Lowermost Amazon River: evidence of late Quaternary sea-level fluctuationsin a complex hydrodynamic system

Sedimentological, geochemical, and high-resolution seismic studies on the l owermost Amazon River (Xingu Mouth-Macapa-Estreitos de Breves) allow recons truction of the sediment dynamics of this large river system in response to late Quaternary sea-level changes. Areas of sediment transport and deposit ion on an older river bottom are delineated by seismic reflectors and are r elated to inundation and incision caused by late Quaternary rising and fall ing of sea level. Older substrates have been identified as the Alter do Cha o Formation (Cretaceous) and the Barreiras Formation (Tertiary). The initia l AMS-radiocarbon dates yielded late Pleistocene to Holocene ages, and indi cate a high sedimentation rate (0.6 cm/yr) for the last 800 yr. Three stage s in the late Quaternary evolution of the Amazon mouth system can be distin guished: (i) relative sea-level fall: probably related to the Last Glacial Maximum with channel incision; (ii) deposition and consolidation of mud fou nd near Afua during relative sea-level rise between late Pleistocene and mi ddle Holocene, corresponding to the "freshwater lake" or the evolution of l arge floodplain areas in Amazonia; and (iii) stable sea-level conditions si nce 6000 yr BP resulting in the modern pattern of deposition and erosion, c ontrolled by complex interactions of fluvial and oceanic processes. (C) 200 0 Elsevier Science Ltd and INQUA. All rights reserved.