Paleoenvironmental Reconstructions in an Area of Rapid Facies Change, Parachute Creek Member of Green River Formation (Eocene), Uinta Basin, Utah

Abstract

Raven Ridge in the northeastern Uinta Basin exposes nearshore deposits of Eocene Lake Uinta. From the northern end of the ridge to the south, units of the Green River Formation change from fluvial to “deep” water lacustrine. The facies changes are well developed in the Parachute Creek Member and are the basis for detailed reconstructions of the depositional history of Lake Uinta. However, the rapid facies changes also make correlation and the establishment of a stratigraphic framework difficult. Correlations between various facies in the Parachute Creek are made possible by the occurrence of sedimentary cycles. These cycles, each consisting of a lower regressive clastic unit and an upper transgressive carbonate unit (with the exception of fluvial facies that have cut-and-fill cycles), record changes in the level of Lake Uinta. Although individual lithologies are restricted laterally, the events that the lithofacies represent can be recognized and correlated from facies to facies. The resulting correlations are precise enough to match individual beds. Analysis of the lithofacies results in the identification of subenvironments, including fluvial channel, floodplain, delta, lagoon, shoal, beach, shoreface, nearshore, and offshore. Lacustrine and fluvial rocks are differentiated largely on the basis of sandstone petrography and fossils. Sedimentary structures and facies relations are used to define the detailed subenvironments.