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doi:10.2204/iodp.proc.325.106.2011

Hole M0057A

Operations

Site 2, Hole M0057A

At 1125 h, the seabed transponder was deployed, and by 1140 h the Greatship Maya had settled on station at Site 2, Hole M0057A. The transponder on the seabed template was changed prior to lowering the template to ~35 m below the sea surface. At 1135 h, API pipe was run to just above the seabed, and a downpipe camera survey was conducted. Further API pipe was then run, tagging the seabed at 1400 h. The API pipe was then set on the elevators and disconnected from the top drive at 1430 h in preparation for running HQ pipe. The seabed was tagged with the HQ pipe at 1520 h, and coring operations began (Table T1).

After Run 3 the hole was caving, possibly because of the string swabbing the hole. Coring operations continued after hole conditioning until 2325 h, when there was a twist off between the crossover sub and the top HQ pipe. Between 2325 h on 3 April and 0100 h on 4 April, the crossover sub was tripped out for repairs and the core barrel was recovered. On the next run, damage to the threads on two HQ rods was sustained while running a new rod in, and two rods were tripped to replace them. Coring then continued until 0440 h, when it was noted that mud was leaking from the repaired crossover sub/HQ pipe joint. To enable safe working on the joint, three HQ rods were tripped out. During the repair period, the hole collapsed. Open-hole drilling and flushing back down to the previously reached depth of 32.38 mbsf ran from 0610 to 0710 h, at which point coring operations restarted.

Operations were temporarily halted between 1040 and 1100 h because of a fire alarm caused by an oil leak dripping onto the exhaust pipe in the engine room. Operations then continued until 1230 h, when the hole was terminated at 41.78 mbsf with an average hole recovery of 45.5%. Between 1240 and 1520 h the HQ rods were tripped, followed by the API pipe. A problem with the iron roughneck delayed lifting of the seabed template, which was completed at 1730 h. The seabed transponder was recovered by 1750 h, and the template and drill floor were secured in preparation for transit to Site 8, Hole M0058A.

Hole M0057A

Hole M0057A is divided into six lithostratigraphic units.

Unit 1: Sections 325-M0057A-1R-1 to 7R-1, 35 cm: coralgal boundstone

The uppermost Unit 1, spanning Sections 325-M0057A-1R-1 to 7R-1, 35 cm, consists of coralgal boundstone with minor amounts of microbialite. The proportion of coral is relatively high (Fig. F110). Coralline algae occur mainly as thick crusts on top of corals (Fig. F111) and are intergrown with vermetids and encrusting foraminifera. In the uppermost Section 325-M0057A-1R-1, coralline algae occur occasionally as frameworks of thin foliose plants (Fig. F112). Microbialites are poorly developed and occur mainly as thin crusts with rough to digitate surfaces on coralline algae and corals (Fig. F113). Some bioclasts are trapped in the microbialite. Lithified internal sediments of Halimeda-rich packstone to rudstone have accumulated in pockets. All boundstone components are bioeroded by bivalves and sponges and contain worm tubes. Soft muddy sediment fills some bivalve borings. Larger foraminifera are absent from the sandy gravels in intervals 325-M0057A-5R-1, 70–75 cm, and 6R-1, 130–135 cm.

Encrusting to submassive Porites or Montipora (often indistinguishable) are common in the upper part of Unit 1. Massive colonies dominate in Section 325-M0057A-2R-1 (Fig. F114) and are increasingly abundant lower in Unit 1. Massive Isopora and branching Acropora dominate the middle of the unit (Figs. F111, F115). Associated corals are branching Acropora and Pocillopora, Faviidae (including Favia), and Poritidae (including Goniopora). Fragments include Isopora, branching Acropora, Seriatopora, Stylophora(?), Agariciidae, Faviidae, Pocilloporidae, and one piece that may be an octocoral(?) (Fig. F116).

Unit 2: Sections 325-M0057A-7R-1, 35 cm, through 8R-1: grainstone to rudstone (whitish)

Unit 2, spanning Sections 325-M0057A-7R-1, 35 cm, through 8R-1, consists of whitish grainstone to rudstone rich in Halimeda, with coral, mollusk, and coralline algal fragments (Fig. F117). Coral fragments, up to 14 cm in size, are encrusted locally by coralline algae (Fig. F118). The rudstone includes intraclasts of a Halimeda rudstone with a greenish muddy matrix that is pitted by bivalve borings infilled with fine-grained internal sediment. Moldic porosity, created by partial dissolution of originally aragonitic components, and geopetal infilling of larger voids occurs throughout the unit. Brown-stained, irregular surfaces within the grainstone suggest subaerial exposure and diagenetic alteration (Fig. F119).

Most corals are tilted or upside down and probably not in growth position. They include massive Platygyra (Fig. F120), massive Montastrea curta (Fig. F118), a solitary Fungiidae (Fig. F121), and some small coral fragments that include branching Acropora.

Unit 3: Sections 325-M0057A-10R-1 through 12R-CC: coralgal boundstone with rudstone/grainstone (yellowish)

Unit 3, spanning Sections 325-M0057A-10R-1 through 12R-CC, consists mainly of yellowish coralgal boundstone alternating with yellowish rudstone. Corals are covered by thick crusts of coralline algae. No microbialite is visible. Corals, mollusks, and Halimeda are the main components of the rudstone, and both corals and coralline algae are intensely bioeroded. Coral skeletons and other originally aragonitic components are extensively dissolved (Fig. F122). Many surfaces in both the boundstone and rudstone have reddish stains, and Section 325-M0057A-10R-1, 48 cm, contains the possible trace of a root (Fig. F123). Interval 325-M0057A-10R-CC, 4–8 cm, contains a spectacular cavity partially filled with small calcitic speleothems.

Although few corals are visible, large corals include massive Montipora, Symphyllia (Fig. F124), and Acropora associated with branching to submassive Pocillopora, Acropora, Stylophora(?), Montipora, Favia, and massive Porites (Fig. F125). Fragments include Seriatopora(?), Acropora, and Isopora.

Unit 4: Sections 325-M0057A-13R-1 to 14R-2, 50 cm: coralgal boundstone

Unit 4, spanning Sections 325-M0057A-13R-1 to 14R-2, 50 cm, consists mainly of a whitish coralgal boundstone. Thin encrusting corals are intergrown with crusts of coralline algae (Fig. F126) containing local concentrations of vermetid gastropods (Fig. F127). The internal sediment is packstone/grainstone to rudstone with grains composed of corals, mollusks, coralline algae, larger foraminifera, and Halimeda. The absence of fragments from overlying units suggests that the lime sand is an original uncontaminated deposit. Extensive partial dissolution of coral and other originally aragonitic components occurs throughout the interval.

Dominant corals are encrusting to massive Montipora (Fig. F126), Agariciidae, and possibly some Porites. Associated corals include Cyphastrea and Leptoseris. Fragments include Acropora, Montipora, and Cyphastrea.

Unit 5: Sections 325-M0057A-14R-2, 50 cm, through 15R-CC: coralgal boundstone (pinkish)

Unit 5, spanning Sections 325-M0057A-14R-2, 50 cm, through 15R-CC, consists of pink-colored coralgal boundstone with internal sediments of packstone to rudstone (Fig. F128). Corals are encrusted by thick coralline algal crusts. Corals, mollusks, and larger foraminifera are main components of the rudstone. Interval 325-M0057A-14R-2, 118–130 cm, contains a concentration of spicules (probably from octocorals). Corals and coralline algae are intensely bioeroded. Sponge borings filled with fine-grained sediments have been exposed by dissolution of the host coral (Fig. F129). Near the top of Unit 5, a brownish calcrete overprints large dissolution cavities in the packstone in interval 325-M0057A-14R-2, 50–85 cm (Fig. F130). Extensive dissolution of corals and other originally aragonitic components is common throughout the unit.

Dominant corals are encrusting to massive Acroporidae (Acropora or Isopora) (Fig. F129) and Agariciidae (including Leptoseris(?) and Pachyseris(?)) (Fig. F130).

Unit 6: Sections 325-M0057A-16R-1 through 16R-2: packstone to rudstone (pinkish)

The lowermost Unit 6, spanning Sections 325-M0057A-16R-1 through 16R-2, consists of pinkish packstone to rudstone (Fig. F131) with large coral fragments. The major components are fragments of corals, mollusks, and coralline algae. Interval 325-M0057A-16R-1, 50–64 cm, contains a concentration of spicules. Coral fragments and other major components are intensely bioeroded, and borings are filled with packstone. Coral skeletons and other originally aragonitic components are extensively dissolved. Dissolution surfaces within the packstone and the coral fragments have pinkish stains.

Corals are severely diagenetically altered. The only identifiable corals are encrusting to submassive Agariciidae (probably Pachyseris) (Fig. F132) and a single solitary Fungiidae.

Physical properties

Hole M0057A was cored to a total depth of 41.78 m DSF-A, of which 19.00 m was successfully recovered (45.48% recovery). A summary of the physical property data for this hole is given in Table T2.

Density and porosity

In Hole M0057A, gamma density varies from 1.00 to 2.38 g/cm³ (Fig. F133). As with most other cores in this transect, gamma density data appears erratic, and a broad range of values is measured within any one core. The reason for these erratic data is that core quality is poor as a result of biscuiting and fragmentation. Consequently, the majority of bulk density data underestimates bulk density at any given point. Discrete moisture and density measurements on 14 samples from Hole M0057A give bulk densities ranging in value from 2.08 to 2.49 g/cm³ (Fig. F134). Overall, these values are at the high end of the range of multisensor core logger bulk density for this hole. Discrete samples are principally coralgal boundstone and a few rudstone samples, with a porosity varying between 14% and 39%. Grain density is between 2.67 and 2.75 g/cm³ for all but two of the samples, which have a higher grain density (2.86 and 2.90 g/cm³).

P-wave velocity

Because of core quality issues, there are no whole-core P-wave velocity data available for Hole M0057A. However, 11 discrete samples were taken from across the hole to be measured on the P-wave logger. Measurements yielded values ranging from 3022 to 5322 m/s (mean resaturated values) (Fig. F135A). These values are appropriate for well-lithified, porous formations such as these. There is a weak (general) positive linear relationship between bulk density and P-wave velocity (Fig. F135B).

Magnetic susceptibility

Hole M0057A has magnetic susceptibility values ranging from –1.27 × 10^–5 to 27.09 × 10^–5 SI. Downhole variation is obvious. The most notable feature is an interval of elevated magnetic susceptibility that increases from ~3 × 10^–5 SI at 18 m CSF-A to 27.09 × 10^–5 SI at 18.5 m CSF-A before falling quite dramatically to ~0 × 10^–5 SI at 18.68 m CSF-A (Fig. F133). However, these values do not appear to be coincident with any significant lithostratigraphic features.

Electrical resistivity

Noncontact resistivity measured on whole cores in Hole M0057A is variable, ranging from 0.87 to 28.91 Ωm (Fig. F133). No systematic downhole trend is obvious from this data set, and certain cores (325-M0057A-2R, 3R, 10R, and 11R) clearly give erratic data, probably as a consequence of core quality (see “Physical properties” in the “Methods” chapter).

Digital line scans and color reflectance

Hole M0057A was one of the two boreholes with relatively high recovery for this transect. All cores were digitally scanned and, where appropriate, measured for color reflectance. Reflectance in Hole M0057A ranges from 38.56% to 83.03% with a homogeneous distribution and dispersion of the measurements with depth (Fig. F136). The interval from 0 to 6 m CSF-A presents a large range of values from 40.98% to 79.84% for L. The range of L* values then narrows to between 59.8% and 80.8%. This unit is largely composed of coralgal boundstone. These ranges of dispersion for L* are maintained throughout most of the hole because of the occurrence of corals within the coralgal boundstone The presence of a coralgal boundstone and packstone (to rudstone) unit below 35.4 m CSF-A makes reflectance values a bit higher than observed toward the top of the hole. Values for a* were mainly positive but close to zero, except for two outliers at 24.66 and 24.95 m CSF-A. These outliers are due to the presence of red stains detected in the cores, which are likely the result intense diagenesis of corals at these depths. The borehole exhibits high dispersion of data at all depths in all three parameters of color reflectance (L*, a*, and b*).

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. F137). Positive susceptibilities were recorded in samples from much of the core, ranging from 0.02 × 10^–8 to 4.00 × 10^–8 m³/kg, with an arithmetic mean of 0.95 × 10^–8 m³/kg, indicating the presence of paramagnetic and/or ferromagnetic minerals. In addition, 22 negative susceptibilities (diamagnetic) were recorded throughout the hole, ranging from –0.02 × 10^–8 to –1.78 × 10^–8, with an arithmetic mean of –0.50 × 10^–8 m³/kg.

Chronology

A calibrated radiocarbon age (12 cal y BP, Core 325-M0057A-5R) (Fig. F138) and one U-Th age (old, Core 15R) (see Table T10 in the “Methods” chapter) are consistent with their stratigraphic positions. The assessment that the U-Th age is “old” is based on the [²³⁰Th/²³⁸U] being in excess of any possible closed-system scenario. Therefore, this hole recovered material from the middle portion of the deglaciation and may well have recovered material from before the Last Glacial Maximum.

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