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In this work, unconformity-bounded stratigraphic (UBS) units are applied to a complex volcanic terrain. UBS units allow us to summarize the spatial-temporal relationships between the cartographic units of a volcanic district or complex, and identify individual successions of volcanic deposits and their probable connection with morphogenetic and/or tectonic phases. The UBS unit approach for the interpretation of significant preserved unconformity surfaces is supported by data from geomorphology, sedimentology, petrology, tectonics, and volcano-tectonics. The result is a stratigraphic scheme that combines the recognized lithostratigraphic units into a series of synthems, in which several phases of geological evolution of Ustica Island are recognized.

The emplacement in the early Pleistocene of the basaltic Monte Guardia dei Turchi lithosome, which represents the emerged part of the volcanic seamount, was followed by hydromagmatic activity and construction of a tuff cone that is part of the Monte Costa del Fallo synthem. Following a period of quiescence, volcanism on the island resumed with a Plinian eruption that deposited a thick sequence of trachytic pumice breccias and was accompanied by a caldera collapse. Subsequently, marine terracing occurred in the southwest, while extrusive volcanics filled the caldera in the northern part of the island. Lava flows associated with this volcanism are present on the western flank of the volcano, whereas along the sublittoral zones, dike swarms provide evidence of phreatomagmatic activity. This volcanic activity was followed by repeated alluvial deposition and marine erosion. Finally, recent subaerial monogenetic centers produced shallow subvolcanic bodies, scoria and tuff cones, and lava flows.

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