Wilson M. Ngecu

Lithostratigraphy, provenance and facies distribution of Archaean cratonic successions in western Kenya

Abstract The greenstone belt of the Tanzanian shield in Western Kenya is composed of two supracrustal successions, which form the Nyanzian and Kavirondian Groups. The Nyanzian Group at the base is composed of mafic tholeiitic basalts, calc-alkaline dacites and rhyolites. The group is unconformably overlain by the Kavirondian Group. During recent field mapping, the Kavirondian Group was divided into three formations. The Shivakala Formation consists of thickly bedded basal conglomerates, which are interbedded with thin sandstone beds. The Igukhu Formation conformably overlies the Shivakala Formation and is composed of thickly and locally thinly bedded greywacke. The uppermost Mudaa Formation is composed of blocky mudstones and thinly laminated shales. A high proportion of volcanic, granitic and chert pebbles in the conglomerates, along with abundant quartz, feldspars and mudstone fragments in the greywacke, indicates a mixed provenance of volcanic, granitic and recycled sedimentary rocks. Primary sedimentary structures and lithofacies associations indicate that the conglomerates were deposited in an alluvial fan/fan-delta setting. The greywackes represent proximal turbidites while the mudstone and shales were deposited mainly as distal turbidites. In the study area there is no evidence of transitional nearshore or shallow marine facies transitional to the continental and deep marine facies.

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 palaeolakes, 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 palaeolake 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.