DUAL MECHANISMS FOR THE FORMATION OF FLUVIAL SILCRETES IN THE DISTAL REACHES OF THE OKAVANGO DELTA FAN, BOTSWANA

Silcretes exposed within river-marginal or valley settings have been d escribed in a number of studies, but few models have been suggested fo r the development of these 'fluvial' silcretes. An exception is that p roposed by McCarthy and Ellery (Journal of Sedimentary Research, 1995, Vol. A65, pp. 77-90) to describe mechanisms of early stage near-surfa ce silica diagenesis in the Okavango Delta, Botswana. This paper descr ibes the characteristics and possible origins of massive surface and s ub-surface silcretes at Samedupe and Bore Junction, beyond the distal margin of the Okavango Delta and further downstream than the sites des cribed by McCarthy and Ellery. Morphological and petrological evidence from surface exposures and three sedimentary cores suggests that othe r modes of formation may also be applicable. A dual model of formation is proposed: surface silcretes are suggested to have developed by sil ica accumulation in seasonal pools remaining after the annual Okavango flood, whilst sub-surface horizons appear to have formed under condit ions of varying pH associated with fluctuating groundwater levels bene ath the channel floor. This model is reviewed in the context of the fl uvial silcrete debate. (C) 1998 by John Wiley & Sons, Ltd.