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Shell Canada Limited geologists and geophysicists have made a study of the stratigraphy of the Tertiary strata in the Tofino and Queen Charlotte basins of Canada's Pacific shelf. Data was obtained from Mesozoic and Tertiary outcrops along the shoreline margins of the basins, 6 Richfield Oil Corporation wildcats on the Queen Charlotte Islands, Shell Canada's aeromagnetic, reflection, and refraction seismic surveys, and 14 offshore wildcats drilled between May 1967 and May 1969.
The pre-Tertiary framework of the shelf consists of a thick and complex sequence of Mesozoic sedimentary, metamorphic, and intrusive and extrusive igneous rocks. Little is known about the early Tertiary history but data from the Tofino basin suggest widespread early-middle Eocene submarine volcanic activity, initial uplift followed by subsidence in late Eocene time, distinct transgressions of Oligocene-early Miocene seas, followed by a middle Miocene period of crustal deformation, uplift, and regression.
There was a major transgression in late Miocene time and a lesser one in early Pliocene time followed by a regressive phase in late Pliocene-Pleistocene time. The early Tertiary volcanism in the Tofino basin spread northward and continued, at least sporadically, in the Queen Charlotte basin to the end of the Miocene and perhaps into the early Pliocene. Tertiary deposition in the Queen Charlotte basin began early in the Miocene and, although interrupted by perhaps 2 periods of uplift and erosion, continued through the Pliocene into the Pleistocene.
The maximum thickness of Tertiary strata is more than 15,000 ft. The strata range from deep-water, open-marine sequences of shale, siltstone, and sandstone in the Tofino basin, through both deep- and shallow-marine deposits in Queen Charlotte Sound, to a thick nonmarine sequence of sandstone, shale, siltstone, and coal in Hecate Strait and the Queen Charlotte Islands. The sandstones in both basins are composed primarily of feldspars and quartz, and those of the Queen Charlotte basin are characterized by high porosity and low permeability values.
There is a wide variety of structural styles including
areas of numerous large anticlines with multiple unconformities and complex growth and fault histories; areas of small, gentle, low-relief anticlines and areas where the Tertiary sediments onlap older volcanic rocks with little or no folding of the strata. Both oil and gas shows have been detected, but no commercial accumulations have yet been found.
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