A Late Devonian Reef Tract on Northeastern Banks Island, N.W.T.

Abstract

ABSTRACT The Upper Devonian Weatherall Formation, outcropping on northeastern Banks Island, N.W.T., contains a 200-ft-thick limestone unit here termed the Mercy Bay Member. The member is Middle to Late Frasnian in age. Gyrfalcon Bluff has been chosen as the type section. Mercy Bay Member outcrops on the extreme northeastern portion of Banks Island, and many excellent exposures permit detailed paleogeographical and paleoecological studies. The member contains numerous organic build-ups and represents a Late Devonian reef tract located in the marine-shelf environment of an exogeosyncline situated between a tectonic highland to the northwest and a stable craton to the southeast. The main facies changes in the Mercy Bay Member occur in an east-west direction. The organic build-ups in the eastern part of the study area are narrow, linear bioherms trending north-south. They are encased in younger terrigenous clastic rocks. To the west the organic build-ups, which are biohermal in the lower part and biostromal in the upper, are more numerous. The lower bioherms trend east-west. Penecontemporaneous interbiohermal strata consist of dark, fine-grained argillaceous limestone. Organic build-ups on the western edge of the outcrop area are bioherms which trend north-south. The lower portion in all organic build-ups consists of corals and tabular stromatoporoids. These are interpreted as biogenetic banks constructed in the quiet and intermediate-energy zones (water depths more than 30 ft). The upper portion is composed of massive stromatoporoids. This facies represents rigid reefs constructed in the high-energy zone (above 30 feet). Successive sea-level rises allowed the reefs to grow upward. Cessation of reef growth was caused by an influx of terrigenous sediment related to the seaward migration of the northern and western shorelines. The outcropping organic build-ups of the Mercy Bay Member are tightly cemented, but frequent bitumen occurrences indicate that they were once oilbearing. Organic build-ups of the Mercy Bay Member probably occur in the subsurface to the west. End_Page 730------------------------