GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 47 Results. We made a detailed log of the exposed section (1:10 scale) including pedostratigraphic description of the colluvial units, structural data acquisition, sampling for radiocarbon, OSL dating and for soil thin-section micromorphology, presently still ongoing (Fig. 2). We defined 5 Log units, including intact and weathered bedrock, a fault breccia and two stacked bodies of colluvial deposits, thickening upslope (Fig. 2), that point to a paleo-valley axis presently infilled by these deposits (Fig. 3). Preliminary age constraints, coming from AMS dating, indicate two main colluvial phases ascribed at ca. 30 kyr BP and Holocene respectively (Fig. 2). The lowermost samples apparently indicate age reversal, thus pointing to reworking of pre-existing colluvial soils and deposits from the surrounding slopes. Moreover, several soil characteristics (e.g., illuviation, weathering, concretions etc.) indicate a strong and prolonged phase and/or phases of pedogenesis, thus supporting this possibility. This preliminary model needs to be validated by further analyses, including thin section soil micromorphology and comparison with OSL datings. Discussion. The preliminary interpretation of the exposed section indicates the presence of a reverse fault zone, likely associated to Monferrato Arc thrusts system, controlling the morphology of the hills surrounding the exposed section (Fig. 3). Moreover, the data collected so far allow to state that this site show evidence of Late Pleistocene and possibly Holocene reverse surface faulting. This is the first site with documented evidence of coseismic displacement of very recent deposits in the whole Monferrato belt anyway. The recent geomorphologic evolution of the basin, including an attested event of drainage piracy (Fig. 2), could be partly caused by tectonic-induced surface deformation together with climatically driven regional phases of gully erosion. A detailed pedostratigraphic study together Fig. 2 – a) Perspective of the analyzed outcrop with the location of samples for different analysis; b) detail on a fault showing reverse slip and displacing the overlying colluvial unit; c) tangent-lineation diagram for fault data and kinematic analysis indicating a NW-SE directed principal stress; d) Log of the analyzed section.