The Geology of Juba and the Surrounding Area,

Central Equatoria

Field Report

By

Samuel Macharyek Adak

10-CN-236

Department of Geology & Mining

College of Natural Resources and Environmental Studies

University of Juba

P. O. Box 82 Juba

August 2015

i

Contents

1. Acknowledgement…………………………………………………………iv

2. Abstract…………………………………………………………………….v

3. Chapter One

1.1 Location and Accessibility ……………………………………………1

1.2 Physiograpghy…………………………………………………………3

1.3 Climate and vegetation………………………………………………...4

1.4 Previous work……………………………………………………..…5-6

1.5 Present work………………………………………………………….6-7

2 Chapter Two

2.0 Regional geology………………………………………………….8-12

3 Chaper Three

3.1 Detail Geology…………………………………………..……………13

3.2 Gneisses……………………………………...……………………13-14

3.3 Gabbro……………………………………………………………...…15

3.4 Dolerite …………………………………………………………....15-16

3.5 Pegmatite ……………………………………………………………..16

3.6 Migmatites………………………………………………………….…17

3.7 Granite…………………………………………………………..…18-19

3.8 Amphibolite ………………………………………………………..…19

3.9 Metaquartzite …………………………………………………………19

3.10 Alluvial deposit ……...………………………………………..…20

4 Chapter Four

4. 0Structural Geology…………………………………………………..21

4.1 Folds…………………………………………………………………..21

4.2 Faults………………………………………………………………22-23

4.3 Crenulation cleavage………………………………………………23-24

4.4 Augen Structure……………………………………………………….24

4.5 Joints………………………………………………………………24-25

4.6 Exfoliation …………………………………………………………....25

4.7 Graded bedding and cross bedding …………………………………..26

4.8 Other intrusions……………………………………………………….26

5 Chapter Five

5.0 Geological History……………………………………………...27-28

6 Chapter Six

6.0 Economic Consideration

ii

6.1 Rocks………………………………………………………………..29

6.2 Clay…………………………………………………………………...30

6.3 Sand…………………………………………………………………..30

6.4 Gold……………………………………………………………….30-31

6.5 Conclusion………………………………………………………...31-32

Refrences…………………………………………………………………33

iii

List of illustrations

1. Figure 1; Map showing the study area……………………………………….2

2. Figure 2; Topography Features of Juba area………………………………...3

3. Figure 3; Average Annual temperature and Precipitation of Juba………...4-5

4. Figure 4; showing vegetation in the Area……………………………………5

5. Figure 5; showing map of the geology of South Sudan…………………….12

6. Figure 6; showing weathered banded gneiss on Jebel Kuruk………………14

7. Figure 7; shows boulder of gabbro found at the foot of Jebel Luri………...15

8. Figure 8; shows dykes in the area of study…………………………………16

9. Figure 9; pegmatitic feldfars vein near the foot of Rejaf East Mountian…..17

10. Figure 10; showing migmatite in Jebel Kuruk……………………………..17

11. Figure 11; showing Garnet spots in Jebel Kuruk Granite…………………..18

12. Figure 12; quartzite at the bank of KhorLadokeji………………………….20

13. Figure 13; chevron fold in mylonite zone in Khor Ladokeji……………….21

14. Figure 14; Macro and Micro folds found on Jebbel Kerek ………………..22

15. Figure 15; view of the fault in north Gumbo NE ministry of telecommunication

and postal services…………………………………………………………23 16. Figure 16; crenulation cleavage found around the dyke in North of Gumbo24

17. Figure 17; open Joints on rocks in area South of Radio Gumbo. ……….…25

18. Figure 18; Fig (18) Set of close joint found in Jebel Kuruk……………....25

19. Figure 19; Exfoliation (onion-skin weathering) In Khor Lado Keji………..25

20. Figure 20; Nature of Jebel Luri and Economic Activities around it………..29

21 Figure 21; showing gravel work in the study area………………………….30

iv

Acknowledgement

I commend Dr David Evuk, Mr. Thomas Oromo, Joseph Lagu and Mr. F. K.

Alex. Your explanations during this field trip have enable me gain more

knowledge about the geology of Juba area. Also your sincere guidance, candid

and insight feedbacks in the field and during the preparation of this report are

unforgettable.

Many thanks to all my classmates specially Mr. Lucky Sebit , Mr. Osfaldo

Emmanuel, Mrs. Monica Santino, Mr. Acieng Akuot Ayat and Mr. Pascal

Kafilondi for providing me with some geological photos and being so

supportive during our discussion on issues relating to this field excursion.

v

Abstract

The department of Geology and Mining university of Juba chose an area of

approximately 100 , extending from J Rejaf East Mountain, Gumbo and

Sherikat in the eastern bank of River Nile to Jebel Kuruk and Jebel Kunufi to

the southwest and West bank of River Nile where field excursion for geology

students was conducted. The objects of the trip were; as part and partial

fulfilment of the curriculum, and to train us (students) on the field investigation

to delineate rock types, structural orientation etc. in an area.

Juba area is generally underlain by metamorphic complex which formed the

Basement Complex intruded by numerous granitiods and doleritic dykes. The

Basement Complex is overlain uncomformably by alluvial and surficial deposits

that vary in thickness from one location to another.

This study reveals the following types of rocks; Gneisses which comprises of

banded gneiss, augen gneiss and biotite gneiss. They are abundant in the area

and dominate the country rock and occur as large exposures as well. Granites

occurs as large intrusions as seen in Jebel Kuruk and Jebel Kunufi. Both gneiss

and granites are cut by either doleritic or gabbroic dykes which trend EW.

1

Chapter One

1 Introduction

1.1 Location and Accessibility

The study area covers eastern and western part of Juba, the capital of the

government of South Sudan and Central Equatoria State as well. It lies on the

latitude , 45’, 34.2’’ and 52’, 06.6’’ N and longitude 21’,

12.8’’ and , 39’, 27’’ E . An area of approximately 100 , extending

from Jebel Rejaf East, Gumbo and Sherikat in the eastern bank of River Nile to

Jebel Kuruk and Jebel Kunufi to the southwest and West bank of River Nile

were covered.

Juba is accessible by Land, air, and by river transport. Due to improving roads

and bridges, the study area is easily reachable and the regional trade and social

networks are reshaping as well. There are both local roads connecting the city

with other state capitals, and International Highway like Juba-Kampala

Highway. The city has modernized airport and active river port.

Because of rapid urbanization and settlement in the area, some sites will not be

accessible for studies in some years to come. It is important that some specific

sites are preserved by the government in collaboration with University of Juba

as study areas.

During the study, a bus and land cruiser were use to access the sites but it also

involves walking from one location to another in the field. This is because of

the roughness of the terrain in some areas and the tall grass and unpassable

roots.

2

Figure (1)

Map showing the study area

3

1.2 Physiography

Juba has spectacular landscape rugged topography, high grounds and plains. The

mountains cover the east, west and Jebel Nyakenyi locally known as Jebel Lado to the

north. They represent areas of high relief that range from more than 600 plus to 1200 m

above sea level. The general ground has the average altitude, 468 meters above sea level

with gentle slope to the north. The mountain ranges, located west and east of Juba

provide a picturesque backdrop for the town with its sharp scenic peaks and wide

valleys.

The drainage pattern in the area is dendritic which is typical in mountainous areas. The

streams flow in different direction with respect to their origin and eventually drained to

River Nile they include streams such as Khor Rumula, Khor Ladokeji etc. The Nile

River creates a lush greenway that spans into Greater Juba and supports a diversity of

local flora and fauna, creating a rich ecological zone.

Numerous rocky outcrops are widely exposed throughout the area. They range from

small-sized to large mountains like Jebel Kuruk, Jebel Luri, Rejaf East. Dikes occur as

linear features trending generally East-West direction.

A B

4

Fig (2) Topography Features of Juba area (A) showing some mountains. (B) showing the main stream the River

Nile

1.3 Climate and Vegetation

The climate of Juba is tropical climate with average yearly high temperature of 34oC

and average yearly low temperature 21oC. Maximum temperature is experience during

the month of January to March and minimum temperature during April to July.

Rainy season starts from April-October. Average yearly total rainfall is 953.7 mm. The

dry season with small amount of rain start from November and intensifies from

December to March.

The type of vegetation is more biologically diverse and is not generally very dense.

Examples of common trees are Teak, shrubs, mango trees etc.

The rate of weathering is very high due to the combination of both high temperature

and rainfall, this condition facilitates hydrolysis, oxidation and reduction and physical

type of weathering such as the action of the plant’s roots.

(A)

5

(B)

Fig (3) Average Annual temperature and Precipitation of Juba. (A) Average min and max temperature. (B)

Average annual precipitation.

(A) (B)

Figure (4) showing vegetation in the Area (A) around Luri (B) western bank of the River Nile

1.4 Previous work

In general speaking, South Sudan is one of the areas in East Africa with little geology

data, Whiteman (1971) summarised brief descriptions of the igneous and metamorphic

rocks at selected localities. Much of geological investigations in the country were

generally done by correlation and extrapolation with the geology of the neighbouring

countries. For instance, in 1974, Professor Vail of the University of Khartoum

prepared –a geological map of Sudan using all available information, and extrapolating

6

data from adjacent countries. The project area had received little detailed study by the

students of the department of geology and mining, University of Juba.

However, limited systematic work was carried out in the central and parts of eastern

equatorial region. For example, Geologists from University of Khartoum studied the

gold occurrences in the Luri River basin and Kapoeta district. They also made

observations on the geology along the roads between Juba, Yei and Mundri ( Eyobo,

1973, Badri El Din).

Hunting Geology between 1976 and 1980 contracted by the regional government of

then Southern region conducted study aim at evaluating mineral resources that includes

metallic, radioactive and industrial minerals of the Juba area, and they produced a

1:250,000 scale geological map of the project area.

In 1986-1997, Legge published his findings on copper mineralization in Eastern

Equatoria, and the importance of Aswa shear lineament. He also described the geology

of the area as covered by basement of rocks invaded by Precambrian intrusion.

1.5 Present work

The study was conducted on Monday, August 31st

to September 3rd

2015 and it lasted

for 4 days. The object of the study is as follows:

1. It is part and partial fulfilment of the curriculum

2. To train us (students) on the field investigation to delineate rock types, structural

orientation etc. in an area.

7

Methods used during this study includes; observations, sample description of

geological features, and sampling. Also measurement of strike, dips, orientation of

structures were done plus snapshots were taken. An introduction to mapping was

conducted on the last day of the trip.

The tools used comprises of; geological hammer, goggles, clinometer and lens, GPS,

mathematical set, and notebook.

Accessibility to most sites of study is difficult because of the vegetation cover which

in some cases obscure detail of rock exposures which is the main constraint

encountered.

8

Chapter Two

2.0 Regional Geology

South Sudan is underlain by metamorphic and granitic rocks belonging to the northern

portion of the Tanganyika Craton bordered by gneissic rocks of heterogeneous Sudan

that were deformed during end-Precambrian Pan African orogenic event (Hunting

1980).

However, the Southern and North-western regions of the South Sudan are mainly

underlain by Archaean and Proterozoic rocks which are generally of medium to high

metamorphic grades.

The Basement Complex in these Regions can be discuss in four main contexts

identified in the adjacent neighbouring countries.

1) The Madi Sequence

The Madi rock Sequence was recognised by Berry and Whiteman in 1968 and by

HUNTING Geology in 1976. The Madi Sequence was originally described in northern

Uganda (Mathews, 1952, Hepworth and Macdonald, 1966), in the Abu Satta Hills and

to the northwest of Ragor along the boundary with the Central African Republic.

The Madi Sequence of northern Uganda continues into southern Sudan and is exposed

on the Juba - Yei road about 53 km southwest of Nimule. The Madi Sequence consists

of muscovite quartzites interbanded with muscovite schists,

quartz-feldspar-biotite-hornblende gneisses and amphibolites. The Madi Sequence

originated as a succession of arenaceous and argillaceous clastic sediments with minor

calcareous beds and probably with some volcanic rocks. Deposition age predates the

Miriam and Chuan deformations, but probably postdates Aruan (HUNTING, 1975).

9

(2) Karasuk Group (Miriam Group, Hepworth, 1967)

The term Karasuk Group was used in East Africa by Macdonald (1961), Almond

(1969) and applied to the Mozambique Belt of Eastern Africa (Holmes, 1951). The

Karasuk Group of the Precambrian basement rocks occupies a broad area covering the

eastern extremity of South Sudan and extends northwards as far north as Daga Post

(HUNTING. 1976). The rocks of the Karasuk Group are an assemblage of both

sedimentary and igneous origin, and include amphibolites, biotite-hornblende gneisses,

marbles, quartzites, chlorite and graphite schists, altered granites, ultrabasic rocks and

metagabbros. According to Almond (1969), most of the Karasuk rocks fall within the

amphibolite facies metamorphism, while the rest are of the greenschist facies regional

metamorphism.

(3) Gneiss Group

According to HUNTING Geology (1976), this group includes:

a -, massive weathered foliated granites and poorly banded gneisses,

b - biotite-hornblende gneisses with minor amphibolite and quartzite gneisses,

c - muscovite-biotite gneisses.

The gneissic rocks cover much of South Sudan of the Basement Complex. On the

basis of field observations (HUNTING, 1976), all the rocks of the Boya Hills area in the

Yambio region and areas northwest of Yambio, the Lafit mountains, areas west of

Mundri, near Wau, along areas west of Raga, southwest and southeast of Juba, and west

of Nyamlell, are included in this group.

10

(4) Strongly banded rocks, gneisses, migmatites and locally metasedimentary

rocks

This unit occurs in three areas to the east, north and west of the Imatong Mountains

and occupies broad zones within the Raga region west of longitude 26° 30' E

(HUNTING, 1976). The migmatitic rocks occur between Maridi and Mundri and

southeast of Tambura. Rocks of this group include quartz-feldspar-biotite garnet

gneisses, graphite-sill imanite gneisses and migmatites, in addition to quartzites, schists,

marbles, etc. of metasedimentary sequences.

In the study area, Precambrian basement includes the Nile gneisses occupying east

bank of the Nile which can be trace westward near Jebel Kuruk, the Madi

metasediments around Jebel Kuruk extending west beyond Jebel Kunufi in Luri area

metamorphosed to high and medium grade metamorphism and Kenyeti-metasediment.

According to Hunting 1980, the basement under when four tectono-thermal activities.

They include Watian 2.88 Ga, Aruan, Mirian and Chuan recognized in the neighbouring

Uganda.

The basement Complex during the Palaeozoic era was intruded by granitoids of post

tectonic era. They comprise of two granitic composition exposures at Jebel Kuruk and

Kunufi along with the nepheline syenite of Jebel Narkenyi (Lado) and Jebel Bilinyang.

During Tertiary the area was subjected to tensional tectonics that opened EW fractures

which were subsequently filled by doleritic dike cutting both the basement and the

major intrusions in the area.

11

In the region where there are Precambrian outcrop, lateritic soil develop and occurs at

irregular intervals. The laterite is composed of reddish brown, sandy soils and duricrust.

The duricrust forms broken blocky outcrops composed of yellow, reddish brown or

black, spongy and more massive hydrated iron oxides with scattered included detrital

silicate grains and larger angular rock fragments.

Umm Rwaba formation; this cover is found north of Juba and it is widely

distributed in South Sudan, and the Sudan, almost totally covering the Upper Nile

region and partly the Equatoria, Bahr el Ghazal, Darfur, Kordofan and Blue Nile

regions. It is characterize by low topography, smooth texture, variable thickness and

small number of drainage courses. Composition of Umm Rwaba is clay, sand and

gravels. (El Haga et tal, 2013-2014).

This formation contains very few fossils and therefore not much can be said about age.

It is considered of Quaternary to Tertiary age by Whiteman and as Quaternary by Vail.

Conditions of deposition for the Umm Rwaba are thought to have been fluviatile and

lacustrine, with sediments laid down in a series of land deltas similar to those existing in

the Sudd at the present day. The age of sediments of the formation are considered to

range from the Tertiary to Quaternary (vail 1978).

The Umm Rawaba formation may be divided into three units (Shafie, 1975):

Lower unit: some 99 m thick, it is characterised by individual grains of epidote.

Authigenic minerals are represented by the presence of many limonite grains;

Middle unit: some 149 m thick, it is characterised by abundant epidote, the increased

quantity of which forms the bottom of the cross-section;

12

Upper unit: some 157 m thick, the quantity of the epidote grains (in general) decreases

in this unit.

The Umm Rawaba Formation is also characterised by the presence of a considerable

amount of feldspars which may reach up to 40 % of the light fraction, and ilmenite

which reaches up to 77 % of the heavy fraction.

Much of the area overlain by the Umm Rawaba Formation has surface deposits

consisting of heavy clays, dark grey to chocolate in colour with kankar nodules. In the

Muglad and Umm Rawaba area (Quz), sands directly overlie the clays.

According to oil exploration data from central and southern Sudan, the maximum

drilled thickness of the Umm Rawaba Formation is higher than 15,000 feet (about 4,570

m). According to geophysical data the maximum recorded thickness is higher than

27,000 feet (about 8,225 m) in some places.

Other unconsolidated deposits lie as a thin blanket of varying thickness over the

Precambrian basement. Extensive alluvial deposits underlie the flood plain of the Nile

and many of its tributaries within the study area and toward the north of Juba.

Figure (5) showing map of the geology and mineral deposit of South Sudan

13

Chapter Three

3.0 Detail Geology

3.1 Introduction

Generally, the study area is underlain by metamorphic complex which formed

the Basement Complex intruded by numerous granitiods and doleritic dykes.

The Basement Complex is overlain uncomformably by alluvial and surficial

deposits that vary in thickness from one location to another. The detailed

description of the above rock categories will be discuss below.

3.2 Gneisses

Gneisses are abundant in the area of study, they dominate the country rock and

occurs as large exposures as well. Several types of gneiss that include banded

gneiss, biotite gneiss and augen gneiss are clearly exposed.

Banded Gneiss; are abundant in areas South of Gumbo, Rejaf East Mountain

and Jebel Kuruk in which banded gneiss occurs inform of large angular

fragments. They are high grade metamorphic rocks that occur as parent rock

that host many intrusions such as granitic plutons and dykes.

The banded gneiss has clear bands of dark and light colour minerals. They are

composed principally of quartz, feldspar mica and pyroxenes with subordinate

amphiboles. Its weathered surface is light brown and fresh surface is light grey.

Foliation and quartz vein are highly visible. Texture is medium to coarse

grained.

14

Fig (6) showing weathered banded gneiss on Jebel Kuruk.

Augen gneiss occurs in Rejaf East Mountain and an area North of Radio

Gumbo. In Rejaf East the augen forms very large exposure associated with

quartz-feldspathic. Some gneiss around Rejaf East Mountain is wholly

porphyroblastic. Weathered surface of this gneiss is greyish to brownish and

fresh surface is light grey.

Those Augen gneisses in area North of Radio Gumbo are coarser than those on

Rejaf East Mountain. This probably; means that the grade of metamorphism had

increase North-ward from Rejaf East Mountian. The main constituents of

Augen are the quartz, feldspar or amphiboles.

Biotite gneiss: occurs intermittently around Rejaf East Mountain and form the

main outcrop of Jebel Lemon, they looks grey on the fresh surface and dark on

weathered surface. It is composed of biotite, amphiboles, hornblende, quartz,

feldspar and it has a coarse-grained texture.

15

3.3 Gabbro

Occurs in form of dyke, generally oriented EW. Its main outcrop is a linear

feature found south of Gumbo, parallel to Juba-Nesitu highway. Also gabbro is

found incorporated with granite at Jebel Kunufi in southeast portion of the

outcrop adjacent to the quarry. It is produced as a result of assimilation of the

felsic magma by the country rock.

It is composed of few light minerals and composes dominantly of biotite,

minor pyroxenes, plagioclase and amphiboles. It has a coarse-grained texture.

Fig (7) shows boulder of gabbro found at the foot of Jebel Luri.

3.4 Dolerite

Dolerite are common inform of dikes that cut across gneiss in most areas, they

also cut granitic intrusion in areas around Jebel Luri, and Jebel Kuruk. They are

composed dominantly of mafic minerals and have fine-grained texture.

16

Most doleritic dikes races parallel to each other with orientation that is

generally EW direction. Doleritic and gabbroic intrusions occurs as linear dikes

that have gradational contact such as those found in an area along Khor Lado

keji and others have sharp contact in the same area and extend everywhere in

the area of study. In areas South of Gumbo, North of Radio Gumbo, the dikes

occurred as large intrusions with no visible contact with the parent rocks.

The general EW trending nature of the dikes across the area of study suggest

that there was extensional force oriented North-South that acted across the

region, resulting into formation of many fractures that were then filled with the

dykes.

A B

Fig (8) shows dykes in the area of study A. the dyke in area south of radio Gumbo. B. view

of the main doleritic dyke in area NE of Ministry of Postal services and Telecommunication.

3.5 Pegmatite:

There is occurrence of pegmatite around the gabbroic intrusion in area North

of Radio Gumbo, The pegmatite is composed of microcline (pinkish); it looks

the same with orthoclase. Also there is a pegmatitic feldspar composed of

feldspar and quartz and occur in form of lineament at approximately 20m away

from the foot of Rejaf East Mountain.

17

Fig (9) pegmatitic feldfars vein near the foot of Rejaf East Mountian.

3.6 Migmatites:

Migmatites found in the study area are granitic in composition. They occur

in association with granitic plutons such as those on Jebel Kuruk. The

migmatites have large crystals of feldspars, microcline or mica, this is

because of the slow cooling and low pressure. They have nebulitic texture.

Fig (10) showing migmatite in Jebel Kuruk

18

3.7 Granite

The granite invaded the gneiss in areas of Jebel Luri, Jebel Kuruk and an area

along Juba – Nesitu road south of Gumbo. Those granites at Jebel Luri and

Jebel Kuruk occur as large, linear intrusions composed of large boulders. The

granite in the study area can be discussed in two forms:

1. Garnetiferrous granite

2. Post-tectonic granite.

Garnetiferrous granite is confined to Jebel Kuruk, it bear garnet spots and

are granitic in composition, characterise by weak foliation and medium grained

texture. Since garnet is a high temperature metamorphic mineral, their

occurrence on Jebel Kuruk may be attributed to partial melting that resulted into

migmatites which are common on that mountain. Also existence of xenoliths

may suggest assimilation of the country rock that eventually led to formation of

garnet.

Figure (11) showing Garnet spots in Jebel Kuruk Granite

Post-tectonic granite is found in Jebel Luri, it has pinkish brown weathered

surface and its fresh surface is grey. The largest exposure is trending north-

19

-South. No observable foliation, there is even distribution of minerals. Its

texture is medium to coarse grain. Composition; light colour mineral dominated

by potash feldspar, and some quartz microcline, plagioclase and muscovite. And

dark colour minerals including; biotite, amphibole and hornblende. The dark

colour cannot be easily removed when scratched this is a characteristic of

amphibole.

According to (Hunting, 1980) the accessory minerals present include

sphene, apatite, zircon, opaque minerals, chlorite and secondary calcite.

3.8 Amphibole:

Amphibole is clearly seen as lenses with thickness of about 3-4 meters and

approximate length of about 1m on Jebel Kuruk in banded gneiss, it also occurs

as spots on granitic boulders called garnet ferrous granite. They are composed

of dominant hornblende and plagioclase plus small quartz.

However, the amphiboles found in other locations occur in form of trace

elements in the gneisses and in the doleritic and gabbroic dykes.

3.9 Metaquartzite;

Metaquatzite occurs inform of lens on eastern bank of Khor LadoKeji, it is

light brown to whitish in colour. It is highly fractured and is dominantly made

of quartz. Observing the fresh surface, a mosaic texture is seen, which is a

characteristic of quartzite. This distinguishes it from quartz vein which is

normally not crystallized.

The metaquartzite here is inter-bedded with dark colour rocks. It is probably

part of Madi-metasediment which is reported in literature that it is found on

both sides of Luri Hills. The mafic rock is meta-gabbro composed of pyroxenes

and plagioclases. Some parts of meta-gabbro are metamorphosed to

amphibolite.

20

Fig (12) quartzite at the bank of KhorLadokeji

3.10 Alluvial Sediments:

This is confined to areas along the streams such as Khor Ladokeji, river Nile

and many others. They occur as unconsolidated sediments on top of the streams

and as consolidated sand at the bottom of the streams. Their main constituents

are sands and some clay deposits.

During the deposition, they form several sedimentary structures like graded

bedding and cross bedding.

21

Chapter Four

4.0 Geological structures.

The rocks in the study area are mildly deformed, the geological structures in

the area consist of both ductile structures which include; folds, foliation. And

crenulation cleavage, and brittle structures which includes joints and faults.

Geological structure is a definable shape or fabric in a rock. The geological

structures are divided into primary structures which are structures formed

during or shortly after deposition (sedimentary) or formation (igneous) of rocks

while secondary structures formed after its host rocks are formed.

4.1 Folds

Jebel Kuruk is a host to many folds. The folds occur with tilting appearance,

micro-folds occurs at the base of the mountain, while, macro-folds occurs at the

top of the mountain.

In mylonitic zone in khor Ladokeji, chevron fold are seen. They have average

amplitude of 11 cm.

22

Fig (13) chevron fold in mylonite zone in Khor Ladokeji

A B

Fig (14) A Macro folds found on Jebbel Kerek (Photo by Lucky Sebit) B. Micro folds, also on

Jebel Kerek.

4.2 Faults

Faults occurs less commonly, the few that occurs are oriented E-W, implying

that there was regional extension that acted perpendicularly E-W. A normal

fault that has EW orientation and a displacement of 54 cm is found North of

Radio Gumbo at elevation 472 meters above sea level. The fault zone is

enriched with silica and characterised by presence of slicken sides.

At the foot of Rejaf East Mountain, There is what appears to be a fault zone

because of the followings:

- There are reddish materials from ferroginzation.

- Whitish material implying enrichment of silica because of high pressure

in the zone.

- Angular fragment from brecciation

- Striae

23

- Groove and steps

It appears that movement of the fault was dextral.

Fig (15) view of the fault in north Gumbo NE ministry of telecommunication

and postal services ( Orange arrow = fault direction, black arrow =displacement (54 cm) direction)

4.3 Crenulation and foliation

The country rocks in the study area are foliated. Some of the granitic intrusions

are also foliated. Crenulation occur in areas were pegmatization took place for

24

example in an outcrop found in area NE of ministry of Telecommunication and

postal services.

Fig (16) crenulation cleavage found around the dyke in North of Gumbo

4.4 Augen structures.

These structures were found in some sites namely; Jebel Kuruk, Rejaf East, and

Gumbo. These are structures of metamorphism in which the effects of crushing

of rocks under pressure are clearly associated with recrystallization. In these

structures some resistant minerals get deformed into thin, lenticular lenses like

shapes giving a characteristic appearance of eyes.

4.5 Joints

Close joints are the most characteristic structures found on all outcrops in the

studied area. There are few open joints observed. They are either vertical or

horizontal joints on both the country rocks and intrusions.

25

Fig (17) open Joints on rocks in area South of Radio Gumbo. Fig (18) Set of close joint found in Jebel

Kuruk

4.6 Exfoliation

A type of physical weathering that result due to unloading, it is vivid clear in

areas along Khor Lado keji and on dyke found NE of ministry of

Telecommunication and postal services. It is found on plutons that were uplifted

by tectonic activity in the area.

Fig (19) Exfoliation (onion-skin weathering) In Khor Lado Keji (Photo by Osfaldo

Emmanuel).

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4.7 Graded bedding and cross bedding: are sedimentary structures found in

khor Lado keji. They are formed by the deposition of the alluvial sediments on

both sides of khor lado keji and Luri river basin.

4.8 Other Intrusions

Granitic intrusions on western bank of the Nile have a gradational contact with

parent gneiss. Areas of migmatization and pegamatization are characterised

by presence of large crystals of feldspar mica or microcline that shows perfect

cleavage.

27

Chapter Five

5.0 Geological History

The study area falls in a Precambrian shield underlain by metamorphic rocks

compose dominantly of gneisses and granitic rocks that are of medium to high

grade metamorphism. They were deformed during end-Precambrian Pan

African orogenic event.

During the Palaeozoic era, the basement Complex was intruded by granitoids

of post tectonic era. They comprise of two granitic composition exposures at

Jebel Kuruk and Kunufi along with the nepheline syenite of Jebel Narkenyi

(Lado) and Jebel Bilinyang.

During Tertiary the area was subjected to tensional tectonics that opened EW

fractures which were subsequently filled by doleritic dike cutting both the

basement and the major intrusions in the area.

According to Hunting 1980, the basement under when four tectono-thermal

activities. They include Watian 2.88 Ga, Aruan, Mirian and Chuan recognized

in the neighbouring Uganda. The area South and South West of Juba is

underlain by Archean and Proterozoic rocks. The basement complex here

include: - the Watian which is the oldest Archean tectno-thermal events

recognised in Uganda and dated at 2.88 billion years ago. It is recognised in

Imatong Mountain area and characterised by massive unfoliated Charnokites

rocks forming mass of the upland area, there are extensive fractures. They can

also be found South of Lainya as small inliers of Charnokitic rocks.

Aruan: - the younger Archean event recognised in North Uganda. They include

the Nile gneiss West of Imatong Mountain and east of the Juba-Nimule road.

Here the Nile gneisses are composed principally of gneisses of amphibolite

28

facies with subordinate metasediments and amphibolite. Foliation trends are

poorly defined but generally in Northerly direction.

Miran: - recognized in the banded gneisses between Yei and Lainya. This

trend can be traced to the east-northeast through the madi metasediments and

across the Aswa shear zone towards Juba.

Chuan: - in South Sudan and in Uganda is represented by a series of refoliation

zones similar in photo geological expression and age and to the Mozambiquan

but with more north-westerly trend. The best established is the Aswa zone,

trending from the upper reaches of the River Assua in Uganda, along the line of

the Nile from Nimule towards Juba and Continuing as far as Mvolo.

From Palaeozoic to present, South Sudan underwent uplift and erosion during

Palaeozoic period as there is no evidence of sedimentation during that period.

The basement rocks were intruded by post tectonic bodies such as Jebel Luri,

Jebel Kerek and so on.

The laterisation and tilting of the land took place during the Tertiary. The area

to the South was uplifted and eroded whereas to the north of Juba the Umm

Rwaba sediments accumulation in the Sudd Region. Evidence of tilting and

more extensive laterite cover can be seen in the high level remnants of laterite

found in the Koda watershed where it has been elevated to approximately 600

meters above the present level of Juba.

Cycles of physical and chemical weathering have been occurring. The

products of weathering include silici-clastic fragments, sands and other

unconsolidated materials. These unconsolidated materials are being deposited

into area north of Juba and into the River Nile. The area north of Juba marks the

periphery that lead to Umm Rwaba Formation toward northern part of the

country.

29

Chapter Six

6.0 Economic Potentials

6.1 Rocks

The study area is an important source of crashed-gravels as there are abundant

rocky outcrops and mountains that are fairly accessible by land. Although there

have been some quarry companies operating on large scale as well as local

people on small scale on Jebel Kerek and Jebel Luri, the rock in the area are still

unexhausted. The products (crashed-gravels) are locally used for construction of

roads, bridges, buildings etc.

Fig: (20) Nature of Jebel Luri and Economic Activities around it, it is composed of boulders of granitic

rocks. At the foreground of this photo, there are machines for a Quarry company operating on this

mountain.

30

Fig (21) showing gravel work in the study area. (photo from collection by Francis K Alex)

6.2. Clay

The clay deposit along the streams, such as Kabur Aboba, and River Nile is

very crucial in bricks lying. This generates a lot of income and the bricks are

used for construction. Currently, both small scale and large scale brick work

takes place along the streams in Juba.

6.3. Sand

Streams such as Luri River, khor Rumula are important source of sand as it

is deposited as alluvial sediments on their banks. Sand is an important mixture

in the concrete used in construction too.

6.4. Gold

There is occurrence of alluvial gold in the Luri river basin as it was studied by

geologists from university of Khartoum. This could be of economic importance

in the future when its source is identified and mined.

31

The area provide important field of study and research, the knowledge gained

here will hopefully be of economic value to the country.

6.5 Conclusion

This field excursion have enable me gain more understanding about basement

complex of Juba area, numerous intrusions and in general, the types of rock

present in the study area.

The basement complex of Juba is gneiss that range from medium to high grade

metamorphism, its chief constituents are mafic and felsic rocks, both banded

and unbanded gneiss occurs in the area, different types such as Augen gneiss,

grey gneiss etc. exist, the gneiss is invaded by several doleritic, gabbroic and

granitic intrusions, the invasion occurs as plutons, and others are inform of

doleritic, grabbroic dikes that have East-West trending.

Most commonly occurring types of rocks in the area are; gneiss, granites,

dolerite gabbro, etc. meanwhile ultra-mafic rocks, migmatites, metaquartzite

etc. less commonly occur in the area. Most of the surface in this area is covered

with sillici-clastic deposits, other unconsolidated materials and numerous rocky

outcrops.

There are some faults and many joints; this indicates that the intruding actions

of the plutons and/or effect of the tectonic forces occurred on parent rocks at

relatively shallow depths and moderate temperature, hence depicting the brittle

behaviour of the parent rocks.

Folding is less common, they are found in few areas, such as in Jebel Kuruk

and area along Khor Ladokeji.

Weathering is inevitable, due to tropical climate of the area; the effect of

weathering can be seen as in the change of surface colour (usually dark or

32

reddish brown) of most rocks, exfoliations and other types of physical

weathering such as action of trees’ roots are common.

The area is drained by river Nile and several other streams such as khor

ladokeji and many others that are connected to the River Nile. The banks of

River Nile and these streams are covered with clay or mud and others with

sands. They are important areas for brick lying. Also, the presence of large

plutonic mountains such as Jebel Kuruk, Jebel Luri etc provides a great source

for crashed-gravels useful in constructions.

33

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Clemens, John (1998). "Observations on the origins and ascent mechanisms of granitic

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