Geoscience Reference
In-Depth Information
Figure 18.7. Dissected 30-million-year basalt flows near the headwaters of the Blue
Nile, Semien Highlands, Ethiopia.
through Cenozoic basalts (
Figure 18.7
), Mesozoic and Palaeozoic sedimentary rocks
and Precambrian basements rocks to form a spectacular canyon that is more than
350 km long, up to 20 km wide and up to 1.5 km in depth. McDougall et al. (
1975
)
obtained the first potassium-argon ages (27-23 Ma) for the horizontal basalt flows
through which the river had cut. They also estimated that 100,000
50,000 km
3
of
rock had been eroded from the gorge, which drained an area of around 275,000 km
2
.
This amounts to a mean denudation rate of 15
±
7. 5 m
3
km
−
2
yr
−
1
. This value
is very low for a tectonically active region of high relief and is more consistent
with rates from undisturbed forested tropical lowlands (McDougall et al.,
1975
).
Modern erosion rates in the headwaters amount to at least 120-240 m
3
km
−
2
yr
−
1
which is an order of magnitude faster than the mean geological rate. The volume of
sediment stored in the Nile Delta and its much larger submerged cone in the eastern
Mediterranean is 150,000
±
50,000 km
3
, which is very similar to the estimated
volume of rock eroded from the Blue Nile gorge, allowing for changes in bulk density
(Nile deltaic sediments: 1.5 g cm
−
3
; bedrock eroded from Ethiopia: 2.8 g cm
−
3
), as
well as losses in solution (estimated at 30 per cent). Relative to the volume of the
Nile cone, the amount of alluvial sediment stored in the Nile flood-plain is trivial,
amounting to 100-600 km
3
±
along the main Nile, 800 km
3
in the Atbara Fan and
1,800 km
3
in the Gezira Fan between the Blue and White Nile rivers in central
Sudan.
