On average, smectite is the most abundant mineral in the clay-sized fraction at Sites 1173 and 1177, followed by illite and chlorite (+ kaolinite). At Site 1174, illite is the most abundant clay mineral, followed by smectite and chlorite (+ kaolinite). The smectite is dioctahedral, which is consistent with its origin as an alteration product of silicic volcanic rock and ash.
Data from Sites 1173, 1174, and 1177 show downhole increases of smectite in Shikoku Basin deposits. Part of this increase was caused by changes in detrital influx. The transfer of smectite to Shikoku Basin and Nankai Trough was higher during the Miocene and decreased progressively through the Pliocene and Quaternary. In situ alteration of disseminated volcanic glass added even more authigenic smectite to the clay assemblage as burial depths and temperatures gradually increased.
Data from Sites 1173 and 1174 show downhole depletions of smectite in the lower Shikoku Basin deposits over the same depth ranges as increasing percentages of illite in I/S mixed-layer clays. An absence of this reaction at Site 1777 is consistent with its lower geothermal gradient. A kinetic model of illite-smectite reaction progress matches the diagenetic profile for Site 1173 fairly well using the present-day values of temperature, burial time, and potassium availability. Modeling indicates that rapid burial and/or tectonic thickening at Sites 1174 and 808 caused illite-smectite diagenesis to lag behind the prevailing temperature conditions by 15%-20%.
The maximum abundance of bulk smectite in mudstones from Site 1173 is ~35 wt%; at Site 1174 the maximum reaches nearly 30 wt%. Most such values, however, are <25 wt%. Mudstones from Site 1177 (Ashizuri Transect) are significantly different because bulk smectite consistently reaches 30-50 wt%, especially in the lower Shikoku Basin. This amount of smectite should be enough to lower the coefficient of friction of the sediment relative to coeval deposits in the Muroto Transect. Smectite probably affects mudstone compressibility and permeability at all three sites.