B.J. Witzke, R.R. Anderson, and R.H. Hammond

The Geological Society of America
Special Paper 302, 1996, p. 433-456
The Manson Impact Structure, Iowa: Anatomy of an impact crater
C. Koeberl and R.R. Anderson, eds.


The Crow Creek Member is one of several marl units recognized within the Upper Cretaceous Pierre Shale sequence of South Dakota and Nebraska. Its geographic extent is largely coincident with the Sioux Ridge, a long-lived paleotopographic feature in the eastern margin area of the Cretaceous Western Interior Seaway. Crow Creek strata are unique within the Pierre Shale in containing appreciable amounts of silt- and sand-sized detritus, particularly in its basal part. Shock-metamorphosed quartz and feldspar grains represent a component of impact-derived ejecta recognized throughout the member, and shocked ejecta grains comprise up to 3% of the rock volume in the basal unit. Abundant detrital Paleozoic grains may also have an impact origin. The Manson Impact Structure is considered the most likely source of the Crow Creek ejecta. Recent interpretations of Manson impact-triggered tsunami sedimentation for the Crow Creek Member are regarded as equivocal. Sedimentologic and paleontologic features are consistent with more gradual shallow-shelf sedimentation associated with eastward incursion of a transgressing sea. The sub Crow Creek regional unconformity surface is related to an episode of sea-level lowstand, which was onlapped by transgressive deposits of the succeeding Bearpaw marine cycle. Ejecta grains within the Crow Creek are interpreted to be reworked from impact deposits associated with the regional unconformity surface, and the impact event is suggested to have generally preceded Crow Creek deposition.