Print this pageCordilleran Fold-Thrust Belt and Foreland Basin System of Northwestern Montana
At the latitude of northwestern Montana a continuous evolution of the Cordilleran retroarc fold-thrust belt is recorded in strata of the Middle Jurassic-Eocene foreland basin system. Detrital zircon ages and modal sandstone petrography suggest that by Bajocian times (~170 Ma) miogeoclinal strata were being deformed and eroded in hinterland regions. Oxfordian-Kimmeridgian sandstones of the Swift and Morrison Formations contain detrital zircons derived from the Intermontane belt to the west. This early record of the onset of orogenic deformation is preserved in relatively thin, tabular, shallow marine and fluvial deposits that possibly accumulated in a back-bulge depozone. By late Early Cretaceous time the fold-thrust belt had propagated eastward and a classic foredeep was deposited on top of distal Middle Jurassic deposits. Pre mid-Cretaceous movement of major thrust sheets in hinterland regions involving Proterozoic Belt Supergroup rocks is constrained by provenance data and cross-cutting relations of stitching plutons. By Campanian time the Lewis and related thrust sheets were active and being eroded, and previous foreland basin deposits were cannibalized. The last stage of evolution of the fold-thrust belt deformed Paleozoic and Mesozoic strata in the footwall of the Lewis thrust and produced the Sawtooth Range, in part an exhumed duplex active until early Paleogene time. A new balanced cross-section indicates a minimum shortening of 125 km for the frontal part of the fold-thrust belt of northwestern Montana.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009