Revised Lithostratigraphy of the Lower Pierre Shale Group (Campanian) Of

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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.)

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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 relatively limited lithostratigraphic utility. Extensive geological and paleontological investigations of the lower Missouri River Trench indicate that a number of previously described units should be subdivided. In particular, the lowest described unit of the Pierre Shale along the Missouri River, the Sharon Springs, exhibits three distinct disconformity-bounded lithostratigraphic units that are newly designated as members. The lowermost unit is distinguished 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 middle 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 characterized 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 designated 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

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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 sporadically 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 formational 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. The upper Gregory, a marl, was and Carlson, 1950), Platte (Carlson, 1950), Lucas (Baker and Carl- designated the Crow Creek by Gries and Rothrock (1941) and son, 1951), Dixon (Baker, 1951), Iona (Baldwin and Baker, 1952), was considered a formal member of the Pierre Shale by Crandell Chamberlain (Petsch, 1952), Stephan (Curtiss, 1951), Lower (1950, 1952). Crandell (1950) also revised the Sully Member to Brule (Petsch and Curtiss, 1950a), Fort George Butte (Petsch include the DeGrey Member, subjacent to the VerendryeMember. and Curtiss, 1950b), Oahe (Crandell, 1955), Canning (Crandell, No matter the nomenclature, each member-rank unit, except the 1954b), Pierre (Crandell, 1954a), Fort Bennett (Petsch and Bolin, Crow Creek, has been mapped by the South Dakota Geological 1950), No Heart (Stevens, 1952a), Artichoke Butte (Stevens, Survey and the U.S. Geological Survey on the basis of distinct 1952b), Cheyenne Agency (Mickelson, 1952), Mouth of Moreau lithologies, and therefore these units qualify as formation-rank (Mickelson and Baker, 1950), Mobridge (Baker, 1952), and Pol- entities. lack (Baldwin, 1951). Therefore, owing to the distinct lithologies Nomenclatural problems are principally found among mem- of the various members of the Pierre Shale (Crandell, 1958) and bers of the lower Pierre that were originally considered the Gre- their suitability for mapping (North American Commission on gory Member by Searight (1937). When the name Sharon Springs Stratigraphic Nomenclature, 1983, p. 858), most members should was extended into South Dakota (Moxon et al., 1938), the Gre- be raised to formational status (Fig. 3) and should include the gory Member was restricted to the upper marl and sandstone, later Sharon Springs Formation, the Gregory Formation, the DeGrey considered the Crow Creek Member (Crandell, 1950). Gries and Formation, the Verendrye Formation, the Virgin Creek Forma- Rothrock (1941) noted a marl below the Crow Creek and included tion, the Mobridge Formation, and the Elk Creek Formation. this marl and the intervening shale in a newly defined Gregory Because the Crow Creek Member is thin (3 m), it cannot be Member. As a result, the original Gregory Member of Searight mapped like the other Pierre units and should be regarded as a (1937) now consists of the Sharon Springs, Gregory, and Crow member of the DeGrey Formation. Similar situations occur in Creek units. western South Dakota, eastern Wyoming, and elsewhere where spe427-02.qxd 10/9/07 12:42 PM Page 12

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described as the Pembina beds, whereas the Sharon Springs was formally designated a member. Moreover, the long historical, widespread, and common usage of the Sharon Springs terminology in the United States, coupled with the equivalent subdivisions in South Dakota and the type area in Kansas, resulted in our deci- sion to retain the Sharon Springs name. The Sharon Springs Member was initially designated in Logan and Wallace counties, Kansas, by Elias (1931) for black shales superposed above the Niobrara chalk. Later investigations revealed three subdivisions of the member, including a basal soft, organic-rich shale unit successively overlain by a vertically weathered organic shale and a phosphatic nodular unit (Gill et al., 1972). The soft, organic-rich shale contains relatively few bentonites and lies gradationally above the Niobrara chalk. Based upon bentonite correlations, this lower unit may be equivalent to the basal Pierre Shale unit, the Gammon Ferruginous Shale of the Black Hills area. This lower unit in Kansas is 27.4 m thick Figure 3. Nomenclature of the Pierre Shale Group as revised herein. and is disconformably overlain by the vertically weathered shale. This vertically weathered unit is of the same approximate thickness and contains layers of dense carbonate concretions near the top. The upper phosphatic shale unit conformably over- separate successions of members have been described, and these, lies the vertically weathered unit. The upper unit is 3–3.5 m too, should be elevated to formational rank. Therefore, the Pierre thick and contains as many as 10 layers of phosphatic nodules Shale should be raised to group rank and considered to be the (Gill et al., 1972). Pierre Shale Group. As a consequence, the Montana Group could The Sharon Springs Member of Kansas was extended into be raised to supergroup status (North American Commission on the Missouri River breaks of South Dakota by Searight (in Moxon Stratigraphic Nomenclature, 1983, p. 859), but its relatively little et al., 1938) and was later extended into the region surrounding lithostratigraphic utility suggests that this concept should be aban- the Black Hills (Moxon et al., 1939). Here, the lower Pierre Shale doned. Another reason for abandonment is that the base of the has been subdivided into the Gammon Ferruginous, Sharon Montana lithostratigraphic unit has traditionally been placed at the Springs, and Mitten Black Shale members (see Gries and Martin, base of the Pierre Shale. However, the similarity of marine depo- 1985, for historical review). The Sharon Springs Member around sition and paleofaunas between the subjacent Niobrara Formation the margin of the southern Black Hills contains a basal interval, and the Pierre Shale suggests that this boundary would not serve characterized by numerous bentonite beds, that has been consid- as that of a supergroup. On these bases, therefore, we propose ered informally by us as the Ardmore bentonite succession. This abandonment of the Montana Group of Eldridge (1888, 1889). interval is similar to that in central South Dakota and northeastern North Dakota, and geochemical fingerprinting of bentonites LOWER PIERRE SHALE GROUP confirms this correlation (see Bertog et al., this volume). The main body of the Sharon Springs Member in the Black Hills area The lower Pierre Shale Group is the major focus of this con- normally lies disconformably above the bentonitic-layered unit tribution (higher occurring units will be reviewed, and new units and contains at least three layers of dense carbonate concretionary delineated in later contributions). This portion of the Pierre Shale layers. This unit weathers vertically because of its highly organic, Group occurs within the Missouri River Trench within South siliceous nature. Athird, softer organic shale may disconformably Dakota and is exposed in North Dakota, Colorado, around the overlie the vertically weathered unit. This softer shale has thin, Black Hills of South Dakota and Wyoming, and elsewhere. In interbedded bentonites and is disconformably overlain by the particular, the Sharon Springs Formation has been found to darker, iron-manganese shales and concretions of the Mitten include distinctive lithologies that may be considered as new Black Shale Member, named by Rubey (1930). members. In northeastern North Dakota this lithostratigraphic As previously mentioned, the Sharon Springs equivalent in interval has been termed the Pembina Member of the Pierre Shale northeastern North Dakota has been regarded as the Pembina (Gill and Cobban, 1965), extending the member terminology Member of the Pierre Shale but should be considered as the from Canada. However, identical member subdivisions occur in Sharon Springs. Interestingly, three lithological subdivisions may North Dakota, as in South Dakota and Kansas; therefore, the also be recognized, although they are thicker than those ex- Pembina Member should be considered the Sharon Springs For- posed in central South Dakota. These subdivisions include super- mation in North Dakota. Although it may be argued that the Pem- posed units of a lower shale characterized by bentonite layers, a bina has priority by a single year (Kirk, 1930), the Pembina was middle unit of vertically weathering organic-rich shale, and an spe427-02.qxd 10/9/07 12:42 PM Page 13

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upper phosphatic nodular unit. These subdivisions may be re- more localized extent than the lithostratigraphic members that garded as members of the Sharon Springs Formation in North occur at a minimum from Kansas to North Dakota. Dakota. Three distinct lithologies at both macroscopic and micro- scopic scales (Martin et al., 2004; Patrick et al., 2004, and this vol- Sharon Springs Formation ume) occur within the Sharon Springs Formation along the Missouri River in south-central South Dakota (Fig. 4) and north- The Sharon Springs Member, a distinctive black, highly eastern North Dakota. The basal unit is characterized by numer- organic, fissile shale with numerous bentonites (Gries and Mar- ous distinct beds of yellow-weathered bentonite. The middle unit tin, 1985), originally described in Kansas and correlated into is characterized by few thin bentonites within a fissile, highly South Dakota by Searight (in Moxon et al., 1938), is herewith organic, somewhat siliceous gray shale (Schultz et al., 1980) that revised and elevated to lithostratigraphic formational rank. weathers to a vertical profile (also termed buttress-weathered by Gill et al., 1972). The upper unit is a black bentonitic shale char- Type section of formation. Near center of section 8, T. 12 S., R. acterized by numerous, poorly layered, white to pink, gypsifer- 36 W., Logan County, Kansas, 2 mi (3.3 km) east of McAllaster ous, phosphatic concretions. These three units are herein Buttes near the abandoned town of Lisbon (Gill et al., 1972). considered members of the Sharon Springs Formation, and in the Missouri River Trench they represent disconformably bounded Original author. Elias (1931); see also Gill et al. (1972). packages as well as distinct lithological entities. Therefore, the units in their type area along the Missouri River qualify as both lithostratigraphic and allostratigraphic units. The lower litho- Etymology. Named for the town of Sharon Springs in Wallace stratigraphic member is herein named the Burning Brule Member, County, western Kansas. the middle interval the Boyer Bay Member, and the upper unit the Nicholas Creek Member, all named from exposures south of the Lithology. Black to gray, highly organic claystone with a fissile town of Chamberlain, South Dakota, along the banks of the Mis- parting, commonly with concretions and numerous yellow- souri River. Allomember designations could likewise be applied, weathered bentonite beds, particularly near the base (Gries and utilizing the same geographic name, but the allomembers are of Martin, 1985).

Figure 4. Section of Sharon Springs For- mation near the type section of the Burn- ing Brule Member. White bars denote contacts. The Niobrara Formation is seen in the lower part of the photograph, the Burning Brule Member contains the banded bentonites, the Boyer Bay Mem- ber weathers vertically, the Nicholas Creek Member is gray and less resistant, and the upper part of the section includes the light tan shales and marls of the lower Gregory Formation. spe427-02.qxd 10/9/07 12:42 PM Page 14

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Age. Medial Campanian: Baculites obtusus through Baculites (Fig. 5) represents the thickest, most completely known succes- perplexus Ammonoid Range Biozones (Cobban, 1993; Obrado- sion of bentonites in central South Dakota; most other areas vich, 1993; Grandstein et al., 1994). exhibit differential erosion of the upper contact. The bentonites vary extensively laterally, and the entire interval may be Distribution. Northern Great Plains, including the Missouri degraded. In some areas, thick bentonites split into two bentonite River Trench of central South Dakota. beds; in other areas, bentonites fade laterally. Distinctive chan- neling can be observed within the Burning Brule Member, and Synonymy. Pembina Member, Canada. these channels may represent disconformable surfaces noted elsewhere within the Sharon Springs Formation (Martin, 1996). The base overlies a disconformable surface on which occurs a Boundaries. Underlain disconformably by the Niobrara Forma- basal lag composed principally of large vertebrate remains in a tion in some areas or conformably by the Gammon Ferruginous clayey sand matrix. Formation in others.

¹⁄₄ ¹⁄₄ Members. Burning Brule Member successively overlain by Type section. SW NE section 30, T. 104 N., R. 71 W., Brule Boyer Bay Member and Nicholas Creek Member (newly named County, South Dakota (Appendix 1). herein). Etymology. Named for the area known as Burning Brule south Description. The Sharon Springs Formation is composed of of the town of Chamberlain along the Missouri River, Brule gray to black, organic-rich claystones that weather into well- County, central South Dakota. parted shales. These shales are characterized by numerous bentonite layers and concretionary zones. The Sharon Springs Lithology. Black, organic-rich shale with numerous yellow- Formation differs from other Pierre Shale Group formations in weathered bentonite interbeds; in some areas isolated concre- its high radioactivity, resistance to erosion, and darker color tions occur and may be associated with bentonite layers; basal owing to very high organic content (Gill et al., 1972). In most lag deposits occur sporadically that are sometimes composed of areas, bentonite beds characterize the lower portion of the vertebrate bone conglomerates. Sharon Springs Formation, including the Ardmore bentonite bed (Spivey, 1940; Bertog et al., this volume), whereas organic- rich, siliceous shale with resistant carbonate concretionary Thickness. 0–4.5 m in type area. zones characterizes the middle portion of the formation. The upper portion of the formation is less resistant owing to dissem- Age. Medial Campanian: Baculites obtusus Ammonoid Range inated bentonite and is characterized by phosphatic nodules. Biozone (Gill et al., 1972; Cobban, 1993; Obradovich, 1993; Grandstein et al., 1994); Ar/Ar determination from sanidine from Burning Brule Member of Sharon Springs Formation the Ardmore succession in the Elk Basin of Wyoming yielded 80.54 0.55 Ma (Obradovich, 1993); another date, also from In central South Dakota the Pierre Shale Group lies with sanidine, of the Ardmore bentonite bed near Redbird, Wyoming, disconformity over the Niobrara Formation, composed of chalk revealed a date of 80.04 0.4 Ma (Hicks et al., 1999). in its upper part. However, in western South Dakota–eastern Wyoming, the Gammon Ferruginous Formation intervenes. In Distribution. Central South Dakota west to the Black Hills both regions the basal portion of the black siliceous shale of the region and into eastern North Dakota. Sharon Springs Formation is characterized by bentonites, including the Ardmore bentonite bed of Spivey (1940), exposed in the southern Black Hills. Numerous bentonites occur near the base Equivalents. The same member occurs in the Black Hills region, of the Sharon Springs that prompted Martin et al. (1996) to con- where it includes the Ardmore bentonite succession, and in sider informally the entire interval around the Black Hills as the northeastern North Dakota, where it is also characterized by Ardmore succession, including a distinctive bentonite couplet numerous bentonite layers. (0.3 m each) near the top of the interval. In central South Dakota south of the town of Chamberlain in an area known as Boundaries. Both the upper and lower boundaries are discon- Burning Brule (named because the organic-rich Pierre Shale formable. In the type area the lower contact lies disconformably caught fire and smoked for many years), 12–13 bentonites occur above the Niobrara chalk; a basal lag may be present. The upper within an 8 m interval of black siliceous shale (a similar number contact with the suprajacent member is likewise disconformable, occurs in about half the thickness in the Black Hills area; see and the disconformity may have completely cut out the Burning Bertog et al., this volume). The type section at Burning Brule Brule Member. spe427-02.qxd 10/9/07 12:42 PM Page 15

Revised lithostratigraphy of the lower Pierre Shale Group (Campanian) 15

Figure 5. Type section of the Burning Brule Member of the Sharon Springs Formation. The individual is holding a grid unit that is 0.5 m on a side. The base of the grid unit lies on top of the Niobrara Formation.

Boyer Bay Member of Sharon Springs Formation Type section. NW¹⁄₄SE¹⁄₄ section 5, T. 102 N., R. 71 W., Brule County, South Dakota (Appendix 2). The Boyer Bay Member of the Sharon Springs Formation appears to be a widespread unit and can be identified from west- Etymology. Named for Boyer Bay south of the town of Cham- ern Kansas through South Dakota, westward into Wyoming, and berlain in Brule County, South Dakota, where the type section north into northeastern North Dakota. The member is character- occurs. ized by highly organic-rich, siliceous shales with relatively few thin bentonites. The resistant nature of the shale results in a vertical- weathering profile (Fig. 6), and even with extensive weathering Lithology. Black to gray, highly siliceous, organic-rich shale the unit retains relatively steep slopes of gray fissile shale. Ben- with vertical or steep weathering profiles, with distinctive, thin tonites in the type area of central South Dakota are thin but strik- white- or yellow-weathered bentonite beds. ing, owing to the contrast of the white or yellow bentonite with the black shale. The lowest bentonite is 1.5 cm thick, occurs 1 m Thickness. 6.5–9.7 m in type area. above the contact, and is distinctive because of its siliceous nature. This white bentonite has been informally termed the por- Age. Medial Campanian: top of Baculites obtusus through Bacu- cellanite bentonite owing to its brittle nature. Laterally, this ben- lites asperiformis Ammonoid Range Biozones (Gill et al., 1972; tonite may grade into a typical yellowish-white clay. Higher in the Cobban, 1993; Obradovich, 1993; Grandstein et al., 1994). section, two more bentonites occur and are separated by 1 m. The lower of the two is 2.5–3.0 cm thick, whereas the higher of the two is relatively thick, 4.0–7.5 cm (Appendix 2). Distribution. Western Kansas through South Dakota into As previously mentioned, the base of the Boyer Bay Mem- Wyoming and north into North Dakota. ber lies directly on the Niobrara Formation in some areas and is found to have cut down into various levels within the Burning Equivalents. The same member occurs in the Black Hills region Brule Member in other areas. In all cases where observable, the and in northeastern North Dakota, where it overlies the Ardmore lower contact is disconformable. The upper contact is sharp but bentonite succession (Burning Brule Member). This member also disconformable. may also extend into the type Sharon Springs area of western spe427-02.qxd 10/9/07 12:42 PM Page 16

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gory Formation. The upper contact with the Gregory Formation is sharp and marked by a distinct color change from black to brown and a lithological change from bentonitic shale to bedded argillaceous shale. The base of the Gregory Formation was placed by Gries and Rothrock (1941) and Crandell (1950) at the base of a 0.5-m-thick, yellowish red marl for convenience. However, the brown, bedded argillaceous shales of the Gregory Formation lie below the marl and above the black, massive Nicholas Creek Member. There- fore, the basal contact of the Gregory Formation is placed low enough to include brown argillaceous shale and bentonites (Appendix 3).

Type section. NE¹⁄₄SW¹⁄₄ section 11, T. 102 N., R. 72 W., Lyman County, South Dakota (Appendix 3).

Etymology. Named for Nicholas Creek, south of the town of Chamberlain, western shore of the Missouri River, Lyman County, South Dakota; the type section occurs just south of the mouth of Nicholas Creek.

Lithology. Black bentonitic shale containing numerous poorly layered to scattered white to pinkish gypsiferous, phosphatic concretions.

Thickness. 2.2–3.7 m in type area. Figure 6. Type area of the Boyer Bay Member of the Sharon Springs For- mation. The member is characterized by vertically weathered, organic- Age. Medial Campanian: top of Baculites asperiformis to top of rich shale. The individual’s feet are at the basal contact. Baculites sp. (smooth) Ammonoid Range Biozones (Gill et al., 1972; Cobban, 1993; Obradovich, 1993; Grandstein et al., 1994). Kansas, where it is represented by the buttress-weathering middle unit of Gill et al. (1972). Distribution. Western Kansas, central South Dakota, northeastern North Dakota. Boundaries. Both the upper and lower contacts are bounded by disconformable surfaces. Equivalents. The Nicholas Creek Member is found in northeastern North Dakota and also extends into the type Sharon Springs Nicholas Creek Member of Sharon Springs Formation area of western Kansas, where it is represented by the phosphatic nodular unit of Gill et al. (1972). The uppermost member of the Sharon Springs Formation in central South Dakota is a black, bentonitic shale characterized by RESULTS white to pinkish phosphatic concretions (Fig. 7). These concretionary bodies form a layer at the base of the member (Fig. 8), In view of years of geological and paleontological investiga- appearing almost as a basal lag. These relatively small (10 cm tions along the Missouri Trench in central South Dakota and else- long 5 cm high) concretions appear to be scattered through the where,wehaveraisedthePierreShaleofMeekandHayden(1862) remainder of the member. In some areas the concretions appear to group status, essentially restoring the status that was originally to be somewhat aligned into discrete layers, but these layers are proposed by Meek and Hayden. The Montana Group is herewith not as well developed as the basal layer of concretions. This unit abandoned because of its relatively limited utility. The former can be distinguished by its soft bentonitic character in compari- members of the Pierre Shale (except the Crow Creek Member) are son with the highly resistant lower members. Nevertheless, it elevated to formational rank, and new members are described for appears more similar to the black-gray shales of the lower mem- distinctive lithologies within the Sharon Springs Formation.Addi- bers than to the brown, argillaceous units of the overlying Gre- tional members will be named for other distinctive lithologies spe427-02.qxd 10/9/07 12:42 PM Page 17

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Figure 7. Type section of the Nicholas Creek Member of the Sharon Springs Formation. The individuals are collecting a mosasaur jaw in a plaster jacket that lies on the basal contact of the Nicholas Creek Member. Note the phosphatic concretions in poorly organized layers above this level. The upper contact is demarked by the distinct change in color and lithology. The light band across the middle of the photograph is the marl formerly utilized as the base of the Gregory Forma- tion; note that typical Gregory deposits underlie the marl, resulting in a down- ward shift of the formational contact.

within the DeGrey Formation in a later contribution. However, the Dakota the only previously considered member to retain Crow Creek unit is retained at member status and considered a its member status is the Crow Creek Member owing to its member of the DeGrey Formation. Consequently, the following thinness, which does not allow the unit to be mapped at listing represents the changes proposed herein: appropriate scales. Therefore, in central South Dakota the following superposed formations should be recognized: 1. The Pierre Shale of formational rank is elevated to group Sharon Springs Formation, Gregory Formation, DeGrey status and should be considered the Pierre Shale Group. Formation, Verendrye Formation, Virgin Creek Forma- 2. The Montana Group that included the Pierre Shale should tion, Mobridge Formation, and Elk Creek Formation. In be abandoned. western South Dakota–eastern Wyoming the following 3. Former members of the Pierre Shale in western Kansas, superposed formations should be recognized: Gammon central and western South Dakota, North Dakota, and Ferruginous Formation, Sharon Springs Formation, Mit- southeastern Wyoming should be elevated to formational ten Black Shale Formation, Red Bird Silty Formation, a status. A similar situation may be proposed for other areas lower unnamed shale unit, Monument Hill Bentonitic For- of the Pierre Shale and its equivalents, but those decisions mation, Kara Bentonitic Formation, and an upper unnamed should be made by local investigators. Within South shale unit (see Gries and Martin, 1985). In northeastern spe427-02.qxd 10/9/07 12:42 PM Page 18

18 Martin et al.

Figure 8. Basal phosphatic layer of the Nicholas Creek Member of the Sharon Springs Formation. Scale in centimeters.

North Dakota and into Canada, the Pembina Member relative with the Ardmore succession to the west and the should be considered the Sharon Springs Formation. basal bentonite-rich unit in northeastern North Dakota. In 4. The Crow Creek Member, a tan siltstone, should be con- central South Dakota the member disconformably overlies sidered a member of the DeGrey Formation, because the Niobrara Formation, composed principally of chalk. other, similar tan marls have been recognized in the sec- 7. The Boyer Bay Member is another organic-rich siliceous tion interbedded with typical dark gray bentonitic clay- black-gray shale but exhibits only a few very thin ben- stone of the lower DeGrey Formation. tonites and retains a near-vertical weathering profile, and 5. The Sharon Springs Formation exhibits three distinct at places is considered a buttress-forming shale. This lithologies that are named as separate, disconformity- member, too, has a disconformable base that appears bounded members: the Burning Brule, Boyer Bay, and incised into different levels of the subjacent member. Nicholas Creek members. This unit also occurs in western Kansas, northeastern 6. The Burning Brule Member is characterized by black North Dakota, around the southern Black Hills, and will organic-rich shale interbedded with numerous relatively probably be correlated farther into Colorado, Montana, thick, yellow-weathered bentonites that appear to be cor- and elsewhere. spe427-02.qxd 10/9/07 12:42 PM Page 19

Revised lithostratigraphy of the lower Pierre Shale Group (Campanian) 19

8. The Nicholas Creek Member is a bentonitic black shale APPENDIX 2: STRATOTYPE SECTION (FIG. 6) FOR THE BOYER ¹⁄ ¹⁄ characterized by white to pink gypsiferous phosphatic BAY MEMBER, SHARON SPRINGS FORMATION (NW ⁴ SE ⁴ SECTION 5, T. 102 N., R. 71 W., BRULE COUNTY, SOUTH DAKOTA) concretions. The concretions are normally concentrated at the base of the member and occur in poorly organized lay- Contact with Nicholas Creek Member ers or scattered throughout the member. This member also Boyer Bay Member occurs in western Kansas and northeastern North Dakota. 220 cm black organic-rich, vertically weathered shale 9. Other members of superposed formations will be desig- 4 cm yellow weathered bentonite nated in subsequent contributions, particularly those for 126 cm black organic-rich, vertically weathered shale the DeGrey Formation. 2.5 cm yellow weathered bentonite 220 cm black organic-rich, vertically weathered shale 0.2 cm yellow weathered bentonite 181 cm black organic-rich, vertically weathered shale APPENDIX 1. STRATOTYPE SECTION (FIG. 5) FOR THE BURNING BRULE MEMBER, SHARON SPRINGS FORMATION 3 cm yellow weathered bentonite (SW¹⁄⁴NE¹⁄⁴ SECTION 30, T. 104 N., R. 71 W., BRULE COUNTY, 845 cm black organic-rich, vertically weathered shale SOUTH DAKOTA) Burning Brule Member Thick- Color 2.8 cm yellow weathered bentonite ness Description codes* 3.5 cm black organic-rich shale Boyer Bay Member 18.5 cm bluish gray bentonite 6 cm silicified bentonite, “porcellanite” bentonite 5YR4/1 2.5 cm black organic-rich shale 45 cm black, vertically weathered organic-rich shale N4 11 cm bluish gray bentonite 5.2 cm black organic-rich shale Burning Brule Member 0.3 cm bluish gray bentonite 8 cm siliceous bentonite N3 4.6 cm black organic-rich shale 2 cm black silicified organic-rich shale N3 3 cm bluish gray bentonite 1 cm weathered bentonite N3 8.8 cm black organic-rich shale 28 cm organic-rich shale w/scattered Fe-stained concretions 5YR4/1 Contact with Niobrara Formation 9 cm weathered bentonite N3 23 cm organic-rich shale N3 15 cm indurated ferruginous sandstone N6 41 cm organic-rich shale N4 14 cm weathered bentonite 5Y8/4 25.9 cm organic-rich shale N3 APPENDIX 3. STRATOTYPE SECTION (FIG. 7) FOR THE NICHOLAS CREEK MEMBER, SHARON SPRINGS 27.5 cm weathered bentonite 5Y7/2 FORMATION (NE¹⁄⁴SW¹⁄⁴ SECTION 11, T. 102 N., R. 72 W., 16.5 cm organic-rich shale N1 LYMAN COUNTY, SOUTH DAKOTA) 5 cm indurated bentonite 5Y8/4 27 cm organic-rich shale N2 Top not preserved 12.7 cm bentonite N9 Gregory Formation 27 cm organic-rich shale N2 20 cm reddish yellow ferruginous concretionary layer 20 cm indurated bentonite N7 100 cm brownish black shale 3.5 cm organic-rich shale N3 25 cm reddish yellow ferruginous concretionary layer 4 cm indurated bentonite N8 150 cm brownish black shale 7.8 cm organic-rich shale N5 28 cm brownish tan, Fe-stained siltstone normally present 4 cm coarse-grained bentonite 5Y7/2 (previously considered base of Gregory Formation) 47 cm organic-rich shale 5YR2/1 57 cm brownish black shale 6 cm bentonite N6 1 cm yellow weathered bentonite 4.3 cm organic-rich shale N4 72 cm brownish black shale 28 cm weathered bentonite 5Y8/4 4 cm reddish pink Fe-stained siltstone 14 cm organic-rich shale N2 Nicholas Creek Member of Sharon Springs Formation 3.5 cm weathered bentonite N6 290 cm bentonitic black shale with scattered cobble-sized, 6 cm organic-rich shale 5YR2/1 compressed phosphatic concretions; concretions occur also 2 cm gypsiferous shale 5YR2/1 in distinct basal layer Contact with Niobrara Formation Boyer Bay Member of Sharon Springs Formation Note: Top not preserved. 100 cm black organic-rich shale *Munsell standard colors. Base covered by talus spe427-02.qxd 10/9/07 12:42 PM Page 20

20 Martin et al.

ACKNOWLEDGMENTS Curtiss, R.E., 1951, Areal geology of the Stephan Quadrangle: South Dakota Geo- logical Survey map, scale 1:62,500. This contribution has evolved over the past 20 years of field Eldridge, G.H., 1888, On some stratigraphical and structural relations of the country about Denver, Colorado: Colorado Scientific Society Proceedings, investigations whose primary goal was to produce high-resolution, v. 3, p. 86–118. stratigraphically controlled vertebrate and invertebrate collec- Eldridge, G.H., 1889, Some suggestions upon the methods of grouping the formations for biostratigraphic purposes. Therefore, we sincerely thank tions of the middle Cretaceous and the employment of an additional term in the numerous colleagues, students, and volunteers who have con- its nomenclature: American Journal of Science, v. 38, p. 313–321. tributed greatly to our success. Land-managing agencies, the Crow Elias, M.K., 1931, The geology of Wallace County, Kansas: Kansas Geological Survey Bulletin 18, 254 p. Creek Sioux Tribe, and local farmers and ranchers have been ex- Gill, J.R., and Cobban, W.A., 1965, Stratigraphy of the Pierre Shale, Valley City tremely kind in allowing us access to their lands and for allow- and Pembina Mountain areas, North Dakota: U.S. Geological Survey Pro- ing important specimens to be retained in the public trust. The fessional Paper 392-A, 20 p. widespread discussions and critical reviews with geologists too Gill, J.R., and Cobban, W.A., 1973, Stratigraphy and geologic history of the Mon- numerous to mention have greatly benefited our research. In par- tana Group and equivalent rocks, Montana, Wyoming, and North and South Dakota: U.S. Geological Survey Professional Paper 776, 37 p. ticular, the manuscript reviews by J. Foster Sawyer, South Dakota Gill, J.R., Cobban, W.A., and Schultz, L.G., 1972, Stratigraphy and composition Geological Survey, and James E. Fox, Department of Geology of the Sharon Springs Member of the Pierre Shale in western Kansas: U.S. and Geological Engineering, South Dakota School of Mines and Geological Survey Professional Paper 728, 50 p. Technology, have been extremely helpful. Grandstein, F.M., Agterberg, F.P., Ogg, J.G., Hardenbol, J., van Veen, P., Thierry, J., and Huang, Z., 1994, A Mesozoic time scale: Journal of Geophysical Research, v. 99, p. 24,051–24,074, doi: 10.1029/94JB01889. Gries, J.P., 1942, Economic possibilities of the Pierre Shale: South Dakota Geo- REFERENCES CITED logical Survey Report of Investigations 43, 79 p. 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