Internal structure and morphology of a foredune blowout (Peró Beach - RJ, Brazil).

Matrim Moulton 1, Thais Rocha2, Israeli Santos1, Guilherme Fernandez1
1Universidade Federal Fluminense, Niterói, Rio de Janeiro., Brazil, 2Universidade Estadual do Rio de Janeiro, Rio de Janeiro, Rio de Janeiro., Brazil

Blowouts are hollowed depressions formed by wind erosion on pre-existing sand deposits. Often enough blowouts are found over foredunes, altering the morphology and depositional dynamics of these costal dunes. Although foredunes have been largely studied, the interpretation of the internal structure of blowouts over this type of dune is still very incipient. The aim of this work is to examine the internal structure and morphology that indicates the array of depositional processes that occur on foredunes associated with blowout establishment. In the Coastal plain of Peró (Brazil) the occurrence of Blowouts is restricted to the North central segment of the continuous foredune ridge, where a complex morphology takes place. Blowout depressions are found in different stages of development and exhibit different shapes. Ground penetrating radar (GPR) allied with detailed morphological data was used in other to obtain insights into the internal structure of the dune, this methodology has been proven to be a suitable tool for the interpretation of subsurface depositional processes. Therefore, this particular segment of the Peró foredunes, found to be under the influence of active blowouts, was survived using ground-penetrating radar (GPR), along with detailed three-dimensional topography by Differential Global Positioning System and Two-dimensional profiles using total station. Three-dimensional topography was made from the deflation plain up to the beach shoreline, covering the whole GRP survey area. The results of the three-dimensional topography showed the detailed morphology of the blowouts over the foredune ridge, with small depressions having a saucer shape and bigger blowouts having a more elongated form. The two biggest blowouts of the survey area had their internal structure analysed. Both GPR profiles were made inside the blowout following a straight line oblique to the foredune ridge (direction of the predominant North-east wind). GPR profile A-A', was made inside the biggest blowout depression (10m wide, 20m long and 4m high sidewalls) and GPR profile B-B’, was made inside the second biggest blowout depression (4m wide, 8m long and 2m high sidewalls), both showed similar internal structures. GPR interpretation showed a sequence of five different radarfacies established: Incipient dune (f1); Foreslope accretion (f2); Foredune core (f3); Deflation basin (f4); and Depositional Lobe (f5). The results show that depositional lobes in these blowouts exhibit foreset deposition and moderate apparent dip in the south-west direction (in alignment with the predominant wind). Irregular mounds found within the deflation basin, presumed to be formed by the differential erosion of the wind, are in fact the result of deposition, and the depressions in both cases were deeper than it is at present time. Therefore, deposition not only occurs on the depositional lobe, but also inside the blowout and deflation basins show both erosive and depositional processes.