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Site C00091

Expedition 319 Scientists2

Background and objectives

Integrated Ocean Drilling Program (IODP) Site C0009 (proposed Site NT2-11B) is located in the Kumano forearc basin ~20 km northwest of IODP Site C0002, drilled during IODP Expeditions 314 and 315 (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009) (see Figs. F1, F2, F3 in the "Expedition 319 summary" chapter). The purpose of this site was to secure a location for a long-term monitoring system in the hanging wall directly above the plate boundary where it is hypothesized to be interseismically locked, conduct downhole measurements and wireline logging, and collect core and cuttings within the forearc basin and underlying units. One key motivation for collecting core in the lower part of the borehole was to obtain samples for shore-based geotechnical and mechanical analyses near the depth of the potential observatory installation.

Based on seismic intepretation and the results of drilling at Site C0002, the anticipated geology from the top of the section to the planned total depth (TD) was a thick sequence of flat-lying to gently dipping turbidites, hemipelagic muds, and volcanic ash layers to ~825 m seismic depth below seafloor (SSF), deposited in the forearc basin. Below this, a slope deposit of fine-grained mudstone analogous to lithologic Unit III at Site C0002 (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009) was anticipated to overlie a section composed of slope basin deposits and/or older accreted trench wedge sediments comprising the accretionary wedge down to the intended TD of ~1600 meters below seafloor (mbsf). Several prominent seismic reflection surfaces can be identified near Site C0009 and traced regionally in the three-dimensional (3-D) seismic reflection data (see Fig. F4A in the "Expedition 319 summary" chapter). Seismic Surface S1 within the basin fill separates continuous, high-amplitude reflectors above and lower amplitude, less continuous reflectors below and can be traced to Site C0002 just below the seafloor. Seismic Surface S2 represents a downlap surface at the base of the well-stratified basin deposits. Unconformity UC1 is a local angular unconformity at Site C0009. Unconformity UC2 is an unconformable boundary marking a change from clear seismic reflectors to discontinuous low-amplitude reflectors near the base of the proposed borehole. Seismic reflectors in the upper section at Site C0009 (including surfaces such as seismic Surface S1 mentioned above) can be traced to Site C0002, and they suggest widely divergent sediment deposition rates between the two sites during basin history (cf. Fig. F6B in the "Expedition 319 summary" chapter).

The scientific and operational objectives for Site C0009 were to drill, sample, log, and case the formation to 1600 mbsf. Specifically, our scientific goals at this site were to

  • Characterize the lithology, depositional history, physical properties, chemistry and composition, and deformation of the Kumano Basin sediments and underlying units through analysis and integration of cuttings, core, mud gas, measurement while drilling (MWD), and wireline logging data;

  • Directly measure stress and pore pressure conditions throughout the section from wireline logging Modular Formation Dynamics Tester (MDT) measurements;

  • Characterize the velocity structure in the rock volume around and below the borehole by conducting zero-offset, circular, and "walkaway" vertical seismic profile (VSP) experiments; and

  • Case the hole to ~1500 mbsf in preparation for later placement of an observatory, which will monitor deformation, seismicity, pore pressure and temperature.

1Expedition 319 Scientists, 2010. Site C0009. In Saffer, D., McNeill, L., Byrne, T., Araki, E., Toczko, S., Eguchi, N., Takahashi, K., and the Expedition 319 Scientists, Proc. IODP, 319: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.319.103.2010

2Expedition 319 Scientists' addresses.

Publication: 31 August 2010
MS 319-103