GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.2 103 tectonic process started in the Early Miocene (Maillard et al. , 1992) with the opening of the Valencia trough, related to the clock-wise rotation of the Balearic promontory. The rotation of the Corso-Sardinian microplate took place between 30 and 15 Myr (Cherchi and Montadert, 1982) producing the compressional phase of the Apennine Chain, while the north-east sector of the microplates was deformed in the present Calabria massif. The west Mediterranean lithosphere is characterized by a variable thickness, as shown by seismic and gravimetric studies, which furnish values from 60 km in the oceanic basins to 80 km in the continent. The crustal thickness ranges between 10-15 km in the oceanic basins and 20-30 in the continent. About 16 Myr the Valencia through and the Ligure – Provençal basin were completely opened. The Ligure-Provençal Basin [or Sardo-Provençal Basin, as called by Geletti et al. (2014)] is bounded by two continental margins that show an asymmetry due to the different sedimentary supplies from the French-Spanish and from the Corsica-Sardinian rivers (Geletti et al. , 2014). Messinian Salinity Crisis and MES. The MSC occurred in the whole Mediterranean Sea during the Late Miocene (5.33-6 Myr). The event was probably originated by the progressive approach of the African and European plates that caused the closure of the Strait of Gibraltar. This event determined a massive evaporation of water and the creation of a hypersaline lake. A salt layer precipitated with a thickness varying from 1.5 to almost 3 km, and an estimated volume of 1 million km 3 . In the Sardo-Provençal basin the MSC deposits correspond to an average 1.6 -2.1 km thick sequence of mainly evaporitic lithologies. They are divided, starting from below, in Lower Evaporite (LU), Salt/Mobile Unit (MU) and Upper Evaporite (UU) (Geletti et al. , 2014). Clauzon et al. (1996) suggested that the evaporite precipitation took place in two steps: initially in the peripheral basins, after the first lowering of the sea level; then, a massive precipitation of halite (1.5 km) occu r red in the deepest part of the basins. The MES (Margin Erosional Surface, as defined by Lofi et al. , 2011) is the erosional truncation that affects the upper slope, the continental shelf and the onshore. The UU is covered by the Pliocene sequence that deposited after the end of MSC, when the Strait of Gibraltar re-opened and the water flood again in the Mediterranean basins. The unconformity MES developed during the emersion of the Messinian or, more often, pre- Messinian sediments. In the Sinis Peninsula, which is the northern onshore of the Oristano Gulf, Messinian evaporitic deposits have been divided by Cornée et al. (2008) in four formations: 1) the Basal Marls and 2) the Capo San Marco Formations, both consisting in marls with foraminifers and bioclastic limestones deposited in marine conditions; 3) the Sinis Limestone Formation consisting in micritic limestone deposited under hypersaline lagoon conditions and 4) the Torre del Sevo Limestone Formation, consisting in limestone and dolomitic limestone, interpreted as a fully marine deposit. This last formation was affected by an emersion episode that determined an erosional surface. This sequence is overlain by the Pliocene marine unit. Plio-Quaternary sequence. The Plio-Quaternary sediments of the studied area are fluvial- alluvial type and overlay the MES. Geletti et al. (2014) measured a sediment thicknesses from about 400 ms, in the Sardinia slope, to more than 1600 ms, in the deep basin. The Plio-Quaternary sequence is usually characterized by two typical seismic facies: 1) at its base the Lower Pliocene semi-transparent pelagic marly sequence with volcanic pebbles and 2) the Upper Pliocene/ Quaternary sequence which shows an increased reflectivity (Geletti et al. , 2014) due to the sediment coarsening. The PQ covers the MES unconformity, or the top of a thin basaltic plateau that was erupted at the end of the MSC (Lustrino et al. , 2000). The Pliocene basalts are dated back to 5.5 Myr and are related with an extensional tectonics associated with the opening of the southern Tyrrhenian basin (Beccaluva et al. , 1987; Fais, 1996). After the Lower Pliocene transgression the area, already involved in the southern branch of the Oligo-Miocene Sardinian Rift, was partially covered by fluvial-alluvial sediments related to the start of the Campidano Graben rifting. A Middle Pliocene-Quaternary prograding complex