an engineering geological characterisation of tropical clays - GBV

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The Nairobi phonolite consists of a number of flows. This is evidenced by age determinations
made on feldspars from the rock, and which range from 5,2 to 10,2 Ma, suggesting the
possibility of more than one age for the lava. Observations in quarries have shown the various
flows to be generally between 30-46 m thick. Further evidence is provided by borehole
records, which show the thickness of the rock unit to be 79 m (BH 2023) at Embakasi in the
south –eastern part of the area; and 65 m (BH C201) at Wilson Airport in the western part of
the area. The base of the flow commonly consists of pumiceous and dribbly lava overlying a
slightly uneven surface of agglomeratic tuff (Saggerson, 1991).
Borehole evidence and flow directions recorded in the field suggest the lavas of Nairobi
phonolite having derived from the Muguga area, to the west of Nairobi city and present area.
The lavas flowed in a south-easterly stream across Nairobi and the present area, before
consolidation. Maximum thicknesses of the lavas therefore occur at/ and around the Jomo
Kenyatta International Airport located in the eastern part of the study area. The lavas
generally thin out north-westwards, as well as farther south-eastwards and southwards up to
Athi River township, located beyond and outside the present area (Saggerson, 1991).
Aerial photographs of the plain at Jomo Kenyatta International Airport reveal sub-circular
patches with crudely linear arrangements beneath the black cotton soil cover. These patches
probably represent the areas of maximum weathering at intersections of master joint-planes
(Scott, 1963).
Parts of the Nairobi National Park between Bushy Vale and Rocky Valley show the Nairobi
phonolite separated from the underlying Mbagathi phonolitic trachytes by some thickness of a
few feet of dark grey tuff, which belongs to the Athi Tuffs and Lake Beds Series. Outcrops
generally reveal minor differences in the Nairobi phonolites. They are characterised by a
marked fissility and platy texture developed locally at and/ or near the surface, and which is
attributed to flow phenomena. They also reveal the vesicular character of the upper portion of
a number of flows. However, the rocks are seldom amygdaloidal (Saggerson, 1991).
Fresh hand specimens of the rock are black or blue-grey to blue in colour. However, deep
weathering has altered much of the upper portions of the rock located near the ground surface.
The rock is also lacking in large phenocrysts, the phonolite having undoubtedly resulted from
the rapid cooling of an already viscous lava.
Weathering usually converts the Nairobi phonolites to pale brown and reddish ferricrete
deposits (murram), which are observed to overly the rocks in much of the study area. In other
parts, however, especially in the eastern and south-eastern sections of the present area, the
lava surface has been autobrecciated to form a boulder-bed consisting of rounded and angular
fragments of phonolite cemented in a light brown, weathered base of the same material
(Saggerson, 1991).
Thin sections of Nairobi phonolite exhibit long tabular phenocrysts of sanidine in a finegrained
groundmass. The groundmass consists of patches of soda-amphibole which include
cossyrite and kataphorite, while bright green aegirine and/ or aegirine-augite are found
associated. Nepheline is found occurring as small micro-phenocrysts, or as crystals in the
groundmass; larger crystals of nepheline commonly alter into yellowish zeolite, sodalite,
natrolite, analcime and calcite. Strongly pleochroic biotite with magnetite inclusions, orangebrown
mica flakes as well as occasional zeolite-filled vesicles may also be found (Fairburn,
1963). The matrix composition is also summarised in Table (3.4). Table 3.2 gives the results
of chemical analyses of Nairobi phonolite, as well as other rocks for comparison purposes.