AREIAS RECENTES DOS RIOS DA BACIA AMAZONICA: COMPOSIÇÕES PETROGRÁFICA, TEXTURAL E QUÍMICA

ELENA FRANZINELLI, PAUL EDWIN POTTER

Resumo


Thin section and chemical study of 95 sand samples from the Amazon River system in Brazil, Peru, Ecuador, and Colombia, a river system with great contrasts in climate and rock types, reveal that sands derived from the Andes are lithic arenites whereas sands derived from Precambrian terrains are much richer in quartz, as are sands in rivers draining Tertiary molasse. Sand becomes more mature dowstream along the main stream of the Amazon-Solímões-Marañón so that a sample analyzed at the mouth does not accurately reflect headwater composition. Dilution by quartz-rich tributaries and quartz-rich Tertiary outcrops, possible differential elimination of roch fragments, and even possible weathering on the modem plain of the Amazon contribute to enhanced maturity downstream. River sands of Precambrian watersheds are coarser than those derived dírectly from either the Andes of from those derived from Tertiary molasse for two probable reasons: Precambrian granites have coarser quartz than the reworked quartz of a major geoyncline and, secondly, tropical weathering eliminates quartz-feldspathic rock fragments much more rapidly than quartz.

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