FACIES ACUSTICHE E STRUTTURE DI FONDO DELL’ULTIMO CICLO EUSTATICO NEI CANALI DELL’ARCIPELAGO DI LA MADDALENA (SARDEGNA SETTENTRIONALE)

Acoustic facies and morphologies of the last eustatic cycle in the La Maddalena Archipelago (North Sardinia, Italy) A high-resolution Chirp acoustic survey of 250 nautical miles performed in the La Maddalena Archipelago provided a scheme of the acoustic facies of the recentmost sediments and a frame of the large-scale bedforms linked to the last eustatic cycle that characterize the channels of the Archipelago. Five main acoustic facies have been recognized and mapped. Due to the good coherence with the sediment texture map of the area, a rough interpretation of the acoustic facies in sedimentary terms is also possible. The Archipelago is characterized by a complex system of inlets, promontories, bays and channels ruled by the main tectonic trends of the region. A main, 20 km-long channel (also known as Bucinara Channel) with numerous straits and some shoals, roughly oriented WNWESE,divides the northern Sardinia coast from the main islands of Spargi, La Maddalena, Santo Stefano and Caprera. A shorter and larger, W-E trending channel occurs in the northern part of the Archipelago. Two minor, roughly N-S trending sea branches connect these channels. The bathymetry of the area reveals that the channels generally have a flat bottom and steep flanks, their depth ranging from – 30 and – 60 m, with the exception of saddles and shoals. The sedimentary regime of the area is controlled by storm-induced currents and by the high carbonate production of the Posidonia Oceanica seagrass, a marine Phanerogama that populate the flanks of the channels of the Archipelago. A multitude of morphologic forms can be attributed to the phases of falling, low-standing, rising and high-standing of the last Pleisto-Holocene eustatic cycle. Angular unconformities,gullies and sediment-incised palaeo channels along with abrasion platforms in the granitic basement witness the Würmian sea-level fall and low-standing. These forms now lie at depths ranging from -33 m to -70 m and are covered by a thin veneer of sediments of the Holocene transgression. They have all been formed when the sea-level was about -110 m below the present datum and the Archipelago was a landscape of rocky hills separated by large, flat valleys scoured by torrent-like rivers. The Holocene rising of sea-level (Versilian transgression) caused the flooding of the landscape and the deposition of the thin sedimentary cover imaged by the acoustic profiles. The rising was punctuated by some sea-level still stands evidenced by terraced surfaces at depths comprised between -33 m and -47 m which are coherent with the mapped and dated beachrocks of the Bocche di Bonifacio strait. The slowdown of the rising between 6000 and 5000 years B.P. was responsible for the present sedimentary and hydrodynamic conditions of the area which is dominated by an eastward (Tyrrenian) trending sea current, and subordinately by a westward trending current. In the two main WNW-ESE and W-E trending channels of the Archipelago, the current flows formed some longitudinal sand ridges up to 3 km long and 5 m high, while in the N-S oriented channels, transversal sand banks up to 10 m thick occur. Sea-floor undulations, interpreted as sand waves, are present in some restricted areas close to the Sardinia coast in underflow position with respect to the headlands. Their wavelength (L) ranges from 10 to 40 m and the height(H) is up to 1,5 m, the ratio H/L being 1:20 to 1:30. Those of symmetric shape are due to wave motion during stormy seas. The asymmetric ones, instead, are likely to originate from bottom currents. Their steeper side faces to the East, according to the direction of the dominant current.