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THE SEDIMENTARY BEDFORMS IN THE CHANNELS OF LA MADDALENA ARCHIPELAGO (NW SARDINIA, ITALY): A PRODUCT OF THE HOLOCENE SEA LEVEL RISING
No full textThe use of a Chirp high-resolution acoustic source made it possible the differentiation of acoustic facies in the recentmost sediments of the channels of the La Maddalena Archipelago and the recognition of large scale bedforms linked to the last eustatic cycle.
The acoustic facies, distinguished on the basis of their reflectivity characters, show a substantial coherence with the distribution of the sedimentary facies described by previous Authors.
In the channels of the archipelago flat bottom sediments with medium to poor reflectivity correspond to sands, gravels and muds mainly of bioclastic composition deriving from the high CaCO3 productivity of the Posidonia Oceanica seagrass that populates the flanks of the channels down to about - 40 m.
On the contrary sediments on the flanks, which are mostly non-reflective, are mainly represented by silico-clastic sands resulting from the degradation of the granitic structural highs.
Sediments of the most proximal and protected bays are characterized by higher reflectivity and correspond to finer deposits such as silts and sandy silts.
The Chirp survey highlighted the occurrence of large scale, low-H/L-ratio bedforms not evidenced on SSS survey because of their too low relief.
In the channels roughly oriented W-E sedimentary bodies up to 3 km long and some meter thick occur in axial position.
Two larger and thicker transversal bodies are present in two N-S trending channels, where they act as bathymetric thresholds between the adjacent islands.
All these acoustically opaque bedforms, classified as sand ridges or sand banks are younger than the underlying sediments.
They are mainly composed of bioclastic sands and gravels and represent the product of the present hydrodynamic regime.
Seafloor undulations of sand wave type occur in the main channel along the Gallura coast down-flow to the headlands with respect to the dominant, eastward directed current.
Symmetric forms may be attributed to stormy wave motion, while the asymmetric ones to the seabed current.
Sand ridges and sand waves are significant sedimentary bodies in the archipelago depositional system as they witness the importance of the currents in the hydrodynamic regime.
These features are overprinted by smaller sedimentary features such as comet marks, small scale sand waves, sand ribbons and ripple mark fields, which indicate the axes and directions of the sedimentary transport.
The large-scale sedimentary bedforms of the channels of La Maddalena archipelago are the result of the hydrodynamic regime set up after the slackening of the sea-level rise, mainly between 6 -5 ky B.P.
This event brought to the stabilization of the sea level and the formation and maintenance of the bedforms
Records acustici nell’area patagonico-fuegina: prospettive per un’indagine paleoclimatico-ambientale
No full textFACIES ACUSTICHE E STRUTTURE DI FONDO DELL’ULTIMO CICLO EUSTATICO NEI CANALI DELL’ARCIPELAGO DI LA MADDALENA (SARDEGNA SETTENTRIONALE)
No full textAcoustic 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
Acoustic facies and seabed features of the mixed carbonate-siliciclastic deposits of the last eustatic cycle in the La Maddalena Archipelago (North Sardinia, Italy)
No full textA 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 charac - terize 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/
Seismostratigraphy and sedimentology of the South Chile margin: State of the art and future prospects
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