Stable isotopes in sulphate minerals from the Salina formation in southwestern Ontario

Abstract

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.