| Summary: | The Ribkwo area is divided into two contrasting east and west parts by the Kito Pass fault system, This has upfaulted the area to the west of the Chepanda Hills and Sigatgat Hill exposing 6,000 to 7,000 feet of phonolite and tuffaceous sediments with subordinate basaltic lavas (the Tugen Hills Group of Miocene to middle Pliocene age) resting on metamorphic basemento The phonolites in the Chepanda Hills form the flanks of a large dominantly phonolite volcano with its centre in the Sigatgat Hill area. The Barpelo Basalts followed the Tugen Hills Group and were erupted from several minor centres in the area to the east of the Kito Pass fault system c 1 came after its first major movement in the middle Pliocene. Flood trachytes followed the basalts and occur on both sides of the fault system in tho south of the area. They are overlain by fossiliferous sediments of upper Pliocene age which were deposited in a tectonic basin to the east of the Chepanda Hills, Further movements on the Kito Pass fault system preceded the build up of Ribkwo, a large dome shaped upper Pliocene volcano of phonolitic trachyte, which covers most of the eastern part of the area. Its south-eastern flank is overlain by thin formations of basalt, mugearite, and tuff which range from upper Pliocene to Pleistocene age. A tectonic history, consisting of major and minor faulting episodes, is described and is broadly similar to other areas in the northern part of the Rift Valley. The faulting is entirely of the normal type and is consistent with tensional tectonics. The petrography of the lavas is described and their chemistry and mode of evolution is discussed.
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