Chronostratigraphy of the Miocene-Pliocene Sagantole Formation, Middle Awash Valley, Afar rift, Ethiopia

The Sagantole Formation comprises more than 200 m of lacustrine, alluvial, and volcaniclastic sediments, plus compositionally bimodal tephras and basa ltic lavas, exposed in a domelike horst named the Central Awash Complex in the southwestern Afar rift of Ethiopia, The Sagantole Formation is widely k nown for abundant vertebrate faunas, including the 4.4 Ma primitive hominid Ardipithecus ramidus. New Lithostratigraphic data are used to subdivide th e Sagantole Formation into the Kuseralee, Gawto, Haradaso, Aramis, Beidaree m, Adgantole, and Belohdelie Members, in ascending order. The members are d efined on the basis of lithologic differences and laterally continuous boun ding tephras. Ar-40/Ar-39 dating of 12 intercalated volcanic units firmly establishes the age of the Sagantole Formation to be 5.6 to 3.9 Ma, significantly older th an previous proposals based on erroneous correlations. Magnetostratigraphic data reveal eight paleomagnetic polarity zones, which can be correlated un ambiguously with the Thvera, Sidufjall, Nunivak, and Cochiti Subchrons of t he Gilbert Chron, Thus, by reference to the geomagnetic polarity time scale , seven additional chronological datums can be placed in the Sagantole Form ation. With a total of 19 such datums, the age resolution anywhere in the S agantole Formation is better than +/-100 k.y., making this the best-dated M iocene-Pliocene succession in Africa.