Evolution of the Nile River since 70 Ma: insights from surface processes and anorogenic reliefs controlled by mantle dynamics
Résumé
Anorogenic reliefs (plateaus and plains) made up about 70% of the total emerged reliefs on Earth. They are characterized by nearly flat erosional surfaces upstream bounded by escarpments, called pediments/pediplains. Africa’s landforms are constituted by very long (several thousands of kilometers) wavelength relief of broad “basins” (depressions) and swells (Holmes, 1944) possibly controlled by mantle dynamics. This "basin" and swell pattern caused Africa to be consisted of numerous endorheic and exorheic systems. Consequently, the Nile River, the longest river on Earth and main object of this study, crosses today a set of two former endorheic systems (Ugandese and Sudanese “Basins”) and one exorheic system (Egyptian Margin) along its courses to the Mediterranean Sea. Our objective is here to unravel the paleorouting systems of the Nile River through relief growth, tectonic, and climate since the uppermost Cretaceous.
Several generations of stepped pediments, proxies of relief growth, were characterized and mapped on DEM and satellite images and dated using their geometrical relationships with dated magmatic rocks. To better constrain periods of relief uplift and the deformation wavelength through time, the stratigraphic record of the sedimentary basins located in between two swells were studied using biostratigraphy and sequence stratigraphy. The originality of the approaches is to integrate data of all the source-to-sink (S2S) systems to produce a coherent scenario of the evolution of the Nile.
We proposed the following model for the evolution of the Nile River. First, a main large pediplain is formed during the uppermost Cretaceous (?75-66 Ma), acting as a base of any kind of landforms that would be formed afterwards, bounded westward by the Darfur-Ennedi crest. Second, a major marine flooding during late Paleocene time (58-57 Ma) reached a subtle high bounding northward the endorheic Sudanese “Basin”. Third, Pre-Eonile started to form during the uppermost Eocene (~37 Ma) with a divide limited to the Egyptian Margin. Fourth, the Eonile was incised during late Miocene (~10 Ma) at time of a major uplift at the scale of north-east Africa. Finally, the Nile captured first the Sudanese endorheic system in the Early Pliocene (~4 Ma) and altered the Ugandese one in the Middle-Late Pleistocene (less than 1 Ma).
