Petrology, geochemistry and environmental significance of silcrete-calcrete intergrade duricrusts at Kang Pan and Tswaane, Central Kalahari, Botswana

摘要

Calcrete-silcrete intergrade duricrusts are an important component of the Kalahari Group sediments of central southern Africa and yet have neither been analysed systematically nor in any detail. In this study, the petrological and geochemical characteristics of suites of calcrete, silcrete and intergrade duricrusts from two fresh, relatively deep exposures at Kang Pan and Tswaane (adjacent to the Okwa Valley) in the central Kalahari, Botswana, are described. The duricrust profile at Kang Pan consists of a highly indurated crystalline non-pedogenic calcrete which has been extensively silicified by chalcedony, and. rarely. cryptocrystalline silica or microquartz. Silicification is most extensive in lower parts of the profile, where replacement is related to groundwater fluctuations, and in upper sections due to periodic flooding by ephemeral surface water. The exposure at Tswaane consists of a sequence of pale green glauconitic non-pedogenic silcrete and cal-silcrete overlain by non-pedogenic calcrete, all of which have formed within sediments situated upon granitoid-gneiss bedrock. The siliceous duricrusts are dominated by cryptocrystalline silica cements and appear to have developed through the replacement of a preexisting non-pedogenic calcrete. These siliceous duricrusts have also been calcified at a later date during the formation of the overlying calcrete to produce a complex range of silica-carbonate cements. At both sites, the style and type of silicification present appears to be determined by the duration of wetting and the permeability of the precursor calcrete. Geochemical evidence indicates a lack of chemical weathering profile development within the granitoid-gneiss bedrock and considerable differences between the chemical signature of the bedrock and combined duricrusts from Kang and Tswaane. This suggests that bedrock made a minimal contribution in terms of silica and carbonate species to duricrust formation, and that the majority of cementing agents were non-local. It would therefore appear likely that the geomorphological context of each site had a major influence upon the development of calcrete, silcrete and intergrade duricrust cements. Copyright (C) 2004 John Wiley Sons. Ltd.