Caratteristiche geoambientali delle dune costiere del golfo di Pistis (Sardegna sud-occidentale)

Abstract

The western coast of Sardinia on the South of the Gulf of Oristano, is characterized by the presence of dune ridges, which extend inland about 1 km from the beach of Pistis (see Fig. 1). These dunes cover Palaeozoic metamorphic rocks, ancient terraced alluvial deposits, and residual outliers of late glacial (Wormian) cross-bedded eolian sandstones and slope debris. The geomorphologi-cal, sedimentological and geochemical features of the dunes are identified on the basis of detailed field surveys using aerial photographs taken at various times and the analysis of sand samples from profiles normal to the beach. Morphologically, the dunes have shapes like parabolic tongues, which are accompanied by sand accumulations around obstacles, eolian ripple fields and pebble pavements. Under the microscope, sand is formed of prevailing quartz grains with subordinate grains of K-feldspar and mafic minerals. Bioclasts and lithic fragments are also present. Grain size distribution curves show a better sorting in dune sand than in berm sand, where a wind-selected coarse fraction (pavement or lag) is present. Morphoscopic analysis indicate that dune deposits depend on beach sources with Ba and Sr local anomalies linked to either the source or littoral and eolian sedimentary processes. The increased number of tourists in latest years and the lack of protected areas have caused the retreat of the already discontinuous vegtation, favouring the widening of openings in the dune tongues and the formation of scours and holes.