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doi:10.2204/iodp.proc.311.110.2006 SummaryNew seismic data from the northern Cascadia margin were acquired from 1999 to 2004 in support of an IODP proposal that resulted in drilling Expedition 311 in 2005. The aim was to study the quantification and systematic changes of gas hydrate formation through a series of sites within the gas hydrate stability field up the continental slope. The data comprise sleeve and air gun data recorded either on the 1140 m long COAMS MCS streamer or on the Teledyne single-channel system. Data processing of the COAMS data included localization of the hydrophone depths and offsets because of the strong curvature of the streamer and an otherwise conventional processing scheme. The Teledyne data processing was similarly simple without localization and offset-dependent processing steps. Some areas were covered with densely spaced seismic lines, thus enabling 3-D binning and data analysis. Our data description focused on six sites, from the trench into and through the gas hydrate stability field up the continental slope. The first site, CAS-04B, lies just off the deformation front outside the hydrate stability field, where the hardly deformed sediments are about to be accreted. Equivalent sediments were drilled already during Leg 146, whereas the remaining sites were drilled during Expedition 311. Site U1326 is located on the frontal ridge where a BSR indicates gas hydrates; the ridge area shows presumably normal fault structures, identified by scarps on the seafloor, with extension parallel to the margin. In the slope basin landward of the seafloor ridge, Site U1325 overlies a buried ridge; adjacent to the drill site is a reflectivity wipe-out zone, and the BSR appears weak. To the northeast, Site U1327 is located between two topographic highs over a strong BSR and acts as a tie hole, as with Site 889. The most landward or end-member site of the transect through the gas hydrate stability field is Site U1329, located over a weak BSR on a steep portion of the slope that is near slumping. Finally, Site U1328 is located at a prominent cold vent, identified in the seismic data as one of several blank zones, but adjacent to a bright spot region where sediment reflectors just above and below the BSR are very strong. |
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