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ABSTRACT: Cratonic Platform and Foredeep Response to Plate Margin Convergence: Devonian through Mississippian Subsidence History in Western Montana and East-Central Idaho

S. L. Dorobek, S. K. Reid, M. Elrick, G. C. Bond, M. A. Kominz

Devonian and Mississippian sedimentary rocks of western Montana and east-central Idaho were deposited on a cratonic platform that faced a northern extension of the Antler foredeep. Subsidence analyses of this sequence and isopach maps illustrate regional patterns of subsidence related to convergence along the western North American plate margin. Tectonic stresses affected deposition on platform areas which were hundreds of kilometers inboard from the ancient continental margin. Wavelengths of paleostructural elements, tectonic inversion of these structures (i.e., transition of a paleohigh into a depocenter), and time scales involved in the inversion process cannot be attributed solely to flexure or to vertical displacements by in-plane stresses but suggest reactivation of Precambrian structural trends.

Late Devonian (Frasnian) platform sedimentation began during a brief interval of increased subsidence across western Montana. This interval of increased platform subsidence is greater than a Late Devonian eustatic sea level rise (determined from subsidence analyses of Devonian strata from stable cratonic areas) and suggests some tectonic event must have influenced subsidence in Montana. Thin uppermost Devonian strata contain numerous unconformities that may be related to flexure of the platform plus eustatic sea level fluctuations.

Rapid subsidence across Montana during the Early Mississippian (Kinderhookian) resulted in a condensed platform sequence, which is overlain by deep water shaly carbonates. Rapid subsidence continued into the Osagean, then slowed, allowing progradation of carbonate platform facies across Montana. A regional karst surface on top of the Meramecian platform coincides with conglomerate deposition and increased subsidence rates in the foredeep; unconformity durations on the platform also increase to the east. These relationships suggest that unconformity development may be partially related to flexure across the platform and not entirely to eustatic sea level fall.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990