No abstract

The Nankai Trough Seismogenic Zone Experiment is a multiexpedition project carried out in several stages by the Integrated Ocean Drilling Program. The first stage of the experiment included two cruises during which coring took place (Expeditions 315 and 316). We have studied grain size distribution in more than 600 specimens of cores from Sites C0001, C0002, C0004, and C0006-C0008 and correlated the distributions to the lithologies recovered, logging and logging-while-drilling data in nearby holes, and the overall tectonic setting. The two sites in the upper slope and Kumano Basin (Sites C0001 and C0002, respectively) show the smallest mean grain sizes, which are smaller in the accreted strata compared to the forearc basin or slope sediments. Further downslope (Sites C0004 and C0008), grain size variability is larger and lithologies range from slope apron, mass transport deposits, accreted mud/mudstones, and fault zone material, to underthrust hemipelagics. Surprisingly, the mass transport deposits at Site C0004 are fine grained, whereas their counterparts at Site C0008 slightly downslope are the coarsest within the recovered succession there. The two sites near the toe of the accretionary prism (Sites C0006 and C0007) provided the coarsest material recovered, namely the trench-slope transition facies and the uppermost accreted trench wedge facies. Mudstones from the underlying upper Shikoku Basin facies are well sorted, uniform, and fine grained. In general, the study provides a solid basis for more specialized sedimentological research and further reveals that (1) results of the measurements taken with the Coulter Counter LS200 laser particle analyzer are in good agreement with earlier onboard visual core description; (2) accuracy and reproducibility of the method is satisfactory given the very narrow error bars of repeat measurements on aliquots of a larger sediment sample; and (3) the mass transport deposits have distinct particle size distribution patterns, whereas fault gouge and other material from structural features cannot easily be distinguished from surrounding sediment by grain size analysis alone.