Abstract: Offscraped strata within the toe of Nankai accretionary prism display an overall facies pattern of thickening and coarsening upward. Detrital clay minerals within the Quaternary trench-wedge facies are dominated by illite; chlorite is the second-most abundant clay mineral, followed by smectite. Relative mineral percentages change only modestly with depth. The hemipelagic clay-mineral population is virtually identical to clays washed from turbidite matrix, and different size fractions (<2 µm and 2–6 µm) show nominal amounts of mineral partitioning. Smectite content increases beneath the trench-wedge deposits, where the upper subunit of the Shikoku Basin stratigraphy (late Pliocene and early Pleistocene) includes abundance of volcanic ash. Syneruptive, subaerial chemical weathering of volcanic source rocks, together with in situ alteration of disseminated glass shards, caused the increase in smectite. Smectite begins a monotonic transformation to illite/smectite (I/S) mixed-layer clay at ∼555 mbsf and an estimated temperature of ∼65 °C. Ordered (R = 1) I/S interlayering first appears at ∼1220 mbsf (<2 µm size fraction) and ∼1100 mbsf (<0.2 µm size fraction). The illitization gradient coincides with a reduction in pore-water chlorinity, but depth-related changes in bulk mudstone geochemistry (K2O, Rb) are subtle. The absolute abundances of discrete smectite and I/S appear to be insufficient to account for the magnitude of pore-water dilution via in situ dehydration reactions. Instead, pore water probably was transported to Site 808, either from sources located deeper in the accretionary prism, where bulk mudstone porosities are lower, or from strike-parallel sources where mudstones originally deposited in the Shikoku Basin might contain higher percentages of smectite.