Eolian influence on soils developed in a chronosequence of basalts of Victoria, Australia

Abstract

Development of five soils in basalt flows of ages ranging from 6,200 to about 4 million years in Victoria, Australia was investigated in conjunction with a radioisotope retention study reported elsewhere. The particle size distribution became coarser with the progressive development of concretion-like, amorphous, ferruginous nodules in the A horizons (up to 46% of the soil greater than 2 mm) with increasing age of the basalt. In contrast, dithionite extractable Fe 2O 3 decreased in the surface horizons. The quartz contents of the surface soils varied (7 to 45%) with geographic proximity to eolian source of the quartz and the age of the basalt in which the soil was formed, even though the basalt was shown to be quartz-free both petrographically in thin section and by the absence of quartz in the H 2SiF 6 residues of the rocks. This is in keeping with the near absence of quartz in the argillaceous C horizons of the soils highest in surface soil quartz. The quartz is predominantly concentrated in the fine sand to coarse silt (250-20 μm) range, in contrast to known aerosolic quartz sizes (10-1 μm). Eolian origin of the quartz blown from beach ridges and quartz sand sheets a few km to the west (windward) is indicated by the remarkable constancy of the oxygen isotope abundance in the provenance of δ 18O = 14.2 to 14.80/00 in the medium silt (20-5 μm diameter) separated not only from the soils developed in the basalts but also from surface horizons of a laterite paleosol and sandy soils of the region, all eolian-influenced. Fluvial admixture of the quartz appears to be limited, in view of the topography and geomorphology.