Sorted bedforms over transgressive deposits along the continental shelf of western Sardinia (Mediterranean Sea)

• Multibeam data revealed the occurrence of active sorted bedforms at 43–70 m depth.
• Bedforms are associated with prograding wedges at four depth ranges.
• An evolutionary model of wedge formation and sorted bedform evolution is proposed.
• Sorted bedforms became persistent at depth > 40 m.

Multibeam echosounder data collected along the shelf of the western Sardinia margin (western Mediterranean Sea) revealed the occurrence of isolated dunes at a water depth of 35–40 m, linear and crescent-shaped sorted bedforms in the depth range of 43–70 m and crescent and irregular-shaped dunes at deeper sectors of the shelf, at a 73–87 m depth range.The morphodynamics of the seabed was analyzed by coupling multibeam backscatter data calibrated with ground truth sediment samples. The linear and crescent-shaped sorted bedforms in the depth range of 43–70 m are formed by a sequence of slight depressions characterized by coarse-grained sediments which alternated with elongated bodies, composed of finer sand, spaced ten to hundred meters apart and < 1 m high.The shelf sector, where the sorted bedforms occur, shows a terraced morphology, with planar sectors connected by gentle slopes. The sorted bedforms are generally oriented southwest to northeast, forming sequences of parallel crests which are located at the edge of the terraces. The analysis of very high-resolution seismic records suggests that the terraces are transgressive depositional sedimentary bodies forming prograding wedges.The analysis of the threshold hydrodynamic conditions for the motion of the sediment which forms the bedforms and the comparison of multibeam and sidescan sonar data collected over a 7-year time span revealed that the sorted bedforms migrate, and they are subjected to a regular forcing by the most severe storm waves from the NW. Sorted bedforms were not found in shallower water, where they are probably ephemeral features rapidly degraded by multi-directional waves and currents. At deeper sectors, > 70 m, wave energy is not strong enough to transport sediments and mud deposition prevails.We proposed an evolutionary model of the formation of prograding wedges and sorted bedforms off the western Sardinia shelf during the last sea-level rise. The landward migration of the shoreline subsequent to pulses of sea-level rise caused the deposition of prograding sandy wedges at different depths. Based on the present-day depth distribution, we can infer that the formation of persistent sorted bedforms over the edges of the terraces started after the sea level rose to a depth of 40 m over the seafloor where these bedforms occur.