Unconformity-bounded units and terraced marine deposits as stratigraphic tools at Ustica (Southern Tyrrhenian Sea).

The Unconformity-bounded units are employed for the reconstruction of the geological evolution of the Ustica Island as the result of the interaction between volcanic activity, tectonics, and sea-level fluctuations. The Ustica Island is located in the Southern Tyrrhenian sea in the eastern sector of the Sicilian-Maghrebian Chain. It mainly consists of volcanic products, basaltic to trachitic in composition and belonging to a sodic-alkaline series and, subordinately, of marine deposits (conglomerates, sands and fossiliferous sediments) belonging to a well-preserved series of terraces intercalated within the volcanic succession. Seven successive raised paleo-shorelines are recognized at elevation ranging from 115 m to 8 m a.s.l and are attributed to major sea-level highstand peaks by means of stratigraphic relationships with dated volcanic products, relative dating of fossils and relative chronostratigraphic attribution: paleoshoreline I is attributed to marine oxygen-isotope stage (MIS) 13, paleoshoreline II and III to distinct peaks during MIS 9, paleoshorelines IV, V and VI to MIS 7, paleoshoreline VII order (a, b, c) to MIS 5. Following modern stratigraphic procedures to stratigraphic analysis in volcanic areas, marine deposits are adopted as a powerful means of correlation by adopting the unconformity-bounded concept. They are interpreted as transgressive deposits formed during main sea-level highstands and are bounded by sub-horizontal unconformities: the basal unconformity is the marine erosion surface formed during the sea-level raising, whereas the top unconformity is the surface of subaerial exposure and erosion developed during the following sea-level lowering. In addition, several unconformities formed in relation to endogenous processes of modification of the volcano (quiescence periods, shifting of eruptive centres, ecc.) are introduced at Ustica and hierarchized according to their areal extendibility. First-order unconformities UI and UII bounding MIS 5 marine deposits have a potential regional stratigraphic significance and are documented at Ustica after their first definition across the Aeolian archipelago. Then, five second-order (and one third-order) unconformities with stratigraphic significance for the whole Ustica Island are introduced. The resulting unconformity-bounded stratigraphy allows reconstruction of the geological evolution of Ustica as the result of four successive constructive, relatively short-lived stages of volcanic activity (eruptive epochs), which are separated by longer inter-eruption periods of quiescence when erosion and mass-wasting processes in subaerial and marine environments become prevailing. Most ancient volcanic products dated at more than 750 ka are represented by subaerial pyroclastic deposits emplaced from dilute pyroclastic density currents coming from an eruptive centre located SWwards offshore the Ustica Island. Then, the second and third eruptive epochs document the progressive transition from submarine volcanism (hyaloclastites and pillow lavas) to subaerial eruptive activity with the emplacement of lava products, and of pyroclastic deposits from both dilute and concentrated pyroclastic density currents originated from eruptive centres located in the central sector of Ustica. The fourth and most recent volcanic activity, between 200 and 124 ka, determines the construction of the eccentric Falconiera tuff cone, located in the NE portion of the island. The Ustica Island is inactive during the last 124 ka, and is charaterized by diffuse and recurrent subaerial erosion and reworking.