Composition and trace-element geochemistry of detrital clay and heavy-mineral suites of the lowermost Amazon River: A provenance study

Compositions of clays and heavy minerals as well as trace element geochemis try of the lowermost Amazon sediments reflect source regions and the intens e chemical weathering conditions. Illite and montmorillonite indicate mount ainous sources under physical weathering conditions, whereas kaolinite is r elated to the soil composition under humid tropical conditions, where chemi cal weathering predominates. The suite of unstable heavy minerals, on the o ther hand, reflects some sediment transport from the Andes, more supply fro m tributaries draining the basic and metamorphic rocks from the South easte rn Guiana Shield, and strongest influence of the upstream Amazon and Solimo es areas. These source signals are damped by transitional storage and inten se 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 ref lect recycling, Rare earth and trace-element data from the clay and heavy-m ineral fractions pro vide additional clues to the source areas: in the down stream direction, superimposed upon provenance of sediments from an undiffe rentiated are, the Andes, is provenance from strongly weathered continental crust, the Precambrian shields, as well as sedimentary recycling. Small-sc ale provenance can be delineated by the most immobile elements, Heavy rare earth elements in the heavy-mineral fraction increase downstream toward Est reitos (to the Para River).