Previous   |    Next

doi:10.2204/iodp.proc.325.105.2011

Hole M0050A

Operations

Site 4, Hole M0050A

The downpipe camera was retained inside the API pipe while the vessel moved 4 m under dynamic positioning to Site 4, Hole M0050A. A precoring seabed survey was completed by 1650 h on 22 March 2010, and the first run (extended nose corer) was started (Table T1). Standard rotary corer coring (Core 2R downhole) continued until 2120 h, when the hole was terminated at 10.5 mbsf after encountering a change in lithology interpreted to be the older Pleistocene deposits, with an average recovery of 17.8%. The API pipe was tripped to just above the seabed, and a downpipe camera survey was conducted between 2200 and 2220 h. Between 2220 and 2245 h, the API pipe was tripped until there was ~60 m hanging beneath the drill floor. The seabed transponder was then recovered, and at 2305 h the vessel began moving under dynamic positioning 78 m closer to the modern reef to Site 3.

Sedimentology and biological assemblages

Hole M0050A is divided into two lithostratigraphic units.

Unit 1: Sections 325-M0050A-1X-1 through 1X-CC: coralgal boundstone and sand

The uppermost Unit 1, spanning Sections 325-M0050A-1X-1 through 1X-CC, consists of coralgal boundstone fragments mixed with lime sand containing fragments of Halimeda, mollusks, benthic foraminifera, and coralline algae. These sediments are locally consolidated to form packstone/rudstone. The boundstone is mainly composed of an open framework of thin foliose coralline algae (algal bindstone). Most boundstone fragments have brownish staining. The presence of recently living coralline algae at several levels in Section 325-M0050A-1X-1 indicates mixing of recent and older sediments in the core. Medium to coarse sand from interval 325-M0050A-1X-1, 4–9 cm, includes many well-preserved specimens of Cycloclypeus, Amphistegina, and Operculina.

The only corals in Unit 1 are rare fragments of Porites, Acroporidae, Faviidae, and Agariciidae.

Unit 2: Sections 325-M0050A-2R-1 through 6R-1: coralgal-microbialite boundstone

The lowermost Unit 2, spanning Sections 325-M0050A-2R-1 through 6R-1, consists of fragments of slightly bioeroded coralgal-microbialite boundstone. Coralline algae occur as thick crusts on top of corals or form open frameworks of thin foliose plants. A branching coralline algae occurs at the base of Section 325-M0042A-3R-1 (Fig. F20). Thick microbialite crusts are brown and poorly to clearly laminated (stromatolitic) and contain trapped bioclasts. The surfaces of a few corals have reddish stains. Well-preserved specimens of Amphistegina are common in medium to coarse sands from interval 325-M0050A-2R-1, 15–20 cm.

The main corals are small pieces of Acroporidae, Agariciidae (including Leptoseris), Porites, Montipora(?), and Goniopora(?). Occasional coral fragments include Agariciidae (Leptoseris(?)) and Porites.

Physical properties

A total of 1.87 m of core was recovered from Hole M0050A, which was drilled to 10.50 m DSF-A (17.81% recovery). Physical property data are summarized in Table T2.

Density and porosity

In Hole M0050A, whole-core multisensor core logger (MSCL) measurements range from 1.72 to 2.43 g/cm³ (Fig. F21). Core quality may have compromised data quality. Unfortunately, there were no discrete samples taken for moisture and density measurements.

P-wave velocity

P-wave velocity MSCL measurement of Hole M0050A cores yielded very few data points (because of core quality issues), which range from 1507.04 to 1845.36 m/s (Fig. F21). As with other P-wave data sets in this transect, values at the lower end of the range (close to 1500 m/s) should be treated with caution. Similar to the density and porosity measurements, no samples were available for discrete analysis with the P-wave logger. The lack of data in this hole makes it impossible to comment on downhole trends.

Magnetic susceptibility

In Hole M0050A, magnetic susceptibility increases overall with increasing depth (Fig. F21). Values obtained by measurement offshore on the MSCL range from 2.07 × 10^–5 to 31.60 × 10^–5 SI.

Electrical resistivity

Whole-core noncontact resistivity measurements on Hole M0050A cores range from 0.63 to 1.50 Ωm (Fig. F21). Despite the small data set available for this hole, electrical resistivity clearly increases with depth.

Digital line-scans and color reflectance

All cores from Hole M0050A were digitally scanned, and, where appropriate, measured for color reflectance. Color reflectance varies between 45.08% and 74.93% for L* (Fig. F22). Variations in color reflectance parameters show high dispersion in the uppermost meter of the hole. This can be attributed to heterogeneity in the lithology at this depth, comprising lime pebbles, coralgal boundstones, and coral fragments, with the dispersion in the color reflectance values indicating the presence of fragments of different material. Lithologies from 2 to 7 m CSF-A are composed of a heterogeneous mix of lime pebbles of coralgal-microbialite boundstones. The variation in lithology creates a more dispersed pattern in the color reflectance data. Measurements taken from 6.5 to 6.7 m CSF-A are grouped in a similar way, with an average value of 60% for reflectance. This average value indicates that, despite the presence of both lime pebbles and microbialite, the color is more homogeneous in this section than in previous ones. The presence of coralgal boundstones makes the measurements taken between 8 and 9.61 m CSF-A more consistent, with reflectance values varying from 65% to 68%.

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. F23). The record shows positive susceptibilities throughout the entire length of the core, with susceptibility values ranging from 0.60 × 10^–8 to 11.79 × 10^–8 m³/kg, with an arithmetic mean of 3.97 × 10^–8 m³/kg. In addition, a prominent peak at 2.73 mbsf has a value of 11.79 × 10^–8 m³/kg. These positive susceptibility values indicate the presence of paramagnetic and/or ferromagnetic minerals.

Chronology

This hole has one calibrated radiocarbon age of 11 cal y BP from Core 325-M0050A-2R (Fig. F24). There are only four cores beneath this 11 cal y BP section, indicating that this hole probably only contains material from the last deglaciation.

Top of page   |    Previous   |    Next