Diagenesis of an Upper Devonian carbonate-evaporite sequence: Birdbear formation, southern interior plains, Canada

The Frasnian Birdbear (Nisku) Formation is a carbonate-evaporite succession formed on a shallow restricted shelf along the eastern side of the Western Canada Sedimentary Basin. Low δ ¹³C values of the shelf limestones and dolostones (1‰ to -5‰) relative to coeval open marine settings reflect the restricted paleoenvironment of the shelf. Limestones are present only in the eastern part of the study area, and were altered by meteoric and burial diagenesis and do not retain marine δ ¹⁸O values. δ ¹⁸O values of replacement dolomites have limited range (-3‰ to -5‰) and may indicate dolomitization by hypersaline marine waters having low δ ¹⁸O values. ⁸⁷Sr/ ⁸⁶Sr ratios of the replacement dolomites are variable (0.7082-0.7085), but also suggest that reflux of Late Devonian seawater, possibly Famennian, was the principal dolomitizing mechanism in the Birdbear shelf. Most bedded anhydrites have δ ³⁴S values (22‰ to 24‰ CDT) and ⁸⁷Sr/ ⁸⁶Sr ratios (ca. 0.7081) that record Frasnian seawater. Dissolution and reprecipitation of sedimentary sulfates resulted in sulfate cements having up to 2‰ higher δ ³⁴S values and slightly higher ⁸⁷Sr/ ⁸⁶Sr ratios (ca. 0.7083). Fluids associated with anhydrite dissolution may have been derived from the overlying Famennian shelf. Late diagenetic cements in the Birdbear Shelf precipitated during the Late Cretaceous and early Tertiary during maximum burial from hot, evolved residual evaporitic brines. Residual evaporitic brines that originated on the shallow eastern Upper Devonian shelves were likely important agents of diagenesis throughout the basin.