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doi:10.2204/iodp.proc.338.105.2014 Site C00181M. Strasser, B. Dugan, K. Kanagawa, G.F. Moore, S. Toczko, L. Maeda, Y. Kido, K.T. Moe, Y. Sanada, L. Esteban, O. Fabbri, J. Geersen, S. Hammerschmidt, H. Hayashi, K. Heirman, A. Hüpers, M.J. Jurado Rodriguez, K. Kameo, T. Kanamatsu, H. Kitajima, H. Masuda, K. Milliken, R. Mishra, I. Motoyama, K. Olcott, K. Oohashi, K.T. Pickering, S.G. Ramirez, H. Rashid, D. Sawyer, A. Schleicher, Y. Shan, R. Skarbek, I. Song, T. Takeshita, T. Toki, J. Tudge, S. Webb, D.J. Wilson, H.-Y. Wu, and A. Yamaguchi2Background and objectivesDuring Integrated Ocean Drilling Program (IODP) Expedition 338, a slope basin seaward of the megasplay fault was logged with logging while drilling (LWD)/measurement while drilling (MWD) at Site C0018. The slope basin, characterized in 3-D seismic data by stacked mass transport deposits (MTDs) (Strasser et al., 2011) was drilled and sampled during IODP Expedition 333 in Hole C0018A (Fig. F1) (Expedition 333 Scientists, 2012a). The primary goals of operations at Site C0018 were to establish the stratigraphy of Quaternary mass-movement events and to sample the distal part of an exceptionally thick MTD for analyzing its rheological behavior to constrain sliding dynamics and tsunamigenic potential. Site C0018 (proposed Site NTS-1A; water depth = 3084.35 m) is located ~5 km southwest of IODP Sites C0004 and C0008, which were drilled and cored during IODP Expedition 316 (Screaton et al., 2009) (Figs. F1, F2). Site C0018 is located within a lower slope basin that (1) represents the depocenter for downslope mass transport, (2) is characterized by stacked MTDs that were seismically identified as acoustically transparent to chaotic bodies with ponded geometries (Fig. F2), and (3) includes a large (as thick as 182 m) MTD (Strasser et al., 2011). Hole C0018A was drilled at a location where the MTD bodies wedge out and basal erosion is thought to be minimal. Coring to ~314.15 meters below seafloor (mbsf) allowed sampling of the MTDs across this stratigraphic succession. Sediment cored in Hole C0018A are divided into two lithologic subunits. Lithologic Subunit IA (0–190.65 mbsf) is primarily composed of hemipelagic mud (i.e., silty clay) with interbedded volcanic ash layers and is affected by MTDs, whereas lithologic Subunit IB (190.65–313.65 mbsf) is a sandy turbidite sequence (Expedition 333 Scientists, 2012a; Strasser et al., 2012). Six MTDs (numbered 1–6 from top to bottom) ranging in thickness from 50 cm to 61 m are identified within Subunit IA (Expedition 333 Scientists, 2012a; Strasser et al., 2012) (Fig. F3). These MTDs are composed of chaotic and convolute bedding with intervals of coherent bedding and commonly have bases defined by shear zones. A mass-movement event related to deposition of the lowermost and most prominent MTD 6 is younger than 1.05 Ma (because an ash layer immediately below the thick MTD 6 is identified as the one dated on land as 1.05 Ma) but older than 0.85 Ma, as constrained by another marker ash layer overlying MTD 6 (Expedition 333 Scientists, 2012a; Strasser et al., 2012). Another mass-movement event related to the uppermost MTD 1 occurred later than 0.291 Ma, which is the age indicated by nannofossils ~10 m below this MTD (Expedition 333 Scientists, 2012a; Strasser et al., 2012). The ages of other MTDs are not well constrained. During Expedition 338, LWD in Hole C0018B was conducted as a contingency operation. It provided a high-resolution data set of natural gamma radiation (NGR) and resistivity as well as resistivity images to 350 mbsf, enabled us to correlate and integrate these data with core and seismic data of the MTDs, and hence enabled us to understand the comprehensive nature of MTDs and their bearing on sliding dynamics and tsunamigenic potential.
Publication: 13 January 2014 |
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