The Lemudong’o Formation is defined here as part of a late Miocene to Late Pleistocene sequence of stratified lavas, air-fall and waterlain tuffs, lacustrine, alluvial, and fluvial sediments, and paleosols, that crop out over an approximately 25*50 km area on the western margin of the southern Kenyan Rift Valley, approximately 100 km west of Nairobi. The study area is deeply incised by three major permanent river systems that expose sediments of three late Neogene lake basins. The Lemudong’o Formation comprises deposits of the second paleolake basin, which formed during the late Miocene. Stratigraphic sections in several localities are described and correlated in this report, the Lemudong’o Formation is defined, and a basin sedimentary history and environmental reconstruction is proposed. The Lemudong’o Formation has three main phases of sedimentation with a total thickness of 135 m. Phase 1 is represented by predominantly lacustrine and lake-margin siltstones, mudstones,and sandstones. Phase 2 comprises paleosols in the basin center, and fluvial and alluvial sediments on the eastern basin margin. Phase 3 comprises mainly waterlain tuffs and silts, capped by a welded tuff. Phase 2 may reflect a more arid climate, or a lower basin-overflow elevation. Four tuffs in upper phase-1 mudstones in Lemudong’o Gorge are dated to 6.12 – 6.08 Ma. The main fossil-bearing horizons at Lemudong’o Gorge Locality 1 lie between, and immediately above, the dated tuffs. Fossils are associated with beach and/or deltaic sands and fine gravels, and silty and sandy claystones representative of an intermittently flooded lake margin.Stanley H. Ambrose, Christopher M. Nyamai, Eliud M. Mathu and Martin A. J. William
