Dune morphology and dynamics in the Murcia continental shelf (Western Mediterranean Sea)

Abstract

Subaqueous dunes are pervasive bedforms on many continental shelves worldwide generated by strong tidal or unidirectional currents (e.g. geostrophic currents or winddriven flows). Dunes can be stationary bedforms or migrate at different rates providing valuable information about local and regional current patterns and sediment transport. In the Murcia continental shelf (SE Iberian Peninsula), a large field of subaqueous dunes has been observed in the middle to outer shelf, off Águilas city. Here, we present a detailed morphologic and sedimentological analysis of these bedforms with the aim to discuss their possible mechanism of formation; and to assess their potential mobility. These findings are based on high-resolution multibeam data from repeated surveys (2003 and 2013), subbottom seismic profiles and sediment samples. Dune migration rates have been determined from the net displacement of the crest positions deduced from a large set of cross sectional bathymetric profiles. Dunes are widely distributed between 60 and 110 m depth covering an area of 23 km2 and showing a NW-SE orientation, perpendicular to the main trend of the shoreline and the shelf edge. The mean height is 0.29 m but it can be up to 1.28 m. The spacing of individual dunes, measured from trough to trough, ranges from 18 to 140 m, with a mean spacing of 69 m. The area occupied by the subaqueous dunes is composed mainly by fine to coarse sand (0.131-0.479 mm), coarser than the surrounding areas (0.025-0.115 mm). Seabed samples collected in the crests and troughs of individual dunes revealed that the crests are dominated by coarse sand, whereas the dune troughs consist of fine to medium sand with a higher content of fines. Dunes have asymmetrical profiles (average asymmetry ratio of 0.065) with the lee slope facing towards the southwest that reveals a net southwest sediment transport. Indeed, observed crest displacements indicate that dunes migrate at low rates towards the southwest (<1 m yr-1). This sediment transport pattern is consistent with the present-day circulation of the margin, thus suggesting that the predominant geostrophic current flowing southwards along the shelf break could be responsible for the maintenance of these bedforms as dynamic morphological features