| Description |
Detailed lithofacies analysis of over 4,500 m of measured sections of most of the Eocene portion of the fluvial and lacustrine Green River Formation in the south-central Uinta Basin documents twelve lithofacies: Sa (nonsinuous trunk streams), Sb (meandering delta distributary channels), Sc (amalgamated delta mouth bars), Sd (regressive deltaic sandsheets), Se (overbank and shallow-lacustrine sandsheets), Sf (crevasse channels and splays), C (passively-filled abandoned channels), Mg (subaqueous mudflats), Mr (subaerial mudflats), L (lake-margin carbonate flats), and la and lb (openlacustrine deposits). The lower half of the study section consists of the carbonate marker unit (130 m) and the overlying Sunnyside delta interval (375 m), which record marginal-lacustrine sedimentation along the southern shore of Lake Uinta within and adjacent to a large, fluvially-dominated lacustrine delta informally named the Sunnyside delta. Meandering delta distributary channels on the delta plain cannibalized most delta mouth bars. Shallow water depths in the southern half of Lake Uinta attenuated waves and wave-generated currents, thereby preventing the development of shoreface ravinement, beaches, barriers, or significant siliciclastic bars. Evaporative pumping on subaeriallyexposed mudflats produced brines which altered detrital clay minerals to analcime. The combination of moderately high local subsidence and variations in lake volume produced low amplitude (less than 12 m) fluctuations in relative lake level which, due to the gentle gradient of the delta plain, produced shifts in shoreline position on the order of 40-50 km. During periods of rapid relative lake level rise most siliciclastic sediment was trapped upstream, resulting in iv lake level which, due to the gentle gradient of the delta plain, produced shifts in shoreline position on the order of 40-50 km. During periods of rapid relative lake level rise most siliciclastic sediment was trapped upstream, resulting in the development of extensive, but thin, transgressive shallow-lacustrine carbonates. The regressive portion of each depositional cycle consists of upper and lower delta plain, shallow-nearshore lacustrine, and relatively rare delta front deposits. The upper half of the study section consists of the transitional interval (200 m), which records a major, but gradual, expansion and deepening of the lake, and the overlying upper member (300 m), which consists of dark mudstone and dolostone and relatively rare hummocky cross-stratified storm deposits produced by combined flows and nearshore-lacustrine sandbodies that accumulated in a generally quiet, but not necessarily very deep, relatively siliciclastic-poor, open-lacustrine setting. |
