Petrography and phase equilibria involving the minerals trona (Na 2CO3•NaHCO3•2H2O), nahcolite (NaHCO3), and shortite (Na2CO3•2CaCO 3) 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 Bridger Basin (BB), 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 BB 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 early Eocene atmosphere. New experiments (temperature vs. pCO2) in the NaHCO3 -Na2CO3-CO2-H 2O system show that nahcolite forms at a minimal pCO 2 concentration of ~ 680 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 early Eocene atmosphere at ~ 680 ppm.
Shortite formed diagenetically during burial in the BB 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 Na 2CO3 via the reaction: Na2CO3•CaCO3 •2H2O(pirssonite) + CaCO3(calcite) = Na2CO3•2CaCO3(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 BB 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.
|Advisor:||Lowenstein, Tim K.|
|Commitee:||Carroll, Alan R., Demicco, Robert V., Jenkins, David M., Pietras, Jeffrey T.|
|School:||State University of New York at Binghamton|
|Department:||Geological Sciences and Environmental Studies|
|School Location:||United States -- New York|
|Source:||DAI-B 75/11(E), Dissertation Abstracts International|
|Subjects:||Sedimentary Geology, Paleoclimate Science, Geochemistry|
|Keywords:||Evaporites, Green river formation, Nahcolite, Shortite, Sodium carbonates, Trona|
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