Pliocene-Pleistocene evolution of the upper central Po Plain (Northern Italy)

The Po Plain (Northern Italy) subsurface preserves the almost continuous Quaternary and Pliocene sedimentary succession, but outcrops are sparse and difficult to correlate among them. For this reason, data from the few deep boreholes, scanty for the upper portion, are the main indicators to reconstruct the Pliocene-Quaternary stratigraphy of the basin. The upper central Po Plain with its high density of water wells, provides a suitable location for studying the Lower Pleistocene and Pliocene marine sediments and their paleoenvironmental evolution. We present data from two transects north of Milan comprising 18 wells, 8 of them sampled for sedimentological, petrographic and biostratigraphic analyses on perforation cuttings. From top to bottom, the eastern transect in the Adda-Molgora Valley and the western one in the Seveso-Lambro Valley include a continental succession documenting the transition from gravel-dominated fluvioglacial deposits to braidplain conglomerates and sandy gravels. Both sequences include coarse clasts of alpine and local origin. Below, fine sediments with continental gastropods and wood fragments are reported; they indicate deposition in a meandering alluvial plain. The thickness of the continental deposits increases from east (ca. 40 m) to west, where it reaches ca. 150 m. These deposits have been attributed to the end of Calabrian to Late Pleistocene (Baio & Violanti, 2021). Continental sediments lie on the top of a regional unconformity, which is related to several and diachronous erosional events, and separates the upper continental deposits from the marine/transitional-marine succession. The latter were correlated through micropaleontological data to the neritic domains of the Pliocene-Pleistocene regressive sequence of Northern Adriatic Sea and Po Plain. The marine sequence presents different setting in the two transects. The western transect reports sediments with foraminiferal assemblages typical of delta, beach, and infra-circalittoral environments, dated to the Gelasian and Calabrian stages. Petrographic analysis on interspersed gravel layers indicates sources from alpine and local drainage basins. The eastern transect shows the presence of Calabrian and Piacenzian sediments at similar depth in adjacent wells. The Piacenzian succession includes infra-circalittoral and beach environments with the occurrence of lithodome bores on gravel clasts, derived from local drainage basins. Foraminiferal assemblages in the Calabrian sequence resembles those from the western transect. The investigations in the two transects show diachronous Piacenzian-Calabrian marine deposits and indicate that from the Piacenzian to the Late Pleistocene the area was characterized by different paleoenvironments, evolving from marine domains to outwash systems. Additionally, the occurrence of Piacenzian and Calabrian sediments at the same depth may suggest local tectonic deformation during the Early-Middle Pleistocene.