Geoscience Reference
In-Depth Information
impeded efforts to establish a coherent chronology of past climatic fluctuations. The
thoughtful review by Yang et al. (
2011b
) enlarges on these difficulties. Even when
the dunes contain pedogenic carbonates sandwiched between dune sands, indicative
of episodically wetter conditions, as in the Badain Jaran Desert, there are well-
known problems in obtaining reliable radiocarbon ages for pedogenic carbonates (see
Chapter 6
). One viable but very time-consuming option is to date the quartz grains
enclosed within the carbonate horizons using OSL (Singhvi et al.,
1996
). In the sandy
lands in the wetter eastern semi-deserts, the fossil soils within the now stable dunes
are red kraznozems (Yang et al.,
2011b
), which are comparatively rich in clay and,
presumably, contain some organic carbon (see
Chapter 15
). It should therefore be
possible to obtain a reliable AMS
14
C chronology for these paleosols in the future.
8.14 Desert dunes of the Namib and Kalahari
The Namib Desert covers 34,000 km
2
and extends from south to north for about
2,000 km along the west coast of southern Africa between 23
S. The
present-day sand sea is underlain by the Tsondab Sandstone Formation (TSF), a
fossilised sand sea of pre-late Miocene (Ward,
1988
) or Oligocene age (Besler,
1991
).
The TSF was eroded to a gently undulating surface during the late Miocene. Pliocene
uplift and fluvial incision coincided with the development of the modern sand sea. The
Kalahari Sands were mapped by Cooke (
1958
) and occupy a vast area extending from
beyond the Congo River in the north to the Orange River in the south (
Figure 8.13
).
Dingle et al. (
1983
, p. 293) suggested a tentative age of Mio-Pliocene for the oldest
of the Kalahari dunes. If all of the Kalahari Sands are indeed eolian, then they are
almost certainly diachronous in age, with the oldest sands forming first in the north
as southern Africa drifted northwards into dry tropical latitudes during the early to
middle Cenozoic, a topic discussed at greater length in
Chapter 18
.
Besler (
1983
;
1991
) and Lancaster (
1989
) have studied the morphology of the
Namib dunes and the processes responsible for their formation. However, relatively
few of the inland dunes have been dated by OSL. Lancaster and Teller (
1988
)have
described the four main types of inter-dune deposits in the Namib, comprising of
coarse, poorly sorted eolian sands, calcareous lake sediments, coastal salt marshes
and alluvial silts, but the precise ages of these deposits are still unknown.
Chase and Thomas (
2007
) obtained OSL ages for thirty-five sand samples from
six dune cores along a 300 km transect close to the coast, extending from Cape Town
in the south to the Namib Sand Sea in the north. They found five distinct peaks
suggestive of sand dune activity at 73-63, 49-43, 33-30, 24-16 and 5-4 ka, and
concluded that sand movement was more closely linked to wind strength and sand
supply than to periods of increased aridity. Earlier work by O'Connor and Thomas
(
1999
) on late Quaternary degraded linear dunes in western Zambia also concluded
that sediment supply, in this case from the Zambezi River, had played a pivotal role
in dune development in this region. However, they also considered that a reduction
°
S and 28
°
