The oxygen and sulphur isotopic compositions of anhydrite and minor gypsum from the Salina formation in the Michigan Basin of southwestern Ontario were investigated to obtain information on depositional environments. The data base was also used to compare the isotopic compositions of these Late Silurian sulphates with global "age" curves for evaporites.The morphological and petrographical appearance of the sulphates suggests a shallow-water, possibly sabkha-type environment of deposition of the Salina anhydrites throughout most of the basin. δ18O values are rather consistent throughout the Salina but with short and substantial isotopic shifts in specific horizons. These rapid changes reflect the importance of "redox cycling" in shallow, restricted environments. No significant correlation between δ18O and δ34S is observed in massive and bedded anhydrites but is recognized in nodular and some chickenwire anhydrites. Biological reduction of sulphate in the environment of deposition must account for this.Comparison of 18O and 34S data with those from older and younger evaporites shows that the Salina sulphates are slightly depleted in 18O, if compared with the global, extrapolated curves for marine sulphates, but are enriched in 34S, possibly reflecting a major and rapid change in marine sulphate composition at the transition from the Silurian to Devonian times.

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