Organic-walled microfossils from the Proterozoic Thule Supergroup, Northwest Greenland

Abstract

The Proterozoic rocks of the intracratonic Thule Basin on the northern margin of the Canadian–Greenlandic shield have recently been referred to as the Thule Supergroup. The succession comprises unmetamorphosed, unfolded sedimentary and magmatic rocks, which are, with few exceptions, not deformed. Five groups have been defined and fine-grained siliciclastic samples from the uppermost three (Baffin Bay, Dundas and Narssârssuk groups), yielded moderately well-preserved acanthomorphic and non-acanthomorphic acritarchs, prasinophytes and filamentous microfossils. The age of the supergroup is quite poorly constrained, at the base by a 1268 Ma basaltic sill, and at the top by a basic dyke swarm with a K/Ar age range between 630 and 725 Ma, that cuts the entire succession. Biostratigraphical studies further constrain the age of the Thule Supergroup and suggest that the succession is of a late Mesoproterozoic/earliest Neoproterozoic age. This age assignment is corroborated by independent stable isotope data and correlation with the Bylot Supergroup to the south. This leads us to propose that the Thule Supergroup was deposited between c. 1300 and c. 1200 Ma. Microfossil taxa previously interpreted as exclusively occurring in pre-Varangerian, Neoproterozoic rocks (i.e. Simia annulare Jankauskas, Tasmanites rifejicus Jankauskas and possibly Vandalosphaeridium varangeri Vidal) are consequently re-evaluated. Process-bearing acritarchs, herein attributed under open nomenclature to ? Comasphaeridium sp., ` Trachyhystrichosphaera truncata' and ? Vandalosphaeridium sp., are of special palaeobiological importance since acanthomorphic acritarchs have rarely been reported from pre-Neoproterozoic successions.