Facies and Diagenesis of Some Nisku Buildups and Associated Strata Upper Devonian, Alberta, Canada
The Nisku formation in the Alberta subsurface displays three distinct, time-equivalent facies domains along the edge of the so-called Outer Shelf (or bank): basinal, reefal, and bank facies. The reefal facies occurs in small buildups enclosed in the basinal facies, and fringing the bank-edge. The bank facies is here designated the Dismal Creek Member, and its lithology, log characteristics, and relationship to the other facies domains are demonstrated.
Reefal deposition commenced usually in Lobstick time in the northeast, and in Bigoray time further southwest, on top of a carbonate platform in probably moderate water depths below wave base. Most buildups contain little or no framework and are variably fossiliferous, mostly coral-bearing mudmounds. Reefal deposition ceased toward the end of Nisku time.
The diagenesis is very complex. The few partially dolomitized buildups display a multiple succession of limestone-diagenetic fabrics. On the other hand, (a) dolomitization, (b) dissolution of calcific remnants, and (c) anhydritization are the most striking processes detectable in the completely dolomitized buildups. The degree of dolomitization and anhydritization increases down structural dip. Dolomitization is interpreted to have taken place in an intermediate burial environment. The exact source and nature of the dolomitizing fluids are presently not known. Anhydritization followed dolomitization, and may have happened at intermediate or deep burial. The ultimate source of the anhydrite is not known yet, but a second generation of anhydrite found as pore-filling cements is probably locally derived from the first-stage anhydrite. The sour gas, mainly found at the down-structural dip end of the study area, is possibly derived from the scavenging of anhydrite by hydrocarbons. This process could have created tertiary, post-anhydrite porosity.