Principali tipi di falesia nella Penisola Sorrentina e nell’isola di Capri: caratteri e lineamenti morfoevolutivi

Rocky coasts in the Surrentine Peninsula and in the Isle of Capri (Italy) are subject to different rates of recession in terms of time and space. Two major morphologies are recognised: sloping shore platform (type-A1 and type-A2 platforms) and plunging cliffs. The recent evolutionary history, when the sea level has reached its present position after the Versillian transgression (6 ky b.p.), is not only connected to mechanical wave erosion that cuts back the cliffs, since sea cut notches are not well developed. The cliff recession phenomenon is mainly due to a range of processes such as wave erosion, physical and chemical weathering, bio-erosion, that are collectively responsible for mass movements. The occurrence probability of these movements increases usually during winter time. This may be explained by the greater intensity of the processes which affected the cliffs, such as the strong wave action at their base and the large amount of rain which penetrates the weathered rocks at the top or along the cliff. The morphotype of coast characterised by a type A1 shore platform frequently developed in the investigated area at the expenses of weak and/or vulnerable to weathering lithologies: highly fractured limestones, weathered tuffs, terrigenous successions, terrigenous deposits overlain by limestones. As a consequence, the shore platform is 200 m to 500 m wide and gently sloping (<10%); the base of the cliff is characterised by little depth or by narrow pocket beaches. The types of mass movement phenomenon giving rise to parallel retreat of cliffs are mainly rock falls, in limestone and flysch, toppling in tuff and wedge failure in limestone. In terrigenous deposits the cliff morphoevolution is due to shortening; this type of rocky coast is characterizes the tip of the Surrentine Peninsula, with lithology sometimes subjected to significant weakening of strength. The wave erosion as well as the physical-chemical weathering concurred to model these slopes; in particular, slopes of this kind are frequently affected by mass movements (e.g. rotational slide). Along the southern and eastern coasts of the Isle of Capri as well as in the carbonate headland bordering the Bay of Sorrento, type A2 cliffs are well developed, resulting from the evolution of a plunging cliff. Such cliff is formed by resistant lithology, even if it usually shows a complex network of fissures and fractures, due to processes occurred in the severe climatic conditions of the last glaciation. During this period, therefore, the state of fracturing could trigger the instability along the slope, and the consequent deposition of significant debris at its base. The rise of sea level that followed, caused the gradual erosion of such talus, the parallel retreat of the cliff, locally associated to slope decline, and the development of a narrow shore platform (wideness maximum 100 m and sloping: 10-30%). At Punta Campanella, near the tip of the Surrentine Peninsula, and in the western side of Capri the coast is characterised by plunging cliffs, likely controlled by faults. In these coasts the water depth at the base of the cliff is greater than the breaker depth; incident waves do not break, but are reflected from the cliff face resulting in standing waves. The low rate of recession of these cliffs is also determined by rocks that are very strong and highly resistant to weathering. However, where fractures belonging to different sets intersect, intense rock falls are often recognised. In few littoral stretches of the Surrentine Peninsula a specific type of rocky coast is observed, this is characterised by the presence of an emerged sea terrace, remodelled during the last sea level rise. The terrace was subject to recession, so at present the paleo-cliff is located landward. The edge of such terrace represents the face to breaker waves. In the submerged area a narrow and steep shore platform (type A2) is recognised.

Il sistema costiero nella Penisola Sorrentina e nell’isola di Capri è caratterizzato prevalentemente da falesie orlate da piattaforme costiere più o meno ampie (morfotipo A di Sunamura, 1992) o da falesie strutturali (plunging cliff). Nell’area in studio il morfotipo A è stato ulteriormente suddiviso in due sottoclassi, A1 e A2, il primo caratterizzato da una piattaforma ampia e poco inclinata mentre il secondo costituito da una piattaforma stretta e acclive. Le discontinuità strutturali ed i processi di degradazione meteorica predispongono le falesie a movimenti di massa; il modello evolutivo è per arretramento parallelo e la piattaforma costiera risultante è del tipo A1. Similmente per i sistemi costieri caratterizzati dalla presenza di depositi terrigeni, la falesia è soggetta ad arretramento per shortening. In corrispondenza di sistemi costieri caratterizzati da rocce poco erodibili e dislocati da faglie dirette il morfotipo caratterizzante è la plunging cliff; in questo caso l’azione erosiva delle onde sulla falesia è praticamente annullata mentre agiscono processi subaerei che conducono a processi di tipo rock falls. Più marcati processi erosivi di tipo subaereo si sono invece attivati nel corso dell’ultima glaciazione; le falesie abbandonate venivano sottoposte a severe condizioni di degradazione che determinavano l’accumulo al piede dei materiali degradati. La successiva risalita del livello del mare ha attivato una graduale erosione marina dei talus deposizionali, determinando un arretramento della falesia per processi di tipo parallel retreat, e localmente per processi di tipo slope decline, cui corrispondeva il modellamento di una piattaforma costiera poco ampia ed acclive del tipo A2.