Browsing Faculty of Science by Author "Vink, B.W."
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Ringrose, S.; Kampunzu, A.B.; Vink, B.W.; Matheson, W.; Downey, W.S. (John Wiley & Sons, Ltd. http://www3.interscience.wiley.com/journal/117935722/grouphome/home.html, NaN, 2002)[more][less]
Abstract: Quaternary sedimentation in the Moshaweng dry valley of southeastern Botswana is evaluated on the basis of geomorphological evolution and sedimentological analyses. Stratigraphic evidence reveals an 'upper surface (1095 m) containing abundant sil-calcrete, an intermediate surface (1085 m) in which sil-calcrete underlies nodular calcrete and lower (1075 m) surface in which sil-calcrete and nodular calcrete are interbedded. This subdivision is reflected in the geochemical composition of the sediments which show an overall trend of decreasing Si02 content (and increasing CaC03 content) with depth from the highest to the lowest surface levels. The calcretes and sil-calcretes represent modifications of pre-existing detrital Kalahari Group sand and basal Kalahari pebbles which thinned over a Karoo bedrock high. Modification took place during wet periods when abundant Ca++ -rich groundwater flowed along the structurally aligned valley system. With the onset of drier conditions, water table fluctuations led to the precipitation of nodular calcretes in the phreatic layer to a depth of about 20 m. A major geochemical change resulted in the preferential silicification of the nodular calcrete deposits. Conditions for silica mobilization may be related to drying-induced salinity and in situ geochemical differentiation brought about by pebble dissociation towards the top of the sediment pile. As calcretization and valley formation progressed to lower levels, silica release took place on a diminishing scale. Thermoluminescence dating infers a mid-Pleistocene age for sil-calcrete formation suggesting that valley evolution and original calcrete precipitation are much older. Late st~ge dissolution of CaC03 from pre-existing surface calcretes or sil-calcretes led to the formation of pedogenic case-hardened deposits during a time of reduced flow through the Moshaweng system possibly during the upper or late Pleistocene. URI: http://hdl.handle.net/10311/310 Files in this item: 1
Ringrose_ESPL_2002.pdf (3.095Mb) -
Ringrose, S.; Harris, C.; Huntsman-Mapila, P.; Vink, B.W.; Diskins, S.; Vanderpost, C.; Matheson, W. (Elsevier www.elsevier.com/locate/sedgeo, NaN, 2009)[more][less]
Abstract: Trace elements togetherwith some O and C isotope analysiswere undertaken on duricrust strandline deposits in the palaeo-Makgadikgadi sub-basin (PMSB) to provide insight into palaeo-climatic conditions through the interpretation of calcrete, silcrete–calcrete intergrade and silcrete deposits. Trace element content and relative abundance suggest that the duricrust origins are associated with the long-term weathering of the Karoo Large Igneous Province which underlies the PMSB. This work shows that duricrust origins are related to Ca2+ and Si (and associated trace elements) being transported mainly through the groundwater and then subsequently precipitated at different strandline elevations over time. Local groundwater feeding in towards the pan margin and accumulating in near-neutral pan-marginal pools, appears to facilitate Si concentration and permeation of pre-existing calcretes. The silica precipitates as the pH drops when renewed freshwater enters the pools. Hence the inferred palaeo-climatic regime for silcretisationmay be similar to that occurring in Botswana at present being dry semi-arid with lowseasonal rainfall. In contrast the extensive calcrete precipitation in the strandlines results from abundant Ca2+ in adjacent waters which appear to be derived from both local and regional sources. The arrival of Ca2+ from regional sources (shown by trace element evidence) infers heavy rainfall in the upper catchment suggesting a major humid event followed by regional drying. Palaeo-climatic inferences suggest the juxtaposition ofmajor humid events interspersedwithmore normal semi-arid palaeo-climateswith an exception obtained from isotope data, of drier and cooler conditions than usual for the region around 80–90000 years ago. Whereas trace element data can greatly assist in the interpretation of complex deposits such as duricrusts, care should be taken over the use of particular ratios (such as Yb/Gd ratio) which may produce spurious results. URI: http://hdl.handle.net/10311/560 Files in this item: 1
RINGROSE2009Makgadikgadi Pans.pdf (3.500Mb) -
Ringrose, S.; Huntsman-Mapila, P.; Kampunzu, A.B.; Downey, W.; Coetzee, S.; Vink, B.W.; Matheson, W.; Vanderpost, C. (Elsevier www.elsevier.com/locate/palaeo, NaN, 2005)[more][less]
Abstract: This work considers new evidence for palaeo environmental change taking place during the Pleistocene in northern Botswana. Duricrusted strandlines along the northeastern margin of Sua Pan provide palaeo-environmental data pertaining to the Makgadikgadi subbasin (MSB) with inferences regarding the larger Makgadikgadi–Okavango–Zambezi (MOZ) rift depression. Field, XRD and geochemical data show that MSB strandlines comprise calcretes (LU1 type), MgO-rich calcretes with silica (LU2 type), sil-calcrete (LU3 type) and silcrete (LU4 type). Early freshwater episodes appear to have been followed by calcrete-dominated drying phases interspersed with repeated silcretisation. Calcretisation through pan littoral sediments may have been both biogenically and environmentally induced. Calcite precipitation was in part controlled by the Mg/Ca ratio of pore water in the pan littoral zone suggesting closed basin type evaporative conditions, which were followed by a major desiccation interval. Phases of silcrete precipitation appear to be related to periods when the geochemistry of the lake littoral more closely resembled present-day Na–CO3–SO4–Cl-type brines. Silica saturated acidic, moderately saline groundwater preceded Si precipitation which took place as the pH reduced. Si mobilisation occurred (inter alia) as a result of quartz grain dissolution enhanced by diatoms, bacteria and algal growth in the moist pan littoral. SiO2-rich pore waters migrated through cracked and desiccated calcrete into areas of lower salinity and lower pH resulting in preferential calcite removal and silcrete precipitation. Approximate TL dates imply that exposed littoral sand underwent calcretisation during the drying phases of extensive palaeo-lakes which occurred prior to 110 ka, 80–90 ka and 41–43 ka. These wet periods compare fairly well with Vostok core chronologies for southern Africa. URI: http://hdl.handle.net/10311/794 Files in this item: 1
Kampunzu2006Sedimentological.pdf (3.498Mb)
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