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.
to evaluate the oil-shale, nahcolite (NaHCo 3), dawsonite (Na~Al (Co 3)o.2Al (OH),>, and major, minor, and trace element concentrations of the northeastern part of the Piceance Creek Basin (figs. 1 and 2). Nahcolite and dawsonite may possibly be important byproducts derived from a large scale oil-shale industry. The Laramie Energy Technical Center, U.S. Department of Energy determined the shale-oil content of the cores, utilizing the modified Fischer assay method. The results of these assays have been published previously (Donnell and Pitman, 1975). The top and base of rich and lean oil-shale zones delineated on the shale-oil histograms in Donnell and Pitman (1975) are also depicted on figure 3 in this report. A representative cut of each sample used for shale-oil assay, in the parts of the core believed to contain nahcolite and dawsonite from the two coreholes, were analyzed for water-soluble sodium and acid-extractable alumina. The interval sampled for water-souble sodim was determined by visual inspection of the core. The interval sampled for acid-extractable alumina was comparable to the intervals known to have significant concentrations of acid or caustic extractable alumina in other cores. About 900 feet of core from corehole CR-1 and approximately 1,000 feet of core from corehole CR-2 were analyzed for acid-extractable alumina (fig. 3). About 300 feet in corehole CR-1 and about 60 feet in corehole CR-2 were analyzed for water-soluble sodium. All the samples were analyzed by Wayne Mountjoy, Harriet Neiman, and Carol Gent of the U.S. Geological Survey, Branch of Analytical Laboratories in Denver, Colorado. The results are shown in table 1, table 2, and figure 3. The concentration of the mineral nahcolite in the core may be determined by multiplying the percent of Na 2 0 reported in table 1 and table 2 by 2.71. Most of the acid-extractable alumina is derived from dawsonite and an unidentified amorphous sodium aluminum oxide mineral; however, the alumina in some zones such as the interval Table 1.-Acid-extractable alumina and water-soluble sodium analyses for U.S. Geological Survey corehole CR-1 [Leaders, no analysis] Acid-extractable alumina Water-soluble sodium Sample interval A1 2 03% Na 2 0%
sonseaig sft al meipoge gfena-irod sit eo | abtes sgnlu sdf bas Obs 70[SD. ,nreed seat bf un-2-4 ie slaeT -vijos suoizey to einumdeiiqnesos ovissdrinae®
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.