Genesis and geomorphological evolution of deep-seated gravitational slope deformations: an example of dating in Western Sicily

Abstract

The study area lies in the westernmost part of the Sicani Mountain range where a superposition of SE-trending, stacked, geological bodies crops out. this structure is the result of transpressive tectonic phases of Miocene and Lower Pliocene age. The neo-autochthon outcrops are made up of clay-marly deposits of the Middle-Upper Pliocene, and of calcarenites of Pleistocene age. The extensional Quaternary tectonic phases later dissected and dismembered the original structure, thus involving the calcarenites that show clear, mainly NE-SW oriented dislocations. Such calcarenites, which have a total thickness of about 80 metres, overlap the Belice marly-arenaceous formation. This determined favourable conditions for Deep-seated Gravitational Slope Deformations (DSGD), which affected the western plateau of St. Margherita Belice causing deep landslides. The trigger agent of these landslides has been correlated with the seismic activity that has affected this area since ancient times. The Deep-seated Gravitational Slope Deformations considered here have peculiar geomorphologic characteristics such as disjointed blocks, scarps, landslides and areas with double ridges. On top of the tilted bodies, wide trenches are frequently observed, whereas the base of the collapsed masses sometimes shows no real area of accumulation. As a result of the partial rotation of the masses involved in the collapse, the calcarenitic blocks show counter-tilting, and in some cases are covered by «talus»-type deposits of uneven thickness. These deformations can currently be considered inactive. The discovery of an archaeological site of late-Roman and Byzantine age (VII-VIII centuries AD) has proved that the landsliding activity has been inactive since before the area was inhabited by humans. However, the dislocation of rocky tombs belonging to the middle Bronze Thapsos culture (XV-XIII century BC) suggests that these landslides occurred after the necropolis was built. Data from an archaeological study of the earthquakes involving the city of Selinunte, located just south of the study area, has allowed us to date the onset of the above-mentioned instability.