Glaciation and post-glacial history of the continental shelf off southwestern Vancouver Island, British Columbia

Abstract

Bathymetry, sediment distribution and seismic stratigraphy are used to interpret the pattern of glaciation of the continental shelf off southwestern Vancouver Island, Canada. Four seismostratigraphic units are distinguished and mapped in relation to sediments and sedimentary rocks present on the shelf. They are: (1) undivided Tertiary bedrock; (2) a unit composed mainly of Pleistocene outwash; (3) Pleistocene morainal deposits; and (4) Holocene marine sediments. A large piedmont glacier is envisaged. Formed by the coalescence of glaciers emanating from Juan de Fuca Strait and Barkley Sound, it cut a system of deep troughs into the Tertiary bedrock, built up a complex of morainal ridges, and built out an extensive outwash plain on the unglaciated shelf to the northwest. During glacial maxima, the piedmont glacier extended right across the shelf. Particularly high discharge of debris from the seaward ends of the glacial troughs may have caused the erosion of adjacent submarine canyons. During the final deglaciation, the ice withdrew somewhat faster than the sea advanced although masses of downwasting ice may have persisted in many of the troughs. The glacial topography including moraines and Tertiary bedrock were levelled by the advancing sea but many of the troughs were preserved from burial, perhaps because they were still ice filled or because the coastal system was sediment starved. Strong currents sweep the present shelf, redistributing sand and mud, winnowed from the tops of the truncated moraines, into low-lying areas and leaving concentrations of gravel behind.