Geoscience Reference
In-Depth Information
Figure 18.8. Generalised cross section across southern Africa. (After Thomas and
Shaw,
1993
.)
coastal plains (Thomas and Shaw,
1993
)(
Figure 18.8
). The Kalahari Basin is an
internally drained structural depression, with the major rivers flowing from the Angola
highlands in the north-west down to the Okavango and Makgadikgadi Basin described
in
Chapter 11
. The Kalahari sand sea covers about 2.5 million km
2
and is the largest
sand sea on earth.
Measurements of
10
Be and
26
Al indicate geological erosion rates in the hyper-arid
Namib Desert ranging from about 3 to 5 m/Ma, consistent with minimal change in the
Namib margin since the Eocene (Fujioka and Chappell,
2011
). The Namib Desert lies
between the Namaqua Highlands and the coast. This coastal desert is about 2,000 km
long and 200-300 km wide. It owes its extreme aridity to three main factors. First,
the presence offshore of the cold Benguela Current and associated cold upwelling
coastal waters means that, apart from coastal fog, there is very little precipitation from
westerly air masses, for the reasons outlined in
Chapter 2
. Second, the semi-permanent
anticyclone located over the south-east Atlantic is associated with subsiding air and
minimal convection. Third, the Namib Desert lies in the rain shadow of the Namaqua
Highlands and therefore receives almost no rain from easterly sources.
As Siesser (
1978
) pointed out, the inception of extreme aridity in the Namib was
determined by the onset of strong coastal upwelling. Sediment accumulation rates in
marine cores off the coast of Namibia increased rapidly in the late Miocene, about
10 Ma ago, as did diatom productivity, isotopic evidence of colder temperatures and a
sudden increase in phosphate formation (Siesser,
1978
). All of these phenomena are
consistent with enhanced upwelling, although there are some indications of earlier
mild coastal upwelling starting in the late Oligocene. There is persuasive evidence
that the progressive build-up of ice in Antarctica was the primary agent controlling the
location and strength of the cold Benguela Current, with maximum ice accumulation
in Antarctica at the end of the Miocene coinciding with a substantial increase in
the upwelling of the Benguela Current (Siesser,
1978
;Dingleetal.,
1983
; Coetzee,
1980
).
Shackleton and Kennett (
1975
) were the first to identify a major drop in Southern
Ocean temperature at the Eocene-Oligocene boundary based on isotopic analysis of
