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doi:10.2204/iodp.proc.338.206.2016

Introduction

The Nankai Trough region, offshore southwest Japan (Fig. F1), has been sampled by scientific ocean drilling several times over the past four decades (Karig, Ingle, et al., 1975; Kagami, Karig, Coulbourn, et al., 1986; Shipboard Scientific Party, 1991, 2001). 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; also see the “Expedition 338 summary” chapter [Strasser et al., 2014a]). 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, deposits during the Miocene were enriched in smectite, whereas proportions of detrital illite and chlorite increased steadily during the Pliocene and Quaternary.

Integrated Ocean Drilling Program (IODP) Site C0002 is located near the seaward edge of Kumano Basin (Fig. F1). IODP Expedition 338 extended Hole C0002F by riser drilling through the upper accretionary prism to 2004 meters below seafloor (mbsf) (see the “Expedition 338 summary” chapter [Strasser et al., 2014a]). The sampled interval contains nannofossils that are Miocene in age (>5.59 Ma). The common lithology is clayey siltstone (hemipelagic mudstone), with variable percentages of medium silt to fine sand (turbidites). A lithologic boundary between Units IV and V occurs at 1740 mbsf, as defined by a sharp reduction of sandstone content (Fig. F2). Stratigraphic details are obscured, however, by drilling technology that placed logging tools between a 12¼ inch drill bit at the base of the bottom-hole assembly and a 20 inch reamer bit above. That configuration of two cutting tools resulted in thorough mixing of cuttings over a vertical interval of at least 43.8 m.

One of the goals of NanTroSEIZE has been to document the abundance and hydration state of clay minerals (especially the smectite group) within accreted Nankai strata. That task 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 37 cuttings samples extracted from Hole C0002F. We focus on common clay minerals (smectite, illite, chlorite, and kaolinite) and on indicators of clay diagenesis.

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