News: Earthworks in progress

Gravity survey and the origin of mankind

The Kalahari occupies an exceedingly flat tableland taking up much of the central part of southern Africa.  When the national gravity survey of Botswana was mooted in 1970, absolute heights above seal level for this remote, dry and thinly-populated area were unavailable and precision primary levelling was still in progress.  Earlier levelling work in pursuit of potential engineering work on river courses (Du Toit, 1926; Brind, 1955) had proved inconclusive.  A by-product of the national gravity survey (Reeves & Hutchins, 1976) was the first rough topographic contour map of Botswana dervied from about 2500 heights determined by barometric altimeters tied to the few available reliable datum points.  The lowest point in the inland drainage basin in the Makgadikgadi salt flats was determined as 903 (+/- 5) metres.  Almost all of northern Botswana lay below 1000 m with the only escape for water from the annual flooding of the Okavango delta in the NW of the country at about 930 m, located near the administratrive centre of Maun.

More recently, Shuttle imaging radar (SIR) has determined the topographic surface with centimetre accuracy. Geomorphological studies, linked to SIR data, have defined old shorelines at 973 m outlining a lake that occupied most of northern Botswana - an area about half the size of France - in earlier pluvial times.  Archeological studies of these ancient shorelines have revealed copious stone tools, relics of a Stone Age civilisation living in a much more clement climate than today's (Moore & Larkin, 2001).  In a recent Nature publication (Chan et al., 2019) it is claimed that studies of mitochondrial DNA lead back to this earthly lakeshore paradise hosting the origin of Man.  Migratory pulses are interpreted to have left for east Africa 130 000 years ago, and for the south 110 000 years ago.

The Mwembishi fault zone (see previous news item, below) separtaes the ancient lake from the present-day watercourses of three large rivers, the Okavango, the Cuando and the Zambezi, all drawing their water from the Angola highlands that, even today, enjoy higher levels of rainfall than the Kalahari.  It would seem that movements on the Mwembishi shear zone in the Early Cretaceous set up the topogrpahy of southern Africa, much of which persists to the present day.  Perhaps we should all be grateful for that!

In the figure, Karoo rocks are indicated in beige and Kalahari sand cover in pale yellow.  International boundaries are in red and the paleo-lake is shaded in blue.

2019 October 31

Brind, W.G., 1955. The Okavango delta: report on the 1951-3 field surveys.

Chan, E.K.F., et al., 2019. Human origins in a southern Africa paleo-wetland and first migration.  Nature. https://doi.org/10.1038/s41586-019-1714-1

Du Toit, A, 1926. Report of the Kalahari reconnaissance of 1925.  Union of South Africa, Department of Irrigation.

Moore, A.E., and Larkin, P.A., 2001.  Drainage evolution in south-central Africa since the break-up of Gondwana.  South Africa journal of Geology, 104, 47-68.

Reeves, C.V. and Htuchins, D.G., 1976. The national gravity survey of Botswana, 1972-73.  Bulletin No. 3, Geological Survey Department, Botswana.

Gravity survey and the origin of mankind