Geoscience Reference
In-Depth Information
The discrepancy between the modern erosion rates and the long-term geological
rates suggests that there has been episodic uplift of the Ethiopian Plateau interspersed
with periods of prolonged tectonic stability. Pik et al. (
2003
;
2008
) used thermo-
chronology to test models of Ethiopian landscape evolution. They obtained apatite
helium ages showing partial resetting of pre-existing basement rock ages resulting
from burial of the basement rocks beneath a thick mantle of Trap Series flood basalts
around 30 Ma ago, and they concluded that erosion of the Blue Nile gorge began as
early as 25-29Ma ago, confirming the results of McDougall et al. (
1975
), with erosion
along the scarps flanking the highlands starting after 11 Ma (Pik et al.,
2003
). The
major volcanic/tectonic divides in Ethiopia date to 30-20 Ma and were formed before
the rifting and break-up of the original Ethiopian volcanic plateau, which commenced
after 20 Ma (Pik et al.,
2008
; Corti,
2009
).
Gani et al. (
2007
) used a digital elevation model to reconstruct initial topography
before erosion of the Blue Nile gorge and compiled potassium-argon ages for the
volcanic rocks from present-day eroded volcanic remnants. They concluded that the
Blue Nile and its tributaries had eroded at least 93,200 km
3
of rock from the Ethiopian
Plateau since 29 Ma. Uplift occurred in three phases (29-10, 10-6 and 6-0 Ma), with
erosion rates accelerating at around 10 and around 6 Ma. The inferred rapid increase
in erosion at 6 Ma (Gani et al.,
2007
) is the same age as that of renewed volcanism in
the Afar Rift and renewed movement along the Dead Sea transform fault. Adamson
and Williams (
1987
) speculated that the movement of the Dead Sea transform fault
at this time may have been triggered by the repeated loading and unloading of the
Mediterranean seabed during the Messinian Salinity Crisis of 6.2-5.3 Ma. The results
of Gani et al. (
2007
) support the concept of episodic uplift and erosion put forward
by McDougall et al. (
1975
) and raise the possibility that much of the sediment in the
Nile cone may be of Pliocene age and younger.
We can therefore conclude that the elevated Ethiopian Plateau, which ultimately
controls the hydrology of the Blue Nile and its related river systems, has been in
existence since the late Oligocene. The Blue Nile and Atbara rivers have been ferrying
sediment across the lowlands of Sudan and Egypt and into the eastern Mediterranean
throughout the last several million years, and possibly at intervals over the last 30
million years. No other model seems able to account for the equivalence in volume
of the Nile cone and the bedrock eroded from the Ethiopian headwaters of the Blue
Nile and Atbara rivers, first estimated nearly forty years ago by McDougall et al.
(
1975
). Nevertheless, there are persistent claims for alternative courses for the Nile
either across to the Red Sea or into the Chad Basin. Such claims lack supporting
sedimentary evidence and so remain speculative working hypotheses. Some of the
proposed early courses of the Nile in Egypt show a complete reversal in flow direction
to the south-west (Goudie,
1985
). While the possibility of periodic disruptions to the
Nile drainage network cannot be ruled out, the most compelling evidence seems to
indicate that the long-term supply of water and sediment to the main Nile over the
past 30 million years was primarily from the Ethiopian Plateau.
