--> ABSTRACT: Cretaceous Strata of Northwestern Montana, by T. S. Dyman, S. V. Foster, D. D. Rice, W. A. Cobban; #91002 (1990).

Datapages, Inc.Print this page

ABSTRACT: Cretaceous Strata of Northwestern Montana

T. S. Dyman, S. V. Foster, D. D. Rice, W. A. Cobban

Cretaceous strata in northwestern Montana record sedimentary-tectonic relationships and relative sea-level fluctuations along the western shoreline of the Western Interior seaway. These intertonguing marine and nonmarine strata represent facies of the Cordilleran foreland basin. Tectonic influences on sedimentation include periodic minor uplift of the Sweetgrass arch, regional volcanism, and thrusting. Thrusting has telescoped Cretaceous rocks in the Disturbed belt. Composite surface sections from the Sun River Canyon area south of Glacier National Park and the Great Falls-Highwood Mountains area were studied as part of the Western Interior Cretaceous Project of the Global Sedimentary Geology Program.

In the western part of the Western Interior seaway south of Glacier National Park, Cretaceous strata include the Aptian(?) to Albian Kootenai Formation, the late Aptian(?) to Albian Mount Pablo Formation, the Albian to Cenomanian Blackleaf Formation, the Cenomanian to Santonian Marias River Shale, the Santonian Telegraph Creek Formation and Virgelle Sandstone, the Campanian Two Medicine Formation, the Campanian to Maastrichtian Bearpaw Shale, and the Maastrichtian Horsethief Sandstone, and St. Mary River and Willow Creek (part) formations. Near Glacier National Park, Cretaceous rocks are more than 7000 ft thick.

Farther east in the Great Falls-Highwood Mountains area, Cretaceous strata include the Kootenai and Blackleaf formations, the Marias River Shale, the Santonian to Campanian Eagle Sandstone, and the Campanian Claggett Shale and Judith River Formation. The total thickness of preserved Cretaceous rocks in the eastern part of northwestern Montana is approximately 4000 ft.

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