The Phlegrean Fields beneath the sea: seismo-stratigraphic and marine magnetic evidence on the Gulf of Pozzuoli (Naples)

A grid of Sparker Multitip seismic profiles recorded in the Gulf of Pozzuoli in the frame of research projects of submarine geologic cartography has been interpreted to give new insights on the seismic stratigraphy of the Gulf of Pozzuoli, i.e. the submarine elongation of the Phlegrean Fields volcanic complex. One aim of this paper is to provide a new seismo-stratigraphic framework for the Gulf of Pozzuoli based on geological interpretation of recently acquired data in order to delineate an improved geological setting through seismo-stratigraphic concepts. New stratigraphic and volcanological studies, mainly onshore, dealing with the definition of volcanological units during the field survey and specific volcanological features, petrographic, geochemical and petrological studies, coupled with geophysical models have been recently presented. These studies have allowed us to delineate the problems related to stratigraphy and volcanology of active volcanic districts of southern Italy, referring in particular to the Naples Bay. A sketch geomorphological map of the Pozzuoli area has been constructed to improve the geological knowledge of the onshore area. Main morphological lineaments are represented by lava domes, crater rims, collapse rims, Late Holocene sea cliff, faults older than the marine ingression, faults affecting La Starza terrace, rims of subaerial terraces and local elevation of terraces. Holocene marine terrace of La Starza is bounded by an abandoned sea cliff. The succession exposed in this cliff overlies a volcanic substratum of yellow tuffs, including fossiliferous littoral deposits alternating with subaerial pyroclastic deposits and paleosols. The lowest marine interval exposed in the cliff has been radiometrically dated to 10.5 ky B.P,; an overlying paleosol gave an age of about 8 ky B.P. and the uppermost beach deposits an age of 4.6 ky B.P. The continental intervals of the succession were deposited during periods of uplift having rates greater than sea-level rise. The final uplift of the terrace (about 30 m in a few centuries) was probably related to the caldera resurgence, accompanying the onset of a new period of strong volcanic activity of the Phlegrean Fields. The Gulf of Pozzuoli represents the submerged border of the Phlegrean caldera, resulting from the volcano-tectonic collapse induced from the pyroclastic flow deposits of the Campanian Ignimbrite (35 ky B.P.). Main submerged physiographic units include the inner continental shelf, the central basin, the submerged volcanic banks and the outer continental shelf. The stratigraphic relationships between the Quaternary volcanic units related to offshore caldera border and the overlying deposits of the Late Quaternary depositional sequence in the Gulf of Pozzuoli are shown. 14 seismic units, both volcanic and sedimentary in nature appear to be tectonically-controlled by coeval folding and normal faulting. Magma uprising in correspondence to extensional structures is evidenced by volcanic dykes. A large field of tuff cones next the Nisida inlet is related to the emplacement of the Neapolitan Yellow Tuff deposits. A thick volcanic unit crops out at the sea bottom off the Capo Miseno volcanic edifice and is genetically related with the Bacoli-Isola Pennata-Capo Miseno yellow tuffs, occurring in the Northern Phlegrean district.