The tropics of South America hold the largest plant diversity in the world, yet the origins and processes leading to this high diversity still remain elusive. The Neogene sedimentary and fossil record of Amazonia could contain important clues to understanding the evolution of the Amazonian forest. Here, we study the pollen and spore record of two drill cores of the Neogene Solimoes Formation taken in northwestern Brazil, east of the Iquitos Arch. We studied 41 palynological samples and used several techniques to analyze the results, including Unitary Associations, a quantitative biostratigraphic technique, and Multidimensional Scaling Analysis. We describe 109 species, 51 of which are new, and seven new combinations. The biostratigraphic analysis indicates that the age of the Solimoes Formation in the study area is Late Miocene/Early Pliocene. This age is younger than that of nearby sections located west of the Iquitos arch, suggesting that the Iquitos Arch was active during the accumulation of the Solimoes Formation. The pollen/spore assemblages indicate that the Solimoes accumulated in fluvial deposits. We did not find evidence of either marine/tidal-flat deposits or extensive lakes.
