Geology

The Geological Evolution of the Main Nile

Five river phases have been identified with the Nile system however the ancient beginnings of the river has its origin in systems that traversed the Afro-Arabian swell as early as the Late Cretaceous (Whiteman, 1971). The five sequential phases include the Eonile in the Upper Miocene, Palaeonile in the Upper Pliocene and the Proto- Pre- and Neonile during the Pleistocene (Said, 1981). The various phases of the river, and their associated climate, erosion, sedimentation, are illustrated in Table 1. It has been suggested that Lake Tana was first formed during the early stages of river system development in the Pliocene with the Little Abbay canyon excavated during Upper Pliocene to early Pleistocene. Although the Blue Nile was integrated into the Nile system within the mid Pleistocene during the Prenile phase at around 130,000 yrs BP, it was originally initiated in isolation towards the end of the Miocene, around 8 Ma BP (Sultana, 2003)

Figure 1. Geological Map. Click map to enlarge.

Summary of the Geology of Ethiopia

The basement rocks of Ethiopia consist of Precambrian igneous and metamorphics containing several orogenic episodes. It is in these rocks or from these rocks that the majority of economic mineral deposits exits. These include gold deposits of the northern, western and southern provinces, Coper, zinc and lead sulphides in greenschist facies metamorphics, talc and nickel and platinum in the Welega Province. The basement rocks are relatively impermeable and thus water resources are in general only associated with fracturing and faulting resulting from rift development of the Red Sea and East African-Ethiopian Rift Valley. For much of the early Palaeozoic, Ethiopia was in a state of steady uplift which caused widespread erosion in all but the northern provinces where deposits partly of glacial origin have been noted. Subsidence followed in the Mesozoic with a large shallow sea spreading initially over the Ogaden province eventually extending further north and west as the land continued to subside. This sequence was followed by general uplift and drying out of lake beds to leave gypsum and anhydrite precipitates. Similar cycles continued through the Tertiary. Regional tectonic activity associated with rifting events in the Red Sea, Gulf of Aden and East African Rift Valley during the late Tertiary caused faulting and fracturing together with widespread volcanism. Vast quantities of basaltic lava was extruded over the western half of Ethiopia. This was accompanied by ash and coarser tephra forming a sequence know as the Trap Series. Several shield volcanoes consisting of alkali basalts and tephra developed at this time around the eastern edge of the Lake Tana depression.

Figure 2. Recent seismic activity in Ethiopia. Click map to enlarge.

Quaternary deposits are mainly confined to those associated with large depressions and lakes. Seismic and volcanic activity continues today along the Ethiopian Rift valley system as can be seen by maps of recent earthquake activity (Figure 2).