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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. Alternating organic-rich laminae and primary aragonite and calcite laminae were identified from the lower Laney Member in the Bridger Basin, Wyoming. Criteria for identifying primary lacustrine aragonite include its purity, preservation of well sorted, prismatic 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 well sorted equant blocky polyhedra, ∼10μm in size. Primary calcite and aragonite in the lower Laney Member have d18O 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 than in the underlying Wilkins Member. The top of the Wilkins Peak Member contains heterogeneous laminae of calcite and dolomite. Evaporites associated with these layers suggest deposition in underfilled, evaporative lakes. Carbonate mineral textures include well-sorted euhedral primary-precipitated dolomite crystals >15μm and interlocking diagenetic mosaics of calcite and dolomite 20-70μm in size. Electron microprobe analyses indicate diagenetic overgrowths of Fe-rich dolomite on cloudy Fe-poor cores. d18O 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.