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doi:10.2204/iodp.proc.310.106.2007 Sedimentology and biological assemblagesThe eastern transect drilled in the region of Maraa (southwest Tahiti) includes, with increasing distance from the island, Sites M0017, M0015, M0018, and M0016, ranging in depth from 56.45 to 81.8 meters below sea level (mbsl). Last deglacial sequence (Unit I)
The thickness of the last deglacial sequence (lithologic Unit I) ranges from 30 m in Hole M0015A at 72.15 mbsl to 40 m in Hole M0018A at 81.8 mbsl. The base of the unit has been recovered from 96 mbsl at the inner sites to 121 mbsl at the outer sites. In Hole M0017A, the top of the last deglacial sequence comprises a well-consolidated packstone-grainstone composed mainly of Halimeda segments associated with foraminifers (benthic and encrusting foraminifers), bryozoans, echinoids, and mollusks. Rhodoliths are observed in Core 310-M0017A-1R. In Hole M0016A, the top of the last deglacial sequence consists of rubble of coral fragments (encrusting agariciids, Porites, and Montipora) and rhodoliths. The surfaces of the skeletal fragments are usually stained and bored. This sequence is primarily composed of coralgal-microbialite frameworks commonly interlayered with loose skeletal sediments, including coral and algal rubble. Coral and algal rubbles are mostly composed of accumulations of fragments of corals (mostly composed of encrusting Acropora and Montipora and branching Pocillopora and Porites), microbialite crusts and mollusks, Halimeda segments, and rounded lithoclasts (e.g., skeletal sandstone rich in volcanic grains) (Figs. F1, F2) (e.g., intervals 310-M0017A-12R-1, 138–146 cm, and 310-M0018A-21R-1, 18–32 cm); benthic foraminifers are usually scarce. Coral fragments are commonly coated with thin crusts of nongeniculate coralline algae. The surface of the coral fragments usually displays evidence of extensive bioerosion and, locally, reddish staining. Pebbles of basalt and sand-sized volcanic grains occur locally. Skeletal sand corresponds to Halimeda sand and is rich in fragments of corals (e.g., branching Porites), echinoids, and mollusks. The frameworks that form the bulk of the last deglacial sequence include three subunits, each displaying distinctive coral assemblages and internal structure. Subunit IA
Subunit IA primarily comprises locally well lithified coralgal bindstone that forms the upper part of the last deglacial sequence in all holes: from 56.45 to 68.1 and 80 mbsl in the inner holes (Holes M0017A and M0015B, respectively) and from 81 to 97 mbsl in the outer holes (Holes M0016 and M0018, respectively). In Holes M0015B and M0018A, the top of the subunit is characterized by the occurrence of a hardground and yellowish-reddish staining of the rock surface; bioerosion is extensive (Figs. F3, F4) (e.g., intervals 310-M0015B-1R-1, 0–10 cm, and 310-M0018A-1R-1, 0–6 cm). Coral assemblages are dominated by encrusting colonies of Montipora, agariciids represented by Pavona, Porites, Leptoseris, and Leptastrea, and foliaceous colonies of Pachyseris (Figs. F5, F6, F7, F8, F9, F10, F11, F12) (e.g., intervals 310-M0015B-1R-1, 20–40 cm, and 2R-1, 35–50 cm, 310-M0016A-7R-1, 40–50 cm, 310-M0017A-3R-CC, 1–10 cm, and 4R-1, 7–17 cm, and 310-M0018A-1R-1, 16–29 cm, 1R-1, 79–90 cm, and 3R-1, 60–80 cm). Associated corals include massive colonies of Porites, branching colonies of Porites, Pavona, and Pocillopora (including robust branching colonies), and tabular colonies of Acropora in the lower part of this subunit (Fig. F13) (e.g., interval 310-M0018A-7R-1, 70–80 cm). Corals are usually thinly encrusted with nongeniculate coralline algae, except in the lower part of the subunit where the crusts are significantly thicker (up to 2 cm thick) and include vermetid gastropods and serpulids. The last stage of encrustation over coral colonies corresponds to thick microbialite crusts dominated by massive laminated fabrics. Some primary cavities are partly filled with well- and moderately consolidated skeletal sands or are veneered with microbialites; locally the walls of the cavities exhibit brownish staining. Subunit IB
Subunit IB is mostly composed of coralgal-microbialite frameworks in which coral assemblages are dominated by tabular colonies of Acropora (Figs. F14, F15, F16, F17) (e.g., intervals 310-M0015A-7R-1, 20–35 cm, and 10R-1, 70–80 cm, 310-M0015B-19R-1, 53–69 cm, and 310-M0017A-12R-1, 57–65 cm). This distinctive subunit occurs only at the inner sites of the transect (i.e., in Holes M0017A [68–85 mbsl], M0015A [79.4–85.7 mbsl], and M0015B [80–86.44 mbsl]). Associated corals include encrusting colonies of Montipora (locally dominant [e.g., Cores 310-M0015B-14R through 16R]), Porites, Leptastrea, Montastrea, and agariciids, massive colonies of Porites (e.g., Sections 310-M0015B-9R-2 and 10R-1 and Core 11R) and Montastrea, and branching colonies of Porites, Pocillopora, and Acropora (Figs. F18, F19, F20, F21) (e.g., intervals 310-M0015A-9R-1, 28–40 cm, 310-M0015B-9R-1, 0–12 cm, and 310-M0017A-9R-1, 10–17 cm, and 14R-1, 87–103 cm). Robust branching colonies of Acropora and Pocillopora occur in the lower part of this subunit where they are locally dominant (Figs. F22, F23) (e.g., intervals 310-M0015B-21R-1, 85–105 cm, and 310-M0017A-13R-1, 21–36 cm, and Cores 310-M0017A-15R through 17R). Corals are generally coated with thick crusts of nongeniculate coralline algae, up to a few centimeters, in which vermetid gastropods and serpulids occur. Very thick microbialite crusts form the last stage of encrustation and are mostly composed of massive laminated fabrics locally overlain by dendritic accretions (Figs. F24, F25, F26) (e.g., intervals 310-M0017A-13R-1, 3–12 cm, 310-M0017B-16R-1, 87–98 cm, and 310-M0015B-21R-1, 84–92 cm). Coral colonies usually exhibit traces of bioerosion; brownish-yellowish staining is conspicuous on some surfaces. Large primary cavities are partly filled with coarse skeletal sand rich in Halimeda segments and fragments of mollusks and echinoids. Subunit IC
Subunit IC comprises coralgal-microbialite frameworks dominated by branching colonies of Porites (Figs. F27, F28, F29, F30) (e.g., intervals 310-M0015A-20R-1, 55–68 cm, and 30R-1, 74–84 cm, and 310-M0015B-33R-1, 112–139 cm, and 34R-1, 13–34 cm). The top of this subunit occurs at 85 mbsl in the inner holes (Holes M0017A, M0015A, and M0015B) and deeper than 90 mbsl in the outer holes (Holes M0016A, M0016B, and M0018A). The base of Unit I was recovered from 94 mbsl at the inner sites to 121 mbsl at the outer sites. A well-diversified coral assemblage is associated with branching colonies of Porites (Figs. F31, F32) (e.g., intervals 310-M0016B-11R-1, 20–34 cm, and 310-M0017A-18R-1, 77–91 cm) and includes branching and robust branching colonies of Pocillopora and Acropora (e.g., Cores 310-M0015A-14R, 16R, 17R, 27R, and 34R, 310-M0015B-26R and 31R, and 310-M0016A-26R and 27R), massive colonies of Montastrea, encrusting colonies of Porites, Montipora, Leptastrea, Millepora, agariciids (Pavona and Leptoseris), and Psammocora, and tabular colonies of Acropora (Figs. F33, F34, F35, F36, F37, F38, F39, F40, F41) (e.g., intervals 310-M0015A-23R-1, 18–42 cm, and 26R-1, 8–31 cm, 310-M0015B-21R-1, 85–90 cm, and 25R-1, 40–65 cm, 310-M0016A-22R-1, 13–20 cm, 23R-1, 0–16 cm, 29R-1, 38–60 cm, 30R-1, 14–24 cm, and 21R-1, 52–66 cm, and 310-M0018A-13R-1, 25–35 cm). Massive colonies of Porites dominate in Cores 310-M0016A-33R through 36R (e.g., interval 310-M0016A-3R-1, 23–33 cm). Coral colonies are generally coated with thick crusts of nongeniculate coralline algae locally associated with vermetid gastropods and serpulids. In those frameworks, microbialites (laminated and thrombolitic microbial fabrics) form massive crusts (up to 20 cm thick) over corals and coralline algae and usually represent the major volumetric and structural component of the frameworks (Fig. F42) (e.g., interval 310-M0015A-14R-1, 23–49 cm). They usually correspond to a two-stage encrustation composed of very thick laminated fabrics overlain by thrombolitic accretions that usually represent the last stage of encrustation. Large primary cavities are partly filled with micritic sediments, including Halimeda segments and fragments of mollusks and echinoids (Figs. F43, F44, F45) (e.g., intervals 310-M0015A-14R-1, 48–60 cm, and 21R-1, 19–31 cm, and 310-M0015B-26R-1, 50–75); brown crusts occur in some cavities. Some surfaces of the reef rocks display yellowish to reddish staining. Corals exhibit alteration in Cores 310-M0018A-12R and 14R; yellowish, brownish to reddish staining is conspicuous on some surfaces in Cores 18R through 20R. In Hole M0015B, the base of the last deglacial sequence comprises an algal bindstone composed of crusts and branches of nongeniculate coralline algae. Volcanic silt and sand and occasional granules and pebbles appear only as minor components in some of the carbonate units at the base of Holes M0015A and M0015B. In Hole M0015A, volcanic sediments first appear in Section 310-M0015A-35R-1; in Hole M0015B, they only appear in Section 310-M0015B-38R-2. Older Pleistocene sequence (Unit II)
The older Pleistocene sequence (lithologic Unit II) is primarily composed of irregular alternations of yellow, gray to beige poorly sorted skeletal limestone (grainstone to rudstone-floatstone) and coralgal frameworks with local intercalations of coral and algal rubble that display a conspicuous alteration. Postdepositional diagenetic processes of the older Pleistocene sequence is indicated by the diagenetic alteration of coral skeletons and the occurrence of large solution cavities that display yellow, brown to red-brown staining and cement lining (Figs. F46, F47, F48, F49) (e.g., intervals 310-M0015A-40R-1, 10–24 cm, and 41R-1, 105–118 cm, 310-M0015B-38R-3, 92–102 cm, and 310-M0015A-40R-1, 30–38 cm). Some solution cavities are partly filled with internal sediments. Grainstone, rudstone, and floatstone are rich in fragments of corals (e.g., robust branching Pocillopora, branching and massive Porites, branching Acropora, and encrusting Montipora), coralline algae, echinoids and mollusks, and Halimeda segments (Figs. F50, F51, F52, F53, F54, F55, F56, F57, F58) (e.g., intervals 310-M0015A-37R-1, 40–55 cm, 39R-1, 63–80 cm, 39R-1, 115–129 cm, 40R-1, 29–32 cm, and 41R-1, 25–35 cm, 310-M0015B-38R-3, 40–55 cm, 310-M0016B-22R-CC, 10–24 cm, 24R-1, 57–70 cm, and 24R-1, 79–94 cm, and 310-M0017A-21R-3, 92–119 cm). Skeletal elements are usually extensively bioeroded. Sand-sized volcanic grains are commonly associated with the skeletal grains. Intraclasts are common. Coralgal frameworks are composed of encrusting and branching colonies of Porites, tabular colonies of Acropora, encrusting and massive colonies of Montipora, and encrusting colonies of agariciids (Figs. F59, F60, F61) (e.g., intervals 310-M0015A-38R-1, 63–75 cm, and 310-M0017A-21R-1, 98–118 cm, and 22R-2, 32–64 cm); massive colonies of Porites occur locally. These corals are thickly encrusted with nongeniculate coralline algae associated with vermetid gastropods and serpulids. |
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