A Preliminary Report on the Bentonite Beds of the Lower Virgin Creek Member of the Pierre Shale, Stanley County, South Dakota

Abstract

The Virgin Creek Member of the Pierre Shale has been divided by earlier workers into lower and upper zones based on weathering and shale differences Within the study area in Stanley County, South Dakota, the lower Virgin Creek contains 49 bentonite beds, whereas only seven bentonite beds have been observed in the upper Virgin Creek Of the bentonite beds of the lower Virgin Creek, the Government Draw Bentonite Beds, a doublet bentonite bed with a row of concretions, and bentonite bed 20 are the best markers for correlation from stream valley to stream valley However, because of the variation in number and thickness of both shale and bentonite beds from site to site, it is difficult to use the other bentonite beds as correlation tools 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 As a result of the variation, the exact position of the upper and lower contacts of the Virgin Creekland the contact between the upper and lower Virgin Creek with respect to a selected marker bed IS quite variable 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 Creek, and a set of concretions at the contact between the upper and lower Virgin Creek