yy a a ie. gts : 2 eo Pores Dea a ibe DAMEN Se HARVARD UNIVERSITY LO uf ES LIBRARY OF THE Museum of Comparative Zoology ca — ~ —s —" as _ — ae - a - — = a 7 ‘ F sto OCT:1 8 1960! pie a fH) Peabody Museum of Natural Histo Wi SITY | Yale University Cae Bulletin 15 Stratigraphy and Micropaleontology of the Thermopolis Shale by Don L. Eicher PEABODY MUSEUM OF NATURAL HISTORY YALE UNIVERSITY BULLETIN 15 Stratigraphy and Micropaleontology of the Thermopolis Shale BY DON L. EICHER Department of Geology, University of Colorado NEW HAVEN, CONNECTICUT 1960 Printed in the United States of America HAR: il Copyright Peabody Museum, Yale University New Haven, Connecticut, 1960 CONTENTS Abstract Introduction Area The problems Nomenclature Stratigraphy Summary of previous work Nomenclature Stratigraphic descriptions Thermopolis shale Rusty beds and the basal contact Lower shale Middle silty shale Upper shale Type section Fossils Muddy sandstone Fossils Shell Creek shale Fossils Bentonite beds Regional relationships Relations to the Colorado Front Range Thermopolis shale Muddy sandstone Shell Creek shale Relations to the Black Hills Thermopolis shale Muddy sandstone Shell Creek shale Relations to western Wyoming Thermopolis shale Muddy sandstone and Shell Creek shale Paleogeography Thermopolis shale Rusty beds Basal portion of the rusty beds Lower shale Middle silty shale Summary of pre-upper shale deposition Upper shale Foraminiferal ecology Summary of upper shale deposition Muddy sandstone Shell Creek shale The Mowry contact SNNNN eS ew iv STRATIGRAPHY OF THE THERMOPOLIS SHALE Paleontology Previous work Summary of microfaunas Field and laboratory methods Sample data Systematic descriptions Locality register Measured sections References cited Index Plates 123 at back of book ILLUSTRATIONS Figures “ID Ot — OOF DD = 8. 9. Thermopolis faunal changes between Big Horn Basin and northern Black 10. HL: 2: Tables ie 2; Plates i D Ot Of HO . Index map . History of terminology . Isopach map of the Shell Creek shale . Type locality of Shell Creek shale and reference locality of Muddy sandstone . Correlation of a Shell Creek bentonite bed . Correlation between Big Horn Basin and Colorado Front Range foothills . Thermopolis faunal changes between Big Horn Basin and Colorado Front Range foothills Correlation between Big Horn Basin and Black Hills Hills Drainage direction in western Kansas prior to Lower Cretaceous marine transgression Size distribution of Trochammina depressa Size distribution of Bimonilina variana Content of samples from half-foot intervals below and above five bentonites Tabulation of stratigraphic intervals of samples studied for microfossils Exposures of the Cloverly, Thermopolis and Shell Creek formations at back of book . Variations in rusty beds and correlatives at back of book . Saccamminidae, Tolypamminidae and Lituolidae at back of book . Lituolidae and Textulariidae at back of book . Verneuilinidae and Rzehakinidae at back of book . Rzehakinidae, Trochamminidae and Theophormididae at back of book ee he? “7 oy a i ORES, (2 ee . envi rete tt ‘ bs mT? it Aa i , y4 1 O)e8? 9 ay we tiiean =F F é bmhity ioe: note iii f are ats 1 ony a?» has of ? f oooh Cee op Saath i iper | lees? §.¢ ‘ f ; ht Vir idee eye i) rr | st Ongar ‘Meee, ip—e Yaa) ¢@ «'? B 5 q tf ayeey i iv. Ofa¢ § 7s 4 aten 0 han ~ ie mois Ae4 ‘ ‘v lar SV of - + med i ¢ ay ABSTRACT In the Big Horn Basin of Wyoming, the Thermopolis shale (Lower Creta- ceous) of U. S. Geological Survey usage consists of three mappable units; a lower sequence of black shale and prominent siltstones, the Muddy sandstone, and an upper sequence of black shale and a few bentonites. This paper restricts the term “Thermopolis shale” to only the lowest of these three units in conformity with local usage, proposes the new name Shell Creek shale for the upper unit, and recognizes all three, the Thermopolis, the Muddy, and the Shell Creek, as forma- tions. In the Big Horn Basin, the restricted Thermopolis shale consists of four informal members: the rusty beds at the base, a lower black shale, a thin silty shale, and an upper black shale. The uppermost few feet of the underlying Cloverly formation immediately beneath the contact with the rusty beds com- monly contain tiny siderite spherulites which weather to iron oxide. Identical spherulites occur regionally at a similar stratigraphic position; they formed in environments accompanying the initial transgression of the Cretaceous sea. Stratigraphic evidence indicates that the three lower members of the Ther- mopolis were deposited in an arm of the boreal sea which extended deep into the western interior. The upper shale of the Thermopolis contains twenty-four species of arenaceous foraminifera; five of these are new and the remainder include some of the Gulf Coastal province and others of the boreal province, indicating a joining of the two seas. For the first time, an interior seaway extended the length of the North American continent. The limited number of kinds of foraminifera, as well as molluscs, in the seaway was caused by low salinity. The two distinct biofacies in the seaway may also have resulted from differing salini- ties. A paucity of individuals in some marginal areas of the seaway resulted from a lack of oxygen on the sea floor. The Muddy sandstone locally consists predominantly of shale and siltstone. In spite of great lateral variations in thickness, in sequence, and in rock type, the Muddy is a laterally persistent, widespread unit which occupies just one stratigraphic interval and which records a single depositional episode. It was deposited over a large area in a variety of local, very brackish, extremely shallow water environments. The Shell Creek shale represents renewed transgression of the boreal sea. It contains a new radiolarian species and twelve species of arenaceous foraminifera; four are new and the remainder are known only from the boreal province; three also occur in the Thermopolis. The depositional environment was probably similar to that of the upper shale of the Thermopolis. Its contact with the over- lying siliceous Mowry shale is laterally persistent; the two units are not facies of one another. South and east of the Big Horn Basin, the Shell Creek shale thins greatly and has not been mapped, although it can be distinguished in good exposures and on electric logs. In southern Wyoming, it thins out and is over- lapped by the siliceous Mowry shale. INTRODUCTION It is a pleasure to acknowledge the following persons and organizations whose help in making this study possible is sincerely appreciated: Dr. K. M. Waagé suggested the problem, directed the research, and generously gave many hours of encouraging guidance and capable advice. Dr. C. O. Dunbar and Dr. J. T. Gregory gave helpful suggestions on various aspects of interpreta- tion and presentation. Audrey Eicher ably assisted both in the field and in the laboratory, and in addition, typed the manuscript. D. S. Barker and A. T. Oven- shine each assisted for a few days in the field. A National Science Foundation Fellowship provided financial support for the study throughout its duration, and the Carter Oil Company lent electric logs. This report was presented as a dissertation for the degree of Doctor of Philos- ophy at Yale University. AREA In Wyoming, the rocks that lie above the Cloverly formation and below the Mowry shale of Darton are included in the Thermopolis shale by the U. S. Geo- logical Survey. These rocks are Lower Cretaceous in age, and they represent the earliest marine deposits of the great Cretaceous interior sea in this region. Throughout most of their outcrop area, the gray and black shales of the Ther- mopolis are poorly exposed, forming valleys or vegetated slopes. In a few excep- tional places such as the northeastern flank of the Big Horn Basin, however, they crop out in barren badlands where detailed study and thorough sampling are possible. Here their total thickness is about 600 feet. Most of the field work was concentrated in this area and in the area of the type Thermopolis on the southern flank of the basin. Field work was also extended out of the basin to help establish correlation with other areas. The field studies occupied the summer of 1956 and part of the summer of 1957. All localities visited are shown in figure 1; most represent only partial sections, and most were sampled for microfossils. THE PROBLEMS NOMENCLATURE The first problem encountered in the sequence of beds between the Cloverly formation and the Mowry shale is that of an entangled nomenclature. The multi- plicity of meanings for the names has nearly defeated its basic purpose—to replace briefly and clearly a lengthy descripition. As of now, when using the name Thermopolis, for example, one must either specify whose usage he is following, or else leave his reader with a question as to just what rocks he is talking about. Perhaps there is no one best way to apply the names within this sequence, but a standardization—any standardization—of the names would have the obvious advantage of precision in thought and expression. If, in addition, the named units were lithogenetic as well as utilitarian, regional geologic history would be more easily comprehensible. INTRODUCTION 3 SOUTH ase DAKOTA *GREYBULL x 24 BUFFALO 226 eA TENSLEEP 15 3 CLIFTON * JACKSON xHOT SPRINGS 17 . *ARMINTO *DOUGLAS ,ALCOVA 3) 28 | . | | © 10 20 30 40 —— MILES 29 ° *RAWLINS *ROCK RIVER "ROCK SPRINGS IRON MTN. » CHEYENNE COLORADO e *FORT COLLINS Figure 1. Index map. Localities visited are marked by dots. X’s represent towns. For key to localities, see locality register, p. STRATIGRAPHY The following are the principal problems which will be discussed in this report: The Thermopolis shale contains a microfauna which has not been previously described. Inasmuch as macrofossils are generally rare in the Lower Cretaceous marine rocks of the western interior and especially so in most of Wyoming, the microfossils are of particular value in filling certain gaps in the knowledge of the paleogeography. During Early Cretaceous time, the western interior of North America was occupied by the arms of two great seas; one extended south from the boreal regions, and the other extended north from the area of the Gulf of Mexico. The two seas were inhabited by distinctly different contemporaneous faunas, so that their distinctive fossils help to chart their respective movements. Which was the first to reach the western interior? When were the two seas first joined? What was their nature? The detailed stratigraphic relationships of the Thermopolis shale of the Big Horn Basin with the rocks in other areas have not been previously studied. A disconformity indicating the first transgression of the Early Cretaceous sea was previously recognized in the Colorado Front Range foothills (Waagé, 1955). Above it lie marine and marginal marine rocks deposited under the influence of the transgressing sea; below it lie rocks deposited entirely in nonmarine environ- ments. This two fold arrangement provided a natural framework for mapping 4 STRATIGRAPHY OF THE THERMOPOLIS SHALE the strata of the Front Range Dakota group. Might not a similar arrangement of marine and marginal marine rocks above and nonmarine rocks below continue northward across Wyoming, and even over a large part of the western interior? If so, it would help provide a meaningful classification of the rocks. The Muddy sandstone, which lies near the middle of the Thermopolis shale of the U. S. Geological Survey, is an important unit recognizable over a large area. Yet, many workers have relegated the Muddy to member, or even lesser status. Perhaps this is due, in part, to the stigma attached to formal recognition of a term which has never been formally proposed and defined, but it may be due also to the suspicion by some geologists that the term Muddy has been indis- criminately applied, not to a single stratigraphic unit, but to any prominent lens or tongue of sandstone in the entire Thermopolis sequence. Thus, the nature of the Muddy and its relationship with the Thermopolis need clarification. The black shale unit above the Muddy sandstone and below the siliceous Mowry shale of Darton has been previously mapped separately in some areas, but in other areas where it is less easily distinguished, it has been included with the overlying siliceous Mowry shale and their gradational relationship has been stressed. The exact relationship of these units is a problem. Might they be in part contemporaneous facies? SUMMARY OF PREVIOUS WoRK The nomenclature of the Thermopolis shale of the U. S. Geological Survey usage and its regional correlatives has become unnecessarily complicated. In some places other than the Big Horn Basin this is a result of too many names or of inept application of ‘foreign’? names, many of which, to make matters worse, are surrounded with confusion in their native areas. In and around the Big Horn Basin, however, most nomenclatorial complications are the result of a number of conflicting usages for a few names which have been in use for a long time. In order to see how these different usages came about, to properly evaluate the nomenclatures currently extant, and to place the stratigraphic problems in historical perspective, it is desirable to review the evolution of the terms, as well as some of the accompanying ideas. This summary pertains chiefly to the Big Horn Basin and immediately adjacent areas. History of usage of terminology is summarized in figure 2. Eldridge (1894), in a reconnaissance report, was the first to refer specifically — to the rocks of the Big Horn Basin now known to be Lower Cretaceous. He recognized a Dakota formation which he described as a commonly iron-stained quartzose sandstone of variable thickness beneath which lay 400 to 600 feet of Jura strata, dominantly shale and (1894, p. 21) “wholly of marine origin.” The beds above his Dakota, Eldridge placed in the Colorado formation, separating within it the Benton and succeeding Niobrara formations. His Benton consisted of 300 to 600 feet of gray and black shales with a few thin beds of limestone and numerous small lenticular ironstones, and his Niobrara consisted of 300 to 800 feet of light gray shales with some impure shaly limestones and, locally, well- developed sandstones. He notes (1894, p. 23) that the shales “are of extremely close grain and of fine even texture, indurated in a marked degree, and which, though gray on fresh fracture, weather a characteristic milky white.” He de- scribed his Niobrara “limestones” in detail: “The limestones are not of a pro- nounced character but rather are calcareous layers of shales, which are held together with greater tenacity than portions containing less lime. These layers INTRODUCTION 5 are rarely over a foot of two thick, are very earthy and are usually of fine texture.” Most of Eldrige’s Niobrara was later to be called the Mowry shale, and his “lime- stones,” bentonites. Near Five Springs Creek, Eldridge believed that the black shales of his Benton rested directly on the variegated clays of his Jura and that his Dakota was entirely absent as a result of non-deposition. HEWETT ELORIDGE | DARTON |WASHBURNE| HINTZE | LUPTON & PIERCE MILLS THIS PTON 1894 1906 1908 1915 1916 Sane 1948 1956 PAPER MOWRY MOWRY MOWRY SHALE SHALE SHALE “SEs SEE Ir NIOBRARA “| ForMATION MOWRY SHALE SHALE (MEMBER) (MEMBER) wWwRY “UPPER THERMO- FORMATI (0) BENTON SHALE SHALE POLIS" LOWER BENTON SHALE OF COLORADO GROUP 9 _____ n” o} — ‘“mupoy | — © wmuooy ss 4 sand" | 4 5 MEMBER [=) °o ° o WwW FORMATION a x = - ¢ a a ° ° z ° = i = = = x 5 “baxota sut|m migore ary ° w o ——|5 . w o x= © LOWER jy “RUSTY |_y w E oO __ SHALE =x be a = fo} JO -rysty CLOVERLY |W ‘RUSTY Eas Sh a Ore F r FORMATION|= 8e0S"_|q RUSTY I BEDS |) =| DAKOTA SS. GREYBULL SNO| GREYBULL SANO|GREYBULL SS, CRE ereMBER | GREYBULL SS. |= = = CLOVERLY | CLOVERLY | CLOVERLY | CLOVERLY | CLOVERLY | MORRISON CLOVERLY | CLOVERLY - FORMATION] FORMATION | FORMATION | FORMATION] FORMATION] FORMATION| FORMATION| FORMATION Figure 2. History of terminology of Lower Cretaceous rocks within the Big Horn Basin. N. H. Darton made the first detailed stratigraphic studies in the Big Horn Mountain area during the years 1901 through 1905. Darton recognized the Morrison formation which shortly before he had correlated into the Black Hills from its type area in Colorado. Above the Morrison, he found a sequence of beds which he believed to be equivalent to his Lakota and Fuson formations of the southern Black Hills. Because overlying lithologic equivalents of his Black Hills Dakota sandstone were absent, however, Darton (1904, p. 398) deemed it best to give this unit a new name, the Cloverly formation, taken from Cloverly Post Office on the east side of the Big Horn Basin where the strata were 113 feet thick. Above the Cloverly, Darton (1904, p. 399) included about 1300 feet of domi- nantly shaly strata in the Benton formation. Later he used the term “Colorado for- mation,” stating (1906b, p. 7), “The Colorado formation in the Bighorn region comprizes the Benton and Niobrara formations of the Rocky Mountain and Great Plains region farther south and east. . . . The upper beds of the formation are gray shales, which probably represent the Niobrara formation.” Darton (1904, p. 399) mentioned that, “The basal member of the Benton consists of dark gray shales, in part sandy and of rusty brown color, with occa- sional thin beds of brown sandstone.” He called this interval the “rusty series” (1906a, p. 54) and mentioned that it was about 200 feet thick with a persistent 6 STRATIGRAPHY OF THE THERMOPOLIS SHALE horizon about 100 feet above the base, characterized by round phosphatic con- cretions up to two inches in diameter. These concretions have since been shown to consist of the mineral dahllite (McConnell, 1935). Darton described the lower contact of the Benton west of Cloverly Post Office as abrupt on top of the Cloverly formation but without sign of unconformity. He suspected that either the rusty series or some of the thin sandstones at the top of the Cloverly possibly repre- sented his Dakota sandstone (the Fall River sandstone of Russell, 1927) of the Black Hills (1904, p. 399). Because the base of the Dakota was, to Darton, synony- mous with the base of the Upper Cretaceous, he tentatively placed the Lower- Upper Cretaceous boundary in the upper part of the Cloverly formation. Above the rusty series, Darton reported 600 feet or more of black fissile shales which contained occasional carbonate and iron concretions and, near the middle, a light-colored sandstone. This unit was overlain in turn by 150 to 300 feet of hard, light gray, fish-scale-bearing shales and thin-bedded sandstones which weathered to light gray barren ridges. Darton (1904, p. 400) designated these “the Mowrie beds . . . from Mowrie creek northwest of Buffalo.” Later, he changed the spelling to Mowry beds (1906a, p. 54), and elsewhere (1906b, p. 7) he referred to it as the Mowry member, noting that it could not be Niobrara because it lay below beds containing diagnostic Benton fossils. Fisher (1906) made a few additions to Darton’s stratigraphic descriptions on the east flank of the Big Horn Basin, and carried all of the stratigraphic units over to the west flank. He noted the bentonite beds in the upper part of the soft black shale interval below the Mowry. Washburne in 1908 published the first report on oil and gas in the Big Horn Basin. From the area of Greybull, he introduced the term “rusty beds” for the thin-bedded sandstone and shale beds at the base of the Colorado formation. Washburne interpreted the rusty beds as a basal transgressive unit of the Creta- ceous sea, considering them unconformable on the Cloverly formation and placing the Lower-Upper Cretaceous boundary at their base. He explained the discontinuity of Cloverly sandstones beneath the rusty beds as a result of erosion prior to the marine transgression and suggested that where the Cloverly sand- stones were absent, the entire formation had been eroded away leaving the rusty beds in contact with the underlying Morrison formation. The lower portion of Washburne’s (1908, p. 350) generalized section of the Colorado formation follows: Shales, gray and black, with many fossil fish scales; contain numerous layers of hard flinty shale, and one 3-foot bed of bentonite; the Mowry shale ....................0055 200 Shales, dark bluish and black, with a few beds of volcanic ash and white clay (bentonite) NAN BRST PPPOE SAUNA oo hea alam we, ale nin tune a)n oven wie we a/e c-ciareya's alalalamiteie -jelefeveiane nee 250 Shales, black, carbonaceous, in many places oily, locally containing one or more lenses gieandstone in) the Tower 100 feet |... .5)ece/. 5: 0.5,0, 5/015 j0i0is 6 aise vinnie mie eile eseln eh 300 Sandstone, thin beds 3 to 18 inches thick, weathering brown, which are separated by pattings of black shale 1 to 12 inches thick; the ‘rusty beds’ .:.........<.-..-:seeeeee 20-100 His mention of bentonite and volcanic ash in the dark shale below the Mowry is interesting. Bentonite had been reported from this interval previously by Fisher (1906), but it was nine years before Hewett’s (1917) paper advocating the volcanic origin of bentonite appeared. As many bentonite beds contain volcanic shards, perhaps Washburne distinguished his “beds of volcanic ash” on this cri- terion and restricted his term “white clay (bentonite)” to beds in which shards were not visible. If this is so, however, the origin of bentonite must have been INTRODUCTION 7 apparent to him because of the similarity of the bentonite beds, with or without obvious shards. In his detailed section of Mesozoic and Tertiary rocks on the west side of the basin near Cody, Wyoming, Hewett (1914) recognized a lower member of the Colorado formation 1026 feet thick resting on the Cloverly formation. Hewett distinguished no members, but he mentioned an impressive number of bentonite beds in a 392-foot soft black shale unit that lay between a 20-foot buff sandstone containing vertebrate fossils below and a sequence of sandstone and fish-scale- bearing shales above. He included 60 feet of thin-bedded sandstone in the top of his Cloverly formation, most or all of which was equivalent to the lower por- tion of the rusty beds on the east side of the basin. This practice has since been customary on the west flank of the basin where the rusty beds contain much more sandstone in their lower part. In the Basin-Greybull area, Hintze (1915) divided the Colorado group into upper and lower formations which he correlated with the Benton and Niobrara formations of the plains. He subdivided the Benton into four members, the lowest of which included the 900 feet of strata between the base of the “Rusty beds” and the top of the Mowry shale. He noticed, as had Washburne, that the rusty beds had a much more uniform character and distribution than the under- lying Cloverly. Hintze believed that the contact between the rusty beds and the underlying Cloverly formation was either a disconformity or a slight angular unconformity, but he granted that the intermittent occurrence of sandstone in the upper part of the Cloverly could have resulted from local deposition instead of erosion prior to rusty beds deposition. Hintze concluded that the subsurface “Greybull sand” of drillers in the area was one of these local sandstone beds in the upper part of the Cloverly formation, and he noted the intermittent occur- rence of such beds in the subsurface as well as in outcrop. Near the middle part of his Lower Benton shale member, Hintze described a white sandstone 25 to 40 feet thick and mentioned that in the subsurface this unit was locally called the “Muddy Sand.” He mentioned its importance in the subsurface as an economic unit and as an excellent marker bed; although he found and examined its out- crops east of Greybull, he did not give it a formal name. Hintze thought it noteworthy that the Dakota, which is so prominent in Colorado, is not recognizable in its typical form in the Big Horn Basin. He stated (1915, p. 31), “The Cloverly was thought by some to represent the Dakota but it has more recently been proved to be of Lower Cretaceous age,” inferring that because of its age it was ineligible for consideration as a Dakota equivalent. This belief, that the term “Dakota” should be restricted to rocks of Upper Cretaceous age, had previously induced others to revise the nomenclature in the Black Hills and southeastern Colorado. The consequences of this mis- leading concept of the Dakota as a rock unit restricted to a particular time span has plagued the terminology ever since (Waagé, 1955, p. 18). Hintze’s statement is the first indication that it had reached the Big Horn Basin. In order to explain the greater thickness of black shale between the Cloverly and the Mowry in northern Wyoming compared to this interval in southern Wyoming, Hintze entertained the interesting possibility that the original trans- gression of the Benton Sea came from the north and extended nearly to Colorado where it was later met by the Dakota transgression from the south. Lupton, in 1916, introduced the stratigraphic classification of the Colorado group that has been used ever since by the U. S. Geological Survey. He raised the 8 STRATIGRAPHY OF THE THERMOPOLIS SHALE Mowry to the rank of formation, and he applied the name Thermopolis shale to the 700-foot sequence corresponding to that part of Hintze’s Lower Benton shale member which lay below the Mowry. He stated (1916, p. 168) that the Thermopolis was conformable with the upper sandstone bed of the Cloverly below and with the Mowry shale above. Lupton noticed the variable thickness of the medial Muddy sand and stated that near Basin, Wyoming, it ranged from 15 to 55 feet. The name Thermopolis was taken from a town in the southern part of the basin “near which it is well exposed,” but Lupton gave no section of the Thermopolis from the type area. His generalized section (1916, p. 167) from the Basin oil field follows: Mowry shale: (in part) feet Sandstone, Octh Louie sand of drillers; yields a little oil ...............-..4- 25 160 Thermopolis shale: Shale, hard: ‘contains lenses of sandstone ....... 2.6.60 606 S550 oe + aeiaimente 230 SHANE, SOL CATR Oo silo Sin ele ele leow cee tials oo SEN INS whale) oe teal ee te 170 Sandstone, Muddy sand of drillers; contains a little gas ............++++++0: 35 riales SOLE) athe <2 26 ie 52. dic bo Wid oie sales cie'sye b 5nd [als Met a perg he mre 275 710 Cloverly formation: (in part) Light-buff or tan-colored sandstone (Greybull sand of drillers) ............... 20 Lupton took his Cloverly section from Darton’s (1906a, p. 52) description of the type Cloverly. In applying the term Greybull sand to the upper 20 feet of the type section, Lupton was unhesitatingly following Washburne (1908) in corre- lating the discontinuous Greybull sand of the subsurface with the discontinuous sandstone beds at the top of the Cloverly on outcrop. The following year, Hewett and Lupton (1917, p. 19) formally designated “the Greybull sandstone member of the Cloverly formation, from the town of Grey- bull, near which it is typically exposed.” They stated that it consisted of 10 to 20 feet of yellowish-gray sandstone at the top of the Cloverly formation. Hewett and Lupton mentioned no specific type locality, and because the unit is totally absent over much of the Greybull area, as Washburne (1908) and Hintze (1915) had previously ascertained, the merit of its formal designation was questionable. Though Hewett and Lupton made the astute observation that the sandstone © near the middle of the Thermopolis shale is unusually persistent, they made no effort to give it the formal status it warranted, but continued to refer to it (1917, p. 19) as “termed by the drillers the ‘Muddy sand.’”’ This is paradoxical in view of the formal designation, on the same page, of the Greybull—a unit long known to be notoriously discontinuous. Ziegler (1917a, 1917b) traced the Thermopolis shale northward to the Byron oil field on the Shoshone River and westward across the Big Horn Basin to the Oregon Basin field. He noted the uniform occurrence of the rusty beds in the lower part of the Thermopolis, and because of variation in the Cloverly beneath, he considered the contact to be disconformable. In both the Byron and Oregon Basin areas, Ziegler found a sandstone unit near the middle of the Thermopolis shale but was not aware that it represented the Muddy sand. He interpreted the rusty beds as deposits of a transgressing sea and the overlying Thermopolis and Mowry shales as deposits of quiet, but relatively shallow water. INTRODUCTION 9 From a study of the floras, Stanton (1922, p. 264-267) correctly postulated that the Muddy sand of central Wyoming and the Newcastle sandstone of the Black Hills were equivalent to the uppermost sandstone of the Dakota formation of the Colorado Front Range. He suggested that the portion of the Thermopolis shale underlying the Muddy, and the Skull Creek of Collier (1922) underlying the Newcastle, were equivalent to the middle shale of the Dakota formation in the Front Range. He also pointed out that marine pelecypods from this shale could be compared as convincingly to Early Cretaceous species as to Late Creta- ceous species, but concluded tentatively that they were probably Late Cretaceous and that the sea did not extend north of southern Colorado and central Kansas during Early Cretaceous time. The following year, Reeside (1923) described, from the dark middle shale of the Dakota formation of northern Colorado and southern Wyoming, the small molluscan fauna previously mentioned by Stanton but regarded it of Early Cretaceous age, the same as faunas from the Purgatoire and equivalent Lower Cretaceous rocks of southern Colorado and adjacent areas. He proposed that the Lower Cretaceous sea invaded at least as far north as southern Wyoming. Because the Dakota of northern Colorado contained only three species and lacked eight others occurring in the Purgatoire and its equivalents farther south, Reeside concluded that the three species common to both areas were possibly hardier and could endure the more unfavorable environmental conditions that presumably existed to the north. He pointed out that Stanton (1922, pl. 5) had previously shown a progressive change in fauna, marked by the disappearance of one species after another, northwestward from New Mexico and Oklahoma. A continuation of this trend, he believed, might account for the paucity of Purga- toire species in northern Colorado. The Late Cretaceous age of the flora of the Muddy, Newcastle, and upper- most sandstone of the Front Range Dakota formation, which Stanton (1922) had previously correlated, was still accepted, and hence the Lower-Upper Cretaceous boundary was placed at the base of these sandstone units, where it remained for over a quarter century. Lee (1923) called the Dakota formation of northern Colorado the Dakota group and divided it into five informal subunits, which he traced northward across Wyoming into the Big Horn Basin. Later, Lee (1927, p. 57) compared the Cloverly-Thermopolis-Muddy sequence at Thermopolis with the five subunits of his Dakota group. He correlated the Cloverly, which consisted of a lower con- glomeratic sandstone, overlying red sandy shale, and upper interbedded sand- stone and variegated shale with his lower sandstone, lower shale and middle sandstone of northern Colorado, respectively. The dark shale containing rusty beds at the base, and the overlying Muddy sand, he correlated with his upper shale and upper sandstone, respectively. These correlations were essentially cor- rect, but Lee’s terminology has not since been used, presumably because his lower subdivisions lack lateral continuity. Lee paid little attention to some stratigraphic details, and his inconsistencies have understandably confused later workers. For example, he (1927, p. 58) said that at a locality two miles northwest of Thermopolis, Wyoming, “the Thermopolis shale includes near the base brown sandy material that constitutes the so-called ‘rusty beds.’ ”’ Several lines further, we read that above colored sandy shales lie “the ripple-marked, rusty-brown sand- stone and thin layers of dark shale which some have called the ‘rusty beds.’ Above these ‘rusty beds’ is a thick mass of dark shale—the Thermopolis shale.” Still 10 STRATIGRAPHY OF THE THERMOPOLIS SHALE further on, he states (1927, p. 64) that north of Greybull, “The Greybull sand- stone consists of many layers of rusty-brown sandstone and shale, which grade up- ward into the Thermopolis shale without sharp demarkation,” confusing the rusty beds with the Greybull sandstone, perhaps intentionally because here, “The beds below the rusty sandstone consist of highly colored material characteristic of mid- dle Cloverly.” Thus, in those areas where he found no local upper Cloverly sand- stone beds, Lee apparently invoked as convenient correlatives to his middle sandstone subunit the rusty beds, effectively confusing the terms “rusty beds” and “Greybull sandstone.”” Additional carelessness is illustrated by the statement (1927, p. 18) that certain correlations are made with “the Muddy sand, a name derived from Muddy Creek, in northern Wyoming.” Later Wilmarth (1938, p. 1448) stated, “There are at least 8 Muddy Creeks in Wyo., but, although the sand outcrops, it is not known to be exposed on any Muddy Creek.” Lee knew better, for further on (1927, p. 23) he used the phrase, “the Muddy sand of the oil men of Wyoming.” The Cloverly -Thermopolis contact has been difficult to pick consistently in mapping. Ever since the name Greybull sandstone was given to a local bed at the top of the Cloverly, the tendency has been to draw the Cloverly-Thermopolis contact arbitrarily above the most prominent sandstone beds below the dark shales of the Thermopolis. This practice included a considerable portion of the rusty beds in the Cloverly on the west side of the basin, where they contain more sand than they do on the east side. Pierce and Andrews (1941, p. 117), who arbi- trarily utilized the lowest conspicuous dark gray shale of the Thermopolis in the area south of Cody, emphasized that though it was expedient, it was inconsistent, and that Cloverly thickness variations “are not due to true thickening or thin- ning of the formation but to the inclusion in the Cloverly of greater or smaller thicknesses of the overlying or underlying strata, depending upon the lithology at the particular place examined.” Similarly, in the area immediately north of the Big Horn Basin, Knappen and Moulton (1930) included with the Cloverly formation thinly bedded shaly sandstones of the lower part of the rusty beds, in the belief that this was the Greybull sandstone and that it had simply become much thicker to the north. They noted, however, that this unit was sharply distinct from the underlying light-colored, more massive strata, and that its upper part indicated marine conditions which continued uninterruptedly upward into the overlying black — shales. They mapped this “Greybull” separately. Love and others (1945), on a correlation chart covering a large area of central Wyoming, included the rusty beds in the Cloverly formation intentionally, be- cause “nonmarine Lower Cretaceous fossils, identical with those found in the underlying variegated claystone zone, are present in the ‘Rusty beds’,” and because “In some places sandstones in the ‘Rusty beds’ intertongue with the underlying variegated claystones.” Dahllite and ironstone concretions occur in some of their sections in the lower part of their Thermopolis shale and in other sections in the upper part of their Cloverly formation. They considered the Thermopolis shale minus the rusty beds unsatisfactory for mapping, because the upper contact with the Mowry shale was gradational, and because they thought the Thermopolis included at the base of the Muddy sandstone an important time horizon, the Lower-Upper Cretaceous boundary. On the east side of the Big Horn Basin, Pierce (1948) and Rogers and others (1948) distinguished the rusty beds but intentionally mapped them in the INTRODUCTION 11 Cloverly formation for convenience. In Pierce’s area, the base of the Cloverly was difficult to map, because the conglomeratic sandstone on which it was defined was absent. Pierce therefore mapped the base of the “Greybull sandstone” as this contact. Rogers and others followed Pierce and used the same contacts on their adjoining map, but in much of their area the discontinuous conglomeratic sandstone which marks the base of the Cloverly was apparently present, for they mapped it separately in the middle of their Morrison formation. Inasmuch as sandstones at the top of the Cloverly are commonly totally absent in these areas, the rusty beds must, in places, make up the entire unit mapped as Cloverly. It follows that, in places, the entire sequence equivalent to the type Cloverly was mapped in the Morrison. Reeside (1944), in a summary of the western interior Cretaceous, showed on his correlation chart a Muddy sandstone member of the Thermopolis shale. Desig- nation of the Muddy sandstone as a member has since been followed by the U. S. Geological Survey. For over twenty years, the Muddy sandstone had been accepted as a wide- spread unit seemingly worthy of formational status. Because of the structure of the pre-existing terminology, however, formational designation of the Muddy would have necessitated other nomenclatorial changes at the same time, perhaps the most logical of which would have raised Thermopolis to a group name and separately named the black shale units below and above the Muddy. This revision must have appeared drastic, for it was never made, but it would have prevented much subsequent confusion. In the Wind River Basin, Love (1948, p. 106) proposed, for the purposes of a guidebook, that the term Thermopolis shale be limited to the interval of black shale between the Cloverly formation and the Muddy sandstone, that the Muddy be considered a formation, and that the interval of soft black shale between the Muddy and the Mowry shale be included with the Mowry. This was not imme- diately adopted in other areas. The following year Downs (1949, p. 48), in the Powder River Basin, still referred to the Muddy as a member of the Thermopolis shale and considered the Thermopolis to include the soft black shale above the Muddy. By 1952, however, the Muddy was widely enough accepted as a forma- tion to influence Cobban and Reeside (1952, p. 1030) to say of the Thermopolis, “Many geologists now prefer to restrict the name to the lower part of the dark shales, to designate the sandstone the ‘Muddy sandstone’ and view it as a for- mation, and to include the upper part of the dark shale in the Mowry formation. The compilers think this restricted application of Thermopolis much more useful at many places than the older usage.” Today this seems by far the most popular classification, although the U. S. Geological Survey uses the term Thermopolis in its original sense and considers the Muddy sandstone a member near the middle. Crowley (1951) elevated the Lower-Upper Cretaceous boundary from the base to the top of the Newcastle sandstone in the Black Hills on the basis of fora- miniferal faunas. He found that the faunas of the Newcastle and the underlying Skull Creek shale were similar. Because he did not find the fauna in the overlying Mowry shale, he concluded that the Newcastle was more closely related to the Skull Creek and thus probably the same age. Cobban and Reeside (1951) reported Lower Cretaceous ammonites of the boreal genera Gastroplites and Neogastroplites from the Mowry shale in Wyo- ming, Colorado and Montana, but they later considered that all properly be- 12 STRATIGRAPHY OF THE THERMOPOLIS SHALE longed only to Neogastroplites (Imlay and Reeside, 1954, p. 225). These forms enable correlation with rocks of northern Canada which are considered upper Albian or uppermost Lower Cretaceous in age. Thus, Cobban and Reeside con- cluded that the Lower-Upper Cretaceous boundary correctly belonged at the top of the Mowry shale. Their conclusions were not accepted by Yen (1952, 1954) who dated pre-Mowry equivalents in western Wyoming as lowermost Upper Cretaceous by means of non-marine molluscs which he compared to similar faunas from Europe. Both Eldridge (1894) and Darton (1906b) had used the term Colorado group in the Big Horn Basin, and had followed White (1878) in placing its boundary at the top of the Cloverly formation. Knechtel and Patterson (1956, p. 17) recently mentioned a change of this lower boundary of the Colorado group as a result of the latest faunal evidence bearing on the position of the Lower-Upper Creta- ceous boundary. Knechtel and Patterson stated that the Colorado “is still gener- ally understood to include only Upper Cretaceous rocks. Therefore, the Geological Survey’s classification has recently been modified to exclude the Thermopolis and Mowry beds from the Colorado group.” This classification also appears in Cobban and Reeside’s (1952, pl. I) correlation chart, where the strata between the base of the Fall River sandstone and the top of the Mowry shale are the only Creta- ceous beds not included in any group. But why should our latest age determina- tions, which changed previous notions of the age of parts of the Colorado group, be allowed to alter the physical boundaries of the Colorado group? The term “Colo- rado shale” was first applied by White (1878, p. 21, 22, 30) to a sequence of rocks which included the lowest black shales in the Cretaceous sequence on the upper Missouri River. The term “Coloradoan” has since come to be used as a stage name and applied to rocks deposited during the Colorado shale time. Exactly what strata of the European type section were laid down during the same time was unknown, but they were long believed to be entirely Upper Cretaceous. Now that this is no longer thought to be true, the impulse is to change our strati- graphic boundaries to conform to the previous incorrect notion of age and thus to keep the Colorado group entirely within the Upper Cretaceous European stages. But to what extent is the age of a rock unit a part of its definition? Can we ever be certain that our knowledge of the Lower-Upper Cretaceous boundary is subject to no additional modification, or do we intend to periodically readjust the stratigraphic boundaries of the Colorado group to conform to our newest evidence on where in our rock sequence the time boundaries between the Euro- pean stages lie? Recently, there has been a trend toward using Black Hills nomenclature for the Lower Cretaceous rocks of central Wyoming. Above the Cloverly, on the east flank of the Big Horn Mountains, Hose (1955) mapped the Skull Creek and Newcastle formations, possibly to avoid the ambiguity that the use of the term Thermopolis might convey and to denote unequivocal formational status for the Muddy sandstone in an area where only member status had been previously implied. Hose (1955, pl. 9) demonstrated that the Skull Creek and Newcastle strata of the Black Hills were indeed equivalent to the intervals to which he applied these names on the east flank of the Big Horn Range. The unit underlying the Skull Creek, however, changes facies markedly between the Black Hills and the Big Horn Mountains. Whereas the soft, shaly rusty beds are included in the Thermopolis shale to the west in the Big Horn Basin, their correlative in the a A ct INTRODUCTION 13 Black Hills, the harder, more resistant Fall River sandstone, was always mapped separately and was never included in the Skull Creek shale. Correct extension westward of the term ‘Skull Creek” therefore excludes the underlying equiva- lents of the Fall River. These equivalents are the rusty beds and a few feet of overlying strata, including the dahllite concretion-bearing beds. These must now either be considered as a separate unit or included with the underlying Cloverly formation. Hose included them with the Cloverly, thereby expanding Darton’s Cloverly to embrace 150 feet of strata that overlie it in the type area just across the Big Horn Range. The base of the Cloverly he drew at the base of a sandstone, 15 to 45 feet thick, underlying the rusty beds. The boundaries of his Cloverly thus were essentially the same as those used for expedience by Pierce (1948) and Rogers and others (1948) on the west side of the Big Horn Range. Mills (1956, p. 19), in a symposium of the Wyoming Geological Association, considered the Thermopolis shale in the subsurface of the Big Horn Basin to consist only of the dominantly black shale unit below the Muddy sandstone. He included the rusty beds in its lower part and he recognized a silty unit near its middle. He considered the Muddy sandstone a separate formation and assigned the soft black shale unit above the Muddy to the Mowry but discussed it by the informal term “Upper Thermopolis.” In much of central and eastern Wyoming, Burk (1956, p. 30), Faulkner (1956, p- 40), and Peterson (1956, p. 46) proposed that the Black Hills term “Inyan Kara group” be used for the strata previously included in the Cloverly formation and the rusty beds, because the term “Cloverly” has often been misused in the subsurface. In the western part of the Wind River Basin, Burk (1956, p. 31) included the rusty beds in the lower portion of the Thermopolis shale, and in the eastern part, he included them in the upper portion of the Inyan Kara group with the Thermopolis lying above. In the western Powder River Basin, Faulkner (1956, p. 40) considered the term “Thermopolis” synonymous with the term “Skull Creek,” consequently excluding the rusty beds from the Thermopolis shale. Everywhere in central and eastern Wyoming, the authors of the 1956 Wyo- ming Geological Association symposium included the soft black shale interval between the Muddy sandstone and the siliceous Mowry shale of Darton with the Mowry shale. However, they recognized it separately on all electric log corre- lations and in all text discussions, where they informally referred to it either as the ‘Upper Thermopolis shale” or as the Nefsy shale. NOMENCLATURE The term “Thermopolis shale” is currently used in three principal ways. The U. S. Geological Survey applies it to all the strata between the Cloverly forma- tion and Mowry shale of Darton and considers the Muddy sandstone a medial member. The Wyoming Geological Association and most petroleum geologists use the term Thermopolis only for the shale and siltstone unit below the Muddy, recognize the Muddy sandstone as a separate formation, refer the shale unit above the Muddy and below the Mowry of Darton to the Mowry in theory, and call it by an informal name in practice. Finally, a few have excluded the rusty beds from the basal part of the Thermopolis and have included them with the nonmarine Cloverly formation. How the names should be applied has thus been a matter of divided opinion, and many convictions have been partially governed by the particular area in which they were formulated. The status of the Muddy sandstone appears to be the key to much of this nomenclatorial muddle, for if it is not worthy of formational status, the old usage of the Thermopolis of Lupton would seem to be adequate. If, on the other hand, the Muddy is significant and important enough to be considered a formation, then the usage of Lupton is obsolete and should not be allowed for a moment to obstruct a more meaningful classification. Two different stratigraphic concepts of the Muddy sandstone are prevalent. In the first, the Muddy is viewed as consisting of separate and discontinuous sandstone lenses or tongues of dubious correlation which lie isolated within and surrounded by a thick body of black shale. This view implies that the sandstone deposits are of minimal importance in the geologic history, and it would not encourage recognition of the Muddy as a formation. It seems to be exemplified by Sielaff’s (1952, p. 133) statement that, “In general the name is applied to any sandstone in the Thermopolis section.” In the second concept, the Muddy is viewed as a single, persistent unit of highly variable lithology which occupies a single stratigraphic interval and records an important historical episode of basin-wide deposition. Subsurface stratigraphic data presented by Burk and others (1956) strongly support this con- cept, as does all stratigraphic and paleontologic evidence gathered in the present study. These studies indicate that the Muddy is, indeed, a significant and widely occurring, though variable unit that warrants formational designation. Another excellent indication that the Muddy can be justifiably considered a formation is that it has been successfully mapped in, and adjacent to the Big Horn Basin at scales ranging from 1:48,000 to 1:125,000 (Knappan and Moulton, 1930; Pierce and Andrews, 1941; Pierce, 1948; Rogers and others, 1948; Richards and Rogers, 1951; Hose, 1955). A third and equally significant reason for recognizing the Muddy as a forma- tion is that many geologists now follow this usage, and it has received popular acceptance as well. Paradoxically, there appears to be a surprising amount of popular resistance toward formally recognizing the Muddy and establishing a type section for it. Such resistance does not seem to result from any suspicions or convictions that the Muddy is not worthy of formational status, but rather it seems to stem from NOMENCLATURE 15 feelings that the Muddy has been informally recognized for so long that it has attained sufficient eminence to preclude any need for formal recognition. This is suggested, at least, by informal statements from two petroleum geologists who, when asked for comments, issued the following: “As far as establishing a ‘type’ locality for the Muddy sandstone, I certainly feel this is unnecessary and would add nothing towards its formal recognition as a widespread correlative unit, as the term is widely accepted and well established in the literature,” and “I am not impressed personally with the necessity or convenience of setting up a type locality for Muddy sand. On the other hand, the Wyoming Geological Association has a volume, 1956, on Wyoming Stratigraphy in which the Muddy sand is carefully defined and correlated by means of stratigraphic and electrical logs, and at the bottom of page 19, Norman K. Mills states, ‘Nomenclature of the Muddy sandstone is considered adequate.’ ” There is no legal necessity for formally designating a formation. Indeed the Stratigraphic Commission (1956, p. 2007) says only that, “Groups and formations are customarily given formal names.”” By popular preference, the Muddy will perhaps always retain its current informal formational status. For future workers who may be disturbed by the lack of a type locality for the Muddy, one might suggest that they call the Muddy the Newcastle formation. The Newcastle has a type locality (at Newcastle, Wyoming), and the name has already been applied as far west as the east flank of the Big Horn Mountains (Hose, 1955). However, such a procedure would probably be little recompense. One might alternatively suggest that the future workers visit outcrops nearest the Greybull oil field where the name Muddy was first used. This is the course followed here, and a good exposure of the Muddy near Greybull in NY Sec. 36, T. 53 N., R. 93 W. (figure 4) is recommended as the reference locality. Recognition of the Muddy as a formation divides the old Thermopolis shale of Lupton into three parts: the Muddy itself, an underlying unit of black shale containing prominent siltstone beds, and an overlying unit of shale containing a few prominent bentonite beds. Conventional evolution of terminology suggests the elevation of the term Thermopolis shale to a Thermopolis group embracing three formations, but actual usage has already established a different precedent. Workers who have previously considered the Muddy to be of formational rank (Love, 1948; Thompson and others, 1949; Burk and others, 1956) have always restricted the term Thermopolis to the unit of dominantly black shale beneath the Muddy. It seems desirable to follow this precedent here. Thus the term Thermopolis shale is applied only to that part of Lupton’s Thermopolis shale which lies below the Muddy sandstone. The rusty beds are marine or marginal marine sediments which overlie a regional disconformity formed by the initial transgression of the Lower Cretaceous sea into the western interior. They are, therefore, much more closely related to the overlying black shales than to the underlying light-colored massive non- marine rocks of the Cloverly formation, and they are here considered the lower part of the Thermopolis shale. Sandstone beds in the basal part of the rusty beds probably represent deposi- tion in estuarine, deltaic, and other local marginal environments produced dur- ing the marine transgression. Sandstones in the uppermost part of the underlying Cloverly were deposited mainly in stream channels. The term Greybull sandstone member (Hewett and Lupton, 1917, p. 19) was once given to some sandstone beds in this general part of the section, and inasmuch as the type locality was not 16 STRATIGRAPHY OF THE THERMOPOLIS SHALE designated accurately, it is not now possible to ascertain whether the term re- ferred only to channel sandstones in the upper part of the Cloverly, or to sand- stone beds at the base of the rusty beds, or to some of each. Neither the Cloverly nor the rusty beds’ sandstones, however, would constitute a unit that would be practical to map, and the inclusion of all the sandstones in this general interval in one unit would involve ignoring the significance of the regional boundary between nonmarine sediments and marine and marginal marine sediments which is marked by a zone of spherulitic siderite. Hence, the term Greybull should not be used beyond the confines of the Greybull oil field where originally it was in- formally applied to an oil sand. Recognition of the Muddy sandstone as a formation and concomitant re- definition of the term Thermopolis to apply only to the shale unit below the Muddy leaves the soft black shale unit, which was originally included by Lupton (1916, p. 168) in the upper part of his Thermopolis shale, without a name. What should it now be called? Two courses are open: one would give the unit a new name; the other would include it in the Mowry shale, thus extending the term Mowry down to the top of the Muddy sandstone. As a guide to the most desirable course, we might examine what has been done in the past with the soft black shale unit whenever the Muddy was considered to be of formation rank. Most workers who have considered the Muddy a formation have followed Love’s (1948, p. 113) suggestion that the term Mowry be extended downward to include the soft black shale sequence. Love first applied this classification in the 100' ~~ wiNo RIVER a ve BASIN _~ A O.. 20. 40 60 a —_—— MILES fsa eb Figure 3. Isopach map of Shell Creek shale in eastern Wyoming, showing decrease in thickness over the mountains which border the Big Horn Basin on the east and south. PE ——— NOMENCLATURE 7 Wind River Basin, and it has been extensively used here and in the Powder River Basin. Exactly where this suggestion originated is important, because in the Wind River Basin, and in the Powder River Basin as well, the soft dark shale unit is much thinner than in the Big Horn Basin. Figure 3 illustrates its rapid decrease in thickness southeastward over the mountain uplifts which separate the Big Horn Basin from the Wind River and Powder River Basins. Even in the Big Horn Basin, many workers now follow this classification, but others still find it preferable to continue to recognize the Muddy as a member of the old Thermopolis shale of Lupton rather than to recognize it as a forma- tion and, as a consequence, to so drastically redefine the Mowry shale of Darton. This course fails to recognize the significance of the Muddy sandstone, but the former course expands the Mowry of Darton to a thickness more than twice as great as the beds to which Darton originally applied the term, thus rendering it completely different from the Mowry in parts of Montana where it is still recognized as a siliceous shale. The issue becomes clearer when one examines more closely what has hap- pened in actual practice whenever the soft black shale sequence has, at least in theory, been included with the Mowry. Even in the Powder River and Wind River Basins where the soft black shale is nearly everywhere considered too thin to map separately on the surface, several informal names have sprung up to distinguish it, in discussions, measured sections, electric logs, and correlation charts, from the siliceous Mowry shale of Darton. This response to necessity is the most convincing evidence possible that separate recognition of this unit is practical in descriptive work. The name most commonly applied to it is “Upper Thermopolis shale,” and less commonly, “Lower Mowry” or “Black Mowry.” Even the long-abandoned Black Hills term, “Nefsy shale,” appeared on a re- cently published stratigraphic cross-section (Faulkner, 1956, chart). If the term Thermopolis shale is to be firmly restricted to strata below the Muddy, certainly the use, even in an informal sense, of the term “Upper Thermopolis shale” for the soft black shale above the Muddy should be discontinued. In the Big Horn Basin, the soft black shale is a distinct, mappable unit be- cause of its greater thickness. It has been mapped by many of the same workers who have mapped the Muddy—(Pierce and Andrews, 1941; Pierce, 1948; Rogers and others, 1948). In addition, it is easily distinguishable on electric logs (Mills, 1956, p. 20) where the regional persistence of its contact with the siliceous Mowry shale is especially apparent, verifying that the southeastward thinning results from overlap or convergence within the soft black shale itself and not from facies changes with the overlying siliceous Mowry shale. If the Mowry shale of the Big Horn Basin were redefined to include the soft black shale as well as the siliceous shale to which the name was originally applied, this would not only result in two different regional usages for the term Mowry, but if the units are ever to be distinguished, it would necessitate the ultimate proposal of two new member names. The alternative course, and the one proposed here, is to accept the Mowry shale as it was originally defined by Darton and to consider as a separate forma- tion the underlying soft black shale sequence which was originally included in the upper part of the Thermopolis shale of Lupton. Shell Creek shale is the new name proposed for this unit. Its type locality is on the southwest flank of Sheep Mountain anticline about six miles northwest of Greybull, Wyoming (figure 4). The locality is selected for its good exposures, 18 STRATIGRAPHY OF THE THERMOPOLIS SHALE which include well exposed upper and lower contacts, and for the comparative lack of slumping (plate la). Although Shell Creek itself flows only within six miles of the type locality, it is one of the nearest geographic features with an un- occupied name. In addition, Shell Creek borders extensive outcrops of the shale, which are prominently visible north across its valley from U. S. Highway 14 between Shell and Greybull, Wyoming, southwest of the type locality. TYPE SECTION —\ | SHELL CREEK SHALE 23 / SANDSTONE a Lene Figure 4. Index map showing type locality of Shell Creek shale and reference locality of Muddy sandstone. The Shell Creek shale is a mappable unit in the Big Horn Basin. It is also mappable in areas outside the basin to the north and northeast where its thick- ness is no less than inside the basin (Knappen and Moulton, 1930; Richards and Rogers, 1951). South and east from the basin, however, the Shell Creek equivalent thins out toward the margin of the sea in which it was deposited and is over- lapped by the siliceous shale of the Mowry. In these areas, it is thin, and although it is distinguishable in good exposures and on electric logs, it is not generally considered mappable at the scale on which most field mapping has been done. Therefore, although the Shell Creek equivalent is recognized south and east of the Big Horn Basin for purposes of discussing the historical geology, sufficient work has not been done to extend the name formally into these areas at the present time. It remains for future work to determine whether refined scales of mapping will require its formal recognition outside the basin to the south and east. Meanwhile, in these areas where the thin Shell Creek equivalent occurs, all the shale above the Muddy preferably should not be lumped under the term Mowry shale with no explanation, for this is misleading. The Shell Creek equiva- lent should be differentiated, at least informally, wherever it is found. — STRATIGRAPHIC DESCRIPTIONS ‘THERMOPOLIS SHALE The Thermopolis shale, as defined here, includes only that part of Lupton’s (1916, p. 168) Thermopolis shale which lies below the Muddy sandstone and is about 300 feet thick in the Big Horn Basin. It consists of black shale, the basal member of which contains tan and rusty-weathering interbedded siltstones and sandstones about 120 feet thick, commonly called the rusty beds. The shale sequence above the rusty beds is divided near the middle into lower and upper parts by a tan-weathering silty shale about 30 feet thick. The lower shale, below the middle silty shale, contains a few thin ironstones, siltstones, and silty lime- stones throughout, and dahllite concretions in its lower part. The upper shale, above the middle silty shale, is generally darker and contains nearly no siltstone or limestone. These four informal members—the rusty beds, lower shale, middle silty shale and upper shale—are easily recognizable, even from a distance, by their weathering characteristics. The rusty beds and middle silty shale weather to tan and brown slopes with small ridges or benches caused by thin siltstone or limestone beds. ‘The lower shale locally contains a few resistant beds, but, like the upper shale, it weathers to dark gray slopes. RUSTY BEDS AND THE BASAL CONTACT The rusty beds are composed mainly of interlaminated and very thinly inter- bedded dark shales and gray siltstones, which range from lenticular beds show- ing delicate lamination and cross-lamination to laminae, broken and contorted on a very small scale. They contain much carbonaceous material and the castings and trails of burrowing organisms. The rusty beds also generally contain some brown shale and a few thin beds of ironstone, commonly associated with cone-in- cone structures, and flaggy or cross-bedded sandstone and sandy limestone. Typically, these thin beds are very extensive and some can be traced several miles. Thicknesses of individual lithologic units, distinguishable in measuring, and of intervals between key beds, are remarkably consistent over wide areas. In their basal part, the rusty beds commonly contain thicker, brown-weathering sand- stone beds which are more local in extent. Two excellent exposures of the rusty beds near Thermopolis, Wyoming ex- emplify the lateral uniformity of the entire sequence and the persistence of even the very thin ironstone beds within it. One of these is the type section, locality 2 (figure 1) on highway 20, about three miles north of the town. The other is locality 8 on the east side of the Big Horn River, immediately southeast of the town. The exposures are about three miles apart. At both places, the sequences are nearly identical even with respect to the sandstone beds in the basal portion. The most impressive similarity, however, is the occurrence of four, half-foot iron- stone beds in the shale and siltstone sequence above the basal sandstone beds. In both localities, the ironstones occur at intervals of 9, 15, 28, and 42 feet above the base of the interlaminated shale and siltstone unit. Thus, these thin beds are remarkably persistent and extensive. The upper contact of the rusty beds is chosen at the top of a flaggy siltstone unit below a 30-foot shale containing a bed of dahllite concretions. Above this 20 STRATIGRAPHY OF THE THERMOPOLIS SHALE shale is another 5-foot brown-weathering siltstone unit. All of these beds can be traced widely in the Big Horn Basin. The siltstone and sandstone beds in the upper part of the rusty beds do not interfinger with the overlying lower shale. Many criteria differentiate the rusty beds from the underlying Cloverly forma- tion. The rusty beds consist of dark-colored, laterally persistent, generally thinly bedded strata, whereas the Cloverly consists mostly of light-colored, highly vari- able, massive claystones, siltstones, and sandstones. The contact at the type locality is illustrated in plate 1b. In spite of their gross differences, however, the precise contact between the rusty beds and the Cloverly is difficult to select con- sistently in the Big Horn Basin. Typically, the rusty beds grade downward near their base into an interval containing sandstone beds several feet thick. Unlike most beds in the main part of the rusty beds above, the beds in this interval are variable locally and show lateral changes in thickness and sequence. In some places sandstone beds overlie or are interbedded with rusty beds similar to those above them. In other places, sandstones, or beds of black carbonaceous shale, or slabby siltstone like those in the rusty beds may actually underlie beds of light- colored reddish or tan claystones and siltstones, which appear more like the underlying Cloverly. The variable sequence comprises an interval which may be as much as 35 feet thick below the more typical rusty beds strata. Elsewhere in the western interior where Lower Cretaceous transgressive sedi- ments overlie Lower Cretaceous nonmarine sediments, tiny iron oxide pellets occur in the rocks, particularly the claystones, immediately below the contact. In outcrop, the pellets appear as profuse tiny orange dots in a light matrix. Analysis of unoxidized pellets from drill cores in the Black Hills has shown them to be spherulites composed of siderite. Here the spherulites occur in the non- marine rocks immediately below the base of the transgressive Fall River sand- stone, and in the Colorado Front Range foothills, they occur in the uppermost part of the Lytle formation immediately below its disconformable contact with the transgressive South Platte formation (Waagé, 1958, p. 75). Along the south- eastern flank of the Big Horn Mountains, spherulites occur in the strata im- mediately below the rusty beds-Cloverly contact (MacClintock, 1957, p. 45). They also occur immediately below the contact on the east flank of the Big Horn Mountains at locality 26 and in the northern part of the Wind River Basin at locality 17. The spherulites in all these areas presumably were formed in like environments, proximal to the transgressing Lower Cretaceous sea. Thus they are believed to mark a contact of regional extent, upon which lie rocks deposited during or after the marine transgression. In the Big Horn Basin, spherulites occur below the thin variable sequence at the base of the rusty beds, and they are the most useful criterion in selecting a reasonably consistent contact between the Cloverly formation and the Thermop- olis shale. They generally occur throughout an interval three to five feet thick, and they are best preserved and most common in claystones, although they also commonly occur in siltstones and sandstones. Claystones containing them com- monly weather reddish-orange. The strata immediately above the spherulite- bearing rocks are not everywhere marine; most were probably deposited in transitional environments marginal to the transgressing sea. However, the change at the contact is most significant. The rocks above it were deposited under en- tirely different conditions from the rocks below it. This is one of the principal reasons that the rusty beds should not be placed in the Cloverly formation. Although siderite spherulites are distributed widely in the interior region, eect ET ett STRATIGRAPHIC DESCRIPTIONS 21 their occurrence in many local areas is spotty. When absent, they can usually be found in a short distance laterally along the outcrop. The contact can usually be chosen with moderate accuracy, even where the spherulites cannot be found, by comparing rock types which consistently occur above the contact elsewhere. Com- paratively thick sandstone units in the variable interval below more uniform thinly bedded rusty beds generally belong with the rusty beds, but local channel sandstones in the uppermost part of the Cloverly are important exceptions. Sandstones above the contact are generally hard, fine-grained, and weather rusty-brown, whereas those beneath the contact are commonly friable, light gray, silty, and fine- to coarse-grained. Very thinly bedded shaly siltstones which weather to light gray slabs belong in the basal part of the rusty beds; they have never been found below the contact. Peat beds and other dark-colored strata with much carbonaceous material generally belong in the rusty beds, but some Cloverly sediments, especially those adjacent to channel deposits, contain profuse carbonaceous material. Additional boundary criteria can generally be recognized locally, but regionally, the spherulites provide the most consistent criterion. In the Colorado Front Range foothills, spherulites occur below a sharp dis- conformity, but in the Big Horn Basin, the change they depict seems gradual in most places, apparently because deposition here was more continuous. The transi- tional change from nonmarine to marginal conditions in the basin and the ap- parent lack of a clear-cut break in many places makes the spherulites particularly valuable. In some places on the northeastern flank of the basin, not just one, but two beds contain spherulites. These have been found as much as 25 feet apart. They obviously represent a repetition of environmental conditions at the sea’s margin, and they are another indication that deposition was probably more continuous here than in most other areas. Where two spherulite-bearing beds occur, the contact is drawn on the lower one, inasmuch as this is the first indication of environmental change. That this lower zone, where two occur, might be locally absent, however, illustrates that the spherulites are not altogether infallible as a precise boundary criterion in a single outcrop. They are an environmental in- dicator and have no time value within the limits of time required for the transgression. Siderite spherulites occur locally in other Cretaceous rocks in the interior. K. M. Waagé (personal communication) reports finding them 40 feet below the top of the Cloverly formation at one locality in the Big Horn Basin and in the Newcastle formation at a few localities in the Black Hills. Merriam (1957, p. 12) reported siderite pellets from the Cruise sandstone and Huntsman shale in Kansas. However, the unusually wide persistence and concentration of the spherulites at the base of transgressive Lower Cretaceous sediments gives them considerable value for consistent lithogenetic correlation. LOWER SHALE The member above the rusty beds is about 90 feet thick in the Big Horn Basin. It consists predominantly of fissile, commonly silty shale that generally contains a few very thin sandstone, ironstone, or sandy limestone beds. The shale weathers to a medium-gray, commonly crusty surface which differentiates it from the rusty-weathering strata below. A five-foot siltstone unit about 30 feet above the base of the unit weathers tan. Round dahllite concretions, commonly the size of golf balls, occur in the shale below this siltstone. In the northern part of the a2 STRATIGRAPHY OF THE THERMOPOLIS SHALE basin, dahllite concretions also commonly occur in one or two other horizons higher in the lower shale. These higher concretions, however, are commonly botryoidal rather than round. Bentonite beds are rare, but a single greenish half- foot bed in the upper part can be traced from Tensleep (locality 15) about 60 miles northwest to Five Springs Creek (locality 20). MIDDLE SILTY SHALE About 30 feet of tan- and brown-weathering strata similar to the rusty beds separate the gray slopes of the lower shale from those of the upper shale. This unit contains gray and tan silty shale, and thin siltstone and silty limestone beds. It is somewhat more resistant to erosion than the shales above and below, and it commonly forms a slight ridge or bench. Beds within it, like those of the rusty beds and lower shale, are persistent over wide areas. The contact with the upper shale is abrupt (plate Ic). At Muddy Creek on the eastern flank of the Big Horn Mountains (locality 26), the middle silty shale is exceptionally thick, 45 feet, and consists of three striking cyclic repetitions of beds. Each cycle consists of silty shale in its lower part which grades into thinly bedded siltstone in its upper part. Two of the cycles are capped by beds of silty limestone. UPPER SHALE The uppermost member of the Thermopolis shale is about 110 feet thick in the northern part of the basin and about 75 feet thick in the southern part. It consists of fissile black shale which contains an extraordinarily small amount of silt. It generally contains some iron-stone beds but very few siltstone beds. Com- monly, one or two very thin bentonite beds occur in the upper 20 feet. It is overlain by the Muddy sandstone. In some places in the Big Horn Basin, the contact with the overlying Muddy sandstone is sharp, and in other places, it is somewhat gradational. The shale at the extreme top of the upper shale commonly becomes very silty. Several feet of interbedded black shale and light gray siltstone may lie below more prominent siltstone or sandstone beds of the Muddy. Where the interbedded siltstone and shale beds occur, the Muddy contact is chosen at their base, because this horizon depicts the first change that heralds Muddy conditions, and because it is the most consistent one available. Certainly the base of the lowest sandstone is not satis- factory as a contact, because in some places the Muddy contains almost no sand- — stone beds at all. Where the basal unit of the Muddy comprises non-resistant siltstone and shale beds, the weathered contact is generally obscure and must be exposed with a shovel. In most of these places, there seems to be no break in sedimentation between the Thermopolis and the Muddy. Nowhere, however, are these units known to intertongue with each other. - TYPE SECTION When Lupton (1916, p. 168) named the Thermopolis shale for exposures in the area of the town of ‘Thermopolis, Wyoming, he designated no specific type section but presented a supplementary lithologic description generalized from subsurface samples from near Basin, 50 miles north. A detailed description of the Thermopolis in its type area has never been published, and actually, the entire sequence is not well exposed in any one place near the town of Thermopolis. The lower part of the redefined Thermopolis shale is best exposed in outcrops ete ne men il STRATIGRAPHIC DESCRIPTIONS 23 about three miles north of the town on U. S. Highway 20 and the upper part, northeast across the Big Horn River on both flanks of Lucerne anticline. FOSSILS Macrofossils are un¢ommon in the Thermopolis shale of the Big Horn Basin. Fossil leaves occur in the extreme lower, variable portion of the rusty beds just above the contact at some localities, and a few poorly preserved casts of what may be nonmarine pelecypods and gastropods occur in a basal sandstone at locality 20. Love and others (1945) mentioned nonmarine fossils from the rusty beds of the Wind River Basin and these, too, undoubtedly came from the extreme lower variable portion. Fragments of vertebrae, ischium and armor plate of a crocodile from the lower shale at localities 15, 24, and 20 were identified by J. T. Gregory, Yale University, as closely resembling Coelosuchus reedi Williston. Inoceramus comancheanus Cragin was collected from the upper shale about 25 feet below the Muddy sand- stone at locality 21 by K. M. Waagé (personal communication), and prismatic Inoceramus shell material occurs in the upper shale at other localities on the northeastern flank of the basin. Trails and castings of burrowing organisms are abundant throughout the rusty beds and are common in the middle silty shale, but actual remains have never been found. No foraminifera were found in samples from the rusty beds, lower shale, and middle silty shale. However, one microfossil, a tintinnid about 0.10 mm. long, nearly always flattened in preservation, is common throughout these three mem- bers. This tiny marine protozoan also occurs in the lower portion of the upper shale. Arthur S. Campbell, St. Mary’s College, examined specimens of this tin- tinnid from the rusty beds and concluded that it is similar to Calpionella, but probably represents an undescribed genus. Teeth and bone fragments of fish also occur in the rusty beds, lower shale, and middle silty shale and less commonly in the upper shale. The uppermost portion of the upper shale in some places contains a few sponge spicules. No ostracods were found in the Thermopolis shale. Arenaceous foraminifera occur throughout the upper shale. The following 24 species have been found: Alveolophragmium linki (Nauss) Ammobaculites euides Loeblich and Tappan Ammobaculites fragmentarius Cushman Ammobaculites obliquus Loeblich and ‘Tappan Ammobaculites petilus n. sp. Ammobaculites subcretaceus Cushman and Alexander Ammobaculites tyrrelli Nauss Ammobaculoides phaulus Loeblich and Tappan Ammobaculoides whitney: (Cushman and Alexander) Ammomarginulina cragini Loeblich and Tappan Bimonilina variana n. sp. Glomospira reata n. sp. Glomospira tortuosa Nn. sp. Haplophragmoides gigas Cushman Involutina kiowensis (Loeblich and Tappan) Lituotuba sp. 24 STRATIGRAPHY OF THE THERMOPOLIS SHALE Miliammina ischnia Tappan Miliammina inflata n. sp. Miliammina cf. M. sproulei Nauss Saccammina alexanderi (Loeblich and Tappan) Spirolocammina subcircularis (Tappan) Trochammina depressa Lozo Verneuilinoides kansasensis Loeblich and Tappan Verneutlinoides hectori (Nauss) All but one of these species occur in the upper part of the upper shale, but only the following six species occur in the lower part. The first of these has been found only in the lower part, the next three are much more common in the lower part than in the upper part, and the last two are common throughout: Miliammina ischnia Miliammina inflata Miliammina cf. M. sproulei Spirolocammina subcircularis Trochammina depressa Verneuilinoides kansasensis Of the 18 species that have been described previously, the following nine are known only from the Kiowa shale of Kansas or Lower Cretaceous rocks of Texas: Ammobaculites euides Ammobaculites obliquus Ammobaculites subcretaceus Ammobaculoides phaulus Ammobaculoides whitneyi Ammomarginulina cragini Saccammina alexanderi Trochammina depressa Verneutlinoides kansasensis The following eight previously described species are known only from the Joli Fou shale and its correlatives in western Canada or Lower Cretaceous rocks of northern Alaska. In Canada, the Joli Fou and equivalent strata which con- tain most of these species are referred to as the Haplophragmoides gigas zone — (Wickenden, 1941, p. 153): Alveolophragmium linki Ammobaculites fragmentarius Ammobaculites tyrrelli Haplophragmoides gigas Miliammina sproulei Miliammina ischnia Spirolocammina subcircularis Verneutlinoides hectori One species, Involutina kiowensis, has been reported both from Kansas and from northern Alberta. The intimate mixture of Gulf Coastal and boreal faunas in the upper shale indicates that the Gulf Coast and boreal seas were connected and that the upper shale is coextensive with the Kiowa shale of Kansas and the Joli Fou shale of STRATIGRAPHIC DESCRIPTIONS 25 Alberta. All 24 species which occur in the upper shale, including those of boreal provenance, those of Gulf Coastal provenance, and those described as new or undetermined, are considered to constitute the fauna of the Haplophragmoides gigas zone in Wyoming. Strata in the lower part of the upper shale, which con- tain only six of the species, are a biofacies of the strata which contain all but one of the species and will be referred to as the Verneuilinoides kansasensis biofacies of the Haplophragmoides gigas zone after their most common species. ‘The strata in the upper part, which contain the nearly complete H. gigas fauna, including the most common species of the Kiowa and Joli Fou shales, will be referred to as the Ammobaculites euides biofacies of the H. gigas zone after their most com- mon species. The H. gigas zone thus embraces the upper shale member of the Thermopolis shale, although the species H. gigas itself does not occur throughout the entire interval in Wyoming. Southward across the Big Horn Basin, the Verneuilinoides kansasensis biofacies comprises an increasingly greater propor- tion of the upper shale. Muppy SANDSTONE Above the black shales of the Thermopolis and below the black shales of the Shell Creek lies a highly variable unit consisting of siltstone, sandstone, shale, and bentonite beds, and locally containing beds of lignite and chert pebble con- glomerate. In many places, the term Muddy sandstone is actually a misnomer inasmuch as the unit does not consist predominantly of sandstone. In some places, sandstone beds are almost altogether absent. The Muddy is generally 30 to 50 feet thick in the Big Horn Basin, but its extreme range is from a few feet to more than 100 feet. Thus, it is a distinctive but laterally variable unit which contrasts with the dominantly shale units below and above, whose beds are notably per- sistent and uniform. However, the Muddy as a heterogeneous unit is equally widespread. The mistaken belief that it must everywhere be a sandstone has led some workers to the erroneous concept that it occurs sporadically. In the southern part of the Big Horn Basin, the Muddy is more variable, both in thickness and lithology, than in the northern part. Great variations of thickness commonly take place within only a few hundred feet along the outcrop. Local thick lenses of blocky-weathering sandstone account for the more spectacu- lar variations. Localities 14 and 17 (figure 1), for example, are easily accessible areas where prominent sandstone lenses about 50 feet thick and a few hundred feet wide are well exposed. In spite of variations in thickness and lithology, the Muddy is everywhere present, although in places it lacks sandstone beds which give it topographic expression. The Muddy in the northern part of the basin is basically different. Its striking white or very light gray color is caused by a greater content of bentonite. Bentonite occurs in distinct though rarely pure beds and as matrix for sand- stones and siltstones. Scattered rounded and polished chert and quartzite pebbles up to two inches in diameter commonly weather out of the white bentonitic sandstones. Thick, lenticular, hard, blocky-weathering brown sandstone beds ap- pear to be absent. Thicknesses and rock types are somewhat more consistent laterally here than in the southern part of the basin, and facies changes are more gradual. The contact of the Muddy with the overlying Shell Creek shale is commonly gradational through a thin interval, but nearly everywhere it is easily selected at a horizon above which soft black shale predominates. 26 STRATIGRAPHY OF THE THERMOPOLIS SHALE FOSSILS Several kinds of fossils occur sparsely in the Muddy. Richard A. Paull (per- sonal communication) has collected specimens of vertebrates, marine inverte- brates, and plants. The plant remains, identified by Erling Dorf, Princeton Uni- versity, are: Cinnamomum scheuchzeri Heer Eugenia primaeva Lesquereux (?) Salix sp. The vertebrate remains, identified by D. H. Dunkle, United States National Museum, are: shark teeth and spines; Asteracanthus sp. cf. Hybodus sp. cf. Synechodus sp. chimaeroid fish spines; cf. Edaphodon sp. holosteans scales; cf. Lepidosteus sp. Amia sp. teleostei fragments turtle fragments; cf. Glyptops sp. crocodile teeth; Crocodylus sp. Invertebrate fossils found in the Muddy include Inoceramus sp. and other clams which were not determinable. Also, one species of foraminifera, Miliam- mina ischnia, occurs in a shale within the Muddy at locality 7. Silty and sandy shale beds transitional with the underlying Thermopolis shale at localities 19 and 22 contain foraminifera of the Ammobaculites euides biofacies. A silty shale transitional with the Shell Creek at locality 16 contains Spirolocammina sp. Silty shale beds in the uppermost part of the Muddy at locality 29 contain Miliammina ischnia and what appears to be Spirolocammina subcircularis, a Thermopolis shale species. SHELL CREEK SHALE Above the Muddy sandstone and below the siliceous Mowry shale of Darton lies the Shell Creek shale, a unit of dark gray and black shale containing a few ironstone beds and concretions and a few prominent white bentonite beds. Com- monly, some of the shale beds are silty, and a cross-bedded sandstone bed was found at one locality (Gypsum Creek, locality 22) in the Big Horn Basin. Ben- tonite beds are generally less than three feet thick, but beds as thick as nine feet occur. The thicker bentonite beds are widely extensive and are of value in correlation. In most well exposed outcrops in the Big Horn Basin, the contact of the Shell Creek with the overlying siliceous Mowry shale is easily chosen at the base of a silver-weathering ledge of the basal Mowry. Everywhere, it is persistent laterally, and in most places it marks a sharp vegetation change. Locally at the STRATIGRAPHIC DESCRIPTIONS 27 contact, large calcareous concretions with cone-in-cone structure occur. Difficulties in mapping this contact are understandably encountered in some places where it is poorly exposed, but the Mowry shale of Darton is an unusually resistant, distinctively weathering unit, and its peculiarities of hardness, color, and topo- graphic expression easily distinguish it nearly everywhere from the underlying soft dark Shell Creek shale. In general, these units can be distinguished as easily as any two formations of comparable thickness in the Big Horn Basin. The regional persistence of the contact is more apparent on electric log cross- sections (Mills, 1956, chart) than on surface cross-sections. On electric logs, the bed with high resistivity on which the contact is chosen can be traced confidently for great distances. This verifies the conclusion drawn independently from out- crop studies that thickness changes in the Shell Creek shale result from overlap or convergence within the unit itself and not from facies changes with the over- lying Mowry shale. The base of the Mowry shale appears to be a remarkably uniform horizon. In the Big Horn Basin, the Shell Creek shale is about the same thickness as the Mowry shale of Darton, ranging from 200 to more than 300 feet. Southeast- ward, however, it thins rapidly. Much of the decrease in thickness appears to take place across areas of the Big Horn and Owl Creek Mountains where only pre- Cretaceous rocks now occur (figure 3). FOSSILS No macrofossils were found at the type section of the Shell Creek shale, but a few fossils of vertebrates and invertebrates were collected in an area north of Shell Creek at localities 21 and 23 (figure 4). The fauna occurs principally in two beds of ironstone concretions whose stratigraphic positions are 160 and 187 feet above the base of the Shell Creek shale. J. T. Gregory identified the vertebrate fragments as scales of a garpike-like fish (which is either Lepidotes sp. or Lepidosteus sp.) and as pieces of a turtle shell, Glyptops sp. J. B. Reeside Jr., U. S. Geological Survey, identified ammonites as Neogastroplites haasi Reeside and Cobban, and stated (personal communication) that they represent the lowest of five zones of Neogastroplites which he recognizes in the Lower Cretaceous of the western interior. H. B. Roberts, U. S. National Museum, identified two kinds of crabs, Homolopsis sp. and Dakotacancer sp. In addition, this fauna includes at least one unidentified species of Inoceramus. The following 12 species of foraminifera have been found in the Shell Creek shale: Bimonilina variana n. sp. Eggerella sp. Glomospira glomerosa n. sp. Haplophragmoides multiplum Stelck and Wall Haplophragmoides uniorbis n. sp. Miliammina ischnia Tappan Miliammina manitobensis Wickenden Spirolocammina planula n. sp. Trochammina gatesensis Stelck and Wall Trochammina umiatensis Tappan Verneuilina canadensis Cushman Verneuilinoides hectori (Nauss) 28 STRATIGRAPHY OF THE THERMOPOLIS SHALE Of these, the following five have been found only in the uppermost portion of the Shell Creek shale in the northern part of the Big Horn Basin: Eggerella sp. Glomospira glomerosa Haplophragmoides multiplum Haplophragmoides uniorbis Trochammina umiatensis The remainder occur commonly throughout the sequence. The fauna is similar to the fauna of the Thermopolis shale in that it includes many of the same genera, and that all species are arenaceous. Only three species, Bimonilina variana, Miliammina ischnia, and Verneuilinoides hectori, are common to both formations. All previously described species are known only from western Canada or northern Alaska. Thus the fauna of the Shell Creek shale is entirely of boreal province, whereas the fauna of the upper shale member of the Thermopolis be- low represents both the boreal and Gulf Coastal provinces. Radiolaria occur in the upper third of the Shell Creek shale in the Big Horn Basin and range upward into the Mowry shale. The bell-shaped Clathrocyclas irrasa n. sp. is the most distinctive. The remainder are white, opaque spheres of two kinds, 0.05 mm. to 0.19 mm. in diameter. Some are comparatively smooth and finely perforate, and others have a somewhat spiny surface and are more coarsely perforate throughout. In addition, several kinds of siliceous sponge spicules occur commonly throughout the Shell Creek. No ostracods were found in the Shell Creek shale. BENTONITE BEDS Most bentonite beds represent individual falls of volcanic ash which accumu- lated very rapidly. Their basal contacts are sharp. Shards are coarsest in their lower parts and decrease in size upward. Upper contacts are gradational with overlying shale, indicating that the last of the ash settled out slowly, accompanied by increasing proportions of other clastic material. Some beds extend over thou- sands of square miles and thus represent large volumes of ash indicating enor- mous volcanic eruptions. Because individual bentonite beds were deposited everywhere simultaneously, their sharp basal contacts are excellent time horizons and they enable precise correlation. Many Shell Creek shale bentonite beds are regionally extensive, but others, particularly thin ones, are comparatively local. Inasmuch as even the thinnest beds probably fell universally in large areas, portions must have been eroded by currents after deposition and mixed with, and masked by, other clastic material. Some bentonite beds are only partly reworked and contain intermixed dark shale. Bentonite beds with peculiar characteristics are especially valuable, for they can be correlated confidently without physical tracing. One such bed in the Ther- mopolis, Wyoming area is exceptionally thick and has a dark brown-weathering lower portion. It can be traced over much of the southern part of the Big Horn Basin, and, by virtue of its brown-weathering portion, it can be recognized tentatively in the northern part of the Wind River Basin (figure 5). A similar though much thinner bentonite bed whose upper part weathers dark brown, occurs in the upper part of the Shell Creek in the northern part of the Big Horn Basin. STRATIGRAPHIC DESCRIPTIONS 29 10 20 MILES BIGIMORN BIG HORN CRETACEOUS «SHOSHONI WIND RIVER ~~? BASIN BIG HORN R., ! ARMINTO INDEX MAP SHALE =] SILICEOUS SHALE rs ETT] BENTONITE [=] sitstone ae SANDSTONE 12 MILES Figure 5. Tentative correlation of a distinctive Shell Creek shale bentonite bed between the Big Horn and Wind River Basins. Half-foot shale samples were taken immediately below and above five ben- tonite beds in the Shell Creek and Thermopolis shales in different localities, in order to survey the effect of ash falls on the benthonic foraminiferal faunas. Foraminifera were absent above only one of the bentonites and here some sponge spicules occurred. Immediately above three of the bentonites, a few specimens of some of the species which occurred below them were found. This shows that after ash falls had buried the bottom foraminiferal populations, the new bottom sur- faces were repopulated rapidly by some of the same species. The results of the samples taken above and below the fifth bentonite, a bed in the upper part of the Thermopolis shale, were entirely unexpected. No foraminifera occurred immedi- ately below this bentonite, but immediately above it they were profuse. Here the change in bottom conditions must have been distinctly favorable for the ben- thonic fauna. Table 1 summarizes the results of the five sets of samples. STRATIGRAPHY OF THE THERMOPOLIS SHALE 30 (juepunqe 112) sisuaspsuvy “4 vssaidap “I SUD] NILIIQNS *§ 1ajnoads "32 "Ww (I) sapina *p () satnoids asuods sapnoids asuods (9) (8) (1) (F) aaoqe saiadg ‘saivads yova Jo auepunge aanejar ay) SuNneu -rxoidde ‘payeorpur st paysid suautdads jo saquiny ‘saq1u0jUaq JAY aAoqe PUL MOTAq s[BAIAIUT 100J-J[VY Woy sazdures Jo yuaIUOD “| 21q¥,I, sajnoids asuods sisuasvsuDY *f 140329Y “4 pssaadap * I. sisuamory *] DUDILVE. “G7 snjnvyd “Pp GE A sisuasvsuvy *f SILDINILIIQNS *§ sisuamory *] DUDLLDA. *T idaupiym “py sninoyd “py 119440) * snarvja1Iqns "py sapina "p saqnoids asuods pinuvjd “5 sisuaqojiuvU "PW sisuapvuv *4 sisuasaqvo * 7 sisuaqojiuDU "Py sisuapvuvo *4 pjynuvjd *s§ MOTaq satads /L'0 Appa Mopaq ,gI ‘stfodouay yy, :1Z /6'0 :Appny Mopaq OI ‘syjodounsyT, ‘61 a :Appny Mojaq ,gI ‘syodounsyy :/ 40 :AppnyL aaoqe 0G “YP2ID TPYS :L1 4°0 :Appny aaoqe {LET “YI2ID T12yS 1 ssouysIy uorisod a31u0}Uuag Aqipeo0'[T REGIONAL RELATIONSHIPS RELATIONS TO THE COLORADO FRONT RANGE THERMOPOLIS SHALE The Thermopolis shale thins southeastward (figure 6). On the north flank of the Wind River Basin (locality 17) all four members can still be recognized, but farther south the middle silty shale and lower shale rapidly become thin and obscure. The middle silty shale persists possibly as far as Alcova (locality 28), where it may be represented by a resistant 12-foot sandstone and limestone unit, but in outcrops farther southeast, it cannot be discerned, and all the sandy lower part of the Thermopolis is referred to the rusty beds. The upper shale is much more consistent in thickness than the underlying members, although it, too, thins somewhat southeastward. Concomitantly, the Verneuilinoides kansasensis biofacies occupies an increasing proportion of it, and south of the Big Horn Basin the Ammobaculites euides biofacies is absent (fig- ure 7). On the south flank of the Wind River Basin (locality 27), foraminifera are common only in the upper part of the upper shale; in the lower part the shale is sandy, a few sponge spicules occur, and foraminifera are rare. Farther southeast at Alcova (locality 28), the entire upper shale is unusually thin, silty, and exceptionally carbonaceous, and foraminifera are absent altogether. Here only a few tiny tintinnids occur. This locality is unusual in that thinly inter- bedded black shales and gray siltstones of the rusty beds overlie thick, massive, light gray conglomeratic sandstone of the Cloverly with a sharp contact. Cloverly claystones and siltstones which occur farther north are absent. Southeast at Rock River (locality 29), the Verneuilinoides kansasensis biofacies is again present in the upper part of the upper shale. At Iron Mountain (locality 30), the upper few feet again contain the Ammobaculites euides biofacies. The sequence at Bellvue, Colorado (locality 18) is similar, containing A. euides in the upper few feet. The upper shale member is equivalent to the shaly middle por- tion of the South Platte formation in the Front Range foothills. South from Bell- vue these strata become increasingly sandy (Waagé, 1955, figure 17). The upper portion of the upper shale equivalents at Bellvue and at Iron Mountain contains thin sandy limestone beds and abundant fossils. Reeside (1923) described this fauna and reported the following species: Inoceramus comancheanus Cragin Inoceramus bellvuensis Reeside Pteria salinensis White Ostrea larimerensis Reeside Ostrea noctuensis Reeside Anchura kiowana Cragin? Reeside (1923, p. 200) interpreted this to be a northern extension of the Comanchean fauna which Stanton (1922) had shown to become progressively depleted northward from New Mexico and Oklahoma. Farther north in Wyo- ming and in the Black Hills, the upper shale and its equivalents appear to be almost entirely barren of macrofossils with the local exception of Inoceramus STRATIGRAPHY OF THE THERMOPOLIS SHALE 32 ‘asuryY JUOIY Ope1ojoD ay) pur urseg us0 Sig UIayII0U ay1 UVaMIaq aTeYs yaax77 Tyg pue suojspues Appnyy ‘aeys stpodounay.y, ayy Jo donrieass ‘9g oIn31y \ | | ee - at SS eo : a R Prt N — (7) (2) MS9-N CI-EI y3Aly MOB-NZ2-21 MCB-NEC-2 3NA1138 ONIN le "MLA NOU "90¥u SHYUVA “LiAOD Ie vAooI¥ ‘LAOD Sim OLNmUY 8! VIRBOIITVS of 62 OMEN SY B AYCD COVHD 82 3NN3A3HD Y3EUVE ai SNITI09 14 * Gy oqvyol10d ONIWOAAM 3WHS SITOdOWY3HL , fe) SNOILZYONOD BUNIHVO [) S1ISSOJV93RM -3 Os SHIVIONG SISNISVSNYN == 311NO1N38 [:] S3QIONI TINSNY3A = 001 aide S30¥ 4008 2 3IWHS SNOI9ITIS [] JTWHS SMOWORUIHL ~ 3LUNQLN3G NIHL (vy) pals V HLIM 3NOLSONVS 002 ViEWIOIIvY ONY i - M68-N&b-S2 MI6-NOG -12 3NO1S3h7 ABBV7S Dee Sanat N aapeanuaiie sae LiAOD ly d331SN3L =6y UNM HOIND 3SHOH «= ‘NL d33HS "33ND WMSdAD o0¢ TWAYILNI 0343A09 [5] ONv aNousins 4 auvosv3s $\ PETS) 61 zz 1334 $3UNSOdX3 YOOd Pf OEM ae 33 REGIONAL RELATIONSHIPS ‘aSuULY JUOIY OpLAO[OD ay) pue uIseg UIOZY Sig UsayIIOU ayy uvamjaq ayeys taddn ay ur sasueys [euney Surmoys ayeys stfodouray.y, ay Jo uonoas-ssor9 orydesSneng “2 aan8rq S3I1dNVS ON-SN STSSOJONOUW -3 s WAU3LNI 0383A00[9 ] 3Y¥NSOdx3 YOOd [9F] 0 SNOIL3YONOD AUTHVA HLIM 31VHS Ee) 3LINOLN3@ NIHL Vv HLM 31vHs LI) $wI7709 44, 8), ' oavy0109 I Wy BIAVEVT Sait ' Of"~~_ 62 0s 3NOLSONVS [_] \ o ol 3NOLSANI7 ABvIs FY uaasvo, = uzowe = 030038Y31NI 31vHS Soe / m ONY SNOISIIS LJ © S/SWISYSNUM A > ae a os! 3NOLS3WIT NIHL 3O LIWIT AS aoe 2 1334 V HLIM 31VHS oy nS NOLWAYOS 37LA7, $310v4019 ASS 2 Ww BaeMaN ts $IOINZ V 30 LIWIT MS oO maining] - ~ = = — ac of = f —~ 2) a) x Ss BSWHS |= = - zi a Sp | 7) = ees iY, =Te ™ 3700 | 1 Yyyy es Hy i: ele Yffyyy ma / = ° as aL /j.. YR : “ZF Ui, SSNISUSHUN fz Sea Oe pe : me ES Meco Vlsitooo pe IID LLL LL LL EE = crosnusneenvesweanes ese ba at S3TIW #9 S37IN OF = S320IW 99 S327IN Ld i S37'IW B89 S31IW 2€ = suis $ fee NIVLNNOW uBAly Nisva NIVLNNOW 43340 anu138 NOU! ¥90u MAO ORY NOLLNO SITOdOWYSHL 433HS Wnsdad HLNOS 8! of 62 ez 42 61 zz HLYON 34 STRATIGRAPHY OF THE THERMOPOLIS SHALE comancheanus. Still farther north in central Montana, however, Cobban (1951, p- 2176) has reported most of the species described from Colorado by Reeside. Good outcrops of the lower part of the Thermopolis are scarce between the Wind River Basin and the Colorado Front Range foothills. At Rock River the upper shale is well exposed but the thin rusty beds equivalents are totally covered. Near Douglas (locality 31, not shown on figures 6 and 7), however, more than 40 feet of fucoidal and carbonaceous thin- and medium-bedded sandstone and silty shale of the upper part of the rusty beds is well exposed; below it are less well-exposed, thicker, more resistant beds which help support a prominent hogback. South at Iron Mountain on the east flank of the Laramie Range (local- ity 30), rusty beds equivalents consist of about 50 feet of resistant brown-weath- ering interbedded sandstones, siltstones and some shale which form the promi- nent lower ridge of the Dakota double hogback of the area. In northern Colorado, the Plainview sandstone member of the South Platte formation occupies the same stratigraphic position. It is about 30 feet thick, contains fucoidal markings and carbonaceous material, and it weathers rusty brown. Its lithology is very similar to the rusty beds in northern Wyoming, but it contains more sand and, like the rusty beds in extreme southern Wyoming, it forms a ridge. Light-colored non- marine strata immediately below it commonly contain iron oxide spherulites. South of the northern Front Range foothills it is absent (Waagé, 1955, p. 42). MUDDY SANDSTONE South of the Big Horn Basin, the Muddy remains characteristically variable. At some places, it consists mainly of sandstone beds, but at other places, it consists mainly of siltstone and shale beds. Yet the formation is an easily traceable, per- sistent stratigraphic unit, occurring always immediately above the Haplophrag- moides gigas zone of the Thermopolis shale. Its consistent occurrence above these strata and below beds containing the distinctive Shell Creek fauna illustrates that the Muddy everywhere occupies the same position in the stratigraphic sequence, and that as a whole, it is continuous, even though individual beds within it are not. The Muddy can be traced southward into the Front Range foothills where it correlates with the uppermost sandstone of the South Platte formation. SHELL CREEK SHALE The Shell Creek shale becomes thinner southward and pinches out alto- gether in southern Wyoming (figure 6). At Arminto (locality 17) on the north flank of the Wind River Basin, it is only 80 feet thick, and at Alcova (locality 28), where it is only 25 feet thick, the southernmost Shell Creek foraminifera occur. At Rock River (locality 29), the Shell Creek appears to be absent, the siliceous shales of the Mowry resting on thin sandstones and silty shales of the Muddy. The contact between the Muddy and Shell Creek seems to become increasingly gradational southward. At Alcova, it is particularly difficult to pick but is chosen, as elsewhere, at a horizon below which siltstone and sandstone is dominant and above which black shale is dominant. Possibly the Shell Creek includes some silt- stone beds in its lower part near its southern limit. If so, these would probably be included with the Muddy. At Rock River, for example, some of the thin silt- stone and shale beds in the uppermost part of the rather thick Muddy sequence possibly correlate with part of the Shell Creek to the north. However, these strata contain only non-diagnostic foraminifera and a few sponge spicules; sponge spicules are known elsewhere from the Muddy as well as from the Shell Creek. 35 REGIONAL RELATIONSHIPS ‘(6 ‘Id ‘Gg6I) PSOH{ J0IJe payTpour UOTIe[aIIOD soKJINsqngE ‘s[ITH WLI IY} pure urseg UIOH{ BIg oy} UdsaMmiaq aTeYs Y221D []eyS pue suojspues Appnyw ‘aeys stjodousyT, sy} Jo uoNL[a1I0D °g ainsi WAYILNI G3N3A09 (9) SNOILJYONOD ALMHVO [=] $30V50I8 S/SNISYSNUM JLINOLN39 FJ SIOIONITININYTA = $31I0v30I8 S70INI = 31vHS SNOZ9ITIS FJ SILITINIVGOWNY = ° 3IVHS SIOdOWURHL y LINCS HOLeANGe ©) os viuvtoiva anv— \ 030038u3LNI 37VHS ATNO WuaJINIWWuOd anv anoisinis oo! N 31VHS 3349 173HS Nim gave Ge) «=—- WLOVO HLNOS oz NOLLVANYOS —_-— A183A019 $a398 r-] ALSNY = : a: 31 37VHS : =| 43M01 °=) RS ADS W ==|= 31VHS~ = y3addNn SWHS SINMdOWYSHL ‘SS JTLSVOM3N = =——= (9) os rae MS9-NSb-EI (8) (v) AUMOW 2 3143WAWNI» ML9-NOS-BI M2L-N2S-2 SONIYdS LOH NO1LIIND W 13 vuYN3Is | NOSNIGOY 1 LINN NOOV N33u9 ACGNA d331SN3L SITOdOWYSHL €¢ Ze -NYOH DIG = d0D 13d VOVUINY ‘09 SVX3L 92 Gi G-2 36 STRATIGRAPHY OF THE THERMOPOLIS SHALE Farther south, no Shell Creek correlatives appear to exist, and it is probably represented by a hiatus at the top of the South Platte formation in the Colorado Front Range foothills. At Iron Mountain (locality 30) where Shell Creek correlatives are almost cer- tainly absent, the siliceous Mowry shale appears to directly overlie a thick resist- ant quartzite bed in the upper part of the Muddy. Quartzite rarely occurs in the Muddy to the north where it is overlain by the Shell Creek shale, but south of the Shell Creek pinchout, quartzite is common just below Mowry equivalents. In the Colorado Front Range foothills, quartzite beds usually comprise the uppermost portion of the South Platte formation (Waagé, 1955, p. 40) and help support the first great ridge of the Dakota double hogback. RELATIONS TO THE BLACK HILLs THERMOPOLIS SHALE On the east flank of the Big Horn Mountains at Muddy Creek (locality 26), four members of the Thermopolis are as easy to recognize as in the Big Horn Basin (figure 8). The rusty beds are slightly thinner, but the middle silty shale is a few feet thicker and has a threefold cyclic character, each cycle grading upward from shale through siltstone to a limestone bed. Eastward across the Powder River Basin, the rusty beds maintain fairly consistent thickness but be- come progressively more sandy, and in the Black Hills they consist largely of sandstone with some interbeds of shale. The overlying dahllite concretion-bearing shale capped by a six-foot sandstone bed also becomes sandy eastward and gives way similarly to sandstone beds. Together, the equivalents of the rusty beds and overlying 25 feet of the lower shale at Muddy Creek appear to comprise the cliff- forming Fall River sandstone of the Black Hills. Typically, the Fall River contains some thickly bedded and some thinly bedded flaggy sandstone with abundant fucoidal markings and a little inter- bedded shale. Its appearance resembles that of the Plainview sandstone of north- ern Colorado, and like the Plainview, it overlies nonmarine strata which in most places contain iron oxide spherulites immediately below the contact. Toward the southern Black Hills, the middle silty shale and the upper part of the lower shale become thin and obscure and finally appear to pinch out entirely (figure 8). Toward the northern Black Hills, however, these units main- tain their thickness and character, at least in the subsurface (Peterson, 1956, chart), and comprise a significant portion of the Skull Creek shale there. The upper shale thickens somewhat toward the Black Hills. Northeastward the Ammobaculites euides biofacies becomes thicker (figure 9). A complete sec- tion of upper shale was not sampled in the Black Hills, and it is not known exactly what proportion belongs in the respective biofacies of the Haplophrag- moides gigas zone. The upper 60 feet of the Skull Creek shale at locality 37 (figure 9) contains only the A. euides fauna, and the lower 60 feet at Hot Springs (locality 33) contains only the Verneuilinoides kansasensis fauna (figure 8). At Hot Springs, foraminifera occur down into lowermost Skull Creek strata immedi- ately above the Fall River sandstone. This substantiates electric log correlations that the entire Skull Creek in the southern Black Hills correlates with the upper shale of the Thermopolis. Northward in the Hills, the Skull Creek becomes thicker, not as a result of significant thickening in the upper shale, but as a 37 REGIONAL RELATIONSHIPS ‘SITTH (OIG UaYIIOU ay) puL UIseg UIOP{ Sq UaYINOS dy) usamjeq ayeys saddn ay) ur sasueys yeunvy Surmoys ayeys stpodourssy.T, ey} JO UoNIaS-sso19 orydesrsyeis “6 ons NOILVWYOS 7 N SVS