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Sedimentary History of African Rift Basins |
Dept of Geography, Monash University, Clayton, Victoria 3168, Australia
Faculty of Sciences, Addis Ababa University, PO Box 1176, Addis Ababa, Ethiopia
School of Biological Sciences & Quaternary Research Unit, Macquarie University, North Ryde, New South Wales 2113, Australia
The Awash flows from the Ethiopian Highlands via the Ethiopian Rift into Lake Abhe in the Afar Rift. Over 8000 km2 of Neogene fluvio-lacustrine deposits crop out in the Middle Awash valley, of which we have surveyed 300 km2 E of the river between 10°25'N and 10°45'N. Pleistocene sediments adjacent to the Awash floodplain consist mostly of unfossiliferous brown clays with sand and gravel interbeds which contain vertebrate (including hominid) fossils and prehistoric stone artefacts (Oldowan and Acheulian). Palaeocurrent directions in these fossiliferous sands invariably parallel the ephemeral sandbed channels that flow W from the basalt escarpment to the Awash. We interpret the brown clays as palaeo-Awash floodplain deposits, in contrast to the fossiliferous gravel-sands, which are mainly of local provenance. Away from the river, alternating Pliocene clays and diatomites, with minor fossiliferous gravel-sands, are capped by a 3.8–4.0 Ma primary air-fall tuff. Facies changes are more frequent above this ‘Cindery Tuff’, indicating tectonic disruption of the early Pliocene lakes, reflected also in a progressive increase in fluvio-volcanic gravels from Pliocene to early Pleistocene.
