STRATIGRAPHY OF BASIN-SCALE LAKE EXPANSION AND CONTRACTION: A REGIONAL STUDY OF THE LOWER GREEN RIVER FORMATION, UINTA BASIN, UTAH

The Eocene Green River Formation, located in intermontane basins of Utah, Wyoming, and Colorado, is known for its paleoclimate records and economic resources. In the Uinta Basin (Utah), the lower Green River Formation contains an excellent sedimentological record of early Lake Uinta development with deposits from the first widespread lake expansion (the informal Uteland Butte member) and a subsequent basin-scale lake contraction (the informal Castle Peak member). This basin-wide study of the Uteland Butte and Castle Peak members incorporates six outcrop localities, eight cores, and well logs to assess spatial and stratigraphic depositional variation, and considers potential tectonic and climatic depositional controls. Twenty-four lithofacies are identified via measured sections, core descriptions, petrographic analyses, and a large suite of inorganic and organic geochemical data. The ~60-m-thick lacustrine Uteland Butte member is characterized by alternating argillaceous- and carbonate-dominated lithofacies that can be laterally traced across the entire basin, except in locations with direct fluvial input. The lateral continuity of individual lithofacies and the widespread presence of mollusks suggest a relatively shallow lake setting with limited structural variation. The clay-carbonate cycles are interpreted to reflect climatic control on deposition by which more humid periods led to higher siliciclastic input into the basin from updip fluvial sources, and drier periods muted fluvial input which fostered carbonate accumulation. In contrast, the overlying 40–100-m-thick Castle Peak member shows extensive lateral and vertical heterogeneity. The basin margin is characterized by fluvial and terrestrial deposits, whereas the depocenter contains lacustrine limestone and mudstone lithofacies interbedded with isolated distal deltaic sandstones. Overturned beds with soft-sediment deformation (slump deposits) and localized changes in siliciclastic sediment character (provenance) suggest a tectonic influence during Castle Peak time, which may have muted some climate signals in the sedimentary record. The two distinct basin settings, presented in the broader context of early Eocene climate, offer broad insights into climatic and tectonic controls on lacustrine stratigraphy.