Previous   |   Next

doi:10.2204/iodp.pr.314.2008

Site summaries

Site C0001

Site C0001 is located at the seaward edge of the Kumano Basin uplift (outer arc high), where the megasplay fault system branches and approaches the surface (Fig. F7). Drilling at this site during Expedition 314 targeted the uppermost 1000 mbsf to access the thrust sheets uplifted by several branches of the megasplay fault, as well as a thin overlying slope basin cover sequence. The principal objective was to obtain LWD logging and SWD data in this section. In addition, this was a pilot operation for later drilling at this site to 3200 mbsf to access multiple branches of the megasplay fault system during the first riser drilling program of IODP.

Operations

Because of concerns about possible unstable drilling conditions and safe running of the LWD tools with a radioactive source, pilot Hole C0001A was first drilled to a total depth (TD) of 1000 m LWD depth below seafloor (LSF) with the MWD/APWD tools only. Next a short Hole C0001B was drilled to obtain hydraulic piston coring system cores from 0 to 32 m LSF for geotechnical studies related to riser site well planning. Finally, the LWD/MWD BHA was made up with sonic, gamma ray, resistivity imaging, density and porosity, and seismic check shot tools. Hole C0001C was drilled from 0 to 78 m LSF but was terminated because of large hole inclination that developed during downtime for a rig repair and because in part to a change in current direction. Hole C0001D was spudded without retrieval of the drill string to the surface and drilled successfully from 0 to 974 m LSF (Fig. F8). Sonic, resistivity, and check shot data were successfully recovered from memory; however, the density-neutron porosity tool failed upon retrieval to the surface, and only data from 0 to 506 m LSF were successfully retrieved after recovery attempts at Shekou base (China). Logging data quality is generally good, and image logs in particular are of high quality. The sonic log suffers from slow formation velocity and poor data in a zone of enlarged and sticky hole conditions.

Log characterization and lithologic interpretation

Three logging units and eight logging subunits were defined based on data from Hole C0001D (Fig. F9). Logging Unit I (0–198.9 m LSF) is characterized by gradually increasing resistivity and decameter-scale gamma ray cycles. It is interpreted as slope basin sediments composed of silt turbidites and mud. No clear sedimentary features could be recognized in the images. Logging Subunit IB (190.5–198.9 m LSF) is thought to represent an unconformity at the base of the slope sediments and is characterized by positive peaks in the resistivity logs and a broad negative peak in the gamma ray log, suggesting washout of a possible sandy/silty layer. Logging Unit II (198.9–529.1 m LSF) exhibits higher gamma ray and resistivity baselines, suggesting an increase in clay content. The near-continuous breakouts suggest this unit is composed of relatively homogeneous hemipelagic mud and silt turbidites. Logging Unit III (529.1–976 m LSF) shows distinct differences from logging Units I and II. The gamma radiation, resistivity, and velocities are all increased. This unit also contains highly fractured resistive zones and few to no breakouts. Both logging Units II and III are interpreted as accretionary prism sediments.

Physical properties

All LWD resistivity logs show a general downhole increasing trend. A comparison shows that the ring resistivity is systematically higher than the bit resistivity. The profile of resistivity presents a net downward decreasing step at the 200 m LSF logging Unit I/II boundary and two significant zones of lower resistivity values at 529–628 m LSF in a disrupted zone (see below) and at 815–855 m LSF (Fig. F9). The sonic P-wave velocity generally increases with depth. The gradients of velocity with depth are 0.757, 0.976, and 1.793 m/s per meter for logging Units I, II, and III, respectively. The 200 m LSF logging Unit I/II boundary is characterized by relatively low velocities. Five major low-velocity zones (529–600, 660–695, 773–827, 884–894, and 915–965 m) depart from the increasing trend of velocity in logging Unit III, the first zone corresponding to the same disrupted zone.

Because the adnVISION tool failed, the real-time monitoring density (RHOB) and porosity (TNPH) data could only be retrieved from the uppermost 506 m LSF. Those data suffer from large scattering. Real-time density generally increases with depth. At the 200 m LSF logging Unit I/II boundary the log shows a sudden drop of value. Below this boundary the density log gradually increases, with the notable exception of the 470–490 m zone, where the density values decrease. The neutron porosity curve shows the opposite trend and the same anomalous zone. Porosity estimation from the resistivity data was calibrated using the data from Sites 808 and 1173 of ODP Leg 196 at the Muroto transect (Taira et al., 1991; Moore et al., 2001). Its profile presents a net increasing step at 200 m LSF and two zones of increased porosity departing from the general decreasing trend (468–633 and 810–910 m LSF). That resistivity-derived porosity fits only with the lowest values of density-derived porosity calculated from the RHOB log. Crossplots show good correlation between velocity and resistivity-derived porosity, and velocity and resistivity-derived density.

Structural geology and geomechanics

Resistivity images provide constraints on the structural development and stress orientations at Site C0001. Breakouts and associated tensile fractures are a striking feature of the upper half of the borehole and indicate a maximum horizontal stress orientation of 335° (Fig. F9). This horizontal stress is more northerly than both the Philippine Sea–Japan plate convergence vector and the geodetically determined shortening direction on the Kii Peninsula. The maximum horizontal stress is nearly perpendicular to the trend of the accretionary prism and major faults and is consistent with prism-normal shortening. The difference between the maximum horizontal stress at Site C0001 and the orientation of local plate convergence suggests oblique convergence or right lateral strike-slip faulting elsewhere in the forearc region.

Bedding and fracture orientations are consistent with deformation around an axis parallel to the trend of the accretionary prism. That is, poles to bedding form a girdle trending north-northwest. Fractures are more complex with shallower orientations having northwest and southeast dips. A set of near-vertical conductive natural fractures trend north-northwest, are cut by younger normal faults, and may have been dilated by drilling operations, as inferred from the observation of wide conductive zones. Bedding dips are locally steeper at the contact between the slope deposits and the accretionary prism, but faulting is not evident at this apparent unconformity.

A zone of difficult drilling from 529 to 629 m LSF shows patterns of bedding disruption typical of fault zones. Increasing consolidation across this zone of disruption may be interpreted as resulting from normal faulting; however, nontypical geometries of thrust faulting are also consistent with this consolidation state.

Log-seismic correlation

Logging Unit I corresponds to the hemipelagic slope sediments between the seafloor and ~200 m seismic depth below seafloor (SSF) (Figs. F7, F9). Logging Subunit IB corresponds to a thin low-amplitude reflection overlying the strong positive reflection at 200 m SSF that defines the base of logging Unit I. Logging Unit II corresponds to a zone of southeast-dipping, generally low amplitude reflections. Reflections intersect the borehole at the logging Subunit IIA/IIB and IIB/IIC boundaries, but there is no change in the general character of the seismic data across these reflections. The boundary between logging Units II and III correlates with a change in reflectivity from relatively low amplitude above to relatively high amplitude below the boundary. Logging Subunit IIIA corresponds to a series of high-amplitude, laterally continuous reflections that appear on the southeast side of the borehole and are cut off by the inferred fault that intersects the borehole at the base of this subunit.

Check shot data to 635 m LSF provided good constraint on the long wavelength velocity depth conversion. These data demonstrated that the prestack depth-migrated (PSDM) velocity model at the site was quite good. We found it difficult to reconcile the strong reflection at the boundary between the slope basin section and the accretionary wedge below with the log observations in the hole. Synthetic seismograms of this interval fail to reproduce the strong reflection because of the lack, in the logs, of an abrupt increase in acoustic impedance just above the boundary.

In summary, a 976 m interval made up of apparently strongly deformed mudstones with some silty to sandy turbiditic sediments was drilled and logged at Site C0001. This section is the thrust sheet of one main branch of the megasplay fault system, plus the overlying ~200 m of slope basin deposits. Logging and vertical seismic data provide the first information on the physical properties, lithology, structure, and state of stress in this key element of the Nankai subduction system.

Top of page   |   Previous   |   Next