Geometry and evolution of Holocene transgressive and regressive barriers on the semi-arid coast of NE Brazil

An integrated study based on ground penetrating radar (GPR) profiles, vibracore descriptions, water-well logs, and radiocarbon dating in a coastal deposit located in the northern region of Rio Grande do Norte State, northeastern Brazil, allowed us to identify Holocene transgressive and regressive barriers. The construction process for the studied coastal barrier is different from that proposed for the Holocene coastal plains along the eastern Brazilian coast, where the hydraulic barrier set up by large rivers for sediments transported by longshore currents has caused a strongly positive longshore sediment imbalance. In the study area, interpretation of the GPR images, within the constraints of vibracores data, allowed us to interpret five radar facies and four radar boundary sequences for these coastal deposits, which were built up during the Holocene coastal evolution of the region. As a result, the geometry of the coastal barrier was reconstructed. Based on barrier geometry, sediment ages, stratigraphic records, and sedimentation patterns, we propose a barrier evolutionary model for the Holocene for the study region. During the Holocene highstand, a transgressive barrier was deposited and a lagoon extended landward. During the sea-level fall soon after the Holocene highstand, the deposition of a regressive barrier (forced regression) started. This deposition was induced by the coastal geometry and high amounts of eolian sediments supplied by east-northeast winds. Also during this period of sea-level fall, the beach face became wider, and thus more subjected to wind action, facilitating the deposition of the first eolian deposits. These sediments were transported to the nearly formed embayment, providing a surplus for the construction of the regressive barrier. During the regressive phase, tidal channels closed and the lagoon became isolated from the open sea. The geometry of both the regressive and transgressive barriers as well as the stratigraphic relation between the sedimentary deposits suggest that the Holocene highstand in this region was not more than 1.4 m above present-day mean sea level.