Stratigraphy, Devonian Swan Hills reef complexes of central Alberta

Abstract

ABSTRACT Devonian reef complexes in the subsurface of the Swan Hills area of central Alberta are some of the most prolific oil-producing reservoirs in Canada. The complexes, composed of carbonate rocks and collectively forming the Swan Hills Formation, contain rich, varied, and well-preserved stromatoporoid faunas and other associated fossils, including ostracodes. Six discrete reef complexes (Snipe Lake, Goose River, Kaybob, Virginia Hills, Swan Hills, and House Mountain) lend themselves in particular to detailed stratigraphic analysis. From the many available cored sections, the rock sequences can be established readily, and divisions of the sequences correlated from one reef complex to another. Moreover, the rocks of these complexes are negligibly altered, so that various facies can be recognized, and facies relationships established with little difficulty. Nine separate stages of growth can be recognized in the Swan Hills reef complexes, and these are designated conveniently, from bottom to top, Divisions I to IX. A distinct break is apparent at the boundary between Divisions V and VI, expressed in part by a perceptible southwesterly shift in the loci of reef growth. Each division consists of as many as four main facies, which contain characteristic rocks and fossils. Thirty-one species of stromatoporoids have been recognized in the Swan Hills Formation, and five of these (Euryamphipora sp. A, Trupetostroma sp. A, Trupetostroma sp. B, Clathrocoilona sp., and Stromatopora sp.) are new. From examination of the many stromatoporoid colonies present in the rocks, it is apparent that growth forms intergrade, and dendroid stromatoporoids are allied genetically to both subspherical and tabular forms. It is proposed, therefore, that, in contrast to the importance accorded it by many palaeontologists, growth form be relegated to a position of little importance for taxonomic purposes. Three distinct species of Amphipora, (A. ramosa, A. pervesiculata, and A. angusta) can be recognized in the collections from the Swan Hills Formation. A progressive loss of stromatoporoid species occurs at the end of each stage of reef growth (top of each division) except for Division I. The most significant faunal break occurs at the boundary between Divisions V and VI, where seven stromatoporoid species disappear. Significantly, diagnostic Middle Devonian ostracode species also disappear at this level. No ostracodes diagnostic of either Middle or Late Devonian age were recovered from the rocks above this boundary. The shales of the Waterways Formation, which enclose the reef complexes and locally underlie Divisions VI to IX, where these lie to the southwest of the main centres of the complexes, contain a Late Devonian microfaunal assemblage. Lithological and faunal evidence points to a disconformity, marking a considerable time lapse, between Divisions V and VI, and there is little doubt that this hiatus contains the Middle-Late Devonian boundary. Fluctuations in sea level, preceding retreat of the Middle Devonian sea, resulted in the development of Divisions I to V. Similar fluctuations of the Late Devonian sea, after deposition of early Waterways Shale, resulted in the development of Divisions VI to IX. End_Page 444------------------------