Deposits in the Gona Paleoanthropological Research Project (GPRP) area in east-central Ethiopia span most of the last ~6.4 m.y. and are among the longest and most complete paleoenvironmental and human fossil archives in East Africa. The 40 Ar/ 39 Ar and paleomagnetic dates and tephrostratigraphic correlations establish the time spans for the four formations present at Gona: the Adu-Asa (>6.4–5.2 Ma), Sagantole (>4.6–3.9 Ma), Hadar (3.8–2.9 Ma), and Busidima Formations (2.7 to <0.16 Ma). The volcano-sedimentary succession at Gona displays many classic tectono-sedimentary features of an evolving rift basin. The mixed volcanic and fluviolacustrine Adu-Asa Formation is the earliest expression of rifting at Gona, probably deposited in a small half-graben. The Sagantole and Hadar Formations were deposited in a much larger half-graben bounded to the E-NE by an as-yet-unidentified normal fault. The Sagantole and Hadar Formations are both fluvial and lacustrine, reflecting periodic shallow impoundment of a low-gradient paleo–Awash River, perhaps by an accommodation zone north of the Ledi-Geraru project area. Starting at 2.9–2.7 Ma, the character of sedimentation changed dramatically throughout the Awash Valley as bed load coarsened and the meandering paleo–Awash River cyclically cut and filled. Unlike the Hadar Formation, the Busidima Formation thickens westward, suggesting deposition in a half-graben of the opposite polarity compared to Sagantole/Hadar time. Sedimentation rates decreased 5-fold, from 0.25 mm/yr in the Hadar Formation to 0.05 mm/yr in the Busidima Formation, perhaps in response to slowing extension rates and/or opening of the half-graben north of Gona.

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.

Sediments exposed in the Dikika Research Project area form a nearly continuous sequence spanning the period from older than 3.8 Ma to younger than 0.15 Ma. By developing a stratigraphic framework of sedimentary basins, we are able to reconstruct a regional geological history that illuminates environmental changes resulting from tectonic events in the Afar triple junction region. The sequence begins with the Basal Member of the Hadar Formation, which was deposited on a dissected and deeply weathered surface of the uppermost flow of Dahla Series Basalt (8–4 Ma). This contact signals an increase in sediment accumulation rate due to active extension along faults parallel to the Red Sea Rift system. Sediments of the Hadar Formation indicate the progressive infilling of the Hadar Basin and migration of the shoreline northward or northeastward toward the axial depocenter, with several brief transgressions southward. After 2.9 Ma, the Dikika Research Project area was uplifted, and the Hadar Formation was faulted and eroded on an angular unconformity. Subsequent to 2.7 Ma, sedimentation returned, although the character and position of the newly developed Busidima half-graben had changed. This basin was formed by the rotation of an asymmetric marginal half-graben around a border fault that paralleled the western escarpment of the Ethiopian Rift. The Busidima Formation deposited in this basin records the migration of the paleo–Awash River across its floodplain in response to a changing tectonic setting. These local paleoenvironmental changes are primarily the response to regional tectonics and are superimposed on the global and regional records of climate change.

Mapping and description of the Hadar and Busidima Formations in the northern Awash valley, Ethiopia, have been greatly aided by the use of tephrostratigraphy and tephra correlation in the Dikika, Gona, Hadar, and Ledi-Geraru paleoanthropological project areas. The Hadar Formation contains at least nine dated tuffs, many of which have been correlated across the northern Awash project areas, and all of which are easily distinguished from each other on the basis of major-element chemistry. The overlying Busidima Formation contains at least 35 distinct tuffs, many of which are firmly or approximately dated. Because of their discontinuous and compositionally similar nature, many of the Busidima Formation tuffs are not correlated across the northern Awash project areas. Trace-element compositional data or detailed stratigraphic information may be necessary for correlation or relative placement of many of the Busidima Formation tuffs. Differences in the frequency, chemistry, and extent of Hadar and Busidima Formation tuffs preserved in the northern Awash valley may ultimately be related to the tectonic evolution of the region throughout the Pliocene-Pleistocene, as well as to basin-scale geological processes. Despite a number of known issues in tephra correlation, the composite tephrostratigraphy assembled for the northern Awash valley demonstrates the effectiveness of this technique, which has played a key role in ongoing efforts to document the geological history of this unique and important region.

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.

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.