High resolution event stratigraphic and sequence stratigraphic interpretation of the lower Pierre Shale (Campanian) with the description of the new Walhalla and Chamberlain members

The lower Pierre Shale represents a time of significant changes in the Cretaceous Western Interior Seaway, resulting from a complex interplay of tectonic events and eustatic sea level changes. Volcanic activity along the western margin of the seaway produced large volumes of ash that are recorded in the sediments of the basin in the form of bentonites. The bentonites provide information useful in interpreting volcanic activity that was active as well as providing event horizons in the strata that are useful for high-resolution stratigraphic analysis.; Bentonites of the lower Pierre Shale indicate that at least three major sources were active during this period, corresponding to forearc, island arc and backarc magmatism, as indicated by trace element whole rock geochemical signatures. Mineralogical suites of these bentonites indicate that both andesitic and rhyolitic sources were present within these major regions. Biotite phenocrysts further segregated magmatic sources of the bentonites.; Based on the composition of the bentonites, several individual layers of bentonite can be identified in the lower Pierre Shale. These bentonite horizons, combined with biostratigraphic correlation have provided high-resolution detailed stratigraphic evaluation of the lower Pierre Shale across the basin. The newly defined Walhalla Member represents a tectonically influenced sequence that precedes the eustatic sea level rise recognized as the Claggett Cycle in the Western Interior. The Walhalla Member is restricted to the northern part of the basin, where tectonic activity resulted in increased subsidence of the axial basin and the Williston Basin prior to the eustatic sea level rise. Following the Walhalla Member, eustatic sea level rise resulted in the deposition of the Sharon Springs and the Chamberlain members on the eastern stable platform while the Mitten Black Shale Member was deposited in the axial basin. Recognition of these distal sequences is necessary in order to understand the complete basin dynamics.