Late Pleistocene and Holocene deposits in the Venetian underground : Palaeo environments and history

Abstract

Several new cores in the city area of Venice give new data for the interpretation of the palaeoenvironments and depositional history of the last 20,000 years. The multidisciplinary approach is based on sedimentological observations and on palaeoecological information obtained from quantitative analyses of foraminifera. The statistical analysis of the foraminifera content at different depths distinguishes the lagoon paiaeomorphologies, i.e., tidal channels, salt marshes or mud flats. Sedimentological observations describe the pre-transgression deposits sedimented in the late-glacial fluvial-lacustrine plain. Dating is based on 14C measured on peat or carbonate shells. Peats 10-12 m depth result to be linked to the interstadials between 21,750 to 19,000 years BP. The chronology of Jagoonal sediments give an average rate of sedimentation of 1.2 mm/year, very close to the regional Holocenic rate of 1.3 mm/year previously estimated. The overconsolidated layer, known as caranto and corresponding to the stratigraphic gap underlying the lagoon, involves all ancient ffoodplain deposits. Despite the trace findings of river channels beneath the lagoon, the thickness of lagoon sediments in the Venice subsoil does not show elevation differences that would indicate the continuance in the subsoil of the ancient riverbed of the Pleistocene Epoch, North-West of Venice. Between 4,670 years BP and the historical epoch, in the area of St. Mark's, the lagoon shows evidence of tidal creeks and intense exchanges, while to the North and South, lagoon systems with poor exchange developed. Structures and deposition connected with anthropogenic activity have gradually overtaken both.