The Nile's journey through space and time: A geological perspective

Abstract

This synthesis summarizes the geography, geomorphology, and regional tectonics of the Nile drainage system (henceforth the Nile System) as well as outlines the important features and geological events in its different segments. The Nile System assembled from different drainage systems that were shaped by different tectonic and environmental events. The drainage systems at its two sources, the Lake Plateau and the Northwestern Ethiopian Plateau are shaped since ~30 Ma by tectonic uplift related to mantle plume(s) activities, eruption of the Ethiopian Large Igneous Province and the opening of the East African Rift System (EARS). Downstream from its 5000–2000 m high sources, the evolution of the Nile System was largely influenced by the White Nile paleo-lakes (and possibly a smaller paleo-lake within the Sudd Nile – the Lake Sudd) and the development of major low-angle alluvial fans (the Lol, the El Fula, the Bahr el Jebel, the Gezira, and the Abu Habil) within the flat plains of South Sudan and central Sudan. The drainage evolution of these regions of the Nile System ultimately ended with the emergence of the Blue Nile and the White Nile which meet in Khartoum to form the Cataract Nile which carves its way northward until the Sudanese-Egyptian boarder through Precambrian crystalline rocks and Mesozoic sandstones exposed within the Sahara. Here, recent tectonic uplift of the Nubian Swell diverted the river to flow southwestward before resuming a north course forming the Great Bend of the Nile. In Egypt, carving of the Eonile Canyon during the Messinian Salinity Crisis signaled the birth of the Egyptian Nile which possibly connected to the rest of the Nile System only ~2.5–~2.0 Ma ago. This synthesis proposes five phases for the assembly of the Nile System. (1) The period immediately before 6.0 Ma which is characterized by the presence of the Paleo-lake Obweruka along the Western Branch of the EARS, head-ward incision of the Blue Nile (and possibly the Tekeze – Atbara River) into the Northwestern Ethiopian Plateau, and the domination of the Ancestral Egyptian Nile drainage system. (2) The period between 6.0 Ma and 5.4 Ma which witnessed the appearance of the Lake Tana as well as the carving of the Eonile Canyon and its subsequent flooding by the Mediterranean Sea water to turn it into a gulf. (3) The period between 2.5 Ma and 0.5 Ma which witnessed in the Lake Plateau the segmentation of the Western Branch of the EARS turning the Paleo-lake Obweruka into the Lake Albert and the Lake Edward and establishing the eastward directed drainage system (the Kifa, the Katonga, and the Kagera). This period also witnessed the carving of the Blue Nile its way through the Sahara to connect with the Egyptian Nile. (4) The period between 0.5 and 15 ka which witnessed the appearance of the Lake Victoria in the Lake Plateau as well as the intermittent appearance of the White Nile paleo-lakes and possibly the Lake Sudd. It is also possibly the period when the Tushka – Kharga paleo-lakes were formed west of the Modern Egyptian Nile. This period also witnessed the continuation of the development of alluvial fans in the lowlands of South Sudan and central Sudan. Also during this period, tectonic uplift of the Nubian Swell in northern Sudan and southern Egypt produced the Great Bend of the Nile. (5) The period between 15 ka and Present which witnessed drainage re-organization and the most recent overflow of the lakes of the Lake Plateau Nile to re-connect with the Sudd Nile, White Nile, and the Cataract Nile. This tentative model can be refined by better constraints on the tectonic uplift history of the sources of the Nile System, sediment budget of the system, and timing of connectivity between its different segments.