West-East Stratigraphic Transect of Cretaceous Rocks--Southwestern Montana to Western Minnesota

DYMAN, T. S., U.S. Geological Survey, Denver, CO, S. B. ANDERSON, North Dakota Geological Survey, Bismark, ND, E. B. CAMPEN, Consultant, Billings, MT, W. A. COBBAN, U.S. Geological Survey, Denver, CO, J. E. FOX, South Dakota School of Mines, Rapid City, SD, R. H. HAMMOND, South Dakota Geological Survey, Vermillion, SD, K. W. PORTER, Consultant, Bozeman, MT, D. D. RICE, U.S. Geological Survey, Denver, CO, D. R. SETTERHOLM, Minnesota Geological Survey, St. Paul, MN, and G. W. SHURR, St. Cloud State University, St. Cloud, MN

In Montana, North and South Dakota, and Minnesota, Cretaceous strata of the Western Interior foreland basin are preserved today in Laramide structural and cratonic basins. The Western Interior basin was asymmetric: more than 17,000 ft of strata are present in southwestern Montana, less than 1000 ft in eastern South Dakota. Asymmetry resulted from varying rates of subsidence owing to tectonic and sediment loading. Cretaceous rocks consist primarily of sandstone, siltstone, claystone, and shale. Conglomerate is abundant along the western margin, whereas limestone is generally restricted to the eastern shelf. Sediment was deposited in both marine and nonmarine environments as the shoreline fluctuated during major tectonic and eustatic cycles.

A west-east transect of the Cretaceous System from southwestern to east-central Montana, the Black Hills and Williston basin, and eastern South Dakota and western Minnesota includes regional facies relations, sequence boundaries, and biostratigraphic and radiometric correlations. More than 17,000 ft of Cretaceous strata in southwestern Montana typify thick nonmarine facies of the rapidly subsiding westernmost part of the basin. These strata include more than 10,000 ft of synorogenic conglomerate facies of the Late Cretaceous Beaverhead Group. West of the Madison Range, sequence boundaries are at the base of the Kootenai and Blackleaf formations and bracket the Frontier Formation; sequences are difficult to define because the rocks are mostly nonmarine. Cretaceous strata in east-centra Montana (about 4500 ft thick) lie at the approximate depositional axis of the basin and are mostly marine terrigenous rocks. Chert-pebble units in these rocks reflect unconformities to the west. The Cretaceous System in North and South Dakota (1500-2000 ft thick) represents a marine shelf sequence dominated by shale and limestone overlain by coastal sandstone and nonmarine rocks. Major sequence boundaries are at the base of the Lakota Formation, Fall River Sandstone, and Muddy Sandstone and bracket the Niobrara Formation.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)