The geomorphological evolution of the outskirts of Naples during the Holocene: a case study of the Bagnoli-Fuorigrotta depression

Abstract Lithostratigraphic analysis, geologic and geomorphologic surveys, and radiocarbon dating were carried out to elucidate the evolution of the Bagnoli-Fuorigrotta depression. The origin of the feature, located at the outskirts of Naples and confined between the ridge of Posillipo and the Agnano volcanic centre, was triggered by the huge volcanic-tectonic collapse, subsequent to the Tufo Giallo Napoletano emplacement (12 ka BP), which caused the Phlegrean caldera. Since then, its evolution has been controlled by the recurrent volcanic and tectonic events which occurred throughout the Phlegrean Fields as well as by sea-level rise. In particular, the tephra ejected from the vents of the Agnano centre (from 11 to 3.7 ka BP) modelled the depression by aggrading its bottom and shifting the coastline seaward. At present, a short erosion scarp, locally resembling a “palaeofalaise”, separates the plain reach (Coroglio plain) of the depression from the marginal Bagnoli-Fuorigrotta terrace. The depression fill consists of subaerial tephra and marine deposits with intercalations of palaeosols and transitional alluvial and lacustrine-palustrine sediments. Such an assemblage supports the inference that the depression underwent a rather complicated evolution, characterized by an overall subsidence trend in part overshadowed by the counteracting uplifting and tephra input. At Coroglio plain, the sympathetic effects of subsidence and sea-level rise were effective in preserving (apart from a few lacustrine-palustrine phases) a marine environment up to historical times. The final evolution of the plain dates back some 1.8 ka BP and depended upon the deposition of a coastal barrier capable of maintaining scattered swampy areas on the land. Since then, the plain was only affected by subsidence, at a rate inconsistent with further marine ingressions but compatible with the persistence of marshy spots until the complete reclamation of the area was accomplished in the nineteenth century.