ABSTRACT: Cretaceous Rocks of Cedar Creek Anticline Area, Eastern Montana and Southwestern North Dakota

Dudley D. Rice, William A. Cobban, George V. Shurr

More than 4800 ft (1460 m) of Cretaceous rocks are present in the Cedar Creek anticline area. Most of the rocks occur below a cover of Tertiary rocks except along the axis of the anticline where younger Cretaceous are exposed. Cretaceous sediments in this area are mainly of marine origin and were deposited in the central part of the Western Interior seaway. The Cedar Creek structure experienced recurrent tectonic movement from early Paleozoic to middle Tertiary time, including the Cretaceous.

Nonmarine rocks of the Lower Cretaceous Lakota Formation unconformably overlie Upper Jurassic nonmarine rocks. The Lakota is unconformably overlain by a Lower Cretaceous marine clastic sequence comprising the Fall River Sandstone, Skull Creek Shale, and Muddy Sandstone. Most of the Upper Cretaceous section consists of thick marine shales assigned to the Mowry Shale, Belle Fourche Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, Gammon Shale, and Pierre Shale. Depocenters for many of these fine-grained sequences were in the area of the anticline, suggesting recurrent movement. The Greenhorn and Niobrara formations are prominent

carbonate units that were deposited during times of relative sea-level high-stands. A regional unconformity is developed at the base of the Niobrara that extends across the eastern shelf in North and South Dakota. Shelf sandstones equivalent to the Judith River Formation are locally present along the axis implying shoaling along the anticline. The thick shale sequence is overlain by coastal sandstones of the Fox Hills Sandstone and nonmarine rocks of the Hell Creek Formation, both of Late Cretaceous age.

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