Osservazioni preliminari sull’evoluzione geomorfologica della Piana del Sarno (Campania, Appennino Meridionale)

Abstract

The Sarno River coastal plain occupies the southern part of the broad peri-Tyrrhenian Piana Campana Graben. The buried structural setting of this depression is composed of step- fault blocks of Mesozoic limestone units of the Apennine chain overlined by residuals of their Miocene terrigenous cover. The faults are prevailingly oriented NW-SE and NE-SW and their maximum total throw is of some 6 500 m. The age of the graben is not clearly defined at the present stage of knowledge. On the ground of old drilling data and geomorphological consideration we assume that the northern part of the graben started subsiding during the upper Pliocene, while the southern one did so in the lower Pleistocene. The sedimentary infilling of the Piana Campana Graben is made of alternating marine and transitional deposits with localized lateral passages to volcanic (either ancient buried volcanic reliefs or upper Pleistocene - Holocene products of the present Campanian volcanoes, in the upper reaches). The Sarno Plain is limited by the Sarno's Mountains to the East, the Lattari Mountains to the South and the Somma-Vesuvio volcanic plex to the North. The plain appears almost perfectly flat and depressed up to the very base of the step marginal fault scarps. This is likely due both to the occurrence of a relatively recent phase of subsidence and to the flattening effect of the prevailingly pyroclastic aggradation the plain has experienced during the last millennia. The uppermost portion of the infilling, as it appears from drilling data, contains several intercalations of shallow marine deposits: Deposits of the last widespread ingression, which reached the eastern marginal reliefs, are to be found at about 25 m below s.l. They are tentatively ascribed to the last interglacial. Wells drilled on the coastal portion of the plain show the presence of deposits left by younger ingressions (postglacial in age) which can be traced as inland as Pompei. Geomorphological observations carried out in Sarno and Castellamare Stabia - Gragnano areal allowed us to outline the evolution of the tectonic margins of the Sarno Plain. The oldest morphological feature is represented, in both areas, by remnants of a Pliocene paleosurface presently seated at about 1000 m a.s.l. The uplift and the block-faulting of this surface occurred through at least two tectonic phases probably during the Lower and the early Middle Pleistocene. Following each phase of uplift, formation of broad alluvial fans occurred at mouths of valleys dissecting the fault scarps. We were able to recognize four discrete episodes of fan formation; the oldest one appears cut by the second tectonics phase, the second is probably cut by a third moderate tectonic phase or, alternatively, by a deep marine ingression (likely of Tyrrhenian age). The third generation of fans is to be referred to the late Würm for the presence of radiometrically dated pyroclastic intercalations. The Würmian fan are cut at the foot by a Versilian seacliff. At mouth of valleys that cut this cliff a fourth generation of fans is present which grew during historical times. On the ground of the available geomorphological data it seems that the Sarno plain has been substantially stable during the last two millennia.