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doi:10.2204/iodp.proc.310.105.2007

Petrophysics

Recovery at Maraa western transect sites, on the southern side of the island of Tahiti, was low (Hole M0005A = 32%, Fig. F63; Hole M0007C = 36%, Fig. F64), partial (Hole M0005C = 53%, Fig. F65; Hole M0007B = 56%, Fig. F66), and good (Hole M0005B = 74%, Fig. F67; Hole M0005D = 64%, Fig. F68; Hole M0007A = 69%, Fig. F69). Hole M0006A was cored only with hammer coring for microbiology investigation. Multisensor core logger (MSCL) measurements were not possible on the rubble recovered from this hole (Hole M0006A). Cores 310-M0007B-31R and 36R and 310-M0007C-7R were left unsaturated and thus have different data coverage and quality (see the “Methods” chapter for more details). Water depths are as follows: Hole M0005A = 59.13 mbsl, Hole M0005B = 59.13 mbsl, Hole M0005C = 59.63 mbsl, Hole M0005D = 59.63 mbsl, Hole M0005E = 61.34 mbsl, Hole M0007A = 44.45 mbsl, Hole M0007B = 41.65 mbsl, and Hole M0007C = 43.35 mbsl.

Density and porosity

Bulk density at Maraa western transect sites was measured on discrete 1 inch core plugs (MAD) and using the MSCL (GRA). The last deglacial sequence (lithologic Unit I) was recovered in Holes M0005A–M0005C (Figs. F63, F65, F67) where density and porosity values are highly scattered downhole. Density values range from 1.8 to 2.4 g/cm³. The porosity profile also lacks any clear downhole trend with highly variable readings from 20% to 55%. Holes M0007A–M0007C (Figs. F64, F66, F69) have much more continuous data coverage, with densities varying between 1.9 and 2.3 g/cm³ and a mean constant value of ~2.1 g/cm³. Porosity directly follows the density profile, with average porosities of ~35%, a minimum of 20%, and a maximum of up to 55%.

The transition from Unit I to the older Pleistocene sequence (Unit II) is sharp and abrupt (e.g., Hole M0007A, 41 mbsf; Fig. F69) or lacks data coverage because of bad recovery due to poor lithification (e.g., Hole M0007B, 44 mbsf; Fig. F66).

Six distinct intervals are recognized in the older Pleistocene sequence based on density, porosity, and velocity trends. Intervals 2 to 6 were only recovered in Hole M0005D (Fig. F68).

• Interval 1: Holes M0005D (Fig. F68): 23–33 mbsf (Cores 310-M0005D-1R through 6R); M0007A (Fig. F69): 41 mbsf to the bottom of the hole (Cores 310-M0007A-35R through 36R); and M0007B (Fig. F66): 44 mbsf to the bottom of the hole (Cores 310-M0007B-34R through 36R). After a sharp and abrupt increase in density from the overlying last deglacial sequence, density and porosity are constant at 2.4 g/cm³ and ~15%, respectively.
• Interval 2: Hole M0005D (Fig. F68): 33–49 mbsf (Cores 310-M0005D-7R through 16R). Density sharply increases at 33 mbsf, and from 33 to 36 mbsf density continues to increase to 2.4 g/cm³. Variable densities gradually decrease to 1.8 g/cm³, reflecting strong lithification at the top of this interval changing to weak to moderate lithification downcore (~49 mbsf).
• Interval 3: Hole M0005D (Fig. F68): 49–59 mbsf (Cores 310-M0005D-17R through 20R). The underlying unit is composed of massive moderately lithified volcaniclastic sandstone with densities ranging between 1.8 and 2.2 g/cm³. Porosities are equally high, between 35% and 55%.
• Interval 4: Hole M0005D (Fig. F68): 59–71 mbsf (Cores 310-M0005D-21R through 24R). Coralgal frameworks alternate with microbialites and skeletal sand. Density is between 2.0 and 2.4 g/cm³ with a slight decrease toward the bottom of this interval, to ~2.0 g/cm³. Porosity shows a reverse trend, with porosities ranging from 15% to 35% in the top part and averaging 40% at the bottom part of the interval.
• Interval 5: Hole M0005D (Fig. F68): 71–78 mbsf (Cores 310-M0005D-25R through 28R). This interval is poorly recovered, most likely because of poorer lithification, and consists of sandy bioclastic limestone with scattered density observations of ~2.0 g/cm³.
• Interval 6: Hole M0005D (Fig. F68): 78 mbsf to the bottom of the hole (Cores 310-M0005D-29R through 36R). The lowermost interval shows strong diagenetic overprint on sandy coralgal framework and coarse floatstone and rudstone. Large solution cavities are present. Density profile shows continuous high values between 2.1 and 2.5 g/cm³ and porosity varying between 15% and 35%.

MAD measurements are in correspondence with GRA density and porosity values and range from 17% to 42% (average = 24%). Grain density averages 2.72 g/cm³ for lithologic Unit I and increases to 2.74 g/cm³ for lithologic Unit II.

P-wave velocity

P-wave velocities were measured with the MSCL P-wave logger (PWL) on whole cores and the PWS3 contact probe system on discrete 1 inch core plugs.

P-wave velocity profiles in the last deglacial sequence (Unit I) are highly variable with small-scale variations and velocities ranging between 1800 and 4300 m/s (Hole M0007A, Fig. F69; Hole M0007B, Fig. F66). Discrete measurements show similar variation with high velocity values corresponding to dense microbialite matrix sediments (e.g., Hole M0007B, 12 mbsf) and lower velocities in highly porous sand-dominated samples. Samples measured correlate to peaks of velocities in the PWL and sometimes exceed PWL velocities by as much as 300 m/s. The velocity profile in the last deglacial sequence shows that this extreme variation results from the open character of the coralgal-microbialite framework where porosity changes from 50% to 10% occur over a centimeter scale.

Velocity changes abruptly at the lithologic Unit I/II boundary. The velocity profile is constant in the high-velocity domain with values of ~4300 m/s on average and seldom below 4000 m/s (e.g., Hole M0007A, 42 mbsf; Fig. F69). The velocity profile mimics the density and porosity zone subdivision with decreasing velocities from 4200 to 2200 m/s in Interval 2 reflecting the decrease in density and increase in porosity. The massive sandstone in Interval 3 has constant low velocities of ~1800 m/s, as would be expected for moderately lithified sand-size sediments. Interval 4 follows the density profile with values of ~4000 m/s at the top of the interval and a decrease in velocity toward 3000 m/s in the lower part of the profile. Interval 5 shows scattered velocity observations, mostly ~2000 m/s. The last interval, Interval 6, reflects the diagenetic overprint modification toward higher densities also with equally high velocities varying between 3800 and 4300 m/s.

A cross plot of velocity versus porosity for the Maraa western transect shows a general inverse relationship (Fig. F70). For the time-average empirical equation of Wyllie et al. (1956) and Raymer et al. (1980), the traveltime of an acoustic signal through rock is a specific sum of the traveltime through the solid matrix and the fluid phase. Moderately lithified sandstone forms a distinct group in porosity-velocity space, occupying a region of relatively low velocities compared to densities. For carbonates, the general trend appears to be rather linear, with the largest deviations from the general trends between porosities of 0% and 15% and only scattered observations between 35% and 50%. A comparison of the V_P MSCL data with wireline downhole sonic logging data shows clear correlation of high-velocity excursions (Fig. F71). Sonic velocities are ~500 m/s slower, most likely because of scaling effects. Where MSCL measures velocity directly only on matrix sediments, the sonic log provides an average over an interval of 1 ft (~31 cm) in which velocity is averaged over large primary pores containing seawater (~1535 m/s) and rock. Direct shear waves show a range of 800–2800 m/s, closely following trends in compressional wave sonic data.

Magnetic susceptibility

Magnetic susceptibility in the last deglacial sequence is generally <100 × 10^–5 SI units with a few outliers as high as 220 × 10^–5 SI units. No clear downhole trends are observed. Below the boundary with the older Pleistocene sequence, the upper magnetic susceptibility profile changes immediately. Values in Interval 2 are constantly low, <200 × 10^–5 SI units, but in Interval 3 a sharp increase occurs, corresponding to the occurrence of volcaniclastic sandstone. Magnetic susceptibility values range between 0 and 720 × 10^–5 SI units and show small-scale variation. In Intervals 4 and 5, overall magnetic susceptibility values are generally high, ranging between 0 and 500 × 10^–5 SI units, because of the influx of sand and small pebbles of volcanic origin. The lowermost interval, Interval 6, shows continuous high magnetic susceptibility with values above 100 × 10^–5 SI units. The uppermost part of Interval 6 (83–88 mbsf) displays values up to 735 × 10^–5 SI units. Volcanic grains are abundant in this entire interval (see “Sedimentology and biological assemblages”).

Resistivity

The electrical resistivity of rocks depends on several factors, including the presence of conductive minerals such as base metal sulfides or oxides and graphite in the rock. Rocks without these minerals are usually poor conductors, and their resistivities are governed primarily by their porosity, their degree of fracturing, the salinity of the pore water, the degree of saturation of the pore spaces, and, to a lesser extent, the intrinsic minerals that constitute the rock. Diagenetic processes such as silicification and cementation tend to reduce porosity and thus increase the resistivity of the rock. In igneous and metamorphic rocks, the resistivity log is mainly useful for mapping conductive minerals and fracture zones. In sedimentary rocks, the resistivity log is frequently used in lithologic mapping because changes in lithology are often associated with changes in porosity.

During Expedition 310, good resistivity measurements were difficult to obtain. This may have been caused by the following reasons:

• Defective sensor because of entrance of water into the electronics, affecting all sections logged subsequent to Core 310-M0015B-20R (see the “Methods” chapter), and
• Low volume of monomineral rock versus high porosity saturated with seawater, affecting all sections logged.

Rocks are poor electrolytic conductors whose effective resistivity varies with the volume and arrangement of the pores and with the conductivity of the pore fluid. During Expedition 310, all cores were saturated with seawater. When comparing coreline with downhole measurements of electrical conductivity, no clear correlations are visible and absolute values are incomparable. It is thought that drilling disturbances in combination with the low rock:water volume ratios of the measured liners (including the core) resulted in MSCL resistivity values near those of seawater (Fig. F72).

Diffuse color reflectance spectrophotometry

Color reflectance in the last deglacial sequence varies between 40% and 85% L* units. Downhole trends are not present in any borehole. Slightly higher values are present just below the seafloor where modern reef sediment (corals) was recovered. In the interval from Core 310-M0005D-16R to Core 19R within the older Pleistocene sequence, color reflectance has lower L* values of 13%–67% L* units (Fig. F73). This interval corresponds to volcaniclastic sandstone lithofacies. Increases and decreases of L* values in this interval may therefore be indicative of influx of the volcaniclastic grains.

Hole-to-hole correlation

Borehole correlation at the Maraa western transect is straightforward along the Unit I/II boundary. Deeper intervals are only cored in Hole M0005D (Fig. F68), however, and results do not permit any crosshole trends. The top few meters in Hole M0005D (Fig. F68) do not show the same clear petrophysical expression (i.e., high density, low porosity, and high velocity) as in Holes M0007A (Fig. F69) and M0007B (Fig. F66).

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