The geology of the Ribkwo area, Baringo district, Kenya.
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
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.