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Stratigraphic analysis of the 70-m thick Green River Formation in Fossil Basin of southwestern Wyoming reveals fluctuations in the water-column chemistry ranging from fresh to hypersaline. Fresh water dominated the first two-thirds of the lake’s history while hypersalinity dominated the final stages.

Salinity changes are identified by variations in calcite-dolomite ratio, tuff bed mineralogy (authigenic analcime, K-feldspar, and clay minerals), and paleontology. The lower two-thirds of the sequence are dominated by finely laminated carbonates (primarily kerogen-rich micrite), interrupted by 0.3- to 1.0-meter thick beds of massive dolomicrite. Dolomicrites grade laterally into bioturbated calcimicrites. The sequence is not random, and is related to a lithofacies assemblage including (from the base up) kerogen-rich laminated micrite, kerogen-poor laminated micrite, partly burrowed laminated micrite, and dolomicrite. Tuff beds occur randomly in the sequence and are composed of authigenic analcime and K-feldspar. Laterally, they grade into clays.

These data are interpreted as representing a lake fluctuating from fresh to hypersaline, with stages controlled by sudden freshwater expansions of the lake followed by more gradual regressions. However, initiation of dolomite precipitation was relatively sudden al the end of regressive stages. Even then, the lake remained relatively fresh along its margins as indicated by accumulated synchronous shore–phase calcimicrite. Tuff bed mineralogies confirm this conclusion.

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