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.

Located adjacent to the paleoanthropological site of Hadar in Afar, Ethiopia, the Ledi-Geraru project area preserves multiple tephra deposits within the Pliocene sediments of the hominin-bearing Hadar Formation. Tephra deposits of the Bouroukie Tuff 2 volcanic complex (BKT-2) are important regional markers, and here we provide correlations between the Hadar and Ledi-Geraru project areas using major-element glass chemistry, stratigraphic relationships, outcrop characteristics, and 40 Ar/ 39 Ar dates. These correlations greatly expand existing temporal and spatial resolution, aid in interpretations of regional depositional environments, and increase the documented extent of BKT-2 to ~600 km 2 . BKT-2 exposures at Ledi-Geraru are the thickest and most complete yet observed. There, the BKT-2 complex is preserved as two air-fall lapilli layers, BKT-2U (<97 cm thick) and BKT-2L (<9 cm thick), separated by <2.5 m of silts and clays or diatomite that overlie the Green Marker Bed (GMB), a laminated ash tuff. Measured sections were evaluated to create a stratigraphy-based model of paleolandscape variations using BKT-2 tephra as laterally extensive isochronous surfaces. BKT-2 was mainly erupted into lacustrine and nearshore environments. The eastern Ledi-Geraru region was likely located at the depocenter of an expansive fluviolacustrine network. Representing the last major lacustrine phase of the Hadar Formation, lateral facies variations show the westward expansion of a lacustrine setting ca. 2.96 Ma followed by eastward regression initiated sometime prior to the eruption of BKT-2U ca. 2.94 Ma. High-resolution, well-correlated, and temporally constrained stratigraphic records are key to the interpretation of paleoenvironmental variation in East Africa.