The Nankai Trough is an active clastic-dominated convergent margin where the Philippine Sea plate is being subducted beneath the Eurasian plate. Drilling during Leg 190 targeted two transects (Fig. F1). The Muroto Transect included a reference site (Site 1173) that penetrated the prism's toe (Site 1174) and three sites on the landward slope (including Site 1175). Site 1177 was drilled as the reference site for the Ashizuri Transect.
At Sites 1173 and 1174, we cored five major lithostratigraphic units (Figs. F2, F3) (Shipboard Scientific Party, 2001b, 2001c). The trench-wedge facies consists mainly of thick sand and silt turbidites with interspersed hemipelagic mud. The upper Shikoku Basin facies consists mainly of hemipelagic mudstone and volcanic ash. The lower Shikoku Basin facies consists of hemipelagic mudstone with scattered carbonate-cemented nodules and siliceous mudstone. A volcaniclastic unit of variegated mudstone lies atop pillow basalt (Shipboard Scientific Party, 2001b, 2001c).
We cored three units at Site 1175 (Fig. F4). The upper slope-basin facies consists of hemipelagic mud rich in nannofossils, thin silty and sandy turbidites, and volcanic ash. The middle slope-basin facies consists of muddy sand, thin silty and sandy turbidites, hemipelagic mud, and volcanic ash. The strata below 300 meters below seafloor (mbsf) contain carbonate-poor hemipelagic mud, sandy and silty turbidites, and gravelly mudstone (Shipboard Scientific Party, 2001d).
The uppermost 300 m was not cored at Site 1177, and the topmost unit encountered is the upper Shikoku Basin facies. Beneath that facies are the lower Shikoku Basin hemipelagic facies and the lower Shikoku Basin turbidite facies, where sand, silty sand, and hemipelagic mudstone are interspersed with scattered gravel, mudstone-clast conglomerate, and siliceous and carbonate-cemented claystone. The lowermost sedimentary unit is the volcaniclastic-rich facies, which consists of silty claystone, vitric mudstone, and sandy and silty beds (Shipboard Scientific Party, 2001e).