Rift sedimentation and volcanism in the Afar depression

In the Upper Stratoid Series (5 - 1.4 Ma) of the southern Afar depression in Ethiopia, near the village Gadamaitu, a tooth of an Australopithecus cf. afarensis, accompanied by a rich vertebrate fauna, has been detected during a reconnaissance tour under the guidance of the Viennese anthropologist Horst Seidler in February 2000. The fossiliferous lake and fluvial deposits which are intercalated with basic and acidic volcanic rocks and tephra layers are similar to sedimentary successions of the Awash Group, from which many famous fossils of early man have been described. The research area is situated only few kilometers to the east of the tectonically active rift structure (Hertale graben), a continuation of the Main Ethiopian Rift into the Afar depression. The region is intensively dissected by NNE-SSW trending faults. Tilted blocks up to 30 have been observed. The project objectives are the development of a detailed stratigraphic system based on marker beds with radiometrically well dated volcanic layers and tephra deposits. In sections and a three-dimensional model of the sedimentary basin, the changing facies distribution, in space and time, in accordance to falling and rising lake water level will be exhibited. Such stratigraphical, geochronological and sedimentological information is important for successful palaeoanthropological excavations. On the basis of tephra stratigraphy, it should be possible to correlate the successions of the research area with other hominid-bearing sites within the Afar depression and locations outside, far to the south, in the Omo valley and in the region of the Lake Turkana. In the project, a three-year lasting field campaign is planned with extensive facies and tectonic analysis. The interpretation of satellite data (CORONA, LANDSAT, IKONOS, RADARSAT) will support the fieldwork. 40Ar/39Ar whole rock dating on massive basalts as well as 40Ar/39Ar laser-probe dating on single sanidine and plagioclase crystals will form the geochronological backbone of the stratigraphic system. Petrological and geochemical investigations will be helpful in tephra correlations and will give deep insights into the evolution of the geochemically bimodal Afar volcanism. Methods in structural geology will be helpful in reconstructing the former sedimentary basin.

The project provided an international research team of palaeontologists and anthropologists under the leadership of Horst Seidler, University of Vienna, with stratigraphical, sedimentological, volcanological and structural data necessary for their excavation campaigns in Ethiopia. The research area is located in the southern Afar Depression, east of the Awash River, in the district of Gadamaitou village. The area of investigation is characterized by badlands and the table mountain-like Mount Galili represents the most conspicuous elevation of the region. From a geological point of view the area is part of the eastern elevated rift shoulder of the Main Ethiopian Rift, the northernmost continuation of the East African Rift System. The stratigraphic succession comprises about 140 m volcanic and sedimentary rocks. Geochronological and biostratigraphic data give a time range between 4.5 - 3.7 Ma. Magnetostratigraphic investigations on the basalts corroborate these biostratigraphic data because of their reversed polarity (uppermost part of the Gilbert-Reverse Chron). During the exhumation of the rift shoulder, the entire succession was intensively deformed by predominantly N-S-trending steep faults. A new lithostratigraphic unit of Pliocene age, the so-called Mount Galili Formation, has been established and has been subdivided on the basis of volcanic marker beds into five members - from bottom to top: Dhidinley, Godiray, Shabeley Laag, Dhagax, and Caashacado Member. Environmental reconstructions, based on geological mapping and on detailed analyses of 40 lithostratigraphic sections, indicate, that sediments were deposited in a lacustrine and fluvial environment. Stable isotope data of carbon and oxygen from pedogenic carbonates depict an environment composed of mixed vegetation of woods and open grassland (20 - 50 %) along a lake and river landscape. It was the habitat of crocodiles, turtles and fishes as well as of a rich fauna of mammals dominated by bovids, elephants and suids. Among them lived some primates, such as the early man Australopithecus. Several times catastrophic volcanic eruptions interrupted this development. The major volcanic lithologies consist of basalts, ignimbrites, pumice and lapilli tuffs. Ignimbrite layers form the most prominent acid volcanic rocks within the research area. The geochemical composition of the basalts show signatures known from mid-oceanic ridges and oceanic island, which are consistent with the crustal nature of the northern East African Rift.