Large-scale seafloor waveforms on the flanks of insular volcanoes (Aeolian Archipelago, Italy), with inferences about their origin

• Large bedforms are widespread on the submarine flanks of the Aeolian volcanoes.
• These features are related to gravity-driven processes.
• Bedforms are always found downslope of a decrease of slope gradients below 4°–8°.
• Bedform variability is a useful proxy to reconstruct sedimentary processes.
• Bedforms can be interpreted as cyclic steps.

High-resolution multibeam data in the central and eastern part of the Aeolian volcanic arc (Southern Tyrrhenian Sea) reveal widespread large-scale waveforms on the submarine flanks of these insular volcanoes. These features show high variability in wave parameters (wavelength of 60–1600 m and wave height of 4–200 m) and morphology (from sinuous to arcuate in plan-view, and from undulating to stepped, with very steep scarps, in cross-section), indicating that different processes, at variable spatial and temporal scale, are responsible for their genesis. By relating the morphological characters of the different waveforms and the local boundary conditions (morphological setting, regional slope gradients, sediment source and dynamics) where these features form, we were able to distinguish bedforms formed by sedimentary gravity flows from bedform-like features due to gravity instability processes. In detail, bedforms have been interpreted as coarse-grained sediment waves according to their wave parameters, geometry and texture. Bedforms are always found where the slope gradients markedly decrease to values < 4°–8°, promoting a hydraulic jump in the sedimentary gravity flow. Hence, we tested if the recognized coarse-grained sediment waves can be generated as cyclic steps by comparing their morphometric characteristics with the results of experimental evidence and numerical modeling proposed in literature. The results show that cyclic step is a reasonable interpretation for these features, and we discuss the effect of flow depth, flow concentration and discharge as governing parameters affecting their wavelength, wave height and shape.