Quaternary evolution of the Central Po Plain: surface constraints from the San Colombano relief (Lombardy, Italy)

The Quaternary evolution of the Po Basin reflects the complex interaction between the tectonic evolution of the active Apennine thrusts (S), the rebound at the flexured alpine margin (N), the Middle Pleistocene advances and retreats of the Lario-Verbano glaciers, the changes in sediment and accommodation rates. In such almost flat areas, topographic reliefs and terraced landscapes represent the key-sectors to obtain geological constraints to the reconstruction of both evolution and subsurface stratigraphy. The San Colombano hill is one of the topographic expressions of buried Quaternary Apennine-related tectonics scattered in the Southern Po Plain. Nowadays, only regional 1:100.000 scale geological maps exist on this area. Therefore, a detailed cartography and new data are needed to constrain the relative chronologies of the Quaternary tectono-stratigraphic evolution of the Central Southern Po Plain. The hill exposes the uplifted and folded marine Miocene and Lower Pleistocene units (Sant’Agata Fossili Marls and San Colombano Fm.), unconformably overlain by the Upper Pleistocene alluvial and eolian pedogenized and deformed deposits (Cascina Parina and Invernino Units). The sequence is in turn shaped and covered by the Post-Glacial to Holocene processes and sediments along the slopes and within the valley network of the hill. We present a new geological and geomorphological map and a revised evolution of the San Colombano structure, which involves the mainly S-Alps-sourced sediments and the N-Apennines thrust-related tectonics. We applied an integrated approach that combines new geological mapping at 1:10.000 scale with sedimentological, geomorphological and pedological field and laboratory data analyses. In particular, different morphological evidences along the hill, as drainage pattern anomalies, the erosional nature and distinctive pedo- features of sub-planar sectors, nature and recurrent orientations of the escarpments are highlighted. They can be consistently used to derive the complex tectono-stratigraphic evolution of the San Colombano structure during Quaternary as it is suggested below: 1) the main Plio-Early Pleistocene phases of advance of the most external N-Apennine thrusts shaped the erosional unconformity between the transgressive San Colombano Formation (Calabrian) and the folded Sant’Agata Fossili Marls (Upper Miocene). 2) Thrust-folding and uplift persisted until the Upper Pleistocene, involving the Cascina Parina Unit, fed by an alpine glacio-fluvial depositional system. It was terraced by the alluvial deposits and covered by the loess of the Invernino Unit. Evidences of erosion and displacement of the pedologic profiles of Cascina Parina sequence are registered. 3) Transtensional faults dissected the structure close to the end of the Pleistocene. Fine-grained alluvial sediments filled local depocentres in the intervening gentle hangingwall synclines, onlapping the tilted Invernino Unit at the fringing-up of the Alpine fed systems. The river network was reorganized and polycyclic soils developed on gently tilted and partly suspended terraces. 4) Uplift quiescence and development of the post-glacial, NW-SE oriented paleo-drainage. 5) New uplift mainly involves the NNW sector of the structure. Further reworking and entrenchment of the river network, southward shift and entrenchment of the Po river, colluvial deposition and aggradation close to the relief, deposition of the Holocene to historical terraced units of Lambro river. The dataset used to define the geological evolution has been predisposed to a further 3D reconstruction, that will be a way to compare and validate different geometrical and evolutionary hypotheses carried out in this work. At the present state of the research, the proposed evolution gives new geometrical and chronological tectono-stratigraphic constraints to reconstruct the evolution of the Central Po Plain.