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doi:10.2204/iodp.proc.322.206.2014

Introduction

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) was designed for the comprehensive understanding of the repeated mega-earthquake zone along the subduction boundary of the Philippine Sea Plate. Integrated Ocean Drilling Program (IODP) Expedition 322 is a part of the second stage of the NanTroSEIZE project. One of the main purposes of the expedition was to characterize incoming sediment and the upper igneous basement prior to their arrival at the subduction front of the Nankai Trough (see the “Expedition 322 summary” chapter [Underwood et al., 2010]). During the expedition, the R/V Chikyu drilled at Sites C0011 and C0012 in the northern part of the Shikoku Basin in the northwest Pacific Ocean. Site C0012 (32°44.888′N, 13°55.024′E, 3510.7 m water depth) is located near the crest of a prominent basement high known as Kashinosaki Knoll (Fig. F1). At this site, ocean floor sediment and igneous basement rocks were recovered with a rotary core barrel (RCB) system from 60–537.81 and 537.81–576 m coring depth below seafloor (CSF-A), respectively. The uppermost 60 m of sediment was jetted and therefore was not collected. The recovered sediment is mainly composed of hemipelagic claystone to siltstone with many intercalating volcanic and sand layers.

The reconstruction of the detailed deformation process of the Nankai Seismogenic Zone requires a combined stratigraphic approach that includes biostratigraphic methods. According to results of the first stage of NanTroSEIZE, sediment from the Pliocene to late Miocene accretionary complex of the subduction zone bears calcareous microfossils including planktonic foraminifers (Ashi et al., 2009; Hayashi et al., 2011). The sediment composing the accretionary prism was originally deposited as continuous ocean-floor sediment of the Shikoku Basin. The onboard study (see the “Site C0012” chapter [Expedition 322 Scientists, 2010]) revealed that the Pliocene to middle Miocene interval of Site C0012 yields planktonic foraminiferal fossils. Therefore, this site has a good potential for establishing a standard biostratigraphy for the middle to late Miocene interval of the Nankai Seismogenic Zone. Thus, the purpose of this study is to construct a planktonic foraminiferal biostratigraphy of Site C0012 as a reference biostratigraphy for the NanTroSEIZE project.

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