GEOLOGY OF THE NAHCOLITE DEPOSITS AND ASSOCIATED OIL SHALES OF THE GREEN RIVER FORMATION IN THE PICEANCE CREEK BASIN, COLORADO

At the depocenter of Eocene Lake Uinta in the northern part of the Piceance Creek Basin, Rio Blanco County, Colorado, the lacustrine Green River Formation contains a saline facies composed of nahcolite and halite commingled with kerogenous marlstones (oil shales). Stratigraphic and lithologic studies of drill cores from 10 exploratory holes reveal that five-eighths of the nahcolite resource, estimated at 29 x 10('9) metric tons, occurs as crystalline aggregates in marlstone. The remainder of the resource consists of laterally continuous zones of disseminated nahcolite in marlstone and beds of mixed nahcolite and halite.; Sedimentologic data indicate that the marlstones and associated sodium minerals were deposited by pelagic, turbiditic, and evaporitic processes in a permanent alkaline lake. About half of the marlstones of the saline facies are laminated and accumulated slowly by pelagic sedimentation of seasonally precipitated carbonate, organic detritus, and fine-grained siliciclastic sediment supplied by surface currents. Organic-rich marly sediments were swept out into the deeper parts of the lake by episodic density currents. These sediments are represented by blebby and streaked kerogenous marlstones that comprise nearly one-half of the marlstones in the saline facies. Bedded nahcolite and halite precipitated during maximum evaporative stages of the lake.; Lower lake waters and sediments were anoxic and favored high rates of bacterial reduction of sulfate and hydrolysis of fine-grained detrital silicate minerals. These processes resulted in production of bicarbonate and the formation of an authigenic suite of carbonate and silicate minerals devoid of clay and sulfate minerals.; Cyclic, probably seasonal, stratification is recorded by the laminated marlstones and in some units of disseminated and bedded nahcolite and halite. The vertical distribution of total sulfur in the marlstones is also cyclic and may be related to evaporative phases of the lake. Bromine values that increase generally upward through the halitic rocks of the saline facies indicate the existence of residual lake brines and suggest that the lake never evaporated to dryness during Parachute Creek time.