Iodp Expedition 311: Cascadia Margin Gas Hydrates Site U1325 Summary

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Site U1325 (Prospectus Site CAS-02C) is located near the southwestern end of the marginperpendicular transect established during Expedition 311 and is within a major slope basin that developed eastward of the deformation front behind a steep ridge of accreted sediments. Bathymetry data show that the seafloor in the western part of this slope basin is relatively flat with water depths around 2200 m. Around Site U1325 the seafloor becomes gradually shallower before it rises rapidly to the east to form the plateau of the second main accreted ridge at water depths around 1200 m, on which Sites U1327 and 889 are located. A bottom-simulating reflector (BSR) is clearly visible in the eastern part of the slope basin, but it fades to the west (CDP 1180 – 1280 along MCS line 89-08). The BSR also shows the typical frequency dependent reflection strength pattern as observed at all other sites. At Site U1325 the BSR is less strong than at the core of a buried ridge of accreted sediments, which is located about 700 m west of this Site. The BSR appears to be at ~230 ± 5 mbsf based on seismic travel-time depth conversion, using an average P-wave velocity of 1636 m/s. This average velocity was determined at Site 889 to match observed travel-time depths of the BSR and VSP data. We acknowledge that this velocity may not be appropriate in this setting. However, lack of good control in seismic interval velocities from MCS semblance analyses of the lines 89-08 and PGC9902_ODP7 prohibited accurate depth migration.

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تاریخ انتشار 2005