| dc.description.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. | en |
