A thin formation of folded mafic flows resting unconformably on Archean basement is informally named the Spi Lake basalt. Stratigraphic, structural, and textural evidence suggests that these lavas were probably fed from a diabase dike swarm dated at 2250+ m.y. by the Geological Survey of Canada. The Spi Lake basalt is overlain by conglomerate that may represent the base of the Hurwitz Group.

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