PRESERVATION OF PRIMARY LAKE SIGNATURES IN CARBONATES OF THE EOCENE GREEN RIVER WILKINS PEAK-LANEY MEMBER TRANSITIONAL ZONE

Important 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, Wyoming, which reflect evolution of inflow waters, lake waters, and paleoenvironments.

Pristine, unaltered laminae of primary aragonite and calcite at the base of the Laney Member were identified by powder x-ray diffraction, transmitted light and scanning electron microscopy. Criteria for identifying primary lacustrine aragonite include its purity, preservation of prismatic needle-like crystals 5-10μm in length, micro-lamination defined by crystal size variation, and poor cementation. Primary precipitated calcite also forms laminae that are monominerallic, unconsolidated, and lack diagenetic overprints. Calcite crystals are equant blocky polyhedra, ~10μm in size. Primary calcite and aragonite in the lower Laney Member have δ¹⁸O values that decrease upward by ~3‰ over 15 meters of stratigraphic section which suggests (1) source waters changed to high altitude foreland rivers or (2) lake waters underwent less evaporative concentration.

The top of the underlying Wilkins Peak Member contains laminae of pure dolomite and mixed calcite/dolomite. Evaporites associated with these layers suggest deposition in underfilled, evaporative lakes. Scanning electron microscope images of pure dolomite laminae show heterogeneous crystal sizes (5-40μm); electron microprobe analyses indicate diagenetic overgrowths of Fe-rich dolomite on cloudy Fe-poor cores. Laminae of mixed calcite/dolomite comprise interlocking mosaics of amorphous calcite and rhombohedral dolomite crystals 20-70μm in size. δ¹⁸O values of carbonate laminae in the upper Wilkins Peak Member vary by ~6‰ with no covariance, suggesting diagenetic overprinting. 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.