Estudo de proveniência e granulometria dos sedimentos dos rios Tocantins e Itacaiúnas: cidade de Marabá-PA

Abstract

The region between the confluence of the Tocantins and Itacaiunas rivers, located in a stretch that includes the city of Marabá, situated in the southeast region of the state of Pará, stands out for presenting a diversity in its sedimentary fluvial dynamics. These two rivers are the main in the region, which confer abundance of water resources. Starting from the scarcity of sedimentological studies along the rivers, the research intends to identify the provenance through the description of the assemblies of heavy minerals and perform a granulometric analysis of the sediments collected along the Tocantins and Itacaiunas rivers. To obtain the data was used the sampling, classification and description of the sediments of bottom these rivers, after was made the characterization of the granulometric analysis and heavy minerals, estimating their relative abundance, distribution and provenance, correlating jointly with the rocks sources presents in the region. The results of the granulometric analysis showed a sedimentary variation in the three profiles made along the rivers. Profiles I and II, located along the Tocantins River, presented a granulometric variation of fine, medium and coarse sand to very coarse sand. This variation suggests differentiated sediment transport, probably caused by a variation of the river's energy. In the longitudinal profile III, along the Itacaiunas river, there was little granulometric variation, with predominance of the sediments of of coarse sand to very coarse sand sizes, indicating that in this area the river has a high energy with few variations of flow, so that it is able to carry sediments of this granulometry. Various species of heavy minerals were identified in this study: zircon, tourmaline, rutile, staurolite, kyanite, silimanite and garnet. With respect to the provenance of profiles I and II, along the Tocantins River, they presented a mineralogical assembly composed of zircon, tourmaline, rutile, staurolite, kyanite, silimanite and garnet, showing a variety of shapes, degree of roundness, colors and textures, suggesting sources and processes of distinct sedimentary cycles for these minerals as igneous, sedimentary and metamorphic rocks, which are rocks belonging to the study region. The heavy mineral assembly identified in the sediments collected in profile III, along the Itacaiunas River, has a low variation, consisting only of zircon, tourmaline, staurolite, kyanite, sillimanite and garnet, indicating a provenance of weathering only of metamorphic and sedimentary rocks, referring to the geological terrains in the study region. To evaluate the degree of maturity along the analyzed profiles, the ZTR indexes were used, which is the sum of the percentages of zircon, tourmaline and rutile, indicating that in the three analyzed profiles they have a high degree of mineralogical maturity shown in the ZTR calculated along the profiles. In addition, the presence of round grains of zircon, rutile and tourmaline corroborate this claim.