The main sand detrital mode of the Apulia beaches is represented by a carbonaticlastic supply, related to the limestones weathering of the Mesozoic platform of the foreland domain outcropping - as main source rocks -along the coast. However, locally, high amounts of heavy minerals concentrate in dark layers and placers, easily distinguishable from the lighter carbonaticlastic fraction. The petrographic analysis, carried out on the beach sands of the Adriatic coast of Apulia, from the Gulf of Manfredonia to the Otranto promontory, revealed that the darker fraction is mainly composed of volcanic minerals, namely clinopyroxene, amphibole and melanite garnet. Electron MicroProbe (EMP) analyses by Wave Dispersion System (WDS) showed that the composition of the volcaniclastic fraction does not vary from North to South, and that it is identical to that of the minerals occurring in the Ofanto river sediments, whose mouth is located south of the Gulf of Manfredonia. This river drains the sedimentary Southern Apennines units and the Mt. Vulture volcano. The comparison of heavy minerals composition with that of the Vulture volcanic source rocks confirms that detrital clinopyroxene, Mg-hastingsite amphibole and melanite garnet of Apulia sands can be related to a provenance from this volcano. Heavy detrital minerals have been 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 composition and texture of the sampled sediments showed that the grain size of the sediments and the abundance of volcanic minerals are not constant along the coast. The investigated area can be divided in three sectors: 1) a northern sector, about 60 km long, extending from Manfredonia to Trani and including the Ofanto mouth, with prevailing fine sands and high amounts of heavy minerals; 2) a central sector, extending for about 90 km from Trani to Torre Canne, with coarser sands and a sharp decrease of heavy minerals; 3) a southern sector, about 120 km long, extending to Otranto, where grain size decreases and the amount of heavy minerals increases. These variations could be attributed to differences in the coastal bathimetry and isobaths depths: the northern and southern sectors are characterized by a larger platform with a wide surf zone and low energy of the waves, whereas, in the central sector, a narrower platform is responsible of a higher energy environment, preventing the onshore deposition of the heavier sandy fraction. In each sector small- scale variations in heavy minerals contents can be related to erosional processes, too. In the northern sector, where anthropic erosion is very high, the amount of volcanic minerals is higher than the carbonaticlastic fraction, leading to the formation of a "black sand beach", similar to those of volcanic islands. This suggests that the variation of heavy minerals content of coastal sedimentation could be used as an index of coastal erosion.
The Vulture volcanic source rocks control on the Apulia beach sands composition (southern Italy)
Delle Rose M;
2019
Abstract
The main sand detrital mode of the Apulia beaches is represented by a carbonaticlastic supply, related to the limestones weathering of the Mesozoic platform of the foreland domain outcropping - as main source rocks -along the coast. However, locally, high amounts of heavy minerals concentrate in dark layers and placers, easily distinguishable from the lighter carbonaticlastic fraction. The petrographic analysis, carried out on the beach sands of the Adriatic coast of Apulia, from the Gulf of Manfredonia to the Otranto promontory, revealed that the darker fraction is mainly composed of volcanic minerals, namely clinopyroxene, amphibole and melanite garnet. Electron MicroProbe (EMP) analyses by Wave Dispersion System (WDS) showed that the composition of the volcaniclastic fraction does not vary from North to South, and that it is identical to that of the minerals occurring in the Ofanto river sediments, whose mouth is located south of the Gulf of Manfredonia. This river drains the sedimentary Southern Apennines units and the Mt. Vulture volcano. The comparison of heavy minerals composition with that of the Vulture volcanic source rocks confirms that detrital clinopyroxene, Mg-hastingsite amphibole and melanite garnet of Apulia sands can be related to a provenance from this volcano. Heavy detrital minerals have been 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 composition and texture of the sampled sediments showed that the grain size of the sediments and the abundance of volcanic minerals are not constant along the coast. The investigated area can be divided in three sectors: 1) a northern sector, about 60 km long, extending from Manfredonia to Trani and including the Ofanto mouth, with prevailing fine sands and high amounts of heavy minerals; 2) a central sector, extending for about 90 km from Trani to Torre Canne, with coarser sands and a sharp decrease of heavy minerals; 3) a southern sector, about 120 km long, extending to Otranto, where grain size decreases and the amount of heavy minerals increases. These variations could be attributed to differences in the coastal bathimetry and isobaths depths: the northern and southern sectors are characterized by a larger platform with a wide surf zone and low energy of the waves, whereas, in the central sector, a narrower platform is responsible of a higher energy environment, preventing the onshore deposition of the heavier sandy fraction. In each sector small- scale variations in heavy minerals contents can be related to erosional processes, too. In the northern sector, where anthropic erosion is very high, the amount of volcanic minerals is higher than the carbonaticlastic fraction, leading to the formation of a "black sand beach", similar to those of volcanic islands. This suggests that the variation of heavy minerals content of coastal sedimentation could be used as an index of coastal erosion.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
