The geology of Gona, Afar, Ethiopia

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 40Ar/39Ar 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.