Abstract

The wide outcrops along the cliffs of Bonifacio at the southern tip of Corsica expose two stratigraphic series of shallow marine deposits, the reefal Cala di Labra Formation (Upper Burdigalian), and the Bonifacio Formation (Lower Langhian at the youngest). Both series are transgressive on the faulted Hercynian basement. The Bonifacio Formation is divided into two members, the Pertusato Member at the base, and the Bonifacio Member at the top.

The Pertusato Member, with an average thickness of about thirty meters, is composed mainly of siliciclastic marine deposits, and is weakly fossiliferous. The Pertusato deposits exhibit locally hummocky cross stratification at their distal lower part, and are commonly associated with synsedimentary deformations and water escape structures. In their upper part, and laterally, the coarse-grained Pertusato Member deposits are organized into a prograding coastal prism which bear structures of a barrier beach jagged by some local erosional discontinuities. The whole siliciclastic sequence of the Pertusato Member is wave and storm controlled. Due to the absence of precise biostratigraphic markers, the age of the Pertusato Member is not well constrained. However, it exhibits a thin silty clay layer, that suggests a possible link with the volcanic events recognized on the southwest corsican margin [Rossi et al., 1998], which delivered an absolute age of 16.1 ± 0.4 Ma (Burdigalian-Langhian boundary).

The Bonifacio Member (Lower Langhian) is composed of a succession of sandstones and calcarenites over 50 m in thickness, organized into aggrading strata of large 2D and 3D dunes, with several bioturbated levels. The series is interpreted as an offshore tidal system, deposited in a slightly deeper environment (from 25 to 40 m water depth) than the Pertusato Member. The transition from a wave-dominated to a tide-dominated setting might occur as the response of the connection between the basin of Bonifacio and the recently formed Tyrrhenian Sea. The carbonate content of the sediments steadily increases to up to 80% towards the top of the series. At the same time, the skeletal grain associations increase from a Bryomol facies to Rhodalgal, interpreted as the result of an increase in water temperature in favour of carbonate factory.

The transition from wave to tidal control in such a system illustrates the extreme sensitivity of morphologically complex coastal basins to hydrodynamic forcing caused by a transgression.

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