Provenance and dispersion of heavy minerals in the beach sand of the Apulia coast

A peculiar aspect of Apulia beaches sand is the occurrence of important amounts of heavy silicate minerals (mainly pyroxenes, garnets and amphiboles), forming thin, dark layers and placers interbedded with the dominant light carbonaticlastic fraction. A granulometric, compositional and microanalytical study was carried out on the sand of the beaches stretched along the Gulf of Manfredonia to the Otranto promontory, in order to detail provenance and dispersion of the heavy silicate sandy fraction. The rocks outcropping along the Apulia coast are mainly represented by the limestones of the Mesozoic platform of the foreland domain and by the Plio- Pleistocenic sedimentary covers. The origin of the silicate minerals cannot therefore be found in the rocks outcropping in the area, but must be related to a more distant source. The Ofanto river, whose mouth is in the Gulf of Manfredonia, is the main stream feeding the Apulia coast with sediments eroded by the southern Apennines units and the Mt. Vulture volcano. The microanalitical study revealed that the composition of the pyroxene, melanite garnets and Mg-hastingsite amphiboles is identical to that of the same minerals found in the sediments along the Ofanto river and occurring inside the volcanic bedrock of Mt. Vulture. A minor population of colorless grains of garnets can be probably related to the recycling processes from the Oligo-Miocene sandstones of Southern Apennine terrains. Heavy detrital minerals shed by Vulture edifice source rocks have been found in the sandy fraction dispersed along both northward and southward of the Ofanto mouth. This is in accordance with main longshore current depositing them in the Gulf of Manfredonia. Part of the sands, however, is transported southward by the littoral currents of the Adriatic Sea. The sandy heavy minerals fraction in the is not constant along the coastline. The highest contents are in the northern sector, from the Gulf of Manfredonia to Trani, while they almost disappear in the central sector between Molfetta (BA) and Torre Canne (BR). South of Posticeddu (BR) the amount of heavy minerals increases again, though being always subordinate to the carbonaticlastic fraction. These variations are correlated to the coast physiography: the northern and southern sectors are characterized by a larger platform with a wide surf zone and low energy of the waves, while in the central sector a narrower platform is responsible of a higher energy environment, preventing the deposition of the heavy fraction. A coarser grain size also characterizes the beaches of this sector. At a smaller scale, local variations in the heavy minerals abundances are also related to erosional processes due to anthropic factors: an example is the harbor of Margherita di Savoia (BT), which eastern pier obstacles the northward transport of the Ofanto sediments. The beach southward of the pier has relatively low heavy mineral percentages, while northward the carbonate erosion indirectly causes an enrichment in this component.