Geological Society of America 3300 Penrose Place P.O. Box 9140 Boulder, CO 80301 (303) 447-2020 • fax 303-357-1073 www.geosociety.org Chapter 2: “Revised lithostratigraphy of the lower Pierre Shale Group (Campanian) of central South Dakota, including newly designated members” (Martin et al.), in SPE427: The Geology and Paleontology of the Late Cretaceous Marine Deposits of the Dakotas (Martin and Parris, eds.) This PDF file is subject to the following conditions and restrictions: Copyright © 2007, The Geological Society of America, Inc. (GSA). All rights reserved. Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in other subsequent works and to make unlimited copies for noncommercial use in classrooms to further education and science. For any other use, contact Copyright Permissions, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA, fax 303-357-1073, [email protected]. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. This PDF file may not be posted on the Internet. spe427-02.qxd 10/9/07 12:42 PM Page 9 The Geological Society of America Special Paper 427 2007 Revised lithostratigraphy of the lower Pierre Shale Group (Campanian) of central South Dakota, including newly designated members James E. Martin Museum of Geology, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, USA Janet L. Bertog Department of Physics and Geology, Northern Kentucky University, Highland Heights, Kentucky 41099, USA David C. Parris Bureau of Natural History, New Jersey State Museum, Trenton, New Jersey 08625-0530, USA ABSTRACT The Pierre Shale, a member of the Montana Group, is extensively exposed throughout the Northern Great Plains and is well exposed along the Missouri River Trench in central South Dakota. Currently, the Pierre Shale is of formational rank, but herein it is elevated to group status and should be termed the Pierre Shale Group. Most current members of the Pierre Shale should be elevated to formational rank, as they are of distinctive lithology and are mappable throughout the Missouri River area and elsewhere. The name Montana Group should be abandoned because of its rela- tively limited lithostratigraphic utility. Extensive geological and paleontological investigations of the lower Missouri River Trench indicate that a number of previously described units should be subdi- vided. In particular, the lowest described unit of the Pierre Shale along the Missouri River, the Sharon Springs, exhibits three distinct disconformity-bounded lithostrati- graphic units that are newly designated as members. The lowermost unit is distin- guished by bentonites, and the upper two units can be observed in the type area of the Sharon Springs in western Kansas. The lowermost unit is characterized by numerous bentonite beds similar to the Ardmore bentonitic succession in the southern Black Hills, is normally disconformably superjacent to the Niobrara Formation, may be absent where degraded, and is named the Burning Brule Member. The overlying mid- dle unit within the Sharon Springs consists of a siliceous shale that weathers vertically and is termed the Boyer Bay Member, whereas the upper unit is a bentonitic shale char- acterized by gypsiferous phosphatic concretions and is named the Nicholas Creek Member. These three units are herein regarded as new members of a hierarchically elevated Sharon Springs Formation. Other currently recognized members of the Pierre Shale in central South Dakota should likewise be elevated to formation-rank units except for the Crow Creek Mem- ber, a relatively thin tan siltstone. Because this unit is not mappable at recognized Martin, J.E., Bertog, J.L., and Parris, D.C., 2007, Revised lithostratigraphy of the lower Pierre Shale Group (Campanian) of central South Dakota, including newly des- ignated members, in Martin, J.E., and Parris, D.C., eds., The Geology and Paleontology of the Late Cretaceous Marine Deposits of the Dakotas: Geological Society of America Special Paper 427, p. 9–21, doi: 10.1130/2007.2427(02). For permission to copy, contact [email protected]. © 2007 The Geological Society of Amer- ica. All rights reserved. 9 spe427-02.qxd 10/9/07 12:42 PM Page 10 10 Martin et al. scales, it should be retained as a member of the DeGrey Formation. Inclusion of the Crow Creek Member within the DeGrey Formation is based upon another sporadi- cally occurring tan siltstone that lies stratigraphically lower, and the intervening shales are similar to those of the lower DeGrey Formation. The Pierre Shale members should also be elevated to formational status in North Dakota, Kansas, and around the Black Hills in eastern Wyoming and western South Dakota. The elevation of units should probably be made throughout the Pierre Shale depositional area, but those decisions should be made by local investigators. Keywords: Cretaceous, Pierre Shale Group, lithostratigraphy, new members, South Dakota. INTRODUCTION nally in 1817 and eventually named for Pierre Chouteau Jr., which was utilized principally as a trading post and later as a steamboat One of the classically known geological units in the Ameri- destination. The lithostratigraphic unit was designated as the Fort can West is the Pierre Shale, whose type area occurs in central Pierre Group, the fourth of five subdivisions of the Cretaceous of South Dakota (Fig. 1). Lewis and Clark visited the area in 1804 the Great Plains that had previously been numerically designated while ascending the Missouri River (see the introduction to this (Parris et al., this volume, Chapter 1) by Hall and Meek (1855). volume, Parris et al.), and in 1810 Nuttall noticed a pyrite-rich Eldridge (1888, 1889) included the Fort Pierre Shale of forma- unit (Nuttall, 1821) that was destined to become the Sharon tional rank in the Montana Group, a group that originally included Springs Member of the Pierre Shale. The next major investigators the marine Fort Pierre Shale and the estuarine-deltaic Fox Hills were F.B. Meek, J. Hall, and F.V. Hayden, associated with the fed- Formation. Later, the name and lithostratigraphic rank of the unit eral geological survey, although Joseph Nicollet and John Evans was shortened to Pierre Shale. The Montana Group was ex- collected specimens from what was to become the Pierre Shale in panded (Gill and Cobban, 1973) to include all formations from 1839 and 1849, respectively. Meek and Hayden (1862) described the top of the Niobrara Formation through the terrestrial Lance the unit for exposures of interbedded black shale, bentonite, con- and Hell Creek formational equivalents (Fig. 2). Therefore, the cretions, and marl along the banks of the Missouri River in the Montana Group consists of a great range of lithologies, represent- vicinity of old Fort Pierre, an early western fort established origi- ing a diversity of paleoenvironments, ranging from totally marine Figure 1. Index map of South Dakota. Im- portant sites are marked along the Missouri River Trench in central South Dakota. spe427-02.qxd 10/9/07 12:42 PM Page 11 Revised lithostratigraphy of the lower Pierre Shale Group (Campanian) 11 In 1942, Gries noted an intermittent chalky bed below the Crow Creek. The intervening shale between the two tan units appears identical to the lower bentonitic portion of the DeGrey unit. This lower tan unit has been informally considered to be the Fort Thompson beds (see Hanczaryk and Gallagher, this volume). The other major nomenclatural revision concerns Searight’s (1937) Sully Member. As previously described, the basal unit of the Sully Member, a marl, is now considered the Crow Creek Member. Of the three remaining superposed lithologies of the Sully Member—the Agency, Oacoma, and Verendrye shales— only the latter remains unchanged as the Verendrye Member. In 1942, Gries indicated that the Agency and Oacoma shales were not superposed units; therefore, Crandell (1950) redefined the Agency-Oacoma interval as the DeGrey Member. As a result, as recognized in Crandell’s (1950) revision, the Pierre Shale of for- mational rank included the Sharon Springs, Gregory, Crow Figure 2. Original nomenclature of the Montana Group in Montana and Creek, DeGrey, Verendrye, Virgin Creek, Mobridge, and Elk South Dakota. Creek members (Crandell, 1958). PROPOSAL AND JUSTIFICATION to totally terrestrial. Moreover, the group appears to include parts of two separate major transgressive-regressive marine cycles, the Our investigation of the lower Pierre Shale during the last Claggett and Bearpaw cycles (Caldwell et al., 1993). Therefore, 15 yr, in conjunction with detailed vertebrate biostratigraphical few uniting factors may be identified for the Montana Group, and collections, suggests that additional revision is required. Anumber this group term should be abandoned. of discrete lithologies occur within the previously designated Although Meek and Hayden (1862) attempted the first sub- members, and some members should be redefined on the basis of division of the Fort Pierre Group, Searight (1937) formally lithological consideration. These revisions will result in a nomen- named five superposed members: Gregory, Sully, Virgin Creek, clature that will have more utility for detailed geological and pale- Mobridge, and Elk Creek members. The Gregory and Sully mem- ontological studies. Nearly every member of the Pierre Shale has bers were subsequently refined. The Sharon Springs Member, been mapped on a scale of 1 inch to 1 mile (1:62,500) on the fol- originally described in western Kansas by Elias (1931), was lowing South Dakota and U.S. Geological Survey quadrangles: extended into South Dakota by Moxon et al. (1938) for the lower Lake Andes (Stevenson and Carlson, 1951), Bonesteel (Stevenson portion of the Gregory Member.