Past and present provenance of the Amazon River

Abstract

In its modern configuration, the Amazon River is well suited to test the assumptions upon which sediment provenance techniques rely. The scale of the system coupled with harsh weathering conditions and diverse geology allow for direct observation of sedimentary response to transport and environmental processes. Over 3,200 new U/Pb detrital zircon ages for fourteen Amazon River samples indicate that long-distance transport of zircon age populations is possible. In the western catchment, zircon ages are dominantly Phanerozoic, reflecting Andean sources. As sand progresses downstream, the presence of Proterozoic grains increases indicating Amazon Craton input, but even in the lowermost river reaches, 3000+ km from the Andes, ~30% of zircons are Phanerozoic. In spite of long transport distances, age spectra are heterogeneous suggesting that the technique is not a reliable quantitative assessment tool. Comparison of zircon provenance to other indicators shows that signals can vary widely for sediment derived in the same catchment. Specifically, techniques that rely upon chemically labile minerals are biased toward areas of physical weathering and high sediment output, whereas techniques that rely upon stable minerals areas are biased toward areas of chemical weathering and low sediment output. In the Amazon, the rate at which sediment is transported has a strong influence on the ultimate provenance signal; high transport rate yields source similar results while slowly transported sediment is sensitive toward weathering conditions. When exploring provenance of ancient deposits, multiple indicators with different weathering responses should be used to create more complete sedimentary histories. Previous drainage configurations have directed detritus to various locations across Amazonia. Comparison of zircon ages for modern sand to ages and sedimentologic data for Miocene and Cretaceous deposits details the occurrence of continent-scale drainage reorganization. As Gondwana split during the Late Cretaceous, drainage was directed west from a rift-shoulder near the Amazon mouth. During the Miocene, after rifting had ceased and as Andean uplift rates were at a maximum, drainage was split into two sub-basins separated by the Purus Arch. Once the Miocene Andean foreland basin was overfilled and the Purus Arch was overtopped, the modern Amazon River system was established during or after the latest Miocene.