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doi:10.2204/iodp.proc.308.106.2006

Physical properties

Downhole profiles of physical properties were established for Site U1322 where the overburden above the Blue Unit is thin (see “Background and objectives”). Physical properties are compared with those from Site U1324, where overburden above the Blue Unit is thick, and are also compared with those of Site U1323, where there is an intermediate overburden.

Density and porosity

To increase the resolution of moisture and density (MAD) data near seismic Reflectors S20 and S30, extra samples were taken in Cores 308-U1322B-10H, 15H, 16H, and 17H.

Bulk densities determined from the different methods (gamma ray attenuation [GRA], MAD, and image-derived density [IDRO]) are in good agreement (Fig. F34A). LWD porosities were calculated assuming a grain density of 2.7 g/cm³ and a pore water density of 1.024 g/cm³.

MAD bulk densities increase from 1.3 to ~1.8 g/cm³ from the seafloor to 28 mbsf (Fig. F34A). Correspondingly, porosity values decrease from 80% to 56% (Fig. F34C). From 28 mbsf to the bottom of the hole there is little increase in density (and little decrease in porosity). Nevertheless, small shifts from the main trend are observed within this lower interval. The first is a 3% increase in porosity recorded from 28 mbsf (the depth of seismic Reflector S10) to 34 mbsf. This correlates with the silt laminae interval at the bottom of lithostratigraphic Subunit IA (see “Lithostratigraphy”). The next two shifts, up to 5% in porosity, are recorded at the base of lithostratigraphic Subunits IB and ID and are interpreted as MTDs. At these boundaries, the MTDs show higher densities and lower porosities than the nondeformed sediments below them. The boundary between the top of the MTDs and the nondeformed unit above is recorded with only a gradual change in density and porosity.

Noncontact resistivity

Noncontact resistivities (NCR) at Site U1322 increase 0.2 Ωm from the seafloor to 30 mbsf and then remain relatively constant at ~0.6 Ωm until 70 mbsf (Fig. F35A). From that depth downhole resistivity values abruptly increase and become more scattered but center on a mean value of 1 Ωm (Fig. F35A). At 70 mbsf the first high methane concentrations were detected (see “Geochemistry and microbiology”). Thus, the large shift in NCR at this depth and the high scatter in the data are believed to result from gas expansion and the formation of voids and cracks.

NCR in Hole U1322B and LWD-derived resistivity measurements from Hole U1322A do not correlate well. Only a few peaks appear in both logs, but the shapes and magnitudes of the peaks are different. In addition, the resistivities from LWD data are generally higher than those of NCR (Fig. F35B). This might result from measurement conditions, especially temperature contrast between laboratory and in situ conditions.

In the uppermost 15 mbsf, resistivity increases more in the LWD data than the NCR data (Fig. F35A, F35B). The LWD data in this interval are probably affected by the large borehole diameter (see “Downhole measurements”) and may be not as reliable as the NCR data. From 90 mbsf downhole, the LWD data show relatively constant values along the profile, with major excursions in MTD intervals (Fig. F35A). Some of these excursions may reach resistivities as high as 2.5 Ωm.

Magnetic susceptibility

Magnetic susceptibility generally increases slightly from the top to the bottom of the hole (Fig. F36). In the uppermost part of the hole, very low magnetic susceptibility values correspond to increases in the carbonate content (see “Lithostratigraphy”). In lithostratigraphic Subunit IA (0–37.43 mbsf), magnetic susceptibilities increase with depth and then decrease to the base of the unit (Fig. F36). The maximum magnetic susceptibility value in this interval is reached at 31 mbsf, where a thin interval of silt/​sand layers was identified, according to the lithostratigraphic description (see “Lithostratigraphy”). It correlates with seismic Reflector S10.

Magnetic susceptibilities within lithostratigraphic Subunit IB (37.43–58.60 mbsf) sharply increase from 32 × 10^–5 to 42 × 10^–5 SI (Fig. F36). This unit, interpreted as an MTD, is composed of three subunits. The middle unit is characterized by deformed and faulted sediments, whereas the bottom and the top units are less deformed (see “Lithostratigraphy”). Different magnetic susceptibility values are present in each subunit.

Lithostratigraphic Subunit IC (58.6–91.31 mbsf) is characterized by a sharp decrease in magnetic susceptibility values and then increasing values to the base of the unit. Nevertheless, between 78 and 87 mbsf, lower values are measured. Lithostratigraphic Subunit ID (91.31–125.8 mbsf) shows higher values of magnetic susceptibility than the general trend of the profile. This correlates with the presence of silty intervals and mud clasts within the mass transport deposit, particularly near the base of the unit (see “Lithostratigraphy”).

The lowermost part of the profile, corresponding to lithostratigraphic Unit II (125.8–234.5 mbsf), is composed of stacked MTDs (see “Lithostratigraphy”). Magnetic susceptibility values are more scattered within this unit. Magnetic susceptibilities tend to increase with depth. High values of magnetic susceptibility appear to be related to MTDs, mud clasts, and/or faulted sediments within MTD and silt/​sand intervals. Seismic Reflectors S40-1322 and S50-1322 are identified as peaks in magnetic susceptibility.

Thermal conductivity

Between Cores 308-U1322B-21H and 22H, one thermal conductivity measurement per section was performed. Thermal conductivity values at Site U1322 average 1.15 W/(m·K) and increase with depth in the uppermost 20 m of the sediment column (Fig. F37), as would be expected from the decreasing porosity profile (Fig. F34C).

Below 20 mbsf, the thermal conductivity profile has high variability (0.8–1.35 W/[m·K]), probably resulting from the effect of gas expansion in the cores. At the bottom of lithostratigraphic Unit II, the thermal conductivity profile stabilizes and decreases gently from 1.18 to 1.1 W/(m·K).

P-wave velocity

P-wave velocity data from the multisensor track (MST) P-wave logger (PWL) were recorded from near the seafloor to 72.42 mbsf. PWL velocity measurements range from 1478 m/s near the seafloor to 1609 m/s at 60 mbsf (Fig. F38). From the seafloor to 60 mbsf, P-wave velocities increase. From 60 to 70 mbsf, where gas expansion affected measurements, P-wave velocities decrease. Velocities from P-wave Sensors 1 and 2 (PWS1 and PWS2) insertion probes show a similar trend, while the contact probe PWS3 shows slightly higher velocities. Anisotropy within the measured interval is relatively low and centered around a mean value of 0.8%.

Undrained shear strength

At Site U1322 several extra measurements were performed near lithostratigraphic unit boundaries and in deformed sediments (MTD) and faults. The undrained shear strengths measured by the automated vane shear (AVS) system consistently matched those measured with the penetrometer (Fig. F39B). Peak undrained shear strengths increase downhole from near zero at the top of lithostratigraphic Unit I to ~140 kPa at 200 mbsf (Fig. F39A). As depth increases, the undrained shear strength data become more variable.

MTDs generally have higher undrained shear strengths than undisturbed sediments. The shear strength decreases abruptly with depth at the base of the MTDs as shown at the base of lithostratigraphic Subunit IB (Fig. F39). In contrast, the shear strength only increases gradually with depth at the top of MTDs, as shown at the top of lithostratigraphic Subunit IB. There is a particularly abrupt decrease in shear strength (~50 kPa) with depth at the top of lithostratigraphic Unit II (125 mbsf) (Fig. F39).

The residual shear strength profile shows high variability (Fig. F39A). Overall, a linear trend is observed from 0 kPa at the top of lithostratigraphic Unit I to ~40 kPa at 200 mbsf. The profile is similar to that of the peak undrained shear strength, with higher values in slump deposits and lower values in nonslumped deposits. The difference between the two is, however, more subtle. Below 200 mbsf residual shear strength values are relatively low (~20 kPa) and show almost no increase with depth. The sensitivity is generally low (~4) with some intermediate values (from 5 to 10) and a few peak values that indicate high sensitivity (Fig. F39C).

In Figure F40A, peak shear strength is plotted and compared with iso-lines of the ratio between peak shear strength and the vertical hydrostatic effective stress (). σ_vh′ is calculated from the LWD bulk density log and assumes hydrostatic conditions (see “Physical properties” in the “Methods” chapter). The relationship between the ratio lines and peak undrained shear strength gives an indication of the consolidation state of the clay. At Site U1322 peak undrained shear strength is not parallel to a particular ratio line. Within lithostratigraphic Subunits IA and IB, undrained peak shear strength follows the 0.1 ratio line (Fig. F40A). In lithostratigraphic Subunits IC and ID, the undrained shear strength profile moves in between the lower ratio lines of 0.1 and 0.05. Below that depth undrained shear strength is more variable, with slump deposits having values between the ratio lines of 0.1 and 0.05 and nonslump deposits between 0.05 and 0.25.

Summary

At Site U1322 the porosity profile (Fig. F34C) decreases relatively rapidly from the seafloor to 30 mbsf and then more gently to the bottom of the borehole. Shifts from the main trend are observed at MTDs. These shifts have a maximum magnitude of 5%, with lower porosities in MTDs than in undisturbed deposits. These porosity changes are mirrored by the bulk density profile (Fig. F34A).

The undrained shear strength increases linearly with depth. A shift of 50 kPa at 125 mbsf is observed at the base of lithostratigraphic Subunit ID, a 30 m thick MTD. Smaller shifts are also observed at the base of each MTD. MTDs typically have higher undrained shear strengths than undisturbed deposits.

From the main trends in porosity (Fig. F34), undrained shear strength (Fig. F40A), and the relation of the shear strength to the hydrostatic vertical effective stress () (Fig. F40B), it is possible to make some inferences regarding the sediment consolidation state:

• The change in porosity and undrained shear strength at 125 mbsf indicates that lithostratigraphic Unit II is less consolidated than the underlying MTD.
• MTDs tend to be more consolidated. Higher consolidation might result from reformation of the originally loose sediments into a more packed structure by dewatering during the landslide process. This phenomenon has been observed in other areas such as the Amazon Fan (Piper et al., 1997).
• Porosity (Fig. F34) and undrained shear strength (Fig. F40) values at the top of the MTDs do not vary significantly with respect to the undisturbed sediments above. This might imply that dewatering and consolidation preferentially takes place at the base of the slump where shearing is most likely.

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