2014 Rocky Mountain Section AAPG Annual Meeting

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Hypersaline Facies and the Termination of Eocene Lake Uinta, Upper Green River Formation, Uinta Basin, Utah


The early evolution of ancient Lake Uinta has been the focus of significant study due to the enormous hydrocarbon reserve in the Uinta Basin's lower to middle Green River Formation (GRF). In contrast, the upper GRF, which includes strata of 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. We used detailed descriptions and mineralogy from ten cores transecting the basin were used to help delineate these three hypersaline lake phases and better define the events related to the closing of Lake Uinta. Lake Uinta's first hypersaline phase, recorded in the Uinta Basin, occurred during maximum lake level at Mahogany zone time. Saline minerals, mostly nahcolite nodules and small shortite crystals, were deposited in the lake's paleo-depocenter in central Uintah County, but also in a sub-basin in west-central Duchesne County. The second hypersaline phase is represented by a basin-wide small-saline-crystal facies (both nahcolite and shortite), as well as a large-saline-nodule facies (nahcolite), also centered on the lake's eastern paleo-depocenter. This second phase includes the Birds Nest aquifer and is believed to mark a transition to a more restricted lake basin. Near the end of the second hypersaline phase, sediments originating from the east and north began to infill the lake, pushing the paleo-depocenter to the west. The third hypersaline phase is represented by a thick deposit of lacustrine sediments with disseminated saline minerals (nahcolite and shortite) and bedded salts (halite and trona) centered in north-central Duchesne County.