Figure 1: Stratigraphy of the Huronian Supergroup.
(a) Simplified stratigraphy of the Huronian Supergroup1, showing proxy indicators of redox conditions in the atmosphere1,8. The depositional age of the Huronian Supergroup is constrained to between about 2.45 and 2.217 Ga, based on the dating of dyke swarms in basement rocks and the post-Huronian Nipissing diabase, respectively1,2. Detrital uraninite and pyrite and mass-independent fractionation in sulphur (MIF-S) are found in the lower units1,8; whereas, presence of redbeds and disappearance of MIF-S are observed in the upper units1,8. (b) Stratigraphy of the boundary between the Bruce and Espanola formations. The Bruce Formation contains granitic and mafic dropstones of up to cobble size, in a matrix of grey-green sandstone. The boundary is defined as the uppermost occurrence of dropstones.
