Late Quaternary sedimentation and tectonics in the Po Basin: field evidences at the Po Plain-Apennines border (Lombardy)

Topographic reliefs and terraced landscapes represent key-sectors to constrain recent geological evolution and subsurface stratigraphy in almost flat landscapes like the Quaternary Po Plain. Aiming to investigate the role of Late Quaternary tectonics on the complexity of stratigraphic and geomorphological features exposed at the Po Plain-Apennines border in Lombardy, we focus on a structural culmination of the Emilia salient, the San Colombano Hill ramp anticline. Geological and geomorphological mapping at 1:10.000 scale, stratigraphic, sedimentological, paleontological, petrographic and morpho-structural analyses, complemented by C14 and OSL age determinations, show the incremental tectonic imprints on the Quaternary stratigraphy originating the present-day palimpsest landscape. Location of unconformable stratigraphic vs. conformable morphological boundaries, pinch-out and cross-cut relationships among alluvial sedimentary bodies, uplifted paleovalley fills, cannibalism of pre-existing alluvial clastics, colluvial wedges and sediment deformation structures highlight how, where and when tectonic-driven processes controlled the evolution of the Hill and the adjacent plain. The S. Colombano ramp anticline underwent Early-Middle Pleistocene thrusting, which uplifted and folded the Gelasian regional unconformity between littoral Calabrian and deep-marine Miocene formations. Late Pleistocene, alpine-sourced alluvial and glacio-fluvial units, terraced the deformed marine succession through the composite Late Pleistocene unconformity. The mapped synthems progressively wedge, thin and amalgamate S-wards, suggesting the syn-sedimentary confinement by an uplifting mild relief ancestor of the present-day Hill. Relicts of syn-tectonic paleo-valley fills testify the first drainage pattern of this proto-hill, where also polycyclic loess-soil aggraded during early Late Pleistocene. The S. Colombano structure underwent dissection since latest Pleistocene along three fault systems, while LGM glacio-fluvial and alluvial units prograded from the NW. Evidences of Latest Pleistocene fault activity are observed as thickness variations of the LGM synthem, offset of the Late Pleistocene unconformity, paleosol reworking in colluvial wedges on the fault-block hangingwalls, marked by triangular facets and abrupt diversions of the river network. A late-LGM muddy flood plain developed N of the hill, owing to tectonic-induced subsidence. This was cross-cut by the meandering tributaries of the paleo-Po River during the Late Glacial. The post-glacial-Holocene entrenchment of the river network and river anomalies suggest an eventual latest phase of uplift and transtension of the San Colombano structure, plausibly related to the ongoing N-wards propagation of the Emilia salient. The progressing research is integrating the surface and subsurface field geological and evolutionary constraints into 3D(4D) geological models, as the basis for hydrogeological and geohazard applications.