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doi:10.2204/iodp.proc.325.105.2011

Hole M0049A

Operations

Transit to transect RIB-02A

Following regular assessment of weather conditions at all three geographical drilling locations, the Greatship Maya departed Townsville Anchorage (Australia) and began the transit to Ribbon Reef 3 at 2040 h on 19 March 2010, arriving at 1930 h on 20 March. At 2030 h, the Greatship Maya began dynamic positioning (DP) trials ~0.3 nmi seaward from transect RIB-02A, Site 4. These trials continued into 21 March because of problems with the bow thrusters. At 1530 h on 21 March, new DP software was installed, and testing continued until 1645 h, when the Greatship Maya began to move onto station at Site 4.

Site 4, Hole M0049A

At 1900 h on 21 March 2010, the Greatship Maya arrived on station, and the seabed transponder was deployed (Table T1). The anchor was also made ready for fast deployment as per the Environmental Management Plan for the Ribbon Reef sites. Running of American Petroleum Institute (API) pipe commenced at 1950 h. Between 2225 and 2240 h, a downpipe camera survey of the seabed was conducted prior to tagging the seabed at 2300 h.

The first core arrived on deck at 2330 h. However, after two extended nose corer runs (3.5 meters below seafloor [mbsf]), a problem with the water swivel/flush pipe at 0040 h on 22 March necessitated tripping the API pipe to ~7 m above the seabed. The vessel remained on DP and on station during the repairs.

Sedimentology and biological assemblages

Hole M0049A is divided into two lithostratigraphic units.

Unit 1: Section 325-M0049A-1X-1: modern coralgal boundstone

The uppermost Unit 1, consisting only of Section 325-M0049A-1X-1, is composed of coralgal boundstone fragments in a lime sand that is rich in Halimeda and contains mollusks and worm tubes (Fig. F2). Most boundstone fragments consist of thin coralline algal framework (Fig. F3). Brown-stained boundstone fragments and recently living coralline algal fragments at several levels are probably derived from the modern seafloor and indicate mixing of recent and older material within the section. Gravelly fine-grained sand from interval 325-M0049A-1X-1, 6–11 cm, contains generally well preserved specimens of Cycloclypeus, Amphistegina, and Planostegina.

The only corals in Unit 1 are a few pieces of Agariciidae, probably foliaceous Leptoseris.

Unit 2: Sections 325-M0049A-2X-1 through 2X-CC: unconsolidated sediment

The lowermost Unit 2, spanning Sections 325-M0049A-2X-1 through 2X-CC), consists of fragments that appear to have come from the overlying boundstone, corals, mollusks, and Halimeda (of Unit 1) as a result of disturbance during drilling. There are no larger foraminifera in the gravelly coarse sands from interval 325-M0049A-2X-CC, 7–12 cm.

Most coral fragments are too small for identification, but they include Porites, Montipora, and Agariciidae.

Physical properties

Hole M0049A was cored to a total depth of 3.50 m drilling depth below seafloor (DSF-A), of which 0.77 m was successfully recovered (22.0% recovery). Physical property data for this hole are summarized in Table T2.

Density and porosity

In Hole M0049A, bulk density varies from 1.37 to 2.24 g/cm³ (Fig. F4). Owing to the very shallow hole and limited core recovery, only the first core was logged on the multisensor core logger, and it is therefore difficult to comment on downhole trends. Two discrete samples were measured with the pentapycnometer. The porosity of these discrete samples is 38% and 39%, with bulk densities of 1.99 and 2.17 g/cm³ and grain densities of 2.78 and 2.85 g/cm³, respectively (Fig. F5).

P-wave velocity

Whole-core P-wave measurements taken offshore on the first core range from 1505 to 1852 m/s (Fig. F4). Values at the lower end of this range are likely a product of diminished core quality and should therefore be treated with caution (see “Physical properties” in the “Methods” chapter). Discrete measurements were not taken on any material from this hole.

Magnetic susceptibility

Recovered core from Hole M0049A exhibits a small range of magnetic susceptibility values, from 1.28 × 10^–5 to 3.26 × 10^–5 SI (Fig. F4). On the one core measured, no evidence for any downcore trends in terms of magnetic susceptibility exists.

Electrical resistivity

As with the other multisensor core logger measurements, data for electrical resistivity from Hole M0049A material are only available for one core. Electrical resistivity for this core ranges from 0.86 to 1.35 Ωm (Fig. F4), with no evidence of a downcore trend.

Digital line-scans and color reflectance

All cores from Hole M0049A were digitally scanned, and, where appropriate, cores were measured for color reflectance. Color reflectance varies between 52.87% and 63.00% L* (Fig. F6). Owing to core quality, Section 325-M0049A-1X-1 (coralgal boundstone) was the only section measured with the color reflectance sensor. The other core from this hole is not appropriate for this measurement because it is composed of lime granules. Dispersion found in the reflectance and color measurements is small, and they are grouped in relation to the variation in color for the same unit.

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. F7). The record shows two positive susceptibility samples located at 0.37 and 2.18 mbsf with susceptibility values of 0.81 × 10^–8 and 2.58 × 10^–8 m³/kg, indicating the presence of paramagnetic and/or ferromagnetic minerals.

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