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A study of the Pierre Shale was undertaken, focusing on two measured sections along the eastern side of the Missouri River near Fort Thompson, South Dakota. The members of the Pierre Shale represented within these sections are, successively, the Gregory, Crow Creek, DeGrey, and Verendrye. Micropaleontologic samples were analyzed, with factor analysis performed to classify the assemblages. Four foraminiferal biofacies are recognized: two predominantly agglutinated, Boreal in origin, one shallower and the other deeper; one composed of calcareous benthic foraminifers, Tethyan in origin; and a fourth, which represents a mixed assemblage, transitional between the others. 87Strontium/86Strontium age...

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