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doi:10.2204/iodp.proc.343343T.103.2013

Physical properties

In Hole C0019E, physical properties measurements were made to obtain basic information regarding lithology, porosity, strength, and deformation and for correlation with downhole logging data. After capturing X-ray CT images and letting the core reach thermal equilibrium with the ambient temperature of ~20°C, gamma ray attenuation (GRA) density, magnetic susceptibility, NGR, P-wave velocity (PWV), and noncontact electrical resistivity were measured using an MSCL-W. Cores were then split parallel to the core axis. One half was reserved for archiving and one half was for sampling and analysis (working half). A photo image logger (MSCL-I) and a color spectroscopy logger (MSCL-C) were used to collect images of the archive-half split surfaces. Thermal conductivity was measured using a half-space line source method on split working halves. Discrete samples were obtained from the working halves in order to measure moisture and density (MAD). Additional discrete cylindrical samples from working halves were taken to measure unconfined compressional strength (UCS). Electrical resistivity and P-wave velocity measurements for discrete samples (PWVD) were made on 2 cm × 2 cm × 2 cm samples cut from working halves. Some of these cube samples were further tested for P-wave and S-wave measurements under high pressure. Grain size analysis was restricted to Core 343-C0019E-1R, as all other cores were too lithified for the particle size analysis to be useful.

MSCL-W

Gamma ray attenuation density

Bulk density determined by GRA is observed to increase gradually downhole. Maximum values are ~1.3 g/cm3 starting at 176.5 mbsf and increase throughout lithologic Unit 1 to ~1.7 g/cm3 at 184.8 mbsf (Fig. F47). The upper range of GRA density values continues to increase through lithologic Unit 2 and the top of lithologic Unit 3, reaching ~1.9 g/cm3 by 772 mbsf (Fig. F48). Between 785 and 812 mbsf, GRA density decreases slightly before reaching values of ~2.0 g/cm3 from the bottom of Unit 3 through Unit 4. This overall trend of increasing bulk density is consistent with depth-dependent consolidation and porosity loss in sediment. Overall, GRA density varies widely and many measurements are unreliable because of the presence of air and/or water between the core and liner during measurement. However, because the maximum values we observe are in strong agreement with bulk density values obtained from MAD measurements on discrete samples (see “Moisture and density” below), we consider the upper range of these values to be valid.

Magnetic susceptibility

Magnetic susceptibility within Unit 1 appears to increase downhole and reaches peak values of 221.7 × 10–5 SI (Fig. F47). In Unit 2 magnetic susceptibility is generally >100 × 10–5 SI, whereas it is <100 × 10–5 SI in Unit 3. Within both Units 2 and 3, magnetic susceptibility also increases downhole relative to other values in the same unit (Fig. F48). Near the bottom of Unit 3, magnetic susceptibility values spike to values as large as ~330 × 10–5 SI at ~817.6 mbsf, which is considerably larger than maximum values elsewhere within the unit. In Unit 4, consisting of Core 343-C0019E-17R and a major shear zone interpreted to be the plate boundary décollement zone (see “Structural geology”), magnetic susceptibility increases within a ~40 cm interval centered around a peak value of 162 × 10–5 SI at ~822.2 mbsf. Background values elsewhere within the Unit 4 are <75 ×10–5 SI even where density measurements indicate that low values are not a result of insufficient core volume filling the core liner (Fig. F49). Within Unit 5, magnetic susceptibility appears to slightly decrease downhole except for some local high values occurring within 827.1–828.2 mbsf. Magnetic susceptibility within Units 6 and 7 decreases with depth to ~2 × 10–5 SI at ~833.5 mbsf, with the exception of some high values of up to ~187 × 10–5 SI at ~833 mbsf and some values of ~50 × 10–5 SI within Unit 7. Magnetic susceptibility measured with the MSCL-W is generally consistent with bulk magnetic susceptibility of paleomagnetic discrete samples (see “Paleomagnetism”).

P-wave velocity

PWV data recorded by the MSCL-W show significant scatter ranging between 800 and 2000 m/s (Figs. F47, F48). MSCL-W velocity measurements are lower than velocities calculated using the LWD logs (see “Logging while drilling”). This discrepancy is likely due to gaps between the core and core liner and core liner partially filled with air resulting in velocities lower than that of seawater (~1500 m/s). At any particular depth, upper values range between ~1500 and 1900 m/s; these values are generally consistent with values obtained from discrete measurements. However, an expected trend of increasing PWV with depth is difficult to identify.

Electrical resistivity

The electrical resistivity generally increases with depth to ~820 mbsf (Figs. F47, F48), with local variations probably caused by small-scale lithologic changes or variations in degree of compaction. These variations are in general agreement with LWD data from Hole C0019B (see “Physical properties and hydrogeology”). Lower than expected resistivity values in Unit 2 (~630–660 mbsf) are probably due to poor core quality. Lower values observed in Core 343-C0019E-16R (~818 mbsf), just above a major fault, are inconsistent with resistivity values obtained from discrete measurements and logging data. The values may be related to high amounts of drilling-induced fractures filled with drilling fluid. High-resistivity values in the upper portions of Core 343-C0019E-17R, consisting of the interpreted basal décollement zone (see “Structural geology”), may result from large amounts of air between the core material and liner, as suggested by correspondingly low densities (Fig. F49). Between ~821.9 and ~822.5 mbsf within Core 343-C0019E-17R, electrical resistivity values are consistently less than ~10 Ωm with no apparent change where there is a marked increase in magnetic susceptibility (Fig. F49). Below the fault, resistivity values should be taken with caution because they are lower than measurements from LWD and discrete samples. High-resistivity values in the last core, Core 343-C0019E-21R covering depths >836.55 mbsf, are also due to incompletely filled core liners.

Natural gamma radiation

NGR data from the MSCL-W were used to help correlate Hole C0019B with Hole C0019E (see “Logging while drilling” and “Core-log-seismic integration”) and identify lithologic units (see “Lithology”). NGR ranges between ~5 and 20 counts per second (cps) in Unit 1, with the exception of higher values at ~179 mbsf (up to 50 cps). Average NGR increases in Units 2 and 3, averaging 24.7 and 27.5 cps, respectively. Values as high as ~40 cps were observed locally. Significantly higher NGR values (up to 58 cps, averaging nearly 40 cps) were observed within Unit 4. Compared to the overlying unit, this suggests an abrupt change in lithology consistent with structural and lithologic descriptions identifying this unit as the plate boundary décollement zone (see “Lithology” and “Structural geology”). Between ~821.9 and ~822.5 mbsf within Core 343-C0019E-17R and Unit 4, values show a marked decrease to 43.97 cps over a ~20 cm interval centered at 822.2 mbsf, corresponding to the location of a peak in magnetic susceptibility (Fig. F49). Elevated NGR values were also found at the top of Unit 5 before decreasing to levels similar to those observed in Units 2 and 3 (~25–30 cps) at ~826 mbsf. Average NGR drops to below 14 cps in Unit 6, signaling a switch to a lithology lacking in clays.

Moisture and density

MAD measurements provide characterization of grain density, bulk density, porosity, void ratio, and water content in Hole C0019E, despite incomplete core recovery. All MAD data are provided in Table T4. Below, we summarize the most important parameters, which are bulk density, grain density, and porosity. Downhole variations are graphically shown in Figure F50.

Bulk and grain density

In the spot core (343-C0019E-1R) taken below 176.5 mbsf (lithologic Unit 1/log Unit I), bulk densities in the siliceous mudstones vary between 1.53 g/cm3 and 1.69 g/cm3. Grain densities range from 2.45 to 2.57 g/cm3.

In the next continuously cored interval (Cores 2R and 3R; lithologic Unit 2/log Subunit IIb) to 659.7 mbsf, sediment bulk density ranges from 1.72 to 1.88 g/cm3, and grain density ranges from 2.39 to 2.67 g/cm3. Some of the low grain density values in this depth interval may be related to inadequate sampling of rock chips, which were the only available material.

In Cores 343-C0019E-4R–16R, which correspond to lithologic Unit 3 and form part of log Subunit IIb, bulk density values gradually increase downhole. Core 4R shows the lowest bulk density (1.75 g/cm3), whereas values higher than 1.9 g/cm3 are restricted to Core 8R or deeper (deeper than ~720 mbsf). This increase in bulk density probably reflects a higher degree of compaction in the sediment with burial. Grain density in Unit 3 ranges from 2.47 to 2.76 g/cm3, which is a characteristic range for pelagic and hemipelagic silicate minerals.

A single discrete sample taken from the plate boundary fault zone at 822 mbsf (Core 17R; lithologic Unit 4/uppermost log Unit III) before core splitting has a bulk density of 1.98 g/cm3 and grain density of 2.86 g/cm3. Beneath the fault zone, Core 18R through Section 20R-2, 53 cm, which form lithologic Unit 5 and the lower part of log Unit III, have bulk density values ranging from 1.76 to 2.03 g/cm3 and grain density values ranging from 2.58 to 2.74 g/cm3. In Core 21R a single discrete sample of chert representative of lithologic Unit 7 and log Unit IV has a bulk density of 2.01 g/cm3 and grain density of 2.54 g/cm3. No discrete MAD measurements were made for lithologic Unit 6, which showed evidence of invasion by drilling fluid.

Porosity

Porosity and void ratio values show a general downhole decrease in Hole C0019E cores. Samples from the spot core (343-C0019E-1R) encompassing Unit 1 show porosities between 55.3% and 68.7% and corresponding void ratios ranging from 1.24 to 2.05. The cored interval through Unit 2 between 648 and 660 mbsf has porosity values that vary between ~40.5% and 52.5%. This variability is unlikely to reflect petrographic differences between samples but may be related to water uptake in rock chips during core recovery. In contrast, samples from the third cored interval (688.5–729.0 mbsf), containing the upper portion of Unit 3, show a regular downhole decrease in porosity from values slightly >50% to ~45%. The lowest values were found in the fault zone between 719 and 725 mbsf (Cores 8R and 9R) (see “Structural geology”). This may suggest that these sediments have experienced shear-enhanced compaction, resulting in reduced porosity in the sheared lithologies.

Further downhole in the interval between 770 and 818.6 mbsf (Cores 10R through 16R) covering the lower part of lithologic Unit 3, porosity values scatter between 41.9% and 52.7%. In the sample taken from the décollement zone (Core 17R) at 822.49 mbsf, porosity is 48.2%. The underthrust brown mudstones beneath the décollement (Cores 18R through 20R) have porosity values between 35.9% and 52.5%. A chert sample from Core 21R has a porosity of 35.0%.

Unconfined compressive strength

We tested the UCS of nine minicores sampled from the split core working halves. Results are shown in Figure F51 and listed in Table T5. The number of measurements was limited because pervasive fracturing induced by drilling resulted in a lack of adequately sized intact rock pieces suitable for sampling and measurement. UCS values are generally low in the upper portion of Unit 3, with two samples exhibiting strengths of 3.4 and 3.8 MPa and one low value of 1.3 MPa. The low-strength sample was recovered from within a fault zone at 720 mbsf (see “Structural geology”). In the lower portion of Unit 3 (below 800 mbsf), UCS increases significantly to 6.4–7.6 MPa. Within Unit 5, two tested samples show lower strengths of 3.4 and 4.7 MPa. These samples are located 5–10 m below the décollement zone, suggesting that their low strength relative to above the décollement may be related to either a different lithology or near-fault damage. A chert sample from Unit 7 was the strongest tested, with a minimum UCS of 65.3 MPa. This value should be considered a lower boundary estimate because the sample was tested several times due to failure of supporting material surrounding the sample, and therefore likely experienced fatigue. In general, the UCS data show a trend of increasing strength downhole, with low strength values exhibited by samples from near or within fault zones.

P-wave velocity

PWVD ranges from 1400 to 3300 m/s over the cored interval, with a clear trend of increasing velocity with depth (Fig. F50; Table T6). Discrete P-wave samples were only obtained from lithologic Units 2, 3, 5, and 7, with the majority coming from Unit 3. Measured velocity decreases in samples from Cores 343-C0019E-19R and 20R (Unit 5), which are from the footwall of the décollement zone at ~820 mbsf. Velocity reaches a maximum of ~3272 m/s in the cherts of Unit 7.

PWVD has an inverse relationship with porosity (Fig. F52). The majority of the samples follow an approximately linear trend except for the chert sample from Unit 7, which has a much higher velocity. Vertical anisotropy was calculated from resistivity measurements in the x-, y-, and z-directions. Horizontal anisotropy was not calculated because of orientation ambiguity in the x-y plane. Calculated vertical anisotropy in PWVD is small, ranging from ~0% to 6% (Fig. F53). In general, PWVD is nearly isotropic below 810 mbsf, where values are mostly ~2% or lower.

Elastic wave velocity measurements under high pressure

Elastic wave velocity was measured during Expedition 343 on five cubic samples. The sample taken from Core 343-C0019E-4R (688.57 mbsf) within Unit 3 was measured under confining pressures up to 90 MPa and used to estimate how the measured velocity changes temporally at a constant pressure. We found that stable values were reached after 2 h at confining pressures ≥70 MPa. Considering the limited available time for shipboard measurements, the measurements for other samples were conducted using stepwise increases of confining pressure. Pressure steps were 0, 5, 10, 15, 20, 30, and 50 MPa. We also measured velocity while depressurizing at 30, 10, and 0 MPa. Figure F54 shows the results for each sample. Velocity generally increases with increasing pressure. Samples from Unit 3 (Cores 4R, 5R, and 7R) showed a similar range in velocity: 1.5–2.5 km/s for P-wave velocity and 0.8–1.5 km/s for S-wave velocity in the pressure range of 0–50 MPa. A sample from Core 19R in Unit 5 showed similar S-wave velocity to samples from Unit 3, but P-wave values were significantly higher. P-wave velocity also rapidly increased as a function of confining pressure. In contrast, velocities from Core 21R (Fig. F54E) were representative of the cherts within Unit 7 and were remarkably different from other cores. Measured velocity values did not change significantly within the range of confining pressures applied in this experiment, indicating extremely low compressibility. Although we attempted to conduct nine measurements for each sample within the whole applied range of confining pressures, we not always receive complete recordings (e.g., SXz of Core 7R). This was likely due to problems with signal communication, insufficient coverage of the silicon coating on the samples, or coupling of the sensor to the sample surface.

Electrical resistivity

In soft sediments from Core 343-C0019E-1R, resistivity was measured along the y- and z-axis with a four-pin, 2 kHz Wenner array (Fig. F50; Table T7). Resistivity in these samples increases with depth, with an average value of 0.72 Ωm. This value is lower than the deep-resistivity measurements from LWD. This difference may be caused by desiccation and cracking of the core samples during the ~24 h between core splitting and measurement. However, this likely has a greater effect on the soft sediment recovered from the shallow core (Core 343-C0019E-1R).

Below 688 mbsf (Core 4R), the rocks were indurated enough to measure resistivity on cube samples, although the number of samples was limited by core quality (Fig. F50; Table T8). In Units 1 and 3 (170–786 mbsf), resistivity increases with depth, with lower resistivity values in Core 7R (~714 mbsf) possibly related to the presence of ash layers. Resistivity then decreases to 810 mbsf, and then increases again in Core 15R (817 mbsf). Resistivity variations are in good agreement with variations in porosity from MAD measurements. In lithologic Units 5 and 7 (from 826 to 836 mbsf) below the major fault zone, resistivity decreases with depth and then increases in the Unit 7 cherts.

No clear trend in vertical anisotropy is observed downhole, although the magnitude of anisotropy can be large (–31% to 13% through Unit 5) compared to anisotropy in P-wave velocity (Fig. F53). The cherts in Unit 7 have very large values of anisotropy (98% vertical, 69% horizontal); however, similar anisotropy is absent in the P-wave velocity data.

The relationship between porosity and resistivity is illustrated in Figure F55. The trends in the data set provide a first approximation of Archie’s law parameters (see “Logging while drilling”) in the different lithologic units. The logarithm of resistivity for Units 1, 3, and 5 broadly exhibits a linear relationship with porosity that does not include the chert from Unit 7.

Thermal conductivity

Thermal conductivity was measured on the working half of core sections from Hole C0019E at 45 discrete locations (Fig. F50). The mean of all measurements is 1.139 W/(m·K) (standard deviation = 0.118 W/[m·K]; n = 45). Unit 1, interpreted to be slope sediments, has the lowest values (mean = 0.874 W/[m·K]; standard deviation = 0.0141 W/[m·K]; n = 2). These low values are likely related to relatively higher porosity and possible desiccation of the samples (Fig. F50) compared with the deeper mudstones of Units 2–5, which have a mean value of 1.143 W/(m·K) (standard deviation = 0.074 W/[m·K]; n = 34). The clays of Unit 6 exhibit thermal conductivity that is slightly lower, yet comparable to the overlying mudstones, with a mean of 1.086 W/(m·K) (standard deviation = 0.0870 W/[m·K]; n = 3). A very high value of 1.622 W/(m·K) is observed in the chert from within Unit 7, which is expected because of its high density and silica content. Although two measurements were recorded in the depth interval for Unit 7, the lower value was measured in a cobble that was later interpreted as material that was displaced after drilling and fell downhole, possibly from Unit 3.

Particle size analysis

Grain size distribution was measured on seven sediment samples from Unit 1, ranging in depth from 177.5 to 184.7 mbsf in the shallow spot core in Hole C0019E (Fig. F56; Table T9). Below this depth, measurements were impossible because the grains in consolidated sediments could not be dispersed with a dilute alkaline solution of sodium polyphosphates ([NaPO3]6). The mean grain size of the measured sediments ranges from 24.6 to 69.5 μm, except for the sediments from Sections 343-C0019E-1R-5 and 1R-6. The mean grain size range is consistent with the lithologic description of the sediments, which indicate that they are composed of abundant clay- to silt-sized siliciclastic material. However, bimodal size distributions were measured in three samples (Sections 1R-3, 1R-6, and 1R-7; Fig. F56), indicating that grains >0.5 mm exist in the measured samples. After analysis, we confirmed that these large grains were aggregates of clayey siliciclastic materials, indicating dispersion of the sediment using both chemical and mechanical means was incomplete. Thus, the measured grain size may be overestimated.

The effect of H2O2 solution treatment on grain size distribution was evaluated using the sample from Section 1R-1, which contained little organic material. The mean grain size increased slightly from 24.6 to 32.5 μm and kurtosis decreased after H2O2 solution treatment (Table T9).

Color spectrometry

Color reflectance results of split core sections are shown in Figure F57. The L* value represents lightness, from black (L* = 0) to white (L* = 100). The a* value represents color changing from pure green (a* = –127) to pure red (a* = 127), and the b* value represents color changing from pure yellow (b* = –127) to pure blue (b* = 127).

For lithologic Unit 1, L* values range from ~25 to 48, a* values range from –2 to 1, and b* values range from 0 to 7. For Unit 2, the mixed brown and gray mudstone has L* values ranging from ~25 to 50, a* values ranging from –2 to 5, and b* values ranging from 0 to 10. For Unit 3, varying gray mudstone has L* values ranging from ~22 to 45, a* values ranging from –2 to 1, and b* values ranging from –2 to 5.

Below the 822 mbsf fault zone (Unit 4), L*, a*, and b* values all abruptly increase. The brown clay layer (Unit 5) has L* values ranging from 37 to 52, a* values ranging from 2 to 6, and b* values ranging from 5 to 16. Unit 6 has the widest ranges of all values, which can be judged qualitatively by its colorful appearance. L* values range from 32 to 65, a* values range from –1 to 10, and b* value range from 0 to 22. Variations in color spectrometry values are closely related to lithologic changes.