Definizione lito-biostratigrafica delle unità formazionali del tardo Paleogene e Neogene del Nord Sardegna e della Corsica(*)

This work proposes a terminological definition of the lithostratigraphic units in the literature and recently defined ones which make up the Tertiary successions, especially the Miocene, outcropping in northern Sardinia. These units are then correlated with the coeval formal and informal units outcropping in Corsica. The different lithostratigraphic units are also placed within the frame of the most up-to-date zonal diagrams of planktonic Foraminifers and calcareous plankton associations currently recognized in the Mediterranean. Northern Sardinia’s Cenozoic deposits are represented by a sedimentary and volcano-sedimentary succession several hundreds of metres thick, the age of which is included within the Chattian-Aquitanian and the Pliocene. These are sediments that go from silicoclastics to mixed silicoclasticcarbonatic, sometimes richly fossiliferous, in which are inserted volcanic products going from acid to basic-intermediate and having a calc-alkaline composition almost exclusively in the Aquitanian-Burdigalian interval. They refer to three main sedimentary cycles recognized on the basis of lithostratigraphy and on the analysis of benthic macrofauna associations which will be the specific subject of a future paper. Further marine sedimentation occurs in the Early Pliocene. The first cycle evolved between the Uppermost Chattian and Late Burdigalian (N6 Zone); the second cycle began at the Uppermost Burdigalian, in correspondence to the upper part of the Globigerinoides trilobus Zone (N7 Zone) and closes in the Serravallian-Tortonian limit (G. siakensis Zone, G. siakensis - G. obliqua obliqua Subzone); finally, the third cycle in all probability begins at the Late Serravallian and ends in the Early Messinian, in correspondence to the upper part of biostratigraphic N17a Zone. On the basis of a comparative analysis of the autochthonous benthic associations, especially those with Molluscs and of the textural characteristics of the sediments, the prevalent depositional environment is that of a platform and secondly that of a slope, but in some cases fluviolacustrine and deltaic. The major sedimentary, tectonic and volcanic events which took place in north Sardinia between the Oligocene and the Pliocene, also considered in the wider context of the geodynamic evolution of the western Mediterranean area are also synthetically described. A comparison between the geochronological, lithostratigraphic and biostratigraphic arrangement proposed here for the northern Sardinian units and that of Corsica highlights a substantial parallelism. The main sedimentary cycles which evolved in Sardinia and Corsica between the end of the Oligocene and the Early-Middle Miocene are found to be closely connected with the collision and post-collision phases of the northern Apennines, which in various ways and with varying intensity involved the Sardinian-Corsican block. Towards the end of the Messinian and Pliocene, the extensional tectonics of the southern Tyrrhenian is responsible, even in northern Sardinia, for the marine ingression of the Early Pliocene located in the Baronie (Cava Fuile n. Formation) and in the Sinis regions (Nuraghe Baboe Cabitza B n. Formation), as well as for widespread volcanic activity of a prevalently basaltic alkaline nature (the «Plio-Pleistocene volcanic cycle»). The continental deposits of the Middle-Late Pliocene of the Nuraghe Casteddu (Orosei) Formation also document an erosional phase connected with the neotectonic movements of the southern Tyrrhenian area. The units of the second northern Sardinian sedimentary cycle extend to Corsica (the Balistra - Capo Sperone – Cappili - Cala Labra - Capo Pertusato - Bonifacio basins, those of the Piana di Aleria, Aiaccio and Francardo - Saint Florent),with analogousfacies and good geochronostratigraphic and biostratigraphic correspondence.