The Virgin Creek Member of the Pierre Shales has been divided by earlier workers into lower and upper zones based on weathering and shale differences. Of the 49 bentonite beds of the lower Virgin Creek, the Government Draw Bentonite Beds, and bentonite bed 20 are the best markers for correlation from stream valley to stream valley. The variation of number and thickness of shale and bentonite beds is due to bioturbation, current activity, differential compaction, basin subsidence, and merging and splitting of bentonite beds. Three distinctive concretion horizons have the potential of also being used as stratigraphic markers within the study area. They include a nodule zone between two bentonite beds, barite(?) concretions that locally mark the lower contact of the Virgin Crrek, and a set of concretions at the contact between the upper and lower Virgin Creek. -from Author
Additional publication details
A preliminary report on the bentonite beds of the lower Virgin Creek Member of the Pierre Shale, Stanley County, South Dakota ( USA).