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doi:10.2204/iodp.proc.314315316.135.2009

Physical properties

At Site C0007, physical property measurements were made to provide basic information characterizing lithologic units, states of consolidation, deformation, and strain and to correlate coring results with downhole logging data. After capturing X-ray CT images and the core reached thermal equilibrium with ambient temperature at ~20°C, gamma ray attenuation (GRA) density, magnetic susceptibility, natural gamma radiation, P-wave velocity, and noncontact electrical resistivity were measured using a multisensor core logger (MSCL) system on whole-round core sections (MSCL-W). Thermal conductivity was measured using either a full-space needle probe method or a half-space line source method on split working halves. The half-space method was used on lithified sediments deeper in the hole that were impenetrable with the needle probe. Cores were split in two longitudinally, one half for archiving and one half for sampling and analysis. A photo image capture logger (MSCL-I) and a color spectrophotometer (MSCL-C) were used to collect images of the split surfaces of the archive halves. Moisture and density (MAD) were measured on discrete subsamples collected from the working halves as well as from “clusters” adjacent to whole-round samples removed before splitting. Vane shear and penetration experiments were performed on the working halves to 150 m CSF. Additional discrete subsamples from working halves were used to perform electrical conductivity measurements, P-wave velocity measurements, and anisotropy calculations.

Density and porosity

Bulk density values at Site C0007 were determined from both GRA measurements on whole cores and MAD measurements on discrete samples from the working halves of split cores (see “Physical properties” in the “Expedition 316 methods” chapter). A total of 225 discrete samples were analyzed for MAD (100 from Holes C0007A, C0007B, and C0007C and 125 from Hole C0007D).

Holes C0007A, C0007B, and C0007C

MAD wet bulk density increases markedly to ~20 m CSF from ~1.55 to ~1.90 g/cm³, followed by a much more gradual increase with depth to ~150 m CSF (Fig. F51A). The maximum MSCL bulk density values also show similar trends. GRA bulk density is in very good agreement with MAD bulk density for depths less than ~100 m CSF. At depths >100 m CSF, GRA density is generally smaller than MAD bulk density and reveals a larger degree of scatter compared to MAD density. The scatter in MAD bulk density is likely due to lithologic variations among the interbedded sand, silt, and mud at this site. Grain density was also determined via MAD measurements on discrete samples. Grain density decreases slightly from ~2.7 to ~2.68 g/cm³ from 0 to ~34 m CSF (Fig. F51B). Between ~34 and ~110 m CSF, grain density is approximately constant with depth (average = ~2.7 g/cm³).

Porosity was estimated from whole-round core MSCL scan data and calculated from MAD measurements on discrete samples. These porosity values (see “Physical properties” in the “Expedition 316 methods” chapter) generally decrease with depth (Fig. F51C) and vary inversely with MAD bulk density.

Hole C0007D

MAD wet bulk density increases slightly with depth (~1.93 to 2.07) from ~175 to ~320 m CSF (Fig. F51D). Bulk density decreases from ~2.07 g/cm³ at ~320 m CSF to ~1.87 g/cm³ at ~400 m CSF. Maximum MSCL bulk density values at various depths within this range are similar to the MAD bulk density versus depth trend. The scatter in MAD bulk density is likely due to lithologic variations at this site. Grain density values were approximately constant with depth (average = ~2.7 g/cm³) (Fig. F51E).

MAD porosity decreases slightly (from ~42% to ~39%) from ~175 to ~320 m CSF (Fig. F51F) and varies inversely with bulk density (from which porosity data are derived) over that range. At ~320 m CSF, porosity increases with depth (from ~39% to ~50%) from ~320 to ~400 m CSF (Fig. F51F).

Because of the greater scatter in MSCL density and porosity, we focus our description on bulk density and porosity calculated from discrete MAD measurements.

Density and porosity of lithologic units

At Site C0007, a Pleistocene package of muds with interbedded sand and volcanic ash comprises Unit I (see “Lithology”). Bulk density increases markedly with depth in the uppermost 20 m at this site from ~1.55 to ~1.90 g/cm³ (Fig. F51A). Porosity varies inversely with density over this depth range (Fig. F51C).

Unit II is composed of Pleistocene interbedded sand and mud (see “Lithology”). Within Unit II from ~34 m CSF in Hole C0007C to ~320 m CSF in Hole C0007D, bulk density gradually increases from ~1.90 to ~2.07 g/cm³ (Fig. F51A, F51D). This trend is mirrored by a slight decrease in porosity with depth from ~48% at 34 m CSF to ~39% near the bottom of Subunit IIC in Hole C0007D (Fig. F51C, F51F).

In the bottom two-thirds of Unit II (from ~320 to ~400 m CSF), bulk density decreases markedly from ~2.07 to 1.87 g/cm³ (Fig. F51D), whereas porosity increases from ~39% to ~50% (Fig. F51F). At ~400 m CSF, however, bulk density and porosity resume normal trends, increasing and decreasing, respectively, with increasing depth (Fig. F51D, F51F).

Relationships of density and porosity changes to lithologic unit and structural boundaries

In addition to the unit boundaries described above, the structural geology group identified the following zones of concentrated shear deformation as fault zones (see “Structural geology”):

• Fault Zone 1 = ~238 to ~259 m CSF,
• Fault Zone 2 = ~342 to ~362 m CSF, and
• Fault Zone 3 = ~399 to ~446 m CSF.

As indicated above, MAD-derived bulk density and porosity values increase and decrease, respectively, from 0 to ~320 m CSF and reveal no clear discontinuities at depths corresponding to possible faults or at the Unit I/II and Unit II/III lithologic boundaries within that depth range. A discontinuity in the porosity versus depth trend at ~320 m CSF occurs within Unit II.

The zone of highest porosity (~50%) between ~360 and ~400 m CSF may be bounded by fault Zones 2 and 3 (Fig. F51F). This zone of elevated porosity likely results from higher density of fluid-filled microcracks and other fault-related damage. Alternatively, elevated porosity in this zone could indicate that these sediments are underconsolidated because of elevated fluid pressures, which may have localized shear deformation in this region. We note that although mud and silt samples from these zones of elevated porosity appear to be well-indurated chunks of material, they are actually quite friable in hand samples. This observation suggests that these are possible zones of fault-related damage. We also note that the interval of elevated porosity appears to correlate with the lithologic transition from lowermost Unit II to Unit III. Finally, we note that lithologic analyses reveal that Unit III exhibits an overall increase in clay content and decrease in plagioclase content compared to the lowermost portion of Unit II (see “Lithology”). Clays may lead to spuriously high porosity values, if the drying process during MAD analyses removes interlayer water from smectite. Clays may also decrease permeability of the sediments and rocks in this interval, allowing elevated pore fluid pressures to develop more readily.

P-wave velocity and electrical conductivity in discrete samples

At Site C0007, P-wave velocity and electrical conductivity were acquired from discrete samples from ~300 to 450 m CSF. This limited depth range encompasses the Unit II/III boundary, and data show interesting trends that seem to correlate fairly well with findings at nearby Site C0006.

Figure F52 shows the variations of P-wave velocity, electrical conductivity, and their respective in-plane and transverse anisotropies (see “Physical properties” in the “Expedition 316 methods” chapter). P-wave velocity varies between 1800 and 2000 m/s and increases slightly with depth but then decreases between 360 and ~440 m CSF (Fig. F52A). The trend changes sign at the Unit II/III boundary. The scarcity of data makes interpretation difficult. It is possible that the apparent decrease below 360 m CSF is the combination of a P-wave velocity offset (approximately –100 m/s) at the Unit II/III boundary and a velocity reduction associated with thrust faults observed at ~430–440 m CSF. The Unit II/III boundary is associated with a higher transverse velocity anisotropy (Fig. F52B), which could be indicative of horizontal compaction or vertical stress relief, causing hardening parallel to σ₁ or microcracking along the bedding plane.

Electrical conductivity (Fig. F52C) trends are inversely correlated to those observed in P-wave velocity. Electrical conductivity increases slightly to the Unit II/III boundary and then decreases. As with P-wave velocity, the lack of electrical conductivity data hinders interpretation. However, comparisons with data from nearby Site C0006 indicate that the results obtained in the trench-basin transition are consistent and make inferences possible. These comparisons suggest that electrical conductivity may be decreasing to ~330 m CSF and that there may be an offset at the Unit II/III boundary (see “Lithology”) followed by a more normal conductivity decrease trend with depth in Unit III. The progressive offset takes place. Transverse anisotropy (Fig. F52D) is significantly reduced at the Unit II/III boundary, possibly indicating horizontal compaction. This interpretation suggests that Site C0007 results between ~330 and ~370 m CSF represent intervals that were not recovered at Site C0006.

The offset observed in the electrical conductivity signal fits very well with trends observed in MAD porosity values. Below the Unit II/III boundary, higher electrical conductivity and lower P-wave velocity are consistent with the higher porosity discussed above.

Thermal conductivity

Thermal conductivity measurements were conducted on whole-round HPCS and ESCS cores (<316 m CSF) and on split core samples from RCB cores at depths >340 m CSF. Thermal conductivity ranges from 0.94 to 1.58 W/(m·K) (Table T19; Fig. F53A). Thermal conductivity from the seafloor to ~50 m CSF increases rapidly, likely reflecting the decrease in porosity with depth and relatively high sand content. A negative trend in thermal conductivity between ~50 and 75 m CSF likely reflects a decrease in sand content. Between ~190 and 410 m CSF, thermal conductivity values fall in a restricted range between 1.21 and 1.32 W/(m·K).

In situ temperature

In situ temperature was measured using both the advanced piston coring temperature tool (APCT3) and sediment temperature (SET) tool. All measurements were made in calm to moderate seas and were successful (Table T20). Temperature-time series for each temperature measurement are shown in Figure F54. The temperature tool was stopped at the mudline for as long as 10 min prior to each penetration. The average apparent bottom water temperature is 1.65°C (Table T20). Significant frictional heating occurred on all penetrations, and the temperature versus time records exhibit the characteristic probe penetration heating pulse and subsequent decay (Fig. F54). Equilibrium temperature estimates are based on a 1/time approximation from the temperature-time series while the tool is at the bottom. The effective origin time of the thermal pulse associated with tool penetration was estimated by varying the assumed origin time until the thermal pulse decay followed the theoretical curve. A delay of 4–81 s from the initial penetration heating time was required to give a linear 1/time plot. The temperature measurement at 52.6 m CSF shows multiple temperature spikes consistent with tool movement while in the formation (Fig. F54D). As a result, the equilibrium temperature is based on a shorter time series than the other measurements. However, in spite of the short period over which this equilibrium temperature is estimated, it falls close to the thermal gradient defined with the other temperature-depth measurements, giving confidence in this temperature determination (Fig. F53C). Additionally, it is important to note that temperatures derived from the APCT3 and SET tools give consistent results. Equilibrium temperatures as a function of depth are quite linear. The best-fit thermal gradient through the first six measurements is 42°C/km (Fig. F53C).

Heat flow

If heat transfer is by conduction and heat flow is constant, the thermal gradient will be inversely proportional to thermal conductivity according to Fourier’s law. This relationship can be linearized by plotting temperature as a function of summed thermal resistance (Bullard, 1939) (see the “Expedition 316 methods” chapter) (Fig. F53D). The least-squares fit to the temperature and thermal resistance data indicates a heat flow of 53 mW/m² and a bottom water temperature of 2.0°C. The estimated bottom water temperature is 0.4°C different than the measured apparent bottom water temperature (Table T19). Predicted temperature deviates from those measured by <0.2°C. A constant conductive heat flow appears to describe the overall thermal structure quite well. The computed heat flow is anomalously low with respect to other heat flow values in the Kii region (Yamano et al., 2003) but higher than estimated for Site C0006.

Shear strength

Shear strength measurements were conducted using a semiautomated miniature vane shear device and a pocket penetrometer. Measurements were made at discrete locations on the working halves of split cores at a frequency of approximately three measurements per core (generally in sections 2, 5, and 7). Tests were conducted on relatively intact and homogeneous parts of these sections, generally between 50 and 100 cm below the top of the section. To minimize error induced by disturbance of the core by the measurement technique, vane shear tests were conducted first, followed by penetrometer tests 1–2 cm from the site of the vane shear penetration. No vane shear measurements were conducted on whole cores. Both instruments were inserted orthogonally to the surface of the working halves. The rotation rate of the semiautomated vane shear apparatus on board the Chikyu is 71°/min.

Three distinct intervals of increasing shear strength with depth were observed (Fig. F55). The intervals are offset from each other by decreases in shear strength. The first interval extends from the seafloor to ~35 m CSF (the Unit I/II boundary). Shear strength through this interval increases at a much faster rate than at similar depths at Sites C0004 and C0006. At 35 m CSF, shear strength drops by a factor of 2 and increases to 55 m CSF, at which point it decreases to the lower limit of our ability to measure it. Shear strength then increases to 91 m CSF. At depths below this point, the consolidation state of the cores makes further interpretation difficult. As with Site C0006, low shear strength values for lower depths do not necessarily represent in situ shear strength at these depths. Both vane shear and penetrometer measurements are designed for use in clay and thus may underestimate the shear strength of sand.

Color spectrometry

Results from the measurement of color reflectance are presented in Figure F56. The L* values range from ~20 to 50. The a* values range from –2 to 2 and the b* values range from 0 to 4. There are no significant anomalies.

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using whole-round core MSCL in all recovered cores from Site C0007 (Fig. F57). Uncorrected magnetic susceptibility values are presented in this chapter. Magnetic susceptibility varies between ~0 and 600 × 10^–3 SI with a mean of ~137 × 10^–3 SI. Magnetic susceptibility values in Unit I generally increase with depth. Within Unit II, values reach a maximum and then generally decrease with depth. Values within Unit III are low, possibly reflecting lithologic variation.

Natural gamma ray

Natural gamma ray (NGR) results are reported in counts per second (cps). The background scatter, produced by Compton scattering, photoelectric absorption, and pair production was measured at the beginning and subtracted from measured gamma ray values. In general, NGR counts are low and consequently may be affected by the short counting interval and porosity variations. The average and standard deviation of NGR results are 34 and 8 cps, respectively (Fig. F58). NGR values generally decrease from the seafloor to ~150 m CSF. Between ~150 m CSF and TD, NGR values remain relatively constant.

Integration with seismic data

There are no logging-while-drilling (LWD) data at this site, and core-seismic integration between these holes is primarily based on prestack depth-migrated seismic profiles. The distance between Holes C0007A, C0007B, C0007C, and C0007D is 185 m (Fig. F1), and these holes are ~700 m from Holes C0006E and C0006F. Seismic profile Inline 2437 from the CDEX 3-D seismic survey (Fig. F2) passes through Hole C0007D and is ~10 m from Holes C0007A, C0007B, and C0007C. Holes C0006E and C0006F are 50–60 m from this seismic profile and are projected onto it.

The accuracy of core-seismic integration is degraded by both the lack of LWD data in general and low to poor core recovery in some sections of the holes. Nevertheless, NGR and MSCL-W data from core logs are superimposed over the seismic data (Fig. F59) and some key reflections match well with core data, likely imaging key sedimentary facies and faults. The Unit II/III boundary at 362 m CSF corresponds to a high-amplitude reflection with no depth shift. Preliminary interpretations suggest the Unit III/IV boundary at 475 m CSF may tie to the reflector at 450 m CSF, indicating a depth difference of 25 m. Furthermore, the lithology of Core 316-C0007D-25R is sand, consistent with seismic interpretations. However, poor core recovery of this section makes seismic-core integration difficult.

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