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Upper Silurian, Lower and Upper Devonian conodonts are discovered for the first time from rare calcareous beds of the Serra S'llixi Group in the Pala Manna area, in connection with the nucleus of the "Sarrabus syncline" (southeastern Sardinia) where the youngest rocks of the Palaeozoic sequence of the Genn'Argiolas Unit crop out. Consequently, clastic rocks of the conformably overlying Pala Manna Formation, also affected by the main Hercynian folding fase, could be considered Famennian—Lower Carboniferous in age. It also seems reasonable that the same rocks could be compared with the more or less coeval "Culm—type" clastic sediments of the Villasalto Conglomerate, in the nearby Gerrei region.

The activity of the first Hercynian tectonic phase is documented in the Lake Mulargia area (central Sardinia) by the presence of numerous tectonic units. We have examined in particular the southern region of the lake, where one of the upper tectonic units (the Bruncu Maresusus Unit) outcrops. It has at its base a thick silicoclastic succession where we have found Tremadocian Acritarchs in its stratigraphically upper part. This formation is time—equivalent to the Solanas Formation and the Arenarie di San Vito Auct. Another tectonic unit, Bruncu Brevexilis, constitu ted from bottom to top of metavolcanities of probable middle Ordovician age and fossiliferous sediments of Caradocian, Silurian and Devonian age, is also present in the same area and lies under the Bruncu Maresusus Unit.

The subject of this paper is new geological and stratigraphical data regarding the Paleozoic in the Sarrabus area (Southeastern Sardinia). More particularly Tremadocian (Lower Ordovician) acritarchs have been found in samples taken for microfossils research from the weakly metamorphosed terrigenous sediments of the San Vito Sandstone. Until recently only acritarchs from the Middle and Upper Cambrian has been found in the San Vito Sandstone. This new finding provides a more precise dating of the San Vito Sandstone, a more exact correlation between this Sarrabus formation and analogous formations with acritarchs in Central- Eastern Sardinia and also further confirmation of the Intraordovician age of the Sarrabese Phase and of the corresponding Sardic Phase of Iglesiente-Sulcis area (Southwestern Sardinia).

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.

Recent, detailed geological and stratigraphic surveys of a large area of the Marmilla region (central Sardinia) have made it possible to reconsider the lithobiostratigraphic characteristics of the formal and informal units in the literature on the Cenozoic (Late Oligocene – Neogene) which have been in use up to now. Extension of the research to adjoining lands, including Alta Marmilla, Alto Oristanese, Sinis–Planargia, Barigadu, Baronie and Sarcidano, has also made it possible to perform an overall analysis of the entire Cenozoic basin of central Sardinia and, finally, to make significant comparisons with the coeval successions outcropping in southern and northern Sardinia. In this preliminary note we then report on new data concerning the biostratigraphic aspects of the successions considered, while eco-biostratigraphic data and the depositional and synthemic arrangement, now at an advanced stage of definition, will be discussed in a later, more complete work. Some units in the literature are once again proposed, albeit with amendments, as suggested by the new regulations governing the establishment of new formal depositional units and conservation of units in the literature, and with new geochronostratigraphic delimitations (e. g. the Gesturi Formation). Other units change their hierarchical rank (e. g. the Marmilla Group), while for others it was not deemed useful to keep them (e. g. Marne di Ales), since they might cause confusion. Most of the Formations, all of the Members and Groups are proposed in this work for the first time. The formation units outcropping in central Sardinia’s Cenozioc basin are represented for the most part by a sedimentary and volcano-sedimentary succession several hundreds of metres thick, the age of which is included within the Chattian-Aquitanian passage and the Plio-Quaternary. These are sediments from silicoclastics to mixed carbonatic-silicoclastics, in some cases richly fossiliferous, in which are inserted volcanic products from acid to intermediate-basic with a calcalkaline composition, the latter almost exclusively present starting from the Aquitanian up to the Late Burdigalian, with the strongest concentration in deposits of Burdigalian age. As is the case in northern and southern Sardinia, central Sardinia’s Miocene formations can be referred to three main sedimentary cycles recognized on the basis of heir litho-biostratigraphy and the analysis of associations of benthic macrofauna, which will be discussed in a specific article to be published shortly. The first Miocene sedimentary cycle 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, after a clear regression phase, in the Tortonian (or perhaps already in the Uppermost Serravillian) the third cycle began and ended in the Early Messinian, in correspondence to the upper part of biostratigraphic N17a Zone. 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 for the three cycles mentioned previously is that of a platform and secondly of a slope, but in some cases fluvio-lacustrine and deltaic. At the end of the third cycle there is a hiatus and/or erosion, or local continental sedimentation (Nuraghe Baboe Cabitza A n. Formation). A further marine sedimentary cycle locally (Sinis, Orosei) began and ends in the Lower Pliocene (Nuraghe Baboe Cabitza B n. Formation). In an overall biostratigraphic and geochronological frame, the comparison between the most important sedimentary, tectonic and volcanic occurrences in central Sardinia from the Oligocene to the Pliocene and those which occurred in the north and south of the island are pointed out. From the paleogeographic standpoint, the deposits of the first Miocene sedimentary cycle are part of the autochthonous cover of the southern European continental margin; those of the second and third cycles are instead connected with the aperture of the Balearic basin and the northern Tyrrhenian Sea; finally, the Early Pliocene marine succession is referred to the open of the southern Tyrrhenian basin, as well as the widespread volcanic activity of prevalently basaltic alkaline nature (the «Plio-Pleistocene volcanic cycle») towards the end of the Messinian and in the Pliocene.

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.