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doi:10.2204/iodp.proc.310.106.2007

Petrophysics

Recovery at Maraa eastern transect sites, on the southwestern side of the island of Tahiti, was partial (Hole M0016A = 56%, Fig. F62; Hole M0016B = 51%, Fig. F63; Hole M0017A = 56%, Fig. F64; Hole M0018A = 61%, Fig. F65) and good (Hole M0015A = 72%, Fig. F66; Hole M00015B = 71%, Fig. F67). Cores 310-M0015A-12R, 14R, 15R, 16R, and 38R, 310-M0015B-28R, 310-M0016A-02R, 310-M0017A-04R, and 310-M0018A-19R were left unsaturated and therefore have different data coverage and quality (see the “Methods” chapter for more details). Water depths are as follows: Hole M0015A = 72.15 mbsl, Hole M0015B = 72.30 mbsl, Hole M0016A = 80.85 mbsl, Hole M0016B = 80.35 mbsl, Hole M0017A = 56.34 mbsl, and Hole M0018A = 81.80 mbsl.

Density and porosity

Two methods were used to evaluate bulk density at Maraa eastern transect sites. Gamma ray attenuation (GRA) on the MSCL provided an estimate of bulk density from whole cores. Discrete moisture and density (MAD) samples facilitated a second, independent measure of bulk density and provided grain density and porosity data.

From 0 to 37 mbsf (e.g., Cores 310-M0015A-1R through 37R and 310-M0015B-1R through 37R; last deglacial sequence [lithologic Unit I]), density ranges between 1.9 and 2.4 g/cm³. The downhole profile shows small-scale density variations, which are directly reflected in porosity. Porosity ranges between 20% and 50%. In some cases, MAD values deviate as much as 0.5 g/cm³ from GRA bulk density (Fig. F67). This deviation is most likely due to the presence of large pores in the core area over which the density is calculated. Below 37 mbsf (Cores 310-M0015A-37R through 41R and 310-M0015B-38R; older Pleistocene sequence [lithologic Unit II]), MAD bulk density correlates with maximum GRA values. The high GRA bulk densities are always >2.1 g/cm³ with a maximum of 2.45 g/cm³, corresponding to the diagenetically altered older Pleistocene sequence (see “Older Pleistocene sequence [Unit II]”). As a result of diagenesis, porosities are toward lower values in the range of 10%–25% with few outliers to 35%, corresponding to local intercalations of gravel and sand layers. None of the older Pleistocene sequence was recovered in Hole M0016A (Fig. F62).

Grain density has an average value of 2.73 g/cm³ and considerable scatter from 2.67 to 2.80 g/cm³. Values <2.67 g/cm³ are rare and may be the result of grain volume measurement error.

Porosity, calculated from MAD data (see “Moisture and density” in the “Methods” chapter), ranges from 47% at, for example, 8 mbsf (Fig. F62) to <9% at, for example, 37 mbsf (Fig. F66). The lowest porosity (6%–8%) occurs in the hardground between at the last deglacial–older Pleistocene sequence transition (Holes M0015A and M0015B, 37 mbsf; Figs. F66, F67).

P-wave velocity

P-wave velocities were provided through multisensor track (MST) P-wave logger (PWL) on whole cores and discrete measurements on core plugs. In Unit I, the velocity profile is highly scattered, reflecting small-scale variability in velocity as a result of porosity changes. Values range from 1800 to 4900 m/s and show no clear downhole trends. Discrete measurements confirm excursions in velocities toward the higher velocity spectrum. The older Pleistocene sequence shows distinct higher velocities with values mostly >4000 m/s. In Hole M00015A (Fig. F66), at 36–37 mbsf, this hardground shows constant velocities >4500 m/s, indicating tight and dense matrix properties. Discrete measurements reveal velocities as high as 5212 m/s (Hole M0015B, 38 mbsf; Fig. F67). A cross plot of velocity versus porosity for the Maraa western transect sites shows a general inverse relationship (Fig. F68). 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. The general trend appears to be approximately linear, with the largest deviations between porosities of 0% and 15% and scattered observations in the high-porosity domain. Downhole sonic logging data for Hole M0017A (Fig. F64) are available for the interval 15–27 mbsf and display a highly variable velocity profile in correlation with scattered V_P MST observations. This profile correlates with the high abundance of macro pores that makes the interval difficult to core intact; the interval velocities tend to be lower because of the presence of seawater (~1535 m/s) making up to 50% of the medium through which the acoustic wave propagates, which results in a decrease in overall “averaged” velocity values (Fig. F69).

Magnetic susceptibility

Magnetic susceptibility values in the last deglacial sequence are generally low but are still higher than expected for clean carbonates. Values rarely exceed 100 × 10^–5 SI units. The largest excursions can be found in the uppermost 10 mbsf (e.g., Holes M0015A, M0015B, and M0016A; Figs. F62, F66, F67). The zone above the transition to the older Pleistocene sequence appears to contain very few magnetizable minerals with a magnetic susceptibility maximum of ~30 × 10^–5 SI units (e.g., Hole M0018A, 30–33 mbsf; Fig. F65). This same zone can be observed in Hole M0017A at 31–35 mbsf (Fig. F64). The older Pleistocene sequence shows an increase in magnetic susceptibility but only in individual excursions as high as 220 × 10^–5 SI units. Average response is ~50 × 10^–5 SI units.

Resistivity

See "Resistivity" in the "Maraa western transect" chapter.

Diffuse color reflectance spectrophotometry

The downhole trend in Holes M0015A and M0015B (Figs. F66, F67) is a slight increase in L* values from 55 L* units (average = shallower than 10 mbsf) to 62 L* units (average = deeper than 32 mbsf). This trend reflects lithological changes from gray microbialite-dominated lithofacies in the last deglacial sequence to light yellowish/brown rudstone lithofacies in the older Pleistocene sequence. Many peaks of high L* values (up to 80 L* units) in the interval 9.1–11.8 mbsf originate from an above average abundance of corals, such as massive Porites (Fig. F70).

No clear downhole trends are observed in the last deglacial sequences of Holes M0016A and M0017A (Figs. F62, F64). On the other hand, in Hole M0016B (Fig. F63), L* values (up to 80 L* units) decrease from the lower part to the middle part of the last deglacial sequence. This trend corresponds to lithological changes from coralgal framework–dominated lithofacies (including some massive Porites) to microbialite-dominated lithofacies. The decrease of L* values observed at 49.2 mbsf in Hole M0016B (Fig. F63) corresponds to the unconformity between coralgal bindstone in the last deglacial sequence and rudstone in the older Pleistocene sequence.

Downhole variations in L* values are observed in the last deglacial sequence of Hole M0018A (Fig. F65). Three “cycles” are observed that correlate with lithological changes from coralgal framework–dominated (including Porites and Acropora) to microbialite-dominated lithofacies.

Hole-to-hole correlation

Borehole correlation patterns at Maraa eastern transect sites do not have obvious magnetic susceptibility responses. Density, porosity, and velocity profiles are scattered in the last deglacial sequence and do not permit direct correlation. One clear marker is the generally low response in the 4–5 m directly overlying the older Pleistocene sequence. The main correlation surface is the last deglacial–older Pleistocene sequence transition, which centers around 39 mbsf in most boreholes except for Hole M0015A, where the transition can be found at 36 mbsf.

Site-to-site correlation

The last deglacial–older Pleistocene sequence transition, a sharp and abrupt unconformity, is the key correlation surface through all boreholes. Above this transition, the last deglacial sequence comprises an open coralgal-microbialite framework with highly variable density, porosity, and velocity values. Correlation of these properties has proven to be difficult. Magnetic susceptibility correlates well within sites, but over long distances it does not permit site-to-site correlation. The transition is characterized by a sharp and abrupt increase in density and velocity and a decrease in porosity. Subaerial exposures have altered the upper few meters of this sequence, implied by the occurrence of cement crusts, infillings of karst features with younger sediments, and other diagenetic features (see “Sedimentology and biological assemblages”). The depth of this transition is not constant. At Maraa eastern transect ridge sites, this level is present at ~120 mbsl for most sites except for the shallower Hole M0017A (Fig. F64), where the unconformity occurs at 93 mbsl. At Maraa western transect sites, this transition occurs at ~87 mbsl for all boreholes. This would indicate that the last deglacial sequence was deposited on an irregular topography with the underlying older Pleistocene sequence shallowing landward.

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