Still active Sub-Andean foreland deformation is suggested to have syndepositionally modified the fluvial depositional environments in the Peruvian Amazonian foreland basin throughout Neogene-Quaternary time. Modern fluvial aggradation continues to proceed on a large scale (c. 120 000 km2) in two differing depositional systems. Firstly, various multistoried floodbasin deposits are derived from the meandering and anastomosing rivers within the subsiding intraforeland basins. Secondly, in the northern part of the Pastaza-Marañon basin the largest known Holocene alluvial fan-like formation (c. 60 000 km2) composed of reworked, volcaniclastic debris derived from active Ecuadorian volcanoes, has been identified.
The widespread, poorly known, dissected surface alluvium (terra firme) which covers the main part of the Peruvian Amazonian foreland basin shows further evidence of long-term foreland deformation, and terraces indicate both the effects of tectonism and Pleistocene climatic oscillations. In northern Peru, the surface alluvium was deposited by a Tertiary fluvial system with palaeocurrents to the west and northwest into the Andean foreland basin. In southern Peru, the respective surficial alluvium was part of a post-Miocene fluvial system flowing northeast into the main Amazon basin. Both systems were gradually abandoned when the eastward migrating Andean foreland deformation led to the more distinctive partitioning of the intraforeland basins, and the modern drainage system was created.