LATE-PALAEOZOIC TO EARLY-MESOZOIC TECTONOSEDIMENTARY CYCLES IN SARDINIA AND THEIR CORRELATION IN THE WESTERN MEDITERRANEAN AREA

Recent and detailed stratigraphic studies on the main continental Upper Carboniferous-Permian to Middle Triassic deposits of Sardinia as well as on the petrography and geochemistry of the coeval volcanic units, highlighted in the island three main tectonosedimentary cycles. Each one of these sequences, bounded by regional unconformities, mirrors a major geodynamic phase and thus allows correlation with wide sectors of the W Mediterranean (particularly France and Spain). The first cycle can be ascribed to a Late Carboniferous (Westphalian?-Stephanian) - lower Early Permian (“Autunian”) time interval. It is mainly represented by richly fossiliferous alluvial-to-lacustrine siliciclastic sediments and fresh-water limestones, which are interfingered and overlain by large volumes of calcalkaline volcanic rocks. These volcano-sedimentary successions characterise the main intramontane basins (Seui, Perdasdefogu, Escalaplano-Mulargia), as well as reduced outcrops (Guardia Pisano, S. Giorgio, Lu Caparoni). The inception of these troughs can be attributed to crustal attenuation and tearing which affected vast parts of Variscan Europe, giving rise to a large transcurrent megashear zone. The second cycle, spanning hypothetically from the late Early Permian to the late Permian p.p., is characterised by the piling of a Group of azoic reddish alluvial formations, to be considered as UBSU. In Sardinia, these deposits crop out with considerable thicknesses only in NW (Nurra) and scarcely in SW (Guardia Pisano), whereas in Provence they normally attain several hundreds metres and are repeatedly intercalated to alkaline volcanic rocks. In Sardinia, extrusive rocks with this affinity are clearly exposed only in Nurra (Mt. Santa Giusta). Alluvial mature siliciclastics, which testify to a clearly arid period widespread in large part of Palaeo-Europe, evolving to floodplain-to-playa and subsequently to pre-evaporitic marly-dolomitic deposits, characterise the reduced sequences of the third cycle, which age is Olenekian(?) to Anisian. These deposits, commonly referred to the coeval French and German Buntsandstein, occur in NW, SW and SE Sardinia. They mark the onset of the second first-order eustatic cycle during the Phanerozoic and are overlain by the Middle Triassic carbonatics of the Muschelkalk.