An examination of the hypersaline phases of Eocene Lake Uinta, upper Green River Formation, Uinta Basin, Utah

Abstract

The early evolution of ancient Lake Uinta has been the focus of significant study due to the enormous hydrocarbon reserves in the Uinta Basin’s lower to middle Green River Formation. In contrast, the upper Green River Formation, which includes strata recording the lake’s highest level (Mahogany zone), as well as three previously poorly delineated hypersaline phases, is less understood but still important for developing a complete lacustrine system evolutionary model. Detailed descriptions and mineralogy from several cores, as well as examination of geophysical logs from hundreds of oil and gas wells, were used to help delineate these three hypersaline lake phases and better define the events related to the infilling of Lake Uinta. Lake Uinta’s first hypersaline phase, recorded in the Uinta Basin, occurred synchronously with the upper R-6 and Mahogany zone deposition. Evaporite minerals, mostly nahcolite nodules and small shortite crystals, were deposited in the basin’s paleo-depocenter in central Uintah County. The second hypersaline phase is represented by a nearly basin-wide small-evaporite-crystal facies (both nahcolite and shortite), as well as a large-evaporite-nodule facies (nahcolite), also centered on the basin’s eastern paleo-depocenter in central Uintah County. Near the end of the second hypersaline phase, sediments originating from the southeast and north began to infill the lake, pushing the paleo-depocenter to the west. The third hypersaline phase is represented by a thick sequence of lacustrine sediments with disseminated evaporite minerals (nahcolite, shortite, and other more exotic sodium evaporite minerals) and bedded salts (halite and trona) centered in north-central Duchesne County.