Bull. Geol. Soe. Am., Vol. 71 Moberly, PI. 1 THERMOPOLIS SHALE T I

Bull. Geol. Soe. Am., vol. 71 Moberly, PI. 1

THERMOPOLIS SHALE CUOVERLY FORMATION MORRISON FORMATION t i Black bentonitic clayshale HIMES MEMBER _ Red-banded facies Gray lithic wacke J Quartz arenite m Green calcareous muds tone ch. I Variegated day stones with iron and sandstone oxide velnlets SYKES MOUNTAIN FORMATION Olive-gray lithic wacke SUNDANCE FORMATION sm Thinly mterbedded sandstone, LITTLE SHEEP MUDSTONE MEMBER s Glauconitic sandstone and si Its tone, an4 shale, withth'n marine fossils. SOUTHEAST ironstone beds. Conglomeratic sandstone END clsj Variegated bentonitic mudstone PRYOR CONGLOMERATE MEMBER g&U Black-chert pebble conglomerate

NORTHWEST

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VIEW IS TO NORTHEAST. LINE OF SECTION IS SHOWN ON FIGURE I. CLOVERLY- SYKES MOUNTAIN CONTACT IS DATUM. ;x$ /' LITHOLOGIC CHANGES IN UPPERMOST JURASSIC AND LOWERMOST CRETACEOUS ROCKS ALONG NORTHEAST SIDE OF BIGHORN BASIN, WYOMING AND MONTANA View northeast into panel diagram

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MORRISON, CLOVERLY, AND SYKES MOUNTAIN FORMATIONS, NORTHERN BIGHORN BASIN, WYOMING AND MONTANA

By RALPH MOBERLY, JR.

ABSTRACT

The Morrison, Cloverly, and Sykes Mountain formations are the uppermost Jurassic and lowermost Cretaceous sedimentary rocks in the northern Bighorn Basin, Wyoming and Montana. Similar formations were deposited contemporaneously throughout most of the Western Interior. Most studies of nonmarine rocks have been in localities of more rapid sedimentation, whereas the rocks of this study accumulated slowly, under virtually atectonic conditions. The Morrison formation of the Bighorn Basin, as here restricted, includes the earliest- formed nonmarine sedimentary rocks of this sequence, conformably overlying the marine Sundance formation. The rocks are interlensed calcareous quartz sandstones, green mudstones and shales, and subordinate limestones, with locally conspicuous red-banded mudstones. Overlying the Morrison formation, generally with conformity, is the Cloverly formation, as redefined in this report. The lowest of its three members, here named the Little Sheep mudstone members, consists chiefly of bentonitic (montmorillonitic) mudstones in variegated shades of neutral gray, purple, olive, and dusky to pale red. Other typical lithologies include bentonites, cherts, coaly beds, calcareous nodules, and chert-pebble conglomeratic sandstones. The Pryor conglomerate member in the northernmost Bighorn Basin is characterized by black-chert pebbles and rests unconformably on the Morrison formation. Its beds are the stratigraphic equivalent of the mid-Little Sheep conglomeratic sandstones farther south. The upper member of the Cloverly formation, here named the Himes member, comprises three principal lithologies. Commonly at its base is olive-gray and reddish-brown clay-matrixed salt-and-pepper sandstone. Most of the member is variegated reddish- and yellowish-brown and gray kaolinitic claystone and mudstone, containing veinlets and hardpans of iron oxides. Clean quartz sandstones which filled fluvial channels are laced through the claystones. Disconformably overlying the Himes member are sandstones and thinly interbedded, rusty-brown-weathering siltstones, dark shales, and ironstones, here named the Sykes Mountain formation. The Sykes Mountain formation grades into the overlying marine Thermopolis shale. Distinction of stratigraphic units on a lithogenetic basis is believed to eliminate the confusion which existed in previous nomenclature of this sequence. Characteristic primary and secondary structures, clay, accessory, and authigenic minerals, and gross stratigraphic distribution aided the interpretation of the origin and history of these deposits. The Morrison formation accumulated in fluvial, lacustrine, and flood-plain environments from detritus derived chiefly from erosion of sedimentary rocks west of the present Bighorn Basin. The Little Sheep mudstone member and most of the Himes member of the Cloverly formation probably were formed authigenically in seasonal lakes and swamps from weathering of volcanic debris, with their different lithologies due to different drainage conditions and parent ash. The Pryor conglomerate member of the Cloverly formation and lenses of similar conglomeratic sandstones in the Little Sheep member were derived from reworked sedimentary rocks west of the depositional area. Channel-filling clean quartz sandstones of the Himes member were derived from an eastern sedimentary, and perhaps metamorphic, terrain. Thinly interbedded sandstones and shales of the Sykes Mountain formation are tidal-flat and other shallow-water beds deposited at the periphery of the transgressing Early Cretaceous sea. Slow deposition of these well-sorted and mature detrital sediments and close adjust- ment of authigenic minerals to prevailing environments depended largely on the stable tectonic conditions. During Cloverly time, very little aggradation except of volcanic debris took place, so that soils were formed and preserved. 1137

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CONTENTS TEXT ILLUSTRATIONS Figure Page Page 1. Index map 1139 Introduction 1138 2. Interpretations of uppermost Jurassic and General statement 1138 lowermost Cretaceous stratigraphy in Previous work 1138 Bighorn Basin, Wyoming and Mon- Present work 1140 tana 1142 Acknowledgments 1140 3. Type sections of Little Sheep mudstone Stratigraphy 1140 member and of Himes member of General character 1140 Cloverly formation 1146 Historical summary 1140 4. Type section of Sykes Mountain forma- Proposed stratigraphic nomenclature 1142 tion 1149 Morrison formation 1143 5. Probable lithogenetic equivalents 1151 Bighorn Basin 1143 6. Early Cloverly paleocurrents, Bighorn Basin 1156 Elsewhere in Western Interior 1144 7. Pryor conglomerate member paleocurrents Cloverly formation 1145 near Red Dome, Montana 1157 General features 1145 8. Late Cloverly paleocurrents, Bighorn Little Sheep mudstone member 1145 Basin 1158 Pryor conglomerate member 1147 9. Facies changes in Himes member of Himes member 1148 Cloverly formation 1159 Sykes Mountain formation 1149 10. Heavy-mineral provenances 1166 Correlation 1150 11. Clay minerals 1168 Incomplete local fossil evidence 1150 12. Sandstone composition and texture related Lithogenetic equivalents 1152 to maturity and fluidity 1170 Ages 1152 Plate Facing page Petrology 1153 1. Lithologic changes in uppermost Jurassic Sedimentary structures 1153 and lowermost Cretaceous rocks along Stratification 1153 northeast edge of Bighorn Basin, Directional properties 1155 Wyoming and Montana 1137 Secondary structures 1157 Following page Petrography 1159 2. Cloverly and Sykes Mountain formations' Composition and texture 1159 3. Bedding features Special studies 1159 4. Photomicrographs of thin sections \ 1160 Sedimentary rock families 1163 5. Photomicrographs of thin sections, and Petrogeny 1171 tuffaceous mudstone outcrop Morrison formation 1171 Cloverly formation 1171 TABLES Sykes Mountain formation 1171 Tables Page Physical environment 1172 1. Compositional and textural properties of Tectonic environment 1173 Morrison, Cloverly, and Sykes Mountain Geologic history 1173 formations, Northern Bighorn Basin, References cited 1174 Wyoming and Montana 1164

INTRODUCTION rocks described in this study crop out in Big Horn and Park counties, Wyoming, and Carbon General Statement County, Montana, extending along the east side of the basin from the vicinity of Hyattville, The present study analyzes a sequence of ex- Wyoming, for about 90 miles northwest to the posed Mesozoic sedimentary rocks in the vicinity of Red Dome, Montana, and on the northern Bighorn Basin that are the product of west side of the basin from about 10 miles south a predominantly nonmarine physical environ- of Cody, Wyoming, northward for about 40 ment in a stable tectonic environment. These miles to the Montana state line (Fig. 1). The rocks include the Morrison formation, the area is one of excellent exposures and uncompli- Cloverly formation, and the rusty-brown- cated structures. weathering beds heretofore included in the base of the Thermopolis shale, thus involving the Previous Work entire sequence deposited after the last Jurassic marine flooding and before the return of In the early 1900's, U. S. Geological Survey dominantly marine Cretaceous conditions. field parties mapped much of the area (Darton, The upper Jurassic and lower Cretaceous 1906b; Fisher, 1906). Later maps which have

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EXPOSED FORMATIONS, UNDIFFERENTIATED.

LINE OF SECTION, PLATE I

TYPE LOCALITIES SYKES MOUNTAIN FORMATION. HIMES MEMBER, CLOVERLY FORMATION. (D LITTLCLOVERLE SHEEY P FORMATIONMUDSTONE. MEMBER, (Mop modified from Andrews, Pierce, & Eorgle, 1947} FIGURE 1.—INDEX MAP Exposures of Morrison, Cloverly, and Sykes Mountain formations along the northern edges of the Big- horn Basin.

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been useful for this study include detailed ones STRATIGRAPHY near Red Dome and Bowler by Wilson (1936; also manuscript map in the Thorn Library of General Character the Yellowstone-Bighorn Research Association, Red Lodge, Montana), near Cody by Pierce and Three distinct facies are present between the Andrews (1940), near Greybull by Pierce Jurassic Sundance formation and the Creta- (1948), and near Hyattville by Rogers et al. ceous Thermopolis shale within the area (1948). Regional maps recently compiled are studied. The lowest includes about 200 feet of those of the Bighorn Basin (Andrews, Pierce, light-olive-green, lenticular, calcareous silt- and Eagle, 1947), of Wyoming (Love, Weitz, stones, shales, mudstones, and fine-grained and Hose, 1955), and of Montana (Ross, cross-bedded sandstones. Commonly red-brown Andrews, and Witkind, 1955). calcareous mudstones or shales alternate with the olive-green rocks in the upper portion, producing a red-banded aspect comparable with Present Work many of the Rocky Mountain lower Eocene This paper is a portion of a dissertation rocks. Lenses of aphanitic limestone and clay- presented in partial fulfillment of the require- gall conglomerate are subordinate. ments for the degree of Doctor of Philosophy Overlying these beds with a contact varying at Princeton University. Field studies were from conformable to unconformable are varie- made in the summers of 1954 and 1955. Strati- gated red, purple, orange, dark-olive, and gray, graphic sections located on air photographs partly bentonitic, commonly noncalcareous were measured by hand level or by Brunton mudstones about 300 feet thick. The lower mudstones, which weather into low, rounded, compass and tape. Primary structures were gumbo hills, are more purple, drab, and sketched or photographed and their attitudes bentonitic than the upper ones and contain determined. Petrographic studies of textural local chert beds, interbedded thin bentonites, attributes and composition of these rocks and zones of large calcareous concretions. At or included mechanical analyses, thin-section near the base are local tongues and lenses of studies, heavy-mineral separations, and clay- cross-bedded conglomeratic sandstone, with mineral determinations. distinctive black-chert pebbles and sand grains and some thin lignitic beds. The much less A cknowledgments bentonitic variegated claystones and mudstones of the upper member, which weather into cliffs, Laboratory studies were accomplished during are brighter yellow, orange, and light gray, tenure of National Science Foundation and and are considerably impregnated with iron Charlotte Elizabeth Proctor fellowships at oxide as blebs, veinlets, and hardpans. Com- Princeton University. Several faculty members monly, the basal few feet is olive to reddish- of Princeton University have been helpful on brown salt-and-pepper sandstone with a innumerable occasions. I am especially in- swelling-clay matrix. Elongate bodies of cross- debted to Franklyn B. Van Houten and Erling bedded quartzose sandstone are interlaced Dorf, who guided much of the research. The through this upper mudstone unit. Shell Oil Company supported all field expenses The uppermost sequence of strata is about for this study. Gordon Rittenhouse, research 100 feet of rusty-weathering, thinly interbedded geologist with the Shell Oil Company, deserves limonitic siltstones, gray shales, and brown special gratitude for his interest in helping plan sandstones, with associated ironstones at the base. Although the contact with the underlying the investigation, as well as his subsequent variegated mudstones is distinct in most assistance. J. D. Love and K. M. Waage have places, the contact with Thermopolis shale is discussed several of the problems as the study gradational. progressed. Drs. Waage, Rittenhouse, and Van Houten criticized the manuscript and made Historical Summary helpful suggestions. Martin S. Raymond proved an able field assistant; Jean Lawson Moberly, The first formal attempts to describe the Raymond Peck, S. P. Ellison, Jr., S. K. Fox, stratigraphic units exposed in the northern Jr., J. T. Gregory, W. T. Thorn, Jr., Helgi Bighorn Basin were by Darton (Fig. 2). As Dar- Johnson, Richard S. Fiske, and various resi- ton compared the Bighorn Mountains section dents of the Bighorn Basin also helped con- with sections studied earlier in the Black Hills siderably in this study. and the Colorado Front Range, he extended the

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name Morrison to the Bighorn Range, and Barton considered the few thousand feet of introduced the name Cloverly there (Barton, beds overlying the Cloverly formation to be the 1904, p. 398-399; 1906a). Barton described the Colorado shale. Its salient features west of Morrison as chiefly pale-green or maroon soft Cloverly include a basal "rusty" series, about sandy shales and clays which alternate with 100 feet thick, of dark-gray to black shales with layers of fine-grained sandstone, all of fresh- thin beds of brown sandstone, that rests water origin. Barton believed the sandstone abruptly on top of the Cloverly sandstone, but overlying the Morrison beds represented the with no evidence of unconformity. Above this Lakota formation of the Black Hills; the sand- unit are black fissile shales (Barton 1906a, p. stone is overlain by and merges into clays 55). resembling the Fuson formation. Because of Figure 2 shows some findings of subsequent lack of definite evidence as to the equivalency workers which added to or differed from of these beds, and because of the apparent Barton's studies. The most important concepts absence of deposits representing the Bakota relevant to this study include Washburne's sandstone above the clay, Barton proposed the (1909) opinion that the Cloverly locally was separate designation "Cloverly" for this series, partly or entirely absent because of an uncon- a name derived from a now-deserted post office formity below the marine Colorado shale, on the eastern side of the Bighorn Basin. Hewett's (1914) separation of Cloverly from Barton described the Cloverly formation as Morrison above the highest maroon clays, containing conglomeratic sandstones overlain Lupton's (1916) naming of the Thermopolis by clays overlain by sandstones. The lower shale above the Cloverly, and Hewett and sandstone is generally buff or dirty gray, cross- Lupton's (1917) formal designation of the bedded, massive, and coarse-grained, with its higher Cloverly sandstone unit as the Greybull basal portion commonly conglomeratic and sandstone member of the Cloverly formation. locally containing some thin deposits of coaly Ziegler (1917) placed the upper Morrison con- shale. The middle clays are mostly ash-colored tr.ct at the top of the highest maroon clays, and purplish or reddish, with some portions although he recognized that Barton had put pale green and maroon, similar to the under- the contact at the base of a conglomerate below lying clays of the Morrison formation. At some some of these variegated clays. Hares (1917) introduced the term Pryor conglomerate localities a few feet of massive light colored member for the lowest part of the Cloverly sandstone overlies the clays. formation of the northeastern Bighorn Basin. Barton (1906a, p. 52) described what he Of interest in the next decade were the thought to be a typical exposure of the Cloverly transfer of the Rusty Beds to the Cloverly formation as follows: formation in a U. S. Geological Survey correlation chart (Wilmarth, 1925) and Lee's opinion "In the vicinity of Cloverly the formation varies in thickness from 50 to 125 feet, and to the east and (1927) that as much as 53 feet of carbonaceous north of that place it consists of sandstones which shale, sandstone, and black-chert-pebble con- outcrop extensively in cliffs of massive buff-colored glomerate in the eastern Bighorn Basin be- beds, mostly of moderately coarse-grained material. longed properly neither to the Cloverly nor to To the west the middle and lower portions of this sandstone change to a maroon color and some clay the Morrison formations but represented the is intermixed with the sand. This rock weathers coal-bearing parts of the Kootenai formation. into badlands. In recent years stratigraphers in this section "Section of Cloverly formation lj/silts tones.

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Others (e.g., Andrews, Pierce, and Eagle, mapping an area has determined which lithic 1947; Love, Weitz, and Hose, 1955) would map characters are selected for determining forma- the Morrison, Cloverly, and the rusty- tional boundaries. However, as understanding weathering beds as one undifferentiated unit. the origin of a series of sedimentary rocks be-

FIGURK 2.—INTERPRETATIONS OF UPPERMOST JURASSIC AND LOWERMOST

Still others (e.g., Blackstone and Sternberg, comes of more value in the historical interpreta- 1947, p. 226, 231, 244, 248, 250; Spalding, 1952, tion of an area, the lithologic characters of p. 104, 108, 110, 139) have introduced names greatest genetic significance, rather than those from outside the Bighorn Basin, most com- most easily mapped, must be selected for the monly dividing the Cloverly formation into a determination of a formation. basal Lakotu conglomerate, a medial Fuson Definition of the Upper Jurassic and Lower shale, and an upper Dakota "silt" or sandstone. Cretaceous rocks of this study on a lithogenetic basis has great value in describing the events of Proposed Stratigraphic Nomenclature that area. The name Morrison formation is re- A formation is a genetic lithic unit, and be- stricted to the lowest of the three major litho- cause of its genetic and economic significance, genetic divisions of this sequence. These lithic constitution of a sequence is fundamental chiefly greenish mudstones and sandstones, to subdivision into formations (Ashley et al., red-banded in part, had fluvial and lacustrine 1933, p. 430-431). Frequently, expediency for origin on a constantly aggrading flood plain.

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Overlying the Morrison formation, generally the area studied; the more lenticular conglome- with conformity, is the Cleverly formation as ratic sandstones to the south within the Little redefined. One of its three members, chiefly Sheep mudstone are not named. These coarser- variegated bentonitic mudstones, named herein grained rocks represent deeply weathered

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CRETACEOUS STRATIGRAPHY IN BKIIORN BASIN, WYOMING AND MONTANA

the Little Sheep mudstone member of the regolith materials swept into the area by Cloverly formation, resulted from subaqueous streams at a time of a slight tectonic pulse. weathering of volcanic debris in seasonal lakes The uppermost sequence of strata, here and swamps. The highest member, of claystones named the Sykes Mountain formation, of and sandstones channeled with better-sorted rusty-weathering interbedded shales, siltstones, sandstones, here named the Himes member of and sandstones, was deposited in the tidal flats the Cloverly formation, formed from the and lagoons peripheral to the encroaching leaching and subaerial weathering of volcanic Thermopolis seaway. The general stratigraphic debris and other sediments on a better-drained relationships of these formations in the northeastern Bighorn Basin are shown on Plate 1. plain of seasonal lakes crossed by some through- going streams. The name Pryor conglomerate Morrison Formation member of the Cloverly formation is retained for the well-defined basal Cloverly chert- Bighorn Basin.—The name Morrison is pebble-conglomeralcbeds in the northern part of employed in the Bighorn Basin for the non-

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marine formation immediately overlying the similar to the lower Eocene rocks of the Bighorn marine Jurassic Sundance formation for reason Basin and nearby regions. of stratigraphic precedence of over 50 years, A lithologic series grading from calcareous and because the beds can be traced in outcrop medium- and fine-grained sandstone through and by well logs to the Colorado Front Range, very silty or sandy limestones to fairly pure where they constitute a large part of the type limestone is also common in the Morrison Morrison formation. The use of the same formation. The detrital component normally is stratigraphic name in areas far from the type sufficiently impure to cause these sandstones to locality implies similar depositional conditions be classified as wackes. Most commonly, these throughout a wide region and so emphasizes muddy-matrixed calcareous sandstones and the distinctive tectonic and physical environ- sandy limestones are thinly cross-bedded, as ment in which these strata were deposited. well as pseudocross-bedded from ripple marks. The contact of the Morrison formation with Some of the purer limestones occur fairly the underlying Sundance formation is in most commonly as nonlaminated lenses 1 foot or less localities at the base of lenticular green shales thick, especially in the red-and-green banded or mudstones immediately above the highest facies where they cover a few dozen square yards fossiliferous, even coquinoid, beds of the in area. The more sandy or silty limestones of this Sundance formation. Where both formations lithologic series cover a larger area; some of the are greenish calcareous sandstones and shales, calcareous sandstone lenses may extend for the Sundance beds owe their color to a high several acres where they are thickest and most glauconite content, whereas the similarly numerous in the northeastern edge of the Big- colored Morrison rocks have greenish clay horn Basin. Lenticular sandy beds range from matrix around generally lighter-colored grains. nearly white to pale green for the less calcareous In some outcrops glauconite has been reworked muddy-matrixed sandstones, and to pale orange several inches up into Morrison strata. Slight for the cleaner more calcareous sandstones, scouring and channeling of the uppermost or even limestones. Sundance, with clay galls in the overlying basal Variegated clays, coaly beds, and conglom- Morrison, is another local feature of the contact. eratic sandstones assigned to the Morrison Subaerially formed mudcracks and raindrop by some authors are included in the Cloverly marks in the lower Morrison emphasize the formation of this report. change to nonmarine conditions. Fresh-water snails, including locally abun- The Morrison measures about 130 to 280 feet dant Viviparus rcesidei Yen, some lignitic thick, averaging about 190 to 200 feet, with needles, chara oogonia, petrified wood, and thicker sections generally on the western side of worn reptilian bones are fossilized in the the basin. Morrison formation. The pronounced lithologic changes occurring Elsewhere in Western Interior.—The term every few feet vertically or every few dozen Morrison formation was formally applied to yards horizontally result chiefly from the some fresh-water marls and sandstones typi- lenticularity of a few repeated lithologies rather cally developed at Morrison, Colorado, near than numerous different rock types. The most Denver, by Eldridge (in Emmons, Cross, and common single rock type is calcareous, some- Eldridge, 1896, p. 60), although the name had what sandy mudstone or shale, colored shades been used in a folio (Cross, 1894) published of green, gray, olive, and yellow. Near Shell, shortly before the monograph on the region. Rose Dome, the mouth of Five Springs Creek, Because these publications did not give an and elsewhere, most of the formation is greenish unequivocal description of the upper beds and calcareous sandy mudstone. This marly lithol- contact, confusion arose in later interpretations ogy may also be the dominant one in a section of the "type Morrison", so that Waldschmidt containing lenses of sandstones or limestones of and LeRoy (1944) presented a new type section types mentioned hereafter. Greenish mudstone which has been generally accepted. Waage is also present in one particularly characteristic (1955, p. 24-25) reviewed earlier interpretations facies of the Morrison formation in the Big- and concluded from his own observations that horn Basin. Lenses of this facies from a few the upper Morrison contact of Waldschmidt inches to several feet thick are associated with and LeRoy is between 35 and 55 feet higher reddish-brown calcareous mudstones, com- than Eldridge's. Moreover, Waage believes monly somewhat finer-grained than the adjacent that the contact he draws between the Morrison green lenses. This produces a red-banded effect and the overlying Lytle formation of the

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Dakota group corresponds with Eldridge's canic debris includes: bentonites; partly ben- Morrison-Dakota contact. tonitic mudstones and claystones; highly Waage (1955, p. 23) terms as "undoubted siliceous rocks such as cherty siltstones, Morrison strata" the greenish-gray marls and tuffaceous mudstones, and spotted to white claystones of the Front Range section which are cherts; and variegated rocks, especially colored overlain by dominantly red, variegated beds, shades of pale red, light to dark purple, pale with sandstones and conglomerates. The Morri- greenish yellow, and neutral gray. son-Lytle contact is distinct where thick Lytle The Pryor conglomerate member uncon- conglomeratic sandstone lenses rest unconformably overlies the Morrison formation in the formably on "undoubted Morrison" greenish- outcrops from Sykes Mountain northwestward gray marls or where basal Lytle conglomeratic to Red Dome and at some exposures on the west sandstone lenses rest with obvious uncon- side of the basin. This contact is very distinct, formity on the dominantly red variegated upper with some channeling of the upper Morrison Morrison beds, but is obscure where, as Waage surface. The angularity of the unconformity in notes, this northern part of the basin is a few feet per mile. South of Sykes Mountain the Cloverly- "... the entire sequence of beds between the disconformity at the top of the Lytle and the greenish- Morrison contact is conformable, and in some gray claystone of undoubted Morrison consists of instances where variegated bentonitic mud- alternating lenses of sandstone, or conglomeratic stones overlie the Morrison the contact may be sandstone, and variegated claystone." gradational through a few feet. Little Sheep mudstone member.—The Little Cloverly Formation Sheep mudstone member of the Cloverly formation is a series of bentonitic mudstones, General features.—Cloverly formation is the commonly containing some chert-pebble sand- name applied to the middle of the three strati- stone lenses and beds of a few other lithologies, graphic units considered in this report. Because which overlies the Morrison formation and Darton was not consistent in his choice of either underlies the Himes member of the Cloverly upper or lower contacts of the formation, the formation. Locally it overlies the Pryor name Cloverly must be redefined carefully. As conglomerate member of the Cloverly forma- proposed here, the Cloverly formation in the tion. The member is 250 feet thick at its type Bighorn Basin includes those nonmarine sedi- locality on the northeast flank of Little Sheep mentary rocks which lie above the Morrison Mountain about 4 miles south-southwest of formation and below the Sykes Mountain Kane, Wyoming, in sec. 36, T. 56 N., R. 95 W. formation. The Cloverly formation can be (Fig. 3). subdivided into the Little Sheep mudstone The most characteristic and dominant lithol- member, the Pryor conglomerate member of ogy of the Little Sheep mudstone member is local extent, and the Himes member. variegated bentonitic mudstone. Variegated The vicinity of the former Cloverly post here means that a portion of the rock as small office remains a good type locality of the re- as a hand sample shows two or more colors in defined Cloverly formation. A compound type irregular patches perhaps grading into each section there totals 382 feet thick, with the other. This color change does not depend on Himes member (98 feet thick) measured in the bedding, jointing, concretions, or other struc- cliffs west of Cloverly (PI. 2, fig. 1) near the tures. The bentonitic character of these mud- section Darton measured (1906a, p. 52), and stones is evidenced by their rapid color change the Little Sheep mudstone member (284 feet where moistened, followed by a slower process of thick) measured in the buttes southeast of the swelling and disintegration. They weather mouth of Cedar Creek. The Cloverly formation rapidly to "gumbo" (some of the older literature ranges from 150 to 400 feet thick elsewhere in the northern Bighorn Basin, with an average terms it "adobe"). Bentonitic gumbo is char- thickness of about 280 to 300 feet. acterized by the formation of popcorn-shaped The contact of the Cloverly formation with masses of the mudstone during weathering. the underlying Morrison formation is at the Commonly this gumbo mantle, barren of base of the lowest beds which either show vegetation, is a few feet thick above bedrock. evidence of significant additions of volcanic These variegated bentonitic mudstones com- debris or contain pebbles or granules of black monly are shades of gray and red. The grays chert. The former relationship is most common are very nearly neutral, ranging from very light in the area studied. Evidence indicating vol- gray to very dark gray, but rarely including

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some shades of yellowish gray and light olive acterized by layers of large nodules with rough, gray. The reds are most commonly moderate knotty surfaces and individual smaller and reds, pale reds, and reddish browns; in some smoother nodules. All gradations from chert intervals, red-purple shades are very common. nodules to calcareous nodules apparently are

UNITS FEET 20' p- — i^**-. Scole [•> 1 Greenish-gray lithic wocke j of HlMES MEMBER. ("•^tv 1 I0 ! ^"^ II Mudstone^ dark ^^!% gray, bentonitic. 36' ^ °' .° ^^ UNITS FEET Thinly-interbedded siltstones, (cont) % "* J ° 7 Siliceous mudstone, ^ ^ shales, and sandstones of SYKES MOUNTAIN FORMATION. 11 quartz arenite at base) !?S^^™(^ 6. Bentonite (local) i' '° v^ 9. Sandy mudstone, yellowish gray; grades into quartz 31 10. Siliceous mudsfone 1 — .-i— r 4. Sandstone; white ^~'^~ ; arenite channel to north. 4 It gray, tuffoceous. 1' 8. Claystone; variegated gray ^_ quartz arenite w. ^ S brown, contoins iron- . ^j.-r-—. thin green shale "--£•„ ,^N 9 Mudstone; chiefly oxide veliitets. 13 dork gray variegated • beds a some lime- ^ -: 7 Sandstone' channel filling stone lenses. 35* %-i- 0 w. reef, bentonitic. 84' of cross-bedded quartz aren-, ite w. a few shale partings 15

~Kr$~r£rfJ 6. Claystone; yellowish gray w, , I/ A_^ *1 iron-oxide veMets Grades into;6 or_—>T\ 5. Sandy mudstone, light green- ""^—-^^r 3i Mudstone; similar 0=0,. (calcareous nodule -,^*J.;J.VF**| ish gray. 4' "' 1 to units. 12' ^'' ^ horizon) '"'"" 4. Sandstone; pale-grayish-red thinly-bedded quartz wocke to lithic wacke. " S 2. Sandstone; coarse ^ ^ grained quartz aren- ' ^ ite w. abundant ^ black chert grains. \\ °'/-' \ 18 • .^ ~-~^^^T i Mudstone, variegated '-& Sandstone; greenish-yellow - j dork reddish brown, friable lithic wacke. 28' 7 weamering purpte; =0=0 (calcareous nodule bentonitic. 15' >>. ,, horizon)

— ^—^^ Gray to green calcareous ^f^t __ claystone of MORRISON . r >'^= 8. Mudstone; variegated FORMATION S/, greens reddish brown, 92 weathering purple; Gray to olive-brown bentonitic bentonitic. 4$ mudstone of LITTLE SHEEP MUDSTONE MEMBER. °^_ "^

TYPE SECTION OF i • r LITTLE SHEEP T , TYPE SECTION MUDSTONE MEMBER OUi_ OF HlMES MEMBER OF CLOVERLY FORMATION OF CLOVERLY FORMATION FIGURE 3.—TYPE SECTIONS OF LITTLE SHEEP MUDSTONE MEMBER AND OF HIMES MEMBER OF CLOVERLY FORMATION

The mudstones, which range from claystone to present, although across any one horizon very silty mudstones, have various combina- nodules tend to be the same type. Veins of tions of these colors. Purplish and reddish chalcedony about three-eighths of an inch brown are especially prominent immediately thick form septaria of some nodules and, more above and several feet below the zone of commonly, cut through the mudstones. Selenite conglomeratic sandstone. Grays, rarely with crystals, rounded, smooth-surfaced pebbles variegation of pale and moderate reds, are from 2 to 4 inches in diameter ("gastroliths"), present in most localities immediately below bones and bone fragments, and fragments of the conglomeratic sandstone interval and in the silicified wood are also found in these mud- middle and upper parts of the section between stones, especially in the darker-gray mudstones. the conglomerate and the lowest beds of the Sparkly quartz sandstones and thin bentonite overlying Himes member. A very dark-gray beds, commonly less than 2 inches thick but bentonitic mudstone or claystone is generally ranging to more than 2 feet thick, are locally the uppermost unit of the Little Sheep mudstone abundant in the unit of purplish mudstones member. The bentonitic mudstones are char- which in many localities overlies the conglom-

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eratic sandstone unit and underlies the dusky- Cloverly of Jackson Hole, Wyoming; it has brown mudstones which are below the upper related forms in the Peterson limestone near gray mudstones. Cokesville, Wyoming (Yen, 1952). The conglomeratic sandstone is interest- Reptilian bones, teeth, scutes, and claws are ing from the standpoint of its role in the common in the more bentonitic mudstones of stratigraphic nomenclature of these formations the Little Sheep member, but most are too as well as because of the significance which is fragmentary and worn even for general identifi- attached to its peculiar lithology in this gener- cation. A small assemblage of crocodile, ally much finer-grained member. The conglom- dinosaur, and turtle remains was collected from erates and sandstones themselves are the uppermost dark-gray bentonitic mudstone characterized by well-rounded pebbles, gran- of this member near Black Butte. Dr. J. T. ules, and sand grains of black chert, less Gregory, who very kindly examined a portion commonly of brownish-black chert, or black of the collection, stated (Personal communica- chert veined with white. A second typical tion, 1955): feature is angular white-chert grains which are "The turtle scutes . . . belong to the genus Comp- flecked through the rock. These are partly semys and are closer to the type species, C. mcta decomposed and punky. The chief component Leidy, than to any other which has been referred of these conglomeratic beds is coarse- to fine- to it. Cotnpsemys ranges through the Upper Creta- ceous and Paleocene; sometimes the Jurassic grained quartz sand; the chert detritus is Glyptops plicatulus is referred to it, but I believe it generally only a small to moderate, but con- wise to recognize Glyptops as distinct, and might spicuous, proportion of the rock. even question whether some other species referred In many places a few inches of green mud to to Compsemys were cogeneric with the type." shale immediately underlies these conglomerate lenses. This mud characteristically contains He also stated that a caudal vertebra centrum numerous rounded large pebbles or small appeared to be from a stegosaur dinosaur. cobbles of chert, quartzite, and other lithologies. This is an interesting combination: Comsemys In the southeastern part of the region studied, is not recorded from beds lower than Upper from Howard Gulch past Hyattville, a facies of Cretaceous; stegosaurs are known chiefly from dark-brown micaceous shales and siltstones the Jurassic, less commonly from the Lower contains bright-yellow iron sulfate and a Cretaceous (Romer, 1945, p. 600). concentration of black plant debris in beds of a Pryor conglomerate member.—A distinctive fraction of an inch (rarely to a few inches) basal conglomeratic sandstone facies of the thick. These lignitic beds apparently are Cloverly formation on the south and west composed chiefly of macerated twigs, rushlike flanks of the Pryor Mountains has been named plants, and conifer needles. the Pryor conglomerate member of the Cloverly Another lithologic assemblage consists of formation. This unit thins southeastward to a very siliceous rocks, including green cherts feather edge at Sykes Mountain. From extra- dappled white, very light-gray siliceous and polation of the regional studies of Lammers tuffaceous mudstones, and cherty siltstones and (1939, p. 114), and from the current-direction sandstones. Although most of these siliceous studies of this report, the conglomerate at Line deposits are small lenses, one is developed very Creek at the northwest margin of the Bighorn extensively. It is a light-gray, siliceous and Basin probably is part of the Pryor member. tuffaceous mudstone which weathers to a Most other similar-appearing conglomerates in prominent white ledge wherever it outcrops. the central and southern edges of the basin This layer occurs near the top of the member, appear to be individual lenses, although a few generally overlain only by the very dark-gray may be edges of more extensive tongues that bentonitic mudstone that is normally the may thicken southwest of the basin. uppermost layer in the Little Sheep member. The Pryor member consists of several related Limestone lenses above the level of the lithologies, the most conspicuous of which is conglomeratic sandstones yielded gastropods at conglomerate containing rounded black-chert three localities, most abundantly at the west pebbles. All size gradations of the chert frag- face of the butte overlooking Bighorn River at ments exist, from rare pebbles more than 1 inch the mouth of Crystal Creek. These apparently in diameter to sand grains. Angular sand- and are all the one fresh-water species, Reesidella grit-sized grains of white chert are also abun- montanaensis (Stanton), known from the dant. These chert grains are mixed in varying Kootenai of Harlowtown, Montana, and the proportions with light-yellowish-brown to white

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quartz-sand grains, resulting in rock types The contact of the Himes member with the ranging from sandstone to pebble conglomerate. Little Sheep member is marked by channeling Angular blocks of white chert as much as in many localities; in others, a disconformable several inches long are common in the basal relationship, or at least a hiatus, is suspected beds of the member, especially in exposures from inferences about the genesis of the upper- southwest of Big Pryor Mountain. Farther most dark-gray mudstones of the Little Sheep southeast, from Gypsum Creek to the northwest member. end of Sykes Mountain, the basal beds consist Lithologically, the claystones of the Himes of rubble beds composed of blocks of sandstone, differ from the mudstones of the Little Sheep white chert, and fragments of lignitic wood as in that they are typically finer-grained, gener- well as a mass of finely hashed carbonaceous ally nonbentonitic, and contain veinlets, blebs, plant remains, all in a sandy matrix. Presum- and local hardpans of hematite and limonite ably, this rock type occurs at about the level (using those iron oxide names in a general field and locality in which Washburne (1909, p. 168) sense). The blebs weather out to dark-reddish- reported finding a thin coal bed. No bed of coal brown blotches which protect the underlying could be found, although it is possible that in claystone from rainwash so that the cliffs of some localities the carbonaceous trash may be claystone have vertical flutings. The claystones the only component of one of these rubble beds. or mudstones are variegated in shades of reddish The unidentifiable plant fragments were the brown, yellowish brown, and gray; only very only fossils found. rarely are greenish, purplish, or pale-red shades Immediately south of Sykes Mountain, in the found. part of the member near its edge, the black- The sandstones of the Himes member are of chert pebble and -sand component noticeably two distinct types, which differ from the decreases both in volume and grain size. At its lenticular conglomeratic, well-sorted sand- edge the member is a thin, well-sorted quartz stones of the Little Sheep or the sheets of well- arenite, interfingering with the lowermost beds sorted sandstone of the Sykes Mountain. One of the Little Sheep mudstone member of the type, common at the base of the member, Cloverly formation. locally channels slightly into the underlying Himes member.—Between the Little Sheep bentonitic mudstones; it is a lenticular, cross- mudstone member of the Cloverly formation bedded olive-gray sandstone, having a salt- and the Sykes Mountain formation is the Himes and-pepper aspect resulting from lithic grains member of the Cloverly formation. This is a and partly decomposed feldspars and a matrix cliff-forming member of claystones veined with of mud which swells when moistened. This iron oxide and lithic wacke and quartz arenite olive lithic wacke is generally overlain by, sandstones, in contrast to the gently rounded interfingered with, or, in some localities re- gumbo hills of the Little Sheep member and the placed by, a reddish-brown lithic wacke of the sandstone ledges and rusty-brown-weathering same general characteristics. Lenses of cross- slopes of the Sykes Mountain formation. bedded wacke locally grade upward through The outcrops around Little Sheep Mountain sandy mudstones or claystones and into clay- between Himes and Lovell, Wyoming, are well stones with iron oxide blebs which show only exposed and contain some very fresh rock. indistinct bedding or none at all. The type section measured on the west flank of The second major sandstone type consists of Little Sheep Mountain, in sec. 15, T. 55 N., R. shoestring-shaped bodies, flat on their upper 95 W., includes 92 feet of beds assigned to the surface and convex downward, interpreted as Himes member of the Cloverly formation (Fig. fluvial channel fills. This facies of sandstone is 3). Its thickness is about the average thickness composed of fairly clean, medium- to fine- of the member throughout the Bighorn Basin. grained, cross-bedded quartz sand. At the edges Noteworthy exposures in the vicinity of the of the channels, and locally at their bases, thin- type area include a recently abandoned small ner shales and siltstones are interbedded with quarry for building stone from a channel sand- the sandstones. The channels range in width stone within the Himes member immediately from a few yards to 1 mile and in maximum north of the type section and a clay pit operated thickness from a few feet to more than 100 feet. by the Lovell Clay Products Company in the Apart from the iron oxide-blebbed clay- uppermost variegated claystones about 4 miles stones, basal lithic wacke lenses, and the east of Lovell in the extreme northern outcrops quartz arenite channels, a few other lithologies on the west flank of Little Sheep Mountain. are known. In some places the channel-edge

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sandstones and shales contain carbonaceous, separated from the Himes member of the hashed plant remains. Locally, as immediately Cloverly by 15 feet of thinly interbedded silt- southwest of Shell and about half a mile south stones and shales. of the lower Bear Creek exposures, a thick,

shoestring-shaped body of gray sandy shale UNITS FEET and yellow sandstone cuts through the lower wBt9t| Block bentonitic clayshale of claystones, the olive or red lithic wackes, and t£s^SJ THERMOPOLIS SHALE. into the underlying Little Sheep bentonitic IG-Sondstone,- quartz arenitew. , mudstones. In the lower Bear Creek locality ovoid Ironstone concretions. 5, • 15. Ironstone. 1 some thick very calcareous sandstone lenses W. ThWwnterbedded brn.sltstcne. ss., a gray sh. w. considerable which lie gradationally beneath the olive . „ black clayshale. Kj * 13. Ironstone 1 lithic wacke channel into the Little Sheep 12. Chiefly black clayshale, w. considerable brown sillslaie , bentonitic mudstones. In a very few localities, athin quartz arenite beds. 20 chert beds, bentonite beds, and some bentonitic mudstones are found in the Himes member. II. Limestone 8 ironstone; , 1 gray, sandy. 2 No fossils were discovered in this member. lO.Thinly-interbedded brown siltstones, quartz arenites, 8 Assignment of a probable early Cretaceous age gray shales, w. some thin , is based on its stratigraphic position. ironstone beds. 16 3 Similar to unit 10 but also , some block shale Decs. 6 _*+&£$ 8. Simitar to unit II. 3' Sykes Mountain Formation 7. Similar to unit 10. 19' The Sykes Mountain formation is composed of thinly interbedded siltstones, sandstones, and 6 Similar to unit II. 21 shales, commonly containing some thicker 5. Similar to urit 10. 6' blanket-shaped sandstones, especially near the " '""' ' 4. Similar to unit II. I1 3. Similar to unit 10. 25' base, and thin beds of ironstones, all of which -i show some marine and some nonmarine features and weather rusty yellowish brown. These beds lie with very sharp contact, representing a ^ 2. Sandstone; gray to brown hiatus, on the nonmarine variegated claystones 'aijjaij^ia 1 gtz.aren. w' ironstone beds -^ a concretions. ° and channel sandstones of the Himes member of 1. Similar to unit 10. 15 the Cloverly formation. They grade upward and interfinger with the marine black bentonitic Variegdted claystone of '^5 clayshale of the overlying Thermopolis shale. -^ y-TV- * HIMES MEMBER OF THE This formation includes the "Rusty beds", CLOVERLY FORMATION. "Dakota silt", Dakota sandstones, and Grey- TYPE SECTION OF bull member of the Cloverly formation, of SYKES MOUNTAIN FORMATION various authors. As no existing name is ade- FIGURE 4.—TYPE SECTION OF SYKES MOUNTAIN quate, the new name Sykes Mountain formation FORMATION is proposed. This commonly poorly exposed Scale is the same as that for Figure 2. stratigraphic unit crops out in several excellent exposures near Sykes Mountain and Crooked The formation ranges in thickness from about Creek along the northeastern edge of the Big- 100 to about 300 feet in the northern Bighorn horn Basin (PL 2, fig. 2). At its type section Basin. Comparison of the type section with (Fig. 4), it comprises 136 feet of sandstones, nearby sections reveals that interfingering of shales, siltstones, and ironstones. This section the uppermost sandstones, siltstones, and gray northwest of Sykes Mountain is between shales, and ironstones of the Sykes Mountain Crooked Creek and Gypsum Creek about 1 with the lowermost black bentonitic shales of mile south of the Montana State line, in the the Thermopolis results in rapid changes in northeastern quarter of sec. 25, T. 58 N., R. 96 thickness of these two formations. Perhaps the W. Here the formation displays its characteristic Sykes Mountain should be designated only a increase in the proportion of black shale in its basal transgressive member of the Thermopolis upper part. The upper contact is placed above shale, but its distinct lithology and differing the uppermost sandstones, siltstones, and iron- origin argue against that classification. stones. The thickest sandstones, siltstones, and The contact of the Sykes Mountain formation ironstones do not occur at the base but are upon the Himes member of the Cloverly forma-

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tion is very easily selected, the well-bedded ironstone. Some persistent thin beds of sandy thin siltstones, sandstones, and shales con- or silty limestone, locally displaying cone-in- trasting markedly with the underlying red- cone structures, are another authigenic compo- blotched claystones which show no bedding. In nent. a very few localities where a fairly thick lower One or two sandstone units from about 5 to 10 Sykes Mountain blanket sandstone directly feet thick are characteristic of the lower Sykes overlies a channel sandstone in the uppermost Mountain formation in several localities. Some Himes, it is difficult to select the contact, for of these are very persistent, as for example, one the sandstones above and below the contact in the upper Alkali Creek-Five Springs area, may be similar. However, by carefully tracing two at Line Creek, and two west and south of the thinner, sheetlike sandstone beds of the Red Dome. In general, they are fine-grained lowermost Sykes Mountain laterally from an quartz sandstones, showing both bedding and adjacent area past the zone of the channel-edge cross-bedding, micaceous in part, and cemented sandstones, it is possible to pick the contact. commonly by limonite, less commonly by Most likely, it would be impossible to pick the calcite. Their uppermost beds are, in many contact from samples from a well which pene- instances, very well indurated, becoming trated a channel sandstone intimately overlain dark-brown ironstones having a submetallic by sheet sandstones. luster. The color of the sandstones range from Gradation between the uppermost Sykes nearly white to shades of moderate brown, Mountain formation and the lowermost Ther- yellowish brown and grayish orange. It is not mopolis shale, with repetition of Sykes Moun- thought necessary to name any of these tain-type lithologies at various higher levels in members. the section, causes some difficulty in placing the Aside from worm burrows, this formation is upper contact. In this study, the contact is apparently unfossiliferous. Fossils are rare in the normally placed at the top of the highest unit overlying Thermopolis shale, but a specimen of which is at least 1 foot thick, in which limonite- Inoceramus comancheanus Cragin was collected stained siltstones and sandstones, or ironstones, by K. Waage (Personal communication, 1955) are more preponderant than black or dark-gray from the lowest Thermopolis shale above the bentonitic clayshales, as this represents the end line of cliffs west of the old Cloverly post of paralic, or mixed marine and nonmarine, office. Waage (1955, p. 41) considers /. coman- sedimentation. The black shales lower in the cheanus as late Albian, and so it provides an section represent brief periods of more dominant upper time limit for the Sykes Mountain marine sedimentation during the paralic sedi- formation here. mentation. In some localities, as between the lower reaches of Crystal Creek and Salt Creek Correlation (Dry Bear Creek of some maps), this definition of an upper contact places as much as 43 feet of Incomplete local fossil evidence.—Fossil evi- black shale within the Sykes Mountain forma- dence for dating the Morrison, Cloverly, and tion, which occurs here below a local lens of 157 Sykes Mountain formations in the Bighorn feet of the rusty-brown-weathering sandstones, Basin has been meager. Dinosaur remains siltstones, and gray shales. collected from the Cloverly in southern Mon- The most common lithology in the Sykes tana (B. Brown, 1933), and from the Morrison Mountain formation is thinly interbedded near Cloverly Post Office (B. Brown, 1935) limonite-stained siltstones and fine-grained have not been described. Knowlton (1916) sandstones, with gray shales. Most beds are considered some lower Cloverly plants to be between one-eighth and three-quarters of an Kootenai equivalents, and of Cretaceous age. inch thick and have even finer lamination. The The present study has contributed little addi- bedding is considerably disturbed on a small tional fossil information. scale with fragments of one lithology in a bed of To date these Bighorn Basin units between of another, scoured and filled features, and their Oxfordian (uppermost Sundance) and similar structures, many of which appear to be late Albian (lowest Thermopolis) limits, the the results of burrowing organisms; most of ages of the beds they laterally adjoin must be these features indicate deposition in tidal-flat considered. Some further, more cautious, infer- environments. These thin detrital beds, com- ences of age can be drawn from their homotaxial monly micaceous and carbonaceous, also (or syntopogenic) equivalents elsewhere in the contain concretions and thin beds of authigenic Western Interior, recognizing the great hazards

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of equating lithogenetic similarity with age the identically appearing Burro Canyon (Early equivalence over these distances. For a first Cretaceous) and Cedar Mountain formations approximation, Figure 5 may be a correlation which overlie it, composed of variegated benton- chart, although it is designed only as a graphic itic claystone, cherts, calcareous nodule zones, supplement to the following summary of and chert-pebble conglomeratic sandstone apparent lithic equivalents of other well-known lenses (Stokes, 1952) reasonably are related sections. to the Little Sheep member of the Cloverly, Lithogenetic equivalents.—In central Montana with the addition of some Pryor-type sediments. the lithic equivalent of the Morrison formation A subaerially leached (Himes) rock type is not is the nonmarine section below the Jurassic described in the literature. The transgressive plant beds (Lammers, 1939; R. W. Brown, beds of the Dakota formation of this region 1946). These beds and the remainder of the represent deposits laid down peripheral to the Kootenai appear to the writer to represent the encroaching sea as do the Sykes Mountain beds Cloverly, with the Third Cat Creek sand ap- of the Bighorn Basin. parently of the same lithogenetic origin as the Ages.—The late Jurassic age of the Morrison Pryor member, and the First Cat Creek sand formation was firmly established by Simpson the lithogenetic equivalent of the Sykes Moun- (1926). In fact, emphasis on its Jurassic (Port- tain formation. landian) age, coupled with difficulties in select- In a pertinent review of recent stratigraphic ing an upper lithologic boundary, has produced studies near the Black Hills, Waage (1958) a situation whereby age alone has been used to stresses that the Inyan Kara group between the delimit the Morrison formation, either by in- Morrison formation and the Skull Creek shale ference (Baker, Dane, and Reeside, 1936, p. 10, can be divided into two gross lithogenetic units. 31) or by definition (Stokes, 1944, p. 956). This He terms the lower the Fuson-Lakota sequence, practice should be abandoned. abandoning Barton's formations as not ac- J. B. Reeside (in Yen, 1952) believed that ceptable for mapping outside the immediate Morrison deposition ceased possibly before the type area of the Fuson. This sequence of non- end of the Portlandian, or probably before the marine beds includes lenses of sandstone and of Purbeckian, and that the Cloverly deposition partly bentonitic, nonlamcllar, varicolored of basal brown conglomeratic sandstones did claystone, with local chert-pebble conglomer- not begin until at least the Hautrivian or possi- ates, resembling the Bighorn Basin Cloverly. bly the Barremian. This hiatus of deposition of Above a transgressive disconformity lie the the latest Jurassic and earliest Cretaceous dark shale, siltstones, and flaggy sandstones of stages is illustrated graphically in Cobban and the marginal marine Fall River sandstone, the Reeside's correlation chart (1952). lithogenetic equivalent of the Sykes Mountain There is no fossil evidence for such an ex- formation. tensive, single hiatus in the Western Interior. The Lytle formation and the upper Morrison In fact, Yen (1949) considered some of these variegated claystones and conglomeratic sand- beds probably Neocomian, admittedly using stones described by Waage (1955) along the nonmarine mollusks. Moreover, although the Colorado Front Range seem to be the genetic physical evidence in the northern Bighorn equivalent of the Bighorn Basin Cloverly, but Basin suggests that, of the multitude of small as was the case with the Fuson-Lakota sequence hiatuses present, the one represented by the of the Black Hills, it is difficult to make com- sub-Pryor unconformity is most significant, parisons with specific Cloverly members. Below there is no indication that it represents a few the Lytle, the "undoubted Morrison" of European stages. South of the edge of the Waage resembles the Bighorn Basin Morrison Pryor conglomerate, beds of uppermost Morri- as now restricted, and above the Lytle, the son and lower Little Sheep member of the Plain view sandstone member of the South Cloverly are not eroded by that unconformity. Platte formation probably had an origin very In this southern area the Morrison-Cloverly similar to the Sykes Mountain formation. contact is conformable, even gradational in In the Colorado Plateau, the red-and-green part. This contact may be entirely within the banded claystones and lenses of sandstones of Jurassic, if lowest Cloverly correlations to the the Salt Wash, Recapture, and Westwater Jurassic lowest Kootenai and Brushy Basin members of the Morrison formation (Craig lithogenetic equivalents are allowed. et al., 1955) are lithogenetically similar to the From these meager data and from inferences Bighorn Basin Morrison formation. The Brushy about the apparently slow accumulation of most Basin (Jurassic) member of the Morrison, and of this nonmarine sequence and wdth no physical

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or fossil evidence to the contrary, it is tenta- red and green lenses have a few inches of tively proposed that the most reasonable age disturbed bedding with small fragments of one assignments are as follows: lithology included within another. The Morrison formation was deposited be- The less calcareous and cleaner quartz arenite tween Oxfordian and Purbeckian time. Cleverly lenses of the Morrison exhibit true cross-bed- deposition followed immediately, lasting con- ding. Some of the more calcareous sandstones and tinually until the return of near-marine condi- sandy limestones owe their cross-bedded aspect tions. The fine-grained members of the Cloverly to their ripple markings (or migrating rolling formation accumulated exceedingly slowly. strata). Many sandy limestones without ripple Little Sheep time, characterized by seasonal marks are in beds more than 2 feet thick, com- lacustrine weathering of intermittent ash falls, posed of many parallel laminae averaging less was interrupted by a slight warping in the than one-tenth of an inch thick. Some of the northern part of the area and the spread of small, more convex silty limestone lenses, which coarser detritus, perhaps in earliest Neocomian are less than 20 feet or so wide but about 1 or 2 time. The Little Sheep episode ended at some feet in maximum thickness, are a single bed unknown time, possibly in the late Neocomian, with no apparent lamination; other lenses have with the advent of a period of subaerial beds 1 or more inches thick separated by very weathering of ash and coarser epiclastic debris, thin green mudstone beds. represented by the Himes member. The mixed The cross-bedded sandier units of the Morri- marine and nonmarine interval of deposition son formation are, in the terminology of McKee of the Sykes Mountain formation possibly and Weir (1953, p. 386), large to medium scale, commenced in Aptian or early Albian time and low- to very low-angled, and mostly wedge- lasted until late Albian time. shaped. The lower bounding surfaces of sets commonly are curved surfaces of erosion, so PETROLOGY they fit into McKee and Weir's trough cate- gory, even though the troughs are shallow. Sedimentary Structures Oscillation ripple marks are very common in the thinner-bedded calcareous sandstones of the Stratification.—Stratification has been de- Morrison formation. There is no apparent pre- scribed by numerous qualitative terms and used ferred orientation of the marks; one bed may in a great many different ways (Ingram, 1954, p. have ripple marks developed at right angles to 937). Furthermore, thickness terms are not as the ripple marks of the bed it overlies. Some yet scaled well either for describing naturally beds in the Morrison formation have mud occurring strata or for manipulating such data cracks and rain prints. statistically (Bokman, 1956). Following CLOVERLY FORMATION: Bedding in the Bokman, this author uses names for stratifica- Cloverly formation ranges from indistinct in tion only in a genetic sense in the present report. the dark-gray bentonitic mudstones to very Qualifying adverbs such as thinly or thickly distinct in the sandstone and conglomeratic (bedded) are used informally. A bed is a unit of sandstone units. Only the sandstones show any layered rock which appears to have been de- bedding in weathered exposures, as the swelling posited under essentially constant physical of the bentonitic components destroys bedding conditions. A bed may contain one lamina, the features rapidly, leaving only a general layered smallest layer of particles recognizable in the effect from broad color bands, silicified tuffa- sediment, or more. A set comprises two or more ceous mudstones, and the zones of chert and beds of the same lithology. Similarly, there are calcareous nodules. cross-laminae, cross-beds, and cross-sets. All Freshly exposed portions of the bentonitic these may be referred to as strata. interval clearly show bedding, as in an outcrop MORRISON FORMATION: Interlensed sets of in a steep bank on the south side of Bear Creek cross-beds, the most characteristic type of about 3 miles above its mouth. Deeply weath- Morrison formation stratification, are more ered deposits farther from the creek appeared pronounced in the sandstones and coarser silt- as homogeneous, variegated sandy bentonitic stones than in the indistinctly bedded finer mudstones, yet in the fresh cut discrete beds of mudstones and shales. In the red- and green- differing rock types were visible. These in- banded facies of the Morrison, the reddish- cluded pale-purplish-pink and pale-green brown finer-grained lenses are only rarely cross- bentonites in beds less than 1 inch to several bedded and have thinner laminations than the inches thick, yellowish-green very bentonitic greenish-gray lenses. Some contacts between siltstones and mudstones in beds a few inches

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thick, and white to light-gray, muddy-matrixed bedding. In plan view the exposed upper ends fine-grained sandstones, slightly bentonitic, of these beds are crescent-shaped. This type of and containing a few galls of bentonitic mud- cross-stratification is typical of cuspate beaches, stone in finely laminated beds several inches to according to G. Rittenhouse (Personal com- a few feet thick. Except in some of the benton- munication, 1955). Cusps are well displayed ites variegation was rare. in outcrops south of Crystal Creek on the east This type of interbedding of bentonites, flank of Crystal Creek anticline. bentonitic mudstones, and fine-grained wackes, Stratification in the Pryor conglomerate admittedly difficult to detect in weathered member of the Cleverly formation (PL 3, figs exposures, seems to be best developed in the 4 and 5) is similar to that in the pebbly sand- interval of dusky-red (maroon of previous liter- stone lenses of the Little Sheep member. Trough ature), grayish-purple, and lighter-gray, gum- cross-sets are most abundant and commonly bo-weathering mudstones above the interval of have better preferred orientation of axes in any conglomeratic sandstone lenses. The overlying one locality than is present in the unnamed sequence of variegated to gray mudstones lenses of the Little Sheep member. The cuspate- capped with the white siliceous and tuffaceous type bedding was observed only at the south mudstone, in turn overlain by the dark-gray end of Sykes Mountain, the southeastern edge mudstones at the top of the Little Sheep mem- of this tongue. The cusps there are asym- ber, shows no bedding, even in the freshest of metrical, perhaps owing to currents parallel undercut creek banks. (Minor exceptions are to the shore of the lakes in which they were bedding locally in the uppermost few inches, formed. which might be attributed to reworking below Most detectable stratification in the olive- the olive-colored wackes of the Himes member, gray lithic wackes at the base of the Himes and indistinct bedding in the white tuff, which member of the Cloverly formation is cross- may be a secondary feature of selective silicifi- bedding. The reddish-brown lithic wackes gen- cation along several bands.) Some exposures of erally show better stratification. These trough variegated bentonitic mudstones below the cross-sets of reddish-brown sandstone are more level of conglomeratic sandstone also show no shallow and thinner-bedded than those of the bedding in fresh exposures. olive-gray sandstone. Good stratification is produced by concentra- The channels of sandstone in the Himes tion of micaceous minerals and fragments of claystone display several types of bedding. The carbonaceous plant debris on the thin lamina- beds at the base of the largest channels, in sets tions of brown siltstones and shales which 100 feet or more wide, are as much as 4 feet locally occur below the level of the conglomer- thick and contain thinner interbeds of silt- atic sandstones near Hyattville and in Howard stones and shales. Channel-edge contortion of Gulch. the beds into which the channel was cut prob- Distinct stratification in the conglomeratic ably originated as bank slumps. At the edges sandstones is commonly due to both composi- of the wider channels the sandstones are thin- tional and color changes; for example, coarser ner-bedded and interbedded with siltstones beds are chiefly composed of black-chert gran- and shales. In contrast, the beds closer to the ules. Cross-stratification is predominant (PL middle of the channels are dominantly thicker- 3, fig. 3), and the coarser beds most commonly bedded sandstones. The narrower channel sand- are in aligned troughs in which more than one stones have very few or no interbedded finer- cross-set may have approximately the same grained rocks at their edges. The upper beds of axis. Troughs channeling into the underlying the channel sandstones display a distinctive mudstones are especially well-aligned. Most of type of cross-stratification, with sets a few the remainder of the cross-sets of this rock type inches thick of finer-grained beds alternating are also troughs in McKee and Weir's (1953) with cross-sets several inches thick of coarse- terminology, but axes are less well oriented, grained cross-beds (PL 3, fig. 2). especially in the finest-grained sandstones. Several surfaces of subaerial or subaqueous The upper beds near edges of some finer- (fresh-water) weathering are believed to be grained lenses show very shallow-troughed present in the Little Sheep and Himes members. cross-sets 1-2 feet thick and 10-15 feet wide The term diastem (Barrell, 1917) may be ap- and long (exceptionally longer). These beds propriate for these surfaces. The most impor- are 1 inch thick and concave upward, and all tant and widespread diastem is the contact be- have a. similar orientation (PI. 3, fig. 1). The tween these two members, typically developed sandstone between adjacent sets shows no where the olive-gray sandstone at the base of

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the Himes rests with a slightly wavy contact on well exposed Morrison formation suggest that the very dark-gray mudstones at the top of the currents in the northern localities came from Little Sheep member. the north and northwest, whereas those in the The upper contact of the Cloverly, where the southern localities came from the southeast, variegated claystones at the top of its Himes south, and southwest. member are covered by the Sykes Mountain Other than in the interval of conglomeratic formation, is a transgressive disconformity. sandstones, no features which might show direc- Even though the episode of subaerial weather- tion of current flow were observed in the Little ing which preceded Sykes Mountain deposi- Sheep mudstone member of the Cloverly formation was not necessarily any more intense or tion. Cross-bedding, axes of scouring-trough prolonged than any of the earlier periods of cross-sets, and channel-filling of conglomerates weathering recorded in the Himes or Little within the sandstones showed current direction Sheep members, the resulting unconformity in the conglomeratic sandstone lenses of this can be traced extensively because of the marked member. Pebble imbrication is uncommon, difference in rock type of the Sykes Mountain perhaps because of high pebble sphericity. As formation. This unconformity is also marked by the lenses are not uniformly disturbed through- local lag gravels of "gastroliths" and some re- out the area, an efficient sampling plan could working of Himes claystone into the lowest not be employed. The record of all possible few inches of the Sykes Mountain, where the measurements (Fig. 6) reveals that the most claystone is thinly laminated, in contrast to consistent trend of stream direction in the nonbedded leached rock below. southern part of the area is from the southwest. SYKES MOUNTAIN FORMATION: The Sykes The middle area includes part of this southern Mountain formation is characteristically very pattern and part of the pattern of the Pryor thinly interbedded fine-grained sandstone to conglomerate in the northern area. siltstone and shale. The beds, averaging one- Cross-bedding and trends of channels in the quarter of an inch in thickness, range from un- Pryor conglomerate member of the Cloverly disturbed with very delicate laminations and formation show a dominant current direction cross-laminations to greatly disturbed, with from the west or northwest (Fig. 6). The trend convoluted fragments of one lithology within is most persistent in the chert-pebble and gran- the other. Descriptions and photographs by ule beds. Some sandstone beds within the mem- van Straaten (1951; 1954a; 19S4b) and W. ber were transported from other directions, Hantzschel (in Trask, 1939) of modern tidal especially from the east, nearly opposite that of flats and rocks inferred to have been formed in the coarser beds. An example of this change of ancient tidal lagoons show stratification identi- direction is shown clearly in the Red Dome area, cal with that of the Sykes Mountain formation, where the conglomerate beds are more con- with bedding disturbance from burrowing ani- centrated into channels rather than distributed mals, load casting, and reworking in tidal chan- throughout the lower and middle portions of nels and gullies, or prieles. the member. The trend of these channels is The thicker sandstone units of the Sykes northwest-southeast, and the dip of cross-beds, Mountain formation contain mostly simple, plunge of axes of trough cross-sets, and imbrica- rarely planar, cross-sets between sets of beds. tion of pebbles indicate a current dominantly No cuspate or trough types of cross-bedding from the northwest. These channels of con- were observed. glomerate are braided through sandstones The thin, very calcareous sandstones or which below the channels are cross-bedded sandy limestones show some thinly laminated chiefly from the northeast, at the level of the cross-bedding, largely resulting from ripple channels from the east and south and above the marks. Oscillation and current ripple marks are channels from the southwest to northwest. As present both in the thinner beds of the thicker Red Dome is on the Nye-Bowler lineament sandstone units and in the thin, platy-weather- (Wilson, 1936), numerous fault blocks made it ing sandy limestones. Some interference ripples impossible to trace individual channels for more are also present. Current direction deduced than a few hundred yards. However, there is a from ripple marks on any particular bed com- marked parallel orientation of channels, even monly differs from the direction deduced from with these limitations (Fig. 7). cross-bedding or ripple marks in the beds im- Systematic observations of cross-bedding in mediately above or below. the lithic wacke commonly present at the base Directional properties.—The few cross- of the Himes member were limited by the len- bedding observations made in the generally less ticularity of the unit and the obliteration of

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outcrop detail owing to weathering of its tion of flow was almost certainly constant, in- swelling-clay matrix. Cross-bedding in sand- dicating fluvial rather than tidal channels. Most stone generally indicated a current flowing from of the cross-bedded sand was transported by

( area of Fig. 7)

FIGURE 6.—EARLY CLOVERLY PALEOCTJRRENTS, BIGHORN BASIN Arrows indicate direction of currents which deposited cross-beds containing pebbles, granules, or sand grains of black chert in the Pryor conglomerate member or in the Little Sheep mudstone member of the Cloverly formation. Outcrop outline as shown on Figure 1.

the west and northwest; gray sandy shale chan- streams from the northeast. Two of the three nel fills also showed current flow from the north- largest streams came from the east, and there west. The variegated claystones have no flow- were a few from other directions (Fig. 8). direction properties. In the large channel exposed north of the The sandstone channels in the Himes mem- type Cloverly area the successive thickly ber show the most unequivocal direction of flow bedded sandstone lenses are shingled from south in all the formations studied. Their channel to north, as the channel was filled behind a shapes are unmistakable, with parallel sides, northward swing of the main current (Fig. 9). flat tops, and convex-downward undulatory The beginning of a swing southward is shown bottoms. Because the cross-bedding dips only in the upper beds, which are thinner and more one way in each channel, which is in the same consistently cross-bedded. Channel-edge thin sense as the current ripple markings, the direc- shale and siltstone beds alternate with the

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sandstones, representing flood-plain deposition nous accretionary bodies. Some individual nod- south of the main channel in its later stages. ules or concretions occur in scattered zones; Cross-bedding and cross-lamination are other zones are irregularly shaped but continu- present in every thin siltstone and sandstone ous layers. Some of the accretions are more ir-

•*- To Bridger Kt Thermopolis shale and younger. Ksm Sykes Mountain fm. Cloverly formation: Kch Himes member / Kcls Little Sheep mudstone member Kcp Pryor conglomerate member Direction of currents /which deposited the quartz arenites. Limits and direction To Bowler - <* of channels in which /'/JblacDiackK cnerr-peoDichert-pebble gravels were de- norm 7r pposited08^e(j. Jm Morrison formation. Js Sundance formation. Modified from Wilson (1936).

FIGURE 7.—PRYOR CONGLOMERATE MEMBER PALEOCURRENTS NEAR RED DOME, MONTANA Location shown on Figure 6

bed of the Sykes Mountain formation. No at- regularly shaped and not necessarily concen- tempt was made to measure directions of dip, trated in bands. The white-weathering however, for it was assumed that the bedding calcareous concretions or nodules of the Little formed under conditions of constantly rework- Sheep mudstone member of the Cloverly for- ing tidal waters that depend on wind direction mation weather from the soft mudstones and as well as the diurnal changes in tides. conspicuously pave the colored gumbo slopes. Cross-bedding in thicker sandstones dips to These structures range from less than 2 inches the west or west-southwest. The cross-beds also to more than 1 foot in diameter (4 to 8 inches is formed in a paralic environment, but the spe- average). Although some show the concentric cific environment cannot be inferred. The cross- banding of true concretions, others with no bedding is not similar to any of the types de- apparent concentric growth pattern are best scribed on modern beaches (Thompson, 1937; termed nodules. All are grayish red when fresh, McKee, 1951). Perhaps the bedding represents weathering to white, gray, or pink, and have what van Straaten (1951, p. 237) calls lateral botryoidal to rough irregular surfaces. They sedimentation in tidal channels. Even if this is leave insoluble residues of very silty pinkish true, no valid inference may be made, from the mud. Some are scattered throughout the mass present knowledge of modern cross-bedding, of the variegated mudstones, others are con- about the direction of land or the direction of centrated in layers within the mudstone. Very the ultimate source of the sediment, except commonly the surrounding mudstones are non- that the sand must have traveled in general calcareous, especially those mudstones in which down the regional slope (Pettijohn, 1957, p. the calcareous nodules occur in layers. Septarian 581). veinlets of orange to white chalcedony were Secondary structures.—Secondary structures present in some of the nodules, and others were of the Cloverly formation thought indicative in various degrees of silicification leading to of the uncommon mode of origin of this forma- chert concretions. tion include calcareous, siliceous, and ferrugi- Some of the layers of calcareous and siliceous

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rock are continuous but show colloform growth Less widespread cherty beds occur a few feet structures of calcite or silica around and be- above or below the conglomeratic sandstone tween masses of formless calcareous or siliceous lenses.

DETAIL OF GYPSUM CREEK AREA.

0 Z

FIGURE 8.—LATE CLOVERLY PALEOCURRENTS, BIGHORN BASIN Larger arrows locate, and indicate current direction of, wide and thick channel-filling sandstones in the Himes member of the Cloverly formation. Numerals and smaller arrows show quantity and current of small channels in the member. Outcrop outline as shown on Figure 1.

mudstones rather than primary bedding. Com- An extensive, very calcareous unit a few feet monly, these layers could be traced laterally above the conglomeratic sandstones locally and observed to grade into a layer of discon- appears to have primary bedding but most com- nected individual nodules or concretions. monly consists of reniform masses of closely The most conspicuous and widespread of spaced irregularly shaped individual concre- these layers is the nearly white-weathering, tions. very light-gray, siliceous tuffaceous mudstone In some beds of the Little Sheep member are near the top of the Little Sheep member (PI. 5, lenses a few inches thick and several feet in fig. 2). It commonly has orange chalcedony diameter of chert in shades of light yellowish veinlets through it, bone fragments and well- green dappled with white or spotted with rounded pebbles in its upper few inches, and darker green. In thin section the chert is a closely spaced joint sets which dictate its silica-replaced tuffaceous mudstone. characteristic crenulated weathering habit. Very irregularly shaped secondary accretion-

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ary structures also occur in the Little Sheep mudstone member. These cloudy films of calcium carbonate in the dark-gray, usually noncalcareous, bentonitic mudstones at the top of the member permeate the mudstones with no apparent structural control. Their pearly luster results from gray color of the mudstones showing through the thin translucent white film. The most characteristic features of the variegated claystones and mudstones of the Himes member of the Cleverly are the ferruginous veinlets, films, blebs, and layers which pervade it. These are iron oxides and oxyhydrates, chiefly hematite. No rhombs which might have indicated original siderite, were observed by hand lens or in thin section. Most of the ferruginous masses and veinlets have no internal structure, but some pinhead-sized il spherulites are especially common near the top of the formation. The Sykes Mountain formation contains abundant disk-shaped ironstone concretions of 0 limonite after siderite, and some of its black shale beds contain zones of dahlite spherulites. Cone-in-cone structure is present in sandy, finely crystalline limestone beds in the Sykes Mountain formation.

Petrography Composition and texture.-—Particle textures 1.1 of several representative samples of each formation were studied by sieving and pipetting for size sorting, by comparison with visual aids for « shape and roundness, and by direct observa- 3H w < OJ tions for surface textures and orientation of ts fabric. The composition of allogenic and authigenic minerals and of allogenic rock fragments as determined by optical (PL 4, 5) and by X-ray-diffraction methods is summarized in Table 1. Special studies.—Because of broader implications, some analyses of particular compositions are reported in detail. PEBBLES: The rounded chert pebbles, granules, and sand grains of Cleverly rocks are ce mostly black. Some black chert has white-chert o veinlets, and some contain indistinct outlines of fossils, probably bryozoans and corals. Brown and gray chert are of secondary importance, generally a small percentage of the pebbles at any outcrop. Gray-quartzite pebbles are more uncommon. In exposures along the western edge of the Bighorn Basin, and at some localities elsewhere, rare limestone, pink quartzite, and phosphate pebbles occur. According to J.

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D. Love (Personal communication, 19SS) erosion and clastic deposition, so that the fossils found in the chert pebbles of conglomer- punky appearance of the fragments probably is ates at about this level throughout Wyoming due to their having been still opaline during are preponderantly forms from the Madison reworking. limestone. Superficially, the chert of the pebbles Mud and clay galls in the more sandy beds of in the Bighorn Basin is similar to Madison the Morrison formation and the Little Sheep chert, but this is not a certain evidence of its mudstone member of the Cloverly formation origin. apparently were derived from subjacent layers. Angular, punky fragments of white chert are A rare type of intraformational conglomerate is locally present as pebbles and granules and a marl-granule conglomerate in a lower Little ubiquitous as sand-sized grains in these beds. Sheep sandstone lens on the southeast end of These dull or flat white-chert grains which look the east flank of Sheep Mountain. Very silty much like flecks of blackboard chalk commonly microcrystalline calcite pellets and interstitial have smaller quartz or black-chert grains medium- to fine-grained well-rounded quartz pressed into them, or they are molded against sand are partly cemented with patches of other grains. Yet they are hard enough to calcite. scratch steel. They give the X-ray pattern of The chert-pebble conglomerates of the Clo- quartz, and under the petrographic microscope verly formation commonly have a pebble- and are seen to be cloudy cryptocrystalline silica, cobble-studded yellowish-green-clay bed im- with tiny fractures and frothy air spaces, con- mediately below them. Although less common taining clastic-textured, silt-sized quartz and below the Pryor conglomerate, this gravel some limonite and clay. No dolomite rhombs occurs in scattered localities throughout the or fossils were seen. Some of the white chert in area studied and is interpreted as a lag-gravel blocks of cobble size appears identical to chert deposit. The matrix between the pebbles ranges in the lower parts of the Little Sheep mudstone from plastic clay or mud to shale, locally very member of the Cloverly. Very probably all the sandy. Pebbles are of pink and white quartzites, white angular chert is derived from this intra- jasper, white, black, gray, and brown chert, and formational source. Probably the chert formed rarely limestones, quartz, and granite. Pebbles as siliceous hardpans only a short time before are well-rounded, with dull, pitted and etched

PLATE 2—CLOVERLY AND SYKES MOUNTAIN FORMATIONS FIGURE 1.—Cliffs west of Cloverly, Wyoming, looking west at section Darton (1906a) probably measured. Frontier and Mowry formations cap hill; "Muddy sand" is prominent white band in black Thermopolis shale. Top of cliff in middle ground is part of Sykes Mountain formation. Cliff is Himes member of Cloverly formation, with a small channel-sandstone lens at left (talus). Gentle hill in lower center in gray bentonitic mudstone of Little Sheep mudstone member of Cloverly formation. FIGURE 2.—Sykes Mountain formation near type locality. Uncommonly good exposure of typically thinly interbedded yellowish-brown sandstones and siltstones and gray shales, with some calcareous and ferruginous nodules. PLATE 3.—BEDDING FEATURES FIGURE 1.—Crescent-type cross-bedding in upper beds of a quartz and chert sandstone in Little Sheep mudstone member of the Cloverly formation, near Crystal Creek. Interpreted as having been beach cusps (viewer facing "lake"). Light-meter case about 3 inches across. FIGURE 2.—Channel-filled sandstone bedding in Himes member of Cloverly formation. Remnant joint blocks of upper beds of channel-filling sandstone in Himes member on bluff above Crooked Creek. Note persistence of cross-bedding dips within blocks. In bedrock, dips are to west-northwest. FIGURE 3.—Conglomeratic sandstone, Little Sheep mudstone member of the Cloverly formation near Cedar Creek. Cross-bedding dips persistently northeast (left). FIGURE 4.—Pryor conglomerate member of Cloverly formation near Red Dome, Montana, with shallow trough-type cross-sets. View approximately along the axes of the troughs which plunge toward observer. Note angular white chert chips in upper beds. FIGURE 5.—Black-chert-pebble-conglomerate beds channeled through yellowish-brown sandstone in Pryor conglomerate member of the Cloverly formation near Red Dome, Montana.

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CLOVERLY AND SYKES MOUNTAIN FORMATIONS

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surfaces. Those partly weathered out and in Possibly the present random distribution float reveal that they readily become polished of these pebbles through the claystones and under present climatic conditions. mudstones and their concentrations above These pebbles are identical with "gastro- siliceous layers resulted from a diagenetic liths" locally concentrated at the tops of the process of "self-tilling" as will be elaborated, siliceous tuff beds and also scattered through- How the stones arrived in the depositional area out the remainder of the Cloverly formation. is not known. Presumably they were washed in In the Bighorn Basin none were found in the by rivers in times of seasonal flood, but some Morrison formation. No evidence pointed to may have been rafted by floating vegetation, the generally discredited hypothesis that these or may even have been carried in the stomachs were stones used by reptiles to aid their diges- of dinosaurs, tion. Many were similar to rock types in the Belt series and Tensleep, Madison, Amsden, VOLCANIC DEBRIS : and Phosphoria formations. The jasper is Volcanic contributions to the Morrison for- superficially similar to some jasper pebbles mation are not readily evident. Some heavy- found elsewhere in the Cloverly formation of mineral concentrations contained thin, ragged Wyoming which contained fusilinids of a later flakes of reddish biotite and worn brown horn- Permian age than any outcropping Permian blende, suggesting some reworking of volcanic yet discovered in the Western Interior (J. D. materials. Love, 1955, personal communication). When A significant portion of the Cloverly forma- the pebbles are pried from the mudstones, their tion consists of debris clearly of volcanic origin, surfaces generally are rough and dull, but they In fact, the bulk of the formation probably is of also become smooth and highly polished when volcanic origin, the clearest indication of which exposed to weathering conditions. is its bentonite beds. These are most abundant

PLATE 4.—PHOTOMICROGRAPHS OF THIN SECTIONS FIGURE 1.—Ordinary light, X100. Sandy limestone, Morrison formation, of very fine-grained quartz sand in calcite groundmass. A few grains of clastic calcite and a mud grain in upper left. FIGURE 2.—Crossed nicols, X40. Fine-grained calcareous quartz arenite. Morrison formation. Subangular to subrounded quartz grains, rare chert grains, and subordinate muddy matrix, well-cemented with calcite in large poikilitic patches. FIGURE 3.—Ordinary light, X20. Chert-granule conglomerate of Pryor conglomerate member of Cloverly formation. Well-sorted, porous chert granules, with minor interstitial quartz sand and silt. Some patches of silica cement. Some grains sutured or molded to one another. Large grain in center crushes several smaller grains it touches; cracks radiate from points of contact. FIGURE 4.—Crossed nicols, X100. Chert-granule conglomerate, Little Sheep mudstone member of Cloverly formation. Vevy well-rounded chert granules, showing some dolomite rhombs, rimmed by stubby radiate chalcedony; center of pore filled with radiate microfibrous chalcedony. PLATE 5.—PHOTOMICROGRAPHS OF THIN SECTIONS, AND TUFFACEOUS MUDSTONE OUTCROP FIGURE 1.—Ordinary light, X10. Cherty tuffaceous mudstone of Little Sheep mudstone member of Cloverly pictured in outcrop in Figure 2 of this plate. Fragments of feldspar, mudstone (originally lapilli?), quartz, and unidentifiable grains in a groundmass of, and replaced by, cryptocrystalline silica, dusted with opaque iron oxides. FIGURE 2.—Cherty tuffaceous mudstone in upper part of Little Sheep mudstone member of Cloverly formation, showing typical crenulated weathering from closely spaced joints. FIGURE 3.—Ordinary light, X100. Lithic wacke, Himes member of Cloverly formation. Medium-grained rock fragments and some quartz sand in montmorillonite-clay matrix. Streaky laminated, bubbly textured grain (pumice?) in lower center crushed against grain to left. Note trachitic texture of dark grain above center, and felted texture of grain in left center. FIGURE 4.—Ordinary light, X100. Calcareous silty ironstone, Sykes Mountain formation. Black is hematite after siderite (note rhombs in lower center and right center); light gray is angular quartz in very fine-sand and coarse-silt sizes. Slightly darker gray is calcite (veinlet across upper left, patch in lower right).

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through several feet of the Little Sheep mud- a matrix of some sandstones. A common source stone member above the chert-pebble con- of montmorillonite is altered volcanic glass glomeratic sandstones, where individual (Waters and Granger, 1953, p. 6; Grim, 1953, p. bentonites ranging from about 1 inch to about 357; Ross and Hendricks, 1945; Allen, 1944). 2 feet thick weather to the characteristic gumbo The siliceous layers are another possible, hills. Bentonite beds in the Himes member yet inconclusive, indication of volcanic activity usually weather white, with a horizontally in the source area. Much of the silica is still layered aspect contrasting strongly with the isotropic but has a higher refractive index than vertical flutings to which the reddish and yel- opal. Furthermore, variegation in pastel shales lowish-brown claystones weather (PI. 2, fig. 1). of purple, pink, maroon, and green has been In thin section, a number of the lithic frag- attributed to volcanic debris (Waters and ments in the sandstones at the base of the Granger, 1953, p. 2; John Clark, personal com- Himes member are clearly of volcanic origin munication). (PI. 5, fig. 3). These grains show textures The heavy-mineral suite of some of these clearly, but mineral identification is difficult montmorillonite-matrixed sandstones includes owing to alteration. Most phenocrysts which fresh hexagonal biotite flakes, apatite, con- can be identified are twinned plagioclase and siderable magnetite (many octahedron euhe- partly chloritized biotite. Other distinct pheno- dra), abundant euhedral zircon, brown horn- crysts less readily identified probably are un- blende, garnet, and sphene. There is also a twinned feldspars, chloritelike platy minerals, large amount of sodic plagioclase in the light- and brown to green alterations of ferromag- mineral fraction. Admittedly these might well nesian minerals. More common than the grains have been derived from an intrusive acidic having porphyritic textures are those with igneous terrain rather than a volcanic one. Yet unidentifiable microlites (feldspars?) in hyalo- the feldspar, biotite, apatite, euhedral mag- pilitic or, more commonly, trachytic textures. netite and zircon, and hornblende are present In the siliceous mudstones and sandstones of in some of the silicified tuffs. Weeks (1953) the Little Sheep member, and less commonly considers biotite, apatite, and euhedral zircon of the Himes member, silica precipitated around suites in the Brushy Basin member of the the clastic particles preserves their textures. Morrison formation to have been derived from Tuff and lava fragments, feldspar and biotite crystal tuffs. euhedra, and pumice ash and lapilli are visible Cretaceous formations succeeding the Clo- in some specimens. verly formation have received volcanic material In addition to the criteria mentioned, other also. The Thermopolis shale is a black ben- less certain evidences of volcanic additions to tonitic clayshale containing numerous individ- the Cloverly formation exist. Abundant in the ual bentonite beds; its middle "Muddy" sandy sandstones at the base of the Himes member in member contains abundant fresh biotite flakes particular, but also sparingly present through- and lithic volcanic grains. The silica of the out most of the Little Sheep and Himes sandy Mowry shale has been attributed to a volcanic mudstones, are clay masses of sand size with a origin (Rubey, 1929); moreover, the Mowry cellular or bubbly texture similar to what contains thick bentonite beds in the Bighorn Waters and Granger (1953, Fig. 3) called de- Basin. The Frontier formation contains vol- vitrified pumice, now altered to montmoril- canic grains, and the top of its Torchlight lonite. Other lithic fragments have indistinct sandstone member is a lag-gravel bed with textures which are similar to those of welded abundant pebbles and cobbles of andesite. tuffs, trachytic lavas, glass shards, and vitro- Other authors have described volcanic clastic tuffs; however, they may well be fine- contributions to these Bighorn Basin rocks or grained nonvolcanic rock, such as shales or their correlatives (e.g., Pierce, 1948, in south- phyllites. central Big Horn County, Wyoming; Knappen Another suggestion of volcanic debris is the and Moulton, 1931, bentonites and andesitic abundance of the mineral montmorillonite. sandstones and agglomerates in southern Montmorillonite in grains having relict vol- Montana; and Love et al., 1945, thin ben- canic or suspected volcanic textures was al- tonites from central Wyoming). In the Colorado ready mentioned. However, the greatest Plateau Waters and Granger (1953) have amount of montmorillonite as seen in oil-ground described the possible bearing of abundant thin sections occurs as textureless clay, either volcanic debris on the origin of the uranium massively developed in the mudstones or as ores.

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Deposition of the Cloverly formation oc- CLAY MINERALS: curred during a continuing orogeny on the X-ray-diffraction procedures of Johns, Grim, West Coast. The volcanic activity of that and Bradley (1954) were employed to deter- orogeny may well have been a source of much mine the mineral composition of the clay and of the windborne ash. finest silt-sized fraction of about 50 samples HEAVY MINERALS: from the Sundance, Morrison, Cloverly, Sykes Mountain, and Thermopolis formations. The most obvious feature of the minor Results from the same X-rayed slide varied accessory mineral suites of these formations is slightly on repeated runs (perhaps due to the abundance of opaque grains. Except in the interrupted use of the X-ray machine), so that conglomerates, these grains usually total more quantitative analyses by the Johns, Grim, and than 60 per cent of each sample. Next most Bradley method would probably be suspect. common minerals are zircon, tourmaline, and Nevertheless, the qualitative results are plotted garnet (Table 1). on Figure 11 so that the heights of the lines Of particular interest are the heavy minerals representing the various clay minerals are in the Himes member. As pointed out, the proportional to the amplitudes of the X-ray Himes lithic wackes had several accessory peaks for the basal spacings along the c axis minerals which strongly suggest volcanic of the clay minerals. In 2 8 angles for nickel- origin. Much of the sphene and leucoxene is filtered copper radiation, these were 5.2° for encrusted with pale-yellow needles of authi- glycolated montmorillonite, 6.1° for chlorites, genic rutile. These suites closely resemble those 8.8° for the 1-Md muscovites, or illite (Yoder of the middle sandy member of the Ther- and Eugester, 1955, p. 253; Foster, 1956, p. 64), mopolis shale. and 12.3° for kaolinite. No other clay minerals Channel-filling sandstones of the Himes were detected. member which trended east-west (Fig. 8) Nonclay minerals present in the fine fraction contain abundant staurolite, some garnet, and include chiefly quartz, some calcite, and rarely, leucoxene in amounts about equal to black feldspars, cristobalite, and hematite. opaque grains. In contrast, those sandstones Sandstone matrix from the uppermost with sources from the north or south have a Sundance formation, with glauconite carefully high ratio of leucoxene to black opaque grains excluded, contains only kaolinite and mont- and little if any staurolite or garnet. morillonite as layered silicates. If roundness of the suites is arbitrarily In summary (Fig. 11), Morrison clay is defined as the percentage of rounded and mostly illite. Montmorillonite is overwhelm- subrounded (including grains subsequently ingly abundant in the Cloverly formation, broken) tourmaline and zircon in all tourmaline except for the matrix of the chert-pebble con- and zircon grains, most of the samples are glomerates and channel-filling sandstones, and 60-85 per cent rounded. The less rounded for the Himes claystone, which is kaolinite. suites are from some of the Himes channel Samples from the Sykes Mountain formation sandstones, the Sykes Mountain formation, are mixtures of kaolinite and other clays. The the middle sandy member of the Thermopolis Thermopolis shales were mostly montmoril- shale, and especially the basal lithic wackes of lonite. the Himes formation, which are less than 10 Sedimentary rock families.—Some interest- per cent rounded on this arbitrary scale. ing features of the rocks of this study become Abrasion-resistant minerals, such as tourmaline, staurolite, sphene, and garnet (Pettijohn, most apparent when related rock types are 1957, p. 558-561), are more common in these compared with one another as well as with suites than the easily worn ones, such as recent sediments or older rocks described by monazite, kyanite, apatite, and rutile. An other authors. Rock nomenclature used gen- exception again is the basal lithic wackes of erally follows Gilbert (in Williams, Turner, and the Himes member of the Cloverly formation. Gilbert, 1954) except that the arenite-wacke Disregarding the minerals of more question- distinction is placed at 15 per cent matrix and able origin, as well as the authigenic minerals, all silt is considered matrix. most of the heavy minerals of these formations came from pre-existing sedimentary rocks; in a SANDSTONES AND CONGLOMERATES: few instances there were additions from mcta- Sandstones of the Morrison formation are morphic and pegmatitic rocks or from acidic predominantly calcareous quartz arenites to igneous rocks (Fig. 10). calcareous quartz wackes. Compared with the

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SYKES MOUNTAIN Sandstones, very- Poor in disturbed Chiefly angular, but Dull and etched C : quartz fine sand and C : siderite; iron ox- C: leucoxene; black opaque min- FORMATION fine-grained, other bedding ranging from sub- silt; kaolinite ides; calcite erals beds finer angular to sub- P.- illite R; montmorillonite (?); P: zircon; chlorite; rutile rounded; mean R : musco vite ; feld- pyrite; selenite; dah- R: garnet; tourmaline sphericity about spars; heavy min- lite; heavy minerals 0.76 erals; montmorillonite (?) CLOVERLY FOR- MATION Himes member Channel - filling Medium - grained; Very well sorted; Subrounded to sub- Dull and etched; less C: quartz fine sand P; iron oxides East-west North-south sandstones sand not peaked angular; mean commonly pitted R : musco vite ; ka- R: silica; calcite; man- C: leucoxene; C: leucoxene ; sphericities 0.76 to olinite; feldspars; ganese minerals at black opaque black opaque 0.89, average 0.81 heavy minerals tops of channels minerals; minerals staurolite P: tourmaline; zi- P: tourmaline; con zircon R: rutile; stauro- R: rutile; gar- lite; chloritoid; net; kyanite kyanite Mudstones Clay, with silt and Generally poor Sand grains well Some sand grains C : quartz silt and sand C: kaolinite Rare sand admixtures rounded to sub- etched P: feldspar; chert; P: iron oxides common rounded volcanic and other R; montmorillonite rock fragments R: kaolinite (?) Basal lithic Fine sand with 12- Poor; skewed to fine Angular; low to very Dull; nearly all C : quartz; feldspar; C: montmorillonite C: black opaque minerals, espe- wackes 40 per cent silty- fraction low sphericities etched chert; volcanic and P: iron oxides cially euhedral magnetite; leu- clay matrix other rock fragments coxene P: heavy minerals P: apatite; euhedral zircons; biotite; garnet R: sphene; angular zircon; detrital and authigenic rutile; brown hornblende; tourmaline

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/71/8/1137/3431920/i0016-7606-71-8-1137.pdf by guest on 29 September 2021 Pryor conglomerate Chiefly small peb- Good in individual Well rounded except Pebbles and granules C: quartz sand and P: calcite C : leucoxene ; rounded zircons; member bles to medium beds, but poor in that white chert is pitted to smooth; silt; dark-chert peb- R: silica; iron oxides black opaque minerals, especially sand; rare cobbles groups of beds; angular some larger ones bles, granules, and rounded ilmenite; tourmaline, to silt not skewed polished sand especially rounded P: white chert as cob- P: rounded rutile; other zircons; bles to medium sand garnet R: kaolinite; heavy R: staurolite minerals Little Sheep mudstone member Conglomeratic Chiefly granules to Very good in indi- Mostly well rounded, Quartz smooth to C: quartz sand and P: calcite; silica C: rounded zircon; leucoxene; black sandstones fine sand; pebbles vidual beds, fair in except that white etched; chert silt opaque minerals; other zircons uncommon groups of beds; chert is angular smooth to pitted, P: dark -chert granules P: tourmaline; garnet; collophane; skewed to fine with dull surfaces and sand rutile fraction R: white-chert gran- R: staurolite; brookite ules and sand; kaolinite; heavy minerals Mudstones Usually less than 1 Generally good in Ranges from well- C: quartz silt C: montmorillonite Generally rare, but in sand beds: per cent sand; dark beds rounded medium- P: quartz sand P: silica; calcite C: leucoxene; black opaque miner- silt/clay ratio grained sand to R: feldspars; kaolinite; R: iron oxides; barite; als; tourmaline variable angular silt volcanic and other selenite; iron sulfate P: zircon; garnet rock fragments; mus- R: apatite; rutile; staurolite; fluo- covite rite; anatase MORRISON FOEMA- TION Sandstones Fine sand; rarely Detrital grains are Chiefly subrounded Smooth to slightly C : quartz fine sand C: calcite C: leucoxeae; black opaque minerals medium - grained well sorted etched and silt P: iron oxides P: tourmaline; garnet sand or coarser P: chert sand; quartz- R : rutile; zircon ; bio tite ; horn- ite sand; illite blendes; spinel R: muscovite; collo- ph ane ; feldspars; kaolinite; heavy minerals Mudstones Generally silt/clay Fair Angular silt C: illite; quartz silt C: calcite C: leucoxene; black opaque min- ratio is 8 or 9 to 1 R: heavy minerals P: iron oxides erals; hornblendes R: selenite P; tourmaline R: apatite; rutile

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Cellophane, leucoxene, and rounded P rutile. tourmaline, and zircon. hi

O td W

'I in H TOTAL O 40% D 1% a

Tl 1 Thermopolis shale, muddy sand ' O 2 Sykes Mountain formation

3 Cleverly fm., Wimes mbr, channel sandstones from east - " " " " from north or METAMORPHIC AND PEGMATITIC i SOURCE " " basal lithic wackes Chlorite, chloritoid, dumortierite, Little Sheep mds. mbr., ss. and mds. M fluorlte, blue-green hornblende, IGNEOUS SOURCE chert-pebble conglomerates (volcanic or acid intrusive) O kyanite, sitfimanife, spinel, and Apatite, biotite, common and M O stourolite. " " Pryor conglomerate mbr. brown hornblende, sphene, and Morrison formation euhedral zircon. 2J Sundance formation M 01 k-t

X FIGURE 10.—HEAVY-MINERAL PROVENANCES Authigenic and undiagnostic species excluded

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other sandstones studied, those of the Morrison in the uppermost and lowermost beds. Perhaps are characterized by a high percentage of a calcite cement has been removed by present quartz grains and abundance of calcite cement weathering conditions. However, the freshest (PL 4, fig. 1). Compared with the Morrison rock observed, in the southernmost quarry near formation of the Colorado Plateau (Craig et al., Gypsum Creek (Fig. 8) has some silica, but no 1955), these rocks contain more authigenic calcite, cement. Aside from the lack of cement, calcite and less detrital feldspar and chert these sandstones resemble other rocks called than the Salt Wash member and no conglom- quartz arenites by Gilbert (in Williams, Turner, erate facies such as are present in parts of the and Gilbert, 1954) and orthoquartzites by Westwater Canyon and Recapture members. Pettijohn (1957). Sandstones in the Little Sheep mudstone The sandstones of the Sykes Mountain member of the Cloverly formation are chiefly formation are quartz arenites, commonly quartz arenites with varying admixtures of calcareous and ferruginous. These rocks chert pebbles, granules, and sand. Cements, generally resemble the quartz arenites of the where present, include calcite, silica, and iron Himes member of the Cloverly, although they minerals. Some finer-grained sandstones are tend to be better cemented, thinner-bedded, and wackes. Compared with the other rocks of this finer-grained. Sykes Mountain sandstones study, the Little Sheep sandstones are charac- resemble the thicker sandstone members of the terized by the common occurrence of silica South Platte formation (Waage, 1955), al- cement, by an intermediate degree of sorting, though they are apparently thinner and less and by the presence of rounded black- and widespread. angular white-chert grains (PL 4, fig. 4). Their Some sandstones, analyzed by the point- coarser facies closely resemble the Pryor count method, are plotted on Figure 12. member lithologies, of which they would be a part if lateral continuity could be established. MUDSTONES: The conglomerates and sandstones of the Fine-grained detrital rocks of the Morrison Pryor conglomerate member of the Cloverly formation are greenish brown and reddish formation (PL 4, fig. 3) are the coarsest-grained brown, slightly fissile, and universally cal- rocks described in this study and apparently careous. Bedding is common, but secondary are similar to the Cloverly conglomerate structures are very rare. They are similar to (Knappen and Moulton, 1931) north of the the mudstones of the Eocene Willwood forma- Pryor Mountains. Undoubtedly they are parts tion (Van Houten, 1948) and of the lower of the same unit. members of the Colorado Plateau Morrison Sandstones in the Himes member of the formation (Weeks, 1953; Waters and Granger, Cloverly formation are two distinct types, 1953). unlike most other sandstones studied. Lithic Mudstones of the Little Sheep mudstone wackes occur at the base of the member; member of the Cloverly formation are char- quartz arenites were deposited later. The acteristically bentonitic and variegated. Fis- wackes contain a montmorillonite-rich matrix. sility and primary structures are rare, whereas Some of the bubbly textured grains seen in secondary structures are abundant. Mudstones oil-ground thin sections may also be mont- of this type are found only very rarely in the morillonite. units above and below. The upper reddish-brown wackes contain These variegated rocks resemble the Brushy more cement (ferruginous and calcareous) and Basin member of the Morrison formation of less matrix than the lower olive-gray ones. the Colorado Plateau (Stokes, 1944; Craig et al., These sandstones closely resemble the middle 1955) and the Burro Canyon and Cedar sandy member of the Thermopolis shale and Mountain formations of the same area (Stokes, fit descriptions of some Morrison and Chinle 1944; 1952). In their colors and calcareous formation sandstones reported by Waters and nodule content, they resemble parts of the Granger (1953). If these rocks were more Oligocene formations of Nebraska, described firmly indurated and their matrix chloritized, by Schultz, Tanner, and Harvey (1955), who they would be graywackes (Gilbert, in Williams, suggested that these rocks are buried soils Turner, and Gilbert, 1954). (paleosols). Most of the very well-sorted quartz arenites Even more strikingly, these Mesozoic rocks in the Himes member are uncemented, except resemble soil profiles (Mohr and Van Baren, for some iron and manganese oxides, especially 1954) currently forming on slightly permeable

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I00,%*of sample. MONTMORILLONITE (5.2° glycol) \ f CHLORITE (erglycol) ./ -*"^ ix AMI ifcuvr' * M <•«« v^^Hcwhts of hnw proportionof to ^KAOLINITE (I231X 26 value, of principal peak, on X-ray spectrograms, CuKQ rad'n. \_ Key to (ample numbers

• J•IERII40POLIS SYKES MOUNTAIN. M f M 1 M M K K 1 f K it M Li C.L|' M L L i 1 I . .. L 1 _L£ 3._iL 4. hi 5.1 ll 6jJ_ 1 8. i a^ i - HJMES clayst jnes. pVERLY- M i. f I \< 1 K K K }{ M K M L M L M i 10 IL 13 Ii4 18.) | 19. 1 fl 4 quari c areniti>. matrbir -HIMES fcont ) M llthte wacke matrb K i M \ • M j-

I«ll C 2Q_ 21 \ L, 'fa 1 24 25! ilkelhC 2j? j —PRYOR — LITTLE SHEEP K ' K conglomerate K K ss. K! K ^tncrtflx ^ L matrix M 1 L 28M d 30. L 31 L 32 33. 1

M TLE S HEEP M(C jnt.) mudst >nes, M"/-' M | M [M M \ f K i f M K l L L rM, 36 kz 138 31 JJ4L 1 42 43 ORRISON SUNI?ANCE,, L L < h

\ K \ .. N 1 M 44. 45. 4el 47 48. !- J 50. FIGURE 11.—CLAY MINERALS KEY: CLOVERLY FORMATION THERMOPOLIS SHALE Himes Member 1. "Muddy sand", west of Cloverly 10. Claystone, upper Bear Creek 2. "Muddy sand", lower Bear Creek 11. Sandy mudstone, Shell dome 3. Clayshale, Shell dome 12. Claystone, west of Cloverly 4. Clayshale, west of Cloverly 13. Mudstone, Red Dome 14. Bentonitic mudstone, west of Cloverly SYKES MOUNTAIN FORMATION 15. Claystone, south of Cedar Creek 5. "Paper" shale, west of Cloverly 16. Uppermost claystone, Lovell Clay Products pit 6. Shale, southwest of Cloverly 17. Claystone, 2 feet below 16 7. Siltstone, upper Bear Creek 18. Claystone, 9 feet below 16 8. Shale, Shell dome 19. Claystone, 14 feet below 16 9. Basal claystone, Lovell Clay Products pit 20. Quartz arenite, south of Cedar Creek (Continued at the foot of next page)

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volcanic ash, under savannah conditions of clayshales, and yellowish-brown siltstones. As seasonal rainfall and hot temperatures, in pointed out, fragments of one lithology are seasonal lakes and swamps. For example, Mohr commonly incorporated in the other, and these and Van Baren (1954, p. 323-326, Fig. 66) siltstones and shales resemble recent tidal-flat describe a senile profile of black, montmoril- deposits. lonitic, noncalcareous clay, with filmy veinlets Hand samples of mudstones of the Morrison of calcium carbonate and calcareous nodules are commonly of but one lithology and color, (or "kanker") underlain by a white silica- whereas those of the Cleverly are of one lithol- cemented tuff. This association corresponds in ogy but with variegated colors and those of the every detail with the uppermost part of the Sykes Mountain formation are of two inter- Little Sheep mudstone member of the Cleverly mixed lithologies and colors. formation. Such soil does not develop a distinct profile because when the lakes dry up, large CHEMICAL ROCKS: deep cracks form, into which the surface clods, Chemical rocks of the Morrison formation as well as pebbles, bones, and other debris, are primary carbonates, ranging from nearly may fall, and thus be mixed at any horizon. pure limestones to calcareous sandstones and Lower mudstones of the Little Sheep se- mudstones. quence fit descriptions by Mohr and Van In the calcareous sandstones and sandy Baren of other similar soils that are somewhat limestones, calcite crystals poikilitically enclose less mature or were formed under different several quartz grains. In some limestones, conditions of drainage or on different parent polycrystalline calcite grains are about the ash. The pale-gray and pastel colors, the zones same size as the adjacent quartz grains, all of irregular chert or calcareous nodules, poikilitically enclosed by larger calcite crystals gypsum, sandstone or ash interbeds, and the (PI. 4, fig. 2). This texture suggests that much development of montmorillonite are all typical. of the calcite was fragmental and sorted with Equally striking is the similarity of the the detrital quartz. claystones and mudstones of the Himes member Authigenic silica is characteristic of the of the Cleverly formation to present soils also Cloverly formation, although calcite, iron forming under savannah climatic conditions but oxides, and some sulfates (barite and gypsum) developed on more permeable ash in better- are also present. In contrast to the uniformly drained environments (Mohr and Van Baren, 1954, p. 309-322, PL D, Table 98). Characteris- widespread distribution (distribution on a tic features of both are the reddish- and yellow- hand-sample scale is uneven) of calcite and ish-brown colors mottled with gray, the iron oxide in some units, silica is concentrated development of veinlets, threads, and hardpans in layers where it cements and replaces detrital of iron oxyhydrates, the absence of bedding, pebbles, sand, and mud. Similar "welded and the formation of kaolinite as the chief clay cherts" occur in the Morrison formation near mineral. Canon City, Colorado (Fredrickson, De Lay, The fine detrital rocks of the Sykes Mountain and Saylor, 1956; see also Ogden, 1954). Clays formation are gray shales grading to black of Cloverly mudstones are mostly authigenic.

21. Quartz arenite, Horse Center anticline 36. Uppermost mudstone, Cedar Creek 22. Quartz arenite, Cottonwood Creek (east rim) 37. Bentonite, upper Bear Creek 23. Quartz arenite, Gypsum Creek 38. Bentonitic mudstone, upper Bear Creek 24. Lithic wacke, road to Sunlight Basin 39. Siliceous mudstone, Shell dome 25. Siltstone, north end of Sheep Mountain 40. Bentonitic mudstone, Black Butte 26. Lithic wacke, west of Cloverly 41. Carbonaceous shale, Howard Gulch 27. Lithic wacke, Black Butte 42. Gastropod limestone, lower Crystal Creek Pryor conglomerate member 43. Mudstone in "kankar" nodule, Cedar Creek 28. Conglomeratic sandstone, Gypsum Creek MORRISON FORMATION 29. Conglomerate, east of Warren 44. Calcareous quartz arenite 30. Conglomerate, Line Creek 45. Calcareous mudstone, Cedar Creek Little Sheep mudstone member 46. Gastropod marlstone, East Fork of Sand Draw 31. Conglomeratic sandstone, Horse Center anti- 47. Calcareous quartz wacke, Red Dome cline 48. Calcareous mudstone, Sykes Mountain 32. Conglomeratic sandstone, Cedar Creek 33. Sandstone, Cedar Creek SUNDANCE FORMATION 34. Sandstone, Cedar Creek 49. Glauconitic sandstone, lower Bear Creek 35. Bentonitic mudstone, upper Bear Creek 50. Coquinoid sandstone, Sykes Mountain

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t-1 av

O W H S ><

w a

Feldspar grains. Unstable lithic fragments 8t mineral grains. Seleded Bighorn Basin Sandy Rocks: THERMOPOLIS SHALE CLOVERLY FORMATION MORRISON FORMATION HIMES MEMBER LITTLE SHEEP MUDSTONE I MIDDLE SANDY MEMBER W MEMBER KH 1,2. Channel-filling sandstones 23 ( Figure 2 of Plate 4), O w 5,6,7,8. 15 (Figure 4 of Plate 4), 24 (Figure I of Plate 4). o 16, 17, 18, 19, 20, 21. X) Mudstones SYKES MOUNTAIN FORMATION 9, 10. fcd

Basal sandstones PRYOR CONGLOMERATE 3 (Figure 4 of Plate 5), MEMBER II (Figure 3 of Plate 5), 4. 12, 13, 14. FIGURE 12.—SANDSTONE COMPOSITION AND TEXTURE RELATED TO MATURITY AND FLUIDITY (Construction of diagram and nomenclature of sandstones based largely on Figures 96 and 97 of Gilbert in Williams, Turner, and Gilbert, 1954.)

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Limestone occurs as a few primary beds but (1953) for streams in semiarid regions and of more commonly as accretionary bodies. Leopold and Miller (1956) for ephemeral Authigenic iron minerals are very common in streams in New Mexico show velocities for the Sykes Mountain formation. Ironstone beds streams which have the necessary range of are abundant, and the hematite or limonite pres- values. ent is a replacement of earlier siderite (PL 5, Hjulstrom's results are interesting when fig. 4). applied to the lowest beds of the Himes member, which are sandstones lying on a slightly Petrogcny wavy upper surface of Little Sheep mudstones, with no significant erosional interval. These Morrison formation.—Illite in sedimentary lithic wackes could have been transported and rocks may have formed in marine environments deposited by streams whose velocities (about or may have been derived from weathering of 7 cm/sec) would have been much too slow to pre-existing marine shales (Van Houten, 1953). have eroded the underlying dark-gray mud- Keller (1953) has suggested that illite may also stones (about 60 cm/sec necessary). be formed in lakes with unique concentrations Conditions of fluidity which produce cross- of salts and cites the occurrence of illite in bedding also produce good sorting. Yet these lacustrine (charophyte-bearing) beds of the poorly sorted lithic wackes at the base of the Morrison formation of the Front Range. Himes member commonly show cross-bedding. Although a bed containing nonmarine, non- If much of their montmorillonite matrix, as pulmonate gastropods in the Morrison forma- seen today, had originally been vitric lapelli in tion of the present study contained illite, one a better-sorted sediment, this association with gastropods in the Little Sheep mudstone would seem less contradictory. member of the Cleverly contained mont- The resemblance of Cloverly rocks to some morillonite. Moreover, illite occurs in the tropical soils, noted heretofore, probably in- Morrison stream-channel deposits as well as in dicates a tropical-weathering origin. If the the lacustrine beds. This suggests that illite Cloverly rocks are attributed to katamorphism in the Morrison of the Bighorn Basin has been of ash, some further observations of Mohr and reworked from older illite-bearing rocks. Van Baren (1954, Chapters IX, X) about the Probably most of the red pigment of the genesis of present-day soils suggest parent Morrison mudstones was inherited either from material, drainage, climate, and other factors thoroughly weathered rocks in the source area of Cloverly deposition. or from existing red rocks there. Iron minerals Such features as lime concretions, color of the red muds deposited in lakes, swamps, mottling in black, olive-green, yellow, and violet and streams were reduced to green colors, colors, siliceous layers, and montmorillonite whereas red colors were retained in the over- clay are attributed to hot, swampy and lacus- bank beds (see also Van Houten, 1948). trine environments which may (black) or may Cloverly formation.—According to the dep- not (other colors) dry out and be aerated in ositional curves of F. Hjulstrom (in Trask, the dry season when alkaline ground water 1939), and assuming generally no turbidity ascends. This conclusion fits well with the flows, streams that eroded the earlier-formed observed lithologies of the Little Sheep mud- Little Sheep mudstones and transported the stone member. chert pebbles and granules in the Little Sheep With more permeable parent material, such and Pryor members probably moved at veloci- as stream-deposited volcanic or tuffaceous ties greater than 100 cm/sec (3.2 ft./sec.). Some sandstones or coarser pyroclastic rocks in a interbedded fine sands in even laminae pre- hot region of seasonal rainfall and flooding, sumably were deposited at less than 5 cm/sec. having good ground-water movement or wet- A similarly great difference between a 100 season drainage and dry-season ascent pro- cm/sec velocity presumably necessary to ducing "amphibious" weathering, the soils are erode channels in the clays of the Himes characterized (in the terminology of Mohr and member and the 10 cm/sec velocity necessary Van Baren) by streaky threads, hardpans, to permit deposition of sands in these channels veins, and small concretions of iron oxyhy- indicates a large variation of stream discharge, drates, mottled grayish-yellow colors, and the probably of seasonal nature. Data for velocity development of kaolinite. These features are of savannah-type streams are not available, all conspicuous in the Himes member. but the studies of Leopold and Maddock Sykes Mountain formation.-—Primary sid-

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erite, indicated by rhombic outlines of the the climate in the depositional area. There was secondary iron oxides in thin section (PL 5, at least enough moisture for development of fig. 4), indicate a depositional environment in lakes and rivers, with a fauna of dinosaurs, a slightly negative Eh and pH less alkaline crocodiles, and turtles. Plant fossils, normally than 7.8, with considerable ionic or colloidal good climatic indicators, are rare, but there iron present, according to the calculations of were trees over 55 feet high, and swamps Krumbein and Garrels (1952). The interpreta- supported rushlike plants. However, gypsum tion of reducing environment is supported by and calcite indicate times of dryness. If much the presence of pyrite and carbonaceous plant of the Cloverly formation was developed as remains, although some aeration must have successive soil profiles, analogy with present been possible in beds reworked by burrowing soils indicates they were formed in a climate organisms. Of the mixture of clay minerals of hot year-round temperatures and moderately present, kaolinite is most common, as might be high rainfall interrupted annually by a 2- or expected. The montmorillonitic shales of the 3-month dry season, that is, a savannah overlying Thermopolis shale indicate the climate. A postulated climate of this type is change to less-reducing conditions. In either compatible with all other observed features of formation the lack of illite, so common in these formations, whereas proposals of other marine rocks, perhaps was due to a high con- climatic types are inadequate on one or more centration of ferrous iron (or its proxy, mag- points. nesium) or to a lack of potassium ions (Mohr It cannot be determined whether this and Van Baren, 1954, p. 204-209). savannah climate was due to the summer shift Alkalinity and increased oxidation potential poleward of the wet unstable equatorial air during later diagenesis is evidenced by oxide masses alternating with the winter shift replacement of siderite, and later calcite as equatorward of the dry stable subsiding trade cement and veinlets. winds or, as suggested by Sheldon Judson Physical environment.—Morrison and Clo- (Personal communication, 1956), to a monsoon verly-type deposition was extensive, affecting cycle of wet summer air masses moving onto an area about 1400 miles north to south the continent, and dry winter air moving out and about 800 miles east to west. This suggests of the continent. The postulated paleogeog- that the area was near sea level; today's raphy would be acceptable to either theory. hypsometric curve shows very little of the The general environmental conditions out- earth's surface more than a few hundred meters lined occur today in the Gran Chaco of Para- in elevation, and the higher elevations are guay, northern Argentina, and southeastern nearly all areas of rugged relief and erosion. Bolivia. The available descriptions of the Chaco These extensive formations suggest a broad also suggest additional detailed features that area of low relief, with aggradation in shallow may have prevailed during Morrison and lakes and rivers. Cloverly time.1 It is known that the seas existed both to the The Gran Chaco is a vast lowland plain north and to the southeast of the region. The below 700 feet elevation in most places, rising western land area of deeply weathered quartz, westward 8-10 inches per mile to the base of chert, and illite-bearing sedimentary rocks had the Andean foothills, and extends about 1400 behind it an orogenic and volcanic terrain. by 700 miles. It is a region of scrub forest The mature sediments of the Morrison and interspersed with grassy savannahs, lakes, and most of the Cleverly were derived from the fresh-water swamps, but with a pronounced former provenance, the Cleverly and later winter dry season, characterized by parched volcanic pyroclastic and epiclastic sediments and brown landscape and cracked soil. During from the latter. To the east and northeast the this season all but two rivers are intermittent continent was low-lying, probably chiefly and water becomes too saline to drink (Grubb, covered with sedimentary rock, but perhaps 1919). In the middle of the spring the rains the Canadian Shield was exposed in part. There start, and by summer vast areas in the eastern were no "Ancestral Bighorns", because pebbles portions are flooded a. foot or more in depth; of quartzite, phosphate, chert, red sandstone, in the somewhat drier west, flooding occurs granite, and amphibolite, and feldspar grains only near the courses of the few streams. are missing in the easterly derived Himes channel fillings. 1 Lugn (1941) suggested a Chaco-Hke environ- Some direct inferences may be made about ment for some Miocene beds of the High Plains.

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Rivers follow braided and shifting courses, authors have attempted to show a relationship, changing position each year during high there is not yet a firm basis for the existing water, sometimes changing the patterns of schemes relating sedimentation and tectonics their channels radically (James, 1942, p. 308- because none of the current tectonic syntheses 309). Carlson (1936, p. 166) describes the fit the recent discoveries and interpretations Argentine Gran Chaco as follows: about the nature of the Earth's crust and its structures or recently revised opinions of "The Gran Chaco . . . Plain . . . filled with debris from adjacent highlands . . . most . . . apparently geologists about the most probable types of transported from the Andes on the west . . . Only a tectonic processes. Nevertheless, the following few streams make their way across the flat land . . . generalities are valid. The Bighorn Basin Along major rivers are broad swamps, or malezais, deposits of this report accumulated on a stable and lagoons, or lagunas. During the rainy summers the flood waters convert these features into great continental crust in a nonmarine environment lakes and completely obscure the drainage lines. In which prevailed from the retreat of the epeiric places are great depressions, called banados, that Sundance sea until the return of the Cretaceous are filled with water during the time of flood, but seaway. Significantly, during this late Jurassic are baked mud flats in dry seasons. Some of the deeper banados contain water at all times." and early Cretaceous interval, a very intense orogeny was affecting the area 600 or so miles Carlson continues (p. 209) about the Para- to the west. There was no tectonic control of guayan Gran Chaco: sedimentation by structural elements now related to the Bighorn Basin. Shortly after this "The Chaco landscape contains forested areas, time the region was broadly downwarped, and great swamps, extensive plains studded with the later still the Laramide orogeny occurred. fan-leaf palm . . . and areas of desert plants. Most of the region is flat with a mean height above sea- level of about 450 feet. The surface soil is void of GEOLOGIC HISTORY stones; it is sandy-textured in the southern part and heavier, or more clayey, nearer the Paraguay An extensive low-lying region, part of the River. present Western Interior of the United States, "The annual precipitation at Chaco has been estimated at about 54". Drought and abnormal was exposed in late Jurassic time as a result of floods are frequent. In the south the average summer the regression of an epeiric sea in which the temperature is about 72°F., and in the north about Sundance and related formations had been 75°F. However, the temperature in summer often deposited. The present Bighorn Basin was a rises above 110°F., while in winter it falls below freezing. Probably the most distressing climatic small part of this region. Aggradation com- conditions are the hot north winds that blow quite menced in rivers and on their flood plains. frequently during the summer." Fine-grained quartz sands and red muds, derived from lateritic weathering of Paleozoic Ahlfeld (1946, p. 52-53) points out that the marine shales and sandstones exposed to the Chaco in Bolivia is covered with alluvium west, were deposited together with minor derived from the Andes and from the Brazilian additions of volcanic debris. Because the Shield. Some magnesium sulfate lakes exist deposition was fairly continuous, the deposits where rainfall is slight. were buried rapidly enough to inhibit modi- The late Quaternary and Recent sediments fying effects of the depositional environment of the southern Chaco are described by Hager- except for destruction of most organic and the man (1936, p. 159-160, PI. 3) as orange, brown, more labile volcanic components and reduction and in some places reddish, poorly sorted sandy of part of the red coloring. Muds of the over- clays. He believes that some types of strati- bank deposits remained red. The climate was fication indicate uniform, lacustrine sedimen- hot with fairly high rainfall interrupted by a tary conditions, whereas other deposits are brief, but pronounced, dry season during which fluvial. In general these sediments are less than small ephemeral lakes and swamps became 20 m thick and were derived from Triassic highly alkaline, with deposition of calcium red sandstones, with some Andean airborne carbonate aided by algae. volcanic ash. These deposits are underlain by After this early nonmarine period of dep- about 15 m of early Quaternay conglomerates, osition which produced the Morrison forma- which unconformably overlie late Tertiary tion, aggradation diminished significantly, so brown conglomeratic sands and clays washed that only minor amounts of stream-borne from the Andes. sediments reached the area, but the volcanic- Tectonic environment.—Although numerous ash contribution increased. As the drainage

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deteriorated, a system of vast seasonal lakes ended with the first advance of the Cretaceous and swamps developed where the ash weathered sea, represented by paralic deposits of the Sykes to montmorillonite and formed siliceous Mountain formation. These sediments were hardpans and kankar nodules. During the chiefly organic muds, continuously sorted by dry season the soil cracked, and ascending tidal activity into thin separate beds of clay and alkaline solutions deposited gypsum. Few silt, while just as regularly disrupted by organic remains were preserved in this regimen, burrowing organisms and more violent tidal except for plants in peat-forming swamps, some scour. Sands were spread as sheets over parts wood which became petrified, and some dis- of the area from time to time. Circulation to articulated bones and other hard parts of the open sea was restricted; rivers crossing the reptiles. nearby, deeply leached land surface provided The rocks of the Little Sheep mudstone iron-rich waters to a reducing environment in member of the Cleverly formation formed from which siderite and kaolinite were deposited in the subaqueously and amphibiously weathered the fine sand. ash. In the middle of the weathering interval Marine conditions gradually became more there was slight tectonic activity west of the dominant. As the supply of coarse detritus area, so that streams of increased gradient, diminished and deposition of black muds carrying chert pebbles and quartz and chert predominated, a new formation, the Thermop- sand into the area from the west, were also olis, accumulated. able to rework the deposits formed immediately before. These new coarse sediments, part of REFERENCES CITED which are the Pryor conglomerate member of the Cleverly formation, were mixed with Ahlfeld, F., 1946, Geologia de Bolivia: La Plata, quartz sands derived from an eastern source Univ. Nac., Mus., Rev. n. s., torno 3, no. 19, p. 5-370 and partly reworked along the lake shores. Allen, V. T., 1944, Sedimentary and volcanic Additional ash falls and stream deposition processes in the formation of high-alumina of quartzose sands intermittently interrupted clays (Abstract): Econ. Geology, v. 39, p. 85 weathering processes. This episode ended with Andrews, C. A., Pierce, W. G., and Eargle, D. H., the longest period of uninterrupted weathering, 1947, Geologic map of the Bighorn Basin, Wyoming and Montana, showing terrace with the formation of a thick black bentonitic deposits and physiographic features: U. S. soil characterized by an extensive siliceous Geol. Survey, Oil and Gas Inv., Prelim. Map 71 hardpan. Ashley, G. H., et al., 1933, Classification and nomen- A later period of slight aggradation char- clature of rock units: Geol. Soc. America Bull., v. 44, p. 423-459 acterized by a different type of weathering Baker, A. A., Dane, C. H., and Reeside, J. B., Jr., then commenced, forming the Himes member 1936, Correlation of the Jurassic formations of of the Cleverly formation. This change ap- parts of Utah, Arizona, New Mexico, and parently was due to better drainage conditions. Colorado: U. S. Geol. Survey Prof. Paper 183, 66 p. First to be deposited were slurries of volcanic Barrell, J., 1917, Rhythms and the measurements of debris, (including crystal, vitric, and lithic geologic time: Geol. Soc. America Bull., v. 28, tuffaceous sands) mixed with finer ash and p. 745-904 quartzose and chert sand. Succeeding pyro- Blackstone, D. L., Jr., and Sternberg, C. W., Editors, 1947, Guidebook, 2d annual field conference in clastic and epiclastic sediments of the same the Bighorn Basin, August 5-8, 1947: Wyo. types were affected by long periods of weather- Geol. Assoc. and Yellowstone-Bighorn Research ing in seasonal swamps and lakes, but the Assoc.. 277 p. sediment was porous enough so that leaching Bokman, J., 1956, Terminology for stratification in sedimentary rocks: Geol. Soc. America Bull., was more thorough, and kaolinite was formed, v. 67, p. 125-126 with iron oxyhydrates and oxides precipitated Brown, B., 1933, Stratigraphy and fauna of the in veins and hardpans. Fuson-Cloverly formation in Montana, Wyo- From time to time rivers traversed the area, ming, and South Dakota (Abstract): Geol. Soc. America Bull., v. 44, p. 74 generally flowing from the northeast and east, 1935, Sinclair dinosaur expedition, 1934: Nat. carrying first- and second-cycle quartz sands History, v. 36, p. 2-15 from the metamorphic terrain of the Canadian Brown, R. W., 1946, Fossil plant and Jurassic- Shield, and covered Shield sandstones of the Cretaceous boundary in Montana and Alberta: Am. Assoc. Petroleum Geologists Bull., v. 30, Mid-Continent. These sands were deposited p. 238-248 in the channels in clays forming at that time. Carlson, F. A., 1936, Geography of Latin America: This long period of nonmarine sedimentation New York, Prentice Hall, 642 p.

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Ross, C. S., and Hendricks, S. B., 1945, Minerals Front Range foothills, Colorado: U. S. Geol. of the montmorillonite group, their origin and Survey Prof. Paper 274-B, p. 15-51 relation to soils and clays: U. S. Geol. Survey Waage, K. M. 1958, Regional aspects in Inyan Prof. Paper 205-B, p. 23-79 Kara stratigraphy, p. 71-76 in Strickland, Ross, C. P., Andrews, D. A., and Witkind, I. J., J., et al., Editors, 13th annual field conference 1955, Geologic map of Montana: U. S. Geol. guidebook:Wyo. Geol. Assoc., 341 p. Survey Waldschmidt, W. A., and LeRoy, L. W., 1944, Rubey, W. W., 1929, Origin of the siliceous Mowry Reconsideration of the Morrison formation in shale of the Black Hills: U. S. Geol. Survey, the type area, Jefferson County, Colorado: Prof. Paper 154-D, p. 153-170 Geol. Soc. America Bull., v. 55, p. 1097-1114 Schultz, C. B., Tanner, L. G., and Harvey, C. H., Washburne, C. W., Gas fields of the Big Horn 1955, Paleosols of the Oligocene of Nebraska: Basin, Wyoming: U. S. Geol. Survey Bull. Univ. Nebr. State Museum Bull., v. 4, 15 p. 340, p. 348-363 Simpson, G. G., 1926, The age of the Morrison 1909, Coal fields of the northeast side of the formation: Am. Jour. Sci., 5th ser., v. 12, p. Bighorn Basin, Wyoming, and of Bridger, 198-216 Montana: U. S. Geol. Survey Bull. 341-b, Spalding, R. W., Editor, 1952, Guidebook, 7th p. 165-199 annual field conference, Southern Big Horn Waters, A. C., and Granger, H. C., 1953, Volcanic Basin, Wyoming, July 31-August 2, 1952: debris in uraniferous sandstone and its possible Wyo. Geol. Assoc., 180 p. bearing on the origin and precipitation of Stokes, W. L., 1944, Morrison formation and related uranium: U. S. Geol. Survey Circular 224, 26 p. deposits in and adjacent to the Colorado Weeks, A. D., 1953, Mineralogic study of some Plateau: Geol. Soc. America Bull., v. 55, Jurassic and Cretaceous claystones and silt- p. 951-992 stones from western Colorado and eastern 1952, Lower Cretaceous in the Colorado Utah: U. S. Geol. Survey TEI Report 285 Plateau: Am. Assoc. Petroleum Geologists Williams, H., Turner, F. J., and Gilbert, C. M., Bull., v. 36, p. 1766-1776 1954, Petrography: San Francisco, W. H. Straaten, L. M. J. U. van, 1951, Texture and Freeman and Co., 406 p. genesis of Dutch Wadden Sea sediments: Wilmarth, M. G., Compiler, 1925, Tentative corre- 3d Internal. Cong. Sedimentology Proc., lation of geologic formations in Wyoming: p. 225-244 U. S. Geol. Survey Comm. Geol. Names, Chart 1954a, Sedimentology of recent tidal flat Wilson, C. W., Jr., 1936, Geology of Nye-Bowler deposits and the Psammites du Condroz lineament, Stillwater and Carbon counties, (Devonian): Geol. Mijnb., new ser., Jaarg. 16, Montana: Am. Assoc. Petroleum Geologists p. 25-17 Bull., v. 20, p. 1161-1186 1954b, Composition and structures of recent Yen, Teng-Chien, 1949, Review of the Lower marine sediments in the Netherlands: Leid. Cretaceous fresh-water molluscan faunas of Geol. Meded., v. 19, p. 1-110 North America: Jour. Paleontology, v. 23, Thompson, W. O., 1937, Original structures of p. 465-472 beaches, bars, and dunes: Geol. Soc. America 1952, Molluscan faunas of the Morrison for- Bull., v. 48, p. 723-752 mation: U. S. Geol. Survey Prof. Paper 233-B, Trask, P. D., Editor, 1939, Recent marine sediments, p. 21-51 a symposium: Am. Assoc. Petroleum Geologists, Yoder, H. S., Jr., and Eugster, H. P., 1955, Syn- Tulsa, 736 p. thetic and natural muscovites: Geochim. Van Houten, F. B., 1948, Origin of red-banded Cosmochim. Acta, v. 8, p. 225-280 early Cenozoic deposits in the Rocky Mountain Ziegler, V., 1917, The Byron oil and gas field: region: Am. Assoc. Petroleum Geologists Bull., Geol Survey Wyo., Bull. 14, p. 181-207 v. 32, p. 208372126 1953, Clay minerals in sedimentary rocks and UNIVERSITY OF HAWAII, HONOLULU, HAWAII derived soils: Am. Jour. Sci., v. 251, p. 61-82 MANUSCRIPT RECEIVED BY THE SECRETARY or THE Waage, K. M., 1955, Dakota group in northern SOCIETY, JUNE 6, 1958

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