EVALUATION OF THE PALEOGENE TECTONIC, TOPOGRAPHIC, AND CLIMATIC EVOLUTION OF THE SEVIER-LARAMIDE OVERLAP ZONE, SOUTHWESTERN MONTANA

Recent reviews of Cenozoic stable isotope (δ¹⁸O, δ¹³C) data from across the western US have suggested that mid-Eocene time marked the beginning of isostatic crustal rebound of the Cordillera, which initiated extensional reactivation (collapse) of the Cordilleran orogenic wedge and large-scale drainage reorganization throughout the Sevier-Laramide structural province. The tectonic events were coeval with a climatic shift from warm and wet during latest Cretaceous and Paleocene time to cooler and drier during later Eocene and Oligocene time. The interplay of these events modified northern Cordilleran geography and influenced sedimentation throughout the western US, but the relative influence of each is difficult to elucidate.

This study revisits a series mid-Eocene to Oligocene sedimentary successions in southwestern Montana (SWMT) that host a δ¹⁸O signal of km-scale elevation gain. We investigate new and existing stable isotope (δ¹⁸O, δ¹³C) data in the context of detailed physical stratigraphy, sediment provenance data, and paleogeographic reconstructions. Based on these data, we interpret a time-series of tectonic, topographic, and climatic events for the SWMT region including (1) erosional exhumation of the SWMT Cordillera and sedimentary bypass; (2) regional elevation gain of 1.5 to 2.5 km during mid-Eocene time; (3) structural segmentation of the pre-existing paleovalley network; (3) contemporaneous generation of closed lacustrine basins on top of the extant Sevier fold-thrust belt and an interbasinal drainage network in the adjacent foreland region; (4) development of an aridity gradient (or rain shadow) across SWMT; and (5) progressive aridification during later Eocene and Oligocene time, accompanied by significant sedimentation within the basin network.