Middle carboniferous fusulinacean biostratigraphy, northern Ellesmere Island (Sverdrup Basin, Canadian Arctic Archipelago)

Abstract

The Sverdrup Basin is an elongate pericratonic depression underlying much of the Canadian Arctic Archipelago. The oldest rocks in the basin, nonmarine marlstones that constitute the Viséan Emma Fiord Formation, occur in isolated structural depressions along the southern margin and in the centre of the basin. The Borup Fiord Formation, consisting mainly of marine and nonmarine siliciclastics, was the first widespread sedimentary unit to be deposited following initial rifting and collapse of the basin. At its type section on northern Ellesmere Island, the Borup Fiord Formation contains a thin marine limestone with biseriamminid and asteroarchaediscid foraminifers of Serpukhovian (Chesterian) age. This limestone is the only datable unit within the entire Borup Fiord Formation. Continued rifting and subsidence of the Sverdrup Basin led to predominantly marine conditions, which are recorded at the basin margins by a thick succession of bioclastic limestone belonging to the Nansen Formation. The presence of primitive fusulinaceans from the lower several metres of the Nansen in its type area strongly suggests an early Bashkirian (Morrowan) age. Erosional relief, coupled with biostratigraphic evidence, indicates an unconformable contact between the Borup Fiord and Nansen formations. Successively higher deposits within the lower 350 m of the Nansen are characterized by fusulinacean assemblages that range from Bashkirian through early late Moscovian (early Desmoinesian) time. Fusulinaceans are particularly useful in correlating Middle Carboniferous strata in the Sverdrup Basin with standard reference sections in subArctic North America, Russia, and elsewhere in the Commonwealth of Independent States. The Nansen contains species of primitive pseudostaffellins previously documented from the Bashkirian stratotype on the western slope of the south Urals. A lineage for the origin of the Fusulininae is tentatively proposed. Evolutionary inferences suggest that both pseudostaffellin and fusulinin clades arose in early Bashkirian time from the splitting of plectostaffelloid stock. One new species, Plectostaffella reitlingeri Groves, is described.