Dual mechanisms for the formation of fluvial silcretes in the distal reaches of the Okavango Delta fan, Botswana

Abstract

Silcretes exposed within river-marginal or valley settings have been described in a number of studies, but few models have been suggested for the development of these 'fluvial' silcretes. An exception is that proposed by McCarthy and Ellery (Journal of Sedimentary Research, 1995, Vol. A65, pp. 77-90) to describe mechanisms of early stage near-surface silica diagenesis in the Okavango Delta, Botswana. This paper describes the characteristics and possible origins of massive surface and sub-surface silcretes at Samedupe and Boro Junction, beyond the distal margin of the Okavango Delta and further downstream than the sites described by McCarthy and Ellery. Morphological and petrological evidence from surface exposures and three sedimentary cores suggests that other modes of formation may also be applicable. A dual model of formation is proposed: surface silcretes are suggested to have developed by silica accumulation in seasonal pools remaining after the annual Okavango flood, whilst sub-surface horizons appear to have formed under conditions of varying pH associated with fluctuating groundwater levels beneath the channel floor. This model is reviewed in the context of the fluvial silcrete debate.

Epigenetic silicification of the Upper Oxfordian limestones in the Sokole Hills (Kraków-Częstochowa Upland): relationship to facies development and tectonics

“…Ullyott and Nash 2006), under phreatic conditions, presumably close to the fluctuating zones of mixing of ascending solutions with groundwaters (Gradziński et al 2011;cf. Thiry et al 1988;Thiry 1997;Shaw and Nash 1998;Nash and Ullyott 2007;Bustillo et al 2013). Silica leached from the Albian quartz sands might have migrated along the joints and faults (cf.…”

Going the distance: Mapping mobility in the Kalahari Desert during the Middle Stone Age through multi-site geochemical provenancing of silcrete artefacts

“…≠Gi lies at the westernmost edge of this area. Samples were collected additionally at multiple sites within the extensive silcrete localities along the Boteti River and around Lake Ngami to the south of the Okavango Delta (Mallick et al, 1981;Shaw and Nash, 1998). All sampling sites were in primary context; unlike the Cape coastal zone of South Africa, the extremely low gradient of the Kalahari landscape precludes the large scale erosion of outcrops and, hence, the development of secondary sources at distances from their origin.…”

Geomorphic Landforms and Tectonism Along the Eastern Margin of the Okavango Rift Zone, North Western Botswana as Deduced from Geophysical Data in the Area

“…The orientation of the faults on the surface is influenced by pre-existing faults and folds of the basement rocks Kinabo et al, 2007;2008). The MOZ basin comprised of both alluvial fan deposits and deeper palaeo-lake sediments in structural depressions or sub-basins Shaw and Nash 1998). This rift basin hosts the largest in-land alluvial fan on Earth, supporting the largest (~18 000 km 2 ) wetland in southern Africa (McCarthy et al, 1991).…”