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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 estimation yielded 12 usable results. Samples from near the Baculites compressus Biozone yielded an age of ca. 72.43 ± 2 Ma. An average 87Sr/86Sr date of 72.41 ± 2 Ma was determined from fossil shells near a Globidens sp. excavation site within the upper DeGrey Member. More equivocal dates of 74.58, 74.63, and 75.09 ± 2 Ma were determined for the basal Crow Creek Member, a calcareous sandy siltstone within the upper Gregory Member, and the B. gregoryensis Biozone at the base of the section, respectively. Based on this study, the Gregory Member is interpreted as a highstand systems tract but as a part of the regressive pulse of the Claggett depositional cycle; the Crow Creek and DeGrey Members are the result of the Bearpaw transgression (a transgressive systems tract), and the boundary between the DeGrey and Verendrye Members is interpreted as a maximum flooding surface; and the Verendrye Member is a highstand systems tract. The distinct, calcareous sandy siltstone unit in the upper Gregory Member, herein named the Fort Thompson Beds, is interpreted to represent a storm-dominated deposit.

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