Nuovi dati stratigrafici e cronologici sull'evoluzione recente della piana del fiume Alento (Cilento, Campania)

Abstract

Field investigations concerning the Quaternary deposits outcropping in the lower Alento valley, stratigraphical data obtained by analysing numerous pre-existing boreholes and a detailed geomorphological study of the area have permitted to outline some of the main morphoevolutionary stages distinguishing the construction of the Alento Plain during the Upper Pleistocene and Holocene. The chronology of reconstructed events, which was based mainly on stratigraphical and geomorphological criteria, has been integrated by chronological data using the C¹⁴ radiometric method as well as archaeological evidence and historical documents. Micropaleontoiogical analyses of the fossil contents (ostracoda and foraminifera) of clastic sediments were carried out to clarify the depositional environment. Stratigraphical analyses of the Quaternary infilling found in the lower Alento valley revealed a succession of epiclastic deposits of marine, transitional and fluvial environment up to more than 50 m in thickness. In the upper part of the study area examinated borehole cores reveal a sequence made of prevailingly clayley sediments subdivided into two intervals (the upper one known as the Argille Superiori) unit, the lower one as the Argille Inferiori unit), which are attributable to lacustrine and perhaps partly lagoonal environments separated by a layer of coarse-grained fluvial sediments (the so-called Alluvioni Sepolte dell'Alento unit). The Argille Superiori unit, the upper part of which is proved to be of lacustrine origin, is characterized by local fluviatile inputs (Ghiaie di Scifro unit) and may be ascribed to the last interglacial high-stand. The terminal part of the Alento valley owns characters of a coastal plain developed prevailingly on marine, eolian and transitional deposits to be referred to the Versilian marine transgression following the maximum regression of the last Glaciation, which had caused the dissection of the Pleistocene plain. The distribution of the "Versilian" marine and transitional deposits evidence the formation of a kind of ria when the sea reached the maximum transgression and a position up to 2.5 km from the present coast The successive shorelines, concerning phases of progradation of late Holocene age, get gradually more rectilinear and parallel to the present coastThe use of stratigraphical, geomorphological and archaeological-historiographical evidence has permitted to recognize at least six paleo-shorelines, the most ancient of which serving to define more correctly the paieogeographical features of the plain during the period of the Greek settlement of Elea, located on the promontory bordering the plain to the south-east. The most ancient of the above-mentioned shorelines (dated to 2600 years BP) can be traced along the outer edge of the "Zambini" dune ridge, which outcrops about 1.5 km from the present coast. Two successive positions of the shoreline, highlighted by the Paino and Marina di Casalvelino dune-ridges which are situated about 650 m and 3%400 m from the present shoreline respectively, are to be referred to a period prior to 2200 years BP on the basis of archaeological data. Another paleoshoreline, mainly proved by a thick eolian deposit outcropping along the south-eastern border of the plain, can probably be referred to the late Roman period (6°-7° Century?). A subsequent phase of progradation is represented by the polyphasic Pantano dune-ridge situated some hundred metres from the present coast. The period of greatest extension of the coastal plain can be referred to the late Middle Ages (14°-15° Century?) when the coastline occupied a considerably more advanced position than the present one. This is proved by the accumulation of a layer of lagoonal clay (the Lido Tirreno unit) outcropping along the present coastline where anthropically induced marine erosion has destroyed its eolian cover. This latter can be referred as the most recent shoreline to be reconstructed in the study area.