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Evidence from the Crow Creek Member (Pierre Shale) for an impact-induced resuspension event in the late Cretaceous Western Interior Seaway

¹ Paleo-Data Inc., 6619 Fleur de Lis Drive, New Orleans, Louisiana 70124, USA
² Department of Geosciences, University of Nebraska, Lincoln, Nebraska 68588, USA

The 1–3-m-thick Crow Creek Member is a unique marlstone with rip-up clasts and a basal coarse layer in the Upper Cretaceous Pierre Shale in South Dakota and Nebraska. Although the Member has been thought to represent a marine transgression along the eastern margin of the Western Interior Seaway, the presence of impact ejecta from the Manson Impact Structure suggests an impact-induced genesis.

An upper Campanian in situ nannofossil assemblage with a lower Campanian reworked assemblage (from older Niobrara Chalk) occurs in the Crow Creek at most localities. The reworked assemblage decreases in abundance upward through the marlstone, a pattern consistent with an origin involving gravitational settling rather than marine transgression. Gray marlstone clasts in the basal coarse layer have nannofossils derived from the underlying Gregory Member and Niobrara Chalk. The reworked assemblage decreases in abundance with increased distances from the Manson Impact Structure and the Sioux Ridge (a paleotopographic high). The nonuniform geographic distribution of reworking suggests that Crow Creek deposition was linked to the Manson Impact. These observations, and a fining upward trend, the presence of impact ejecta, and coeval deposition with the Manson Impact Structure, support a resuspension-event origin for the Crow Creek Member.

Key Words: Pierre Shale • Manson impact structure • Nannofossils • Cretaceous • Campanian • Tsunami