BGR Bundesanstalt für Geowissenschaften und Rohstoffe

Botswana: The Paleohydrogeology of the Okavango Basin and Makgadikgadi Pan in the Light of Climate Change and Regional Tectonics

Country / Region: Botswana

Begin of project: May 1, 2007

End of project: December 31, 2008

Status of project: December 31, 2008


In Southern Africa large intra-continental depressions (Fig. 1) form nowadays the base level of huge hydrological catchments which are dewatered by mostly ephemeral streams and few persistent rivers. Being former basins of large freshwater lakes and representing therefore ancient recharge areas, these large depressions are receiving an increasing interest as they may have the potential to contain groundwater resources that could be used as drinking water for an increasing population. To understand the recent, and sometimes surprising, discoveries of deep lying, fresh groundwater resources it is therefore necessary to investigate beside of geological structures also the paleohydrogeological conditions and the paleohydrology of these lakes. This needs to be addressed not only under the perspective of climatic change but also considering regional tectonic events.

Fig. 2: Correlation of lake levels and beach barriers with the calcrete depositsFig. 2: Lake levels, beach barriers Source: BGR

In the late Pleistocene the Okavango Delta and the Makgadikgadi Pan in Botswana were covered by lakes reaching levels of 945 m, 936 m, 920 m and 912 m asl which corresponds to water depths of locally more than 60 m. Accordingly, the analysis of lithological borehole logs in the vicinity of the lakes revealed the occurrence of calcretes, indicating that former groundwater levels must have been much higher than in the present. In a first approach all lake levels cited in the literature were compared with results obtained by a digital elevation model (DEM) and transferred into a geographic information system (GIS). During a second step the different lake levels in combination with their barrier beaches and large sand dunes were localised on satellite images (Landsat TM 5). They are clearly visible (Fig. 2) and correlate quite well with literature data. At few locations a false colour analysis of the satellite images revealed consistent boundaries between lake sediments and the barrier beaches on their fringes. The different soil types can be clearly identified on these satellite images and show sharp boundaries between salt soils in the Ntwetwe und Sua Pan (Fig. 2), which lies in the Makgadikgadi Pan, and sediments outside the pan. Finally, all 14C- and luminescence dating of calcretes cited in the literature were compiled and transferred into a GIS.

Already during the early Pleistocene, tectonic movements formed a depression which lead to the nowadays Makgadikgadi Pan. The paleohydrology was controlled by the Okavango- and Zambezi River and the high level stage on 945 m asl developed between 40,000 and 35,000 BP, before the connection with the prominent Zambezi water course was disrupted. Tectonic displacements along NE – SW trending faults lead to a down-warping of the Okavango Delta and split the lake into two basins. One of these lakes was Lake Thamalakane on 936 m asl, which later developed slowly into the present deltaic system. The other remnant lake was located in today’s Makgadikgadi Pan and oscillated between 920 m and 912 m asl. Both lakes were fed by the Okavango and were connected by the Boteti River overflow (Fig. 3).

Fig. 4: Compilation of lake levels known from literature, of the paleoclimate and tectonics in the region examined during the last 45,000 yearsFig. 4: Lake levels, paleoclimate, tectonics Source: BGR

During the Holocene both lakes dried up gradually. Since a direct correlation of the lake levels with South African paleoclimate records is difficult, the influence of over-regional tectonic movements for the history of the lakes becomes more and more evident (Fig. 4). The ancient lakes had obviously a vast catchment which was gradually cut off with time. As the groundwater levels in the past were up to 100 m above the present levels the influence of the ancient lakes on the groundwater recharge was important.

The lithological analysis of borehole data proved that lake water infiltrated into the Ntane Sandstone which forms, due to its intense fracturing, the main aquifer in this region. Hence, the lakes were not only the hydraulic base level for the surrounding huge catchments but served also as important recharge areas. An analysis of soil types revealed that in some areas infiltration was rather more supported than hampered (Fig. 5). Various calcrete layers in borehole profiles indicate that the ancient groundwater level must have been much higher than today. The decline of the groundwater table to the actual level was presumably directly coupled to the slowly desiccation of the lake in the Makgadikgadi Pan.





Literature:

Partner:

  • Department Geological Survey Botswana
  • Leibniz Universität Hannover, Institute of Soil Science (Prof. Dr. Jürgen Böttcher)

Contact:

    
Prof. Dr. Thomas Himmelsbach
Phone: +49-(0)511-643-3794
Fax: +49-(0)511-643-3661

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