At Site 1178, laboratory measurements were made to provide a downhole profile of physical properties through slope-apron sediments and the upper portion of the accretionary complex. With the exception of short (<50 cm) and small-diameter sections, all cores were initially passed through the multisensor track (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. Because of time constraints at this last site, P-wave velocity and natural gamma-ray (NGR) measurements were not made. Voids and cracking caused by gas expansion, noted in cores between the mudline and ~400 mbsf, degraded MST measurements. Biscuiting in XCB and RCB cores also degraded measurements.
In each full core, four samples were selected for index properties measurements from undisturbed core. Measurements of dry volume and wet and dry mass were uploaded to the 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 on split cores or discrete samples 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 between the mudline and 60 mbsf, at which point the cores became too stiff for insertion of the vane shear device. Electrical conductivity measurements were taken at least once per core. Raw data and calculated physical properties data are available from the Janus database for all MST, moisture and density, thermal conductivity, velocity, and shear strength measurements (see the "Related Leg Data" contents list). Because electrical conductivity data are not currently available from the database, they are included in Table T19.
Sediment bulk density was determined by both the GRA method on unsplit cores and the mass/volume method ("index properties") 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 and XCB cores (Fig. F26A, F26B). Both moisture and density measured on discrete samples and GRA density measurements show similar downhole trends. However, GRA densities exhibit considerable scatter and average GRA densities from APC cores are generally higher than core measurements, whereas those from XCB and RCB cores are generally lower than those measured on discrete samples.
Grain densities determined from dry mass and volume measurements are essentially constant throughout lithostratigraphic Subunit IA (upper slope-apron facies), with a mean value of 2.65 g/cm3 (Fig. F26C). At the base of Subunit IA (94 mbsf), grain densities increase sharply to a mean value of 2.72 cm3 and remain nearly constant to the base of the hole. Scatter in grain density values also increases below 94 mbsf (Fig. F26C).
Overall, porosity at Site 1178 is characterized by a typical compaction trend, decreasing gradually with depth from ~63%-70% at the mudline to 26%-35% at 672 mbsf. Porosity values within lithostratigraphic Subunits IB and IC (middle and lower slope-apron facies) are more scattered than within Subunit IA and Unit II. The increased scatter may be attributed to lithologic variations in this relatively sand-rich part of the section.
Deviations from the general compaction trend occur at 70-100 and 140-160 mbsf, where porosity increases. In general, the boundaries between lithostratigraphic units are not characterized by large changes in porosity (Fig. F26A, F26D), although the boundary between lithostratigraphic Units I and II at 199 mbsf does coincide with a short interval of slightly decreased porosity.
Undrained shear strength measurements were made using a miniature automated vane shear (AVS) and were conducted exclusively in fine-grained silty clays. Shear strengths increase gradually downhole from 15 kPa at the seafloor to values >130 kPa at 60 mbsf (Fig. F27). Scatter in the data reflects 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. Below 342 mbsf (Core 190-1178A-37X), insertion of the needle caused fracturing, so a half-space method was used on split cores. Half-space measurements were sparse because of lack of core pieces of sufficient size.
Within lithostratigraphic Subunit IA (upper slope-apron facies), thermal conductivity averages 1.1 W/(m·°C), which is slightly higher than seen at similar depths at other sites. A drop in thermal conductivity is observed at the bottom of this subunit. Between 100 and 340 mbsf, thermal conductivity averages 1.2 W/(m·°C) and does not show any distinct changes at lithologic or stratigraphic boundaries. Between 340 mbsf and the bottom of Hole 1178B, the limited measurements indicate values of 1.5 W/(m·°C) or greater.
A conductive heat flow of ~51 mW/m2 was defined by shipboard thermal conductivity and downhole temperature measurements to ~115 mbsf (see "In Situ Temperature and Pressure Measurements Section"). Using this estimated heat flow and measured shipboard thermal conductivity, projected downhole temperatures reach ~20°C at 400 mbsf, the depth of the BSR, and ~25°C at the bottom of Hole 1178B (Fig. F28B).
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 liner (x-axis). In XCB cores, sample cubes were cut and measurements in all three directions were performed using the PWS3 contact probe system. Poor core quality in RCB cores prevented measurements in Cores 190-1178A-10X through 18X. Subsequent measurements were compromised by fissility, foliation, and fractures. Samples taken below 200 mbsf also exhibited high attenuation, which made determination of travel times difficult. Velocities generally increase with depth (Fig. F29) but display large scatter and some abnormally low values between 320 and 410 mbsf. The BSR lies near the base of this interval, at ~400 mbsf on the depth-converted seismic section.
Measurements were made on APC cores with a four-needle 30-kHz electrode array. On XCB cores, conductivity was measured on the same sample cubes used for P-wave measurements with a 30-kHz two-electrode system. Apparent formation factor measured on unsplit cores is given in Table T20. The electrical conductivity and formation factor measured on the sample cubes are given in Table T19. Values appear more scattered than at the other sites, most notably in the z-direction (Fig. F30A). Vertical anisotropy develops below 220 mbsf, but values are scattered in both the horizontal and vertical planes (Fig. F30B). Most sample cubes were cut relative to the core axis, but bedding dip was generally steeper than at the other sites, and zones of steep bedding dip were not confined to narrow intervals. Furthermore, a dipping foliation, often parallel to bedding, was observed in many cores (see "Structural Geology"). These observations caused us to reconsider our sampling procedure, and subsequent samples (below 651.4 mbsf) were cut parallel to the foliation. Samples cubes cut parallel to foliation or bedding appear in italics in Table T19. These data have a higher and more consistent vertical anisotropy (where the vertical axis is considered to be perpendicular to foliation or bedding) of 35%-50% and a lower horizontal anisotropy. This suggests that changes in foliation and bedding dip may be partially responsible for the data scatter.
Volumetric magnetic susceptibilities were measured on unsplit cores by the MST (Fig. F31). Uncorrected values of magnetic susceptibility from the Janus database were used. Magnetic susceptibility data show wide scatter in the uppermost 20 m. Between 20 and 80 mbsf, peaks of magnetic susceptibility data with values as high as 120 × 10-5 SI are observed. Between 80 and 190 mbsf, the magnetic susceptibility data show low scatter and almost constant values (20 × 10-5 to 40 × 10-5 SI). Between 190 and 360 mbsf, magnetic susceptibilities slightly increase and many peaks (as high as 170 × 10-5 SI) exist. Below 360 mbsf, the scatter of magnetic susceptibility data increases. Values range from 20 × 10-5 to 100 × 10-5 SI between 520 and 560 mbsf. Between 560 mbsf and the bottom of Hole 1178B, magnetic susceptibility values show large scatter.
Porosities at Site 1178 decrease with depth, from values of 63%-70% at the mudline to 26%-35% at 672 mbsf. Deviations from this compaction trend occur at 70-100 mbsf, 140-160 mbsf, and ~200 mbsf. Porosity values within lithostratigraphic Subunits IB and IC are more scattered than in Subunit IA and Unit II and probably reflect either lithologic variations in this sandier part of the stratigraphic section or deposition by slope failure processes. Grain density increases abruptly at the boundary between the upper and middle slope-apron facies (94 mbsf). Velocities and formation factors generally increase with depth and show considerable scatter. In general, there are no obvious differences in physical properties between the slope-apron deposits of Unit I and the underlying accreted sediments of Unit II.