Petrology, Chemistry, and Texture of Modern River Sands, Amazon River System

Abstract

Key Idea: Following the spirit of P. D. Krynine, the sands of the world's largest tropical river system are studied to reveal much about the general origin of sand and sandstone. 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, reveals 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 downstream along the main stem of the Amazon-Solimões-Marañon 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 rock fragments, and even possible weathering on the modern flood plain of the Amazon contribute to enhanced maturity downstream. We speculate that, although data on modern, large subartic and temperate rivers are scant, unlike large tropical rivers, they do not, as a rule, have a marked downstream increase in mineralogic maturity. River sands of Precambrian watersheds are coarser than those derived directly from either the Andes or from those derived from Tertiary molasse for two probable reasons: Precambrian granites have coarser quartz than the reworked quartz of a major geosyncline and, secondly, tropical weathering eliminates quartzo-feldspathic rock fragments much more rapidly than quartz.