EOCENE GREEN RIVER FORMATION EVAPORITE PALEOENVIRONMENTS: CLUES FROM PETROGRAPHY AND PHASE EQUILIBRIA

Petrography and phase equilibria involving the minerals trona (Na₂CO₃•NaHCO₃•2H₂O), nahcolite (NaHCO₃), and shortite (Na₂CO₃•2CaCO₃), from the Eocene Green River Formation provide information on the paleoenvironments that controlled their formation during deposition and diagenesis. Shortite and trona are exclusive to the Wilkins Peak Member (WPM) of the Green River Basin (GRB), WY, whereas nahcolite is the primary Na-carbonate mineral in the contemporaneous Parachute Creek Member of the Piceance Creek Basin (PCB), CO. Trona from the GRB and nahcolite from the PCB are stratigraphically associated with oil shale, suggesting deposition in perennial, density stratified saline lakes. Preserved primary textures of trona and nahcolite show that they formed at the air-water interface as microcrystalline chemical muds, which supports the hypothesis that precipitation occurred in contact with the Eocene atmosphere. New experiments (temperature vs. pCO₂) in the NaHCO₃ -Na₂CO₃-CO₂-H₂O system show that nahcolite forms at a minimal pCO₂ concentration of ~ 780 ppm at 19.5 °C, 1 atm, which is lower than the pCO₂ determined by Eugster (1966) (1330 ppm and 1125 ppm with NaCl added). These new results anchor the minimum pCO₂ of the Eocene atmosphere at ~ 800 ppm.

Shortite formed diagenetically during burial in the GRB as displacive crystals, fracture fills, and pseudomorphous replacements of a precursor Na-Ca-carbonate in carbonate mudstone and oil shale. Experimental results on the thermal stability of shortite in the Na₂CO₃-CaCO₃-H₂O system show that it forms at temperatures > 55 °C, 1 atm, and 1.1m Na₂CO₃ via the reaction: Na₂CO₃•CaCO₃•2H₂O_(pirssonite) + CaCO_3(calcite) = Na₂CO₃•2CaCO_3(shortite) + 2H₂O. The large area over which shortite occurs in the WPM indicates saline pore fluids existed in the buried lacustrine sediments and that, at times, giant Na-CO₃-rich saline alkaline lakes existed in the GRB during WPM time. The thermal stability of shortite, coupled with vitrinite reflectance data and inferred regional geothermal gradients, establish that the WPM was buried to depths of ~ 1,500 m and experienced post WPM erosion of ~ 800 m.