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doi:10.2204/iodp.proc.310.110.2007

Sedimentology and biological assemblages

Sites M0019 and M0020 were drilled at 59.9 and 83.7 meters below sea level (mbsl), respectively. The boundary between the last deglacial and older Pleistocene sequences at the two sites, which is defined on the basis of lithological and diagenetic features, occurs at 82 and 92 mbsl in Holes M0019A and M0020A, respectively.

Last deglacial sequence (Unit I)

• Intervals: Cores 310-M0019A-1R through 10R-1 and 310-M0020A-1R through 7R

Lithologic Unit I recovered at Sites M0019 and M0020 is 21 and 8 m thick, respectively, and displays a very similar composition in the two holes.

The top of the sequence corresponds to a hardground and is characterized by extensive bioerosion and staining of the top 50 cm (Fig. F1) (e.g., interval 310-M0020A-1R-1, 3–9 cm). It is primarily composed of loose coralgal-microbialite frameworks (bindstone) interlayered with beds of coral rubble. The beds are composed of reworked and rounded fragments of coral colonies (branching agariciids and Porites), coralline algal crusts, and microbialites that are strongly bored and stained.

The coralgal-microbialite frameworks are dominated by encrusting colonies of Montipora, agariciids (Pavona?), Acropora, Psammocora, and Echinophyllia associated locally with massive colonies of Porites, Montastrea, and Cyphastrea and encrusting colonies of Leptoseris and by fragments of robust branching Pocillopora, tabular Acropora, and branching Porites in addition to the coral colonies listed above (Figs. F2, F3, F4, F5, F6) (e.g., intervals 310-M0019A-1R-1, 8–13 cm, 2R-1, 74–80 cm, 2R-1, 80–98 cm, and 9R-1, 72–86 cm, and 310-M0020A-2R-1, 57–64 cm, and 4R-1, 20–28 cm). The base of the last deglacial sequence is characterized by the occurrence of in situ robust branching colonies of Pocillopora, massive colonies of Porites (Cores 310-M0019A-9R and 10R), and tabular colonies of Acropora (Core 310-M0020A-7R); bioerosion is extensive in those intervals (Fig. F7) (e.g., interval 310-M0019A-9R-1, 32–38 cm).

Corals are usually coated with thin crusts of nongeniculate coralline algae. Microbialites consist of dark gray, laminated, dense, and thrombolitic fabrics; the latter are usually dominant (Fig. F8) (e.g., interval 310-M0020A-4R-1, 0–10 cm). Large cavities, partly to fully filled with skeletal sand rich in Halimeda segments, commonly occur. Reddish brown to dark staining on the surface of reef rocks is conspicuous in Cores 310-M0019A-2R, 7R, and 8R and 310-M0020A-2R, 3R, 6R, and 7R. Corals display traces of alteration in Core 310-M0020A-5R.

Older Pleistocene sequence (Unit II)

This sequence displays a distinctive composition in the two holes. The uppermost part of the older Pleistocene sequence (lithologic Unit II) is characterized by the occurrence of brown to dark staining.

Hole M0019A

The older Pleistocene sequence recovered at Site M0019 is 43 m thick. This sequence includes three subunits characterized by their distinctive lithology and composition and separated by unconformities at 106.2 (Core 310-M0019A-22R) and 121.12 (Core 310-M0019A-33R) mbsl, respectively.

Subunit IIA

• Interval: Section 310-M0019A-10R-1 through Core 22R

The upper subunit in Hole M0019A is 24 m thick and comprises irregular alternations of beige coralgal and skeletal limestone bearing in situ coral colonies and beds composed of coral rubble.

The coral assemblages forming the coralgal frameworks consist of massive colonies of Porites (e.g., Cores 310-M0019A-14R, 16R, and 18R through 20R), branching and encrusting colonies of Porites, and, to a lesser extent, encrusting colonies of Porites, agariciids, and Montipora, massive colonies of Leptastrea and Montastrea, and tabular colonies of Acropora (Figs. F9, F10, F11, F12, F13, F14) (e.g., intervals 310-M0019A-13R-1, 40–45, cm, 13R-1, 50–60 cm, 15R-1, 51–57 cm, 19R-1, 42–54 cm, 20R-1, 65–75 cm, and 22R-1, 35–50 cm). Fragments of branching Pocillopora occasionally occur. These coral colonies are generally thinly coated with nongeniculate coralline algae, locally associated with microbialite crusts, except in the lower part of the subunit, where coralline algal crusts are thicker and include vermetid gastropods.

Skeletal limestone consists of a well-lithified rudstone including subrounded fragments of coral colonies (massive and branching Porites, robust branching Pocillopora, and tabular Acropora) in many cases encrusted with nongeniculate coralline algae and locally surrounded by cement fringes. Other skeletal grains include Halimeda segments and fragments of echinoids, coralline algae, and mollusks. This limestone locally displays conspicuous alteration; walls of pores in many cases exhibit red-brown staining and in some cases are lined with yellow cements (Fig. F15) (e.g., interval 310-M0019A-10R-1, 70–85 cm).

Rubble beds are composed of fragments of coral colonies (branching and massive Porites, robust branching Pocillopora, and tabular Acropora) and are thinly encrusted with coralline algae and, locally, microbialites.

Subunit IIB

• Interval: Cores 310-M0019A-22R through 26R

Subunit IIB in Hole M0019A is 15 m thick and composed of yellowish brown skeletal floatstone to grainstone including rhodoliths, foraminifers, and fragments of corals (branching and encrusting Porites, robust branching Pocillopora, and tabular Acropora) (Fig. F16) (e.g., interval 310-M0019A-22R-1, 50–70 cm) and coralline algae, echinoids, and mollusks; sand-sized volcanic grains occur in the lower part of this subunit, and their abundance increases downhole. Large coral fragments are usually thinly encrusted with coralline algae (Figs. F17, F18) (e.g., intervals 310-M0019A-22R-1, 102–108 cm, and 27R-1, 38–44 cm). In situ coral colonies (massive and encrusting Porites in Cores 310-M0019A-24R and 25R) occur locally.

Subunit IIC

• Interval: Cores 310-M0019A-27R through 34R

The lower subunit in Hole M0019A is 11 m thick and composed of irregular alternations of coralgal frameworks, skeletal limestone, and rubble beds. Volcanic silt and sand and occasional volcanic gravels and pebbles occur as minor components in the carbonate units.

Coralgal frameworks are dominated by tabular colonies of Acropora, massive colonies of Porites (e.g., Cores 310-M0019A-27R and 28R), encrusting colonies of Porites and Montipora, and robust branching colonies of Acropora (Figs. F19, F20) (e.g., intervals 310-M0019-27R-1, 68–83 cm, and 32R-1, 46–66). These coral colonies are heavily encrusted with nongeniculate coralline algae associated with vermetid gastropods. Sand-sized volcanic grains occur commonly in the matrix. Corals exhibit evidence of diagenetic alteration.

Skeletal rudstone is very poorly sorted and rich in large fragments of coral colonies (tabular Acropora, robust branching Pocillopora, branching Porites, and encrusting agariciids), rhodoliths, Halimeda segments, and fragments of coralline algae, mollusks, and echinoids (Figs. F21, F22, F23, F24) (e.g., intervals 310-M0019A-32R-1, 67–83 cm, 33R-2, 10–35 cm, 33R-2, 97–112 cm, and 33R-3, 82–104). Large fragments of corals are commonly encrusted with coralline algae. Sand- to pebble-sized volcanic grains are commonly present. Reddish to yellowish stains occur locally.

Rubble beds are composed of fragments of coral colonies (branching and massive Porites, robust branching Pocillopora, and tabular Acropora) thinly encrusted with coralline algae, rhodoliths, and basalt gravels and pebbles.

Hole M0020A

• Interval: Cores 310-M0020A-7R through 25R

In Hole M0020A, the sequence is primarily composed of coralgal frameworks locally interlayered with skeletal limestone and rubble beds.

The coralgal frameworks are dominated by massive, branching, and encrusting colonies of Porites, locally associated with encrusting colonies of Montipora and Pavona, and robust branching colonies of Pocillopora (Figs. F25, F26, F27, F28, F29, F30) (e.g., intervals 310-M0020A-10R-1, 40–47 cm, 12R-1, 65–80 cm, 16R-1, 56–70 cm, 18R-1, 49–58 cm, 20R-1, 89–101 cm, and 21R-2, 14–23 cm). These coral colonies are heavily encrusted with nongeniculate coralline algae associated with vermetid gastropods and serpulids and with microbialites (massive laminated fabrics overlain by thrombolitic accretions) (Figs. F31, F32, F33) (e.g., intervals 310-M0020A-12R-1, 34–50 cm, 18R-2, 0–15 cm, and 20R-1, 110–153 cm). Corals display traces of bioerosion and are locally strongly altered. Large vugs are partly filled with internal sediments and are commonly lined with cements. Walls of cavities and vugs display brown, red, to black staining throughout this interval. Large bioerosion cavities are locally filled with microbialite crusts (e.g., Core 310-M0020A-22R).

Massive Porites are dominant in several intervals (e.g., Cores 310-M0020A-14R, 15R, and 22R through 24R); a single colony of massive Porites ~3.5 m thick was recovered in Cores 310-M0020A-22R and 23R (Fig. F34) (e.g., interval 310-M0020A-22R-2, 16–29 cm).

Interlayered skeletal limestone consists of Halimeda wackestone and poorly sorted coral rudstone including fragments of branching and encrusting Porites, robust branching Pocillopora and Acropora, and encrusting Montipora. Other skeletal grains include Halimeda segments (Fig. F35) (e.g., interval 310-M0020A-25R-2, 0–17 cm) and fragments of mollusks and echinoids; volcanic silt to sand grains are locally abundant.

Rubble beds are comprised of fragments of coral colonies (branching Porites, Pocillopora, and Pavona).

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