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doi:10.2204/iodp.proc.325.102.2011

Sedimentological and biological assemblages

Visual core descriptions

Offshore sedimentologists and coral specialists conducted preliminary visual inspections and prepared written descriptions of the intact cores and core catcher material recovered during the expedition. Intact cores were examined visually, either through plastic core liners or uncovered when drilled using metal splits, along with a hand lens when necessary. More detailed observations of individual corals, bioclasts, and other components were made using a binocular microscope. These observations were recorded on visual core description (VCD) forms and then entered into the OffshoreDIS. Section description barrel sheets were generated from the information entered into the OffshoreDIS.

Onshore sedimentologists conducted visual inspections and observations of the archive halves of the cores and prepared written descriptions. Note that a single core can be composed of as many as three sections plus the core catcher. Onshore description of the coral fauna by the coral specialists within each cored interval was based mainly on observations of the archive halves of the cores. Section units were defined and numbered on the basis of general lithological changes and/or changes in the dominant hermatypic coral type (identified to family, genus, and/or growth form). This allowed the coral specialists and sedimentologists to map the preliminary stratigraphic positions of all visible corals on the VCD forms (Figs. F7, F8). The identity, general appearance, growth direction, and context of coral specimens were described from the top to bottom of each core. The following properties were described:

• Core disturbance and intervals of downhole sediment contamination in the upper parts of some cores;

• Lithologies;

• Sedimentary boundaries (e.g., lithological changes, depositional discontinuities, unconformities, erosional surfaces, hardgrounds, etc.);

• Major components;

• Minor components;

• Individual corals;

• Microbialites;

• Coralline algae;

• Mollusks, worms, bryozoans, larger foraminifera, and so on;

• Bioerosion and fossil preservation;

• Diagenesis and other features (e.g., dissolution, cementation phases, staining, geopetal fabrics, etc.).

Core disturbance

Highly disturbed intervals were marked on VCD forms, and disturbance of the core caused by fracturing and/or grinding during drilling was recorded. In the upper part of some cores, sedimentary breccias were clearly identifiable as downhole contamination (Fig. F9).

Lithology

Grain size was defined using terms and symbols from Wentworth (1922):

• cb = cobbles (256–64 mm).
• p = pebbles (64–4 mm).
• g = granules (4–2 mm).
• vc = sand, very coarse (2–1 mm).
• c = sand, coarse (1–0.5 mm).
• m = sand, medium (0.5–0.25 mm).
• f = sand, fine (0.25–0.125 mm).
• vf = sand, very fine (0.125–0.0625 mm).
• M = mud (<0.0625 mm) (true for fine carbonate sediment).
• s = silt (0.0625–0.0039 mm).
• C = clay (<0.0039 mm).

Overall lithology/texture was defined for carbonate sediments following Dunham’s (1962) classification as modified by Embry and Klovan (1972). This classification distinguishes the following lithologies (see Fig. F10 for examples):

• Boundstone = original components organically bound during deposition.

• Bindstone = framework bound together by encrusting organisms.

• Framestone = rigid framework of organisms.

• Bafflestone = bound by organisms that act as baffles.

• Rudstone = granule/pebble-sized grains >2 mm, grain-supported textures with no matrix between the grains.

• Floatstone = granule/pebble-sized grains >2 mm, matrix supported.

• Grainstone = sand-sized grains <2 mm, grain-supported textures with no mud between the sand grains.

• Packstone = sand-sized grains <2 mm, grain-supported textures with mud between the sand grains.

• Wackestone = mud-supported textures with sparse sand-sized grains floating in the mud.

• Mudstone = mud-supported textures with sparse rare (<10%) sand-sized grains.

“Framework” is a lithology with in situ corals encrusted with thick coralline algal crusts or microbialites, and one where corals may make up only a small percentage by volume of the entire rock (see “Major components of a core”). Framework is a generic term that is not based on the most voluminous component (e.g., microbialite).

Any noncarbonate sediments and their lithified equivalents were also mentioned and described.

Sedimentary boundaries

Lithological changes represented by discontinuities, unconformities, bioerosion, boring, and other features (e.g., staining, cementation, or dissolution) associated with such boundaries were marked on the VCD forms.

Major components of a core

The major components of a core are defined as those that contribute to the highest percentage by volume of the sediment and/or rock. Framework lithologies can include corals, microbialites, or coralline algae as major components or may contain a mixture of these organisms. Major component categories used during the Onshore Science Party were modified slightly from those used in offshore descriptions. Terms and codes used to describe major and minor components are listed in Table T3.

Coral descriptions

Coral taxonomy

Both offshore and onshore coral identifications generally followed the usage of Veron (2000) but were usually done only to family or genus, either because species-level taxonomic characters were not preserved or because there was insufficient time for careful examination. Many specimens were sampled for later identification. A few species were easily recognizable, and these were identified to species level. Uncertain identifications pending further study are marked “?”.

The most important taxonomic change from the offshore descriptions involved the common robustly branching to massive frame-building corals that were listed in the offshore reports as Acropora palifera, Acropora cuneata, or simply “massive Acropora.” Recent genetic work (Wallace et al., 2007) demonstrates that these species belong to a different (but related) genus, Isopora Studer, 1878. Because the two species are morphologically very similar, they are described in the onshore reports as either Isopora palifera/cuneata or as “massive Isopora.”

Coral growth forms

Coral growth forms were described as

• Encrusting (<5 cm thick) or massive (>5 cm);

• Foliaceous (free margins <2.5 mm thick) or platy (free margins >2.5 mm);

• Branching-fine (<1 cm average diameter), medium (1–3 cm), robust (3–5 cm), or columnar (>5 cm);

• Tabular or discoid (free-living); or

• Fragments.

Coral context

Special attention was given to establishing the context of each coral within each cored section. A combination of criteria (Blanchon and Blakeway, 2003; Webster and Davies, 2003; Webster et al., 2004) was used to distinguish in situ corals from allochthonous rubble and/or drilling disturbance. These criteria included

• Presence or absence of fresh breakage surfaces;

• Presence or absence of severe surface abrasion and rounding of coral colonies;

• Orientation of well-preserved corallites;

• Orientation of coral skeletal characters, including growth axes;

• Presence or absence of thicker (>5 mm) coralline algal crusts capping upper surfaces;

• Presence or absence of attached substrate at base of coral; and

• Presence or absence (and orientation) of macroscopic sediment geopetals in cavities.

Based on these criteria, each larger individual coral was assigned a context:

• IS = in situ (convincing supporting evidence).

• ISX = not in situ (ISX) = convincing nonsupporting evidence.

• IS? = possibly in situ (IS?) = inconclusive supporting evidence.

• ISN = status not known (ISN) = inadequate evidence either way.

Microbialite descriptions

Some cores had laminated or clotted micritic sediment interpreted as microbialites (Camoin and Montaggioni, 1994; Camoin et al., 1999, 2006). Three general morphologies were distinguished: laminated, dendritic, and undetermined. For microbialites that grew into open cavities, the morphology of the surfaces (knobbly versus smooth) was also distinguished.

Larger benthic foraminifera

The cores were examined for the presence of larger benthic foraminifera (arbitrarily defined as benthic foraminifer tests >2 mm in diameter) using a Stemi 2000 binocular microscope. In this volume, if not otherwise qualified, “larger foraminifera” refers to larger benthic foraminifera (Hallock, 1985). Where present, larger foraminifera were identified to various taxonomic levels from species to order and summarized in tables for each transect. Wherever possible, the degree of preservation was noted.

Diagenesis and other features

Where applicable, occurrences of such features as open cavities, geopetal infills, color staining, cementation, and diagenetic alteration of components were identified. Dissolution cavities were present in some cores. Some of these cavities were filled locally by internal bioclastic (marine) sediment, whereas other cavities were lined by cement. The morphology of this cement (e.g., whether it had a “speleothem” shape, acicular or fibrous morphology, and/or color staining) was described, as these features are indicative of subaerial exposure surfaces.

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