DIAGENESIS OF AN UPPER DEVONIAN CARBONATE-EVAPORITE SEQUENCE - BIRDBEAR FORMATION, SOUTHERN INTERIOR PLAINS, CANADA

The Frasnian Birdbear (Nisku) Formation is a carbonate-evaporite succe ssion formed on a shallow restricted shelf along the eastern side of t he Western Canada Sedimentary Basin. Low delta(13)C values of the shel f limestones and dolostones (1 parts per thousand to -5 parts per thou sand) relative to coeval open marine settings reflect the restricted p aleoenviromment of the shelf. Limestones are present only in the easte rn part of the study area, and were altered by meteoric: and burial di agenesis and do not retain marine delta(18)O values. delta(18)O values of replacement dolomites have limited range (-3 parts per thousand to -5 parts per thousand) and may indicate dolomitization by hypersaline marine waters having low delta(18)O values, Sr-87/Sr-86 ratios of the replacement dolomites are variable (0.7082-0.7085), but also suggest that reflux of Late Devonian seawater, possibly Famennian, was the pri ncipal dolomitizing mechanism in the Birdbear shelf, Most bedded anhyd rites have delta(34)S values (22 parts per thousand to 24 parts per th ousand CDT) acid Sr-87/Sr-86 ratios (ca. 0.7081) that record Frasnian seawater, Dissolution and reprecipitation of sedimentary sulfates resu lted in sulfate cements having up to 2 parts per thousand higher delta (34)S values and slightly higher Sr-87/Sr-86 ratios (ca. 0.7083). Flui ds associated with anhydrite dissolution may have been derived from th e overlying Famennian shelf. Late diagenetic cements in the Birdbear S helf precipitated during the Late Cretaceous acid early Tertiary durin g maximum burial from hot, evolved residual evaporitic brines, Residua l evaporitic brines that originated on the shallow east ern Upper Devo nian shelves were likely important agents of diagenesis throughout the basin.