MINERALOGIC STRATIGRAPHY OF THE EOCENE GREEN RIVER FORMATION, GREEN RIVER AND WASHAKIE BASINS, WYOMING, USA

The Green River Formation of Wyoming represents a complex sequence of lacustrine, fluvial, and playa sediments deposited in and associated with Eocene Age Lake Gosiute. Detailed mineralogical and organic analysis of ten cores from the Green River Basin produced correlatable stratigraphic horizons that can be used to interpret the geochemical and sedimentalogical history of Lake Gosiute. More than thirty minerals were identified in this study, with graphic correlations and geostatistical evaluations defining sixteen major minerals and oil yields, as defined by Modified Fischer Assay. These relationships could be used to define distinct correlatable stratigraphic horizons that, when used in conjunction with known chronostratigraphic volcaniclastic units, could be used to interpret major influences upon the depositional history of Lake Gosiute. The four primary influences upon mineralogical stratigraphy of Lake Gosiute were determined to be: (1) deposition and preservation of organic matter, (2) volcaniclastic materials, (3) periods of evaporation and concentration, and (4) influxes of fresh water accompanied by increased transported detrital material. Climatic variation both on broad and seasonal scales influenced the mineral suite. Wet periods in the history of Lake Gosiute (Tipton and Laney members) were associated with increased organic production and a related mineral suite; while dry periods (Wilkins Peak Member) resulted in decreased organic production and the genesis of a prolific suite of saline and evaporitic minerals. This systematic investigation of the Green River Formation illustrates that applied mineralogy can be used a tool for the interpretation of complex depositional environments and stratigraphy.