THE GEOLOGY OF GONA
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.