To date and characterize depositional environments of the hominin-bearing Hadar Formation, lacustrine sediments from the eastern part of the Hadar Basin (Ledi-Geraru research area) were studied using tephrostratigraphy and magnetostratigraphy. The Sidi Hakoma Tuff, Triple Tuff-4, and the Kada Hadar Tuff, previously dated by 40 Ar/ 39 Ar in other parts of the basin, were identified using characteristic geochemical composition and lithologic features. Paleomagnetic samples were collected every 0.5 m along an ~230-m-thick composite section between the Sidi Hakoma Tuff and the Kada Hadar Tuff. A primary detrital remanent magnetization mostly carried by (titano-) magnetites of basaltic origin was recognized. Consistent with existing data of the Hadar Basin, paleomagnetic directions show a postdepositional counterclockwise vertical-axis tectonic rotation (~5°–10°) and shallowing of paleomagnetic inclination (~5°–10°) related to sedimentation and compaction. Two normal-polarity intervals (chrons 2An.3n and 2An.2n) are recorded bracketing a reversed interval identified as the Mammoth event (chron 2An.2r). Resulting sediment accumulation rates (~90 cm/k.y.) are high compared to existing accumulation-rate estimates from the more western part of the Hadar Basin. The resulting eastward increasing trend suggests that deposition took place in an eastward-tilting basin. Sediment accumulations were constant throughout the basin from ca. 3.4 to 3.2 Ma. At 3.2 Ma, a regional and relatively short-lived event is indicated by significant change in depositional conditions and a large increase in accumulation rate. This disruption may have been related to increased climate variability due to astronomical climate forcing. It provides a possible explanation for changes in the Hadar faunal community and Australopithecus afarensis in particular.

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.