Variação sazonal da sedimentação e morfologia da plataforma continental interna de Fortaleza/CE

Abstract

The objective of this research is the characterization of sediments through granulometric and morphoscopic statistical parameters, and the identification of submerged sedimentary structures of the inner continental shelf of Fortaleza-CE, from bathymetric data collected in the dry and rainy periods of 2018, which are part of the CIRCUNPLACE project (Importance of the Circulation for the Transport and Destination of Organic Contaminants on the Ceará Continental Shelf), supported by FUNCAP. The study area ranges from the breaking zone to the -18 m isobath of the inner platform, comprising important sectors of the city such as the Cocó, Maceió, Ceará rivers' outlets, undersea emissary, as well as the rain galleries, which input material in a smaller scale. Sediments were collected on April (rainy season), November/December (dry season) of 2018 with a van veen dredger, along with a single-beam bathymetric survey, aiming to cover the whole study area of the platform in the two different periods. The samples were analyzed in the Geological Oceanography - LOG laboratory with the standard methodology. SAG software was used and ANASED in the classification of statistical and particle size parameters, in addition to the treatment and correction of bathymetric data, and generations of bathymetric maps by interpolation (kriging) using the GIS environment. With the acquired results, maps were prepared for both periods, with the parameters: average diameter, asymmetry, standard deviation, kurtosis, calcium carbonate content, organic matter, sphericity and roundness of the grains. In both periods, the predominance of fine sediments in the port basin was higher in the area; thick sediments were predominant over the submerged sand bank. In the rainy season, sedimentary data showed a predominance of fine sediment along the eastern shelf, which were also higher than in the dry season. Thus, the Cocó River is an important source of sediment for the platform during this rainy season. The regions of influence of the undersea emissary and Maceió Creek are characterized by fine sedimentation. In contrast to the outlet of the Cocó River and the Ceará River, in which predominate mixed sediment. The highest concentrations of carbonate sediments are associated with river outlets and existing local reefs. The same correspondence happens with the organic matter content. Sub-rounded grains were predominant in the rainy season with high sphericity, while in the dry period were predominant rounded grains with low sphericity. The comparison of bathymetric data in the two periods of the year revealed features associated with rocky outcrops, sandy deposits, among other forms resulting from the action of the migratior cycles of sedimentary material and the intense coastal movement during the year.