Diagenesis of an Upper Devonian Carbonate-Evaporite Sequence: Birdbear Formation, Southern Interior Plains, Canada

Abstract

ABSTRACT 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 d13C 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 d18O values. d18O values of replacement dolomites have limited range (-3 to -5) and may indicate dolomitization by hypersaline marine waters having low d18O 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 d34S 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 d34S 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 th basin.