The geological evolution of the Northern Kamasia Hills Baringo District, Kenya

Abstract

The Kamasia Hills are formed essentially by a line of 'en echelon' tilted blocks within the main rift valley in northern Kenya. The metamorphic basement crops out locally at the foot of the eastern fault-scarps of the range and is overlain by the thickest sequence of the late Miocene plateau phonolites so far known. in Kenya. This sequence, which includes minor amounts of basic and intermediate lavas and thick sedimentary units, is described, together with that of a thick series of Pliocene basalts, trachytes and fossiliforous sediments exposed in the foot-hills to the east of the main range. Isotopic (K-Ar) age determinations from the lavas facilitate the dating if the main geological events. Deformation is by normal faulting, associated with block tilting and fault-bounded flexuring on axes normal to the dominant fault trend. Faulting episodes are identified by consideration of stratigraphic relationships and movement on the largest faults is shown to have occurred repeatedly. Major features of the geomorphology are described and a geomorphde tectonic history is constructed for the area. Volcanism was mainly of the plateau, lava1 type; very few volcanic centres were found in the area but it is apparent that the western limits of both volcanic activity and minor faulting have moved towards the rift centre with time. The petrography of the lavas is summarised and fifty-one representative chemical analyses are presented and discussed, A major faulting episode in the early Pliocene which, from field relations, clearly terminated the phase of phonolite lava extrusion is shown also to define a distinct petrochemical break-with respect to both the mafic and the felsic lavas. Problems of petrogenesis are briefly considered.