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doi:10.2204/iodp.proc.310.103.2007

Sedimentology and biological assemblages

Visual core descriptions

Expedition 310 sedimentologists were responsible for visual descriptions of the archive halves of cores recovered during the expedition. Archive halves were examined by eye and with hand lenses, and for further analysis of corals, bioclasts, and volcanic rock, binocular microscopes were used. Observations were recorded on VCD forms with the high-resolution line scan of the core section already printed on it. Thus, individual elements in these cores could be marked directly, granting later identification of individual corals and other components in the scans. VCD forms were scanned in order to archive them, and data were entered into OffshoreDIS. Section description barrel sheets (Figs. F5, F6) were computer generated from the information entered into Offshore DIS.

Section units were defined on the basis of general lithology changes and/or changes in the dominant hermatypic coral type (genus and/or growth form). One core section can be composed of several core-section units.

The following properties were given attention:

• Core disturbance,
• Overall lithology,
• Major component of unit,
• Detailed list of other components,
• Description of individual corals,
• Description of microbialites,
• Bioerosion intensity,
• Breaks in sedimentation, and
• Other properties.

Core disturbance

Highly disturbed intervals were marked on VCD forms. Strongly crushed intervals were described as “crushed.”

Overall lithology

Overall lithology was defined (for the carbonate sediments following Dunham’s [1962] classification as modified by Embry and Klovan [1972]) by distinguishing between

• Different types of boundstone (e.g., framework, bindstone),
• Carbonate sediment types (rudstone, floatstone, grainstone, packstone, wackestone, mudstone) and their nonlithified equivalents,
• Volcanic sediment and their nonlithified equivalents, and
• Volcanic rocks (e.g., basalt).

A section unit with in situ corals that are encrusted with thick microbialites is termed a “framework,” even where corals only make up a small volume percentage of the entire rock (see “Major component of a section unit”). That is, a generic term is used (framework), and the terminology is not based on the most voluminous component (microbialite).

Unconformities

Unconformities were noted because of their potential significance as boundaries. Bioerosion and other hardground properties were noted, as well as stains and diagenetic properties such as cementation and dissolution.

Other properties

Where applicable, properties such as open vugs, geopetal infills, color stains, cementation including marine cement linings, and diagenetic alteration of components were noted.

Major component of a section unit

The major component of a section unit highlights the component with the highest volume percentage of that section unit. For example, a section unit with an overall lithology of “framework” with a coral framework can be composed of the major component “microbialite.” Common major components are corals, microbialites, and volcaniclastic sediment. Components in the cores included the following components listed for each individual section unit:

• Basalt pebbles,
• Basalt sand grains,
• Bivalves,
• Branching coralline algae,
• Bryozoans,
• Corals (for details on the determination of the corals, see “Description of corals”),
• Coralgal bindstone (CAB; finely interlayered encrusting corals and coralline algae),
• Echinoderms,
• Encrusting coralline algae (ECR),
• Foraminifers,
• Gastropods,
• Halimeda,
• Intraclasts,
• Microbialites (MB),
• Rhodoliths,
• Rubble (carbonate rubble),
• Unknown lithoclasts,
• Unknown bioclasts, and
• Vermetid gastropods.

Carbonate components

Description of corals

The following is a summary of the coral description procedure and the data recorded in the VCD forms and OffshoreDIS.

Identification of corals and coral-bearing horizons

Onshore description of the coral fauna within each cored interval was based on observations of the archive half. The sample half was also examined if coral features were obscured in the archive half. A high-resolution line scan of each core was printed directly onto the VCD sheet. This allowed the coral specialists, along with the sedimentologists, to map the stratigraphic positions of all visible corals precisely on the VCD forms. Each coral specimen was numbered individually (C1, C2, C3, etc.) from top to bottom in the core, and its growth direction, context, and general appearance were described. Core intervals composed of numerous colonies of the same genus were also numbered (C1, C2, C3, etc.) and used to define discrete section intervals that were then entered into OffshoreDIS.

Coral growth forms

Using standard nomenclature (Veron, 2000), coral growth forms were described as massive (>2 cm thick), robust branching (>2 cm maximum branch diameter), branching (<2 cm maximum branch diameter), tabular, foliaceous, and encrusting (<2 cm thick; in contact with basal substrate).

Coral taxonomy

Coral taxonomy followed standard taxonomic references (Veron, 2000). During the Onshore Science Party, coral identifications were carried out at the genus level because taxonomic details were not commonly preserved. All intact corallite surfaces were subsampled for more detailed laboratory investigation and species-level identifications.

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. This included identifying

• Whether the coral displayed 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,
• Whether the tips of branching corals were capped by thicker (>5 mm) coralline algal crusts, and
• Presence of macroscopic sediment geopetals in cavities within the corals.

Description of microbialites

Cores generally show abundant laminated or clotted micritic sediment interpreted as microbialites (Camoin and Montaggioni, 1994; Camoin et al., 1999, 2006). In split cores, three general morphologies were distinguished: laminated, thrombolitic, and dendritic. For microbialites that grew into open vugs, the morphology of the surfaces (knobby versus smooth) was also distinguished. For example, in many cases a succession of laminated followed by dendritic morphology and knobby surface was observed.

Volcaniclastic sediments

Definition and classification

Volcaniclastic sediments contain at least ~50% by volume (vol%) of volcanic lithic components. The terms sand and silt were used in place of “ash,” and sandstone and siltstone were used in place of “tuff,” if lithified. The terms granule and pebble (granule conglomerate and pebble conglomerate, respectively, if lithified) were used in the “lapilli” size category. Cobbles of larger volcanic clasts (“volcanic breccias”) were encountered as components of the rubble in several of the core catchers, between some of the carbonate units, and as occasional components of the carbonate sediments. A large basalt boulder (drilled section = ~65 cm) occurs in Sections 310-M0008A-7R-CC through 8R-1. Lithic fragment size divisions are those of Wentworth (1922) (see Table T1). The main reasons for the use of this set of terrigenous sediment terminology are the following:

• Carbonate debris is ubiquitously intermixed with volcanogenic lithic components.
• Volcaniclastic sediment units are almost always massive and only weakly lithified and in several cases are completely unconsolidated.
• Units contain occasional skeletal-rich, rip-up clasts.
• Units lack volcanic glass shards that are typical of many tephra deposits.
• In the thick sequence of volcaniclastic sediments in Hole M0008A, the bottom volcanic sandstone and siltstone units contain occasional wood fragments and fine, delicate plant roots. These indicate that the volcaniclastic sediments are not primary products of explosive volcanic eruptions but instead are epiclastic sediments produced from erosion of volcanic terrains composed of extrusive volcanics and/or pyroclastics in Tahiti.

The volcaniclastic sediment names we applied were based entirely on hand-specimen observations. As in other sedimentary rocks (see “Overall lithology”), volcaniclastic sediment section units were defined on the basis of general lithologic changes.

VCD forms were used to document each section of the volcaniclastic sediment cores. Based on visual and hand lens/​binocular microscope observation, volcaniclastic sediments were described according to the following sequence of characteristics:

• Structure,
• Lithic fragment size,
• Sorting and grading,
• Sphericity or angularity, and,
• Composition of the components.

Structure was determined by whether the rock is massive or stratified and whether it is composed of unlithified/​nonconsolidated lithics or the lithics are well lithified/​consolidated. As noted earlier, lithic fragment size divisions are shown in Table T1. Sorting refers to variations of size distribution, with unsorted referring to large lithic size variation within an interval and well sorted referring to lithic fragments that have almost the same size. Grading refers to change of size distribution with depth within an interval, with normal grading referring to fragment sizes become coarser toward the bottom and reverse grading referring to the opposite. Sphericity refers to degree of roundness of the lithic fragments, from angular to well rounded. The composition of the component lithic fragments was based on hand-specimen observations using hand lens and binocular microscope.

In order to describe the igneous rock boulder in Hole M0008A, individual cobbles (>64 mm), and pebbles (4–64 mm) that are enclosed within volcaniclastic and carbonate units as well those in the rubble in several core catchers, the following sequence of characteristics were used:

• Size and sphericity of the lithic fragment,
• Granularity or grain size,
• Texture,
• Vesicularity, and
• Alteration.

The size (Table T1) and sphericity of lithic fragments were described to produce an integrated picture of the distance of transport and possible depositional environment of fragments at their respective sites. Sphericity of fragments ranges from angular to rounded. Sizes of mineral components within the rocks are fine grained (<1 mm; used here synonymously to the term aphanitic), medium grained (1–5 mm), or coarse grained (>5 mm). Grain-size variations within units were also noted. Texture is mainly whether a rock contains “phenocrysts,” which is a term used here to describe a crystal that is significantly larger (typically 5 times) than the average size of the groundmass crystals (or matrix) and generally subhedral to euhedral in shape. Based on this term, we used the following descriptors:

• Aphyric (or nonporphyritic): phenocryst content <1% of the volume of the rock;
• Sparsely phyric (or porphyritic): phenocryst content 1%–2%;
• Moderately phyric (or porphyritic): phenocryst content 2%–10%; and
• Highly phyric (or porphyritic): phenocryst content >10%.

If the rock is phyric, then size range (in millimeters), mineral type, amount of alteration of the phenocrysts, and further comments, if appropriate, were noted. Phenocrysts are predominantly the typical ferromagnesian minerals in a basalt (i.e., mainly pyroxenes and olivine) with subordinate amounts of feldspars (plagioclase and alkali feldspar) or feldspathoids. Owing to their generally fine to medium crystal size, slightly to highly altered state, and in particular the known tholeiitic to highly alkalic volcanic rock varieties in Tahiti (e.g., McBirney and Aoki, 1968; Cheng et al., 1993), feldspars and feldspathoids were not differentiated; hence, the term “feldspars/​foids” was used. Vesicles were described based on their abundance, size (in millimeters), and shape (sphericity and angularity). Secondary minerals (amygdules) lining the walls of or completely filling vesicles were also noted. Abundance categories are sparsely vesicular (1–5 vol%), moderately vesicular (>5–20 vol%), and highly vesicular (>20 vol%). Degree of alteration is unaltered (<2 vol% of alteration products), slight (2–10 vol%), moderate (10–40 vol%), high (40–80 vol%), very high (80–95 vol%), or complete (>95 vol%).

Onshore volcaniclastic sediment analyses

Five representative samples of volcaniclastic sandstone/​siltstone, seven volcanic cobbles, and two small portions of the boulder from Hole M0008A were selected for analyses of major and trace element contents using a Spectro XEPOS portable energy dispersive polarization X-ray fluorescence analyzer (EDP-XRF) using the procedure described in Wein et al. (2005). Approximately 10 cm³ aliquots of the volcaniclastic sediments were frozen, freeze-dried to remove water, and ground by hand with an agate mortar and pestle. Rock samples were air dried, broken into smaller chips, and then ground using a Fritsch Pulverisette 7 tumble mill equipped with a zirconium oxide beaker. Approximately 5 grams of dried powder from each sample was poured and then compressed with a pestle in a volumetrically calibrated sample cup consisting of top and bottom rings and 4 µm prolene foil sides and bottom. A total of 5–6 cups of samples were then placed in an autosampler along with 3–4 cups of different rock standards to assess accuracy and precision. Results of the analyses of international and University of Bremen internal standards are presented in Table T2, together with previous analyses of these standards using the same instrument. All sample preparation and analyses were performed at the Department of Geosciences, University of Bremen (Germany).

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