Dispersione dei sedimenti ed evoluzione della costa Ira Capo Spulico e Taranto durante il Quaternario

Sediment dispersion and coast evolution between Capo Spulico and Taranto during the Quaternary

Abstract

The coast stretch from Capo Spulico to Punta Rondinella (near Taranto) is a single continuous system which has not any sedimentary exchange with the adjoining coast stretches, Beyond the present beach, 7 subsequent paleo-beaches terraced at various levels as far as more than 300 m towards the edge of the Apennines chain are recognizable. The present longshore drift, chiefly bound to the action of the winds from S and SE, moves the deposits from Capo Spulico and from Punta Rondinella toward the coast between the rivers Lenne and Patemisco. The longshore drift had the same trend in all the highest paleo-beaches, always keeping the same convergence zone between the rivers Lenne and Patemisco. The presence of gravels of Apennines origin at Punta Rondinella and at Isola di S. Pietro, requires a considerably rectilinear beach between Capo Spulico and this island. Considering the topography and the bathymetry of the present coast, such a beach could hypothetically exist only at a depth between 100 and 200 m, with respect to the present sea level. A virtually rectilinear coast line, as in plate 1, can be attributed to the considerable lowering in the sea level, caused by the Riss Glaciation. As seen, this coast line matches only in part the present day 200 m isobath; at the head of the Taranto canyon, in fact, the present day 200 isobath has retreated of about a dozen kilometers and in connection with the deepest gully of the canyon, an erosion of 700 m at least has been found. Assuming an age of 200 000 years at least for the end of the Riss, a 6 cm per year withdrawing and 3.5 mm per year deepening turn out. This quite high speed is probably due to the fact that the erosion taking place at the head of the Taranto canyon is exploiting the muddy infilling of the “Fossa Bradanica”, as shown by the shape of the isobathes. The coincidence between the walls of the “Fossa Bradanica” and the sides of the Taranto canyon is in fact noticeable (pl. 1). The flat and gently southward deeping bottom of this canyon represents the southward outcropping of the bottom of the “Fossa Bradanica”, reached by oilwells. Five petrographic provinces can be defined on the beach: 1 ) from Capo Spulico to Montegiordano Marina controlled by the supplies of the Ferro and Cardona creeks; 2) from Montegiordano Marina to Policoro Lido controlled by the supply of the Sinni river; 3) from Policoro Lido to Stazione di Marconia controlled by the supply of the Agri river; 4) from Stazione di Marconia to Bosco il Pineto controlled by the supplies of Basento and Bradano; 5) from Bosco il Pineto to Pino Solitario, which is still under the influence of the Sinni river. The change in the gravels composition (tabs. 1 and 5) shows that the degree of maturity of the deposits is more related to the reworking duration than to the transport length. The “apenninic” extremity of each of the seven marine terraces has been uplifted with respect to the opposit extremity leant against the Apulia calcareous massif. This uplift took place with a very uniform gradient from the Apulia extremity until the valley of the Basento river, which lies upon the front of an allochthonous nappe gliding downward the front of the Apenninic chain. Between the Basento and the Sinni valley the uplift was very irregular in the time and in the space.