Cyclic variations in sediment provenance from late Pleistocene deposits of the eastern Po Plain, Italy

A cyclic vertical succession of alluvial, littoral, and shallow-marine deposits is identified within two continuously cored boreholes (187-S1 and 204-S15) drilled to ~180 m beneath the present Po coastal plain, in northern Italy. Integrated sedimentologic, micropaleontologic (benthic foraminifers and ostracods), and geochemical studies allow the reconstruction of the paleogeographic evolution of the study area during the late Quaternary, with a special emphasis on major changes in provenance and sediment dispersal patterns. Transgressive surfaces appear as the most readily identifiable stratigraphic features in the two cores, allowing identification of a series of transgressive-regressive sequences. The transgressive surfaces mark the onset of coastal to shallow-marine conditions, followed by delta and strand plain progradation and the reestablishment of continental environments. This cyclic pattern of facies is paralleled by distinctive cyclic variations in chemical composition of sediments, reflecting a systematic increase in Ni/Al within lower transgressive deposits, followed by a marked decrease in the overlying alluvial plain sediments. At relatively northern locations (core 187- S1), the maximum flooding surfaces identified within shallow-marine deposits on the basis of subtle, but consistent changes in microfaunal assemblages are characterized by anomalously high Mg/Al values. The abrupt peaks in Ni/Al recorded at the transgressive surfaces are interpreted to reflect enrichments in mafic-ultramafic detritus, probably derived from the western Alps and the northwestern Apennines and supplied by the Po River to the coastal areas. These variations took place at the onset of brackish and littoral conditions, when direct connection with the sea favored sediment dispersal from the Po River mouth to lagoonal and coastal environments via the littoral drift. High Mg/Al values within open-marine deposits at the maximum flooding surfaces likely reflect an increasing contribution from eastern Alpine (dolomite-rich) sources at time of maximum shoreline migration. The recurrent changes in geochemical composition recorded across the transgressive surfaces fully support the stratigraphic subdivision of late Quaternary deposits of the Po Basin into transgressive-regressive sequences, rather than depositional sequences. The sequence-bounding unconformities do not display distinctive geochemical signatures.