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doi:10.2204/iodp.proc.314315316.123.2009

Physical properties

Porosity and density

At Site C0001, 638 discrete samples were analyzed for moisture and density (MAD) (Blum, 1997). These were taken in Holes C0001B (15 samples), C0001E (194 samples), C0001F (221 samples), and C0001H (208 samples).

Figure F46A shows porosity versus depth in a composite section of all drill holes at Site C0001. Porosity from discrete samples decreases systematically with depth in lithologic Subunit IA from an average of 63.6% in the uppermost 10 m to an average of 54.7% from 190 to 195 m CSF. Average porosity between ~195 and 205 m CSF (Subunit IB) is 47.7%. The porosity trend is offset in Unit II to higher values and decreases with depth from an average of ~59% at ~213 m CSF to ~50% at the bottom of Hole C0001F (~450 m CSF) (Fig. F46A). Similar porosity trends are evident in data from whole-core multisensor core logger (MSCL) measurements; however, porosity values are highly scattered and larger than those derived from discrete samples (Fig. F46A). If we exclude the outliers at the upper limit of the data set, a corridor of dense data points plots immediately above the data on discrete samples. MSCL-porosity is 60%–70% near the top of the hole and linearly decreases downhole to values between 52% and 62% at the contact with Subunit IC sandy deposits. Porosity values derived from MAD measurements of sand lithologies probably do not reflect true formation index properties because of drilling disturbance of the unconsolidated material. Unit II silty clays and mudstones are offset again to higher values (56%–68% porosity) and then steadily decrease to ~53%–65% near the bottom of Hole C0001F (Fig. F46A). Porosities derived from MSCL data are systematically higher than those from discrete MAD measurements for a variety of reasons, including drilling disturbance and mixing of drilling fluid with the sediment, incomplete filling of the core liner leading to anomalously high void ratios, and gas expansion that generates voids and cracks not present in the undisturbed formation.

Figure F46B 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 densities of discrete samples scatter between ~1.6 and 1.7 g/cm³ just below the seafloor and increase nearly linearly to 1.70–1.75 g/cm³ at 175 m CSF (Fig. F46B), roughly coinciding with the base of lithologic Subunit IA. Subunit IB shows slightly higher wet bulk densities (1.75–1.90 g/cm³). At the top of the Unit II accreted sediments, a shift to lower densities is observed (~1.70–1.80 g/cm³) (Fig. F46B). Bulk densities in the accretionary prism facies show a downhole increase to values ranging between 1.80 and 1.90 g/cm³ at 450 m CSF.

Similar to the porosity data set, bulk densities from the MSCL-W parallel the overall downhole trend for discrete samples but are consistently offset to lower values (Fig. F46B).

Figure F46C shows the grain density trend at Site C0001 versus depth. Average grain density in the upper 195 m (Subunit IA) is 2.71 g/cm³ and increases slightly with depth. Grain density in Unit II shows the same subtle continuous increase with depth. This increase may be attributed to higher clay mineral contents in the accreted strata relative to the overlying hemipelagic mud (i.e., clay with grain densities = ~2.75 g/cm³, quartz = 2.65 g/cm³), the latter of which is more abundant in calcite (see bulk powder XRD analytical results in “X-ray diffraction mineralogy”).

Explanations for the offset in porosity and density between Units I and II (Fig. F46A, F46B) can be divided into two categories: methodological or geological.

Two methodological artifacts cause overestimation of porosity. First, upon oven drying at 105°C, pore water and water contained in some hydrous minerals are removed. In particular, water contained in smectite interlayers is completely removed after oven drying at 105°C for 24 h, at least for mineral compositions generally found in marine sediments (Colten-Bradley, 1987; Brown and Ransom, 1996; Henry, 1997). Because the magnitude of this effect depends on the abundance of hydrous clay, we anticipate a larger porosity overestimate in Unit II than in the overlying slope apron. Second, ESCS and RCB coring may disturb the sediment somewhat and cause a secondary, artificial change in porosity and wet bulk density although samples were carefully chosen from a relatively undisturbed portion.

One possible geological reason for the observed offset in the physical property data is that the consolidation behavior (and therefore compaction state) is highly dependent on sediment grain size distribution and sediment composition. In addition, the accreted material may suffer deformation and a certain degree of lateral shortening in addition to compaction. If this deformation, which is attested by faulting and formation of shear zones (see “Structural geology”), cannot be accommodated by pore space reduction and pore fluid drainage, overpressures may result. Such supra-hydrostatic fluid pressures hinder compaction and are not unknown to accretionary complexes (e.g., see Screaton et al., 1990; Saffer and Bekins 2002). However, given the shallow depth as well as the highly permeable sands overlying the accreted Unit II, it is highly unlikely that these latter materials could support enhanced fluid pressures over significant time spans to counteract compaction.

Shear strength

Results from vane and penetrometer shear strength measurements are presented in Figure F47A. Results obtained in the uppermost 30 m from Hole C0001B (Expedition 314) are included in the plot.

Shear strength generally increases with depth from ~10 kPa at the seafloor to ~150 kPa at 195 m CSF with a clear negative correlation between porosity and strength. Below 195 m CSF, excessive cracking and separation of the core material occurred and the vane shear measurement technique was abandoned. Penetrometer measurements were conducted to a maximum depth of 247 m CSF. Circled measurements on Figure F47A were derived from cores obtained by the ESCS. A high degree of sample disturbance was noted in these cores, and the measured strengths may not reflect actual sediment shear strength values. Penetrometer-based strength exceeds vane-based strength by an average of 37% over a comparable measurement range.

Thermal conductivity

Thermal conductivity measurements were conducted on whole-round HPCS cores (<230 m CSF) with the needle probe and on split samples from RCB cores (>230 m CSF) with the half-space probe.

Thermal conductivities of HPCS cores (<230 m CSF) range from 0.81 to 1.45 W/(m·K). At 200–230 m CSF, core recovery was poor and the sediment disturbed, which resulted in a relatively large variation in thermal conductivity in this depth interval (Fig. F47C). In comparison, thermal conductivities of RCB cores (>230 m CSF) are more tightly constrained, ranging from 1.09 to 1.31 W/(m·K). Uncertainties in the measurements notwithstanding, thermal conductivities generally increase with depth (Fig. F47C).

Color spectroscopy

Color reflectance measurement results are presented in Figure F48. In Subunits IA and IB, values of L* ranged from 30 to 50. Some high values of ~70 correspond to ash layers. Values of a* ranged from –2 to 1, and those of b* ranged from 2 to 6. At the top of this unit, slight decreases in a* and b* values were observed. In Subunit IC, the value of L* is relatively lower than values found in Subunits IA and IB, ranging from 25 to 45. Compared to values in Subunits IA and IB, values of a* are higher, ranging from 0 to 3, and those of b* are lower, ranging from 0 to 4. In Unit II, the value of L* ranged from 30 to 50. Values of a* ranged from –2 to 1, and those of b* ranged from 0 to 4. There is no significant anomaly of these values with depth.

Downhole temperature

Downhole temperature was measured with the APCT3. Starting at a depth of 12 m CSF, measurements were obtained from every third core. In total, seven data points are available to 165.5 m CSF (Table T23). Below this depth, the APCT3 tool was not used when getting close to HPCS refusal in Hole C0001H because this special cutting shoe is not compatible with RCB coring. Data points show a nearly linear increase in temperature with depth, except for one outlier at 123 m CSF. The data delineate a temperature gradient of 0.044°C/m (Fig. F49) when discounting the 6.98°C temperature obtained at 127 m CSF, whereas the gradient would be 0.0043°C/m with this point.

The calculation of heat flow was obtained by using the Excel add-in Program Solver. First total resistance estimated by integration of 1/(thermal conductivity) was used to calculate temperature with the following formula:

T_c = R × Q + T_bw,

where

• T_c = calculated temperature,
• R = total resistance,
• Q = heat flow, and
• T_bw = bottom water temperature.

Thermal conductivity was measured on core samples and heat flow and bottom water temperature are unknown parameters. Heat flow and bottom water temperature were 47 mW/m² and 2.12°C from mean square error minimization. Temperatures were then extrapolated downhole assuming constant heat flow (Fig. F49). Resultant values of heat flow and bottom water temperature were 47 mW/m² and 2.12°C, respectively.

Discrete sample P-wave velocity and electrical conductivity

P-wave velocity and electrical conductivity were measured on discrete samples between 230.19 and 456.83 m CSF. Measurements were performed in three orthogonal directions on cubes. Data tables in Microsoft Excel format are given as supplementary material (see C0001_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, exceptions noted on the log sheets when dipping beds were recognized (see the “Expedition 315 methods” chapter). Variations of electrical conductivity and P-wave velocity with depth are consistent with the evolution of porosity. Porosity and conductivity decrease with depth from 230 to 350 m CSF, whereas P-wave velocity increases, as may be expected from a consolidation trend. From 350 to 400 m CSF, porosity, electrical conductivity, and P-wave velocity remain constant on average. Porosity and electrical conductivity decrease and P-wave velocity increases again from 400 to 456.85 m CSF (Fig. F50). These changes are not correlated with major lithologic changes. Electrical conductivity and P-wave velocity are generally smaller in the z-direction than they are in the x- and y-directions (Fig. F51). However, the anisotropy of the samples in the x, y plane is comparable to the transverse anisotropy (see the “Expedition 315 methods” chapter). This indicates overprinting of the transverse anisotropy resulting from normal basinal compaction by tectonic strain in the accretionary wedge. Shore-based sample reorientation is needed for further interpretation.

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