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doi:10.2204/iodp.proc.308.104.2006

Physical properties

Laboratory measurements at Site U1320 were performed to provide downhole profiles of physical properties at a reference site with assumed hydrostatic pore pressure conditions.

Density and porosity

Porosities at Site U1320 range from 67% at the top of the hole to 40% near the bottom. In general, porosity decreases with depth and increasing vertical hydrostatic effective stress (σ_vh′) (Fig. F26).

In the uppermost part of the section, mostly in lithostratigraphic Unit II, the porosity versus vertical hydrostatic effective stress profile is jagged (Fig. F26). High variability of porosity in this unit is probably associated with sand and silt in this section. Low porosity at the boundary between lithostratigraphic Units II and III at 137 mbsf correlates well with seismic Reflector R30. Lithostratigraphic Unit III (137.5–145.3 mbsf) has similar porosities to those of Unit II. From the top of lithostratigraphic Unit IV (145.3–174.4 mbsf), where the hydrostatic effective stress exceeds 1 MPa, porosity values gradually decrease. In this interval, which consists mostly of clay, porosities decrease ~5%. Nevertheless, a peak is observed near the boundary between lithostratigraphic Units IV and V at 174.4 mbsf that correlates well with seismic Reflector R40.

Gamma ray attenuation (GRA) bulk densities in lithostratigraphic Unit I (0–2.65 mbsf) increase with depth from 1.33 to 1.4 g/cm³. In lithostratigraphic Unit II (2.65–137.50 mbsf), densities are higher and vary from 1.5 to 2.1 g/cm³ (Fig. F27A). These densities depend greatly on the nature of sediments encountered. Poor recovery in lithostratigraphic Subunits IIC (37.76–82 mbsf) and IID (82–124.6 mbsf) make the results more difficult to interpret. However, the trend observed in wireline logging bulk density indicates that the overall pattern of the bulk density curve is preserved both in the moisture and density (MAD) and GRA data. The trend in bulk density mirrors that of porosity. Therefore, lithostratigraphic Unit III shows typically low bulk densities, whereas they are high within lithostratigraphic Unit IV. The boundary between lithostratigraphic Units IV and V is marked by a decrease in bulk density. Bulk density values then gradually increase to TD, approximating an exponential trend. Densities are >2 g/cm³ at the bottom of the hole.

Grain densities have a more constant profile, oscillating between 2.4 and 2.8 g/cm³ (average = 2.72 g/cm³) (Fig. F27B). Wood and high organic content-bearing clays between 3.9 and 5 mbsf are associated with grain densities as low as 2.1 g/cm³. Grain densities within massive sand layers range from 2.65 to 2.75 g/cm³. Greenish clay has a grain density of ~2.7 g/cm³. Grain densities in lithostratigraphic Units III and IV are slightly lower than those of the sediments immediately above and below.

The density profile from wireline logging at Site U1320 is shown in Figure F27A for comparison purposes. In general, the patterns observed in densities measured using the multisensor track (MST), MAD methods, and wireline logging are consistent. However, wireline bulk densities are often higher than those measured by GRA and MAD. GRA and MAD results may reflect gas expansion, unloading (~5%), and disturbance from XCB drilling. The intervals lacking MST and MAD data because of poor core recovery are described on the basis of available wireline logging data.

Noncontact resistivity

Within lithostratigraphic Unit I (0–2.65 mbsf), mainly composed of greenish gray clay, noncontact resistivity (NCR) increases linearly with depth (Fig. F28A). This trend correlates with an increase in GRA bulk densities (Fig. F27A). The general trend in NCR correlates with wireline logging resistivity (deep, medium, and shallow induction resistivity) (Fig. F28B).

Lithostratigraphic Unit III (137.5–145.3 mbsf) consists of mostly clay, which translates to low resistivity values in the logs. High porosities at/near the boundary between Units III and IV (145.3 mbsf) are mimicked by NCR and wireline resistivities. Lithostratigraphic Units IV and V are characterized by relatively constant resistivities (~1 Ωm), only interrupted by peaks below and above no-recovery intervals near the boundary between Units IV and V (174.4 mbsf). This general trend correlates with wireline resistivity, except between 265 and 285 mbsf, where shallow resistivities are slightly lower (probably due to a borehole washout). Wireline resistivities show two significant changes at 180 and 285 mbsf that correlate with seismic Reflectors R40 and R60.

Wireline resistivities vary from 0.2 to 2 Ωm between 62.5 (top of the logged section) and 176 mbsf (average = ~1.2 Ωm) (Fig. F28B). In the same interval, NCR values vary from 0.4 to 1.6 Ωm (average = 1 Ωm) (which includes values near the edge of no recovery) (Fig. F28A). NCR values are consistently lower than wireline resistivities, possibly due to differences between in situ measurement and laboratory conditions (temperature and pressure), as well as the formation of voids due to gas expansion.

Magnetic susceptibility

Magnetic susceptibility values in the uppermost part of the log, corresponding to lithostratigraphic Unit I (0–2.65 mbsf), rapidly increase from 17 × 10^–5 to 50 × 10^–5 SI (Fig. F29). Lithostratigraphic Unit II (2.65–137.50 mbsf) can be divided into several subunits based on magnetic susceptibility data that correlate with lithostratigraphic Subunits IIA–IIE (see “Lithostratigraphy”). Despite low recovery in the lower part of lithostratigraphic Unit II (Cores 308-1320A-10X through 14X), high magnetic susceptibility values correlate with the occurrence of sand-rich turbidite sequences. The base of lithostratigraphic Unit II is characterized by a relatively constant magnetic susceptibility value of ~35 × 10^–5 SI. Magnetic susceptibility values in lithostratigraphic Unit III (137.50–145.30 mbsf) are similar to those from the base of lithostratigraphic Unit II except at its base, where a peak can be correlated with ash Layer Y8 (see “Lithostratigraphy”). At Site U1319, however, ash Layer Y8 correlates with a local minimum magnetic susceptibility value. Within lithostratigraphic Unit IV (145.30–174.40 mbsf), high magnetic susceptibility values can be correlated with silt/sand layers. Magnetic susceptibilities in lithostratigraphic Unit V (174.40–299.61 mbsf) increase from 40 × 10^–5 to 90 × 10^–5 SI. This could indicate a downward change in the mineral composition. Lithology alternates between greenish gray, brownish green, and brownish red clays and black organic-rich clays (see “Lithostratigraphy”).

Thermal conductivity

The average thermal conductivity is 1.2 ± 0.31 W/(m·K) (Fig. F30). The upper part of the profile (lithostratigraphic Units I–IV) has high variability in thermal conductivity (0.8–2.2 W/[m·K]). The high frequency of thick sandy-silty layers in these units can provide scattered values, due to higher thermal conductivities in the sand. Values <1.0 W/(m·K) may reflect low-quality measurements caused by core biscuiting. In lithostratigraphic Unit V, thermal conductivity measurements were performed in mud/clay sediments, as seen in the lithostratigraphic log (see “Lithostratigraphy”). In this part of the log, thermal conductivities increase from 1 to 1.3 W/(m·K) and the trend fits an almost linear increase with depth, as expected from the decreasing porosity profile (Fig. F27C).

P-wave velocity

P-wave velocity measurements performed on split sections resulted in acceptable data for the uppermost 20 m only, probably because of voids and cracks formed by gas expansion (Fig. F27C). Measurements on distilled water were carried out regularly to check that unrealistic negative values were not caused by malfunctioning equipment. Transverse velocities (along the x- and y-axis) and longitudinal velocities (along the z-axis) using the P-wave sensor (PWS) probes produced results for three core sections from 4.18 to 16.71 mbsf (Fig. F27C). Average velocity along the x-axis = 1541.1 m/s, along the y-axis = 1496 m/s, and along the z-axis = 1528.3 m/s. These values are near the velocity of the control measurements in distilled water, indicating high porosities and low consolidation of sediments in these three sections (Fig. F27C).

Shear strength

Undrained shear strength measured using the automated vane shear (AVS) apparatus consistently matched those measured using the handheld penetrometer (Fig. F31A).

Peak undrained shear strengths increase systematically downhole from near zero within lithostratigraphic Unit I to ~100 kPa at 160 mbsf (Fig. F31A, F31B). Considerable scatter in the data appears below ~160 mbsf. As seen in the corresponding measurements at Site U1319, this scatter could partially result from XCB coring. The XCB causes biscuiting and local failure planes in the core. The residual shear strength profile has relatively high values in the upper 80 mbsf. From that point onward the residual undrained shear strength profile shows much lower values. No significant variations are observed until 180 mbsf. Then values gradually increase to ~50 kPa in lithostratigraphic Unit V.

The sensitivity (peak/residual shear strength) is low in the intervals from 0 to 110 and 185 to 300 mbsf (Fig. F31C). In the intermediate interval that composes the lower part of lithostratigraphic Units II, III, and the upper part of IV, higher sensitivities are caused by relatively low values of residual shear strength.

Peak undrained shear strengths measured at this site plot within 0.2 and 0.025 ratios with respect to the vertical hydrostatic effective stress (Fig. F32). These ratios are rather low, especially when compared to the values observed in the uppermost sediment column for conventional piston cores.

Summary

At Site U1320, MST, MAD, and shear strength data show two different types of behavior. The lower part of lithostratigraphic Unit V is similar to the profile recorded at Site U1319. The sequence above has greater variations in bulk density and porosity, which are the result of alternating lithologies encountered at this site (see “Lithostratigraphy”). Overall, Sites U1319 and U1320 show comparable trends in porosity versus vertical hydrostatic effective stress. Clays in lithostratigraphic Unit V (which correspond to those of lithostratigraphic Unit VI at Site U1319) possibly show slightly higher porosities relative to vertical hydrostatic effective stresses. This could suggest that lithostratigraphic Unit V at Site U1320 is slightly underconsolidated.

Undrained shear strength values increase with depth from near 0 kPa at the top of Hole U1320A to almost 200 kPa at the bottom. An interval with relatively constant values of ~100 kPa occurs between 110 and 170 mbsf (lower part of lithostratigraphic Unit II, Unit III, and upper part of Unit IV). This interval also has low residual undrained shear strengths and high sensitivities that coincide with an MTD seen on seismic reflection profiles (see “Lithostratigraphy”). Below this interval, the undrained shear strength profile has a more serrated pattern but increases with depth.

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