A “natural sand plant” at the shelf edge in the low-energy Gulf of Lions, western Mediterranean Sea

Abstract Thick sand deposits at the edge of continental shelves, sometimes covered with large bedforms, are generally considered as relict features inherited from periods of low sea level, but alternative interpretations are possible, even in zones where modern oceanic processes are moderate. Detailed investigations in the western Gulf of Lions (western Mediterranean Sea) reveal the presence of remains of Marine Isotope Stage 4 (MIS 4) and MIS 2 shelf-edge deltas at the head of canyons. The sands were then “cannibalized” by strong rising sea-level currents, forming a 20-km-long, strike-oriented sand body covered by dunes. Presently, the migration of dunes is maintained by strong storm-driven currents. The seasonal reversal of circulation is at the origin of inversion of dune morphology, thus the sand body mimics a tidal sand bank with clockwise circulation of bedforms. In the long term, dunes migrate southward with a negative angle of climb, eroding into the underlying shelf-edge deltas. Acoustic Doppler current profiles and numerical simulations indicate that the sediment is sorted, once in suspension, and is flushed to the deep sea while sand bedload contributes to bedform migration. This relative enrichment of sand without external supply is referred to as a “natural sand plant,” by analogy with industrial sand plants where sand is washed and sieved. It forms a discontinuous sand belt, 5–20 m thick, more than 110 km along-strike, connecting multiple shelf-edge deltas. The geological importance of recognizing this recycling mechanism is indicated by the preservation of similarly large buried dunes formed during the deglaciation leading to interglacial MIS 7 at ca. 240 ka (Termination III).