Paleoenvironmental reconstructions along the southern margin of Eocene Lake Uinta during a period of climatic variability

The Middle to Upper Green River Formation, Uinta Basin, Utah

Abstract

The stratigraphy preserved in the lacustrine Eocene Green River Formation suggests a variety of depositional environments that were subject to pulses of sediment and lake level fluctuations in the Uinta Basin. This paper defines 13 facies within six facies associations observed in outcrop and core from the middle to upper Green River Formation in Gate Canyon, south-central Uinta Basin, and Willow Creek/Indian Canyon, western  Uinta Basin, along the margin of the ancient lake system. The facies associations are defined by dominant lithology, relative siliciclastic sediment supply, relative slope, and lake zonation, and encompass a suite of facies revealing distinct environments across a relatively small region (~35 km apart). In the lower interval, a shift  from a high energy, steeper ramp setting in the east, to a lower energy, shallower ramp setting to the west suggests a transition off the main depositional axis. Correlatable siliciclastic and carbonate dominated packages  reflect the impact of early Eocene hyperthermal events that followed the Paleocene-Eocene Thermal Maxi- mum, and the diverse facies present at the two study areas suggest variable responses to these climatic events.  In the overlying stratigraphy, the facies are consistent in both regions and consist of organic rich and poor carbonate mudstones and siltstones. The interval thickens slightly from east to west, and the overlying Horse  Bench Sandstone transitions from a wave-dominated shoreface sequence to a river-dominated deltaic sequence, indicating an overall shift in the lake depocenter towards the west. This study offers a unique glimpse  at the paleoenvironmental changes along the margin of a continental interior lake system during a time of climatic variability. It also highlights the need to use multiple depositional models to accurately characterize marginal deposits in different regions of large internally-drained lake systems which, unlike typical marine depositional models, are more sensitive to basin geometry, size, and sediment fairways.