STROMATOLITE FORMATION DURING PERIODS OF DRAMATIC HYDROLOGIC CHANGE IN THE EOCENE GREEN RIVER FORMATION REVEALED FROM TRENDS IN PETROGRAPHY AND HIGH-RESOLUTION STABLE ISOTOPE MEASUREMENTS

The Eocene Green River Formation of Wyoming, Colorado, and Utah records changes in regional hydrology from tectonic processes (on tectonic timescales) and cyclical changes in climate (on orbital to multi-annual timescales). Stromatolites contain among the finest-resolution records of hydrologic changes in the basin, with geochemical and textural changes preserved in fine layers. Their analysis provides evidence of the response of lacustrine systems in continental North America to the high-temperature, high-CO₂ Eocene environment, with potential implications for the same region in the Anthropocene. Detailed petrographic analyses and high-resolution microdilling (mm-scale resolution) were followed by stable isotope, major ion, and trace element analyses on an organic-rich stromatolite from the Lower LaClede Bed, Laney Member, Green River Formation, part of the Greater Green River Basin of Wyoming. The major ion and isotope concentrations in the stromatolites suggest multiple dynamic episodes of lake volume increase and decrease (fill and evaporation) in a closed basin setting, culminating in spillover to the adjacent basin as stromatolite growth ceased. The significant shifts in hydrology implied by high-resolution geochemistry are marked by several changes in microfacies—changes in the bundling and thickness of stromatolite laminae are evident at the hand sample scale. This modifies previous interpretations of stromatolites as shoreline facies during simple transgression of the lake. Instead, the transgressive cycle was more complicated and occurred in fits and starts in a hydrologically closed system before becoming overfilled/spillover during stromatolite growth. Given the lower elevation of the stromatolite bed relative to the sill that connected the basin to Lake Uinta to the south during periods of lake spillover, it is apparent that the stromatolites were not exclusively shallow shoreline deposits, but rather deposited in a range of depositional environments from shoreline to deeper lake. Thus, in both ancient and modern settings, lacustrine stromatolites should not be considered simple shoreline facies markers without careful examination, and instead merit further consideration as detailed records of lake depth, volume, and biogeochemical change.