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doi:10.2204/iodp.proc.314315316.118.2009 Expedition 314 Site C00061Expedition 314 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site C0006 (proposed Site NT1-03) (Figs. F1, F2) is located at the frontal thrust of the Nankai accretionary prism near the trench axis. Drilling targeted the main frontal thrust at an estimated depth of ~700 meters below seafloor (mbsf), as well as subsidiary faults and deformed sediments in the hanging wall above that zone and a footwall zone of strong reflectors likely caused by coarse turbiditic trench fill sediments. Overall objectives of drilling this site with logging-while-drilling (LWD)-measurement-while-drilling (MWD) instruments were to characterize the lithology, deformation, stress state, and physical properties of the hanging wall rocks and frontal thrust fault zone. LWD drilling during Expedition 314 contributed in situ resistivity, gamma ray, and ultrasonic caliper logs, as well as borehole imagery for this characterization. Unfortunately, no neutron porosity or lithodensity logs were obtained because of the inability to run a radioactive source at this site, and the sonic tool failed to record useful data for most of the interval below 274 m LWD depth below seafloor (LSF) because of loss of the MWD turbine power system. Based on seismic data and submersible dive studies (Ashi et al., 2002), this frontal thrust system was predicted to have placed moderately consolidated clastic rocks over weak and unlithified upper Quaternary trench section clastic sediments. However, at the final location chosen for Site C0006, it is not clear whether the same older section sampled by submersible is in the thrust sheet or if it is exclusively composed of uplifted trench wedge. Detailed analysis of seismic data suggests that substantial footwall protothrust deformation exists a few hundred meters below the main frontal thrust fault at this site location, implying the existence of a deeper décollement and that strain decoupling is not total across this fault. Reflection amplitude of this fault plane is variable near this site, but generally it is a negative polarity reflector. Characterization of physical properties across the frontal thrust zone, as well as the many subsidiary faults imaged in the thrust sheet, were primary objectives of drilling this site; however, the lack of porosity and velocity data has hampered this effort. Nevertheless, resistivity imaging and other data provides extensive documentation of fracture and stress orientation, lithology, and possible fault repetition of strata. Pilot Hole C0006A was drilled with MWD-annular-pressure-while-drilling (APWD) and gamma ray tool string to total depth (TD) of 885.5 m LSF, and Hole C0006B was drilled to the same TD with the full LWD tool string but without any radioactive source in the adnVISION tool. Drilling was smooth; however, real-time MWD communication was lost at 274 m LSF, as well as the power from the MWD turbine. LWD tools recorded data in memory mode on battery power; however, sonic source transducers were negatively affected and sonic data were of very poor to unusable quality from 274 m LSF to TD (885.5 m LSF). Drilling was initially planned to 950 mbsf but was terminated at 885 mbsf. MWD data showed that drilling was penetrating thick sandy sequences and conditions were deteriorating below ~800 m LSF. Judging that drilling had passed through the main fault reflector position at <700 m, we decided the objective had been met and the hole was terminated. |
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