Indicatori geomorfologici di episodi di stazionamento basso del livello marino in base a dati morfobatimetrici e sismici di alta risoluzione nell’alto morfo-strutturale di Punta Licosa (Tirreno meridionale)

Some geomorphological evidences of the occurrence of sea level lowstand episodes in the morpho-structural high of Punta Licosa (Southern Tyrrhenian sea) based on a densely spaced grid of high resolution seismic reflection profiles (Subbottom Chirp) are here presented. These evidences are represented by terraced surfaces disposed at several water depths under the sea level and incised in the rocky acoustic basement, widely cropping out in the seaward prolongation of the morpho-structural high of Punta Licosa (Southern Tyrrhenian sea). The geological structures and the related seismic sequences, unconformably overlying wide outcrops of the acoustic basement, have been studied with detail. This allowed to analyse the stratigraphic architecture of Quaternary marine deposits. They are well developed in the depocentral areas located between the mouth of the Solofrone river and the town of Agropoli. In the Punta Licosa morpho-structural high the rocky outcrops result from the seaward prolongation of the stratigraphic-structural units, widely cropping out onshore in the adjacent emerged sector of the Cilento Promontory (“Flysch del Cilento” Auct.; CIAMPO et al., 1984; BONARDI et al., 1988). The Cilento Promontory represents a morpho-structural high, interposed between the coastal depressions of the Sele Plain-Salerno Gulf and of the Policastro Gulf. Its reliefs are composed of thick successions of turbidite siliciclastic and carbonatic sequences, dipping landwards into the main carbonatic reliefs of the Southern Apennines (“Alburno Cervati Unit” Auct. ). The terrains cropping out in the Cilento Promontory are formed by siliciclastic rocks, accumulated into deep basins during a time interval ranging from the Late Mesozoic and the Late Miocene. The oldest one of these formations is the North Calabrian Unit, the highest stratigraphic-structural unit in this sector of the Southern Apenninic chain. In the Cilento area this is represented by a formation ranging in age from the Malm to the Oligocene, composed of dark clays, marls and marly limestones, reaching a thickness of 1300 meters. The North Calabria unit is overlain by Early Miocene synorogenic units, showing a degree of deformation minor with respect to the overlying tectonic units. The Cilento Flysch includes the Formations of Pollica, S. Mauro and Monte Sacro, showing an overall thickness of about 1500 meters (BONARDI et al., 1988). In the western sector of the Cilento Promontory several morphological depressions filled by alluvional deposits, whose origin has to be attributed to NNE-SSW (Plain of the Alento) and NW-SE (Plains of S. Maria di Castellabate and S. Marco) trending structural elements, occur. The formation of these depressions has to be attributed to the late Middle Pleistocene (BRANCACCIO et al., 1995); they include transgressive-regressive cycles referred to the glacio-eustatic oscillations of the isotopic stages 9, 7 and 5 (SHACKLETON & OPDYKE, 1973), downthrown of several tens of meters with respect to their original altitude, between the end of the Middle Pleistocene and the beginning of the Late Pleistocene. The Cilento Promontory has been involved by a vertical uplift of more than 400 m during the late Early Pleistocene and the Middle Pleistocene. Absolute estimates of the entity of the Pleistocene uplift which involved the Cilento Promontory have been obtained by the vertical distribution of the Pleistocene marine terraces along the Cilento coasts. In the Northern Cilento the oldest marine terraces (Middle Pleistocene) are located at a maximum altitude of 350 m s.l.m. (CINQUE et al., 1994). At the Bulgheria Mt. (southern Cilento) the marine terraces of the Late Pliocene-Early Pleistocene are lifted at altitudes of 450 m s.l.m.; on the other side, the terraces of the Emilian are lifted at altitudes of 350 m s.l.m. (BAGGIONI et al., 1981; LIPPMANN-PROVANSAL, 1987; BORRELLI et al., 1988). Morphological elements of the coastal areas relative to palaeo-stands of the sea level during the Late Pleistocene (isotopic stages 5e and 5c) evidence an overall tectonic stability of this tract of the shore from the Tyrrhenian stage to present-day (ROMANO, 1992). The lack of relevant vertical movements during the last thousand of years is evidenced by the altimetric position of the Versilian beach deposits; these deposits have been observed in the fluvial coastal depressions, incised during the previous glacial regression, for more than 2 km in the inner of the shoreline (CINQUE et al., 1994). The present-day coastal cliffs are incised in the arenaceous and silty successions of the Pollica Formation. The geological survey of the Quaternary deposits between the towns of Agropoli and Ogliastro Marina has evidenced the occurrence of five sea level palaeostands at altitudes ranging between 25 m and 1.5 m a.s.l. The oldest levels, the Comenale Complex and the S.Antonio-S. Marco sandstones Complex have been tentatively ascribed to the stages 9 and 7 of the isotopic stratigraphy. The successive paleostands (+ 8, + 10, + 4 m a.s.l.) are represented by abrasion terraces and notches, ascribed to the stages 5e and 5c of the isotopic stratigraphy. The stratigraphic units individuated in the adjacent offshore through the seismo-stratigraphic analysis belong to the Late Quaternary Depositional Sequence; in this sequence, the spatial and temporal evolution and the lateral and vertical migration of marine coastal, continental shelf and slope depositional environments of the Late Pleistocene to Holocene glacio-eustatic cycle have been recognised. The stratigraphic succession records the variations of the accommodation space of the Late Quaternary deposits during the last 4th order glacio-eustatic cycle, ranging between 128 ky B.P. (Tyrrhenian stage) and the present-day (isotopic stage 5e). Several local unconformities overlie coarse- grained deposits, filling intra-basinal depressions or palaeo-channels located at the top of the acoustic basement. Ii is worth to underline the polycyclic nature of these unconformities. This suggests that the acoustic basement was involved in several phases of erosion/emersion, development of terraces and successive transgression, as a consequence of both Late Quaternary glacio-eustatic sea level fluctuations and of Pleistocene tectonic uplift. The geologic interpretation of seismic reflection profiles localised in the morpho-structural high of Punta Licosa has evidenced the occurrence of the acoustic basement (unit S), cropping out at the sea bottom nearshore and dipping seawards under the Quaternary deposits, which form the recent sedimentary cover. Remnants of terraced surfaces located at several depths evidence the complex morpho-evolution of the acoustic basement during the Late Quaternary. In particular, four main orders of terraced surfaces have been recognised. The oldest ones are located at water depths ranging between – 50 m and – 43 m and are genetically related with the terraced surfaces disposed at water depths ranging between – 46 m and – 44 m in the Capo Palinuro area. The second order of terraced surfaces has been identified at water depths ranging between – 27 m and – 17 m and is genetically related with the terraced surfaces located at water depths ranging between – 18 m and – 24 m in the Capo Palinuro area. The third order of terraced surfaces has been recognised at water depths ranging between – 10 m and – 14 m and is related with similar surfaces disposed at water depths ranging between – 12 m and – 14 m in the Palinuro Cape. Finally, the terrace rims occurring at – 8 m are coeval, if not precedent, the last interglacial and are related with the last part of the isotopic stage 3. On the basis of high resolution reflection seismics it has not been possible to recognise the Eutyrrhenian paleo-sea level mark or related deposits