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doi:10.2204/iodp.proc.310.110.2007

Petrophysics

Recovery at the two Faaa sites, on the northwestern side of the island of Tahiti, ranges from partial (Hole M0019A = 41%; Fig. F36) to good (Hole M0020A = 70%; Fig. F37). The two main units recovered at Faaa represent the last deglacial (lithologic Unit I) and older Pleistocene (lithologic Unit II) sequences. Cores 310-M0019A-23R, 24R, 26R, and 34R were left unsaturated and therefore have different data coverage and quality (see the “Methods” chapter for more details). Water depths are as follows: Hole M0019A = 58.75 mbsl; Hole M0020A = 83.30 mbsl.

Density and porosity

Bulk density at Faaa sites was computed from gamma ray attenuation (GRA) using unsplit cores on the MSCL and from moisture and density (MAD) measurements on discrete plug samples. GRA bulk density measurements were near-continuous downcore for Hole M0020A (Fig. F37), and two intervals were recognized:

• Interval 1: 0–61 mbsf (Cores 310-M0019A-1R through 31R) and 0–36 mbsf (Cores 310-M0020A-1R through 23R): Data are scattered between 1.9 and 2.2 g/cm³.
• Interval 2: 61 mbsf to the bottom of the hole (Cores 310-M0019A-32R through 34R) and 36 mbsf to the bottom of the hole (Cores 310-M0020A-24R through 25R): The general bulk density trend is a gradual increase down to ~39 mbsf. Small-scale variations in density occur downhole between 39 and 42 mbsf; bulk density is rather uniform at ~2.6 g/cm³.

Density data for Hole M0019A (Fig. F36) from 20 mbsf to the bottom of the hole display high variability (note data gaps at 0–20, 30–40, 51, and 54 mbsf), and no distinct trends are identified in the upper sections. Below 61 mbsf, density shows a clear increase to values of ~2.4 g/cm³.

Grain density averages 2.74 g/cm³ and shows no distinct pattern of variability as a function of depth. MAD density is between 1.95 and 2.43 g/cm³ for the last deglacial sequence and increases to 2.60 g/cm³ for the older Pleistocene sequence (e.g., 63 mbsf in Hole M0019A).

Values in Interval 2 are compatible with cementation and karstification below the Unit I/II boundary (see “Sedimentology and biological assemblages”), whereas Interval 1 has highly variable physical properties in the last deglacial sequence. Volcaniclastic input is common, resulting in higher densities (e.g., Hole M0019A from 61 mbsf down). With the exception of a few outliers, GRA bulk density values exceed MAD values by ~0.1 g/cm³.

Porosity profiles generally reflect a combination of stress history and sedimentological and diagenetic effects, such as variability in compressibility, permeability, sorting, grain fabric, and cementation. Porosity for MAD samples is calculated from the pore water content, assuming complete saturation of the wet sediment sample (see “Moisture and density” in the “Methods” chapter). The porosity curve from the MSCL mirrors the bulk density curve, as it is directly calculated from MSCL bulk density with a constant grain density of 2.71 g/cm³ (see the “Methods” chapter). Porosity at Site M0020 varies between 20% and 55%. No distinct decrease of porosity with depth is observed. No abrupt steps in porosity, a common characteristic of erosional unconformities, are observed within Unit I. A sharp decrease of porosity is observed at the transition in Interval 2 between 36 and 39 mbsf for Hole M0020A and 61 mbsf for Hole M0019A, which might indicate increased cementation. Cementation destroys porosity and increases density, one possible effect for diagenetic alteration attendant with subaerial exposure.

For Hole M0020A in Interval 2, low porosities with average values of ~15%–20% are the result of fine- to coarse-grained cemented skeletal limestone. MAD porosities all follow GRA porosities. Deviations (GRA) may occur because of the method of porosity calculation used, which cannot fully account for the multimineral nature of the limestone with volcaniclastic influx.

P-wave velocity

P-wave velocities were measured with the Geotek MSCL P-wave logger (PWL) on whole cores and the PWS3 contact sensor system on a modified Hamilton frame on ~2–4 cm long 1 inch round discrete samples of semilithified and lithified sediments (see the “Methods” chapter). Velocities in one transverse (x) direction were measured on the plugs. Extreme scatter and unreasonable velocities, likely a function of drilling disturbance, bad coupling, and lack of saturation, call into question the quality of the PWL, and data were therefore filtered for realistic values only. For Interval 1 in Hole M0019A, this results in scattered values and a highly discontinuous velocity profile. Only in Interval 2 is the record more continuous, with average velocities of up to 4700 m/s in Hole M0019A. Subsequent to filtering, Interval 1 consists of a group of relatively low velocities of ~1900 m/s and a group of higher velocities of ~3700 m/s (Fig. F36). Discrete measurements range from 3745 to 4782 m/s. For MAD properties, only samples sufficiently lithified and samples free of large pores were suitable for P-wave velocity measurements. As a result, mainly matrix sediments were sampled and the velocities from plug samples are generally located in the high spectrum of velocities. Values are all >3500 m/s and coincide with peaks in velocity measured with the PWL (Fig. F36). Hole M0020A shows a highly discontinuous and variable velocity profile, with the only distinct change in velocities occurring at ~61 mbsf (Interval 2) where velocity increases. Values of discrete measurements range from 3718 to 4705 m/s.

A cross plot of velocity versus porosity for Faaa sites shows a general inverse relationship (Fig. F38). 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. Porosity and velocity data show a poor match with the time-average equation and display large scatter around the general trend line. For a given density of 2.0 g/cm³, velocity may vary as much as 2000 m/s. No sonic logging data are available for Faaa sites.

Magnetic susceptibility

Magnetic susceptibility values are a function of the mineralogy and concentration of magnetic minerals, with higher concentrations of ferromagnetic minerals such as magnetite, hematite, goethite, and titanomagnetite resulting in higher susceptibilities. The source of this material may be associated with influxes of volcaniclastic material. In the absence of ferromagnetic minerals, magnetic susceptibility displays low values induced by paramagnetic and diamagnetic minerals such as clay minerals and evaporites. Natural gamma radiation (NGR) values are a function of the terrigenous clay content within sediment. Clay minerals, being charged particles, tend to attract and bond with K, U, and Th atoms so that an increasing NGR count typically correlates with increasing clay/shale content. Both MS and NGR contain independent information concerning source provenance and magnetic mineral derivation. In clean carbonates, however, NGR is difficult to predict. Red algae are known to incorporate U, and as a result, intervals with dominant rhodoliths may show increased uranium response.

As described in “Density and porosity,” magnetic susceptibility measured at Faaa sites can be divided into two intervals with distinct patterns.

• In Interval 1, magnetic susceptibility is characterized by nearly constant amplitude from 0 to a maximum of 50 × 10^–5 SI units.
• Interval 2 shows low recovery but generally low susceptibility except for a few outliers at ~3 mbsf (Hole M0019A). Interval 2 shows increased values ranging from 0 to 250 ×10^–5 SI. It is inferred that Interval 2 represents times of enhanced terrigenous input in Pleistocene times, whereas the last deglacial sequence at the Faaa sites has very little volcaniclastic material admixed.

Resistivity

See “Resistivity” in the “Maraa western transect” chapter.

Diffuse color reflectance spectrophotometry

Color reflectance in the last deglacial sequence shows a variation of 38 to 86 L* units (Fig. F39). Downhole trends are not present. High values of L* (>70 L* units) can be observed in the interval 32–35.4 mbsf (Sections 310-M0020A-22R-2 through 23R-2). These high values were measured on an interval of massive Porites. Lowest L* values (40–61 L* units) were measured on grainstone-dominated lithologies containing volcaniclastic grains. These lithologies are found at the base (53–55 mbsf) of the last deglacial sequence (Cores 310-M0019A-25R through 26R).

In the Cores 310-M0019A-33R through 34R (below 62.4 mbsf) within the older Pleistocene sequence, color reflectance has a relatively lower L* value of 43–79 L* units (average = 57 L* units). This interval corresponds to rudstone with volcaniclastic grains (including basalt pebbles).

Hole-to-hole correlation

Hole-to-hole correlation in carbonate reef environments is not an easy task. Heterogeneity in physical properties is large, and often only major unconformities can be identified. At Faaa, the best indication for correlation comes from magnetic susceptibly values together with abrupt changes in density, porosity, and velocity.

For example, Figure F37 shows an increase in magnetic susceptibility below 36 mbsf in Hole M0020A. In Hole M0019A, there is an increase in susceptibility at ~48 mbsf, although the exact change from low to high magnetic susceptibly probably occurs higher than this, within the poorly recovered section.

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