REVISED STRATIGRAPHY OF THE LOWER PIERRE SHALE (UPPER CRETACEOUS) OF CENTRAL SOUTH DAKOTA

The Pierre Shale 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 should be elevated to group status and all members should be elevated to formational rank as they are of distinctive lithology and are mappable throughout the Missouri River area. Extensive geological and paleontological investigations of the lower Missouri River Trench indicate a number of previously described units should be subdivided. In particular, the lowest described unit of the Pierre Shale, the Sharon Springs, exhibits three separate disconformity-bounded lithostratigraphic units that should be newly designated. The lowermost is unique, whereas the upper two units may be observed in the type area of the Sharon Springs in western Kansas. The lowermost unit is characterized by numerous bentonite beds, is normally disconformably superjacent to the Niobrara Formation, and may be absent where degraded. This unique unit requires separate designation. The two units within the Sharon Springs consist of a lower siliceous shale that weathers vertically, whereas the upper unit is more bentonitic and characterized by phosphatic concretions. These two units should be considered as members of a hierarcically elevated Sharon Springs Formation.