THE CROW CREEK MEMBER, PIERRE SHALE (UPPER CRETACEOUS) OF SOUTHEASTERN SOUTH DAKOTA AND NORTHEASTERN NEBRASKA: IMPACT TSUNAMITE OR BASAL TRANSGRESSIVE DEPOSIT?

by
R.H. Hammond, D.K. Watkins, B.J. Witzke, and R.R. Anderson

Geologic Field Trips in Nebraska and Adjacent Parts of Kansas and South Dakota
C.A. Flowerday, ed.
Nebraska Conservation and Survey Division,
Guidebook No. 10, 1995, p. 109-120.

FIELD TRIP ABSTRACT


The Crow Creek Member is one of several marl units recognized within the Upper Cretaceous Pierre Shale of eastern South Dakota and northeastern Nebraska. The member has been interpreted as a basal, transgressive unit of the Bearpaw Cycle, one of several Upper Cretaceous transgressive-regressive cycles. Recently, a significant discovery of impact shock-metamorphosed mineral grains in the Crow Creek was reported by Izett and others (1993) along with a new 40Ar/39Ar age (73.8 Ma) for the Manson Impact Structure (northwestern Iowa) that suggested that the two were coeval. They interpreted shocked grains in the Crow Creek as distal impact ejecta derived from the Manson Structure and further suggested that the member displays evidence of impact-induced tsunami sedimentation triggered by the Manson impact, a tsunami that may account for certain regional unconformities in the Western Interior Basin.

On this field trip we will visit several Crow Creek exposures in southeastern South Dakota and northeastern Nebraska. We will examine the rocks and discuss the characteristics of this unit and how they may be related to interpretations of its depositional history. We will end the trip at an exposure of Niobrara Formation chalk and compare this unit to Crow Creek.


REFERENCE:

Izett, G.A., Cobban, W.A., Obradovich, J.D., and Kunk, M.J., 1993, The Manson Impact Structure: 40Ar/39Ar age and its distal impact ejecta in the Pierre Shale in southeastern South Dakota: Science, v. 262, p. 729-732.