Nuova classificazione biostratigrafica e geocronologica delle unità formazionali oligoceniche e neogeniche della Sardegna. Correlazioni con le omologhe unità della Corsica

Abstract

Recent revisions in classification and the new definitions in the literature of late Paleogenic and Neogenic formations outcropping in the northern, central and southern Sardinia provided by the authors have led to the preparation of a complete biostratigraphic and geochronological picture of the entire Sardinian Tertiary sedimentay basin. The lithostratigraphic units, of which the measured stratigraphic sections of reference will be given in another work, were placed in the most recent and up-to-date zonal classifications of Mediterranean planktonic Foraminifers associations and calcareous plankton. In this proposal for the reclassification of the stratigraphic successions of Sardinia’s Oligocene-Neogene, three main marine sedimentation cycles are recognized. These are silicoclastics and mixed silicoclastic-carbonatic sediments, sometimes richly fossiliferous, in which are inserted volcanic products going from acid to intermediate-basic and having a calcalkaline composition (the «Oligo-Miocene volcanic cycle») almost exclusively in the Aquitanian-Late Burdigalian interval. On the basis of a comparative analysis of 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 of a slope, but in some cases fluvio-lacustrine and deltaic (preliminary data on the paleo-ecological aspects are being prepared for publication). Of the three main sedimentation cycles, the first evolved between the Chattian-Aquitanian limit and the Late Burdigalian (N6 Zone); the second cycle began in the Uppermost Burdigalian, in correspondence to the upper part of the Globigerinoides trilobus Zone (N7 Zone) and closes in the Late Serravallian (G. siakensis Zone, G. siakensis - G. obliqua obliqua Subzone); finally, the third cycle begins in the Uppermost Serravallian and finishes in the Early Messinian, in correspondence to the upper part of biostratigraphic N17a Zone. The successions examined are also considered with reference to the major tectonic and volcanic events which took place in Sardinia from the Oligocene to the Pliocene within the wider context of the geodynamic evolution of the western Mediterranean area. In particular, the first two sedimentary cycles which evolved from the end of the Oligocene and the Early-Middle Miocene would appear to be closely connected with the collision and postcollision phases respectively of the northern Apennines. The northern Apennines collision phase appears to have manifested itself in Sardinia with an important transcurrent tectonic with transpressive and transtensive structures and with the development of subsidence basins of the pull-apart type in correspondence to which the first Miocene sedimentary cycle and the associated calcalkaline volcanism developed. The creation in the Burdigalian of the wide N-S Sardinian rift, in which the second Miocene sedimentary cycle evolved, is to be attributed to the subsequent phase of postcollision distension. The third Miocene sedimentary cycle becomes manifest, following a regressive phase, limited to the most subsident areas of the Logudoro -Sassari, Capo Frasca - Sinis and Cagliari regions. The marine ingression of the Early Pliocene, located in central Sardinia (Cava Fuile and Nuraghe Baboe Cabitza B nn. Formations) and in the south of the island (in drilling) is to be placed in connection with the strong resumption of the southern Tyrrhenian distensive tectonics towards the end of the Messinian and Pliocene, as well as with widespread alkaline, prevalently basaltic, volcanic activity (the «Plio-Pleistocene volcanic cycle»). In the Late-Middle Pliocene, between the Gulf of Cagliari and the Gulf of Oristano, there was the development of the Campidano graben, filled with more than 500 m of continental deposits (the Samassi Formation). The four Corsican basins (Bonifacio, Saint Florent, Francardo, the eastern Plain), in which Miocene and/or Pliocene sediments outcrop, show, starting from the summital Burdigalian, an evolution that is fairly parallel to what has been recognized in the north of Sardinia. The formations of Aghione, Saint Florent, Cala di Labra and Bonifacio are in fact partly referable to the second Sardinian sedimentary cycle. The subsequent phases characterized in Corsica by emersion, marine sedimentation (the Vadina and Urbino units), fluvio-deltaic sedimentation (the Aleria Formation) and further marine sedimentation (the Urbino, Casabianda, Propiano and Aiaccio units), are in Sardinia respectively correlated with the hiatus and/or erosion or continental sedimentation in the Late Serravillian, with the Tortonian part of the third marine sedimentation cycle (the Monte San Michele Formation), with the Messinian part of the same cycle (the Capo San Marco, Sinis and Torre del Sevo Formations) and with the Nuraghe Baboe Cabitza B (Capo San Marco) Pliocene formation