CHANGES IN GEOCHEMICAL CONDITIONS IN EOCENE LAKE UINTA, PICEANCE BASIN, COLORADO, INDICATED BY ANALCIME AND DAWSONITE DISTRIBUTIONS

X-ray diffraction data for seventeen coreholes in the Eocene Green River Formation, Piceance Basin of northwestern Colorado were digitized and analyzed to better define variations in the extent of the saline minerals dawsonite, NaAl(OH)₂CO₃, nahcolite, NaHCO₃, analcime, Na₂(Al₂ Si₄O₁₂)^.2H₂O, and halite, NaCl, through time. Much of this data was originally collected by the former Bureau of Mines. Nahcolite is currently utilized as a source of sodium bicarbonate. Dawsonite is of interest for its potential use as an aluminum ore, and because the breakdown of dawsonite during oil shale retorting yields large amounts of carbon dioxide (CO₂). The distributions of authigenic minerals, such as dawsonite and analcime, are also of interest for the insights they can provide into the geochemical evolution of Eocene Lake Uinta and may be useful for comparing and contrasting with other lacustrine petroleum systems around the world. Alkalinity, salinity, CO₂ concentration, and the aqueous activities of aluminum and silica are all important considerations for understanding changes in the assemblage of minerals present in the Green River Formation during the evolution of Lake Uinta. Two transitions of importance are: (1) a change from the illitic oil shales of the Garden Gulch Member to carbonate-rich, feldspathic oil shales containing saline minerals in the Parachute Creek Member; and (2) a transition within the Parachute Creek Member around R-5 time when massive halite beds were deposited in the basin center. These beds grade marginward into nahcolite. Deposition of these beds is followed by a period (L-5 zone) when albite, NaAlSi₃O₈, is still found, but other sodium-aluminum-bearing authigenic minerals (e.g., dawsonite and analcime) are absent in the basin center and nahcolite deposition was greatly reduced. Analcime reappears in the R-6 and Mahogany zones (R-7), along with minor amounts of nahcolite and rare dawsonite. These transitions mark major changes in the salinity and alkalinity of Lake Uinta, driven in part by climate changes that led to shifts in lake level causing dramatic changes in mineral precipitation. The new mineral distribution maps being produced by the USGS provide more detailed information on these transitions than has been available previously.