The Eocene Green River Formation in Fossil Basin, Wyoming provides a detailed record of the paleoecology and depositional history of ancient Fossil Lake. Fossil Lake was one of three Eocene lakes that formed an extensive lake system in Wyoming, Utah, and Colorado. It began as a flood-plain lake in the southern part of Fossil Basin and expanded northward as the lake evolved. Fossil Lake went through the major stages of lake evolution, including the overfilled (Road Hollow Member), balanced-fill (Fossil Butte Member), and underfilled (Angelo Member) stages. These stages are represented in the sedimentary record by a complete suite of lake-margin to lake-center facies.
This study establishes the Road Hollow Member of the Green River Formation as representing the earliest stage of lake evolution in Fossil Lake. We also revise the boundaries for the Fossil Butte and Angelo Members of the Green River Formation, which clearly delineate the latest two stages of lake evolution. These revisions not only describe and add a previously unrecognized and thick sequence of lacustrine rocks in Fossil Basin, but help us to better understand the depositional systems that existed during each stage of lake evolution.