Coral bioconstruction variability in a small basin: the Burdigalian Bonifacio basin (South Corsica).

Coral-rich deposits and coral bioconstructions characterize the mixed carbonate-siliciclastic systems developed in the Bonifacio Basin during the Early Miocene. Accurate mapping on photomosaics allowed to document the internal organization of coral deposits as well as lateral and vertical facies relationships. Thin sections observation contributes to textural characterization and skeletal components identification. Notwithstanding the small size of the basin (4 x 8 km) at least six types of coral bioconstructions and related depositional geometries are developed. 1) Above gently sloping substrate patch reefs occur in a wedge-shaped system. Siliciclastic facies characterizes the nearshore environment and carbonate production dominates the meso-oligophotic zone, where coral build-up passes basinward to the maerl facies. 2) In the pocket beach oysters and domal coral colonies rapidly colonize gravel and boulders of shoreface setting. 3) On rocky shore, coral buildup is organized in a lens-shaped structure showing a core mainly constituted by a relatively dense coral domestone with a moderate increase of platy corals in the upper part. A coral rubble associated with granitic cobbles and pebbles is locally present at the base of the structure. 4) Coral carpets developed in a patchy pattern on coarse skeletal sand of the shoreface environment. 5) Closer to the granitic basement, coral bioconstruction made up of a coral domestone that may developed on coastal conglomerate evolving basinward to a coarse hybrid sandstone with isolated large domal coral colonies. 6) Cross-bedded coral rudstone to floatstone formed at the base of clinoforms representing the failure of patch reefs developed on the shelf break of an infralittoral prograding wedge system. The inherited topography produced by the Hercynian crystalline basement, together with the oceanography and the sedimentary processes acting in this small basin, are responsible for having produced such coral bioconstructions variability.