BANKS ISLAND, CANADIAN HIGH ARCTIC: REEFS OF THE LATE DEVONIAN(EARLY TO MID-FRASNIAN), PRIOR TO THE F/F MASS EXTINCTION

A pristinely preserved (CAI 1), tectonically undisturbed, ca. 100km long and 40km wide reef tract of the Mercy Bay Fm., Banks Island, western Canadian Arctic, represents the only Late Devonian remnant of the vast Innuitian stable, tropical carbonate platform that extended more than 2500km from Alaska to Ellesmere Island during the Middle Devonian. Reefs were developed on the distal, seaward lobes of a giant, siliciclastic delta complex that extended westwards from Greenland. The Mercy Bay Fm (early-mid-Frasnian) is ca. 130m thick, and forms a small part of the 1400m thick siliciclastic Weatherall Group (late Givetian-Mid-Famennian). It contains three reefal units from base to top, separated by dark grey to black calcareous shales and siltstones with plant remains. The lower carbonate unit is best exposed along the lower Mercy and Manning rivers, and is ca. 20m thick, with reefs up to ca. 150m in diameter and 8m thick. The middle reefal unit, ca. 60m thick, is exposed in areas S of Gyrfalcon Bluff, and along the Mercy and Manning Rivers, and has domed reefs up to about 1km in diameter. The most widespread, and thickest reefal units and carbonate development terminated reef growth in the area, and isolated domed and platform reefs my be found as far east as the M’Clure River, and in the southernmost outcrop belts, 40-50km S of Mercy Bay. All reefs contain abundant cavities, skeletal framework and storm-disturbed accumulations of tabulate and colonial rugose corals. Reefs are locally capped by thick platy stromatoporoids, and commonly feature a rich consortium of encrusting calcimicrobes (Renalcis, Rothpletzella, Spongiostroma, etc.). Mudmounds were absent; no reefs were developed in distal slope settings. Shelly and coral faunas show strong similarities with the Old World of Western Europe. Reefs in this area disappeared ca. 1myr prior to the end Frasnian extinctions .