Pleistocene-Holocene sedimentary deposits of the Solimoes-Amazonas fluvial system, Western Amazonia

摘要

Three levels of Pleistocene-Holocene fluvial terraces, informally referred to as Upper (UT), Intermediate (IT) and Lower (LT) Terraces, are described in the stretch between the Purus and Manacapuru tributaries (Western Amazonia) of the Solimoes-Amazonas river system, using remote sensing, sedimentological, stratigraphic and geochronological data. The terrace levels are located at heights between 15 and 75 in, defining bands parallel and symmetrical to the channel system that extends for tens of kilometers, truncated by paleochannel features. Internally, the intercalations of sand and mud (silt and clay) layers form the Inclined Heterolithic Stratification (IHS) pairs that are related to the migration of scrollbars, with ages from 204 to 0.75 ka determined by Optically Stimulated Luminescence (OSL). In this period, the low stability of the channel system was related to the meandering fluvial pattern that originated an extensive alluvial plain area, with symmetrical fluvial terraces levels, consisting of point bars with IHS and paleochannel features. However, multi-temporal data analysis of the last decades shows that the Solimoes-Amazonas river system has a high degree of stability, indicated by the low migration rate of the channels, and the presence of muddy and phytostabilized islands and marginal bars that define the current anastomosed-anabranching pattern. The time involved in changing the river pattern and the associated processes has been a source of controversy. Geomorphological, current sedimentary and stratigraphic data indicate that the change occurred in the Pleistocene - Holocene transition, probably due to the paleoclimatic, tectonic and glacio-eustatic changes that affected the sea level rise (North Atlantic), consequently, decreasing the gradient and increasing the sedimentary input that, associated with the increasing humidity (forest vegetation), made the channel system more stable.