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Sedimentology and biological assemblages

Holes dominated by volcaniclastic sediments

Hole M0008A

Hole M0008A contains a 35.90 m thick sequence, from 64.15 to 100.05 meters below sea level (mbsl), and includes two lithologic subunits composed of volcaniclastic sediments.

Subunit A
  • Interval: Core 310-M0008A-1R through Section 10R-1

Volcaniclastic Subunit A is black uncemented to poorly consolidated volcaniclastic sand, silt, and clay interlayered with beds of variable thickness that are composed of basalt pebbles, cobbles, and boulders (Fig. F1) (e.g., interval 310-M0008A-8R-1, 80–110 cm) with a granule-rich sandstone/​siltstone subunit member in Section 310-M0008A-8R-CC. At the top of the underlying Section 310-M0008A-7R-CC is a thin (~4 cm thick) granule conglomerate horizon, underlain by a rubble interval.

Sand and silt are unsorted and not graded; clay and subrounded granules are subordinate. They include volcanic lithics, feldspars, feldspathoids, quartz, and iron oxides and hydroxides. Skeletal components include foraminifers and fragments of corals (acroporids), mollusks, echinoids, and Halimeda; in situ roots occur locally.

The sand/​silt sediment in the upper section of the hole is generally gray in color, whereas that in the lower part is orange (10–15 cm horizon transition in Section 310-M0008A-8R-1, 100 cm). (Note that a similar color transition was observed in the bindstone unit in Section 310-M0021B-19R-1.) The transition from gray to orange is interpreted as the result of a change in the relative proportions of iron oxides/​oxyhydroxides with the possible increase in decayed organic matter; brownish color is locally conspicuous. The amount of clays (mud) also increases downward, peaking at the bottom of Section 310-M0008A-16R-1, where the lithology turns to a silty clay (mud) subunit. A large boulder (or lava flow?) is present in the rubble unit from Section 310-M0008A-7R-CC to Section 8R-1, 80 cm, above the gray to orange color transition.

Pebbles, cobbles, and boulders are subangular to rounded and mostly composed of a slightly to moderately altered basalt, sparsely to highly phyric, nonvesicular to sparsely vesicular, with olivine, pyroxene, and feldspars; some subangular pebbles are made of siltstone to very coarse sandstone that includes volcanic lithics, iron oxides and hydroxides, and skeletal grains. Some pebbles and cobbles are encrusted with coralline algae.

Subunit B
  • Interval: Cores 310-M0008A-10R-1 through 19R

Volcaniclastic Subunit B is black, dark brown, to reddish brown and orange volcaniclastic siltstone to sandstone interlayered with volcaniclastic sand and silt and volcaniclastic cobble and rubble.

Siltstone and sandstone are massive, weakly cemented, unsorted, and not graded; they usually exhibit weak and irregular stratifications. They are mostly composed of angular to subangular grains including volcanic lithics, feldspars, feldspathoids, quartz, iron oxides and hydroxides, and skeletal grains (coral fragments, tiny mollusks, and algal fragments). Organic-rich soil horizons occur in Sections 310-M0008A-14R-1 and 15R-1.

Rubble is made of angular to subrounded basalt granules and pebbles, whereas basalt comprises angular basalt cobbles. A few small (approximately a few millimeters to a centimeter in length) wood fragments and rare, delicate, fine (submillimeter diameter) roots are present in the sand/​silt units.

Hole M0022A

  • Interval: Cores 310-M0022A-1R through 6R

A 7 m thick sequence (118.64–125.46 mbsl) of volcanic granules and pebbles, including a medium-grained trachyandesite or mugearite, and fragments of corals (branching Pocillopora) was recovered in Hole M0022A overlying volcanic siltstone (Sections 310-M0022A-12R-1, 12 cm, to 3R-1) that includes subangular to subrounded pebbles of fine- to medium-grained basalt rich in olivine, pyroxene, and feldspars and of medium-grained andesite or mugearite (Fig. F2) (e.g., interval 310-M0022A-2R-1, 23–33 cm). Volcanic siltstone includes skeletal grains and exhibit bioturbation; their tops are encrusted with coralline algae in Core 310-M0022A-2R. A bioclast/basalt conglomerate unit (interval 310-M0022A-4R-1, 0–23 cm) and a volcanic siltstone unit compose the bottom of the hole.

Holes composed of volcaniclastic sediments and carbonates

Last deglacial sequence (Unit I)

In holes near the outer ridge (Holes M0010A–M0014A), the last deglacial sequence (lithologic Unit I) is composed of three successive subunits that were recovered from 78.85 to 117 mbsl.

Subunit IA
  • Interval: Cores 310-M0012A-1R through 16R

Lithologic Subunit IA was recovered only at Site M0012 from 78.85 to 107 mbsl. It is mostly composed of loose sediments (coral rubble and skeletal silt and sand) locally interlayered with volcaniclastic sand rich in skeletal grains.

Coral rubble consists mostly of gravels, pebbles, and cobbles of coral fragments (foliaceous and encrusting Pavona, Leptoseris, Montipora, and Pachyseris and branching Pocillopora and Porites), crusts of nongeniculate coralline algae, microbial crusts, and clasts of coralgal-microbialite frameworks. Those elements are generally well rounded to subangular and display abundant traces of bioerosion. Fragments of bivalves and gastropods and Halimeda segments also occur. Sand-sized volcanic grains are locally associated. Basalt cobbles and smaller pebbles and granules are prevalent rubble components in Sections 310-M0012A-13R-1 and 14R-1.

Skeletal silt to sand is composed of fragments of corals, coralline and green (especially Halimeda) algae, and, to a lesser extent, bryozoans, mollusks, and foraminifers (Amphistegina and Heterostegina).

In situ coral colonies occur in Section 310-M0012A-9R-1 (massive Porites), Core 14R (encrusting Montipora), and Section 15R-1 (robust branching Pocillopora) (Fig. F3) (e.g., interval 15R-1, 17–40 cm).

Subunit IB
  • Intervals: Cores 310-M0010A-1R through 4R, 310-M0011A-1R and 2R, 310-M0012A-7R and 8R, 310-M0013A-1R through 5R, and 310-M0014A-1R through 3R

Lithologic Subunit IB, recovered in an interval ranging generally from 90 to 100–105 mbsl, is dominated by volcaniclastic sediments that are locally interlayered with beds composed of carbonate elements, including coral colonies. The various lithologies recovered follow.

Volcaniclastic and skeletal sand/​silt

Volcaniclastic and skeletal sand and silt usually form the top of the recovered sequences. Skeletal grains are abundant and include Halimeda segments, benthic and planktonic foraminifers, and mollusk fragments. This unit includes beds composed of angular to subangular gravels, pebbles, and cobbles of fine- to medium-grained phyric or vesicular basalt that includes fine laths of feldspathoids and pyroxenes; the groundmass is aphanitic. Alteration of the basalt is variable.

The top ~8 m in Hole M0010A (Sections 310-M00010A-1R-1 through 4R-CC) consists of unconsolidated volcanic sand and silt. The amount and size of lithic basalt and coral debris components increase downward. Basalt cobbles and pebbles were observed in the rubble unit in the upper part of Hole M0011A (from Section 310-M0011A-1R-1 to Section 3R-CC). Thin volcanic sand units appear near the upper part (Sections 310-M0012A-7R-CC and 8R-CC) of Hole M0012A. Rubble at the top of Hole M0013A contains basalt pebbles. This unit is underlain by a volcanic sand/​silt unit. A volcanic silt unit occupies the upper part of Hole M0014A (Section 310-M0014A-1R-1 and interval 2R-1, 0–11 cm).

Carbonate and volcanic beds

Beds are composed of a mixture of carbonate and volcanic elements. Carbonate rubble is mostly composed of coral fragments, pieces of coralline algal crusts, and pebble-sized clasts of microbialite-coralgal frameworks. Coral fragments include robust branching Pocillopora and Acropora, branching Porites, and encrusting Montipora and Pachyseris, some of which are encrusted with coralline algae and/or microbialites (dense and thrombolitic microbial fabrics). Volcanic elements include cobble- and pebble-sized pieces of basalt.

Subunit IC
  • Intervals: Cores 310-M0010A-5R through 9R, 310-M0011A-2R through 9R, and 310-M0014A-3R through 12R

Lithologic Subunit IC, from ~100 to 115 mbsl, is primarily composed of coralgal-microbialite frameworks interlayered with beds composed of

  • Coral rubble and fragments of coralline algal crusts and microbialites mixed with sand- to pebble-sized volcanic elements,
  • Skeletal sand rich in Halimeda segments, foraminifers, and fragments of mollusks and coralline algae mixed with volcanic grains, and
  • Volcaniclastic and skeletal sand and silt.

Frameworks are composed of distinctive coral assemblages dominated by branching colonies of Pocillopora, Acropora, Porites, and Montipora, encrusting colonies of Montipora, agariciids, and faviids (including Leptastrea), and massive colonies of Porites and faviids.

Encrusting colonies
  • Intervals: Cores 310-M0010A-7R and 8R, 310-M0011A-6R through 9R, and 310-M0014A-5R, 9R, 11R, and 12R

This interval comprises encrusting Montipora, agariciids, and faviids and branching Porites (Fig. F4) (e.g., interval 310-M0011A-7R-1, 10–21 cm). These corals are coated with thin nongeniculate coralline algal crusts and microbialites (especially thrombolites) to form a loose framework. Primary cavities are partially filled with skeletal sand and gravels, including abundant volcanic components.

Massive colonies
  • Intervals: Cores 310-M0010A-9R, 310-M0011A-2R, 4R, and 6R, and 310-M0014A-3R, 5R through 7R, and 10R

Massive colonies of Porites and faviids are locally associated with branching Pocillopora and Montipora. Some massive colonies of Porites are as thick as 50 cm. Microbialites may form dense compact layers overlying thin crusts of nongeniculate coralline algae in those frameworks. Some primary cavities in the frameworks are infilled with volcaniclastic sediments.

Older Pleistocene sequence (Unit II)

The boundary between the last deglacial and the older Pleistocene sequences is sharp and was recovered in Holes M0011A (interval 310-M0011A-9R-1, 1 cm; Fig. F5), M0012A (interval 310-M0012A-16R-1, 30 cm; Fig. F6), and M0014A (interval 310-M0014A-13R-1, 45 cm).

Lithologic Unit II is composed of well-lithified gray to light brown coralgal boundstone, coral rudstone, and skeletal sandy limestone interlayered locally with horizons of gravels and rubbles made of that material. Coral assemblages are dominated by foliaceous colonies of Pachyseris, tabular and branching colonies of Acropora, robust branching Pocillopora, encrusting and branching colonies of Montipora, and branching and massive colonies of Porites. Microbialites are abundant and include laminated and thrombolitic fabrics. The matrix of the limestone is rich in Halimeda segments; volcanic grains are locally abundant. Subaerial diagenetic processes are indicated by the alteration of coral skeletons and the occurrence of large solution cavities filled with volcaniclastic and skeletal sandstone and with gravels and rubble, including fragments of branching Pocillopora and basalt pebbles at the top of the sequence.

In Hole M0014A, the base of the older Pleistocene sequence is composed of sandy skeletal grainstone rich in Halimeda segments, shell fragments, and much less common bryozoan skeletons; fine to medium sand-sized volcanic grains are commonly associated (Figs. F7, F8) (e.g., intervals 310-M0014A-13R-1, 26–60 cm, and 13R-1, 105–115 cm). This unit grades downward into poorly bedded volcaniclastic sandstone rich in skeletal grains (Halimeda and corals associated with foraminifers, bryozoans, and gastropods). A volcanic cobble occurs in a coral mudstone unit in Section 310-M0014A-13R-1.