OXYATMOVERSION AT MILLENIAL RESOLUTION ACCOMPANIES YOUNGEST SIDERIAN SNOWBALL EARTH DEGLACIATION

Paleoproterozoic Gowganda Formation of Superior Craton's Huronian Supergroup has long been recognised to expose diverse and exceptional glaciogenic facies and to approximately mark the significant rise of atmospheric oxygen on Earth, with demonstrably primary redbeds only occurring at stratigraphically-higher levels in the Huronian succession. We report several characteristics of Firstbrook Member, Gowganda Formation from a shore-proximal setting, near the northernmost margin of the relatively little-metamorphosed Bruce Mines outcrop subregion, which indicate that significant oxygenation of Earth's terrestrial realm was intimately associated with terminal Gowganda deglaciation and left an advected geochemical record in the shallow marine realm over a short timespan (10-100 kyr).

Unlike Firstbrook Member in deeper, more easterly, and generally more metamorphosed outcrops of the Huronian outcrop belt, near Wakokmata Lake Firstbrook Member is unambiguously glaciogenic, containing dropstones and varves organised into cyclic bundles consistent with multiannual, decadal, and centennial-scale climate forcings. A variety of periglacial facies associations suggest a very nearshore (3-10km), mostly shallow-water depositional setting. A variety of oxidized clasts imply oxygenation of Earth's atmosphere in the nearby terrestrial realm, and primary redbeds are directly associated with terminal deglacial facies, below the level of the well-known, postglacial "purple siltstone" unit. Magnetic susceptibility logging reveals a redox-sensitive metal enrichment spike spanning ~10 kyr within Firstbrook varves, which may correlate to an oxygen-implicating redox metal spike previously recognised elsewhere in the basin in Firstbrook strata of purportedly nonglacial affinity.

While Gowganda Formation remains undated except by igneous cross-cutting relations, potential peperitic textures in high-grade outcrops suggest a similar age and/or only shallow burial by ca. 2.2 Ga. We bolster this inferred relation, but at lower metamorphic grade, by recognising additional peperite of diabase intruding wet, buried Gowganda diamictite near Wakomata Lake, and a vesicular dike altering the first basement outcrop north of the exposed basin margin.