Petrology of Nile River sands (Ethiopia and Sudan): Sediment budgets and erosion patterns

Abstract

Detrital modes of modern Nile sands, together with estimates of sediment volumes trapped in Sudanese reservoirs, allow us to calculate sediment loads of major tributaries (Blue Nile, White Nile, Atbara) and erosion rates in the Nile catchment. A tridimensional array of high-resolution bulk-petrography and heavy-mineral data was obtained on both levee (suspended load) and bar (bedload) deposits, analysed separately for each grain-size subclass at 0.5 Φ intervals. From available information on sediments stored in the Roseires, Khashm el Girba and Lake Nasser reservoirs between 1964 and 1990, the total Nile load is reassessed at 230 ± 20 10 6 t/a, an estimate two to four times higher than figures reported so far, on which previous estimates of sediment yields and erosion rates were based. Of such huge amount of detritus, 82 ± 10 10 6 t/a are contributed by River Atbara, which carries more volcanic rock fragments, brown augite and olivine from basaltic rocks, and 140 ± 20 10 6 t/a by the Blue Nile, which carries more K-feldspar and hornblende from amphibolite-facies basement rocks. The additional ≤ 10 7 t/a of almost purely quartzose sediments supplied by the rest of the Nile catchment, corresponding to insignificant average yields and erosion rates, represent the stable residue which survived extreme subequatorial weathering in southern Sudan swamps (White Nile, Bahr ez Zeraf, and Sobat sands) or fluvial and eolian recycling of ancient quartzarenites in hyperarid climates (Nubian sands). Sediment production is thus markedly focused on Ethiopian rift highlands, where rainfall is concentrated in a single July–August peak. High average yields and erosion rates (800 ± 150 t/km 2 a, 0.29 ± 0.05 mm/a) partly reflect anthropically-accelerated erosion caused by deforestation and intensive land use, and cannot be extrapolated far in the past. Erosion patterns may have changed repeatedly during Quaternary climatic oscillations, and possibly also in the longer term during the multistage rift-related events which, since impingement of the Afar plume and eruption of flood basalts in the Oligocene, caused elevated topography and monsoonal climate in Ethiopia.