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