Famennian Reefs in Alberta, Canada: ABSTRACT

Abstract

The lowermost 200 feet of the Wabamun group of Famennian age may sporadically display an unusually abundant fauna in the subsurface of Alberta northeast of the then slowly submerging Poaco River landmass. This fauna, composed of stromatoporoids, coralline algae and various rod- and tube-shape bioclastic fragments in a sparry cement, is completely absent in nearby boreholes that show primarily carbonate muds with only rare dwarfed Foraminifera, some ostracods, and crinoid stems. Irregular mounds of medium to coarse cystalline dolomite, which contain stromatoporoids (Labechia sp. and Pseudolabechia sp.) and relic tube- and rod-shape fragments suggestive of coralline algae and other potential reef builders are found encased in Wabamun carbonate muds. These mounds are restricted to the region of complete or partial dolomitization of the underlying Graminia formation south of this Poaco River landmass. Identifiable specimens of reefal organisms still stand out, even if they are only moderately common in the severely dolomitized sections; they are completely absent in adjacent pelleted or unpelleted carbonate muds. The muds contain a negligible clastic admixture and are populated only by some individual Foraminifera of the Nodosaria family, Dasycladaceae such as M zzia sp., ostracods, and crinoids, with an extreme scarcity of biogenic remnants being the really paramount feature. The Wabamun dolomite mounds appear to occur preferentially near Wabamun thins, on the slopes of the Wabamun sea floor. Such reefal developments are found on slopes formed by the Wabamun being comparatively thicker over buried Frasnian Woodbend reefs, due possibly to some negative adjustments within or below these older reefs during Graminia and Lower Wabamun deposition. Unlike their undolomitized and tight equivalents, the Wabamun dolomite mounds have been cored frequently and thus are accessible to detailed study; they are economically important as attractive gas reservoirs with porosities to 13 per cent, permeabilities up to 200 millidarcys. Dolomitized biostroms previously reported by others from Palliser outcrops near Banff, Alberta, are exposed equivalents of similar magnitude; t e carbonate mud encasement renders them akin to Silurian reefs on Lake Erie and James Bay in Ontario. End_of_Article - Last_Page 548------------