Preservation of primary lake signatures in alkaline earth carbonates of the Eocene Green River Wilkins Peak-Laney Member transition zone

Abstract

Significant changes in carbonate mineralogy, texture, and stable isotope composition occur at the transition from the Wilkins Peak Member to the Laney Member in the Eocene Green River Formation, Bridger Basin, Wyoming, which reflect evolution of inflow waters, lake waters, and paleoenvironments.The top of the Wilkins Peak Member contains heterogeneous laminae of calcite and dolomite. Evaporites associated with these layers suggest deposition in hypersaline lakes. Diagenetic carbonate mineral textures include euhedral cement overgrowths and interlocking mosaics of calcite and dolomite crystals, 20–70μm in size. Electron microprobe analyses indicate diagenetic overgrowth of Fe-rich dolomite on cloudy Fe-poor cores. δ18O values of carbonate laminae in the upper Wilkins Peak Member vary by ~6‰ with no depth dependent or mineralogic trends, which also suggests diagenetic overprinting.Alternating organic-rich and primary aragonite, calcite, and dolomite laminae were identified from the lower Laney Member. Primary lacustrine aragonite consists of well sorted, prismatic crystals 5–10μm in length, with micro-lamination defined by crystal size variation. Primary precipitated calcite and dolomite laminae are monominerallic, with well sorted polyhedral crystals, ~10μm in size. Primary mineralogy of the lower Laney Member changes from calcite to aragonite and dolomite stratigraphically upward. Along the same 15m thick stratigraphic interval, δ18O values decrease upward by ~3‰, all of which suggests (1) lake waters underwent evaporative concentration, which together with calcite precipitation increased the lake water Mg/Ca ratios and led to formation of aragonite and dolomite, (2) source waters became lower in δ18O, possibly as inflow changed to higher altitude foreland rivers.The results from this study show that understanding the primary lacustrine versus diagenetic origin of Green River carbonate minerals is essential for paleoenvironmental and paleoclimate interpretations.