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doi:10.2204/iodp.proc.339.104.2013

Physical properties

Physical properties at Site U1386 were determined in Holes U1386A and U1386B and the cored sections of Hole U1386C (Cores 339-U1386C-2R, 4R, and 6R through 18R). High-resolution scanning at 2.5 cm intervals on whole-round sections was performed with the WRMSL in all holes, whereas the Special Task Multisensor Logger (STMSL) was only used for Hole U1386B for stratigraphic correlation purposes. Thermal conductivity probes were applied to Section 3 of each core in Hole U1386A. P-wave velocity measurements on split-core sections (working half) were obtained in Hole U1386A for every section; however, reasonable results were only obtained for the upper ~76 mbsf. Moisture and density measurements were determined for every second section of each core from Hole U1386A, downhole below Core 339-U1386B-37X, and for Cores 339-U1386C-2R, 4R, and 6R through 18R. Color reflectance analysis and split-core point-magnetic measurements were taken on every section at 5 cm intervals.

Whole-Round Multisensor Logger and Special Task Multisensor Logger measurements

The STMSL was not used for Hole U1386A because no immediate acquisition of data for stratigraphic correlation was necessary, but the STMSL was used for Holes U1386B and U13686C. Cores were equilibrated for 3 h before measuring with the WRMSL.

Gamma ray attenuation bulk density

Gamma ray attenuation (GRA) density at Site U1386 is highly variable between 1.6 and 2.2 g/cm³ (Fig. F33). Some of the scatter could be explained by incomplete filling of core liners, especially in sandy sections and cores for which the more disruptive XCB and RCB drilling techniques were applied. Besides this, the natural variations in the sand content of the sediment and in its texture are also reflected in the GRA density record. Between 400 and 450 mbsf increased densities correspond to an interval rich in sandy turbidite deposits. This relation is opposite to the general trend observed at this site, that low GRA densities often conform to sand-rich intervals. The contrasting correlations between density and lithology reflect the difference in texture and composition of layers generated by bottom current processes (i.e., contourites) and gravitational processes (i.e., turbidites).

Magnetic susceptibility

Of all physical properties, magnetic susceptibility displays the most marked changes (Fig. F33). Based on WRMSL data, four physical property units can be distinguished that differ in value range and variability. Physical properties Unit I, between 0 and ~150 mbsf, exhibits declining susceptibilities and distinct fluctuations between 10 × 10^–5 and 40 × 10^–5 SI with peaks approaching 55 × 10^–5 SI. For this unit, a remarkable coherence of susceptibility, NGR counts, and spectral reflectance component a* can be noted (Figs. F33, F34). More reddish sediments (i.e., high a*; see Site U1386 visual core descriptions in “Core descriptions”) are found in intervals of high susceptibility with no apparent change in grain size. Whether the color change might be attributed to varying amounts of hematite or other mineralogical changes in the composition of the sediments remains unclear. Below in physical properties Unit II (150–420 mbsf), magnetic susceptibility shows less variability (5 × 10^–5 to 20 × 10^–5 SI).

The entire interval between 0 and 420 mbsf corresponds to lithologic Unit I (see “Lithostratigraphy”). The boundary between lithologic Subunits IA and IB, at ~110 mbsf in Hole U1386B, fits to the general trend of declining magnetic susceptibility. Except for its upper part (110–150 mbsf), the more mud rich Subunit IB has, on average, low and uniform susceptibilities. The top of lithologic Subunit IC, at ~216 mbsf in Hole U1386B, shows a slight increase of susceptibility, which drops to a level similar to that of Subunit IB further downhole. Whereas Subunit IC is very similar to Subunit IA in terms of lithology and mineralogical composition, the latter clearly has the highest and most variable susceptibilities.

Between 420 and 445 mbsf, the very distinct physical properties Unit III can be discerned with highly variable values that markedly increase to maximum peaks of 526 × 10^–5 SI, corresponding to turbidites. Downhole, below 445 mbsf, physical properties Unit IV is defined by magnetic susceptibility data that is dominated by low values close to 10 × 10^–5 SI. Here, debrites and turbidites are the dominant lithofacies. In the lithologic units, the entire interval below 420 mbsf corresponds to Unit II (see “Lithostratigraphy”).

Because the sulfate reduction zone only extends through the upper 12–13 m of sediment and gas flux is significantly lower than at Site U1385, diagenesis is suspected to have a minor influence on the magnetic susceptibility signal. Hence, we assume that the magnetic susceptibility signal is dominated by the primary sediment composition.

WRMSL and split-core magnetic susceptibility data agree well for cores drilled using the APC. For sediments retrieved with the XCB or RCB, WRMSL values are consistently lower. We assume that this difference can be attributed to the incomplete filling of the liner for those cores with lower values, providing less volume for sensor loop integration.

P-wave velocity

Sonic velocities were measured with the WRMSL for Holes U1386A and U1386B, and an attempt was made to determine P-wave velocities on split cores in each section of Hole U1386A (Fig. F33). Because of poor sediment to liner coupling, reasonable results from the WRMSL could only be obtained for the upper ~25 m of sediment. The P-wave velocity profile can be extended to 77 mbsf by using the P-wave determinations on split cores. Although the sediment surface appeared to be smooth and should have provided adequate coupling to the transducers, no clear acoustic signal could be obtained for P-wave velocity determinations at greater depth. An explanation for this might be the relatively stiff and brittle nature of the sediment, which promotes the formation of small cracks that negatively affect signal propagation.

P-wave velocities follow the trend of increasing GRA densities in the upper 12 mbsf, beginning with values of 1450–1500 m/s in the uppermost meters, increasing downhole to 1550–1650 m/s for WRMSL and split core data (only accounting for automatically processed data). Both types of measurement agree well, especially when only considering split-core data with high signal quality (solid symbols in Fig. F33). One measurement at 22.6 mbsf stands out, with a sonic velocity of 1796 m/s measured on a sandy interval.

Natural gamma radiation

NGR scanning on all cores from Site U1386 was performed mostly at low resolution with one position (usually Position II) of the detector array. Exceptions are Cores 339-U1386A-1H and 2H, which were measured in Positions I and II of the detector array. NGR counts fluctuate between 20 and 60 cps, revealing cyclic patterns throughout all cored sections (Fig. F34). Comparatively low NGR counts of not more than 45 cps below 150 mbsf might at least partly be attributed to the lower recovery of core during rotary drilling. A correlation between the dramatic increase in magnetic susceptibility below 420 mbsf and higher NGR counts is apparent, although NGR values start to increase at ~380 mbsf. It is notable that above ~380 mbsf, sandy layers are commonly associated with low NGR values, whereas below ~380 mbsf, coarse-grained sediments are related to intervals dominated by high NGR counts. The change in the depositional style from contouritic to turbiditic apparently affects the mineralogical composition of the sandy layers, which can be interpreted as a change from a silicate-dominated to a carbonate-bearing lithology.

Moisture and density measurements

Determination of moisture and density (MAD) on discrete sediment samples was performed on every section of Hole U1386A (Fig. F35). Sampling was preferentially carried out on the dominant lithology of a section, avoiding small interlayers of differing grain size (e.g., centimeter-scale sand layers). Generally, GRA and MAD methods give consistent bulk density results (Fig. F33). Compared to Site U1385, a steep compaction-related density increase might only be inferred for the uppermost ~12 mbsf, where porosity and water content of the sediments decrease inversely to the increase of bulk density (Figs. F33, F35). Below 12 mbsf, moisture content and porosity slowly decrease toward 20%–25% (moisture content) and ~45% (porosity) at ~460 mbsf, below which no further decrease occurs. This change in consolidation state of the sediment is associated with the significant hiatus identified by biostratigraphic analyses of Cores 339-U1386C-11R and 12R (Fig. F28). Cyclic fluctuations are overprinted on the general trends, with marked increases in porosity and moisture content at about 185, 310, and 370–395 mbsf (Fig. F35). Increased sand content was noted around 310 and 370–395 mbsf, although a similar relation to coarser sediments is not evident for the high moisture content and porosity at ~185 mbsf.

Grain density is relatively constant for the entire interval drilled at Site U1386, fluctuating between 2.7 and 2.8 g/cm³ (Fig. F35). The only exception is the interval between 400 and 450 mbsf, which has on average slightly higher grain density (constantly above 2.75 g/cm³) than the other cored intervals. High magnetic susceptibility, characteristic for this interval, might point to a significant contribution of heavy minerals in this turbidite-rich interval (see “Lithostratigraphy”). The underlying interval between 460 and 480 mbsf characterized by debrites has grain densities between 2.71 and 2.77 g/cm³, which are lower than those of the overlying turbidites.

Thermal conductivity

Thermal conductivity was measured once per core using the full-space probe, usually in Section 3 of all cores in Hole U1386A, except Cores 339-U1386A-12H through 17H because of technical problems (Fig. F36). Thermal conductivity generally follows reduced porosity and water content in the sediment, with a decrease from ~1.7 W/(m·K) in the uppermost interval to 1.1 W/(m·K) at ~100 mbsf. Because interstitial water is a good thermal conductor, this relation matches expectations. The enhanced scatter below 180 mbsf relates to XCB coring, which generates cracks and core disturbances that affect the accuracy of the thermal conductivity measurements.

Summary of main results

Based on physical property data, distinct intervals were identified that are commonly related to boundaries between the defined lithologic units. The upper ~150 mbsf (physical properties Unit I) are characterized by relatively high, fluctuating NGR and magnetic susceptibility values with a persistent covariation of both parameters with sediment color. Between ~150 and 420 mbsf (physical properties Unit II) this variability decreases and the relation to sediment color is lost. Outstanding, especially in terms of very high magnetic susceptibility, is the interval between 420 and 445 mbsf, physical properties Unit III, corresponding to turbidite-prone deposits. Below 445 mbsf (physical properties Unit IV) low NGR and susceptibility values conform to a shift in the lithology to debrites and turbidites below a major hiatus. The assumed occurrence of the mid-Pleistocene revolution discontinuity at ~250 mbsf is not reflected in a notable change in physical properties.

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