The stratigraphic succession in the northern Great Plains is punctuated by many evaporite deposits of varying thickness and areal extent. They are most predominant in rocks of Paleozoic age and particular in the Devonian and Mississippian Systems. The youngest known deposits are associated with redbeds which have been assigned Late Triassic to Early Jurassic ages. Most of the evaporites in the northern Great Plains are composed of calcium sulfate; halite is somewhat less common, and there are small amounts of soluble potassium chlorides.

Recent geochemical investigations of the distributions of Sr⁺² in modern evaporites and brines indicate that the mass ratios of Sr⁺²/Ca⁺² in calcium sulfate deposits may be employed to assist in the identification of the hydrologic environment in which an evaporite was precipitated and indirectly to distinguish depositional settings. The data from these investigations have been applied to selected evaporites in the northern Great Plains and corroborate the interpretations of the depositional settings as obtained from an examination of the lithologic associations.

The brines from which the evaporites in the northern Great Plains were precipitated may be thought of as having existed in one of two hydrologic environments: (1) interstitially in siliciclastic or carbonate sediments, or (2) as standing water in topographic depressions that underwent intermittent replenishment by less saline waters. The depositional settings as interpreted from lithologic associations and the results of geochemical analyses include: (1) arid supratidal flat, (2) barred-marine basins, ranging from structurally

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controlled regional elements to restricted lagoons on the landward side of offshore barrier complexes, and (3) playa lakes.

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