--> Revisiting the Depositional Environment and Provenance of the Wasatch Formation in the Uinta Basin, Utah: Implications for the Early Eocene Paleogeography of Western North America

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Revisiting the Depositional Environment and Provenance of the Wasatch Formation in the Uinta Basin, Utah: Implications for the Early Eocene Paleogeography of Western North America

Abstract

The Wasatch Formation was named by F.V. Hayden in 1869 as Paleocene to Eocene red conglomerate, sandstone, shale, and variegated claystone deposits; widespread over the Rocky Mountain Region. This study integrates outcrop and subsurface datasets across the Uinta Basin, Utah to better define the genetic relationships between different “Wasatch” deposits in the basin and refine paleogeographic interpretations. Early Eocene siliciclastic deposits in the Uinta Basin defined as the Wasatch Fm. on the northern basin margin are sourced from the Uinta Uplift, whereas a large fluvial system supplied siliciclastic sediment to the basin's southern margin. Sandstones sourced from the southern fluvial system are lithic arenites with approximately 25% sedimentary rock fragments and high proportions of chert. These southerly sourced deposits span fluvial, deltaic, and deep lake depositional environments. Sandstones sourced from the Uinta Uplift are quartz arenites and were deposited dominantly in proximal alluvial/fluvial depositional environments. The sediments sourced from the southern fluvial system have a larger aerial and volumetric footprint in the basin, and deep-lake sediment gravity flow deposits sourced from southern fluvial system are found interfingering with northerly sourced alluvial/fluvial deposits near the basin's northern margin. Detailed outcrop and petrographic studies of the Wasatch Fm. in the Uinta Basin have key implications for the Early Eocene paleogeography of North America. Conservative results calculating paleodischarge from the outcrop analysis of southerly sourced fluvial deposits and paleocatchment areas from scaling relationships with maximum paleodischarge (e.g. Syvitski et al., 2014) show that the southerly sourced fluvial system was a regional scale drainage network during the Early Eocene (catchment size approximately 20,000 – 200,000 km2). Detailed petrographic study also demonstrates that much of the sediments supplied by the southern fluvial system were first generation sediments. These stratigraphic and petrographic observations support recent detrital zircon work (e.g. Davis et al., 2010; Dickinson et al., 2012) proposing a paleoriver flowing northward, sub-parallel to the Sevier thrust front, during Early Eocene time. This is in contrast to competing interpretations that the Early Eocene sediments in the Uinta Basin were recycled Cretaceous topcover supplied by small river systems with local catchments (e.g. Wernicke et al., 2011).