Stratigraphy of the Steep Rock Group, northwest Ontario: a major Archaean unconformity and Archaean stromatolites

Abstract

The Archaean Steep Rock Group of northwest Ontario, situated in the Wabigoon Subprovince of the Superior Province, Canada, comprises five formations: Wagita Formation (clastics), Mosher Carbonate, Jolliffe Ore Zone, Dismal Ashrock, and Witch Bay Formation (metavolcanics). Reinvestigation of the geology of the group has shown that the basal clastics of the Wagita Formation (0–150 m) unconformably overlie the Marmion Complex (a massive tonalite – tonalite gneiss terrane, 3 Ga old). Overlying the basal elastics is the Mosher Carbonate (0–500 m), containing diverse stromatolite morphologies. Extensive zones of carbonate breccia occur adjacent to fault zones and mafic dykes. Stratigraphically above the Mosher Carbonate is the Jolliffe Ore Zone (100–400 m), which is divided into a lower Manganiferous Paint Rock Member and an upper Goethite Member. Within the Jolliffe Ore Zone thin layers of "Buckshot Ore" occur. These are horizons of haematitic pisolites and fragments, set in a lighter ferruginous matrix of kaolinite and gibbsite. Overlying the Jolliffe Ore Zone is the Dismal Ashrock, a dominantly high-Mg pyroclastic rock (22% MgO) with minor interbedded lava flows (15% MgO). In contact with the Dismal Ashrock are the metavolcanics of the Witch Bay Formation. This juxtaposition is not exposed in the Steep Rock mine section, and the Witch Bay Formation may be separated from the Dismal Ashrock by a structural break. The Witch Bay Formation is only provisionally included in the Steep Rock Group.The group is interpreted as a sequence deposited in an extensional or rifting environment. The unconformity has regional significance, and it may be possible to define an extensive cratonic nucleus of 3 Ga or older age in northwest Ontario.