Foraminifera and stratigraphy of the upper part of the Pierre Shale and lower part of the Fox Hills Sandstone (Cretaceous), north-central South Dakota

Abstract

Twenty-two stratigraphic sections along and near the Grand and Moreau Rivers in north-central South Dakota depict the stratigraphy of the Virgin Creek, Mobridge, and Elk Butte Members of the upper part of the Pierre Shale and the lower 74 feet of the Pox Hills Sandstone. The Virgin Creek Member, about 60 feet thick, is noncalcareous fissile shale in all but the upper 10 to 15 feet; in the lower 20 feet are numerous thin bentonite beds. The Mobridge Member, between about 200 and 225 feet thick, is typically calcareous shale with numerous concretion horizons and a few bentonite beds. The Elk Butte Member, 103 feet thick along the Moreau River and 192 feet thick along the Grand River, is noncalcareous shale with very few bentonite beds; it becomes very silty in the upper 15 to 30 feet. The lower part of the Fox Hills Sandstone is compacted but nonindurated sandy, clayey silt and silty, clayey sand, which in most places lacks bedding. Correlation of individual sections was made by means of bentonite beds and concretion horizons wherever possible. Two relatively thick bentonite beds in the Mobridge Member were used as datum planes. The contacts between the calcareous shale of the Mobridge Member and the noncalcareous shales of the Virgin Creek Member below and the Elk Butte Member above are gradational and are not uniform in stratigraphic position in sections correlated on the basis of bentonites.