Variability in bulk geochemistry of fluvial terrace sands: consequences for the study of weathering chronosequences

A comparison of bulk geochemistry of alluvial terrace sands from three
basins differing in basin lithology and/or climate has been made, using
factor analysis to study the behaviour of individu al element groups, related
to specific minerals. In the Allier basin in the French Central Massif
underlain by vo1canic and basement rocks temporal changes in the composition
of the supplied sediment outweigh compositional changes due to weathering. In the Dore basin, in the same region, consisting of basement rocks essentially, weathering outweighs compositional change in Quaternary terraces, but older quartz-rich Plio-Pleistocene sediments are still clearly individualized geochemically. In the Caquetá Basin in the Colombian Amazones weathering affected immature terrace sediments of Andean provenance even in low terraces to such an extent that they are difficult to distinguish from quartz rich Tertiary sediments of Amazonian provenanee. Parent material effects, th erefore , must be taken into consideration when studying chronosequences in fluvial terraces.