Desert dunes - Climatic Change in Deserts - page 124

Servant ( 1973 ) identified wind-blown sands in a number of what he considered to be
very late Tertiary stratigraphic sections. He concluded that the onset of aridity and
the first appearance of desert dunes in this part of the southern Sahara was a late
Tertiary phenomenon. Using fossil and sedimentary evidence, Schuster et al. ( 2006 )
have since confirmed that the onset of recurrent desert conditions in the Chad Basin
began at least 7 Ma ago. The sedimentological evidence of Servant ( 1973 ) and of
Schusteretal.( 2006 ) and the pollen evidence of Maley ( 1980 ; 1981 ; 1996 ), indicate
that the onset of climatic desiccation and the ensuing disruption of the integrated early
to mid-Miocene Saharan drainage network (Griffin, 2006 ) was a feature of the late
Miocene.
The major dune fields of the Sahara occupy topographic depressions formed during
the Miocene and earlier phases of volcanism and tectonic activity in the central,
eastern and northern Sahara. It was during this time that volcanic massifs such as the
Hoggar, Tibesti and Jebel Marra were formed. Volcanism was preceded and ac-
companied by uplift of the Proterozoic crystalline basement complex rocks and their
sedimentary cover of Palaeozoic and Mesozoic sandstones and shales, giving rise to
the undulating topography of basins and swells described by Arthur Holmes nearly
fifty years ago (Holmes, 1965 , fig. 763). Eocene deep weathering was followed by
climatic desiccation and by tectonic uplift, triggering a wave of erosion. As the once
well-integrated drainage network of the Sahara became segmented and disorganised,
the alluvial sediments were no longer carried to the sea and instead accumulated within
internally drained depressions, or depocentres. The finer particles were winnowed out
by the prevailing Trade Winds along the southern Sahara and blown out across the
Atlantic as desert dust, bringing nutrients to the Amazon rainforest (see Chapter 9 ). In
the north of the Sahara, the westerlies carried Saharan dust at least as far as the Negev
Desert in southern Israel. The sand-sized particles left behind by this winnowing
process were in turn fashioned into desert dunes and sand plains.
As a very rough rule of thumb, the orientation of the dunes reflects the anticyclonic
wind circulation in the Sahara, but in contrast to central Australia, the pattern is far
from simple and there is considerable evidence of the deflection of wind and dunes
around mountains and other smaller topographic obstacles (Grove, 1980 ; Mainguet
et al., 1980 ; Warren, 2013 ). The age of such major dune fields as the Grand Erg
Oriental (Great Eastern Erg, or sand sea) in southern Algeria is not well-established
but appears to be at least as old as the fluvial deposits that traverse them, some of
which contain Early Stone Age hand-axes (Acheulian bifaces) with a minimum age
of 300 ka and a maximum age of approximately 1.5 Ma (see Chapter 17 ). It is still
an open question as to whether there has been much long-distance movement of sand
across the Sahara or whether certain dune fields are essentially local features, as seems
to be the case with the Qoz Dango in south-west Darfur (Williams et al., 1980 ).
Along the southern margins of the Sahara, as mentioned in Section 8.2 , presently
fixed dunes extend to at least 500 km south of the present limit of active dunes, which