TECTONO-STRATIGRAPHY AND SEISMIC CHARACTER OF SUBDUCTING SHIKOKU BASIN SEDIMENTS, IODP EXPEDITION 322

IODP Expedition 322 drilled Sites C0011 and C0012 near Kashinosaki Knoll as a seaward extension of the Kumano transect, SW Japan, to document the incoming sedimentary strata and igneous basement prior to arrival at the subduction front. These sediments accumulated in the Shikoku Basin, formed by early to middle Miocene seafloor spreading along the backarc of the Izu-Bonin island arc. Combining both drill sites, 6 lithologic units were identified. Starting at the top, the upper Shikoku Basin deposits (Unit I) are Holocene to late Miocene, and consist of hemipelagic mud with thin interbeds of volcanic ash. The middle Shikoku Basin deposits (Unit II) are a previously unrecognized late Miocene unit of hemipelagic silty claystone with interbeds of volcaniclastic turbidites and minor mudstone turbidites. These deposits were likely supplied from the Izu-Bonin arc 8 Ma. Seismic Line 95 from the IFREE mini-3D seismic survey is interpreted to show channelized submarine fans, particularly in Unit II, that appear to preferentially display lateral offset-stacking away from the crest of Kashinosaki Knoll. Core and logging-while-drilling data support the interpretation of channelized amalgamated sandstones with fining-upward trends. The lower Shikoku Basin hemipelagic deposits (Unit III) are middle to late Miocene in age, as mainly hemipelagic silty claystone. Biostratigraphic and paleomagnetic data suggest an abrupt change in the rate of hemipelagic sedimentation within these deposits, coincident with the cessation of anomalous forearc volcanism at ~ 11 Ma. The lower Shikoku Basin turbidite deposits (Unit IV) are middle Miocene in age, and mainly hemipelagic silty claystone with abundant interbeds of mudstone and sandstone turbidites. These older turbidites show a mixed detrital provenance and broadly correlate with quartz-rich Miocene turbidites on the western side of the Shikoku Basin. Lithologic Unit V, volcaniclastic-rich deposits, of early to middle Miocene age, is mainly hemipelagic silty claystone alternating with tuff beds and volcaniclastic turbidites. Unit VI comprises a thin interval of mainly red pelagic claystone in contact with pillow basalt and yields a minimum age of 18.9 Ma. Petrography and geochemistry suggest that the igneous basement is typical Shikoku Basin highly altered basalt.