TEPHROSTRATIGRAPHIC AND GEOCHEMICAL INVESTIGATION OF EARLY PLEISTOCENE SILICIC TEPHRA, MIDDLE AWASH REGION, AFAR RIFT, ETHIOPIA

The Middle Awash region of the Afar Rift, Ethiopia has experienced a protracted history of faulting, basin development, sedimentation, and volcanism. These processes led to the preservation and subsequent exposure of a rich hominid evolutionary record spanning at least six million years. Local and regional volcanic activity contributed to the Middle Awash stratigraphic record with proximal and distal silicic tephra serving as essential chronostratigraphic tools. Faulting and erosion have exposed the stratigraphic and paleontological resources and created a landscape characterized by discontinuous outcrops of intercalated sedimentary and volcanic horizons. The tephra record becomes a principal means to link these non-contiguous stratigraphic packages. Correlation is accomplished primarily through geochemical characterization of silicic volcanic glass, as this constituent typically provides a unique fingerprint of the responsible eruption or magma system.

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.