PHYSICAL PROPERTIES

Measurements on whole-round core sections taken in Holes 1150A and 1150B included magnetic susceptibility, GRA bulk density, P-wave velocity, NGR activity, and thermal conductivity. P-wave velocity, shear strength, and index properties measurements were made on what appeared to be the least-disturbed portions of split-core sections. Hence, sampling was biased and favored indurated sections in XCB (112-723 mbsf) and RCB (703-1180 mbsf) cores. Descriptions of the experimental methods are provided in "Physical Properties" in the "Explanatory Notes" chapter.

Magnetic susceptibility was measured with a 2-cm sampling interval on all cores recovered in Holes 1150A and 1150B. The results of magnetic susceptibility measurements are discussed in "Paleomagnetism".

GRA bulk density was measured with a 2-cm sampling interval on all cores at this site. In addition, a number of whole-round core sections from Hole 1150B (i.e., 42R-4, 43R-5, 44R-2, 47R-2, and 50R-5) were rerun with a 1-cm sampling interval to aid the selection of suitable whole-round core samples for postcruise laboratory tests. The GRA data are most reliable in filled APC and XCB cores, whereas RCB cores generally yield lower values because the core does not completely fill the liner. For the preliminary onboard processing, the GRA data from RCB cores in Hole 1150B were corrected because the cores did not completely fill the core liner (see "Physical Properties" in the "Explanatory Notes" chapter). In general, RCB cores in Hole 1150B had rather constant diameters and few crushed intervals (see the "Core Descriptions" contents list). With a diameter of 57 mm, we obtained a good fit with bulk density measurements on discrete samples and similar magnitudes as the overlapping GRA measurements on XCB cores in Hole 1150A. Finally, the density data were edited by removing values less than 1.0 g/cm³ (i.e., density of pure water). The maximum GRA bulk density is reported here because it is assumed to provide the best estimate of bulk density.

The maximum GRA bulk density values in Hole 1150A have a total range from ~1.3 to ~2.1 g/cm³, and the corrected maximum GRA bulk density values in Hole 1150B have a total range from ~1.5 to ~2.5 g/cm³ (Fig. F53). Generally, these values correspond very well with those derived from discrete samples and logging (see "Comparison of Core and In Situ Physical Properties Measurements"). GRA bulk density increases from ~1.3 to ~1.8 g/cm³ in the upper 70 m of Hole 1150A. This is followed by a gradual decrease to 1.4 g/cm³ at 200 mbsf. Rather constant values, ranging from 1.3 to 1.5 g/cm³, are measured from 200 to ~620 mbsf. The GRA bulk density values increase to a maximum value of 2.1 g/cm³ at ~915 mbsf. The interval from 915 to 1047 mbsf is characterized by lower and rather constant values (1.7-1.8 g/cm³). A shift to higher and more scattered values occurs at 1047 mbsf (i.e., top of lithologic Unit IV, see "Lithostratigraphy"), and GRA bulk density ranges from 1.8 to 2.2 g/cm³ to the base of Hole 1150B.

P-wave velocity in the horizontal direction was acquired with the P-wave logger (PWL) on full sections from the mudline to 65 mbsf and from 82 to 112 mbsf in Hole 1150A. The cores in these intervals were severely disturbed by gas expansion, which resulted in mechanical stretching and micro- to macrofracturing of the cores and explosive breakage of the core liners in two cores (186-1150A-10H and 11H). As a result, no MST measurements were made from 84 to 93 mbsf and from 96 to 103 mbsf. After the preliminary onboard processing (see "Physical Properties" in the "Explanatory Notes" chapter), the results of PWL measurements included data only from the upper 17 m of Hole 1150A. P-wave velocity varies between 1450 and 3237 m/s. Most values range between 1470 to 1500 mbsf (Fig. F54). Higher velocities (>1650 mbsf) are obtained near the edges of core sections and are assumed to be artificial.

NGR activity was measured every 20 cm with 20-s-long counting periods on all cores recovered in Holes 1150A and 1150B and reported in cps with the total background radiation subtracted (12.27 cps). NGR activity has an overall range between 2 and 27 cps, and the downhole variation generally has a width of scatter of 5 to 10 cps (Fig. F55). The maximum values at each depth of NGR activity are reported here.

After a rapid increase in NGR activity from 7 cps at the seafloor to 17 cps at 7 mbsf, there is an increase to 26 cps at ~70 mbsf. Below this peak, NGR activity decreases to ~12 cps at 222 mbsf, the base of lithologic Unit I (see "Lithostratigraphy"). The NGR activity has an oscillating and somewhat decreasing trend between 222 and 598 mbsf (i.e., lithologic Unit II), and maximum values generally range from 10 to 15 cps. Lithologic Unit III (598-1047 mbsf) is characterized by slightly higher and more scattered values (~10-20 cps). Another shift occurs at the top of lithologic Unit IV, and NGR activity decreases from ~20 cps to ~15 cps at 1180 mbsf.

Thermal conductivity was measured at a frequency of one measurement per core down to 419 mbsf in Hole 1150A using the full-space configuration. The mean error associated with these measurements is assumed to be 0.2 W/(m·K). The average thermal conductivity, which was obtained from three measurements, ranges from 0.4 to 0.9 W/(m·K) (Fig. F56; Table T17, also available in ASCII format). The largest variability in values is observed in the upper 112 m of Hole 1150A (i.e., the section cored with the APC system), whereas a more uniform downhole trend is observed from 112 to 419 mbsf (i.e., the section cored with the XCB system). Apart from gas expansion, sediments cored with the APC system are little disturbed, and the variation in thermal conductivity generally has a positive correlation with bulk density and a negative correlation with porosity. On the other hand, poor correlation is obtained for thermal conductivity versus porosity and bulk density in sediments cored with the XCB system. This suggests that the drilling disturbances in XCB cores (i.e., drilling mud slurry) deteriorated the quality of the data.

P-wave velocity was measured in split cores with the PWS1, PWS2, and PWS3 systems. We aimed at a sampling frequency of one measurement per section. However, because of scattering and attenuation of the signal in the sediment specimen, the PWS1 and PWS2 systems could not be used below 10 mbsf, and the PWS3 system was not used from 0 to 11 mbsf and from 58 to 304 mbsf. PWS3 measurements were made through the liner (11-58 mbsf), on indurated pieces (304-710 mbsf), and on cylindrical minicores (710-1180 mbsf). For the latter, P-wave velocity was measured in two or three directions, allowing estimation of acoustic anisotropy. By combining declination data from the same depth interval as the minicores, we derived the orientation with respect to north, which provided an estimate of the azimuthal influence on the magnitude of horizontal velocity. Discrete measurements of P-wave velocity were made close to the location of samples for index properties in soft sediments and on the same piece or cylindrical minicore in more lithified sediments. The cores were much disturbed by gas expansion in Hole 1150A, which often resulted in velocity values less than 1450 m/s; therefore, they were deleted.

Horizontal P-wave velocity ranges from 1450 to 1570 m/s in the upper section of Hole 1150A, and the downhole trend is slightly increasing (Fig. F57; Table T18, also available in ASCII format). The four measurements of vertical P-wave velocity are rather constant and vary between 1510 and 1530 m/s (Fig. F57B; Table T18). Only horizontal velocity was measured from 304 to 710 mbsf; the data are well grouped and increase from ~1560 to 1750 m/s (Fig. F57A). The horizontal velocity drops to less than 1500 m/s below 750 mbsf, which is followed by a generally increasing trend to 1900 m/s at the base of Hole 1150B. The lower part of Hole 1150B (920 to 1010 mbsf) comprises a zone of a wider scatter of values (1460-2000 m/s). The vertical velocity shows a similar trend as that of horizontal velocity, but with slightly lower velocities. From 710 to 1180 mbsf, vertical velocity generally increases from ~1680 to 1880 m/s (Fig. F57B). Maximum horizontal and vertical velocities in Holes 1150A and 1150B (2150 and 2090 m/s, respectively) were measured at 1115 mbsf.

Three values of anisotropy were calculated below 710 mbsf in Hole 1150B (Table T18), namely: maximum vs. minimum horizontal velocity (A_Hh); maximum horizontal vs. vertical velocity (A_HV); and minimum horizontal vs. vertical velocity (A_hV) (see Eq. 7 in "Physical Properties" in the "Explanatory Notes" chapter). Anisotropy in the horizontal direction (A_Hh) generally varies from 0 to 0.05, with maximum values of 0.10 at certain depths (Fig. F58A). The ranges of A_HV and A_hV are -0.04 to 0.13 and -0.08 to 0.09, respectively (Fig. F58B). However, the vast majority of A_HV measurements (95%) and most A_hV measurements (76%) have positive values. The variation of horizontal anisotropy (A_XY) with azimuth was determined using paleomagnetic declination data (see "Paleomagnetism"; see also "Physical Properties" in the "Explanatory Notes" chapter). In total, 138 measurements of declination and horizontal velocity data were obtained from 731 to 1179 mbsf (Table T18). The radius of the 95% confidence circles for the paleomagnetic directions (₉₅) varied from 1° to 51.5°. To avoid using some of the poorly constrained declinations, we selected only directions that had ₉₅ 4.5°. For this data set, a sinuous-shaped correlation is obtained between horizontal anisotropy and azimuth (Fig. F59). The results suggest that maximum anisotropy has an approximately west-northwest-east-southeast orientation (~120°N) and that minimum anisotropy has a north-northeast-south-southwest orientation (~30°N). The results indicate that highest, intermediate, and lowest velocities are horizontal velocities in the west-northwest-east-southeast direction, horizontal velocities in the north-northeast-south-southwest direction, and vertical velocities, respectively. However, note that the principal axes of anisotropy may deviate from purely horizontal and vertical orientations.

For Hole 1150A cores, undrained shear strength was measured from the mudline to 27 mbsf with the vane shear device, and from 27 to 93 mbsf with the pocket penetrometer. Undrained shear strength derived from vane shear and penetrometer measurements generally increases with depth, and ranges from 17 to 69 kPa and from 29 to 118 kPa, respectively (Fig. F60; Table T19, also available in ASCII format). The cores were disturbed by gas expansion, and fractures were often developed during testing. Therefore, the obtained undrained shear strength values probably do not reflect in situ conditions.

Index properties were determined on discrete samples recovered from Holes 1150A and 1150B using gravimetric methods. A dedicated program calculates the index properties from wet and dry mass and dry volume using a salinity of 0.035 and a pore-water density of 1.024 g/cm³. At this site, the salinity and density of the pore water were significantly different (0.034-0.018 and 1.024-1.012 g/cm³, respectively). Index properties were recalculated using in situ values of salinity and pore-water density from linear extrapolation of 39 measurements on interstitial pore-water samples (see "Geochemistry", see also "Physical Properties" in the "Explanatory Notes" chapter). Figure F61 shows variation with depth of the ratio of index properties corrected for in situ salinity and pore-water density over those determined with standard salinity and pore-water density. The different salinity and pore-water density values have only minor influence on water content of total mass, porosity, and bulk density (the ratio ranges from 0.98 to 1.02). However, they have intermediate influence on dry density (the ratio ranges from 0.96 to 1.05) and significant influence on water content of mass of solids, void ratio, and grain density (the ratio ranges from 0.94 to 1.06). Index properties determined from in situ variations of salinity and density of pore water are reported here. Note that index properties included in the Janus database are calculated from standard values of salinity and pore-water density.

To cross-examine the internal quality of the data, porosity, dry density, and void ratio were calculated indirectly from the other index properties (see "Physical Properties" in the "Explanatory Notes" chapter). The maximum differences between direct and two indirect determinations of porosity were 1.5% and -0.4%, respectively. The corresponding differences are 0.012 g/cm³ and -0.03 for dry density and void ratio, respectively. These differences lie within the estimated uncertainty of index properties measurements (± 2% of values), which implies good quality of index properties measurements.

The overall plots of index properties vs. depth indicate that sediments at this site are porous and poorly consolidated. The main results of the downhole variation in porosity, bulk density, and grain density are presented below. Water content and void ratio have downhole trends similar to that of porosity, and dry density has a downhole trend similar to that of bulk density (Fig. F62; Table T20, also available in ASCII format).

There is a rapid change in porosity and bulk density in the upper section of Hole 1150A; porosity decreases from 83% at the mudline to a minimum value of 56% at 113 mbsf, and bulk density increases from 1.26 to 1.70 g/cm³ across the same interval (Fig. F62). This depth coincides with a positive peak in NGR activity and GRA bulk density values. The section from 113 to ~200 mbsf generally has inverse trends of porosity and bulk density; porosity increases to 79% and bulk density decreases to 1.29 g/cm³. Scattered grain density values (2.40-2.66 g/cm³) are measured from the mudline to ~200 mbsf. Slightly inverse trends of porosity and bulk density are measured from ~200-600 mbsf; porosity and bulk density from ~63% to ~77% and 1.56 to 1.32 g/cm³, respectively. Grain density data are more spread than porosity and bulk density data and range from 2.11 to 2.58 g/cm³. The index properties trends are marked with several minor offsets and changes in slope and one interval with slightly inverse trends of porosity and bulk density (Fig. F62). Lithologic Unit III can be divided into an upper (598-915 mbsf) and a lower (915-1047 mbsf) portion on the basis of index properties. Porosity generally decreases from 70% to a minimum value of 50% in the upper interval. The porosity has a positive offset downhole of ~10% at 915 mbsf and decreases to a minimum value of 56% in the lower interval. The bulk density variation with depth mirrors that of porosity; increasing values from 1.48 to a maximum of 1.76 g/cm³ at 915 mbsf are measured. After a negative shift of ~0.2 g/cm³, bulk density increases to 1.63 g/cm³ at 1044 mbsf. Grain density data are more scattered than porosity and bulk density data; grain density ranges from 2.24 to 2.59 g/cm³ in the upper interval, and from 2.27 to 2.48 g/cm³ in the lower interval. The boundary between lithologic Units III and IV is marked by shifts to higher bulk density and grain density and by lower porosity values. In the basal unit of Hole 1150B, porosity is rather constant and generally ranges from 50% to 60%; bulk density increases by ~0.1 g/cm³ over depth and varies from 1.60 to 1.77 g/cm³; and most values of grain density generally vary from 2.42 to 2.55 g/cm³.

Bulk density, porosity, P-wave velocity, and NGR activity were measured both in cores and in logs (in situ) (Fig. F63). Bulk density is measured with gravimetric (split cores) and GRA/absorption methods (whole-round cores and logs). Despite these partly different principles, there is generally an excellent correlation between core and log measurements. The main difference between core and log measurements is that the log data reveal several thin and dense horizons (i.e., dolomite and carbonate-rich horizons) that are not observed in gravimetric measurements but are sometimes seen in the GRA bulk density measurements. Porosity is measured with gravimetric (cores) and neutron absorption (log) methods. The log porosity generally indicates a wider scatter of values than the core porosity. However, the smoothed curve of log porosity correlates well with that of core porosity above 700 mbsf. Below, the core porosity is slightly lower (5%-10%) than the smoothed log porosity curve. P-wave velocity is determined in the vertical direction from the traveltime of sound over a known distance (both in core and log). Overlapping core and log measurements were obtained only from below 700 mbsf in Hole 1150B. Although core and log data have similar trends, the core measurements are generally lower (by ~250 m/s) and more scattered than the log measurements. Thus, the comparison of porosity suggests that core sampling favors less porous and/or fractured sediment, whereas the opposite is suggested from comparison of velocity in core and log. NGR activity is measured using different units in core (cps) and log (gamma-ray American Petroleum Institute [gAPI]). Moreover, log data generally have a smaller statistical error than core data, mainly because the log sensors are exposed to a bigger sample than the core sensors. The NGR activity for the core demonstrates a similar but less variable and a more smoothed curve in comparison to the in situ NGR activity.

The total and effective vertical stresses were calculated from bulk density from porosity data following Equations 13 and 8 in "Physical Properties" in the "Explanatory Notes" chapter. The bulk density data consisted of discrete measurements of bulk density in cores from 0 to 1180 mbsf and hostile environment lithodensity sonde (HLDS) bulk density from 113 to 638 and 745 to 1165 mbsf. HLDS bulk density data were converted into porosity using Equation 9 in "Physical Properties" in the "Explanatory Notes"  chapter. Grain density values were estimated from linear extrapolation of discrete grain density measurements in cores, and pore-water density was assumed to be 1.024 g/cm³. The total vertical and the effective vertical stresses increase linearly with depth to 10.0 and 17.6 MPa, respectively, at 1180 mbsf (Fig. F64).

Quality of Data

In general, the various shipboard analyses of physical properties show similar downhole trends. However, the quality of undrained shear strength and P-wave velocity (PWL and PWS measurements) is degraded by the presence of micro- and macrofractures in many of the APC and XCB cores. These fractures were formed by gas expansion in the cores when they were recovered to the drill floor. Gas expansion could affect GRA bulk density measurements as this method is based on core-unit measurements, whereas index properties that are based on gravimetric methods are not influenced (Blum, 1997). Maximum GRA bulk density values are apparently little affected by gas expansion in Hole 1150A because they correlate well with index properties and log bulk density. Tests of internal consistency of index properties data confirm the good quality. In general, there is good correlation between core and log index properties. Several of the XCB cores consisted of drilling biscuits surrounded by drilling mud slurry. This has resulted in low quality of thermal conductivity data in these cores (i.e., there is no correlation between thermal conductivity and porosity or bulk density).

Downhole Variation of Physical Properties

Lithology at this site is dominated by varying amounts of greenish homogeneous hemipelagic diatomaceous clay and clayey silt that become indurated with depth. The variation of physical properties data with depth is marked with five main breaks, namely at ~70-113, 200, 600, 915, and 1047 mbsf, respectively (Figs. F57, F62, F63). These breaks generally correlate with downhole chemical and lithologic changes and appear to signal variations in hydrologic and mechanic conditions.

The section from the mudline to 222 mbsf of Hole 1150A (lithologic Unit I) is largely composed of diatomaceous ooze and clays. The sediment is of Holocene to latest Pleistocene age from the mudline to 136 mbsf, and of late Pliocene age from 136 to 222 mbsf. The uppermost section is characterized by subsequent compaction of sediments (decreasing porosity and increasing bulk density and P-wave velocity) (Figs. F57, F62), and the sedimentation rate decreases rapidly from ~200 m/m.y. above 55 mbsf to <40 m/m.y at 85 mbsf (see "Sedimentation Rates"). In the interval from ~70 to 113 mbsf index properties and MST data reach peak values (Figs. F53, F55, F62), which are followed by inverse trends of index properties (increasing porosity and decreasing bulk density). The abrupt changes in trends suggest that there could be an unconformity at this depth that coincides with a change in salinity of the pore water (see "Geochemistry") but slightly above the unconformity indicated by biostratigraphy (see "Biostratigraphy"). In addition, the local maximum in GRA bulk density (~1.8 g/cm³) and CaCO₃ content (~22 wt%) supports the presence of some form of change in the depositional environment. The inverse trends of index properties from 113 to ~200 mbsf suggest that the sediments are underconsolidated and that the rapid deposition (100-200 m/m.y.) and impermeable nature of the sediment section (clay and calcareous rich) prohibit dewatering.

Index properties from core and log measurements are rather constant in the interval from ~200 to 600 mbsf (Fig. F63). Minor and gradual changes characterize the lithology across this interval, which is late to early Pliocene in age; diatomaceous ooze and ash layers are common down to 222 mbsf, whereas only a few ash layers and sandy silt layers are observed from 222 to 598 mbsf (Lithologic Unit II). The induration of sediment changes from soft to firm at 424 mbsf and from firm to hard at ~600 mbsf (see "Lithostratigraphy"). Porosity is high (>60%) and bulk density is low (<1.6 g/cm³) relative to the depth of burial. The constancy of index properties values across this section suggests that these deposits are also underconsolidated. Moreover, the change from constant salinity (S = 0.029) in the upper part of the interval to decreasing salinity from ~450 mbsf indicates that the hydrologic and mechanic conditions change in the lower portion of the interval. Geochemistry data indicate that gas hydrates may be present in sediments above 450 mbsf (see "Geochemistry"). Further analyses of seismic reflection and thermal data are required to confirm this hypothesis.

From 598 to 1047 mbsf, the lithology is dominated by lower Pliocene to upper Miocene greenish diatomaceous silty claystone and clayey siltstone with common and moderate bioturbation and a gradual increase in silt and sand particles and volcanic glass (see "Lithostratigraphy"). Porosity and bulk density values are characterized by rather uniform changes to lower and higher values, respectively. Dolomite nodules, glauconite-bearing silt and sand layers, and carbonate-rich layers are observed. Furthermore, fractures, faults, and joints are frequently observed in the cores from 598 to 787 mbsf, and their abundance increases below 787 mbsf. The wider scatter of vertical and horizontal P-wave velocities with depth probably mirrors the increasing amount of structural discontinuities (Fig. F57). The wider scatter in P-wave velocity values from ~920 to 1010 mbsf therefore indicates an increased frequency of fractures, and the interval roughly corresponds to a section with increasing fault observations in the cores (see "Lithostratigraphy"). The section between 787 and 1046 mbsf is probably normally consolidated because when the porosity and density are extrapolated to the surface, the trends match those of the top section of this site (Fig. F62). This would indicate that the mechanical state of sediment changes from underconsolidated to normally consolidated across the interval from 598 to 787 mbsf and that the presence of faults and joints has facilitated dewatering of the sediment. There is a shift to lower porosity and higher bulk density and P-wave velocity at 1047 mbsf, which coincides with the boundary of lithologic Units III and IV. The sediments become harder and have higher resistivity below 1047 mbsf, but there is no major change in the frequency of structural discontinuities, rate of sedimentation, or age of sediments (see "Lithostratigraphy," "Sedimentation Rates," "Biostratigraphy," "Downhole Measurements"). The shift in physical properties probably reflects a downhole increase of induration of the sediment.

The overall downhole trend of NGR activity generally mimics that of porosity, which points to the intimate relationship between physical properties and the mineralogical composition of sediments (Figs. F55, F62). Local peak or trough values in NGR activity generally correlate well with peak or trough values in clay content (smectite, illite, and/or kaolinite and chlorite) of the sediment. It also seems clear that the diatomaceous nature of the sediment results in higher porosity and lower bulk density than in diatom-free lithologies. The peak intensity of the opal-A hump is a measure of the diatom content, and there is a general tendency of porosity to increase with the opal-A hump peak intensity (Fig. F65A). Cementation, which may uncouple the relationship between porosity and consolidation, often occurs in carbonate-rich sediment (e.g., Karig, 1996). At this site, the carbonate content is generally low (see "Geochemistry"). Nevertheless, porosity tends to increase with carbonate content (especially at higher porosities) (Fig. F65B). Local peak values in log and GRA bulk density often correlate with denser dolomite- and carbonate-rich layers that are observed as layers of low resistivity in FMS and resistivity logs (see "Downhole Measurements"). These may act as local seals and are often associated with minor shifts in physical properties data.