ABSTRACT: Cretaceous System on Western End of Sioux Ridge and Southeastern Margin of Williston Basin, Central South Dakota

George W. Shurr, Richard H. Hammond, David K. Watkins

The Cretaceous System in central South Dakota is dominantly marine shale and chalk with marginal marine to nonmarine units at the base (Dakota Formation) and top (Fox Hills and Hell Creek Formations). Between these formations the section includes Graneros Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, and Pierre Shale. The total system thickens from less than 1500 ft (535 m) in Gregory County at the western end of Sioux Ridge to more than 2500 ft (760 m) in Dewey County on the southeastern margin of the Williston basin.

The Dakota Formation is fairly continuous in the subsurface throughout the area. It onlaps upper Paleozoic and lower Mesozoic rocks on the sub-Cretaceous unconformity and represents initial marine transgression onto Sioux Ridge. The Fox Hills and Hell Creek are generally preserved only in the Williston basin and represents the regression that ended Late Cretaceous deposition.

Sediments between the sandy Dakota and Fox Hills were deposited on muddy shelves and carbonate ramps that sloped gently west and north, off Sioux Ridge and into the Williston basin. Chalks in Greenhorn and lower Niobrara mark transgressive maxima, while those in upper Niobrara and Pierre correlate with lowstands. Near Sioux Ridge facies changes indicate shallow water and/or emergence. Within the Carlile, shale and shallow-shelf sandstone in Gregory County pass laterally into marginal marine and nonmarine sediments in and around Davison County, located about 75 mi (120 km) to the east near the crest of Sioux Ridge. In exposures along the Missouri River, the Crow Creek Member of Pierre Shale carries reworked Niobrara nannofossils and may also preserve an inverted Niobrara biostratigraph .

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990