A band of Upper Triassic Vancouver Group, Quatsino Formation limestone up to 1.25 kilometres wide, extends southeastward from the east shore of Upper Quinsam Lake for 6 kilometres to the headwaters of the Iron River, 28 kilometres southwest of the town of Campbell River. The limestone dips to the northeast. It is segmented by a series of north to northwest trending faults. The band is bounded to the northeast by granodiorite of the Jurassic Island Intrusions and clastic sediments of the Cretaceous Comox Formation. Underlying basaltic flows of the Upper Triassic Vancouver Group, Karmutsen Formation outcrop to the southwest. The Argonaut magnetite deposit (092F 075), is hosted in the northwestern end of this limestone band.