Chronostratigraphy of the southwestern Amazon: new U-Pb ages for Neogene deposits in the Upper Acre River

The modern Amazon region holds global significance for its unparalleled biodiversity and ecological importance, both of which were strongly influenced by major Neogene tectonic and paleoenvironmental events. The nature and timing of these events, however, remain poorly constrained due to limited geological data and scarce radiometric dating of the sedimentary units. Here, we present new U-Pb detrital zircon ages from two key fossiliferous sites in southwestern Amazon to improve the chronological framework of the Solimões Formation and its vertebrate assemblages. Maximum depositional ages of 5.573 ± 0.019 and 6.038 ± 0.100 Ma are inferred from the single youngest zircon grains at the Patos and Cavalcante localities, respectively. These ages, within the Messinian (Late Miocene), are consistent with biostratigraphic proposals and recent geochronological evidence, while contrasting with earlier zircon estimates for the Solimões Formation that suggested Tortonian ages. These findings challenge the interpretation of the genesis of the fossiliferous conglomerates in the region as isochronous, related to older Andean tectonic events, and instead support the hypothesis of diachronism among vertebrate-bearing deposits of the Solimões Formation during the Late Miocene. The revised assessment of the Patos vertebrate assemblage further constrains the temporal ranges of several taxa using absolute ages, indicating the persistence of typical giant forms of Proto-Amazonia close to the Miocene-Pliocene boundary. Their co-occurrence at Patos suggests that the high resource productivity and ecological complexity characteristic of the fluvial-lacustrine Acre phase, which followed the decline of the Pebas wetlands during the Late Miocene, endured until at least ∼6 Ma in the southwestern Amazon.