Neogene palaeo-drainage divide in western Amazonia (Madre de Dios Basin, Peru): provenance evidence and implications

Abstract Reconstructing the geological history of the Amazon River and its palaeo-drainage is of prime importance for understanding the control of tectonics and geodynamics processes on Amazonian landscape dynamics and the origin of Amazonian biodiversity. In this study, we report new stratigraphic, sedimentological and provenance constraints on the Neogene sedimentary rocks of the Madre de Dios Basin (South Peru). Our multidisciplinary dataset shows that the Neogene sedimentary rocks of the northern part of the Madre de Dios Basin (12.5–13° lat S) have a different provenance than those of the southern part (13–14° lat S). The sedimentary rocks of the northern part of the basin are characterized by Solimoes-like Ɛ Nd (0) values (between −9.8 and −5.2) for fined grained sedimentary rocks, REE signatures suggestive of authigenic Fe-oxides formed from sulfide oxidation processes and U–Pb age distributions dominated by zircon ages between 0.9–1.3 Ga (Grenville/Sunsas), 0.5–0.7 Ga (Brasilian/Pampean), 290–252 Ma (Permian magmatism) and, to a lesser extent, zircon ages younger than 130 Ma (Andean Arc). We interpret the source of these sedimentary rocks to be the Altiplano and the Eastern Cordillera (EC). In contrast, the Neogene sedimentary rocks of the southern part were sourced from the EC and Sub-Andean Zone (SAZ) with secondary Fe-oxides derived from silicate rock weathering. Combined with previous studies, these contrasted geochemical signatures imply the existence of a palaeogeographic barrier within the Madre de Dios Basin between the Cretaceous and the Pliocene suggesting that the southern limit of the Amazonian palaeo-drainage basin was located within the Madre de Dios Basin.