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doi:10.2204/iodp.proc.314315316.134.2009

Physical properties

At Site C0006, 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 270 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 C0006 were determined from both GRA measurements on the MSCL 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 613 discrete samples were analyzed for MAD (14 from Hole C0006C, 14 from Hole C0006D, 507 from Hole C0006E, and 78 from Hole C0006F).

MAD wet bulk density increases dramatically from 0 to ~38 m CSF and then decreases slightly from ~38 to ~50 m CSF, followed by a gradual increase to ~400 m CSF (Fig. F48A). Maximum MSCL bulk density values (one component of the MSCL logging suite) also show similar trends. MSCL bulk density is generally lower than MAD bulk density, likely because of voids in the unsplit cores, and reveals a larger degree of scatter compared to density from discrete samples. The scatter in MAD bulk density is likely due to lithologic variations among the interbedded sand, silt, and mud at this site. Grain density averages ~2.7 g/cm³ (Fig. F48B) and increases slightly (~0.05 g/cm³) from 0 to 570 m CSF. Grain density values >3 g/cm³ are likely artifacts.

Porosity was estimated from whole-round core MSCL scans and calculated from MAD measurements on discrete samples. MAD porosity (see “Physical properties” in the “Expedition 316 methods” chapter) generally decreases with depth (Fig. F48C) and varies inversely with bulk density. Because of the greater scatter in MSCL density and porosity values, we focus on bulk density and porosity values calculated from MAD measurements.

Density and porosity of lithologic units

At Site C0006, bulk density of the Pleistocene trench-slope transition package of mud and interbedded sand comprising Unit I (see “Lithology”) increases in the upper 5 m from ~1.80 to ~1.90 g/cm³ and then gradually increases from ~1.90 to 1.92 g/cm³ with depth to 27 m CSF (Fig. F48A). As density increases, porosity decreases (Fig. F48C).

Unit II is composed of trench-wedge and trench-basin transitional sand, silt, and mud of Pleistocene age (see “Lithology”). From Subunit IIA through the upper one third of Subunit IID (i.e., from ~27 to ~410 m CSF), bulk density gradually increases from ~1.92 to ~2.07 g/cm³ (Fig. F48A). This trend is mirrored by a gradual decrease in porosity with depth from ~46% at the top of Unit II to ~38% at ~410 m CSF (Fig. F48C). Gaps in the data in Unit II mark regions of low core recovery and likely indicate the presence of fault zones (see “Structural geology”). In the bottom two-thirds of Subunit IID (from ~410 to ~450 m CSF), bulk density decreases rapidly from ~2.07 to 1.94 g/cm³ (Fig. F48A), whereas porosity increases from ~39% to ~49% (Fig. F48C).

From ~450 to ~475 m CSF, bulk density decreases and porosity increases. At ~475 m CSF, however, the bulk density and porosity trends reverse, increasing and decreasing, respectively, with increasing depth (Fig. F48A, F48C). From ~475 to 570 m CSF, bulk density increases from ~1.90 to ~2.10 g/cm³ (Fig. F48A). Similarly, porosity decreases dramatically from ~49% to ~40% from ~475 to ~570 m CSF.

Relationships of density and porosity changes with unit and structural boundaries

MAD-derived bulk density and porosity increase and decrease, respectively, from 0 to ~410 m CSF and reveal no clear discontinuities at depths corresponding to possible faults at 235, 277, and 367 m CSF or at the Unit I/​Subunit IIA, Subunit IIA/IIB, or Subunit IIC/IID lithologic boundaries within that depth range. A discontinuity in the porosity versus depth trend at ~410 m CSF occurs within Subunit IID and coincides with a thrust fault at ~410 m CSF inferred from seismic reflection data and above the top boundary of the shear zone in the interval from 434 to 440 m CSF identified in the split core.

The zone of highest porosity (~50%) between ~450 and ~490 m CSF is bounded by faults. This zone of elevated porosity possibly results from increased occurrences of fluid-filled microcracks and other fault-related damage. Alternatively, elevated porosity in this zone could indicate that these sediments are underconsolidated, reflecting 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 sample. This observation suggests that these are zones of fault-related damage. In either case, this interval appears to be a zone of significant faulting.

We also note that the interval of elevated porosity appears to correlate with an interval of increased clay content. Lithologic analyses of the split core reveal that Unit III exhibits an overall increase in clay content and decreases in quartz and feldspar contents compared to the overlying Subunit IID (see “Lithology”). Clays may lead to spuriously high porosity because the drying process during MAD measurement removes interlayer water from smectite (Brown and Ransom, 1996). Clays may also decrease permeability of sediments and rocks in this interval, allowing pore fluid overpressures to develop more easily.

From ~490 to 570 m CSF, porosity decreases with depth, though porosity values in this interval are still higher than might be predicted based on the overall trend observed from ~5 to 410 m CSF. These higher than expected porosity values are probably explained in part by lithologic and compositional differences between units, which result in different porosity-depth trends. The mismatch may also indicate that rocks in this interval are significantly damaged by the overlying fault or faults from ~434 to ~490 m CSF and the faults from ~533 to ~560 m CSF, identified on the basis of seismic data and structural geology observations. Alternatively, the high porosity could be due to the presence of interlayer water in clays and/or overpressured fluids.

P-wave velocity and electrical conductivity in discrete samples

P-wave velocity and electrical conductivity were measured on cubic samples along x-, y-, and z-directions of the core reference (see “Physical properties” in the “Expedition 316 methods” chapter). Samples were relatively indurated, and no noticeable disaggregation was observed when immersed in seawater. This induration is in marked contrast to the behavior observed in samples from Site C0004 through the shallow megasplay. The electrical conductivity data set is more extensive than that collected for P-wave velocity. The instrument used to measure P-wave velocity requires an applied normal stress of ~120 kPa, which caused deformation to samples from depths shallower than 230 m CSF, voiding the measurement. Conversely, in some deeper cores, P-wave velocity measurements were easier to obtain because of the difficulty of sawing adequate faces for electrical conductivity measurements. Results of electrical conductivity, P-wave velocity, and associated anisotropy measurements are plotted in Figure F49.

Electrical conductivity (Fig. F49A) decreases overall with depth. Although the data are highly scattered throughout the uppermost 300 m, below 300 m CSF electrical conductivity manifests two recognizable trends in the intervals from ~300 to ~400 m CSF and from 470 to 600 m CSF. Both trends show a decrease in electrical conductivity but are offset by ~10% through an increase in electrical conductivity within Subunit IID. These trends and offsets are also observed in LWD resistivity-at-the-bit (RAB) data, which improves our confidence that this pattern is real (see the “Expedition 314 Site C0006” chapter). Additionally, these trends mimic trends observed in MAD porosity (Fig. F48C). MAD porosity values are offset by ~10%, similar to the offset observed in electrical conductivity data. Between 410 and 470 m CSF, poor core condition prevented adequate sampling.

Electrical conductivity anisotropy shows considerable scatter at depths shallower than 300 m CSF (Fig. F49B). At ~200 m CSF, three sets of measurements indicate high anisotropy and are associated with significantly lower magnitudes of electrical conductivity. Between 300 and 400 m CSF, transverse anisotropy decreases with depth and, coupled with the decrease in electrical conductivity (Fig. F49A), might indicate progressive compaction with depth.

P-wave velocity (Fig. F49C) increases from ~1800 m/s a little below 200 m CSF to ~2100 m/s near the bottom of Hole C0006E. At ~300 m CSF and again at ~400 m CSF are two zones that correspond to increased scatter in both P-wave velocity and electrical conductivity measurements. P-wave velocity also decreases by as much as 300 m/s through discrete intervals at 520, 550, and 570 m CSF. Although these velocity differences could potentially be associated with faults, microstructural data are needed to conclusively establish the cause of these low-velocity zones. The anisotropy of P-wave velocity (Fig. F49D) generally decreases with depth with two discrete zones of higher transverse anisotropy at ~400 and ~480 m CSF.

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 from >316 m CSF. Thermal conductivity ranges from 0.51 to 1.61 W/(m·K) (Table T19; Fig. F50A). Thermal conductivity from the seafloor to ~80 m CSF shows a striking positive excursion through Unit I and Subunit IIA that likely reflects higher sand content through this interval. A negative trend in thermal conductivity is observed through Subunit IIA, likely reflecting a decrease in sand content. Between ~80 and 400 m CSF, thermal conductivity generally increases, likely reflecting decreasing porosity. As porosity decreases, thermal conductivity increases as water is forced out of void spaces because the thermal conductivity of sediment is greater than water. Thermal conductivity values in this region show large scatter relative to the mean, likely reflecting the effects of interbedded sands and clays. Thermal conductivity in Unit III increases with depth but is offset to lower values relative to the trend through Subunits IIB and IIC.

In situ temperature

In situ temperature was measured using both the APCT3 and DVTP. Three of six deployments of the APCT3 and three of seven deployments of the DVTP were successful (Table T20). All measurements were made in calm to moderate seas. Temperature-time series for each temperature measurement are shown in Figure F51. 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. F51). Equilibrium temperature estimates are based on a 1/time approximation from the temperature-time series while the tool is in the bottom sediments. 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 15–106 s from the initial penetration heating time was found to be required to give a linear 1/time plot. Because of tool movement in the bottom, the time over which equilibrium temperatures are estimated is variable. Equilibrium temperature estimates are based on relatively short fits at 98.3 and 220.3 m CSF with fitting times of 2.8 and 3.3 min, respectively. However, in spite of the short period over which these equilibrium temperatures are estimated, and with the exception of the measurement at 277.3 m CSF, resulting temperatures form a relatively constant gradient with depth giving confidence to individual temperature determinations (Fig. F50B). Additionally, it is important to note that temperature derived from both the APCT3 and DVTP give consistent results.

Equilibrium temperatures as a function of depth are quite linear, with the exception of the measurement at 277.3 m CSF. The best fitting thermal gradient through the first six measurements is 27°C/km (Fig. F50B). The equilibrium temperature at 277.3 m CSF falls well below the thermal gradient derived above with a temperature only slightly warmer than bottom water. The low thermal gradient coupled with this anomalously cold temperature at depth suggests that colder than normal conditions exist around Hole C0006E.

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. F50C). In the following analysis, the temperature measurement at 277.3 m CSF is not included. The least-squares fit to the temperature and thermal resistance data indicates a heat flow of 33 mW/m² and a bottom water temperature of 2.1°C. The estimated bottom water temperature is only 0.4°C different than the measured apparent bottom water temperature (Table T19). Predicted temperature deviates from those measured by <0.2°C. With the exception of the measurement at 277.3 m CSF, a constant conductive heat flow appears to describe the overall thermal structure quite well. This heat flow value is anomalously low with respect to other heat flow values along the Nankai Trough and in the vicinity of the Kii transect (Yamano et al., 2003).

Two features of the temperature-depth profile at Site C0006 are important. The first is the very low but constant thermal gradient, which gives rise to an anomalously low heat flow value. The second feature is the low temperature measurement at 277.3 m CSF. The heat flow value appears to be a local feature because it is anomalously low relative to heat flow values at Sites C0004 and C0007, as well as regional data determined from gravity-driven probes. Possible explanations for this locally low heat flow value include the dissociation of hydrate and local downward flow of seawater. Because of the low thermal gradient, depths drilled at Site C0006 are all within the hydrate stability zone, precluding the widespread dissociation of hydrate. The lack of widespread hydrate is also confirmed by both interstitial water geochemistry and methane measurements on cores. Inorganic geochemistry results do not indicate downward fluid flow. However, downward fluid flow seaward of Hole C0006E but close to the hole may induce the locally low thermal gradient. The low temperature measurement at 277.3 m CSF is 6.6°C cooler than predicted by the best-fit conductive gradient (Fig. F50B). This temperature measurement coincides with a sharp negative anomaly in both sodium and chlorinity concentrations (see “Inorganic geochemistry”) consistent with the local presence of methane hydrates. However, a 6.6°C temperature drop associated with the dissociation of hydrate seems implausible. Another explanation for the low temperature measurement is the infiltration of cold bottom water through the sediments prior to the temperature measurement because of drilling disturbance. The lithology at this depth is sandy silt and core recovery was poor (recovery = 38%). Thus it is possible that the low temperature reflects intrusion of seawater prior to the temperature measurement. Discerning the cause of the low heat flow will require additional work.

Shear strength

Shear strength measurements were determined 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 in sections 2, 5, and 7. Tests were conducted on relatively intact and homogeneous regions of these sections, generally somewhere 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, followed by MAD sampling. 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.

Results of shear strength measurements from the vane shear and penetrometer methods are presented in Figure F52. Both measurement techniques were applied at regular intervals to 270 m CSF in Hole C0006E. An additional measurement was taken in an apparently unconsolidated sandy layer in Core 316-C0006E-31X at 321 m CSF, but this was not a characteristic lithology for either the core or Subunit IIC as a whole.

We observe a rapid increase in shear strength in the uppermost 40 m CSF in all of Unit I and into the top of Subunit IIA. At 40 m CSF, shear strength drops rapidly and is associated with the prevalence of sandy beds. Both vane shear and penetrometer measurements may differ for fine-grained and coarse-grained sediments because of variations in partial drainage behavior during the test. Deformation of low-permeability clay-rich sediments is likely to be nearly undrained, whereas the deformation of highly permeable coarse-grained sediments is likely to be nearly drained. As a result, direct comparison between values for substantially different lithologies is not straightforward.

The Subunit IIA/IIB boundary is coincident with an increase in shear strength at 72 m CSF. A similar increase at 110 m CSF does not appear to correlate with a significant lithologic change. Shear strength continues to increase to 200 m CSF. Between 200 and 275 m CSF, there is an apparent decrease in shear strength. This decrease will need to be corroborated by shore-based research, as there is some bias in both methods due to changing deformation style as rocks become more cohesive. Vane shear and penetrometer techniques are designed for pervasive deformation and failure of sediments and are not calibrated for measuring materials that fail because of the formation of discrete fractures.

Color spectrometry

Results from the measurement of color reflectance are presented in Figure F53. 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. At less than ~100 m CSF, there is a slight decrease in each index. There is no significant anomaly of these values with depth.

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the whole-round core MSCL in all recovered cores from Site C0006 (Fig. F54). Uncorrected magnetic susceptibility values are presented in this volume. Magnetic susceptibility varies between ~0 and 500 × 10^–3 SI with a mean of ~200 × 10^–3 SI. Magnetic susceptibility values in Unit I are relatively low with the exception of a positive spike at ~37 m CSF. Within Subunit IIA, values generally decrease with depth but oscillations may reflect interbedded sands and clays. Within Subunits IIB and IIC, magnetic susceptibility values are relatively constant with a relatively wide range between ~200 and 400 × 10^–3 SI. Values within Subunit IID decrease with depth and are very low through Unit III. These low values may be erroneous because of incomplete core recovery near the bottom of the hole.

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. F55). NGR values decrease through Unit I and Subunit IIA, increase with depth in Subunit IIB, and remain relatively constant through Subunits IIC and IID and Unit III where there are no significant anomalies.

Integration with seismic and logging-while-drilling data

Distances between Holes C0006B, C0006E, and C0006F are ~25 m and distances between Holes C0006C, C0006E, and C0006D are ~2.5 m (Fig. F2). Core-seismic integration between Holes C0006B, C0006E, and C0006D is primarily based on prestack depth-migrated seismic profiles of Inline 2435 from the Center for Deep Earth Exploration (CDEX) 3-D seismic survey (Fig. F2) and Cross-line 1755 from the Institute for Frontier Research on Earth Evolution (IFREE) 3-D seismic survey. Additionally, the structural and lithologic framework in Hole C0006F relies on Line 89 of the IFREE 3-D seismic survey for correlation. Thick sand layers provide key reflections for correlation. For example, the truncation points indicate an unconformity at the Unit I/​Subunit IIA boundary. However, because each of these holes intersects many faults, dips of strata change with depth, complicating correlations. Additionally, between 110 and 210 m seismic depth below seafloor (SSF) as well as between 330 and 530 m SSF, the seismic phases between holes are discontinuous.

In spite of the low vertical seismic resolution and discontinuous reflectors, core and log data between Holes C0006B, C0006E, and C0006F are integrated based on the comparison between NGR, MSCL P-wave velocity logs, and gamma ray and sonic logs from LWD data sets (Fig. F56A). Correlations between Holes C0006C, C0006E, and C0006D rely on MSCL NGR and P-wave velocity logs (Fig. F56B). The logs are well correlated because these holes are very close to each other. Generally, the estimated uncertainty in correlated depths is <10 cm. Correlations between Holes C0006E and C0006F have greater uncertainty. Core recovery in Hole C0006F was low, and therefore MSCL-W logs are incomplete for this hole. Furthermore, the LWD sonic log from Hole C0006B is absent at coring intervals of Hole C0006F, and comparison of MSCL-W NGR data and the LWD gamma ray log does not provide a unique solution. As a result of these complications, the correlation between Holes C0006B and C0006F relies to a greater extent on bedding and structural dips interpreted from seismic reflection data; borehole images from the RAB tool have also been used to correlate data between holes. This proved effective for some thick sand layers that are well matched between Holes C0006E and C00006F.

Depth-shifted profiles are shown in Figure F56 and depth shifts applied to Hole C0006B are given in Table T21. The depth shifts are based on 70 correlations between the five holes. The depth transfer functions were defined by linear regression of the correlated positions. Generally, the depth offsets were <8 m.

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