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latitudes. The Miocene and later volcanism in the central and southern Sahara that
created the high mountains of the Hoggar, Tibesti and Jebel Marra was preceded by
prolonged deep weathering, during which kaolinitic and bauxitic weathering profiles
up to 45 m thick developed on rocks of Eocene to Precambrian age along the southern
Sahara (Greigert and Pougnet,
1967
; Williams,
2009a
). Uplift in the mid-Cenozoic
resulted in a change from chemical sedimentation to dominantly clastic sedimentation
in what is now the central and southern Sahara (Faure,
1962
; Greigert and Pougnet,
1967
, p. 157). Rejuvenated rivers flowing down from the great watersheds of Tibesti,
the Hoggar and the Aır deposited around the uplands the fluvial gravels, sands and
clays known to French geologists as the
Continental terminal
(Faure,
1962
; Greigert
and Pougnet,
1967
).
Very large, dry, sinuous river valleys are clearly visible on the early Gemini space
photographs of Libya and Tibesti (Pesce,
1968
) and on the later satellite imagery of
the eastern Sahara (Griffin,
1999
; Griffin,
2002
; Griffin,
2006
). One such valley can be
traced north from the Chad Basin into the hyper-arid desert of south-east Libya, where
it is bounded by the Tibesti volcanic massif to the west and three highly dissected
Nubian sandstone plateaux to the east (Williams and Hall,
1965
; Pesce,
1968
). Griffin
(
1999
;
2002
;
2006
) has analysed these valleys in detail. He reviewed the sedimentary
evidence from marine and terrestrial sites in and around the Mediterranean, the Gulf
of Suez and the Red Sea and concluded that the Messinian Zeit Formation (7.04-
5.34 Ma) was laid down during a time of high rainfall and high fluvial sediment yield,
reaching peak monsoonal activity in the Late Messinian (Griffin,
1999
), at a time
when the Straits of Gibraltar were closed and the Mediterranean had dried out and
become a salt desert. This occurred during the Messinian Salinity Crisis, which is now
precisely dated as 5.96-5.33 Ma (Cosentino et al.,
2013
). Griffin called these Late
Neogene rivers the Sahabi Rivers and deduced that they continued to cross central
North Africa until about 4.6 Ma ago, when the monsoon shifted to the south and the
eastern Sahara became drier (Griffin,
2002
).
Post-Eocene uplift in the Sahara triggered a widespread phase of late Cenozoic
erosion within major massifs such as the Tibesti and the Hoggar, as well as in more
isolated ring complexes such as Jebel Arkenu and Jebel 'Uweinat in south-east Libya
or Adrar Bous (A in
Figure 18.3
) in central Niger. The mid-Cenozoic drainage system
in North Africa appears to have been a highly efficient and well-integrated system
which kept pace with the various epeirogenic uplifts across the Sahara.
The Nile cut down through the Nubian Sandstone capping the Sabaloka ring com-
plex to form the Sabaloka Gorge north of Khartoum - one of the many instances of
superimposed Cenozoic drainage in the Sahara (Grove,
1980
; Williams and Williams,
1980
; Thurmond et al.,
2004
). The early Cenozoic mantle of deeply weathered rock
was almost entirely removed from the uplands of the southern Sahara, leaving a
bare and rugged landscape of gaunt rocky pinnacles and boulder-mantled slopes.
Episodic deep weathering, followed by episodic erosion and the exhumation of the
