MAPPING THE CANADA/US TRANSBOUDARY MARINE REGION OF GEORGIA BASIN; PRELIMINARY INTERPRETATION

A multibeam swath-mapping program was developed between the Geological Survey of Canada and Moss Landing Marine Laboratories' Centre for Habitat Studies to increase the present knowledge of the seabed in the Canada/US transboundary region of Georgia Basin. The program was initiated in 2001 covering the southern Strait of Georgia, the Gulf Islands and the San Juan Islands. The seabed in these areas has been mapped at a grid resolution between 2 to 5 m permitting interpretation of habitats, surficial geology and geohazards. This paper presents preliminary data and interpretations for the surficial geology and potential geohazards of the area. As of today, 6 units have been used to map the seafloor in the San Juan Islands area. These units include bedrock (Unit 1), pre-Fraser glacial deposits (Unit 2), till/diamict (Unit 3), glaciomarine (Unit4), post-glacial mud (Unit 5) and mobile sand and gravels (Unit6). The complexity and the rugged morphology of the seafloor is the result of the glacial history as well as the dynamic oceanographic environment (measured current velocities in Haro Strait up to 2.23ms-1 in the water column). During the Late Pleistocene, glaciers from the Fraser glaciation reshaped the region, and sediments deposited throughout the Olympia nonglacial interval were excavated, leaving remnant banks formed during the nonglacial to glacial transition at the beginning of the Fraser glaciation. Due to the present strong oceanographic conditions prevailing in the study area, and the relatively low modern sediment inputs, most of the post-glacial sediments found consist of reworked glacial deposit. Geohazards such as sand dune and sand wave fields, indicating high velocity bottom currents, as well as signs of slope instability and active faults, are of concern in a region with a growing population and expanding infrastructure.