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

The Geological Society of America
29th Annual North-Central Section and South-Central Section
University of Nebraska-Lincoln, Lincoln, NE, April 27-28, 1995
1995 Abstracts with Programs, v. 27, no. 3, p. 55


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 along with a new 40Ar/39Ar age (73.8 Ma) for the Manson Impact Structure (northwest Iowa) that suggested that the two were coeval. The shocked grains in the Crow Creek were interpreted as distal impact ejecta derived from the Manson Structure, and further suggested that the member displayed evidence of an impact-induced tsunami sedimentation triggered by the Manson impact, a tsunami that may account for certain regional unconformities in the Western Interior Basin.

At the end of this North-Central GSA meeting we will lead a field trip to examine these units. Initial investigations of the Crow Creek has led to several critical observations about this unit. These include 1) impact shocked quartz , feldspars, and other minerals are present in abundance in the sandy basal portion of the unit and are also present in the upper marl, however no evidence of impact melt-rock was observed; 2) while older nanofossils, reworked from the underlying Niobrara Formation, are present in the Crow Creek they are most common near the base and occur with a nanofossil assemblage that is unique to the Crow Creek, implying that time was required to accumulate sufficient fossil debris and develop index species; 3) radiolaria and foram assemblages change upward through the Crow Creek marls and into the overlying DeGrey Mbr, indicating normal depositional processes and not deposition by a single event; 4) fragile, thin-shelled benthic mollusks are present as whole-shell fossils, indicating a relatively passive depositional system; and 5) features indicative of long-standing erosion or non-deposition (such as phosphate lag, phosphatized matrix and glauconite enrichment) are abundant in the basal Crow Creek and underlying strata.

The occurrence the sand and marl of the Crow Creek in the normally deep water black shales of the Pierre Shale is not unique. Marls are present in the basal Mobridge, DeGrey, and Gregory mbrs in the region and the Niobrara and Greenhorn formations throughout the basin. The basal Niobrara is also sandy. These and other observations and interpretations argue against, but do not entirely preclude, the tsunamite scenario as an explanation or some of the unusual features of the Crow Creek Member.