Evaporites of the Green River Formation, Bridger and Piceance Creek Basins: Deposition, Diagenesis, Paleobrine Chemistry, and Eocene Atmospheric CO2

Abstract

Sedimentary structures and petrographic textures of evaporites and associated sediments of the Green River Formation, together with evaporite phase equilibria, provide information on depositional and diagenetic conditions. Volumetrically important trona (NaHCO3 ∙ Na2CO3 ∙ 2H2O) and shortite (Na2CO3 ∙ 2CaCO3) occur in the Wilkins Peak Member, Bridger Basin, WY, whereas nahcolite (NaHCO3) occurs in the saline facies of the time equivalent Parachute Creek Member, Piceance Creek Basin, CO. The saline facies of the Parachute Creek Member was deposited in a relatively deep perennial hypersaline lake. In contrast, deposition of the Wilkins Peak Member, Bridger Basin, occurred in shallower perennial saline lakes that periodically desiccated. Trona from the Bridger Basin and nahcolite from the Piceance Creek Basin are stratigraphically associated with oil shale, suggesting evaporite deposition in perennial, density stratified saline lakes. Primary textures in bedded trona and nahcolite indicate that they formed at the air-water interface as microcrystalline cumulate needles. Halite formed concurrently with trona and nahcolite as cumulate layers, and as basin floor crusts. Shortite formed diagenetically during burial of the Wilkins Peak Member in the Bridger Basin as displacive crystals, pseudomorphous replacements of precursor Na-Ca-carbonate minerals, and fracture filling cements. Precipitation of trona and nahcolite necessitates that the inflow waters that fed the Green River lakes contained total carbonate (HCO3 − + CO3 2−) greater than Ca2+ + Mg2+. During evapoconcentration, lake waters evolved into Na+-CO3 2−-HCO3 −-Cl− brines. The formation of trona versus nahcolite can be explained by variations in brine pH, Na+ concentration, temperature, or pCO2. Nahcolite is the stable mineral at elevated atmospheric pCO2 (>1,125 ppm); trona is also stable at high pCO2, but at higher temperatures. Nahcolite formed under elevated atmospheric pCO2 during the Early Eocene Climatic Optimum.