Heavy minerals as indicators of coastal erosion processes: the Apulian coast case study

The heavy volcanic minerals occurring in the sediments of the Ofanto river, mainly represented by pyroxene, amphibole and melanitic garnet, witness the important contribution of Monte Vulture on the sediment production. This sediment is transported by the Ofanto river to the Apulia coast and dispersed northward by the main longshore current, and southward by the littoral currents of the Adriatic Sea. The result is that even the beach sands of the Adriatic coast of Apulia, from the Gulf of Manfredonia to the Otranto promontory, are locally made up of high amounts of heavy minerals concentrated in dark layers and placers, easily distinguishable from the lighter carbonaticlastic fraction. The grain size of the sediments and the abundance of volcanic minerals are not constant along the coast. These variations could be attributed to differences in the coastal bathimetry and isobaths depths, but also to coastal erosion processes. In fact, during the last sixty years an important decrease of load of the Ofanto river has been recorded. This decrease is partly due to climatic variations but above all to the numerous dams built inside the drainage basins. As a consequence of the reduced supply of sediment, the Apulia coast shows a significant retreat, at places intensified as a consequence of anthropic works along the coast (harbours, piers etc). As the erosion mainly affects the light carbonaticlastic fraction, the stretches of coastline affected by the highest rates of erosion are also characterized by the highest amount of heavy volcanic minerals. Also the grain microtextures of the pyroxene vary significantly along the coast, as a result of chemical and mechanical processes during their transport. At the Ofanto mouth most of the pyroxenes are angular or with poorly rounded edges. Mechanical fractures, collision pits, arcuate steps, straight steps and V-shaped pits are common. Chemical weathering features, as small chemical pits and some solution hollows, are rare. Only few crystals show an intense chemical weathering process producing numerous lenticular (almond-shaped) depressions. Moving northward and southward from the mouth the features related to chemical weathering and the roundness of the pyroxene crystals increase, testifying the longer transport. However, the high roundness and weathering of some crystals also close to the mouth could be the effect of continuous reworking, thus witnessing the decrease in the river sediment supply. The results of the study demonstrated that there is a relationship between the presence, abundance and morphology of heavy minerals and the phenomenon of coastal erosion.