The Sturgeon Lake Caldera contains volcanic units that outcrop over from east to west with up to five separate, major ash flow tuff units with thickness ranging from to . The Mattabi pyroclastic flow, with a thickness in excess of and a strike length of at least , is the third and most voluminous eruptive event associated with the Sturgeon Lake Caldera. It hosts the 12-Mt Mattabi massive sulfide deposit which is interpreted to have formed on and below the seafloor, the latter through the processes of pore-space filling and replacement.