Sedimentology of a Macigno turbidite section in the Piombino-Baratti area (northern Apennines, Italy)

The Macigno Formation is representative of the late Oligocene-early Miocene foredeep deposits of the northern Apennines. It crops out from the western coastal areas to the Tuscan-Emilian Apennines and the eastern Chianti region. The more «internal» foredeep deposits of the northern Apennines are found in the Tuscan coastal area and are informally named «Macigno costiero», to distinguish them from the more external and more widely outcropping Macigno s.s. We have studied a vertical section of «Macigno costiero» about 135 m thick in the Piombino-Baratti area. Eight sedimentological facies have been identified. Facies are classified into three groups of genetic meaning and assembled in facies associations. The first group includes deposits of very high density gravitational flows, the second group is made of facies ascribed to very high density turbidity currents, and the third is composed of facies related to high and low density turbidity currents. Five facies associations are detected in the sequence. They correspond to different stages of the turbidite system, and show a general coarsening and thickening upward trend, from distal to more prossimal facies associations, marking a progradation of the turbidite system. A few levels relative to channel-levee facies are intercalated in the sequence, with the thickest ones located at the top of the sequence. In synthesis, we consider the studied sequence as the product of progradation/aggradation of a turbidite system characterized by lobes and minor channels that developed into a major channel system. Subsequently, the turbidite basin was closed by the emplacement of the allocthonous Subligurian Nappe