Sequence Stratigraphy in Mixed Lake Systems, Organic Richness and Climate — Green River Formation, Lake Uinta, Part I, Sequence Stratigraphy

Tanavsuu-Milkeviciene, Kati; Sarg, Rick; Bartov, Yuval

Early to middle Eocene Green River Fm. in Lake Uinta formed in two subbasins, Piceance Creek and Uinta basins and represents a mixed carbonate-siliciclastic-evaporite and organic-rich shale or siliceous dolomitic mudstone lake deposits formed in a deep (10's meters), stratified lake environment. The deposition is characterized by complex upward-deepening small-scale (decimeters to meters) depositional units and large-scale (10's metres) depositional sequences bounded by sequence boundaries and correlative conformities. Deposition is controlled by variations in runoff and vegetation that influence the inflow of siliciclastics and nutrients, and the distribution of facies associations and formation of organic-rich deposits. Organic richness varies vertically and laterally within the depositional units and the richest deposits form dominantly in the beginning or latter part of the rich units. Stacked depositional units form depositional sequences that have a similar depositional pattern, and can be correlated over the entire basin area. They display significant changes in lake regime and are separated by sequence boundaries, defined by sharp, erosive surfaces at the lake margin overlain by carbonates or deltaic or shoreline sandstones, and in the lake center by the conformable tops of organic-rich oil shale and can be overlain by evaporites (nahcolite or halite). Transgressive surfaces and maximum flooding surfaces further define periods of rising and high lake. Thirteen depositional sequences have been defined across the basin. Correlating them with the published age dates suggests that they represent 400 Ky eccentricity cycles. Depositional units appear to represent 100 Ky eccentricity cycles.

Depositional sequences characterize the distribution of facies associations and organic material in the lake. During low lake, organic lean deposits formed in the deeper part of the basin that thin out towards the basin margins. The rising lake is marked by thick siliciclastic and carbonate deposits along the basin margin, and rich oil shale deposits deposited in the deeper part of the basin. During the following high lake, littoral to sublittoral deposits formed along the basin margins, and in the deeper profundal lake environment, rich oil shale deposits were deposited. Marginal lake deposits formed during the rising and high lake are not rich, but they are equivalent to the organic-rich deposits formed at the same time in the deeper part of the basin.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013