Lithostratigraphy and sedimentary environments of the hominid-bearing Pliocene–Pleistocene Konso Formation in the southern Main Ethiopian Rift, Ethiopia

The Pliocene–Pleistocene Konso Formation defined here is >180 m thick and is extensively exposed in the Konso area at the southernmost part of the Ganjuli graben of the Main Ethiopian Rift. The Konso area is known for its abundant vertebrate fossils, including those of Homo erectus and Australopithecus boisei, and its rich Acheulean archaeological assemblages. The fluvial, floodplain, lake margin, and lacustrine sediments of the formation were mostly deposited between 1.9 and 1.4 Ma, based on single-crystal 40Ar/39Ar ages of interbedded marker tuffs. The formation is subdivided into the Sorobo, Turoha, Kayle, and Karat Members, in ascending stratigraphic order. Each member contains dark brown or dark grey clay beds of lake margin and/or lacustrine origin, suggesting the recurrence of lake environments. Most of the fossils and artifacts derive from whitish grey or brown silt, sand, and gravel beds widely exposed between the finer sediments. These beds appear to have been deposited in an emerging marginal floodplain following repeated recession of the palaeo-lake. The depositional history of the Konso Formation reveals aspects of Quaternary rifting at the southern terminus of the Main Ethiopian Rift. Rifting, subsidence, and sedimentation in the Ganjuli graben occurred in the Konso area mainly between 1.9 and 1.4 Ma, while active faulting with associated volcanism during the Pleistocene was mostly confined to the middle part of the graben between Lakes Abaya and Chamo and to the Segen basin east of Konso. A shift of rift-related faulting and subsidence from the Konso area eastward to the Segen basin is likely to have resulted in the erosion and exposure of the fossiliferous Konso Formation.