PRELIMINARY SEDIMENTOLOGICAL IMPLICATIONS FROM THE CROW CREEK MEMBER (PIERRE SHALE): A HYPOTHESIZED IMPACT-INDUCED TSUNAMI DEPOSIT IN THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

The 1-3 m thick Crow Creek Member is a unique marl unit with rip-up clasts and a basal coarse layer within the Upper Cretaceous Pierre Shale in South Dakota and Nebraska. Two hypotheses exist for the origin and deposition of the Crow Creek Member: 1) a marine transgressive deposit of the Bearpaw sea level cycle along the eastern margin of the Western Interior Seaway; and 2) an impact-induced tsunami deposit resulting from the Manson Impact in north-central Iowa. Recent investigation of core and outcrops along the Missouri River and tributaries revealed sedimentological evidence pertinent to a tsunami hypothesis. An outcrop along the White River in South Dakota provided a rare glimpse of the horizontal bedding surfaces and paleocurrent indicators within the basal coarse layer. Bedding surfaces within the marl unit exhibit convolute bedding directly above the coarse layer in an otherwise thinly laminated sequence. Recently acquired core from atop the Sioux Ridge near Montrose, SD contains black sand beautifully grading into a thick (~15 m) marl unit with an upper Campanian autochthonous nannofossil assemblage (indicative of the Crow Creek Member) as well as a lower Campanian allochthonous nannofossil assemblage with species characteristic of the Niobrara Formation. The sedimentology of this unit strongly resembles the Holocene megaturbidites associated with the Mediterranean tsunami triggered by the collapse of the Santorini caldera.