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Sedimentology and biological assemblages

Last deglacial sequence (Unit I)

The last deglacial sequence (lithologic Unit I) is 26 m thick in Hole M0023A, from 67.98 to 93.98 meters below sea level (mbsl), and 30 m thick in Hole M0023B, from 67.58 to 97.5 mbsl. It is primarily composed of coralgal-microbialite frameworks in which microbialites (laminated and thrombolitic microbial fabrics) usually represent the major volumetric and structural component; thrombolites usually represent the last stage of encrustation (Fig. F1) (e.g., interval 310-M0023A-10R-1, 66–103 cm). Primary cavities are locally partly filled with muds or with skeletal sediments comprising abundant Halimeda segments (Fig. F2) (e.g., interval 310-M0023B-4R-1, 143–151 cm), associated with mollusk and gastropod shells, and echinoid spines; volcanic silt- to sand-sized grains usually occur in those infillings, especially in the lowermost part of the sequence. Manganese(?) impregnation and dark staining of corals are reported at the top of the sequence (e.g., interval 310-M0023A-1R-1, 26 cm, and Core 310-M0023A-2R). Basalt pebbles and volcanic silt- to sand-sized grains are abundant toward the base of this sequence (Core 310-M0023A-13R).

The last deglacial sequence is characterized by three successive coral assemblages (Subunits IA–IC).

Subunit IA

  • Intervals: Cores 310-M0023A-1R, 3R, and 4R and 310-M0023B-1R through 5R

Subunit IA is characterized by the abundant occurrence of encrusting colonies of Montipora, agariciids (including Pavona) (Fig. F3) (e.g., interval 310-M0023B-3R-1, 13–36 cm), Porites, and Montipora, foliaceous colonies of Pachyseris associated with branching colonies of Porites, and, to a lesser extent, robust branching colonies of Pocillopora. These coral colonies display evidence of bioerosion and are usually coated by thin crusts of nongeniculate coralline algae. Fragments of branching colonies of Pocillopora, Porites, and Acropora also occur.

Subunit IB

  • Intervals: Core 310-M0023A-2R, Section 5R-1, and Core 6R

Subunit IB includes massive and encrusting colonies of Porites and faviids (including Leptastrea) (Fig. F4); branching colonies of Porites occur locally (e.g., interval 310-M0023A-2R-1, 39–47 cm). These coral colonies are extensively bored; most of the borings are still open.

Subunit IC

  • Intervals: Cores 310-M0023A-7R through 13R and 310-M0023B-6R through 15R

Subunit IC contains branching and encrusting colonies of Porites associated with encrusting colonies of Millepora, Montipora, Psammocora, and Pavona, massive colonies of Leptastrea and Porites, branching and robust branching colonies of Pocillopora, and tabular colonies of Acropora (Figs. F5, F6, F7) (e.g., intervals 310-M0023A-10R-2, 0–26 cm, 11R-1, 0–40 cm, and 13R-1, 51–71 cm). These coral colonies commonly display traces of bioerosion by bivalves. Crusts of nongeniculate coralline algae on coral colonies are usually thin, except in some intervals where they are well developed in association with vermetid gastropods and serpulids (Cores 310-M0023A-13R and 310-M0023B-6R, 8R, and 10R). Branching Porites frameworks are locally fragmented to form rudstone intervals (Cores 310-M0023B-5R and 6R). Coralgal-microbialite frameworks are locally interlayered with beds of rubble composed of branching Porites and Pocillopora fragments and Halimeda segments (e.g., Core 310-M0023B-15R).

Older Pleistocene sequence (Unit II)

  • Intervals: Cores 310-M0023A-14R through 16R and 310-M0023B-16R

The older Pleistocene sequence (lithologic Unit II) is composed of brown well-lithified algal bindstone, microbialites, and coralgal frameworks that exhibit evidence of diagenetic alteration, including the transformation of coral skeletons and the occurrence of solution cavities. The matrix of the coralgal frameworks is composed chiefly of silt to fine sand-sized skeletal grains (Fig. F8) (e.g., interval 310-M0023A-14R-1, 68–87 cm). Basalt pebbles and lithoclasts occur throughout this sequence. Core 310-M0023B-15R recovered coral clasts derived from robust branching colonies of Pocillopora and branching colonies of Porites, which display traces of diagenetic alteration. Solution cavities occur throughout this interval and are filled with several generations of infillings, including well-lithified, pale brownish limestone and poorly lithified dark brown sandy sediments, including skeletal grains.

The coral assemblage includes massive and encrusting colonies of faviids (including Montastrea) (Fig. F9), robust branching and tabular colonies of Acropora, encrusting colonies of agariciids, and branching colonies of Porites (e.g., interval 310-M0023A-15R-1, 17–37 cm). Fragmented coral colonies include branching Pocillopora and Porites and tabular Acropora.