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doi:10.2204/iodp.proc.314315316.124.2009

Physical properties

Porosity and density

At Site C0002, a total of 492 discrete samples were analyzed for MAD. Figure F28A shows porosity evolution at Site C0002 versus depth. Porosity from discrete samples decreases with depth in Unit I, and the average is 60.7%. Porosity decreases slightly with depth in Unit II, from ~50% at 480 m CSF to ~40% at 820 m CSF. Porosity increases slightly near the top of Unit III to ~40% and increases slightly with depth within Unit III to ~42–43% at its base. In the upper ~100 m of Unit IV, porosity decreases with depth from values scattered between 31%–42% at its top to 28%–40% at 1020 m CSF but exhibits a trend of increasing porosity with depth between 1020 m CSF and the bottom of the hole.

Figure F28B shows bulk density on discrete samples and derived from MSCL measurements on whole-round cores (MSCL-W; see the “Expedition 315 methods” chapter for details). Bulk density of the discrete samples scatters between ~1.5 and 2.2 g/cm³ throughout all the units. The bulk density trend mirrors that of porosity. In Unit I, density increases with depth (average = 1.67 g/cm³). In Unit II, density increases slightly with depth (average = 1.95 g/cm³) and then decreases with depth within Unit III (average = 2.00 g/cm³). In Unit IV, density increases with depth above 1020 m CSF and then decreases with depth below 1020 m CSF. The average bulk density in this unit is 2.05 g/cm³.

Figure F28C shows grain density at Hole C0002B versus depth. Average grain density in Unit I is 2.68 g/cm³. Average grain density in Unit II is 2.68 g/cm³ and increases slightly with depth. Grain densities in Unit III and IV are nearly constant with depth, with average values of 2.70 and 2.68 g/cm³, respectively.

The observed discontinuities in porosity and bulk density (e.g., at the Unit III/IV boundary), as well as the reversal of the compaction trend within Unit IV (1020–1050 mbsf), may be explained by a variety of processes. As was the case for Site C0001, these fall into two categories, methodological and geological. There are two main methodological effects that can lead to overestimates of porosity: (1) hydrous minerals dehydrate during shipboard oven-drying (105°C for 24 h), leading to overestimates of porosity in zones with abundant opal or hydrous clays, and (2) drilling disturbance and remolding, despite the fact that samples were carefully chosen from relatively undisturbed portions of the core. There are also two geological processes that could affect porosity: (1) Sediment composition (mineralogical composition, grain size distribution, and degree and type of cementation) are known to affect sediment consolidation behavior, which could result in differences in porosity (and density) trends, and (2) tectonic compaction in the accretionary prism (Unit IV) could cause additional compaction consolidation, and therefore lower porosity than in Units II and III.

Shear strength

Results from penetrometer shear strength measurements are shown in Figure F29A. Shear strength data are highly scattered and do not show any significant trend. No reliable correlation with porosity could be found (Fig. F29B). Although sample depths at this site are deeper than those at Site C0001, shear strengths are significantly lower than those at Site C0001. RCB drilling may also induce a high degree of sample disturbance. Taking these possibilities into consideration, we consider all shear strength data from Hole C0002B to be unreliable.

Thermal conductivity

Thermal conductivity measurements were conducted on whole-round cores (<555.06 m CSF) with the needle probe and on split samples from cores from depths >562.27 m CSF with the half-space probe.

Thermal conductivity increases with depth in lithologic Units I and II to 645 m CSF. This increase may be fit with a linear regression:

k = 0.954 + (5.78 × 10^–4)z

with z in meters and k in W(m·K).

Below 645 m CSF, in lithologic Units II, III, and IV, thermal conductivity remains constant at 1.527 ± 0.083 W(m·K). The cause of the transition is unclear, as it does not correlate with a major lithologic change or with a discontinuity in MAD results. However, this depth corresponds to the depth at which it became possible to cut sample cubes for P-wave velocity and electrical conductivity measurements.

Color spectroscopy

Results from color reflectance measurements are presented in Figure F30. No significant difference in L*, a*, and b* values were observed throughout all the units. L* ranges from 30 to 50, a* ranges from –3 to 2, and b* ranges from 1 to 4.

Downhole temperature

In Hole C0002D, eight in situ temperature measurements were conducted with the APCT3 and six were successful. Best fit equilibrium temperatures and root mean square misfits are given in Table T19. Based on examination of the misfit function, uncertainty on the extrapolated equilibrium temperature is less than ±0.05°C for all measurements, except the first one (Core 315-C0002D-2H), which has a wider range of acceptable values from 2.6° to 3°C. Linear regression of all temperature data yields a gradient of 40°C/km (Fig. F31). Thermal resistance was computed by depth integration of measurement on cores down to 1047.2 m CSF with thermal conductivity data from Holes C0001B and C0001D. A mean square fit of temperature versus thermal resistance yields a heat flow of 39.7 mW/m². The thermal conductivity in the data gap between 201.9 and 485 m was interpolated using the linear regression defined in the thermal conductivity subsection and temperatures extrapolated downhole assuming a 39.7 mW/m² constant heat flow (Fig. F31).

Discrete sample P-wave velocity and electrical conductivity

P-wave velocity and electrical conductivity were measured on discrete samples between 622.89 and 1051.32 m CSF. Measurements were performed in three orthogonal directions on cubes. Data tables are given as a supplementary material (see C0002_DS_VP_EC.XLS in 315_PHYS_PROPS in “Supplementary material”). Cubes were generally taken such that cube axes x, y, and z correspond to the core referential. There were, however, a few exceptions, noted on the log sheets, when dipping beds were recognized (see the “Expedition 315 methods” chapter). Overall, electrical conductivity decreases with depth (Fig. F32A), but a reversed trend is observed between 987.61 m and 1051.32 m CSF. Porosity also increases in this interval. However, P-wave velocity (V_P) increases in this interval (Fig. F32A). This suggests that the increase in porosity and conductivity is more likely caused by intrinsic sediment properties than by a reversed consolidation trend. P-wave velocity could only be measured on four samples in the uppermost part of lithologic Unit IV, from 930 to 1000 m CSF, and these values are remarkably scattered, ranging from 1840 to 5550 m/s. The 5550 m/s value was measured on a hard sandstone piece, which is not representative of the formation. However, the P-wave measurements obtained on these four samples suggests the high reflectivity of the top of the old accretionary complex on the seismic profile may be due to high impedance contrasts at short wavelengths. Electrical conductivity and P-wave velocity is systematically lower in the z-direction than in the x- and y-directions in lithologic Units II and III. This anisotropy is presumably caused by compaction in the forearc basin. As was observed at Site C0001, the difference between measurements in z- versus x- and y- directions is relatively smaller in the accretionary wedge (Unit IV). This may result from a more complex strain history in the accretionary wedge than in the slope and forearc basin sediments. Understanding the anisotropy signal requires postcruise paleomagnetic reorientation of all samples. Samples are currently stored at Kochi core center.

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