Previous   |    Next

doi:10.2204/iodp.proc.339.109.2013

Physical properties

The shipboard physical properties program at Site U1391 included high-resolution nondestructive measurements of gamma ray attenuation (GRA) bulk density, magnetic susceptibility, and P-wave velocity mostly in 2.5 cm steps on the WRMSL. NGR on whole-round core sections was measured at 10 and 20 cm spacing, depending on the time available. The Special Task Multisensor Logger was only used in Hole U1391B for stratigraphic correlation purposes. Thermal conductivity was obtained on Section 3 of each core in Hole U1391A until Core 19H. Discrete measurements of P-wave velocities were determined on working-half sections, one every other section in Hole U1391A. However, reasonable results were only obtained for the upper 50 mbsf. Moisture and density (MAD) samples were measured for every second section of each core in Holes U1391A and U1391C. Color reflectance spectrometry and split-core point-logger magnetic susceptibility were obtained for every section in each hole in 5 cm steps.

Based on the physical property data, we can distinguish four units (Figs. F21, F22). Physical properties Unit I, from 0 to ~155 mbsf, is characterized by a clear positive correlation between NGR, magnetic susceptibility, and a*. The color reflectance measurement a* is generally very variable and includes positive (reddish) values. Physical properties Unit II, between ~155 and 200 mbsf, is characterized by low magnetic susceptibility and a marked negative correlation between NGR and a*. Physical properties Unit III, from 200 to 560 mbsf, exhibits low magnetic susceptibility values and a positive correlation between NGR, magnetic susceptibility, and GRA density. The correlation between magnetic susceptibility and NGR is particularly pronounced. Conversely, color reflectance measurements (L* and a*) show a positive correlation in several peaks but a complex, unclear relationship when compared with NGR. Below 200 mbsf, a* values are less variable and exclusively negative. Physical properties Unit IV can be recognized in Hole U1391C downhole from 560 mbsf as a marked reduction in NGR variability and magnetic susceptibility values.

Coinciding with the major change in physical properties at ~200 mbsf is the boundary between lithostratigraphic Units I and II, which is characterized by more abundant sandy intervals above 200 mbsf (see “Lithostratigraphy”). The color reflectance data is further in agreement with the visual core descriptions, which indicate that alternations of greenish and reddish intervals are only present above ~170 mbsf, whereas reddish intervals disappear below 200 mbsf.

Whole-Round Multisensor Logger measurements

GRA bulk density and magnetic susceptibility were measured using the WRMSL in all core sections at 2.5 cm intervals at Site U1391 after core acclimation for 3 h (Fig. F21).

Gamma ray attenuation bulk density

Measured GRA density at Site U1391 ranges between 1.4 and 2.1 g/cm³ and displays a steady increase in the upper 20 mbsf, probably caused by early diagenetic compaction. Downhole, we observed cyclic variations with average values around 1.85 to 1.9 g/cm³ and distinctive, low-GRA density intervals at 90, 108, 165, and 255 mbsf (Fig. F21A). In Hole U1391C, GRA density reaches a maximum value of 2.4 g/cm³ at 632 mbsf related to the presence of a dolomitic mudstone (see “Lithostratigraphy”) (Fig. F21B).

When GRA density is compared to lithology, no clear correlation can be found. Compared with the other physical properties, a similarity to magnetic susceptibility is present, although it is not as pronounced as observed for the other sites within the Gulf of Cádiz (e.g., Site U1389). In physical properties Unit III, from 200 to 560 mbsf, a moderately strong positive correlation can be found between GRA density and NGR.

Magnetic susceptibility

The most notable aspects of the magnetic susceptibility records in Hole U1391A are a sharp decrease at 28 mbsf (from 65 × 10^–5 SI to ~10 × 10^–5 SI) and cyclic variations ranging from 5 × 10^–5 to 60 × 10^–5 SI between 28 mbsf and the lower boundary of physical properties Unit I. Generally, low values are found in physical properties Units II and III (mostly <20 × 10^–5 SI), and a further decrease can be observed in physical properties Unit IV, with values usually <10 × 10^–5 SI downhole (Fig. F21).

Magnetic susceptibility records at Site U1391 show a clear positive correlation with NGR data. This relationship becomes remarkably good below ~155 mbsf. In physical properties Unit I (0–155 mbsf), magnetic susceptibility is sensitive to the presence of sands, although with a complex pattern. For example, sandy layers in Sections 339-U1391A-6H-4 and 6H-6 exhibit low magnetic susceptibility in their lower parts and an abrupt shift to high values in the upper parts. Sharp compositional changes are described within these two sandy layers (see “Lithostratigraphy”). However, no correlation of magnetic properties can be found in relation to sand layers in the other physical properties units (e.g., sand layers in Cores 339-U1391A-22X and 30X).

Low magnetic susceptibility values through physical properties Units II, III, and IV could be explained by the reduction of fine-grained magnetite to Fe sulfides below the sulfate reduction zone. However, the present sulfate–methane transition zone occurs at a shallower depth of ~20 mbsf (see “Geochemistry”). A link between magnetic susceptibility and diagenetic processes could occur in physical properties Unit IV, where the decrease in magnetic susceptibility might relate to a large increase in alkalinity and the inferred presence of gas hydrate at roughly the same depth.

P-wave velocity

Sonic velocities were measured with the WRMSL in Hole U1391A, and an attempt was made to determine P-wave velocities on split cores in each section of Hole U1391A (Fig. F21A). Because of poor sediment to liner coupling, reasonable results from the WRMSL could only be obtained for the upper ~45 mbsf. The P-wave velocity profile can be extended downhole to 50 mbsf by using the P-wave measurements on split cores. Although the sediment surface appeared to be smooth and should have provided an adequate coupling to the transducers, no clear acoustic signal could be obtained at greater depths. An explanation for this might be the formation of small cracks in the relatively stiff and brittle sediment that negatively affects signal propagation through the sediment. Additionally, small voids between sediment and liner might prevent a sharp signal from being received by the transducer touching the liner.

P-wave velocities follow the trend of increasing GRA density in the upper 10 mbsf. Downhole values range from 1400 to 1650 m/s for WRMSL and split-core data (only accounting for automatically processed data). Values obtained by both types of measurements agree well, especially when only considering split-core data with high signal quality (solid symbols in Fig. F21A).

Natural gamma radiation

Measured values of NGR range from 20 to 55 cps, revealing cyclic patterns that are more abrupt than regular oscillations in physical properties Units I, II, and III (Fig. F22). In physical properties Unit IV, NGR decreases in variability between 30 and 45 cps and lacks distinct peaks (Fig. F22B).

NGR data do not show a clear correlation with grain size variations, which might indicate that detrital potassium and thorium are present independent of grain size variations. Notably, an almost perfect correlation exists between NGR and magnetic susceptibility, and a negative correlation with exists a* in physical properties Unit II.

Moisture and density measurements

Determination of moisture content and density on discrete sediment samples was made on every second section of Hole U1391A (Fig. F23). Samples were consistently taken at ~60 cm intervals. Generally, GRA and MAD methods give consistent density results, with slightly lower values for MAD in the upper ~20 mbsf of Hole U1391A (Fig. F23A).

The compaction-related downhole moisture and porosity decrease follows a characteristic pattern. Rapid compaction can be recognized in the upper 2 mbsf, with a decrease in porosity from 70% to 60% and moisture content from 40% to 30%. More progressive compaction occurs from 2 to ~30 mbsf, where values reach 30% moisture and 55% porosity. This variation in compaction mode can be related to the different sediment composition and texture and might be a result of changes in the sedimentation rate. Below 30 mbsf in physical properties Unit I, we observed two long-wavelength cyclic variations between 25% and 30% in moisture content and 45% and 55% in porosity. Grain density shows an inverse correlation with these cyclic changes, which might indicate that these cycles are controlled by carbonate content.

In physical properties Unit II (155–200 mbsf), two peaks in porosity and moisture content can be recognized. The previously recognized anticorrelation to grain density appear to be more complex in this unit. Throughout physical properties Unit PPIII, high-amplitude cyclicity appears to be superimposed upon other low- and high- amplitude oscillations. Grain density values are highly variable from 420 mbsf to the lowermost core, with sharp variations between 2.85 and 2.65 g/cm³. This interval is characterized by a slightly increased abundance of sandy intervals, which might explain the larger scatter of the grain density data. Data below 500 mbsf are not available because of time constraints at the end of the expedition.

Thermal conductivity

Thermal conductivity was measured once per core using the full-space probe, usually in Section 3 near the middle of sections downhole to Core 339-U1391A-19H (see “Downhole measurements”). Because cores retrieved using XCB drilling are severely disturbed and affected by biscuiting, thermal conductivity measurements were only taken on APC cores. Thermal conductivity varies between 1.1 and 1.6 W/(m·K), which is in the range observed at the other sites. In the upper 30 mbsf, thermal conductivity values show a low range of variation (1.1–1.3 W/[m·K]). Below 30 mbsf, no clear trend is imprinted in the data; however, more prominent variations appear between cores that reach the highest and lowest values. A weak correlation to moisture content and porosity can be recognized between Cores 339-U1391A-4H and 9H, in which relatively high moisture and porosity values appear to correlate with high thermal conductivity values. For the other cores, the relationship between thermal conductivity and other parameters is not evident, although pore water content should have an effect on thermal conductivity.

Summary of main results

Correlation of physical property data allows us to discern four physical properties units at Site U1391 with boundaries at 155, 200, and 560 mbsf. The boundary at 200 mbsf matches with a change in the abundance of sandy beds in the mud-dominated sediment (lithostratigraphic Units I and II). In general, a coherent relationship exists between high magnetic susceptibility, NGR, and GRA density. Especially notable is the excellent correlation between NGR and magnetic susceptibility below ~155 mbsf. The presence of coarse layers is, if at all, inconsistently expressed in the measured physical parameters. However, major changes in physical properties are evident in apparently homogeneous calcareous mud deposits. The varying (i.e., positive as well as negative) correlation of NGR and magnetic susceptibility in relation to color changes suggests that the associated changes in mineralogical composition are of a different nature in this location when compared with previous reddish layers described during this expedition.

Top of page   |    Previous   |    Next