Sampling and seismic profiling in the Tofino basin west of Vancouver Island have disclosed a thick sequence of Tertiary rocks ranging in age from Eocene to Pliocene. Most of these rocks were deposited in deep-water environments and subsequent uplift has exposed them in many areas. Eocene and Oligocene sediments were deposited in a belt along the present shoreline area off Vancouver Island, whereas Miocene and early Pliocene rocks are present farther seaward. Later Pliocene rocks form a regressive sequence overlapping the older Tertiary in most areas.

Several major periods of deformation resulted in faults, folds, and diapirs on the continental shelf. Deformational patterns show a marked change from north to south. North of Brooks Peninsula, including Queen Charlotte Sound and Hecate Strait, sediments are generally undeformed by folding but are truncated by faults along the steep continental slope. The Kyuquot uplift south of Brooks Peninsula exposed Eocene and Oligocene rocks across the shelf. Farther south, Miocene and Pliocene rocks unconformably overlie the uplift. Folding increases southward, culminating in an area of diapirism off Nootka Sound. Elongate diapirs trend parallel or subparallel with the coastline.

Tectonic features observed on the shelf and slope probably can be explained best by a consideration of the configuration and projected relative movements across spreading centers and along transform faults off the coast of British Columbia. The juxtaposition of differing tectonic styles may be due to the presence of a triple-point junction which has subsequently migrated north.

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