EOCENE GREEN RIVER EVAPORITES: DEPOSITIONAL ENVIRONMENT AND PALEOCLIMATE

One puzzling feature of the Eocene Green River evaporites is that nahcolite (NaHCO₃) is the important Na-carbonate in the Piceance Creek basin (PCB), Colorado but trona (Na₃CO₃)(HCO₃)•2(H₂O)) dominates deposits in the Green River basin (GRB), Wyoming. Petrographic and mineralogical (X-Ray Powder Diffraction) studies suggest that basin center evaporites from the PCB formed in a perennial saline lake whereas time-equivalent deposits in the GRB may have lost primary evaporite textures during diagenesis. Evaporites from the PCB contain large, clear halite crystals which display vertical, widening upward growth textures, and linear bands of negative cubic fluid inclusions, all similar to bottom-growth halite found in modern saline lakes. PCB evaporites also contain laminated nahcolite and dolomite-rich mud layers with randomly distributed halite cubes and rafts. Well-sorted layers of cumulate halite crystals display drape textures above clear bottom-growth halite. The chemical mud and cumulate halite layers, and the complete lack of dissolution features resemble textures observed in density-stratified perennial saline lake deposits, such as the Dead Sea. Single-phase fluid inclusions in PCB halite suggest no significant heating has occurred. A single core sample from the GRB in Wyoming contains abundant randomly distributed aggregates of bladed trona and large clear poikilotopic halite crystals with linear arrays of cube-shaped fluid inclusions. The homogeneity of the trona and halite along with the lack of detrital textures and primary crystal growth fabrics suggests a diagenetic origin for the trona and halite in the GRB. Differences in texture between the GRB trona and the PCB nahcolite may thus be explained by original precipitate mineralogy in the PCB versus a diagenetic origin of the GRB trona. Recent studies have suggested that the nahcolite and trona deposits from the Eocene can be used to estimate paleoatmospheric CO₂ levels (Lowenstein and Demicco, 2006). If the nahcolite from the PCB represents the original precipitate, then estimates of the CO₂ concentrations during the Eocene, a period of global warming, can be made.