The relict deposits of the Cilento offshore (Southern Italy): new seismo-stratigraphic data based on high resolution seismic profiles

The relict deposits of the Cilento offshore have been analyzed based on the geological interpretation of new seismo-stratigraphic data, consisting of the Sub-bottom Chirp profiles recorded in the oceanographic cruise GMS03_01 (R/V Urania, National Research Council of Italy) during the scientific and technical activities for the realization of the geological sheet n. 502 "Agropoli" (Cilento offshore, Southern Tyrrhenian Sea). In the Cilento offshore two kinds of relict deposits have been identified based on the geological interpretation of Sub-bottom Chirp profiles, including both the palimpsest deposits and the lowstand deposits.The sedimentary cover of the continental shelves basically comprises two types of sediments, i.e. those in equilibrium with the present-day environmental conditions and those that are not in equilibrium with the present-day environmental conditions, which are the relict sediments. Based on the classical definition of Emery (1968) the relict sediments have been deposited long ago, in equilibrium with the depositional environments therefore, while later, these depositional environments are no longer in equilibrium, even though they have not been covered by subsequent sediments. Thus, the unconsolidated sedimentary cover of the continental shelf basically comprises two types of sediments: those that are not in equilibrium with current environmental conditions (i.e., in a broad sense, relict sediments) and those in equilibrium with these conditions. Emery (1968) defined relict sediments as "sediments deposited long ago, in equilibrium with the environments therefore; later, the environments changed so that these sediments are no longer in equilibrium, even though they have not been covered by subsequent sediments". An important feature in recognizing the relict origin of a sediment is represented by the occurrence of coarse-grained sands, located at a greater distance from the coast and at greater depths than fine-grained sands, which are usually thinner than the fine-grained ones. Another important concept is that of the palimpsest sediments, having characteristics of two sedimentary environments, one older and one more modern. While the relict sediments represent remains of previous depositional environments, the palimpsest sediments correspond with relict sediments, which have been subsequently reworked. Intermediate phases are possible, starting from pure relict sediments, passing through palimpsest sediments, towards modern autochtonous sediments, not preserving the textural, sedimentological and petrographic characteristics of the previous depositional environments. The Cilento offshore (Southern Tyrrhenian Sea, Italy) represents a structural high, resulting from the seawards prolongation of the Licosa Cape structural high, bounded northwards and southwards by two half-graben, the Salerno Valley and the Policastro Gulf. The Salerno Valley is a half-graben basin, whose individuation has been controlled in the Early Pleistocene by the master fault Capri-Sorrento Peninsula, bounding southwards the margin of the Sorrento Peninsula with throws in the order of 1500 m, downthrowing the Meso-Cenozoic carbonate sequences through a step of normal faults. The structural high of Cilento has been the subject of geological and seismostratigraphic studies since the end of the 90s, when the interpretation of the AGIP multi-channel seismic showed extensive high structural zones, characterized by deaf acoustic facies with no acoustic base of internal reflections, alternating with sedimentary basins. Subsequently, the acquisition of deep multi-channel seismic along the Tyrrhenian edge, confirmed this structural set-up, highlighting how the upper Cilento structure is locally complicated by bending, reverse faults and phenomena of basin inversion, documented along the entire eastern Tyrrhenian margin.In the emerged sector of the Cilento Promontory the siliciclastic sequences ascribed to the Cilento Flysch crop out and have been involved by the deformation of the Apenninic chain during the Cenozoic and then by extensional tectonics and tectonic uplift starting from the Late Pliocene. These sequences represent the rocky acoustic basement of the Plio-Pleistocene and Holocene de posits of the continental shelf from the Licosa Cape to the Palinuro Cape. At the sea bottom the rocky acoustic basement crops out in the marine area surrounding the structural high of the Licosa Cape and is characterized by erosive morphologies. The study area represents a wide continental shelf, whose outer margin is located at water depths of 250 m. While the continental shelf northwards of the Licosa Cape has uniform gradient, the marine area surrounding the Licosa Cape represents an E-W trending structural high, characterized by remnants of terraced surfaces, carving the rocky acoustic basement and particularly abundant in the bathymetric interval ranging between 10 and 20 m, but occurring up to water depths of 60 m. An abrupt break in slope, located at water depths ranging between 60 and 80 m marks the passage from the structural high of the Licosa Cape, to the outer shelf, through a steep slope. Towards the Gulf of Policastro the continental shelf is characterized by an articulated morphology and high gradients and shows water depths ranging between 10 m and 110 m. The seismic processing of the Sub-bottom Chirp profiles has been carried out by using the software Seismic Unix, which has allowed for a general improvement of the quality of the seismic sections and producing as a final result, the seismic sections as bitmap images, which have been interpreted based on the criteria of seismic stratigraphy. The geological interpretation of the Sub-bottom profile BL14 is herein described in order to summarize the obtained results. The seismic line has been calibrated with the lithostratigraphic data of the Licosa core, which is a gravity core previously sampled in the Cilento offshore and known in literature data. The rocky acoustic basement (ssi unit) has been recognized, coupled with three main seismo-stratigraphic units, representing the stratigraphic bulk of the Cilento platform. The ssi unit is overlain by a progradational seismic unit interpreted as the beach deposits of the isotopic stages 4 and 5. It exhibits the remnants of old beach systems, genetically related with the isotopic stages 4 and 3. The core calibration and the seismo-stratigraphic interpretation have shown that this unit is overlain by the Sg unit, composed of coarse-grained organogenic sands, rich in Mollusk shells (Arctica islandica, cold host of the Pleistocene). The Sg unit is located at higher water depths, if we consider the sedimentary distribution model, which puts this kind of organogenic sands in the coastal belt. The Sg unit has been interpreted as relict sands and has been correlated with the organogenic sands described by Péres and Picard (1964) in their model of bioclastic sedimentation. This unit has the shape of a sandy ridge, occurring at water depths ranging between 130 and 140 m and interpreted as a part of the submerged beach, genetically related with the last lowstand phase (isotopic stage 2).