On the use of unconformities in volcanic stratigraphy and mapping: Insights from the Aeolian Islands (southern Italy)

The approach to volcanic stratigraphy and mapping in the Aeolian Islands has found the concept of Unconformity-Bounded Units (UBUs) extremely useful for purposes of stratigraphic classification and correlation. Volcanic unconformities (erosive and collapse unconformities) relative to processes of internal modification of a volcano are classified separately from sea level-driven unconformities. The erosive unconformities formed during periods of volcanic quiescence are generally characterized by angular relationships and lithological change between the products below and above an unconformity, and may be associated with paleosols or epiclastic horizons. The collapse unconformities related to calderas and lateral collapses represent surfaces of structural truncation along the collapse borders, passing outside of the collapse to erosive unconformities developed during the corresponding repose periods. Instead, the sea level-driven unconformities are marine and subaerial erosive surfaces bounding in pairs the depositional cycles relative to sea level highstands, which are recorded in raised marine terraces. Most unconformities can be correlated on a variable scale from local (volcanic island) to regional (inter-island) by means of careful stratigraphic analysis targeted to identify the corresponding stratigraphic hiatus in the volcanic successions in different points of observation. This helps to solve the problems arising from the difficult recognition of the unconformities in the field moving away from the type locality due to their variable features. A practical nomenclature scheme for a hierarchy of UBUs to be applied in volcanic areas can be established on the basis of the areal magnitude of the bounding unconformities and their importance in the volcano history, passing through a slight modification of the International Stratigraphic Guide's rules. Combined with lithostratigraphic units (mapped rock bodies) and lithosomes (eruptive centres), the UBUs provide a useful stratigraphic synthesis of a volcanic area by means of a framework of time-stratigraphic unconformable surfaces. Moreover, UBUs can be directly interpreted in terms of volcanic activity units with a time-stratigraphic significance, thus providing the key elements for the eruptive history of a volcano and the definition of its event stratigraphy.