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SOUTH EASTERN KENYA UNIVERSITY
INSTITUTE OF MINING AND MINERAL PROCESSING
DEPARTMENT OF GEOLOGICAL SCIENCES
A GEOLOGICAL FIELDWORK REPORT ON THE
GEOLOGY OF MATUU-MAVOLONI AREA
MAP SHEET NO 149/2 AND 150/1
SGL 308; FIELD GEOLOGY
A DESSERTATION SUBMITTED TO SOUTH EASTERN KENYA UNIVERSITY IN PARTIAL
FULFILMENT OF THE REQUIREMENT FOR THE UNDERGRADUATE DEGREE IN BACHELOR OF
SCIENCE GEOLOGY
BY
DICKSON OMBEVA LWIMBU
REG NO: I13/0180/2013
EMAIL ADDRESS: [email protected]
SUPERVISED BY: MR. LINCOLN K. GITHEYA AND PROFESSOR ELIUD MATHU
JUNE 2016
ACKNOWLEDGEMENT
Regards to South Eastern Kenya University and its management for financial support to,
during the fieldwork.
I acknowledge Professor Eliud M. Mathu, the director of the institute of mining and mineral
processing, South Eastern Kenya University, for his complete dedication, insights and
devotion to the discipline of geology in the whole country as a pioneer of geology to most
students and I to the worth of the career and its applications to the real world situations. He
is really a role model to every geology student.
I thank Dr. Kariuki, the chairman of the department of geological sciences, for his
participation in notifying on the time of the event and informing us on the requirements.
To our Lecturer, Mr. Lincoln Gikenya for his input of geology knowledge and report writing
in Introduction to fieldwork.
I also acknowledge the presence of all my classmates for their academic support and
company during the fieldwork study.
Above all I thank God for His mercy that I have come this far in my academics especially in
geology.
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DEDICATION
I dedicate the work of my report to the rest of upcoming scholars of geology in
my country. I would be glad to witness the triumph of this career in future.
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ABSTRACT.
This geological report describes an area that is around Matuu town towards the North of
Ikaatini and from Katulani towards Mavoloni to the west located in the North of Machakos
County in Kenya. It is bounded by Northings 98 72 and 9884 and Easting 3208 and 3469. It lies
on the east Africa Mozambique belt rock system of Precambrian and early Cambrian ages.
Tertiary volcanic sediments like the tuffs are found in the south eastern part of the map
around Mbingoni and at the foot of Mavoloni hill in the North eastern part of the map. The
quaternary deposits like the limestones and marl overlay the basement system of
metamorphic rocks comprised of Biotite-muscovite schist/gneiss, amphibolite and
Migmatites, the Basement System rocks are metamorphic, and have been in places
granitized to a considerable degree, with the production of granitoid gneisses especially
towards the north of Ndalani, west of Mavoloni and around Ekalakala. The north-western
end of the lava-capped Yatta Plateau passes across the area east of Ol Doinyo Sabuk, and
the surface on which the lava rests is believed to represent a remnant of the sub-Miocene
bevel and comprises of the Kapiti phonolites and trachytes with tuffs. It is believed the
basement system to present the original sedimentary series of limestone,shale and
sandstone into which the basic magma has been intruded. Most of the outcrops in the south
western area are consist of folded basement system gneisses and schists. The type of the
soil in the area depend on the drainage, sand soil and murram form in well drained regions
while black cotton soils develop in poorly drained areas. The metamorphic rocks grain size
range from fine grained schists to course grained gneisses rich in minerals such as biotite,
muscovite and hornblende.
The area is economically rich with volcanic sediments and deposits such as phonolites, tuffs
and sand, which are used in building and in road construction. The gneisses are also used in
road construction amongst other uses. These activities provide job opportunities to the local
people around the area.
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TABLE OF CONTENTS.
ACKNOWLEDGEMENT ................................................................................................................................... 2
DEDICATION ...................................................................................................................................................... 3
ABSTRACT. ......................................................................................................................................................... 4
TABLE OF CONTENTS. .................................................................................................................................... 5
Table of figures ..................................................................................................................................................... 7
TABLE OF SLIDE IMAGES ..................................................................................................................................... 7
CHAPTER ONE. .................................................................................................................................................. 8
INTRODUCTION .................................................................................................................................................. 8
1.1 Geographic setting. .................................................................................................................................. 8
1.1.1 Location ............................................................................................................................................ 8
1.1.2 Communications and accessibility .................................................................................................. 8
1.1.3 Administration ................................................................................................................................. 8
1.1.4 Physiography.................................................................................................................................... 9
1.1.4.1 Drainage.................................................................................................................................... 9
1.1.5 Climate and Vegetation ................................................................................................................... 9
1.1.6 Rainfall ............................................................................................................................................. 9
1.1.7 Temperature .................................................................................................................................... 9
1.2 GEOLOGICAL SETTING. ............................................................................................................................ 9
1.2.1 Regional Geology ............................................................................................................................. 9
1.2.2 Local Geology ................................................................................................................................. 10
1.3 PURPOSE AND OBJECTIVE OF THE FIED WORK. ................................................................................... 10
1.3.1 Methodology.................................................................................................................................. 10
1.3.1.1. Planning. ................................................................................................................................ 10
1.3.1.2. Mapping. ................................................................................................................................ 10
1.4 CHALLENGES ENCOUNTERED. ............................................................................................................... 11
1.5. PREVIOUS GEOLOGICAL WORK............................................................................................................ 11
1.5.1 Local geological works. .................................................................................................................. 11
1.5.2 Regional geological work. .............................................................................................................. 11
CHAPTER TWO. ............................................................................................................................................... 12
2.0. GEOLOGY. ................................................................................................................................................. 12
2.1. Geological Summary. ........................................................................................................................... 12
2.2 Metamorphic Units. .............................................................................................................................. 12
2.2.1 Meta sediments; ............................................................................................................................ 12
2.2.2. Biotite gneiss ................................................................................................................................. 12
2.2. 3 Mica schist..................................................................................................................................... 13
2.2.4 Quartzite ........................................................................................................................................ 14
2.3 Metamorphosed Intrusives into the basement system. ...................................................................... 15
2.3.1 Amphibolite. .................................................................................................................................. 15
2.4. Granitized sediments. .......................................................................................................................... 16
2.4.1. Granitoid Gneiss. .......................................................................................................................... 16
2.4.2 Schistose gneiss ............................................................................................................................. 17
2.5 Migmatites. ........................................................................................................................................... 18
2.6. Intrusives. ............................................................................................................................................. 19
2.6.1. Veins. ............................................................................................................................................. 19
2.6.2. Pegmatites. ................................................................................................................................... 19
2.7 Tertiary Volcanics. ................................................................................................................................. 19
2.7.1 Phonolite. ....................................................................................................................................... 19
2.7.2 Tuffs................................................................................................................................................ 20
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2.7.2.1 Trachytic tuff. ......................................................................................................................... 20
2.7.2.2 Crystal/Welded tuff. ............................................................................................................... 20
2.7.2.3 Ordinarytuff. ........................................................................................................................... 20
2.8 Quaternary/Recent Deposits. ............................................................................................................... 21
2.8.1 Sands .............................................................................................................................................. 21
2.8.2 Soils. ............................................................................................................................................... 22
2.8.3. Laterites ........................................................................................................................................ 22
CHAPTER THREE. ........................................................................................................................................... 23
3.0. STRUCTURES. ............................................................................................................................................ 23
3.1 Unconformities. ..................................................................................................................................... 23
3.1.1 Non-conformityNo table of figures entries found. ........................................................................ 23
3.2 Folds....................................................................................................................................................... 23
3.2.1. Micro-folds. ................................................................................................................................... 23
3.3 Boudins. ................................................................................................................................................. 24
3.4 Veins. ..................................................................................................................................................... 24
3.5 Exfoliation.............................................................................................................................................. 25
3.6 joints ...................................................................................................................................................... 26
3.7 Foliation. ................................................................................................................................................ 26
3.8 Lineations and Mallions. ....................................................................................................................... 27
3.9 Faulting .................................................................................................................................................. 28
3.10 Beddings .............................................................................................................................................. 28
3.11 Mudcracks ........................................................................................................................................... 28
3.12 Shearing ............................................................................................................................................... 29
CHAPTER FOUR. ............................................................................................................................................. 30
4.0. GEOLOGICAL HISTORY AND STRATIGRAPHY. .......................................................................................... 30
4.1. Geological History. ............................................................................................................................... 30
4.2. Stratigraphy. ......................................................................................................................................... 31
CHAPTER FIVE. ............................................................................................................................................... 32
5.0 ECONOMIC GEOLOGY. ............................................................................................................................... 32
5.1 Economic rocks. ..................................................................................................................................... 32
5.1.1. Tuff. ............................................................................................................................................... 32
5.1.2 Phonolite. ....................................................................................................................................... 32
5.2 Sands...................................................................................................................................................... 32
5.3. Laterite ................................................................................................................................................. 32
CHAPTER SIX. .................................................................................................................................................. 33
6.0 ENVIRONMENTAL GEOLOGY. .................................................................................................................... 33
6.1. Exposed quarries. ................................................................................................................................. 33
6.2. Natural Hazards. ................................................................................................................................... 33
6.2.1. Flooding......................................................................................................................................... 33
6.2.2 Gulley Erosion. ............................................................................................................................... 33
6.3. Water. ................................................................................................................................................... 34
6.3.1. Surface water. ............................................................................................................................... 34
6.3.2 Ground water. ................................................................................................................................ 35
CHAPTER 7 ........................................................................................................................................................ 36
7.0 RECOMMENDATIONS AND CONCLUSION. ................................................................................................ 36
7.1. Recommendations. .............................................................................................................................. 36
7.2. Conclusion ............................................................................................................................................ 36
REFERENCES................................................................................................................................................. 36
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Table of figures
Figure 1.1 Map of Kenya showing the area of study ...................................................................... 8
Figure 2.1 A biotite-muscovite schist near Sofia ......................................................................... 14
Figure 2. 3 An Amphibolite outcrop near Katulani ....................................................................... 15
Figure 2.4 Granitoid gneiss outcrop at Kwa Ndolo hill ................................................................. 17
Figure 2. 5 A schistose gneiss outcrop along Thika-Garissa road near sofia ................................... 17
Figure 2.6 A Migmatite gneiss with a layer of salt at Ndalani river ............................................... 18
Figure 2.7 A Migmatite gneiss showing shearings and microfolds, it is highly deformed. .............. 19
Figure 2.8 a phonolite boulder broken from an outcrop West of Mamba. .................................... 20
Figure 2.9 An ordinary tuff excavated from a quarry at the foot of Mavoloni hill. ........................ 21
Figure 2.10 Strata of recent deposits overlying the old basement rock, muscovite schist .............. 21
Figure 2.11 Laterite in a domant quarry near Sofia ...................................................................... 22
Figure 3.1 recent sediments overly old Cambrian rocks showing unconformity ............................ 23
Figure 3.2 Shows a fold in schist. ................................................................................................ 24
Figure 3.3 boudins in a deformed migmatite. .............................................................................. 24
Figure 3.4 a depiction of a sugar like felsic vein ........................................................................... 25
Figure 3.5 Depicts an exfoliation weathering in a meta diorite .................................................... 25
Figure 3.6 A Transverse joint in a Migmatite outcrop along Ndalani river ..................................... 26
Figure 3.7 Foliations in a hornblend gneiss .................................................................................. 27
Figure 3.8 Lineations in a biotite-muscovite gneiss ...................................................................... 27
Figure 3.9 A depiction of a shear fault in a granitoid gneiss outcrop............................................. 28
Figure 3.10 S- like shape shear zone structure ............................................................................. 29
Figure 4.1 A Stratigraphic series of rocks in Matuu-Mavoloni area ............................................... 31
Figure 5.1 Sand harvesting along Chania River. .......................................................................... 32
Figure 6.1 A dormant quarry filled with stagnant water .............................................................. 33
Figure 6.2 A gulley eroded by surface water ................................................................................ 34
Figure 6.3 A barrier set up to harvest surface/river water. .......................................................... 34
Figure 6.4 Boreholes and wells dug to supply water to the residents ........................................... 35
TABLE OF SLIDE IMAGES
Slide Image 1 A microscopic image showing minerals in a Mica schist. ......................................... 13
Slide Image 2 A microscopic image showing minerals in a Quartzite. ........................................... 14
Slide Image 3 A microscopic image showing minerals in an Amphibolite. ..................................... 15
Slide Image 4 A microscopic image showing minerals in a Granitoid Gneiss. ................................ 16
Slide Image 5 A microscopic image showing highly oriented minerals in a Migmatite. .................. 18
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CHAPTER ONE.
INTRODUCTION
1.1 Geographic setting.
1.1.1 Location
Matuu is a town located in Eastern province of Kenya, county of Machakos, Upper Yatta sub-county.
It is bounded by latitude 10 14’-10 17’ S. and longitude 37054’-37060’ E. It is about 105 km East NE of
Kenya’s capital, Nairobi. The elevation of the town is 3,734 ft. The study covers an area of
approximately 276 km2. Figure 1.1 shows the study area on the map of Kenya.
Figure 1.1 Map of Kenya showing the area of study
1.1.2 Communications and accessibility
The area is generally well networked by a road system i.e. Thika- Garissa highway and several
murram roads and paths which makes most of the area accessible, both by bus and on foot, only
some of the regions are inaccessible by a vehicle due to the hilly topography and during heavy rains.
Most of the roads network in the interior especially the Mavoloni area, Ikaatini, Mwambathana,
Katulani, Kilango and Ndalani area are murram surfaced roads. The laterites and the murram, with
their cementing abilities are proving to be a conquest in the infrastructure development in this area.
Other communication services like postal services are only offered mostly in Matuu town.
1.1.3 Administration
The entire Matuu area is being administered from Machakos at county level, Matuu at district level
in the Upper Yatta sub county.
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1.1.4 Physiography
The area is dominated by hilly terrain especially in the North Eastern part reaching the elevation of
1,520 m [Mavoloni hill] above sea level, other hills like the Nzukini 1460m and Mwambathana are
found in the Western and North Western region. The flat volcanic plains of the lava flow on the Yatta
plateau in the North west region is the lowest elevation of 1165m.
1.1.4.1 Drainage
The main drainage system is comprised of a river system. The main river is R. Thika/Chania, most of
the rest are seasonal rivers and streams. The river system is in turn dependent on the rock type and
the geomorphological unit of the stream is passing through. There is no dominant direction of flow
as the rivers flow in all directions guided by the topography. The rivers have numerous tributaries
which join them at various angles forming a dendritic pattern. Most of the rivers are intermittent
only flowing during the rainy period.
1.1.5 Climate and Vegetation
The area experiences semi-arid conditions characterized by high temperatures during the day and
relatively low temperatures at night. Humid conditions and seasonal rainfall are also characteristic of
this climate.
The vegetation that characterizes this area comprises of short acacia trees on the plains with short
grass together with other short and sturdy shrubs which are all drought resistant. On the highlands
and along river channels, tall trees, which include Grevillea (Grevillea robusta) and other weeds such
as stinging nettle, are present.
In the farms, onions and small scale growing of maize, cassava and beans is practiced. On the
highlands, much better maize crop is dominant, small scale farming of coffee and arrowroot is
witnessed. Green vegetables are also seen to do better here.
1.1.6 Rainfall
Apart from the highlands, rainfall in this area is low and also unreliable. The total rainfall ranges
between 400 mm and 800mm. The precipitation can be termed as bimodal with long rains falling in
the March-May period and short rains between October and December. (Moore 1979;
porter1965).The hilly areas have better crop cover indicating that the conditions there are different
from those in the lowlands.
The cold season is experienced in the months of June-August while the harvesting season is usually
between February and May.
The rains are distributed over a long (March-May) and a short (October –December) rainy season,
separated by a significant dry season.
1.1.7 Temperature
Mean annual temperatures range between 150C to 250 C while the average monthly maximum
temperature varies between 22.20 C and 27.30C and the minimum temperature varies between
11.10C and 15.20.
1.2 GEOLOGICAL SETTING.
The geology of Matuu area can be discussed under two perspectives;
 Regional Geology
 Local Geology
Reference to previous geological works in this area enables a comprehensive comparison.
1.2.1 Regional Geology
The study area is located within the Mozambique belt which lies east of the rift valley in Kenya. This
is a broad belt that defines the southern part of the East African Orogen and essentially consists of
medium-to-high-grade gneisses and voluminous granitoids. The belt is also composed of igneous
rocks consisting of phonolites and tuff. Paleosands and current sands are also a common feature. It
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extends south from the Arabian-Nubian shield into southern Ethiopia, Kenya, Somalia, via Tanzania
to Malawi and Mozambique and also includes Madagascar.
1.2.2 Local Geology
Locally, the geology of the area can be said to comprise only two of the three major types of rocks
(metamorphic, Igneous and sedimentary). The dominant rock type is the metamorphic, which
comprises of the mica schist, gneisses and granitoids.
The igneous system is composed of phonolites and tuffs. The tuffs are further classified as lapilli,
trachytic or welded tuff.
Paleosands, recent sands and kunker limestone are also present as tertiary deposits though limited
in occurrence.
1.3 PURPOSE AND OBJECTIVE OF THE FIED WORK.
This field work project is aimed at introducing the Geology student to field mapping exercise which is
a prerequisite for further mapping in practice. Also, in fulfillment of the requirements for the course,
one must have been introduced to field mapping as a unit.
The main objective of the field work was to;
I. Introduce the geology student to field mapping by exposure to all practicality possible so that
theory learnt in the lecture room is actually put into practice and prepare him/her for future field
mapping projects.
II. Map geologically the Matuu area and establish the lithological units in the area and generally
describe the geology of the area.
1.3.1 Methodology
The whole field mapping exercise was done to completion in twostages: Planning stage
 Mapping stage
The mapping exercise took place from the 6th to the 18th of June, 2016.
1.3.1.1. Planning.
This was done in preparation for the field work. Some of the planning steps are documented below.
 A reconnaissance visit to the study area was undertaken by the course coordinators to
reconnoiter with the topography, routes, and camping facilities and obtain authorization
from the local administration.
 Costing of the whole project and contributing towards the same.
 Preparation of base maps from topographical sheet no. 149/4.
 Issuance of the field equipment.
 Purchasing of the required items such as pencils, colored pencils, erasers and notebooks.
1.3.1.2. Mapping.
The mapping exercise involved driving in the university bus and making stops where outcrops were
observed. These included sites such as river cuttings, road cuttings, quarries and also on extensive
rock exposures on the surface. Six (6) Groups of between 7 and 9 individuals were created, each
group tasked with taking measurements of dip and strike on foliation and joints and generals
description of the site and outcrop and collecting rock samples for lab analysis. Each group would be
assigned specific days for leading the others.
Traversing on foot was done to establish the extent of the outcrops and to get a broader picture of
the area. This would involve traversing the area with respect to the general strike of the rock units
(across the strike), along rivers channels, through bushes and thickets and climbing hills and valleys.
This way, field relations and contacts of the rock units would be established so that generalization
would be minimized.
Notes were taken at every stop, recording of evidence by taking photographs and collecting samples.
At the same time, strike and dip measurements would be taken for every stop made and where
applicable. Rock description, which included grain description, texture and color, would lead to
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assigning a name to the rocks. Where doubt or controversy presented itself, a sample for lab analysis
was carried. These samples were promptly put in plastic bags and labeled to avoid confusion and
mix-ups.
Geological structures and conspicuous outcrop features such as micro-folds, veins, joints and
lineation were captured in photographs as well as sketches in notebooks.
Each one was issued with a base map on which to label the stops made. This was to enable easier
tracing of boundaries between rock units. The maps would then be shaded in different agreed upon
colors to distinguish the various units upon completion of the field mapping exercise.
1.4 CHALLENGES ENCOUNTERED.
Although the field work was successful with the objectives being achieved, various challenges were
encountered;
 Inadequate equipment slowed down the field work as everyone struggled to get a glimpse of
how they work and how to use them too.
 Absence of aerial photographs hindered effective study of geological structure that could
not be identified in the base maps. This denied us the easy time we would have experienced
during mapping.
1.5. PREVIOUS GEOLOGICAL WORK.
This area, as mentioned earlier, being within the Mozambique belt, has attracted a good number of
researchers in the pre and post-colonial eras. These works can be discussed under local and regional
perspectives.
1.5.1 Local geological works.
Fairburn (1963) in his report on the geology of the North Machakos-Thika area showed that the
basement rocks of the area had suffered high grade metamorphism that included granitoids of
replacement origin. The gneisses and granitoids observed agree well with these findings as is later
discussed in chapter two.
Opiyo (1981) observed that the rocks had suffered some form of deformation and cataclasis, but
cataclastic textures are rarely visible in hand specimen. He also observed that migmatites and
granitoids gneisses of the belt had been formed by varying degrees of granitization and
metasomatism. Migmatites were however not observed in this area but the granitoid gneisses were
observed and their metamorphism is as he observed.
Patel (1972) in his thesis “paleomagnetic studies of kapiti phonolites of Kenya” determined and
established that the kapiti phonolites had two polarities on geological history. Magnetic properties
of the rocks were however not studied in this field work but the phonolites are the same.
Mathu (1980) described the Mozambique belt as being characterized by metamorphic rocks such as
pelitic gneisses and schists, psammitic granulites, calc-silicates and marbles. He reported that most
of the rocks appeared as metamorphosed sediments, and this area is a good representative with the
gneisses and the schists.
Nyamai et al. (1993) observed that gemstones of high quality form part of the mineral wealth in the
Kenyan segment of the Mozambique belt. This was done on a much larger scale and this area is
exceptional since it lacks in gemstones as far as this report examines.
1.5.2 Regional geological work.
Arthur Holmes (1951) documents the Mozambique belt as a polycyclic Proterozoic mobile belt with
a dominant northerly structural trend. He introduced the term “Mozambique belt”. This is the term
that has continued to be used to date and also is referred to in this report in several occasions.
Almond (1983) suggested that the Mozambique belt of east Africa and the Arabian-Nubian shield of
north east Africa developed on structural and metamorphic continuity along the pan-African
orogeny(900-500 Ma). This belt is regarded as the Neo-Proterozoic collision suture between the
west and east Gondwana fragments. This age of the Mozambique belt is what is referred to in this
report.
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CHAPTER TWO.
2.0. GEOLOGY.
2.1. Geological Summary.
Matuu region lies within the Mozambique belt in the East of the Kenyan rift valley. The region has a
basement system that represents an original series of sand stone, shales and limestone where basic
magma has intruded. This magma intrusion causes compression and increase in temperature thus
metamorphosing the country rock into a folded metamorphic series of varying grades controlled by
the distance from the intrusion. This is however on a smaller scale, otherwise, the metamorphism is
regional. The Tertiary volcanics and deposits are also found in the area. The metamorphic rocks will
hence differ in grain size, as in fine grained-schists and course-gneisses.
This chapter discusses the individual rock types found in the study area. The various grouping of the
rocks simplifies the description of mineralogy, correlation and inference of the facies represented by
the mineral assemblages for the metamorphic rocks. Easier comparisons of the different rock units
are also made possible this way.
METAMORPHIC UNITS OF THE BASEMENT SYSTEM.
Meta sediments
 Biotite gneiss.
 Mica schist.
Metamorphosed Intrusives into the basement system.
 Amphibolite.
Granitized sediments.
 Granitoid gneiss
 Granitoid gneiss porphry
Migmatites
INTRUSIVES
 Veins
 Pegmatites
VOLCANIC UNITS
Tertiary volcanic
 Phonolite
 Tuff
 Trachyte
QUATERNARY SEDIMENTS
 Paleosands
 Laterites
 Soil and sands
 Kun-ker limestone
2.2 Metamorphic Units.
2.2.1 Meta sediments;
2.2.2. Biotite gneiss
The fresh surface of a sample has a general pink-grey coloration. Another sample has a general
coloration of white and black. The different colorations have been segregated into light and dark
bands. This is a characteristic texture of this rock type, commonly called gneissosity. The samples
have a common granoblastic-porphyroblastic texture, feldspar and micas readily noticeable in hand
a hand sample using an X10 magnification hand lens.
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Several minerals are common in gneisses. In thin section, Biotite appears in shades of brown in PPL
with its basal/perfect cleavage well distinguishing from hornblende. Its unusually weakly pleochroic.
Some few Hornblendeare also present. Orthoclase is among the feldspars and is colorless and
anhedral in PPL but turns pale grey in XP, shows inclined extinction and has no twinning. It is only
distinguished from quartz by its negative relief. A few crystals of microcline showing its
characteristic cross hatched twinning (a double set of polysynthetic twins at about 900) are
observed. Plagioclase feldspars are represented by Andesine which is determined using the Michel
Levy‟s method for albite twinning in plagioclases. Albite/polysynthetic twinning is diagnostic. The
opaque mineral is Ilmenite. It remains opaque throughout only transmitting very little light along
parts of its edges. Ilmenite is a common accessory mineral in a variety of metamorphic and volcanic
rocks.
The different varieties of gneiss are named after the type of rock from which they have been formed
(as granite gneiss and diorite gneiss) or after a mineral in which the rock is unusually rich (as Biotite
gneiss and hornblende gneiss).
These are the most abundant ancient rocks in the area, (Fairburn 1963). From the map obtained, it
still comes out as the most abundant rock in the study area. Biotite gneisses vary from well foliated
non Granitized varieties to highly Granitized types that approach granitoid gneiss. Both Granitized
and non Granitized varieties occur in this area. Its occurrence in the study area is most dominant in
the northern and western including Ndalani, Ekalakala, Mavoloni, Kilango and Mbembani on the
Eastern side.
2.2. 3 Mica schist.
A variety metamorphic rocks in which the crystals of the predominating mineral are aligned in
parallel layers, forming a large number of close, well-developed foliations. The rocks are easily
broken along a lamination, or schistosity, into thin, flaky plates (slaty cleavage). It is composed of
mica, usually in the form of biotite and muscovite, and quartz and feldspars when viewed in the field
using an X10 magnification hand lens. The various schistose rocks are named and characterized
according to the predominating mineral that produces the foliation. The most common schistose
rock in the area is mica schist. The dominant schist in this area, as would be expected in the
Mozambique belt is the Biotite schist proving further the P-T conditions in this belt.
Biotite
Muscovite
Quartz
K- Feldspars
Slide Image 1 A microscopic image showing minerals in a Mica schist.
Most of the schistose rocks are mainly confined to the river channels where they occur in close
association with the biotite gneiss. The outcrops in the various locations vary from muscovite-rich
varieties to Biotite-rich varieties but the Biotite tends to dominate. The following is a biotitemuscovite schist near Sofia.
13
Figure 2.1 A biotite-muscovite schist near Sofia
2.2.4 Quartzite
Quartzite is a hard,non-foliated metamorphic rock which was originally pure quartz sandstone.
Sandstone is converted into quartzite through heating and pressure usually related to tectonic
compression within orogenic belt. The quartzite we found are shades of red to pink although some
were pure and were white. They are red to pink due to various amounts of iron oxide (fe2o3). When
the sandstones are cemented to quartzite, the individual quartz grains recrystallized along the
former cementing material to form an interlocking mosaic of quartz crystals. Metamorphism
resulted into these quartzites which are glass in appearance. Minor amounts of former cementing
materials, iron oxide, silica, and carbonate migrated during recrystallization and metamorphism. This
is why the quartzite has streaks and lenses.
Sillica
[quartz]
Slide Image 2 A microscopic image showing minerals in a Quartzite.
Quartzite is very resistant to chemical weathering and this explains why we found them as broken
rock fragments in some places.
14
2.3 Metamorphosed Intrusives into the basement system.
2.3.1 Amphibolite.
Amphibolites are rocks that are containing primarily of amphibole minerals. They are formed as
metamorphosed intrusives at moderate pressures and medium temperatures. These intrusives could
be have been emplaced among the basement system sediments before the metamorphism began,
this is according to Fairburn (1963). They have been as well metamorphosed together with the
bounding gneiss. Amphibolites occur in this area as bands within the Biotite gneiss especially near
Katulani junction about 7 km from Matuu on the river cutting and are localized in occurrence with
the bands not so extensive. Has also biotite, quatz and feldspars minerals.
Plagioclase
Amphiboles.
[Hornblende]
Biotite
HHHH
Slide Image 3 A microscopic image showing minerals in an Amphibolite.
Figure 2. 3 An Amphibolite outcrop near Katulani
15
2.4. Granitized sediments.
Best (1986), granitization is a type of metamorphism or metasomatism in which considerable
amounts of material is introduced into a solid rock body, other materials necessarily being removed,
changing its chemical composition, but all in the solid state. The materials are usually fluids
circulating within the system. The granitization sequence between unaltered metamorphic rocks and
Granitoid gneisses can be summarized as follows:
2.4.1. Granitoid Gneiss.
This rock occurs as a homogeneous, unbedded massive outcrop that is more felsic at 60% than mafic
at 40% with its outline parallel to the strike. It has a porphyroblastic texture made up of quartz,
feldspar and hornblende. The surface may also experience exfoliation weathering forming large
exfoliation surfaces or outcrops as large slabs or blocks of great enough dimensions to be detected
by aerial photographs. There is no mineral banding on its fabric and also absent are mineral lineation
and foliation surfaces which are a common attribute of lower grade metamorphic rocks. This is a
confirmation of high grade metamorphism which obliterates such features.
Most of the minerals observed in the Biotite gneiss are also present in the granitoid. Only those that
had not been encountered and discussed are discussed here.
Plagioclase
Biotite
quartz
Slide Image 4 A microscopic image showing minerals in a Granitoid Gneiss.
In thin section, under crossed polars, a mineral like Staurolite is pleochroic from nearly colorless to
pale yellow in this section. The sieve structure imposed by many inclusions (mineral inclusions) of
allother minerals are as described in the Biotite gneiss. The modal mineralogical composition
together with the granitic texture leads to the interpretation of the rock as a granitoid gneiss
(granitic gneiss). Granitoid gneiss is the rock forming the hills of the Mavoloni, Nzukini hill, Kwa
Ndolo hill and Nthunguni hills in the North.
16
Figure 2.4 Granitoid gneiss outcrop at Kwa Ndolo hill
2.4.2 Schistose gneiss
These are medium to course grained metamorphic rocks composed of laminated, often flaky parallel
layers of chiefly micaceous minerals. They have a schistose texture, which is the mode of foliation
that occurs in schist metamorphic rocks as a consequence of the parallel of platy and lath shaped
mineral constituents. It reflects the intensity of metamorphism. They are flat sheet like grains in a
preferred orientation and nearby grains are roughly parallel it is defined by having more than 50%
platy and elongated minerals,often finely interleaved wit quartz and feldspars. This flat and planner
minerals include micas,chlorite, talc, hornblende, garnet and other platy minerals.
Schist is normally garnetiferous. Schistose normally forms at a higher temperature and has larger
grains than phylite, which is formed at a lower temperatures than schistose. The schistose we found
include, biotite-muscovite schist, schistose gneiss and muscovite schist. The individual minerals
grains in schist, drawn out into flaky scales by heat and pressure can be seen by naked eye. Schistose
is foliated and the individual mineral grains can be easily split off into flakes and slabs. This schist are
derived from muds and clays which have passed through series of metamorphic process involving
the products of slates and phylites as intermediate steps. Schistose can also be derived from fine
grained igneous rocks such as basalt and tuff.
Figure 2. 5 A schistose gneiss outcrop along Thika-Garissa road near sofia
17
2.5 Migmatites.
An outcrop of metamorphic nature with light and dark bands distinctively segregated. Intense microfolding in the form of Ptygmatic and kinked micro-folds is exhibitive. This results from plastic or
ductile deformation of the gneissic texture. The ductile state of the rock is due to high temperature
close to those of melting during prograde metamorphism. Migmatites are composed of two
components; a dark gneissic matrix (melanosome), consisting mainly of dark minerals, such as
biotite, hornblende, cordierite, garnet, sillimanite, and others and lighter felsic portions (leucosome),
mainly made up of more light minerals (quartz and/ or feldspars).
Garnet
Sillimanite
Hornblende
Quarts
Slide Image 5 A microscopic image showing highly oriented minerals in a Migmatite.
Migmatites mainly occur as tightly folded dyke-lets, veins and segregation of lighter granitic
composition within the dark colored amphibole and Biotite rich material. Their formation in
metamorphic terranes is attributed to anatexis and metasomatic processes, i.e. to the addition of
emanations, although not precisely described, are supposed to have supplied alkalis, in some terrain
especially sodium, whereas in others mainly potassium. The picture below shows a migmatite near
the bridge along Ndalani River whose surface has a layer of sodium chloride due to its Na+
composition.
Figure 2.6 A Migmatite gneiss with a layer of salt at Ndalani river
18
Figure 2.7 A Migmatite gneiss showing shearings and microfolds, it is highly deformed.
2.6. Intrusives.
2.6.1. Veins.
Veins are typically a result of hydrothermal solutions that precipitate minerals, most especially
feldspar and/or quartz, in openings/cracks in the rock mass. Veins range from microscopic to a few
inches in thickness. They are found in almost every metamorphic rock in the area that has
undergone metamorphism past schist towards gneiss. Veins are usually more resistant to
weathering than the host rock since it is made up of more resistant mineral as mentioned earlier.
2.6.2. Pegmatites.
These are unusually coarse grained rocks mainly of magmatic origin that intrude into other preexisting rocks, mainly in openings in them such as cracks or foliation gaps. In the area of study, they
occur mostly in granitoids gneisses and sometimes in gneiss. Their composition is mainly quartz and
K-rich feldspar.
2.7 Tertiary Volcanics.
2.7.1 Phonolite.
Several outcrops in the field of a hard, fine grained volcanic rock, dark grey in color and mainly
composed of sanidine or anorthoclase as microcrystalline groundmass or phenocrysts. Nepheline is
also present as groundmass and as phenocrysts too. Some are vesicular and porphyritic with the
phenocrysts which make up to 15% of the rock by volume reaching an average length of 2.5 cm.
Phonolites are specially silica deficient rocks. They occur in the area of study in the North western
part, west of Mavoloni, boulders reaching up to a few meters in some areas.
19
Figure 2.8 a phonolite boulder broken from an outcrop West of Mamba.
Mineral
Sanidine
Nepheline
Orthoclase
Mafic minerals
Accessory
TOTAL
Composition %
50
25
15
8
2
100
2.7.2 Tuffs.
These are pyroclastic rocks which are formed when volcanic ash ejected during a volcanic eruption
consolidates and is cemented together by lithic material. They typically have fragmental textures
since they comprise mixtures of fragments of rocks, crystals and glass. Glass, or devitrified glass is
often an important constituent in tuffs.
Three types of tuff were identified in this area;
 Trachytic tuff
 Crystal tuff
 ordinary tuff
2.7.2.1 Trachytic tuff.
This sample of tuff has a uniform fabric. Lineation of phenocrysts gives a clue of stress and/or ash
pyroclastic material flow direction giving a preferred orientation. The fabric has grey groundmass of
volcanic ash at around 80% and pyroclastic tephra at about 19% and glass shards at about 1%. It has
a composition of pumice fragments, quartz and sanidine phenocrysts. This composition makes it
compact. It is found in Mbingoni south western part.
2.7.2.2 Crystal/Welded tuff.
In an ash flow deposit, the glass fragments may initially be plastic enough to be partly or wholly
welded together by compaction caused by overlying material. This forms a type of tuff consequently
called welded tuff. The rock sample is massive, 95% groundmass and only 5% mafic or felsic
phenocrysts of seriate texture with some glass shards, it was found at the foot of Mavoloni hill.
2.7.2.3 Ordinarytuff.
Has no crystals and is formed as a result of deposition and consolidation of volcanic ash. The
hardness varies from rock to rock depending on the period and amount of pressure the rock has
20
been exposed to. Ordinary tuff is the most common type used as building stones. They are found in
Mavoloni and Mbingoni in the Western and south Eastern regions.
Figure 2.9 An ordinary tuff excavated from a quarry at the foot of Mavoloni hill.
2.8 Quaternary/Recent Deposits.
These include soils and alluvial sand deposits. Soils are residual weathering deposits whose
composition is controlled by the physical conditions of formation. Alluvial sands include river
deposits from the hills. The deposits encountered in the area include
Sands
Soils
Laterite
Figure 2.10 Strata of recent deposits overlying the old basement rock, muscovite schist
2.8.1 Sands
The sand in the area can be distinguish into two categories, paleosands and recent sands.
Paleosands are in limited occurrence in the area north of Ndalani. They compost mostly of fine
21
grained quartz. They appear white in color, silt sized grains and the fine texture hinting considerable
transportation by the rivers, and are exploited for construction industry. Recent sands on the other
hand are red brown in color and are silt-sized.The fine grain texture indicates some degree of
transportation by rivers where they are harvested at the river beds. It has a characteristic gritty
texture.
2.8.2 Soils.
Three types of soils were identified as the most dominant in the area. These are;
 Sandy soil.
 Red volcanic soil.
 Black cotton soil.
Sandy soils are mostly found in areas where the basement rocks are dominant. Their occurrence can
thus be explained as a result of weathering of the basement rocks which are rich in quartz and
feldspars. Some of the sand has also been washed down by rivers from the hills. Advanced
weathering turns the sandy soil to a red-brown color due to oxidation effect on the iron content in
the other minerals such as Biotite and the clays formed.
The red volcanic soils are as a result of the weathering effects on volcanic rocks in the area, which
include the phonolites and tuffs. The red coloration is the result of formation of iron oxide from the
iron bearing minerals such as pyroxenes, amphiboles and also clays that are formed. These soils also
show signs of contamination by contents of the basement rocks. They are mostly found overlaying
tuffs in most areas and where they are considerably thick, maize and coffee farming is mostly
practiced.
The black cotton soils are not encountered as much. In Sofia the soils are grey-black in color. In other
places, it is overlain by the red soils.
2.8.3. Laterites
This is a type red soil developed as result of intensely weathered material rich in hydrated iron and
aluminum oxides which show between 40 and 50% iron oxide and between 20 and 25% of both
alumina and silica that have been leached from the above layers deposited and accumulated at the
bottom layers. They are located 2 km to Sofia from Matuu town.
Figure 2.11 Laterite in a domant quarry near Sofia
22
CHAPTER THREE.
3.0. STRUCTURES.
3.1 Unconformities.
These are boundaries separating rock units of different ages that typically signal great gaps in time.
3.1.1 Non-conformityNo table of figures entries found.
It is a contact in which an erosion surface on plutonic or metamorphic rock has been covered by
younger sedimentary or volcanic rock. Like the one found at the foot of Mavoloni hill where the
gneisses are overlain by the late tertiary volcanic sediments, the tuffs and the laterites that overlay
the biotite-muscovite schists at the dormant quarry 2 km to Sofia from Matuu town.
Younger sedimentary
deposits; laterite and soils
Unconformity
Older metamorphic rock; schist.
Figure 3.1 recent sediments overly old Cambrian rocks showing unconformity
3.2 Folds.
Folds result from the earth’s crust response to compressive forces. This results in the crustal rocks
being folded. Folds can be classified on the basis of the scale on which they occur. Most of the folds
which occur in the area of study can be regarded as micro-folds.
3.2.1. Micro-folds.
These are folds that occur within a particular rock unit/body, which in this area are granitoids and
foliated schists and in some gneisses and migmatites. This is as a result of compressive forces during
granitization where in some instances the rocks are in ductile state due to the high temperatures
reached. Rocks showing mineral banding exhibit these folds wellof than those without. Several of
these micro-folds were observed in the area of study as depicted in figure 2.6 in a migmatite above.
23
The crest of a
symmetrical fold
Figure 3.2 Shows a fold in schist.
3.3 Boudins.
Otherwise called pinch and swell structures, boudin structures are formed when rocks are subjected
to extensional/stretching forces. When a rigid rock body is stretched and deformed amidst less
competent surroundings such as a pegmatite vein in a schist, the rigid and competent vein will break
up or thin out at several points, forming sausage-shaped structures, and thus the name Boudins.(
“Boudin” is French for sausage).Most of the boudins in this area are of this nature, that is sheared
veins characterized by stretching along the foliation planes and shortening perpendicular to it,
mostly occurring in the schists, gneisses and granitoids in various locations where these rocks
occur.Were discovered in a migmatite along Ndalani River.
Figure 3.3 boudins in a deformed migmatite.
3.4 Veins.
A vein is a tabular mass of mineral matter, deposited in the fissure, crack, or crevice of a body of
rock, and differing in composition from the substance in which it is embedded. Most veins are the
result of the gradual precipitation of substances, (mostly when minerals precipitate from
hydrothermal solutions) which were carried by underground waters or gases after the formation of
24
the enclosing material. Veins vary in size from tiny streaks, which may be entirely contained in a
small rock specimen, to masses thousands of feet in extent. Veins of quartz and other minerals may
also form when magmatic fluids are injected into fissures opened by intrusion of large bodies of
igneous rock.Most of the veins occur in all the metamorphic in the area of study.
Sugar like
crystals felsic
vein.
Figure 3.4 a depiction of a sugar like felsic vein
3.5 Exfoliation.
When a rock body is exposed to the surface, temperature differences on the surface and the
subsurface are bound to be experienced. During the day, the rock surface is exposed to direct
heating by the sun but the absorption level on the surface and the interior of the rock will be
different, with the surface responding by expansion more than the subsurface. At night and during
the cold times, the rock will release the heat to the surrounding also differentially with the surface
losing more heat faster than the subsurface. This leads to differential expansion and contraction of
the rock. The expansion and contraction of the outer layer more than the interior causes it to peel
analogously to an onion due to sheet joints that develop parallel to the outer surface. This process is
termed exfoliation and the structure thus formed on the surface is known as exfoliation surface. This
gives the rock a rounded outlook. In Matuu these features were found in granitoids and
metadiorites.
Figure 3.5 Depicts an exfoliation weathering in a meta diorite
25
3.6 joints
A joint is a break/crack in the continuity of a rock where no relative movement has occurred in
within the rock body. Joints are the result of brittle failure when the tensile strength of a rock is
exceeded. They end up forming sites for mineral precipitation in the formation of veins.
Joints may occur randomly in a rock, in sets with angles between them or even as parallel joints and
all these are present in the study area. Joints are important to quarriers as it makes it easy for them
to extract boulders from which they cut dimension stones. This is mostly useful in the tuff quarries in
the area.
Figure 3.6 A Transverse joint in a Migmatite outcrop along Ndalani river
3.7 Foliation.
Foliation can be defined as the planar arrangement of dimensionally oriented minerals in
metamorphic rocks which is mainly composed of micas or generally the phyllosilicates. It occurs
because certain minerals in a parent rock naturally form in parallel planes. Foliation may also occur
when different minerals are sandwiched together and compressed, or when rock is fractured along
parallel lines. In most cases, the foliation plane is what used to be bedding planes for sedimentary
rock that has now been metamorphosed. Foliation is highly exhibited in schists and very limited in
granitoids. In granitoids, all that remains are relicts of the foliation planes after they have been fused
during increased pressure- temperature levels in metamorphosis. It is worth noting the trend shown
by foliation in the area. Fairburn (1963), reports that the foliation lies parallel to the boundaries
between different rock units and to preferred orientation of the micas and also the banding
orientation in the gneisses. This was found to be the case in this area in most of the schistose rocks
during this field work.
26
Figure 3.7 Foliations in a hornblend gneiss
3.8 Lineations and Mallions.
This is the orientation of the planers of mica flakes and feldspathic minerals on the surface of the
rocks as linear structures and its characteristics of many metamorphic rocks including gneisses,
schist and migmatites. The parallelism of these minerals gives the rocks a planar fissility along the
foliations and some mica schist could easily be parted along the foliations.
Granitoid gneiss rarely exhibits a linear fabric but characterized by crystallization foliation. The linear
orientation of the elongated pebbles of the conglomerates mixed with sands and quartz in the dried
riverbed is approximately north to south. Mallions are lineations that are thick or rod-like.
Figure 3.8 Lineations in a biotite-muscovite gneiss
27
3.9 Faulting
A fault is a planar fracture or discontinuity in a rock mass or structure, across which there has been
significant displacement along the fractures as a result of earth movement. Faults form a
discontinuity that have a large Influence on the mechanical behavior of soil and rock masses. On the
Eastern side of the Mavoloni hill, a normal fault was noted.
FAULT
LINE
DISPLACED
DYKE
Figure 3.9 A depiction of a shear fault in a granitoid gneiss outcrop
3.10 Beddings
A bedding is the smallest division of a geologic formation or stratigraphic rock series marked by welldefined divisional planes separating it from layers above and below. A bed, whether tabular or
lenticular, has some lithologic or structural unity which sets it apart from other strata with which it is
interleaved. Beds of unlithified cobbles and pebbles of conglomerate, quartz marbles mixed with
materials of other fragments of quaternary rocks and silt.
3.11 Mudcracks
They are sedimentary structure formed as muddy sediment dries and contracts they may also form
in clayey soils as a result of a reduction in water content. Naturally occurring mud cracks starts as
wet,muddy sediment desiccatescausing contraction. A strain is developed because the top layer tries
to shrink while the material below stays the same size.
When this strain become large enough and channel cracks form in the desiccated surface material,
retrieving the strain. Individual cracks join up forming a polygonal interconnected network. These
cracks may later be filled with sediment and form casts on base of the overlying bed.
28
3.12 Shearing
Are S like structures formed when a rock that has been exposed to high temperature and pressure
conditions and is acted upon by two opposite compressional force. Because the rock is ductile under
this conditions it doesn’t form faults instead it bends to form shear zone structures.
Figure 3.100 S- like shape shear zone structure
29
CHAPTER FOUR.
4.0. GEOLOGICAL HISTORY AND STRATIGRAPHY.
4.1. Geological History.
Matuu-Mavoloni area lies in the Mozambique belt which is believed to have formed in the
Precambrian period, 500-600 Ma (Neoproterozoic). During the Paleozoic era, Eastern Africa was
occupied by folded mountains which were undergoing denudation and sedimentation,[Baker
(1963)]. After sedimentation, the rocks showed metamorphism and tectonism. The directional fabric
of rocks in the area is clear evidence of regional metamorphism together with structures such as
mineral banding, pinch and swell structures and Ptygmatic folds.
The basement system which is made up of gneisses (granitoid gneisses, Biotite-muscovite gneiss and
schistose gneiss) and schists is believed to represent an original series of sedimentary limestones,
shales and sandstones, Fairburn (1963). Basic magma then intruded into these rocks to be later
metamorphosed together with the sedimentary rocks. This is evidenced by the amphibolite dyke
near Katulani junction about 7km from Matuu town.Intense compression with rising temperatures
resulted in the rocks being transformed into highly folded metamorphic rocks. This has been
witnessed with the observation of micro-folds in the gneiss, schists and granitoids. During
compression, the basement system of sedimentary series was affected by granitization. Alkali
metasomatism led to microcline-rich rocks as mentioned earlier and the more active fluid
reconstituted the original minerals to give rise to granitoid gneiss. As such, the metamorphic rocks
found in the study area form the roots of the original fold mountains. The occurrence and processes
of the basement rocks is followed by the formation of the volcanic rocks on the older Mozambique
belt rocks. These volcanic rocks in the area were formed during the eruption of Ol Doinyo Sabuk
which is of Miocene age after the opening of the East Africa.The late tertiary volcanism which is
associated with the formation of the rift valley system replaced the dominant Archean terrain which
had persisted until then. The activities in the rift valley during the Miocene brought about
outpouring of the Kapiti phonolite and deposition of pyroclastic such as the tuffs which are of
significant thicknesses. The phonolites and tuffs of this area are evidence of this event. The sanidine
phenocrysts in the phonolites show the directional trend of the lava flow. Then later was followed by
the formation of the Pleistocene sediments contained of quartzites and sandstones which were
formed due to chemical and physical weathering, erosion, denudation of the older archean rocks
and then re-deposited. Up to the presenttimes, the area is still undergoing active erosion to attain a
new equilibrium (peneplain). Such deposits include soils, laterites and recent sands.
Chronological summary of the geologic events.
I.
Archean……………………basement system of sedimentary rocks
II.
Neoproterozoic………………basement system of metamorphics
 Basic intrusions.
 Compression and folding of the Basement System coupled with metamorphism and
granitization.
 Injection of granite with pegmatite and quartz veins.
III.
Tertiary ……………………volcanics and sediments.




Periods of uplift and erosion.
Formation of the unconformities
Uplift and rejuvenation.
Formation of the sub-Miocene peneplain.
30
IV.
 Emplacement of Tertiaryvolcanic from the eruption of Mount Kenya
Pleistocene …………………sediments including laterite.
V.
Recent ………………………soils, alluvial deposits
4.2. Stratigraphy.
Stratigraphy is the study of layered rocks and their temporal implications. In the field we applied
stratigraphy on the rock layers exposed at one side of a river bank and the same rock exposed on the
other side of the river bank. This explain that the rock layers on both sides were at one time joined
together but due to tectonic activities and processes like erosion acting on our planet have led to
deformation of the layers, this in account to the principle of uniformitarianism.
On account of the principle of superposition which explains that the rock layers deposited or formed
first are found at the bottom and the young layers are found at the top. In our area of study we
found phonolite overlying gneisses. The gneisses were the Basement System rocks and were the
oldest while the phonolites formed due to volcanic activities which were formed after the gneisses.
Stratigraphy of Matuu-Mavoloni region
Recent sands/ Laterite
RECENT
Uncomformity
LATE
TERTIARY ;
MIOCENE
Lapilli tuff
Trachytic tuff
Crystal tuff
Phonolites
Unconformity2
TERTIARY
Palaeosands
NEOPROTEROZOIC
Unconformity
Biotite-muscovite schist/ gneiss
Biotite gneiss
Schistose gneiss
Granitoid gneiss
Figure 4.1 A Stratigraphic series of rocks in Matuu-Mavoloni area
31
CHAPTER FIVE.
5.0 ECONOMIC GEOLOGY.
5.1 Economic rocks.
5.1.1. Tuff.
Tuffs are the most exploited rocks in this area and its mining is one of the major income generating
activities. Almost all the quarries in the area are tuff quarries. The favorite variety of tuff here is the
trachytic tuff as evidenced by location of quarries where it occurs. Welded tuff which resembles
phonolites in hardness and color sometimes, is also mined and crushed for ballast or cut into
building stones. Mining is done manually by hand making use of the joints. The rocks are excavated
and cut into dimension stones ready for sale to the builders. This method of mining is referred to as
open cast mining.The ordinary tuff is cut into building stones especially at the foot of Mavoloni hill
and Mbingoni area.
5.1.2 Phonolite.
These are found in the western part of the study area (North western part of Mavoloni). The rock is
quarried and crushed in most cases for use as ballast. The locals here use it in the construction of
roads and buildings. In the hilly areas along the slopes, they use it to build gabions to curb soil
erosion. The sale of this ballast earns them income to supplement that from other sources.
5.2 Sands.
Paleosands which are as a result of weathering of schists and gneisses in the area around Mbondoni
Bridge are used in construction industry. Recent sands are more abundant in this area than the
Paleosands. They are found on the river channels where they have been deposited by the rivers. The
locals scoop the sand using shovels and deposit it on the banks waiting for it to dry before
transportation.
Figure 5.1 Sand harvesting along Chania River.
5.3. Laterite
The area has laterite covering most of the surface of volcanics. Laterite is main material used in the
area for surfacing the roads in the area.It’s also used in brick making due to its rapid hardening
property when exposed to the atmosphere. It is being extracted and used on the local roads and
also transported to other areas.
32
CHAPTER SIX.
6.0 ENVIRONMENTAL GEOLOGY.
The extractive industry is the main concern as far as the environment is concerned. This is because it
degrades the land leaving it completely changed. In this area, the quarrying of tuff and phonolites is
the main extractive activity.
6.1. Exposed quarries.
As narrated by a resident in Mbongoni near an open pit of a dormant quarry, many quarriers
abandon exhausted quarries and move into new ones. The exhausted quarry is left uncovered,
unfenced off and un-rehabilitated. This has had a serious impact in the area such as accidents where
children and livestock fall into them and suffer injuries.Others have converted into mosquito
breeding grounds once water is held stagnant in them after the rains. This promotes the prevalence
of malaria in the area.
Figure 6.1 A dormant quarry filled with stagnant water
6.2. Natural Hazards.
6.2.1. Flooding.
Matuu-Mavoloni area has a fairly hilly topography, those areas that are down hills and the river
valleys tend to experience flooding effect during heavy down pour. Although the region receives an
averagely low rainfall yearly, April and November is accompanied by heavy rains which may result to
stagnant water. It habours breeding mosquitos which cause malaria. Information from the locals
indicates that this is a common occurrence every rainy season.
6.2.2 Gulley Erosion.
Erosion in some areas is extensive and gullies are left behind. These gullies have been left unattended and as a result, they continue to grow deeper. This leads to more adverse effects such as
road destruction vandalizing piping systems underground during heavy rains.
33
Figure 6.2 A gulley eroded by surface water
6.3. Water.
Water is a natural resource that much contributes to the environmental setup of any area.
6.3.1. Surface water.
The surface water in this area comes from the many tributaries that are in the area. Though most of
the rivers are seasonal, only flowing during the rainy season and some flowing for a few weeks after
the rains and few others like river Chania and Kianguni are permanent,however, several dams have
also been constructed in the area. These help to hold floods during rainy seasons and also trap river
water for storage and use during the dry seasons especially across river Kianguni.
Figure 6.3 A barrier set up to harvest surface/river water.
34
6.3.2 Ground water.
Several boreholes have been sunk in the area and show a consistent flow according to a resident, he
explained though the water is salty. Fairburn records that boreholes sunk on the volcanics have
higher yields than those done on the basement rocks. Besides boreholes, some springs are also
evident in some areas. The water oozes out of the surface after percolating through a permeable
rock, which in this area is tuff. When it encounters am impermeable basement rock, it flows on it in
the subsurface and oozes to the surface where there is a termination in the continuity of the
formation especially at the foot of Mavoloni hills.
Figure 6.4 Boreholes and wells dug to supply water to the residents
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CHAPTER 7
7.0 RECOMMENDATIONS AND CONCLUSION.
7.1. Recommendations.
The government should coordinate with the local authorities to train and bring awareness to the
local population about the safety requirements in the mining industry as applies to the mining of tuff
and phonolites in the area.
More field mapping should be carried out to collect more information on foliation, joints and
lineations to enhance better interpretation of both the structural and deformational features in the
area especially around Mavoloni.
More research on the situation of ground water in the area should be carried out. The government
and other organizations should then invest in boreholes sinking. This would assure the residents of a
sustainable, continuous and affordable clean water supply.
The dormant quarries or the open pits should be rehabilitated and law enforced about the same
especially on people who abandons the tuff quarries open.
7.2. Conclusion
The whole area was mapped and a geological map showing the different rock units obtained in the
scheduled time. A lot that entails field mapping was learnt during this field work. Use of various
equipment was grasped through hands-on exposure. Most of the foliation strikes in the North West
as established by [Fareburn 1963].
The geology of the area is composed of metamorphic rocks which form the basement system,
volcanics which include phonolites and tuffs, and sands (recent and Paleosands). Several
deformational structures such as folded veins and Migmatites provided evidence of tectonics in the
region.
REFERENCES.
Baker,B.H.,-“Geology of the southern Machakos district”.Report No.27,geol.Surv.Kenya.
Best, M.G., 1986. Igneous and Metamorphic petrology, CBS publishers & distributors, New Delhi,
India.
Ehlers, G.E and Blatt H., 1999, PETROLOGY Igneous, Sedimentary and Metamorphic. CBS
publishers & distributors, New Delhi, India.
Fairburn W.A., 1963. Geology of North Machakos-Thika area, report No 59, Geological survey of
Kenya.
Mathu E.M., 1980, Polymetamorphic Textures and Structures of the Migwani area, Kitui.
36