Acoustic facies and seabed features of the mixed carbonate-siliciclastic deposits 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 its channels. Five main acoustic facies have been recognized and mapped. Due to the good coherence with the sediment texture map of the area, an interpretation of the acoustic facies in sedimentary terms is presented. 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 (Bucinara Channel) with numerous straits and some shoals divides the northern Sardinia coast from the main islands of Spargi, La Maddalena, Santo Stefano and Caprera. A shorter and larger channel occurs in the northern part of the Archipelago. Two minor sea branches connect these channels. The bathymetry of the area reveals that the channels generally have a flat bottom and steep flanks, with water depth mostly ranging between -35 and -60 m, with the exception of saddles and shoals. The sedimentary regime of the area is controlled by storm-induced currents and by a high carbonate production associated to the widespread presence of the Posidonia Oceanica seagrass, a marine Phanerogama that populates the flanks of the channels of the Archipelago. A multitude of morphologic forms can be attributed to the phases of falling, lowstanding, rising and high-standing of the last Pleistocene-Holocene eustatic cycle. Angular unconformities, gullies and sediment-incised palaeo channels along with abrasion platforms in the granitic basement witness the last sea level fall and low-stand. These forms now lie at water depths ranging between -33 m and -60 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 rise 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 lying at depths between -33 m and -47 m which are coherent with the mapped and dated beach-rocks of the Bocche di Bonifacio strait. The decrease of sea level rise at 5000-6000 years B.P. and the following stabilization of sea level have triggered the present sedimentary and hydrodynamic conditions which, in turn, favoured the formation and maintenance of the bedforms. The Archipelago is dominated by an eastward directed sea current, and subordinately by a westward current. In the two main WNW-ESE and W-E trending channels of the Archipelago, the current flows have 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 downflow position with respect to the headlands. Their wavelength (L) ranges between 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.