Geological mapping as a tool for understanding the historical landscape evolution of an Alpine valley: the Piuro case study (Bregaglia Valley, SO)

Mountain valleys are characterized by an intense geological and geomorphological activity, representing territories in continuous evolution. In the Alps, this evolution is tightly connected with the human exploitation that has to coexist with the natural processes. Geological mapping of these territories hence covers a fundamental role for the understanding of the active geological processes and the assessment of hydrogeological risk, but it is also the key tool to interpret their palimpsest landscape evolution. The case study here presented is located in the area of Piuro, a small village of the Italian Bregaglia Valley (Central Alps) famous for the catastrophic landslide that in 1618 erased the entire ancient town. Within the Interreg project A.M.AL.PI.18 (Interreg V-A IT-CH 2014-2020 Cooperation Program – Axis 2 “Cultural and natural enhancement”, Id 594274 - A.M.AL.PI.18 “Alpi in Movimento, Movimento nelle Alpi. Piuro 1618-2018”) we realized a 1:10.000 geological map and two boreholes (S1: 76 m b.g.s.; S2: 25 m b.g.s.) (Pigazzi et al., 2022) with a stratigraphic, structural and geomorphological approach, principally aimed to the reconstruction of the extent of the landslide deposits and of the source area. Coupling these data with field and remote geomorphological analyses performed on a high resolution 0.50 m DTM (Marotta et al., 2021), radiocarbon age determinations from borehole samples and archaeological and historical observations we constrained the relative depositional chronology of the lithosomes that compose the valley bottom. We then proceeded with the sub-surface interpretation of the single sedimentary bodies that were progressively subtracted from the actual topographic surface to obtain a 3D reconstruction of the interpreted pre- and immediately post-1618 event topographic surfaces. We also proposed a plausible interpretation of the chronological evolution of the valley bottom of Piuro from the post-Last Glacial Maximum to present, that shows how the evolution of this section of the valley has been predominantly dominated by gravitational and torrential related processes for the last ca 10.000 years.