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The Wilkins Peak Member of the Eocene Green River Formation is estimated to contain about 100 billion tons of trona (Na2CO3 · NaHCO3 · 2H2O) and halite (NaCl) in more than 40 beds at depths as great as 3,500 ft in a 1,300-sq-mi area in the Green River basin of Wyoming. Incomplete data on these beds indicate that trona comprises 80-85% of the total, and that individual beds range up to 40 ft in thickness and up to 900 sq mi in area. Trona, a major source of sodium carbonate, is mined at 4 localities. At least 30 billion tons of halite-free trona, possibly as much as 40 billion tons, are contained in beds that are at least 6 ft thick under an area of at least 25 sq mi. Halite is present either intermixed with trona, or i terlayered with trona (in layers up to 20 ft thick), or rarely as halite beds with little or no trona. Halite is present only in the southwest part of the trona-halite area, only in 14 beds in the lower half of the Wilkins Peak Member, and only as part of, or continuation of, a trona bed.
The beds of trona and halite are evaporite deposits from a large alkaline lake that occupied the southern end of the Green River basin at the beginning of Wilkins Peak deposition, then gradually expanded northward. Beds of trona, or trona-halite, were deposited across successively larger areas until the middle of Wilkins Peak deposition, when trona deposition shifted northward and halite deposition ceased.
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