DEPOSITIONAL FACIES AND CARBONATE DIAGENESIS OF THE DOWNSLOPE REEFS IN THE NISKU FORMATION (UPPER DEVONIAN), CENTRAL ALBERTA, CANADA (SEDIMENTOLOGY, DOLOMITE, ISOTOPES)

The Nisku Formation (Frasnian) in Alberta was deposited during four stages of basin infill, corresponding to deposition of the Lobstick, Bigoray, Cynthia, and Wolf Lake members. Stages are regionally correlatable, and are bounded by shaly carbonates. Lobstick and Bigoray carbonates were deposited on a ramp in moderate water depths. Rises in sea level resulted in the backstepping of Bigoray carbonate deposition higher onto the ramp and the formation of a shelf margin. Coral mounds formed downslope in water depths of around 50 m. Argillaceous carbonates of the Cynthia and Wolf Lake members infilled the basin between the reefs. Reefs average 2 km in diameter and 100 m in thickness, and exhibit an overall shoaling-upward sequence. The tops of some reefs were subaerially exposed, resulting in the formation of shallow meteoric lenses and minor calcite dissolution and cementation. Reef growth was terminated by drowning.; Marine diagenesis included micritization and extensive cementation by Mg-calcite with inclined extinction. Calcic dolomite formed penecontemporaneously with deposition at the tops of the reefs. Following deposition and through the Mississippian, the reefs were buried to depths of 0.5 to 1 km. Stabilization of marine precipitates and precipitation of minor amounts of calcite cement occurred in seawater modified during burial at elevated temperatures. An active hydrologic system developed during the Pennsylvanian and through the Early Cretaceous in response to the westward tilting of the craton. Replacement of limestone by fabric-selective and nonfabric-selective dolomite and calcite dissolution occurred at depths of 0.5 to 1 km and temperatures around 5(DEGREES)C. Magnesium was derived from updip migrating, burially modified seawater. Mass balance calculations indicate that insufficient connate fluid existed in the basin, and in order to dolomitize the reefs and platforms, regional recharge of seawater must have occurred. Thermal convection is proposed as the driving mechanism for this recharge. Baroque dolomite, calcite, and anhydrite formed during deep burial from saline brines during the Late Cretaceous and Early Tertiary.; Carbon isotopic data indicate a rock-buffered carbon system. Progressively later calcites and dolomites are depleted in ('18)O and enriched in ('87)Sr because of increasing burial temperatures and increasing reaction of detrital silicates. The susceptibility of these carbonates to burial modification is attributed to deposition in a downslope setting with little early meteoric alteration.