Lithofacies, stable isotopic composition, and stratigraphic evolution of microbial and associated carbonates, Green River Formation (Eocene), Piceance Basin, Colorado

Lacustrine carbonates of the Eocene Green River Formation crop out on the western margin of the Piceance Basin and the eastern margin 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 cycles that start with sharp-based grainstones and packstones followed by stromatolites or thrombolites and capped by fine-grained stromatolites and/or oil shale deposits. The vertical succession of carbonate deposits correlates with evolutionary lake stages. The succession starts with 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 5 m (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 δ¹⁸O and δ¹³C values covary and range from -8%o to +0.8%o and -3%o to +5%o, respectively. The δ¹⁸O values indicatecarbonate- precipitating water evolved from fresh to saline and became less saline in the upper Green River. Negative excursions of δ¹³C values correspond to lake level rises, and positive excursions of δ¹³C values occur during lake level falls. Syndepositional to burial diagenesis modified carbonate porosity. Early dissolution is followed by burial compaction and fracturing. Compaction and late calcite cements occluded primary and secondary porosity.