Origin and palaeo‐environments of calcareous sediments in the Moshaweng dry valley, southeast Botswana

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 SiO 2 content (and increasing CaCO 3 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 CC -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 stage dissolution of CaCO 3 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.

Application of Object Based Classification and High Resolution Satellite Imagery for Savanna Ecosystem Analysis

“…African savannas in particular are projected to be under risk of extensive change largely due to changes in climate patterns [9], which may exacerbate the challenges presently facing humans living in this region. The discussion of ecological change in African savannas focuses on shifts in tree and shrub cover, specifically the decline in tree cover and change in spatial arrangement of trees, which impacts the productivity of the system, modifies availability of resources for both wildlife and humans, and could have large impacts on earth-atmosphere interactions [10][11][12]. It is thus imperative that the quantification and characterization of tree canopies in savannas is improved to inform management policies aimed at ensuring sustainable use of resources in savanna regions and to better understand the impact of changes in savannas at multiple scales [6,13].…”

Origins and palaeo-environments of Kalahari duricrusts in the Moshaweng dry valleys (Botswana) as detected by major and trace element composition

“…Detailed descriptions of the sampled morphological surfaces are given in Ringrose et al (2002). Ground level varies from 1100 to 1150 m a.s.l.…”

“…One hundred and fifty major element analyses of duricrusts from Letlhakeng (25°04 0 -25°05 0 E; 24°08 0 -24°09 0 S) yielded SiO 2 contents between $4-86 wt% and CaO abundances in the range $5-53% (Gwosdz and Modisi, 1983). Morpho-structural features of the Moshaweng duricrusts are described by Ringrose et al (2002). The fossil valleys are up to 1 km wide with vertical duricrust cliffs up to $7 m. Waterberg Group conglomerates outcrop in the area leading to the deposition of extensive gravel trains which are the source of calcified and loose quartz pebble conglomerates.…”

“…The fossil valleys are up to 1 km wide with vertical duricrust cliffs up to $7 m. Waterberg Group conglomerates outcrop in the area leading to the deposition of extensive gravel trains which are the source of calcified and loose quartz pebble conglomerates. Thermoluminescence (TL) dates were obtained from two samples from the lower terrace at 1075 m from location C1 (Ringrose et al, 2002). The TL age date for quartz particles embedded in calcrete is 254,442 ± 19,682 ka years BP.…”

“…A genetic link between calcretes and silcretes is required by their field relationships (e.g. Ringrose et al, 1999Ringrose et al, , 2002 but the processes involved during the deposition of intergrade duricrusts and associated calcretes, particularly with respect to their trace element composition, are not fully understood. The objectives of this paper are: (1) to present major and trace element compositions of duricrusts exposed along the Moshaweng dry valleys, in the southern Kalahari (Botswana).…”