Sedimentology and sulfur isotope geochemistry of Green River Formation (Eocene), Uinta Easin, Utah Piceance Creek Basin, Colorado

The Green River (Eocene), Uinta (Eocene), and Wasatch (Paleocene-Eocene) Formations in the Uinta and Piceance Creek Basins represent sedimentary rocks deposited in fluvial and lacustrine environments associated in and around ancient Lake Uinta, Study of the sedimentologic characteristics of 271 polished slabs of oil shale and other fine-grained rocks demonstrates that eleven descriptive classes of primary stratification and six classes of secondary structures are common, Primary and secondary structures vary with lith- ology and stratigraphic position, A correlation exists between the stratification characteristics of oil shale and its organic content. Pyrite, marcasite, and pyrrhotite are the most common sulfide minerals, Pyrrhotite is present only in oil shale. Marcasite was the most common sulfide mineral deposited with oil shale, but has generally recrystallized to pyrite. Sulfur isotopic ratios of the sulfides show a range of delta34S values from -12,6 to 66.6 permil, a total spread of 79.2 permil. Sulfides in fluvial and nearshore lacustrine rocks have wide delta34S distributions and mean values near 15 34 permil, Sulfides in oil shale have narrow delta34S distributions and average about 35 permil. No correlation was found between the delta34S values of the sulfides and the sulfide mineralogy. A poor correlation was found between the delta34S values of sulfides and the mineralogy of the host rocks. A good correlation was found between the delta34S of sulfides and the organic content of the host oil shale. Values of with lateral position. The variation is most pronounced in sandstone and poorly stratified marlstone, and is least pronounced in oil shale. Sulfides show delta34S cation intervals, and correlate with variations in lithology suggesting environmental control. Stratigraphic variation of delta34S through the Parachute Creek Member of the Green River Formation shows that sulfides are most enriched in 34S in the Mahogany zone. Sulfides in fluvial rocks of the Wasatch Formation, the Douglas Creek Member of the Green River Formation, and the Uinta Formation in the Piceance Creek Basin are the most depleted in 34S. Lacustrine rocks in the Green River Formation, especially in the Parachute Creek Member, have the most 34S-enriched sulfides. A depositional model for Lake Uinta is generated to explain the delt34S variations from one environment to another (mudflat to offshore lacustrine) by the activity of sulfate- reducing bacteria existing in areas with steady and unsteady sulfate supplies.