This paper documents the lithology and geochemistry of vitric tephra deposits from the Pliocene-Pleistocene Hadar and Busidima Formations from the early hominin site of Hadar in Ethiopia. Vitric tephras of the Hadar Formation (ca. 3.45–2.9 Ma) are limited to certain facies of the Sidi Hakoma Tuff, the Kada Hadar Tuff, and the Bouroukie Tuff 2 (BKT-2) Complex, the latter of which is discussed in detail in this study. In contrast, this systematic study identified at least 12 distinct vitric tephras preserved in the Busidima Formation at Hadar (ca. 2.7–0.81 Ma), which are represented by no less than 20 chemical modes. These analyses are used to construct the first tephrostratigraphic-based sequence for the highly complex and discontinuous Busidima Formation deposits preserved at Hadar. Busidima Formation correlations have also been established between Hadar and neighboring project areas, specifically Dikika and Gona. Artifact Site Tuff 3 (AST-3), the Inaalale Tuff, and the Ken Di Tuff are correlated between Hadar and Dikika. AST-1, AST-3, the Ken Di Tuff, the Dahuli Tuff, and several localized tuffs of the Busidima Formation are correlated between Hadar and Gona. However, tuffs associated with the earliest archaeology in the two regions, namely AST-2 from Gona and BKT-3 from Hadar, were not identified outside their respective project areas. Nonetheless, the sequence of tephra provides important information for the placement and relationship of archaeological and paleontological sites both within Hadar and between Hadar and adjacent project areas.

The Pliocene Hadar Formation (Ethiopia) preserves a rich geological and paleontological record germane to our understanding of early hominin evolution. At the Hadar Research Project area, ~155 m of Hadar Formation strata span the interval from ca. 3.45 to 2.90 Ma and consist of floodplain paleosols (dominantly Vertisols), fluvial and deltaic sands, and both pedogenically modified and unmodified lacustrine clays and silts. Clays and silts constitute the majority of the Hadar sediments. In the absence of clear lacustrine indicators, most of these fine-grained sediments are interpreted as fluvial floodplain or delta-plain deposits that exhibit varying degrees of pedogenic modification. Lacustrine and lake-margin deposits are represented by laminated mudstones, gastropod coquinas, limestones, and certain pedogenically modified and unmodified strata preserving gastropods, ostracods, and aquatic vertebrate remains. Most sands can be attributed to channel and point-bar deposits of a large-scale meandering river system or associated crevasse-splay and distributary-channel deposits. Fluvial-deltaic deposition predominated at Hadar. The lacustrine depocenter was located east and northeast of Hadar, but lacustrine transgressions into the region were a regular occurrence. Evidence presented here suggests that during lacustrine-dominated intervals, lake water depths at Hadar were most likely relatively shallow and included repeated regression events across a low-gradient shoreline. Vertebrate remains at Hadar are disproportionately recovered from fluvial and deltaic sands and silts. This is most likely a taphonomic effect related to the low preservation potential of bones in Vertisols, which are common at Hadar, as opposed to their original distribution across the paleolandscape.