SEDIMENT TRANSPORT AND STORAGE ALONG AMAZON RIVER VALLEYS (Invited Presentation)

Large-scale, long-term sediment transport from orogens to basins involves both storage and maturation of sediment and associated materials (weatherable minerals, carbon, and radionuclides used for geochronology). Recent progress in dating sediments stored along large river valleys (using U-Th, ¹³⁷Cs, ¹⁴C, ¹⁰Be, ⁹Be) yields average values of sediment age that require explanation and stimulate curiosity about (a) mechanisms involved in transport and storage of various grain sizes along valleys; (b) environmental controls on these processes; (c) potentials for interpretation of sediment ages and conditions and for prediction.

Mechanistic sediment budgets have been created through a combination of empirical and computational methods along the Amazon River in Brazil (2000 km) and one of its tributaries, the Beni River (400 km) crossing the foreland basin east of the Bolivian Andes. The sediment budgets involve average annual rates of sediment exchanges between successive channel reaches and their floodplains. These annual fluxes were converted into a stochastic model of long-term particle transport and storage. The resulting model was used to estimate storage times and long term particle transport rates through each river valley, and their sensitivity to variations in the sediment exchange processes and their environmental controls. The model currently represents steady-state conditions, but can also be used to explore relaxations of that constraint.