A HOLOCENIC HIGHSTAND CARBONATE-TERRIGENOUS CONTINENTAL SHELF IN THE GULF OF CAGLIARI, SARDINIA

By means of 11 new echographic lines and 100 surface sediment samples it has been possible to individualize on the continental shelf of Cagliari Gulf several depositional facies associations between foreshore and shelf edge. These associations document a suite of peculiar environments that diversified and stabilized during the Holocene eustatic rise. From shore to shelfbreak we first encounter a shoreface, gently sloped and distally sea-grass covered which ends in a trough parallel to the coast. Toward the open shelf is a relief, topping at -12-15 meters, site of the Posidonia oceanica main bank, followed by a wide foreslope. At a depth of 30 m the vegetation becomes rare, and starting from -35-40 m a large belt of sand waves appears. At the depth of 50 m a flat bottom prevails, followed by submerged shoreline relieves at -55 m. Externally to such structures a narrow outer shelf occurs and the shelf edge is observed at a depth of 115-125 m. The inner shelf trough can be interpreted as a paleo-river valley during the MIS 4-2 and previous lowstands, combined with terrigenous inputs insufficient to fill the accommodation space produced by the Holocene sea-level rise. Towards the middle shelf the trough is bounded by paleo-reliefs where the Posidonia oceanica bank at present overgrows, weakly aggrading and prograding landward and seaward. Compositional and grain-size analysis of samples allowed us to identify eight depositional facies related to present day environments. Shoreface sediments are represented by terrigenous coarse to medium sands in the eastern sector of the gulf, and mixed terrigenous-bioclastic medium to fine sands on the western sector. Two distinct groups of lithic facies are present near Cape S.Elia and near Zavorra Point, due to sediment dispersion from the limestone and andesite outcrops along the cliffed coast. The western inner-shelf trough is characterized by mixed quartzose-bioclastic sandy muds with bivalves and peloidal grains. Mixed facies of biogenic production and relict terrigenous sediments are present on the Posidonia bank, while through the sand wave belt a transition is observed to bioclastic, poorly sorted gravely sands with red algae and foraminifers. Rare breakthroughs of mixed sediments are present in proximity of the -55 m paleo-shores. Starting from depths of 75 m, sediments are represented by fine bioclastic-quartzose sands with bivalves and foraminifers. The depositional facies described document this sedimentation as of a mid-latitude mixed carbonate-terrigenous shelf. The present day sedimentation is highly siliciclastic in the inner shelf from fluvial-continental feeding, and bioclastic in the rest of the shelf with a maximum productivity along the middle-shelf Posidonia bank and its foreslope. In the middle and outer shelf the siliciclastic component is related to previous eustatic phases. The sequential analysis of 3.5 kHz lines show the Holocenic trasgressive systems tract blanking over a maximum flooding surface, or over a trasgressive ravinement surface or lowstand erosional truncations. The Holocenic TST, except for the siliciclastic shoreface, is mostly carbonatic, owing to the highstand-related terrigenous starvation and to a good bioclastic productivity of molluscs, red algae, briozoans, foraminifers and echinoids. Analogous depositional contexts should be attributed to the preceding highstand phases of the Middle-Upper Pleistocene. Otherwise, during glacio-eustatic phases of falling, lowstand and rising sea-level, the shelf has been mostly interested by terrigenous continental or shoreface deposits that are currently resedimented in the middle and outer shelf. In summary, the depositional model of this shelf is characterised by alternated phases of carbonate-terrigenous mid-latitude sedimentation with poorly rainy, warm Mediterranean climate during highstand conditions, and terrigenous phases during lowstands with rainy, cold Mediterranean climate.