ORGANIC-WALLED MICROFOSSILS FROM THE ~1 GA BYLOT SUPERGROUP, BAFFIN ISLAND

The Mesoproterozoic–Neoproterozoic transition is marked by gradual oxygenation of shallow marine environments, tectonic upheaval, and the radiation of crown-group eukaryotes, yet the number of studied stratigraphic units is limited. The Bylot Supergroup, exposed in the Borden Basin, Arctic Canada, provides a key archive of paleontological information from this time.

To evaluate the diversity of early eukaryotes in this poorly studied interval, we have conducted high-resolution paleontological and paleoenvironmental analyses of 69 samples. Well-preserved microfossils occur throughout more than 1000 m of latest Mesoproterozoic siliciclastic and carbonate rocks. Recent Re-Os radiometric ages constrain the maximum age of the middle Bylot Sprgp. to c. 1050 Ma. Organic-walled microfossils (OWM) were extracted from the rock via palynological maceration in hydrofluoric acid and studied with various microscopic techniques (TLM, SEM). The Arctic Bay, Iqqittuq, and Angmaat formations contain diverse microfossils (by Mesoproterozoic standards) of prokaryotic and eukaryotic affinities, including Culcitulisphaera revelata, Fabiformis baffinensis, Microlepidopalla mira, Satka favosa, Squamosphaera colonialica, various leiosphaerids and symplassomorphs, and several new form-species. OWM show complex morphologies including various sculpture patterns, organic scales, and processes. Some morphotypes appear ~200 myr earlier than previously recorded. The stratigraphic range of the characteristic late Tonian C. revelata–M. mira association is hereby extended into the terminal Mesoproterozoic.

Marine mudstones of the Arctic Bay Fm. represent the deeper part of the basin, and yield a moderate OWM diversity consisting predominantly of leiosphaerids. Shales interbedded with dolostones of the overlying Iqqittuq Fm. represent a shallow to outer ramp setting and host the most diverse OWM assemblage. The nadir of OWM abundance and diversity co-occurs with a positive δ¹³C_carb excursion (-0.3 to +4‰) in the Iqqittuq Fm.

The new high-resolution biostratigraphic record provides a valuable tool for the correlation of the Bylot Supergroup with late Mesoproterozoic–early Neoproterozoic successions elsewhere in North America, as well as in Congo, Mauritania, and Siberia.