The Effects of the Inception of Amazonian Transcontinental Drainage During the Neogene on the Landscape and Vegetation of the Solimões Basin, Brazil

Amazonia covers nearly 50% of Brazil. The history of this biome is marked by many changes in the landscape caused mainly by the Central and Northern Andes uplift. This event has influenced the palaeogeography of the Amazon and therefore its biogeography and possibly biodiversity. Herein we present palynological and lithostratigraphical results from the Solimões Formation in the well 1AS-33-AM. It was drilled in the Solimões Basin, Amazonas State, Brazil, reaching 405 m in depth. We identified 152 palynomorphs, in 32 samples, among them some biostratigraphical markers, such as Crassoretitriletes vanraadshoovenii, Fenestrites spinosus, Cichoreacidites longispinosus, Ladakhipollenites? caribbiensis and Echitricolporites mcneillyi, allowing us to recognise four biozones sensu Lorente (1986): Crassoretitriletes Interval Zone (399.10–276.70 m), Asteraceae Interval Zone (276.70–262.00 m), Psilatricolporites caribbiensis Interval Subzone (239.90–70.00 m) and Echitricolporites-Alnipollenites Interval Subzone (70–32 m). The palynological and lithostratigraphical results indicate two environmental phases. The palynomorph association presents a change marked by the sudden appearance and predominance of Grimsdalea magnaclavata from 239.90 m upwards and a general increase in the number of species. The lithostratigraphy shows from approximately the same depth greater amounts of sandstones towards the top of the well, suggesting a change from a paralic to a fluvial environment of higher energy. The first phase can be associated with the Pebas/Acre depositional systems, wetlands composed by lakes and swamps with seasonal floods that existed from 23 to 7–5 Ma. The second phase indicates a fluvial system that may be interpreted as a record of the modern Amazon transcontinental fluvial system, which was already established in the Pliocene.