The physical properties program at Site 1168 included MST and thermal conductivity measurements of whole-round cores and compressional wave (P-wave) velocity, moisture and density (MAD), and vane shear-strength measurements of split cores. The Adara tool was deployed four times in Hole 1168A for in situ temperature measurements.
All core sections from Holes 1168A, 1168B, and 1168C were routinely measured on the MST (for magnetic susceptibility, P-wave velocity, GRA density, and natural gamma ray) at 2-cm intervals. P-wave velocities were recorded at 2-cm intervals in Hole 1168A sections to a depth of ~111.8 mbsf. P-wave velocities were not recorded in the XCB-cored sections. Natural gamma-ray measurements were made on cores from Hole 1168A at 5- to 20-cm intervals down to 291.4 mbsf. Because the counts were barely above the background, these measurements were omitted below 291.4 mbsf.
GRA density data and discrete wet bulk density data both show a gradual downhole increase resulting from sediment compaction and dewatering with increased overburden (Fig. F36A). In addition to the overall downhole trend, the GRA data show distinct variations in relation to the lithologic changes at several stratigraphic boundaries (e.g., Miocene/Pliocene and Oligocene/Miocene boundaries; Fig. F36A).
Some offsets, however, are evident between the MST and discrete density data. For example, GRA values are higher than the discrete density values between ~20 and 240 mbsf. The higher GRA values are explained by the relatively high carbonate content, porosity, and moisture content of the sediments for this interval. As the calibration procedure for the MST is optimized for mixed sediments, the GRA method overestimates the density in the carbonate-rich sediments of lithologic Subunits IA and IB.
Magnetic susceptibility (Fig. F36A) is negatively correlated with GRA and wet bulk density (Fig. F36B) in both carbonate- and clay-rich intervals of the hole. A good correlation is observed between magnetic susceptibility and color reflectance measurements, mainly the lightness value (L*) and the chromaticity coordinate b* (see "Lithostratigraphy" and "Composite Depths"). Magnetic susceptibility and color reflectance reveal a pronounced cyclicity, which may be useful to identify astronomically controlled depositional processes in this region.
Compressional velocities were obtained on the split-core sections at a sampling interval of one per section down to ~330 mbsf (Figs. F37, F38A). The discrete measurements were performed with the digital sediment velocimeter to a depth of ~240 mbsf. Below ~240 mbsf, we used the modified Hamilton Frame (PSW3) velocimeter. Signals were completely attenuated at a depth of ~350 mbsf, except for some cemented sandstones several centimeters thick in the lowermost part of Hole 1168A (in Sections 189-1168A-75X-6 and 86X-1; Fig. F38B). A comparison of the continuous velocity profile obtained with the MST and the discrete values is shown in Figure F37. As expected for the sediment type encountered at this site, velocities vary between 1550 m/s in the soft-surface sediments and 1900 m/s in the more consolidated sediments (Table T20). Downhole, the PSW3 (x- [across core] direction) velocities increase up to 2100 m/s at ~340 mbsf (Fig. F38A), similar to values seen in the downhole logging data (see "Downhole Measurements"). The velocity measurements perpendicular (PSW1; z- [along core] direction) and parallel to the bedding (PSW2; y- [perpendicular to the core] direction) show similar readings, indicating that no velocity anisotropies are present in the y- and z-directions of the sediments. There is an offset of PSW1 and PSW2 velocities compared to the PSW3 velocities, which increases sharply below ~100 mbsf. This offset is attributed to tension cracks in the sediments and fine cracks created in the sediments by the insertion of the transducers, thereby reducing P-wave propagation and resulting in lower P-wave velocities overall than obtained from the PSW3 transducer. The PSW3 transducer is nondestructive and appears to produce more accurate data below ~100 mbsf. PSW3 results are comparable to both MST and downhole logging data.
Thermal conductivity was measured on Section 3 of each core in Hole 1168A to a depth at which induration prevented insertion of the needles (~112 mbsf; Fig. F39). Values decrease with depth, corresponding to an overall decrease in porosity with depth (Fig. F40).
The Adara tool was deployed four times in Hole 1168A. The temperature at the seafloor (2.5°C) was determined using the mudline stops. Three of these deployments yielded acceptable temperature records. The temperature record from Core 189-1168A-12H (a typical deployment record) shows a well-developed thermal decay after the penetration (Fig. F41). The successful penetration of the Adara tool into the bottom-hole formation results in an instantaneous rise of temperature as a result of the frictional heating of the penetration, which is followed by a gradual, exponential decrease during the dissipation of the heat.
The geothermal gradient was determined using the four points of the temperature profile (Fig. F42). They can be reasonably fitted with a linear least-squares regression. The solution gives a geothermal gradient of 58°C/km compared to 27°C/km in the Cape Sorell No. 1 exploration well on the continental shelf 100 km to the northeast (Willcox et al., 1989). The average of the thermal conductivities measured from 0 through 160 mbsf at Site 1168 is 1.25 W/(m·K) (Table T21). Calculations using the average conductivity and the geothermal gradient yield a heat flow of 72.5 mW/m2. This heat-flow value is nearly two times higher than values reported from sedimentary basins and slopes near western Tasmania north of the site and Mesozoic continental margins in the mid-Atlantic (~40 mW/m2; Nagihara et al., 1996; Paull, Matsumoto, Wallace, et al., 1996).
Vane shear strength was measured once per section on Hole 1168A cores to the depth at which induration prevented insertion of the vane for shear strength (~260 mbsf). The results are displayed in Figure F43 and Table T22. On average, values show a good overall correlation with the GRA density (Fig. F43), indicating a strong relationship between lithology, degree of cementation, and shear strength.
Discrete moisture and density measurements are presented in Figure F40 and Table T23. Downhole trends reflect increased compaction and dewatering with depth. All of the first-order variations in moisture and density correlate well with the lithologic and stratigraphic (e.g., mid-Miocene, Oligocene/Miocene boundary, and Eocene-Oligocene transition) units. The discrete density data (Fig. F40) correlate very well with the GRA data and can be used to calibrate and correct GRA density (Fig. F36A).
Wet bulk density and porosity variability correlate well with silica content (see "Biostratigraphy" and "Inorganic Geochemistry") in lithostratigraphic Subunit IB of Hole 1168A. Water content and porosity mirror magnetic susceptibility (Fig. F36B) in the Oligocene and Eocene sections, reflecting the increased clay content in the deeper intervals at Site 1168. The carbonate-rich lithostratigraphic Subunit IB can be subdivided into an upper interval (from 45 to 125 mbsf) with relatively high density, P-wave velocity, shear strength, and thermal conductivity and low porosity and water content, and a lower interval (from 125 to 260 mbsf) characterized by low density, P-wave velocity, shear strength, and thermal conductivity but high porosity and water content (cf. Figs. F36A, F38A, F39, F40, F43).
The coring technique was changed from APC to XCB at 112 mbsf, which often results in spurious P-wave data because of disturbance resulting from core sampling. The P-wave velocity values measured both in downhole logs (starting at 120 mbsf) and on the cores, however, correlate very well around this interval. Therefore, it seems improbable that the drilling and splitting techniques have influenced the quality of the core measurements. The physical properties data for this interval merely reflect carbonate/silica variations in the sedimentary section.