Depositional Cycles and Sequences in an Organic-Rich Lake Basin: Eocene Green River Formation, Lake Uinta, Colorado and Utah
J. Frederick Sarg, Kati Tänavsuu-Milkeviciene, Yuval Bartov, "Depositional Cycles and Sequences in an Organic-Rich Lake Basin: Eocene Green River Formation, Lake Uinta, Colorado and Utah", Sequence Stratigraphy: The Future Defined, Bruce Hart, Norman C. Rosen, Dorene West, Anthony D’Agostino, Carlo Messina, Michael Hoffman, Richard Wild
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Green River Formation lacustrine deposits in the eastern portion of Lake Uinta formed in two sub-basins (the Piceance basin and the Uinta basin) and represent mixed siliciclastic-carbonate and organic-rich lake deposits deposited during the Eocene climate optimum. The formation is comprised of organic-rich and organic-poor mudstone, siliciclastics, and carbonates, formed in a shallow to deep (tens of meters), stratified lake environment. Integrated sequence stratigraphic analysis using gamma logs, Fisher Assay plots, core, and outcrop has resulted in a predictive framework for organic-rich oil shale distribution, reservoir characterization, and hydrocarbon systems analysis.
Lacustrine strata are characterized by three types of (meter to decimeter) depositional cycles: (1) Type 1 cycles formed in a littoral/sublittoral zones and comprise progradational siliciclastic-rich deposits that pass upward into progradational to aggradational carbonate shoal and microbial carbonate and are capped by mud-to silt-sized sublittoral deposits. In the profundal zone, two types of depositional cycles occur: (2) Type 2 cycles start with lean oil shale, pass upwards into siliciclastic turbidites, and are overlain by rich oil shale deposits. (3) Type 3 cycles initiate with evaporites and mixed lean and rich oil shale that is overlain by rich oil shale. Stacked depositional cycles form depositional sequences meters to tens of meters thick. Eleven upward-deepening depositional sequences have been described and are divided into periods of low, rising, and high lake that are separated by sequence boundaries, transgressive surfaces, and main flooding surfaces, respectively.
The development of depositional cycles and sequences in these lacustrine basins appear to be strongly affected by climate changes and respective inflow; i.e., during times of low inflow (low lake level) siliciclastic and nutrient input into the lake decreased. In contrast, the highest input of siliciclastics and nutrients occurred during increased and high inflow (rising and high lake level). Low lake level is marked by thin marginal deposits and lean oil shale and at times, evaporite deposition in profundal areas. Increased runoff is marked along basin margins by sharp-based sandstones and carbonates. In the profundal area, rich oil shale overlay lean oil shale.