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doi:10.2204/iodp.proc.322.101.2010

Summary

Figure F35 shows the provisional correlation of lithologic units between Sites C0011 and C0012. The approximate ages of unit boundaries are also shown, using the respective composite age-depth models for the two sites. In general, there is good agreement between the two data sets. In particular, a close match exists for the lithologic Unit I/II boundary, which marks the base of the upper Shikoku Basin facies. Based on its age and volcaniclastic sand content, Unit II is unique to the Kumano transect area and has been designated herein as the middle Shikoku Basin facies. Equivalent deposits were not cored within either the Muroto or Ashizuri transects, and the closest volcanic source at the time was the Izu-Bonin arc. The stratigraphic transition into the lower Shikoku Basin seems to correlate with a similar transition to a dominantly hemipelagic lithology at other sites in the Shikoku Basin and accretionary prism toe, but the ages are different. At all other localities across the Nankai margin, what has been classified as the upper part of the lower Shikoku Basin consists of monotonous heavily bioturbated hemipelagic mudstone, ranging in age from ~3.5 to ~6.0 Ma. At Sites C0011 and C0012, the hemipelagic facies occurred earlier in the basin's evolution during the middle–late Miocene (~9.1 to ~12.7 Ma). Below that stratigraphic interval, the sandy turbidite facies of the lower Shikoku Basin seems to match up with broadly coeval packets of siliciclastic turbidite that were recovered at Site 1177 along the Ashizuri transect. Detailed petrographic work will be required to quantify petrofacies, compare the detrital provenance, and piece together the regional system of sediment dispersal on both the west and east sides of the basin.

Recovery of basal pelagic deposits in contact with pillow basalt constitutes a major achievement at Site C0012. We know that the age of the basement is older than ~18.9 Ma, based on nannofossil assemblages in the overlying pelagic sediment. Radiometric age dating will be needed to establish the eruptive age of the basalt.

When viewed as a pair of stratigraphic columns, it is clear that the condensed section at Site C0012 displays significant reductions in unit thickness for all correlative parts of the two stratigraphic columns, including the sand-rich intervals (Fig. F36). This is another important discovery. Although relief on the bathymetric high may have been enhanced by inversion at some point in the late Miocene or Pliocene, the basement clearly modulated sedimentation rates throughout the history of the Shikoku Basin. Relief on the seafloor, however, was never high enough to completely prevent the transport and deposition of sandy detritus at the crest of the bathymetric high (Kashinosaki Knoll). Deposition of sandy detritus at Site C0012 may have resulted from thick turbidity currents and/or upslope flow of gravity flows.

Although some important coring intervals are missing, the composite stratigraphic succession at Sites C0011 and C0012 (Fig. F36) captures all of the ingredients that we need to complete an assessment of changes in geologic properties down the subduction zone to seismogenic depth. This composite provides the template upon which all of the postexpedition laboratory results will be placed, particularly the details of composition, fluid chemistry, geotechnical properties, frictional properties, and porosity/permeability.

In addition to material properties, profiles of interstitial water geochemistry for Site C0012 represent the closest we have seen to a true geochemical reference site for the Nankai Trough. Unlike Site C0011, where a freshening trend is obvious in the interstitial water, Site C0012 yields no evidence for significant in situ dehydration reactions or movement of freshened fluids updip to the crest of the bathymetric high (Fig. F37). Instead, chlorinity increases toward the basement because of hydration reactions and diffusional exchange with fluids in the underlying basalt. In essence, that site represents the presubduction geochemical reference site for the Nankai subduction zone, with pore fluids unaffected by the effects of focused flow and diagenesis associated with rapid burial beneath the trench wedge and frontal accretionary prism. These geochemical data, together with the precise fingerprints of isotopic analyses, will be crucial for evaluating the evolution of fluid-rock interactions from the distal reaches of the Shikoku Basin through the frontal accretion zone, and finally into the seismogenic zone.

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