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 r shale zone shale zone zone beds be m Oacoma .a Oacoma Oacoma Oacoma me zone zone zone s zone a y a a "3 a Agency Agency a Agency- Agency r CO _>> shale "3 shale shale "3 shale "3 CO CO CO upper Gregory marl Gregory marl Crow Creek membe sand and marl ch y Sull shale zone r so g t upper 0 a marl zone mem membe s y Sharon Springs lower member 'C s O. upper CO r Gregor (=S 0 c Spring 1-1 lower har n m a membe J3 lower CO i Sharo REVISION OF PIERRE SHALE OF CENTRAL SOUTH DAKOTA 2341 member.