“9!) Pa ‘ v‘llé‘f EFFECTS OF COAL MIN GEOLOGICAL SURVEY PROFESSIONAL PAPER 1164’ \ COVER.—Effects of ipast and present coal mining in the western Powder River Basin. as discussed in this report. EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA — FRONTIS‘PIECE.—Aerial oblique views showing surface sub- sidence effects above abandoned coal mines 10—15 km north of -Sheridan, Wyo. A, Aerial oblique view of sub- sidence depressions and pits above the Old Monarch mine in operation from 1904 to 1921 (May 1978). Rectangular depressions, some of which are bounded by pits or locally include pits, are evident at right. Some of the pits are sealed at the bottom and provide sufficient moisture to support trees (foreground). Overburden thickness is estimated to be approximately 10—15 m. The depressions occur where much of the coal is removed and the remaining coal cannot support the weight of the overburden. Pits at the margins of the depressions commonly are caused by piping failure and the local flow of surface water to underground mines via subsidence cracks. B, Subsidence pits and troughs, above the Dietz coal mines (October 1976). The Dietz mines were operated from the 1890’s to the 1920’s. Coal was mined from three different beds. The mine workings, which were abandoned in the early 1920’s, are locally superimposed. The overburden comprises weak claystones, shales, and local thin, soft sandstones. Its thickness is estimated to range from about 5 m along the margins of the subsidence area (right side of photograph) to as much as 45 m (Darton, 1906, p. 111). Pits and troughs located in draws draining into Goose Creek (near background) disrupt or divert surface water to old mine workings. Bighorn Mountains are in far background. A summary of geolog, subsidence, and other effects of past and present mining as related to the environment, coal resource management, and land use Supplemental information is included from Beula/z, N Dale, Decker, Mont, Somerset, Colo., and Raton, N. Mex. UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication data Dunrud, C. Richard. Effects of coal mine subsidence in the Sheridan, Wyoming, area. (Geological Survey Professional Paper 1 164) Bibliography: p. 48 Supt. of Docs.no.: 119.1621164 ‘ 1. Coal mines and mining—Environmental aspects-Wyoming—Sheridan region. 2. Mine subsidences —Wyoming—- Sheridan region. I. Osterwald, Frank W., joint author. II. Title. III. Series: United States Geological Survey Professional Paper 1 1 64. TN805.W8D86 557.35 [333.73] 79-607120 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024—001-032881 CON TE N TS Page Abstract 1 Introduction - 1 Acknowledgments 5 Geology and physiography - 8 Bedrock and coal deposits - 8 Surficial deposits 9 Bedrock structure 9 Geotechnical studies - 12 Surface subsidence effects - 1 7 Sheridan, Wyoming, area -- ‘ - 17 Beulah, North Dakota, area - 20 Subsidence processes - 21 Modern coal mining - 28 Coal mine fires - 3O Seismic activity - a - 41 Environmental consequences of coal mining 41 Hazards in relation to land use 46 Summary and conclusions ----- 47 References cited -- 48 \ ILLUSTRATIONS Page FRONTISPIECE Aerial oblique Views showmg surface subSidence effects above abandoned coal mines FIGURE 1 Map of the Powder River BaSin and adjacent areas -- - 2 Photographs showing present and past mining in the Big Horn mine area -- -- 3 Westward aerial obhque photograph of the west Decker strip mine near Decker, Mont --------------------------------- 4 4 Map showmg the locations of abandoned underground mines and surface mines in the Sheridan Wyo area - - ----- - ----- 7 5 Compos1te geologic section and geotechmcal properties of the bedrock and coal in the Tongue River Member of the Fort Union Formation from two core drill holes near the Big Horn coal mine --------------- 10 6—13. Photographs: 6. Westward aerial oblique View of the highwall at the abandoned Plachek surface mine near the Big orn mine ------------------- - 13 7 Southward aerial obhque View of subSidence depressmns, pits, and cracks above the abandoned room- an -pillar Acme mine - - ------- 14 8 Southeastward aerial View showmg the effects of past and present coal mining along the Tongue River --------- - --------------- 16 9 Ground Views showmg tensile and compresswe features caused by subs1dence ---------------------------------------- 18 10 T ee holes in scil above subsidence crack in bedroc - -- ----- 2O 1 1 Recent subSidence pit above the Old Monarch mine showmg connecting underground caVity ---------------- 21 12 Oblique aerial View of subSidence features located north of Beulah N D ---------------------------------------------- 22 13 Eastward aerial View showmg the effects of hgnite mining near Beulah N Dak -------------------------------------- 23 14 Subs1dence processes caused by mimng different amounts of coal beneath overburden of different strengths and thicknesses 24 ————"7 VI CONTENTS Page FIGURE 15. Maximum subsidence for selected coal mining areas in the United Kingdom and in the Western United States - 16-20. Photographs: 16. Eastward aerial oblique view of the surface effects of an underground coal mine fire in the northern part of the Acme mine 17. Northeastward aerial oblique view of the firepit located above the northern part of the Acme mine ------- 32 18. Southward views of firepit above the northern part of the Acme mine - - 34 19. Closeup views of the advancing fire front in the firepit - 37 20. Firepit above the east-central part of the Acme mine 38 21. Subsidence and fire above the northwest part of New Monarch mine - ----- 39 22—24. Photographs: 22. Effects of subsidence and fire above the New Monarch mine - 40 23. Some pitfalls of filling subsidence pits -- 42 24. Fire in the southern part of the Acme mine -- 44 25. Graph showing the daily number of small earth tremors at a seismometer station located near the firepit in the northern part of the Acme mine - - ------ 45 / METRIC—INCH—POUND EQUIVALENTS Metric unit Multiply by To give inch-pound equivalent / Centimeter (cm) 0.39 Inch (in) Meter (m) 3.28 Feet (ft) Kilometer (km) .62 Mile (mi) Calories per gram (call g) 1.8 British thermal unit per pound (Btu/1b) Meganewton per square 145 Pounds per square inch (psi) meter (MN/m2) Grams per cubic 62.4 Pounds per cubic foot (lb/ft“) centimeter (glcm’) EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA By C. RICHARD DUNRUD and FRANK W. OSTERWALD \ ABSTRACT Analyses of the surface effects of past underground coal mining in the Sheridan, Wyoming, area suggest that underground mining of strippable coal dep051ts may damage coal as they create more cavities, more subsidence, and more cracks and pits through which air can circulate. monly is much greater than that of underground mining pro- cedures. ' operations. This report discusses (l) the geology and surface and underground effects of former large-scale underground coal mining in a 5O—km2 area 5—20 km north of Sheridan, Wyo., (2) INTRODUCTION Thick coal deposits, representing hundreds of billions of tons of low-sulfur subbituminous coal, are present in the Powder River Basin of ground mining. Surface mining operations disturb the total land surface of the mining area during the mining cycle (figs. 2, 3). The topsoil and remaining over- burden commonly are removed by earth-moving scrapers or by large draglines; the coal is then the land is revegetated (figs. 2, 3). In a few years or a few decades, depending on the climate and on HE SHERIDAN, WYOMING, AREA EFFECTS OF COAL MINE SUBSIDENCE IN T 105° ___._— _._—.__ POWDER HARTVILLE 7cUF’UFT I 4 I . r’ I 300 KILOMETERS o 100 200 i o 100 200 MILES EXPLANATION ecambrian crystalline rocks ”TtgrfusgiptaultnSaw teeth show direction Precambrian metamorphic rocks-Dashes show 3 Anticline dominant structure trend; Precambrian rocks in part of the Laramie, Owi Creek, and +Syncline in feet, on top of tern) are undifferentiated / Structure contour, --Datum is mean Hartville upiitts (no pat the Dakota Sandstone Contact AAA-“4"“ High-angle tautt--Rake teeth, where present, sea level. 1 tt=0.3048 m show direction Of dm § Study area-Area shown in figure 4 and adjacent areas. Modified from the tectonic map of the United States (US. Geol. Survey FIGURE 1.—Map of the Powder River Basin and Am. Assoc. Petroleum Geologists, 1961). !_______ INTRODUCTION FIGURE 2.—Present and area. A, Eastward face mine ( about 50 m high a moved, ultimately will be as much as 75 m high in the ac- tive mining cycle. Note the variable thickness in rocks sep- arating the coal beds. Geotechnical tests on core were made from drill holes located in left and middle back- ground of photograph. Haul roads and spoil, in early stages of reclamation and revegetation, can be seen in the foreground. B, View of an underground mine opening of the abandoned Dietz N o. 8 mine (fig. 4) in the Monarch coal bed (one of the beds mined in the Big Horn surface mine (right middle ground of A», December 1971. Only about 2 m of a 17~m~thick coal bed was removed in past under- ground mining. Photograph (July 1972) courtesy of Big Horn Coal Co. ———i 4 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA the physico-chemical and geotechnical properties of the overburden, the land surface normally can be restored to its original use or it can be used for other purposes (Hardaway, 1976, p. 1—76). Underground mining, on the other hand, ap- pears at first to disturb only a small part of the mining area. Mine portals, coal processing and loading facilities, and mine support buildings oc- cupy only a small part of the area underlain by mine workings. However, beneath the surface the mine workings comprise a labyrinth of under- ground entries and rooms that are connected by coal haulageways and airways to mine portals or shafts at the surface (cover). The unmined coal pillars may be left to support the overburden, or FIGURE 3.——Southward aerial View of the west Decker strip mine near Decker, Mont. (May 1978). The overburden is removed by large draglines. The coal, which is about 17 m thick, is removed by power shovels and trucks. The topsoil is removed by earthmoving scrapers and is stockpiled prior to stripping. The spoil is graded, covered with top- soil, and revegetated (green strips). The overburden com- prises soil, colluvium, and soft bedrock of the Tongue River Member of the Fort Union Formation and ranges in they may be extracted near the end of the mining cycle, which causes a local lowering of the ground surface. In modern mining operations, the pillars are extracted where possible, in order to make more efficient use of energy resources. In the western Powder River Basin and in other areas, such as in underground mining areas in the Williston Basin of western North Dakota, studies by the authors revealed that the long-range ef- fects of past underground mining, which include a local lowering of the land surface and attendant deformation, collectively termed subsidence, can cause environmental problems. In addition, coal fires in abandoned mine workings have locally damaged the land surface more severely than thickness from 8 to 37 m. Coal is hauled by trucks along radially oriented haul roads to the unit-train loading facil- ity at left. Production of low-sulfur subbituminous coal from this mine totaled 9.3 million t (metric tons) in 197 7, or about 1.5 percent of the total coal production of the United States in 1976 (based on a total of 609 million t (World Almanac, 1978, p. 97)). Bighorn Mountains and Cloud Peak are in right background. 4—— i ACKNOWLEDGMENTS 5 have surface mining activities on nearby land. Big Horn and Decker now reside in Sheridan and Subsidence continues to be a problem many years other nearby communities. The purpose Of this report is: (1) to discuss and clarification. Subsidence is defined as the local evaluate the subs1dence effects caused by past lowering of the Earth’s surface caused by subsur- coal mining in the Sheridan, Wyo., area in under- face removal or compaction of material. Coal bedrock and surficial deposits 5—60 m thick; (2) to ground surface and all deformation processes compare some 0f the long-range enVironmental ef' within the overburden and at the surface that are fects 0f .the underground and surface .mining produced by the movement of rock and surficial methods in these areas; and (3) to make this infor- material into underground coal mine openings. These investigations are part Of a series 0f duced by flexure of strata, compressive strain engineering geologic StUdieS _0f the western associated with compression arching, shearing Powder River Basm, in connection With the U.S. across lithologic boundaries due to flexure of Geological Survey’s env1ronmental studies of strata, and even upward movements that locally Energy Lands. Detailed subsidence investiga- occur near depressions. The term overburden tions Of a 50—km2 area 5‘20 km north 0f Sheridan, means all earth material that overlies the coal bed Wye. (fig. 4)! were supplemented by a ground and being mined. Coal mine openings include all aerial reconnaissance study (of a 5—km.2 mining underground cavities, entries, or workings area 8—10 km west of Sheridan along Big Goose created as coal is removed. land impacted by underground and surface min- ACKNOWLEDGMENTS ing near Beulah N Dak. Other investigations were made of strip mines in the northern Great The authors’ work was aided by many in- arch, and Dietz coal beds located between about 5 provided drill core for geotechnical testing, maps, and 60 in beneath the ground surface (fig. 4). charts, and other useful information. Valuable in- Underground mining began in about 1892 formation, mine mapping, and assistance in lo- (Kuzara, 1977, p. 55) and ended in 1953. The old eating and identifying old coal mines were pro- mining towns or camps of Acme, Monarch, Kooi, vided by S. A. Kuzara, Ray Bottomly, and W. F. Riverside, Kleenburn (Carneyville), and Dietz Welch. Information on subsidence and coal mine (Higby) are abandoned, demolished, or in the pro- fires was given by G. L. Mooney, Department of cess of being demolished. In 1907 the population Environmental Quality, State of Wyoming; H. F. Riverside were established. About 1,600 men Geologic information on the western Powder were employed at the Dietz, Carney, and Mon- River Basin was provided by W. J. Mapel, S. P. arch mines (Taff, 1909, p. 125). Many of the per- Kanizay, B. E. Law, and B. E. Barnum, U.S. Geo- sonnel employed at the current surface mines at logical Survey. Subsidence information from New — ~.3m. 4 , . fl. fl/mmzswfifi .meF x ,_ .23» zomhmavq .259 / 2mm? _,__ ~ N s _ 39$ ‘ ‘ ‘ in , y , , ,, . , N, k/\: zom RC who” ACKNOWLEDGMENTS £52“me .ooodofiH NAMtram :8. 280 .m.D Sod emum #3: «Exam : :ofiwQ .Ambt msotgw .mew 559:; $5.32 mo Emma wfifiobs mo 83m Gauging 9:3 :80 .Amwumu mach? .856 .3595. mwinm 18%380 .m.D MES—35 98.58 3253 9:. 42TH: d 68 :mwusm .m.D .Amamw ,gmm.m ‘2,va /_z_w§‘.k ,. mm PE. 20 as H .828 _ 8 x. em .5583 mmmem ‘gwmf 5)me W MES ”mix $6.3. _, .. .mwn¢v VmeF WEE OZDOIOIm—DZD 5505...: 9:5 *0 ucwuxw14wZI_ 53.50:: 9523 Umthc 505.: “two 30% mwmwcucwgmmlmDm—m 4 _— r———l CL Coal (Dietz 3); black, brittle, conchoidal fractures 70 70 CL . - . l___————_—- H Sandstonevery tine grainedsfltstone, a i——l CL shaly, argillaceous sandstone lenses; ———-— —— H CL-ML light to brown gray, calcareous to —— CL limy, carbonaceous laminae -——-—— _— H H CL-ML 80 _I H SC ————— —- r—l CL Coal (Monarch), black, brittle, oonchoidai fractures 90 —— H CL lnterbedded argillaceous siltstones, - —— H CL l sandstones, mudstones at bottom; .___ P_’4 CH i very fine to fine-grained sand, light to —. 100 dark gray, calcareous to —_ — ML 100 .1 m non'calcareous, carbonaceous "l E laminae, contorted laminae a H CL 1101‘ g —— l—‘l CL 1 10 ‘b l Coal (Carney); black, brittle,conchoidal ‘ I— i-‘i CL fractures, shale partings l’ ' K Ea— on CL ’ ' 120 CL Siltstone, argillaceous, becoming ___—__—_—_— »——i 120 sandstone,very line to tine-grained —— _— SC GEOLOGY AND PHYSIOGRAPHY 1 l POINT LOAD POINT LOAD ESTIMATED COMP. ESTIMATED COMP, UNCONFINED COMP. a) g STRENGTH STRENGTH Amsomopy STRENGTH STRENGTH STRENGTH 1 a: E I u MN/m2 I 1 MN/m2 I J./I u ””652” RANGE 2i MN/rn2 I5 u S ' s ’ S S MN/m MN/m Lu 2 0.5 1.0 1.5 0.5 1.0 1.5 1 2 3 4 20 4o 20 40 1o 30 50 E 1O 20 3 20 30 8‘0 1 .5 EH 40 § 50 49.0 i 60 30 70 20.0 80 \ 13.5 90 10.0 100 8.1 17.2 ti I? 110 H 120 57'0 H 120 48.0 [4 {3“ 13.0 ——77 12 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA placed by discontinuous faults that commonly , discontinuities is zero, or nearly zero. Therefore, dip steeply to vertically and trend east-north- bedded, jointed, and faulted rock masses derive eastward (B. E. Law, oral commun., 1976). strength only by confining stress and the result- Joints and lineaments in the bedrock are ex— ant friction along the various fractures. This con- pressed as individual fractures on outcrops or by fining stress commonly is greatly reduced or the trends in stream drainage, and are revealed as altered by surface and underground mining ac- lineaments on aerial photographs. Dominant tivities. trends are northwest, east-northeast, west, and Other factors that tend to weaken rock masses, north. Based on studies of the trends of drainages particularly over periods of years or decades, are on aerial photomosaics and faults and joints the effects of weathering, wetting, and drying by mapped by high-resolution photogrammetric fluctuation and movement of ground water and plotters, a close correlation exists among (1) the exposure to air. An example of how rock masses trends of joints and faults in the Fort Union are weakened by the movement of surface and rocks within the Powder River Basin, (2) the ground waters can be seen in surface mining dominant trends of the uplifts, anticlines, and operations in the western Powder River Basin. flexures adjacent to the Powder River Basin, and Highwalls ranging from 7.5 m to more than 30 m (3) the trends of foliation and joints and faults high commonly stand nearly vertical for periods within the Precambrian rocks of the Bighorn of weeks or months in operating surface mines uplift (fig. 1). (figs. 2, 3), but they weaken and fail by rockfalls, landslides, or other mass-gravity movements over GEOTECHNICAL STUDIES periods of years or decades under the effects of alternate wetting, drying, freezing, and thawing The geotechnical and geological properties of of water along fractures and bedding surfaces rock above and below the coal beds are important (fig. 6). factors to consider in planning mining activities, The stable slope angle of fractured and jointed because they control the behavior of the bedrock bedrock on strip mine highwalls ultimately may in response to surface or underground mining and be less than the stable slope angle of a broken- also the behavior of the stripped overburden or up pile of the same rock, because open joints and spoil after restoration of surface-mined lands. The tension fractures behind the rims of highwalls properties of the rocks at the outcrop commonly provide avenues for water to flow, as well as to are so altered from their natural state that freeze and thaw, whereas the broken counterpart geotechnical test results on rock samples from of the bedrock in spoil piles at the angle of repose outcrops can be misleading. Most weak, soft appears to be less permeable and therefore less siltstones, shales, mudstones, and coal in out- susceptible to the effects of water. Graded spoil crops are not preserved well enough to be tested material, however, might absorb surface water and appear much weaker than core samples, readily and fail, unless the graded slopes are de- whereas most outcropping sandstones are ce- signed in accordance with soil engineering prop- mented more completely by near-surface ground erties of the broken-up and mixed overburden water and therefore are stronger at the outcrop material (Terzaghi and Peck, 1967, p. 31—35; than in unaltered bedrock. Therefore, it is often Lambe and Whitman, 1969, p. 33—38). Results of necessary to test drill cores in order to obtain these tests should be considered, in addition to in- realistic geotechnical results (fig. 5). Some geo— place soil properties, potential land use, and exist- technical properties of the coal and rock mass ing climatic conditions, before benching and final also can be determined from calibrated, downhole grading requirements of highwalls and require- geophysical logging that includes gamma densi- ments for grading restored spoil in surface mines ty, caliper, and sonic velocity measurements. are specified. Rock masses commonly are much weaker than As an aid in evaluating the behavior of the Fort strength tests on core samples indicate, because Union bedrock in response to past, present, and bedding planes, joints, and faults weaken the future coal mining, field and laboratory tests rocks greatly. Cohesion across most of these were conducted on cores from two drill holes in 4...— i GEOTECHNICAL STUDIES Westward aerial oblique View of the highwall at the abandoned Plachek surface mine near the Big Horn mine (April 1978). Rockfalls, landslides, and mass wasting caused by wetting, drying, freezing, and thawing may ultimately reduce the highwall slope below the angle of repose of the rocks of Similar lithology and structure when broken up and mixed by surface mining. A coal fire, located in the black area near the west end of the highwall (middle ground at the right), started in about 1975 by spontaneous combustion. Since then, the fire has spread southward and eastward into coal near the base of the highwall. the Big Horn mine area in March 1976 (figs. 2, 4, Few tests were conducted on the coal from the 5). The core, which was made available for testing Big Horn cores, because the coal was broken up by Peter Kiewit Sons’ 00., was stored in unsealed and put in bags before geotechnical testing freezing and thawing. with a black, shiny luster, which break into small Results of the field and laboratory tests on the angular blocks and fragments with hackly to con- Big Horn cores showed that the bedrock was choidal surfaces when mined. The coal, however, weak to very weak, except for local thin lenses of turns a dull~black to gray color and breaks readily dolomite. Characteristically the bedrock might be into small fragments and dust when exposed to classified as overconsolidated muds, clays, and moisture and air. According to E. W- Temple (Big of 1975 (R. A. Farrow, written commun., 1977) re- eXposed to air and water. unless the fine dust is vealed the same thing. removed. Analyses of coal samples, on an as- — *7 14 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA received basis, collected by Taff (1909, p. 135) from the Carney, Monarch, and Dietz coal beds, reveal that the coal is subbituminous in rank with heat values of about 5,000—5,400 cal/g (9,000—9,700 Btu/lb). Sulfur and ash contents ranged from about 0.2 to 0.8 percent and 3 to 6.5 percent by weight, respectively, for the Carney and Monarch coal beds, and from about 0.4 to 1.2 percent and about 4.5 to 8 percent by weight, respectively, for the Dietz coal beds. The field tests, which were conducted in a mobile laboratory, included lithologic identifica- tion, point-load strength index, and slake durabil- FIGURE 7 (above and facing page).——Southward aerial oblique view of subsidence depressions, pits, and cracks above the south part of the abandoned room-and-pillar Acme mine (May 1976). The mine was operated from the early 1900’s to 1943. The location and geometry of subsidence depres- sions and pits correspond to mining areas and to in- dividual mine openings. A, General view of subsidence features (foreground), abandoned strip mine (left back- ground), and currently operating Big Horn surface mine 4.— ity tests (fig. 5). The point-load strength index test (Broch and Franklin, 1972, p. 669-693) is a simple, inexpensive method of estimating the ten- sile and compressive strengths of rocks. The point-load strength index approximates the ten- sile strength of the rocks, whereas an uncon- fined compressive strength can be estimated by multiplying the point-load strength by 15 and 35, based on tests by D’Andrea, Fischer, and Fogel- son (1965, p. 7), Broch and Franklin (1972, p. 690), and on various tests run by the authors. Point- load index strength tests were run parallel and perpendicular to bedding in order to determine along the Tongue River (center and left background). Dragline—dumped spoil piles in old abandoned strip mine are stable at the angle of repose; the vertical highwall in alluvium also is stable. Note that depressions enclosing pits (left middle ground) are rectangular in form near the abandoned strip mine, where overburden is about 12 m thick, to elliptical in form in left foreground, where the overburden is about 25 m. Some pits (left center and right center of photograph) are deeper than the reported i GEOTECHNICAL STUDIES 15 anisotropy or the ratio of strength perpendicular approximately 1 (fig. 5). The slake durability in- to bedding to strength parallel to bedding. The dices of the siltstones, mudstones, and shales, erosion, and transport by streams. Two cycles ranged from 25 to 85 percent in the first cycle and were run on most samples to more accurately 15 to 75 percent in the second cycle (fig. 5). Slake simulate wetting and drying. durability indices, according to Franklin and Characteristically, the point-load strength indi- Chandra (1972, p. 337) of bedrock range from ces of the siltstones ranged from 0.01 to 1 MN/m2 very low to high. perpendicular to bedding, with an anisotropy of 2 Laboratory tests conducted at U.S. Geological to 10; the sandstones, 0.5 to 1 MN/m2 parallel and Survey laboratories in Golden, Colo., included perpendicular to bedding, with an anisotropy of grain-size distribution, density, unconfined com- thickness of coal mined. On cold mornings steam is visible cracks. Pits of markedly different ages are present, rang- above many cracks and pits, indicating underground fires. ing from bowl-like depressions blanketed by green grass, The subsidence and fire that occurred above ground on many years or decades old, to small pits with vertical or January 22, 1979, were located a few meters to the right of overhanging walls, only a few months or years old. Sub- the road along the spoil where the road curves to the right. sidence depressions, pits, and cracks occur sporadically on The grass is greener in depressions and pits where road in left foreground and present problems to stock and moisture accumulates. B, Closer view of subsidence to vehicle travel. if 16 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING. AREA FIGURE 8.——Southeastward aerial view showing the surface and troughs in middle of photograph are in alluvium. Note effects of past and present coal mining along the Tongue that the pits near the road and left of the draw do not occur River (July 1977). Subsidence pits, troughs, depressions, in any noticeable depression; these pits are located above and cracks have formed above the south part of the Acme collapsed areas in the main haulageway or in spur mine. Dam in right foreground across Hidden Water Creek haulageways of the Acme mine, where adjacent coal pillars ruptured due to subsidence. The water now is diverted into are strong enough to support the overburden. Alluvium is subsidence depressions, pits, and cracks upstream from being stripped at the Big Horn surface coal mine to extract the dam. Garbage from the town of Acme was dumped into the coal (background). Grading is beginning near the river the large pit at the left of the photograph. Subsidence pits (right background). pressive strength, and Atterberg limits (fig. 5). were too weak even to be prepared for the uncon- Grain-size and density test results showed that fined compressive strength test; such rocks were the rocks are primarily argillaceous siltstones, treated as strong soils. claystones, shales, and local sandstones with den- Liquid limits and plastic limits were measured sities ranging from about 1.7 to 2.3 g/cm3. The on ground up samples of the core. Plasticity in- density of the coal ranged from 1.1 to 1.3 g/cm3. dices, which are the numerical difference between Results of the unconfined compressive strength liquid limit and plastic limit, provide a basis for tests reveal that, with the exception of the local, the unified soil classification (fig. 5; Lambe and strong dolomite beds and a few other beds, com- Whitman, 1969, p. 33—38). Based on this classifi- pressive strength of the remaining bedrock rang- cation, the siltstones and mudstones contained ed from 10 to 50 MN/mz. Many of the weaker clays of from slight plasticity (ML) to low to rocks, which were tested for point-load strength, medium plasticity (CL), the local shales and 4.— * SURFACE SUBSIDEN CE EFFECTS I 7 claystones beneath the coal beds contained clays of high plasticity (CH), and the sandstones com- prised poorly graded sand-clay mixtures (SC) and sand-silt mixtures (SM). SURFACE SUBSIDENCE EFFECTS SHERIDAN, WYOMING, AREA Surface subsidence effects caused by under- ground mining in the western Powder River Basin comprise local depressions, pits, troughs, tension cracks, and compression bulges (cover; frontispiece A, B; figs. 7, 8). These effects also were observed and recorded in the Illinois coal fields in the early 1900’s by Young (1916) and in the United Kingdom, for example, by Piggott and Eynon (1978, p. 749—780). The depressions overlie a number of rooms in room-and-pillar mining areas, where the remaining coal was not strong enough to support the weight of the overburden either because the coal pillars were partly re- moved or because the initial coal pillars were too small to support the load. In areas where the overburden is less than about 15 m thick, the depressions range in depth from about 30 cm to 2.5 In; in plan view many are square or rec- tangular (frontispiece A) with rounded corners that outline the extent of the former mining areas. In overburden thicker than about 25 m, the corners of the depressions become more rounded, and the general outline of the depressions is more circular or elliptical in plan view. The areas en- compassed by the surface depressions appear to be slightly larger than the mining areas beneath the depressions. Tension cracks occur at the margins of sub- sidence depressions as a result of convex bending and associated stretching of the ground surface (figs. 7, 8, 9A; Dunrud, 1976a, p. 4—5). Compres- sion ridges or bulges occur in the depressions where the ground surface is subjected to concave bending and attendant shortening of the ground surface (fig. QB; Young, 1916, p. 36), although this damage is not as evident as the tension cracks because the soft bedrock and alluvium compress with less visible effect and compression features are not enhanced by erosion. Holes measuring 5 cm to as much as 3 m wide are pre- sent locally in soil and colluvium above tension cracks a few centimeters to a few meters wide that occur in the shale and claystone overburden rocks below the soil and colluvium (fig. 10). In these areas the soil and colluvium stretched without fissuring, as the bedrock cracked in response to tensile stresses along the margins of local subsidence depressions. Consequently, soil and colluvium cover the underlying fissures ex- cept where holes were initiated by local piping or by the activities of man or animals. At the sur- face these holes look very much like the initial pits above individual mine cavities; however, below ground the initial pits commonly widen out into large cavities and are 3—4 m deep. The holes continually enlarge with time under the forces of erosion and small mass gravity movements, until the slope becomes stable or until the cracks are filled with material. Subsidence pits and troughs occur above in- dividual mine openings or above the intersections of mine openings. Field evidence indicates that the pits result from intermittent, sequential col- lapse of the overburden or an upward stoping pro- cess that is initiated by the collapse of mine roofs. In some areas the upward stoping process can be observed in subsidence pits that are adjacent to underground cavities. The roofs of these cavities commonly comprise arches and the floors consist of caved, broken rock or unconsolidated material (fig. 11). The cavities migrate upwards as roofs collapse and the caved material accumulates on the floor of the cavities. Troughs, which occur abovelelongate mine openings or rooms, probably form by the coalescence of individual pits. Col- lapse of mine roofs is governed by the width of mine openings, the strength of mine roofs, and the adequacy of roof support system used. he occurrence of pits at the surface, therefore, depends on the time necessary for mine roofs to fail, on the thickness and strength of overburden materials, and on the width of mine openings. In the Acme area, the overburden consists of weak shales, mudstones, and local sandstones ranging in thickness from about 5 to 60 m. Pits commonly are more abundant in areas where mine openings trend northwest or northeast, parallel to the pre- dominant trends of joints in the bedrock. New pits may form among old pits many years or many decades after initial pit formation (fig. 7). The occurrence of pits at the surface, even where overburden is of constant thickness and strength, ——7 18 EFFECTS OF COAL MINE SUBSI -Ground views showing tens westward view of a large tension crack at the margin of a sub FIGURE 9 (above and facing page). above individual underground mine openings. DENCE IN THE SHERIDAN. WYOMING, AREA ile and compressive features caused by subsidence. A, North- sidence depression. Local pits to the right of the crack occur Snow and water accumulate in the larger cracks, which in turn enlarges the cracks by erosion. B, View of buckled and bulged siltstone and mudstone in the eastern part of the Acme mine above an underground fire. The bu coal mine fires because subsidence amounts are greater where was stained a maroon color by oxi tends to be sporadic because collapse of mine roofs is sporadic. The initial pits caused by sequential collapse of mine roofs commonly are nearly circular holes 1—3 m wide and 2—5 m deep which have vertical to overhanging sidewalls (example of appearance in figs. 10, 14A). The pits widen and the slope angle becomes smaller through the processes of ero- sion, mass wasting, and deposition. In this area, these processes may continue for 10—25 years or more, depending on geologic conditions and geo- technical properties of the overburden. The bases and walls of the pits support range grass, vines, and woody plants a few years or decades after for- lge is about 1.5 m high. The effects of shortening are most pronounced in subsiding ground above more of the coal is removed than in past mining. The bedrock dation of iron caused by heat from the fire below mation of the initial pits, depending on the nature of the near-surface material. The geotechnical properties of the material in which the pits, troughs, and cracks form control the behavior of the rims and walls and their rate of failure. Pits, troughs, and cracks in sandstone or in alluvium, consisting of well-drained sands and gravels commonly exhibit vertical rims and walls for many years or even decades. However, the rims and walls of pits and cracks in poorly drained siltstones, mudstones, claystones, and shales—that contain clays of medium to high plasticity—may slake, slab, slough, or flow read- ily. These failures occur by mass gravity move- * SURFACE SUBSIDEN CE EFFECTS 1 9 ments and mass wasting until pits and cracks attain a saucer shape or flattened trench shape in a few years or decades and begin to support vegetation. Subsidence pits are present in the local depres- sions as well as in areas above room-and-pillar mine workings where no noticeable depression ex- ists—or at least where depressions cannot be de- tected without making precise, periodic surface measurements (figs. 7, 8). Most pits within the depressions are not as deep as pits outside the depressions. The total depth of the depressions and the pits within them is roughly equal to the depths of pits that are present where no notice- able depression exists. On the basis of mine reports, the total depth of both types of these sur- face collapse features commonly is nearly equal to the thickness of coal mined. Locally, however, the total subsidence is greater than the reported thickness of the coal mined. In these areas some of the collapsed material probably spreads, or is transported, laterally into adjacent mine open- ings or compacts more than the original state of the material, aided by the presence of water. Most pits caused by roof collapse occur where the overburden is less than about 10 times the orig- inal height of the mine workings, an upper limit suggested by studies in the United Kingdom by Piggott and Eynon (1978, p. 764—765). However, pits might occur in overburden as much as, or more than, 15 times the height of the mine work- ings in areas where the caved material moves laterally. Pits within the local depressions usually form many years or many decades after the depres- sions, depending on geologic factors and mining conditions previously discussed; therefore, local depressions can serve as warnings to possible future pit collapse within them (figs. 7, 8). However, the elevation of the ground surface ——i 20 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA may not noticeably change prior to pit collapse above mine openings that are bounded by pillars strong enough to support the overburden (figs. 8, 11). In these areas the ground surface may col- lapse suddenly with little or no advance warning unless it is periodically monitored for minute movement. Pits and cracks are only beginning in some parts of the Dietz mine area, where the over- burden is about 45 m thick (Darton, 1906, p. 111) above mines that were worked from the early 1900’s to the early 1920’s (frontispiece B; fig. 4). BEULAH, NORTH DAKOTA, AREA Spectacular surface subsidence features occur above abandoned underground coal mines in western North Dakota, particularly in the Beulah area (fig. 12). The subsidence features look like those in the western Powder River Basin except that subsidence troughs are more common. The overburden is similar in lithology and age to the coal-bearing rocks near Sheridan, Wyo., except that the bedrock appears to be somewhat softer and that glacial deposits—comprising clays, sands, silts, gravels, and local erratic boulders—locally are present in the upper few meters (D.E. Trimble, written commun., 1977). Room-and-pillar mines operated in lignitic coal beds 2.5-9 m thick from 1884 to 1966. The area shown in figure 12 was mined by underground methods from 1918 to 1952 (Parker, 1973, p. 40). The coal bed is about 5 m thick and the over- burden thickness averages about 25 m (Pollard and others, 1972, p. 21). The geometry of the room-and—pillar mine workings is vividly por- trayed by subsidence pits and troughs. As in the Sheridan, Wyo., area, the troughs are believed to overlie elongate rooms and apparently formed by coalescence of individual pits. Much of the lignite in the Beulah area was produced by surface mining in the past; cur- rently lignite is produced only by surface mining (fig. 13). Restoration of surface-mined lands, al- though not done in the past, is now underway (Hardaway, 1976, p. 107 —112) in compliance with the Mined Land Reclamation Law, 1969, which essentially requires that land mined by surface methods be restored in a manner that will minimize adverse economic and aesthetic effects FIGURE 10,—Three holes in soil above a subsidence crack in bedrock. The holes are 30—45 cm in diameter and are located in soil and weathered bedrock approximately 1.2 m thick. Soil may initially bridge the tension crack without rupturing as fissure forms in bedrock below. Later, the holes form, either by natural piping associated with wet- ting and drying or by surface activities of animals. Note grass is greener and more prevalent on either side of crack than along it because crack dewaters soil and retards plant growth. and will maximize the future use of the land (Pollard and others, 1972, p. 7—8). Based on an aerial reconnaissance study by the authors, oper- ations appear to be hindered more by near-sur- face ground water than in similar surface mining operations in the Powder River Basin. Depres- sions may occur on restored land in the Beulah area, however, that could affect drainage because much of the land is nearly flat. There are fewer hills than in the Powder River Basin from which material can be used to compensate for the volume of coal removed (figs. 12, 13). i SUBSIDENCE PROCESSES 21 SUBSIDENCE PROCESSES the previous section. In this section, stresses and deformations are briefly described (1) within and Studies of deformations within underground near the mine workings, (2) in the mine over- coal mine workings and at the surface in the burden, and (3) at the land surface, in order to FIGURE 1 1,—Recent subsidence pit above the Old Monarch mine ( fig. 4) showing connecting underground cavity. Fencepost, about 2 In long, was left dangling in mid-air by collapse. The road from which photograph was taken collapsed in 1974 and had to be filled and graded in order to be used again. The cavity is migrating upward by intermittent collapse (stoping) of the mine roof. Original mine workings are estimated to be about 10 In below the surface. Note that the . . b ——'i 22 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA the coal and rocks near the mine openings than they were before any opening existed in order to bear the stress previously borne by the material removed from the openings. The shapes of the mine openings—in addition to such factors as overburden thickness, lith- ologic and structural character of the coal and associated rocks, and hydrologic conditions—con- trol the state of stress around them. For example, tensile stresses in the roofs and floors of common rectangular or trapezoidal mine! openings, can be nearly as great as the overburden stress, whereas compressive stresses two to three times greater than the overburden stress can occur on the ribs (Schoemaker, 1948, p. 4). However, stresses around elliptical openings, where the long axis of FIGURE 12.—Oblique aerial view of subsidence features located 1.5-6.5 km 11 1976). Pits and troughs occur above elongate rooms in room-and-pillar mine workings in lignitic coal that was mined from the ellipse is oriented in the direction of max- imum stress, are primarily compressive accord- ing to Fenner’s analysis (presented in Schoemak- er, 1948, p. 6-8). Elliptical openings therefore are much more stable than the common rectangular openings because, as shown in the geotechnical properties section (fig. 5), rocks are much stronger in compression than in tension. Elliptical zones of compressive stress com- monly occur around individual mine openings (fig. 14A). They also occur around larger cavities created by extraction of pillars in room-and-pillar mining areas (Dunrud, 1976a, p. 23—30) or by longwall mining unless the elastic limit of the coal or rock is exceeded and viscoelastic or plastic deformation occurs, or the cavities are sufficient- orth of Beulah, N. Dak., looking south (October 1918 to 1952. The overburden averages about 15 m thick and is composed of poorly consolidated claystones, siltstones, and lenticular sandstones overlain by local glacial deposits. The land surface above mine workings is of little use because of subsidence hazards. Wheat farming, the major industry in the area, is hazardous in the old mining areas because the vibra- tions and extra weight of equipment might trigger further collapse. Cultivated areas at right probably overlie areas of unmined coal. ‘— i 23 SUBSIDEN CE PROCESSES FIGURE 13.—Eastward aerial view showing the effects of lignite mining near Beulah, N. Dak. (October 1976). Farmlands used for raising wheat are disrupted by subsidence pits and troughs (middle ground), spoil piles, and highwalls remaining from past underground and strip mining operations (foreground and background). Restoration, including grading and revegeta- tion of spoil, has begun on later surface mining operations (background). Wheat is being grown between two abandoned strip mines in the foreground but not above underground mines (middle and near background). ly wide so that the compression ellipses migrate where height-to-width ratios are near 1 (fig. 14A, to the ground surface (fig. 14A, D). Whittaker enlarged View; 148, C), whereas the rock strata and Pye (1977, p. 306—307) discussed the transfer may only break one to three mining thicknesses of overburden stresses above mine openings to above mine openings with very small height-to- and indicated that the height of the relaxed or downward into mine openings as more or less con- destressed zone is approximately equal to the tinuous units only a few mining heights above width of the mine opening. mine openings; with very small height-to-width ——'7 24 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA FIGURE 14 (above and facing page).—Subsidence processes caused by mining different amounts of coal beneath over- A burden of different strengths and thicknesses. A, Block diagram showing subsidence pits above caved mine open- ings where adjacent coal is sufficiently strong to support plot of tilt, horizontal displacement, curvature, and strain the weight of the overburden (left), and subsidence depres- caused by differential settlement is shown below the sub- sions and subsidence pits where the remaining coal yielded sidence depression on right. «#1 , limit or draw angle; [1‘ , partially to the weight of the overburden (right). An break angle; +, tensile strain; -, compressive strain. B, enlarged view of the elliptical zone of compressive stress Caved opening above an entry in the Riverside mine (fig. 4) around a rectangular mine opening shows buckled and exposed on a surface mine cut (May 1978).Bedrock is soft, heaved floor and caved roof rocks that are common in silty claystone with local thin carbonaceous zones. Entry zones of reduced stress within the zone of compression. is estimated to be about 3 m wide. The original height of Subsidence pits, which are the end result of successive col- the entries reportedly was about 2 m; the current limit lapse of the mine roofs, are deeper where no subsidence of caving is about 9.5 m above the mine floor. C, depression occurs than are pits within the depression. A Caved opening of an entry in the southern part of the SUBSIDEN CE PROCESSES 25 ' (MAXIMUM TILT\_\ I, l / MAXIMUM TENSILE ' / \ STRAIN \ i “W ZERO STRAIN / 'lkAamwwkd l ' ' STRAIN I INFLECTION POINTS 7? _ _ \ / \ \ ~BL/ \ / \ \* ’A i/ \\ \fl¢ CRITICAL —EXTRACTION WIDTH — SUPERCFIITICAL EXTRACTION WIDTH 7 D \ 1‘ Acme mine (fig. 4) (January 1979). Mine entry is about 3 m wide and 3 m high. Bedrock is moderately well cemented sandy siltstone. Note that the caved fragments are much larger and occupy a larger volume at the Acme mine than do the caved fragments at the Riverside mine. D, Block diagram showing subsidence depression above mine open- ing where overburden is thicker than about 60 m or greater than about 10—15 times the thickness of coal mined. Sub- sidence profile shows tilt, tensile and compressive strain, inflection points, and critical and supercritical mining wid- ths above an opening where the coal was completely removed. 26 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA the rock units that are within or above the com- pression zones from those that are the compres- sion zone, in (fig. 14A). Subsidence commonly occurs above these ellip- tical zones of compressive stress because the weaker material, such as the coal, shale, and mudstone within the zone, yields to the increased compressive stresses (Dunrud, 1976a, p. 22-25). Surface subsidence therefore commonly begins above mining areas before the elliptical zones of compressive stress reach the surface. The zones may migrate upward after all mining activities have ceased, because of either time-dependent failure of the rock or coal within the zone of com- pressive stress or failure due to wetting and desiccation of the coal and rock with seasonal and . other cyclical fluctuations of the position of the water table. When the zones of compressive stress reach the surface, the ground surface either settles downward to form subsidence depressions (fig. 14A, D) or it collapses to form pits, depending on the height-to-width ratios of the underground cavities (fig. 14A, left). Pits eventually form at the surface in areas where these ratios are close to 1, unless the overburden is thick enough to contain cavities after they have become filled with caved debris (fig. 143, C). Depressions commonly occur where the height- to—width ratios of the mine cavities are much smaller than 1 (fig. 14A, right; 14D). The area mined necessary to cause maximum surface subsidence is called the critical area (fig. 14D; National Coal Board, 1975, p. 2—3). Critical area is dependent on overburden thickness. The thicker the overburden, the greater the mined area must be for the compres- sion arch to reach the surface and allow max- imum subsidence. Maximum subsidence com- monly occurs when the horizontal dimensions of the mine cavities are greater than 0.8-1.2 times the overburden thickness (fig. 15). An area smaller than the critical area, where compression arches may still be present above the mine cavity, is called subcritical and a mining area larger than critical is called a supercritical area. The areas mined commonly are rectangular but most sub- sidence depressions are rounded (fig. 14A, D), ap- parently because of the formation of an elongate, horizontally oriented elliptical zone of com- pressive stress. Rounding of the corners of a rec- tangular mining panel was observed by Dunrud in western Colorado in 1976 as the roof rocks broke and collapsed behind the roof support jacks in a longwall coal mining operation. S, IN PERCENT OF t 100 O 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 W/d FIGURE 15.—Maximum subsidence for selected coal min- ing areas in the United Kingdom and in the Western United States. Graph shows maximum subsidence (S), in percent of thickness of coal mined (t), versus the ratio of mining panel width (W) to overburden depth (d) (see inset for details) for longwall mines in the United King- dom (upper solid line for backfilled (stowed) mines; lower solid line for caved or strip-packed longwall mines; data from Bell, 1975, p. 116; Shadbolt, 1978, p. 729) and for room-and-pillar mining at Somerset, Colo. (dashed and solid lines through circled data points), in rugged terrain underlain by moderately strong mudstones and sandstones of the Mesaverde Formation of Late Cretaceous age. The dashed curve shows surface subsidence upon completion of mining; the solid curve through circled data points shows sub- sidence amounts when the surface appears to have stabilized; difference between the two curves equals residual subsidence. The points enclosed by the square and diamond are for measurements made by Gentry and Abel (1978, p. 204) on a ridge and adjacent draw above a supercritical longwall panel in the York Canyon coal mine, in rugged topography near Raton, N. Mex., where the overburden consists of moderately strong mudstones and sandstones; the subsidence was 25—30 percent less beneath a draw than it was beneath an ad- jacent ridge (Gentry and Abel, 1978, p. 203—204). é SUBSIDENCE PROCESSES 27 Although complicated by mining and geologic factors, such as adjacent mining areas, mining of vertically superposed coal beds, topography, lithology, and structure, the differential lowering of the overburden strata and the ground surface above subcritical, critical, and supercritical min- ing areas produces within the overburden and at the surface: (1) vertical displacement, (2) horizon- tal displacement, (3) tilt, (4) convex (positive) and concave (negative) curvature, and (5) tensile and compressive strain and local rupture (fig. 14A, D). As vertical displacements occur within sub- sidence depressions, tensile stress and attendant strain occur at the margin of the depression due to positive curvature and reach a maximum at the point of maximum curvature (fig. 14A, D). The tensile strain reaches zero at the point of in- flection, where the ground tilt reaches a max- imum. The stresses and resulting strains become compressive inward from the point of inflection due to negative curvature and reach a maximum at the point of maximum negative curvature. Above critical or supercritical mining areas, negative curvature and tilt decrease inward and are zero where the curvature is zero. Compressive stresses and strains may be greater above sub- critical mining areas than they are above critical or supercritical areas because two zones of com- pressive stress commonly are superimposed. Tensile stresses and strains often are greater above coal barriers between two adjacent mine areas where tensile stresses, caused by positive curvature at the margins of the two adjacent depressions, are superimposed (Dunrud, 1976a, p. 4—5; Shadbolt, 1978, p. 727—728). As mining proceeds, the strata and surface are subjected to all these types of deformation until the mining no longer influences the surface. Shear stresses and strains also are generated along major lithologic boundaries by flexure of strata caused by down- warping of the rock mass into mine cavities. Fur- ther information on deformations produced by subsidence can be found, for example, in Shad- bolt (1978, p. 705—748), in the Subsidence Engineers Handbook of the United Kingdom (N a- tional Coal Board, 1966, 1975), Dunrud (1976a), Peng (1978, p. 281—342), Singh (1979, p. 92—112), Salamon (1978, p. 187—208), and Pottgens (1978, p. 267—282). Subsidence depressions may not occur, or at least may not be evident without the use of pre- cision surveying instruments, above room-and- pillar mine areas where only a small percentage of the coal is removed (figs. 8, 11, 14A, B, C). In these areas pits may eventually form if the over- burden is not thick enough for the cavities to close by bulking of the caved debris. Subsidence pits can occur suddenly and without warning where depressions do not occur. These pits com- monly are deeper than the pits within depres- sions. Pits within subsidence depressions indi- cate that coal pillars or parts of pillars are present in the mine openings below, but that these pillars are not strong enough to support the weight of the overburden completely (figs. 7, 8, 14A). Con- sequently, the overburden and surface subside, but not as much as if all the coal were removed. Pits form later, sometimes decades later, when the zones of compressive stress above individual mine openings or above the intersections of two mine openings migrate to the surface. The total depth of the depressions and pits may be nearly as deep as or sometimes even deeper than the height of the mine workings, because the col- lapsed material may spread laterally into adja- cent mine openings, particularly where water is present in the mine workings to promote it. The surface area affected by subsidence com- monly is greater than the area mined where all or much of the coal is removed (fig. 14D; A, right side), and particularly where the overburden is thicker than about 60 m. The draw angle or limit angle 95 is defined as the angle, referenced to the vertical, made by drawing a line from the margins of the depressions or affected surfaces at the sur- face down to the margins of the mine area causing the subsidence. Studies in abandoned coal mine areas north of Sheridan, Wyo., indicate that the draw or limit angle is steep to nearly vertical where the overburden is less than about 60 m thick. This angle is nearly 0 above barrier pillars between two adjacent mining panels and ranges from 10° to 25° above solid coal barriers in mod- erately strong Upper Cretaceous Mesaverde rocks in the Somerset district, Colorado (Dunrud, 1976a, p. 34). However, the limit angle might range from 0° to 35° in many mines of the West, depending on overburden thickness, mining it 28 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA methods, geology, and topography. In the Lim- burg coal mining area of the Netherlands, the limit angle is approximately 45° (Pottgens, 1978, p. 267), whereas it averages 35° in British coal fields, but may be less in deeper mines, according to Shadbolt (1978, p. 726). The limit or draw angle probably changes attitude with lithology, struc- ture, strength, and depth of overburden. The angle of break, or break angle [3 ——which is the angle of inclination of a line connecting points of maximum tensile stress and strain, from the edge of the mine workings to the surface—is less than the angle of draw. The break line—the line connecting points of maximum tensile stress and strain—normally is not straight, but steepens in thick, strong rocks and flattens in thin, weak strata. In the Sheridan, Wyo., area and in the Beulah, N. Dak., area, the break line appears to be nearly vertical to perhaps slightly negative; that is, dipping steeply away from the mine open- ing (fig. 14A). In the Limburg area of the Netherlands, the break angle is about 20° (Pott- gens, 1978, p. 269). Knowledge of limit angle and break angle is important in land-use planning because these angles determine the limits of deformation. Information regarding the rate and amount of subsidence and attendant surface strain that can be expected for mined-out areas of various widths and heights and for various overburden thick- nesses of more than about 60 m can be found, for example, in Zwartendyk (1971); in the Subsidence Engineers’ Handbook (National Coal Board, 1966, 1975); in Salamon (1978, p. 187-208); Dunrud (1976a, p. 3-36); Brauner (1973a, b); and Pottgens (1978, p. 267—282). However, little or no specific information is available on the rupture limits in tension and compression of various types of bedrock or surficial material in any sources known by the authors. Studies to 1978 in the western Powder River Basin show that the rupture limit of bedrock (limit of strain at which rupture occurs)—consisting of weak, soft silt- stones, claystones, shales, lenticular sandstones, and coal beds—is significantly lower than that of the soil and colluvium overlying the bedrock. Cracks as wide as 15—20 cm were observed in bedrock but were not observed in overlying col- luvium or soil cover except where holes caused by piping failure or the activities of man or animals identified the underlying cracks (fig. 10). ‘— Deformation of the soil and colluvium under tension was observed at the margins of sub- sidence depressions (figs. 7, 8, 9). Cracks were common in soil and colluvium where differential vertical settlement was more than about 0.8 m within a lateral distance of 3—6 m in overburden 9—25 m thick. Cracks were rare in soil and col- luvium at the margins of subsidence depressions where differential vertical settlement was less than about 0.6 m within 3—6 m in horizontal distance. It is not known whether or not the underlying bedrock is cracked beneath the soil and colluvium. MODERN COAL MINING Subsidence caused by modern underground coal mining beneath overburden less than about 60 m thick, or less than about 10-15 times the thickness of coal mined, cannot be evaluated in the Powder River Basin and in western North Dakota, because no underground mines are cur- rently operating in these areas. As far as is known by the authors, nearly all current mining in thin overburden is done by surface methods in the United States as well as in the rest of the world, because it is more feasible from an operational and economic standpoint. The recent Subsidence Engineers’ Handbook from the NOE (National Coal Board of the United Kingdom, 1975, p. 9), for example, does not predict sub- sidence amounts where overburden above long- wall coal mines is less than about 50 m thick. Subsidence information from NCB also does not apply to pillar and stall (room-and—pillar) mining (1975, p. 40) because coal pillars remaining after mining as well as mine roofs and floors may fail for many years due to stress concentrations and (or) wetting of claystones and shales susceptible to deterioration by water. Information on the ef- fects of modern coal mining by underground methods in overburden less than about 60 m thick appears to be rare or nonexistent. In compliance with recently enacted Federal Coal Mining Operating Regulations (Federal Register, 1976, 30 C.F.R. 211, May 17, 1976, p. 20261—20273), underground mining of coal on Federal lands may be required in areas where the overburden is of variable thickness because of topography or structure, is locally less than MODERN COAL MINING 29 about 60 m thick, but commonly is too thick to mine by surface methods. One primary purpose of the Coal Mining Operating Regulations (Federal. Register, 1976, 30 C.F.R. 211.1(b), May 17, 1976, p. 20261) is to: “. . . assure the orderly and efficient prospect- ing, exploration, testing, development, mining, preparation and handling operations, and pro- duction practices without avoidable waste or loss of coal or other mineral resources or damage to coal-bearing or other mineral- bearing formations. . . .” The first part of the section on “Underground mining, maximum recovery” (Federal Register, 1976, 30 C.F.R. 211.30, May 17, 1976, p. 20268) stated that: “Underground mining operations shall be con- ducted so as to yield a maximum recovery of the coal deposits consistent with the protection and use of other natural resources, sound economic practice, and the protection of the en- vironment—land, water, and air.” Accordingly, if appropriate from an environ- mental and resource protection standpoint, mining plans for particular areas may include provisions for mining coal by underground methods beneath overburden less than about 60 m as adjacent coal beneath overburden thicker than about 60 m is mined. In these areas, unless a company planning underground mining operations could negotiate with a surface mining company to mine the coal beneath the thinner overburden or unless the underground company had surface mining equip- ment, the coal beneath overburden less than about 60 m thick might be mined by underground procedures. In order to evaluate the overall effects on the environment and on resources caused by under- ground coal mining beneath thin overburden, future mining sites should be studied to deter- mine such factors as (1) limit or draw angle and break angle, (2) subsidence amount and rate, and (3) local deformation. Studies by the authors (Dunrud and Osterwald, 1978a, b) and others (for example, Gentry and Abel, 1978, p. 202—203; Shadbolt, 1978, p. 729) in areas where the over- burden is thicker than about 60 m show that sub- sidence above modern underground mines com- monly equals 60—90 percent of the thickness of coal mined. In the Limburg coal mining area of the Netherlands, maximum subsidence is as much as 96 percent of the extracted coal thickness (Pottgens, 1978, p. 269). Deformations, in the form of tension cracks, commonly occur at the margins of subsidence depressions, and compres- sion bulges locally occur within the depressions provided that the areas mined are at least as wide and as long as the overburden is thick (critical mining area) and that the maximum amount of coal that can be safely and economically ex- tracted is removed (fig. 15; Dunrud, 1976a, b). Topography, lithology, structure, and amount of water present in overburden rocks control the rate and amount of subsidence as well as the nature of surface deformation. However, as in- dicated previously, the mining thickness, areal extent of mining, mine geometry, and overburden thickness commonly are the dominant controlling factors. Tension cracks commonly are wider, more abundant, and more extensive near cliffs and steep terrain than they are in flat or gently rolling topography (Dunrud, 1976a, p. 12). Results of subsidence measurements above a supercritical longwall mining area in the York Canyon Mine near Raton, N. Mex., where rugged canyon topography is underlain by moderately strong mudstones and sandstones less than about 225 m thick, show that cracks are more extensive and abundant and the horizontal movement was much greater on steep slopes where mining pro- gressed from beneath thicker towards thinner overburden than it was on slopes of similar grade which progressed from beneath thinner towards thicker overburden. In addition, the total amount of subsidence was from 25 to 30 percent greater on ridges than it was in valleys (fig. 15; Gentry and Abel, 1978, p. 203—204). Horizontal surface strain was as much as twice as great as predicted from studies in the United Kingdom. A similar effect of topography on surface crack- ing was observed in the fall of 1977 in the Somerset, Colo., area, where moderately strong sandstones, mudstones, and shales 100—200 m thick underlie very rugged topography. Numer- ous tension cracks, some as much as 50 cm wide, occurred on a steep slope facing in the direction of pillar extraction above a room-and-pillar mining area where coal was mined from beneath a ridge towards a canyon bottom, whereas cracks were 30 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA rare and only a few centimeters wide on a similar slope facing in a direction opposite to the direc- tion of pillar extraction, where mining progressed from the canyon bottom toward the next ridge. In overburden consisting of strong rocks, such as well-cemented sandstones, the draw or limit angle steepens and the total amount of subsi- dence sometimes is less, compared to weaker shales and mudstones (Dunrud, 1976a, p. 5—7). Strong rocks, such as well-cemented sandstones, siltstones, and limestones, may also break into larger fragments than shales or mudstones. Sub- sidence amounts may be less in strong, massive rocks than in weak rocks because of higher bulk- ing factors in caved zones near the mine workings. Structural features, such as joints and faults, commonly localize subsidence movements. Open cracks can occur along joints in well-cemented sandstones and other strong rocks, particularly near cliffs (Dunrud, 1976a, p. 8-12). Subsidence can be localized and concentrated along faults, even in thick overburden. Extensive offset sub- sidence cracks were observed by the authors in Carbon County, Utah, in 1977; they occurred in strong overburden, along fault projections as much as 700 m above the elevation of room-and- pillar mines that were driven in the late 1950’s and early 1960’s. Backfilling mine openings with mine tailings or other available material during mining might reduce the amount of subsidence by about 50 per- cent (fig. 15) and perhaps reduce the rate of subsidence as well, which also commonly would reduce surface damage. This practice, however, is costly, particularly after the mines are aban- doned. Abandoned coal mines that have not yet been affected by subsidence cover some 170,000 hectares (418,000 acres) in the 25 coal-producing States. Costs to backfill these mines have been estimated at $12.5 billion by the US. Bureau of Mines (Johnson and Miller, 1979, p. 9). The amount of water present in, or available to, the overburden from the surface also may control the amount and rate of subsidence. In areas where large amounts of ground water are re- ported to be common in the overburden, such as in the Netherlands (Pottgens, 1978, p. 267) and the United Kingdom (Piggott and Eynon, 1978, p. 752), subsidence amounting to 90—96 percent of the thickness of coal mined is common in supercritical mining areas, whereas in the Somer- set, Colo., and Raton, N. Mex., areas—where small amounts of ground water are present in the overburden—subsidence amounts to about 70 percent of the mining thickness. Rock fragments in caved zones above the mine workings, par- ticularly fragments of weak rocks such as shales and mudstones, may be significantly smaller where submersed in water than in dry or slightly wet zones. COAL MINE FIRES Coal mine fires are common in abandoned mine workings in the Sheridan, Wyo., area. The fires threaten both the environment and adjacent un- mined coal deposits (fig. 16). According to the US. Bureau of Mines (Johnson and Miller, 1979, p. 19), about 250 uncontrolled fires are burning in abandoned underground coal mines in 17 States located primarily in the east and the west, of which about 195 are located in the Western States and approximately 150 in the States of Montana, Colorado, Wyoming, and North Dakota. Studies by the authors show that fires are burning in about a 3-km2 area in parts of at least five abandoned coal mines 10—20 km north of Sheridan, Wyo. Other areas of various under- ground mines may be on fire that have not yet been detected. Most of the fires apparently started in these abandoned mines by spontaneous ignition when oxygen and water were introduced to the mine workings through subsidence cracks and pits and unsealed portals or shafts. Ignition probably oc- curred initially in piles of coal consisting of fragments of various sizes and fine dust that re- mained in the abandoned mines where oxygen and water were available in proportions con- ducive to combustion. Reports published during the past 50 years of laboratory studies-on spon- taneous heating and ignition of coal (Kim, 1977, p. 2—6) reveal that (1) availability and flow of oxy- gen, (2) particle size, (3) rank of coal, (4) changes in moisture content, and (5) other factors, such as temperature, pyrite content, geologic structure, and mining practice, are the prime factors caus- ing spontaneous heating and ignition. Coal rank and changes in moisture content appear to be two of the more important factors. COAL MINE FIRES Field studies indicate that, once the coal ignites, the fire can support combustion and spread by drawing fresh air in through open subsidence cracks and pits and exhausting gases via other cracks and pits. Voids created as the coal is burned produce further ground settlement, addi- tional tension cracks, and more pits, which in turn provide more oxygen to the fire. Noxious gases, smoke, and steam produced by the fires are a major source of air pollution in the area. A reconnaissance sampling of the gases that are exhausted along with smoke and steam % 31 through tension cracks and pits was conducted by the authors in the Acme mine area in April 1976 (fig. 4). Analyses by M. E. Hinkle, US. Geological Survey, and R. L. Kaplan, MSHA, revealed the presence of carbon disulfide, carbon oxysulfide, and an unknown sulfur compound. Methane in excess of 1 percent by volume was detected in a crack near one of the more intense fire areas. The exhausted gases also commonly contained considerably less oxygen and more helium and carbon dioxide than did the normal at- mosphere. Carbon monoxide, ranging from a FIGURE 16.—Eastward aerial oblique View of the surface effects of an underground coal mine fire above northern part of the Acme mine (November 1975). The firepit, shown in greater detail in figures 17 , 18, 19, and 20, is the brown spot in the left center of picture that is exhausting blue smoke and is bounded on the front (west) by plumes of steam. Steam fumaroles can be seen above subsidence pits and cracks throughout the valley. Most of the grass and trees in the foreground were burned when the fire first reached the surface. A coal bed that crops out in the draw about halfway between the road and the firepit (left foreground), will eventually be ignited by the advancing fire unless it is controlled. 32 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA FIGURE 17 (above and facing page).—Northeastward aerial oblique views of the firepit located above the northern part of the Acme mine. A, Position of firepit in November 1975. B, Position of firepit in April 1978; fire has advanced about 40 m dur- ing this period; total length of firepit in 1978 was about 110 111. Note anomalous green grass adjacent to scorched area around firepit. Fire, blue smoke, and steam issue from branching and offset cracks caused by subsidence above the burning coal. Crenulate cracks within and marginal to the pit divert surface runoff and ground water. The firepit area is within a larger subsidence depression that contains numerous old pits and is bounded by tension cracks as much as 3 In wide. trace to as much as 0.35 percent by volume, also was locally detected. Fires in the abandoned underground mines locally have breached to the surface or are close enough to the surface to produce ground temper- atures that melt snow c0ver readily during the winter. Bedrock and surficial material weakened by steam and heat generated by fires in coal mines may collapse into underlying cavities without warning, particularly in areas where (1) coal fires have started recently, (2) the over- burden is less than about 10—15 times the height of the mine workings, and (or) (3) thermal gra- dients have not been established and there are no elevated ground surface temperatures. An underground fire above the northern part of the Acme mine breached to the surface in 1972 and started a large grass fire that also burned a grove of juniper and pinon trees (figs. 4, 17, 18, 19). In the breached area, which is locally called “the firepit,” a vertical column of fire and molten rock 1.5—3 m in diameter and an estimated 15 m high supported combustion in the spring of 1976 by burning other coal beds in the section or by burning the combustible products of the coal bed gasified by the fire, or by both processes (fig. 20). Temperatures in flame-filled cracks and chimneys averaged 925°C, as measured with an optical pyrometer. Sulfur deposits are common along smoking cracks with a strong sulfur odor where COAL MINE FIRES 33 the temperature ranged from 90° to 225°C. Near the firepit, temperatures were as much as 150°C in cracks at the margins of scorched and dying grass. Temperatures in steaming cracks in green grass a few meters from the firepit were measured at about 60°C. Another firepit, which formed above the east- central part of the Acme mine from 1974 to 1977, afforded a good chance to study the processes of firepit formation (figs. 4, 20A, B). Elevated ground temperatures and fumes had begun to kill the vegetation in November of 1974 when the area was first visited by the authors. During 1975 and early 1976 the ground began to sink into an elongate trench as much as 2.5 m deep, 15 m wide, and 25 m long. Temperatures as high as 150°C were measured in cracks 30 cm down. At the collapse site in April 1976, a strong sulfur odor was detected from tension cracks along the margins of the depression. The initial collapse occurred in May 1977 near a large tension crack on the east end of the depres- sion. The resulting pit was a jagged hole measur- ing about 2.5 m long and more than 3 m deep. The pit was encompassed by crenulate, roughly con- centric cracks. Collapse is the culmination of suc- cessive stoping of overburden rocks above cavities; it operates much like the stoping process above mine cavities that are not burning, except that the process probably is accelerated by the continual enlargement of the underground open- ing and by effects of high temperatures on the overburden material. The collapse occurred near a tension crack that had the highest temperatures and produced the most sulfur fumes in the area during 1976. Fire and sulfur fumes were issuing from the jagged hole. The hole may continue to enlarge, now that the initial pit has formed (fig. 203). A coal bed is believed to occur between the abandoned mine 34 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA workings, which are on fire, and the surface. This bed, if present, also is on fire. A 1979 subsidence episode and fire in the north- west corner of the New Monarch mine (figs. 4, 21A, B) yielded considerable detailed eyewitness information about subsidence processes related to mine fires. Subsidence began with a single pit (pit 1, fig. 21A) about 2 m wide and 6 m deep that occurred in 1977 (H. F. Alley, oral commun., 1979). About a year after the hole was discovered, a fence was built around the hole as a safety precaution. Steam was first observed in the pit by H. F. Alley in mid-November 1978. On December 1, 1978, DEQ (Department of En- vironmental Quality), State of Wyoming, re- ceived an air pollution complaint from a local resi- dent concerning large quantities of acrid smoke FIGURE 18 (above and facing page).——Southward views of the firepit above the northern part of the Acme mine. A, Bare ground surrounded by snow 0.25—1 m deep shows that the underground fire is much more extensive than the firepit and steam in the Monarch area (G. L. Mooney, written commun., 1978). Subsequently the fire area was inspected by G. L. Mooney, DEQ, D. L. Donner, USBM (US. Bureau of Mines), and L. Hendrickson, OSM (US. Office of Surface Min- ing) on December 19, 1978 (fig. 21A, B). Steam and smoke were emanating from the initial hole (fig. 21A, pit 1) on the site, which had enlarged to about 14 m in maximum diameter and 9 m deep. About half of the fence had collapsed due to enlargement of the pit. In addition, two new sub- sidence pits were found (fig. 21A, pits 2, 3). Pit 3 was located about 7 m north of a high voltage powerline serving the towns of Ranchester and Dayton and adjacent rural areas (T. P. Ham- mond, Electrical Superintendent, Montana- Dakota Utilities, oral commun., 1979). area (March 197 8). B, The scorched, cracked, and collapsed ground of the firepit area is in stark contrast with the sur- rounding lush green grass (May 1978). Other bare spots can be seen in background where either high ground COAL MINE FIRES 35 On January 2, 1979, an explosion shook the P. L. Vine residence located about 600 m south of the subsidence area (P. L. Vine, oral commun., 1979). Black smoke and orange flames were visi- ble above a 30-m-high ridge located between the subsidence area and the Vine residence. The flames appeared to be coming from pit 2. A second explosion was heard and felt by Mr. Vine about 5 hours later. Apparently, either methane present in the mine had suddenly ignited or steam in the mine workings was suddenly re- leased under pressure. On January 3, another steaming and smoking pit (fig. 21A, pit 4) appeared about 30 m west of pit 3 and 3 m north of the powerline (T. P. Hammond, oral commun., 1979). This pit was oblong in plan, measuring about 4 X 8 m and as temperatures or noxious fumes inhibit growth of vegeta- tion. Blue smoke and fire issue from the firepit and adja- cent branching and offset subsidence cracks. Steam is emitted from cracks about 2 m ahead of the fire front in much as 6 m deep (T. P. Hammond, oral commun., 197 9). At this time, fire could be seen at ground level in pit 2. Flames 10-12 m high were observed by T. P. Wollenzien, geologist, Peter Kiewit Sons’ 00., the evening of January 3 (fig. 22A; T. P. Wollenzien, oral commun., 1979). G. L. Mooney, DEQ, visited the fire area on the afternoon of January 4 in response to another air pollution complaint from a resident living about 6 km southeast of the fire area. Large quantities of smoke and steam were erupting from pit 2, which now was about 7 m in diameter (G. L. Mooney, written commun., 1979; fig. 223). Only small amounts of smoke and steam were coming from pit 1. Steam and smoke also were emanating from pit 3, where no emissions were noted on December 19. At this time, pit 4 was emitting right foreground and also along ridge in background. Craters within the pit are either explosion craters caused by sudden releases of steam and other gases or are local secondary collapse features. 36 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA large amounts of steam and smoke, second in volume only to pit 2. Another pit (pit 5) was ob- served about 15 m north of pit 2. It was esti- mated that the fire was burning in about three rooms of the abandoned mine (fig. 21A). Fire control attempts began on January 6, when a local contractor began filling pit 4 to protect the powerline from fire damage (T. P. Hammond, oral commun., 1979). During the day, personnel walked across the ground surface be- tween pits 3 and 4. The following morning a steaming pit (pit 6) had formed where persons had walked the previous day (fig. 21A, B; T. P. Hammond, oral commun., 197 9). The snow, which was about 0.5 m deep, had not melted nor was there any other indication of impending collapse (fig. 22D). At this time the contractor withdrew his equipment because of the hazard. It was decided to use an emergency fund supplied by OSM to contract for (1) drilling and gamma log- ging to define the mine workings and solid coal and (2) filling the holes after the mine workings and coal pillars were defined. The fire area was first visited by Dunrud on January 9, 1979. Large volumes of steam were emanating from pit 5 with lesser amounts from pits 3, 4, and 6 (fig. 21A, B). A small amount of smoke was present in pit 1; pit 2 was devoid of either smoke or steam. Pit 2 and the north side of pit 4 showed obvious signs of intense heat (fig. 22C). Bedrock and surficial materials in the pit were baked and stained red 1-3 cm inward from the surface of pit 2. The surface was black- ened with soot a few millimeters thick. Sagebrush around pit 2 was burned or scorched. Intercon- necting underground cavities 1—3 m high with arched roofs, located 1—4 m below the ground sur- face, were observed in pits 2, 4, and 6. Pit 4 had ”enlarged southward within the past few hours (fig. 22D). A map prepared by D. L. Donner, G. L. Mooney, and C. R. Dunrud, using information from 10 drill holes and a map of the underground mine workings, revealed that the pits and inter- connecting cavities occurred above north- trending rooms about 7.5 m wide that are sep- arated by pillars 10—20 m long and 7.5 m wide; crosscuts between pillars are 3—5 m wide (fig. 21A). The mine workings, which are 5—6 m high, are in the Monarch coal bed (fig. 213). Results of drilling, gamma-ray logging, and inspection of the pit walls revealed that another 5.5 m of coal, containing five rock partings 50—75 cm thick, oc- curs above the mine workings; this coal in turn is overlain by 10-22 m of silty claystone with thin local siltstone lenses and a coal bed about 1 m thick located approximately 5 m above the base. Surficial deposits, 1—5 m thick, overlie the bedrock. Filling of the pits began again on January 10, using a large bulldozer. By midafternoon, pits 4 and 6 were filled. However, a pit suddenly devel- oped beneath the bulldozer during the filling of pit 3, causing it to tip sideways (fig. 23A). Inspec- tion of the hole beneath the bulldozer revealed that it was connected northward toward pit 2 by an arched underground passageway 1—2 m high, 2-3 m below the surface. The hole was filled using another bulldozer and backhoe (fig. 23B). New pits continued to develop near the loca- tions of the six original pits after repeated filling. On January 11, steaming pits were observed near, or above, all three pits filled the previous day. On January 12 pits were again observed near the south edge of pit 4 and the north edge of pit 6. Rumbling sounds were heard near pit 4. On J an- uary 24 seven new pits, ranging from 1 to 3 m in maximum width and depth, were observed by Mooney and Dunrud near the original pits (fig. 21A). Two more steaming and smoking pits were observed north of pits 3 and 5 on March 1, 1979. In addition, cracks 1—20 cm wide were observed around the original pits 1, 3, 4, and 6. By mid- May three additional pits had developed—one cir- cular pit immediately south of pit 2 and two elongate pits east of the original subsidence area (fig. 23C). A subsidenc‘e pit and fire also occurred above the south part of the abandoned Acme under- ground coal mine about 35 m west of a highwall of the Big Horn surface mine on the night of January 22, 1979 (figs. 4, 7A, 24). Flames were visible about 10 m above ground level for a few hours. Large volumes of steam and yellowish- gray smoke emanated from a rapidly enlarging pit on January 24 and 25 when Dunrud Visited the site (fig. 24A). The diameter of the pit en- larged from about 2.5 to 5 m and the volume of steam and smoke tripled or quadrupled in about 20 hours. Acrid steam and smoke filled the fire column is hot enough to melt the bedrock. Tongue River Valley for a few kilometers upstream and downstream from the fire until the pit was filled on January 25. In contrast to the fire above the New Monarch mine, the fire above the Acme mine produced flames above ground within minutes after the pit formed, perhaps because large quantities of air were available to the mine workings from mine openings exposed on the highwall of the nearby surface mine (fig. 243). Like the fire above the New Monarch mine, large volumes of foul- smelling steam and smoke were emitted from the pit after the fire subsided below ground level. Steam was present in greater proportions at the Acme site than at the Monarch site, perhaps because more water was available. The large quantities of steam probably contributed to the rapid enlargement of the pit. Fires caused by spontaneous combustion also locally occur on highwalls of abandoned surface mines or on exposed coal in active surface mines (Hertzberg, 1978, p. 47—49) in the Powder River Basin. The coal outcropping at water level in the abandoned Plachek surface mine near the cur- rently active Big Horn mine caught fire, ap- parently by spontaneous combustion, in the early 1970’s (fig. 6). During 1975 and 1976 the fire had spread along the outcrop and was beginning to COAL MINE FIRES 39 A 0 1 O 20 30 METERS Z EXPLANATION Caved bedrock, coal, and surflcial material; tire where shaded Surficial material; weathered bedrock and soil 0 20 4O METERS Claystone. soft, silty, with local lenses of soft siltstone l I .__ —— Coal, Monarch coal bed overlain by coal and rock interbeds EXPLANATION mun $ Power line-~Montana-Dakota Utilities, 46,000 volts FIGURE 21.—Subsidence and fire above northwest part of © New Subsidence 9“ mapped 3‘1'79 New Monarch mine. A, Idealized block diagram showing fl Subsidence pit-Dashed hachured lines where filled with subsidence pits, lithology of overburden, and underground ‘4 earth material by 1-17-79; small ellipses, new pit v . . mine workings in the fire area on January 9, 1979. B, Map \0/ mapped 1‘24‘79' “ad‘s “WM “"5' mapped 3‘1‘79 showing relation of room-and-pillar mine workings to sub- Q" Coal pillar-Dashed line where probably burned, solid sidence pits, filled pits, new pits, and estimated fire area ""9 Where “mar “what?” '“ ”'8“ as °f January 1979 (outlined by heavy dashed line), January 1979. Compiled . r-«e, Coal pillar, mined by 1953 by D. L. Donner, USBM, G. L. Mooney, DEQ, and C. R. ‘8‘ . . . . . Dunrud, based on taped traverses, drilling, geophysical 0 Drill hole, drilled 1-(8-9l-79, v0ld encountered ln coal logging, and inspection of pits. Voids 0.6 m high were en- '8 countered in drill holes 6 and 7; a void 6 m high and fire @ were encountered in drill hole 4. Drill hole; drilled 1-(8-9l-79, no void encountered Subsidence pit as old as or older than pit 1 <1 FIGURE 20.—Firepit above the east-central part of the Acme mine (May 1977). Broken, cracked, and locally blackened ground and scorched grass provide a stark contrast to the lush green grass surrounding this new firepit. A, Eastward View across firepit area. Meadowlands adjacent to the Tongue River are in the background. B, Southeastward view showing the jagged pit, which is bounded by crenulate, roughly concentric cracks. The depression (mid- dle) is about 2.5 m deep. The pit, which is more than 3 m deep, contains fire and hot rocks, although they are not visible in picture because of the sunny day. Temperatures in the pit were measured at about 850°C with an optical pyrometer. —’—' 40 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA FIGURE 22.—Effects of subsidence and fire above the New Monarch mine (January 1979). A, Steam and smoke (left) and fire (right) erupting from pits 1 and 2, respectively, at 7:00 p.m. on January 3 (fig. 21A). Flames are about 10 m high. Powerline is visible in foreground. Photograph cour- tesy of T. P. Wollenzien, geologist, Peter Kiewit Sons‘ Co. B, Eastward View showing column of smoke and steam erupting from pit 2 (fig. 21A) on January 4, 1979. Base of column is about 7 m in diameter. Photograph courtesy of G. L. Mooney, DEQ, State of Wyoming. C, Northward View showing pits 1 (left middle ground), 2 (middle ground), and 4 (foreground). Note areas free of snow near pits 2 and 4, where heat from fire melted snow; fire also blackened the rim of pit 2 and the north edge of pit 4. D, Southward View of pit 4 showing melted area (foreground), overhanging rim, and freshly caved snow and soil near steaming area. Note that the snow shows no sign of melting at cave line in background on either side of steam. fi ENVIRONMENTAL CONSEQUENCES OF COAL MINING 41 advance underground. Fires such as this can sup- port combustion underground for considerable distances by intaking air and exhausting gases via tension cracks caused by collapse of the over- burden into voids left when the coal burned. Steps should be taken to insure that adequate amounts of noncombustible material protects the coal from possible ignition during abandonment and restoration procedures in any future surface mining activities. Fires also are a serious problem in currently active underground mines. Mine deformation studies in coal mines in Utah and Colorado by the authors (Dunrud and Osterwald, 1978b, p. 58) show that fires are common. The fires apparently start by spontaneous ignition, particularly in areas where stresses are high and heat is pro- duced by resulting deformation of the coal and rock. The areas continue to burn even years after they have been sealed. Steps should therefore be taken to control fires during operation and aban- donment of modern underground mines. SEISMIC ACTIVITY Small earth tremors commonly are generated by caving and stress readjustments in over- burden rocks above underground mine workings and coal fires in the Sheridan, Wyo., area. The authors and other personnel of the US. Geologi- cal Survey monitored seismic activity above the Acme mine with a temporary seismic network between September 30 and November 4, 1975. The network consisted of 10 vertically oriented seismometers with a natural frequency of 1 Hz, which were connected by wires to a mobile recor- ding laboratory. Ground motion velocity was recorded both on magnetic tape and on visual seismograms. Magnification of the system, with respect to ground-motion velocity, was about 200,000 at 10 Hz. One seismometer was installed within about 185 m of the firepit that breached to the surface in 1972 (figs. 16, 17, 18, 19) during the first half of the recording period and within 30 m of the firepit during the last half of the period. In addition to earth tremors generated by blasting in the Big Horn and Decker surface mines (figs. 2, 3), 20—90 small earth tremors were recorded per day by the seismometer when it was located about 185 m from the firepit, whereas 550—800 small earth tremors per day were re- corded when the seismometer was moved to within about 30 m of the firepit (fig. 25). Later seismic studies, during a 6-week period in the fall of 1976 above the Acme mine, showed a large in- crease in the seismic activity, which indicated that the fire was accelerating at a rapid rate. At the site of the firepit, as well as in other areas of the Acme mine, most of the tremors ap- peared to be caused by breaking and caving of overburden rocks above and adjacent to areas where fires are burning intensely and perhaps have ignited overlying coal deposits. Another source of local seismic activity might be small underground explosions caused by steam sudden- ly released under high pressure. A seismic network with audio recording capa- bility and six vertical seismic stations linked to the mobile recording laboratory was installed above the fire area in the New Monarch mine on March 16, 1979. Preliminary results of the audio monitoring reveal thumping, rumbling, and hiss- ing noises that sound like breaking and caving of the overburden and also movement and sudden releases of steam and air within the mine cavities. ENVIRONMENTAL CONSEQUENCES OF COAL MINING Adverse effects on the environment caused by underground mining or surface mining, in addi- tion to potential hazards to life and property and effects on population growth, commonly com- prise (1) disruption of the Earth’s surface, bedrock, or other mineral deposits; (2) diversion or pollution of surface or underground water; and (3) spontaneous ignition of underground fires with the attendant land disturbance, water and air pollution, and surface fires. Past mining by both procedures has produced all of these effects in the Powder River Basin. However, in all cases known to the authors, the surface mining activi- ties have produced less severe and shorter term problems than past underground mining. This is primarily because fire hazards are reduced by the extraction of most of the coal in the surface min- ing areas (compared to only a small percentage of the coal in the underground mining areas), and because problems such as damage to the surface, $£=:S“F“"" .: V 2.335, a”: v m, a 2&2“ :, 2: av .. 32;.» £33 ’” v mg? “ mace: , .2 '3 “'5- ‘55 $2, . ,2: f2 «‘2 .— «35.3 22.2., 5:222" x. WYOMING, AREA ., . x 4». «We...» Jam 8 ..:,..m» imam $3.3m. fa M3392“ fimmmwhmfim $5.. 5‘.- 2 «3525,22,. 2:32; a: «2-2-2 2 Y5 . a: a 1: “3:4... 2.2 .x‘ :2.’ ,3: «.‘f ,. z :2 2v. "5.2.23; , , . . . 2mm 2‘ saw , . um“ up. maanmmumwmx . fimfimg 2 :muwuumu: , 2”.” .v nummfinfiwmmmwx gm ,. 2...“? 3. 'f 1233, EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, "mmmm ”mu... 42 ENVIRONMENTAL CONSEQUENCES OF COAL MINING 43 FIGURE 23 (facing page and above).—Some pitfalls of filling subsidence pits. A, Track of large bulldozer (D—9 Cater- pillar) fell into pit, which suddenly developed above an underground cavity (January 1979). B, Cavity beneath bulldozer initially was as much as 4 In deep. water diversion, and pollution are open to view and can be examined, assessed, and corrected. In the underground mines, however, these problems may be hidden for many years or even many dec- ades after mining is completed (figs. 2A, 7—11, 16—24). Modern surface mining activities in the Powder River Basin include well-planned sequential removal of topsoil, bedrock, and coal followed by replacement of spoil, topsoil, and revegetation (figs. 2, 3). Thus the land can be restored and put back into use in a few years or decades. Unstable conditions, locally common at the sites of older surface mines (fig. 6), and possible unstable conditions in restored spoil can be minimized Backfilling operations beneath track took about an hour (January 1979). C, Southward aerial view of the subsidence and fire area (May 1979). Cracks and pits occur near the pits that were filled in January. Also new elongate pits occur to the east. by grading and compaction in accordance with design specifications based on detailed, on- site geologic and geotechnical investigations of the bedrock and of broken and mixed rock in mine spoil. The fire hazard can be minimized by assuring that all coal outcrops and other remain- ing piles of coal are adequately blanketed by non- combustible material. Surface mine operators, therefore, can have good control of restoration procedures and can minimize long-range environ- mental damage if mining activities are properly planned and implemented from the initial cut to the final restoration of the highwall. Many of these provisions and others are part of the Coal Mining Operating Regulations (Federal 44 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA Register, 1976, 30 C.F.R. 211.1 to 211.4, May 17, 1976, p. 20261—20264), in the National Environ- mental Policy Act of 1969 (Public Law 91—190, 42 U.S.C. 4321—4347, Jan. 1, 1970), as amended (Public Law 94—83, Aug. 9, 1975), and the Surface Mining Control and Reclamation Act of 1977 (Public Law 95—87) as implemented by the Sur- face Mining Reclamation and Enforcement Provi- sions (Federal Register, 1977, Part II, 30 C.F.R. 700—725, December 13, 1977, p. 62639—62712). Changes, diversion, and pollution of water are under study by other governmental agencies, but it appears to the authors that these effects can be minimized by proper planning based on on-site studies of geologic and hydrologic conditions. In any case, these effects are, in the opinion of the authors, less damaging than those often caused by modern underground mining of thick beds beneath thin overburden, wherein surface and ground-water flows may be changed and (or) diverted by surface depressions, tension cracks, and subsidence pits or troughs, and they may be locally contaminated by garbage and other refuse being placed in subsidence pits or troughs. Depressions resulting from extraction of thick coal beds by surface methods in the Powder River FIGURE 24.—Fire in the southern part of the Acme mine (January 1979). A, Southward View of the fire area show- ing emission of a large column of steam and yellowish-gray smoke from a newly formed pit. Maximum height of spoil pile (background) is about 15 In. B, Westward View of highwall in Big Horn mine, located about 35 m east of steaming pit. Height of highwall is about 15 m. Two en- tries of the Acme mine (fig. 4) are exposed; the roof of the smaller one on the left (fig. 140) has begun to collapse, whereas the wider one on the right still is intact. Basin can perhaps be eliminated or minimized, in areas where topographic conditions and owner- ship or lease areas are amenable, by borrowing material from adjacent hills to compensate for the volume of coal removed, as is currently planned by Big Horn Coal Co. (figs. 2, 7, 8). Environmental hazards caused by past under- ground mining in the Sheridan, Wyo., area in- clude subsidence of the ground surface, diversion of surface and ground water, coal mine fires and attendant surface subsidence, and pollution of the air and water (frontispiece A, B; figs. 7, 8, 16—20). Grasses and plants commonly die or are stunted near subsidence cracks, bulges, and along the margins of subsidence pits where the root systems are dewatered. Subsided lands are of limited use, and commonly are not useful for livestock grazing because stock can fall into pits and cracks (fig. 4). Also the noxious fumes emanating from cracks above burning coal mines have killed trapped cattle, in some cases, after ex- posure of less than an hour (Dan Scott, Padlock Ranch, Dayton, Wyo., oral commun., 1976). Grass fires and forest fires also have been started when underground coal fires reached the surface. In 1972, a grass fire was ignited when a ENVIRONMENTAL CONSEQUENCES OF COAL MINING 45 firepit in the northern part of the Acme mine first breached to the surface (figs. 16—20). The fire burned about 1.5 km2 of winter range grass and trees before it was brought under control. All these environmental hazards limit the value of the land above these abandoned mine workings for agricultural, residential, or industrial develop- ment. Damage is compounded when the value of the lost coal is added to the damage done to the surface and to any overlying coal beds that often catch fire. Periodic geologic and seismic studies of the Acme and New Monarch mine areas show that the fires are increasing in size and are spreading into unmined coal. It is estimated that at least five different fires are burning over a 3—km2 area (fig. 4). Other fires, such as reported in the Black Diamond mine in 1911 (Kuzara, 1977, p. 215—216), may still be burning although there presently is 800 700£ 600 I I 500 400 300 200 NUMBER OF SEISMIC EVENTS 100 29 1 5 1o 15 20 25 30 1 OCTOBER 1975 TIME(DAYS) FIGURE 25.——Graph showing the daily number of small earth tremors at a seismometer station located near the firepit in the northern part of the Acme mine. The station was located about 185 In from the firepit from September 29 to October 19, 1975, then was moved to within about 30 m of the firepit. Note the approximate tenfold increase in number of events when the station was moved closer to the firepit. The earth tremors apparently are caused by the breaking and collapse of overburden material and perhaps by small underground explosions. no surface evidence of a fire. Unmined coal beds of economic value locally overlie mine areas that are on fire, and they are threatened by the spreading fires below (figs. 4, 16—24). Seismic studies in the Acme and New Monarch mine areas also indicate that collapse and possibly underground explosions are increasing rapidly in these areas. Fires of this magnitude, in the opin- ion of the writers, may best be controlled by con- structing a strip mine firebreak, isolation barrier, or isolation trench (Johnson and Miller, 1979, p. 18—21) around the burning area in order to cut off the supply of coal available to the fire. Care- fully planned strip mining of the coal around fire areas, using mining procedures that protect per- sonnel from subsidence and fire hazards, may be the best method to control fires in underground mines less than about 60 m, or about 10—15 times the coal thickness, below the surface and also might be economic in areas where large amounts of coal remain in place. Many local ranchers, coal operators, and others also favor timely strip min- ing around and within fire areas as the most effec- tive control measure (figs. 16—24). Water introduced from the surface seems only to increase the intensity of fires in this sub- bituminous coal. Other methods of control, such as hydraulically backfilling a slurry of retardant material through drill holes to the fire areas or operations to seal the fire, as done by the US. Bureau of Mines in several areas (for example, Whaite and Allen, 1975, appendix by Carlson), probably would not be cost effective on a fire the size of the Acme mine fire. Moreover, it would not recover any remaining coal. Studies of coal mine fires suggest that the potential fire hazard also should be considered in planning in-place coal gasification or oil shale retorting activities. It would seem that advanc- ing fire fronts Can only be controlled as long as the supply of oxygen and withdrawal of gases are under controlled conditions. The presence of underground voids could create subsidence and cracks in the overburden that might allow the fire to intake air and exhaust gases independently from the designed system which, in turn, could result in out-of-control fires. The key factor is to control subsidence by either backfilling or de- signing the gasification or retorting operation 46 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA such that the cavities are small enough and the overburden is deep enough to provide a stable overburden above stable rubble-filled cavities (fig. 14A). (See Shoemaker and others, 1979, p. 140—153, for examples of coal gasification model studies.) HAZARDS IN RELATION TO LAND USE The hazards to people, animals, and structures caused by coal mining activities can significantly affect future uses of the land because the severity of subsidence hazards may govern how mined lands are developed. Two of the basic hazards caused by coal mining—subsidence and land- slides—also are identified as geologic hazards in “Warning and preparedness for geologic-related hazards, April 12, 1977” (Federal Register, 1977a, Proposed procedures, p. 19292—19296). A geologic hazard is defined in this document as: “. . . a geologic condition, process, or potential event that poses a threat to the health, safety, or welfare of a group of citizens or to the func- tions or economy of a community or larger governmental entity . . .” (p. 19292). Coal mine subsidence hazards to people include the sudden collapse of the ground surface above mine openings (figs. 7, 8, 11, 12, 13), above ten- sion cracks bridged by soil (figs. 9, 10), or collapse above cavities created by underground coal fires (figs. 16—20). Subsidence also can occur in spoil of surface mines, particularly in areas where in- creased surface loading and (or) rising water tables cause a reduction of porosity (Charles and others, 1978, p. 229—251). The greatest hazard to life and property in the Sheridan, Wyo., area is the sudden collapse of the ground surface into underground fires. In addition to ground col- lapse, the effects of tension, compression, and tilt due to the formation of depressions can damage structures beyond repair. Larger structures with small tolerances for deformation—such as multi- story apartment buildings (Thorburn and Reid, 1978, p. 87—99), schools (Stephenson and Aughen- baugh, 1978, p. 100—118), factories, and powerplants—commonly will sustain the greatest damage unless they are designed to withstand the stresses and deformations caused by sub- sidence. Landslides, which include rockfalls, slides, slumps, and earthflows on surface mine highwalls or reclaimed spoil material, can be a hazard to people and equipment during mining operations and also to people and structures after restora- tion, abandonment, and subsequent development of the mined land, unless proper grading, compac- tion, and vegetation procedures are followed. Small landslides on subsidence pits and cracks could damage adjacent structures or perhaps be a hazard to people and animals in the area. Land- slides also may be triggered by subsidence on unstable slopes or, conversely, the increased sur- face loading from landslides above unstable mine openings may cause further subsidence. Subsidence pits and cracks caused by under- ground mining and underground fires currently are a hazard to only a few persons traveling in the area and to animals. However, should lands underlain by underground mines be developed for either residential or industrial use, a hazard to many people and structures could exist in much of the area above abandoned underground mines (fig. 4). Abandoned coal mines with surface subsi- dence features are located in other areas in the western Powder River Basin that are not in- cluded in figure 4. Maps showing such aspects as present subsidence areas, fire areas, areas underlain by coal mines, thickness of overburden, and thickness of coal mined are needed to more precisely delineate current and potential hazard areas. (See also Ivey, 1978, p. 163—174.) Struc- tures for residential or industrial use should not be sited above abandoned mines unless existing underground voids are located by systematically compiling appropriate subsidence and land-use maps and by conducting on-site engineering geo- logic studies, by drilling and geophysical studies where needed (Ivey, 1978, p. 163-174), or by other methods, and stabilized by such procedures as backfilling or grouting. Surface structures also might be designed to withstand the effects of subsidence in lieu of stabilization procedures. (For examples see National Coal Board, 1975, p. 64-95; Bell, 1978, p. 562—578; Johnson and SUMMARY AND CONCLUSIONS 47 Miller, 1979; Geddes, 1978a, b; Shadbolt, 1978, p. 739—744; and Wood and others. 1978. SUMMARY AND CONCLUSIONS Subsidence studies in the western Powder River Basin and in western North Dakota indi- cate that, in overburden less than about 60 m thick or less than about 10-15 times the thick- ness of coal mined, the land surface can be re- claimed and returned to its original use, or to some other use, more quickly, easily, and cheaply if thick coal beds are mined by surface methods than if they are mined by underground methods, provided that proper restoration procedures are followed. Subsidence above room-and-pillar work- ings driven 25—80 years ago is still hazardous to man, animals, and the environment, and may con- tinue to be a hazard for many years or many decades to come. Studies of subsidence above cur- rent underground coal mines indicate that subsi: dence effects—such as depressions, cracks, and bulges—would be caused by modern under- ground mining procedures, where the amounts of coal reserves extracted are acceptable in terms of economics, conservation, and hazard reduction. Depressions and cracks also disrupt the normal flow of surface and ground water. Some subsidence pits, particularly in areas where the most dramatic and potentially hazard- ous collapses occur, may be deeper than the orig- inal height of the mine openings, because ( 1) the material spreads or is transported laterally into adjacent mine openings as it collapses, particu- larly in water-filled mine openings, (2) it compacts more than the state of the original material due to wetting and drying, or (3) it is subjected to both of these processes. Coal recovery from thick coal beds in the weak rocks of the Tongue River Member of the Fort Union Formation is much greater, and long-range environmental problems are smaller, when modern surface mining procedures are employed rather than underground room-and-pillar mining, where the overburden is less than about 60 m thick, or where the overburden thickness is less than about 10—15 times the mining thickness. In View of current coal mining technology and the subsidence effects from modern underground coal mining, surface mining may be the best way to produce coal in any area where coal thickness and overburden conditions make the operation economical, provided that proper restoration pro- cedures are followed. Coal mine fires, which often start by spon- taneous ignition in the subbituminous and lignitic coal fields of Montana, Wyoming, and North Dakota, increase the damage to the en- vironment manyfold compared to normal sub- sidence damage. The fires also consume large amounts of valuable energy resources as they create unsightly and hazardous steaming and smoking cracks and firepits, and pollute the air and water. Strip mine firebreaks appear to be the most effective way to control large underground fires beneath thin overburden, such as the large fires in the Acme and New Monarch mines. Most of the fires start spontaneously with the entry of oxygen and water through subsidence cracks. Once started, the fire spreads by creating larger cavities, increased subsidence, and additional cracks to intake oxygen. Careful consideration should therefore be given to subsidence and possible cracking in planned in-place gasification or retorting of combustible hydrocarbons such as oil shale. Results of this report reveal a paradox. The land devoid of subsidence features, and therefore the most desirable for development, may be the most hazardous to develop if the area (1) is underlain by room-and-pillar mine workings, (2) the overburden depth is less than about 60 m thick or about 10—15 times the original height of the mine workings, and (3) sufficient coal remains underground adjacent to unstable mine openings to support, or partially support, the overburden. In these areas the land surface may be unaffected by subsidence for many years, or even hundreds of years, depending on the strength of the rock and the hydrologic conditions in the overburden. However, when and if surface collapse occurs as a result of successive collapse above the mine open- ings, the resulting pits can be much more of a threat to people, animals, and structures than the normal subsidence depressions and boundary cracks that commonly occur above mine work- 48 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA ings where all, or nearly all, the coal was removed. In the opinion of the writers and others (for ex- ample, Ivey, 1978, p. 174), guidelines are needed to assure that all available mining and mine sub- sidence information is systematically assembled, evaluated, and made available to land-use plan- ners before mined lands are developed for residen- tial or industrial use or other uses involving the public welfare. REFERENCES CITED Barnum, B. E., 1974, Preliminary geologic map and coal resources of the Ranchester quadrangle, Sheridan County, Wyoming and Big Horn County, Montana: US. Geological Survey Open-File Report 7 4—35, 4 p., 2 sheets. 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LL,, -,L«LLL..-,-L,ELLL »L7 L»LLLLL.,.EL.L.LLL,,ELLLLLL .LLL L.,,LE .L.L,, L.LLL ,. m.» ,L ,.L M ,L.. .LLLL.LLL4 LLLLL. -L LLLL x L L a ELL -. E -» E-ELLL ELL LEM LLL .,,L, ..,-,'L ,L77L ELL,L,E.ELL, ,.L, ,,L E ,L.. L. LLL L,L,,LLL , LL,.L ,.,LL , 7L? E L? EL ‘2‘ LE .L.L-L7... LL.LL... .. 7.L..- ELL MEL: WW» Differentiation of a Gabbro Sill in the Oregon Coast Range by Crystallization-Zone Settling By NORMAN s. MacLEOD GEOLOGICAL SURVEY PROFESSIONAL PAPER 1165 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1981 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Doyle G. Frederick, Acting Director Library of Congress catalog-card No. 81-6000M9 For sale by the Superintendent of Documents, US. Government Priming Office Washington, DC. 20402 CONTENTS _ Page Abstract ........ Order of crystallization ........................................................................ 8 Introduction ..... Petrochemistry ........... . . 8 Geologic setting ....... Composition of the chilled border ............................................ 8 General features ........ . Composition of rocks from the interior of the sill 9 Petrography .. ........ . . . 3 Compositional variations with height ............................................ 9 Chilled basalt ............... 3 Bulk composition of the sill ................................................. lO Gabbro 3 Residual liquid compositions ..................... 12 Ferrogranophyre ............. . 3 Chemical variation .............................................. l4 Pegmatite _____ ' ............. 6 Process of differentiation ..................................................... 15 Granophyre ........................................................................................ 6 Summary ___________________________________________________ 21 Mineralogical data _. i .. 6 References cited ...................................................................................... 22 Textural relations ...................................................................................... 7 ILLUSTRATIONS Page FIGURE 1. Map showing distribution of Oligocene gabbro sills in Euchre Mountain and Valsetz quadrangles, Oregon .................. 2 2. Diagrammatic section showing distribution of rock types in gabbro sill ________________________________________________________________________________________ 3 3-12. Diagrams showing: 3. Composition of pyroxene from Stott sill ________________________________________________________________________________________________________________________________________________ 7 4. Vertical variation in modes, mineral compositions, major oxides, and oxide ratios in lower gabbro zone ..... __ 10 5. Amount of major oxides in rocks and bulk residual liquids at various stages in solidification of gabbro sill , 14 6. Silica variation for rocks from gabbro sill ____________ . ......... 15 7. FMA variation ____________ ........................... 15 8. Progressive solidification of gabbro sill _____________________________________________________________________________________________ l7 9. Inferred crystallization and settling of mineral in lower zone of crystallization _ 18 10. Calculated settling rates of minerals _____________________________________________________________________________________ . 18 11. Proportion of liquid that added to rock duplicates composition of residual liquid _____________________________________________________ 20 12. Relative distance that olivine could settle as a function of height ....................................... 20 TABLES TABLE 1'. Modal, mineralogical, and chemical data for rocks from the lower gabbro zone 2. Chemical analyses and norms of rocks from the gabbro sill _________ . 3. Chemical analyses of chilled gabbro _. .................... 4. Average compositions of rocks ............ _____ . _____________________ ll 5. Compositions of successive residual liquids ______________________________________________________________ l2 6. Comparison of calculated residual liquids to compositions of mixtures of analyzed rocks and silicic liquids .................... 13 III DIFFERENTIATION OF A GABBRO SILL IN THE OREGON COAST RANGE BY CRYSTALLIZATI ON -ZON E SETTLIN G BY NORMAN S. MACLEOD ABSTRACT At Stott Mountain in the Oregon Coast Range, an iron-rich granophyric gabbro sill, 90 to 229 m thick, is strongly differentiated but shows no rhythmic layering or crystal lamination. Gabbro in the lower half is progressively more mafic upward from the base to the center of the sill. Above the center the rocks are progressively more felsic, and ferrogranophyre and granophyre are abundant. At the top of the sill, gabbro like that near the base forms a thin zone below the upper chilled margin. The bulk composition of the sill and the composition of the chilled margins are virtually identical. Thus differentiation occurred in place without significant assimilation of country rock or influxes of magma of different composition. Differentiation probably resulted from settling of crystals that were precipitating in two zones that moved inward from the bottom and top of the sill as it solidified. In the lower zone, crystals formed over a thermal gradient and settled according to their height of nucleation within the zone and their density and size. As the lower crystallization zone progressed continuously upward, differentiation became progressively more pronounced because the thermal gradient within’the zone decreased, consequently increasing the thickness of the zone and the time available for settling. The minerals changed composition upward because an increasingly greater proportion of relatively silicic residual liquid was displaced upward out of the lower zone and thus was not ~ available for reaction. In the upper zone crystals also settled, but they were apparently resorbed as they passed into the higher temperature liquid interior of the sill. The ferrogranophyre and granophyre in the upper part of the sill formed from late residual magma. INTRODUCTION A gabbro sill in the central part of the Oregon Coast Range contains rocks that range widely in composition; for example, their SiO2 content ranges from 51 to 76 percent, and Fe203+FeO from 2 to 18 percent. The bulk composition of the sill is nearly the same as that of the chilled margins; thus differentiation must have occurred in place. The. chemical, modal, and mineralogic composition of rocks collected at intervals in the lower half show that the sill is progressively more mafic upward from the base to the center. This part of the sill is massive with no crystal layering or lamination, and rock textures are not obviously cumulate. Nevertheless, the cryptic vertical change in composition appears to ‘ ’be the result of crystal settling. In succeeding sections of this report the petrography, mineralogy, and textures of the rocks, the order of crystallization of the minerals, andthe vertical modal, mineralogic, and chemical variations are described. In the last section they are shown to be consistent with differen- tiation by crystal settling within crystallization zones that moved progressively toward the sill’s interior as it cooled and solidified. This report is a product of geologic mapping and petrochemical studies of volcanic and intrusive rocks of the Oregon Coast Range carried out by P. D. Snavely, Jr., and the author. Snavely kindly made available several chemical analyses of the gabbro - sill which he obtained prior to this study, as well as analyses of related sills farther south. I am indebted to him for providing the opportunity for this study and for his encouragement and advice. GEOLOGIC SETTING Granophyric gabbro sills underlie many of the higher mountains in a 5,000-km2 area of the Oregon Coast Range between lat 43°45’ N. and 45° N. (Snavely and Wagner, 1961; Baldwin, 1964; Snavely and others, 1976). Three K-Ar ages on minerals from these sills range from 29.9 to 31.6 million years (Tatsumoto and Snavely, 1969; P. D. Snavely, Jr., unpub. data) and indicate an Oligocene age. The sills are typically 120 to 170 m thick; the thickest, at Marys Peak, near Corvallis, Greg, is about 335 m (Roberts, 1953; Baldwin, 1955; Clark, 1969). The granophyric gabbro sill near Stott Mountain, the subject of this report, is the northernmost of this group of sills (MacLeod, 1969). It was formerly more widespread, but now crops out only as erosional remnants on higher ridges and peaks in the Valsetz and Euchre Mountain 15-minute quadrangles (fig. 1). N o feeder dikes were found near Stott Mountain, but two large dikes to the south (fig. 1) are similar in composition to the Stott sill and either may formerly have been connected to it. The gabbro sill intrudes sandstone and siltstone of 2 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE 124° 123°30' Area of figure 1 OREGON 0 5 KILOMETERS EUCHRE MOUNTAIN VALSETZ 45 a“: . m K , i.\\ \ Q \“. i) 4/1345 Figure 1. Distribution (cross-hatched areas) of Oligocene gabbro sills, dikes, and inclined sheets in Euchre Mountain (Snavely and others, 1976) and Valsetz (Baldwin, 1964; MacLeod, 1969) 15-minute quadrangles, Oregon. Dashed line outlines Stott Mountain sill; dikes and inclined sheets that may have fed Stott sill trend west-northwest and north-northeast from Diamond Peak. Solid circle shows location of samples described in table 1. the middle Eocene Tyee Formation and siltstone with interbedded tuff, basaltic sandstone, and glauconitic sandstone of the Yamhill Formation of middle and early late Eocene age. These deep—water marine sedimentary rocks overlie basalt breccias, pillow flows, tuffs, and massive flows of the lower and middle Eocene Siletz River Volcanics, which are the oldest rocks exposed in the Coast Range (Snavely and others, 1968). The Stott Mountain sill ranges in thickness from 90 to about 220 m; several small sills, less than 1 m to about 45 m thick, occur in a few places above and below the main sill. Locally the sill becomes an inclined sheet with discordance to sedimentary bedding of 5° or more. The sedimentary rocks are baked as far as 10 m from the sill. Hornfels with incipient andalusite and, less commonly, cordierite porphyroblasts have resulted from baking of siltstone in a few places; sandstone appears bleached, but only its matrix min- erals are recrystallized. No evidence of assimilation of these sedimentary rocks was found within the sill. GENERAL FEATURES The chilled borders of the sill range from 0.5 m to 3 m in thickness. The chilled basalt grades inward to fine— and medium-grained gabbro. The interior of the sill is divided into three intergradational zones (fig. 2). The thick lower and thin upper gabbro zones are separated by a silicic zone composed of ferro- granophyre, granophyre, and pegmatite. The sill was systematically sampled from base to top along a small ravine on the north side of Stott Mountain (fig. 1) where the sill is about 140 m thick. Here the lower gabbro zone is about 98 m thick, the silicic zone, about 35 m, and the upper gabbro zone is about 7 m thick. Gabbro in the lower and upper zones is massive in appearance. Field and thin-section studies show that it is not rhythmically layered and that it lacks crystal lamination. The silicic zone is heterogeneous. It is composed PETROGRAPHY 3 } Baked sedimentary rocks ) Chilled border Ga ro ] Upper gabbro zone Granophyre, , pegmatite, ‘ ferrogranophyre, J / l’ \’ silicic abbro m '- .. . g . as \\- \ ,‘W SlllClC zone gradahonal <:D§E31>\ masses and “ f , ‘ z ¢ transgressive I =’ ‘L // bodies _ \\ ) ’\/ f \ \ \’\ 77, / \ / a , \ \ \I l’ / :\ f '\ 50 METERS \\/\ \\/\/// / \ \/ \/ \/I \\ /\ \ ll “ V‘ \I: Lovver Gabbro \/ : \ / \ / - gabbro 1 zone 0 Approximate scale ; }ChiHed border } Baked sedimentary rocks Figure 2. Distribution of rock types in gabbro sill. principally of ferrogranophyre but also contains granophyre, pegmatite, and silicic gabbro. Trans- gressive bodies composed of ferrogranophyre, granophyre, and pegmatite occur in the upper part of the silicic zone, and some extend into the upper gabbro zone. Most of the transgressive bodies parallel the upper contact of the sill, extend laterally less than 50 m, and are 2—10 m thick. In detail they are very irregular and commonly bifurcate. Contacts of the bodies with the enclosing rock are sharp but show no evidence of chilling. The transgressive bodies are interpreted as resulting from injection of late residual magma produced by differentiation within the sill into its upper part. The limited horizontal extent of individual bodies suggests that the residual magma was derived nearby. PETROGRAPHY CHILLED BASALT The chilled borders of the sill are composed of dark- gray very fine grained basalt. The grain size gradually increases from less than 0.1 mm for larger crystals at the contact to about 1 mm at 3 m from the contact. The chilled basalt has an intersertal or intergranular and variolitic texture. Most is aphyric, but some contains rare plagioclase micropheno- crysts less than 1 mm long. The chilled basalt is composed of fine radiating sheaves of plagioclase , and minute grains of titaniferous ferroaugite, actin- olite, biotite, opaque minerals, and clay minerals. The chilled basalt is extensively altered. Pla- gioclase is clouded with fine-grained albite, chlorite, and (or) clay minerals that surround a few remaining relict patches of primary sodic labradorite. Former glass(?) is replaced by clay minerals, actinolite, and biolite. GABBRO The mafic rocks that form the bulk of the the sill are not readily classified and are here called gabbro to avoid a cumbersome nomenclature. The SiO 2 content of 51 to 59 percent is greater than that of most gabbros, but the rocks have an unusually high iron content (13 to 18 percent as FeO) that results in a color index between 30 and 40, and the plagioclase is mostly labradorite. Similar rocks from other areas have been variously called ferrogranophyre, granophyre, granophyric ferrodiabase, quartz dolerite, or leucogabbro. The gabbro is medium dark gray and weathers to light gray or reddish brown. Grain size is typically 0.3—1 mm, and the largest crystals are as much as 3 mm across. The average grain size increases only. slightly from the base toward the sill center. The texture is granophyric and hypidiomorphic—granular and locally slightly subophitic. The gabbro in both the lower and upper zones is massive, and neither layering nor crystal lamination is present. The gabbro changes composition vertically, as discussed later, but this variation is not obvious in the field. The gabbro is composed of plagioclase, ferroaugite, iron-rich olivine, apatite, and opaque minerals surrounded by quartz and alkali feldspar (both as intergrowths and isolated crystals) and patches of phyllosilicate minerals (tables 1 and 2). Iron-rich hornblende locally occurs as overgrowths on ferroaugite; it is also associated with iron—rich nontronitic clay minerals, biotite, and chlorite which together form the phyllosilicate patches. Olivine occurs in equilibrium with quartz in the gabbro as well as in all other rocks from the sill because it has a very high iron content (folw). Olivine also occurs with quartz in late-stage rocks of the Skaergaard intrusion, Greenland (Wager and Brown, 1968) and quartz-Fe-olivine-Ca-Fe-clino- pyroxene are stable coexisting phases at moderately low pressure (Smith, 1971). FERROGRANOPHYRE The ferrogranophyre is mottled medium gray or brownish gray and is lighter colored than gabbro, DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE TABLE l.—Modal, mineralogical, and chemical noon. ANALYSIS Height abova has: of 5111 (meters) -- 4-1/2 9 13-1/2 24-1/2 29 35 42-1/2 49 62-1/2 70 81 91 Rock nunbox -----‘- H203 H204 11205 11206 14207 H208 14209 14210 14211 H212 M213 14214 Plaqiocllla -- -- 33.0 32.6 36.0 35.2 34.9 36.6 37.2 37.2 36.3 39.4 33.7 33.0 Q-F intezqrawthz -° 27.6 27.0 24.9 24.6 21.4 21.8 20.6 19.0 17.2 16.7 17.9 21.8 Qulxtlz ------ --— 2.9 2.9 2.3 3.4 5.1 3.7 2.6 3.2 2.9 3.0 2.9 4.3 Alkali feldspnrz ‘- 1.7 1.7 1.1 2.1 3.0 1.5 1.5 1.3 2.2 1.6 1.9 4.7 Apltlte ----------- .8 .6 .7 .9 1.3 1.2 1.3 1.4 1.6 1.6 1.7 2.0 Pyroxene -"—‘ ------ 9.6 10.2 9.9 10.4 10.1 10.0 10.0 9.9 10.7 9.8 9.5 6.7 Olivine ------ —---- 10.3 9.5 11.3 10.2 11.6 11.7 11.6 12.9 13.6 12.5 12.4 11.5 Waq'ue minerals -— 2.2 2.2 2.9 3.0 2.9 3.3 3.3 3.2 3.7 4.3 4.0 3.7 Hornblende ----- --- .3 1.6 2.0 1.5 1.4 1.7 1.1 .1 .4 .2 .6 .7 Phylloailicatel --- 11.6 10.9 8.9 0.7 9.3 3.5 10.8 11.0 12.2 10.8 11.5 11.4 C010! indnx ----—-‘ 34.0 34.4 35.0 33.8 34.3 35.2 36.8 37.9 40.8 37.7 38.0 34.0 MINERAL DATA 011111115 (percent E0) 13. - 12. 17. 18. - 17. 18. 18. 18. 19. 19. 1.721 1.721 1.721 1.721 1.721 1.721 1.720 1.720 1.720 1.720 1.719 1.719 Pyroxena: 52. 51. 52. 50. 51. 51. 52. 51. 51. 52. 52. 50. Plagioclale (percent an in core!) ------- 51. 51. 54. 54. 56. 55. 54. 51. 54. 54. 54. 54. CHEMICAL ANALYSES (RECALCD'MTED HZO-FREB 1‘0 100 PENENT) $102 ------ - ------- 57.6 57.5 56.9 55.4 55.4 55.0 54.7 53.7 53.5 53.6 52.0 54.6 ”203 -----— ------- 13.5 13.3 13.4 13.7 13.2 13.2 13.3 13.4 12.9 13.5 13.1 13.4 P6203 ----- --— ----- 2.9 2.6 1.4 2.3 2.3 3.2 3.3 3.6 3.0 3.7 2.7 3.8 PeO ------------- -- 11.5 11.9 13.0 13.4 13.2 12.9 12.9 12.7 13.7 13.6 15.3 12.5 Mg!) --------------- 1.4 1.4 1.6 1.6 2.0 1.9 2.1 2.1 2.2 2.2 2.3 1.9 can --------------- 6.0 6.1 6.5 6.5 6.5 6.8 7.0 7.3 7.5 7.2 7.3 6.9 11820 ------- ‘ ------ 2.9 3.0 2.7 2.7 2.7 2.6 2.5 2.7 2.7 2.8 2.5 2.6 K20 --------------- 1.9 1.5 1.7 1.3 1.5 1.5 1.4 1.3 1.3 1.0 1.2 1.4 “02 ______________ 1.5 1.7 1.7 1.3 1.8 1.9 1.9 2.0 2.0 1.6 2.3 l-9 9205 -------------- .67 .72 .73 .93 .85 .83 .87 .88 .86 .68 1.0 .55 11110 --------------- .27 .26 .28 .29 .30 .29 .31 .30 .32 .20 .33 .30 TABLE 2.—Chemical analyses and lower Upper Silicic zone zone m. qabbro gabbro Gabbro Pertoqranophyre Peqmatite Granophyre Column number ---- 1 2 3 4 5 6 7 B 9 1D 11 12 13 14 15 Rack number ------ 61-66 60—56 66-124 66-125 67-216 67-218 67-219 66-145 60-62 67-215 66-144 66-136 61-65 66-142 66-135 HDDAL ANALYSES Plaqiocl'ase ------ 33 41 34 34 41 35 31 34 31 29 32 33 (too fine grained 10 9—? interg'tOWth -— 24 22 27 26 19 26 I 32 38 30 26 45 29 to determine 86 Quart: ----------- 3 2 3 2 ‘4 4 3 3 7 8 7 2 mode) - Alkali feldspar -- 1 1 2 2 3 3 3 4 3 10 2 7 - Apatite ---------- 1 1 1 1 2 1/2 1/2 - - - - 1/2 - Pytoxene --------- 13 1| 9 1D 9 10 10 6 10 8 5 6' Z Olivine ---------- 9 B 9 7 6 6 12 7 5 2 5 2 - tpaque minerals -- 3 3 z s 2 a 2 2 2 2 1 5 1/2 I-brnblende ------- 1 1 2 1 1 2 1/2 1 - 3 1 1/2 - Phyllosilicates -- 12 10 11 12 12 7 7 5 12 12 2 6 2 Color index ------ 38 33 33 35 30 29 31 21 29 26 14 19 4 MINERAL DATR Olivine (percent f0) — - - - 16 15 10 9 - - 4 - - ----- 1.718 V 1.722 - 1.723 1.722 1.724 1.723 1.734 1.725 1.724 1.737 1.735—8 1.739 Pytoxene: 2V ----- 51. 50. - 50. 51. 52. 50. 54. 53. 51. 54. 52-56 55. Plagioclase (percent An in cores) ------ 53. 54. 54. 54. 54. 51. - 51. 45. - 51. - 2. CHEMICAL ANALYSES [RECAICULATED WATER-FREE TO 100 PERCENT) $1.02 ------------- 51.7 54.7 55.5 55.7 56.6 57.7 59.2 60.6 61.2 63.1 63.4 65.7 72.4 74.9 76.1 A1203 ------------ 13.1 12.9 13.9 13.3 14.9 13.3 13.3 13.6 13.0 13.4 13.4 14.3 13.9 13.0 12.6 F2203 ------------ 4.1 3.3 3.4 3.0 4.0 3.3 3.5 2.7 5.6 4.0 5.3 1.8 1.9 2.7 .38 Feb -------------- 14.3 13.2 11.9 12.3 9.3 11.1 10.2 9.8 7.6 7.7 5.8 5.4 1.9 .53 1.2 M90 -------------- 1.9 1.2 1.6 1.7 1.3 1.4 1.1 .93 .49 .68 .53 .49 .27 .05 .11 C30 -------------- 6.6 7.0 6.7 6.9 5.7 6.2 5.9 5.4 4.5 4.4 4.6 3.8 .54 .33 .76 Nazo ------------- 2.7 2.9 2.6 2.7 3.1 2.7 2.8 3.0 3.1 3.0 3.4 5.4 6.2 3.5 4.1 K20 -------------- 1.5 1.5 1.6 1.5 1.6 1.7 1.9 2.2 2.1 2.2 2.5 2.0 2.4 4.1 4.3 TiOz ------------- 2.7 2.2 2.1 2.1 1.6 1.7 1.4 1.3 1.5 1.0 .81 .84 .37 .29 .31 P205 ----------- 1-0 .55 .67 .76 .61 .67 .57 .40 .58 .32 .26 .17 .04 .00 .02 24110 -------------- .35 .32 .23 .22 .27 .29 .26 .17 .25 .23 .14 .04 .06 .09 .06 data for rocks from the lower gabbro zone PETROGRAPHY M0081. ANALYSES Height above bass of “n (“a") —- 4-1/2 9 13-1/2 24-1/2 29 35 42-1/2 49 62-1/2 70 81 91 Rack number -— ----- M203 M204 M205 H206 M207 M208 M209 M210 M211 M212 M213 M214 NORHS (CALCULATED 11121-1 56203 = 10 PERCENT Tom. F- OXIDE) g n--- ——————— .--—_- 12.6 13.4 12.9 11.8 10.7 10.5 10.5 8.4 7.8 7.5 7.1 10-1 or ———--- ——————— -—— 11.2 8.9 10.0 7.7 8.9 8.9 8.3 7.7 7.7 5.9 7-1 8-3 ab ----- -------- -—- 24.5 25.4 22.8 22.8 22.8 22.0 21.2 22.8 22.8 23.7 21.2 22.0 In ....... _-- —————— 10.2 10.4 19.4 21.4 19.5 19.9 20.9 ' 20.6 19.2 21.3 21.0 20.3 1110 ------- --—-- 3.0 3.0 3.4 2.2 3.7 3.5 3.4 4.1 5.2 4.2 3.7 3.3 .11. .n _- ........ -- .5 .5 .6 .4 .7 .7 .7 .8 1.0 .8 .7 .6 Es ------ ------ 2.8 2.9 3.1 2.0 3.2 3.1 3.0 3.6 4.5 3.7 3.2 3.0 en -------- ---— 3.0 3.0 3.4 3.6 4.3 4.1 4.5 4.4 4.4 4.7 5.0 4.1 by: is ---------- -— 17.8 17.6 17.4 20.2 18.7 19.5 19.7 19.2 19.0 21.2 21.9 20.0 M -.- ———————— -.—_- 2.1 2.1 2.0 2.3 2.2 2.3 2.3 2.4 2.4 2.5 2.6 2.4 11 .-_. —————————— _- 2.3 3.2 3.2 3.4 3.4 3.6 3.5 3.94 3.8 3.0 4.4 3.6 .p _____ ——————————— 1.5 1.7 1.7 2.2 2.0 2.0 2.1 2.1 2.0 1.6 2.4 2.0 CHEMICAL ANALYSES (ORIGINAL); 5102 --- ------ ----- 56.1 55.7 55.5 54.3 54.1 53.6 53.1 52.0 51.7 51.3 50.7 53.0 31203 ----- -- ------ 13.1 12.9 13.1 13.4 12.9 12.9 12.9 13.0 12.5 12.9 12.8 13.0 86203 ------- ------ 2.8 2.5 1.4 2.2 2.2 3.1 3.2 3.5 2.9 3.5 2.6 3.7 no ---------- ——--- 11.2 11.5 12.7 13.1 12.9 12.6 12.4 12.3 13.2 13.0 14.9 12.1 ”go - ______________ 1.4 1.4 1.6 1.6 1.9 1.8 2.0 2.0 2.1 2.1 2-2 1.8 can ————— —-- ------- 5.9 5.9 6.3 6.5 6.6 6.6 6.8 7.1 7-2 6.9 7.1 6.7 11.120 ----- --- ------ 2.8 2.9 2.5 2.6 2.6 2.5 2.4 2.6 2.6 2.7 2.4 2.5 x20 ......... -_.——— 1.3 1.5 1.7 1.3 1.5 1.5 1.4 1.3 1.2 1.11 1.2 1.4 1120. ——----_ ——————— 1.2 1.3 1.1 .91 .92 1.2 1.4 1.4 1.3 1.5 1.1 1.4 1120+ -.-- ———————— —_ 1.4 1.3 1.2 1.1 1.3 1.3 1.3 1.6 1.9 2.0 1.3 1-0 T102 ------- ——————— 1.5 1.5 1.7 1.8 1.8 1.8 1.8 1.9 2.1 1.5 2.2 1.8 12205 -- ------- ——- .55 .70 .71 .91 .83 .81 .85 .85 .83 .65 1.0 .83 11:10 ---- ------- ---- -26 i _fl -25 i -2_9 _-_3E i _-_3" _-E ._'33 +29 ’Ibtal ----~------- 100.0 99.5 99.9 100.0 99.8 100.0 99.9 99.8 99.8 99.2 99.8 99.9 ‘samplea collected along mall ravine in center 58c. line 25-26, 2‘. 7 5., R. 9 11., Valsetz 15' quadrangle. zmdividual quartz and alkali feldspar and intetgrown quartz and laldapaz counted separately. 30180111381 nmlyiel wen made by s. Betta, 11. saith, 3. Chloe, 1.. Artis, .1. Kelsey, .1. Glenn, 0. Elmore, 8nd I. Barlow using methods «scribed in u.s. Geological Survey Bulletin 1144A, .upplemented by atomic abaotpuon. auple and an aadxtlonal 2,000 point: for opcqut nun-nu and apatite. norms of rocks from the gabbro sill Moan. malysu an band on 2,000 points or more per me: Upper sluclc mm mm Iona gabbro gnbbx'o anbxo nrrogrnnophyre Peqmtite Granophyta Colunn number ---- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 neck nunbar -- 61-66 50-56 66-124 66-125 67-216 67-218 67-219 66-145 60—62 67-215 66-144 66-136 61-65 66-142 66-135 11011145 (cmuum 1111-11 P2203 - 10 PERCENT Mu. Pa) 9 --- -------— 6.2 9.7 11.4 11.6 10.9 14.3 15.6 16.3 18.6 20.6 18.3 15.6 223.3 335.9 33.2 or -----—--------- 8.9 8.9 9.5 8.9 9.5 10.1 11.2 13.0 12.4 13.0 14.8 11.8 14.2 24.2 25.4 lb ---------—--—-- 22.9 24.5 22.0 22.9 26.2 22.9 23.7 25.4 26.2 25.4 28.8 45.7 52.5 29-6 34.7 In --------------- 19.2 17.8 21.5 19.7 22.0 19.2 18.1 17.1 15.4 16.6 13.9 8.9 2.4 1.6 3.3 o ----—------ 3.0 4.8 3.1 4.0 3.0 3.0 3.1 3.0 1.3 1.3 3.0 3.7 - - .2 41. :11 ---------- 0.5 0.6 0.5 0.7 0.5 0.5 0.4 0.4 0.1 0.1 0.3 0.4 - - - n ------- --- 2.7 4.7 2.8 3.6 2.8 2.8 3.0 2.9 1.4 1.3 3.1 3.6 - - .2 h)“ an - ------ --- 4.2 2.4 3.5 3.5 2.8 3.0 2.3 1.9 1.1 1.6 1.1 .8 .7 .1 .3 £1 ---------— 22.4 18.2 18.3 17.4 15.9 17.5 16.7 14.9 17.3 15.8 13.3 6.4 5.3 4.8 1.9 nt ------- ----- --- 2.7 2.4 2.2 2.2 1.9 2.1 2.0 1.8 1.9 1.7 1.6 1.0 .5 .5 .2 11 ------ --------- 5.1 4.2 4.0 4.0 3.0 3.2 2.7 2.5 2.9 L9 1.5 [-5 .7 .5 46 up —-— ------- ---—- 2.4 2.0 11.6 1.8 1.4 1.6 1.3 1.0 1.4 .8 .6 .4 .1 - .1 cmlcu. ANALYSIS (ORIGINAL-)1 5102 - -------- ---- 50.0 53.2 53.7 53.6 54.8 56.0 57.5 59.0 58.8 61.3 61.6 54.5 72.0 73.1 75.3 A1203 ------------ 12.7 12.5 13.4 12.8 14.4 12.9 12.9 13.2 12.5 13.0 13.0 14.0 13.8 13.3 12.5 "203 - -------- --- 4.0 3.2 3.3 2.9 3.9 3.2 3.4 2.6 5.4 3.9 5.1 1.8 1.8 2.6 .38 no ------------- - 13.8 12.8 11.5 11.8 9.0 10.8 9.9 9.5 7.3 7.5 5.6 5.3 1.9 .52 1.2 H90 -------------— 1.8 1.2 1.5 1.6 1.3 1.4 1.1 .91 .47 .66 .52 .48 .27 .05 .11 can -----— ------ -- 6.4 6.8 6.5 6.6 6.5 6.0 5.7 5.3 4.3 4.3 4.5 3.7 .54 .52 .75 11.20 --«--------- 2.6 2.8 2.5 2.6 3.0 2.6 2.7 2.9 3.0 2.9 3.3 5.3 6.2 3.4 4.1 :20 -- ------ ------ 1.4 1.5 1.5 1.4 1.5 1.6 1.8 2.1 2.0 2.1 2.4 2.0 2.4 4.0 4.2 1.120- ------------- 1.3 1.2 1.2 1.3 1.5 1.5 1.3 .79 1.4 1.2 1.0 .59 .41 .81 .16 1120+ -——---------- 1.7 1.0 2.0 2.0 1.6 1.3 1.1 1.7 2.0 1.4 1.8 1.2 .85 1.6 .69 1102 ------------- 2.6 2.1 2.0 2.0 1.5 1.6 1.4 1.3 1.4 1.0 .79 .82 .37 .28 .31 11205 ---- ------ -—— 1.0 .83 .65 .73 .59 .65 .55 .39 .56 .31 .25 .17 .04 .00 .02 5"“ """““"" _-3‘_ A ____-21 _'Zl J 43.2 J .J _'2‘. J A .42 _'°5 ___.__-°9 '05 Tot-1 ——---------— 99.6 99.4 100.0 99.5 99.9 99.8 99.6 99.9 99.4 99.8 100.0 99.9 100.6 100.3 99.8 ‘mmcal ugly... won :48. by s. Batu, a. Smith. 8. Chloe, 1.. nus, a. Kelsey, .1. Glenn, 1:. um", and 1. Blzlow using Ilathodl donatibed 1n u.s. Geologic-1 805"}, nuctln 114411, mpplnented by .tmlc ahmption. 5 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE owing largely to the greater abundance of interstitial quartz and alkali-feldspar. Grain size is variable even in individual samples, but larger crystals are typically 1—3 mm long. The rock is composed of the same minerals as the gabbro but in different proportions (table 2). Because of the greater abundance of lateformed granophyric intergrowths of quartz and alkali-feldspar, crystals of plagioclase, pyroxene, and olivine tend to be more euhedral than in gabbro. Plagioclase is less abundant in the ferro- granophyre than in the gabbro. Crystal-core compositions are similar, but progressively zoned rims are larger in the ferrogranophyre and the bulk plagioclase is more sodic. Olivine and apatite are also less abundant than in gabbro, but the content of opaque minerals and hornblende is about the same. Variable amounts of phyllosilicates (iron-rich clay minerals, chlorite, and biotite) occur with quartz- feldspar intergrowth. Olivine is fayalitic, and the pyroxene is ferroaugite or ferrohedenbergite. PEGMATITE Small bodies of pegmatite, a coarse—grained (as large as 5 cm) variety of ferrogranophyre, occur within the silicic zone. They form masses that grade into ferrogranophyre and also form transgressive bodies. Some pegmatite contains as much as 17 percent olivine, but most contains less than 7 percent. Olivine and ferroaugite are very rich in iron (table 2). Plagioclase displays marked progressive zoning and ranges from sodic labradorite or andesine in crystal cores to calcic oligoclase on the rims. GRANOPHYRE Granophyre, which constitutes less than 5 percent of the sill, occurs as large masses that grade into ferrogranophyre, as transgressive bodies as much as 10 m thick that cut ferrogranophyre, and as small veins. The granophyre is light gray or brownish gray, and the grain size is typically less than 0.1 mm. Texture is micrographic (granophyric) to aplitic. In addition to quartz and alkali-feldspar, the granophyre typically contains about 10 percent sodic plagioclase, 0.3 percent each of ferrohedenbergite and olivine or its alteration products, and less than 0.5 percent opaque minerals. Most of the olivine is completely altered to clay minerals, but the high Fe/Mg ratios of these rocks suggest that it was fayalite. MINERALOGICAL DATA The approximate compositions of the pyroxenes were determined from optical properties using the diagram of Carmichael (1960). Analyzed pyroxenes from other chemically similar Coast Range sills have optical properties and chemical compositions that fit this diagram better than other published diagrams relating composition and optical properties. The pyroxene ranges in composition from ferroaugite to ferrohedenbergite; no calcium-poor pyroxene was noted in any of the rocks. The pyroxene becomes progressively richer in iron (higher refractive index, N y) in the series gabbro-ferrogranophyre-gran- ophyre (tables 1 and 2). The trend in pyroxene composition (fig. 3) paral- lels that of Ca—Fe pyroxenes from the Skaergaard intrusion (Brown and Vincent, 1963) but, at least on the basis of optical properties, is slightly more calcic. The trend is considerably more calcic than that of Ca- Fe pyroxenes from the Red Hill Dolerite of Tasmania (McDougall, 1961). All these trends, however, are toward a final composition of about Ca40.43Fe60_57 on the ferrosilite-wollastonite join, a composition similar to that of synthetic ferrohedenbergite in equilibrium with quartz and fayalite at pressures of 1 and 2 kbar (Lindsley and Munoz, 1969). Olivine compositions were determined on mineral separates using the X-ray diffraction method (d130) of Yoder and Sahama (1957). The olivine in all rocks is rich in iron, as would be expected considering that it occurs in equilibrium with quartz. In gabbro the olivine is f0”.19 (ferrohortonolite), and in ferro- granophyre, pegmatite, and granophyre it is fo4_9 (fayalite). In some granophyre olivine may be richer in iron than fo4, but it is quantitatively minor, extensively altered, and consequently difficult to separate. Olivine is richer in iron than coexisting pyroxene in all rocks (fig. 3). Olivine is variably altered to iron-rich nontronitic clay minerals Plagioclase compositions were determined by universal stage using the albite twin law curves of Uruno (1963). In two gabbro samples plagioclase crystal cores are an56; in all others they range only from an51 to an54. Cores constitute about 70 volume percent of individual crystals and are faintly oscillatory zoned; zonal differences are generally less than anl. The progressively zoned outer rims are sodic andesine to calcic oligoclase. Plagioclase cores are an,,5_51 in ferrogranophyre, an30_51 in pegmatite, and an2_10 in granophyre; progressively zoned rims in ferrogranophyre and pegmatite reach oligoclase. The structural state of plagioclase is intermediate in all rocks. Some plagioclase is altered to clay minerals along crystal cracks, particularly in more silicic rocks. In most rocks, however, alteration is slight. All euhedral quartz crystals have 3 quartz shape; no evidence of former tridymite was seen. The TEXTURAL RELATIONS 7 4‘ en to Ferroougife Ferrohedenbergite \ 80 90 Olivine ("/0 1‘0) Figure 3. Inferred composition of pyroxene from Stott sill based on optical properties. Analyzed pyroxenes are labeled x; tie lines connect coexisting pyroxene and olivine pairs. wo, wollastonite; en, enstatite; fs, ferrosilite; f0, forsterite; fa, fayalite. Skaergaard (Sk) and Red Hill Dolerite (RH) pyroxene trends are shown by dashed lines. composition of intergrown feldspar is difficult to determine optically because of its fine grain size. TEXTURAL RELATIONS The rocks contain subhedral to euhedral crystals of Fe-olivine, Ca—Fe pyroxene, plagioclase, opaque minerals, and apatite that are surrounded by intergrown quartz and alkali feldspar and patches of phyllosilicate minerals. The resulting granophyric and hypidiomorphic-granular texture changes from the most mafic gabbro to granophyre. In the former, the earlier minerals tend to be more commonly subhedral, and the amount of quartz-feldspar intergrowth is low (as little as 16 percent). In ferrogranophyre and granophyre, the earlier minerals tend to be more euhedral because the late- formed quartz-feldspar intergrowth is more abundant. Cores of olivine, pyroxene, plagioclase, opaque minerals, and apatite crystals in gabbro of the lower zone are considered to be cumulus in origin. Rims on these minerals are interpreted as adcumulus growths, and the quartz-feldspar intergrowth, phyllosilicate minerals, and hornblende are intercumulus. Crystal settling is inferred to have occurred but produced neither crystal layering nor lamination, for reasons discussed later. Ferroaugite in gabbro is equant to irregular in shape, is in places molded in subophitic fashion on smaller plagioclase laths, and projects with euhedral form into quartz-feldspar intergrowth. In some ferrogranophyre and pegmatite, the ferroaugite or ferrohedenbergite occurs as sets of parallel elongate crystals that poikilitically enclose plagioclase along their ragged borders; single crystals reach lengths of as much as 5 cm. Individual sets interlock with sets with a different orientation, producing a randomly ordered mosaic. Olivine with the same habit also occurs in these same rocks. This texture may be the result of nucleation at points within sets of inter- connected Fe—Si polymers in the magma. Olivine crystals are subhedral or anhedral where they are in contact with plagioclase and subhedral to euhedral against pyroxene; some olivine occurs as small crystals enclosed in pyroxene. Although they clearly overlap in their period of crystallization, plagioclase occurs as small crystals within olivine whereas the reverse relation is rarely seen; where observed it may result from two-dimensional View in thin section. Olivine is euhedral where bordered by quartz-feldspar intergrowth and, because of its iron- rich composition, has no reaction relation with 8 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE quartz. The opaque minerals, about 90 percent of which are ilmenite and the remainder pyrrhotite, are typically much smaller (commonly 0.1—0.2 mm) than the other minerals except apatite. They occur enclosed in all other minerals except apatite and are particularly abundant within olivine and pyroxene crystals. The opaque minerals contain inclusions only of apatite. Most of the ilmenite appears homogeneous in polished section; only rarely are exsolution lamellae developed. Pyrrhotite occurs as small irregular masses within ilmenite and as small isolated grains. Apatite occurs principally as 0.05 by 0.4 mm euhedra, but some very small, highly elongate crystals are also found within quartz-feldspar intergrowth. The larger apatite crystals occur within all other minerals and appear to be the first phase to begin crystallization; smaller crystals in intergrowth crystallized late. Quartz-feldspar intergrowth and phyllosilicate patches fill in between crystals of plagioclase, pyroxene, olivine, opaque minerals, and apatite. The quartz—feldspar intergrowth consists of wavy parallel quartz rods with cuneiform cross sections that are enclosed in alkali-feldspar. Viewed parallel to quartz rods, some intergrowth appears myr- mekitic; viewed perpendicular to the rods, it is micro- graphic. In gabbro the quartz rods are typically 0.01—0.02 mm wide and 0.2—0.3 mm long; in ferrogranophyre and pegmatite they reach 0.2—0.3 by 0.8—1 mm. Quartz and alkali-feldspar make up approximately equal proportions, but the inter- growth is so fine that it is difficult to measure their relative proportions. The intergrowth is generally finer grained where bordered by plagioclase, pyroxene, and olivine than it is near centers of intergrowth. Near centers, quartz, and less commonly feldspar, occurs as subhedral to euhedral crystals in optical continuity with some of the surrounding intergrowth and is euhedral where it extends into phyllosilicate patches. Boundaries of intergrowth with plagioclase, pyroxene, and olivine are sharp, and no replacement of earlier minerals is indicated. The phyllosilicate patches consist predominantly of clay minerals (probably iron-rich nontronite) with some chlorite(?) and iron-rich biotite. Ilmenite, pyrrhotite, and hornblende occur within some patches. The iron-rich clay minerals are not stable at magmatic temperatures (Ernst, 1966) so the patches were probably originally iron-rich glass that devitrified to clay minerals as the rocks cooled. This glass may have been immiscible with the residual liquid that formed the quartz-feldspar intergrowth, but if so, later crystallization has obscured any textural indications of immiscibility. However, immiscibility has been demonstrated for liquids with bulk compositions similar to those of the Stott Mountain phyllosilicates plus quartz-feldspar inter- growth (Roedder, 1951; McBirney and Nakamura, 1974; McBirney, 1975; and Irvine, 1975). Also, a 3-m- wide basalt dike south of Stott Mountain that is compositionally similar to the gabbro in the sill, and probably related to it, contains an iron-rich glass (chlorophaeite) and silicic glass in the residuum that do show textural relations of immiscibility similar to those reported by De (1974). Iron-rich hornblende occurs principally as a marginal replacement product of pyroxene, probably as a result of reaction of pyroxenes with the residual fluids that ultimately formed the phyllosilicate patches. ORDER OF CRYSTALLIZATION The order of crystallization of the minerals in gabbro is inferred from textural relations. During much of the crystallization, plagioclase, olivine, ferroaugite, opaque minerals, and apatite formed together. However, of inclusion-bearing crystals, most larger plagioclase crystals contain only apatite and opaque minerals in their cores; olivine crystals contain small crystals of plagioclase, apatite, and opaque minerals; and pyroxene crystals contain small crystals of plagioclase, olivine, opaque minerals, and apatite. Opaque minerals contain apatite; crystals of apatite do not contain inclusions. Quartz-feldspar intergrowth and phyllosilicate patches fill volumes between the other minerals. These relations indicate that the order of first crystallization of the individual minerals in each rock was apatite, then opaque minerals, plagioclase, olivine, and pyroxene. Quartz-feldspar intergrowth and the phyllosilicate patches formed later. Determining the order of first crystallization of apatite on the basis of texture may be misleading owing to its ubiquitous occurrence as euhedra in most rocks, including those from the Stott sill. However, apatite crystals that are only partially enclosed in other early-formed minerals commonly are larger where they protrude. Also, the high P205 content of the rocks (typically 0.7 percent) is consistent with apatite as an early liquidus phase. PETROCHEMISTRY 'COMPOSITION OF THE CHILLED BORDER Four analyses of chilled border rocks all have nearly the same chemical composition (table 3) except that differing amounts of alteration of COMPOSITIONAL VARIATIONS WITH HEIGHT 9 TABLE 3.—Chemical analyses of chilled gabbro [Chemical analyses were made by S. Botts, H. Smith, G. Chloe, L. Artie, J. Kelsey, J. Glenn, P. Elmore, and I. Barlow using methods described in 11.5. Geological Survey Bulletin 1144A, supplemented by atomic absorption.) Chemical analyses recalculated HZO-free to 100 percent Average of Column number--— 1 2 3 4 columns 1—4 Rock number ----- $68 $84 M202 M220 SiO2 ----------- 57.1 57.2 57.2 . 57.9 57.4 A1203 ---------- 13.3 13-4 13.3 13.4 13.4 F2203 ---------- 3.1 4.2 2.1 2.3 2.9 Feo ............ 12.3 10.7 13.4 12.4 12-2 M90 ------------ 1.5 1.2. 1.4 1-7 1-5 CaO ———————————— 4.8 5.2 5-2 4-8 5-0 Nazo ——————————— 4.9 3.2 2-9 4-2 3-3 K20 ———————————— .41 1.9 2.1 :83 1-3 T102 ___________ 1.7 1.9 1.7 1.7 1-7 p205 ----------- .71 .72 .67 .70 -7O MnO ———————————— .23 .31 .24 .28 -27 Chemical analyses (original) 5102 ___________ 54.8 55.4 55.5 55-7 111203 —————————— 12.8 13.0 12.9 12.9 11.3203 ---------- 3.0 4.1 2.0 2.2 FeD ____________ 11.8 10.4 13.0 11-9 M90 ———————————— 1.4 1.2 1-4 1-6 CaO ............ 4.6 5.0 5.0 4.6 N620 ——————————— 4.7 3.1 2.8 4-0 x20 ____________ .39 1.8 2.0 -80 1.120— ——————————— .75 .38 .72 1.4 1120+ ——————————— 2.7 2.0 1.8 2-2 T1102 ___________ 1.6 1.8 1.6 1.6 p205 ——————————— .68 .70 .65 -67 Mno ............ .22 .30 .23 .27 co ............ .05 .28 .05 .05 feldspar and former glass(?) produce variation in the 030, NaZO, and K20 content. The analyzed chilled samples are from sites as much as 8 km apart, and one is from a 7-m-thick, very fine grained sill below the main sill. The magma was apparently uniform in composition over a large area. The chilled rocks are unusual in that they have a very high total iron oxide, TiOz, and P205 content considering their relatively silicic nature. COMPOSITION OF ROCKS FROM THE INTERIOR OF THE SILL The rocks in the sill show a large variation in chemical composition (tables 1 and 2). SiO2 ranges from 51.7 to 76.1 percent, and total iron oxide from 1.6 to 18.4 percent. The combination of low MgO content (maximum 2.3 percent) and relatively high iron oxide content in all of the rocks produces an unusually high mafic index (100x(FeO+Fe203)/(FeO+Fe203+MgO)) that ranges from 88 to 98. Compared to the chilled margins, the least silicic rocks are enriched in total iron oxide, MgO, CaO, TiO2 and P205 and depleted in NaZO and K20. In contrast, the granophyres are enriched in alkalies and silica, depleted in other major oxides, and have a granitic composition. The norms were determined on water-free analyses after recalculating the variable FezOg/(FeO+Fe203) to 1/ 10 which approximates that of the least oxidized rock. A low initial oxidation state is indicated by an absence of magnetite in these iron-rich rocks; the increase in the FezOs/FeO ratio resulted from deuteric alteration. All rocks in the sill are quartz- normative. Modal quartz is greater than normative quartz (even when calculated as volume percent) because some of the normative hypersthene is represented in the rock by olivine and quartz. The normative pyroxenes plot in the iron-rich side of the pyroxene quadrilateral (fig. 3) in the “forbidden zone.” Under moderately low pressure, liquids of this composition crystallize to Ca—Fe clinopyroxene, fayalitic olivine, and quartz; calcium-poor pyroxene is not stable (Smith, 1971). COMPOSITIONAL VARIATIONS WITH HEIGHT The modal, mineralogical, and chemical data indicate that the lower gabbro zone has a cryptic vertical variation in composition. For instance, the proportions of minerals in the zone vary syste- matically upward from the base to the center of the sill (fig. 4A—D). Olivine, opaque minerals, apatite, and plagioclase increase in abundance upward, quartz and alkali feldspar decrease, and ferroaugite, phyllosilicates, and hornblende are nearly constant (fig. 4A). Olivine, opaque minerals, apatite, and plagioclase are most abundant near the center of the sill. Mineral compositions also change vertically (fig. 4B). Ferroaugite shows a slight progressive increase in M g/ Fe ratio (shown by slight decrease in Ny) from the base to a maximum near the center even though the amount of ferroaugite does not change significantly. Olivine is also progressively richer in magnesium, ranging from f012_13 near the base to fo19 near the center. Plagioclase crystals are zoned, so it is difficult to determine their vertical variation in bulk composition, However, crystal cores in the lowest two samples are an“, whereas near the center of the sill most are an54. This apparent upward increasein core anorthite content is paralleled by an, upward increase in normative an/an+ab from 42—43 near the base to 46—50 near the sill center. The chemical composition of the rocks also changes vertically (fig. 4C—D). From the base to the center of the sill, total iron oxide , MgO, CaO, TiOz, and P205 increase; A1203 remains nearly constant; 10 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE Silicic zone 100- ' ——M215 —_T'-""“ F_‘_" “"I ___.'____ _______|_ ""‘ cl 1M214 60— t I —M213 I CenteII ' 60- I. I —M211 1 | —M21O 40- I I —M209 i —M208 ' —M207 0 -M206 20— I U7 g y —M205 g o -M204 J M203 ; 0 I I I I I | I I I I I I I J o 10 20 30 10 20 o 5 0 2 10 2o 30 4o 20 30 40 E % PVROXENE OLIVINE OPAGUES APATITE PHVLLO- PLAGIOCLASE QUARTZ‘ALKALI E m A HE'IICBIIEPDE WSW D O 2 Lu LA) (I) at) Silicic zone g _ ——M215 g _______________ _ _ __ _ $ 2 I “ “ <( —M214 ._ 5 I, —-M213 I Center sill —M211 -M210 “M209 — M208 “M207 -M206 ' M205 — M204 -' M203 1. I ,_.Ll I I l L I I 25 35 30 40 50 60 O 110 2017125172092 90 8'8 510 5'5 40 50 COLOR TOTAL 0le ~PLAG OLIVINE / F0 PVROXENE MAFIC INDEX XL CORES NORM INDEX 0pm .Ap PLIAG OCLASE /. AN B 0 Figure 4. Vertical variation in lower gabbro zone. A, Modes. B, Mineral compositions and chemical data. C, Major oxides. D, Oxide ratios. and SiOZ, NaZO, and K20 decrease. The differ- entiation index (Q+or+ab), mafic index, and felsic index decrease systematically from the base to the center of the sill (fig. 4B—D). The differentiation index parallels the change in amounts of quartz and alkali feldspar, the mafic index in part reflects the change in Mg/Fe of ferroaugite and olivine, and the change in felsic index correlates with plagioclase compo- sition and amount of alkali feldspar. The modal, mineral, and chemical data indicate that composition changes systematically from the base to above the'center of the sill and that the center is most mafic (femic). Most other differentiated dolerite or gabbro sills are most mafic at a much lower position, usually nearer the base than the center. Most rocks in the silicic zone in the area where the Silicic zone 100 —M214 ~M213 08mg” .. .. . ”75,1212 —M211 — M210 M209 I 40 — "M208 — M207 - M206 20 — Y J J. | I I l J q I ? —- M205 I | l I | | I | | I 1 1 —M20«: —IM203 050 55 501214 112 14 15 1 SIO; AI203 FeOsFegoa MnO SAMPLE NUMBER 100— —M215 — M21 HEIGHT ABOVE BASE 0F SILL, IN METERS an o I ‘M213 . TM212 —M211 "M210 40 — a M209 — M208 — M207 — M206 20 " — M205 — M204 — M203 0 I l I I I I I I l l I l | I I I 0 1 2 0.5 1 0.5 1 0.3 0.4 0.5 40 50 60 0.4 0.5 20 30 40 TIOz P205 F90 N320‘K20 Omnah inf unhysmhli D FeO+FeZO3 Na70¢K20‘Ca0 amab (Felsic index) rocks of table 1 and figure 4 were collected are weathered and only four were suitable for analysis (table 2, cols. 5, 6, 7, and 10); three are silicic gabbro and one is ferrogranophyre. The weathered rocks there and elsewhere are mostly ferrogranophyre. The remaining analyses of ferrogranophyre, pegmatite, and granophyre shown in table 2 are from the silicic zone at other locations at and near Stott Mountain. They show that rocks in the silicic zone contain more SiOz, and K20 and less total iron oxide, MgO, CaO, TiOZ, and P205 than does the rest of the sill. Elements that are enriched in the silicic zone are those that are impoverished in the lower gabbro zone compared to the chilled margins. BULK COMPOSITION OF THE SILL The bulk composition of a differentiated sill must approximate the original chilled margin composition if chemical variation resulted from closed-system differentiation of magma in place. As with many BULK COMPOSITION OF THE SILL 11 TAIL}: 4,—Avemge compositions of rocks Column number---- 1 2 3 4 5 6 Lower Upper Lowest gabbro silicic gabbro Chilled , three Bulk zone1 zone2 zone2 gabbros3 gabbros4 composition5 $102 ------------ 54.7 64.6 56.5 57.4 57.3 57.3 A1203 ----------- 13.3 13.6 13.5 13.4 13.4 13.4 FeO + 0.9 Fe203 — 16.0 9.6 14.9 14.8 14.2 14.4 Mgo ------------- 1.9 0.67 1.5 1.5 1.5 1.6 CaO ------------- 6.9 3.9 6.0 5.0 6.3 6.1 Nazo ------------ 2.7 3.7 2.8 3.8 2.9 2.9 K20 ------------- 1.4 2.5 1.6 1.3 1.7 1.7 ‘I'io2 ------------ 1.9 1.0 1.9 1.7 1.6 1.7 P205 ------------ .83 .33 .70 .70 .71 .70 MnO ------------- .29 .17 .25 .27 .27 .26 1Average of analyses in table 1 weighted according to interval thickness represented by each analysis. 2Average of all analyzed rocks in zone. 3Average of four chilled gabbro analyses listed in table 3. 4Average of rocks 203-205, table 1. 5Weighted average of three zones according to interval thickness represented by each zone 0 gabbro and dolerite sills, however, the original composition of the chilled margins of this sill has been modified slightly by deuteric alteration and weathering. Alteration has caused variations in alkalies and CaO but little change in the other major oxides. Alteration effects are best seen by comparing analyses of the chilled margin rocks to those of little altered rocks from close to the chilled margins (table 4, cols. 4 and 5). The bulk composition of the sill can be calculated from compositions and thicknesses of the three zones that make up the sill. The composition of the lower gabbro zone, which forms 70 percent of the sill, can be determined accurately because rocks within the zone show a systematic vertical variation in composition defined by 12 analyzed rocks. The bulk composition of this zone (table 4, col. 1) was obtained by adding the compositions of the individual intervals weighted according to the interval thickness. The composition of the silicic zone, which forms about 25 percent of the sill, is not so readily obtained because of its heterogeneity. The average compo- sition of all analyzed rocks from the silicic zone is assumed to be approximately equal to the bulk composition of this zone. This average composition (table 4, col. 2) is similar to an analyzed ferrogranophyre (table 2, col. 10) that is typical of the most abundant rock in the silicic zone. The bulk composition of the sill (table 4, col. 6) was determined by adding appropriate proportions of the average composition of each Zone. For instance, 70 percent of the average SiO2 of the lower zone plus 25 percent of the average SiO2 of the silicic zone and 5 percent of the SiO2 of the upper gabbro zone equals the bulk SiO2 composition of 57.3 percent SiOz. The average chilled margin composition (table 4, col. 4) is virtually identical to the calculated bulk composition. This result indicates that the variations 12 IJIFWVEIIEPJTTIYFBDPJ()F.A.ChAI3BIU3 Sllli CHIECHDPJCNDIXSTTILAPHSE TABLE 5.—Compositions of successive residual liquids [Residual liquid compositions were determined using bulk composition of sill (table 4, col. 6) equal to. composition of sill at time of emplacement (0 percent solidified) and successively subtracting compositions or rocks starting at the base of the sill and progressing inward with each rock analysis weighted according to interval thickness it represents. For purposes of the calculation the average gabbro analysis from the upper gabbro zone was used for all rocks in that zone. The average composition of all rocks in the silicic zone is presumed to approximately equal the composition of the residual liquid when the silicic zone first began to form (75 percent solidified) and the average granophyre to approximately equal the liquid at 95-100 percent solidified.] Percent solidified- 0 2O 25 30 35 43 50 59 67 75 95-100 5102 ----- -— ------- 57.3 57.4 57.5 57.7 58.0 58.5 59.4 60.3 62.3 64.6 74.5 A1203 ------------- 13.4 13.4‘ 13.4 13.4 13.4 13.4 13.5 13.5 13.6 13.6 13.2 FeO + Fe203 ------- 14.4 14.3 14.2 14.1 13.9 13.6 13.1 12.4 11.1 9.6 2.8 Mgo --------------- 1.6 1.6 1.6 1.6 1.5 1.4 1.3 1.2 .94 .67 .14 CaO --------------- 6.1 6.1 6.0 6.0 5.9 5.7 5.4 5.1 4.6 3.9 .54 NaZO -------------- 3.0 3.0 3.0 3.0 3.1 3.1 3.2 3.3 3.4 3.7 4.6 K20 --------------- 1.7 1.7 1.7 1.7 1.7 1.8 1.9 2.0 2.2 2.5 3.6 T1102 -------------- 1.7 1.7 1.6 1.6 1.6 1.5 1.5 1.4 1.2 1.0 .32 P205 -------------- .70 .69 .67 .66 .65 .62 .58 .56 .45 .33 .02 Mno --------------- .26 .25 .25 .25 .24 .23 .22 .22 .20 .17 .07 NORMS (Calculated with Fezo3 = 10 percent total iron) Q ---------------- 12.3 12.4 12.6 12.9 13.1 13.7 14.5 15.5 17.9 19.7 30.8 or ——————————————— 10.1 10.1 10.1 10.1 10.1 10.6 11.2 11.8 13.0 14.8 21.3 ab ——————————————— 25.4 25.4 25.4 25.4 26.2 26.2 27.1 27.9 28.8 31.3 38.9 an --------------- 18.1 18.1 18.1 18.1 17.6 17.3 16.9 16.1 15.4 13.1 2.6 we ---------- 3.2 3.2 3.1 3.1 3.1 2.9 2.6 2.3 1.9 1.7 — di: en ---------- .6 .6 .6 .6 .5 .5 .4 .4 .3 .2 — fs ---------- 2.9 2.9 2.8 2.8 2.8 2.7 2.4 2.2 1.8 1.7 - en ---------- 3.4 3.4 3.4 3.4 3.2 3.0 2.8 2.6 2.1 1.5 .4 hy: fs ---------- 17.4 17.2 17.3 17.2 16.8 16.7 16.1 15.4 14.0 12.1 4.0 mt --------------- 2.1 2.1 2.1 2.0 2.0 2.0 1.9 1.8 1.6 1.4 .4 i1 --------------- 3.2 3.2 3.0 3.0 3.0 2.9 2.9 2.7 2.3 1.9 .6 ap --------------- 1.7 1.6 1.6 1.6 1.5 1.5 1.4 1.3 1.1 .8 - in composition of the sill resulted from differ- entiation of magma in place. Had there been multiple injection of magma (for which there is no evidence), then either the magmas were similar in composition or the amount of later magma of different composition was small. Furthermore, no significant amount of sedimentary rocks could have been assimilated after the chilled margins formed. RESIDUAL LIQUID COMPOSITIONS Residual liquid compositions were determined by subtracting the compositions of successive intervals of rock starting at the margins of the sill and progressing toward the interior. The bulk residual liquids can be calculated only for the interval during which the lower and upper gabbro zones solidified because these show systematic changes in rock compositions whereas the order of formation of rocks within the silicic zone is poorly known. Compositions of the residual liquidsiare shown in table 5. Although the calculated residual liquid compositions define the gross changes in the bulk magma during solidification, they do not necessarily represent the liquid compositions near the sites of crystallization because of crystal growth and settling and because the liquid in the sill could not have been perfectly homogenized. Furthermore, the residual liquid compositions that were calculated are those of the RESIDUAL LIQUID COMPOSITIONS TABLE 6.—Comparison of calculated residual liquids to compositions of mixtures of analyzed rocks and silicic liquids 13 Height (meters) ------ 13-1/2 24-1/2 29 35 42-1/2 Percent solidified --- 10 19 22 27 32-1/2 Factor1 -------------- 0.062 0.278 0.287 0.371 0.467 205 205 206 206 207 207 208 208 209 209 Liq.2 Calc.3 Liq. Calc. Liq. Calc. Liq. Calc. Liq. Calc. SiO2 ----------------- 57.3 57.3 57.4 57.4 57.5 57.5 57.6 57.6 57.9 57.9 A1203 ---------------- 13.4 13.4 13.4 13.7 13.4 13.3 13.4 13.3 13.4 13.5 FeO + Fe203 ---------- 14.4 14.1 14.3 14.4 14.3 14.2 14.2 14.3 14.0 14.0 Mdo ------------------ 1.6 1.6 1.6 1.4 1.6 1.7 1.6 1.6 1.6 1.7 CaO ------------------ 6.1 6.4 6.1 5.9 6.1 6.2 6.0 6.0 6.0 6.0 Na20 ----------------- 3.0 2.8 3.0 2.9 3.0 2.9 3.0 2.9 3.0 2.9 K20 ------------------ 1.7 1.8 1.7 1.6 1.7 1.7 1.7 1.8 1.7 1.8 T102 ----------------- 1.7 1.7 1.7 1.6 1.7 1.6 1.6 1.7 1.6 1.6 P205 ----------------- .70 .71 .69 .80 .68 .73 .67 .69 .66 .70 MnO ------------------ .26 .27 .25 .26 .25 .27 .25 .26 .25 .27 Height (meters) ------ 49 62-1/2 70 81 91 Percent solidified -—- 37-1/2 48 53 62 70 Factor --------------- 0.693 1.018 1.264 2.600 5.670 210 210 211 211 212 212 213 213 214 214 Liq. Calc. Liq. Calc. Liq. Calc. Liq. Calc. Liq. Calc. SiO2 ----------------- 58.2 58.2 59.1 59.1 59.7 59.7 61.1 61.1 63.1 63.1 A1203 ---------------- 13.4 13.5 13.5 13.3 13.5 13.6 13.5 13.5 13.6 13.6 FeO + Fe203 ---------- 13.8 13.6 13.3 13.1 13.5 13.6 13.5 13.5 13.6 13.6 MgO ------------------ 1.5 1.5 1.3 1.4 1.3 1.4 1.1 1.1 .85 .85 CaO ------------------ 5.8 5.9 5.5 5.7 5.3 5.4 4.9 4.8 4.4 4.4 NaZO ----------------- 3.1 3.1 3.2 3.2 3.2 3.3 3.4 3.4 3.5 3.5 K20 ------------------ 1.7 1.8 1.9 1.9 1.9 1.8 2.1 2.1 2.3 2.3 TiO2 ----------------- 1.6 1.6 1.5 1.5 1.5 1.3 1.3 1.4 1.1 1.1 P205 ----------------- .64 .65 .60 .60 .57 .49 .51 .52 .41 .41 MnO ------------------ .24 .25 .22 .24 .22 .18 .21 .21 .19 .19 1Factor is the amount of liquid with composition of the silicic zone (sioz=64.6) added to each rock to match the Sio2 of the calculated mixture with the residual liquid composition. 2Interpolated values from table 5. 3SiO2 of residual liquid and calculated mixture are by definition the same, all other oxides of the mixture are calculated using the factor determined to balance the SiO2 contents. bulk liquid plus all crystals suspended in it. The residual liquid compositions can be closely duplicated by combining each analyzed rock and an appropriate proportion of liquid more silicic than the calculated residual liquid. In table 6, the residual liquid compositions are compared to calculated mixtures of each rock and an appropriate amount of liquid whose composition is that of the average silicic zone rock (table 4, col. 2). The calculations were made so as to duplicate the SiO2 content of the residual liquid (as for example, for rock 21 1, SiO2 of rock = 53.5, Si02 of residual liquid at the time of formation of rock 14 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE 211 = 59.1, as extrapolated from table 5, SiO2 of silicic zone = 64.6, thus W:591 andx:1018 ]-+}( . , . . All other oxides of the mixture were calculated using 1 part of rock 211 and 1.018 parts of composition like that of the silicic zone rock). Similar results can be obtained using liquids more or less silicic than the average silicic zone rock. The similarity of the mixtures to the residual liquid compositions indicates that removal of relatively silicic liquid residues from crystal accumulations can produce the observed rock compositions. CHEMICAL VARIATION The composition of the analyzed rocks and cal- culated successive bulk residual liquids are plotted on a solidification diagram (Wager, 1960) in figure 5. It shows the composition of the rocks that formed at various stages during the solidification of the magma and the corresponding gross changes in the magma’s bulk composition. The successive residual liquids began to differ appreciably from the initial composition when the sill was about 20 percent solidified. With respect to the initial bulk composition, the liquid was enriched in Si02, NaZO, and K20 and depleted in all other major oxides except A1203, which remained nearly constant. The liquid progressively changed composition until the last liquid crystallized and, at this end stage, differed greatly in composition from the initial liquid. The liquid trends are smoother than the rock trends because the former were determined using successive rock analyses, which tend to smooth out the small scatter shown by the rock analyses. Much of the scatter in this and succeeding diagrams is within the range of analytical error for rapid-rock analyses. The CaO content of the rocks reached its maximum when the sill was 50 percent solidified, and FeO+Fe203, MgO, Ti02, and P205 were maximum when the sill was about 60 percent solidified. This stage in the solidification cor- responds to the formation of the upper part of the lower gabbro zone and approximately to the crystallization of rock M213. The rock composition trends for all oxides cross the bulk composition lines at a time corresponding to 75 percent solidification. Thus rocks that formed at about this stage (for example, table 2, col. 6) may have compositions similar to the bulk composition and the chilled margins, although much different from the residual liquid from which they crystallized. | I I I 70 — _. SiO 2 60— sq- 18 _ Fe0+Fe103 skate/M 10; N - _ 0210 O 4 - N _ Fe0+ F810J ,94 — Fe0+Fe203+Mgo — .92— _ .0 .AVU O / V . O .90 4 _ N310 MAJOR OXIDE CONTENT, IN WEIGHT PERCENT O I 3 " TiO, _ 2 _ o _ ." U U W 1‘ \ 0 — P205 ‘ 1— o . fl 0' V W o I l l I 0 20 40 60 80 100 SOLIDIFICATION, IN PERCENT Figure 5. Solidification diagram showing amount of major oxides in rocks and bulk residual liquids at various stages in solidification of gabbro sill. Rock trends are shown by a solid circle and liquid trends by an open circle. Horizontal lines show bulk composition of sill; vertical line at 75 percent solidification indicates beginning of formation of silicic zone. PROCESS OF DIFFERENTIATION 15 14—— 3% l _ ’_ 1. o l ~4————-— M213 —————> 5‘ p— 2 Lu _ _ U a“. a. 2“ . — E 9 O” ‘ LU 2— _ ; MgO Z 0_ o — ’5 LL] 8— . _ r— o O Z . ~ 0 — _ U . “9" C00 4— — >< . O — _ CK O O 9., o— + + — < E 6'— . .— 2— + 4— — 4— ’ o O _ K20 _ . M _ 0— o + + - 2— .' f“ ._ TiOz MM 0~ + + 0 ° — 2— _ P204 ' 0‘. o 50 60 7o SILICA CONTENT, IN PERCENT Figure 6. Silica variation for rocks from gabbro sill. Bulk com- position shown by x, residual liquid compositions are shown by solid lines, and composition of residual liquid at time of crystallization of gabbro M213 is shown by open circle. The rocks show a concomitant increase in N aZO and K20, relatively constant A1203, and decrease in total iron, MgO, CaO, TiOz, and P205 with respect to increase in silica (fig. 6). The rock analyses plot along smooth curves except for those with high SiOZ content, which show some scatter, especially with respect to Na20 and K20 (total alkalies, however, plot along a smooth curve). As the sill solidified, the newly formed rocks had a progressively lower silica content starting at 57 percent SiO2 (chilled margin and bulk composition) and decreasing to 52 percent- A \l M Figure 7. FMA (F=Fe0+0.9Fe203+MnO, M=MgO, A=N20+K20) diagram of analyzed rocks and residual liquids of Stott sill. Open circle shows composition of residual liquid at time of formation of rock M213; solid line shows variation in composition of residual liquids; and dashed line Sk shows Skaergaard trend. Si02 (sample M213, table 1) when the sill was about 60 percent solidified. After that, the rocks were successively enriched in SiO2 until formation of the granophyre with 76 percent SiOZ. The analyzed rocks all have high Fe/ Mg ratios and plot along a nearly linear trend on an FMA diagram (fig. 7). Unlike many other differentiated dolerite or gabbro sills, the rocks from the Stott Mountain sill show no marked early change in the uniformly high Fe/Mg ratio. On the basis of 200 analyses of rocks from related sills in the Oregon Coast Range (P. D. Snavely, Jr., and N. S. MacLeod, unpub. data), however, this unusual F-A trend is characteristic, and none of the sills show an F-M trend. The bulk composition of the Stott sill, with its high Fe/Mg ratio, is much like that of late-stage rocks of many differentiated dolorite sills. Rock compositions changed initially to lower relative alkali content and slightly lower Fe/Mg ratios until about 40 percent residual liquid remained, at which time sample M213 crystallized, and then showed a marked relative increase in alkalies. In contrast, the residual liquid trend is toward continuously higher alkali content. PROCESS OF DIFFERENTIATION Any explanation ofthe processes of differentiation of the gabbro sill must take into account the following: (1) The sill is thin compared to other gabbro, diabase, or dolerite sills that show similar degrees of 16 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE differentiation. (2) Agreement between the calculated bulk compo- sition and the chilled margin composition shows that no large amount of material could have been assimilated by the magma within the sill and that there could not have been large additions of magma of different composition. The sedimentary rocks marginal to both the Stott Mountain sill and the related Marys Peak sill also have compositions (Snavely and others, 1969) grossly different from the sill rocks (that is, the sedimentary rocks have more MgO and A1203). Also, no field evidence was found for assimilation of sedimentary country rock or multiple intrusion. Furthermore, lead-isotope ratios of gabbro, granophyric diorite, pegmatite, and granophyre from the Marys Peak sill are uniform and differ from isotope ratios of the sedimentary‘ rocks intruded by the sill (Tatsumoto and Snavely, 1969). (3) Neither rhythmic layers nor crystal lamination were observed. The gabbro is uniformly homo- geneous in gross aspect even though the lower half of the sill shows vertical chemical and mineralogic variations. (4) Differentiation of the magma produced asymmetric trends in rock compositions, mineral proportions, and mineral compositions. Rocks show, a progressive increase in MgO, total iron oxide, CaO, TiOZ, and P205, and decrease in Si02 and alkalies, from the base toward the center of the sill. Plagioclase, olivine, opaque minerals, and apatite increase in abundance, and pyroxene and olivine are progressively richer in magnesium over the same interval. The most mafic rock occurs somewhat above the center of the sill, and silicic rocks occur at a high level. (5) The sill was intruded under a thick cover of sedimentary rocks in middle Oligocene time. Much of the cover has been eroded, but farther west, Eocene and lower Oligocene marine sandstone and siltstone have a thickness of about 1.5 km (Snavely and others, 1976), and the sill was intruded near the base of this sequence. Owing to the insulating cover, conductive heat loss should have been about the same from the base and top of the sill during solidification (Jaeger, 1957, p. 312), unless convection, crystal settling, or some other process of heat transport was involved. Convection and crystal settling both cause asymmetric heat loss resulting in final solidification in the upper part of a sill rather than at its center (Irvine, 1970). (6) The chilled margins are aphyric, and the magma was therefore near or above liquidus temperature. (7) The last minerals to crystallize in early-formed rocks have bulk compositions grossly similar to the composition of late-formed rocks. (8) The parent magma had an unusual composition (that is, high SiOz, P205, and total iron, and low MgO). A differentiation process that appears to account for the distribution of rock types, the mineralogic and chemical variations, and absence of rhythmic layers and crystal lamination combines the progressive inward crystallization proposed by Hess (1960) with crystal settling within zones of crystallization at the floor and roof of the liquid interior of the sill. Differentiation as a result of crystal settling within crystallization zones has been discussed by Jackson (1961), Wilshire (1967), and Irvine (1970). Richter and Moore (1966) report such a transient downward- moving zone of crystallization beneath the developing crust of the Kilauea lava lake. According to the suggested model, as the sill solidified, one crystallization zone moved continu- ously upward from the base and another downward from the top (fig. 8). Inferred relations in the lower zone of crystallization are shown in figure 9. The order of nucleation of the minerals shown by textural relations suggests that the minerals first nucleated at different heights within the zone. Thus, the top of the lower zone contained magma plus apatite crystallites, the first mineral phase to nucleate. Progressively lower parts contained (1) magma, apatite, plus opaque minerals; (2) magma, apatite, opaque minerals, plus plagioclase; (3) magma, apatite, opaque minerals, plagioclase, plus olivine; (4) magma, apatite, opaque minerals, plagioclase, olivine, plus ferroaugite; and (5) at the base of the zone, apatite, opaque minerals, plagioclase, olivine, ferroaugite, plus quartz-feldspar intergrowths. Even though the various minerals first nucleated at different heights in the zone of crystallization, in successively lower positions they continued to grow and react with residual liquid. The early-formed minerals in the lower zone of crystallization were denser than the enclosing magma and thus would have settled. In order broadly to define relative settling velocities (fig. 10), the approximate viscdsity and density of the residual bulk magmas were calculated from their chemical compositions using the methods of Shaw (1972) and Bottinga and Weill (1970), respectively. Crystals were assumed to have the same volumes as they do in the rocks but, for simplification, were assumed spherical; Jackson (1971) shows that elongate crystals will settle about 10 percent more slowly than spheres of the same volume. According to the model, PROCESS OF DIFFERENTIATION 17 Baked sedimentary rocks \\7\\7\\ __/\\/\\’_ {\7’\/\7\/\ /\1\7\\’. f\7k\7\/\ 7\\7\\; —— \\7\\73\ 7\\/\\/ ;\7\\7\\\ , \\/\\ -— \\/\ 7 l\;\\;\/ {\7\/\7\\ 7\\7\\// 7\ \;\\7 \\’\\’\ \\/\\/\\ / / / / 7\\7\\7 WW? \\;\\7\ \\/\\7\\ 7\\7\\7 7\’\7\\7- \\;\\;\ \\//\\7\\ /\\/\\/ / / \/ \\ \ / , /\, I / / / / /\ \\ \ -’\ A\:.r,,-,-=\g>;[zl:- ’:l':"l‘ I‘: ‘)’l '. Baked sedimentary .. . .. . rocks ~ Time of Successwely later times 9 Time of initial final intrusion solidification Figure 8. Progressive solidification of gabbro sill. Horizontal lines delimit upper and lower zones of crystallization; arrows indicate direction of solidification. the crystal size would increase downward in the lower crystallization zone; if the crystals grew continuously in size during settling, their initial velocity would be much lower and average settling velocity would be only one-third that of crystals with constant size, assuming constant viscosity. Thus, the calculated rates for various temperatures and liquid compositions (fig. 10A, B) are somewhat greater than the actual rates would have been if crystals settled as individuals. Settling of crystal aggregates formed as a result of heterogeneous nucleation (Campbell, 1978), on the other hand, would be more rapid than settling of individuals. Numerous laboratory experiments show that crystallization of basaltic magmas commonly occurs over a 100°—200°C temperature interval. No data are available for rocks with compositions similar to those in the sill, but a similar temperature interval appears reasonable; considering the unusual compo- sition, the interval may even have been greater. The temperature interval over which the various minerals crystallized and settled would change as the lower crystallization zone migrated upward because the bulk residual magma became pro- gressively more silicic in composition. Crystals nucleating in the upper part of the crystallization zone would not settle until they reached a size large enough to overcome the yield strength of the magma. Upon reaching sufficient size, they would begin to settle, and the rate would increase as their size increased at progressively lower positions in the zone. At even lower positions, the rate would decrease because the remaining residual liquid at successively lower positions would have had a continuously higher viscosity (more silicic composition and lower temperature). To a certain extent, the downward decrease in settling rate caused by increasing viscosity may have been counteracted by an increasing tendency for crystals (because of higher crystal content) to settle as aggregates at a rate higher than they would as individuals. V The combined effect of changing temperature and liquid composition (arbitrarily related) on settling rate are shown diagrammatically in figure IOC. Without liquidus temperature determinations it is not possible to speculate on the thickness of the crystallization zone nor on the heights within the zone at which the various minerals would nucleate. Figure 10C does indicate, however, that the settling 18 EARLY 1 LATER Sill top | 2 --w o> 3|— op — 79° 9 m-E‘ 2 0P 0352‘” c «87,2- ———->pl 3.9—8.» CWEJL’“ ol 0 ww>m 1: Cr-UE 2 “=0 f 5 8.1299 0 “Dwu a— U «0 c.5758 Z"—W- U Highest nucleation of: :— Cap 2 OP 0 I .58 P <—# =0 oI EN .1: f 2‘ U .3 <—- ChiIIed base —> 9 Sediment % w Figure 9. Inferred crystallization and settling of minerals in lower zone of crystallization at two different periods in solidi- fication of gabbro sill. ap, apatite; op, opaque minerals; pl, plagioclase; 01, olivine; f, ferroaugite. rates would have decreased by an order of magnitude from the upper to the lower part of the zone even if it '\ represented a temperature interval of only 100°C. The settling rates would have been so low near the base even for crystal aggregates that the crystals would effectively become frozen in position at a height higher than the point at which an intact crystal framework (about 50 percent crystals) would itself inhibit further settling. Thus in the absence of a well-defined floor and because of the extremely low settling velocity in the lower parts of the zone, crystal lamination could not result from crystal settling, nor would the resulting textures be obviously cumulate. The calculated settling rates appear to be generally consistent with the model of crystallization zone settling, considering the apparent orders of nucleation of the minerals and the amount of upward increase they show in the rocks of the lower gabbro TEMPERATURE. IN DEGREES CELSIUS SILICA CONTENT/ TEMPERATURE DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE I I I I I I I 1300 _ ap pl op o 1200 — I “0° sao2 : 57% | I I 1 000 58— I 62- 66— T=1100°C 70 '- SILICA CONTENT, IN PERCENT 74 1 57/1300 57/1200 I 64/1100 — 74/1000 l I 1 10 100 1000 SETTLING VELOCITV, IN METERS PER YEAR 001 .01 Figure 10. Calculated settling rates of apatite (ap), plagioclase (pl), opaque minerals (op), olivine (01), and ferroaugite (f), as a function of temperature and silica content of residual liquid. A, Decrease in settling rate with decrease in temperature. Calculated viscosity ranges from 6.7x102p at 1300°C to 3.6x104p at 1000°C and density increases from 2.56 to 2.60 over same interval. B, Decrease in settling rate as a function of increasing SiO2 content of residual liquid with viscosity of least silicic liquid of 7.8x103p increasing to 2.3x106 for most silicic and density decreasing from 2.58 to 2.33 over same range. C, Marked decrease in settling rate that results from decreasing temperature and increasing silica content of liquid. Settling rates were calculated based on constant crystal size; actual settling rates would have shapes shown by dashed line if crystal growth were taken into account. zone. Apatite, the first phase to nucleate, shows the greatest relative upward increase in amount in the lower gabbro zone. As the calculated settling velocity for apatite is very low (small crystal size), for it to have accumulated in large amount by settling requires that it settle from a very high position relative to the other minerals. Thus, for much of the zone, only apatite could have been present in the magma. Apatite crystals commonly do occur enclosed in olivine so they would have in part settled as aggregates at a higher rate, but much of the apatite must have settled as individual crystals because it increases in amount upward more than does olivine. Early crystallization of apatite is consistent with the very high P205 content of the magma (bulk composition has 0.7 percent P205). Opaque minerals nucleated after apatite. Because PROCESS OF DIFFERENTIATION 19 they have a much higher calculated settling rate, yet increase in abundance upward nearly as much as apatite, the height at which opaque minerals became large enough to settle must have been far below that of apatite. Plagioclase nucleated after apatite and opaque minerals, thus at a lower position in the crystallization zone, but its calculated settling velocity is very low (low density contrast with the magma), consistent with the minor upward increase it shows. The calculated settling velocity for plagioclase is the least reliable, for its density contrast with the liquid is so low that slight differences between the actual and calculated magma density greatly affect settling rate. However, aggregate grains including plagioclase would settle at rates higher than those calculated. Olivine nucleated next and increases upward more than plagioclase because it has a settling rate an order of magnitude greater. Ferroaugite, the last cumulus phase to nucleate, has the highest calculated settling velocity yet remains about constant in amount throughout the lower gabbro zone. Apparently at its low position of nucleation and growth in the zone, the viscosity of the remaining residual liquid was high, and ferroaugite settled much less than the other minerals. Even though ferroaugite does not significantly change in abundance upward, it nevertheless must have settled to some extent because the bulk residual magma composition was changing: the absence of ferroaugite settling would correspond to an upward decrease in ferroaugite. Compaction of the crystal mush in the lowest part of the crystallization zone may have been sufficient to increase the proportion of ferroaugite slightly compared to the amount produced by an increasingly more silicic magma with less potential ferroaugite. Residual magma displaced upward by crystal settling within the lower zone would be depleted in components of the early-formed minerals and less dense than the magma overlying the zone of crystal- lization. The displaced residual magma may have diffused into the overlying magma in response to density, chemical, and thermal gradients. That part of the residual magma which remained trapped in the lower part of the zone ultimately formed adcumulus rims on crystals and the intercumulus quartz-feldspar intergrowth and phyllosilicate patches. ' The phyllosilicate patches initially decrease in abundance upward and then increase toward the center of the sill to a value similar to that near the base. This increase may result from a possible immiscible relation between the iron-rich liquid from {which these patches were formed and the silicic residual melt that crystallized as intergrowth. Perhaps the residual liquids within the lower crystal- >lization zone were initially displaced before immiscibility could occur, whereas during later stages the residual liquid unmixed and only the light silicic residual liquid was displaced upward. ~ On the basis of the calculated magma Viscosity and the thickness of the sill, it seems likely that magma within the interior of the sill would convect (Bartlett, 1969, fig. 1). No evidence indicating convection was found, but the lower crystallization may have lain quiescent below the convecting interior, such as Jackson (1961, p. 97) postulated for the Stillwater Complex of Montana. Convection in the interior of the sill would have tended to homogenize displaced residual magma with magma in the interior. When the lower zone of crystallization was near the base of the sill, crystal settling would have been minor because the zone was thin (steep thermal gradient) and its base (surface of complete solidi- fication) moved rapidly upward. The .early crystals in the zone would have reacted with entrapped residual interstitial liquid, and the bulk composition of the rock would be virtually unchanged from that of the original magma. As a sill cools, isotherms progress inward from the contacts approximately in proportion to the square root of time (J aeger, 1957, p. 79). Therefore, the distance between any two isotherms, such as bounded the lower zone of crystallization, would increase as they migrated upward (figs. 8 and 9). The distance from top to bottom of the zone would increase even though the bounding isotherms would change as the zone migrated upward as a result of changing residual magma composition. Consequently, because increasing time would be available as the crystallization zone moved upward, the efficacy of crystal settling would increase and progressively more residual liquid would be displaced. Such'a process is suggested by the data of table 6 in which residual liquid compositions are closely approximated by mixtures of analyzed rocks and relatively silicic liquids. The proportion of displaced silicic liquid (factor of table 6) increases with the degree of solidification and height (fig. 11). Although the compositions of the displaced residual liquids are not known and probably changed as the zone of crystallization progressed upward, reasonable compositions more silicic than the residual liquid at the time each rock formed can produce similar results with different amounts. 20 DIFFERENTIATION OF A GABBRO SILL, OREGON COAST RANGE 100 | | 80 _ / / - / 80— V ‘60 60— _ '2 / e (D E j E g ” -40: z 2 E. J 940' LT. l I _ g C 1 m [- —20 NT ,0 l I l l I 0 1 2 3 4 5 e FACTOR Figure 11. Proportion (factor) of liquid with composition of average silicic zone rock that added to rock duplicates composition of residual liquid (see table 6). The relative distances that crystals of olivine could settle within the crystallization zone as a function of height above the base of the sill are shown in figure 12. At progressively greater height, the zone of ‘ crystallization would be progressively thicker, and thus more time would be available for individual crystals to settle. Thus, the efficacy of settling increases as the zone moves upward and as its thickness increases. At the last stage of formation of the lower gabbro the effect of settling decreases as the increasing viscosity of the bulk residual magma in the interior begins to have a strong influence on settling rate. Although the curve shown is based on calculated distances of settling, no scale for distance is shown on figure 12 because no data are available on the probable temperature interval over which olivine would have been on the liquidus. For simplification in the calculation, the bounding isotherms were assumed to remain the same, whereas they actually would have decreased upward c0100 80 60 40 20 HEIGHT ABOVE BASE 0F SILL, IN METER RELATIVE DISTANCE SETTLED Figure 12. Relative distance that olivine could settle as a function of height within sill. As crystallization zone moved upward it became progressively thicker allowing more time for settling. Late decrease in settling rate results from increas~ ing viscosity of residual liquid. as the bulk liquid became more silicic. The loss of relatively silicic residual liquid (displaced upward) and the addition of settled crystals would cause the bulk composition (crystals plus remaining liquid) of the lower part of the crystallization zone to be more refractory. Therefore, the crystals would have a more refractory composition when crystallization was completed at successive intervals than if no settling had occurred. Such an upward increase in refractory components of minerals is shown by the upward increase in Mg/ Fe in olivine and ferroaugite, the slight absolute increase in Ca/ Na in plagioclase, and the increase in Mg/Fe and Ca/Na of rocks from the base toward the center of the sill. As the rocks became progressively more mafic, the bulk residual magma became more silicic. "If crystallization zone settling was the major SUMMARY 21 . process of differentiation of the gabbro sill, the residual magma, displaced by settling crystals, must have been mixed with overlying magma. The displaced residual liquid would have had a lower density (more silicic composition) than the overlying, probably convecting, magma and thus may have risen buoyantly and have been homogenized with it. If the displaced residual liquid was not mixed, but settling still occurred, a strong chemical gradient would have been locally established and chemically graded rhythmic layers probably would have been produced (Jackson, 1961, p. 94-98). The upper zone of crystallization progressed continuously but slowly downward during solidifica- tion of the sill (fig. 8). In contrast to the lower zone, settling in this upper zone would result in removal of crystals from the zone. These crystals would sink toward the interior of the sill into magma hotter than in the upper zone of crystallization where they formed. Therefore, the settled crystals would be progressively resorbed during their downward transport if sufficient heat was available. Heat released in the upper zone by crystallization would be removed from the interior of the sill by resorption. Thus crystallization, settling, and resorption probably would produce a net upward transport of heat, and as a result, high temperatures would be maintained near the top of the sill, causing it to solidify less rapidly from the top than from the base (Irvine, 1970). This process would largely account for the smaller thickness of the upper compared to the lower gabbro zone. The crystals that settled from the upper zone would, upon being resorbed, add refractory components to the interior magma. At the top of the upper zone of crystalli- zation, the residual magma would have an increased viscosity (cooler and more silicic as a result of downward settling of early minerals) and settling would be impeded; thus this zone would move slowly downward. If the crystals that settled downward from the upper crystallization zone had not been resorbed in the interior, but had settled through the sill into the lower zone of crystallization, a rhythmic layering or sharp increase in minerals with a high settling rate (for example, olivine) would be expected in the lower part of the sill. No such rhythmic layering or marked increase in any of the early minerals was observed. In addition, the first crystals to settle through the sill to the bottom would have had the more refractory composition, but no such compositions are found in the lowest part of the sill. Indeed, the most refractory mineral compositions occur near the center of the sill. Because the upper zone progressed downward much less rapidly than the lower zone moved upward, the displaced silicic residual magma accumulated high in the sill where it formed the rocks of silicic zone. The variations exhibited by rocks in the silicic zone most likely are the result of several processes including filter pressing (compaction of crystal mush), crystal settling, and possibly alkali diffusion. Crystal settling would have been much less efficient in producing compositional changes in the magma in this zone because viscosity was much greater (more silicic magma) than at an earlier stage when the lower gabbro zone was formed. Trans- gressive ferrogranophyre and granophyre bodies probably are the products of magma that was filter pressed from a somewhat lower part of the silicic zone (where the last part of the sill, other than the transgressive bodies, crystallized). The magma was injected into the upper part of the silicic zone and in places into the upper gabbro zone, perhaps along vertical and horizontal joints. The wide variation in NaZO/ K 20 in the granophyres, whose composition is otherwise similar, suggests that alkali diffusion may have occurred during the last stages of crystalliza- tion. SUMMARY Crystal settling in zones of crystallization that moved inward from the margins of the Stott sill as it solidified appears to account for the variation in mineralogy and petrochemistry in the sill. In the lower zone of crystallization, the amount of settled individual minerals was controlled by their height of nucleation and settling rate. Viscosity increased downward so that settling rates were very low near the base of the zone and no well-defined floor was present. Thus lamination or layering was not produced. The amount of settling increased as the zone moved upward because the zone became increasingly thicker and more time was available for settling. Liquid displaced upward by settled crystals was probably homogenized with magma in the interior of the sill. Crystals in the upper zone of crystallization settled downward into the interior of the sill where they were resorbed. As a result of the effects of heat of crystallization and resorption, high temperatures were maintained in the upper part of the sill, and the upper crystallization zone moved downward much more slowly than the lower crystallization zone moved upward. Thus the last part of the sill to solidify was in the upper part. Differentiation by settling in zones of crystal- lization should be particularly pronounced where the constituent minerals crystallize over a wide tempera- ture range. Such a wide temperature range may be 22 much more important for this type of differentiation than thickness of the sill and consequent greater time of solidification. Also important is the magma viscosity because of its effect on settling rate. Other sills much thicker than the Stott sill may have undergone a similar history but show much smaller degrees of fractionation. 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W., 1976, Geologic map of the Cape Foulweather and Euchre Mountain quadrangles, Oregon: U.S. Geological Survey Miscellaneous Investigations Series Map I—868, scale 1:62,500.. Tatsumoto, M., and Snavely, P. D., Jr., 1969, Isotopic composition of lead in rocks of the Coast Range, Oregon and Washington: Journal of Geophysical Research, v. 74, p. 1087-1100. Uruno, K., 1963, Optical study of the ordering degree of plagio- clase: Tohoku University Science Reports, 3d series, v. 8, no. 2, p. 171—220. . Wager, L. R., 1960, The major element variations of the layered series of the Skaergaard intrusion [Greenland] and a re estimation of the average composition of the hidden layered series and of the successive residual magmas: Journal of Petrology, v. 1, no. 3, p. 364-398. Wager, L. R., and Brown, G. M., 1968, Layered igneous rocks: 1 Edinburgh and London, Oliver and Boyd, 588 p. Wilshire, H. G., 1967, The Prospect alkaline diabasepicrite intrusion, New South Wales, Australia: Journal of Petrology, v. 8, p. 97163. Yoder, H. S., Jr., and Sahama, T. G., 1957, Olivine X-ray determina- tive curve: American Mineralogist, v. 42, p. 475-491. GPO 789-036/60 77.:.77.JJ.J,JJ,.77...~,J.:7, ,. , 71-...9... s7 7 7.319;. ,. , . _, :JJ .7....J771s 1‘17“ . . 99.9,... 97...... 7,7 JJ,-._.771.J‘7 ‘.7 1J1 77,13“ 7 J-7-17717.1J,1.J7-,-77,:J.1 . ,, 9,9 . .1» .77. ..,. , 7 J. .,., ,,.,,“ L x , , .. J . £777 7..,..:-.:.,;1,7:-7,17.7‘7‘77,_71::.:~1.717 ‘ . ,7 x1...7...77,:. .J:,.,.,,, .2717... .J,» .,:-27 : «,.. 7 , .7 : .,, :,»-17.: 7 7 J35, .,,....5 199., .. 7J7:777 J 7.. 71 {s J J. . ‘777J.1J1717§17.::1.z7»J7»§.7717.7....771J;:..J:,, 7JJ ' ..7.7... 7...: , 7.1.5799. .7.7.7J 7 .J. J,1..J .J . .6 77.-2:71“ ..,,J3-9.2P.,:;J77,.J,.17,7.7.:71,1m-7.J.~»,J71.7777 .7.. ,. J . 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RICHARD DUNRUD With a section on EARLY MAN IN THE SUNNYSIDE AREA By jAMES o. DUGUID, JR. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1166 Description of the geologic setting and economic potential of an east- centml Uta/z coal-mining district UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1981 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES C. WATT, Secretary GEOLOGICAL SURVEY Doyle G. Frederick, Acting Director Library of Congress Cataloging in Publication Data Osterwald, Frank W. Bedrock, surficial, and economic geology of the Sunnyside coal-mining district, Carbon and Emery Counties, Utah. (Geological Survey Professional Paper 1166) Bibliography: p. 63 Supt. of Docs. no.: I 19.1621166 l. Geology—Utah—Carbon County. 2. Geology-Utah—Emery Co. I. Maberry, John 0.,joint author. II. Dunrud, C. Richard, joint author. III. Title. IV. Series: United States Geological Survey Professional Paper 1166. QE170.C37084 557.92’566 79—607177 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 CONTENTS Page Abstract ................................................ 1 Introduction ............................................ 1 Fieldwork and acknowledgments ........................ 3 Physiography and general geology ...................... 4 Early man in the Sunnyside district, by James O. Duguid, Jr .............................. 5 Early exploration and development of coal in east-central Utah ................................... 7 Stratigraphy ............................................ 7 JurassicSystem.....................................;8 Carmel Formation ................................. 8 Entrada Sandstone ................................ 8 Curtis Formation ................................. 9 Summerville Formation ............................ 9 Morrison Formation ............................... 9 Salt Wash Sandstone Member .................. 11 Brushy Basin Shale Member ................... 12 Cretaceous System .................................. 13 Cedar Mountain Formation ........................ 13 Buckhorn Conglomerate Member ................ 13 Unnamed shale member ........................ 14 Dakota Sandstone ................................ 14 Mancos Shale .................................... 15 Mesaverde Group ................................ 17 Blackhawk Formation ......................... 17 Aberdeen Member ......................... 18 Kenilworth Member ........................ 18 Lower mudstone member ................... 19 Sunnyside Member ........................ 19 Upper mudstone member ................... 19 Castlegate Sandstone .......................... 20 Price River Formation ......................... 20 Lower unnamed member .................... 21 Bluecastle Sandstone Member ............... 21 Cretaceous and Tertiary Systems, undivided ............. 22 North Horn and Flagstaff Formations ............... 22 Tertiary System ..................................... 23 Eocene Series .................................... 23 Colton Formation ............................. 23 Green River Formation ........................ 24 Quaternary System .................................. 24 Pleistocene Series ................................ 25 Sediments of early Pleistocene age .............. 25 Sediments of pre-Wisconsin(?) age ............... 25 Boulder deposits .......................... 25 Alluvium of Bull Flat ...................... 26 Pediment gravels .......................... 26 Page Stratigraphy—Continued Quaternary System—Continued Pleistocene Series—Continued Sediments of early Wisconsin(?) age ............. 28 Cemented conglomerate .................... 28 Upland slope mantle ....................... 29 Sediments of late Wisconsin(?) age .............. 29 Gravel along canyon walls .................. 29 Sand and silt ............................. 30 Alluvial-fan deposits ....................... 32 Terrace gravel ............................ 36 Alluvium of late(?) Wisconsin age ............ 37 Holocene Series .................................. 37 Talus and alluvial-fan deposits .................. 37 Alluvium of Holocene(?) age .................... 38 Man-induced talus ............................ 38 Mine dumps ................................. 38 Quaternary history .................................. 40 Structural geology ...................................... 43 Folds .............................................. 43 Joints .............................................. 45 Northwest- to north-northwest-trending joints ........ 45 Northeast- to north-northeast-trending joints ......... 45 Faults ............................................. 46 Sunnyside fault zone .............................. 48 East-northeast- to northeast-trending and east- trending faults ................................. 49 Subsurface fault ................................. 51 West-northwest-trending fault belt .................. 51 Economic geology. ; ..................................... 52 Coal ............................................... 52 Coal-mine bumps ................................. 55 Sunnyside coal bed ............................... 56 Structures in the coal ............................. 58 Analysis of the coal .............................. 59 Reserve estimates ................................ 59 Gypsum ............................................ 60 Water .............................................. 60 Petroleum-series compounds ........................... 61 Asphalt-impregnated sandstone .................... 61 Oil ............................................. 61 Natural gas ..................................... 62 Uranium ........................................... 62 Metallic minerals .................................... 63 References cited ........................................ 63 Index ................................................. 67 III IV CONTENTS ILLUSTRATIONS Page PLATE 1. Generalized geologic map and cross section of the Sunnyside coal-mining district, Carbon and Emery Counties, Utah ................................................................................ In pocket 2. Bedrock and surficial geology of Sunnyside coal-mines area, Carbon County, Utah .......................... In pocket FIGURE 1. Index map of eastern Utah and western Colorado ............................................................. 2 2—34. Photographs of: 2. Building of probable early Pueblo culture southeast of Sunnyside, Utah ................................... 6 3. Small early Pueblo storage bin southeast of Sunnyside, Utah ............................................ 6 4. Pictograph above early Pueblo storage bin southeast of Sunnyside, Utah .................................. 7 5. Carmel Formation below ledges of Entrada Sandstone .................................................. 8 6. Entrada Sandstone and associated strata ............................................................ 10 7. Summerville Formation ........................................................................... 11 8. Railroad tunnel in Summerville Formation ........................................................... 12 9. Morrison Formation .............................................................................. 12 10. Buckhorn Conglomerate Member, Cedar Mountain Formation ........................................... 14 11. Channel-fill sandstone deposit of Dakota Sandstone ................................................... 15 12. Mancos Shale and associated strata ................................................................. 16 13. Kenilworth and‘ Sunnyside Members of Blackhawk Formation ........................................... 18 14. Sunnyside Member of Blackhawk Formation ......................................................... 19 15. Surface plant of the Book Cliffs mine, with rocks above and below Sunnyside coal bed ...................... 20 16. Channel sandstone beds in upper mudstone member, Blackhawk Formation ............................... 21 17. Alluvium and bedrock strata (Castlegate Sandstone and higher beds) in Little Park Wash ................... 21 18. North Horn and Flagstaff Formations ............................................................... 22 19. Uniform bedding in Flagstaff Limestone ............................................................. 23 20. Panoramic View of Colton Formation ................................................................ 24 21. Bedding in oldest pediment gravel .................................................................. 27 22. Middle pediment gravel unit below oldest unit ........................................................ 27 23. Caliche layer in middle pediment gravel .............................................................. 28 24. Cemented conglomerate of early Wisconsinl?) age ..................................................... 28 25. Slope mantle in lower part of Bear Canyon ........................................................... 29 26. Landslide debris at Sunnyside water-supply dam ...................................................... 30 27. Alluvial gravel of late Wisconsin(?) age .............................................................. 31 28. Gravel of late Wisconsin(?) age overlain by talus and alluvial-fan debris ................................... 32 29. Stratified silty and clayey alluvium of late Wisconsin(?) age ............................................. 32 30. Pebbles, cobbles, and boulders in alluvium of late Wisconsin(?) age ....................................... 32 31. Alluvium overlapping slope mantle .................................................................. 33 32. Alluvial sand and silt of late Wisconsin(?) age entrenched by Grassy Trail Creek ........................... 34 33. Channel of Range Creek ........................................................................... 34 34. Smooth surface of alluvial sand and silt of late Wisconsin(?) age, Dragerton, Utah ......................... 34 35. Topographic map of Horse Canyon alluvial fan .............................................................. 35 36—46. Photographs of: 36. The Cove, view westward from point above Book Cliffs Mine ........................................... 36 37. Shallow valley in oldest pediment gravel ............................................................. 36 38. Steep scarp near mouth of Whitmore Canyon ......................................................... 36 39. Thick silty and clayey alluvium of late(?) Wisconsin age. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .‘ .................. 37 40. Settler's home near Marsh Flat Wash ............................................................... 37 41. Holocene(?) talus overlapping alluvium of late(?) Wisconsin age .......................................... 38 42. Man-induced talus north of Dragerton, Utah ......................................................... 38 43. Mine-waste dumps near Sunnyside .................................... _ .............................. 39 44. Alluviated erosion surfaces in Whitmore Canyon ...................................................... 40 45. View southwest toward mouth of Whitmore Canyon ................................................... 42 46. Remnants of pre-Wisconsin pediment near Book Cliffs ................................................. 44 47. Stereograms of joint poles in Sunnyside district ............................................................... 46 48—61. Photographs of: 48. Conjugate shear joints in Sunnyside Member, Blackhawk Formation ..................................... 48 49. Erosional scarp along fault in Mancos Shale .......................................................... 48 50. Buckhorn Conglomerate Member draped over faults ................................................... 48 51. Sunnyside fault in Sunnyside No. 1 Mine ............................................................ 49 52. Fault cutting Castlegate Sandstone in Slaughter Canyon ............................................... 50 53. Steep dip in fault zone ............................................................................ 51 54. Alluvial-fan debris overlapping faulted Ferron Sandstone Member and overlying part of Mancos Shale ........ 51 55. Coke plant at Sunnyside, Utah, before 1910 .......................................................... 53 FIGURE TABLE 56. 57. 58. 59. 60. 61. 1. Ultimate and proximate analyses of nine samples of Sunnyside coal CONTENTS V Page Surface plant at Sunnyside, Utah, before 1908 ........................................................ 54 Unit-train loader of the Sunnyside mines ............................................................. 55 Coke-oven operation at Sunnyside, about 1950 ........................................................ 55 Sunnyside coal bed, cropping out in Book Cliffs ....................................................... 56 Megascopic features of Sunnyside coal .............................................................. 58 “Eye” coal from the Sunnyside No. 1 Mine ........................................................... 58 TABLES ............................................ 59 2. Proximate analyses of range of five samples of Sunnyside coal from the Sunnyside No. 1 Mine, and average of two samples of Sunnyside coal from the Columbia Mine ....................................................... 59 3. Analysis of one sample of coke made from Sunnyside coal ....................................................... 59 BEDROCK, SURFICIAL, AND ECONO MIC GEOLOGY OF THE SUNNYSIDE COAL-MINING DISTRICT, CARBON AND EMERY COUNTIES, UTAH By FRANK W. OSTERWALD, JOHN O. MABERRY, and C. RICHARD DUNRUD ABSTRACT The Sunnyside mining district is in the western Book Cliffs of Utah, at the north end of the Colorado Plateau. The region has been inhabited by humans since pre-Basket-Maker time. Bituminous coal, which forms the economic base for much of east-central Utah. has been mined extensively from the district since about 1900. Most of the coal is used to make metallurgical coke for the steel industry~ in the Western United States. Mining is difficult in much of the dis- trict, but the importance of the coal to the steel industry provides the stimulus for mining it. Among the difficulties that hamper mining in the district. the most important is coal-mine bumps (rock- bursts in the coal), which are continuing hazards to life and property in the mines. A sedimentary sequence more than 10,000 feet (3.050 meters) thick. ranging in age from Jurassic to Eocene, crops out in the district and dips gently northeastward into the Uinta Basin. Rocks ranging in age from Precambrian to Early Jurassic in the district are known only from drill records. The exposed sedimentary rocks represent deposition in both continental and marine environments, under a wide variety of local conditions. The Blackhawk Formation. the major coal-bearing unit, was deposited in a dominantly deltaic environment during the retreat of the Late Cretaceous sea. Most Tertiary rocks were formed in continental depositional environ- ments. A large freshwater lake occupied much of the area during Tertiary time. Sedimentation largely ceased by the end of late Eocene time. A complex sequence of nonindurated sedimentary materials of Quaternary age yielded much information on the geomorphic and structural history of the district. These materials range from stream alluvium of probable early Pleistocene age high in the Book and Roan Cliffs to man-induced talus that formed after 1959. The lower Pleistocene alluvial deposits probably were deposited in old strike valleys by strong southward-flowing streams of an earlier drainage pattern. Range Creek. Little Park Wash, and parts of Whitmore and Horse Canyons probably are remnants of this earlier pattern. Scattered boulder deposits near the top of West Ridge may be remnants of pre-Wisconsin glacial deposits. Three levels of pedi- ments were cut along the base of the Book Cliffs during pre-Wis- consin time, following a great amount of erosion when the lower Pleistocene alluvium and glacial materials were deposited. Surficial deposits of early Wisconsin(?) age are widespread at high elevations in the district. The most widespread is a unit composed of slope mantle, colluvium. and landslide debris. Some Pleistocene land- slides in this unit were reactivated by modern construction activ- ities. Materials of late Wisconsin(?) age consist mostly of various types of stream alluvium related to major modern drainage courses. Some of these alluvial materials are valuable sources of ground water and forage for livestock, although much alluvium in the southern part of the district, derived mostly from Mancos Shale, is mostly dry and nearly barren of vegetation. Surficial deposits of Holocene age consist mostly of small talus cones along tributary drainages, small alluvial fans. and alluvium along modern streams. A few large masses of talus north of Dragerton formed after 1959 as a result of tremors and ground motion related to mining. These Quaternary materials indicate that many of the structural features observed underground in the coal mines may have resulted from dif- ferential stresses set up by erosion during and after the cutting of pre-Wisconsin pediments. These stresses have a direct relationship to occurrence of coal-mine bumps. Three large mine-waste dumps constitute another Holocene surficial unit. The geologic structure of the district is simple. The beds dip northeastward less than 20°. except in some fault zones. Steeply dipping joints occur in three major sets, one trending west- northwest, one trending north to north-northwest, and one trending east-northeast. The most consistent orientation of faults is nearly parallel to these joint directions. Stratigraphic separation on all faults exposed at the surface in the mining area is less than 200 ft (60 m); horizontal separation on a fault west of the mining area is about one-half mile (0.8 km). A pronounced belt of west-northwest- trending faults in the central part of the district probably is the crest of a collapsed anticline. Most folds in the district are broad and gentle. Locally, steepened dips of the coal beds near the Book Cliffs suggest folding as a result of elastic rebound of Mancos Shale during erosional unloading. Minor structural features in the coal, such as cleavages and fracture zones, which are known to be impor- tant factors controlling coal-mine deformation, probably were formed by the same stress system that caused the joints. Coal is the only commodity now mined in the district, but gyp- sum. methane. and asphalt-impregnated sandstone are present in potentially economic quantities. Water is a valuable commodity in the district, because it is so scarce. A few shows of oil have been found in wells in the district and uranium prospecting formerly was widespread, but no commercial production of either commodity is known. Minor occurrences of metallic sulfide minerals were found along a few faults. INTRODUCTION The Sunnyside coal-mining district, as defined here, is in east-central Utah and is a part of the Book Cliffs Coal Field which extends from Castle Gate, Utah, east- ward for about 150 miles (240 km) to Palisade, Colo. (fig. 1). The Book Cliffs Coal Field is geologically coex- tensive with the Wasatch Plateau Coal Field, which extends southward from Castle Gate for about 75 miles (120 km); these fields are separated mainly for convenience (Spieker, 1925, p. 17). Fisher (1936, p. 3) referred to all of the Book Cliffs Coal Field west of the 1 2 SUNNYSIDE COAL-MIN IN G DISTRICT, UTAH F 111° 110° 109° | I 1 \Castle Gate In . o_"' "— _T‘ x Q ’9 o ‘ —e- \00 o . Ir- ‘3 *- \ 4’ ‘7 Sunnysnde C VD"! Lelper \\\ 4’ mining district § E|o #1 l i ° '11“! \\ Ag/lorfint DE UINTA I 43 /L / K. Dragenonljsunaysiee BASIN lo I } Wellington ®H(® 19/? ) Mounds ‘gé § 351 (/Huntington 0 5t“ 2 I 4: Canyon f,“ (18 0 I Z flConnelsville \O ’8) 63; (abandoned) \_:“ ° Huntington l3) \ “j/ Woodside 1 3,, L é” ceda’_ TAVAPUTS san’?%e/ to Mountaln . PLATEAU a \ x“ \ A. 39° — 0 10 0 10 20 La Sal Mountains 20 30 MILES 30 40 KILOMETERS FIGURE 1.—Index map of eastern Utah and western Colorado. Green River (fig. 1) as the “Sunnyside district,” a usage at variance with ours. The coal-mining area near Castle Gate, 25 miles (40 km) west of Sunnyside, is separate geologically, geographically, economically, and technologically from that near Sunnyside and is not considered to be within the district. The Sunnyside district, from which high-volatile bituminous coking coal is mined from the Blackhawk Formation of Cre- taceous age, includes the Sunnyside mines of Kaiser Steel Corp., the Geneva (formerly Horse Canyon) and Columbia Mines of United States Steel Corp., and the Book Cliffs Mine of the Book Cliffs Coal Co., and is within the Sunnyside and Woodside 15-minute topo- graphic Quadrangles (fig. 1). Nearly all the coal currently mined (1972) in the Sunnyside district is shipped to steel plants at Provo, Utah, and Fontana, Calif; these plants depend upon the mines as major sources of metallurgical coke. Mining difficulties in the district have increased stead- ily because of the ever-increasing depths to which mining is pursued so that large tonnages of coal can be produced rapidly. The value of the remaining coal adds economic incentive to mine coal in the district, in spite of many physical difficulties. Among the difficulties which have plagued mining in the Sunnyside district are coal-mine bumps. Bumps— which are spontaneous and commonly violent releases of energy stored in coal or rock in mine ribs, floors, roofs, and faces, resulting in sudden ejections of coal and rock—have been a continuing problem in the dis- trict since the mines were first opened in 1899. Bumps are frequent in the mines of the district, and even small ones may slightly damage equipment or injure per- sonnel. Deaths have occurred when single football-size INTRODUCTION 3 pieces of coal were suddenly ejected from highly stressed ribs. Large bumps have caused large areas of some mines elsewhere in central Utah to be closed and sealed (John Peperakis, Kaiser Steel Corp., oral commun., 1965). Our investigations were concerned primarily with bumps and began in 1958 at the request of and in coop- eration with the US. Bureau of Mines. Members of the Bureau staff had studied engineering problems related to coal-mine bumps for many years, but wanted supplemental information concerning geologic factors that might influence the occurrence of bumps. Because of its economic importance, history of frequent bumps, and accessible surface geology, we selected the Sunny- side No. 1 Mine for our initial study. The study later was expanded to include most mines in the Sunnyside district. Active cooperation with the Bureau of Mines continued until 1961; after 1961 the work was carried on by the Geological Survey, with continuing informal cooperation with the Bureau in the field. This report presents the results of our surface geo- logic investigations in the district from 1958 through 1969 and serves as an: introduction to a series of reports on various specialized facets of our studies. Much additional research will be needed before the mechanics of coal-mine bumps can be fully understood and before all the geologic, engineering, and topo- graphic factors that influence them can be evaluated fully. The application and extension of our research to the study of mine failures in other areas as described in this series of reports may lead eventually to general principles which can be used to make mining safer and more economical than at present. Application of geo- logic studies to design of mines will effectively increase the minable reserves of many commodities. The geo- logic features discussed in this report provide the basic information for our engineering-geologic studies of bumps that will be presented in future reports. FIELDWORK AND ACKNOWLEDGMENTS Underground and surface geologic mapping of the Sunnyside No. 1 Mine area was begun by Osterwald and R. E. Eggleton in 1958. Few, if any, guidelines were available for this work, so mapping scales were made large enough to depict any possible but obscure relations between geologic features and failure of coal and rock in underground workings. Underground map- ping of selected areas in which details of coal failure could be closely studied was at the scale of 1 in. to 40 ft (1:480). After pertinent features were delineated by this detailed mapping, large areas of the mine were mapped, using standardized mining engineering maps as bases, at the scale of 1 in. to 200 ft (1:2,400). Surface mapping in the field was done on enlarged aerial photo- graphs, delineating all beds more than 10 ft (3 m) thick. Information plotted on the photographs in the field later was transferred in the office by photogrammetric methods to specially prepared topographic base maps having a scale of 1 in. to 500 ft (1:6,000), and a 20-ft (6- m) contour interval (Osterwald, 1961, 1962a; Oster- wald and others, 1969; Dunrud and Barnes, 1972). The bases were prepared by extending horizontal and ver- tical control from mining company surveys, so that maps of underground workings could be fitted accu- rately to the surface geologic maps. The surface and underground mapping of the Sunny- side No. 1 Mine area was completed in 1959 by Oster- wald and Harold Brodsky. Brodsky (1960) also inde- pendently studied the stratigraphy of the Mesaverde Group and the coal beds at Sunnyside. During the 1960 field season a simple mechanical three-component seismograph was operated in the office building of Kaiser Steel Corp. to determine whether earth tremors, which are commonly felt in the region, could be re- corded and related to known bumps. Brodsky con- tinued his stratigraphic work in 1960, and both men collected additional surface geological information for the Sunnyside No. 1 Mine-area map (Osterwald, 1961, 1962a). Vertical and horizontal control also was ex- tended from mining company surveys in preparation for mapping parts of the Sunnyside No. 2 and Columbia Mines. Osterwald and Dunrud began mapping surface and underground geology of the Sunnyside No. 2 and Columbia Mines, at a scale of 1 in. to 200 ft (122,400), in 1961. The surface geology was mapped on enlarged aerial photographs and transferred to a special topo- graphic base having a scale of 1 in. to 500 ft (126,000) and a 20-ft (6-m) contour interval. This work was largely completed in June 1962, when construction of a fixed seismic-monitoring network encompassing the entire district was begun. James O. Duguid, Jr., Barton K. Barnes, and Jerome Hernandez helped with construction of the network and installation of the instruments. Dunrud and Duguid measured strati- graphic sections near the Geneva Mine in preparation for geologic mapping in that area. Maberry con- structed structure-contour and overburden-thickness maps for the area of the Sunnyside No. 2 and Columbia Mines, and participated in the final surface and under- ground work, which began in the fall of 1963 (Oster- wald and others, 1969). Modification and debugging of the seismic system and interpreting records occupied most of our time during 1963, although Dunrud and Barnes began to map surface geology at the Geneva Mine. Mapping at 4 SUNNYSIDE COAL-MIN IN G DISTRICT, UTAH the Geneva Mine was done at the same scales as at the Sunnyside No. 1 mine but with emphasis on different details because of the different geology and mining practices. A topographic base map also was specially prepared (Dunrud and Barnes, 1972). Mapping at the Geneva Mine was essentially completed in 1966, although subsidence cracks and fault movements were measured at irregular intervals after that time. The seismic recording system was completely rede- signed and modified by Electronics Engineer John B. Bennetti, J r., in 1963. Bennetti also designed and built special transistorized preamplifiers for the system in 1964, and in 1965 he installed a 14-channel FM mag- netic-tape-recording system, using specially modified equipment. The seismic system operated almost con- tinuously from 1963 to 1977, largely through the ef- forts of Jerome Hernandez, and was a valuable tool in avoiding casualties from bumps, as well as a valuable research tool. Dunrud, Barnes, and Hernandez inter- preted most of the seismic records (Barnes and others, 1969; Dunrud and others, 1970, 1973). A continuously recording tiltmeter was installed at the seismic recording station near Sunnyside in 1962. Bennetti modified and redesigned both the transducer and recorder sections of the tiltmeter several times. A microbarograph and two recording thermographs were operated continuously until 1977 at the recording sta- tion to determine whether ground tilt or changes of seismic activity patterns are related to changes of air pressure or temperature (Osterwald and Dunrud, 1966, p. 104-107). Maberry began an independent study of sedi- mentary structures, stratigraphy, and trace fossils in the Blackhawk Formation in 1966, to determine whether such features could be used to predict mining conditions before actual mining. His results are con- tained in a separate report (Maberry, 1971). Osterwald began mapping the geology of the Wood- side 15-minute Quadrangle in 1962 so that regional structural and stratigraphic changes could be related to deformational patterns in the coal mines. This work was recessed in 1963 and 1964 because of the large amount of time required to operate the instrumenta- tion systems, but was resumed in 1965 and completed in 1968. Maberry, aided in 1968 by J. L. Stevenson, mapped the eastern part of the quadrangle, where much unmined coal exists, in an attempt to predict mining conditions on the basis of work at Sunnyside. Responsibility for the material in this report is di- vided among us. Osterwald assembled much of the material gathered from various facets of the fieldwork and wrote most of the sections on Quaternary geology, structure, and economic geology. Maberry wrote the material pertaining to pre-Pleistocene stratigraphy and to characteristics of the coal, utilizing his own measured sections as well as those by R. E. Eggleton, Harold Brodsky, Dunrud, and Osterwald, and V. H. Johnson (written commun., 1962). Dunrud supplied much information on stratigraphy and structure of the Horse Canyon area, and contributed importantly both to the investigations of structural features in the coal and its associated rocks, and to the interpretation of many features. The work upon which this report is based could not have been done without the help and interest of many persons. We are particularly indebted to David J. Varnes of the Geological Survey, who originally out- lined possible geologic problems that could be investi- gated and who planned the original work at the Sunny- side No. 1 Mine. His continuing interest and guidance in later phases of the work were invaluable. Many employees of the mining companies con- tributed greatly to the investigation through discus- sion of problems and by providing logistical and moral support. John Peperakis of Kaiser Steel Corp. and R. M. von Storch of United States Steel Corp. gave ready access to the properties under their control and were always ready to discuss problems or new develop- ments. Members of the engineering staffs of their com- panies, particularly R. J. Bowen, J. T. Taylor, Lynn F. Huntsman, and J. B. McKean, were very helpful in contributing their knowledge of the area to our study. 'We greatly appreciate the many courtesies and the assistance given by many miners and local residents. We are particularly grateful to the Ray Wilcox, Waldo Wilcox, and Don Wilcox families, who gave us free access to their lands along Range Creek. Elwin Ras- mussen of Dragerton, Utah, devoted much time, energy, and skill to keep our field vehicles and other mechanical equipment in good operating condition. PHYSIOGRAPHY AND GENERAL GEOLOGY The Sunnyside district is along the southern margin of the Uinta Basin (Fenneman, 1931, p. 304), which is formed by the Book Cliffs (figs. 1, 2), an im- posing southwestward- and westward-facing escarp- ment of alternating siltstones and sandstones of Late Cretaceous and Tertiary age. These rocks make up a series of light-brown cliffs which are separated by narrow slopes of nonresistant siltstones and mud- stones. Above the Book Cliffs, a series of reddish cliffs and slopes made up mostly of sandstones, siltstones, and mudstones of early Tertiary age constitutes the Roan Cliffs. The Roan Cliffs are capped by an irregular surface which is called the Tavaputs Plateau. The Tavaputs Plateau, cut into extremely rough topog- raphy by many canyons, slopes gently northward and INTRODUCTION 5 eastward into the central part of the Uinta Basin. The desert floor at the base of the Book Cliffs is as much as 4,500 ft (1,370 m) below the level of the Tavaputs Plateau. This desert floor, made up of the Castle Valley and the Clark Valley, is a lowland underlain by as much as 3,500 ft (1,070 m) of Mancos Shale of Cre- taceous age. Early explorers named the Book Cliffs for the open- book appearance of the evenly bedded Cretaceous strata between reentrants. The Roan Cliffs and Roan Plateau were named for the reddish-brown color of the Tertiary rocks that form their features. The Book Cliffs and Roan Cliffs are a rough, remote, mountainous area. Steep cliffs and deep canyons make many places difficult to reach, even on foot. Local thick forests of aspen and spruce alternate with open areas covered by sagebrush and grasses at high ele- vations. Thick stands of juniper, mountain mahogany, and pinon-pine commonly grow on slopes in the Book Cliffs and on extensive pediments that extend from the base of the cliffs. Thickets of mountain mahogany at intermediate elevations are extremely dense and hard to pass through on foot. In many of these thickets indi- vidual plants are large and commonly reach the size of trees, in contrast to typically small mountain mahog- any shrubs in many other western localities. The climate in the district is arid to semiarid and generally is mild except for a few weeks in midsummer when high temperatures are common. Temperatures above 100 °F (38 °C) are rare, however, except for local areas beneath west- or southwest-facing cliffs. Spring and fall seasons are commonly dry and mild. Rainfall is about 6 in. (15 cm) per year at low elevations and as much as 20 in. (51 cm) per year above 8,000 ft (2,440 m); most of the precipitation occurs during winter and spring. During the summer months violent thunderstorms cause local floods that sometimes con- stitute hazards to field parties working in valleys and gulches. Cool winds, known locally as “canyon winds,” resulting from convective overturns, blow down many canyons at night, particularly near the town of Sunny- side. As a result of these winds, local nighttime tem- peratures as low as 45°F (7°C) are common during the summer months. US. Highways 6 and 60 traverse the lowland near the base of the Book Cliffs, and connect Price, Utah, with Provo, Utah, and Grand Junction, Colo. Utah Highways 123 and 124, built during World War II, connect Sunnyside and Dragerton to US. Highways 6 and 50 and also link the Geneva Mine and Columbia to Dragerton. A paved county road extends southwest- ward from the Geneva Mine to US. Highways 6 and 50. Except for a few access roads built by mining com- panies, most other roads in the district are steep, rough, and poorly maintained. EARLY MAN IN THE SUNNYSIDE DISTRICT By JAMES O. DUGUID, JR. Artifacts and campsites left by prehistoric man are common in the Sunnyside district and surrounding areas. Most of the campsites were opened by amateur pot hunters, and, as a result, much historically val- uable material has been lost. A few new sites were found during fieldwork, and the following discussion is based on my examination of articles from these sites and of identifiable articles from the Prehistoric Museum at Price, Utah. Only a few of the most diag- nostic remains are described. All dates referred to these remains are approximate and were determined by analogy with similar artifacts from other localities; the dates are presented only to give the general range of ages. Campsites are common under overhanging cliffs in many of the canyons at locations that now are far from known sources of water. Similarly, a large campsite extends for several miles along Grassy Trail Creek westward from the mouth of Whitmore Canyon near Sunnyside. The size, abundance, and locations of the campsites suggest that water was much more abun- dant when the sites were occupied than it is at present and that rainfall was formerly much greater. The oldest known artifact found in the Sunnyside district is an Agate Basin—type weapon point discov- ered by the Wilcox family on their ranch on Range Creek. These points represent a culture more than 9,000 years old, but this specimen may have been carried into the area by a later people. Most campsites and artifacts appear to be of a Basket-Maker culture, which ranges in age from 1,500 to 3,500 years B.P. (before present). These people prob- ably were similar to the Pinto Basin (Uncompahgre) people (Alice Hunt, 1956) who inhabited the Uncom- pahgre Plateau region of western Colorado during this time. Most sites are under overhanging sandstone ledges, many of which are smoke blackened. We found three red pictographs of human figures under a large overhang in Whitmore Canyon, and numerous picto- graphs can be seen at the base of the cliffs at the mouth of Whitmore Canyon. An overhang in Horse Canyon has fire-blackened walls, and the floor is thickly covered with juniper bark. A pictograph of an arrow passing through a circle can be observed be- neath an overhang east of Range Creek near the mouth of Sheep Canyon, about 1 mile (1.6 km) east of the east- ern boundary of plate 1. We found several bone imple- ments, some chipped-stone artifacts, and part of an atlatl shaft beneath the Range Creek overhang. Near the overhang we found two metates, one mano, and two broken comer-notched weapon points, which, with the atlatl shaft, suggest a Basket-Maker culture. 6 SUNNYSIDE COAL-MINING DISTRICT, UTAH The most recent archeological remains in the Sunny- side district are of Pueblo culture, which ranges in age from historic time to about 1,500 years B.P. Artifacts of this culture, reported to be from the Sunnyside area, are in the Prehistoric Museum at Price. We found ruins of a few crudely constructed buildings (fig. 2), probably of an early Pueblo culture, beneath a large overhanging cliff of Bluecastle Sandstone Member of the Price River Formation southeast of Sunnyside. Crude cleared areas on a nearly level surface about 100 ft (30 m) above the ruins may be sites of former fields, although no water is nearby at present. If these cleared areas are former fields, rainfall was probably much greater at the time they were worked than it is at present. Factory sites, where projectile points were made, are common near Little Park Wash on dip slopes of Bluecastle Sandstone Member. The sites are marked by abundant chips of quartzite, quartz, and chert, and broken projectile points are common. A small storage bin under an overhanging sandstone ledge in the Sunnyside Member of the Blackhawk Formation in a wash about 2 miles (3.2 km) east of Sunnyside is shown in figure 3. This bin was made of sandstone slabs cemented together with crude adobe mortar. A pictograph above the bin (fig. 4) shows a crude atlatl shaft or arrow impacted in the center of an arch-topped figure. This pictograph was painted on the FIGURE 2,—Crudely constructed building of probable early Pueblo culture beneath overhanging cliff of Bluecastle Sandstone Member of the Price River Formation, 2.5 miles (4 km) south- east of Sunnyside, Utah. Building outline marked by one stand- ing wall, fallen sandstone blocks, and juniper logs. rock face in red, yellow, and black; the colors persist, having been protected from weathering by the overhang. We were shown outlines of several unexcavated early Pueblo pit houses along Range Creek, about 1.5 miles (2.4 km) east of the Woodside quadrangle, by the Wilcox family. The abundant archaeological sites indicate that the district has been inhabited for a very long time and FIGURE 3.—Small storage bin beneath ledge of Sunnyside Member of the Blackhawk Formation, 2.5 miles (4 km) southeast of Sunnyside. A, Top view, showing shape and size of bin. B, Side view, showing crude mortar construction. Pick handle is 18 in. (46 cm) long. STRATIGRAPHY 7 FIGURE 4.—-Pict0graph of atlatl shaft or arrow impaling a figure, found on sandstone face above storage bin, about 2.5 miles (4 km) southeast of Sunnyside, shown in fig. 3. Shaft or arrow is red and is about 25 in. (64 cm) long; interior of figure and box- like areas on its top are yellow: outlines are black. that it supported a large population. The reasons are probably similar, except for coal mining, to the reasons that attract people to the Book Cliffs and Roan Cliffs today—pleasant climate, scenic views, good hunting, and abundant berries and pinon-pine nuts—although water probably was more abundant in prehistoric times. There is no evidence, however, that in pre- historic times the inhabitants used coal in any way. EARLY EXPLORATION AND DEVELOPMENT OF COAL IN EAST-CENTRAL UTAH Many of the early Spanish and American exploring expeditions passed through the lowlands at the base of the Book Cliffs because they were easy to travel. The main route of the Spanish Trail from Santa Fe, N. Mex., to California passed a few miles south of the Woodside Quadrangle. That trail is now marked by a Utah State Historical Society sign. Some of the early explorers probably noticed the coal beds, but the members of the Gunnison expedition in 1853 (Beckwith, 1855, p. 65) made the first mention of the coals in what is now east- central Utah. Beckwith and James Schiel, a geologist, examined and described coal which probably came from Rock Canyon in what is now known as the Wasatch Plateau Coal Field (fig. 1). The first mines in the area were opened at Connelsville, in upper Hunt- ington Canyon (western Emery County, fig. 1), in 1874 (Morton, 1877). Economic development of east-central Utah, of which coal mining is an integral part, was closely related to development of good transportation systems. The first railroad in east-central Utah, the narrow-gage Utah and Pleasant Valley, was built in 1878 from Springville to newly opened coal mines near Scofield, in northwestern Carbon County. This railroad was absorbed by the narrow-gage Denver and Rio Grande Western Railway, which was completed from Ogden, Utah, to Grand Junction, Colo. in 1883 (Beebe and Clegg, 1962, p. 372). The D&RGW Ry. became the . Rio Grande Western in 1889 and was rebuilt to stand- ard gage in 1890, when about 10 miles (16 km) of track in the canyons of Price River and Grassy Trail Creek in the southern part of the district were rerouted because of numerous floods (Denver and Rio Grande Western Railroad, written commun., 1965). Remains of the abandoned grade can still be seen in the river canyons. The Carbon County Railroad, originally a subs'diary of both the Utah Fuel Co. and the Rio Granlde Western Railroad, was built from Mounds Sta- tion td Whitmore Canyon in 1899 as a result of the dis- covery of coal (Beebe and Clegg, 1962, p. 374). This subsidiary became the Sunnyside branch of the Denver and Rio Grande Western Railroad, which was formed in 1908 by consolidation of the Rio Grande Western with the Denver and Rio Grande; the Sunnyside branch was extended to Columbia in 1924. During World War II, part of the Sunnyside branch was re- alined to reduce grades and was extended southward from Columbia to the Geneva Mine. The track from Columbia Junction near Dragerton to the Geneva Mine presently (1977) is operated by the Carbon County Railway, a subsidiary of United States Steel Corp. STRATIGRAPHY Rocks exposed at the surface in the Sunnyside dis- trict were deposited in continental and marine environ- ments, and range in age from Middle Jurassic to Eocene (pl. 1). N onindurated sediments of Pleistocene and Holocene age cover large areas, but deposition was not continuous in the area throughout the Pleistocene and Holocene. Rocks as old as Mississippian age occur in the subsurface above Precambrian basement rocks. Deposition of the rocks exposed near Sunnyside took place in varied environments. Flood plains and re- stricted lagoonal and paludal areas occupied the region during Late Jurassic time, receiving sediments from a low positive area in west-central Colorado (McKee and others, 1956, pl. 9). These conditions progressed to marine deposition with the advance of a Cretaceous seaway that flooded most of the Western Interior of 8 SUNNYSIDE COAL-MININ G DISTRICT, UTAH the United States (Reeside, 1957, p. 506). Later with- drawal of the sea caused offlap deposition of marine, transitional, and continental sediments. There was no significant sedimentation in the area after late Eocene time, although younger sedimentary rocks occur a few miles to the north. Rocks of the Colton Formation of early and middle Eocene age are the youngest de- scribed in this report, although rocks of the Green River Formation of Eocene age crop out a few miles north and east of Sunnyside (pl. 1). Chemical composition of the rocks described harem varies widely because of their different depositional environments; lithologically, however, the rocks are sandstone, coal, mudstone, and limestone. Gypsum, silica, clay, and calcium carbonate are common ce- menting agents, and some formations contain distinct layers of gypsum, clay, and limestone. Composition of the rocks governs their physiographic expression. Erosion forms strike valleys in mudstones and promi- nent dip slopes, ledges, bluffs, and cliffs in sandstones. Limestones generally are resistant to erosion, forming a series of retreating ledges having valleys or stripped surfaces between them. Where they were burned exten- sively at their outcrop, most coal beds are indicated by red baked zones in the Book Cliffs. Rock color in the Sunnyside district covers a wide spectrum. Jurassic rocks are dominantly red, with sec- ondary shades of green, brown, white, and yellow. Cre- taceous mudstones and most limestones are gray, whereas sandstones tend to be yellowish except near burned coals, where they are red and white. Tertiary rocks mostly are variegated pastel shades of red, gray, brown, and yellow. Many formation names in the area are based on litho- logic similarity to units that were established else- where. As a result, some names do not carry the same age connotation as they do at other locations, but to explore the history and correlation of each name is beyond the scope of this report. Excellent and compre- hensive nomenclature histories are contained in Stokes and Holmes (1954) (Jurassic); Fisher, Erdmann, and Reeside (1960) (Cretaceous and Tertiary); and C. B. Hunt (1956) (Cenozoic). Many Upper Cretaceous units are named for towns and coal mines in the Wasatch Plateau and Book Cliffs Coal Fields. JURASSIC SYSTEM CARMEL FORMATION The Carmel Formation is of Middle Jurassic age (Imlay, 1952, p. 963); it comprises the oldest rocks that crop out in the Sunnyside district. The Carmel consists of thin beds of evaporite, mudstone, siltstone, sand- stone, and limestone that form low, rolling topography having low bluffs, ledges, and stripped dip slopes. The Carmel was named by Gilluly and Reeside (1928, p. 73) during their investigations of oil and gas possibilities in the San Rafael Swell. The formation consists of two general facies, a lower arenaceous limestone facies and an upper silty sand- stone facies containing mudstone and anhydrite. The lower facies of the Carmel is composed of about 80 ft (24 m) of gray, flaggy, somewhat oolitic limestone, interbedded with thin lenses of gray mudstone, silt- stone, and sandstone (fig. 5). Fossils of shallow-water pelecypods are common in the limestone beds, indi- cating that the lower facies of the Carmel was de- posited in shallow marine water. The upper facies of the Carmel consists of 150—200 ft (45—60 m) of soft, easily eroded, gray to white gypsum and anhydrite, interbedded with red and some gray, shaly mudstone and sandstone. The rocks of the upper facies were deposited in shallow coastal lagoons when the rate of regional subsidence was only slightly slower than the rate of sedimentation. Some Carmel gypsum beds are of economic signifi- cance. The Carmel is exposed in the San Rafael Swell, in the southwestern part of the district. ENTRADA SANDSTONE The Entrada Sandstone, of Middle Jurassic age, is a bright pastel-red series of sandstone beds that forms rounded bluffs and dip slopes in the southwestern part of the area. The sandstone was named by Gilluly and Reeside (1928, p. 76) for a locality about 25 miles (40 km) southwest of Sunnyside, in the San Rafael Swell. In the map area of this report, the Entrada is 300 ft (90 m) thick; it is as thick as 850 ft (260 In) in the section described by Gilluly and Reeside. FIGURE 5.—N0rthward view of the Carmel Formation in T. 18 S., R. 13 E., unsurveyed. Gently rolling slopes of Carmel (Jca) are below resistant ledges of Entrada Sandstone (Je). JURASSIC SYSTEM 9 The lower part of the sandstone is impure, silty, and clayey, but is better sorted upward and becomes nearly pure quartz sand, cemented by iron oxide and silica. Quartz grains are well rounded and frosted, and the Entrada is almost universally regarded to be of eolian origin. The lower part is softer and less resistant to erosion than the upper part and so forms rounded bluffs along .its outcrop. The upper part forms bold cliffs and steep escarpments (fig. 6). The upper surface of the Entrada is cut by stream-channel deposits and shows the ef- fects of subaerial erosion prior to deposition of the Curtis Formation. CURTIS FORMATION The Curtis Formation, of Middle Jurassic age, is a soft slope-forming series of sandstone and shale beds of pastel-green hues. The formation was named by Gil- luly and Reeside (1928, p. 78) for exposures at Curtis Point in the San Rafael Swell, about 30 miles (48 km) south of Sunnyside. At this type locality, the Curtis is 190 ft (60 m) thick; it thickens northward over a dis- tance of 2 miles (3 km) to 250 ft (75 m) at Summerville Point. This increase in thickness over a short lateral distance is due to deep erosion of the Entrada surface upon which the Curtis was deposited. The Curtis thins rapidly to the south and east. Rocks of the formation were deposited in shallow marine waters; Gilluly and Reeside (1928, p. 79) col- lected crinoid and crustacean fossils from the Curtis. In addition, they found sedimentary structures such as a discontinuous basal pebble conglomerate, bicuspate ripple marks, and foreset crossbedding. These features indicate shallow-water conditions during deposition. Curtis rocks are soft and relatively easily eroded; they form rounded knolls and gently rolling dip slopes atop the Entrada bluffs. SUMMERVILLE FORMATION The Summerville Formation, of Middle Jurassic age, consists of interbedded sandstone and laminated gypsiferous mudstone. The overall color of the unit is dark brownish red. The formation crops out as rounded bluffs in the southwestern part of the Sunnyside dis- trict. The Summerville was named by Gilluly and Reeside in their 1928 study (p. 80) for exposures at Summer- ville Point in the San Rafael Swell, about 25 miles (40 km) south of Sunnyside. They included it as the upper formation in their San Rafael Group (pl. 1). Gypsum is ubiquitous throughout the formation, both as intergranular cement and as discrete beds. The bedded gypsum commonly is somewhat silty. Bedding is remarkably even and persistent in the Summerville (fig. 7). The alternation of beds probably is due to a form of cyclic sedimentation in shallow water. Stokes and Holmes (1954, p. 38) postulated that the depo- sitional environment of the Summerville was a playa lake or shallow embayment. Fossils are extremely rare in the Summerville, indicating that hostile ecologic conditions persisted in the depositional environment. We believe that hot, dry, sometimes subaerial condi- tions existed and that the water of Summerville depo- sition was usually hypersaline. The Summerville is a facies in a continental to marine sedimentational progression. The unit becomes much sandier southward, and passes eastward into the beach sandstone of the Moab Tongue of the Entrada Sandstone (Stokes and Holmes, 1954, p. 38). The contact between the Summerville and the over- lying Morrison Formation is gradational (Gilluly and Reeside, 1928, p. 79), and is without apparent uncon- formity. Although the Summerville appears to be non- resistant to weathering and structurally weak in out- crops, an abandoned narrow-gage railroad tunnel built in about 1883 along the Price River, about 3.25 miles (5.2 km) northwest of the Silvagni Ranch, was still in good condition in 1977. Timbering was used only near the south portal, and only minor roof caving had oc- curred elsewhere. Trenching of the tunnel floor about 6 ft (2 m) deep since 1969 for irrigation water has weakened the sills, and some inward movement of the lower walls was observed in 1977 . This inward move- ment has caused some related fracturing in the rocks above the tunnel roof (fig. 8). MORRISON FORMATION The Morrison Formation, of Late Jurassic age, com- prises a series of sandstone and mudstone beds of many pastel colors. These strata form slope-and-bluff topography near the Price River in the western part of the Sunnyside district, where they are about 400 ft (122 m) thick. The Morrison was named originally by G. H. Eldridge (in Emmons and others, 1896) for a partly ex- posed section near Morrison, Colo. The formation sub- sequently was revised by Waldschmidt and LeRoy (1944, p. 1097). The Morrison is one of the most widespread suites of rocks in the Western Interior of North America. It crops out in the United States from Montana to Ari- zona and from Texas to North Dakota. It underlies al- most the entire Colorado Plateau. The lower part of the Morrison contains important uranium and vanadium deposits, and many of the uranium districts of the eastern part of the Colorado 10 SUNNYSIDE COAL-MIN NG DISTRICT. UTAH JURASSIC SYSTEM 1 1 FIGURE 6 (facing page).—Views of the Entrada Sandstone and associated strata. A, Northward view of a bluff in sec. 29, T. 17 S.. R. 13 13.. showing erosional forms of resistant sandstone and shale beds of Entrada Sandstone (Je), overlain by a thin ero- sional remnant of the less well cemented Curtis Formation (Jc). B, Northeast view from top of the bluff (A) showing the soft, crossbedded, easily eroded sandstones of the Curtis Formation (Jc) overlying Entrada Sandstone (Je). Shales of the Summer- ville Formation (Js) underlie sandstones of the Salt Wash Mem- ber of the Morrison Formation (J ms) in the Sugarloaf (SL). Mouth of Horse Canyon in Book Cliffs and Roan Cliffs shown in background. Photographs by Vard H. Johnson. Plateau are in this part. The formation also contains many deposits of well-preserved.”dinosaur and other vertebrate-animal remainsfflm _ The Morrison is composed of two members in the Sunnyside district, the Salt Wash Sandstone Member (Lupton, 1914, p. 117) at the base and the overlying Brushy Basin Shale Member (Gregory, 1938, p. 59). Age-diagnostic fossils are rare in the Morrison, and this lack of fossils has led to a long controversy over the age of the formation. From its definition in 1896 - y, until 1936, the Morrison was considered to be Early Cretaceous or Late Jurassic in age. Baker, Dane, and Reeside (1936, p. 31) included “under the name Morri- son all the Jurassic continental sediments deposited subsequent to the deposition of the San Rafael Group.” Cobban (1945, p. 1270) found distinctive algae and ostracodes indicative of Late Jurassic age in the Morrison, and Imlay (1952) included the formation in the Upper Jurassic of North America. SALT “ASH SANDSTONF. MEMBER The Salt Wash Sandstone Member in the Sunnyside district comprises 50—200 ft (15-60 m) of sandstone containing minor lenses of mudstone. The sandstones dominantly are brownish red in color, and the enclosed mudstones are shades of red, brown, gray, and green. The member as a whole forms cliffs, ledges, and bluffs along its outcrop in the southwestern part of the dis- trict. Fluviatile channel-fill deposits having prominent crossbedding are the dominant sedimentary units, and crossbedding studies of the channel fills indicate depo- FIGURE 7.—Summerville Formation (Js) on the east side of Camel Wash in sec. 27 . T. 17 S., R. 13 E., showing Summerville shales capped by thick gypsum beds (J msg) that are at the base of the Salt Wash Sandstone Member of the Morrison Formation (J ms). Gypsum beds (gb) showing pinch-and-swell structure and thin impure gypsum veins (gv) are characteristic of the Summerville. Section above top of Summerville Formation is measured section of Gilluly and Reeside (1928, p. 80). Photograph by Vard H. Johnson. 12 FIGURE 8.—South portal of abandoned narrow-gage railroad tunnel in cliff of Summerville Formation along Price River, about 3.25 miles (5.2 km) northwest of Silvagni Ranch. Inward movement of walls and timber sills, resulting from digging of irrigation ditch, allowed arch-shaped tension fractures (arrows) to form in rocks as much as 10 ft (3 m) above tunnel roof. Headgate for irrigation ditch is in distance, near north portal. sition by east-flowing streams from highlands or mountainous regions in western Utah and eastern Nevada (Cadigan, 1967, p. 44). The mechanism of transport and deposition of the Salt Wash was a system of aggrading braided streams (Cadigan, 1967, p. 46) flowing across a broad alluvial plain. Grain sizes in the individual channels vary from pebbles to fine sand and become finer toward the upper part of the channel fill. BRUSHY BASIN SHALE MEMBER The Brushy Basin Shale Member of the Morrison in the Sunnyside district is a unit about 200 ft (60 In) thick, composed of interbedded mudstones, siltstones, SUNNYSIDE COAL-MIN IN G DISTRICT, UTAH FIGURE 9.——Views of the Morrison Formation. A, Northwest view, sec. 4, T. 17 S., R. 13 E., of rounded slopes (foreground) charac- teristic of the easily eroded Brushy Basin Member of the Morri- son Formation. Blocks on slope at right are derived from Buck- horn Conglomerate Member of the Cedar Mountain Formation, which caps cuesta to right. B, Southward view of east-dipping channel-fill deposits (cf) (outlined) of chert-pebble conglomerate in the Brushy Basin Member of the Morrison Formation, crop- ping out above the Price River near Silvagni Ranch in sec. 23, T. 17 S., R. 13 E. These conglomerates closely resemble the over- lying Buckhorn Conglomerate Member, which, however, con- tains relatively few pebbles in this vicinity. Compare those re- sistant ledges with the easily eroded beds in the Brushy Basin Member in A. claystones, and limestone lenses (fig. 9A). The most conspicuous feature of the member is its variegated colors. Limestone layers generally are gray, and other lithologies are various bright shades of red, green, brown, yellow, and gray. Thin discontinuous channel siltstones indicate continued deposition by very sluggish streams on a nearly featureless but extensive flood plain. Much of the claystone is bentonitic, the bentonite having been deposited by ashfall on the Mor- rison flood plain and mixed with terrigenous sedi- CRETACEOUS SYSTEM 1 3 ments. The limestone lenses formed in small lakes scat- tered over the low-lying flood plain. A group of channel-fill deposits of chert-pebble conglomerate occurs in the Brushy Basin on the Price River, near Sil- vagni Ranch in sec. 23, T. 17 S., R. 13 E. (fig. 9B). Fragments of mineralized Wcur in many outcrops of the Brushy Basin Member through- out central Utah. Excellent We con- tained in the Brushy Basin a out 20 miles (30 km) southwest of Sunnyside, in the San Rafael Swell. Many complete skeletons of carnosagiarrdjngsaprs were re- covered from this locality‘lStokes, 1944, p. 964). At Humbug Wash (pl. 1), Gilluly and Reeside (1928, p. 81) recovered poorly preserved fossil gastropods. Tectonism in the Sunnyside area, as in most of the Colorado Plateau during Jurassic time, was limited mostly to slow subsidence of the region. Gilluly (1929, p. 111) reported that the sandstones of the Salt Wash Member thin to as little as 5 ft (1.5 m) on the west side of the San Rafael Swell, and this thinning has been found to be due to a change of facies from sandstone to mudstone. During our field investigations, we ob- served channel directions in the Salt Wash that deviate from the normal streamflow direction, which was from the northwest across the Morrison coastal plain. These anomalous, arcuate, convex-northeast- ward trends of relict channel structures indicate deflec- tion of streamflow by a topographically high area. These trends suggest that a slight topographic high on an otherwise relatively featureless plain resulted from slight arching of the swell during deposition of the Salt Wash Member. CRETACEOUS SYSTEM Our field studies indicate that between deposition of the Morrison Formation and deposition of the Cedar Mountain Formation the Sunnyside district was part of a slowly subsiding low-lying coastal plain having little relief. No widespread erosion or sedimentation took place here, and only a few sluggish streams meandered eastward to the sea. In western Utah, the Sevier Arch and other north-trending positive areas began to re-emerge after earlier erosion. These areas furnished sediment eastward to the encroaching sea- way throughout Cretaceous time. The San Rafael Swell apparently was quiescent dur- ing much of the Cretaceous, because Cretaceous rocks do not thin noticeably toward the swell. Later uplift and erosion removed all Upper Cretaceous rocks from the structure, and so there is no direct sedimentational evidence of tectonism in the swell during the Cre- taceous. The swell probably arched slightly in Cam- panian time. Sedimentation in the region resumed quietly; emer- gence of highlands to the west brought surges of depo- sition of a widespread pediment-gravel veneer on the Morrison sediments, followed by more variegated silts and muds on a coastal plain (Katich, 1954, p. 42). CEDAR MOUNTAIN FORMATION The Cedar Mountain Formation was originally as- signed group status (Stokes, 1944, p. 958) but was later designated a formation by Stokes (1952, p. 1774). The type locality is on the southwest flank of Cedar Mountain, about 10 miles (16 km) west of Woodside. The formation is composed of two members in the dis- trict: the Buckhorn Conglomerate Member and an overlying unnamed shale member. BUCKHORN CONGLOMERA'I‘E MEMBER The Buckhorn Conglomerate Member crops out in patches atop the Morrison Formation and forms a dis- tinctive lithologic break from the fine-grained Brushy Basin deposits. The type section (Stokes, 1944, p. 966) is at Buckhorn Flat in sec. 9, T. 18 S., R. 10 E., in Emery County, Utah. In the Sunnyside district it varies in thickness from 0 to 60 ft (0 to 18 m) and forms ledges, ridges, and long dip slopes on the east flank of the San Rafael Swell (fig. 10A). The Buckhorn is a conglomerate composed of chert pebbles, feldspar, quartz, quartzite, siliceous lime- stone, and rare dinosaur-bone fragments. We observed no multilithic rock fragments. Silica and clay are the most common cements. The conglomerate occurs both as extensive sheet deposits and as channel deposits (fig. 103). It is widespread in the northern part of the district but is less abundant in the southern part. Channel forms are discontinuous; they show graded bedding and crossbedding and contain large pieces of silicified plant material (fig. 10C). Field measurements of internal sedimentary structures indicate that the dominant transport direction was eastward. Channel, structures are preserved in a matrix of fine- to coarse- grained clay-cemented sandstone. Leaf imprints and silicified small plant fragments occur in the sandstone. Individual channel structures are common in the lower part of the member, and many channel and tabu- lar bodies coalesce in the upper part. Lithologies change abruptly both laterally and vertically through- out the member. Stokes (1944, p. 976) determined that the Buckhorn was derived from upper Paleozoic rocks to the west and southwest of the San Rafael Swell. Stokes (1952, p. 1774) also postulated that this unit is a Cretaceous pediment gravel on the basis of compari- son with Holocene pediment gravels. 14 SUN NYSIDE COAL-MIN IN G DISTRICT, UTAH UNNAMED SHALE MEMBER The best exposure of the unnamed shale member is on Cedar Mountain, about 10 miles (16 km) west of Woodside, in the northern part of the San Rafael Swell. The shale member unconformably overlies the Buck- horn Member, as shown by channels and other erosion FIGURE 10.—Views of the Buckhorn Conglomerate Member of Cedar Mountain Formation. A, Southeast view of the Buckhorn Conglomerate Member in a tributary of the Price River, sec. 23, T. 17 S., R. 13 E. Buckhorn (ch) is overlain by the unnamed shale member of the Cedar Mountain Formation (Kcs); higher Cretaceous units in the Book Cliffs are on the skyline. B, South- east view of conglomeratic channel deposit cut into sandstone in the Buckhorn Conglomerate Member, sec. 14, T. 17 S., R. 13 E. Conglomerate (Cgl) tongues out just beyond the locality of the right edge of the photograph (southwest), and is wholly re- placed by sandstone facies (ss). Most of the pebbles in the con- glomerate facies are chert. C, Large silicified log in Buckhorn Conglomerate Member. Orientation of log is N. 70° W. Log di- ameter above pick is 19 in. (48 cm). Most of pebbles in con- glomerate are chert. Photograph taken in north-trending tribu- tary to Price River, sec. 14, T. 17 S., R. 13 E. features at the top of the Buckhorn. The member is 272 ft (83 m) thick at Cedar Mountain, but thins eastward and is only 25 ft (7.6 m) thick in western Colorado (Stokes, 1944, p. 989). In the Sunnyside area, the mem- ber forms smooth slopes between ledges of more re- sistant rocks (fig. 10A). The unnamed shale member consists of variegated mudstones and lenticular sandstones and siltstones that commonly contain nodules of chert or siliceous limestone. The unnamed shale may have been derived from reworked Morrison sediments under conditions of deposition similar to those prevailing during Morrison deposition. The two units are very similar in composi- tion and color, but whereas Morrison colors are domi- nantly bright red and purple, the colors of the un- named shale member are pastel shades of those colors. Field comparisons showed that nodules of chert and siliceous limestone are more rounded in the unnamed shale than in the Morrison. Rare fossils in the unnamed shale member indicate late Early Cretaceous (Aptian-Albian) age (Katich, 1954, p. 44). Katich postulated that the member was formed on an alluvial plain, where slow, uniform mud and silt deposition occurred between shallow channels of sluggish streams. A few freshwater lakes and ponds dotted the plain, and vegetation was limited to grass, rushes, and low-growing bushes; only a few large frag- ments of plants occur in the member. Climatic condi- tions during deposition of the unnamed shale member probably were warm and humid, or subtropical. The upper contact of the unnamed shale member is placed at the t0p of the highest variegated mudstone. This surface is interpreted to be on an unconformity, because it is irregular and incised by fluviatile chan- nels of the Dakota Sandstone. DAKOTA SAN DSTON E The Dakota Sandstone was extended into central Utah by Richardson (1909, p. 14), who applied the CRETACEOUS SYSTEM 1 5 name to an interbedded, ledge-forming, coal-bearing sequence of sandstones and mudstones below the Mancos Shale. In the Sunnyside district the Dakota forms cuestas or hogbacks, or caps mesas of rocks less resistant than itself to erosion. The Dakota is of Early(?) and Late Cretaceous age in the Sunnyside district. The Dakota varies in thickness from about 20 to 65 ft (6 to 20 m) in the Sunnyside district and is composed of two facies: a lower conglomerate and coarse-grained sandstone and an upper medium- and fine-grained sandstone interbedded with shale. The lower facies consists of 5—15 ft (1.5—4.5 m) of siliceous chert- and quartzite-pebble conglomerate and arkosic coarse-grained sandstone. The beds commonly are cemented by silica and comprise channel-fill de- posits (fig. 11). This facies closely resembles the con- glomerate in the Buckhorn Member of the Cedar Mountain Formation. Katich (1954, p. 45) found that Dakota conglomerate consists of 70 percent quartzite and 30 percent chert. The upper facies consists of 15-50 ft (4.5—15 m) of greenish-brown medium- to fine-grained arkosic sand- stone that contains interbeds of light-gray to pale- greenish-gray shale. The shale commonly forms small strike valleys between low sandstone cuestas. The sandstone beds are crossbedded and have irregular upper and lower boundaries. Although we observed no distinct channel-fill structures during our study, we in- terpret the overall environment of deposition of the Dakota to be fluviatile. Fossils in the Dakota are uncommon, and only plant fossils are known from the Dakota in the Sunnyside district. Richardson (1909, p. 14) collected Dakota FIGURE 11.—Channel-fi]l sandstone deposit of Dakota Sandstone (Kd), cut into unnamed shale member (Kcs) of Cedar Mountain Formation as preserved on a ridge crest in sec. 24, T. 17 S., R. 13 E., above the Price River near Silvagni Ranch. View is east- ward and direction of sediment transport in the channel was southeastward. Scale is indicated by thickness of Dakota (about 20 ft or 6 m). plant fossils near Woodside, and Rushforth (1969) found several genera of fossil ferns in the Dakota. The ferns seem to be good indicators of paleoenvironment, inasmuch as they are related to modern taxa of ferns that grow only in tropical areas, under climatic condi- tions of high rainfall and humidity. The uppermost part of the Dakota becomes increas- ingly finer grained upward, and is conformable with the Mancos Shale. MANCOS SHALE The Mancos Shale has been recognized over vast areas of the Western Interior. Because the Mancos in- terfingers with other formations, its age range is dif- ferent in different areas. Mancos sediments were de- posited in the Western Interior from latest Early Cre- taceous to late Late Cretaceous time. In the Sunnyside area, deposition of the Mancos took'place throughout most of the Late Cretaceous; it correlates with the middle and upper parts of the Colorado Group and the lower part of the Montana Group reference section of the northern Western Interior (Cobban and Reeside, 1952). The Mancos is as thick as 4,400 ft (1,340 m) near Sunnyside. Thickness throughout the area is variable, however, owing to varied physiographic conditions during deposition and local postdepositional erosion prior to or concurrent with deposition of Mancos sedi- ments, and to the intertonguing of the upper contact. Abundant fossils and trace fossils indicate that the formation was deposited in offshore marine environ- ments in the Western Interior seaway. Trace fossils constructed by worms and other benthonic organisms are abundant in the shales, and cephalopod and pele- cypod shells are common in ferruginous Mancos con- cretions (J. R. Gill, oral commun., 1969). The entire Mancos fauna indicates marine deposition. As a distinctive lithologic unit, the Mancos extends westward as broad outcrop bands across the Colorado Plateau from the Rocky Mountains, and northward from northeastern Arizona to northern Colorado. Al- though the Mancos in Utah contains several members, near Sunnyside only the Ferron Sandstone Member (Lupton, 1913, p. 16-17) exists as a discrete mappable unit. We concur with R. C. Moore (in Longwell and others, 1923, p. 15) that the Tununk Shale (Gilbert, 1877, p. 4), Ferron Sandstone, and Blue Gate Shale (Gilbert, 1877, p. 4) Members of the Mancos in the Henry Mountains area correlate approximately with the entire Mancos section in east-central Utah. There- fore, our map (pl. 1) shows only the main body of the Mancos Shale, the Ferron Sandstone Member, and the lower and upper sandstone members. 16 SUN NYSIDE COAL-MIN IN G DISTRICT, UTAH The Mancos Shale in the Sunnyside district forms a wide valley between the San Rafael Swell and the Book FIGURE 12.—Physical characteristics of the Mancos Shale and associated strata. A, View eastward toward the Book Cliffs of badland topography carved in the Mancos Shale, southern part of the district. Resistant ledge, Kba, in middle ground is tongue of Aberdeen Member of Blackhawk Formation; Kbk, Kenil- worth Member, and Kbs, Sunnyside Member of Blackhawk For- mation; Kc, Castlegate Sandstone, Km, Mancos Shale. B, Oblique aerial view of the cliff front near Columbia, Utah, show- ing steep slopes of Mancos Shale (Km) protected from erosion by the overlying Blackhawk Formation (Kb) and Cretaceous and Tertiary rocks (T K). QC, colluvium derived from Mancos and Blackhawk sediments. Town of Columbia in lower right corner. Eastward View, sec. 20, T. 15 S., R. 14 E. C, Ferron Sandstone Member (Kmf) bluff on the east flank of the San Rafael Swell, near Cedar Siding. The Ferron is cut by a west- northwest-trending zone of faults, having total upward relative displacement of 110 ft (34 In) to the north (left in the photo- graph). Denver and Rio Grande Western Railroad track passes through the bluff in the fault zone (arrow). Bluff in the middle foreground is Mancos Shale (Km) that was protected from ero- sion by a remnant of alluvial fan gravel (Qf). Faults cut the Ferron Member behind the middle foreground bluff. Southeast- ward viewin sec. 2, T. 17 S., R. 13 E. Cliffs (fig. 12A), where it has been eroded into exten- sive and intricately carved badlands and broad swales (fig. 128). Badland topography becomes more com- mon near the Price River. Badland formation is strongly influenced by salts, particularly mirabilite (NaZSO,-10H20) and thenardite (Na2S04), which inhibit plant growth on the Mancos and also undergo hydra- tion and dehydration as humidity changes. These hydration changes cause swelling and contraction of the sediment particles and contribute much to the weathering process (Buss, 1956, p. 19). In the badlands area, ephemeral streams that drain into the Price River from the Book Cliffs have cut deep, vertically walled trenches in the Mancos. Farther north, however, the Mancos forms broad, gently s10ping valleys that ex- tend to the foot of the cliffs. At the foot of the Book Cliffs, the Mancos Shale is protected from erosion by overlying sandstones, and it rises in steep slopes above the valley floors (fig. 123). Although seemingly soft and able to easily absorb water, the Mancos contains much very fine grained mixed-layer clay that allows water only slight penetration below the surface and forces much of the precipitation to run off. For example, during heavy rains Mancos surfaces nor- mally are impassable to vehicles, due to formation of mud on the surface, but the rocks may be dry a few inches down. The surfaces generally dry out within a few hours after a drenching rainstorm. The lower part of the Mancos Shale comprises about 500 ft (150 m) of medium-gray to bluish-gray, locally fissile mudstone that contains discontinuous lenses and stringers of ferruginous siltstone and claystone. This lower part of the Mancos also contains stringers and veinlets of crystalline gypsum (selenite), and CRETACEOUS SYSTEM 17 weathered masses of crystals are abundant on mud- stone slopes where they sparkle and shine in the sun- light. Thin discontinuous sandy zones occur locally and form low cuestas or support low knolls. The Ferron Sandstone Member is 800 ft (250 m) thick at its type locality, about 50 miles (80 km) south- west of Sunnyside, near the Wasatch Plateau. In the Sunnyside district, however, the Ferron is only 20-50 ft (7—15 m) thick, where it comprises thin beds of inter- bedded reddish-brown siltstone and very fine to fine- grained sandstone. The Ferron forms rounded bluffs on the flanks of the San Rafael Swell (fig. 120). We found only trace fossils in the Mancos in this area. We have found trace fossils of the cruziana and zoophycos facies of Seilacher (1964, p. 310) in some of the siltstone and sandstone beds. According to Seilacher, these fossils indicate an offshore marine en- vironment of deposition for this part of the Ferron; therefore, this unit may be interpreted as the offshore facies of a beach sand that was deposited farther to the west. The arenaceous sediments were transported east- ward into the Sunnyside district during a limited re- gression of the Cretaceous sea. Following this, west- ward advance of the shoreline resulted in deposition of offshore deep-sea muds above the Ferron. The upper part of the Mancos Shale resembles the lower part. In lithology, color, and other details, the units generally are similar, but have minor local varia- tions. The upper part contains more gypsum crystals and veins than the lower part; it also contains calcium carbonate and other salts; weathered slopes that look as though they were dusted with flour are common throughout the upper part of the Mancos as a result of weathering of the gypsum and calcium carbonate. Dis- continuous tongues of thin sandstone and siltstone, probably offshore extensions of the Aberdeen Sand- stone Member of the Blackhawk Formation that is present west of the Sunnyside district, crop out in the Mancos in the southern part of the district near the base of the Book Cliffs. The Mancos grades upward into and intertongues with sandstone units of the Blackhawk Formation. The contact is conformable, indicating continuing sedi- mentation during regression of the sea. Because of the intertonguing in these regressive deposits of the Blackhawk and Mancos, the upper contact of the Man- cos is placed at successively higher stratigraphic posi- tions (or levels) as one progresses southeastward across the district. MESAVERDE GROUP The term “Mesa Verde Group” originally was ap- plied to a group of coal-bearing sandstone and shale units between thick marine shale units (Holmes, 1877, p. 245). The type locality is near Mesa Verde in south- western Colorado, and the name was used for similar rocks throughout a large part of the Western Interior. So indiscriminately has the name been used that “Mesaverde” is now applied to many rock units in dif- ferent parts of the Western Interior that are lithologi- cally similar to those at the type locality but that may be of different ages. In the Sunnyside district the Mesaverde Group com- prises three Upper Cretaceous formations. In ascend- ing order, these are the Blackhawk Formation, Castle- gate Sandstone, and Price River Formation. Spieker and Reeside (1925, p. 445) named the Blackhawk and Price River (in which they included the Castlegate as Castlegate Sandstone Member) Formations, and Fisher, Erdmann, and Reeside (1960, p. 14) raised the Castlegate to formation rank. The basal contact of the Mesaverde Group is grada- tional and intertongues with the Mancos Shale. Be- cause of this relationship, the contact rises strati- graphically eastward into Colorado so that none of the Mesaverde units at Sunnyside are present at the east- ern terminus of the Book Cliffs. Thus, all of the Mesa- verde Group in western Colorado is younger than any of it at Sunnyside (Fisher and others, 1960, p. 11). BLACKHAWK FORMATION As the Mancos sea retreated, a vast deltaic complex prograded eastward across much of central Utah (Howard, 1966, p. 31). In the Book Cliffs region, one such delta (Maberry, 1971) deposited a suite of sedi- ments that is now called the Blackhawk Formation (Spieker and Reeside, 1925, p. 443). This formation comprises several depositional sequences of similar genesis which were laid down in an offlap relationship. Each sequence represents a progression from marine environments of deposition on the west to continental environments on the east. Each sequence also ended with a relatively minor transgression of the sea, fol- lowed by the next regressive sequence. The Blackhawk forms the main scarp of the Book Cliffs in the Sunnyside district. It is composed of thick cliff-forming sandstone beds that are resistant to ero- sion, and weakly resistant slope-forming mudstone beds between the sandstones. The Blackhawk Formation contains several exten- sive coal deposits, which are the basis for much of the economy of central Utah. Because of the economic im- portance of the coals, stratigraphers have subdivided the Blackhawk into members, each member containing one particular coal bed somewhere in the Wasatch Plateau or Book Cliffs Coal Fields. Various workers did not always use the same criteria for distinguishing 18 members, however; historically, these criteria de- pended upon the goals of the individual investigators and upon the locations of their investigations. Spieker and Reeside (1925, p. 442) named the Star Point Sand- stone, a unit that can be separated from the overlying Blackhawk on the basis of lithology and topographic expression. The Star Point occurs only in the Wasatch Plateau. Clark (1928, p. 11) distinguished two mem- bers of the Blackhawk (in ascending order): the Aber- deen Sandstone Member and the coal-bearing member; Young (1955) used topographic expression and inferred genesis in separating the Blackhawk into six members, including Clark’s Aberdeen Member. Maberry (1971) redefined two of Young’s members on the basis of lithology and unit mappability, and pointed out some of the stratigraphic problems inherent in naming rock strata of the Book Cliffs. Only the Aberdeen Member, Kenilworth Member, upper mudstone member, lower mudstone member, and Sunnyside Member are present in the Sunnyside district (Maberry, 1971, p. 24). Members of the Black- hawk stratigraphically below the Aberdeen crop out farther west and tongue out into the Mancos Shale be- fore reaching Sunnyside. Members stratigraphically above the Sunnyside occur to the south and east of the area of this report. Maberry (1971) described both the Kenilworth and Sunnyside Members as being composed of marine sandstone. He described an upper mudstone member, above the Sunnyside, and a lower mudstone member, separating the Sunnyside and Kenilworth. ABERDEEN MEMBER The Aberdeen Member was named by Clark (1928, p. 18) for an outcrop in the western Book Cliffs, at the Aberdeen Mine near Kenilworth. In the Sunnyside dis- trict the member is only a thin zone of weakly re- sistant, shaly, fine-grained sandstone and siltstone that crops out discontinuously. It is underlain and overlain by the Mancos. Its base is some 200 ft (60 m) stratigraphically below the Kenilworth Member. South of Horse Canyon, rocks mapped as Aberdeen(?) form a low cuesta at the foot of the Book Cliffs. The eastern part of the Aberdeen contains the zoophycos- facies trace fossils and is interpreted as the offshore and prodelta slope facies of the deltaic beach-complex sandstone body that makes up the type Aberdeen. The Aberdeen Member in the Sunnyside district is barren of coal. KENILWORTH MEMBER Named by Young (1955, p. 183) for the type locality near its outcrop at Kenilworth, Utah, the Kenilworth Member at Sunnyside comprises three distinct sand- stone tongues, each overlain by a shale sequence (fig. SUN N YSIDE COAL-MI NIN G DISTRICT, UTAH 13). Each sandstone tongue begins and ends farther east and south than the tongue it overlies. The entire Kenilworth was deposited in a three-cycle rhythmic succession of marine-to-continental-to—marine sedimen- tation. In the southern part of the district near the Price River, only the two upper tongues persist. We placed the base of the Kenilworth at the lowest persistent sandstone bed in the Blackhawk Formation. The base thus defined rises stratigraphically to the south and east but remains physically consistent. This lithologic break is expressed physiographically as a sheer cliff composed of a series of sandstone ledges and is distinctly mappable. The member ranges in thick- ness from 110 to 220 ft (35 to 65 m). The sandstone tongues of the Kenilworth were de- posited as prograding beach and barrier complexes (Maberry, 1971). The vertical gradation from mud- stone at the base to well-sorted sandstone at the top in- dicates progressive shoaling of the sea floor as the sea withdrew eastward. As the shoreline migrated sea- ward, coarser grains were deposited in the Sunnyside area as finer sediments were carried out to sea. Mudstone units between sandstones of the Kenil- worth and Sunnyside and above the Sunnyside com- monly contain coal lenses and carbonaceous horizons. Most of these mudstones indicate positions of coal beds farther west and east which are paleoecologic in- dicators of low-lying. swamps landward from the beach complex. The coal-bearing part of each mudstone unit is overlain by marine mudstone deposited when the sea made limited transgressions. In ideal cyclothemic stratigraphy, the coal-bearing units should be included FIGURE 13.—Cyclic sedimentation units in Kenilworth and Sunny- side Members of the Blackhawk Formation at the mouth of Fan Canyon, 3 miles (5 km) east of Sunnyside (pl. 1). Km, Mancos Shale. Blackhawk Formation: Kbk, Kenilworth Member; Kbml, lower mudstone member; Kbs, Sunnyside Member; Kbmu, upper mudstone member; Kc, Castlegate Sandstone. This out- crop is the principal reference section of the Kenilworth Mem- ber (Maberry, 1971). Scale indicated by 1,000-ft (305-m) thick- ness from base of Kenilworth Member to top of Castlegate Sandstone. CRETACEOUS SYSTEM 1 9 with the sandstones at the top of a cycle, the marine part as the base of the next succeeding cycle. However, for convenience in mapping, the contacts at the bases of the sandstones were used. LOWER MUDSTONE MEMBER Above the Kenilworth Member is a 150—200-ft (45—60-m)-thick, slope-forming sequence of coal-bear- ing, generally dark-gray, clayey mudstones, shales, and sandy siltstones designated the lower mudstone member of the Blackhawk (Maberry, 1971, p. 26). The thin discontinuous and lenticular Rock Canyon coal bed lies on top of the tongue of the Kenilworth Mem- ber, but the coal tongues out southward into a very thin carbonaceous zone near Woodside. Trace fossils and inorganic sedimentary structures of continental affinity are common throughout the lower half of the lower mudstone member, but the upper part contains biogenic and inorganic structures indicative of marine deposition. The lower continental mudstone was deposited landward of the beach com- plex. When the sea encroached upon the land the conti- nental sediments were covered by sea-floor muds. SUNNYSII)E MEMBER Young (1955, p. 185) gave the name Sunnyside Mem- ber to a sequence composed of a massive basal sand- stone tongue and the overlying coal-bearing rocks “which are replaced eastward by barrier-bar sand- stones.” This sequence is difficult to recognize at every point along the Book Cliffs; therefore, Maberry (1971, p. 27) redefined the Sunnyside Member, applying the name to the continuous cliff-forming sandstone unit about 200 ft (60 m) above the Kenilworth Member. The Sunnyside contains a 100-190-ft (30-58-m)—thick sequence of interbedded sandstone and siltstone at the base, grading upward through fine-grained sandstone to thick-bedded, medium-grained sandstone at the top (fig. 14). The upper boundary of the member is at the top of the uppermost sandstone bed in the sequence. The Sunnyside sandstones were deposited as beaches and barriers as the Cretaceous sea withdrew from the area toward the east. The lower part of the member contains trace fossils and sedimentary struc- tures that indicate a quiet offshore depositional envi- ronment. Upward in the section crossbedding is com- mon, and the trace fossils are those of organisms that lived in shallow offshore marine environments. In the upper part of the member, only trace fossils of or- ganisms that lived in a beach and near-shore littoral environment were found, and crossbedding and ripple marks indicate deposition in shallow water having high kinetic energy (Maberry, 1971, p. 27—30). FIGURE 14.—Characteristic topographic forms and sedimentation units in the Sunnyside Member of the Blackhawk Formation, east side of Slaughter Canyon near the canyon mouth, sec. 32, T. 14 S., R. 14 E. Blackhawk Formation: Kbk, Kenilworth Mem- ber; Kbml, lower mudstone member; Kbs, Sunnyside Member; Kbmu, upper mudstone member; KC, Castlegate Sandstone. UPPER MUDSTONE MEMBER The Sunnyside Member is overlain by 100—200 ft (30—60 m) of mudstone and discontinuous sandstone beds of the upper mudstone member. The thick and economically important Sunnyside coal is at the base of the member, directly above the thick sandstone of the Sunnyside Member (Maberry, 1971, p. 30). Above the coal are mudstone, siltstone, and sandstone beds that were deposited in back swamps and coastal plains westward of the beach area. A thick sandstone, the lateral equivalent of the upper mudstone member (Maberry, 1971, p. 31), over- lies the Sunnyside coal in the southern part of the dis- trict (fig. 15). This overlying sandstone, which begins near Horse Canyon and persists southward into the Beckwith Plateau where the Sunnyside coal bed pinches out, was designated the Grassy Member of the Blackhawk by Young (1955, p. 186). Fluviatile channel sandstones, the landward lateral equivalent of the Grassy, also crop out in Water Canyon and in Fan Canyon (sec. 8, T. 15 S., R. 14 E.), about 6 miles (10 km) north of the limits of the Grassy (fig. 16). Large channel sandstones in the upper mudstone member are the lateral equivalent of estuarine and beach deposits farther east. These channel deposits are more common in the upper part of the upper mudstone member, and many merge or branch to form larger or smaller structures. The largest of these channel sand- stones in the Sunnyside district is 150 ft (45 m) wide and 24 ft (7 m) thick at its thalweg. Internal crossbed- ding and ripple marks in the channel sandstones indi- 20 SUNNYSIDE COAL-MINING DISTRICT, UTAH FIGURE 15.—Surface plant of the Book Cliffs Mine, abandoned in 1967. showing relation of the Sunnyside coal bed (at the level of the upper building) to the underlying sandstone beds of the Sunnyside Member (Kbs). Strata from the top of the Sunnyside Member to the top of the photograph are in the upper mudstone member of the Blackhawk Formation. The sandstone bed (88), which overlies the coal, was designated the Grassy Member by Young (1955). Photograph by Vard H. Johnson. cate that the principal flow direction was east- southeast. Thin discontinuous lenses and pods of impure coal . are common throughout the mudstone interval. These deposits are the remains of local short-lived swamps and bogs (back-swamp deposits) landward from the shore. Such coals are neither thick enough to be of com- mercial interest nor extensive enough to be useful for correlation. The upper contact of the Blackhawk Formation is an unconformity, but is the product of a local diastem rather than a regional unconformity. Stream channels of the Castlegate Sandstone were cut into the soft muds and friable sands after deposition of the Black- hawk. These channels were filled in by coarse sands of the Castlegate Sandstone. Slump structures resulting from failure of channel banks locally are common in the base of the Castlegate, and thin deposits of carbonized plant debris are common along the contact between the mudstone and Castlegate Sandstone. (IAS’l‘lJ'XlA'I‘E SANDS'I‘ONE The name Castlegate Sandstone Member was ap- plied by Spieker and Reeside (1925, p. 445) to the lowest cliff-forming sandstone member of the Price River Formation. At the former town of Castle Gate, Utah (fig. 1), the sandstone in Price River Canyon is eroded to buttresses and towers, reminiscent of castles and forts. Fisher, Erdmann, and Reeside (1960, p. 14) elevated the Castlegate to formation status on the basis of its areal extent and unique lithology. At Castle Gate, Utah, the formation is about 550 ft (170 m) thick and is conglomeratic. The conglomerate grades laterally eastward into finer grained deposits. In the Sunnyside district the lower 150 ft (45 m) of Castlegate Sandstone is generally medium and coarse grained, comprising quartz, feldspar, chert, and rock fragments in a calcareous clay matrix. Above this is a fine- to medium-grained, moderately well-sorted quart- zose sandstone unit about 50 ft (15 m) thick. In the northwestern part of the Sunnyside district, the upper part is fine-grained, well-sorted quartzose sandstone. Most of the Castlegate in the Sunnyside district was deposited by a stream-channel network debouching onto the coastal plain area behind the Blackhawk beaches. The most common sedimentary structure in Castlegate channel deposits is large-scale crossbed- ding formed by dunes, antidunes, and point bars. Large-scale foreset crossbedding dips at right angles to channel flow. Three-dimensional field measurements of crossbedding indicate that overall flow direction was east-southeast, although individual channel flow varied as much as 90° from the major direction. Dis- continuous beds of thinly interlaminated sandstone and siltstone occur locally at the base of the Castle- gate. These beds are here interpreted to be deposits of sluggish tributary streams. The Castlegate forms a prominent cliff above the Blackhawk Formation throughout the Book Cliffs (figs. 13, 14, 16). Units above the Castlegate form a series of retreating ledges and slopes parallel to the trend of the cliffs behind the Castlegate. Deposition of the Castlegate Sandstone ended as the western highland was eroded. After the period of force- ful, turbulent Castlegate sedimentation and the time of erosion of the Castlegate surface, deposition was slow and quiet as muds and silts accumulated on the alluvial plain during the final recession of the sea. PRICE RIVER FORMATION Spieker and Reeside (1925, p. 445) named the Price River Formation for outcrops in Price River Canyon, near Helper, Utah. Fisher, Erdmann, and Reeside (1960, p. 14) restricted the Price River Formation by removing the Castlegate Sandstone, and we mapped the Price River in two members (pls. 1, 2). The lower member is unnamed and consists of interbedded mud- stones, claystones, siltstones, and fine-grained sand- stones; the upper member is the Bluecastle Sandstone Member. The Price River sequence west of the Wasatch Plateau is a thick conglomerate which has a deeply eroded upper surface. Progressing eastward, the Price River thins and grades into rocks composed largely of clay-size materials in western Colorado. The formation .is as much as 600 ft (180m) thick in Price River ’CRETACEOUS SYSTEM 21 FIGURE 16,—Fluvial channel sandstone beds in the upper mud- stone member of the Blackhawk Formation, Water Canyon, sec. 9, T. 16 S., R. 14 E. Upper limit of the Sunnyside coal bed is at the level of the old mine portal (arrow). Fluvial sandstone facies, the products of shifting stream-channel networks on the coastal plain, pinch out laterally into continental shale and mudstone facies. Kc, Castlegate Sandstone. Canyon, about 35 miles (55 km) west of the Sunnyside district (Spieker, 1946, p. 120), but it thins eastward across the Sunnyside district and intertongues in east- ern Utah with marine Mancos Shale units. We found part of a fossil gastropod in the unnamed member of the Price River Formation on the east side of Whitmore Canyon, a quarter of a mile (400 m) south of the mouth of Pasture Canyon. This fossil was identi- fied by D. W. Taylor of the US. Geological Survey (written commun., 1966) as the operculum of the fresh- water snail Reesidella nana (White). Taylor stated: White (1886, p. 32) described this species (as “Viviparus nanus”) from “Wasatch strata, near Wales, Utah, and in equivalent strata at several localities in the Wasatch Mountains.” Later collectors have not found the species in either the Flagstaff Formation or Colton Formation, so it may have come from either the North Horn or the Price River Formation. The present occurrence is the first confirmation of the species in the Price River Formation. LOWER UNNAMED MEMBER The lower unnamed member of the Price River For- mation consists of about 150—300 ft (45—90 m) of argil- laceous fine-grained sandstone deposited on a coastal plain. Dominant colors are gray and brown. Crossbed- ding is common locally in siltstones and fine-grained fluviatile sandstones. Thin discontinuous impure coal beds occur at many horizons in the unnamed member. These coals indicate accumulation of organic and inor- ganic debris in restricted, short-lived upland swamps and bogs. Pods and lenses of sandstone occur at inter- vals throughout the member, indicative of channel cut- ting and infilling by slow-flowing streams. Fossils are uncommon in the member, but channel sandstones locally contain freshwater pelecypods, and we re- covered incomplete remains of an unidentified large in- vertebrate from the member in Horse Canyon. The unnamed member generally is soft and easily eroded because most individual strata are poorly cemented. The sediments contain large amounts of or- ganic material and clay. The unnamed member in the Sunnyside district commonly forms low slopes behind the Castlegate cliffs. In the southern part of the dis- trict, where faulting and differential erosion have locally removed the overlying Bluecastle Sandstone Member, the drainage of Little Park Wash (pl. 1) formed along the strike of the unnamed member (fig. 17). BLUECASTLE SANDSTONE MEMBER The Bluecastle originally was designated as a bed of the Neslen coal-bearing member of the Price River For- mation, a transitional marine unit in the Book Cliffs east of the Green River (Fisher, 1936, p. 18). V. H. Johnson of the Geological Survey recognized the Blue- castle as the upper 300 ft (90 m) of the Price River For- mation in the Woodside Quadrangle (Fisher, Erdmann and Reeside,‘ 1960, p. 17), and it was subsequently (Fisher and others, 1960, p. 11, 14) raised in strati- graphic rank to the upper member of the Price River Formation, west of the Green River and as an upper member of the Neslen Formation east of the Green FIGURE 17 .—Northward view of the upper reach of Little Park Wash, showing the alluvium-filled strike valley (Qa3); the upper part of the cliff, and dip slopes of the Castlegate Sandstone (KC); lower unnamed member of the Price River Formation (Kpl), into which Little Park Wash is incised (steep canyon walls in lower left part of picture); and cliff and part of the dip slopes of the Bluecastle Sandstone Member of the Price River Forma- tion (Kpb). High cliffs and slopes in the background are the Roan Cliffs, composed of the North Horn, Flagstaff, and Colton Formations. Skyline in left half of picture is top of Book Cliffs. 22 SUN NYSIDE COAL-MIN IN G DISTRICT, UTAH River. In mapping the Sunnyside district we found that the Bluecastle is a prominent mappable unit throughout the western Book Cliffs (pl. 1) and that the member becomes thinner and more fine grained east- ward and intertongues with the upper part of the N eslen Formation. The N eslen Formation and the Price River Formation, composed of the lower un- named member and the Bluecastle Sandstone Member, thus are considered to be time-equivalent facies of the same depositional episode. The Bluecastle Sandstone Member consists of fine- to medium-grained quartz sandstone, cemented to varying degrees by silica, ferruginous clay, and carbo- nate. It is thickest and most coarse grained in the northwestern part of the Book Cliffs. Local case- hardening by ferruginous silica cement is common throughout the unit, and desert varnish is common on weathered cliff faces. The Bluecastle is 10—300 ft (3—90 m) thick in the Sunnyside district, and forms an abrupt vertical cliff between the lower unnamed member and the overlying North Horn Formation, particularly north of Sunny- side (pls. 1, 2), where it attains maximum thickness. Stripped dip-slope surfaces are common on top of the Bluecastle and result from downdip erosion of the softer North Horn. The Bluecastle is made up of multi- story and multilateral crossbedded, interbedded chan- nel sandstone bodies, interspersed with thick, flat- bedded sandstone beds. We believe that the channel sandstones were formed by lateral and vertical migra- tion of mobile stream channels at the lower end of an alluvial plain derived from the western highland. Spieker (1946, p. 132) correlated the Price River For- mation with “the postorogenic gravels of the western districts”; the channel sandstones and flat-bedded sandstones probably formed in response to the chang- ing hydrologic regimen in the western highland. Sedi- mentary structures in channel bodies, such as ripple marks, dunes, flute casts, and prod marks, indicate for- mation in the lower flow regime of a stream system flowing generally east-southeast. Graded bedding locally is present in the flat-bedded sandstones, and in- ternal structures of these beds include scour-and-fill crossbedding, chaotic and convolute lamination, and foreset crossbedding. The upper surface of the Bluecastle has been postu- lated to be an unconformity (Young, 1955, p. 180), but the only evidence we have found to support this postu- lation is local concentrations of silicified wood on the upper surface and local lateral thickness variations in the unit. In most places the top of the Bluecastle ap- pears to be conformable with the North Horn Forma- tion. CRETACEOUS AND TERTIARY SYSTEMS, UNDIVIDED NORTH HORN AND FLAGSTAFF FORMATIONS The North Horn Formation was named by Schoff (1937, p. 379) for exposures on North Horn Mountain in the Wasatch Plateau. Spieker (1946, p. 133) meas- ured 1,650 ft (500 m) of North Horn at the type 10- cality, and recognized that the unit consists of alter- nating beds of la‘wstrine origin and of overbank flood- plain and fluviatile channel origin. The North Horn is of Late Cretaceous and Paleocene age. Cretaceous dinosaur remains were found in the lower part of the unit, and Palgyeji£_mamrna,l’rgnains_ occur in the upper part, but there is no known clearcut boundary between rocks of the two ages (Cobban and Reeside, 1952, p. 1028). Whether a sharp boundary is of great importance is problematical, however. Spieker (1946, p. 142—149) concluded that any time boundary within the North Horn probably should be regarded as “an arbitrary device, founded *** on phenomena of natural significance, but hardly expressive of any comprehen- sive principle. ” In the Sunnyside district the North Horn inter- fingers with and is transitional into the overlying Flag- staff Limestone (fig. 18). In this area the North Horn consists of interbedded claystones, mudstones, lime- stones, siltstones, and sandstones. The claystones and mudstones generally are brownish gray to blue gray and are highly plastic. Most contain much carbo- naceous detritus and contain from 5 to 15 percent silt- FIGURE 18.—Southward view of transition between and inter- tonguing of North Horn (T Kn) and Flagstaff (Tf) Formations in sec. 9 (unsurveyed), T. 17 S., R. 15 E. Ledges in the middle of the hill are beds of Flagstaff Limestone; most of the slope is mudstone of North Horn Formation; the ledge at the crest of the hill is Flagstaff Limestone. TERTIARY SYSTEM and sand-size quartz. Limestones generally are blue gray to reddish brown and commonly contain abun- dant freshwater pelecypod and gastropod fossils. Most siltstones and sandstones are brownish, lenticular, and contain much carbonaceous detritus and scattered fos- sils. Locally, the North Horn near Sunnyside contains fossil algae. The Flagstaff was named Flagstaff Limestone Mem- ber of the Wasatch Formation by Spieker and Reeside (1925, p. 450). Spieker (1946, p. 135) later elevated the Flagstaff to formation rank. The type locality is on Flagstaff Peak in the Wasatch Plateau, where the unit is nearly 1,500 ft (460 m) thick. The formation is ex- pressed both as ledges of varying thickness and as dip slopes throughout the Sunnyside district. Flagstaff sediments were deposited during middle and late Paleocene and early Eocene time in a freshwater lake. The lake was formed by intermittent basin downwarp- ing in central Utah (Weiss, 1969, p. 1116) between phases of flood-plain deposition of North Horn and Colton sediments. The Flagstaff is thick enough and lithologically dis- tinct so that it may be traced northward continuously from Flagstaff Peak and around the Book Cliffs to a position between the town of Columbia and Horse Canyon. Here the formation thins and intertongues with the North Horn and Colton Formations. The Flagstaff is the most distinctive lithologic unit in the northern part of the district (pl. 2). It can be differen- tiated with difficulty in detailed stratigraphic sections in the central part of the district, and only individual interfingering beds of Flagstaff lithology are dis- cernible in the southern part of the district (fig. 19). Flagstaff beds are distinguished by the wealth of con- tained fossils (La Rocque, 1960), and individual strata in the southern part of the Sunnyside district may be differentiated on this basis. North Horn sediments were deposited on a base-level plain upon which environments alternated between lacustrine and alluvial plain. When North Horn deposi- tion began, lacustrine conditions dominated near . Sunnyside, and sediments were deposited in deltaic, near-shore, and offshore environments in the lake in re- sponse to changing climatic conditions. Weiss (1969, p. 1115) showed that the Flagstaff Lake persisted in Utah over a long period of time. He also showed that the upper surface of the Flagstaff is conformable with the Colton Formation (early and middle Eocene) or with the Green River Formation (early and middle Eocene). The following fossils that we found at Sunnyside were identified by D. W. Taylor (written commun., 1966). He believes that these fossils indicate early 23 FIGURE 19.—Characteristic uniform bedding in Flagstaff Lime- stone in sec. 12, T. 16 S., R. 15 E. Scale indicated by motor scooter, 3.5 ft (1.1 m) high. Eocene (Gray Bull) age. The upper part of the Flagstaff in the Wasatch Plateau is equivalent to the upper part of the North Horn Formation and lower part of the Colton Formation in the Sunnyside district. Freshwater species Class Pelecypoda Plesiellip tio sp. Class Gastropoda Valvata bicincta Whiteaves H ydrobia utahensis White Bellamya n. sp. “Campeloma " limneaformis (Meek and Hayden) Physa cf. P. bridgerensis Meek Cleopatra tenuicarinata (Meek and Hayden) Hydrobia sp. Pleurolimnaea tenuicosta (Meek and Hayden) Bulinus (Pyrogophysa) sp. Valvata sp. Continental (land) species Class Gastropoda Discus? sp. Helminthoglyptidae? TERTIARY SYSTEM EOCENE SERIES COLTON FORMATION The Colton Formation is a series of variegated sand- stones and mudstones, ranging in thickness from 900 ft (275 m) to about 3,000 ft (900 m) in the Sunnyside district. The unit was named by Walton (1944, p. 120) for outcrops at Colton, Utah, about 40 miles (65 km) northwest of Sunnyside. Colton strata thicken to the southeast, reaching 3,400 ft (1,050 m) in thickness in the cliffs north of the town of Green River, Utah, but thin north of Sunnyside and grade laterally into the 24 SUN N YSIDE COAL-MIN IN G DISTRICT, UTAH Green River Formation. In the Sunnyside district, the Colton forms the Roan Cliffs, standing in north-trend- ing steep slopes and sheer cliffs that face westward. Near Woodside, the Colton Formation changes strike to swing eastward toward the Green River. The change of strike occurs some distance before the Blackhawk and Price River Formations change strike, and the de- parture of the Roan Cliffs from the Book Cliffs is a con- spicuous physiographic feature. In the northern part of the district, we divided the Colton, for mapping pur- poses, into a lower and an upper unit (pl. 2). The lower unit is mostly mudstone containing some sandstone channel deposits; the upper unit contains dominantly fluviatile sandstone deposits and is more resistant to erosion than the lower unit. The lower unit forms slopes and occasional ledges; the upper unit forms cliffs and bluffs having minor small slopes between. The upper unit of the Colton, called Wasatch Forma- tion by Holmes, Page, and Averitt (1948), contains beds of bituminous sandstone (“tar sands”) north of Sunnyside. Interbedded fluviatile, overbank, flood-plain, and lacustrine beds make up the Colton (fig. 20). Bedding is expressed as channel-fill structures and in tabular and lenticular bodies of small individual areal extent. In the Sunnyside district, the Colton was deposited dur- ing a local interlacustrine period between the most ex- tensive coverage of Lake Uinta, of which the middle and late Paleocene and Eocene Flagstaff Lake (C. B. Hunt, 1956, p. 76) is here considered a part, and the Eocene Green River Lake (C. B. Hunt, 1956, p. 78). Colton sediments are here thought to be a fluvial and overbank flood-plain facies of a depositional episode that elsewhere produced lacustrine deposits of the Flagstaff and Green River Formations. In the Sunnyside district, most of the Colton is red, but strong hues of red, reddish brown, yellow, green, and gray alternate throughout the formation. The Roan Cliffs were named for the red-brown coloration given them by the Colton Formation. The Colton originally was correlated with the lower part of the Wasatch Formation; historically, however, there is some disagreement among geologists concern- ing the “Wasatch problem” (Spieker, 1946, p. 137—139). In recent times the trend has been toward the use of Wasatch much in the same manner as Mesa- verde, that is, to let the name indicate a typical assem- blage of lithologies. Used in this way, the Colton For- mation certainly may be called early Wasatch age. GREEN RIVER FORMATION In the Sunnyside district, the Green River Forma- tion consists of a 0—125-ft (0—38-m)—thick sequence of freshwater limestone and marlstone, siltstone, sand- stone, and shale. It occurs as the highest bedrock unit in the stratigraphic section and caps topographically high ridges and knobs. It is present only in the north- eastern part of the district but was outside the area mapped during field investigations. The Green River is transitional with the underlying Colton Formation. QUATERNARY SYSTEM Surficial materials of Quaternary age are distributed widely throughout the Sunnyside district. These mate- rials range from high-level deposits of stream alluvium of probable early Pleistocene age along the Book Cliffs and along the slopes above Whitmore Canyon to H010- cene alluvium along modern stream courses and to man-induced talus formed since 1959 (fig. 41). The FIGURE 20.—Northward panoramic View showing topographic forms produced by lenticular and intertongued sandstones in the Colton Formation south of Horse Canyon in sec. 24, T. 16 S., R. 14 E. The Colton Formation overlies the TKfn, North Horn and Flagstaff For- mations undivided; TCI, IOWer unit, Colton Formation; Tcu, upper unit, Colton Formation. PLEISTOCENE SERIES 25 Quaternary materials were of considerable interest to our investigations, because their depositional history provides clues to the diastrophic and erosional his- tories of the region, which bear importantly on the stress history of the coal and on the distribution of strain energy stored in the coal. Some sedimentary units of Quaternary age are correlated tentatively with similar units recognized by Richmond (1962) in the La Sal Mountains, Utah, about 130 miles (210 km) south- east of Sunnyside. These correlations are tenuous at best, because they are based largely on inferences drawn from the topographic positions of the units and from their relationships to the regional geomorphic de- velopment. Other units, on even less definitive evi- dence, are correlated with units elsewhere in the Colo- rado Plateau, as summarized by C. B. Hunt (1956, p. 27, 38). PLEISTOCENE SERIES SEDIMENTS OF EARLY PLEISTOCENE AGE Stream-worn gravels and boulders indicative of former stream terraces are widely scattered on inter- fluvial divides along the face of the Book Cliffs and along valley slopes above Whitmore Canyon (Qa1 on pl. 2). These terrace remnants are found at several levels, but some that tentatively can be correlated indi- cate a gradual depositional gradient to the south. Cobbles and boulders in all remnants that we examined were derived from the Bluecastle Sandstone Member of the Price River Formation, sandstones of the North Horn Formation, the Flagstaff Limestone, and sandstones of the Colton Formation. Still higher and older remnants along the top and flanks of Patmos Mountain (pl. 1) overlie the Green River Formation and slope about 2° south; all fragments in these rem- nants were derived from the Green River Formation. In general, the rock types in each remnant are similar to the rocks near the stratigraphic level at which the remnant occurs; this similarity suggests that the former streams by which the rocks were deposited flowed southward in a series of strike valleys, as do the streams presently flowing in Range Creek, Little Park Wash, and in parts of Whitmore and Horse Canyons (pl. 1). Range Creek and Little Park Wash probably are survivors of the drainage pattern in which the gravels were deposited. Streams flowing westward down the face of the ancestral Book Cliffs, as the cliffs retreated to their present position, probably captured some of the older and higher streams (Osterwald and others, 1971, p. 13). The age of these remnants is unknown, but they clearly are older than the Book Cliffs. The old streams probably existed during early Pleistocene time, but they possibly could have been of Pliocene age. Several feet of stream alluvium covers the top of West Ridge (pl. 1; Qt on pl. 2). Much of this alluvium is dark brown and deeply weathered and consists of silt, clay, and some sand. A few lenses of flat to oblate, rounded sandstone gravels and cobbles, as well as a few sandstone boulders, are interbedded with the fine- grained material. The alluvium fills low places in a gently rolling topography on top of the ridge. Small hills underlain by sandstones of the Colton Formation project upward through the alluvium in some places. Siltstones of the Colton beneath the alluvium are deeply weathered whereas sandstones are not, al- though they are less tightly cemented than the silt- stones. In some places the alluvium is slightly plastic but is nonswelling. On the south part of West Ridge the alluvium fills a broad but shallow valley that may be an ancient stream course (pl. 2). SEDIMENTS OF PRE-WISCONSIN(?) AGE BOULDER DEPOSITS In the Sunnyside district two separate areas of scat- tered, deeply weathered boulders (Qm on pl. 2) are the only units of Quaternary age that may be of glacial ori- gin. These two areas, each several hundred feet long and a few hundred feet wide, are near the crest of West Ridge at altitudes of 8,640 to 8,800 ft (2,633—2,682 m). They are about 800 ft (245 m) above the floor of an ancient valley in which Whitmore Canyon is en- trenched (p. 41; pl. 2), or about 1,750 ft (530 m) above the present floor of Whitmore Canyon. The boulders are rounded to subangular and range in size from 1 or 2 m to the dimensions of small houses. No matrix sur- rounds the boulders, presumably because it has long since been eroded. Bedding planes in the boulders trend at widely diverging attitudes, and only in a few is bedding parallel to the strike and dip of the under- lying Colton Formation. The boulders were deposited on stream alluvium of probable early Pleistocene or Pliocene age. Although the boulders are similar litho- logically to sandstones of the Colton Formation at lower elevations in West Ridge, no logical source for such boulders is present on West Ridge; therefore, they probably were transported to their present posi- tion by ice from one or more modified cirquelike de- pressions in the west side of Bruin Point at an eleva- tion of about 10,000 ft (3,050 m) 4 miles (6.5 km) to the northeast, or from Mount Bartles (fig. 1) at a slightly lower elevation 7 miles (11 km) to the north. Whatever 26 SUN N YSIDE COAL-MINING DISTRICT, UTAH the source, the glacier could not have flowed on any presently existing topography because all the possible sources are separated from West Ridge by large canyons. The exact age and correlation of the boulder masses cannot be determined directly. Because of their topo- graphic distribution and general characteristics, the boulder masses may be correlative with some of the pre-Wisconsin glacial deposits in other parts of the Colorado Plateau, as summarized by C. B. Hunt (1956, p. 35—36). The boulder masses, which clearly antedate Whitmore Canyon, also may be correlative with Rich- mond’s (1962, p. 25—35) lower member of the Harpole Mesa Formation in the La Sal Mountains, Utah, which occupies a similar topographic and physiographic posi- tion. Richmond correlated the Harpole Mesa Forma- tion with the N ebraskan, Kansan, and Illinoian Glacia- tions of the midcontinent region. ALLUVIUM ()F BULL FLAT A large area of alluvium (Qa2) underlies Bull Flat, a large and gently sloping bench 1,000 ft (300 m) below the top of West Ridge and 800 ft (245 m) above the floor of Whitmore Canyon (fig. 31, pl. 2). Smaller berms along the inner gorge of Whitmore Canyon but below the elevation of Bull Flat also are covered with alluvial materials. At the point where the unimproved road to West Ridge crosses the eastern edge of Bull Flat (pl. 2), the alluvium consists of about 1 ft (0.3 m) of dark-brown soil, rich in organic material, which is covered by as much as 20 ft (6 In) of reddish-brown silty alluvium containing some sand and many rounded cobbles, gravels, and boulders mostly derived from sandstones of the Colton Formation. Some of the boulders are as much as 15 ft (4.5 m) in diameter. The central part of Bull Flat is a broad shallow depression that apparently is an old southward-trending stream course partly reexcavated, and so the modern drainage is northward and into Whitmore Canyon (pl. 2). Some of the landforms along this depression apparently are meander scars (Harold Brodsky, US. Geological Sur- vey, oral commun., 1959). Some of the alluvial deposits may be of the same age as those along the face of the Book Cliffs (pl. 2), but they are clearly younger than the alluvium at the top of West Ridge. The alluvium seemingly is overlapped at its west edge by light- reddish-brown slope mantle and colluvium of probable early Wisconsin(?) age which contains flat subangular plates of sandstone as large as small boulders. (See “Upland slope mantle”) The alluvium of Bull Flat probably is of pre-Wisconsin age. PEDIMENT GRAVELS Three series of pediment gravels formed at the base of the Book Cliffs between pre-Wisconsin(?), and H010- cene(?) time in the Sunnyside district. Gravels, which contain pebble- to boulder-size fragments in a clay- to sand-size matrix and become coarser near the cliffs, cap the various pediment surfaces. The three series represent different ages and generally can be distin- guished by their topographic position, degree of ero- sion, cementation, lithologic characteristics, and de- gree of weathering of included fragments. These three series of gravels in the district may be equivalent to the three or more units mapped by Fisher (1936, p. 6, pl. 9) at the base of the Book Cliffs in eastern Utah and western Colorado. Oldest pediment gravel—Remnants of the oldest pediment gravel (on on pl. 1) are distributed along the base of the Book Cliffs throughout the district at elevations between 6,000 and 7,000 ft (1,830 and 2,135 In). These gently sloping, nearly planar remnants, which cap pediments cut in Mancos Shale, are large and abundant in the northern part of the district, where they are one of the most striking features of the landscape. They stand as isolated buttes and mesas as much as 300 ft (91 m) above surrounding lowlands, or as extensive high-level sloping plains as much as 6 miles (10 km) long and 3 miles (5 km) wide. Upper sur- faces of the remnants appear to have a uniform dip away from the Book Cliffs, but the dip actually de- creases gradually away from the mountains and there- fore longitudinal sections of the surfaces approximate logarithmic curves. The dip, however, is not always di- rectly away from the present face of the Book Cliffs; some remnants near the canyon of Bear Creek, about 5 miles (8 km) northwest of Dragerton, dip nearly south at a place where the front of the Book Cliffs trends northwest. The upper surface of a large remnant 4.5 miles (7 km) southwest of Dragerton (pl. 1) dips gently eastward toward the Book Cliffs, although a nearby surface from which the remnant was separated by ero- sion dips westward. These divergent surfaces are not related to mouths of major canyons, where streams de- bouched laterally at different times, indicating pri- mary divergence of surfaces, as is commonly found in southern Utah (Fred Peterson, written commun., 1973). The oldest pediment gravel (pl. 1) consists of sub- rounded to subangular rock fragments set in a matrix of pale-reddish-brown sand and silt. The material is crudely bedded (fig. 21), firmly cemented with calcium carbonate, and is more resistant to erosion than the r’i PLEISTOCENE SERIES 27 FIGURE 21.—Crudely bedded boulders, cobbles, sand, and silt in = oldest pediment gravel. View northeastward one-quarter mile (0.4 FIGURE 22.—Remnants of middle pediment gravel (me) about 400 km) south of Dragerton along Utah Highway 130. ft (120 m) below remnants of oldest pediment gravel (on). Fore- ground is alluvial-fan material of Late Wisconsin(?) age (Qf). Mancos Shale which it overlies. The pebbles, cobbles, Northwest view from Utah Highway 124. about 3 miles (5 km) by and boulders consist of pieces of Flagstaff Limestone, road north °f Geneva Mine' sandstones of the Castlegate Sandstone, Colton For- mation, and the Green River Formation, and scarce geomorphic sequence of the region makes a pre-Wis- fragments of reddish hardened shale of the Blackhawk consin age most likely. Formation which were derived from burning of coal Middle pediment gravel—Remnants of the middle beds in the Book Cliffs. Many boulders are stained pediment gravel (0pm on pl. 1) in the Sunnyside dis- white with caliche, particularly in the upper part of the trict stand at levels of 4,7 00-6,200 ft (1,433—1,890 m), unit. The sand and silt probably were derived mostly about 400 ft (120 m) below nearby remnants of the from various units of the Blackhawk and Price River oldest pediment gravel (fig, 22), Remnants of the Formations. The unit was deposited on an undulating middle pediment gravel are smaller and more widely surface of Mancos Shale and is as much as 40 ft (12 m) distributed than remnants of the oldest pediment thick in some stream channels. North of Sunnyside. gravel but do not form as prominent features of the some pediment gravels extending eastward into landscape as do remnants of the oldest unit. Crudely canyons in the Book Cliffs are coextensive with early bedded materials within the unit consist of rounded to alluvial fills in the canyon bottoms (fig. 41). subangular pebbles, cobbles, and boulders of most of The exact age and correlation of the oldest pediment the resistant stratigraphic units above the Mancos gravel unit are unknown. The unit underlies alluvial Shale in the Book Cliffs, in a sandy and silty matrix. sand and silt that may be of late Wisconsin age. Rich- Fragments of rocks of the Mesaverde Group are, how- mond (1962, p. 10) described similar units in the La Sal ever, more abundant here than in the oldest pediment Mountains (fig. 1) which he thought were of early and gravel. The middle pediment gravel is tan, is cemented middle Pleistocene age. High-level pediments and the with calcium carbonate, and characteristically con- fanglomerates on them in the Henry Mountains and tains a foot or two (less than 1 m) of white caliche over- along the foot of the Book Cliffs were stated by C. B. lain by a thin soil at the top of the unit (fig. 23). This Hunt (1956, p. 38) to “appear to be of pre-Wisconsin unit is less than 30 ft (9 m) thick and was deposited on age.” Soils on some of the pediments near the Book a slightly irregular channeled surface cut in Mancos Cliffs also were thought by C. B. Hunt (1956, p. 72) to Shale. be of pre-Wisconsin age. Flint and Denny (1958, p. 156) We found no direct evidence of the age and correla- thought that the two earliest gravels of a similar series tion of the middle pediment gravel, except that rem- of pediments around the Aquarius Plateau, about 100 nants projecting through alluvial-fan sand and silt miles (160 km) southwest of Sunnyside, were of pre- near the mouth of Horse Canyon suggest that it may Sangamon age. We found no direct evidence for the age be older than late Wisconsin in age. The middle gravel of the oldest pediment gravel, but its position in the may be of early or middle Pleistocene age. Flint and —fi— 28 SUNNYSIDE COAL-MINING DISTRICT, UTAH FIGURE 23.—Middle pediment gravel showing thin caliche layer (white) overlain by thin soil. View northeastward along Utah Highway 124, about 2.5 miles (4 km) north of Geneva Mine. Whit- more Canyon is notch in cliffs (arrow) in left middle distance. Denny (1958, p. 156) thought that similar pediment gravels in a series nearthe Aquarius Plateau were of pre-Sangamon age. The middle pediment gravel is off- set by faults of little separation in the southern part of the district (pl. 1). Youngest pediment gravel—Remnants of the third or youngest pediment gravel (pr on pl. 1) are most abundant in the southern part of the district, where they stand about 50 ft (15 m) below adjacent remnants of the middle pediment gravel, at elevations between 4,800 and 5,300 ft (1,463 and 1,615 m). Youngest gravel remnants cap rounded but generally flat-topped hills and are made up mostly of weathered sandstone boulders, pebbles, and cobbles partly surrounded by uncemented sand and silt. Most of the clasts were de- rived from the Blackhawk and Price River Formations, but some fragments of Flagstaff Limestone and sand- stones of the Colton and Green River Formations are present. No evidence for the age of the youngest pediment- gravel was found. Remnants of the unit in the lowlands south of The Cove (pl. 1) may be younger than late Wis- consin age, because The Cove was formed by erosion of a large remnant of the oldest pediment gravel and of alluvial-fan sand of late Wisconsin(?) age deposited upon it. The Cove, however, may have been partly ex- cavated before the alluvial-fan sand was deposited at the mouth of Horse Canyon, because erosion dissected the oldest pediment surface at Whitmore Canyon be- fore deposition of other similar alluvial fans. SEDIMENTS OF EARLY WISCONSIN(?) AGE Two types of sediments of early Wisconsin(?) age are present in the Sunnyside district. Both of these units cover the lower slopes of canyon walls and therefore are probably younger than the last phase of canyon cutting. The older of these deposits is present only as scattered remnants of cemented conglomerate on canyon walls; the younger consists of upland slope mantle and associated colluvium and landslide debris and is widely distributed throughout the district at ele- vations of 7,000 ft (2,135 m) and above. CHM ENTEI) CONGLOMERATE Small remnants of tightly cemented conglomerate (009 on pl. 2), scattered throughout the northern part of the district, contain fragments of nearly all strati- graphic units above the Mancos Shale. These frag- ments are subangular to well rounded, are pebble to boulder size, and are cemented firmly in a silty and sandy matrix (fig. 24). Most of the cement is calcite, but one remnant on the north wall of Whitmore Canyon near its mouth is cemented with silica and con- tains a few thin veinlets of silica. Some remnants con-_ tain so much calcite that they resemble tufa. Most remnants are only a few hundred feet (less than 150 In) above the present valley floors and probably were formed since the most active phase of canyon cutting. Somewhat similar deposits containing interlayered spring deposits are found along canyon walls in south- FIGURE 24.—Cemented conglomerate of early Wisconsin(?) age near the mouth of Whitmore Canyon, several hundred feet (about 155 m) above the present valley floor. Randomly oriented sand- stone blocks of Blackhawk Formation are cemented by calcareous material that resembles tufa. Scale indicated by notebook in upper left of photograph. PLEISTOCENE SERIES 29 central Utah (C. B. Hunt, 1956, p. 38). These deposits probably are of early Wisconsin age; they formed after the major phases of canyon cutting when ground-water levels were high during a time of maximum rainfall (C. B. Hunt, 1956, p. 38). The cemented conglomerate remnants in the Sunnyside district probably are of similar age and origin because of their topographic position and because they bear no possible relationship to presently active springs. UPLAND SLOPE MANTLE One of the most widespread Quaternary units in the district is a thick cover of colluvial material composed of dark-gray to light-reddish-brown clay, silt, and sand containing large amounts of black organic material (08 in fig. 31, pl. 2) that forms a widespread mantle of col- luvium on slopes above 7,200 ft (2,195 m) in elevation, particularly in upland areas on north-facing slopes. This unit also contains variable amounts of subangular to angular rock fragments as large as several feet (less than 2 m) in diameter, derived locally from underlying bedrock. Some of the sand facies are crudely layered, having small flat rock fragments parallel to the layer- ing. Much of the clay is somewhat plastic, and organic- rich parts, particularly those formed from shale or mudstone, are compressible. Some of the clays are highly expansive when wetted. The mantle forms con- vex slopes through which only a few prominent bed- rock units project. Smaller areas of similar mantle are scattered along the face of the Book Cliffs, particularly on slopes facing north or northwest. Some masses ex- tend downward into the lower gorge of Whitmore Canyon and its tributaries (pl. 2; fig. 25) and hence FIGURE 25.——-Convex deposits of upland slope mantle (OS) of early Wisconsin(?) age extending into the lower part of Bear Canyon, about 3.4 miles (5.5 km) northeast of Sunnyside. Mantle is over- lapped by alluvium of late Wisconsin(?) age (0213) in valley floor. Modern drainage (arrows) is entrenched into alluvium about 8 ft (2.4 In). were formed since the last major episode of canyon cut- ting. Locally, some of the mantle masses were dis- turbed by landsliding in Pleistocene time. In places, large areas of the lower east-facing slopes of Whitmore and other large canyons are covered by landslide debris that closely resembles slope mantle; locally, the debris merges imperceptibly into mantle. These land- slides apparently moved at about the same time as the mantle was being formed, and they are mapped with the mantle as one unit. Some of the slides are active currently, and most masses of mantle are easily made unstable by excavation for roads and other structures (fig. 26). The exact age of the slope mantle cannot be deter- mined. No exact stratigraphic correlation can be made, but, because of its topographic distribution and the ex- ternal form and internal characteristics of individual mantled slopes, the unit resembles the solifluction mantle facies of the Placer Creek Formation of early Wisconsin age described by Richmond (1962, p. 48, 87) in the La Sal Mountains. Because of these similarities, the slope mantle in the Sunnyside district also may have formed from solifluction debris of early Wiscon- sin(?) age. The upland slope mantle and colluvium apparently are older than the alluvial fill in valley floors, because wherever streams cut into mantle slopes, dark-gray clay and silt are exposed in cut banks below the level of reddish-brown alluvium. SEDIMENTS OF LATE WISCONSIN(?) AGE GRAVEL ALONG CANYON WALLS Small remnants of the oldest alluvial gravel (09 on pls. 1, 2) are found 100 ft (30 m) or more above modern stream courses along the lower slopes of large canyons and tributaries, particularly along Whitmore Canyon (figs. 27, 28) and Horse Canyon (pl. 1) and also along Neversweat Wash in the San Rafael Swell (pl. 2). Some of these remnants retain terracelike flat upper surfaces that were dissected before most of the alluvial valley fill was deposited. Modern stream courses and tribu- tary gulches also are incised into the gravel remnants, and deposits of younger talus overlap some of them. A gravel-capped terrace of late Wisconsin(?) age near the portal of Sunnyside No. 1 Mine (pl. 2) was deeply eroded. Materials within the remnants consist of about 80 percent subrounded to rounded pebbles, cobbles, and small boulders as much as 2 ft (0.6 m) in diameter in a 20-percent matrix of silt, sand, and clay. All of the fragments appear to be derived from sedimentary units of Cretaceous and Tertiary age that crop out in the present slopes and ridges upstream from the remnants. 30 SUNNYSIDE COAL-MIN IN G DISTRICT, UTAH FIGURE 26,—Landslide debris, probably derived from upland slope mantle of early Wisconsin(?) age, on east-facing slope of Whitmore Canyon at the dam for the Sunnyside domestic water-supply reservoir (pl. 1). Colton Formation at top right of photograph dips about 4° toward observer; dam extends across the canyon, away from the viewer, in lower center foreground of the photograph. Excavation for dam abutment and roadway has reactivated landslide. Dark areas in slide are water seeps. Automobile shows scale. We know of no direct evidence of the age of the alluvial gravels along Whitmore and Horse Canyons, and Neversweat Wash. They clearly are younger than the last phase of canyon cutting but are older than ex-_ tensive alluvial valley sand and silt, because some, par- ticularly near the mouth of Whitmore Canyon (figs. 27, 28), are partly overlapped by alluvium of probable late Wisconsin age. The thickness, elevation above stream courses, and general composition of the alluvial gravel remnants are similar to those of Richmond’s alluvial gravel facies of the Placer Creek Formation (Rich- mond, 1962, p. 46—47), which is of early Wisconsin age (Richmond, 1962, p. 87), although the gravels in the Sunnyside district are slightly thicker and higher above the stream courses than those in the La Sal Mountains. SAND AND SILT Most of the larger valleys in the Sunnyside district are floored by alluvial sand and silt (Qa3 on pl. 2) or by the lower part of the alluvium (Qa on pl. 1) that is as much as 50 ft (15 m) thick in Little Park Wash (pl. 1) and at least 20 ft (6 m) thick in Whitmore Canyon (pl. 2). Some of the alluvial fills probably are much more than 50 ft thick, but their bases are concealed. The upper part of the alluvial sand and silt is light brown and contains dark organic-rich layers and layers rich in limonite (fig. 29). Locally, lenses of crudely bedded gravel and cobbles occur in the sand and silt (fig. 30); many of these lenses in Whitmore Canyon are about 5 ft (1.5 m) below the flat upper surface of the alluvium, but we saw no evidence of an unconformity or of an erosion surface at the level of these gravels. PLEISTOCENE SERIES 31 FIGURE 27,—Large mass of alluvial gravel of late Wisconsin(?) age (beneath water tanks) along south side of Whitmore Canyon. Gravel overlaps Kenilworth and Sunnyside Members of Blackhawk Formation, and is overlapped by younger alluvium of late Wisconsin(?) age along the canyon floor. Structures in left center of picture are the machine shop (left), backfill plant, and tipple (right) for the Sunnyside coal mines. Cliffs behind structures are the Castlegate Sandstone. Grassy Trail Creek is incised several feet (less than 2 m) into the younger alluvium to left of and in front of railroad buildings in center of picture. Boulders in left lower corner are remnant of alluvium of late Wisconsin(‘?) age. Long structure having many arches is railroad tunnel under construction (1968) for use in continuous loading of coal trains. Tunnel, which was built partly on artificial fill an struction because the fill was only loosely compacted. View is nor The sand and silt clearly is younger than the cutting of the inner gorge of Whitmore Canyon and seemingly overlaps the lower parts of some solifluction mantle masses in the upper reaches of Whitmore Canyon and its branches (fig. 31). Many tributary valleys have de- posited small alluvial fans upon the surface of the sand and silt (figs. 29, 31) since its deposition. Small talus cones along canyon walls also overlap the alluvial sand and silt. Modern streams flowing in the alluviated valleys are entrenched as much as 15 ft (4.5 m), and most of them meander extensively. Examples of such entrenched streams are Grassy Trail Creek in Whit— more Canyon (fig. 32), Range Creek (fig. 33), and Little Park Wash (fig. 17). Locally, modern streams also have formed small flood plains and terraces below the sur- d partly on alluvial gravel, was unstable for a long time after its con- theastward across Whitmore Canyon near its mouth. face of the sand and silt. All available evidence indi- cates that the sand and silt was deposited by streams having greater flow than the modern streams. We know nothing directly of the age of the alluvial sand and silt in the Sunnyside district; no fossils or other materials were found from which an age deter- mination could be made. Most of the critical relation- ships at canyon mouths are strongly disturbed by sur— face operations of coal mines, by residential develop- ments, and by farming. Sand and silt from alluvial valley fills apparently accumulated on the upstream parts of extensive pediment surfaces after erosion had cut into the surfaces as much as 50 ft (15 m). Alluvium was deposited in a stream channel cut into the oldest pediment surface along Grassy Trail Creek in the town 32 FIGURE 28.—Small remnants of gravel (09) of late Wisconsin(?) age overlap Mancos Shale and are overlain by talus and alluvial-fan debris (Qnt) of Holocene age. Westward view of the north side of Whitmore Canyon, near its mouth. Grassy Trail Creek flows in small gully in lower left foreground. FIGURE 29,—Upper part of alluvium of late Wisconsin(?) age, ex- posed in bank of Grassy Trail Creek, showing stratification of silty and clayey alluvium, with soil in the top 2 ft (0.6 m). Flat area beside stream is veneer of Holocene alluvium. Westward view in Whitmore Canyon, about 2 miles (3 km) above the Sunny- side No. 1 Mine. of Dragerton (fig. 34). The older parts of the alluvial sand and silt, however, may have been disturbed dur- ing the cutting of some of the youngest pediments. SUN N YSIDE COAL-MINING DISTRICT, UTAH FIGURE 30.—Lens of subangular to subrounded pebbles and cobbles and a few boulders in alluvium of late Wisconsin(?) age in Bear Canyon. Gravel is crudely bedded and cross-stratified. Cross-strata dip to right which is upstream. Bank is about 8 ft (2.5 m) high. The alluvial sand and silt in the Sunnyside district is probably of late Wisconsin(?) age. Its composition, tex- ture, distribution, and relationships to topography closely resemble similar facies of the Beaver Basin For- mation of Pleistocene and Holocene age in the La Sal Mountains as described by Richmond (1962, p. 60—61, 87). The smooth upper surfaces, uniform composition, and topographic setting of the alluvial sand and silt also are similar to those of the alluvium of late Wis- consin age in the Colorado Plateau as summarized by C. B. Hunt (1956, p. 38—39). Al.l.UVlAlxl“/\N l)lil’()SI'l'S Large older alluvial fan deposits dominantly com- posed of sand and silt, but having small local lenses of gravel, extend outward from the mouths of Horse Canyon and Whitmore Canyon (Qfo on pl. 1). Pebbles, cobbles, and small boulders are more abundant within 0.5 mile (0.8 km) of the canyon mouths and in the cen- tral part of the fans than elsewhere. Smaller, less dis- tinct fans containing relatively more gravel and less sand than the large fans extend outward from a canyon near the town of Columbia, Utah, and from Water Canyon. The fan near Columbia (pl. 1) originates in a valley about 150 ft (45 In) below the large erosional remnants of the oldest pediment surface in the district. Areas underlain by alluvial-fan sand are more perme- able and hence bear a much more lush flora, consisting of tall grass, pinon-pine, and many types of Wild PLEISTOCENE SERIES 33 FIGURE 31.—Alluvium, Qa3, overlapping upland slope mantle (convex slope, 05, left center of photograph). View southwest in Bear Photograph also shows an alluvial fan Canyon, a branch of Whitmore Canyon. (Qf) deposited on the alluvium from a tributary watercourse to Bear Canyon. Bull Flat (arrow) is covered with pre-Wisconsin alluvium. Top of West Ridge (skyline, right side of photo- graph) is another alluviated surface, older than Bull Flat. flowers, than do nearby areas underlain by Mancos Shale or by alluvium derived from the Mancos and therefore are valuable grazing areas for livestock. They also are much more easily traversed by vehicles when wet than are adjacent areas underlain by Mancos Shale, which become almost impassable when wet. The alluvial fan extending outward from the mouth of Horse Canyon is the largest in the district; remnants overlap upturned sedimentary rocks of the San Rafael Swell near Grassy Trail Creek. 8 miles (13 km) south- west of the canyon mouth. The fan is as much as 6 miles (10 km) wide. Its surface slopes to the west about 200 ft (60 m) per mile, but is markedly convex, as shown by topographic contours (fig. 35). Small inter- mittent water courses as much as 10 ft (3 m) deep flow down the fan; the creek flowing in Horse Canyon is perennial, but the flow disappears within the alluvial sands. The fan clearly is younger than the oldest and middle pediment gravels. One large remnant of the oldest pediment gravel stands about 250 ft (75 m) above the fan surface near the mouth of Horse Canyon. Mancos Shale crops out beneath the gravel and boul- ders capping the pediment and above the fan surface (pl. 1). Smaller remnants of the middle pediment gravel extend through the alluvial-fan sand in the northern part of the fan, apparently along old drainage divides. Streams have deeply eroded the southern edge of the fan and the Mancos Shale beneath it, forming a very prominent steep scarp known as The Cove (fig. 36), which is as much as 400 ft (120 m) high. The fan’s west- ern edge is being cut into isolated remnants by Grassy Trail Creek and its tributaries. The western or down- slope end of the fan filled an older broad valley into which the present valley of Grassy Trail Creek was cut. The older alluvial-fan deposits at the mouths of Whit- more and Horse Canyons, therefore, are older than the valley of Grassy Trail Creek and its alluvial fill of late Wisconsin(?) age. FIGURE 32.—Northward View of the inner gorge of Whitmore Canyon, from a point just north of the Sunnyside No. 1 Mine portal. Nearly flat valley surface is floored by alluvial sand and silt of late Wisconsin(?) age that is entrenched about 8 ft (2.5 m) by Grassy Trail Creek. East (right) side of canyon has large, buttresslike headland between Pole and Bear Canyons. Broad, flat bottom of older U-shaped valley is outlined by upper surface of headlands in upper right, ahd by edge of Bull Flat, in upper left corner of photograph. Kc, Castlegate Sandstone; Kpb, Bluecastle Sandstone Member of Price iver Formation; Tan, North Horn and Flagstaff Formation; Tc, Colton Formation. FIGURE 33,—Entrenched channel of Range Creek, 0.5 mile (0.8 km) upstream from mouth of Sheep Canyon. Bank in left middle ground is stratified alluvium of late Wisconsin(?) age. Bank at ex- treme right rear is west margin of entrenched channel. Flat area beside stream is modern alluvium. The younger alluvial-fan deposits extending west- ward from the mouth of Whitmore Canyon were de- posited in a shallow stream valley cut into a large rem- nant of one of the oldest pediment gravels in the dis- trict (fig. 37) and extend about 4.5 miles (7 km) west- SUN N YSI DE COAL-M IN IN G DISTRICT, UTAH FIGURE 34.—View west along Grassy Trail Creek in Dragerton, Utah. Smooth surface of alluvial sand and silt of late Wisconsin(?) age is farmed; houses are on adjacent pre-Wisconsin(?) pediment remnants. ward down the slope of the pediment from the canyon mouth. The fan is about 3 miles (5 km) wide; the north- ern margin is indistinct and merges with various seg- ments of the oldest pediment gravel. This fan contains much more coarse material, at least in its upstream part, than does the one in Horse Canyon. The western end of the fan is a featheredge that merges nearly im- perceptibly with the oldest pediment gravel. The southern margin of the fan is being eroded rapidly by Holocene streams (Icelander Creek and its tributary drainages) along a steep scarp about 500 ft (150 In) high cut into underlying Mancos Shale (fig. 38). The scarp, however, is not entirely of Holocene age, be- cause the smaller compound alluvial fan at the mouths of Water Canyon and another canyon near the town of Columbia, which presumably is of about the same or slightly older geologic age as the one in Whitmore Canyon, overlaps the lower slopes of Mancos Shale at the base of the scarp (pl. 1). The upstream section of the fan west of Whitmore Canyon, near the base of the Book Cliffs, and part of the northern margin are covered partly by coalescing younger alluvial fans formed near the mouths of small canyons cut by streams flowing down the face of the cliffs. The upper surface of the alluvial fan at the mouth of Whitmore Canyon is considerably more altered by ero- sion than is the surface of the fan at the mouth of Horse Canyon, probably because Grassy Trail Creek and its tributaries have a much larger perennial flow than does the creek in Horse Canyon, which has only a trickle of water in dry seasons. Grassy Trail Creek has cut a narrow gulch about 5—10 ft (1.5—3 m) deep into the fan at the mouth of Whitmore Canyon near the town of Sunnyside, but about 1.5 miles (2.4 km) west, at Dragerton, this creek has cut a valley about 1,000 ft 35 PLEISTOCENE SERIES T. a :25 , .. . _ t l , , . v imam/t. 4, is . a, a x v V. , H .. .A. x? / NU/Lfl‘f)‘ ‘ , < , id x? , , a? 1 10°22’30” 4 K|LOMETERS 25' ,thwmgwfil , . 1:62 S. Geological Survey 500 Woodside 1949 Base from U ?) age cap hills in( lSCOl’lS e modified from US. Geo- ) age in Horse Canyon. Bas f the oldest pediment gravel (on) of pre—W in by remnants of Cretaceous (K). 9 ( lsconsm cutting steep-sided gulches as much as 15 ft (less than 5 In) deep into the fan. eW' Remnants o Smaller , 1948. ,500 to the fan h and east of Dragerton are :62 of the alluvial fan (Qf) of Lat m) deep in Quadrangle, 1 is underla and ic map of part ide FIGURE 35.——Topograph logical Survey Woods above the fan surface, (300 m) wide and 20 ft (6 intermittent streams nort 36 SUN NYSIDE COAL-MININ G DISTRICT, UTAH FIGURE 38.—Steep scarp eroded in Mancos Shale (Km) near the mouth of Whitmore Canyon by Icelander Creek and its tribu- taries. Oldest pediment gravel of pre-Wisconsin(?) age (on) forms skyline. Alluvial-fan debris of late Wisconsin(?) age (Qf) from Water Canyon and other canyons covers slopes in fore- ground. Icelander Creek issues from Whitmore Springs (informal name) below the pediment gravel in the right center of the photo- graph. View northward across Utah Highway 123 and Carbon County Railway track. FIGURE 36.—Westward view of The Cove (fig. 35, pl. 1) from a point above the Book Cliffs Mine. The Cove is an erosional depression having a steep scarp carved in Mancos Shale at the north edge. early pediment surface and may be contemporaneous Pre-Wisconsin pediment gravel (on) forms the rim above the With the fan at the mouth of Whitmore Canyon. Mancos Shale (Km) amni‘d m°.St Of The Cove. Alluwal'fan sand’ Age and correlation of the alluvial-fan deposits are Silt, and gravel of late Wisconsm(?) age (Qfo) surround a remnant _ , , , of pediment material and extend to the eastern edge of The Cove. poorly known' D IStrlbutlon and eXt’ent 0f the alluvml' fan debris in the Sunnyside district, however, closely resemble Richmond’s descriptions (1962, p. 46—47) of alluvial-fan gravel in the La Sal Mountains that he as- signed to the Placer Creek Formation of early Wis- consin age. Because of its position with relation to other Quaternary units, however, we think that the younger alluvial-fan deposit is of late Wisconsin age. The alluvial-fan materials presumably are at least slightly older than alluvial sands and silts along canyon floors, as described below; if they were younger than the sands and silts, much of the material along the canyon floors would have been removed by the in- creased flow of water necessary to deposit the alluvial- fan debris. The alluvial sand and silt may have been de- posited during the decline of streamflow following deposition of alluvial-fan sand, silt, and gravel; if so, FIGURE 37,—Shallow, broad valley in oldest pediment gravel of materials in the fans may be correlative With alluVial' pre-Wisconsin(?) age (on) filled with alluvial-fan material of late fan gravel facies Of the Beaver Basin Formation Wisconsinm age (qu). Mancos Shale (Km) cr0ps out below pedi- (Pleistocene and Holocene) in the La Sal Mountains (as ment gravel. View east along Denver and Rio Grande Western described by Richmond, 1962, p. 47). The fans once Railroad, about 4 miles (7 km) west of Dragerton. Patmos Moun- 0 t d thr' i hi tor'c h m n l t re S de- tain on right skyline, Whitmore Canyon mouth in right center. supp r e a_ _ 1V 11g pre S 1 u a cu u ’ a scribed earlier in this report. North of Dragerton, alluvium at the mouths of TERRACEGRAVEL several canyons forms a compound fan that merges Flat-topped dissected remnants of stream-terrace with the one at the mouth of Whitmore Canyon to the gravel and of gravel-capped benches line the valley of south. The compound fan was deposited on part of an the Price River and its tributaries in the southern part HOLOCENE SERIES 37 of the district (Qt on pl. 1) for many miles. Pebbles, cobbles, and boulders in these terraces are subrounded to rounded, and most were not locally derived. The ter- race gravels are as much as 20 ft (6 m) thick and are at elevations about 150 ft (45 m) above the present stream level. The age of these terrace gravels is not known, but presumably they are correlative with simi- lar gravels, along the Green River that were described by Hansen (1965, p. 174). Associated with terrace gravels west of the Price River, about 1.5 miles (2.5 km) upstream from Silvagni Ranch (pl. 1) are some well-defined meander scars, also about 150 ft (50 m) above the stream. ALLUVIUM OF LATEO) WISCONSIN AGE Modern stream courses are bordered by considerable amounts of alluvium, which comprises mostly locally derived silt and clay, lesser amounts of sand, and little gravel (Qa on pls. 1, 2). Upper surfaces of this alluvium generally are smooth. Silty clay, derived mostly from Mancos Shale, forms broad flats in the southern part of the district (pl. 1). Such flats are particularly abun- dant near the Price River where they are as much as 1 mile (1.5 km) wide (fig. 39), but they contain much greater amounts of sand, derived mostly from sand- stones of Cretaceous and Tertiary age many miles up- stream. Subrounded to rounded pebbles, cobbles, and boulders make up poorly defined, discontinuous beds in the alluvium along the Price River and also are abundant in other drainages within a few miles of the Book Cliffs. The alluvium is deeply gullied by modern FIGURE 39.—Thick silty and clayey alluvium (Qa) of late(?) Wis- consin age, derived mostly from Mancos Shale along Price River. Cliffs on skyline are Blackhawk Formation; slopes of Mancos Shale (Km) are below cliffs and above oldest pediment gravel (me) of pre-Wisconsin(?) age. View southeast across Price River, about 2 miles (3 km) east of Woodside. watercourses—impassable steep-walled trenches make travel across some alluvial flats extremely difficult. As much as 15 ft (4.5 m) is exposed in gullies, but the base is not exposed and the maximum thickness could not be determined. The alluvium generally resembles Wisconsin alluvium in the Colorado Plateau, as summarized by C. B. Hunt (1956, p. 39). Although we found no direct evidence of the age of the alluvium in the Sunnyside district, it may be of late(?) Wisconsin age. Most of the alluvial flats near Price River support little vegetation. Attempts were made in the past, ap- parently by early settlers, to farm some of the flats along Price River and Marsh Flat Wash near Woodside (fig. 40, pl. 1). The settlements and adjacent fields were abandoned long ago, probably because of barren soils and lowered water tables in the alluvium as a result of gullying. HOLOCENE SERIES TALUS AND ALLUVIAL-FAN DEPOSITS Small natural talus cones and alluvial fan deposits (Qnt on pls. 1, 2) accumulated at the mouths of small drainages tributary to major canyons (pl. 1) and also along the front of the Book Cliffs. Material in these cones and deposits is locally derived and ranges from clay and silt to large boulders. Coarser pieces range from subrounded to angular, and most are only slightly weathered. Lower parts of many cones and de- posits consist mostly of boulders. Many overlap alluvial sand and silt in canyon floors (fig. 41). Ac- cumulations of gravel and boulders along the bases of FIGURE 40.—Ruins of an early settler’s home near Marsh Flat Wash, about 4 miles (6.5 km) northwest of Woodside. Living space was made by roofing over a rectangular excavation in silty and clayey alluvium of late(?) Wisconsin age derived from Mancos Shale. View northeast, toward Book Cliffs. 38 SUNNYSIDE COAL-MINING DISTRICT, UTAH FIGURE 41.—Holocene(?) talus (Qnt) overlapping alluvium of late(?) Wisconsin age of valley floor. Ledge (Kpb)'at side of road in left center of picture is Bluecastle Sandstone Member of Price River Formation. Prominent cliff (Tcu) on skyline is upper part of Colton Formation. N orthward view of east side of Whitmore Canyon, near mouth of Pole Canyon. steep slopes below pediment-surface remnants in Clark Valley are derived entirely from material weathered from gravel and boulder pediment caps (pl. 1). These accumulations probably are equivalent in age to the talus and alluvial fans along the cliff front and in the canyons. Material is still being added to most of the de- posits as debris rolls down or is washed from above during storms. The deposits are of Holocene(?) age and may be equivalent to similar facies of the Gold Basin Formation in the La Sal Mountains (Richmond, 1962, p. 78—80). ALLUVIUM OF HOLOCENE(?) AGE Deposits of young alluvium (Qa on pls. 1, 2) in the Sunnyside district are abundant along modern stream courses in the Clark Valley, particularly north and west of Woodside (fig. 1). The alluvium, consisting of pebbles, cobbles, and boulders mixed with silt, sand, and clay, is strewn along flats bordering the streams but is entrenched as much as 10 ft (3 m) by modern watercourses. The upper surfaces of these deposits are rough and undulating and have as much as 5 ft (1.5 m) of relief. The exact age of the alluvium is unknown, but it is younger than the time at which the modern drain- age system was established and is Holocene(?). Some of the alluvial deposits appear to be rudimentary pedi- ment gravels forming during the present erosional cycle. MAN-INDUC ED TALUS Mining activities are causing accumulations of talus at several places along the Book Cliffs in the northern part of the district. These accumulations (th on pl. 2) consist of angular boulders, many of huge size, derived from sandstones of the Blackhawk an’d Castlegate. Most of the debris falls from cliffs on the sides of large headlands, above areas where coal is actively mined, probably as a result of subsidence into the mined-out voids and of the shaking due to numerous earth tremors (Barnes and others, 1969; Osterwald, 1961). An example of such talus accumulation can be easily seen along a cliff face 2.5 miles (4 km) north of Drager- ton (fig. 42). We traversed the slopes below this cliff several times in 1958 and 1959 during geologic map- ping (Osterwald, 1962a), at which time little or no debris had fallen. Mining began beneath the headland in 1959, and within 2 years the cliff became unsafe for travel due to unpredictable debris falls. Other large talus accumulations are found along the Book Cliffs between Columbia, Utah, and the Geneva Mine, and south of the Geneva Mine near Lila Canyon. MINE DUMPS Mine dumps consist of refuse from mining opera- tions, such as clinker and coke breeze from old mining and coking operations mixed with later debris consist- ing of bony coal, coaly shale, rock, and heavy minerals, as well as pieces of mine timber, fragments of metal, and miscellaneous trash. This refuse, which makes up as much as 30 percent of the current total output of the Sunnyside mines, is removed from the mined coal in the washer. These mine dumps have been used for as long as 70 years; the dumps in Water Canyon and Fan Canyon (fig. 43A, B) now are abandoned and inactive, but the one near Sunnyside at the head of Icelander Creek is enlarged daily, and also served as the town garbage dump for many years (fig. 43A). FIGURE 42.—Large area of man-induced talus (th) from cliffs of Castlegate Sandstone (KC) and ledges of sandstones of the Black- hawk Formation (Kb) 2.3 miles (3.7 km) north of Dragerton, Utah. Photograph taken in October 1968. HOLOCENE SERIES 39 FIGURE 43.—-Mine waste dumps near Sunnyside. A, Present dump of Sunnyside mines showing steep face of dump filling valley above Carbon County Railway near the head of Icelander Creek. B, Abandoned dump (arrow) of Sunnyside No. 2 and No. 3 Mines along the north side of Water Canyon. The Water Canyon and Fan Canyon dumps were con- structed by dumping mining debris along one side of the canyons and smoothing the upper surface with heavy equipment (fig. 433). Both dumps are limited in their lateral extent by the narrow canyon walls, and as a result are long narrow masses having extremely steep sides. The Sunnyside dump is constructed by dumping mining debris into a gully between two pediment rem- nants until the dump reaches the level of the pedi- ments. The dump is then built laterally and vertically along the pediment surface and over the front of the pediment. This dump is not physically limited in lateral extent, and so forms a high, long, lobate struc- ture having extremely steep sides. The upper surface is planed regularly by heavy equipment to allow for the addition of more debris. Three trucks of approximately 10-ton (9,000-kg) capacity constantly haul debris from the washer to the dump while the mines are in opera- tion. The Sunnyside and Water Canyon dumps were afire in 1971. These fires probably started by spontaneous combustion of volatile and flammable material in the dumps. Of the two, the Sunnyside dump had the more active fires. Smoke and occasional flames emanated from the dump during the day, and many areas of flam- ing debris were seen at night. The almost daily influx of new combustible material added to the potential fire area and enabled the flames to continue. Fire is not so apparent at the abandoned Water Canyon dump; the oxidation of combustibles beneath the surface causes some smoke, but we have never seen flames at this dump. Several near-surface hot areas exist, however, and these are marked at the surface by warm, viscous, tarlike substances, by soft, puffy ground, and locally by small blister and spatter cones formed by partial melting of dump material. Such burning waste dumps elsewhere in the United States are known to be very hazardous, because they are prone to local but violent explosions when wet, and because the surface crust is thin, allowing persons to fall into the burning material (McNay, 1971, p. 12-13). The dumps contribute smoke, suspended solids, and odor to the atmosphere, and minerals to the streams and ground water. A fork of Icelander Creek runs along the west side of the Sunnyside dump, and the drainages of Fan and Water Canyons run along the south and southeast sides of those dumps. The gar- bage on the Sunnyside dump harbors rats and other rodents less than a mile (1.6 km) from town, and may contribute trash and pollutants to Icelander Creek. Al- though we made no measurements, we feel that the very steep sides of all dumps pose a landslide threat. The CarbonCounty Railway track is below the west end of the Sunnyside dump (pl. 2). We believe, how- ever, that the dumps are no great hazard to life and property because all are situated far from main roads and are topographically or geographically far below or away from towns. Because of the sparse vegetation near the dumps, we believe that no fire hazard exists from the dump fires. They are potential hazards to occasional visitors and to animals because they are burning beneath a thin but solid-appearing crust. The dumps in Water Canyon and Fan Canyon also may be hazardous because of potential slumping onto mine- access roads (fig. 433). Only the Sunnyside dump is readily visible; it may be seen from State Highway 123, south of Sunnyside, and from much of the town. The dumps contribute to environmental pollution through smoke and its attendant suspended solids, and through mineral pollution of Icelander Creek and 4O SUNNYSIDE COAL-MINING DISTRICT, UTAH the surface and underground drainages of Fan and Water Canyons. QUATERNARY HISTORY The Sunnyside district has a complicated Quater- nary history of interrelated events consisting of ero- sion and deposition, structural deformation, drainage changes, and probable climatic variations. Geomorphic development of the district and its surrounding re- gions was strongly influenced by a master drainage system along the Green River. Times of erosion and deposition were controlled by climatic changes (varia- tions in precipitation and glaciation), as well as by tec- tonic events. Tectonic events, by changing relative heights not only within the district but elsewhere within the surrounding regions, also may have influ- enced strongly the climate of the district. The sequence of Quaternary events summarized below enabled us to decipher the stress history of the coals in our study of coal-mine bumps. Particularly, the sequence has yielded information on timing and rate of unloading of stress due to removal of overburden and on the timing and nature of diastrophic stresses. The influence of the Quaternary history on stress changes in the coal beds is discussed elsewhere. The sequence of events leading to the present land- forms of the region surrounding the Sunnyside district actually began when the present drainage system of the Green River was established across the Tavaputs Plateau, presumably by late Pliocene time (Hansen, 1969, p. 63—65; C. B. Hunt, 1956, p. 84—85). Although C. B. Hunt (1956, p. 84) suggested that upper Tertiary rocks were deposited in the Clark Valley, we found none, and if they were deposited they were completely removed later by accelerated erosion resulting from uplift of the Colorado Plateau (C. B. Hunt, 1956, p. 85). Some poorly sorted beds of cemented conglomerates in Neversweat Wash (pl. 1) which underlie alluvial-fan material may be of late Tertiary age, but we think that the beds probably are of Pleistocene age, perhaps equivalent to the cemented conglomerates (early Wis- consin(?)) near Sunnyside. Isolated remnants of alluvial gravels containing bedded pebbles, cobbles, and boulders, most of which are rounded to well rounded, are found on several sur- faces on high, narrow divides between tributaries to Whitmore Canyon, as well as on the summit of West Ridge, on Patmos Mountain, and on Bull Flat. Three major levels of these surfaces are well defined along Whitmore Canyon, one slightly above the base of the Green River Formation, one in the Colton Formation which includes the surface of West Ridge, and one at about the level of the Flagstaff Limestone which in- cludes the alluviated surface of Bull Flat (fig. 44). FIGURE 44.—Southwestward View from Patmos Mountain, above Bear Canyon southeast of Bruin Point. showing remnants of alluviated erosion surfaces in Whitmore Canyon at top of West Ridge (1), on Bull Flat and on corresponding surfaces above east side of inner gorge (2), and at bottom of inner gorge (3). South end of West Ridge is in the right middle distance, San Rafael Swell in the background. QUATERNARY HISTORY 41 Other small alluviated remnants are found between these three, as well as below the Flagstaff Limestone surface. Nearly all of the clasts in these gravels are composed of sandstone from the Colton Formation. These clasts and the distribution of the surfaces imply that a strong stream flowing generally southward in a strike valley began cutting downward from the level of Bruin Point and Patmos Mountain at least as soon as early Pleistocene time, judging from the position of glacial deposits of possible early Pleistocene age. It may, however, have begun in late Pliocene time, when the Green River began flowing southward (C. B. Hunt, 1956, p. 82). Downcutting probably was interrupted several times, perhaps during times of continental glaciation. There is little evidence concerning the early Pleisto- cene history of the district except for the high-level alluvial materials. Deposits containing exotic boul- ders, high on West Ridge at an altitude of more than 8,500 ft (2,590 m) (pl. 2), possibly may indicate pre- Wisconsin glaciation, perhaps during Nebraskan time. The source of the hypothetical glacier is not known. Many pediments also formed along the base of the Book Cliffs in pre—Wisconsin time. (See the section on “Pediment gravels”) According to C. B. Hunt (1956, p. 38), the gravels capping these pediments may have been derived from glacial or periglacial deposits; if so, the pediments probably formed during or shortly after a major phase of pre-Wisconsin glaciation in an arid cli- mate. They are much younger than the glacial debris on West Ridge, however, because much erosion oc- curred after the glaciations and before the pediments were formed. The earliest pediments were dissected, probably when streamflow increased during a phase of glacial recession, but late pediments also may be of pre—Wisconsin age, suggesting a return to arid condi- tions because of a readvance of ice in the surrounding mountains. The later pediments were in turn dissected by increased streamflow, and the soils that developed on top of the pediments indicate a moist climate (C. B. Hunt, 1956, p. 72). Fragments of clinker in the oldest pediment gravel indicate that coal outcrops in the cliffs were burned before or during cutting of the pedi- ment. These clinkers may indicate that the Book Cliffs region was forested before the pediments formed, be- cause similar burnings of coal-bed outcrops at Grand Mesa, 0010., probably were started by forest fires (Lee, 1912, p. 216—217). Most prehistoric forest fires in a similar environment at Mesa Verde, 0010., were deter- mined to have resulted from lightning (Erdman and others, 1969, p. 17—19). This origin of the clinkers at Sunnyside seems more applicable than spontaneous combustion, because experiments in abandoned coke ovens by Kaiser Steel Corp. were unsuccessful in find- ing any means of generating spontaneous combustion (J. T. Taylor, oral commun., 1959). Extensive erosion in the mountains behind the Book Cliffs followed the pre—Wisconsin glaciation, before and during the times that pediment surfaces below the cliffs were being cut and dissected. The erosional events are well illustrated in Whitmore Canyon, which separates West Ridge from the main part of the Roan Cliffs (fig. 1, pl. 1). Furthermore, the front of the Book Cliffs north of Sunnyside has not retreated appreciably since the pre-Wisconsin pediments were cut, because deposits of Wisconsin age along the front were laid di- rectly upon the pediments (pl. 2). Whitmore Canyon apparently was formed in two or more stages. At high levels along the canyon walls, particularly near'West Ridge, the canyon has a broad, flattened, U-shaped cross section that later was deeply eroded in the bottom to a V-shaped cross section. Walls of the V-shaped canyon were extensively eroded by tributary streams, so that only remnants of the walls remain. ,We found many remnants of former stream terraces on top of narrow buttresses along the walls of the main canyon. The largest remnant along which a few old meander scars are visible is on Bull Flat, which probably was part of the bottom of the old U-shaped valley. Other remnants, recognizable from their grassy surfaces and their position on narrow east-trending ridges, can be identified southward from Bull Flat beyond the present mouth of Whitmore Canyon. If this broad U-shaped valley was the course of an ancient stream, it has since been tilted, because valley rem- nants within 2 miles (3 km) south of Sunnyside are higher than remnants to the north and south. The stream may have joined the original drainage of Horse Canyon and flowed southward into Little Park Wash (pl. 1). The stream flowing in ancestral Whitmore Canyon apparently was captured near the town of Sunnyside by a more active stream flowing down the face of the Book Cliffs. Capture is suggested by Grassy Trail Creek, which flows southward through most of its course in Whitmore Canyon but turns abruptly west- ward at the Sunnyside coal mines and emerges from the Book Cliffs (fig. 45, pls. 1, 2). Similar capture of southward drainage by a stream flowing down the face of the Book Cliffs took place at Horse Canyon and on a tributary to Little Park Wash, about 3.5 miles (5.5 km) southeast of Horse Canyon. Further capture of Little Park Wash is imminent (Osterwald and others, 1971, p. 13), 4 miles (6.5 km) southeast of Horse Canyon (pl. 1). Since capture of the original Whitmore Canyon drainage, a narrow V-shaped gorge about 800 ft (250 m) deep was eroded into the bottom of the original 42 SUN N YSIDE COAL-MIN IN G DISTRICT, UTAH FIGURE 45,—View southwest toward the mouth of Whitmore Canyon showing the abrupt bend in Grassy Trail Creek near the Sunnyside No. 1 Mine before the creek emerges from the canyon. Small saddle (arrow) on ridge above coal tipple probably is a former channel of Grassy Trail Creek. Excavations on slope in right center of photograph for unit-train loading facility caused Kenilworth Member of Blackhawk Formation to become un- stable. 1, Portal of Sunnyside No. 1 Mine; 3, portal of Sunnyside No. 3 Mine; and b, stacking belt for unit-train loader installed in 1968. Number Two Canyon in left center of photograph. Kbk, Kenilworth Member; and Kbs, Sunnyside Member of Blackhawk Formation. broad valley. The time of capture of the Whitmore Canyon drainage cannot be determined accurately, but deposits of early Wisconsin age are found on the walls of the gorge near the mouth of the canyon, and the gorge antedates an alluvial fill that probably is of late Wisconsin(?) age, so the capture and gorge sculpture are probably of pre-Wisconsin age. We found no evidence of glaciation younger than the West Ridge boulder areas (pm-Wisconsin(?)) in the Sunnyside district. Pre-Wisconsin glacial features are not known in the Wasatch Plateau, 25 miles (40 km) west of Sunnyside, although younger glacial features are abundant (Spieker and Billings, 1940). This lack of evidence for pre-Wisconsin glaciation may indicate that the plateau was uplifted as much as 2,000 ft (610 m) during Pleistocene time (Spieker and Billings, 1940, p. 1194—1196; C. B. Hunt, 1956, p. 61). Glacia- tion equivalent in intensity to that in the Wasatch Plateau might have occurred in the Roan Cliffs during Wisconsin and later times if the Wasatch Plateau had remained at its pre-Wisconsin altitude during the Pleistocene (Spieker and Billings, 1940), but during and after Wisconsin time most of the precipitation from eastward- and northeastward-moving storms probably was trapped by the Wasatch, as it is now. Cemented Pleistocene conglomerates along the lower canyon walls may be remnants of outwash from glacia- tion of highlands during early Wisconsin(?) time. A moist climate during that time is indicated by abun- dant calcium carbonate cement, which probably re- sulted from a high ground-water level. Abundant up- land periglacial mantle, probably formed by solifluc- tion during a glaciation of early Wisconsin(?) age, also indicates deep weathering and abundant moisture. Alluvial gravel and boulder terraces in the lower parts of the canyons (fig. 28) may be of late Wisconsin(?) age and perhaps were derived from outwash during subse- quent times of high streamflow. Deposition of alluvial- fan debris at canyon mouths, followed closely by alluvial sand and silt in canyon floors during late Wis- consin(?) time, probably indicates another time of high streamflow, which may have resulted either from melt- ing snow and ice on the high mountains or from in- creased precipitation. The region surrounding the Sunnyside district was the site of considerable tectonic activity during Pleistocene time. Some of the oldest pediment surfaces formed in pre-Wisconsin time were warped before the beginning of the Wisconsin, because remnants of the youngest pre-Wisconsin pediment gravels are not noticeably warped. At two places in the southern part of the district, however, remnants of the middle series of pediments are offset a few feet (less than 2 In) along steeply dipping faults (Osterwald and Maberry, 1974). The ancestral course of Whitmore Canyon is at a higher elevation . just south of Sunnyside than it is farther north or south, presumably as a result of local warping. Thirty miles (50 km) west of the Sunnyside district, Pleistocene deformation was much stronger; in the Wasatch Plateau, early Pleistocene drainages were offset by faults, and glacial cirques of Wisconsin age were faulted (Spieker and Billings, 1940, p. 1 192). Evidence of late Pleistocene faulting in surrounding regions was mentioned by Hansen (1969, p. 117), who found faults offsetting glacial-outwash gravels of Bull Lake age (early Wisconsin) along the south margin of the Uinta Mountains, about 75 miles (120 km) north of Sunnyside. Alluvial silt of Wisconsin age along the west side of Marsh Flat Wash seemingly was cut by a west-northwest-trending fault. The smooth upper sur- face of the alluvium is not visibly offset, but a promi- nent 1-ft (0.3-m)-wide zone of efflorescent salts, which marks the trace of the fault in nearby Mancos Shale, continues several hundred yards into the alluvium and can be seen easily on aerial photographs. Any surface offset which may have been present probably was de- stroyed by sheetwash during modern storms. Seismic evidence indicates that some faults in east-central Utah may still be active (Osterwald and others, 1971). Holocene sediments in the Sunnyside district indi- cate a time of increased aridity, times of rapid alluvia- tion due to increased runoff, and times of rapid erosion. STRUCTURAL GEOLOGY 43 Rudimentary pediments along the courses of modern streams indicate a cold and arid period, probably dur- ing a temporary return to glacial conditions in sur- rounding highlands. Glacial features of probable Holo- cene age were found nearby in theLa Sal Mountains (Richmond, 1962, p. 75-84) and in the Wasatch Plateau (Spieker and Billings, 1940, p. 1188). Talus and alluvial-fan debris along cliffs and canyons prob- ably resulted from a climate that was originally cold, then warmed and became more moist as ice and snow melted. Alluvium along stream courses also indicates a time of abundant water following Holocene glaciation in nearby regions. Alluvium of late Holocene age along modern stream courses in the district (pl. 1) may repre- sent either the prepottery Holocene alluvium (about 2,000-4,000 yrs B.P.) or the historic alluvium (about 1500 A.D. to 1600 A.D.) of C. B. Hunt (1956, p. 38—39), or it may be a combination of both. Abundant camp- sites of early man along the course of Grassy Trail Creek indicate that some of the alluvium is equivalent to Hunt’s prepottery Holocene alluvium. We found no evidence in the Sunnyside district of a cycle of arroyo- cutting (gullying) about 1200 A.D., as mentioned by C. B. Hunt (1956, p. 39) for the Colorado Plateau, but modern gullies as deep as 15 ft (less than 5 m) have been cut in Holocene alluvium. These gullies may be the result of an erosional cycle that began in 1880—95 (C. B. Hunt, 1956, p. 39), because the Denver and Rio Grande Western Railway had considerable trouble from flooding and washouts along Grassy Trail Creek during the 1880’s (Denver and Rio Grande Western Railroad, written commun., 1965). The cycle of arroyo- cutting probably resulted from a drying climate, com- bined with frequently intense flash-flooding in and along the mountains. STRUCTURAL GEOLOGY The geologic structure of the Sunnyside district is simple. Except for small areas in fault zones, the beds dip less than 20°, generally to the east and northeast toward the Uinta Basin. These gentle eastward and northeastward dips indicate that the district is on a flank of the San Rafael Swell, a major north- to north- east-trending, flat-topped anticlinal uplift in central Utah (Kelley, 1955; fig. 1). The gently dipping beds ex- tend northeastward for many miles beyond the district toward the axial part of the basin. The beds generally dip 6°—18° east and northeast near the Book Cliffs; they commonly dip no more than 4° one mile (1.6 km) northeast and east of the cliffs (Osterwald and Dunrud, 1966, p. 99). Rocks in the district are cut by at least three sets of steeply dipping joints, one trending north to north-northwest about parallel to the strike of beds, one trending west-northwest, and one trending north- east to east-northeast, about parallel to the dip of beds. The most consistently oriented faults in the district are about parallel to the important joint directions, al- though the east-northeast-trending faults vary in strike and locally trend nearly east. A few east-trend- ing faults cut the beds in the Geneva Mine area (Dunrud and Barnes, 1972), and some in the south- western part of the district (pl. 1; Osterwald and Maberry, 1974) trend north-northwest to north-north- east. Stratigraphic separation on all the faults at the surface within the mining area is less than 200 ft (60 m); separation on most is only a few feet. As much as 4,000 ft (less than 1200 m) of stratigraphic separa- tion on a fault cutting subsurface rocks of Paleozoic age is known from seismic refraction studies (Tibbetts and others, 1966, p. D136) and horizontal separation of the Ferron Sandstone Member of the Mancos Shale at the surface is about three-quarters of a mile (1.2 km) along an east-northeast-trending fault, near Cedar. FOLDS Most folds in the Sunnyside district are broad and gentle and related spatially and genetically to the San Rafael Swell. The Woodside anticline and an adjacent syncline plunge north-northeastward into the southern part of the district (pl. 1) from the northeastern part of the swell. A few broad but very gentle anticlinal noses and synclines plunge northeastward beneath the Book Cliffs, east of the town of Sunnyside (Clark, 1928, pl. 22). A few other small open anticlines, having only a few feet of structural relief, trend eastward across the eastern boundary of the San Rafael Swell in the south- western part of the district; examples are in the area west of the confluence of Grassy Trail Creek and Price River (pl. 1). Other local folding in the Sunnyside district may have resulted either from elastic rebound of Mancos Shale during erosional unloading as the Book Cliffs were eroded eastward and northeastward toward their present position or from monoclinal folding parallel to the present cliffs. Several lines of evidence indicate that the present attitudes of beds along the cliffs are related partly to the present topography. Dip of the coal bed in the Sunnyside No. 1 Mine steepens gradually toward the cliffs from the lowest parts of the mine (pl. 2), which suggests an upward bending of the beds near the front of the cliffs. Geologic mapping at the surface (Osterwald, 1962a) indicates that dips along the cliffs are as much as 19°, but underground in the Sunnyside N0. 1 Mine and along Whitmore Canyon, about 2 miles (3 km) east. of the cliffs in the northern part of the district, dips are as little as 4°. 44 SUN N YSIDE COAL-MININ G DISTRICT, UTAH Strikes of sandstones in the Sunnyside Member of the 'Blackhawk Formation along the north side of the right fork of A Canyon (pl. 2) change from north-north- west on the east to almost west-northwest on the west, parallel to the topographic surface at the mouth of the canyon (Osterwald, 1962a). The sandstones at the head of the canyon dip 9° to the east, but on the west, at the mouth of the canyon, they dip as much as 19°. Al- though attitudes of the sandstones on the north slope of the ridge in the left fork of A Canyon cannot be deter- mined accurately because of thick surficial cover (pl. 2), we think that strikes may trend more nearly north along the central part of the ridge and that clips are greater on the west end of the ridge than they are to the east. The patterns produced-by these variations in attitudes along the flanks and ends of some ridges in the Book Cliffs suggest that broad, gently plunging synclines underlie some of the ridges and that some of the large canyons transverse to the cliffs may be along the crests of gentle anticlinal noses. The steepening of dip near the front of the Book Cliffs may be due to upward elastic rebound of the Mancos because of erosional unloading, monoclinal folding having anticlinal and synclinal bends trending northwest, or a combination of these two processes. Upward rebound of the Mancos is suggested by atti- tudes of beds around large promontories where the beds dip beneath the promontories from the sides as well as at the front (pl. 2). A cross section drawn through the main slope of the Sunnyside N o. 1 Mine in- dicates a flattening of clip a short distance southwest- ward from the cliff front; this change in dip suggests that the present cliff front is near the anticlinal bend of a monocline (pl. 2). The parallel, though variable, trends of northwestward joints and faults and their near-parallel alinement with the Book Cliffs and with the strike of beds in the Sunnyside No. 1 Mine also suggest monoclinal folding. Rotated joints along the Book Cliffs, however, and conjugate shear joints indi— cate late movement and vertically directed compo- nents of compressive stress after initial folding and erosion of the cliffs to their present position, probably because of rebound. The steepening of dip of beds near the Book Cliffs does not resemble closely the steepening of dip that would result entirely from bending of beds on a mono- cline, because: 1. The “synclinal bend” is not at the front of the Book Cliffs, as observed in many monoclines of the Colorado Plateau, but is about 2,000 ft (600 m) east of the cliffs. 2. Dips in the Mancos Shale and Kenilworth Member of the Blackhawk Formation are less westward from the face of the Book Cliffs (zone of steep dips) than they are east of the steep dip zone (pl. 2). 3. Beds that strike nearly parallel to the trends of major ridges in the Book Cliffs and that dip inward be- neath the ridges are more compatible with an origin of local uplift (elastic rebound) than they are with an ori- gin related to monoclinal folding. Uplift along the cliffs may have proceeded as wide- spread pediments were cut in Mancos Shale at the base of the cliffs in pre-Wisconsin time. (See the section on “Pediment gravels.”) Some pediments, particularly those in the northern part of the district, steepen rapidly toward the cliffs (fig. 46), and at the base of the cliffs attain slope angles as steep as 40 percent. This steepening of the original slope may have resulted in part from upward rebound of the Mancos when the re- treat of the cliffs was slowed during a climatic change, and in part from the decreased sediment load carried by streams (Lobeck, 1939, p. 557). Such slopes seem much too steep to have been formed entirely by either decreased sediment load or accelerated erosion and deposition near the cliffs, as is common on pediment surfaces elsewhere. Elastic rebound of the Mancos Shale as the actual cause of the pre—Wisconsin uplift along the cliffs seems likely, however, because of the known physical be- havior of the Mancos and similar rocks. Dips near the outcrop of coal beds in the Grand Mesa, Colo., coal field steepen similarly; these increases are attributed by Lee (1912, p. 65) to “relief of pressure as the super- incumbent rocks were eroded away.” A few feet of elas- tic rebound, equivalent to an expansion of about 10 percent, in the very similar Pierre Shale at Oahe Dam, FIGURE 46.—View southeast near mouth of A Canyon, north of Dragerton, Utah. Steeply sloping remnants of pre-Wisconsin pediment (arrows) near the Book Cliffs, separated from cliffs by late Pleistocene erosion and by some talus of late Pleistocene age (Qnt). Bank in foreground is wall of gulch about 20 ft (6 m) deep that resulted from late Pleistocene erosion of pediment surface. Km, Mancos Shale; Kbk, Kenilworth Member, Blackhawk Forma- tion; Kbs, Sunnyside Member, Blackhawk Formation; Kc, Castle- gate Sandstone; Kpb, Bluecastle Sandstone Member of Price River Formation. STRUCTURAL GEOLOGY S. Dak., occurred during excavation of about 150 ft (46 m) of overburden (Underwood, 1957; Underwood and others, 1964). Similarly, a cut slope in tuffaceous rocks about 125 ft (40 m) high, in the State of Wash- ington, failed massively as a result of rebound amount- ing to 0.5 ft (0.15 m) horizontally and 1.5 ft (0.5 m) ver- tically (K. R. Meadaris, oral commun., 1973). This re- bound was equivalent to more than 10-percent expan- sion vertically. We observed cores of Mancos Shale tongues in the Blackhawk Formation, taken from a drill hole in Whitmore Canyon, that expanded longi- tudinally (upward) and broke into many small disks when withdrawn from core barrels. Expansion of less than 10 percent in the 3,000—4,000-ft thickness of the Mancos Shale in the Sunnyside district probably could account for the observed changes in bedding attitudes near the cliffs. JOINTS Joints at most localities in the district occur in three principal sets, although one or more of the sets locally may not be clearly discernible, and at some localities additional sets may appear. Nearly all joints in the dis- trict dip steeply, but slight variations in both dip and strike were noted at many places. Upper-hemisphere pole diagrams of joints show distinct girdle patterns, local statistical concentrations representing the major sets (fig. 47). These statistical concentrations vary slightly in the different parts of the district, probably as a result of different structural patterns. Many joints were rotated slightly since their formation, probably during Wisconsin time. Blocks of rock bounded by joints commonly fall from cliff faces and probably con- tributed significantly to retreat of the cliffs to their present position. On the crest of West Ridge (pl. 2), outward rotation of large sandstone joint blocks which are loose but have not yet fallen has produced gaping troughs, partially filled with soil, that are several feet wide and deep and several tens of feet long. NORTHWEST- TO NORTH-NORTHWEST-TRENDINGJOINTS Joints of the northwest- to north-northwest-trending set are more variable in attitude than are joints of the west-northwest set, probably because the strike of beds, which the northwest to north-northwest joints nearly parallel in trend, changes from nearly north in the southern part of the district to northwest in the northern part. In addition, these joints apparently were rotated to different angles during local uplift of the Book Cliffs, because the joints trend almost at right angles to most of the prominent west-trending ridges forming the front of the cliffs (pl. 1). In some areas along the cliffs, as in the Geneva Mine area, dif- 45 ferential rotation along faults (Dunrud and Barnes, 1972) apparently produced two or more joint attitudes that diverge in trend and dip steeply southwest (fig. 47E). Locally, joints of the northwest- to north-north- west-trending set occur in zones a few feet wide con- taining individual joints less than 1 ft (0.3 m) apart; the zones themselves are a few feet to 20 ft (6 m) apart. Movement has occurred along many joints of this set, particularly near the northwest-trending Sunny- side fault zone, and many have as much as a few feet of stratigraphic separation. A few joint surfaces, particu- larly near Columbia, are coated with thin films of cal- cite and a few others are coated with gypsum. NORTHEAST- TO NORTH-NORTHEAST-TRENDINGJOINTS A set of joints trending northeast to east-northeast, which nearly parallels the direction of dip of beds, varies considerably in spacing of fractures but is rather con- sistent in trend between the various parts of the dis- trict. The set generally forms a nearly orthogonal pair with the west-northwest set, striking nearly northeast in the northern part (fig. 47A, B) and in the extreme southern part of the district (fig. 47H). Near the Geneva Mine the strike of joints is about east-north- east (fig. 47E, H), probably because the strike of beds near the mine is nearly north, and suggesting that the joints formed in response to stress that produced the folding. South of the Geneva Mine, beds strike more north, and the northeast joints strike more east than at the Geneva Mine (pl. 1, fig. 47E, F, H). Apparent con- jugate sets of generally northeast-trending joints along the Book Cliffs near the Sunnyside No. 1 Mine (fig. 47A) and near the Columbia Mine (fig. 470) may have resulted from opposing dips of beds on opposite sides of northeast-trending ridges which probably were caused by vertical stress components resulting from upward elastic rebound of Mancos Shale as deep canyons were cut into the cliffs. Varying attitudes of northeast-trending joints near the Geneva Mine (fig. 47E, F) may have resulted from differential movement along fault blocks (Dunrud and Barnes, 1972) as a con- sequence of similar rebound. Some conjugate sets of steeply dipping, northeast- trending, vertically curved shear joints locally cut sandstones of the Sunnyside Member along the cliffs, particularly between the towns of Sunnyside and Columbia (fig. 48). These shear joints, which probably formed in response to vertical stress components be- cause their acute bisectors are nearly vertical (fig. 48), also may have resulted from upward elastic rebound of Mancos Shale during erosional unloading. Similar joint sets were not observed in any sandstone units above the Sunnyside Member. 46 SUNNYSIDE COAL-MINING DISTRICT, UTAH §“““‘\\ \ 3’ 15.5 B S D 10.5 ' S FIGURE 47 .—Stereograms of joint poles in the Sunnyside coal mining district, plotted on upper hemisphere of equal-area net showing attitudes of major statistical joints. A, 78 joints along the Book Cliffs, Sunnyside No. 1 Mine area, maxima (black) at 7 percent, con- tours at 4, 2, and 0.5 percent. B, 319 joints behind Book Cliffs, Sunnyside N o. 1 Mine area, maxima (black) at 19 percent, contours at 15, 10, 5, and 1 percent. C, 323 joints along Book Cliffs, Columbia area, maxima (black) at 40 percent, contours at 10, 6, 3, and 1 percent. D, 96 joints behind Book Cliffs, Columbia area, maxima (black) at 20 percent, contours at 15.5, 10.5, 5.0,‘and 2.0 percent. E, 437 joints FAULTS cr0ps along the Book Cliffs and in steep canyon walls, because individual fractures are discontinuous and be- Most faults and fault zones in the Sunnyside district cause the stratigraphic separation along individual are obscure at the surface, in spite of excellent out- faults commonly is less than the vertical variations in STRUCTURAL GEOLOGY 47 N. 88° W. 90° ”/1 ’J ”fiz/Z‘ all/l" 9 S F H l 3 along Book Cliffs, Geneva Mine area, maxima (black) at 9.5 percent, contours at 9, 7, 5, 3, and 1 percent. F, 118 joints behind Book Cliffs, Geneva Mine area, maxima (black) at 15 percent, contours at 12, 8, 6, and 4 percent. G, 50 joints east part of Woodside 15-minute quadrangle south of Geneva Mine, maxima (black) at 10 percent, contours at 8, 4, and 2 percent. H , 84 joints, northeast part San Rafael Swell, maxima (black) at 10 percent, contours at 8, 3, and 1 percent. stratigraphic positions of rock contacts. Some faults, than 15 m) wide. Fault traces in hard sandstones be- especially those in the northern part of the district, are hind the cliff front are best discerned on high-altitude indistinct because small amounts of movement are dis- (1:60,000-scale) aerial photographs. Fault traces in the tributed along joints over zones a few tens of feet (less Mancos Shale of the low-lying Clark Valley are best 48 SUNNYSIDE COAL-MIN IN G DISTRICT, UTAH FIGURE 48.—View southwestward of conjugate shear joints in sandstones of Sunnyside Member of Blackhawk Formation along south side of Fan Canyon, north of Columbia, Utah. Joints strike northeast; acute bisector of joint planes is nearly vertical. Sand- stone ledge is about 25 ft (7.5 m) thick near center of the photo- graph. Sunnyside coal bed is exposed by bulldozer out just above ledge. discerned as straight lines on 1:20,000-scale aerial photographs. Subsequent field examination in the low- lying areas commonly revealed erosional scarps as much as 40 ft (12 m) high resulting from retreat of soft, weathered shale from underlying thin, resistant beds (fig. 49). A few faults and fault zones are silicified, iron stained, or filled with calcite and gypsum, and thus can be easily traced, but most are unmineralized, un- cemented, and indistinct. Changes in strike and dip of faults are common within short distances along the faults. Most move- ment along faults in the district was vertical, but along east-northeast-trending faults in the northern part of the district, horizontal slickensides, gash fractures, and obliquely intersecting shear fractures in fault zones locally indicate horizontal components of move- ment. One east-trending fault north of Horse Canyon apparently moved horizontally (pl. 1). Faults in the northern part of the district are nearly vertical, but most faults in the Geneva Mine area (central part of the district) dip less steeply, and some dip 45° or less. Faults in the southern part of the district dip nearly vertically. Small gash fractures filled with silica, cal- cite, and iron oxides are common in clearly exposed fault zones along the flank of the San Rafael Swell (fig. 1, pl. 1) and indicate considerable components of hori- FIGURE 49.—Erosional scarp (arrows) along fault cutting Mancos Shale in the southern part of the Sunnyside district, about 1 mile (1.6 km) west of Grassy siding on Denver and Rio Grande West- ern Railroad. Stratigraphic separation is about 40 ft ( 12 In) up to the right in the photograph. Dark material in lower left center is spoil from small excavation. View southeast toward Book Cliffs. ch Kd FIGURE 50.—Buckhorn Conglomerate Member (ch) of Cedar Mountain Formation draped over faults cutting Brushy Basin Member (J mb) of Morrison Formation about 2 miles (3 km) north- west of Cedar. U, upthrown side; D, downthrown side. Flat in fore- ground is covered by upper Wisconsin(?) terrace gravel (Qt). Ridge on skyline is capped by Dakota Sandstone (Kd) underlain by shale member (Kcs) of Cedar Mountain Formation. View west- ward. zontal movement. Beds of Jurassic and older age in the northeastern part of the San Rafael Swell are draped over major faults and fault zones (pl. 1, fig. 50), indicat- ing large components of vertical fault movement. In general, stratigraphic separation on major faults in the district is large in Jurassic and Lower Cretaceous rocks, moderate in Upper Cretaceous rocks, and small in Tertiary rocks. SUNNYSIDE FAULT ZONE Most of the north-northwest-trending faults in the Sunnyside district are part of the Sunnyside fault STRUCTURAL GEOLOGY 49 zone, which extends from the Geneva Mine area to West Ridge (pl. 1). Near the Geneva Mine the zone is as much as 1.5 miles (2.5 km) wide (Osterwald and Dunrud, 1966, p. 99), but it is only~10 ft (3 m) wide in the Sunnyside No. 1 Mine (Osterwald, 1962b, p. 64). Individual faults within the zone dip steeply. Average stratigraphic separation of coal in the zone is about 30 ft (9 m) in the northern part of the district at the Sunnyside No. 1 Mine (Clark, 1928, p. 25), and about 40—60 ft (12-18 m) in the southern part of the district at the Geneva Mine. Most faults within the zone are nearly parallel to the trend of the zone, but some diverge at small angles and merge with other faults in the zone that are parallel with the zone boundaries. The map relationships of the faults within the zone suggest that the diverging faults may be gash frac- tures resulting from small components of horizontal movement in which the block northeast of the zone moved relatively northwestward. Thus, the total dis- placement across the zone may be much greater than the 30 ft (less than 10 m) of stratigraphic separation. At a few localities, for example in the north side of Number Two Canyon (pl. 1), strongly fractured zones in the Castlegate Sandstone show no stratigraphic separation and may have resulted from predominantly horizontal motion. Many faults in the zone are parallel to north-northwest-trending joints, and less than 1 ft (0.3 m) of stratigraphic separation can be measured on joint planes at many localities near the fault zone. The zone varies widely in its internal characteristics. At some localities in the northern part of the district, for example in the southernmost bleeder slopes of the Sunnyside No. 1 Mine (Osterwald, 1962a), no single fault plane is present in the zone, and the separation is distributed along numerous minute fractures across a width of several feet. Elsewhere, as in the main slope of the Sunnyside No. 1 Mine (Osterwald, 1962a), the posi- tion of the zone is marked by gouge, breccia, and frac- tured rock (fig. 51). The fault zone is easy to trace at the surface in the northern part of the district only where it is locally silicified and iron stained. In the southern part of the district, where the zone is broad, it consists of widely spaced individual faults. EAST-NORTHEAST- TO NORTHEAST-TRENDING AND EAST- TRENDING FAULTS Faults that trend east-northeast and east are widely distributed throughout the district but are more numerous near the Geneva Mine. These faults trend about parallel to the dip of beds but vary considerably in strike and dip (Dunrud and Barnes, 1972). Most of these faults dip steeply in the northern part of the dis- trict but dip less steeply farther south, and at the Geneva Mine they dip 45° or less. East-northeast- FIGURE 51.—Gouge, breccia, and fractured rock along Sunnyside Fault, in southeast rib of old right-side manway, Sunnyside No. 1 Mine, Utah. View is southeast along strike of fault. Stratigraphic separation is about 13 ft (4 m); downthrown block is on right side of picture. Photograph by J. C. Witt. trending faults offset faults of the Sunnyside fault zone and hence may be younger than the zone, both in the Sunnyside No. 1 Mine area (Osterwald, 1962a) and in the Columbia area (Osterwald and others, 1969), al- though we realize that offsets of fault sets may not in- dicate the relative ages of the sets (McKinstry, 1948, p. 354—360). Stratigraphic separation on individual east-north- east-trending faults is as much as 150 ft (46 m) in the Book Cliffs, but on most faults it is much less. Separa- tion on many of the faults is greatest near the cliffs but decreases eastward away from the cliffs, particularly about 1 mile (1.6 km) north of the town of Sunnyside (fig. 52). Separation also is less along these faults in the Colton Formation than it is in older rocks, indicating either that movement began during Cretaceous time and continued with decreasing energy into early Ter- tiary time, or that the fault separation diminished to the east. Horizontal separation of the Ferron Sand- stone Member of the Mancos Shale along one such 50 SUNNYSIDE COAL-MINING DISTRICT, UTAH FIGURE 52.—East-northeast-trending fault cutting Castlegate Sandstone (K0) in Slaughter Canyon, north of Sunnyside, Utah, illustrating decreasing stratigraphic separation east of Book Cliffs. A, Fault cutting Castlegate in west side of canyon about 1,500 ft (460 m) east of Book Cliffs; separation is greater than thickness of Castlegate. Kss, sandstone of Sunnyside Member of Blackhawk Formation; us, upper split, and Is, lower split, Sunnyside coal bed. B, Same fault cutting Castlegate in east side of canyon, about 4,500 ft (1,400 m) east of Book Cliffs; separation is less than thickness of Castlegate. fault about 1 mile (1.6 km) north of the Cedar railroad siding, nearly 6 miles (10 km) west of the Book Cliffs, is approximately three-quarters of a mile (1 km). Many small east-northeast- or northeast-trending faults follow joint planes and have only a few tenths of a foot (less than 15 cm) to a few feet (less than 2 m) of stratigraphic separation. Locally, as in the Colton For- mation on the east side of Whitmore Canyon between Bear Canyon and Pole Canyon (pl. 2), several of these small faults along joint planes show as much as 4 ft (1.2 m) of separation across a zone 40 ft (12 m) Wide. Rock along some of the small faults is brecciated, and individual sand grains are broken, although the strati- graphic separation on each fault may be as little as 0.5 in. (1.3 cm); locally, the brecciated rock is recemented with calcite (pl. 2). Other faults belonging to this set have thin veins of calcite and gypsum less than 0.5 in. (1.3 cm) wide, and one contains a few grains of a copper sulfide mineral. Movement on most of the east-northeast- and north- east-trending faults apparently was dip slip. Locally, as in the now inaccessible part of Sunnyside No. 3 Mine beneath the mouth of Schoolhouse Canyon (pl. 1), horizontal slickensides in the coal indicate strike-slip movement, but slickensides along a calcite vein in the Bluecastle Sandstone Member of the Price River For- mation at the surface in the same locality plunge 24° southeast, indicating components of dip slip. We be- lieve that the first movement along the east-northeast faults was dip slip but that subsequent motion (after some faults were filled with calcite) may have been ver- tical, horizontal, or somewhere in between. STRUCTURAL GEOLOGY SUBSURFACE FAULT A fault having about 2,600 ft (800 m) of strati- graphic separation in subsurface rocks of Paleozoic age trends northwestward beneath the south-central part of the district and was detected by seismic methods (Tibbetts and others, 1966, p. D136; pl. 1). Strati- graphic separation in overlying rocks decreases as the age of the rocks decreases, and the fault dies out up- ward in the Mancos Shale. We found no evidence of this fault at the surface or in mine workings, but the fault pattern of the district differs on opposite sides of the surface projection of its subsurface position. North-northwest-trending faults (Sunnyside fault zone) are much more abundant northeast of the subsur- face fault, and east-northeast-trending faults are more uniformly spaced and vary less in strike and dip south- west of the subsurface fault than they do to the north- east. These differences suggest that slight movements along the subsurface fault may have caused stress re- adjustments in the Upper Cretaceous and Tertiary rocks, thus influencing the distribution and attitude of younger faults. The fault probably is the southwestern margin of an uplift that was raised in Paleozoic time (Heylmun, 1959, p. 17 2—173). Based on the log of a hole drilled for oil in SW%SW% sec. 25, T. 16 S., R. 15 E. (several miles south of the section in pl. 1), the Mancos Shale is thicker northeast of the fault than it is in sec- tions measured below the Book Cliffs. WEST—NORTHWEST-TRENDING FAULT BELT A belt of numerous west-northwest-trending faults crosses the district south of the Geneva Mine (pl. 1). Individual faults within this belt are only a few miles (less than 10 km) long, but in reconnaissance work the belt was traced from the confluence of Range Creek and the Green River 10.4 miles (16.1 km) east of the Woodside 15-minute Quadrangle for a distance of 32 miles (51 km) to near Desert Seep Wash, 12 miles (19 km) south-southeast of Price, Utah (fig. 1). Most faults in the belt clip steeply or vertically and strike nearly parallel to the trend of the belt; directions of stratigraphic separation on the steeply dipping faults indicate that most are normal faults. Upper Cretaceous and Tertiary rocks within the fault belt are not tilted or brecciated, but beds of Jurassic age within the belt dip northward as much as 57° about 1.5 miles (2.5 km) south of Cedar (pl. 1, fig. 53). Stratigraphic separation seemingly is greater in rocks of Jurassic age than in those of Tertiary age, in- dicating that movement along faults in the belt began during or after the Jurassic rocks were deposited and continued intermittently into the Tertiary. Some pedi- ment surfaces of pre-Wisconsin age also are offset a 51 FIGURE 53.——View west of steep southward dip of Brushy Basin Member of Morrison Formation in fault zone about 1.5 miles (2.5 km) south of Cedar. Skyline in distance is capped by thick conglomeratic sandstone lens in the upper part of the Brushy Basin, which dips gently northeastward. FIGURE 54.—View north about 2 miles (3 km) northeast of Cedar of alluvial-fan debris (Qf|)of late Wisconsin(?) age from mouth of Horse Canyon overlapping faulted Ferron Sandstone Member (Kmf) and the overlying part of Mancos Shale (Km). Alluvial-fan debris shows no visible offset. U, upthrown side; D, downthrown side. Book Cliffs near Sunnyside, Utah, in background; aban- doned embankment for US. Highway 6 in middle ground. few feet (less than 2 m) by faults within the belt in the south-central part of the Woodside Quadrangle (pl. 1; Osterwald and Maberry, 1974), but alluvial-fan mate- rial of late Wisconsin(?) age 3 miles (5 km) southeast of Cedar overlaps faulted Ferron Sandstone Member of Mancos Shale without visible offset (fig. 54). Structure sections drawn through the fault belt indi— cate that it is at the crest of a broad anticline which has collapsed (Osterwald and Maberry, 1974). The length, surface pattern of faulting, and trend of the belt sug- 52 SUNNYSIDE COAL-MINING DISTRICT, UTAH gest that it is similar to but less strongly developed than the well-known anticlines near Moab, Utah, and Naturita, Colo., about 90—150 miles (145—240 km) southeast of Sunnyside, that resulted from diapiric movement of salt beds at depth (Cater, 1955; Jones, 1959). Some of these anticlines extend northwestward to the vicinity of Green River, Utah (Jones, 1959, p. 1890; Cohee and others, 1961), about 40 miles (64 km) southeast of Sunnyside and only about 20 miles (32 km) from the confluence of Range Creek and Price River. The fault pattern in the collapsed anticline (pl. 1) closely resembles the fault patterns in the upper parts of salt anticlines as described by Baars (1966, p. 2107) and Stokes (1948) and as illustrated by Jones (1959, p. 1872). We infer, therefore, that the fault belt is the surface expression of a salt anticline at depth, probably near the northern margin of a basin of salt deposition during Pennsylvanian time. Some faults in the belt may still be active, because some earth tremors recorded by us at Sunnyside, Utah, originated within the belt. ECONOMIC GEOLOGY Several commodities of actual or potential economic value are present within the Sunnyside district, but coal is the most abundant and of the most economic value. Gypsum, present in the Carmel Formation in the southwestern part of the district, is a potential source of plaster and wallboard if economic conditions in central Utah become favorable for its exploitation. Underground water, although rare, emerges as springs at various places within the district and is used locally for domestic and stock water. A cold-water geyser, charged by carbon dioxide probably derived from deeply buried Paleozoic carbonates, is near the Price River at Woodside. The geyser, which discharges periodically, is used only as a tourist attraction. It is the result of a well drilled many years ago by the Den- ver and Rio Grande Western Railroad for a source of boiler water. Asphalt-impregnated sandstone, quarried from the Colton Formation northeast of Sunnyside (Holmes and others, 1948), formerly was used to pave streets and roads, but the quarry has long been idle. Petroleum and carbon dioxide are produced from the Grassy Trail Oil Field about 8 miles (13 km) west-southwest of Dragerton. The Jurassic and Lower Cretaceous rocks in the southwestern part of the district were exten- sively prospected for uranium, but little ore is known to have been produced. Other commodities in the district are only of scien— tific interest as mineral occurrences but have no present economic value. Most of these occurrences con- sist of grains of metallic minerals scattered along some faults. COAL Coal at Sunnyside was discovered in 1898 by J effer- son Tidwell (Peterson and others, 1956, p. 206). Natu- ral coke also was found in 1898 where the coal outcrop had been burned (Lewis and Varley, 1919, p. 67). The Sunnyside mines were opened in 1899 (Harrington, 1901) by Daniel Harrington and James Westfield, and 1,200 men soon were at work (Peterson and others, 1956, p. 206). Shortly afterward the mines were ac- quired by’the Utah Fuel Co., a subsidiary of the Rio Grande Western Railway (Athearn, 1962, p. 194—195), largely to provide a source of locomotive fuel (Storrs, 1902, p. 455). Because the Sunnyside coal could be used to make metallurgical coke, it became too valu- able to use only for locomotive fuel, and by 1919 the largest beehive coke-oven plant in the United States was at Sunnyside (fig. 55; Lewis and Varley, 1919, p. 68). Kaiser Steel Corp. bought the Sunnyside mines in 1950 as a source of coking coal for their steel plants in California. The Columbia Mine (pl. 1) was opened by Utah Fuel Co. in 1924 and later was acquired by the Columbia- Geneva Steel Corp. as a source of coking coal for mills near Provo, Utah. Columbia-Geneva Steel Corp. later was acquired by United States Steel Corp., the present owner of the Columbia Mine. Small coal prospects in Horse Canyon known as the Carlson and Woodard Mines were opened during the 1930’s on leased Federal lands. These prospects were acquired by the Defense Plant Corp. during World War II, and, through a con- tract with the Columbia-Geneva Steel Division of United States Steel Corp., were developed into the Horse Canyon coal mine (also known later as the Geneva Mine). Columbia-Geneva bought the property outright after World War II. The Book Cliffs Mine, also known as the Murray Mine and adjoining the Horse Canyon (Geneva) Mine, was developed during World War II and was operated by the Book Cliffs Coal Co. until 1966. Mining ceased in the Columbia Mine in 1967 because of many faults which caused dan- gerous mining conditions. The three principal settlements in the district are Sunnyside, Columbia, and Dragerton. Sunnyside, the oldest settlement, originally was in the lower part of Whitmore Canyon near the present portal of the Sunnyside N o. 1 Mine; it was a company town owned by Utah Fuel Co. During World War II, additional modern houses and business buildings were built in the town of Sunnyside near the mouth of Whitmore ECONOMIC GEOLOGY 53 FIGURE 55.—Coke-oven plant at Sunnyside, Utah, probably before 1910. Each small opening along the brick wall at left center of photo- graph is an individual coke oven. Each oven along the brick walls was operated manually by one man (see fig. 58). Photograph courtesy Library, State Historical Society of Colorado. Canyon by the Defense Plant Corp., in an area known locally as Sunnydale, and were leased to Utah Fuel Co. Following World War II, Sunnydale was sold to Utah Fuel 00., and subsequently, the entire Sunnyside prop- erty (mines, surface plant, and townsite), was sold to Kaiser Steel Corp. Some of the World War 11 buildings and a few cement-block houses built by Kaiser Steel re- main in Sunnyside. Columbia was built about 1924 as a company-owned town by Utah Fuel Co. to provide housing for miners at the Columbia Mine; later it was sold to Columbia-Geneva Steel Corp. and subsequently was acquired by United States Steel Corp. During the 1950’s, the buildings in Columbia were sold to indi- vidual residents; the town now is privately owned. Dragerton, the business center of the district, also was built during World War II by the Defense Plant Corp. to provide modern dwellings and business facilities for miners at the 'Horse Canyon (Geneva) Mine. Subse- quently it was sold to United States Steel Corp. and was resold to individual owners and shopkeepers at the same time as Columbia was sold. In 1972, about 700 persons lived in Dragerton, 150 in Columbia, and 200 in Sunnyside. Columbia and Dragerton were incor- porated as East Carbon City about 1974. Most mining in the Sunnyside mines was done by room-and-pillar methods, at first by individual men using hand-mining methods and later by using various electrically driven cutting and loading machines. Early machine mining was done by undercutting the face and blasting down the coal. Much mining at Sunnyside, however, does not require blasting because the coal is stressed and fails continuously in a series of small bumps. Old miners report that, in some places, a pick thrown at the face at the beginning of a work shift caused an immediate bump so that the remainder of the shift could be devoted to loading broken coal. All room-and-pillar work in the district at present (1974) is done by using continuous-mining machines that feed automatically into rubber-tired self-propelled shuttle cars. These shuttle cars transport mined coal to under- ground loading points where it is dumped into mine cars hauled by electric motors. Longwall mining meth- ods were first used in the Sunnyside mines during the early 1920’s by a simple modification of mining pro- 54 SUN N YSIDE COAL-MIN IN G DISTRICT, UTAH cedures in which cutting machines and loading equip- ment were adapted to operation along a continuous face rather than to a series of gradually progressing rooms and pillars (James Westfield, oral commun., 1961). Modern longwall mining methods were intro- duced in the Sunnyside mines in 1963, using automatic equipment. Mining plans in the Sunnyside mines are designed to permit complete collapse of the roof over mined-out areas (John Peperakis, oral commun., 1958). Entries are driven approximately along the strike of the coal bed, to the right and left of haulage slopes which are about parallel to the dip of the coal. The entries inter- sect bleeder slopes near the boundaries of each mine; the bleeder slopes provide continuous ventilation cir- cuits and also serve as escapeways should the main slopes be blocked. Rooms are driven up the dip of the coal bed from each entry, leaving a BOO-ft (90-m) or 275- ft (84-m) barrier pillar beside the bleeder slope. Only a few rooms are started at one time, and pillars separat- ing rooms are reduced progressively in size until only a small remnant remains. These remnants are then either removed by mining or blasted to destroy their strength so that the roof will cave completely. Modern longwall faces are oriented about parallel to the dip of the coal bed. Roofs adjacent to longwall faces are sup- ported by movable hydraulic props, and are allowed to collapse a few feet from the face as soon as the props are advanced. This progressively retreating method of mining allows nearly all of the mining work, except for actual working of the faces, to be done in solid, undis- turbed coal, and is much safer and more economical than traveling through partially developed or worked- out blocks of coal. Surface operations at the Sunnyside mines became more efficient as changes were made in underground FIGURE 56.—View southeastward of Utah Fuel Co.’s surface plant at Sunnyside, in Whitmore Canyon, probably before 1908. Miners’ houses are in foreground. Smoke is from coke ovens near mouth of Whitmore Canyon. Compare figures 45 and 57. Photograph courtesy of the State Historical Society of Colorado. ECONOMIC operations. A coal-fired electric plant provided power for underground and surface machinery soon after the mines were opened (Clark, 1922, p. 212), and large preparation plants and loading tipples were built (fig. 56). The miners’ living quarters remained primitive, however (fig. 56). The preparation plant, washer, and tipple were enlarged and modernized by 1958, but rail- road cars still were loaded individually and manually rolled into the load yard, much as they were 50 years earlier. Car loading was mechanized in 1968, when a unit-train loader that commonly fills 80 railroad cars of 125-ton (113,400-kg) capacity in about 1 hour was in- stalled near the mouth of Whitmore Canyon (fig. 57). Beehive coke ovens were first operated at Sunnyside in 1903 (Allen, 1925, p. 2). When the Sunnyside No. 2 Mine was acquired by Kaiser Steel Corp., additional coke ovens were built near the Columbia yards of the Carbon County Railway (pl. 1). Operation of the bee- hive coke ovens was an individual piecework procedure (fig. 58), requiring a large force of men. The beehive ovens were abandoned in 1958; since then all the coke has been produced in modern byproduct ovens at the Kaiser Steel plant in Fontana, Calif. Coal from the Columbia and Geneva Mines is hauled by rail to Well- ington, Utah, where it is sized and washed. It is then reloaded and hauled by rail to Provo, Utah, where it is coked in byproduct ovens. Coal mining is normally a hazardous occupation, and mining in Utah is no exception. Fatalities in Utah coal mines historically are numerous, and several major disasters, defined by the US. Bureau of Mines as hav- FIGURE 57.—Unit-train loader of the Sunnyside mines, in lower Whitmore Canyon. The train backs through the tunnel, then pulls forward continuously so that about 80 cars of 125-ton (113,400- kg) capacity are loaded in an hour. Coal is loaded through trap doors in the tunnel beneath the coal pile. Bulldozer pushes coal up to crater at top of pile while loading. Coal pile and part of the tunnel are supported by artificial fill. View southeastward across Whitmore Canyon from a point above and north of Sunnyside. Photograph taken in 1968. Compare figures 45 and 56. Sand- stones of Kenilworth and Sunnyside Members of Blackhawk For- mation in cliff above leading locomotive. Ledges of Castlegate Sandstone are on slope above the coal pile. GEOLOGY 55 FIGURE 58.—Coke-oven operation at Sunnyside, about 1950. Coal was delivered to individual ovens by the track—mounted hopper car above, the coal was coked, then it was removed into a wheel- barrow by the oven operator and dumped into railroad cars. Ovens were individually closed and opened by operators using bricks and mortar. Photograph courtesy of W. R. Muehlberger. ing five or more fatalities, have occurred. For example, the fatality rate per million man-hours of employment in Utah coal mines during 1941 was the highest in the United States (Adams and Geyer, 1944a, p. 6). Two hundred men were killed at Scofield, Utah, in 1900, and 171 'men were killed at Castle Gate in 1924 in mine-gas explosions (Adams and Geyer, 1944b, p. 120). Over the years, many other miners were killed by numerous small explosions and by falls of faces or ribs, including bumps, in various Utah mines. Twenty-three miners died in a gas explosion in the Sunnyside No. 1 Mine in 1948 (Harrington and others, 1950, p. 28). We believe that geologic features and processes underlie many of the physical phenomena involved in such mine failures and that a knowledge of failure characteristics of mine rocks and coal beds can yield useful information to better understand and eventually control the violent release of energy in such disasters. COAL—MINE BUMPS Bumps, which have been a major hazard to coal min- ing in the district, are the same physical phenomena as rock bursts which occur in many noncoal mines. They also are known by many other names, such as crumps, bounces, mountain shots, and pounces, but all these terms fundamentally refer to the same process—the sudden, sometimes catastrophic release of stress stored in the rock or coal. All such failures are referred to as bumps in this report, although miners in the dis- trict most commonly use the term “bounce.” Our usage of the term “bump” (Osterwald, 1970, p. 2083—2084) follows the definitions of Holland and 56 SUNNYSIDE COAL-MIN IN G DISTRICT, UTAH Thomas (1954, p. 3), but varies from the usage of Obert and Duval (1967, p. 582), who restricted “bump” to the noise or shock wave resulting from a rock or coal failure (burst). Mine faces that bump continuously during mining are common in the district. Clark (1928, p. 80), a pio- neer geologist in the Book Cliffs Coal Field, noted that in 1911 “The unweathered coal is brittle and hard to pick and has a metallic ring when struck with the ham- mer. While mining is going on the working faces are continually snapping and splintering.” Such small bumps actually are beneficial to some degree, because they make mining easier and reduce the need for blast- ing; however, they are hazardous because unwary per- sonnel may be killed or injured by even small bumps. Modern room-and-pillar mining in the district is done by various types of continuous-mining machines. While these machines are being operated, small to moderately large pieces of coal are ejected con- tinuously from many faces, accompanied by explosion- like reports as loud as those from a large-caliber rifle and by perceptible tremors in the flOor. During these ejections, slabs of roof rock may fall onto the machines, ribs may become active and crumble, and floors may heave. Such activity continues at an ap- parently decreasing rate when mining is temporarily discontinued. Many coal miners believe that they can foretell the probability of large bumps by the actual mining condi- tions at the face. Presumably, such bumps are more apt to occur when the coal becomes “hard to cut.” Whether or not such a correlation actually exists is dif- ficult to determine. Bumps may occur without such premonitory sensations, and “hard cutting” coal is not always followed by a bump. SUNNYSIDE COAL BED The Sunnyside coal bed varies in thickness from a few inches in the west near Kenilworth, Utah, to as much as 24 ft (7 In) in a single bed in parts of the Sunnyside district. The coal commonly splits into two beds, with as much as 75 ft (23 m) of rock intervening. Both seams are exploited where multilevel mining is practical. The Sunnyside coal bed has been mined ex- tensively in the northern and central parts of the dis- trict, but most coal in the southern part of the district has not been exploited, and only a few prospect pits have been opened south of the Book Cliffs Mine (pl. 1). Clark (1928, p. 2) named the Sunnyside coal bed for the Sunnyside Mine, where the bed was first worked. He distinguished the “Lower Sunnyside” and “Upper Sunnyside” beds and traced the “Lower Sunnyside” from a point between the canyons of Soldier Creek and Coal Creek (T. 13 S., R. 11 E.) to Horse Canyon (T. 16 S., R. 14 E.) (fig. 1). During our field investigations, we found that the Sunnyside bed also extends southward continuously along the Book Cliffs beyond the Price River into T. 16 S., R. 15 E. In compiling the following discussion, we have drawn freely upon the works of Brodsky (1960) and V. H. Johnson (written commun., 1951), as well as upon our own work. ' The Sunnyside coal interval consists of locally thick coal accumulations interrupted at places by variably thick rock partings (fig. 59). Field studies have shown that no single parting persists throughout the lateral extent of the coal interval, and that most partings are open to the east and south. Data from drill cores indi- cate that partings are variable and of only local extent. It is therefore probable that peat accumulation never was simultaneously interrupted by inorganic sedimen- tation throughout the district, and consequently the “Upper” and “Lower” Sunnyside beds are not sepa- rate but are splits of the same bed (Brodsky, 1960; Maberry, 1971, p. 30). The lower coal split of the Sunnyside bed is about 3 ft (1 m) thick in outcrops in the canyon of Soldier Creek, about 16 miles (25.5 km) northwest of Sunny- side, and it thickens eastward to as much as 24 ft (7 m) in the Sunnyside N 0. 2 Mine. The upper split is not as extensive as the lower, but is discontinuously present from Dugout Canyon (T. 14 S., R. 12 E.) to a point southeast of Whitmore Canyon. The stratigraphic in- terval between the upper and lower splits is variable due to local fluvial conditions in the swampy area in which the coal-forming material was deposited (Brod- sky, 1960, p. 26; Maberry, 1971). During deposition of coal-forming materials, anastomosing streams flowed seaward across an area of low-lying coastal swamps and lagoons. FIGURE 59.—Sunnyside coal bed, cropping out in the Book Cliffs in sec. 26, T. 16 S., R. 14 E. Lower coal split is shaly and bony; 3-ft (1- m) sandstone parting at level from man’s waist to near his ankle; upper coal split is pure. Thick channel-fill sandstone directly over- lies the coal at this locality. Photograph by V. H. Johnson. ECONOMIC GEOLOGY 57 The streams cut channels and filled and abandoned them, cutting new channels at the same time, in the same manner as streams in present-day deltas. The channel-fill sand bodies became interconnected and formed multilateral and multistory channel-fill de- posits. These sand bodies, with some overbank de- posits of silt and organic debris, and surges of sedi- ment deposition locally interrupted peat concentration in the swamps. Trace fossils indicative of marine to brackish-water environments are found in sandstone bodies that we in- terpret to be channel-fill deposits of tidal inlets and delta distributaries overlying the coal bed. Micro- scopic examination of the coal shows that it consists of fragments and leaves of small plants and grasses. Pieces of large plants in the Sunnyside coal are rare. Invertebrate shell fossils which we found slightly above the Sunnyside coal bed are the brackish-water forms (Crassostrea, Anomia micronema Meek, Brachi- dontes, and “Corbula”), according to W. A. Cobban (written commun., 1968). The main Sunnyside coal bed overlies a thick marine-transitional sandstone. These lines of evidence suggest that coal accumulated in a backwater swamp and lagoonal area near the shore of the Late Cretaceous sea. Both open and closed partings occur in the coal. In open partings the coal beds join in one direction only; in closed partings the coal surrounds the rock. Closed partings may be lenticular pods of clastic sediment or they may be channel-fill deposits; open partings were formed by lateral migration of stream courses and by deposition of overbank deposits in a wedge of sedi- ment. Local thickness variations in the coal are due mostly to differential compaction and to channel-fill sand- stone bodies that cut the coal from above to various depths. These sandstone bodies occupy relict stream channels, abandoned as the coastal plain prograded seaward. In Utah only the Sunnyside coal is known to contain the qualities necessary for the production of metal- lurgical coke, and it is the most important source of coking coal in the Western United States (Averitt, 1966, p. G23). The Sunnyside coal produces a poor- quality, weak, granular coke owing to the lack of fusion between particles (Gray and Schapiro, 1966, p. 55). Consequently, the Sunnyside coal is blended with coking coal from other areas to improve the quality. The Sunnyside consists largely of attrital coal, al- though it contains some vitrain. The lack of fusion of the coke is due mostly to the low degree of meta- morphism of these vitrain particles. The coal contains 68—69 percent vitrain particles, and the degree of their metamorphism during coalification determines the ability of the coal to form coke (Gray and Schapiro, 1966, p. 55). Names for the rank of coal, such as lignite, bitumi- nous, and anthracite, are relatively familiar to most geologists. Names for types of coal are less familiar, al- though the type of coal is a significant criterion for de- termining the depositional environment of the coal. Some confusion exists as to the meaning of certain terms regarding type of coal, however, because terms are variously defined in different classifications. For clarity, the terminology of coal and its petrographic features are briefly reviewed below. The US. Bureau of Mines divides megascopic types of coal into (a) banded, consisting of “bright,” semi- splint, and splint coal, and (b) nonbanded, consisting of cannel and boghead coal and peat (Schopf, 1960, p. 28). Microscopically, coal consists of anthraxylon, attritus, and fusain (Parks and O’Donnell, 1956). Anthraxylon is derived from the woody tissues of plants, including both wood and bark, and forms the brilliant strips and lenses in coal. It appears in thin section as orange to red or brown bands and commonly shows well-pre- served cell structure. Attritus is a mixture of minute particles of vegetable debris and contains, among other materials, spores, pollen, seeds, cuticles, and resi- nous bodies. Attritus appears megascopically as dull gray bands and streaks. Fusain is soft and lusterless, resembling charcoal. It contains a high percentage of carbon with little hydrogen and oxygen. In thin sec- tion fusain is opaque, having distinct cell walls and fibrous structure. The terms vitrain, clarain, and durain are used in Europe for major coal types. Vitrain and clarain are closely analogous to “bright” and semisplint coal, and durain is similar to splint coal (Francis, 1954, p. 261). “Bright” coal, splint, and semisplint types are found in Sunnyside beds. According to Raistrick and Marshall (1939, p. 195): There is little doubt that whereas clarain resulted from the accumu- lation of vegetable débris derived from generations of plants which grew in situ, and so gave rise to the coal peat, the durain layers have been formed during periods when the peat was inundated by water. Then, instead of normal peat formation, there would be produced a mud of the finest plant and mineral debris resulting from the ac- cumulation of washed-in plant fragments and sediment, together with material derived from the coal peat. When the waters re- treated, normal peat accumulation was resumed. This view as to the origin of durain is supported by the high proportion of sedimentary mineral matter which is characteristic of that type of coal. Figure 60 shows a comparison. of some megascopic features of the Sunnyside coal, including brilliant bands of clarain and Vitrain (“bright”) material and dull fusain. Splint coal (durain) in the Sunnyside mines is ran- dom and discontinuous in the coal measures, and what 58 SUNNYSIDE COAL-MIN IN G DISTRICT, UTAH FIGURE 60.—Megascopic features of Sunnyside coal. Left piece is horizontally banded clarain and vitrain (brilliant bands) coal; it has a nearly vitreous luster. Right piece is “bright" coal on each end, with a band of fusain (dull) in the middle. Scale is in centi- meters (1 in. equals 2.54 cm). little there is probably formed as a result of fluviatile inundation, rather than by sea-level rise. STRUCTURES IN THE COAL Structures in the Sunnyside coal include banding and a particular type of cleavage known as “eye coal,” as well as cleavage, joints, and fracture zones. Banding in coal is due to alternating layers of different texture or composition. In the Sunnyside coal, banding varies from microscopic size to layers several inches thick. The origin of banding in coal is problematical. Some coal geologists believe that it has an environmental significance. White and Thiessen (1913, p. 29) at- tributed banding to changes in water level and to the influence exerted by bacterial solutions and oxygen content of the water on the rate of decomposition of plant debris. Davis (1946, p. 17) believed that banding was inherited from original differences in plant con- stituents. Lahiri (1951, p. 89—93) observed that banded structure is usually absent in low-rank coals such as cannel and lignite and concluded that banding is due to differentiation during compaction when constituents were separated on the basis of their chemical mobility and were segregated into bands. J. M. Shopf of the US. Geological Survey (oral commun., 1960), however, doubts that the cell structure could be preserved in the vitrain component if segregation during meta- morphism had occurred. The variations in thickness and extent of individual bands in the coal in the Sunny- side mines point to the conclusion that banding in the Sunnyside coal is due to original differences in com- position and in plant types that accumulated in the swamp during deposition. Eye coal derives its name from numerous smooth and shining, crudely equidimensional to elongate spots on nearly parallel cleavage planes (fig. 61). The size of individual eyes in the Sunnyside coal ranges from 1 in. (2.5 cm) to more than 6 in. (15 cm) in diameter. The eyes have no third dimension. The origin of eye-coal cleavage is not well under- stood. Stutzer (1940, p. 253) stated that it is the same as slaty fracture in rocks, which is produced in a direc- tion normal to the direction of pressure. It is possible also that eye coal may originate with shrinkage during coalification. Eye-coal cleavages were mapped in the Sunnyside No. 1 Mine (Osterwald, 1961, p. C349) and at coal outcrops (Osterwald, 1962a). Small structural features, including cleavages and vertical shatter zones, in the Sunnyside coal bed, simi- lar to those described in underground workings of the Sunnyside No. 1 Mine (Osterwald, 1962b), were meas- ured at the outcrop to determine the relationships be- tween these features and the joint and fault patterns. Cleavages, which commonly were the best defined of these small structural features, were mapped at many coal outcrops (pl. 2; Osterwald, 1962a; Osterwald and others, 1969). We were also able to map nearly vertical FIGURE 61.—Eye coal from the Sunnyside No. 1 Mine. Eye spots are smooth, and many are highly reflective. The eyes occur on cleavage planes in the coal. Scale is in centimeters (1 in. equals 2.54 cm). ECONOMIC GEOLOGY 59 shatter zones, which probably are important in the oc- currence of bumps in underground coal mines (Oster- wald and Brodsky, 1960) at a few localities. In general, where we could map cleavages at the outcrop, they were nearly parallel to northwest- and east-northeast- trending joints in nearby sandstone ledges, indicating that the cleavages were formed by the same stress sys- tem that caused jointing. The shatter zones also are nearly parallel to these same joint sets. ANALYSIS OF THE COAL Generally, the Sunnyside coal is high in volatiles and fixed carbon, low in sulfur and other impurities, and has a good heating value (tables 1, 2). Coke from the Sunnyside coal is high in fixed carbon, has low ash and very low sulfur contents, and has a relatively high heating value (table 3). Coke made from the Sunnyside coal yields about two-thirds its weight in furnace-size coke pieces, and these contain many lateral and trans- verse fractures (Averitt, 1966, p. G24). When the coal is blended with higher rank coals and coked at a fast rate of heating, the fusion of the coke is improved and the yield is metallurgical-grade coke (Gray and Schapiro, 1966, p. 71). RESERVE ESTIMATES Clark (1928, p. 100—103) estimated that the Sunny- side coal bed contained 1,811 million short tons (1.643><1012 kg) of coal in the Sunnyside Quadrangle. His estimate was based on the assumption that a coal bed extends laterally under overburden for approxi- mately the same distance that it extends along the out- crop, that it will have an average thickness the same as at the outcrop, and that it thins at the same rate under overburden as it thins along the outcrop. V. H. J ohn- son (written commun., 1951) estimated reserves of the Sunnyside bed in the Woodside Quadrangle to be TABLE 1.—Ultimate and proximate analyses of nine samples ofSunnyside coal [Modified from Clark, 1928, p. 83-86] Condition of coal Run-of—mine Dry (percent) (percent) Moisture ...................... 4.1—9.0 2.4—5.1 Volatiles ...................... 31 8—39 9 33.2—40.8 Fixed carbon .................. 47.7-52 7 48.7—54.3 Ash .......................... 4.7—8.2 4.9-8.5 Sulfur ........................ 46—1.73 .47—1.79 Hydrogen ..................... 5.0—5.7 4.7—5.6 Carbon ....................... 62.2—71.9 64.9—73.8 Nitrogen ...................... 1.25-1.6 1.3—1.6 11.8 22.9 10.8-20.1 0 13,0 0 13 4 TABLE 2.—Proximate analyses (in percent) of ranges of five samples of Sunnyside coal from the Sunnyside No. 1 Mine, and average of two samples of Sunnyside coal from the Colum- biaMine [R. F. Abernethy. US. Bureau of Mines. analyst (1958). Leaders (---), not applicable] Volatile Fixed Moisture matter carbon ASh Sunnyside No. 1 Mine As received .......... 1.8—2.2 36.4—40.1 55.7—58.4 2.1—4.9 Moisture free ........ 37.4—40.9 56.8—59.7 2.3—5.0 Moisture and ash free ............... 38.4—41.8 58.2—61.6 Columbia Mine fl As received .......... 2.25 40.5 59.2 7.6 Moisture free ........ 41.7 50.6 7 7 Moisture and ash free ............... 45.3 54.7 about 410 million short tons (3.7 X 10” kg) in beds more than 14 in. (36 cm) thick under less than 3,000 ft (900 m) of overburden. Johnson’s estimate was based on zones of relative confidence of thickness and extent of coal beds. Of his 410-million-ton (3.7X1011-kg) fig- ure, Johnson estimated 391 million tons (3.55X 10“ kg) in the category of measured reserves (BO-percent confi- dence), 15 million tons (1.4><10lo kg) in indicated re- serves (about 50-percent confidence), and 5 million tons (4.5x 109 kg) of inferred reserves (about 25-percent con- fidence). Kaiser Steel Corp. in 1965—66 drilled explora- tory holes in the Sunnyside Quadrangle on land that was completely unexplored for coal in Clark’s time, and cores recovered by that drilling program show that Clark’s concepts were approximately correct. Although a comprehensive drilling program would be necessary to prove coal reserves in the southern part of the district, Johnson’s estimate seems to be realistic because of the continuity and regular thick- ness variations of the coal bed. The main seam of the Sunnyside coal bed, for example, is about 15 ft (4.5 m) thick in the southernmost part of the Geneva Mine (sec. 24, T. 16 S., R. 14 E.) and is about 17 ft (5 In) thick directly updip at the outcrop, more than a mile (1.6 km) away. TABLE 3.—Averages of 20 proximate analyses of coke samples made from Sunnyside coal, Sunnyside and Columbia Mines [From Reynolds and others, 1946, p. 45] ff/ Number of samples Data True specific gravity .................... 6 1.90 BTU ................................. 20 1 2,946 Volatiles .............................. 20 11.84 Fixed carbon .......................... 20 188.33 Ash .................................. 20 19.83 Sulfur ................................ 20 ‘.82 |In percent. 60 SUN N YSIDE COAL-MIN IN G DISTRICT, UTAH GYPSUM A nearly horizontal bed of apparently pure gypsum crops out over large areas in the southwestern part of the district and in adjacent areas to the south and east. The gypsum, which is at least 10 ft (3 In) thick, is near the base of the upper part of the Carmel Formation. The gypsum bed is irregular, having large pinches and swells, probably as a result of differential hydration of anhydrite. The gypsum, cropping out in an area of gently rolling topography dissected by numerous small gulleys, weathers to a distinctive reticulate pat- tern in the soil as a result of minor differential leach- ing. Local ponding of surface-water runoff in small basins produces locally derived gypSum-rich silty and clayey soils (gypsite), which weather to similarly pat- terned surfaces. Several small prospect pits were opened in the gyp- sum in SE%, T. 18 S., R. 13 E. (unsurveyed), about 2 miles (3 km) south of the southwest corner of plate 1. Although a few tons apparently were removed, prob- ably for testing, no gypsum was commercially pro- duced. The deposit is about 15 miles (24 km) from the nearest railroad by a rough unimproved road. Gypsum also occurs at the top of the Summerville Formation and locally is as much as 50 ft (15 m) thick (V. H. Johnson, written commun., 1951). Our field in- vestigations showed that this gypsum is more silty than that in the Carmel, and it crops out mostly along steep canyon walls. Other thin beds, lenses, and veins of impure gypsum are common throughout the Sum- merville (fig. 7). N o prospect pits or other exploration works for gypsum in the Summerville are known in the district. WATER Potable water, from surface and underground sources, is a scarce and valuable commodity in the Sunnyside district. The only permanent streams in the district containing water suitable for human consump- tion are Range Creek and Grassy Trail Creek. Domes- tic and boiler water for the town of Sunnyside and for electric power in the mines formerly was obtained from Range Creek by an elaborate pumping and pipeline system that raised the water about 1,400 ft (430 m) to the crest of Patmos Mountain, then dropped it about 2,600 ft (800 m) to Sunnyside (Clark, 1922, p. 212). This system was replaced in 1952 by a dam and reser- voir in the upper part of Whitmore Canyon on Grassy Trail Creek, although the Range Creek pumps and di- version ponds were left in place and ruins of the pipe over the mountain remain as a standby system. Water from Price River is unfit for human consumption but is used as stock water and for irrigation water at Silvagni Ranch. During 1977 some water pumped from the Sunnyside N o. 3 Mine was used to supplement domes- tic supplies. The only other sources of potable water in the district are small springs near the base of sand- stones in the Colton and Green River Formations and in limestone beds in the North Horn and Flagstaff For- mations. Several large springs issue from the bases of alluvial fans near Horse Canyon and Whitmore Canyon. A few wells in surficial material contain potable water, mostly near Grassy Trail Creek. The alluvial-fan sand, silt, and gravel at the mouth of Whitmore Canyon is one of the most valuable sources of water in the region. Springs issue from the base of the alluvial fan or from the base of the pediment gravel beneath it at several places along the southern and western margins of the fan. Whitmore Spring (in- formally named), the source of Icelander Creek, half a mile (0.8 km) south of Dragerton, formerly was used as a source of industrial water. Other springs along the perimeter of the fan, such as Big Spring (informally named), about 4.3 miles (7 km) southwest of Dragerton (pl. 1), as well as shallow wells within the fan, are im- portant sources of domestic, irrigation, and livestock water for various ranches. Water from Big Spring for- merly was piped about 10 miles (16 km) to Cedar for use in railroad-locomotive boilers. The springs prob- ably are supplied by flow within the fan, which is re- charged from Grassy Trail Creek and from under- ground flow within the alluvial sand and silt of Whit- more Canyon. Coon Spring, at the southwestern edge of the alluvial fan at the mouth of Horse Canyon, about 1 mile (1.6 km) southeast of Cedar (pl. 1), is a valuable source of stock water. A few other small springs derived from underground flow within the Horse Canyon fan are also usable as sources of stock water. Water from all other springs within the lowland area west of the Book Cliffs contains large amounts of gypsum and alkali salts (V. H. Johnson, written commun., 1951). Water pollution is a serious problem in much of the Sunnyside district. Streams and springs in the Colton and Green River Formations generally are pure, except where locally polluted by livestock, and also are free from high concentrations of dissolved salts. Streams crossing the Mesaverde Group and older rocks, how- ever, rapidly acquire high concentrations of sulfates and alkali salts. Industrial operations also contribute large amounts of pollutants to Grassy Trail Creek, Ice- lander Creek, and to the intermittent stream in Horse Canyon. Although much wash water from the coal preparation plant at Sunnyside is recycled, as is much mine water pumped from the Sunnyside No. 3 Mine, large amounts go to the Sunnyside Mine dump (pl. 1), where additional organic and chemical pollutants are acquired that eventually flow into Icelander Creek. ECONOMIC GEOLOGY 61 Other water from the Sunnyside mines that contains much dissolved iron, as well as salts, is used for irriga- tion in the town of Sunnyside. Ground water in the alluvial fan at the mouth of Whitmore Canyon, which is a valuable source of domestic, livestock, and irrigation water, probably is prone to industrial, commercial, and domestic pollu- tion. Most of the town of Sunnyside, as well as Drager- ton and various outlying business establishments, is on the fan. Surface water bearing waste from these localities percolates easily into the alluvial fan and into the alluvium along Grassy Trail Creek which is en- trenched into it. The Sunnyside sewage-treatment plant also is on the fan; effluent from the plant is used to irrigate a golf course on the fan. Water percolating downward into the basal part of the fan from these varied operations probably mingles with the natural underground flow and eventually could contaminate springs along the fan margin. PETROLEUM-SERIES COMPOUNDS Deposits and occurrences of solid, liquid, and gaseous compounds belonging to the petroleum series are widely distributed in the Sunnyside district. None of these deposits and occurrences are of commercial value at the present time, but asphalt-impregnated sandstone was quarried from the northeastern part of the district for many years, and small quantities of oil are produced from the Grassy Trail field in the eastern part of T. 15 S., R. 13 E., about 8 miles (13 km) west- southwest of Dragerton (fig. 1). The variety and wide distribution of such compounds in the district have stimulated much exploration and several attempts at commercial extraction in the district. ASPHALT-IMPREGNATED SANDSTONE Several layers and lenses of asphalt-impregnated (or bituminous) sandstone in the upper part of the Colton Formation and in the lower part of the Green River Formation are known to crop out on steep west-facing cliffs (pl. 1). Asphaltic sandstones crop out near the crest of Patmos Mountain from the head of Pasture Canyon to the head of the left fork of Whitmore Canyon, a horizontal distance of about 12 miles (19 km) and in a vertical stratigraphic interval of about 1,000 ft (300 m); they probably extend much farther south and west (Holmes and others, 1948, fig. 2). The thickest deposits and the largest number of asphaltic layers, however, are on the south and west faces of Bruin Point (Holmes and others, 1948, figs. 1, 2), about 6 miles (10 km) northeast of Sunnyside. Bituminous material partly fills pore spaces in the asphaltic sandstone and amounts to as much as 13 per- cent by weight of the rock. Sandstone containing con- siderable amounts of asphaltic material is black when freshly broken but weathers to a characteristic medium gray. This weathering, however, extends only a fraction of an inch into the rock (Holmes and others, 1948). Richly impregnated sandstones, when exposed to hot summer sun, exude asphalt which can be ignited with a match and burns with a smoky orange flame. Asphaltic sandstone is resistant to erosion; it forms steep cliffs and commonly contributes boulders of richly impregnated sandstone to the alluvium along Grassy Trail Creek in Whitmore Canyon. We found a few of these boulders in pre—Wisconsin pediment gravels several miles from the mouth of Whitmore Canyon. The asphaltic sandstones near Bruin Point were quarried intermittently from 1892 to about 1950. Total production was estimated to be about 335,000 tons (3.04><108 kg) until 1948, nearly all of which was used for paving material (Holmes and others, 1948). All shipments contained more than 9 percent asphalt. The deposits were estimated to contain 1,600,000 cubic yards (yd3) or 1.2X106 cubic meters (m3), of which 900,000 yd3 (6.9>< 105 m3) was measured or indicated re- serves, and 700,000,000 yd3 (5.4X108 m3) was inferred (Holmes and others, 1948). About half of the total re- serves was estimated to contain more than 9 percent asphalt. Much of the asphaltic sandstone produced from the quarries on the south face of Bruin Point was trans- ported to the county road in Whitmore Canyon by a spectacular aerial tramway about 3 miles (5 km) long having a vertical drop of about 1,750 ft (530 m). The material was transferred to trucks at the lower end of the tramway and hauled to a railroad loading facility near Sunnyside. Several attempts were made by oil companies to mobilize the asphalt sandstone in place by injecting hot water or steam into long drill holes so that it could be pumped up other drill holes. At the time of our last examination in 1969, small quantities of asphalt were extracted, but the process apparently had not proved to be commercially feasible. OIL Several small shows of oil are known in the Sunny- side district. Several feet of oil-saturated sandstone core was obtained in 1959 from an exploratory hole drilled for coal in Pasture Canyon (pls. 1, 2), probably from a sandstone-filled channel in the Blackhawk For- mation above the Sunnyside coal seam (J. T. Taylor, oral commun., 1961). Oil began to seep into a mine opening from a channel-fill sandstone in the roof in 1964 during development work in the Sunnyside No. 3 62 SUN NYSIDE COAL-MINING DISTRICT, UTAH Mine (Maberry, 1971, p. 40—41). Although the seeping has long since stopped, enough oil oozed into mine workings to alert mining personnel to possible com- mercial exploitation. The oil probably was derived from organisms that inhabited the coal-forming marshes or swamps or from the carbonaceous sedi- ments that surround the coal seam and probably migrated as discrete particles into the porous channel sands above the coal after deep burial (Maberry, 1971, p. 40). Compaction of silts, clays, and muds adjacent to the sand-filled channels probably was greater than that of the stratigraphic intervals containing the chan- nels, hence the more porous sandstones became reser- voirs into which the oil could move after being expelled from adjacent finer grained sediments. Although the possibility of finding commercial quan- tities of oil in the Blackhawk Formation at Sunnyside probably is small, any future shallow exploration should be concentrated on sandstone-filled channel systems above the coal, especially where such sand- stones pinch out into less permeable mudstones (Maberry, 1971, p. 40). The oil in the Sunnyside No. 3 Mine is,not an isolated occurrence; similar incidents have ‘occurred elsewhere in the Book Cliffs Coal Field. One such locality in the Castlegate Mine north of Helper, Utah (fig. 1), leaked enough oil to make move- ment of haulage locomotives difficult (D. J. Varnes, oral commun., 1959). NATURAL GAS Methane is common in coal mines throughout the world, mostly as a result of gas exsolved from organic molecules in the coal. Concentrations of methane con- stitute a major explosion hazard to coal mining, and extensive ventilation systems are necessary to reduce concentrations in mine atmospheres to safe levels. Early miners in the Sunnyside mines noticed that con- siderable gas was evolved from the coal during mining (Taff, 1906, p. 295). Although average methane con- tent in the Sunnyside mines is now kept to 0.5 percent or less, local concentrations do occur that are above this level. Additional methane is added to the atmosphere in the Sunnyside mines from sandstones in the roof and floor. We observed methane bubbling upward through a pool of water at the bottom of the exhaust air shaft for the Sunnyside No. 1 Mine, in Whitmore Canyon, apparently coming from the thick sandstones of the Sunnyside Member below the Sunnyside coal bed. Large amounts of methane flowed into the Sunnyside N o. 3 Mine in 1966. These inflows, resembling smoke because of the different indices of refraction of air and methane, were easily visible with miners’ cap lights. According to miners, the gas came from a sandstone- filled channel above the coal, and was in sufficient quantity to cause evacuation of the mine (John Peperakis, oral commun., 1966). The large inflows ceased after a few weeks when the supply of methane in the channel was exhausted. URAN I UM Much of the district, particularly the western part, has been extensively prospected for uranium since 1943 (Johnson, 1959, p. 50). Most of this prospecting was done during the 1950’s, but hundreds of additional claims were staked during a minor “rush" in 1968. During the earlier prospecting period, as many as a few thousand claims for uranium were staked and many miles of access roads' built in, the western part of the Sunnyside district and in adjacent areas, probably be- cause the outcrop belt of sandstones of the Salt Wash Member of the Morrison Formation crosses the dis- trict around the margin of the San Rafael Swell (fig. 1, pl. 1). The Salt Wash and other members of the Morri- son Formation are favorable host rocks for uranium de- posits throughout much of the Colorado Plateau (for example, Finch, 1967, p. 39). Many of the claims prob- ably were staked because of locally increased radioac- tivity or because small amounts of secondary uranium minerals were found on outcrops. None of the claims we examined contained any visible uranium minerals, although we made no attempt to examine all the claims. Less than 100 tons (9X 104 kg) of ore apparently was shipped from the Rock Island group of claims north of the Price River, about 0.75 mile (1.2 km) upstream from its junction with Grassy Trail Creek (pl. 1), near the center of sec. 8, T. 17 S., R. 13 E., before June 1955 (Johnson, 1959, p. 51). This group of 13 claims was staked on several sandstone-filled channels in the Brushy Basin Member of the Morrison Formation, about 1,000 ft (300 m) north of a prominent east-trend- ing fault that dips steeply north. At the time of our examination in May 1971, radioactivity equal to about twice normal background was measured on a lens of iron-stained clay about 1 ft (0.3 m) thick and 15 ft (4.5 m) long, but no other radioactivity or visible uranium minerals could be found. Renewed prospecting for uranium in 1968 resulted in exploration for possible deposits in sandstone-filled paleostream channels in the Salt Wash, where they were thought to curve northward and northwestward around the north end of the San Rafael Swell (James Osburn, oral commun., 1968). This exploration also was stimulated by the fact that the Salt Wash is only 5 ft (1.5 m) thick on the west side of the swell (Gilluly, REFERENCES CITED 63 1929, p. 111) but is as much as 200 ft (60 m) thick on the east side. Johnson (1959, p. 35—36, 50), however, in- dicated that thick channel systems in the Salt Wash trend northwest, across the axis of the swell. Many of the claims staked during this exploration period were located on outcrops of Mancos Shale or on Quaternary units, in the hope that economic deposits could be found at shallow depths by drilling. Many holes were drilled along the flank of the swell northwest from the vicinity of Coon Spring to beyond the boundary of plate 1. No development work followed the drilling; hence we assume that the drilling was unsuccessful. METALLIC MINERALS We saw a few occurrences of metallic minerals in the district. Coarse-grained siltstones in the lower part of the Colton Formation northwest of the mouth of Bear Canyon (sec. 20, T. 14 N., R. 14 E.) contain small clots and veinlets of pyrite and a few scattered grains of a light-colored gray sulfide mineral. Field tests with HCl suggest that the light-colored mineral contains copper. The siltstone is moderate brownish gray in color, tightly cemented, and contains veinlets of calcite. A semiquantitative spectrographic analysis by J. C. Hamilton of the US. Geological Survey of green mud- stone from the Colton Formation near the summit of the Horse Canyon—Range Creek road (pl. 1) indicated that the rock contained 0.015 percent copper.1 The copper content of other mudstones from the Colorado Plateau ranges from 0.002 to 0.0051 percent (Newman, 1962, p. 418—426, tables 33, 34), suggesting that the Colton mudstone contains more copper than many similar rocks in the region. We found small amounts of sulfide minerals along a large fault about 1 mile (1.6 km) northwest of Cedar. Near the eastern end of the exposed trace of the fault, a shaft, now caved, was sunk in a light-tan con- glomeratic sandstone bed, probably part of the Dakota Sandstone. The shaft is about 20 ft (6 m) deep and has only a small dump. Conglomeratic sandstone in the up- thrown block of the fault is brecciated and silicified, is weakly impregnated with iron oxides and pyrite, and contains scattered grains of chalcopyrite, galena, and unidentified dark metallic minerals. About three-quar- ters of a mile (1.2 km) west of Cottonwood Creek, con- glomerates and sandstones of the Buckhorn Con- glomerate Member in the upthrown block of the same fault are in contact with the Brushy Basin Member of the Morrison Formation to the south, where a small ‘This value indicates only that the copper content was within a range of values. Such semiquantitative results are reported in percent to the nearest number in series such as 1, - 0.7, 0.5. 0.3, 0.2, 0.15, 0.1, which represent approximate midpoints of group data on a geo- metric scale. The assigned groups for semiquantitative results include the quantitative value about 30 percent of the time. prospect pit was sunk along the fault in sandstone of the Buckhorn. Near the fault, the sandstone is bleached white, locally stained with iron oxides, and brecciated. Black metallic mineral grains are scattered in the matrix of the rock and along small fractures. REFERENCES CITED Adams, W. W., and Geyer, L. E., 1944a, Coal-mine accidents in the United States 1941: US. Bureau of Mines Bulletin 456, 131 p. ——1944b, Coal-mine accidents in the United States 1942: US. Bureau of Mines Bulletin 462, 140 p. Allen, C. A., 1925, Coal mining in Utah: US. Bureau of Mines Tech- nical Paper 345, p. 1—11. Athearn, R. G., 1962, Rebel of the Rockies: New York and London, Yale University Press, 395 p. Averitt, Paul, 1966, Coking-coal deposits of the Western United States: US. Geological Survey Bulletin 1222—G, 48 p. Baars, D. L., 1966, Pie-Pennsylvanian paleOtectonics—key to basin evolution and petroleum occurrence in Paradox Basin, Utah and Colorado: American Association of Petroleum Geologists Bulletin, v. 50, no. 10, p. 2082—2111. Baker, A. A., Dane, C. H., and Reeside, J. B., Jr., 1936, Correlation of the Jurassic formations of parts of Utah, Arizona, New Mexico, and Colorado: US. Geological Survey Professional Paper 183, 66 p. Barnes, B. K., Dunrud, C. 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A Page Aberdeen Member, Blackhawk Formation 18 Abstract ............................... 1 Access roads .................... Air pollution . . ....... 39 Algae ......................... 11 Alkali salts . ................ 60 Alluvium ....................... 40, 42, 43 Bull Flat ................... 26 fan deposits ...... , . 32, 37 Wisconsin age. . ........... 37 Analysis, coal .. . . ......... 59 Anthraxylon . .. ................... 57 Anticlines .......................... 43 Aquarius Plateau .............. 27 Archeology ................... 5 Arroyo-cutting ........ 43 Artifacts, prehistoric . . . ........ 5 Ash content, coal .............. 59 Asphaltic sandstone .................... 52, 61 Attritus ............................... 57 B Back-swamp deposits ................. 20 Banded coal ...................... 57 origin .. . . 58 Basket-M aker culture ................... 5 Beaver Basin Formation . . . . 36 Beehive coke ovens .............. 55 Big Spring, water supply. . ..... 60 Bituminous coking coal ................. 2 Bituminous sandstone .. 24 Blackhawk Formation .................. 2, 17, 27, 28 oil potential .................... 62 structure .......................... 44 Bluecastle Sandstone Member, Price River Formation .............. 21,25 Blue Gate Shale Member, Mancos Shale ...................... 15 Boghead coal .................... 57 Bogs ........................... 20 Book Cliffs ........... 4 Book Cliffs Coal Co ............. 2 Book Cliffs Coal Field . ........ 1 Book Cliffs Mine ....................... 2, 52 Boulder deposits, pre-Wisconsin ......... 25 Bounces ......................... . . 55 Bright coal ............................ 57 Brushy Basin Shale Member, Morrison Formation ................ 12 Buckhorn Conglomerate Member, Cedar Mountain Formation ......... 13 Bumps ................................ 2, 55 C Carmel coal ............................ 57 Carbon County Railroad ................ 7, 55 Carlson Mine ..................... 52 Castle Gate, Utah ................. 1 Castle Valley ............ 5 Castlegate Sandstone ............ 17, 20, 27 faulting ...................... 49 Cedar Mountain Formation ............. 13 INDEX [Italic page numbers indicate major references] Page Cemented conglomerate ................. 28 Cephalopods ..................... 15 Chalcopyrite ..................... 63 Channel sandstone . .. 19 Chemical composition . . r ........ 8 Clarain ......................... 57 Clark Valley . 5 Climate ...... 5, 40-42 prehistoric ................. 6 Clinker ........................ 41 Coal ............... 52 analysis .................. 59 cleavage .. .......... 58 origin ........................ 57 Coal~mine bumps . ................ 55 Coke .................................. 55, 59 Colton Formation ................... 8, 23, 25, 27, 28 Columbia-Geneva Steel Corp ..... 52, 53 Columbia Mine ............... 2, 52, 53 Composition .. .. ....... 8 Concretions . .. .................... 15 Conglomerate .......................... 28 Coon Spring, water supply ..... 60 Copper ....................... 63 Cove, The ...................... . 28, 33 Cretaceous Blackhawk Formation ........ 2, 17,27, 28 oil potential ....................... 62 structure .......................... 44 Cretaceous Mancos Shale ............... 5, 15, 27 faulting .................. 43, 51 structure ................. 44 Cretaceous System . 13,22 Crinoids ...................... 9 Crumps ................ 55 Crustaceans ........................... 9 Cruziana facies, Mancos Shale .......... 17 Curtis Formation ....................... 9 D Dakota Sandstone ...................... 14 Defense Plant Corp, .................... 52, 53 Denver and Rio Grande Western Ry ...... 7 Diastrophism .......................... 40, 42 Dinosaurs .................. 11, 13, 22 Disasters .................. 55 Dumps ......... 38 Durain .............................. 57 E Early exploration ....................... 7 Early man ........................ 5 Economic development . ........... 7 Economic geology ................... 52 Efflorescent salts .................. 42 Entrada Sandstone ...... . . 8 Eocene Colton Formation ,. .. 8, 23,25, 27, 28 Eocene Flagstaff Lake .................. 24 Eocene Green River Formation 8, 23, 24, 25, 27, 28 Eocene Series .......................... 23 Eolian deposition, Entrada Sandstone ................. 9 67 Page Exploration ............................ 7 Eye coal .............................. 58 F Fault zones ............................ 43 Faults ................... 46 Ferns, fossils ........................... 15 Ferron Sandstone Member, Mancoa Shale ...................... 15 Fire hazard ............................ 39 Flagstaff Lake ......................... 24 Flagstaff Limestone ........... 22, 25, 27, 28 fossils . . ., ................ 23 Folds ....................... 43 Fontana, Calif, steel plants ............. 2 Fossils ................................. 23, 57 algae . . . . ........................ 11 cephalopods ....................... 15 concretions ........... 15 crinoids .................... 9 crustaceans ...... 9 dinosaurs .. ................. 11, 13, 22 -’ ferns .............................. 15 Flagstaff Limestone ............ . . 23 gastropods ........................ 13, 21, 23 leaf imprints ............... 134‘_ MW” orth Horn Formation ...... 23 2 Z ostracodes ............. 11 pelecypods . . . . ............ 8, 15, 21, 23 trace ................. 15, 17 Fusain ............................ 57 G Galena ................................ 63 Gas ............ 62 Gastropods . .. 13, 21, 23 Geneva Mine ....................... 2,52, 53 Geyser ............................... 52 Glaciation ............................. 41 Grassy Trail Creek, water supply ....................... 60 Grassy Trail Oil Field .. ............. 52 Gravel ............................. 29, 36 Green River Formation .......... 8, 23, 24, 25, 27, 28 Green River Lake .................. r . 24 Ground water ....... 52, 60, 61 Gullying ...................... 43 Gypsum ................. 51, 60 Mancos Shale ...................... 16 Summerville Formation ............. 9 H Harpole Mesa Formation ............... 26 Hazards ............................... 55, 62 High-volatile coal ...................... 2 Highways ...................... 5 Holocene alluvium ........... 38, 43 Holocene series ..... 37 Horse Canyon .................... 5 Horse Canyon Mine .................... 2,52, 53 68 l,J,K Icelander Creek, pollution ............... water supply . r . . Illinoian Glaciation . Introduction ......................... Joints ............ Jurassic system ........................ Kaiser Steel Corp ...................... Kansas Glaciation ...................... Kenilworth Member, Blackhawk Formation .. ....... structure .......................... Lake Uinta . . . Landslides . . . , Leaf imprints ................... Lithology .............................. Lower Mudstone Member, Blackhawk Formation .............. Lower Unnamed Member, Price River Formation .............. M Mancos Shale .......................... faulting . . . structure ......... Mantle, upland slope .. Mesaverde Group ..... Metallic minerals ............. Metallurgical coke .............. Methane ........ Mining ..... disasters . dumps ........ hazards ........ Mirabilite .................... . . Moab Tongue, Entrada Sandstone .. Morrison Formation .................... Mountain shots ........................ N,0 Natural gas ............................ Nebraskan Glaciation ................... Neslen Formation . . . Nonbanded coal . . . North Horn Formation . Ostracodes ............................. P Paleocene Flagstaff Lake ................ Palisade, Colo. .......... Paving material ....... 2, 3, 53, 59 26 18 44 24 13 19 21 24 52, 61 INDEX Page Peat deposits .......................... 57 Pediment gravels ................ 26, 41 Pelecypods, fossils ............... 8, 15, 21, 23 Petroleum ........ 52, 61 Physiography .......................... 4 Pictographs, prehistoric . .......... 5 Placer Creek Formation . .......... 29, 30, 36 Pleistocene Series ..... 25 Pollution ............... 39 Pounces ................ 55 Prehistoric man ........................ 5 Price River, water supply ............... 60 Price River Formation ............. 17, 20, 27, 28 Provo, Utah, steel plants. ., ........ 2 Pueblo culture .......... . , 6 Pyrite ................................ 63 Q, R Quarry, sandstone ...................... 52, 61 Quaternary System ..................... 24, 40 Railroads .............................. 7 Rainfall, prehistoric .................... 6 Reserve estimates ...... 59 Rio Grande Western Ry. ,, 7 Roads ................. 5 Roan Cliffs ................ 4 Colton Formation ...... 24 glaciation .......................... 42 Roan Plateau .......................... 5 Rock bursts ............................ 55 S Saltwash Sandstone Member, Morrison Formation ................ 11 San Rafael Swell ................... 11, 43 Sand ........................ 30 Sandstone quarry . . . .......... 52, 61 Sedimentation ..... 8 Semisplint coal ............... 57 Sevier Arch ................... , 13 Sheep Canyon ......................... 5 Silt ................................... 30 Slope mantle ., ........... 2.9 Splint coal .................... 57 Springs .............. 52 Star Point Sandstone ............ 18 Steel plants ..................... 2 Stratigraphy . . . .................. 7 Stream capture .................. 41 Structural geology ..... 43 Sulfide minerals ............... 63 Sulfur content, coal ...... 59 Summerville Formation ................. 9 Sunnyside coal bed ..................... 56 Sunnyside fault zone ................... 48, 51 Page Sunnyside Member, Blackhawk Formation .............. 19 structure . . ..................... 44 Swamps ............. 20 Synclines ........................... 43 T Talus deposits ......................... 37 Tar sands ...................... 24 Tavaputs Plateau .. ............. 4 Tectonism ......... 13, 40, 42 Terrace gravel ................ 36 Tertiary System ............... . 22, 23 Thenardite ............................ 16 Trace fossils . . 15, 17, 19, 57 Transportation ......................... 7 Tununk Shale Member, Mancos Shale ...................... 15 U, V Uinta Basin ..................... 4 Uncompahgre Plateau ........ . . . 5 Underground water ..................... 52 United States Steel Corp, .............. 2, 7, 52, 53 Upland slope mantle ..... 29 Uplift ................................. 40, 43 Upper Mudstone Member, Blackhawk Formation .......... . 19 Uranium ........................ 52, 62 Morrison Formation ................ 9 Utah and Pleasant Valley Railroad ...... 7 Utah Fuel Co. ......................... 7, 52, 53 Vanadium, Morrison Formation ......... 9 Vegetation . . . ................... 5, 32, 37 Vitrain ................................ 57 W, Z Wasatch Formation .................... 23, 24 Wasatch Plateau Coal Field ........ .. 1 Water ................................. 60 penetration, Mancos Shale .......... 16 pollution ...................... 39, 60 resources ............... 52 Weapon points, prehistoric ..... 5 Whitmore Canyon, development . 41 Whitmore Spring, water supply . . 60 Wisconsin alluvium ..................... 37 Wisconsin conglomerate ................. 29 Wisconsin Glaciation . . . 26 Woodard Mine ............... . . . 52 Woodside anticline ..................... 43 Zoophycos facies, Blackhawk Formation .......... . 18 Mancos Shale ...................... 17 n 0.5. 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A Comparison of Two Atlantic -type - Continental Margins By JOHN 5- SCHLEE GEOLOGICAL SURVEY PROFESSIONAL PAPER 1167 Stratigraphy and structure of the continental margin of eastern North America and West A frica—their history as restricted rift basins and later as broad troughs UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1980 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication Data Schlee, John Stevens, 1928- A comparison of two Atlantic-type continental margins. (Geological Survey professional paper ; 1167) “Stratigraphy and structure of the continental margins of eastern North America and West Africa—their history as restricted rift basins and later as broad troughs.” Bibliography: p. Supt. of Docs. no. : I 1916:1167 1. Continental margins—Atlantic coast (North America) 2. Continental margins—Atlantic coast (Africa) 3. Geology—Atlantic coast (North America) 4. Geology—Atlantic coast (Africa) I. Title. II. Series: United States. Geological Survey. Professwnal paper ; 1167. GC84.Z.N65S34 551.46’08’093 80-607047 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 CONTENTS Page Abstract 1 Margin south of Long Island _____________________ Introduction _ _ _ 1 Margin between Long Island and Newfoundland _____ Acknowledgments -1 _ 3 Margin north of the Newfoundland lineament _______ West Africa ______ 4 Discussion Structural elements of the region _____________________ 4 Similarities and dissimilarities in the structure of Division of the continental margin ____________________ 5 matching margins _ Transform margin _____________________________ 5 Stratigraphic sequences ____________________________ Divergent margin ______________________________ 7 Oceanic-margin ties _ North America 10 Divergent-transform margins _______________________ Structural elements of the region _____________________ 10 References cited _ Division of the continental margin ____________________ 11 FIGURE 1. 2. 3. S" ILLUSTRATIONS Index maps showing geography and continental margin bathymetry of eastern North America and West Africa ___________ Generalized geologic map of West Africa north of Liberia showing major structural framework _________________________ Map of the continental margin off Liberia and parts of Sierra Leone and Ivory Coast showing major oceanic fracture zones, in— ferred and mapped mafic intrusive rocks, coastal and shelf sedimentary basins, and the modifications of trend in bathymetry caused by the intersections of the fracture zones with the margin Cross sections and location map of a shelf basin off Liberia Cross section across the continental margin from West Sahara northwest to the eastern Canary Islands __________________ Schematic cross sections through the West African and eastern North American margins at the locations indicated in figures 2 and 8 Map showing structural and stratigraphic elements of the eastern North American continental margin and the adjacent western Atlantic basimCape Hatteras to Newfoundland _ Isopach map of J urassic(?) and younger sediments on the Atlantic continental margin, Chesapeake Bay to Newfoundland ____ Page 12 14 15 15 15 17 1'7 18 A COMPARISON OF TWO ATLANTIC-TYPE CONTINENTAL MARGINS By JOHN S. SCHLEE ___.__——— ABSTRACT The tectonic framework and, deposits of the northwest African and eastern North American continental margins are similar; they reflect the complex interplay of differential subsidence, buildup of carbonate deposits, deposition of elastic sediments, and local tectonism. Early history was marked by block faulting or downwarping to form a series of basins at the margin edge in which continental clastic sediments were deposited. Where climatic and oceanographic conditions permitted, these beds were covered by evaporite deposits and shallow-water limestone. Off Liberia, volcanic rocks and elastic sediments dominate, whereas further northwest (Guinea-Bissau, Morocco), evaporite deposits and limestones are also in the section; a similar sequence of evaporite de- posits and red beds of Late Triassic to Middle Jurassic age characterized the eastern Canadian margin. Salt forms diapirs, mainly ofl" Canada and West Africa. The evaporite deposits are interbedded with or are overlain by limestone and dolomite that formed platforms and reefs adjacent to the newly formed Atlantic Ocean. The reef complexes are discontinuous features that appear to have acted as partial sediment dams during the Jurassic and Early Cretaceous as the North Atlantic Ocean widened. For some margins, these platforms and reefs have provided the foundation to which later physiographic features of the shelf and slope broadly conform. Fine—grained elastic sediments from an- cient deltas overwhelmed the carbonate deposits during the Cretaceous to prograde the Atlantic margins seaward and build a gentle slope to the deep ocean. This slope was cut and steepened during the Tertiary and Quaternary. The intersection of oceanic fracture zones with the continent can have different structural formats, depending on whether the margin has undergone divergent or transform movement. In areas where transform motion is dominant, fracture zones intersect areas marked by faulting, intrusion, and shallowly buried Precambrian rocks; narrow fault basins can be tucked in the shelf-slope areas between the intersection of these zones with the continent. On divergent margins, areas of present margin offset seem to be related to ancient continen- tal zones of crustal offset and faulting, and the present offsets appear to be associated with zones of volcanism in the oceanic crust. These relations lend support to the idea that marked irregularities in the outline of the crustal break (leading to transform motion of crustal blocks) can lead to the formation of zones of weakness in the newly formed oceanic crust and then to the formation of seamounts and volcanic islands. INTRODUCTION In this paper, I examine the structure and stratigraphy of Atlantic-type continental margins (as described by Mitchell and Reading, 1969) along eastern North America and West Africa (fig. 1) to see the similarities and dissimilarities in patterns of continental-margin response to crustal separa- tion. Also of interest are the possible ties between oceanic and continental structures and the different effects divergent and transform motion have had on the tectonic framework of the margin. Since 1965, many offshore studies of West Africa and eastern North America (Beck and Lehner, 1974; Behrendt, Schlee, and Robb, 1974; Behrendt, Schlee, Robb, and Silverstein, 1974; Delany, 1971; Emery and others, 1975; Given, 1977; Institute of Petroleum and Geological Society of London, 1965'; Jansa and Wade, 1975a, b; Lehner and de Ruiter, 1976, 1977; Mattick and others, 1974; Schlee and others, 1974, 1976; Sheridan, 1974a, b; Uchupi and others, 1976; Van der Linden and Wade, 1975) have revealed much about the margin structure and stratigraphy. Major summaries of the regional geology ashore (Dillon and Sougy, 1974; Fisher and others, 1970; Van Houten, 1977; Zen and others, 1968) in the two areas have aided in a comparison of the margins. Broad geophysical surveys of the eastern Atlantic (Emery and others, 1975; Uchupi and others, 1976) have added much new data on oceanic crustal structure there. Stratigraphic well data have been compiled for the eastern Canadian margin (Jansa and Wade, 1975a) and have been correlated to reflectors on multichannel seismic- reflection profiles (Given, 1977); such correlation is underway for the US. Atlantic margin (Smith and others, 1976; Scholle, 1977). Multichannel re- flection profiles (Beck and Lehner, 1974; Lehner and de Ruiter, 1977 ; Schlee and others, 197 6; Grow 1 A COMPARISON OF TWO ATLANTIC-TYPE CONTINENTAL MARGINS s’o‘h 15°w 10°w 5°w / \ / \ S/ l 4000 m | (H ,3. 'D/rtzumm 514x 6 _ Q 0 ’ - : .' ._'_ 350N 0014/\ QL a a way + 070 m+' ' ' '\‘ 400 m 0 00 . / O flew/Egg 0 74.. Casablanca l) 0% G ' . \ I}? 4000 MM /‘~5 '\-—-\,_,a 2000 m _. K J A 1i -MOROCCO/? 0 + ' ’ //¢-|( — 30°N @Canary Islandé. 100!“ I// '5’ a . <9 319+———‘L ALGERIA 5’ , , St. Johns X 7 WEST I \ " , THE GRAND $900 .SAHARéJ \ BAN/(5‘ 0F " ~ {‘_—— \\ NEWFOUNDLAND | + __ _ 25°N I | \\ J ‘ \\ I’- ‘ \ ‘ l \ ‘ \ \ \ ___2 l\ f: '. + + l‘ + — 20°N . MAURITANIA ‘ .‘ l‘ M A L I "“\ l . \\ 7 \t/ ‘$~¥—_——+ //C15°N ) I CEEEGAL, // 0° \4 A /\ 11, \ x - \A) ) UPPER ' Irv-x / V 7 A , $5 . ~ 5 / VOLT E \\ x'f ; ~GUINEA )L ("-- GUINEA , . \_/- § (T ' ‘ ABysSAL_ 0 121352234" "‘“~,fr"‘\ + L) b +\/\}‘ - 10°N s / DH N \4 IERRA—fiIVORY §// 5 pLA’ $> “NEONENI S- COAST /\ o / a 3 "Tag. .I/ IRE-fill ‘ I] 9”“ E 7000 - ' . GICIANA a 0) ...... . - 0% f N + :""I.H£‘.OA.S.‘T )- 5°N ‘0 RY 945', V . .. x 1V0 V 000 m STATES .‘_ ., V0Ry_coA5 4 \/ <9 6* - \ 7 AREA OF FIGURE 3 0 ' 3. zoNE 11/ \\washingtglj,_: 00 m RoMANCHE FRACTURE gag/é: o 100 200 300 406 KIOMETS 7 Cape —%m% + — 0°N EECE . ' ”Hatteras SE / I | l l A B FIGURE 1.—Index maps showing geography and continental margin bath ymetry. A, Eastern North America. B, West Africa. Base modified from Uchupi (1971). and others, 1979) have allowed much to be in- ferred about deep structures beneath the shelves 01f West Africa and off the Eastern United States. In addition, refraction surveys (Grow and Sheridan, 1976; Keen and others, 1975), magnetic studies (Behrendt and Klitgord, 1976; E 1970; Haworth, 1975; Klitgord Taylor and others, 1968), and gravity data (Grow and others, 1975) have contributed to the picture of crustal-margin configuration. mery and others, and others, 1976; INTRODUCTION 3 20° 15°N 10° + FIGURE 2.-—Generalized geologic map of West Africa north of Liberia showing major structural framework. Com- piled from the Association of African Geological Surveys and United Nations Educational, Scientific, and Cultural Organization (1968), and Uchupi and others (1976, figs. 3 and 35). Locations of African sections (figures 5 and 63 and D) are indicated here. ACKNOWLEDGMENTS Many of my colleagues have contributed to this paper through their comments and discussions with me: W. P. Dillon, K. D. Klitgord, J. A. Grow, and J. C. Behrendt of the US. Geological Survey; Elazar Uchupi and K. 0. Emery of the Woods Hole Oceanographic Institution; and L. H. King of the Bedford Institute of Oceanography, Dartmouth, Nova Scotia. David A. Ross of the Woods Hole Oceanographic Institution and James E. Case and William P. Dillon of the US. Geological Survey read the paper and helped it to attain its final form. 4 A COMPARISON OF TWO ATLANTIC-TYPE CONTINENTAL MARGINS Kevin King and Michael Roy prepared the illus- trations. WEST AFRICA STRUCTURAL ELEMENTS OF THE REGION The major structural elements of West Africa (figs. 2 and 3) are the West African Shield, the Mauritanides, and the coastal basins that border the Mauritanides or West African Shield. Major structural features transverse to the margin are the Alpine fold belt of Morocco (fig. 2), a zone of intrusions associated with the Canary Islands, and faults through the Guinea marginal plateau (fig. 18). The West African ,, Shield consists of Precam- brian metamorphic rocks and granite, and it ex- tends from the Ghana-Ivory Coast-Liberia area north to the Anti-Atlas Mountains of Morocco (Dil- lon and Sougy, 1974) and westward to near the coast in a few places, although it is covered in some areas by platform deposits of Paleozoic age (fig. 2). The main outline of the West African Shield had achieved its present size and shape by 1,000 my. (million years) ago. Subsequent epeirogenic move- ments have warped it to create the Reguibat uplift of northwest Africa, and the Anti-Atlas Mountains of Morocco and Algeria. Adjacent to these uplifts, the shield is covered by varying thicknesses of ma- rine platform deposits of late Precambrian and Paleozoic age in several broad basins (Dillon and Sougy, 1974; Association of African Geological Surveys and United Nations Educational, Scientific, and Cultural Organization, 1968). Adjacent to the shield and its associated basins 1 4°W 1 2°W 10°W 8°W . H - . l I . .'- L, Freetown a. “$0 % '. O ‘3 I a I 8°N — 7000 3 o. l, ’6 a. ‘. ' .‘ '. ". SHERBRO ”3’ a. '._ '°. ISLAND % —' é ‘— 6°N 4- EXPLANATION — _ Oceanic fracture zone ’— lnferred and mapped mafic — intrusive rocks _ i _ Inferred faults; arrow shows direction of fault-plane dip Coastal and shelf sedimentary % basins 4°N — ; ."”4000 m": Depth below mean sea level, ' given in meters ST. PAUL FRACTURE ZONE ——I’ 0 100 L l 200 300 KlLOMETERS l | FIGURE 3.—Continental margin of Liberia and parts of Sierra Leone and Ivory Coast showing major oceanic frac- ture zones, inferred and mapped mafic intrusive rocks, a1 and shelf sedimentary basins, and the modifications inferred faults and the direction of fault-plane dip, coast- of trend in bathymetry caused by the intersections of the fracture zones with the margin. Area of figure is shown in figure 13. WEST AFRICA 5 are the Mauritanides, a belt of discontinuous folded mountains that crop out from Algeria in the north to Sierra Leone in the south. Sediments as young as Late Devonian in age were deformed during the Hercynian orogeny (Carboniferous); the presence of volcanic detritus and ophiolites caused Dillon and Sougy (1974) to speculate that the Maurita- nides may have been part of a middle Paleozoic subduction zone, prior to the Hercynian orogeny. In the south, the Mauritanides split into two branches (Dillon and Sougy, 1974); the western branch goes along the southern side of the Mauri- tania-Senegal basin and the other goes into Sierra Leone. North of the South Atlas fault (fig. 2), the Her- cynian orogenic belt continues in the High Atlas and the Moroccan Meseta (fig. 2), where thick eugeosynclinal sediments were folded and thrust in an orogenic wave that moved east during the Devonian and Carboniferous. In contrast, miogeo- synclinal sequences make up the Mauritanides south of the South Atlas fault. West Africa and its margin are crossed by two major structural lineaments that divide the area into three subregions, both physiographically and tectonically. In the north, the South Atlas fault and the Alpine fold belt associated with it separate the fragmented crust (basins and adjoining mesetas) to the north from the series of seaward-opening coastal basins to the south (fig. 2). The same zone also marks a change in width of the shelf from an average 54 km in the south to 37 km in the north. South of the Mauritania—Senegal basin (fig. 2), the coastal area and margin are characterized by fault- ing, arching, and intrusion near the intersection of the Guinea fracture zone. A zone of mafic intrusive rocks (dolerites) trends northeast from the coast of Guinea (fig. 2). The South Atlas fault (a right-lateral fault) is the latest expression of an ancient structural zone of weakness along which movement has taken place from the early Paleozoic to the present age (Rod, 1962). It juxtaposes eugeosynclinal rocks of early Paleozoic age and miogeosynclinal rocks of the same age. Indeed, the fault area may well have been a shelf edge during the early to middle Paleozoic (Dillon and Sougy, 1974). In the Tertiary, the fault zone formed the southern limit of orogenic move- ments that created the Atlas Range, Rif Mountains, and Tell Range of Morocco and Algeria. Rod (1962) speculated that the fault zone continued offshore toward the Canary Islands. The transverse struc- tural zone off Guinea shows up as (1) an offset of the continental margin of about 230 km (Krause, 1964) in an east-west direction, (2) a narrowing of the Mauritania-Senegal basin against a broad east— trending basement rise thought to underlie the southern edge of the Guinea marginal plateau (Lehner and de Ruiter, 1976) near the margin ex- tension of the Guinea fracture zone, and (3) a change in the pattern of magnetic anomalies (Mc- Master and others, 1971) under the shelf off of where the northeast-trending zone of mafic in- trusive rocks ashore intersects the coast. South of Guinea (fig. 13), in Sierra Leone, Liberia, and the Ivory Coast (fig. 3), the West African Shield ex- tends to the coast, where it is bordered by narrow sedimentary basins (either beneath the Continental Shelf or on land). Offshore, the West African margin (fig. 13) con- sists of several kinds of Atlantic—type margins be- cause the continent has been involved both in trans- form motion relative to South America and in crust- al divergence away from North America. The transform motion took place along the southern part of West Africa (Liberia, Ivory Coast, Ghana) and resulted in the rifting of the West African Shield. DIVISION OF THE CONTINENTAL MARGIN TRANSFORM MARGIN The Liberia-Ivory Coast part of West Africa (figs. 3 and 4) was the northernmost area affected by the separation of Africa and South America during the Early Cretaceous. Oceanic fracture zones approach the coast at low angles, subparallel to the trend of the southern bulge of West Africa. Their orientation to the coast suggests an oblique ' separation of the two crustal blocks. How has this oblique separation affected the shape of the coastal basins, and what are the ties between the equatorial oceanic fracture zones and coastal geology? The northernmost of the equatorial fracture zones associated with the translation of Africa past South America is the St. Paul fracture zone, a com- plex of three zones (Behrendt, Schlee, and Robb, 1974) that intersect Liberia and the Ivory Coast near Cape Palmas. Where they intersect the coast, the margin is faulted in a series of broad crustal blocks that are tilted seaward. The blocks trend east, appear to be intruded, and underlie much of the Continental Slope as broad ramplike steps which strike parallel to the fracture zone and deepen toward the west. The Continental Shelf adjacent to Cape Palmas is probably Precambrian crystalline rocks covered by a thin veneer of sediments 6 A COMPARISON OF TWO ATLANTIC-TYPE CONTINENTAL MARGINS A S N Seafloor 0 jg LL? ____—_- ,_ z 9— _ 2 45/2: I— I ’6" d81- ’7 _ is n: :s g; 2- _ E 31 _ 0 rx—I 0') 2 Sea floor 20 KILOMETERS VERTICAL EXAGGERATION OF TOPOGRAPHY ABOUT X10; SUBSURFACE REFLECTORS X5 OR LESS 100 KILOMETERS FIGURE 4.—Cross sections and location map of a shelf basin off Liberia. Note the block—faulted basement and the sea- ward-thickening prism of sediment. The Modified from Schlee and others (1974). generally less than 0.5 km thick. Onshore near Cape Palmas, a few mafic intrusive bodies cut isoclinally folded Eburnean paragneisses (~2,000 my. old) which strike east-northeast, subparallel to the trend of the intersecting fracture zones (Behrendt, Schlee, and Robb, 1974; Behrendt, Schlee, Robb, and Silver- stein, 1974) ; in addition, coastal metamorphic rocks are faulted in the area where the St. Paul fracture zone is inferred to intersect the coast. The other main tectonic trend in coastal Liberia is northwest as shown by the mafic intrusive rocks (fig. 3) that range in age from 176 my. old to 192 my. old. These diabase dikes and sills were intruded parallel to the magnetic grain created by the Pan African thermotectonic event (~55O m.y. ago; Hur- ley and others, 1971); rocks metamorphosed in that event are found in a narrow belt along the coast (White and Leo, 1969). An aeromagnetic survey of Liberia (Behrendt and Wotorson, 1970), revealed the mafic rocks to be associated with two broad northwest-trending belts of —50— to —150—gamma linear anomalies, one in the coastal area and shelf, and the second associated with dikes, 90 km inland. Tensional forces connected with crustal rifting and the regional grain of Precambrian crystalline rocks appear to have affected the trend of mafic rocks intruded during the separation of crustal blocks (May, 1971). Shelf basins adjacent to the intersection of the St. Paul fracture zone and the margin are elongate narrow features that open in a seaward direction over a block—faulted basement beneath the slope. To the east, the Ivory Coast basin (Spengler and Del- teil, 1966; Arens and others, 1971; Lehner and WEST AFRICA 7 de Ruiter, 1977) trends east parallel with the coast (fig. 13) ; the basin is bordered on the north by a major east—trending coastal fault (inner shelf con- tinuation of St. Paul fracture zone) and on the south by the Romanche fracture zone (figs. 13 and 3). Beneath the Ivory Coast Shelf, Cretaceous and Ter- tiary sediments more than 5 km thick form a south- plunging monocline that thickens under the Con- tinental Slope and thins seaward of the slope. Northwest of Cape Palmas, another sediment— filled shelf basin also parallels the coast (fig. 3). Off central and northwest Liberia, it is built over an irregularly faulted basement (fig. 4) of presumed crystalline rocks, Paleozoic sedimentary rocks, and volcanic rocks (Schlee and others, 1974). The basin is faulted seaward, and at least as much or more sediment is under the rise and lower slope as is under the shelf. As can be seen from figure 4A, seismic waves passing through sediment in the coastal basin have two—way travel times of about 2 s; if the aver- age velocity is assumed to be 2 km/s, this sediment is 1—2 km thick. The shelf basin extends northwest to the Liberia—Sierra Leone border; off southern Sierra Leone, two smaller isolated basins have been out- lined by McMaster and others (1975, fig. 6). Jones and Mgbatogu (1977) inferred that Cretaceous and younger sediments are 2 km thick west of Freetown (fig. 3). The Sierra Leone—Guinea area bridges the change from transform motion in the south to divergent motion in the north (described in the next section “Divergent Margin”). Ashore, it is intruded by a northeast-trending belt of intrusive rocks, and off- shore, it is marked by east—west offsets of the Con- tinental Slope (see the 200- and 2000-m isobaths, figs. 2 and 3) where the Sierra Leone and Guinea fracture zones intersect the margin (McMaster and others, 1973, McMaster and others, 1975). McMaster and others (1971) postulated that at least two nor- mal faults are transverse to the margin; one of these faults intersects the shelf edge near where Krause (1964) postulated that the Guinea fracture zone projects along the slope (fig. 2). Lehner and de Ruiter (1976, 1977) inferred that‘at least 6 km of sedimentary rocks (Paleozoic and younger) is above an irregular Precambrian basement (acoustic ve- locity of 6.1—6.5 km/s) beneath the Guinea mar- ginal plateau; the basement rises to within 3 km of sea floor beneath the slope south of the plateau. An extensive carbonate platform covered the pla- teau during the Mesozoic. . To summarize, in the Liberia-Ivory Coast part of West Africa, the main responses to crustal separa- 7 tion of a shield area where Africa and South Amer- ica moved away from each other at a low angle were: (1) the formation of narrow coastal and shelf sedimentary basins adjacent to the slope areas between fracture zones; they are built over block-faulted shield rocks and thicken in a seaward direction; (2) faulting and the intrusion of mafic rocks parallel to axes of the basins; and (3) the formation of a structurally complex ridge of tilted fault blocks and intrusions where extensions of the ocean fracture zones intersect the coast. The struc- tural response of the Liberia-Ivory Coast area to rifting appears to lend support to Kinsman’s (1975) concept of what a margin subjected to trans- current separation should be like. The margin is narrow, is occupied by narrow basins, and lacks a thick postrifting sedimentary wedge. DIVERGENT MARGIN North America and West Africa, north of Sierra Leone, have been diverging from each other during the past 180 my. In response to the separation, several coastal basins (50—400 km wide) have formed; these basins contain Cretaceous and Ceno- zoic sediments deposited over a block-faulted crust of Paleozoic and older rocks (Aymé, 1965; Spengler and others, 1966). A map of two-way travel time of seismic waves to basement (fig. 2) shows that postrift sediment is in linear basins beneath the West African Shelf and Slope and that two-way travel time through this sediment increases seaward to more than 4 s. Data used to draw basement contours were compiled by Uchupi and others (1976) from published literature for land isopachs and from their marine survey for offshore isopachs. Land data can be converted to thickness in kilometers by multiplying time figures by 3 km/s. This computation indicates that thicken- ing exists underneath the seaward parts of the basins. Sediment thickness follows, in a general way, basin outcrop pattern; the thickness is less where the shield areas approach the coast or inter- sect it (as off Guinea-Bissau). The trend and values of the contours change significantly at zones of crustal weakness (extension of the South Atlas fault) or where oceanic fracture zones approach the coast. The most pronounced change is in the vicinity of the Canary Islands, a zone of weakness along which Tertiary volcanic rocks were extruded (fig. 5). Adjacent to this zone and north of it, coastal basins are more restricted, the continental margin (particularly the shelf) is narrowed, and the onshore geology is characterized by block-faulted 8 A COMPARISON OF TWO ATLANTIC-TYPE CONTINENTAL MARGINS West Sahara U) 'U C z 2 E’ 1’ 8 NW ATlANTIC E 5 00 n EAN KILOMETERS 8 E o F“ n' .71- 10 . 1‘1‘/|/. ~ -\,i~ \ I l \ I- -\,| ‘1'? [\i’. 200 ./." r"‘ \-f | l '— KILOMETERS EXPLANATION Tertiary Mesozoic - Oceanic basement basalt Continental basement Tertiary volcanic rocks FIGURE 5.—Cross section across the continental margin from West Sahara northwest to the eastern Canary Islands. Taken from Beck and Lehner (1974). Location of profile is shown on figure 2. basins (of Late Triassic and Early Jurassic age) and basement blocks (Van Houten, 1977). Along the offshore extension of the South Atlas fault, in- truded faulted continental basement rises in a manner similar to the continental basement south of the Guinea marginal plateau (Lehner and de Ruiter, 1976). Sedimentary rocks within the coastal basins (figs. GB and D) are mainly continental clastic rocks of Mesozoic and Cenozoic age toward the east; they change to marine shale, limestones, and evaporites marked by conspicuous unconformities and diapirs (Dillon and Sougy, 1974; Aymé, 1965; Templeton, 1971) in the west. The salt is inferred to be Triassic or Early Jurassic in age (Uchupi and others, 1976; Beck and Lehner, 1974), and it intrudes Cretaceous and younger strata in the southern part of the Mauritania-Senegal basin, and off Morocco and West Sahara. On land, outcrops of Late Triassic age are red beds of conglomerate, sandstone, and mudstone as- sociated with some salt. They are interlayered with basalts in the upper part and are restricted mainly to Morocco and Algeria (Van Houten, 1977); drill. ing in West Sahara has penetrated evaporite de- posits of Triassic age. Dolerites intruded between the Carboniferous and Late Triassic are associated with the West African Shield (Mauritania, Mali, Guinea, Ivory Coast). In the northern basins, the sedimentary section changes from continental red beds of Early J uras- sic age to marine carbonate and evaporite deposits of Middle and Late Jurassic age (Dillon and Sougy, 1974). Jurassic and younger limestones formed carbonate platforms off Senegal and Guinea; during the Jurassic, the platforms flanked evaporite basins (Lehner and de Ruiter, 1976), but the banks per- sisted into the middle Cretaceous, unlike those flanking the basins further north. The lower Ter- tiary sedimentary rocks in Senegal are thin beds of phosphatic glauconitic limestone, chert, and shale. Off part of Morocco, effective deposition ceased after the Eocene or Oligocene time (Summerhayes and others, 1971) because of upwarping which shifted the loci of deposition to the slope and rise. The main response of the West African margin (north of Sierra Leone) to crustal divergence is a central zone of coastal basins built over a block- faulted basement of deformed Paleozoic rocks; broad highs of Precambrian crystalline rocks sepa- rate these depocenters. The basins range widely in their width and appear narrowed where the West African Shield and Mauritanides are close to the coast. North of the Canary-South Atlas lineament, the margin is intricately faulted and deformed (figs. 3 and 6) in the part of Africa that has inter- acted with the European plate. In the south off Guinea, the change from a divergent margin to one affected by transform motion is marked by a nar- rowing of the basins, a lessening of subsidence, the absence of evaporite sequences, and a rise in the continental basement. WEST AFRICA N KILOMETERS s 0—. ‘4le L" \“zz-n'. ' I/<_ _i a a 6? 5 Lu 3 5E LIE-5 9 THICK- DESCRIPTION 33 E 8 S ,5 FORMATION g “£25.28 Bedding terminology after McKee and Weir (1953) as U’ 5 u.i § tn ._ RS modified by Ingram (1954) u.i ‘0 O 3 m u. in Z 0 PUV'SIma Formation Very thick bedded yellowish-gray tuffaceous and :l 2 150+ diatomaceous siltstone with thick interbeds of _°' 3 _-,_ bluish-gray semifriable andesitic sandstone .9 ' E c o .9 g 8 § % Medium- to thick-bedded and faintly laminated pale- ._ o yellowish-brown siliceous mudstone with scattered E '0 Santa Cruz Mudstone 0—2700 spheroidal dolomite concretions; locally gradesto g E sandy siltstone a. c D .‘E I: ‘5 Lu » . . . 2 E . Very thick bedded and thickly crossbedded yellcwvish- U-l Santa Margarita Sandstone 0.130 gray to white friable arkosic sandstone 8 Uncanform/ty S a: g 5: Medium- to thick-bedded and laminated olive-gray 2 -— . subsiliceous organic mudstone and sandy siltstone 2 é’ M°"“’"’Y F°"“at'°" 81° with few thick dolomite interbeds (I! E ._ Th‘ck-b dd d to s' e ellowish- a a ko ic 2 Q Lompico Sandstone 50.240 s'andsfonee mas W Y gr y r s Unconformab/e on Butano and — . . _, _ . . , : underlying racks 185 Thin- to medium-bedded and faintly laminated olive- a g gray to dusky-yellowish-brown organic mudstone u, a, Lambert Shale ; . . . U Thick-bedded to massive yelIOWish-gray arkOSlC _ 350+ sandstone; contains a unit, as much as 60 m thick, of I“ 5 Vaqueros Sandstone pillow-basalt flows , "24 'E // Thick- to very thick bedded yellowishorange arkosic 0 g 550 sandstone with thin interbeds of green and red 8 g '3 Zayante Sandstone siltstone and lenses and thick interbeds of pebble and :l 8 cobble conglomerate o .9 A . . . . . . . . g ;\~ : 8 Rice: Mudstone 275 Masswe medium—light-gray fine-grained arkosic sand- __ 5 g a fig Member stone 5 3 c 3 Twobar Shale Member 60 Very thin bedded olive-gray shale g t? SE a, 26 : Z . . . . I“ 8 ——— Thin- to very thick bedded medium-gray arkosw z :3 c 0 Upper sandstone 980 sandstone with thin interbeds of medium-gray Lu 3 c member siltstone o .2 9, O E in W 5 E 8 —_:— o i Thin- to medium-bedded nodular olive-gray pyritic £13 5 Middle siltstone member 75—230 siltstone E ‘5 ‘1’ m v th'kbdddt‘ ' ll 'h k‘ ery ic e e o masswe ye OWIS -gray ar osic I-I-I Lmzesgigistone 460+ sandstone with thick to very thick interbeds of sandy E g c pebble conglomerate in lower part 0 E 8 § 5 —- Not in contact within area — _i E >- i . . Nodular olive-gray to pale-yellowish-brown micaceous E n. Locatelli Formation J 270 siltstone; massive arkosic sandstone locally at base Unconformab/e an crystalline complex of Ben Lomond Mountain FIGURE 2.—Composite stratigraphic section of Tertiary rocks of the central Santa Cruz Mountains northeast of San Gregorio fault. STRATIGRAPHY NORTHEAST OF SAN GREGORIO FAULT . 9 Megafossils are locally common in the basal sand— stone beds of the Locatelli Formation and are listed on table 1. Cucullaea mathewsonii, Perissolax tricar- natus, Pholadomya nasuta, T urritella infragranulata, and T. pachecoensis have all been recorded by Weaver (1953, p. 28—29) from his Vine Hill Sandstone of the typical Martinez sequence in Contra Costa County and together with Cidaris martinezensis are con- sidered on the Pacific Coast as diagnostic of a Paleo- cene (“Martinex”) age. Associated with these Paleocene megafossils in the basal sandstone beds is a predominantly calcareous foraminiferal fauna, whereas the superjacent siltstone section is barren of megainvertebrates and yields foraminiferal assemblages that are almost exclusively arenaceous (Clark 1966a, table 2; Brabb and others, 1977, table 5). The Locatelli contains such characteris- tic early Tertiary benthonic species as Anomalina regina, Dentalina alternata, Pseudouvigerina naheo- lensis, Robulus aff. R. midwayensis, and Vaginulina suturalis. Most of the arenaceous foraminifers in the siltstone section range from Late Cretaceous into the early Tertiary. Ammodiscus glabratus, Haplophragmoides coronata, and H. excavata have their highest reported occurrence in the Bulimina escavata Zone of the Ynezian Stage (Mallory 1959, p. 26). The upper strati— graphic range of these arenaceous species together with the lower range of many of the calcareous taxa suggests an Ynezian Age for the Locatelli Formation. A more restricted zonal assignment is uncertain because of conflicting range data; however, the simi- larity of the fauna to that of the Vine Hill Sandstone TABLE 1.-—Paleocene megainvertebrates from the Locatelli Formation [USGS localities are listed under “Fossil Localities") USGS Cenozoic loc. Fossil 5 3 8 g 8 {D ‘9 ‘0 O O <' V V lfl I!) E E E 2 E Gastropods Perissolax tricarnatus Weaver ....................... Surculities(?) sp ................................. Turritella cf. T. infragranulata G Turritella pachecoensis Stanton . Turritella cf. T. pachecoensis Stanton Pelecypods Acila sp ..................................................... . Cucullaea mathewsonii Gabb. Cucullaea mathewsanii Gabb( Glycymeris sp.. Lucinomu sp Nuculana sp Pholadomya nasuta a Pinna n. sp ....................... Scaphopod Dentalium sp ................................................ Echinoids Cidaris martinezensis Kew X X Periaster n. sp. Pericosmus sp ...... echinoid spines... Arthropod Raninoides sp ............................................................................................. X , .. XXX X suggests an assignment to the Silicosigmoilina cali- fornica Zone. Planktonic foraminifers identified by Berggren and Aubert (1977, p. 2—3) from the Locatelli Forma- tion include Subbotina triloculinoides, S. triangu- laris, S. velascoensis, Acarinina mckannai, A. acari- nata, and Morozovella aequa and are diagnostic of late Paleocene (P4—P5 Zone) age. Of stratigraphic significance is’the association within the same sand- stone bed (USGS loc. M4669) of late Paleocene planktonic foraminifers with Ynezian Stage benthonic foraminifers and Cucullaea mathewsonii, a so-called Martinez Stage mollusk. Paleontological evidence indicates that the Loca- telli Formation was deposited under progressively deepening marine conditions. The basal sandstone beds, locally containing abundant echinoids and thick-shelled mollusks, were deposited in a neritic environment. The foraminiferal assemblages of these beds are characterized by a diversity of nodosariids with a relatively large number of species but few individuals of each. This diversity together with a paucity of miliolids, a group common in warm shallow water, suggests that the sandstone beds were deposited at neritic depths probably greater than 30 m. This depth estimate is supported by the local abundance of the pelecypod Cucullaea, a genus that today in the Indo—Pacific province commonly occurs between depths of 17 and 170 m (Nicol, 1950, p. 343). The presence of Pinna and species of Turritella and the diversity of the foraminiferal assemblages suggest tropical to subtropical temperatures. The superjacent siltstone beds of the Locatelli Formation, which are characterized by arenaceous ’ foraminifers, were deposited in deeper water. The development of this arenaceous assemblage may have been favored by muddy bottom conditions, and the paucity of calcareous benthonic foraminifers suggests cold-water conditions, such as might have existed at bathyal to abyssal depths. Dominantly arenaceous assemblages are found living at such depths today, as for example in the Peru-Chile trench, where arenaceous forms compose over 90 percent of the population below 3,000 m (Bandy and Rodolfo, 1964, p. 833), and in the Gulf of California, where arenaceous specimens average 85 percent of the population at depths greater than 2,750 m (Phleger, 1964, p. 389). The presence of Glomospira charoides, Haplophragmoides coronata, and Hyper- ammina cf. H. elongata, species that today favor a bathyal to abyssal habitat, also supports a deep- water interpretation for the siltstone beds. Pelagic globigerinids, although rare throughout the forma- tion, indicate the existence of open-sea connections. 10 STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA EOCENE TO LOWER MIOCENE SEDIMENTARY SEQUENCE The Eocene to lower Miocene sequence consists of sedimentary rocks ranging in age from Penutian to Saucesian, which are poorly exposed in the northern part ofthe Felton quadrangle and locally exposed on the south slope of Ben Lomond Mountain. The sequence includes the Butano Sandstone, the San Lorenzo Formation, the Zayante Sandstone, the Vaqueros Sandstone, and the Lambert Shale. BUTANO SANDSTONE To the massive sandstone beds exposed on Butano Ridge, 11 km north of the area, Branner, Newsom, and Arnold (1909) gave the name “Butano Sand- stone.” Subsequent field mapping, supported by micropaleontology, has demonstrated that the Butano Sandstone is more extensively distributed in ‘ the central Santa Cruz Mountains than shown in the Santa Cruz folio, and the beds mapped as Butano in this report were assigned to the Vaqueros Sandstone by those early workers. The Butano Sandstone that crops out in the struc- turally complex northern part of the Felton quad- rangle is here subdivided into three informally named members: LOWER SAN DSTONE MEMBER The lower sandtone member crops out in the northeastern part of the area south of the Zayante fault, where best exposures are along the upper part of Bean Creek and along Zayante Road. This sand— stone is very thick bedded to massive, very light gray to yellowish gray, poorly to moderately sorted, medium to coarse grained, and arkosic. The terrig- enous mineral composition of this sandstone as determined by point count (200 points per specimen) is given in table 2. Orthochemical minerals include dolomite cement, authigenic pyrite, and leucoxene from the alteration of sphene and ilmenite. Medium- dark-gray siltstone clasts that average 2 cm in diameter and locally are elongated as much as 1 1 cm are common in the sandstone. This member also contains a few thin interbeds of olive-gray, pyritic siltstone. Characteristic of the lower part are thick to very thick interbeds of conglomerate composed of well- rounded quartzite and varicolored porphyritic vol- canic pebbles and cobbles and of more angular granitic boulders as long as 2.8 m. Thin carbonaceous laminae are common in the upper part of this member. Between Bean Creek and the conformable contact with the overlying middle siltstone member, only the upper 460 m of the lower sandstone member is exposed. An additional 1,060 m of the lower part of this member is exposed to the east in the Laurel quadrangle, where its conglomeratic beds appear to rest upon granitic basement along the West Branch of Soquel Creek (Dibblee and others, 1978). The sandstone is barren of mollusks. Stratigraphi- cally undiagnostic, arenaceous foraminifers, includ- ing large Bathysiphons and Trochammina sp., and pyritized radiolaria and plant stems(?) are sparse in the siltstone interbeds. Microfossils from the super— jacent middle siltstone member indicate that the lower sandstone member is Penutian (early Eocene) or older. The apparent unconformable relationship of a lithologically similar section of the Butano Sandstone to the underlying Locatelli Formation on the north flank of Ben Lomond Mountain (Cum- mings and others, 1962, p. 183) suggests that the lower sandstone member is younger than Ynezian. MIDDLE SILTSTONE MEMBER Gradationally above the lower sandstone member lie the less resistant nodular beds of the middle siltstone member. Exposures of this member are commonly slumped and are restricted to the Zayante canyon-Mountain Charlie Gulch area south of the Zayante fault. The siltstone is thin to medium bedded and nodu- lar; it is olive gray where fresh and weathers moder- ate yellowish brown. It is pyritic and includes scat- tered, disk- and rod-shaped siderite concretions that weather dark yellowish orange. Thin interbeds of laminated and graded, medium- to fine-grained, arkosic sandstone are rhythmically interbedded in the lower part. These sandstone interbeds (L49, table 2) are cemented by sparry calcite and are litholog- ically similar to those of the lower sandstone member. TABLE 2,—Terrigenous mineral composition, in percent, of the Butano Sandstone [Sample locations are shown on plate 2] Lower Middle Upper Informal subdivision of sandstone siltstone sandstone Butano Sandstone ........................................ member member member Sample No _____________________ L44 L129 L49 L12 Quartz" ..... 36 38 45 42 Orthoclase 14 18 15 24 Microcline _____ 7 6 9 3 Plagioclase(An12.n) ________ 10 11 10 10 Granitic rock fragments ........... 20 12 15 2 Silicic volcanic rock fragments _ 12 10 2 2 Low-grade metamorphic rock fragments 2 1 <1 Sedimentary rock fragmen ~ <1 <1 Biotite ____________________________ <1 <1 7 Muscovite _ <1 <1 1 Chlorite, <1 <1 4 Zircon <1 Apatite . <1 S ‘- <1 <1 <1 Rutile ,. < 1 Epidote ___________________________________ --- <1 1 Garnet .............. ~ <1 ~ Opaque minerals <1 2 4 STRATIGRAPHY NORTHEAST OF SAN GREGORIO FAULT ll Along Zayante canyon, the middle siltstone mem- ber is between 180 and 230 m thick. To the west along an intermittent tributary to Zayante Creek (SW1/ 4 sec. 36, T. 9 S., R. 2 W.), this member is poorly exposed and is estimated to be about '7 5 m thick. Poorly preserved and commonly pyritized organic remains, including small plant stems(?), occur in this siltstone unit. Calcareous planktonic foraminifers are common locally and are associated with a few arenaceous and diverse calcareous benthonic speci- mens. Small, thin-shelled pelecypods, including Lucina sp., Nuculana sp., and Propeamusium(?) sp., are rare. The lowest diagnostic microfauna occurs approxi— mately 30 m stratigraphically above the contact with the subjacent lower sandstone member (Clark, 1966a, table 3; Brabb and others, 1977, table 4). At that locality, the abundant planktonic foraminiferal fauna is diagnostic of an early Eocene age, and the joint occurrence of the calcareous benthonic species, Anom- alina garzaensis, Anomalina aff. A. regina, Gyroidina orbicularis var. obliquata, Hoeglundina eocenica, and Siphonina wilcoxensis, suggests a late Penutian age. Calcareous nannoplankton from this same strati- graphic level are also diagnostic of a Penutian age (F. R. Sullivan, written commun., 1974). As Sullivan believes that nannoplankton collected from near the base of the superjacent upper sandstone member are also of Penutian age, a late Penutian age is indicated for the entire middle siltstone member, although the lower 30 m is not well dated. UPPER SAN DSTONE MEMBER Conformably above the middle siltstone member lies the upper sandstone member, which is correlative with at least the upper part of the Butano Sandstone in its type locality to the northwest. This member is discontinuously exposed in a narrow band south of the Zayante fault from Zayante Creek westward to the northern boundary of the area. North of the Zayante fault, the upper sandstone member is exposed along Newell Creek where it grades upward into the Twobar Shale Member of the San Lorenzo Formation. In general, this sandstone member becomes thinner bedded and finer grained upward. The lower part as exposed along Zayante and Lompico Creeks is medium- gray, moderately sorted, granular, mediumgrained arkosic sandstone with a few interbeds of sandy pebble conglomerate. Higher in the section along Love Creek, but still south of the Zayante fault, the sand- stone is well sorted and fine grained. Thin interbeds of medium-gray siltstone that contain foraminifers are scattered through the section south of the fault. In Newell Creek canyon north of the Zayante fault, thin to medium sandstone beds are graded and alternate with thin foraminifer-bearing siltstone beds, which become more numerous near the contact with the superjacent San Lorenzo Formation. Biotite is com- mon in these fine-grained arkosic sandstone beds (L12, table 2), which are cemented by sparry calcite. Because of faulting and discontinuity of exposures, the thickness of the upper sandstone member is difficult to estimate. Approximately 740 m of this member crops out south of the Zayante fault, and about 240 m is exposed in Newell Creek north of the fault. Nannoplankton from a siltstone interbed near the base of the upper sandstone member east of Zayante Creek are diagnostic of a Penutian age, whereas siltstone interbeds along Lompico Creek yield Ulati- sian nannoplankton (F. R. Sullivan, written commun., 1964). Associated with the nannoplankton are strati- graphically less diagnostic foraminifers, and at one locality along Lompico Creek are several specimens of the nuculid bivalve Acila (T runcacila) of. A. (T.) decisa, a species that on the Pacific coast has been recorded from beds of Paleocene and Eocene age (Schenck, 1936, p. 53). These specimens of Acila were the only megafossils found in the upper sandstone member. The joint occurrence of the foraminifers Bulimina corrugata and Gyroidina soldanii var. octo- camerata in the Love Creek section indicates that at least the upper 110 m of the upper sandstone member that crops out south of the Zayante fault is of late Ulatisian or early Narizian age. Diagnostic Narizian foraminifers were collected by Brabb (1960a, p. 35) from outcrops of this member about 3 km to the northwest along Bear Creek south of the Zayante fault. Thus, the Butano beds truncated by the Zayante fault are progressively younger to the west. In the upper sandstone member, within the Felton quadrangle, the lowest stratigraphic occurrence of foraminifers diagnostic of Narizian age is along Newell Creek north of the Zayante fault, where Planul- aria markleyana and Uuigerina garzaensis occur approximately 185 m and 135 m, respectively, below the contact with the superjacent San Lorenzo Forma- tion. '1‘H1(:KM:ss The total estimated thickness of the Butano Sand- stone in the northern Felton quadrangle is about 1,670 m, but the amount of section that is cut out by the Zayante fault is not known. An additional 1,060 m of the lower sandstone member that crops out to the east in the Laurel quadrangle produces a composite section of approximately 2,730 m. This thickness is similar to 12 STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA the 2,750 m estimated by Cummings, Touring, and Brabb (1962, p. 186) for the Butano Sandstone of the northern Santa Cruz Mountains, of which about 1,830 m crops out in the type area of Butano Ridge. AGE The Butano Sandstone of the Felton quadrangle ranges from Penutian (early Eocene) to N arizian (middle and late Eocene) age. The lower sandstone member is not well dated; it is late Penutian or older. As the arenaceous foraminiferal fauna of this member does not resemble that of the Locatelli Formation, and a correlative section to the northwest appears to overlie the Locatelli Formation unconformably, the lower sandstone member is probably younger than Ynezian. The middle siltstone member yields nanno- plankton and benthonic foraminifers diagnostic of a late Penutian age. Most of this member that crops out south of the Zayante fault contains microfossils diag- nostic of Ulatisian to N arizian age, whereas benthonic foraminifers from the upper 185 m of section along N ewell Creek are diagnostic of N arizian age. Planktonic foraminifers identified by Berggren and Aubert (1977, p. 3) from the middle siltstone member and the lower part of the upper sandstone member as exposed along Zayante and Lompico Creeks include Subbotina linaperta, S. turgida, Acarinina acarinata, A. coalingensis, A. wilcoxensis, and Morozovella sub- botinae, which are diagnostic of an early Eocene (P8 Zone) age. The Butano Sandstone was provisionally referred to the Oligocene by Arnold (1906, p. 16) because of its conformable position below the megafossiliferous San Lorenzo Formation. Foraminifers diagnostic of a late Eocene age were subsequently reported from the upper part of the Butano (Schenck, 1936, p. 69; Sullivan, 1962, p. 247). Cummings, Touring, and Brabb (1962, p. 186) report that faunas older than Narizian have not been collected from the Butano Sandstone in the northern Santa Cruz Mountains but that the lower 1,850 m is not well dated. The older section of the northern Felton quadrangle, therefore, has provided new information on the lower and middle Eocene succession of the Santa Cruz Mountains and has resulted in an extension of the age of the Butano Sandstone to include the early and middle Eocene. BUTANO(?) SANDSTONE In correlating the three informally named members of the Butano Sandstone that are mapped in the northern part of the Felton quadrangle with other sections in the Santa Cruz Mountains, the writer (1968a, p. 170—174) suggested that a section in the Davenport quadrangle that is exposed along San Vicente canyon on the southwest slope of Ben Lomond Mountain may be correlative with the lower sandstone member. This steeply dipping section consists predominantly of yellowish-gray medium-grained arkosic sandstone in beds 1 to 10 m thick that commonly grade upward to greenish—gray sandy pyritic mudstone. The section includes very thick interbeds of sandy cobble conglom- erate, containing well-rounded dark silicic porphyry and light-colored quartzite pebbles and cobbles with granitic boulders as much as 1 m long. Light-yellow- gray sandy mudstone clasts as much as 30 cm long occur in both the sandstone and conglomerate beds. Although the base of this Butano(?) section is not exposed along San Vicente canyon, structural atti- tudes suggest that the section overlies the quartz diorite that crops out upcanyon. In a railroad cut on the west side of the canyon, 42 m of section is exposed and is unconformably overlain by a bituminous sand- stone bed of the Santa Margarita Sandstone. Arenaceous foraminifers are rare in the mudstone and include large specimens of Bathyisphon eoceni- cus, Trochammina(?) sp., and Haplophramoides(?) sp. Small pyrite clusters may represent internal molds of globigerinids. Although this fauna is stratigraph- . ically undiagnostic, fauna] and lithologic similarities suggest a correlation with the lower sandstone mem- ber of the Butano Sandstone. SAN LORENZO FORMATION The San Lorenzo Formation was named by Arnold (1906, p. 16) for exposures of shale and fine-grained sandstone along the San Lorenzo River, about 4 km north of the town of Boulder Creek. Arnold assigned an Oligocene age to this formation—the first unit to be so dated in California—because of its conformable position beneath the Vaqueros Sandstone and its megafossil fauna, which he listed (1906, p. 17) and later described (1908a). Foraminifers diagnostic of Narizian (late Eocene) age were subsequently reported by Sullivan (1962) from the lower part of the forma- tion. Brabb (1964) subdivided the San Lorenzo For- mation into a lower Twobar Shale Member of N arizian (late Eocene) age and an upper Rices Mudstone Member of Refugian and Zemorrian (late Eocene and Oligocene) age, with the type section along Kings Creek about 7 km north of Boulder Creek, and postu- lated a possible disconformity between these two members. Best exposures of the San Lorenzo Formation with- in the area were previously along Newell Creek, where the Twobar Shale and Rices Mudstone Members cropped out discontinuously along the canyon bottom. STRATIGRAPHY NORTHEAST OF SAN GREGORIO FAULT 13 This section is now covered by the water of Loch Lomond. Two kilometers to the west along Love Creek, only the Rices Mudstone Member is exposed, and the Twobar may be faulted out of the section (pl. 2). TWOBAR 51 [ALE MEMBERS The Twobar Shale Member is conformable above the Butano Sandstone. The gradation from the sand- stone beds of the Butano to the less resistant shale beds of the overlying San Lorenzo Formation is marked by a broadening of Newell Creek canyon. The shale is typically very thin bedded and laminated; it is olive gray where fresh and weathers moderate yellow- ish brown. The lower part contains a few thin inter- beds of well-sorted fine—grained sandstone and light- colored phosphatic lenses. The upper 20 m is nodular and includes several thin to medium glauconitic interbeds. This member is about 60 m thick along Newell Creek. Foraminifers occur throughout the shale section, but calcareous forms are generally leached and poorly preserved in surface exposures. Some fresh samples yield abundant specimens of the planktonic forami— nifer Globigerina sp. Other microfossils include radio- laria, which are locally common to abundant, and a few sponge spicules. Large fish scales are common. The only mollusk collected from this member is the mud pecten Delectopecten peckhami, external molds of which are common in the upper part. The joint occurrence of Bulimina corrugata and Uuigerina churchi in the lower part of the Twobar Shale Member along Newell Creek is diagnostic of a Narizian age and indicates that the contact between the Butano Sandstone and the San Lorenzo Forma- tion falls within the Narizian Stage. RICES MUDSTONE MEMBER Along Newell Creek, the contact between the Two- bar Shale and Rices Mudstone Members is mapped at the abrupt change from thin-bedded nodular mud- stone to massive, more resistant sandstone. There, the Rices Mudstone Member is composed of well- sorted, very fine to fine—grained, biotite—bearing, arkosic sandstone with scattered round carbonate concretions that average 30 cm in diameter. The sandstone is medium light gray where fresh and pale yellowish brown where weathered. At the base of this member is a 20-cm-thick bed of slightly granular medium-grained glauconitic sandstone. A few thin interbeds of pebble to cobble conglomerate occur near the top of this member and represent tongues of the overlying Zayante Sandstone. The Rices Mudstone Member is estimated to be as thick as 275 m along Newell Creek. Along Love Creek, approximately 240 m of this member crops out be- tween the Zayante fault to the south and the contact with the superjacent Zayante Sandstone to the north. Although in the type area to the northwest the Rices Mudstone Member is conformably overlain by the Vaqueros Sandstone, along both Love and Newell Creek canyons this member is conformably overlain by the Zayante Sandstone. Organic remains generally are not so abundant in the Rices as in the Twobar. Foraminifers are rare to absent in this coarser grained member, whereas mol- lusks are locally common. Arnold (1908a, p. 371-372) recorded Priscofusus hecoxae from the San Lorenzo Formation on Love Creek, and the present writer collected from the lower part of the Rices Mudstone Member on Love Creek the following molluscan fauna: Acila sp. (?)Pleurotoma perissolaxoides Arnold Tellina cf. T. lorenzoensis Arnold Yoldia sp. Along Newell Creek, mollusks are locally abundant and include: Dentalium sp. Lucinoma sp. Modiolus sp. Panopea sp. Pitar sp. Solen sp. In addition, Balanus sp. occurs in the upper part of this member. From approximately 15 m above the base of the Rices Mudstone Member along Newell Creek, Brabb (1960a, p. 184) collected the foraminifer Uvigerina gesteri. As he found this species to be diagnostic of a Zemorrian age in the Big Basin area (1960a, fig. 20), he suggests that Refugian strata may be missing along Newell Creek. Although other stratigraphically diag- nostic foraminifers have not been recorded from this member along Newell Creek, its stratigraphic position beneath the Zayante Sandstone, which intertongues to the north with the Vaqueros Sandstone of Zemor— rian to early Saucesian age, indicates that the Rices is not younger than Zemorrian. Along Bear Creek, 5.4 km northwest of the Newell Creek section, where the Rices Mudstone Member is overlain conformably by the Vaqueros Sandstone, Kleinpell (1938, p. 111) assigns at least the upper 300 m of the San Lorenzo Formation which is synonymous with Brabb’s Rices Mudstone Member, and the lower 600 m of the superja- cent Vaqueros Sandstone to the lower Zemorrian Stage. Thus, although the lower part of the Rices l4 STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA along N ewell Creek may be Refugian, most of this section probably falls within the lower Zemorrian Stage. ' ZAYANTE SANDSTONE In the northern part of the Felton quadrangle, the Vaqueros Sandstone as mapped by Branner, N ewsom, and Arnold (1909) includes in its lower part a probable nonmarine unit of pebbly sandstone, conglomerate, and sandy siltstone. This unit was differentiated from the marine Vaqueros Sandstone and named by the writer (1966b) the Zayante Sandstone, for exposures on Zayante Creek. The type section extends along Zayante Creek from the axis of an anticline (N W1/ 4 sec. 31, T. 9 S., R. 1 W.) northward to the contact with the overlying Vaqueros Sandstone (SW1/ 4 sec. 30, T. 9 S., R. 1 W.) The exposed Zayante Sandstone extends . for about 10 km from Love Creek on the west near the boundary with the Castle Rock Ridge quadrangle eastward to Mountain Charlie Road in the western part of the Laurel quadrangle. The Zayante Sandstone is shown as parts of the “Lower Miocene marine” (Vaqueros Sandstone) and “Eocene marine” (Butano Sandstone) on the San Francisco sheet of the Geologic Map of California (Jennings and Burnett, 1961). The Zayante Sandstone consists predominantly of thick to very thick beds of moderately to poorly sorted, pebbly, medium- to coarse-grained, biotite—bearing, arkosic sandstone. The sandstone is bluish gray where fresh but weathers yellowish orange. Thick interbeds, lenses, and pods of conglomerate contain well-rounded varicolored porphyritic volcanic and quartzite pebbles and cobbles and more angular gra- nitic cobbles and boulders, which locally are as much as 1.2 m in diameter. Distinctive thin interbeds of grayish-olive, poorly sorted, slightly granular, sandy, chloritic siltstone that is locally mottled with various hues of red and green are common. The large-scale heterogeneity, the poor sorting of individual beds, the mottled greenish and reddish coloration, and the local channeling at the base of conglomerate beds both at the base and within the formation, together with the complete absence of marine fossils, suggest that the Zayante Sandstone is nonmarine. This formation at the type section is about 500 m thick. Structural attitudes along Lompico Creek sug- gest that approximately 550 m of the Zayante Sand— stone is present between the contact to the southwest with the subjacent Rices Mudstone Member of the San Lorenzo Formation and the contact to the northeast with the superjacent Vaqueros Sandstone. The Zayante Sandstone conformably overlies the Rices Mudstone Member along both Love and Newell Creek canyons. This lower contact is placed at the base of the lowest interbedded sequence of poorly sorted, medium- to coarse-grained sandstone, con- glomerate, and greenish siltstone. A few thin inter- beds and lenses of conglomerate occur below the contact as mapped along Love Creek and along the ridge separating Newell and Lompico Creeks (sec. 26, T. 9 S., R. 2 W.). The Zayante Sandstone is conform- ably overlain by and locally intertongues with the Vaqueros Sandstone; the upper contact is drawn at the base of the stratigraphically lowest, thick to massive, light-colored, moderately to well sorted, fine- to medium-grained sandstone bed. Where exposed, this upper contact is sharp, and the basal sandstone bed of the Vaqueros locally contains greenish siltstone clasts that resemble the siltstone of the underlying formation. The conformable position of the Zayante Sandstone above the Rices Mudstone Member of the San Lorenzo Formation of Refugian(?) and Zemorrian (Eocene? and Oligocene) age and below and partially inter- tonguing with the Vaqueros Sandstone of Zemorrian and early Saucesian (Oligocene and early Miocene) age brackets the Zayante Sandstone as Zemorrian (Oligocene). In Major Creek canyon in the southwest corner of the Felton quadrangle, a sequence of beds that is lithologically similar to the Zayante Sandstone is exposed for a distance of about 300 m and is tenta- tively referred to the Zayante Sandstone (pl. 2). The finer grained beds of this sandstone and siltstone sequence display the characteristic greenish hue of the formation, but conglomerate interbeds are fewer and contain smaller clasts than in the type area. Although contacts along Majors Creek are not ex- posed, these beds tentatively assigned to the Zayante Sandstone appear to overlie the granitic basement and to be overlain by the Lompico Sandstone of middle Miocene age. VAQUEROS SANDS'I‘ONE “Vaquero sandstone” was the name given by Hamlin (1904, p. 14) to “a rather coarse uniformly gray, white or light-yellow quartzose sandstone” that is well developed in Los Vaqueros Valley on the eastern slope of the Santa Lucia Range. Hamlin reported that in the type area this formation rests nonconformably on the crystalline basement and on pre—Miocene (“older than the N eocene”) sedimentary rocks and is overlain by the Monterey Shale. The name “Vaquero” was soon extended to strata in the Santa Cruz Mountains 120 to 160 km northwest of the type area by Haehl and Arnold (1904), who applied it to lithologically similar rocks that appeared to occupy STRAITGRAPHYTQORTHEAST(H7SADIGREGORKDFAULT 15 the same stratigraphic position and to contain a fauna similar to the one listed by Hamlin (1904, p. 14). Strata originally included in the Vaqueros Sand- stone in the Santa Cruz Mountains by Branner, Newsom, and Arnold (1909) are now known to range in age from Paleocene to Pliocene. The name is now restricted to marine strata of Zemorrian (Oligocene) and Saucesian (early Miocene) age. In the Felton quadrangle, this formation is restricted to the north— eastern part, where it crops out north of the contact with the subjacent Zayante Sandstone and south of the contact with the superjacent Lambert Shale on the south limb of the San Lorenzo syncline and reappears on the overturned north limb of this fold (pl. 2). The Vaqueros is primarily thick-bedded to massive, moderately sorted, slightly granular medium- grained to well-sorted fine-grained arkosic sandstone. The sandstone is light gray to medium gray where fresh and yellowish gray to moderate yellowish brown where weathered. A few thick to very thick interbeds of olive-gray to dusky-yellowish-brown mudstone are in the upper part. A lignite interbed in coarse-grained sandstone along Love Creek just north of the mapped area (SE 1/4 sec. 21, T. 9 S., R. 28., Castle Rock Ridge quadrangle) was mined for local use around the turn of the century. Approximately 76 to 80 m stratigraphically above the contact with the underlying Zayante Sandstone is a series of pillow basalt flows. These flows are best exposed to the west side of Zayante Road (SW1 / 4 sec. 30 T. 9 S., R. 1 W.), where they are approximately 60 m thick and form a prominent east-trending ridge between Zayante and Lompico Creeks. The basalt flows are discontinuously exposed for about 8 km from an isolated occurrence in the Castle Rock Ridge quadrangle about 200 m beyond the northern limit of the mapped area southeastward to exposures in the Laurel quadrangle about 21/2 km beyond the eastern boundary of the area. The basalt flows contain pillows, from 15 to 30 cm in diameter, interstratified with minor amounts of flow breccia, consisting of angular pebble-size basalt fragments cemented by sparry calcite. The basalt contains phenocrysts of plagioclase (labradorite) as much as 5 mm long, euhedral phenocrysts of olivine (usually altered to iddingsite, “bowlingite,” and magne- tite), and a groundmass composed of microlites of plagiocl ase (labr adorite-andesine) partly enclosed in augite, with subordinate amounts of glass (altered to celadonite), magnetite and (or) ilmenite, and apatite. The texture ranges from subophitic to inter- sertal, and the basalt is locally vesicular and amygdaloidal; the amygdules are composed mainly of opal and chalcedony. Spheroidal weathering is common, and the basalt is deeply weathered to a characteristic moderate-yellowish-brown to reddish- brown soil. These basalt flows are lithologically similar, occupy a similar stratigraphic position, and are partially contemporaneous with the Mindego Basalt of Dibblee (1966), the type section of which is near the town of La Honda, 26 km to the northwest (fig. 1). As these basalt flows do not appear to have been laterally continuous with those mapped by Dibblee, the name “Mindego” is not applied to them. In the northeastern part of the area, the Vaqueros Sandstone conformably overlies the Zayante Sand- stone and is conformably overlain by the Lambert Shale. In this area, the Vaqueros, including the intercalated basalt flows, is betwen 350 and 440 m thick. To the north in the Castle Rock Ridge quad- rangle, where the Zayante Sandstone is absent along the overturned north limb of the San Lorenzo syn— cline, the Vaqueros Sandstone conformably overlies the Rices Mudstone Member of the San Lorenzo Formation and is approximately 920 m thick. Directly overlying the basalt flows in the Vaqueros Sandstone along Zayante canyon are molluscan bioherms, composed largely of the pelecypod genera Dosinia and Crassatella and locally containing a varied gastropod fauna (table 3). Except for a few basalt pebbles, these bioherms are composed almost entirely of shells and shell debris that are solidly cemented by calcium carbonate, and their occurrence is restricted to the top of the basalt flows. Similar bioherms or even individual mollusks were not observed elsewhere in this formation. Foraminifers TABLE 3.—Megainvertebrates from the Vaqueros Sandstone [USGS localities are listed under “Fossil Localities"] USGS Cenozoic loc. Fossil M5049 M5050 Gastropods Astraeu aff. A. morani Loel and Corey ............... Bruclarkia santacruzana (Arnold) Calyptraea cf. C. inornata (Gabb) Conus owenianus Anderson ..... Cymatium n. sp. Oliua sp. ,. Oliuella cf. 0. Pedroana var. subpedroana Loel and Corey.... Scaphander(?) sp. .. Searlesia sp. Turritella inezana Con Pelecypods Anadara(?) sp. . ............................. Chione latilaminosa Anderson Chlamys sespeensis (Arnold) .. Crassatella granti (Wiedey) ...... Dosinia margaritana Wiedey .. Dosinta margaritana var. projecta Loel and Corey... Glycymeris sp. Macrochlamis magnolia (Conrad) Myttlus cf. M. expansus Arnold X xxgx xxxxxx xx xix xxxxx xxxxxxxxx Trachycardium uaquerosense (Arno X Vertipecten of. V. perrini (Arnold) ..... X Barnacle Balanus sp. ................... X Echinoids .................................... X 16 STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA are common and fish scales sparse in the mudstone interbeds in the upper part of the formation. Foraminiferal faunas diagnostic of Zemorrian (Oligocene) age have been recorded from the Vaqueros Sandstone of the adjoining Castle Rock Ridge and Laurel quadrangles (Kleinpell, 1938, p. 111; Burchfiel, 1964, p. 403; Clark, 1966a, p. 60; Fairchild and others, 1969, p. 19 and p. 23—24; and Brabb and others, 1977, table 7). Within the Felton quadrangle, the bioherms above the basalt flowswithin the Vaqueros Sand- stone (table 3) yield Crassatella granti, Dosinia margaritana var. projecta, Macrochlamis magnolia, Turritella inezana, 3.3., and other stratigraphically diagnostic molluscan representatives of the Turritella inezana Zone, which Kleinpell (1938, fig. 14) corre— lates with his Zemorrian Stage. This faunule, together with the noteworthy absence of Turritella ocoyana, is diagnostic of an “Upper” or “Uppermost Vaqueros horizon” of Loel and Corey (1932, p. 136—138) and of the “Vaqueros Stage” of Addicott (1972). Along Zayante Creek, mudstone interbeds within the Vaqueros Sandstone that are 30 to 45 m strati- graphically along the top of the basalt flows yield abundant calcareous foraminifers. The joint occur- rence there of Eponides nanus, Siphogenerina klein- pelli, S. mayi, and Uvigerinella obesa is diagnostic of the Siphogenerina transversa Zone of the Saucesian Stage and indicates that at least the upper 185 m of the Vaqueros Sandstone on the south limb ofthe San Lorenzo syncline is of early Saucesian (early Miocene) age. A potassium-argon date of24. 1 my on plagioclase feldspar from the basalt flows within the Vaqueros Sandstone that was obtained by J. F. Evernden (Clark, 1966a, p. 59) has recently been revised by Turner (1970, p. 100—101) to 23.17/a0. 7 my This date is significant because the preceding mega- and micro- fossil data indicate that these flows along Zayante canyon are upper Zemorrian and stratigraphically . close to the Zemorrian-Saucesian Stage boundary. LAMBERT SHALE The Lambert Shale was named by Dibblee (1966) for a thick sequence of shale, siltstone, and mudstone exposed along Peters Creek in the Mindego Hill quadrangle, 19 km north ofthe mapped area (fig. 1). In the type area, this formation conformably overlies the Vaqueros Sandstone and is overlain by the Monterey Shale and yields foraminifers diagnostic of Zemorrian and Saucesian ages (Dibblee, 1966). The Lambert Shale is poorly exposed along the axis of the San Lorenzo syncline in the northeast corner of the Felton quadrangle. Better exposures are to the north along Zayante Creek and to the east along Mountain Charlie Gulch, where this formation is discontinuously exposed along the overturned north limb of the syncline. These beds were mapped as Monterey Shale by Branner, N ewsom, and Arnold (1909). The Lambert Shale is typically thin- to medium- bedded and thinly laminated, olive-gray to dusky- yellowish-brown, organic mudstone that is locally semisiliceous. The mudstone contains silt to very fine sandstone grains of quartz, plagioclase, micro- cline, orthoclase, biotite, basalt fragments, and glau— conite. Most of the terrigenous particles are angular, and the feldspar grains are generally fresh. The matrix of the mudstone consists of clay, collophane, iron oxides, and finely disseminated bituminous matter, with microcrystalline carbonate (dolomite?) lower in the section and opaline silica locally higher in the section. A distinctive feature is the occurrence in the lower part of the formation of graded pale- yellowish-brown phosphatic laminae and lenses that give the mudstone a banded appearance. A few, thin, laminated, well-sorted, fine-grained and thick, graded, medium-grained arkosic sandstone interbeds occur in the upper part. Approximately 450 m of the Lambert Shale crops out along Mountain Charlie Gulch just east of the area. About 185 m of this formation is preserved along the San Lorenzo syncline in Zayante Creek to the north. The Lambert Shale is conformable above the Vaqueros Sandstone. The Vaqueros-Lambert contact is well exposed on the steeply dipping, slightly overturned, north limb of the San Lorenzo syncline both along Zayante Creek (SW1/4 sec. 19, T. 9 S., R. 1 W., of Castle Rock Ridge quadrangle) and along Mountain Charlie Gulch (SW 1/4 sec. 29, T. 9 S., R. 1 W., Laurel quadrangle). This contact is placed at the lowest occurrence of well-bedded mudstone in a pre- dominantly mudstone section. To the south along Mountain Charlie Gulch, the Lambert Shale is uncon- formably overlain by the Santa Cruz Mudstone. Organic matter is abundant in the Lambert Shale. In addition to that in the matrix, bitumen occurs as very thin, wispy inclusions that are elongate to 2 mm parallel to the lamination. Calcareous foraminifers are common in the lower part of the section, where most of the tests are crushed and filled with sparry calcite, whereas arenaceous varieties are more numerous in the upper part. Diatoms and sponge spicules are less common lower in the formation but more abundant higher, where the mudstone is semi- siliceous. Phosphatic fish scales and fragments are common throughout. Mollusks are notably absent STRATIGRAPHY NORTHEAST OF SAN GREGORIO FAULT 17 from this entire unit. Foraminifers from the Lambert Shale along Moun— tain Charlie Gulch (Clark, 1966a, table 5; Brabb and other, 1977, table 7) include Dentalina quadrulata, Siphogenerina kleinpelli, S. transversa, and Uvigeri- nella obesa var. impolita, which are diagnostic of Saucesian (early Miocene) age, although those from the upper 120 m of this section are not definitely diagnostic. DEPOSITIONAL ENVIRONMENT OF EOCENE TO LOWER SEDIMENTARY MIOCENE SEQUENCE The Butano Sandstone and the Twobar Shale Member of the San Lorenzo Formation were probably laid down at bathyal depths throughout the northern and central Santa Cruz Mountains. The development of the arenaceous foraminiferal assemblages from the siltstone interbeds of the lower sandstone member of the Butano Sandstone may have been controlled by muddy bottom conditions, but the complete absence of calcareous forms, the uniformity of the fauna, and the abundance, locally in flood proportions, of large specimens of Bathysiphon suggest deposition in cool, deep (bathyal-abyssal) water. Bandy and Rodolfo (1964, p. 833) report that, in the Peru-Chile trench, large specimens of Bathy- siphon “are significant members of the abyssal foraminiferal faunas.” The foraminifers from the middle siltstone member suggest bathyal depths. The more abundant benthonic species include such deep-water forms as Asterigerina crassaformis, Bulimina cf. B. pyrula of Mallory (1959), and large specimens of Bathysiphon eocenicus. The relative abundance of the nodosariids, on the other hand, suggests depositional depths no greater than bathyal. A common species of nodosariid, Dentalina consobrina, favors a bathyal habitat in the present—day seas. The upper sandstone member of the Butano Sand- stone and the overlying Twobar Shale Member of the ' San Lorenzo Formation yield deep-water foramini- fers, including costate buliminids and costate uviger- inids. The paucity of mollusks throughout the coarse clastic part of this Eocene section is consistent with a deep-water interpretation. Clark and Nilsen (1972) have recently interpreted the Eocene paleogeography of central California as consisting to the west of an irregular continental borderlands of islands separated by deep marine basins. They postulate that the Butano Sandstone of the Santa Cruz Mountains was deposited as a large deep-sea fan by turbidity currents that transported sediment northward from the vicinity of the present- day Monterey Bay. The relative abundance of potas- sium feldspars (table 2) suggests that this sediment was derived mainly from a potassium-rich granitic terrane. While deep-water conditions continued to the north ‘ of the mapped area (Brabb 1964, p. 676), shallowing from bathyal to neritic depths occurred during deposition of the San Lorenzo Formation in the northern Felton quadrangle. Along Newell Creek, the Rices Mudstone Member is coarser grained than contemporaneous beds to the north and locally con- tains in abundance such shallow-water mollusks as Panopea, Pita, Solen, and Modiolus. Shallowing continued during Zemorrian time, and in the north— ern Felton quadrangle marine conditions were suc- ceeded by the terrestrial conditions under which the Zayante Sandstone was deposited. Because of the limited areal extent and coarseness of the nonmarine Zayante Sandstone and the total absence of the marine San Lorenzo and Vaqueros Formations to the south of the Zayante fault, Clark and Rietman (1973) have postulated a regional uplift and emergence of the terrane south of this fault during Zemorrian time. The Zayante and Vaqueros Sandstones represent orogenic deposits that were derived from this emergent and largely crystalline terrane to the south of and proximal to the Zayante fault. The Zayante Sandstone was probably deposited as an alluvial fan along the fault, whereas 11/2 to 3 km to the north, bathyal marine sedimentation continued without interruption during deposition of the San Lorenzo and Vaqueros Formations. During the later phase of Zayante Sandstone deposition, subsidence began in the northern part of the area, and a southward transgression of the sea resulted in deposi- tion of the Vaqueros Sandstone. Lignite formed locally under marginal marine conditions. Into this transgressive sea, basalt flowed, locally producing banks upon which shallow-water molluscan repre- sentatives of the Turritella inezana Zone thrived. Continued subsidence together with a decrease in the influx of coarse elastic detritus resulted in deposi- tion of the organic mudstone beds of the Lambert Shale. A bathyal environment for these beds is suggested by the common to abundant occurrence of Bolivina marginata, Siphogenerina transversa, Uvigeri- nella obesa, and less common species that Bandy and Arnal (1969, p. 788—791) assign to their upper, middle and lower bathyal biofacies. The association of the flat, unornamented species Bolivina margi- nata with these laminated strata is in accord with Harman’s (1964, p. 90) observation that “Flat boli- vinid species in laminated sediment generally lack apical spines, costae, and marginal keels in contrast 18 STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA to those in homogeneous sediment.” He postulates that Bolivina marginata and morphologically similar bolivinids favor an oxygen-deficient habitat, which could account for the absence of mollusks in the Lambert Shale. MIDDLE MIOCENE SEDIMENTARY SEQUENCE Resting with marked angular discordance upon the previously deformed Eocene to lower Miocene sequence are the local deposits of a marine sedi- mentary cycle of middle Miocene (Relizian to Luisian) age. This middle Miocene sedimentary sequence consists of a widely transgressive basal sandstone unit, the Lompico Sandstone, and an overlying organic mudstone unit, the Monterey Formation. LOMPICO SANDSTONE A transgressive sandstone unit at the base of the middle Miocene sequence was named the Lompico Sandstone by Clark (1966b), for exposures in Lompico Creek, where this formation is about 150 m thick in the vicinity of the community of Lompico. The type section is 1.6 km southeast of Lompico, where the lower 82 m of this formation is well exposed along the northwest side of Zayante Road (SW1/4 sec. 36, T. 9 S.., R. 2 W., Felton quadrangle). On the north limb of the Scotts Valley syncline, the Lompico Sandstone is exposed discontinuously east of Lompico Creek, but to the west it can be traced almost continuously to the northern boundary of the Felton quadrangle (pl. 2). On the east slope of Ben Lomond Mountain, this formation is exposed almost continuously for about 10 km from the vicinity of Boulder Creek to south of the town of Felton. On the southwest slope of Ben Lomond Mountain, the Lompico Sandstone crops out along Baldwin, Majors, and Laguna Creek canyons (pl. 1—2). The Lompico Sandstone as here mapped was referred to the Vaqueros Sandstone by Branner, N ewsom, and Arnold (1909) and is shown as “Lower Miocene marine” and thus not differentiated from the Vaqueros Formation on the Santa Cruz sheet (Jennings and Strand, 1958) and on the San Fran- cisco sheet (Jennings and Burnett, 1961) of the Geologic Map of California. Although Brabb (1960a, p. 68) properly interpreted the stratigraphic rela- tionship of these sandstone beds near Boulder Creek and included them within his “Formation A of the Monterey Group,” he assigned the correlative beds that crop out along Majors Creek to the Vaqueros Sandstone, as had Page and Holmes (1945). At the type section, the Lompico Sandstone contains 1.5 m of basal conglomerate with a few granitic and more abundant dark porphyry and light quartzite pebbles in a medium-grained sandy matrix. The lower third of this section consists of thick beds of light-gray, moderately sorted, granular medium- grained, biotite-bearing arkosic sandstone. Calcar- . eous concretionary interbeds include abundant barnacle fragments and a few pelecypod valves. The upper two-thirds of the type section is massive yellowish-gray, buff-weathering, well-sorted, fine- grained arkosic sandstone. The terrigenous mineral composition of the Lompico Sandstone as determined by point count (200 points per specimen) is given in table 4. The feldspars are fresh to highly altered; in general, the microcline is least altered and the plagioclase the most. Orthochemical minerals include sparry calcite, which cements these sandstones; potassium feldspar occurs locally as overgrowths on feldspar grains; and sphene is altered locally to leucoxene. The Lompico Sandstone is approximately 150 m thick along Zayante, Lompico, and Newell Creeks. Along strike, this formation thins both to the east and to the west and is about 60 m thick along the San Lorenzo River at the northern limit of the Felton quadrangle. On the east slope of the Ben Lomond Mountain, structural attitudes indicate that the Lompico Sandstone thickens to the south from approxi- mately 60 min the Boulder Creek-Clear Creek area to about 100 m in the Marshall Creek-Manson Creek area. On the southwest slope of Ben Lomond Moun— tain this formation reaches a maximum thickness of about 240 m along Majors Creek canyon, where it forms conspicuous cliffs. To the west, the Lompico Sandstone thins, and the upper part of the formation intertongues with siltstone beds of the superjacent Monterey Formation. Along San Vicente canyon, the Shell Davenport core hole No. 1 penetrated about 135 m of this sandstone before reaching granitic basement. Although the contact of the Lompico Sandstone with the underlying Butano Sandstone is not exposed at the type section or elsewhere along the north limb of the Scotts Valley syncline, a pronounced angular unconformity is indicated by the great discordance in strike, almost opposite direction of dip of the two formations, and westward overlap of all three members of the Butano Sandstone by the Lompico Sandstone. Along Ben Lomond Mountain, the Lompico Sand- stone rests noncomformably upon the crystalline basement, and locally as along Fall Creek, rounded granitic boulders form a thick conglomerate at the base of the exposed sandstone section. The contact with granitic rocks is exposed in Clear Creek near STRATIGRAPHY NORTHEAST OF SAN GREGORIO FAULT 19 Brookdale and to the south along Baldwin Creek. The Lompico Sandstone locally rests unconformably upon the Locatelli Formation and along Majors Creek canyon appears to overlie the beds tentatively referred to the Zayante Sandstone. The Lompico Sandstone is conformably overlain by the Monterey Formation. This gradational contact is well exposed along Lompico Creek (SW1/4 sec. 35, T. 9 S., R. 2 W.), Where the sandstone becomes progressively finer grained upward, and the upper contact is placed at the lowest occurrence of bedded mudstone. In the type section, the sandstone likewise decreases in grain size upward, but the contact with the overlying Monterey Formation is covered. On both flanks of Ben Lomond Mountain, the Lompico Sandstone and the Monterey Formation intertongue, and the contact is mapped at the base of bedded mudstone where mudstone predominates in the section. From sandstone beds that are included here in the type section of the Lompico Sandstone, Arnold (1906, p. 83) collected the type of Leptopecten andersoni, where it occurs with Chlamys (Hinnites) multi- rugosus, Chlamys sespeensis var. hydei, and Balanus cf. B. estrellanus. On the east slope of Ben Lomond Mountain, the Lompico Sandstone is rela- tively unfossiliferous. ArnOld (1906, p. 83) recorded Leptopecten andersoni from “near Felton,” and his unpublished field notes of 1902 confirm that this species together with Dosinia mathewsoni was col- lected from this formation in the town of Felton. Along Manson Creek, the writer collected Panopea sp., indeterminate gastropods, and numerous plant fragments; to the south along Gold Gulch, a coquina bed in the Lompico is composed of Balanus frag- ments and a few specimens of Ostrea. Megafossils are more common and diverse in the Lompico Sandstone on the southwest slope of Ben TABLE 4.—Terrigenous mineral composition, in percent, of the middle Miocene sequence [Pointcount analyses by C. A. Wallace. Sample locations are shown on plates 1 and 2; L79 is from San Lorenzo River in Boulder Creek, 60 m north of Felton quadrangle] Monterey Rock Unit ........................................................ Lompico Sandstone Formation __'__.__—— ____ Sample No ______ L75A L75B L79 L95 L70 L10 Quartz ......................... 55 53 57 61 61 66 Plagioclase (Amen) . .. Orthoclase ...... Miocene ................. Granitic rock fragments ........ Metamorphic rock fragments Volcanic rock fragments ...... Sedimentary rock fragmentsf?) .. Apatite . Epidote(?) ..... Tourmaline Garnet ..... Clinozoisit ... ... <1 <1 Lomond Mountain, and those collected by the writer from scattered exposures along Baldwin, Majors, and Laguna Creek canyons are listed in table 5. The large, bell-shaped echinoid Vaquerosella coreyi is abundant through a 7.7 m thick section of thick to very thick, medium-grained sandstone beds that are conspicuously exposed along the steep canyon walls. These echinoid beds can be traced northward from the southern part of Majors Creek canyon to where they dip beneath the exposed section to reappear 2.4 km to the northeast along a westward-flowing tributary of Majors Creek. Siltstone interbeds within the Lompico Sandstone along Majors Creek yield a limited foraminiferal fauna with numerous specimens of the shallow- water taxa Ammonia beccarii and Buccella oregonensis. Anadara rivulata, Leptopecten andersoni, Lyro- pecten crassicardo, Vaquerosella andersoni, and V. coreyi are diagnostic of middle Miocene age and of the “Temblor Stage” of Addicott (1972). On the north limb of the Scotts Valley syncline from Bean Creek west to Newell Creek, benthonic foraminifers from the lower part of the superj acent Monterey Formation are diagnostic of late Relizian age and thus restrict the latest age of the underlying Lompico Sandstone to Relizian. To the west in the vicinity of Boulder Creek, where foraminiferal faunas from the lower part of the overlying Monterey include Valvulineria miocenica and Siphogenerina cf. S. reedi, which are diagnostic of Luisian age, the age of the Lompico Sandstone probably extends into early Luisian. In Siltstone interbeds of this formation along Majors Creek, the joint occurrence of Bolivina imbri— cata, Florilus incisus, and Valvulineria williami is diagnostic of Relizian age, as a similar fauna from the overlying Monterey Formation along Laguna Creek. Thus, on the southwest slope of Ben Lomond Mountain, the Lompico Sandstone appears to be restricted to the Relizian Stage, whereas to the east of Ben Lomond Mountain, this formation probably ranges from Relizian into the Luisian Stage. The Lompico Sandstone extends eastward into the Laurel quadrangle, where fossiliferous beds are exposed along Vinehill Road and along Blackburn Gulch, about 3 km east of the Felton quadrangle (Dibblee and others, 1978). These are the eastern- most known of this formation. In the northern Santa Cruz Mountains, sandstone beds that are locally found at the base of the Monterey Formation, designated the Woodhams Shale Member by Cummings, Touring, and Brabb (1962), are corre- lative with the Lompico Sandstone but were not differentiated by these workers. Transgressive sand- 20 stone beds that rest on quartz diorite on the southern slope of Montara Mountain in the Half Moon Bay quadrangle (fig. 1) are probably correlative with the Lompico Sandstone because they have a similar lithology, fauna, and stratigraphic position. MONTEREY FORMATION The Monterey Formation was named by Blake (1856) for exposures of diatomaceous and siliceous shale near the town of Monterey about 50km south of the area of this report. In the type area this formation is of Luisian, Mohnian, and Delmontian (middle and late Miocene) age (Kleinpell, 1938, fig. 14). Organic mudstone strata within the central Santa Cruz Moun- tains that were originally mapped as the Monterey Shale by Branner, Newsom, and Arnold (1909) are here assigned to the Lambert Shale, Monterey Formation, and Santa Cruz Mudstone. The Monterey Formation in the area of this report is restricted to a succession of organic mudstone beds of late Relizian and Luisian (middle Miocene) age that are conformable above the Lompico Sandstone. This formation crops out on the east slope of Ben Lomond Mountain and underlies the Scotts Valley syncline, where it is well exposed along N ewell and Zayante Creeks. On the southwest slope of Ben Lomond Mountain, the Monterey Formation is discon- tinuously exposed along Smith Grade and is poorly exposed along Laguna, Yellow Bank, and East Branch of Liddell Creeks. As exposed along Newell, Lompico, and Zayante Creeks on the north limb of the Scotts Valley syn- cline, the Monterey Formation consists of semisili- TABLE 5.—Megainuertebrates from the middle Miocene se STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA ceous organic mudstone that grades upward into micaceous and carbonaceous sandy siltstone. The lower mudstone section has a maximum thickness of about 580 m and is typically medium to thick bedded and irregularly laminated. Only locally is the mud- stone sufficiently fissile to be properly termed shale. Upon exposure, this olive-gray mudstone weathers to light gray, probably from the leaching or oxidation of bituminous matter in the matrix, and becomes lighter in weight and punky. Angular, silt to very fine sand-size grains of quartz, plagioclase, orthoclase, microcline, and biotite are scattered throughout the mudstone and locally are concentrated in thin, discontinuous laminae. Very fine grains of glauconite are scattered throughout, and pyrite occurs as finely disseminated, angular grains that locally fill chambers and replace walls of calcareous foraminifers. Collophane occurs in the matrix and as discrete light-colored rounded inclu- sions that are as much as 3 cm long and are most abundant about 150 m above the base of the forma- tion; from this level the inclusions progressively decrease in size and abundance upward. Several thick interbeds of grayish-orange-weather- ing dolomite occur in the lower part of the mudstone section (fig. 3). At least four bentonite interbeds that are from 4 cm to 30 cm thick and consist ofrelatively pure montmorillonite are also in the lower part of this section. Sandstone interbeds are absent from the lower mudstone section along Newell, Lompico, and Zayante Creeks. The mudstone grades upward into micaceous and carbonaceous sandy siltstone that is best exposed near the axis of the Scotts Valley syncline along quence on the southwest slope of Ben Lomond Mountain [USGS localities are listed under “Fossil Localities"] Monterey Rock unit .......................................................................... Lompico Sandstone Formation s a a a a ' w s s s a a a .7; USGS Cenozmc locality ...................................................... g a a B 3 ”c; 3 g g g 1% g 8 2 E E E 2 2 E E E 2 2 E 2 Gastropods Calyptraeu filosa (Gabb) ...................................................... Nassarius(?) sp. ............... Ocenbra cf. 0. topangensis Oliua sp. ............. Polim'ces(?) sp ................... Turritella ocoyana Conrad Turritella cf. T. ocoyana Conr Turritella n. 5p. Pelecypods Amusium lompacensis (Arnold) .......... Anadara (Cunearca) rivulata (Wiedey) Anadara(?) sp .................................... Chione aff. C. latilaminosu Anderson & Martin Crassostrea cf. C. titan Conrad .. Clycymeris sp ............................ Leptopecten andersoni (Arnold) Lyropecten crassicardo (C Macoma sp ................... Ostrea sp. Pecten sp. . Indeterminate pelecypods ................... Echinoides Vaquerosella andersoni (Twitchell) . Va uerosella coreyi Durham Barnac e Balanus sp. STRATIGRAPHY NORTHEAST OF SAN GREGORIO FAULT 21 Newell and Zayante Creeks, where it is 245 m thick (fig. 4). The siltstone is very thick bedded and irregularly laminated. In many exposures the bedding is indistinct, and the rock characteristically displays a flaky weathering. Coarse silt to fine sand constitutes as much as 50 percent of the siltstone, and a few very coarse granitic grains are scattered in this section. Biotite, commonly altered to chlorite and iron oxides, is abundant and together with finely disseminated bituminous matter colors the siltstone olive black. Outcrops commonly are covered with a thin yellowish film of jarosite. The siltstone section contains a few thick interbeds of arkosic sandstone that commonly are friable and grade continuously upward from medium sand to silt. Thick interbeds and biscuit—shaped concretions of microcrystalline dolomite are conspicuous in this part of the Monterey Formation and locally produce rapids and falls where they strike across streams. Westward from the Newell-Zayante Creek area, the entire Monterey Formation becomes coarser grained and less well bedded and contains thick to very thick arkosic sandstone interbeds, which are mineralogically similar to the underlying Lompico Sandstone (spls. L70 and L10, table 4). Thus, the Monterey beds that crop out on the east flank and on the southwest slope of Ben Lomond Mountain are FIGURE 3.—Mudstone beds in lower part of Monterey Formation on north limb of Scotts Valley syncline. View west across Zayante Road (NE 1/; sec. 2, T. 10 S., R. 2 W., Felton quadrangle). lithologically more similar to the upper siltstone section of the Newell-Zayante Creek area than they are to the lower mudstone section. On the north limb of the Scotts Valley syncline, the Monterey Formation is as much as 810 m thick. Along Laguna Creek on the southwest slope of Ben Lomond Mountain, structural attitudes suggest that about 370 m of this formation is preserved along the axis of a shallow east-west-trending syncline. About 5 km farther west, the Shell Davenport core hole No. 1 penetrated more than 600 m of mudstone with interbedded sandstone of the Monterey before reaching the Lompico Sandstone. The Monterey Formation conformably overlies the Lompico Sandstone and is unconformably overlain by the Santa Margarita Sandstone. This uncon- formable contact is well exposed at numerous locali- ties along the Scotts Valley syncline, where the basal conglomeratic beds of the Santa Margarita rest with angular discordance and with local relief upon the more steeply dipping beds of the Monterey (fig. 5). South of Boulder Creeek, sandstone interbeds within the lower part of the Monterey locally contain Leptopecten andersoni. Mollusks from the lower 150 m of the Monterey Formation on the north limb of the Scotts Valley syncline include: Anadara (Scapharca?) obispoana (Conrad) Anadara (Scapharca?) obispoana subsp. perdisparis (Wiedey) Cyclocardia montereyana (Arnold) Yoldia impressa Conrad naticid gastropods _ Arnold (1908a, p. 380—381) collected the type of Mactra montereyana from the lower part of the Monterey section along Love Creek (NW1/ 4 sec. 33, T 9 S., R. 2 W.) and the type of Cyclocardia montereyana from the correlative section along Newell Creek (NI/2 sec. 34. T. 9 S., R. 2 W.). The type of Anadara obispoana perdisparis also comes from this lower part of the Monterey Formation, as Reinhart (1943, p. ’72) reports the type locality is “Near center of south line of SE1/4 sec. 36, T. 9 S., R. 2 W.” Anadara obispoana and Yoldia impressa are locally so numerous that disarticulated molds of these pele- cypods cover the entire surface of slabs of mudstone that are split parallel to the stratification. Higher in the Monterey section of the Scotts Valley syncline, the molluscan fauna is characterized by the common occurrence of the mud pecten Delecta- pecten peckhami. Mollusks are absent from the upper 250 to 300 m of this Monterey section. In this same area, calcareous benthonic foramin- ifers are common to abundant in the lower 600 m of the formation, where they are concentrated locally in 22 STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF thin discontinuous laminae. Higher in the section, these calcareous microfossils decrease in relative abundance and are succeeded by a few arenaceous forms in the upper 150 to 210 m of section along Newell and Zayante Creek. Plant remains occur throughout the Monterey Formation, and some slabs of mudstone that are split parallel to the bedding display beautifully preserved imprints of leaves. Three Monterey samples from along Newell and Lompico Creeks yield abundant gymnosperm pollen, mostly pine, and a variety of angiosperm pollen, with hickory and oak most common (W. R. Evitt, oral commun., 1961). Other organic remains include phosphatic fish scales and vertebrae, which are scattered throughout the formation, and a few isolated shark teeth. Diatoms and sponge spicules are also present through- out but are more common in the upper part of the ‘formation. Nowhere are the diatoms sufficiently concentrated to form diatomite. The common mollusks ofthe Scotts Valley syncline CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA section are missing from the Monterey Formation on the southwest slope of Ben Lomond Mountain. Mol- lusks collected by the writer from siltstone beds and sandstone interbeds that crop out along Laguna Creek canyon are listed in table 5 (USGS locs. M5056, M5057, M5054). The foraminiferal faunas of these two areas also differ markedly, for calcareous foraminifers are rare in the siltstone beds on the southwest slope of Ben Lomond Mountain, where along Laguna Creek and in the Shell Davenport core hole N o. 1 they are dwarfed and include numerous shallow-water representatives. Between N ewell and Bean Creeks on the north limb of the Scotts Valley syncline, the joining ocCurrence of Bulimina cf. B. pseudoaffinis, Lenticulina hughesi, L. simplex. L. smileyi, and Valvulineria califomica obesa in the lower 60 m of the Monterey Formation is diagnostic of late Relizian age. Valvulineria califor— nica 3.3. and V. miocenica, both of which do not range below the Luisian Stage, occur higher in the section but are absent from this lower part, indicating that the FIGURE 4.—Siltstone beds in upper part of Monterey Formation on north limb of Scotts Valley syncline. View west across Newell Creek Road (NW% sec. 3, T. 10 S., R. 2 W., Felton quadrangle). STRATIGRAPHY NORTHEAST OF SAN GREGORIO FAULT 23 Relizian-Luisian Stage boundary falls within the lower 60 to 120 m of the Monterey Formation of this area. A minimum of 500 m of section along Newell Creek and of 400 m of section along Lompico and Zayante Creeks can be referred to the Luisian Stage on the highest occurrence of F lorilus costiferus, which does not range above this stage. The highest occur— rence of Valvulineria depressa further restricts 340 m of Monterey section along Newell Creek and 275 m of section along Lompico Creek to the lower Luisian. The uppermost 220 m of the formation along Zayante Creek lack stratigraphically diagnostic foraminifers but are tentatively referred to the Luisian. In the vicinity of Boulder Creek and on the east flank of Ben Lomond Mountain, foraminifers from the lowest part of the Monterey Formation commonly include Valvulineria miocenica and are thus diag- nostic of Luisian age, indicating that locally to the west the lower part of this formation is younger. The Monterey Formation on the southwest slope of Ben Lomond Mountain is not so well dated. Along Laguna Creek from about 180 m above the base of the formation, a foraminiferal fauna that includes Bolivina imbricata, Cibicides americanus, Florilus incisus, and Valvulineria williami is diagnostic of Relizian age (Clark, 1966a, table 11, 10c. JC62—15). The Monterey Formation extends eastward from the area into the western part of the Laurel quad- rangle, where it loCally contains Anadara obispoana and foraminifers diagnostic of Luisian age. This formation is also discontinuously exposed in the northern Santa Cruz Mountains, where it has been referred to the Woodhams Shale Member of the Monterey Formation by Cummings, Touring, and Brabb (1962, p. 194—195) and mapped as Monterey Shale by Dibblee (1966). _ fie FIGURE 5.-—Unconformable contact between the Monterey (Tm) and Santa Margarita (Tsm) Formations on north limb of Scotts Valley syncline (NWM: sec. 6, T. 10 S., R. 1 W., Felton quadrangle). 24 STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA DEPOSITIONAL ENVIRONMENT OF MIDDLE MIOCENE SEQUENCE The Lompico Sandstone represents the transgres- sive basal part of the middle Miocene sequence. Relatively shallow-water mollusks, including Lepto- pecten, Ostrea, and Panopea, and abundant frag- ments of the barnacle Balanus, together with the local occurrence of shallow-water foraminifers, indicate that these sandstone beds were laid down in a near- shore, shallow-marine environment. They were de- posited as the area resubmerged after an extensive interval of uplift and erosion. The mudstone section in the lower part of the Monterey Formation accumulated as subsidence pro- gressed; approximately 600 m of fine clastic sediment was laid down in progressively deeper water. The molluscan fauna in the lower part of this mudstone is characterized by the common occurrence of Anadara obispoana. This arcid pelecypod, which at several localities shows a complete gradation from juvenile to adult forms, some of which have articulated valves, suggests relatively shallow depths. Reinhart (1943, p.12) notes that “most of the Arcidae . . . inhabit the littoral and sublittoral zones and the shallower part of the neritic zone,” and A. M. Keen (oral commun., 1963) reports that a recent homeomorph in the Gulf of California has been dredged from depths down to 110 m. Higher in this mudstone section, the foraminiferal fauna is dominated by inflated Valvulinerias of the Valvulineria californica group, and the molluscan fauna by the mud pecten Delectopecten peckhami. This section was deposited in deeper water but pro- bably still within neritic depths. Kleinpell (1938, p. 14) refers to “the presence of Valvulineria closely related to the Reliz Canyon species in shallow water off the western coast of central Mexico,” and Bandy and Rodolfo (1964, p. 825) report that inflated Valvuliner- ias that are similar to the Miocene forms of California are the most abundant foraminifers in their shallow- est sample from a depth of 180 m in the Peru-Chile trench. The genus Delectopecten suggests depths greater than 55 m, as Woodring and Bramlette (1950, p. 93) record that this genus has been dredged from depths 55 to 180 In off the California coast and has also been found in deeper water. Basinal deepening probably continued during depo- sition of the highest part of the mudstone section that is exposed in the Newell-Zayante Creek area. There, foraminiferal faunas are characterized by the com- mon occurrence of the striate uvigerinid U vigerinella cf. U. obesa, whereas inflated Valvulinerias and mollusks are absent. Although Bandy and Arnal (1969, p. 789—790) assign Uvigerinella obesa to their middle bathyal fauna (600 mi300 m), they (1957, p. 2048) earlier report that striate uvigerinids are one of the characteristic forms of the outer shelf fauna off the west coast of Central America. The upper siltstone section probably records a time when more rapid sedimentation outpaced subsidence and the sea became shallower. The impoverishment of the fauna in this part of the section possibly resulted from a restriction that produced toxic bottom condi- tions in the local basin. On the southwest slope of Ben Lomond Mountain, shallow-water conditions probably persisted after depo- sition of the Lompico Sandstone through deposition of the siltstone beds of the Monterey Formation. The Monterey section in the Shell Davenport core hole N o. 1 yields only shallow-water foraminiferal faunas that include such characteristic forms as Ammonia bec- carii, Buccella oregonensis, Buliminella elegantis- sima, and Elphidium hughesi. The westward coarsening of the Monterey Forma- tion from the N ewell-Zayante Creek area and the appearance of numerous arkosic sandstone interbeds in the sections on both flanks of Ben Lomond Moun- tain indicate that the crystalline complex of Ben Lomond Mountain was a topographic high, possibly an island, that supplied clastic detritus to adjacent areas of deposition. The marked faunal contrast in the lower part of the middle Miocene sequence on opposite sides of the mountain suggests that this high sepa- rated these two areas of deposition, at least during Relizian time. On the north limb of the Scotts Valley syncline, the lower part of the Monterey Formation becomes younger toward the west, suggesting that east of Ben Lomond Mountain transgression was from east to west, and the Ben Lomond topographic high was probably transgressed during Luisian time. UPPER MIOCENE TO PLIOCENE SEDIMENTARY SEQUENCE Unconformably overlying the middle Miocene sequence is a succession of sedimentary rocks that records a marine sedimentary cycle that was initiated in late Miocene (Mohnian) time. This upper Miocene to Pliocene sedimentary sequence consists of a shallow— water transgressive sandstone unit, the Santa Mar- garita Sandstone, a deeper water siliceous organic mudstone unit, the Santa Cruz Mudstone, and a shallow-water unit, the Purisima Formation. SANTA MARGARITA SANDSTONE Branner, N ewson, and Arnold (1909, p. 5) assigned the “distinctive formation consisting of pure white sand overlain by white shale” that crops out in the vicinity of Scotts Valley in the eastern part of the area to the Santa Margarita Formation. The northward STRATIGRAPHY NORTHEAST OF SAN GREGORIO FAULT 25 extension of this name to the Santa Cruz Mountains by these early workers (1909, p. 5) was “based upon the stratigraphic, lithologic, and paleontologic similarity” of these beds to the type Santa Margarita of the upper Salinas Valley, about 240 km to the south. The Santa Margarita Sandstone is most exten— sively developed along the Scotts Valley syncline between the community of Ben Lomond and Scotts Valley. Excellent exposures are in the several commer- cial sand pits of this area, where the sand locally is being quarried for construction use. Sandstone beds that discontinuously crops out on the southwestern slope of Ben Lomond Mountain and are locally bituminous are assigned here to the Santa Margarita Sandstone. These beds were included incor- rectly in the Vaqueros Sandstone by Branner, New- son, and Arnold (1909), were assigned a Vaqueros and (or) Monterey “age” by Page and Homes (1945), and are shown as “Lower Miocene marine” (Vaqueros Formation) on the San Francisco sheet of the Geologic Map of California (Jennings and Burnett, 1961). Best exposures are in the several abandoned quarries along the ridges between Majors, Baldwin, and Laguna Creeks, where the bituminous sandstone was worked for road material from 1878 to 1915, and along Sandy Flat Gulch, where the sand is being actively quarried for construction use. The Santa Margarita is typically very thickbedded to massive, thickly crossbedded, well-sorted, slightly granular medium to fine arkosic sand (fig. 6). In fresh exposures the sand is yellowish gray but appears brilliant white along ridges. Large, nearly vertical burrows, as much as 5 cm in diameter, are common in the sand and stand out with slight relief on weathered outcrop surfaces. The Santa Margarita is generally friable but is locally calcareous and firm where fossiliferous. Along San Vicente canyon where it overlies marble of the crystalline basement, this sandstone is solidly ce- mented by sparry calcite and displays columnar jointing. On the southwest slope of Ben Lomond Mountain, the sandstone is locally bituminous, and the bitumen content varies both laterally and verti- cally (for details, see Page and Homes, 1945). The terrigenous mineral composition as determined by point-count analysis (200 points per specimen) is given in table 6. The feldspar grains range from fresh to highly altered; microcline generally is the least altered and plagioclase the most. Orthochemical min- erals include sparry calcite, which locally forms large poikilitic crystals cementing the sandstone, and potas- sium feldspar, which commonly mantles detrital feld- spar grains as clear overgrowths and locally cements the rock. A few glauconite pellets and rare phosphatic bone fragments occur in some specimens. Cobble- and pebble-bearing gravel beds and lenses are common in the lower third of the Santa Margarita that crops out on the south limb of the Scotts Valley syncline. Near Mount Hermon, these gravel beds are as much as 8 m thick and contain numerous abraded vertebrate bones and teeth. The percentage composi- tion of the gravel as determined by thin—section analysis of 100 pebbles from a very thick bed along Lockhart Gulch (sample L37; SE1/4 sec. 1, T. 10 S., R. 2 W.) is: Granitic (leucocratic granite to quartz diorite) 20 Biotite—quartz schist" .. 6 Quartzite (metaquartzite and quartzarenite).,.,.. ...12 Meta-arkose and metasubarkose ..... 8 Arkosic sandstone.............. , 6 Sandy siltstone and mudstone (Monterey) ..... 6 Chert (including Franciscan?) .. 2 Silicic volcanic (virtric tuff, rhyolite, rhyodacite, and porphyry) 37 Basalt 3 100 In general, the granitic, schist, and siltstone clasts predominate among the cobbles and are rounded, whereas the Silicic volcanic, quartzite, and basalt clasts are restricted to pebble size and are typically well rounded. The sandy siltstone and mudstone cobbles commonly display molluscan burrows. The Santa Margarita Sandstone reaches a maxi- mum thickness of 130 min the Olympia area along the axis of the Scotts Valley syncline. To the south along Bean Creek, this formation is about 98 m thick and thins to the southeast, nearly pinching out along Redwood Drive near the eastern limit of the area. Along the Glenwood syncline in the northeastern part of the area, the Santa Margarita is less than 12 m thick and thins to the east. There, limited exposures and the present map scale do not permit its differen- TABLE 6.—Terrigenous mineral composition, in percent, of the Santa Margarita Sandstone and Santa Cruz Mudstone [Point-count analysis by C. A. Wallace. Number in parentheses gives number of specimens point-counted in composite sample; sample locations shown on plates 1 and 2] Santa Margarita Santa Cruz Rock unit ................................ Sandstone Mudstone Sample No. ______________________________ L1(2) L63(6) L123 L126 L128 L132(2) Quartz ...................... 57—68 30—59 11 68 70 60- 66 Plagioclase (Among) 11712 7—16 31 2 7 6—8 Orthoclase_.., 8-18 9—17 9 ll 12 12 Microcline ______________________ 2—5 6—12 40 6 3 <1 Granitic rock fragments _. 5—8 8—30 7 9 4 0-3 Metamorphic rock fragments 0»<1 <—4 <1 2 <1—4 Sedimentary rock fragments ...... <1 0»<1 <1 2 <1 0m<1 Volcanic rock fragments .......... 2 2-5 <1 1 2 Muscovite ............. ._ 0—<1 O—<1 -<1»6 Biotite _____ <1»2 0—<1 3 <1 <1 0—<1 Chlonte 0-< 1 <1 0—2 Sphene <1 O—< 1 Zircon ......... <1 0-<1 <1 <1 <1 0-< N >< :r >< :- >< cr North Pacific diatom zone and subzone Diatoms: Denticula cf. D. kamtschatica Sabelina ................................. Lithodesmium minusculum Grunow .. Notzschia sp. (aff. N. miocenica Burckle? Nitzschia reinhaldii Kanaya and Koizum Nitzschia sp Rouxia californica Peragallo Synedra jouseana Sheshukova-Poretzkay Thalassiosira antiqua (Grunow) CleveAEuler Thalassiorsira convexa var. aspinoisa Thalassiosira lineata Jouse .................. Thalassiosira miocenica Schrader Thalassiosira nativa (of Schrader) Silicoflagellates: Dictyocha aspera clinata Bukry . . Distephanus boliuiensis frugalis Bukry . ................................ IX-XXX lX><)<><- XX ->< x-x-xxx-x-x- ->< x-x-x‘ xxx-x. x-xxx- - x . bilis. The genus Glycymeris ranges from the intertidal zone to a depth of 365 m along the West Coast today (Keen, 1963, p. 105). Solariella peramabilis is distrib— uted along the West Coast from San Diego to Alaska (Grant and Gale, 1931, p. 839), where its bathymetric range is from 5 to 247 m and averages 27 to 37 m. Although the modern genus Patinopecten may indi- cate modern depths (Durham and Addicott, 1965, p. A18), P. healeyi is commonly associated with rela- tively shallow—water mollusks and echinoids in other Pliocene deposits of California. Calicantharus is an extinct genus, but its closest living relatives are inner neritic (W. O. Addicott, oral commun., 1974). This limited molluscan evidence suggests an inner neritic depositional depth for the upper part of the Santa Margarita Sandstone. The lateral variation in the thickness of the Santa Margarita Sandstone resulted from transgression over an irregular topographic surface; the formation thins markedly and locally pinches out onto ancestral highs. The continuity of the Astrodapsis beds from the Olympia area near the axis of the Scotts Valley syncline, where the sandstone is about 130 m thick, to the north limb of the syncline, where it is between 15 and 37 m thick, demonstrates that, in this area at least, the thinning occurs in the lower part of the formation. The coincidence of the thickest secticin of the Santa Margarita with the axis of the Scotts Valley syncline suggests that downwarp may have been active during deposition of the sands, while nearby granitic areas remained as highs, perhaps islands, in the transgressive sea. The thinning of the sandstone onto such a granitic high is clearly seen along Red— wood Drive in the southeastern part of the Felton quadrangle (sec. 30 and 31, T. 10 S., R. 1 W.), where the formation thins northward from 24 m to 5 cm in a distance of slightly more than 1.6 km. The pebble bed near the top of the Santa Margarita Sandstone probably represents a submarine lag gravel. This bed records the beginning of slower deposition with the development of a bored zone at its base together with a local concentration of marine mammal bones and glauconite. Progressively less coarse elastic detritus reached the area as basinal subsidence began. The Santa Cruz Mudstone was deposited as the area submerged below the zone of effective wave and current action. During deposition of this formation, the supply of coarse elastic detritus was greatly reduced, pyroclastic debris began reaching the basin, and diatoms flourished. Lucinoma cf. L. annulata in the lower part of the mudstone is similar to the living species that today ranges from San Diego northward to Kodiak Island and lives in depths from 27 to 68 m 36 off Puget Sound and from 15 to 247 m off San Pedro. This species thus suggests shelf conditions. The limited foraminiferal fauna from theupper part of the type section (USGS 10c. Mf2187) is also diagnostic of shelf conditions, and Buliminella elegantissima sug- gests inner neritic depths. Rhyolitic ejecta followed by andesitic debris reached the basin in significant quantities during deposition of the Purisima Formation to produce shallower condi- tions. The mollusks from the area suggest inner neritic depths, and those collected by the writer from the adjacent Laurel quadrangle include such shallow- Water representatives as Siliqua lucida and Solen sp., which are diagnostic of depths of less than 45 m. The Purisima Formation is more widespread in the Santa Cruz Mountains than the Santa Cruz Mudstone and records a Pliocene continuation of a marine trans- gression that began with deposition of Santa Mar- ' garita Sandstone in late Miocene time. STRATIGRAPHY SOUTHWEST OF SAN GREGORIO FAULT Sedimentary strata exposed southwest of the San Gregorio fault (fig. 1) are of Mesozoic and Cenozoic age and differ markedly from the strata northeast of the fault. The Mesozoic section consists of more than 2,600 m of marine clastic sedimentary rocks, assigned to the Pigeon Point Formation of Late Cretaceous age. These Cretaceous strata crop out along the coast north of the mapped area and have been penetrated in the subsurface of the Ai'io N uevo quadrangle. Uncon- formable upon and locally faulted against the Pigeon Point Formation are more than 520 m of elastic sedimentary and volcanic rocks that range in age from Oligocene (Zemorrian) to Holocene. This Ceno- zoic section includes the Vaqueros(?), Monterey, and Purisima Formations, which are complexly folded and faulted in exposures north and east of Afio Nuevo Point. ‘ PIGEON POINT FORMATION The Pigeon Point Formation was the name applied by Hall, Jones, and Brooks (1959) to Upper Cretaceous strata that are discontinuously exposed along the coast from Pescadero Beach in the southwest corner of the San Gregorio quadrangle southward to midway between Franklin and Afio Nuevo Points in the Franklin Point quadrangle (fig. 1). These strata do not crop out in the area of the present report but were penetrated in the Richfield Steele core hole No. 1, about 11/2 km north of ABO N uevo Point. In the coastal exposures this formation consists of a heterogeneous sequence of interbedded sandstone, STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA mudstone, and conglomerate, which displays a variety of sedimentary structures. Lowe (1972), who has studied the sedimentology of this sequence, informally subdivides it into two parts: a lower 1,800 m of thick units of interbedded sandstone and mudstone, and an upper 900 m of bedded conglomerate, of interbedded conglomerate, pebbly mudstone, and coarse-grained sandstone, and of crossbedded fine-grained sand- stone. He interprets the lower part as having been deposited in part by turbidity currents on the conti- nental slope or on the uppermost continental rise, and the upper part as representing submarine canyon or slope fill and shallow shelf sediments. The sand grains are angular to subangular and composed predominantly of quartz (45 to 80 percent), feldspar (20 to 50 percent), and lithic fragments (10 to 30 percent), of which volcanic and granitic clasts are more abundant than metamorphic and sedimentary types (Tyler, 1972, p. 544). The conglomerate consists mainly of well-rounded pebbles and cobbles of silicic volcanic, plutonic (mostly quartz diorite), and meta- sedimentary rocks. This lithology suggests that the sediments were derived largely from the crystalline rocks of the Salinian block and from a silicic volcanic terrane. The porphyritic rhyolite that apparently under- lies the Pigeon Point Formation about 16 km north of the mapped area and west of the San Gregorio fault may represent a part of this volcanic terrane. The relative abundance of lithologically similar volcanic clasts and the postulated northeast to southwest paleocurrent movement for the sediments of the Pigeon Point (Tyler, 1972, p. 553) are consistent with this source interpretation. The base of the Pigeon Point Formation is not exposed. At Pescadero Beach, this formation is over- lain with marked angular discordance by conglom- erate and sandstone beds that were referred to the Vaqueros(?) Formation by Hall, Jones, and Brooks (1959) and were included in the Mindego Formation by Cummings, Touring, and Brabb (1962). To the south and inland, the Pigeon Point Formation appears to be overlain unconformably by the Purisima Formation of Pliocene age. In sea cliff exposures 2.2 km north of Afio Nuevo Point, very thick sandstone beds of the Purisima rest with clear angular discordance upon the more steeply dipping beds of the Pigeon Point. This unconformity was probably penetrated in the Rich- field Steele core hole, as Upper Cretaceous strata were intersected at a depth of 49 m beneath the Purisima Formation. . Estimates of the thickness of the Pigeon Point Formation range from more an 2,600 m (Hall and others, 1959, p. 2856) to 3,0001m (Branner and others, 1909), but the larger estimate included 230 m of the STRATIGRAPHY SOUTHWEST OF SAN GREGORIO FAULT 37 Monterey Formation that crops out at Afio Nuevo Point. The Richfield Steele core hole penetrated about 780 m of the Pigeon Point Formation but failed to reach its base. Fossils listed by Branner, Newsom, and Arnold (1909) and by Hall, Jones, and Brooks (1959) are diagnostic of Campanian and probably Maestrich- tian age. Foraminifers identified by R. L. Pierce (written commun., 1963) from the Richfield Steele core hole range in age from “probably Goudkoff's D—l zone” (Maestrichtian) downward to “Goudkoff’s F—2 and (or) G—l zone” (Campanian, Santonian, and (or) Coniacian). The lower and upper age limits of the Pigeon Point Formation have not been determined. VAQUEROSQ) FORMATION Strata referred to the Vaqueros(?) Formation by Hall, Jones, and Brooks (1959, fig. 2) crop out north and east of Ano Nuevo Point. About 1.6 km north of this point and 460 m north of the mapped area within the Franklin Point quadrangle, beds of this forma— tion are exposed at low tide below the beach for a distance of 210 m. There, the Vaqueros(?) consists predominantly of olive—gray to dusky—yellowish- brown bioturbated siltstone; bedding is defined by a few thick, graded, very fine to fine-grained arkosic sandstone interbeds. In limited exposures near the upper part of the beach, the siltstone includes thin phosphatic lenses, which were not observed in the fresh outcrops at low tide, and is locally cut by numerous thin sandstone dikes. Approximately 85 m of section is discontinuously exposed and stratigraphically isolated below the beach, where the beds are folded into a syncline, the northern limb of which becomes vertical and is locally overturned. Locally these siltstone beds contain abundant calcareous foraminifers charac— teristic of lower bathyal depths (Brabb and others, 1977, table 8). Siphogenerina nodifera is diagnostic of Zemorrian (Oligocene) age, and the joint occur- rence there of S. nodifera with S. mayi in the upper part of this section is diagnostic of late Zemorrian age. The uppermost part of this section may extend into Saucesian (early Miocene) age, for the strati- graphically highest faunal assemblage is of late Zemorrian or early Saucesian age. About 370 m east of Afro Nuevo Point, the Vaqueros(?) Formation is exposed along the beach and sea cliffs near the axis of an anticline, the central part of which is occupied by volcanic breccia. This formation consists there of laminated dusky- yellowish-brown phosphatic mudstone interbedded with medium- to fine-grained glauconite-bearing arkosic sandstone. East of a volcanic headland, where about 12.5 m of the Vaqueros(?) is exposed in the sea cliffs, sandstone predominates in the lower part of the section, whereas contorted and sheared mudstone makes up most of the upper part (fig. 13). West of the volcanic outcrops, where about 13 m of this formation is discontinuously exposed below the beach, the lower part of the Vaqueros(?) includes dark—yellowish—brown burrowed siltstone beds that resemble those exposed to the north in the Franklin Point quadrangle. The volcanic breccia is highly altered and cut by siliceous and dolomitic veins. One rock is fine grained and microporphyritic, with microphenocrysts of plagio clase (labradorite) as much as 1.4 mm long and of pyroxene ghosts that range from 0.80 mm to 1.2 mm and are completely altered to clay, probably mont- morillonite, and calcite. The groundmass consists largely of a felted mass of plagioclase microlites, most of which appear skeletal, and of clay, which is probably an alteration product. Quartz occurs locally between feldspars and in interstices. Calcite is common and is probably a replacement and altera- tion product; locally it fills amygdules. The texture, feldspar composition, and tentative identification of pyroxene suggest that this rock is probably a tholeiitic basalt. Other volcanic blocks are more felsic and are probably andesite. On the east flank of the anticline, sandstone beds of the Vaqueros(?) Formation locally appear to rest upon the volcanic breccia. Two large blocks of mud- stone are included in the breccia just west of this contact. . The mudstone beds of the Vaqueros(?) Formation are faulted against the siliceous beds of the Monterey Formation 'on the east flank of the anticline (fig. 13), whereas to the west the Vaqueros(?)-Monterey contact appears to be gradational. Mudstone beds within this Vaqueros(?) section yield deep—water calcareous foraminifers diagnostic of Saucesian and Relizian ages (Brabb and others, 1977, table 8). The stratigraphically lowest faunas are from the discontinuous intertidal exposures just west of the volcanic rocks and include Bulimina alligata, Cibicies dohertyi, C. floridanus, Dentalina quadrulata, Siphogenerina multicostata, S. trans- versa, and Uvigerinella obesa impolita, which together are diagnostic of Saucesian, probably early Saucesian age. The sheared mudstone beds from the upper part of the section east of the volcanic rocks yield foraminifers diagnostic of Relizian age. Siphogenerina cf. S. hughesi from the lower part of this sheared section is suggestive of early Relizian age. Higher in the sheared section taxa characteristic of the Pseudosaucesian facies of Beck (1952) are 38 STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA associated with middle Miocene (probably Relizian) species. The foraminiferal dating thus suggests that the Vaqueros(?) Formation that crops out north of Afio Nuevo Point is older than most, if not all, of the section east of the point. The base of the Vaqueros(?) Formation is not exposed in the Vicinity of A50 Nuevo Point. It probably rests unconformably on the Pigeon Point Formation in the subsurface, for the conglomerate and sandstone beds at Pescadero Beach in the San Gregorio quadrangle that were mapped as Vaqueros(?) Formation by Hall, Jones, and Brooks (1959) and yield Macrochlamis magnolia, which is diagnostic of late Zemorrian or early Saucesian age (W. O. Addicott, written commun., 1977) lie with marked angular discordance on steeply dipping beds of the Pigeon Point. At Pescadero Beach, these beds of the Vaqueros(?) Formation appear to be overlain by volcanic breccia that is more highly altered but petrologically similar to that east of Afio Nuevo Point. MONTEREY FORMATION The Monterey Formation is well exposed in the sea cliffs north and east of Ai’io Nuevo Point and on Ai'io Nuevo Island. This formation consists predomi- nantly of thin-bedded and thinly laminated, olive- gray to dusky-yellowish-brown, siliceous mudstone. The siliceous beds are brittle and fractured and locally alternate with thin beds of less siliceous mudstone and slightly glauconitic siltstone. In the eastern sea cliff exposures, a color banding is pro- duced by a few dolomitic mudstone interbeds that weather pale yellowish brown. More than 215 m of the Monterey Formation is gently folded between Ano Nuevo Island and the contact with the underlying Vaqueros(?) Formation to the east. Farther east of the east flank of the anticline, about 67 m of this formation crops out between the Ano Nuevo fault and the contact with the overlying Purisima Formation. The contact with the subjacent Vaqueros(?) Formation appears to be gradational, whereas the contact with the super- jacent Purisima Formation is clearly unconformable. This upper contact is well exposed in the sea cliffs about 640 m east of Aim Nuevo Point, where a 1.2—1.5-m-thick basal conglomerate bed of the Purisima composed largely of siliceous Monterey cobbles rests with slight angular discordance on the siliceous beds of the Monterey (fig. 12). The Monterey Formation contains a few cetacean bones, fish scales, and a diverse microfauna. Diatoms are locally common, siliceous sponge spicules and radiolaria are less abundant, and calcareous foramin- ifers occur locally, where they are typically poorly preserved and rarely silicified. The foraminifers are characteristic of the upper bathyal biofacies (300:185m) of Bandy and Arnal (1969) and are diagnostic of middle Miocene age (Brabb and others, 1977, table 8). The joint occurrence of Bolivina advena var. striatella, Valuulinera cf. V. californica var. obesa, and of V. cf. V. depressa is diagnostic of late Relizian to early Luisian age. Florilus incisus suggests that at least part of this section is restricted to the upper Relizian.~ PURISIMA FORMATION The Purisima Formation that crops out east of A130 Nuevo Point and west of the San Gregorio fault is informally subdvided into a mudstone member and a sandstone member. The mudstone member is exposed above a beach east of its unconformable contact with the underlying Monterey Formation (fig. 12) and west of the Green Oaks fault that brings it into contact with the sandstone member. The lowermost part of the mudstone section is medium to thick bedded, but to the east it is nodular and highly fractured and bedding is not apparent. The mudstone is light olive gray where fresh but weathers with a light-brown limonite coating. It contains thin laminae of sand and is slightly glauconitic. The mudstone contains abundant diatoms, a few fish fragments, and a few small pelecypods. A sample collected from near the base of the sea cliff that is approximately 15 m stratigraphically above the base of this mudstone section provided the following diatoms identified by J. A. Barron (written commun., 1977): Denticula kamtschatica Sabelina Nitzschia fossilis (Frenguelli) Kanaya Nitzschia of. N. reinholdii Kanaya and Koizumi Thalassiosira antiqua (Grunow) Cleve-Euler Thalassiosira oestrupii (Ostenfeld) Proshkina- Lavrenko , Barron believes that this flora correlates with North Pacific diatom Zone IX of early Pliocene age, which, however, is younger than the traditional early Plio- cene of the West Coast megainvertebrate chronology. This correlation indicates that these mudstone beds of the Purisima are younger than the lower tuff- aceous and diatomaceous siltstone beds of the Purisima that crop out south of Scotts Valley and in Santa Cruz (table 7). About 76 m of this mudstone is exposed above the unconformity with the underlying Monterey Forma- tion. It is unconformably overlain by Pleistocene terrace deposits. Because the mudstone and sand- STRATIGRAPHY SOUTHWEST OF SAN GREGORIO FAULT 39 FIGURE 12.—Thick basal conglomerate bed of the mudstone member of the Purisima Formation (Tpm) resting unconformably upon siliceous beds of the Monterey Formation (Tm) about 640 m east of A'fio Nuevo Point. deposits. A few cetacean bones and vertebrae are enclosed within the carbonate concretions of this member. Mollusks are more common and locally abundant in the sea cliff exposures southeast of Afio Nuevo Creek, where they are concentrated in lenses as much as 10 cm thick. Mollusks and irregular echi- noids are also abundant in sandstone beds of the Purisima along Afro Nuevo Creek about 60 m south stone members are in fault contact, their strati- graphic relationship is uncertain. The sandstone member consists of well-sorted fine- grained lithic sandstone that is typically thick to very thick bedded and locally crossbedded. The sandstone is olive gray where fresh but weathers pale yellowish brown to grayish orange. Small, irregular carbonate concretions are scattered in the sandstone and locally form thin discontinuous inter- beds. Other carbonate concretions are elongate as much as 1 to 1.5 In parallel to the bedding. As much as 110 m of the sandstone member is discontinuously exposed between the Green Oaks fault to the west and the San Gregorio fault to the east, but this sea cliff section is folded and locally faulted. Neither the base nor the top of this member is exposed. It probably rests unconformably upon the Pigeon Point Formation because lithologically similar beds of the Purisima are unconformable upon the Pigeon Point above 2 km to the north. It is unconformably overlain by Pleistocene terrace of the northern boundary of the quadrangle (USGS loc. M5154). The sandstone member of the Purisima is sepa- rated into two faunally distinct sections by an unnamed fault that cuts the sea cliff about 600 m west of the mouth of A'no Nuevo Creek (pl. 1). The fauna from the section west of the fault (Arnold’s loc. 139, 1908a) was believed by Branner, Newsom, and Arnold (1909, p. 6) to be similar to that of the type Purisima to the north. The eastern sandstone section that is truncated by the San Gregorio fault to the east is more fossili- 40 STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA ferous. Because Branner, Newsom, and Arnold (1909) believed that its fauna (Arnold’s loc. 140, 1908a) was younger and similar to that of the lower part of the type Merced Formation, they incorrectly mapped these eastern beds as “Merced.” From a locality (USGS loc. M5154) about 1 km north of the mouth of A710 Nuevo Creek that is probably on strike with the eastern sandstone section, the writer collected Dendraster gibbsi (Rémond). This echinoid species is reported by Touring (1959, p. 185) to be common in one bed in the upper part of the upper Pliocene San Gregorio Member of the Purisima Formation of Cummings, Touring, and Brabb (1962). This species has not been reported from the type Merced Formation but occurs in the Etchegoin Formation (W. O. Addicott, written commun., 1969). These correlations suggest that the sandstone ‘ member is, at least in part, oflate Pliocene age in the provincial megainvertebrate chronology and is correla- tive with both the Purisima and Merced Formations in their type areas. QUATERNARY DEPOSITS MARINE TERRACE DEPOSITS Five prominent emergent marine terraces indent the seaward slope of Ben Lomond Mountain between Santa Cruz and A'no Nuevo Point. Each terrace consists of remnants of a wave-cut platform and sea cliff, which are cut into the crystalline and Tertiary sedimentary rocks of the mountain. The elevations of the inner edges of the platform surfaces from lowest to highest are: 27 to 38 m, 80 to 111 m, 116 to 135 m, 167 to 197 m, and approximately 255 to 260 m (modified from Bradley, 1965, fig. 13—1). These surfaces are overlain by a thin, discontin- uous cover of marine and nonmarine sediments (pls. 1—2). The terrace deposits consist typically of massive to locally crossbedded, moderate-yellowish-brown, well sorted, fine to medium sand. Locally at the base of the sand is a thin pebble and cobble gravel bed that contains rounded clasts of mudstone, silicic volcanic rocks, granitic rocks, and black and red chert. Adjacent to the emergent sea cliffs, these deposits commonly include semiconsolidated, poorly sorted, sandy granule conglomerate composed mainly of mudstone clasts. These deposits are best preserved on the lowest emergent platform, where they are generally from 1.5 to 6 In thick and reach a maximum thickness of about 24 In along the coast above Greyhound Rock in the Afio N uevo quadrangle. The upper terraces have been deeply dissected and their surficial deposits largely removed by subsequent erosion. From 3 to 4 m of sand is preserved only locally on remnants of the highest emergent marine terrace. Where the platform surfaces bevel the Santa Cruz Mudstone or the Purisima Formation, the bedrock surface is commonly pitted by molluscan borings. Fossil shells are preserved, however, only in the deposits on the lowest emergent terrace of the coastal area. Mollusks are locally abundant at the base of these deposits near Point Santa Cruz and north and east of Afio Nuevo Point, where Arnold (1908a, p. 355-356) records 32 gastropods and pelecypods. Addicott (1966) lists 101 larger invertebrate taxa, mainly mollusks, 27 taxa of foraminifers, and 18 species of ostracodes from these two areas and assigns a late Pleistocene age to these fossil assemblages. Mollusks from this lowest terrace yield uranium- series dates of 68,000 to 100,000 years (Bradley and Addicott, 1968) and amino-acid dates of 130,000;L 50,000 years (Lajoie and others, 1975). Presumably all of the terraces are of Pleistocene age. Lajoie, Weber, and Tinsley (1972, p. 107) postulate that the highest terrace “would be between 700,000 and 800,000 years old,” whereas Bradley and Griggs (1976, p. 444) estimate the age of this oldest platform to be 1,000,000 to 1,200,000 years. Bradley (1 965) believes that these terraces were cut by wave erosion when sea level was rising and that the marine sediments were laid down as beach deposits when sea level was falling. These marine sediments were then locally covered by a relatively thin veneer of fluvial and colluvial sediments. This sequence of erosion and deposition must have been repeated at least five times during uplift of the Ben Lomond Mountain area. The lowest and thus youngest emergent terrace was probably out by the high stand ofthe sea during the Sangamon Interglaciation of the mid-continent. Addicott’s (1966) analysis of the mulluscan assem- blage from this terrace suggests that the late Pleisto- cene marine climate in the area was cool temperate compared to the modern temperate climate. RIVER TERRACE DEPOSITS Along the San Lorenzo River, terraces are locally mantled by weakly consolidated sandy pebble to ' cobble gravel and pebbly fine to medium sand. The topographically highest terrace deposit on the east slope of Ben Lomond Mountain is exposed along the Felton-Empire Road at an approximate elevation of 275 to 290 m. These deposits are more numerous along the river to the south where they occur locally QUATERNARY DEPOSITS 41 at elevations of 120 to 145 m, 92 to 100 m, 62 to 80 m, and possibly from 43 to 49 m. These gravels and sands were laid down by the river when the region was lower during Pleistocene time. LANDSLIDE MATERIAL Landslide conditions are widespread in the area. Although all of the mapped rock units have been affected locally, landslides are most commonly developed in the northern and western parts of the area where steeper slopes are underlain by one of the mudstone units. The highly fractured rocks of the Monterey Formation and of the Santa Cruz Mudstone have been most susceptible to landsliding, and exten- sive creep of these strata has resulted in unreliable structural attitudes on slopes and along narrow ridges. Only the larger definite landslide deposits that could be readily mapped in the field are shown on plates 1 and 2. A preliminary landslide map of Santa Cruz County based on photointerpretation by Cooper— Clark and Associates (1975) covers most of the area of the present report and should be consulted for land-use planning. ALLUVIUM Alluvial deposits of Holocene age are discontin- uously distributed along the valley bottoms of the larger streams and consist of unconsolidated moder— ately sorted silt, sand, and gravel derived from the drainage areas of the respective streams. This allu- vium is a meter to several meters thick but is locally thicker near the coast. A well drilled along the San Lorenzo River near the northern limit of the city of Santa Cruz is reported to have penetrated 29 m of gravel (Alexander, 1953, p. 20). Some of the thicker coastal deposits may be as old as late Pleistocene, for detrital wood fragments from 15 m below the surface of alluvial fill in the valley of Ain Nuevo Creek yield a radiocarbon date of 10,200i 300 years B.P. (Wright, 1971). Soil and colluvium have not been mapped but cover much of the area and hinder precise mapping of the bedrock geology. STRUCTURE The Santa Cruz Mountains are traversed by two major active faults, the San Andreas fault to the northeast and the San Gregorio fault to the southwest (fig. 1). The San Andreas fault, forming the north— eastern boundary of the Salinian block, passes 5 km northeast of the mapped area. To the west within the area, the San Gregorio fault has juxtaposed two major tectonic blocks with markedly different stratig- raphies. The tectonic block between the San Gregorio and San Andreas faults is broken into two smaller blocks with interdependent histories by the northwest- trending Zayante fault. South of the Zayante fault, crystalline rocks of the Salinian block are extensively exposed or occur at shallow depths below a thin cover of Tertiary strata that are folded into broad open structures. North of this fault, the basement is not exposed and is deeply buried under a very thick section of Tertiary strata that are more strongly deformed and locally overturned. SAN GREGORIO FAULT The onshore trace of the San Gregorio fault was originally mapped by Branner, Newsom, and Arnold (1909) as a single fracture extending from the coast about 3 km east of Aho Nuevo Point northwestward for 27 km to coastal exposures near San Gregorio (fig. 1). Recent onshore and offshore mapping has demon- strated that the fault as traced by these early workers is part of a wider zone of deformation that includes numerous shorter subparallel faults (Brown, 1972). East of Ano Nuevo Point, this zone is about 3 km wide and includes at least five northwest-trending faults from the Ano Nuevo fault on the west to minor faults at Greyhound Rock to the east (pl. 1). The Ano Nuevo fault is well exposed in the sea cliff abut 430 m east of the point, where it strikes N. 40° W. and 38° NE. (fig. 13). The lowest emergent marine terrace is displaced about 5.2 m by this fault, and siliceous beds of the Monterey Formation are thrust over unconsolidated marine terrace sand and gravel. In the sea cliff west of this fault, phosphatic mud- stone beds of the Vaqueros(?) Formation are highly sheared for a distance of about 17 m, but this shearing does not appear to have affected the overlying terrace deposits. About 500 m east of the Ano Nuevo fault, the Green Oaks fault is poorly exposed above the beach, where it has juxtaposed nodular beds of the mudstone member of the Purisima Formation against beds of the sandstone member. Dips in the sandstone beds steepen westward toward the fault, suggesting that the east side is relatively downthrown. This fault probably continues northwestward 2.2 km to sea cliff exposures north of A'no Nuevo Point, where a gouge zone separates slightly overturned siltstone of the Vaqueros(?) to the south from steeply dipping beds of the Pigeon Point Formation to the north. About 700 m farther to the east, a nearly vertical fault has dropped Quaternary alluvium down 42 STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA against sandstone beds of the Purisima to the west and separates two faunally distinct Purisima sections. Charcoal from a deformed bed of this alluvium has yielded a carbon-14 age of 9,510i140 years B.P. (Weber and Lajoie, 974). This fault strikes N. 30° W. across the lowest terrace, where it has produced an alinement of topographic scarps. The major change in stratigraphy occurs across the main strand of the San Gregorio fault (fig. 14). This fault intersects the coasts east of Afio N uevo Creek at a notch in the sea cliff that trends N. 15° W. There, fossiliferous sandstone beds of the Purisima are faulted against the Santa Cruz Mudstone. In sea cliff exposures west of the fault, the Purisima is cut by numerous minor high-angle faults, whereas for 150 m southeast ofthe fault the Santa Cruz is highly fractured. The lowest emergent terrace appears to be vertically offset about 5 m by this fault (W. C. Bradley, written commun., 1970). In the sea cliffs east of Greyhound Rock, the Santa Cruz ,Mudstone and terrace deposits are vertically FIGURE 13.—Ai‘10 Nuevo fault has thrust siliceous beds of the Mon Formation ('IV?) and over unconsolidated marine terrace deposits (Qm) about 430 m east of A710 Nuevo Point. displaced a total of 10.5 m by three closely spaced northwest-trending faults (W. C. Bradley, written commun., 1970). The easternmost fault appears to offset the upper surface of the Pleistocene terrace deposits (Hall and others, 1974). These faults at Greyhound Rock may mark the eastern boundary of the San Gregorio fault zone. Seismic profiling south of the Aho N uevo coast has delineated two northwest-trending parallel faults that bound a deformed zone (Greene and others, 1973). The eastern offshore fault is alined with the main strand of the San Gregorio fault, whereas the western fault trends toward the Green Oaks fault. Greene, Lee, McCulloch, and Brabb (1973) have traced the San Gregorio fault zone southward across Monterey Bay and postulate that it continues on- shore near Point Sur in the Santa Lucia Range as the Serra Hill and Palo Colorado faults. These workers also suggest that this fault zone trends northward from San Gregorio to join the onland Seal Cove fault in San Mateo County and then continues offshore to terey Formation (Tm) over sheared mudstone beds of the Vaqueros(?) STRUCTURE 43 Afio Nuevo Point *Lower (?) and upper Pliocene: sandstone and mudstone (180+m) \ M Middle Miocene: siliceous mudstone (215+m) \ Oligocene to middle Miocene: sandstone, siltstone, volcanic breccia, and phosphatic mudstone (100+m) - M *Upper Cretaceous: sandstone, mudstone, and conglomerate (2600+m) M Porphyrific rhyolite ‘ ? PACIFIC OCEAN *Unit abuts San Gregorio fault *Upper Miocene and lower Pliocene: siliceous mudstone and sandstone (0—2800 m) Middle Miocene: mudstone, siltstone, and sandstone (1050 m) Eocene: sandstone and conglomerate (1430 m) M Paleocene: siltstone and sandstone (270 m) *Granitic basement D Santa Cruz FIGURE 14.—Contrasting stratigraphic sections across the San Gregorio fault in the central Santa Cruz Mountains. Offshore trace of San Gregorio fault from Greene, Lee, McCulloch, and Brabb (1973). join the San Andreas fault north of the Golden Gate near Bolinas in Marin County (see Jennings, 1975, for a recent compilation of these faults). If these postulated extensions of the San Gregorio fault zone are correct, this zone would be a major branch of the San Andreas fault with an overall length of at least 200 km. The San Gregorio is an active fault zone. The topographic expression of probable fault breaks and anomalous stream patterns along this fault zone in San Mateo County are cited by Brown (1972) as evidence for active faulting. The deformation of Holocene beds east of A'fio Nuevo Point confirms recent movement. Recent earthquake epicenters cluster near this zone in Monterey Bay and are scattered along this zone northward into the area of the present report and beyond (Greene and others, 1973, pl. 2). A magnitude 4.6 earthquake of May 22, 1963, appears to have been associated with the onshore San Gregorio fault zone in the Santa Cruz Mountains (Bolt and others, 1968, p. 1738). Fault- plane solutions for this and recent Monterey Bay earthquakes along this zone are interpreted as showing nearly vertical faults with right-lateral strike-slip motion (Bolt and others, 1968; Greene and others, 1973). The more west—trending, oblique orienta- tion of minor fold axes Within the Monterey Forma- tion near Afio Nuevo Point IS also consistent with right-lateral movement. The marked contrast in the stratigraphic sections across the San Gregorio fault in the Santa Cruz Mountains (fig. 14) suggests past large-scale lateral displacement. Upper Cretaceous strata are notably absent in the block to the east of the fault, and the thick Santa Cruz Mudstone section that abuts the fault to the east is represented by an unconformity in the block to the west. Although Monterey strata of middle Miocene age are mapped in both blocks, these rocks differ in lithology fauna and bathymetry. Hill and Dibblee (1953, pl. 1) included the San Gregorio fault with faults that have or possibly might have a substantial strike-slip component of displacement. Matching pairs of offset geologic features, Graham and Dickinson (1978) have recently postulated about 115 km of post-early Miocene right slip along this fault zone. But the past history of this fault is still to be worked out. ZAYANTE FAULT The Zayante fault was originally mapped by Branner, Newsom, and Arnold (1909) from the vicinity of Boulder Creek slightly south of east to the east boundary of the Santa Cruz 30-minute quad- rangle. Because of the dense vegetation and deep weathering in the area, this fault is poorly exposed, and its trace is mapped largely from structural and 44 STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA stratigraphic discordances of adjacent beds. Where the Zayante fault is exposed along Bear Creek Road about 0.7 km north of the mapped area, the Butano Sandstone and Vaqueros Sandstone are separated by a 6-m—wide, nearly vertical shear zone. Although the throughgoing strand of this fault is confined to a narrow zone, branching lineaments in the vicinity of Newell Creek occupy a zone as wide as 2 km. Only locally is the Zayante fault expressed topo- graphically. East of Zayante Canyon, Mountain Charlie Gulch flows southward until it reaches the fault and then turns abruptly westward to follow the fault to Zayante Creek. The relatively straight course of this fault across the canyons and ridges of the area indicates that the fault plane is nearly vertical. Along Mountain Charlie Gulch, the Zayante Sand- stone and the lower sandstone member of the Butano are juxtaposed by the fault, indicating about 1.2 km of ' dip separation with the north side relatively down- thrown. Model studies of the gravity field across the area suggest that the crystalline basement is displaced approximately 2 km by this fault (Clark and Rietman, 1973, p. 12—14). The Zayante fault can be mapped southeastward from the area for about 17 km to the vicinity of Corralitos, where it becomes covered by Quaternary sediments. A gravity investigation (Clark and Riet- man, 1973) demonstrates the continuity of this fault beneath this cover into the Vergeles fault at the northern margin of the Gabilan Range, about 44 km southeast of the area. The Vergeles fault in turn continues southeastward into the San Andreas fault south of San Juan Bautista. The Zayante fault has been mapped northwest from the town of Boulder Creek to its juncture with the Ben Lomond fault (fig. 1). Because the suggested amount and sense of displacement along a fault on the north flank of Ben Lomond Mountain are of the same order of magnitude as that along the Zayante fault to the east, Clark and Rietman (1973) believe that the north- western part of the Ben Lomond fault of earlier mappers is the westward continuation of the Zayante fault. The Zayante-Vergeles fault, including this western segment, is 82 km long. The Oligocene (Zemorrian) shallowing that is recorded by the upper part of the Rices Mudstone Member of the San Lorenzo Formation along N ewell and Love Creeks probably denotes initial movement along the Zayante fault. Continued uplift along this fault resulted in emergence of the crystalline base- ment to the south, in deposition of the terrestrial Zayante Sandstone along and north of the fault, and in restriction of marine conditions to an embayment to the north. Allen (1946) reports a remarkably similar history along the Vergeles segment of the fault in the San Juan Bautista quadrangle to the southeast. The Zayante fault ceased to be an important struc- tural feature by early Miocene (Saucesian) time. The fine clastic deposition of the Lambert Shale north of the fault suggests that the uplifted block to the south had been reduced to a lowland, which was subse- quently transgressed by the middle Miocene seas with deposition of the Monterey Formation. Two small patches of Monterey Formation that are preserved as tectonic slivers along the fault provide evidence for post-Miocene displacement. This movement may have occurred in post-early Pliocene time, for the Purisima Formation is offset along this fracture to the east of the area. Some segments of the Zayante fault may be active. Recent movement east of Corralitos is suggested by a scarp of probable Holocene age and a deformed late Pleistocene surface alined with this fault (Hall and others, 1974). These workers also postulate that the Zayante fault east of Corralitos may be connected to the San Andreas by a diffuse system of possible fault breaks, which they define as the Corralitos fault complex. They suggest late Pleistocene and possible Holocene activity along this fault complex. Griggs (1973) plots numerous earthquake epicenters in the central Santa Cruz Mountains between the western segment of the Zayante fault and the Butano fault to the north. It is not known, however, which fault or faults may have produced this activity. MINOR FAULTS The Ben Lomond and Bean Creek faults are rela— tively minor dislocations that displace middle Miocene (Luisian) strata. As originally defined, the Ben Lomond fault was an arcuate fracture that extended west of Boulder Creek (Branner and others, 1909). The name “Ben Lomond” is now restricted to the fault that trends southeastward from near Boulder Creek, through the community of Ben Lomond, to the vicinity of Felton. Near Boulder Creek, this fault has brought the Monterey Formation into contact with the granitic rocks of Ben Lomond Mountain and to the south has locally juxtaposed the Monterey and Lompico Sand- stone, suggesting a dip separation of less than 200 m. Model studies of the gravity field in the Felton area suggest that the crystalline basement is vertically offset less than 350 m by the Ben Lomond fault (Clark and Rietman, 1973, pl. 1). The youngest strata clearly displaced by this fault are of the Monterey Formation of middle Miocene (Luisian) age. The Quaternary alluvium along the San Lorenzo River does not appear to be affected. STRUCTURE . 45 Movement on the Bean Creek fault has deformed the Monterey strata near Mount Hermon. Chevron drag folds in strata on the south side of this fracture indicate that this side is relatively upthrown. As the beds on both sides of the fault yield similar foramin— ifers diagnostic of Luisian age, dip separation appears to be minor. East of the mapped trace of the Bean Creek fault, the Santa Margarita Sandstone is not displaced. ‘ Numerous small faults occur in isolated exposures of the area, but only those that could be traced beyond a single exposure or that are inferred to juxtapose different rock units have been mapped in the present study. FOLDS South of the Zayante fault, the Tertiary strata and crystalline basement are deformed into three broad, northwest—trending structures—the Scotts Valley syn- cline, the Ben Lomond high, and the Davenport syncline (fig. 1). These structures record several periods of deformation. The Scotts Valley syncline extends from Boulder Creek eastward through Scotts Valley. This fold appears to die out farther east in the Laurel quad- rangle, where beds of the Purisima Formation rest on shallow crystalline basement. The Monterey strata are more strongly folded along this structure than the overlying beds of the upper Miocene to Pliocene sequence, documenting late middle or early late Miocene deformation. Downwarp of the Scotts Valley syncline probably continued during deposition of the Santa Margarita Sandstone, for the thickest sand— stone section coincides with the axis of this structure. Gently folding of the Purisima Formation along this syncline records post-early Pliocene deformation. The crystalline rocks of Ben Lomond Mountain are exposed along a northwest-trending anticline that is herein referred to as the Ben Lomond structural high. Beds of the middle Miocene and upper Miocene to Pliocene sequences rest nonconformably upon these rocks and generally dip to the northeast and south- west away from the core of the mountain. The Ben Lomond structural high appears to have been delineated by middle Miocene time. On the east flank, the middle Miocene sequence coarsens toward the west, and on both flanks of the mountain, the Monterey Formation contains thick sandstone inter- beds. These facies changes and marked differences in the middle Miocene faunas on opposite sides of the mountain suggest that the Ben Lomond high had formed by middle Miocene (Relizian) time. This high was probably later uplifted along the Ben Lomond fault before deposition of the Santa Marga- rita Sandstone, which subsequently transgressed this crystalline complex. The unconformable relationship of the Purisima Formation to the underlying Santa _ Cruz Mudstone in the sea cliffs west of Point Santa Cruz records positive movement along the Ben Lomond axis in latest Miocene time. The emergent wave—cut terraces on the western slope of Ben Lomond Mountain have been uplifted and tilted in a seaward direction, documenting that uplift has continued into Quaternary time (Bradley and Griggs, 1976). West of Ben Lomond Mountain, the Santa Cruz Mudstone is broadly folded along the Davenport syncline, which trends westward into the San Gre— gorio fault north of the mapped area (fig. 1). As the emergent terraces appear to be undeformed by this structure, folding did not continue into the Quater- nary. The opening folding south of the Zayante fault contrasts with the tight folding to the north. Along the north limb of the San Lorenzo syncline, which can be traced 16 km to the northwest (fig. 1), the Vaqueros Sandstone and Lambert Shale are locally overturned. This overturned section is poorly exposed in the northeastern part of the area but well exposed to the east along Mountain Charlie Gulch. The Glenwood syncline, along which the Purisima Formation is folded, can be traced for 20 km to the east to the vicinity of Corralitos. After the Eocene to lower Miocene sequence was deformed into the San Lorenzo syncline during middle or late Miocene time, beds of the upper Miocene to Pliocene sequence unconform— [ably overlapped this older structure, and they were subsequently synclinally folded in post-early Pliocene time with the axis of folding to the south of the earlier axis. The two periods of deformation recorded by the synclinal downwarps north of the Zayante fault probably coincide with the late middle or early late Miocene and post—early Plicoene deformations of the Scotts Valley syncline. These northern synclines are separated from the Scotts Valley syncline by an unnamed anticline, along the axis of which the Zayante Sandstone is exposed. The contrast in the intensity of folding north and south of the Zayante fault probably results not from a difference in the type of basement underlying the two areas but rather from a difference in the thickness of the overlying sedimentary sections. To the south where the sedimentary section is much thinner, the rigidity of the relatively shallow crystalline basement has resulted in less intense deformation of the over- lying strata. 46 STRATIGRAPHY, PALEONTOLOGY, AND GEOLOGY OF CENTRAL SANTA CRUZ MOUNTAINS, CALIFORNIA FOSSIL LOCALITIES [M are megafossil localities; Mf microfossil locality; LSJU are Stingord University localities. Localities are shown on plates 1 an USGS Cenozoic localities (Menlo Park register) Sand pit on east side of intersection of Glen Canyon Road and Redwood Drive, SW1/4 sec. 31, T. 10 S., R. 1 W., Felton quadrangle. Santa Margarita Sandstone, upper Miocene. Collected by J. C. Clark and C. A. Repenning, 1962, 1963, and 1964. Slope at north edge of clearing, west of Bean Creek, sec. 13, T. 10 S., R. 2 W., Felton quadrangle. Upper part of Santa MargaritaSandstone, upper Miocene. Same stratigraphic position as M1037; same locality as M5053. Collected by J. C. Clark and C. A. Repen- ning, 1963. Cut at sharp curve of abandoned dirt road, about 30 m east of Bean Creek Road, Felton quadrangle. Upper part of Santa Margarita Sandstone, upper Miocene. Same stratigraphic position as M1036; same locality as M5052. Collected by J. C. Clark and C. A. Repenning, 1963. High vertical cut in sand pit on west side of old Highway 17 in Scotts Valley, elevation about 215 m, Felton quadrangle. Upper part of Santa Margarita Sandstone, upper Miocene. Collected by J. C. Clark, 1960. Kaiser sand pit, south of Bean Creek, Felton quad- rangle. Lower part of Santa Margarita Sandstone, upper Miocene. Collected by C. A. Repenning,, 1963. Cut on east side of Nelson Road, NE% sec. 13, T. 10 S., R. 2 W., Felton quadrangle. Upper part of Santa Margarita Sandstone, upper Miocene. Collected by J. C. Clark and C. A. Repenning, 1964. Palisades on east side of Branciforte Drive in De Laveaga Park, NWIA sec. 7, T. 10 S., R. 1 W., Santa Cruz quadrangle. Santa Margarita Sandstone, upper Miocene. Collected by C. A. Repenning, 1964. Large exposure at rifle range on west side of Peasley Gulch canyon, elevation 24 In, Santa Cruz quad- rangle. Santa Margarita Sandstone, upper Miocene. Collected by J. C. Clark, 1963. Cliff on east side of Moore Creek canyon, below Western Drive in Santa Cruz, elevation 55 m, sec. 15, T. 11 S., R. 2 W., Santa Cruz quadrangle. Upper part of Santa Margarita Sandstone, upper Miocene. Collected by J. C. Clark and C. A. Repenning, 1964. Glauconitic bed in sea cliff below intersection of Delacoste Avenue and West Cliff Drive in Santa Cruz, SE11: sec. 23, T. 11 S., R. 2 W., Santa Cruz quadrangle. Base of Purisima Formation, upper Miocene. Collected by J. C. Clark and C. A. Repenning, 1963. Cut on northeast side of abandoned section of Smith Grade, 10 m northwest of new road and 30 m east of fork of Majors Creek, 51/2 sec. 29, T. 10 S., R. 2 W., Felton quadrangle. Lower part of Locatelli Forma- tion, Paleocene. Same locality as LSJU 3401A. Collected by J. C. Clark, 1960. North side of deep out along Smih Grade, approxi- mately 300 m southwest of where Smith Grade joins Empire Grade, Felton quadrangle. Lower part of Locatelli Formation, Paleocene. Collected by J. C. Clark, 1960. M1035 M1036 M1037 M1038 M1039 M1105 M1106 M1107 M1108 M1109 M4667 M4668 M4669 M5049 M5050 M5052 M5053 M5054 M5055 M5056 M5057 M5058 M5060 M5061 M5062 M5063 M5064 M5065 Bed in unnamed westward-flowing tributary to San Lorenzo River, about 530 m east of where tributary joins river, in Henry Cowell Redwoods State Park, Felton quadrangle. Lower part of Locatelli Forma- tion, Paleocene. Collected by J. C. Clark, 1960. Molluscan bioherm on hillside, approximately 150 m ‘ west of Zayante Road, elevation of 245 m, SW14 sec. 30, T. 9 S., R. 1 W., Felton quadrangle. Above basalt flows within Vaqueros Sandstone, Oligocene. Col- lected by J. C. Clark, 1960. Molluscan bioherm just east of Zayante Road and west onayante Creek, SW‘A sec. 30, T. 9 S., R. 1 W., Felton quadrangle. Above basalt flows within Vaqueros Sandstone, Oligocene. Collected by J. C. Clark, 1960. Cut at sharp curve of abandoned dirt road, about 30 m - east of Bean Creek Road, Felton quadrangle. Upper part of Santa Margarita Sandstone, upper Miocene. Same stratigraphic position of M5053, same locality as M1037. Collected by J. C. Clark, 1961. Slope at north edge of clearing, west of Bean Creek, sec. 13, T. 10 S., R. 2 W., Felton quadrangle. Upper part of Santa Margarita Sandstone, upper Miocene. Same stratigraphic position as M5052, same locality as M1036. Collected by J. C. Clark, 1961. Bed or unnamed tributary to Laguna Creek, about 125 m east of Smith Grade, elevation about 195 m, NW‘A sec. 31, T. 10 S., R. 2 W., Davenport quadrangle. Sandstone interbed in Monterey Formation, middle Miocene. Collected by J. C. Clark, 1962. Bed of Majors Creek, below U-curve in Smith Grade, Felton quadrangle. Lompico Sandstone, middle Miocene. Collected by J. C. Clark, 1962. East bank of Laguna Creek, about 90 m upstream from city water reservoir, SW14 sec. 30, T. 10 S., R. 2 W., Davenport quadrangle. Lower part of Monterey Forma- tion, middle Miocene. Collected by J. C. Clark, 1962. Near base of large exposure on northeast side of straight stretch of Smith Grade, Davenport quad- rangle. Monterey Formation, middle Miocene. Col~ lected by J. C. Clark, 1962. East bank of Laguna Creek, elevation about 125 m Davenport quadrangle. Lompico Sandstone, middle Miocene. Collected by J. C. Clark,_71962. Concretion in west bank above Majors Creek, 30 m N. 60° W. of where Majors Creek forks, Felton quad— rangle. Lompico Sandstone, middle Miocene. Col- lected by J. C. Clark, 1962. Near base of high cliffs on west rim of Majors Creek canyon, elevation about 150 m, Felton quadrangle. Lompico Sandstone, middle Miocene. Collected by J. C. Clark, 1962. Cliffs on west side of Majors Creek canyon, elevation 185 In, Santa Cruz quadrangle. Lompico Sandstone, middle Miocene. Same stratigraphic position as M5061. Collected by J. C. Clark, 1962. Cut on west side of abandoned logging road on west side of Majors Creek canyon, elevation 170 In, Santa Cruz quadrangle. Lompico Sandstone, middle Miocene. Approximately same stratigraphic position as M5061 and M5062. Collected by J. C. Clark, 1962. West bank of Baldwin Creek across stream from small clearing at end of logging road, elevation 140 m, Santa Cruz quadrangle. Near base of Lompico Sand- stone, middle Miocene. Collected by J. C. Clark, 1962. North bank of Gold Gulch, about 90 m upstream from M5066 M5067 M5068 M5069 M5086 M5112 M5154 M5155 Mf2187 Mf3675 Mf3676 Mf3677 Mf3678 FOSSIL LOCALITIES 47 where stream passes under Highway 9, Felton quad- rangle. Lower part of Locatelli Formation, Paleocene. Collected by J. C. Clark, 1962. Bed in Gold Gulch, approximate elevation 160 m, Felton quadrangle. Lower part of Locatelli Forma- tion, Paleocene. Collected by J. C. Clark, 1962. Bed of Laguna Creek, about 215 m downstream from M5058, elevation about 115 m, Davenport quad— rangle. Lompico Sandstone, middle Miocene. Col- lected by J. C. Clark, 1963. Southeast bank of Baldwin Creek canyon, below clearing, elevation 150 In, Santa Cruz quadrangle. Lompico Sandstone, middle Miocene. Collected by J. C. Clark, 1963. Southeast bank of Baldwin Creek canyon, 30 m west of M5068, elevation 140 m, Santa Cruz quadrangle. Lompico Sandstone, middle Miocene. About 6 m stratigraphically below M5068. Collected by J. C. Clark, 1963. Astrodapsis bed in high cut near top of Kaiser sand pit on east side of Zayante Road, elevation 170 m, Felton quadrangle. Santa Margarita Sandstone, upper Miocene. Same stratigraphic position as M5155. Collected by J. C. Clark, 1961. Astrodapsis bed in cliff beneath water fall along unnamed tributary to Zayante Creek, about 280 m east of Zayante Creek, NWM: sec. 30, T. 9 S., R. 1 W., Felton quadrangle. Santa Margarita Sandstone, upper Miocene. Collected by J. C. Clark, 1960. Bank of Ano Nuevo Creek, 10 m below small dam on creek, Ano Nuevo quadrangle. Sandstone member of Purisima Formation, Pliocene. Collected by J. C. Clark, 1968. Astrodapsis bed on ridge known as “sand dollar hill,” NE M: sec. 1, T. 10 S., R. 2 W., Felton quadrangle. Santa Margarita Sandstone, upper Miocene. Same strati- graphic position as M5086; same locality as LSJU 3995. Collected by J. C. Clark, 1960. Wave-cut below sea cliff below intersection of Delacosta Avenue and West CliffDrive in Santa Cruz, SE11: sec. 23, T. 11 S., R. 2 W., Santa Cruz quadrangle. Santa Cruz Mudstone, upper Miocene. About 3 m strati- graphically below unconformity with overlying Purisima Formation. Collected by J. C. Clark, 1963. North side of unimproved road in new subdivision, elevation 120 m, NW‘A NW‘A sec. 6, T. 11 S., R. 1 W., Felton quadrangle. Tuffaceous siltstone bed of Purisima Formation, lower Pliocene. About 18 m above contact with Santa Margarita Sandstone. Collected by J. C. Clark, 1976. Cut on east side of parking lot, 30 m south of Water Street in Santa Cruz, SWM: sec. 7, T. 11 S., R. 1 W., Santa Cruz quadrangle. Diatomaceous siltstone bed of Purisima Formation, lower Pliocene. About 6 to 9 m above contact with Santa Cruz Mudstone. Col- lected by J. C. Clark, 1976. Cut on north side of Highland Avenue just north of sharp curve in Santa Cruz, NW‘A sec. 13, T. 11 S., R. 2 W., Santa Cruz quadrangle. Diatomaceous siltstone bed of Purisima Formation, upper Miocene. About 6 to 9 m above contact with Santa Cruz Mudstone. Col— lected by J. C. Clark, 1976. Cut on west side of hill behind Callaway house, east of Glen Canyon Road, about 21/2 km south of Scotts Valley, Felton quadrangle. Tuffaceous siltstone bed of Purisima Formation, lower Pliocene. About 1 m above beds of Santa Cruz Mudstone, which here is not differentiated from Purisima because of map scale, and about 4.5 m above contact with Santa Margarita Sandstone. Collected by J. C. Clark, 1976. California University of Paleontology (Berkeley) localities (UCMP) V4004 Santa Cruz Aggregates sand pit, north of Bean Creek, SE‘A sec. 14, T. 10 S., R. 2 W., Felton quadrangle. Lower part of Santa Margarita Sandstone, upper Miocene. Santa Cruz Aggregates sand pit, north of Bean Creek, SE‘A sec. 14, T. 10 S., R. 2 W., Felton quadrangle. Santa Margarita Sandstone, upper Miocene. About 60 m above base, and 25 m below top; about 46 m stratigraphically above V4004. Graham sand pit east of Nelson Road, Felton quad- rangle. Lower part of Santa Margarita Sandstone, upper Miocene. V5244 V5555 REFERENCES CITED Addicott, W. 0., 1966, Late Pleistocene marine paleoecology and zoogeography in central California: U.S. Geol. Survey Prof. Paper 523—C, 21 p. _.___1969, Late Pliocene mollusks from San Francisco Peninsula, California, and their paleogeographic significance: California Acad. Sci. Proc., ser. 4, v. 37, no. 3, p. 57—93. 1972, Provincial middle and late Tertiary molluscan stages, Temblor Range, California, in The proceedings of the Pacific Coast Miocene biostratigraphic symposium: Soc. Econ. 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Survey if ”’ DAYs. :76 MM I FLOODS IN KANSAS CITY, MISSOURI AND KANSAS, SEPTEMBER 12—13, 1977 pared jointly by the U.S. Geological Survey Report pre heric Administration and the National Oceanic and Atmosp U.S. DEPARTMENT OF THE INTERIOR 0 U.S. DEPARTMENT OF COMMERCE Y M 9,3 \98\ GEOLOGICAL SURVEY PROFESSIONAL PAPER 1169 '_ DEPOSV“ FLOODS IN KANSAS CITY, MISSOURI AND KANSAS, SEPTEMBER 12—13, 1977 By LELAND D. HAUTH and WILLIAM J. CARSWELL, JR., US. Geological Survey, and EDWIN H. CHIN, National Weather Service, National Oceanic and Atmospheric Administration GEOLOGICAL SURVEY PROFESSIONAL PAPER P1169 Report prepared jointly by the US. Geological Survey and the National Oceanic and Atmospheric Administration UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1981 UNITED STATES DEPARTMENT UNITED STATES DEPARTMENT OF THE INTERIOR OF COMMERCE JAMES G. WATT, Secretary MALCOLM BALDRIGE, Secretary NATIONAL OCEANIC AND GEOLOGICAL SURVEY ATMOSPHERIC ADMINISTRATION Doyle G. Frederick, Acting Director James P. Walsh, Acting Director Library of Congress Cataloging in Publication Data Hauth, Leland D Floods in Kansas City, Missouri and Kansas, September 12—13, 1977. (Geological Survey professional paper; 1169) “Report prepared jointly by the U.S. Geological Survey and the National Oceanic ond Atmospheric Adminis- tration.” Bibliography: p. Supt. of Docs. no.: I 19.16:1169 1. Kansas City metropolitan area—Flood, 1977. I. Carswell, William J ., joint author. II. Chin, Edwin H., joint author. III. United States. Geological Survey. IV. United States. National Oceanic and Atmospheric Ad-' ministration. V. Title. V1. Series: United States. Geological Survey. Professional paper; 1169. GB1399.4.M8H38 551.48’9’09778411 80—607048 For sale by Distribution Branch, Text Products Section, US. Geological Survey, 604 South Pickett Street, AIexandria, Va 22304 FIGURE CONTENTS Page Glossary ___________________________________________________________________ IV Conversion of inch-pound units to International System of units (SI) ____________ V Abstract __________________________________________________________________ 1 Introduction ______________________ _ ________________________________________ 1 Purpose and scope _____________________________________________________ 1 Acknowledgments _______________________________________________________ 2 Description of the flood area _________________________________________________ 2 Meteorological setting and precipitation distribution ___________________________ 2 Precipitation estimations using satellite infrared imagery __________________ 15 Description and measurement of floods ________ A ______________________________ 20 Flood damages _________________________________________________________ 20 Flood hydrographs _____________________________________________________ 24 Flood-crest profiles and inundated areas _________________________________ 24 Measurement of flood discharges _________________________________________ 25 General sediment deposition _________________ _ ____________________________ 25 Magnitude and frequency of floods ____________________________________________ 37 Flood volumes __________________________________________________________ 37 Comparative magnitude of floods ____ ______________________________________ 37 Summary __________________________________________________________________ 37 References cited _____________________________________________________________ 47 ILLUSTRATIONS Map showing location of area of flooding in Missouri and Kansas ____________________ Map showing flood area and location of flood-determination points ___________________ Maps for 0600 c.s.t., Sept. 12, 1977, showing: surface analysis, 700-mb analysis, and 500-mb analysis ___________________________ 4. Rawinsonde profiles at Topeka,vKans., 0600 and 1800 c.s.t., Sept. 12, 1977 ____________ 5. Graph showing temperature, dew point,and pressure at Kansas City International Airport on Sept. 12, 1977. Wind speeds and directions near 6 p.m. are shown ______ 6. Schematic showing surface fronts, moist tongue, and 500 mb jet, 1800 c.s.t., Sept. 12, 1977 ________________________________________________________________ Isohyetal map, storm of Sept. 12—13, 1977, Kansas City and vicinity _________________ Isohyetal map of total rainfall for Sept. 11—13, 1977, in southeastern Nebraska, northeastern Kansas, and north-central Missouri ________________________________ 9. Mass rainfall curves at four selected raingages Sept. 11—14, 1977 ____________________ 10. Depth-area analysis for 36-hour duration for storm precipitation, and composite rainfall mass curve derived from satellite infrared imagery, Sept. 12—13, 1977, and composite rainfall mass curve derived from satellite infrared imagery, Sept. 11—13, 1977 _____ 11. GOES infrared imagery for first and second storms ________________________________ 9’59!" 12—16. Photographs showing: 12. Brush Creek, after flood crest, looking north on J. C. Nichols Parkway at Ward Parkway, at Country Club Plaza, Kansas City, Mo ____________________ 13. Brush Creek, after flood crest, looking east along Ward Parkway, at Wornall Road at Country Club Plaza, Kansas City, Mo _________________________ 14. United States Post Office, after Brush Creek flood crest, on Ward Parkway at Country Club Plaza, Kansas City, Mo ________________________________ 15. View of 600 block of West 48th Street, after Brush Creek flood crest, at Country Club Plaza, Kansas City, Mo ________________________________ 16. Aerial view after flood crest, looking west along Blue River between 23rd Street and Truman Road, Kansas City, Mo ___________________________________ Page 2 3 4 7 9 13 14 15 18 20 21 22 23 24 m a IV CONTENTS Page FIGURES 17—29. Discharge hydrographs at U.S. Geological Survey gaging station, for flood of Sept. 12—13, 1977, at: 17. Line Creek at Riverside, Mo .__________________________________________;___ 25 18. Blue River near Stanley, Kans ___________________________________________ 26 19. Indian Creek at Overland Park, Kans _____________________________________ 28 20. Tomahawk Creek at Overland Park, Kans _________________________________ 28 21. Blue River near Kansas City Mo _________________________________________ 29 22. Brush Creek at Main Street, Kansas City, Mo _____________________________ 29 23. Round Grove Creek at Raytown,, Mo ______________________________________ 30 24. Rock Creek at Independence, Mo _________________________________________ 30 25. Shoal Creek at Claycomo, Mo ____________________________________________ 31 26. Little Blue River below Longview Road Damsite in Kansas City, Mo ________ 31 27. East Fork Little Blue River near Blue Springs, Mo ________________________ 32 28. Little Blue River near Lake City, Mo ____________________________________ 32 29. Sni-A-Bar Creek near Tarsney, Mo __________________________________________ 33 30. Profiles of water surface of Blue River, floods of Sept. 12—13, 1977, and Sept. 1961 _______ 34 31—34. Profile of water surface, flood of Sept. 12—13, 1977, of: 31. Rock Creek (Kans.) ______________________________________________________ 36 32. Brush Creek (Kans.) ____________________________________________________ 38 33. Rock Creek (Mo.) _________________________________________________________ 40 34. Little Blue River _________________________________________________________ 42 35. Map showing boundary of Sept. 12—13, 1977, flood along Brush Creek between Main Street and Jefferson Street, Kansas City, Mo _________________________________________ 44 36. Photograph showing recently deposited gravel and scattered coarse material on left (north)Brush Creek flood plain approximately 1,000 ft downstream from Woodland Avenue bridge, Kansas City, Mo ____________________________________________ 45 37. Photograph showing channel-bed slabs and car in Brush Creek stream channel immedi- ately downstream from Rockhille Road bridge, Kansas City, Mo _________________ 45 38. Graph showing particle-size distribution of sediment deposits in Brush Creek in the vicinity of Woodland Avenue bridge in Kansas City, Mo __________________________ 45 39. Graph showing comparison of Sept. 12—13, 1977, peak discharges to upper limits of known floods in Missouri-Kansas area and in the United States __________________________ 47 TABLES Page TABLE 1. K index and 12-hour net vertical displacement at Kansas City vicinity, September 1977 ____________________ 7 2. Supplementary rainfall data, storms, of Sept. 12—13, 1977, in northeastern Kansas, northwestern Missouri, and southeastern Nebraska ________________________________________________________________________ 12 3. Summary of peak stages and discharges for Kansas City area floods, Sept. 12—13, 1977 ____________________ 27 4. Summary of flood volumes for Kansas City area floods of Sept. 12—13, 1977 ________________________________ 41 GLOSSARY Bed material. The sediment mixture of which the moving bed is composed. Continuousrecord station. A site on a stream where continu- ous records of discharge are obtained. Cubic feet per second (ftsls). The rate of discharge; one ft3/s is the rate of discharge of a stream having a cross- sectional area of 1 square foot and an average velocity of 1 ft per second: 1 fta/s : 0.646 million U.S. gallons per day, 28.32 L/s or 0.02832 mS/s. Dew point (or dew point temperature). The temperature to which 'a given parcel of air must be cooled at constant pressure and constant water-vapor content in order for saturation to occur. Fine material. That part of the total stream sediment load composed of sizes not found in appreciable quantitiesin the bed material; normally, the silt and clay sizes (less than 0.062 mm). Flood hydrograph. A graphical representation of a stream’s fluctuation in flow (in cubic feet per second) arranged in chronological order. Flood peak. The highest value of the stage or discharge at- tained by a flood. Flood profile. A graph of the elevation of water surface of a river in a flood, plotted as ordinate, against distance, measured in the upstream direction, plotted as abscissa. Flood stage. The approximate elevation of the stream when overbank flooding begins. CONTENTS V Front. The interface or transition zone between two air masses of difl‘erent density. Isohyetal map. A map showing lateral distribution of precipi- tation and drawn as contours of equal rainfall depths. Jet stream. Relatively strong Winds concentrated within a narrow stream in the atmosphere. Miscellaneous site. A site where data pertaining to a specific hydrologic event are obtained. Moist tongue. An extension or protrusion of moist air into a region of lower moisture content. N-year precipitation (rain). A precipitation amount which can be expected to occur, on the average, once every N years. Particle size. The diameter of a particle measured by settling, sieving, micrometric, or direct measurement methods. NGVD. National Geodetic Vertical Datum of 1929; a geodetic datum derived from a general adjustment of the first- order level nets of both the United States and Canada. In the adjustments, sea levels from selected tide stations in both countries were held as fixed. Particle-size distribution. The relative amount of sediment sample having a specific size, usually in terms of percent by weight finer than a given size, D percent. Radiosonde. A miniature radio transmitter that is carried aloft (as by an unmanned balloon) with instruments for broadcasting humidity, temperature, and pressure. Rawinsonde. A radiosonde tracked by a radio direction-finding device to determine the winds aloft. Rainfall mass curve. A graph of the accumulated rainfall depth, plotted as an ordinate, against time or duration of storm, plotted as abscissa; the curve represents total precipitation depth throughout the storm. Recurrence interval. As applied to flood events, recurrence interval is the average number of years within which a given flood peak will be exceeded once. For example, a 50-year flood discharge will be exceeded on the average of once in 50 years. In terms of probability, there is a 2-percent chance that such a flood will occur in any year. Ridge. An elongated area of relatively high atmospheric pres- sure. Saturation.~The condition in which the partial pressure of water vapor is equal to its maximum possible partial pressure under the existing environmental conditions. Sediment. Solid particles usually derived from rocks or earth material that have been or are being transported laterally or vertically from one or more places of origin. Sounding. A single complete radiosonde observation of the upper atmosphere. Squall line. Any nonfrontal line or narrow band of active thunderstorms; a mature instability line. Total Total Index. A measure of air mass static stability, TT, given by: TT : Tsso + Tam ~—2Tsoo where T and Ta are temperature and dew point, respec- tively, in degrees Celsius; and the subscripts denote pres- sure level in millibars. A Total Total Index exceeding 50 is favorable to the occurrence of severe thunderstorms. Through. An elongated area of relatively low atmospheric pressure. Vapor pressure. The pressure exerted by the molecules of a given vapor; in meteorology, this term is used exclusively to denote the partial pressure of water vapor. CONVERSION OF INCH-POUND UNITS TO INTERNATIONAL SYSTEM OF UNITS (SI) Most units of measure used in this report are inch-pound units. The following factors may be used to convert inch-pound units to the International System of Units (SI). Multiply inch-pound units inches (in.) feet (ft) yards (yd) miles (mi) nautical miles (nmi) knots (kn) acres acres square miles (mi’) acre-feet (acre-ft) acre-feet (acre-ft) cubic feet per second (ft3/s) gallons (gal) degrees Fahrenheit (°F) by 25.4 0.3048 0.9144 1.609 1.85 1.85 4,047 0.4047 2.590 1,233 1.233 x 10-3 0.02832 3.785 x 10-3 5/9 (F — 32) To obtain SI units millimeters (mm) meters (m) meters (m) kilometers (km) kilometers (km) kilometers per hour (km/ h) square meters (m2) hectares (ha) square kilometers (km’) cubic meters (111“) cubic hectometers (hms) cubic meters per second (ma/s) cubic meters (m3) degrees Celsius (°C) FLOODS IN KANSAS CITY, MISSOURI AND KANSAS, SEPTEMBER 12—13, 1977 By LELAND D. HAUTH and WILLIAM J. CARSWELL, JR., U.S. Geological Survey, and EDWIN H. CHIN, National Weather Service, National Oceanic and Atmospheric Administration ABSTRACT The storms of Sept. 12—13, 1977, delivered as much as 16 in. of rain, with average rainfall exceeding 10 in. in the Kansas City metropolitan area. Twenty-five lives were lost, many were left homeless, and damages exceeded $80 million. Data obtained by the National Weather Service and the U.S. Geological Survey indicate that two record-setting rain- storms occurred within 24 hours. The first storm, in the - early morning, thoroughly soaked the local drainage basins. The second storm, centered along the Brush and Round Grove Creek basins, resulted in a devastating flash flood. Peak dis- charges were determined during and after this major flood at gaging stations and selected miscellaneous locations. Streamflows and flood volumes in many locations far ex— ceeded estimated values for the 100-year flood. INTRODUCTION Outstanding floods occurred on streams in the Kansas City, Mo.—Kans., area as a result of two sep- arate rainfall events occurring 8—12 h apart, each of which exceeded the 100-year 24-h rainfall. A total of up to 16 in. of rain fell in some sections of the metro- politan area. The first storm saturated the ground and caused a greater part of the second rainfall to run off, resulting in peak discharges well in excess of the 100-year recurrence interval in some areas. These storms extended over parts of western Mis- souri, northeastern Kansas, and southeastern Nebraska. However, the heaviest rainfall was in Kansas City and vicinity. The metropolitan area re- ceived an average rainfall in excess of 10 in. for the two events. The heaviest rain fell east of the city, just south of Independence, Mo., and along Brush Creek to the south and west, including its headwater areas in Kansas. Peak discharges were computed for many locations by indirect methods, because the rapid rise and fall of the floodwaters did not permit real-time measure- ment with current meters. These computations were made after the flood from data that was obtained by carefully measuring high-water marks and the geom- etry of the hydraulic structures and channels. Twenty-five persons lost their lives and damages exceeded $80 million. Although many homes and businesses suffered losses throughout the storm area, the major economic damage occurred in the Brush Creek basin of Missouri and Kansas and within the lower Blue River basin downstream from the mouth of Brush Creek (see fig. 2). Upstream from the U.S. Geological Survey gaging station on Brush Creek, the Country Club Plaza received national attention because of extensive flood damages to its numerous shops. PURPOSE AND SCOPE This report is one of a continuing series of joint flood reports undertaken by the National Weather Service in the National Oceanic and Atmospheric Administration of the Department of Commerce, and by the U.S. Geological Survey in the Department of the Interior. Data collected by the National Weather Service document the meteorological settings associated with the extreme precipitation and the distribution of rainfall. Materials presented in this report include related weather maps, atmospheric soundings, rain- fall mass curves, and ishoyetal analyses. Streamflow data collected by the U.S. Geological Survey present surface runoff including rates of flow and total flood volume. These data include peak stages and discharges, discharge hydrographs, water-surface profiles of selected stream reaches, and flood volumes. Elevations are referred to National Geodetic Vertical Datum of 1929 (NGVD). Compilation of all pertinent meteorological and hydrological analyses related to the flood in this one report is intended to provide convenient reference for hydraulic planning. Analysis of such outstanding I fl 2 FLOODS IN KANSAS CITY, MO. flood events can aid in promoting prudent develop- ment within any river basin where the threat of se- vere flooding exists. ACKNOWLEDGMENTS Elevations for water-surface profiles of Brush Creek, Rock Creek (Kans.), Blue River, and Little Blue River were provided by the US. Army Corps of Engineers, Kansas City District. The Corps also pro- vided personnel for field assistance. Photographs of the Plaza area were taken by Frederick Solberg, J r., and William H. Batson, Kansas City Star photogra- phers. DESCRIPTION OF THE FLOOD AREA This report encompasses the Kansas City metro- politan area and extends 15 mi eastward. This area comprises about 1,000 mi2 (fig. 1). The region most affected by the storm consisted of drainage basins of 95° AND KANS., SEPT. 12-13, 1977 the Blue River with Brush Creek as one of its tribu- taries, the Little Blue River, and Sni-A-Bar Creek, all emptying into the Missouri River within a 40-mi reach (fig. 2). Streamflows shown in this report reflect runoff from both urban and rural areas. The metropolitan area includes approximately 60 percent of the area shown in figures 1 and 2. The Brush Creek basin (main site of the Plaza area damage) is completely urbanized, while the Sni-A-Bar Creek basin is rural. The other drainage basins included in this report lie somewhere between these extremes, with only part of each basin being urbanized. METEOROLOGICAL SETTING AND PRECIPITATION DISTRIBUTION At 1800 est, Sept. 11, 1977, a weak Low was lo- cated over western Kansas. A warm front associated with the Low was about 300 mi southwest of Kansas 94° 93° l I ‘x‘ O“) )0 St. Joseph C a f sh°° ’6 \ «e e 7 42;? k-Qk ‘ _ , R Marshall 0 0 3’ a F 39°— liq/V843 V‘ KANs_' Rwer /\,e CITY 0 KANS I ( Lawrence W l 6\ocywo l STUDY Elm AREA (5’ O 9. . 6 Marc’s deg (>1 St. Louis I I 000’ River ' an E ‘0 Index map showing :4' g ’* location of study area I I U 20 40 M ES IL-U " ' I I I I l I U 20 40 60 80 KILOMETEHS FIGURE 1.—Location of area of flooding in Missouri and Kansas. METEOROLOGICAL SETTING AND PRECIPITATION DISTRIBUTION MISSOURI .I KANSAS Index map showing location of study area EXPLANATION A U S G S gaging station 0 Miscellaneous discharge measurement site (Number corresponds to that in tables 3 and4) it) 1'5 MILES | 15 20 KILOMETEHS FIGURE 2.—Flood area and location of flood-determination points. City. During that night, the wind field over the southern Great Plains was characterized by a low- level jet with strongest southwesterly winds imping— ing aloft on the sloping frontal surface south of the Kansas City area. Strong warm advection brought in warm moist air below the 700-mb level into eastern Kansas and western Missouri. By 0600 c.s.t., Sept. 12, the warm advection had extended to the 500-mb 4 FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 level. The arrival of the warm moist air mass was in- dicated by a very small dew-point depression of 1°C at both 850-mb and 700-mb-1evels in the vicinity of Kansas City in the morning. As the northwesterly flow of colder air from the mountain states met the warm moist gulf air coming from the southeast, a cold front was formed. This cold front was associ- ated with the Low centered over western Kansas. The Low with its associated frontal system gradually progressed eastward towards Kansas City. The sur- face, 700-mb, and 500-mb analyses at 0600 c.s.t. are shown in figures 3A, 3B, and 30, respectively. Temperature, moisture, and wind distributions in the upper atmosphere are routinely probed by ra- winsonde at selected stations. The rawinsonde station nearest to Kansas City is at Topeka, Kans. Upper-air soundings there for 0600 and 1800 c.s.t., Sept. 12, are shown in figure 4. Figure 4A shows that the air was moist through a deep layer from the surface up to 570 mb, capped by a dry layer. Strong warm advec- tion existed from the surface to above 700 mb. The inversion below 800 mb indicated a diifuse frontal transition zone. This was consistent with the fact that the surface front was south of Topeka at that 450 725° 16 EXPLANATION —‘—‘-— Cold front —‘--— Warm front -—12-— lsobars—12 stands for 1012 millibars H Center of high pressure L Center of low pressure FIGURE 3.—A, Surface analysis 0600 c.s.t., Sept. 12, 1977. —i— METEOROLOGICAL SETTING AND PRECIPITATION DISTRIBUTION 5 ) ¢ ° . s)- V / I, 9 0° ‘ “ i s“ ‘ to" .‘ Q” 6’5 ' fl ‘ 0 ' %' V r ‘ , I 53 a “ ° ' <\/’ ‘5 ° ’ 60 ° ’ 0 I D . e5 — y l O n\ \ \ \ p [I 10‘ a \ ‘ : . ' — ‘ h ' \ \ \ I 3 o 0: \ . I \ N” ‘ \ , , —- \ N ‘ 306 Q9 77 \ r / _ g’ ‘\ ° I , ‘ . (SI \ — ' ’ Q’ o I 1' ‘ \ i 4&‘ ~ \ \ = —— ’ / 12 a I - \ \ — A I 10 / c I < \ ’ ‘ I ’ / I ‘ \ 315 \\ L \ 4 _ _ - _I \ .- ‘ l \ \ I/I’ nag o 7 .' --~- — r” (5° 25° \ | , , 318 \ '1 ‘ I - ,, \ H\\ —— ____ -L_- ‘ \ § \ \. I g, \ x ‘ l | I " W l ‘ - \\ _ _.__l 30 Q i v \ J- V p '1’ W . ° 5 0%.“ \\f-\\ I a. - a l ‘ 25° / \ +10 // I ’ \ \ / \ [I EXPLANATION —306— Contour in decameters H Center of contour height 306 stands for 3060 meters ——+10—- Temperature, degrees Celsius FIGURE 3.—Continued. B, 700-mb analysis 0600 c.s.t., Sept. 12, 1977. ime (fig. 3A). Figure 4B shows a moderately moist lower and midlayer capped by an inversion at about 670 mb and a very dry layer above. These soundings indicated that the low level southerly flow brought in moist air from the Gulf. Aloft the advection of much drier air from the northwest by the midtro- pospheric jet stream provided another ingredient for generation of instability. The presence of an in- version around 670 mb at 1800 c.s.t. prevented deep convective overturning initially so that a progressive increase of instability could occur. Instead of being inhibiting factors, these are, in fact, contributing factors favorable to the development of intense con- vection. Even though Topeka, Kans, was not at the center of storm rainfall, the soundings should repre- sent atmospheric structure in the fringe area during the latter part of the first storm (fig. 4A) and prior to the beginning of the second storm (fig. 4B). FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 P. f 0 0° 0 s . w“ 9 ‘60 i ‘2’ v? 4:0 fl ‘ (f Q: . , 3 0 60° 800 V ‘ a 85° \ \ 90 a \ ’ I ‘5: ’ a I \ —J‘2 ’f” I ’— — \\ I ° " l \ I Q? _‘ __ “ (Y. :15 o I . ‘ : ob. 7 ‘/ / e“) l I ‘ o I I x I ’ 10 ’0 ~l - " I 93 . ‘. ‘ - x 6’ (b ‘r " fl \ I 1 ’/ .— \ ‘ ’ "" \ I r \ \ 1’ g o ..._ 1 f , fbo \ ,7 / l x J/ ‘ I L ' .\ TT \ -"' \ \ i \ f '\ I \ " :/_.l ‘ 30 Q ffii o _ v a °° s Q :9 . P \f‘\ ,r 9, . a 25° EXPLANATION —576— Contour in decameters 576 stands for 5760 meters —-10—— Temperature, degrees Celsius FIGURE 3.——Continued. C, 500-mb analysis 0600 c.s.t., Sept. 12, 1977. The integrated 12-h vertical displacement of the air parcel terminating at 700 mb indicates the degree of large-scale atmospheric vertical motion. Sinking motion, or subsidence, is generally associated with good weather. On the other hand, vigorous rising motion provides a favorable environment for storms to develop. The K Index is a measure of the air-mass moisture content and static stability, and is given by: K : (T850 —‘ T500) + Td,850 ~ (T700 — Td,700) where T and Ta are temperature and dew point, re- spectively, in degrees Celsius; and the subscripts de- note pressure level in millibars. The larger the K Index of the air mass, the more unstable it is. In gen- eral, a K Index greater (less) than 35 (20) is asso- ciated with numerous (no) thunderstorms. Both parameters are computed using the NWS Three- Dimensional Trajectory Model in a 24-h forecast mode. The evolution of these parameters in the vicin- ity of Kansas City over the 48-h period is shown in table 1. ._— METEOROLOGICAL SETTING AND PRECIPITATION DISTRIBUTION 7 400 6' ¥ 7 I I \ I I l l I a ‘ — > v ’ ’ ’ 450 - \ - \ 20 \ 19 500 - \\ - 18 ~ ~ ~ ~ ~ ~ ~ ~ ~ 17 "DJ 550 — - 16 E ‘g 15 E g 600 - — 14 3:1 _: 13 "-' g EXPLANATION E 650 - . , _ 12 (n 2 LI. Lifting Index 11 (ED u—J‘ T.T. Total Total index '5: n: 700 " LCL Lifting condensation level ' 10 o a ——— Dew-point temperature 9 E g 750 - — Air temperature .. 8 g E Wind direction and velocity: < 800 _ V Earbs on shaft indicate wind speed, - 7 i?) m knots. Long barb=10 knots; short 6 US 850 _ barb =5 knots. Flag on shaft=50 knots _ 5 :5 4 900 ' L.|.=—1 " 3 950 - T.T.=48 LCL _ 2 1000 - .. o o 1050 I I I I I I l I l —50 "40 ~30 —20 ‘10 0 (C) 10 20 30 40 50 l I I I T | I l T l ‘50 —40 ~30 —20 —10 0(F) 10 20 30 40 50 60 70 80 90100110120 TEMPERATURE, IN DEGREES CELSIUS (°C) AND FAHRENHEIT (°F) FIGURE 4.—A, Rawinsonde profile at Topeka, Kans., 0600‘ c.s.t., Sept. 12, 1977. TABLE 1,—K Index and 12—h net vertical displacement at Kansas City vicinity, September 10—12, 1977 [All times in c.s.t.] Sept. 10 Sept. 11 Sept. 12 1800 0600 1800 0600 1800 K Index (°C) 10 12 20 36 41 Vertical Displace- ment ——10 <20 20 >20 >40 (mb/ 12 hr) <40 Note: Upward (downward) vertical displacement is poeitive (negative). The atmosphere in the Kansas City area became progressively unstable on Sept. 12, 1977, as the data indicates (table 1) . At the same time, the atmosphere became increasingly more moist. The mean relative humidity from the surface to approximately 490 mb for an air column increased from 25 to 85 percent over a 48-h period ending at 0600 c.s.t., Sept. 12, 1977. Precipitable water is defined as the total at- mospheric water vapor-contained in a vertical col- umn of unit cross-sectional area extending between the surface to a specific level, usually 500 mb. The precipitable water over Kansas City increased from 0.4 in. to about 1.8 in. during the same period, ap- proaching the climatological maximum of 2.0 in. in the first half of September. The monthly mean pre- cipitable water there for September is 0.95 in. (Lott, 1976), about one-half of the observed amount. The mixing ratio is defined as the mass of water vapor per unit mass of dry air in the mixture. The inter- polated mean mixing ratio in the lowest 100 mb at * 8 FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 E A o E 8 400 ._. :: \\\ I I I I I I I 7+}: 23 \ 22 450 _ \\ _ W — 21 L-‘ 6_ - 20 m 500 “~“ ~19 g ‘rfi”’ _ ‘1‘ '-18 I: \“‘ - 17 5 m 550 — ‘~ - 5' - 16 < a ,x r _ I _ u.I E 600 I — 4 14 I Q \ V} 13 % E 650 — <\_____ _ —12 0 Z -_ \ — 11 E 35‘ 70° ' EXPLANATION ' W‘” 2 E 5,) 750 — K.| K Index ‘ < 35’ T.T. Total index g o. 800 '- ——- Dew-point temperature _ < Air temperature 5 350 " Wind direction and velocity: ‘ 05 . Barbs on shaft indicate wind speed, \ - 900 _' V in knots. Long barb: 70 knots; short barb \ ‘ D 950 _ =5 knots \ _ K.I.=4I \ 1000 * T.T.=52 - 1050 l l I I I I l I ‘ I ‘50 ~40 —30 -20 ‘10 0 (C) 10 20 30 40 50 I I I I I I I I I I I I I I I I I T ‘50 —40 —30 —20 ~10 0 (F) 10 20 30 40 50 60 70 80 90 100 110 120 TEMPERATURE, IN DEGREES CELSIUS (°C) AND FAHRENHEIT (°F) FIGURE 4.—Continued. B, Rawinsonde profile at Topeka, Kans., 1800 c.s.t., Sept. 12, 1977. 0600 c.s.t., Sept. 12, in the Kansas City region was 13.8 g/kg. For comparison, a value ranging from 12 to 14 g/kg is usually associated with a typical Great Plains severe storm (Maddox, 1976). With the air increasingly moist and unstable, and with strong low-level wind impinging on the frontal surface aloft in the Kansas City vicinity, the first burst of heavy rain began in the very early morning of September 12. The rate of rainfall was intense at some gages: in north Kansas City, 2.20 in. of rain fell in a one-half hour period ending at 0150 c.s.t. This first storm lasted about 6 to 7 hours. The ob- served 6-in. maximum amount exceeded the 100-yr 6-h rainfall of 5.8 in. (Hershfield, 1961). The areal average near Kansas City also exceeded 5 in. Rain- fall from this first storm ended by 0700 c.s.t.; it com- pletely saturated the soils of the local drainage basins, but by itself did not cause any damage. Near the end of the first rain period and during the initial period of no rain, fog was observed. This indicated the existence of a temperature inversion with its base near the ground. This condition was brought about by the warm front that was then just south of Kansas City. Evaporative cooling by rain- drops falling into initial cooler air near the ground raised the dew point until fog was formed. Since the surface wind was light, the fog persisted until di- urnal heating raised the temperature into the 70’s (°F) in the early afternoon. This rain-cooled air mass extended from central Missouri to eastern Kan- sas; its southern boundary became a zone of convec- tive activities and was situated south of Kansas City at 1800 c.s.t. Between storms there was an interlude of sunshine; and the atmospheric pressure began to change. At the Kansas City International Airport it fell from 1012.4 mb at 0700 to 1003.5 mb at 1800 fir METEOROLOGICAL SETTING AND PRECIPITATION DISTRIBUTION 9 101 6 I 1 | I EXPLANATION 1014 _ P Pressure _ Wind direction and velocity: Barbs on shaft indicate wind speed, 1012 in knots. Long barb: 10 knots; short _ barb =5 knots 1010 1000 1008 PRESSURE, IN MILLIBARS 1006 1004 1002 00 O 75— 70 65 IN DEGREES FAHRENHEIT TEMPERATURE AND DEW POINT, I I 0 AM 6 AM NOON I I I EXPLANATION T, Temperature Td, Dew-point temperature I I J 6 PM MIDNIGHT FIGURE 5.—Temperature, dew point, and pressure at Kansas City In- ternational Airport on Sept. 12, 1977. Wind speeds and directions near 6 p.m. are shown. c.s.t. ; then it began to rise. Temperature also dropped 5°F in only 45 minutes, ending at 1830 est. In the same period, surface wind shifted from 140° at 5 kn to 340° at 20 kn gusting to 32 kn (fig. 5). These ob- servations indicated a cold front passage, just prior to 1830 c.s.t. Figure 6 shows the positions of surface fronts and 500 mb jet at 1800 est, Sept. 12, 1977—- the time just prior to the beginning of the second major storm. Areas with 1000—mb dew point temper- ature equal to or greater than 70°F are shaded. .Dashed line encloses an area with K Index greater than 36, indicating high instability. Kansas City was —<— 10 FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 . 3 g . , 9 e ‘ ‘l V is . ‘ / .\ ‘ . . \h / _- 6- ' a, , ' 70 -- ‘: ‘ ’ k 4'4 ~ , fl . " " t a:- ‘ . ’3‘ ‘ '_ f ‘ _- . > ,_ 1.; “l - __ - - ( . ,. _ t - . I . -\. ‘. \ ~ _ / 3 __' ’f/ i it i... -‘ e—a ' / \ 't’ ' . . , Cl ,/ ' ,2" A ' - , . ‘ , . I. . \ ‘ . . I \\I. L . . I ’_—EX_ '_ ' . \\ 70° , _J__.._._.—— _ -— . § .. ‘ . -_ /fi 0 ‘ \ > X . \ : _, a- Q 75° 0 . o 750 15' ( 0 70°.-— p \ V o EXPLANATION —‘36“ K, Index 70° Surface dew-point temperature °F Cold front 40 Knots Wind velocity of 500 mb jet -.——A— Warm front MCI Kansas City International Airport FIGURE 6.—Surface fronts, moist tongue, and 500 mb jet, 1800 c.s.t., Sept. 12, 1977. Crosshatched areas denote areas with surface dew-point temperature exceeding 70° or 75°F, as shown. also located just north of the warm sector and at the tip of a moist tongue. All the ingredients necessary for the occurrence of significant convection were present. The upward ver- tical motion associated with the tropospheric jet passing just to the north of Kansas City provided a triggering mechanism for releasing the instability. Convective clouds grew rapidly in this favorable en— vironment. Cumulonimbus tower soon grew to a height exceeding 15 km (9.3 mi) with cloud top tem- perature dropping to below —80°C (—112°F) and rain began to fall over soil already saturated by the morning storm. By 2000 c.s.t., torrential rain had been falling over the metropolitan area for 2 hours. A more detailed description of both storms using in- formation derived from satellite imagery is pre- sented in a later section. Isohyetal maps of total storm rainfall for the Kan- sas City vicinity and for affected areas of Missouri, Kansas, and Nebraska are shown in figures 7 and 8, respectively. Rainfall mass curves for selected rain gages are shown in figure 9. A maximum depth-area analysis for storm duration of 24 hs is shown in fig- ure 10A. The metropolitan area is well covered by —»— METEOROLOGICAL SETTING AND PRECIPITATION DISTRIBUTION 11 52 - . . CI 0. MISSOUH City ID:- 0) ' Liberty \,\ O / ’I 9 TN“ (9 7| 9dstone 0/, I Fort Osage -I— 9 \550 '5 \ 3 ‘ 73K _ Rive ido A n I II nun/n ('1 ,dAll'l' 2| 7 Bethel 5 Kan Smmp sa Ci P i 4 “It .., Buckner a 3 , PP Cm“ \ 9 70 e —- h . rin 70 1° " uke Cnty 7 udsvil a 8 ‘ In ca 12 — ardsvi e IN“ 6 H ,_ \ I 32 Kansas City \N \"L "a, ‘4 S / ‘0 2 2 ~ 7 (14/ 9 a n 'b Mo. c. 7 03 ram Valley On 6’ ._ ' 5° l. kc Tit/tun ”(a 7 wnee "y R o a " 9 I/ — 6 \n h l 74 Lake Jar \ 6 3 m I y. - ”I 5 i ' 73‘“. as , ~/ er L in .. Lab 9, __4 7 7| 70 0' Bell r ' 35 ‘ “E 9 -\ ee’s Summit :50 ra w 3 O 6, char - e aur AFB Olathe / 7 [— a2 \ I \ 6 I I o1 b (a) EXPLANATION N 75 —8-— ‘Isohyet showing precipitation, in inches I 0 5 10 MILES I I I I l I 4| f I I 0 IO KILOMETERS FIGURE 7.-—Isohyetal map, storm of Sept. 12—13, 1977, Kansas City and vicinity. rain gages. At the Kansas City International Air— port, 8.82 in. fell on September 12, while the total storm rainfall there was 9.39 in. over a 36-h period. This latter amount exceeded the 100-year 2—day rain- fall of 8.8 in. Maximum storm rainfall of 16.15 in. observed to the south of Independence over approxi- mately a 36-h period exceeded the 100-year 10-day rain of 13.0 in. (Miller, 1964). Supplementary rain- fall data were collected through an intensive bucket survey conducted by the National Weather Service, Central Region, helped by personnel from the Na- tional Severe Storm Forecasting Center and the US Army Corps of Engineers, Kansas City. These data are listed in table 2. h 12 FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 TABLE 2.—Supplementary rainfall data, storms of Sept. 12—13, 1977, in northeastern Kansas, northwestern Missouri, and southeastern Nebraska R031 OF THE RAIN OCCURRED IN ABOUT 25 NOURS EJ" INDEPENDENCE.HO.RECORDERV 3 RILES NORTH OF THE 16 INCH CENTERVINOICAVED 11.03 IN. FRO" 1 AR SE'T 1.2 TO 2 AR 15 SEP'IOUV OF A STORM TOTAL OF IIHII INCHES. THE RAIN IN THE HETROPOLI'AN KANSAS CITIES OCCURRED PRIHARILV IN THO BURSTS. THE FIRST FROM 1 T0 6 AR ON TNE IZTH AND THE SECOND FROM ABOUT 6 Al TRE 12TH T0 2 AR THE IJTR. THE BAND OF REAVV RAINFALL EXTENDED FROR SOUTH OF GRAND ISLANDlNASTINGS NE) TO NEAR COLUMBIA HO. UITH HAX AROUNTS AND DAHAGE IN THE KANSAS CITY AREA AND EASTHARD TO NEAR ODESSA ND. TNE REPORTS ARE PROVIDED IN THO LISTINGS (I) FEDERAL ERPLOVEES RRIHARILT NHS C OF E AND NESS AND (2) FROM PUBLIC REPORTS RECEIVED 8V RAIL. $700!! RAIN sronn RAIN COUNTV STATE THP RGE SECTION TOTAL CAGE REnARKS~OasuL0CAnON COUNTV STATE THP RGE SEc7ION TOTAL SAC: REKAKK. LocAnoN CLAV no 51N 33H 27 SE 1/« 7.07 CL-vu ALEXANDER. «25 NH 50m JOHNSON HS 125 25: 20 SE 1/« SE 110 12.09 TAYLOR 5603 H. 77711 7:11. cLAV no 51H 33.. 1 NHI/« 9.00 HEDGE ARNs.GLAD570NE 3N JOHNSON KS 12S 23E 21 NEI/« 12.63 HEDGE 7912 ELnON": CLAY nO 51N 33H 25 Sum 9.15 HEDGE AUDSLEV. 5610 HANOR DR JOHNSON Ks 125 25: 22 SH1/«NE1/0 13.0 7600 CHADHICK CLAv no 51N 32H 31 NE 1/« NH1/« 0.60 HEDGE OOSHELL. N.KC JOHNSON KG 125 25: 29 321A 1«.A0 5 IN 0027 HALHEA CLAV no son 33H 11 NH 1/« 0.50 5 IN HALKs7ED.N.KC JOHNSON KS 125 25: 30 N01/« SE1/« 11.50 « IN 0316 H 010? CLAY no 51N 31H 7 NE 1/« 10.0 uSDA LIBERTY no. ToHN so. JOHNSON Ks 12S 25: 32 SE1/«5E1/0 11.«0 SPF 5600 H 92ND PL CLAV no 51N 33H 13 SE 1/« 9.11 NOHLER.721« N.HOODLANO JOHNsON Ks 135 2«E 2 NH 1/« 11.65 11 IN. 10210 H.92ND PLACE CLAV no 51N 33H 25 521/0 NH1/« 9.22 HEDGE PHILLIPS.5003NHOOOLAND JOHNSON KS 13s 2«E 2 NE1/« 5.03 HEDGE 10110 H 907H CLA7 no 51N 32H 29 SH 1/« 0.75 HEDGE ZAJDEL. N.Kc JOHNSON Ks 13S 2«E 3 SHI/u 9.02 15019 H 102ND 57 JACKSON no «0N 33H 7 NE1/« 10.5 ALL-H BALKE. 7151' AJEFFERSON JOHNSON Ks 13s 2«E 5 SE1/« SH1/« 11.00 HEDGE 6017 H 10157 JACKSON no 07N 33H 5 NH 1/« 9.13 HEDGE BASKIN.112 E 10! IBM JOHNSON KS 13s 2«E 36 MEI/0 2.30 , JACKSON no HAN 32H 9 Sum 13.15 5 IN DARRAH.RA770HN JOHNSON no 135 252 27 NE1/A 0.2 5 IN 12707 OVERBRODK RD. JACKSON no «9N 32H 9 SE1/«SE1/« 12.00 HEDGE GAVLORD JACKSON no «9N 33K 12 SH 1/« 12.5 sTD JOHNSON.2526 OAKLEY HVANDOTTE Ks 10$ 2«E 27 SE1/«SE1/« 9.05 RAIN-R 6«20 PARKvIEH JACKSON no «7N 33H 17 NH1/« NH1!“ 0.95 HEDGE KANE.GRANov1EH IVANDOTTE KS 11s 23E 23 9.31 usDA 1313 EOHARovaLLE DR JACKSON no «9N 32H 33 NH 1/0 NE1/« 13.00 HEDGE KERR.511 NORINERN HYANDOTTE KS 115 25: 20 SH1/«NE1/« 11.15 VICTOR 2100 5.107H JACKSON no «on 32H 15 NE1/« SH1/« 12.90 HEDGE KONOPASEK HVANDO7TE KS 115 25: 3« sum 11 ACU-R «600 FISHER ST JACKSON no «9N 52H 15 SH 1/« 12.07 HEDGE LEKDN.3201 ENGLEHOOD 1' JACKSON no «7N 33H 1« NH1/Io 0.07 HEDGE LENOH.700 DUTCH RD. CLAv no 51N 320 30 SE1/«NH1/« 0.25 5907 N HDHARD JACKSON no «0N 32H 30 NE 1/« 11.75 5 IN nATHEHS.0«07 EAST 9157 :1," no 51p. 32.. :3 551,0 10.5 «912 HINCnEstR JACKSON no «0N 33H 29 SH 1/0 9.35 HEDGE HONDSCHEIN.9701 HALNur CLAY no 51N 33H 2 SE1/« 9.27 10 IN. 0016 N.CHARLOTTE JACKSON no «0N 30H 23 SE 1/« NH I/lo 10.95 ALL-H HULLER.GRAIN v. as CLAv no 51N 33H 0 NH1/« 0.7 6 IN. 0791 N.HIGHLAND JACKSON no «9N 32H 26 NH1/« 16.15 HEDGE NELSON.LEES sunnn CLAV no 51N 33H 12 NH1/«NE1/« 9.73 VICTOR 0920 N.PROSPEcr JACKSON no «7N 32H 2 NE1/« 9.75 HEDGE NEunAN.LEEs sunnn 5H cLAv no 51N 33H 23 NEI/«NHI/« 7.50 7010 N.HOLnEs JACKSON no «0N 33H 30 N: 1/0 9.27 5 TN REED. 616 H 09m JACKSON no «9N 32H 3 NE1/« 10.99 cL-vu ROCK CR. NET.CE JACKSON no «7N 30H 1 NE1/HNE1/« 0.0 LONE JACK. no JACKSON no «9N 32H 11 SE1/« {12.03 cL-vu ROCK CR. NET.CE JACKSON no «7N 31H 25 SE1/« NEI/« 7.0 3900 SEOUOIA JACKSON no «9N 32H 15 NE1/« 13.06 CL-vu ROCK CR. NET-CE JACKSON no «7N 32H 19 NH 1/« 0.95 700-: 131«6 SVCAnORE JACKSON no «9N 32H 16 SH1/0NH1/« 11.30 cL-vu ROCK CR. NET.CE JACKsoN no «7N 33H 3 SH1/« SE1/« 0 51N 10612 HALRoNo JACKSON no «9N 32H 22 SE1/« 10.22 cL-VU ROCK CR. NEI.CE JACKSON KO «7N 33H 25 SH1/« 7.50 5 IN. 129m A 71 NH JACKSON no «0N 32H 19 NC 1/0 13.05 ROCKHOOD.RAVIOHN JACKSON no «on 31H 1 NE1/« 12.7 GIN 1312 SKvLINE DR JAcKSON no «on 31H 3« 5H1» NH1/« 10.0 HEDGE RUCKER.LEES sunnn JACKSON no «0N 32H 19 NE1/« 11.65 7Ru-C 0912 JAnES A REED RD JACKSON no «7N 33H 3 NH1/« SEA/0 0.00 HEDGE SARGER7.«009RED BRIDGE JACKSON n0 «0N 35H 3 NE1/« 12.700 5 IN 5929 KENSING70N JACKSON no «9N 31H 26 SE 110 13.77 cL-VU' SPITTLER-BLUE SPRGS JAcKSON no «0N 33H 5 SE 1/« 12.95 VICTOR 5619 ROCKHILL RD JACKSON no «9N 32H 13 SH1/« 10.5 STD SUEENEY. 3«00 S.NOCKER JACKSON no «0N 33H 0 SE1/« 13.52 SIN 7023 HoLnES JACKSON no «0N 52H 3 SE1/«NH1/« 10.53 CL-vu VOCHAuER.12201 E 51 7 JACKSON no «0N 33H 11 NH1/« 15 11 IN. SHOPE PK. GREENHOUSE JACKSON no «0N 33H 31 NE 1/« 10.15 HEINRICH. 1100 H.100 7 JACKSON no «0N 33H 16 SH1/« 10.00 0900 EUCLID JACKSON no «0N 33H 0 SH1/0 13.32 HEDGE HERINAN.«05E.71S7.7ER JACKSON no «0N 33H 17 N:1/« 1«.00 5 IN. 920 E.767n TER. JOHNsoN KS 12s 2«E 27 SH1/«NH1/« 10.0 ALL-H CALAORESE.0200 NOLAND JACKSON no «0N 33H 10 NH1/9 12.70 oHIO 1105 H T'ITH STREET JOHNSON KS 125 25: u. NH1/« 11.23 5 1H. CAS70.ROELANo pK JACKSON no HON 33H 10 SH1/« NH1/« 0.5 7909 HARD PARKHAV JOHNSON KS 125 20: 1 NH1/‘0 10.50. HEDGE CRAIG. SHAHNEE JACKsoN no «on 33H 21 MEI/A 11.21 TRU-C 0601 GARFIELD JOHNSON KS 11S 2«E 25 Run SH1/« 9.10 HEDGE OECAIGNV.1S«2S.51SI JACKSON no «0N 33H 31 NEI/« 10.50 5 IN. 9015 JARBOE JOHNSON KS 135 2«E 10 NH 1/« 9.02 cL-VU GRAV. OVERLAND PK JACKSON no 00N 33H 32 NE 1/« NH 1/« 10.5 5090 10020 HALNUT DR JOHNSON RS 125 2«E 11 NH 1/0 0.95 RRG HAHN. SHAHNEE JACKSON no H9N 30H 3 SH1/uSH1/« 11.0 BUCKNER MS JOHNSON KS :35 2«E 1 SH1/« 10.99 NALEs JACKSON no «9N 30H 35 10.50 5 IN. GRAIN VALLEV. no JOHNSON K5 125 25: 15 SH 1N 19.3 CL-vu NUGHES. n1SSION HILLS JACKSON no «9N 32H 32 SE 1/« NE 1/« 11.5 5 IN. 115 N. OVERTON AvE JOHNSON KS 13S 25: « NH1/« 11.3 CL-VU KNUDSEN.OVERLANO PK JACKSON no «9N 32H 32 SE1/«NH1/« 15.50 HEDGE 0905 E. 557R JOHNSON Ks 125 25: 33 SH1/« 11.31 HEDGE LEE LARSON 957mm): JACKSON no «9N 33H 1 CENTER 13.0 10 IN. 1052 FULLER JOHNSON Ks 13S 25E 5 Hum 11.12 CL-VU nEAux.7500 H 95m TER JAcKSON no «9N 33H 25 NE 1/« 1«.0 5 IN. 7061 E.«7IH 57 JOHNSON KS us 250 « NH 1/« 11.0 ALL-H 08707. «012 H. 97 TER JACKSON no «9N 33H 32 NE1/«NE1/« 12.73 6 IN. 5««1 HOLnES JOHNSON KS 125 2«E 27 SH1/«NH1/A 10.0 ALL-H PROENzA.021|I HAUSER JACKsoN no «9N 33H 36 SE1/« 13.03 vIC. 6501 E. 507H JOHNSON Ks 12s 25: 31 NH1/« 13.51 6 IN SCHOENI.052« H 00m T. JACKSON n0 50N 30H 22 NE 1/« 10.0 CL-vu DUCKNER DANK JOHNSON KS 13S 20: 2 SH1/« 9.01 HEDGE sEDOvIc ‘ JACKSON no 50" 31H 30 11.60 HEDGE 1700« REDHOOD DR. JOHNSON KS 12s 25: 29 NE 1/« 1«.70 HEDGE HEISS JACKSON no son 32H 32 SH1/« SH1/« 12.2 TAYLOR 0720 RODERIS PLATTE no 51N 3«H 23 NH 1/« 9.67 cL-vu HENDERSON.PARKvlLLE 2N JACKSON no 50N 33H 32 SH1/« 9.10 5 IN. 307 N. GRAND PLATTE no 51N 30H 21 SH 1/« 9.67 HEDGE JOHNSTON.PARKVILLE 2H JACKSON no 50M 33H 35 321/9 9.33 0 IN. “0‘0 N HARDESTV PLATTE no 50N 33H « NE1/«S1/2 0.50 HEDGE KNIPP.«00 HODDLAND RD JACKSON no 51N 30H 30 5H1/«SH1/« 30.0 SIBLEV 1H PLAI'7E n0 53N 35H 9 NH1/« 0.71 cL-vu LONGSDORF.HES70N 6E JACKSON no 51N 32H 29 NE1/« SH1/H 9.35 SPF 5209 E.507H 7En. PLA77E no 51N 3«H 11 SH 1/« 0.«~ 5 IN REHBDLDTuHEATNERBY L. JACKSON no 51N 32H 31 SE 1/« 9.5 390« NE «91H 7:11. PLA77E no 51N 33H 17 NE 1/« SH 1/« 0.01 HEDGE HILLIAHS. HAuKonIs HVANDOYTE RS 105 23: 26 NH1/9 NE1/« 9.17 HEDGE SHITH.2906 N567H JOHNSON no ««N 27H « 6.1 S IN. HOLDEN 65E JOHNSON KS 12; 20; . Nu,“ 101 10 m ““5 u «0 7“ JOHNSON no «N 20H 27 NE 1/« 5.0 5 IN. HOLDEN 3N JOHNSoN KS 12s 29: 11 SEI/« 12.00 VICTOR 10715 H 615T JOHNSON KS 125 2«E 12 NH1/« NE1/« 11.00 5 IN 5713 KESSLER LAvFAvEITE no «0H 26! 15 SH1/« 9.51 JOHNSON KS 125 20: 2« NEI/K NH1/Io 10.01 5 IN 7013 “mm/1“ LAVFAVETTE no MN 260 17 NH1/«SH1/« 10.5 ODESSA 0E5: JOHNSON Ks 125 2«E 25 Sum 9.50 .. 1N DSTHAFARLEV UV‘IVET'E "0 “N 2“ 27 531/“ 10.23 0 1"- JOHNSON Ks 125 2«E 33 NE1/« 10.50 5 IN 0966 PARK LAVFAVETTE no «0N 29H 3 NE1/« 13.0 BATES CI" 1.5H JOHNSON Ks 12s 25: 2 NH1/'0 "up. ”,5 5 m 0931 GLENDALE RD LAFAYETTE no «9N 25H 5 NEST 0.15 HIGGINSVILLE HAIER PLT JOHNSON KS 125 25E 0 sum 10 5 m «101 H 53 7:0 LAVFAVETTE no «9N 27H 13 MEI/0 9.00 HAVVIEH-HO. JOHNSON KS 125 25E 5 SCI/Io 11.6 5 IN 5631 BEVERLY LAVFAVETTE no U’N 20H 3 SH1/A 9.5 ODESSA 5NNH JOHNSON KS 125 25: g NH1/« My“. ”,9“ no.5 6320 H 5., "m LAYFAYETTE n0 «9N 20H 15 SCI/0 12.0 ODESSA 3NNH JONNSON K5 125 25: 9 NH 110 ' 13.10 VICTOR 51000 “on LAVFAVETYE no 50M 2" 15 SUI/“SE11! 9.00 HELLINGTON R0. JOHNSON KG 125 25E 10 N 1/« SH 1 13 3 N. 60«1 N SOR JOHNSON Ks 125 25: 10 N51,. “1,." 12:59 11 1 ‘9“ $33.30" L25; PLATTE no 51N 33H 20 NE1/« 7.7 HEDGE 5906 HUTSON RD. JgnngN KS 12S 25: 17 n.1,. “'15 Sp; 22,: “my. PLANE no 51N 3«H 25 SCI/«521M 0.75 5 IN. 1131 N. 367H 5!. JNNSN KS 125 25: 175 I NH1 - A H 7H E ‘ I. I. 15 “R G ‘ Ill ' ‘ 1' RA! HO 51N 29M 23 SEA/'0 9.60 BANK OF ORRICR RAv no 52H 29H 1 0.9 5 IN. EXCELSIOH SPGRS 5: I POSSIBLE OVERELOII 0R SPLASH OUT RAIN GAGES CL'VU«ALL-l '0 INIDIAUFVNNELQAPPIOXO 12 IN. CAPACITV PLASTIC RRG UNIV. aECD‘IDER STD STANDARD NHS NON-RECORDER HEDGE APPROX. E IN. PLASTIC 5 IN. PLASTIC 5 IN. CAPACITY ACU-R ACU-RITE OHIO ONTO TNERN. RAIN GAGE RAIN-N RAINHASTER SPF SPRINGFIELD TAYLOR TRlI-C TRUE CHECK VIC. VICTORY VICTOR-u!» IN CAPACITV 6098 / AIR-G. AIRGUIDE 13 METEOROLOGICAL SETTING AND PRECIPITATION DISTRIBUTION .muuommfia Guanoofifica «in .mamgM Eon—muofinc: .S—maunoz E33338 5 .52 in: .32». 5 $32 33 no $8 3033!.» 5.ch I am . 417‘s: .. . ., a a _ I d i... ll:— 1 ‘ . .0 q | f A g arm V 05 .1“! «V r \ r/ 94» d 3 :32. \ “mm 0 «.5 2.33.: .18 ||w|| 3:2: 5 sum; maufvufiiwfia 1.5%. 1 . 50233696 mctsocm $2.0m. \ 6 3‘“ 3.3% In...» a 1:! , .2 . 3...... o «E 28 wmfiw 95sz Bugs. :- u I Iwmmm .52.”. o . ..<.<.m Id 939. .2m23m $5me own 4 4 69m 52m. cozmhomgw . 9.6 65:20 E3 Rom—mm 305:0 ¢ + .cozmgfioui cl. anflongm . . . cozfioagm . a .— :mwwmwwflm‘. o o ucm 5:538on EBSmQEB co. m m o :9“thth .9 9 “En conmgfluwi 4 52:95 0 o conga cozflifiwi _ oz. uz_ uz. wz_ )) namoom: nomoomz I208: Iomoomm I202 I202 ZOF104 [1.) of the electromagnetic spectrum, thin, nonprecipitat- ing clouds become fairly transparent and raindrops can be detected. This differentiation is not possible in the visible (0.55 to 0.70 ,u) and infrared channels 10.5 to 12.6 ,u.) sensed by the GOES Visible and In- frared Spin Scan Radiometer (VISSR). One of the earlier attempts to obtain a quantita- tive rainfall estimation was made by Follansbee (1973). Using one afternoon cloud’s coverage picture from a polar-orbiting satellite, he made rainfall esti- mates over an area the order of 105 km2 (38,600 mi?) by considering the summed ratios of coverages by cumulonimbus, nimbostratus, and cumulus congestus, each weighted by an empirical coefficient. This method is only appropriate where and when rainfall is due to convective storms produced by diurnal 16 FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 heating. In general, however, the evolution of cloud system is too swift for one picture to be representa- tive for a whole day. During recent years, several improved schemes have been devised. It should be mentioned that Cheng and Rodenhuis (1977) found that the instantaneous state of clouds in a satellite imagery and region of precipitation, as indicated by radar echoes, was not well correlated. Woodley and his group at the National Hurricane and Experimental Meteorology Laboratory (of N 0AA) did much of the groundwork in rainfall esti- mation using satellite data (1972). They found that the rain—producing clouds are bright and cold on the satellite image by virtue of their greater thickness. They also pointed out that instead of a snapshot, a complete “time-exposure” is needed to track a cloud mass of interest. Since the same brightness or cloud top temperature can indicate quite different rainfall, an accurate estimate of rainfall amount depends on whether the cloud mass is growing or decaying. The life stages of the cloud system can be identified only from a sequence of satellite pictures. Clouds with expanding cold tops in the infrared (IR) or bright clouds in the visible imagery correspond to the incipient and mature stage of storm development and produce more rainfall than those not expanding. Clouds with contracting bright area or cold top cor- respond to the decaying stage of a storm and are associated with little or no rainfall. Most of the sig- nificant rainfall occurs in the upwind at the anvil— level portion of a convective system. The highest and coldest clouds form where the thunderstorms are most vigorous and the rain heaviest. These cold clouds get thinner downwind and become warmer as the anvil material blows away from its origin over the updraft (Woodley and others, 1972). Rain estimates using satellite imagery have been made and verified in Florida using gage-adjusted radar estimates of precipitation as ground truth (Griffith and others, 1978). The results show con- siderable overestimation by the satellite method. But accuracy appears to be a function of the total time period under consideration. Both error and standard deviation of estimates decrease when estimates are accumulated for 6 h or longer. Based on the relation- ships between rainfall and satellite cloud imagery found by Woodley’s group, Schofield and Oliver (1977) proposed an empirical method for making quantitative estimations of half-hourly rainfall, mainly from infrared imagery that can be applied operationally in near real-time. Satellite rainfall esti- mation made in the current study (fig. 108) is based on this empirical method. The extreme rainfall that caused the Kansas City flash flood of September 1977 came as two storms, each of about 6- to 8-h duration and separated by a period of no rain of 8—12 h. Within each storm, rain- fall intensity varied considerably. Selected GOES in- frared imagery pictures for the two storms are shown in figure 11. Characteristics of these two storms as revealed by these pictures are summarized separately. Specific features of the infrared imagery of the first storm (fig. 11A, B, and C) : 1. Kansas City was located near the edge of the anvil at 0100 c.s.t., Sept. 12. Anvil is defined as that portion of a cumulonimbus (Cb) cloud system where cloud top temperature T, is less than -—32° (—25.6°F). 2. Protruding Cb turrets with cloud top tempera- ture Tt colder than ——80°C (—112°F) existed near Kansas City from 0100 to 0230 c.s.t. They propa- gated eastward afterwards and disappeared by 0400 c.s.t. 3. Turrets went through considerable expansion between 0130 and 0200 c.s.t., indicating occurrence of intense rainfall in this period. 4. Maximum cloud top temperature gradient —V T, reached near Kansas City was at 0230 c.s.t. estimated to be 39°C/50 km (70°F/30 mi) toward the northeast. 5. By 0330, active turret had moved to the east of Kansas City and T, over Kansas City increased to between —58°C (—72°F) and —62°C (—80°F), signifying the beginning of storm decay and reduc- tion of rainfall intensity there. 6. Dissipating stage continued past 0400 c.s.t., but with little rain indicated after 0500 c.s.t. Specific features of the infrared imagery of the second storm (figs. 11D, E, and F) : 1. At 1800 c.s.t., Sept. 12, Kansas City was located at the edge of the anvil topping the cumulonimbus cloud mass to the north. 2. Protruding turret with cloud top temperature T, less than —80°C (—1120F) was already in exist- ence at 1800 c.s.t. Area of turret remained the same until 1830, but then started to expand. 3. The most rapid expansion occurred between the period 1930 to 2030 c.s.t.; this corresponded to the time of most vigorous convective activity and most intense rainfall. 4. The anvil level wind was southwesterly about 50 kn. Between 1930 and 2030 c.s.t., the turret area f METEOROLOGICAL SETTING AND PRECIPITATION DISTRIBUTION actually expanded upwind. This signaled an abun- dant moisture inflow and vigorous rising motion to the southwest of Kansas City—over Johnson County and Brush Creek headwaters in Kansas. 5. Maximum cloud top temperature gradient —§7Tt reached near Kansas City was at 2030 c.s.t. estimated to be 48°C/25 km (84°F/15 mi) toward the north. This gradient was more than twice as steep as that reached in the first storm. Quantitative relationship between ——VTt and rainfall has not been established, but qualitatively there is a positive cor- relation between them, and the maximum rainfall intensity of the second storm was greater than that of the first storm. 6. Cold turret began contraction after 2030 c.s.t. This contraction initially affected the Kansas portion of the storm system. The turret over and to the northeast of Kansas City, Mo., was maintained past 2200 c.s.t. When locating the highest and, therefore, coldest top of thunderstorm on a map, a correction for dis- placement error of satellite—sensed cloud top must be made. This correction is necessary because, except at the subpoint, the line of sight from the satellite sensor to the cloud top is slanted. The amount of cor- rection depends on the height of the cloud top and its longitude and latitude differences from those of the subpoint. For example, at 2030 c.s.t., Sept. 12, the coldest top north of Kansas City (fig. 118) was esti- mated at a height of 53,000 ft, to map its true geo- graphical position, it should be displaced toward southeast by a distance of 12 miles. Based on the analysis of the infrared characteris- tics of the storm and using the method in NOAA Technical Memorandum NESS 86, we constructed a composite rainfall mass curve (fig. 103). From the users’ point of view, the quality of an infrared imagery is characterized by three parameters: the spatial resolution or instantaneous field of View, the temporal resolution or the time between consecutive looks at the same spot, and temperature resolution or the ability to discern a cloud against the earth back- ground by virtue of the temperature difference be- tween the two. These three parameters are not mu- tually independent. For example, to increase the spa- tial resolution by reducing the field of view would degrade temperature and temporal resolutions. Thus, the design of the Visible and Infrared Spin Scan Radiometer (VISSR) aboard GOES represents an optimal trade-off among these three parameters. The present GOES infrared imagery has a spatial resolution of 9 km (5.6 mi) at the satellite subpoint 17 at 0.5° N., 75.0° W. This resolution degrades the farther the sensed area lies from the subpoint. Therefore, the derived mass curve (fig. 10B) repre- sents the time distribution of rainfall over an area approximately 100 km2 (about 39 miz) where rain- fall was heaviest. It should not be compared with any mass curve of specific gages in figure 9, but in- stead should be compared with depth-area-duration analysis. A storm depth-area analysis for a duration of 36 hours is shown in figure 10A. This is based on the isohyetal analysis of “ground truth” data from rain gages and a bucket survey. Comparison of fig- ure 103 to rainfall amounts at 39 mi2 in figure 10A shows that figure 10B underestimated the maximum rainfall by 1.4 in. This represents a 10 percent un- derestimation. The possible reasons for this are not discussed here. It is evident, however, that the com- posite mass curve derived from GOES infrared imagery (fig. 103) is not inconsistent with the con- ventional depth-area-duration analysis (fig. 10A). Weather radar coverage for the Kansas City area was provided by a WSR—57 radar at the National Severe Storm Forecast Center in Kansas City and by a WSR—74C radar at Topeka, both operated by Na- tional Weather Service. The radar Video Integrator and Processor (VIP) provides automatic contouring of the varying radar echo intensity which is used to estimate instantaneous rainfall rate. Detailed time sequence of radar scope displays are archived in film form at NOAA’s National Climatic Center at Ashe- ville, NC, and are not presented here. Radar data indicated that precipitation of the second storm first began to the north and west of metropolitan area and then moved southward. This is consistent with the satellite pictures in figures 11D, E, F. The storm reached Kansas City downtown area by 1830 c.s.t. In the Great Plains, it is not uncommon for a flood to result from new rainfall on succeeding nights. Normally, such subsequent heavy rainfall has a tend- ency to occur in an area downwind from the earlier events. What distinguished the 1977 Kansas City flood was that the two bursts of heavy rains fell on nearly the same location. In fact, in the satellite in- frared imagery, the cumulonimbus turret actually propagated upwind toward southwest in the early stage of the second storm. During about 61/; h, this second storm released up to 7 in. of rainfall on the previously saturated area. This amount again ex- ceeded the 100-year 6-h rainfall value. All streams and creeks rose rapidly and almost simultaneously as the rain fell. It was this second burst of heavy rain that caused havoc. Even with drainage basins as small as those involved, a shift of the heaviest rain of 18 FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 /A ..........nnu o a a Q Q A.—0100 c.s.t., Sept. 12, 1977. B.—0200 c.s.t., Sept. 12, 1977. ,...c~.ioq-Ov"’ C.—0300 c.s.t., Sept. 12, 1977. FIGURE 11,—GOES infrared imagery for first storm (11A, B, C) and second storm (11D, E, P). All pictures are enhanced by use of M3 curve to increase the contrast of cloud top temperature differences. To convert different shad- ing into temperature ranges, use the scale shown in figure 113, where A: —— 32—) — 41°C; B: — 41—) — 52°C; C: — 52—> — 58°C; D: — 58—> — 62° C; E: -- 62—) — 71°C; F: — 71-—> — 80°C; G: below — 80°C. —>— METEOROLOGICAL SETTING AND PRECIPITATION DISTRIBUTION 19 m n : r‘~u.ucuoto . m» ' ' ' ' ‘ " ' “ ‘ " '3 -' D.—1830 c.s.t., Sept. 12,1977. _, . , ' nu...” .v...~.n at. , . . _ n. a ""’I.an~0i~* o...- E.—1930 c.s.t., Sept. 12, 1977. }«nufiuu|‘.*. a 0 a: Itntaslo. ’9"*, v ‘ ' F.—2030 c.s.t., Sept. 12, 1977. FIGURE 11.—Continued. 20 FLOODS IN KANSAS CITY, MO. either storm of only a few miles apart would have greatly reduced the intensity of the flood. The fact that the axis of heavy rain in the second storm also lay WSW-ENE, approximately along the direction of Brush Creek and the adjacent small basins, further aggravated the severity of the resulting flash flood. DESCRIPTION AND MEASUREMENT OF F LOODS FLOOD DAMAGES Water damage was widespread in nearly every drainage basin in the area. Because of the high rain- fall intensities that occurred during the storm, hill- side runoff flooded garages and basements of build- ings in some areas located well above the flood plains. High-rainfall intensity, however, was not the sole cause of flood damage in the study area. Only 10 +— AND KANS., SEPT. 12—13, 1977 hours apart, both rainfall periods exceeded the 100- year 24-h rainfall frequency depth. The first rainfall period saturated the ground. This saturated soil, to- gether with large impervious urbanized areas forced much of the second storm rainfall to become surface runoff, since it could not infiltrate the ground. All these conditions, together with the high rainfall in- tensities, added to the magnitude of the total runoff. The greatest damage to commercial property oc- curred in the Brush Creek and lower Blue River basins. In the Brush Creek basin, many shoppers had parked their cars in underground sections of multi- level parking lots and along streets that were inun- dated during the flood. Mission Shopping Center in Mission, Kans., was inundated by floodwaters from steep hillside slopes and overbank flow from Rock Creek, a tributary in the Brush Creek basin. Shops and stores in the Country Club Plaza of Kansas City, FIGURE 12.—Brush Creek, after flood crest, looking north on J. C. Nichols Parkway at Ward Parkway, at Country Club Plaza, Kansas City, Mo. —>— DESCRIPTION AND MEASUREMENT OF FLOODS 21 FIGURE 13.—Brush Creek, after flood crest, looking east along Ward Parkway, at Wornall Road at Country Club Plaza, Kansas City, Mo. Mo., just upstream from the US. Geological Survey exceeded $14 million, according to Federal Disaster gaging station at Main Street, were flooded by as Assistance Administration officials in Kansas City. much as 6 ft of water, as evidenced in figures 12 to The Small Business Administration reports that 16. Floodmarks can be seen in the Window of the about $31 million in Missouri and $8.5 million in US. Post Oflice at Plaza Center (fig. 14). Kansas has been awarded in a total of 3,000 small The industrialized lower Blue River flood plain business loans, as a result of the flood. The US. was damaged, also. Railroad tracks were undermined Army Corps of Engineers damage survey indicates as much as 3 to 4 ft deep along bridge approaches. that, overall, the Rock Creek (Mo.) and Brush Creek Figure 16 shows the general flooding in the indus- basins, where the highest rainfall depths and intensi- trial area in the Blue River basin between 23rd ties were recorded, sustained more than $80 million Street and Truman Road. in damages. Residents along Rock Creek in Independence, Mo., According to Federal Insurance Administration felt the brunt of the flood when 11 houses were com- oflicials in Kansas City, there were 1,539 flood insur- pletely destroyed. Other houses were left in the mid— ance policies in force Within the metropolitan area of dle of streets after ceiling-level floodwaters floated Kansas City, M0.—Kans. at the time of the flood. FIA them from their foundations. reports that there were 2,142 flood insurance policies A 10-county area was affected by the floodwaters. in force during 1978 (metropolitan Kansas City), Public damage awards made in the Kansas City area With an increase of over $32 million in coverage. fi 22 FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 FIGURE 14. —United States Post Office, after Brush Creek flood crest, on Ward Parkwa Mo. y at Country Club Plaza, Kansas City, —7—7 DESCRIPTION AND MEASUREMENT OF FLOODS 23 Club Plaza, Kansas City, Mo. FIGURE 15.—View of 600 block of West 48th Street, after Brush Creek flood crest, at Country fi 24 FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 FIGURE 16.—Aerial view after flood crest, looking west along Blue River between 23rd Street and Truman Road, Kansas City, Mo. FLOOD HYDROGRAPHS Figures 17 to 29 are flood hydrographs for the storm period at selected US. Geological Survey gag- ing stations in the area. A rainfall mass curve is also shown for Round Grove Creek (fig. 23) where rain- fall was recorded simultaneously with river stage in a dual gaging system. The gage structures on Rock Creek at Independence, M0., and Brush Creek at Main Street at Kansas City, M0., were damaged dur- ing the flood peak and parts of the flood hydrographs were computed on the basis of high-water marks, earlier flow records, and the stage record, prior to gage failure. Estimated hydrograph segments are shown as dashed lines. Discharge hydrographs were developed from stage records and from the relationship between stage and stream discharge at each site (figs. 17 to 29). Other flood hydrographs for streams outside the area of maximum precipitation depths show the vari- ation of storm runoff over the area. FLOOD-CREST PROFILES AND INUNDATED AREAS Water-surface profiles of the flood on Blue River, Brush Creek, Rock Creek (Kans), Rock Creek (Mo.), and Little Blue River are shown in figures 30 to 34. Where information is available, elevations of bridge floors are shown to indicate road overflow. The profile of the September 1961 flood on the Blue River is shown for comparison (fig. 30). The effect of inflow of Brush Creek at mile 11.3 is evident. —7—, DESCRIPTION AND MEASUREMENT OF FLOODS 25 Commercial and residential areas along Brush Creek underwent extensive flood damage, especially at and around the Country Club Plaza shopping cen- ter. Shown in figure 35 are the flood boundaries in the Plaza area upstream from the U.S. Geological Survey stream-gaging station at Main Street. The aera of inundation within the flood lines shows that the major flooding in the Plaza area is restricted to the low left-bank flood plain. The flooded area is shown from Main Street upstream to Jefferson Street. The Main Street roadfill was not overtopped dur- ing the flood, and the total flood discharge was con- fined to the bridge opening (fig. 35). Elevations of high-water marks at the bridge showed a 2.5 ft dr0p in water surface between the upstream and down- stream sides of the bridge. Flood boundaries of the Sept. 12—13, 1977, flood in other areas where flood-crest profiles have been de- termined are delineated on topographic maps (scale 124,000) and are available from the U.S. Geological Survey, 1400 Independence Road, Rolla, Mo. 65401. Table 3 lists the discharge measurement sites in downstream order. A summary of location descrip- tions, as well as other basic data, is provided in an earlier report by Hauth and Carswell (1978). MEASUREMENT OF FLOOD DISCHARGES Hydrologists were in the area measuring storm runoff from the two-storm flood by noon of Sept. 13. Although floodwaters had receded from small drain- age systems before daylight, useful discharge meas- urements were obtained from streams of greater drainage areas. At the same time, hydrologists were selecting sites and identifying floodmarks for indi- rect determinations of discharge. Peak discharges were determined at 31 sites (table 3 and fig. 2). These included indirect determinations of discharge made at 13 continuous-record stations and 13 miscellaneous sites using methods described in the reports, “Techniques of Water-Resources In- vestigations of the U.S. Geological Survey,” (Dalrymple and Benson, 1967; Matthai, 1967; Bodhaine, 1968; Haulsing, 1968). One current-meter measurement was made on Little Blue River at Lake City, Mo. Peak discharges were obtained from stage- discharge relationships developed at five sites. Fig- ure 2 shows the location of the sites and the drainage system. Peak discharges were measured both in the fringe area (areas of low rainfall depth) and in areas of maximum rainfall. 04000— 2 o 0 Lu (I) CC LIJ O. f— L“; LL3000— 2 CD :> 0 LL 0 (I) 3 <2°°°' (I) 3 o I 1— Z ui 5.? <1ooo— \ 6 g \ D \ \ 011111111111 0 48121620244 812162024 SEPT. 12 SEPT. 13 TIME, lN HOURS FIGURE 17.—Discharge hydrograph at U.S. Geological Survey gaging station on Line Creek at Riverside, Mo., flood of Sept. 12, 1977. GENERAL SEDIMENT DEPOSITION Floodwaters in the Kansas City area left very few deposits of fine sediment (silts and fine sands) on the flood plains and streets because of the high veloci- ties experienced during the flood. Large amounts of fine sediment, however, were deposited in the base- ments and on the ground-floor levels of many resi- dential and commercial buildings. Much of the fine sediment, especially in the Brush Creek basin, was transported all the way to the Blue River. Although fine-sediment deposits were sparse, deposits of large- sized sediment were numerous. Figures 36 and 37 depict sediment that was transported in Brush Creek. The apparent source for much of the fluvial mate- rial shown in figure 36 is the Kansas part of the basin in the Mission Hills area. However, some of the material came from retaining walls and other struc- tures that were torn out during the flood. Figure 38 —<_ 26 FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 g 300 ment samples can be considered representative of the 8 large-sized material transported in the Woodland g Avenue reach on Brush Creek. 5 Flood-plain deposits in the Woodland Avenue |°_‘ reach were also sampled. The pebble-count method 3 (Guy and Norman, 1970) was used to obtain a rep- : 200— resentative measure of the sediment sizes. Flood— g plain deposits depicted in figure 36 had been put in a o shallow pile by cleanup crews after the photograph 5 was taken. A grid was laid out over the pile and the (8 particles at the intersection points were measured. 2 The intermediate dimension was recorded foreach cg ‘00 _ particle. The sizes measured ranged from 91.4 to 960 0 mm with a median size of 218 mm. Although the E flood-plain deposits were not sampled in their origi- Z nal state, the results are considered to be a repre- g sentative measurement. g , I , . , . 1 , , , , . | , , , . , 1 ‘ , , , , l , Many of the very large, in-channel deposits were § 12 13 14 15 imbricated (fig. 37). Such large particles probably 5 TlME. IN HOURS were deposited soon after the flood peak. The con- FIGURE 18.——Discharge hydrograph at U.S. Geological Survey gaging station on Blue River near Stanley, Kans., flood of Sept. 12—13, 1977. shows the particle-size distribution of two deposits sampled near Woodland Avenue bridge. These sedi- crete slabs shown in figure 37 had lined Brush Creek where it was channeled through the Country Club Plaza. 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AND KANS., SEPT. 12—13, 1977 DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECOND DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECOND \j\ ‘1 \ 0 0 0 4 8 12 16 20 24 4 8 12 16 20 24 0 4 8 12 16 20 24 4 8 12 16 20 24 Ep _ SEPT. 13 SEPT. 12 SEPT. 13 S T 12 TIME, IN HOURS TIME, IN HOURS FIGURE 19.—Discharge hydrograph at U.S. Geological FIGURE 20.—Discharge hydrograph at U.S. Geological Survey gaging station on Indian Creek at Overland Survey gaging station on Tomahawk Creek at Over- Park, Kans., flood of Sept. 12—13, 1977. land Park, Kans., flood of Sept. 12—13, 1977. DESCRIPTION AND MEASUREMENT OF FLOODS 29 18 20 _ 17 16 g 18—— __ 15 8 o $ 16 — 3 cc 0 14 LL, m o. _ U) m 13 '— — m E 14 n- ”' _ ,_ 12 g E 1 _ 8 2 (:2 11 m ”- — :) o 0 10 U) 10 ~ u. E O 9 < — U) \ (I) g 8 8 8 — < I I ‘3 I I— — o 7 Z I i 6 — l— \ 6 5 _ Z \ in: ui 5 N011E: I 4 _ g Dashed line indicates 8 < \ estimated discharge a — I 4 U \ ‘2 I 2 — o \ 3 I \ 2 0 I I I I | I I I I I I I I I I | I | I I I \ 0 4 8 12 16 20 24 4 8 12 16 20 24 \ SEPT. 12 SEPT. 13 1 f \ \ TIME, IN HOURS 0 I J . , 0 4 8 12 16 20 24 4 8 12 16 20 24 FIGURE 21.—D1scharge hydrograph at U.S. Geologlcal Sur- SEPT. 12 SEPT. 13 vey gaging station on Blue River near Kansas City, Mo., flood of Sept. 12—13, 1977. “ME 'N HOURS FIGURE 22.—Discharge hydrograph at U.S. Geological Survey gaging station on Brush Creek at Main Street, Kansas City, M0,, flood of Sept. 12—13, 1977. 30 FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 . Dashed Iined estimated (I) LLI I U E Z :If I— 0. LLI Q __I .J < LL Z < C: DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECOND DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECOND 024681012141618202224 SEPT.12 TIME, IN HOURS 11 12 13 TIME, IN HOURS FIGURE 24.—Discharge hydrograph at U.S. Geological Sur- vey gaging station on Rock Creek at Independence, Mo., flood of Sept. 12—13, 1977. FIGURE 23.——Discharge hydrograph and mass curve of rain- fall at U.S. Geological Survey gaging station on Round Grove Creek at Raytown, Mo., flood of Sept. 12—13, 1977. DESCRIPTION AND MEASUREMENT OF FLOODS 31 I I I I | I I I | I DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECOND | | | I I o 4 8 12 16 20 24 4 8 12 16 20 SEPT. 12 SEPT. 13 TIME, IN HOURS FIGURE 25.—Discharge hydrograph at U.S. Geological Sur- vey gaging station on Shoal Creek at Claycomo, Mo., flood of Sept. 12—13, 1977. 18 DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECOND 15—- 14— 13— ‘ I I I I I 8 12 16 20 24 4 8 12 16 20 24 SEPT. 12 SEPT. 13 TIME, IN HOURS FIGURE 26.—Discharge hydrograph at US. Geological Sur- vey gaging station on Little Blue River below Longview Road Damsite in Kansas City, Mo., flood of Sept. 12—13, 1977. 00 N DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECOND ‘0 4 8 12 16 20 24 4 8 12 16 20 24 SEPT. 12 SEPT. 13 TIME, IN HOURS FIGURE 27.——Discharge hydrograph at U.S. Geological Sur- vey gaging station on East Fork Little Blue River near Blue Springs, Mo., flood of Sept. 12—13, 1977. FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 18IIIIIIIIIII 17- — 16- _ 15— _ 14— -— 13— — 12— — 11— —— 10—- - DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECOND 0 IIIIIIIIII 0 816 24 816 24 816 24 816 24 SEPT. 12 SEPT 13 SEPT. 14 SEPT. 15 TIME, IN HOURS FIGURE 28.—Discharge hydrograph at U.S. Geological Sur- vey gaging station on Little Blue River near Lake City, Mo., flood of Sept. 12—13, 1977. DESCRIPTION AND MEASUREMENT OF FLOODS 33 16 I DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECOND 0 4 8121620244 812162024 SEPT. 12 SEPT. 13 TIME, IN HOURS FIGURE 29.—Discharge hydrograph at US. Geological Sur- vey gaging station on Sni-A-Bar Creek near Tarsney, Mo., flood of Sept. 12—13, 1977. FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 34 I 3 _ r _ _ _ 1 z ’ I : I 2 cm >3: .ij,” 1 x35 swam / I ’ II 1 c 1 ,— - O - o , 1 mm 2:8“. €825. 1.. mm 086:2”. cum \I / um I $33.6 _ I 9 93¢ c>>9>mm _ _ /_ Ion >>>I 9899c. .wj I # 7 I 8 8 >3: .ij w & I Mr I _/ = m. 625 Eme _ B 5 I . ,Mt % I 6 . T 1 :9 m .32“. .mammcocmEI :7: M Umom cmEE... _/ W. 395 EEI O / | / | 4 x mm 228; 303:2 Aww / | Q>< wocwucwamuE W/ // I 3 4w «v /_ Rom :8__>>I¢AM> | mm 9:841:03: oww/P I 2 mm EwcSow >20 mmmcmv. I g a I , I 1 a a / _ ~ _ _ 7 _ 0 0 0 o 0 0 0 0 0 0 9 8 7 6 5 4 3 2 1| 7 7 7 7 7 7 7 7 7 mm? “.0 5.3.55 ._ UPwOOmO ._Om_< Hum“. Z_ .ZO_._.<>m:m FIGURE 30.—-Profile of water surface of Blue River MILES UPSTREAM FROM MOUTH OF RIVER 35 DESCRIPTION AND MEASUREMENT OF FLOODS .l E o:_o£-_58m=2 l1 x85 55:... umom 552:8 20 | . K >>>I .wj Isl . vmom 2639.90 U>_m 309:0 outta ~08 fun. 895w 62m 2% p mm 058115822 I / a , / g / /’ ” —’/ wgl/ ’ 1 E w SE? /—- —’ 27 25 mNmF _ O 9 7 810 — 800 — ”.0 55.510 ._ U_._.m_QOwO 40m< hum”. _ o 8 7 0 6 7 _ 0 5 7 _ 0 0 4 .Z_ .ZO_._.<>m._m_ MILES UPSTREAM FROM MOUTH OF RIVER for Sept. 12—13, 1977, and Sept. 1961 floods. FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 36 Horse _to>__:o _|_\ /_ _ _ _ _ a g _ _ _ a 8 R 02.5 vcfloom / x I 1. W oocmbcw 65:: W F / 6 0 $230 35.3 .. .I 1. H QES 388 mm: >31 .03 W aEE mmmoom as >31 .wj / O 4 M x | , 1 M 0 80¢ :mntocw , l 2 M I 1 A E R / T S o D: I I 1 U r S E / m [8 M a 0 2.6 2.2 2.0 / two: co_mw__>_ / 04 , Umom {mm 92m / _ _ _ _ _ / _ mmmw _ _ _ _ _ ~ _ _ _ 0 0 0 0 0 0 0 w m m m 5 M w m 1 m % 00° 7 m w 0 9 9 9 9 9 9 9 9 9 8 8 8 8 8 1 “.0 55.56 ._ UFmDOmO ._Om< .rmmE Z_ .ZO:.<>m..m_ Profile of water surface of Rock Creek (Kans.), flood of Sept. 12—13, 1977. FIGURE 31. DESCRIPTION AND MEASUREMENT OF FLOODS 37 MAGNITUDE AND FREQUENCY OF FLOODS Knowledge of the magnitude and probable fre- quency of flood recurrence is useful in the design and location of many types of hydraulic structures and in the development of criteria for flood-plain manage- ment. Techniques for deriving flood-frequency relations are those described by the US. Water Resources Council (1977) and by Hauth (1974). Peak discharges of the Kansas City flood were combined with data for other floods to obtain re- gional flood-frequency relations. No attempt was made to adjust for the effects of urbanization. Fre- quencies of flood discharge are estimated for recur- rence intervals of 100-years or less. For greater dis- charges, recurrence intervals are noted only as “greater than 100-years.” FLOOD VOLUMES During recent years, the nationwide construction of flood-control reservoirs and the allocation of ca- pacity in multipurpose reservoirs for flood control have increased at a higher rate than other reservoir uses. Growth in numbers and capacity of small flood— storage projects in urban areas across the nation has created a need for flood—volume data from typical urban watersheds. Table 4 is a tabulation of flood volumes in acre-ft and in. of runoff, as computed from streamflow hy- drographs available for the Kansas City area. See figures 17 to 29. Recurrence intervals of flood vol- umes given in table 4 are based on relationships de- veloped by Skelton (1973) for rural areas. For a given amount of rainfall, greater flood volumes will occur in an urban area than in a rural area because of the greater area of impervious surfaces. Thus, for significantly urbanized basins, the recurrence inter- vals shown in table 4 may be too high because the estimates are based on rural conditions. Until more data are collected in urban areas of Missouri, there is no way to accurately adjust these frequency esti- mates. COMPARATIVE MAGNITUDE OF FLOODS Peak discharges of the Kansas City Sept. 1977 flood, the July 1951 flood in Missouri and Kansas (US. Geological Survey Water-Supply Paper 1139, 1952, p. 199), and miscellaneous flood discharges ex- perienced in Missouri and Kansas are plotted against their respective drainage areas for comparison (fig. 39). A curve developed by Crippen and Bue (1977) on the basis of maximum known floods in the United States defines their approximate upper limit through 1974 (fig. 39). The other curve also shown here was developed by Crippen and Bue using maximum floods in the Missouri-Kansas area, including metropolitan Kansas City. Peak discharges encountered in the Sept. 197 7 flood do not approach the maximum flood experience in the United States. Curve values are more than double the highest peak flows of Sept. 12—13, 1977. However, the Sept. 1977 flood does ap- proach maximum flood experience in the Missouri- Kansas area. SUMMARY Two record-setting rainstorms occurred in the metropolitan Kansas City area on Sept. 12—13, 1977. Storm runoff claimed the lives of 25 people and caused over $80 million in damages. The US. Geo- logical Survey and the National Weather Service cooperated to document the meteorological and hy- drological aspects of the flood. Average rainfall over the metropolitan area ex- ceeded 10 in. with a maximum of more than 16 in. Total storm rainfall of 9.39 in. at Kansas City Inter- national Airport exceeded the 100-year 2-day amount of 8.8 in. Maximum storm rainfall of 16.15 in., in approximately 36 hr, exceeded the 100-year 10-day rain of 13.0 in. The combination of meteoro- logical factors associated with the storms are dis- cussed. Isohyetal maps of storm rainfall are pre- sented. Rainfall estimates, using satellite infrared imagery, are found to be consistent with the result of depth—area analysis using conventional data. FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13. 1977 38 _ _ fl _ _ _ _ _ .l. / 30m 0:: 83w mmmcmv.-_.50wm_§ m 35.6; .885 l x w / m / e / uwmbw wwwm II / I m I0. I. / f l mzcm> >>2>m=mm I: e >Mx>xm gecmoml [1. .mu I / & nmom =mEo>> ll , I / “ Umom 30:22 .0 .6' I .l. 62% £22 I 1|. / _ | 525 xmo ll . I nmom 55.03. / I x W// wzcw>< $09... I / 8mm; 9: l/ 1 Oowv/ l / @9/ l / Awe/fl wscw>< u:m_noo>> , .I. / a / // H/ / /, wzcm> omamo. 1 / my I < H n. / V 1 z I 1 xwmho ____>_ flvz n>_m_ 5:35 am. wzcm>< b:m_m>m_o H/ I II R. I E. B p M u mzcw>< quSEE fi/ 1 W— S. m: o_:o§-_5omm_s_ 1|, " * _ _ _ _ , _ _ _ o o O O o 0 0 0 o w w m a m m m m n mm? “.0 _>_D._. UFmOOmO ._Om< kme Z_ ~ZO_._.<>m._m_ MILES UPSTREAM FROM MOUTH OF RIVER FIGURE 32.—Profi1es of water surface of Brush 39 DESCRIPTION AND MEASUREMENT OF FLOODS I— - Bridge floor elevations // / Emhw 5mm // / / z / womtoh ucmb / “wwhw 3K / x 8:: 65.3. ”—0330 EEC—JP wscw>< mom nmom meLmEE. m», ww/ «V N/ I My cw «7/ °\ / 52m 28. (my / 95.. :39: l 984 :Euc. / v.36 goo: , / / two: fan. 32m \l / x / / I Box 85 2:58 2:: 8525. || , / umom wc: 38w 3237:2035 _ _ _ 0 0 0 6 m m 8 980 — 940 — 920 — 0 8 9 9 8 8 mmmF ”.0 EDP/‘0 4 UFmDOwO ._0m< hum“. Z_ .ZO_._.<>m:m_ 820 11 10 MILES UPSTREAM FROM MOUTH OF RIVER Creek (Kans.) flood of Sept. 12—13, 1977. _ _ _ _ A _ _ _ _ _ _ _ _ _ _ _ _ _ A // $95 28 .4; 505m am PM //1 62% £8 I I n I 5 x .m / mzca>< 300552 N. 7 u. oscm>< EoELo> m 7 r w mzcm>< V m .. 9:55 H B x e _ z 9 2 d 1 Em>m_:om / M. T“ l Eoztoz / _ | 4 P x I. E S «60¢ “‘59me //x ., 625 BR 1 / S / M N A mscm>< noo>>>22 K // D x N I I 3 A mscm>< 555.3 0. / // M I / I: / Y 9/ 6v T ,1: e m 9\ I xv? / . 9 S «\d «7 A umom :mEEk W, I S I I 2 N 4v / A aw, // K Aw / Ox m V S / D r o x 0 93¢ 5cE>> I L F I , I 1 a / Rom com=>> I //1 mscm>< 503:3. I x _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 0 0 O O 0 0 0 0 O O 0 0 O 0 0 0 0 0 0 0 0 m m w w m % % M w w m m m n W m B M n n 0 4 mwmr “.0 55:3 ._ UFmQOmO 40m< Puma Z_ ~ZOFaSmfi. MILES UPSTREAM FROM MOUTH OF RIVER FIGURE 33.—Profile of water surface of Rock Creek (Mo.), flood of Sept. 12—13, 1977. DESCRIPTION AND MEASUREMENT OF FLOODS TABLE 4.——Summary of flood volumes for Kansas City area floods, Sept. 12—13, 1977 41 Map Volume of flood event Recurrence interval, No. Permanent Drainage Flows in acre-ft Total storm in years I (see station area fig. 2) No. Stream and place of determination (miz) Sept. 12 Sept. 13 Acre-ft In. Sept. 12 Sept. 13 1 06821280 Line Creek at Riverside, Mo _____ 19.2 1,770 2,973 4,743 4.60 7 13 b 6 06893080 Blue River near Stanley, Kans. ___. 46.0 ____ ____ 164 .06 ___ <2 b 8 06893300 Indian Creek at Overland Park, Kans _________________________ 26.6 1,558 3,817 5.375 3.79 2 18 9 06893350 Tomahawk Creek near Overland Park, Kans ___________________ 23.9 371 1,716 2,087 1.64 ___ 70 10 06893500 Blue River near Kansas City, Mo _ 188 7,423 22,049 29,472 2.94 _-_ 7 16 06893560 Brush Creek at Main Street at Kansas City, Mo ______________ 14.8 1,910 6,254 8,164 10.34 13 >100 17 06893570 Round Grove Creek at Raytown Road at Kansas City, Mo ______ 5.87 1,206 1,662 2,868 9.16 40 100 19 06893600 Rock Creek at Independence, Mo _ 5.20 1,540 968 2,508 9.04 >100 25 20 06893670 Shoal Creek at Claycomo, Mo ___- 29.8 4,078 4,530 8,608 5.42 12 13 24 06893793 Little Blue River at Longview Road Damsite at Kansas City, Mo ___________________________ 50.7 ___- 9,940 9,940 3.68 ___ 19 28 06893890 East Fork Little Blue River near Blue Springs, Mo _____________ 34.4 ___- ---- 7,898 8.06 ___ 20 29 06894000 Little Blue River at Lake City, Mo ___________________________ 184 --..- --.... 26,514 2.70 ___ 10 b 30 06894680 Sni-A-Bar Creek at Tarsney, Mo .. 29.1 9,548 3,772 13,320 8.58 (c) 15 I Based on relationships for rural areas. 9 Volume includes total storm period. e Flood volume-frequency relationship not available for given storm duration. FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 42 _ _ _ _ mmmn>m K >31 .mj 1 8 >3: .m.: on >3: 83295 .mj Dmom mam—0m >E 02m 2:] f0”. ummw 52% Sam I\ mm >>>I .mj KN Umom cmEE... 1 93¢ scomuczm mm 2:08-:835 _ _ I mm oc_omn_-_5omw_s_ Ill _ _ 3 >3: .0: \ll Umom w__:>_ m:_m || Rom $36 8:22 m: mu. Scum vcm mxmooficoflsog 1 Box 30:01 Baum: m w a m S ///// 22 23 24 25 | I | 11 12 13 14 15 16 17 18 19 20 21 MILES UPSTREAM FROM MOUTH OF RIVER 10 _ 0 9 7 _ _ O O 0 3 2 .I 8 8 8 800 — am? no 5.3...45 n_ UFmQOwO ._Om< meu. Z_ .ZO_._.<>m4m_ _ 0 8 7 _ 0 7 7 _ o 6 7 _ O 5 7 FIGURE 34.—Profi1es of water surface of 43 DESCRIPTION AND MEASUREMENT OF FLOODS _ _ _ _ w E _ _ _ _ _ _ _ vmom >>m_>mco._ mtm Ema >>m_>mco._ umom B0359: 900 mm Ucflm. xoombmmoEo I'll / / tmom 65:53 lll/ , z 98: 23oz Ill/ om >>>I .mj , , m. B m 1 1. E w. my mm o_:omn_-_50mm__>_ a... S umom mam 233 I‘ A 2 ;\ a “flu, P at, Umom :EEsm 83 wmma>m K >>>I .m.D / _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 0 0 O 0 0 0 0 0 0 0 0 0 0 0 0 987654321098765 oo 8 8 8 8 8 8 oo oo 8 7 7 7 7 7 mm? ”.0 55kg”. ._ UFmOOmO ._m_._m 37 38 39 4O 41 42 32 33 34 35 36 MILES UPSTREAM FROM MOUTH OF RIVER 28 29 3O 31 Little Blue River, flood of Sept. 12—13, 1977. 27 25 26 740 44 FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 7 94°35’28" 94°35’1 1" Vim—ifl.‘ . “2:. 39°02 ’32" 4' / I § "u i ° ° NICHMS _ /]75 I. r. g .. ‘ My , ’ A] C“ it; " ‘v 0d 0 " . t] al‘ amufiit /W“” ' y A?“ f (MW ¢ ' we . A/W430db0mi’a’V9 l; ‘3 ‘9 l_ W £377-, ~ :er :2: mm: PAnKWAv xx :3“ ,. M;/,/ / n V 1 C :2: , A, //:n e ///o;/Q) 93“ 39°02’21 ” AOUFEET ] 0 f H" iZJ ‘ |__.|_'_l__l_l___J jl /a: :57 T u 1UOMETERS 4' g I é ' D 3M FIGURE 35.—Boundary of Sept. 12—13, 1977, flood along Brush Creek between Main Street and Jefferson Street, Kansas City, Mo. Base map by US. Army Corps of Engineers, Kansas City district FIGURE 36.—Recently deposited gravel and scattered coarse material on left (north) Brush Creek flood plain approxi- mately 1,000 ft downstream from Woodland Avenue bridge, CUMULATIVE PERCENT OF TOTAL WEIGHT Kansas City, Mo. DESCRIPTION AND MEASUREMENT OF FLOODS bridge, Kansas City, Mo. 45 FIGURE 37.—Channe1-bed slabs and car in Brush Creek stream channel immediately downstream from Rockhill Road 100 90— 80— 70— 60— 50- 40'— 30— 20” 10 I I 0 I 1.0 PARTICLE SIZE, IN MILLIMETERS City, Mo. 1000 FIGURE 38.—Particle-size distribution of sediment deposits in Brush Creek in the vicinity of Woodland Avenue bridge in Kansas 46 FLOODS IN KANSAS CITY, MO. AND KANS., SEPT. 12—13, 1977 100— 10'— DISCHARGE, IN THOUSANDS OF CUBIC FEET PER SECOND EXPLANATION 0 Peak discharge: ' Peak discharge: July 1951 ‘ Previous peak discharge in — Missouri-Kansas area September 12—13, 1977 FIGURE 39,—Comparison of Sept. 12—13, 1977 1.0 10.0 DRAINAGE AREA, IN SQUARE MILES the United States. 100 1000 , peak discharges to upper limits of known floods in Missouri-Kansas area and in REFERENCES CITED 47 Thirteen U.S. Geological Survey stream-gaging stations in operation during the flood provided in- formation on flood volumes and peak discharges. Re- currence intervals of flood volumes experienced in areas of greatest rainfall, such as Brush Creek and Round Grove Creek, were greater than the regional 100-‘year estimates. In surrounding areas, the de- creasing magnitude of recurrence intervals reflected the basins’ distance from the center of the two storms. The second storm caused the greater flood damage because the first storm had saturated the drainage basins. Peak discharges were determined at 31 locations in the Kansas City, Mo.-Kans. area. Recurrence in- tervals exceeded 100-years in many areas. Floodflows occurring on Sept. 12—13, 1977, are considered comparable to other major floods which have been documented in Missouri and Kansas, but are not as great as those experienced in other areas of the United States. REFERENCES CITED Bodhaine, G. L., 1968, Measurement of peak discharge at cul- verts by indirect methods, chap. A3 of bk. 3, Applications of hydraulics: U.S. Geological Survey Techniques of Water-Resources Investigations, 60 p. Cheng, N. and Rodenhuis, D., 1977, An intercomparison of satellite images and radar rainfall rates: University of Maryland, Meteorology Program, Publ. 77—166, 60 p. Crippen, J. R., and Bue, C. D., 1977, Maximum floodflows in the conterminous United States: U.S. Geological Survey Water-Supply Paper 1887, 52 p. Dalrymple, Tate, and Benson, M. S., 1967, Measurement of peak discharge by slope-area method, chap. A2 of bk. 3, Applications of hydraulics: U.S. Geological Survey Tech- niques of Water-Resources Investigations, 12 p. Follansbee, W., 1973, Estimation of average daily rainfall from satellite cloud photographs: National Environmen- tal Satellite Service, NOAA, Tech. Memo NESS 44, 39 p. Griflith, C. G., Woodley, W. L., Grube, P. G., Martin, D. W., Stout, J., and Sidkar, S. N., 1978, Rain estimation from geosynchronous satellite imagery—visible and infrared studies: Monthly Weather Review 106, 8, 1153—1171. Guy, H. P., and Norman, V. W., 1970, Field methods for meas- urement 0f fluvial sediment, chap. CZ of bk. 3, Applica— tions of hydraulics: U.S. Geological Survey Techniques of Water-Resources Investigations, 59 p. Haulsing, Harry, 1968, Measurement of peak discharge at dams by indirect method, chap. A5 of bk. 3, Applications of hydraulics, U.S. Geological Survey Techniques of Water—Resources Investigations, 29 p. Hauth, L. D., 1974, Technique for estimating the magnitude and frequency of Missouri floods: U.S. Geological Survey Open-File Report, 20 p. Hauth, L. D., and Carswell, W. J., Jr., 1978, Floods in Kansas City, Missouri and Kansas, September 12—13, 1977: U.S. Geological Survey Water-Resources Investigations 78—63, 36 p. Hershfield, D. M., 1961, Rainfall frequency atlas of the United States for duration from 30 minutes to 24 hours and re- turn periods from 1 to 100 years: U.S. Weather Bureau Technical Paper 40, 61 p. Lott, George A., 1976, Precipitable water over the United States, vol. 1, monthly means: U.S. Department of Com- merce, National Oceanic and Atmospheric Administra— tion, National Weather Service, NOAA technical report NWS 20. Maddox, R. A., 1976, An evaluation of tornado proximity wind and stability data: Monthly Weather Review 104, p. 133—142. Matthai, H. F., 1967, Measurement of peak discharge at width contractions by indirect methods, chap. A4 of bk. 3, Ap- plications of hydraulics: U.S. Geological Survey Tech- niques of Water-Resources Investigations, 44 p. Miller, J. F., 1964, Two- to ten—day precipitation for return periods of 2 to 100 years in the contiguous United States: U.S. Department of Commerce, Weather Bureau, Wash- ington, D.C., Technical Paper no. 49. Schofield, R. A., and Oliver, V. J., 1977, A scheme for estimat- ing convective rainfall from satellite imagery: U.S. De- partment of Commerce, National Oceanic and Atmos- pheric Administration, National Environmental Satellite Service, Washington, D.C., NOAA Technical Memoran- dum NESS 86, 47 p. Skelton, John, 1973, Flood-volume design data for Missouri streams: Missouri Geological Survey and Water Re- sources, Water Resources Report 28, 28 p. U.S. Department of Commerce, 1974, Climates of the States: National Oceanic and Atmospheric Administration, pub- lished in 2 vols. by Water Information Center, Inc., Port Washington, N.Y. U.S. Geological Survey, 1952, Kansas-Missouri floods of July 1951: U.S. Geological Survey Water-Supply Paper 1139, 239 p. U.S. Water Resources Council, 1977, Guidelines for determin- ing flood flow frequency: Washington, D.C., U.S. Water Resources Council Bulletin 17A, 163 p. Woodley, W. L., Sancho, Briseida, and Miller, A. H., 1972, Rainfall estimation from satellite cloud photographs: NOAA Technical Memorandum ERL 0D—11, Boulder, 0010., 43 p. it U.S. GOVERNMENT PRINTING OFFICE: I98I — 34I-6H/183 475 75' f6 12- He 6: PM“ PROFESSIONAL PAPER 1166 UNITED STATES DEPARTMENT OF THE INTERIOR PLATE 1 GEOLOGICAL SURVEY “I ”0030’ R. 13E. 110°i5’ " // "CORRELATION OF MAP UNITS T135. ' : ' - ' , , _ , _ Hd ] Holocene(?) V , . _ ‘ W , __ ' ‘ - I, IT.13S. UNCONFORMITY Ont I , _ Upped?) B - - Point Wisconsin _ A102 Upper Wisconsin(?) $QUATERNARY > Pleistocene Y 0a Os UNCONFORMITY Lower 01a ch Wisconsin(?) UNCONFORMITY Middle or lower 03 0pm Pleistocene ' . : UNCONFORMIIY Op T.14S.- ~ , .7 I , ' .‘ , Lower __ , , V 7 . _ . y , \ , ‘ T. 14 S De on Pleistocene ‘ UNCONFORMITY ’ Tgr Eocene Tc LTERTIARY TKfn Paleocene < 39°35’ 39°39 UNCONFORMII Y Upper > Cretaceous > CRETACEOUS Upper and Lower(?) Cretaceous — ~ -—-— Lower(?) UNCONFORMTTY Cretaceous - < UNCONFORMIIY UNICONFORMITY Upper Jurassic LJURASSIC Middle ,1 . ‘ V ‘ , - Jurassic T.155. .' A . : '- _, ‘ ‘ . _. V x - _ ' T. 15 S. 1 LIST OF MAP UNITS P I Hd ‘ MINE WASTE DUMP (HOLOCENE) Ont NATURAL TALUS (HOLOCENE AND UPPER? WIS— CONSIN) 0a ALLUVIUM UNDIFFERENTIATED (HOLOCENE TO LOWER PLEISTOCENE) GRAVEL (UPPER? WISCONSIN) 0c COLLUVIUM (UPPER? WISCONSIN) TERRACE DEPOSITS (PLEISTOCENE TO UPPER ? WISCONSIN) ALLUVIAL FAN DEPOSITS (UPPER WISCONSIN?) Undifferentiated Youngest Middle Oldest Os SLOPE MANTLE, COLLUVIUM, AND LANDSLIDE DE- BRIS (LOWER WISCONSIN?) ch CEMENTED CONGLOMERATE (LOWER WISCON— SIN?) PEDIMENT DEPOSITS (MIDDLE OR LOWER PLEIS— TOCENE AND LOWER PLEISTOCENE) T‘ 16 3. OP Undifferentiated T. 16 s. QPY Youngest (Holocene and upper? Wisconsin) me Middle (middle or lower Pleistocene) OPO Oldest (lower Pleistocene) Tgr GREEN RIVER FORMATION (EOCENE) Tc COLTON FORMATION (EOCENE) ‘TKfn FLAGSTAFF AND NORTH HORN FORMATIONS UNDIF- FERENTIATED (EOCENE TO UPPER CRETACE- OUS) PRICE RIVER FORMATION (UPPER CRETACEOUS) E Kpb Bluecastle Sandstone Member m Mudstone member — CASTLEGATE SANDSTONE (UPPER CRETACEOUS) —Kbsc— CASTLEGATE SANDSTONE AND SUNNYSIDE MEMBER, UNDIFFERENTIATED BLACKHAWK FORMATION (UPPER CRETACEOUS) Kbs Sunnyside Member - Kenilworth Member Aberdeen Member MANCOS SHALE (UPPER CRETACEOUS) Main body Upper sandstone member Ferron Sandstone Member Lower sandstone member DAKOTA SANDSTONE (UPPER AND LOWER? CRETA— CEOUS) CEDAR MOUNTAIN FORMATION (LOWER CRETACEOUS) Shale member Buckhorn Conglomerate Member 7‘ 3 c m MORRISON FORMATION (UPPER JURASSIC) Brushy Basin Shale Member Salt Wash Member - SUMMERVILLE FORMATION (MIDDLE JURASSIC) - CURTIS FORMATION (MIDDLE JURASSIC) - ENTRADA SANDSTONE (MIDDLE JURASSIC) - CARMEL FORMATION (MIDDLE JURASSIC) CONTACT ——I-— FAULT—Bar and ball on downthrown side, dashed where . T‘ 18 3' inferred, dotted where concealed SUBSURFACE FAULT—Found by seismic methods. U, up- thrown side; D, downthrown side SYNCLINE AXIS—Showing dip of beds ANTICLINE AXIS—Showing clip Of beds —‘— STHKE AND DIRECTION OF DIP OF BEDS SSOIIIJOBU' I, R 13 E ' ' " ‘ _ : , , , I R 14 E 20’ R 15 E. Km 11001331015, Base from US. Geological Survey, 14" S - Price, 11250000 raised relief edition IIIIII V GALE 1-62 500 Geology by F' W' Osterwald, J' 0' Maberry I 5 L 1—1 1—12 H H O 1 a 3 4 MILES and C. H. Dunrud, 1958-72, assisted by E g I————I a UTAH H. E Eggleton, 1958, Harold Brodsky, 1959—60, 2 é’ 1 .5 0 1 2 3 I J. O. Duguid, 1962, and B. K. Barnes, 1962-67 E (5 H H H. a . % fl4 KILOMETERS S momma MW NATIONAL GEODEIIC VERTICAL DATUM OF 1929 QUADRANGLE LOCATION DECLINATION 1981 A METERS % A, 3000 _ .x Forest Oil Co. E? C FEET § . 25—1 Arnold « -‘ 2 ~10 000 2500 _ 0 Projected 4724 m (15,000 It.) 3 § 5 ' E BOOK northwest into section '3‘ U D- _ 9000 H CLIFFS 2 8’ w 2000 — % E g 6%; — 8000 g THE R: O — 7000 — 6000 — 5000 — 4000 — 3000 - 2000 T 1000 —SEA LEVEL 5 _ 00 Pre-Morrison rocks Surficial deposits not shown Inf d b rf f l erre su su ace aut\ VERTICAL EXAGGERATION X 0.8 GENERALIZED GEOLOGIC MAP AND CROSS SECTION OF THE SUNNYSIDE COAL MINING DISTRICT, EMERY AND CARBON COUNTIES, UTAH _ SEA LEVEL PROFESSIONAL PAPER 1166 UNITED STATES DEPARTMENT OF THE INTERIOR PLATE 2 GEOLOGICAL SURVEY ”0025 'c {a 33 E0 531; 1‘3 4 6, 000 F 48,000 R. l 4 E, N 64000 N63,,IOOO INDEX MAP OF UTAH N6?,OOO '7. NDIIOOO Neoooo 0 20 40 00 80 100 120140 100 KILOMETEHS 0 20 40 60 80 IOU MILES T. I 3 S. N59,000 ' N58,000 “0025' .1394" I 5 >- o E 5 u 0 39°40 g e APPROXIMATE MEAN N 57,000 DECLINATION 1931 70) 1700 N56,000 CORRELATION OF MAP UNITS _ -‘ » ' Q5 Holocene UNCONFORMITY E 5 51000 0 Upper Qa . . Wisconsin(?) UNCDNI-‘DRNIHY 551939213: UNCONFORMITY f, b :3 V .Lower ? Pleistocene r QUATERNARY a Wisconsm(?) UNCONF RMITY N54,000 Pre-Wisconsin(?) Lower UNCONFORMITY Pleistocene Oa1 J J / UNCONFORMITY T N Eocene > TERTIARY N 53000 Paleocene A Upper Cretaceous r CRETACEOUS / J N 52,000 LIST OF MAP UNITS COLTON FORMATION (EOCENE) —— CONTACT Tcu Upper member u . MAN—INDUCED TALUS (HOLOCENE) —'—'D FAULT—Dashed where approxrmately located; Lower member dotted where concealed. U, upthrown side; D, Ont NATURAL TALUS (HOLOCENE AND UPPER - downthrown side WISCONSIN?) FLAGSTAFF FORMATION (EOCENE) Qa4 YOUNGEST ALLUVIUM (HOLOCENE AND NORTH HORN FORMATION (EOCENE TO ——> FORMER STREAMCOURSES UPPER WISCONSIN? YOUNGER EARLY ALLZJVIUM (HOLOCENE UPPER CRETACEOUS) R CRETA : MINE PORTAL W000 '2 PRICE RIVER FORMATION UPPE - ~ , AND UPPER WISCONSIN" CEOUS) ( J’— STRIKE AND DIP OF BEDS ALLUVIAL-FAN DEBRIS (UPPER WISCONSIN?) ALLUVIAL SAND AND SILT (UPPER WISCON Blue‘mfle sandsme Member 15— STRIKE AND DIP OF JOINTING—In degrees SIN?) gfig Unnamed lower mudston e member Box on both sides Indicates vertIcal Jomtlng ALLUVIAL GRAVEL AND BOULDERS (UPPER STRIKE AND DIP OF CLEAVAGE IN COAL BEDS CASTLEGATE SANDSTONE (UPPER CRE- SLOWISMONSIf) COLLUVIUM AND LAND - TACEOUS) Lil Dipping less than 90 degrees PE ANTL , , _ SLIDE DEBRIS (LOWER WISCONSIN?) BLAEXCPéngg) FORMATION (UPPER CRE' »—~ Vertical _ '2 V STREAM ALLUVIUM (PRE WISCONSIN) Kbs , Sunnyside Member—Contains Sunnyside coal _A_A.. BASE 01: COAL BED—Teeth indicate coal burned MORAINE (PRE-WISCONSIN?) beds at outcrop 0 00,000 - Kenilworth Member—Contains Rock Canyon STREAM TERRACE (LOWER PLEISTOCENE) 7 coal bed " MANCOS SHALE (UPPER CRETACEOUS) Oa1 OLDER EARLY ALLUVIUM (LOWER PLEIS- TOCENE) Base from U.S. Geological Survey, 1958—59 SCALE 1:6 000 Geology by F. W. Osterwald, 1958—72 and J. 0, Maberry, 1970—72, Coordinate System by Utah Fuel 00-. about 1900: 500 3 500 1000 1500 2000 2500 3000 FEET assisted by H. E. Eggleton, 1958, and Harold Brodsky. 1959—60 origin south and west of map area :0 l—-—-l I—I l—————-———-l I , ' 1 “149.000 c; fi 500 fi l____i 10100 METERS CONTOUR INTERVAL 20 FEET NATIONAL GEODEHC VERTICAL DATUM or 1929 ll0°22’3il” T, 14 S. A A’ METERS LLI 2500— West Ridge ”0" 2 Tel 9 8 z '_ Bull Flat E 5‘ FEET l m E 8000 Tf _ c _ 2 — 7500 C (U o L g Air shaft 760 ft _ E (230 m) to top 1:: _ 3 _ — 7000 ‘ 2000 _ —6500 L0000 N0 VERTICAL EXAGGERATION BEDROCK AND SURFICIAL GEOLOGY OF SUNNYSIDE COAL MINES AREA, CARBON COUNTY, UTAH UNITED STATES DEPARTMENT OF THE INTERIOR PROFESSIONAL PAPER 1168 GEOLOGICAL SURVEY \ ‘ PLATE 1 122°22'30” P‘ 1700” R 4 w 15' 1230" 9 10’ R 3w R 2w 122°07’30" 3700713011 , ’ y, , , e x _ , , ‘ , 1 . , y i , _ _ f . » , .. {g , , 37°07’30/, ’3 Richfield Steele core‘hole l #0 (T. D. 2675’) Om CORRELATION OF MAP UNITS Qal Os OIs Holocene QUATERNARY Qm Pleistocene Unconformity 6 SOUTHWEST OF NORTH EAST OF SAN GREGORIO FAULT SAN GREGORIO FAULT Tps - Pliocene 5, _ Tpm Unconformity Miocene _ — TERTIARY :- Oligocene Unconformity _ Eocene(?) Nonconformity —_ Upper Cretaceous _ CRETACEOUS : MESOZOIC OR J PALEOZOIC DESCRIPTION OF MAP UNITS SURFICIAL SEDIMENTS ‘Qal ALLUVIUM—Unconsolidated gravel, sand, and silt Os SAND DUNES Ols LANDSLIDE MATERIAL—Half arrows show direction of downslope movement Om MARINE TERRACE DEPOSITS—Unconsolidated moderate—yellowish-brown fine sand and granular gravel T P lett" l exas o 1 ' SOUTHWEST OF SAN GREGORIO FAULT (T. D. 9197') PURISIMA FORMATION (Pliocene) Tps Sandstone member—Thick— to very thick bedded olive—gray fine-grained lithic sandstone with thin interbeds of carbonate concretions Tpm Mudstone member—Medium- to thick-bedded light-olive-gray nodular mudstone Tm MONTEREY FORMATION (Miocene)—Thin-bedded and thinly laminated olive-gray to dusky—yellowish-brown siliceous mudstone TV? VAQUEROSI?) FORMATION (Oligocene and Miocene)—Dusky-yellowish-brown j; phosphatic mudstone and fine- to medium-grained glauconite-bearing arkosic I sandstone Volcanic breccia——Altered andesitic and basaltic blocks at base of exposed section PIGEON POINT FORMATION (Upper Cretaceous. Shown in cross section only)— lnterbedded sequence of brownish—gray fine— to coarse—grained sandstone, dark— gray sandy siltstone, sandy pebble to cobble conglomerate, and pebbly mudstone NORTHEAST OF SAN GREGORIO FAULT ; Upper Miocene to Pliocene sedimentary sequence Tsc SANTA CRUZ MUDSTONE ( upper Miocene)—Medium— to thick—bedded and faintly laminated olive-gray to pale—yellowish-brown blocky siliceous mudstone and A nodular sandy siltstone 2 30 _ SANTA MARGARITA SANDSTONE I upper Miocene)—Very thick bedded to massive light-olive—gray to white medium— to fine—grained calcareous arkosic sandstone; locally calcareous; locally bituminous Middle Miocene sedimentary sequence . Tm MONTEREY FORMATION—Thin- to medium-bedded olive-gray to medium-gray sandy siltstone and siliceous mudstone. Includes a few very thick medium-grained arkosic sandstone interbeds LOMPICO SANDSTONE—Thick- to very thick bedded medium-gray to grayish— orange medium— to tine-grained calcareous arkosic sandstone Eocene to lower Miocene sedimentary sequence BUTANO(?) SANDSTONE (Eocene?)—Very thick bedded yellowish-gray medium- grained arkosic sandstone, commonly grading upward to greenish-gray sandy mudstone. Includes very thick sandy cobble conglomerate interbeds Crystalline plutonic and metamorphic rocks QUARTZ DIORITE GRANITE AND ADAMELLITE GNElSSlC GRANODIORITE HORNBLENDE-CUMMINGTONITE GABBRO METASEDIMENTARY ROCKS—Mainly pelitic schist and quartzite MARBLE—Locally contains interbedded schist and calc-silicate rocks M_Y__ Contact, approximately located—Triangle where well exposed; queried where uncer— tain — Fault—Dashed where approximately located; short dashed where inferred; dotted where concealed. U, relatively upthrown side; D, downthrown side ‘ __ Anticline—Dashed where approximately located; dotted where concealed 4___ Syncline—Showing direction of plunge; dashed where approximately located; dotted where concealed Strike and dip of beds I5 _I_ Inclined _,_ Vertical @ Horizontal _I__ Estimated 1 Strike and dip of foliation Fossil localities M5049 x Megainvertebrate V5555 x Vertebrate Mf3675 x Microfossil l 137 X Lithologic sample locality 37°00 _ o Abandoned exploratory oil well 37000, Half arrows show direction of downslope movement A A, B B’ METERS METERS 300 — — 800 — x _ _ FEET _ § FEET 600 — — r 2000 500 _ :3 — 2000 — — — 1000 — Tsm 2 qd —1000 400 ‘ — —1200 400 — E l1200 s _ _ _ _ O _ - ANO NUEVO FAULT — 800 — 200 — A"° GREEN OAKS FAULT SAN GREGORIO FAULT — _ 200 — 13 _ 800 Gal - _ 400 — <0” — 400 SEA LEVEL — — SEA LEVEL SEA LEVEL — :3 .'// — SEA LEVEL _ ’— _ . é ‘/ _ —400 ¢'/-/' ”400 2 — “ 200 — ” ‘ ' _ ' 00 _ F800 _ T/eaéfié —800 _ g: Q/ / //‘ _ 400 _ _ 1200 400 __ ”LET/Zr” / _— 1200 — —1500 — / ///L — 1600 600 —— 2000 600 F 2000 SCALE 1:24 000 I 1 1 MILE é . '5 1 .5 1 KILOMETER E l—l I—I I———I I——I l—{ I—,:_——-—_] APPROXIMATE MEAN CONTOUR INTERVALS 20 AND 40 FEET QUADRANGLE LOCATION DECL'NAT'ON“98‘ DASHED LINES REPRESENT ZU-FOOT CONTOURS NATIONAL GEODETIC VERTICAL DATUM OF 1929 DEPTH CURVES IN FEET—DATUM IS MEAN LOWER LOW WATER SHORELINE SHOWN REPRESENTS THE APPROXIMATE LINE OF MEAN HIGH WATER THE AVERAGE RANGE OF TIDE IS APPROXIMATELY 4 EEET Humble Scaroni 1 (T. D. 885') GEOLOG IC MAP AND SECTIONS OF THE AN 0 NUEVO-DAVEN PORT AREA SAN MATEO AND SANTA CRUZ COUNTIES CALIFORNIA , 9 30°57'30' ‘ ; I I ' 36°57'30” 12202230,, 20 17130" 1230" 10/ , INTERIORiGEOLDGICAL SURVEY, RESTON, VA7I9817679684 12200730,, Base from US. Geological Survey Ario Nuevo and Davenport, 1955; Santa Cruz, 1954 Geology mapped 1960—69 or; PLATE 2 '75' Pb milk? UNITED STATES DEPARTMENT OF THE INTERIOR Pith 3" PROFESSIONAL PAPER 1168 GEOLOGICAL SURVEY 122°07’30” , I I II , 122°00’ 37°07’30" , , . , 5 230 , , __ , C ._ _ .sz ,\ _ , , , ._ _ . 37°07'30" 1 l "‘ ‘ ' ' CORRELATION OF MAP UNITS Cal 015 : Holocene — - - QUATERNARY Ot Om - Pleistocene Uncontormity T T Tp :F Pliocene Tsc Tsm Unconformity - Miocene Unconfrmoity — TERTIARY — Oligocene T 9 S _ - Eocene T 9 S 1 T 10 S Unconformity -Paleocene z J Nonconformity qd ga gd hcg - CRETACEOUS T 10 S sch, ‘_ MESOZOIC OR J PALEOZOIC le Qt Om Tv Tbs Ti qd 2'30” — 2,30,, ga gd thg sch *1— __.(.__ FL— M5049 x V5555 X Mf3675 x L37X ¢ ; T108 TIIS 37°00’ 37°00' Mf3676 T H S 57/30"— 7 , ’ .- : y , _ — 57'30" 36°56’30” l l 36°56’30” 122007301, 5’ 2’30" 122°00’ Base from US. Geological Survey JQW SCALE 1:24 000 Geology mapped 1960—69 Felton, 1955; Santa Cruz, 1954 I E 1 % 0 1 MILE Igneous and metamorphic geology of Ben Lomond 1% 5 :_ . - . . . ' . L Mountain area modified after Lee (1961) 2 E “’79 1 .5 0 1 KILOMETER E 5 I-—t I—I l—I I——1 I—T { S CONTOUR INTERVALS 20 AND 40 FEET APPROXIMATEMEAN NATIONAL GEODEIIC VERTICAL DATUM or 1929 QUADRANGLE LOCATION DECL'NAT'ON’ ”81 DEPTH CURVES IN FEETvDATUM IS MEAN LOWER LOW WATER SHORELINE SHOWN REPRESENTS THE APPROXIMATE LINE OF MEAN HIGH WATER C fl"; THE AVERAGE RANGE OF TIDE IS APPROXIMATELY 4 FEET I— I— o _l METERS .2 a, k 3 > I. < .2 600 3 § '5 § LL § Lu 8 35’ U 5 x ‘3‘ E < z o if N S O > .1 400 5 -° 2 2 § 0, o 8 E a + + + + + sch U 92 o I: E ‘5. (n Tm 2 + + + + + + + + + + / , qd % 31 " 0 ‘° E 200 + + + + + + + + + + + + + + + + + + / \ ’ r / v u‘ . T'° g E 8 T‘ + + + + + + + + + + + + + + + + + \ , / / \ /\ ’\ 03' ‘0 m + + + + + + + + + + + + + + + + + + / / / / V V /\ V w SEA LEVEL + + + + + + + + + + + + + + + + + + \ I/ V 7 I/ < + + + + + + + + + + + + + + + + + + / 4 A + + + + + + + .1 / / \ / \ < 7 V 4- \/ b l/ 400 \/ /\ v 500 .:< D g g D ' E METERS U 52 Tlss FEg METERS 400 E ‘9 1200 400 g 2° 6 “““““““ 800 200 to 200 400 SEA LEVEL SEA LEVEL SEA LEVEL 200 400 800 200 400 1200 DESCRIPTION OF MAP UNITS SURFICIAL SEDIMENTS ALLUVIUM—Unconsolidated gravel, sand, and silt LANDSLIDE MATERIAL—Half arrows show direction of downslope movement RIVER TERRACE DEPOSITS—Unconsolidated sandy pebble and cobble gravel and dark—yellowish-orange fine to medium sand MARINE TERRACE DEPOSIT—Unconsolidated moderate—yellowish—brown fine sand and granular gravel UPPER MIOCENE TO PLIOCENE SEDIMENTARY SEQUENCE PURISIMA FORMATION (upper Miocene and Pliocene)—Very thick bedded yellowish—gray tuffaceous and diatomaceous siltstone with thick interbeds of bluish—gray semifriable fine-grained andesitic sandstone. Includes Santa Cruz Mudstone east of Scotts Valley and north of Santa Cruz SANTA CRUZ MUDSTONE (upper MioceneI—Medium— to thick—bedded and faintly laminated blocky—weathering pale—yellowish-brown siliceous organic mudstone. Includes Santa Margarita Sandstone along Glenwood syncline SANTA MARGARITA SANDSTONE (upper Miocene)—Very thick bedded to massive thickly crossbedded yellowish—gray to white friable granular medium- to fine— grained arkosic sandstone; locally calcareous MIDDLE MIOCENE SEDIMENTARY SEQUENCE MONTEREY FORMATION—Medium— to thick-bedded and laminated olive-gray to light-gray subsiliceous organic mudstone and sandy siltstone. Includes few thick dolomite interbeds LOMPICO SANDSTONE—Thick-bedded to massive yellowish—gray medium- to fine» grained calcareous arkosic sandstone; locally friable EOCENE T0 LOWER MIOCENE SEDIMENTARY SEQUENCE LAMBERT SHALE (lower MioceneI—Thin— to medium-bedded and faintly laminated olive-gray to dusky—yellowish—brown organic mudstone with phosphatic laminae and lenses in lower part VAQUEROS SANDSTONE (Oligocene and lower Miocene)—Thick—bedded to mas- sive yellowish—gray arkosic sandstone BasaltflSpheroidal-weathering pillow basalt flows in upper part ZAYANTE SANDSTONE (OligoceneI—Thick— to very thick bedded yellowish-orange arkosic sandstone with thin interbeds of greenish and reddish siltstone and lenses and thick interbeds of pebble and cobble conglomerate SAN LORENZO FORMATION Rices Mudstone Member (Eocene and Oligocene)—Massive medium-light—gray fine— to very fine grained arkosic sandstone; thick bed of glauconitic sandstone at base Twobar Shale Member (Eocene)—Very thin bedded and laminated olive—gray shale BUTANO SANDSTONE (Eocene) Upper sandstone member—Thin— to very thick bedded medium—gray tine- to medium-grained arkosic sandstone with thin interbeds of medium—gray siltstone Middle siltstone member—Thin- to medium-bedded nodular olive—gray pyritic siltstone Lower sandstone member—Very thick bedded to massive yellowish—gray granular medium— to coarse-grained arkosic sandstone. Conglomerate—Thick to very thick interbeds of sandy pebble conglomerate in lower part of lower sandstone member PALEOCENE SEDIMENTARY SEQUENCE LOCATELLI FORMATION—Nodular olive—gray to pale—yellowish—brown micaceous siltstone Sandstone—Massive medium-gray fine— to medium-grained arkosic sandstone locally at base CRYSTALLINE PLUTONIC AND METAMORPHIC ROCKS QUARTZ DIORITE—Grades to granodiorite south and east of Ben Lomond Mountain GRANITE AND ADAMELLITE GNEISSIC GRANODIORITE HORNBLENDE-CUMMINGTONITE GABBRO METASEDIMENTARY ROCKS—Mainly pelitic schist and quartzite MARBLE—Locally contains interbedded schist and calc-silicate rocks Contact, approximately located—Triangle where well exposed; queried where uncer— tain Fault—Dashed where approximately located; short dashed where inferred; dotted where concealedU, relatively upthrown side; D, downthrown side Anticline—Dashed where approximately located; dotted where concealed Syncline—Showing direction of plunge; dashed where approximately located; dotted where concealed Strike and clip of beds Inclined I Vertical Horizontal Estimated Strike and dip of foliation Fossil localities Megainvertebrate Vertebrate Microfossil Lithologic sample locality Abandoned exploratory oil well Half arrows show direction of downslope movement FAULT —l < ZAYAN TE El \( Carbonera Creek 400 INTERIURiGEOIOGICAL SURVEY, RESTON, VAv19817679584 GEOLOGIC MAP AND SECTIONS OF THE FELTON-SANTA CRUZ AREA, SANTA CRUZ COUNTY, CALIFORNIA FEET 1600 1200 800 400 SEA LEVEL 400 000 1200 500 2000 FEET 1200 800 400 SEA LEVEL 400 800 1200