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doi:10.2204/iodp.pr.322.2009

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 volcanic sand content, Unit II is unique to the Kumano transect area and has been designated middle Shikoku Basin facies. Equivalent deposits were not cored within either the Muroto or Ashizuri transects, and the closest volcanic source is the Izu-Bonin arc. The apparent age of the Unit II/III boundary is ~1 m.y. older at Site C0012 than at Site C0011, but this difference could be due to incomplete recovery. The stratigraphic transition into the lower Shikoku Basin seems to correlate with a similar transition in 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 ~2.5 to ~7.0 Ma. At Sites C0011 and C0012, cessation of both volcanic ash (by air fall) and sand/silt (by turbidity currents) occurred earlier in the basin's evolution during the middle–late Miocene (~9.1 to ~12.8 Ma). Below that interval, the sandy turbidite facies of the lower Shikoku Basin seems to match up with broadly coeval siliciclastic turbidite intervals 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 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, 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 atop 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 coring intervals are missing, the composite stratigraphic succession at Sites C0011 and C0012 (Fig. F36) captures all of the important ingredients that need to be assessed with respect to 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 with respect to details of composition, geotechnical properties, frictional properties, and porosity/permeability.

In addition to material properties, profiles of pore 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 pore 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 diffusion, probably in response to both volcanic ash and basement alteration. In essence, that site represents the 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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