Facies and Geometry of Swan Hills Reef Member of Beaverhill Lake Formation (Upper Devonian), Goose River Field, Alberta, Canada

Abstract

The petrography, paleontology, and geometry of the Swan Hills Member (limestone reef) of the Late Devonian Beaverhill Lake Formation in the Goose River field, Alberta, were studied in 11 wells. The Swan Hills Member is divisible into a lower Dark Brown submember (stromatoporoid reef platform) and an upper Light Brown submember (stromatoporoid biohermal reef complex). Twenty-two rock types and eight environmental facies were distinguished within the reef complex and indicate an atoll-like reef development. Geomorphic features including outer slope, reef rim, reef crest, and central lagoon are evident. Biocalcarenite and biocalcirudite with well-developed porosity are present in the reef-rim positions, whereas micritic rock types with low porosity are present in the central part of the reef complex. The following zonation of fossils is recognized: (1) large stromatoporoids, brachiopods, and crinoids are present in forereef positions; (2) organic reef and backreef accumulations consist of Stachyodes and large stromatoporoids; and (3) the lagoonal parts are dominated by Amphipora with a few occurrences of ostracods, Foraminifera, and calcispheres. Stylolites are very common and indicate a reduction in reef thickness of 9 percent or more. A thin green shale break is believed to represent an interruption in reef growth.