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doi:10.2204/iodp.pr.314.2008

Site summaries (continued)

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. F12). Drilling at this site was 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 is the only available scientific data set 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.

Operations

Hole C0003A was spudded at 1300 h on 20 October 2007. The LWD/MWD drilling/logging operations were conducted from the seafloor (2481.5 m DRF) to a TD of 3007 m DRF (525.5 m LSF), where the drill pipe became stuck and the BHA became separated from the string because of accidental disconnection between two drill collars. In spite of numerous efforts to recover the bottom part of the BHA (including all of the MWD-APWD-LWD tools), the fishing operation was not successful and the hole was cemented on 30 October, leaving behind the tools and 19 drill collars. Because the LWD-MWD-APWD tools were not recovered, the available data set is limited to the MWD-APWD and a limited set of LWD data transmitted in real time, including density, porosity, ultrasonic caliper, natural gamma radiation, and sonic velocity. Except for the depth interval 75–155 m LSF, characterized by major washouts (sand-rich interval), and 390–460 m LSF, characterized by a complex washout and bridge pattern, hole conditions are good. Scalar logging data quality is generally good. Some of the automatically picked sonic compressional arrival times and coherence show a surprisingly good fit with the interval velocity data derived from the real-time check shot data.

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. F13). 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 consistent 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 C0003, some physical properties appear to be significantly affected by hole conditions. For example, where severe washouts were detected, density decreased and neutron porosity increased dramatically; therefore, these measurements are not reliable (Fig. F13). Even ring resistivity is affected in logging Unit II; however, bit resistivity seems to remain 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 resistivity and density/porosity measurements; the latter occur later because of the position of the tools in the BHA. The zone is more likely to correspond to clay-rich material. The high importance of the washouts may suggest that the material was already damaged.

Structural geology and geomechanics

Because of the lack of image data and the poor quality of the density and sonic logs (Fig. F13), we derived structural interpretations from the caliper, gamma ray, and resistivity logs. A number of washouts (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 gouge. 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 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. F12, F13). 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 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 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 attributed to 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 and clearly heavily deformed by multiple faults and associated brittle fractures.

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