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doi:10.2204/iodp.proc.325.106.2011

Hole M0054B

Operations

Site 7, Hole M0054B

Operations in Hole M0054B commenced at 0545 h on 30 March 2010 following recovery of the blocked bottom-hole assembly (BHA) that led to the termination of Hole M0054A. The HQ pipe was run down the API string, tagging the bottom of the borehole at 0830 h. The first core was recovered at 0915 h and consisted of wash material from the caving of Hole M0054A (Table T1). Coring then continued until 1910 h, when Core 325-M0054B-12R was recovered and the total depth of 33.2 mbsf was achieved. The average recovery for Hole M0054B was 29.63%

Following recovery of this core, the drill floor was prepared for wireline logging operations. Through-pipe gamma logging started at 2330 h and was completed at 0145 h on 31 March. The HQ string was tripped to swap the BHA with a shoe to allow the open-hole logging sondes to be run. At 0315 h, a hydraulic fluid leak occurred, which delayed the tripping of the HQ string. At 0520 h, tripping recommenced, and the HQ BHA was recovered to deck at 0645 h. Following removal of the crossover sub from the top drive, half an API pipe was run in and the crossover reattached. At 0815 h, the HQ string was run back into the hole, and flushed with mud and water. At 1530 h, the HQ string was put into heave compensation. At 1600 h, the conductivity sonde was run, followed by the acoustic borehole image (1745 h), optical borehole image (1900 h), spectral gamma (2145 h), sonic (2330 h), magnetic susceptibility (0110 h on 1 April), Idronaut (0155 h) and caliper (0240 h) sondes. By 0335 h, all wireline tools were recovered to the deck and the logging rooster box was demobilized.

At 0530 h, tripping of the HQ string commenced and continued until 0800 h, followed by tripping of the HQ pipe to just above the seabed to allow deployment of the downpipe camera for posthole survey. Tripping was interrupted by a further hydraulic fluid leak on the iron roughneck from 0840 until 0855 h, when tripping recommenced. By 0920 h, the API pipe was positioned just above the seafloor, and the seabed template was lifted to 7 m above the seafloor to allow the camera survey to commence. At 0940 h, the camera was deployed. It is thought that the camera was caught on the steps inside the bumper sub, blowing the bulb and making it impossible to complete the survey.

The camera was recovered to deck by 1000 h. The remaining API pipe was tripped until the bumper sub was level with the drill floor for inspection by 1120 h. By 1140 h, the seabed frame had been lifted to 20 meters below sea level (mbsl) and the seabed transponder recovered to allow transit to the next hole. At 1200 h, the vessel commenced transit to Hole M0055A.

Sedimentology and biological assemblages

Hole M0054B is divided into four lithostratigraphic units.

Unit 1: Sections 325-M0054B-1W-CC through 8R-2: microbialite boundstone

The lowermost Unit 1, spanning Sections 325-M0054B-1W-CC through 8R-1, consists of microbialite boundstone (Fig. F46) containing relatively low proportions of coral. Coralline algae occur as thick crusts over the corals and as thinner foliose plants (Fig. F47). Microbialites are usually dark colored and laminated (Fig. F48). Some reach several centimeters in thickness, grow in several phases (Fig. F49), and develop microdome shapes. Microbialite is structureless at contacts with coral and coralline algae. Halimeda-rich internal sediment is trapped in microbialites or accumulated as packstone (Fig. F50) within the boundstone. Digitate growths may occur on the upper surfaces of microbialite crusts. All components of the boundstone are bioeroded by bivalves and sponges (Fig. F51).

The dominant corals are encrusting, platy, or submassive Montipora (Fig. F46) and Porites (Fig. F52), although their relative proportions cannot be estimated because they are often indistinguishable in small exposures. Associated corals are massive Isopora, fine-branching Acropora, Stylophora, Pocilloporidae, Tubipora musica (Fig. F53), and (in Section 325-M0054B-8R-2) submassive to massive Faviidae, including Cyphastrea (Fig. F54).

Unit 2: interval 325-M0054B-9R-1, 0–69 cm: lime sand with Halimeda

Unit 2, consisting of interval 325-M0054B-9R-1, 0–69 cm, is composed of medium to coarse lime sand and pebbles rich in Halimeda, plus mollusk, coral, and coralgal fragments. Although some downhole contamination may have occurred, the scarcity of fragments of overlying facies suggests that the lime sand is an original uncontaminated deposit. Well-preserved specimens of Amphsitegina are present but rare in very coarse sand from interval 325-M0054B-9R-1, 45–50 cm.

Two large corals, a massive Porites with black growth bands (Fig. F55) and a branching (corymbose) Acropora (Fig. F56) are present near the top of the interval. Fragments include Acropora, Seriatopora, and Tubipora musica.

Unit 3: Sections 325-M0054B-9R-1, 69 cm, through 10R-1: rudstone

Unit 3, spanning Sections 325-M0054B-9R-1, 69 cm, through 10R-1, consists mainly of dark gray rudstone (Fig. F57). The major components are Halimeda, gastropods, bivalves, larger foraminifera, coral fragments, and coral coated by coralline algae.

A fragment of boundstone containing medium-thickness branching Acropora forms the top of the unit. Other corals include branching Acropora, massive Porites, and Seriatopora fragments.

Unit 4: Sections 325-M0054B-11R-1 through 12R-1: lime sand with larger foraminifera

The lowermost Unit 4, spanning Sections 325-M0054B-11R-1 through 12R-1, consists of medium to coarse lime sand and pebbles rich in fragments of larger foraminifera, Halimeda, mollusks, coral, and bryozoans. The sand is dark colored because of the presence of black grains. The scarcity of fragments from the overlying Unit 3 suggests this lime sand represents an original uncontaminated deposit. Medium to coarse sands from interval 325-M0054B-12R-1, 10–15 cm, commonly include fragmented specimens of Calcarina, Operculina, and Amphistegina.

There are few recognizable corals apart from occasional fragments of Acroporidae and Pocilloporidae (including Seriatopora) and a small piece of the fragmenting fungiid Diaseris contorta.

Physical properties

Hole M0054B was cored to a total depth (TD) of 33.20 m DSF-A, of which 8.25 m was successfully recovered (29.63% recovery). Petrophysical data from these cores are summarized in Table T2.

Density and porosity

Multisensor core logger (MSCL) bulk density varies from 1.02 to 2.37 g/cm³ in cores from Hole M0054B (Fig. F58). The cores are characterized by biscuited and rubbly sections, which compromise the quality of the whole-core MSCL measurements. This quality is reflected in fluctuating bulk density values. Measurements for moisture and density were taken on eight discrete samples between 15 and 31 m CSF-A in Hole M0054B (Fig. F59). Porosity increases with depth from 25% to 49%, as the lithologies change downhole from a microbialite boundstone to lime pebbles to lime sand to a rudstone unit at the base of the measured interval. Grain density varies between 2.74 and 2.82 g/cm³, whereas bulk density for the samples falls within the range of 1.91 to 2.34 g/cm³. Discrete bulk density measurements display good agreement with the higher MSCL bulk density values.

P-wave velocity

Successful P-wave velocity MSCL measurements are limited to the bottommost two cores in Hole M0054B. Values range from 1502 to 1678 m/s (Fig. F58). The proximity of these values to the velocity of seawater (1500 m/s) suggests that core quality issues (see “Physical properties” in the “Methods” chapter) may have compromised the data quality. Four core plugs were taken from Hole M0054B and measured with the P-wave logger. Data for these samples range from 3538 to 4093 m/s (mean resaturated values). This fits well with the samples being composed mainly of microbialite boundstone (Fig. F60A). In discrete samples measured from Hole M0054B, no clear correlation between P-wave velocity and bulk density exists (Fig. F60B).

Magnetic susceptibility

Hole M0054B magnetic susceptibility values range from –1.09 × 10^–5 to 22.91 × 10^–5 SI (Fig. F58). The lower end of this range dominates in the bottom third of the hole (from ~21 m CSF-A to TD). From ~15 to 21 m CSF-A, a zone of higher magnetic susceptibility exhibits obvious variation downhole, but no evidence for a systematic trend exists. This relative high coincides with the microbialite boundstone unit.

Electrical resistivity

Hole M0054B noncontact resistivity measurements fall between 0.56 and 25.28 Ωm (Fig. F58). The highest resistivity values are encountered from 15 to 22 m CSF-A. The downhole trend is very similar to that encountered in the magnetic susceptibility data set, with higher values in the interval from 15 to 21 m CSF-A and lower values below 21 m CSF-A.

Digital line scans and color reflectance

All cores from Hole M0054B were digitally scanned, and, where appropriate, cores were measured for color reflectance. The dispersed trend obtained for color reflectance measurements in Hole M0054B is consistent with the dispersion in Hole M0054A. This consistency is in keeping with the close proximity of these two holes. Hole M0054B has higher recovery than Hole M0054A. However, only a small region of the cores overlap and can be compared. Color reflectance in Hole M0054B varies between 40.12% and 79.36% for L* (Fig. F61). The composition of Hole M0054B from 11 to 20 m CSF-A is mainly a microbialite boundstone unit that is coincident with a relatively similar composition (i.e., coralgal microbialite boundstone) in Hole M0054A below 14 m CSF-A. As with Hole M0054A, measurements taken in this unit show a highly dispersed pattern (40.12% to 79.36% for L*). Values of a* and b* for microbialite are always positive. At 24.3 m CSF-A, the presence of a rudstone unit is coincident with a narrowing of the range of L* (54.81%–62.09%). Values from 27.24 m CSF-A to TD are dispersed because of the presence of medium to coarse pebbles. This bottom of the borehole shows mainly negative a* values.

Paleomagnetism

Measurements of low-field and mass-specific magnetic susceptibility (χ) were performed on samples taken from the working half of the recovered core (Fig. F62). Positive susceptibilities were recorded in samples throughout much of the core, ranging from 0.01 × 10^–8 to 15.25 × 10^–8 m³/kg. The arithmetic mean of the positive measurements from the core is 4.71 × 10^–8 m³/kg, indicating the presence of paramagnetic and/or ferromagnetic minerals. Higher values in magnetic susceptibility are mainly concentrated in the stratigraphic interval from the top of the core to 21.71 mbsf. Susceptibility measurements of the core from 21 mbsf to TD show a negative (diamagnetic) portion ranging from –0.04 × 10^–8 to –0.36 × 10^–8 m³/kg with an arithmetic mean of –0.23 × 10^–8 m³/kg.

Chronology

Two calibrated radiocarbon ages (13 cal y BP, Core 325-M0054B-1W; 18 cal y BP, Core 3R) (Fig. F63) and one U-Th age (20 cal y BP, Core 4R) (see Table T10 in the “Methods” chapter) are consistent with their stratigraphic positions. The U-Th age is unaffected by corrections of initial ²³⁰Th. This hole recovered material from the Last Glacial Maximum interval, as well as the early deglacial. This hole has a substantial amount of recovery from the Last Glacial Maximum interval and potentially older Pleistocene because there are eight cores below Core 325-M0054B-4R, which is dated to 20 cal y BP.

Downhole measurements

Hole M0054B represented the first hole where geophysical wireline operations could be performed in a HQ (“logging”) hole. It was therefore possible to run the high-priority imaging tools (see “Downhole logging” in the “Methods” chapter for information on the HQ logging set-up and logging tool details). Geophysical wireline logging operations were completed in Hole M0054B to a maximum depth of 33.23 m wireline log matched depth below seafloor (WMSF) with the ANTARES Spectral Natural Gamma Probe (ASGR) sonde. However, open-hole measurements were only made between ~17.52 and 26.02 m WMSF at the largest interval. Recovery in Hole M0054B was just over 29.63%; therefore, as continuous measurements, the downhole logging data can be used to fill in data gaps where there is an absence of core or low core recovery. Chronologically, the following tools were run:

1. ASGR 256 (through API [conductor pipe] and HQ pipe),

2. Induction Conductivity Probe (DIL45) (conductivity in open hole),

3. Acoustic Borehole Televiewer (ABI40) (acoustic imaging tool)

4. Optical Borehole Televiewer (OBI40) (optical tool),

5. ASGR 256 (Spectral Gamma Ray in open hole),

6. Sonic (communication issues experienced, resulting in 50% data loss),

7. EM51 (magnetic susceptibility in open hole),

8. IDRONAUT (borehole fluid analyzer), and

9. Caliper (CAL3) (borehole diameter).

Hole stability was relatively poor, and consequently there was some hole infilling during wireline operations, reducing the section available for open-hole logging.

Two logging units can be identified in the measured section of Hole M0054B (Fig. F64):

1. Unit I (17.52–22.36 m WMSF; Cores 325-M0054B-7R through 9R) is characterized by relatively high total gamma ray (TGR) counts ranging from 33 to 40.5 cps. Conductivity is relatively low in Unit I, ranging between ~40 and 181 mmho, although conductivity gradually increases toward the base of this unit. Magnetic susceptibility wavers around 0.7 mSI throughout the unit, and the caliper shows the borehole to be in gauge throughout. Acoustic images provide virtual hardness visualization, and within Unit I the large majority of the formation appears “hard.” The acoustic data provide a virtual borehole view (Fig. F65), which clearly shows the quality of the borehole. Optical images also show some of the main cavities in the borehole wall. However, data are of low quality because of the amount of suspended sediment in the borehole fluid. Lithostratigraphic units identified within this logging unit include (coral)algal/microbialite boundstone (Sections 325-M0054B-5R-1 through 8R-1) followed by lime sand (with Halimeda) unit (Section 9R-1), and finally rudstone (Section 9R-1, 69 cm, and below). Clearly the boundstones lend themselves to aiding borehole stability; hence, the borehole is in gauge throughout this logging unit.

2. Unit II (22.36–32.99 m WMSF; Cores 325-M0054B-10R through 12R) is defined by lower TGR counts compared with Unit I, with values ranging from ~14 to 20 cps. Conductivity values are consistently high in the unit, with values consistently around 380 mmho. There is a minor decrease in magnetic susceptibility compared with Unit I, with values around 0.6 mSI. The caliper data indicate that the hole width is greater than the upper unit. Hole diameters ranging from 12 to 16 cm were measured, indicating two main areas of hole washout and probable hole instability at 22.82 and 23.70 m WMSF. Acoustic images clearly show a major change in lithology with a shallowly dipping boundary at 22.36 m WMSF. The borehole wall exhibits lower “hardness” in the acoustic image throughout this logging unit, and the traveltime image shows an elevated number of cavities and borehole wash-outs. Lithostratigraphic units encountered in this logging unit include dark gray rudstone (Section 325-M0054B-10R-1) that passes into lime sand (with large benthic foraminifera) unit.

Lithostratigraphic and logging unit boundaries do not perfectly concur. Coring and wireline logging use different methods of measuring depth, so further work will be needed to integrate wireline logging and core data. However, the current “raw” relationships can be seen on Figure F66.

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