Multisensor track (MST) measurements of magnetic susceptibility (MS), gamma-ray attenuation (GRA) bulk density, P-wave velocity, and natural gamma radiation (NGR) were made on APC and XCB whole cores from lithostratigraphic Units I, II, and III (Holes 1179A, 1179B, and 1179C). Index properties and P-wave velocities of these materials were also measured on discrete samples, and P-wave velocities and bulk densities were measured on 42 basalt samples from Unit V (Hole 1179D). In general, the measured physical properties correlate well with lithostratigraphy. P-wave velocities in the sediments are typically 1530-1550 m/s. Average bulk densities of the individual sedimentary units range from 1.265 to 1.450 g/cm3, and average porosities range from 67% to 83%. Average bulk densities and velocities in the basalts are 2.745 g/cm3 and 5002 m/s, respectively. Within Unit I, bulk density, thermal conductivity, and NGR decrease with depth in the upper 150 m of the section; porosity increases to >85%. We suspect that these seemingly paradoxical trends in density and porosity are caused by a progressive increase in the relative abundance of diatom fragments, which have low grain densities and contain large volumes of intragranular pore space, with depth. The contact between Units I and II is marked by small changes in the physical properties, but the transition from Unit II to the pelagic clays of Unit III is dramatic. Densities increase, porosities fall to <70%, and there are marked increases in NGR and MS. Marked changes in NGR and MS at a depth of ~265 mbsf also suggest a compositional change within Unit III. The temperature gradient and average thermal conductivity in the upper 60 m of the sediment column are 0.0610° ± 0.002°C/m and 0.76 ± 0.01 W/(m·K), respectively. The conductive heat flow at Site 1179 is 46.40 ± 0.01 W/m2.
For the purposes of physical properties, the geologic section at Site 1179 can be viewed as consisting of three intervals, a 283-m sequence of oozes and clays (Units I-III) cored using the APC and XCB, a 92-m section of interbedded cherts and softer sediments (Unit IV), and igneous basement (Unit V) below 375 mbsf. Cores from the first of these intervals (Holes 1179B [Cores 191-1179B-1H through 6H] and 1179C [Cores 191-1179C-1H through 26X]) were scanned on the MST. MS, GRA bulk density, and P-wave velocity were measured through the core liner at 2-cm intervals, and NGR was counted for 60 s at intervals of 20 cm. Thirty-six measurements of thermal conductivity were made on whole-round sections. One hundred seventy-one discrete measurements of index properties (wet bulk density, grain density, and porosity), 153 sonic velocities (PWS1, PWS2, and PWS3), and 94 automated vane shear (AVS) strength tests were made on the split cores. These data are summarized in Tables T12, T13, and T14. Measurements of sonic velocities using the PWS1 and PWS2 were discontinued after Core 191-1179C-3H (60 mbsf) because the sediments cracked when the transducer probes were inserted. Similarly, AVS measurements were abandoned when the cores split consistently during the test after Core 191-1179C-18H. No physical properties measurements were made on samples from the chert-bearing sediments because only a few chert cobbles were recovered in each core; softer sediments, which we believe are present in situ, were apparently washed away by the coring process. However, Wilkens et al. (1993) have made a detailed study of the properties of north Pacific deep-sea cherts. They report porosities of chert samples in the range 5.5% to 13%, with average bulk density and P-wave velocity of 2.506 g/cm3 and 5265 m/s (at 10 MPa confining pressure), respectively. Forty-two wet bulk densities and P-wave velocities (PWS3) were measured in 2-cm3 cubes cut from the basalt cores recovered from the basement section (Unit V). These data are summarized in Table T15. The measured physical properties from Site 1179 are plotted with the lithostratigraphic column in Figures F57 and F58. In these profiles, a 19-point median filter has been applied to the MST data. The average properties of Units I-III and V are summarized in Table T16.
The P-wave velocity profiles are shown in Figure F57. The PWS1, PWS2, and P-wave logger (PWL) velocities are in good agreement in the upper 25 m of the section. Below that depth, velocities measured on the split cores using PWS1 and PWS2 are lower than the PWL velocities. There is good agreement between PWS3 and the PWL velocities below ~80 mbsf. PWS3 velocities, which are measured through the core liner in a direction normal to the core axis, are distinctly higher than velocities measured using either PWS1 or PWS2, which are also measured on split cores. We believe that the difference between the PWS3 and PWS2 velocities is caused by cracks that form in the core when the PWS1 and PWS2 probes are inserted in the sediment. We made several PWS2 and PWS3 measurements in the same intervals of Core 191-1179C-3H and found a systematic difference of 50 m/s. Consequently, PWS1 and PWS2 measurements were discontinued after Core 191-1179C-18H. PWS3 P-wave velocities in the sediments from Units I, II, and III are summarized in Table T12.
Within Units I and II, P-wave velocities increase very gradually from ~1535 m/s at a depth of 50 m in Unit I to ~1560 m/s at the boundary between Units II and III. Because of the discrepancy between velocities in samples from Unit III measured by PWL and PWS3 (Fig. F57), it is unclear whether sonic velocities are higher or lower in Unit III than they are in Unit II; PWS3 velocities are distinctly lower, with an average near 1530 m/s, whereas PWL velocities are slightly higher.
Sonic velocities and bulk densities were also measured on 42 ~8-cm3 cubes cut from the cores of tholeiitic basalt (Unit V). The velocity measurements were made parallel to the core axis (z-direction) at bench pressure using the PWS3 velocimeter. Measurements were made only in the z-direction for two reasons. First, velocity anisotropy in oceanic basalt, diabase, and gabbro samples is generally minimal. For example, velocity anisotropy in gabbros recovered from Hole 735B is typically <3% (Iturrino et al., 1991) and the average is not statistically significant. Hence, only one measurement is needed to characterize these rocks. Second, the cubes tend to be slightly irregular in shape because opposite faces are often not quite parallel. The first cuts, made across the core perpendicular to the core axis, are most nearly parallel. Another consideration is that velocities in crystalline rocks are known to increase rapidly with increasing effective pressure, which is roughly equal to the difference between the confining pressure acting on the rock and the pore pressure. However, the effective pressure at the top of oceanic basement must be near zero, and the pore pressure is very likely to be near hydrostatic, at least in the upper part of the igneous crust. Thus, the in situ effective pressure is unlikely to be more than ~0.2 MPa (~300 psi) at a subbasement depth of 100 m. Measurements made at bench pressure are probably sufficient for estimating the properties of the basalt at sample scale. The measured properties of the basalt samples are summarized in Table T15, and the average velocity is 5002 m/s (Table T16). In situ velocities are almost certainly lower than this average owing to the presence of large-scale cracks and voids in the formation.
The index properties of sediments recovered from Holes 1179A, 1179B, and 1179C are summarized in Table T13, and the depth profiles are shown in Figures F57 and F58. The approximate bulk densities of the basalt samples are given in Table T15. These values are approximate because the sample volumes were estimated from the measured dimensions of the cut samples, which are slightly irregular.
The GRA sediment densities are distinctly lower than the discrete sample densities, probably because the GRA density system is calibrated for aluminum and water as opposed to clays or siliceous sediments (Blum, 1997). The difference is slightly <1 g/cm3. The depth trends, on the other hand, are very similar, suggesting that the pattern of density variations is well represented by both data sets. The porosity trend parallels the density profile, because bulk density depends largely on the porosity and grain density. Average grain densities in Units I and II are near 2.47 g/cm3.
Although the average densities of samples from Units I (1.27 g/cm3) and II (1.33 g/cm3) are similar (Table T16), there is a clear correlation of density and porosity with lithology (Fig. F57). Densities in Unit II are nearly constant, but the densities of samples of the stiff clays from Unit III are higher, at ~1.54 g/cm3. We were able to measure the dry volumes of only two samples from the Unit III clays because their gas permeability is apparently so low the pycnometer failed to reach equilibrium during the time allotted by the software for the measurement of the dry volume. Six additional measurements of density and porosity in samples from Unit III were estimated from the dimensions and wet and dry weights of "plug" samples taken from the cores (see Table T13).
An interesting feature of Unit I is its very high porosity, which averages >80% to a depth of >220 m. Another interesting feature of the density and porosity profiles through the radiolarian-bearing diatom oozes of Unit I is that densities actually decrease from 1.3 g/cm3 near the surface to near 1.2 g/cm3 at a depth of ~150 mbsf. There is a corresponding increase in the total porosity to ~86% near 140 mbsf. Below 150 mbsf, densities gradually increase to ~1.3 g/cm3, with a corresponding decrease of porosity to near 80% at the bottom of Unit I. Grain densities (shown in Fig. F59) show a similar pattern, deceasing from 2.6 g/cm3 at the seafloor to 2.3 g/cm3 at 150 mbsf. These observations lead us to speculate that the high porosities we observe in Unit I reflect the abundance of diatom fragments in the sediment. Diatoms are composed of opaline silica, with a bulk density of 2.01 to 2.16 g/cm3, and their lacy morphology includes abundant small holes that are not filled with clay or other materials. A gradual increase with depth of the relative abundance of diatoms in the sediment would account for both the increase of porosity and the decrease of grain density in the upper 150 m of Unit I.
The thermal conductivity, NGR, and MS profiles are shown in Figure F58. Thermal conductivity measurements were made in the third section of each core recovered from Holes 1179A, 1179B, and 1179C, through Core 191-1179C-26X. No measurements were made of thermal conductivity in the cherts (Unit IV) or basalts (Unit V). Thermal conductivities in Units I through III are summarized in Table T13 and range from 0.67 to 0.95 W/(m·K). Average values for Units I through III are listed in Table T16. Thermal conductivity increases with depth, and the three lithostratigraphic units have statistically distinguishable thermal properties that increase from 0.75 ± 0.01 W/(m·K) in Unit I to 0.80 ± 0.01 W/(m·K) in Unit II and 0.91 ± 0.01 W/(m·K) in Unit III. Worth noting is the fact that the variation of thermal conductivity in Unit I initially decreases with depth and reaches a minimum near 150 mbsf, mimicking the pattern we observed in the variation of bulk density and porosity. The thermal conductivity profile probably reflects the variation of porosity with depth.
Temperatures were measured with the Adara tool on several APC cores. Temperatures at the seafloor and at depths of 30 and 60 mbsf are 2.00°, 3.84°, and 5.66°C, respectively; the temperature gradient is 0.0610 ± 0.002°C/m. The average thermal conductivity of samples from the same depth interval (0-60 mbsf) is 0.76 ± 0.01 W/(m·K), and the conductive heat flow at Site 1179 is 46.40 ± 0.01 mW/m2.
Strictly speaking, NGR is not a physical property. The intensity of natural gamma radiation is a measure of the abundance of potassium, thorium, and uranium in the cores. NGR was counted for 60 s at intervals of 20 cm in all APC and XCB cores from Holes 1179A, 1179B, and 1179C. The NGR profile shown in Figure F58 has been filtered using a 19-point median filter. Through Unit I, NGR shows a pattern similar to porosity, density, and thermal conductivity; NGR decreases gradually with depth to ~150 mbsf then begins to increase. Unit II does not appear to be distinguishable from Unit I, but there is a clear increase in NGR at the contact between Units II and III. NGR decreases abruptly at a depth of ~265 mbsf within the red-brown pelagic clay of Unit III. This change suggests a change in the mineral content, possibly a decrease in clay content, in Unit III that is not indicated by the physical properties, with the exception of magnetic susceptibility.
MS measurements were made for 3 s at 2-cm intervals on APC and XCB cores from Holes 1179A, 1179B, and 1179C. A 19-point median filter has been applied to the profile shown in Figure F58. Susceptibility correlates well with the lithostratigraphy of the sediments cored at Site 1179, increasing slightly at the contact between Units I and II and more strongly between Units II and III. There is also an abrupt increase in MS at ~265 mbsf. Like the change in NGR noted above, this suggests a change of mineral content within Unit III that is apparently not evidenced elsewhere.
Vane shear measurements were made near the bottom of each section of the APC cores from Holes 1179A, 1179B, and 1179C until the cores began to split, invalidating the shear strength measurements. Vane shear measurements were therefore discontinued after Core 191-1179C-18H. All vane shear measurements were made in cores recovered from Unit I. Vane shear strengths are summarized in Table T17, and the peak strength profile is shown in Figure F60. There is considerable scatter in these data. Shear strength is low (~5 kPa) in the shallow sediments, as expected, increasing to an average of ~60 kPa at a depth of 100 mbsf. Below that depth, peak strengths increase very gradually to average values of perhaps 70 or 75 kPa at a depth of 190 mbsf.
Surveys by the Hakuho Maru in August of 1996 (cruise KH96-3-1) produced the two seismic multichannel profiles (2-1 and 2-4) across Site 1179 that are shown in Figures F61 and F62. In addition to the seafloor reflection, there are three prominent reflectors in the seismic section at ~0.15, ~0.17, and ~0.22 s one-way traveltime and a slightly weaker reflector near 0.25 s. There are also numerous weak and often discontinuous reflectors between the seafloor and the 0.15-s reflector.
We attempted to correlate the observed reflections with the lithostratigraphic column by calculating traveltimes from depths to possible reflecting horizons based on average P-wave velocities (PWS3 and PWL) measured in the laboratory, but that produced a set of implausible one-way traveltimes. Because the sediments at Site 1179 have very high porosities, we next corrected the measured velocities for the local temperature profile, using dV/dT for seawater = 3.2 m/s/°C. We then made another forward calculation of the traveltimes to possible reflectors. The best correlation model generated by this method is summarized in Table T18. The model accounts for many of the weak reflections above the Unit II/III transition, but the correlations are problematical for a number of reasons. One is that the interval velocities are highly variable, and some are unreasonably high or low. More serious problems are that neither the strong reflection at 0.172 s nor the very strong reflection at 0.22 s corresponds to any horizon in the lithostratigraphic column, and the top of the chert (Unit IV) does not correspond to a reflection.
As an alternative to using the shipboard data, we applied an empirical relationship between one-way traveltime and depth based on measured traveltimes and drilled depths to basement at DSDP sites worldwide (Carlson et al., 1986). According to the model,
where
The root-mean-square error reported by Carlson et al. (1986) is 26 m, and the corresponding velocity-depth function is
This model was applied by computing depths from the observed traveltimes from both profiles (which are slightly different). The results are summarized in Table T19. Despite the fact that this correlation model is based on a global average time-depth model, the results are quite good. The model accounts for all of the major reflectors; from the line 2-1 traveltimes, it predicts the depths to all of the unit boundaries to within 10 m or less, and the major lithostratigraphic boundaries—the Unit II/III boundary (~246 mbsf), the top of the chert (~283 mbsf), and the top of igneous basement (375 mbsf)—correspond to the strongest reflections (Fig. F10). The success of this model suggests that P-wave velocities measured in the cored sediments can be too low by as much as 150 to 200 m/s. The probable cause of this rather large discrepancy is disruption of the grain-to-grain contacts by expansion of pore water when the cores are brought to the surface. From a depth of 5500 m, the volume expansion is nearly 2.5%.