Abstract

Individual members of the Green River Formation in the Greater Green River Basin of southwestern Wyoming and northwestern Colorado, U.S.A. record a wide range of lake conditions. Even within this one basin, lake systems changed from freshwater to hypersaline, open to closed hydrology, and clastic to carbonate to evaporative dominated. Each of these systems left a distinctive stratigraphic and geochemical record amenable to detailed analysis based on outcrop expression and abundant subsurface data. Variations in sedimentary and geochemical records are seen at many scales in these rocks: from millimeter-thick annual varves to kilometer-thick basin-fill packages. Detailed and thorough observations of the sedimentology and stratigraphy of the lake stratal packages is essential to deciphering the lithological and geochemical variations— and a full integration of these observations with biofacies data is key to reconstructing lake-system processes and composing predictive models.

We discuss three representative units to illustrate the distinct variations in lithofacies architecture and sequence-stratigraphic expression within the Green River Formation: Luman Tongue, Wilkins Peak Member, and Laney Member (early to middle Eocene).

Considering the high degree of variability inherent in lacustrine rocks, analogues from both modern and ancient environments must be carefully selected, since small changes in the depositional and preservational environment (recorded at the meter scale) can fundamentally alter the character of lacustrine strata. From the geologic and geochemical data, however, it is apparent that the Green River Formation contains a wide variety of depositional settings that can be used as analogues for comparison and prediction in many different lake settings.