The presence of volcanic ashes and siliceous claystones was used to help define the boundaries of major lithostratigraphic units. The deepest volcanic ash layer with abundant, recognizable volcanic glass shards or pumice was defined as demarking the lowermost bed of the upper Shikoku Basin facies. However, this lithostratigraphic unit boundary is not a sharp diagenetic boundary, since alteration of volcanic ashes is common in the lower part of the upper Shikoku Basin facies. Recognizable glass shards or pumice fragments are present in some of the siliceous claystones in the lower Shikoku Basin facies and the volcaniclastic facies (or volcaniclastic-rich facies; Site 1177). The boundary between the upper and lower Shikoku Basin facies occurs at different depths (Site 1173: 344 meters below seafloor [mbsf]; Site 1174: 661 mbsf; and Site 1177: 401 mbsf) and in sediments of different ages (Site 1173: Pliocene; Site 1174: Pliocene; and Site 1177: late Miocene). The lithologic distinction between the upper and lower Shikoku Basin facies results from differences in ash diagenesis and variability in pyroclastic input (Moore, Taira, Klaus, et al., 2001).
Abundant volcanic ashes are interbedded with hemipelagic clays in the upper Shikoku Basin facies. Less commonly, ashes are interbedded with hemipelagic clays and silt or sand turbidites in the trench-wedge facies. Volcanic ashes with recognizable glass shards are also present in the volcaniclastic or volcaniclastic-rich facies, where they are interbedded with hemipelagic mudstones. Siliceous claystones are interbedded with hemipelagic mudstones and are found in the lower Shikoku Basin facies, the lower part of the upper Shikoku Basin facies, and the volcaniclastic (or volcaniclastic-rich) facies.
Layers of volcanic ash are mostly very thinly bedded (1-3 cm thick) to medium bedded (10-30 cm thick); however, beds may be up to a few meters thick. The basal contacts of beds are sharp and irregular or plane-parallel, and upper contacts are gradational and irregular. Color varies considerably between different ashes and within individual beds. Colors recorded include whites, pinks, grays, and greens. Particle size varies from clay/silt to coarse sand and gravel-grade lapilli. Many of the beds fine upward, although some beds include basal units that coarsen upward before fining upward. Diffuse plane-parallel laminae are present in some volcanic ash beds. The upper gradational top passing into hemipelagic clays is often affected by bioturbation.
Siliceous claystones show a number of similar sedimentological characteristics to the volcanic ashes. Beds are very thinly bedded to medium bedded. Lower bed contacts are sharp and plane-parallel to irregular, and upper bed contacts are gradational and irregular. Although most of the siliceous claystones are gray or green, white and pink beds are also present. Most particles are clay/silt sized, although coarse sand-grade material is rarely present in the base of some beds. Sedimentary structures were mostly absent from the siliceous claystones, although rare faint lamination was observed. A number of the beds had been affected by bioturbation.
Initial results reveal that the volcanic ashes are predominantly vitric or pumiceous tuffs with a rhyolitic to rhyodacitic composition. Vitric tuffs contain variable amounts of secondary components such as pyrite, quartz, feldspar, biotite, or amphiboles and, in some cases, would be classified as crystal-rich vitric tuffs. In the trench-wedge facies and the upper part of the upper Shikoku Basin facies, volcanic ashes are predominantly unaltered. Low amounts of hemipelagic clays intermixed with volcanic ashes are difficult to distinguish from minor alteration of silt/clay-sized glass particles. It is therefore difficult to define the onset of ash alteration, and some of the results where volcanic ashes show between 0% and 30% alteration of clay/silt-size glass particles may be spurious (Tables T1, T2, T3). Diagenetic alteration of volcanic ashes typically increases downhole, on passing deeper into the upper Shikoku Basin facies. Within this "zone" of ash alteration, finer-grained lithologies are typically more diagenetically altered than coarser-grained ashes, and the finer-grained beds may be almost completely altered to siliceous claystones. Partially altered coarse-grained volcanic ashes may show between 30% and 65% alteration of clay/silt-grade glassy particles, 0% to 30% alteration of sand/gravel-grade glassy particles, and remaining glassy particles often have irregular scalloped margins due to dissolution.
Siliceous claystones are composed of opal-CT, clay minerals, and zeolites, as well as some of the secondary minerals found in the unaltered ashes. Clay minerals within the siliceous claystones include smectites and mixed-layer clays, based on preliminary X-ray diffraction (XRD) analysis undertaken on board the ship. Many of the siliceous claystones contain minimal or no recognizable glass particles, and it was only on the basis of their sedimentological characteristics and/or inclusion of secondary minerals that they were recognized as altered ash beds. Siliceous claystones that do contain recognizable glassy particles typically show 65% to 100% alteration of silt/clay-sized glassy material and between 35% and 90% alteration of sand/gravel-sized glassy particles. Any remaining glassy particles tend to have highly irregular, scalloped margins due to dissolution. Feldspar crystals present in the siliceous claystone often show partial or near-complete alteration to clay minerals, particularly along cleavage planes.