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The Parachute Creek Member of the Green River Formation in the southern Piceance Creek basin, Colorado, and the eastern Uinta basin, Utah, displays four major lacustrine depositional facies: sandstone (bottom), stromatolite, marlstone, and oil shale (top). In a vertical sequence these facies record the transgressive history of Lake Uinta during its most expansive stage. The sandstone facies and the stromatolite facies represent deposition in a marginal-lacustrine environment, while the marlstone facies and oil-shale facies were deposited in an open-lacustrine setting. Sulfur-isotope values (δ34S) were determined for iron-sulfide minerals (pyrite, marcasite, and pyrrhotite) from all facies except the marlstone facies. Values were also determined for pyrite from the Uinta Formation in the eastern Piceance Creek basin.

The sulfide minerals demonstrate a total range of about 72 permil and show strong enrichment in 34S when compared with sulfides from marine rocks. Maximum 34S enrichment was found in sulfides from oil shale and marlstone beds of the oil-shale facies, averaging about 35 permil (CD). Sulfides from the stromatolite facies and the sandstone facies show less enrichment in 34S, and average about 17 permil. The Parachute Creek Member shows progressive and uniform upward enrichment in 34S which culminated during deposition of the rich oil shale beds of the Mahogany interval of the oil-shale facies.

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