NEW MEMBERS OF THE DeGrey FORMATION, PIERRE SHALE GROUP (UPPER CRETACEOUS) FROM CENTRAL SOUTH DAKOTA

For over twenty years, lithostratigraphical and biostratigraphical studies have been undertaken concerning Late Cretaceous marine rocks and assemblages of South Dakota under the direction of the authors. During these investigations, distinct lithostratigraphic units were noted that extend over great areas and appear more than simple lithofacies. As a result in 2007 (Martin et al.), the Pierre Shale, which had been considered of formational rank, was elevated to group status, and most of the previously named members were elevated to formational status. One of the new formations of the Pierre Shale Group is the DeGrey Formation, which had a varied nomenclatural history and was finally well established in 1950 (Crandell). This formation is well exposed along the Big Bend of the Missouri River where five members can be recognized. Of Crandell’s original DeGrey designation, two distinct, laterally extensive lithologies have been noted that will be named as new members: a lower bentonitic gray shale with thin yellow concretionay layers and an overlying light gray siliceous shale unit containing relatively numerous interbedded bentonites and in some areas characterized by abundant FeMn concretionary layers. Normally, the Crow Creek Member, a tan, siltstone/marl occurs at the base of the gray bentonitic shale. The Crow Creek was considered a member of the Pierre Shale and recently was assigned as a member of the DeGrey Formation (2007). During our investigations, we noted another tan unit similar to the Crow Creek Member lying higher in the section occurring intermittently along the Missouri River. This unit was informally termed the Fort Thompson beds (Hanczaryk and Gallagher, 2007), and this calcareous, sandy siltstone will be formalized as a member of the DeGrey Fm., as well as the bentonitic shale unit that intervenes between the two tan siltstone units. The type sections of these new members occur in the Big Bend area of the Missouri River, although the members can occur throughout the Missouri River breaks of central South Dakota.