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Sedimentology and biological assemblages

Last deglacial sequence (Unit I)

On the outer ridge of Tiarei, the last deglacial sequence (lithologic Unit I) was recovered from 81.7 to 122 mbsl. The thickest continuous sequence, reaching 30 m, was recovered in Holes M0021A, M0021B, and M0024A.

The last deglacial sequence is mostly composed of coralgal-microbialite frameworks locally interlayered with

  • Volcanic sediments, including coarse sand displaying skeletal elements (foraminifers, Halimeda segments, fragments of mollusks, bryozoans, and corals, including branching Porites).
  • Rubble and sand consisting mostly of coral fragments (dominated by branching colonies and, to a lesser extent, tabular colonies) associated with crusts of nongeniculate coralline algae and microbialites (Fig. F1) (e.g., interval 310-M0009B-9R-1, 5–20 cm), chunks of microbialite crusts, Halimeda segments (Fig. F2) (e.g., interval 310-M0024A-15R-1, 0–20 cm), and mollusk shells and fragments (e.g., Cypraea and other gastropods); rhodoliths occur locally. Intraclasts display bioeroded surfaces and encrusting worm tubes. Volcanic pebbles are locally mixed with carbonate elements.
  • Skeletal packstone to floatstone rich in Halimeda segments and fragments of corals and mollusks (Fig. F3) (e.g., interval 310-M0026A-6R-1, 88–96 cm).

The top of the last deglacial sequence is characterized by the widespread development of microbialite crusts that display laminar and knobby morphologies; thin thalli of nongeniculate coralline algae (Fig. F4) (e.g., interval 310-M0021A-1R-1, 10–23 cm) are usually associated with those crusts.

Extensive bioerosion, dark brown staining, and hardgrounds are features that have been locally observed within the top 2–3 m of the sequence (Fig. F5) (e.g., interval 310-M0021B-1R-1, 0–11 cm).

Coralgal-microbialite frameworks are characterized by distinctive internal structure and coral assemblages (Subunits IA–IE). Some of the coral assemblages listed below are intergradational, both laterally and vertically. However, the succession reported in all holes displays a general consistent trend, including Subunits IA–IE.

All coralgal-microbialite frameworks are characterized by the widespread development of dark gray microbialite crusts that extend into primary and bioerosion cavities. Microbialites usually represent the major volumetric and structural component of the frameworks (Figs. F6, F7) (e.g., intervals 310-M0021A-5R-1, 58–71 cm, and 8R-1, 18–31 cm). The thickness and morphologies of the microbialite crusts are closely related to the shape and size of the cavities in which they developed. In bindstone formed by encrusting coral assemblages, microbialites are generally dominated by thrombolitic fabrics, whereas in frameworks made of branching and massive coral colonies, they are characterized by the development of compound crusts, up to 15 cm thick, in which multiple generations formed by the succession of laminated and thrombolitic fabrics may be closely packed and merged (Fig. F8) (e.g., interval 310-M0021B-5R-1, 63–80 cm). Thrombolites usually represent the last stage of encrustation (Fig. F9) (e.g., interval 310-M0021A-6R-1, 9–20 cm) and consist of closely spaced micritic masses that range from narrow millimeter-sized upward-radiating shrubs to broader dendritic clusters up to 1 cm high. These masses are intergradational, both vertically and laterally.

In Hole M0025B, the basal part of the last deglacial sequence is composed of an algal bindstone (Core 310-M0025B-12R).

Subunit IA

  • Intervals: Cores 310-M0009A-1R through 6R-1 and 8R-1, 310-M0009B-1R through 4R, 310-M0009C-4R through 6R, 310-M0009D-1R through 4R, 310-M0009E-1R through 6R, 310-M0021A-1R and 6R, 310-M0021B-1R through 3R-1, 310-M0024A-1R through 3R, 310-M0025A-1R through 4R, and 310-M0025B-1R

Subunit IA comprises coralgal-microbialite frameworks (bindstone) dominated by encrusting colonies of Montipora, Montastrea, Leptastrea, Pavona, and Porites and encrusting and foliaceous colonies of agariciids (Pachyseris and Leptoseris) (Figs. F10, F11, F12) (e.g., intervals 310-M0009B-2R-1, 20–30 cm, 310-M0021B-1R-1, 65–83 cm, and 310-M0025A-1R-1, 34–61 cm). Other corals are less abundant and include branching colonies of Porites and Pocillopora (Fig. F13) (e.g., interval 310-M0021A-5R-1, 88–107 cm) and massive colonies of mussids (Acanthastrea?) (e.g., interval 310-M0021B-1R-1, 30–50 cm).

Coral colonies are usually thinly coated with nongeniculate coralline algae and microbialites to form loose, cavernous frameworks, including large primary cavities that are partially filled with skeletal sand and gravels composed of Halimeda segments and mollusk fragments; volcanic grains are associated locally. Bioerosion is locally extensive, especially at the top of the subunit.

Volcanic sand and occasional volcanic granules and pebbles were observed in vugs and small sedimentary pockets in the carbonate units and in the interstitial spaces in coralgal frameworks and crushed framework intervals in Section 310-M0021-2R-1.

Subunit IB

  • Intervals: Section 310-M0009A-8R-2, Cores 310-M0009B-5R through 8R and Section 13R-1, Cores 310-M0009C-7R through 10R and 14R through 16R, Cores 310-M0009D-5R, 6R, and 8R and Section 9R-1, Cores 310-M0009E-7R through 10R, Section 310-M0021A-6R-2 through Core 19R, Section 310-M0021B-3R-1 through Core 9R and Cores 11R through 19R, Core 310-M0024A-3R through Section 10R-1, Cores 310-M0025B-2R, 4R through 8R, 11R, and 12R, and Core 310-M0026A-1R

Subunit IB contains coralgal-microbialite frameworks dominated by branching and encrusting colonies of Porites and Montipora; associated corals include encrusting and branching colonies of Pavona and, to a lesser extent, robust branching colonies of Pocillopora, tabular colonies of Acropora, and encrusting colonies of agariciids (Figs. F14, F15, F16, F17, F18, F19, F20, F21) (e.g., intervals 310-M0021A-10R-1, 102–117 cm, and 12R-1, 30–50 cm, 310-M0021B-7R-1, 70–86 cm, and 15R-1, 46–66 cm, and 310-M0024A-3R-1, 72–91 cm, 5R-2, 8–20 cm, 9R-1, 19–49 cm, and 9R-2, 74–90 cm).

Coral colonies are coated with crusts of nongeniculate coralline algae up to 2.5 cm thick, locally associated with vermetid gastropods and serpulids (Figs. F22, F23, F24, F25, F26) (e.g., intervals 310-M0009B-13R-1, 3–18 cm, 310-M0021B-9R-1, 0–12 cm, and 310-M0024A-9R-1, 78–90 cm, 9R-2, 6–19 cm, and 10R-1, 0–6 cm). Nongeniculate coralline algal crusts are generally overlain by microbialites. Traces of bioerosion usually occur on coral colonies, but their abundance is variable.

Pockets of skeletal sand that include Halimeda segments (Fig. F27) (e.g., interval 310-M0025B-8R-1, 52–63 cm) and fragments of mollusks, bryozoans, and echinoids occur, usually within primary voids of the frameworks; silt- to sand-sized volcanic grains and basalt gravels are associated locally.

Subunit IC

  • Intervals: Cores 310-M0009A-7R, 310-M0009B-11R, 12R, and 14R, 310-M0009C-11R through 13R, 310-M0009D-6R and 7R and Section 9R-2 through Core 11R, Cores 310-M0009E-7R and 9R, Sections 310-M0021A-6R-1 and 310-M0024A-10R-1 through Core 13R, and Cores 310-M0025A-6R and 8R through 10R, 310-M0025B-5R and 8R through 10R, and 310-M0026A-3R, 4R, 6R, and 7R

Subunit IC comprises a coral assemblage dominated by massive colonies of Porites, some of which display traces of bioerosion (Figs. F28, F29, F30, F31) (e.g., intervals 310-M0009D-7R-1, 10–30 cm, 310-M0021A-6R-1, 59–71 cm, 310-M0024A-12R-2, 10–15 cm, and 310-M0025A-6R-1, 72–90 cm). Associated coral colonies include encrusting and branching Porites, robust branching Pocillopora, and encrusting Montipora.

Coral colonies are usually coated with thin to moderately thick crusts of nongeniculate coralline algae, associated locally with vermetid gastropods and serpulids, and overlain by very thick and massive microbialite masses.

Interlayered volcaniclastic sediments include sand and basalt pebbles (Core 310-M0009A-7R). Limestone clasts and/or shells and skeletal sand rich in Halimeda segments and volcanic grains occur locally between massive coral colonies.

Subunit ID

  • Intervals: Cores 310-M0009A-12R and 13R, Section 310-M0009B-13R-2 through Core 15R, Section 310-M0009E-10R-2, Core 11R, and Section 12R-1, Sections 310-M0024A-10R-1 and 11R-2 and Core 15R, and Cores 310-M0025B-6R and 8R

Subunit ID includes a coral assemblage dominated by robust branching colonies of Pocillopora, encrusting colonies of faviids, and encrusting and branching colonies of Montipora (Figs. F32, F33) (e.g., intervals 310-M0024A-10R-1, 65–85 cm, and 310-M0025B-6R-1, 20–37 cm).

Coral colonies are coated with thick crusts of nongeniculate coralline algae, locally associated with vermetid gastropods and serpulids, and overlain by massive microbialite coatings.

Subunit IE

  • Intervals: Cores 310-M0009C-17R and 18R, 310-M0009D-12R, and 310-M0026A-4R

Subunit IE displays an assemblage composed of encrusting and massive colonies of faviids (Favia and Leptastrea) locally associated with encrusting colonies of Montipora and Psammocora. Associated coral rubble is composed mostly of fragments of branching Porites, Acropora, and Pocillopora (Fig. F34) (e.g., interval 310-M0009C-18R-1, 90–107 cm). Coral colonies are coated with thick crusts of nongeniculate coralline algae overlain by microbialites. Pockets of skeletal sand that includes Halimeda segments and fragments of mollusks occur, usually within the primary voids of the frameworks.

Older Pleistocene sequence (Unit II)

The contact between Units I and II is characterized by an irregular unconformity typified by the abundance of large solution cavities partly filled with unconsolidated skeletal and volcanic sand that includes fragments of fruticose, coralline algal branches, and Halimeda segments (Section 310-M0009C-20R-CC and Core 310-M0009D-11R) (Fig. F35) and with coral clasts (fragments of Pocillopora and Montipora; Core 310-M0009D-12R). Some cavities are partly filled with skeletal and volcanic sand and gravels and stalagmite crusts (Figs. F36, F37, F38) (e.g., intervals 310-M0009D-14R-2, 48–54 cm, and 24R-1, 14–25 cm, 310-M0021B-19R-1, 38–52 cm, and 310-M0025A-10R-1, 78–100 cm). These solution cavities are seemingly related to the karstification of the top of the older Pleistocene carbonate sequence. The top of that sequence (lithologic Unit II) is locally characterized by the occurrence of multiple bored and encrusted surfaces (hardgrounds; e.g., Cores 310-M0021B-19R and 310-M0025A-10R and interval 310-M0025A-10R-1, 22–37 cm) (Fig. F39) and several unconformities (Fig. F40) (e.g., interval 310-M0009C-20R-1, 84–104 cm).

The older Pleistocene sequence comprises four major lithologic subunits that are locally interlayered.

Subunit IIA

  • Intervals: Cores 310-M0009A-14R and 16R, 310-M0009B-15R through 18R, 310-M0009C-18R through 21R, 310-M0009D-13R through 20R, 23R, and 24R, 310-M0009E-12R, 310-M0021A-19R through 22R, 310-M0021B-19R and 20R, 310-M0024A-15R and 16R, 310-M0025A-10R and 11R, and 310-M0026A-19R and 20R

Subunit IIA is made up of well-lithified gray to brown coralgal frameworks associated with coral rudstone-floatstone and skeletal limestone. Some skeletal grains display brown staining. Volcanic gravels and sand-sized grains are locally abundant; volcanic pebbles and cobbles occur as a component in the coral boundstone subunit beneath the major unconformity at Section 310-M0024A-15R-1, 70 cm. A volcanic siltstone horizon occasionally containing volcanic pebbles and granules is present from Section 310-M0009A-16R-1, 28 cm, to the bottom of the hole in Section 310-M0009A-18R-CC. Holes M0025A and M0025B both terminated in a volcanic siltstone interval, from Section 310-M0025A-11R-1, 120 cm, in the former and from Section 310-M0025B-12R-1, 70 cm, in the latter.

In coralgal frameworks, coral assemblages are dominated by encrusting colonies of Leptastrea, Pachyseris, Montipora, and Psammocora and are locally associated with robust branching colonies of Acropora and Pocillopora, tabular colonies of Acropora, and massive colonies of Porites (Figs. F41, F42, F43, F44, F45, F46, F47) (e.g., intervals 310-M0009A-14R-1, 5–23 cm, 310-M0009D-18R-1, 13–31 cm, 310-M0021A-22R-2, 60–80 cm, 310-M0021B-20R-1, 60–68 cm, and 20R-2, 0–10 cm, and 310-M0025A-10R-1, 108–128 cm, and 11R-1, 50–65 cm). Nongeniculate coralline algal crusts are abundant and locally include vermetid gastropods and serpulids. Algal crusts are generally encrusted with columnar laminated and dendritic microbialite fabrics that may form very thick crusts and represent the last stage of encrustation. The matrix of the frameworks is generally composed of skeletal sediments rich in Halimeda segments and mollusk fragments and including volcanic grains. Reworked basalt pebbles occur locally in those beds.

Coral rudstone-floatstone is composed of fragments of corals (massive Porites, robust branching Pocillopora, and encrusting Echinophyllia, Montipora, Millepora, and Pachyseris) locally encrusted with nongeniculate coralline algae and microbialites (Fig. F48) (e.g., interval 310-M0009B-16R-1, 35–60 cm); the matrix is rich in Halimeda segments and echinoid fragments mixed with volcanic sand-sized grains and pebbles and cobbles of basalt.

Skeletal wackestone to packstone is rich in Halimeda segments, foraminifers, and fragments of corals, mollusks, echinoids, and nongeniculate coralline algae (encrusting and fruticose); sand-sized volcanic grains and basalt pebbles and cobbles are usually associated with the skeletal grains.

This subunit is characterized by abundant diagenetic alterations, including the recrystallization and transformation of coral skeletons, and by the occurrence of large solution cavities that usually display brown staining on their walls. Those cavities are filled with several generations of infillings, some of them displaying a geopetal structure, consisting of dark gray to brownish volcaniclastic skeletal silt and sand overlain by microbialite crusts. Some cavities are locally rimmed with multiple generations of cement crusts.

Subunit IIB

  • Intervals: Cores 310-M0009A-14R and 15R, 310-M0009D-21R through 23R, 310-M0021A-20R through 22R, 310-M0024A-15R and 16R, and 310-M0026A-7R and 8R

Subunit IIB contains horizons composed of coral fragments, limestone clasts, basalt pebbles, and reworked coral colonies (branching Porites and robust branching Acropora). Coral fragments include robust branching or tabular Acropora, robust branching Pocillopora, branching Porites, encrusting Montipora, and massive Porites; associated skeletal grains include Halimeda segments and bryozoan colonies. These coral fragments usually display abundant traces of bioerosion. Some of them are encrusted with nongeniculate coralline algae, whereas other clasts are partly embedded in Halimeda packstone.

Subunit IIC

  • Interval: Core 310-M0009D-25R

Subunit IIC is a sandy packstone/​grainstone with fragments of corals (branching Porites, Acropora, and Pocillopora) (Fig. F49) (e.g., interval 310-M0009D-25R-1, 0–11 cm) overlying a coral boundstone consisting mainly of massive Porites and encrusting coral colonies (Fig. F50) (e.g., interval 310-M0009D-25R-1, 52–62 cm) associated with tabular Acropora colonies (Fig. F51) (e.g., interval 310-M0009D-25R-1, 70–95 cm).

Subunit IID

  • Intervals: Cores 310-M0009A-16R through 18R, 310-M0025A-11R through 13R and 16R, 310-M0025B-12R (bottom) and 13R, and 310-M0026A-8R (bottom)

Subunit IID comprises massive poorly consolidated and cemented dark grayish to brownish volcaniclastic siltstone to sandstone that includes fragments of corals (branching Pocillopora and encrusting Montipora) and nongeniculate coralline algal crusts, as well as skeletal components (tiny mollusk shells and Halimeda segments). This siltstone to sandstone is interlayered with sandy limestone and granule- to pebble-sized pieces of aphanitic dense basalt (Figs. F52, F53, F54, F55, F56) (e.g., intervals 310-M0009A-16R-1, 4–9 cm, 310-M0025A-11R-1, 72–86 cm, 11R-1, 85–107 cm, and 12R-2, 70–85 cm, and 310-M0025B-13R-1, 65–75 cm). Bioturbation products are locally abundant and infilled with coarse sediments, including gastropods and Halimeda segments.