TEPHROSTRATIGRAPHIC AND GEOCHEMICAL INVESTIGATION OF EARLY PLEISTOCENE SILICIC TEPHRA, MIDDLE AWASH REGION, AFAR RIFT, ETHIOPIA
Between 3.5 and 1.5 Ma, Afar and Main Ethiopian Rift silicic volcanism contributed numerous tephra units to the Middle Awash geologic record. However, the period between ~2.5 and 1.8 Ma contains a dearth of documented large and widely dispersed eruptions. We report here the results of tephrostratigraphic and geochemical studies focused on a set of localities that preserve a record of deposition and volcanic input during this time interval. Correlations between deposits are suggested by field studies and further explored using grain-specific and purified bulk glass major and trace element geochemistry. This approach allows for (1) correlation of many separate outcrops into several distinct tephra horizons spanning >20 km of aerial extent and incorporating multiple important fossil hominid and archeological localities, (2) correlation of multiple tephra horizons to previously studied deposits in other portions of the Middle Awash, and (3) correlation of multiple tephra horizons with deposits described in the Omo and Turkana regions of Ethiopia and Kenya, respectively. The latter two results help to better frame the Pleistocene chronostratigraphic record of the Middle Awash and also provide evidence for the eruption of large silicic magma systems in the northern East African Rift system during the ~2.5 to 1.8 Ma time period.