Studio delle associazioni a cisti di dinoflagellati e sporomorfi di alcune successioni marine mioceniche della Sardegna e della Calabria

The Miocene Epoch (from 23.03 to 5.33 Ma) is largely documented in Sardinia and in Calabria through fossiliferous sedimentary and volcanic sequences that resulted from tectonic, magmatic and sedimentary processes in these regions. The geodynamic processes related to the evolution of the Mediterranean area in the Tertiary comprise ocean opening and closure (Ligurian basin and Tyrrhenian basin), development of collision margins and thrust and fold belts with opposite vergences, rise of a volcanic arc and progressive migration of the subduction margin (Calabrian-Peloritan Arc), opening of back-arc basins (Algerian and Liguro-Provencal basins, Alboran Sea) and co-presence of both extensional and compressional features related to the same geodynamic setting. A close relationship between the Sardinian-Corsican block, the Northern Apennines and the Calabrian-Peloritan block, all characterized by typical European palaeo-margin sequences, is accepted by many authors. It appears that published models often lack accurate biostratigraphic control for the reconstruction of the relationships of the two terranes (Sardinian-Corsican and Calabrian-Peloritan blocks) with the Northern Apennines (e. g., Principi & Treves, 1984; Rosenbaum & Lister, 2004; Scandone, 1979). This thesis represents an investigation of organic-walled dinoflagellate cyst assemblages with the aim to increase the amount and improve the quality of biostratigraphic data available from the Sardinia and the Calabria Miocene deposits. This will serve to better correlate the two basins and give a contribution to the geodynamic reconstruction of the Central-western Mediterranean basin evolution. We want to propose a correlative scheme of the successions, through recognition of bio-events calibrated at the Western Mediterranean basin scale, and to investigate the paleoenvironmental features of the two areas during the Miocene. This study has been carried out within the activities of the multidisciplinary Italian National Research Project (PRIN) “Analysis of stratigraphic, palaentological and structural features of the Hercynian basement and the Tertiary cover in Sardinia and Calabria, as a contribution to the geodynamic reconstruction of the Central-western Mediterranean basin”. In this thesis new biostratigraphic and paleoecological data from the Miocene of Sardinia and Calabria are reported.Six outcrop sections and two boreholes were investigated in Sardinia: three sections from the Marmilla region in central-southern Sardinia (Ales, Villanovafranca-Gesico, Barumini), one from the Campidan region (Sestu quarry) in southern Sardinia, one from Logudoro region (Florinas) in northern Sardinia and one from the Sinis Peninsula in western Sardinia. The two boreholes were drilled in the Logudoro region (Bonorva), northern Sardinia. Seven outcrop section were investigated in Calabria: four from the Serre Massif (Valle dello Stilaro dx, Stilo-Stignano, Valle dello Stilaro sx, Prachi) and two from the Aspromonte Massif (Bovalino, Madonna del Carmine), in the southern sector of the Calabrian-Peloritan Arc; one from the Sila Massif (Caloveto) in the northern sector of the Calabrian-Peloritan Arc. Between 10g and 15g of sediment were processed for palynological preparations, using cold HCl (10% and 30%) and HF (40%). The residue was sieved at 15 μm using a nylon mesh, mixed with glycerine, and mounted on microscope slides. At least 200 dinoflagellate cysts were counted for each sample analysed and slides were also checked for rarely occurring taxa. Dinoflagellate cyst taxa nomenclature generally follows Fensome & Williams (2004). The time-scale of Gradstein et al. (2004) is used throughout. The dinoflagellate cysts assemblages have been studied quantitatively and biostratigraphic events have been recognized, allowing comparisons with previously studied successions in other Mediterranean areas (e. g., Langhe region in Northwest Italy; Marche region in Central Italy) (e. g., Powell, 1986a, b; Biffi and Manum, 1988; Brinkhuis et al., 1992; Zevenboom, 1995; ENI internal report, 2001). A total of 102 dinoflagellate cyst taxa have been recognized, which include biostratigraphical markers as Exochosphaeridium insigne, Cordosphaeridium cantharellus, Cousteaudinium aubryae, Cerebrocysta poulsenii, Labyrinthodinium truncatum, Unipontidinium aquaeductum, Achomosphaera andalousiensis. They provide supplementary information on the age of Miocene sediments from Sardinia and Calabria. This is the first ever through study on Sardinia and Calabria Miocene sequences which uses dinoflagellate cysts as tools. Paleoenvironmental interpretations based on dinoflagellate cysts provide interesting insights. The relationships between cysts and their paleoenvironmental indications have been investigated since the 1970s (e. g., Downie et al., 1971; Wall et al., 1977; Harland, 1983). Because many Miocene dinoflagellate cysts are still found in present-day sediment samples results obtained from dinocyst assemblage studies of recent sediments (Marret & Zonneveld, 2003 and others) can be used to interpret the Miocene record in terms of paleoenvironmental reconstructions. The dominating dinocyst group found in the Sardinia samples (Spiniferites spp.) is known to be characteristic of neritic environments (Brinkhuis, 1994; Wilpshaar & Leereveld, 1994). Moreover, the great abundance of pollen and terrestrial plant debris found in the same samples suggests that the depositional environment in the Ales, Barumini, Villanovafranca-Gesico sections must have not been far from the coast. In the Florinas section the dinocyst assemblages indicate a nearshore, inner-shelf paleoenvironment (15-20 m water depth). The assemblages of Sestu and Capo San Marco sections display greater dinocyst species diversity. The abundance and species diversity of Spiniferites spp., associated with oceanic taxa such as Impagidinium spp. and Nematosphaeropsis spp. may suggest the establishment of open neritic conditions. The Calabria sections (Valle dello Stilaro dx, Stilo-Stignano, Madonna del Carmine) represent inner neritic facies overlain by open neritic to bathyal and oligotrophic facies (Valle dello Stilaro sx section). In general, the dinocyst assemblages of the Calabrian sections represent a great diversity of palaeoenvironments ranging from continental or transitional (freshwater/brackish water) to bathyal during the Middle and Late Miocene. The Sardinia sections investigated here cover almost entirely the Miocene from the Aquitanian with a probable extension to the uppermost Oligocene (NP25 biozone) to the lower part of Messinian. The biostratigraphic interval best represented extends from NN1 to NN2 Nannoplancton biozones (Aquitanian-early Burdigalian). The equivalent of NN7-NN10 Nannoplancton zones (early-middle Tortonian) is not represented in the studied material. The Calabria sections of this study represent almost entirely the Miocene record, from the Aquitanian to the lower Messinian. The interval equivalent to nannofossil zones NN1-NN3 (Aquitanian-middle Burdigalian) is particularly well represented. If we consider the paleoenvironments as reconstructed by dinocyst assemblages, and the possible relationships between the Sardinia and the Calabria basins, we conclude that there is a great similarity between the Stilo-Capo d‘Orlando Fm. in Calabria and the earliest Miocene formations in the southern-central part of Sardinia (Ales Marls Fm. and Marmilla Fm.) during the Aquitanian-Early Burdigalian interval.