Depositional environments of oil shale in the Green River Formation, Douglas Creek Arch, Colorado and Utah

Douglas Creek arch in western Colorado and eastern Utah separates the Uinta and Piceance Creek basins. During deposition of the lower part of the Eocene Green River Formation, the arch provided a physical barrier dividing ancient Lake Uinta into two segments. As the lake expanded, the arch became a shoal area where marginal-lacustrine and mud flat sediment was deposited. Near the end of Green River deposition, a major lacustrine transgression submerged the arch and deep-water open-lacustrine deposition prevailed. As a whole, rocks of the Green River Formation on Douglas Creek arch provide a comprehensive view of the initiation, evolution, and expansion of Lake Uinta in the Uinta and Piceance Creek basins. Oil shale on Douglas Creek arch is present in two stratigraphic intervals separated by approximately 250 m of sandstone, siltstone, marlstone and lacustrine limestone (micrite, boundstone, grainstone). The lower sequence, in the Garden Gulch Member, represents an early lacustrine phase of Lake Uinta and consists of approximately 29 m of interbedded clay-rich oil shale, gastropod grainstone and packstone, ostracode grainstone, algal boundstone and calcareous silty claystone. Fish fossils, plant debris, and pelecypods are locally present. Detrital clay minerals (smectite, chlorite and illite) are common and calcite abundance is greater than dolomite. The second oil-shale interval is located in the upper 10 m of the Parachute Creek Member. The Mahogany ledge crops out in the lower 15 m of this sequence. Oil shale in the Parachute Creek Member is a kerogenous dolomicrite (dolomite greater than calcite). Analcime, quartz, K-feldspar, and albite are also common. Illite is the dominant clay mineral. Fossils include insects, insect larvae, and plant debris. Marlstone and volcanic tuff are interstratified with oil shale. The two oil-shale types have different mineral contents, fauna and flora, stratification sequences, and sedimentary structures. Clay-rich oil shale in the Garden Gulch Member was deposited in a shallow, open-lacustrine environment characterized by fresh to slightly saline water. Oil shale in the Parachute Creek Member, in contrast, is the product of deposition in a deep, chemically-stratified, saline, alkaline, open-lacustrine environment.