Scanning electron microscope analyses of sediment samples collected from the frontal portion of the Nankai accretionary prism and Shikoku Basin during Ocean Drilling Program Leg 190 show systematic changes in clay microstructure and microfabric that correlate with depth, lithology, sediment deformation, and changing physical properties. Heterogeneous, open sediment textures found in the shallower lithologies, such as the upper Shikoku Basin facies, are characterized by random distributions of coarse particle aggregates and fine-grained clay minerals distributed throughout the matrix. The degree of clay mineral preferred orientation (CPO) generally increases with depth and adjacent to deformation structures but is retarded in an anomalous zone of high porosities at the reference Site 1173. Clay microstructures in and below the anomalous zone are very similar and provide little evidence that clays serve as the cementing agent. Deeper sediments, including most of the lower Shikoku Basin facies, exhibit more homogeneous distributions of dispersed and uniformly sized clay particles and commonly exhibit a high degree of CPO. The transition in texture appears to occur at the protodecollement horizon at the reference Site 1173 and slightly shallower in the section within the protothrust zone. The coincidence of this transition with noticeable downhole decreases in smectite abundance and compressional wave velocity (Vp) at Site 1173 highlight an important diagenetic boundary that may help to localize slip along the accretionary decollement as it propagates seaward.