Common origin of coexisting sediment undulations and gullies? Insights from two modern Mediterranean prodeltas (southern Italy and northern Morocco)

Depositional and erosional bedforms that shape prodelta deposits in the Gulf of Patti (southern Tyrrhenian Sea) and in the Al-Hoceima Bay (northern Morocco) were investigated using multibeam bathymetry as a basis for morphometric analysis. The two study areas have comparable structural settings and oceanographic regimes and share the occurrence of sediment undulations and closely spaced gullies. The resulting bedform fields are confined on prodelta deposits, from the inner to the outer shelf. The infrequent coexistence of these two types of bedforms on other Mediterranean margins makes our case study an ideal opportunity to investigate the processes that may control the combined or independent genesis of each type of bedform. Whereas gullies are more confidently interpreted as the result of erosion of the seafloor by hyperpycnal flows generated at river mouths and/or triggered by settling plumes and small-scale instabilities on delta front, unveiling the genetic mechanism behind sediment undulations is more difficult, and makes identifying the underlying processes, ranging from bottom currents to gravity flows or even internal waves, more ambiguous. Quantitative measurements of the main morphometric parameters of sediment undulations and gullies were computed to resolve relationships between their variability and their distribution over the prodelta area. Particular attention was paid to statistical analysis of sediment undulations using multivariate analysis based on principal component analysis (PCA) and k-mean fuzzy clustering. The individual and comparative analyses suggest that shelf gradients, location and distance from the river mouth, and sediment supply are keys in controlling the growth and scale of sediment undulations with respect to their distribution downslope. Although from our analysis, sediment gravity flows stand out as the most likely mechanism triggering the generation of sediment undulations, alternative causes (i.e., soft-sediment deformation or sediment remobilization by oceanographic processes) cannot be ruled out. Our results provide new insights into the possible origin of sediment undulations under the control of river-driven sediment gravity flows in prodelta settings, indicating that their concomitant occurrence with gullies is not accidental.