A 24 kilometre long band of limestone of the Upper Triassic Quatsino Formation (Vancouver Group) extends northward along the west side of Bonanza River and Bonanza Lake to the Tsulton River, where it is truncated by a northeast trending fault. The limestone is overlain by argillite of the Upper Triassic Parsons Bay Formation (Vancouver Group) and volcanics and sediments of the Lower Jurassic Bonanza Group. Basaltic flows of the Upper Triassic Karmutsen Formation (Vancouver Group) conformably underlie the unit. The sequence strikes north-northwest and dips gently to the west. The limestone is 300 metres thick in the vicinity of Bonanza Lake.

The lower portion of the unit is comprised of white and grey, fine-grained limestone. In the middle, the deposit becomes darker in colour and dolomitic in some beds. The upper portion consists of black limestone with scattered 5 to 15 centimetre thick lenses of black chert. A 61 metre long sample comprised of chips taken at 3.0 metre intervals across mixed layers of black and white limestone just west of the north end of Bonanza Lake analyzed 54.65 per cent CaO, 0.22 per cent MgO, 1.88 per cent insolubles, 0.35 per cent R2O3, 0.09 per cent Fe2O3, 0.006 per cent MnO, 0.03 per cent P2O3, 0.02 per cent sulphur and 42.94 per cent ignition loss (Minister of Mines Annual Report 1968, page 318, Sample 28).

A portion of the deposit was staked in 1989 by Industrial Fillers Ltd. during a search for white limestone.