Holocene Coastal Evolution of the Rio Açu Area (Rio Grande do Norte, Brazil)

The coastal development of the Rio Açu area (Rio Grande do Norte, Brazil) and the response of the river mouth to the Holocene sea level rise were reconstructed from seismostratigraphy. A sharply defined stratigraphic boundary between the Pleistocene and Holocene deposits was recognised. The seismic data reveal that, during sea level lowstand, a valley up to 9 km wide was cut into the Pleistocene deposits of the shelf platform. This valley extends perpendicular to the shoreline and is preserved in the recent morphology. It narrows at ∼20 km offshore to a 1km wide channel, which then widens to its former size a few hundred meters further offshore. Several incised valleys and cut and fill structures up to 2500 m wide and 12 m deep were observed in the Pleistocene deposits. These extend to 45 m below sea level, and indicate that the ancient Rio Açu meandered. A sediment sequence with sigmoidal structures increases in thickness offshore to 12 m, is composed of reworked material, and is interpreted as an embankment. A tilted block composed of more solid material was identified over a distance of at least 800 m. Holocene sediments thin out above this block, indicating postglacial neotectonics in the area.