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Continuous seismic reflection profiling in the Strait of Georgia, British Columbia Tiffin, Donald Lloyd

Abstract

Approximately 790 kilometers of continuous seismic reflection data were obtained with a 5000 joule Sparker in the Strait of Georgia, southwestern British Columbia. The Strait is a geological boundary between Upper Cretaceous Nanaimo Group rocks of the Vancouver Island area and Late Cretaceous-Early Tertiary continental rocks found in scattered outcrops on the southern mainland. Coast Intrusives form mountains on the mainland northeast of the Strait. The Fraser River has built a large submarine delta across the Strait and is the main source of Recent sediments. Deposition is occurring mainly on the delta front and in deep basins to the northwest. In the basin adjacent to the delta, flat-lying bottomset beds average about 200 meters in thickness. An older layer of bottomset beds in this basin overlies bedrock and extends under the present foreset beds. Thinner sedimentary layers of possible hemipelagic origin overlie Pleistocene banks and ridges along the mainland north of the delta. No significant amounts of Recent sediment are presently accumulating in the Strait south of the delta. Erosion of possible Late Pleistocene deltaic sediments has deepened the Strait in that area. Pleistocene deposits of probable drift, till and interglacial sediments occur mainly along the northeast side of the Strait. One extensive stratified deposit, possibly correlated with exposed Pleistocene deposits on nearby shorelines, may reach 550 meters in thickness. Below the Pleistocene, stratified reflectors, suspected to be Late Cretaceous-Early Tertiary bedrock, unconformably overlie Coast Intrusive bedrock along the mainland shore. The reflectors dip seaward at 8 degrees or more. Along the southwest Island coast Upper Cretaceous bedrock dips into the Strait. Deformation, most severe in the south, decreases northward. Dips of bedrock reflectors become less in mid-Strait before disappearing under delta deposits toward the mainland. Some synclinal and anticlinal folding occurs near mid-Strait.

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