Effects of Late Pleistocene synsedimentary tectonics on alluvial architecture at the Po Plain-Apennines border (N-Italy)

The stratigraphic architecture of the Quaternary Po basin fill records the interplay between local and remote controls on alluvial sedimentation: Apennine thrusting acted at the southern basin margin; Pleistocene dynamics of alpine glaciers controlled the Alpine margin; relative eustasy forced the eastern base-level, the Adriatic Sea, to fluctuate. Multiple base-levels, fluvial discharges, sediment textures, inflow and accommodation rates changed through space and time under these forcing factors, which determined the incremental geo-history and the alluvial architecture of the basin fill. In this frame, the role of Late Pleistocene-Holocene synsedimentary tectonics has been investigated at the Po Plain-Apennines border in Lombardy, where a structural culmination of the Emilia salient (San Colombano Hill ramp anticline) exposes the Mio-Quaternary stratigraphy. Geological mapping, stratigraphic, sedimentological and petrographic analyses, complemented by C14 and OSL age determinations, show the tectonic imprint on alluvial architecture: composite unconformities, pinch-out of alluvial sedimentary bodies, cross-cut relationships among alluvial terraces, uplifted palaeo-valley fills, cannibalism of pre-existing alluvial clastics, fault-related colluvial wedges and soft-sediment deformation structures. A high-rank, Middle-Late Pleistocene angular unconformity truncates the Gelasian regional unconformity and the local intra-Calabrian and Early-Middle Pleistocene unconformities, due to N-ward thrusting increments along the Emilia salient. This composite unconformity bounds alpine-sourced alluvial and glacio-fluvial units: Cascina Parina Synthems 1 and 2 (CPS1 and CPS2, Late Pleistocene, bracketed by OSL data to the MIS5-MIS4 time span and bounded respectively by intermediate-rank unconformities S0 and S1), Invernino Synthem (INS, Latest Pleistocene-LGM, bounded by the intermediate-rank erosional unconformity S2). They terrace the folded Miocene-Calabrian marine succession on the uplifting hill. Progressive wedging and S-ward thinning, recurrent amalgamation, petrographic changes and soft-sediment deformation structures of CPS1, suggest that the system was confined by an uplifting mild relief, ancestor of the present-day Hill. CPS2 glacio-fluvial system, fed from the N-western Verbano-Lario glacial amphitheatres, fringed-out towards the western hill sector, while a N-S flowing, distal braided glacio-fluvial system eroded the structural culmination to the East, originating the planation surface S1 at present uplifted at the hilltop. On the hill, CPS2 sediments fill relicts of syn-tectonic paleo-valleys, i.e. the first drainage pattern of the Late Pleistocene San Colombano relief. S2 unconformity heralds the S-ward progradation of the LGM glacio-fluvial depositional system (INS). A local, lateral ramp-related transtensional regime triggered differential uplift and tilting of the INS terraces bounding the hill, causing the progressive shifting of INS depositional systems on the lowered hangingwall. The increasing energy of the relief enhanced colluviation along the steep fault-slopes. A late-LGM muddy flood plain developed N of the hill, owing to tectonic-induced subsidence during glacial retreat. This was cross-cut by the meandering streams of the paleo-Sillaro Synthem during the Late Glacial, while paleo-Po River large meanders were carved on the SW flank of the hill. Post-glacial-Holocene entrenchment of the river network led to deposition of the Po Synthem, proposing a link among the origin of this lowermost terrace, the Holocene-to-recent river network anomalies and a phase of uplift and transtension related to the ongoing N-wards propagation of the Emilian salient.