ABSTRACT: Cretaceous Rocks North of the Sioux Ridge, Eastern South Dakota and Western Minnesota

Richard H. Hammond, George W. Shurr, Dale R. Setterholm, David K. Watkins

The Cretaceous System is relatively well preserved north of the Sioux Ridge in eastern South Dakota and western Minnesota. Marine units of the Western Interior reference section show dramatic facies change to the south where they onlap the flanks of the Ridge and a more subtle change eastward where they onlap low-relief basement surfaces in Minnesota.

In northeastern South Dakota approximately 150 mi (240 km) north of the Sioux Ridge crest, marine units onlap a small basement high. Dakota Formation and Graneros Shale thin from 100 ft (30 m) to zero over the high; Greenhorn Formation and Carlile Shale are locally found directly on crystalline basement and thicken to more than 200 ft (60 m) away from the high. Niobrara Formation and Pierre Shale are incompletely preserved in the area. The Dakota and Graneros thin eastward for 100 mi (160 km) to central Minnesota, where calcareous shale equivalent to Greenhorn overlies thin nonmarine siltstones, shales, and kaolinitic saprolith formed on crystalline basement.

Immediately adjacent to Sioux Ridge in east-central South Dakota, the Graneros through Pierre marine units grade southward into nonmarine and nearshore sandstones, claystones, and spiculites of the Split Rock Creek Formation along the embayed north flank of the Ridge. These sediments suggest that parts of the Proterozoic quartzite Sioux Ridge remained emergent even during peak highstands in the Late Cretaceous. Approximately parallel to the ridge crest, marine units grade eastward into transitional and nonmarine rocks in Minnesota. In addition to onlapping a low-relief surface developed on Archean crystalline rocks, units equivalent to the Greenhorn and Carlile may onlap an east-thickening wedge of nonmarine sediments.

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