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doi:10.2204/iodp.proc.314315316.206.2011

Introduction

Integrated Ocean Drilling Program Expedition 315 was conducted to investigate detailed physical properties and stratigraphy at two pilot sites for future riser drilling in the Nankai Trough Seismogenic Zone off the Kumano region, Kii Peninsula, southwest Honshu, Japan (see the “Expedition 315 summary” chapter [Ashi et al., 2009]). During the expedition, the D/V Chikyu drilled seven holes at Site C0001 and two holes at Site C0002. Site C0001 is located at the slope basin on the Nankai accretionary prism (33°14′N, 136°42′E; 2198 m water depth) (Fig. F1). At Site C0001, ocean bottom sediments to 458 m coring depth below seafloor (CSF) were obtained using the hydraulic piston coring system, extended shoe coring system, and rotary core barrel system. With respect to onboard visual description of the obtained cores, the lithology is divided into two units: Unit I is Quaternary slope apron deposits, and Unit II is Miocene to Quaternary accretionary prism sediments. These sediments are mainly composed of hemipelagic silty clay to clayey silt with many intercalating ash and sand layers. The boundary between the units is unconformable and marked by a basal sand layer.

To reconstruct the detailed deformation history of the NankaiTrough Seismogenic Zone, we need a combined stratigraphic approach including biostratigraphic methods. In previous drilling of the Nankai area at the Ashizuri and Muroto transects, ~200–250 km westward of the present site (Fig. F1), the stratigraphic framework of the accretionary prism was mainly established by calcareous nannofossil biostratigraphy (Deep Sea Drilling Project Legs 31 and 87 and Ocean Drilling Program Legs 131 and 190) (Karig, Ingle, et al., 1975; Kagami, Karig, Coulbourn, et al., 1986; Hill, Taira, Firth, et al., 1993; Moore et al., 2005). However, because of dissolution processes beneath the calcium carbonate depth (CCD), planktonic foraminifers are poorly preserved and sparse at these sites (Ujiié, 1975; Lagoe, 1986).

According to onboard study during Expedition 315, sediments from Site C0001 bear calcareous microfossils including planktonic foraminifers (see the “Expedition 315 summary” chapter [Ashi et al., 2009]). Therefore, this site is suitable for establishing a standard biostratigraphy for the Nankai Trough Seismogenic Zone. In addition, continuous foraminiferal data from the present site have a great potential for aiding paleoceanographic reconstruction of the Western Boundary Current of the northwestern Pacific Ocean (the Kuroshio Current; Fig. F1). The purpose of this study is to construct a detailed planktonic foraminiferal biostratigraphy of Site C0001 as a reference biostratigraphy for the Nankai Trough Seismogenic Experiment (NanTroSEIZE) project.

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