The Pliocene Hadar Formation in the west central Afar sedimentary basin, Ethiopia, is noted for important discoveries of Australopithecus afarensis and what may be the world's oldest artifacts. It consists of over 250 m of terrestrial sediments subdivided into four members, the Basal, Sidi Hakoma, Denen Dora and Kada Hadar, on the basis of laterally extensive interbedded tuffs and bentonites that have been radiometrically dated. Four complexly interfingering lithofacies associations (A, B, C, and D) have been identified that represent (1) alluvial fan, braided stream, and braid delta (A), (2) meandering stream and associated floodplain (B), (3) marginal and shallow lacustrine (C), and (4) delta plain with distributary channel, marsh and floodplain deposits (D), respectively. Lithofacies association B documents 11 fining upward cycles of major axial meandering stream systems draining the Ethiopian highlands and flowing northwards within the Ethiopian Rift toward the Red Sea-Gulf of Aden. This lithofacies is interrupted at least three times by lithofacies association C that records lacustrine conditions forming during a period of rapid basin subsidence resulting from downward displacement along major normal faults bounding the Ethiopian Rift. Lithofacies association A interfingers laterally with lithofacies associations B and C. It represents relatively short, transverse, braided streams, deposited primarily during periodic high discharge. These flowed from the Ethiopian Escarpment into the rift and formed alluvial fans along its margin and coarse braid deltas as they emptied into lakes or coarse tongues where they joined the axial meandering streams. Lithofacies association D overlies and interfingers with lithofacies association C and documents the lateral transition from fining upward fluvial cycles of meandering streams to deltas prograding into and filling the lakes;Recently, a major disconformity was recognized stratigraphically above the Bouroukie Tuff (BKT2) in the middle part of the Kada Hadar Member. Analysis of the sandstones of the Hadar Formation indicates that augite and magnetite are the dominant heavy minerals throughout the unit. However, an increase in augite occurs just above the unconformity. The heavy mineral assemblage is distinctly volcanic and was derived from the adjoining Western Ethiopian Escarpment. The increase in augite records the relative uplift of the escarpment and the unroofing of basaltic rocks. Petrographic studies of rock fragments also indicate that the sandstones below the unconformity contain a higher concentration of acidic volcanic rocks reflecting unroofing of the Oligocene volcanic rocks of the Western Ethiopian Escarpment. In contrast, above the unconformity, granule to pebble conglomerates of the upper part of the Kada Hadar Member are characterized by high concentrations of mafic volcanic rocks indicating their probable derivation from the pre-Oligocene basalts of the Western Ethiopian Escarpment;Most of the significant in situ discoveries of Australopithecus afarensis are from facies that represent the floodplain of a major north-flowing meandering stream in the Denen Dora and the lower portion of the Kada Hadar members. The "Lucy" skeleton was recovered from a thin sheet sandstone that extends laterally as "wings" from the ribbon-like Kada Hadar # 1 sandstone (KH-1). This sheet sandstone represents a crevasse splay deposit that spilled out across the floodplain from the KH-1 channel. The remains of the "First Family" were recovered from a siltstone that represents a floodplain associated with a subjacent distributary channel of the Denen Dora Member sandstone (DD-2). (Abstract shortened by UMI.)