ABSTRACT.

Most of the oil reserves of Western Canada have been found in Upper Devonian reefs and Mississippi; :arbonate Sediments. Reefs deve ionse to rapid subsidence along the margins of limestone banks and on shoals within t¿ few reefs that have escaped complete dolomitization show that sedimentbinding algae aitu JLE uiiiaruporoids were the framework builders of the biohermal margins, behind which bioclastic ai accumulated. The dolomitiz d to be mainly epigenetic, caused by < reefs from adjacent and deeper com Mississippian epieontinentai seas shallowed towards the partly emergent Canadian Shield, and the sedimentary facies belts moved back and forth in response to sea level changes. The genesis and diagenesis of the resulting cyclic sequence of limestones and dolomites are discussed in terms of the typical sucerssion exposed in the Moose Dome inlier of the foothills of Southern Alberta. The Banff formation at the base consists predominantly of argillaceous, cherty, pasty limestones formed by the anaerobic rotting of skeletal detritus below wave base. Bioclastic lime-sandstones (Pekisko formation) appeared as the seas shallowed, first with interstitial pasty matrix and then clean-washed as the energy of the depositional environment increased. Thin oolitic beds are the first sign of increased salinities. Continued shallowing segmented the seas into lagoons where lime-muds were precipitated to form the lithographic limestones of the Shunda formation. Associated dolomite-muds accumulated in local, more saline lagoons. A return to open shelf conditions started the second cycle with the deposition of clean-washed lime-sandstones (Turner Valley formation) composed predoi ossicles and bryozoau fragments. Epigenetic dolomitizatiou is common in these rot iose formed from the more heterogeneous bioclastic detritus. Cryptocrystalline dolomites, brecciated by the solution 01 associateci annyarite, occur in the overlying Mount Head formation, which represents the evaporitic phase of the second cycle. :h Mg+-/Ca++ ratio escapii minantly of crinoid zks, particularly in tl .... .I

RESUME.

Les principales réserves de pétrole de l'Ouest canadien ont été trouvées dans des récifs du Dévonien supérieur et dans des sédiments carbonatés bioclastiques du Mississipien. Les récifs se sont développés par suite de l'affaissement rapide le long des bords des banc calcaires et sur les hauts-fonds à l'intérieur de ces bancs. Les quelques récifs qui ont échappé à une dolomitisation complète montrent que les algues agglomérant les sédiments et les strom

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