Lacustrine carbonates of the Eocene Green River Formation crop out on thewesternmargin of the Piceance Basin and the easternmargin of the Uinta Basin, in western Colorado. This area allows tracing of vertical and horizontal facies variation over hundreds of meters. Limestone beds consist of littoral to sublittoral lithofacies: bioclastic and oolitic grainstones, oolitic wackestone, intraclastic rudstone, stromatolites, and thrombolites. Facies form upward-deepening cy- cles that start with sharp-based grainstones and packstones followed by stromatolites or thrombolites and capped by fine-grained stro- matolites and/or oil shale deposits. The vertical succession of carbonate deposits correlates with evo- lutionary lake stages. The succession startswith grainstone deposits rich in ostracods and gastropods that correspond to an initial freshwater lake. Thrombolites capped by laminated stromatolites or coarse-agglutinated stromatolites correlate with a higher-salinity transitional lake.Deepening-upward cycles, as much as 5m(16 ft) thick, of thrombolites, agglutinated stromatolites, and fine-grained stromatolites occur in the highly fluctuating lake. The upper section is dominated by laminated stromatolites that correspond to a rising lake. Stable isotope d18Oand d13C values covary and range from−8‰ to +0.8‰and −3‰to +5‰, respectively. The d18Ovalues indicate carbonate-precipitating water evolved from fresh to saline and be- came less saline in the upper Green River. Negative excursions of d13C values correspond to lake level rises, and positive excursions of d13C values occur during lake level falls. Syndepositional to burial diagenesis modified carbonate po- rosity. Early dissolution is followed by burial compaction and frac- turing. Compaction and late calcite cements occluded primary and secondary porosity.
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