Unveiling the origin of enigmatic seabed morphologies on the Cape Licosa ridge (Italy)

High-resolution marine geophysical surveys conducted in 2004 revealed submarine morphologies offshore Cape Licosa (Tyrrhenian Sea, Italy), whose origin remains debated for years. Remote and direct data collected within the CORSUB project between 2024 and 2025 identified a field of subcircular to polygonal mounded structures distributed between approximately 75 m and 90 m of water depth along a ridge. These bedforms were analyzed using multibeam bathymetry and backscatter data, side-scan sonar, sub-bottom profiling, and video observations. CHIRP profiles indicate that these morphologies occur above a rugged acoustic basement attributed to the Miocene Cilento Flysch, which is draped by a thin, discontinuous Holocene sedimentary cover. Morphometric analysis delineated 565 discrete features with average dimensions of 8.6 × 6.1 × 0.4 m and a prevailing northeast to southeast orientation. They exhibit a distinctive high-reflectivity ring surrounding a lower-reflectivity core, producing a regular beehive-like seabed texture. Videos document a wavy-profiled seascape characterized by coarse-grained biogenic sediments, including abundant boxwork rhodoliths, and localized encrustations of coralline algae and bivalves on rocky outcrops. The data suggest that an inherited, complex paleo-topography of the Flysch substrate formed during subaerial exposure of the ridge at the Last Glacial Maximum. Holocene sedimentation draped this surface, involving both sediment accumulation driven by intense hydrodynamic activity and benthic colonization of sparse rocky substrates, thereby preserving this complexity and resulting in a wavy seafloor profile. These findings highlight the control of paleo-topography and post-glacial sedimentary dynamics in shaping mesophotic seabed morphology and distinctive seabed landforms along Mediterranean continental shelves.