{75 Z 742474 zDA{ Aeromagnetic, Bouguer Gravity, and Generalized Geologic Studies of the Great F alls-Mission Range Area, Northwestern Montana GEOLOGICAL SURVEY PROFESSIONAL PAPER 726—A Aeromagnetie, Bouguer Gravity, and Generalized Geologic Studies of the Great F alls-Mission Range Area, Northwestern Montana By M. DEAN KLEINKOPF and MELVILLE R. MUDGE GEOPHYSICAL FIELD INVESTIGATIONS GEOLOGICAL SURVEY PROFESSIONAL PAPER 726—A Interpretation of aeromagnetic and gravity anomalies in terms of fault tectonics, basement roe/e units, and regional geology UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTONzl972 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 73—184625 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 2401—2075 CONTENTS 1—.— Page Page Abstract ___ A1 Geophysical studies . A9 Introduction 1 Field measurements General geology 2 ROCk properties Magnetic field configuration ____________________ 10 Geologic structure 6 Gravity field configuration _____________________ 10 Precambrian rocks 6 Interpretation of geophysical data ______________ 10 Paleozoic and Mesozoic rocks ___________________ 7 Structure section __________________________ 16 Mesozoic and early Cenozoic igneous activity ___- 8 Conclusions 16 References cited 16 ILLUSTRATIONS [Plates are in pocket] PLATE 1. Generalized geologic and aeromagnetic lmap of the Great Falls-Mission Range area. 2. Complete Bouguer gravity map of the Great Falls-Mission Range area. 3. Structure section and magnetic and grzjvity profiles across the Great Falls-Mission Range area. 1 Page FIGURE 1. Index map of study area ______ L ______ A1 2. Generalized chart of rock-stratigraphic units _____________________________________________________ 4 l III {7'37" 3 GEOPHYSICAL FIELD INVESTIGATIONS AEROMAGNETIC, BOUGUER GRAVITY, AND GENERALIZED GEOLOGIC STUDIES OF THE GREAT FALLS-MISSION RANGE AREA, NORTHWESTERN MONTANA By M. DEAN KLEINKOPF and MELVILLE R. MUDGE ABSTRACT Aeromagnetic and gravity data provide new information on the structural framework and the distribution of near- surface crystalline rocks in northwestern Montana. A variety of trends and anomaly types in the magnetic data reflect the diverse geology of the area. Complex mag- netic patterns of the Great Plains are attributed to a hetero- geneous crystalline basement, which is probably a meta- morphic complex of schist and gneiss with numerous magnetite-rich intrusives. In addition to the well—known Scapegoat-Bannatyne trend of northeast—alined fractures, the magnetic and gravity data suggest two other parallel trends of suspected fracturing and possible faulting of the crystalline basement. One northeast trend extends from Brown Sandstone Peak to near Brady; the other extends from Stonewall Mountain Lookout Station to Power. North- and northwest-trending Tertiary structures in the mountains are indicated by the gravity and magnetic data as high gra- dient zones. Gravity minimum anomalies coincide with dOWn- thrown fault blocks of the Swan River valley and the valley of the South Fork of the Flathead River. Fractures along the Scapegoat—Bannatyne trend may have been the loci for emplacement of two concealed batholithic masses, one near Scapegoat Mountain and the other just north of Augusta. Similar batholithic masses are inferred also beneath the Redhead Peak-Larch Hill area and the east- ern part of the Swan Range. Prominent positive magnetic anomalies are in the southern part of the area. Some reflect exposed quartz monzonite or somewhat more mafic stocks. Similar isolated magnetic anomalies are in areas where igneous rocks are not exposed, but these anomalies probably reflect near-surface intermedi- ate stocks. A large positive anomaly lies east of the stocks. It may indicate a gabbroic mass at a depth computed to be about 6,000 feet. The Adel Mountain volcanic field produces a distinctive complex magnetic pattern. Plutons along a structure section are inferred, on the basis of computer model studies, to be in, and slightly above, the crystalline basement. The model that best agrees with the gravity and magnetic data shows the basement to be deep, with an irregular surface and with the plutons pro— jecting up into the lowermost Belt sedimentary rocks. INTRODUCTION Aeromagnetic and gravity interpretations were made in conjunction with regional structural and stratigraphic studies of part of the northern Rocky Mountains and Great Plains in northwestern Mon- tana (fig. 1). The purpose of the study was to pro- vide information about the regional structural frameworkof the area and the distribution of near-surface igneous rocks. The area extends north— west of the Big Belt Mountains about 90 miles and west of Great Falls about 135 miles to the South Flathead valley. The data coverage extends across the South arch of the Sweetgrass arch; the Saw- tooth, Lewis and Clark, Swan, Mission, and Garnet mountain ranges; and several mining districts in the Lincoln area. Earlier geological and geophysical studies of the southeastern part of the Lewis and Clark Range which were modified in the compila- tion of plate 1' follow: Barnett (1916) ; A. F. Bate- man (written commun., 1967, unpub. data); Bier- wagen (1964); Clapp (1932); C. F. Deiss (unpub. data) ; C. F. Deiss and C. H. Clapp (unpub. data) ; Dobbin and Erdmann (1955); Fox (1966); J. L. Funk (written commun., 1967), unpub. data); Gwinn (1961) ; Harrison, Reynolds, Kleinkopf, and Patee (1969); Holcombe (1963); Johns (1964); Kleinkopf and Mudge (1968); Kleinkopf, Mudge, and Harrison (1968); Lange (1963) ; Lyons (1944) ; McGill and Sommers (1967) ; Melson (1964); Mudge (1965; 1966a, b, c; 1967; 1968); Mudge, Rob- inson, and Eaton (1966); Mudge, Erickson, and Kleinkopf (1968) ; H. J. Prostka (written commun., 1967; unpub data); G. D. Robinson and W. B. Myers (written commun., 1967 ; unpub. data) ; Ross, Andrews, and Witkind (1955); Schmidt (1963); Schmidt, Swanson, and Zubovic ( 1964); R. G. Schmidt (written commun., 1967; unpub. data); Sommers (1966); Stebinger (1918) ; Viele (1960); and Viele and Harris (1965). The areas of plate 1 not covered by the above maps have been covered by either ground reconnaissance or aerial photo- A1 A2 1.16’ 1 15° GEOPHYSICAL FIELD INVESTIGATIONS 49 __ _ __ __ 114' 113 112° CANADé_ 111°__ __ 110’ f 1, TUNITED STATES for ‘\°t- (g KEVIN—SUNBURST \1; DOME \ m \\ 2 ° t“ \E‘n m ‘5. 0“ 6 A oKalispell \a flggfipx‘. 5;, \ \z,_/ 7 \ \ ’ “3“ w— \ e.- s t 0 (fix \ Flathead Lake \‘ \ ’35, ‘7’ \ |\ \ e‘xx \ “ \\ «826 ‘7 l ‘ \9. Crag, ’90 'y HIGHWOOD \ | \Qég/s /y 12‘ MOUNTAINS S x ‘ 0‘» (QSBURN p \\ «5. AULT LITTLE BELT Q; 0 204/ MOUNTAINSC (‘ vi» 0 47°_ O\\A ...... _ 7A7“); oMissoula 0 O 50 MILES L__._1__i_|_|_l FIGURE 1.—Map showing relation of the study area to major geologic features in northwestern Montana. Thrust faults indicated by sawteeth (on upper plate), normal faults, by bar and ball (on downthrown side). graph interpretation. The broad regional aspects of the gravity data have been discussed by Smith (1970). The Boulder batholith, on which an aero- magnetic survey was made by Johnson, Henderson, and Tyson (1965), joins the study area on the south, near the Blackfoot mining district. The authors thank the many persons who have contributed to this project. Many of the gravity data were made available through the courtesy of the 1381st Geodetic Survey Squadron, Air Photo- graphic and Charting Service, US. Air Force. Thanks are given to many US. Geological Survey colleagues for their assistance. G. P. Eaton and D. L. Peterson prepared the Bouguer gravity map from Air Force data. D. L. Peterson also made ad— ditional gravity measurements in the field. A. F. Bateman and his staff provided the borehole data shown on plate 1. Most of the geologic data for the southeast corner of the area was prepared by Rob- ert George Schmidt; some were provided by G. D. Robinson and W. B. Myers. The geologic data for the southwest corner were taken from a compilation by H. J. Prostka. A preliminary geologic map of the area northeast of Lincoln was kindly supplied by J. L. Funk, who was at the time of the study affiliated with the University of Missouri. GENERAL GEOLOGY The area shown on plate 1 encompasses the east- ern part of the northern Rocky Mountains and the GREAT FALLS-MISSION RANGE AREA, NORTHWESTERN MONTANA western part of the Great Plains. It includes major Basin-and-Range-type fault blocks to the west, part of the disturbed belt, and the southern part (the South arch) of the Sweetgrass arch to the east. The outcropping sedimentary rocks range in age from Precambrian (Belt) to Quaternary. The central part of the area is complicated by numerous thrust faults, some folds, and large normal and small transverse faults. The southern part of the area is complicated by effects of the complex igneous activ- ity during the Late Cretaceous and Tertiary, when the sedimentary rocks were intruded by a variety of igneous rocks—sills, stocks, dikes, and possibly lac- coliths—and in part were covered by volcanic debris of various ages. A variety of metallic mineral de- posits—the most valuable, gold, silver, lead, zinc, and copper—are found in the mapped area (pl. 1). Most deposits are in or near plutons. The mining districts and some of the outlying mines are dis- cussed in detail by Pardee and Schrader (1933) and partly by Sahinen (1959, p. 136), and therefore they will not be discussed here. Figure 2 is a generalized chart showing the rock- stratigraphic units in the area and their correla- tions as used by various authors. Overlap, facies change, and intertonguing stratigraphic relations are merely suggested, and the time span of each formation is shown only in a general way. The Widespread unconformities are shown on the chart, whereas the local unconformities within units have been omitted. The descriptions of the rock units are given by the authors cited for each of the areas and are not repeated here. The tectonic framework in which the Pre- cambrian and Paleozoic sediments were deposited is illustrated by Sloss (1950, figs. 1, 3) and by Mc- Mannis (1965, p. 1806) and is discussed by Mudge (1970). The western part of the study area was the eastern part of the Cordilleran geosyncline in which elastic and carbonate sediments were deposited in shallow seas. The southern part of the area was in the Central Montana trough (Sloss, 1950, p. 427—428). Much, if not all, of the Sweetgrass arch area may have been positive during Precambrian Belt sedimentation. The pronounced eastward thin- ning of the Belt rocks in the disturbed belt is dis- cussed by McGill and Sommers (1967), Mudge (1970), and several earlier authors. If Belt sedi- mentary rocks were deposited over the arch, as sug- gested by Sloss (1950, p. 430), they would have been extremely thin. The arch and nearby areas were uplifted and eroded before being in ‘ndated by the Middle Cambrian sea. A3 During Paleozoic sedimentation the eastern part of the area was probably part of the craton as suggested by Sloss (1950, fig. 1), and the Cordil- leran miogeosyncline probably lay to the west. To the south a narrow zone of thicker Paleozoic sedi- mentary rocks that were deposited in the Central Montana trough trends east (Sloss, 1950, fig. 1 and p. 426). The tectonic pattern of the Mesozoic probably began during Middle Jurassic; at that time the western part of Montana was a marine basin (Imlay, 1957, p. 471). The Sweetgrass arch area was a broad low uplift in the marine basin that was just below sea level (Cobban, 1945, p. 1286—1287). Later, the area was exposed, eroded, and again in- undated by marine water. In Late Jurassic time the sea retreated and continental deposition followed (Cobban, 1945, p. 1290). Cretaceous sedimentation occurred in both nonmarine and marine environ- ments. During this period, westernmost Montana was a highland that shed much detritus to the east (Reeside, 1957, p. 505—506). The thickest sediments accumulated in the western and southwestern parts of the outcrop area. During Early Cretaceous, the eastern edge of this highland may have extended into or near the present Lewis and Clark Range (Mudge and Sheppard, 1968). Plutonism, volcanism, and tectonism very likely began in Late Cretaceous time in the southern part of the area inasmuch as similar events occurred in the Boulder batholith re- gion to the south as described by Robinson, Klep- per, and Obradovich (1968). At the close of the Mesozoic, regional uplift resulted in doming and broad folding (Erdmann, 1944). The events of the Cenozoic caused the landscape of today. The last major orogeny occurred during the early part of the Tertiary. Uplift of the western part of Montana may have caused a slide mass to move eastward across the Mesozoic basin to form a northeastward-trending belt of closely spaced thrust fau‘ts, some longitudinal normal faults, and many folds (Mudge, 1970). To explain similar structures in Alberta, Canada, Price and Mountjoy (1970) provide an alternate hypothesis of lateral gravita- tional spreading from a zone of upwelling in the hot, mobile, metamorphic infrastructure of the core of the eastern Cordillera. The large normal faults in the western part of the mapped area (pl. 1) may have formed during or shortly after thrusting. Ero- sion, sedimentation in the intermontane basins, vol- canism, and local plutonism took place in middle and late Teritary. Multiple periods of glaciation, A4 GEOPHYSICAL FIELD INVESTIGATIONS 2 SOUTHWESTERN AREA SOUTHEASTERN AREA\ NORTHERN AREA 4 N th t f th M' ul _ Wolf Creek-northeastern part of Little Groff (1963); Ross, Andrews, and Wit- ERA E9. SERIES ‘gmfnalzoflip‘fififpzbmgzefsghi Belt Mountains. , kind (1955); Mudge (1965,1966a,b, g ‘f' d f G ff 1963) d f H Schmidt (1963); Schmidt and Strong c, 1967, 1968); Cobban, Erdmann, m 1 19 mm '0, ( 8“ mm - (1968); Groff (1963); and Robinson, Lemke, and Maughn (1959); and s. Prostka (Wfltten commn 1967) Klepper, and Obradovich (1968) Madge, Ericson, and Kleinkopf (1968) Pliocene g _ Undiffeg'efnftiatfii ibasin sedimtentsl, 31:; Gravels - lclc u , as ang omera es, a 2 E Miocene and flood-plain deposits; locally some O < dacite N )—( . g E Oligocene 9 a a Basalt and andesité, mainly flows; m1- Eocene nor intrusive bodies; some‘volcanic breccxa Paleocene s l Adel 1&0th Willow Creek Formation q u a St. Mary River Formation St. Mary River Formation Montana Group Homethief Sandstone Horsethief Sandstone sedimentafiofk/{JL/J“ n. :3. L2 Bearpaw Shale ’ ' Volcanic a v a ‘ be - - 2 /\/~/‘ ,1 /\/~/‘ 92 mm ,- Wfli‘fififim a Two Medicine Formation Golden Spike facies of Elkhorn g g Mountains Volcanics g g 2 Virgelle Sandstone E Virgelle Sandstone Upper Telegraph Creek Formation Telegraph Creek Formation Carter Creek Formation i Kevin Shale Member i Kevin Shale Member 3 w (I) o 5'3 . ’53 . 8 BE Ferdlg Shale Member 5 Ferdlg Shale Member ‘1 Jens Formation a '3 Cone Member g Cone Member or. . a. a O Coberly Formation 3°: 2 Floweree Member ’8’ E F'loweree Member 2 i O 'w' ' O 0 ° a g g I E Boot egger m 2 = 3 '= M b {:1 o .9 o .9 V hn em er 2 0 i3 Vaughn Member 0 g; 8118 E E _Member Vaughn Member 0 Blackleaf Formation E E Lo 5 Taft Hill Member 5 Taft Hill Member wer f) .31 s 3 “3 Flood Member 93 Flood Member Kootenai Formation Kootenai Formation Kootenai Formation WW?me Morrison Formation Morrison Formation Morrison Formation Upper :2 Swift Sandstone 1:. Swift Sandstone 9. Swift Formation _ 5 WNW a ,1 .~ ---~ 5 WW 2 (.25 Rierdon Formation 5 V Rierdon Formation 5 Rierdon Formation % .91 .3 h .93 a ‘ a: E Sawtooth Formation E Sawtooth Formation E Sawtooth Formation 3 Middle FIGURE 2.—Generalized chart GREAT FALLS-MISSIO RANGE AREA, NORTHWESTERN MONTANA A5 SOUTHWESTERN . AREA Northeastern part of the Miss ula- SERIES Drummond—Phillipsburg area. od- ified from Groff (1963) and from H. S. Prostka (written commun. SYSTEM Phosphoria Formation IAN Quadrant Quartzite Amsden Formation PEN NSYL— VANIAN MISSISSIPPIAN Madison Limestone PALEOZOIC Jefferson Dolomite DEVONIAN Maywood Formation Red Lion Formation Pilgrim Dolomite Park Shale Hasmark Limestone CAM BRIAN Middle Silver Hill Formation I Flathead Sandstone Pilcher Quartzite Garnet Range Quartzite McNamara Argillite Bonner Quartzite Miller Peak Argillite PRECAMBRIAN Belt Supergroup Helena Formation Ravalli Group of rock-stratigraphic units. SOUTHEASTERN AREA NORTHERN AREA Wolf Creek-northeastern part of Little Groff (1963); Ross, Andrews, and Wit- Belt Mountains. kind (1955); Mudge (1965, 1966a, b, Schmidt (1963); Schmidt and Strong c, 1967, 1968); Cobban, Erdmann, (1968); Groff (1963); and Robinson, Lemke, and Maughn (1959); and Klepper, and Obradovich (1968) Mudge, Ericson, and Kleinkopf (1968) Quadrant Quartzite Amsden Formation Heath Formation Otter Formation Kibbey Formation Big Snowy Mission Canyon Limestone Castle Reef Dolomite Madison Group Lodgepole Limestone Allan Mountain Limestone Three Forks Shale Three Forks Formation Birdbear Member Birdbear Member Jefferson Dolomite Jefferson Formation Unnamed member Unnamed member Maywood Maywood Formation Snowy Range Formation Pilgrim Limestone Devils Glen Dolomite Park Shale Switchback Shale M e a 8h er Limestone Steamboat Limestone Pagoda Limestone Dearborn Limestone Wolsey Shale Damnation Limestone Gordon Shale Flathead Sandstone Flathead Sandstone amara Argillite Bonner Quartzite ' Shields Shepard Formation Marsh Snowslip Formation Helena Dolomite Helena Dolomite Belt Supergroup Empire and Spokane Formations and Spokane Belt Supergroup Greyson Shale Greyson Shale A6 erosion, and sedimentation characterized the Qua- ternary. PRECAMBRIAN ROCKS Precambrian metamorphic and igneous rocks un- derlie the eastern part of the area, whereas Precambrian sedimentary rocks are exposed in the western part. Metamorphic and igneous rocks con- stitute the basement beneath Paleozoic rocks in the South arch of the Sweetgrass arch (Alpha, 1955a, b). These rocks, known from seven boreholes, range from gneissic granite, granitic gneiss, and quartz diorite to diabase, trachyandesite porphyry, and porphyritic andesite. The top of the basement is from 3,300 to 4,200 feet beneath the surface (Alpha, 1955a) and is within a broad anticline, as shown by Dobbin and Erdmann (1955). The composition of the older Precambrian rocks in the western part of the map area is not known. Such rocks are not exposed and have not been pene- trated by drilling. We assume that they are similar to the rocks in the basement in the Sweetgrass arch and to the gneiss and schist that locally have been intruded by the diorite exposed in the Little Belt Mountains (Weed, 1899) just southeast of the area shown on plate 1. All Precambrian rocks that are exposed in the area belong to the Belt Supergroup. The Belt con- sists of slightly-metamorphosed sedimentary rocks; locally, they are intruded by many sills and a few dikes of diorite that are Precambrian. The Belt rocks consist of fairly thick clastic units and one moderately thick carbonate unit that were deposited in very shallow marine waters. Although this sequence does not change markedly in lithology in the area, it does vary considerably in thickness. The exposed strata are about 20,000 feet thick in the southern part of the Mission Range (Harrison and others, 1969), as much as 23,000 feet thick in the Swan Range (Walcott, 1906), 15,000—17,000 feet in the Lincoln area, and 5,000—7,500 feet in the southern part of the Lewis and Clark Range (Mudge and others, 1968), and they are absent from the Sweetgrass arch (Alpha, 1955a). The var- iation in thickness is partly a result of depositional thinning of the units to the southeast and to the east and partly of deep pre-Middle Cambrian ero- sion in the eastern part. In those areas the Flathead Sandstone was deposited on an erosional surface in which successively older strata had been truncated to the east (Deiss, 1935, pl. 8; Mudge, 1971). To the west near the Continental Divide, the Flathead ov- erlies the Garnet Range Formation, whereas in the GEOPHYSICAL FIELD INVESTIGATIONS easternmost exposure of the Flathead in the Dear- born River canyon, it overlies the Greyson Shale, a stratigraphic difference of at least 8,500 feet (fig. 2). The erosional edge of the Belt rocks is very likely at, or near, the eastern edge of the disturbed belt, as shown on plate 1. Its position is inferred from the rate of eastward thinning of Belt rocks—the amount of pre-Middle Cambrian erosion of the Belt rocks—and from the knowledge that the Belt rocks are absent from the Sweetgrass arch. A distance of 46 miles separates the westernmost well that bot— toms in the Precambrian crystalline rocks in the arch from the Belt outcrop in the Dearborn River canyon. Sills and one or more dikes of diorite intrude var- ious Belt rocks in the western part of the area (pl. 1). The sills are as much as 900 feet thick. In the southern part of the Swan Range they intrude the Spokane Formation, but northward they rise and cut across about 10,000 feet of strata to the Snows- lip Formation. Similar positioning of the sills was observed in the eastern part of the Lewis and Clark Range by Mudge (1966b, c). In the northern part of that range, sills are emplaced in units as young as the Mount Shields Formation. The only dike ob- served in the eastern part of the area transects the Greyson Shale in the Dearborn River canyon. The age of the diorite has been determined by potas- sium-argon methods as 750:25 m.y. (million years) by J. D. Obradovich (oral commun., 1966). The ig- neous rocks may be correlative with some of the buried igneous rocks on the Sweetgrass arch. PALEOZOIC AND MESOZOIC ROCKS Sedimentary rocks of the Cambrian, Devonian, Mississippian, Pennsylvanian, and Permian Systems of the Paleozoic Era, and the Jurassic and Creta- ceous Systems of the Mesozoic Era crop out in the area (pl. 1). Pennsylvanian and Permian rocks crop out only in the southern part (fig. 2). Clastic rock (Willow Creek Formation) of Late Cretaceous and Paleocene age is present east of the Dearborn River canyon (Viele and Harris, 1965, p. 412) . The Paleozoic rocks are about 3,100 feet thick in the north-central part of the area, about 4,100 feet in the northwestern part, and about 5,100 feet in the southwestern part. The lower and middle parts of the section consist typically of very thick se- quences of carbonate rock that alternate with thin sequences of clastic rock. The upper part of the sec- tion is elastic rock and thin beds of carbonate rock. The Mesozoic rocks have an aggregate thickness GREAT FALLS—MISSIOI‘L RANGE AREA, NORTHWESTERN MONTANA A7 of about 9,200 feet in the north-centr l outcrop area, 12,100 feet in the northwestern art, and 16,700 feet in the southwestern part. The typically are thick sequences of mudstone interbedded with thin sequences of sandstone and some confomerate. MESOZOIC AND EARLY CENOZOIC IGNEOUS AC IVITY Igneous activity in, and adjacent to, the area began early in Cretaceous time and continued pe- riodically through the Tertiary. The activ ty is rep- resented in part by sedimentary rocks deiived from igneous rocks in, or adjacent to, the ar a and in part by igneous rocks exposed in the area. Most of the intrusive and extrusive rocks are exp sed in the southern part. Many of them are proba%ly geneti- cally related to similar rocks in the Boul er batho- lith area (Robinson and others, 1968). The sequence of igneous events is as follows, from oldest to youngest: L 1. Very Late Jurassic or earliest Cretac ous (Ap- tian) extrusive and shallow intrusive activity in or near the study area. Indicate by peb- bles and cobbles of granite, quartz Eonzonite, granodiorite, and quartz diorite, as ell as sil- icic lava and tufl’, in the conglomerate locally present in the lower and upper pa ts of the Kootenai Formation in the southevE part of the Sawtooth Range (Mudge and heppard, 1968). 2. Late Early Cretaceous (Albian) volc nic activ- ity in or near the study area. In icated by dacite and rhyolitic welded-tuff pe bles and cobbles and volcanic-rich sedimentary rocks, including tuff which contains accre ionary la- pilli locally, in the Vaughn Mem er of the Blackleaf Formation in the southe n part of the Sawtooth Range (Mudge and Sheppard, 1968). 3. Late Cretaceous A. Two Medicine (early Campanian) volcanic activity. Volcanic-rich sedimentary rocks, tuff, and rhyodacite nd latite lavas in the lower part of the fwo Medi- cine Formation in the Wolf C eek-Dear- born River area (Viele and Harris, 1965; Schmidt and Stro g, 1968; Schmidt, 1963; Schmidt and others, 1964). These rocks are equi alent, at least in part, to the Elkhorn ountains Volcanics of the Boulder ba holith re- gion, as described by Klepp r, Weeks, and Ruppel (1957, p. 31) ; Sme es (1966, p. 21)‘; and Robinson, KleppT, and Ob- radovich (1968, p. 563, 566—569). Peb- bles of dacite and andesite lava are in conglomerate in the lower, middle, and upper parts of the Two Medicine (Mudge and Sheppard, 1968). B. Emplacement of the Boulder batholith and its satellites (much of Campanian and early Maestrichtian; 78—68 m.y.; Tilling and others, 1968, p. 687 ; Robinson and others, 1968); emplacement of grano- diorite and quartz monzonite (Klepper and others, 1957; Knopf, 1963; Smedes, 1966). The Marysville granodiorite stock has been dated as 78 my. by Baads— gaard, Folinsbee, and Lipson (1961, p. 697). The Blackfoot, Granite Butte, Sil- ver Bell, McClellan Gulch, and other nearby stocks probably were emplaced at the same time. C. Horsethief volcanic activity (late Cam- panian). Pebbles and cobbles of por- phyritic trachyandesite and andesite in the Horsethief Sandstone in the vicinity of Augusta and Sun River (Viele and Harris, 1965, p. 411; Mudge and Shep- pard, 1968). D. Intrusion of trachyandesite, syenite, and diorite sills, dikes and irregularly shaped bodies in Lower Cretaceous rocks in the southeastern part of the area and in the Sawtooth Range (possibly late Maestrichtian or early Paleocene) (Gwinn, 1961; Mudge, 1966a, b, c, 1967). E. Adel Mountain Volcanics and related in- trusives (late Maestrichtian or possibly Paleocene) that cover an area about 32 miles long and as much as 18 miles wide (Lyons, 1944; Schmidt and Strong, 1968; Schmidt, 1963; Schmidt and oth- ers, 1964; Fox, 1966). They consist of about 3,200 feet of potash-rich volcanics and related intrusives, which are dikes, sills, and stocks mainly of trachybasalt, syenogabbro, and monzonite (Lyons, 1944). 4. Tertiary A. Deposits of Eocene age in southwestern part of the area. Basalt and andesitic flows; some volcanic breccia, minor in- trusive bodies, and numerous mineral- ized veins (H. J. Prostka, written com- mun., 1967). A8 B. Deposits of Oligocene, Miocene, and Pliocene age in the southwestern part of the area. Rhyolite, dacite, welded tuff, latitic flows, and porphyritic intrusions (Gwinn, 1961; Bierwagen, 1964; Melson, 1964; H. J. Prostka, written commun., 1967). C. Emplacement of Haystack Butte intrusive, southeast of Augusta (Miocene, 20:3 m.y., J. D. Obradovich, written com- mun., 1969). Rhyodacite porphyry that resembles a volcanic plug. GEOLOGIC STRUCTURE The map exhibits three major structural fea- tures: the Sweetgrass arch in the northeastern part, the disturbed belt in the central part, and the large normal faults in the western part (pl. 1). They all formed during the Cenozoic. Some older structural events are evident from the many unconformities shown in figure 2. The early Tertiary orogeny con- sisted of folding, thrust faulting, and normal fault- ing; it probably occurred between Paleocene and late Eocene (Russell, 1951; Bossort, 1957; Me Mannis, 1965; Bally and others, 1966; Price and Montjoy, 1970). The large normal faults in the western part of the area may represent a distinct stage of Cenozoic orogeny that has been dated in nearby areas to have begun in the Oligocene and to have climaxed in the Pliocene (Pardee, 1950, p. 360; Russell, 1951, p. 69; Cook, 1960, p. 199). Some of the faults possibly have had recurrent movements on them throughout the Pleistocene (Mansfield, 1923, p. 269). The disturbed belt in northwestern Montana, as defined by Mudge (1970) and used herein, is the area of intense deformation that extends from the central part of the Lewis and Clark Range to about Augusta, Mont. (pl. 1). This definition differs from that of Stebinger (1918) and Alpha (1955b, p. 137), who defined the west edge of the Belt as the mountain front. They defined the east edge as it is used in this report. The disturbed belt contains two distinct types of structural features (Mudge, 1970). In the western part (Lewis and Clark Range) large widely spaced low-angle thrusts and large longitudinal normal faults displace Precambrian and Paleozoic rocks. In the eastern part (Sawtooth Range and western Great Plains) the thrust faults are relatively small and closely spaced, and they form a broad imbricate zone. The western Great Plains are characterized by folds with a few very small thrust faults. In the GEOPHYSICAL FIELD INVESTIGATIONS northern part of the map area these structural fea- tures trend almost north, but in the southern part they trend northwest. The Sweetgrass arch is a broad northwest-plung- ing flexure with two echelon structural features— the South arch and Kevin-Sunburst dome (Alpha, 1955b, p. 138; Dobbin and Erdmann, 1955). Only the South arch is present in the area shown on plate 1. This arch has been an active tectonic fea- ture at various times from the Precambrian to the present, but it achieved its present size and configu- ration largely during the early Tertiary orogeny (Alpha, 1955a, b). The Scapegoat-Bannatyne trend is a northeast- ward-trending series of structural anomalies that were first recognized by Dobbin and Erdmann (1955) and discussed by Alpha (1955a). As shown on plate 1, its southWest terminus is in a transverse fault zone that displaces thrust faults. As discussed by Alpha (1955a, p. 133), this trend parallels the structural grain of the pre-Paleozoic and early Pa- leozoic features of the Canadian Shield in Canada and Minnesota and the pre-Paleozoic structural fab- ric in the subsurface in central and eastern Mon- tana and in North Dakota. The Pendroy fault zone, at the north end of the South arch (fig. 1), has a similar orientation. On the Sweetgrass arch the Scapegoat-Banna- tyne trend is marked by high areas on the Pre- cambrian surface against which the Cambrian and basal Devonian rocks wedge out; these areas have a known relief of more than 1,400 feet (Alpha, 1955a, p. 133). The higher parts of the trend were subse- quently raised probably during pre-Jurassic, Late Cretaceous, and early Tertiary times. The north-trending Augusta syncline, just north- west of Augusta, lies west of the Sweetgrass arch, across the eastern part of the disturbed belt (Alpha, 1955b). The west side of the syncline con- tains numerous small folds and some small faults (Stebinger, 1918, pl. 24). The east side forms the west flank of the Sweetgrass arch. In the southwestern part of the area there are numerous southeast-plunging folds not shown on plate 1 that are along the Montana lineament, as discussed by many authors, including Calkins and Jones (1931) ; Clapp (1932); Langton (1935) ;Poul— ter (1959); Gwinn (1961); and McMannis (1965). The largest normal faults in the western part of the area are along the east side of the Swan River valley (Swan fault) and the east side of the South Fork of the Flathead River (pl. 1). The displace- GREAT FALLS-MISSI?N RANGE AREA, NORTHWESTERN MONTANA ment of the Swan fault is probably more than 20,000 feet. The displacement of the outh Fork Flathead fault is about 16,000 feet (Sommers, 1966, p. 84). L The Mission Range, at the western dge of the map area, is a structurally simple ran e of block- fault mountains (Harrison and others, 1969). The block is bounded on the west by the northward- trending Mission fault and on the ast by the north-northwest-trending Swan fault (1 l. 1). The block is tilted eastward at about 250 ; th's tilting re- sults in a homoclinal dip from the c‘irest of the range eastward into Swan River valley. A large monoclinal flexure at the range crestJflattens the beds to the west, where they have b en dragged down to about horizontal on the footwal of the Mis- sion fault. Most of the northeast-trending faEIts in the west-central part of the area are tear o transverse faults; some are normal faults. The nort east-trend- ing faults are younger than the thrust faults, and their strike is virtually parallel to the Scapegoat- Bannatyne trend; similar faults in the western part of the area are undated. GEOPHYSICAL STUDIES FIELD MEASUREMENTS L The total intensity aeromagnetic dat shown on plate 1 involved two surveys. One sur ‘ey of about 7,800 square miles was flown in a no beast-south- west direction with traverses spaced about 2 miles apart. A smaller survey on the west, w ich covered about 1,800 square miles of the Swan nd Mission Ranges, was flown east-west at a flight line spacing of about 1 mile. The data from the smjaller survey have been used in geologic studies of the Mission Mountains primitive area by Harriso , Reynolds, Kleinkopf, and Pattee (1969). Both s rveys were flown at a barometric elevation of 9,0 0 feet, with deviations to 10,500 feet for clearance f mountain peaks. Measurements were made with a continu- ously recording ASQ—lO fluxgate magnFtometer in- stalled in a Convair airplane. Topographic maps were used for pos'tion control. The flight paths were recorded by a gilrostabilized 35-mm continuous-strip camera (Evenaen and oth- ers, 1967). Flight paths were recovere by Doppler navigation for the survey over the Sw n and Mis- sion Ranges. Base lines, which were clo ed on them- selves, were flown normal to the trave se lines for correction of diurnal and instrument 1 drift. The calibration of the magnetometer was makintained by frequent comparisons with a proton m gnetometer. A9 The total intensity data were reduced to an arbi- trary datum at a contour interval of 20y (gammas). The overall precision of the magnetic map is esti- mated to be about 57. Gravity data for 430 stations were obtained through the courtesy of the Air Force, and 125 sup- plemental points were set by the US. Geological Survey (pl. 2). The gravity measurements were ad- justed to the absolute datum of the Woollard (1958, p. 533) airport base, station WA124, at Great Falls, Mont. The data were reduced to complete Bouguer gravity values by use of an assumed rock density of 2.67 g/cm3 (grams per cubic centimeter). The con- tour interval of 5 mgal (milligal) seems to be real- istic for most of the area. Terrain corrections were made through the H zone of Hammer (1939) with hand templates and out to 167 km (kilometer) by means of a digital computer (Plouff, 1966). Many of the gravity stations in the mountains were reached by helicopter, and the elevations at the stations were determined by altimetry or by es- timates made from topographic maps—such eleva- tions may be in error by 50—100 feet. Most station elevations on the plains where US. Geological Sur- vey or US. Coast and Geodetic Survey bench marks are used for reference are probably accurate to 1 foot. Consequently, the accuracy of the Bouguer gravity map is estimated to vary from less than 1 mgal on the plains to 6 mgal in the mountains. ROCK PROPERTIES Magnetic susceptibility and density data were ob- tained from 22 samples analyzed in the laboratory and from data on similar rock types in surrounding areas (M. R. Klepper, written commun., 1969; Bur- feind, 1967; Davis and others, 1965). The density and susceptibility values used in the interpretations are assumed averages for the whole area and should be considered only approximations for specific prob- lem areas. Rock densities are assumed to average from 2.7 to 2.8 g/cm3 for the Precambrian Belt and Paleozoic rocks, 2.6 g/cm3 for Mesozoic rocks, and 2.3 g/m3 for Cenozoic rocks. The Precambrian diorite sills are about 2.9 g/cm3 (Harrison and others, 1971). Knopf (1963) reported an average density of 2.66 g/cm3 for the Upper Cretaceous Priest Pass Leuco- monzonite mass at the north end of the Boulder batholith. Values of 2.7—2.8 g/cm3 seem to be the range for the Upper Cretaceous granodiorite stocks in the southern part of the area (Knopf, 1963). The magnetic susceptibility measurements show significant differences of magnetic properties among A10 the various crystalline rock types. On this basis, these rocks could be expected to produce detectable magnetic anomalies. In laboratory determinations, the Tertiary trachyandesite sills show an average value of 0.001 emu/cm3 (electromagnetic units per cubic centimeter), whereas the Precambrian dior- ite-gabbro sills average about 0.003 emu/cm3. Sam- ples of granodiorite from the Boulder batholith av- erage 0.0058 emu/cm3. Quartz monzonite varies from 0.0008 to 0.002 emu/ems; gabbro is near 0.0024 emu/cm3. A magnetic susceptibility range of 0.006—0.008 emu/cm3 for intrusive rocks in the Three Forks region to the south was determined by Davis and others (1965), by means of a field method described by Hyslop (1945). MAGNETIC FIELD CONFIGURATION The first-order relief across the magnetic map from northeast to southwest is about 1,000y; this relief reflects the southwest-dipping gradient of the earth’s normal field of about 7y per mile. The north- west trend of the regional magnetic strike is locally deflected to the north by major early Tertiary structural trends. Some of the large normal and thrust faults shown on plate 1, for example, those along the west side of the Mission Range and those along the east side of the Sawtooth Range, are rep- resented by high magnetic gradient zones. The com- plexities of the magnetic field over the Great Plains are exhibited by diverse trends and by variations in anomaly shapes and sizes. Northwest, north-north- west, and northeast alinements are evident in the magnetic patterns. These alinements contrast mark- edly with expressions over the mountains, where the more gentle gradient zones are distorted mainly by shorter wavelength anomalies that have been produced by igneous intrusives. Sharp high-ampli- tude positive and negative anomalies are character- istic of the igneous rocks of the Adel Mountain vol- canic field. GRAVITY FIELD CONFIGURATION The regional gravity field dips to the southwest at about 1 mgal per mile, within a 100-mgal range of —80 mgal over the Great Plains to —180 mgal over high topography in the southwestern part of the area. The gravity field over the Great Plains (pl. 1) is dominated by a northwest-trending high-gradient zone located just east of the disturbed belt (pl. 1). The high-gradient zone is distorted by several southwest plunging maximum noses. A large north-trending 10-mgal minimum is lo- cated just east of the Sawtooth Range. Major GEOPHYSICAL FIELD INVESTIGATIONS north-northwest-trending minima correlate with valleys of the Flathead and Swan Rivers. The edge of a broad north-south-trending zone of high-grav- ity gradient occurs along the major normal fault that bounds the west side of the Mission Range. An- other prominent gravity feature is a nearly east- west-trending maximum ridge of 10- to 15-mgal amplitude that extends west from about lat 47° N., long 1120 W. INTERPRETATION OF GEOPHYSICAL DATA Magnetic data were interpreted qualitatively by gradient and configuration studies and semiquanti- tatively by the methods of Vacquier, Steenland, Henderson, and Zietz (1951), Zietz and Henderson (1956) , and Zietz and Andreasen (1967). Remanent magnetism, where important, is assumed to be par- allel to the earth’s field; thus, total magnetizations (induced plus remanent) of all rocks are assumed to be everywhere parallel. The source must reflect some type of crystalline rock, inasmuch as the sedi- mentary rocks are nonmagnetic, except for two sandstone beds (Upper Cretaceous Virgelle and Horsethief Sandstones), which locally contain at least 30 percent magnetite but which are less than 10 feet thick. A traceable zone of high-magnetic gradient correlates with the outcrop of the Virgelle Sandstone (pl. 1). The Bouguer gravity data are broadly spaced, and elevation control on many of the stations is poor. These data, as they relate to the magnetic in- terpretations, are therefore discussed only in a qualitative fashion. The variety of trends and anomaly types in the magnetic data reflects the diverse geology of the area. The complex magnetic patterns of the Great Plains are attributed to a heterogeneous crystalline basement, probably a metamorphic complex of schist and gneiss and numerous magnetite-rich in- trusions. Depths to anomaly sources, believed to be at or near the crystalline basement level, were calculated from measurements of magnetic gradients (method of Vacquier and others, 1951). The results for the Great Plains agree within 10 percent of values ob- tained from a combination of structure and isopach maps. The basement sea-level datum was computed to be about —1,000 feet at Great Falls and -2,500 feet at Choteau. In the disturbed belt, the paucity of discrete magnetic gradients that could be measured with confidence prevented constructing a coherent basement surface-configuration map. GREAT FALLS-MISSI N RANGE AREA, NORTHWESTERN MONTANA The north- and northwest-trending e rly Tertiary structures in the Lewis and Clark a d Sawtooth Ranges are reflected in both the magne ic data and the gravity data by high-gradient zone of varying degrees of continuity. The high-gradi nt zones of interest are distorted and compounde in several areas by shorter wavelength anomali s that are caused by dioritic sill complexes (pl. 1 . The large magnetic anomalies labeled 21 and 22 (pl. 1) also mask definition of unique high-gradien zones that are associated with northwest structural rends. The sills shown on the geologic map netic anomalies in the form of elongated xhibit mag- short-wave- length positive and negative closures and noses of less than 100y amplitude. Presence of other sills, buried at shallow depths, is strongly suggested by the magnetic data. The gravity data are not of suf- ficient detail to reflect either the exposed sills or the buried sills. The Precambrian diorite sills give a greater magnetic response than the Upper Creta- ceous or lower Tertiary trachybasalt greater response can be attributed to magnetite content (as much as 15 perc Precambrian diorite sills (Knapp, 1963). sills. This the greater ent) of the The possible geologic significance of the more prominent magnetic and gravity anomalies is dis- cussed. The geophysical data add greater subsurface definition to known geologic features an basis for postulating the presence of known. The following paragraph numbe to the prominent magnetic and gravity the maps (pls. 1, 2) and are discussed counterclockwise order, beginning near Anomalies numbered 18, 21, 22, and same anomalies referred to as numbers and 5, respectively, in an earlier report Erickson, and Kleinkopf (1968). d provide a those un- r's are keyed features on in roughly Great Falls. 38 are the 14, 10, 11, by Mudge, 1. The complex magnetic patterns of the Great Plains, north and west of Great Falls, are at- tributed to a heterogeneous crystalline base- ment, presumably of Precambrian schist and gneiss like that near Neihart in the Little Belt Mountains (Weed, 1899). have penetrated the basement r Seven wells ocks on the Sweetgrass arch and Kevin-Sunburst dome. The rocks are gneissic granite, granite gneiss, diabase, quartz diorite, trachyandes- ite porphyry, 12, T. 24 N., R. 2 W.; sec. 30, T. W.), between Collins and Dutton trachyandesite porphyry. Some and porphyritic (Alpha, 1955a, p. 133—134). Two andesite wells (sec. 25 N., R. 1 , penetrated of the ig- A11 neous rocks may be much younger than Pre- cambrian. 2. A magnetic and gravity trend extending north-northwest from the north end of the Little Belt Mountains to about 35 miles north of Great Falls closely approximates the trend of the anticlinal axis that passes through Great Falls (pl. 1). The trend probably re- flects an extension of the north-plunging structural features of the Little Belt Moun- tains. The sharpness of the magnetic trend may be the manifestation of a buried base- ment block uplifted on the west or a north- trending unit in the basement that has a magnetic contrast to the adjacent rocks. The persistence of the north-trending zone of high magnetic gradient is demonstrated by the fact that it terminates several east and east-northeast magnetic trends. 3. The magnetic anomalies marked 3A and 3B probably reflect buried plutons. The plutons are very likely of intermediate composition, inasmuch as stocks and laccoliths of such composition are exposed in the northern end of the nearby Little Belt Mountains (Weed, 1899; I. J. Witkind, oral commun., 1969). 4. The most pronounced northeasterly magnetic 5.A and gravity alinement is the Scapegoat-Ban- natyne trend (pl. 1). It is manifested in the magnetic data as a series of residual posi- tives and northeast contour alinements. On the Great Plains, the gravity expression of the Scapegoat-Bannatyne trend is a south- west-plunging maximum nose which has a residual maximum closure of about 10 mgal at its intersection with the axis of the South arch. The trend may have regional structural implications. The fracture system may have been the structural control for the emplace- ment of two magnetic anomaly sources, one about 8 miles north of Augusta (pl. 1, anom- aly 18) and the other at Scapegoat Mountain in the Lewis and Clark Range (pl. 1, anom- aly 21) . northeasterly magnetic trend parallel to the Scapegoat-Bannatyne extends from Brown Sandstone Peak past the north end of the large magnetic positive at the edge of the plains to about 5 miles north of Brady. The trend is manifested in both the gravity and the magnetic data as a series of alined highs and lows and by parallelism of the contours. A12 GEOPHYSICAL FIELD INVESTIGATIONS The gravity data suggest that the trend rep- resents a basement fault that extends 10 miles northeast and southwest of Brady. The northwest side is probably upthrown. 6. About 15 miles southeast of the Scapegoat-Ban- natyne trend, a northeasterly trend in the magnetic and gravity data reflects strong contour deflections and alinements. One northeast alinement along the trend is the magnetic negative whose axis passes about 12 miles southeast of Augusta. The trend ex- tends southwest through the small magnetic closure over Cunifl' Basin (anomaly 7) and Blowout Mountain to Stonewall Mountain Lookout Station. It extends east—northeast as far as the secondary axis of the South arch (pl. 1), crossing the arch near the town of Power in the form of a high-gradient zone. The trend is well indicated in the gravity data from the edge of the disturbed belt on the plains southwest to Blowout Mountain. Here, there are sufl‘icient gravity stations to define a contour alinement. . 7. The small magnetic anomaly over Cuniff Basin reflects the High Bridge stock, which, ac- cording to Viele and Harris (1965, p. 410), is a trachyandesitic to rhyodacitic mass that closely resembles volcanic pipe breccia. 8. The three small-amplitude positive magnetic re- siduals at Crown, Shaw, and Cascade Buttes reflect the trachyandesite sills and feeder dikes discussed by Lyons (1944) and Fox (1966). Square Butte nearby does not show a residual on the aeromagnetic map, but this is probably due to lack of data, for this butte too has a sill and feeder dike (Fox, 1966, pl. 1). 9. The northwesterly trend of magnetic contours appears to extend from anomaly 38 through Haystack Butte (anomaly 10) to anomaly 17. Three wells were drilled along this trend (pl. 1). The two westernmost wells penetrated diorite sills(?) in Cretaceous rocks. The sills(?) in the well (NE. cor. T. 18 N., R. 6 W.) northeast of Cuniff Basin are at depths of 4,410—4,650 feet, 4,670—4,715 feet, 5,170—5,320 feet, 5,650—6,490 feet, and from 6,580 to the bottom of the hole at 6,880 feet. In the well east of Cuniff Basin (SW. cor. T. 18 N., R. 5 W.), sills(?) were penetrated at depths of 2,490—3,120 feet, 3,460—4,420 feet, 4,920—5,340 feet, and 7,000—7,100 feet. The instrusives are very likely correlative with the sills exposed at anomaly 17. In the area between Haystack Butte (anomaly 10) and anomaly 38 there is a relatively broad band of exposed volcanics cut by sills in the Two Medicine Formation (Viele, 1960; Viele and Harris, 1965). Sills and possibly volcanic rock may be in the subsurface in the area be— tween Haystack Butte and anomaly 17, as suggested by the northerly extension of the trend of magnetic anomaly 9. If this is true, the igneous rocks do not extend as far east as the well (SW. cor. T. 20 N., R. 7 W.) just north of Haystack Butte, which down to a depth of 9,327 feet did not penetrate igneous rocks. Volcanic-rich sedimentary rocks of the TWO Medicine Formation, however, are re- peated at least 11 times by thrust faults in the upper 6,000 feet of the well, and these may contribute to the magnetic anomaly trend. 10. Haystack Butte is a prominent landmark east of the mountains that towers 1,800 feet above the plains. The butte, of rhyodacite porphyry, resembles a volcanic plug; dikes extend west and south from it (Viele, 1960, p. 146). It lies between two flight lines and is not reflected on the magnetic map. A gravity station near the southeast edge of the butte detected no anomaly, which suggests that the intrusive may be a vertical mass. 11. A high magnetic gradient zone extends from 5 miles southwest of Great Falls in a west- northwesterly direction for 30 miles. This high-gradient zone seems to mark the buried southeastward extension of the zero edge of Belt rocks from the Sun River Canyon area (Mudge and others, 1968). The magnetite- rich source for the gradient zone is un- known. 12. The positive magnetic anomalies labeled 12A and 123 in the northeast corner of the map area very likely reflect mafic bodies in the crystalline basement. 13. The most pronounced gravity expression on the plains is a high-gradient zone about 15 miles wide and 30 mgal in amplitude that extends northwest from Great Falls through Cho- teau. The zone of high gradient is part of an extensive northwest-trending regional fea- ture shown on the Gravity Map of the United States by Woollard and Joesting 17. A northwest-trending anomaly in GREAT FALLS-MISSIO‘N RANGE AREA, NORTHWESTERN MONTANA (1964) which may reflect an pper-crust transition between the Great Plans and the Rocky Mountains. Crustal model studies of Smith (1967) show, by means osteismic re- fraction and regional gravity da a, that the high-gradient zone may be cau ed by the wedging out of high-density mat rial in the upper crust. Inasmuch as the magnetic data show no correlative anomaly, th high-den- sity source can be assumed to be 0 closer to the surface than the Curie point evel (Vac- quier and others, 1951) . ’1 14. A 5— to 10-mgal gravity minimum extends north-northwest for about 35 iles from Haystack Butte, along, and party beneath, the mountain front. The minimu suggests a structural low which may be b oader than that shown by Stebinger (1918, pl. 1). 15. A fan-shaped set of magnetic anomalies reflects the complex Adel Mountain Volcanics and as- sociated intrusive rocks. The larg st anomaly within this volcanic field is about 00y in am- plitude. It may reflect a mode (ater large near-surface mafic mass. The pregence of' nu- merous small intrusive bodies wi in the vol- canics suggest that many of‘ the other smaller anomalies are also near-strface mafic masses. The northwest-trending anomaly may reflect one or more dikes in he area, as shown by Lyons (1944). At the orth end of the volcanic field,'in T. 18 N., R. 3 W., there are five small syenogabbro laccoliths and as- sociated feeder dikes (Lyons, 1944, p. 459). 16. A small northwest-trending anomajy, southeast of Augusta, is produced by a st ck, possibly of syenogabbro, that correlates w'th the crest and north side of the maximum f the anom- aly. the Pretty Prairie area reflects a magnetite-rich trachy- andesite sill, as much as 600 fee thick, that is folded and repeated many tim s by thrust faults. The sill extends north to‘form Sheep Reef where it dips steeply to the west and is not repeated by faulting. Near Sheep Reef it has no magnetic expression. 18. A magnetic anomaly, underlying more than six townships east of the disturbed belt and north of Augusta, probably reflects a gab- broic body at a computed depth of 6,000: 1,100 feet below the surface (anomaly 14 of Mudge and others, 1968, p. E16). A well 19. The 20. 21. 22. 23. A13 (NW. cor. T. 21 N., R. 5 W.) within the area of anomaly (pl. 1) bottomed in the Steam- boat Limestone (Middle Cambrian) at a total depth of 6,775 feet. The lower Paleozoic rocks probably rest unconformably on pre- Belt basement rocks here (Mudge and others, 1968, p. E16), and the gabbroic body is pre- sumably within the basement. southeast-trending noselike magnetic anomaly that extends from Deer Creek (19A) to the upper reaches of Smith Creek (190) reflects a diorite-gabbro sill; this sill is as much as 500 feet thick and is locally re- peated by thrust faults. Along Wood Canyon (19B) the sill contains as much as 15 percent magnetite (Knapp, 1963). The closed anom- aly in the Deer Creek area very likely re- flects repetition of the sill by thrust faults. A magnetic anomaly, on the west side of the North Fork of Sun River valley, reflects the east edge of the outcropping Precambrian sedimentary rocks of the Lewis and Clark Range block and their contained sills. The minor trend depicted by the contours is probably influenced more by the sill than by the Belt rocks. The magnetic anomaly southwest of Scapegoat Mountain, in the area of Concord Mountain and Evans Peak, is discussed by Mudge, Er- ickson, and Kleinkopf (1968, p. E15) as anomaly 10. It is interpreted to represent a buried pluton of intermediate composition at a computed depth of 10,500:1,200 feet below the surface. Magnetic anomaly 22 extends over the Redhead Peak-Larch Hill area. Mudge, Erickson, and Kleinkopf (1968, p. E15, E16, anomaly 11) believe that this anomaly reflects a large plu- ton at a computed depth of 9,500: 1,000 feet. Magnetic anomaly 23, in the Camp Creek Cay- use Mountain area, is of small amplitude. The anomaly is the one labeled 8 by Mudge, Erickson, and Kleinkopf (1968, pl. 2). Only Belt sedimentary rocks are exposed in the area. To the north, 1 mile east of Big Prairie Ranger station, a diorite sill, 77 feet thick, intrudes these rocks (Sommers, 1966, p. 74); this sill apparently has no magnetic expres- sion. The anomaly may represent a buried dike or small pluton. 24. A north-trending zone of high magnetic gra- dient extends from Sugarloaf Mountain to A14 the north edge of the map area. The zone, which is over the valley of the South Fork of the Flathead River, on the west side of the Lewis and Clark Range, may be caused by basement faulting. A normal fault of about 16,000 feet displacement has been mapped along the east side of the valley (Sommers, 1966). The gravity data are sparse, but they do show a broad 5— to 10-mgal minimum over the valley area of Paleozoic rocks, the downthrown block. 25. A large 80-y magnetic anomaly centered over the west edge of the Swan Range suggests a source buried about 20,000 feet (16,000 ft below sea level). The position of the anomaly over the Swan fault suggests the presence of a buried pluton in the Precambrian crystal- line basement (pl. 3). 26. To the east of anomaly 25, a narrow elongate 100-y positive anomaly on the east flank of the large anomaly reflects a highly magnetic Precambrian sill and dike complex of diori- tic-gabbroic composition that crops out for several miles along the crest of the Swan Range (pl. 1). 27. The gravity minima at Swan valley probably are produced by two local sedimentary bas- ins, one centered near the Condon Ranger station, and the other, at Seeley Lake. Near the Ranger station the Cenozoic sediments may be 2,000—3,000 feet thick, as suggested by the 10— to 15-mgal minimum. At Seeley Lake the gravity minimum is attributed to Belt rocks that are preserved along the Swan Range fault or to a buried silicic pluton that is related to the Garnet Range on the south. The total displacement of the normal fault along the east side of the valley is at least 20,000 feet (Mudge, 1970). The fault lacks gravity expression, because only Belt rocks are involved on both sides of the fault. 28. A north-trending high magnetic gradient zone, about 2 miles wide and of 807 amplitude, is parallel to the west edge of the Mission Range and to the large north-trending Mis- sion fault described by Pardee (1950). The fault dips about 45° W. and may have a dis- placement of as much as 17,000 feet (Nobles, 1952). The rocks exposed along the west side of the range are of the Ravalli Group (Johns, 1964; Harrison and others, 1969); exposures of igneous rocks are not recorded. GEOPHYSICAL FIELD INVESTIGATIONS The source for the high magnetic gradient zone appears to have a maximum depth of about 1 mile. The Mission fault may have controlled the west edge of an igneous intru- sion. The Mission fault was not reflectedin the gravity data, inasmuch as the displace- ment involves only Belt rocks. 29. The southward positive nosing of the magnetic contours from lat 47°20’ N. to lat 47°30’ N., near McDonald Lake, may reflect near-sur- face igneous rocks. The small outcrop of Ter- tiary quartz diorite about 5 miles to the north along Elk Creek has no magnetic ex- pression (Harrison and others, 1969), but quartz diorite may extend downward to the south and may attain dimensions that would produce the magnetic nosing. 30. The southeasterly trend of the magnetic con— tours parallels the trace of the Swan fault on the north side of Kleinschmidt Flat. This fault with the southwest side downthrown has been traced southeast as far as the Lin- coln area. As shown by the trend of the con- tours, the fault may very likely extend be- neath the younger volcanics at Stemple Pass (between anomalies 35 and 36) and connect with the normal fault shown southeast of the pass. This major normal fault may extend much farther southeast to form the north margin of Little Prickly Pear valley (G. D. Robinson, oral commun., 1969). The south- easterly trend of contours in this area is vir- tually parallel to the Central Montana Trough as visualized by Sloss (1950, fig. 3) and McMannis (1965, p. 1806). 31. In the Arrastra Mountain-Mineral Hill area, north of Lincoln, the southeast-trending magnetic anomaly reflects two thick diorite- gabbro sills of late Precambrian age. 32. The magnetic anomaly at Dalton Mountain, a few miles east of Browns Lake, is probably caused by a buried pluton. The pluton may be related to stocks exposed along the moun- tain ridge, including Granite Butte. If so, the pluton may be quartz diorite to quartz mon- zonite. 33. Southeast of Dalton Mountain, this magnetic anomaly trends northeast along the crest of the range. No stocks are exposed in the area, but basalt and andesite are present and may be associated with a buried intrusive. GREAT FALLS-MISSION RANGE AREA, NORTHWESTERN MONTANA A15 34. The exposed quartz diorite stock at the head of McClellan Gulch (SW. cor. T. 13 N., R. 8 W.) was referred to as the Dalton Mountain stock by Melson (1964, fig. 2), but Dalton Mountain is about 13 miles to the northwest. (See pl. 1, anomaly 32.) The stock at the head of McClellan Gulch is slightly more mafic than the Silver Bell and Granite Butte stocks (Melson, 1964, p. 37). The magnetic anomaly not only includes the McClellan Gulch stock in the northern part but also ex- tends farther west than the McClellan Gulch stock. A short distance to the north a group of small stocks mapped by Melson (1964, p. 37) do not show anomalies on the aeromag— netic map—probably because the stocks are small and are located between flight lines. 35. These closely spaced contours are over the ex- posed part of a granodiorite and quartz mon- zonite stock at Granite Butte south of Stem- ple Pass. The western projection of the magnetic nose includes the exposed Silver Bell stock. The lack of a more pronounced anomaly over it is possibly explained by Mel- son (1964, p. 36) : “the Silver Bell stock, in contrast to the Granite Peak stock, shows considerable evidence of late magmatic alter- ation and mineralization by water-rich vola- tiles.” 36. The two small magnetic anomalies north of Stemple Pass and Virginia Creek may reflect buried granodiorite or quartz monzonite stocks like the Granite Butte and Silver Bell stocks. The area is overlain by LOGO—1,500 feet of latitic to andesitic volcanic rocks, some of which is welded tuflF; locally, there are rhyolite domes (Melson, 1964, p. 46). Melson (p. 46—47) noted that the volcanic rocks rest unconformably on the contact au- reoles of the southern stocks. 37. The broad magnetic nose south of Rogers Pass is produced by a large diorite-gabbro por- phyry sill that crops out in Shave Gulch and its tributaries, north of the Mike Horse mine (Pardee and Schrader, 1933, p. 88). Pardee and Schrader (p. 90) noted that dikes of trachyte porphyry—granodiorite and rhyo- lite—younger than the sill are present in the Mike Horse mine and nearby are s. A phon- olite body, also younger than the sill, is just north of the mine. 38. This large magnetic anomaly is mostly east of Rogers Pass in drainages of the Middle and South Forks of the Dearborn River. Belt and Paleozoic rocks, in large low-angle thrust- fault plates, cr0p out over the western half of the anomalous mass. These rocks are thrust on the Two Medicine, St. Mary River, and Willow Creek Formations, which are ex- posed over the eastern half of the anomalous mass. Gabbro and monzonite sills and small quartz monzonite stocks are exposed near the western edge of the area of the anomaly. The anomaly is probably produced by a large mafic mass at a calculated depth of 6,000: 1,000 feet beneath the surface (Mudge and others, 1968, p. E15). 39. This magnetic anomaly, west of Marysville, re- flects the Marysville stock, Which is exposed in the eastern part of the area of the anom- aly. The stock of diorite intrudes the Belt rocks and has a contact aureole %—2 miles wide; it broadens downward into an irregu- lar pyramidal form (Pardee and Schrader, 1933, p. 64). The anomaly indicates that the bulk of the stock is buried south of the expo- sure (pl. 1). 40. This magnetic anomaly is produced by the Blackfoot stock and nearby smaller intru- sives, which are quartz monzonite. The anomaly covers much more area than the ex- posed stock, suggesting that the intrusive complex broadens with depth. 41. The magnetic anomaly at the head of Warm Springs Creek may reflect a pluton of inter- mediate composition that is buried by Ter- tiary basalt and andesite. The western part of the anomaly is overlain by Paleozoic car- bonate rock that has been altered to skarn. 42. The extensive 15—20-mgal gravity minimum which embraces the area of the stocks and circular anomalies 32—40 may represent a buried extension of the porphyritic grano- diorite and (or) Priest Pass Leucomonzonite bodies (Knopf, 1963) that are exposed about 30 miles south of Lincoln near Austin. The stocks that have been described as granodio- rite may be apophyses of the parent body. Barrell (1907, p. 81) suggested that stocks such as those near Marysville and Granite Butte are upward prolongations or cupolas of the northern extension of the Boulder batholith. The gravity minimum extends south over the batholith. A16 Structure Section The main geologic features of the study area are depicted on structure section A—A’ (pl. 3). Profiles of the observed magnetic and gravity data, With the regional field removed, have been drawn in the same plane. The magnetic profile calculated from two-dimensional model studies is compared with the observed magnetic data. The gravity control is not of sufficient quality or detail to obtain meaningful results from modeling the complex density section Which involves density contrasts in the sedimentary rocks as well as in the crystalline basement. Several of the large positive magnetic and gravity anom- alies were previously interpreted as representing buried pluton masses of high magnetic and density contrasts (Mudge and others, 1966, 1968; Harrison and others, 1969). The model studies of section A—A’ provide additional speculations on the possi- ble size, shape, and depth of burial of the plutons and the configuration of the crystalline-basement surface beneath the sedimentary rocks. Through trial and error, magnetic susceptibility values of 0.0008—0.0025 emu/cm3 were assigned to the in— ferred plutons. These values fit compositions rang- ing from quartz monzonite through granodiorite to gabbro. The crystalline basement was assumed to be Precambrian gneissic rock similar to exposures in the Little Belt Mountains. Magnetic susceptibility values of 0.0001—0.0005 emu/cm3 were assigned, which suggests a low, but varying, magnetite con- tent of the Precambrian gneiss. The basement configuration and distribution of plutons shown in section A—A’ gives a theoretical magnetic profile which is a reasonably good fit to the observed magnetic profile (pl. 3). In the west half of the section, the Precambrian basement is considered to be deeply buried, with plutons possi- bly projecting through it into the lower part of the Belt sedimentary rocks. The gravity data confirm the presence of plutons postulated from the magnetic model studies. Other gravity anomalies probably have sources above the basement. For example, the gravity maximum anomaly of about 6 mgal over the eastern Sawtooth Range is attributed to the concentration of imbri- cate thrust sheets above the basement. A broad gravity high Which correlates with the positive magnetic anomaly over the Swan Range pluton, is distorted by a 14-mgal gravity minimum anomaly attributed to low-density sediments of Swan River valley. GEOPHYSICAL FIELD INVESTIGATIONS CONCLUSIONS Aeromagnetic and gravity data provide additional dimensions to geologic investigations in the Great Falls-Mission Range area, northwestern Montana. Use of the aeromagnetic data in defining the struc- tural framework and distribution of near-surface crystalline rocks permits more detailed interpreta— tion of the geology, particularly in the mountainous areas of limited access. In the southern part of the area, magnetic data provide evidence of buried stocks similar to those associated with the ore deposits of the area. The anomaly at the head of Warm Springs Creek (pl. 1, 41) may reflect an intermediate stock that is over- lain by Tertiary basalt and andesite. Magnetic model studies of structure section A—A’ provide a basis for visualizing a deeply buried ir- regular basement surface of Precambrian crystal- line rocks with much younger plutons possibly projecting into the lower part of the Belt sedimen- tary rocks. REFERENCES CITED Alpha, A. G., 1955a, The Genou Trend of north central Mon- tana, in Am. Assoc. Petroleum Geologists Rocky Mtn. Sec. Geological record, Feb. 1955: p. 131—138; slightly revised, World Oil, V. 142, no. 1, p. 79—82, 84, 1956. 1955b, Tectonic history of north central Montana, in Billings Geol. Soc. Guidebook 6th Ann. Field Conf., Sept. 1955: p. 129—142. Baadsgaard, Halfdan, Folinsbee, R. E., and Lipson, J. I., 1961, Potassium-argon dates of biotites from Cordilleran granites: Geol. Soc. America Bull., v. 72, no. 5, p. 689—701. Bally, A. W., Gordy, P. L., and Stewart, G. 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M., 1964, Progress report of geologic investiga- tions in the Kootenai-Flathead area, northwest Montana —[Pt.] 6, Southeastern Flathead County and northern Lake County: Montana Bur. Mines and Geology Bull. 42, 66 p. Johnson, R. W., Jr., Henderson, J. R., and Tyson, N. S., 1965, Aeromagnetic map of the Boulder batholith area, southwestern Montana: U.S. Geol. Survey Geophys. Inv. Map GP—538. Kleinkopf, M. D., and Mudge, M. R., 1968, Aeromagnetic and Bouguer gravity anomalies and related geology of parts of Cascade, Teton, and Lewis and Clark Counties, Mon- tana, in Abstracts for 1967: Geol. Soc. America Spec. Paper 115, p. 429. Kleinkopf, M. D., Mudge, M. R., and Harrison, J. E., 1968, Aeromagnetic and gravity studies across the northern disturbed belt in northwestern Montana [abs.]: Am. Geophys. Union Trans., v. 49, no. 1, p. 330. Klepper, M. R., Weeks, R. A., and Ruppel, E. T., 1957, Geol- ogy of the southern Elkhorn Mountains, Jefferson and Broadwater Counties, Montana: U.S. Geol. Survey Prof. Paper 292, 82 p. [1958]. Knapp, G. F., 1963, A diorite sill in the Lewis and Clark Range, Montana: Massachusetts Univ. M.S. thesis. Knopf, Adolph, 1963, Geology of the northern part of the Boulder bathylith and adjacent area, Montana: U.S. Geol. Survey Misc. Geol. Inv. Map I—381. Lange, S. S., 1963, The geology of the Lewis and Clark Pass area, Lewis and Clark County, Montana: Missouri Univ. unpub. M.A. thesis. Langton, C. M., 1935, Geology of the northeastern part of the Idaho batholith and adjacent region in Montana: Jour. Geology, v. 43, no. 1, p. 27—60. Lyons, J. B., 1944, Igneous rocks of the northern Big Belt Range, Montana: Geol. Soc. America Bull., v. 55, no. 4, p. 445—472. Mansfield, G. R., 1923, Structure of the Rocky Mountains in Idaho and Montana: Geol. Soc. America Bull., v. 34, no. 2, p. 263—284. McGill, G. E., and Sommers, D. A., 1967, Stratigraphy and correlation of the Precambrian Belt Supergroup of the southern Lewis and Clark Range, Montana: Geol. Soc. America Bull., v. 78, no. 3, p. 343—351. McMannis, W. J., 1965, Résumé of depositional and struc- tural history of western Montana: Am. Assoc. Petro- leum Geologists Bull., v. 49, no. 11, p. 1801—1823. Melson, W. G., 1964, Geology of the Lincoln area, Montana, and contact metamorphism of impure carbonate rocks: Princeton Univ. unpub. Ph. D. dissertation, 194 p.; abs., in Dissert. Abs., v. 25, no. 10, p. 5864—5865, 1964. Mudge, M. R., 1965, Bedrock geologic map of the Sawtooth Ridge quadrangle, Teton and Lewis and Clark Counties, Montana: U.S. Geol. Survey Geol. Quad. Map GQ—381. 1966a, Geologic map of the Patricks Basin quadran- gle, Teton, and Lewis and Clark Counties, Montana: U.S. Geol. Survey Geol. Quad. Map GQ—453. A18 1966b, Geologic map of the Pretty Prairie quadrangle, Lewis and Clark County, Montana: U.S. Geol. Survey Geol. Quad. Map GQ—454. 1966c, Geologic map of the Glenn Creek quadrangle, Lewis and Clark, and Teton Counties, Montana: U.S. Geol. Survey Geol. Quad. Map GQ—499. 1967, Geologic map of the Arsenic Peak quadrangle, Teton, and Lewis and Clark Counties, Montana: U.S. Geol. Survey Geol. Quad. Map GQ—597. 1968, Bedrock geologic map of the Castle Reef quad- rangle, Teton and Lewis and Clark Counties, Montana: U.S. Geol. Survey Geol. Quad. Map GQ—711. 1970, Origin of the disturbed belt in northwestern Montana: Geol. Soc. America Bull., v. 81, no. 2, p. 377—392. 1971, Pre-Quaternary rocks in the Sun River Canyon area, northwestern Montana: U.S. Geol. Survey Prof. Paper 663—A. Mudge, M. R., Erickson, R. L., and Kleinkopf, M. D., 1968, Reconnaissance geology, geophysics, and geochemistry of the southeastern part of the Lewis and Clark Range, Montana: U.S. Geol. Survey Bull. 1252—E, 35 p. [1969] Mudge, M. R., Robinson, G. D., and Eaton, G. P., 1966, Pre- liminary report on regional aeromagnetic anomalies in northwestern Montana, in Geological Survey research 1966: U.S. Geol. Survey Prof. Paper 550—B, p. Bill—B114. Mudge, M. R., and Sheppard, R. A., 1968, Provenance of ig- neous rocks in Cretaceous conglomerates in northwest— ern Montana, in Geological Survey research 1968: U.S. Geol. Survey Prof. Paper 600—D, p. D137—D146. Nobles, L. H., 1952, Glacial geology of the Mission Valley, western Montana: Harvard Univ. unpub. Ph. D. disser- tation, 123 p. Pardee, J. T., 1950, Late Cenozoic block faulting in western Montana: Geol. Soc. America Bull., v. 61, no. 4, p. 359—406. Pardee, J. T., and Schrader, F. C., 1933, Metalliferous de- posits of the greater Helena mining region, Montana: U.S. Geol. Survey Bull. 842, 318 p. Ploufl", Donald, 1966, Digital terrain corrections based on geographic coordinates [abs]: Geophysics, v. 31, no. 6, p. 1208. Poulter, G. J., 1959, Structural synthesis of an area in southeastern Granite County, Montana, in Billings Geol. Soc. Guidebook 10th Ann. Field Conf., Aug. 1959: p. 22—33. Price, R. A., and Mountjoy, E. W., 1970, Geologic structure of the Canadian Rocky Mountains between Bow and Athabasca Rivers—a progress report, in Structure of the southern Canadian Cordillera: Geol. Assoc. Canada Spec. Paper 6, p. 7—25. Reeside, J. B., Jr., 1957, Paleoecology of the Cretaceous seas of the western interior of the United States, chap. 18 of Ladd, H. S., ed., Paleoecology: Geol. Soc. America Mem. 67, p. 505—541. Robinson, G. D., Klepper, M. R., and Obradovich, J. D., 1968, Overlapping plutonism, volcanism, and tectonism in the Boulder batholith region, western Montana, in Studies in volcanology—A memoir in honor. oquowel Williams: Geol. Soc. America Mem. 1.16, p. 5574576. GEOPHYSICAL FIELD INVESTIGATIONS Ross, C. P., Andrews, D. A., and Witkind, I. J., compilers, 1955, Geologic map of Montana: U.S. Geol. Survey, 2 sheets; repr., 1958. Russell, L. S., 1951, Age of the front-range deformation in the North American Cordillera: Royal Soc. Canada Trans, 3d ser., v. 45, sec. 4, p. 47—69. Sahinen, U. 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America Mem. 47, 151 p. Viele, G. W., 1960, The geology of the Flat Creek area, Lewis and Clark County, Montana: Utah Univ. unpub. Ph. D. dissertation, 292 p.; abs., in Dissert. Abs., v. 21, no. 4, p. 853, 1960. Viele, G. W., and Harris, F. G. 3d, 1965, Montana Group stratigraphy, Lewis and Clark County, Montana: Am. Assoc. Petroleum Geologists Bull., V. 49, no. 4, p. 379—417. Walcott, C. D., 1906, Algonkian formations of northwestern Montana: Geol. Soc. America Bull., v. 17, p. 1—28. Weed, W. H., 1899, Description of the Little Belt Mountains quadrangle [Montana]: U.S. Geol. Survey Geol. Atlas, Folio 56, 11 p., 4 maps. GREAT FALLS-MISSION RANGE AREA, NORTHWESTERN MONTANA A19 Woollard, G. P., 1958, Results for a gravity control network Zietz, Isidore, and Andreasen, G. E., 1967, Remanent magne- at airports in the United States: Geophysics, v. 23, no. tization and aeromagnetic interpretation, in Mining geo- 3, p. 533. physics——v. 2, Theory: Tulsa, Okla., Soc. Explor. Geo- Woollard, G. P., chm., and Joesting, H. R., coordinator, 1964, physicists, p. 569—590. Bouguer gravity anomaly map of the United States (ex- Zietz, Isidore, and Henderson, R. G., 1956, A preliminary clusive of Alaska and Hawaii): U.S. Geol. Survey, 2 report on model studies of magnetic anomalies of three- sheets. dimensional bodies: Geophysics, v. 21, no. 3, p. 794—814, U. S. GOVERNMENT PRINTING OFFICE : 1972 0 - 454-215 , V , _ ‘ , ‘ h 2.: , : . ,..:.§.i:...§x.1z.....siléi:§$ ._s.,.,.:..,?¥«£25. , .. ‘ . , PROFESSIONAL PAPER 726—A PLATE 1 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY lli°00’ 48° 35’ 132030 REW. iS’ RVSW. 48”}.5’ 4 - ._ _ ; _ _ ' i ‘ ' I) TWTZQN T2§ Nfi Ranch'_ “ \ / / , > 35,“ \ ‘ 1““ n; L‘ \“Tianclh‘ i, T V , » Graham Ra nah fie‘rna‘at R9,:1Mi’; MM name Comm. CV , as“ i i I sneaky”; (3 ’Rghch g‘gfl‘ ' ("7 BrowueltScheot ‘ 5.x ’ x x 0\¢?//.T«L\Rar1t:n ‘Nug - m/ on SCMOIXX I /\“ iiem ; \ Giirwei Id . ~ * RTE?“ 38“" \m a. M, n0 \ .Raan , gigging; Ranch {f . -' TEE/0% . ‘\ _ \ .A I”, .4 _ iffirlrimae’fignotf l, , ’35; Ranch, 48“£§§)' “i“ 15’ a 20w . . T26 N. , .. R 7 W i Ii ,- V‘fpfiggefl COUNTER?“ ’ 2400 “’ TETETC‘WWYF ,' , , 03‘ LAKE CO 4 Angel Point Hoods [fay ; ’ 1 3 ° senseless _ FLA'QIEAoCQUNTv‘ _ ‘ y _ »- / . ~ « i , A. _ , ,/ -, _- ,, x , . , LAKECQUNTY ,5 \ .‘ 5 p ,- f x; , ,~ ~ p f _: ._ ~~ { «x x , . I - _ , j . ~ _ , _ , \. , Table \ m ; i ‘ ”4me/‘XEHLIELWJ/ ‘ i is: < l, .. d ”g, W a. M W M m FLATHEAD NATFONAL FOREST FLA THE/«10 LAKE Wildfiorxe 2892 ARIA) id TRAN, 5,, .4 f A“ /V;:f-3lawzz>7w «- - Lilo» TZON. ‘ '3’62‘.-:" .2; A. (J : x\criz{_ <2 {3 Ga efitmcli I Be: «Ifier r‘<. \ /\ , 1,; v4( 476306 R ’ a ’i - - , 114“}.‘3‘ ‘ 4: V REYW. RIGW. 15’ .33 W. Base from U.S. Geological Survey SCALE 1:250 000 Geology compiled by M. R. Mudge, 1969, 5 O 5 10 15 20 25 MILES from sources listed In text. T ,___, _, %—-————4 I—I I Aeromagnetic survey flown at 9000 feet barometric elevation, 1967 and 1968, L H J 5 10 15 20 -315 K'LOMETERS and compiled by US. Geological Survey CONTOUR INTERVAL 200 FEET WITH SUPPLEMENTARY CONTOURS AT lOO-FOOT INTERVALS DATUM IS MEAN SEA LEVEL I972 MAGNETIC DECLINATION VARIES FROM 20" EASTERLY FOR THE CENTER OF THE WEST EDGE TO l8“ EASTERLY TO THE CENTER OF THE EAST EDGE EXPLANATION All line symbols approximately located 09,“ xix i ' ' Stratigraphic contact Measured maximum or minimum intensity Pliocene to Oligocene Pliocene to Oligocene Tertiary volcanic rocks Tertiary intrusive rocks , within closed high or closed low lake and stream de— latitic flows, welded Probably Eocene —"—h———‘— In gammas posits tuffs, rhyolite domes, Thrust fault and porphyritic in— Sawteeth on upper plate _____ — — trusions Flight path - Fault Showing location and spaCing of data u A A , T E2, N 0 Line of structure section and magnetic and Upper Cretaceous dio- Normal fault ' gravity profiles shown on plate 3 _ , 7 rite, Syenite, and KS U,upthrown side; D, downthrown Side . . gabbro sills _J 45' Ksif, sills repeated by Cretaceous intrusive \— 19A _ 9 \ thrust faults rocks Tear fault . . _ W “I?“ M all be as young ‘13 Mostly quartz monzonite Arrows show direction of horizontal movement Anomaly dlscussed 1n text _ EDI“??? Paleocene Sedimentary and volcanic rocks and gabbro stocks ._ . KJ r, undifferentiated Cretaceous and Jurassic _——-}——>— _I_>. _g___ rocks . . . . . ~ Kav, Adel Mountain Volcanics of Lyons (1941;) Antldme 8312:1326 overturned antmhne T.“ N; Kai, intrusive rocks related to Adel Mountain Volcanics of Lyons (1944) Ktmv, volcanic member of Two Medicine F or- mation (Big Skunk Formation of Viele and , Harris, 1965) _I_l_ _|_|_ _|_l.. _|._|_ Showing crestline or troughline, direction of dips of limbs, and direction of plunge Kv, Virgelle Sandstone; mapped separately Edge of disturbed belt only m eastern area Hachures point in direction of faulted and folded rocks 419g? 114° 112., 110° 02948M We“ KALISPELL CUT BANK SHELBY “HON. Showing depth of hole, in feet, and oldest rocks penetrated: M, Mississippian; D, Devonian; C, Cambrian; pC, Precambrian 1960—68 1954—67 PaleOZOIC sedimentary “E'Ck? _ crystalline rocks. Queried where age of rocks uncertain 48” WA LLACE' Permian, Pennsylvanian, MisSiSSippian, I 9 5 6— 6 6 ' Devonian, and Cambrian rocks 60 0" RE T FALLS a \\ O 4» (2) @W) 47° ”axe, 6’OHZ‘ ‘ 3 ‘WHITE Sh . t t l . t -tMagnet1c-C(;nt\ '— l-I K:0.0005 40,000'7 “i ‘l \( X l‘ ‘l 1 40,000' i I k A I i I 45000’ 45,000' Line of sectlon shown on plate 1 SCALE 1:250 000 O 5 10 15 20 25 MILES I I I I I I ,,I O 5 IO 15 20 2J5 KlLOMETERS EXPLANATION —'—-—_~“ FTWWAA FT‘I—TTT‘T—I—‘a . I V I V Cc _ j 1F?” ‘ p l_ l“ K:0.0001 “l Cretaceous and Jurassic Paleozoic Precambrian Belt Supergroup Precambrian Contact, Fault, approximately located, Inferred extension Well Pluton which represents the hypothetical mass used sedimentary and igneous rocks sedimentary rocks Includes upper Precambrian crystalline rocks approximately located showing direction of movement of thrust fault in modeling to fit observed data sills Post/ulated from model studies of magnetic data. K: magnetic susceptibility, in electromagnetic units per cubic centimeter, assumed in crystalline rocks. Mag- netic susceptibility for sedimentary rocks assumed to be zero STRUCTURE SECTION AND MAGNETIC AND GRAVITY PROFILES ACROSS THE GREAT FALLS-MISSION RANGE AREA, NORTHWESTERN MONTANA 454—215 0 — 72 (IN POCKET) \/ 7 DAY EARTH SCIENCES Gravity and Magnetic Evidence @0157" of Lithology and Structure Vii/”‘51 in the Gulf of Maine Region GEOLOGICAL SURVEY PROFESSIONAL PAPER 726—B Prepared in cooperation with the National Oceanic and Atmospheric Administration ‘ gfifififlj ,_ Gravity and Magnetic Evidence of Lithology and Structure in the Gulf of Maine Region By M. F. KANE, M. J. YELLIN, K. G. BELL, and ISIDORE ZIETZ GEOPHYSICAL FIELD INVESTIGATIONS GEOLOGICAL SURVEY PROFESSIONAL PAPER 726—B Prepared in cooperation with the National Oceanic and Atmospheric Administration A study of regional gravity and aeromagnetic maps of the Gulf of Maine using anomaly-lithology correlations established in nearby land areas UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1972 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 70—188161 For sale by the Superintendent of Documents, U.S. Government Printing Oflice Washington, D.C. 20402 - Price $1.25 (paper cover) Stock Number 2401—2078 CONTENTS Page Page Abstract _ ___ Bl Geophysical interpretation—Continued Introduction 1 Local gravity anomalies—Continued Acknowledgments _ 1 Gulf of .Maine region ______________________ BS Previous investigations _____________________________ 1 Aeromagnetlcs , 11 - Regional magnetic field ___________________ 11 Geologic framework 4 Land areas 11 Geophysical interpretation _________________________ 5 Gulf of Maine region _____________________ 13 Gravity and aeromagnetic surveys -------------- 5 Geologic implications of the gravity and magnetic Lithic units and anomalies ____________________ 5 anomalies 15 Quantitative analysis __________________________ 6 Lithology and ages _______________________ 15 Regional gravity _____________________________ 7 Structure ___ 17 Local gravity anomalies _______________________ 7 Crustal structure of the Gulf of Maine _____________ 18 Land areas - 7 Summary ___ 20 Coastal areas _ 8 References cited ___ 20 ILLUSTRATIONS Page PLATE 1. Bouguer gravity and geologic map of the Gulf of Maine region In pocket 2. Aeromagnetic and geologic map of the Gulf of Maine region In pocket FIGURE 1. Regional geologic map of part of the northern Appalachian region 32 2. Bathymetric and location map of the Gulf of Maine __ _ 3 3. Provisional lithologic map of the in iurated bedrock of the Gulf of Maine 16 4. Crustal model showing regionalized Bouguer gravity field, maximum horizontal gravity gradients, and hypothetical density distribution of the Gulf of Maine-Maine region 19 III h- V “f4, < mm 3.‘ 4;. GEOPHYSICAL FIELD INVESTIGATIONS GRAVITY AND MAGNETIC EVIDENCE OF LITHOLOGY AND STRUCTURE IN THE GULF OF MAINE REGION By M. F. KANE, M. J. YELLIN,1 K. G. BELL, and ISIDORE ZIETZ ABSTRACT The aeromagnetic and gravity fields over the Gulf of Maine are characterized by well-defined regional zones and widespread intense anomalies. The dominant trend of the fields is northeast like that of the adjoining Appalachians. Most anomalies arise from igneous plutonic and volcanic sources, many of which are mafic in composition. Two ad- joining parallel magnetic belts, one featureless and the other made up of intense anomalies, trend northeast through the gulf and extend to adjoining land areas of Maine and Mas- sachusetts. The belt of intense anomalies defines a zone which is underlain in land areas, and presumably in the gulf region, by a high concentration of mafic rocks. It apparently marks the location of a major crustal fault zone. The cen- tral part of the gulf is underlain by indurated bedrock in which igneous plutons. are abundant; gravity data indicate that moderate thicknesses of sedimentary rocks, possibly de— posits of Carboniferous age, may also be present in this area. Correlations of areal geology with anomaly patterns and levels in land areas suggest that much of the crystalline bedrock underlying the gulf may be early Paleozoic or Pre- cambrian in age. Discontinuities in regional anomaly pat- terns indicate the presence of major faults along the north- west and northeast margins of the gulf. Analysis of the regional gravity field shows that the crustal block underly- ing the gulf is distinguished from that of the surrounding land by high-density rock at relatively shallow crustal depths. The region is in near isostatic equilibrium, and at least part of the equilibrium is achieved by broad masses Within the earth’s uppermost crust. INTRODUCTION The Gulf of Maine, a broad shallow depression in the east margin of the North American Continent (fig. 1), is bordered landward on three sides by the Appalachians, and seaward by the shallow banks of the outer continental shelf. The bottom of the gulf is a complex of basins, swells, and ridges (fig. 2) which contrast with the smooth tablelike bathym- etry of the bordering banks. The thicknesses and at- titudes of the sediments that mantle the gulf bottom have been investigated extensively by continuous seismic profiling (Uchupi, 1966), but the lithology and structure of the underlying indurated bedrock are largely unknown. In this report mag- 1National Ocean Survey, National Oceanic and Atmospheric Adminis- tration, Groton, Conn. netic and gravity surveys of the gulf region are an- alyzed in terms of structural trends, probable lith- ology of the indurated bedrock, and crustal structure. The use of gravity and magnetic surveys to infer the geology of completely obscured bedrock involves considerable uncertainties, partly because of the in- herent ambiguity of the geophysical methods and partly because of the manyfold correspondences be- tween anomalies and rock types. Geophysical inves- tigations of the bedrock in regions surrounding the Gulf of Maine are, however, comparatively ad- vanced (for example, Garland, 1953; Griscom and Bromery, 1968; Kane and Bromery, 1968) and pro- vide considerable data on empirical correlations be— tween gross geologic units and their associated geo— physical fields. The interpretation in this report makes use of a few well-established lithology—anom- aly correspondences but relies to a great extent on the empirical correlations in the nearby land areas. This practice leads to a degree of uncertainty Which is reasonably small for predictions of gross lithol- ogy but which may be fairly large for speculations about the structure and age of the bedrock. These factors require consideration in evaluating any par- ticular conclusion of the report. ACKNOWLEDGMENTS We are indebted to John R. Kirby, whose knowl- edge of the aeromagnetic data was a major re- source, and to Andrew Griscom for his counsel on the interpretation of the aeromagnetic maps. D. Duncan Arnold and David N. Packham generously provided able navigation and transportation to the islands off southern Maine in the summer of 1968. This work stems in large part from a dissertation done with the encouragement and guidance of Pro- fessor Otto W. Nuttli of St. Louis University, St. Louis, Missouri (Kane, 1970) . PREVIOUS INVESTIGATIONS In a study of the shallow stratified rocks underly- ing the Gulf of Maine, Uchupi (1966) synthesized the Bl B2 66° “$5,311 .‘1019 v ’4; \\/\( \ — a /I/\ —‘7-"\7 " \,I‘ «V; \l,>’:\\‘/I’T\, \ - I— \‘ ’¢\-’lI:L\/\.\<\£‘\l/—\:\ \ \ I- " Mr Azwzi—u 251),: r) J, \ —‘\‘/\J:L\I{-\l’| / I\-\‘|\'>’\‘/ /\Z"\ I \ / ll \1‘ I \ — \ / /\I:\l"l ‘\ A Grand Manan Island GULF OF MAINE CAPE ANN 42° 41° 74‘ 70° 66° EXPLA Carboniferous \ Devonian to Cambrian Quaternary TriaSsic FIGURE 1.——Regional geologic map of part of the northern Appalachian region. Geology modified mainly from King (1969); additions from White (1968); and generalized structure in Quebec, Maine, New Hampshire, and Ver- mont from Cady (1960, 1967), in Vermont from Doll, findings of earlier investigations with his results and reported the following conclusions: Strata of Triassic age underlie a broad irregular zone which extends from the Bay of Fundy 120 kilometers southwest into the gulf. Strata of Cretaceous age |/\:/|J’ \’/\|/? / x~ \ ,l‘//’\/_I’\\\ GEOPHYSICAL FIELD INVESTIGATIONS . 58° 62 \L ~ \ \/ — I i‘u‘7' ~’ / I —k ’ \ \J \7\‘\/\ I, \\—‘\/l,~/\_ " _\_\ \\—x\/ ,—\/'v/\/\,\\/ \‘ \ GULF OF LA WRENCE 4 ,z’ o" ’ 4",” 1K O PRINCE EDWARD 0 l 0 I l I I ISLAND I I 44° 42° 300 NAUTICAL MlLES l I 200 KILOMETERS l_l_Ll_l_l—_L_.____l 41" 56° 62° N A T l O N I/\ / ’l\‘/\Il 33/2: (I: c. > — — \ . . . H75 \ $15.15 Antlclinonum Precambrian May include some rocks ____‘____ of younger age ' . Syncllnonum ————/000—— —— Fault IWmewr water depth Cady, Thompson, and Billings (1961), in New Hamp- shire from Billings (1955), in Maine from Osberg, Moench, and Warner (1968), and in Connecticut from Dixon and Lundgren (1968). are restricted to Georges Basin and Georges Bank (Emery and Uchupi, 1965), except possibly for Some erosional remnants in Cape Cod Bay' (Hoskins and Knott, 1961). Strata of Tertiary age underlie the area of Georges Bank and are present in a con- GRAVITY AND MAGNETIC EVIDENCE OF LITHOLOGY AND STRUCTURE, GULF OF MAINE 69" B3 68‘ 67° 56" \ l l \, NEM‘ l nnmsn‘m: [— Q h BOSTON 4 } l :% MASSACHUSETTS 42° 3 . at ,1» - ‘ l s l /‘ PROVIDENCE CAPE COD NAWVUC/(fr ' /: sauzvo mmms ° 6 NANTUCKET vmsuno ‘ ° b ISLAND .1 _ 1, 9&5)“ / l‘w ISL- ND “Ea" é V'.1L 4.” g 43° Q? J 4 ‘ n f a \ “m BROWNS BANK mu TON swELL % 70" 69“ FIGURE 2.—Bathymetric and location map of the Gulf of Maine. Modified from Uchupi (1965) ; contour interval 40 meters. tinuous blanket over the western quarter of the gulf, including Cape Cod Bay. These strata and post-Tertiary deposits elsewhere are underlain by a highly reflective horizon, which is probably a sur- face on metamorphic and igneous rocks of pre-Cen- ozoic age. A seismic-refraction survey by Drake, Worzel, and Beckmann (1954) showed that much of the gulf bottom is underlain at a shallow depth by an inter- face Which is characterized in most places by a com- pressional-wave velocity of about 6.0 kmps (kilome- ters per second). In two areas in the western part of the gulf, the velocity along this interface ranges from 4.6 to 5.5 kmps. Drake, Worzel, and Beckmann (1954) also described a 3,000-meter-thick unit hav- ing a velocity of 4.5 kmps in the area west of Yar- mouth, Nova Scotia, and units having a velocity of 4.0 kmps and thicknesses of as much as 500 m in two localities north and east of Cashes Ledge (fig. 2). They concluded that the velocity of 6.0 kmps was anomalously high and related to a “subbase— ment” rock of an unknown nature. The velocities of 4.5 to 5.5 kmps were considered typical for a wide variety of igneous and metamorphic rocks. By com- B4 parison with measured velocities in other areas, the rock unit with a velocity of 4.0 kmps was believed to be most probably of Triassic age (Drake and oth- ers, 1954). Uchupi (1966), however, observed that this rock unit did not have the seismic-reflection characteristics typical of Triassic rocks elsewhere and suggested that it is “mildly metamorphosed” rock of Paleozoic age. Drake, Ewing, and Sutton (1959) showed that, in contrast to the gulf, the neighboring banks are underlain by as much as 3,000 m of low-velocity strata. These strata appear to be thickest near the central part of the banks and thin gradually seaward but more rapidly land- ward. Malloy and Harbison (1966) reported detailed magnetic and continuous seismic profile measure- ments over a large area in the northeastern part of the gulf. They used magnetic anomalies and bedrock morphology to infer the distribution of “tightly folded crystalline Paleozoics” and Triassic sedi- ments and to delineate several fault zones in that part of the gulf. In particular they identified an ar- cuate magnetic anomaly which they attributed to a ring dike like the ones of Maine and New Hamp— shire. Schlee and Pratt (1970) studied the distribution of pebbles of distinctive lithology obtained in hauls from the gulf bottom. They concluded that gross bed- rock types could be predicted for parts of the gulf. Their results were as follows: Igneous and meta- morphic rocks are prevalent in the western gulf, felsic rocks being present near southeastern Massa- chusetts and mafic rocks near Cape Ann, Mass, and the New Hampshire coast; coarsely crystalline pink granite occurs southwest of Mount Desert Island, Maine; sedimentary rocks similar to those of Trias- sic age extend from the Bay of Fundy to Mount Des- ert Island; spotted schist, probably correlative with rock of Ordovician age in Nova Scotia, is present in the offshore region near Yarmouth, Nova Scotia; felsite occurs west of Yarmouth and extends north to St. Marys Bay. In the central part of the gulf, the gravel in the hauls was too sparse to give indic- ative results. The only known sample of crystalline rock from outcrop in the gulf was obtained by a dive to Cashes Ledge (fig. 2) the shallowest feature of the central gulf. Toulmin (1957) identified the rock as peralkaline granite, probably of Mesozoic or Paleozoic age. Submarine gravity profiles by Worzel and Shur— bet (1955) revealed that the average Bouguer grav- ity field over the gulf is about 20 milligals higher than that (about 0 mgal) of the surrounding land GEOPHYSICAL FIELD INVESTIGATIONS area. They concluded that the Gulf of Maine is a flooded continental block. A preliminary analysis of a deep seismic—refrac- tion profile from the center of the gulf to the inte— rior of Maine by Steinhart and others (1962) indi- cated that a range of velocity models of the crust could be derived from the traveltime relations of the compressional—wave velocities. All the models showed a velocity of about 6 kmps in the upper crust, a velocity of about 7 kmps in the lower crust, and a Mohoroviéié-discontinuity velocity of 8 kmps at a depth of about 40 km. It was demonstrated that the velocity changes, including that at the crust-mantle interface, could be either steplike or smooth. GEOLOGIC FRAMEWORK The northern Appalachians (fig. 1) form a broad belt of stratified geosynclinal rocks between the Pre- cambrian cratonic core of North America on the northwest and the Atlantic Ocean on the southeast. Most of the strata are of early to middle Paleozoic age and have been regionally metamorphosed. A wide variety of igneous rocks, mainly intrusive masses of felsic composition, has been emplaced in the stratified rocks throughout the region. Regional geologic trends, shown by the axes of major folds and the areal distribution of rock units, are mostly northeast, {in accord with the predominant trend of the Appalachians. Rocks of Precambrian age form the cores of structural uplifts in the western part of the region and underlie several widely separated areas in the east. Rocks of Carboniferous or younger age, some of which are locally metamor- phosed, occur in the southeast and over a broad area around the Gulf of St. Lawrence. The broad expanse of eugeosynclinal rocks of New England provide few clues to the geology of the Gulf of Maine except that it may have been a source region for part of the sediments of the eu- geosyncline (Zen, 1968). The exposures of rocks of late Precambrian or early Paleozoic age northeast and southwest of the gulf suggest that the inter- vening region may be underlain by rocks of similar age. In this setting the bedrock of the gulf would form a structural high along the oceanward side of the eugeosyncline much as does the Green Mountain- Sutton Mountain anticlinorium on the opposite (cratonic) side. In the coastal areas of Maine, extensive exposures of volcanic rocks indicate that volcanism was prev- alent during some periods along the northwest mar- GRAVITY AND MAGNETIC EVIDENCE OF LIT HOLOGY AND STRUCTURE, GULF OF MAINE gin of the gulf (Hussey, 1968; Boucot, 1968; Gates, 1969). Chapman (1962) concluded that a complex of mafic and felsic rocks occupying most of the east- ern Maine coast extends as much as 30 km into the gulf. In southern New England, major rock units strike into the gulf and may emerge in Maine, but positive ties between the two regions have not as yet been made. In Nova Scotia, regional trends par- allel the northwest coastline, but broad folds on the west trend southwest to south into the gulf. In an early report on the coastal region of New England, Johnson (1925) postulated that a major fault system was located within the Gulf of Maine. Bell (1967) described a great imbricate overthrust fault trending northeast into the gulf in the vicinity of Cape Ann, Mass. Wilson ( 1962) used the location of faults in Newfoundland, Nova Scotia, Prince Ed- ward Island, and Massachusetts, and Johnson’s (1925) Gulf of Maine fault system as the basis of proposing the Cabot fault (fig. 1), “an Appalachian equivalent of the San Andreas Fault.” He suggested that the structure may have been linked to the Great Glen fault of Scotland as a continuous feature before continental drifting began. Indications of displacement on the fault were considered meager, although there was some evidence of left-lateral movement. Belt (1968) described a series of thick basin de- posits of Carboniferous age that extend in a narrow sinuous zone from Newfoundland to the Bay of Fundy and that are near or coincident with the Cabot fault. On the basis of the geometry and char- acter of these deposits, he proposed the existence of a major rift system that was active from Late De- vonian to Triassic time. Webb (1968) assembled more detailed data on the individual elements of the Cabot fault; he concluded that it was active during the same period as Belt’s rift system and has as much as 190 km of right-lateral movement. Gates (1969) established the beginning of major deforma- tion in the region of the Cabot fault as Early Sil- urian, on the basis of associated volcanic rocks of that age in Maine and New Brunswick. From a regional standpoint, therefore, it seems reasonable that a major structure of continental di-‘ mensions passes through the Gulf of Maine. The precise nature of the structure is not clear, but the study of Belt (1968) indicates that Carboniferous deposits are associated with it, and therefore these deposits may be present in the gulf. This conclusion is supported by the presence of Carboniferous de- posits in southern New England, which are on re- gional strike with those of the Bay of Fundy. A B5 major implication of the conclusions of Wilson (1962) and Webb (1968) is that the Gulf of Maine region may not be directly related to the Appa- lachian region to the northwest because of major offset on a transcurrent fault separating the two re- gions. GEOPHYSICAL INTERPRETATION GRAVITY AND AEROMAGNETIC SURVEYS Gravity data at sea are based on both bottom measurements and surface-ship measurements as described by Yellin (1968). North- and east-ori- ented ship tracks were spaced at about 16 km, and bottom stations were measured on an underlying 16-km grid that generally coincided with the ship tracks. Data from bottom stations cover about 80 percent of the area of the surface measurements. Surface data were averaged for 5-minute (time) in- tervals, which is equivalent to about a 2-km spac- ing; these data are internally consistent within about 2 mgal. There is a 3-mgal discrepancy be- tween surface and bottom measurements which seems to be a measure of systematic error in the surface-ship survey (Yellin, 1968). An elevation correction, based on a density factor of 2.8 grams per cubic centimeter, was used to reduce the data to the Bouguer gravity field. Contours are drawn on surface data supplemented by bottom data in areas outside the surface network. Land data are from Kane and Bromery (1966), Bromery (1967) , Joyner (1963), and from unpublished measurements made by Kane on islands in the summer of 1968. Aeromagnetic data for the Gulf of Maine and the adjoining United States are adapted from an aeromagnetic map of the eastern continental mar- gin of the United States (Taylor and others, 1968) . Flight-line spacing was about 8 km; flight altitude was 200 m over ocean and 500 to 800 In over land. Estimated accuracy of flight-line position is :15 km, and the maximum error is 3 km. Aeromagnetic data for parts of eastern Canada were compiled from the series of maps published by the Geological Survey of Canada (1968). Flight lines for these maps were 0.8 km apart and at altitudes of 150 to 300 m; some coverage was taken from shipboard surveys. LITHIC UNITS AND ANOMALIES Relations between physical properties and rock types are generally ambiguous, but under some conditions the correspondence between geophysical anomalies and specific rock types is fairly well es- tablished. As an example, well-defined gravity lows B6 (15 to 50 mgal in amplitude) in regionally meta- morphosed terrane are frequently associated with felsic plutons, and sharp gravity highs (greater than 15 mgal in amplitude), with mafic or ultra- mafic plutons. This correspondence holds between Bouguer gravity anomalies and geologic features in Maine (Kane and Bromery, 1968) and in some parts of Nova Scotia and New Brunswick (Garland, 1953). Elsewhere in Nova Scotia and New Bruns- wick, however, gravity lows are caused by relatively low-density sedimentary rocks of Triassic and (or) Carboniferous age, and by a deposit of salt in one place. For the anomalies cited above, the critical in- terpretive guide was the identification of the source with bedrock exposures or with drill-hole informa- tion. Such a guide is obviously lacking in water-cov- ered areas so that a twofold ambiguity must be rec- ognized for gravity lows in the Gulf of Maine. Bott (1962) has proposed anomaly-shape criteria to dis— tinguish between the two types of gravity lows, but his method requires a greater precision than is in- herent in the gravity data for the gulf. The plan shapes of some anomalies may provide insight into their causes, although such insight islunlikely to be definitive. In contrast to interpretation of gravity lows, the identification of a mafic or ultramafic mass as the source of a steep-gradient high-amplitude gravity high is relatively unambiguous. The causes of low-amplitude low-gradient gravity anomalies can be interpreted with less assurance. Kane and Bromery (1968) have concluded that most low-amplitude highs over the regionally meta- morphosed bedrock underlying Maine are caused by volcanic rocks of mafic composition. In one area of Maine moderate gravity lows are associated with volcanic centers typified by felsic rock (Rankin, 1968). Where porosity is a density factor, low-am- plitude gravity lows may indicate the presence of low-density sediments. An established practice for the interpretation of aeromagnetic maps is to assume that anomalies are caused by igneous rocks. The primary magnetic mineral giving rise to the anomalies is magnetite, although anomalies of moderate amplitude are sometimes caused by other minerals, including sul— fides. As a rule, magnetite is associated more fre- quently with mafic minerals than with felsic miner- als so that intense anomalies (amplitude greater than 500 gammas at an altitude of 300 m) are usually attributed to mafic and ultramafic rocks. Al- though felsic plutons are generally nonmagnetic, they commonly have magnetic aureoles which are caused by more magnetic border phases or by al- GEOPHYSICAL FIELD INVESTIGATIONS tered country rock (Allingham, 1961). Sedimentary rocks are typically nonmagnetic so that flat mag- netic fields are associated with thick sedimentary deposits. An iron-formation, however, gives rise to very intense anomalies particularly where it is in- truded by igneous rocks. Regional metamorphism is also a factor in rock magnetism (Reed and others, 1967), but the relationship is as yet incompletely understood. There are many exceptions to the mag- netism-lithology relations noted here so that they constitute, at best, very general guidelines for inter- pretation of geophysical data. At present, the corre- spondences between magnetic anomalies and rock units that are observed in the region of study seem to offer the best aids to the interpretation of aero- magnetic maps. This practice is followed in this report. QUANTITATIVE ANALYSIS As a primary step in the interpretation of grav- ity anomalies, Kane and Bromery (1968) have sug- gested the calculation of the maximum depth to top of source (Bott and Smith, 1958; Bancroft, 1960) and the minimum thickness of source. The maxi- mum depth is given by A Dm‘ax : K v where Dmax = maximum depth to top of source, in kilometers, K = constant, less than 1, depending on shape of source, A = anomaly amplitude, in milligals, and ,S'max = maximum horizontal gradient of anomaly, in milligals per kilometer. The minimum thickness of source is given by A = 0.042d’ where Tm... = minimum thickness, in meters, min A = anomaly amplitude, in milligals, and d = maximum density contrast, in g/cm‘s. Calculation of these quantities is a preliminary step in any gravity analysis, and subsequent refinements usually require either well-defined gravity gra- dients, reliable density data, additional information on the anomaly source, or a combination of these. The knowledge of gravity gradients and probable density contrasts for the Gulf of Maine is fairly limited so that the above formulas alone are gener- GRAVITY AND MAGNETIC EVIDENCE OF LITHOLOGY AND STRUCTURE, GULF OF MAINE ally sufficient for the quantities that may be de- duced from the gravity data. Analysis of aeromagnetic anomalies in terms of shape and magnetic properties of the source is more difficult than analysis of gravity anomalies, partly because of the complexity of the magnetic field aris- ing from a distribution of dipoles and partly be- cause both remanent and induced components of magnetism may be present. An empirical depth re- lation (Vacquier and others, 1951), similar to that for the gravity method, is used for a few magnetic anomalies in areas where magnetic rocks seem to be deeply buried. REGIONAL GRAVITY The Bouguer gravity field increases in value from the surrounding coastal areas into the interior of the Gulf of Maine where, except for three locations, it is everywhere positive (pl. 1). The background grav- ity field of about 20 mgal is highlighted on plate 1 by shading of the +15- to +25-mgal contour inter- val. The shaded area persists over' the whole sur- veyed region of the gulf and is generally character- ized by a sparsity of contours. Superposed on the background gravity is a complex group of positive and negative anomalies which taken together have an overall northeast trend. The general appearance of the anomalies is similar to that on land, suggest- ing similar causes—near—surface masses of felsic and mafic igneous rocks, and possibly sedimentary rocks, set in a matrix of metamorphic rocks. The lack of contours in the shaded area indicates that the bedrock underlying it has a relatively uni- form density. The uniform background gravity level of the gulf, though higher, is similar in geometry to the three regional gravity levels described for the interior of Maine by Kane and Bromery (1968), who showed that the maximum depth to the tops of the sources giving rise to the change in gravity lev— els was less than 6 km. No specific cause was ad- vanced for the change in gravity levels, but it was pointed out that, in the southwestern part of the State (northwest of Portland), these levels were re- placed by a rather uniformly sloping regional gra- dient. Kane (1968) suggested that the uniformly dipping regional gravity field is related to the lithol- ogy of the rocks in the high-grade metamorphic (sillimanite) zone that underlies the area. The average background gravity level of the coastal area of Maine was estimated by Kane and Bromery ( 1968) to be about —5 mgal. The gravity gradient between the background gravity level over the coastal area and that over the gulf can be seen B7 most clearly northeast of Portland (on pl. 1), and has a value of about 1 mgal/km. The maximum depth to the density contrast causing the change in gravity levels is calculated to be about 8 km, on the basis of the formula on page B6 where K = 1/7r (plate-shaped source), Smu=1 mgal/km, and A=25 mgal (+20 mgal over the gulf and —5 mgal on land). Thus the higher gravity level over the gulf is caused by a rock mass whose top is shallower than 8 km and whose density is greater than the crustal rocks at similar depths in the adjoining land region. A regional gradient like that over the high-grade metamorphic zone of southwestern Maine is not ap- parent in the gulf, suggesting that a similar zone is not present in the gulf. Gravity values on the west coast of Nova Scotia exceed +20 mgal, indicating that the high gravity level over the southwestern two-thirds of the gulf extends over the unsurveyed area to the east. An onshore example of high gravity level, other than levels over exposures of mafic rock, is in southeast- ern New Brunswick (northeasternmost quadrant of pl. 1) where a level greater than +15 mgal is asso- ciated with bedrock of Precambrian or early Paleo— zoic age. Gravity levels exceeding +15 mgal also are present over broad areas of exposures of similar age in northeastern Nova Scotia (Garland, 1953). The correlation of broad areas having high gravity levels with bedrock of early Paleozoic or Pre- cambrian age is not conclusive, but it does suggest a plausible explanation for the high gravity level of the gulf and the shallow depth of the source of the gravity level. Presumably, the crustal block underly- ing the gulf is a structural high ,in which older basement (Precambrian?) and deeper denser crust is uplifted relative to the surrounding land regions. Other explanations are possible, including a gulf crustal block that has a different composition from that of the land regions, or bedrock that has a com— paratively high proportion of mafic lithic units. Whatever the precise cause of the high gravity level, it differentiates the gulf region as a distinc- tive crustal unit. LOCAL GRAVITY ANOMALIES Land Areas Many examples of the gravity anomaly—lithology correlations, cited in the section on gravity analysis, are shown in the land regions around the gulf. Gravity lows are present over most of the areas of felsic bedrock except for the area between Cape Ann, Mass, and Portland, Maine. Gravity highs are present over mafic exposures in eastern Maine and B8 northeastern Massachusetts. In New Brunswick, relatively lower gravity is associated with a region underlain by porous sedimentary rocks of Carboni- ferous age but, in Rhode Island and Massachusetts, no apparent anomaly is associated with indurated and partly metamorphosed strata of similar age. The absence of anomalies over exposures of felsic and mafic rocks is assumed to imply that the mapped rocks are relatively thin and lack sufl‘icient volume to cause appreciable anomalies. The absence of an anomaly over the rocks of Carboniferous age in Rhode Island and Massachusetts is probably due to the lack of porosity caused by induration and met- amorphism and the consequent elimination of an effective density contrast. Gravity anomalies in the gulf therefore indicate the presence of igneous rocks only where they are thick and of sedimentary rocks where they are both thick and porous. Coastal Areas A series of gravity highs generally associated with underlying exposures of mafic rock occur along the coastal margin of the gulf from Lubec, Maine, to Cape Cod (fig. 3). Although a high gravity level is present over the gulf, the coastal anomalies can be differentiated from it by their closure. The specific occurrences are the following: The gabbroic phase of the Bays of Maine igneous complex (Chapman, 1962), underlying much of the eastern coast of Maine, and associated gravity highs; mafic expo- sures on Monhegan Island, Maine, and a gravity high extending into the gulf; a limited exposure of gabbro and a much broader gravity high at Cape Neddick, Maine; a gabbro exposure at Cape Ann, Mass., and a probable lateral extension of it at depth shown by an extensive gravity high; and an unexposed body shown by gravity and magnetic highs (Griscom and Bromery, 1968) in western Cape Cod. A gravity high is also indicated southeast of Portland, Maine, but it may be a partial manifes- tation of the high gravity level underlying the gulf. The rate of incidence of the mafic masses at the coast seems too high to be coincidental, especially as major mafic masses are relatively rare inland. The range in age of the plutons is large, probably as great as from early Paleozoic to Mesozoic. The pres- ence and time range of the plutons indicate that a major crustal structure, persisting from Pre- cambrian to present, occurs in the vicinity of the coast. The structure may be the result of adjust- ments between the distinctive crustal block underly— ing the gulf (described in an earlier section) and the crust of the surrounding land regions. GEOPHYSICAL FIELD INVESTIGATIONS Gulf of Maine Region The local gravity anomalies of the gulf region have relative amplitudes as great as :30 mgal and gradients as large as 6 mgal/km. In order to stay above the margin of probable error, the following analyses are limited to those anomalies that are at least :10 mgal in amplitude. To facilitate discus- sion, anomalies are labeled in Roman numerals, I through V for negative anomalies, and X through XV for positive anomalies; subdivision of groups of anomalies is shown by suffixed letters. Note that the term “amplitude,” when used below, refers to the peak value of an anomaly relative to the back- ground gravity field. Reasonable approximations for the densities of probable bedrock types are as follows: Carbonifer- ous sedimentary rocks, 2.5 g/cm3; felsic rocks, 2.6 to 2.7 g/cm3; mafic rocks, 3.0 g/cm3; and metamorphic host rock, 2.8 g/cm3 (Kane and Bromery, 1968) . The maximum probable density contrast is 0.3 g/cm3, which leads to a minimum thickness—amplitude re- lation (using the minimum-thickness formula given on p. B6) of 80 m/mgal. Because the density con- trast enters as a simple factor, the relation may be adjusted for a preferred density contrast by using a simple ratio. The gravity effect of'unconsolidated or/semicon- solidated strata in the survey region may be as much as 5 mgal, but the effect is much less in most places. This effect is considered to be a negligible factor in the discussion which follows. The amplitudes and shapes of the gravity anom- alies over the gulf appear to be similar to those on land, except that contours are somewhat smoother and gradients are less'steep over the gulf. The dif- ference in appearance is due, at least in part, to the larger spacing and lesser precision of the marine gravity measurements. Because of the established correlations on land, the gravity highs are assumed to be caused by masses of mafic igneous rock, and gravity lows, by masses of felsic igneous rock or of porous sedimentary rock. It is also assumed that the widespread Acadian (Devonian) regional meta— morphism of the northern Appalachians extends throughout the gulf, so that low-density (porous) sedimentary strata are restricted to a Carbonifer- ous or younger age. The investigation by Uchupi (1966) seems to rule out any thick deposits of low- density strata of post-Paleozoic age, except at two locations (near the mouth of the Bay of Fundy and in Georges Basin) which are outside the region of the gravity survey. Consideration of sedimentary sources for gravity lows is thus restricted to rocks GRAVITY AND MAGNETIC EVIDENCE OF LITHOLOGY AND STRUCTURE, GULF OF MAINE of late Paleozoic age and, because of the lack of rocks of known Permian age in the region, further restricted to rocks of Carboniferous age. Anomaly I, the broadest negative anomaly in the gulf, is a composite feature, consisting of three clo- sures set in a large irregular low. Part of the irreg- ularity reflects some small positive anomalies within the low which do not show at the 5-mgal contour in- terval. The amplitude at the lowest closure is more than —20 mgal, indicating that the minimum thick- ness of the source (sedimentary) in that area is more than 1,600 m. For a felsic igneous source hav- ing a density contrast of about 0.15 g/cm3, the mini- mum thickness would be about 3,000 m. The hori- zontal outline of the anomaly is more complex than outlines over individual felsic plutons, although the complexity might be attributed to a group of adja- cent plutons like those in the eastern coastal area of Maine. In particular, the narrow linear part of the low is suggestive of a rift structure filled in with low~density sedimentary rocks. The cause may be a multiple source consisting of sedimentary rocks de- posited in a terrane underlain by felsic plutons. Several linear aeromagnetic highs (shown on pl. 2 along with trace of gravity closures), trending southwest, are present within the gravity low. A small gravity high (not apparent at the 5-mgal con- tour interval) correlates with the most intense of the aeromagnetic anomalies, indicating the presence of a correspondingly small linear mafic or ultra— mafic mass. The anomalies are not well enough de- fined to estimate the dimensions of the high-density source precisely, but the aeromagnetic anomaly in- dicates that it is near the surface, and the gravity anomaly indicates that it may be more than 1 km in Width. The magnetic field is generally flat in the area of the three negative closures in the gravity field. The general appearance of the magnetic field over the gravity low is not like that over most felsic masses in Maine, although individual magnetic lin- eaments occur within some felsic plutons—for ex- ample, the Katahdin batholith (Boucot and others, 1964). The area east of the 0-isogal closure is underlain by a rock unit that is about 500 m thick and has a compressional-wave velocity of about 4.0 kmps (Drake and others, 1954). This velocity is low for felsic igneous rock but is typical for many types of sedimentary rock. The amplitude of the gravity anomaly would imply a thicker section of sedimen- tary rocks, but the seismic profile may have crossed an area where they are thinner. In general, most evidence favors sedimentary rocks as the source, 454-534 0 - 72 - 2 B9 but additional exploration is needed to resolve the cause or causes of the gravity feature. Anomaly II is a broad steep-gradient feature with an amplitude of more than —30 mgal, the largest negative amplitude in the gulf. The anomaly opens to the southwest beyond the limits of the gravity survey. The magnetic field is flat near the central part of the low but is anomalous along its margins. As pointed out earlier (p. B6), magnetic aureoles are commonly observed on the peripheries of felsic plutons. The large amplitude also suggests a felsic igneous pluton, although sedimentary rocks cannot be ruled out. The anomalies in the regions marked 111a and IIIb resemble gravity anomaly I in their irregularity and amplitude. Magnetic anomalies are present but are not as prevalent and intense as over gravity anomaly I. Part of the low, north of Cape Cod between the 5- and 10-mgal lines, coincides with an area of high seismic reflectivity outlined by Hoskins and Knott (1961). The source could be sedimentary rocks, a felsic pluton, or possibly a composite of both. Anomalies IVa through IVf are similar in size, shape, amplitude (—10 to —20 mgal), and gradient. The shape similarities are in part the result of in— complete data, for two are only partly defined and two are crossed by only one profile. The center of gravity low IVa is magnetically low and relatively flat; some small magnetic anomalies are grouped on its periphery. A seismic profile (Drake and others, 1954) on the northwest rim of the gravity low shows the presence of 400 to 500 m of rock having a compressional-wave velocity of 4.0 kmps. The pat- tern of the magnetic field favors an igneous source, whereas the seismic data favor a sedimentary source, at least in part. The stations in the area of anomaly IVb are not adequate to define it com- pletely, but a lower gravity field is indicated by sev- eral measurements along the margins of the gravity anomaly. The magnetic field is anomalous through- out the gravity low, particularly along its southeast side. Joyner (1963) reported the presence of gneis- sic granite from islands in the southwestern part of the low, which would appear to confirm a felsic igneous source. Anomaly IVc is defined by gravity measurements made on a group of islands. A mag- netic low, which coincides with the center of the gravity low, is bordered by a semiarcuate magnetic high of moderate amplitude. The islands are under- lain by felsic intrusive rock so that the source of the gravity low is a felsic pluton. The magnetic high may arise from a more mafic border phase or from altered host rock. B10 Anomaly IVd is a well-defined steep-gradient low on the south but is poorly defined in the north. The magnetic field is flat over the southeastern two- thirds of the gravity anomaly but shows southwest- trending linear features to the northwest. Anom— alies IVe and IVf are broad gravity features based mainly on single profiles. A low-gradient linear magnetic high trends southwest through the center of IVe; the south part of a broad magnetic high oc- curs over part of IVf. The geophysical data in the areas of IVd, IVe, and IVf are generally not ade- quate to distinguish between a felsic igneous and a sedimentary rock source. The equidimensional shape of the anomalies favors an igneous source, but the shape is at least partly the result of insufficient data. Anomaly V is an elongate feature of moderate negative amplitude. The magnetic field over gravity anomaly V is relatively low and flat, having several small-amplitude highs striking transverse to the trend of the gravity anomaly. The elongate charac- ter of the gravity anomaly is more typical of folded sedimentary rocks than of felsic plutons, although at least one example of a felsic pluton having such a shape occurs in Maine (Pavlides, 1965). As a group, the gravity highs are simpler in form and more discrete in setting than are the gravity lows. These characteristics of the group, coupled With the close correspondence of mafic exposures and gravity highs on land, make interpretation more straightforward. Anomaly X is a type gravity anomaly for a large mafic pluton. It has a relative amplitude of more than 20 mgal and a coincidental broad magnetic anomaly about 2,000 gammas in amplitude. The gravity anomaly is closely similar in amplitude, hor- izontal shape, and areal extent to that over a gab- bro pluton in central Maine (Kane, 1960; Kane and Bromery, 1968). On the basis of a density contrast of 0.2 g/cm3, the minimum thickness is calculated to be nearly 3,000 m; the steep gradients indicate that the top of the source is near the surface of the bedrock. The peaks of both the gravity and magnetic anomalies are over the southeastern part of Cashes Ledge, a submarine ridge whose shallowest peak is the source of a sample of peralkaline granite (Toul- min, 1957). The mafic source, clearly indicated by the gravity and magnetic data, apparently is over- lain by felsic rock, possibly a differentiate. The magnetic anomaly extends considerably farther west than the gravity anomaly—indicating that an additional source, possibly moderately magnetic fel- GEOPHYSICAL FIELD INVESTIGATIONS sic rock, contributes to the magnetic high. The geol- ogy of the area of Cashes Ledge may be similar to that of Cape Ann, where granite overlies what ap- pears to be a broad subsurface extension of the Salem Gabbro—Diorite. The double-peaked broad gravity high underlying the area marked XI is much larger in areal extent than any positive anomaly observed in nearby land areas. The relative amplitudes of the peaks are high (more than 20 mgal), but the gradients are some- what low. Linear magnetic anomalies, mostly of low or moderate amplitude, are present in the area of the high, but they seem too small in area to be caused by the same source. The unusual appearance of the anomaly, the lack of an associated magnetic anomaly, and the low gradients suggest that the source may not be directly related to near-surface geologic features but may be deeper in the crust. Anomaly XII is a broad, elongate, in part steep- gradient feature having an amplitude of about 20 mgal. It is separated from the gravity highs over gabbroic rocks of the Bays of Maine igneous com- plex by an area of moderately low gravity (indi- cated by saddles between the land highs and gulf highs). A zone of intense magnetic anomalies corre- sponds with the gravity high, the broadest and most intense of the magnetic features being over the northeast part of the gravity high. This intense magnetic anomaly is shown by Malloy and Harbison (1966) to be a ringlike feature that coincides with a faint morphologic ringlike structure in the bedrock. The morphology, magnetic anomaly, and gravity anomaly of this area are similar to those over the Merrymeeting stock in New Hampshire (Joyner, 1963), although the Merrymeeting stock is some- what larger in area. The mapping of anomaly XII is based wholly on sea-floor measurements spaced on a 16-km grid, so details of the anomaly are lacking. The present measurements suggest that it may be a composite of two broad equidimensional highs which are 20 mgal in relative amplitude and alined along a northeast axis. The northeasternmost peak would fall near the ringlike feature mapped by Malloy and Harbison (1966). The source of the gravity anomaly appears to be either a broad mafic mass elongate to the northeast or two circular masses which are alined in that direction. The highest Bouguer gravity value measured in the Gulf of Maine is the peak of anomaly XIII. It is a well-defined isolated high with a relative ampli- tude of 30 mgals, and it coincides with a sharp magnetic high which is elongate northeastward. To GRAVITY AND MAGNETIC EVIDENCE OF LITHOLOGY AND STRUCTURE, GULF OF MAINE the northeast a second partly defined gravity high of about 20-mgal amplitude correlates with a mod- erate elongate magnetic anomaly. The magnetic anomalies are much smaller in areal extent than the gravity anomalies and may reflect near-surface ex- tensions of larger deeper masses. The anomalies most probably indicate mafic intrusions. Anomaly XIV is a broad northeast-trending fea- ture between two gravity lows. The relative ampli- tude is about 15 mgal but appears higher because of the adjoining lows. A group of small magnetic anomalies is present along the northwest flank of the high, but the magnetic field is flatter and at a lower level in the central and southeast parts. The anomaly could be caused by a broad mafic pluton of moderate thickness or by a sequence of stratified rocks that contains a considerable percentage of mafic volcanic units. Anomalies XVa through XVd are all of relatively low amplitude (10 to 15 mgal). The linear south- western part of anomaly XVa correlates closely in trend and location with a fairly intense linear mag- netic high, whereas the northern part of the gravity anomaly is marked by a series of low-amplitude magnetic features. The northern part of the gravity anomaly extends to the gabbro exposures of Monhe- gan Island, so the source of the entire high is prob- ably a mafic pluton. Anomaly XVb encompasses a group of magnetic highs of moderate intensity, in- dicating the presence of mafic plutonic or volcanic rocks or possibly both. A small gravity high (not la- beled) just east of XVb coincides with an intense magnetic high and indicates the presence of a small mafic pluton. Anomaly XVc is a fairly broad elongate feature in the area of a low-level and slightly anomalous magnetic field. Anomaly XVd is an areally small fairly sharp high with no apparent associated mag- netic feature. The geophysical characteristics of these anomalies are not critical enough to indicate their sources, although the elongate shape of XVc suggests a volcanic source and the equidimensional shape of XVd suggests a plutonic source. AEROMAGNETICS Regional Magnetic Field The main component of the earth’s geomagnetic field has been removed from the aeromagnetic data (Taylor and others, 1968, p. 755) so that magnetic levels may be compared over widely separated areas (pl. 2). The magnetic field of the Gulf of Maine re- gion has the highest average level and the most in- tense anomalies of any region along the Atlantic B11 coast of the United States (Taylor and others, 1968). As a group, the anomalies have a northeast trend approximately parallel to the principal trend of the Appalachians, though single anomalies and small groups of anomalies commonly have divergent trends. Some broad anomalies have lateral extents of as much as 50 km; amplitudes relative to nearby magnetic levels reach a maximum of more than 2,000 gammas. Distinctive contrasts in magnetic ex- pression are apparent between large areas of the gulf region, which indicates corresponding contrasts in the geology of the bedrock underlying these areas. On plate 2, areas of major contrast in mag- netic expression are outlined in solid heavy lines; some subordinate divisions are shown by dashed lines. In the following discussion, the anomaly-lith- ology relations observed in the land areas around the gulf serve as the principal guidelines in inter- preting the magnetic anomalies in the water-cov- ered areas. Land Areas A highly anomalous high-level magnetic belt ex- tends from southern New Brunswick (north- easternmost corner of map) to Mount Desert Island, Maine. North of the eastern part of the Bay of Fundy, the high-level magnetic field (greater than 1,000 gammas) and local intense positive anomalies coincide with an area underlain by rocks of Precambrian age. These rocks and rocks of simi- lar age, described in a later section, are made up of a wide variety of volcanic and plutonic igneous rocks as well as of metamorphosed sedimentary rocks. Farther north, a southwest-trending linear high (enclosed by 1,000-gamma contours) corre- sponds on the northeast to a structural uplift cored by Paleozoic volcanic rocks and on the southwest to a linear zone of rocks of Precambrian age. West of St. John, New Brunswick, the high-level magnetic field correlates with a major pluton which is pri— marily felsic in composition but which is made up in part of mafic rocks. Gravity data (pl. 1) indicate that the pluton extends to considerable depth and that its top is only partly exposed. Most of the local intense anomalies of the magnetic field over the plu- ton are associated with areas of mafic rock or with inliers of stratified rocks. A moderately intense lin- ear anomaly, which trends southwest across the central part of Passamaquoddy Bay and thence south along the Maine—New Brunswick boundary, correlates with an anticlinal structure in volcanic rocks of Silurian age. In the eastern coastal area of Maine, intense equidimensional positive anomalies B12 within the high-level magnetic belt correspond to mafic plutonic rock and, in some places, to felsic rock (Mount Desert Island area and west of Passa— maquoddy Bay). Gravity anomalies, however, indi- cate that the felsic rock is thin and that it may be underlain by rock that is mafic in composition. At the southwest end of the gulf, broad magnetic anomalies of high level are present at Cape Ann, Mass, and Cape Neddick, Maine, in the vicinity of relatively small areas of mafic rock. Gravity data, however, indicate that mafic masses widen with depth and attain horizontal extents that are compa- rable with the areas of the magnetic anomalies. Inland in Maine a broad moderately intense mag- netic high extends southwest from the vicinity of Bangor to the coast. The anomaly corresponds gen- erally to the Knox Gneiss of Perkins and Smith (1925) which they mapped as a distinct unit be- cause of “abundant intrusions,” mostly of small areal extent. A broad magnetic high extends on- shore in southwestern Rhode Island and southeast- ern Connecticut where it is underlain by stratified rocks out by sheetlike felsic igneous bodies (Richard Goldsmith, oral commun., May 1969). An elongate southwest-trending magnetic anomaly of moderate intensity in central Massachusetts (westernmost magnetic high) overlies a belt of stratified rocks of Cambrian and Ordovician age which is character- ized by intense migmatization (Hansen, 1956). It is not clear whether the group of anomalies described in this paragraph arises from the cumulative effect of many small magnetic sources or from a larger related magnetic source at depth. The association of the highly intruded areas of rock with broad moder- ately intense positive magnetic anomalies, however, seems to be a common one. The flat parts of the magnetic field in the north- easternmost part of the map area (New Bruns- wick) are generally coincident with areas underlain by sedimentary rocks of Carboniferous age. To the south, the flat low-level magnetic field is associated with the Triassic strata of the Bay of Fundy re- gion, an association which is characteristic of Triassic basins (see, for example, Bromery and Griscom, 1967). The flat magnetic field presumably reflects the thick sequence of weakly magnetic rocks that typically underlies the Triassic basins. Parts of the moderately intense positive anomalies over Grand Manan Island correlate with rocks of Pre- cambrian age and apparently are caused by base- ment protruding through the Triassic strata. The magnetic field over the major felsic pluton underly- ing much of the Nova Scotian peninsula is low level GEOPHYSICAL FIELD INVESTIGATIONS and virtually not anomalous, whereas the field over surrounding stratified rocks of Ordovician age, though also low level, shows linear anomalies of moderate positive amplitude. The linear anomalies correlate with areas of black slate and schist which overlie an older unit made up of graywacke, quartz- ite, gneiss, and minor slate. The correlation indi- cates that the black slate and schist are moderately magnetic, whereas the older unit is virtually non- magnetic. A noteworthy aspect of the magnetic field in Nova Scotia is the abrupt termination of the lin- ear magnetic anomalies where the stratified rocks are cut off by the felsic pluton. In Maine, a southwest-trending zone of low-gra- dient low-level magnetic field extends from about 67°3'O’ W., 45°00’ N. to about 69°30’ W., 44°00’ N. The zone is underlain by several large felsic plutons and by intervening areas of pelitic rocks in which volcanic rocks are typically scarce (Gates, 1969; P. H. Osberg, oral commun., May 1969). A similar zone in southeastern New Hampshire and north- eastern Massachusetts is underlain by pelitic rocks in which “volcanic rocks are distinctly subordinate” (Billings, 1956). In southeastern Massachusetts and eastern Rhode Island, the magnetic field over strati- fied rocks of Carboniferous age and felsic plutonic rock of Precambrian or early Paleozoic age (Quinn and Moore, 1968) is generally low level and not anomalous. A fairly fiat, southwest-trending, low- level magnetic zone also extends from northeastern Massachusetts to northern Rhode Island and north- eastern Connecticut and is underlain for the most part by felsic plutonic rocks. Some particular anomaly-lithology correlations are illustrated by examples in the area near Port- land, Maine. A broad magnetic low, centered at 70° W., 44° N., is underlain in part by a group of small felsic plutons. A gravity low (pl. 1) indicates that these plutons are exposed parts of a larger continu- ous subsurface felsic mass which may also give rise to the magnetic low. The broad low-gradient mag- netic high that adjoins the low on the southwest is over a large expanse of felsic rock which, because of the lack of an associated gravity low, is assumed to be thin. In addition, ground magnetic profiles in this area show that numerous strongly magnetized dikes intrude the felsic rock. The magnetic high is probably caused either by the cumulative effect of the dikes or by a deeper magnetic source to which the dikes may be related. Farther south, a sharp magnetic high of moderate amplitude is over a small alkalic pluton of Mesozoic age; this associa- tion is typical for small plutons of similar age in GRAVITY AND MAGNETIC EVIDENCE OF LIT HOLOGY AND STRUCTURE, GULF OF MAINE the New England region (Griscom and Bromery, 1968). Small linear anomalies in the bay area east of Portland correlate with major folds in stratified rocks that are mostly volcanic in origin (Hussey, 1968). The anomaly-lithology correlations in the land areas show that the principal causes of magnetic anomalies in this region are igneous rocks. Intense broad magnetic anomalies correlate for the most part With large masses of mafic plutonic rock. Broad anomalies of moderate amplitude appear to be associated with areas that are characterized by small but abundant intrusions. Linear anomalies correlate in many places with broad folds in vol- canic rocks, where the width of the anomaly is re- lated to the scale of the fold. Small but intense equi— dimensional anomalies are present over small intrusions of alkalic or mafic composition. Flat magnetic fields usually correlate with areas under- lain either by thick stratified rocks, in which vol- canic rocks are typically scarce, or by felsic intru- sions of considerable thickness. In some places the correlations have depended on the use of gravity data to evaluate the depth extent of exposed rock units. Gulf of Maine Region A major contrast is apparent between the mag- netic field southeast of the easternmost three en echelon solid lines on plate 2 and that of the re- mainder of the region. The magnetic field of the southeastern part is differentiated primarily by the low gradients of the anomalies, which indicate that magnetic rock is relatively deep. The broad mag- netic highs at the south-central and southeast mar- gins of the map (within the conspicuous 1,000- gamma contours, pl. 2) are parts of the east coast magnetic high which is apparently related to the boundary between the oceanic and continental crust (Keller and others, 1954; Drake and others, 1963; Taylor and others, 1968; Emery and others, 1970). Part of Nantucket Shoals, Georges Bank, and the southwestern part of Browns Bank lie between the en echelon lines and the east coast magnetic high. (The eastern part of the east coast magnetic high is parallel to but about 40 km seaward of Georges Bank.) The uniform character of the magnetic field indicates that the basement (magnetic rock) is deep throughout this area. Estimates of depth of base- ment below sea level (Vacquier and others, 1951) are 2.8 km at a (0.06 km of water) and 1.8 km at ,8 (0.04 km of water). These depths are in close agreement with thicknesses of low-velocity strata B13 calculated for nearby locations from seismic refrac- tion measurements (Drake and others, 1959, fig. 35, Portland section, sta. 66 and 59, respectively). Depths may, however, be greater in other parts of this area. In contrast to the anomaly sources in the area of the banks, the sources of magnetic anom— alies for most of the remainder of the gulf region appear to be near or at the sea bottom, within the limit of resolution provided by the present magnetic data. Magnetic anomalies, although subdued in ampli- tude and gradient, are present in the area of the banks. These anomalies imply that the surface un- derlying the low-velocity strata measured by Drake, Worzel, and Beckmann (1954) may be a terrane of igneous and metamorphic rocks like that in the more anomalous magnetic areas to the northwest. On a broad scale, the transition from high-gradient magnetic field (shallow magnetic rocks) to a low- gradient field (deep magnetic rocks) is fairly ab- rupt and suggests that a fault zone may be present along the inner edges of the banks. The present magnetic data, however, are not sufficient to quan- tify the width of the transition closely, and the slope between shallow and deep magnetic rock could be of low magnitude. Northwest of the easternmost three en echelon lines (pl. 2), the magnetic field contains an irregu- larly shaped central zone of highly anomalous char- acter (outlined by solid line) bordered on the east, southeast, and southwest by zones of relatively sub— dued character. On the northwest, part of the highly anomalous central zone is over land, and the entire zone is bordered by a band characterized by a low-level magnetic field in which anomalies are con— spicuously absent. A secondary boundary (dashed line) within the highly anomalous central zone sepa- ratesa northwest belt of intense closely spaced anom- alies from a broader area in which anomalies are generally less intense and spaced at wider intervals. Although the gross contrast between parts of the magnetic field throughout the Gulf of Maine is’rea- sonably clear, the precise location of boundaries is somewhat arbitrary. As a rule, location of magnetic boundaries in the gulf region was drawn with the intent of minimizing structural implications. From a regional standpoint, there is a general correlation between magnetic-field expression and sea-bottom topography. The highly anomalous cen- tral magnetic zone includes most of the relatively shallow area of the central gulf, though it also in- cludes much of Wilkinson Basin, the northern part of Murray Basin, and the deep western part of J or- B14 dan Basin. A locally flat or low magnetic field coin- cides with the southern parts of Murray and Wilk- inson Basins, with much of Rogers Basin, Franklin and Crowell Basins, the central part of Georges Basin, and the eastern part of Jordan Basin; the lack of anomalies in these areas indicates that they could contain substantial thicknesses of nonmag- netic stratified rocks. The belt of intense anomalies along the north- western part of the central magnetic zone extends to land, where it coincides with the Bays of Maine igneous complex (Chapman, 1962). According to Chapman, early volcanism in this area was suc- ceeded by a plutonic sequence in which an extensive sheet of gabbroic rocks was later intruded by stock- like masses of felsic composition. The magnetic and gravity anomalies in the land area reflect both the mafic and felsic plutonic phases of the complex but indicate that the mafic plutonic phase predominates in the coastal area of Maine. The magnetic and gravity anomalies along the magnetic belt in the gulf are similar to those on land—that is, an inter- mixed group of pronounced highs and lows—and imply that the complex extends southwest into the gulf. Southwest of Penobscot Bay the magnetic belt as a whole bends sharply south but returns to its original southwest trend farther southwest. The in— dividual magnetic anomalies, however, maintain their dominant southwest trend even within the southward flexure. West of 69° W. the magnetic anomalies are predominantly linear and may indi- cate that broad folds of volcanic rocks are common in this area. The magnetic belt and associated grav- ity anomalies appear to resume on land in the Vicin- ity of Cape Ann, Mass., which supports Chapman’s suggestion that the Bays of Maine igneous complex might extent through the gulf and emerge in the Massachusetts region (Chapman, 1962) . Two linear anomalies of exceptionally high ampli- tude occur on the southeast side of the belt, one 3,000 gammas in amplitude centered along 68° W., and the other 2,500 gammas in amplitude centered just west of 69° W. These amplitudes are signifi- cantly higher than those observed over mafic rocks in nearby areas and may indicate the presence of ultramafic rocks. Anomalies in the remainder of the central mag- netic zone (southeast of the dashed line, pl.2) are generally less intense and more widely spaced than those to the northwest. Three anomalies of broad horizontal extent and fairly high amplitude are in this area—one each in the northeast, east, and southwest extremities of the zone. The anomaly in GEOPHYSICAL FIELD INVESTIGATIONS the northeast extremity is the magnetic feature which has been attributed by Malloy and Harbison (1966) to a ringdike. The general appearance and trends of anomalies of this entire area are more random than elsewhere in the gulf, although the predominant orientation is still southwest. In gen- eral, both the magnetic and gravity anomalies indi- cate that plutonic rocks of mafic and felsic composi- tion are abundant and that several of the plutons are of broad horizontal extent. The individual plu- tons appear to be more widely spaced, however, and broad folds of volcanic rocks more scarce, than in the magnetic belt to the northwest. The east-central part of the zone is similar in some respects to the magnetic field over the broad zone of rocks of Pre- cambrian age in the coastal area of New Bruns- wick. Some magnetic anomalies, particularly the double-peaked intense high east of 43°N., 68°W., are similar to those over small alkalic plutons of Mesozoic age in Maine and New Hampshire. A broad zone of flat magnetic field extends south- west from the Bay of Fundy and thence south to the vicinity of 43°20’ N. Trends of anomalies are southwest on the north, but change to south in the south-trending part of the zone. East of Grand Manan Island, anomalies are scarce; but over Grand Manan Island and to the west and southwest, scat- tered anomalies of low to moderate amplitude are present; these anomalies coincide for the most part with local bathymetric highs. Uchupi (1966) con- cluded that the character of seismic reflections over much of this zone was typical of strata of Triassic age. Similarly, the magnetic characteristics of this zone resemble those over other Triassic basins, where a flat magnetic field is underlain by stratified continental deposits and anomalies are underlain by exposures of diabase or by basement protruding through the stratified rocks (for example, the Buck- ingham area of Pennsylvania, Bromery and Gris- com, 1967). The flat magnetic field extends beyond the boundaries of the Triassic strata as outlined by Uchupi (1966), indicating that these strata were deposited on a surface underlain by nonmagnetic rocks. In a similar setting in the Triassic basin, in southeastern Pennsylvania, 3. flat magnetic field that lies at the margin of the Triassic strata is as- sociated with metamorphosed carbonate rocks, shales, and clastic rocks of early Paleozoic age (Bromery and Griscom, 1967) . The estimated depth to magnetic rock at the loca- tion marked 'y on plate 2 is about 3.0 km below sea level (0.14 km of water), which agrees with the thickness of rocks measured in this area by Drake, GRAVITY AND MAGNETIC EVIDENCE OF LITHOLOGY AND STRUCTURE, GULF OF MAINE Worzel, and Beckmann (1954) having a moderate velocity (4.5 kmps). These rocks may be related to a broad syncline that underlies the west coast of Nova Scotia and trends gulfward in this area. The abundant anomalies between the area marked y and Nova Scotia are apparently caused by the upper member of folded stratified rocks of Ordovician age which gives rise to the prominent linear anomalies on the Nova Scotian mainland. A zone characterized by numerous anomalies, mostly of low amplitude and invariably of south- west trend, extends from northwest of Browns Bank southwest to the vicinity of 71° W. This zone is separated from the subdued field to the northeast (over the Triassic basin and Nova Scotia) by an ab- rupt change in anomaly trend (indicated by a dashed line on pl. 2) from south to southwest. Northeast of Franklin Swell the magnetic anomalies are typically elongate features of small area and low intensity; gravity anomalies in the surveyed part of this area are also small in horizontal extent. Both the magnetic and gravity anomalies indicate that igneous units may be common in the area be- tween Franklin Swell and Browns Bank but that in— dividual plutonic units are small in areal extent compared With those in area to the northwest. The elongate magnetic anomalies suggest that broadly folded magnetic stratified rocks may also be pres- ent. An area of notably low magnetic gradients is centered near 67° W., 42°30’ N., and extends south- west. Seismic measurements indicate that low- velocity strata in this area are as thick as 1 km (Uchupi, 1966). Southwest of Franklin Swell the subdued mag— netic zone between the banks area and land contains several broad magnetic anomalies of moderate am- plitude and several areas of flat magnetic field. Gravity anomalies in the part of the zone that was surveyed are also of considerable horizontal extent. The flat magnetic field southeast of 69° W., 42° N. coincides with the major gravity low that was at- tributed to a felsic pluton in a previous section. Magnetic anomalies along the margins of the grav- ity low may be caused by altered country rock or by smaller igneous bodies that are related to the felsic pluton. The broad arcuate magnetic low centered northeast of 70° W., 41° N. coincides with a zone of low-velocity (5.0-kmps) basement that was mapped by Drake, Ewing, and Sutton (1959). The velocity is distinctly lower than that of the surrounding area (about 5.8 kmps), but it is not sufficiently dis— tinctive to classify closely the probable rock type. The circular nature of the anomaly suggests a plu- 315 tonic source, but the velocity is more typical of rocks of sedimentary origin. Low-velocity strata in the area northwest of 70° W., 41° N. are nearly 1 km thick (Drake and oth- ers, 1959, fig. 35, Woods Hole section, sta. 3.2), which explains the broad gradients on the extensive magnetic high of this area and, possibly, the ab- sence of magnetic anomalies in the area northwest of the high. The fairly intense amplitude and broad extent of the high indicate that the source is a mafic pluton.Small magnetic sources may be present in the area of flat magnetic field to the northwest, but large sources are apparently absent. The linear magnetic anomalies of moderate amplitude in the Cape Cod area suggest that igneous rocks are abun- dant in the subsurface. Only anomalies of low ampli- tude and small horizontal extent occur throughout the area north and northeast of Cape Cod, indicating that magnetic rocks are present only as minor con- stituents. A narrow belt of low—gradient flat magnetic field forms the northwest boundary of the central mag- netic zone. Apparent extensions of the belt on land to the northeast and west are underlain by felsic plutonic rocks and by stratified rocks in which vol- canic rocks are a minor constituent. A broad grav- ity low in the south-trending part of the belt in the Gulf of Maine indicates the presence of a felsic plu- ton. A gravity high, however, having minor associ- ated magnetic anomalies, is present farther south- west along the belt. In general, the flat magnetic 'field of the belt indicates the absence of major amounts of volcanic rock or of mafic plutonic rock. Under this interpretation, the presence of the grav- ity high is puzzling, but it may be caused by a deep source, as was suggested in the section on gravity (p.B10). Linear magnetic highs of low to moderate ampli- tude are abundant in the gulf area just off the southern coast of Maine. The probable sources of these anomalies are folded belts of volcanic rocks. GEOLOGIC IMPLICATIONS OF THE GRAVITY AND MAGNETIC ANOMALIES Lithology and Ages The results of the geophysical interpretation are summarized in figure 3 where distinctive lithologic units, deduced mainly from gravity anomalies, are superposed on the zones which correspond to re- gional contrasts in the magnetic field. It is esti- mated that all the indicated lithologic units are at least 1 km thick and that many are as thick as 3 km or more. Most of the units are probably igneous plutonic‘ masses, but some may be composed of vol- B16 GEOPHYSICAL FIELD INVESTIGATIONS 7 . 45.? 7'0 ~$ . 41“ 71° 70° 69° 68, 67. 66° 1 UNITED TATEs ICANADA ~ fi’ x/\ 6:: fl ° / 43“ 50 NAUTICAL MILES 25 0 25 50 KILOMETERS LLL1_1_1___L_J | l 68“ 67° 41° 66° EXPLANATION Area of pronounced gravity high indicating presence of mafic plutonic rock, or possibly in some places, ultramafic rock Area of pronounced areally large, aeromag~ netic high indicating presence of mafic plutonic rock Area of moderate gravity high indicating presence of mafic rock, probably of volcanic origin Area of pronounced gravity low probably Caused by felsic plutonic rock. Broad area in central parts of zones B and C may be underlain in part by stratified rocks Area of moderate gravity low indicating presence of thin bodies of felsic composi- tion, or of moderate thicknesses of strati- fied rocks which are at most partly metamorphosed A Magnetic zone discussed in text Principal magnetic zone boundary Subordinate magnetic zone boundary XXX Pronounced linear aeromagnetic high, prob- ably caused by ultramafic rock FIGURE 3.—Provisional litholog'ic map of the indurated bedrock of the Gulf of Maine, derived from gravity and aeromagnetic data. GRAVITY AND MAGNETIC EVIDENCE OF LITHOLOGY AND STRUCTURE, GULF OF MAINE canic rocks or rocks of sedimentary origin. The uni- form background gravity field shown in figure 3 im- plies that the host rock in the intervening areas has a grossly uniform density which is probably similar to that of average metamorphic rock. Some characteristics of the host rock in the individual zones are described below. Note that figure 3 differs from a conventional geologic map in that the outlines of the lithologic units do not represent contacts at a single horizon. The interpreted sources of the anomalies are three- dimensional bodies whose lateral boundaries can be located, for the most part, only within very broad limits. The outlines of the lithologic units may best be interpreted as very approximate estimates of the maximum lateral extents of the units. Part of the upper surface of most of the units, however, proba- bly coincides with the metamorphic and igneous rock horizon mapped by Uchupi (1966). This con- clusion is based on the observation that gravity anomaly sources in the surrounding land areas un- derlain by metamorphic terrane are almost always partly exposed. The exposures are most commonly found near the peaks of the anomalies. The flat character of the magnetic field of zone A shows that it is underlain by magnetite-poor rocks. In Maine, the zone correlates with stratified rocks that are mapped as predominantly Cambrian and Ordovician in age and with felsic intrusive rocks that are Devonian in age. A possible extension of the zone in Massachusetts (zone A’) is associated with rocks that may be Silurian in age (Billings, 1956), but the extension must be considered specu- lative because of the discontinuity in the vicinity of Cape Ann. In the Gulf of Maine, the zone is proba- bly underlain by volcanic-poor stratified rocks of Cambrian and Ordovician age which are intruded in places by felsic plutons of Devonian age. The pres- ence of mafic rock in the southwestern 'part of the zone in the gulf is, as stated earlier (p. B10), not consistent with the magnetic character of the zone, but the mafic rock may be deeply buried. In contrast to zone A, the rocks of zone B are magnetite rich, a characteristic which apparently reflects an extensive history of igneous activity. Correlations on land imply that the major plutonic components of the zone are Devonian in age and that the volcanic rocks are mostly Silurian in age (Gates, 1969). Mafic plutons predominate in the eastern Maine coastal area, but large felsic plutons are present in the extension of the zone in New Brunswick and, most probably, within the gulf. Some B17 linear anomalies might be caused by elongate blocks of Precambrian rocks. Magnetite-rich rocks are abundant. in zone C but are more widely separated than those in zone B. The general appearance of parts of the magnetic field of this zone resembles that over the region in New Brunswick underlain by rocks of Precambrian age. Because of this resemblance, the bedrock is tentatively identified as probably Precambrian in age, at least in part. Rocks of Carboniferous age may be' present within zones B and C, in the areas which have a flat magnetic field. Zone ~D encompasses almost all the area that Uchupi (1966) showed as being underlain by strati- fied rocks of Triassic age. As pointed out earlier, (p. B12) , the magnetic aspects of this zone are very similar to those of the Triassic basin in southeast- ern Pennsylvania (Bromery and Griscom, 1967). As in Pennsylvania, some of the flat magnetic field pe- ripheral to the indicated area of strata of Triassic age may be underlain by stratified rocks of Carboni- ferous or older age. Furthermore, rocks of Triassic age may possibly extend beyond the limits of zone D, inasmuch as some of the anomalies of the northern part of zone C are similar to those over extensive exposures of diabase in the Pennsylvania area. The circular magnetic anomaly discovered by Malloy and Harbison (1966), for example, could be caused by an arcuate emplacement of diabase. Zones E and F are characterized by widely- spaced anomalies of moderate amplitude, a condi- tion which permits a considerable range of possible correlations. The magnetic character of zone E is somewhat like that of the stratified rocks of Ordovi- cian age in Nova Scotia and may indicate an exten- sion of these rocks to this part of the gulf. The widely spaced broad anomalies that make up much of zone F do not clearly resemble the magnetic field over any of the nearby land areas. Structure The dominant gravity and magnetic trends of the Gulf of Maine indicate that the main structural grain is northeast, in accord with that of the Appa- lachians as a whole. Direct gravity and magnetic evidence of discrete structures underlying the gulf is generally precluded by the wide spacing of the measurements. Discontinuities in the patterns of the magnetic field in some areas, however, suggest abrupt changes in gross bedrock units which can best be explained by major faults. The most obvious discontinuity is between the linear magnetic zones A and B in the gulf and their extensions A’ and B’ B18 in southern New England. If these zones were in fact once continuous, then a major fault offsets them in the vicinity of Cape Ann. A plausible mech- anism for the offset is the great imbricate thrust fault described by Bell (1967) which strikes into the gulf north of Cape Ann. A conclusive demon- stration of this relationship, however, would require both land and sea surveys in considerably more detail. Near the east margin of the gulf, sharp changes in the character and trends of the magnetic field suggest the presence of a major north-trending structural discontinuity. On the north, the transi- tion from a very flat magnetic field over the Bay of Fundy to an anomalous field over and southwest of Grand Manan Island indicates a fairly abrupt change from a broad basin uniformly underlain by a thick sequence of nonmagnetic stratified rocks to a region where broad blocks of magnetic rock (Pre- cambrian?) are near or at the sea bottom. To the south, linear magnetic anomalies that strike south- west on the Nova Scotian peninsula bear sharply south in the western and southern coastal areas. Apparent extensions of these linear anomalies in the gulf and broad anomalies in the western part of zone D also have a southward alinement. Just north of 43° N., the southward trends terminate and are replaced to the south by southwestward trends. Taken together these changes in the magnetic field suggest a structural break between the Bay of Fundy—N ova Scotia region and the eastern Gulf of Maine. Realinement of flat parts of the magnetic field indicates that there may be a northward com- ponent of movement with left-lateral sense and as much as 50 km of displacement. Although the present gravity and magnetic data provide no direct evidence of the Fundian fault sys- tem of Johnson (1925) or of a gulfward extension of the rift system of Belt (1968) , the interpretation of these data is compatible with the location of struc- tures like these within the gulf. Gates (1969) inter- preted the setting and structural style of volcanic rocks in eastern Maine as evidence of a crustal zone of weakness along which major horizontal displace- ments took place in late Paleozoic and early Meso- zoic time. Presumably, magnetic zone B is a mani- festation, at‘ least in part, of an extension of the volcanic rocks within the gulf. A linear magnetic high, which extends from the northeast end of the volcanic belt described by Gates (1969) to the Gulf of St. Lawrence (Bhattacharyya and Raychaudhuri, 1967), is underlain by structural highs in the bed— rock geology. Thus a linear magnetic high, appar- GEOPHYSICAL FIELD INVESTIGATIONS ently marking the location of a major crustal fault, extends from the Gulf of St. Lawrence to the south- ern New England coast. This interpretation sup- ports the presence in the Gulf region of a north- east-trending fault of continental dimensions as proposed by Wilson (1962) and by Webb (1968). The presence of rift structures, like those described by Belt (1968) , is also compatible with the interpre- tation of some of the gravity lows and with some of the areas of very flat magnetic field within the cen- tral part of the gulf. CRUSTAL STRUCTURE OF THE GULF OF MAINE The uniformly high Bouguer gravity field, the abundance of mafic rock units, and the compara- tively high velocity of shallow bedrock (Drake and others, 1954) are evidence that the crust underlying the Gulf of Maine differs substantially from that of the surrounding areas. Some gross properties of that crust and their influence on the formation of the gulf are indicated by several aspects of the gravity field. The average free-air gravity field over the gulf (Yellin, 1968) and over Maine is slightly positive, indicating that the region is near isostatic equilib- rium; that is, the differential surface loads caused by regional variations in the topography of the land and sea floor are balanced by subsurface masses of offsetting density contrast. Accordingly, the mass load represented by the rise in bedrock surface from the gulf to the interior of Maine must be balanced by subsurface masses of negative density contrast. As shown by analyses of the Bouguer gravity field, these balancing masses are located, at least in part, in the uppermost crust. Part of the isostatic equilib- rium may be achieved by changes in crustal thick- ness, but changes in crustal thickness are not re- quired to explain the regional variations in the Bouguer field. The close correspondence between the shore of the gulf and the landward edge of the high gravity level (pl. 1) suggests that the depression of the bed- rock surface and the source of the gravity level are causally related. More specifically, isostatic theory implies that the shallow high-density source of the gravity level causes an effective negative buoyancy and thereby restrains the bedrock surface below sea level. The uniformity of the high gravity level also suggests that the crust beneath the gulf may act as a single block or plate in response to whatever surface or body forces are present. A sim- ilar block geometry was described for the crust un- GRAVITY AND MAGNETIC EVIDENCE OF LITHOLOGY AND STRUCTURE, GULF OF MAINE derlying Maine (Kane and Bromery, 1968) and would appear to apply equally well to the major peninsula that makes up southwestern Nova Scotia, where the crustal block is presumably buoyed posi- tively by the relatively low-density pluton that un- derlies much of the peninsula (Garland, 1953). The major inference of this concept of crustal blocks is that the relative mass of extensive bodies of near- surface rocks may provide at least a partial control on the vertical position and attitude of the crustal block in which they occur. Figure 4 is a crustal model that accounts for both the horizontal gradients and the amplitudes of the regional Bouguer gravity field by changes in the density and thickness of stratified rocks of Paleo- zoic age overlying a high-density basement of Pre- cambrian age. The model is based on changes in level of the smoothed regional Bouguer gravity field, maximum depths to the density contrasts giv- ing rise to these changes, an evaluation of the den- sity of metamorphosed rocks of pre—Devonian age (Kane and Bromery, 1968), and recent specific gravity measurements on samples of stratified rocks of Devonian age from northwestern Maine. The vertical contacts of the crustal masses are arbi- trary, except to the extent that the maximum-depth B19 calculations are based on mass boundaries which have this attitude. If the boundaries sloped appreci- ably less than 90°, then the maximum depths would be correspondingly less. The critical constraints im- posed by the gravity data are as follows: (1) Maxi- mum depths to the shallowest density contrasts at junctures A, B, and C range from 5 to 8 km, and (2) the density of the medium on the southeast side of the juncture is always higher; that is, the grav- ity field is always higher to the southeast. The in- corporation of the geologic and density data leads to a model in which these lateral contrasts in density are caused by vertical displacements of crustal blocks (increasing uplift to the southeast) ; in the blocks, density increases with depth, and subsequent erosion has resulted in the present lateral density contrasts. The geologic aspects of the model are speculative, but they fit well with other information such as the high compressional-wave velocity of the “subbasement” beneath the gulf (Drake and others, 1954). The physical aspects (density, depth, vol- ume) of the model are more certain, however, and any model based on these data must have a density distribution that is similar to that shown here. The identification of shallow intracrustal sources for variations in the regional Bouguer gravity field ‘2' .‘2 3‘ '30 3o 1% 31,2 0 $ max=1 mgal/km a so 0 inf-E S max=l.3 mgal/km 0‘3 3‘30 —30 B Smax=2.3 mgaI/km .5 E ‘60 —60 .9 DD 0) :1: Northwest Maine Central Maine Coastal Maine Gulf of Maine g NW A B 0 SE 7; Devdr'iialnfé'ézjgs/cifi? Lower Paleozonc (7 =28 g/cm3 \ 0 1’ m ' Lower Paleozoic \ \ 3 B «7 =2.8 gl/cm3 \ \ U) a \ 3 g . Precambrian «r :29 g/cm3 2 _° ' l 8 '2 5 3 5° 50 D a =Density S max=Maximum horizontal 0 50 100 NAUTICAL MILES gravity gradient | l l 4 l l l 0 so 100 KILOMETERS A, B, C =Junctures of crustal blocks I I | FIGURE 4.—Regionalized Bouguer gravity field, maximum horizontal gravity gradients, and hypothetical density distribution of the Gulf of Maine—Maine region based on the assumption that the gravity-field variation is caused by changes in thickness of the Paleozoic stratified rocks. B20 supports the finding of Pakiser and Steinhart (1964) that the simple empirical relation between regional Bouguer gravity and crustal thickness is not generally valid. Weaver (1967) reported a simi- lar conclusion for Newfoundland where a high re- gional Bouguer gravity level correlates with a thick (42-km) high-velocity crust and a high-velocity mantle, and a low gravity level correlates with a thin (31-km) low-velocity crust and a low-velocity mantle. The seismic-velocity model of the crust in the Gulf of Maine (Steinhart and others, 1962) is generally intermediate to these two models, al- though the Bouguer gravity level is significantly higher than the high gravity level of the Newfound- land region. These results suggest that the present data are insufficient for speculation about the over- all nature of the deeper crust in the gulf region. SUMMARY The geologic results of this investigation of the Gulf of Maine represent a provisional description and distribution of gross units of indurated bedrock and an outline of some of the major structures. The principal conclusions are that (1) the indurated bed- rock contains abundant mafic rocks and is proba- bly mainly of early Paleozoic or Precambrian age and (2) major faults are along at least some mar- gins of the gulf. The crust that underlies the gulf appears to be distinctly different from that of the surrounding areas, but it does not clearly resemble any of the known conventional models of the crust. Two important properties of the crust are that high-density rock (2.9 g/cm3) seems to occur at very shallow crustal depths and that isostatic equilibrium is achieved, at least partly, by major rock masses at shallow crustal depths. Gravity surveys in the near-coast areas (repre- sented by dashed contours on pl. 1) and over the surrounding continental shelf are needed to extend and possibly modify the conclusions presented here. Detailed magnetic and continuous seismic profiling of the type done by Malloy and Harbison (1966) would be especially useful in providing details about the bedrock. These surveys would substantially fill the hiatus between this study and that of Uchupi (1966) and would provide more detailed informa- tion on the gross features described herein. More seismic measurements of velocities of the bedrock are needed, particularly in areas where gravity anomalies indicate the presence of either felsic plu— tonic rock or porous stratified rock. The findings of this study bearing on crustal structure suggest that gravity and magnetic sur- GEOPHYSICAL FIELD INVESTIGATIONS veys can provide useful guides to the location of deep-probing seismic-refraction profiles. Interpreta- tion of this type of seismic data would be more straightforward if the profiles were not located so as to cross the boundaries between contrasting crus- tal blocks. REFERENCES CITED Allingham, J. W., 1961, Aeromagnetic interpretation of zoned intrusions in northern Maine: U.S. Geol. Survey Prof. Paper 424—D, p. D265—D266. Bancroft, A. M., 1960, Gravity anomalies over a buried step: J our. Geophys. Research, v. 65, p. 1630—1631. Bell, K. G., 1967, Faults in eastern Massachusetts [abs]: Geol. Soc. America, Northeastern Sec., 2d Ann. Mtg., Boston, Mass., 1967, Program, p. 14. Belt, E. S., 1968, Post-Acadian rifts and related facies, east- ern Canada, in Zen, E-an, White, W. S., Hadley, J. B., and Thompson, J. B., Jr., eds., Studies of Appalachian geology—~northern and maritime: New York, Intersci. Publishers, p. 95—113. Bhattacharyya, B. K., and Raychaudhuri, B., 1967, Aero- magnetic and geological interpretation of a section of the Appalachian belt in Canada: Canadian Jour. 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W., Jr.,.1968, Structure of eastern Connecticut, in Zen, E—an, White, W. S., Hadley, J. B., and Thompson, J. B., Jr., eds., Studies of Appa- lachian geology—northern and maritime: New York, In— tersci. Publishers, p. 219—229. Doll, C. G., Cady, W. M., Thompson, J. B., Jr., and Billings, M. P., compilers and eds., 1961, Centennial geologic map of Vermont: Montpelier, Vt., Vermont Geol. Survey, scale 1:250,000. Drake, C. L., Ewing, W. M., and Sutton, G. H., 1959, Conti— nental margins and geosynclines—the east coast of North America north of Cape Hatteras, in Ahrens, L. H., Press, Frank, Rankama, Kalervo, and Runcorn, S. K., eds., Physics and chemistry of the earth, Volume 3: Lon- don, Pergamon Press, p. 110—198. Drake, C. L., Heirtzler, J. R., and Hirshman, Julius, 1963, Magnetic anomalies ofl" eastern North America: Jour. Geophys. Research, v. 68, p. 5259—5275. Drake, C. L., Worzel, J. L., and Beckmann, W. C., 1954, Geo- physical investigations in the emerged and submerged Atlantic Coastal Plain—Part 9, Gulf of Maine: Geol. Soc. America Bull., v. 65, p. 957—970. Emery, K. 0., and Uchupi, Elazar, 1965, Structure of Georges Bank: Marine Geology, v. 3, p. 349—358. Emery, K. 0., Uchupi, Elazar, Phillips, J. D., Bowin, C. 0., Bunce, E. T., and Knotts, S. T., 1970, Continental rise ofl" eastern North America: Am. Assoc. Petroleum Geolo- gists Bull., v. 54, p. 44—108. Garland, G. D., 1953, Gravity measurements in the Maritime Provinces: Ottawa, Dominion Observatory Pub., v. 16, p. 185—275. Gates, Olcott, 1969, Lower Silurian—Lower Devonian vol- canic rocks of New England coast and southern New Brunswick, in Kay, Marshall, ed., North Atlantic—geol- ogy and continental drift: Am. Assoc. Petroleum Geolo- gists Mem. 12, p. 484—503. Goldsmith, Richard, 1964, Geologic map of New England: U.S. Geol. Survey open-file map, scale 1 :1,000,000. Griscom, Andrew, and Bromery, R. W., 1968, Geologic inter- pretation of aeromagnetic data for New England, in Zen, E-an, White, W. S., Hadley, J. B., and Thompson, J. B., Jr., eds., Studies of Appalachian geology—north- ern and maritime: New York, Intersci. Publishers, p. 425—436. Hansen, W. R., 1956, Geology and mineral resources of the Hudson and Maynard quadrangles, Massachusetts: U.S. Geol. Survey Bull. 1038, 104 p. Hoskins, Hartley, and Knott, S. T., 1961, Geophysical inves- tigation of Cape Cod Bay, Massachusetts, using the con— tinuous seismic profiler: J our. Geology, v. 69, p. 330—340. Hussey, A. M., 1968, Stratigraphy and structure of south- western Maine, in Zen, E-an, White, W. S., Hadley, J. B., and Thompson, J. B., Jr., eds., Studies of Appa- lachian geology—northern and maritime: New York, In- tersci. Publishers, p. 291—301. Hussey, A. M., Chapman, C. A., Doyle, R. G., Osberg, P. H., Pavlides, Louis, and Warner, Jeffrey, compilers, 1967, Preliminary geologic map of Maine: Augusta, Maine, Maine Geol. Survey, scale 1:500,000. Johnson, D. W., 1925, The New England—Acadian shoreline: New York, John Wiley & Sons, 608 p. B21 Joyner, W. B., 1963, Gravity in north-central New England: Geol. Soc. America Bull., v. 74, p. 831—858. Kane, M. F., 1961, Structure of plutons from gravity meas- urements: U.S. Geol. Survey Prof. Paper 424—C, p. 0258—0259. 1968, Metamorphic and igneous events reflected in the gravity field over the New England Appalachians [abs.]: Geol. Soc. America Spec. Paper 121, p. 152—153. 1970, Geophysical study of the tectonics and crustal structure of the Gulf of Maine: St. Louis, Mo., St. Louis Univ., Ph.D. dissertation, 106 p. Kane, M. F., and Bromery, R. W., 1966, Simple Bouguer gravity map of Maine: U.S. Geol. Survey Geophys. Inv. Map GP—580, scale 1:500,000. 1968, Gravity anomalies in Maine, in Zen, E-an, White, W. S., Hadley, J. B., and Thompson, J. B., Jr., eds., Studies of Appalachian geology—northern and maritime: New York, Intersci. Publishers, p. 415—423. Keller, Fred, Jr., Meuschke, J. L., and Alldredge, L. R., 1954, Aeromagnetic surveys in the Aleutian, Marshall, and Bermuda Islands: Am. Geophys. Union Trans., v. 35, p. 558—572. King, P. B., compiler, 1969, Tectonic map of North America: U.S. Geol. Survey, 2 sheets, scale 1:5,000,000. Malloy, R. J., and Harbison, R. N., 1966, Marine geology of the northeastern Gulf of Maine: U.S. Coast and Geod. Survey Tech. Bull. 28, 15 p. Osberg, P. H., Moench, R. H., and Warner, Jeffrey, 1968, Stratigraphy of the Merrimack synclinorium in west- central Maine, in Zen, E-an, White, W. S., Hadley, J. B., and Thompson, J. B., Jr., eds., Studies of Appa- lachian geology—northern and maritime: New York, In- tersci. Publishers, p. 241—253. Pakiser, L. C., and Steinhart, J. S., 1964, Explosion seismol- ogy in the western hemisphere, in Odishaw, Hugh, ed., Research in geophysics—Volume 2, Solid earth and in- terface phenomena: Cambridge, Massachusetts Inst. Technology Press, p. 123—147. Pavlides, Louis, 1965, Geology of the Bridgewater quadran- gle, Aroostook County, Maine: U.S. Geol. Survey Bull. 1206, 72 p. Perkins, E. H., and Smith, E. S. C., 1925, A geological sec- tion from the Kennebec River to Penobscot Bay: Am. Jour. Sci., 5th ser., v. 9, p. 204—228. Quinn, A. W., and Moore, G. E., Jr., 1968, Sedimentation, tectonism, and plutonism of the Narragansett Basin re- gion, in Zen, E-an, White, W. S., Hadley, J. B., and Thompson, J. B., Jr., eds., Studies of Appalachian geolo- gy—northern and maritime: New York, Intersci. Pub- lishers, p. 269—279. Rankin, R. W., 1968, Volcanism related to tectonism in the Piscataquis volcanic belt, an island arc of Early De- vonian age in north-central Maine, in Zen, E-an, White, W. S., Hadley, J. B., and Thompson, J. B., Jr., eds., Studies of Appalachian geology—northern and mari- time: New York, Intersci. Publishers, p. 241—253. Reed, J. 0., Jr., Owens, J. P., and Stockard, H. P., 1967, In- terpretation of basement rocks beneath the Atlantic Coastal Plain from reconnaissance aeromagnetic data [abs.]: Geol. Soc. America Spec. Paper 115, p. 182—183. Schlee, John, and Pratt, R. M., 1970, Atlantic Continental Shelf and Slope of the United States—Gravels of the northeastern part: U.S. Geol. Survey Prof. Paper 529—H, 39 p. B22 Steinhart, J. S., Green, R., Asada, T., Rodriguez, B. A., Ald- rich, L. T., and Tuve, M. A., 1962, Seismic studies: Car- negie Inst. Washington Year Book 61, 1961—62, p. 221—234. Taylor, P. T., Zietz, Isidore, and Dennis, L. S., 1968, Geo- logic implications of aeromagnetic data for the eastern continental margin of the United States: Jour. Geophys. Research, v. 33, p. 755—780. Toulmin, Priestly, 3d, 1957, Notes on a peralkaline granite from Cashes Ledge, Gulf of Maine: Am. Mineralogist, v. 42, p. 912—915. Uchupi, Elazar, 1965, Maps showing relation of land and submarine topography, Nova Scotia to Florida: U.S. Geol. Survey Misc. Geol. Inv. Map I—451, scale 1:1,000,000. 1966, Structural framework of the Gulf of Maine: Jour. Geophys. Research, v. 71, p. 3013—3028. Vacquier, Victor, Steenland, N. C., Henderson, R. G., and Zietz, Isidore, 1951, Interpretation of aeromagnetic maps: Geol. Soc. America Mem. 47, 151 p. Weaver, D. F., 1967, A geological interpretation of the Bouguer anomaly field of Newfoundland: Ottawa, Do- minion Observatory Pub., v. 35, p. 223-251. GEOPHYSICAL FIELD INVESTIGATIONS Webb, G. W., 1968, Palinspastic restoration suggesting late Paleozoic North Atlantic rifting: Science, v. 159, p. 875—878. Wilson, J. T., 1962, Cabot fault, an Appalachian equivalent of the San Andreas and Great Glen faults and some im- plications for continental displacement: Nature, v. 195, p. 135—138. White, W. S., compiler, 1968, Generalized geologic map of the northern Appalachian region, in Zen, E-an, White, W. S., Hadley, J. B., and Thompson, J. B., Jr., eds., Studies of Appalachian geology—northern and mari- time: New York. Intersci. Publishers, scale 1:2,500,000. Worzel J. L., and Shurbet, G. L., 1955, Gravity anomalies at continental margins: Natl. Acad. Sci. Proc., v. 41, p. 458—469. Yellin, M. J ., 1968, Gravity survey of the Continental Shelf, seabottom and seasurface survey, Gulf of Maine: U.S. Coast and Geod. Survey Operational Data Rept. C and GSDR—2, 12 p. Zen, E-an, 1968, Introduction, in Zen, E-an, White, W. S., Hadley, J. B., and Thompson, J. B., Jr., eds., Studies of Appalachian geology—northern and maritime: New York, Intersci. Publishers, p. 1—5. U. 5. GOVERNMENT PRINTING OFFICE: 1972 O - 454-534 UNITED STATES DEPARTMENT OF THE INTERIOR PREPARED IN COOPERATION WITH GEOLOGICAL SURVEY PROFESSIONAL PAPER 726—8 THE NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION PLATE 1 72° 46 6506“ 7 ° . 66° 4 l 69" 68° NEW BRUNSWICK @ CANADA 5° NITED S’ITAT ~_—-_—.-_- FI'I'V'IT BANGOR. / \ 9 / / HHIHSdIAIVH AMEN GRAND MANA ._._. ISLAND v4r >e< >< A .. YVJ . 1))A>>l\;7~l<> —— I '— L 4~>v4Vr1F / 1/ —— v PLA< \ v / ‘ T/ V v>4F>< 4 s 4v< v . NOVA SCOT .. 7 " — < <7 )qLJA'>.,(q" >A< 7 ~ “ . 7 Jvr><>A P1 [-4 3‘ : Quaternary Felsic rocks mostly of Devonian age Triassic Alkalic rocks of Mesozoic and Paleozoic age a, land; b, water covered Mafic rocks mostly of Devonian age Carboniferous . . STRATIFIED AND Devonian and SIlurian PLUTON I C ROCKS Exclusive of marine areas \\ \ o \ - 45 N 45° _____ Precambrian A Ordovician and Cambrian May include rocks of younger age / 0 < ’l \ v ‘ ’ ~— ______ @ @ BANGOR. r (b - “’-_fld: J .— Aeromagnetic contours < \A Values are given in hundreds of gammas (for example 10: 1000 L -‘ ———————————— gammas) relative to arbitrary datum. Interval 100 gammas. Hachures indicate areas of relatively lower aeromagnetic field. Data sources are given in text Principal magnetic field boundary Subordinate magnetic field boundary a Location of basement depth estimate from aeromagnetic data +25 C: , ~ , -— g — __ 3 Q q: wwwwwwwww _ Relative gravity anomalies from plate 1 . Interval 5 mgal. — ” .. v - - Hachures indicate negative anomalies 72° 71° 41° 40° 72° 0 7 66° 65° 71 70° 69° 68° 67° . I I I f H Geolo from White (1968) with addItlons In Maine rom .ussey and offliers (1967), in southern New England from GoldsmIth (1964), 25 O 25 50 NAUTICAL MILES and in the Gulf of Maine from Uchupl (1966) I l I l I l l 25 O 25 50 KILOMETERS I AEROMAGNETIC AND GEOLOGIC MAP OF THE GULF OF MAINE REGION 7 DAY Gravity and Magnetic Features if as Related to Geology in the Leadville 80-Minute \Quadrangle, Colorado é 726-6; GEOLOGICAL SURVEY PROFESSIONAL PAPER 726—C Gravity and Magnetic Features as Related to Geology in the Leadville 80-Minute Quadrangle, Colorado By OGDEN TWETO and J. E. CASE GEOPHYSICAL FIELD INVESTIGATIONS GEOLOGICAL SURVEY PROFESSIONAL PAPER 726—C A summary and correlation of geologic and geophysical data on the most productive part of the Colorado mineral belt UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 72—600032 For sale by the Superintendent of Documents, US. Government Printing Oflice Washington, DC. 20402 Stock Number 2401—2145 CONTENTS Page Page Abstract ..................................................................................... Cl Gravity features ....................................................................... 016 Introduction ............................................................................... 1 Gravity survey ........................................... 16 ' Geology 3 Errors in the Bouguer anomalies... 17 Major features ......... 3 Interpretation of the gravity anomaly map. 18 Precambrian rocks ........................................................... 3 Mineral-belt gravity low ................................................. 18 Sedimentary rocks ............................................................ 4 Underlying batholith as source .............................. 18 Upper Cretaceous and Tertiary intrusive rocks. 4 Deep crustal or subcrustal source ......... 20 Tertiary volcanic rocks. 7 Local anomalies along the mineral-belt low ................. 20 Structure .......................................................... 8 Gravity low over the Arkansas valley graben ............. 22 Precambrian Homestake shear zone ..................... 8 Other bedrock anomalies ....................................... 22 Laramide and younger structure ........................... 8 Aeromagnetic survey ........................... 24 Geology of the Arkansas valley bottom... 9 Interpretation of the magnetic map .............. 24 Ore deposits ....................................................................... 11 General features of the magnetic map ......................... 24 Densities and magnetic properties of the Anomalies over the Arkansas valley ............................. 25 major rock units ........................................................... 12 Anomalies related to the Homestake shear zone.. 25 Densities ............................ 13 Anomalies along the Sawatch Range ............................ 25 Magnetic properties .............. 14 Anomalies in the southern Gore Range ........................ 26 Precambrian gneiss .................................................. 15 Anomalies along the Mosquito-Tenmile Range... 26 Precambrian granitic rocks .................................... 15 Anomalies in the eastern part of the quadrangle ....... 27 Paleozoic sedimentary rocks ................................... 15 Summary and conclusions ....................................................... 27 Upper Cretaceous and Tertiary intrusive rocks. 16 References cited ........................................................................ 29 ILLUSTRATIONS Page PLATE 1. Geologic and Bouguer anomaly map and sections of the Leadville 30-minute quadrangle, central Colorado ......................................................................................................................................................... In pocket 2. Aeromagnetic map and profiles of the Leadville 30-minute quadrangle. .................................................. In pocket FIGURE 1. Index map of Colorado ........................................................................................................ 02 2. Sketch map showing major structural features... .......................................................... 6 3. Histogram showing densities of major rock units .................................... 12 4. Histogram showing magnetic susceptibilities of major rock units ................................................................................. 13 5. Regional Bouguer anomaly map ......................................................... .. .. 1’7 6. Section showing interpretation of regional gravity anomaly ........................................................................................ 19 '7. Section showing gravitation effects of a deep crustal-mass deficiency beneath the Colorado mineral belt ............ 21 8. Map and section showing residual gravity anomaly over the Arkansas valley graben and its interpretation.... 23 TABLES Page TABLE 1. Generalized geologic column, Leadville 30-minute quadrangle ...................................................................................... 05 2. Summary of magnetic properties of the major rock units ............................................................................................. 15 III GEOPHYSICAL FIELD INVESTIGATIONS GRAVITY AND MAGNETIC FEATURES AS RELATED TO GEOLOGY IN THE LEADVILLE 30-MINUTE QUADRANGLE, COLORADO By OGDEN TWETO and J. E. CASE ABSTRACT Regional gravity and aeromagnetic surveys were made in the Leadville 30-minute quadrangle, Colorado, to supplement and extend geologic studies in a highly productive segment of the Colorado mineral belt. The quadrangle lies at the inter- section of the mineral belt with the Arkansas valley graben, a northern extension of the Rio Grande and San Luis Valley graben system of New Mexico and southern Colorado. Almost the entire quadrangle is a part of the eastern flank of the huge Sawatch anticline. The Sawatch Range, in the western part of the area, is carved in the Precambrian rocks in the core of this anticline. The Arkansas River valley, at the foot of the range, is an elongate downfaulted block, or graben, high on the eastern flank of the anticline. The Mosquito and Tenmile Ranges, which occupy a broad central belt through the length of the quadrangle, constitute an upthrown fault block east of the Arkansas valley graben. They consist of Precambrian rocks capped by Paleozoic sedimentary rocks. The sedimentary rocks dip eastward into South Park, a broad structural basin. The Precambrian rocks and the Paleozoic sedimentary rocks are cut by numerous bodies of intrusive porphyry of Late Cretaceous and Tertiary age. These porphyry bodies and re- lated mineral deposits—such as those of Climax, Leadville, and several other mining districts -— characterize the Colorado mineral belt, which here forms a broad northeast-trending belt that occupies much of the quadrangle. A deep fill of upper Tertiary alluvial deposits and mantling Pleistocene glacial deposits in the Arkansas valley covers the bedrock along the axial trend of the mineral belt for several miles southwest of Leadville. The dominant gravity anomaly is a huge low of 30—50 milligals which trends northeast across the quadrangle. From analysis of steep gradients on the flanks of the anomaly, the low is inferred to be caused by a shallow batholith of rela- tively low density. This batholith is concluded to extend to within a few thousand feet from the surface and to be the source of the numerous Upper Cretaceous and Tertiary intru- sive bodies. Superimposed on the main gravity anomaly is a low of 5-20 milligals over the Arkansas valley graben. This anomaly is attributed to a density contrast of 0.4—0.5 grams per cubic centimeter between the bedrock and fill and indicates a maxi- mum thickness of about 4,000 feet of fill in the graben. A general magnetic low occurs along the same trend as the main gravity low, from Climax southwest to the Twin Lakes stock of Tertiary porphyry. Magnetic highs and lows occur both over Precambrian granitic and gneissic rocks and over porphyry intrusives. The magnetic susceptibilities of these rocks have overlapping ranges; thus, many of the anomalies can be confidently interpreted only where the sources are exposed. Many magnetic highs correlate with topographic highs developed in crystalline rocks, but other highs are nearly independent of topography, and reflect strong mag— netization of the rocks. The Precambrian Homestake shear zone, which trends northeast in the northwestern part of the quadrangle, is ex- pressed by a strong magnetic gradient that reflects magnetic biotite gneiss and migmatite within, and northwest of, the shear zone, and less magnetic amphibole and quartz gneiss southeast of the shear zone. The gradient indicates both con- tinuity and location of the shear zone beneath areas covered by Paleozoic sedimentary rocks. Magnetic and gravity highs are found over a fault block near Sheep Mountain and Round Hill, reflecting dense and magnetic rocks, possibly mafic intrusives, in the block. Gravity and magnetic lows are found in the southeastern part of the quadrangle, where Paleozoic rocks are several thousand feet thick. Neither the Climax nor the Leadville mining district has a gravity or total-intensity-magnetic expression, but both are marked by weak residual magnetic lows, probably reflecting alteration of the crystalline rocks. Many of the mining dis- tricts are located near the axis of the main gravity low, and, thus, it may be postulated that mineralization arose from the apical region of the batholith at depth. INTRODUCTION The Leadville 30-minute quadrangle is in the most productive part of the Colorado mineral belt, a belt of Upper Cretaceous and Tertiary intrusive rocks and ore deposits that extends diagonally across the mountains of Colorado and contains most of the State’s ore deposits (fig. 1). The intrusive rocks and ore deposits that characterize the belt generally have been regarded as manifestations of an underlying batholith (Crawford, 1924; Lovering and Goddard, 1938; Tweto, 1968a, p. 570—571). Location of the belt and presumably of the underlying batholith coincides Cl C2 GEOPHYSICAL FIELD INVESTIGATIONS 41°109° 108° 40~L RIVER PLATEAU 39° ~ 0 0 <1: 0: O .1 O O 38°~ MOUNTAINS you 1 06° 105° 1 104° NORTH PARK LNOH“ MIDDLE LEADVILLE QUADRANGLE 0 Colorado Springs on "u... one”. OCanon City fl, u. “I“ .. "ciN"‘<'i Area of figure 1 COLORADO 50 100 MILES 1 l FIGURE 1. — Index map of central and western Colorado, showing location of the Leadville 30—minute quadrangle in relation to the Colorado mineral belt and the Rio Grande trough. with a system of Precambrian shear zones in the basement rocks (Tweto and Sims, 1963). In the quadrangle the mineral belt intersects basin-and- range faults of the Rio Grande trough or graben sys- tem and widens abruptly, as compared with its northeastern part (fig. 1). A program of gravity and magnetic surveys was begun in 1962 in this area of intersection and south- ward along the basin—and-range fault system of the upper Arkansas River valley. These geophysical stud- ies were undertaken to complement and extend the geologic studies by defining more clearly the nature GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30—MINUTE QUADRANGLE, COLORADO of the mineral belt in depth, and to serve as an aid in the geologic and economic appraisal of an areally significant segment of the belt that is buried beneath thick alluvial deposits of the downfaulted upper Arkansas River valley. This report on the Leadville 30-minute quadrangle presents the results of the geophysical surveys in the northern part of the area investigated. The Leadville quadrangle is centered over the Mos- quito-Tenmile Range, an element of the Park Range, in the heart of the mountain province in Colorado. (The Mosquito-Tenmile Range is a single range, but the part of it south of the Continental Divide is called the Mosquito Range, and the part north of the Divide is called the Tenmile Range.) In the northern part of the quadrangle, the Tenmile Range is overlapped on the west by the south end of the Gore Range, also an element of the Park Range (fig. 1). The western part of the quadrangle, west of the valleys of the Arkansas and Eagle Rivers, is in the Sawatch Range. The southeastern corner of the quadrangle is in South Park, and the northeastern corner is on the western toe of the Front Range. The crests of both the Mosquito—Tenmile and the Sawatch Ranges are generally above 13,000 feet in elevation, and they include many peaks that exceed 14,000 feet. The intervening valleys of the Arkansas and Eagle Rivers are at elevations of 9,000—10,000 feet, as is South Park. Thus, most gravity observa- tions were subject to large terrain corrections, and aeromagnetic flight lines, flown at a constant baro- metric elevation of 14,500 feet, varied greatly in height above the ground. GEOLOGY MAJOR FEATURES Geologically, almost the entire area of the Lead- ville quadrangle is a part of the eastern flank of the huge Sawatch anticline. The Sawatch Range, which extends both west and south beyond the quadrangle (fig. 1), is carved in Precambrian rocks in the core of this anticline. The eastern flank of this crystalline— rock core is cut longitudinally by a major graben that constitutes the valley of the upper Arkansas River. The graben and valley, deeply filled with Tertiary deposits, extend far to the south of the quadrangle, and the graben continues southward through the San Luis Valley (fig. 1) to a continuation with the Rio Grande graben of New Mexico, a major arm of the basin-and-range structural province. The graben tapers to an end in the Leadville quadrangle, but the fault system along its east side continues northward beyond the quadrangle. C3 The upthrown block on the east side of the graben constitutes the Mosquito—Tenmile Range. This range consists of Precambrian rocks capped by Paleozoic sedimentary rocks. As part of the Sawatch anticline, the sedimentary rocks dip eastward into South Park, forming a homoclinal eastern flank of the Mosquito Range. South Park is a broad structural basin separating the Mosquito and Front Ranges. Precambrian rocks occupy about half of the quad- rangle area (pl. 1). In the other half, Paleozoic and younger sedimentary rocks, having a preserved thickness of as much as 10,000 feet, overlie the Precambrian rocks. Both the Precambrian and the sedimentary rocks are cut by many bodies of hyp- abyssal intrusive rocks of Late Cretaceous and Tertiary age and of granodioritic to granitic com- position. Upper Tertiary alluvial deposits and Pleis- tocene glacial deposits occupy a large part of the upper Arkansas valley, and glacial deposits mantle many of the valley bottoms elsewhere in the quad- rangle. PRECAMBRIAN ROCKS The Precambrian rocks are divided, for geophysi- cal analysis, into metasedimentary gneisses and gra- nitic rocks. The metasedimentary gneisses are largely biotite gneiss and migmatite, but in the northern part of the quadrangle they include hornblende gneiss, calc—silicate gneiss, and impure quartzites that are markedly less magnetic than the bordering biotite gneiss and migmatite. Most of the biotite gneiss is biotite-quartz-plagioclase gneiss, but sillimanite— and garnet—bearing varieties also are common. Gneisses of the Tenmile Range were described by Koschmann (1960) and by Bergendahl (1963). Those elsewhere in the quadrangle are generally similar to the gneiss- es in the Front Range, as described by Sims and Gable (1964, 1967). The granitic rocks are of several varieties, but are principally the St. Kevin Granite in the west-central part of the area, the Denny Creek Granodiorite Gneiss in the southern part, and the Silver Plume Granite in the northeastern part. The St. Kevin Granite consists of at least five distinct textural and compositional varieties (Tweto and Pearson, 1964), but most of it is an even—grained to markedly por— phyritic granitic rock that compositionally is near the borderline between granite and quartz monzonite. It is characterized by the presence of muscovite, in addition to biotite. Other varieties range from this composition to one near the borderline between quartz monzonite and granodiorite. The Denny Creek Gra- nodiorite Gneiss (Barker and Brock, 1965) ranges in composition from biotite granodiorite to biotite- C4 GEOPHYSICAL FIELD INVESTIGATIONS quartz diorite, and it generally contains some horn- blende and unevenly distributed augen of microcline perthite. Silver Plume Granite, as applied in the Climax-Alma area (Butler and Vanderwilt, 1933; Singewald and Butler, 1941), is a porphyritic biotite- muscovite-quartz monzonite (Koschmann, 1960) . The Denny Creek belongs to the group of older, syntec- tonic Precambrian granites of Colorado, exemplified by the Boulder Creek Granite of the Front Range, 1.7 by. (billion years) in age (Peterman and others, 1968). The St. Kevin and Silver Plume belong to a middle, late syntectonic—early posttectonic group, about 1.4 by. in age (Pearson and others, 1966). SEDIMENTARY ROCKS The pre-Tertiary sedimentary rocks are grouped in two units on the map. The lower unit consists of quartzites and dolomites that range in age from Cambrian through Mississippian (table 1). It is only 500—1,000 feet thick, but it is of prime interest eco- nomically because it contains the host rocks of major ore deposits, as at Leadville. The upper unit consists principally of sandstones, conglomerates, and shales of Pennsylvanian and Permian age and is as much as 10,000 feet thick. Near Breckenridge the lower unit is absent, and the upper unit consists only of a thin sequence of Pennsylvanian or Permian rocks and of overlying Mesozoic formations. The contrast in the sedimentary sequence in the Breckenridge area as compared with that in the rest of the quadrangle reflects the effects of the late Paleozoic and early Mesozoic Front Range highland, an element of the so-called ancestral Rockies. Most of the Leadville quadrange is in the area of a basin of sedimentation on the southwest side of this high- land. This area is characterized by a thick sequence of Pennsylvanian and Permian rocks constituting the Belden, Minturn, and Maroon Formations, as well as by the presence of pre-Pennsylvanian sedi- mentary rocks (table 1). The Breckenridge area, in contrast, was on the edge of the highland; conse- quently, the pre-Pennsylvanian rocks were stripped from it in Pennsylvanian time, and only a few hun- dred feet of the Maroon and Chinle Formations are preserved beneath the Morrison Formation there (Lovering, 1934; Singewald, 1951). As judged from the distribution of pre—Jurassic rocks, the boundary of the highland extended generally westward from the Breckenridge area to the vicinity of Copper Mountain (pl. 1), where it turned north-northwest- ward. Thus, only a few square miles in the northeast corner of the Leadville quadrangle was on the high- land proper. The Cenozoic sedimentary rocks, shown as a single unit on the map (pl. 1) , consist at the surface of the upper Tertiary Dry Union Formation and extensive Pleistocene glacial deposits (Tweto, 1961). This unit reaches a geophysically significant thickness only in the Arkansas valley, although the glacial deposits are widespread. In the Arkansas valley, unexposed older Tertiary deposits may lie beneath the Dry Union Formation. The Dry Union, of Miocene and Pliocene age, is known—from surface exposures, mine shafts, and drill holes—to be at least 1,000 feet thick in the area just southwest of Leadville. Farther out in the Arkansas valley, the Tertiary sediments may be as much as 4,000 feet thick, as indicated by the geo- physical data presented in this report. It is unlikely that all these strata belong to the Dry Union Forma- tion. They may include equivalents of the Miocene Browns Canyon Formation of the Salida area (Van Alstine, 1969) and possibly even of the Oligocene Antero Formation of Stark and others (1949) , which occurs on the east side of the Mosquito Range, in South Park (Stark and others, 1949; DeVoto, 1964). N ear-surface stratigraphy of the fill in the Arkan— sas valley is discussed in a following section on geology of the Arkansas valley bottom. UPPER CRETACEOUS AND TERTIARY INTRUSIVE ROCKS Intrusive igneous rocks of Late Cretaceous and Tertiary age occur in stocks, plugs, crudely lacco- lithic bodies, sills, and dikes. The sills and dikes num- ber in the thousands, but owing to limitations of scale, only the largest among them are shown on the map (pl. 1). Most of the intrusive rocks are porphy- ritic, and rocks of the group as a whole are generally referred to as porphyries. Compositionally, the por- phyries range from diorite to granite, but most of them are granodiorite or quartz monzonite. More than 50 different varieties of porphyry have been distinguished in the quadrangle. Many of the porphyries fit into an empirical age sequence as determined from geologic relations (Tweto, 1960). Radiometric dating by the K—Ar (potassium-argon) method has established the pres- ence of two main age groups among the porphyries —an early one that is Late Cretaceous and early Tertiary (Paleocene) in age (Pearson and others, 1962) , and a later one that is middle Tertiary (Oli- gocene) in age (Tweto, 1968a; Wallace and others, 1968). A few radiometric dates corresponding to an Eocene age raise a question as to whether intrusion was fairly continuous from Late Cretaceous to the end of the Oligocene, or whether the intermediate age values reflect argon leakage from Late Cretaceous and Paleocene intrusives by heating during the Oli— gocene intrusive episode. This question is thus far GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30—MINUTE QUADRANGLE, COLORADO C5 TABLE 1.—Generalized geologic column, Leadville 30-minute quadrangle Age Unit Tfiifel‘elzfss Character and remarks Alluvium, morainal deposits, Quaternary and outwash gravels 0—500+ Loosely consolidated sandy silt, sand, and '2 Dry Union Formation 0_2’000(?) gravel. Mlocene and Pliocene 1n age; may be S underlain by unexposed older Tertlary units 8 in Arkansas valley. (50 Tertiary Porphyries of many varieties; mainly quartz monzonite, but range from granodiorite to Intrusive igneous rocks granite in composition. Mainly of two gen- eral ages: (1) Late Cretaceous and early Tertiary and (2) Oligocene. Pierre Shale Max1mum Of 500 Shale ft preserved Niobr ar a Formation 350 Calcareous shale and lime- Cretaceous stone 0 E Benton Shale 360 Shale Present only in g Dakota Sandstone 125_175 Sandstone, quart21te, and area near 2 shale Breckenridge. Morrison Formation 200—300 Sandstone, shale, and lime- Jurassic stone Entrada Sandstone 20—50 Sandstone Triassic Chinle Formation 250—400 Siltstone Permian and Maroon Formation 1,000—4,500 Sandstone, grit, conglomerate, and shale. Pennsylvanian _ Minturn Formation 3,600—6,000 Grit, conglomerate, sandstone, limestone. Pennsylvanian Belden Formation 0-1,200 Shale, sandstone, limestone. o . _ . '8 Mississippian Lea dville Dolomite 0_210 Dolomlte, Gllman Sandstone Member at base, 8 15—25 ft. 0 '6 . . Dyer Dolomite Member at top 0—80 ft; Parting D Ch if F t 0-135 ’ A evonian a ee orma lon Quartzite Member at base, 0-55 ft. , _ Harding Sandstone 0—40 Sandstone, quartzite, and shale. Ordov1c1an Manitou Dolomite 0—150 Cherty dolomite. , Peerless Formation 0—100 Sandstone and dolomite. Cambrlan Sawatch Quartzite 0—190 Quartzite. g St.p11{:;1;n(:::n:::er Quartz monzonite and granite. b4 .9 . . E Denny Creek Granodiorlte Granodiorite and quartz diorite. g Gneiss and related rocks 0.) fi Metasedimentary gneisses 1 ? Biotite .gneiss, migmatite, hornblende gneiss, calc-sxllcate gnelss, and impure quartz1tes. unresolved. Minor late intrusive activity, suggested by geologic relations to be late Tertiary, produced small rhyolite dikes and, at Leadville, rhyolitic ex- plosion breccia pipes (Emmons and others, 1927). Porphyries of the older group occur principally as widespread sills or subconcordant sheets in the sedi- mentary-rock terrane (pl. 1) and as a few small stocks. The earliest of all the porphyries, the Pando Porphyry (including the White and Mount Zion Por- phyries of earlier usage), is the most widespread. It occurs throughout the length of the quadrangle as sills or sheets, some of which are more than 1,000 feet thick, and as a small stock just north of Lead- ville (Prospect Mountain stock, fig. 2). Other major porphyries that occur principally as sills are the Elk Mountain Porphyry of the Kokomo area, the 466—308 0 - 72 - 2 C6 GEOPHYSICAL FIELD INVESTIGATIONS 393186 030, 15' 106°OO’ / // I 6) / 3 f / Breckenridgeo 0. 9‘ f f/ // f ““133 o " / / ff $70 oPando ‘* 3d 0 HUMBUG 0 /Holy Cross / G// o STOCK ft @? / o . K. / ' y 9% / 6‘36 / district Q, / ‘{~ '\9 oKokomo 4. //// 6 / 99"/_/ / ff” /¢o‘“/ “(Ami-”ff 4MISSOURI Climax CHALK MOUNTAIN A . 7 CREEK STOCK STOCK Clirgax r dIstrIct ‘ Position of Climax stock, concealed .06" -§‘ BUCKSKIN b STOCK PROSPECT MOUNTAIN St. Kevin STOCK district oAlma Sugarloaf 15' — d'Str'Ct OLeadville — Leadville district Horseshoe district TWIN LAKES STOCK BLACK MOUNTAIN STOCK 39°OO’ 'Tv‘i/inxLakes district _. ' Twin Lakes Reservoir ROUGH AND TUMBLI NG STOCK 5 10 MILES 1 FIGURE 2. — Major structural features, stocks, and mining districts in the Leadville 30-minute quadrangle. Bar and ball symbol indicates downthrown side of fault. GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30-MINUTE QUADRANGLE, COLORADO Lincoln and Sacramento Porphyries of the areas north and east of Leadville, the Evans Gulch and Johnson Gulch Porphyries of the Leadville district, and the various unnamed porphyries in the Alma and Breckenridge districts. Chemically, all these porphy- ries are near the borderline between granodiorite and quartz monzonite, although the Pando Porphyry is abnormally siliceous (72 percent Si02) for such a composition. The composite Buckskin stock, west of Alma, has been shown by Singewald (1932) and by Kuntz (1968) to consist of successively intruded quartz diorite, granodiorite, and quartz monzonite inferred to have been derived from a quartz diorite body at depth. The quartz monzonite facies was cor- related with the Lincoln Porphyry by Singewald (1932). If the porphyries that have yielded intermediate or Eocene K—Ar ages are, in reality, older, as noted in the preceding paragraphs, then the older group of porphyries also includes the two largest stocks in the quadrangle —- the Twin Lakes stock and the Humbug (or Bald Mountain) stock (fig. 2). The Twin Lakes stock, only part of which lies within the quadrangle, consists of coarsely porphyritic grano- diorite (Wilshire, 1969) that chemically resembles the Pando Porphyry in its high content of Si02 and low content of Fe303, FeO, MgO, and CaO. The Twin Lakes stock is cut by fine-grained white dikes that closely resemble the Pando. Biotite from the grano- diorite porphyry has been dated as 41.7:12 m.y. (million years) in age by the K—Ar method, and 49.7 24.5 m.y. by the Rb—Sr (rubidium-strontium) method (Obradovich and others, 1969), and also as 56:10 my by the Rb—Sr method (Moorbath and others, 1967). The Humbug stock occupies an area of 4—5 square miles on the western slope of the Tenmile Range, about 5 miles north of Climax. The rock of the stock has been described as quartz monzonite by Crawford (1924) and Bergendahl (1963). Four K—Ar age determinations made for the Climax Molybdenum Co. (V. E. Surface, written commun., 1970) yielded disparate results ranging from 35 to 47 my If the stock is older than the Lincoln Porphyry, as indi- cated by Bergendahl (1963, p. 15 and map), its true age should exceed 64 my (Pearson and others, 1962). Porphyries of middle Tertiary age occur mainly in crosscutting bodies, such as stocks, plugs, and dikes, but also in a few domed sills of limited extent. Most notable of these younger intrusives is the Climax stock, to which the great Climax molybdenum deposit is genetically related (Wallace and others, 1968) . The C7 stock is not exposed at the surface, but it is known from subsurface exploration to be about 3,000 feet in diameter and to be a sodic granite in composition. Several K—Ar determinations indicate that its age is about 30 my (Wallace and others, 1968). Rhyolite or nevadite porphyry of the Chalk Mountain stock, just west of Climax, has a K—Ar age of 27:19 my, and a plug of rhyolite porphyry a few miles to the north has been dated as 35.2:14 my in age (V. E. Surface, written commun., 1970). Other intrusive bodies, assigned to this age group are (1) numerous dikes of “late white porphyry” in the Mosquito Range east of Climax (Singewald and Butler, 1941, pl. 1) and east of Leadville (Behre, 1953, pl. 1) ; (2) late leucocratic porphyries in the intrusive complex in the eastern part of the Leadville district (Tweto, 1968b, p. 700) ; (3) a rhyolitic breccia plug near the head of Lake Fork, in the Sawatch Range; and (4), tentatively, two rhyolite porphyry stocks near the southeast corner of the quadrangle, in South Park. Possibly also of this age, but more likely of younger age still, are rhyolitic dikes and a small elongate plug along a north-trending fault on the lower slopes of the Sawatch Range northwest of Leadville. The change with time from granodioritic to gra- nitic or rhyolitic compositions among the exposed porphyries suggests progressive differentiation of the underlying source batholith, and this implies that the batholith itself is compositionally zoned. Its up— per part may approximate granite in composition, and the composition probably changes progressively downward to quartz diorite or diorite at a depth of a few miles. Evidence of a composition as mafic as diorite is seen in the early quartz diorite facies of the Buckskin stock, and in scattered dikes and small plugs and sills of diorite as mafic as augite diorite. For purposes of geophysical computations in later sections of the report, the average composition of the batholith is assumed to be that of the early porphyries — that is, granodiorite near quartz mon- zonite. TERTIARY VOLCANIC ROCKS Near the southeast corner of the quadrangle, Ter- tia‘f‘y volcanic rocks cover an area of about 1 square mile (pl. 1). These rocks are a part of a small vol- canic field at Buffalo Peaks on the crest of the Mos- quito Range, 1/2—1 mile south of the quadrangle. The volcanic rocks, described by S. F. Emmons (in Cross, 1883), consist of hypersthene andesite, hornblende andesite, tufl’s, and breccias that have an aggregate C8 thickness of as much as 1,500 feet. They lie hori- zontally on tilted Paleozoic rocks and on Precambrian granite. The volcanic rocks apparently cover strands of the Weston fault (pl. 1; also Gould, 1935), but the distribution of the hypersthene andesite, as com- pared with the other volcanic rocks described by Emmons, would permit a major fault through the volcanics. No direct data on the age of the volcaniCs —— within the Tertiary — are available. The rocks are tentatively classed as outliers of the extensive Thirty- nine Mile volcanic field of southern South Park, which is Oligocene in age (Epis and Chapin, 1968). STRUCTURE PRECAMBRIAN HOMESTAKE SHEAR ZONE Aside from an intricate fold structure in the meta- sedimentary gneisses, not considered in this report, the principal Precambrian structural feature in the quadrangle is the Homestake shear zone (fig. 2). This master shear zone, which has a pronounced magnetic expression, consists of many individual shear zones of northeast trend in a belt 7—8 miles wide. The individual shear zones range in width from a few feet to several hundred feet. Collectively, the shear zones are characterized by a wide variety of cataclastic rocks, ranging from completely recrystal- lized biotite gneisses through phyllonites, blastomy— lonites, pseudotachylyte, and mylonites to gouge (Tweto and Sims, 1963). Some of the individual shear zones contain scattered small lenses of horn- blendite, and the master shear zone is bordered on the southeast by a swarm of metalamprophyre dikes. The Homestake shear zone disappears beneath Paleo- zoic sedimentary rocks east of the Eagle River, but elements of it reappear in the Precambrian rocks of the Gore Range, north of the Leadville quadrangle (Tweto and others, 1970). LARAMIDE AND YOUNGER STRUCTURE Structural features considered in this report are principally the faults. The sedimentary rocks are folded only locally, near faults, and have a regional easterly dip (pl. 1, sections A—A’ and B—B’) that re- flects the location of the quadrangle on the eastern flank of the huge Sawatch anticline. A complex system of faults, only the largest of which are shown on plate 1, characterizes the Mos— quito Range and both sides of the Arkansas valley. Many of these faults originated during the Laramide orogeny, but they were reactivated later to serve as elements of the basin-and-range fault system. Some, however, seem to be principally Laramide, and some may be principally basin-and-range in time of major movement. GEOPHYSICAL FIELD INVESTIGATIONS The master fault along the west side of the Mos- quito—Tenmile Range, from Iowa Gulch northward to the quadrangle boundary and beyond, is the Mos- quito fault. This is an early Laramide west-dipping normal fault that was in existence before intrusion of the porphyries. A concentration of intrusive rocks in a belt along the fault, particularly on its west side, suggests that it was a conduit at depth for porphyry magmas (Tweto, 1968a, b). Near the north boundary of the quadrangle, the fault shows indications of Pre- cambrian ancestry (Tweto and Sims, 1963), as does the intersecting Gore fault (fig. 2), but no indica— tions of this early history are known farther south. Near Leadville, total vertical displacement on the Mosquito fault and on the many subsidiary faults to the west of it is about 12,000 feet. Near Climax, the displacement may be as much as 14,000 feet (pl. 1, section A—A’). Of this amount, a significant part is post-Laramide. On the basis of one interpretation of faulted ore bodies, Wallace and others (1968) esti- mated that 9,000 feet of displacement has occurred on the main strand of the Mosquito fault since the mineralization stage in Oligocene time. Alternative interpretations might be made, but they would still indicate a post-Laramide displacement of at least a few thousand feet. Two miles south of Climax, slices of the Dry Union Formation (too small to be shown on pl. 1) occur in the Mosquito fault zone, indicating that much of the late displacement was Pliocene or younger. These slices are at an elevation of 11,900 feet, nearly 2,000 feet higher than the Dry Union of the Arkansas valley; thus, they also indicate exten- sive displacement on subsidiary faults between the Mosquito fault and the valley, as do faults at Lead- ville (Tweto, 1968b). Near Iowa Gulch the north-northeast-trending Mosquito fault intersects the north-northwest-trend- ing Weston fault. The main line of Laramide dis- placement extends south-southeastward along the Weston fault from this locality, and the line of younger fault displacement, possibly following Lara- mide faults, continues southward along the Mosquito fault trend to a connection with the border faults of the Arkansas valley (pl. 1). The Weston fault, a steep reverse or vertical fault, has a displacement of about 4,000 feet, with the east side upthrown (pl. 1, section B—B’). The displacement decreases south of Weston Pass, where the fault crosses the present drainage divide of the Mosquito Range. At the south edge of the quadrangle, the fault is in several strands that have displacements only in the hundreds of feet, and some of these fault strands are evidently covered by the Oligocene ( ?) volcanic rocks of Buffalo Peaks. The principal fault on the east side of the Mosquito Range is the London fault. From an intersection with GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30-MINUTE QUADRANGLE, COLORADO C9 the Mosquito fault, northeast of Leadville (pl. 1; fig. 2), this fault extends south-southeastward for many miles into South Park. The London is an eastward- dipping reverse fault that has a displacement of about 3,000 feet (pl. 1, section B—B’). The Mosquito, London, and Weston faults all have had stocks intruded along them (fig. 2), and their relations to the oldest porphyries indicate that they existed before porphyry intrusion began. Many of the faults subsidiary to these main faults, as at Leadville, are somewhat younger, having formed during the period of intrusion of the early porphy- ries (Tweto, 1960, 1968b). Many of these subsidiary faults, particularly those of near northward trend, and the Mosquito fault evidently underwent repeated movements in later time, in conjunction with the sinking of the Arkansas valley graben. Major move- ment on them occurred in Miocene or earlier time, forming one flank of the structural valley in which the Dry Union and possibly older formations accumu- lated. Still later movements displaced the Dry Union Formation and even the glacial deposits (Tweto, 1961) . Evidence of such a history on the west side of the graben is not as clear because rocks younger than Precambrian are virtually absent. However, one of the north-trending faults characteristic of the slope of the Sawatch Range contains dikes that indicate a generally similar history. This fault, which extends from Sugarloaf Mountain to West Tennessee Creek (pl. 1), contains local dikes of two or three different porphyries, all of which are crushed and altered, in- dicating movement on the fault since their emplace- ment. One of the porphyries is the Lincoln Porphyry, which indicates that the fault is at least as old as the early porphyries. An irregular dike of undeformed rhyolite also follows the fault. This dike is charac- terized by wide chill zones of flow-banded vitrophyre, suggesting that it was emplaced close to the present surface and, hence, that it is very young. GEOLOGY OF THE ARKANSAS VALLEY BOTTOM The Arkansas valley has a graben structure (fig. 2) that constitutes the northern tip of the great Rio Grande—San Luis Valley graben system of New Mex- ico and southern Colorado (fig. 1). Near the south edge of the Leadville quadrangle, a local high block of Precambrian rocks forms a ridge across the bot- tom of the graben, thus creating a subbasin that extends from the ridge northward to the tip of the graben, about 3 miles northwest of Leadville. This basin is deeply filled with sediments of the Dry Union Formation and mantling glacial deposits. Similar de- posits fill the valley south of the constricting ridge, which extends for about 1 mile south of the quad— rangle boundary. The geology of the valley bottom is of special concern because the bedrock along a 12-mile-Iong axial segment of the mineral belt is buried beneath the thick fill. The fact that the Leadville and Sugar- loaf-St. Kevin mineralized areas are directly across the valley from each other (fig. 2) and extend to the edge of the valley fill suggests that some potential for mineralization exists beneath at least that part of the valley. Geology of the valley bottom is dis- cussed here in somewhat greater detail than for other areas or geologic units because of its pertinence to any physical or geophysical exploration that might be undertaken in this covered area. Most of the fill in the valley consists of the Dry Union Formation plus whatever unexposed older Tertiary units may be present beneath it. The Dry Union (Tweto, 1961) is varied in character, but its principal component is brown sandy silt, or pebbly silt, which occurs in structureless layers 5—50 feet thick. Lenses and beds of coarse sand and gravel are scattered among the silt beds, and some of these are cemented to a tough sandstone or conglomerate by the calcium carbonate of old caliche zones. Across most of the Width of the valley, the Dry Union con- sists of assorted materials derived, in part at least, from the Mosquito Range; only in a narrow zone along the west side of the valley does it consist ex- clusively of materials derived from the Precambrian rocks of the Sawatch Range. Wherever seen in out- crop, the Dry Union dips westward. Even as far west as the slope north of the western, or upper, lake of Twin Lakes Reservoir, less than a mile from the western border fault of the valley, the Dry Union dips 9° W. After extensive fault movements that placed Dry Union against bedrock in many places (Tweto, 1961) and, also, after deep stream dissection of the Dry Union, extensive parts of the valley were covered by an ancient till (till No. 1). Except in the vicinity of Twin Lakes, where it contains abundant volcanic rocks derived from the Sawatch Range near the head of Lake Creek, this till is characterized by the pres- ence of quartzite, phenocrystic quartz crystals from porphyries, and other resistant materials derived from tlhe east side of the valley. The till is also characterized in most places by a thick, intensely weathered zone at its top that consists of tough reddish-brown gumbo. After deposition of the No. 1 till, extensive pedi- ments formed on the sides of the valley, cut on till No. 1 and the Dry Union Formation. These pedi- ments have subsequently been displaced by faults, giving an impression of terrace remnants at various 010 levels. After some dissection of the pediments, an extensive glacial advance produced till No. 2 in mas- sive moraines, and also a very extensive blanket of outwash gravel. This gravel, called the Malta Gravel (Tweto, 1961), is more than 300 feet thick in the Leadville area, where it fills old valleys and caps the earlier, somewhat eroded pediments. The gravel also formed a thick blanket over the bottom of the Arkan- sas valley westward to the slopes of the Sawatch Range. Large parts of the present valley bottom from the mouth of the East Fork Arkansas River south- ward are floored on this gravel. After an erosion interval during which till No. 2 and the Malta Gravel were weathered and dissected and were also displaced by fault movements, succes- sive glacial advances produced tills No. 3, 4, 5, 6, and 7. Stratigraphic relations of these tills are evident from the positions of various moraines but are best shown in test holes drilled by the US. Bureau of Reclamation in the Turquoise Lake and Twin Lakes areas. Test holes 500 feet deep on the large moraine enclosing Turquoise Lake showed as many as four superposed tills to a depth of 380 feet, below which is the Dry Union Formation. Each till is marked by a weathered and stained top. In places in the Twin Lakes area, black soils are present at the tops of some weathered zones. In this same area, tills Nos. 4 and 5 are separated by as much as 225 feet of laws— trine ashy silts and sands and blue clay, indicating both a pronounced break in glacial deposition—pre- sumably, the break between the Bull Lake and Pine— dale Glaciations—and, possibly, the occurrence of local volcanism at that time. Data on depth and configuration of the bedrock surface beneath the valley fill are scant, except along the borders of the valley bottom. In the area just west and southwest of Leadville, scattered shafts and drill holes and a single small outcrop of Lead- ville Dolomite at the pediment surface (pl. 1) indi- cate that the depth to bedrock ranges from zero to more than 600 feet, and that the bedrock surface is extensively faulted. No information is available on the depth or character of the bedrock in the area from the edge of the pediment on the east side of the valley westward to Lake Fork and Turquoise Lake, except that the depth probably considerably exceeds 500 feet. “Bedrock” was reported by the drillers at average depths of 40—60feet in 88 churn- drilled placer test holes bored in this area in 1946—47, but it was interpreted at the time by Tweto as prob- ably being hard layers in what is now known as the Dry Union Formation. Partly on the basis of the 1Swartz, J. H., Farnham, F. C., Scharon, J. L., and Raspet, R., 1943, Report of a geophysical survey of proposed drainage-tunnel routes in the Leadville mining district, Colorado: US. Bur. Mines unpub. report, 18 p. GEOPHYSICAL FIELD INVESTIGATIONS reported bedrock, refraction seismic data obtained by the US. Bureau of Reclamation (Conwell, 1950) were interpreted to indicate bedrock at depths of 25—35 feet beneath the valley bottom due west of Leadville, as P-wave velocities of 7,000—8,000 feet per second were obtained for a layer at this depth. Subsequent drilling by the US. Bureau of Reclama- tion has proved that the placer and seismic “bed- rock” is the gumbo at the top of till No. 1. The drilling also established that various other seismi- cally refractive layers in the moraine area at Turquoise Lake, distinguished by P—wave velocities of as much as 10,500 feet per second, are till sheets with weathered tops. Detailed gravity, magnetic, and seismic surveys made in 1969 by Earth Sciences, Inc., of Golden, Colo. (Duane Bloom, written commun., 1970), indi- cated faults in the bedrock and (or) overlying ma- terials in the part of the valley discussed in the preceding paragraph. These data support the in- ferred north-northwest-trending faults that pass just west and east of Malta (pl. 1). At Lake Fork just below Turquoise Lake, several drill holes proved that the Malta Gravel, till No. 1, and the Dry Union Formation are faulted against Precambrian rocks along the main western border fault of the Arkansas valley. The fault there is com- plex and consists of at least three or four strands in a zone a few hundred feet wide. A mile to the south, a vertical drill hole penetrated outwash gravel that is tentatively correlated with till No. 3 and bottomed in a gouge zone 40 feet thick in shattered Precam- brian rocks. A few hundred feet southwest of this locality, a ditchbank exposure at the foot of the slope of the Sawatch Range shows the Malta Gravel to be in fault contact with Precambrian rocks. Refraction seismic investigations, largely experi- mental, were made by the US. Geological Survey in 1941—42 in the area between Leadville and the mouth of Iowa Gulch.1 Many of the interpretations of sub- surface geology based on the seismic survey were questionable even at that time (J. H. Swartz, oral commun., 1942), and, viewed in light of the subse- quent drilling and seismic work at Turquoise Lake, they are now even more questionable. However, the survey did show a high-velocity refracting surface (12,500—15,800 feet per second) at a depth of about 1,300 feet near the mouth of Iowa Gulch that prob— ably represents bedrock, presumably either Precam— brian rock or lower Paleozoic quartzites and dolomites. The data also showed abrupt changes of as much as 750 feet in the level of this surface, sug- gesting fault scarps or deep, steep-sided canyons in the bedrock surface. GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30-MINUTE QUADRANGLE, COLORADO Test holes drilled by the US. Bureau of Reclama- tion near the Twin Lakes Reservoir established a complex fault pattern in the bedrock and valley fill in that area. Although a canyon topography in the bedrock surface cannot be precluded completely, the drill-hole data strongly suggest that the fill is deeply downfaulted against Precambrian rocks east and south of the lakes. Within the area of deep fill is at least one elevated block of Precambrian rocks, be- neath the eastern part of the eastern lake. Granite is exposed at the surface in a small area along the north shore of the lake (pl. 1), but drill holes a few hundred feet to the west and north bottomed in the Dry Union Formation at depths of as much as 150 feet. A drill hole on the east shore of the lake reached Precambrian bedrock at a depth of 271 feet. Possibly, this locality is part of the block that crops out at the north edge of the lake. Half a mile north of this hole, however, and half a mile east of the granite outcrop, a 503«foot hole bottomed in till No. 1 (or possibly an even earlier till). This till, which was continuous from a depth of 75 feet to the bottom of the bore hole at 503 feet, seems to be in a block bounded by Precambrian bedrock on one side and by the Dry Union Formation on the other. The block is inter- preted as a fault block, although it conceivably could be a prism of fill in a steep-walled canyon. The ridge that is about 500 feet high on the north side of the Twin Lakes Reservoir has long been classed as a huge moraine (Westgate, 1905; Capps, 1909; Ray, 1940). In reality, however, it is a valley wall cut in the Dry Union Formation. This valley wall is only thinly mantled by moraines, as shown by both sur- face exposures and drill holes. Large areas of the valley wall have been modified by intraglacial and postglacial landslides. ORE DEPOSITS The Leadville 30-minute quadrangle is in the heart of the most productive part of the Colorado mineral belt, and all parts of it, except the southeast corner, show evidence of mineralization to some degree. Pro— ductive areas constituting generally recognized but vaguely defined mining “districts” are indicated in figure 2. Somewhat mineralized areas between these districts, as well as the numerous districts them- selves, constitute a large and promising hunting ground for exploration targets of the future. Most of the districts have been described in detail in various reports that are referred to in the following brief outlines. Climax district. —— The Climax district, or molyb- denum deposit, occupying an area of no more than 1 square mile, is the premier district of the quad- Cll rangle in terms of production and of known produc- tion potential. Total value of molybdenum produced from the district through 1969 probably exceeded $1.5 billion; in addition, significant amounts of by- product tungsten and pyrite and a small amount of tin were produced. The Climax deposit (described by Wallace and others, 1968) is in Precambrian rocks above a small stock of sodic granite, of Oligocene age, in the footwall of the Mosquito fault. The ore is in stockworks in silicified and feldspathized rocks in three overlapping dome—shaped ore bodies related to three phases of intrusion of the underlying Climax stock (Wallace and others, 1968). Leadville district. —— Prior to the period of great productivity at Climax, which began in 1940, Lead- ville was the leading district in the quadrangle and, indeed, the most productive mining district in Colo- rado. Production from the district from 1859 through 1963 was about $512 million (as valued at time of production) in silver, zinc, lead, gold, copper, and various minor metals (Tweto, 1968b). Mining opera- tions in the district closed in 1957, but in 1969 de- velopment work in preparation for relatively large scale operations was started in the eastern part of the district. Ore bodies at Leadville are mainly re- placement bodies in the pre-Pennsylvanian dolomites (table 1), but some are in veins and in minor stock- works. The sedimentary rocks are complexly intruded by porphyries and broken by numerous faults, many of which had both preore and postore movement. The district was described by Emmons (1886), by Emmons, Irving, and Loughlin (1927), and by Tweto (1968b). Bordering areas to the east and south were described by Behre (1953). Breckenridge district. —- The Breckenridge district has produced about $35 million in gold, zinc, lead, and silver; nearly half of the total was in placer gold. The lode deposits are principally veins in Cretaceous rocks (table 1) and in porphyries that intrude these rocks. The district was described by Ransome (1911) and by Lovering (1934), and the area to the south of the district was described by Singewald (1951). Kokomo district. — The Kokomo district has pro- duced about $25 million in lead, zinc, and silver. The ore deposits are principally replacement deposits in limestone beds in the Minturn Formation, but a few are in veins. The districts also has some low—grade molybdenum deposits—thus far unexploited —— in porphyry. Geology and ore deposits of the district were described by Koschmann and Wells (1946) and by Bergendahl and Koschmann (1971). Holy Cross City district. —The Holy Cross City mining district is a small district characterized by 012 small gold-bearing feldspathic veins that cut Pre- cambrian rocks. Production probably has not ex- ceeded $1 million (Ogden Tweto and R. C. Pearson, unpub. data). Alma district. — The ore deposits of the Alma dis- trict (Singewald and Butler, 1941; Singewald, 1947) are scattered over a wide area on the eastern slope of the Mosquito Range, opposite the Climax and Leadville districts, but 70 percent of the lode-mineral production has come from a small area along the London fault. Total production from the district is approximately $45 million, of which $35 million was in gold, silver, and lead from lode deposits and about $10 million in placer gold (including placer produc- tion from downstream, in South Park). The prin- cipal deposits are veins in porphyries and Paleozoic sedimentary rocks, but significant production was also made from replacement deposits in the sedimen- tary rocks. Placer and minor lode deposits in the area east of Alma were described by Singewald (1942, 1950). Horseshoe district—The Horseshoe district (Singewald and Butler, 1941) has produced silver, lead, and zinc ores from vein and replacement de- posits in the Paleozoic dolomites. Total production is variously estimated to be $2—$5 million. The dis- trict was being actively explored in 1969. St. Kevin and Sugarloaf districts. — The St. Kevin and Sugarloaf districts, west of Leadville, are credited with a production of $10—$15 million, pri- marily in silver, from veins in Precambrian rocks. The ore deposits were described by Singewald (1955) and the geology was mapped in detail by Tweto and Pearson (1958). Weston Pass district. — The Weston Pass district PRECAM BRIAN ROCKS PALEOZOIC SEDI MENTARY ROCKS GEOPHYSICAL FIELD INVESTIGATIONS (Behre, 1932) is a minor district that has produced silver, lead, and zinc with an estimated value of about $125,000 from replacement deposits in the Leadville Dolomite. Granite district—The Granite district, thus far not studied in detail, has produced somewhat over $1 million from gold placer deposits west of the town of Granite, and probably less than $1 million in precious-metal ores from veins in the Precam- brian rocks east of the town. Twin Lakes district. — Widely scattered mines in the valley of Lake Creek, constituting the Twin Lakes district, have made a small output of gold- silver ores from veins in Precambrian rocks or in the granodiorite of the Twin Lakes stock. Low-grade molybdenum deposits in the part of the district just west of the quadrangle have not been mined. The ore deposits were briefly described by Howell (1919). DENSITIES AND MAGNETIC PROPERTIES OF THE MAJOR ROCK UNITS Densities and magnetic properties were measured on samples collected from some of the major rock units in the Leadville quadrangle and vicinity to aid interpretation of the geophysical maps. Sampling was not systematic, and the samples may not rep- resent true values of all the geophysicially significant units; nonetheless, some useful information on the values of these properties was obtained for the crystalline rock units. Densities were measured by W. E. Huff and Lee Peck. Magnetic properties were measured by W. E. Huff; measurements were con- ducted by standard techniques, and the results are shown in figures 3 and 4. LATE CRETACEOUS AND TERTIARY INTRUSIVE ROCKS All pre-Minturn 3Olllll|l|l| Biotite gneiss and migmatite . (20 samples) 20 l | I | | l l I | l l l I I I | I I m I I “I I —' I E I l I : < 15 — ' . . . I I _ U) wDenny Creek Granodlorite Gnelss I , . _ . _ LL and similar rocks (12 samples) IMOStIy S'I'C'C porphyries O | l (64 samples) 5 10 _ I Sandstone,quartzite,and | I; ——St. Kevin and Silver Plume I ffécareoazsfuartmte | ' 2 sam D _ Granites( 3 samples) I Limestone and dolo— I z 5 _ fl I mite (6 samples) I Amphibole gneiss ‘I I (4 samples) I 0 lll‘illir- l l m o l!) o m o in o In In 0 o m 0 M5 0 In 0 m 0 LO 0 m o In 0 m mownnwwmeOH nmqgnwew. ssenevwn N N N N N N N N N m m' m N N N N N N N N N N N N N N N N DENSITY, IN GRAMS PER CUBIC CENTIMETER FIGURE 3.—Densities and total samples of major rock units. GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30-MINUTE QUADRANGLE, COLORADO 50||l|l||l|lll .51 _ /Precambrian gneissic rocks (biotite gneiss, migmatite, and amphibole gneiss) 40 l 35 //St. Kevin and Silver Plume Granites 30 eé/Denny Creek Granodiorite Gneiss 25 and similar rocks — 20 NUMBER OF SAMPLES 15 ;/Paleozoic sedimentary rocks 10 i O O O ‘ .o.o202¢29’£= Tertiary porphyries\ i ll lllllll llllllllWZEEl <- m LO rx no ow o' o' 0' o' 0' o' a N to v I!) SUSCEPTIBILITY X10'3, IN CENTIMETER—GRAM—SECOND UNITS oo o co 0 0900' o 9:03... ’ ’ s o :0 A.A O o O 202 Q ’9’. 0. 0. o 0090 0.0 O 0.0: Q 2920’ «are 00 FIGURE 4. — Magnetic susceptibilities of rocks from the Leadville quadrangle and upper Arkansas River val- ley areas, Colorado. The Precambrian granitic rocks are not sub- divided on the geologic map (pl. 1), because of a lack of data for doing so in many places. However, the Denny Creek Granodiorite Gneiss and similar rocks differ significantly in both density and mag- netic properties from the St. Kevin and Silver Plume Granites. Consequently, the Denny Creek and the St. Kevin and Silver Plume are treated separately in the discussions that follow. In general, the data for Denny Creek rocks will apply to the granitic rocks of the southern and south-central parts of the quadrangle, and the data for the St. Kevin and Silver Plume rocks will apply to the granitic rocks of the west-central and northeastern parts of the quad- rangle. DENSITIES The Precambrian rocks consist of four major 'lithic units, each of a different density. The follow- ing tabulation lists average densities determined for 013 each of these units and the estimated volume pro- portion of each in the Leadville quadrangle, from which a weighted average density of 2.76 g/cm3 (grams per cubic centimeter) is calculated for the Precambrian rocks of the quadrangle as a whole. Num er Avera e Estimated Fr tion 1 Rock ofb densitgy pr‘b‘ggxr‘ztligon dile‘nsitya samples (g/cms) “ (percent) product St. Kevin and Silver Plume Granites ............ 23 2.64 20 0.53 Denny Creek Granodiorite Gneiss 12 2.71 25 .68 Biotite gneiss and migmatite .................... 20 2.75 40 1.10 Amphibole gneiss ............ 4 3.00 15 .45 Weighted-average density .................. 2.76 ‘g/cm“, grams per cubic centimeter. Only four samples of amphibole gneiss were col- lected from the Leadville quadrangle, but an average density of 3.00 g/cm3 is nearly the same as, or slightly greater than, samples of similar rocks from the Front Range (Tooker, 1963, table 18; Sims and Gable, 1964, table 19). If the average density of the amphibole gneiss is 2.9, the weighted average den- sity of all Precambrian rocks would be 2.74 g/cm3. Densities of 15 samples of lower Paleozoic quartz- ites, sandstone, and calcareous sandstone average 2.63 g/cm3, and those of six samples of limestone and dolomite average 2.80 g/cm3. Inasmuch as these two general lithologies are about equally abundant in the lower part of the stratigraphic section, the weighted average density for the pre-Belden rocks, thus, is about 2.71 g/cm3. Although the number of samples measured is small, this value seems reason- able. Perhaps the point to be emphasized is that the density of this thin unit (500-1,000 ft.) is not greatly different from the average for the Precam- brian rocks. The elastic rocks of the Belden, Minturn, and Maroon Formations are quartzo-feldspathic sand- stones and siltstones containing some shales. Den- sities of these rocks were not measured. The lithology is generally similar to that of the Cutler Formation of the Colorado Plateau, for which mea- sured dry densities average 2.50 g/cm3 (Byerly and Joesting, 1959, p. 41). Densities of the Upper Cretaceous and Tertiary porphyries present some uncertainties, primarily because completely fresh samples of many varieties are unobtainable. The weighted average density of porphyries in the Leadville quadrangle is about 2.62 g/cm3, as calculated from 53 samples shown in the following tabulation. Cl4 N m v Estimated . 1’03;er “orb“ 6.5359 “gym“ F3325?“ samples (g/cm°)' (percent) product Pando Porphyry . 4 2.61 20 0.52 Lincoln Porphyry .. . 9 2.61 18 .47 Sacramento Porphyry .. . 3 2.63 8 .21 Elk Mountain Porphyry ...... 5 2.57 7 .18 Johnson Gulch Porphyry 2 2.59 7 .18 Twin Lakes stock .................. 10 2.65 15 .40 Humbug stock ........................ 5 2.64 5 .13 Breckenridge-Alma area ...... 5 2.67 10 .27 All others ................................ 10 2.63 10 .26 Weighted-average density ...................... 2.62 *g/cm”, grams per cubic centimeter. The average density of 64 porphyry samples from 37 localities elsewhere in the Arkansas valley region is 2.64 g/cm3. Most of these samples are from the Mount Princeton batholith, which represents the largest exposed sample of the batholith presumed to underlie the mineral belt. Perhaps an average density of 2.63 g/cm3 is reasonable for the porphy- ries of the region. Little information is available on densities of the Dry Union Formation and of glacial deposits. A sample of typical sandy silt from the Dry Union in drill core had a dry density of 1.81. Another sample from the same core, but containing abundant frag- ments of Pando Porphyry, had a density of 2.02. As reported by Birch, Schairer, and Spicer (1942, tables 2—6), sandy silts from the Fort Union Formation range in density from 1.81 to 2.15 g/cm3, and sand- stones of the Fort Union range from 2.14 to 2.59 in saturated bulk density. Pakiser, Kane, and J ack- son (1964, p. 23—24) assumed an average density of 2.2 to 2.3 g/cm3 for Cenozoic deposits in the Owens Valley, Ca1if., region. A mean density of 2.3 g/cm3 was assumed for 7,000—8,000 feet of beds in the Rio Grande trough near Albuquerque, N. Mex., by Joesting, Case, and Cordell (1961). Perhaps a mean saturated density of 2.2 g/cm3 is reasonable for the Dry Union Formation. As judged from limited seismic data, the older glacial deposits may have similar densities; however, in most places the glacial deposits are too thin to influence the gravity in- terpretations. To summarize, the weighted average density of the Precambrian rocks is about 2.75 g/cm3, although the St. Kevin and Silver Plume Granites average only about 2.64 g/cm3. The average density of the porphyries is about 2.63 g/cm3, making a density contrast of about 0.12 g/cm3 between average Pre- cambrian rocks and the Cretaceous and Tertiary porphyries. The older sedimentary rocks have aver- age densities very near those of the Precambrian rocks, and the thick upper Paleozoic rocks have as- sumed densities of about 2.50 g/cm3. The Dry Union Formation is estimated to have a saturated density GEOPHYSICAL FIELD INVESTIGATIONS of 2.2 g/cm3, making a density contrast of —0.4 to ——0.5 g/cm3 with the adjacent crystalline rocks along the Arkansas valley. MAGNETIC PROPERTIES Numerous samples of the Precambrian rock units and of the porphyries were collected for measure- ment of induced and remanent magnetization. Such measurements provide merely a rough idea of the magnetization of a given rock unit. Where good ex— posures of the rock units are available, their ex- pression on the magnetic map is a better indication of combined magnetic properties. These properties can then be used as a guide for interpreting anomalies from buried sources. From analysis of the magnetic map and from laboratory measurements, the three major magnetic units—Precambrian granitic rocks, Precambrian metamorphic rocks, and the porphyries—have a wide range of magnetiza- tion. Some of them are virtually nonmagnetic, and others are very magnetic. Sedimentary rocks of the region are effectively nonmagnetic. Influence of surficial weathering on the magnetic properties of rock units should be kept in mind in evaluation of the magnetic data presented in figure 4. Magnetite, the commonest accessory magnetic mineral in crystalline rocks, is readily oxidized to such iron oxides as hematite, limonite, or goethite, which are only weakly magnetic. The rock samples collected were of varying degrees of freshness, so the measured magnetic susceptibilities collectively are probably lower than in rocks at depth, and some may be abnormally low because of surficial weather- ing. This may account for the peak of susceptibility values below 0.1)(10‘3 cgs (centimeter-gram-second) units shown on the histogram (fig. 4). Another major cause of low susceptibility is deuteric or hy- drothermal alteration of rock samples in which magnetite may be converted to weakly magnetic iron minerals. This significant effect can be observed in samples collected from altered and mineralized zenes. From the combined analysis of the aeromagnetic map and the laboratory determinations, the follow- ing generalizations can be made: (1) rocks whose susceptibility is less than 1.0)(10'3 cgs units are nonmagnetic to weakly magnetic; (2) rocks whose susceptibilities range from 1.0 to 3.0)(10'3 cgs units are moderately magnetic and cause anomalies of a few tens of gammas to several hundred gammas; and (3) rocks whose susceptibilities are greater than 3.0)(10'3 cgs units are moderately to strongly magnetic and cause anomalies of several hundred GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30-MINUTE QUADRANGLE, COLORADO 015 TABLE 2.—Summary of magnetic properties of the major rock um’ts [cgs units, centimeter-gram-second units] K J J Rock unit Nugébel‘ susceabtglgliigitg 10—3 Nuggber magngéiezlzaggrgf 10—3 Nugi‘ber Q =fi' H = 0'56 oersteds samples Range Average samples Rang? u l sAVemge samples Range Average Upper Cretaceous and Tertiary porphyries .................................. 29 0.46 —2.67 1.58 21 0.032-1.52 0.23 21 0.02— 1.27 0.21 Precambrian Silver Plume and St. Kevin Granites .................... 19 .03 —1.64 .32 16 .022— .601 .44 16 .87—17.93 5.1 Precambrian Denny Creek Granodiorite Gneiss and similar rocks .............................. 8 .073—2.07 .86 7 .016— .159 .084 7 .14— 0.86 .40 Precambrian gneisses ........................ 24 .015—4.52 1.37 18 .001— .854 .278 18 .25-— 2.70 1.30 Paleozoic sedimentary rocks ............ 25 .012— .06 .02 Generally weak. ........ gammas, as observed at the flight level of the air- craft. The magnetic properties of major rock units are summarized in table 2. PRECAMBRIAN GNEISS Susceptibilities of Precambrian gneiss samples show a bimodal distribution (fig. 4). Sixteen samples have susceptibilities below 1.0)(10'3 cgs units and eight samples have susceptibilities above 3.O><10'3 cgs units. In the Leadville 30-minute quadrangle, this bimodal distribution is reflected by the aero- magnetic expression of the lithologic units: biotite- rich gneisses and migmatites in and northwest of the Homestake shear zone are more magnetic than the amphibole and quartz gneisses southwest of the shear zone. The average susceptibility of all the gneisses is 1.37 cgs units. Remanent magnetization of the gneiss is ex— tremely variable in both intensity and direction. Intensities range from 0.001 to 0.854><10'3 cgs units. Remanent orientations fall generally in northeast or southwest quadrants, but the azimuths are widely scattered. The average Q value for 18 samples is 1.3. On the average, remanent magnetization exceeds in- duced magnetization by a small amount. However, most of the samples with a high total magnetization have low Q values; therefore, magnetic anomalies shown on the aeromagnetic map reflect, for the most part, the induced component of total magnetization. Because the specimens were not demagnetized, the data are not suitable for paleomagnetic analysis. PRECAMBRIAN GRANITIC ROCKS Magnetic susceptibilities of the Denny Creek Granodiorite Gneiss and similar rocks average about 0.86><10’3 cgs units, and those of the St. Kevin and Silver Plume Granites average 0.32x10'3 cgs units. All but one sample of the St. Kevin and Silver Plume Granites have susceptibilities less than 1><10‘3 cgs units and that one has a susceptibility less than 2><10'3 cgs units. Remanent magnetizations of the Denny Creek trend north to northwest, and the azimuths are widely scattered. Inclinations are generally steep. Remanent magnetizations are low and range from 0.016 to 0.159><10'3 cgs units. Q values range from 0.14 to 0.86, and average about 0.4. Thus, remanent magnetization makes very little contribution to the total magnetization. Remanent magnetization of the St. Kevin and Sil- ver Plume Granites ranges from 0.022 to 0.601X10‘” cgs units. The magnetization generally has a re- versed, low inclination, and the direction in most samples is northeast. Q values range from 0.87 to 17.93, but only two of the 16 samples have values below 1.0, and the rest exceed 2.1. The average Q is 5.1; thus, remanent magnetization is clearly domi- nant for these rocks. It should be emphasized that none of the Denny Creek rocks show reversed remanent magnetization, whereas the St. Kevin and Silver Plume rocks do exhibit reversed magnetiza- tion. Magnetic data may aid in distinguishing rocks of the St. Kevin and Silver Plume Granites from those of the Denny Creek Granodiorite Gneiss. Three mag- netic features may serve as a supplement to petro- graphic, chemical, and field criteria: (1) Q values of rocks of the St. Kevin and Silver Plume are much greater than those of the Denny Creek. (2) The magnetic susceptibilities of biotite-quartz monzonites and granodiorites of the Denny Creek are somewhat greater than those of the granites of the St. Kevin and Silver Plume. (3) Reversed remanent magneti- zation is commonly present in rocks of the St. Kevin and Silver Plume and seems to be rare in rocks of the Denny Creek. These potentially significant correlation techniques merit much further investigation by concerned spe- cialists. PALEOZOIC SEDIMENTARY ROCKS Table 2 shows that the Paleozoic sedimentary rocks have magnetic susceptibility values of less 016 than 0.1)(10'3 cgs units and, thus, that these rocks are effectively nonmagnetic. Therefore, the sedi- mentary rocks can be excluded as a significant source of magnetic anomalies. UPPER CRETACEOUS AND TERTIARY INTRUSIVE IGNEOUS ROCKS These rocksmthe porphyries—have a wide range of magnetization. Fresh porphyries generally have moderately high values of magnetic suscepti- bility, but altered porphyries have low values. Thus, both magnetic highs and lows are found over the porphyries; magnetic lows may reflect either altera- tion or a low content of magnetite in fresh rock. Remanent magnetization of Tertiary porphyries is generally weak and variable; n0 samples have reversed directions of remanent magnetization. Most remanent directions are northeastward. Their in- clination is somewhat greater than that of the earth’s present field in the region. Q values are vari- able and range from 0.02 to 1.27. GRAVITY FEATURES One of the largest regional negative gravity anomalies in the United States trends northeast across Colorado, from the San Juan Mountains to the Front Range, near Boulder. This low, as shown on the US. Bouguer anomaly map (Woollard and J oesting, 1964), is interrupted by a saddle near the Gunnison River. Thus, two main negative anomaly areas can be recognized: one centered over the upper Arkansas valley area, and one over the San Juan Mountains. The origin or source of this regional negative anomaly and its relationship to the C010- rado mineral belt constitute one of the major geo- physical problems of the Rocky Mountain region. Regional gravity maps of Colorado have been pre- sented by Holmer (1954), and an isostatic anomaly map and analysis based on Holmer’s data were pre- pared by Qureshy (1960, 1962), who considered the low to be caused by crustal thickening. Behrendt (1968) identified the low as the most negative gravity anomaly in the conterminous United States, and Case (1965) interpreted the low as caused by a batholithic mass of low density. GRAVITY SURVEY Gravity stations were established by Case during 1961—65 along nearly all roads accessible by vehicle and, also, by foot traverse along many ridges. Worden portable gravity meters with scale constants of about 0.5 mgal (milligal) per scale division were used throughout the survey. A master gravity base GEOPHYSICAL FIELD INVESTIGATIONS network was established which extends from Lead- ville through Buena Vista to Poncha Springs and Monarch Pass, from Buena Vista through Fairplay to Breckenridge, and from Beckenridge through Climax to Leadville (fig. 5). The value of observed gravity at Leadville is 979.1855 gals, as determined by D. J. Stuart, of the US. Geological Survey (oral commun., 1961), in connection with establishment of a Southern Rocky Mountains gravity profile tied to the international gravity network (Stuart and Wahl, 1961). All gravity values in the Leadville quadrangle were determined with respect to the Leadville value. Locations of gravity stations were plotted on the Holy Cross, Mount Lincoln, and Mount Elbert 15- minute quadrangles; the Mount Sherman, Fairplay West, South Peak, and Jones Hill 71/2-minute quad- rangles; the Tenmile mining district (scale 1 : 12,000) and Leadville mining district (scale 1:9,600) special topographic sheets (pl. 1, index map); and preliminary 71/2-minute maps covering parts of the Holy Cross and Mount Lincoln quad- rangles prepared by the US. Army Map Service. Locations of gravity stations are considered to be correct to within 0.1 mile for most stations. Elevation control was provided by bench marks of the U. S. Geological Survey and US. Coast and Geodetic Survey, by photogrammetric elevations shown on the modern topographic maps, by leveling data along the main highways provided through the courtesy of the Colorado Highway Department, and by surveyed “spot elevations” shown on the older topographic maps. A few elevations were determined by altimetric surveys. Elevations shown on older maps, especially the Mount Lincoln and Mount El- bert quadrangles, are subject to greater uncertainty than those shown on the modern maps. As a result, the greatest errors in Bouguer anomalies are to be expected in the Mount Lincoln and Mount Elbert quadrangles. Gravity values were reduced to Bouguer anoma- lies by standard methods. (See Oliver, 1965, p. 218.) A density of 2.67 g/cm3 was assumed in the reduc- tions. Terrain corrections, determined for all sta- tions, ranged from 2.4 mgal at lower stations in the open flat valley southwest of Leadville to 52.3 mgal at Mount of the Holy Cross. Many corrections were in the 3- to 10-mga1 range. The largest corrections were required at the higher, sharper peaks and in the floors of the deep, relatively narrow glacial val— leys. Inner corrections through Hammer’s (1939) zone H were obtained through use of the Hammer template system. Corrections for the outer zones were obtained by a computer method devised by Plouff (1966) which utilizes digitized terrain on a GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30-MINUTE QUADRANGLE, COLORADO geographic coordinate basis of 1 minute for Hammer zone I (2.6146 km) (kilometers) through 21.9 km, and a 3-minute basis for 21.9—166.7 km. ERRORS IN THE BOUGUER ANOMALIES In mountainous regions where topographic relief is great, as in the Leadville quadrangle, errors in the Bouguer anomalies attain a maximum. Principal sources of error are the terrain corrections and the station elevations. Elevation errors. — Elevation errors range from less than 1 foot at the better bench marks to as much as 50 feet, the map-contour interval, at spot elevations shown on the older quadrangles. An error of 50 feet is equivalent to an error in the computed Bouguer anomaly of about 3 mgal. Most elevations are believed to be correct to within 20 feet, equiva- lent to errors of 1.2 mgal. Terrain correction errors. — Errors in terrain corrections are exceedingly difficult to estimate, but they probably do not exceed 10 percent of the total correction. Thus, these errors could range from 0.24 mgal for stations in the flat valley southwest of Leadville to as much as 5.2 mgal at Mount of the Holy Cross. Location errors.—Virtually all locations are cor- rect to Within 0.1 mile, equivalent to errors of about 0.1 mgal or less in the latitude correction. Errors in observed gravity. —— The principal errors in observed gravity arise from instrumental drift and earth tides which occurred during daily tra- verses. Normal drift rate of the gravity meters did not exceed 1 mgal per daily traverse, and commonly was less than 0.5 mgal. During daily traverses, re— peats were made at base stations and intervening stations where possible. Where such repeats were possible, relatively precise drift corrections could be applied; therefore, the values of observed gravity for these traverses are probably correct to within 0.2 mgal. On long mountain foot traverses, Where re- peats were made only at the end of the traverse, the error may be as much as 0.5 mgal. The normal expected error in the Bouguer anomaly value at an individual station is about 2 mgal, as a subjective estimate. The Bouguer anomaly map (pl. 1) has been contoured with an interval of 2 mgal, because a 1-mgal map would reflect the many small errors unavoidably present in the data. No sig- nificance can be attached to apparent anomalies of 2 or 3 mgal, particularly in the high mountains, as these values are too close to the possible error in— herent in the data. The gravity anomaly contours were controlled by the values at the stations and were drawn as smooth 017 lines between control points, without any special attempt to reflect known geologic features. 106°30’ 106.00, 3? LEADWLLEQUADRANGLE 30’ 5 ., 9‘ q ‘90 / ‘\\ WUBreckenridgg o9 ‘ s \ v (l a is Q £5 FairplayC El») 900 TVVIN LAJ- x-—~ / —J zx’ < < .1 E < 0 -310 — ——20 3 z 9 < 3 K g -320- —-30 x 0 Residual gravity anomaly g 3 \ / < O \\ / m -330 — \ / — -4o 0 \ LIJ \ X, .— \X\ // 3 \\ D. \ '340 — ‘X~‘___X__,// Computed gravity anomaly ~-50 g o -350 — ——60 E o '92 C 3 .2 g 0’ 20,000' — 3 E g — 20.000' 3 E 3 GORE a .3 MOSQUITO RANGE — _ AWAT AN E _ _ s CH R GE 8 E RANG [3 3;: SOUTH PARK _ 10,000' — — 10.000' SEA _‘ '_ SEA LEVEL — . LEVEL 10,000' —_ / _ ,\ H '— 10,000' - \ ,\I>L\L\\INFERR s \ /,\—\ — : \//—\/\;\l/I\l\/\/'\l\/\/_\/:l/,\l\ \/\/ /\/'\_—/ I ' I \ — / \ / \ /\ ’ I \ / \ \ _ , 20,000 i ‘ \— /|\//'\I\/\l/\(Ap=_0_12 g/cma),,\/\\I\\/\\/_ ,\/_\/>/,\ 720,000 /‘__l_\/‘\/l\—\/\ .,\/\—\\ /\/\ //\/\/\/\ 3 ‘5?” \I\’l>\/’:\/\7<‘/\\\’\/V/\/l:0 (I \/l/\\/_\///—/l — 30.000' : / \/\\\\ \\/\/\/\/\ ’ ’/\‘/\// \ /\\ /\ Z _ _ ’ \— '— — \ i / _ a \ 40.000' ’ 40.000' FIGURE 6. —— Regional gravity anomaly and its interpretation along profile C—C’. Assumed regional gradient is about 0.6 mgal per mile. Line of profile shown on plate 1. CZO sumed to be two dimensional — that is, their lengths greatly exceed their width. Obviously, this assump- tion is not completely correct. Moreover, the gravity field related to the mass causing the mineral-belt low is somewhat influenced by the thick low-density fill in the Arkansas valley, west of Leadville. How- ever, this gravitational disturbance can be no more than a few milligals along the line of profile, which is 5—6 miles northeast of the region of thick fill. The observed gravity anomalies over the con- cealed low-density mass are shown in figure 6. If a linear regional gradient of 0.6 mgal per mile is assumed, the residual profile can be matched very closely by the computed anomaly of the model. A geologically plausible model is a batholithic mass that averages 15—20 miles in width, that extends to depths on the order of 40,000 feet below sea level, whose apex is within a few thousand feet of the surface, and whose average density contrast is about ——0.12 g/cm3, which yields a computed anomaly that matches the observed anomaly both in ampli- tude and in steepness of gradient. Such a batholith could consist in part of the St. Kevin and Silver Plume Granites, inasmuch as these granites are only slightly more dense than the porphyries, but the continuity of porphyry intrusion along the mineral— belt low strongly suggests that the underlying mass is a continuous batholith of Late Cretaceous and Tertiary age. No attempt was made to construct a model whose gravity anomaly exactly matches the observed anom- aly. The discrepancy between the observed and the computed anomalies at specific points is generally less than 2 mgal, which is on the same order of magnitude as the error inherent in the Bouguer anomalies. Note, however, that the observed anomaly could be matched by other models; in particular, the bath- olithic mass could extend to greater depth if the average density contrast is less than —0.12 g/cm3, or a somewhat smaller mass at greater depth could be present if the density contrast is greater than —0.12 g/cm3. The closure of the gravity contours in the north- eastern part of the area, near Breckenridge, and the generally narrow width of the mineral-belt low indicate that the batholithic mass may become small- er or that its roof may plunge downward toward the northeast. DEEP CRUSTAL OR SUBCRUSTAL SOURCE A general thickening of the crust beneath the high mountains of Colorado is indicated by the fact that the Bouguer anomalies increase with increasing mean topographic elevation, as is well illustrated GEOPHYSICAL FIELD INVESTIGATIONS by a map by Gilluly, Waters, and Woodford (1968, p. 176). From analysis of seismic data from a mine blast at Climax, Jackson and Pakiser (1965) deter- mined a crustal thickness of about 50 km (30 miles) beneath the Southern Rocky Mountains in Colorado, as contrasted to a crustal thickness of 35—40 km in the Colorado Plateau and the Uinta Mountains to the west. However, they found little contrast in the crustal thickness between the mountains and the plains to the east. The thickening implied by the regional gravity and seismic data applies to much broader areas than the mineral—belt low. The steep gravity gradients of the low and the narrow width of the low, about 15 miles, strongly suggest a local, shallow feature whose effects are superposed upon those of a region- ally thickened crust. On the basis of only a few gravity stations, Qureshy (1960, 1962) concluded that the low was caused by local crustal thickening to about 52 km across a horizontal distance of 30 miles or more, but this width is too great. The average width of the anomalous mass causing the low cannot be greater than the distance between the positions of the steepest gradients on the flanks of the anomaly, or about 15 miles. The requirement of a shallow position within the crust of the body causing the anomaly can be illustrated by comparison of observed gravity gra- dients with the calculated effects of a deep body. If a body 10 km thick, 20 km wide, and having a density contrast of —0.3 g/cm3 were located at a depth of 30 km within the crust (fig. 7), the anomaly at the surface would be only —9.4 mgal, measured from a point at the center of the body to a point 30 km from the center. It is evident from the com- puted points shown in figure 7 that such a body would produce only a small anomaly, and that the gradients would be much flatter than those ob- served. A “root” of similar dimensions at the base of the crust would have even smaller effect, unless an unreasonable density contrast between crust and mantle (on the order of 1.5 g/cm3) were assumed. From seismic studies, Jackson and Pakiser (1965, p. D90) concluded that “no pronounced crustal root [exists] under the Southern Rocky Mountains.” In summary, a deep crustal or subcrustal source of the anomaly (mineral-belt low) seems to be precluded. The source is concluded to lie at relative- ly shallow depth within the crust and to be super- posed on a broader regional low related to the thickened crust of the mountain province as a whole. LOCAL ANOMALIES ALONG THE MINERAL-BELT LOW Local gravity anomalies, both relative highs and lows, are superimposed on the main mineral-belt GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30-MINUTE QUADRANGL'E, COLORADO C21 >: n o m -260— _ _5 l o w , E3 _| A: 2‘ 5 < ,— _ o -270 — —————— L15 g j :1 __ ““““ Computed anomaly of |ow~density mass """" 5 E :’ ‘9‘ ______ C 43—" a z z 280 31‘ _ Z ' — —25 E _ E j 8 < —290 — _ 2 0 Z < —300 - _ '35 Bouguer anomaly Lu 3 8 —310— _ O m -320— _ C C’ 15000? SAWATCH RANGE GORE RANGE MOSQUITO RANGE SOUTH PARK lo‘oooli/WN— 0 sooo’é SEA j _ SEA LEVEL LEVEL ”‘ - 5 — ASSUMED NORMAL CRUST ‘10 E _ o —15 ‘53 m Lu I- Lu 2 O -_J _ —20 x E :E .— O. — — 25 '3 If 20 km _ ~30 _ _35 40 O 5 1'0 MILES O 5 10 KILOMETERS l_i_i_i_l_|__.___.._l FIGURE 7. — Gravitational effects of a deep crustal-mass deficiency beneath the Colorado mineral belt. low. Some, such as the low over the Arkansas valley altered Precambrian rocks. There seems to be little graben, discussed in the following section, can be correlation of the low with the position of the Mos- readily explained, but the origin of others is obscure. quito fault. The eastern limit of the low is poorly A small low near Climax occurs, in general, over controlled, and additional gravity observations might the Climax stock and adjoining areas of intensely establish that the —320 contour at Climax joins 022 the ~320 contour in the area along the Continental Divide, to the east. The fact that the low at Climax occurs over a variey of rock types suggests that this low may reflect a concealed igneous body of low density. A conspicuous gravity high of about 4—5 mgal splits the main mineral-belt low where it crosses the Tenmile Range, northeast of Climax. This high may be caused by relatively dense gneissic (amphi- bolitic) rocks within the sequence of Precambrian gneiss, or it may be an “apparent” high between the two flanking lows, which presumably are caused by largely concealed silicic bodies of relative .low den- sity. For example, the Humbug stock may spread southward at shallow depth. The high is defined by only a few stations in a region where the elevations may be subject to large error. Thus, no attempt should be made to quantitatively interpret the anom- aly unless additional gravity stations and more pre- cise elevations are obtained. A small low is found over a plug or stock of Pando Porphyry just north of Leadville. However, this low is controlled by only two gravity stations, and it cannot be readily isolated from the effects of the main source of the mineral-belt low or of the fill in Arkansas valley. A well-defined low of about ——10 mgal is corre- lative with the Twin Lakes stock in the southwestern part of the quadrangle. The low is interrupted by a gravity high that coincides with a prong of Pre- cambrian rocks projecting into the stock from Quail Mountain to La Plata Peak. GRAVITY LOW OVER THE ARKANSAS VALLEY GRABEN In the Arkansas valley graben, the low-density fill of alluvial and glacial materials adds to the effects of the mineral belt low, creating the most conspicuous low in the quadrangle and, indeed, the deepest low known in the conterminous United States. Identification of that part of the low due to the fill is difficult, but necessary, in order to evaluate the thickness of the fill, which is otherwise unknown. The problem is compounded by lack of knowledge of the bedrock beneath the valley fill. For purposes of analysis, it was assumed that the buried bedrock consists primarily of Precambrian rocks. If fault blocks of sedimentary rocks are present, as they may be, they probably are mainly lower Paleo- zoic rocks, of about the same density as the Pre- cambrian rocks. If the bedrock consists in large part of intrusive porphyry of low density, such as that of the Twin Lakes stock, its effect would be to reduce the calculated thickness of the Cenozoic fill in the valley. GEOPHYSICAL FIELD INVESTIGATIONS A residual low over the Arkansas valley can be isolated by assuming near-linear variations in the Bouguer anomaly field between bedrock areas on the two sides of the valley. Bouguer anomalies over bedrock areas decrease from about —320 mgal in the area 3—5 miles northeast of Leadville to —325 to —330 mgal in the area near the Twin Lakes stock. Thus, one can assume, as the simplest situa- tion, a near-linear decrease in minimum values along the axis of the mineral-belt low across the Arkansas valley graben. Similarly, Bouguer anomalies over bedrock decrease from about —300 mgal, near Ten- nessee Pass, to —310 mgal at the south-central edge of the quadrangle, 5 miles south of Twin Lakes. Values over bedrock along the eastern and western flanks of the graben are about —310 mgal. These regional Bouguer anomaly values were used to aid in constructing a smooth regional map (fig. 8). The differences between gravity values on the smooth map and the Bouguer anomaly map were then ob- tained and contoured to prepare a residual anomaly map (fig. 8) over the graben. This method yields a maximum residual anomaly of about —20 mgal. As an alternative method of obtaining a regional profile, one may smooth the anomaly values across the valley between inflection points of the profile (fig. 8). This method yields a residual anomaly of about 17 mgal along profile B—B”. This residual was fitted by a model in which the thickness of valley fill is about 3,200 feet for a density contrast of —0.45 g/cm3. If the maximum residual anomaly in the valley is 20 mgal, the depth of fill would be some- what greater, perhaps 3,800 feet. Because of the uncertainties —— the regional grav- ity field, the average density contrast, and the exact location of the steep residual gravity gradients on the flanks of the anomaly—the model shown must be regarded merely as a geologically and gravimet- rically plausible model. The thickness of valley fill could be in error by as much as 25 percent. A thick- ness of 3,000—4,000 feet is adopted here as a reason- able estimate. OTHER BEDROCK ANOMALIES A gravity high of 6—8 mgal is centered over Round Hill, on the London fault, in the southeastern part of the area. From this high, a positive nose extends northwestward along the trace of the London fault at least as far as Sacramento Gulch. Farther north- west, a line of bending in the contours suggests a continuation of the nose, but this is more likely a reflection of the lows at, and east of, Climax. The gravity high and positive nose along the London fault can be attributed, in some part, to the 023 GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30-MINUTE QUADRANGLE, COLORADO .H 8R3 swim 550% we 95 mo 9:3 :gwumoB mafia :MIM 058m 553.» mvdl mo umahzoo mime—ow 9:32 =c 38280 WuO .5552535 m“: was 53.2» 22:; msmcavi< 23 ago fiafiozw 33.3w Husgmwmld "5“:ch S'IVSH'IIW S'IVBI'I'IIW «6.3 $333 532 &§ §s§s $393M gs Sufi ~§u§ 3:3.“me €333 wvgwsungw £38m Ewwzmfi 5 $12.55“ 3?qu 33.62 M5393 mun—3.80 Q\I Bay—:5 5 .Eofi 3:3?! can—Emma Mats—i PBS—50 ZO_Fm; 4w>m4 (mm (mm .000m 600m .ooodH .OoodH «EQ\M 3%.? H adv 6006 a .ooo.m H :m m ON: 29:95 53.2w UBJQEoo Owl M _I .I 2. 07 m V _I o \ l I I Ix o S 32:95 5395 3:331 ovmu 2:95 Zancm .36an $9530 owmu 0mm. 0mm- w _H lllll ‘ _H 8m- \ \ \ lllll I I am 9 \ \ \ V. I I W o a macaw; 053mm gm: 2: k _ . u < ofiml S OOMI 00ml _I mw:_:>_ m 0 .m H amm .OMeooH C24 contrast between the relatively dense Precambrian and lower Paleozoic rocks on one side of the fault and the relatively light Pennsylvanian rocks and porphyries on the other side (pl. 1, section B—B’). However, this difference seems to be insufficient to be the entire cause. The anomaly may reflect a line of concealed mafic intrusions, as discussed further in the section on interpretation of magnetic anomalies. AEROMAGNETIC SURVEY Aeromagnetic surveys were flown in a twin—engine aircraft at an elevation of 14,500 feet above sea level. Total intensity of the magnetic field was mea- sured by an ANASQ/12A fluxgate magnetometer mounted in a retractable “stinger,” or boom, on the tail of the aircraft. Surveys were flown under the supervision of J. L. Meuschke and F. A. Petrafeso in 1963, and data were reduced under the super- vision of J. R. Kirby and Jean Blanchett. Flight lines were flown east and west and were spaced 2 miles apart. The flight paths, controlled by strip film, were plotted on topographic base maps, scale 1262,500 and 124,000. The data were compiled on an enlargement to a scale of 1 : 125,000 of the Lead- ville 1° by 20 quadrangle. The aeromagnetic map (pl. 2), contour interval 20 gammas, was compiled by standard methods (Balsley, 1952). Aeromagnetic surveys have been conducted in adjacent quadrangles to the north, east, and south of the Leadville quad- rangle. A detailed aeromagnetic survey of the Climax area was described by Meyer (1968); the flight elevation was 14,000 feet, and lines were spaced about one-quarter mile apart. The resulting magnetic map shows many details not shown on plate 2. INTERPRETATION OF THE MAGNETIC MAP GENERAL FEATURES OF THE MAGNETIC MAP [All anomaly letter designations refer to plate 2] The magnetic map is contoured with respect to an arbitrary datum. A general northward increase in total magnetic intensity is caused by the earth’s main magnetic field, which increases about 8.3 gam- mas per mile toward the north-northeast, according to the U.S. Coast and Geodetic Survey (1955, chart 3077F). Local magnetic anomalies, caused by topo- graphic relief and by contrasts in rock magnetiza- tion, are superimposed on the earth’s main field. In regions where rugged topography has been carved in magnetic rocks, the aeromagnetic map must, to some extent, reflect variations in the distance from the flight elevation of the aircraft to the rocks below. This effect will be superimposed on GEOPHYSICAL FIELD INVESTIGATIONS the variations in the magnetic field resulting from contrasts in rock magnetization. An example of a rather crude, general terain effect is shown on plate 2, profile A—A’, the southernmost aeromagnetic profile in the Leadville 30-minute quadrangle, which ex- tends from south of La Plata Peak and Mount Hope, near Quail Mountain and eastward across the valley, south of Clear Creek Reservoir, and across the Mosquito Range, to the region south of Pole Gulch. Magnetic values increase from about 2,250—2,260 gammas over the low point in the Arkansas valley to 2,596 gammas over the high part of the Mosquito Range—a magnetic increase of about 340 gammas across topographic relief of about 3,000 feet. A similar increase is found to the west of the valley, where magnetic values reach 2,616 gammas at Quail Mountain and 2,615 gammas near La Plata Peak, at elevations of 3,000—4,000 feet above the valley. Gran- itic rocks at Quail Mountain and over the Mosquito Range are dominantly biotite-quartz monzonites or granodiorites related to the Denny Creek Granodiorite Gneiss, and the average susceptibility contrast is about 2~3><10’3 cgs units. The flight elevation of the aircraft was 14,500 feet; therefore, the magnetic rocks along the mountain ranges were 500—2,500 feet below the aircraft, and those in the valley were 5,500 feet below the aircraft. Yet, variations in amplitude of the magnetic anomalies over the high Sawatch Range are nearly as great as the amplitude variation from the Arkansas valley to the crests of the ad— jacent ranges. The west half of the northernmost profile (pl. 2, profile B—B’) shows relatively little correlation be- tween the topography and the variations in magnetic field. The magnetic high over the gneissic rocks northwest of the Homestake shear zone persists as a high, even though several thousand feet of relief is present in the terrain. The steepened magnetic gradient along the shear zone crosses several promi- nent ridges, and the lower values southeast of the shear zone are not reflected in the topography. Many of the anomalies, thus, are independent of topographic relief and arise predominantly from contrasts in magnetization of the crystalline rock units. Perhaps half of the closed anomalies are the result of topographic relief. As indicated in the dis- cussion of magnetic properties, the various igneous and metamorphic rock units have a wide range of magnetization. Positive anomalies are found over some of the Tertiary porphyries, Precambrian gneisses, and Precambrian granitic rocks. Relative magnetic lows may be found over the same units as a result of the low original magnetite content or of the alteration of magnetite. GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30-MINUTE QUADRANGLE, COLORADO In the following discussion, specific anomalies are identified by letters keyed to plate 2. The amplitudes assigned to the anomalies generally refer to the difference in magnetic intensity from the crest or trough of the anomaly to the adjacent regions of flat gradient, near the flanks of the anomaly. One should bear in mind that the contours are controlled only along the positions of the flight lines; hence, contours between flight lines are approximations. ANOMALIES OVER THE ARKANSAS VALLEY The general magnetic low over the Arkansas valley is expectable from the relatively low topo— graphic setting and the great thickness of glacial and alluvial deposits in the valley. The minimum anomaly in the region is found north and west of the Hayden Ranch, suggesting that the greatest thickness of valley fill is in the southwest quarter of T. 10 S., R. 80 W., and the northwest quarter of T. 11 S., R. 80 W. As noted previously, the minimum gravity values are in the area immediately south of Malta. The combined gravity and magnetic data suggest that the maxi- mum thickness of fill lies in the west half of T. 10 S., R. 80 W. However, it is also possible that the low magnetic values over the valley might be caused, in part, either by nonmagnetic crystalline rocks or by reversely magnetized rocks, such as the St. Kevin Granite, at depth beneath the fill. An apparent constriction of the magnetic low over the valley is present in the area due west of Lead- ville (anomaly N) . ‘This small positive anomaly may mark an area of major thinning of the valley fill, but it could also reflect the presence in the bedrock of a fine-grained granodioritic facies of the St. Kevin Granite. This granite is exposed in broad dikelike bodies southwest of Turquoise Lake and extends beneath the moraines. A similar constriction west of the town of Granite (anomaly P) coincides with a fault block of granitic rock projecting into the valley fill. ' Calculation of the magnetic effects of a model across the Arkansas valley is not feasible, because the susceptibilities, as well as the depth, of the crystalline rocks beneath the valley would be only a guess. The area of the Leadville mining district is mag— netically flat. If a residual magnetic map were con- structed, the principal mines would lie in a magnetic low (anomaly T) on the southern flank of a weak magnetic high. ANOMALIES RELATED TO THE HOMESTAKE SHEAR ZONE Much of the Homestake shear zone exposed in the Leadville quadrangle cuts through gneissic rocks. A CZ5 prominent magnetic high (anomaly M) is present over the region northwest of Homestake Creek, in the vicinity of Mount of the Holy Cross and Whitney Peak. This anomaly is evidently caused by the mag- netic migmatite and biotite gneiss, as well as by the high elevations of this part of the range. In many places in this area, the migmatite is so magnetic as to make a compass unreliable or even unusable. To the southeast of the main zone of shearing, the gneissic rocks consist of more weakly magnetic amphibole gneiss, calc-silicate gneiss, and impure quartzite or quartz gneiss. Unlike the amphibolites of North Park, Colo., which are magnetite-bearing and magnetic (Behrendt and others, 1969) and are probably of igneous origin, the amphibole— bearing gneisses southeast of the Homestake shear zone are magnetite-poor rocks of sedimentary origin. Thus, most of the magnetic high and the prominent northeast strike of the magnetic contours are caused by the contrast between the high susceptibility of the migmatite and biotite gneiss and the low sus- ceptibility of the amphibole gneiss and accompany- ing rocks. However, the shear zone itself may be magnetic relative to bordering rocks. The magnetic ridge, or high, along the northwest side of the shear zone and the line of steepened magnetic gradient that borders it extend across the region of nonmagnetic Paleozoic sedimentary rocks northeast of Camp Hale and into the Gore Range, where the shear zone is again exposed (Tweto and others, 1970). Thus, the shear zone can be inferred to persist beneath the sedimentary rocks east of the Eagle River. In the area of concealment, the width of the zone of steepened magnetic gradient does not indicate the width of the shear zone, but only pro— vides a general indication of its location. ANOMALIES ALONG THE SAWATCH RANGE In the southwestern part of the quadrangle, positive anomalies over high parts of the Sawatch Range are separated by a magnetic low (anomaly I") over Lake Creek and the Twin Lakes stock. This low projects as a lobe from the main Arkansas valley magnetic low. The low is caused in part by low topography, but it also reflects a low magnetite content of the main body of the Twin Lakes stock, as contrasted with its border zones, particularly the north border (Wilshire, 1969). South of Lake Creek, positive anomalies A, C, and E form a line along the high ridge from La Plata Peak to Quail Mountain. This line of magnetic anomalies coincides with a gravity high over Precambrian gneisses that evi- dently project as a deep-rooted septum into the Twin Lakes stock. Negative anomaly B, between La Plata 026 Peak (anomaly A) and Mount Hope (anomaly C), seems to reflect a projecting arm of porphyry of the Twin Lakes stock. The magnetic saddle (anomaly D) coincides with a topographic saddle between Mount Hope and Quail Mountain (anomaly E) and may be caused, in part, by alteration along the faults in this area. In the area of Halfmoon Creek, between Mount Elbert and Mount Massive, is a deep magnetic low (anomaly H). The cause of this anomaly is not im- mediately evident. The anomaly may reflect alter- ation, inasmuch as the rocks throughout much of the area weather yellowish brown, suggesting widely dis- seminated pyrite, especially in the north fork of Halfmoon drainage. The moderately productive Mount Champion gold mine, on main Halfmoon Creek, is only 1 mile west of the quadrangle boundary. The low might also be interpreted to re- flect a concealed porphyry body that is similar in magnetic properties to the Twin Lakes stock or that is related to it. However, the magnetic low coincides approximately with a gravity high of several milli- gals. Assuming that the magnetic and gravity anomalies are due to the same cause, this would preclude an intrusion of the composition of the Twin Lakes stock, which generates a pronounced gravity low. Clearly, further geologic and geophysi- cal investigation of this area is warranted, especially as there may be direct economic applications. A magnetic high marked by positive anomalies G and I, separates the low just discussed from that of the Twin Lakes stock (anomaly F). The high is over Precambrian gneisses in a topographically high area, and it stands out mainly because of its contrast with the bordering lows. Positive anomaly J is a long magnetic high that coincides approximately with the high topographic ridge that extends north-northwestward from the crest of Mount Massive. Hence, the anomaly is at least partly topographic in origin. However, it is in an area of the St. Kevin Granite, which—owing to strong reversed remanent magnetization—should produce magnetic lows rather than highs. The anomaly is coextensive with a system of strong north-northwest-trending faults, some of which con- tain lenticular plugs or large dikes of quartz por- phyry. The fault zone may overlie an intrusive body at shallow depth, as indicated diagrammatically on plate 1, section B—B’. Such an intrusive body might account, in part, for the magnetic anomaly. Negative anomaly K, a magnetic low over the valley of Lake Fork and the low hills of the St. Kevin mining district, probably reflects three factors GEOPHYSICAL FIELD INVESTIGATIONS in combination: (1) low topography, (2) reversed remanent magnetization in the St. Kevin Granite, and (3), extensive alteration of the rocks in the St. Kevin district. The north end of the Arkansas valley magnetic low, north of the constriction at anomaly N, is continuous with anomaly K, and may be due to these same causes, supplemented to some degree by the effects of the valley fill. Negative anomaly L, near the forks of Homestake Creek, straddles a topographically high ridge in an area characterized by: ( 1) actinolitic quartzite and calc-silicate gneisses in a vertically standing belt nearly 1 mile wide, (2) numerous irregular bodies and very large dikes of leucocratic St. Kevin Granite, (3) many rusty zones reflecting sparsely disseminated iron sulfides in some calc-silicate layers, and (4) numerous lamprophyre and diorite dikes. The low most likely reflects a body of weakly magnetic, or reversely polarized(?), St. Kevin Granite at shallow depth, especially as it coincides with a small, but distinct, gravity low. The weakly magnetic gneisses might supplement the effect of the granite, but the swarm of mafic dikes should have the opposite effect. Conceivably, a Laramide intru- sive similar to the Missouri Creek stock 2 miles to the north could lie beneath the magnetic low, but no porphyry dikes exist at the surface, and the iron sulfide mineralization in the gneisses predates the lamprophyre dikes, which are known to be Pre- cambrian in age. ANOMALIES IN THE SOUTHERN GORE RANGE Other than the zone of steepened magnetic gradient related to the Homestake shear zone in the buried Precambrian rocks, the dominant magnetic anomaly in the southern part of the Gore Range is a very prominent magnetic low (anomaly Q) associated with the Chalk Mountain stock, west of Climax. This stock consists of rhyolite porphyry that contains very little iron in any form and, thus, is virtually nonmagnetic. The magnetic low is centered a little to the south of the stock, over a slight gravity high, and includes an area of Minturn Formation exten- sively intruded by Lincoln Porphyry (Tweto, 1956, section 0-0). This area is characterized by strong alteration and widespread weak mineralization. Thus, the magnetic low might be caused in part by altered basement rocks, as well as by the body of rhyolite porphyry. ANOMALIES ALONG THE MOSQUITO-TENMILE RANGE A large magnetic high (anomaly R), the most conspicuous anomaly of the quadrangle, is centered GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30-MINUTE QUADRANGLE, COLORADO over the Humbug stock of quartz monzonite por- phyry, but extends into adjacent areas of Precam- brian rocks in the Tenmile Range. Samples collected from this stock indicate moderately magnetic to magnetic rock. It seems evident that this porphyry body is not severely altered and, thus, is not an encouraging site for prospecting. Elsewhere along the Tenmile Range, a general magnetic high, closely following the crest of the range, is found over Pre- cambrian gneissic and granitic rocks. Positive anomaly S is apparently caused by the Buckskin stock, which consists of porphyries that range in composition from quartz diorite to quartz mon- zonite. The high continues southeastward to Alma, indicating that the magnetic unit extends at depth in that direction. The Climax district has no distinctive magnetic expression that is evident on plate 2, but it is located in the zone of steep magnetic gradient between the high along the crest of the Tenmile Range and the low of the Chalk Mountain stock (anomaly Q). This zone of steep gradient marks the Mosquito fault, and it extends north and south of Climax along the fault. A detailed aeromagnetic survey of the Climax area (Meyer, 1968) showed a slight trough in the total- magnetic-intensity contours over the Climax mine area, but no closed negative anomaly. However, second derivative and residual maps yielded closures. A general magnetic high (anomaly U) extends south along the crest of the Mosquito Range from the latitude of Leadville to the south edge of the quadrangle. The positive anomaly varies in ampli- tude along the range and reflects variations in topography, as well as zones of higher or lower susceptibility in the Precambrian granitic rocks that make up most of this part of the range. The Weston fault is locally expressed by a steepened magnetic gradient (for example, west of Peerless Mountain and Horseshoe Mountain), and sedimentary rocks west of the fault are expressed as a magnetic bench. The Union fault—the southern continuation of the Mosquito fault—is also marked by a line of steepened gradient. The small stock of Tertiary porphyry on the Weston fault at the line between Tps. 11 and 12 S., R. 78 W., apparently is nonmagnetic to weakly magnetic, as it causes no deflection of the magnetic contours. Except for the Humbug and Buckskin stocks, none of the porphyry bodies in the Mosquito and Tenmile Ranges are marked by magnetic highs. This reflects the generally altered character of many of the por- phyry bodies and, also, the fact that most of these bodies are sills and, hence, have a limited vertical dimension. 027 ANOMALIES IN THE EASTERN PART 01‘ THE QUADRANGLE A weak magnetic high (anomaly V) is found over the area of closely spaced porphyry intrusions north- east of Breckenridge, and a small positive anomaly (anomaly W) occurs over similar intrusive rocks a few miles southeast of Hoosier Pass. Thus, in their magnetic expression, these porphyry intrusions re- semble the Humbug and Buckskin stocks to the west. The most prominent anomaly along the west margin of South Park is the high (anomaly X) over Sheep Mountain and Round Hill. The ridgelike mag- netic high coincides with a ridgelike gravity high, although the peak of the magnetic high is over Sheep Mountain, and the peak of the gravity high is over Round Hill. Both the gravity and the magnetic highs are probably caused, in part, by the crystalline rocks comprising the core of the fault block elevated along the northeast side of the London fault. However, farther northwest the fault does not show, or only weakly shows, such gravity and magnetic features, although the displacement is the same (Singewald and Butler, 1941, p. 26). A relatively dense and magnetic body of rock must underlie Sheep Mountain and Round Hill along the London fault. Such a body could be either a Precambrian rock or a younger intrusive. If Precambrian, it may be an intrusive rock, such as diabase or gabbro, along an ancestral London fault, or perhaps an isolated body of dense and magnetic gneiss, such as amphibolite. If a younger intrusive, it would seem to be unrelated to the exposed igneous rock at Sheep Mountain, as this is typical porphyry of quartz monzonitic composition. Such porphyry, like that of the Humbug and Buck— skin stocks, could be the source of the magnetic anomaly, but not of the gravity anomaly. Signif- icantly, the rhyolitic porphyry stock at Black Moun- tain, just south of Round Hill, has no magnetic expression, but coincides with a slight gravity low. Further studies in this area will be necessary before the anomalies along the London fault can be fully understood. A large magnetic low (anomaly Y) in the south- east corner of the quadrangle is undoubtedly caused by the relatively greater depth to the crystalline basement beneath the thick cover of sedimentary rocks in this area. SUMMARY AND CONCLUSIONS The segment of the Colorado mineral belt that extends northeastward across the Leadville quad- rangle is characterized by numerous porphyry in- trusives of Late Cretaceous and Tertiary age and by major ore deposits; it coincides with a deep gravity 028 low, or gravity valley. This gravity low is inter- preted to reflect an underlying batholith. From the combination of gravity data and known geologic relations, this batholith is visualized to be greatly elongate, extending beyond the northeast and south- west corners of the quadrangle; to be 15—20 miles wide; to have an apex within a few thousand feet of the surface; to extend to depths of at least 10 miles; to be Late Cretaceous and Tertiary in age; and to consist largely of granodiorite-quartz monzonite, al- though grading upward to compositions near granite at the apex and downward to quartz diorite or diorite at depth. This batholith was the source of the hypabyssal sills, stocks, plugs, dikes, and sublac- colithic bodies that are exposed at the surface, and of the associated ore deposits, such as those of Lead- ville, Climax, Kokomo, Breckenridge, and Alma, as has been predicated previously on purely geologic grounds (Crawford, 1924; Tweto and Sims, 1963). Except for the Twin Lakes stock, none of the exposed intrusive bodies in the quadrangle have a marked gravity expression—that is, none affect more than one or two gravity contours. The evident reason for this is that the individual porphyry bodies are small in volume, and their density contrast with their surroundings is not great. The Twin Lakes stock, by far the largest exposed intrusive body in the quadrangle, has volume and density contrast large enough to affect the gravity contours, and it coincides with a pronounced negative gravity anomaly. In the Arkansas valley graben, a deep fill of low- density Tertiary sediments and overlying Pleisto- cene glacial deposits accentuates the mineral—belt gravity low. Gravity data suggest that this fill may be 3,000—4,000 feet thick in the deepest part of the graben, 1—2 miles west of the Arkansas River. This thickness may be less if (as is possible) the gravity low over the valley is caused, in part, by a large intrusive body of porphyry in contact with Pre- cambrian rocks in the bedrock beneath the fill. Data from shafts, drill holes, and limited seismic surveys near the margin of the valley fill indicate the pres- ence of many faults that cut the valley fill materials and, therefore, must displace the underlying bedrock. In light of the mineral productivity of the area of intrusion and gravity low, from Leadville north- eastward, the covered valley southwestward along the axis of the low is worthy of consideration for exploration. If exploration is undertaken, an uneven, block-faulted bedrock, with relief of at least many hundreds of feet, is to be expected. More detailed geophysical surveys, particularly resistivity and seismic, would be required to delineate the bedrock GEOPHYSICAL FIELD INVESTIGATIONS surface in advance of physical exploration. However, limited seismic exploration in the valley has shown that fossil caliche zones in the Tertiary sediments and the weathered tops of superposed tills present problems for the seismic method. A few miles south of the Leadville quadrangle, resistivity soundings have proved effective in determining the depth to bedrock acording to W. D. Stanley of the US. Geo- logical Survey (oral commun., 1970). The largest magnetic feature in the quadrangle is a low over the Arkansas valley. This low is attrib- uted mainly to the combination of low topography and deep valley fill, but, inasmuch as it coincides approximately with the gravity low, it could also reflect the presence of such rocks as the St. Kevin Granite or porphyry similar to that of the Twin Lakes stock. Both of these rocks have densities and magnetic properties that cause gravity and magnetic lows. A zone of steep magnetic gradient characterizes the Homestake shear zone, which extends northeastward across the northwestern part of the quadrangle. This gradient is attributed to difference in the Precam- brian rocks on the two sides of the shear zone, and possibly to the magnetic properties of the shear zone itself. Gravity contours parallel the magnetic con- tours along the shear zone, reflecting the concealed batholith and the control of the northwest side of this batholith by the shear zone. The porphyry intrusive bodies show a wide range in magnetic expression. A few, such as the Humbug and Buckskin stocks, are magnetic highs. Others, such as the Twin Lakes and Chalk Mountain stocks, are magnetic lows. Most of the intrusive bodies, however, have no magnetic expression. The reasons for this are varied. Many of the bodies are altered— deuterically or hydrothermally, or both — with con- sequent destruction of the magnetite in them. Many are in sills which, although thick by the standards for sills, have a negligible vertical dimension. A few, such as the stocks in the southeast corner of the quadrangle, evidently have a mineralogic composition that produces magnetic characteristics indistinguish- able from those of the surrounding rocks. Neither the highly mineralized and intruded Climax district nor the Leadville district has a distinctive magnetic expression at the scale of the survey reported here. The Climax district is in a zone of steep magnetic gradient along the Mosquito fault, and the Leadville district is in a magnetically flat area at the edge of the Arkansas valley magnetic low. A small, high-amplitude magnetic low in the Half— moon Creek area may reflect rock alteration, pos- GRAVITY AND MAGNETIC FEATURES, LEADVILLE 30-MINUTE QUADRANGLE, COLORADO sibly in combination with a concealed intrusive body, and is worthy of further investigation. An elongate magnetic high along a fault zone to the north, on Mount Massive, possibly reflects a buried intrusive body, though quartz porphyry dikes exposed at the surface probably are not magnetic. A pronounced magnetic high over the Humbug stock suggests that the rock in the stock is unaltered. Thus, if any part of the stock is mineralized, the mineralization would likely be “primary,” in the sense of being of the same age as the intrusive rock. 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G., 1969, Mineral layering in the Twin Lakes granodiorite, Colorado, in Larsen, L. H., ed., Igneous and metamorphic geology: Geol. Soc. America Mem. 115, p. 235—261. Woollard, G. P., chm., and Joesting, H. R., coordinator, 1964, Bouguer gravity anomaly map of the United States (exclusive of Alaska and Hawaii) : Am. Geophys. Union and U.S. Geol. Survey. U. S. GOVERNMENT PRINTING OFFICE: 1972 O - 466-308 UNITED STATES DEPARTMENT OF THE INTERIOR PROFESSIONAL PAPER 726~C PLATE 1 GEOLOGICAL SURVEY Total magnetic intensity BOUGUER ANOMALY, IN MILLIGALS TOTAL MAGNETIC INTENSITY, IN GAMMAS Bouguer anomaly / A I MOSQUITO RANGE 15,000' MT OF THE MT LINCOLN 15,000’ HOLY CROSS CHALK MOUNTIAN SAWATCH RANGE Suuth Plaiie River Eagle River ” m 10000 v‘ ‘A m 10,000’ \A m U? Wwf‘ocn WWI/fl q;;<>r:.-:A 3b :0 N O 2 177 mmwv~¢~ Homestacke Creek wwwwm Bouguer anomaly Total magnetic intensity BOUGUER ANOMALY, IN MILLIGALS TOTAL MAGNETIC INTENSITY, IN GAMMAS MOSQUITO RANGE B / 15,000’ MOUNT MASSIVE SAWATCH RANGE HORSESHOE MOUNTAIN \NCS TKi VALLEY 7 T“ ‘ - SOUTH PARK UNION FAULT WESTON FAULT LONDON FAULT SHEEP MOUNTAIN ARKANSAS A rkansas River 50 O’ INTERIORiGEOLOGICAL SURVEY, WASHINGTON. D.C.~I972—G71265 O EXPLANATION 015 Contact, approximately located Upper Cenozoic sedimentary and glacial deposits 106°30’ 106.00, Dashed where inferred; dotted where concealed 39.30, Tv . . "‘ 106°30’ 106°C ' Tertiary volcanlc rocks Shear zone 39a 30' O SOURCES OF GEOLOGIC DATA References at end of report _ H _ ,, _?_. ., _ .. . Ogden Tweto, and R. C. Pearson Tenmile (unpub. map). Inferred fault in Cenozoic deposits . . T Tertiary and Upper Cretaceous intrusive rocks Some faults, or parts of them, concealed by young HOLY 3:33: : 03:: 513:2 (unpub map)_ deposits. Bar and ball on downthrown scde CROSS . Koschmann and Wells (1948). MOU NT . Bergendahl (1963). Leadville {0U NCOI—N . Ransome (1911). mining . Lovering (1934). Permian and Pennsylvanian sedimentary rocks Gravity contours district - Singewald (1951). $— . . Butler and Vanderw1lt (1933). Includes Mesozoic rocks in small area near Breckenridge Dashed where approximately located or uncertain. T t (1956) . . . . we 0 . , 1 SCALE 1.125 000 Geology compiled by Ogden Tweto. 1969 Bouguer anomalies were computed wzth a reductwn Base from U'S' GeOIOgIcaI Survey V - factor corresponding to a density of 2.67g/cm3. In— Q/V. ' E:§:g:?ggggarson (1958A Leadville, 1:250,000,1956—62 , 4 8 10 MILES from sources listed 4 . . . ]__—_._1 Gravity survey by J_ E. Case, 1962—64 terval szllzgals V . Singewald and Butler (1941). . . . . . . MOUNT Singewald (1942). 4 6 8 10 KILOMETERS Mlss1s51pplan through Cambrian sedimentary rocks ELBERT . Behre (1953). . Emmons, Irving, and Loughlin (1927), . Singewald (1955). CONTOUR INTERVAL 200 FEET DCg , , . Howell (1919) DATUM 1S MEAN SEA LEVEL Grav1ty contour enclosmg area of low grav1ty . Wilshire (1966). Precambrlan granitlc rocks 39., 00, - Ogden Twetp (unpub. reconnaissance Gravity station INDEX TO TOPOGRAPHIC QUADRANGLES . 3532.932). GE OLOGIC AND BOUGU ER AN OMALY MAP AND SECTIONS OF THE ngn AND SPECIAL MAPS . Chronic (1954). LEADVILLE 30—MINUTE QUADRANGLE, CENTRAL COLORADO Precambrian metasedimentary gneisses C—C’ is line of profile Shown in text figure 6 39°00' : 2211:??3331161'5 (1949). IQLIF NURT—I PROFESSIONAL PAPER 726—C UNITED STATES DEPARTMENT OF THE INTERIOR PLATE 2 GEOLOGICAL SURVEY \ r 1 i’lt - , , i. M 7 M , ll: 7/ Wu H r. ~ ‘ H _ — r r I M “ ,. , L ‘ //, X ‘ K i L. ‘- l l i c; l 'r"; ‘ " / a I u» ,, v” ,x’ l O c/ / _ r ‘ l a m I '3 t , . , I 7 [M [W ~ , ' .4 II VI/ 0 ‘ T i ‘ - ) / , i ‘ ( T “S » I 4 / f l at ’ a/ /l \ I 41/. r. t .f i - ' ~.; , \‘ . -rngn l 2’ x / ’ I \ t , - J / fl ' é ‘1 ( / / /\ ‘ t f / i , i / , — » , / S I \ / / xl . t T - ‘ 9 I / / a ~ ~ , . ‘ ‘ o - O 2 ’ ' / g / i , , ‘ ' , EXPLANATION . f i 4 v , ~ / ‘ I “ :9. / ' ' o \ i I 5 fl , Ozs \ _, / " » »- ' l, . f ' - ‘ " r \ ' Upper Cenozoic sedimentary and glac1al depos1ts ~ I ll- “ («J Tv «M i H _« N , , \ . f , \ Tertiary volcanlc rocks \I \ fr 8 7/ / 'I ‘ L,‘ L w» r I, , ‘ ' ' C3 i‘ \ \ v k L r ~ ‘_,. w ’ 2' 3 \ ~ 4;" \ , ‘1 TKI /' l x a /7 —. ' V, r I \ ' ' ‘j, .7 " / » , Tertiary and Upper Cretaceous 1ntrus1ve rocks , f / . _ {I \ I _ v . i -\ ~. ~ . \ \ . / H .. ~_: \ , , ‘\ ~‘ ‘ _ PIPS ’ ; . - o ‘ " ‘ ». . » " . 2, f \ . .. / ‘. i: Permian and Pennsylvanian sedlmentary rocks ( x” / ,L N s w— ‘ ~ :"1/1 Li - _ 5 Includes Mesozoic rocks in small area near Breckenridge i w L // \ ._ [Ey/Aé/ / I ' (7 /”/{r , / ‘ , i » o x l! /'[l 3 i ‘ ’ l ' ’ ,J ‘ 1 \‘7‘ \ o / MCS ’ 5:23 2,\ ‘~ f z i] \C j." J Ozéfso t ' K . r ’ " V , I 0 ._ ‘\ /’ ’ , «I as M ’ ' i 5 I \w P , ° i " O Mi i i ' n h h ambrian sedimenta rocks . ,2 I‘- I Kl ~ o / l M) (’7 . 555‘; C 233‘? 1 .1 . ,2” a, . , ss ss ppia t roug C ry b («a I a l \ ( I _ . I _ I IL [I I < p 3‘ . ‘ , r 3 ._ ‘r" """T ’ i j I J \ " \K \\ v ' . {x 0 I peg I) [a , t s A ‘ \ If? _. . . _ . ' , l ;l r f ‘” \‘ V / ' ' A I » \E L ~ :\ > T " X ' V ‘1 \ \ Precambrian granitic rocks / 1’ ) K jg \Q/ \5 I , ( .. . '. fl _ 4 " r , _ , ,,,,,,,,,,, ‘ ‘ ‘» ngn w . : ,5 I fir": _ _. ‘ Precambrian metasedimentary gneisses i Lg R r- /:'.q T \ ------- _ ’ k e I .. (5:37 , i -~ _ T N * I F” m a ~ _ ‘ Contact, approximately located a V -. ' ‘ r ' : PIPs \ p , ‘ 3:“! V5._,/ ‘\ ' __—.—__ ...... \ I - :a-n: :3 \\%, , . ' \l (3 Q \\ {I \J“ \\\ )I ‘ Fault \ V w LU b l I ’ \ O \- 1 I Dashed where inferred; dotted where concealed , \ a... ._____—L \ .. J w I . VVVVVV m ~ [0: \ . .50. q ”a- / I ' . K, . , ~ , ., 2:31:11. r ; ‘ 1 a / , , . \ ., ' ‘ «. .~ ' ~ ‘ “—4 (a i T, f I? x A r"? ' a ‘0 \ , __ .// , ,9 _a V3st T' a Shear zone * , ~ 0 a v: x , ' ' . . : o (I am?- < ' B I O L o'.‘ f (I m 9 G if;- _..__..—L.. n.— ’ \J E” 13 .- ‘ , - t ‘ s‘ T - __. . R‘l Inferred fault in Cenozmc deposrcs ‘ \ i i _ ‘1 '- ' / ’ " . T < I I \\ Some faults, or parts of them, concealed by young / ‘ I ‘ 1 , x G O ' *tx ' deposits. Bar and ball on downthrown side I ~ no / I “ - 2 ' - / . 1 \~ * I O ‘ N ‘ . \ \ x . » « /. , S n t n \_ > , \ ~ * g , O . , Q m x . i ‘ x ‘/ . / 1 \ ,- . he .i -- , ‘ - , - . / 2 - m C" ’ \ : g fir _ M; . C w ~~~ ... . _ L M) __ - Oz _ m L, , ,,,,, , _ .- -. t \_ L ‘ 4/ f] 2500 s ‘7 “‘2. 5 - // - _ K ~~ ‘ )1 V. I """" Magnetic contours / . "1 / l 2“ \‘ ' . \ » "A N L‘ cw "w w I , P Showing total intensity magnetic field of the earth in x‘ ' ”b I / x \\ 3,5 ‘ . - _ ' \‘ -\ gammas relative to arbitrary datum. Hachured to - l I . 0 O y B’ indicate closed areas of lower magnetic intensity. 6 _ \ D a c; I Contour interval 20 and 100 gammas B i ' / » . . m —» vvvvv O \\ . o . , ' [2 O TI \ I): N / O \l ‘1 \ n . 160 o S - . 5 i l . ,, ‘_ x . i 2i) 8 é," "- " I , _ - 7‘ . ; I . l‘v‘ 13/ Measured maximum or minimum intenSIty ‘ Q " __ ; . ‘ _...~ .d l m ...... within closed high or closed low = / L , \ “7 __________________ \,,‘ I _ ._ 01$ . \ I (9/? . . Flight path . * . . o \ \ ». , > % Showing location and spacmg of data . ‘ / A . ,_ .. ,. (J §I / \ 3 2 _ A l / s - f _ O l l i _ ‘ I I l I ‘ , /' I a) I , q Magnetic anomaly discussed in text a u :\ Q T \ c X \ f \y ...... ‘ ~* I \ \ \ J}; ;\\\ 4 ‘1 L \ _ 1 . . t x w M - ,_ fl 99 N \ / \\\ , . _/\ 1): J . t “1’” / “I 21$: a?“ I _ . , ’/ L, i 7 H 5' R9} ‘ \, it r ' »:’ i 110’ (33 \ . i / V b / \ - , _‘ l ,’l T, ll 7 ., . ~_ 4 “ ' , ,— f . J 190 ?8 er!» N” . ‘ - - " "‘ 47/N , ‘3 .2 . _, .. Minx“ »\ 9' / 001' I ‘ ,g 0 I” ~ , I _ -3/ Tu e I. A / Q ~ ~ «« / , ~ 0 . _, a ‘5 p ..... ‘ x99 - m , / 3 r x , ~; - ‘ kg / / O F L T‘\‘ o , ‘ / r C / rwxr _ \ . d g I, . ‘ "b g / , q , I . , _\P’u i :‘\ a ‘ ’ It, ’ PIP \‘ L p ‘KQI .3 I F l ' - » 9.x: a ‘ "' 9:) \(9 2’2 ‘-. o ’ i " " 3‘ S I/ \ \ ' l I i - j. ‘\,\J/\ Q g L .._\ . 1 <’\ , . 72/ . w. 0 \ . , . . . . . , . _,,_ / . g ‘ N . ,, . . t \ i , -_ l ‘ " ' ' 01$ \ j . . \\ \ ‘ ’\ I ' . 3 \ v X [0/ / \ 9% O x l 940 \ —. kl ' ‘ \ Q I i / \I T O \ I' 2 € 0 \ / <5 \ \ .. (\I . . ff _ — . ., 0 ‘ \ . J , ., . . . , _, , , I m \ i 6 ,7 , r. " - - V x , ’ ~~ ,. J \ .. ’x. -, N - ' x° '\ ~ ‘ “ ' * \ .. ;V\\ “M‘- . , 4. ,4: 5/ ' \‘ . \ \_\ _ , ‘ - i” f F I,” V ,/-,\\ l ,« LT :W X E 1L; gt / \C \r»/ “a > » / x ‘ I: -. “ 0 ‘ .~- 2 \ , ”‘ ’ ’ "" ‘ "'2 I ’ ”j s H x , 'l / \z‘ T . " - " - , ~. ‘ '- / r' '7 f? _, X K Law- 2 ,\ ”Lei ,w q 7/ “ 11’ . ",1 1’"; It In _ x “ \ \k‘rw ”X \ , .... I . \6i :0 . _, 2: , 1,; 2' .91 it ~ I: m . :2 E" 7’33 F? f 7 u; : ;-« . Base from US. Geological Survey 13.1/2° SCALE 1:125 000 Aeromagnetic survey flown at Leadville, 1:250,000, 1956—62 2 O 2 4 6 8 10 MILES 14,500 feet barometric elevation, E L_._i ,__i }—————-—l . : 1963, and compiled by U.S. Geological g Survey. Northeast quarter from m 2 o 2 4 6 8 10 KILOMETERS resurvey 1957 3 i—--l ' l l——l [—1 v - r H Geology from plate 1, this report * CONTOUR INTERVAL 200 FEET ”PROX'MAIE MEAN DATUM IS MEAN SEA LEVEL DECLINATION, 1972 3300 - r" 3300 3200 -l — 3200 3100 J — 3100 (I) 3000- — 3000 U) 2 2900— — 2900 E < 4: Cl 2800"~ _ 2800 (5 2700 — - 2700 2600‘ — 2600 2500— 2500 WEST EAST 15,000! MT OF THE Flight level of aircraft, 14,500 feet 15,000: CL C o s H v R SSAWATCH G o R E TENMILE RANGE 10,000’ 10,000' 5000' 5000’ PROFILE ALONG NORTHERNMOST FLIGHT LINE 2700 2700 2600 2600 w (I) g 2500 2500 ‘2: <2): 2400 2400 (<25 (.5 2300 2300 2200 2200 WEST EAST 15.000' ____________________________________ F Ehfiflfiifleflfifflfgeg____‘_______________.__________________ 15,000' LA PLATA PEAK MOUNT HOPE GUAM MOUNTAW MOSQUITO 10,000, ARKANSAS VALLEY 10,000, 5000' 5000' PROFILE ALONG SOUTHERNMOST FLIGHT LINE AEROMAGNETIC MAP AND PROFILES OF THE LEADVILLE 30—MINUTE QUADRANGLE, CENTRAL COLORADO 466-308 0 ’ '72 (In pocket) UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY 115.00 R IQE R (II, E P :35 I? ; E R 23 E 30 R 21w R 20 w 3.3M) , II I _. 'I f: I / , I ( II: j J E% I » . ”It I I . . / / . o ‘ l4rlmez _ _é'__' ’\\ k‘ ,, 'LS if,” lrEQ ¢_/,. W-.. I a _ " q x: ., _;/._-_/ i f —I / - ' a -- c .‘ ‘ ‘ " 3 “ -shers{anding/vw -./ _ -._\2\50 I s \ QTa ' . 1‘ 0: I \ an " ' \\ a . 7.11 A ‘ .7 ' \ /,- .1 -.“- " - , q. I\I /r~—~/ \ '/v' \ ‘ II V .- [j :3 I“ T\ \. /.. <fl%//o / 'I , \ . /' - / G. a 4 \_ ,/“ / Iii Inflow « , gamma I '- . r \ as Ix // x : , \ D I mg \ \ l\/-) ', // . './ y/ ¥M a Q. \\ a / I ,’ - '.I. _.,«:-—/ /./ _St ”I / 3/ .' . A k / 0 ”at w'rO v I \ .I \ .A, I. . I / . 1V ; d . n /;O T \ 3 / V” / -/I /§ ,' _ ‘ O /(§~._ [Hr ,1 t‘ / \ / m, 9“, \a /‘ ° ”V" iQT , a, ./ ,~ ——+ v . .. ' : 1 IT. .¢ J’ ‘1 \ Q 4‘36 J ‘ ~. \ , ‘j‘ -‘ J T‘ a» ‘ it?“ ’ I . I“ ‘1'? ( N \\ I ' I”; ' ' o ' \j‘ c a 4 I :v ‘ 7‘ J! I , a we I ,- _ rd .. . _ _ an ,/ . - “'57 +- If K . x‘ \ ’7’ E! .... ' , ' I l . 1"; 9,) e,“ u cm x ‘ . ‘—‘ at K. \ ._ q f4)“ ,, 1/ 5 I #44” .: g ' \ w ’ ‘ I I < z r a”; I . “V" — . ‘ a . ’ \ " '~ a ' ., “!‘» a q .1». Sta on \ I 2 .i I N L ‘ .. A _ ‘ IV V _ 5 J M . V j 3 ' {'2’ 7 'I .ra‘. My 1' ‘f” ' ’ S A “ ' T .2 I ' To a; lo - < i ‘ 45' '///( x > x > \ '&.L- 5 _ 5% 3. .T.8§ .(AHCQRNIAxI’ _ _ . Q iv "TE-Tim CAI II’OPN‘A ‘JI; " -. ur\\ q ‘ " > \()r nul. [era] \‘ I , I. .fi_ \ c V— . f1 Base from US. Geological Survey rr‘ El Centre, 1958 19 . \\ ’ _. vboleT‘D'" \~-\_\_\ ‘\ WV . C—vN “A . K3 I a; ”J \ I Ir / '53? -' QTaITT’N T 9 s A, " -_+_j _;_._-. K z‘I . \| ;[ ICOCE . (. / \ I INDI I' I / ; I - / / I < I‘ I .r , c I , Q ' . c am I/ V / f. 1- 402 . _/ s ' -' \ ‘7 Ins 7, : .V /I ° Q ‘ ‘ 5 ’ K’ '3 T 10 s I ‘ I K I’T—_ ~ IS °. 9 III/S (‘3 I " N . ‘~-_ _‘ ,_.\- I III; '1‘ 0’” .-K; I :3 /‘ Yuma Number ,Ir “([344 ‘ " ‘ ”vi - 3" 5r: 2 #6.. 9° 32.30. 3/ Tn . ‘ :1“ R :5 w G ‘90 R. Q NEG .1 . ' \ S g I. 7‘ ‘0. I 3 .111" SCALE 1:250 000 5 O 5 10 15 MILES R. ZI W- E L—I l—I I——¢ % :L j . E ' . g 5 o 5 10 15 KILOMETERS s E E: Z 9‘ _ CONTOUR INTERVAL 200 FEET APPROXIMATE MEAN DECLINATION, 1973 WITH SUPPLEMENTARY CONTOURS AT lOO—FOOT INTERVALS DATUM IS MEAN SEA LEVEL GEOLOGIC AND BOUGUER GRAVITY MAP OF THE YUMA AREA, ARIZONA AND * U-S. GOVERNMENT PRINTING OFFICE: 1973—515—659/42 PROFESSIONAL PAPER 726—D PLATE I EXPLANATION “ § § 6 8 Alluvium and windblown sand g “ v§ Alluvial sand, gravel, silt, and clay, ll. it: and well-sorted fine to medium sand of eolian origin in dunes and in thin sheets on Conglomerate of Chocolate § Bouse Formation Mountains .§ Younger marine sedimentary rocks Conglomerate and gravel composed ‘3': consisting chiefly offossiliferous chiefly ofvolcanic detritus. Ranges clay, silt, and fine sand. Exposed in age from Pleistocene to Miocene only in one area, 2—3 miles south- east of Imperial Dam g”??? N onmarine sedimentary rocks . §N§ s Clastic sedimentary rocks ranging VOICaHIC I‘OCkS § : § from megabreccia andfanglomerate Pyroclastic and flow rocks ranging g a 3 to mudstone and shale (in part, of in composition from rhyolite to lacustrine origin). Locally inter- basaltic andesite or basalt. Locally bedded with volcanic rocks (TV) interbedded with nonmarine sedi— mentary rocks (Tn) 7 We? Consolidated rocks Include Tertiary volcanic and non- Basement complex marine sedimentary rocks and pre- Platonic, metamorphic; and dike Tertiary basement complex rocks. Granitic rocks, gneiss, and schist are most abundant Contact Dashed where inferred —A—#—&g ::_ ._ _ _7 ........ Fault Dashed where inferred; dotted where concealed; queried where location or trend doubtful. Sawteeth indicate upper plate of thrust fault. U, upthrown side; D, downthrown side. Arrows indicate inferred direction of horizontal displacement Gravity contours Dashed where approximately located. Hachured contours enclose areas of low gravity. Contour interval 2 milligals Gravity station 27 ¢ Drill hole or test well All numbers are preceeded by “DH-" in text Numbered or lettered profiles are illustrated in figures or on plates with this report / a a Resistivity profile A A’ Gravity profile 8 m Seismic-refraction profile . , location of shotpoint Seismic-reflection profile CALIFORNIA TERTIARY PRE— TERHARY TERTIARY AND QUATERNARY UNITED STATES DEPARTMENT OF THE INTERIOR PROFESSIONAL PAPER 726—D GEOLOGICAL SURVEY PLATE 2 LITHOLOGIC RESISTIVITY ACOUSTIC VELOCITY (ft) LOG (Short-normal) merino-m 0 10,000 20,000 fps 0 I I I | I 100 H L Coarse clean gravel , 200 — * 30 — — 0 L ITH 0 L0 G Y Gravel and sand, sandy gravel 400 — — Sand and gravel, gravelly sand 500 — g — Sand and scattered gravel E Cemented gravel f" <=( Sand 600 — ~( Sand and silt, silty sand 700 _ '3‘? 1 It} # Silt, sand and clay, sandy clay Clay and gravel, clay, sand, and gravel 800 - L Clay, silty clay, clayey silt 900 _ _ Sandy limestone 1000 * L Megabreccia _IL “6 S E C ,9 ‘g Tuff(?) 1100 —— S _ I— 1200 — L C .Q ‘5 E E Q) (I? 3 O m 1300 — L 1400 * L 1500 — L g D E >‘ E E' (1) E '0 3 1600 — g — 5 E C 0 Z 1800 I I I I I BOREHOLE GEOPHYSICAL LOGS OF WELL DH-17 (USGS LCRP 26), YUMA AREA, ARIZONA * U.S. GOVERNMENT PRINTING OFFICE: 1973—515—659/42 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY \ ‘\; ,' l § ; l Ii f“; \ i ,L I ,, t _ a: / ’1 I; , Elmo |.___.._..r S QTa Base from U.S. Geological Survey El Centro, 12250000, 1958 SAN L U / S 4 NO ‘7 , M (5‘ _ a All’ — >_ o: to 9 0: <2 QTa § § 5 o E .§ 3.2 P g m Alluvium and windblown sand of £1 E E O /éé?7 w E . . §fi s S Nonmarme sedl- *TV 3 § 3; IE mentary rocks Volcanic 2 Lu rocks I” Basement complex W—j PRE— TERflARY Contact U —“——_7.... D ‘.__ Fault Dashed where inferred; dotted where concealed; queried where location or trend doubtful. U, upthrown side; D, downthrown side. Arrows indicate inferred di- rection of horizontal displacement Magnetic contours Showing total intensity magnetic field of the earth in gammas relative to arbitrary datum. Hachured to indicate closed area oflower magnetic intensity; dashed where data are incomplete. Contour interval 10 gammas 0 O p X Measured maximum or minimum intensity within closed high or closed low Flight path Showing location and spacing of data 20 4} Drill hole or test well All numbers are preceded by “DH-” in text Numbered or lettered profiles are illustrated in figures or on plates With this report A A ’ Gravity profile 7 Seismic-refraction profile ' , location of shotpoint Seismic-reflection profile Aeromagnetic data were obtained along 29 flightlines spaced about 1/2 mile apart, 1,000 feet above the ground surface, and oriented approximately N. 37" E. Area flown in 1964. PROFESSIONAL PAPER 726—D PLATE 4 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY SOUTH Mesa anomaly Yuma anomaly NORTI: 0 A F—~_—‘—b——\ 2 —10 — I ' ’ ' — ~10 < 8 —20 — ‘ ‘20 _J Bouguer gravity anomaly / g —30 — — ‘30 40 I 40 3300 — — 3300 3200 — —- 3200 (I) ‘5‘ 3100 » — 3100 2 PICACHO—BARD BASIN 5 3000 — . _ _ Mesa anomaly Yuma anomaly — 3000 Magnetic mtensrty Q: N 2900 I}; g - 2900 > e 2800 YUMA BASEMENT HIGH E D -2800 m A o s B a 5 S: 2 B ’ m 1000? g a gig a g “Q: a) E; Cargo Muchacho Mts. “1000, L'- l 3 SAN LUIS BASIN g 2' E E Aiiuvium Alluvium § 3 5c f A ‘ V LEEA M TD 79 It Chiefly alluvium, but :VVA “JEEP—‘7‘?" _ . Alluvium .' " Bouse(?) Formation “: LEE/EL EL probably contains « )V “Mfg/(‘5, -, 4 H" L“; «J \ Tertiary nonmanne x:- ' at 323 ft Alluvium r4,» rocks of transition 7 7r L > 4 U V P a”) :< r < Contact from seismic- ,‘\ sedimentary rocks Basalt xxx T0 360 ft _" ‘ > . zone and Bouse For- 4 r > V I" v ) " (”L a 4 4 ’ V 4 " refraction profile 7 L t—L \\(breccia and conglomerate) ____—’___,_$V_ ) v ‘ ) t 7 Pre—Tertiary IOOO’—I , Alluwum > mation and possibly r" Pre-Tertiary basement complex 4 4 :v)\ _ . _I/<: 4: AIL) v z > r r L v A ,4 basement complex —1000’ (deposits of the Gila and Colorado Rivers) older Tertiary sedi— A ’8 (porphyritic quartz monzonite) 4 “(\r Contact from seismic- A V , A ., v r > P > < A ‘V r Contact from seismic- 4 ‘ v r 1 4 N\ refraction profile 4 /{r 3 < |~ " A v " refraction profile 4 —i V 7 A " V r '\ r 7 > ,_ /// r , , _ , < 7 ,.\.\, /v I, 7 " _ 1 2000 ____ __,/ t > Contact from grawty profile C C 7 P A» < A a LETiZ—VFContact estimated 2000 7 from gravity data Transition zone 3000’ 4 < — 3000’ _ r: > 'I h h 4000' I ‘ ~ 4000' Pre-Tertiary 5000,: TD 4868 ft basement complex —5000’ < 7 L F 6OOO’A -~ H 6000’ TD 6007 ft A V 1 Tertiary sedimentary rocks ‘fczmrtact from DH—25 to DH—3O computed from NOTE: The small southward displacement of the magnetic anomalies 7000; Fr 14 gravity data and seismic—refraction profile 8 relative to the grayity-anomalies are attributed to the inclination of —7000’ t r the earth's magnetic field ——59" in the Yuma area. The contacts are i 1090 20‘00 FEET dashed where inferred or where geophysical data are doubtful. 8000' — 0 5000 M ETERS — 8000' VERTICAL EXAGGERATION X4 9000’— — 9000’ 7 Pre—Tertiary basement complex 10,ooo'— ’ —10,000' 11,000’— ~11,000’ GRAVITY AND MAGNETIC PROFILE B-B’ ALONG APPROXIMATE AXIS OF YUMA BASEMENT HIGH, YUMA AREA, ARIZONA AND CALIFORNIA ' “ 7 12,000' 12,000’ V * U-S. GOVERNMENT PRINTING OFFICE: 1973—515—659/42 e 6 PROFESSIONAL PAPER 726—D o O? UNITED STATES DEPARTMENT OF THE INTERIOR PLATE 5 eye 61, GEOLOGICAL SURVEY R. 23 E. 114030, 11445, 2120002521 E I 111','v Irrgr , .L‘ m. . . 40’ (PICACHO PEAK) ' ' I - I . , ‘I ‘ 260000 FLT (AW’II I ‘R' 22W 32045“ 3245f . ; cw Kr \ - .- _ m , “I I , ,. , gs ITWEF-fi': I» 23 I ‘ o 3 4' .. I ~ U7HFI‘2N 777777777777 :12 {BB .212 ‘5 ‘ KN, '. j k 313‘ ___‘_x ’LT 7 . ii ” 1 ‘ . a 0 »\ ~ ’ ‘ I- I a .V *2 W95 :6 21000:) FEEI_W' I“ “ W V: (CALIF I . vs ‘ . , $330 000 FEET (ARIZ) 3623mm: T 16 5 fl . T. 8 S E 8 E 1‘3 :2 1 a o E;- k E um E : o, '0 HOIDIIBI 13.9 _ ,4 x 'm ' #52 . , 5 33 1, 5E i I 40’ I l I l , l . I- _____ T 9 S T. 9 5 ~ 0 o O I I!) ‘3 E E D >. y . : (I Z 3 L W 0 it E E ' Gin .— In? 3hr 0U) "Q $3 ”I : 35’ . 35’ T ‘0 S T. 10 S ‘ I ~ I .L E X P L A N A T I O N I . I W Numbered or lettered profiles 29 I 28 O f b are iillustrated in figures or V I UtCI‘OD 0 asement rock on pllates with this report I W“ 20m... > I, 28 I {/z/ W... ........... M d d ’ E__//;;E ‘1? / / ‘ I TTTTTTTTTTTTTTTTTTTT I— TTTTTT GraVity COHILOUI‘S Resistivit rofil E «4/ 7f: \ x .. .. 11 Contour interval 1 milligal yp e My, ”5/ I - C C ’ ‘ I 2 I a 3 Gravity station . . __./ ‘ I V Grav1ty profile 33.9mm." ‘ I 27 L . l ‘ <> 8 130 000 FEET 550 ooo FEET 5 _ /_ ______ 4_____-__ Drill hole or test Well Seismic-refraction profile /___(CAL|F-) (ARIZTI— 7 I All numbers are preceeded . , location of shotpo'mt 0) / by “DH—” m text ,9 INS l DJ . T “““““ 7“ . /3 +115. I 5 I o 4 Seismlc-reflectlon profile / K I ' ‘I J | 1 323301 L ‘ I F. 7* . I . I:I ,. IL ‘ \l\1. I \IESJ I V I 320301 1140451 ‘ (YUMA /‘.'/25 000) R- 23 W I I 35/ I I 1140301 00‘ Base from U.S. Geological Survey, 1940 1.1" SCALE 1:62 500 If) 10,0007f00t grid based on Arizona coordinate system,west zone, 1 V2 0 1 2 3 4 MILES ‘-\ and California coordinate system, zone 6 g I=LH H H H ; fi‘ 9 fil 7 ARIZONA 09? 1000-meter Universal Transverse Mercator grid, g 1 .5 O 1 2 3 4 KILOMETERS cl zone 11 5 Eu: H H H .L j I— a ‘1 APPROXIMATE MEAN CONTOUR INTERVAL 25 FEET QUADRANGLE LOCATION DECLINATION,1973 DATUM IS MEAN SEA LEVEL DETAILED BOUGUER GRAVITY MAP OF THE'SOUTHERN PART OF THE YUMA BASEMENT HIGH SOUTH OF YUMA, ARIZONA * U~S. GOVERNMENT PRINTING OFFICE: 1973*515-—659/42 70A? EARTH scream-3 ll BRARY LE7; Geophysical Studies in the 7:” Yuma Area, Arizona and California GEOLOGICAL SURVEY PROFESSIONAL PAPER 726-D AHN DUL-UEVEERHS HM 1’ (:11; AUG 10 7973 "IU‘Insl‘p‘z ”MW: 1"! 51"1A'1Hrfifi‘g'x #“N V ‘ ‘ 1 Na g’:{\v’ {1' 1" I/ I ; \ \ \g i, .- xi. 71175.21 :1. Geophysical Studies in the Yuma Area, Arizona and California By R. E. MATTICK, F. H. OLMSTED, and A. A. R. ZOHDY GEOPHYSICAL FIELD INVESTIGATIONS GEOLOGICAL SURVEY PROFESSIONAL PAPER 726-D Geophysical studies, including gravity, aeromagnetic, seismic-refraction, seismic-reflection, and resistivity surveys, of the Yuma area, Arizona and California, in support of a geohydrologic‘investigation UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1973 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 73-600114 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20102 — Price $1.75 domestic postpaid or $1.50 GPO Bookstore . Stock Number 2401—00327 CONTENTS Page Page Abstract ................................................. D 1 Geophysical surveys — Continued Introduction ............................................. 1 Seismic-reflection surveys ............................. D10 Geology ................................................. 2 Interpretation of structural features ........................ 11 Physical features ..................................... 2 General structural pattern ............................ 11 Major rock units ..................................... 3 Gila, Laguna, and Chocolate Mountains ............... 11 Regional structure .................................... 5 Fortuna basin ........................................ 15 Physical properties of major rock units ..................... 7 Picacho-Bard basin ................................... 18 Density .............................................. 7 Cargo Muchacho Mountains .......................... 20 Magnetic properties .................................. 8 Pilot Knob .......................................... 22 Electrical resistivity .................................. 8 Yuma basement high ................................. 22 Acoustic velocity ..................................... 8 Boundary basement high .............................. 28 Geophysical surveys ...................................... 9 Yuma trough ........................................ 29 Gravity survey ....................................... 9 San Luis basin ....................................... 31 Aeromagnetic survey ................................. 9 Algodones fault and related faults ..................... 32 Resistivity survey .................................... 9 References cited .......................................... 36 Seismic-refraction survey ............................. 9 ILLUSTRATIONS [Plates are in pocket] PLATE 1. Geologic and Bouguer gravity map of the Yuma area, Arizona and California. 2. Borehole geophysical logs of well DH-17. 3. Geologic and aeromagnetic map of part of the Yuma area. 4. Gravity and magnetic profile B—B’ along approximate axis of Yuma basement high. 5. Detailed Bouguer gravity map of the southern part of the Yuma basement high. Page FIGURE 1. Map of the lower Colorado River region, showing location of the Yuma area ........................................ D 2 2. Chart showing stratigraphic relations of the major rock units in the Yuma area ................................. 3 3. Map of the Yuma area, showing inferred extent and configuration of the top of the Bouse Formation ............. 6 4. Graph showing average density contrast between basement rock and Cenozoic sediments ........................ 7 5. Seismic-refraction profile 1 across gap between basement outcrops of the Chocolate and Laguna Mountains ....... 13 6. Seismic-refraction profile 2 across the Colorado River gap just upstream from Laguna Dam ...................... 14 7. Seismic-refraction profile 3 across the Gila River gap between the Gila and Laguna Mountains .................. 15 8. Gravity profile A-A' across the Fortuna basin from DH-25 to the Gila Mountains ............................... 17 9. Resistivity profile a-a' across the Fortuna basin along the southerly international boundary, showing interpreted results of four vertical electrical soundings ............................................ 18 I0. Seismic-refraction profile 4 from near the north end of the Yuma basement high to a basement outcrop east of the Cargo Muchacho Mountains ...................................................................... 21 ll. Resistivity profile b-b' across part of Picacho-Bard basin ...................................................... 23 12. Seismic-refraction profile 5 from Pilot Knob to the Cargo Muchacho Mountains ................................ 24 l3. Seismic-refraction profile 6 from Pilot Knob west across Pilot Knob Mesa ...................................... 25 l4. Seismic-refraction profile 7 along Pacific Avenue in Yuma between two basement outcrops ...................... 26 15. Observed and computed gravity anomaly on profile C-C’ across the gravity saddle between the Yuma and Mesa anomalies ......................................................................... 26 16. Resistivity profiles c-c ' through f—f’across the Mesa anomaly .................................................. 27 17. Seismic-refraction profile 8 on the south flank of the Yuma basement high ..................................... 27 18. Observed and computed gravity anomaly on profile D-D ’. across the Yuma trough ............................... 31 19. Seismicmefraction profile 9 from Pilot Knob eastward to near the north end of the Yuma basement high ......... 33 20. Observed and computed gravity anomaly on profile E-E’ from well DH~25 to the center of the San Luis basin . . . . 34 2|. Resistivity profiles g-g’through k-k' across the inferred trace of the Algodones fault southeast of the Yuma basement high ............................................................................. 35 III IV CONTENTS TABLES Page TABLE 1. Depths of Tertiary and pre-Tertiary horizons in wells in the Yuma area ............................................ D 4 2. Summary of resistivity data from selected wells and electrical soundings ........................................ 8 3. Acoustic velocities of major rock units penetrated in well DH-17 ................................................ 9 GEOPHYSICAL FIELD INVESTIGATIONS GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA By R. E. MATTICK, F. H. OLMSTED, and A. A. R. ZOHDY ABSTRACT During 1963-67, gravity, aeromagnetic, seismic-refraction, seismic- reflection, and resistivity surveys were made in the Yuma area, Arizona and California, in support of a geohydrologic investigation by the US. Geological Survey. The primary object of the geophysical study was to investigate the regional geology, particularly the gross dis- tribution and thickness of Cenozoic sediments, which contain the ma- jor aquifers. The following major structural features in the area were revealed by the study. 1. Shallow channels into bedrock beneath the Gila River where it crosses the Gila and Laguna Mountains at the Gila River narrows and beneath the Colorado River where it crosses the Chocolate and Laguna Mountains between the Laguna and Imperial Dams. Between these dams on the Colorado River, a channel has been incised into rocks of the basement complex to a depth of about 600 feet and filled with Cenozoic sediments. At the Gila River narrows on the Gila River, a channel has been incised into Ter- tiary breccia and conglomerate (of undetermined thickness) to a depth of 320 feet, and the fill consists chiefly of alluvium. 2. A significant trough west of the Gila Mountains, designated the For- tuna basin, that is estimated to contain about 16,000 feet of Cenozoic sediments, the lower seven-eighths of which is pre- Colorado River marine and nonmarine deposits. 3. A shallow extension of the Fortuna basin, designated the Picacho- Bard basin, that extends northwest between the Cargo Muchaco and Chocolate Mountains and has considerable basement relief. 4. A multiple-crested basement high, designated the Yuma basement high, extending roughly southward from Yuma and comprising two main parts: the northern part, associated with basement out- crops, and the southern part, associated with basement rock at depths of less than 100 feet. Geophysical data in the vicinity of the Yuma basement high show that considerable basement relief, probably resulting from a highly complex fault pattern traversing a buried erosion surface, is associated with this high. 5. A basement high centered on a row of basement outcrops on the southerly international boundary, designated the Boundary base- ment high, and separated from the Yuma basement high by a deep, broad saddle. 6. A deep basin along the southerly international boundary, south of the Yuma basement high and west of the Boundary basement high, estimated to contain about 13,500 feet of Cenozoic sediments and designated the San Luis basin. 7. A trough west of the Yuma basement high designated the Yuma trough. In addition, the top of the Bouse Formation (Pliocene) was mapped in most of the studied area. The top of the Bouse Formation is impor« tant because it is an indicator of the maximum amount of deformation in post-Bouse time (the time since the Colorado River entered the Yuma area) and because this horizon may be considered as the effec— tive floor of the main part of the ground-water reservoir. A lineament in the magnetic data and a gravity nose north of Somer— ton are interpreted as reflecting right-lateral movement along the Algodones fault, a major northwest-trending fault in the Yuma area and a possible branch of the San Andreas fault system, A westward offset of the southern part of the Yuma basement high relative to the northern part of the high can also be explained by right-lateral move- ment on a parallel fault. Seismic-refraction data at the south end of the Yuma basement high reveal that the basement on the southwest side of the Algodones fault is downthrown 350 feet, and gravity data about one~half mile farther south suggest an additional 1,000 feet of throw on a parallel fault. lNTROl)UC'l‘lON Beginning in 1963 and continuing intermittently through 1967, geophysical surveys were made to obtain subsurface information in support of a geohydrologic in- vestigation of the Yuma area by the US. Geological Survey. Results of the geohydrologic investigation are given by Olmsted, Loeltz, and Irelan (1973); results of the geophysical surveys are summarized in the present report. The area studied is in the southwest corner of Arizona and the southeast corner of California, about 70 airline miles north-northeast of the mouth of the Colorado River (fig. 1). An initial regional gravity survey was made in 1963, and an aeromagnetic survey was made in 1964. In the years 1964-67, seismic-refraction and seismic- reflection profiles, resistivity soundings and profiles, and detailed gravity surveys were made at selected places to complement the earlier regional work. Gravity obser- vations cover an area of about 900 square miles (most of the area of investigation shown in fig. 1), and the aeromagnetic survey covers an area of about 300 square miles in the southwest corner of Arizona. The gravity and aeromagnetic surveys were directed by l). R. Mabey, and the seismic—refraction surveys by D1 D2 GEOPHYSICAL FIELD INVESTIGATIONS 117' 116° 115' 114” I l flown/mo l I RIVER .. 9: Kg / g! ‘33 l 4’ i \ : Area of 33° , \ ,7 \ O M‘ investigation _ < o\ lidiifguo EH-PCOU‘TE"? \ ) 6‘ 11, MTSX‘I': 3 M15 , IMPERIAL DAM \ o ’0 ° ‘2 ’+‘G:.~; \ ’1’ 4;. . ;. San Diego ’ ‘ ' UNITEflTfilE—L — - "’ ‘ CAEFORNLA— _ _ _. .——-— -' TVIEXIQO O Mexicali — .— “A cALIFORNIA \ \\ \ \\ g, °I \ 87 32° \{\ O 25 50 MILES l_1_1___1_l_i____—i O 25 50 75 KILOMETERS l__._‘_._]____|—J FIGURE 1. —— Map of the lower Colorado River region, showing location of the Yuma area and part of the boundaries (dashed lines) of the Salton Trough and Sonoran Desert sections of the Basin and Range physiographic province (from Fennernan, 1946). R. E. Mattick and J. S. Watkins. The resistivity surveys were made and interpreted by A. A. R. Zohdy and D. B. Jackson, and part of the seismic-reflection survey was made by J. C. Roller. Some of the gravity observations were made by Arthur Conradi, Jr., and D. L. Peterson. Geologic studies were made by F. H. Olmsted as part of the geohydrologic investigation. The gravity and aeromagnetic surveys were used primarily to investigate the regional geology, par- ticularly the gross distribution and thickness of the Cenozoic sediments, which contain the major aquifers. Seismic and resistivity surveys were used to obtain more detailed information in local areas or along profiles. Test wells drilled for or by the US. Geological Survey, the U.S. Bureau of Reclamation, and various oil companies provided valuable information on the thickness and character of the geologic units and were used as control in interpreting the geophysical data. In addition to the geophysical studies by the US. Geological Survey, seismic-reflection surveys were made in 1965 and 1966 by General Atomic Division of General Dynamics Corp. (in 1966 subcontracted to Rogers Ex~ plorations, Inc.), under contract to the US. Bureau of Reclamation. These surveys were made in support of a special joint geohydrologic study by the US. Geological Survey, the US. Bureau of Reclamation, and the US. Section of the International Boundary and Water Com- mission. Results of the joint study were described by Herschel Snodgrass (written commun., 1965) and R. D. Davis (written commun., 1966), and their data form the basis of some of the interpretations in the present report. GEOLOGY PHYSICAL FEATURES The area of investigation lies along the east side of the Salton Trough section and along the west margin of the Sonoran Desert section of the Basin and Range physiographic province (Fenneman, 1946). (See fig. 1.) The city of Yuma, near the center of the studied area, is located near the junction of the Colorado and Gila Rivers, near the apex of the Colorado delta. The area is characterized geomorphically by broad desert plains and river flood plains, above which rise low but rugged mountains. The main features, described briefly in the following paragraphs, are shown in figure 1. The Colorado River enters the main part of the Yuma GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA D3 area through a gap about 1 mile wide between the Laguna and Chocolate Mountains and then flows across its recent flood plain, which widens southwest of uma to form the broad, low, fan-shaped subaerial par of its delta. The Gila River enters the main part of the area through a similar gap between the Gila and Laguna Mountains and then flows westward across its flood plain to join the Colorado Rovier just east of Yuma. The river flood plains are bordered by terraces and piedmont slopes —— the broad desert plains. The desert plains and river flood plains range in elevation from about 90 feet at the southwest corner of Arizona to about 1,000 feet at the base of the Chocolate Mountains. With a few exceptions, the southwestern part of the Sonoran Desert east of Yuma is characterized by elongate low mountain ranges trending generally about north-northwest (N. 20°-40° W.). This average struc- tural grain seems to continue farther west; but regional subsidence has occurred in the Salton Trough and bordering area, and as a result, only the summits of some of the mountain blocks now extend above the surround- ing alluvial fill; other blocks are completely buried and are revealed only by geophysical data and test drilling. Average summit elevations Of the main mountain masses are less than 2,000 feet above sea level; the max- imum elevation within the area of investigation is 3,150 feet, in the southeastern Gila Mountains. Although the total relief is small compared with that of many other parts of the Basin and Range province, slopes on ex- posures of plutonic, metamorphic, and volcanic rocks in the mountains are steep, locally exceeding 40°. Data from test wells and geophysical surveys indicate that some ofthe buried slopes of these rocks are equally steep. MAJOR ROCK UNlTS For the purpose of the present study the rocks are grouped into the following major rock units: (1) Base- ment complex, composed of metamorphic, plutonic, and dike rocks (pre-Tertiary); (2) nonmarine sedimentary rocks (Tertiary); (3) volcanic rocks (Tertiary); (4) older marine sedimentary rocks (Tertiary); (5) Bouse Forma- tion (younger marine sedimentary rocks of Pliocene age); (6) transition zone (Pliocene); (7) conglomerate of Chocolate Mountains (Tertiary and Quaternary); and (8) alluvium deposited by the Colorado and Gila Rivers and their tributaries, and minor windblown deposits (Pliocene to Holocene). The inferred stratigraphic relations of these units are shown in figure 2, and their surface extent, on plate 1. The pre-Tertiary basement complex consists of a variety of igneous and metamorphic rocks, of which granitic rocks and various kinds of gneiss and schist are most abundant. The entire complex has been invaded by dikes and sills composed chiefly of pegmatite, aplite, and now-altered fine-grained mafic rocks. A distinctive AGE SUBSURFACE EXPOSURES > . a .2 E Windblown g z u sand i: . . u; 2 w Rlver Local I; 3 E deposits Alluvium deposits 3 a O O = g Transition zone Conglcgrfnerate E Bouse Formation Chocolate Mountains Older marine sedimentary rocks Nonmarine sedimentary rocks Miocene >. a: S i- :2 it, Volcanic rocks _7_ II E I) .§ 2. E Nonmarine sedimentary rocks > MAJOR UNCONFORMITY fi '5; ; Basement complex: a. E Metamorphic, plutonic, and dike rocks p. FIGURE 2. — Stratigraphic relations of the major rock units in the Yuma area. porphyritic quartz monzonite, locally gneissic and con- taining large phenocrysts (or porphyroblasts) of potassium feldspar and irregular patches and streaks of fine-grained biotite, is perhaps the most widespread basement rock type. This rock constitutes virtually all the detritus in some of the coarse breccia and con- glomerate in the Tertiary nonmarine sedimentary rocks. The ages of most of the basement rocks are unknown, although all these rocks appear to predate the Late Cretaceous to early Tertiary Laramide orogeny. The only two radiometric dates available are for the porphyritic quartz monzonite from a locality at Yuma. Biotite from this rock gave a rubidium-strontium age of 73 m.y. (million years) (Wasserburg and Lanphere, 1965), but zircon from the same locality yielded a uranium-lead age of 1,440 m.y. (L.T. Silver, written commun., 1968). The zircon age (Precambrian) probably reflects the time of original crystallization, whereas the biotite age (Late Cretaceous) appears to indicate Laramide metamorphism. Some of the dikes and sills, and possibly some irregular small intrusive bodies, may be earliest Tertiary but, for convenience, are grouped with the pre- Tertiary basement complex. The Tertiary nonmarine sedimentary rocks are ex- posed in parts of the mountains and hills, and they un- D4 TABLE 1. — GEOPHYSICAL FIELD INVESTIGATIONS Depths of Tertiary and pre- Tertiary horizons in wells in the Yuma area [Location of wells shown on pl. 1. Leaders ( ....... ) indicate not penetrated; query (7) alone indicates that penetration is questionable; with figures, that depth is questionable. USGS LCRP, US. Geological Survey lower Colorado River project test well; USBR, US. Bureau of Reclamation test well] Depth, in feet, to top of unit Land surface Total Older marine Nonmarine elevation depth Transition Bouse sedimentary sedimentary Basement Well No. Name of well or owner (ft) (ft) zone Formation rocks rocks complex Remarks DH-l USGS LCRP14 . 155.1 505 209 471 ........ 2 USGS LCRP 23 . 143.8 715 548 687 (7) Possible basement complex at 703 ft. 3 Tanner Paving Co 195 396 ....... (7) ........ Possilal; gonmarine sedimentary rocks at 2 4 Gila Valley Oil & Gas Co. 145 2,140 ......... 422 ........... 482 ........ Kamrath 1. 5 SBR CH-G ............... 140.5 501 ......... 422 ........... 497 ........ 6 B Palon .................. 423 603 ...................................... 563 7 ............ do ........... 395 1, 085 ......... 700 ...................... 1,082 8 USBR CH- 704; USGS LCRP 150.8 1 997 794(7) 1,045 1,396 ................... Top of transition zone may be 29. a . 9 USBR CH-BRD ............ 128.5 360 ......... ('2) .............................. Basalt 34f? -360 ft. Possible Bouse Formation at 328 10 San Carlos Hotel .......... 145 173 ........................... 173 ........ Granite boulder at 173 ft. 11 Yuma School District N o. 1 . 175 404 ........................... (7) ........ Possible nonmarine sedimentary rocks at . 12 Abe Marcus Pool .......... 175 478 ................ 470 “Granite” at bottom. Possible nonmarine sedimentary rocks at 300 ft. 13 S & WRa ncesh ............ 141 191 ....... 190 Do. 14 Stagdust Hotel; USGS LCRP 197 1,090 ....... 1,085 Porplfioyritic quartz monzonite at ttom. 15 Sinclair Oil Co. Kryger 1 . . . . 120 1,400 (7) ........... 1,243 1,398 “Granite” at bottom. Possible Bouse Formation at 972 ft. 16 Yuma County Fairgrounds . . 212 306 .................. 292 . 17 USGS LCRP 26 ........... 125.4 1,777 1,115 1,380 ........ 18 Arizona Public Service Co. . . 117 978 .......................... Bottom in alluvium or transition zone. 19 USGS LCRP 28 ........... 118.7 2,466 ..................................... 20 Colorado Basin Associates 110 3.277 ............................. 1,431 Drilled 1,846 ft into granitic Elliott 1. basement. 21 Old 011 test ................ 118 730 ...................................... 730 Reported in Kovach, Allen, and Press (1962 l 22 USBR CH-21YM .......... 188 235 ...................................... 267 Cored porphyritic quartz monzonit/e. 23 USGS auger hole .......... 195 90 ........................... 90 24 ....................... 196 79 .................. 79 25 USBR CH- 20YM .......... 196 64 .................. 47 Cored porphyritic quartz monzonite. 26 USGS LCRP 25 ........... 204 6 2,318 2,101 ..................................... 27 Colorado Basin Associates 170 6,007 2,367 3,112 3,802 4 937(7) ........ Top of nonmarine sedimentary Federal 1. rocks may be at 4,302 ft 28 M. P. Stewart Co. 181 3,660 (7) (?) (7) ................... Possible transition zone at Federal 1. 1,748 ft; possible Bouse Formation at 2,515 ft, 29 USGS LCRP 17 ........... 94 2,946 2,514 ..................................... 30 YuMma Valley1 Oil & Gas Co. 90 4,868 2,525 3,395 4,350 ................... “SKID ve 31 USBR CH 28YM .......... 577.5 1,427 1,285 .................. ‘ ................... derlie most of the alluvium beneath the desert plains and river flood plains. This unit is composed of strongly to weakly indurated clastic rocks ranging from mudstone and shale (in part, of lacustrine origin) to megabreccia and boulder conglomerate. Fanglomerate, composed of angular to subrounded clasts of igneous and metamorphic rocks of local origin, seems to be the most widespread type. The Tertiary nonmarine sedimentary rocks have an upper surface with at least several hundred feet of local relief and overlie a basement surface of even greater relief. The maximum thickness of this unit is not known, but at least 5,000 feet of the unit is exposed in both the Chocolate and the Laguna Mountains, and the aggregate stratigraphic interval exposed in these mountains may be more than 10,000 feet. The Tertiary volcanic rocks, exposed most extensively in the Chocolate Mountains, are interbedded with the nonmarine sedimentary rocks. Included in this unit are tuffs and flows ranging in composition from basalt or basaltic andesite to rhyolite. Although an aggregate thickness of more than 2,000 feet is exposed in the Chocolate Mountains, the only known subsurface oc- currence of this unit is an altered basalt penetrated at a depth of 342- 360 feet in well DH- 9, 3 miles north of Yuma (table 1). Potassium-argon dates for several of the volcanic rocks from the Chocolate and Laguna Mountains range from 23 to 26 my (Olmsted and others, 1973); a middle Ter- tiary age is therefore indicated for the volcanic rocks and associated nonmarine sedimentary rocks. The older marine sedimentary rocks are composed of somewhat indurated fine-grained sandstone and in- terbedded siltstone and claystone. Their age is uncer- tain, but their stratigraphic position suggests that they probably intertongue with the upper part of the non- marine sedimentary rocks of Tertiary age. The older marine sedimentary rocks occur entirely in the subsur~ face in the Yuma area; they have been penetrated in wells DH-8, DH-27, and DH-30, and possibly in DH-28 (table 1). The maximum known thickness of this unit within the area of well information is about 1,000 feet. GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA The Bouse Formation is a younger marine unit ex- posed in parts of the Colorado River valley north of the Yuma area (Metzger, 1968), but except for one small ex- posure near Imperial Dam (fig. 3), it occurs only in the subsurface within the area of the present investigation. The Bouse Formation is more extensive than the un- derlying older marine sedimentary rocks and represents the deposits of a marine embayment that existed after the mountains and basins had assumed approximately their present outlines (fig. 3). The unit consists of silt and clay, which contain thin interbeds of find sand, hard calcareous claystone, and, locally in the basal part, limy sandstone or sandy limestone, tuff, and, possibly, con- glomerate of local provenance. Invertebrate fossils in- dicate a marine to brackish environment but are not diagnostic as to age. However, stratigraphic position (Metzger, 1968; Olmsted and others, 1973) indicates that the Bouse Formation is Pliocene. Because of the usefulness of the top of the Bouse For- mation as an indicator of the maximum amount of defor- mation in post-Bouse time (the time since the Colorado River entered the Yuma area) and because this horizon may be considered as the effective floor of the main part of the ground-water reservoir, a special effort was made to determine the configuration of this surface (fig. 3). Subsurface data from test wells, summarized in table 1, were supplemented by interpretations of geophysical data discussed later in this report. The known thickness of the Bouse Formation ranges from zero, where it pinches out or has been removed by erosion, to about 1,000 feet, in the southwestern part of the area. Throughout the central and southern parts of the study area, the Bouse Formation is overlain by a transi- tion zone of intertonguing marine and nonmarine (fluvial) strata. The base of this transition zone is marked by the lowest stratum of identifiable alluvium from the Colorado River; the top, by the uppermost bed of fossiliferous marine clay or silt. The greatest known thickness of the transition zone, in the southwestern part of the area, exceeds 850 feet. The conglomerate of the Chocolate Mountains occurs on the flanks of the Chocolate Mountains, near the north-central margin of the study area. Volcanic detritus makes up most of the unit, which includes strata probably equivalent to both the upper part of the Ter- tiary nonmarine sedimentary rocks and the lower part of the alluvium. This unit has not been identified in any wells in the area. The alluvial deposits range from clay to cobble and boulder gravel; sand is the predominant fraction at most places. Clay and silt constitute less than 20 percent of the total thickness at most places, but they are somewhat more abundant in the lower part of the unit, below depths of 1,500 feet. Cementation is uncommon, except in some of the beds of well-sorted gravel and D5 coarse sand in the southwestern part of the area and in finer grained deposits beneath parts of the “Upper Mesa” (pl. 1) in the southeast. The alluvium contains most of the usable ground water of the Yuma area and includes gravel beds that yield copious quantities of water to irrigation, drainage, and supply wells. REGIONAL STRUCTURE The area of investigation includes parts of both the southwest margin of the Sonoran Desert and the northeast margin of the Salton Trough, a landward ex— tension of the Gulf of California (fig. 1). These two physiographic units differ somewhat, both in the trend of their structural features and in the time of their latest structural activity. The southwestern part of the Sonoran Desert east of Yuma is characterized generally by long, narrow moun- tain ranges separated by more extensive desert plains. The mountain ranges, most of which are oriented north- northwest, are elevated or tilted blocks bounded by steep faults; the intervening desert plains are basins con- taining thick Cenozoic fill (Thornbury, 1965). The mountains are composed chiefly of pre-Tertiary plutonic and metamorphic rocks, although Cenozoic volcanic and minor sedimentary rocks are locally extensive (Wilson, 1960). The mountains and basins of the southwestern Sonoran Desert appear to have been outlined by struc- tural activity consisting chiefly of extensive faulting and tilting in middle Tertiary and earlier time. Later movements have consisted chiefly of minor warping and normal faulting and of probable regional subsidence near the west margin of the area, adjacent to the Salton Trough. The Salton Trough, which contains most of the Colorado River delta, is a deep basin which subsided rapidly during Cenozoic time and accumulated as much as 20,000 feet of fill, most of it nonmarine (Biehler and others, 1964). In contrast to the Sonoran Desert, the Salton Trough has been tectonically active to the pres- ent time. The trough is traversed at actue angles by northwest-trending faults of the San Andreas system, on which the major component of movement has been right- lateral (Crowell, 1962). The Gulf of California — the southern extension of the Salton Trough — has been in- terpreted as having been formed by oblique rifting across the fault system (Hamilton, 1961) and probably also by ocean-floor spreading (Larson and others, 1968). The trend of the faults of the San Andreas system is somewhat more northwesterly than that of the moun- tains of the Sonoran Desert and their bordering faults, but the two trends may converge toward the southeast, in Sonora, Mexico. The northeasternmost major fault in the'San Andreas system in the Yuma area, which was identified and named the Algodones fault by Olmsted, Loeltz, and D6 114°45’ GEOPHYSICAL FIELD INVESTIGATIONS 114°15’ \ \ 32° \\ . — Pilot 45 \ Knob I @ ’ UNITED £33225 — -—-/') fl _ _ mEXICO I l —2500: Q0 4 $/ é/ , l I l (’3 °( w \ 3‘“ C 5’ I 32° _ Q '- 30' .—3305 ‘ N \ \l‘ ~ ‘52) s EXPLANATION Mmfig‘ZLA —— Inferred subsurface margin of Bouse Formation ',-.;.+245 Exposure of Bouse Formation +245, elevation, in feet, of top of formation .-2942 Well penetrating Bouse Formation —2942, elevation, in feet, of top of formation 0—2400: Well showing estimated top of Bouse Formation Elevation, in feet, estimated from geophysical data for well penetrating only the overlying transition zone . pTc Well penetrating pre-Bouse rocks beneath alluvium Tn, Tertiary non'marine sedimentary rocks pTc, pre— Tertiary crystallirw rocks +500 0 —5oo—— Structure contours Drawn on top of Bouse Formation, Contour interval 500 feet Fault offsetting Bouse Formation Dashed where uncertain \\\\\ Area of shallow basement m Hills and mountains O 5 10 MILES l 1 I 1 l l l O 5 10 KILOMETERS |_L_L___|__;L__._l DATUM IS MEAN SEA LEVEL FIGURE 3. — Map of the Yuma area, showing inferred extent and configuration of the top of the Bouse Formation. GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA Irelan (1973), crosses the area of the present study diagonally (pl. 1). This fault, which may be a continua- tion of the San Andreas fault to the northwest, as inter- preted by Dibblee (1954), is a feature of major hydrologic significance in the Yuma area. PHYSICAL PROPERTIES OF MAJOR ROCK UNITS Physical properties of rocks pertinent to the geophysical investigation include density, magnetic properties, electrical resistivity, and acoustic or seismic velocity. Laboratory measurements of these properties were not made as part of the present investigation, but field measurements and other sources of data were available from which reasonable estimates could be made; these estimates are summarized in the following paragraphs. DENSITY Density measurements have been made by the US. Geological Survey and the US. Bureau of Reclamation as part of their ground-water studies of the Yuma area, and data are also available from nearby areas. Kovach, Allen, and Press (1962, table 2) reported that the den- sities of 33 surficial samples of plutonic and metamorphic rocks of the basement complex from the Colorado delta region range from 2.40 to 2.92 g cm ‘3 (grams per cubic centimeter) and average 2.67 g cm ‘3. Two cores of porphyritic quartz monzonite from wells DH—22 and DH-25 (table 1), south of Yuma, have densities of 2.7 and 2.6 g cm ‘3, respectively (U.S. Bur Reclamation, unpub. data, 1966) — close to the average value for basement rocks reported by Kovach, Allen, and Press (1962). The bulk density of saturated Cenozoic sediments, from five wells in the Colorado River delta region, averages 2.37 g cm‘3 for well samples obtained from depths of 04,000 feet, 2.44 g cm‘3 for samples from 4,000-8,000 feet, and 2.47 g cm‘3 for samples from 8,000- 12,000 feet (Kovach and others, 1962, table 2). The average density value of 2.37 g cm'3 for samples from 0- 4,000 feet appears to be substantially higher than the average density value for surface or near-surface uncon- solidated alluvium of the Yuma area; in general, the data of Kovach, Allen, and Press (1962) probably apply to deposits deeper than about 1,000 feet rather than to surface or near-surface unconsolidated alluvium. In the Yuma area, 25 samples of sand, silt, and clay obtained by the Geological Survey from near-surface alluvium have a grain density that ranges from 2.66 to 2.81 g cm '3 and averages about 2.7 g cm '3 (Olmsted and others, 1973); the bulk density ranges somewhat widely, owing to variations in porosity, but probably averages about 2.0 g cm '3 (which corresponds to an average porosity of about 41 percent). This value of 2.0 g cm"‘ for the near- surface alluvium in the_Yuma area agrees with the average value for similar deposits obtained by the Bureau of Reclamation in the Phoenix, Ariz., area 150 D7 miles to the northeast. According to Earl Komie (oral commun., 1969), near-surface samples of unconsolidated alluvium in the Phoenix area have densities that range from 1.71 to 2.20 g cm ‘3 and average 2.01 g cm '3. The density data are summarized in figure 4; the graph shows the average density contrast, as a function of depth, between basement rock (assumed density of 2.67 g cm'”) and Cenozoic sediments. DENSITY CONTRAST, IN GRAMS PER CUBIC CENTIMETER 00 0.1 0.3 0.5 0.7 I ' | ‘ _L___.—-1 z”, I / / / 2000— _ 6000 _ l DEPTH BELOW SURFACE. IN FEET 8000 — _ 10,000 — _ 12,000 I 1 FIGURE 4. — Average density contrast, as a function of depth, between basement rock (assumed density of 2.67 g cm‘“) and Cenozoic sediments. Data for 1,000-12,000 feet are from the Colorado River delta region, as reported by Kovach, Allen, and Press (1962). Dashed lines indicate sufficient data are not available to show density contrast. For purposes of interpreting thicknesses of Cenozoic sedimentary deposits, we have used the vertical density distribution shown in figure 4. Although these data rely heavily on the work of Kovach, Allen, and Press (1962), who were dealing with stratigraphic units that differ somewhat from those present at depth in the Yuma area, the density distribution in figure 4, which appears to be attributable to compaction of Cenozoic sediments with depth, probably is applicable to the Yuma area. D8 The effects of lateral density variations within the Cenozoic sediments are neglected in the present report, even though this neglect may give rise to local errors in depth estimations. The effects of lateral density variations within the pre-Tertiary basement rocks are considered linear along the various profiles and are removed or partly removed in subtracting a linear regional gradient along profiles that extend from base- ment outcrop to basement outcrop. MAGNETIC PROPERTIES Magnetic properties of rocks in the region have not been measured, but depth analyses based on magnetic data (Vacquier and others, 1951) do not involve assump- tions of the magnetization and are not strongly depen- dent on the type of magnetization. For interpretive pur- poses, the sedimentary rock in this area is assumed to be virtually nonmagnetic, and all magnetic anomalies are attributed to the basement complex or to Tertiary volcanic rocks. ELECTRICAL RESISTIVITY Correlation of electric and geologic logs of test wells with electrical soundings indicates that resistivity values for unconsolidated sandy and gravelly alluvium generally range from about 20 to 40 ohm-m (ohm- meters). Electric logs indicate values of 50-60 ohm-m for beds of somewhat cemented gravel and 100-250 ohm-m for more strongly cemented gravel beds in test wells DH- 29 and DH-30, southwest of Somerton, Ariz. Local zones of about 1,000 ohm-m have been mapped by electrical soundings beneath parts of Yuma Mesa, but these values are exceptional and represent dry sand and cemented gravel above the water table. Electric-log data from wells penetrating the Bouse Formation indicate that this unit is highly conductive; resistivity values for the clay and silt (the predominant fractions) range from 2 to 5 ohm-m and average about 3 ohm-m. Electric logs of wells DH-8, DH-27, and DH-30 show that the siltstone and claystone beds in the un- derlying older marine sedimentary rocks have even lower resistivities (commonly in the range of 1-3 ohm-m), probably owing to the more saline water generally pres- ent in this unit, but the interbedded soft sandstone has resistivities of about 8-15 ohm-m. Because sand (or sandstone) is much more abundant in the older marine sedimentary rocks than in the Bouse Formation (more than half the total thickness, as compared with less than one-fifth), the average resistivity of the older marine sedimentary rocks is greater than that of the Bouse For- mation, despite the fact that the older unit probably contains more highly saline water. The resistivity of the Tertiary nonmarine sedimentary rocks varies widely, depending on the salinity of the in- terstitial water. In much of the northern part of the area, electric logs of test wells and electrical soundings GEOPHYSICAL FIELD INVESTIGATIONS recorded values of 15-30 ohm-m (pl. 2) — approaching those cited for the sandy and gravelly alluvium. The cementation and lower porosity of the Tertiary deposits (which tend to increase the formation resistivity) largely offset the effect of the slightly more saline water‘they contain (which tends to decrease the formation resistivity). Farther south, however, the nonmarine sedimentary rocks contain warm saline water, and the formation resistivity is correspondingly low, despite the overall coarseness of the unit. In well DH-27, the coarse- grained beds (probably fanglomerate) that were penetrated near the bottom of the hole, below a depth of 5,800 feet, have an average resistivity of about 3 ohm-m; a few thin beds, probably cemented, have a resistivity of about 6 ohm-m. Rocks of the basement complex generally have a resistivity of more than 300 ohm-m, as recorded on resistivity curves from deep electrical soundings. These data are summarized in table 2. TABLE 2. — Summary of resistivity data from selected wells and electrical soundings Resistivity (ohm-m) Rock unit or subunit Range Average Alluvium ............................................ 30, with local variation Clay ................................ 3-10 Sand and gravel ...................... 20-40 Partly cemented ...................... 50-60 Strongly cemented .................... 100-250 Sand and cemented gravel (above water table) ................. > 1,000 .............. Bouse Formation ....................... Clay and silt ......................... 2-5 Older marine sedimentary rocks . . . . Siltstone and claystone ....... Sandstone .................. Nonmarine sedimentary rocks . . . Basement complex ...................... ACOUSTIC VELOCITY Velocity data for the rocks of the Yuma area are available from the seismic—refraction surveys and from acoustic-velocity logs of wells DH-17 (about 3 miles west of Yuma) and DH-31 (near the southeast corner of the study area). Data from the log of DH-17 (pl. 2) are sum- marized in table 3. These velocities are in good agreement with velocity data obtained from a seismic-refraction survey about one-half mile south of DH-17, which recorded velocities of 6,700 fps (feet per second) for the alluvium and 12,800 fps for the fanglomerate and megabreccia. (See fig. 19.) (Owing to probable anisotropy, one would not expect the acoustic-log velocities, which are measured in a vertical direction, to correspond precisely with the seismic- refraction velocities, which are measured along horizon- tal paths.) Because the Bouse Formation is characterized by an average velocity lower than that of the overlying alluvium, the top of this important unit could not be delineated by seismic-refraction techniques. For this reason, seismic-reflection surveys were used to map this horizon. - GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA TABLE 3. — Acoustic velocities of major rock units penetrated in well DH -1 7 Acoustic velocity (fps) Degath Rock unit or subunit ( t) Range Average Alluvium .......................... 0-1,033 5,200-10,200 7,500 Transition zone ..................... 1,033-1,115 6,400-10,400 8,000 Bouse Formation: Clay, silt, and sand ............... 1,115-1,343 5,500- 7,400 6,500 Wm?) .......................... 1,343-1,352 8,500- 9,300 9,000 Limestone ............... 1,352-1,380 9,500-11,400 10,500 Nonmarine sedimentary rocks: Fanglomerate .................... 1,380-1,665 10,600-15,500 13,000 Megabreccia ..................... 1,665-1,777 10,100-20,400 13,700 Basement velocities of 14,000-18,000 fps were measured along several seismic—refraction profiles in the Yuma area. Basement outcrops in the vicinity of these profiles imply that velocities of 14,000-16,000 fps- probably represent granitic rocks and that velocities of 17 ,000-18,000 fps probably represent gneiss and schist. GEOPHYSICAL SURVEYS GRAVITY SURVEY In the vicinity of Yuma about one gravity station per square mile was established. This density was adequate to define most of the larger gravity anomalies. Outward from Yuma the station density decreases, and over much of the surveyed area only the more extensive gravity features are defined (pl. 1). About 7 miles south of Yuma the density was increased to about two stations per square mile to define a local basement high. Most of the gravity stations are located where eleva- tion control is available from topographic maps or at bench marks, section corners, or road intersections. Positional control for the remaining stations was ob- tained by either transit or altimeter surveys. The gravity data were reduced to the simple Bouguer anomaly by standard techniques. The data are referenced to base station WU 7 in Golden, Colo. (Behrendt and Woollard, 1961). A density of 2.67 g cm‘” was assumed in making the Bouguer correction. Terrain corrections have not been applied, but the terrain effect is not significant except at a few stations on or adjacent to the larger mountains. The relative Bouguer anomaly values probably are accurate to within 0.5 mgal (milligal). AEROMAGNETIC SURVEY Aeromagnetic data were obtained along 29 flightlines spaced about one-half mile apart, flown 1,000 feet above the ground surface, and oriented N. 37° E. The magnetic survey was made with an ASQ-8A magnetometer in the tail boom of a Convair aircraft. The flightpath of the aircraft was recorded by a gyrostabilized continuous-strip-film camera. The magnetic data were compiled relative to an arbitrary datum at a 1:62,500 scale, and the contour map was reduced to a 1:125,000 scale (pl. 3). D9 RESISTIVITY SURVEY The resistivity equipment consisted of a 2.5—kilowatt generator for power supply, a pulser for stepping up the impressed voltage and regulating the current, a poten— tiometric chart recorder for recording the potential difference, and more than 20,000 feet of cable for con- necting the electrical equipment to the electrodes, which were stainless steel rods 2 feet in length and three- fourths inch in diameter. The maximum electric current put into the ground was about 1.2 amperes, and the smallest potential difference measured was about 0.01 millivolt. Electrical soundings were made using both symmetric Schlumberger and bipole-dipole equatorial arrays (Ber- dichevskii and Petrovskii, 1956; Dakhnov, 1953; Kunetz, 1966; Zohdy and Jackson, 1968). Electrode spacings reaching 10,000 feet allowed exploration to depths ex- ceeding 7,000 feet. To detect lateral variations, horizon- tal profiling was employed. Resistivity profiles across a basement high south of Yuma were made for comparison with the other geophysical surveys of this feature (profiles c-c ' through f—f’ on pl. 5). Profiles were also made across the inferred trace of the Algodones fault south of Yuma (profiles g-g’ through k-k' on pl. 5). In addition, lines of electrical soundings were made near Bard, Calif. (profile b-b' on pl. 1), and along the border with Sonora, Mexico (profile a-a' on pl. 1). SEISMIC-REFRACTION SURVEY Seismograms were recorded on photographic paper using a 12-channel HTL 7000B seismograph, and for most of the fieldwork, a constant geophone spacing of 820 feet was used. Profiles ranged in length from about 1,000 feet to 8 miles; for the longer profiles the scheme for shooting each profile was as follows: First, a 9,020- foot geophone cable with 12 geophones was laid along one end of the profile, and the P—waves propagated from buried dynamite charges that were exploded at each end of the profile were recorded. Then the geophone cable was moved forward 9,020 feet, and the previously used shotpoints at each end of the profile were reloaded and reshot. This procedure of moving the cable and reshooting at the same shotpoints was continued until the entire distance along a profile was covered. In addi- tion, intermediate shots at 9,020-foot intervals were used to record velocity changes in the near-surface materials. The dynamite charges ranged from 5 to 100 pounds and were loaded in holes drilled to depths of 10-40 feet. In general the resulting field seismograms were of good quality and showed easily identifiable first arrivals. The traveltimes from shotpoint to seismometer were picked to the nearest 0.001 second, and traveltime curves were constructed for each profile. Velocities were determined D10 by visual fitting of straight-line segments to the traveltime data, and because there was little relief along the profiles, no elevation corrections were applied. The intercept time of the first velocity horizon was between 0.010 and 0.100 second on all the traveltime curves. This intercept time, or weathering correction, is attributed to a thin surface layer of dry, unconsolidated, low-velocity material often referred to as the weathering layer. Corrections for the weathering layer, assuming a velocity of 2,000 fps, were applied to the traveltime data prior to making depth calculations. The base of the weathering layer probably corresponds to the top of the water table. The graphical interpretation method of Slotnick (1950) and the time-depth method of Hawkins (1961) were used in calculating depths and dips of the refracting horizons. Some of the final interpretations were checked by fitting theoretical ray paths to the computed models. The seismic—refraction surveys were made to supple- ment the information obtained from the gravity and magnetic surveys and, particularly, to determine the nature and thickness of sedimentary fill in alluvial gaps through which ground water moves. Nine profiles were made, and these were numbered 1 through 9 for easy reference (pl. 1). Six of the profiles are across alluvial gaps: (a) between the Laguna Mountains and a low base- ment ridge east of the Yuma Proving Ground Head- quarters (labeled “Yuma Test Station” on pl. 1) (profile 1); (b) between the Laguna and Chocolate Mountains at Laguna Dam (profile 2); (c) between the Gila and Laguna Mountains at the Gila River narrows (profile 3); (d) between Pilot Knob and the Cargo Muchacho Moun- tains, through which ground water now moves westward from the Yuma area toward Imperial Valley (profile 5); (e) between two basement outcrops in Yuma (profile 7); and (f) between Pilot Knob and an outcrop of Tertiary nonmarine sedimentary rocks (breccia and con- glomerate) near the north end of the Yuma basement high (profile 9). Profile 4 is along the suspected northward continuation of a basement high in the vicinity of Yuma. Profile 6, westward from Pilot Knob, was made in an attempt to delineate the buried base- ment slope and the position and nature of the Algodones fault. Profile 8 is on the south flank of a basement high near Somerton and crosses the inferred trace of the Algodones fault. All the profiles were reasonably successful; the results are described in the section “Interpretation of Structural Features.” SEISM l(‘-REFLE(‘TION SURVEYS In early 1965 an attempt was made to record seismic reflections using the same equipment used in the refrac- tion survey. However, these tests produced reflections of very poor quality that were of no value in this investiga- tion. The tests did indicate that more sophisticated in- strumentation using nonexplosive energy sources might GEOPHYSICAL FIELD INVESTIGATIONS produce useful data. Later in 1965 the Bureau of Reclamation contracted with the General Atomic Divi- sion of General Dynamics Corp. to make a trial seismic- reflection survey. The purposes of the survey were ( 1) to determine the applicability of that corporation’s seismic-reflection system to the problems in the Yuma area, and (2) to make a profile extending southwest from well DH-26, delineating reflecting horizons such as the top of the Bouse Formation (at that time known as “estuarine sediments”). The General Atomic system used arrays of surface- mounted vibrating sources with controlled frequency and pulse width. In the survey, the source arrays were located at l/2-mile intervals along the survey line, and geophones were placed midway between the array positions; frequencies used for the seismic signals ranged from 32.5 to 152.5 hertz (Herschel Snodgrass, written commun., 1965). This procedure was repeated until the entire survey line was covered. Three contiguous or nearly contiguous lines were run southeast of Yuma, starting near well DH-26 (pls. 1, 3, 5). Unfortunately, this well penetrates only the top of the transition zone, not the top of the Bouse Formation nor deeper horizons (table 1). However, by using the observed velocity distribution recorded at well DH-17 (pl. 2) a synthesized seismogram was constructed which permitted a reasonable interpretation of the actual seismogram recorded near DH-26. The top of the Bouse Formation was inferred to be at a strong negative reflec- tion (decrease in velocity below the reflecting interface) at a calculated depth of about 2,500 feet, about 200 feet below the bottom of the well. Despite some difficulties during the trial survey, the results were encouraging; and in 1966 the US. Geological Survey, the US. Bureau of Reclamation, and the US. Section of the International Boundary and Water Commission recommended an additional seismic- reflection survey. The primary goals of the second survey were to define more adequately the configuration of the top of the Bouse Formation (the effective base of the main ground-water reservoir), the position and nature of the Algodones fault and other faults that might be hydrologically significant, and the configuration of the basement surface. The Bureau of Reclamation con- tracted with General Dynamics Corp., who then subcon- tracted with Rogers Explorations, Inc., to make the ac- tual survey. Herschel Snodgrass of General Dynamics Corp., who was in charge of the 1965 survey, assisted with the 1966 survey, which used the same type of equip-_ ment and the same general field procedures as. were used in the 1965 survey. : I The 1966 survey, which was reported by R. D. Davis and Herschel Snodgrass (written communs., 1966), was generally sucCe‘ssful, although at some places no usable data were obtained. Profiles were made west of Yuma GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA from well DH-17 to well DH-19, east of Yuma from near well DH-26 to near well DH-2, and south of Yuma in the vicinity of the 1965 survey. As in the 1965 survey, relationships of time to depth were based on velocities observed at well DH-17 and interpolated to well DH—26; as a result, actual depths at locations away from those wells are somewhat uncertain. However, additional well control (such as at wells DH-8 and DH-19) was available at the time of the second survey, and therefore the iden- tity and depth of the reflecting horizons are reasonably well known. The tops of the Bouse Formation, the transi- tion zone, and several other reflecting horizons were successfully traced along the profiles, and the inferred Algodones fault and several other parallel(?) or en echelon(?) faults were identified (pl. 1). INTERPRETATION OF STRUCTURAL FEATURES GEN ERAL STRUCTURAL PATTERN Major structural features of the Yuma area are delineated by the gravity and aeromagnetic data (pls. 1, 3). Gravity highs are associated with all known ex- posures of the pre-Tertiary basement complex, and gravity lows, with all areas known to be underlain by thick Cenozoic sedimentary fill. However, large variations in Bouguer anomaly values not related to the distribution of Cenozoic deposits are apparent and are attributed both to variations in the density of the base- ment rock and to a regional anomaly associated with the Salton Trough. Bouguer anomaly values of —10 mgal occur at or near basement outcrops at Pilot Knob, at Yuma, and in the central Gila Mountains near the Fortuna mine, but values at other stations at or near basement outcrops (northern Gila Mountains, Muggins Mountains, and southern Gila Mountains) range from — 18 to —30 mgal. This range of Bouguer anomaly values for stations over bedrock must reflect, at least in part, mass anomalies within the basement complex. For example, near the Fortuna mine, where the gravity values are about —10 mgal, presumably dense hornblende schist and gneiss are abundant; whereas farther south, less dense leucocratic granite is exposed, and the gravity values for stations near basement outcrops decrease to about —30 mgal. Gravity surveys in the Colorado River delta region (Kovach and others, 1962; Biehler and others, 1964) have revealed a major regional gravity high presumably associated with a thinning of the crust under the Salton Trough. This regional gravity anomaly probably extends into the Yuma area and gives rise to an east- northeastward decrease in anomaly values across the surveyed area. The regional gradient however, does not appear to be significant, at least in comparison with the amplitudes of local anomalies in the Yuma area. D11 Superimposed upon the regional anomaly related to the Salton Trough and the gravity anomalies produced by variations in the density of the basement rock are anomalies produced by the density contrast between the Cenozoic sedimentary deposits and the denser, older rocks. In some parts of the area, however, the anomalies produced by Cenozoic rocks are difficult to distinguish from those reflecting intrabasement features, and this difficulty seriously limits the usefulness of the gravity data in studying the subsurface geology, particularly in making quantitative interpretations. Within the study area the major gravity anomalies and associated structural features revealed by the gravity survey (pl. 1) consist of the following: 1. An elongate north-northwest-trending gravity high coextensive with the Gila, Laguna, and Chocolate Mountains, with a slight gravity saddle at the Gila River narrows and a more pronounced gravity saddle at the Colorado River narrows at Laguna Dam; these saddles reflect alluvial gaps through which ground water moves toward the Yuma area. 2. A pronounced gravity trough west of the Gila Mountains; the associated structural feature is designated the Fortuna basin. 3. An extension of the Fortuna basin anomaly, but of much smaller amplitude, that extends northwest between the Cargo Muchacho and Chocolate Mountains; the associated structural feature is designated the Picacho-Bard basin. 4. A gravity saddle between the Cargo Muchacho Mountains and Pilot Knob that reflects a significant alluvial gap through which ground water now moves westward from the Yuma area toward Imperial Valley. 5. A multiple-crested gravity high extending roughly southward from Yuma and comprising two main parts -— the northern part, called the Yuma anomaly, and the southern part, called the Mesa anomaly; the overall associated structural feature is designated the Yuma basement high. 6. A gravity high centered on basement outcrops on the border with Sonora, Mexico, and separated from the Yuma basement high by a deep, broad gravity saddle; the associated structural feature is designated the Boundary basement high. 7. A gravity trough between Yuma and Pilot Knob; its associated structural feature is designated the Yuma trough. 8. A gravity low along the southerly international boundary 5-15 miles east-southeast of San Luis; its associated structural feature is designated the San Luis basin. In the area covered, the aeromagnetic map (pl. 3) reveals the same major structural features as the gravity map. The Yuma basement high, the Fortuna basin, the Yuma trough, and the San Luis basin all are clearly shown by the magnetic data. GILA, LAGUNA, AND CHOCOLATE MOUNTAINS The east margin of the area of investigation is oc- cupied by a mountain chain comprising, from south to north, the Gila, Laguna, and Chocolate Mountains. The Gila Mountains are formed chiefly of pre-Tertiary base- ment rock: granite in the southern and northern parts, gneiss and schist in the central part. Tertiary sedimen- tary rocks are exposed extensively in the Laguna Moun- tains. Tertiary volcanic rocks, of which basaltic andesite D12 or basalt is most extensive, make up most of the ex- posures in the southern Chocolate Mountains (pl. 1). The entire mountain chain is characterized by an elongate gravity high caused by a density contrast between the older rocks in the ranges and the less dense younger fill in the adjacent basins. At the few stations near the south end of the Chocolate Mountains, gravity values are lower than at stations farther south, probably owing, in part, to the presence of vesicular basaltic andesite flows overlying breccia and conglomerate in the Chocolate Mountains; both of these Tertiary units are less dense than the pre-Tertiary basement and the Ter- tiary sedimentary rocks. exposed farther south in the Laguna and Gila Mountains. The gravity high coexten- sive with the Gila, Laguna, and Chocolate Mountains is intersected at the Colorado River and Gila River gaps by gravity lows, of which the one on the Colorado River at Laguna Dam is the more prominent. The northward decrease in gravity values at the northeast corner of the gravity coverage may reflect a basin under the Castle Dome Plain; if so, more extensive gravity coverage is needed to adequately define this basin. Between the Laguna Mountains and the Muggins Mountains to the east a gravity low of about 6 mgal suggests a thickness of about 1,000 feet of Cenozoic deposits. This thickness is based on the density data of figure 4. Irregularities in the gravity pattern over the area im- mediately adjacent to the Gila, Laguna, and Chocolate Mountains suggest a basement surface having con- siderable local relief. The gap between basement outcrops in the Laguna Mountains and a southeasterly extension of the Chocolate Mountains was explored with seismic- refraction profile 1 (fig. 5). The profile lies about 11/2 miles east of the Yuma Proving Ground Headquarters (labeled “Yuma Test Station” on pl. 1) and has a total length of 12,250 feet. The southwesternmost shotpoint is just northeast of an exposure of porphyritic quartz mon- zonite, and the northeasternmost shotpoint is about 350 feet southwest of an outcrop of gneiss. Gneiss similar to that near the northeast end of the profile crops out in a small hill near the center of the profile. Two distinct velocity layers were recorded on profile 1; the average recorded velocity through the upper layer is about 5,500 fps between shotpoints 1 and 3, southwest of the central outcrop, and about 6,300 fps between shot- points 4 and 6, northeast of the outcrop. The thickness of the upper layer is calculated to be about 540 feet near the center of the northeastern trough and about 400 feet near the center of the southwestern trough. The upper part of this 5500. to 6,300-fps layer is unconsolidated alluvium, but the marine clay and silt of the Bouse Formation may GEOPHYSICAL FIELD INVESTIGATIONS be present in the lower part, as indicated by exposures of this unit about 1 1/2 miles northwest of the profile, at the Yuma Proving Ground Headquarters. The velocity of the underlying layer is computed to be 12,500 fps between shotpoints 1 and 2, 14,200 fps between 2 and 3, and 13,700 fps between 4 and 6. These velocities could represent either highly fractured or strongly weathered pre-Tertiary basement rocks or non- marine sedimentary rocks of Tertiary age. Although the velocities are generally lower than those recorded for pre- Tertiary basement rocks elsewhere in the Yuma area, they are within or at least partly within the range of velocities for coarse-grained nonmarine sedimentary rocks of Tertiary age (breccia and conglomerate). Such Tertiary rocks are not exposed along the profile but do crop out less than a mile northwest of the northeast end of the profile (pl. 1). Fractured and brecciated basement does occur at several places within the Laguna Moun- tains. Seismic-refraction profile 2 (fig. 6) extends across the Colorado River gap between the Laguna and Chocolate Mountains just upstream from Laguna Dam. Slightly to moderately fractured and weathered porphyritic quartz monzonite basement rock is exposed at both abutments of the dam, about a mile apart. Test well DH-l, drilled to a depth of 505 feet near the center of the profile, penetrated alluvium to a depth of 209 feet, Bouse For- mation from 209 to 471 feet, and fanglomerate (Tertiary nonmarine unit) from 471 feet to the bottom of the hole. Because shotpoints 1 and 2 are close to basement out- crops, all arrivals shown on the time-distance plot are assumed to represent basement refractions. A standard time-delay method was used to calculate the basement velocity and basement depth at each geophone (Hawkins, 1961). The basement velocity was calculated to be 15,700 fps, and a channel-fill velocity of 6,500 fps was measured near the center of the profile by means of an additional shotpoint not shown. The resultant cross section is shown in figure 6. The thickness of Cenozoic fill near the center of the alluvial gap is computed to be about 600 feet. An intermediate-velocity layer cor- responding to the Tertiary nonmarine sedimentary rocks (fanglomerate) was not identified in the seismic data; therefore, the computed depths to basement, at least near the center of the profile, probably are too shallow. The small gravity low at the Colorado River gap between the Laguna and Chocolate Mountains (residual anomaly of about 3 mgal on pl. 1), however, is consistent with the GOO-foot thickness of Cenozoic fill interpreted from the seismic data. There is no gravity evidence of a significantly greater thickness of fill in the area between Laguna Dam and Imperial Dam. Seismic-refraction profile 3 (fig. 7) was recorded across the Gila River gap between the Gila and Laguna Moun- D13 GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA @1me €963 .33. $3358 833:? £286 Suwanee—«m ”mm .mgofimscuamm 65.20 9:55 «85> we 385:8 mfiwussoz «.5qu ES 83895 2: mo 389:5 30833 5953 man 883 H 0589 somuuwaouémfimmom I .m 5505 whzamboxw _ _ _ _ _ 4 mmwhmz OOmH OOOH 00m 0 _ _ _ _ _ _ _ _ A hmmu 000m 000v 000m OOON Doom 0 22.88% n v m w 82. n. W .3 m 83 s 11. 3 0 N a S 89° oced SGNOOSS NI ‘SWIJ. .mn: ASN.2 mg 08.3 m9 oomdg 2: 88 < E:_>::~ EEEU EJermomwmoEo wtcwwmficoh< o mm m aw N mm H mm wt). uthOOIO mt). (2304] hmm_._ (um _OOOH D14 NORTHWEST 1000’ CHOCOLATE MTS SEA LEVEL A A A Quartz monzonite A A A A A Alluvium Formation _ __ _, / TD 505 ft Bottomed in nonmarine GEOPHYSICAL FIELD INVESTIGATIONS SOUTH EAST LAGUNA MTS A Quartz monzonite A A A A A 15,7 _ 1000, 00 fps sedimentary rocks 0.600 m o z o 0 Lu ‘0 o.3oor _ E m. E [.— 0.000l SHOTPOINTS 2 O 1000 2000 3000 FEET 0 800 METERS l—I—J FIGURE 6. — Seismic-refraction profile 2 across the Colorado River gap between the Laguna and Chocolate Mountains just upstream from Laguna Dam. The contact between basement rock and overlying sedimentary rocks is dashed where seismic-refraction interpretation is doubtful. SP, shot- point; TD, total depth; dots, bedrock arrivals. tains. Megabreccia and boulder conglomerate (Tertiary nonmarine sedimentary rocks) are exposed at the west end of the profile; pre-Tertiary gneiss, schist, and migmatite are exposed near the east end. A low hill of porphyritic quartz monzonite lies near the center of the profile, on the east side of the Gila River flood plain. Dissected alluvial gravels of local derivation occupy the area between the quartz monzonite hill and the main mass of the Gila Mountains to the east; well DH-3 was drilled to a total depth of 396 feet in the western part of this area. Beneath the Gila River flood plain, between shot- points 1 and 2, the velocity of the alluvium is assumed to be about 6,000 fps; the velocity of the underlying refrac- tor is computed to be about 12,000 fps. The maximum thickness of alluvium (which may include the Bouse For- mation) between shotpoints 1 and 2 is computed to be about 320 feet. The 12,000-fps layer probably is the brec- cia and conglomerate exposed west of shotpoint 1. If this interpretation is correct, the contact of the breccia- conglomerate unit and the quartz monzonite must dip steeply and most likely is a fault, as shown in figure 7. The large amount of scatter about the average velocity lines on the time-distance plot (fig. 7) probably reflects a buried surface of Tertiary breccia and conglomerate that is highly irregular. In the eastern part of the profile, between shotpoints 3 and 5, the near-surface velocity of 6,000 fps undoubtedly represents the exposed coarse alluvium. An underlying layer having a velocity of 10,700 fps probably represents semiconsolidated Tertiary nonmarine sedimentary rocks, which are exposed less than a mile north of the profile and are believed to have been penetrated in well DH-3 below a depth of about 207 feet (table 1). The deepest refractor between shotpoints 3 and 5 has a calculated velocity of about 15,500 fps and a computed dip of about 12° SE. However, the calculated velocity and dip are based on an updip velocity of 22,400 fps recorded on only a very short segment of geophone spread; hence, the calculated velocity and dip may be erroneous. But even if this calculated velocity is erroneous, the 12,500-fps velocity recorded in the down— dip direction is less than the velocity of either gneiss or quartz monzonite, and therefore the basement surface must dip eastward. Near the east end of the profile the eastward slope probably terminates against a steep fault that appears to be a continuation of a fault exposed to the northeast (pl. 1, fig. 7). GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA NORTHWEST 1000' LAGUNA MTS Gila River SP 1 A A A I Megabreccia and conglomerate A SEA LEVEL Chiefly alluvium 6000 fps /\ 12,000 fps AA . . /\ APorphyntIc quartz / monzonite /\ D15 SOUTHEAST GILA MTS Boflomed in conglomerate (Tertiary non marine sedimentary rocks); well is projected 350 ft into line of section SP 4 SP 3 DH—3 Alluvium 6000st Nonrnarine sedimentary rocks X 10,700 Ips ///{3neiss‘ /\ , /\ A A A A schist, and 1000, A 15,500 fps n, migmatite 0.600 0.600 m w 2 S O O o o E‘n‘ a 0.300 0.300 E z oi oi E 2 v— 7—" 0.0001 2 0.000 4 SHOTPOINTS SHOTPOINTS O 1000 2000 3000 4000 5000 FE ET I l l l I I I I I I 0 500 | l I J I | 1000 1500 M ETERS l | FIGURE 7. — Seismic-refraction profile 3 across the Gila River gap between the Gila and Laguna Mountains. The Gila River flood plain occupies the northwestern part of the profile. SP, shotpoint; TD, total depth; circles, alluvium arrivals; triangles, conglomerate or megabreccia arrivals; dots, porphyritic quartz monzonite arrivals. FORTU NA BASIN The most extensive and highest amplitude gravity anomaly within the surveyed area is an elongate low west of the Gila Mountains. This low extends south-southeast from U.S. Highway 80 to the international boundary (pl. 1) and appears to continue into Mexico. The gravity low is interpreted as reflecting a deep basin, here designated the Fortuna basin, which is filled with Cenozoic sedimentary rocks. The Fortuna basin gravity low actually consists of two closed minimums separated by a northeast-trending saddle. Gravity profile A-A’ (fig. 8) extends directly east from DH-25 on the Yuma basement high, crosses the north gravity low, and terminates in the Gila Mountains (pl. 1). The maximum amplitude is about —39 mgal. The maximum depth to basement along A-A’ is com- puted to be 16,000 feet, assuming a density contrast between Cenozoic sediments and pre-Tertiary basement of 0.44, 0.30, 0.23, 0.20, and 0.19 g cm ‘3 for depths of 0- 1,000, LOGO-3,000, 4,000-8,000, 8,000-12,000, and>12,000 feet, respectively. (These assumed density contrasts agree with the data of fig. 4.) Similarly, the south gravity low, which has an amplitude of about —-32 mgal relative to the gravity values over the boundary basement high to the west, probably reflects about 13,000 feet of Cenozoic fill. If the assumed density contrast is correct, the For- tuna basin contains the greatest thickness of Cenozoic fill within the area investigated. By comparison, the greatest depth to basement in the Salton Trough, west of the Yuma area, is estimated to be more than 20,000 feet (Biehler and others, 1964) — roughly 25 percent greater than the depth of the Fortuna basin. In the Salton Trough, however, the bulk of the overlying fill consists of alluvial and deltaic deposits of the Colorado River (Muffler and Doe, 1968), whereas in the Fortuna basin, as shown by DH-26 (table 1), only the upper one-eighth of the fill includes equivalent alluvial deposits — the lower seven-eighths consists of pre-Colorado River marine and non-marine deposits. The steep gravity gradients which bound the Fortuna basin anomaly to the east, west, southwest, and north probably reflect highly complex fault patterns in these areas. Over most of the Fortuna basin the gravity data are sparse (pl. 1). Only the approximate amplitude and configuration of the anomalies were recorded, and additional detailed obser- vations would be required to define detailed structure of the basin. D16 Three lines of gravity stations, including the stations of A-A ', cross all or part of the east margin of the Fortuna basin (pl. 1). Faults are inferred by the local steepening of gradients, but continuity is doubtful because the profiles are too far apart. Probably one or more con- tinuous fault zones occur within a few miles of the front of the Gila Mountains (pl. 1, fig. 8). The strike of the fault or fault zone shown on plate 1 is assumed to be north-northwest, the same direction as the trend of the steep gravity gradients and the strike of the nearby faults at the Fortuna mine. The exact location and nature of the faults bounding the Fortuna basin west of the Gila Mountains must remain speculative until more data are obtained. The exceptionally steep gravity gradient directly west of the Fortuna mine is interpreted as reflecting a local area of high-density basement rock rather than a fault. A considerable amount of dark hornblende schist and gneiss exposed near the mine (Wilson, 1933, p. 189-193) may be the high-density rock causing the steep gradient. Along the west margin of the Fortuna basin, in the vicinity of the basement outcrops of the northern part of the Yuma basement high, steep gravity and magnetic gradients imply a pattern of north- to north-northwest— trending faults (pls. 1, 3), whereas in the vicinity of the shallow basement rocks of the southern part of the Yuma basement high, gravity, magnetic, and seismic-reflection data suggest a pattern of northwest-trending faults (pls. 1, 3). These fault patterns are discussed further in the section on “Yuma Basement High.” Two of these faults, which are associated with steep gravity gradients, are shown in figure 8. The inferred Algodones fault (pl. 1), probably the ma- jor fault bounding the southwest margin of the Fortuna basin, is associated with an exceptionally steep gravity gradient in the vicinity of the Boundary basement high. Because of the decrease in gravity values at the north end of the Fortuna basin, the basement is inferred to rise generally northward toward the Laguna Mountains and, to a lesser degree, toward the Picacho-Bard basin — a shallower continuation of the Fortuna basin. The sinuous gravity pattern at the north end of the Fortuna basin implies that the basement surface has con- siderable local relief in that area. The isolated outcrop of basement rock (porphyritic quartz monzonite) located on'the prominent gravity nose south of the Gila River and enclosed by the -18-mga1 contour appears from the gravity data (pl. 1) to be the high point on a mostly buried ridge of basement extending south from the Laguna Mountains to about the position of US. Highway 80. A prominent magnetic high (pl. 3), a little more than a mile south of the porphyritic quartz monzonite outcrop (peak value 3196 ‘Y ), is only partly defined and could _ represent a body of more magnetic basement. Because of GEOPHYSICAL FIELD INVESTIGATIONS the proximity of this magnetic high to the gravity nose, . they may be related to the same feature. An additional possibility is that this magnetic high and a second magnetic high 41/2 miles farther west, just east of the confluence of the Gila and Colorado Rivers, reflect buried masses of volcanic rock — as was shown to be'the source of the prominent magnetic high at DH-9 (pl. 4). Seismic-reflection data (R. D. Davis and Herschel Snodgrass, written commun., 1966), together with infor- mation obtained from wells DH-4 through DH-8, show a fairly rapid shallowing of reflecting horizons toward the north and northeast sides of the Fortuna basin. The structure contours on the top of the Bouse Formation (fig. 3) illustrate this trend. Two faults were identified on a seismic-reflection profile about 1 and 2 miles south of test well DH-8. The data reveal downthrow to the south on both faults but do not show the strike of the faults; in the absence of other evidence, we assume that these are continuations of inferred range-front faults west of the Gila Mountains trending generally northwest or north- northwest (pl. 1, fig. 3). In the vicinity of test well DH-26, the seismic- reflection survey recorded the top of the Bouse Forma- tion at an approximate depth of 2,550—2,600 feet (about 2,400 ft below sea level). Farther south, at the inter- national boundary, test well DH-31 penetrated the top of the transition zone at a depth of 1,285 feet (707 ft below sea level) —— substantially shallower than it occurs at DH-26 (table 1). Near the east end of the seismic- reflection line, 41/2 miles south of DH-26, R. D. Davis (written commun., 1966) reported westerly dips of reflec- ting horizons —— in the opposite direction from the gravity gradient. We interpret these apparently anomalous conditions as indicating that the Tertiary sedimentary rocks older than the Bouse Formation thin rapidly westward against an east-dipping basement sur- face and that the center of the basin in Bouse and post- Bouse (alluvial) time was northwest of the basement trough. (Compare pl. 1 and fig. 3.) An additional geophysical interpretation of the west margin of the Fortuna basin is shown by profile a-a’ (fig. 9), constructed from four deep electrical soundings across the basin eastward from the Boundary basement high. Resistivity data reveal a highly conductive layer of about 3.6 ohm-m resistivity at a computed depth of about 1,200-1,300 feet (fig. 9). The top of this layer appears to be the top of the transition zone, penetrated at 1,285 feet in DH-31; the recorded value of 3.6 ohm-m is in good agreement with the average Bouse Formation resistivity of 3 ohm-m shown in table 2. A maximum depth to basement of perhaps 10,000 feet at a point about 1 mile west of DH-31 (fig. 9) is inferred from projection of the four electrical sounding depths. Comparison of this depth with the depth of 13,000 feet as calculated from the gravity data farther northeast and D17 433:3. mm 5532985 82:» gamma mm 83:00 .523 mastom 2.: £83 ‘Véy mica 538.0 I .w 553E mIMPuZ ooov OOON O _ _ _ _ ._.m_m_I._ ooodm ooodfi o A ooo‘oN M I I R H I O I I F I m l Iooo_m~ C I vaEoU “cwEvmnn bfltohéi waEoo «:oEwmmn ifltwhéi G I 1 3 .I A I .I. Iooodfl A I N I N H .5 22950 n... I 0 a I Z I I. I m I A F I88 A H I A, - H m I I I \ I 25:32 gal \ :9: ~52me <22, nnIto 0 835m 959.0 M hw 50,000 fps, respectively, was recorded. The deeper velocity horizon, with a computed depth of 150 feet at shotpoint 4, is assumed to be the top of the base- ment complex. Its true velocity, therefore, is probably about 17,800 fps as measured on profile 9, which extends east from Pilot Knob (fig. 19). The more compacted alluvial layer (velocity of 8,200 fps) probably continues beneath the entire profile even though it was not recorded between shotpoints 2 and 4; its absence can be explained by the particular geometry of the problem. Calculated arrivals from the assumed continuation of this layer are shown by a dotted line on the time-distance plot northeast of shotpoint 3. Ap- parently, energy arrivals from this layer would appear as second arrivals on all but one geophone. Inspection of the appropriate seismograms in the time zone calculated for these second arrivals, however, indicates that the energy level of the first arrivals is too high to distinguish possi- ble second arrivals. An alternative though less likely explanation for the absence of the 8,200-fps layer between shotpoints 2 and 4 is displacement along the Algodones fault. Reference to the geologic map (pl. 1) shows that the seismic profile crosses the projected strike of the Algodones fault at about shotpoint 2. This explanation is less probable because, if this were the case, the top of the 8,200-fps layer should be higher between shotpoints 2 and 4 than GEOPHYSICAL FIELD INVESTIGATIONS between shotpoints 1 and 2 and would distinctly appear on the time-distance plot. Although the major move- ment on the Algodones fault is horizontal, the upthrown side of the fault, at least near Pilot Knob, is to the east. Assuming that the 8,200-fps layer continues across the profile, the apparent dip of the basement complex from Pilot Knob southwestward beneath the seismic profile is calculated to be about 27°. YUMA BASEMENT HIGH The highest Bouguer anomaly values and the highest magnetic intensity were observed in an area extending southward from Yuma for about 10 miles (pls. 1, 3). Out- crops of pre-Tertiary basement and numerous drill holes penetrating the basement at shallow depths clearly show that the high geophysical anomalies are caused by a basement high which is herein called the Yuma base- ment high. The general correlation between the gravity and magnetic data over the Yuma basement high is well il- lustrated in profile B-B' (pl. 4). From low values over the southerly international boundary, both the gravity values and the magnetic intensity increase to a mul- ticrested maximum over the Yuma basement high. The small southward displacements of the magnetic anomalies relative to the gravity anomalies are at— tributed to the inclination of earth’s magnetic field (59° N. in the Yuma area); the magnetic data represent total magnetic field, whereas the gravity data represent only the vertical component of the total gravity field. Both the gravity and the magnetic data reveal that the Yuma basement high anomaly includes two principal features: (1) a northern high, referred to as the Yuma anomaly and consisting of three maximums, is related to a row of basement outcrops; and (2) a southern high, referred to as the Mesa anomaly, is related to an area where holes drilled after the discovery of the feature by the gravity survey penetrated basement at depths of less than 100 feet. The two anomalies are separated by a gen- tle saddle, and as indicated on the gravity and magnetic maps (pls. 1, 3), they differ in the trend of their elonga- tion. The summits of the Yuma basement high — the present outcrops — are composed of the ubiquitous porphyritic quartz monzonite and are alined about N. 20° W., approximately parallel with the trend of the Yuma anomaly and with the northern Gila Mountains to the east. North of the quartz monzonite outcrops are two outcrops of Tertiary breccia and conglomerate, made up entirely of quartz monzonite detritus, between which the present channel of the Colorado River follows a super- posed course. . . Although several basement outcrops occur within the area of the Yuma anomaly (pl. 1), only two of these out- crops (the two southernmost) are associated with in- D23 GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA .H SEQ so 850% mica we on: .quoUm mo 8: 8.: $3 Good “scam US$315 :8: mm: «.59 :85 .uEUuzom ~85on 1851:; 1mm; 550.5 1§>Emmmou 2.: 259m :39? mm wEoE 332» w .mmmonusa 3:989:00 .55— .Eman Bum—6:085 9: we En $80me 0505 533$QO 1;: “.5505 all] $355.5. N H o _ _ 1 _ mug: N H o A< )A1;V nuyrn >§.__.._L JMUH >14qu < «47.141.31.333- 4 r V P > b )r< «44) v < (1 r Hv‘ uqv> br Iv <4r>v>4rgv1LAcv<u...m U.:~ .Eom .Ucww . 1.. . IoooH . a 1 u o .. O a: a a .> > D > um w i H ¢ m Bu; Man a $3 wm> mm> 95 my m m m. I.‘ a W Oml ( :ozaum 339.6 wNI 29:95 .9333. M “H cm! W V 1 vml S NNI 1a a 1133:! NI ‘HldBCI D24 GEOPHYSICAL FIELD INVESTIGATIONS SOUTH Quam C M h NORTH a 1000, Pilot Knob monzonite rfigungginascho SP1 spz SP3 5” SP5 sps\ 3'77 Sift; SP9 | SEA . ' ' ’ ' < V > 4 v Alluvium 5600 fps " ’ V n _x . rtz LEVEL Gnelss . h ”I L < Qua . - .3 Alluvnum 6700 fps /,/—/’[ A 1000’ ‘ I x .asfgibo olgplex Basement complex 11700 fpsi l Basement complex 15,200 fps l 2:: 1.200 1.200 2 0'00" ’05 TIME, IN SECONDS 0.900 0.600 __ 0.300 3 0.000 8 9 SHOTPOINTS SHOTPOINTS 0 2000 6000 10,000 FEET | . I . 1 l I 1000 2000 —O 3000 M ETERS l l FIGURE 12. — Seismic-refraction profile 5 from Pilot Knob to the Cargo Muchacho Mountains. The higher basement velocity recorded along the southern two-thirds of the profile may represent gneiss, in contrast to quartz monzonite beneath the northern one-third of the profile. The higher velocity, however, more likely reflects the greater shotpoint-to-geophone dis- tance employed along the southern part of the profile. SP, shotpoint; circles, alluvium arrivals; dots, basement arrivals; dashed lines, arrivals weak or inferred. dividual gravity closures. Similar closed highs possibly occur over the remaining outcrops, but the station den- sity was insufficient to differentiate individual closures. Bouguer anomaly values are decidedly lower on the out- crops of breccia and conglomerate than on the outcrops of quartz monzonite, a fact which reflects the lower den- sity of these Tertiary nonmarine sedimentary rocks as compared with the quartz monzonite basement. Although the magnetic data reveal a general high associated with the Yuma anomaly, the details are different from those of the gravity pattern. Instead of the elongate high oriented about north-northwest as in- dicated by both gravity and outcrop patterns, the magnetic high is more nearly equidimensional and is centered on a point slightly east of the quartz monzonite outcrop traversed by US. Highway 95 in Yuma. The reason for the difference in pattern is not known; however, it may be caused by a local mass of more magnetic basement east of the porphyritic quartz mon- zonite outcrop. The Yuma anomaly is separated from the Mesa anomaly to the south by a gravity saddle and a magnetic saddle (pls. 1, 3). Although no basement rock is exposed within the area of the Mesa anomaly, the gravity and magnetic values suggested a shallow depth to basement on the crest of the Mesa anomaly, and subsequent drill- ing confirmed the presence of basement rock near the surface. A detailed gravity map of the Mesa anomaly at a contour interval of 1 mgal (pl. 5) reveals three closed highs which apparently represent buried basement peaks. Test wells DH-25 and DH-24, drilled at the two highest Bouguer anomaly values, penetrated porphyritic quartz monzonite at depths of 47 and 79 feet, respec- tively. Two other test wells (DH-23 and DH-22), drilled at gravity stations having Bouguer anomaly values 0.7 mgal lower than those at the previous two sites, penetrated similar rock at depths of 90 and 267 feet, respectively. Additional information is provided by two oil test wells on the flanks of the basement ridge: DH-20, 2 miles northwest of the high at DH-22, penetrated granitic basement (probably the quartz monzonite) at a depth of 1,431 feet; and DH-21, an old well reported by Kovach, Allen, and Press (1962), penetrated granitic rock at a depth of 730 feet (table 1). In all these wells alluvium appears to directly overlie porphyritic quartz monzonite basement. Both gravity and magnetic patterns imply that the buried basement ridge causing the Mesa anomaly has a more northwesterly trend than the ridge responsible for the Yuma anomaly. The general subsurface configuration of the Yuma basement high is shown on profile B-B' (pl. 4), which ex- tends from DH-30 near the international boundary, across the Yuma basement high, to an outlier of the GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA D25 WEST Approximate location PilotEAST 10005-1 of Algodones fault Knob 1000’ SP 1 Pilot Knob Mesa SILVA” American Canal SP 3 SP 4 ,4 x SEA 1 I I K Gnelss SEA LEVEL — Unconsolidated alluvium x x x x LEVEL 6750035 xxxxxxx — 7... .__7_ —-— —?— — 1000’— c cted II , 1000’ omnaazoo fansuwum Pie—Tertiary basement complex 2000’ 1 17,800 fps 2000, 1.500 Calculated arrivals from BZOO-lps hovilon 1.200 ‘ - >50,000 fps :0 S o 0.900 - — 0 Lu (n E g 0.600 — : 0.300 *6? —— c° 65 0.000 ° d a SP 1 SP 2 SP 3 SP 4 SHOTPOINTS 0 5000 10,000 FEET l | 1 I 1 l J O 1000 2000 METERS FIGURE 13. — Seismic-refraction profile 6 from Pilot Knob west across Pilot Knob Mesa. The basement velocity is assumed to be 17,800 fps as measured on profile 9, which extends east from Pilot Knob. SP, shotpoint; circles, alluvium arrivals; dots, basement arrivals. Cargo Muchacho Mountains. The subsurface geology in the vicinity of the San Luis basin was constructed from gravity data and the data of wells DH-27 and DH-30 (table 1). The interpretation in the vicinity of the Picacho-Bard basin is based on the previously discussed data of seismic-refraction profile 4 (fig. 10) and the gravity data of plate 1. The interpretation in the central part of plate 4, in the vicinity of the Yuma basement high, is based on the following seismic-refraction sur- veys, detailed gravity surveys, and resistivity surveys: (1) Seismic-refraction profile 7 (fig. 14), extending between two of the quartz monzonite outcrops in the area of the Yuma anomaly; (2) gravity profile C-C’ (fig. 15), extending across the gravity saddle between the Yuma and Mesa anomalies; (3) a detailed resistivity sur- vey in the area of the Mesa anomaly (fig. 16); and (4) seismic-refraction profile 8 (fig. 17), on the south flank of the Yuma basement high. These four surveys are dis- cussed in detail in the following paragraphs. The depth and configuration of the basement surface between two of the quartz monzonite outcrops associated with the Yuma anomaly were determined by seismic- refraction profile 7 along Pacific Avenue in Yuma (fig. 14); the location of profile 7 is shown on plate 5. The time-distance plot shows two distinct layers: the upper layer is correlated with alluvium, and the lower layer is correlated with the quartz monzonite basement exposed beyond the two ends of the seismic spread. The P-wave velocity in the alluvium is about 5,200 fps — indicative of unconsolidated deposits. The greatest thickness of alluvium, about 620 feet, is near the center of the profile. The downdip and updip velocities of the basement are about 10,000 fps and 22,500-35,500 fps, respectively, and the true velocity is calculated to be about 14,900 fps, about average for granitic rocks in the Yuma area. The seismic data do not show any evidence of Tertiary non- marine deposits beneath the alluvium. Well data confirm the seismic-refraction results; semiconsolidated to consolidated Tertiary nonmarine deposits are absent except on the lower flanks and in the northern part of the Yuma anomaly. Wells DH-12 through DH-16, drilled in the vicinity of the Yuma D26 SOUTH "sex NORTH 500’ SP 3 | SEA _~, , LEVEL > ‘A J“ .i Forphyritic quartz monlonite 14,900 fps 1 V > V ’T Porphyritic quam" monzonite 14,900 fus< Alluvium, 5200 fps 500' 0.600 22. «k I (I) o S 5 8 W“ (I) 0.300_ \6\ E Lul 2 fig ,: z 9:- o o 0 o 0.000 1 2 3 SHOTPOINTS o 2000 FEET l—.___l 0 500 METERS L_._l_—l FIGURE 14. — Seismic-refraction profile 7 along Pacific Avenue in Yuma between two basement outcrops. SP, shotpoint; circles, alluvium arrivals; dots, basement arrivals. GEOPHYSICAL FIELD INVESTIGATIONS anomaly, all bottomed in granitic basement rock — probably the porphyritic quartz monzonite exposed in the area (table 1). Of these wells, only possibly DH-12, which is near the exposures of Tertiary breccia and con- glomerate (pl. 1), and DH-15 penetrated the Tertiary nonmarine sedimentary rocks between the alluvium and the basement complex. Well DH-14 penetrated “granite” (probably porphyritic quartz monzonite) at a depth of 1,085 feet beneath predominantly fine—grained alluvium. Well DH-15, farther down the western flank of the basement high from DH-14, appears to have penetrated the Bouse Formation and about 155 feet of underlying Tertiary fanglomerate between the alluvium and the basement complex, which was penetrated at a depth of 1,398 feet (table 1). The area between the Mesa and Yuma anomalies was explored by gravity profile C-C’ (fig. 15). C-C' extends north-northeastward from DH-24, across the gravity saddle between the Yuma and the Mesa anomalies, to the southernmost basement outcrop on the Yuma anomaly (pl. 5). If we assume a variable density contrast in accord with the data of figure 4 (0.44 g cm‘3 for depths of 0-1,000 ft and 0.30 g cm‘3 for depths greater than 1,000 ft), the computed model has a maximum Assumed resi onal gravity -10 MILLIGALS I ,_. u: -20 SOUTH-$0 UTHWEST Basement complex Computed anomaly Measured Bouguer anomaly NORTH-NORTHEAST DH-24 0 K. at 79 ft Ground surface Basement ,_ 1000 — Lu Lu “- 2000 — 3000 4000 SOJOO F E ET 1000 2000 M ETERS FIGURE 15. — Observed and computed gravity anomaly on profile C-C' across the gravity saddle between the Yuma and Mesa anomalies. Maximum sediment thickness is about 2,600 feet near the center of the trough. GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA D27 0H M-M ETERS SECTIONS Depth to basement loo-200 feet PLAN 0 Depth to basement less than 100 feet 1 MILE L—.__1_—l FIGURE 16. — Resistivity profiles c-c’ through f-f across the Mesa anomaly. qualitative interpretation / Sections show a of the resistivity profiles assuming a basement composed of ver- tical blocks (crosshatched); dashed lines show boundaries of blocks in which basement rock is interpreted to be shallow. VES, location of ver- tical electrical sounding. Lines of profiles shown on plate 5. SOUTH NORTH SP 2 SP 1 /Ground surface DH_25 E 500— Alluvium r " ’ _ m 7000 fps Por - ‘ phyntlc “' 1000~ Aquartz monzonite‘ V 1500 _ _ «~16,500t|5s i 0.600 34.000 “’5 m D z O 0 Lu ‘0 0.300 - E e id 000, E 00 .: O O 0.000 S? 1 SP 2 0 1000 2000 FEET o 500 METERS L__1—l FIGURE 17. — Seismic-refraction profile 8 on the south flank of the Yuma basement high. SP, shotpoint; circles, alluvium arrivals; dots, porphyritic quartz monzonite arrivals. sediment thickness of about 2,600 feet; this thickness probably includes some relatively dense Tertiary sedimentary rocks (both marine and nonmarine?) in the basal part of the fill overlying the basement. The steep gravity gradient about 700 feet northeast of DH-24 suggests a basement fault; the computed model, which assumes a vertical fault with about 400 feet throw, gives a good fit to the observed anomaly. Note that this model was computed using two-dimensional analysis, but the actual layout of profile C-C ' (pl. 5) indicates that a two- dimensional analysis is not completely justified. The results, therefore, should be considered approximate. The area of the Mesa anomaly was surveyed with four resistivity profiles, and depth information along these profiles was provided by three electrical soundings (fig. 16, pl. 5). The data reveal the general complexity of this area. A simple qualitative interpretation assuming a basement composed of vertical blocks is shown in figure 16. At VES (vertical electrical sounding) 6, which is about 1,450 feet S. 64° E. from the basement high at DH-25, the depth to basement is computed to be about 115 feet. In addition, by matching the recorded sounding curve with a set of theoretical VES curves over a dipping contact (Dakhnov, 1953), we estimated that the base- ment surface dips about 40° S. This slope is in reasonable agreement with the slope of 30° estimated from seismic-refraction profile 8 (fig. 17) described next. The area of steep magnetic and gravity gradients on the southwest flank of the Mesa anomaly was explored 'D28 with seismic-refraction profile 8 (fig. 17). The north end of profile 8 is at the center of the southernmost gravity closure of the Mesa anomaly, where test well DH-25 penetrated porphyritic quartz monzonite at 47 feet, the highest known point on the buried basement ridge. In the computed model, the basement surface dips about 30° S. along the line of profile. About 1,600 feet south of DH-25, a significant offset on the time-distance plot probably reflects a steeply dipping fault which offsets the basement surface. Assuming the fault to be vertical or nearly vertical, we computed a throw of about 350 feet. The considerable relief associated with the Yuma basement high probably‘ results from a complex fault pattern traversing a buried erosion surface as rugged and complex as the slopes in the exposures of basement rock in the Yuma area. Although the general fault pattern can be delineated from the geophysical data (pl. 1), the exact location and nature of these faults must remain speculative, at least until more data are obtained. Along the east flank of the Yuma anomaly the gravity and magnetic gradients imply a complex pattern of faults that probably trend north (fig. 8); in contrast, along the northeast side of the Yuma anomaly, at the north end of the Yuma basement high, gravity and magnetic gradients suggest a northwest-trending fault or faults. The gravity gradients on the northwest side of the Yuma anomaly suggest north-northeast- to northeast- trending faults; one such fault, as indicated by the aline- ment of fault intercepts on seismic-refraction profile 9 (fig. 19) and gravity profile D-D' (fig. 18), may bound the Yuma basement high on the northwest (pl. 1). The gradients on the west and northwest flanks of the Yuma anomaly are less pronounced than those on the east flank, presumably because the bordering west basin (Yuma trough) is shallower than the Fortuna basin to the east. On the west flank of the Yuma anomaly, close to the northern basement outcrops of the Yuma base- ment high, the gravity gradients suggest a north- northwest-trending fault or faults, as shown on plate 1 for the easternmost fault recorded on gravity profile D- 0’. Southwest of the Yuma basement high, the seismic- refraction data recorded a fault with about 350 feet of throw (fig. 17). This fault is interpreted as the Algodones fault (Olmsted and others, 1973). About 4,000 feet farther south, down the flank of the Yuma basement high, the steep gravity gradient indicates an additional 1,000 feet of throw on a parallel fault (pl. 4, fig. 20). The magnetic and resistivity data are consistent with these results insofar as they confirm that the south flank of the Yuma basement high dips steeply into the San Luis basin, and the linearity of the southwest sides of the GEOPHYSICAL FIELD INVESTIGATIONS gravity and magnetic anomalies associated with the high implies that the faults or fault zone trends northwest. Along the northeast flank of the Mesa anomaly the trend of the gravity and magnetic gradients probably reflects a northwest-trending fault, as does the aline- ment of fault intercepts on profiles C-C’ (fig. 15) and A- A’ (fig. 8). An additional fault to the east, recorded dur- ing the seismic-reflection survey and inferred from gravity data of profile A-A’ (fig. 8), was assumed to strike northwest. This interpretation is supported by seismic-reflection data to the south, where two seismic- reflection profiles revealed several faults (Herschel Snodgrass and R. D. Davis, written communs., 1966). These faults were interpreted on the basis of offset reflec- tors, local steep dips, and zones of missing record. Although strike was not determined, the faults are in- ferred to strike northwest because of the alinement of fault intercepts on mutually perpendicular profiles. One of these faults appears to be along the trace of the Algodones fault, and the other are parallel or en echelon to the Algodones fault (pl. 1). The complex structure inferred for the Yuma base- ment high relative to other areas of the survey probably reflects the greater abundance of geophysical data in this area of near-surface basement rather than anomalously complex structure. BOUNDARY BASEMENT HIGH A pronounced gravity high is centered on a line of out- crops of porphyritic quartz monzonite that straddles the southerly international boundary (pl. 1). This gravity high is interpreted as reflecting a basement ridge, which is herein called the Boundary basement high. The out- crops (most are located south of the border in Mexico and are not shown on pl. 1), which are the summits of this mostly buried basement ridge, trend about N. 45° W. As seen on the gravity map (pl. 1), the gravity high is elongated in the same direction and is separated from the Yuma basement high 20 miles to the northwest by a broad saddle about midway between the two highs. The maximum Bouguer value is —20.0 mgal on the northern- most basement outcrop, on the United States side of the border; the Bouguer anomaly in the saddle is not well defined, owing to sparse control, but appears to be about —37 mgal. Using the density data shown in figure 4, the difference in gravity values reflects a depth to basement of 4,000-5,000 feet in the saddle. The gravity data (pl. 1) indicate that from the Boun- dary basement high the basement surface slopes steeply into the Fortuna basin and somewhat more gently into the San Luis basin. Basement outcrops to the southeast, in Sonora, Mex- ico, imply that the Boundary basement high continues in that direction. GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA YUMA TROUGH The Yuma trOugh is a north-northeast-trending trough between the Pilot Knob basement high on the west and the Yuma basement high on the east. The gravity data (pl. 1) indicate that the axis of the trough is about midway between the basement outcrop at Pilot Knob and the one in Yuma and that a gentle saddle crosses the trough west of Yuma. Bouguer gravity values decrease north-northeastward and south-southwestward from this saddle. As a first approximation, an idealized model was com- puted from the gravity data of profile D-D', which ex- tends from Pilot Knob to Yuma across the saddle (fig. 18). Near the west-central part of the profile the depth to basement is about 2,700 feet, as estimated from the magnetic data by using the method described by Vac- quier, Steenland, Henderson, and Zietz (1951). Ad- ditional basement control was available from well DH- 15, which penetrated the basement complex at a depth of 1,398 feet (table 1). On the basis of the preceding depth information and the density data of figure 4, we computed an idealized gravity model — a nearly symmetrical trough with about 3,500 feet of Cenozoic sediments at the center (fig. 18). Two faults near the east end of the profile were required to give a reasonably good fit to the observed gravity data. Seismic-refraction profile 9 (fig. 19) extends across the Yuma trough approximately along the west-flowing reach of the Colorado River from Yuma to Pilot Knob. Pre-Tertiary gneiss of the basement complex is exposed at Pilot Knob, near the west end of the profile, and Ter- tiary breccia and conglomerate composed of porphyritic quartz monzonite detritus forms the outcrop near the north end of the Yuma basement high, just east of the profile (pl. 1). Subsurface control is provided by wells DH-10, DH-17, and DH-18, all near the profile. The time-distance plot for profile 9 (fig. 19) reveals a simple two-layer configuration. The P-wave velocity recorded in the upper layer ranges from about 6,500 to 7,000 fps and averages about 6,700 fps. In the western part of the profile, between shotpoints 1 and 4, the velocity of the lower layer is calculated to be 17,800 fps. In the eastern part, between shotpoints 4 and 8, however, the velocity of the lower layer is only 12,800 fps. Interpretation of these velocities is facilitated by the acoustic-velocity and lithologic logs of DH-17, about one-half mile north of the profile. (See pl. 2 and table 2.) These logs recorded three distinct seismic layers (a fourth, deeper layer — the basement — was not penetrated by the well): (1) alluvium (including the un- derlying transition zone); (2) Bouse Formation (the clay, silt, and sand part); and (3) nonmarine sedimentary rocks (including the tuff(?) and limestone at the base of D29 the Bouse Formation). According to the acoustic log, the recorded velocities for these layers are about 7,500, 6,500, and 13,000 fps, respectively. Comparison of these velocities with the seismic- refraction data reveals the following: (1) The velocity of the upper layer as measured by seismic refraction is about 10 percent less than the weighted-average velocity of the equivalent alluvium, transition zone, and major part of the Bouse Formation recorded by the acoustic- velocity log; (2) seismic-refraction methods cannot differentiate the lower velocity Bouse Formation from the overlying higher velocity alluvium and transition zone; and (3) the lower layer along the east half of the seismic-refraction profile corresponds to the fanglom- erate (Tertiary nonmarine sedimentary rocks unit), and the velocities recorded by the two methods for this unit agree closely. (The top of the fanglomerate probably dips south between well DH-17 and the seismic profile, as shown by the fact that this horizon was penetrated at a depth of 1,380 ft in the well and that, at a point on the seismic profile directly south of the well, the computed depth to the top of the fanglomerate is 1,550 ft.) The discrepancy between the velocities measured in the upper layer by the two methods probably can be ex- plained by anisotropy; the velocity is measured ver- tically by the acoustic log and horizontally by the seismic-refraction technique. In the seismic com- putations, if an average velocity of 7,500 fps is used, in- stead of 6,700 fps, the computed thickness of the upper layer is increased by about 10 percent. Between Pilot Knob and shotpoint 3 the 17,800-fps layer is assumed to be gneiss like that exposed at Pilot Knob, and its surface is computed to dip west 61/2 °. Well DH-18, less than one-half mile south of the profile, bot- tomed in alluvium (or possibly in the transition zone) at a depth of 978 feet, suggesting that the Cenozoic fill overlying the basement, at least near the west end of the profile, is chiefly or entirely unconsolidated alluvium. At shotpoint 5, about 0.6 mile southeast of DH-17, the top of the Tertiary fanglomerate is computed to dip east about 2%°. Projection of the dips of the pre-Tertiary basement and Tertiary fanglomerate surfaces implies that the Tertiary fanglomerate overlaps the basement and pinches out near shotpoint 4, as shown by the dashed lines in figure 19. The seismograms recorded between shotpoints 4 and 6 were studied for later arrivals that could reveal the at- titude of the basement complex beneath the Tertiary fanglomerate, but any possible later arrivals were masked by the high energy level of the first arriving waves. A raypath analysis on the computed cross sec- tion, however, indicates that east of shotpoint 5 the base- ment surface must lie at a depth of at least 1,000 feet D30 _5_ MILLIGALS GEOPHYSICAL FIELD INVESTIGATIONS Assumed regional gravity G 2 U 0 '§ 3 ST 2 WE i Approximate location of Pilot Knob a I/magnetic depth determination X X 01° Alluvium ‘.o 1‘. ‘ Gneiss X x ‘°‘-—' Bottom in alluvium '- x TD 978 ft or transition zone x Cenozoic fill Lu m 2000 — X X LL X 4 Pre-Ter‘tiary basement complex O 2000 4000 8000 FEET l | | I l | l J O 500 1000 1500 M ETERS L.L_J._|_1_l_.—l__.—j below the top of the Tertiary fanglomerate; otherwise, the first arrivals recorded between shotpoints 5 and 6 and shot from shotpoint 3 would have been basement arrivals. Possibly, the contact between the Tertiary fanglomerate and basement complex is actually a fault plane; however, because of the lack of gravity or magnetic expression, this alternative interpretation is unlikely. The shelf of Tertiary nonmarine sedimentary rocks ex- tending west from the Yuma basement high to about shotpoint 6 (fig. 19) is probably associated with the Yuma basement high. The fault shown on the cross sec- tion just west of shotpoint 6 is required to explain the discrepancy in depth to the top of the Tertiary rocks as computed in shooting east and west from shotpoint 6. As shown on plate 1, this fault may be a continuation of a fault that intercepts gravity profile D-D' (fig. 18). If so, the fault strikes north-northeast and bounds the northwest side of the Yuma basement high. At the south end of the Yuma trough the gravity and magnetic data (pls. 1, 3) suggest that the depth to base- ment increases southward toward the San Luis basin. GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA D31 Measured Bouguer anomaly —15 Computed anomaly G G ‘6 fl) '§ 5 E | I YUMA EAST Ground surface a BASEMENT HIGH 31-" to Alluvium-Bouse(?) /\ /\ 0 a Top of basement complex Jl .' Formation at 972 ft !\ TD 1393 ft '£;Nonmarine sedimentary rocks h f‘ Porphyritic quartz monzonite _ 2000 FEET 4000 FIGURE 18. — Observed and computed gravity anomaly on profile D-D’across the Yuma trough. The idealized model was computed on the basis of the density data of figure 4, test well DH-15 (projected 3,000 ft), and a magnetic depth deter- mination. TD, total depth. SAN LUIS BASIN From the extreme southwest corner of Arizona both magnetic intensity and Bouguer anomaly values rise toward the north-northeast, and a resulting zone of low magnetic and gravity values trends east-southeastward along the border with Sonora, Mexico. This magnetic and gravity low is interpreted as reflecting a low on the basement surface, which is herein called the San Luis basin. The San Luis basin is bounded on the northeast by a largely buried ridge that includes the Yuma base- ment high and the Boundary basement high. Although the magnetic coverage is incomplete near the inter- national border, the center of the basin appears to strad- dle the border about 11 miles west-northwest of the Boundary basement high. Although no wells south of the Yuma basement high have penetrated the pre-Tertiary basement, three oil tests — DH-27, DH-28, and DH-30 (pl. 1) — provide minimum depths to basement which are significant to the geophysical interpretation (table 1). DH-27, the deepest drill hole in the Yuma area, bottomed in Ter- tiary nonmarine sedimentary rocks at a depth of 6,007 feet (5,837 ft below sea level); DH-28 and DH-30 were drilled to depths of 3,660 and 4,868 feet, respectively, D32 without reaching the Tertiary nonmarine sedimentary rocks. Figure 20 shows a model computed from the gravity data along profile E-E' from well DH-25, at the south end of the Yuma basement high, to the center of the San Luis basin, at the international border. The depths to basement at the extreme north end of this profile (including the northernmost fault) are based on the results of seismic-refraction profile 8 (fig. 17). A second fault, about 4,000 feet down the flank of the Yuma base- ment high from the prior fault, is based on a local steepening of the gravity gradient; the throw of this fault is estimated to be about 1,000 feet. The trend of the gravity and magnetic gradients at the south end of the Yuma basement high (pls. 1, 3) suggests that these faults probably strike northwest. If the computed model is cor- rect, the basin fill — including alluvium, rocks of the transition zone, Bouse Formation, and Tertiary non- marine sedimentary rocks — is about 13,500 feet thick at the border with Sonora, Mexico. The model was com- puted using the density contrasts shown in figure 4. ALGODONES FAULT AND RELATED FAUL’I‘S A principal branch of the San Andreas fault system extends along the northeast side of the Salton Trough to a point near the southeast shore of Salton Sea, southeast of which it is concealed by apparently unaffected alluvium and windblown sand. This branch has been identified as the Banning-Mission Creek fault (Allen, 1957) or as the San Andreas fault (Dibblee, 1954). Biehler (1964; oral commun., 1967) inferred that the trace of the fault, which is indicated by a strong aline- ment of gravity lows, extends from the Salton Sea southeastward beneath the Sand Hills on the East Mesa of Imperial Valley to the Colorado River south of Pilot Knob. The gravity data are supported by information from seismic-refraction profiles (Kovach and others, 1962), which indicate downthrow of the basement sur- face southwest of the fault. A continuation of this fault (or possibly a parallel fault) which extends southeastward across the Yuma area from a point on the Colorado River about 2 miles south of Pilot Knob to the southerly international boun- dary 26 miles east of the Colorado River (pl. 1) was named the Algodones fault by Olmsted, Loeltz, and Irelan (1973). Those authors presented five kinds of evidence for this fault, which may be summarized as follows: 1. Anomalous topography 0n the “Upper Mesa.” Ex- posures of older alluvial deposits in the so-called “Upper Mesa” in the south-central part of the area of investiga- tion contain an anomalous northwest-trending drainage approximately perpendicular to the normal, consequent drainage from the Gila Mountains. The inferred trace of the fault is along the anomalous drainage at the foot of GEOPHYSICAL FIELD INVESTIGATIONS an eroded northeast-facing alluvial escarpment; the mesa surface southwest of the escarpment is elevated 30- 60 feet relative to the northeast side. Lineaments ap- parent on aerial photographs suggest that the fault trace has a branching pattern near the middle of the area, as shown on plate 1. 2. Groundwater-barrier effect and displacement of the water table. Shallow test wells drilled in the area of the “Upper Mesa” indicate a sharp displacement in the water table along the inferred fault trace. Water levels northeast of the fault are more than 30 feet higher than those southwest of the fault (near the northwest edge of the “Upper Mesa”) and are about 7-8 feet higher near the southerly internationalboundary. Measurements in private irrigation wells and government observation wells in the southeastern part of the Yuma Mesa show that the water-table offset continues about 3 miles northwest of the edge of the “Upper Mesa,” although this part of the fault trace is concealed. The displace— ment of the water table may be caused by the fault acting as a barrier to ground-water movement. Electrical-analog data indicate that observed changes in water level on both sides of the fault are best modeled by assuming that the transmissivity of the fault barrier is less than one one-thousandth that of the alluvial deposits on either side. 3. Magnetic gradients. A steep magnetic gradient along the southwest flank of the Mesa anomaly and ex- tending across Yuma Valley to the Colorado River somewhere between 1 1/2 and 4 miles south of Pilot Knob (pl. 3) suggests a northwest-trending fault or series of faults along which the basement is downthrown to the southwest. 4. Gravity patterns. Gravity data along the southwest flank of the Mesa anomaly indicate a steep gradient at about the same position as the magnetic one. In addi- tion, the gravity map (pl. 1) shows a very steep gradient on the northeast flank of the Boundary basement high, which suggests downthrow of the basement surface toward Fortuna basin along a fault about in the position of the fault inferred from topographic and hydrologic evidence. 5. Ground-water-temperature anomalies. Although the fault is concealed beneath Yuma Mesa and Yuma Valley, its position throughout much of its extent across these features is revealed by an elongate body of anomalously warm ground water, mostly on the northeast side of the inferred fault trace. Similar temperature anomalies farther southwest probably reflect parallel or subparallel faults. The higher temperatures in these areas probably result from upward movement of deep warm water induced by the barrier effects of the faults. However, the effect of the buried basement ridge at the Mesa anomaly may in part ac- count for the temperature anomaly along the Algodones fault. D33 GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA .vmbomE 8 433 flazbw 6:: gamut 6132.8 “5583 ago—u 6.85.8 33:2:wa mafia—55: bflfioE £2»:me ”mmafibm 5535:“ £288 usuaov Raou— dfi 35039.3 Mm .932 ignofimvem ~33in «5.8.55: 2: «Eu mmmwcm m5 £3332 83:8 2: 25% 8 Buscwvwfiuuw 3% =28a¢2-25mmum.mv_oo~ zumucofimvom 95255: bflfifih mm @88535 E m3 oow.§ «o 3623 3% .no:& 8an “a @0898 $5 3.: mmmesw 3.8352 baanoa mac cowIS «o 3629» SE. .nmE Eofimman «NE—cw 2: mo 98 £5: 2.: :3: 3 Eagmmw no:& ”—25 89¢ a e595 :osuaafiéfimmow I .3 553E _ _ fl _ _ _ q A mmwhmz 000m OOON OOOH O — _ _ _ _ — _ hum“. ooodfi 000m O ”#2 .0thIw a N 08.0 I OOmd ea 35 08.0 H. W .3 m I 000.0 S 3 3 O N I cow.“ w I OOmA oowé 3. RES 38.5%.“... E: 880523.: = $2 E. J. an. Sui .OOON 9.3. 9.2: 3:258" act-SEE is...» 9.8. b.2258 2:252. 22:3 Id «I I Al I U H II I7 II onES .5538 855.com : m: an 33:32. 9.2.: .52253 02:5 I. n.” I III. IN! C mg m... .08H 2.35:2. fuzz—Evan 0535.3: 32:: _ — me 83 . rue: a K EE>==< we 83 33.; 4u>m4 . a 552.5 . I _ _ a . _ w . a _ . «mm mmm m 5mm wan mmm H «mm «mm mem dam 08H :9: _ ._. ._. . 5259:. m z a soc: 8:“. i; m m m Eu; 9: m. m m h an m w W m. m D34 GEOPHYSICAL FIELD INVESTIGATIONS Assumed regional gravity _20_ MILLIGALS ~30 —40 Measured Bouguer anomaly .‘Computed anomaly —50 — _ NORTH YU MA BASEMENT HIGH SOUTH Ground surface DH_25 0 SAN LUIS 5000 _ BASIN _ i3 Cenozoic fill 141 u. 10’000 _ Pre-Tertiary basement complex _ 15,000 0 5000 10.000 15,000 20,000 FEET I l | l l 0 1000 3000 5000 M ETERS FIGURE 20. — Observed and computed gravity anomaly on profile E-E’ from well DH-25, at the south end of the Yuma basement high, to the center of the San Luis basin, at the international border. The depths to basement at the ex- treme north end of the profile (including the northernmost fault) are based on the results of seismic-refraction profile 8 (fig. 17). To further define the location and nature of the Algodones fault and possible parallel or en echelon faults, the postulated trace of the fault zone was crossed at several places by seismic and resistivity profiles (pl. 1). Several faults, including the probable Algodones fault, were delineated in the seismic-reflection survey of 1966; the following interpretations are summarized from Herschel Snodgrass (written commun., 1966). A seismic- reflection profile along the Colorado River southeast of Pilot Knob indicated a steeply dipping fault along the east bank of the river about 21/2 miles south of Pilot Knob — within the zone indicated by the magnetic data and about in alinement with the fault to the northwest postulated by Biehler (1964; oral commun., 1967). The top of the Bouse Formation was estimated to be downthrown 500 feet to the south or southwest. Throw on the basement surface was not determined but is es- timated to be even greater. An area of shallow basement believed to be a southeasterly nose of Pilot Knob was found about 1-2 miles north of the fault. Southeast of the Mesa anomaly two seismic—reflection profiles revealed several faults. These faults were inferred from offset reflectors, local steep dips, and zones of missing record. Although strike was not determined, alinement of fault intercepts on mutually perpendicular profiles suggests that the faults strike generally northwest. One of these faults appears to be along the trace of the Algodones fault inferred from water-level data; the others ap- parently are en echelon or parallel faults. Five resistivity profiles across the inferred trace of the Algodones fault southeast of the Mesa anomaly (pl. 5, fig. 21) failed to yield near-surface direct evidence for the fault. The large anomalies on profiles g-g' and h-h', which could be explained by the presence of highly resistive fault gouge, are interpreted instead as represen- GEOPHYSICAL STUDIES IN THE YUMA AREA, ARIZONA AND CALIFORNIA 114°37’30” D35 114°35’ 5 E m 5 J j ’ 5 32'35'— t: 20 El ~52 10 5. o o 11 ohm-m 2 N .-. I OHM-METERS O I w 70 130 g“ 1 120 u, 50 110 :w /\ /l me. 530} 32 ohm-m W U V V E 90$ 20 80 m 10 7o 5. 0 V2 1 2 MILES I 1 2 KILOMETERS L____;_l_—__l FIGURE 21. — Resistivity profiles g-g’ through k-k’ across the inferred trace of the Algodones fault southeast of the Yuma basement high. Lines of profiles shown on plate 5. ting strongly cemented gravel tongues with a high resistivity (about 1,000 ohm-m). Evidence for this inter— pretation is the fact that these large anomalies do not appear on profiles i-i', j-j', and k-k', along the presumed trace of the fault; rather, the anomalies are oriented northeast, parallel to known trends of gravel tongues in the area (Olmsted and others, 1973), instead of northwest, along the inferred fault. Although the resistivity surveys did not yield an- ticipated information about the position and nature of the fault or fault zone, the hydrologic data obtained later in the same area delineated the trace of the concealed barrier within narrow limits but did not provide informa- tion about its precise attitude and nature. Inter- pretations of the barrier’s dip, width, and cause must await detailed exploration by means of test drilling or excavation. Unequivocal data on the direction and amount of movement on the Algodones fault are likewise difficult to obtain. Until it can be shown that several features along the trace of the proposed fault all are offset, we can only speculate about the amount and type of movement. By analogy with other faults in the San Andreas system, the movement probably has been chiefly right lateral. The gravity nose north of Somerton (pl. 1) may reflect a part of the Yuma basement high displaced northwestward by right-lateral movement. The westward offset of the Mesa anomaly relative to the northern part of the Yuma base— ment high (the Yuma anomaly) could also be explained by right-lateral movement, on a parallel fault to the northeast. Vertical components of displacement on the Algodones fault are more apparent. In the northwestern part of the area near the Colorado River, seismic- reflection data indicate that the top of the Bouse Forma- tion is downthrown 500 feet on the southwest side of the fault. Seismic-refraction data along the southwest flank of the Mesa anomaly (fig. 17) indicate that the basement is downthrown 350 feet on the southwest side of the fault, and gravity data 4,000 feet farther south suggest an ad- D36 ditional 1,000 feet of throw (fig. 20). Farther southeast, however, geophysical information suggests throw in the opposite sense ~ the Fortuna basin northeast of the fault is downthrown relative to the boundary basement high southwest of the fault. In the same area the topographic offset of the “Upper Mesa” surface also suggests downthrow to the northeast. Similar reversals in throw along the strike of the fault are common along other faults in the San Andreas system which have had predominantly strike-slip movements. Time of the last significant movement on the Algodones fault can be established qualitatively from geologic and topographic evidence. The near-surface alluvial deposits beneath both the Holocene flood plain of the Colorado River (Yuma Valley) and a river terrace of probable late Pleistocene age (Yuma Mesa) are un- affected, but the fault is clearly exposed across the older alluvial surface of the “Upper Mesa” farther southeast. The age of the “Upper Mesa” surface has not been es- tablished but is almost certainly older than latest Pleistocene. Significant movement on the Algodones fault therefore ceased sometime during the Pleistocene. The parallel or en echelon faults are inferred to be even older, owing to the apparent absence of significant hydrologic effects (other than thermal effects) caused by these faults. By contrast, most of the faults crossing the Salton Trough farther west are still active, reflecting the much more recent deformational activity of that region as compared with the Sonoran Desert. REFERENCES CITED Allen, C. R., 1957, San Andreas fault zone in San Gorgonio Pass, southern California: Geol. Soc. America Bull., v. 68, no. 3, p. 315- 350, Behrendt, J. C., and Woollard, G. P., 1961, An evaluation of the gravity control network in North America: Geophysics, v. 26, no. 1, p. 57-76. Berdichevskii, M. N., and Petrovskii, A. D., 1956, Methods of bilateral equatorial soundings; Prikladnaya Geofizika, v. 14, p. 97-114. (English translation by Ivan Mittin, 1965, U.S.G.S. Library, Denver, 0010., 37 p.) Biehler, Shawn, 1964, A geophysical study of the Salton Trough, Southern California: California Inst. Technology Ph. D. thesis; Ann Arbor, Mich., University Microfilms, Inc., 145 p. Biehler, Shawn, Kovach, R. L., and Allen, C. R., 1964, Geophysical framework of northern end of Gulf of California structural GEOPHYSICAL FIELD INVESTIGATIONS province, in Marine geology of the Gulf of California — a sym- posium: Am. Assoc. Petroleum Geologists Mem. 3, p. 126-143. Crowell, J. C., 1962, Displacement along the San Andreas fault, California: Geol. Soc. America Spec. Paper 71, 61 p. Dakhnov, V. N., 1953, Electrical prospecting for petroleum and natural gas deposits: Moscow, Gostoptekhizdat, 497 p. Dibblee, T. W., Jr., 1954, Geology of the Imperial Valley region, California, in chap. 2 of Jahns, R. H., ed., Geology of southern California: California Div. Mines Bull. 170, p. 21-28. Fenneman, N. M., 1946, Physical divisions of the United States, with Characteristics [of the sections], by Fenneman, N. M., and John- son, D. W.: U.S. Geol. Survey 127,000,000-scale map. Hamilton, Warren, 1961, Origin of the Gulf of California: Geol. Soc. America Bull., v. 72, no. 9, p. 1307-1318, Hawkins, L. V., 1961, The reciprocal method of routine shallow seismic-refraction investigations: Geophysics, v. 26, no. 6, p. 806- 819. Kovach, R. L., Allen, C. R., and Press, Frank, 1962, Geophysical in- vestigations in the Colorado delta region: Jour. Geophys. Research, v. 67, no. 7, p. 2845-2871. Kunetz, G., 1966, Principles of direct current resistivity prospecting: Berlin, Gebruder Borntraeger, 103 p. Larson, R. L., Menard, H. W., and Smith, S. M., 1968, Gulf of Califor- nia — A result of ocean-floor spreading and transform faulting: Science, v. 161, no. 3843, p. 781-784. Metzger, D. G., 1968, The Bouse Formation (Pliocene) of the Parker- Blythe-Cibola area, Arizona and California, in Geological Survey - research 1968: U.S. Geol. Survey Prof. Paper GOO-D, p. D126- D136. Muffler, L. J. P., and Doe, B. R., 1968, Composition and mean age of detritus of the Colorado River delta in the Salton Trough, southeastern California: Jour. Sed. Petrology, v. 38, no. 2, p. 384- 399. Olmsted, F. H., Loeltz, O. J., and Irelan, Burdge, 1973, Geohydrology of the Yuma area, Arizona and California: U.S. Geol. Survey Prof. Paper 486-H (in press). Slotnick, M. M., 1950, A graphical method for the interpretation of refraction profile data: Geophysics, v. 15, no. 2, p. 163-180. Thornbury, W. D., 1965, Regional geomorphology of the United States: New York, John Wiley & Sons, Inc., 609 p. Vacquier, Victor, Steenland, N. 0., Henderson, R. G., and Zietz, Isidore, 1951, Interpretation of aeromagnetic maps: Geol. Soc. America Mem. 47, 151 p. Wasserburg, G. J., and Lanphere, M. A., 1965, Age determinations in the Precambrian of Arizona and Nevada: Geol. Soc. America Bull., v. 76, no. 7, p. 735-758, Wilson, E. D., 1933, Geology and mineral deposits of southern Yuma County, Arizona: Arizona Bur. Mines Bull. 134, 236 p. -—- 1960, Geologic map of Yuma County, Arizona: Arizona Bur. Mines, scale 1:375,000. Zohdy, A. A. R., and Jackson, D. B., 1968, Ground-water exploration using the resistivity method on the Hawaiian Islands of Oahu and Hawaii: U.S. Geol. Survey open-file rept., 34 p. * U.S. GOVERNMENT PRINTING OFFICE: 1973—515—659/42 @575 P0 7 DAY v.7aa~£ Gravity and Aeromagnetic Study of Part of the Yakima River Basin, Washington GEOLOGICAL SURVEY PROFESSIONAL PAPER 726—E SCIENCES W N, . .h‘..m~. . ~ A JAN 6 1976 ‘ 1 'VHfi'SHV Ul- CAHMIW‘M man. A Gravity and Aeromagnetic Study of Part of the Yakima River Basin, Washington By s. L. ROBBINS, R. J. BURT, and D. o. GREGG GEOPHYSICAL FIELD INVESTIGATIONS GEOLOGICAL SURVEY PROFESSIONAL PAPER 726-E A gravity survey of the Toppenish Creek basin and surrounding area in the northwest part of the Columbia River Plateau 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 Robbins, Stephen L. Gravity and aeromag‘netic study of part of the Yakima River basin, Washington. (Geophysical field investigations) (Geological Survey Professional Paper 726—E) Bibliography: p. E6. Supt. of Docs. No.: I 19.162726—E 1. Gravity—Washington (State)—Yakima River watershed. 2. Magnetism, Terrestrial—Washington (State)——Yakima River watershed. I. Burt, R. J ., joint author. II. Gregg, Dean 0., joint author. III. Title. IV. Series. V. Series: United States. Geo- logical Survey. Professional Paper 726—E. QB335.U6R6 52‘6’ .7'09797 55 74-26970 For sale by the Superintendent of Documents, US. Government Printing Oifice Washington, DC. 20402 Stock Number 024-001—02741-1 CONTENTS Page Abstract _____________________________________________________________________________ E1 Introduction _________________________________________________________________________ 1 Geologic setting _____________________________________________________________________ 1 Rock units ___________________________________________________________________________ 2 Yakima Basalt ___________________________________________________________________ 2 Ellensburg Formation ___________________________________________________________ 3 Tieton Andesite _________________________________________________________________ 3 Touchet Beds _____________________________________________________________________ 3 Loess ___________________________________________________________________________ 3 Alluvium and cemented gravel ___________________________________________________ 3 Geophysical data _____________________________________________________________________ 4 Gravity interpretation _______________________________________________________________ 4 Aeromagnetic interpretation _________________________________________________________ 5 Structural significance of data _________________________________________________________ 5 References cited _____________________________________________________________________ 6 ILLUSTRATIONS Page PLATE 1. Gravity, magnetic, and structural maps of part of the Yakima Basin, Washington, and a cross section ______________ In pocket FIGURE 1. Simple Bouguer gravity map of south-central Washington and north-central Oregon, and outline of area covered in this report ________________________________________________________________________________________________________ E2 TABLE Page TABLE 1. Composite stratigraphic sequence in the northwestern Columbia Plateau __________________________________________ E3 III GEOPHYSICAL FIELD INVESTIGATIONS GRAVITY AND AEROMAGNETIC STUDY OF PART OF THE YAKIMA RIVER BASIN, WASHINGTON By S. L. ROBBINS, R. j. BURT, and D. O. GREGG ABSTRACT A gravity survey of the Toppenish Creek basin, tributary to the Yakima River basin, and surrounding area in the northwest part of the Columbia River Plateau shows a broad, 6-milligal gravity low over much of the basin. The low, which is somewhat smaller than expected, is probably due to about 300 m (1,000 ft) of the Ellensburg Formation and younger sediments that overlie the Yakima Basalt within the structural basin. Analysis of the gravity data suggests that: (1) the Toppenish Creek basin has a relatively flat bottom; (2) the basin is enclosed on the southeast by a broad, buried ridge between Toppenish Ridge and Snipes Mountain; (3) several nearby valley areas contain very little sediment; (4) the thickness of basalts within the study area varies considerably; and (5) as many as four small silicic volcanic cones of pumicite deposits may be buried within the younger Yakima Basalt along a possible north-south fracture zone. Differences in pattern between the main part of Toppenish Creek basin and the shallow valley areas on the aeromagnetic map lend support to the gravity interpretation. INTRODUCTION The Yakima River basin is in south-central Washing- ton in the northwest part of the Columbia River Plateau. The part of this basin covered by gravity sur- vey (fig. 1) (principally the Toppenish Creek basin) lies between Naches Valley on the north, Satus Creek on the south, the foothills of the Cascade Range on the west, and the town of Sunnyside on the east. The survey was made in the fall of 1971 in conjunction with an ongoing comprehensive water-resources study of the Yakima Indian Reservation carried out by the US. Geological Survey. In the central part of the Toppenish Creek basin, no wells penetrate the entire thickness of sedimentary rocks belonging to the Ellensburg Formation, which mostly overlies basalt of the Columbia River Group. No other data were previously available to indicate the thickness of the Ellensburg Formation or the con- figuration of its lower surface. The primary purpose of the gravity survey was to determine both the configuration of the upper surface of Columbia River basalt Within the Toppenish Creek basin and the thick- ness of the water-bearing sedimentary deposits overly- ing the basalt. Without this information, an appraisal of the aquifer system and development of computerized management models of the system could be based only on a hypothetical projection of the basalt surface from the ridges bordering the basin. We thank Professor Z. F. Danes and the University of Puget Sound in Tacoma, Wash., for gravity base station information and for the loan of a Worden gravity meter. Elmer Hauer and the US. Air Force Aeronautical Chart and Information Center in St. Louis, Mo., pro- vided us with gravity station data and elevation data. The complete gamma-gamma log for Rattlesnake Hills well No. 1 was graciously provided by D. J. Brown of the Atlantic Richfield Hanford Company. GEOLOGIC SETTING The Yakima Basalt of the Columbia River Group and the sedimentary Ellensburg Formation, both of middle Tertiary age, underlie most of the area covered by this report. Older Tertiary volcanic rocks possibly occur be- neath the Yakima Basalt in this area (Brown, 1970, p. 180), but they do not crop out. A tongue of Quaternary andesitic lava forms the southwest side of the Naches River valley in the northern part of the map area. The Cascade Range to the west is underlain chiefly by Ter- tiary igneous and metamorphic rocks. Regional folding during the Pliocene Epoch warped the basalt and the interlayered and overlying Ellens- burg Formation into a series of east-trending anticlinal ridges. From north to south these ridges include Cow- iche Mountain, Ahtanum Ridge, and Toppenish Ridge. The Ahtanum Ridge anticline continues east of Union Gap as the Rattlesnake Hills anticline. These anticlinal ridges are generally asymmetrical, with steep slopes on the north and gentler slopes on the south. During the folding, the ancestral Yakima River continued its southeasterly course, bisecting the uplifting ridges and forming steep-walled canyons, such as Union Gap. Most structures in the region trend east-west, except for the northeast-trending ridge flanking the southwest side of Medicine Valley and the east-southeast-trending Sniper Mountain between Granges and Sunnyside. The major ridges are separated by wide synclinal valleys, the structural bottoms of which are partly filled by the Ellensburg Formation, alluvium, and lacustrine silt. E1 E2 122° l GEOPHYSICAL FIELD INVESTIGATIONS 120n 118° [ l ,‘9 ____ _ _______ II—I _ r’/ /‘@ / 0 R E G 0 N l 100 MILES I r1I l I I J 50 100 150 KILOMETRES FIGURE 1.—Simple Bouguer gravity map of south-central Washington and north-central Oregon from Woollard and Joesting (1964) and outline of area compared in this report with complete Bouguer anomaly values. Contours are labeled at 50-mgal intervals, and selected intermediate contours at 10-mgal intervals. Hachures indicate gravity lows, and plus signs indicate gravity highs. ROCK UNITS YAKIMA BASALT The Yakima Basalt (fig. 2, pl. 1), which comprises the lava flows in the upper part of the Columbia River Group, is exposed on the ridges and underlies all of the other exposed rock formations in the study area. The Yakima, of middle and late Miocene and early Pliocene age (Fiske and others, 1963, p. 63; Snavely and others, 1973), consists of many individual lava flows, several of which have distinctive petrographic characteristics over wide areas of the region. The lowermost named sedimentary deposit interbedded with the flows of the Yakima Basalt is the Vantage Sandstone Member; be- low this the Yakima is generally undivided because of lithologic similarity between flows and the rarity of thick interbeds of sedimentary rocks. Overlying the Vantage is about 400 In (1,200 ft) of basalt, subdivided into members of the Yakima Basalt. The thickness of these basalt members and interbeds differs from place to place. The upper flows interfinger with sedimentary rocks assigned to the Ellensburg Formation. In general, the Yakima Basalt is a hard, dense, mi- crocrystalline rock, black or gray on fresh surfaces. Upper parts of flows are vesicular except where eroded; many central parts exhibit columnar, irregular, or platy jointing. Individual flows are a few to more than 30 m (100 ft) thick. Pre-Vantage interbeds of sedimentary rocks are rare but locally significant. For example, a pumicite deposit, capped by basalt flows tentatively identified as of pre- Vantage Sandstone age by D. A. Swanson (oral com- mun., 1972) on the basis of chemical characteristics, was found by Burt and Gregg just south of the gravity- surveyed area along Satus Creek adjacent to State Highway 97 in secs. 21, 28, and 33, T. 7 N., and R. 18 E. The'pumicite is 30 m (100 ft) thick or more, but the areal extent is unknown. The bulk density of most particles is GRAVITY STUDY, YAKIMA RIVER BASIN, WASHINGTON less than that of water. The deposit grades downward into a well-cemented basaltic conglomerate containing sporadic quartzite pebbles. The upper members of the Yakima Basalt and as- sociated interbedded sedimentary materials are briefly described in table 1, which shows a composite strati- graphic sequence for the northwest Columbia Plateau. ELLENSBURG FORMATION The Ellensburg. Formation (fig. 2, pl. 1) of late Miocene and early Pliocene age (Foxworthy, 1962, p. 19; Bingham and Grolier, 1966, p. G3), overlies and is in- tercalated with upper flows of the Yakima Basalt. The Ellensburg consists of as much as 600 m (2,000 ft) (Bin- gham and Grolier, 1966, p. G13) of generally poorly sorted, loosely consolidated lacustrine and fluvial sedi- ment derived primarily from upper Miocene and Pliocene volcanic rocks and much older metamorphic rocks of the Cascade Range, as well as from the Yakima Basalt itself. The formation underlies the valley areas and is occasionally exposed on the flanks of the anticli- nal ridges, but it has been eroded from the crests of the ridges, exposing the basalt. The top of the Ellensburg Formation is not well defined, as it grades upward into reworked sedimentary deposits. TIETON ANDESITE The Tieton Andesite is a tongue of andesitic lava (fig. E3 2, pl. 1), which overlies the Yakima Basalt and Ellensburg Formation northwest of the city of Yakima. The andesite occupies valleys eroded in the Ellensburg Formation and Yakima Basalt and is of Pleistocene age (Foxworthy, 1962, p. 21). It is lighter in color than the basalt, and porphyritic. TOUCHET BEDS The Touchet Beds (fig. 2, pl. 1) (Flint, 1938, p. 493—499) are Pleistocene varved deposits consisting of as much as 11 m (35 ft) of silt and sand. The Touchet covered much of the lowlands below about the 365-m (1,200-ft) elevation level in the Yakima River basin (Laval 1956, p. 91) but has been partly stripped away by the meandering Yakima River and its tributary streams in the central part of the basin. LOESS Loess is the wind-deposited silt of Holocene age that forms mounds over much of the upland benches and ridges. The deposit is not differentiated on the geologic map. ALLUVIUM AND CEMENTED GRAVEL The alluvium and cemented gravel (fig. 2, pl. 1) consists of gravel, sand, and silt derived primarily from the Ellensburg Formation, the Yakima Basalt, and to a lesser extent from the igneous and metamorphic rocks of the Cascade Range. The alluvium varies in thickness TABLE 1.—Composite stratigraphic sequence in the northwestern Columbia Plateau (from Bingham and Grolier, 1966, p. G3) M E M BE Fl SYSTEM SERIES GROUP FORMATION AND BED LITHOLOGY AND THICKNESS m Ellensbur Ellensburg Ellensburg Formation undifferentiated:1.800 ft of pebble conglomerate,sand, : g _ and mudflows; upper 500 ft basaltic. contains coarse brown send; 3 Formation Formation remainder andesitic, with beds of fine ash in lower part .2 undifferentiated Beverly Member: As much as 300 ft of pumicite, quartzite-bearing con— CL _ glomerate, and tuffaceous sand, silt, and clay COLUMBIA RIVER GROUP _ E “ g ‘ Beverl Yakima Basalt: : c V Saddle Saddle Mountains Member: One or more basalt flows; total thickness as 8 x6 8 Mountains great as about 400 ft. Basalt is black to light gray, dense, fine to very fine 9 9 Member Member grained; some flows are sparsely porphyritic. Small columns or hackly 2 E jointing are common, but some flows are composed of agglomerate or >. _____ pillows in places Priest Rapids Priest Rapids Member: Four basalt flows; total thickness as great as 220 ft. D: Member Basalt is grayish black where fresh, mottled greenish brown where Q , weathered; coarse grained and nonporphyritic. Very large columns as much < D‘ t thCYB d as 10 ft in diameter are common to e _ '8 pm: Quincy Diatomite Bed: Diatomite as thick as 35 ft: contains a few lenses of silt and clay |— a, Hoza Member Roza Member: Two basalt flows; total thickness as great as 200 ft. Basalt 3 is dark blue gray or dark reddish gray where fresh; weathers deep red EC 9 LJ brown: coarse grained and porphyritic. Phenocrysts are not numerous but 0 a Squaw Creek are present in nearly all outcrops. Phenocrysts are lath shaped and average 1 L“ g g 3 Diatornite Bed cm in length. Large columns which break into plates and chips are common |_ 3 2 Frenchman Springs Member: As many as six flows; total thickness as great 0 E as 375 ft. Basalt is dark gray to black, medium to fine grained and sparsely s m a Frenchman Springs porphyritic. Phenocrysts are roughly equidimensional, shattered, yellowish '5 E Member white. and average 1 cm in diameter. Some large columns are present, but E % irregular jointing is common. Pillow zones are common in lowermost flow 3 >. Squaw Creek Diatomite Bed: Diatomite as thick as 17 ft; grades west- 8 Vantage Sandstone ward to sandstone, fine conglomerate, siltstone, or clay Vantage Sandstone Member: Sandstone, as thick as 35 ft. Blue or green Member where fresh, pale yellow where weathered. Consists of medium-grained quartz-feldspar—mica sand, or a tuffaceous sand, silt,and clay Lower basalt flows: Total thickness generally more than 1,000 ft. Basalt Lower basalt flows is dark gray, fine grained, and well jointed. Columns 1-2 ft in diameter are common. Pillows and spiracles more common than in overlying basalt members E4 from a few to many tens of feet and is generally deeper and coarser on the north side of the valleys, where the Yakima River has built up broad, gentle alluvial fans caused by an increase in gradient as the river emerges from the water gaps. The bottom of the alluvium grades into the top of the Ellensburg Formation, making differentiation between the two difficult. GEOPHYSICAL DATA A simple Bouguer gravity map of the region surrounding the lower Yakima River basin is shown in figure 1. The complete Bouguer gravity map (fig. 2, pl. 1) is based on observations made at 462 gravity stations, spaced at intervals of 11/2—3 km (1—2 mi). LaCoste- Romberg gravity meter G17 was used at 300 of these stations, and LaCoste-Romberg meter G8 was used at another 58 stations. A Worden gravity meter, having a sensitivity of 0.09277 mgal/scale division, was used at 75 stations located in the central part of the basin. The remaining 29 stations were observations from the files of the Aeronautical Chart and Information Center. All of the observed gravity values are relative to an absolute value of 980,617.74 mgals (Z. F. Danes, written commun., 1971) at a base station in Yakima, Wash. The data were reduced using the assumed average crustal density of 2.67 g/cm3. The terrain effect was removed by using the US. Coast and Geodetic Survey system (Swick, 1942, p. 67) through Zone F (2.29 km), and the values for zones G through 0 (166.7 km) were calculated using a computer program described by Plouff (1966). The resultant complete Bouguer anomaly values were contoured at 2—mgal intervals. The aeromagnetic map (fig. 3, pl. 1) is based on 31 north-south traverses flown in 1959 by the US. Geological Survey, using a continuously recording modified AN—ASQ/3A airborne magnetometer. The flight lines are 1.6 km (1.0 mi) apart and 150 m (500 ft) above the ground surface. The map, compiled by Jack Kirby and others with a location error of the flight lines of 0.4 km (1% mi), shows variations in the total intensity relative to the International Geomagnetic Reference Field of 1965 (Fabiano and Peddie, 1969). GRAVITY INTERPRETATION The interpretations of the gravity anomalies seen in figure 2 (pl. 1) are based on the following assumptions: 1.—The Ellensburg Formation and the other more recent sedimentary deposits, including loess and al- luvium, have a combined average density of 2.3 g/cm3. This assumption is based on (a) models computed to fit Foxworthy’s (1962) interpretation of the structure of Ahtanum Valley and sediment thicknesses determined from drill holes in Ahtanum Valley and Toppenish Creek basin, using Bott’s (1960) two-dimensional gravity-mass computation computer program for a GEOPHYSICAL FIELD INVESTIGATIONS one-density contrast model (cross section A—A’, fig. 6, pl. 1); and (b) one uncalibrated gamma-gamma log from a well on the north side of Toppenish Creek basin (well No. 5 in fig. 7, pl. 1). , 2.—The Yakima Basalt and older basalts that may underlie the Yakima River basin have an average density of 2.8 g/cm3. This value is based on a gamma-gamma log from Rattlesnake Hills No. 1 (D. J. Brown, written commun., 1971), a 3,248-metre-deep (10,655-ft) well drilled by Standard Oil Company of California in 1957 and 1958 on Rattlesnake Ridge about 10 km (6 mi) to the east of this study area. The well bottomed in basalt of probable Eocene age (Brown, 1970, p. 179), and only about the uppermost 600 to 1,200 m (2,000 to 4,000 ft) of basalt belongs to the Yakima Formation. 3.—-—The rocks underlying the basalt are unknown as to type and composition but are assumed to have an average crustal density of 2.7 g/cm3 (Bromery and Snavely, 1964, p. N3). 4.—Most of the “regional” gravity field represents the variation in the total thickness of the basalts, although it could also be caused partly by lithologic changes in the older basement rocks. In the attempt to determine the “regional” gravity field, upward continuation of the complete Bouguer gravity field (fig. 2, pl. 1) was computed using the method described by Henderson (1960). The computa- tion is basically a low-pass wave number filter equation (Byerly, 1965, p. 283), and the results of this continua- tion based on a 1.6-km (1.0-mi) grid to an elevation of 5 km (3 mi) above the surface are shown in figure 4 (pl. 1). Computations were also made for elevations of 1.6 and 3.2 km (1 and 2 mi), but the resultant anomaly field was not smooth and appeared still to contain anomalies caused by relatively small shallow sources. The anomalies shown in figure 4, therefore, represent the gravity attractions caused by large deep bodies and quite possibly the bottom configuration of the basalts, although part of the continued gravity field may be due to lithologic differences within the underlying base- ment rocks. However, the primary focus of this study is the anomalies caused by the shallower bodies overlying or within the Yakima Basalt. A map of the residual gravity field (fig. 5, pl. 1) was obtained by subtracting the “regional” field (fig. 4) from the Bouguer field (fig. 2). It is assumed that the anomalies seen in this figure are caused by basin fill (Ellensburg Formation and younger sedimentary de- posits) and by structural and lithologic differences within the Yakima Basalt. The most significant features are: 1.—Relatively steep gradients on the flanks of the east-west-trending topographic and structural ridges. GRAVITY STUDY, YAKIMA RIVER BASIN, WASHINGTON 2.—A north-northwest-trending gravity high west of Yakima Valley. 3.—A large gravity high centered just south of Ahtanum Ridge in the vicinity of Union Gap and partly within the north end of the Yakima Valley. 4,—Broad gravity lows over most of the valley areas. 5.—A broad gravity high over the Yakima Basalt south of Yakima Valley. Quantitative interpretation of features 1, 4, and 5 was performed along the north-south cross section A—A’ (fig. 6, pl. 1) using the two-dimensional gravity- interpretation computer program of Talwani, Worgel, and Landisman (1959). AEROMAGNETIC INTERPRETATION The interpretation of the aeromagnetic map (fig. 3, pl. 1) is qualitative, based only upon visual comparisons with the gravity anomalies. Aeromagnetic data are not available for the entire area of the gravity survey, but in the area of coverage, the most significant features are (1) magnetic highs over Toppenish and Horse Heaven Ridges (the contours over Ahtanum Ridge are dashed because the flight lines are not continuous over the ridge), and (2) broader features over the rest of the area. There is a weak northeast-southwest lineation of the magnetic pattern (fig. 3) in much of the Toppenish Creek basin and part of the large area of exposed basalt to the south, but east of this area the pattern appears to be alined northwest-southeast. High-level aeromagne- tic data in Swanson (1971) indicate northwest- southeast trends for the entire area. These lineations probably represent both local and regional deformation trends within (or beneath) the basalt, since remanent magnetization vectors in undefined Tertiary rocks are basically the same or opposite to that of the present field of the earth. It is the variations of these vectors with respect to the earth’s field that produce anomaly patterns. STRUCTURAL SIGNIFICANCE OF DATA Gravity observations in part of the Ahtanum Valley, in which the thickness of sediment fill and configuration of the basin are better defined (Foxworthy, 1962, pl. 1) than in the Toppenish Creek basin, are included in this study so comparisons could be made. About 3 km (2 mi) southeast of the center of the gravity low (fig. 5, pl. 1) the log of well No. 2 (fig. 7, pl. 1) shows that the upper surface of the Yakima Basalt is at a depth of more than 400 m (1,300 ft). At the gravity minimum, the depth is possibly 100 to 130 m (300 to 400 ft) greater, although part or all of this interpreted increase in depth may be due to buried low-density rocks as discussed later in this section. This gravity low is larger than the low over the Toppenish Creek basin, Where calculations based on the E5 gravity data show the Ellensburg Formation to be only about 300 In (1,000 ft) thick. The structural contours on the buried upper surface of the Yakima Basalt within the valley areas (fig. 7) de- rived from well-log and gravity data indicate that the sediment thickness is quite uniform within the Top- penish Creek basin and that the north and south sides of the basin are bounded by relatively steep slopes. These slopes and the associated ridges are believed by most geologists to have been formed by folding with only minor faulting in some areas (Waters, 1955, p. 666). According to D. A. Swanson (oral commun., 1973), sev- eral investigators presently working on the Columbia plateau are suggesting that block faulting and tilting within the basement rocks beneath the basalts may have caused the folding within the basalts and the asymmetry seen in the ridges. Since the basement rocks are probably of only slightly lower density (0.1 g/cm3) than the basalts, it would be difficult to observe this faulting and tilting in the gravity data. South of White Swan a 2- to 3-mgal, nearly circular, partly closed gravity low (fig. 5) is located almost wholly within the mapped Yakima Basalt. Because a large landslide (3X3 km, 2X2 mi) has been observed in this area, the low may reflect an area of structural weak- ness. However, it is equally possible that a different rock type of lower density may be buried beneath the area, for none of the several other smaller landslides in the study area appear to be associated with gravity lows. North of this gravity low, on the north side of Yakima Valley centered along section A—A’ (fig. 6, pl. 1) is a circular low that cannot be entirely attributed to a greater thickness of (sedimentary deposits (Ellensburg Formation). Still farther north (fig. 5) are two more circular lows (including part of the Ahtanum Valley low) that, although located over sediments, are difficult to model relative to the surface geology. The circular shape, the north-south alinement, and the proximity of the Yakima Basalt to the gravity lows plus the indica- tion of relatively shallow sources for the 2- to 3-mgal anomalies suggest the presence of local accumulations of low-density rocks covered by the Yakima Basalt and emplaced along a fracture zone. In the Tieton River area, just northwest of our study area, Swanson (1967, p. 1080) has observed cones of intermediate and silicic volcanic rocks of early or middle Miocene age, and such rocks are common along the east part of the Cascade Range. It therefore seems possible that andesitic or dacitic cones or perhaps local pumicite deposits, such as those described earlier along Satus Creek, lie below or interbedded with the younger Yakima Basalt. The northwest part of the “regional” gravity map (fig. 4, pl. 1) shows a flat gravity field along with the lowest gravity values. Within the study area, the basalt is probably thinnest in the northwest, perhaps as thin E6 as 0.6 to 1.0 km (2,000 to 3,000 ft). However, as the field increases to the south (fig. 6, pl. 1; a minimum depth of 1.0 km Was assumed for this computation) and east, the basalt probably exceeds 7 km (4 mi) in thickness, espe- cially to the east of this survey area where there is a very large gravity high (fig. 1). This area is recognized by others as having the thickest sections of basalts within the Columbia River Plateau (Swanson, 1971, p. 3351). The areas with large gravity highs in figure 5, specifically (1) the long northwest-southeast-trending high southwest of White Swan, (2) the broad high south of Toppenish Ridge, and (3) the large magnitude high just south of Union Gap, are seen as areas of gravity “ridges” on the “regional” map (fig. 4). There are several possible interpretations for these “ridges”: the areas may contain thicker sections of basalt; the basalt may be denser than in some of the surrounding areas; dense intrusions may occur beneath the highs; or the “ridges” may result from a combination of these. The northwest-southeast trend of the gravity high south- west of White Swan is similar in direction to structural trends seen in pre-Yakima Basalt rocks by D. A. Swan- son (oral commun., 1972) in the Tieton River area which are on strike with this high. The gravity high located just south of Union Gap and Ahtanum Ridge is partly within the northernmost part of Yakima Valley. Possi- bly, the Yakima River south of the water gap has eroded away some of the southern part of the ridge, leaving this part of the valley with only a very thin layer of fill (fig. 7). Snipes Mountain has been described by Laval (1966, p. 101) as a “simple doubly-plunging anticline.” Analysis of both the gravity field (fig. 5) and the aeromagnetic expression (fig. 3, pl. 1) over this feature support this description, as the anomalies do not seem to extend much beyond the mountain itself. The gravity field (fig. 5) also suggests that Snipes Mountain may be part of the same anticlinal system as Toppenish Ridge, and that the valley sediments southwest of Snipes Mountain are quite thin, perhaps only 30 to 50 ‘m (100 to 150 ft) thick. Both the gravity and magnetic data indi- cate that the valley on the north side of the mountain contains more sediment (possibly 60 to 90 m, 200 to 300 ft). Thus, the basin part of Toppenish Creek basin may be closed on the southeast, ending along a line between Snipes Mountain and the east end of Toppenish Ridge. Immediately south of the line between Snipes Moun- tain and Toppenish Ridge are two very small magnitude gravity lows. These lows suggest shallow basins of about 60 to 100 m (200 to 300 ft) depth that are undula- tions in the top of the thick Yakima Basalt field between Toppenish Ridge and Horse Heaven Hills and possibly define an eastward extension of the Dry Creek syncline. GEOPHYSICAL FIELD INVESTIGATIONS The aeromagnetic pattern also suggests that the basins are shallow (fig. 3) for the pattern is similar to the rest of the basalt to the south, whereas in the Toppenish Creek basin the much broader pattern reflects a greater depth to the basalt. Toppenish Ridge and the Horse Heaven Hills have anomalies with short wavelength and great- er magnitude than the basalt field between these ridges, probably because the airplane was closer to the basalt over the ridges and because the structure within the ridges is more deformed and complex. Medicine Valley has only a very thin cover of Ellens- burg Formation. Figure 5 shows about a l-mgal flexure, which suggests about 30 to 50 m (100 to 150 ft) of cover. This agrees with the depth at which basalt was penet- rated, 33 m (110 ft) in well No. 4 drilled by the U.S. Geological Survey in 1972. West of the gravity high southwest of White Swan is a gradient that ends in a very large gravity low centered over the Cascade Range west of the study area (fig. 1). The low is probably caused by low-density Pleistocene volcanic rocks and an “acidic batholith” (Danes, 1969, p. 549). REFERENCES CITED Bingham, J. W., and Grolier, M. J., 1966, The Yakima Basalt and Ellensburg Formation of south-central Washington: U.S. Geol. Survey Bull. 1224—G, 15 p. Bott, M. H. P., 1960, The use of rapid digital computing methods for direct gravity interpretation of sedimentary basins: Royal As- tron. Soc. Geophys. Jour., v. 3, no. 1, p. 63—67. Bromery, R. W., and Snavely, P. D., J r., 1964, Geologic interpretation of reconnaissance gravity and aeromagnetic surveys in north- western Oregon: U.S. Geol. Survey Bull. 1181—N, p. N1—N13. Brown, R. E., 1970, Some suggested rates of deformation of the basalts in the Pasco Basin, and their implications, in Proceedings of the second Columbia River Basalt Symposium: Eastern Washington State College, Cheney, Wash., p. 179—187. Byerly, P. E., 1965, Convolution filtering of gravity and magnetic maps: Geophysics, v. 30, no. 2, p. 281—283. Danes, Z. R, 1969, Gravity results in western Washington: EOS, v. 50, no. 10, p. 548—550. Fabiano, E. B., and Peddie, N. W., 1969, Grid values of total magnetic intensity IGRF—1965: U.S. ESSA Tech. Rept. no. 38, 55 p. Fiske, R. S., Hopson, C. A., and Waters, A. C., 1963, Geology of Mount Rainier National Park, Washington: U.S. Geol. Survey Prof. Paper 444, 93 p. Flint, R. F., 1938, Origin of the Cheney-Palouse scabland tract, Washington: Geol. Soc. America Bull., v. 49, no. 3, p. 461—523. Foxworthy, B. L., 1962, Geology and ground-water resources of the Ahtanum Valley, Yakima County, Washington: U.S. Geol. Sur- vey Water-Supply Paper 1598, 100 p. Henderson, R. G., 1960, A comprehensive system of automatic compu- tation in magnetic and gravity interpretation: Geophysics, v. 25 no. 3, p. 569—585. Huntting, M. T., Bernett, W. A. G., Livingston, V. E., J r., and Moen, W. S., 1961, Geologic map of Washington: Wash. Div. Mines and Geology, scale 1:500,000. Laval, W. N ., 1956, Stratigraphy and structural geology of portions of south-central Washington: University of Washington, Seattle, Ph.D. thesis, 223 p. GRAVITY STUDY, YAKIMA RIVER BASIN, WASHINGTON 1966, Engineering geology of Tertiary formations and struc- tures, south-central Washington, in Proceedings of the 4th An- nual Eng. Geol. and Soils Eng. Symposium: Univ. of Idaho, Mos- cow, Idaho, p. 91—111. Ploufi“, Donald, 1966, Digital terrain corrections based on geographic coordinates [abs]: Geophysics, v. 31, no. 6, p. 1208. Snavely, P.’D., J r., MacLeod, N. S., and Wagner, H. C., 1973, Miocene tholeiitic basalt of coastal Oregon and Washington and their relations to coeval basalt of the Columbia Plateau: Geol. Soc. America Bull., v. 84, no. 2, p. 387—424. Swanson, D. A., 1967, Yakima Basalt of the Tieton River area, south- central Washington: Geol. Soc. America Bull., v. 78, no. 9, p. 1077—1110. 1971, The area west of the Idaho batholith, in Zietz, Isidore, Hearn, B. C., Jr., Higgins, M. W., Robinson, G. D., and Swanson, 1:» U.S. GOVERNMflVT PRINTING OFFICE: 1975-0-690-036/35 E7 D. A., Interpretation of an aeromagnetic stripcacross the north- western United States: Geol. Soc. America Bull., v. 82, no. 12, p. 3347-3372. Swick C. H., 1942, Pendulum gravity measurements and isostatic reductions: US. Coast and Geod. Survey Spec. Pub. 232, 82 p. Talwani, Manik, Worzel, J. L., and Landisman, Mark, 1959, Rapid gravity computations for two-dimensional bodies with applica- tion to the Mendocino submarine fracture zone: Jour. Geophys. Research, v. 64, no. 1, p. 49—59. Waters, A. C., 1955, Geomorphology of south-central Washington, illustrated by the Yakima East quadrangle: Geol. Soc. America Bull., v. 66, no. 6, p. 663—684. Woollard, G. P. and Joesting, H. R., 1964, Bouguer gravity anomaly map of the United States: US. Geol. Survey map, scale 1:2,500,000. S6 a £3. g, £er . m UNITED STATES DEPARTMENT OF THE INTERIOR PROFESSIONAL PAPER 726-E GEOLOGICAL SURVEY PLATE 1 lBt‘OO’ ,o y.” iflioo my; 46ka b so 1) out, its ‘36 $5,) 7 I .t‘: a I EXPLANATION EXPLANATION \llttvium tutti cemented _ A” Alluxium and cemented l E gruxelx ot Hint (I935), < < ghouls oi I lint (I )m‘iI. / < .7 £6 .7 :5 louchct Beds v E lottehet Beth V I lacustrinc dcpmits N I— Lllcll‘s‘ll‘ltlc \IL‘DO‘HIS :— I k V)‘ Tiettm Andexitc E Tieton Andexite z < < EIIcnsbtlrg I‘ormzttion i FIIL‘HNhIH'g I'ormzttion I ; Yakima Basalt F“ Yakima Basalt J 7‘ ,, Geologic contact Geologic contact Anticltnc /" Anticitnc Synoline ' Synuline x/i /, , 'l / TAN M ' ‘A' - ‘4‘ w NUM \ t ' . 1' -’ ’ \t , r' ‘\ 2.06 7 . - ~ _ \ . . , _ ( y 9,“) Air/“Hm“ - #41“, / , AMWHW , - _ .. v. r In“ E‘, \¥ 444 / - - 7% — ~6 30 / , —— 4630' w ~16 80' :éta/g‘} '7 .I o 5 mass: 0 «0 , 0° ° 0 asaoflz 0.399.533. apato 007809911§032Q 0° go 0° 0 87.64 , 71 weasg . 273m V ’\ I, V“, {I ' ' l ””‘ /‘/ . ,, - x 2 "8-0 _ ’ “7 \/ VH7; ' - ( A " 4.696 , . 88.5 ' ' ' Kigali” / c <7 ‘ a " ' - ’ 4’ / x’ \ ' L, eébgia" 3145952 0 P 589.95 :89 72 _ . . :86567 ‘ . _ ‘ ¢i\;x'\ .0 o :3 O 0/ o o ibis)“: O 88.13 787.30 —86.82 - 8‘1, I. ‘a:29 ‘ 3 D. :8920 589.16 :8754 586.71 585.49 _ 788414 we. 7 / a ' —88 22 _—a7 a? :86 15 a: at: 586.77 ’87-07 I 58555 V8532 _—85.60 [784.97 “8540 A ‘ o g o ( ’ “bunmfiidér‘ at ' "I o e Z“ "/8/ K; ‘4 .2.02 . fl” 4 = 76 'l wy/ ,, // r, , , , . , . / .> If _ '/ , \ . 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" GrandVIew ' ' / ’ / // /«’ / v, / / / / / // ‘ Grandvtew . / / // L/ / / / / / l/ (1/ / /,» /,/ // / / 1/ , / , / / / / . / (r , ,r , I , / 1/ / I / f y / , / g x/ // z/ 1/, / /,, / / / I / // ,, / ,/ , / / / , / / V / ,,/ /// / // ’/ / , / /’ / / //// // // /. / // /////// // / I, / / ./ ’ / / / / 1/ /’ , / /« / / / / « / / / / / , ,/ r / / / / / / / //, //’ //// /’, /,//,// / \\\>\ / 1/ /, /// / // // // ///z / / \ \ TTLSQ / ’ '/ ’1 '/ / T\\*r\‘:;: / // / ”// ,/ / / /////C / /' // / / , CONTOUR INTERVAL = 100 GAMMAS / ‘ Flown at 500’ above surface / 1GR Field Removed / / CONTOUR INTERVAL = 2 MILLIGALS / ,/ x/ / ,_ / 1/ // / I / /,« / 7 / s X , l. ,/ / / , t / r / / / / / y/@ , / / / x ' ' / - / / / // / ' P3 / / / , /.. , / ,, , / / / / , , / , . , / Fllght path // / / , / , 269/ / ./ / / / / i' / / ' '1 / ’ V = / Showing location and spacing of data ’ / / ,/ / / / é) / GCOIOgy mapped by Gregg and Burt, 1973; / / / , / ////// , / / / / / / / Lavel,l956;Waters,I955;Schmincke,l964; / / // / / // LaVeI,1956;Waters,1955;Schmincke,l964; // // ////:,/ / // I/ / / // /,/ )// //// /,/ // / Foxwortliy.1962;Huntting,andothers196],l ,7 L // // I / ,// / 1/ // / I// / , Foxworthy, l962;Huntting,/and others 1961.| m‘nri' ' so ’ 12000 121‘00’ 45’ 15' 120 <<' Base from U,S_ Geological Survey Base from U.S. Geological Survey Sourcefrom]. Irby andothersfromdataobtained Yakima 1:250,000 Yakma 1:250,QOO in 1959. FIGURE 3. —Aeromagnetic and geologic map ofpart ofthe Yakima Basin, Washington. 1” 12100 wow ,‘ IQVOO' 45’ 30' if" wow ‘6 3 ' 4‘3' 7', ,, ~:7 Ltb‘VlfV , _ - n, x b t i I \ I \ / \t \\ ‘ .t , , I EXPLANATION Sew /> EXI’LANAI ION * V -- i >- Allttvtttmnnd cemented T! I : AIIttHumtinil_cemetitetl I :Z J gravel. of Him (I938). < :5 gl'mcim m Hint um». < 2 __ D on .. :1; ‘ 'I' ‘h‘ ,1. WM 0 ‘ Iottchct Beds v LU l\_/ */\/\l\‘//J’ 0W U 8"” _ " Lu 0 Leicttstrme depmits I— \/‘I\ /‘~/‘/:,\}“\’\ w Locustrtne depostts E- t r" \” .‘rt/ \ \ /" > (t \‘t‘AQZfii‘ VIN/0w > V: V l‘icton Antlesitc Z i )T ‘ /,\,/ llcton Andcsttc g -7 < « , _« \ ; Ellenshurg Formation E _ , - Ellenshurg Formation ; t 7 ‘ I‘L‘ / \V1 I a L/ 1 \"akimzthhalt I" 1 _ wmgj‘§ thktmuBusalt ~ «77-4; R ' \V I \ '- t . ,,,,,,,,,,,,,, _ Geologic contact ’JV§ Geologic contact +——-— Anticlinc w \nticlinc / / / / / ,// / (9‘9 / M E VALL mAHTANUM k V! CW“ ”'3 ‘ Ahtuntml 46 30' — 46‘30 -— 4630’ t C nV" Q l// ‘4‘ " M N \\l_ \/ ‘,:\)‘\, \\~‘\§ l\/’/:‘//\/*«I\/\\>\\ " §‘,\I/\1(\/t“ / x o /,, 5 / lit/77$ / ‘t,/|\ \\/ , I 0 . (/0 . E 0 o o 6 0CD OP 0 ’ O 00 C ‘ . 0 C O 3 ()0 C o 0 ' v\ - - . V . , Garoysfidfi o “N. O o 0 0 "$14,111? S’tde“ ‘ Vowcvqoc i Q'kkk Aux/OOOQ“ ‘\ / V Qt WWI \— \ / o, / i / // , x , / / ' / / , l’ ’ , / // ,/ / / , ,» / / 4 ' / / y/ / x \ A 4615' L / / / , / / / , /, , / M / y l, / // / , / ,, / ' 1615’ 4645' [I 'i I « / / / / l / // xi‘andview GrandVIeW / \ / \’§:¥\\L / / / / / t » / , _ 4, //,// /,/.« ,/ W // / CONTOUR INTERVAL — 2 MILLIGALS , //// / CONTOUR INTERVAL : 2 MILLIGALS ,. / // / / / /// // / , /’ / / / , /” / / / / x/ / / // / / / I , / //" ,x/ t/ / f / ’ ////// // /, / I /// / / I ’ x / , ,r 4 / / ’ / / / ’ / // /// / ’/ // /,/ // / , / / / ,/ / / / ///////// / 4/ / V / , / . // / /// / / / / //// / / / ‘ / y/ / / / / // /,, ,/ // , // / ,/ / r / / , / / [r /, .// // / / ./ ,’ x / / ///, / / / , /// , l/Cg/ Geology mapped by Greg and Burt,1973; ////'// / // / /// / / / Q“ 7 / / / / /i / / /’/ / / / / / / // /' / / ,/// , // /// // / / // Lavel,1956;Waters,1955;Schmincke,1964; / / / // / / / / / // /” / // // / /// /’ Lavel,l956;Waters,195§;Schmtncke, 196413; ‘ ////’// // /// / // / / /// /’ / // / I/ /,/ 1/ / / // / // // // / /’ // /|// / / V Foxworthy,1962:Hunttmg.andothersI961.) L //L/ // / / /, / / / / / // I/ // // / / / Foxwonhy.1962,Huntt1ng,dndothersl96 .l ‘21‘00’ 45' 30' 25' 130-900: WWW :15‘ 30’ 120 00' Base from U.S. Geological Survey Base from U.S. Geological Survey Yakima 1:250,000 Yakima l:250,000 FIGURE 4. — Upward continuation gravity map computedfor the 3—mile level (Yakima Basin, Washington). FIGURE 5. —— Residual gravity map after removal of3—mile continuedfield (Yakima Basin, Washington). wet 00' 45' 30' 15' ‘PO‘OU’ K > 16‘ 33' I 3‘35 :3 A A OBSERVED GRAVITY ANOMALY *80— _ B (‘5 3 COMPUTED 3-MILE UPWARD E CONTINUATION GRAVITY ANOMALY ,90_t __ 4680' _ 46330' / 1° //////////.5/45‘/" / // /o745'/.35%39/ RESIDUAL GRAVITY / /'7/1.0// / / 9 /\ ./COMPUTED GRAVITY ANOMALY / / / / 3‘) 0 o ' . ANOMALY .__. // / // 2' . °\,/ \ / . . . .z. ,._,;/'/ 730' // /— E . ”—1. °\O_—'/ 670 / 0/ \ 0/ 0/ _10 I U, 1000 / f \ AEROMAGNETIC < 900 / ANOMALY 2 ~ 2 am / g 700 600 I“ V V g Sunnysrde LLI _ (5 ct: g 3: c: a) it _ E E g ~ 2 K < n. 1— (I, l— O ._ In I ._ 5 WELLNUMBER1(FIG.7) < E AHTANUM VALLEY WELL NUMBERS (FIG. 7) P: 22 7 - ‘. YAKIMA VALLEY 'fl ? 7. L SEA _/ p, 2.3 _ LEVEL P: 28 1’36‘15’ '46‘l5’ / N -' p: 2-8 Grandview // / EXPLANATION / m a , A = asa ts " p 2'7 COMPUTED BOTTOM/ / : OFBASALT D th ft fb alt 't tdf II h 3/ «0.9 745' ep 0 opo as sinerpree romwe stat E g EXPLANATION \\ . have penetrated the basalts U) E S’ ’2‘ . —262' \ 9‘35 Ellensburg Formatton 0 Depth ofwells that have not reached basalts ET? and younger sediments E 3 Basalts \ ‘ I 73— Older rocks of unknown origin and composition \ \ CONTOUR INTERVAL = 200 FEET _4_ .0: 2.7 \ _ g ‘ \ Geology mapped by Greg and Burt, 1973; O Lavel, 1956; Waters, 1955; Schmincke, 1964;/ \ L l Foxworthy, 1962; Huntting, and others 1961. l 5 \ i3; mi 45' 30' t5‘ 12000 0 5 MILES Base from U.S. Geological Survey 0 8 K'LOMETRES Yakima i:250,000 FIGURE 6. — Cross section A —A’ (Yakima Basin, Washington). FIGURE 7. —Structural contour map on the top ofthe Yakima Basalt surface in the Toppenish Creek basin andpart oftheAhtanum Valley based on gravity data. Interior—Geological Survey, Reston, Va.—l975 — G74 289 SCALE 1:250 000 5 O 5 10 15 20 25 MILES t—--t l--——l : . -— , - 1 l 5 O 5 10 15 20 25 3O 35 KILOMETRES L—l l——l t— 1 _, L j . . :— 1 l GRAVITY, GEOLOGIC, AEROMAGNETIC, AND STRUCTURAL CONTOUR MAP AND MAP SECTIONS OF PART OF THE YAKIMA BASIN, WASHINGTON 1? U.S. GOVERNMENT PRINTING OFFICE: 1975-0-690-036/35 Gravity, Magnetic, and Seismic Studies of the Silver Cliff and Rosita Hills Volcanic Area, Colorado By M. DEAN KLEINKOPF, DONALD L. PETERSON and ROBERT E. MATTICK GEOPHYSICAL FIELD INVESTIGATIONS GEOLOGICAL SURVEY PROFESSIONAL PAPER 726-F Interpretation of gravity, aeromagnetic, and seismic anomalies in terms of volcanic geology and fault tectonics UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1979 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication Data Kleinkopf, Merlin Dean Gravity, Magnetic, and Seismic Studies of the Silver Cliff and Rosita Hills Volcanic Area, Colorado (Geophysical field investigations) (Geological Survey Professional Paper 726—F) Bibliography: p. F16 1. Volcanic ash, tuff, etc.—Colorado—Custer Co. 2. Gravity—Colorado—Custer Co. 3. Magnetism, Terrestrial—Colorado— Custer Co. 4. Seismic refraction method. 1. Peterson, Donald L., joint author. II. Mattick, R. 13., joint author. 111. Title. IV. Series. V. Series: United States Geological Survey Professional Paper 726—F QE461.K64 551.2'1'0978852 78—11687 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024-001-03255-4 CONTENTS Page "11 ,_. Abstract; ........................................................................ Introduction .................................................................... Geology ........................................................................ Collection of data ................................................................ Gravity survey .............................................................. Aeromagnetic survey ........................................................ Seismic-refraction survey ..................................................... Interpretations of geophysical data ............................................... Regional gravity features ..................................................... Regional magnetic features ................................................... Detailed gravity map ........................................................ Detailed aeromagnetic map ................................................... Geological significance of local gravity and magnetic anomalies .................. Seismic interpretations ....................................................... 10 Gravity modeling ............................................................ 13 Conclusions and recommendations ................................................ 16 References cited ................................................................. 16 qthAAwwmep—a ILLUSTRATIONS Page PLATE 1. Bouguer gravity and simplified geologic map of the Silver Cliff and Rosita Hills volcanic areas, Custer County, Colorado ................................................................................................ 1,, pocket ”2. Aeromagnetic and simplified geologic map of the Silver Cliff and Rosita Hills volcanic areas, Custer County. Colorado In pocket FIGURE 1. Location of the Silver Cliff and Rosita Hills study area ........................................................... F2 2. Reconnaissance gravity map ................................................................................... 5 3. Regional aeromagnetic map ................................................................................... 6 4. Geological section A—A ' constructed from seismic refraction data with time-distance plots, gravity, and aeromagnetic profiles ................................................................................................... 1 1 5. Geological section B—B’ constructed from applying gravity modeling and seismic-refraction data to geological surface extensions ................................................................................................ 1 2 6. Residual negative gravity anomaly over Silver Cliff volcanic area ................................................. 14 7. Gravity model of configuration of volcanic rocks, Silver Cliff volcanic area ......................................... 15 TABLE Page TABLE 1. Physical properties of rocks in the Silver Cliff and Rosita Hills volcanic area, Colorado .............................. F7 III GEOPHYSICAL FIELD INVESTIGATIONS GRAVITY, MAGNETIC, AND SEISMIC STUDIES OF THE SILVER CLIFF AND ROSITA HILLS VOLCANIC AREA, COLORADO By M. DEAN KLEINKOPF, DONALD L. PETERSON, and ROBERT E. MATTICK ABSTRACT Gravity, aeromagnetic, and seismic refraction data provide new information about the thickness distribution of volcanic rocks and geometry of volcanic structures in the Silver Cliff and Rosita Hills volcanic areas, south-central Colorado. Geophysical surveys con- sisted of gravity measurements at 318 stations, low-level . aeromagnetic coverage flown at 0.8 km spacing, and two seismic- refraction profiles across the Silver Cliff volcanic area. The gravity patterns are dominated by negative anomalies which mainly show the distribution of Tertiary volcanic rocks. At the Silver Cliff and adjacent Johnson Gulch areas, prominent negative anomalies of nearly 10 mgals amplitude were interpreted to be caus- ed by thick sequences of low-density breccias, tuff, and glasses preserved in volcanic subsidence features in Precambrian crystalline terrane. Three-dimensional modeling of the gravity data gave 1,000-1,200 m thickness of volcanic rocks in the deepest parts of these areas. The negative gravity anomalies in the Rosita Hills are smaller, indicating more restricted volcanic subsidence, although this may be complicated by a greater percentage of higher density volcanic rocks, such as andesites and trachytes, compared with the Silver Cliff area. The magnetic data reflect both lithologic variations of Precam- brian crystalline gneisses and distribution of the Tertiary volcanic rocks. The anomaly patterns are somewhat similar to the gravity features. In the Silver Cliff and Johnson Gulch areas, negative magnetic anomalies generally are broad and pronounced and cor- relate with the negative gravity anomalies across the subsidence areas. In addition. negative magnetic anomalies over adjacent Precambrian terrane suggest the possibility of incipient volcanic features not evident in the surface geology. A prominent magnetic low across the southeastern Bull Domingo Hills probably reflects a nonmagnetic phase of Precambrian gneiss as suggested by magnetic susceptibility measurements. However, the presence of a small breccia pipe at the old Bull Domingo Mine may point to a more extensive volcanic feature in the subsurface. The magnetic patterns at the Rosita Hills consist mainly of low-relief short-wave- length positive and negative anomalies that reflect compositional variations of the volcanic rocks. A major exception occurs along the southwest side of the Rosita Hillarwhere a broad negative magnetic anomaly suggests geologic conditions somewhat similar to that at the Silver Cliff area. Two north-south seismic-refraction profiles across the Silver Cliff volcanic area have provided more specific quantitative data that complement the gravity and magnetic studies in defining the con- figuration of the volcanic subsidence areas and in delineating pos- sible associated local vents. INTRODUCTION Gravity, magnetic, and seismic surveys were made by the authors at the Silver Cliff and Rosita Hills volcanic areas of south-central Colorado in conjunction with geologic studies of the volcanic centers and ore deposits (fig. 1). The purpose of the geophysical studies was to provide information about the thickness distribution of volcanic rocks, the geometry of volcanic structures, and possible relationships of volcanism to mineralization in and around the Silver Cliff and Rosita Hills volcanic areas. Geophysical and geological studies of the region have been conducted by various workers. Published areal reconnaissance maps of gravity data (Peterson and others, 1974; Behrendt and Bajwa, 1974) and high- level aeromagnetic surveys (Zietz, 1972) cover the Silver Cliff and Rosita Hills volcanic areas. Regional interpretations of the gravity and aeromagnetic data covering the Pueblo 1 ° X2° quadrangle, which includes the study area, were made by the authors and Richard B. Taylor and William N. Sharp (unpub. data, 1970). Preliminary interpretations of the geophysical data at Silver Cliff and Rosita Hills have been made (M. Dean Kleinkopf, R. B. Taylor, D. L. Peterson, R. E. Mattick, and W. N. Sharp, unpub. data, 1970; Kleinkopf, Mat- tick, and others, 197 0). The geology and ore deposits of F1 F2 GEOPHYSICAL FIELD INVESTIGATIONS the Silver Cliff and Rosita Hills area have been studied by Cross (1896), Emmons (1896), Gabelman (1953), and Sharp (1978). Geophysical techniques have been used instudies of other volcanic centers of the southern Rocky Moun- tains. At the Cripple Creek mining district, about 72 km to the northeast of Silver Cliff, the volcanic area showed a 10-mgal (milligal) negative gravity anomaly upon which were superimposed local gravity lows believed to be related to deep mineralized fissure zones of fractured and loosely packed volcanic breccia (Kleinkopf and others, 1970b). In addition, the pro- nounced negative magnetic anomaly across the Cripple Creek center provided information about the depth and configuration of the altered breccia fill contained in the so-called “volcanic subsidence basin” (a usage after Koschmann (1949)). In the Bonanza area, about 64 km west-northwest of Silver Cliff (Karig, 1965), a closed gravity low, along with geologic data, indicated an ellip- tical volcanic structure with near-surface horizontal dimensions of 12.8 km and 16 km. The feature was estimated to contain about 2,400 m of low-density material (Karig, 1965). The local geology (pls. 1 and 2) was simplified from the detailed interpretive geologic map of Sharp (1978). Three major rock divisions are shown, Precam- brian rocks, Tertiary volcanic rocks, and Tertiary and Quaternary alluvium and sedimentary rocks. The Ter- tiary volcanic rock unit includes lithologies mapped by Sharp that generally were not of sufficient contrast in physical properties to be individually discriminated by the geophysical surveys, although some minor magnetic anomalies reflect variations of volcanic lithology in the Rosita Hills. Most of the faults and eruptive centers mapped by Sharp are included since in many cases they had a direct bearing on the geophysical interpretations. We thank Mr. Joseph Chellini of the Callahan Min- ing Company, Mr. D. W. Fieldman of the Congdon and Carey Company, and personnel of the Cleavenger Land and Cattle Company for allowing access to properties in the Silver Cliff and Rosita Hills area. We are in- debted to William N. Sharp of the U.S. Geological Survey for the geologic information that he provided, for logistical help during the surveying, and for many helpful suggestions in preparing the report. The geology shown on plate 1 was generalized from Sharp (1978). GEOLOGY The Silver Cliff and Rosita Hills volcanic area lies on the western flank of the Wet Mountains near the east edge of Wet Mountain Valley (fig. 1). The Wet Moun- tains in this area consist mainly of Precambrian metasedimentary, migmatitic, and granitic gneisses, and schists. The Precambrian rocks are part of an en echelon extension of the ancient Front Range highland that was exposed by erosion following uplift of the Wet Mountains during the Laramide revolution (Christman and others, 1959; Lovering and Goddard, 1950; Cross, 1896). o Bonanza COLORADO 37° FIGURE 1.—Location of the Silver Cliff and Rosita Hills study area, south-central Colorado. The Wet Mountain Valley is a tectonic basin that separates the Wet Mountains from the Sangre de Cristo Range on the west and appears to be related to the Rio Grande rift system of buried en echelon horsts and grabens. Scott and Taylor (1975) described the valley fill as composed of a variety of post-Paleocene Tertiary rocks and Quaternary volcanic debris that record the volcanic history of the region. The Eocene rocks consist of Echo Park Alluvium and of prevolcanic boulder alluvium equivalent to the Huer- fano Formation of Huerfano Park. In Oligocene time, the basin fill changed abruptly from a sedimentary composition to an assortment of lava flows, ash flows, lahars, and volcanic-rich fluvial deposits derived from eruptions at the Rosita Hills volcanic center and other small volcanic fields in the area. The uppermost Ter- tiary deposit is a salmon-pink, basin-fill alluvium, which Scott and Taylor have called the Pliocene and Miocene Santa Fe(?) Formation. The Silver Cliff and Rosita Hills volcanic centers form separate areas connected by a small isthmus and SILVER CLIFF AND ROSITA HILLS VOLCANIC AREA, COLORADO F3 are surrounded by outcrops of Precambrian crystalline rocks. The emplacement of the Tertiary volcanic rocks was probably controlled by deep-seated, northwesterly trending, crustal fractures of Precambrian age reac- tivated during Tertiary uplift of the Wet Mountains (Siems, 1968). The Rosita Hills area consists of an early volcanic center surrounded by rhyolitic domes that formed by forceful intrusiOn of magma along ring faults (Siems, 1968). The probable boundaries of the early crater and the caldera as interpreted by Sharp (1978) are shown on plate 1. The volcanic rocks are made up of a complex structural association of flows, tuff, and ash that includes rhyolite, rhyodacite, latite, trachyandesite, and andesite. Siems (1968) described several phases: extrusives, subvolcanic intrusives, for- mation of stock and radial dikes, subsidence, and final- ly doming in the later stages to form a small, in- completely developed, resurgent cauldron. At Silver Cliff, early eruptions of rhyolite tephra, ash flows, breccias, and glass were accompanied by subsidence and formation of rhyolite domes along ring faults. As a result of subsidence, the volcanic rocks have been preserved from erosion. The ore deposits occur mainly within the two volcanic areas, although many mineral occurrences are known in the adjacent Precambrian crystalline rocks. Deposits of silver, gold, lead, zinc, and copper associated with the Tertiary volcanic rocks have been mined. Emmons (1896) described three main types of ore bodies based largely on structural and alteration control. One type is the well-defined, fissure-vein deposits that occur along fault planes in the Rosita Hills district. A second type, mainly in the Silver Cliff district, is cavity fillings and replacements of highly altered, confining rock. A third, specialized type of ore body found in breccia pipes provided the most produc- tive mines. The Bassick Mine, located on the south flank of Mount Tyndall, was in a breccia pipe compos- ed of fragments of andesite with some granite and gneiss. Similarly, the Bull Domingo Mine in the Silver Cliff district was in a breccia pipe within Precambrian gneiss terrane. COLLECTION OF DATA GRAVITY SURVEY Gravity measurements were made at 318 stations in the study area using a gravity meter with a scale con- stant of about 1 mgal per dial division, which could be read to one-hundredth of a dial division. Station eleva- tions were obtained from bench marks, transit and rod surveying, and photogrammetric control shown on 71/2 minute topographic maps. The gravity values were referenced to base station WU7 at the Colorado School of Mines, Golden, Colo. (Behrendt and Woollard, 1961). The measurements were reduced to complete Bouguer gravity values using an assumed rock density of 2.67 g/cm3. Terrain corrections were made for each station through the H zone (2,615 m) of Hammer (1939) with hand templates and from the H zone to 167 km by means of a digital computer (Plouff, 1966). The preci- sion of the contoured gravity map is estimated to be 0.5 mgal. AEROMAGNETIC SURVEY The magnetic data were collected by the US. Geological Survey in 1970 as part of a larger survey (US. Geological Survey, 1978) that extended northwesterly to the Arkansas River. Measurements were made with a continuously-recording ASQ-lO flux- gate magnetometer along flight lines oriented northwest-southeast and spaced 0.8 km apart. The flight elevation was 2,895 m above sea level. Topographic maps were used for position control of the aircraft and flight paths were recorded by a gyrostabilized, 35-mm continuous-strip camera (Evenden and others, 1967). Base lines were flown nor- mal to the traverse lines for correction of diurnal and instrument drift. The magnetic contours (pl. 2) show the total intensity magnetic field of the Earth in gam- mas relative to an arbitrary datum. The gradient of the Earth’s normal field was not removed from the data. SEISMIC-REFRACTION SURVEY Seismic-refraction surveys were made to supplement the information obtained from the gravity and magnetic surveys and, particularly, to determine the thickness of the alluvium and volcanic rocks. Two north-south seismic-refraction profiles were shot. A—A extended through Westcliffe and B— B passed through the east edge of Silver Cliff (pls. 1 and 2). Seismograms were recorded on photographic paper using a 12-channel HTL 7000Bl seismograph. For most of the field work a constant geophone spacing of 198 m was employed. The geophones were attached to a 2,180-m cable. In order to increase the profile length beyond 2,180 m, the following procedure was used. First, the 2,180-m geophone cable with 12 geophones was laid out along one end of the profile, and the P-waves propagated from the buried dynamite charges that were exploded at each end of the profile were recorded. The geophone cable was moved forward in in- crements of 2,180 m, and the previously used shot- points at each end of the profile were reloaded and reshot. This procedure of moving the cable and 'Use of brand names in this report is for descriptive purposes only and in no way constitutes endorsement by the U. S Geological Survey. F4 GEOPHYSICAL FIELD INVESTIGATIONS reshooting at the same shotpoints was repeated until the entire distance along a profile was covered. In addi- tion, intermediate shots at 2,180-m intervals were used to record velocity changes in the near-surface rocks. The charges consisted of 23 to 91 kg of nitrate ex- plosive that was detonated with a combination of elec- tric blasting caps, primacord, and stick dynamite. The charges were loaded and covered to a depth of about 3 m in pits dug with a backhoe. In general the resulting seismograms were of good quality and showed easily identifiable first arrivals on profile A-A’; some difficulty was encountered in pick- ing first arrivals on profile 3—3., The traveltimes from shotpoint to seismometer were picked to the nearest 0.001 second, and traveltime curves were constructed (fig. 4). Velocities were determined by visual fitting of straight-line segments to the traveltime data. Since there was little relief along the profiles, no elevation corrections were applied. The intercept time of the first recorded velocity horizon was between 0.010 and 0.100 second on all traveltime curves. This intercept time, or weathering correction, was attributed to a thin surface layer of dry, unconsolidated, low-velocity material. Corrections for the weathering layer, assuming a velocity of 330 mps, were applied to the traveltime data prior to making depth calculations. The base of the weathering layer probably corresponds to the top of the water table. The graphical interpretation method of Slotnick (1950) and the time-depth method of Hawkins (1961) were used in calculating depths and dips of the refracting horizons. Final interpretations were made by fitting theoretical ray paths to the com- puted models. INTERPRETATIONS OF GEOPHYSICAL DATA The geophysical data give insights about the third dimension of the geology. The Bouguer gravity and magnetic anomalies provide information on the pos- sible subsurface distributions of rock masses of various densities and magnetic susceptibilities. The seismic-refraction profiles directly provide quan- titative data about the depths and thicknesses of various rock units based on their seismic velocities. REGIONAL GRAVITY FEATURES In order to show the broad gravity setting of the region covering the study area a small scale recon- naissance gravity map contoured at 5 mgals is inclu- ded (fig. 2). Within the study area the contours were aproximated from the detailed gravity data shown at the larger scale (pl. 1). The reconnaissance map shows a northwesterly trending gravity high related to Precambrian crystalline rocks that comprise the uplifted Wet Mountains block. The associated regional gravity field dips toward the Sangre de Cristo Moun- tains rather uniformly in a southwesterly direction across the study area at about 2 mgal/km (Peterson and others, 1974), except for an increase of gradient across the low-density sedimentary rocks preserved in the Wet Mountain Valley basin. Local perturbations of the regional field can be observed and reflect anomalies attributed to the distribution of volcanic rocks in the Silver Cliff and Rosita Hills area. REGIONAL MAGNETIC FEATURES To show the regional magnetic setting, a small map (fig. 3) is included which shows high-level aeromagnetic survey data for the immediate region of the volcanic centers. The regional map shows a broad, complex magnetic high over the southern part of the Wet Mountains. The striking characteristic of the magnetic map is the plateau-like feature that is delineated by high gradient zones with a change of 150 gammas. The gradient zone along the southeastern flank of the Wet Mountains trends northeasterly and nearly bisects the Rosita Hills volcanic area. Perhaps this represents a lithologic boundary in the Precam- brian subsurface between rock units of much different magnetic properties, such as gabbroic rocks, and less magnetic granitic rocks at San Isabel to the south (M. Dean Kleinkopf, R. B. Taylor, D. L. Peterson, R. E. Mattick, and W. N. Sharp, unpub. data, 1970). The coincidence of the volcanic complex with the magnetic gradient zone suggests that the volcanic activity may have been, in part at least, controlled by this postulated lithologic boundary. The regional magnetic map shows a magnetic trough, defined by contour closures 2,220 and 2,240 gammas, which correlates with volcanic rock ex- posures located immediately southeast of the crater complex. This magnetic trough coupled with a nearly coincident gravity trough (fig. 2) may indicate a thick section of volcanic rocks preserved in a southeasterly trendind graben or volcanic-subsidence feature. Alter- natively, the troughs could indicate a southeasterly trending lithologic unit of Precambrian rocks of low magnetization and relatively low density. To the north, the major magnetic high defined by the 2,500- and 2,600-gamma closures probably represents a block of Precambrian gneiss that separates the volcanic centers at Silver Cliff and the Rosita Hills. The gravity data show a corresponding high, indicated by westerly nosings of the -215 and -220 mgal con- tours. Although the reconnaissance aeromagnetic data are dominated by east-northeasterly trending features probably related to Precambrian lithology and struc- .l SILVER CLIFF AND ROSITA HILLS VOLCANIC AREA, COLORADO F5 105330' i05“15‘ v 5 cgfifgi‘“ rfls" . flaw 3* K 7- K fl. A ‘3 38°15 ' 38°00” D 5 10 KILOMETERS EXPLANATION ) GRAVITY CONTOURS—Showing relative gravity val- + FAULT—Dashed where uncertain; U, upthrown side; D, a @/ j ues in milligals. Hechures show closed areas of lower downthrown side _ 0/ gravity values. Contour interval 5 mgal VOLCANIC ROCKS SC, Silver Cliff area RH, Rosita Hills area JG, Johnson Gulch area 0 GRAVITY STATION —— CRATER COMPLEX AT ROSITA HILLS FIGURE 2.—Reconnaissance gravity map, modified from Peterson and others (1974). ture in the subsurface, slightly negative expressions southeast of Silver C ' f village, where contours of attributed to the volcanic deposits were observed over 2,520 and 2,540 ga 5 show negative noses over the Silver Cliff center. This was most evident just volcanic rocks. F 6 GEOPHYSICAL FIELD INVESTIGATIONS WI .30' 38‘00’ 0 5 10 KLOMETERS |___—l__.___l EXPLANATION MAGNETIC CONTOURS-Showing total intensity U . . . _ magnetic field of the Earth in gamma: relative to D FAderEPBShed yghere unoertam, U' upthrown sIde, 0' 02600 arbitrary datum. Hachured to indicate closed areas of rown 8' 6 lower magnetic intensity. Flight lines 4420 m above sea level, at 1.6 km spacing. Contour interval 20 vglécgri‘llilirficfigKasrea 9mm“ RH, Rosita Hills area {00 LOCATION OF MEASURED MAXIMUM on MINIMUM JG' J°hns°n Gm" 8'“ x INTENSITY, IN GAMMAS, WITHIN CLOSED HIGH 0R __ CRATER COMPLEX AT RosrrA HILLS CLOSED LOW FIGURE 3.—Regional aeromagnetic map, modified from US. Geological Survey (1970). parent on the 5 mgal map. The gravity pattern is dominated by high-gradient zones and by negative The Bouguer gravity map (pl. 1) is contoured at 1 anomalies expressed as closed lows and residuals in the mgal intervals and shows many local features not ap- form of contour nosings. The gross northwesterly DETAILED GRAVITY MAP SILVER CLIFF AND ROSITA HILLS VOLCANIC AREA, COLORADO F7 trend of the contours reflects the regional gravity field observed on the reconnaissance map. On the basis of correlations with known geology, most of the gravity anomalies can be interpreted in terms of lateral and vertical distribution of low-density Tertiary volcanic rocks in contact with high-density Precambrian gneisses (table 1) and are not related to lithologic varia- tions in the Precambrian rocks. This is exemplified at the Silver Cliff and Johnson Gulch areas, where promi- nent negative anomalies apparently reflect thick se- quences of highly altered volcanic breccias preserved in volcanic subsidence features. Conversely, the pro- bable boundary of the early crater complex at the Rosita Hills, as outlined from geologic studies (pl. 1), is not clearly discernible in the gravity data. Instead, several smaller anomalies of less amplitude and areal extent may reflect a pattern of more restricted sub- TABLE 1.—Physical properties of rocks in the Silver Cliff and Rosita Hills volcanic area, Colorado Sample Density' 2Magnetic susceptibility (X 10—4) Felsic gneiss 1 2.61 0.58 2 2.61 * 3 2.62 * 4 2.61 * 5 2.63 .24 Average 2.62 0.16 Mafic gneiss 1 3.04 0.32 2 2.91 121.87 3 2.81 12.20 4 2.87 .53 5 2.94 18.74 Average 2.91 30.89 Ta“ 1 2.04 * 2 1.95 65 Average 2.00 32 Andesite 1 2.73 0.48 2 2.96 10.97 Average 2.85 5.73 Rhyolite 1 2.46 0.43 2 2.51 .32 Average 2.49 0.38 ‘In grams per cubic centimeter. 7In electromagnetic units per cubic centimeter. ‘. values less than 10—6 emu/cm3, the threshold of the measuring equipment. sidence coupled with a greater percentage of denser volcanic rocks, including andesites and trachytes. DETAILED AEROMAGNETIC MAP The detailed aeromagnetic map (pl. 2) is contoured at 50 gamma intervals and shows a number of prominent anomalies which reflect both lithologic and structural boundaries in the Precambrian gneissic complex as well as distribution of Tertiary volcanic rocks. Com- pared to the high-altitude survey (fig. 3), the informa- tion content of the low-altitude survey is considerably greater, since it was flown nearly 1,525 m closer to the ground surface and at one-half the flight-line spacing. The high-level data show broad smooth anomalies related to Precambrian rocks and the anomalies can be recognized on the detailed map as complex features complicated by the magnetic influence of the volcanic rocks. A good example is the major easterly trending positive anomaly that extends nearly across the map. Compare the smooth 2,500-gamma closure on the inset map of figure 3 with the rather contorted 2,500-gamma anomaly on the large-scale map. The magnetic patterns are more complex than the gravity anomalies. The gross magnetic grain is northeasterly as would be expected since this trend is pronounced in the study area on the regional aeromagnetic map. In the northwestern part of the map, prominent negative anomalies occur over the volcanic deposits at Silver Cliff and Johnson Gulch. In- terpretations of other negative features over adjacent Precambrian terrain will be discussed in the following section of this report. As was the case for the gravity data, the probable boundary of the early crater com- plex at the Rosita Hills is not distinctive in the magnetic data. The magnetic pattern is a rather nondescript mixture of low-relief highs and lows, ex- cept for a broad but well-defined negative anomaly along the southwest edge of the Rosita Hills. The positive features in the extreme southern and southwestern parts of the area are attributed to lithologic variation in the Precambrian basement rocks. GEOLOGIC SIGNIFICANCE OF LOCAL GRAVITY AND MAGNETIC ANOMALIES The gravity and magnetic features of interest are often referred to as “anomalies” in the discussions to follow. “Anomaly” is used in the customary way as a local deviation of geophysical values, such as milligals or gammas, above or below a regional field or background. An “anomaly” may be one of two types, first a high or low expression of various configurations portrayed by closed contours, or second, an expression portrayed by nosings or bowings in the contours in F8 GEOPHYSICAL FIELD INVESTIGATIONS which no closed contours occur, but which nevertheless indicate a mass irregularity representing a geological disturbance of interest. The latter is referred to in the text as a “residual anomaly,” or the local expression that occurs above or below the background field. The interpretations were focused on the gravity data, which proved to be more diagnostic than the magnetic data in identifying and studying volcanic features in the third dimension. The magnetic data provided valuable qualitative information that often corroborated and thus strengthened the gravity inter- pretations. Because the magnetic patterns showed no indications of reversals, the magnetic anomalies were assumed to be caused by sources magnetized by induc- tion in the Earth’s field, and effects from remanent magnetization not in the direction of the field were con- sidered negligible. Possible correlations between topographic features and the gravity and magnetic anomalies were examin- ed. The topographic relief of the area varies from low to moderate with a total relief of about 610 m: elevations range from less than 2,340 m at De Weese Reservoir to over 2,950 m at Mount Robinson in the Rosita Hills. Qualitative evaluations of the gravity and magnetic data across large topographic changes in both volcanic and granitic rocks showed no influences large enough to detract from the interpretation presented. Density and magnetic susceptibility measurements were made (table 1) and showed marked contrasts be- tween major rock units that are significant to the inter- pretations. However, in many cases, it was useful to assume average density and magnetic susceptibility values since the Tertiary volcanic rocks and Precam- brian gneisses were a mixture of several lithologies. Gravity anomaly 1.—The most prominent gravity feature is the large negative anomaly that corresponds to the Silver Cliff volcanic field in the west-central part of the area (pl. 1). The amplitude of the negative anomaly is nearly 10 mgals; two areas along the grav- ity trough where the gravity data indicate that the less dense rocks or volcanic rocks are thickest are located approximately by the — 224 and —225 mgal closed con- tours. The deepest parts of the gravity trough define the Silver Cliff graben, which extends nearly to the Rosita Hills. On this basis, coupled with geologic field studies, the anomaly is interpreted to represent an area of substantial subsidence of Precambrian blocks, which resulted in accumulation and preservation of the volcanic rocks observed in outcrop. The gravity trough is terminated near the Rosita Hills by a cross-trending gravity high over near-surface Precambrian crystalline rocks that separate the Silver Cliff and Rosita Hills volcanic areas. The magnetic data show a major negative anomaly that corresponds to the gravity feature. Particularly striking is the south side of the magnetic anomaly where it is elongated in a northwesterly-southeasterly direction as an axial low that correlated with the grav- ity trough. The magnetic trough is also terminated just west of the Rosita Hills, but a fingerlike extension of the negative anomaly suggests faulting that is not observed in the gravity data. The magnetic data have geologic significance in confirming the gravity inter- pretation of a structural depression filled with volcanic rock. The magnetic data indicate that most, if not all, of the volcanic debris are relatively nonmagnetic types, or that most of the magnetic character has been destroyed by alteration associated with later stages of the volcanism. Gravity anomalies 2 and 3.—These anomalies are subsidiary negative residual anomalies formed from nosings in the contours along the steep gravity gra- dient zone that marks the north and northeastern flank of the major Silver Cliff feature, anomaly 1. The anomalies correlate with two volcanic vents, Upper Chlorite vent (anomaly 2) and Ben West volcano (anomaly 3). The eruptive vents are composed of chaotic volcanic debris that apparently is sufficiently porous to cause the gravity lows. Another center, Geyser vent, has been mapped in geologic field studies (pl. 1), but exhibits no gravity expression. The magnetic data are significant. Magnetic low 2,185 nearly centers over Ben West volcano, but no observable magnetic expression of Upper Chloride vent or Geyser vent exists. The magnetic data strong- ly suggest that the volcanic alteration was more inten- sive at Ben West volcano, compared to the Upper Chloride and Geyser vents, where the magnetic character of the breccias apparently was not, greatly changed. The seismic-refraction profile A—A (fig. 4) across Ben West volcano confirmed the presence of low-velocity rocks in the subsurface and will be discussed subsequently in the section on seismic inter- pretations. Gravity anomaly 4.—-This is a negative residual anomaly that appears as a definite nosing in the con- tours over Precambrian outcrops. The amplitude of the anomaly is about 1 mgal. This could indicate an in- cipient volcanic feature in the subsurface, in which the magnetic properties of the Precambrian gneiss may have been destroyed by hydrothermal activity associated with the volcanism. Although the magnetic and surface data do not confirm such a feature, the anomaly is of interest because of the proximity of the main volcanic areas and the nearly adjacent reentrant of volcanic rocks along the northeastern end of the Silver Cliff graben. Gravity anomaly 5.—Along the south side of the Silver Cliff graben, Precambrian crystalline rocks have SILVER CLIFF AND ROSITA HILLS VOLCANIC AREA, COLORADO F9 been mapped in outcrop (pl. 1); and gravity anomaly 5 suggests that the crystalline rocks are in the near sub- surface in a zone at least 2 km wide along the south side of the Silver Cliff graben. In fact, the seismic data along profile A-A’ show crystalline rocks ranging in depth from 150-250 In between the graben and the Westcliffe fault. Magnetic high 2,965 generally cor- relates with gravity anomaly 5, and the apex of the magnetic anomaly is about 1 km southwest of the top of the gravity anomaly, as would be expected for a uniformly magnetized block. The gravity and magnetic highs appear to represent a unit of Precam- brian gneiss of rather uniform lithology. Gravity anomaly 6.—-The Westcliffe fault is located along the southwestern margin of the study area (pl. 1) and shows subtle geophysical expressions. Scott and Taylor (1975) attached considerable importance to the fault and stated that it was one of the faults along which late Tertiary uplift uplift of the northern part of the Wet Mountains occured. A local steepening of the gravity gradient occurs along the fault northwest of Aldrich Gulch; to the southeast, the gravity contours cross the fault with no noticeable deflection. The broad magnetic anomalies which were interpreted to reflect lithologic units in the Precambrian crystalline basement cross the fault without interruption. However, 1 km to the southwest of the fault, the magnetic gradients are noticeably broader, thus indicating that sources of the anomaly are deeper beneath the sedimentary rocks of this part of the Wet Mountain Valley. The seismic-refraction data do show about 30 m of offset, down to the southwest, near the end of profile A—A (fig. 4). The lower velocities recorded in Precambrian rocks south of the Westcliffe fault suggest a change in the lithology of the crystalline Precambrian rocks. This is compatible with the magnetic data, which show evidence of decreasing magnetic susceptibility from magnetic high 2,965 southward to the negative magnetic trough (pl. 2). Gravity anomaly 7.—This negative residual anomaly may represent an incipient volcanic center possibly containing brecciated and altered volcanic rocks. The anomaly, in the form of a nosing in the gravity con- tours, correlates with an apparent surface mantle of volcanic rocks over Precambrian crystalline rocks; and a similarity with the expression at the Bassick pipe (anomaly 13) exists both in configuration and in loca- tion, relative to the Rosita Hills volcanic area. The magnetic data show correlative low values in a saddle located between two prominent magnetic highs. A negative magnetic response might be expected from an incipient volcanic feature as postulated here. Gravity anomaly 8.-This negative gravity anomaly is expressed by the -218 mgal contour closure and a trough-like extension, to the north. It is in an area of rhyolite flows, the source of which may have been the vent area at Rattlesnake Hill. The -218 mgal contour closure may reflect the thickest deposits of rhyolite. This possibility is reinforced by the magnetic data, which show a coincident negative feature elongated in an east-northeasterly direction along the axis of the gravity anomaly. Gravity anomaly 9.—The shape of this negative gravity anomaly suggests a graben-like feature of less dense volcanic rocks in an area of predominantly rhyolite flows. The gravity trough extends to the southeast and is part of the regional gravity trough (fig. 2) that was discussed earlier in the section on regional features. Magnetic low 1,960 is located on the southwest edge of the volcanic complex of Rosita Hills, and it appears to be related in part to gravity anomaly 9. There is a general similarity between magnetic low 1,960 and magnetic low 2,185, which centers over Ben West volcano and covers most of the Silver Cliff volcanic area. The east-west magnetic troughs at both the north and south edges of the anomaly resemble the patterns of the magnetic low associated with the Silver Cliff graben. Magnetic low 1,960 may represent a composite source ranging from volcanic rocks on the north to Precambrian lithologies of low magnetic intensities on the south. However, because of its location and its similarity to the magnetic anomaly at Silver Cliff, the locality of magnetic low 1,960, particularly the 2,000 gamma closure, seems worthy of detailed examination in the field for possible evidences of subsidence. Gravity anomaly 10.—The pronounced nosing in the gravity contours results in a positive residual an- omaly, about 2 mgals in amplitude, that correlates with an eruptive vent composed largely of Miocene Pringle Latite (Sharp, 1978; Siems, 1968). North- northeasterly trending faulting enhances definition of the western flank of the anomaly. The positive magnetic feature, magnetic anomaly 2,365, correlates with the gravity anomaly; the apex is offset to the southwest as would be expected, but it may in part be related to dike rocks along the fault zone. The north- east side of the anomaly is indistinctly bounded by the Rosita fault, which becomes a complex zone in its northwesterly trend, where the gravity data suggest ‘that it connects with faulting along the north side of the Silver Cliff graben. Whether the Rosita fault may have been a controlling crustal fracture related to emplacement of the volcanic rocks (Siems, 1968) is a matter of speculation. Gravity anomaly 11.—Over the eruptive vent map- ped on geologic evidence (Sharp, 1978), a weak negative gravity residual is suggested, although it is based on one gravity station on the edge of the struc- F10 ture. No local magnetic anomaly is evident, although the vent is on the edge of a large magnetic trough, magnetic low 2,000. This suggests that the volcanic feature may narrow rapidly in a conelike fashion to a small diameter pipe or that the volcanic rocks within the center were not subjected to extensive alteration or brecciation sufficient to lower the density or magnetic susceptibility of the rocks. Gravity anomaly 12.—The breccia vent mapped about a kilometer west of the townsite of Querida is ex- pressed as a positive gravity residual or ridge. The ridge may be artificial and may have resulted from gra- dients associated with the lows to the east (anomaly 13) and to the west. No anomaly was detected in the magnetic data, which suggests, as in the case for anomaly 11, that there probably was not enough brec- ciation or alteration associated with the eruptive center to produce a density or magnetic susceptibility contrast with the surrounding rocks. In fact, the grav- ity data suggest the presence of somewhat denser rocks, possibly dacites or andesites at the eruptive center. Gravity anomaly 13.—A pronounced negative grav- ity residual anomaly of 3—4 mgals amplitude correlates with the Oligocene breccia pipes located on the south flank of Mount Tyndall. The anomaly is part of a northeasterly trending gravity trough located along the northeast edge of the main volcanic field of the Rosita Hills. The anomaly is caused partly by volcanic agglomerates that form a reentrant into adjacent Precambrian gneiss outcrops. The Bassick pipes area does not exhibit a correspon- ding magnetic low, but is located about 1.5 km northwest of the low point of a broader magnetic feature, magnetic low 1,940. It is surprising that there is not a sharp, high-amplitude magnetic low over the Bassick area because of the strong alteration expected with formation of the explosive breccia pipes. Gravity anomaly 14.—The gravity expression over the monadnock-like mass of the Bull Domingo Hills is a broad elongate high that is poorly controlled. A prominent magnetic feature, magnetic low 2,060, centers over the southwest half of the hills, which con- sist of Precambrian rocks predominantly of horn- blende and biotite gneiss. The magnetic low across this topographically high mass of dark-colored gneisses was unexpected. However, indications from two samples tested for magnetic susceptibility suggest that the Precambrian rocks of the southeastern hills may be relatively nonmagnetic compared to surround- ing Precambrian terrane. The two samples averaged less than 1.0X10—4 emu/cm3 compared with an average of 30.89X10-4 emu/cm3 obtained for mafic gneisses of the study area (table 1). The possibility of a GEOPHYSICAL FIELD INVESTIGATIONS volcanic source for the magnetic anomaly is highly unlikely, but should be mentioned, since there is a small breccia pipe at the Bull Domingo Mine near the south end of the hills. The pipe could be the surface manifestation of volcanic activity that destroyed, through alteration, the magnetic properties of a large volume of rocks in the subsurface. If this were the case, it seems likely that there would be other areas of ven- ting or alteration at the surface. None have been reported. Magnetic low 2,060 is part of a broad pattern, a pro- nounced arc-like expression, that includes low 2,185 over Ben West volcano, low 2,195 just east at Johnson Gulch and low 2,235, some 6 km farther east. The lows at Ben West volcano and Johnson Gulch are both associated with gravity lows and correlate with areas of suspected volcanic subsidence. It is possible that associated with magnetic low 2,060 there may be a gravity low that was not detected in the recon- naissance control, owing to the broad spacing of the gravity stations in this area. Gravity anomaly 15.—About 3 km east of the Bull Domingo Hills another volcanic feature has been iden- tified along Johnson Gulch (Sharp, 1978; Kleinkopf and Peterson, 1976). The nearly coincident gravity and magnetic (2,195) lows (pls. 1 and 2) over outcrops of volcanic ash,coupled with geologic evidence (William N. Sharp, written commun., 1976), suggested the presence of a cauldron subsidence feature. Field evidence for subsidence was evident along the Precambrian-Tertiary tuff contact, where a zone of coarse breccia was composed of Precambrian rocks, along with some volcanic blocks, all tightly packed and angular with abundant iron staining. Three- dimensional modeling of the gravity anomaly in- dicated a column of low-density rocks extending about 1,000 m below the surface. Gravity anomaly 16.—A negative gravity residual defined by a rather pronounced nose in the gravity con- tours is located about 3 km east of Ben West volcano. No surface evidence of volcanism has been reported in this area of Precambrian crystalline gneisses, Tertiary gravels, and conspicuous faults. However, a signifi- cant factor is the corresponding negative magnetic residual anomaly formed by a sharp nosing in the magnetic contours and controlled by two flight lines. Thus, gravity and magnetic expressions call attention to an area not far removed from the volcanic sub- sidence at Silver Cliff, and they may reflect a subsur- face volcanic source that did not reach the present level of erosion. SEISMIC INTERPRETATIONSI , The two seismic-refraction profiles (A—A and 3—3, figs. 4 and 5) shot across the major subsidence feature GAMMAS MILLBALS MILLISALS METERS TIME IN MILLISEDONDS SILVER CLIFF AND ROSITA HILLS VOLCANIC AREA. COLORADO A NORTH 2900 — 2800 —_ / 2700 — \37/ _ 8‘0} 2600 — / 2500 — / A I SOUTH / ‘I'I‘I'I 'I'I I § I -210 -215 - 220 IIIIIIIIIIIIIIIIIIIIIII| -230 Gravity station IIIIIIIVIIIIIIIIIIIIIIII] RESIDUAL GRAVITY PROFILE 00315195 600 ‘ QUATERNARY ALLUVIUM TERTIARY VOLCANICS PRECAMBRIAN CRYSTALLINE ROCKS ‘F‘ FAULT—Arrows show direction of movement 1400 — — 1200 — 7 @‘36 _ ’20 'S‘ 1000 MP8 fi é _ ‘1‘ _ .9" 49" \ 5% 800 fi 680 MP5 '5 K 998‘" “ 5% 099 ‘I {a “I «2% 600 — 41 ‘69" ‘x‘ 5790 \ ‘tx _ PS 5°53“ {5 W 6155 MP8 910 400 _ 9 4695 M ‘5 _ 8v; \A '5 K3 9% “33‘ 200 MP3 65'3" o 3% s 4% Q5 ,3, 4% “ 5% «8" *0 .59 3350 MP8 § $9 6‘ Q‘ ‘ ‘3‘ 2630 MP8 .n w 0 7 6 5 4 2 1 U 3000 METBS I__1__1—_J F11 EXPLANATION —o—o— QUATERNARY ALLUVIUM —I—o— TERTIARY VOLCANICS —O—o——- PRECAMBRIAN CRYSTAL- LINE ROCKS SHOT POINTS FIGURE 4.—Geologic section A-AI, constructed from seismic refraction data with time-distance plots, gravity, and aerornagnetic profiles. F12 GEOPHYSICAL FIELD INVESTIGATIONS AEROMAG NETlC GAMMAS L 3 - \~_._ _A2lm*9_reg_i°nal wm______________- ‘2 —215 ~ _ _ — — _ — - 2 2 - _ -220 — — -225 0— _ (fl _ _ < ~ _ g -5 — Gravity value computed — g _ from model below _ -10 Drill hole reaches Prewmbrian 0 crystalline rocks at 219 meters 250 Dotted line computed from sen E2 500 refraction data E 5 750 - line indicates depth 1000 omputed from gravity - data 1250 0 500 1000 1500 METERS |_._l_—_l_._l FIGURE 5.—Geologic section B—BI, constructed from applying gravity modeling and seismic-refraction data to geologic surface extensions. SILVER CLIFF AND ROSITA HILLS VOLCANIC AREA, COLORADO at Silver Cliff (anomaly 1) provided quantitative infor- mation about the thickness and distribution of volcanic and alluvial deposits relative to Precambrian crystalline basement. The results from profile A—A showed that the p-wave velocity measured in the Precambrian crystalline rocks varied from 3,350 mps to 6,250 mps, in the volcanic rocks from 2,680 mps to 3,930 mps, and in the overlying Tertiary sedimentary rocks from 2,290 mps to 2,440 mps. The notably low value of 3,350 mps velocity recorded for one area of outcropping Precambrian rocks was attributed to a high degree of fracturing. Disregarding this velocity measurement, the velocity in the Precambrian complex varied from 5,120 mps to 6,250 mps. , Seismic records from shotpoints 1-3 on profile A—A (fig. 4) were of excellent quality, and interpretation was a straightforward procedure. Disregarding the thin weathering layer with a velocity of 330 mps and vary- ing in thickness from 7 to 15 m, three refracting horizons were recorded with calculated velocities of 2,290-2,440 mps, 2,680 mps, and 5,120—5,640 mps. The three velocities were interpreted as representing sedimentary, volcanic, and crystalline rocks, respec- tively. Calculated depths to crystalline basement are about 300 m, 150 m, and 230 m at shotpoints 1, 2, and 3, respectively. A small offset on the time plot, located about 1,500 m south of shotpoint 2, was interpreted as indicative of the Westcliffe fault with displacement of about 30 m. On the upthrown side of the fault the velocity of the basement rock was calculated to be 5,640 mps and the velocity on the downthrown side was calculated to be 5,120 mps. This difference in velocity could be caused by a change in lithology, moderate alteration, or possibly a change in amount of fracturing and weathering. The quality of the records obtained from shotpoints 4—7 (fig. 4) was not as good as from those obtained along the southern segment. Nonetheless a reasonable interpretation could be made. Between shotpoints 6 and 7, where Precambrian crystalline rock is exposed, the time-distance plot indicates two distinct velocity layers within the Precambrian complex with velocities of 5,760 mps and 6,250 mps. Near shotpoint 4 a maximum velocity of 3,660 mps was measured. This velocity, not typical of Precam- brian gneiss, is interpreted as reflecting a thick section of volcanic rocks that corresponds to the Silver Cliff graben shown on the gravity map (pl. 1). Similarly, the area just north of shotpoint 4, where velocities no greater than 3,930 mps were recorded, appears to cor- respond to the Ben West volcano. Between the two volcanic centers at the Silver Cliff graben and Ben West volcano the near-surface velocity varied from F13 3,350 to 3,960 mps and the deeper velocity was 5,180 mps. The thickness of the relatively low velocity sur- face volcanics is calculated to be at least 150 m; the surface volcanics overlie higher velocity material, which is interpreted to represent fractured crystalline rock of the Precambrian complex. Along profile B—B’a schematic structure section was made from analyses of gravity modeling and the seismic-refraction data (fig. 5). The time picks of the seismic arrivals were variable in quality, but enough reliable information was obtained to construct the geologic section in conjunction with the gravity model- ing. The volcanic rocks thicken from about 120 m near the north end of the profile to about 1,220 m in the trough, as calculated from the seismic-refraction data and the two-dimensional gravity modeling (fig. 5). The calculated thickness of the volcanic section was con- firmed at one point by an existing drill hole that bot- tomed at the top of the Precambrian rocks at 219 m. The time-distance plots along profile B-B [suggest an average velocity of 5,790 mps for Precambrian rocks and 3,110 mps for the volcanic rocks—in good agree- ment with ,the corresponding velocities obtained on profile A—A. GRAVITY MODELING Three-dimensional iterative modeling of the Silver Cliff volcanic area was done to obtain quantitative in- formation about the thickness and configuration of the volcanic material. The gravity modeling involved several steps. First, a residual map was prepared by subtracting from the complete Bouguer gravity data (pl. 1) a regional field that was a smoothed approxima- tion of the Bouguer gravity data. Figure 6 shows the arcuate dipping regional and the resulting residual gravity anomaly. The residual map then was gridded at a 0.5 km interval to obtain digitized data for input to a computer program (Cordell and Henderson, 1968). The horizontal reference plane chosen was the ground surface. After five iterations the solution was obtained as shown by the isopleths in figure 7. A single average density contrast of 0.5 gms/cm3 between Tertiary volcanic rocks and Precambrian gneisses was assumed based on the physical property determinations (table 1) coupled with the abundance of low-density tuffaceous and brecciated rocks in the volcanic suite at Silver Cliff. This order of magnitude of density contrast was confirmed by the large velocity differences measured during the seismic-refraction surveys. Along profile A-A’ (fig. 4), velocities for volcanic rocks ranged from 2,680 to 3,960 mps and from 5,120 to 6,250 mps for Precambrian gneisses. The F14 GEOPHYSICAL FIELD INVESTIGATIONS 105°22’30" 105°30' 38° , , , 12, 27130 2'5 ml! EXPLANATION \\\ GRAVITY CONTOURS—Showing residual gravity anomaly in milligals. Hachures show closed areas of lower gravity values. Contour interval 1 mgal ——10—— GRAVITY CONTOURS—Showing southwesterly dip- ping regional field in milligals. Contour interval 2 mgal Bull Domingo Hills 10’~ 38° 07' — 30’ O l 2 3 KILOM ETEBS l l | l FIGURE 6.—Residual negative gravity anomaly over Silver Cliff and Johnson Gulch volcanic area. Anomaly derived by subtracting the southwesterly dipping regional field (shown by contours 0 to 12 in increments of 2 mgals) from the Bouguer anomaly (pl. 1). SILVER CLIFF AND ROSITA HILLS VOLCANIC AREA, COLORADO 105°ao' 33° 2730' 25' F15 105°22’3lf’ 12' | | Bull Domingo Hills . ' Outcrop of volcanic rocks Johnson GuIch area 38° 07' — U 1 2 3 KILOMETBiS I l I l 0 FIGURE 7.—Gravity model of configuration of volcanic rocks, Silver Cliff volcanic area. Thicknesses shown by isopleths of loo-meter inter- vals derived from 3-dimensional iterative modeling of the gravity data. (See fig. 6.) F16 results from profile B—B’(fig. 5) were in good agree- ment; 3,110 mps for volcanic rocks and 5,790 mps for Precambrian gneisses. The value of 0.5 gins/cm3 is pro- bably a maximum density contrast to be expected and would give minimal thicknesses of less dense volcanic rocks, since one might anticipate the contrast to decrease with depth. The results of the modeling (fig. 7) show a wedge of low-density volcanic material with thicknesses reaching 900 and 1,200 m in the deepest parts of the Silver Cliff trough. These areas correspond to the—224 and—225 closures on Bouguer gravity map (pl. 1). The order of magnitude of the thickness values is in general confirmed by seismic-refraction profile A—A (fig. 4) and the schematic-structure section made from seismic- refraction and two-dimensional modeling of detailed gravity data along profile B—B'(fig. 5). It is noteworthy that across Ben West volcano and Silver Cliff trough the seismic-refraction data did not resolve the bottom of the volcanic features, and it is probable that the gravity data show a minimum thickness since the depth extent apparently exceeds the width of the volcanic feature. Earlier results at Johnson Gulch (Kleinkopf and Peterson, 1976) indicated the presence of a low-density column of rocks shaped like an in- verted and truncated cone which extended to a depth of approximately 1,000 m beneath the volcanic tuff outcrops. The Johnson Gulch gravity model is inclu- ded here since the northeasterly trends in the gravity data, as well as the model results (fig. 7), suggests a possible connection in the subsurface between the Silver Cliff and Johnson Gulch areas. This is highly speculative, and such indication was not apparent from field geologic studies of the outcropping Precam- brian gneiss, which showed no evidence of northeaster- ly trending structures. In fact, several northwesterly trending faults were mapped across this area (pl. 1). However, the gravity and magnetic evidence for a possible incipient volcanic vent occurring between the two areas should not be overlooked (gravity anomaly 16 with corresponding negative magnetic residual). CONCLUSIONS AND RECOMMENDATIONS Several exposed volcanic eruptive and cauldron sub- sidence features in south-central Colorado show well- developed negative gravity and magnetic anomalies. Examples are the Silver Cliff graben, Ben West volcano, and the Johnson Gulch features. For the study area in general. the relative amplitudes of the gravity and magnetic anomalies gave information about the degree of brecciation and intensity of altera- tion of volcanic rocks as well as their thicknesses. Variations in velocities recorded in the seismic refrac- GEOPHYSICAL FIELD INVESTIGATIONS tion surveys were indicative of buried Tertiary volcanic deposits and highly fractured Precambrian rocks. Several other localities besides those examples men- tioned have distinct negative gravity or magnetic anomalies, possibly indicative of volcanic deposits in the subsurface; and these also seem worthy of careful studies. Additional geophysical and geological work seems warranted, in order to obtain more evidence of subsidence and alteration, possibly associated with mineralizaton. In particular the following anomalies are recommended for further considerations: 1. Gravity anomaly 7, located along the north side of the main Rosita Hills volcanic area. 2. Gravity anomaly 15, located at Johnson Gulch. 3. Gravity anomaly 16, located about 3 km east of Ben West volcano. 4. Magnetic low 2,235, located about 6 km east of J ohn- son Gulch (gravity anomaly 15). 5. Magnetic low 2,060, associated with the southeast- ern part of the Bull Domingo Hills. 6. Magnetic low 1,960, located along the southwestern boundary of the main Rosita Hills volcanic area. REFERENCES CITED Behrendt, John C., and Bajwa, La Cretia Y., 1974, Bouguer gravity map of Colorado: US. Geol. Survey Geophys. Inv. Map GP—895. Behrendt, J. C., and Woollard, G. P., 1961, An evaluation of the gravity control network in North America: Geophysics, v. 26, p. 57—76. Christman, R. A., Brock, M. R., Pearson, R. C., and Singewald, Q. D., 1959, Geology and thorium deposits of the Wet Moun- tains, Colorado—A progress report: U.S. Geol. Survey Bull, 1072—H, p. H491-H535. Cordell, Lindrith, and Henderson, Roland, G., 1968, Iterative three- ; dimensional solution of gravity anomaly data using a digital computer: Geophysics, v. 33, no. 4, p. 596—601. Cross, Charles Whitman, 1896, Geology of the Silver Cliff and Ro- sita Hills, Colorado: US. Geol. Survey 17th Ann. Rept., pt. 2, p. 263—403. Emmons, S. F., 1896, The mines of Custer County, Colorado: US. Geol. Survey 17th Ann. Rept., pt. 2, p. 405—472. Evenden, G. I., Frischknecht, F. C., and Meuschke, J. L., 1967, Digital recording and processing of airborne geophysical data, in Geological Survey research 1967: US. Geol. Survey Prof. Paper 575—D, p. D79—D84. Gabelman, John W., 1953, Definition of a mineral belt in south central Colorado: Econ. Geology, v. 48, no. 3, p. 177—210. Hammer, Sigmund, 1939, Terrain corrections for gravimeter sta- tions: Geophysics, v. 4, p. 184—194. Hawkins, L. V., 1961, The reciprocal method of routine shallow seismic refraction investigations: Geophysics, v. 26, no. 6, p. 806—819. Karig, Daniel E., 1965, Geophysical evidence of a caldera at Bon- anza, Colorado, in Geological Survey research 1965: US. Geol. Survey Prof. Paper 525—B, p. B9—B12. SILVER CLIFF AND ROSITA HILLS VOLCANIC AREA, COLORADO Kleinkopf, M. Dean, and Peterson, Donald L., 1976, Geophysical evidence for a new cauldron subsidence feature near silver Cliff, Colorado: Geology, v. 5, p. 445—447. Kleinkopf, M. Dean, Mattick, Robert E., Sharp, William N., and Peterson, Donald L., 1970. Geophysical studies in the area of the Silver Cliff mining district, Colorado: Geophysics, v. 35, no. 6, p. 1165. Kleinkopf, M. Dean. Peterson, D. L., and Gott, Garland, 1970, Geo— physical studies of the Cripple Creek mining district, Colorado: Geophysics, v. 35, no. 3, p. 490-500. Koschmann, A. H., 1949, Structural control of the gold deposits of the Cripple Creek district. Teller County, Colorado: U.S. Geol. Survey Bull. 955—B, p. 19—60. Lovering. T. S., and Goddard, E. N ., 1950, Geology and ore deposits of the Front Range, Colorado: U.S. Geol. Survey Prof. Paper 223, p. 319. Peterson, D. L., Kleinkopf, M. D., and Wilson, D. M., 1974, Gravity map of the Pueblo 1°X2° quadrangle, Colorado: U.S. Geol. Survey Open-File Report 74—146. Plouff. Donald, 1966, Digital terrain corrections based on geogra- phic coordinates [abs]: Geophysics. v. 31, no. 6, p. 1208. F17 Scott, Glenn, R., and Taylor, Richard B., 1975, Post-Paleocene Tertiary rocks and Quaternary volcanic ash of the Wet Moun- tains Valley, Colorado: U.S. Geol. Survey Prof. Paper 868, 15 p. Sharp, William N., 1978, Geologic map of the Silver Cliff and Ro- sita volcanic centers, Custer County, Colorado: U.S. Geol. Survey Misc. Geol. Inv. Map I—1081. Siems, P. L., 1968, Volcanic geology of the Rosita Hills and Silver Cliff district, Custer County, Colorado. in Cenozoic volcanism in the Southern Rocky Mountains: Colorado School of Mines Quart, v. 63, no. 3. p. 89—124. Slotnick, M. M., 1950, A graphical method for the interpretation of refraction profile data: Geophysics, v. 15, no. 2, p. 163—180. U.S. Geological Survey, 1978, Aeromagnetic map of the Westcliffe- Royal Gorge area, Custer and Fremont Counties. Colorado: U.S. Geol. Survey Geophys. Inv. Map GP-929. U.S. Geological Survey, 1970. Aeromagnetic map of the Cripple Creek-Saguache area, south-central Colorado: U.S. Geol. survey Open-file report, Zietz, Isidore. and Kirby, J. R., Jr., 1972, Aeromagnetic map of Colorado: U.S. Geol. Survey Geophys. Inv. Map GP—880, scale 1:1,000,000. 9U.S. GOVERNMENT PRINTING OFFICE: l9794677-026/l9 PROFESSIONAL PAPER 726—F PLATE 1 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY EXPLANATION QUATERNARY AND TERTIARY ALLUVIUM AND SEDIMENTARY ROCKS Tv . TERTIARY VOLCANIC ROCKS PRECAMBRIAN Y AND X CRYSTALLINE ROCKS M w -/“' VOLCANIC VENT—Site of eruption of local rhyolite and andesite flow or ash BRECCIA PIPE—Developed by explosive venting; generally mineralized as a late stage process “*‘T'T—LTL'TT'J‘ BOUNDARY OF MAJOR ERUPTIVE VENT OR VOLCANIC PLUG ----—------— PROBABLE BOUNDARY OF EARLY CRATER COMPLEX AND SUBSI- DENCE AT ROSITA CAULDRON WW ~~~~~~~ m CONTACT —-——L-—-—~————?—-—— FAULT—Dashed where approximately located or concealed; queried where questionable. Bar and ball on downthrown side 7 OUTER RING FAULT GRAVITY CONTOURS—Dashed where approximately located. Hachured contours enclose areas Of low gravity. Contour interval 1 milligal. A density of 2.67 grams/cm3 was assumed in reducing the data to the complete WESTOLIFFE GRABEN ,, / ‘ _ _ _ _205// Bouguer anomaly I breccia '~ _ A ‘_ . ._ , _‘ y ' . I ‘ - ' . ‘ , ‘ ‘ ~ ~ . . GRAVITY STATION 1 ' v ’ * B B, SEISMIC REFRACTION, MAGNETIC, AND GRAVITY PROFILES—Shown V ‘ ' ' . I _ _ ’ 4 y on figures 4and 5 I ' 1 1 ANOMALOUS FEATURE—Discussed in text R‘ MINE a MINE SHAFT >r“ ADIT PROSPECT PIT X GRAVEL PIT brecia ' _ 4 . _ , , 7 . . . “ i REFERENCES I) 1> pies T \Eg , YX Sharp, William N., 1978, Geologic map of the Silver Cliff and Rosita volcanic centers, Custer Co., Colorado: US. Geol. Survey Misc. Inv. Series Map [—1081. VOLCAN c CENTE' // n’ PRINGLE VOLCANIC CONDUIT SCALE 1.24 000 Geology modified from William N. Sharp. Geology of the 1 V2 ‘ . . . C.) 1 MILE Rosita volcano modified by Sharp after Siems 11968). , , , # COLORADO Postvolcano deposits and volcaniclastic deposits from Scott and Taylor (1975) and Sharp (1978). Gravity I survey by D. L. Peterson and M. D. Kleinkopf 1968—73 l .5 0 1 KILOMETER I=L l—I I—l l—l |--—l CONTOUR INTERVAL 20 FEET E § E NATIONAL GEODETIC VERTICAL DATUM OF 1929 nus. GOVERNMENT PRINTING OFFICE: I9794677-026/19 BOUGUER GRAVITY AND SIMPLIFIED GEOLOGIC MAP OF THE SILVER CLIFF AND ROSITA HILLS VOLCANIC AREAS, CUSTER COUNTY, COLORADO Base from U. S. Geological Survey Westcliffe, 1955; Mount Tyndall, 1954; Aldrich Gulch, 1957; and Rosita,1954 TRUE NORTH PROFESSIONAL PAPER 726—F PLATE 2 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY GUL “CAULDRON .fr EXPLANATION QUATERNARY AND TERTIARY ALLUVIUM AND SEDIMENTARY ROCKS ‘ TERTIARY VOLCANIC ROCKS PRECAMBRIAN Y AND X CRYSTALLINE ROCKS VOLCANIC VENT—Site of eruption of local rhyolite and andesite flow or ash m C, BRECCIA PIPE—Developed by explosive venting; generally mineralized as a late stage process BOUNDARY OF MAJOR ERUPTIVE VENT OR VOLCANIC PLUG r . PROBABLE BOUNDARY OF EARLY CRATER COMPLEX AND SUBSI- DENCE AT ROSITA CAULDRON CONTACT FAULT—Dashed where approximately located or concealed; queried where questionable. Bar and ball on downthrown side OUTER RING FAULT MAGNETIC CONTOURS—Showing total intensity magnetic field of the @ Earth in gammas, relative to arbitrary datum. Hachured to indicate closed 0 areas of lower magnetic intensity. Contour interval 50 gammas. Flight elevation 2895 meters WESTCLI GRABE ' / , 2250 Ash 1, . ; _ ,_.. _ (5S MEASURED MAXIMUM OR MINIMUM INTENSITY WITHIN CLOSED f3 , . - ‘ J 7- ‘ H x» .V v, 1:," ,, y , V , _ X HIGH OR CLOSED LOW I \ " ' , ,4: 2' ‘. I L ‘ I I , i ,, , ,, ~ ”w , , FLIGHT PATH—Showmg location and spacing of data é? MINE KGeyser _, vent n MINE SHAFT ,, I , >—~ ADIT " ,;: PROSPECT PIT , ,x, GRAVEL PIT REFERENCES Breccia pipes Sharp, William N., 1978, Geologic map of the Silver Cliff and Rosita volcanic centers, Custer Co., Colorado: US. Geol, Survey Misc. Inv. Series Map I—1081. Geology modified from William N Sharp. Geology of the Rosita volcano modified by Sharp after Siems (19681 COLORADO Postvolcano deposits and volcaniclastic deposits from Scott and Taylor (1975) and Sharp (1978). Aeromagnetic survey flown and compiled by US Geological Survey, SCALE 1:24 000 1 MILE 1970 V2 0 1 KILOMETER I Base from Ur S Geological Survey Westcliffe, 1955; Mount Tyndall, 1954; I Aldrich Gulch, 1957: and Rosita, 1954 E g 1 Z O U E 25 1 .5 O E g 1—1 l——-i l—-—l I—i 1—1 5 CONTOUR INTERVAL 20 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 PUTS. GOVERNMENT PRINTING OFFICE: 1979—677-026/19 APPROXIMATE M EAN DECLINATION, 1980 AEROMAGNETIC AND SIMPLIFIED VOLCANIC AREAS, CUSTER COUNTY, COLORADO GEOLOGIC MAP OF THE SILVER CLIFF AND ROSITA HILLS