Stratigraphic Evidence for Tectonic and Current Control of Upper Devonian Reef Sedimentation, Duhamel Area, Alberta, Canada

In the Duhamel area of central Alberta, the Upper Devonian section under study comprises in ascending order: (1) Cooking Lake formation, (2) basal calcilutite of the Duvernay formation, (3) Leduc reef, and (4) Ireton formation. In off-reef sections, the Duvernay consists of basal calcilutites and argillaceous beds overlain by bioclastic limestones and an upper sequence of brown and black shales and calcilutites, overlain by the Ireton formation. The Cooking Lake formation is composed mainly of pelletoid (pseudo-oolitic) calcarenites and calcilutites formed probably by precipitation on a widespread bank. Calcarenites were concentrated along the western side of an interpreted shoal, the margin of which trended north to north-northeast approximately along the locus of later Leduc reef growth at Duhamel. The basal Duvernay calcilutite unit is less than 10 feet thick above the Cooking Lake shoal, but increases in thickness abruptly to 50 feet directly west of the shoal margin. It is interpreted that these facies and thickness changes occurred along a tectonic hinge line caused by possible fault movement in the Precambrian basement that determined the locus of later Leduc reef growth. Leduc reef growth at Duhamel commenced essentially after the basal Duvernay calcilutite was deposited. Earliest or incipient patch-reef growth was confined to the relatively positive eastern side of the hinge line during latest Cooking Lake sedimentation. However, with differential subsidence along the aforementioned hinge line, the reefs grew laterally westward, overlapping the thickest section of basal Duvernay calcilutites, and the subsequent main reef buildup occurred on the interpreted downwarped, western side of the hinge line. The bioclastic limestone unit of the Duvernay increases in thickness toward reefs, suggesting a derivasion by erosion of actively growing reefs. The thickness distribution of bioclastic deposits indicates dominant currents from the northeast that transported a greater amount of debris to the southwestern (leeward) side of the reefs; however, thicker deposits at the southeast may have resulted from longshore current transport southward along the fore-reef side. During upper Duvernay deposition, increased subsidence resulted in mainly vertical reef growth and reduction in amount of reef erosion; only calcium carbonate and argillaceous muds accumulated more than one mile away from the reef locus. The muds were deposited in relatively deep water, resulting in dark bituminous and partly laminated rocks with a sparse benthonic fauna. Brown and black shales accumulated in greater amounts in the more protected and stagnant waters southwest (leeward) of the reefs, and only in minor amounts as far as 10 miles north and east of the Duhamel reef front. In lower Ireton time, further increase in subsidence rate removed the restricting effects of the biohermal reefs and gray-brown and green-gray calcareous shales and calcilutites were deposited in off-reef areas. Reef growth was probably terminated during lower or middle Ireton deposition. Facies and isolith maps of pertinent units in the Cooking Lake and Duvernay formations may be used as an effective new tool in the search for petroleum-bearing Leduc reefs in central Alberta.