The geology of Gona, Afar, Ethiopia

Deposits in the Gona Paleoanthropological Research Project (GPRP) area in east-central Ethiopia span most of the last similar to 6.4 m.y. and are among the longest and most complete paleoenvironmental and human fossil archives in East Africa. The Ar-40/Ar-39 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.