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doi:10.2204/iodp.proc.308.108.2006

Physical properties

A downhole profile of physical properties was established for Site U1324 where the overburden above the Blue Unit is thickest relative to Sites U1322 and U1323. Physical properties are compared with those from Site U1322, where overburden above the Blue Unit is thinnest, and Site U1323, which has an intermediate overburden.

All cores were processed through the multisensor track (MST) before splitting. Initially, noncontact resistivity (NCR) and magnetic susceptibility measurements were taken at 2 cm intervals. P-wave logger (PWL) and gamma ray attenuation (GRA) densitometer measurements were taken at 4 cm intervals. All MST measurements were taken at 6 cm intervals from Core 308-U1324B-5H. PWL measurements were aborted after Core 308-U1324B-26H because of voids and cracks created by gas expansion.

Moisture and density (MAD) samples were selected from undisturbed cores at regularly spaced intervals ~50 cm from the top of each section. They were used to calculate water content, bulk density, grain density, and porosity. An extra sample was taken ~100 cm from the top of each section from Cores 308-U1324B-46X through 52X. The dry volume of these extra samples was not measured and, hence, grain density was not calculated because of time constraints and equipment capacity. Instead, the grain density of the sample from the MAD measurement at 50 cm in the same section was used to estimate the bulk density and porosity.

The discrete P-wave velocity (PWS) measurements only provided good data for cores above Core 308-U1324B-28H due to extensive voids and fractures in the cores.

Density and porosity

Bulk density was determined from GRA on whole sections and from MAD measurements on discrete samples. Image-derived density (IDRO) from LWD data are discussed for comparison. Bulk densities determined from different methods are in good agreement in lithostratigraphic Unit I (Fig. F34A). There was considerable scatter of the LWD (IDRO) bulk density in lithostratigraphic Unit II. This is interpreted to be due to borehole washout when drilling through sandy-silty layers. The LWD (IDRO) densities had low values because it was partially measuring the water density when the borehole was washed out. LWD porosities were calculated assuming a grain density of 2.7 g/cm³ and a pore water density of 1.024 g/cm³. MAD porosities and LWD porosities are generally in good agreement.

MAD bulk densities rapidly increase from 1.27 to ~1.7 g/cm³ from the seafloor to 35 mbsf. Consequently, porosity values decrease from 80% to 55%. A decrease in bulk density and increase in porosity at 35 mbsf correlates with seismic Reflector S10. From 35 to 160 mbsf, bulk density increases slightly from 1.7 to 2.0 g/cm³. Correspondingly, porosity decreases from 55% to 45% (Fig. F34C). The sharp density decrease reflected by both the MAD and LWD data at ~160 mbsf may be related to the silt layer above seismic Reflector S30 (see “Lithostratigraphy”). Below seismic Reflector S30 there is little increase in bulk density with depth; values range from 1.95 g/cm³ immediately below seismic Reflector S30 to 2.05 g/cm³ at the bottom of the hole. Porosity from MAD and LWD data shows little variation below seismic Reflector S30, with values decreasing gently from 48% to 40% with depth. Slight variations from this trend are observed at the top of lithostratigraphic Subunit IIA, with a small sudden increase in density, and within lithostratigraphic Subunit IIC, with a negative peak.

Noncontact resistivity (NCR)

NCR increases with depth from 0.36 Ωm at the seafloor to ~1.5 Ωm at the bottom of the borehole (Fig. F35). NCR values increase with depth in lithostratigraphic Subunits IA and IB from 0.36 Ωm at the seafloor to 0.80 Ωm at 59.7 mbsf. In lithostratigraphic Subunits IC to IIB (59.7–481.9 mbsf), NCR values are more scattered than within the overlying lithostratigraphic units; these values range from 0.60 to 1.20 Ωm. The scatter is more evident between 240 and 400 mbsf, which probably reflects gas voids and cracks, in addition to lithologic variability. The high variability in these intervals correlates with high methane concentrations in the pore water (see “Geochemistry and microbiology”).

The lowest part of lithostratigraphic Subunit IIC (550–578.9 mbsf) has the highest resistivity values, while the sandy/​silty layers within Subunit IID (578.9–608.0 mbsf) have NCR values similar to those of the upper part of Subunit IIC.

The resistivities from wireline logging data show a similar trend to that of MST measurements. However, wireline logging resistivities are generally higher, which probably results from measurement conditions, especially temperature contrast between laboratory and in situ conditions.

Magnetic susceptibility logger

Interpretations are based on uncorrected magnetic susceptibility (Fig. F36). Near-zero values or extremely high values were observed at the top and bottom of each section. These values are not included in the description of the data.

Lithostratigraphic Subunit IA shows an increase of magnetic susceptibility with depth from 0 to 22 mbsf that correlates with a rapid increase in bulk density (Fig. F34). From 22 mbsf to the base of lithostratigraphic Subunit IA at 43.9 mbsf, a slight decrease in magnetic susceptibility can be observed (Fig. F36).

Magnetic susceptibility within lithostratigraphic Subunits IB (43.9–59.7 mbsf), IC (59.7–107 mbsf), and ID (107–116.8 mbsf) increases rapidly with depth until 160 mbsf and then, in lithostratigraphic Subunit IE (116.8–264.6 mbsf), decreases. The maximum value of magnetic susceptibility is correlated with a thin silt-rich interval that corresponds to seismic Reflector S30 (Fig. F36) almost at the top of lithostratigraphic Subunit IE. The lowest part of lithostratigraphic Unit I, including Subunits IF (264.8–288 mbsf) and IG (288–364.7 mbsf), shows again an increase in magnetic susceptibility from ~65 × 10^–5 to 95 × 10^–5 SI. Magnetic susceptibility in lithostratigraphic Subunit IIA (364.7–445.5 mbsf) continues the trend of the interval above. This unit, corresponding to a thick interval of fining-upward silt/​sand-rich turbidites alternating with muddy intervals (see “Lithostratigraphy”), does not display excursions as might be expected from the alternating lithologies.

Lithostratigraphic Subunit IIB (445.5–481.9 mbsf) is mainly composed of material deformed and remolded within MTDs (see “Lithostratigraphy”). This unit is characterized by very low magnetic susceptibility values, on the order of 45 × 10^–5 SI (Fig. F36). However, the rest of the MTDs at Site U1324 do not show similar excursions. The top of lithostratigraphic Subunit IIC shows values on the order of 100 × 10^–5 SI, comparable to those in the lower part of Subunit IIA, and has decreasing values with increasing depth.

Thermal conductivity

Thermal conductivity increases rapidly with depth within lithostratigraphic Subunit IA and continues to increase with depth but less rapidly in lithostratigraphic Subunits IB and IC. Below lithostratigraphic Subunit IC, at 107 mbsf, there is almost no increase in thermal conductivity with depth, although the profile oscillates between 1 and 1.4 W/(m·K), centered around a mean of 1.2 W/(m·K) (Fig. F37). The trend in thermal conductivity mirrors that of the porosity: a rapid porosity decrease through lithostratigraphic Subunits IA–ID and a relatively constant porosity profile thereafter (Fig. F34).

P-wave velocity

The data from the PWS probes and the PWL correspond well (Fig. F38). The velocity rapidly and smoothly increases from 0 to 60 mbsf and then increases more moderately from 60 to 190 mbsf (Fig. F38). No major shifts in P-wave velocity are observed when crossing from one lithostratigraphic unit to another. Velocity peaks in PWS data are relatively small in the uppermost 100 m and, from this depth downhole, the profile shows a more jagged pattern. The most important excursion recorded by PWS and PWL data is related to seismic Reflector S30, at ~160 mbsf. This low-velocity excursion is also evident in wireline logging data (Fig. F38).

P-wave velocities from wireline logging in Hole U1324B show substantial disagreement above seismic Reflector S20. Below this reflector the wireline logging values converge with PWS and PWL data (Fig. F38). This mismatch most probably results from the large borehole diameter above seismic Reflector S20, at 107 mbsf. The caliper log from LWD data in Hole U1324A shows a large hole diameter above 120 mbsf.

Below 220 mbsf, the wireline logging data show significant deviations from the smooth increase in velocity observed above. The first occurs between 220 and 260 mbsf, where wireline P-wave velocity decreases by ~50 m/s (Fig. F38). The caliper tool in Hole U1324A measured a large diameter in this interval, indicating that this decrease may be at least partially due to borehole washout. From 260 to 350 mbsf, near the top of lithostratigraphic Unit II, P-wave velocities continue to increase smoothly (Fig. F38). The wireline P-wave velocity log shows more irregular behavior from 350 mbsf downhole (lowest part of lithostratigraphic Subunit IG and lithostratigraphic Unit II). This is probably related to the alternating silt-sand-clay layers within this unit. Nevertheless, two intervals can be observed in Unit II. In the upper one (350–425 mbsf), velocities decrease from 1675 to 1650 m/s. The second one shows increasing velocities from 1675 to 1750 m/s.

P-wave velocity anisotropy was derived from PWS measurements. It is relatively small, with values ~1%, within lithostratigraphic Subunits IA, IB, and IC but increases in lithostratigraphic Subunit ID, especially below seismic Reflector S20, with mean anisotropies of ~2%.

Undrained shear strength

The undrained shear strengths measured by the AVS consistently match those measured with the pocket penetrometer (Fig. F39). Below 240 mbsf, the values measured by the penetrometer are systematically higher than those measured by the AVS, probably because the penetrometer measurements do not generate fractures whereas those of the AVS do.

Peak undrained shear strength increases downhole from near zero at the top of lithostratigraphic Unit I to ~250 kPa at 220 mbsf (Fig. F39). The undrained shear strength is more scattered in lithostratigraphic Unit II. This is interpreted to be associated with the voids and fractures formed before and during measurements and the highly variable lithology. The upper part of the MTDs (440–480 mbsf) has higher undrained shear strength. This may result from a higher consolidation state of the sediments due to recompaction during the landslide process. An overall trend of decreasing shear strength with depth is shown in lithostratigraphic Subunits IIB and IIC. The shear strength increases from 540 to 580 mbsf and then decreases to the bottom of the hole. The decrease in shear strength at the base of lithostratigraphic Subunit IIB is correlated to the presence of silty layers, whereas within lithostratigraphic Subunit IID, the low shear strengths correlate with the presence of sand layers within clay (measurements were always performed on the mudstone lithology).

Residual shear strength is highly variable (Fig. F39). This is partly explained by the difference in failure pattern after the shear vane reaches the undrained peak shear strength. Brittle failures isolate a plug around the shear vane, and the undrained residual shear strength is then very low. If the failure is caused without fracture the residual shear strengths are observed to be generally higher.

The sensitivity is generally intermediate (5–10) with a few peak values that indicate high sensitivity (Fig. F39). These peak values are associated with low residual shear strengths and reflect brittle failure rather than low disturbed shear strength.

In Figure F40, the peak shear strength is plotted and compared with iso-lines of the ratio between peak shear strength and the vertical hydrostatic effective stress (see “Physical properties” in the “Methods” chapter). The relation between the iso-lines and the peak undrained shear strength gives an indication of the consolidation state of the clay. At Site U1324 the peak undrained shear strength is not parallel to a particular iso-line. Within lithostratigraphic Subunits IA and IB, the undrained peak shear strength follows the 0.1 iso-line (Fig. F40). In Lithostratigraphic Subunit IC the peak shear strength moves toward the lower iso-lines 0.05 and 0.025.

Summary

Variations in physical properties correlate with lithostratigraphic units (see “Lithostratigraphy”). The interbedded silt, sand, mud, and MTDs in lithostratigraphic Unit II are characterized by highly variable bulk density and porosity. Physical properties show much less scatter within the uniform hemipelagic mud and clay in lithostratigraphic Unit I.

Bulk density, porosity, noncontact resistivity, thermal conductivity, and P-wave velocity data are mutually dependent. Porosity decreases rapidly from the seafloor to 35 mbsf and then slowly to 160 mbsf. There is only a slight decrease in porosity below 160 mbsf to the bottom of the hole. This trend is also reflected on the bulk density, NCR, P-wave velocity, and thermal conductivity data. Almost no decrease in porosity with depth below 160 mbsf suggests that these sediments are underconsolidated and overpressured. This explanation is supported by the high sedimentation rates described in “Biostratigraphy.”

There is no significant difference in physical properties between the MTDs and the nondeformed sedimentary successions in lithostratigraphic Unit I at Site U1324, suggesting that these MTDs remained relatively intact during transport and probably have not moved significant distances from their original deposition location. This explanation is supported by the mild deformation observed in these intervals (see “Lithostratigraphy”).

Undrained peak shear strength shows larger oscillations within lithostratigraphic Unit II. It generally increases with depth from a low at the boundary between lithostratigraphic Units I and II until the bottom of the hole. However, major reductions in shear strength occur at ~520 mbsf, at the base of a major MTD, and at ~590 mbsf.

Lithostratigraphic Subunits IB and ID correspond to MTDs at Sites U1322 and U1324. The trends in porosity and undrained shear strength are similar at both sites. However, the porosity and undrained shear strength profiles at Site U1324 (Figs. F34C, F40B) show more subdued variations (or no variation at all) between MTDs and undisturbed units than those at Site U1322 (Figs. F34C, F40B). It is possible that at Site U1324, upslope on the Mississippi Canyon levee, the velocity of landslides might have been lower than that at Site U1322, in the center of the basin, and thus the amount of shearing and deformation might also have been lower. Higher shearing at Site U1322 would then translate into a higher degree of consolidation.

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