Comparison of 3-D seismic line 215 with Site 1173 physical properties and lithology data suggests a direct correlation between seismic stratigraphy and lithostratigraphy (Fig. F54). Unit I (trench-wedge facies) extends from the seafloor to the strong reflection at 4860 m (core depth of 102.14 mbsf) and is characterized by high-amplitude, laterally continuous reflections. The trench wedge thickens landward and onlaps the seaward flank of the trench. Unit II (upper Shikoku Basin facies) consists of wavy stratifications that are laterally continuous from the base of Unit I to 5108 m (343.77 mbsf core depth). Reflection amplitudes diminish in the lower third of the unit. The base of Unit II is sharp, dips toward the trench, and is angularly unconformable with the underlying unit. Unit III (lower Shikoku Basin facies) has mounded, laterally continuous, moderate-amplitude reflections in the upper part, is reflection free in the middle, and has laterally continuous, low-amplitude reflections at its base. Its thickness varies along strike. Unit IV (volcaniclastic facies) is a very thin, distinct reflection that drapes the basement reflection and varies in thickness laterally.
The Shikoku Basin facies (Units II and III) are deformed by numerous subtle faults (Fig. F54). Individual reflections are offset less than one seismic wavelength (<30-40 m). Faults extend neither to basement nor above the top of the Unit I/II boundary. Fault offsets are predominantly normal, although reverse faults are also present. The faults are traceable laterally up to 700 m in the 3-D seismic data set.