This work presents new results of palaeoenvironmental interests from the area of La Maddalena archipelago, the southern part of the Strait Bonifacio between the islands of Sardinia and Corsica. The data have been acquired in the frame of a cooperation program in marine geology researches between the Universities of Cagliari and Trieste. The data consist of about 250 nautical miles of echographic profiles acquired with Chirp technology in water depths not greater than – 60 m using two motorvessels provided with a differential GPS positioning system. The profiles have been correlated with 150 bottom samples collected by means of grab samplings and scuba divings. The sedimentological, paleontological and isotope geochemical content of the analyzed samples provided an interpretative link with the palaeoclimatic and palaeoenvironmental events of the late Pleistocene to Holocene. The very high Chirp resolution signal, despite a penetration limited to about ten metres in the sandy sediments of the area, allows the recognition of the sedimentary deposits and their related morphologies linked to the Versilian transgression. These deposits lie over a non-reflective bed-rock characterized by an irregular topography corresponding to the Sardinia pre-Carboniferous granitic basement. Dikes and lodes, typical of the cristalline basements,are arranged along N-S and W-E, with subordinate NE-SW and NW-SE trends that control the morphostructural and hydrographic pattern of the entire region. During the Würmian sea level low stand the hydrographic network was overdeepened into the cristalline basement. The successive Holocenic sea level rise drowned the palaeocoastlines and caused the sedimentary infilling of the previous hydrographic network with the deposition of the sedimentary bodies illustrated in this work. Five main acoustic facies have been distinguished on the echographic profiles. Their correlation with the analysed samples provided the following interpretation: 1 - Bed-rock facies due to the outcropping of the granite and its dikes and/or to quaternary and pre-quaternary consolidated sediments. 2 - Transparent or semi-transparent facies over the bed-rock. This facies occurs near the coastlines and represent medium to coarse sands deposited over the bed-rock during the late Holocene sea level high stand, i.e.the late 5000 years B.P. 3 - Semitransparent facies with sparse internal irregular reflectors. This facies is found along the northern coast of Sardinia in front of and near the present mouths of the so-called "rias", rivers with torrential regime incised in the granitic basement. This facies would represent palaeo-fluvial and palaeo-deltaic environments developed during the late-Holocene high standing conditions. Thicknesses up to 20 m are observed on some profile near the coast. 4 – Semitransparent to opaque facies over stratified horizontal facies. These are found in water depths generally deeper than -30 m and correspond to very coarse sands deposited over stratified coarse to medium sands. Within the channels between the Sardinian coast and the islands of La Maddalena and Spargi, bottom currents of the Strait of Bonifacio sometimes shape the upper facies into acoustically opaque, upward convex lenses that at least on bottom surface correspond to very coarse biogenic sands and gravels. The contact with the underlying stratified facies is sometimes of erosional type. There is evidence of weak angular unconformities and channels that have been incised for a couple of metres into the stratified facies and infilled by the overlying semitransparent-to-opaque facies. We interpret these two superposed facies respectively as the transgressive and the high standing depositions of the Versilian transgressive event. 5 – Very well stratified facies, occuring only in the area of Porto Liscia, Porto Puddu and the inner part of the gulf of Arzachena, represents a fine to very fine sediment deposited in low-to-very low energy environments like restricted or protected bays.

ACOUSTIC FACIES AND MORPHOLOGIES OF THE HOLOCENE DEPOSISTS WITHIN THE LA MADDALENA ARCHIPELAGO (BOCCHE DI BONIFACIO – NW SARDINIA, ITALY)

DEMURO, SANDRO;
2003

Abstract

This work presents new results of palaeoenvironmental interests from the area of La Maddalena archipelago, the southern part of the Strait Bonifacio between the islands of Sardinia and Corsica. The data have been acquired in the frame of a cooperation program in marine geology researches between the Universities of Cagliari and Trieste. The data consist of about 250 nautical miles of echographic profiles acquired with Chirp technology in water depths not greater than – 60 m using two motorvessels provided with a differential GPS positioning system. The profiles have been correlated with 150 bottom samples collected by means of grab samplings and scuba divings. The sedimentological, paleontological and isotope geochemical content of the analyzed samples provided an interpretative link with the palaeoclimatic and palaeoenvironmental events of the late Pleistocene to Holocene. The very high Chirp resolution signal, despite a penetration limited to about ten metres in the sandy sediments of the area, allows the recognition of the sedimentary deposits and their related morphologies linked to the Versilian transgression. These deposits lie over a non-reflective bed-rock characterized by an irregular topography corresponding to the Sardinia pre-Carboniferous granitic basement. Dikes and lodes, typical of the cristalline basements,are arranged along N-S and W-E, with subordinate NE-SW and NW-SE trends that control the morphostructural and hydrographic pattern of the entire region. During the Würmian sea level low stand the hydrographic network was overdeepened into the cristalline basement. The successive Holocenic sea level rise drowned the palaeocoastlines and caused the sedimentary infilling of the previous hydrographic network with the deposition of the sedimentary bodies illustrated in this work. Five main acoustic facies have been distinguished on the echographic profiles. Their correlation with the analysed samples provided the following interpretation: 1 - Bed-rock facies due to the outcropping of the granite and its dikes and/or to quaternary and pre-quaternary consolidated sediments. 2 - Transparent or semi-transparent facies over the bed-rock. This facies occurs near the coastlines and represent medium to coarse sands deposited over the bed-rock during the late Holocene sea level high stand, i.e.the late 5000 years B.P. 3 - Semitransparent facies with sparse internal irregular reflectors. This facies is found along the northern coast of Sardinia in front of and near the present mouths of the so-called "rias", rivers with torrential regime incised in the granitic basement. This facies would represent palaeo-fluvial and palaeo-deltaic environments developed during the late-Holocene high standing conditions. Thicknesses up to 20 m are observed on some profile near the coast. 4 – Semitransparent to opaque facies over stratified horizontal facies. These are found in water depths generally deeper than -30 m and correspond to very coarse sands deposited over stratified coarse to medium sands. Within the channels between the Sardinian coast and the islands of La Maddalena and Spargi, bottom currents of the Strait of Bonifacio sometimes shape the upper facies into acoustically opaque, upward convex lenses that at least on bottom surface correspond to very coarse biogenic sands and gravels. The contact with the underlying stratified facies is sometimes of erosional type. There is evidence of weak angular unconformities and channels that have been incised for a couple of metres into the stratified facies and infilled by the overlying semitransparent-to-opaque facies. We interpret these two superposed facies respectively as the transgressive and the high standing depositions of the Versilian transgressive event. 5 – Very well stratified facies, occuring only in the area of Porto Liscia, Porto Puddu and the inner part of the gulf of Arzachena, represents a fine to very fine sediment deposited in low-to-very low energy environments like restricted or protected bays.
palaeoenvironmental; La Maddalena archipelago; facies; Holocene Deposists
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11584/10236
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