Distribution, geometry and palaeogeography of the Frasnian (Late Devonian) reef complexes of Banks Island, NWT, western arctic, Canada

Abstract

Abstract Following the collapse of the >2000 km long Givetian (Middle Devonian) Inuitian/Ellesmere carbonate platform factory in arctic Canada, within the 0° to 10° equatorial palaeolatitudes north, the only Frasnian reefs in high arctic Canada retreated westwards, confined to northeastern Banks Island. These reefs, numbering well over 130, and dominated by corals and stromatoporoid sponges, were spread over c . 5000 km 2 , within the 220 m thick Mercy Bay Formation. Reefs were developed at four different stratigraphic levels (termed the A, B, C and D levels) during early and middle Frasnian time, periodically smothered by intervening siliciclastics during sea-level lowstands, and were finally buried by thick siliciclastic sands, silts and muds derived from the east during the late Frasnian. The Frasnian–Famennian boundary is masked within continental, deltaic facies bearing plant remains. The Banks reef and carbonate succession is preserved as horizontally stratified, in situ limestones, with the succeeding reefs backsteppping towards the east, in response to cyclic Frasnian sea-level rises. The Banks reefs were developed on the distal lobes of a megadelta, periodically covered by extensive lowland forests that stretched more than 2000 km west from Greenland and Ellesmere Island. Reefs are three-dimensionally preserved, unaffected by diagenesis, dolomitization, major tectonics, vegetative cover or glaciation, and thus display some of the finest, pristine Late Devonian reef complexes known. The reefs represent a range of geometries, from small circular to oval patch reefs a few tens of metres in diameter and <5–10 m thick, to larger prominent tabular (‘reef platform’ style) and domal reef structures >2 km across, or 300–400 m diameter, and 40–60 m thick. Coral and stromatoporoid-rich biostromes also formed a significant part of the reef seascape. Morphology of the reefs leads to an estimation of penecontemporaneous relief of 10–20 m above the prevailing sea floor, with reef-core facies generally dominated by stromatoporoid sponges, and flank facies by a variable mix of colonial tabulate and rugose corals, as well as stromatoporoids. Bryozoans and calcimicrobialites were rare, in contrast to other Late Devonian reefs such as those of the Canning Basin (Australia), or the Guangxi platform (South China); mudmound reef facies, such as seen in Belgium were limited.