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Tahiti geology and core lithology

Tahiti is the largest of the Society Islands in French Polynesia and is composed of twin shield volcanoes (Fig. F1A) that were active from 1.367 ± 0.016 to 0.187 ± 0.003 Ma (Cabioch et al., 1999). The island is surrounded by modern discontinuous fringing reefs that grade locally into a chain of barrier reefs commonly interrupted and locally enclosing a narrow lagoon. The barrier reef complex includes, from land seaward: a backreef zone that corresponds to a 1 km wide bay, reaching a maximum depth of 20 m; a relatively narrow reef flat zone (130 m in maximum width); and an outer-reef slope that consists of coral-built spurs and grooves. In the northwest, the reef foreslope gently deepens seaward to depths of 15 m and then steepens sharply to 50 m and forms an almost vertical wall between 50 and 100 m (Fig. F2).

Cored material shows that the reefs around Tahiti are composed of two major lithologic units: a late deglacial carbonate sequence (Unit I) and an older Pleistocene sequence (Unit II) (see the “Expedition 310 summary” chapter) (Figs. F2, F3).

Unit I is primarily composed of coralgal-microbialite frameworks (Fig. F3A) commonly interlayered with skeletal limestone and loose skeletal sediments, including coral and algal rubble and skeletal sand. The coralgal-microbialite frameworks that form the bulk of this unit are characterized by widespread development of microbialites (Fig. F3A, F3B), which locally represent the major structural and volumetric component of the reef rock. They developed within the primary cavities of the reef framework, where they generally overlie crusts of nongeniculate coralline algae (Fig. F3B). Microbialites generally comprise a suite of fabrics including two end-members represented by laminated fabrics and thrombolitic accretions; laminated fabrics generally correspond to the most abundant fabric. The late deglacial sequence at Maraa and Tiarei displays a difference in levels of volcaniclastic component, with the reef at Tiarei containing a greater volumetric component (Fig. F3C–F3F). This can be observed in the core and downhole magnetic susceptibility logs and quantified by mineralogical analyses.

Lithologic Unit II is a coralgal framework interbedded with skeletal grainstone to packstone rich in volcanic grains (Fig. F3G, F3H). Coralgal frameworks exhibit evidence of diagenetic overprints, including the alteration of coral skeletons and the occurrence of solution cavities. Large solution cavities display yellow and brown to reddish brown staining. Some solution cavities are partly filled with sediments. Noncarbonate grains can be found as individual grains of olivine, pyroxene, and plagioclase, but millimeter-sized subrounded basaltic lithoclasts are also common (Fig. F3G, F3H).