The Pretare-Piedilama rock block deposit: evidence of a further case of Quaternary rock avalanche in Central Apennines, Italy

In the last two decades large clastic deposits in Central Apennines with specific morphological and sedimentological features have been interpreted as the result of Quaternary rock avalanche events (e.g., Di Luzio et al., 2004; Bianchi Fasani et al., 2014; Schilirò et al., 2019; Antonielli et al., 2020). The analysis of such deposits, that are located within intermontane basins and narrow valleys bounded by high mountain ridges, have improved the knowledge about this kind of massive rock slope failures, also clarifying their relationship with Deep-seated Gravitational Slope Deformations.The present study then describes a multidisciplinary analysis carried out on a huge rock block deposit which crops out within the Pretare-Piedilama Valley, in the piedmont junction area of the Sibillini Mountain range (Central Italy), where Mesozoic basinal carbonates overthrust Miocene foredeep deposits.Specifically, we performed sedimentological, stratigraphical and morphometric analyses on the clastic deposit; results support the interpretation of the event as a rock avalanche body. The accumulation area shows a T-like shape with a wide, E-W-oriented, proximal part and a N-S channelization in the central and lower sectors. The evidence suggests erosional events and tectonics as controlling factors on rock flow deposition. In this respect, the area was involved in the 2016 central Italy seismic sequence and was tectonically active during Quaternary times (Tortorici et al., 2009).As regards on the deposit genesis, considering the geometric characteristics of a sub-rectangular detachment area located on the southern edge of the Sibillini Range, an original mechanism of rockslide failure involving about 8·106m3 of Early Jurassic limestone was inferred. Here, the post-failure geomorphic features behind the main scarp are considered for the evaluation of hazard conditions.Finally, well-log analysis of the clastic sequence filling the Pretare-Piedilama Valley evidenced additional Quaternary landslide events occurred before the rock avalanche, thus testifying to a long history of large slope instabilities in the area controlling the landscape development.