Morphological and sedimentary evolution during the last 5000 years of the Bagnoli volcano-tectonic coastal plain (Naples, Italy)

Analysis of six boreholes drilled in the lower part of the Bagnoli coastal plain permitted the reconstruction of its morphological evolution, consisting in three main phases. The first one is limited upward by a pomiceous level related to the Plinian eruption of Agnano-Monte Spina, 4400 yr BP; it is characterised by an alluvial area landward and a coarse sandy beach seaward. The second phase is represented by lagoon environments, as testified by many peat levels, increasingly recent towards northwest; their age ranges from 3700+/-55 to 1850+/-50 yr BP. The last phase is mainly characterised by coastal dune ridges, and subordinately by an alluvial area landward. The reconstruction of stratigraphic architecture shows that an important phenomenon occurred during the second phase. In fact, the peats are deposited at sea level or slightly above; by contrast, the data from boreholes indicate a depth range from 7.7 to 1.3 m b.s.l. This condition suggests a ground displacement, well known in the Phlegrean Fields since Roman times, and interpreted as bradyseismic movements. Subsidence mean rate, considering also the sea level changes, is here estimated around 4-5 m from 3700 to 1850 yr BP. These latter data, previously unknown, as well as the identification in this area of bradyseismic phenomena, already defined only in Pozzuoli and its suburbs and recently in the western area, provide interesting new details in the evolution of this volcano-tectonic coastal plain.