Environmental and Microbial Influence on Chemistry and Dolomite Formation in an Ancient Lake, Green River Formation (Eocene), Uinta Basin, Utah
Maxwell Pommer*, J. Frederick Sarg, Forrest McFarlin
Department of Geology and Geological Engineering, Colorado School of Mines
maxwell.pommer@gmail.com
*currently at Premier Corex, University of Colorado Boulder
Abstract:
Integrated stratigraphic, petrographic, and geochemical data allow interpretation of biogeochemical and mineralization processes in paleoenvironmental context of ancient lacustrine environments. These indicate lake chemistry, microbial processes, and organic matter (OM) strongly influenced dolomite formation in near-surface environments throughout deposition of the Green River Formation (Eocene, Uinta basin, Utah).
The lower Green River Formation consists of interbedded fluvio-deltaic siliciclastics, paleosols, carbonate mud, coated-grain carbonates, mollusc and ostracod limestones, and microbialites all landward of profundal OM-bearing illitic mudrocks. Calcite, dolomite, Fe-dolomite, and authigenic feldspars are common. Carbonate δ18O and δ13C are covariant, and positive excursions of carbonate δ13C (up to 6.9‰VPDB) and organic-matter δ15N (up to 13.9‰V-AIR) occur in profundal OM-bearing mudrocks.
The upper Green River Formation consists mainly of laminated OM-lean and OM-rich dolomitic muds (i.e., “oil-shales”). Zoned dolomite crystals with Mg-calcite centers and Fe-dolomite rims are widespread in addition to authigenic feldspars, and Na-carbonates. Carbonate δ13C-enrichment (up to 15.8‰VPDB), and organic-matter δ15N-enrichment (up to 18.4‰V-AIR) occur in these OM-rich dolomite muds. Organic-matter δ13C is relatively invariable (mean = -29.3‰VPDB) and does not covary with carbonate δ13C.
Trends in mineralogy, organic-matter abundance, and stable isotopes result from changing hydrologic systems, paleoclimate, lake chemistry and microbial processes coincident with the Early Eocene Climate Optimum. The lower Green River Formation paleo-lake was smaller in area and volume, heavily influenced by meteoric fluvial input, variably oxygenated, and ranged from neutral and fresh to alkaline and saline. Especially in littoral environments with abundant microbialites, dolomite formed through recrystallization of precursor carbonate involving both replacement of precursor carbonate and direct precipitation as cements and overgrowths. The upper Green River Formation paleo-lake was more expansive with widespread low-oxygen, nutrient-rich, and alkaline saline environments with increased planktic organic-matter productivity. Microbial decay of organic matter in low-oxygen environments produced alkaline lake waters through methanogenesis, possible denitrification, and microbial sulfate reduction to a limited degree. This favored precipitation of widespread dolomite, as well as Na-carbonates, authigenic feldspars, and analcime from lake water and phreatic pore water. Extracellular polymeric substances (EPS) excreted by microbial communities provided favorable nucleation sites for Mg-carbonate, allowing kinetic barriers of low-temperature dolomite formation to be overcome. Cycling of pH due to turnover of organic matter and associated microbial processes potentially bolstered EPS generation and abiotic environmental conditions favorable to dolomite precipitation. It is likely that metastable precursor carbonate was recrystallized to ordered dolomite, but it is possible that direct precipitation occurred. Fe-dolomite overgrowths precipitated after dolomite where microbial Fe reduction occurred in stagnant, oxygen-depleted, alkaline pore waters.
Pommer, M., Sarg J.F., McFarlin, F., Environmental and Microbial Influence on Lake Chemistry and Dolomite Formation in the Green River Formation (Eocene), Uinta Basin, Utah. Journal of Sedimentary Research, v.93, no.4, 2023, p. 213-242.
Biography: Max is a geologist who evaluates fluid-reservoir systems and Earth’s history with integrated stratigraphic, petrologic, and geochemical methods. His research spans a broad range of topics including pore-system evolution, linking diagenesis and sequence stratigraphy, the origin of dolomite, as well as paleoenvironmental and biogeochemical dynamics.
Currently, he is a senior Geological Advisor and Lead Sedimentologist at Premier Corex, as well as a Faculty Lecturer at the University of Colorado Boulder, where he instructs a course in Petroleum and Transitional Reservoir Geology. He holds a bachelor’s degree from the University of Colorado Boulder, a master’s degree from The University of Texas at Austin, and a doctorate from Colorado School of Mines. Previously he was a Postdoctoral Researcher at Colorado School of Mines and has been a consultant in industry since 2011.
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