RESOLVING THE EFFECTS OF CLIMATE, TECTONICS, AND GEOMORPHIC PROCESSES ON A CLOSED LACUSTRINE SYSTEM, GREEN RIVER FORMATION, WYOMING

The Wilkins Peak Member of the Green River Formation of southwest Wyoming is a type example of an evaporative lake system. It was deposited in a hydrologically closed system during the intense warmth of the early Eocene climatic optimum, and therefore the environmental changes recorded in it provide an important window into greenhouse climate and geomorphic change. The Wilkins Peak Member contains repeating intervals of lacustrine carbonate and alluvial siliciclastic facies that have been hypothesized to record orbital forcing. The specific mechanisms responsible for this cyclicity and its relation to the marine record have been the subject of debate. Nine named siliciclastic beds thicken towards the southeastern edge of the basin and contain well developed paleosols along the basin margin and less developed inceptisols in the basin center. These siliciclastic beds are interpreted to have been deposited during lowstands that allowed fluvial processes to dominate and are hypothesized to reflect eccentricity minima. Lacustrine intervals, in contrast, record multiple precession-scale alternations between deep lacustrine and salt pan conditions, suggesting higher variance in insolation resulting from amplified precessional variation during high eccentricity. We correlated the Wilkins Peak Member to laterally equivalent basin margin fluvial strata along a basin to margin transect using facies analysis, gamma ray spectrometer, and handheld XRF. We measured channel height, width, grain size, and grain composition of fluvial bed forms to streams entering the basin. Two sources of siliciclastic detritus are apparent in the basin: 1) quartzose, derived from catchments that drained northward from the Uinta Uplift; and, 2) an arkosic source derived from streams that drained crystalline uplifts in Colorado. Our preliminary findings suggest that: 1) streams from these two sources met and mixed along the east side of the Bridger basin depocenter during alluvial dominated intervals, and deposited their sediment in geographically distinct locations during lacustrine phases; and, 2) the paleohydrolgic response of local, high gradient Uinta Uplift-sourced streams differed significantly from larger trunk streams headed along the western slopes of the Eocene Rockies.