Intertonguing of the lower part of the Uinta Formation with the upper part of the Green River Formation in the Piceance Creek Basin during the late stages of Lake Uinta

Abstract

During most of middle Eocene time, a 1,500-mi 2 area between the Colorado and White Rivers in northwestern Colorado was occupied by the Piceance lobe of Lake Uinta. This initially freshwater lake became increasingly saline throughout its history. Sediments accumulating in the lake produced mostly clay shale, limestone, and dolomite containing varying concentrations of organic matter. At the time of the maximum extent of the lake, the organic-rich Mahogany bed of the Green River Formation was deposited throughout the area. Shortly after its deposition, stream deposits began infilling the lake from the north through a series of contractions interspersed with minor expansions. This fluctuation of the shoreline resulted in the intertonguing of the stream sediments of the lower part of the overlying Uinta Formation with the lacustrine sediments of the upper part of the Green River over a distance of about 40 mi; construction of regional stratigraphic cross sections show the pattern of intertonguing in considerable detail. The data utilized in this study, which covered parts of Rio Blanco, Garfield, and Mesa counties, was derived from (1) geologic mapping of thirty-four 7½-minute quadrangles and stratigraphic studies by geologists of the U.S. Geological Survey, and (2) shale-oil assay information from numerous cores. As a result of this previous work and the additional effort involved in the compilation here presented, more than a dozen Green River Formation tongues have been named, some formally, others informally. Middle Eocene strata above the Mahogany bed in the northern part of the study area are dominantly coarse clastics of the Uinta Formation. The sedimentary sequence becomes more calcareous and organic-rich to the south where, in a 400-mi 2 area, a 250 ft-thick sequence of oil shale above the Mahogany bed contains an average of 16 gallons of oil per ton of shale and is estimated to contain 73 billion barrels of oil.

Chemistry and age of groundwater in bedrock aquifers of the Piceance and Yellow Creek watersheds, Rio Blanco County, Colorado, 2010-12

“…Detailed descriptions of the geology of the Green River and Uinta Formations can be found in Cashion and Donnell (1974), Hail (1990), Donnell (2009), Brownfield and others (2010), and Johnson and others (2010). In general, sediments of the Parachute Creek Member were deposited in Eocene Lake Uinta and consist of calcareous and dolomitic marlstone, limestone, calcareous sandstone, siltstone, and mudstone, with alternating lean and rich oil shale zones ( fig.…”

The systematic geologic mapping program and a quadrangle-by-quadrangle analysis of time-stratigraphic relations within oil shale-bearing rocks of the Piceance Basin, western Colorado

“…Additionally, the lowest volcaniclastic unit of the Uinta Formation, which is present near the mouth of Yellow Creek ( fig. 2), has been placed at two different stratigraphic positions by different workers: (1) at the Mahogany marker, a tuffaceous bed within the Mahogany zone just above the Mahogany bed (Johnson, 1981a;Donnell, 2008;Self and others, 2010, their pl. 13) or (2) somewhat higher, just above the top of the Mahogany oil shale zone (Hail, 1973(Hail, , 1988).…”

“…The oil shale interval above A-groove was not named by Cashion and Donnell (1972), but they traced several oil shale marker beds through that interval. Donnell (2008) correlated 44 individual oil shale beds in the interval above A-groove across much of the Piceance Basin and the eastern part of the Uinta Basin ( fig. 7).…”

“…5 and 11). Donnell (2008) determined approximately where most major Green River and Uinta tongues fall stratigraphically in the continuous oil shale section preserved in the southern part of the basin (figs. 10 and 11).…”

“…The B-marker through I-marker beds are from Chancellor and others (1974). Stratigraphic nomenclature for oil shale zones from Donnell and Blair (1970), Cashion and Donnell (1972), Donnell (2008); stages from Johnson (1985a) Garden Gulch …”