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doi:10.2204/iodp.proc.341.106.2014

Physical properties

Physical properties measurements were taken at Site U1420 to provide basic information for characterizing the drilled section using whole-round cores, split cores, and discrete samples. After cores were divided into sections, all whole-round core sections longer than ~30 cm were measured with the gamma ray attenuation (GRA) bulk densitometer and magnetic susceptibility loop on the Whole-Round Multisensor Logger (WRMSL) at 2.5 cm intervals with 5 s measurements. P-wave velocity was not measured on the WRMSL for this site, as RCB core diameter is insufficient to fill the liners (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014]) for a discussion of technical limitations of the P-wave logger system on the WRMSL). After WRMSL scanning, the whole-round sections were logged for natural gamma radiation (NGR) at 10 cm intervals. Color spectrometry, color reflectance, and point magnetic susceptibility were measured on the split cores using the Section Half Multisensor Logger at 2.5 cm resolution. Discrete measurements of P-wave and shear strength were made using the working-half sections of split sediment cores using the Section Half Measurement Gantry. Moisture and density (MAD) were also measured on 10 cm³ plugs collected from the working halves. A summary of all the physical properties measured at this site, except for vane shear strength, is provided in Figure F19.

Gamma ray attenuation bulk density

Variations in GRA bulk density in the recovered sections of Site U1420 likely reflect changes in mineralogy/lithology, consolidation, and porosity, which are overprinted by variable core recovery. Whole-round GRA bulk density averages ~1.8 g/cm³ in the RCB cores and displays downhole variability on the order of ~0.3 g/cm³ (Fig. F19). The WRMSL GRA bulk densitometer is calibrated for a core diameter equivalent to 6.6 cm (see “Physical properties” in the “Methods” chapter (Jaeger et al., 2014), but this diameter of cored sediment was not recovered at Site U1420. Consequently, the densities recovered via the WRMSL measurement system should be regarded as minimum estimates, and the range in observed densities is likely influenced by the highly variable diameter of the recovered cores.

Magnetic susceptibility

As is the case for the WRMSL GRA, the loop magnetic susceptibility meter is calibrated assuming the diameter of the core equivalent to the interior width of the core liner. Thus, although magnetic susceptibility data were collected in a raw form, the absolute values reflect a minimum estimate of magnetic susceptibility. The relationship between loop magnetic susceptibility and point-source magnetic susceptibility is likely to be driven by the volume of sediment in the WRMSL measurement window and cannot be expected to meaningfully reflect instrument reproducibility.

We normalize loop magnetic susceptibility for changes in sediment recovery and compaction by dividing the volumetric susceptibility by the GRA bulk density after smoothing with a Gaussian filter of 10 cm (±3σ) to correct for the differing response functions of the instruments. This generates a mass magnetic susceptibility (χ) with units of cubic centimeters per gram. (Fig. F20). This normalization by GRA bulk density reduces downcore variance in χ by ~15%, relative to the equivalently smoothed raw WRMSL magnetic susceptibility data normalized by the mean core density. Mass magnetic susceptibility averages ~54.3 cm³/g downhole at the site (Fig. F20). Several successions of variability between 25 and 70 cm³/g are recovered in the core deeper than 550 m CSF-A, and susceptibility appears to increase deeper than ~940 m CSF-A, although limited core recovery hinders interpretation.

Compressional wave velocity

P-wave measurements using the P-wave caliper (PWC) tool (see “Physical properties” in the “Methods” chapter (Jaeger et al., 2014) were taken at Site U1420 when core recovery allowed. Because of poor core recovery, only one PWC measurement was taken shallower than ~449 m CSF-A. Much of the recovered material deeper than ~449 m CSF-A consists of clast-rich or clast-poor diamict (see “Lithostratigraphy”). The nature of this material caused uneven contact with the PWC, so velocity measurements could not be obtained with regularity. PWC values were automatically picked where possible and manually picked when the automatic picker encountered errors because the calipers did not have sufficient contact with the sample because of abundant clasts, soft sediment, or bad coupling with the liner. PWC values show no significant overall trend with depth and vary widely, sometimes within the same section, likely because of the varying amounts and lithology of clasts. Though values generally range from ~1500 to ~2000 m/s, occasional measurements of velocities >2200 m/s were also observed (Fig. F21). All discrete measurements at this site were taken within the dominant matrix lithology of the recovered interval.

Natural gamma radiation

NGR was measured at 10 cm intervals on all whole-round core sections that exceeded 50 cm in length (Fig. F19). NGR measurements show recurring downcore fluctuations between 13 and 35 counts per second (cps) with a mean and standard deviation of 27 and 3 cps, respectively. Downhole variations in raw NGR values are influenced by changes in core recovery volume and consequently parallel changes in GRA bulk density.

As with WRMSL magnetic susceptibility, we calculate an equivalent activity of the sediment by normalizing to the WRMSL GRA bulk density after smoothing the data sets with a Gaussian filter of 50 cm (±3σ) to accommodate for the varying response functions of the instruments. This normalization by GRA bulk density reduces downcore variance in NGR by ~45%. Low-frequency variability in equivalent NGR activity is limited in the recovered cores (Fig. F22), although an increase in activity deeper than 940 m CSF-A parallels changes in normalized magnetic susceptibility (χ) and may reflect a change in lithology.

Moisture and density

Bulk density values in Hole U1420A were calculated from mass and volume measurements on discrete samples taken from the working halves of split cores (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014]). Depending on core recovery, quality, and lithology, one to three samples were taken per core. A total of 93 samples were analyzed for MAD.

Bulk density values range from 2.0 to 2.4 g/cm³. The intervals between ~40 and 440 m CSF-A and between ~490 and 540 m CSF-A were not recovered. Deeper than ~450 m CSF-A, MAD densities decrease downhole from ~2.2–2.3 to ~2.1 g/cm³ by ~570 m CSF-A (Fig. F23A). At ~580 m CSF-A, densities rise to 2.3 g/cm³ and remain relatively steady to ~650 m CSF-A. From ~650 to 700 m CSF-A, densities increase from ~2.2 to ~2.4 g/cm³. Deeper than ~750 m CSF-A, densities increase from ~2.0–2.4 to ~2.3–2.4 g/cm³ (Fig. F23B). Bulk grain density values display scatter from ~2.7 to 3.0 g/cm³ downcore and do not appear to correspond with lithostratigraphic trends or patterns (Fig. F24).

Porosity measured on discrete samples generally decreases with depth, with deviations toward higher values. These higher values were identified as mud intervals based on lithology (see “Lithostratigraphy”). Porosity decreases from ~30%–48% to ~25%–40% from ~450 to 1030 m CSF-A. Mud densities are higher than diamict, which may reflect the influence of clasts on the consolidation of the matrix lithology (Fig. F24).

Shear strength

Shear strength measurements were performed on working section halves from Hole U1420A using the automated vane shear testing system (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014]). Efforts were taken to avoid the locations of obvious drilling disturbance or cracks in the half-core sample. Measurements were taken as close as possible to the positions of the MAD samples and PWC measurements. We obtained only four shear strength measurements, all shallower than 40 m CSF-A, because of poor recovery from 0 to ~449 m CSF-A. When recovery increased at ~449 m CSF-A, the recovered material cracked upon penetration of the vane and we halted automated vane shear measurements. Shear strength measurements indicate that sediments within the shallowest ~40 m CSF-A are soft (~20 kPa). All samples reflect the dominant lithology of the recovered sediment.

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