At the Benson Mountain occurrence, six coal seams are present in the lower part (the Early Campanian Northfield Member) of the Upper Cretaceous Extension Formation, Nanaimo Group. Only one seam, the Wellington seam, is of any significance. The seam averages 2.56 metres in thickness and the coal is high volatile bituminous A in rank. The seam occurs on Wolf Mountain and has been mined (092F 322) out in the Whisky Lake area. Mining has also been carried out in the McKay Lake area. The roof and floor of the Wellington seam in this area is a coarse sandstone. In addition to the Wellington seam, 5 other seams occur in the area. They are thin and tend to be of poor quality. Rapid lateral thickness changes are common. Refer to Bebans mine (092GSW026) for an explanation on the Wellington Seam in the Nanaimo Coalfield.
Total indicated resources at Benson Mountain (Wellington seam) are 4.768 million tonnes of high volatile bituminous A rank coal; 2.944 million tonnes south of the fault and 1.824 million tonnes north of the fault (Coal Assessment Report 169). The coal is low in sulphur (0.33 to 0.56 per cent), has ash contents ranging from 5.85 to 20.65 per cent, fixed carbon from 46.18 to 57.04 per cent, volatile matter 31.40 to 36.49 per cent and a calorific content ranging from 11,401 to 13,416 BTU per pound. The resource potential of the north block and the remainder of the south block is very limited.
Structurally, the area is characterized by gently dipping (mainly to the east) strata within a number of gently warped and tilted fault blocks. Faults are the most prominent structural elements in the area. They trend northwest and northeast and are generally downthrown to the northeast and southeast respectively. The northwest trending faults are considered to be longitudinal normal faults (Muller and Jeletzky, 1970).
Few folds are present in the area. A large open southeast trending syncline, east of Boomerang Lake and a minor open anticline to the east are the only folds within the north block. In the south block a southeast trending faulted syncline is present in the Whisky Lake area. To the east of the syncline is the large northwest trending extension anticline.
