LATE DEVONIAN MERCY BAY REEF TRACT, BANKS ISLAND, ARCTIC CANADA: FACIES GEOMETRY, FAUNAL COMPOSITION, AND REEF SUCCESSION

Late Devonian (early to mid-Frasnian) reefs of northeastern Banks Island, Arctic Canada, grew on the seaward margins of a westward-prograding delta. Reef mapping in 2000 and 2003 found reefs at four stratigraphic levels, each probably corresponding to a delta lobe. Positions of the reefs in the four levels indicate backstepping of the delta lobes under rising sea levels. The four levels of reefs, with intervening siliciclastic sediments, have been designated the Mercy Formation, within the dominantly siliciclastic Weatherall Group. Mercy Fm. total thickness is approximately 145 m, exposed over an area roughly 50 km by 100 km. Major reefs are platform reefs 30-40 m thick, up to 500 m wide and 1.5 km long, while small patch reefs are as small as 5 m thick and 50 m diameter.

Dominant fauna are branching and platy tabulate corals, domal to multilobate rugose corals, and domal, tabular, and encrusting stromatoporoid sponges. Faunal diversity is fairly low, as expected for the mid-late Frasnian. Calcimicrobes are important in the lowest reef level only. Tree trunks encrusted by corals and other epifauna are common in the inter-reef and back-reef bituminous carbonates. Banks Island reefs show consistent patterns of reef succession. Branching corals initially stabilized siliciclastic muds or sands, followed by thamnoporid branching tabulate corals and branching phaceloid rugose corals, mostly Smithiphyllum, multilobate alveolitid corals, platy alveolitid and coenitid tabulate corals, and domal cerioid rugose corals, mostly Argutastrea and Chuanbeiphyllum, and rarely, the phaceloid stromatoporoid Euryamphipora. By contrast, the reef core facies of the reef at nearly all localities is dominated by domal, tabular, and laminar stromatoporoids, usually complete, and sometimes with corals and calcimicrobes. Flank facies contain both corals and stromatoporoids. Inter-reef and back-reef carbonate facies consisting of bituminous packstones and papery micrites lie east and south of the principal reef tract, especially in the uppermost level, while siliciclastic sediments occupy most of the area between reefs at the lower stratigraphic levels.