Sedimentological charecteristics and littoral dynamics of the Is Arenas beach (Narbolia) on the west coast of Sardinia

The study area is occupied almost entirely by the Is Arenas dune system, which extends for more than 7 km2, penetrating for over three kilometres inland in a SE direction. Anthropic pressure on this part of the coast is confined to the northern portion as the beach is relatively inaccessible from the other parts. Given its geo-structural characteristics the area can be described as a small “tectonic low” emplaced on the lower-middle Miocene marine succession, situated between the Montiferru volcanic complex to the East and the “horst” structure of the Tortonian-Messinian shelf of the Sinis to the West (CHERCHI et al., 1978). Furthermore, interpretation of continuous seismic reflection profiles collected on the continental shelf in front of the area, (LECCA et al., 1982), together with the stratigraphic data obtained from a number of cores drilled in the dune field, rule out any continuity southwards with the “Campidano Graben". This structural situation drove the evolution of the sedimentary processes, probably as early as the Late Pliocene. These processes have shaped the actual stratigraphic setting of the Quaternary cover. The stratigraphy of the cores indicate the Is Arenas Quaternary cover to be composed of a succession of sandstone-conglomeratic sandstone from 20 to 50 m thick, of alternating coastal and continental marine sedimentary episodes that formed in a depositional environment governed by the eustatic variations in sea level that took place during the Middle-Late Pleistocene and during the Holocene (CARBONI & PALA, 1998). Following a depositional phase started at the end of the Middle Pleistocene-beginning of the Late Pleistocene, the variations recognized can be attributed to the brief episodes of eustatic sea level changes that occurred during the interglacial stage 5 (SHACKLETON, 1987) (sub-stages 5e, 5c and 5a, “Tyrrhenian” s.l.), and/or to paleomorphological changes in a coastal environment. The transition to sediments less marine and more lagoonal in nature and lastly the onset of the continental depositional environment are evidence of the progression of the Post-Tyrrhenian regressive trend. The uppermost part of the succession contains the sediments deposited during the eustatic sea level rise of the Versilian transgression. These consist of a series of recent and less recent coastal dunes, of variable texture and structure, containing thin organic carbon-rich strata and layers of peat. Their depositional episodicity bears witness to the different climate phases that characterized the Holocene age. The sands are composed chiefly of volcanic lithic particles, with subordinate amounts of quartzose sands, containing few bioclastic components. During the final phase of the sea water rise in the Early Holocene a sand body built up along the coast, also favoured by the morpho-structural features of the basement. This body was well supplied by the major sediment input in the northern part of the continental platform from the river Temo (Bosa) and from the channels draining the Montiferru volcanic district already during the Middle-Late Pleistocene. The Is Arenas beach stretches for roughly 6 km between the promontories of Torre di Scau ‘e Sai to the SW and Torre Su Puttu to the NE, occasionally incised and supplied in the northern portion by two intermittent streams. The transverse profile of the beach is similar to many others on the West coast of Sardinia with a wide surf zone comprising bar and trough system, shorface, backshore and a well developed dune system The morphology of the dune system, as the anemometric data for the period 1962-1990 (National Meteorology and Aeronautical Climatology Centre) show, has been moulded by the prevailing north-westerly winds. The elongated parabolic dunes extend perpendicular to the coast and are interspersed with depressions. They are of irregular height, no more than 25-30 m in the westernmost part, where the most recent dunes overlie relict dune formations. In the southernmost part of the area, where the sandy cover, stabilized by natural vegetation, presumably overlies older morpho-structural units of different geological and lithologic nature, the dunes attain heights of 40-45m. The coastal dune system in the immediate backshore consists of foredunes, stabilized by fencing and a forest plantation established in the 1950’s covering about 750 hectares of the dune system, which prevent incipient blowouts from extending inland. Backshore width averages 30-35 m and shoreline features include widely spaced cusps in the central-southern part. The bar and trough zone extends for no more than 470 m. In the autumn three lines of submerged bars have been observed.. Annual mean energy transferred to the shore by wave motion (R.A.S.,1987, ATZENI & GINESU, 1993), has been calculated at 13.4 x 106 tm/m, the highest for Sardinia’s coasts. A NE energy flow direction is indicated for the sector comprising this unit.