THE GEOLOGY OF GONA

Gona archives nearly 6 million years (from ~6 to <0.5 Ma) of deposition and hominid prehistory. The oldest fossil-bearing formations are exposed in the western side of the project area, west of the As Duma Fault. Radiometric and paleomagnetic dates place the youngest of these sediments between 4.5 to 4.0 Ma, whereas the oldest may be 5-6 Ma, based on biostratigraphic constraints. The sediments were deposited by small rivers and lakes confined between large expanses of Dahla Series basalts. All these deposits are richly fossiliferous and have yielded abundant hominid remains, including those of Ardipithecus ramidus .

Extension beginning 4-3.4 Ma activated the east-dipping As Duma normal fault and the Awash graben to the east. Sediments all associated with the paleo-Awash river and fringing alluvial fans poured into the graben, first as the Hadar Formation (3.4 to 2.9 Ma) and later the Busidima Formation (2.7 to <0.5 Ma). Low-gradient rivers and lakes covered the area during Hadar Formation time, whereas higher-energy axial river and alluvial fan deposits compose the Busidima Formation. Radiometric and paleomagnetic constraints place the base of the Busidima Formation at ~2.7 Ma and the top above the Brunhes/Matuyama boundary (0.78 Ma). The Silbo Tuff (0.74 Ma) known from Kenya and the Gulf of Aden is also present just above this paleomagnetic boundary. The Busidima Formation contains a rich archeological (Oldowan (2.58-1.7 Ma) and Acheulian (1.7 to <0.5 Ma)) record largely associated with paleo-Awash Rivers deposits.