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The main scope of Integrated Ocean Drilling Program Expedition 311 was to evaluate gas hydrate formation models in subduction zone accretionary prisms (“Expedition 311 summary” chapter, this volume). For this purpose a transect of four sites (from southwest to northeast, Sites U1326, U1325, U1327, and U1329) was cored through the northern Cascadia margin off Vancouver Island, representing different stages in the evolution of gas hydrate formation in an accretionary prism from its most recent stage at the deformation front in the southwest (Site U1326) to its final stage in shallower waters in the northeast (Site U1329). Additionally, a cold vent (Site U1328) with active fluid flow was investigated.
Corresponding to the tectonic setting, a vertical and lateral highly variable lithology was cored, with frequent intercalation of turbiditic sediments and extensive active faulting near the deformation front (Davis and Hyndman, 1989). Both turbidites and faulting are important for the gas hydrate distribution in the sediment column, whereas faults or fractures act as a fluid conduit; as proposed especially for Site U1328 (“Expedition 311 summary” chapter, this volume; Riedel et al., 2006), coarse-grained turbidites may provide the pore space needed for gas hydrate accumulation. Qualitative assessment of the bulk mineralogy could therefore give additional information about gas hydrate formation processes and might be useful for the study of authigenic carbonates that formed associated with gas hydrates (e.g., Bohrmann et al., 1998).