DETRITAL ZIRCON GEOCHRONOLOGY AND QUARTZ OSL TRACK THE PROGRESSIVE PLEISTOCENE FILL AND SPILL OF PALEO BEAR RIVER BASINS OF UTAH AND IDAHO

The present-day Bear River of Utah, Idaho, and Wyoming is the largest-discharge internally drained fluvial system in North America. However, the timing and mechanisms that drove the Bear River to be captured away from its former course via the Snake River to the Great Salt Lake basin (i.e. Lake Bonneville) remain debated. This study leverages new outcrop level stratigraphy, detrital zircon U-Pb geochronology, and quartz optically stimulated luminescence (OSL) to track the Quaternary changes in fluvial-lacustrine depositional facies and the sediment dispersal pathways of the Bear River across three sedimentary basins upstream of the Great Salt Lake. Both detrital zircon and quartz OSL sensitivity results from deposits are compared against a suite of modern river sand samples to pinpoint changes in sediment provenance from locally derived sources to the more complex and integrated fluvial network of the Bear River. Local sedimentary sources include Miocene volcaniclastic deposits that are characterized by unimodal Cordilleran (0-250 Ma) detrital zircon age peaks and low quartz OSL sensitivities, and Ediacaran-Cambrian quartzites with Yavapai-Mazatzal (1.6-1.8 Ga) zircon age component and relatively high quartz OSL sensitivities. These locally derived sources greatly differ from the Bear River signature of moderate quartz OSL sensitivities and detrital zircon ages that exhibit a more complex, multimodal age distribution with dominant Cordilleran (0-250 Ma), Grenville (1.0-1.2 Ga) and Yavapai-Mazatzal (1.6-1.8 Ga) age components, followed by minor Appalachian (250-500 Ma) and Archean crust (>2.5 Ga) ages. New preliminary OSL ages provide stratigraphic age control for the appearance of paleo Bear River sediments in the key sedimentary basins. Our results indicate that from 55 to 18 ka the Bear River progressively filled each upstream basin followed by a spill-over event into the next subsequent downstream basin. The capture of the Bear River is driven by both a diversion by a volcanic center, and relative baselevel fall during paleo Great Salt Lake regressions.