O sistema fluvial Solimões-Amazonas durante o Quaternário

Abstract

The Amazon River has been considered as having a predominantly anastomosing pattern with increasing stability from upstream to downstream. Meandering and even straight patterns have been recognized in the upper Amazon as well as meandering reaches identified in the Solimões River in Brazil. Additionally, meandering features in the form of scroll bars and oxbow lakes have been also identified in different anastomosing reaches of the Amazon. Theses meandering features have been considered rare or absent in anastomosing rivers. In this context, it is clear that the Amazon, with its meandering features, is different from many other anastomosing rivers that have laterally stable, non-meandering channels. The limited information on the Amazon River provided mainly by remote sensing data and the lack of field data make difficult to understand the characteristics of this fluvial system, to establish a clear relationship between the meandering and anastomosing features and also to determine to what extend in time and space the anastomosing pattern is developed. The lack of information increases respect to the channel pattern characteristics of the Amazon River prior to present-day conditions. Channel pattern features during Pleistocene have been scarcely described, with a few studies that suggest a main single-channel meandering pattern during that time. In contrast, a broad discussion about the origin of the Amazon River as a transcontinental eastward flow has been developed in recent years with no agreement on the exact time this event took place and very scarce sedimentologic data on the deposits of central Amazonia. To our knowledge, no research has yet been undertaken to understand: 1) the origin of the Amazon’s anastomosing pattern and the development of its meandering features; 2) the behavior of the upper reaches of the Amazon, and specifically whether this reaches develop only meandering features, or whether they share the characteristic main anastomosing pattern with meandering secondary channels of the Amazon River in Brazil; 3) the characteristics of the Amazon River during Pleistocene and whether the river was different from the present-day pattern. To reach these goals, the current channel pattern of the Amazon River was studied between Manaus and the mouth of the Madeira River. Sedimentary processes were studied using field data, channel changes were analyzed trough a temporal analysis using remote sensing data, morphology was studied and optically stimulated luminescence (OSL) dates were obtained. The river in this area was found to be anastomosing with extensively distributed scroll bars. These scroll bars are mainly formed by sinuous secondary channels and related to subrecent and present-day migration. Avulsion, midchannel bar formation and chute cut-offs are the formative mechanisms of anastomosis in this system. The main anastomosing pattern and the meandering secondary channels have coexisted at least since 7.5 ± 0.85 ka. A reach of the Colombian Amazon River (upper Amazon River) was studied to more precisely define differences with the middle Amazon River with respect to channel pattern, and also to identify the triggers for its channel pattern development. Channel changes were analyzed trough a temporal analysis using remote sensing data, morphology studied and discharge data was analyzed over three different periods. The river in this area was found to be a multichannel meandering pattern that corresponds to a laterally active anabranching river. Mid-channel bar formation and chute cutoffs are the formative mechanisms of the multichannel pattern in this system. Variations in discharge seem to be responsible for the deposition and erosion dynamics found from remote sensing analysis. To determine for how long the current channel pattern of the Amazon River has existed, and to understand the paleogeographic conditions prior to the Holocene, Pleistocene deposition was studied. Pleistocene deposition (Iça Formation) is very restricted between the upper and the middle reaches of the Amazon River, with the Coari area having the best outcrops. Sedimentary processes were studied using field data, clay and heavy minerals analyzed and optically stimulated luminescence (OSL) dates were obtained. A mainly meandering system was found in this area, developing at least since 133.8± 20.9 ka. This system seems to be characterized as a multichannel meandering pattern similar to present-day conditions of the Colombian Amazon River.