Paleoenvironments of the Eocene Green River Formation (Laney Member) in the Anvil Wash area, Southwestern Wyoming

The study of abundant conchostracans in the Laney Member of the Green River Formation, Wyoming, has contributed to the resolution of the controversy over the depositional environment of finely laminated sediments. Conchostracans that typically live in ephemeral pools, pulmonate gastropods that require oxygenated water and freshwater ostracods indicate deposition of finely laminated sediments in a well oxygenated, shallow, freshwater lake. Mineralogical evidence indicating that Lake Gosiute was primarily fresh during deposition of the upper portion of the Laney Member includes predominance of calcite over dolomite and the clay composition of diagenetically altered tuffs. The minor amount of dolomite is detrital and was transported into the lake by sheet flooding on fringing mudflats. Evidences for rapid sedimentation in an oxygenated lake include excellent preservation of fish, coprolites, conchostracans and undisturbed, laminated sediments. Rapid sedimentation of laminae would be expected to occur during sheet floods associated with storms and seasonal precipitation within the hydrographic basin. Vertical fossil variations occur in repetitive calcareous/ siliceous rock sequences that are interpreted as having been deposited in response to climatic fluctuations. Conchostracans and gastropods occur in calcareous rocks deposited during more arid periods; whereas, ostracods occur in siliceous rocks deposited during periods of increased rainfall. Evidence presented by this research supports the concept of a shallow, freshwater, well-oxygenated, perennial lake with a fringing mudflat during deposition of the upper portion of the Laney Member