Clay mineral composition of river sediments in the Amazon Basin

Clay minerals are important in evaluating the maturity of suspended sediments, weathering intensity and source area. However, there are processes that can change the mineral assemblage such as river transportation, deposition, remobilization and tributary inputs. In terms of water discharge and sediment yield, the Amazon is one of the largest rivers in the world. Most of the suspended sediments come from the Andes, crossing the lowlands before reaching the ocean. This study measures the spatial distribution of clay mineral assemblages over the entire Amazon basin. The results obtained show the main features of the Amazon River main stem and larger tributaries from their sources to their confluence. Clay mineral composition highlights the evolution of the Madeira and Maranon-Solimoes River, which start in the Andes with high illite+chlorite content. Downstream, smectite contents increase. Moreover, all shield tributaries show high kaolinite content. The lower Amazon River is characterized by relative high smectite content, different from the Andean sources. The clay mineral results show that suspended sediments of the Amazon River have three main sources: 1) the Andes mountains; 2) the Amazon shields and 3) the Piedmont basins, especially the Pastaza alluvial megafan and the Fitzcarrald Arch basin. Lateral bank erosion plays also a significant role, by the introduction of more mature sediments into the river, enriched in smectite. (C) 2007 Elsevier B.V. All rights reserved.