Sequence and geochronology of the Kenya rift volcanics

Abstract

The Upper Cenozoic volcanic formations associated with the Kenya rift valley are divided into seven main stratigraphic groups representing widespread ‘plateau’ formations, and a number of central volcano groups of more localised occurrence. 75 K/Ar isotopic age determinations including 30 unpublished dates are listed and the age-range of most of the volcanic groups is given. A tabulation of the sequence of volcanic groups is provided and their distribution is illustrated.

The Kenya rift volcanics were erupted nearly continuously from Early Miocene to Holocene times. Miocene volcanism was mostly of nephelinites, alkali basalts and phonolites. Pliocene activity was trachytic, phonolitic and nephelinitic in central and southern parts of the rift valley, while contemporaneous basaltic volcanism took place along its whole length. The only extensive rhyolitic activity occurred in northwest Kenya in Pliocene times. Late Pliocene and Quaternary volcanism was mainly trachytic in the rift valley floor and basaltic to the east.