INTERMONTANE VALLEY AND BASIN DEVELOPMENT LINKED TO THE TECTONIC, CLIMATIC, AND TOPOGRAPHIC EVOLUTION OF THE SEVIER-LARAMIDE OVERLAP ZONE IN SOUTHWESTERN MONTANA

The Paleogene Renova Formation (~47-21 Ma) is the earliest record of sedimentation following the end of Sevier-Laramide deformation in southwestern Montana (SWMT). The variety and distribution of depositional facies within the Renova Formation indicate that SWMT was characterized by rugged topography during Paleogene time. Paleocurrent and compositional data from basin-margin deposits document radiating paleodispersal away from high-relief uplands into adjacent intermontane basins. Unlike the series of large Laramide lake basins that existed to the south, most of the Renova basins were not internally drained for significant lengths of time. Rather, they formed an integrated fluvial drainage network that flowed eastward across the Sevier fold-thrust belt and around extant Laramide intraforeland uplifts, similar to the modern hydrography.

Continued studies of the Renova Formation have revealed a complex series of interrelated tectonic, topographic, and climatic events that led to decay of the northern Cordillera. Regional, kilometer-scale elevation gain coupled with subtropical climatic conditions resulted in deep exhumation of the orogenic wedge during Paleocene-early Eocene time. Significant fluvial incision occurred along zones of tectonostratigraphic weakness, generating deep (≥2 km relief), elongate, intermontane paleovalleys that paralleled regional structure and served as long-lived sedimentary basins during Cenozoic time. Beginning in mid-Eocene time, lithologic data indicate the presence of a significant rain shadow across the region. Continued climatic aridification was accompanied by widespread sedimentation. Late Eocene-Oligocene crustal extension, focused west of the Sevier thrust front, beheaded parts of the mid-Eocene drainage network and formed small, enclosed basins on top of the fold-thrust belt. A through-going drainage network persisted east of the thrust front through Miocene time. Subsequent Neogene Basin and Range- and Yellowstone hotspot-related extension further reactivated and modified the Cenozoic landscape.