Lithostratigrapy and distribution of Pleistocene sediments of the Munyu wa Gicheru Formation in South Kenya Rift Valley

Abstract

Pleistocene sediments of the eastern African Rift Valley have been subjected to intensive study over the last few years, because they are very useful in the study of palæolakes, climatic changes and the study of lacustrine diatomite deposits, which are a source of industrial raw materials. In this paper, the results of field study on the lithostratigraphical divisions and distributions of Pleistocene sediments in South Kenya Rift Valley is documented.

The palæolake beds at Munyu wa Gicheru lie in a trough, which occurs on a platform at the eastern flank of the South Rift Valley of Kenya. The beds have been divided into three lithostragraphical members, which form the Munyu Formation. At the base of the Gicheru Sequence, the lower member is composed of basal massive obsidian layers, which are overlain by gravel beds. The upper parts of the member consist of a massive diatomite bed, which contains intercalations of greyish clay. The middle member is composed of massive beds of clean white diatomite with varying amounts of silt. The unit is capped by pumiceous volcanic ash. The upper member, which experienced several breaks in sedimentation, consists of reworked, silty diatomite overlain by a phonolite layer. Within the diatomite beds are irregular and discontinuous lamitations of tuff. Diatom species and lithological relationships indicate that the lacustrine sediments were deposited on a westdipping platform bounded by the Kedong Fault on the west and the Kiriri Fault in the east.