The geology of the Kavirondian group of sediments

Abstract

The Kavirondian sediments form the Kavirondian Group and are exposed both in Kakamega and South Nyanza Districts in western Kenya. This lithological Group forms one of the three stratigraphical Groups which constitute the Nyanzian Shield. The other two Groups are the Nyanzian Group and the Dodoman Group. The latter Group has not been identified in western Kenya and therefore the Kenyan part of the Shield is composed only of the Nyanzian and the Kavirondian Groups. The Nyanzian Group is composed of the tholeiitic and calc-alkaline volcanic rocks and it is unconformably overlain by clastic sediments of the Kavirondian Group. In this work, the stratigraphy, provenance and environments of deposition of the Kavirondian sediments have been investigated. An area of about 2,360 square kilometres which is covered by the greenstone rocks of the Nyanzian Shield was geologically mapped using base maps, aerial photographs and spot images. The base maps were at a scale of 1:50,000 but the geological maps compiled were at a scale of 1:25,000. Rock samples were also collected for petrographical, geochemical and geochronological analysis. Petrographic investigations of the Kavirondian sediments were carried out through the preparation and microscopic study of thin sections of samples from the study area. Samples for the geochemical work were crushed, milled and pulverised. The powders were then converted into pellets by pressing in collapsible aluminium cups. Analyses were made using a combination of techniques including X-ray fluorescence (XRF) spectrometry, atomic absorption spectrometry (AAS) and induced couple plasma (lOP). One set of rock samples was crushed and milled for isotope studies. Rubidium-strontium whole rock analysis was carried out in order to provide radiometric ages. The result of petrographical and chemical analyses of the sediments show that the Kavirondian sediments were derived from a multiple of sources which include older granite rocks, volcanic rocks and recycled sedimentary rocks. The radiometric dating indicates that the Kavirondian sediments are about 2,500 Ma old. The age of the granite intrusions which occur at the margins of the sedimentary formations is about 2,400 Ma. These radiometric ages indicate that in the Nyanzian Shield of western Kenya, the Nyanzian Group rock suites which form an angular unconformity with the Kavirondian Group sediments are the oldest rocks in the Shield. The Kavirondian sediments are younger than the Nyanzian Group volcanic rocks and finally the granite intrusives which occur on the margins of the sedimentary formations are the youngest. All the three rock suites were formed between middle Archaean and early Proterozoic. Field studies of the lithological relationships and the primary structures indicate that the Kavirondian qroup can be divided into four formations. These include the Shivakala Formation which is...stratigraphically lowest in the succession, the Igukhu Formation, the Mroda Fop!lation and the Mudaa Formation which is stratigraphically uppermost in the succession. Lithological relationships and primary structures also revealed that the basal conglomerates were deposited in a sedimentary basin as alluvial fan deposits while the resedimented conglomerates, greywackes and mudstones were deposited by turbidity currents in relatively deep water on the upper, middle and lower fans. It was similarly noted that there exists a sedimentological gap between the terrestrial sedimentation of basal conglomerates and the marine sedimentation of the turbidites. The gap is indicated by lack of transitional sediments between the terrestrial and marine sediments. Lithological correlation of the sediments was attempted. However, due to the sudden changes of lithofacies and the diachronous nature of the lithological boundaries, lithostratigraphical correlation at regional scale was not possible. Therefore, lithostratigraphic correlation was quite local in nature. Geochronological results were, however, used to infer the stratigraphic position of the Kavirondian sediments in relation to the Nyanzian rock suites and the surrounding granite intrusions. The results were also used to' propose chrono-correlations between the Kavirondian sediments and Archaean sediments found in other Archaean terraines in the world.