Abstract

Modification of sediment composition by chemical weathering depends both on the overall intensity of weathering and on its duration. At Macuapanim Island, Solimboes River (Amazon main channel), Brazil, a number of alluvial deposits have been exposed to intense tropical weathering for differing lengths of time. In the very fine sand fraction, the detrital component shows a steady and marked increase in compositional maturity with increasing sediment age: immature sands of arkosic to litharenitic composition show a progressive loss of lithic fragments, feldspars, and accessory minerals, with a concomitant enrichment in quartz. This process produces first-cycle sands whose detrital component is of quartz arenite composition. Among the salts, quartz is enriched over time relative to more unstable phases, particularly plagioclase. This enrichment is probably largely caused by the destruction of plagioclase through chemical weathering, but silt-size quartz may also be "produced" through the breakdown of sedimentary and metasedimentary lithic fragments in the sand fraction. In the clay fraction, an increase in the relative abundance of smectite accompanies the breakdown of volcanic lithic fragments in the sand fraction, and probably reflects the degradation of volcanic glass. The clay fractions of the oldest samples examined contain less smectite, somewhat more kaolinite, and markedly more vermiculite as compared to younger samples, but an extensively-leached, cation-poor clay fraction analogous to the quartz-dominated silts and sands is never attained. The compositional maturity of the clay fraction is everywhere lower than that of associated silts and sands, perhaps because of a continual influx into the clay fraction of immature cation-rich clays derived from the physical breakdown of lithic fragments in coarser size fractions. This effect is enhanced by the translocation of fines, whereby younger, less-weathered fine material is juxtaposed with older, more mature coarse material in the lower levels of the alluvial stratigraphy. Reworking of floodplains during channel migration results in the reincorporation of older material stored on the floodplains into the sediment load. If sediments have been intensely weathered during storage on the floodplain, then the result will be a net dilution and replacement of younger, relatively immature sands transported by the river by older, more mature material from the floodplain. The relative proportion of the older sands in the bedload will increase downstream, resulting in a progressive downstream increase in the compositional maturity of the bedload of the river.

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