ABSTRACT

The Cooking Lake carbonates were deposited on a widespread Late Devonian bank that extended from southern Alberta to the Edmonton area of central Alberta. A distinct bank margin existed directly west of the present Leduc-Morinville reefs of Leducage throughout Cooking Lake deposition, whereas in late Cooking Lake time a northern bank margin was developed north of the Smoky Lake and Duvernay areas.

Bioclastic and pseudo-oölitic (or pellet) calcarenites, biostromal limestones and minor calcilutites were formed on the bank, whereas dominantly argillaceous rocks and calcilutites were laid down beyond the bank margins. The large bank was characterized by prominent shoals on which proportionately greater amounts of shallow-water carbonates were laid down in comparison with the “inter-shoal” areas. During lower Cooking Lake deposition the most prominent shoals existed in the Redwater to Beaverhill Lake, Smoky Lake, and Duvernay areas; during upper Cooking Lake deposition, shoals existed in the Redwater, Smoky Lake-Duvernay, and Ardrossan areas. Along the Leduc-Morinville reef trend, dolomitization has destroyed much of the original limestone character of the Cooking Lake formation and and the original shoal outline is not evident, although part of the dolomitized belt appears to have represented a depositional shoal. Biostromes and associated bioclastic debris are typical rocks formed on the shoals and were particularly concentrated on the north sides of the Redwater and Smoky Lake-Duvernay shoals during upper Cooking Lake deposition. Southward, increasing amounts of pseudo-oiilitic (precipitated) limestones formed on the bank proper. Toward the southeast (Vegreville-Plain Lake areas) the presence of primary anhydrite indicates increasingly saHne conditions.

Subsequently, vertical reef growth took place on, or near the margins of, these shoals (e.g., Redwater, Willingdon-Pinedale and Leduc-Morinville reefs of Leduc age).

During early Duvernay sedimentation, lowering of the depositional profile caused the biostromes to grow upward from their established sites on certain shoals. Although initially the off-reef deposits consisted of poorly sorted bioclastic material, they were followed by increasing proportions of shales and argillaceous calcilutites. Thence, during later Duvernay sedimentation restriction of marine waters by prominent reef shoals generally resulted in increased deposition of brown and black shale as compared with gray shale. From a regional standpoint, relatively less brown to black shale was deposited at the extreme north of the area of study, although these shales appear to have been laid down in the northeastern and eastern areas for some time after cessation of deposition of similar rocks on the west. However, partly because of effects of nearby reef shoal environments there was considerable local variation in the relative amounts of brown (or black) and gray (or green) shales deposited.

Detailed mapping of carbonate facies in the Cooking Lake represents a basic method of approach to Leduc reef exploration; in addition, the pattern of distribution of permeable and relatively non-permeable rocks in units such as the Cooking Lake may reveal areas favorable for entrapment of petroleum.

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