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Downhole logging

Hole M0023B

Hole M0023B (67.58 mbsl) was logged for geophysical parameters. Drilling depth was 30.12 mbsf, and geophysical wireline operations were completed from 28.97 mbsf upward with data coverage from all slimhole tools without repositioning the open shoe casing (fixed at 4.63 mbsf) under open and very hostile borehole conditions. The superb quality of the image logs allows clear identification of lithologies forming eight distinct intervals:

  • Interval 1 (9.22–28.97 mbsf; Cores 310-M0023B-12R through 15R) contains relatively high natural radioactivity where elements Th and K contribute most to the total counts measured (Fig. F16). The quality of the optical images of the lowermost 3 m is affected by murky borehole fluids and enlarged borehole diameter. From 25.90 mbsf, in situ open frameworks composed of branching coral colonies can be observed in borehole wall images. The minimum in total counts at 24.42 mbsf correlates with the occurrence of a branching colony of Porites, as observed in the optical image. At 23.62 mbsf, a (sharp) change from branching colonies to more encrusting coral growth forms (Porites and faviids) occurs. Resistivity values were very low (~1.65 m), the temperature of the borehole fluid was ~27.2°C, pH values were ~7.92, and electrical conductivity increased from 56.09 to 56.64 mS/cm (0.1765–0.1783 m). Sonic P-wave velocities (VP) decreased from 2418 to 1709 m/s from base to top in the second sequence. Proper sonic Stoneley wave velocities could not be measured.
  • Interval 2 (20.81–23.00 mbsf; Core 310-M0023B-11R) is characterized by a local total counts maximum in the gamma ray log at 22.9 mbsf that can be correlated with a very large borehole diameter and a thin sand interval with fragments of coralgal material just above. It shows an increase in resistivity from 1.22 to 2.47 Ωm, an increase in VP from 1709 to 3456 m/s, and a decrease in natural radioactivity (Fig. F16). The temperature of the borehole fluid was ~27.37°C, pH values were ~7.95, and electrical conductivity increased from ~56.71 mS/cm (0.176 Ωm).
  • Interval 3 (18.51–20.81 mbsf; Core 310-M0023B-10R) has a top-bounding surface marked by a relatively thick algal crust. Some less pronounced surfaces are present within this section (e.g., erosional contact between a thin-branching coral colony and an interval consisting of coral fragments at 19.43 mbsf). Resistivity increased slightly from 2.11 to 2.65 Ωm, VP increased at the base from 2382 to 3456 m/s but became highly variable toward the top, gamma radiation decreased, and borehole fluid properties remained constant. In the middle part of the last deglacial sequence (13.93–18.51 mbsf; Cores 310-M0023B-8R and 9R), formation resistivity increased over a couple of steps from 2.23 Ωm to a maximum of 3.9 Ωm. These steps can be correlated with lithological changes.
  • Interval 4 (17.50–18.51 mbsf; Core 310-M0023B-8R) shows coral fragments on top of an encrusted surface just below a cavity (Fig. F17). A marked difference in the thickness of the coral branches can be observed when comparing this lithofacies with the same lithofacies in the section below. Furthermore, a marked increase in the intensity of microbialite encrusting can be observed with higher amplitude values in the acoustic borehole image (ABI40) and directly in the optical borehole image (OBI40). Upsection, the coral framework becomes more open, coral branches become thinner, and microbialite encrusting is less intensive. At the very top, some thinly encrusting corals can be observed. Within this interval, VP decreased from 3314 to 2166 m/s.
  • Interval 5 (13.93–17.50; Cores 310-M0023B-8R and 9R) comprises rubble in which a pinkish/​orange-colored fragment of Pocillopora can be observed on the color-calibrated optical image at 17.25 mbsf (Fig. F17). Branching coral colonies are dominated by Pocillopora in the lower part, and the volumetric contribution of microbialite is >50%. Microbialite encrusting remains very abundant upsection, resulting in the highest formation resistivity values in Hole M0023B. A subtle increase in borehole fluid pH values occurred along with a decrease in fluid conductivity and fluid temperature within this subsequence (14.55 mbsf). The upper part of Hole M0023B displayed a decrease in resistivity to a minimum of 1.3 Ωm at 9.10 mbsf; below that point, resistivity remained fairly constant at ~2.1 Ωm.
  • Interval 6 (11.69–13.93 mbsf; Core 310-M0023B-7R) contains intervals of nearly homogeneous microbialite encrustations up to 25 cm thick (e.g., 12.88–13.93 mbsf). Branching colonies of corals also occur, and from 12.11 mbsf the topmost horizon consists mainly of foliaceous corals encrusted by microbialite crusts. The uppermost 45 cm consists of less massive coral assemblages with a more open framework and less pronounced microbialite encrustation. Formation resistivity decreased in this upper interval (Fig. F16).
  • Interval 7 (9.22–11.69 mbsf; Cores 310-M0023B-6R and 7R) consists of branching and more massive coral assemblages, where a decrease in the thickness of branches and in microbialite encrusting can be observed upsection. VP data coverage is not optimal, but a general decrease (3566–1567 m/s) in VP can be observed.
  • Interval 8 (4.63–9.22 mbsf; Cores 310-M0023B-3R through 5R) contains two cavities at the base. Above these, an interval of very open framework composed of branching, foliaceous, and encrusting coral assemblages can be observed. The abundance of microbialite encrustations decreases upsection.