THE GEOLOGY OF OKARARA AND ITS ENVIRONS

UZOCHUKWU CHIDINMA

Department of Geology, University of Calabar

Calabar, Nigeria

ABSTRACT

The Oban Massif is one out of the only two basement complex in the whole of

southeastern Nigeria. It is located in Cross River State and is a western

extension of Adamawa Plateau. Oban Massif is bounded between 80 02I and 80

54I E longitude and latitude 50 00I and 50 50I N. Exposed in this basement is

crystalline rocks from phyllites to amphibolites, charnockite and other

igneous intrusive. While phyllites and schist enclaves are more extensive in

the western part, the eastern part is more dominantly migmatite gneiss and

granite gneiss country (Ekwueme, 1990). Western Oban Massif is also

dominated by a syntectonic granitoid – Uwet granodiorite (Ekwueme, 2000);

Rahman et al. 1981). In this project however, a section of the Oban Massif,

Okarara, bounded in-between 80 37I 0II and 8044I 0II E longitude and latitude

50 21I 0II and 50 26I 0II N was reviewed. Exposed in Okarara are such mappable

metamorphic rocks like gneisses, quartzites and charnockites; and only one

igneous rock which is pegmatite. These rocks, especially the gneisses are

intruded by both quartz and pegmatite veins. Structural features like foliation,

joint and lineation are also found on them. The area is also endowed with

network of streams and rivers which however does not hold any economic

prospects. The area is intensely affected by weathering and erosion which

makes the soil infertile to agriculture leading to low production of food in the

area.

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LIST OF FIGURES

FIGURE 1: Map of southeastern Nigeria basement complex showing the

Study area

FIGURE 2: Drainage and sample location map of Okarara and its environs,

Oban massif, southeastern Nigeria

FIGURE 3: Geologic map of Okarara and its environs, Oban massif,

southeastern Nigeria

LIST OF PLATES

PLATE 1: Calabar-Oban road

PLATE 2: Okarara road leading to Okarara town

PLATE 3: Footpath within Okarara

PLATE 4: Cross section of Ibe River showing rounded boulders and Pebbles

PLATE 5: Cross section of weathered gneiss

PLATE 6: Cross section of the vegetation of Okarara

PLATE 7: Field occurrence of biotite gneiss

PLATE 8: Field occurrence of granite gneiss

PLATE 9: Field occurrence of migmatic gneiss

PLATE 10: Field occurrence of charnockite

PLATE 11: Field occurrence of quartzite

PLATE 12: Field occurrence of pegmatite

PLATE 13: Cut sample of quartzite

PLATE 14: Photomicrograph of biotite gneiss (UC/GLG/L02)

PLATE 15: Photomicrograph of charnockite (UC/GLG/L33)

PLATE 16: Photomicrograph of pegmatite (UC/GLG/L45)

PLATE 17: Photomicrograph of granite gneiss (UC/GLG/L51)

PLATE 18: Photomicrograph of magmatic gneiss (UC/GLG/L68)

PLATE 19: Cross section of quartz vein in gneiss

PLATE 20: Cross section of pegmatite vein in gneiss

PLATE 21: Cross section of foliated gneiss

PLATE 22: Cross section of banded gneiss

LIST OF TABLES

TABLE 1: Average modal composition of the rocks in the study area

TABLE 2a: Strike and dip values of foliations in the study area

TABLE 2b: Frequency and strike range of foliations in the study area

TABLE 2c: Histogram of foliations in the study area

TABLE 2d: Rose diagram of foliations in the study area

TABLE 3a: Strike and dip values of joints in the study area

TABLE 3b: Frequency and strike range of joints in the study area

TABLE 3c: Histogram of joints in the study area

TABLE 3d: Rose diagram of joints in the study area

TABLE OF CONTENTS

CERTIFICATION

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

LIST OF FIGURES

LIST OF PLATES

LIST OF TABLES

CHAPTER ONE

1.0 Introduction

1.1 Geology of the Nigerian Basement Complex

1.2 Location and accessibility

1.3 Aims and objectives of project

1.4 Methodology of project

CHAPTER TWO

2.0 Geomorphology

2.1 Drainage and relief

2.2 Weathering and erosion

2.3 Vegetation

2.4 Climate

CHAPTER THREE

3.0. Petrology

3.0.1. Metamorphic petrology and metamorphism

3.0.1.1. Field occurrence of metamorphic rocks

3.0.1.1.1. Field occurrence of gneiss

3.0.1.1.2. Field occurrence of quartzite

3.0.2. Igneous petrology and differentiation

3.0.2.1. Field occurrence of igneous rocks

3.1. Petrography

3.1.1. Petrography of metamorphic rocks in the study area

3.1.2. Thin section petrography of metamorphic rocks in the study area

3.1.2.1. Quartzite

3.1.2.2. Biotite gneiss (UC/GLG/L02)

3.1.2.3. Charnockite (UC/GLG/L33)

3.1.2.4. Granitic pegmatite (UC/GLG/L45)

3.1.2.5. Granite gneiss (UC/GLG/L51)

3.1.2.6. Migmatic gneiss (UC/GLG/L68)

CHAPTER FOUR

4.0. Structural geology

4.1. Planar structures

4.1.1. Fractures

4.1.1.1. Joints

4.1.1.2. Faults

4.1.1.3. Mineral veins

4.1.2. Foliations

4.2. Linear structures

4.3. Structural analysis

CHAPTER FIVE

5.0. Applied geology

5.1. Engineering geology

5.2. Economic and environmental geology

5.3. Hydrogeology

CHAPTER SIX

6.0. Summary and conclusion

REFERENCES

CHAPTER ONE

1.0 INTRODUCTION

The Okarara area located north-east of Akamkpa Local Government Area,

Cross River State. It forms part of the Oban Massif which is one out of the only

two basement complex in the whole of southeastern Nigeria (see figure 1).

The study area (Okarara) occupies about 67.07km2 in the Oban massif. It is in

the eastern part of Oban massif and as such is dominated by migmatite gneiss,

granite gneiss, charnockite and pegmatite. Quartzites are also found though

they are not as much as the gneisses. These rocks are intruded by pegmatites

and quartz veins. Intrusion of veins is dominant in the gneisses.

The basement rocks in the southeastern part of Nigeria have only recently

started to receive some attention. The thick tropical rain forest and the rugged

topography of Oban massif especially in Okarara area have remained a barrier

to detailed geological studies. In this field work, however, attempts were

made to characterize the drainage pattern, petrologic and structural

relationships of the Okarara area with a view to presenting a geologic and

drainage map of the study area, detailed thin section description of rock

samples collected from the study area, economic potentials of rocks/minerals

in the study (if any) and history of the study area.

FIGURE 1: MAP OF SOUTHEASTERN NIGERIA BASEMENT COMPLEX

SHOWING THE STUDY AREA

1.1 GEOLOGY OF THE NIGERIAN BASEMENT COMPLEX

The study was facilitated with an extensive review of the literature of geology

of the Nigerian basement complex. In geology, the term basement and

crystalline basement is used for the rocks below a sedimentary platform or

cover, or more generally any rock below sedimentary rocks or sedimentary

basins that are metamorphic or igneous in origin. The Pre-Cambrian rocks of

Nigeria collectively known as the basement complex, occupy nearly half the

total area of the country. The other half is covered by the Cretaceous and

younger sedimentary rocks. The basement complex is one of the three major

Okarara

litho-petrological components that make of the geology of Nigeria. The

Nigerian basement complex forms a part of the Pan-African mobile belt and

lies between the West African and Congo Cratons, and south of the Tuareg

Shield (Black, 1980). It is intruded by the Mesozoic calc-alkaline ring

complexes (Younger Granites) of the Jos Plateau and is unconformably

overlain by Cretaceous and younger sediments. The Nigerian basement was

affected by the 600 Ma Pan-African orogeny and it occupies the reactivated

region which resulted from plate collision between the passive continental

margin of the West African Craton and the active Pharusian continental

margin (Burke and Dewey, 1972; Dada, 2006). The basement rocks are

believed to be the results of at least four major orogenic cycles of deformation,

metamorphism and remobilization corresponding to the Liberian (2,700 Ma),

the Eburnean (2,000 Ma), the Kibaran (1,100 Ma), and the Pan-African cycles

(600Ma).

Rahaman (1988) following Rahaman (1976) has subdivided the Nigerian

basement complex into:

Migmatite-gneiss complexes

Older metasediments (Schist belt)

Younger metasediments

Older granites

Younger granites alkaline ring complexes and volcanic rocks

The migmatite gneiss complex is the commonest rock type in the Nigerian

basement complex. It comprises two main types of gneisses: the biotite gneiss

and the banded gneiss. The biotite gneisses are widespread and are fine

grained with strong foliation caused by the parallel arrangement of

alternating dark and light minerals. The banded gneiss show alternating light

coloured and dark bands and exhibit intricate folding of their bands. The

migmatite gneiss complex is the oldest basement rock, and is believed to be of

sedimentary origin but was later profoundly altered into metamorphic and

granite conditions.

The older metasediments were also among the earliest rocks to form on the

Nigerian Basement Complex. Initially of sedimentary origin with a more

extensive distribution, the older metasediments underwent prolonged,

repeated metamorphism; and now occur as quartzites (ancient sandstones),

marble (ancient limestones), and other calcareous and relics of highly altered

clayey sediments and igneous rocks.

Most parts of the Basement Complex are underlain by belts of roughly north-

south trending, slightly metamorphosed ancient Pre- Cambrian sedimentary

and volcanic rocks known as the younger metasediments. The major rock

types are ancient shaly rocks which are now referred to as quartz-biotite-

muscovite schist. These change laterally into coarse grained feldspar-bearing

micaceous schists. Schist with graphite, phyllite and chlorite are common.

Ferruginous quartzites and tale schists also occur. The younger

metasediments contain most of the gold deposits in Nigeria in the northwest

around Maru and Anka, and at Zuru; near Kaduna, and also at IIesha in

southwestern Nigeria.

The older granites are widespread throughout the Basement Complex and

occur as large circular masses within the schists and the older migmatite

gneiss complexes. The older granites vary extensively in composition. It is

composed of tonalitic to granitic plutons and charnockites. Is strongly foliated

to almost unfoliated and is considered to have been emplaced during the Pan-

African orogeny.

The younger granite complexes in Nigeria are found mainly on the Jos

Plateau, forming a distinctive group of intrusive and volcanic rocks that are

bounded by ring dykes or ring faults. Other occurrences approximate a north-

south belt towards the middle Benue in the south where the ages are younger

and towards Niger Republic in the north where the younger granites are

older. There is enormous variety in the granite composition of these rocks.

The Oban massif in which the study area is located, as mentioned earlier, is

one out of the only two basement complex in the whole of the southeastern

Nigeria (see figure 1). The Oban massif is unconformably overlain to the south

by the Calabar Flank which consists of Cretaceous- tertiary sediments. It is

separated to the north from the Obudu plateau by the Ikom-Mamfe

Embayment which consists of Cretaceous sediments and basic

volcanic/intrusives. It is thought that Oban massif and Obudu plateaus were

continuous Pre-Cambrian basement feature before the depression and

deposition of sediments in the Ikom-Mamfe Embayment during the

Cretaceous (Petters et al 1987).

1.2 LOCATION AND ACCESSIBILITY

The study area lies roughly in-between latitudes 50 21ᶦ 0ᶦᶦ N and 50 26ᶦ 0ᶦᶦ N of

the equator; and longitudes 80 37ᶦ 0ᶦᶦ E and 80 44ᶦ 0ᶦᶦ E of the Greenwich

meridian. It is therefore situated dominantly in the equatorial rainforest and

covers a total landmass of about 67.07km2 in the Oban Massif.

The study area is accessible through the Calabar-Oban road (plate 1). East of

this road is an untarred road (plate 2) called Okarara road with a distance of

about 6.86km that leads to Okarara town. This is the only road in the study

area yet not accessible mainly to vehicles because of the rugged topography.

Accessible roads within Okarara are foothpaths (plate 3) and as such are

accessible only to motorcycles.

PLATE 1: CALABAR-OBAN ROAD

PLATE 2: OKARARA ROAD LEADING TO OKARARA TOWN

PLATE 3: FOOTHPATH WITHIN OKARARA

1.3 AIMS AND OBJECTIVES OF PROJECT

AIMS

The overall aim of the project is to show on a flat sheet what the geology

of Okarara and its environs is by looking at all the evidence seen on the

ground e.g. rock outcrops, weathered material, soils and by deducing

from such evidence what the solid geology is.

To provide basic information concerning mineral, energy and water

resources in the study area (if any).

To provide information about the types, distribution and spatial

relationships of rocks and rock materials at or near the surface of the

earth and representing this information’s in one way or another on map.

To develop skills in project management.

To produce a geologic and hydrogeologic map of an area having been

provided with a topographic map.

OBJECTIVES

In order to achieve the above aims, the following objectives are guiding

field work:

Carrying out a detailed literature review of work done on Oban Massif.

Carrying out detailed field study of the area to access the economic

viability and technical performance.

Collection of fresh rock samples for geological analysis to access the

kinds of rocks that dominant the area and the minerals they contain.

Noting the flow directions of streams and rivers to facilitate the

production of hydrogeology map and to understand the drainage

pattern of the area.

1.4 METHODOLOGY OF PROJECT

In this final year project, I used three major steps to implement project

starting from observation, measurements and recording.

In observation, especially at each exposure, I ensured I took a detailed study

noting the rock type (if possible in the field), mineral composition (that can be

recognized in hand specimen), texture (that is the size, shape and orientation

of the particles that make up the rock), whether the rock is folded, banded,

foliated, faulted or jointed, and contacts which were mainly inferred.

In measurement, I ensured I took measurement of strike and dip of exposed

planes of joint and foliation, plunge of lineation, direction of flow of streams

and rivers, traverse direction as well as width of veins found in the field.

In recording, I ensured I penned down the above measurements into my field

notebook. My recording also included plotting exposures and traverses in the

topographic map. Pictures of outcrops and structures on outcrops were also

taken.

The following equipments were used to facilitate this project:

A topographic map of the study area

Contours of base maps provide several means of plotting outcrops accurately

(Compton, 1965). The contours also delineate the drainage and topography

accurately.

I used the base map to indicate rock exposures and their measured strike and

dip; traverse lines between locations, streams, rivers, roads as well as special

features like school and church in the study area.

Compass clinometers

I used this instrument (calibrated in degrees) to measure the strike and dip of

exposed planes of planar structures as well as the dip direction, trends of

minerals and veins, direction of flow of streams and rivers, and traverses.

Global Positioning System (GPS)

This is the only digital instrument that was used in the field. The instrument

gives the longitude and latitude of a place therefore I used it to locate myself

in the map. I also used it to find the distance from one point to another,

altitude of a place, locations and elevations of outcrops and to trace my way

back when lost in the bush.

Sledge hammer and a geologic hammer

These instruments were used to break fresh rock samples for geological

analysis.

Sample bag

This I used for collection of rock samples from the field to the base camp.

Camera

This I used to take pictures of outcrops, structures, vegetation and roads in

the field.

Measuring tape

This I used mainly to measure width of veins in the field.

Field notebook

This I used to write down observations and measurements that were made in

the field.

CHAPTER TWO

2.0 GEOMORPHOLOGY

Geomorphology is the study of the earth’s surface landforms and processes

and provides an understanding of landform change and evolution, as well as

the processes and materials involved. In other words, it is the scientific study

of landforms and the processes that shape them.

For many years, erosion, weathering and general land degradation have

ravaged many parts of the study area. The land is well drained such that fertile

agricultural land is lost leading to low production of food in the area to the

extent that there is no market in the whole of Okarara because there is no

farm produce to sell. The study area is endowed by streams and rivers and the

topography of the area influence the flow of these rivers and streams as they

take their rise from the shallow hills. The streams and rivers are shallow with

the streams resembling large gullies that perennially contain water and the

rivers resembling streams that increases or decreases in size during raining

and dry season respectively.

2.1 DRAINAGE AND RELIEF

As rivers flow across the landscape, they generally increase in size, merging

with other rivers. The network of rivers thus formed is a drainage system.

Network of stream channels and rivers constitute the drainage of the study

area. The stream and river channels is characterized by irregular branching of

tributary streams flowing in many directions and at almost any angles,

although usually at less than a right angle forming a dendritic drainage

pattern. The main rivers in the area are Ikpan, Ibe and Eku Rivers; Eku River

however, is out of the study area. The Ikpan River rises from the highlands

and flows southwest. It has two tributaries but its major tributary is the Ijan

stream which flows southeast. A major stream in the study area is the Ajamji

stream which flows in the southeast direction. It has tributaries which are not

named (see figure 8 below). The rivers and streams are not deep and as such

bridges are not constructed over them since people can walk directly through

them. Ibe River on the other hand is large enough that you can hear the sound

of flowing water as you come near. The river is filled with boulders and

pebbles of different rock types (plate 4) some of which are there as a result of

transportation. For instance, a schist boulder was found in the river whereas

the surrounding outcrops were gneisses. Even though Ibe river is deep

(especially in some parts), no bridge is constructed over it because the

boulders that fill the river (especially in parts that are not deep) serve as a

suitable platform for people, including myself, to walk on. The floor of other

rivers as well as streams in the study area is covered with sandy soil and

pebbles.

The relief of the study area is between 250m to 190m. The southeastern and

the northeastern part of the map area are characterized by undulating rocky

low lands. This can also be observed from the contoured map of the area. The

central southwestern part of the map of the area is hilly. The rocky hills

especially those near Okarara town have scattered broken rocks and rock

outcrops which render them unsuitable for arable farming.

PLATE 4: CROSS SECTION OF IBE RIVER SHOWING ROUNDED BOULDERS

AND PEBBLES

2.2 WEATHERING AND EROSION

Weathering and erosion slowly chisel, polish and shaped the landscape in the

study area. The processes are definitively independent, but not exclusive.

Weathering is the mechanical and chemical hammer that breaks down and

sculpts the rocks while erosion transports the fragments away.

Weathering involves two processes that often work in concert to decompose

rocks. Both processes occur in place. No movement is involved in weathering.

Chemical weathering involves a chemical change in at least some of the

minerals within a rock. Water is perhaps the most powerful agent of chemical

weathering: over time, it can dissolve many kinds of rocks into a solution that

has a different makeup than the original substance. Other types of chemical

weathering involve more complicated chemical reactions with oxygen, carbon

dioxide, water or other compounds. Rainwater also mixes with chemicals as it

falls from the sky, forming acidic concoction that dissolves the rock.

Mechanical weathering involves physically breaking rocks into fragments

without changing the chemical makeup of the minerals within it. Changes in

temperature, the freezing and thawing of water and plant growth are forces of

mechanical weathering.

As soon as a rock particle (loosened by one of the two weathering processes)

moves, we call it erosion if it moves by some flowing agents such as air, water

or ice; or mass wasting when it involves movement down slope due to gravity.

Erosion therefore is the process of transporting weathered materials and it

always involves deposition, or the deposit of the weathered sediment in a new

location.

Along with weathering, erosion and deposition are responsible for the

continual reshaping of the study area. Weathering processes as observed in

the field are enhanced by both chemical and mechanical processes. The extent

of the weathering depends on the composition of the rock. Gneiss formations

in the study area from field observations is the most weathered of all the

rocks (plate 5) especially along Okarara road. The quartzite formation, on the

other hand, from field observation is the most resistant of all the rocks. The

main mineral of quartzite is quartz (SiO2) and quartz has a high percentage of

silica content; this makes quartzite very resistant to weathering. However, as

the silica content of the rock mineral decreases, they become more susceptible

to chemical alteration. The numerous jointed rocks in the study area also

enhance chemical weathering as they provide access for passage of rainwater

through the rocks.

PLATE 5: CROSS SECTION OF WEATHERED GNEISS

2.3 VEGETATION

Vegetation is a very general term for the plant life without specific reference

to particular taxa, life forms, structure, spatial extent, or any other specific

botanical or geographic characteristics; it refers to the ground cover provided

by plants.

Equatorial or tropical rainforests are found in the equatorial zone 10 degrees

either side of the equator or we can say they found in the tropics belt which is

an area around the equator which is located between the Tropic of Cancer

(north of the equator) and the Tropic of Capricorn (south of the equator). The

study area falls between 00 and 100 north and south of the equator and as

such, the vegetation can be classified as Equatorial Rainforest. They receive

more than 60 inches of rain a year. They have temperatures that remain

between 70-80 degrees year around. They also have days that are evenly

divided between sunlight and darkness, hence, the vegetation of the study

area is indeed that of equatorial rainforest. The high temperature and heavy

rainfall produce luxuriant vegetation in these areas. These forests always look

green as there are no prescribed seasons for growing, flowering and shedding

of trees. The deciduous tree shed their leaves at sometime during the year, but

is always possible to find many deciduous trees in leaf. The most remarkable

feature of the vegetation of the study area is the great variety of trees.

Sometimes, several varieties of trees are found in a very small area. The forest

is dense. As the trees struggle for sunlight, they grow to a tremendous height.

The trees form canopies and sunlight is prevented from reaching the forest

floor. Tree ferns and palm trees are dominant. These palm trees serve as

major source of income for the people of this study area as they utilize them in

the production of palm oil. People come from other places to purchase palm

oil especially from their oil palm refinery located at Keba along Okarara road.

Equatorial Forests (that of Okarara inclusive) are not so much of commercial

importance as those of the evergreen forests of temperate regions. This is

because of several reasons: these forests are dense. The ground is wet and

swampy. Construction of roads is almost impossible. There are formidable

transport difficulties. The forests are not found in pure stands of a single

species (see plate 6). On each acre, there are two or more species of trees.

Such great varieties make it very difficult to collect any one type of trees,

which may be in particular demand. People in this forest area (Okarara for

instance) are backward and have no sufficient capital to invest for the

exploitation of forest.

There are an immense number of varieties of insects such as sun flies,

mosquitoes, spiders, ticks, butterflies, termites, tsetse flies etc. Many of these

insects are of the stinging and disease carrying types. Because of lack of grass,

there are very few land animals. Animals of the carnivorous types are also

very few. But animals like bats, tree frogs, tree lizards as well as some species

of snakes spending their time on trees are found in abundance in the study

area.

PLATE 6: CROSS SECTION OF THE VEGETATION OF OKARARA

Because of the vegetation of the study area, is one of the most thinly

populated parts of the world. The people are cut off from the areas of great

progress. They eke out their living as hunters, good gatherers and palm tree

cultivators. Their houses are mere tiny huts, built of branches, the framework

being semicircular. This framework is thatched with leaves and sometimes,

plastered with mud. Despite these harsh conditions, the people of this area are

comfortable with their state of life; to them, is paradise.

2.4 CLIMATE

Climate encompasses the statistics of temperature, humidity, atmospheric

pressure, wind, precipitation, atmospheric particle count and other

meteorological elemental measurements in a given region over long periods.

Climate can be contrasted to weather which is the present conditions of these

elements and their variations over shorter periods.

The study area has an equatorial/tropical type of climate with two distinct

seasons viz wet and dry. The wet season spans a period of about six months

(May to October) and the dry season lasts from November to April. The

temperatures throughout the year are very high and average between 250 to

300C. The range of temperatures between the coldest and warmest months is

very small about 30 or so. Night temperatures usually fall to around 250 or 260.

Thus, there is little seasonal change and only the night is regarded as the

winter of this region.

The humidity is generally very high, which causes very enervating damp

heat, making one feel very uncomfortable. There is also much evaporation

which makes one thirsty most of the time. The rainfall is heavy and falls

throughout the year. It averages between 150 to350cm. The rainfall is of

convectional type. The sun heats the earth’s surface, the warm surface then

heats the air above it. Hot air always rises so this newly heated air does so. As

it rises, the air cools and begins to condensate. Further rising and cooling

causes a large amount of condensation to occur and rain is formed.

The enervating and oppressively damp heat of the study area and the

uniformly monotonous weather causes physical and mental indolence. These

conditions lead to several diseases like malaria, yellow fever and sleeping

sickness. Because of continuous rain, the soils are leached and have poor

drainage conditions.

CHAPTER THREE

3.0. PETROLOGY

The word petrology is from two Greek words: ‘petra’ meaning rock and ‘logos’

meaning study. Petrology therefore is the branch of geology that studies the

origin, occurrence, composition, structure and history of rocks. It has been

defined by Ekwueme (1993) as a science that is concerned with the study of

rocks that is made up of definite mineral assemblages from which the earth is

built. Petrology utilizes the classical fields of mineralogy, petrography, optical

mineralogy, and chemical analyses to describe the composition and texture of

rocks. There are three branches of petrology corresponding to the three types

of rocks: igneous, metamorphic, and sedimentary. Igneous petrology focuses

on the origin, occurrence, history, composition and texture of igneous rocks

(rocks such as granite or basalt which have crystallized from molten rock or

magma). Sedimentary petrology focuses on the origin, occurrence, history,

composition and texture of sedimentary rocks (rocks such as sandstone, shale,

or limestone which consist of pieces or particles derived from other rocks or

biological or chemical deposits, and are usually bound together in a matrix of

finer material). Metamorphic petrology focuses on the origin, occurrence,

history, composition and texture of metamorphic rocks (rocks such as slate,

marble, gneiss, or schist which started out as sedimentary or igneous rocks

but which have undergone chemical, mineralogical or textural changes due to

extremes of pressure, temperature or both). Rocks, however, observed in the

field area are mainly metamorphic rocks with only one igneous rock occurring

both as an outcrop and as vein intrusions in the metamorphic rocks (see

figure 21 below).

3.0.1. METAMORPHIC PETROLOGY AND METAMORPHISM

Metamorphic rocks such as slate, marble, gneiss, quartzite or schist are rocks

which started out as sedimentary or igneous rocks but which have undergone

chemical, mineralogical or textural changes due to extremes of pressure,

temperature or both. Metamorphic rocks arise from the transformation of

existing rock types in a process called metamorphism, which means change in

form or solid-state recrystallization of pre-existing rocks due to changes in

physical and chemical conditions, primarily heat, pressure, and the

introduction of chemically active fluids (mineralogical, chemical and

crystallographic changes can occur during this process but changes at or just

beneath Earth’s surface due to weathering and/or diagenesis are not

classified as metamorphism). The original rock (protolith) is subjected to heat

(temperatures greater than 150 to 2000 C) and pressure (1500 bars), causing

profound physical and/or chemical change. The protolith may be sedimentary

rock, igneous rock or another older metamorphic rock.

Metamorphic rocks make up a large part of the Earth’s crust and are

classified by texture and by chemical and mineral assemblage (metamorphic

facies). They may be formed simply by being deep beneath the Earth’s surface

subjected to high temperatures and the great pressure of the rock layers

above it. They can form from tectonic processes such as continental collisions,

which cause horizontal pressure, friction and distortion. They are also formed

when rock is heated up by the intrusion of hot molten rock called magma from

the Earth’s interior. The study of metamorphic rocks (now exposed at the

Earth’s surface following erosion and uplift) provides information about the

temperatures and pressures that occur at great depths within the Earth’s

crust.

Metamorphic minerals are those that form only at high temperatures and

pressures associated with the process of metamorphism. These minerals,

known as index minerals, include sillimanite, kyanite, staurolite, andalusite,

and some garnet. Other minerals such as olivines, pyroxenes, amphiboles,

micas, feldspars, and quartz, may be found in metamorphic rocks, but are not

necessarily the result of the process of metamorphism. They are formed

during the crystallization of igneous rocks and are stable at high temperatures

and pressures and as such may remain chemically unchanged during the

metamorphic process. However, all minerals are stable only within certain

limits, and the presence of some minerals in metamorphic rocks indicates the

approximate temperatures and pressures at which they formed.

3.0.1.1. FIELD OCCURRENCE OF METAMORPHIC ROCKS

As said earlier, the study area comprises mainly of metamorphic rocks with

only one igneous rock which occurred both as outcrop and vein intrusion.

Metamorphic rocks observed in the field are mainly gneisses, charnockite and

quartzite. Most of the exposures are found occurring along the Okarara Road,

within Okarara town and various foot paths; and are highly deformed and

intensely weathered especially the gneisses. This made collection of rock

samples (for geochemical analysis) at some locations very difficult. The rock

exposures in the study area are not massive, majority lie flat on the ground.

3.0.1.1.1. FIELD OCCURRENCE OF GNEISS

The term gneiss signifies a large and varied series of metamorphic rocks,

which mostly consist of quartz and feldspar (orthoclase and plagioclase) with

muscovite and biotite, hornblende or augite, iron oxides, zircon and apatite.

There is also a long list of accessory minerals which are present in gneisess

with more or less frequency, but not invariably, as garnet, sillimanite,

corderite, graphite and graphitoid, epidote, calcite, orthite, tourmaline and

andalusite. The term gneiss is used to include all moderately coarse-grained

quartzo-feldsparthic rocks of diverse origin. Like schist, they occur in great

varieties and are named in various ways. They are formed by deep-seated,

high-grade regional metamorphism. They all posses a more or less marked

parallel structure (plate 3) or foliation, which is the main feature by which

many of them are separated from the granites, a group of rocks having nearly

the same mineralogical composition and closely allied to many gneisses.

Gneisses are generally confined to eroded fold mountain belts and

Precambrian terrains.

Majority of rocks observed in the study area are gneisses and the various

varieties of gneisses observed in the field are biotite gneiss, granite gneiss and

magmatic gneiss. They do not occur as massive exposures but rather they

occur as low-lying outcrops more or less flat on the ground (plate 3).

Magmatic gneiss however has a large outcrop but not massive. These gneisses

are fine to medium grained, weakly to strongly foliated rock and strikes

predominantly in a NE-SW direction with a steep dip averaging 620 in both

directions. The foliation is as a result of alignment of mineral grains which

have narrow thickness that range between 0.5 and 1cm. The leucocratic

quartz and k-feldspar rich streaks have a wider thickness that range from 2 to

3 cm.

Biotite gneiss occurred along foot path as a flat lying outcrop which is greatly

fractured as shown in plate 7 below

Plate 7: FIELD OCCURRENCE OF BIOTITE GNEISS

Granite gneiss occurred along stream channels (Ijan stream) and is banded

as shown in plate 8 below. Is neither flat lying or massive.

Plate 8: FIELD OCCURRENCE OF GRANITE GNEISS

Magmatic gneiss also occurred along water channels (Ibe River). Of all the rocks observed in the field, is the most massive and shows strong lineations as shown in plate 9 below. The pattern of the lineations shows textural evidence that significant portions of the rock existed as a melt at one time. Intrusion of igneous rock (pegmatite) in the magmatic gneiss further shows that part of the rock recrystallized from a melt. Those melts are of granitic composition.

Plate 9: CROSS SECTION OF FIELD OCCURRENCE OF MIGMATIC GNEISS

3.0.1.1.2 FIELD OCCURRENCE OF CHARNOCKITE

Charnockite is applied to any orthopyroxene-bearing quartz-feldspar

metamorphic rock, composed mainly of coesite, perthite or antiperthite and

orthopyroxene (usually hypersthene) formed at high temperature and

pressure. The charnockite series includes rocks of many different types, some

being felsic and rich in quartz and microcline, others mafic and full of

pyroxene and olivine, while there are also intermediate varieties

corresponding mineralogically to norites, quartz-norites and diorites. A

special feature occurring in many members of the group is the presence of

hypersthene. Charnockite occurs all over the world, most often in deeply

eroded Precambrian basement rock complexes. Charnockite is a particularly

widespread form of granofels which are one of the few non foliated rocks to

form under relatively high temperatures and pressures. It is formed mostly

from the granite clan of rocks, or occasionally from thoroughly reconstituted

clays and shales.

Charnockite in the field occurred along foot path. Is a flat lying exposure and

has marked lineations as shown in plate 10 below

Plate 10: FIELD OCCURRENCE OF CHARNOCKITE

3.0.1.1.2. FIELD OCCURRENCE OF QUARTZITE

Quartzite is a hard non-foliated metamorphic rock which was originally

sandstone. Sandstone is converted into quartzite through heating and

pressure usually related to tectonic compression within orogenic belts. Pure

quartzite is usually white to gray, though quartzite often occur in various

shades of pink and red due to varying amounts of iron oxide (Fe2O3). Other

colours, such as yellow and orange, are due to other mineral impurities. When

sandstone is metamorphosed to quartzite, the individual quartz grains

recrystallize along with the former cementing material to form an interlocking

mosaic of quartz crystals. Most or all of the original texture and sedimentary

structures of the sandstone are erased by the metamorphism. Minor amounts

of former cementing materials, iron oxide, silica, carbonate and clay, often

migrate during recrystallization and metamorphosis. Quartzite is very

resistant to chemical weathering and often form ridges and resistant hilltops.

The nearly pure silica content of the rock provide little for soil therefore,

quartzite ridges are often bare or covered only with very thin layer of soil and

(if any) little vegetation.

Quartzite in the study area does not have many exposures like the gneisses.

Also, unlike the gneisses which occur as low-lying outcrops, they are well

exposed though not massive (plate 11). Quartzite in the study area are the

most resistant and least affected by weathering probably due to the high silica

content.

PLATE 11: FIELD OCCURRENCE OF QUARTZITE

3.0.2. IGNEOUS PETROLOGY AND DIFFERENTIATION

Igneous rocks such as granite or basalt are rocks which have crystallized from

molten rock or magma in a process called differentiation. They are classified

using crystal size and chemistry. The simplest classification uses three

chemical descriptors; basic, intermediate and acidic; and two grain sizes; fine

and coarse. The chemistry of magma alters as it cools according to Bowen’s

Reaction Series. Igneous rocks form geological bodies such as dykes, sills and

veins as intrusive structures; and volcanoes and lava flows as extrusive

features. Intrusive bodies are formed when magma is injected into existing

rock layers. Igneous rocks have many textures which tell us about their

cooling histories and/or chemistry. In general, rocks which have cooled

rapidly are fine grained while rocks that have cooled slowly have larger

grains. This is because, the slower the cooling the more time grains have to

grow and amalgamate. Igneous petrology is the study of these rocks.

Pegmatite, which is the only igneous rock observed in the study is an

extreme igneous rock that forms during the final stage of a magma’s

crystallization. They are extreme because they contain exceptionally large

crystals and they sometimes contain minerals like lithium and beryl that are

rarely found in other types of rocks. Most pegmatite, like the one observed in

the field, have a composition that is similar to granite with abundant quartz,

feldspar and mica. These are sometimes called ‘granitic pegmatite’ to indicate

their mineralogical composition. During the early states of magma’s

crystallization, the melt usually contains a significant amount of dissolved

water and other volatiles such as chlorine, fluorine and carbon dioxide. Water

is not removed from the melt during the early crystallization process, so its

concentration in the melt grows as crystallization progresses. Eventually

there is an overabundance of water and pockets of water separate from the

melt. These pockets of superheated water are extremely rich in dissolved ions.

The ions in the water are much more mobile than the ones in the melt. This

allows them to move about freely and form crystals rapidly. This is why

crystals of a pegmatite grow so large. In the early stages of crystallization, the

ions that form high temperature minerals are depleted from the melt. Rare

ions that do not participate in the crystallization of common rock forming

minerals become concentrated in the melt and in the excluded water. These

ions are what form the rare minerals that are often found in pegmatite.

3.0.2.1. FIELD OCCRRENCE OF IGNEOUS ROCK

Pegmatite in the field occurred both as outcrops and as vein intrusions in the

gneisses. The pegmatite veins are unmappable (see plate 20).

Pegmatite outcrops found occurring in Okarara town were observed to be

rounded and some what smooth at the surface probably due to erosion as

shown in plate 12 below

Plate 12: FIELD OCCURRENCE OF PEGMATITE

3.1. PETROGRAPHY

Petrography is a branch of petrology that focuses on detailed descriptions of

rocks. It is the description and systematic classification of rocks, aided by the

microscopic examination of thin sections. Petrographic descriptions start with

the field notes at the outcrop and include megascopic description of hand

specimens. The mineral content and the textural relationship are described in

detail. Note that the detailed analysis of minerals in thin section and the

micro-texture and structure are critical to understanding the origin of the

rock.

3.1.1. PETROGRAPHY OF METAMORPHIC ROCKS IN THE STUDY AREA

Some of the quartzite observed in the field is pure white having a sugary

texture (plate 5) and can be easily mistaken to be a marble (a carbonate rock).

However, when tested with acid, no reaction was observed proving that it is

quartzite (a non carbonate rock). Marble which is a carbonate rock containing

primarily calcium carbonate (calcite) is expected to react with acid. Quartzite

in the study area are the most resistant and least affected by weathering

probably due to the high silica content.

Plate 13: CUT SAMPLE OF QUARTZITE

Biotite gneiss at the outcrop is observed to be black in colour while the

weathered portion is brownish in colour. Minerals observed in hand specimen

include quartz and biotite. The rock is medium grained. No structural feature

was observed.

Charnockite is also dark coloured and hard to break as seen in the field.

Lineation is observed in some locations. The rock is medium grained. The

weathered part appears rusty or brownish in colour. Minerals observed in

hand specimen include biotite, quartz and hornblende.

Pegmatite is a dark rock with no structural feature. The rock is also a coarse

grained rock. Minerals observed in hand specimen include quartz, feldspar

and biotite. Is highly weathered in some locations and easily broken. The

weathered part appears brownish.

Granite gneiss is light coloured, medium grained and banded. The observed

felsic bands are as a result of segregation of minerals. The felsic bands consist

of quartz and feldspar minerals. Joints were seen on some outcrops.

Magmatic gneiss is light coloured, banded and coarse grained. The banded

gneiss contains mafic and felsic bands. The mafic bands are composed of

biotite and hornblende while the felsic bands consist of quartz and feldspar

minerals.

3.1.2. THIN SECTION PETROGRAPHY OF ROCKS IN THE STUDY AREA

3.1.2.1. QUARTZITE

Quartzite is composed entirely of quartz. Quartzite in the study area is

composed mainly of milky quartz which may be the most common variety of

crystalline quartz and can be found almost everywhere. The white colour may

be caused by minute fluid inclusions of gas, liquid, or both, trapped during the

crystal formation. The cloudiness caused by the inclusions effectively bars its

use in most optical applications; hence, no photomicrograph of quartzite is

attached.

3.1.2.2. BIOTITE GNEISS

As expected, the rock is dominated by biotite which makes up to 47% of the

rock. The biotite is light to dark brown in colour and has an anhedral form. Is

pleochroic from brown to black (see plate 14 below) and cleavage is observed

in some crystals. It lacks extinction, twinning and zoning. Next to biotite in

abundance is quartz. The quartz is clear with low relief. It has an anhedral

form and lacks pleochroism, cleavage, extinction, zoning and twinning. The

plagioclase observed in the rock is light to dark gray in colour. It is anhedral

in form. The relief is low and it lacks both pleochroism and extinction. The

muscovite is found intergrown with biotite mica (see plate 14). Is clear in PPL

but appears pinkish in colour under CPL. It has a medium relief and lacks

pleochroism, twinning and zoning. Cleavage is observed in some crystals.

Orthoclase is the least mineral observed in the rock and is blue in colour. It

has a high relief, anhedral form, cleavage and pleochroism.

The minerals are all randomly oriented in the rock. This probably resulted in

the non-banding nature of the rock.

Plate 14: Photomicrograph of biotite gneiss, slide UC/GLG/L02 under CPL, x5

magnification

3.1.2.3. CHARNOCKITE

From the photomicrograph (plate 15) we can see mostly hypersthene

dominating the left part making up the highest composition of minerals in this

rock. The hypersthene is bronze to brown in colour, pleochroic, euhedral in

form and is closely packed. It also has a high relief and lacks extinction and

zoning. Twinning of some crystals are observed. Quartz is the second

abundant mineral in the charnockite dominating the central left portion of the

photomicrograph (plate 15). The quartz is clear with low relief, but there are

also inclusions of plagioclase in some of the crystals. It has a subhedral form

and lacks pleochroism, cleavage, extinction, zoning and twinning. Other

minerals identified in the thin section of charnockite are; garnet which is

reddish brown in colour (see the lower left portion of plate 15). The garnet is

closely packed with moderate developed crystal (subhedral) and has a high

Plagioclase

Biotite

Quartz

Orthoclase

Muscovite

relief but lacks pleochroism, extinction, twinning and zoning. Grains of light to

dark gray plagioclase feldspar concentrate at the bottom left hand corner of

the photomicrograph and also as inclusions in the quartz crystals (plate 15). It

has an anhedral form. The relief is low and it lacks both pleochroism and

extinction. Charnockites are formed in high pressures in almost water free

conditions hence it has only small amount of hydrous phases (biotite and

hornblende). Only 10% of biotite and hornblende is observed in the rock. The

biotite is pale brown and has an anhedral form. Is pleochroic and lacks

cleavage. It also lacks extinction, twinning and zoning. Hornblende is

brownish green with a subhedral crystal form. It has a low relief but lacks

cleavage, pleochroism, extinction, twinning and zoning.

Plate 15: Photomicrograph of charnockite, slide UC/GLG/L33 under CPL, x5

magnification

Hornblende

Quartz

Hypersthene

Biotite

Garnet

Plagioclase

3.1.2.4. PEGMATITE

The rock is dominated by quartz. The quartz is clear dominating the central

portion of the photomicrograph (plate 16) with low relief. There are also

fibrous inclusions of oligoclase grains. It has a subhedral form and lacks

pleochroism, cleavage, extinction, zoning and twinning. Plagioclase in the

form of oligoclase is found and is light yellow in colour. It has low to medium

relief and is subhedral in form. Some of the crystals are fibrous forming long

slender fibers as inclusions in the quartz (see plate 16). Biotite is dark brown

to black in colour (see central left portion of plate 16) and has an anhedral

form. Is pleochroic and lacks extinction, twinning and zoning. Microcline is

gray in colour (see upper central portion of plate 16). It has a low relief and an

anhedral form. It lacks extinction and zoning. Orthoclase is blue in colour and

is observed more at the corners of the photomicrograph (plate 16). It has a

low to medium relief, pleochroism and anhedral form. Beryl is also found but

in a small percentage. It is greenish black in colour dominating the central

middle portion of the photomicrograph (plate 16). It has a low relief, no

pleochroism and is subhedral in form.

Plate 16: Photomicrograph of pegmatite, slide UC/GLG/L45 under CPL, x5

magnification

3.1.2.5. GRANITE GNEISS

The granite gneiss is dominated by several grains of quartz (see plate 17).

The quartz is clear with low relief but some of the crystals have inclusions of

microcline. It has a subhedral form and lacks pleochroism, cleavage,

extinction, zoning and twinning. Next to quartz in abundance is the

microcline feldspar. Is gray in colour scattered all over the slide (plate 17)

but more concentrated in the left portion. It has a low relief and an anhedral

form. It has cleavage but lacks extinction, twinning and zoning. Next to

microcline in abundance is orthoclase feldspar which is blue in colour

dominating the central portion of the slide (plate 17). It has a high relief,

Orthoclase

Microcline

Beryl

Biotite

Quartz

Oligoclase

anhedral form, cleavage but lacks pleochroism. The hornblende is light

brown in colour dominating the upper central portion of the photomicrograph

and no clear crystal form is observed (anhedral). It experiences no

pleochroism. Biotite is dark brown to black in colour as seen in the upper

right portion of the photomicrograph with no clear crystal form (anhedral). Is

pleochroic but lacks extinction, twinning and zoning.

Plate 17: Photomicrograph of granite gneiss, slide UC/GLG/L51 under CPL, x5

magnification

3.1.2.6. MIGMATIC GNEISS

The rock is dominated by quartz. The quartz is clear and is more concentrated

in the central upper portion of the photomicrograph (see plate 18) with low

relief. Some parts of the quartz crystal have inclusions of orthoclase,

microcline and biotite. It has a subhedral form and lacks pleochroism,

cleavage, extinction, zoning and twinning. K-feldspar in the form of microcline

and orthoclase were also observed in the rock. While orthoclase are seen in

the upper part, microcline are concentrated more on the lower left part of the

Hornblende

Quartz

Biotite

Orthoclase

Microcline

photomicrograph (plate 18). Microcline feldspar is gray in colour. It has a

low relief and an anhedral form. It lacks extinction and zoning. Orthoclase

feldspar is blue in colour. It has a high relief, anhedral form, cleavage but

lacks pleochroism. Plagioclase in the form oligoclase is also present in the

rock. Is light yellow in colour dominating the central left part of the

photomicrograph (plate 18). It has low to medium relief and is subhedral in

form. Mafic minerals in the form of hornblende and biotite are also found and

they contribute to the darker bands that are observed in migmatic gneiss. The

hornblende is green in colour with high relief and well developed crystal

faces- euhedral (as seen in the lower right part of the photomicrograph of

plate 18). It has an imperfect cleavage and is non-pleochroic. The biotite is

dark brown to black in colour (see the central lower part of the

photomicrograph in plate 18). It has a high relief and an anhedral form. Both

the hornblende and biotite lacks twinning and extinction.

Plate 18: Photomicrograph of magmatic gneiss, slide UC/GLG/L68 under CPL,

x5 magnification

The percentages of minerals in the rocks of the study area are summarized in

the table below

Minerals (%) Rocks

Hy Q G B P Ho Mi Or Ol Be Mu

Biotite gneiss 22 47 15 6 10 charnockite 50 20 10 5 10 5 pegmatite 45 10 8 15 15 7 Granite gneiss 35 5 10 30 20 Magmatic gneiss

37 20 10 6 12 15

Orthoclase

Quartz

Hornblende

Oligoclase

Biotite

Microcline

Table 1: Average modal composition of rocks in the study area

Hy = hypersthene

Q = quartz

G = garnet

B = biotite

P = plagioclase

Ho = hornblende

Mi = microcline

Or = orthoclase

Ol = oligoclase

Be = beryl

Mu = muscovite

CHAPTER FOUR

4.0. STRUCTURAL GEOLOGY

Structural geology deals with the origin, geometry and kinematics of

formation of structures. It is similar to architecture in that both require an

ability to visualize objects in three dimensions. Structural geology is the three

dimensional distribution of rock units with respect to their deformational

histories. The primary goal of structural geology is to use measurements of

present day rock geometries to uncover information about the history of

deformation (strain) in the rocks, and ultimately, to understand the stress

field that resulted in the observed strain geometries. This understanding of

the dynamics of the stress field can be linked to important events in the

regional geologic past; a common goal is to understand the structural

evolution of a particular area with respect to regionally widespread patterns

of rock deformation due to plate tectonics. Tectonic structures are produced

in rocks in response to stresses generated, for the most part, by plate motion

within the Earth, and include different kinds of faults, joints and folds, along

with other structures.

It is generally agreed that the Nigerian basement has undergone at least two

phases of deformation which are thought to be prior to or contemporaneous

with the metamorphic episodes (Oyawoye, 1972; McCurry, 1976; Rahaman,

1976). The geologic mapping of the study area delineates structural features

such as fractures (joints and faults) and foliations. Mineral veins and mineral

lineations were also observed in the study area.

4.1. PLANAR STRUCTURES

4.1.1. FRACTURES

Fractures include joints and faults. They are outcome of certain phenomena

that the rocks went through in their geological past and they may be regularly

distributed in a rock mass.

I choose to believe that fractures observed in the study area have been

caused by natural hydraulic fracturing. This is because the study area is

endowed with network of rivers and streams. Hydraulic fracture is tensile

failure of rock at depth in the earth aided mainly by the pressure of fluids

(water) particularly where the water exceeds the horizontal rock stresses.

These fractures are often vertical and facilitate the upward migration in

connate water. In the earth crust under dry conditions, the maximum

principal stress (σ1) is greater than the minimum principal stress (σ3) by

about 20% or more, that is, the maximum value of the minimum principal

stress known is about 80% of the maximum principal stress. Means (1976)

and Brisbin (1986) are of the opinion that the minimum principal stress is

approximately one third of the maximum principal stress and this relationship

goes down into the crust to a depth of about 13km. Below this depth, you get

into the ductile domain where the minimum principal stress and the

maximum principal stress are essentially equal. Vertical stresses tend to be

higher than horizontal stress within the earth. In the presence of fluids say

fluids in pores of rocks or expelled from rocks and grade of metamorphism,

the distribution of stress will be affected such that if fluid pressure continues

to increase, eventually over a time the minimum principal stress will become

zero. This means that there is no effective horizontal stress and when this

happens, the rock is just poised to crack up. The only thing that would prevent

the rock from cracking at this condition is the tensile strength of the rock but

the tensile strength of any rock is not much. Rocks are weaker in tension but

stronger in compression. The value of the tensile strength of any rock is

usually below 2 and 10 Mpa (Handin 1966) for example, quartzite or

sandstones has the tensile strength of -3.6 and a uniaxial compressive

strength of 50. Thus once the fluid pressure continues to increase beyond this

condition, it is easy for it to overcome the tensile strength of the rock. When

this happens, there is development of bedding normal fractures and these

fractures are the hydraulic fractures.

Hydraulic fracturing is responsible for development of small scale fractures

such as fracture cleavage, joints, faults and mineral veins.

4.1.1.1 JOINTS

The joint was initially used by miners to express the observation that part of a

rock mass appears to be joined together across a certain crack or

discontinuity. A joint is a fracture along which there has been no appreciable

displacement parallel to the fracture and only slight movement normal to the

fracture plane. Joints form in solid hard rock that is stretched such that its

brittle strength is exceeded which is the point at which the rock breaks. When

this happens, the rock fractures in a plane parallel to the maximum principal

stress and perpendicular to the minimum principal stress. As such, joints form

in a plane not subject to shear.

Joints are the most common of all structures present in all settings in all

kinds of rocks; this I also observed in the study area. Most of the observed

outcrops in the study area are jointed but not all display excellent planes for

measurements. The dominant trend of joints in the study area is in the NE-SW

direction. In some places, joint filling is not common and joint opening is

about 1.5cm in average whereas in other places, these joints are filled or

healed to form veins. The joints in the study area cut across foliations and as

such, from cross-cutting relationship, are younger than the observed

foliations.

4.1.1.2 FAULTS

A fault is a fracture having appreciable movement parallel to the plane of the

fracture. No major fault was observed in the field. There are many joints than

faults in the field. This is because; there are criteria for the development of

fault. Criteria for faulting include

1. Initiation,

2. Propagation and

3. Arrest.

Coulomb (1776) coined a term ‘internal friction’ in his development of a

fracture criterion. His mathematical formulation is τ = τ0 + σntanφ. This

expression states that shear fractures, that is, fault initiation will occur

whenever the shear stress (τ) on any potential plane exceeds or equal to the

sum of the cohesive shear stress (τ0) of the rock and the product of the normal

stress (σ) across a plane and the co-efficient of the internal friction (tanφ).

Shear stress is required to cause the fracture while normal stress tries to lock

up the fracture. The fracture planes that develop are usually inclined at an

angle less than 450 to the maximum principal stress (σ1).

The coulomb criterion was intended to apply to shear fractures or fault

initiation without propagation. But faulting in the geological sense requires

that considerable slip must occur after initiation. Thus, some other functional

relationships are required to produce geological fault. Hence, Rankin (1857)

magnified coulomb criterion to apply to material with no tensile or cohesive

strength (such as fractured rocks or sands) as τI = σntanφI. For post-fracture

slide, tanφI = µI which is the co-efficient of sliding friction and τI = σn µI is often

known as Amouton’s law. Typical crustal rock will have a static frictional co-

efficient (µ0) of about 0.75 while co-efficient of sliding friction (µI) is usually

less. Sometimes between the creation of fractures or joints and occurrence of

slip, some cohesion is re-established across the fracture zones such that the

criterion returns to an altered form of τ = τ0* + σntanφ*. τ0* is the newly

established cohesive shear strength which although is less than τ0 of Coulomb

equation, is still, not insignificant. It must be overcome if there is going to be

sliding of that plane and a lot of energy is required to produce a fault at this

stage. Tanφ* is the new coefficient of internal friction that has been

established because of minerals. When joints develop, you need τ = σntanφ to

develop fault. But if the joint stays for long and there is mineralization on it, it

will require a higher energy (τ = τ0* + σ tanφ*) for faults to develop and as

such, we have many joints in the field than faults.

Initial rifting of the southern Nigeria continental margin in the Mesozoic Era

produced 2 principal sets of faults trending NE-SW and NW-SE. the NE-SW

fault bound the Benue Trough while the NW-SE faults define the Calabar flank.

The observed fault in the study area trend NW-SE.

4.1.1.3. MINERAL VEINS

These are fractures filled with precipitate commonly quartz, calcite, barite,

pegmatite etc. Veins form when mineral constituents carried by an aqueous

solution within the rock mass are deposited through precipitation. Veins are

classically thought of as being the result of growth of crystals on the walls of

planar fractures in rocks, with the crystal growth occurring normal to the

walls of the cavity, and the crystal protruding into open space. This certainly is

the method for the formation of some veins. However, it is rare in geology for

significant open space to remain open in large volumes of rock, especially

several kilometers below the surface. Thus, there are two main mechanisms

considered likely for the formation of veins: open space-filling and crack-seal

growth.

For open-space filling to take effect, the confining pressure is generally

considered to be below 0.5 GPa, or less than 3-5 kilometers. Veins formed in

this way may exhibit a colloform, agate-like habit, of sequential selvedges of

minerals which radiate out from nucleation points on the vein walls and

appear to fill up the available open space. Often evidence of fluid boiling is

present.

When confining pressure is too great, or when brittle-ductile rheological

conditions predominate, vein formation occurs via crack-seal mechanisms.

The crack seal mechanism involves multistage crack opening due to build-up

of elastic strains in the rock in response to rising fluid pressure often induced

by poor or limited rock permeability. Precipitation of solute species from the

fluid leads to the crack infill, until eventually; the crack is more or less

completely sealed. Once sealed, the permeability of the rock becomes reduced

once again. Vein widening is observed in this process. Assuming there is

continual replenishment of fluid, fluid pressure will rise once more until the

critical value at which the rock fails again and further cracking is induced and

the process of crack infill is thus repeated but the initial vein formation

strongly influences the location of subsequent veins due to the planar

anisotropy set-up. The regular growth increments shown by crack-seal veins

give strong evidence for hydraulic fracturing related to changing fluid

pressure.

Two mineral veins were observed associated mainly with the gneisses in the

study area; they include quartz veins (plate 19) and pegmatite veins (plate

20).

PLATE 19: CROSS SECTION OF QUARTZ VEIN IN GNEISS

PLATE 20: CROSS SECTION OF PEGMATITE VEIN IN GNEISS

These veins are formed by crack-seal mechanism as they do not exhibit a

colloform, agate-like habit, of sequential selvedges of minerals which radiate

out from nucleation points on the vein walls. Regular growth increments were

also observed in the field especially in the pegmatite veins. The increment

ranges from 1.5 - 16cm. These veins form in extension and tensile fractures

but the most frequently occurring is those in extension fractures. Extension

fractures are parallel to the maximum principal stress and perpendicular to

the minimum principal stress. As mineral grains grows from the wall of the

fracture, it grows perpendicular to the maximum principal stress and parallel

to the minimum principal stress as such less energy is required for the growth

of the mineral hence, extension fractures accumulate more mineralization in

the field. Tensile fractures on the other hand are parallel to the minimum

principal stress and perpendicular to the maximum principal stress. Growth of

mineralization in the tensile fracture is parallel to the maximum principal

stress and perpendicular to the minimum principal stress and therefore

require a lot of energy to accumulate mineralization and as such, it rarely

contain mineralization.

4.1.2 FOLIATIONS

Foliation is a planar arrangement of structural or textural features in any rock

type, but particularly that resulting from the alignment of constituent mineral

grains of a metamorphic rock of the regional variety (e.g. schist, slates and

gneisses) along straight or wavy planes. Foliation in the study area was

observed in gneisses only. The foliation was easily observed in outcrops and

runs planar through the rock; that is, it all runs in the same direction (plate 9).

PLATE 21: CROSS SECTION OF FOLIATED GNEISS

Foliation in the study area trend in the NE-SW direction. They strike NE and

dip in the SE direction.

The most intense form of foliation is mineral banding. At the highest grade of

metamorphism, minerals begin to segregate into separate bands. The

micaceous minerals separate from quartz and feldspars. Banding in the study

area was observed in the hornblende gneiss. The banding is defined by

alternation of a light colour containing chiefly quartz and k-feldspar and a

brown colour containing hornblende (plate 10).

PLATE 22: CROSS SECTION OF BANDED GNEISS

4.2 LINEAR STRUCTURES

Any structure that can be expressed as a real or imaginary line is a linear

structure or lineation. Like foliations, linear structures are either penetrative

or non-penetrative. The later kind is confined to isolated surfaces in a rock

mass. Linear structures observed in the study area are non-penetrative as

though they were designs made on the surface of outcrops (see plate 11

above).

4.3. STRUCTURAL ANALYSIS

Statistical analysis of the planar structures in the study area have been carried

out

TABLE 2a: STRIKE AND DIP VALUES OF FOLIATIONS IN THE STUDY AREA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

STRIKE 056 036 068 026 020 044 090 050 052 070 072 052 084 018 090 022 054 028 030 032 028 024 080 080 042

DIP 66 0SE 50 0SE 80 0SE 82 0SE 88 0SE 90 0SE 78 0SE 48 0SE 540SE 780SE 540SE 640SE 740SE 640SE 800SE 520SE 680SE 680SE 340SE 480SE 560SE 450SE 740SE 700SE 780SE

TABLE 2b: FREQUENCY & STRIKE RANGE OF FOLIATION IN THE STUDY

AREA

STRIKE RANGE 0-10 11-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90

FREQUENCY 0 2 6 2 3 4 2 3 3 TOTAL 25

% FREQUENCY 0 8 24 8 12 16 8 12 12

From the above table, it can be seen that foliation planes in the study area has

a strong preferred orientation around 021 to 060 degrees.

TABLE 2c: HISTOGRAM OF FOLIATIONS IN THE STUDY AREA

0

5

10

15

20

25

% F

REQ

UEN

CY

STRIKE AZIMUTH 10 20 30 40 50 60 70 80 90

TABLE 2d: ROSE DIAGRAM OF FOLIATIONS IN THE STUDY AREA

TABLE 3a: STRIKE AND DIP VALUES OF JOINTS IN THE STUDY AREA

1 2 3 4 5

STRIKE 142 140 134 140 146

DIP 800SW 800SW 780SW 880SW 520SW

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

178 154 162 100 168 174 174 160 164 146 168 092 134 166 142 116 108 134 124

880SW 780SW 700SW 700SW 600SW 540SW 500SW 760SW 420SW 460SW 620SW 780SW 620SW 580SW 480SW 700SW 520SW 860SW 840SW

TABLE 3b: FREQUENCY & STRIKE RANGE OF JOINTS IN THE STUDY AREA

STRIKE RANGE 91-100 101-110 111-120 121-130 131-140 141-150 151-160 161-170 171-180

FREQUENCY 2 1 1 1 5 4 2 5 3 TOTAL 24

% FREQUENCY 8.3 4.1 4.1 4.1 20.8 16.6 8.3 20.8 12.5

From the above table, joints in the study area have a preferred orientation

around 131 to 170 degrees.

TABLE 3c: HISTOGRAM OF JOINTS IN THE STUDY AREA

0

5

10

15

20

25 %

FR

EQU

ENC

Y

STRIKE AZIMUTH 90 100 110 120 130 140 150 160 170 180

TABLE 3d: ROSE DIAGRAM OF JOINTS IN THE STUDY AREA

CHAPTER FIVE

5.0 APPLIED GEOLOGY

This is the application of various fields of geology to economic, engineering,

water-supply or environmental problems; geology related to the human

activity. People working in this field are concerned with naturally occurring

geological hazards including earthquakes, landslides, volcanic eruptions and

land erosion. They also address environmental problems like waste disposal

and groundwater contamination, and how to manage and access natural

resources such as groundwater, petroleum, coal and minerals.

5.1 ENGINEERING GEOLOGY

This is the application of the geologic sciences to engineering practice for the

purpose of assuring that the geologic factors affecting the location, design,

construction, operation and maintainance of engineering works are

recognized and adequately provided for.

The landscape in the study area is undulatory and has been intensely affected

by weathering and erosion and as such is not suitable for any large scale

engineering structure. Construction of roads is almost impossible. The ground

is wet and swampy in some places and therefore has a poor rock mass quality,

thus, cannot respond well to the force field of the physical environment.

Quartzite is found in the study area. This is a quartz-rich metamorphic rock

often formed from quartz-rich sandstone. Quartzite is a more compact and

stronger rock than the original sandstone because quartz crystals are tightly

interlocked in quartzite. This makes quartzite more resistance and a good

resource for construction purposes but they are however, not in a commercial

quantity in the study area.

5.2 ECONOMIC AND ENVIRONMENTAL GEOLOGY

Economic geology deals with the economic potential of an area. It is

concerned with earth materials that can be used for economic and/or

industrial purposes. These materials include precious and base metals, non

matallic minerals, construction-grade stone, petroleum minerals, coal and

water.

Equatorial Forests (Okarara inclusive) are not so much of commercial

importance as those of the evergreen forests of temperate regions. This is

because of several reasons: these forests are dense. The ground is wet and

swampy. Construction of roads is almost impossible. There are formidable

transport difficulties. The forests are not found in pure stands of a single

species (plate 11). On each acre, there are two or more species of trees. Such

great varieties make it very difficult to collect any one type of trees, which

may be in particular demand. People in this forest area (Okarara) are

backward and have no sufficient capital to invest for the exploitation of forest.

Granite gneiss when crushed at a commercial quantity offer good source of

aggregates for building and road construction. However, granite gneiss in the

study area does not occur in a commercial quantity. They are only utilized

locally for sharpening of knives.

Lateritic soil covers a vast area of the study area. They are products of an in-

situ weathering process of a basement rock. The gneisses in the study area

result in typical lateritic weathering profiles, often having a considerable clay

layer on top. Lateritic soils are used as construction material for embankment

dams, roads, etc. they are equally important source of many metal ores,

particularly nickel, cobalt, manganese, iron and aluminum. But the lateritic

soils in the study area are also not of economic quantity. They are therefore

locally utilized in construction of their dwelling mud houses.

Lateritic soil is rich in iron and aluminum (though not of economic quantity

in the study area) but poor in nitrogen, potash, potassium, lime and organic

matter. This reduces the fertility of the soil and as such arable farming is not

common in the study area since the lateritic soil do not favour agriculture.

Lateritic soil is impermeable and as such even during the raining season,

water will fall on the land but will not infiltrate for plants to use, rather they

will drain across the land into stream channels in a process known as

overland flow. In the study area, the soil, however, supports the growth of

palm trees. These serve as their major source of income as they produce palm

oil in large quantity for sell.

5.3 HYDROGEOLOGY

This is the branch of geology dealing with the waters below the earth’s

surface and with the geological aspects of surface waters. Groundwater within

the study area (and Oban massif as a whole) is controlled by structural

discontinuities such as fractures, joints, fissures and regolith. Rates and levels

of recharge to porous media in the study area suffer impedance due to the top

lateritic cover characteristic of the area. This is consequent of the high clay

contents of the top lateritic soils, hence their low permeability.

The study area is endowed with network of rivers and streams which serve

as the only source of water for domestic purposes. These streams and rivers

are however shallow and as such not capable of generating hydroelectricity

power. Basement hydrology, as was observed in the study area, is controlled

by natural hydraulic fracturing and this result in the variation of the size of

the surface water with seasons. By hydraulic fracturing, water comes up to the

surface from the point of intersections of fractures and as such water will leak

along both fractures into the groundwater system. During the rainy season,

the leakage of water through these fractures is compensated by precipitation

and overland flow hence the size of surface water in both streams and rivers

remain fairly normal. During the dry season, however, the leakage of water

from fractures is not compensated; hence the size of surface water in both

streams and rivers reduces.

No working water borehole was observed in the study area due to the

limitation of aquifer. The Okarara area, however, has one free aquifer (spring)

which fell out of the map of the study area. The spring serves as the only

source of potable water for the people of Okarara. The strategic location of the

spring further facilitates the reduction of surface waters within the Okarara

town especially during the dry season. The continuous outflow of water at the

spring lowers the water table and this alters the direction of groundwater

movement. Water is then drawn from the streams and rivers into the

groundwater system to recharge the aquifer. This also reduces the amount of

stream flow, hence, in the study area, except with careful observation, you can

hardly notice that the streams are flowing.

CHAPTER SIX

6.0 SUMMARY AND CONCLUSION

Okarara is in the southeastern part of Akamkpa Local Government Area of

Cross River State. It forms part of the Oban Massif occupying an area of about

67.07km2. The vegetation is that of Equatorial Rainforest and the climate is

hot making one feel very uncomfortable. The area is intensely affected by

weathering and erosion. The area is dominated by metamorphic rocks mainly

gneisses, quartzites and amphibolites. These rocks, especially the gneisses are

intruded by veins mainly quartz and pegmatite veins. There are also

structural imprints like foliations, lineations and joints on these rocks. The

rocks especially the granite gneisses are of economic value but are not of

commercial quantity in the study area. Vast area of the land is covered by

lateritic soil and this reduces the fertility of the soil making arable farming in

the area very difficult. The soil however favours the growth of palm trees

which serve as the major source of income for the people of this area as they

produce palm oil in large quantity for sell.

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