Measurements of physical properties at Site 1126 followed the procedures outlined in "Physical Properties" in the "Explanatory Notes" chapter. These included nondestructive measurements of P-wave velocity (every 4 cm; Table T10, also in ASCII format), GRA bulk density (every 4 cm; Table T11, also in ASCII format), MS (every 8 cm; Table T12, also in ASCII format), and NGR (every 16 cm; Table T13, also in ASCII format) using the MST. The P-wave logger (PWL) was activated only on APC cores. Thermal conductivity (Table T14, also in ASCII format) was measured in unconsolidated sediment at a frequency of one per core with three samples per core analyzed after deployments of the Adara and DVTP tools. A minimum of two discrete P-wave velocity measurements per section were made on the working half of the split cores (Table T15, also in ASCII format). Measurement frequency was increased to five per section after the PWL was turned off. Standard index properties (Table T16, also in ASCII format) and undrained shear strength (Table T17, also in ASCII format) (only in unconsolidated sediments) were measured at a frequency of one per section.
The following sections describe the downhole variations in sediment physical properties and their relationships to lithology and downhole logging measurements. Variations in MS are described within "Paleomagnetism" and "Composite Depths".
At Site 1126 an offset was seen between the discrete bulk density measurements and the GRA densiometry measurements of the MST (Fig. F21). This offset was corrected using the equation of Boyce (1976) as described in "Index Properties" in "Physical Properties" in the "Explanatory Notes" chapter. Low core recovery below 160 mbsf hindered the ability to investigate petrophysical properties within the middle and lower parts of the sedimentary section (Fig. F22). Despite this, physical properties measurements at Site 1126 can be divided into five units on the basis of variations in measured parameters (Fig. F22).
Physical properties Unit (PP Unit) 1 (0-45 mbsf) is characterized by high NGR (5-15 cps) with an overall trend of increasing P-wave velocities (1.5 to 1.75 km/s) and bulk density (1.7 to 1.82 g/cm3) (Fig. F23). Within PP Unit 1 porosity (50%-61%) is generally inversely correlated with bulk density and P-wave velocity (Fig. F23). Physical properties Unit 1 has been further divided into three subunits (Fig. F23). Physical properties Subunit 1A (0-6 mbsf) is characterized by increasing trends in bulk density (1.70-1.76 g/cm3), P-wave velocity (1.6-1.65 km/s), and NGR (4-12 cps). Physical properties Subunit 1B (6-11 mbsf) has bulk density and P-wave velocity trends opposite to those in Unit 1A, whereas NGR remains nearly constant (~11 cps). Physical properties Subunit 1C (11-45 mbsf) is characterized by increasing bulk density (1.7-1.84 g/cm3), increasing P-wave velocity (1.6-1.74 km/s), and nearly constant NGR values (~10 cps). The base of PP Unit 1 coincides with the upper Pliocene/Pleistocene boundary, the bottom of lithostratigraphic Subunit IA, and the base of seismic Sequence 2 (see "Seismic Stratigraphy").
Physical properties Unit 2 (45-81 mbsf) coincides with seismic Sequence 3A and is characterized by high NGR (5-14 cps), increased P-wave velocities (1.6-2.0 km/s), and relatively low porosity (42%-55%) (Fig. F22). Physical properties Unit 2 has also been divided into three subunits (Fig. F23). Physical properties Subunit 2A (45-60 mbsf) is characterized by an increasing trend of P-wave velocity (1.67-1.84 km/s), a variable trend in bulk density averaging near 1.72 g/cm3, and nearly constant NGR near 9 cps. The top of PP Subunit 2B (60 mbsf) occurs at the same depth as lithostratigraphic Unit II, characterized by the first occurrence of slumping in the sedimentary sequence. The top of PP Subunit 2C (68 mbsf) marks the beginning of a decreasing trend of NGR to the bottom of Unit 2 (14-3 cps). Within this subunit bulk density (1.8 g/cm3) and P-wave velocity (1.75 km/s) are nearly constant. The top of PP Subunit 2C also marks the start of a generally increasing trend in porosity that continues throughout Unit 3 (Fig. F22). At the base of PP Unit 2 (81 mbsf) NGR values decrease to an average of 3 cps (Figs. F22, F23). This decrease is coincident with the loss of aragonite, which is often high in uranium and thus a source of NGR. This conclusion is supported by spectral gamma-ray data from downhole logging, which show a trend of decreasing uranium values to 81 mbsf (see "Downhole Measurements"). Despite the limited overlap between core measurements and downhole logs, trends in NGR correlated well between the two data sets in the upper 80 mbsf of the section. The bottom of PP Unit 2 coincides with the lower Pliocene/upper Miocene boundary.
Physical properties Unit 3 (81-160 mbsf) is characterized by nearly constant P-wave velocities (~1.7 km/s) and NGR (3 cps) (Figs. F22, F23). These low NGR values are barely above detection levels and reflect the high carbonate and low terrigenous content of sediments within PP Unit 3. The GRA and discrete bulk density values both show cyclic variations between 1.6 and 1.9 g/cm3 (Fig. F23). The origin of these cyclic variations is not obvious, as they were not seen in other data sets. The bottom of PP Unit 3 corresponds to the base of seismic Sequence 3 and the base of lithostratigraphic Unit III. Distinct shifts in several downhole measurement logs at 180 mbsf indicate that the base of PP Unit 3 may actually be deeper than 160 mbsf, although poor core recovery prevents confirmation of this hypothesis.
Poor recovery in PP Unit 4 (160-404 mbsf) limits the ability to characterize unit-wide changes in geophysical properties (Fig. F22). With the exception of NGR, there is a high variability in all physical properties parameters. This arises as a consequence of lithologic alternations of unindurated (ooze) and indurated sediment (porcellanite and chert). P-wave velocities range from 1.65 to 5.4 km/s, bulk density from 1.62 to 2.15 g/cm3, and porosity from 40% to 62% (Fig. F22).
Physical properties parameters change dramatically at the top of PP Unit 5 (404-463 mbsf), which corresponds to an ~35-m.y. hiatus between Cretaceous? sandstones and Tertiary marine carbonate sediments. This boundary is marked by a sharp increase in NGR from 5 to 30 cps caused by greater amounts of radioactive elements within minerals in the sandstones. P-wave velocities in PP Unit 5 have a wide range of values (1.8-4.8 km/s) as a result of variable lithification (Fig. F22). Poor recovery limited our ability to collect measurements and discern trends in bulk density and porosity.
Natural gamma-ray data collected at Site 1126 was the most useful data set for correlation, and it showed a strong inverse correlation to color reflectance data. This relationship was well documented at all sites during Leg 166 and results from a decrease in NGR with increased carbonate content (high reflectance) and an increase in NGR with increased terrigenous minerals (low reflectance) (Eberli, Swart, Malone, et al., 1997).
Overall, trends in P-wave velocity, porosity, and bulk density at Site 1126 do not solely reflect compaction, but rather indicate the importance of diagenesis in the upper 150 m of the sedimentary section. This diagenesis is reflected in part by the presence of extensive chert layers and more indurated carbonate sediments.
Shear strength was measured from 1 to 250 mbsf at Site 1126 (Fig. F24; Table T17). Shear strength increases from an average of 10 kPa near the top of the succession to 80 kPa near the base of PP Unit 3 (160 mbsf) and then generally decreases below 160 mbsf to values averaging 30 kPa (Fig. F24). Shear strength variations exhibit discrete intervals of high values superimposed on an overall increasing trend resulting from compaction. Some of these intervals of increased shear strength occur near PP unit boundaries and are probably a result of diagenetic alteration (Fig. F24).
At Site 1126 thermal conductivity values increase from 1.0 W/(m·K) near the surface to 1.3 W/(m·K) at the base of Subunit 2A. Below this values decrease to an average of 1.1 W/(m·K) for the remainder of the measurements (Fig. F25; Table T14). Thermal conductivity is primarily dependent on variations in sediment bulk density, which is related to other sediment physical properties such as velocity; thus, these data sets are well correlated (Fig. F25).
Four in situ temperature measurements were made at Site 1126, three using the Adara tool and one using the DVTP. All tool runs gave reproducible estimates for mudline temperatures, although none of the measurements made within the sedimentary section proved to be reliable because of the influence of ship heave following emplacement (see "Operations").