Abstract

Compositions of clays and heavy minerals as well as trace-element geochemistry of the lowermost Amazon sediments reflect source regions and the intense chemical weathering conditions. Illite and montmorillonite indicate mountainous sources under physical weathering conditions, whereas kaolinite is related to the soil composition under humid tropical conditions, where chemical weathering predominates. The suite of unstable heavy minerals, on the other hand, reflects some sediment transport from the Andes, more supply from tributaries draining the basic and metamorphic rocks from the southeastern Guiana Shield, and strongest influence of the upstream Amazon and Solimoes areas. These source signals are damped by transitional storage and intense lateral sediment exchange between channel and floodplain. The Brazilian Shield contribution (via the Xingu River) seems to be of minor importance, although the Alter do Chao and the Barreiras formations, which are products of erosion of the shields, supply suites of stable heavy minerals that reflect recycling. Rare-earth and trace-element data from the clay and heavy-mineral fractions provide additional clues to the source areas; in the downstream direction, superimposed upon provenance of sediments from an undifferentiated arc, the Andes, is provenance from strongly weathered continental crust, the Precambrian shields, as well as sedimentary recycling. Small-scale provenance can be delineated by the most immobile elements. Heavy rare earth elements in the heavy-mineral fraction increase downstream toward Estreitos (to the Para River).

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