CHRONOSTRATIGRAPHIC AND PETROLOGIC INFORMATION DERIVED FROM TEPHRA OF THE MIDDLE AWASH REGION, AFAR, ETHIOPIA

The Middle Awash region of the Afar Rift, Ethiopia has experienced a protracted history of faulting, basin development, sedimentation, and volcanism. These processes are responsible for preservation of a rich evolutionary record that spans at least 6 m.y. and includes numerous hominid species. Local and regional volcanic activity contributed to the Middle Awash stratigraphic record with proximal and distal fallout deposits (tephra) serving as essential chronostratigraphic tools and as windows into regional magma evolution processes. Ongoing faulting and erosion are responsible for exposing the paleontological resources and for creating a landscape with patchy and discontinuous outcrops. In this terrain, the tephra record becomes a primary means to link non-contiguous stratigraphic packages and sequences. While physical and petrographic characteristics of tephra are useful, the key to robust tephra correlation is geochemical characterization of volcanic glass (pumice and shards), as this constituent typically provides a unique fingerprint of the responsible eruption/magma system.

Between 1.5 and 3.5 Ma, regional explosive silicic volcanism contributed numerous tephra deposits to the Middle Awash geologic record. These deposits range from a few centimeters to several meters thick and preserve evidence of compositionally homogeneous and heterogeneous eruptions, the latter suggesting the presence of zoned magma chambers. Major element analyses of individual glass shards by electron microprobe provide first-order characterizations and correlations that guide separation of bulk glass aliquots for major and trace element determinations using DCP-OES and ICP-MS. Tephra correlations are based on evaluation of a full suite of elements, but particular attention is paid to major constituents FeO, TiO₂, CaO, and Al₂0₃ and relatively alteration resistant trace elements Zr, Nb, Zn, Y, Sc, Hf, and the REE. This approach has allowed us to correlate 26 outcrops into 8 distinct tephra horizons, spanning up to 20 km of aerial extent and incorporating important paleontological localities. The geochemistry also provides a window into regional magma system evolution over the 2 m.y. interval in question