Geoscience Reference
In-Depth Information
(a)
(c)
Wadi sediments and aeolian sands
Evaporites
6
60
60
6
60
60
3
30
30
3
30
30
0
0
0
0
Wadi sediments and
aeolian sands
Evaporites
3
30
30
3
30
30
6
60
60
6
60
60
EARLY CAMBRIAN
MID PERMIAN
(b)
(d)
Wadi sediments and
aeolian sands
Evaporites
6
60
60
6
60
60
3
30
30
3
30
30
0
0
0
0
Wadi sediments and
aeolian sands
Evaporites
3
30
30
3
30
30
6
60
60
6
60
60
LATE DEVONIAN
LATE TRIASSIC
(e)
6
60
60
3
30
30
0
0
Wadi sediments and
aeolian sands
Evaporites
3
30
30
6
60
60
EARLY CRETACEOUS
Figure 3.1
Arid zones from the geological record (after Glennie, 1987, Figure 26.1).
3.2.2
The development of aridity in the Mesozoic
and Cenozoic
The uplift of the Tibetan plateau in the Pliocene at 2-3
Ma had a fundamental effect on atmospheric circulation.
This created the opportunities for both global cooling and
atmospheric CO
2
drawdown via weathering (Ruddiman
and Kutzbach, 1991), and the development of an upper
atmospheric easterly jet stream that provided dry sub-
siding air to eastern Asia, Arabia and the Saharan belt
(summarised in Williams
et al.
, 1993). The growth of
both Antarctic and northern hemisphere ice caps in this
period (Shackleton
et al.
, 1984) also allowed for steeper
pole-equator temperature gradients and resultant stronger
trade winds, increasing the capacity for aeolian sand trans-
port (Servant
et al.
, 1993). Broadly, it is these events that
saw the change in global conditions during the warm
Tertiary to the conditions favouring opportunities for
glaciations and for aridity in continental interiors and the
Many of the 'modern' deserts, which have in turn fluc-
tuated in extent in association with Quaternary cli-
mate changes, owe their origins to global tectonic
changes that commenced in the Mesozoic (from 245 Ma)
and continued to develop into the Cenozoic era (from
66 Ma).
The division of Gondwanaland from c.176 Ma pro-
vided ultimately for the creation of Antarctica and ice
cap growth, the development of atmospheric circulation
patterns allowing the growth of subtropical high-pressure
belts, and continentality and structural contexts, e.g. in
southern Africa and Australia, that ultimately allowed for
the development of the Kalahari and Australian deserts
