Large‐scale sediment bodies and superimposed bedforms on the continental shelf close to the Strait of Gibraltar: interplay of complex oceanographic conditions and physiographic constraints

Abstract

A previously unknown fi eld of large-scale sedimentary bodies has been mapped and studied on the continental shelf off the Cape Trafalgar near the Strait of Gibraltar with particular emphasis on the relationship between large-scale sediment bodies and the superimposed bedforms. This study is based on a grid of 975 km of high-resolution seismic profi les collected at water depths ranging between 15 and 60 m.High variability of large-scale sedimentary bodies is attributed to the complex interaction of hydrodynamic agents. The most prominent sedimentary features are sand banks and ridges that indicate long-term southwest and southward-directed sediment transport patterns, possibly due to the interplay of two dominant current systems fl owing southward and westward. These sediment bodies evolve laterally to distinct external geometries, such as sand shoals in shallow water and sand sheets in the vicinity of larger sand banks that indicate moderate current velocities. In addition, pre-existing physiography is considered to play a role in the generation of certain sediment bodies, developed over inclined surfaces or confi ned laterally by elevations.Relationships between superimposed bedforms (mostly very large dunes) and underlying sediment bodies vary across the study area. Most superimposed bedforms occur over the complex mosaic of sediment banks and sheets, suggesting the interaction of several high-energy currents with different directions, such as tidal and/or wind-driven currents.

Bedforms: views and new perspectives from the third international workshop on Marine and River Dune Dynamics (MARID3)

“…They show that as the salt wedge density underflow migrates upstream and passes from a flat bed to dune roughness, the underflow becomes both in-phase with the bedforms and supercritical. Lobo et al (2010) also use the internal structure of large-scale sand banks and superimposed bedforms to assist in understanding their formation and dynamics. Such interactions between dunes and density flows are argued to be dependent on the friction Richardson number, internal Froude number and relative bedform height, that define the effective 'blockage' presented by the roughness to the stratified flow.…”

“…Once this occurs, flow mixing is shown to be greatly enhanced, and dominates mixing of the current in comparison to that generated by shear at the top of the flow. Lobo et al (2010) detail the high level of variability in the size and scales of these larger sand banks, and relate this to the complex interactions between two currents moving in differing directions. Future investigations of the interaction between form-induced roughness and density stratification may prove a very useful environment in which to investigate the topology and effects of such large-scale macroturbulence.…”

“…Investigations of the relationships between process, morphology and product are ripe for further research, especially where technological and methodological innovations are beginning to allow significant advances in substantive understanding. Lobo et al (2010) also use the internal structure of large-scale sand banks and superimposed bedforms to assist in understanding their formation and dynamics. They detail a field of largescale sandbanks on the continental shelf off Cape Trafalgar, Straight of Gibraltar, and highlight relationships between these larger-scale forms, smaller superimposed bedforms and sand sheets.…”

“…They detail a field of largescale sandbanks on the continental shelf off Cape Trafalgar, Straight of Gibraltar, and highlight relationships between these larger-scale forms, smaller superimposed bedforms and sand sheets. Lobo et al (2010) detail the high level of variability in the size and scales of these larger sand banks, and relate this to the complex interactions between two currents moving in differing directions. They also identify the locations of such interactions and attempt to relate these to higher-energy, less persistent currents across the zone, such as tidal and/or prevailing wind-driven currents.…”

Measurements and dynamics of multiple scale bedforms in tidally energetic environments

“…Off (1963) believed there should be a relation between sand waves and tidal current ridges, however, due to a lack of accurate surveys, this relation could not be quantified. Further work on tidal ridges similarly hypothesized that offshore tidal ridges are made from the accretion of cross-beds formed by dunes (Lobo et al, 2010;Reynaud and Dalrymple, 2012). More recently, Lefebvre et al (2013) suggested that smaller bedforms may be influencing the dunes on which they reside.…”