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 Member, a relatively thin tan siltstone. Because this unit is not mappable at recognized 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.

Biogenic apatite crystals in living organisms contain relatively high concentrations of carbonate, sodium, and other species, making the crystallite relatively soluble and reactive. During fossilization, apatite composition changes from a metastable carbonate hydroxylapatite to a thermodynamically more stable fluorapatite. Calcium, sodium, carbonate, and hydroxyl are replaced by fluoride, REE, and trace elements during diagenesis. The total REE concentrations in osteological material are generally <20 ppm; in fossil bones, however, the concentrations may be >1000–10,000 ppm. More than 95% of REE in fossil bone is incorporated during diagenesis. The fossilization process occurs within a few thousand years, and thus the chemical composition of the fossils records the composition of early diagenetic fluids. Apatite becomes a “flight recorder” for the environment of early diagenesis and can be used to interpret the condition of the paleoenvironment. REE were analyzed from marine reptile (Mosasauridae) bones collected from five superposed formations (Sharon Springs, Gregory, Crow Creek, DeGrey, and Verendrye) of the Upper Cretaceous Pierre Shale Group at localities along the Missouri River in central South Dakota. Fossil vertebrates from each lithostratigraphic unit sampled of the Pierre Shale Group have different REE signatures. Fossils from the Sharon Springs Formation have distinctive REE signatures that may be further subdivided into three superposed members that correspond with the upper, middle, and lower Sharon Springs Formation. REE signatures are distinctive from each stratigraphic unit; therefore, fossils eroded from their stratigraphic context may be assigned to their proper depositional unit on the basis of REE signature comparisons. Differences in REE compositions of fossil bones among lithostratigraphic subdivisions appear to have resulted from differential mixing of oxygenated and anoxic seawaters. If differences in mixing are interpreted as depth differences, the lower Sharon Springs member was deposited in deep, anoxic water; water depths decreased for the middle and upper Sharon Springs, and the overlying Gregory and Crow Creek units were deposited in even more shallow water. Finally, the overlying DeGrey and Verendrye Formations were deposited in progressively deeper marine waters, but not as deep as for the lower Sharon Springs. These interpretations are generally consistent with those based on faunal diversity and eustatic sea level curves.

The Missouri River and its tributaries in central South Dakota have eroded into the Pierre Shale, exposing a vertical sequence of late Campanian strata, from older to younger: the Crow Creek Member of the DeGrey Formation, upper DeGrey Formation, and Verendrye Formation, deposited during the transgressive phase of the Bearpaw cyclothem. The DeGrey Formation ( Baculites compressus Ammonite Range Zone) is predominantly gray bentonitic shale with interbeds of thin and relatively pure cream-colored bentonite and bioturbated, fossiliferous black manganese-iron carbonate concretions. Fossils are molds, casts, and fragmented shell material from a diverse epifauna of marine bivalve mollusks dominated by byssate, cemented, and unattached inoce-ramids, and ostreids. Inoceramids include Inoceramus convexus Hall and Meek, I. perplexus Whitfield, I. pertenuis Meek and Hayden, I. proximus Tuomey, I. sagensis Whitfield, I. tenuilineatus Hall and Meek, Platyceramus vanuxemi (Meek and Hayden), and Endocostea barabini (Morton). The larger specimens are commonly encrusted with the oyster Pseudoperna congesta (Conrad). Ostrea patina Meek and Hayden, a solitary oyster, is also present. External casts of articulated valves of Pholadomya hodgii Meek, a burrower-borer, are abundant as are those of Pteria ( Pseudopteria ) sublevis Whitfield, P. linguaeformis (Evans and Shumard), and P . sp. B. Anomia subtrigonalis Meek and Hayden and A. oblique Meek and Hayden complete the bivalve fauna. Less abundant are the gastropods Anisomyon borealis Morton, A. shumardi Meek and Hayden, Ampullina ? paludinaeformis Sohl, and Acmaea occidentalis (Hall and Meek). The Late Campanian ammonite index fossils Baculites compressus Say, and Jeletzkytes nodosus (Owen) and J. brevis (Meek), are abundant. Bivalves of the DeGrey Formation lived on a low-energy substrate where bentonite-rich clay was accumulating. Abundant epifauna and extensive bioturbation of the manganese-iron carbonate concretions suggest aerobic conditions. Adjacent deeper, anoxic basin waters periodically transported iron and manganese upward into the oxic zone, where it was precipitated with carbonate. The fragmentary nature of most of the bivalve shells, disruption of the sediment, and abundance of possible coprolitic material suggest predation by sediment-probing and shell-crushing predators.

The first marine reptile from the American West was collected long before the currently accepted lithostratigraphic nomenclature was established. The reptile, Mosasaurus missouriensis (Harlan), was collected from the Big Bend area of the Missouri River from what is now considered central South Dakota and taken to Germany by Prince Maximilian of Weid. Parts of the same specimen were described in 1834 and 1846, but its lithostratigraphic source could be determined only as the Late Cretaceous Pierre Shale Group, although later authors suggested its source as the Virgin Creek Formation of the Pierre Shale. Recent examination of the preservation of the holotype and associated invertebrates indicated derivation from the upper concretionary portion of the DeGrey Formation of the Pierre Shale rather than the Virgin Creek Formation. However, an independent method was sought to confirm this conclusion. Rare earth element (REE) analysis of vertebrate fossils in the Pierre Shale Group has been used successfully in interpretations of original diagenetic environments, including interpretations of paleodepth, identification of fossil provenance, paleoenvironmental interpretations, and stratigraphic correlation. REE signatures and trace element concentrations in fossil vertebrates from stratigraphic units are sufficiently distinctive to allow identification of the original unit or location in which fossilization occurred. Comparative REE analysis from numerous specimens from each unit of the lower Pierre Shale Group along the Big Bend of the Missouri River confirmed the lithostratigraphic source of the mosasaur as the upper DeGrey Formation.

The eastern margin of the Western Interior Seaway was located on the stable North American craton. Epeirogenic tectonism in continental lithosphere was sufficient, however, to influence sedimentation and generate geologic structures in south-central South Dakota. Paleotectonic and post-Cretaceous tectonism affected lithosphere blocks that are bounded by fault zones in the Precambrian basement and are marked at the surface by linear features and lineament zones visible on satellite images. Regional blocks outlined by lineament zones reflect the eastern side of a Proterozoic convergent margin, have expression on surface and subsurface cross sections, and subdivide the southern margin of the Williston basin into a mosaic of lithosphere blocks. Individual linear features within the regional blocks and lineament zones further subdivide south-central South Dakota into a series of broad, anticlinal blocks separated by narrow synclinal troughs and linear features. Paleotectonic movements on the regional blocks are documented by thickness and lithologic variation in Cretaceous depositional cycles found in the Dakota Sandstone through the Pierre Shale. Outcrops along the Missouri River expose thickness and facies changes in the upper Niobrara Formation and lower Pierre Shale across specific linear features that separate local blocks. In particular, the Crow Creek Member of the Pierre Shale has differences in texture and composition that correspond to differences in block position. Postdepositional tectonism included vertical displacements that produced the characteristic pattern of broad anticlinal blocks separated by narrow synclines and linear features. In addition, assemblages of folds, faults, and joints provide evidence for horizontal, strike-slip displacements. Along the Missouri River near Chamberlain, small faults parallel a linear feature trending N35°W and folds and extensional joints lie oblique to the linear feature, suggesting left-lateral displacement. Farther south along the river near Platte, a subtle monocline is found in lower units of the Pierre Shale. Small faults trending east-west generally parallel the monocline axis, which is marked by a linear feature. Joint patterns have modes oblique to the linear feature and are interpreted to show right-lateral displacement. The post-depositional tectonism is possibly post-Miocene because structural patterns in Cretaceous rocks seem to extend upward into Miocene-age units. The identification of lithosphere blocks in south-central South Dakota could influence construction projects and exploration for natural resources. Perhaps more important, the area serves to characterize a structural style which is common in continental lithosphere. That structural style consists of broad blocks separated by zones of basement weakness; blocks are continuously reactivated as they are jostled by asthenosphere flow.