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doi:10.2204/iodp.proc.348.202.2016

Introduction

Scientific ocean drilling has targeted the Nankai Trough region, offshore southwest Japan (Fig. F1), several times over the past four decades (Karig, Ingle, et al., 1975; Kagami, Karig, Coulbourn, et al., 1986; Shipboard Scientific Party, 1991; Shipboard Scientific Party, 2001; Moore et al., 2005). The latest drilling efforts are part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) (Ashi et al., 2009; Screaton et al., 2009; Tobin et al., 2009; Underwood et al., 2010; Expedition 333 Scientists, 2012). Previous investigators demonstrated that hemipelagic mud(stones) throughout the Nankai region change in composition largely as a function of depositional age (Cook et al., 1975; Chamley, 1980; Chamley et al., 1986; Underwood et al., 1993a, 1993b; Masuda et al., 1996, 2001; Steurer and Underwood, 2003; Underwood and Steurer, 2003; Guo and Underwood, 2012; Underwood and Guo, 2013). As a rule, Miocene strata throughout the region are enriched in smectite, whereas Pliocene and Quaternary deposits contain more detrital illite and chlorite.

Integrated Ocean Drilling Program (IODP) Site C0002 is located near the seaward edge of the Kumano Basin (Fig. F1). IODP Expeditions 338 and 348 used riser drilling at Site C0002 to reach intermediate depths of the accretionary prism (Strasser et al., 2014; see the “Expedition 348 summary” chapter [Tobin et al., 2015a]). The interval that was cored during Expedition 348 extends from 2163 to 2218 meters below seafloor (mbsf) (Fig. F2). These are the deepest cores ever recovered from an accretionary prism. Beds within that interval dip steeply, generally at angles of 60°–90°. The common lithology is clayey siltstone (hemipelagic mudstone) with thin interbeds of medium silt to fine sand (turbidites) and black bands that were initially thought to contain organic matter. Scanning electron microscopy, coupled with energy dispersive spectroscopy, subsequently showed that the black bands are composed of pyrite (see Song and Underwood, submitted). The cored interval contains nannofossils that are Miocene in age (9.56–10.73 Ma) (see the “Site C0002” chapter [Tobin et al., 2015b]).

One of the broader goals of NanTroSEIZE has been to document the abundances and hydration state of clay minerals (especially the smectite group) within accreted Nankai strata. This is important because of the clay’s likely influence on fluid production within the accretionary prism, as well as along the landward-dipping plate interface (e.g., Saffer et al., 2008). This report summarizes the results of X-ray diffraction (XRD) analyses of 23 core samples extracted from Hole C0002P. We focus on the common clay minerals (smectite, illite, chlorite, and kaolinite) and provide compositional data for whole-round (WR) specimens that other scientists collected for shore-based studies of hydrogeological, frictional, and geotechnical properties.

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