A high-resolution acoustic survey of 250 nautical miles has been performed in the La Maddalena Archipelago with the aim of investigating the connections between the morpho-sedimentary characters and the sea-level changes of the last eustatic cycle. A complex system of inlets, promontories, bays and channels ruled by the main tectonic trends of the region, characterizes the Archipelago in which the effects of the Pleisto-Holocene regressions and transgressions have left evidence both in the emerged and submerged morphology. A main, 20 km-long channel with numerous straits and some shoals, roughly oriented WNW- ESE, divides the northern Sardinia coast from the main islands of Spargi, La Maddalena, S. 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 the saddles and the shoals. 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. The slowdown of the rising at 5000-6000 years B.P. was responsible for the present sedimentary and hydrodynamic condions of the area which is dominated by an eastward (Tyrrenian) trending sea 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 10 m-thick sand banks occur. Sea-floor undulations, interpreted as sediment 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. Their shape is symmetric or slightly asymmetric according to the direction of the current (steeper side to the East).
MORPHOLOGY AND SEDIMENTARY FORMS OF THE LAST EUSTATIC CYCLE IN THE LA MADDALENA ARCHIPELAGO (NORTH SARDINIA, ITALY) FROM HIGH-RESOLUTION ACOUSTIC DATA
DEMURO, SANDRO
2009-01-01
Abstract
A high-resolution acoustic survey of 250 nautical miles has been performed in the La Maddalena Archipelago with the aim of investigating the connections between the morpho-sedimentary characters and the sea-level changes of the last eustatic cycle. A complex system of inlets, promontories, bays and channels ruled by the main tectonic trends of the region, characterizes the Archipelago in which the effects of the Pleisto-Holocene regressions and transgressions have left evidence both in the emerged and submerged morphology. A main, 20 km-long channel with numerous straits and some shoals, roughly oriented WNW- ESE, divides the northern Sardinia coast from the main islands of Spargi, La Maddalena, S. 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 the saddles and the shoals. 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. The slowdown of the rising at 5000-6000 years B.P. was responsible for the present sedimentary and hydrodynamic condions of the area which is dominated by an eastward (Tyrrenian) trending sea 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 10 m-thick sand banks occur. Sea-floor undulations, interpreted as sediment 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. Their shape is symmetric or slightly asymmetric according to the direction of the current (steeper side to the East).
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