The Gona area includes many rich fossil localities that are of great consequence to the study of human evolution. The Adu-Asa Formation, containing the oldest of these fossils, consists of nearly 200 m of fossil-bearing sedimentary rocks in thin (≤;30 m), laterally variable sections interlayered with abundant basaltic lava flows. These volcanic and sedimentary rocks dip gently to the east and are repeated by north-northwest–trending, mostly west-dipping normal faults that accommodate extension in the Afar Rift. The volcanic rocks in the Adu-Asa Formation are strongly bimodal. Basaltic lavas and tuffs are abundant, but we have also identified a rhyolite center and seven different silicic, or dominantly silicic, tuffs. Of these tuff units, we were able to identify four major tuffs across the Adu-Asa Formation at Gona by combining geochemical comparisons with detailed stratigraphic sections through fossil-bearing deposits: the Sifi, the Kobo’o, the Belewa, and the Ogoti Tuffs. New 40 Ar/ 39 Ar dates of these and other tuffs, as well as basalt flows, indicate that the formation spans the period from 5.2 Ma to 6.4 Ma, although the oldest deposits within the Gona Paleoanthropological Research Project (GPRP) area have yet to be thoroughly surveyed. Known fossil localities within the Adu-Asa Formation at Gona are grouped into three temporal clusters, ranging in age from ca. 6.4 Ma to ca. 5.5 Ma.

Ardipithecus fossils found in late Miocene and early Pliocene deposits in the Afar region of Ethiopia, along with Sahelanthropus tchadensis from Chad and Orrorin tugenensis from Kenya, are among the earliest known human ancestors and are considered to be the predecessors to the subsequent australopithecines ( Australopithecus anamensis and Australopithecus afarensis ). Current paleoenvironmental reconstructions suggest a wooded habitat for both Ardipithecus kadabba and Ardipithecus ramidus but more open and varied environments for other hominids living in Africa during the late Miocene and early Pliocene. To further evaluate the environmental context of Ardipithecus , we present stable carbon isotope data of 182 fossil herbivore teeth from Ardipithecus -bearing fossil deposits in the Gona Paleoanthropological Research Project area, in the Afar region of Ethiopia. The sampled teeth include representatives of all major fossil herbivore taxa and the majority of the mammalian biomass that lived in the same time and place as the hominids. When compared to extant herbivores from East Africa, the spectra of isotopic results from herbivores found in late Miocene Ar. kadabba and early Pliocene Ar. ramidus sites at Gona are most similar to isotopic values from extant herbivores living in bushland and grassland regions and dissimilar to those from herbivores living in closed-canopy forests, montane forests, and high-elevation grasslands. The tooth enamel isotopic data from fossil herbivores make it clear that Ardipithecus at Gona lived among a guild of animals whose diet was dominated by C 4 grass, and where there is no record of closed-canopy vegetation.

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.

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.