PETROLOGY, GEOCHEMISTRY, AND PALEONTOLOGY OF FOSSILIFEROUS CONCRETIONS FROM THE CRETACEOUS PIERRE SHALE, SOUTH DAKOTA

Two calcareous concretions collected from the Pierre Shale near Wasta, South Dakota were analyzed for petrographic, geochemical, and paleontological information. They both are classified as biomicrites and are composed of approximately 75% calcite by weight. X-ray diffraction shows that cement in the matrix and ammonite chambers is calcite and that ammonite shell contains original aragonite. The δ18O composition of matrix calcite (-2.4 to -3.8‰) is lighter than typical marine values and not useful in determining the water temperatures of the Western Interior Seaway. The δ13C composition of matrix calcite (-22.4 to -25.8‰) is isotopically light relative to marine values and indicates sulfate reduction of organic matter during matrix diagenesis. Sparry cements in ammonite cavities have δ18O values (-9.9 to -10.7‰) consistent with formation from either 1) late-stage, δ18O-depleted meteoric fluids or 2) marine water at elevated temperatures.

An abundant molluscan fauna is represented in both concretions including the pelecypods Inoceramus sp. and Pteria sp., the cephalopod ammonites Baculites sp. and Hoploscaphites nodus, the gastropods Margaritella flexistriata, Amauropsis sp., Drepanochilus sp., Anisomyon sexculcatus and one unidentified genera, the scaphopod Dentalium gracile, as well as the coral Micrabacia americana. Abundances of complete individuals of each genus were recorded for each concretion. While both concretions were collected within 100 meters of each other in the same sedimentary horizon, concretion A has a greater abundance of fossils and higher biodiversity than concretion B. Specifically, concretion A has abundant Pteria sp. and more juvenile specimens, especially of Baculites sp. and Hoploscaphites nodus. This suggests considerable lateral variation in paleoecological conditions on the sea floor of the Cretaceous Interior Seaway in this setting.