Le Miocène du bassin de Saint-Florent(Corse): Modalités de la transgression du Burdigalien supérieur et mise en évidence du Serravallien

Abstract

The Miocene series of Corsica deposited after the anti clockwise rotation of the Corsica-Sardinia block,between 18 and 21 Ma (Réhault 1981; Réhault et al. 1984). The study of the Saint-Florent basin (NW corsica) allows to document the conditions of the Miocene transgression and the evolution of the basin. The Saint-Florent sedimentary serie consist of 3 marine formations lying between 2 continental ones. At the base, the continental formation of Fium Albino (150 m) in the northern part of the basin, stratigraphically lies on the Upper Nebbio alpine nappe. It consists of well stratified, red, colluvial deposits from local origin, with interbedded palaeosoils remains. We have very few paleontological evidences, however from geometrical evidence, these series are clearly post nappe and ante Upper Burdigalian. This formation is described here for the first time. At the upper part, the continental formation of Saint-Florent, characterized by well rounded pebbles of rhyolites, lies with an erosional surface truncating different terms of the marine series. It is probably of Lower Tortonian age. The 3 lithostratgraphic units of the marine series are, from the base to the top: the Torra formation (50 m) is characterized by coastal deposits (conglomerate, siltstone, sandstone layers, with debris of oyster, sands with scutella) with sparse micro-fauna and - flora, suggesting an Upper Burdigalian (upper N7/lower N8 Blow's zones); the Monte San Angelo Formation (250 m) is a white calcarenite with rhodophytes, its different components - fine and coarse terrigenous detritus, benthic or planktonic bioclasts-having built hydraulic dunes under the action of varying currents in a low water environment. Planktonic foraminifera - rich levels indicate a Langhian age (from N8 to the upper part of the N9 Blow's zones); the Farinole Formation (100 m), described here for the first time, is observed only in the northern part of the basin. It consists of marls and clayey sandstones deposited in circalittoral environment. Rich microfauna of benthic and planktonic foraminifera, and rich calcareous nannoplankton evidence the Serravallian (NN7-NN6, Martini's zones). To the east, the basin is limited by a meridian normal fault. On the one hand, synsedimentary deformations in the Torra formation and in the lower part of the Monte San Angelo Formation are located to the north of the basin near the fault. On the other hand, thick subtidal environment deposits of the Torra Formation occure in a shallow water. Thus the subsidence of this Formation is probably controled by the syn-sedimentary activity of the meridian normal fault. Extensional fractures indicating a NE-SW direction of extension.