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doi:10.2204/iodp.proc.310.105.2007 Sedimentology and biological assemblagesThe western transect drilled offshore Maraa (southwest Tahiti) includes, from land oceanward, Sites M0007, M0005, and M0006, which range in water depth from 41.65 to 81.58 meters below sea level (mbsl). Modern sediments
Modern sediments were recovered at all sites along this transect. They consist of a few decimeter-thick beds composed of rhodoliths, skeletal sand, and gravels rich in Halimeda segments, mollusk shell fragments, foraminiferal tests, and nongeniculate coralline algal crusts (Fig. F1) (e.g., intervals 310-M0005E-2M-1, 1–6 cm, and 310-M0006A-1M-1, 0–8 cm). They include clasts of Halimeda packstone and coral clasts (especially branching and encrusting corals) that are commonly encrusted with nongeniculate coralline algae and worms (Fig. F2) (e.g., interval 310-M0007A-1M-1, 8–16 cm). The surfaces of some clasts are extensively bored and display brown staining. The cores of the rhodoliths generally correspond to coral fragments that may exhibit traces of bioerosion. Last deglacial sequence (Unit I)
The thickness of the last deglacial sequence (lithologic Unit I) ranges from 33 to 42 m at Sites M0005 and M0007, respectively. The top of the last deglacial sequence is characterized by the abundance of thin crusts of nongeniculate coralline algae and extensive bioerosion features (e.g., Core 310-M0005A-1R); reddish and brownish staining is conspicuous on the upper surface of the last deglacial rocks and in large borings. This sequence is primarily composed of coralgal-microbialite frameworks commonly interlayered with skeletal limestone and loose skeletal sediments, including coral and algal rubble and skeletal sand. Coral and algal rubble are mostly composed of accumulations of fragments of corals (branching and robust branching Pocillopora, Acropora, Porites, and Pavona), nongeniculate coralline algal crusts, rhodoliths, and lithoclasts that are usually extensively bored. Skeletal sand corresponds to Halimeda sand and is rich in echinoid and mollusk grains (Fig. F3) (e.g., intervals 310-M0007B-18R-1, 0–15 cm, and 31R-1, 6–12 cm). Skeletal limestone comprises grainstone and is rich in Halimeda segments and coralline algal fragments (including fruticose forms) (Fig. F4) (e.g., interval 310-M0005C-11R-1, 0–10 cm). The coralgal-microbialite frameworks that form the bulk of the last deglacial sequence are characterized by the widespread development of microbialites, which locally represent the major structural and volumetric component of the reef rock. They develop within the primary cavities of the reef framework, where they generally overlie nongeniculate coralline algal crusts. Microbialites generally comprise a suite of fabrics, including two end-members represented by laminated fabrics and dendritic accretions; the laminated fabrics are generally the most abundant (Figs. F5, F6, F7, F8, F9) (e.g., intervals 310-M0005A-2R-1, 12–24 cm, 310-M0005B-3R-1, 22–36 cm, and 310-M0005C-6R-2, 2–16 cm, 8R-1, 65–79 cm, and 9R-1, 115–134 cm). The reef sequence is characterized by a general succession of distinctive coral assemblages, although many of them are intergradational, both laterally and vertically. Two successive subunits displaying distinctive coral assemblages and internal structure can be identified, from top to base. Subunit IA
Subunit IA, as thick as 20 m, is primarily composed of coralgal-microbialite frameworks dominated by encrusting coral colonies. In the upper part of this subunit, the coral assemblage includes abundant encrusting colonies of Montipora and Porites and, to a lesser extent, Pavona, Psammocora, Astreopora, agariciids, and faviids (including Leptastrea and Montastrea) (Figs. F10, F11, F12, F13) (e.g., intervals 310-M0005A-1R-1, 48–59 cm, 310-M0005B-1R-1, 36–46 cm, and 310-M0005C-2R-CC, 10–31 cm, and 3R-1, 11–23 cm). Massive colonies of Porites, Montipora, Astreopora, and faviids occur locally (Fig. F14) (e.g., interval 310-M0007A-6R-1, 20–34 cm). Branching colonies of Montipora, Pocillopora (including robust branching colonies), and Acropora occur as fragments. These coral colonies are coated with thin crusts of nongeniculate coralline algae overlain by massive microbialite coatings (laminated and thrombolitic fabrics) to form loose frameworks (bindstone) (Fig. F15) (e.g., interval 310-M0007A-9R-CC, 1–11 cm). Multiple generations of coral colonies and thin encrustations of nongeniculate coralline algae may alternate locally. Large primary cavities are partly filled with skeletal sand and gravels, including fragments of corals, coralline algae and mollusks, Halimeda segments, echinoid spines, and foraminifers; rhodoliths occur locally (e.g., Core 310-M0005B-1R). In the lower part of this subunit, the coral assemblage is mostly composed of encrusting coral colonies of Montipora, faviids, and agariciids (Figs. F16, F17, F18) (e.g., intervals 310-M0007A-8R-1, 51–70 cm, 8R-1, 72–84 cm, and 9R-1, 23–45 cm) associated with branching colonies of Porites that are locally dominant (e.g., Cores 310-M0007B-16R through 18R), robust branching colonies of Pocillopora (e.g., Cores 310-M0007A-15R and 310-M0007C-15R and interval 310-M0007A-15R-1, 30–45 cm) (Fig. F19), tabular colonies of Acropora (e.g., Cores 310-M0007B-12R, 15R, and 16R, and 310-M0007C-7R through 9R), and massive colonies of Porites, Montipora, Astreopora, and faviids; some sections of massive Porites colonies are up to 60 cm thick (Fig. F20) (e.g., Cores 310-M0005B-1R and 3R, 310-M0007A-6R through 9R, 11R, 13R through 15R, and 18R, and 310-M0007B-5R-2, 6R, 8R, 9R, and 11R and interval 310-M0007A-7R-1, 18–30 cm). In this part of the subunit, frameworks usually exhibit a denser internal structure and are characterized by thicker crusts of nongeniculate coralline algae. Microbialite crusts display widespread development in those frameworks. Subunit IB
Subunit IB, as thick as 25 m, comprises coralgal-microbialite frameworks made of tabular and robust branching coral colonies that are usually heavily encrusted with nongeniculate coralline algae, locally associated with vermetid gastropods and serpulids, and overlain by very thick and massive microbialite crusts (Fig. F21) (e.g., interval 310-M0005B-6R-1, 9–39 cm). Large primary cavities are partially filled with skeletal sand and are rich in Halimeda segments and coral fragments (Fig. F22) (e.g., interval 310-M0007C-22R-1, 10–26 cm). The coral assemblage is dominated by tabular colonies of Acropora associated with robust branching colonies of Pocillopora, Acropora, and Pavona (especially in the lower part of this interval) (Figs. F23, F24, F25, F26, F27, F28, F29) (e.g., intervals 310-M0005B-4R-1, 19–41 cm, 310-M0005C-8R-1, 38–62 cm, 11R-1, 48–62 cm, and 13R-1A, 66–85 cm, 310-M0007A-30R-1, 7–20 cm, and 31R-1, 45–65 cm, and 310-M0007B-30R-1, 35–56 cm). Other corals include massive colonies of Porites (Figs. F30, F31) (e.g., Cores 310-M0005A-12R, 310-M0005B-5R and 6R, 310-M0007A-19R-2 and 20R-1, and 310-M0007B-20R and 21R and intervals 310-M0005B-4R-1, 87–101 cm, and 310-M0007B-20R-1, 86–106 cm), Montipora, and Montastrea, branching colonies of Porites (locally dominant), and encrusting colonies of Montipora, Porites, Leptastrea, Pavona, Montastrea, Millepora, Favia, and agariciids (Figs. F32, F33, F34) (e.g., intervals 310-M0005C-10R-1, 33–46 cm, and 15R-1, 1–15 cm, and 310-M0007B-34R-CC, 0–20 cm). At Site M0007, the base of the coralgal-microbialite frameworks of the last deglacial sequence is characterized by lithified skeletal grainstone that contains fine sand-sized volcanic grains; a 30 cm thick interval composed of branching coralline algae occurs in Section 310-M0007B-34R-CC. Older Pleistocene sequence (Unit II)The top of the older Pleistocene sequence (lithologic Unit II) is located at 86 and 92 mbsl at Sites M0005 and M0007, respectively. This sequence was drilled down to 161.56 mbsl in Hole M0005D. The older Pleistocene sequence is composed of eight distinctive lithologic units, which are discussed in order from top to base. Subunit IIA
Subunit IIA is made up of beige to gray well-lithified limestone comprising coralgal frameworks associated with rudstone-floatstone beds. The coral assemblage is dominated by encrusting colonies of Montipora, tabular colonies of Acropora, and branching colonies of Pocillopora. Fragments of Acropora and Porites branches are commonly associated. Coral colonies are bounded by very thick crusts of nongeniculate coralline algae in which vermetid gastropods and serpulids occur (Fig. F35) (e.g., interval 310-M0007A-35R-1, 90–110 cm). Associated rudstone-floatstone beds consist of large fragments of corals and nongeniculate coralline algae in a matrix that includes a mixture of skeletal (Halimeda segments and fragments of mollusks and echinoids) and volcanic grains (Fig. F36) (e.g., interval 310-M0007B-35R-1, 82–102 cm). Postdepositional diagenetic processes are indicated by the recrystallization and transformation of coral skeletons and the occurrence of abundant centimeter-sized solution cavities. Cement crusts in those cavities are usually stained reddish. The contact between this limestone and the underlying subunit corresponds to an unconformity; distinctive diagenetic features characterize these subunits. Subunit IIB
Subunit IIB contains well-lithified skeletal floatstone-rudstone rich in rhodoliths, fragments of corals (branching Pocillopora and Porites, encrusting Montipora and Porites, and massive Porites) and mollusks, and Halimeda segments; volcanic grains are common (Figs. F37, F38, F39, F40, F41) (e.g., intervals 310-M0005D-9R-1, 42–53 cm, 10R-1, 8–18 cm, 10R-1, 68–76 cm, 15R-2, 7–16 cm, and 16R-1, 2–10 cm). Abundant centimeter-sized cavities as well as dissolution of skeletal grains (Halimeda segments, fragments of mollusks, and corals that form the core of rhodoliths) and recrystallization and transformation of coral skeletons indicate that these limestones were subject to postdepositional diagenetic processes. Subunit IIC
Subunit IIC comprises poorly sorted and ungraded unlithified volcaniclastic silt to sand, including scattered skeletal grains (Halimeda segments, fragments of mollusks and echinoids, and foraminifers), reworked pebbles of Halimeda floatstone, and angular clasts of siltstone of the same composition (Fig. F42) (e.g., interval 310-M0005D-16R-1, 97–105 cm). Sand-sized volcanic grains are angular to subangular. Bioturbation is locally abundant, and burrows are filled with Halimeda floatstone. These volcaniclastic sediments are locally interlayered with skeletal floatstone rich in Halimeda and grade downward (from Core 310-M0005D-18R) through skeletal siltstone to sandstone rich in fragments of corals (massive Porites and encrusting agariciids). Subunit IID
Subunit IID contains beige to brownish coralgal frameworks in which corals are thinly coated with nongeniculate coralline algae and then heavily encrusted with microbialites composed of laminar and thrombolitic fabrics (Fig. F43) (e.g., interval 310-M0005D-21R-2, 0–12 cm). Large colonies of massive Porites were recovered in this subunit; other corals include robust branching colonies of Pocillopora and Pavona, tabular colonies of Acropora, and encrusting colonies of Montipora, Leptastrea, Porites, and agariciids (Fig. F44) (e.g., interval 310-M0005D-21R-1, 0–15 cm). Corals contain evidence of severe bioerosion and diagenetic alteration (Fig. F45) (e.g., interval 310-M0005D-21R-1, 38–48 cm). Volcanic grains commonly occur in the matrix of the frameworks (Fig. F46) (e.g., interval 310-M0005D-21R-1, 65–75 cm). Solution cavities are usually filled with a mixture of skeletal and volcanic grains. Associated beds include skeletal packstone to grainstone, including foraminifers and fragments of mollusks, coralline algae and echinoids, and rhodolith floatstone that includes volcanic grains (Fig. F47) (e.g., interval 310-M0005D-23R-1, 10–15 cm). Subunit IIE
Subunit IIE is a sandy skeletal grainstone to packstone rich in volcanic grains, large coral fragments of massive Porites, and skeletal fragments of nongeniculate coralline algae and corals (Fig. F48) (e.g., interval 310-M0005D-29R-1, 5–14 cm). Some in situ massive colonies of Porites (e.g., Cores 310-M0005D-27R-1 and 29R-1) and encrusting colonies of agariciids (Fig. F49) (e.g., interval 310-M0005D-29-1, 50–70 cm) occur in this subunit. Subunit IIF
Subunit IIF contains coral and algal rudstone to floatstone composed of rhodoliths (Fig. F50) (e.g., interval 310-M0005D-29R-1, 116–131 cm), lithoclasts, and fragments of corals (massive, encrusting, and branching corals), coralline algae, echinoids, and mollusks in a coarse sandstone matrix rich in volcanic and skeletal grains (Figs. F51, F52) (e.g., intervals 310-M0005D-29R-1, 80–110 cm, and 30R-1, 17–26 cm). Subunit IIG
Subunit IIG consists of coralgal frameworks interbedded with skeletal grainstone to packstone rich in volcanic grains and sandstone (Figs. F53, F54, F55) (e.g., intervals 310-M0005D-31R-2, 0–12 cm, 32R-1, 109–133 cm, and 32R-3, 15–27 cm). The abundance and grain size of the volcaniclastic intervals increase downhole (Fig. F56) (e.g., interval 310-M0005D-34R-1, 28–40 cm). Coral assemblages included are dominated by massive colonies of Porites and Montastrea associated with tabular colonies of Acropora, robust branching colonies of Pocillopora and Pavona, and encrusting colonies of Porites and faviids (Figs. F57, F58, F59, F60, F61) (e.g., intervals 310-M0005D-30R-1, 117–128 cm, 30R-2, 30–46 cm, 30R-2, 79–97 cm, 30R-2, 95–110 cm, and 33R-3, 0–25 cm). Corals are diagenetically altered. Subunit IIH
Subunit IIH comprises skeletal grainstone to floatstone rich in fragments of corals (robust branching Pavona and encrusting Montipora), echinoids and coralline algae, and volcanic grains (Fig. F62) (e.g., interval 310-M0005D-35R-2, 70–97 cm). Strong diagenetic imprints are manifest because coral skeletons have been altered and large solution cavities occur. The base of this subunit includes poorly lithified carbonates rich in volcanic grains. |
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