentative frame- work for the Paleozoic stratigraphy beneath north- ern Florida and southern Georgia and Alabama. A general outline of the regional setting has been provided by Applin (1951), and a preliminary at- tempt at correlating the Paleozoic rocks was made by Bridge and Berdan (1952); the latter report, however, requires revision because of subsequent refinements in the identification of some of the fossils and more recent determinations of micro- fossils. Because the petroleum companies drilling the wells from which the data on the subsurface Paleozoic were obtained were looking for oil in the overlying Mesozoic section, most of the wells did not penetrate very far into the Paleozoic rocks. Probably in part as a result of the short amount of section penetrated by most wells, no well has encountered more than one assemblage of megafossils, and few have cut more than one lithologic unit. 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Hz:H .MH .H. .nH H 62 280 .HH .H. .oH ZOQ~HO HHQQHS 6 H .oZ .ucH £2013on ewes—gob .m 31ch H 62 acmEHowdnamiHflo .w H .02 .980 ooqueHH K H 62 .25.?th Hasuoauwm 6 VUOHOHHBHA ZOQ~HO HHMHBOAO H 62 HEwHHHHHvH .aH 4% .m H .02 HHSEHHSH .mH .0 .¢ H .02 mEdHJH 32 .m H .02 :Swuqu stuHH .N wDOHHmHnHHAwaOm .ZOQ~HO @0304 @ H 62 ES .2 5.5 .H * 803:. 72 QHZOHEZHE mAAHB ZH QHHHNMHHHZHAH mM0O~H ZOE.© 4 SUBSURFACE PALEOZOIC IN FLORIDA AND GEORGIA graphic position of the rocks penetrated by the wells must be inferred from the fossils; and in cores where fossils are nondiagnostic or absent, correlation be- tween wells has been based to a large extent on the lithology. Of more than 50 wells penetrating Lower Paleozoic sedimentary rocks, 18 have yielded either megafossils or acid-resistant microfossils. Fossils are scarce in the lower part of the sequence, and in many instances there is only one fossil per well; only six wells have yielded moderately diversified faunas, and all but one of these are in the part of the section dated as Silurian or Devonian. GEN ERAL SETTING The sedimentary Paleozoic rocks beneath northern Florida and adjacent parts of Georgia and Alabama comprise a sequence of quartzitic sandstones and micaceous shales, dark—gray shales, and red and gray siltstones ranging in age from Early Ordovician to Middle Devonian. Those in northeastern Florida and southeastern Georgia range in age from Early Ordovician to Early Devonian and occupy a roughly triangular area on the crest of the Peninsular arch (Applin, 1951, p. 3) ; this area was named the “Suwannee River Basin” by Braunstein (1958, p. 511) Which was shortened to Suwannee basin by P. B. King (in Flawn and others, 1961, p. 90). The southeastern boundary of this area extends diagon- ally across the State from about the north end of Old Tampa Bay on the west coast to a point between Jacksonville and St. Augustine on the east coast (Applin, 1951. p. 13, fig. 1). This boundary roughly parallels a northeast-trending series of magnetic lows (King, 1959, fig. 1). The northern boundary, less well defined and more irregular, trends generally east-west. This main area of sedimentary Paleozoic rocks is separated from a smaller area at the junction of Florida, Georgia, and Alabama by a deep trough or sag in the pre-Cretaceous surface which was first recognized by Dall and Harris (1892, p. 111) as the “Suwannee Strait,” a term later revised by Applin and Applin (1967, p. G30—G31) to Suwannee Saddle. The oldest rock penetrated in this trough appears to be Triassic in age (Applin and Applin, 1965, p. 16, fig. 3). These rocks have been discussed by Applin (in Reeside and others, 1957, p. 1486—1489). They appear to overlap the Paleozoic rocks on both sides of the trough, but as yet no wells in the middle of the trough have passed through the Mesozoic rocks into Paleozoic strata. The youngest Paleozoic rocks in the western, tristate area are Middle Devonian, younger than those on the Peninsular arch. LOWER ORDOVICIAN SANDSTONES The oldest Paleozoic sedimentary unit consists of quartzitic sandstones and highly micaceous shales characterized by vertical Skolithos borings and in- articulate brachiopods. This unit is present in more than 20 wells and is the most widely distributed of any of the Paleozoic units, occurring in wells from Marion County, Fla., to Houston County, Ala. Howell and Richards (1949, p. 35—36) described one of the linguloid brachiopods from Sun Oil Co. Hazel Lang- ston No. 1 (fig. 1, well 2, sec. 8, T. 8 S., R. 14 E., Dixie County, Fla.) as Lingulepis floridaensis and considered it possibly Late Cambrian to Early Ordo— vician in age and probably not younger than Early Ordovician. The most reliable date for at least part of this unit is Early Ordovician, based on the occur- rence of graptolites in Union Producing Co. E. P. Kirkland No. 1 (fig. 1, well 5, sec. 20, T. 7 N., R. 11 W., Houston County, Ala.). These graptolites have been identified by W. B. N. Berry (written commun., 1958) as Didymograptus deflexus Elles and Wood and ?D. protomdentus Monsen. According to Berry, neither of these graptolites is known from elsewhere in North America, but they do occur together in the zone of Phyllograptus densus (=3 b y of the Lower Didymograptus Shale) in Norway. He states that this zone is about the equivalent of Ross’ trilobite zone H in Utah and the Cotter and (or) Powell Formations in the Ozark sequence. While Didymo- graptus deflexus is not known elsewhere in North America, it is found in the lower Arenigian rocks of Wales and northern England, where it is the name giver of the lowest identified Ordovician graptolite zone (Williams and others, 1972, p. 11). The Kirkland well is correlated with most other wells in peninsular Florida primarily on the basis of the presence of Skolithos borings and on general lithologic similarity; it also contains large oboloid brachiopods. The cores from the Langston well and from two other wells in peninsular Florida, the Sun Oil Co. Alto Adams N0. 1 (fig. 1, well 3, sec. 15, T. 9 S., R. 15 E., Gilchrist County, Fla.) and Humble Oil and Refining Co. C. E. Robinson No. 1, (fig. 1, well 4, sec. 19, T. 16 S., R. 17 E., Levy County, Fla.) likewise contain fragments or specimens of large oboloid brachiopods. One well, the Humble Oil and Refining Co. Fore- most Properties Corp. No. 1 (fig. 1, well 9, sec. 4, T. 6 S., R. 25 E., Clay County, Fla.), which unfortu- nately has yielded no body fossils, cored more than 1,500 ft of Skolz'thos-bored sandstone, and another well, Stanolind Oil and Gas Co. and Sun Oil Co. Per- GENERAL GEOLOGIC SETTING 5 petual Forest, Inc., No. 1 (fig. 1, well 6, sec. 5, T. 11 S., R. 11 E., Dixie County, Fla.), penetrated more than 2,000 ft of the same rock. These thicknesses imply that beds of Cambrian age might be present in the lower part of the section, but because of the lithologic similarity from top to bottom of the Skolithos unit, the entire thickness is tentatively con- sidered Early Ordovician. The Foremost Properties well penetrated about 300 ft of white and reddish sandstone above the Skolithos quartzitic sandstone. The former unit is believed to occur in several other wells, in one of which, Ohio Oil Co. Hernasco Corp. No. 1 (fig. 1, well 7, sec. 19, T. 23 S., R. 18 E., Hernando County, Fla.), more than 750 ft of the unit was penetrated. No fossils have as yet been found in this unit, but it also is tentatively considered Early Ordovician in age because of its presumed stratigraphic position between the Skolithos beds and the next youngest unit. Carroll (1963, p. A9—A12) recognized three assemblages of heavy minerals in samples from this unit and the Skolithos unit; her mineral assemblage A is most common in this unit, although not re- stricted to it. Carroll (1963, p. A12) has suggested that her mineral assemblages may be useful for cor- relation; the occurrence of her assemblage A in the Robinson well in quartzite below assemblage C in Skolithos beds suggests the possibility that this unit and the Skolithos unit are interbedded. Cores from the Foremost Properties well also suggest this pos- sibility, although, because of the spacing of the cores studied for heavy minerals, this is not apparent in Carroll’s report. The presence of Carroll’s assem— blage A in sandstones above volcanic rocks in the Sun Oil Co. Henry N. Camp No. 1 (fig. 1, well 1, sec. 16, T. 16 S., R. 23 E., Marion County, Fla.) adds weight to the interpretation that these sandstones are Paleozoic and equivalent to the white sandstone unit above the Skolithos unit. The Skolithos unit is not present in this well; its absence may be due to faulting. MIDDLE OR UPPER ORDOVICIAN CLASTIC ROCKS A sequence of dark-gray to black shales, with some interbedded gray sandstone, presumably overlies the white sandstone unit. One well, Hunt Oil Co. J. W. Gibson No. 2, (fig. 1, well 10, sec. 6, T. 1 S., R. 10 E., Madison County, Fla.) penetrated 757 ft of dark- gray (N 3) shale and white to gray quartzitic sand- stone, with shale predominating. Core 2 from this well, from a depth of 5,154 to 5,162 ft, or 231 ft above the bottom of the hole, is a dark-gray shale which contains the trilobite Colpocoryphe exsul Whittington, 1953. Whittington (1953, p. 1) consid- ered this trilobite to be of Llanvirnian-Llandeilian Age or early Middle Ordovician. The same core con- tains a species of Conulam'a and numerous small phosphatic brachiopods which appear to be obolids. Whittington and Hughes (1972, p. 245) have stated that Colpocoryphe exsul belongs in their Selenopeltz's faunal province, which also occurs in Czechoslovakia and northern Africa. Core 2 was the lowest core taken in this well; gray and white quartzite appear in the rotary cuttings at a depth of 5,200 to 5,210 ft, and the amount of quartzite in the cuttings increases to the bottom of the hole. This may indicate that the contact of the black shale sequence with the under- lying white sandstone unit was crossed by this well, but without cores it is not possible to determine un- equivocally whether the contact was crossed or, if it was, whether the contact is sharp or gradational. Milton (1972, p. 19—20) reported fragments of diabiase in the cuttings from 5,200 to) 5,210 ft; diabase also occurs at the top of the Paleozoic sec- tion in this well. At least five other wells have penetrated Ordo- vician shales, Three of these, Sun Oil Co. Earl Odom No. 1 (fig. 1, well 12, sec. 31, T. 5 S., R. 15 E., Su- wannee County, Fla.), Humble Oil and Refining Co. Squire Taylor No. 1 (fig. 1, well 11, sec. 25, T. 3 S., R. 13 E., Suwanee County, Fla.), and Hunt Oil Co. Superior Pine Products Co. No. 3 (fig. 1, well 14, lot 532, Land District 13, Echols County, Ga.) have yielded conodonts identified by John W. Huddle (oral commun., 1973) as Drepanodus sp.; the genus ranges throughout the Ordovician, but species of this type are most common in the Middle and Upper Ordovician. The Odom-well conodonts came from core 8, at a depth of 3,060 to 3,080 ft, and are asso- ciated with phosphatic brachiopods. In addition, Andress, Cramer, and Golds-tein (1969) have de- scribed chitinozoans from the Odom well from a depth of 3,040 to 3,161 ft, which is the total thickness of Paleozoic rock penetrated by this well. They consider the chitinozoans of their bottommost sample to “indicate a geologic age for this sample which falls at an undeterminable position within the time span from late Arenigian to early Caradocian.” (Andress, Cramer, and Goldstein 1969, p. 369). This age range would include that of Colpocoryphe exsul from the Gibson well. The conodont in Superior Pine Products Co. No. 3 came from a depth of 3,670 to 3,680 ft. The cores at this depth are red micaceous shale and sandstone and contain fairly large phosphatic brachiopods. Lower cores in this well, from a depth of 3,892 to 3,895 ft, are dark-gray micaceous shale with some 6 SUBSURFACE PALEOZOIC IN FLORIDA AND GEORGIA interbedded fine sandstone and contain small phos- phatic brachiopods. Two other wells, Hunt Oil Co. Superior Pine Products Co. No. 1 (fig. 1, well 13, lot 364, Land District 13, Echols County, Ga.) and Hunt Oil Co. Superior Pine Products Co. No. 4 (fig. 1, well 15, lot 219, Land District 13, Echols County, Ga.) also contain phosphatic brachiopods of probable Middle Ordovician age. In Superior Pine Products Co. No. 1 the brachiopods are in a dark-gray mica- ceous shale, but in Superior Pine Products Co. No. 4, which penetrated only 5 ft of Paleozoic rocks, they are in red micaceous shale and sandstone like the upper part of the section in Superior Pine Pro-d- ucts Co. No. 3. SILURIAN AND DEVONIAN SHALES Presumably overlying the Ordovician shales in peninsular Florida and adjacent parts of Georgia is a sequence of black to dark-gray shales dated by fossils as Silurian to Devonian in age. As yet, no well .can be demonstrated to have passed from the Silurian—Devonian shales into the Ordovician shales, and the nature of the contact is unknown. In all, seven wells have penetrated shales of Silurian to Early Devonian age; however, two of these, Sun Oil Co. M. W. Sapp N0. 1A (fig. 1, well 21, sec. 24, T. 2 S., R. 16 13., Columbia County, Fla.) and Humbie Oil and Refining Co. Bennett and Langsdale No. 1 (fig. 1, well 18, lot 146, Land District 12, Echols County, Ga.) have been dated only on the basis of acid-resistant microfossils identified by J. M. Schopf (written commun., 1959). He identified Angochz’tina filosa Eisenack, Micrhystm'dium pavimentum De— flandre and Hystrichosphaeridium ramusculosum Deflandre from core 38 (4,171—4,173 ft, bottom) of the Bennett and Langsdale well, but Cramer (1973, fig. 2) has indicated that A. filosa occurs in three of his four chitinozoan zones for the Florida Paleozoic, ranging from Late Silurian into the Early Devonian. The relationship of the rocks penetrated in the Georgia well to those in wells in Florida is not cer- tain, except that they are probably Silurian or Early Devonian. The 65 ft of Paleozoic section penetrated in the Bennett and Langsdale well contains a consid- erable amount of sandstone, unlike the sections in other wells in the Silurian-Devonian interval, and the sandstone in some cores is crossbedded and shows horizontal trails to the extent that it was originally considered part of the Lower Ordovician Skolithos unit (Bridge and Berdan, 1952, p. 35). The M. W. Sapp No. 1A penetrated only 7 ft of shale, which was gray to pink at the top of the sec- tion and dark gray to black in the lower part and at the bottom of the hole. Schopf (written commun., 1959) did not specifically identify any microfossils from this well, but stated that the assemblage that he had recovered from core 22 (3,306—3,311 ft) was the same as that from Gulf Oil Corp. Kie Vining No. 1 (fig. 1, well 20, sec. 2, T. 4 S., R. 15 E., Colum- bia County, Fla.) from core 43 (3,450—3,470 ft). The Kie Vining well, which cored 117 ft of dark-gray Paleozoic shale, contains arthropod megafossils in addition to the acid-resistant microfossils. Kjel- lesvig—Waering (1955) described the eurypterid Pterygotus (Acutiramus) suwanneensis and the archaeostracan Ceratiocam's berdanae from core 43 of this well, but no other groups of megafossils have as yet been found. Goldstein, Cramer, and Andress (1969, p. 379) listed the chitinozoans from the Kie Vining well from a depth of 3,350 to 3,450 ft, which is above core 43, and considered them the same assemblage as one they found in the Hilliard well at a depth of 4,640 to 4,824 ft. Cuttings from the Hilliard well in the interval 4,700 to 4,800 ft have yielded a fragment of eurypterid integument with semilunar scales suggestive of Pterygotus (Applin, 1951, p. 15), which agrees with the correlation be- tween wells proposed by Goldstein, Cramer, and Andress on the basis of the chitinozoans. Cramer (1973, p. 284—285, fig. 2) suggested that the chitino- zoans in the Hilliard and Kie Vining wells fall in the age range of Wenlockian to basal early Gedinnian. The Hilliard and Kie Vining wells were originally tentatively correlated by Bridge and Berdan (1952, table 1) with Sun Oil Co. J. H. Tillis No. 1 (fig. 1, well 19, sec. 28, T. 2 S., R. 15 E., Suwannee County, Fla.) on the basis of the eurypterid remains. Gold- stein, Cramer, and Andress (1969, fig. 2), on the basis of chitinozoans, indicated that the Hilliard and Kie Vining assemblages were either younger or older than the assemblage that they found in the Tillis well, and later Cramer (1973, fig. 2) showed the Tillis microfauna as considerably younger than the Hilliard and Kie Vining chitinozoans. The Tillis well passed through 10 ft of brownish-gray, reddish- brown, and red and lavender variegated shale before penetrating 68 ft of black shale; apparently the chitinozoans were obtained both from the variegated shale (core 43, 3,494—3,502 ft) and the lower part of the black shale (core 44, 3,552—3,568 ft) accord— ing to Goldstein, Cramer, and Andress (1969, p. 377). The pelecypods described in this paper are all from core 44 in the black shale; no megafossils were found in the variegated shale. In addition to the pelecypods Nuculites sp. A, Nuculites sp. B, Ari— saigia cf. A. postomata, Arisaigia sp., Actinoptem'a GENERAL GEOLOGIC SETTING 7 sp. A, Actinoptem'a sp. B, Pterinopectenl Modio- morpha sp., Eoschz'zodus ?, Pleurodapis sp., Prothy- ris sp., and pholadomyacean, core 44 contains Ten- taculites, the gastropod Plectonotus, bolliid ostra— codes of an undetermined genus, smooth ostracodes, and Pterygotus floridanus, described by Kjellesvig- Waering (1950). The fossils are preserved as im- pressions in the shaie, with the exception of some tentaculitids, which are slightly pyritized. The dif- ferences in the megafaunal assemblages of the Kie Vining and Tillis wells suggest that Cramer’s sep- aration of these wells on the basis of Chitinozoa is probably correct. The two other wells in peninsular Florida which penetrated the Silurian-Devonian shales are Humble Oil and Refining Co. J. P. Cone No. 1 (fig. 1, well 16, sec. 22, T. 1 N., R. 17 E., Columbia County, Fla.) and Coastal Petroleum Co. J. B. and J. T. Ragland No. 1 (fig. 1, well 17, sec. 16, T. 15 S., R. 13 E., Levy County, Fla.) . The Cone well cored through a thick- ness of 950 ft of dark-gray to black shale and diabase sills (Milton, 1972, p. 13—18). In addition to the pelecypods Panenka sp., Dualina secunda, Mytilmca cf. M. longior, Lunulacardium excellens, Lunulw cardium spp., Cheioptem’a bridgei, Cheiopteria ?, Leptodesma carens, and Actinoptem‘a migrans, the Cone well megafauna also includes orthoconic cepha- lopods with the ornamentation of Parakionocems and Dawsonocems (core 122, 3,562—3,589 ft, core 127, 3,678—3,703 ft), two crinoids, one identified by Edwin Kirk as Periechocrmus? sp. (core 126, 3,653— 3,678 ft), the other identified by Porter Kier as a member of the Flexibilia which appears to repre- sent a new genus (core 126, 3,653—3,678 ft, middle), small rhynchonellid brachiopods (core 127, 3,678— 3,703 ft, top; core 128, 3,703—3,720 ft, middle) and an entomid ostracode (core 135, 3,863—3,888 ft). The most common fossils in the shale, extending as low as core 160 (4,414—4,439 ft), or just above the bottom of the well, are smooth, conical, straight shells which may be either hyolithids or orthoconic cephal-opods, but which are not sufficiently well pre- served to identify more closely. All the fossils are preserved as impressions in the shales, with the exception of the crinoids, some of which are pre- served as calcite. The Cone well penetrated approxi- mately 9 ft of variegated pale-reddish-purple to light-brownish-gray shale which contains an ortho— conic cephalopod (core 107, 3,485—3,487.5 ft) im- mediately overlying the black shale typical of the rest of the well. Goldstein, Cramer, and Andress (1969, p. 378) recognized three zones of chitinozoans in the Cone well: Zone A from 3,482 to 3,630 ft, Zone B from 3,768 to 3,994 ft, and Zone C from 4,156 to 4,444 ft T.D. Most of the megafossils listed above come from the interval 3,562—3,720 ft, and the pelecypods de- scribed in this paper come from the interval 3,562— 3,653 ft. The listed megafossils thus overlap part of chitinozoan Zone A and partly fill the gap between Zones A and B. Cramer (1973, fig. 2) has suggested that the chitinozoan zone present in the Kie Vining and Hilliard wells occurs in the gap between Zones A and B in the Cone well. In view of the difference between the arthropod megafauna of the Kie Vining well and the dominantly molluscan fauna of the Cone well, this hypothesis seems unlikely, and the sugges- tion of Goldstein, Cramer, and Andress (1969, p. 379) that the fauna of the Kie Vining well might be younger than that of the Tillis well is preferred. Unfortunately, the Kie Vining well has not as yet yielded any megafossils suitable for precise age de- termination. The Coastal Petroleum Co. J. B. and J. T. Ragland No. 1 (fig. 1, well 17, sec. 16, T. 15 S., R. 13 E., Levy County, Fla.) penetrated about 40 ft of very dark gray to black shale, of which only the bottom 10 ft was cored (Berdan and Bridge, 1951, p. 69). The bottom 10 ft (core 15, 5,840—5,850 ft) contains the pelecypods Butovicella migrans and Actinopteria sp., crinoid columnals, and poorly preserved orthoconic cephalopods. One of these has the ornamentation of Dawsonocems, and others are smooth and resemble the smooth straight shells in the Cone well. The core in the Ragland well was originally correlated With the upper part of the section in the Cone well be- cause of the general similarity of the faunas in the two wells (Berdan and Bridge, 1951, p. 69). Gold- stein, Cramer, and Andress (1969, p. 377) obtained chitinozoans from the Ragland well but did not discuss them because of their poor preservation. The black shale in the Ragland well is overlain by about 18 ft of unctuous yellow, gray, lavendar, and pink variegated well-stratified shale, of which 5 ft was cored (core 14, 5,791.5—5,796.5 ft). No fossils have been found in this variegated shale, but, because of its lithologic similarity to the shale at the top of the Paleozoic section in the Cone well, it is also consid— ered to be Paleozoic. MIDDLE(?) DEVONIAN CLASTIC ROCKS Three wells near the junction of the boundaries of Florida, Georgia, and Alabama, on the northwest side of the Suwannee Saddle and about 80 miles (128 km) west of the main area of Paleozoic rocks, have 8 SUBSURFACE PALEOZOIC IN FLORIDA AND GEORGIA penetrated shales, fine-grained sandstones, and silt- stones of probable Devonian age. One of these wells, Humble Oil and Refining Co. C. W. Tindel No. 1 (fig. 1, well 23, sec. 8, T. 5 N., R. 11 W., Jackson County, Fla.) cored 738 ft of gray and grayish-red shales, siltstones, and fine-grained sandstones containing plant fragments from 8,526 to 9,165 ft (cores 101— 138). The plant fragments are sporadically distrib— uted, and many of the cores show crossbedding. J. M. Schopf (written commun., 1959) reported that a sample from core 120 (8,737—8,742 ft) contained Dicrospom Winslow and Hymenozonotm'letes Nau— mova and is probably Middle Devonian in age. Schopf also noted that an abundant fossil mycelium present in the same core indicated an in-situ ter- restrial deposit. Fragments of plant megafossils have been identified by F. M. Hueber (oral commun., 1973) as Hostimella, which is most common in the Lower and Middle Devonian. One well in Georgia, Mont Warren et al. A. C. Chandler No. 1 (fig. 1, well 25, lot 406, Land District 26, Early County, Ga.), penetrated 459 ft of dark- gray to black shale, reddish-brown shale, and gray siltstone between 6,781 and 7,240 ft (Applin and Applin, 1964, p. 136—148). Core 4 (corrected depth 6,995—7,015 ft) from this well contained Chevro- leperditia chevronalis Swartz (1949, p. 319—321), undescribed smaller ostracodes (Swartz, 1945, p. 1205), linguloid brachiopods, and small pelecypods described by Palmer (1970) and discussed in this paper. Schopf (written commun., 1958) identified spores from this core as Archeozonotriletes of Nau- mova and considered the sample not older than Middle Devonian. Core 5 (corrected depth 7,015— 7,036 ft) contains the same smaller ostracodes pres— ent in core 4 but to date lacks the other elements of the assemblage. Swartz (1949, p. 320) suggested that Chevroleperditiu was Late Ordovician to Early Silur- ian in age because the combination of characters on which he based the genus was intermediate between those of Ordovician and those of Silurian genera. Swartz’s opinion led Bridge and Berdan (1952, table 1) to classify the black shales in the Chandler well as Silurian or Late Ordovician and to suggest (Bridge and Berdan, 1952, p. 34, 35) that the 80 ft of white, quartzitic sandstone penetrated beneath the black shale in this well might be Early Ordo- vician and equivalent to the white sandstone unit overlying the Lower Ordovician Skolithos unit in other wells. This correlation was queried by Applin and Applin (1964, p. 148) and, with the age of the black shale now considered as Middle Devonian, it appears far less probable. Applin and Applin (1964, p. 146—147) report 181 ft of Middle Devonian(?) weathered(?) reddish- brown and greenish-blue shale and some sandstone overlying the black shale in the Chandler well. As yet no identifiable fossils have been found in this shale, and its age remains uncertain. Another well, Anderson et al. Great Northern Paper Co. No. 1 (fig. 1, well 24, lot 387, Land Dis- trict 26, 2 miles southwest of Cedar Springs, Early County, Ga.) about 1 mile southeast of the Chandler well, has yielded plant fossils which have been de- scribed by McLaughlin (1970). McLaughlin (1970, p. 3) has discussed material from the lowermost 214- foot interval of this well, which he has described as consisting of alternating zones of light- and dark- gray silty shale or siltstone and light- to medium- gray medium- to coarse-grained sandstone. Mega- fossils consist of psilophytacean fragments and microfossils include acritarchs, microspores, and possibly smaller tasmanitoids. McLaughlin (1970, p. 7) has concluded that some elements of this flora indicate a marine environment of deposition, and others were derived from terrestrial plants, and he considered that this part of the section represents a nearshore, shallow-water low-energy marine de- posit into which land plants were washed by streams. The age of the assemblage is believed to be late Early or early Middle Devonian (McLaughlin, 1970, p. 7) and is thus probably slightly older than the assem- blage in the Chandler well, although possibly equiva- lent to that in the Tindel well. Red shales or variegated shales with reddish tints overlie the Paleozoic black shales of various ages in the Chandler, Tillis, Hilliard, Cone, Ragland, Ben- nett and Langsdale, M. W. Sapp, Superior Pine Products No. 3, and other wells. The red shale in Superior Pine Products No. 4 is considered to belong in this group, although, as only 5 ft of Paleozoic was penetrated, the presence of black shale beneath the red is not certain. These red and variegated shales lie just below the unconformity between the Paleo- zoic and overlying Mesozoic rocks and appear to be part of the Paleozoic sequence, as fossils have been found in them in the Cone well and in Superior Pine Products No. 3. Because of their position be- neath the unconformity and because different ages of Paleozoic are represented, they are believed to represent possible alteration of formerly black shales at the unconformity. However, Carroll (1963, p. A15) has suggested that they represent a different environment of deposition from that of the black shales in that they had accumulated rapidly in an GENERAL GEOLOGIC SETTING 9 oxidizing environment with little or no organic matter. IGNEOUS ROCKS Southeast of the main area of Paleozoic rocks in peninsular Florida the Mesozoic section is underlain by volcanic and other igneous rocks (Applin, 1951, fig. 1) which have been described by Bass (1969), Milton and Grasty (1969), and Milton (1972). In general, wells just southeast of the Paleozoic terrane have encountered volcanic rocks of rhyolitic com- position; Milton (1972, p. 45, fig. 27) has noted the presence of a fragment of an undetermined fossil in ash from a depth of 3,879—3,881 ft in Sun Oil Co. H. E. Westbury et al. No. 1 (fig. 1, well 26, sec. 37, T. 11 S., R. 26 E., Putnam County, Fla.). Bass (1969, p. 290—293) considered that Sun Oil Co. Powell Land Co. No. 1 (fig. 1, well 27, sec. 11, T. 17 S., R. 31 E., Volusia County, Fla.) passed through a diorite sill and ended in a hornfels derived from a “clayey volcanic-quartzose sandstone” (Bass, 1969, p. 290). Three wells farther south, one each in Lake, Orange, and Osceola Counties, Fla., entered rock listed by Applin and Applin (1965, p. 10—11) and Bass (1969, p. 289) as granite but considered by Milton (1972, p. 8) to be possible arkose altered by contact metamorphism. South and west of the wells ending in granitic rocks several wells penetrated volcanic rocks similar to those in the northern part of the igneous terrane. Applin and Applin (1965, fig. 3) have shown the volcanic rocks as surrounding a triangular area of granite and diorite, and they also indicate a sep- arate area of altered igneous rock on the southeast- ern coast. This represents Amerada Petroleum Cor- poration Cowles Magazines No. 2, (sec. 19, T. 36 S., R. 40 E., St. Lucie County, Fla.) which has been studied by C. S. Ross (in Applin and Applin, 1965, p. 17—18) and Bass (1969, p. 293—299). Bass (1969, p. 293) considered this well unique in Florida in that it penetrated “schist, gneiss, and amphibolite typical of a regionally metamorphosed terrane.” North of the main area of Paleozoic sedimentary rocks a number of wells have penetrated various kinds of “basement” rocks, which have been de- scribed by Ross (1958), Milton and Hurst (1965), and Milton and Grasty (1969). Although there is not complete agreement about the type of rock en- countered in some of the wells (Milton and Hurst, 1965, p. 1), in general the wells in southern Georgia immediately north of the sedimentary Paleozoic rocks are rhyolitic tuffs and some basalts that are apparently little metamorphosed. Farther north, in Pierce and Coffee Counties, Ga., three wells entered rock described as either altered granite or arkose. Even farther north, in the belt of counties south of the edge of the Coastal Plain sediments, wells en- countered metamorphic and igneous rocks similar to those exposed in the Piedmont area of Georgia. Obviously, any interpretation of the structural relationships of the Paleozoic sedimentary rocks de- pends to a considerable extent on the age of the volcanic terranes to the north and south. Milton and Grasty (1969, table 2) list whole-rock potassium- argon ages for samples from five wells in Florida and three in Georgia, and Bass (1969) has provided both potassium-argon and rubidium-strontium ages for samples from three Florida wells, two of Which were also dated by Milton and Grasty. Mil-ton (1972, table 2) has summarized all the available evidence bearing on the radiometric dating of these rocks. In general, there appear to be two groups of dates, one ranging from 147 to 191 my. (Jurassic to Triassic), mostly obtained from basalts or diabases, and the other centered about 530 my. (Cambrian), obtained from various igneous and metamorphic rocks. The older dates have all been obtained from central and southern Florida, with the exception of an age of 303i15 my. (late Carboniferous) determined by Grasty for a hornblende schist from a well in Cus- seta, Chattahoochee County, Ga., which is consid- ered by Milton and Hurst (1965, p. 16) to be “true basement,” that is, an extension of the rocks under- lying the Piedmont. The older rocks in Florida all fall Within the areas shown by Applin and Applin (1965, fig. 3) as (1) altered igneous rocks of un- known age and (2) Precambrian(?) granite and diorite. Of the younger dates only one, from core at a depth of 8,781—8,7811/2 ft from Humble Oil and Refining Co. W. P. Hayman No. 1 (fig. 1, well 28, sec. 12, T. 31 S., R. 33 E., Osceola County, Fla.) is from rhyolitic rocks; this core gave an age of 173i4 m.y. (Jurassic) (Milton and Grasty, 1969, table 2). The two groups of dates indicate two periods of igneous activity, one during the Triassic or Jurassic, and one during the Cambrian. Diabase sills intrud- ing lower Paleozoic sedimentary rocks are probably Triassic in age and belong to the same period of volcanism that produced some of the dated diabases and basalts in the volcanic terrane. Milton and Grasty (1969, p. 2489) consider that the diabases and rhyolites are related and contemporaneous; however, Bass (1969, p. 308) suggests the presence of a rhyolitic belt, undeformed and essentially un- metamorphosed, extending from North Carolina to 10 SUBSURFACE PALEOZOIC IN FLORIDA AND GEORGIA Florida with a minimal age of 408:40 m.y. (Ordo- vician or Silurian), and Sundelius (in Milton and Hurst, 1965, p. 14) noted the similarity of some of the rhyolitic rocks of Georgia to those of the Caro- lina slate belt, which are as old as Cambrian accord- ing to Saint Jean (1965). Additional radiometric dates are needed to determine the age of the rhyoh lites. The Camp No. 1 passed through presumed Lower Ordovician sandstone and ended in volcanic agglomerate and rhyolitic welded tuif (Milton, 1972, p. 49). Although this might indicate that the vol- canic rocks are older than Early Ordovician, the absence of more than a thousand feet of the Sko- lithos unit suggests that a fault of considerable mag- nitude passes through or near this well. STRUCTURAL RELATIONSHIPS Except for the cross-bedded siltstones and clay- stones in the Tindel well, bedding in all cores from Paleozoic rocks is perpendicular to the axis of the cores, which suggest horizontal bedding or very low dips. The only indication of metamorphism in any of the cores examined by Carroll (1963, p. A12) seems to be low grade and produced by pressure of the overlying rocks. The cluster of wells in Columbia and Suwannee Counties, Fla., which penetrated De- vonian and Silurian shales are bordered by wells in Middle Ordovician shales, which in turn are bord- ered by wells in Lower Ordovician quartzites. This suggests a possible concentric subcrop pattern in the Paleozoic which is offset slightly to the north- west of the Peninsular arch and may indicate a gentle north or northwest dip for the Paleozoic sedimentary rocks in this area. The Silurian cores from the Bennett and Langsdale well in Echols County, Ga., might be explained either by a gentle fold in the Paleozoic sedimentary rocks or by fault- ing. The Silurian and Devonian shales penetrated by the Ragland and Hilliard wells, respectively, which are on the flanks of and on opposite sides of the Peninsular arch, may be most easily explained by postulating the presence of downdropped normal fault blocks on each side of the arch. The structural relationships of four wells, three in the Devonian and one in the Lower Ordovician, located near the Florida-Georgia-Alabama bound- aries, are not clear because of inadequate data. SUMMARY The sedimentary Paleozoic rocks beneath Florida and adjacent parts of Georgia and Alabama lie at a minimum depth of 3,000 ft in a roughly wedge- shaped area bounded both to the north and to the south by volcanic rocks of uncertain age—possibly Cambrian to Precambrian, possibly much younger. The rocks dated by fossils range in age from Early Ordovician to Middle Devonian, the latter being in southwestern Georgia and the Florida panhandle. A composite section based on the logs of individual wells suggests a minimum thickness of 4,000 ft. The rocks are entirely elastic and the lower part of the section, especially the Early Ordovician, shows evi— dence of shallow—water marine deposition. The most abundant megafossils in the Ordovician part of the section are phosphatic brachiopods and, in the Silurian to Lower Devonian part of the sec- tion, mollusks and arthropods. Graptolites are pres- ent in only one Lower Ordovician well. Possible upper Lower Devonian and Middle Devonian shales and siltstones separated from the main area of Paleozoic subcrop are dominated by plant fossils and ostracodes. Bedding is generally at right angles to the cores, and metamorphism is low grade ex- cept in the vicinity of diabasic intrusive rocks of probable Triassic and Jurassic age. SYSTEMATIC PALEONTOLOGY The systematics of the Florida and Georgia sub- surface Paleozoic pelecypods is dealt with well by well. Pelecypods have been recovered from the Chandler, Tillis, Cone, and Ragland wells (fig. 2). The Turkish specimens are assigned to the species Cheioptem‘a bridgei, which also occurs in the Cone well core, and are discussed under that species. The Bolivian collection is treated separately following the discussions of the American and Turkish ma— terial. Following is the synoptic classification to the level of genus of the pelecypods considered in this section: Phylum Mollusca Cuvier Class Pelecypoda Goldfuss Subclass Palaeotaxodonta Korobkov Superfamily Nuculacea Gray Family Praenuculidae McAlester Genus Deceptrix Fuchs Subgenus Praenucula Pfab Superfamily Nuculanacea Adams and Adams Family Malletiidae Adams and Adams Genus Arisaigia McLearn Genus Nuculites Conrad Genus Palaeoneilo Hall and Whitfield Subclass Isofilibranchia Iredale Superfamily Mytilacea Rafinesque Family Modiomorphidae Miller Genus Modiomo'rpha Hall and Whitfield Family Butovicellidae Km Genus Butovicella Krii Subclass Pteriomorphia Beurlen Family Praecardiidae Hornes Genus Panenka Barrande SYSTEMATIC PALEONTOLOGY 84° 11 83° 82' 81° 32' Chandler N? — ~__ Tallahassee 31‘ rA \\ Jacksonville I ' 20 0 2o 40 6O 80 MILES l I J; l L I I l l l l l 20 o 20 40 so so KILOMETRES 30' Gainesville FLORIDA Ragland V FIGURE 2.——Location of the four Florida and Georgia wells which have yielded Silurian and Devonian pelecypods. Class Pelecypoda Goldfuss—Continued Subclass Pteriomorphia Beurlen—Continued Family Antipleuridae Neumayr Genus .Dualina Barrande Superfamily Ambonychiacea Miller Family Ambonychiidae Miller Genus Mytilarca Hall and Writfield ?Family Lunulacardiidae Fischer Genus Lunulacardium Munster Superfamily Pteriacea Gray Family Pterineidae Miller Genus Actinopteria Hall Genus Cheioptem’a n. gen. Genus Leptodesma Hall Superfamily Pectinacea Raflnesque Family Pterinopectinidae Newell Genus Pterinopecten Hall Subclass Heteroconchia Hertwig Superfamily Trigoniacea Lamarck Family Myophoriidae Bronn Genus Eoschizodus Cox Superfamily Carditacea Fleming Family Permophordae van de Pohl Genus Pleurodapis Clarke Class Pelecypoda Goldfuss—Continued Subclass Heteroconchia Hertwig—Continued Superfamily Solenacea Lamarck Family Orthonotidae Miller Genus Prothyris Meek Subclass Anomalodesmata Dall Superfamily Pholadomyacca Gray Pholadomyacean genus and species indet. FLORIDA AND GEORGIA WELLS CHANDLER WELL This well was drilled near the tristate boundary of Georgia, Florida, and Alabama (Mont Warren et al. A. C. Chandler No. 1, lot 406, Land District 26, Early County, Ga.). Only one new pelecypod specimen (pl. 4, fig. 7) from this well was available to us; we have ‘ also examined the specimens from this well (pl. 4, figs. 11, 13, 14) which were figured by Palmer (1970). Pelecypods are known only from Core 4 at a corrected depth of 6,995—7,015 ft. 12 SUBSURFACE PALEOZOIC IN FLORIDA AND GEORGIA Genus MODIOMORPHA Hall and Whitfield, 1869 Modiomorpha? sp. Plate 4, figures 7, 11, 13, 14 Material.—Five specimens, four right valves (PR1 27632, 27633, 27636; USNM 203225) and one left valve (PRI 27633). The left valve (pl. 4, fig. 14, lower part) is 14.7 plus mm long and 7 mm high. The best preserved right valve (pl. 4, fig. 7) is 9.5 mm long and 4.5 plus mm high. Discussion—Palmer (1970) called this species Anthraconauta cf. A. phillipsii (Williamson), and regarded it as a fresh-water Carboniferous form. Although we cannot unequivocally rule out the fresh- water interpretation of these specimens, they are not well preserved and, on the basis of shape and ornament, could also be assigned to the marine genus M odiomorpha Hall and Whitfield. M odiomorpha? sp. occurs with leperditiid ostracodes which are known to be marine in their occurrence; also present in the sample are plant spores. The age of M odiomorpha? sp. is most likely Middle Devonian rather than Carboniferous. Leperditiid ostracodes are not known from rocks younger than Devonian, and Schopf (written commun., 1958) re- garded the plant spores that occur with M odio- morpha? sp. to be of Middle Devonian age. TILLIS WELL This well was drilled in north-central peninsular Florida (Sun Oil Co. J. H. Tillis No. 1, sec. 28, T. 2 S., R. 15 E., Suwanee County, Fla.) Pelecypods are known only from core 44 at a depth of 3,552— 3,568 ft; 12 taxa are recognized. Genus NUCULITES Conrad, 1841 Most Tillis specimens of this genus have a broad poorly defined posterior buttress (pl. 3, figs. 2, 5) in addition to the sharply defined anterior buttress (pl. 3, figs. 2, 5). Species of this sort are sometimes placed in the genus Ditichia (sensu Clarke, 1909) ; however, the type species of Ditichia Sandberger lacks the posterior buttress (McAlester, 1968). As redefined by McAlester (1969) the genus Nuculites includes Paleozoic palaeotaxodonts with an anterior buttress. So far as known all forms with both anterior and posterior buttresses are Late Silurian—Early De- vonian in age, for example, Nuculites elliptica (Maurer) from the Lower Devonian Moose River Sandstone of Maine (assigned to Ditichia by Clarke, 1909); Nuculz'tes afm'comus (Sharpe) from the Lower Devonian of Antarctica, Africa, Bolivia, Uruguay, and Brazil (McAlester, 1965) ; Nuculites n. sp. from the Upper Silurian Moydart and Mc- Adam Brook Formations of Nova Scotia (Bambach, 1969) ; and probably Nuculites unisulcus (assigned to Cleidophorus Hall by Korejwo and Teller, 1964) from the Early Devonian of the Chelm borehole of eastern Poland (M onograptus uniformis Zone). The Tillis specimens of Nuculites are not placed in any named species. There are many described species of Nuculites from the lower Paleozoic and the genus is currently under study by R. K. Bam- bach, Virginia Polytechnic Institute and State Uni- versity, Blacksburg, Va. Nuculites sp. A Plate 3, figure 10 Material.—One left valve (USNM 203231), meas- uring 22.5 mm long and 13.7 mm high. Discussion—The single known specimen of this species has a very weak posterior buttress, a postero- ventrally directed anterior buttress which does not exceed two-thirds of the height of the shell, and a less oblique posterior umbonal slope than does Nuculites sp. B. The pits on the posterior part of the umbo of the specimen are not part of its morphology. Nuculiles sp. B Plate 3, figures 2, 5, 12 Material.—-Four right and four left valves (USNM 203232—35); the best preserved specimen (pl. 3, fig. 12) measures 9.4 mm long and 6 mm high. Discussion—This form differs from Nuculites sp. A in having a more sharply defined posterior buttress, a more oblique posterior umbonal slope, and a longer anterior buttress which is vertical or directed anteroventrally. Genus ARISAIGIA McLeam, 1918 This genus has previously been reported from the Upper Silurian Doctors Brook and McAdam Brook Formations of Nova Scotia, the Lower Devonian part of the Stonehouse Formation of Nova Scotia (Bambach, 1969), and the Lower Devonian of the Chelm borehole of eastern Poland (Monograptus uniformis angustidens Zone; Korejwo and Teller, 1964). The Polish specimens were previously placed in the genus Parallelodon Meek and Worthen by Korejwo and Teller, 1964. Arisaigia cf. A. poslornata McLearn Plate 3, figures 4, 6 Material.——A deformed right valve (pl. 3, fig. 6) and a well-preserved left valve exterior (pl. 3, fig. SYSTEMATIC PALEONTOLOGY 13 4) ; the latter measures 10.9 mm long and 6.2 mm high. USNM 203236, 203237. Discussion—The Florida specimens have the fine overall radial ribbing and subdued posterior plicae of this species; they are smaller than the median height and length of A. postomata (Bambach, 1969), but well within the known size range. Distribution—This species has previously been reported only from Nova Scotia where it occurs in the Upper Silurian Doctors Brook and McAdam Brook Formations. Arisaigia sp. Plate 3, figure 3 Material—One posterior fragment (USNM 203238). Discussion—This fragment has stronger ribbing than A. cf. A. postomata, and appears to be a part of a more elongate shell; however, it may be a frag- ment of A. cf. A. postomata which has undergone some dorsoventral compression. Genus ACTINOPTERIA Hall, 1884 Actinopleria sp. A Plate 3, figure 13 Material—One deformed left valve having a length of 17 plus mm and a height of about 17.7 mm (USNM 203239). The specimen is preserved on the opposite side of the chip on which the specimen of Plearodapis sp. (pl. 3, fig. 11) is preserved. Discussion—This is an erect quadrate shell with well-developed anterior and posterior auricles which are clearly separated from the body of the shell. Actinopleria sp. B Plate 3, figures 8, 14 Material—One left (USNM 203240) and one right valve (USNM 203241) ; the right valve (pl. 3, fig. 14) measures 5.5 mm long and 4.8 mm high. Discussi0n.—These small specimens have un- usually coarse ribs for their size and do not have the anterior and posterior auricles as clearly sep- arated from the body of the shell as does Actinop- tem'a sp. A. Genus PTERlNOPECTEN Hall, 1883 Pterinopeclen'! sp. Plate 3, figure 16 Material and discussion—Many small shells of a pectinacean of the Ptem’nopecten type superimposed upon one another on one chip (USNM 203242). Genus MODIOMORPHA Hall and Whitfield, 1859 Modiomorpha sp. Plate 4, figure 8 Material and discussion—One crushed right valve measuring about 20 mm long with a modioliform shape and prominent ornament (USNM 203243). Genus EOSCHlZODUS Cox, 1951 Eoschizodus? sp. Plate 3, figure 1 Material and discussion—Four crushed or dis- torted specimens (USNM 203244, 203245) of which one is articulated; specimens have a schizodiform shape and commarginal sculpture. The figured speci- men is 28.6 mm long and 23 plus mm high. commarginal Genus PLEURODAPIS Clarke, 1913 Pleurodapis 31:. Plate 3, figures 9, 11 Material—Five incomplete specimens (USNM 203246—48), of which three are right and two are left valves. USNM 203247 is on the opposite side of the chip on which Actinoptem'a sp. A (pl. 3, fig. 13) is preserved. Discussion—This genus is characterized by the presence of strong angular posterior plicae and in some species a single prominent anterior plica (pl. 3, fig. 15). The Florida material does not show the anterior plica. Distribution—In addition to the Florida occur- rence, Plearodapis has been reported fro-m the Upper Silurian of Bolivia (Branisa, 1965, pl. 29, fig. 3) and the Lower Devonian of Bolivia (Branisa, 1965, pl. 29, figs. 1, 2), Brazil (Clarke, 1913), Ghana (Saul, Boucot, and Finks, 1963), Germany (Mauz, 1933), and Belgium (Maillieux, 1936). Genus PROTHYRIS Meek, 1871 Prothyris 51:. Plate 4, figures 5, 6, 9 Material—Five specimens, four right valves and one left valve (USNM 203251—54). The best pre- served specimen (pl. 4, fig. 9) measures 11.4 mm long and 5 mm high. One specimen is preserved on the same chip as the pholadomyacean shown on plate 4, figure 3, and another on the same chip as Arisaigia cf. A. postornata (pl. 3, fig. 4). Discussion—The Tillis specimens have the char- acteristic shape, ornament, and anterior ridge and auricle of this genus. Prothym's is known primarily from upper Paleozoic rocks, but has been reported from the Devonian by several authors including Hall (1885), Whidborne (1896), and Clarke (1913); it is not known from rocks older than the Devonian. Pholadomyacean genus and species indet. Plate 4, figures 2—4 M atem’al and discussion—The incomplete anterior ends of a right (USNM 203255) and a left (USNM 203256) valve; the latter preserves both part and 14 SUBSURFACE PALEOZOIC IN FLORIDA AND GEORGIA counterpart. These specimens are elongate shells with the characteristic rugose commarginal orna- ment found in Paleozoic pholadomyaceans. CONE WELL This well was drilled in north-central peninsular Florida (Humble Oil and Refining Co. J. P. Cone No. 1, sec. 22, T. 1 N., R. 17 E., Columbia County, Fla.). Pelecypods are known from the 3,562—foot level (core 122) to the 4,330—foot level (core 155), although they are identifiable to genus only from the 3,653-foot level (core 126) to the 4,281-foot level (core 152). The Cone well is remarkable because of the sim- ilarity of all pelecypod taxa to those of the Upper Silurian of central Bohemia; the two regions have several species in common. Genus PANENKA Barrande, 1881 Panenka sp. Plate 2, figure 6 Material.—One incomplete right(?) valve from core 126 (USNM 203262), top 5 ft of the 3,653— 3,678-foot interval. Discussion—This specimen belongs to the P. humilis—P. bohemica species group, which has prom- inent commarginal sculpture that is closely spaced in the mature part of the shell. Species of this group are known from the Upper Silurian (upper Lud- lovian)—Lower Devonian (lower Lochkovian) of B0- hemia (Barrande, 1881), France (Babin, 1966, pl. 4, fig. 14), and Morocco (Termier and Termier, 1950, pl. 169, fig. 15). Genus DUALINA Barrande, 1881 Dualina secunda Barrande, 1881 Plate 1, figures 4, 17 Mat'erial.—Three specimens, one each from the bottom of core 126, 3,653—3,678 ft (USNM 203263) ; bottom of core 138, 3,936—3,953 ft (USNM 203264) ; and top of core 152, 4,256—4,281 ft (USNM 203265). Only USNM 203264 is well preserved (pl. 1, figs. 4, 17); it measures 11.9 mm long and 12.9 mm high. Discussion—USNM 203264 shows fine radial rib- lets on the major ribs (pl. 1, fig. 17) , a feature which also occurs on the Bohemian subspecies D. secunda reticulata Barrande; the other Florida specimens are not well enough preserved to determine whether or not these riblets are present. Distribution—In Bohemia this species is known only from the Upper Silurian Kopanina and Pridol Formations (Ludlovian and Pridolian) ; the sub- species D. secmzda reticulum is known only from the Kopanina Formation (Ludlovian). D. secunda also occurs in the Upper Silurian of Bolivia and a similar form, D. convexa Korejwo and Teller (1964), has been reported from the Lower Devonian (Mono- graptus uniformis angustidens Zone) of eastern Poland. Genus MYTILARCA Hall and Whitfield, 1869 Mytilarca cf. M. longior (Barrande, 1881) Plate 1, figure 13 Material.—Two left valves (USNM 203266, 203267) one each from core 127, third 5 ft of the 3,67 8—3,703-foot interval, and top of core 128, 3,7 03— 3,720 ft. The best preserved of the two specimens is figured; it measures 10 mm long and 12.2 mm high. Discussion.—Mytilarca has a relatively simple ex- ternal morphology (Pojeta, 1966), and because of this and the numerous named species it is difficult to assign small samples to a species; in general shape the Florida specimens are most like M. longior of Bohemia. Distribution—M. longior occurs in the Pridol (Upper Silurian) and Lochkov (Lower Devonian) Formations of Bohemia. The species M. lata Korej wo and Teller and M. pressera Korejwo and Teller (1964) from the Lower Devonian of Poland (M 0120- graptus uniformis angustidens Zone) are closely similar to M. longior. Genus LUNULACARDIUM Munster, 1840 Lunulacarclium excellens Barrande, 1881 Plate 2, figures 1-4, 8, 12 Material.——Over 50 specimens from the second and fourth 5—foot intervals and the general interval in core 127, 3,678—3,703 ft. Most of the specimens are incomplete, but there is a good size range, from under 5 mm to about 14 mm in length, and there are chips covered with juveniles (pl. 2, fig. 4). USNM 203268—78. Discussion—L. excellens shows a high degree of variability; it is characterized by having two to five secondary ribs between pairs of primary ribs. In both Florida and Bohemia the species is gregarious, covering bedding-plane surfaces. Distribution—This species is known only from Florida and Bohemia. In Bohemia it occurs only in the uppermost Silurian (Pridolian). Lunulacardium spp. Plate 2, figures 4 (arrow), 5 Material.—Many specimens from several levels; bottom of core 126, 3,653—3,678 ft; core 127, 3,678—3,703 ft; top of core 128, 3,703—3,720 ft; SYSTEMATIC PALEONTOLOGY 15 core 131, 3,768—3,790 ft; top of core 132, 3,790— 3,815 ft; top and middle of core 133, 3,815—3,838 ft; and top and middle core 134, 3,838—3,863 ft. Most of the specimens are incomplete, but there is a wide size range from 2 mm to 13 mm in length. Some levels are almost entirely juveniles (3,768— 3,790 ft and 3,790—3,815 ft. USNM 203279—90. Discussion—The specimens placed in Lumda— cardium spp. have ribs of equal strength and are most like L. bohemicum Barrande and L. eximium Barrande, both of which occur in the uppermost Silurian (Pridol‘ian) of Bohemia. A juvenile is fig- ured in the lower left corner of plate 2, figure 4 (arrow). Genus CHElOPTERlA Poieta and Kid; 11. gen. Type species—Cardin”). glabrum Goldfuss, 1837 (p. 218, pl. 143, fig. 8a, b); non Munster, 1840, is herein designated the type species of the new genus Cheioptem‘a. This species name is usually credited to Munster (1840, p. 66, pl. 12, fig. 11), and in fact Goldfuss credits the species to Miinster. Munster (1840, p. 66) noted that he had given specimens from Prague to Goldfuss for inclusion in the latter’s “Petrefacta Germaniae,” and at the same time noted that he had decided that the German material from Elbersreuth was a different species than the Czech material from Prague: Cardium glabrum *** Von Elbersreuth. Ich habe friiher eine bei Prag im Orthoceratitenkalk vorkommende ahnliche Bivalve mit dieser Art verwechselt und die Prager Examplare an Goldfuss fiir Petrefacten-Werk mitgetheilt, spater aber mich iiberzeugt, dass sie wesentlich verschieden sind und der Prager Muschel wohl zu den Posidonomyen oder Avicula (Monotis) gehoren mo‘chte. Translated: (Cardium glab’rum *** from Elbersreuth. I have earlier mixed up with this species a similar bivalve found near Prague in the Orthoceras Lime- stone, and conveyed the samples from Prague to Goldfuss for the Petrefacten-Work, but was later convinced that they are essentially different, and that the Prague shell could belong to the posidomomyids or Avicula (Monotis).) Goldfuss’ “Petrefacta Germaniae” was published over a period of 18 years from 1826 to 1844. The Neues J‘ahrb. Mineralogie Geognosie, Geologie u. Petrefakten-kunde for 1838, p. 106—109, gives the date of publication of V. 2, p. 141—244, pls. 122-146, as 1837. These plates and pages include Cardium glabrum. Because Goldfuss used the name Cardium glabrum 3 years before Miinster, we regard him as the author of the species. Goldfuss clearly states that Cardium glabrum occurs at Prague and Elbersreuth and his figures show specimens conspecific with those which occur at Prague (pl. 1, figs. 6, 11 herein). The specimen figured by Munster (1840) under the name Cardium glabmm is quite different from those fig- ured by Goldfuss (1837). Description—Small equivalved pteriaceans with rugose commarginal ornament; radial ornament absent; auricles only vaguely separated from the body of the shell. Internal features unknown. Etymology.—Cheiw—a hole in the ground; Ptem’a—a genus of pelecypo-ds. Comparisons—Several names have been proposed for Silurian—Devonian pteriaceans which lack radial ornament. Leptodesma Hall and Joachymz'a Rfiiiéka (pl. 1, fig. 12) have well-developed auricles, of which the posterior is often elongated into a wing. Pterochaem’a Clarke (pl. 1, fig. 7) is sometimes classified as a pteriacean; it has a prominent anterior rib which separates the anterior auricle from the body of the shell. Cheioptem’a is much like N ewsom- ella Foerste in external shape and commarginal ornament; however, Newsomella has radial orna- ment between the commarginal rugae of the right valve. Actinodesma Sandberger has both auricles elongated into wings and a Malleus-like form. Ptem‘nea Goldfuss has the anterior auricle sharply delimited from the rest of the shell and has a poster- ior wing. Distribution—In Florida the genus is known only from the Cone well. In Bohemia it occurs in the Upper Silurian Kopanina Formation (Ludlovian), and in Turkey it comes from rocks of probable Wenlockian or Ludlovian Age (W. T. Dean, written commun., 1973). Cheiopteria glabra (Goldfuss, 1837) Plate 1, figures 6, 11 1837. Cardium glabrum Goldfuss, p. 218. 1840. [mm] Cardium glabrum Munster, p. 66. 1881. Avicula glabra Munster. Barrande, pl. 228. 1949. Pterochaenia (Pterochaenia) glabra (Munster). Rfiziéka, p. 4. Remarks and comparisons—We figure two topo- types of this species (USNM 203260, 203261) from the upper part of the Kopanina Formation, Ortho— ceras quarry, southwest of Lochkov, Bohemia, Czechoslovakia. This locality is near Prague, and the species occurs widely at this level in the Barrandian. Ri’iziéka (1949) placed the species in the genus Pterochaem’a Clarke. Chez’opteria (widget n. sp. is the only other species presently placed in the genus; C. glabm differs from this species in having a prominent byssal sinus be- low the anterior auricle and in that the commarginal rugae are less prominent and angular than in C. bridgei (pl. 1, figs. 2, 3). Cheiopteria bridgei Pojeta and 10%, n. sp. Plate 1, figures 2, 3, 5, 14, 15 Description.—Cheioptem'a lacking a prominent 16 SUBSURFACE PALEOZOIC IN FLORIDA AND GEORGIA byssal sinus below the anterior auricle and having prominent angular commarginal rugae. Etymology.—This species is named for the late Josiah Bridge. Types—The holotype (pl. 1, fig. 3, USNM 203291) is from the middle of Cone well core 128, 3,703— 3,720 ft. Paratypes from the Cone well occur at the top 7 ft of core 128, 3,703—3,720 ft (USNM 203293, pl. 1, fig. 2); middle of core 128, 3,703—3,720 ft (USNM 203292 not figured); bottom of core 128, 3,703—3,720 ft (USNM 203294; 10 specimens not figured) ; bottom of core 135, 3,863—3,888 ft (USNM 203295; 1 specimen not figured); and bottom of core 142, 4,019—4,039 ft (USNM 203296; 5 specimens not figured). In addition there are a large number of paratypes (well over 50) from Halevikdere, near Tufanbeyli, a Village in the Taurus Mountains about 140 km north of Adana, Turkey (fig. 3), probably from the Yukariyayli Formation. The Turkish specimens were collected by Necdet Ozgiil of the Maden Tetkik ve Arama and were brought to our attention by W. T. Dean of the Geological Survey of Canada. The Turkish specimens are shown on plate 1, figures 5, 14, 15 (MTA). Age of the Turkish Materrial.—W. T. Dean (writ- ten commun., 1973) noted that the Turkish speci- mens occur with an encrinurid trilobite which indi- cates a Silurian age, probably Wenlockian or Lud- lovian. Dimensions—The holotype (pl. 1, fig. 3) is 7 mm long and 6.9 plus mm high; the figured Florida para— type (pl. 1, fig. 2) is 7.9 mm long and 7 plus mm high. Cheiopteria? sp. Plate 1, figure 1 Materials and remarks—This form is known from one specimen (USNM 203297) from the middle of core 135, 3,863—3,888 ft. The ornament and out- line of this specimen are similar to Cheioptem‘a glabm, except that the intersection of the anterior and posterior margins with the dorsal margin are more angular. Genus LEPTODESMA Hall, 1883 Leptodesma carens (Barrande, 1881) Plate 1, figures 10, 16 Material.—Five left valves from the bottom of core 126, 3,653—3,678 ft (USNM 203298—300). The larger of the two figured specimens (pl. 1, fig. 16) measures 17.4 mm long and 11.3 mm high. Discussion—The Florida specimens have the same general shape and coarse commarginal ornament as those from Bohemia. Distribution—In Bohemia this species occurs in the Upper Silurian (Pridolian) and Lower Devo— nian (Lochkovian). ‘ Halevikdere Diyarbakit 50 O 50 500 100 150 Mamas O 200 MILES 50 100 150 200 KILOMETRES FIGURE 3.——-Location of the specimens of Cheioptem‘a from Turkey. SYSTEMATIC PALEONTOLOGY 17 Genus ACTINOPTERIA Hall, 1884 Actinopteria migrans (Barrande, 1881) Plate 1, figures 8, 9 Material.—Seven left valves, one from core 126, 3,653—3,678 ft (USNM 203301, this specimen is on the same chip with Periechocrinus) ; one from the third 5 ft of core 127, 3,678—3,703 ft (pl. 1, fig. 8, USNM 203302) ; and five from the middle of core 128, 3,703—3,720 ft (pl. 1, fig. 9, USNM 203303; 203304). All specimens are incomplete or deformed, and none were measured. Discussion—The Florida specimens are similar to those from Bohemia in shape, ribbing pattern, and the sharp definition of the anterior auricle. Distribution—In Bohemia this species occurs in the Upper Silurian (Pridolian) and Lower Devo- nianz (Lochkovian) ; it has the same distribution in Poland (Korejwo and Teller, 1964). RAGLAND WELL This well was drilled in northwest peninsular Florida (Coastal Petroleum Co. J. B. and J. T. Rag- land No. 1, sec. 16, T. 15 S., R. 13 E., Levy County, Fla.). Pelecypods are known only from core 15, at a depth of 5,840—5,850 ft; two taxa are represented. Genus BUTOVICELLA Kiii, 1965 Butovicella migrans (Barrande, 1881) Plate 2, figures 9—11 Material.—Three left valves (USNM 203226—28), the best preserved of which measures 4.9 mm long and 3.6 mm high (pl. 2, fig. 11). Discussion.——All of the Florida specimens show the modioliform shape of the species, and the char- acteristic ornament which consists of a radially ribbed anterior lobe and moniliform radial ribs over the body of the younger part of the shell. Distribution—In Bohemia this species occurs in the upper Liteii (Wenlockian) and Kopanina (Lud- lovian) Formations of the Silurian; it is especially abundant on either side of the Wenlockian-Ludlovian boundary. Outside of Bohemia it is known from the Wenlockian of Poland, and the Ludlovian of Ger- many, Italy, Sweden, Great Britain, France, and Portugal (Krii, 1969, and herein). Genus ACTINOPTERIA Hall, 1884 Actinopteria sp. Plate 2, figure 7 Material.—Five left valves (USNM 203229, 203230), the best preserved of which measures 27.8 plus mm long and 20 plus mm high (pl. 2, fig. 7). Discussion.—All Ragland well specimens of this form are fragmentary or poorly preserved, and we do not assign them to a species; their association with Butovicella migrans shows that Actinopteria occurs in rocks as old as the Wenlockian-Ludlovian part of the Silurian. BOLIVIA COLLECTION All of the Bolivian material discussed herein is from the Pampa Shale at Huari, Oruro Department, about 110 km south of the City of Oruro (fig. 4). In discussing the age of the Pampa Shale, Branisa, Chamot, Berry, and Boucot (1972, p. 27) noted: The Pampa Shale of the Llallagua region has not yet yielded diagnostic fossils, but it is concluded to be of late Llandovery and Wenlock age because of its stratigraphic position. The new collection from Huari contains the pele- cypod Dualina, which elsewhere is not known to occur in rocks older than Ludlovian; its presence suggests that at Huari the Pampa Shale is Late Silurian in age. Genus DECEPTRlX Fuchs, 1919 Subgenus PRAENUCULA PM), 1934 Deceptrix (Praenucula) sp. Plate 5, figures 3, 6, 11, 13 M atem’al.—Twenty-two single valves, 11 right and 11 left, and 2 articulated specimens (USNM 203305— 9). The specimen shown on plate 5, figure 6, is 8 mm long and 6.6 mm high. — 10" 60° 15° B 0 L I V I A . La Paz .Oruro . Santa Cruz Huari ‘ OSucre Potosi. 20° .Tarija ) 50 O 50 100 200 MILES 50 0 50100 200K|LOMETRES FIGURE 4.——Location of the collection of Silurian pelecypods from Bolivia. 18 SUBSURFACE PALEOZOIC IN FLORIDA AND GEORGIA Discussion—This form differs from Decept’rix (Praenucula) pulchella (Clarke, 1899), in being more equilateral and in having a shorter anterior tooth row. D. pulchella is from the Trombetas For- mation of Brazil, which according to Lange (1972) is early Llandoverian in age and thus probably old— er than the Pampa Shale collection from Huari. Genus NUCULlTES Conrad, 1841 Nuculites sp. Plate 5, figure 4 Material.—A crushed articulated specimen (USNM 203310), one left valve (USNM 203311), and one right valve (pl. 5, fig. 4; USNM 203312). The figured specimen measures 21 plus mm long and 14.6 mm high. The articulated specimen pre- serves some muscle scars. Discussion—As noted previously there are many named species of this genus, and the genus is under study by Bambach. The Huari specimens are most similar to the specimen of N. pacatus Reed from the Devonian of Brazil shown by Clarke (1913) on plate 10, figure 23. The other specimens of N. pacatus figured by Clarke are different in having an extremely wide anterior buttress. Genus PALAEONEILO Hall and Whitfield, 1869 Palaeoneilo sp. A Plate 5, figures 1, 2, 5, 8 Mate7"z'al.—Eleven left valves, 17 right valves, and 10 articulated specimens (USNM 203313—16). The specimen shown in figure 2, plate 5, measures 9.7 mm long and 5.6 mm high. Discussion—This is an elongate species in which the dorsal and ventral margins are nearly parallel, the anterior tooth row comes down the anterior face to about the middle of the height and the posterior tooth row occupies at least two-thirds of the dorsal margin posterior to the umbonal peaks. None of the forms described by Clarke (1899, 1900, 1913), Kozlowski (1923), or Branisa (1965) suggests af- finities to this species. Palaeoneilo? Plate 5, figure 7 Material.—Three left and three right valves (USNM 203317, 203318). The best preserved speci- men is figured on plate 5, figure 7, and measures 18.9 mm long and 12 mm high. Discussion—This form has the general shape and ornament of the specimen of P. rhysa Clarke from the Devonian of Parana, Brazil, figured by him (1913) on plate 11, figure 5. Genus DUALlNA Barrande, 1881 Dualina secunda Barrande, 1881 Plate 5, figures 15—20 Material.——Two articulated specimens, three fiat valves, and six convex valves (USNM 203319—24). The best preserved specimen is shown on plate 5, figure 15; it measures 14.9 mm long and 13.3 mm high. Discussion—The Bolivian specimens have the shape, size, and ornament of specimens of this spe- cies from Bohemia and Florida; they show the pres- ence of pelecypod faunal elements of the Silurian Mediterranean Biogeographic Province in the Upper Silurian of South America. Genus ACTINOPTERIA Hall, 1884 Actinopteria 5]). Plate 5, figure 10 Material.—One small incomplete external mold of a left valve (USNM 203325). Discussion—This specimen has the shape and ribbing of the genus Actinopteria. It is small and had to be photographed under very oblique light; although the figure suggests a right valve, it is an impression of the exterior of a left valve. Genus MYTlLARCA Hall and Whitfield, 1869 Mytilarca 5]). Plate 5, figures 9, 12, 14 Material.——One incomplete left valve (pl. 5, fig. 14; USNM 203326) and two right valves (pl. 5, figs. 9, 12; USNM 203327, 203328). The right valve shown in plate 5, figure 12, measures 20.5 mm long and 21.5 mm high. Discussion—None of these specimens is well pre- served. They have the general shape of ambony— ch‘iids, and because they lack radial ornament are best assigned to Mytilarca; however, they are un- usually long and quadrate for that genus. BIOSTRATIGRAPHY A major difficulty with the biostratigraphic in- terpretation of the Silurian-Devonian pelecypods of the Florida and Georgia wells is that none of the wells penetrates more than one major faunal unit; thus we do not know the superposition of the units from any single well. In drawing biostratigraphic interpretations from samples of this kind, it is nec- essary to compare them with those from other areas where the superposition of the faunal zones is known; then a composite picture of the stra- tigraphy of the four southeastern American wells can be made. BIOSTRATIGRAPHY 19 The age of the Chandler well fauna presents the biggest problem. Palmer (1970) interpreted the pelecypods to be fresh-water forms and Carbonifer- ous in age. Swartz (1949) felt the leperditiid ostra- code Chevroleperditia chevronalis suggested a Late Ordovician or Early Silurian age. The pelecypods occur with the leperditiid ostracodes, a group which is not known from rocks younger than Devonian and which is known only from marine facies. Schopf (written commun., 1958) noted that the plant spores associated with the pelecypods and ostracodes were no older than Middle Devonian in age. It seems best to regard the 6,995—7,015-foot level of the Chandler well (which contains the pelecypods, ostracodes, and plant spores) as Devonian, rather than older or younger, and probably Middle Devonian in age. While we cannot entirely rule out the fresh-water interpretation of the Chandler pelecypods, and cer- tainly fresh-water and marine forms can be mixed in either facies, shells of this shape do occur in marine Devonian environments and are usually placed in the genus Modiomorpha. On the basis of the Middle Devonian spores, we regard the Chand- ler pelecypods as the youngest forms in the four wells studied herein. Because of the occurrence of Butom'cella migrans in the Ragland well, the 5,840—5,850-foot level of this well is herein regarded as Ludlovian or late Wenlockian (Silurian) in age. Butom‘cella migrans is most widely distributed in rocks of Ludlovian Age (Late Silurian), but is also known to occur in rocks of late Wenlockian Age (Middle Silurian) in Bohemia and Poland. The Ragland well pelecypod fauna is the oldest from the four wells studied. Thus, the total range of the pelecypods in the four wells is Wenlockian or Ludlovian to Middle Devonian. Pelecypods from the Tillis well occur in a 16-foot interval (3,552—3,568 ft). The total aspect of the fauna is Devonian, and several taxa are present which are not known from rocks older than the Devonian, for example, Prothym‘s, Ptem‘nopecten‘h and Eoschizodus ?. Forms which are known to range into the Late Silurian are also present—Arisaigia, Pleurodam’s, and N uculites with a posterior buttress. These taxa are not known from rocks younger than Early Devonian. This mixing of Devonian taxa with genera not known to occur above the Lower Devon- ian suggests that the Tillis fauna is most likely Early Devonian in age. The mixing of forms which are not known from pre-Devonian rocks with taxa that range across the Silurian-Devonian boundary suggests that sedimen- tation in the Florida Paleozoic was continuous across the Silurian-Devonian boundary. This sug- gestion is also supported by the fauna from the Cone well, which is probably latest Silurian in age. An alternative explanation is that the 3,552—3,568- foot interval of the Tillis well crosses the Silurian~ Devonian boundary. At the present time this seems unlikely, as the Tillis forms which are known to range into the Late Silurian also have Early Devon- ian occurrences elsewhere. Other mollusks occurring in the Tillis well are the bellerophontacean Plectonotus (Tritonophon) sp. (pl. 4 fig. 1) and the cricoconarid Tentaculites sp. (pl. 3, fig. 7). The bellerophontacean was iden- tified by J. S. Peel and E. L. Yochelson, who noted (written commun., 1970) that species of the taxon occur throughout the Silurian and into the Early Devonian. Fisher (1962) gave the range of Ten- taculites as “?L. 0rd, L. Sil. (Llandov.)-—U. Dev. (Mid. M. Frasn.) .” The Cone well fauna is dominated by taxa that are best known from Bohemia, Czechoslavakia. Of the nine taxa recognized from the Cone well, seven occur in Bohemia and an eighth has a closely allied species in Bohemia. The Cone well fauna is Late Silurian (Pridolian) in age and is probably high in the Late Silurian, just below the Silurian-Devonian boundary. This age is indicated by the presence of Lunulacm‘dium excellens, which in Bohemia is known only from the late Pridolian Stage, but is best documented by a comparison of the ranges of the taxa in the Cone well with the occurrence of the same taxa in B0- hemia (fig. 5). In Bohemia some of the taxa cross the Silurian-Devonian boundary, but none of them is known to occur only in the Devonian, and most taxa are concentrated near the boundary. Other mollusks in the Cone well are two frag- mentary specimens of nautiloid cephalopods. One is from core 122, 3,512—3,587 ft (pl. 4, fig. 12), and in ornament is most like the Silurian genus Para- kz’onoceras Foerste. The other is from core 127, 3,678—3,703 ft (pl. 4, fig. 10), and is ornamented like Dawsonoceras Hyatt, a widely distributed Silurian genus. In the four Florida-Georgia wells which have yielded pelecypods, we see a composite section rang- ing in age from late Wenlockian or early Ludlovian to Middle Devonian, with each of the four wells having pentrated rocks of a different age. The Rag- land well fauna is the oldest, late Wenlockian or early Ludlovian; the Cone well fauna is the next oldest, Pridolian in age; the Tillis well fauna is Early Devonian, about Lochkovian; and the Chand- 20 Bohemia Panenka of P. humilis-bohemica group Dualina secunda Mytilarca cf. M. longior Lumtlacard’ium excellens Lunulacardium spp. Cheiopterm glabra Leptodesma corms Actinopteria migrans Devonian Lochkovian a) an as a m x: as :1 o '1: .— L: f-‘u Ludlovian Stage SUBSURFACE PALEOZOIC IN FLORIDA AND GEORGIA Pammka of P. humilis-P. bahemia group Dualim secunda Mytilarca cf. M, longior Lunulacardium excellens Lunulacardium spp. Cheiaptem'a bm'dgei Leptodesma carens Actinopteria mvlgrans FIGURE 5.—Occurrences of pelecypod taxa in Bohemia and the Cone well of Florida ler well fauna is Middle Devonian in age. Kjellesvig—Waering (1950) on the basis of the occurrence of a new species of the eurypterid Pterygotus regarded the 3,552—3,568—foot interval of the Tillis well to be Late Silurian in age. Bridge and Berdan (1952) regarded the Tillis fauna to be Late Silurian or Early Devonian in age, the former being more probable, and Berdan (1970) considered that the Cone fauna was more likely younger than the Tillis fauna. Cramer (1973), however, discussed the chitinozo‘an stratigraphy of four Florida wells, including the Cone and Tillis, and he considered the Tillis fauna to be younger than the Cone fauna, an opinion with which we agree. Both the Tillis and the Cone fauna of this paper fall in Cramer’s Zone of Ancyrochitina fragilis, which he noted (p. 285) is older than late Gedinnian (Early Devonian) ; he also noted (p. 284) that the top of the Florida suc- cession is late Ludlovian [Pridolian?] or earliest Gedinnian. We have concluded that the Tillis pele— cypod fauna is early Early Devonian in age be— cause forms which are not known below the De- vonian are mixed with forms which first occur in the Late Silurian. Also, we conclude that the Cone pelecypods are late Late Silurian, because of their similarity to late Late Silurian species from Bo- hemia. In effect, the two wells are on opposite sides of the Silurian-Devonian boundary. As noted, the Late Silurian age of the Bolivian faunule is based on the occurrence of Dualina se- cunda, which elsewhere is not known to occur in rocks older than Ludlovian. The Turkish material is thought to be Wenlockian or Ludlovian in age on the basis of its occurrence with an encrinurid tri- lobite (W. T. Dean, written commun., 1973). We regard the single Turkish species to be Cheioptem‘a bridgei, which is also known from the Cone well, where it is Pridolian in age. PALEOECOLOGY AND ENVIRONMENT OF DEPOSITION Ecology of the various pelecypods.—Stanley (1970) recognized seven life—habit groups in pele— cypods. The pelecypods from the American wells, Bolivia, and Turkey, belong to three of these life- habit groups: byssally attached, burrowing, and re- clining. Among byssally attached forms Stanley (1972) distinguished endobyssate (semi-infaunal) and epibyssate species—endobyssate forms being those which are partly buried in the substrate and epibyssate forms being those which are entirely PALEOECOLOGY AND ENVIRONMENT OF DEPOSITION 21 epifaunal. The endobyssate category included vir- tually all forms with a prominent rounded anterior lobe or auricle, a shallow byssal sinus, and an oblique shell; thus many pteriomorphs and isofilibranchs previously regarded as epifaunal are considered to be semi-infaunal by Stanley. Kauffman (1969) did not regard all these forms as being semi-infaunal and explained the external shell morphology of Modiolus as an adaptation to an epifaunal mode of life. Stanley’s 1972 paper is a bold synthesis with broad deductive conclusions, but it suffers from a small empirical base (Gordon and Pojeta, 1975) which is largely limited to a few species of living mytilids and the pinnids. His conclusions of the modes of life of many extinct forms need verification from field evidence of fossil pelecypods found pre- served in living position. Herein we have used Stanley’s endobyssate category with the understand- ing that some of these forms may ultimately be shown to have been epibyssate. In the Ragland well both known species are bys— sate forms. Kriz (1969) regarded Butovicella mi- grans as epifaunal, although it has a prominent an— terior lobe and is modioliform in shape. He based his conclusions on the observations that the species is mos-t often found in shelly limestones, as the shells provide surfaces for attachment, and in graptolitic shales associated with the alga Prototaxites. He re- garded the soft bottom in the latter environment as being unsuitable for benthos and suggested that Butom'cella was epiplanktonic, being attached to floating algae which drifted into the area of grap- tolitic shale deposition. The other Ragland species, Actinoptem'a sp., is known only from left valves which are highly oblique and have a small posterior auricle; they have the general morphology of Stanley’s (1972, p. 185) en- dobyssate pteriaceans. Of the 12 pelecypod taxa known from the Tillis well, 8 are infaunal burrowing forms: Nuculites sp. A, Nuculites sp. B, Arisaigia cf. A. postornata. Eoschizodus?, Pleurodam’s sp., Prothyris sp., and pholadomyacean genus and species indet.; the first 4 are palaeotaxodonts comparable to living deposit feeders, and the last 4 are suspension feeders. The remaining species are byssate. Actinoptem’a sp. A is an erect shell with a fairly prominent byssal sinus and may have been epifaunal. Actinopteria sp. B is more oblique with a broad rounded anterior auri— cle and a poorly developed byssal sinus; it would fall into Stanley’s endobyssate category. Modiomorpha sp. has the shell shape of some living endobyssate mytilaceans. Pterinopecten? sp. was probably a byssally attached epifaunal pectinacean (Stanley, 1972, p. 192). In the Cone well there is one infaunal burrowing species, Panenka sp. Seven species are byssate: Mytilarca cf. M. longior, Lunulacardium excellens, Lunulacardium spp., Cheioptem‘a bridgei, Cheiop- tem’a sp., Leptodesma carens, and Actinoptem'a mi- grans. Of the byssate species only Mytilarca of. M. longior fits Stanley’s criteria of an epifaunal form. Also occurring in the Cone well is Dualina se- cunda, a markedly inequivalved, inequilateral form which can be right or left convex and which prob- ably was a reclining species. Stanley (1970, p. 8) defined the reclining life habit as: “Occupying a position on or partially buried in a soft substratum and lacking the capacity for attachment.” In Dualma both valves are convex although one is markedly more so than the other. As in the chamids either valve can be the more convex although there is no sign of cementation in Dualina. Thus, either valve of Dualina probably could be lowermost and the com- missural plane was probably horizontal. There are a number of inequivalved pelecypod groups which rest on the substrate with the com- missural plane horizontal. These include the ce— mented spondylids, ostreids, chamids, rudists, pseudomonotids, and plicatulids; the byssally at- tached buchiids, most anomiids, isognomonids, most inoceramids, some pteriids, and many pectinids; and the reclining species of gryphaeids, anomiids, and inoceramids. The great bulk of the forms which have the commissural plane horizontal are pteriomorphs. Dualina, and antipleurids in general, show no sign of cementation, although they are probably ana- logous to some chamids in that they could have either valve more convex and lowermost. Little is known of the details of muscle—scar morphology of Dualina and antipleurids in general. There is no external sign of a byssus, but its presence cannot be entirely ruled out until we have knowledge of the muscle scars of Dualz’na. Because there is no sign of cemen- tation 0r byssal attachment we regard Dualina as a reclining form analogous to Placuna and some gry- phaeids and inoceramids. The Bolivian fauna described herein has four infaunal deposit-feeding species: Deceptm’x (Prae- mwula) sp., Nuculites sp., Palaeoneilo sp. A, and Palaeoneilo ?. There are two byssally attached species: Mytz'larca sp. and Actinopteria sp., the former was probably epifaunal and the latter is too poorly preserved to apply Stanley’s criteria. Dualina secunda is also present in Bolivia and as noted above was probably a reclining epifaunal species. 22 SUBSURFACE PALEOZOIC IN FLORIDA AND GEORGIA Tables 1 and 2 and figure 6 summarize ecological data for the pelecypods from the Tillis and Cone wells and from Bolivia. The Cone well fauna is dominated by endobyssate forms, whereas the Tillis well collection and the one from Bolivia are domi- nated by infaunal burrowing forms. However, only the Tillis well collection has a strong infaunal sus— pension-feeding element which forms almost 50 per— cent of the individuals in the fauna. TABLE 1.—Snmmary of the inferred life habits, by number of species, of the pelecypods from the Ragland, Tillis, and Cone wells and from Bolivia. [Note the low occurrence of epifaunal byssate and reclining species. In the Cone well fauna byssate forms dominate, whereas in the Tillis well and Bolivia collections burrowing forms dominate] Infaunal Byssate burrowing Locality Epiby- Endoby- Deposit Suspen- Reclining ssate ssate feeding £8213); g Ragland well 1 1 0 0 0 Tillis well 2 2 4 4 0 Cone well 1 5 0 1 1 Bolivia _______ l 1 ‘3 4 0 1 TABLE 2.—Percentage distribution of the life habits of the pelecypods from the Tillis and Cone wells and from Bolivia [Note that each locality has a different ecologically dominant group] Tillis well Cone well Bolivia Number Number Number Life habit Percent of speci- Percent of speci- Percent of speci- of total mens of total mens of total mens Epibyssate ___ 3.3 1, plus 2 2 3.5 3 several superim- posed Pterino— pecten Endobyssate __ 12.1 4 94 94 1.2 1 Reclining _____ 0 0 3 3 12.8 11 Infaunal deposit feeders ____ 36.3 12 0 0 82.6 ’71 Infaunal suspension feeders ____ 48.4 16 1 1 0 0 Total __ 100.1 33 100 100 99.1 86 Environment of deposition of sedimentary se- quences in the American wells.—In the four Ameri- can wells considered herein, the entire sedimentary sequence is clastic; hard slaty black to dark-gray micaceous fissile shale dominates, although there are some thin siltstones (Applin, 1951). Carroll (1963) studied the petrography of the wells; black shales were examined from the Cone and Chandler wells, and red shales from the Tillis and Ragland wells. None of the pelecypods seen by us occur in red shale. For the depth of 3,562—3,587 ft in the Cone well Carroll gave the following analysis of the min- erals observed in thin section (1963, p. A7) : Shale consists of very fine grained angular quartz and carbonaceous matter interlaminated with well-crystallized siderite and minor calcite. The siderite is slightly oxidized in places so that reddish—brown rims occur at the edges of the laminae. The clay material occurs as very fine micaceous EXPLANATION Epibyssate Endobyssate Reclining Infaunal deposit feeders Tillis well \‘m ‘ V Cone well Infaunal suspension feeders Bolivia FIGURE 6.—Percentages of various pelecypod ecologic types for the Tillis and Cone wells and the collection from Bolivia. laths and as indefinite aggregates that exhibit aggregate polarization. A little carbonate also occurs as minute irregular patches in the clay matrix. X-ray examination of the clays showed both kao- linite and illite are present in almost equal propor- tions. The only heavy mineral from the black shale PALEOECOLOGY AND ENVIRONMENT OF DEPOSITION 23 of the Cone well was pyrite; the Chandler well yielded only Opaques. Bridge and Berdan (1952, p. 29) noted that the Paleozoic strata appear to have been deposited in shallow water. Schopf (1959, p. 1671) considered that the presence of pyrite and abundant carbona- ceous material signified deposition in an euxinic en- vironment and postulated a sargassoid environment for microfossils which he had obtained in part from the Cone and Tillis cores. Berdan (1970, p. 150) concluded that the presence of crinoid calices in the Cone fauna suggests fairly normal marine salinity, whereas the eurypterids in the Tillis may indicate a lagoonal environment. Cramer (1973, p. 280—281) stated that The presence of the miospores indicates a sedimentary realm not far removed from land; the absence of coarse sediments in the Silurian of Florida suggests an essentially shallow sea in which the areas inhabited by the spore-producing plants must have been flat islands or shoals. Cramer (1973 p. 286) further commented The shales that make up the succession were probably de- posited in a fully marine environment, not in a lagoonal one as suggested before. The presence of miospores plus the black shale lithology suggests a low-relief, low energy depositional environment*** To date, most workers have interpreted the cores of the Tillis and Gone wells as representing a shal- low water normal marine environment. Bretsky (1969, p. 50—51) noted that pelecypods in the Pale— ozoic are most abundant and diverse in nearshore situations. The information derived from the study of the pelecypods of these wells agrees with these broad environmental generalizations. It is likely that the Cone and Tillis well faunas represent near-life assemblages buried with little transportation of the shells. For the Cone fauna this interpretation is supported by the lack of fragmen- tation of many of the specimens, the well-preserved surface ornament, and the diversity of the fauna at several levels; also, the occurrence of spatfalls on the same bedding plane with larger specimens indi- cates a lack of size sorting (pl. 2, fig. 4). The Tillis fauna likewise shows a general lack of fragmenta- tion of the specimens, the ornament is well pre— served, the fauna is highly diverse, and there are one or two articulated specimens. Although the evi- dence that we are dealing with near-life assem- blages is not overwhelming, it is certainly sugges- tive of that situation. The pelecypod faunas from the Ragland, Tillis, and Gone wells and Bolivia and Turkey indicate normal marine environments. They contain several groups of pelecypods which are not known to occur in brackish- or fresh-water environments, including palaeotaxodonts, pteriaceans, and pectinaceans. The occurrence of cephalopods, crinoids, tentaculites, and bellerophontaceans likewise indicates the marine na- ture of these deposits. As previously noted, the pelecypods from the Chandler well have been re- garded as fresh water (Palmer, 1970), but it is our opinion that they are probably marine, as they occur with leperditiid o‘stracodes. A low—energy environment for the rocks pene— trated by the American wells is indicated by the small grain size of the black shale, the lack of sort- ing of shells by size, the lack of abrasion of the shells, and the well preserved surface ornament of the shells. The depth of the water is difficult to estimate on the basis of the pelecypods. Cramer felt that the presence of miospores in the cores indicates a shal- low sea near low-lying landmasses. The presence of mica in the rock could suggest fairly close proximity to a crystalline terrane but does not help in estimat- ing depth. McAlester and Rho-ads (1967) have dealt with the problem of how to estimate water depth from the pelecypods present in a fossil assemblage. They came to the conclusion that the three primary environmental factors limiting pelecypod distribu- tion are temperature, salinity, and the nature of the substrate, and that the problem of pelecypod bathy— metry is one of relating one or more of these primary limiting factors to water depth. Salinity of the ma- rine environment of the sediments in the American wells was normal, and the micaceous organic-rich black shale can occur in both shallow and deep wa- ter. Temperature is discussed below. The total aspect of a pelecypod fauna can give an indication of the depth at which the animals lived. Thus, in the Cone well, the presence of many byssate species, the reclining Dualina, the L’zmulacardium spatfalls and the diversity of the fauna suggest shallow water. Most byssate and reclining pelecypods occur in water of less than 100 fathoms depth. Al- though the Tillis fauna is quite different from that of the Cone well, one-third of the species are byssate, and they occur with various burrowing forms, many of which have numerous shallow—water representa— tives today. At the present time, deposit-feeding palaeotaxodonts are most common in nearsho-re fine- grained organic-rich sediments (Bambach, 1969) comparable to the black carbonaceous shales of the American wells. McAlester and Doumani (1966) analyzed an Early Devonian pelecypod fauna from Antarctica which contained palaeotaxodonts, Modiomm-pha, 24 SUBSURFACE PALEOZOIC IN FLORIDA AND GEORGIA and Prothyris and was thus similar to the Tillis fauna; however, the percentages of the various taxa were quite different, and the Antarctic fossils oc- curred in a dark, poorly sorted, coarse sandstone. They came to the conclusion that in the Early De- vonian the Antarctic was a region of relatively cool temperatures. This conclusion is supported by recent reconstructions of Early Devonian paleogeog— raphy which show the portion of Antarctica from which their fossils came was about 60° S. lat (Smith, Briden, and Drewry, 1973; Cocks and McKerrow, 1973). For various reasons explained in the paleobio- geographic section of this paper Pojeta and Ki‘iz regard Florida as having been at about 60° S. lat in the Late Silurian and Early Devonian and thus as having had a relatively cool climate. This climate could have been ameliorated by a warm-water cur- rent which would have come by the shores of central Europe and produced the similiar faunas of Bohemia and Florida in the Late Silurian (figs. 7, 8). The Bolivian fauna would be a cold—water assemblage, as it would have been quite close to the South Pole. PALEOBIOGEOGRAPH Y Paleobiogeographic considerations herein are largely limited to the plotting of our data on the maps of the Late Silurian—Early Devonian published by Cocks and McKerrow (1973) and Smith, Briden, and Drewry (1973) ; Mercator and South Polar pro— jections are used (figs. 7, 8). On these maps we have included probable surface paleocurrents. These are generalized and obviously depend upon the loca- tion of landmasses, which is still an intensely de- bated subject. As in modern seas we have an equa- torial current and a general clockwise circulation of currents in the northern hemisphere and a general counterclockwise movement of currents in the south- ern hemisphere; no equatorial countercurrent is shown in figure 8. Figures 7 and 8 show that there is a strong simi- larity between the pelecypod faunas of Bohemia and Poland and that of Florida in the Late Silurian. Of the eight genera known from the Cone well (Pa- nenka, Dualma, Mytilarca, Lunulacardium, Cheiop- tem’a, Leptodesma, and Actinopteria) all occur in Bohemia and all except Panenka are known from Poland. The Tillis and Cone wells have only the genus Actinopteria in common; however, four gen- era known from the Tillis well also occur in Poland: Nuculites, Arisaigia, Actinoptem'a, and Pterinopec- ten?. Of this Late Silurian—Early Devonian assem- blage Dualina is also known from Sweden, Bolivia, and north Africa; Panenka occurs in France, Spain, north Italy and north Africa; Lunulacardz’um occurs in North Africa and France; Cheioptem‘a is known from Turkey; and Arisaigia occurs in Nova Scotia. Mytz'larca is the most widespread genus, occurring in Nova Scotia, New York, Maryland, Ontario, Si- beria, Bolivia, north Africa, and Antarctica (Poj eta, 1966). Mytilarca represents an old stock beginning perhaps in the Ordovician, which could explain its wide distribution. Leptodesma and Actinoptem'a have distributions similar to Mytilarca (although Leptodesma is not known from Antarctica) and are not plotted in figures 7 and 8. The genus Pleurodapis occurs in Florida, Brazil, Bolivia (Branisa, 1965), western Europe, and west Africa (Saul, Boucot, and Finks 1963). If, as is assumed in the 1973 paleogeographic re- constructions used herein, Europe north of the Mediterranean Sea wereadjacent to North America and straddled the paleoequator and if eastern North America were in the same position relative to the rest of the continent as it is now, then warm-water currents passing Europe and heading south would have warmed the eastern coast of North America and carried pelecypod larvae southward (figs. 7 and 8). This circulation pattern would explain the strong similarity of the Late Silurian—Early Devonian pele- cypod assemblage of central Europe and Florida and the presence of some part of this assemblage in Nova Scotia; presumably more of the assemblage should be found in Nova Scotia in the future. Dualina in Bolivia could be explained as a eurythermal taxon carried into very high south paleolatitudes by sur- face currents (figs. 7 and 8), whereas most of the species of the Bohemia—Poland assemblage were not able to tolerate the cold temperatures near the Late Silurian—Early Devonian South Pole. Pleurodapis has a distribution comparable to Dualina and could be explained the same way. The occurrence of some elements of the Bohemia- Poland-Florida fauna in north Africa and Turkey is more difficult to explain, but could have been accom- plished by dispersal along the shoreline of South America and thence to west Africa and down the shores of west Africa and eastern South America; the two continents are thought to have been nearly contiguous in the Late Silurian—Early Devonian. Alternatively, there could have been a seaway con- nection between Bolivia and Brazil through Para- guay, and there may have been a dispersal across South America. 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