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doi:10.2204/iodp.proc.314315316.124.2009 Expedition 315 Site C00021Expedition 315 Scientists2IntroductionIntegrated Ocean Drilling Program (IODP) Site C0002 (proposed Site NT3-01B) targeted the uppermost 1400 meters below seafloor (mbsf) at the seaward flank of the Kumano Basin (Figs. F1, F2). The uppermost 1400 m to be drilled during this expedition provides an opportunity to penetrate the Kumano Basin sedimentary sequence and the underlying accretionary prism materials, the nature of which are unknown. Acoustic features characterized by irregular discontinuous horizons of this section suggest that it may be composed of chaotically deformed accretionary wedge sedimentary mélange transported from significantly greater depths. It shows a good contrast with the overlying well-ordered Kumano Basin sediments. This site also provides an opportunity to compare trench-to-basin transition ages on both sides of the outer arc high. The principal objective during IODP Expedition 315 was to reveal the stratigraphic, physical, structural, and chemical features of materials obtained from the accretionary prism and the overlying forearc basin. An integrated interpretation is proposed based on these findings combined with seismic and log data obtained during IODP Expedition 314. Coring at this site also provides critical pilot hole information for future riser drilling. To achieve the ~6000 m total depth objective using riser and weighted drilling mud involves setting multiple casing strings, with the depth of each depending on the least principal stress, the fracture strength of the formation, and the pore fluid pressure gradient. The key part of this casing plan is the “top-hole” portion, where tolerances on mud weight are tight. Planning the casing program, therefore, requires excellent information on physical properties in the uppermost 1000 m. Additionally, the upper ~50 m of mechanical strength is critically important to the deployment of the riser blowout preventer. In light of this, our strategy is to utilize riserless coring for this section during a future stage of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). Furthermore, downhole (in situ) temperature measurements were conducted as deeply as possible, which will enable estimation of deeper temperatures by extrapolation. These measurements will provide critical information for well planning of the 6 km riser hole and will aid in the design of sensors and telemetry of the long-term borehole observatory planned for a future NanTroSEIZE stage. |
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