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doi:10.2204/iodp.proc.325.104.2011

Hole M0048A

Operations

Site 12, Hole M0048A

The Greatship Maya came onto station over Hole M0048A at 0220 h on 12 March 2010 (Table T1). The seabed transponder was deployed, and an additional API pipe was run to take the pipe to just above the seabed by 0305 h. A downpipe camera survey was completed by 0325 h, and the first standard rotary corer core was recovered by 0405 h. Standard rotary corer coring continued until 0610 h, when the decision was made to halt operations after four runs because of winds in excess of 35 kt and heave >2.5 m. As a result, Hole M0048A was terminated at 7.1 mbsf with an average recovery of 9.7%.

The API pipe was tripped by 0805 h, and the seabed template was secured by 0850 h. The seabed transponder was then recovered and secured on deck by 0910 h.

Waiting on weather and port call, Townsville

The Greatship Maya departed Site 12 (Hole M0048A) at 0910 h on 12 March 2010 to transit to sheltered waters within Hydrographer’s Passage in order to conduct a ship-to-ship transfer of equipment, including a bumper sub and drilling mud from the PMG Pride, which commenced at 1530 h. Because of the failure of previous HQ attempts using the seabed template suspended in the moonpool or water column as a guide for the HQ string and API “casing,” a bumper sub was manufactured to facilitate further attempts at recovering HQ cores.

At 1645 h, the Greatship Maya departed Hydrographer’s Passage to transit northward using the inner passage because of deteriorating weather conditions offshore. At 1510 h on 13 March 2010, the Greatship Maya dropped anchor at the Townsville Anchorage (Australia) to wait on weather. Preparations continued to be made in advance of Cyclone Ului; all containers, inside equipment, cables, winches, and banners were securely fastened while monitoring the weather every 6 h.

At 1916 h on 15 March, the Greatship Maya weighed anchor, coming alongside in Townsville at 2100 h. Following rebunkering and a GC Rieber marine crew change, the Greatship Maya departed Townsville at 2035 h on 16 March and arrived at the Townsville Anchorage at 2205 h to continue waiting on weather. At 1600 h on 17 March a pilot boat came alongside with additional spares. The Greatship Maya continued to wait on weather at the Townsville Anchorage until 19 March.

Sedimentology and biological assemblages

Hole M0048A consists of one lithostratigraphic unit.

Unit 1: Sections 325-M0048A-1R-1 through 3R-CC: coralgal boundstone and lime sand with larger foraminifera

Unit 1, spanning Sections 325-M0048A-1R-1 through 3R-CC, is composed of fragments of coralgal boundstone in lime sand and mud. Some boundstone fragments are pieces of thin coralline algal framework (Fig. F82), whereas others consist of coral, coralgal boundstone, and bryozoans encrusted by blackened coralline algae. Some fragments are stained brown and likely to be material from the modern seafloor. Loose sediment includes examples of larger foraminifera, Halimeda, and mollusks. Larger foraminifera are rare fragmented or encrusted specimens of Cycloclypeus, Amphistegina, and Alveolinella in gravelly medium sands from interval 325-M0048A-1R-1, 0–5 cm, and a few well-preserved specimens of Amphistegina in muddy coarse sands from interval 3R-CC, 7–12 cm. The presence of modern seafloor crusts at the base of Section 325-M0048A-2R-1 suggests stratigraphic mixing of fragments while coring.

All corals are fragments and include Leptoria, Leptoseris, Pachyseris speciosa(?), Porites(?), Montipora(?), Faviidae, and solitary Fungiidae.

Physical properties

Hole M0048A was drilled to a depth of 7.10 m DSF-A. A total of 0.69 m of core was recovered (9.72% recovery). As with the other holes in this transect, petrophysical data from this hole are summarized in Table T2.

Density and porosity

Bulk density values from whole-core multisensor core logger measurements were taken on one section from Hole M0048A (Fig. F83). Values range from 2.02 to 2.16 g/cm³ and are relatively consistent downsection. Three discrete samples were measured for moisture and density in this hole. Bulk density values range from 1.79 to 2.17 g/cm³, whereas porosity measurements fall between 38% and 57%. Grain density presents a small variation with a range of 2.77 to 2.85 g/cm³ for all three measurements (Fig. F84).

P-wave velocity

P-wave velocity offshore measurements on whole cores were only taken on one core section, and values ranged from 1520 to 1581 m/s (Fig. F83). These values are similar to P-wave velocity for water and are probably a result of diminished core quality (see “Physical properties” in the “Methods” chapter). No discrete samples were measured on the P-wave logger for this hole.

Magnetic susceptibility

The one section (325-M0048A-1R-1) that was measured on the multisensor core logger yielded magnetic susceptibility data in the range of –0.08 × 10^–5 to 1.15 × 10^–5 SI.

Electrical resistivity

In Hole M0048A, resistivity ranges from 0.89 to 0.99 Ωm. Data are from one core section only, and it is therefore difficult to observe any downhole trends (Fig. F83).

Digital line scans and color reflectance

Only three small cores were recovered in Hole M0048A. All cores were digitally scanned and measured for color reflectance. L* had a minimum of 47.26% and a maximum of 77.46% (Fig. F85). The three cores were composed of very heterogeneous unconsolidated material including lime pebbles, coral fragments, fine sand, and lime mud. Unsurprisingly, the sections showed in all cases a high dispersion of values in all three parameters of color reflectance (L*, a*, and b*).

Thermal conductivity

The one thermal conductivity point taken for Hole M0048A gives a thermal conductivity value of 1.09 W/(m·K) at 3.09 m CSF-A.

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. F86). Two positive susceptibility values were measured in samples located at 0.02 and 3.03 mbsf with values of 0.09 × 10^–8 and 0.87 × 10^–8 m³/kg. Low negative susceptibility was recorded in a sample located at 4.51 mbsf with a value of –2.02 × 10^–8 m³/kg.

Chronology

This hole has only one calibrated radiocarbon age of 14 cal y BP from Core 325-M0048A-2R (Fig. F87). There are only two cores beneath this 14 cal y BP section, indicating that this hole probably only contains material from the last deglaciation.

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