Abstract

The Wabamun group, consisting mainly of carbonate rocks and anhydrite, increases in thickness westward from less than 500 ft. in the Leduc and Stettler areas to approximately 1,800 ft. in part of the Rocky Mountains of Alberta. E. of Leduc and N. of Stettler, the Wabamun has been irregularly truncated by pre-Cretaceous erosion. For purposes of facies analysis the Wabamun is subdivided into 4 units: 1) basal unit including the lowermost evaporitic interval and lateral marine carbonate equivalents; 2) lower middle Wabamun representing the overlying interval to the top of the Crossfield dolomite in the Calgary area; 3) upper middle Wabamun including post-Crossfield and pre-Big Valley beds; and 4) upper Wabamun representing the Big Valley formation and approximate lateral equivalents. The evaporites and dolomites are dominant in the southeastern part of the map-area, limestones in the northwestern part (and locally at the SW.), and fine to coarse crystalline dolomite in the intervening area. Evaporites and associated extremely fine-textured dolomites exhibit most widespread distribution in the basal Wabamun. The main evaporites extend NW. through Stettler to Leduc whereas the extremely fine-textured dolomites also occur farther NW. in the middle Wabamun, the evaporites terminate a relatively short distance NW. and N. of Stettler because of probable removal by solution and pre-Cretaceous erosion. Presumably middle Wabamun evaporites originally extended N. beyond their present limit. During deposition of the basal and upper Wabamun units, a subsidiary evaporite province occurred in the foothills belt W. of Calgary and may have extended N. as far as the Clearwater River. Limestones range from calcilutites to calcarenites and exhibit incipient dolomitization that commonly imparts a mottled appearance to the rock. The calcarenites are generally of the pseudo-ooelitic or pelletoid type, although bioclastic limestone is characteristic of the uppermost, and locally, of lower beds. The pelletoid limestones and calcilutites were deposited on a widespread bank on which slight restriction of the shallow waters promoted precipitation of calcium carbonate. Anhydrites and cryptocrystalline dolomite were precipitated in the Stettler-Drumheller area near the bank margin at the E., and the associated calcareous muds were diagenetically dolomitized to microsucrosic dolomite. Coarser (very fine to medium crystalline) dolomites are characteristic of a belt between the limestone province at the NW. and evaporites at the SE. Local vuggy dolomite sections in this belt are interpreted to be possible reefs. Petroleum has accumulated in 2 main types of stratigraphically controlled traps within the Wabamun group. Facies change from porous secondary dolomites to relatively nonpermeable equivalents approximately up regional dip (mainly eastward) is responsible for gas entrapment in the Okotoks-East Calgary-Olds trend of fields. In the Edmonton area, oil and gas has been localized in erosional hills made up of porous Wabamun dolomites at the pre-Cretaceous unconformity.

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