Estudo sedimentológico dos depósitos eólicos inativos do campo de dunas do Baixo São Francisco em Alagoas

Abstract

The coastal zone presents dynamics that are different from other landscapes, where conditioning factors intensify the changes identified in them. These changes are identified through the interactions between geology and geomorphology. It is in this region that we find several models, including the dunes, which are produced through the action of winds linked to the availability of sediments. The study area has a great social and economic importance for the region, and is currently an Environmental Protection Area. The present study aims, through methodologies linked to sedimentology, to understand the inactive dune field of Piaçabuçu and the coastal dynamics through the textural variations of the granules and their distribution in the landscape. The proposed methodology was based on the steps of literature review, cartographic data survey, field work and laboratory procedure. As a result, it was identified that the Dunes Field presents different degrees of conservation of its morphologies, comprising patterns of residual mounds, without defined eolian morphology, sand sheets and parabolic dunes with dissected and partially conserved morphologies. Other parameters observed were wind speed and direction, current and past, important data in na attempt to understand wind dynamics. In this sense, it was observed that the past morphological pattern of the inactive dunes is concentrated in the southeast-northwest direction, different from the current ones, showing a probable change in the direction and intensity of the winds. The current trade winds reach the region and the orientation of the regional wind trend, where the active Barcanoides dunes present their axial axis in the current direction of the wind regime, northeast-southwest. When preparing the stratigraphic profiles, in sample areas, the presence of levels of organic matter, roots and leaves was identified, with massive stratification and crossover alternations, evidencing distinct moments in the migration with well-selected grains, as well as changes in the pattern of kinetic energy. In sedimentology, 6 samples were analyzed, where 16.66% were classified as very fine sand and 83.33% as coarse silt. In the selection degree, the six samples presented very well selected grains.In the asymmetry parameter, only one sample was different from the others, where 5 had positive asymmetry, and one approximately symmetrical. In the kurtosis parameter, which considers the flattening of a curve in relation to the curve of the normal distribution, 4 samples were classified as platykurtic, 1 as mesokurtic and 1 as very platykurtic. In morphoscopy, the rolling index was analyzed, where all samples obtained the same result, classified as Sub-angular. In sphericity, which refers to the degree of roundness of a particle, in all particles there were grains with a spheroidal shape. The results suggest that there was a change in the wind intensity and direction regimes along the Late Quaternary, evidenced by the differences in the axial axes when comparing active and inactive dunes. The sedimentological analyzes show that even having a fluvio-marine origin, the sediments were well worked by the wind action in the area, masking the characteristics of hydraulic transport. Corroborating the understanding of the effectiveness of the wind action in the area and the interpretation that the sediments were exposed to alternating cycles of transport and deposition before fixation.