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.

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.

Detailed lateral study of strata associated with the A.L. (Afar Locality) 333 hominin locality provides paleoenvironmental information at geographic scales of hundreds of meters to kilometers as well as insights regarding alluvial deposition and pedogenesis in the middle Denen Dora Member of the Hadar Formation. A.L. 333 is dated at ca. 3.2 Ma and has produced over 260 surface and excavated specimens of Australopithecus afarensis . It represents an unusual source of high-resolution information about the paleoenvironmental context of this hominin. The in situ hominin fossils are associated with the final stages of filling of a paleochannel and were buried prior to the formation of overlying paleosols. Preserved bedding structures in the fine-grained hominin-producing strata provide evidence that the abandoned channel continued to aggrade prior to the onset of sustained pedogenesis. Pedogenic carbonates associated with the hominin level thus postdate the death and burial of the hominins, possibly by centuries to millennia. The reconstructed paleodrainage of the DD-2 sandstone (DD-2s) is oriented south to north and consists of a trunk channel, ~40 m wide and 3–5 m deep, connecting a tributary system south of A.L. 333 to a distributary system to the north, which likely ended on the deltaic plain associated with the basin’s depocenter. The hominin concentration occurs in the upper part of the fill of the trunk channel. The burial of the hominin remains involved fine-grained deposition indicating low-energy, seasonal flood events, and there is no sedimentological evidence for a high-energy, catastrophic flood that could have caused the demise of the hominins.