PROVENANCE MAPPING THE EOCENE RIVERS THAT FED A GREEN RIVER FORMATION LAKE USING SANDSTONE PETROGRAPHY AND HANDHELD XRF ANALYSIS

Closed lake systems can exhibit vast changes in base level that act to intercalate fluvial and lacustrine facies at a fine scale. In the Wilkins Peak Member of the Green River Formation in southwest Wyoming, nine named siliciclastic marker intervals are regularly interbedded with carbonate-rich lacustrine strata that contain bedded evaporites. This cyclic alternation between facies has been attributed to eccentricity based on U-Pb and 40Ar/39Ar geochronology. Understanding the interplay between sediment advection and local climates requires better understanding the source for clastic material within clastic marker beds. We investigated two of the most prominent intervals (beds D and I) using detailed stratigraphy, sandstone petrography, and handheld XRF analysis. We use sandstone petrography to calibrate regression models for mineral abundance from XRF results to more broadly map different petrofacies. Results and comparisons to detrital zircon U-Pb geochronology suggest that the vast bulk of the arkosic material within Wilkins Peak Member alluvial beds was derived from a paleoriver that drained the west slopes of the Park-Gore uplift and north slopes of the Sawatch Uplift. In contrast, quartzose to sub-lithic sediment was delivered by steep streams that drained the Uinta Uplift, represents only a minor fraction of accumulated sediment, restricted to only the immediate mountain front. A third, more quartzose petrofacies occurs along the northern edge of the Wilkins Peak Member depocenter, and likely represents sediment delivered by streams draining the Sevier fold and thrust belt to the west of the lake. Near modern Green River, WY, sediments from this third source appear just prior to lacustrine transgression. This finding is consistent with Sr isotope evidence (Baddouh et al., 2016) that precipitation in the Cordillera was the primary driver of lacustrine highstands within the Wilkins Peak Member. These results suggest that clastic marker beds were deposited during periods of low, relatively stable lake level, suggesting decoupling between precipitation-evaporation, as recorded by lake level, and sediment transport, as recorded by basinward advection of clastic sediments by a fluvial distributary network headed in the Laramide highlands of Colorado.