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doi:10.2204/iodp.proc.314315316.114.2009 Expedition 314 Site C00021Expedition 314 Scientists2Background and objectivesAt Integrated Ocean Drilling Program Site C0002 (proposed Site NT3-01B), we planned and carried out logging-while-drilling/measurement-while-drilling (LWD-MWD) operations to 1400 meters below seafloor (mbsf) through the Kumano forearc basin section and the underlying formations, interpreted as older rocks of the accretionary prism and/or early slope basin sediments deposited prior to the development of the megasplay fault and forearc basin (Fig. F1). Scientific objectives included investigation of the outer forearc basin depositional systems, especially documentation of the turbidite depositional record, the physical properties of these basinal sediments and the underlying preexisting rocks, likely gas hydrate deposits, and a bottom-simulating reflector (BSR) within the forearc basin sediments. We were able to drill and log the entire section from 0 to 1401 mbsf with complete success. An excellent suite of logs and vertical seismic profile (VSP) data reveal the structure, physical properties, and stress state of the forearc basin, the gas hydrate–bearing zone, and the underlying deformed rocks of the inner accretionary prism. The LWD program at this site is the deepest LWD penetration in Ocean Drilling Program (ODP)/Integrated Ocean Drilling Program history to date, as well as the deepest penetration of any type into the interior of an accretionary prism. Site C0002 is slated for deep drilling across the entire plate boundary system to >5500 mbsf during later stage riser drilling (Tobin and Kinoshita, 2006), and the location was selected primarily to meet deep riser drilling objectives to the plate boundary and megasplay reflectors rather than based on the shallower targets drilled during Expedition 314. However, these shallower targets are important primary objectives of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Stage 1 in their own right. Operations during Expedition 314, along with later coring, had the following goals:
As planned, drilling was successful to total depth (TD) of 1401 mbsf and included continuous logging of the entire sedimentary section through the Kumano Basin and several hundred meters of the underlying rocks. A principal objective was to penetrate through this seismically imaged unconformity to sample the underlying formation. Characterization of seismic velocity, density, porosity, stress, rock strength parameters, and pore pressure in both tectonic domains (basin and prism) were all high priorities. Based on the seismic reflection data, the uppermost ~950 m in the Kumano Basin domain was expected to be primarily turbiditic and hemipelagic sediments deposited in the forearc basin, with progressive tilt caused by development and displacement of the megasplay system. Several discrete packages of basin sediment bounded by unconformities are evident in this section. Within this sequence, a prominent BSR interpreted as the base gas hydrate reflection is evident at ~400 mbsf (Fig. F2). Log data were used to estimate the presence and distribution of gas hydrate and possible free gas in the sediment porosity at Site C0002 (see “Log characterization and lithologic interpretation” and “Physical properties”). Between the base of the Kumano Basin at ~936 mbsf and the shallowest branch of the megasplay fault system at ~4000 mbsf lies an interval of rock interpreted as frontally accreted material of the accretionary prism. By analogy with land-based geology and ODP Leg 190 Site 1178, where the interior of the Nankai accretionary prism was drilled in the Muroto region (Ujiie et al., 2003), we anticipated a sequence of variably dipping and disrupted clastic strata cut by numerous small faults, locally forming mélange. Drilling and sampling of this domain affords the first-ever penetration of the deep, older interior of an active accretionary prism, addressing long-standing questions on the fate of offscraped material, the evolution of accretionary mélanges, and role of fluids in prism interiors. Logging to detect fracture and bedding orientations and variability, porosity and density, and state of stress in this environment was therefore a major objective as well. |
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