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doi:10.2204/iodp.proc.307.103.2006

Physical properties

The physical properties measured on the cores from Site U1316 include magnetic susceptibility measured with the MSCL and with the multisensor track (MST), gamma ray attenuation (GRA) density, natural 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 velocity sensor (PWS), moisture and density (MAD), shear strength, and thermal conductivity. Good results were obtained with all types of equipment, except for the PWL measurements below ~60 mbsf. The acquisition of PWL data became unreliable with the use of XCB or RCB drilling because the gap between sediment and core liner blocks the acoustic signal. Shear strength measurements were only carried out on sediments from 0 to 50 mbsf in Hole U1316A; below 60 mbsf the sediment became too indurated to use the Torvane tool.

As recovery in the upper parts of the holes (<~60 mbsf) was very good, nearly continuous records of the parameters were acquired. In the lower levels, however, recovery was less complete, and even after the integration of the data from the different holes, important data gaps persist (see also “Stratigraphic correlation”). The resulting spliced curves, expressed in meters composite depth (mcd) and constructed after splicing the data from the three holes at this site, are presented in Figure F30.

Magnetic susceptibility

Overall, the magnetic susceptibility measurements range from 5 × 10^–5 to 128 × 10^–5 SI units. The overall trend of the magnetic susceptibility depth curve shows a very gradual increase in the upper ~34 mcd of the cores, although several sawtooth intervals of different strength and periodicity exist. A significant break in the overall trend is observed at ~22 mcd with a drop of ~20 × 10^–5 SI units. From a depth of 36 mcd downhole, the magnetic susceptibility increases more rapidly and demonstrates more short-period variability with higher amplitudes. At 49 mcd, the magnetic susceptibility drops sharply to ~30 × 10^–5 SI units, followed by a more gradual reduction to 60 mcd, reaching values as low as 3 × 10^–5 to 4 × 10^–5 SI units. Below this boundary, the magnetic susceptibility remains very low, largely varying between 3 × 10^–5 and 10 × 10^–5 SI units.

Gamma ray attenuation, bulk density, and porosity

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 (upper 60 mcd of the core) according to the equation described in “Physical properties” in the “Methods” chapter. Density increases in the upper 6 mcd of the site from 1.8 to 2 g/cm³. From there the values gradually decrease to 1.7 g/cm³ and increase again to a clear maximum (2.1 g/cm³) at a depth of 16 mcd. A second maximum is visible in the density data at 22 mcd that coincides with the sharp drop in magnetic susceptibility. Below 22 mcd, density remains fairly constant until 30 mcd, after which the values show an increasing amount of scatter. This scatter in GRA data below 29 mcd occurs at a high frequency but gradually increases toward broader cycles and larger amplitudes with depth. Below 64 mcd, the average density stays very constant at ~1.9 g/cm³ with a slightly increasing trend downcore. GRA data shows more scatter than MAD data in the lower interval as a result of the core disturbance (biscuits) in the XCB cores in Hole U1316A.

Natural gamma radiation

The NGR depth curve at Site U1316 shows an overall gradual increase in the uppermost 16 mcd but is also characterized by high-frequency and high-amplitude variations, probably due to the short measurement time of five periods of 1 s. After a slight decrease to 21 mcd, the overall trend stays constant at ~50 cps until a depth of 47 mcd, from where the NGR decreases to 30 cps at 60 mcd. Below this depth, the average values stay lower than in the upper part and the high-frequency variations have lower amplitudes. Some large-scale cycles are present (e.g., 90–96, 102–110, and 116–124 mcd); however, they are in many cases not completely surveyed because of gaps in the core recovery.

Shear strength

The shear strength, measured with the Torvane apparatus, shows three clear trends:

1. An increase from ~0 to ~1.6 10³ kg/m² in the upper 7 mcd.
2. A nearly constant value between 7 and 27 mcd
3. A fairly steep increase to 3.5 10³ kg/m² at 50 mcd. At this point, the shear strength drops sharply to values comparable to those near the seafloor.

P-wave velocity

Although the acquisition of PWL data was sometimes problematic, especially in the deeper cores, there are still trends that can be derived from both the PWL and PWS data. Overall, the sonic velocity increases irregularly in the upper 7 mcd. Between 7 and 17 mcd, both data sets show a pattern comparable to the GRA and bulk density measurements, culminating in a sharp maximum at 16 mcd. The secondary maximum at 22 mcd found in the GRA and bulk density values, however, is not visible in the P-wave data. From 16 mcd there is a very gradual increase in the sonic velocity, from 1550 to 1630 m/s at 47 mcd. Between 47 and 60 mcd, the PWS data especially show considerable scatter and an overall increasing trend from 1600 to 1650 m/s. A jump in P-wave velocity is observed at ~60 mcd. Below 60 mcd, data density is limited, but another gradually increasing trend can be seen, albeit with a lower gradient. The velocity in this interval is ~1700 m/s, but sharp spikes occur at several distinct horizons. Finally, at a depth of ~130 mcd, the sonic velocity increases sharply to 2000 m/s and more. The PWS shows a deviation from the PWL data of ~100 m/s, and there is a considerable offset between the PWS measurements of Hole U1316A and U1316C, which is probably caused by differences in coring techniques. The XCB cores have a lower velocity (by ~200 m/s) than the RCB cores probably because of the ground-up sediment between and surrounding biscuits in the XCB cores.

Thermal conductivity and in situ temperature measurements

Deployment of the advanced piston corer temperature tool provided insight into the subsurface temperature distribution (Fig. F31). Two discrete temperature measurements in Hole U1316A were taken at 26.3 and 54.8 mbsf. The data show temperature gradients of 42°C/km.

Interpretation

Based on the overall physical property trends as described above, several physical property (PP) units can be defined, which correspond to major lithostratigraphic units (Fig. F30).

Unit PP1 comprises the subsurface and reaches to ~6 mcd. It is characterized by increasing shear strength, P-wave velocity, and density. Its lower boundary is marked by a negative peak in density, natural gamma radiation, and high P-wave velocities and by the end of the first sawtooth cycle in the magnetic susceptibility data. The interval illustrates the natural trends of bulk density, shear strength, magnetic susceptibility, and velocity as sediments become buried. The sharp drop in the different parameters that indicate its lower boundary, however, corresponds to a shell layer and an interval of silty material in lithostratigraphic Subunit 1A that otherwise is characterized by homogeneous muds. The upper part of the seismic profile displays high-frequency and high-amplitude reflectors interpreted as drift deposits.

The definition of Unit PP2 is based on the characteristic pattern in the density and P-wave velocity, which shows a decrease followed by a sharp maximum at 17 mcd and a secondary peak at 22 mcd. These intervals again correspond to siltier layers in Subunit 1A. The seismic expression of this unit is relatively low in amplitude compared to the surrounding seismic facies.

Unit PP3 (22–26 mcd) is defined by the absence of major features. Most parameters have a nearly constant value or a limited overall increase along a low gradient. It represents a homogeneous muddy sequence without major boundaries or changes in lithostratigraphic Subunit 1A.

The slightly concave natural gamma radiation profile in these upper three units (PP1–PP3) represents the high clay content. On seismic profiles this unit is characterized by a moderate amplitude and frequency and the reflector morphology is slightly wavy.

The clear change in trend of the shear strength at 26 mcd marks the onset of Unit PP4, which extends to 47 mcd. In the lower part of this unit, the magnetic susceptibility values increase strongly and the amplitudes of the cyclicity in the GRA density data become larger. This unit corresponds to lithostratigraphic Subunit 1B. The spikes in magnetic susceptibility coincide with sandy laminations in Subunit 1B. The P-wave velocity in this unit shows some peaks corresponding to coarser-grained layers. Unit PP4 corresponds in the seismic profile to a high-frequency and high-amplitude parallel reflectors. The frequency of the reflectors is higher than in the overlying facies.

Unit PP5 is marked by a sharp drop in magnetic susceptibility and shear strength and corresponds to sequences containing coral fragments. This drop in P-wave velocity can be correlated with low-amplitude seismic facies without internal reflectors on the seismic profiles of the site survey (Line P000660). The increase in carbonate content causes the decrease in magnetic susceptibility and also a gradual decrease in NGR because of the limited clay content in these intervals. The irregularity of the coral layers causes the scattering and partial reduction of the PWS values. The unit is sharply defined at 60 mcd by a further reduction in magnetic susceptibility to values <10 × 10^–5 SI units, a boundary identified as a major hiatus by biostratigraphy. The lower boundary is characterized by high P-wave velocity and a significant increase in density. These physical parameters of the sediments cause the high-amplitude reflector in the seismic profiles, which is interpreted as a significant erosional event of possible overconsolidated sediments.

Low magnetic susceptibility and fairly constant background levels of velocity and bulk density values are the main characteristics of Unit PP6. This unit corresponds to lithostratigraphic Unit 3, comprising Miocene nannofossil-rich sandy to clayey silts with a carbonate content between 30 and 50 wt%. The low magnetic susceptibility values are caused partly by the high CaCO₃ content of the material and by the limited amounts of magnetic minerals in the sediments. Cycles of natural gamma radiation counts have a negative correlation with the cyclicity of P-wave velocity. The increased carbonate content also results in a general reduction in NGR. Peak acoustic velocities have been measured up to 2500 m/s and correspond with the high-amplitude seismic reflectors interpreted as the top of sigmoidal bodies in the lower drift deposit. The density of these layers is relatively high and the porosity low.

Unit PP7 is located below 130 mcd and is characterized by the sharp increase in PWS values to 2000 m/s and more. This increase in velocity is correlated to the high-amplitude reflector at the base of the sigmoidal bodies. Unfortunately, good PWL data were not obtained at this depth. Natural gamma radiation counts decrease, which suggests a lower clay content and increase in carbonate content. There is also no significant increase in density, which could be related to this velocity increase.

Relationship between physical properties

Statistical analysis and correlation tests have been performed on the different numerical data sets in order to evaluate the quality of the data, to make direct comparisons between different laboratory equipment, and to draw potential relationships among the physical property parameters. The statistical data analysis, including normality tests and confidence intervals for each individual core, assisted in the identification of outliers, which were quite numerous in the pycnometer results (~10%) and less frequent in the MST data (~5%). These data have been filtered out before further processing. Figure F32 shows a matrix plot of the correlation between the physical parameters of the cores to aid in the identification of the most important physical parameters causing the seismic reflectors.

A clear change in the physical properties of the sediment can be seen at the boundary between Units PP5 and PP6 (~60 mcd, Cores 7–8). Bulk sediment GRA density data and direct measurements (MAD bulk density) present a fairly good positive correlation from Units PP1 to PP5 (Cores 1–8) with both P-wave velocity measurements. Below 60 mcd, in Units PP6 and PP7 (Cores 8–21), sediment density remains rather stable; however, a slight but constant increase in the P-wave velocity (PWS) is present throughout both units. Natural gamma radiation shows a negative correlation with P-wave velocity for Units PP2–PP5 (Cores 2–7), and there is no linear relation for Units PP6 and PP7 (Cores 8–21). Shear strength in Units PP1–PP4 (unconsolidated sediments) (Cores 1–6) shows a good positive correlation with the P-wave velocity. A gradual increase in shear strength in Unit PP4 (Cores 4–6) is less visible in the P-wave logs.

Measurements taken for P-waves using PWL (MST) and PWS (sample) were compared in their mean, median, and distribution patterns to check for consistency in both data sets. It was noted that a constant shift from 50 to 100 m/s was present from the PWS to the PWL. This might be due to different operations. From these correlations, it seems that the acoustic velocity is influenced by the density, clay content and in shear strength in unconsolidated sediments.

Stratigraphic correlation

Stratigraphic correlation at Site U1316 was carried out with the help of the Splicer software, as described in “Composite section development” in the “Methods” chapter. An mcd scale was constructed from the physical property data of Holes U1316A, U1316B, and U1316C (Tables T9, T10) as well as a spliced depth curve for most of the parameters (Fig. F30) at this site. The main physical properties used to construct the mcd scale were magnetic susceptibility from the MSCL, GRA density for the sequence above ~60 mcd, and NGR for the interval below that. Information about biostratigraphy and lithologic contacts also aided in the construction of the mcd scale and spliced records. The correlation task was fairly straightforward for the upper sequence (correlation between Holes U1316A and U1316B), as many features appeared in the data that could be used as tie points (spikes, sawtooth trends, etc.). Below ~60 mcd, correlation between Holes U1316A and U1316C became more difficult because of limited recovery, the very low signal of some parameters (e.g., MSCL), and the use of different drilling techniques causing slight differences in the data. Overall, this results in a continuous spliced depth curve for most parameters above 60 mcd, whereas below this boundary some data gaps persist. The overall extension of the mcd scale versus the mbsf scale (as an approximation of the core expansion) is 9%.

An additional difficulty in the creation of continuous spliced depth curves was the sampling of long whole-round cores for microbiological analysis. Ideally, these microbiological sampling intervals should avoid overlap with the gaps between cores in the adjacent hole. In situations with fast core recovery, it is difficult to monitor this in real time, even when using data from the MSCL. Furthermore, the Splicer software is not adapted to allow the use of two separate portions of the same core in the spliced curve. Therefore, only either the interval above or below the microbiological sampling could be used for the spliced curves instead of both.

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