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Four major evaporite cycles are widely present in Lower and Middle Permian strata of western Oklahoma, interstratified with clastic sediments in a sequence 4,000 feet thick. Named in ascending order the Wellington, Cimarron, Blaine (including salt beds that underlie and overlie it in subsurface), and Cloud Chief, they consist almost exclusively of anhydrite and rock salt. Thin beds of dolomite also are present, but potassium salts have not been found. The cycles diminish in thickness upward, as the maximum thickness of the lower two is 1,000 feet, the next younger 600 feet, and the youngest 100 feet.
The uppermost or Cloud Chief evaporites consist only of massive anhydrite, whereas each of the three lower cycles is divisible into individual subcycles, some of them traceable over a region covering more than 15,000 square miles. A single cycle of anhydrite overlain by salt characterizes each of these major units. Dolomite-anhydrite couplets in the 150-foot thickness of the Blaine also are outstanding for their demonstrable continuity and persistence.
Continuous cores of nearly pure rock salt 150 feet thick in the lower part of the Cimarron evaporites likewise show pronounced cyclical development, which is reflected in the isomorphous potassium of the halite, in the range of 100 to 400 ppm. Low potassium values in the lower part of each cycle contrast with high values in the upper part, producing geochemical discontinuities at the boundaries between successive cyles.
Also arranged in cycles are the clastic sediments of the framework that contains the cyclic evaporites. Particularly conspicuous are the interbedded brownish red and gray-green shales of the Flowerpot, Blaine, and Dog Creek Formations. Investigations of boron thus far suggest a marine environment for the shales, and in one area the highest shale values (250 ppm boron) are associated with as much as 4.5 per cent copper.
Cyclicity in the Permian evaporites of Oklahoma are related to cyclical changes in environment during the progressive sinking of a sedimentary basin. Roles are doubtless played by tectonism and climate, in forming the basin itself as well as in producing the aridity necessary for the precipitation of evaporites, but eustatic change in level of the sea must also have been a prominent factor during Early Permian time, when continental glaciation in the southern hemisphere was repeatedly raising and lowering the sea.
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