an engineering geological characterisation of tropical clays - GBV

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Chapter 3
Geology
3.1 Introduction
Geological records show that much of central Kenya, of which Nairobi and the present study
area are part, underwent peneplanation on more than one occasion in the period of late
Precambrian to Tertiary (Saggerson, 1991). Doming, linear warping and erosion of the then
sub-Miocene erosion surface was succeeded by the extrusion of the first lava flows which
flooded the eastern plains, including Nairobi and the present area, during mid-Miocene time.
This was followed by intermittent volcanicity which continued until Recent time. The
volcanicity was genetically associated with tectonic movements and the development of the
Rift Valley to the west of these areas. The western parts of Nairobi and sections of the present
study area have also been affected by faulting on at least three occasions, resulting in
spectacular topography. The lavas and pyroclastics originated mainly from fissure eruptions
of the Rift region. Two central volcanoes, Ngong and Ol Esayeiti, were also active during this
period and contributed their share of lavas and pyroclastic material to the eastern plains. The
volcanoes are located to the south-west of Nairobi city.
The geology of Nairobi and the present study area is, therefore, characterised mainly by a
succession of volcanic rocks and pyroclastics of Cainozoic age (Saggerson, 1991). Underlying
the volcanic rocks is a foundation of folded crystalline metamorphic rocks (gneisses and
schists) of Precambrian age which belongs to the Mozambique Belt. The metamorphic rocks
are only occasionally exposed and some of their fragments have been noted in agglomerates
previously derived from the former Ngong volcano, to the west of the area.
The volcanic rocks of Nairobi and its environs, including the study area, are a part of a wider
East African alkaline suite characterised by a dominance of soda over potash. The rocks are
distinguished into two groups; the first of which is a strongly alkaline series represented by
feldspathoid-bearing phonolites, basanites, tephrites and more basic varieties. The second
group is a mildly alkaline series and includes feldspathoid-free rocks containing soda-rich
amphiboles and pyroxenes. Differentiation of members of the two series is accompanied by
an increase in silica content, giving rise to trachytes, rhyolites and obsidians. On the contrary,
however, volcanic rocks associated with the Western Rift Valley (to the west, outside Kenya)
belong to the potash-rich alkaline series (Saggerson, 1991).
The rock formations are covered and overlain by thick deposits of soils and gravels of
Quaternary age. However, areas within the Rift are characterised by loess and lacustrine
deposits, some containing diatomaceous beds; and all of which serve to reflect major changes
in climatic conditions (Saggerson, 1991).
A number of authors have assigned various ages to the volcanic rocks of Nairobi region. A
majority of the formations are suggested to be Tertiary to Recent. Gregory, on the other hand,
was of the view that the earliest lavas of the Kapitian series were Cretaceous (Gregory, 1921).
Potassium-argon age-dating of the rocks was done by Curtiss, Evernden and Miller, of the
University of California. Their findings indicate volcanicity as having began in Upper
Miocene, and had almost completely ceased by the end of Pleistocene (Evernden et al, 1964).
The dating was performed on whole rock specimens as well as specific minerals
(anorthoclase, sanidine, feldspar, nepheline), and shows the ages of the rocks as varying from
0,8 Ma (Plateau trachyte series, Magadi road) to 13,40 Ma (Kapiti phonolite, Stony Athi