GEOLOGICAL DEVELOPMENT OF THE OMO-TURKANA BASIN DURING THE PLIOCENE AND PLEISTOCENE EPOCHS

Pliocene and Pleistocene deposits of the Omo-Turkana Basin extend from 6°N latitude to about 2°N latitude (~450 km), and laterally from about 35.5°E to 36.5°E. Sediments are brought to the basin by the Omo River which rises in Ethiopia, and by the Turkwel and Kerio Rivers which rise in Kenya and Uganda. Pliocene deposition began ~4.3 Ma ago, and is recorded in the Mursi, Nkalabong, Usno, and Shungura Formations in southern Ethiopia, and by the Koobi Fora, Nachukui, Lothagam, and Kanapoi Formations in neighboring Kenya. The composite stratigraphic thickness of exposed Pliocene strata is on the order of 400 m. The stratigraphy of the basin is well integrated through tephrostratigraphic correlations, and deposits are well-dated through 40Ar/39Ar and K/Ar analyses of lavas and reworked ash layers. In addition the paleontological record in the basin is tied firmly to marine records in the Gulf of Aden. Paleoclimatic changes are recorded not only through isotopic compositions of paleosols but also in the distribution of strata of particular ages. Pleistocene strata have a composite thickness of about 400 m, but there is a gap in the record from about 0.7 Ma to 0.2 Ma. Seismic work in the greater basin suggests that a more complete record exists beneath Lake Turkana, where combined Pliocene and Pleistocene strata appear to be ~4000 m thick. Pleistocene history is recorded in the Shungura, Koobi Fora, Nachukui, Kibish, and Galana Boi Formations. Of particular interest is that the locus of deposition in the basin appears to have controlled to some extent by the same climatic factors that are responsible for development of sapropels in the eastern Mediterranean Sea. For example, deposition of the Kibish Formation near the northern end of the basin is episodic, and new dates suggest that each episode of deposition can be correlated with particular sapropels in the Mediterranean sequence. As the sapropels are related to Nile discharge, this is understandable because the Omo River shares a drainage divide with the Blue Nile. Tectonic and volcanic events have also played a role in the development of the basin during the Pliocene and Pleistocene.