Abstract

The Green River Formation of Wyoming was deposited in a large lake, Lake Gosiute, during Eocene time. Throughout Lake Gosiute's existence, nearby volcanism supplied debris to the lake basin. Tuffs are abundant and widespread in the formation and were highly reactive units. Reaction between glass and the lake waters produced authigenic clay minerals, mordenite or clinoptilolite, analcime, and potassium feldspar. Some tuffs are almost entirely replaced by authigenic aluminosilicate minerals.

The alteration of the tuffs shows a systematic pattern with the clay mineral and zeolite alteration restricted to facies formed in relatively fresh water, and the feldspar alteration is confined to a hypersaline facies. We infer that the lake water ranged from fresh or brackish (SiO2 = 10 ppm, K = 50 ppm, Na = 1,000 ppm) at a pH of 8.0 during the “fresh-water stage” to hypersaline (SiO2 = 1,000 ppm, K = 5,000 ppm, Na = 100,000 ppm) at a pH of 9.0 to 10.0 during the most saline and alkaline stage. Brines were formed primarily by evaporative concentration. Organisms, reflected by abundant high-yield oil shales, thrived during the freshwater episodes, but were apparently absent or of minor significance during the hypersaline episodes; this is reflected by the presence of thick beds of trona and halite.

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