Stratigraphy and geochronology of the late miocene adu-asa formation at gona, ethiopia

Lynnette L. Kleinsasser, Jay Quade, William C. McIntosh, Naomi E. Levin, Scott W. Simpson, Sileshi Semaw

Research output: Contribution to journalArticle

16 Scopus citations

Abstract

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 from5.2 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.

Original languageEnglish (US)
Pages (from-to)33-65
Number of pages33
JournalSpecial Paper of the Geological Society of America
Volume446
DOIs
StatePublished - Jan 1 2008

Keywords

  • Adu-Asa Formation
  • Ardipithecus kadabba
  • Ardipithecus ramidus
  • Gona
  • Tephrostratigraphy

ASJC Scopus subject areas

  • Geology

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