ENVIRONMENTAL AND MICROBIAL CONTROLS ON LAKE CHEMISTRY AND DOLOMITIZATION IN THE GREEN RIVER FORMATION (EOCENE), UINTA BASIN, UTAH

Lake chemistry, microbial processes, and organic matter strongly influenced dolomitization in near-surface environments throughout deposition of the Green River Formation (Eocene, Uinta Basin, Utah). Understanding these processes in ancient analogues within a paleoenvironmental and stratigraphic framework helps to understand biogeochemical processes in extreme environments.

The lower Green River (D-Marker and below) is comprised of interbedded fluvio-deltaic siliciclastics, paleosols, carbonate mud, coated-grain carbonates, molluscan and ostracod-bearing limestone, and microbialites landwards of profundal OM-bearing illitic mudrocks. δ¹⁸O and δ¹³C in carbonates are variable and covariant, however, positive excursions of δ¹³C in carbonate and δ¹⁵N in OM occur in profundal OM-bearing mudrocks. Variable amounts of calcite, dolomite, Fe-dolomite, authigenic feldspars, and clay comprise these mudrocks. The lake expanded in the upper Green River (above D-Marker), which is dominated by laminated dolomitic muds with variable organic content (i.e. “oil-shales”). δ¹³C-enrichment in carbonate (up to 15.8‰PDB), and δ¹⁵N-enrichment (up to 18.4‰V-AIR) occurs in OM within these OM-rich dolomite muds.

These trends resulted from changing hydrologic systems, paleoclimate, lake chemistry and microbial processes. The transition from the lower to upper Green River is coincident with the Early Eocene Climate Optimum. The lower Green River lake was smaller, influenced by meteoric fluvial input, and contained variable oxygenation, alkalinity, and salinity. The climate is interpreted to have been warm, arid, and monsoonal. Dolomite replaced precursor carbonate and precipitated directly. The upper Green River lake was more expansive with widespread low-oxygen, nutrient rich, and alkaline saline environments with increased OM productivity. Microbial decay of OM through methanogenesis and denitrification in low-oxygen environments contributed prolific amounts of alkalinity into the system, resulting in widespread dolomite, as well as Na-carbonates, authigenic feldspars, and analcime. OM surfaces provided favorable nucleation sites for Mg-calcite and dolomite precipitation. Fe-dolomite overgrowths precipitated after dolomite in stagnant, oxygen-depleted, alkaline pore waters containing Fe-reducing microbes.