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doi:10.2204/iodp.proc.307.104.2006

Physical properties

Physical properties measured on the cores of Site U1317 include magnetic susceptibility measured with the “Fast Track” multisensor core logger and multisensor track (MST), gamma ray attenuation (GRA) density, natural gamma radiation (NGR), and P-wave velocity measured with the MST P-wave logger (PWL) and at discrete positions on split cores with the P-wave sensor (PWS), moisture and density (MAD), and thermal conductivity. Good results were obtained with all core logging sensors, except for the PWL measurements that became unreliable due to the high coral content. The use of XCB or RCB drilling also precluded PWL measurements because the air buffer between the sediment and core liner blocked the acoustic signal. Shear strength measurements were only carried out on the upper sections (0–120 mbsf), as below the mound base the sediment became too indurated.

The holes at this site were drilled in the upper flank of the mound going upslope from Hole U1317A to U1317C, with the deeper Hole U1317D just south of the profile between Holes U1317B and U1317C. Hole U1317E was drilled on top of the mound and penetrated the mound base (Fig. F22). Continuous physical property records have been produced due to the almost 100% recovery at Site U1317 inside the mound facies (lithostratigraphic Unit 1). Recovery was less complete, however, in RCB-cored Hole U1317D, which penetrated the units below the mound base. The physical property records demonstrate similar trends in variations and cycles in all holes. Nevertheless, no composite depth profile was made because of the significant lateral variation in the stratigraphic thickness of the layers in the different holes.

Based on the seismic stratigraphy and sedimentology (see “Lithostratigraphy”), the Site U1317 succession was divided into two units. Physical property (PP) Unit PP1 is a Pleistocene rudstone-wackestone unit that comprises the mound itself (Hole U1317A, 0–130 mbsf and Hole U1317D, 0–146 mbsf). Unit PP2 is siltstone below the mound base. Figures F22, F23, F24, F25, and F26 show the physical property measurements of the different holes at Site U1317.

Magnetic susceptibility

The values of magnetic susceptibility are in general very low throughout the cores. Nevertheless, two major units can be identified based on the absolute values of the magnetic susceptibility. Magnetic susceptibility is slightly higher in physical property Unit PP1 (the upper part) than in Unit PP2 (the lower part) and shows a few peaks. Unit PP1 coincides with mound lithostratigraphic Unit 1 (see “Lithostratigraphy”). In Unit PP2, magnetic susceptibility values are very low and show some sawtooth variations. The average value for magnetic susceptibility in Unit PP1 is ~6 × 10^–5 SI with peaks of ~40 × 10^–5 SI (Figs. F22, F23, F24, F25, F26). In Unit PP2, the magnetic susceptibility drops to a background signal of ~3 × 10^–5 SI with peak values of ~12 × 10^–5 SI (Fig. F25). Three narrow peaks have been observed in Unit PP1 at ~1, 8, and 21 mbsf. This pattern is observed in all holes. Between ~21 and 80 mbsf, the values are continuously low with occasional episodes of minor high-frequency variability. At ~20–30 m above the mound base, an increase in magnetic susceptibility is observed before the values drop toward the mound base. The mound base itself has relatively high values. In Unit PP2, the magnetic susceptibility is very low, but troughs and peaks in the curves appear cyclic.

Natural gamma radiation

In Hole U1317A on the flank of the mound, 10 NGR cycles have been identified. Toward the crest of the mound, 11 cycles appear (Figs. F22, F23, F24, F25, F26). The peaks in NGR seem to coincide with floatstones and the lows with wackestones containing more coral fragments (see “Lithostratigraphy”). These cycles can be correlated but do not show the same thickness across the holes because of their different locations along the transect from the slope to the top of the mound. Below the mound base in physical property Unit PP2, the values are slightly higher than in Unit PP1 and show higher-frequency variations. The parallel trend of the NGR and the magnetic susceptibility curve in Unit PP1 is remarkable and probably reflects the contribution of clay in the sediments.

Gamma ray attenuation, bulk density, and porosity

Both GRA corrected density and MAD bulk density measurements display parallel trends. MAD measurements were obtained only from Holes U1317A and U1317D. GRA densities deduced from the MST are corrected in the unconsolidated section of the cores in the mound facies according to the equation described in “Physical properties” in the “Methods” chapter. The overall pattern shows a gradual increase in density throughout the core. The largest increase in density is observed at the mound base. In physical property Unit PP1, density decreases from 2 g/cm³ at the surface to 1.7 g/cm³ at ~14 mbsf. Density increases again to 2 g/cm³ at ~20 mbsf. Density values increase gradually to 60 mbsf and remain stable to 100 mbsf. Values increase sharply at the mound base. The upper part of physical property Unit PP2 has a density of ~2.1 g/cm³ and drops at the base to 1.7 g/cm³ ~15 m below the mound. The porosity is calculated to be ~50% in Unit PP1 and ~40% in Unit PP2.

Shear strength

Shear strength measured with the Torvane apparatus in Hole U1317A shows the following trend:

1. An increase from ~0 to ~7 kg/m² in the upper 60 mbsf.
2. A decrease between 60 and 75 mbsf with a high variability between 0–8 kg/m².
3. A drop to 1 kg/m² in a narrow interval of 5 m between 70 and 75 mbsf.

Between 95 and 102 mbsf, the shear strength decreases gradually from 7 to 1 kg/m². The values increase gradually from 1 to 8 kg/m² in the 102–108 mbsf intervals and drop sharply at ~115 and 120 mbsf just above the mound base.

P-wave velocity

Although the acquisition of PWL data was problematic at this site, there are still trends that can be derived from the data obtained and from the PWS data collected in Hole U1317A. The sonic velocity drops to 1600 m/s and remains constant between 20 and 55 mbsf. Between 60 and 80 mbsf, higher velocities have been measured (up to 1800 m/s), which coincide with floatstones (see “Lithostratigraphy”). The velocity then increases toward the mound base to 1750 m/s (Fig. F22). Below the mound base in physical property Unit PP2, the velocity falls to ~1600 m/s (Fig. F25). In the lower part of Unit PP2, clear peaks can be recognized with speeds of ~2300 m/s with values of 1800 m/s between peaks. About eight intervals of high PWS peaks have been identified in Unit PP2.

The P-wave velocity curve parallels the density and NGR of physical property Unit PP2 in Hole U1317D.

Thermal conductivity and in situ temperature measurements

The deployment of the advanced piston corer temperature (APCT) tool provided insight into the subsurface temperature distribution. Three discrete temperature measurements in Hole U1317A were taken at 25.5, 63.5, and 111.0 mbsf (Fig. F27). The temperature measurement at 25.5 mbsf is high in comparison to the other measurements at the site and is considered an outlier. The data show temperature gradients of 50°C/km, which is a normal gradient for a passive continental margin.

In Hole U1317A, thermal conductivity values increase with depth. Thermal conductivity is primarily dependent on variations in sediment bulk density, which is related to other sediment physical properties such as velocity. Thus, these data sets are well correlated (Fig. F28).

Interpretation

Based on the overall trends of the physical properties, only two main physical property units can be identified. Physical property Unit PP1 comprises the mound sedimentological facies (lithostratigraphic Unit 1). It is characterized by cyclicity (10–11 cycles) in NGR and magnetic susceptibility, GRA density, and P-wave velocity. The low values of magnetic susceptibility indicate higher carbonate and lower siliciclastic contents. NGR parallels the magnetic susceptibility with values reflecting increased clay content. Floatstones containing higher clay content and a lower carbonate fraction generally correspond to peaks in the NGR and magnetic susceptibility. Density and P-wave velocity in these intervals are relatively high in comparison to the measurements at the other sites at these depths. The lower boundary of physical property Unit PP1, or mound base, is characterized by an increase in density, NGR, and magnetic susceptibility. The corresponding seismic facies is acoustically transparent, possibly due to the scattering effect of high coral content and the homogeneous seismic character of the sediments. The internal layers identified in the physical property measurements are probably not laterally consistent or do not cause a significant enough change in impedance to give internal reflectors inside the mound facies.

Physical property Unit PP2 is characterized by very low magnetic susceptibility values and eight peaks of high GRA density, P-wave velocity, density, and magnetic susceptibility that coincide with more lithified layers and sandier layers. These layers are correlated with the high-amplitude sigmoid reflectors observed in the seismic profiles.

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, make direct comparisons between different laboratory equipment, and draw potential relationships among the physical property parameters within the described units. The statistical data analysis, including normality tests and confidence intervals for each individual core, assisted in the identification of outliers. Possible erroneous GRA-corrected density values from the uppermost 5 cm of a few cores were identified (<1%) and checked against direct MAD bulk density measurements. These data were filtered out before further processing. A full set of correlation tables and associated graphs were utilized to aid in the identification of the most important physical parameters causing the seismic reflectors. Figure F29 shows a matrix plot of the correlation between the physical parameters in the cores. NGR (influenced by the clay content), density, porosity, shear strength, and magnetic susceptibility were compared with P-wave velocity.

Bulk density data, both from GRA and direct sample measurements (MAD), largely increase with depth throughout the mound and show a very good linear correlation ratio (Pearson’s correlation coefficient [PCC] = 0.75). Both density data sets also correlate well with the P-wave velocities measured on the split cores, except for the interval between 70 and 80 mbsf (Cores 307-U1317A-8H and 9H), where an increase in the sonic velocities, with a peak value of 1840 m/s at ~75 m, is not reflected in the sediment density values, which remain rather constant or slightly decrease with considerable scatter. This increment in sound velocity is reflected in low shear strength values over the interval from 75 to 80 mbsf and low porosity values for the whole interval.

There is a remarkably good correlation between NGR and shear strength throughout physical property Unit PP1 (PCC = 0.5), especially in the uppermost 100 mbsf. In the interval between 86 and 92 mbsf, high NGR values, probably induced by an increase in clay content, are associated with peaks in shear strength values.

Stratigraphic correlation

The holes of Site U1317 were drilled along a depth transect from the upper mound flank to the top of the mound. No splicing was performed at this site to maintain the layer thickness of identified subunits in the mound along the transect that carry important information on mound development. Figure F30 illustrates the lateral continuity of the lithologic layers in the mound by correlation of the NGR records from each hole. This parameter best characterizes the stratigraphic cycles.

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