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doi:10.2204/iodp.proc.325.106.2011

Hole M0052B

Operations

Site 6, Hole M0052B

Coring (standard rotary corer) recommenced at 0115 h on 27 March 2010 at the same location as Hole M0052A (Table T1). The first core was recovered at 0200 h. The bit was found to be cracked, and the hole collapsed back to 1 mbsf. Coring continued for another three runs until 0430 h. However, weather conditions and large sea swells caused the bit to bounce on the bottom of the hole, as well as dynamic positioning deviation. At 0430 h the barrel became stuck in the bottom-hole assembly (BHA) and could not be recovered without tripping the API pipe, so Hole M0052B was terminated at 6.9 mbsf. The barrel was recovered at 0745 h, and the bit was found to be bent and catching on the BHA landing ring.

Hole M0052B

Hole M0052B is divided into two lithostratigraphic units.

Unit 1: Section 325-M0052B-1R-1, 0–17 cm: muddy Halimeda sand

The uppermost Unit 1, consisting only of interval 325-M0052B-1R-1, 0–17 cm, is composed of unconsolidated muddy Halimeda sand containing common bioclasts of mollusks, larger foraminifera, echinoids, and corals. The muddy, very coarse sand from interval 325-M0052B-1X-1, 10–15 cm, contains well-preserved specimens of Amphistegina, Alveolinella, and Sphaerogypsina. Coral fragments include Seriatopora and encrusting Agariciidae.

Unit 2: Sections 325-M0052B-1R-1, 17 cm, to 3R-1, 5 cm: pebbles of corals, mollusks, and bioclastic packstone

The lowermost Unit 2, spanning Sections 325-M0052B-1R-10, 17 cm, through 3R-1, consists of pebbles of coral, mollusks, and bioclastic packstone associated with coarse to very coarse lime sand (Fig. F7). Some coral clasts are covered with thin encrusting nongeniculate coralline algae. Other bioclasts include Halimeda and larger foraminifera.

Larger coral pebbles are mainly submassive to massive fragments of Isopora, Acropora, Porites, Agariciidae, and Faviidae. Smaller fragments include Montipora(?), Isopora, Acropora, Porites, Seriatopora, and Agariciidae.

Physical properties

A total of 0.46 m of core was recovered from Hole M0052B, which was drilled to 6.90 m DSF-A (6.67% recovery). Only one core recovered from Hole M0052B was long enough to be measured on the multisensor core logger (MSCL) (>15 cm). Data from this core are summarized in Table T2.

Density and porosity

In Hole M0052B, whole-core MSCL measurements taken on Core 325-M0052B-1R range from 1.06 to 1.98 g/cm³ (Fig. F8). No obvious downcore trend in the gamma density data set exists. One core plug was obtained from this hole, with moderately high porosity (48%), grain density of 2.80 g/cm³, and bulk density of 1.95 g/cm³ (Fig. F9). The discrete bulk density measurement agrees well with the MSCL bulk density measurement.

P-wave velocity

There are no P-wave velocity data for Hole M0052B either from the MSCL or from discrete measurements.

Magnetic susceptibility

In Hole M0052B, magnetic susceptibility values obtained offshore by measurements on the MSCL range from 1.79 × 10^–5 to 4.32 × 10^–5 SI (Fig. F8). Values tend to decrease downcore.

Electrical resistivity

Whole-core noncontact resistivity measurements on Hole M0052B cores range from 0.96 to 1.49 Ωm (Fig. F8). Similar to the gamma density, no downhole trend is evident.

Digital line scans and color reflectance

Cores from Hole M0052B were digitally scanned and, where appropriate, measured for color reflectance. Color reflectance in Hole M0052B ranges from 51.24% to 69.91% (Fig. F10), although only 40 cm was measured for this hole. The last section of this borehole (Section 325-M0052B-3R-1) was not measured for color reflectance because it was mainly composed of pebbles, corals, mollusks, and bioclastic packstone. The values in the uppermost 17 cm correspond to an interval of muddy Halmeida sp. sand.

Chronology

This hole has one calibrated radiocarbon age of 14 calibrated years before present (cal y BP; years before 1950 AD) from Core 325-M0052B-1R (Fig. F11). Limited recovery beneath this 14 cal y BP core indicates that this hole probably only contains material from the last deglaciation.

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