Depósitos sedimentares Pleistocenos-Holocenos da zona de confluência dos rios Solimões e Purus, Amazônia Ocidental

Abstract

The Solimões River, at the confluence zone with the Purus River (Western Amazonia) features 3 levels of Pleistocene-Holocenos river terraces, informally called Superior (TS), Intermediário (TI) and Inferior (TInf), defined in this study based in remote sensing data, sedimentology, stratigraphy and geochronology. In general, the terraces display symmetrical distribution, form parallel tracks to the river and extend for tens of kilometers. Located between the elevations 15-75 m, display aged 750-204,000 years BP and internally consist of interbedded sand and mud layers (silt and clay) forming pairs of Stratification Heterolytic Leaning (EHI). The TS (55,000 to 204,596 years BP) is characterized by raised ridges and depressions, showing dendritic drainage patterns to subdendritic and traces of old lines of lateral accretion. The terraces TI (28,400 to 37,240 years BP) and TInf (750-9315 years BP) are part of the active floodplain of the river, subject to seasonal flooding. TI provides relief plan with gentle undulations, displaying rows of lateral accretion and dozens of lakes with elongated shapes, rounded and curved, while the TInf is the closest unit to the main channels, making the marginal bars and islands, stands out for most density lines accretion and lakes. The symmetry of the levels of the terraces of the Solimões River, as well as the features of paleochannels and the presence of EHI, show your past meandering fluvial style that prevailed in the Late Pleistocene, leading to the conclusion that the migrated channel for tens of kilometers. However, recent data show that this river has a high degree of stability, defined primarily based on the channel low migration rate (around 0.32% / year), and by the presence of islands and marginal bars and muddy fitoestabilizadas that define the current anastomosing-anabranching style. The change of fluvial style in the Pleistocene-Holocene boundary can be mainly related to climatic variations, tectonics and glaciostatic. The rise in the sea to the current level, between 18,000 and 6,000 and years BP, blocked the mouth of Solimões-Amazonas river system, causing a decrease in slope and greater input of fine sediments, which in combination with the increased humidity (which allowed phytostabilization the islands and margins) made this system more stable and allowed the installation of the current anastomosing-anabranching style. The predominance of this style can be extended to the east, to the confluence region with the Madeira River.