Physical and biological controis on lhe formalion of carbonate and siliciclaslic bedforms on lhe norlh-easl Brazilian shelf

Abstract

The continental shelf of the State of Rio Grande do Norte, Brazil, is ani openi shelf area located 5°S and 35°W. It is influenced by strong oceanic and winddriven currents, fair weather, 15-m-high waves and a mesotidal regime. This work focuses on the character and the controls on the development of suites of carbonate and siliciclastic bedforms, based on Landsat TM image analysis and extensive ground-truth (diving) investigations. Large-scale bedforms consist of: (i) bioclastic (mainly coralline algae and Halimeda) sand ribbons (5—10 km long, 50—600 m wide) parallel to the shoreline; and (ii) very large transverse siliciclastic dunes (34 km long on average, 840 m spacing and 3—8 m high), with troughs that grade rapidly into carbonate sands and gravels. Wave ripples are superposed on ali large-scale bedforms, and indicate an onshore shelf sediment transport normal to the main sediment transport direction. The occurrence of these large-scale bedforms is primarily determined by the northwesterly flowing residual oceanic and tidal currents, resulting mainly in coastparallel transport. Models of shelf bedform formation predict sand ribbons to occur in higher energy settings rather than in large dunes. However, in the study area, sand ribbons occur in an area of coarse, low-density and easily transportable bioclastic sands and gravels compared with the very large transverse dunes in an offshore area that is composed of denser mediumgrained siliciclastic sands. It suggests that the availability of different sediment types is likely to exert an influence on the nature of the bedforms generated. The offshore sand supply is time limited and originates from sea floor erosion of sandstones of former sea-level lowstands. The trough areas of both sand ribbons and very large transverse dunes comprise coarse calcareous algal gravels that support benthic communities of variable maturity. Diverse mature communities result in sediment stabilization through branching algal growth and binding that is thought to modify the morphology of dunes and sand ribbons. The occurrence and the nature of the bedforms is controlled by their hydrodynamic setting, by grain composition that reflects the geological history of the area and by the carbonate-producing benthic marine communities that inhabit the trough areas.