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doi:10.2204/iodp.proc.310.109.2007

Petrophysics

Recovery at Tiarei marginal sites, on the northeastern side of the island of Tahiti, was mostly low (Hole M0008A = 24%, Fig. F9; Hole M0010A = 30%, Fig. F10; Hole M0012A = 25%, Fig. F11; Hole M0013A = 11%, Fig. F12) and partial (Hole M0011A = 49%, Fig. F13; Hole M0014A = 46%, Fig. F14; Hole M0022A = 57%, Fig. F15). Cores 310-M0010A-1R, 2R, 3R, and 10R, 310-M0012A-2R, 310-M0013A-3R, 310-M0014A-5R, 6R, and 10R, and 310-M0022A-2R, 3R, and 4R were left unsaturated and therefore have different data coverage and quality (see the “Methods” chapter for more details). Water depths are as follows: Hole M0008A = 62.65 mbsf, Hole M0010A = 89.53 mbsl, Hole M0011A = 101.34 mbsl, Hole M0012A = 77.05 mbsl, Hole M0013A = 90.55 mbsl, Hole M0014A = 99.25 mbsl, Hole M0022A = 117.54 mbsl.

The units recovered from these sites are dominated by volcaniclastic clay and sand and basalt gravels, pebbles, and cobbles; the last deglacial sequence (lithologic Unit I) is closely associated and interlayered with volcaniclastic sediments; the older Pleistocene sequence (lithologic Unit II) comprises coralgal skeletal sandy limestone locally abundant in volcanic grains.

As recovery was generally low and recovered intervals were often disturbed by drilling or left unsaturated because of the un- to semilithified character of the deposits, MSCL observations are highly discontinuous. No discrete sample measurements are available for Holes M0008A and M0013A (Figs. F9, F12).

Density and porosity

Bulk density at Tiarei marginal sites was measured with gamma ray attenuation (GRA) on the MSCL and on discrete samples (MAD). Hole M0008A (Fig. F9) only recovered volcaniclastic sediments and basalt (see “Sedimentology and biological assemblages”). Volcaniclastic sands were semilithified and have low densities of ~1.8 g/cm³. The one basalt section cored, however, has a high density of 3.0 g/cm³. Porosity calculations reveal high porosities up to 60% for volcaniclastic sands. This has to be interpreted with care, however, because porosity is calculated using a constant grain density of calcium carbonate: 2.71 g/cm³. The basalt section is very low in porosity and records values near zero.

At Sites M0010–M0014, the last deglacial and older Pleistocene sequences are composed of reef rocks with interbedded volcaniclastic sediments. Densities for the last deglacial interval are variable but concentrate around 2.0 g/cm³ with a minimum of ~1.8 g/cm³ and a maximum of 2.2 g/cm³. The older Pleistocene sequence is characterized by a step increase in density toward an average value of ~2.2 g/cm³. Densities in this interval are variable but are generally >2.0 g/cm³ and are never >2.4 g/cm³. Porosity in both intervals ranges between 20% and 45%, but is ~30% on average. Grain density averages 2.76 g/cm³, and MAD values range from 1.87 to 2.46 g/cm³ with two unreliable outliers of 1.43 g/cm³ (Hole M0012A; 30.17 mbsf) and 3.9 g/cm³ (Hole M0012A, 29.89 mbsf; Fig. F11).

In Hole M0022A, mainly volcaniclastic sediments were recovered, which show consistent density values of 2.2 g/cm³. Porosity is 30% overall but ranges from 20% to 45%. Discrete density measurements range from 2.90 to 3.06 g/cm³, and high porosities of 29% to 41% were measured in this lithology (Fig. F15).

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. P-wave velocity profiles in the last deglacial sequence and in volcaniclastic-dominated holes are highly discontinuous because of low recovery and lack of saturation. Hole M0008A contains intervals with PWL-measured sections that consist predominantly of volcaniclastic sand with low velocities of ~1800 m/s, which can be expected for an unconsolidated sand (Fig. F9). The one basalt section recovered (Core 310-M0008A-8R) has high velocities of ~5700 m/s, which is near the matrix velocity for the minerals making up the bulk of this rock, olivine and pyroxene, and agrees well with previous fresh unweathered seafloor basalts, 5940 m/s (Hyndman and Drury, 1976).

Only the older Pleistocene sequence has generally better recovery, showing consistent velocities of ~4000 m/s (e.g., Cores 310-M0012A-13R through 19R). Discrete velocity measurements range from 3107 to 4741 m/s and show no clear downhole trends. No sonic logging data are available for Tiarei marginal sites.

Magnetic susceptibility

The nearby Tiarei River delivers a substantial amount of weathered volcaniclastic material to the reef system at Tiarei. Bathymetry maps reveal a complex system of river discharge over the submerged part of Tahiti Island (Hildenbrand et al., 2006). At Tiarei, this influx of volcaniclastic material produces a complex pattern of interlayered volcaniclastic sediments and carbonate reef intervals. Holes M0008A and M0022A have no reef material at all and wholly consist of volcaniclastic material (Figs. F9, F15). Magnetic susceptibility in Hole M0008A is generally high with a maximum up to 2550 × 10^–5 SI units (Fig. F9). Volcaniclastic sediments were recovered in Hole M0022A and show an overall high magnetic susceptibility response with values up to 1650 × 10^–5 SI units and generally between 200 and 400 × 10^–5 SI units (Fig. F15). Sites M0010–M0014 have generally low recovery in the last deglacial sequence but show generally high values for magnetic susceptibility, with values up to 700 × 10^–5 SI units and lower limits above 200 × 10^–5 SI units. The older Pleistocene sequence has overall lower magnetic susceptibility, with values <200 × 10^–5 SI units (e.g., Cores 310-M0010A-16R through 20R), but locations with up to 500 × 10^–5 SI units occur (e.g., Hole M0012A; 30 mbsf).

Resistivity

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

Thermal conductivity

Thermal conductivity measurements were carried out on suitable materials comprising sand, silt, and muddy sediment from Sites M0008 and M0011–M0013 in the Tiarei area and Site M0005 in the Maraa area. Mean values are low and range between 0.9 and 1.3 W/(m·K). This range lies just below the range for silt (1.4–2.1 W/[m·K]). Low values obtained from Tahiti sediments may represent the effect of the mixing of lithologies or complications from measurement of loosely consolidated sediments. Unconsolidation also causes difficulties in obtaining good thermal coupling between sediment and probe (see the “Methods” chapter).

Diffuse color reflectance spectrophotometry

Color reflectance data in the last deglacial sequence in each hole are highly discontinuous (Fig. F16) because of low recovery. Color reflectance values are low (15–41 L* units) in all cores in Hole M0008A, which consists only of volcanic sandstone with basalt gravels (Fig. F9). Hole M0010A shows a general increase of L* values from top to bottom (from 18 to 38 L* units) (Fig. F10). This trend reflects the lithological change from volcaniclastic sand-dominated lithologies in the upper part (e.g., Cores 310-M0010A-1R through 3R) to coralgal framework–dominated lithologies facies in the basal part (Section 10R-1, 0–100 cm). Low L* values (17–24 L* units in Cores 310-M0011A-2R and 3R) in the upper part of Hole M0011A originate from basalt pebbles (Fig. F13). In the last deglacial sequence of Hole M0012A, low L* values (26–52 L* units) in Cores 310-M0012A-2R, 7R, 12R, and 13R correspond to terrigenous sand to silt, volcaniclastic sand, and basalt pebbles (Fig. F11). On the other hand, high L* values (63–76 L* units) in Cores 310-M0012A-9R and 11R correspond to the occurrence of massive Porites corals.

Color reflectance (L*) trends in Hole M0014A show a similar pattern to that observed in Hole M0012A (Figs. F11, F14). In Hole M0014A, however, the abrupt downward shift corresponds to the disconformity between coral rudstone in the last deglacial sequence and Halimeda packstone with volcaniclastic sand in the older Pleistocene sequence (Section 310-M0014A-13R-1, 50 cm; 18.78 mbsf), where L* values shift from 50 to 24 L* units.

Color reflectance patterns are equal for the basal part of the last deglacial sequence and the upper part of the older Pleistocene sequence in Holes M0010A, M0011A, and M0012A (Figs. F10, F11, F13, F16), although lowest L* values can be observed in some intervals containing basalt pebbles. In Hole M0022A, L* values (average = 19–58 and 31 L* units) are low because basalt and volcaniclastic siltstone are the dominant lithofacies (Fig. F15).

Hole-to-hole correlation

Borehole locations at Tiarei marginal sites are not directly related and form a group of holes that do not directly correspond to specific assembly of target locations in the reef system. In addition, overall recovery at this site averaged 34%, which makes correlation difficult. Hole-to-hole correlation is therefore not possible.

Site-to-site correlation

The last deglacial/older Pleistocene sequence boundary, a sharp and abrupt unconformity, is the key correlation surface through all boreholes. Above this boundary, the last deglacial sequence is composed of open coralgal-microbialite framework with highly variable density, porosity, and velocity values. Correlation using these properties proves to be difficult. Magnetic susceptibility allows good correlation within sites, but it does not permit site-to-site correlation. The boundary is characterized by a sharp and abrupt increase in density and velocity and a decrease in porosity. Emergence and subaerial exposure have altered the upper few meters of this sequence, confirmed by cement crusts, infillings of karst features with younger sediments, and other diagenetic features (see “Sedimentology and biological assemblages”). The depth of the boundary is not constant. At Tiarei outer ridge sites, the boundary is present at ~118 mbsl. At Tiarei inner ridge sites, the boundary occurs at ~94 mbsl. This would indicate that the last deglacial sequence was deposited on an irregular antecedent topography.

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