Preservation of Primary Lake Signatures in Carbonates of the Eocene Green River Wilkins Peak-Laney Member Transitional Zone
John Murphy and Tim K. Lowenstein
Important changes in carbonate mineralogy, texture, and stable isotope composition occur at the transition from the Wilkins Peak Member (WPM) to the Laney Member (LM) in the Eocene Green River Formation, Wyoming, which reflect evolution of inflow waters, lake waters, and paleoenvironments. Pristine, unaltered laminae of primary aragonite and calcite at the base of the LM were identified by powder x-ray diffraction, transmitted light microscopy, and scanning electron microscopy. Criteria for identifying primary lacustrine aragonite include its purity, preservation of prismatic needle-like crystals 5-10?m in length, micro-lamination defined by crystal size variation, and poor cementation. ?18O values from aragonite laminae are low, suggesting they formed from (1) lake waters sourced by high altitude foreland rivers (e.g. Carroll et al., 2008) and/or (2) the lakes contained fresh water that underwent little evaporative concentration. Primary precipitated calcite also forms laminae that are monominerallic, poorly cemented, with no diagenetic overprints. Calcite crystals are well developed blocky polyhedra, ~10?m in size. ?18O values of primary calcite are higher than the primary aragonite, suggesting changes in lake water temperature, salinity, source, or evaporation. Primary aragonite and calcite in the Laney Member may have precipitated during 'whiting' events, analogous to modern day Pyramid Lake, Nevada, for aragonite (Galat and Jacobsen, 1985) and Lake Zurich, Switzerland, for calcite (Kelts and Hsu, 1978). In contrast to the calcite and aragonite of the basal LM, the top of the underlying WPM is predominantly dolomite with lesser calcite and evaporites, deposited during underfilled, evaporative lake conditions. Pure dolomite laminae contain a range of crystal sizes (5-50?m) and textures, indicating multiple generations of diagenetic crystallization. Laminae of calcite comprise interlocking mosaics with cement overgrowths of crystals ranging between 20 to 70?m. These laminae also contain dolomite crystals up to 60?m in size. Carbonate ?18O values from the upper WPM are variable, perhaps from diagenetic overprinting. The results from this study show that understanding the lacustrine versus diagenetic origin of Green River carbonate minerals is important before using them for paleoclimate interpretations.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013