ABSTRACT: Cretaceous Rocks of North-Central Montana

Dudley D. Rice, William A. Cobban, Donald L. Gautier, George W. Shurr

Cretaceous rocks in north-central Montana, as much as 5000 ft (1520 m) thick, represent sediments deposited mainly in marine environments on the western side of the Western Interior seaway adjacent to Cordilleran highlands that were the main source of terrigenous sediments. Most of the Cretaceous sequence is exposed in and around the Bearpaw and Little Rock Mountains and has been penetrated by numerous oil and gas wells.

Nonmarine rocks of the Lower Cretaceous Kootenai Formation unconformably overlie Upper Jurassic nonmarine rocks. The rest of the Lower Cretaceous section, comprising the Fall River Sandstone, Skull Creek Shale, and Muddy Sandstone, unconformably overlies the Kootenai and is an eastward-thinning marine clastic sequence that was deposited during the first invasion of Cretaceous seaway from the Gulf of Mexico and Arctic Ocean.

Rocks of early Late Cretaceous age are represented by marine strata of the Mowry Shale, Belle Fourche Shale, Greenhorn Formation, Carlile Shale, and Niobrara Formation. The Greenhorn and Niobrara are widespread calcareous units that were deposited during times of relative sea-level high-stand. A sandy section in middle part of Carlile Shale was probably deposited on regional unconformity following major sea-level drop in late Turonian time. Above the Niobrara, the Upper Cretaceous section consists of eastward-thinning deposits of coastal sandstones and nonmarine rocks--the Eagle Sandstone, Judith River Formation, and Fox Hills Sandstone--and intertonguing westward-thinning wedges of marine shale--the Claggett and Bearpaw Shales. Nonmarine rocks of the Hell Creek Formation, which overl e the Fox Hills Sandstone, are sporadically preserved but were probably deposited over the entire area.

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