Lionel E. Jackson Jr., Geological Survey of Canada (Canada)
David Van Zeyl, Simon Fraser University (Canada)
Reginald L. Hermanns, Norges Geologiske Undersøkelse (Norway)
Douglas Stead, Simon Fraser University (Canada)
Andree Blais-Stevens, Geological Survey of Canada (Canada)
Courtney E. Jermyn, Geological Survey of Canada (Canada)
Multibeam swath bathymetry surveys by the Canadian Hydrographic Service have made it possible to examine the submarine topography of many fiords along the British Columbia coast, including Howe Sound, a broad fiord directly northwest of Vancouver. Hummocky submarine topography up to 275 m off shore of Bowen Island and adjacent to the 720 m high west slope of Mt. Gardner, was identified from multibeam imagery. This was interpreted as a possible landslide run-out feature. Preliminary traverses on adjacent slopes of Mt. Gardner recognized evidence of past and present slope movement. A land and sea investigation was initiated to evaluate the stability of the west slope of Mt. Gardner, and the history and genesis of adjacent sea floor sediments. The marine survey included shore-parallel and -normal seismic reflection profiles, remotely operated vehicle sea floor traverses, and piston coring, in and around areas of hypothesized run-out from terrestrial slope failures.
The terrestrial geotechnical survey documented a number of very likely inactive rockslides < 1 Mm3 in altered volcanic and sedimentary rocks located above persistent mainly bedding-parallel failure surfaces, a few of which daylight in glacially eroded rock bluffs down slope. Antislope scarps up to 10 m high form the upper margins, and transverse joint-controlled gullies mark the lateral margins of most slide masses. Where bluffs exist, tension cracks up to 4 m wide delineate displaced blocks, and abundant talus deposits (some supporting likely > 200 year old cedars) record the collapse and run-out of past rockslides, a few of which extend below the shoreline.
Seismic reflection records from the marine survey show that the area of hummocky submarine topography coincides with chaotic deposits up to several metres thick that have loaded underlying stratified marine mud. These chaotic deposits are capped by less than 1 m of younger sediments. Dredge returns from the chaotic deposits comprised angular to subangular rock fragments likely from the Bowen Island Group. Marine cores basinward from the hummocky topography encountered muddy diamictons containing angular and water-worked clasts entirely from the Bowen Island Group. These have less than 1 m of marine mud capping them. Our working hypothesis, in advance of radiocarbon ages, is that these diamictons are submarine mudflows triggered by the sudden loading of marine mud and local submarine colluvial fans by a subaerial rock slide that entered the sea from the west slope of Bowen Island. Radiocarbon dating of marine mollusc shells from cores will place these events in time.