TEPHROSTRATIGRAPHY OF THE OKOTE COMPLEX, OMO-TURKANA BASIN, KENYA AND ETHIOPIA

Pliocene and Pleistocene tephra layers in the Omo-Turkana Basin of Kenya and Ethiopia provide excellent time-stratigraphic markers for regional correlations for geologic mapping, studies of climate change, and late Neogene stratigraphy. Tephrostratigraphy has been relatively straightforward through much of the Omo Group, that ranges in age from 4.1 to 0.7 Ma. Between ~1.55 and 1.45 Ma tuffs of the Omo Group are numerous, of similar composition, and of discontinuous distribution so that placing them in order has proven difficult. At Koobi Fora, this group of tuffs has been termed the Okote Complex. Extensive 40Ar/39Ar ages on this group of tuffs show that most were deposited during a very restricted time interval (~1.45–1.55 Ma) in the Omo-Turkana Basin. At least 35 tuffs lie in this interval, so eruptions occurred on average every 3,000 years, an eruptive rate that appears to be elevated well above background. Several of these tuffs correlate with tuffs in the Konso Formation at the southern end of the Ethiopian Rift Valley, which is significant to paleogeographic reconstructions in the region. Between 1.471 and 1.564 Ma, coincident with deposition of many tuffs of the Okote Complex, no sapropels are recorded in the Mediterranean Sea. During this time Lake Turkana (or its predecessor) was at a lower level, which is evident from the nature and location of the tuffs. During this time fluvial deposits extend far south of the present northern shoreline of the lake. Also, at this time dust flux to the Gulf of Aden was considerably elevated, and dust peaks appear to be offset from sapropels by ~10 ka. Many hominid fossils from the Koobi Fora and Nachukui Formations are associated with tuffs of this complex, and the new geochronological results, in concert with the tephrostratigraphic correlations, provide much better control on their ages. In addition, the results provide information on the paleogeography of the areas which they inhabited. Further, the results are essential to correctly correlate cyclic features in the stratigraphic record in the Omo-Turkana basin with the deep-sea records of the Mediterranean, Gulf of Aden, and Arabian Sea.