BULLETIN OF THE AMERICAN ASSOCIATION OF PETROLEUM GEOLOOISTS VOL. 34, NO. 12 (DECEMBER, 1930), PP. 2337-2346. 2 FIGS.

REVISION OF PIERRE SHALE OF CENTRAL SOUTH DAKOTA1 DWIGHT R. CRANDELL2 New Haven, Connecticut ABSTRACT The stratigraphic subdivision of the Pierre shale in South Dakota is still in a stage of development many revisions having been made of the original units proposed by Searight in 1937. In the present revision a classification is presented that conforms to rules of the Stratigraphic Code. Three sub­ divisions of the Sully member, as used by the South Dakota Geological Survey, have been raised to member rank and the term Sully has been discarded. The Agency-Oacoma zone of Gries has been redefined and renamed the DeGrey member of the Pierre shale.

INTRODUCTION Recent geologic quadrangle mapping, by the United States Geological Survey, of parts of the Missouri River Valley in South Dakota has accentuated the need of a generally acceptable terminology for the subdivisions of the Upper Cre­ taceous Pierre shale of central South Dakota. The South Dakota Geological Survey subdivided the Pierre shale in 1937, and has since made several revisions of names and of boundaries between units. This paper is a contribution to the nomenclature of several of the lower units of the Pierre shale.

SUMMARY OF UPPER MEMBERS OF PIERRE SHALE The Pierre shale of South Dakota was originally named the Fort Pierre group by Meek and Hayden (1862, pp. 419, 424). Meek and Hayden made a rough subdivision of the Fort Pierre group but gave these subdivisions no formal names. The first comprehensive attempt to subdivide the Pierre shale in South Dakota was made by W. V. Searight in 1937. He defined five members, differen­ tiated chiefly on the basis of lithology, as follows. Pierre shale Elk Butte member Mobridge member Virgin Creek member Sully member Gregory member

The revisions of the Pierre shale presented in this paper are primarily con­ cerned with the Gregory and Sully members of Searight (1937). Searight's Elk Butte member, Mobridge member, and Virgin Creek member are accepted

1 Published by permission of the director of the United States Geological Survey. Manuscript received, April 17, 1950. 2 United States Geological Survey. During two field seasons spent in quadrangle mapping near Pierre (pronounced pier), South Dakota, the writer has studied the several members of the Pierre shale exposed in that and adjacent areas. The writer wishes to acknowledge with gratitude the sug­ gestions of J. B. Reeside, Jr., and W. A. Cobban, both of the United States Geological Survey, and of Karl M. Waage of the department of geology, Yale University, made during the writing of this manuscript. David H. Dunkle of the United States National Museum identified the vertebrate fossils.

2337 233& DW1GHT R. C RAX DELL as originally defined and a short description of each, taken from Searight, is here given. Elk Butte member.—The Elk Butte member (the uppermost part of the Pierre shale) includes the beds between the top of the Mobridge member and the base of the Fox Hills sandstone. The member includes at its base non-calcareous shale that overlies the calcareous shale of the Mobridge member. At the type section the Elk Butte member is 270 feet thick and consists predominantly of gray shale. The name Elk Butte is taken from Elk Butte in eastern Corson County, South Dakota, though it appears that Searight mistook Rattlesnake Butte for the true Elk Butte that lies 5 miles west of Rattlesnake Butte.3 The type section is here designated as the section along Federal Highway 12, between 15 and 5 miles west of Wakpala. Mobridge member.—The Mobridge member of the Pierre shale is a succes­ sion of highly calcareous shale and marl beds4 that lies above the "gumbo- forming" shale of the upper part of the Virgin Creek member and below the non- calcareous shale of the Elk Butte member. The Mobridge member forms a wide buff-colored strip on the outcrop between dark-colored beds above and below. At the type section the member is 136 feet thick. The Mobridge member is named for the city of Mobridge, South Dakota. The type locality is in south­ eastern Corson County, at the west end of a highway bridge across the Missouri River at Mobridge (Searight, 1937, p. 44). Virgin Creek member.—The Virgin Creek member of the Pierre shale in­ cludes all beds between the Verendrye shale zone of Searight (Verendrye mem­ ber of this paper) below and the calcareous Mobridge member above. The con­ tact of the Virgin Creek member and the Verendrye member is marked by beds of bentonite. The member at the type section is between 185 and 225 feet thick (Searight, 1937, p. 43), and consists of gray clay and shale, the upper part of which has a zone of limestone concretions containing the fauna of the Sage Creek formation of Douglass. The Virgin Creek member was named for an ex­ posure in the valley wall of Virgin Creek, 15 miles south of Promise in north­ eastern Dewey County, South Dakota.

EVOLUTION Or TERMINOLOGY OF LOWER PART OF PIERRE SHALE Searight (1937, p. 21) defined the Sully member as the "body of shales lying between the chalky beds of the upper Gregory and the base of the highly bentonitic beds which lie at the base of the Virgin Creek member." He further divided the Sully member (1937, p. 22) into three units "readily recognizable over the area of outcrop. These . . . are so characteristic in lithologic expression that names are applied to them here." These units were designated as here shown.

3 W. A. Cobban, oral communication. 4 The term "marl" is here used to denote a calcareous clay, in this case marine. REVISION OF PIERRE SHALE OF CENTRAL SOUTH DAKOTA 2339

Sully member Verendrye shale zone (Verendrye shale, zone, beds! Oacoma zone (Oacoma mangapiferous zone) Agency shale (Agency shale zone) The terms in parentheses represent the alternative names by which Searight designated the units of the Sully member. The lower part of the Sully member had been previously described and named Agency shale by Russell (1930, p. 5). The uppermost subdivision of the Sully member, the Verendrye (pronounced Ver-en-dree) shale zone of Searight, was described as a succession of clay and shale locally containing "clay ironstone" concretions. It underlies the Virgin Creek member. Searight (1937, p. 25) took the name Verendrye from "the ex­ posure under and above the Verendrye monument at Fort Pierre, in Stanley County." Searight described the Oacoma zone as consisting of a succession of highly bentonitic shales and clays typically exposed at Oacoma in southeastern Lyman County, South Dakota. The most distinctive characteristic of the Oacoma unit at the type section is the high content of iron-manganese concretions that weather from the shale. From Oacoma 60 miles northwestward to Pierre the quantity of concretions diminishes; they are comparatively uncommon at Pierre. In this same direction the Oacoma assumes a banded, "stairstep" character caused by thin dark non-resistant bentonitic clay beds occurring between more resistant light-colored shales. The Agency shale, according to Searight, consists of hard light gray siliceous shale lying above the Gregory member and below the Oacoma zone. It crops out in the walls of the Missouri River Valley continuously southward from the type section at Cheyenne Agency, South Dakota, to the confluence of Crow Creek and the Missouri River in Buffalo County, South Dakota. South of Crow Creek the Agency shale was not originally recognized by Searight, and the Oacoma zone was thought to rest directly on the Gregory member as first defined. Searight divided the Gregory member into two units on the basis of their lithology and faunas. He described (1937, p. 11) the lower part of the Gregory member as "dark, bentonite bearing, bitumenous [sic] shales which contain numerous fish scales" and the upper part as "a thin but characteristic zone of chalk, chalky shale, argillaceous chalk, or marl. From the vicinity of the mouth of White River in Lyman County to the northernmost exposures of the member near DeGray [DeGrey] in Hughes County, the lower and upper Gregory are separated by a very thin but persistent bed of sandstone." The sandstone was included in the upper Gregory. The total thickness of the member as defined by Searight was 53 feet. Searight later (Moxon, Olson, Searight, and Sandals, 1938) noted the simi­ larity of part of his Gregory member to the Sharon Springs member (Elias, 1931) of the Pierre shale in northwestern Kansas. He imported the term "Sharon Springs" to include all but the upper marl and sandstone unit of his Gregory TABLE I SUMMARY OP CHANGES IN STRATIGRAPHIC NOMENCLATURE OF SUBDIVISIONS OF PIERRE SHALE IN CENTRA

Searight Searight Moxon and Others Gries and Rothrock 1937 1938 1939 1941 Elk Butte member Elk Butte member Elk Butte member Elk Butte member Elk Bu Mobridge member Mobridge member Interior member Mo >ridge member Mobrid

Virgin Creek member Virgin Creek member Virgin Creek member Virf rin Creek member Virgin C Verendrye Verendrye Verendrye Verendrye shale zone shale zone zone beds be r Oacoma .a Oacoma Oacoma Oacoma zone zone zone s zone a

y me m a a a "3 Agency a Agency Agency Agency- CO _>> shale "3 shale shale "3 shale "3 CO CO CO upper Gregory marl Gregory marl Crow Creek sand and marl ch

shale zone upper so g 0 a marl zone mem t s membe r Sull y Sharon Springs lower member 'C O. upper CO Gregor y (=S 0 lower c har 1-1 m

a membe r J3 lower CO i Sharo n Spring s REVISION OF PIERRE SHALE OF CENTRAL SOUTH DAKOTA 2341 member. The name Gregory was applied to the marl and sandstone and it was added to the Sully member as its basal subdivision. In the following year, Searight (Moxon, Olson, and Searight, 1939, p. 20) subdivided the Sharon Springs member in South Dakota, on the basis of lithol- ogy, into a lower unit and an upper unit and stated: "In South Dakota all beds above the Niobrara formation and below the Gregory marl are included in the Sharon Springs member." Subsequent work by Gries and Rothrock (1941) demonstrated that the beds comprising the "upper" unit of Searight's Sharon Springs member lie above a marl; thus the original Gregory member of Searight included two marls. In an attempt to clarify the situation Gries and Rothrock changed the name of the upper part of the Sharon Springs to Gregory member, and introduced the name Crow Creek sand and marl to include Searight's Gregory marl at the base of the Sully member and the thin sandstone directly beneath it. The Crow Creek sand and marl takes its name from exposures at the mouth of Crow Creek, Buffalo County. This latter usage of the names Sharon Springs, Gregory, and Crow Creek is followed in this paper, except that the Crow Creek has been raised to the rank of a member of the Pierre shale. In 1942 Gries recognized that the Oacoma unit at the type section included beds below what was taken to be the base of the same unit at Fort Pierre. He suggested that the lower beds of the Oacoma unit south of Crow Creek increase in thickness northward and form the Agency unit. Searight (1938) anticipated this relationship in a faunal study of the Sully member which indicated that the Agency unit "formerly supposed to pinch out south of the Great Bend [Big Bend, see Fig. 1], is represented in a thin zone between the Gregory marl [Crow Creek sand and marl of Gries and Rothrock] and the Oacoma." Gries (1942, p. 17) further noted that a distinctive bentonite bed at the base of the Oacoma unit at Fort Pierre was recognized 72 feet below the top of the Agency unit at Cheyenne Agency, and concluded that "the Oacoma zone as defined by Searight included not only the light and dark banded beds of Russell, but also part, if not all, of the Agency zone. Inasmuch as it is not possible to draw the line between the true Agency and the banded beds in the type locality of Searight's Oacoma zone, it is here suggested that both terms be retained, and that all beds lying between the Crow Creek chalk and the base of the Verendrye zone be hereafter referred to as the Agency-Oacoma zone."

REDEFINITION OF AGENCY-OACOMA ZONE OF GRIES The present writer considers retention of Gries' term "Agency-Oacoma" inadvisable. The type sections of the Agency shale and of the Oacoma zone of Searight are nearly 100 miles apart. The base of the Agency is not exposed at its type section, and at Oacoma there is but a very attenuated representative of the Agency. Thus neither at Cheyenne Agency nor at Oacoma is there a representative section of the complete Agency-Oacoma zone of Gries. A new name, the DeGrey member of the Pierre shale, is therefore proposed 2342 DWIGHT R. CRANDELL

WA3HA- BAUGH; MELLETTE

] ] TRIPP B^~ | TODD ! |GREGORY^.heeler

FIGURE I.- INDEX MAP OF CENTRAL SOUTH DAKOTA

Scale 25 50 75 Miles

FIG. I REVISION OF PIERRE SHALE OF CENTRAL SOUTH DAKOTA 2343 for this unit. The type locality of this new member is representative of the sec­ tions at both Cheyenne Agency and Oacoma. It is located on the west edge of the NW. I of Sec. 8, T. 109 N., R. 75 W., Hughes County, South Dakota (Fig. 2). The section is in an old bluff of the Missouri River and is marked by a prominent knob about 50 yards east of an unimproved road. In the type columnar section of the DeGrey member here given, the units numbered 1 through 6 are about 100 yards east of the rest of the section. Correla­ tion between outcrops was based on a distinctive bentonite bed. Color values of the various beds (when dry) are based on the Rock-Color Chart (Goddard and others, 1948).

Pierre shale (in part) Feel Inches Verendrye member (basal part) 1. Shale, light olive-gray (5Y5/2) to dark gray (N3); weathers to "gumbo"; lacks apparent bedding; forms smooth slopes 5 + DeGrey member 2. Shale, gray; contains six or more bentonite beds, one of which contains biotite flakes; (upper micaceous bentonite of Gries, 1940) 4 ir 3. Bentonite, grayish yellow (5Y8/4); (big bentonite bed of Gries, 1940) n 4. Shale, gray; outcrop of upper 8 feet covered with iron-manganese carbonate concretions; contains several thin bentonite beds in upper part 12 5. Shale, gray; contains many bentonite beds 13 6 6. Bentonite, yellowish gray (5Y7/2); contains concentration of biotite flakes; (lower micaceous bentonite of Gries, 1940) 3 7. Shale, gray; contains several thin bentonite beds 6 8. Shale, gray; outcrop of upper 10 feet stands nearly vertically, and is stained a "rusty brown" on surface 26 6 9. Shale, gray; contains two 6-inch bentonite beds bearing biotite flakes 5 6 10. Shale, gray, non-calcareous; contains several bentonite beds 12

Thickness 81 7 Crow Creek member n. Marl, or calcareous clay; unoxidized color, light gray (N7); oxidized color, yellowish gray (5Y7/2) 7 12. Sandstone, calcareous, stratified, slabby; grades upward into marl; unoxidized color, yellowish gray (5Y7/2); oxidized color, dark yellowish brown (10YR4/2). 1 6

Thickness 8 6 Gregory member (upper part) 13. Shale, gray, non-calcareous 20+ The units in the DeGrey member, including numbers 2 through 7, crop out as a succession of steps formed by resistant thin-bedded shale alternating with non-resistant bentonite and bentonitic clay beds. The color of this portion of the section varies from light gray (N7) to medium gray (N5). The contacts between the DeGrey member and the Verendrye member and between the DeGrey member and the Crow Creek member are gradational, whereas the contact between the Crow Creek member and the Gregory member is sharp.

RECLASSIFICATION OF SULLY MEMBER The Lexicon of Geologic Names (Wilmarth, 1938, p. 1657) designates the Pierre shale as the basal formation of the Montana group in the Missouri River Valley. As the Pierre shale is of formational rank, its subdivisions, in accordance 2344 DWIGHT R. CRANDELL with the rules of stratigraphic nomenclature, are members. It has been noted above that the South Dakota Geological Survey has further subdivided the Sully member into units designated as zones. According to the Stratigraphic Code (Committee on Stratigraphic Nomenclature, 1933), a zone is a subordinate unit embracing the rocks deposited during the time of existence of a particular faunal or floral assemblage. It may have the magnitude of a bed, a member, a

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\ \ •*. \ \ * J* ^^ \ \ • 0, \ * \\N \ <^ \ » .OE GREY MEMBER . \ ^V ' II 'TYPE SECTION £-\ HUGHES \ >s. ! \ ^. COUNTY , \ 7 '1 8 •*> ^ re \ "*• *WjP XJ ^

Scolt in f«et \v 0 tooo 4000 \ Scolt in mtl«s \ \ 0 1/2 1 \ \ \

FIG. 2.—Location of type section of DeGrey member of Pierre shale. formation, or even a group. The term "zone" was originally used by Searight in a lithologic sense, as it has been by Gries and Rothrock, and correspondence to faunal zones was not implied. The Stratigraphic Code states that subdivisions of members should not re­ ceive formal names. This ruling automatically excludes naming the subdivisions REVISION OF PIERRE SHALE OF CENTRAL SOUTH DAKOTA 2345 of the Sully member. To preserve the identity of these subdivisions the writer proposes raising the Verendrye unit, the Agency-Oacoma unit (the DeGrey member of this paper), and the Crow Creek unit to the rank of members for the reasons here given. 1. The Verendrye, DeGrey, and Crow Creek members are lithologically dis­ tinct in the area of the present investigation, as shown both by personal observa­ tions and by the reports of the State survey. The "gumbo-forming" shale of the Verendrye member, the banded beds and iron-manganese carbonate concretions of the upper part of the DeGrey member and the siliceous shale of the lower part of that member, and the marl and sandstone beds of the Crow Creek member are readily mappable units. 2. The Verendrye member is typically non-fossiliferous, except that Baculites compressus Say fragments occur locally near its base, near Pierre. The DeGrey member contains a varied fauna, only a part of which has been found near Pierre. In addition to a varied invertebrate fauna of microfossils, cephalopods, and mollusks, near Pierre several marine vertebrates have been found in the upper part of the member. Two mosasaur specimens have been identified as Platycarpus cf. P. brachycephalus Loomis, and a turtle as Archelon cf. A. ischyros Wieland. The only fauna of the Crow Creek member known to the writer con­ sists of microfossils. 3. Mapping of the three members may have economic significance. The bento- nitic beds in the upper part of the DeGrey member render it particularly sus­ ceptible to slumping, which makes this member a matter of importance in high­ way engineering. The sandstone in the basal part of the Crow Creek member forms an excellent key bed for structural mapping. Of probable future impor­ tance is the presence of a large amount of manganese, in the upper part of De- Grey member from the type section southward, which occurs in the iron- manganese concretions.

SUMMARY OF LOWER MEMBERS OF PIERRE SHALE Verendrye member.—The Verendrye member consists of clay and shale beds lying between the bentonites in the basal Virgin Creek above and the DeGrey member below. The lower part of the Virgin Creek member commonly retains a shaly appearance where it crops out. The shale and clay of the underlying Verendrye break down to "gumbo" on outcrops. At the type locality the member is about 160 feet thick. The thickness of 170-180 feet, as reported by Searight (1937, p. 31), was perhaps due to his inclusion of a basal portion of the section that had slumped. DeGrey member.—The DeGrey member consists of clay, shale, and bentonite beds. The top of the member is arbitrarily placed at the contact of the "gumbo- forming" shale and clay of the Verendrye member and the "step-forming" shale of the DeGrey member. This contact coincides approximately with the upper limit of abundant iron-manganese carbonate concretions in the area south­ east of Pierre. The base of the member is defined as the contact of the non-cal- 2346 DWIGHT R. CRANDELL careous shale of the DeGrey member and the calcareous beds of the Crow Creek member. At the type locality the member is about 82 feet thick; the thickness increases toward Pierre to about 160 feet and decreases toward Chamberlain to about 40 feet. The DeGrey member of the Pierre shale is named for the DeGrev Post Office and store in southeastern Hughes County, South Dakota. The type section of the member is 2 miles southeast of DeGrey in an old cut-bank of the Missouri River. Crow Creek member,—The term "Crow Creek member" is here given to the sandstone and calcareous shale and marl beds lying between the non-calcareous shales of the Gregory member below and the DeGrey member above. This usage follows that of Gries and Rothrock (1941), except that the unit has been raised to rank of member. The member ranges in thickness from a few feet to 15 feet. The type locality of the Crow Creek member is at the mouth of Crow Creek, Buffalo County, South Dakota. Gregory member.—The Gregory member of the Pierre shale, as here adopted, is the Gregory member of Gries and Rothrock (1941), the greatly restricted Gregory member of Searight (1937). The member includes all the beds between the Crow Creek member above and the Sharon Springs member below. The member is 34 feet thick at the type section, and consists of beds of light gray to buff-colored shale with many concretions and calcareous layers, and locally, as in Gregory County, basal marl beds. The name Gregory (Searight, 1937, p. 10) was taken from excellent exposures of this member along the Missouri River in eastern Gregory County at "the cut bank at the south end, Gregory County end, of Rosebud bridge south of Wheeler, Charles Mix County." Sharon Springs member.—The Sharon Springs member, introduced by Elias (1931), is here restricted to the fish-bearing bituminous shale that overlies the Niobrara formation and underlies the Gregory member. The member is about 35 feet thick at Rosebud bridge south of Wheeler, Charles Mix County.

REFERENCES Committee on Stratigraphic Nomenclature, 1933, "Classification and Nomenclature of Rock Units," Bull. Geol. Soc. America, Vol. 44, pp. 423-59. ELIAS, M. K., 1931, "The Geology of Wallace County, Kansas," Kansas Geol. Survey Bull. 18. GODDARD, E. N., AND OTHERS, 1948, Rock-Color Chart, Nat. Research Council, Washington, D. C. GRIES, J. P., AND ROTHROCK, E. P., 1941, "Manganese Deposits of the Lower Missouri Valley in South Dakota," South Dakota Geol. Survey Rept. Inv. 38. GRIES, J. P., 1942, "Economic Possibilities of the Pierre Shale," South Dakota Geol. Survey Rept. Inv. 43- MEEK, F. B., AND HAYDEN, F. V., 1862, Proc. Philadelphia Acad. Set., Vol. 13. MOXON, A. L., OLSON, 0. E., SEARIGHT, W. V., AND SANDALS, K. M., 1938, "The Stratigraphic Dis­ tribution of Selenium in the Cretaceous Formations of South Dakota and the Selenium Content of Some Associated Vegetation," Amer. Jour. Botany, Vol. 25, No. 10, pp. 794-809. MOXON, A. L., OLSON, O. E., AND SEARIGHT, W. V., 1939, "Selenium in Rocks, Soils, and Plants," South Dakota Agric. Exper. Sta. Tech. Bull. 2, pp. 5-94. RUSSELL, W. L., 1930 "The Possibilities of Oil and Gas in Western Potter County," South Dakota Geol. and Nat. Hist. Survey Rept. Inv. 7. SEARIGHT, W. V., 1937, "Lithologic Stratigraphy of the Pierre Formation of the Missouri Valley in South Dakota," South Dakota Geol. Survey Rept. Inv. 27. , 1938, "The Microfauna of the Sully Member of the Pierre," Proc. Iowa Acad. Sci., Vol. 45, pp. 135-37. WILMARTH, M. GRACE, 1938, "Lexicon of Geologic Names," U. S. Geol. Survey Bull. 8g6.