HIGH SPATIAL-RESOLUTION PROVENANCE DISCRIMINATION OF SILICICLASTIC MARKER BEDS OF THE WILKINS PEAK MEMBER OF THE EOCENE GREEN RIVER FORMATION, WYOMING
The fluvial rock record is key to understanding ancient fluvial processes, sediment transport and dispersal, the influences of tectonics, and the effects of deep-time climate changes on landscapes. Obtaining this information is challenging, however, due to the low temporal resolution and difficulty in lateral correlation that is typical of fluvial deposits. We examined a series of nine well-dated, laterally-continuous fluvial bedsets within the Wilkins Peak Member of the Green River Formation that offer an unprecedented opportunity to monitor the dynamic evolution of a megafan distributary network at ~100 ky temporal resolution based on previous radioisotopic dating of volcanic tuffs contained in interfingering lacustrine facies. Changes in megafan dynamics were elucidated via sedimentary provenance, based on exposures in the southeastern Bridger basin. We used a handheld XRF device to measure elemental concentrations of more than 1000 samples, which were compared to petrographic point counts and an existing database of detrital zircon (DZ) ages. Principal component analysis of XRF results and synoptic maps of each bed define several distinct chemofacies that correspond well to DZ populations and modal mineral composition. Siliciclastic bedsets in the center of the basin contain Al-, K- and Rb-rich arkosic sandstone sourced from the Aspen paleoriver, which drained Yavapai-Mazatzal basement exposed along the northwestern slopes of the Rocky Mountains in Colorado. These interfinger with quartzose sandstone sourced from the Uinta Uplift along the southern margin of the basin, and with sublithic sandstone sourced predominantly from Paleozoic and Mesozoic strata exposed in the Sevier fold-thrust belt to the west. Synoptic reconstructions of sandstone composition across each of the nine beds show that distributary lobes migrated laterally on the 2-8 km scale from bed to bed. They also reveal a long-term basinward advance of the distributary networks corresponding to the Uinta Uplift and fold-thrust belt sources, and reciprocal retreat of distributaries of the Aspen paleoriver.