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doi:10.2204/iodp.proc.340.104.2013

Physical properties

The physical properties of Holes U1394A and U1394B were correlated with lithologic variations, including composition, grain size, and lithification. Bioclastic-volcaniclastic turbidites and thick tephra layers were distinguished from background hemipelagic sediment by most physical properties. Variable shear strength could be due to the variable facies observed within turbidite and hemipelagic sediment, resulting in different types of sediment consistency. Stratigraphic correlation reveals a systematic decrease in density and magnetic susceptibility values for Hole U1394A, where the XCB was primarily used, versus Hole U1394B, where the APC was the primary tool for extracting core.

Stratigraphic correlation between Holes U1394A and U1394B

We used both magnetic susceptibility and gamma ray attenuation (GRA) density data to correlate depths between Holes U1394A and U1394B (Fig. F7), with magnetic susceptibility providing the most robust correlation. Correlation is best in the uppermost 7 m of the holes, where both holes had high recovery rates. In the uppermost 7 m we see clear correlation of turbidite units between the two holes. Between 7 and 15 mbsf, some discrepancy exists between the holes despite the use of the APC in both holes over this interval. From 15 to 140 mbsf, correlation is more difficult and less consistent, and we attribute this inconsistency to using different coring tools at these depths and low recovery in Hole U1394A or to changes in lithology. Between 140 and 160 mbsf, both holes have higher recovery rates, and correlation is good despite different coring tools being used at these depths (XCB in Hole U1394A; APC in Hole U1394B). GRA density and magnetic susceptibility values are consistent between 140 and 160 mbsf; however, values in Hole U1394A are systematically lower than those in Hole U1394B, a difference we attribute to smaller diameter cores recovered via XCB drilling in Hole U1394A. The largest depth shift for the correlation picks is 2.05 m, with most shifts <1 m. All 18 correlation pick depth shifts are listed in Table T4.

Gamma ray attenuation density, magnetic susceptibility, and P-wave velocity

We measured magnetic susceptibility, GRA bulk density, compressional wave (P-wave) velocity, and natural gamma radiation (NGR) on all core sections, including those with water, air pockets, cracks, and loose clasts (Fig. F8). In the uppermost 25 m cored with the APC, these measurements are reliable and agree with spot measurements of density and P-wave velocity measured on the split core. Where the holes were drilled with the XCB, whole-core logger measurements are less reliable because the core has a smaller diameter (usually 0.5–1.5 cm smaller) than the core liner. Many of the cores deeper than 25 mbsf in Hole U1394A therefore have systematically low density and magnetic susceptibility. Measured whole-core P-wave velocities should only be reduced a few percent because of XCB drilling because water velocity (~1500 m/s) is comparable to shallow marine sediment velocities and the calipers squeeze the core liner, often displacing fluid from the gap between the liner and core. Additionally, the Whole-Round Multisensor Logger (WRMSL) only obtains P-wave velocities when good coupling exists across the core liner, with no gas or sharp water/sediment contacts. Below 25 mbsf, high-end P-wave values are typically ~1600 m/s, consistent with spot measurements of P-wave velocity on the split core. For Hole U1394B, the XCB was used between 15 and 60 mbsf, and magnetic susceptibility and GRA density measured on whole cores are systematically low at these depths. The rest of Hole U1394B was cored with the APC.

In the uppermost 25 m (Units A and B), magnetic susceptibility averaged 740 × 10^–5 SI with a maximum of 3890 × 10^–5 SI, P-wave velocity averaged 1614 m/s with a maximum of 1956 m/s, density averaged 1.68 g/cm³ with a maximum of 2.27 g/cm³, and NGR averaged 9 counts per second (cps) with a maximum of 15 cps. Although absolute values of density, magnetic susceptibility, and P-wave velocity below 25 mbsf may underestimate true values, spatial variations nevertheless identify different lithologies. Typical values of magnetic susceptibility are 200 × 10^–5 to 400 × 10^–5 SI. Occasional excursions to 2000 × 10^–5 or 4000 × 10^–5 SI occur at 4.6, 11.4, 23, 66, 100, 109, 170, and 173 mbsf.

Holes U1394A and U1394B have very similar physical properties (Fig. F8). In all units, the four physical properties measured by the WRMSL correlate well and appear to be modified by similar lithostratigraphic variations. Physical properties of hemipelagic sediment show little variation and have low magnetic susceptibility and P-wave velocity and relatively high NGR values. Physical properties are more variable in turbidites. Single (bioclastic-)volcaniclastic turbiditic units throughout the cores can be traced by their monotonically decreasing magnetic susceptibility, GRA density, and P-wave velocity uphole, mimicking their grading in grain size (e.g., 11–14 mbsf). A sharp drop in each of these values occurs at the turbidite unit boundaries. Where sufficiently thick, ash layers give positive magnetic susceptibility peaks, such as the one at 145 mbsf.

In the interval between 10 and 15 mbsf, GRA density, magnetic susceptibility, and NGR values are systematically different between the two holes and correspond to differences in lithology. In Hole U1394A, dark and dense turbidites with andesitic composition dominate; in Hole U1394B, lighter colored pumice-rich deposits dominate. Between 12.5 and 14.8 mbsf, a block of deformed hemipelagic sediment exists in Hole U1394A but is absent in Hole U1394B (Fig. F8A).

Thermal conductivity

Thermal conductivity was measured on all sections with fine-grained sediment, but only about a third of the measurements were high quality. In the uppermost 25 m (Fig. F8A), poor measurements may be due to water convection in pore space. In the remainder of the core, pervasive cracks and the high strength of the core made it difficult to be sure that the probe did not encounter cracks or create small cracks upon entry. The mean thermal conductivity of 44 reliable measurements was 1.03 W/(m·K) with a standard deviation of 0.07 W/(m·K) and a standard error on the mean of 0.01 W/(m·K).

Shear strength

Handheld penetrometer strengths ranged from 7 to >220 kPa (the upper value being the maximum that can be measured by the probe). Measurements for sandy units are not reliable because the grain size was large enough that the test was probably not probing undrained conditions. Excluding the unreliable measurements, the mean for the upper 7 m is 7 kPa, whereas shear strength varies between 70 and >220 kPa at greater drilled depths (Fig. F8).

Higher shear strength measurements are obtained from the handheld penetrometer compared to those provided by the automated vane shear (AVS) apparatus and the fall cone. The AVS could only be used in relatively soft, cohesive sediment where insertion of the probe did not create cracks, constraining the measurements to muddy, soft, fine-grained sediment.

The fall cone recorded shear strengths as low as 10 kPa at 14 mbsf and as high as 300 kPa at 149 mbsf. Because of scattered data for the uppermost 100 m, no trend could be identified. We did not observe a linear increase in shear strength with depth found in normally consolidated marine sediment. The fall cone and the AVS provided highly variable values between 20 and 125 kPa from 110 mbsf to the bottom of the holes.

P-wave velocity

In the split cores we obtained 128 measurements of x-axis and 26 measurements of z-axis P-wave velocity using the caliper apparatus (Fig. F8). Caliper measurements agree with P-wave measurements made on the full core (WRMSL); however, caliper velocity values in coarse-grained volcaniclastic sediment are generally higher. Higher caliper P-wave velocities on coarse-grained sediment compared to the WRMSL may be the result of dewatering following splitting.

We conducted x-axis caliper P-wave measurements on most split cores. However, z-axis measurements could only be carried out in loose sediment, where the bayonet could enter the split core without damaging the core structure. Z-axis measurements were commonly noisy, with most values close to 1680 m/s. Z-axis P-wave measurements do not always match x-axis and WRMSL values. In the uppermost 25 m, P-wave velocities increase significantly from ~1600 to 2000 m/s across volcaniclastic turbiditic units. In each unit, the base is coarse grained and has the highest P-wave velocity, whereas the fine-grained upper parts have the lowest P-wave velocities.

Moisture and density

We collected 48 moisture and density (MAD) samples (31 from Hole U1394A and 17 from Hole U1394B) (Fig. F8), including 28 from hemipelagic sediment, 18 from turbidites, and one from carbonate sand. Porosity of hemipelagic sediment ranges from 48% to 66%. Turbidites have porosities between 42% and 60%. The porosity of hemipelagic sediment generally decreases with depth.

Bulk density of hemipelagic sediment ranges from 1.52 to 1.85 g/cm³; bulk density of turbidites ranges from 1.65 to 2.08 g/cm³. No systematic difference is apparent in porosity and bulk density between volcaniclastic and mixed bioclastic-volcaniclastic turbidites.

Grain density of hemipelagic samples typically ranges between 2.63 and 2.79 g/cm³, with three outliers between 2.10 and 2.28 g/cm³. Grain density of turbidites ranges between 2.6 and 2.9 g/cm³.

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