THE DECLINE OF LATE DEVONIAN CANADIAN HIGH ARCTIC REEFS PRIOR TO THE END FRASNIAN GLOBAL MASS EXTINCTIONS

Frasnian reefs of the Mercy Fm., Banks Island (73?N latitude, Canadian western high arctic) were developed as part of the Mercy Platform, at four stratigraphic levels (labeled A, B, C, D) within a ca. 300m thick unit of carbonates and siliciclastics. During Late Devonian time (ca. 380Ma), this area was located some 15? north of the equator, on the northwestern flanks of Laurentia, similar geographically today to the Rowley Shoals (NW Australian shelf). The time over which these Banks Island reefs were deposited was about 2 million years. The Banks Island reefs were initiated as a series of smaller patch reefs and reefal mudmounds, constructed on the distal margins of a giant delta complex extending some 1500km westwards from Greenland and Ellesmere islands. Intercalated with reefs, and capping reefs, were dark grey to black siliciclastics carrying abundant woody pteridophyte trunks up to 15cm diameter, some of which also occur within reefal limestones. Reef types include small patch reefs up to 10m diameter and 2-3m thick, grading into large reefs over 1-2 km diameter and 30-70m thick, and reef clusters or tracts spread over tens of km. Reef diversity was low in the region, limited to ca. 30 species of tabulate and rugose corals and stromatoporoids. These are the only Frasnian age reefs in the Canadian arctic, as the 2500km long Middle Devonian reef shelf had vanished. Reefs were lacking in the late Frasnian, buried by encroaching sands and silts from the east due to the rising Caledonian mountains, and restrained by sealevel lowstands. The F/F boundary occurs within wood and spore-bearing terrestrial siliciclastics.