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doi:10.2204/iodp.proc.338.205.2016

Introduction

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) was designed to investigate the fault mechanics and seismogenesis of the repeated mega-earthquake zone along the subduction boundary of the Philippine Sea plate. Site C0002 is the centerpiece of this project to access the seismogenic zone beneath the Kumano forearc basin. Integrated Ocean Drilling Program (IODP) Expedition 338 was conducted as part of the third stage of the NanTroSEIZE project. The main purpose of the expedition was to extend and case riser Hole C0002F from 856 to 3600 m core depth below seafloor (CSF-A). However, because of damage to the riser system under severe weather conditions during the expedition, riser operations were canceled. Contingency riserless coring at Site C0002 was conducted to sample the upper part of the forearc basin sediment (see the “Expedition 338 summary” chapter [Strasser et al., 2014a]).

Precise stratigraphic correlation is required to reconstruct the deformation processes of the Nankai Seismogenic Zone. Planktonic foraminiferal biostratigraphy is a powerful tool for correlating marine strata. In previous NanTroSEIZE expeditions, planktonic foraminifers provided stratigraphic datums (e.g., Hayashi et al., 2011, 2014). For Site C0002, 14 biohorizons were recognized in Hole C0002B from 478.28 to 1052.40 m CSF-A during Expedition 315 (Expedition 315 Scientists, 2009). For Hole C0002D of Expedition 315, detailed chemostratigraphy was constructed from 0 to 100 m CSF-A, reaching marine isotope Stage (MIS) 23 (Matsuzaki et al., 2015).

During Expedition 338, the D/V Chikyu drilled at Site C0002 (33°18′N, 136°38′E; 1937.5 m water depth) in the southern part of the Kumano forearc basin in the northwest Pacific Ocean (Fig. F1). In Holes C0002K and C0002L, a total of 35 cores were cut from 200 to 505 m CSF-A. Two of those cores were recovered with the hydraulic piston coring system (HPCS), four with the extended punch coring system (EPCS), and 29 with the extended shoe coring system (ESCS). The average recovery for the holes was 69% and 79%, respectively. This coring interval was chosen to provide data within the gap (200–500 m CSF-A) between Holes C0002D and C0002B.

According to onboard stratigraphic analyses, Holes C0002K and C0002L are constrained in age between 1.34 and 0.903 Ma and include the Jaramillo Subchron (0.988–1.07 Ma) in the upper part (Fig. F2). Onboard geochemical analyses detected a gas hydrate zone (200–400 m CSF-A) and a minimum concentration zone of salinity, chlorinity, and Na⁺ in pore water (300–500 m CSF-A) resulting from dissociation of methane hydrate (see the “Site C0002” chapter [Strasser et al., 2014c]). The purpose of this study is to describe the planktonic foraminiferal assemblages obtained from Holes C0002K and C0002L as a basic data set for the stratigraphy of the Kumano Basin region.

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