Influence of Proterozoic Belt-Purcell Rift System from Phanerozoic Facies, Isopachs, Structures, and Hydrocarbon Prospectivity in Montana, Usa

James W. Sears
Geosciences, University of Montana, Missoula, MT

The Proterozoic Belt-Purcell basin of NW USA and SW Canada originated at a triple-rift junction where three continental rift zones intersected to define a triangular graben. The complex internal structure of the triangular rift basin included two deep grabens and a central horst. The architecture of the rift system influenced Phanerozoic sedimentation and tectonics. Two of the rift arms evolved into the Cordilleran miogeoclinal margin, with separation of a conjugate continent, likely the Siberian craton. The third rift arm defines the ESE-trending Montana-Tennessee lineament (MTL). Segments of the MTL now define the Lewis and Clark line, Helena embayment, and Central Montana trough. Tectonic adjustments along the MTL, evident in isopach and facies maps of Phanerozoic periods, indicate that the cratonal blocks on opposite sides were alternately uplifted or depressed as they responded independently to far-field tectonic loads. At times the system was extended as the blocks separated, and at times it was compressed as the blocks converged. These shifts permitted accumulation of evaporites and hydrocarbon source rocks along the Central Montana trough. The NE block uplifted in response to the Columbian orogeny, with deep middle Jurassic erosion of a broad fore-bulge, then subsided under the load of the foreland basin as the Rocky Mountain fold-thrust belt propagated eastward. Late Cretaceous granitic batholiths injected east along the MTL into central Montana. The SW block was imbricated into the Rocky Mountain foreland basement uplifts. In detail, the shifting of the rift components of the original Belt-Purcell basin reactivated old faults and compartmentalized the Montana Rocky Mountain fold-thrust belt. The contrasting geologic histories resulted in significantly different hydrocarbon prospectivity on either side of the MTL.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas