The Amazon paleoenvironment resulted from geodynamic, climate, and sea-level interactions.

The northern South American landscape evolved in response to complex interactions between geodynamic processes, climate, and sea-level oscillations, culminating in formation of the Amazon River. Although the history of this drainage basin was investigated in previous studies, the controlling factors on its depositional environment before Amazon River formation remain elusive. Based on existing sedimentologic and paleontologic observations and new numerical models, we show that Andean mountain building, evolving climate, and eustatic sea-level oscillations controlled the transition from intermittent marine to lacustrine environmental conditions in Amazonia before the onset of the transcontinental river. Our numerical simulations show that the incorporation of an evolving climate contributed to the shortening of the time interval between the end of marine influence in Western Amazonia and the onset of the transcontinental Amazon River during the late Miocene. Additionally, our numerical scenarios indicate that sea-level oscillations created intermittent marine incursions in central and western Amazonia only when negative dynamic topography induced by mantle convection was taken into account. After the establishment of the transcontinental river, the numerical scenarios produced no further marine incursions in central and western Amazonia, despite continued sea-level oscillation. We propose that this environmental evolution influenced regional biodiversity and the sedimentation history in northern South America, and explains the formation and disappearance of aquatic environments before the onset of the transcontinental Amazon River.