Structure, Kinematics and Foreland Basin Evolution in the Montana Disturbed Belt, Northwestern U.S.A.

Facundo Fuentes, Peter G. DeCelles, and Kurt Constenius
University of Arizona, Tucson, AZ

The Montana Disturbed Belt (MDB) forms the external part of the Cordilleran thrust belt in NW Montana. Its western part is characterized by the several-km thick, rigid thrust sheets of Precambrian and Paleozoic strata brought to the surface by the Lewis and related thrusts; to the east, the MDB is composed of closely-spaced imbricated panels of Paleozoic to Early Cenozoic deposits. Branch line patterns suggest that the eastern imbricate belt is a large hinterland dipping duplex that fed slip into the overlying Lewis thrust, which served as the roof fault for the duplex. This geometry is similar to that of the Waterton duplex in the Lewis thrust salient. We present new total shortening estimates from this part of the thrust belt based on balanced regional cross sections. A foreland basin system was established in this region by Late Jurassic time. Through time the foreland basin migrated progressively eastward, depositing an upward coarsening 3 km thick sequence of mainly fluvial and shallow marine deposits. Preliminary data suggest that the upper Jurassic Morrison Fm. represents the back-bulge deposits, and the foredeep depozone consists of the lower Cretaceous Kootenai through Maastrichtian-early Paleocene Willow Creek Fm. No evidence exists for the preservation of a wedge-top depozone in this part of the thrust belt. Fluvial paleoflow directions are consistently E-NE. A combined structural-stratigraphic approach, combining balanced cross sections, provenance data, thermochronology and geochronology is being used to constrain the kinematic history of the MDB and associated foreland basin in this region.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005