LOWERMOST AMAZON RIVER - MORPHOLOGY AND SHALLOW SEISMIC CHARACTERISTICS

The riverbed morphology of the lowermost Amazon River was characterize d by high-resolution acoustic profiling in 21 areas. This technique ha s been used for the first time in this area, and allowed the discovery of a variety of reflectors, which delineate areas either of sediment transport or of deposition on an older river bottom. Records obtained with the Finger system made it possible to distinguish three main kind s of reflectors, both surface and sub-bottom: (1) flat, (2) wavy, and (3) irregular reflection patterns, which can be interpreted as strata deposited in a variable, mostly high energy setting, or reworked struc tures of initially flat or wavy strata that have been changed. Mapping of these different reflectors shows that they are not randomly distri buted. The constant association of a given reflector with a particular morphological setting is -indicative of its lithological significance (supported by riverbed sampling), and erosional/depositional processe s. Depositional areas are characterized by transparent units consistin g of soft silt overlying a highly reflective sub-bottom, related to ol d substrates (Alter do Chao or Barreiras Formation). This may result f rom inundation and incision caused by Quaternary sea-level changes. Pr eviously eroded areas are covered by a wavy layer consisting of very f ine to fine sand (unit FS), which can change locally to medium-coarse sand (unit MS). Reflectors are more abundant downstream. A minimum of 3 units were indicated by seismic records and could be related to sea level fluctuations and changes in the river discharge. Moreover, the d etection of fault scarps, interpreted to result from neotectonic activ ity, shows that neotectonic features can be important locally and shou ld not be ignored as a possible reason for modern river instability. T he dynamic setting of the lowermost Amazon area results in patterns of deposition and erosion probably controlled by complex interactions of fluvial and oceanic processes. The effect of the Gurupa arch may resu lt in a zone of sediment bypass in the central part and a zone of incr eased deposition in the distal downstream part. (C) 1998 Elsevier Scie nce B.V. All rights reserved.