EARLY EOCENE DRAINAGE EVOLUTION OF THE IDAHO PALEORIVER, GREEN RIVER BASIN, WYOMING

Lacustrine strata offer a unique window into past tectonic, magmatic, climatic, and geomorphic processes that acted across broad areas of the continents. This perspective is underutilized, however, due in part to uncertainties regarding the extent of paleowatersheds. The Green River Formation is particularly interesting because its deposition overlapped the warmest period of prolonged warm climate in the Cenozoic, the Early Eocene Climatic Optimum. This study focuses on fluvial sandstone in the Green River basin (GRB) of southwest Wyoming, the site of paleolake Gosiute. The GRB watershed spanned three distinct tectonic provinces: the western U.S. Cordillera hinterland, Sevier fold-thrust belt, and Laramide broken foreland. A combination of sandstone petrofacies and new detrital zircon data (11 samples, n=3052) reveals that this watershed evolved dynamically through time, in 4 distinct phases. During phase 1 (ca. 54.0-52.5 Ma) it encompassed parts of central Idaho, southwest Montana, and northwestern Wyoming. The Idaho paleoriver drained southward into the northwest corner of the GRB and deposited alluvial facies of the Wasatch Formation main body. Detrital zircon sources included the Atlanta lobe of the Idaho Batholith, Paleozoic-Mesozoic strata of the Sevier fold-thrust belt, reworked Harebell Formation and Pinyon Conglomerate, Belt Supergroup, and Archean basement of the Beartooth, Wind River, and Tobacco Root uplifts. During phase 2 (ca. 52.5-51.5 Ma) the watershed expanded to include the Bitterroot lobe of the Idaho Batholith, and also incorporated juvenile volcanic detritus from the Absaroka volcanic field. Widespread expansion of fluvial-lacustrine and fluctuating profundal lake facies occurred during this phase. During phase 3 (ca. 51.5-49.0 Ma) the Idaho paleoriver was diverted away from the GRB by uplift within or near the Gros Ventre Range, resulting in a sudden reduction in sand deposition and an abrupt shift to evaporative lake facies. During phase 4 (ca. 49.0-47.0 Ma) the GRB recaptured the Idaho paleoriver, which then primarily carried juvenile volcaniclastic sediment derived from the actively erupting Challis volcanic field. This influx initially refreshened Lake Gosiute, causing it to spill southward into Lake Uinta, and ultimately filled the GRB with deltaic and alluvial facies.