ABSTRACT: Cretaceous Rocks of Southwestern Montana

T. S. Dyman, P. G. Decelles, J. C. Haley, D. J. Nichols, R. C. Pearson, W. J. Perry, Jr., R. G. Tysdal, R. K. Schwartz

Cretaceous strata in southwestern Montana represent deposition in the Cordilleran foreland. Thick nonmarine facies typify the westernmost and more rapidly subsiding half of the basin, whereas thinner intertonguing marine and nonmarine facies occur eastward. Some of the strata in each of these areas are time-equivalent sediments deposited in different environments. Correlation problems have been difficult owing to the paucity of datable material in nonmarine facies, although palynology has contributed to the resolution of some problems. Late Cretaceous and possibly early Tertiary thrust faulting has brought the two composite measured sections presented here (near Lima and in the Madison Range) closer together than when the sediments were deposited.

In the western part of the foreland basin, Cretaceous strata include the Aptian to Albian(?) Kootenai Formation, the Albian to Cenomanian Blackleaf Formation, the Cenomanian to Turonian Frontier Formation, and the Coniacian to Maastrichtian (and early Tertiary?) Beaverhead Group. In addition, Upper Cretaceous volcanic and volcaniclastic strata in the eastern Pioneer Mountains are equivalent to and younger than the Frontier Formation. The Beaverhead Group is composed of several unconformity-bounded formations, each with limited areal extent and laterally variable alluvial facies. Near Lima, Cretaceous rocks may exceed 20,000 ft in thickness.

To the east in the Madison Range, in the distal marine facies of the foreland basin, the Kootenai Formation is overlain by approximately 10,000 ft of strata assigned to the Albian Thermopolis Shale and Muddy Sandstone, the Cenomanian Mowry Shale, the Cenomanian to Turonian Frontier Formation, the Coniacian Cody Shale, the Santonian Telegraph Creek Formation and Virgelle Sandstone, the Santonian to Campanian Evens Formation, the Campanian to Maastrichtian(?) Livingston Formation, and the Maastrichtian Sphinx Conglomerate.

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