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doi:10.2204/iodp.proc.310.108.2007 PetrophysicsRecovery at Tiarei outer ridge sites, on the northeastern side of the island of Tahiti, was partial (Hole M0009A = 43%, Fig. F57; Hole M0009C = 51%, Fig. F58; Hole M0009D = 54%, Fig. F59) and good (Hole M0009B = 66%, Fig. F60; Hole M0009E = 72%, Fig. F61; Hole M0021A = 74%, Fig. F62; Hole M0021B = 65%, Fig. F63; Hole M0024A = 83%, Fig. F64; Hole M0025A = 74%, Fig. F65; Hole M0026A = 71%, Fig. F66; Hole M0025B = 58%, Fig. F67). Cores 310-M0021A-16R, 310-M0021B-5R, 310-M0024A-15R and 16R, 310-M0025A-11R, 12R, and 13R, and 310-M0025B-12R and 13R were left unsaturated and therefore have different data coverage and quality (see the “Methods” chapter for more details). Water depths are as follows: Hole M0009A = 99.71 mbsl; Hole M0009B = 100.31 mbsl; Hole M0009C = 99.85 mbsl; Hole M0009D = 103.18 mbsl; Hole M0009E = 94.94 mbsl; Hole M0021A = 82.30 mbsl; Hole M0021B = 81.70 mbsl; Hole M0024A = 90.44 mbsl; Hole M0025A = 105.40 mbsl; Hole M0025B = 100.84 mbsl; and Hole M0026A = 107.30 mbsl. The last deglacial sequence (lithologic Unit I) ranges in thickness from 10 to 30 m on the outer ridge of Tiarei, depending on the water depth at which the holes were drilled. The last deglacial sequence was deposited on an irregular topography as confirmed by different depths of the last deglacial–older Pleistocene sequence (Unit I/II) transition/unconformity (see “Sedimentology and biological assemblages”). Density and porosityAt Tiarei outer ridge sites, gamma ray attenuation (GRA) bulk density ranges between 1.5 and 2.4 g/cm3 and discrete sample moisture and density (MAD) bulk density ranges between 1.9 and 2.76 g/cm3 in the last deglacial sequence. MAD density values can be as much as 0.3 g/cm3 higher than the maximum GRA density values. This difference is the result of intracore spaces and undersized cores. MAD density increases to 2.35 g/cm3 for the older Pleistocene sequence (e.g., from 21 mbsf in Hole M0009B; Cores 310-M0009B-15R through 18R). In all boreholes, density profiles show large variation downhole with an abrupt increase at the Unit I–Unit II transition. Lower density readings correspond with large primary porosity preserved (e.g., Core 310-M0009B-6R). Two specific intervals can be recognized in the GRA measurements for Unit I:
In Holes M0021A (Fig. F62), M0021B (Fig. F63), and M0026A (Fig. F66), a relationship between density and lithologic units is less apparent. Around 12 mbsf (Hole M0021A; Fig. F62), density decreases gradually from 2.3 to 1.9 g/cm3, with an average MAD density of 2.19 g/cm3. Grain density values average 2.75 g/cm3, range between 2.70 and 2.84 g/cm3, and do not show any clear downhole trend. Porosity values range from 10% to 50% with few outliers above this maximum, most likely in areas where core is undersized with respect to the liner. The porosity profile with depth mirrors the bulk density profile. Porosity is highly variable at the top of the borehole in Interval 1, 30% on average, and increases to 40% on average for Interval 2. The overall porosity profile shows no clear downhole trend, suggesting that porosity is mainly controlled by large-scale primary porosity. At the Unit I–Unit II transition, a step in density is observed toward higher values. Average density lies at ~2.3 g/cm3 for this interval with equally lower porosities of ~20% on average. Both density and porosity show downhole variation with porosity values between 10% and 30% with outliers toward 50%. No clear subdivision in lithologic units, as recognized in “Sedimentology and biological assemblages,” is apparent in core log data. P-wave velocityP-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 are highly discontinuous as result of low recovery (e.g., Hole M0009A; Fig. F57) or drilling disturbance. Values range from 1700 to 4700 m/s and show no clear downhole trend. Velocity inversions are sharp and abrupt and occur commonly. Discrete measurements range from 2453 to 4746 m/s. A sharp increase in velocity occurs at the lithologic Unit I/II boundary. Velocity profiles are more continuous and less variable in Unit II as result of better cementation and subsequent better recovery. Velocities are generally between 4000 and 5000 m/s with a few lower values corresponding to lower density values (e.g., Hole M0009E; Fig. F61). Discrete samples generally correspond to zones of high velocity with a maximum of 5287 m/s (Hole M0024A, 29 mbsf; Fig. F64). Velocity values increase with increased density and decreased porosity. A cross plot of velocity versus porosity for Tiarei outer ridge 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. However, porosity and velocity data do not match the time-average equation but show 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. This may have two causes:
Comparison of MSCL velocity data with downhole sonic logging data shows scattered MSCL VP data mostly corresponding with high-velocity excursions in the downhole data (Fig. F69). This indicates that mainly matrix, cored-rock intervals have been directly measured with MSCL with an underrepresentation of slower sediments or rock intervals with extreme high macropores. The latter deliver slow velocity data because of the large contribution of open pore space filled with seawater (~1535 m/s). Magnetic susceptibilityMagnetic susceptibility at this site shows a clear subdivision into three intervals corresponding to those given in “Density and porosity.” The upper two intervals correspond to the last deglacial sequence. Interval 3 corresponds to the older Pleistocene sequence. Interval 1 has highly variable magnetic susceptibility with values from 0 to a maximum of 750 × 10–5 SI units. Variations occur over short distances, and individual peaks of locations with a high influx of volcaniclastics have pronounced signatures with spikes toward higher values. Interval 2 reveals variable but lower magnetic susceptibility values, averaging ~200 × 10–5 SI units with lower limits of 0 units and maxima up to 400 × 10–5 SI units. Interval 3 is made up of the older Pleistocene sequence and has a highly variable contribution of magnetic minerals with values ranging from 0 to a maximum of 750 × 10–5 SI units. At most sites, the transition from Interval 1 to Interval 2 is sharp (e.g., Hole M00025A, 6 mbsf; Fig. F65). In Hole M0024A (Fig. F64), in contrast, the change occurs at ~10.5 mbsf but shows a gradual decrease between 10.5 and 20 mbsf toward values lower than 200 × 10–5 SI units. In Holes M0021A (Fig. F62) and M0021B (Fig. F63), the subdivision is less pronounced with higher values in the upper part of the last deglacial and slightly lower values, on average 200 × 10–5 SI units, toward the lower part of the unit. The higher magnetic susceptibility values may be attributed to the proximal location of the Tiarei River, which brings in higher concentrations of magnetic minerals within generally fine to coarse sand-sized volcaniclastic material. Through time, however, the course of the river and the amount of influx of volcaniclastic material is expected to have varied, causing the trends observed at these sites. ResistivitySee “Resistivity” in the “Maraa western transect” chapter. Diffuse color reflectance spectrophotometryNo clear downhole trends are observed in Unit I in any hole drilled in this area (Fig. F70). Spikes of higher values (up to 91 L* units) characterize coral skeletons and calcareous algae (e.g., coralline algae). Color reflectance values of the older Pleistocene sequence are generally lower in L*, which may be indicative of influx of terrigenous clastics such as basaltic grains. In Cores 310-M0009A-16R through 18R (below 18.6 mbsf), 310-M0024A-16R (below 30.8 mbsf), 310-M0025A-12R through 13R (below 30.8 mbsf), and 310-M0025B-13R (below 19 mbsf) of the older Pleistocene sequence, color reflectance has a lower L* value, down to 17 L* units (Fig. F70). This interval consists of volcaniclastic sandstone. Hole-to-hole correlationAll holes at the Tiarei outer ridge sites are along the reef (i.e., there are no upslope or downslope holes). Last deglacial–older Pleistocene transitions can easily be correlated and show that the depth of the older Pleistocene sequence is not always constant, indicating that the last deglacial sequence was deposited on an irregular topography. Two distinct zones are recognized in Unit I that can be correlated through specific trends in density, porosity, velocity, and magnetic susceptibility. This trend is more pronounced at the sites toward the northwest (Sites M0025, M0009, and others) and becomes less distinct toward the southeastern sites (Sites M0021 and M0026). |
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