STRATIGRAPHIC CONTROLS ON THE MORPHO-TECTONIC ELEMENTS OF THE GORGE OF THE NILE, ETHIOPIA

This work presents new results of remote sensing and field studies aimed at understanding the evolution of the Gorge of the Nile in the Ethiopian Plateau and possible stratigraphic controls on its incision history. This 1.6 km deep canyon was carved on the Ethiopian Plateau by the Blue Nile which makes a ~150 km loop (by flowing SE then S then SW then NW) navigating its way through shield volcanoes from its headwaters at Lake Tana towards the lowlands of the Sudan. The gorge exposes a ~1.1 km thick Mesozoic sedimentary section overlain by Oligocene-Quaternary flood basalt and underlain by Precambrian crystalline rocks. Previous studies have shown that the Blue Nile's incision started ~30 Ma with significant increase in the incision rate at ~10 and ~6 Ma, possibly due to pulsed plateau uplift associated with the dynamics of the Afar Mantle Plume. Results of this study (which focuses on the SW-flowing segment of the Blue Nile) show the following: (1) Between 30 and 10 Ma (a period characterized by slow incision rate) the Blue Nile incised for only ~100 m on the upper part of the flood basalt. Between 10 and 6 Ma (where moderate incision rate dominated) the river incised for an additional ~200 m to reach the upper part of the Mesozoic sedimentary section. Much of the Blue Nile incision and the formation of the Gorge of the Nile occurred between 6 Ma and present. During this period the Blue Nile incised for an additional ~1100 m through the entire Mesozoic sedimentary section. (2) During its incision on the Ethiopian Plateau, the Blue Nile maintained a uniform width, except when it was incising through the flood basalts and the lower part of the Mesozoic sedimentary rocks. In these units the river channel widens significantly. The channel widening within the flood basalt is attributed to the slow rate of incision whereas the channel widening within the lower part of the Mesozoic sedimentary rocks is explained as due to the presence of easily eroded shale units. (3) The Blue Nile maintained a highly-symmetrical channel throughout its incision history on the Ethiopian Plateau, except during the period when it incised through the Mesozoic shale units. Here, the river is highly asymmetrical suggesting northwestward channel migration. This might be due to the presence of the highly-erodible shale units combined with rift-flank uplift of the Main Ethiopian Rift.