I SEDIMENTI TERRIGENO-CARBONATICI ATTUALI DELLA PIATTAFORMA CONTINENTALE DEL GOLFO DI CAGLIARI

Abstract

Lecca L., De Muro S., Cossellu M. & Pau M., Modern terrigenous-carbonate sediments of the continental shelf of the Gulf
of Cagliari. (IT ISSN 0394-3356, 2005).
Echographic and sedimentological investigations were undertaken on the shelf of Cagliari with the purpose of underlining the relationships between modern terrigenous and bioclastic sediments and the behaviour of the sedimentary system during the Holocene eustatic rising. The Gulf of Cagliari is a sub-tropical, semi-arid Mediterranean area located in the southern part of the NW striking OligoMiocene Sardinian Rift, within which the Pliocene Campidano Graben is superimposed. The Quaternary continental shelf developed transversally to this tectonic trough, being fed by terrigenous sediments derived from the Palaeozoic metamorphic basement and from Tertiary sedimentary and volcanic rocks. The new echographic survey, consisting of 11 lines performed transversally from the coast to the shelfbreak, integrated previously available data and allowed us to divide the shelf into different sedimentary environments. From shore to shelfbreak we first encounter a shoreface, gently sloped and distally seagrass-covered (close to the fair-weather wave base), which ends in a wide trough parallel to the coast. Toward the open shelf is a relief, site of the Posidonia oceanica main bank, reaching a minimum depth of 13 meters under the sea level, followed by its external slope. At a depth of about 30 meters the vegetation becomes rare, as less luminosity limits the growth of Posidonia, and starting from depths of 35÷40 meters a large belt of sand waves appears, with gradually decreasing heigths and increasing frequencies basinward. At the depth of 50 m a flat bottom prevails, followed by several reliefs stretching roughly NE–SW at 55 m depth, interpreted as submerged shore lines. Externally to such structures a narrow outer shelf occurs and the shelf break is observed at a depth of 115÷125 m. Sampling sites were located along the echographic lines and on the foreshore; 98 samples were taken with a 2 dm_ grab-sampler. Compositional and grain size analysis allowed us to identify 8 groups of facies related to present day depositional environments. Shoreface sediments are represented by almost completely terrigenous coarse to medium sands in the eastern sector, and finer mixed terrigenous-bioclastic sands on the S. Gilla shoreface.
Two distinct groups of lithic facies are present near Capo S. Elia and near P.ta Zavorra, due to sediment dispersal from the limestone
and andesite outcrops along the cliffed coast. The western inner-shelf depression is characterized by mixed quartzose-bioclastic sandy
muds with bivalves. Mixed facies of relict terrigenous sediments and present day biogenic production (mollusks, bryozoans, red algae,
echinoids and foraminifers) are on the Posidonia oceanica bank’s fore slope, where the siliciclastic component is found between -30
and -50 meters with decreasing importance seaward. Through the sand-wave belt a transition is observed to bioclastic, poorly sorted
gravely sands of red algae and foraminifers. Here the sediment is completely bioclastic with rare breakthroughs of mixed sediments in
proximity to the –55 m paleo-shores. From a depth of 75 meters, sediments are represented by fine bioclastic-quartzose sands with
bivalves and foraminifers. A peculiar feature of this shelf is the depression in the inner shelf that can be interpreted as the product of paleo-river erosions produced during the “Würmian” low stand (MIS 4-2), associated with terrigenous inputs insufficient to fill up the
available space during the post-glacial sea level rise. Towards the middle shelf the trough is bounded by paleo-reliefs where the
Posidonia bank at present overgrows, weakly aggrading and prograding. The depositional facies described document the present day
sedimentary system as of a temperate water mixed terrigenous-carbonate shelf. The 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 intermediate shelf’s
Posidonia bank and its foreslope. In the middle and outer shelf the siliciclastic component is related to previous eustatic phases and
attributable to continental and nearshore deposits lying underneath the middle-shelf Posidonia bank. Analogous depositional contexts
must be attributed to the preceding high stand phases of the Middle-Upper Pleistocene. Otherwise, during glacio-eustatic phases of
falling, low-stand and rising sea level, the shelf has been affected by terrigenous continental sedimentation or shoreface sequences
that are currently resedimented in the middle and outer shelf. In summary, the depositional model of this shelf is characterised by alternating phases of terrigenous-carbonate sedimentation of middle latitude, with a poorly rainy, warm Mediterranean climate during highstanding sea-level conditions, and terrigenous phases during low stands with rather rainy, cooler Mediterranean climate.