Pockmarks and Associated Shallow Gas, English Bay, Vancouver, Canada
Abstract
A field of more than 200 conical pockmarks, ranging in size from 15 to 100 m in diameter and 5 to 15 m deep, was discovered on the seafloor in water depths between 18 and 65 m during a study of English Bay, Vancouver, B.C. Forming the entrance to the Port of Vancouver, the bay is the site of intense commercial and recreational shipping activities and the pockmark field is a potential marine hazard. The study comprised multibeam and sidescan sonar surveys, high-resolution seismic profiling as well as core and water sampling. Whereas the majority of pockmarks are randomly distributed, some form linear chains, raising the possibility of an association with bedrock control. High resolution seismic and 3.5 kHz profiles reveal that the pockmarks formed at the top of a thick, transparent postglacial mud deposit that contains shallow gas. Piston cores taken from areas where the gas is located within 5 m of the seabed contain up to 9.5% by volume of methane. Gas is absent in some areas of highest pockmark density, suggesting that it has been efficiently vented through the pockmarks. A repeat multibeam survey showed no significant changes in pockmark distribution or dimensions over a one-year period and no active venting was observed during acoustic and video surveys, suggesting that the pockmarks must form intermittently. Given the high sedimentation rate in the bay, it is not likely that the pockmarks would remain open for long periods. It is hypothesized that the venting occurred in the recent geological past as a result of sediment liquefaction during an earthquake event, possibly the last mega-thrust earthquake of 1700 AD.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFMOS32A..03H
- Keywords:
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- 3004 Gas and hydrate systems;
- 3022 Marine sediments: processes and transport;
- 3045 Seafloor morphology;
- geology;
- and geophysics