Research articles (Dept of Geology)
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Huntsman-Mapila, P.; Ringrose, S.; Mackay, A.W.; Downey, W.S.; Modisi, M.; Coetzee, S.H.; Tiercelin, J.-J.; Kampunzu, A.B.; Vanderpost, C. (Elsevier, www.sciencedirect.com, NaN, 2006)[more][less]
Abstract: Sediment samples from a continuous 4.6m profile in the dry bed of Lake Ngami in NW Botswana were analysed for geochemistry and dated using both 14C and TL methods. Certain units in the profile were found to be diatom rich and these, with the geochemical results, were used as indicators of high and low lake levels within the basin. The Lake Ngami sediments contain a high proportion of SiO2 (51–92.5 wt%, avg. 72.4 wt%) and variable levels of Al2O3 (2.04–17.2 wt%, avg. 8.88 wt%). Based on elevated Al2O3 and organic matter (LOIorgC) results, lacustrine conditions occurred at ca. 42 ka until 40 ka and diatom results suggest that relatively deep but brackish conditions prevailed. At 40 ka, the lacustrine sedimentary record was terminated abruptly, possibly by tectonic activity. At ca. 19 ka, shallow, aerobic, turbulent conditions were prevalent, but lake levels were at this time increasing to deeper water conditions up until ca. 17 ka. This period coincides with the Late Glacial Maximum, a period of increased aridity in the central southern Africa region. Generally, increasing Sr/Ca ratios and decreasing LOIorgC and Al2O3, from ca. 16 to 5 ka, suggest decreasing inflow into the basin and declining lake levels. Based on the enrichment of LREE results, slightly alkaline conditions prevailed at ca. 12 ka. Diatom results also support shallow alkaline conditions around this time. These lake conditions were maintained primarily by local rainfall input as the region experienced a warmer, wetter phase between 16 and 11 ka. Lake levels rose rapidly by 4 ka, probably in response to enhanced rainfall in the Angolan catchment. These results indicate that lake levels in the Lake Ngami basin are responding to rainfall changes in the Angolan catchment area and local rainfall. The results confirm that the present-day anti-phase rainfall relationship between southern Africa and regions of equatorial Africa was extant during the late Quaternary over the Angolan highlands and NW Botswana. URI: http://hdl.handle.net/10311/891 Files in this item: 1
Modisi2006SedimentaryRecord.pdf (1.674Mb) -
Alemaw, B.F.; Shemang, E.M.; Chaoka, T.R. (Elsevier Science Ltd, http://www.elsevier.com/locate/pce, NaN, 2004)[more][less]
Abstract: Botswana is a semi-arid country with limited recharge to groundwater resources. Groundwater however is a major source of water for domestic, industrial and agricultural uses. There is therefore the need to ensure that the groundwater resources are of drinking quality, by protecting them against pollution sources such as landfills, pit latrines, industries, agricultural pesticides and waste. In this study a preliminary assessment of vulnerability of groundwater to pollution in Kanye well field in SE Botswana is undertaken. In order to assess the vulnerability of this aquifer to pollution, soils types, mapped geology and information from 82 boreholes were input into a GIS system to classify pollution vulnerability using Theissen polygons of the study area. Results indicate that 58% of the well field area is highly or very highly vulnerable to pollution, 34% is moderately vulnerable to pollution, and only 9% has low vulnerability. Estimates of the aquifer media characteristics in areas with only a few observations or few geological logs were considered to have higher uncertainty than areas with many observations. However, the use of the polynomial estimator enabled consideration of simple uncertainty for the aquifer media characteristics and pollution vulnerability modelling process. URI: http://hdl.handle.net/10311/888 Files in this item: 1
Alemaw_PCE_2004.pdf (375.0Kb) -
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) -
Mapeo, R.B.M.; Armstrong, R.A.; Kampunzu, A.B.; Modisi, M.P.; Ramokate, L.V.; Modie, B.N.J. (Elsevier www.elsevier.com/locate/epsl, NaN, 2006)[more][less]
Abstract: The Segwagwa Group of southeastern Botswana, a correlate of the Pretoria Group of the Transvaal Supergroup of South Africa, consists of a major sequence of siliciclastic sedimentary rocks, minor carbonates and basaltic to andesitic lavas and tuffs straddling the Western and Central Domains of the Kaapvaal Craton. The Segwagwa Group unconformably overlies the Taupone Dolomite Group, a correlative of the South African Chuniespoort/Ghaap Groups of the Transvaal Supergroup. SHRIMP U–Pb analyses of 123 detrital zircons from the top, middle and bottom of the Segwagwa Group sedimentary rocks include 96 concordant to nearconcordant zircons defining three main age groups: >3.0–2.9 Ga (n=12), 2.8–2.5 Ga (n=27) and 2.45–2.20 Ga (n=57). The ≥2.90 Ga zircons were sourced from granitoids emplaced before and around 2915±12 Ma and are related to the amalgamation of the Western, Northern and Central Domains of the Kaapvaal Craton. Concordant zircons with a mean age of 2781±8 Ma originate from the Gaborone Igneous Complex. The detrital zircons in the range 2.7–2.5 Ga were likely sourced from the Kalahari continental fragment made up of the Kaapvaal Craton, Limpopo Belt and the Zimbabwe Craton, specifically from the Limpopo Belt and/or the Zimbabwe Craton where igneous rocks in this age range are widespread. The igneous sources for the Palaeoproterozoic (ca. 2.45–2.20 Ga) zircons are difficult to identify since igneous rocks in that age are not widely known or documented by reliable dates in the Kalahari Craton. The youngest zircons of ca. 2.2 Ga occur in all the sandstones and form the main group (N90%) in the sample from the top of the Segwagwa Group. The youngest detrital zircon of 2193±20 Ma sets the maximum time of deposition of the Segwagwa Group. Published data suggest that the minimum deposition age of Chuniespoort/Ghaap Group sedimentary rocks is 2431±31 Ma [D.R. Nelson, Compilation of SHRIMP U-Pb zircon Geochronological Data, 1996 Record 1997/2, pp. 189, Western Australia Geological Survey, 1997.]. Therefore, the unconformity between the Lower and Upper Transvaal represents a ∼200 Ma hiatus, and the lithostratigraphic units on the two sides of the unconformity should not be grouped in the same supergroup. Detrital zircon ages suggest that the time of deposition of the Segwagwa/Pretoria Group which ranges from ca. 2.40 to 2.20 Ga is coeval with the Palaeoproterozoic global glacial deposits in North America, Australia and Fennoscandia; and with sedimentary rocks from the Palaeoproterozoic Magondi Belt. Therefore, the Segwagwa/Pretoria Group and the Magondi metasedimentary succession were deposited during the first global glacial period, are possibly related to the same geodynamic cycle, and should be part of the same supergroup. URI: http://hdl.handle.net/10311/742 Files in this item: 1
Mapeo2006Transvaal Groups.pdf (1.993Mb) -
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)