REVISITING THE ~2.0 GA BELCHER GROUP WITH HIGH-RESOLUTION CARBON ISOTOPE CHEMOSTRATIGRAPHY AND ZIRCON GEOCHRONOLOGY

The Belcher Group is a 7-10 km thick succession deposited ~2.0-1.8 Ga along the margin of the Superior Craton. It contains a rich sedimentary record of this time interval, including thick, well-preserved carbonate units, and the first cyanobacteria fossils. Despite this, the Belcher Group has been hardly studied since the 1980s, and its geochemistry remains unexplored. Here, we present stratigraphic sections measured through the entirety of the Belcher Group, two new zircon U-Pb age constraints, and a high-resolution carbon isotope curve. The Belcher Group comprises seven depositional stages: a shallow subtidal-intertidal transgressive carbonate platform, a flood basalt phase, a second transgressive carbonate platform buildup, progradation to a restricted basin, a thick submarine basalt phase, a turbidite flysch, and distal molasse. Two new zircon age constraints are presented: a maximum depositional age of 2123.5 ± 1.1 Ma from detrital zircons in the lowermost Belcher Group, and a depositional age of 1855.2 ± 1.3 Ma from a tuffaceous shale at the base of the turbidite flysch. The carbon isotope record from Belcher Group carbonates spans a range from -2 to +3‰, and records fluctuations up to 2‰ superposed on a trend toward isotopically heavier values up-section. This shift toward heavier δ¹³C values is distinct from the carbon isotope record of the subsequent “boring billion,” an interval generally characterized by small fluctuations around 0‰, and may be related to the assembly of the supercontinent Nuna. Critically, the absence of high δ¹³C values in the lowermost Belcher Group constrains it to younger than ~2060 Ma. Combined with new age constraints presented here, this places the Belcher Group in a global stratigraphic framework between two extremes in Earth history, the Lomagundi-Jatuli Excursion and the “boring billion”.