ABSTRACT: Tectonic Significance of Jurassic-Cretaceous Strata in the Western Interior Foreland Basin of Northwest Montana and Southern Canada

GILLESPIE, JANICE M., California State University, Bakersfield, CA, and PAUL L. HELLER, University of Wyoming, Laramie, WY

Basin-modeling studies combined with field observations of nonmarine Jurassic-Cretaceous strata can be used to compare the evolutionary history of the foreland basin in southern Canada with that of northern Montana. Asymmetric thickening and facies relationships within the Upper Jurassic Kootenay Group in Canada suggest it was deposited in a subsiding foreland basin. However, similar evidence indicates that the age-equivalent Morrison Formation in northwest Montana was not. These findings suggest that early tectonic loading by the Sevier/Columbian thrust belt of North America was not widespread but was confined to specific segments of the orogen. The Late Jurassic subsidence event in Canada cannot be traced south of lat. 48 degrees N, which suggests that it may have resulted from eith r (1) deformation associated with the late Middle Jurassic collision of the Intermontane terrane with the continental margin in Canada or (2) that inhomogeneities in the underlying crust caused the continental margin north of this latitude to respond differently to imposed stresses than the area to the south. In Montana, rapid foreland basin subsidence began in early Late Cretaceous time, concurrent with deposition of the Upper Kootenai Formation or the Colorado Group. At the same time, rapid subsidence in the Canadian foreland basin was renewed, suggesting that thrust loading along the entire length of the Cordilleran thrust belt was occurring by Late Cretaceous time. Thrust loading in Montana may have occurred in response to gravity spreading associated with the emplacement of plutons n the Idaho batholith or to accretion of the Blue Mountains Terrane in Western Idaho.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)