Geochemical and mineralogical recognition of the bentonites in the lower Pierre Shale Group and their use in regional stratigraphic correlation

The lower Pierre Shale consists primarily of the Sharon Springs Formation, which has been correlated regionally throughout Kansas, Colorado, Nebraska, Wyoming, South Dakota, and North Dakota. The unit represents distal sedimentation in a tectonically active foreland basin. Correlation of the lower Pierre Shale Group is complicated by the application of a single name, the Sharon Springs, to a wide range of facies. Bentonite correlation provides an independent framework for verifying the age equivalence of various facies of the Sharon Springs Formation. Bentonite correlation involves using a variety of unique chemical characteristics to differentiate individual beds. A combination of whole rock rare-earth-element geochemistry, phenocryst composition, biotite geochemistry, and stratigraphic position has been used to correlate bentonites of the lower Pierre Shale and equivalent units across the Western Interior of the United States.