CHRONOFACIES CONVERGENCE AND DYNAMIC DRAINAGE EVOLUTION IN AN EOCENE BROKEN FORELAND: BRIDGER BASIN, WYOMING, USA

Major marine-terminating river systems carry water and sediment away from continental interiors, acquiring tributaries along the way that average sedimentary provenance signals across progressively larger areas. In contrast, rivers that drain into nonmarine basins can convey more diverse provenance signals related to specific localized watersheds. Based on a compilation of recently acquired detrital zircon ages (N=44, n=10,447), early- to mid-Eocene nonmarine strata in the Bridger basin comprise at least eight distinct chronofacies, which correlate to sandstone petrofacies. Five of these chronofacies were derived from source regions located hundreds of km distant from the basin, encompassing the eastern slope of the North American Cordillera in Utah and Idaho and basement-cored Laramide ranges in Colorado. These chronofacies thus record the convergence of drainage across an area spanning ~1000 km northwest to southeast, bolstering arguments for relatively low Eocene elevation of the Bridger basin. Tectonic and magmatic perturbations within the watersheds of these paleorivers induced stepwise changes to the hydrologic state of paleolake Gosiute, recorded as abrubt, basin-wide, non-Waltherian facies transitions (xenoconformities). For example, tectonic diversion of the Idaho paleoriver away from the Bridger basin at ~51.5 Ma caused an abrupt shift from fluctuating profundal to evaporative lacustrine facies within the Green River Formation. Fish and other macrofauna disappear across this transition due to elevated paleosalinity, and are replaced by bedded evaporite. The Idaho river was later recaptured by the Bridger basin, refreshening the lake and ultimately filling it in with newly-erupted detritus from the Challis volcanic province. Nonmarine Bridger basin deposits thus provide a unique, highly resolved record of the tectonic, magmatic, and geomorphic evolution of the surrounding region. Conversely, the evolution of its watershed directly impacted the dynamic state of downstream lake systems, and therefore the manner in which paleoclimate information was transferred into the sedimentary record.