MINERALOGIA E GEOQUÍMICA DOS SEDIMENTOS DE PRAIAS (BARRAS EM PONTAL) E BARRANCOS DO RIO ACRE E SUA RELAÇÃO COM A FERTILIDADE AGRONÔMICA

Abstract

The State of Acre is situated in the southwestern portion of the Amazon region and occupies 3.16% of its area. Two systems of drainage cross it: Juruá and Purus rivers. During the dry season, some beaches (point bars) are formed in the concave parts of the meanders. An example of this is observed on the Acre River, tributary of the Purus River. Incidentally, this area is of great historical importance for the region, as it was the site of a battle between Brazil and Bolivia for ownership of the State of Acre. Those beaches, which occur on the edges of the main rivers of the Acre, are intensely used for agriculture of subsistence of short cycle (beans (Vigna unguiculata (l) Walp), maize (Zea- Mays) and watermelon (Citrullus Lanatus) by the local peasants. These cultures occupy small areas, up to 2 hectares, on these beaches. Such situation is different from those on beaches from other regions of the Amazonia and Brazil. With the purpose of understanding the source of these sediments and the origin of this fertility, the present work was developed.The main unity of the State Acre is Solimões Formation, that occupies more than 80% of its surface (SEHA, 2000). It is formed mainly by argilites with carbonized organic matter (Radambrasil, 1976), and some fossils fragments of vertebrates and invertebrates. Locally, argillaceous siltites, calcareous rocks, ferruginous arenites and polimithics conglomerates are found (Radambrasil, 1976). On this Formation it was deposited holocenic sediments which constitute in part unstable abrupt declivities and, currently, beaches in both the edges of the rivers.The beach sediments, mainly in the cultivated areas, are predominantly siltics with unimodal distribution. The grains are angular to sub-angular and are constituted mainly of quartz, followed by smectite, illite, kaolinite and feldspar. The chemical composition shows high values of SiO2 (84,0 ± 4,7%, n = 10), and moderate to low values of the oxides (1 to 6%): Al2O3 (6,1 ± 1,7%, n = 10), Fe2O3 (2,7 ± 0,6%, n = 10), K2O (1,3 ± 0,2%, n = 10); Na2O (0,43 ± 0,12%, n = 10), CaO (0,28 ± 0,08%, n = 10) and MgO (0,4 ± 0,15%, n = 10), compatible with the mineralogical assembly identified, and therefore very different from those of other fluvial beaches, which are very poor in these components. Mineral constituents, such as feldspar and smectite and the angular form of the grains beach sediments, indicate immature evolution being Solimões Formation the source of these sediments.The fertility show that these sediments are comparable the eutrofic cambissols, with high capacity of exchange of cations (~ 11.01 Cmolc/Kg) and percentage of saturation of bases (v) above 90, indicating that they are in fact appropriate for agriculture. This fertility is related to clay minerals (smectite and illite) and its siltic and loosely bound grains.