Late Miocene continental sedimentation in southwestern Amazonia and its regional significance: Biotic and geological evidence

Fossil content (vertebrate paleofauna and palynology) indicates that the sediments of the Solimões Formation in Acre (SW Brazilian Amazonia) are continental, having been deposited by avulsive fluvial belts in a floodbasin–floodplain environment. The main source area was the Andes chain. Widespread lacustrine swampy deposits, stacked channel deposits, and paleosoils are typical elements that characterize the Solimões Formation sediments that outcrop in southwestern Brazilian Amazonia. New data on fossil vertebrate assemblages and palynology corroborate the Late Miocene age suggested previously and assign the fossils to the Huayquerian mammalian biozone, spanning 9–6.5 Ma. These geological and paleontological data show that the existence of an intracontinental seaway through SW Amazonia during the Late Miocene (11–10 Ma), connecting the Caribbean Sea with the Parana Basin as previously proposed is unsustainable, because the sediments used by previous authors to propose the seaway were deposited in a continental environment and are younger than 11–10 Ma.