Terraços fluviais quaternários das regiões de Careiro-da-Várzea, Manaquiri, Careiro-Castanho e Autazes, Amazonia central

Abstract

In Careiro-da-Várzea, Careiro-Castanho, Manaquiri and Autazes areas, near Manaus, three asymetric quaternary fluvial terraces levels overlains the Alter do Chão (Cretaceous) and Novo Remanso (Miocene) siliciclastic sediments. Locally, these geological basement outcrops as an elongated stripe about 150 kilometers length toward NE-SW, in heights between 100 and 50 meters sustained by three chemically distinct lateritic paleosoils levels. The quaternary fluvial terraces developed between 240.000 and 2.000 years BP extends more than 100 kilometers between 80 e 10 meters heights composing the Amazonas-Solimões Fluvial System alluvial plain on the studied reach. Generally, these deposits present scroll-bars morphology, with low drainages density, characterized by sparse plain secondary channels and rounded lakes. Internally, they are constituted by pairs of layers made of sand and mud (silt and clay) forming Inclined Heterolitic Stratification (IHS) within the meander point bars. The TS deposits presents pink to reddish colors and shows pedogenesis features, while the TI and TInf deposits tends to present brownish to greyish colors due the rich organic matter and bioturbation features. The TS deposits occupies de south-central portion of the study area extending around 100 kilometers being constituted mainly of mud deposits, commonly used by the locals to brick and tiles manufacture. The TI and TInf deposits occur as narrow and elongated stripes of maximum 30 kilometers width, composing the active floodplain of the actual Solimões-Amazonas system. The point bars with IHS deposits, the terraces asymmetric distribution and the register of paleochannels features near the mouth of Purus River, suggests the predominance of a meander pattern in the study area between 240.000 and 6.000 years BP. The Solimões-Amazonas Fluvial System developed the last 6.000 years to an anastomosed-anabranching stable river related to the rise of sea level during the Late Holocene and the climatic changes that boosted the development of the rainforest, furthering the stabilization of margins on this fluvial system.