Quadro stratigrafico-deposizionale dei depositi regressivi nell’area di Irsina (Fossa bradanica)

According to the «Carta Geologica d'Italia» the regressive deposits belonging tu the «Bradanic cycle» are considered, beginning from the stratigraphic contact with the «Argille subappennine» Formation, as a series of formations organized into vertical aggradational-type sandy and conglomeratic depositional systems. These formations, Pleistocene in age, are in stratigraphic relationship one another, are bounded by horizontal surfaces and form tabular bodies. In particular, the «Sabbie di Monte Marano» Fm., marine in origin, and the «Conglomerato di Irsina» Fm., continental and/or marine in origin, represent the most characteristic deposits of the Bradanic cycle. In the neighbourhood of the Irsina village, a stratigraphic and sedimentological study has been carried out on these deposits. This area is located in the central-western sector of the Bradanic trough: because of its isolated outcrops, Irsina relief represents a witness-area for studying the filling up successions of the Bradanic trough. This kind of study allows to make some important paleoenvironmental consideration, and new formational distinctions. The upper part of the Irsina relief is represented by a group of sandy-conglomeratic deposits, 60 meters in thickness, having lithostratigraphic and facies characteristics laterally and vertically variable. Internally this group of sediments shows a progradational geometry. with NW-SE trending low-and high angle foreset beds. In particular, it is possible to distinguish in the northern zone sandy or sandy-conglomeratic beach sequences, and in the southern zone conglomeratic delta sequences. The beach sequences lie on the «Argille subappennine» Fm. by intergrading: the conglomeratic delta sequences overlie erosively the underlying «Argille subappennine» Fm. The depositional model here suggested shows a very different stratigraphic architecture as regards that well-known in literature. In fact, it is not any more possible to consider the "Conglomerato di Irsina" Fm. as deposits in superposition on the "Sabbie di Monte Marano" Fm. For these reasons we propose to abandone the term "Sabbie di Monte Marano" and to replace it by the formal name "Formazione di Monte S. Marco", indicating the sandy-conglomeratic beach sequences: we propose to maintain the term "Conglomerato di Irsina" to indicate the conglomeratic delta sequences. Furthermore, we introduce the new formal unit "Conglomerati di Fosso Macello", represented by reddish continental gravels erosively overlying the «Formazione di Monte S. Marco» and the «Conglomerato di Irsina» and previously included into "Conglomerato di Irsina", "Sabbie dello Staturo" and «Argille calcigne» Fms.