Lowermost Amazon River: morphology and shallow seismic characteristics.

Vital, Helenice, Stattegger, Karl, Posewang, Jörg and Theilen, Friedrich (1998) Lowermost Amazon River: morphology and shallow seismic characteristics. Marine Geology, 152 (4). pp. 277-294. DOI 10.1016/s0025-3227(98)00099-1.

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The riverbed morphology of the lowermost Amazon River was characterized by high-resolution acoustic profiling in 21 areas. This technique has 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 Pinger system made it possible to distinguish three main kinds 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 structures of initially flat or wavy strata that have been changed. Mapping of these different reflectors shows that they are not randomly distributed. 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 processes. Depositional areas are characterized by transparent units consisting of soft silt overlying a highly reflective sub-bottom, related to old substrates (Alter do Chão or Barreiras Formation). This may result from inundation and incision caused by Quaternary sea-level changes. Previously eroded areas are covered by a wavy layer consisting of very fine 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 detection of fault scarps, interpreted to result from neotectonic activity, shows that neotectonic features can be important locally and should not be ignored as a possible reason for modern river instability. The 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 result in a zone of sediment bypass in the central part and a zone of increased deposition in the distal downstream part.

Document Type: Article
Keywords: seismic reflection Amazon River fluvial processes riverbed morphology sea-level fluctuations
Research affiliation: Kiel University
Refereed: Yes
DOI etc.: 10.1016/s0025-3227(98)00099-1
ISSN: 0025-3227
Date Deposited: 02 Feb 2012 09:17
Last Modified: 28 Mar 2018 10:59
URI: http://oceanrep.geomar.de/id/eprint/16376

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