VARIATION IN BASIN GEOMETRY AND ACCOMMODATION DURING THE LATE CRETACEOUS, NORTH AMERICAN CORDILLERA, MONTANA AND WYOMING

Accommodation in the Western Interior Basin was primarily controlled by contractional deformation driven by the subduction of the Farallon plate beneath the North American plate during the Late Cretaceous. Subduction of the conjugate Shatsky Plateau resulted in the flattening of the Farallon plate, which in turn resulted in the transition from thin-skinned (Sevier) to thick-skinned (Laramide) deformation. Previous studies based on seismic tomographic modeling have suggested the flat slab and associated dynamic subsidence reached Wyoming by the Late Campanian. These studies, however, do not consider deformation observed in southwestern Montana as early as the early Coniacian, approximately 10 Ma earlier than predicted. Late Cretaceous strata in basins in Wyoming, Colorado, and Montana were deposited synchronously with the transition from Sevier to Laramide deformation, providing the opportunity to examine the effects of the transition on accommodation development and depocenter movement along strike of the axis of the basin. Petroleum well logs, sequence stratigraphic correlations, and chronostratigraphic tops from public databases and published literature were integrated to make seven regional isopach maps. The maps cover the time periods from approximately 100 to 65 Ma in five-million-year steps and show depocenters moved from southwestern Wyoming to southern Montana by the late Coniacian (85 Ma), coeval with a transition from a narrow and deep to broad and shallow depocenter geometry. Small scale variations in this pattern are attributed to flexural loading by individual Laramide uplifts. Results support a model in which dynamic subsidence induced by flat subduction reached southwestern Montana by approximately 85 Ma, earlier than predicted by previous models. This could indicate the conjugate Shatsky Plateau was located further north than previously suggested or changes in the effective elastic thickness of the lithosphere allowed for the focusing of the end-loading associated with flat subduction in Wyoming and Montana. These processes allowed for the dramatic changes in basin geometry over time and earlier development of dynamic subsidence in Wyoming and Montana.