Geoscience Reference
In-Depth Information
the southern piedmont. In southern Darfur, deposition of fluvial sands, mainly eroded
from the Basement Complex but with minor volcanic inputs, provided sands that
were reworked by wind to form the Older Qoz (dunes). Soils formed and stabilised
the Older Qoz, which was breached by rivers during a wetter late Pleistocene phase.
A delta formed within the caldera lake that was at least 25 m deep. The delta was
later exposed and eroded and a wave-cut bench developed at 8-5 m above the caldera
floor. Algal limestone deposits on the bench have calibrated
14
C ages of 22.5-19 ka.
Maley (
2000
) obtained similar
14
C ages for lake and river sediments within Tibesti
volcano and attributed them to changes in the Subtropical Jet Stream during the LGM.
A sequence of alluvial terraces formed to the west and south of the massif at intervals
during the late Pleistocene, with the presence of Middle and Late Stone Age artefacts
indicating episodic fluvial deposition between 300 ka and around 15 ka. The Older
Qoz sands were reworked by wind to form the (?) late Pleistocene Younger Qoz, which
was then breached by rivers flowing from Jebel Marra piedmont. In southern Darfur,
there was widespread deposition of a fining-upwards alluvial sequence, followed by
the formation of a late Pleistocene 4 m silt terrace with LSA artefacts on the surface.
During the Holocene, there was continued explosive volcanism and formation of a
younger crater within the main caldera. Alternating silt deposition and river incision
created the 3 m and 1.5 m terraces to the west and a 2 m terrace to the south of the
massif. In southern Darfur, dark cracking clays (vertisols) were deposited during the
early Holocene, when the climate was wetter than today. Incision ensued, followed
by eolian sand and fluvial silt deposition. In the far south, large paleochannels were
progressively filled with alluvium, and underfit channels were formed in the mid-
Holocene. Incision below the modern channel beds was followed by deposition of
coarse bed load sands within the channels along the western and southern piedmonts.
The most striking feature of the Jebel Marra alluvial record is the presence of oil
palm fossils. The southern piedmont of Jebel Marra is today semi-arid, but the oil
palm fossils show that tropical rainforest flourished there at some time between about
1.5 and 0.8 Ma. Another unusual feature is the former deep freshwater lake west
of the massif, with its 5.5 m of finely laminated diatomites (Philibert et al.,
2010
).
A thin layer of sandy alluvium with sporadic basalt and trachyte gravels overlies
the diatomite. The artefacts on top of the exposed lake bed are typologically similar
to the stone tool assemblages excavated at Olorgesailie in Kenya and from Olduvai
Bed IV in Tanzania, which straddle the Brunhes-Matuyama paleomagnetic boundary
(0.78 Ma) (see
Chapter 6
,
Figure 6.2
), which means that an age range of around
0.8
0.3 Ma is likely for the stone tool assemblage (Isaac,
1982
; Gowlett,
1984
;
Owen et al.,
2008
). Acheulian occupation sites elsewhere in northern and eastern
Africa are always associated with the presence of freshwater lakes, rivers or springs
(Clark,
1980
).
There have been relatively few studies of older lake deposits in the Sahara, and they
have often proved hard to date (Karim,
1968
; Williams et al.,
1980
; Petit-Maire,
1982
;
±
