Measurements of physical properties at Site 1128 followed the procedures outlined in "Physical Properties" in the "Explanatory Notes" chapter. These included nondestructive measurements of P-wave velocity (every 4 cm; Table T11, also in ASCII format), GRA bulk density (every 4 cm; Table T12, also in ASCII format), MS (every 8 cm; Table T13, also in ASCII format), and NGR (every 16 cm; Table T14, also in ASCII format) using the MST. The P-wave logger (PWL) was activated only on APC cores. Thermal conductivity was measured in unconsolidated sediment at a frequency of one per core (Table T15, also in ASCII format), with three samples per core analyzed after deployments of the Adara temperature tool and the DVTP (Table T15). A minimum of two discrete P-wave velocity measurements per section were made on the working half of the split cores (Table T16, also in ASCII format), and the measurement frequency was increased to five per section after the PWL was turned off. Standard index properties (Table T17, also in ASCII format) and undrained shear strength (only in unconsolidated sediments) (Table T18, also in ASCII format) 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".
Sediment physical properties at Site 1128 closely reflect lithologic variations observed in the recovered sediments and provide essential data for core-log correlation (Figs. F27, F28). An offset was seen between the discrete bulk density measurements and the GRA densiometry measurements of the MST in the upper 138 mbsf of the sediment section (Fig. F28). This offset was corrected using the equation of Boyce (1976) as described in "Index Properties" in "Physical Properties" in the "Explanatory Notes" chapter.
A close correlation was seen between the downhole logging data (see "Downhole Measurements") and sediment physical properties (Fig. F29). Natural gamma radiation values from both whole core and downhole logging measurements show an excellent correlation that supports the integrity of both data sets (Fig. F29). Index properties and GRA bulk densities have similar patterns to the downhole logging data, although values are generally lower (Fig. F29). This difference probably results from the fact that in situ density includes the influence of sediment overburden and hydrostatic pressure, whereas laboratory measurements do not. A similar effect is seen in the P-wave velocities (Fig. F29), particularly above 250 mbsf, where in situ velocities are higher than those measured on discrete samples. Similar data trends have been observed during other ODP legs (e.g., Leg 166; Eberli, Swart, Malone, et al., 1997)
Physical properties data can be divided into five units on the basis of trends in the measured parameters. Physical properties Unit (PP Unit) 1 (0-69 mbsf) is characterized by high variability in all data sets and corresponds to a lithostratigraphic sequence dominated by turbidites and debris flows (see "Lithostratigraphy"). Values of NGR average ~5 cps, with distinct peaks of as much as 60 cps (39-43 and 45-69 mbsf; Fig. F28). The upper peak (25 cps) occurs with a corresponding peak in MS (140 SI units) and a distinct peak in NGR in the downhole logs (see "Downhole Measurements"). This downhole spectral gamma peak appears to be mainly caused by high thorium concentrations, indicating that terrigenous sediments in the section are the cause of the NGR and MS shifts. The lower and much larger NGR peak (Figs. F27, F28) corresponds to a significant interval of debrites within lithostratigraphic Subunit IB. This interval also has increased bulk density (as much as 1.9 g/cm3), increased MS (80 SI units), and variable P-wave velocity (1.5-1.8 km/s) (Figs. F27, F28). A distinct peak in bulk density (as much as 1.85 g/cm3), correlated to decreased porosity (48%), is seen near 32 mbsf (Fig. F27). Sediments in this interval have increased cohesiveness, as expressed by greater shear strength (Fig. F30). No change in lithology can be identified to explain this density increase. The lower limit of PP Unit 1 is characterized by abrupt shifts in all physical properties parameters measured and marks the base of the debrite unit (see "Lithostratigraphy").
Physical properties Unit 2 (69-139 mbsf) is characterized by low variability in all data sets, punctuated by distinct offsets (1-5 m) corresponding to sharply bounded turbidites. The three most prominent turbidite layers occur at 94, 115, and 132-139 mbsf (Fig. F27) and are characterized by reduced NGR (5 cps), reduced MS (0 SI units), increased bulk density (1.8 g/cm3), and increased P-wave velocity (1.76 km/s). All these changes are characteristic of intervals of high calcium carbonate content (~90%; see "Organic Geochemistry"). Between the turbidite layers, NGR values range from 5 to 20 cps, MS is nearly constant at 30 SI units, bulk density ranges from 1.47 to 1.57 g/cm3, and P-wave velocity is nearly constant at 1.55 km/s (Figs. F27, F28). The base of the unit occurs at the bottom of the thickest turbidite layer between 132 and 139 mbsf (Fig. F28).
Physical properties Unit 3 (139-231 mbsf) is characterized by an interval of low and nearly constant values for all physical properties parameters measured (NGR = ~12 cps; MS = ~5 SI units; bulk density = ~1.5 g/cm3; and P-wave velocity = ~1.55 km/s) (Figs. F27, F28). This PP unit corresponds to lithostratigraphic Unit II, a sequence of monotonous olive-green clay (see "Lithostratigraphy"). Sediment homogeneity was also reflected in downhole logs (Fig. F29) (see "Downhole Measurements") as an interval of low variability in all parameters.
An increase in NGR (8-20 cps), MS (5-55 SI units), bulk density (1.5-1.8 g/cm3), and P-wave velocity (1.6-4.3 km/s) and a decrease in porosity (65%-25%) occur at the upper boundary of PP Unit 4 (231-363 mbsf), below which all parameters show increased variability reflecting alternations of indurated and unindurated sediment (Fig. F27). This unit correlates with logging Units 2-4 (see "Downhole Measurements"), and lithostratigraphic Subunit IIC, Unit III, and Subunit IVA (see "Lithostratigraphy"). Incomplete core recovery within this interval made it difficult to further divide PP Unit 4. Within PP Unit 4, sediments are lithified above 282 mbsf and partially lithified below 284 mbsf (see "Lithostratigraphy"). This decrease in lithification is reflected in sediment physical properties by reduced variability of P-wave velocity and an overall decrease in bulk density (1.7-1.52 g/cm3) (Fig. F27). Decreased lithification was accompanied by a decrease in calcium carbonate content, which resulted in increased NGR (6-23 cps). The base of PP Unit 4 is marked by a shift to lower values in bulk density, MS, and NGR (Fig. F27).
The upper boundary of PP Unit 5 (363-452 mbsf) correlates well with the transition to lithostratigraphic Subunit IVB (see "Lithostratigraphy") and logging Unit 5 (see "Downhole Measurements"). Within this unit all measured parameters show a constant trend for the rest of the recovered interval, reflecting the homogenous nature of the sediments (Fig. F27). This constant trend is also seen in downhole logging data (see "Downhole Measurements").
Undrained peak and residual shear strength were measured on unconsolidated sediments from 0 to 150 mbsf (Fig. F30). Shear strength at Site 1128 shows an overall downhole increase (5-240 kPa) caused by compaction, punctuated by increased variability below 50 mbsf. This variability is caused in part by alternations in sediment lithification. However, in some intervals it may also result from drilling disturbance and cracking of the sediment before sediment failure, resulting in lower values for peak strength. A prominent layer of low shear strength occurs between 132 and 139 mbsf in PP Unit 2 within a turbidite interval (Fig. F30).
An estimate of sediment consolidation can be made by comparing the range of shear strengths measured at Site 1128 to those of a gravimetrically or "normally" consolidated sedimentary section by calculating the ratio of undrained shear strength (Su) to overburden stress (P'o). Overburden pressure is calculated from bulk density and overlying sediment thickness assuming hydrostatic pore fluid pressures. The Su/P'o ratio in normally consolidated sediments is usually between 0.22 and 0.25. All sediments at Site 1128 fall below this range and, thus, appear to be underconsolidated (Fig. F30). Sediment underconsolidation can result from processes such as retarded dewatering of the sediment in response to rapid deposition and low interparticle friction or cohesion.
Thermal conductivity values at Site 1128 range from 0.62 to 1.28 W/(m·K) (Fig. F31; Table T14). In general, thermal conductivity data correlate well with other sediment physical properties, particularly bulk density (Fig. F31). Physical properties Unit 1 is an exception to this relationship, having only a moderate correlation to other physical properties data sets (Fig. F31) caused by high sediment variability occurring as a result of slumping within the upper sedimentary section (see "Lithostratigraphy").
Three Adara and one DVTP in situ temperature measurements were made at Site 1128. There was some variation in estimates of mudline temperatures, which ranged from 1.52° to 2.48°C with an average of 1.92°C (Table T19, also in ASCII format). Measurements made at Core 182-1128C-4H were affected by postemplacement movement of the probe. Despite this disturbance, a value for in situ temperature was determined using data before and after the disturbance.
In situ measurements provide a linear temperature-depth relationship that defines a geothermal gradient of 53°C/km (r2 = 0.985; Fig. F32). This relatively high value can be explained by the low thermal conductivity of the sediments at Site 1128. The geometric mean of thermal conductivity between 0 and 200 mbsf (Fig. F31) was used to determine heat flow (0.94 ± 0.05 W/[m·K]). Using this value and the geothermal gradient determined above, heat flow is estimated to be 49.8 mW/m2. This value is close to the global average for continental and oceanic heat flows and is close to the values of 56 and 63 mW/m2 reported for the Ngalia and Alice Springs/Amadeus Basins of the Central Australia Archean Shield by Lambeck (1986).