Dinamiche gravitative, flussi canalizzati e forme di fondo in scarpata superiore (Margine Meridionale Sardo)

The definition of natural hazards in the deep sea (submarine landslides, fluid escape, submarine canyon bedforms migration, etc.) is realized by high-resolution mapping of hazard features. The geomorphological study of the upper slope in the Sardinian southern margin, described in this work, was carried through the analysis and interpretation of geophysical data acquired during three oceanographic cruises: Canale di Sardegna 2009, Canale di Sardegna 2010 and ARPAS2017. The study area is located in the southern Sardinian continental margin. Morphostructural characters that control the southern Campidano affect the structure of the continental shelf in front. The southern part of the Sardinian rift, with the superimposed Campidano Graben structure, continues within the sea in the Cagliari Gulf, both in the continental shelf and in the upper slope. In this area the morphology shows important tectonic features that follow the main regional tectonics. In particular the eastern shelf edge is oriented parallel to an important tectonic feature, resulting in a steep (> 40°) fault wall exposure. The continental shelf reaches a maximum width of about 2 km and is characterized by sub planar morphology with a slightly steep ground (about 3-4%). Along the eastern edge, an area characterized by the short distance between the shelf edge and the coastline (d ~1000 m) has been studied. Onshore, the Rio Geremeas is present, characterized by the presence of vast source areas in hyper fractured granites, and a steep river equilibrium profile (L=15 km, H=1000 m). Here, in case of extreme rainfall event (rainfall > 200 mm/h), an important phenomena of mud/debris-flow and debris flood can be generate. The sediment that reaches the sea, can get to the shelf edge in the form of hyperpycnal flow and trigger gravity flows to overload, putting, as a consequence, the coastal environment life at risk of abnormal waves back. The geomorphology of the slope is characterized by submarine canyons and several tributary channels, landslides are present inside of them. The significative mass movement deposits mapped along the continental slope and within the main canyon, whose head due to retrogressive trend approach the coastline, represent extremely interesting geo environmental emergencies for local and regional assessments of geo-hazard. The bedforms that characterize both the continental shelf and the upper slope have been analyzed, like submarine creep and pockmarks. Inside Foxi Canyon head, characterized by retrogressive erosion, particular bedforms have been detected, characterized by a wave length of dozen of meters and a height of several meters, with the ridge lines arranged approximately perpendicular to maximum slope. The comparison between the morphobatimetric data of 2009 and those of 2017 have shown that these sediment waves are migrating upslope in response to downslope-moving turbidity currents. Based on this morphological character, we interpret these bedforms as cyclic steps.