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An orientation survey was undertaken in northwest Ngamiland to evaluate, among other criteria, an optimum size fraction for trace element analysis in the Kalahari sandveld regolith samples for a regional geochemical mapping project in the area. The most prominent geomorphological features of the region are the longitudinal rolling sand dunes and floodplain sediments adjacent to the unique, mid-continent Okavango Delta. Most of the dune sands were deposited in the Late Quaternary, and they show evidence of both pluvial and arid environments. Presently the a1ab dunes support varied shrub/grass vegetation with intermittent tree cover. Extensive forest fires have affected the region for generations, and the occurrences of remnants of large trees indicate that the region may have also supported large forests at some stage.

The sandveld regolith comprises 70% quartz sand (-1000 + 63 mu m) and 10-20% silt/clay ( < 63 mu m) material. There are no size variations between material from dune crests, dune depressions and down the soil profile to a depth of a metre. The process of regolith formation is relatively young and dynamic, being widely churned by a variety of boring animals, termites, ants and other insects. Hence no distinct zonation of soil horizons in the top 100 cm of the soil catena were observed. Remarkably, the significant proportion of the silt/clay traction in the regolith indicates progressive weathering and possible deposition of precipitates.

Trace element contents are invariably enhanced in the fine fractions, a characteristic widely observed elsewhere in residual soils. On the Xaxa-Cheracheraha traverse Zn values range from 2 to 98 ppm (mean 9 ppm); Pb varies from 1 to 101 ppm (mean 27 ppm) and Cu from 0 to 18 ppm (mean 5 ppm) in the -75 mu m fraction. Contents in the -180 + 75 mu m are, however, significantly lower than in the fine fractions. The variations in element concentrations along each of the orientation survey traverses at Qangwa-Nokaneng, Nxaunxau-Qurube and Nxaunxau-Ghane, in the coarse and fine fractions, generally remain sympathetic, showing that the enhanced values in the fines are a result of secondary dispersion, rather than mere compositional differences in the sandveld regolith. (C) 2000 Elsevier Science Limited. All rights reserved.