Abstract

A chronology for upper Quaternary process changes in the Middle Amazon area is based on the study of the topographic forms and the stratigraphic record, with special attention to the subsurface structure of the slopes. Climatic and eustatic factors seem to have interacted in rather complex ways and sometimes in opposite senses. The upper Quaternary of the Middle Amazon area may be subdivided in at least five phases: (1) Pre-Wisconsin—aggradation by clayey material. (2) Wisconsin—channel degradation related to lowering sea level. Aggradation by mass movement processes in lower order river valleys. Climatic conditions drier than the present ones. (3) Pleistocene post-glacial—aggradation related to the rising sea level and climatic conditions possibly drier than the present ones. (4) Pleistocene Holocene boundary—climatic change toward the present humidity. (5) Holocene—differential aggradation. The rivers rich in suspended load build up their floodplains, while the underloaded autochthonous rivers are subjected to a process of drowning.

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