Sedimentologia e processi deposizionali della piattaforma continentale di Cagliari dal Last Glacial Maximum all'attuale

An acoustic and sedimentological survey was performed on the shelf of Cagliari, Southern Sardinia, to describe morphology, modern sediments and depositional processes of the shelf from the last post glacial sea level rise. The Gulf of Cagliari is a sub-tropical, semi-arid Mediterranean area located in the southern part of the NW striking Oligo-Miocene Sardinian Rift, over which the Pliocenic Campidano Graben is superimposed. The Quaternary continental shelf developed transversally to this tectonic trough, fed by terrigenous sediments derived from the Palaeozoic metamorphic basement and from Tertiary sedimentary and volcanic rocks. The paleo-morphology influenced the depositional processes and the modern configuration of depositional environments. The shelf is at present characterized by a structural relief in the intermediate shelf, overgrown by a bioclastic bank that shelters the inner sector and can be considered similar in function to a tropical carbonate barrier. The echographic survey, consisting of 20 lines performed transversally to the coast, integrated previous acoustic and seismic data (lines and mosaic SSS, lines SBP and Sparker) and allowed, together with sedimentary data, to identify several morphological/sedimentological features. From the shore to the shelfbreak a suite of environments was detected, each characterized by distinct sedimentation and processes. The shoreface, weakly sloped and distally seagrass-covered, that ends in a wide depression (10x2 Km) roughly parallel to the coast. A relief in the middle shelf, overgrown by the Posidonia oceanica main bank, reaching a minimum depth of 13 meters under the sea level. A middle shelf slope (depth >30 m) where a transition is observed to bare bottoms, (light limit for the growth of Posidonia). A large sand wave belt, from depths of 35÷40 meters, with gradually decreasing heigths and increasing frequencies seaward. Flat bottoms at the depth of 55 m, for the reaching of the Storm Wave Base (SWB) and vanishing of the structures. Several reliefs, stretching roughly NE–SW at 55 m depth, interpreted as submerged paleo-shorelines. Finally, a narrow outer-shelf, externally to these structures, and the shelfbrake at a depth of 115÷125 m. Sampling sites were positioned to the aid of the echographic dataset and 201 samples were taken with a 5 dm³ grab-sampler. Compositional and grain size analysis allowed to identify 9 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 western shoreface (see figure). Two groups of lithic facies are present near distinct outcrops of limestone and andesite along the cliffed coast. The inner-shelf below the fair-weather wave base is characterized by mixed quartzose-bioclastic sandy muds with bivalves while the depression is filled by a 40m thick Holocenic sequence of organic-carbonate mud and fine sands (grey on the map). Mixed facies of relict shoreface terrigenous sediments along with a present-day biogenic component (mollusks, bryozoans, red algae, echinoids and foraminifers) are found in the Posidonia oceanica bank (-15÷35 m). Seaward, a coarser, different siliciclastic component is found between -35 and -50 meters, related to depositional processes precedent to the Last Glacial Maximum (LGM). Through the sand-wave belt a transition is observed to more bioclastic, poorly sorted gravely sands of red algae and benthic foraminifera, below the SWB. The sediment is completely bioclastic but rare breakthroughs of terrigenous material in proximity to the –55 m paleo-shores show the presence of relict sediments underneath a limited surface cover. From a depth of 75 meters fine bioclastic-quartzose sands with bivalves and foraminifers are present due to and open marine reduced sedimentation during the last phases of the post-LGM sea level rise. The interpretation of these morphological and sedimentological context was based on the present-day knowledge of Pleistocene sea-level fluctuations. High-amplitude variations controlled by 20Ky precession cycles are known to have taken place in the Glacial Pleistocene (800Ky), while smaller amplitude 40Ky obliquity cycles variations were dominant during the Lower and Middle Pleistocene. During this early time the actual inner shelf was probably mostly emerged and continental processes prevailed while the sea level oscillated around a mean value of -60m ca. (external paleo-shores). Only during the Middle-Late Pleistocene were the oscillations wide enough to submerge the inner shelf during highstands. An important feature attributable to these continental processes is the inner shelf depression, that can be interpreted as the product of paleo-river erosions, lastly reactivated during the “Würmian” lowstand (MIS 4-2). The terrigenous input was then insufficient to fill up the available space during the post-glacial sea-level rise. The middle-shelf paleo-relief (under the Posidonia bank) is interpreted as the result of the highstands most often reached during the obliquity dominant transgressive cycles of the Lower Pleistocene. The climatic-geomorphologic context determines the present day sedimentation, that is 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 Posidonia bank and its foreslope. A second important productivity is present at greater depths (50-65m) with the rhodalgal biocenosys. In the middle and outer shelf the relict siliciclastic component is related to previous eustatic phases and attributable to continental and nearshore deposits lying underneath the actual shelf’s buildup. Analogous depositional contexts must be attributed to the precedent highstand phases of the Middle-Upper Pleistocene. Otherwise, during glacio-eustatic phases of falling, lowstand and rising sea level, the shelf was characterized by terrigenous continental sedimentation or shoreface sequences that are currently resedimented in the middle and outer shelf. Finally, the depositional model of this shelf alternates between phases of terrigenous-carbonate sedimentation of middle latitude, with a poorly rainy, warm Mediterranean climate during highstands, and terrigenous phases during lowstands with rather rainy, cooler Mediterranean climate.