A triple junction of three lithospheric plates at the British Columbia continental margin has had considerable influence on structure and sedimentation in marginal basins. South of the triple point, compressional forces from subduction of oceanic plate beneath the continent have resulted in deformation mainly by folding and block faulting, but north of the triple point stresses are relieved by strike-slip movement along transform faults with only broad folding taking place in Tertiary sediments.

Tofino basin, south of the triple junction, has undergone major uplift, linear en echelon folding, and elongate diapirism on the outer shelf. More than 12,000 ft of Tertiary mudstone and siltstone has been drilled adjacent to such structures by Shell Canada Ltd. The fine clastics in these distal facies are not conducive to petroleum accumulation. However, potentially productive reservoir beds may exist in the proximal turbidite sequences nearer shore. Subsequent uplift and erosion of the latter also may have resulted in clean second-cycle wedges west of the uplift boundary. In the Tofino basin, as in other areas of the west coast, hydrocarbon prospects appear to have the highest potential in stratigraphic traps.

North of Brooks Peninsula, structural style is dominated by shelf-edge faulting which, west of Queen Charlotte Islands, is transform movement between the Pacific and North America plates. The Queen Charlotte basin has undergone net subsidence of several thousand feet with late Tertiary nonmarine sediments over Tertiary and Mesozoic volcanic basement and Paleozoic intrusives in the north, and late Tertiary marine sediments over Tertiary volcanic rocks in the south. Sediments reach 15,000 ft in thickness. Permeabilities are reduced by silts and clays, but facies changes between interfingering marine and nonmarine depositional sites should make good stratigraphic traps.

Winona basin at the base of the slope is folded only gently and broadly in the north but more highly deformed into prominent sedimentary ridges in the south. Three to six km of sediment fills the deepest point under the base of the slope. The oldest sediments in the flank of the basin are Pliocene. The present deepwater basin undoubtedly has received a high proportion of second-cycle clastic deposits from the uplifted older Tertiary belt.

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