The 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.
THE SEDIMENTARY BEDFORMS IN THE CHANNELS OF LA MADDALENA ARCHIPELAGO (NW SARDINIA, ITALY): A PRODUCT OF THE HOLOCENE SEA LEVEL RISING
DEMURO, SANDRO
2011-01-01
Abstract
The 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.
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