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The Juan de Fuca plate is being subducted obliquely beneath the North American plate along the Washington, Oregon, and northern California continental margins. The Cascadia accretionary prism evolved in response to this oblique subduction. It is composed of folded and faulted abyssal plain turbidites and hemipelagic sediments (Kulm and Fowler, 1974; MacKay et al., 1992). In the northern section of the Cascadia margin, a gas hydrate–related bottom-simulating reflector occurs in a 30 km wide band parallel to the coast beneath much of the continental slope (Fig. F1A). Integrated Ocean Drilling Program Expedition 311 follows Ocean Drilling Program Legs 146 (north Hydrate Ridge) and 204 (south Hydrate Ridge) drilling campaigns in the Cascadia margin area to characterize gas hydrate processes. To constrain the distribution and concentration of gas hydrate in this region, four sites (U1326, U1325, U1327, and U1329) were drilled along a southwest–northeast transect during Expedition 311, from an uplifted ridge near the base of the continental slope (Site U1326) to near the edge of the continental shelf (Site U1329); a fifth site (U1328), representing a cold seep site with active fluid and gas flow, was also drilled (Fig. F1B) (see the "Expedition 311 Summary" chapter). This data report describes the clay assemblages identified by X-ray diffraction of 186 turbiditic sediments, hemipelagic sediments, and authigenic carbonates. The aim was originally to investigate potential modifications of clay mineral assemblages through sites, lithology, depth, and gas hydrate occurrence.