an engineering geological characterisation of tropical clays - GBV

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River). The dating also involved rock specimens from Nairobi metropolitan and present study
areas, and included Nairobi trachyte (3,17-3,45 Ma), Nairobi stone (4,84-5,67 Ma), Nairobi
phonolite (5,2-10,22 Ma) and Kapiti phonolite (12,90-13,40 Ma). There is also the likelihood
that volcanic activity continued into Recent times in areas to the west of Nairobi. This is
evidenced by the occurrence of weathered tuffs and ash layers within soil profiles,
representing the eruptive periods of the later times (Saggerson, 1991).
A summarised discussion of the major rock types underlying the present study area is given in
the following sections.
3.2 Precambrian rocks of the Mozambique Belt
The crystalline metamorphic rocks of Precambrian age outcrop in areas where the volcanic
cover has been removed by erosion. They are especially noticeable in the southern and southwestern
parts of the study area where they occur as isolated exposures in shallow stream
courses, or as float on the stream edges. They are generally small highly weathered outcrops
consisting mainly of layered fine-grained schists and coarse gneisses that have been invaded
in some places by pink quartzo-feldspathic pegmatites. Also found but to a lesser extent are
biotite-garnet-epidote gneisses, hornblende gneisses and quartz-feldspar gneisses.
The structures revealed in the metamorphic rocks generally indicate north-south trending
folds. However, the rocks also show some variations in strike and dip due to generally smallscale
folds. They also exhibit lineations which plunge at approximately 14° - 21° to the southwest;
and which are most probably related to the north-east to south-west fold trends that have
been recognised in neighbouring areas (Saggerson et al, 1960). The north-south and north-east
to south-west fold trends are typical of much of southern Kenya, including the Nairobi area,
and serve to point to the fact that the underlying Precambrian rocks have been subjected to a
minimum of two phases of deformation (Saggerson, 1991).
In other areas, the metamorphic rocks are unexposed and separated from the overlying
volcanic rocks by a plane of unconformity which represents an erosional surface formed in
early Miocene. Evidences of the erosional surface occur in the form of thin layers of
weathered metamorphic materials separating the Precambrian and volcanic rocks, and
exposed in some of the streams in the southern part of the study area. However, cases of soil
materials (clays, sands, detritus) directly overlying the Precambrian rocks also occur, and is
evidenced by a number of borehole records in the present area. According to Sikes (1939), the
presence of a former topography across the study area is also revealed by borehole records,
which show the crystalline rocks as occurring at different elevations above sea-level.
Borehole records of the Nairobi Station (BH 1937c; 37° 00`E, 1° 24`S), for instance, show the
erosional surface to have been penetrated at an elevation of about 838 m above sea-level. In
other places, the crystalline rocks were intercepted at elevations of 1519 m above sea-level
(BH 2655) in the south-eastern part of the area; 91 m depth below ground surface at Wattle
Blossom Farm, southern part of the area; and 1509 m above sea-level in the area around
Nairobi National Park. Records also reveal the crystalline metamorphic rocks to have
undergone cyclic peneplanations in early Tertiary. Evidence of earlier sedimentation is,
however, absent. The latest metamorphic events to affect the rocks are dated at 480-530 Ma,
based on the age of Phlogopite in crystalline limestone from Turoka, in southern Nairobi area
(Saggerson, 1991).
Early studies on soil samples collected from around Embakasi in the south-eastern part of the
present area (Saggerson et al, 1968; Stephen et al, 1956) showed them to contain typical