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doi:10.2204/iodp.proc.339.108.2013 Physical propertiesPhysical properties at Site U1390 were determined in Holes U1390A–U1390C. High-resolution scanning on whole-round sections was performed with the WRMSL at 2.5 cm intervals. NGR was determined for all holes at a high resolution of ~10 cm with two positions of the detector array and occasionally at lower resolution (~20 cm) with one position of the detector. The Special Task Multisensor Logger (STMSL) was only used for Holes U1390B and U1390C, for stratigraphic correlation purposes, at a scanning interval of 2.5 cm. Thermal conductivity probes were applied on Section 3 downhole to Core 339-U1390A-8H and from Core 339-U1390C-9H to 19H. P-wave velocity on split-core segments (working half) was obtained for APC Core 339-U1390A-1H through Section 5H-1. Moisture and density (MAD) measurements were determined for every second section of each core in Hole U1390A. Color reflectance analysis and split-core point-magnetic measurements were performed for every segment at 5 cm intervals. Based on the physical property data, three main units were defined at Site U1390 (Figs. F23, F24, F25). Physical properties Unit I spans from 0 to 70 mbsf (base of Core 339-U1390A-8H), physical properties Unit II from 70 to 295 mbsf, and physical properties Unit III from 295 mbsf to the base of Hole U1390A at 350 mbsf. At this site, the general trends in physical properties show consistent relationships with lithology over the uppermost 220 mbsf. In particular, beds of coarser grain size correspond to higher gamma ray attenuation (GRA) density, magnetic susceptibility, and a* (i.e., more reddish color) (Figs. F23, F24). From 220 to 295 mbsf, the relationship between measured parameters and lithology is inconsistent. Varying correlations between physical properties and lithology occur below 295 mbsf. Physical properties Unit I shows a positive correlation between GRA density, magnetic susceptibility, NGR, and a*. An exception is the uppermost 20 mbsf, where a negative correlation between GRA density and NGR with respect to magnetic susceptibility and a* is apparent (Figs. F23, F24). This interval is also marked by a rapid decline in porosity and moisture content caused by consolidation in the upper ~20 m (Fig. F25). Physical properties Unit II is characterized by cyclic high-amplitude changes in magnetic susceptibility, GRA density, a*, and L* (Figs. F23, F24). The general trends of this cyclicity show consistent positive correlation between GRA density, magnetic susceptibility, NGR, and a* downhole to ~220 mbsf. At small scale, excursions are more complex; GRA density correlates negatively with magnetic susceptibility and a* does not correlate with any other physical property. Below 220 mbsf, the correlation between sandy layers and GRA density is no longer clear and, in general, the relationship between lithology and physical properties is inconsistent. The hiatus noted at 230 mbsf (see “Biostratigraphy”) appears to correlate with this change in the physical property characteristics within physical properties Unit II. Physical properties Unit III corresponds to lithologic Subunit IB (see “Lithostratigraphy”) and is generally marked by a decrease in magnetic susceptibility and NGR and by a change in the frequency and amplitude of the cycles observed in most of the physical properties (Figs. F23, F24). Downhole logging also indicates a more distinct variability in the standard (total) gamma ray (HSGR) data in this interval (see “Downhole measurements”), interpreted as an increase in the abundance of thick sandy layers. The discontinuity previously defined by Llave et al. (2001, 2007, 2011) and Hernández-Molina et al. (2006) as the mid-Pleistocene revolution discontinuity is estimated at 295 mbsf and correlates with the physical properties Units II/III boundary. Therefore, a good correlation seems to exist between climatic changes and cyclical changes in the lithology reflected by physical properties, but further detailed work is required for confirmation. Whole-Round Multisensor Logger and Special Task Multisensor Logger measurementsThe STMSL was not used in Hole U1390A because no immediate acquisition of data for stratigraphic correlation was necessary, but its use was resumed in Holes U1390B and U1390C. Temperature equilibration before starting with the WRMSL was at least 3 h. Gamma ray attenuation bulk densityGRA density at Site U1390 shows highly variable values between 1.6 and 2.1 g/cm3, with a general increase downhole (Fig. F23). In the upper 220 mbsf, the natural variations in grain size and sediment texture are clearly reflected in the GRA density record, in which increases in grain size commonly correspond to increases in GRA density. This matches the observation that sandy beds are generally poorly sorted (see “Lithostratigraphy”), leaving less open pore space than well-sorted sand and therefore increasing bulk density. Below 220 mbsf, the relation between density and grain size deteriorates. Long-term variations in GRA density show three different trends:
Magnetic susceptibilityMagnetic susceptibility data in Hole U1390A show a cyclic pattern, especially from 70 to 295 mbsf within physical properties Unit II, and declining oscillations between 5 × 10–5 and 40 × 10–5 SI in the lower part of the hole (Fig. F23). As discussed above, a remarkable coherence of susceptibility and GRA density can be observed for the coarse-grained layers, mainly in physical properties Units I and II, suggesting a relatively high content of magnetite or other magnetic minerals in the fine fraction of the sand layers. In Unit III, the magnetic susceptibility does not appear to vary with sand content. P-wave velocityThe WRMSL was used to gather sonic velocities for all holes at Site U1390, and an attempt was made to determine P-wave velocities on split cores in each section of Hole U1390A (Fig. F23). Because of poor sediment to liner coupling, reasonable results from the WRMSL could only be obtained for the upper ~30 m of cores retrieved with the APC. The P-wave velocity profile can be extended downhole to 35 mbsf by using the P-wave determinations on split cores in Hole U1390A. Although the sediment surface appeared to be smooth and should have provided adequate coupling to the transducers, no clear acoustic signal could be obtained greater depth. The formation of small cracks in the relatively stiff and brittle sediment might have negatively affected signal propagation through the sediment. P-wave velocities follow GRA densities in the upper 30 mbsf, with values close to 1600 m/s in the uppermost 20 mbsf and then decreasing to 1450 m/s (Fig. F23). Because 1450 m/s is lower than the sonic speed in water, such low values are most likely an underestimation of the true speed, caused by cracks and voids in the sediment. The same problem occurs when measuring split cores. The pressure needed to provide a good contact for the transducers on the upper and lower sides of the sediment specimen leads in some cases to small cracks that increase the traveltime of the acoustic signal through the sediment. Sonic velocities measured for the same intervals by the continuous WRMSL are lower than those obtained individually by selective measurements on split cores. These latter velocities seem to be more reliable from 20 to 30 mbsf (Fig. F23). Natural gamma radiationNGR scanning was performed in all holes at Site U1390. NGR counts fluctuate mostly between 20 and 45 cps, exhibiting cyclic patterns for the cores retrieved with the APC and XCB (Fig. F24). These patterns are also consistent with the logging data (see “Downhole measurements”). At 70 mbsf, a peak of 50 cps is observed, and a notable positive correlation of NGR with GRA density, magnetic susceptibility, and a* is present between 70 and 220 mbsf, where the cyclicity is more evident and oscillates between 30 and 45 cps (Fig. F24). Below 295 mbsf, the correlation with the other studied parameters is less consistent as the amplitude of the variations increases. The relation of the sandy layers and NGR data is complex, with high NGR counts in physical properties Unit I and potentially also in Unit II (upper part) and low NGR values in Unit III. A possible explanation could be that the sands within physical properties Unit III appear to have lower clay content than those above (see “Lithostratigraphy”). Moisture and densityDetermination of MAD on discrete sediment samples was performed on every section in Hole U1390A (Figs. F23, F25). Generally, GRA density and MAD measurements give consistent results when taken on APC and XCB cores. Grain densities in the uppermost 20 mbsf of Hole U1390A oscillate between 2.75 and 2.8 g/cm3 and between 2.8 and 2.65 g/cm3 downhole. Moisture and porosity show parallel variations downhole. Porosity decreases rapidly from 70% to 50% in the uppermost 20 mbsf and then to values around 40% at 295 mbsf, overprinted by wide oscillations. Below 295 mbsf, no further porosity decrease is observed (Fig. F25). Moisture content decreases from 40% to 30% in the uppermost 20 mbsf and from 30% to 20% with broad oscillations over the next 270 m, with no further systematic downhole change. Thermal conductivityThermal conductivity was measured once per core in all cores in Hole U1390A using the full-space probe, usually in Section 3, near the middle of the section (see “Downhole measurements”). Because cores retrieved with the XCB are severely disturbed and affected by biscuiting, thermal conductivity measurements were only taken on APC Cores 339-U1390A-1H through 11H. Thermal conductivity varies between 1.0 and 2.1 W/(m·K), which is in the range of thermal conductivity values observed at the other sites. No clear trend is apparent. A relation to moisture content and porosity is also not evident, although pore water content should have an effect on thermal conductivity. Summary of main resultsThe physical property data allow us to define three main units at Site U1390, with boundaries at 70 and 295 mbsf. In general, a coherent relation exists between high magnetic susceptibility, NGR, and GRA density correlating with coarser intervals downhole to ~220 mbsf. These intervals are mainly composed of contourites, which are believed to be exclusively responsible for the formation of these coarse-grained layers. This would explain the more uniform relation between lithology and physical properties noted in this section. The boundary between physical properties Units II and III corresponds to a hiatus determined by biostratigraphy, with a duration between 0.60 and 0.47 m.y. Color reflectance indicates that sandy layers are mostly more reddish in color, whereas muddy intervals tend to be more greenish. |
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