Seismic Observations of a Possible Carbonate Mound on the Continental Slope off Vancouver Island

A large carbonate or mud mound was identified about 5 km west of ODP Site 889 on the Vancouver Island continental slope. At its base, the mound is ~1 km wide perpendicular to the margin and ~2 km long parallel to the margin. In dives with the remotely-operated submersible ROPOS in May 2001, we observed widespread carbonate pavement was observed which made it impossible to collect sediment push cores. Piston coring also failed to penetrate the pavement or collect sediment samples. ROPOS also observed several biological communities of tubeworms and clams, indicating that venting of methane was likely active. To interpret the structure of this carbonate mound, a high resolution single channel seismic survey was carried out in summer 2001. The source was a 40 cu.in. sleeve airgun, fired by distance at an interval of 12.5 m. The grid consisted of 80 lines, each 3 km in length and separated by 25 m. The seismic lines, recording coherent reflectivity down to about 400 m beneath the seafloor, provide excellent images of this carbonate/mud mound and of the BSR beneath it. The single channel data form a pseudo-3D single channel seismic grid with 25 m,e 25 m square bins, and so we can use 3D migration techniques to image the complex structures. The mound is bounded on its landward and seaward sides by faults that dip toward Vancouver Island. The faults appear to connect to a particularly strong section of BSR, which may trap locally large concentrations of gas. The faults intersect the surface on either side of the mound, and may provide pathways for methane and other fluids from the gas reservoir to vents at the surface.