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The Coppermine River Group was formed when vast volumes of basaltic lava paved over a large area of the northwestern Canadian Shield about 1,267 million years ago. These basalts form flood basalts that reach thicknesses ranging from to and consists of about 150 lava flows, each about 10–25 m thick. Except for the lowermost flows, which contain evidence of interaction with water, the entire sequence was erupted subaerially. Eruption of plateau lavas near the Coppermine River, built an extensive volcanic plateau about 1,200 million years ago with an area of about representing a volume of lavas of at least . These flood basalts have been interpreted as contemporaneous with the giant Mackenzie dike swarm and with the Muskox intrusion.