Age and rates of denudation of Trap Series basalts at Blue Nile Gorge, Ethiopia

BETWEEN Lake Tana and the Sudan border the Blue Nile has cut a deep gorge across the Ethiopian Highlands. The first 150 km of its course lies on lavas of the Tertiary Trap Series, after which the river flows through horizontal Mesozoic and Palaeozoic sediments until the Precambrian is reached 330 km from Lake Tana1. About 280 km below the lake the Addis Ababa–Debre Markos road crosses the gorge at 10° 05′N, 38° 10′E, where 250 m of Trap Series basalts overlie at least 1,150 m of sediments the base of which is not exposed. The gorge is 1,400 m deep and 20 km wide and is incised into a plateau 2,600 m high.