At Site 1173, laboratory measurements were made to provide a downhole profile of physical properties at a reference site seaward of the accretionary complex. Data from tectonically undeformed sediments at the reference site allow comparison of physical properties with those from Site 808 (Leg 131) and from other sites drilled during Leg 190.
With the exception of extremely short (<50 cm) sections, all cores were initially passed through the MST before being split. Gamma-ray attentuation (GRA) and magnetic susceptibility measurements were taken at 4-cm intervals with 2-s acquisition times for all cores. P-wave velocity logger (PWL) measurements were taken at 4-cm intervals with 2-s acquisition times for APC cores. The PWL device yielded extremely scattered values on the APC cores and was turned off for XCB cores. These data are not discussed further. Natural gamma ray (NGR) was counted every 20 cm for 20-s intervals. Because abnormally slow data acquisition on the NGR caused delays, the interval was increased to 40 cm between 493.74 and 570.74 mbsf. Voids and cracking caused by gas expansion were noted in cores between 7 and 83 mbsf and degraded the MST measurements. Biscuiting in XCB cores also degraded measurements.
Moisture and density samples were selected from undisturbed core at regularly spaced intervals of two per section (75-cm resolution) between the mudline and 168.64 mbsf. Below this depth, time constraints and equipment capacity limited the sample frequency to one sample per section. Measurements of dry volume and wet and dry mass were uploaded to the ODP (Janus) database and were used to calculate water content, bulk density, grain density, porosity, void ratio, and dry bulk density. P-wave velocities were measured at a frequency of two to three per core. Measurements were taken in three directions when core conditions permitted.
Shear strength measurements were made near the P-wave core measurement locations from the mudline to 234 mbsf, at which point XCB coring began and the cores became too stiff for insertion of the vane shear device. Resistivity measurements were taken at least once per core, with additional measurements made if time permitted. Raw and calculated physical properties data are available from the Janus database for all MST, moisture and density, velocity, thermal conductivity, and shear strength measurements (see the "Related Leg Data" contents list). Because electrical conductivity and formation factor data are not currently available from the database, they are included in Tables T22 and T23, respectively.
Sediment bulk density was determined by both the gamma-ray attenuation (GRA) method on unsplit cores and the mass/volume ("index properties") method on discrete samples (see "Physical Properties" in the "Explanatory Notes" chapter). The GRA density data and the bulk densities determined by the mass/volume method are in good agreement for APC cores (Fig. F35A, F35B). On average, the GRA bulk densities are 0.1 g/cm3 lower than those determined from discrete samples because of the smaller diameter and biscuited nature of XCB cores from below 250 mbsf. Both moisture and density measured on discrete samples and GRA density measurements show similar downhole trends.
Grain densities determined from dry mass and volume measurements exhibit a shift at 340 mbsf from an average value of 2.70 g/cm3 above 340 mbsf to 2.77 g/cm3 below (Fig. F35C). The calculated porosity profile (which accounts for grain density) is shown in Figure F35D. Changes in porosity or the porosity-depth gradient generally correlate with the major lithostratigraphic boundaries identified at Site 1173 (see "Lithostratigraphy").
Porosities within lithostratigraphic Unit I (outer trench-wedge facies) are characterized by a general decrease with depth, from 72% to 79% at the seafloor to 56%-66% by 100 mbsf. Porosity values within lithostratigraphic Unit I show considerable scatter, which reflects lithologic variability within the turbidite-rich trench wedge. Typically, the lower porosity values (<55%) represent sands and silty sands, whereas the silty clays maintain consistently higher porosities at shallow depths (Fig. F35D). Additional scatter within the silty sand samples may reflect subtle differences in grain size and composition that were not distinguishable in hand specimens.
Although there is some scatter, porosity increases slightly within lithostratigraphic Unit II (upper Shikoku Basin facies; 102.14-343.77 mbsf), from 57% to 65% at ~102 mbsf to 62% to 69% at ~340 mbsf. This is a significant deviation from both normal compaction trends for silty clays (e.g., Hamilton, 1976; Athy, 1930) and the porosity trend within Units I and III above and below.
At 340 mbsf, porosity drops sharply to ~50%. This depth coincides with the boundary between lithostratigraphic Units II and III (upper and lower Shikoku Basin facies). The correlation between the sharp porosity decrease at ~340 mbsf and the boundary between lithostratigraphic Units II and III suggests that this porosity shift is controlled by lithology. Within the lower Shikoku Basin facies, porosities generally follow compaction trends for fine-grained marine sediments (e.g., Hamilton, 1976), decreasing to below 41% at 680 mbsf (Fig. F35A, F35D). A decrease in porosity is observed below 680 mbsf within lithostratigraphic Unit IV (volcaniclastic facies). However, core recovery was insufficient to determine if these values are representative.
Undrained shear strength measurements were made using a miniature automated vane shear (AVS) and were conducted exclusively in fine-grained silty clays. Shear strength was measured on samples above 234 mbsf (Core 190-1173A-25X). Below this depth, samples were sufficiently indurated that insertion of the AVS caused fracturing of the sediments. Shear strength increases systematically downhole from near zero at the seafloor to >100 kPa at 200 mbsf (Fig. F36). Scatter in the data increases below ~120 mbsf as a result of fracturing of sediment and opening of fractures at the tips of the AVS vanes during some measurements. For this reason, actual sediment strength is probably best reflected by the highest measured values.
Thermal conductivity was measured using one of two methods depending on core condition. For shallow, nonindurated samples, a needle probe was inserted into the unsplit core for a full-space conductivity measurement. For samples from below 285 mbsf, insertion of the needle caused fracturing, so a half-space method was used on split cores. Thermal conductivities range from 0.85 to 1.71 W/(mˇ°C) and generally increase with depth (Fig. F37A). Between the mudline and ~320 to 340 mbsf, thermal conductivities range from 0.85 to 1.2 W/(mˇ°C) with no distinct trend. Conductivities increase sharply below 340 mbsf and range from 1.32 to 1.71 W/(mˇ°C) to the base of the borehole. The sharp change in thermal conductivity at ~340 mbsf correlates with the abrupt drop in porosity at ~340 mbsf (Fig. F35D). This relationship is expected because the thermal conductivity of sediment grains is higher than that of pore fluid.
Assuming one-dimensional vertical conductive heat flow, shipboard thermal conductivities and downhole temperature measurements to 284 mbsf (see "In Situ Temperature and Pressure Measurements") define a heat flow of 180 mW/m2. Using this estimated heat flow and measured thermal conductivities, projected downhole temperatures reach ~110°C at 734 mbsf (Fig. F37B).
In APC cores, P-wave velocities were measured using the P-wave sensors 1 and 2 (PWS1 and PWS2) insertion probe system along the core axis (z-axis) and across the core axis (y-axis), respectively. The PWS3 contact probe system was used to measure P-wave velocities across the core liner (x-axis) (Fig. F38A). Measurements in more than one direction could rarely be obtained in the same interval because of unfavorable core conditions. Gas expansion within the sediment from the upper 83 m produced cracks in the cores, which increased attenuation and probably resulted in the very low and dispersed velocity values between 25 and 83 mbsf. In XCB cores, sample cubes were cut, and measurements in all three directions were performed using the PWS3 contact probe system.
Between 83 and 230 mbsf, velocities are ~1600 m/s and show no significant trend. Velocity progressively increases between 230 and 390 mbsf, whereas porosity does not decrease below ~102 mbsf until it drops sharply at 340 mbsf. The velocity profile does not exhibit the sharp changes seen in porosity and resistivity data at ~340 mbsf. At ~390 mbsf, there is a small (50 m/s) but sharp decrease in velocity that does not correlate with a porosity change or a lithostratigraphic boundary. However, this transition coincides with the décollement-equivalent horizon (see "Development of the Decollement Zone, Muroto Transect" in "Summary of Scientific Results" in the "Leg Summary" chapter).
In Figure F38B, data acquired from Hole 1173A are compared to a velocity-porosity relationship based on a weighted average approach with parameters appropriate for clayey silt (Lee et al., 1996). As expected for data acquired without confining pressure, velocities from Units I and III generally plot near or below this reference curve. However, the anomalous behavior of the lower part of Unit II is apparent on this velocity-porosity crossplot, where velocity is slightly higher than expected for the measured porosity. P-wave anisotropy appears significant only in lithostratigraphic Unit III (Fig. F38C). Between 340 and 460 mbsf, anisotropy in the vertical plane is constant at ~2% and then increases to 4%-6% at the bottom of the hole. Over this interval, porosity decreases from 45%-55% to 35%-40%.
Measurements were made on APC cores with a four-needle, 10-kHz electrode array. On XCB cores, conductivity was measured on the same sample cubes used for P-wave measurements with a two-electrode, 30-kHz system. Different frequencies were used in an attempt to minimize out-of-phase potentials that result from electrode polarization and dielectric effects. These effects were small in both cases, and tests performed during the cruise showed that measured conductivity was independent of frequency at least over the 5- to 50-kHz range.
Electrical conductivity and formation factor (see "Physical Properties" in the "Explanatory Notes" chapter) measured on the sample cubes are given in Table T22. For the needle-probe measurements, only the apparent formation factor is given. Needle-probe measurements in the first 260 m of the hole yield formation factors mostly between 3 and 5, with some values up to 7 (Fig. F39A). Coarser lithologies, which include silt, sand, and ash layers, appear more resistive. Between 50 and 250 mbsf, formation factor decreases slightly with depth. This is probably caused by the anomalously high porosity in the upper Shikoku Basin facies. Formation factor increases between lithostatigraphic Units II and III, following a porosity decrease (Fig. F39B). Formation factor increases steadily with depth below 450 mbsf.
Because measurements were performed along all three axes, conductivity anisotropy could be assessed. The most striking feature is the high conductivity anisotropy in lithostratigraphic Unit III, in which the sediment is generally more conductive across than along the core (Fig. F39C). Anisotropy is small (~5%) in Unit II and increases sharply to ~20% across the transition to lithostratigraphic Unit III between 320 and 360 mbsf. Anisotropy increases progressively downward to reach ~60% in the lower part of the section. The electrical conductivity anisotropy thus follows the same trend as P-wave anisotropy but is approximately 10 times larger. The conductivity anisotropy of the 8-cm3 cubes is much larger than the dispersion of the conductivity measurements on individual samples. This suggests that sample-scale anisotropy may cause formation-scale anisotropy in Unit III.
Anisotropy increases as porosity decreases (r = -0.853 on all data), suggesting that anisotropy may be related to clay compaction fabric. Tortuosity anisotropy that is caused by preferential orientation of clay particles has been proposed to explain the permeability anisotropy in clays (Arch and Maltman, 1990). The same mechanism could be applied to resistivity anisotropy. Note, however, that this explanation only takes into account the geometry of the porous network, and that surface conductivity of clays may also contribute significantly to the anisotropy of clayey sediments.
Volumetric magnetic susceptibilities were measured in all recovered cores from Site 1173 (Fig. F40). Uncorrected values of magnetic susceptibility from the Janus database were used. Large magnetic susceptibility values and large scatter between 0 and 100 mbsf are correlated with the occurrence of sand-rich turbidites. Less scatter occurs below 100 mbsf, and a minimum occurs at ~200 mbsf. Below 200 mbsf, magnetic susceptibility increases continuously to 400 mbsf. The hemipelagic mudstones between 400 and 540 mbsf are characterized by three cycles of increasing magnetic susceptibility up to 150 × 10-5 SI. Magnetic susceptibility data show a slight decrease from 540 to 734 mbsf.
NGR results are presented in counts per second (cps) (Fig. F41). The background scatter, produced by Compton scattering, photoelectric absorption, and pair production, was measured at the beginning (6.39 cps) and subtracted from the measured gamma-ray values. In general, NGR counts are low and are consequently likely to be affected by the short counting interval and by porosity variations. Between 0 and 70 mbsf, NGR shows an increase from ~7 to 17 cps. There is considerable scatter between 70 and 100 mbsf. It is not clear what causes the downhole increase of radioactivity in these turbidites containing clays and fine to medium sands. Between ~100 and 340 mbsf, NGR is characterized by a continuous decrease from 10 to 3 cps and considerably less scatter. NGR exhibits an abrupt increase at 340 mbsf to ~10 cps, followed by a decrease to ~6 cps down to ~405 mbsf. From 405 to 734 mbsf, NGR shows an increase to 15 cps, with increasing scatter downhole. This variation does not correspond to any observed variations in porosity or density. Because these silty claystones show only slight changes in composition, the high scatter in count rates may be produced by an insufficient counting rate or changes in clay mineralogy.
Bulk-density, porosity, gamma-ray, and velocity values derived from core measurements can be compared with wireline logging measurements obtained with the triple-combo tool (see "Downhole Measurements").
Bulk-density data measured on cores and logging data show a reasonably good agreement (Fig. F42A). In the upper section (100-340 mbsf), bulk-density values from logging data show slightly higher values (~0.05 g/cm3) than those derived from core measurements. The sharp increase of bulk densities at 340 mbsf is observed in both the core and logging measurements. The logging data from 360 to 435 mbsf are characterized by a higher scatter, possibly as a result of downhole conditions (see "Downhole Measurements"). At this depth, the bulk densities measured on cores are ~0.1 g/cm3 higher than the logging data.
The velocity data measured on cores (z-axis) and those logged by the DSI show a similar trend (Fig. F42B), although the logging data are characterized by higher values (~100 m/s) than the core data. Higher downhole velocities are not surprising because of the greater confining stress within the borehole. A comparison of the MST NGR data with downhole logging gamma-ray measurements shows that there is almost no agreement (Fig. F42C). These differences may be caused by problems with the counting rate on the MST.
Variations in physical properties correlate well with lithostratigraphic units. The turbidites of the outer trench wedge are characterized by highly variable porosity, which generally decreases with depth. Porosity increases at the boundary between the outer trench-wedge and the upper Shikoku Basin facies and continues to increase slightly with depth. These elevated porosities deviate from a typical compaction profile for silty clays. At the boundary between the upper and lower Shikoku Basin facies (~340 mbsf), grain densities increase slightly and porosities decrease sharply. This porosity decrease is accompanied by increasing thermal conductivity, electrical resistivity, and P-wave velocity.
The anomalously high porosities within the upper Shikoku Basin facies may be caused by slight cementation, which could support the overburden and thus maintain high porosities. This explanation is supported by the observed increase in P-wave velocity with depth below ~220 mbsf, which is not accompanied by any porosity decrease. Alternatively, the apparent high porosity may be an artifact of shipboard measurements caused by a significant component of water-bearing minerals, such as smectite (e.g., Brown and Ransom, 1996). The increase in grain density at ~340 mbsf may indicate a primary lithologic difference or a diagenetic boundary (e.g., Nobes et al., 1992) across the upper to lower Shikoku Basin facies boundary.
Shipboard measurements indicate a sharp increase in thermal conductivity at 340 mbsf that corresponds to the sharp porosity decrease observed at this depth. Using the shipboard thermal conductivity measurements and shallow (<284 mbsf) temperature measurements that indicate a basal heat flow of 180 mW/m2, temperatures are projected to the base of the borehole at 734 mbsf. This extrapolation assumes only vertical conductive heat flow and predicts a bottom-hole temperature of ~110°C.
The physical properties of sediments drilled at Site 1173 are consistent with the results of previous DSDP and ODP sites in the region (Sites 582 and 808). Site 582, in the Nankai Trough southwest of Site 1173, penetrated the outer trench-wedge and the upper Shikoku Basin facies. Data from Site 582 show the same pattern of decreasing porosity with depth in the outer trench wedge and a gradual increase of 4%-8% in porosity into the upper Shikoku Basin facies as seen at Site 1173 (Kagami, Karig, Coulbourn, et al., 1986). Site 808, located ~13 km landward of Site 1173 and ~3 km landward of the deformation front, penetrated the outer trench-wedge and the upper and lower Shikoku Basin facies. As at Site 1173, a decrease in porosity was observed at the top of the lower Shikoku Basin facies (Taira, Hill, Firth, et al., 1991). At Site 808, however, porosities decrease with depth within the upper Shikoku Basin facies, indicating an increase in consolidation with depth that is not observed at Site 1173. This may be attributable to tectonic loading and deformation at Site 808 and could also reflect deformation and collapse of weakly cemented sediments.
At Site 808, the décollement zone occurs within the lower Shikoku Basin facies. Based on physical properties data, it is difficult to distinguish a potential protodécollement horizon. Most of Unit III shows continuously decreasing porosities and increasing velocities. At ~390 mbsf (the décollement-equivalent horizon), P-wave velocities decrease.