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doi:10.2204/iodp.proc.307.105.2006

Physical properties

The physical properties measured on the cores of Site U1318 include magnetic susceptibility measured with the “Fast Track” multisensor core logger (MSCL) and multisensor track (MST), gamma ray attenuation (GRA) density, natural gamma radiation (NGR), P-wave velocity measured continuously with the MST P-wave logger (PWL) and at discrete positions on the split cores with the P-wave sensor (PWS), moisture and density (MAD), shear strength, and thermal conductivity. PWL measurements were unreliable with XCB drilling because the gap between sediment and core liner blocks the acoustic signal and the piece-wise recovery (“biscuits”) gives large variations in the recovered sediment column. Shear strength measurements were only carried out on core sections from 0 to 90 mbsf. Below 90 mbsf, the sediment became too indurated.

Recovery at Site U1318 was very good, so nearly continuous records of the parameters were acquired. The resulting curves, expressed in meters composite depth (mcd) and constructed after splicing the data from the three holes at this site, are presented in Figure F22.

Magnetic susceptibility

Overall, magnetic susceptibility ranges from 3 × 10–5 to 160 × 10–5 SI units. The trend of the magnetic susceptibility depth curve shows a gradual increase in the upper ~38 mcd of the cores to a local maximum of 130 × 10–5 SI units at ~38 mcd, although several intervals with high-amplitude oscillations occur. For example, significant breaks in the overall trend are observed at ~20 and 35 mcd, with drops of ~25 × 10–5 and 30 × 10–5 SI units, respectively. From a depth of 38 mcd downhole, the magnetic susceptibility gradually decreases until ~62 mcd with a local minimum of 34 × 10–5 SI units; however, over this interval values show more short-period oscillations with higher amplitudes. From 62 to 80 mcd, the magnetic susceptibility increases rapidly with high-amplitude oscillations >90 × 10–5 SI units. At ~82 mcd, the magnetic susceptibility drops drastically to ~20 × 10–5 SI units followed by a limited increase and several sharp spikes, the last one marking a clear boundary at 92 mcd. Below this boundary, the magnetic susceptibility remains very low, largely varying between 3 × 10–5 and 12 × 10–5 SI units.

Natural gamma radiation

The NGR depth curve at Site U1318 shows a gradual increase in the uppermost 22 mcd, characterized by regular and high-amplitude oscillations. This overall trend stays constant until ~82 mcd, where NGR decreases from ~50 to 20 cps at 83 mcd. The interval from 83 to 92 mcd is characterized by high-amplitude oscillations in a very short range with remarkably high peak values >100 cps at ~92 mcd. Below this depth, the curve follows a smooth increase until ~192 mcd. Values in the interval below 192 mcd stay, in general, lower than in the upper part, ranging from 15 to 50 cps. Some-well defined large-scale cycles are present (e.g., 168–191 and 191–240 mcd) in the lower part of the curve.

Gamma ray attenuation density, bulk density, and porosity

The bulk density measurements (i.e., GRA-corrected density and MAD measurements) display parallel trends. GRA densities deduced from the MST are corrected in the unconsolidated section of the cores according to the equation described in “Physical properties” in the “Methods” chapter. The density increases in the upper 15 mcd of the site from 1.7 to 2.1 g/cm3. From there the values decrease sharply to 1.8 g/cm3, remain low until 21 mcd, and increase again to a maximum of 2.2 g/cm3 at 29 mcd. This is followed by an interval of fairly constant density, which finally reaches a local peak at 54 mcd, followed by a minimum of ~1.9 g/cm3 at 60 mcd. From this point, GRA density becomes more irregular with an overall increasing trend until 92 mcd. This depth represents a major break in the density depth curve with a reduction in density from ~2.2 to 1.9 g/cm3. Between 92 and 132 mcd, the density stays relatively constant with low-amplitude oscillations; however, some broad cycles can be distinguished. Another sharp reduction in density can be observed at 132 mcd, and the depth curve evolves in a sawtooth pattern. From 192 mcd, the amplitudes and frequencies of these oscillations become more irregular, whereas the average density value increases slightly.

Shear strength

Shear strength, measured with the Torvane apparatus, shows four distinctive trends in the upper 92 m of Hole U1318A:

  1. Shear strength increases from ~0 to ~6 × 103 kg/m2 in the upper 32 mcd.
  2. After a drop to 4 × 103 kg/m2, values increase between 38 and 58 mcd, reaching a local maximum of 7.5 × 103 kg/m2. This interval concludes with a drop in values from 7.5 × 103 to 5.5 × 103 kg/m2.
  3. This drop is followed by a fairly steep increase to 9 × 103 kg/m2 at ~72 mcd and remains constant downhole.
  4. The last interval measured, from 82 to 92 mcd, shows an irregular pattern with larger amplitudes in the oscillations and some gaps in the data.

P-wave velocity

PWL data were consistent with PWS data in the uppermost 130 mcd. Deeper than 130 mcd, small gaps are evident in the curve and the data generally become sparser. Overall, sonic velocity increases irregularly in the upper 15 mcd, with local peaks at 2, 5, and 7 mcd. Between 15 and 22 mcd, sound velocities show a pattern comparable to GRA and bulk density measurements, culminating in a low at 21 mcd. This is followed by a sharp increase to ~1630 m/s at ~26 mcd. A zone of slight increase in the values with some small cycles in between occurs until 55 mcd, where it reaches a local maximum of ~1700 m/s. Between 55 and 91 mcd, the curve drops to ~1560 m/s at 61 mcd and begins an increasing trend with higher amplitude cycles until a peak of 1720 m/s at 91 mcd, where there is a clear boundary in the GRA and NGR data sets. This is followed by a long interval, from 92 to 122 mcd, characterized by a moderate increase in sound velocity and low-amplitude (<100 m/s) cycles with highs between ~100 and 112 mcd, which are very comparable to the trends observed in the GRA density curve. Deeper than 130 mcd, gaps in the PWL data become more evident; however, a decreasing trend can be discerned in the PWS curve until ~186 mcd, with a low of 1560 m/s. Below 186 mcd, sound velocities increase irregularly in several cycles to reach a maximum of 1700 m/s at 211 mcd and below.

Thermal conductivity and in situ temperature measurements

The deployment of the advanced piston corer temperature tool provided an insight into the subsurface temperature distribution. Four discrete temperature measurements in Hole U1318A were taken at 28.2, 56.7, 85.2, and 113.7 mbsf (Fig. F23). The data show a temperature gradient of 46°C/km, which is a normal value for passive continental margins.

Interpretation

Based on the overall trends of the physical properties described above, a set of physical property (PP) units can be defined that correspond to major lithologic units (Fig. F22).

Unit PP1 comprises the upper 16 mcd and is characterized by increasing P-wave velocity and density. Magnetic susceptibility and NGR also increase gradually. A sharp drop in density and P-wave velocity, a minimum in NGR, and a change in character of the magnetic susceptibility signal (more high-amplitude excursions) mark the lower boundary. The interval is part of lithostratigraphic Subunit 1A and consists of silty clays with a coarser layer toward its base. It has the same characteristics as Unit PP1 in Hole U1316A. In the seismic profile, it corresponds to a package of chaotic reflections of moderate to high amplitude.

Unit PP2 (16–24 mcd) corresponds to a low-density and P-wave velocity interval and an increase in the amount of high-amplitude variability in magnetic susceptibility. NGR values slightly increase. The unit shows the same characteristics as Unit PP2 at Site U1316 and also consists of clayey materials with faint laminations. Again, this unit falls within lithostratigraphic Subunit 1A. The seismic expression of this unit is relatively low in amplitude compared to the surrounding seismic facies. The lower boundary is erosive and high in amplitude.

Unit PP3 comprises the interval between the sharp increase in density and velocity at 24 mcd and a significant increase in magnetic susceptibility at 60 mcd. The unit itself is characterized by minimal variations in the trends of most physical properties, although shear strength and magnetic susceptibility show local maxima at 33 mcd that correspond to a 24 cm thick graded layer of fine sand or silt on an erosive boundary forming the base of lithostratigraphic Subunit 1A. The lower part of Unit PP3 corresponds to lithostratigraphic Subunit 1B, which is characterized by dark silty clay horizons with strong lamination. Acoustically, Unit PP3 is characterized by parallel high-amplitude reflectors and, in the lower part, moderate- to low-amplitude reflectors.

The significant increase in magnetic susceptibility, together with a change in P-wave velocity (PWS) trend and a local minimum in shear strength at 62 mcd, marks the upper boundary of Unit PP4, which extends to 84 mcd. Unit PP4 overlaps lithostratigraphic Subunit 1C, which contains abundant laminations and slightly coarser material than the subunits above and may correspond to Unit PP4 at Site U1316. At Site U1318, Unit PP4 corresponds in the seismic profile to a high-frequency, high-amplitude facies of parallel reflectors. The frequency of the reflectors is higher than in the overlying facies. Toward the lower boundary of Unit PP4, the reflectors are lower in amplitude.

Unit PP5 (84–92 mcd) is marked by a sharp reduction in magnetic susceptibility, NGR, and shear strength in the uppermost sections and a gradual increase in these values toward the base, although with some sharp spikes. Unit PP5 corresponds to lithostratigraphic Unit 2 and contains interbedded sands and silty clays with abundant shell fragments and pebble-sized clasts. The thickest sand layers occur toward the uppermost sections, where the magnetic susceptibility and NGR are minimal. The lower boundary is formed by a 5–10 cm thick shell bed identified as a major hiatus in the biostratigraphy and causing a strong magnetic susceptibility, NGR, P-wave velocity, and GRA density signal. These physical properties of the sediment create a high-amplitude reflector in the seismic profiles, and this erosive reflector has been tentatively identified as the mound base reflector. Unit PP5 is seismically characterized by high-amplitude reflectors.

Low magnetic susceptibility and fairly constant P-wave velocity and density values with low-amplitude broad cycles are the main characteristics of Unit PP6 (92–132 mcd). P-wave velocity and density have a remarkable parallel pattern. Unit PP6 corresponds to lithostratigraphic Subunit 3A, comprising greenish gray silty clays and silts and coinciding with moderate- to high-amplitude parallel reflectors. The lowermost reflector bounding Unit PP6 is a high-amplitude reflector marking an erosive boundary.

Unit PP7 is located between 132 and 192 mcd, with the lower boundary marked by a local maximum in NGR followed by a reduction in the values of NGR and an increase in P-wave velocity. Magnetic susceptibility values stay very low in this unit, although slightly higher than in Unit PP6, whereas P-wave velocities are lower than in the unit above. The pattern of GRA density in Unit PP7 shows more sawtooth cycles of higher frequency than in Unit PP6, while the overall value is lower. The lithostratigraphic description mentions Subunit 3B, consisting of homogeneous silty clays bound by sandier intervals in the uppermost and lowermost sections. On the seismic profiles, the Unit PP7 shows a fairly low amplitude homogeneous facies with a couple of high-amplitude reflectors that coincide with the more sandy layers.

The lowermost Unit PP8 comprises the interval below 192 mcd. It shows lower NGR values, an increase in P-wave velocities (including some extreme values reaching >3000 m/s), and a slight increase in density in combination with increased amplitudes of high-frequency variations. The high acoustic velocities correspond to lithified layers, whereas the unit itself is identified in the lithostratigraphy as Subunit 3C, consisting mainly of silty clay and fine sands. Unit PP8 coincides with high-amplitude, high-frequency parallel reflectors and corresponds to the sigmoidal unit in Hole U1316A.

Relationship between physical properties

Statistical analysis and correlation tests were performed on the different numerical data sets in order to evaluate the quality of the data, make direct comparisons between different laboratory equipment, and draw potential relationships amongst the physical properties parameters. The statistical data analysis, including normality tests and confidence intervals for each individual core, assisted in the identification of possible outliers. These data were filtered out before further processing. A full set of associated graphs was utilized to aid in the identification of the most important physical parameters causing the seismic reflectors. Figure F24 shows a matrix plot of the correlation between the physical parameters measured in the cores.

Measurements taken for P-wave velocity using the PWL (MST) and PWS (split core) were compared in their mean, median, and distribution patterns to check for consistency. Both data sets have a very similar range in velocity values; however, a slight shift in the PWS measurements of approximately –30 m/s was noted. P-wave velocity on split cores is compared with NGR (influenced by the clay content), GRA density, porosity, shear strength, and magnetic susceptibility. There is a moderate correlation between sound velocity and density measurements in Units PP1–PP5 (Cores 1–10).

There is a remarkably good correlation between magnetic susceptibility and NGR throughout Hole U1318A, which is particularly high in Unit PP6 (Pearson's correlation coefficient [PCC] = >0.8). Magnetic susceptibility also correlates fairly well with shear strength measurements taken over the interval between 0 and 92 mcd (PCC = ~0.75), except from 50 to 66 mcd (Cores 6 and 7).

Stratigraphic correlation

Stratigraphic correlation at Site U1318 was carried out with the help of the Splicer software, as described in “Physical properties” in the “Methods” chapter. An mcd scale was constructed from the physical property data of Holes U1318A, U1318B, and U1318C (Tables T9, T10) as well as a spliced depth curve for most of the parameters at this site (Fig. F22). The main physical properties used to construct the mcd scale were magnetic susceptibility, GRA density, and NGR. Information about biostratigraphy and lithologic contacts also aided in the construction of the mcd scale and spliced records. Drilling was continued below 160 mcd only in Hole U1318B; therefore, the spliced data below that level are fully based on this hole (Table T10). As the recovery was excellent, however, a nearly continuous depth curve could be constructed for the different physical property parameters. In general, the core expansion at Site U1318, estimated from the extension of the mcd scale versus the mbsf scale, is ~3.5%, which is fairly low.

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