Abstract
Sulfate evaporites of mid-Paleozoic age are exposed or can be found within several hundreds of meters below the surface across much of southeastern Iowa. In addition, Mesozoic (Jurassic?) evaporites have been mined from thick beds in north-central Iowa. This study reports stable isotope analyses performed on twenty-four samples, representing each of the major evaporite units in the state. Values of δ 34S in all samples are consistent with those of sulfates of the same age on the global sulfur “time line”, defined by previous investigators. Some samples, however, have δ 18O-values that are as much as 6‰ lighter than sulfates of the same age on the global oxygen “time line”. These anomalies were observed in both Devonian and Mississippian samples, but only in a small region near Ottumwa. The cause of the anomalous values is not clear, although circumstances suggest a chemical interaction with migrating fluids. In this part of Iowa. Devonian and Mississippian strata host major aquifer systems whose waters may have disturbed the isotopic compositions of the evaporites, or may have served in the past as conduits for other migrating fluids. Results of this survey suggest, therefore, that δ 34S-values of Iowa evaporites carry an undisturbed record of the isotopic composition of Paleozoic seawater, but that δ 18O has been affected by postdepositional changes and is no longer a reliable measure.
Published Version
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