Within the Oyster River to Tsolum River area four formations and numerous members of the Nanaimo Group may be recognized. The basal Comox Formation contains three members. Coals of economic interest are concentrated in the middle Cumberland member. Towards the north of the area a shallow synclinal feature, striking northwest, is observed in the sediments. Coal seams outcrop along higher elevations to the southwest where they dip to the northeast at 10-15 degrees and reach depths of 600 metres in the area of the eastern coastline. To the east, near the coast, the sediments dip at 0-3 degrees. A major wrench fault with lateral and downward movement has been identified along the western edge of the area. Several high angle east-dipping faults have been identified by a high-resolution seismic program and by drilling. The contact of the Nanaimo Group with underlying pre-Cretaceous basement rocks, the Karmutsen Formation, is unconformable and in places is marked by substantial relief.
The presence of four seams has been identified by drillholes in the area. The number 1 seam occurs near the top of the Dunsmuir Member of the Upper Cretaceous Comox Formation. In this area it consists mainly of carbonaceous shale and is 0.5 - 0.8 m thick. The middle Cumberland Member contains seams 2, 3 and 4. The underlying Benson Member consists chiefly of conglomerate and sandstone.
The Cumberland Member consists mainly of carbonceous mudstone and siltstone with thick coal beds and occasional channel-sands. Coals in the Cumberland Member range in thickness from 0.8 m to 3.3 m for seam 2; 1.5 to 3.3 m for seam 3; and up to 1.4 m for seam 4. Seam 4 may be missing entirely. Seams 2 and 3 contain thick shale partings in the thicker seam intervals. In the Oyster River - Tsolum River area the Cumberland Member is 50 to 95 m thick, thickening to the southwest and thinning to the northeast. Thickness of this member is controlled mainly by the paleotopography and by postdepositional erosion at its contact with the overlying Dunsmuir Member.
Samples of coal from outcrop along the Oyster River had float/sink tests performed. A specific gravity of 1.4 produced a coal of less than 10 per cent ash and less than 1 per cent sulfur with heating values over 7,000 cal/gm and yields of about 70 per cent. Positive seam identification was not possible. The number 2 seam from drillcore adjacent to the Oyster River had float/sink tests with a specific gravity of 1.45, ash of less than 10 per cent and sulfur of 1 per cent. Heating value was over 7300 cal/gm (high volatile B bituminous coal) with a yield of 85 per cent, and an F.S.I. of 2 1/2.
