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doi:10.2204/iodp.proc.325.106.2011

Hole M0054A

Operations

Site 7, Hole M0054A

The Greatship Maya was on station over Hole M0054A at 0425 h on 29 March 2010, and the seabed transponder was deployed. The downpipe camera survey began at 0440 h but took until 0530 h to complete, as an additional API pipe had to be run in, requiring recovery and redeployment of the camera system. Time was needed to repair hydraulic fluid leaks on the roughneck and to conduct maintenance on the main power pack. After lowering the seabed template, seabed was tagged at 1515 h, followed by washing down to set the API pipe in the hole. HQ coring commenced at 1600 h and continued until 0425 h on 30 March (Table T1).

The hole was terminated at 0425 h at a depth of 18.72 mbsf after three consecutive runs recovered no material. It was considered necessary to recover the bottom-hole assembly to deck, where it was discovered that 60 cm of cored material had become wedged in the bottom-hole assembly and was preventing the barrel from latching. The average recovery for Hole M0054A was 23.98%.

Sedimentology and biological assemblages

Hole M0054A is divided into three lithostratigraphic units.

Unit 1: Sections 325-M0054A-1R-1 to 2R-1, 6 cm: modern reef pebbles

The uppermost Unit 1, spanning Sections 325-M0054A-1R-1 to 2R-1, 6 cm, consists of fragments of corals, mollusks, and branching species of nongeniculate coralline algae. Some coral clasts are encrusted by thin nongeniculate coralline algae. Coral fragments include Acropora, Stylophora, Platygyra, Montipora, and Porites.

Unit 2: Sections 325-M0054A-2R-1, 6 cm, through 3R-1: coralgal boundstone

Unit 2, spanning Sections 325-M0054A-2R-1, 6 cm, through 3R-1, consists of coralgal boundstone. Corals are dominated by branching forms, and the tips of some coral branches have thick crusts of with nongeniculate coralline algae (Fig. F38). Unconsolidated fine lime sand with bioclasts partly to fully fills constructional voids.

Corals are dominated by fine branching Seriatopora (Fig. F38) and fine branching (corymbose?) Acropora. Associated corals are Tubipora musica and massive Favia and Galaxea fascicularis (Fig. F39). Fragments include foliaceous Montipora, branching Acropora and Stylophora, massive Porites, solitary Fungiidae, and Tubipora musica.

Unit 3: Section 325-M0054A-4R-1: coralgal-microbialite boundstone

The lowermost Unit 3, consisting only of Section 325-M0054A-4R-1, is composed of coralgal-microbialite boundstone (Fig. F40). Corals are covered with nongeniculate coralline algae, which in turn are encrusted with microbialites. Microbialites have laminated and, rarely, thrombolitic fabrics.

The dominant coral is fine branching Seriatopora (Fig. F40) with associated branching Acropora and encrusting Porites(?) or Montipora(?).

Physical properties

Hole M0054A was cored to a total depth of 18.72 m DSF-A, of which 2.23 m was successfully recovered (11.91% recovery). Table T2 summarizes physical property data for this hole.

Density and porosity

Multisensor core logger (MSCL) bulk density in Hole M0054B varies from 1.01 to 2.22 g/cm³ (Fig. F41). The cores recovered that were sufficiently long (>15 cm) to be logged on the MSCL were composed of rubble, and as such, it is likely that bulk density data underestimates the actual values of the core material. Two discrete samples were available for measurement from Hole M0054A (Fig. F42). Bulk densities for these samples are 2.22 and 2.42 g/cm³. The corresponding porosities are 35% and 20%, and grain densities were 2.85 and 2.76 g/cm³, respectively. As expected, bulk densities given by discrete samples are higher than values from core logging.

P-wave velocity

As a result of the quality of recovered cores for Hole M0054A, P-wave velocity MSCL data for whole cores are not available (Fig. F41). However, two discrete samples were taken and measured, giving values of 2994 and 3890 m/s (mean resaturated values) (Fig. F43). These are appropriate values for well-lithified porous formations such as these microbialite boundstone samples. The small data set makes it difficult to comment on trends.

Magnetic susceptibility

Magnetic susceptibility measurements for Hole M0054A vary from 0.21 × 10^–5 to 43.95 × 10^–5 SI, with no obvious downhole trend (Fig. F41). The maximum magnetic susceptibility was reached at 11.12 m CSF-A.

Electrical resistivity

Noncontact resistivity measured on whole cores ranges from 0.93 to 66.18 Ωm (Fig. F41). Overall, resistivity decreases with depth in Hole M0054A. However, owing to the quality of the core measured, it would be wise to treat this data set with caution.

Digital line scans and color reflectance

Cores from Hole M0054A were digitally scanned, and, where appropriate, cores were measured for color reflectance. Color reflectance in Hole M0054A varies from 39.63% to 77.13% for L* (Fig. F44). Approximately 4.5 m (from ~11 to 15.5 m CSF-A) of sediment was measured for Hole M0054A. The scattered nature of color reflectance values in these cores is due to the presence of variable lithologies within the same section (e.g., fragments of mollusks and corals, pebbles, and coralgal boundstone). The last section of this borehole corresponds to a coralgal-microbialite boundstone unit. This lithostratigraphic unit shows different colorations, and in general, as observed in previous boreholes, it shows a lack of consistency between measurements in the same section. No trends were detected in the color indexes a* and b* or in the a*/b* ratio. Values of a* were low but mainly positive (red coloration), and values of b* were always positive (yellow color).

Thermal conductivity

One thermal conductivity value of 1.055 W/(m·K) was measured for Hole M0054A at 27.6 mbsf.

Paleomagnetism

Measurements of low-field and mass-specific magnetic susceptibility (χ) were performed on all samples taken from the working half of the recovered core (Fig. F45). Positive low susceptibilities occur, with an arithmetic mean of 5.26 × 10^–8 m³/kg. Core susceptibilities range from 1.56 × 10^–8 to 12.73 × 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 below ~13 mbsf to the base of the core. Two negative susceptibilities at 9.42 and 12.99 mbsf have values of –2.81 × 10^–8 and –0.06 × 10^–8 m³/kg.

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