Abstract The Brazilian tropical north shelf (BT N shelf) and the Brazilian tropical northeast shelf (BT NE shelf) along the Atlantic Ocean display unique conditions for tropical passive margins. Together they encompass approximately 3000 km in length, extending from Cape Orange in the north to Abrolhos Bank in the south. Both the north and NE shelves are very shallow and highly energetic systems. The first one is subject to energetic forcing from a number of different sources, including near-resonant semi-diurnal tides, large buoyancy flux from the Amazon River discharge, wind stress from the northeasterly trade winds and strong along-shelf flow associated with the North Brazil Current. The second one is subject to the full strength of the westerly-flowing South Equatorial Current, combined with high winds, moderate–high tidal range and/or waves. The BT N shelf is the largest shelf in Brazil, and is mostly covered with siliciclastic mud and sands because of the enormous water and sediment discharge from the Amazon River. In contrast, the BT NE shelf is narrow and open, and almost entirely covered by carbonate sediments due to the small amount of freshwater and sediment input.

Abstract The continental shelf adjacent to the northern coast of Rio Grande do Norte State, northeastern Brazil, represents a modern, highly dynamic mixed siliciclastic-carbonate shelf system developed in shallow water (< 60 m). Sediment dynamic processes are driven by waves, winds, tides, and the west- to northwest-directed North Brazil Current. This high-energy setting is associated with the presence of highly variable seabed features. This paper describes the spatial distribution of the mapped bedforms, in order to relate them to sedimentary processes and physical characteristics of the environment, and to the sedimentary history of this shoreline-shelf system. In a novel approach, the integrated data set presented here includes remote sensing, bathymetric, oceanographic, hydroacoustic, and sedimentologic data supported by diving. In addition, the high-energy mixed siliciclastic-carbonate shelf system contains a different sedimentary inventory than most well-studied siliciclastic or carbonate shelf environments. A variety of bedforms, which range from kilometers to tens of centimeters in scale, is present in the study area. Sediment reworking is accountable for the recent formation and migration of mobile bedforms which are superimposed on older relict structures. The integrated approach led to the identification of six main groups of large-scale bedforms: (I) very large longitudinal dunes, (II) very large transverse dunes, (III) small dunes, (IV) isolated shallow-marine sand bodies, (V) submerged beachrock chains, and (VI) incised-valley systems. The continental shelf off Rio Grande do Norte has experienced regressions and transgressions during the last glacial cycles. Outside the incised valley of the Rio Açu a continuous strong reflector, identified in all boomer profiles, represents the most recent sea-level lowstand surface that separates Pleistocene deposits below from early Holocene deposits above. During the Holocene sea-level rise, at least two beachrock lines formed at 20 m and 10 m of modern water depth.