Tectonics and Depositional History of the Continental Margin Off Vancouver Island, British Columbia

Abstract

Sampling and seismic profiling in the Tofino Basin west of Vancouver Island show there is a thick sequence of Tertiary rocks ranging in age from late Eocene to Pliocene. The rocks are mainly mudstones containing abundant foraminifera indicating a bathyal depositional environment throughout most of the Tertiary. Subsequent uplift has exposed the deep water sediments on the shelf over much of the area. Eocene-Oligocene sediments occur in a belt along the inner shelf, while Miocene and Pliocene rocks lie seaward of this. Pliocene rocks form a regressive sequence overlapping the older Tertiary, with the greatest thickness in the south.At least two major periods of deformation resulted in faulting, folding, and diapirism on the continental shelf. Deformational patterns show a marked change from north to south. North of Brooks Peninsula sediments are undeformed by folding but are truncated by faulting along the steep continental slope. The Kyuquot Uplift south of Brooks Peninsula exposes Eocene-Oligocene sediments across the shelf. Farther south Mio-Pliocene sediments unconformably overlie the uplift. Folding increases southward culminating in an area of diapirism off Nootka Sound. Elongate diapirs trend parallel or subparallel to the coastline.Tectonic features on the shelf and slope appear to be related to present and earlier configurations of nearby offshore spreading centers, plates, and transform faults. Crustal plate movements may have been responsible for the observed shelf and slope deformations.

Gas hydrate on the northern Cascadia margin: regional geophysics and structural framework

“…The Crescent Terrane provides the landwarddipping backstop to the accretionary prism that has accumulated since the Eocene (e.g., Davis and Hyndman, 1989;Hyndman et al, 1990). The Tofino Basin, containing as much as 4 km of gently deformed Eocene and Holocene sediments, was deposited over the accretionary wedge and the two accreted terranes beneath the continental shelf of southern Vancouver Island (e.g., Tiffin et al, 1972;MacLeod et al, 1977;Dehler and Clowes, 1992). The results of petroleum exploration wells drilled into the shelf in the 1960s were described by Shouldice (1971Shouldice ( , 1973.…”

Interpretation of Gravity and Magnetic Measurements across the Continental Margin of British Columbia, Canada

“…The continental margin off British Columbia is a region of structural transition as evidenced by seismic reflection profiles (Strivastava et al 1971;Tiffin et al 1972;Chase and Tiffin 1972). The regional structure is shown in Fig.…”

Biostratigraphy

“…Assuming a constant envelope of maximum and minimum rates of diatom productivity (tuned to climatic extremes) in this region during the early Miocene through Pleistocene interval suggests that a major tectonic and/or sedimentologic event must have occured simultaneously over a wide area at about the Miocene-Pliocene boundary, diluting the steady-state rain of biogenous sediment with a major influx of terrigenous material. It is now well established that an episode of major tectonic activity commenced around the North Pacific margin during the late Oligocene through mid-Miocene interval manifested by the formation of numerous Neogene marine basins along the Pacific Coast Allison (1964), Smith (1970), Ingle (1967Ingle ( , 1971Ingle ( ,1972Ingle ( , 1973, Asano et al (1969), andTiffin et al (1972). Note that absolute thicknesses are not given on this diagram.…”