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doi:10.2204/iodp.proc.314315316.111.2009

Site summaries

Site C0001

Site C0001 is located at the seaward edge of the Kumano Basin uplift (outer arc high), where the mega-splay fault system branches and approaches the surface (Fig. F9, F10). 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 cover sequence. The principal objective was to obtain LWD logging and SWD data in this section. In addition, this was a pilot operation for planned later riser drilling at this site to 3200 mbsf to access multiple branches of the megasplay fault system during the first riser drilling program of IODP.

Log characterization and lithologic interpretation

Three logging units and eight logging subunits were defined based on data from Hole C0001D (Fig. F11). Logging Unit I (0–198.9 m LWD depth below seafloor [LSF]) is characterized by gradually increasing resistivity and decameter-scale gamma ray cycles. It is interpreted as slope 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 a basal coarse sediment layer and 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. Gamma ray values, resistivity, and velocities are all increased. This unit also contains highly fractured resistive zones and few to no breakouts. Logging Units II and III are both interpreted as accretionary prism sediments forming part of the megasplay fault thrust sheet.

Physical properties

All LWD resistivity logs show a general downhole increasing trend. A comparison shows that ring resistivity is systematically higher than bit resistivity. The profile of resistivity presents a net downward decreasing step at the 198.9 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. F11). Sonic P-wave velocity generally increases with depth. 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 198.9 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 aforementioned disrupted zone.

Because the adnVISION tool failed during drilling and LWD operations, 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 logging Unit I/II boundary the log shows a sudden drop of value. Below this boundary, density values gradually increase, with the notable exception of the zone from 470 to 490 m, where density values decrease. The neutron porosity curve shows the opposite trend and the same anomalous zone. Porosity estimation from resistivity data was calibrated using the data from Sites 808, 1175, and 1176 of ODP Leg 196 at the Muroto transect (Taira et al., 1991; Moore et al., 2001). Its profile presents a net step increase 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, probably a result of poor log quality in an enlarged borehole. Crossplots show good correlation between velocity and resistivity-derived porosity and between velocity and resistivity-derived density.

Structural geology and geomechanics

Resistivity images provide constraints on 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. F11). 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 (Heki, 2007). The maximum horizontal stress is nearly perpendicular to the strike 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. F9, F11). 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 the boundary to relatively high amplitude below. 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 primarily 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, including an 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, the upper 480 mbsf of which was cored on subsequent Expedition 315 (see the “Expedition 314 Site C0001” chapter).

Site C0002

Site C0002 is located at the Kumano forearc basin off the Kii Peninsula (Fig. F12). The goal of drilling at this site was to log the ~1000 m thick Kumano forearc basin section and several hundred meters of 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. This site is also slated for deep riser drilling across the entire plate boundary system to >5500 mbsf during a later stage. We were able to drill and log the entire section from 0 to 1401 mbsf with complete success. An excellent suite of logs and seismic VSP data reveals the structure of the forearc basin, the gas hydrate–bearing zone, and the underlying deformed rocks of the inner accretionary prism.

Log characterization and lithologic interpretation

Four logging units were defined based on the trends and character in the full suite of LWD log responses (Fig. F13). Each logging unit is bounded by dip discontinuities interpreted as angular unconformities. There is also an angular unconformity within logging Unit III but not a sufficient change in log character to require division of the logging unit. Overall, logging Unit I is interpreted to be slope basin deposits. Logging Units II and III are interpreted as thick basin fill dominated by repeating turbidite deposits, as seen clearly on the log responses, particularly the gamma ray log. Within logging Unit II are two zones of particular interest defined by changes in log responses. Zone A (218.1–400.4 m LSF) is interpreted as a gas hydrate–bearing zone based on the resistivity profiles, with gas hydrates concentrated in the sandy horizons of the turbidite deposits. Zone B (481.6–547.1 m LSF) is interpreted as a potential gas-bearing interval, again within the sandy horizons of the turbidite deposits. The presence of gas is based on the sonic log response across the zone. Logging Unit III is a homogeneous clay-rich interval of mudstone, immediately overlying the top of the older accretionary prism section that forms basement to the basin at 936 m LSF. Logging Unit IV, from 936 to 1401 m, corresponds with the accretionary prism imaged on the seismic profiles, exhibiting a distinct change to much more variable responses in virtually all of the logs.

Physical properties

Physical properties in Hole C0002A show very different behaviors for different lithologies, which are classified based not only on the main logging units (slope sediment units and underlying prism) but also on different stratigraphic zones identified. Almost all physical properties are relatively well determined in the forearc basin sediments, whereas they exhibit a wide range of scatter in the underlying prism section, probably as a result of poor borehole conditions. However, properties not sensitive to hole condition (e.g., bit and deep resistivity) suggest that the accretionary prism is denser and more compacted compared to the forearc basin (Fig. F13). The lower part of the forearc basin below 400 m LSF at the BSR is characterized by physical properties that suggest a relative undercompaction: slightly decreasing or nearly constant resistivity with depth while velocity increase is insignificant and changes in density and porosity are relatively limited. Above the BSR (Zone A), a signature of gas hydrate is indicated by a strongly increasing overall resistivity trend with depth, showing numerous local spikes of high resistivity, whereas changes in neutron porosity and density are minor. Gas hydrate–bearing formations are characterized by a unique velocity and resistivity relation because of a significant increase in resistivity but a muted increase in velocity with depth.

Structural geology and geomechanics

Structure is distinct between the forearc basin and underlying accretionary prism at Site C0002 (Fig. F13). However, orientations of bedding planes and borehole breakouts are consistent (northeast–southwest and northwest–southeast, respectively) throughout the entire borehole. Bedding dips are gentle (<10°–15°) in the forearc basin with some variation in dip direction with depth, which agrees well with seismic reflection data. Dips increase to values of ~30°–60° toward and within logging Unit IV, the older accretionary prism. Natural fractures are rarer than at Site C0001 and have scattered orientations within the basin but a more consistent northeast–southwest trend within the prism. All fractures dip between 30° and 85° with no significant difference between basin and prism. Basin fractures have three dominant trends: northeast–southwest, east–west, and northwest–southeast. All three orientations can be identified in seismic data. A few basin fractures offset bedding in a normal sense, consistent with the seismic-scale normal faulting. Resistivity of the fractures varies but includes several wide aperture (10–30 cm) and highly resistive (mineralized or cemented) fractures in the prism. Logging Unit III, at the basin–prism transition, is characterized by increasing bedding dips and a fractured base. Borehole breakouts indicate a northeast–southwest oriented S_Hmax, perpendicular to that at Site C0001. Breakout azimuth gradually rotates clockwise with depth and breakout width increases in the prism relative to the basin. Stress magnitude estimated from breakout width and empirical rock strength parameters is consistent with normal faulting in the basin, as observed, but are more ambiguous in the prism. Fracture and fault orientations, breakouts, and state of stress collectively support margin-normal extension of this part of the forearc. We suggest this is driven in part by uplift of the megasplay/​outer-arc high causing gravitationally driven extension of the inner wedge. This contrasts with Site C0001 in the active outer prism, where convergence-related compression dominates.

Log-seismic correlation

The portion of the Kumano Basin drilled at Site C0002 includes a series of unconformity-bounded sequences. The shallowest is seismic Unit 2/3/4 undifferentiated, which correlates closely with logging Unit I (Figs. F12, F13). The increase in velocity across this boundary is consistent with a transition from higher porosity sediments to older more compacted and perhaps sandier Kumano Basin stratigraphy. Seismic Unit 5 through uppermost seismic Unit 11 corresponds to logging Unit II.

Within logging Unit II there are two zones of interest. Zone A lies above the BSR (~404 m LSF) and shows elevated resistivity and slightly increased velocities. Velocity and density decrease at the BSR, suggesting that the reflection may be generated by a contrast between sediments bearing free gas below and gas hydrate–bearing ones above. At the top of Zone B, a strong, dipping negative polarity reflection within the seismic data correlates with a second region of low velocities. The regional observation that some reflectors brighten in amplitude as they approach the BSR from below, coupled with interpreted sandy turbidite layers based on gamma ray logs of Zone B, supports the suggestion that this reflection may represent sands bearing small amounts of free gas.

The top of the accretionary prism is a high-amplitude reflection and a clear boundary in all of the logs. A sequence of low gamma ray values just below the top of the prism suggest that a sandier interval may contribute to this bright reflection.

A complete check shot profile of 72 shots defined a clear velocity profile for the Kumano Basin stratigraphy and drilled portion of the accretionary prism. This curve generally agrees well with the sonic log, with the exception of slightly lower velocities from 550 to 675 m LSF and also agrees very well with the PSDM velocities.

Site C0003

Site C0003 is located in the midslope region and targeted a major thrust fault of the megasplay fault system, as well as the overlying thrust sheet and underlying footwall to the thrust (Fig. F14). Drilling at this site was designed to begin the downdip transect of the megasplay fault system by sampling a relatively shallow, presumably aseismogenic point on the fault zone at ~800 mbsf. The objectives of taking logging data from this site were to characterize the material properties, deformational features, and conditions in the fault zones, wall rocks, and sediments. Unfortunately, poor drilling conditions in the highly unstable thrust sheet only permitted us to penetrate to 525.5 m LSF at this site before the drill string became irretrievably stuck and the BHA was lost in the hole and ultimately cemented in place. The MWD-transmitted real-time data from the LWD tools and rig floor parameters are the only available data sets from this site. Fortunately, these real-time data are of sufficient quality that we were still able to obtain a great deal of useful information from the interval 0–525.5 m LSF.

Log characterization and lithologic interpretation

Three broad units were defined for the interval of relatively good quality real-time data available (~55–509 m LSF). Logging Unit I (55–76.6 m LSF) is interpreted as muddy to sandy slope basin sediments, exhibiting moderately high gamma ray baseline values and fairly constant resistivity values (Fig. F15). Logging Unit II (76.6–151.5 m LSF) is characterized by a large degree of hole washout and low gamma ray values. This suggests a formation dominated by unconsolidated and porous sandy beds. Based on seismic reflection data, it is interpreted as a deformed part of the thrust sheet. Logging Unit III (151.5–509 m LSF) exhibits a broad increasing gamma ray trend with fairly constant resistivity and density values. Logging Unit III contains several zones of apparent deformation characterized by changes in the resistivity and density. Overall, logging Unit III is interpreted as clay-rich sediments showing no significant compaction trend with depth, as part of the thrust sheet of the megasplay fault branch.

Physical properties

In Hole C0003A, some physical property measurements were significantly affected by hole conditions. For example, where severe washouts were detected, density decreased and neutron porosity increased dramatically and therefore these measurements are not reliable (Fig. F15). Even ring resistivity is affected in logging Unit II; however, bit resistivity seems to remain relatively unaffected. In the major washout zone between ~400 and ~450 m LSF, density and porosity (both neutron and density-derived porosity) are significantly affected by hole conditions but no notable decrease in resistivity was found. This could be explained by worsening hole conditions between the time of the resistivity and density/​porosity measurements; the latter are recorded later because of the position of the tools in the BHA. The pronounced occurrence of enlarged borehole, despite lack of evidence for sandy sediment, may suggest that this is a zone of caving of brecciated rock, perhaps from a fault zone.

Structural geology and geomechanics

Because of the lack of image data and the poor quality of the density and sonic logs (Fig. F15), we derived structural interpretations from the caliper, gamma ray, and resistivity logs. A number of washouts or caving zones (high-caliper intervals) occur in logging Unit III. Because the gamma ray and resistivity logs do not decrease through these intervals they probably represent poorly consolidated argillaceous material, or most likely, fault breccia. Three prominent washout zones occur between 416 and 451 m LSF, with several others within tens of meters above and below this interval. This area of the borehole also corresponds to bright tilted reflectors, further strengthening the interpretation of these washout zones as intervals of caving of brecciated rock from faults within the hanging wall thrust sheet of the major fault that was the target of drilling.

Log-seismic correlation

Logging Unit I corresponds to the slope basin, and the logging Unit I/II contact is likely the positive polarity reflection separating the base of the slope basin and top of the wedge-shaped upper thrust sheet sequence (Figs. F14, F15). The logging Unit II/III contact is the negative polarity reflection at the base of the wedge-shaped sequence. Logging Unit III spans the low-reflectivity sequence including the fault zone to the total depth (TD) of 525.5 m LSF. The reflection separating the slope basin sediments from the wedge-shaped sequence is positive polarity; however, the density log decreases sharply across this boundary. We suggest that washouts in sandy Unit II decreased the density values measured in this unit. The combination of a sand-rich Unit II with higher densities and a low-density interval ~3 m below the base of the Unit II/III boundary may explain the negative polarity reflection. Fault-related reflections in logging Unit III can be associated with washout zones or damaged zone associated with the faults.

We used real-time check shot data from 10 stations ranging in depth from 86 to 506 m LSF. The first arrival waveforms from these stations are of high quality. The data were used to obtain long wavelength interval velocities in the hole and to correct the PSDM seismic section. Because good-quality P-wave velocity data are not available, we calculated synthetic seismograms using interval velocities from check shot data and real-time density data. The density data used for the synthetic seismogram are modified in logging Unit II because of the low reliability in this interval. The synthetic seismogram can be basically correlated with the major features of PSDM seismic section, but we cannot exactly fit the synthetic seismogram to the PSDM section in some intervals, probably because of the low overall quality of the limited data set available for this purpose.

When the broken section of drill pipe was recovered to the rig floor, it was plugged with cuttings and numerous large blocks (as large as 5–8 cm in diameter) of cavings that had come from an unknown position in the hole. This material had a nannofossil age of late Miocene (5.5–7.2 Ma). It is remarkably well indurated for material from <530 mbsf. This is consistent with the thrust sheet having been uplifted from older parts of the accretionary complex.

In summary, the portion of the thrust sheet penetrated at Site C0003 was a largely homogeneous material, most likely silty to clayey hemipelagic muds and turbidites, clearly heavily deformed by multiple faults and associated brittle fractures.

Site C0004

As an alternative to Site C0003, which was unsuccessful in reaching the megasplay primary target, Site C0004 (proposed Site NT2-01I) was drilled at a location where the megasplay was predicted to lie at only ~280 mbsf (Figs. F16, F17). To test drilling conditions prior to risking the LWD-MWD tool string, pilot Hole C0004A was drilled without any of the LWD-MWD tools to the planned TD of 400 mbsf, and then Hole C0004B was drilled with the full LWD tool string except the nuclear components of the adnVISION tool (neutron porosity/​density). Drilling was smooth and reached TD quickly, and data quality was good, particularly for sonic velocity.

Log characterization and lithologic interpretation

Three logging units were defined based on the different trends and character of the LWD log responses (Fig. F18). Logging Unit I (0–67.9 m LSF) is characterized by variable gamma ray values and low resistivity and is interpreted as slope basin deposits. Logging Unit II (67.9–323.8 m LSF) was defined for the thrust sheet and associated complexes and divided into four subunits. Logging Subunit IIA is characterized by increasing gamma ray value, increasing resistivity baselines, and increasing velocity and is interpreted as chaotic or deformed sediments, possibly emplaced through gravitational processes (slumping). Logging Subunit IIB is characterized by continuous high-frequency fluctuation in gamma ray value, constant resistivity baseline, and increasing velocity. Logging Subunit IIC is characterized by cyclic changes in gamma ray value, variable resistivity, and again increasing velocity. Logging Subunit IID is characterized by decreasing gamma ray value and repeated intervals of decreasing resistivity and velocity. The upper part of this subunit is strongly deformed and the lower part is weakly deformed. The base of logging Unit II is interpreted as a weakly localized deformation zone. Logging Unit III (323.8 m LSF to TD) is characterized by decreasing gamma ray value, variable resistivity, and relatively constant sonic velocity and is interpreted as sediments underthrust beneath the splay fault.

Physical properties

Given the lack of a neutron log, porosity and density were estimated using five different resistivity logs (ring; bit; and deep, medium, and shallow button) and the sonic velocity obtained from the traveltime (DTCO) measurements (Fig. F18). No radioactive source was used at Site C0004, and accordingly neither the neutron porosity nor bulk density logs were available. Analyzed physical properties were compared with identified fracture zones (see “"Structural geology and geomechanics”). Shallow button resistivity is significantly lower than the other two button readings, most likely a sign of generally enlarged hole conditions seen in caliper data. Estimated porosity, calculated from bit and ring resistivity corrected with the estimated temperature profile, shows a slowly decreasing trend with depth below the slope cover. The sonic P-wave velocity log is of good quality and its response appears to reflect the effects of fracture zones and logging units, identifiable as low-velocity zones.

Structural geology and geomechanics

Three structural domains were defined by structural characteristics, including fractures, breakouts, and texture in the borehole images. Structural Domain 1 (0–96 m LSF) is characterized by a lack of fractures, weak breakouts, and little variation in the sediments (Fig. F18). Structural Domain 2 (96–292 m LSF) includes heavily or moderately fractured conductive zones and intensive borehole breakouts. Structural Domain 3 (292–396 m LSF) has few fractures and narrower breakouts relative to the overlying domains. Bedding planes in structural Domain 1 are consistent and mostly strike northeast–southwest and dip 30°–40° to the south. The beds in structural Domain 2 are more scattered both in dip and azimuth but generally trend northeast–southwest and dip at 20°–70° to the north. Structural Domain 3 shows similar bedding strike as Domain 2 (northeast–southwest) but the dips are generally gentler to the north. Most fractures we identified are conductive and only found in Domains 2 and 3. Fractures in structural Domain 2 are scattered both in strike and dip but with a dominant strike of northeast–southwest, steeply dipping to the north. Fractures in structural Domain 3 show northeast–southwest strike and gentler dip (10°–20°) to the north. Fractured zones were defined by intense development of fractures and wide conductive breakouts and were classified as “major” or “minor.” In structural Domain 2, three major fractured zones and four minor fractured zones are identified, whereas structural Domain 3 includes only a minor fractured zone.

Borehole breakouts indicate a consistent northwest–southeast oriented S_Hmax axis throughout the borehole, which is between the S_Hmax direction at Site C0001 and the convergence direction. Breakout width is narrow in Domain 1, wide in Domain 2, and slightly reduced again in Domain 3. Stress magnitude was analyzed from breakout widths, but stress regime is unclear because of uncertainty in rock strength. Domain 1 consists of logging Units I and IIA, which could be interpreted as two sedimentation stages of slope deposits because of small changes in physical properties. Deformation characteristics at the boundary of structural Domains 2 and 3 suggest a narrow (few meters) transition zone between the hanging wall and footwall of the main thrust. Structural features identified in borehole images, bedding, and fractured zones are well correlated with the structural style in the reflection seismic profiles and are compatible with convergence related deformation.

Log-seismic correlation

The base of the slope sediment section corresponds to the logging Unit I/II boundary (Figs. F16, F18). Sonic velocity increases at the boundary from little more than drilling fluid velocity to about 1600 m/s. There are similar increases in the gamma ray and resistivity logs. The transition in each of the logs is gradual rather than abrupt. These gradual changes likely account for the relatively low frequency character of the reflection at the base of the slope basin sediments.

Logging Subunit IID corresponds to a thick zone of roughly parallel northwest-dipping reflections interpreted as the signature of a system of faults along which accretionary prism rocks have been thrust over slope basin sediments. Seismic reflections on both the inline and cross-line adjacent to the hole show considerable variation on a scale of 50–100 m. Thus, we do not expect an exact correlation between log values in a single hole and the seismic data that smear the image laterally on a scale of 20–40 m, and it was in fact difficult to correlate the depth of the apparent fault zones in logs to reflector position.

Velocity in the upper half of the dipping reflection package decreases from ~1900 to ~1800 m/s from 243 to 291 m LSF. The sonic log begins a significant increase in interval velocity, from ~1900 to 2100 m/s at 291 m LSF. Velocity remains high to ~313 m LSF before dropping back to ~2000 m/s. This high velocity corresponds to the base of the broad trough and the top of the basal peak of the dipping reflection section. There is a thin layer with dramatically lower velocity at ~306 m LSF that is within the basal bright peak of the dipping sequence. The nearly flat horizons below have velocities varying between 2000 and 2100 m/s and form a series of bright peaks and troughs.

Usable check shot data were acquired at 21 depths in Hole C0004B that provided reasonable velocity information. Beyond the general increase of interval velocity from 1500 m/s at the seafloor to 2100 m/s at 400 m, there is not an exact match between the check shot velocity curve and the sonic log values.

Site C0005

Site C0005 was drilled without any logging tools. No measurements were made after failure to reach TD in the first pilot Hole C0005A (proposed Site NT2-01E) because of poor hole condition and in the second pilot Hole C0005B (proposed Site NT2-01G) because of ROV problems right at the beginning of drilling (Fig. F17). Site C0005 was then abandoned for lack of time in favor of higher priority objectives at Site C0006.

Site C0006

Site C0006 is located at the frontal thrust of the Nankai accretionary prism near the trench axis, and drilling targeted the main frontal thrust at ~700 mbsf, subsidiary faults and deformed sediments above that zone, and a footwall zone of strong reflectors likely caused by coarse turbiditic trench fill sediments (Fig. F19). Overall objectives of drilling this site with LWD-MWD instruments were to characterize the lithology, deformation, stress state, and physical properties of the wall rocks and the frontal thrust fault zone. Pilot Hole C0006A was drilled with a MWD-APWD and gamma ray tool string to a 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. Because the water depth was beyond the ROV limit of 3000 meters below sea level, both holes were drilled without ROV monitoring. Drilling was smooth; however, real-time MWD communication was lost in Hole C0006 at 274 mbsf, as well as power from the MWD turbine. The LWD tools recorded data in memory mode on battery power; however, the 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).

Log characterization and lithologic interpretation

Four logging units were defined based on the different trends and character of the LWD log responses (Fig. F20). Logging Unit I (0–197.8 m LSF) is characterized by variable gamma ray values and high-amplitude fluctuation of resistivity and is interpreted as sandy and muddy deposits. Logging Unit II (197.8–428.3 m LSF) is characterized by a gradually increasing trend in the gamma ray log with occasional thick (5 m) low-gamma layers. This unit is interpreted as mudstone with thick sandstone beds. Possible repeated stratigraphic sections are recognized in this logging unit. Logging Unit III (428.3–711.5 m LSF) was defined based on inferred alternating beds of mudstone and sandstone and divided into two logging subunits. Logging Subunit IIIA is characterized by high gamma ray values with thin (1 m) low gamma ray layers. Logging Subunit IIIB is characterized by high gamma ray values with a large number of thin (1 m) low gamma ray layers and repeated increasing trends in resistivity. The base of logging Unit III is interpreted as both a fault zone as well as a distinct lithologic boundary. Logging Unit IV (711.5 m LSF to TD) is characterized by low gamma ray value and resistivity and interpreted as being composed of abundant sandstones.

Physical properties

Porosity and density were estimated using five different resistivity logs (ring; bit; and deep, medium, and shallow button) and the sonic velocity obtained from DTCO measurements (Fig. F20). No radioactive source was used in Site C0006, and accordingly neither the neutron porosity nor density logs were available. The deep and medium button resistivity logs show very good agreement. Shallow button resistivity is significantly lower than the other two button logs in logging Units I and IV. Estimated porosity, calculated from bit and ring resistivity using the estimated temperature profile, shows a slowly decreasing trend with depth from 0 to ~650 m LSF and shows increasing trend with depth below 650 m LSF. Because of the malfunction of the sonic tool, P-wave velocity data were only measured to ~160 m LSF. Velocity appears to be nearly constant or slightly increasing over this shallow interval. Velocity and resistivity are generally in good agreement, and high velocity zones correspond to high resistivity zones.

Structural geology and geomechanics

Good quality borehole resistivity images provide information on orientation of bedding, fractures, and breakouts at Site C0006 (Fig. F20). Bedding dips are shallow to moderate with most dips <30°. The mean bedding dip is west–northwest in shallow parts of the hole and more northerly dips are formed in logging Units III and IV. Fractures at Site C0006 are notably not as clustered into zones as at other Expedition 314 sites. Fractures strike mostly northeast–southwest in the upper two lithologic units (above 428 m LSF). Fractures in the deeper two lithologic units (429–853 m LSF). Overall bedding and fracture orientations at depths below 429 m LSF (logging Units III and IV) are consistent with northwesterly directed shortening. In general, deformation at Site C0006 is weaker than at other active prism sites, with less fracturing and no major deformation zones.

Borehole breakouts occur from 188 to 729 m LSF but are in general much more weakly developed than at other sites drilled during this expedition. Breakouts are not readily discernible at greater depths. The lack of discernible breakouts below 729 m LSF occurs just below the transition to lithologic Unit IV, which is dominated by sand, where conductive washouts may obscure breakout evidence. Breakouts show a mean S_Hmax orientation of 330°, consistent with that observed at Sites C0001 and C0004 but divergent from the S_Hmax expected from the far-field convergence of the Philippine Sea plate and Japan.

Log characterization and lithologic interpretation

Logging units do not correlate well with seismic reflection data, probably because the section is strongly faulted. Several faults that intersect the well location are visible on the seismic data (Fig. F20). Several features in the LWD logs do, however, correlate with features on the seismic data. For example, the sandy layers (low gamma ray values and resistivity) at ~220, 240, 300, and 335 m LSF correlate with strong reflections and appear to be parts of the same unit that have been repeated as a result of thrusting (Fig. F20). Several seismically defined thrust faults also correlate with features in the resistivity images, such as the conductive fractures at 360 and 381 m LSF. The basal décollement correlates with a fracture at 657 m LSF.

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