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doi:10.2204/iodp.proc.334.104.2012

Physical properties

At Site U1379, physical properties measurements were made to provide basic information characterizing lithostratigraphic units. After sediment cores reached thermal equilibrium with ambient temperature at ~20°C, gamma ray attenuation (GRA) density, magnetic susceptibility, and P-wave velocity were measured on whole-round core sections using the Whole-Round Multisensor Logger (WRMSL). For basement cores, only GRA density and magnetic susceptibility were measured. After WRMSL scanning, the whole-round sections were logged for natural gamma radiation (NGR). Thermal conductivity was measured using the full-space method on sediment cores and the half-space method on split lithified sediment and basement cores. A photo-image capture logger and a color spectrophotometer were used to collect images of the split surfaces of the archive-half cores on the Section Half Imaging Logger and Section Half Multisensor Logger (SHMSL), respectively. Discrete P-wave measurements were made on split sediment cores and on cubes subsampled from lithified sediment and basement working-half cores on the Section Half Measurement Gantry (SHMG). Moisture and density (MAD) were measured on discrete subsamples collected from the working halves of the split cores.

Density and porosity

Bulk density values at Site U1379 were determined from both GRA measurements on whole cores and mass/volume measurements on discrete samples from the working halves of split cores (see “Physical properties” in the “Methods” chapter [Expedition 334 Scientists, 2012]). Samples were chosen from relatively undisturbed portions and preferentially from clay sediments rather than silty or sandier sections. A total of 530 discrete samples were analyzed for MAD.

In general, wet bulk density values determined from whole-round GRA measurements agree with measurements from discrete samples (Fig. F29A). Wet bulk density values increase with depth. This increase is likely due to dewatering caused by overburden pressure and is well described by a linear trend between 100 and ~880 mbsf. Lack of good core recovery deeper than 880 mbsf made GRA measurements difficult. Bulk density values in the brecciated basement have a density of ~2.3 g/cm³.

Grain density measurements were determined from mass/volume measurements on discrete samples. Values are relatively constant with depth to ~430 mbsf, with an average value of 2.67 g/cm³ and generally ranging between 2.6 and 2.8 g/cm³ (Fig. F29B). Below 430 mbsf, data are more scattered, with maximum values >2.75 g/cm³ reflecting a change in the nature of grains. At the bottom of the sedimentary section and just above the basement, the scatter is significantly reduced, possibly indicating a more homogeneous formation, and grain density is 2.75 g/cm³. On the whole, these values suggest terrigenous composition.

Porosity values (see “Physical properties” in the “Methods” chapter [Expedition 334 Scientists, 2012]) were determined from mass/volume measurements on discrete samples using MAD Method C on sediment cores and Method D on basement cores. Within the sedimentary section, porosity is inversely correlated with bulk density. In the slope sediments, porosity decreases linearly from ~62% to 40%, with several intervals showing higher values. Porosity within the brecciated basement is ~30% (Fig. F29C).

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the WRMSL and point measurements were made on the SHMSL for all recovered cores at Site U1379. Uncorrected values of magnetic susceptibility are presented in Figure F30. Magnetic susceptibility values measured by these two methods are in good agreement. Overall, magnetic susceptibility in the sedimentary sequence is low, indicating an abundance of non-iron-bearing clays. Magnetic susceptibility in the uppermost 100 mbsf is variable, with values to ~0.8 SI. With the exception of some notable excursions, magnetic susceptibility values between ~100 and 880 mbsf are relatively low and more uniform. Conspicuous excursions occur between ~100 and 200 mbsf. These excursions exhibit high wave number variability and may be due to lithologic variations between silt and clay. In the 470–520 and 800–840 mbsf intervals, high magnetic susceptibility values correspond to sand and tephra layers.

Magnetic susceptibility values in the basement are generally greater than those in the slope sediments. The apparent variability is an artifact of incomplete recovery.

Natural gamma radiation

NGR results are reported in counts per second (cps; Fig. F31). NGR counting intervals were ~10 min per whole-core interval. Despite the short counting time, NGR counts are considered reliable.

NGR values in the sediment section are between 15 and 35 cps and show high wave-number variability, likely due to edge effects that have not been filtered. The highest values are at ~280 mbsf. From ~630 mbsf to the top of basement, the downhole decrease of NGR is consistent with a downhole increase of both sandy sediments (see “Lithostratigraphy and petrology”) and grain density (Fig. F29). NGR values in basement are generally low, which is consistent with sandy lithologies described in this interval (Fig. F31).

P-wave velocity

Unconfined measurements of P-wave velocity at Site U1379 were determined from measurements on sediment whole cores and on discrete samples from the working halves of sediment and basement split cores (see “Physical properties” in the “Methods” chapter [Expedition 334 Scientists, 2012]). In general, whole-core measurements yielded reliable values at depths shallower than 80 mbsf (Fig. F32). P-wave velocity increases with depth and varies between ~1535 and 1620 m/s. Deeper in the hole, P-wave velocity values from the WRMSL, SHMG, and bayonet yielded unreliable values. Based on visual inspection of the core, this unreliability is attributed to the low water content of the clays.

P-wave velocity SHMG measurements in the z-direction were measured on discrete samples below 500 mbsf. P-wave velocity is generally slower in the z-direction than it is in the x- and y-directions, and measurements in the other two directions could not be made. Between 500 and 850 mbsf, P-wave velocity ranges between 1400 and 1800 m/s, with no clear trend. Values below 1600 m/s are unreliable but may be a consequence of measuring P-wave velocity in the z-direction. At ~850 mbsf, a sharp increase of P-wave velocity is observed, but the increase is not correlated with a sharp increase of bulk density or a sharp decrease of porosity. The sharp increase of P-wave velocity in the basement reflects the low porosity and high consolidation of the rocks.

Thermal conductivity

Thermal conductivity measurements were performed on sediment whole-round cores using the needle-probe method and on lithified split cores using the half-space method (see “Physical properties” in the “Methods” chapter [Expedition 334 Scientists, 2012]). In general, thermal conductivity increases with depth and is inversely correlated to porosity (Fig. F33). In the uppermost 100 mbsf, variability is significant and may reflect cracking in the cores and changing lithologies. Relatively high values of thermal conductivity are present between ~50 and 90 mbsf and may be correlated with a section of siltstone. Below ~100 mbsf, thermal conductivity increases linearly with depth.

Downhole temperature

Downhole temperature was measured using the APCT-3. Six measurements were attempted between 30.2 and 90.2 mbsf in Hole U1379C (Table T9; Fig. F33). All measurements were made in lithostratigraphic Unit II. Sediment Temperature Tool measurements were not attempted at this site.

All measurements were made in a good sea state (<1 m swell), and all temperature-time series were recorded with a sample interval of 1 s. The temperature tool was stopped at the mudline for as long as 10 min prior to each penetration. The average bottom water temperature was 22.6°C (Table T9). Temperature-time series for each temperature measurement are shown in Figure F34. Significant frictional heating occurred on all penetrations of the APCT-3, with the temperature-time records exhibiting characteristic probe penetration and subsequent decay. Tool movement while the probe was in the sediment is minimal. The effective origin time of the frictional heat pulse was estimated by varying the assumed origin time until the thermal decay pulse best fit a theoretical curve. All of the measurements appear to be reliable. Equilibrium temperatures plotted as a function of depth are relatively linear; coupled with the average bottom water temperature, they give a least-squares gradient of 41.6°C/km (Fig. F33).

Heat flow

Because thermal conductivity appears relatively constant over the depth of temperature measurements, we computed the heat flow as the product of the thermal gradient and thermal conductivity. Using the mean thermal conductivity of 0.97 W/(m·K) gives a heat flow of 40 mW/m². This value is consistent with forearc values of heat flow.

If heat transfer is by conduction and heat flow is constant, the thermal gradient will be inversely proportional to thermal conductivity according to Fourier’s law. This relationship can be linearized by plotting temperature as a function of summed thermal resistance (Bullard, 1939),

where

• T = temperature,

• z = depth,

• T_o = bottom water temperature,

• q = heat flow, and

• [Δz_i/k(z)_i] = thermal resistance.

We assume a linear increase of thermal conductivity with depth and extrapolate temperatures to the sediment basement interface (Fig. F33). The estimated temperature at this depth is 48°C.

Vane shear

Undrained shear strength increases with depth (Fig. F35). The trend is approximately linear in the uppermost 80 mbsf, with a maximum value of 152 kPa. Below 80 mbsf, shear strength suddenly decreases and displays more erratic behavior. This change occurs near the depth where the coring method changed from APC to XCB.

Color spectroscopy

Color reflectance measurement results are presented in Figure F36. L* values are generally between 20 and 40. Regions of lower values correspond to ~40 and 200 mbsf. a* and b* values are inversely correlated. a* values are generally between –5 and 0 and b* values generally vary from –5 to 10.

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