STRATIGRAPHIC RESPONSE TO CENOZOIC EXTENSION IN THE CORDILLERAN HINTERLAND: A CASE STUDY FROM THE NORTHEAST BASIN AND RANGE, IDAHO-UTAH-WYOMING

This study seeks to provide insights into Earth surface responses to changes in tectonic regime, namely the onset of extension following the cessation of Sevier and Laramide shortening, which are recorded in a series of Eocene to modern basin deposits in the Northern Basin and Range of Utah, Idaho, and Wyoming. We conducted field sedimentology that involved measuring stratigraphic sections, paleoflow analysis, conglomerate clast composition measurements, and sampling for sandstone petrography, detrital zircon U-Pb geochronology, and optically stimulated luminescence. We integrated these new sedimentary datasets to track changes in depositional environment and sediment source areas during the Eocene tectonic mode shift, and Oligocene to Pleistocene extensional basin development. The Eocene hinterland basins are characterized by early, large fluvial systems that transition up-section into lacustrine depocenters, which were topographically confined by relict N-trending Sevier age folds and the E-W Unita basement block uplift. Detrital zircon Th/U geochemistry indicates a change from more evolved magmas from the Jurassic-Cretaceous Cordilleran arc to more depleted values of the Eocene volcanics coeval with hinterland basin formation. Oligocene half-graben basins were infilled with fluvial to volcaniclastic deposits that were spatially isolated along W-dipping basin-bounding normal faults that reactivated thrust ramps within the stratigraphic cover of the Sevier thrust front. Whereas the Miocene basin systems were organized into fault-bounded grabens that reactivated structurally deeper, basement-involved thrust ramps of the Sevier hinterland. The Miocene basins include fluvial, lacustrine, and volcaniclastic deposits characterized by the strong volcanic input by the Picabo complex of the Yellowstone Hotspot to the north. The Pleistocene sedimentary system consisted of a series of isolated basins infilling Miocene topography that sequentially connected through fill and spill integration driven by climate-induced pluvial lake formation and river course deflection caused by local volcanism. Our results highlight the complex interactions among tectonics, magmatism, and climate that controlled extensional basin system evolution within antecedent contractional orogens.