The oxygen and carbon isotopic composition of the carbonates of the Upper Silurian Salina formation of the Michigan Basin was investigated to aid in interpretation of depositional environments.13C results indicate that a change from generally anoxic bottom conditions to oxic conditions occurred during deposition of the B evaporite unit. The organic-rich A carbonate units were deposited in a shallow-water, evaporitic setting, most likely adjacent to a sabkha-type environment. A positive water balance maintained the anoxic conditions and buffered the carbon isotopes.Above the B evaporite, the isotopic composition suggests that the development of a similar depositional environment, a subaerial prograding sabkha, occurred over wide areas of the basin.18O results support the conclusion that Silurian oceans were depleted in 18O with respect to modem oceans by 5–6‰.

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