KSD-VP-1/1, a partial skeleton of Australopithecus afarensis, was excavated from Pliocene strata at Korsi Dora, 3.3 km southeast of the confluence of the Waki and Mille rivers in the northwestern part of the Woranso-Mille paleoanthropological research site. A tuff collected from ~2.7 m below the fossil horizon, at the bottom of a trench dug 25 m to the east of the fossil excavation, yielded an 40Ar/39Ar age of 3.60 ± 0.03 Ma for anorthoclase feldspar. Strata in the trench and the fossil excavation site comprise a single normal magnetozone interpreted as part of the normal subchron C2An.3n, immediately above the Gauss/Gilbert paleomagnetic transition. Geologic mapping and tephrochemical analyses combined with paleomagnetic data place the fossil horizon and the trench section into local and regional stratigraphic context by constraining the partial skeleton to be younger than the Kilaytoli tuff (KT), a ~4 m thick vitric ash with an anorthoclase feldspar age of 3.570 ± 0.014 Ma. This unit is widely recognized at Korsi Dora, in collection areas north of the Waki-Mille confluence and outside the field area. The KT correlates with the Lokochot Tuff of the Omo-Turkana Basin in Kenya. Sedimentological features of the mudstone and sandstone in and near the excavation site are consistent with deposition in a floodplain or floodplain lake proximal to a stream channel.