Depositional Cycles and Sequences In An Organic-Rich Lake Basin: Eocene Green River Formation, Lake Uinta, Colorado and Utah, U.S.A.

Abstract: Early to middle Eocene Green River Formation lacustrine deposits in the eastern portion of Lake Uinta formed in two subbasins, the Piceance basin and the Uinta basin, and represent mixed siliciclastic–carbonate and organic-rich lake deposits formed during the time of the Eocene climate optimum. The formation consists of organic-rich and organic-poor mudstone i.e., oil shale, siliciclastic, and carbonate, formed in a shallow to deep (tens of meters), stratified lake environment. Lacustrine strata are characterized by three types of (decimeter to meter) depositional cycles: (1) type 1 depositional cycle formed in marginal areas. Cycle type 1 starts with siliciclastic-rich deposits, passes upward into alternating carbonate shoal and microbial carbonate, and is covered by mud- to silt-size sublittoral deposits. In the deeper, profundal zone of the lake, two types of depositional cycles occur: (2) type 2 depositional cycles start with lean oil shale and pass upwards into siliciclastic turbidites, and are sharply followed by rich-oil-shale deposits. (3) type 3 depositional cycles begin with evaporites and lean oil shale with evaporites that are sharply overlain by rich oil shale. Stacked depositional cycles form depositional sequences meters to tens of meters thick. Altogether, ten upward-deepening depositional sequences have been defined and are correlated over the Douglas Creek Arch from the Piceance basin into the Uinta basin. Depositional sequences characterize significant changes in lake regime and are divided into periods of low, rising, and high lake that are separated by sequence boundaries, transgressive surfaces, and main flooding surfaces, respectively. These depositional sequences indicate that the two basins were most probably connected during most of the first part of the Green River Formation. In the middle of the Green River Formation the arch acted as a partial sill between the two basins, when thick marginal deposits formed at the arch. In the later part of the Green River Formation, profundal deposits covered the arch. It is suggested here that the development of depositional cycles and depositional sequences in these lacustrine basins is strongly affected by climate changes and respective inflow, i.e., during times of low inflow siliciclastic and nutrient input into the lake decreased. In contrast, the highest input of siliciclastics and nutrients into the lake occurred during increased and high inflow. Low runoff resulted in low lake level and is marked by thin marginal deposits and lean oil shale in the profundal area. At times, evaporite deposition could occur in the deeper part of the basin. In the change to a wetter period, increased runoff is marked along basin margins by sharp-based sandstones. In the profundal area rich oil shale sharply overlies lean oil shale. The evolution of Lake Uinta went through uniform regional and climate changes characterized by, (1) forming of highly cyclic units in the middle of the Green River Formation, at the time when the Douglas Creek Arch acted as a partial sill between the two lakes, (2) an overall deepening-upward trend, and (3) filling of basins with siliciclastic deposits at the end of Green River Formation time. Depositional trends that occur both in the Piceance basin and in the Uinta basin also occur in the Greater Green River basin in Wyoming. Therefore, the regional correlation model for all three Green River Formation lacustrine basins has been suggested here in order to show how different basins with similar tectonic and climate background changes could be correlated with each other.