Top-cliff boulder deposits as geomorphological markers of Last Interglacial extreme wave events in the Mediterranean: evidence from south-eastern Sicily

Top-cliff boulder deposits represent one of the most extreme and debated geomorphological expressions of high-energy coastal processes, as their emplacement requires sustained overtopping of cliffs during coastal flooding. Occurring several metres above mean sea level and well beyond the reach of ordinary wave run-up, top-cliff boulder deposits are particularly sensitive indicators of extreme wave events. In this study, we investigate top-cliff boulder deposits atop a 10‑meter‑high cliff in south-eastern Sicily by integrating geomorphological observations with hydrodynamic modelling for both present and Last Interglacial forcing conditions. Hydrodynamic modelling was used to simulate extreme wave events that can cause coastal flooding and wave flow under tropical-like cyclone and tsunami scenarios. To evaluate the geomorphological effects of these extreme wave events, we modelled the current scenarios under the present-day sea level. In contrast, Last Interglacial scenarios incorporate elevated relative sea level and intensified hurricane and tsunami forcings to evaluate wave flow needed for top-cliff deposit emplacement. The results reflect a scenario with a Last Interglacial post-highstand regressive phase, highlighting the role of sea-level-controlled boundary conditions in enabling extreme coastal flooding and inland boulder transport. Our results indicate that Mediterranean top-cliff boulder deposits reflect the effectiveness of extreme waves acting under specific boundary conditions, rather than the absolute magnitude of the waves themselves, with relative sea level exerting a first-order control on coastal impact.