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‫اﻟﻤﻤﻠﻜﺔ اﻟﻌﺮﺑﻴﺔ اﻟﺴﻌﻮدﻳﺔ‬
‫ﺟﺎﻡﻌﺔ اﻟﻤﻠﻚ ﺱﻌﻮد‬
‫آﻠﻴﺔ اﻟﻌﻠﻮم‬
‫ﻗﺴﻢ اﻟﺠﻴﻮﻟﻮﺟﻴﺎ‬

GEOPHYSICAL CHARACTERISTICS OF
THE SUBSURFACE STRUCTURES IN
NORTHEASTERN AL-MADINAH ALMUNAWARAH (HARRAT AL-AQUL)
CENTRAL ARABIAN SHIELD

Prepared By
Omar Allafouza Loni
(B.Sc. Geophysics)

Submitted in partial fulfillment of the requirement for
Master’s degree in Geophysics at the Department of
Geology, College of Science
King Saud University

Riyadh, Saudi Arabia
Muharram,1426 A.H.
March,2005 A.D.

This work is dedicated to:
My father who struggled and did his utmost to bring up and
to educate his children.
My mother, may Allah have mercy on her soul who spent
sleepless nights taking care of her children.
My brothers, wife, sisters and children.
My teachers, professors and all those who shared their
knowledge with me.

ABSTRACT
An

integrated

geological,

hydrogeological

and

geophysical

investigation was conducted in Al-Aqul area, northeastern Al-Madinah AlMunawarah to determine subsurface structural characteristics and volcanic
layer (basalt) sequences. Magnetic data were used to show the variations
between volcanic layer sequences and to locate subsurface structures, while
resistivity measurements were used to delineate hydrostratigraphy.
The study consists of six magnetic profiles, eight Vertical Electrical
Soundings (VES) and eighteen Horizontal Electrical Profiling (HEP). The
total magnetic intensity map was reduced to the pole (RTP), which was
then processed to smooth out field variations caused by various noise
sources and to facilitate the qualitative and quantitative interpretations. The
data acquired from resistivity method were used to construct three
geoelectrical cross-sections, four HEP contour maps and 3D geoelectrical
models.
The structural elements, deduced from the magnetic maps, show the
same structural trends of the study area. The geoelectrical cross-sections
revealed six layers with different physical characteristics. The upper layer
consists of weathered basalt, associated with high resistivity (ρ >1000
Ohm-m). The second layer consists of wet fresh basalt associated with
resistivity ranging from 500 – 1000 Ohm-m. The third layer has an average

i

thickness of 48m and consists of fractured basalt with gravelly sand
associated with resistivity ranging from 100-300 Ohm-m. This layer
probably indicates the main water bearing layer in the study area. The
fourth layer consists of fractured basalt with clay which is associated with
resistivity ranging from 10-100 Ohm-m. The low resistivity in the fourth
layer is probably attributed to the clay content and saline water. The fifth
layer consists of weathered basement associated with resistivity ranging
from 280-500 Ohm-m. The lower layer consists of fresh basement with
resistivity ρ >1000 Ohm-m and depth range from 121m to ∞.
Analysis of magnetic and horizontal electrical profiling (HEP)
suggests that the middle segment of Wadi Al-Ehileene is existing buried
under lava sill. The average thickness estimated from magnetic and
resistivity data of the basaltic lava flow is about 184 and 166m
respectively.

ii

ACKNOWLEDGMENT
Praise and thanks be to Allah, who enabled me to start and finish this
thesis.
I am indebted to the Geology Department, College of Science, King
Saud University for offering me the opportunity to carry out this study
under their supervision, for the degree of Master of Science in Geophysics.
I wish to express my sincere thanks and deep gratitude to, in
particular, to my supervisors Professor Dr. Abdullah M.S. Al-Amri,
Professor of Geophysics and Dr. Hesham M. Al-Araby, Associate
Professor of Geophysics for their guidance, helpful orientation, discussion
and criticism during this study.
My special thanks are due to Dr. Nasir S. Al-Arifi, for his continuous
support and encouragement.
Deep and special gratitude’s are rendered to Mr. EL-Taher Edris, for
his whole-hearted co-operation and assistance in office, field work.
Gratitude and deep thanks are also to Dr. Mohammed Tahir Hussein,
Dr. Mohammad E. Al-Dabbagh, Mr.Hussein Salem, Saad Al-Amri and Mr.
Habib Annakhli of Geology Department for their help and discussions.
I am grateful to the Research Center staff of K.S.U. for submitting
the field work.

iii

I would like to thank, Dr. Saleh Al-Athel, Dr.Turky Al Suad, Dr.
Abdullah Al-Rasheed, Dr. Abdullaziz Al-Sowailem, Dr. Hassan Al-Ayd
and Dr. Abdullgader Al-Sari , of KACST for their help.
My special thanks are due to Dr. Omar Al-Harbi, Dr. Mohammad
Nabeel Shalibi, Dr. Adnan Niyazi, Dr. Tareq Al-Khalifah, Dr.Gholam
Hussein, Mr. Ibrahim Al Sagabi, Mr.Abdullah Al-Khalid ,Mr. Hamad AlSafiran,Eng.Qusi Al-Jasem and Mr. Saleam Akhter, from KACST, Dr
Hamdi Ismaeel from KAU and Dr.Saad Al-Moqren,Dr.Abdullah AlAtass,Dr.John Roobol,Mr.Abdullah Al-Shuweel and Mr.Adil Al-Shareef
from SGS for their assistance and help.
I am deeply indebted to Eng. Mohammed S. Fargaly and Eng. Azam
Marshad,Eng.Fahad Al-Ahmadi from Ministry of Water and Electricity,
Al-Madinah Al- Monawarah, for providing the Al Aqul field borehole data.
Last but not the least; deep thanks are also due to my Father and all
other family members for their sacrifices and continuous encouragement
and patience which made the completion of this thesis possible.

iv

CONTENTS
Page
ABSTRACT……………………………………………………….…

i

ACKNOWLEDGMENT………………………………………….….

iii

CONTENTS……………………………………………………….…

v

LIST OF TABLES…………………………………………………...

viii

LIST OF FIGURES……………………………………………….….

ix

CHAPTER 1: INTRODUCTION………………………………….…

1

1.1. Definition of the Problem……………………….…………….....

1

1.2. Objectives……………….………………..……………....….…..

2

1.3. Study Area……….……………..…………..…………………....

2

1.3.1. Location…..…………….…...............……………….........

2

1.3.2. Geomorphology……..….………………….…...................

5

1.3.3. Climate…………….……………………...........................

5

1.4. Previous Work…...….…...………………………………...….....

6

1.5. Data Acquisition……...………………...................……………..

7

CHAPTER 2: GEOLOGY OF THE STUDY AREA……….….........

9

2.1.General Geology of Arabian Peninsula……..………………….....

9

2.1.1. The Arabian Shield …………………………………….…

9

2.1.2. The Arabian Shelf……………..…..…………………....…

11

2.1.3. Mobile Belt ………………………………………...…......

13

2.2. Geology of the Study Area…….………………..…………….…

13

2.2.1. Hulayfah Group…………………………………...……....

15

2.2.2. Al Ays Group……….………………….………………....

15

2.2.2.1. Farshah Formation……………….........................

16

2.2.2.2. Urayfi Formation……………………....……...….

16

2.2.2.3. Difayrah Formation …………………........……...

17

2.2.3. Furayh Group……………………...........………….……..

17

v

2.2.3.1. Murayr Formation………………………...............

18

2.2.3.2. Qidirah Formation……....…………………….…..

19

2.2.3.3. Dawnak Formation………………….……....….....

19

2.2.4. Tertiary and Quaternary Basalt…………………………...

20

2.2.4.1. Harrat Rahat…………………..………….…….....

20

2.2.5. Structure………………………………………............…..

28

CHAPTER 3: METHODOLOGY AND FIELD WORK….....……...

32

3.1. Magnetic Method………….…………..…..……………….....…

32

3.1.1. Introduction……………...………..…….………………...

32

3.1.2. Magnetic Survey……………...……...……..…………….

35

3.2 Electrical Methods…………………………….…………………

41

3.2.1. Introduction……………......……………………...………

41

3.2.2. Mathematical Background……….…......….……………...

43

3.2.2.1. Wenner Array………………………………….…

44

3.2.2.2. Schlumberger Array……………………………....

45

3.2.3. Resistivity Survey……………...…………...….…………

47

CHAPTER4:DATA PROCESSING AND RESULTS …….………..

54

4.1 Magnetic Data…………………………………...……………….

54

4.1.1 Ground Magnetic Map…………...…………………………….

56

4.2 Resistivity Data….…………………….………………….......….

72

CHAPTER 5:INTERPRETATION AND DISCUSSION…….……..

76

5.1.Magnetic Method…………………...…………..……...…………

76

5.1.1Qualitative Interpretation….….……………………….….

76

5.1.1.1 Aeromagnetic Map……..…….………….……......

76

5.1.1.2 Ground Magnetic Map………….…………….......

79

5.1.2 Quantitative Interpretation….……………………...…...…

95

5.1.2.1. Magnetic Data…………………..………..…….....

95

5.2. Resistivity Methods……………...…………….………...............

99

vi

5.2.1. Vertical Electrical Sounding………………......……...…

99

5.2.2. Horizontal Electrical Profiling …………….............……

110

CHAPTER 6:CONCLUSIONS AND RECOMMENDATIONS........

117

6.1.Conclusions………………………...…….………………………

117

6.2.Recommendations………………….……….……………………

119

REFERENCES……………………....……..………………………...

120

Appendix (A)………..……………..…………………………………

129

1. Magnetic Maps…………………………………………...….

129

2. Magnetic Profiles……………………………………………

142

3.Magnetic Analysis……………………………………………

146

a.Frequency Methods………………….……………………… 146
b.Slope Methods……………………………………………… 150
4. Magnetic Elements Data.……………………………………

154

a.Table………………………………………………….…….

154

b. Lineaments Maps……………………................…...…….

160

5. Magnetic Data.…………………………………….………...

164

Appendix (B)…………………………………………………...…….

191

Resistivity Data………………………...………………………

191

vii

LIST OF TABLES
Table
Page
2.1 Generalized Lithostratigraphic Divisions of Al-Madinah
Area………………………………………………………..
14
3.1

Application of Geomagnetic Surveys……………………..

34

3.2

VES and HEP Coordinate…………………………………

51

4.1

Results of Magnetic Profiles ……………………………..

55

4.2

Depth Estimation Using Slope and Power Spectrum
Methods…………………………………………………..

62

Information of Wells………………………………………

103

5.1

viii

LIST OF FIGURES
Figure
1.1
Location map of Al Madinah Al Munawarah
1.2

Page
3

Topographic Map of the Study Area Showing the
Location and Access Roods………………………………

4

Arabian Plate Boundaries, Adjacent Plats and Geologic
Setting of the Arabian Peninsula……………....................

10

Sketch Map of the Arabian Shield Showing Terranes and
their Boundaries…………………………………………..

12

2.3

Geologic Map of the Study Area…………………………

22

2.4

Rough Basaltic Surface in the Study Area………………..

23

2.5

Sketch Map of the Arabian Shield Showing Major
Structural Element………………………………………..

30

Folding in Tiyyam Mountain (Furayh Group), NE of the
Study Area……………………………………………...

31

3.1

Preliminary Survey and Marked Stations………………...

36

3.2

The Scintrex Envi-Mag Magnetometer Used in the Study
Area……………………………………………………….

36

3.3

Magnetic Base Station Measures Data Automatically…...

38

3.4

Measurements of the Magnetic Data……………………..

38

3.5

Sketch Map of Magnetic Survey Locations………………

39

3.6

Electrode Configurations in Wenner Arrangement………

46

3.7

Electrode Configurations in Schlumberger Arrangement..

46

3.8

ELREC-T Instrument Used in this Study………………

48

3.9

VES Field Work in the Study Area……………………....

50

2.1
2.2

2.6

ix

Figure
3.10 HEP Field Work in the Study Area………………………

Page
50

3.11

Sketch Map of Resistivity Field Work Locations………...

52

3.12

Satellite Image of the Study Area…….............…………..

53

4.1

Magnetic Profile No. 0(a) Before Smoothing and (b)
After Smoothing………………………………………….

58

4.2

Interpretation of an Energy Spectrum…………………….

60

4.3

Average Energy Spectrum Analysis for Total Magnetic
Map……………………………………………………….

61

4.4a

World Magnetic Inclination Chart………………………..

69

4.4b

World Magnetic Declination Chart………………………

69

4.4c

World Total Magnetic Intensity Field Chart……………..

70

5.1

Total Intensity Residual Aeromagnetic Map of the Al
Madinah Quadrangle,Sheet 24D,Reduction to the Pole
and Upward Continuation to 800m (a.g.l.)……………….

78

5.2

Total Magnetic Intensity Map……………………………

80

5.3

Reduced to the Pole (RTP) Map After Removal of
Regional Field (Residual)………………………………...

81

5.4a

RTP Data After Application High-Pass Filter……………

82

5.4b

RTP Data After Application Low-Pass Filter…………….

83

5.5a

RTP Data After Application Upward Continuation to
100m……………………………………………………...

84

RTP data After Application Upward Continuation to
200m……………………………………………………...

85

Frequency-Azimuth Rose-Diagrams Showing the
Dominant Trends of the Structural Elements Deduced
from (a) the Residual and (b) RTP Map………………….

89

5.5b
5.6

x

Figure
Page
5.7
Frequency-Azimuth Rose-Diagrams Showing the
Dominant Trends of the Structural Elements Deduced
from (a) Upward Continuation to 100m and (b) Upward
Continuation to 200m…………………………………….
90
5.8

Frequency-Azimuth Rose-Diagrams Showing the
Dominant Trends of the Structural Elements Deduced
from the High-Pass……………………………………….

91

5.9

Geoelctrical Cross-Section (No.1) NS Direction…………

101

5.10

Lithilogy of Drilling Wells(Modified from Ministry of
Water Al-Madinah Al-Munawarah)..............................

102

5.11

Geoelectrical Cross-Section (No. 2) EW Direction………

105

5.12

Geoelectrical Cross-Section (No.3) NE-SW Direction…..

107

5.13

3D Chargeability Geoelectrical Inversion …….…………

109

5.14

Water level Contour Map in the Study Area……………..

111

5.15

Distribution Contour Map of TDS in the Study Area…….

113

5.16a

HEP Slice and Contour Map at Difference Depths………

115

5.16b

HEP 3D Chargeability Inversion at Difference Depths…..

116

xi

1

CHAPTER 1
INTRODUCTION
1.1 Definition of the problem
Saudi Arabian cities have been grown very rapidly both in population
and land use. One of these is Al-Madinah Al-Munawarah. Much concern has
been expressed about the situation of the city itself and the region around it.
The growth of the population is permanents in search of economic and social
opportunities. Another influential factor is the ever-increasing number of
pilgrims and visitors to the Holy Mosque in the city. The main aquifers of
Al-Madinah region are primarily Tertiary and Quaternary basalts. This
problematic situation, if left to develop without control, could cause severe
damage to the aquifers and environment.
In order to solve the aforementioned situation, integrated
geophysical techniques namely, magnetic and electrical methods were
carried out.
The results and recommendations of this study can be utilized by:
• The Ministry of Water and Electricity.
• The Ministry of Agriculture.
• The Ministry of Municipality and Rural Affairs.
• The Ministry of Transportations.
• Earth Science Departments.

2
• Saudi Geological Survey (SGS).
• King Abdulaziz City for Science and Technology (KACST).
• Private sectors who deals with geotechnical and environmental
o problems.

1.2 Objectives
The main objective of this study is to investigate the subsurface
structure characteristics and volcanic layer (basalt) sequences in
northeastern Al-Madinah Al-Munawarah (Harrat Al-Aqul) applying the
available geophysical techniques namely, magnetic and electrical resistivity
methods.

1.3 Study Area
1.3.1. Location
Al-Aqul area is located about 30 Km in the northeastern part of the
Holy city of Al-Madinah Al-Munawarah, and north of the Harrat Rahat,
between latitudes 24 o 25` 12`` N and 24 o 30` 00`` N and longitudes 39 o
44` 24`` E and 39 o 52` 48`` E and. It is accessible through Al-Qassim - Al
Madinah highway, about 5 km south of the highway, and about 10 km to
the southeast of Al-Aqul town ( Fig. 1.1and 1.2).

3

Study Area

Fig 1.1:Location Map of Al-Madinah Al-Munawarah

4

39o 30`

40o 00`E
24o40`N

24o 40`

Study area

24o 15`

24o 15`N
39o 30`

Scale 1:250,000

40o 00`E

Fig.1.2: Topographic Map of the Study Area Showing the Location and Access
Roads. (DMMR ,1981)

5
1.3.2 Geomorphology
Al-Madinah area is bounded by Harrat Rahat to the west with deep
valleys and in the east by peneplain of Cambrian age (Al- Harbi et al.,
1998). The drainage patterns of the area are controlled by the geological
features in the outcrop. The drainage pattern of Harrat Rahat which consists
of basalt, has a dendritic drainage pattern controlled by the slopes of the
basalt cover (Italconsult, 1979a). The valleys in the north flow in dendritic
drainage system. Most of the wadis (valleys) in the region flow towards the
Red Sea (Bokhari, 1993). The study area (Al-Aqul) is located in the
northern part of Harrat Rahat, it made historic lava flow immediately west
part of the historic vents. A Hawaiian lava (young lava with a rock surface
of loose angular sheets) with some channels of pahoehoe (Fig. 1.2). The
surface of lava flow is very rough with many loose slabs of tilted basalt
(Roobol, 1998). The important valleys in the area are Wadi Al-Bitan and
Wadi Al Khanaq.

1.3.3 Climate
The Arabian Peninsula lies between two of the world’s hottest
regions, namely, Sahara in the African continent and the northwest Indian
subcontinent which provide heat reservoirs (Al-Ahmadi and Sen, 1989).
The study area is characterized by an arid climate, it has low rate of rainfall
and high temperatures. Average daily temperatures are 28 o - 42 o C. during

6
July and August, and 11

o

- 24

o

C. during December and January (Al-

Madinah Municipality, 2004). Rainfall occurs generally as irregular storms
preferentially in November-December and March-April. At Al-Madinah,
the average rainfall is about 40 mm/yr, but it is highly variable and some
years may be completely dry (Ministry of Agriculture and Water, 1988).

1.4 Previous Work
The geological work in Al-Madinah area was started in 1960, by
Directorate General of Mineral Resources. Several geologial investigations
including Brown and Jackson (1960a) who presented a general study of the
Arabian Shield, Bhutta (1961a;b) and Brown et al. (1963) evaluated the
reported mineral prospects of the area. A geographic map of the
northeastern area was published by Brown and Jackson (1958b).The same
map was revised and published in (1968c) with scale 1:50,000. Geologic
detailed mapping of the area was carried out by Hummel (1967). A
hydrologic study of Harrat Rahat basalt plateau was done by Daessle and
Durozoy (1972) and Daessle (1973). The detailed geologic mapping for
western Saudi Arabia and Al-Madinah area were prepared by Pellaton
(1981) that results of mineral exploration in the same were published by
Brosset (1976). The study for water supplies was done by Sogreah
Grebobloise (1968). Detailed investigations of Al-Madinah region for

7
evaluating the groundwater resources provided for a geophysical surveying
by Italconsult (1979b).
Adam (1981) investigated groundwater potentiality in the south of
Madinah Al-Munawwarah. A regional investigation of the Cenozoic
volcanic rocks and Harrat Rahat of Saudi Arabia was explained by
Coleman et al. (1983).
Andreasen and Petty (1974) studied the total intensity Aeromagnetic
map of the northeastern Hijaz Quadrangle, and the total intensity
Aeromagnetic maps of the Arabian Shield were carried out by Georgel et
al. (1985). Determined depth by the 1976 Aeromagnetic survey of Harrat
Rahat by Blank and Sadek (1983). The Cenozoic lava field of Harrat Rahat
by Camp and Roobol (1991). Bokhari( 1988 ; 1993) has used LANDSAT
images to generate drainage pattern in Al-Madianh area. Bokhari (1991);
Bokhari and Khan (1992) studied the groundwater quality and modeling in
Al Madinah area. Al Harbi et al. (1998) investigated the geophysical and
hydrological characteristics of the northwestern part of Al-Madinah area
(Wadi Malal). A view of the Pan-African evolution of the Arabian Shield
was studied by Nehlig et al. (2002).

1.5 Data Acquisition
Data acquisition for analysis was acquired from six sources:
1. Comprehensive geophysical field work namely, magnetic and

8
electrical resistivity survey covering the whole study area in two
months period in 2003.
2. Aeromagnetic map surveying (1976) for Harrat Rahat compiled
by Blank and Sadek. The geographic and geologic maps are
from the Saudi Arabian, Deputy Ministry for Mineral Resources
(now the Saudi Geological Survey) and its associates the French
Bureau Reserches Geologiques et Minieres (BRGM) and the US
Geological Survey (USGS),in (1983).
3. Topographic maps (scale 1:25,000) from the Ministry of
Municipality and Rural Affairs (MMRA), Deputy Ministry for
Town Planning,1409 A.H.
4. Satellite image type of (SPOT 4) from the SPOT processing system
in Saudi Arabia, King Abdulaziz City for Science and Technology
(KACST).
5. Borehole data from Ministry of Water and Electricity branch AlMadinah Al-Munawarah.
6. Personal communications and published works.

9

CHAPTER 2
GEOLOGY OF THE STUDY AREA
2.1 General Geology of the Arabian Peninsula
The Arabian Plate is located in the southwest of the Asian continent,
and is bounded by various active tectonic boundaries (Fig 2.1):
1. Taurus – Zagros thrust fault in the east and northeast.
2. Dead Sea left lateral transform fault to the north and northwest part.
3. Red Sea and Gulf of Aden spreading centers to the west and southsouthwest of the Arabian Peninsula.
4. Owen fracture zone which is right lateral transform fault to the southsoutheast.
The Arabian Peninsula is situated in the southern part of the Arabian
Plate and geologically, can be divided into three major structural provinces
(Power et al., 1966). The first province contains the Arabian Shield, the
second is the Arabian Shelf and the third is the great mobile belt (Fig.2.1).
2.1.1 The Arabian Shield
The Arabian Shield is exposed over an area of 610,000 km2, and is
equal to about 1/3 of the Arabian Peninsula. The Arabian Shield is located
in the western part of the Arabian Peninsula. According to Nehlig el al.
(2002) and Camp and Roobol (1991), the Red Sea began to open at about
25 to 30 Ma, the Arabian Shield was before a part of a larger geologic unit

10

Fig 2.1: Arabian Plat Boundaries, Adjacent Plats and Geologic Setting of the
Arabian Peninsula (Johnson, 1998)

11
known as Arabian-Nubian Shield. To the west, the Shield is separated
from the African Shield by Red sea depression and rift and to the east the
Precambrian rocks of the Shield are overlain by veneer of Phanerozoic
sedimentary rocks . The major geologic province of western Saudi Arabia
is a Precambrian Shield boardered on the east by platform of gently dipping
Cambrian rock covered with younger sedimentary rocks and to the west by
lava fields (Harrats) that was formed during the Tertiary and the
Quaternary, contemporaneously with Red Sea rifting, approximately 30 Ma
ago (Gettings et al., 1986; Camp and Roobol, 1991).
The Shield is divided into five terranes (Asir, Hijaz, Midyaen, Afif and
Rayn) which are separated by four suture zones namely, Bir Umg, Yanbu,
Nabitan and Al-Amer (Stoeser and Camp, 1985).
The western Shield is composed of at least three intraoceanic islandarc terrane (Asir, Hijaz and Midyan) where as the eastern contains one
terrane of continental affinity (Afif) and another terrane of possible
continental affinity (Ar Rayn);(Fig. 2.2)

2.1.2 The Arabian Shelf
According to Power et al. (1966), the Arabian Shelf was formed after
solidification and mature peneplanation of the Arabian Shield and buried in
the Tethys Sea beneath thin sheets of almost flat-lying sediments. A vast
dominantly Precambrian complex is of igneous and metamorphic rocks.

12

Fig 2.2: Sketch Map of the Arabian Shield Showing Terranes and their Boundaries
(Stoeser and Camp,1985 ).

13
The Arabian Shelf can be subdivided into three major provinces as
follows:1. The interior Homocline.
2. The interior Platform.
3. Several basins.
2.1.3 Mobile belt
Encircling the stable interior region is a mobile belt of young mountains
and thin foreland area deformed contemporaneously with the main range.
These mountains are Zagros of Iran and Oman mountains and small link in
the much longer Alpine-Himalayan systems.

2.2 Geology of the Study Area
Al Madinah quadrangle is covering an area around 16550 km2, and is
covered for almost 75 percent by basalt of Harrat Rahat. The other 25
percent is made up of Precambrian basement rocks occupying the west
edge and the northeast corner of the quadrangle and represented by distinct
units (Brosset, 1976). The quadrangle is underlain by Precambrian
metasedimentary, metavolcanic and igneous rocks, sandstones belonging to
the Cambrian-Ordovician age, Quaternary and Tertiary basaltic flow and
surficial deposits (Pellaton,1981). The upper Proterozoic of Precambrian
age (Table 2.1) is divided into three groups: Hulayfah Group, Al- Ays
Group and Furayh Group (Al-Harbi et al., 1998). The Al-Ays Group and

14

Table 2.1Generalized lithostratigraphic Divisions of Al-Madinah Area
(Pellaton,1981).

Era

Age
(in m.y.)

Units

Basalt flows

0

Trachyphonolytic

to

extrusions

40

Unconformity

Sandstones

561
to

FURAYH

Unconformity
Murayr
Conglomerate
Qidirah
Mafic Volcanic
Dawnak

742

Microgranite;
Granophyre
Peralkalic
Granite
Granite;
Granodiorite

Sandstones

673

ALAYS

UPPER PROTEROZOIC

Intrusive
rocks

Formation

Cambro-Ordovician

PALEOZOIC

CENOZOIC

Group

Lithology

Difayrah

Volcanic-sedimentary

Urayfi

Rhyolitic-sedimentary

Farshah

Andesitic

Granodiorite;
Tonalite;
Diorite;
Gabbro

15
the overlying Furayh Group are equivalent to the Murdama Group.

2.2.1 Hulayfah Group
Hulayfah Group is mainly volcanic of andesitic composition and
pyroclastic rock (rhyolitic tuff and breccia, andesiteioc tuff and tuffite) and
the sedimentary rocks of Nugrah Formation (arkose, chlorite and sericite
shcist and intercalations of small marble layers) are separated by Tertiary
basalt. The rocks look like as if they were of Urayfi Formation of the Al
Ays Group but the field correlation is not possible at any part of the
quadrangle.
Hulayfah group is located in the northwestern corner of the
quadrangle.

2.2.2 Al Ays Group
Al Ays Group, which lies in the northern half of the quadrangle, also
occurs in the southwest. The name Al Ays Group, has been defined by
Kemp (1980) and Pellaton (1979) in the Wadi Al Ays quadrangle to the
northwest. The Ays Group is predominantly mafic to silicic volcanic rock
and derivative epiclastic and detrital sedimentary rock. It is cut by
numerous igneous intrusions and is overlain by the Furayh Group to the
south and by Cambrian-Ordovician sandstone and Tertiary basalt to the
northeast. The Al Ays Group constitutes three different Formations as
follow:

16
2.2.2.1 Farshah Formation
Farshah Formation comprises several thousand meters thickness of
andesite lava and subordinate pyrolastic rock, but the base is not seen. It is
conformably overlain by the Urayfi Formation and shows a gradational
contact on both sides of the Jabal Farshah anticlinorium’s.
The lower part of the Farshah Formation consists of massive gray,
dark green or violet andesite and basalt flow predominantly of aphyric or
porphyritic lava and more rarely of amygdaloidal and microlitic types
where are the upper part more clearly defined and the lava is more
definitely amydgolidal with amydgules of quarts, chlorite and epidote. The
sequence can be seen clearly on the east flank of the Jabal al Farshah
anticlinorium . Several ryholite layers are interbedded in this part of the
section which indicates pertological change to the silicic rock of the
overlying Urayfi Formation.

2.2.2.2 Urayfi Formation
The Formation named from Jabal al Urayfi north-northeast of the
city of Al Madinah, where the alternation of the different facies represents
the upper part of the Al Ays Group and consists of several thousands
meters of epiclastic volcanic breccia and interbedded volcanic rock
(sandstone and pyroclastic rocks).They are clearly formed with the layers
of ryholite, ignimbrite, ryholite tuffs and ductile in the northeast of the

17
quadrangle. A detrital sedimentary member has been also identified in
Wadi Atammah area in the northwest of the quadrangle. At the top of this
Farshah Formation, gradational changes from andesitic tuffs to
heterogeneous tuffs are present.

2.2.2.3 Difayrah Formation
The Formation is named from Jabal ad Difrayah and covers a small
area on the western side of the quadrangle. It lies at the top of the Al Ays
Group and correlates with the upper part of the Urayfi Formation. The
Formation is composed mainly, of alternating rhyolitic volcanic rocks
intercalated with fine-grained decrial rocks which were deposited under
water.
The sedimentary rocks comprise fine grained greywacke siltstone
epiclasic sandstone with thin layers of chert and marble,which are
commonly boudinized.
The Formation is bounded to the north and east by granitic intrusions
and to the south by the Assaadin graben which puts it into fault contact
with the Urayfi Formation (Pellaton,1981).

2.2.3 Furayh Group
The Furayh Group is named from Jabal Furayh in Al Hissu
quadrangle, where it is given the same age as the Murdama and Shammar
Groups. It is covering a large part of the southern half of the

18
quadrangle,and underlain by Furayh Group which probably also extends
beneath the Harrat Rahat basaalt. The group generally forms low hills
surrounded by Quaternary cover. It unconformably overlies the Al Ays
Group but is not clear everywhere, particularly where Qidirah Formation
rocks overliesitic facies of the Urayfi Formation. The contact is only
identifiable by the presence of volcanic dikes in the Urayfi Formation.
The group is divided into three Formations:
2.2.3.1 Murayr Formation
The Formation is named from Jabal Al Murayr on the eastern margin
of the quadrangle. Where an anticline exposes several hundred meters of
polymictic conglomerate and sandstone. They are overlain by the Qidirah
Formation on each side. The conglomerate contains pebbles and boulders
as much as 60 cm in size of rhylite, andesite, granite and jasper. The matrix
is sand sized. The color of sandstone is green purplish brown.
The Murayr Formation reappears on the western side of the
quadrangle overlying the Urafi Formation north of Bir Faraj, 44 km
southwest of Al-Madinah. About 400m thickness of conglomerate
containing poorly rounded, poorly sorted boulders and composed of rholite
and ignimbrite of the same type as occur in the underlying Urafi Formation.

19
2.2.3.2 Qidirah Formation
The Qidirah Formation was named after Jabal Al Qidirah east of
Harrat Rahat. It forms the lower part of the Furayh Group except where the
Murayr Formation is developed and is conformably overlain by the
Dawank Formation in the southeast of the quadrangle. The Formation is
consists of mafic volcanic rocks, varying in thickness from several
thousand meters in the southwest of the quadrangle to several hundred
meters near Suwaydarah in the northwest.
The main outcrop of the Qidirah Formation is west of Harrat Rahat,
where thick layers of detrital sedimentary rocks are interceded and have
been mapped as a distinct member(Pellaton,1981).
The Qidirah Formation is essentially composed of andesite and darkgray to green amygdaloidal basalt containing chlorite and epidote.

2.2.3.3 Dawnak Formation
The Formation is named from Jabal Durus ad Dawnak to be far away
about 20 km west – southwest of Jabal Al Murayr in the southeast of the
quadrangle and represent the top of Furayh Group. And more restricted
north and west of Harrat Rahat. It consists of sandstone with conglomerate,
tuff and marble. The main outcrop in the southeast of the area is
represented by thick sequence of several rocks, including graywack,
siltstone, lithic sandstone, sandstone, conglomerate and marble. In the east

20
of Al Madinah Airport the sedimentary rocks are bounded to Harrat Rahat
and lithologically is similar to Furah Group. The outcrop shows an
alternation of amygdaloidal basalt and layered tuff, which may represent
the Qidirah Formation. It is also believed that the intrusions are represented
by batholiths and plutonic calc-alkalic granites. These are considered
younger than Furah Group. The thickness of the Formation has not been
determined but is probably several thousand meters.

2.2.4 Tertiary and Quaternary basalt
The western part of the Arabian Peninsula and the north-south part
of Al-Madinah quadrangle contains poorly – known lava fields (Harrats)
that formed contemporaneously with Red Sea rifting (Camp and Roobol,
1991). They constitute one of the worlds largest alkali basalt provinces
with an approximate area of 180,000 km2 (Coleman et al., 1983). The
basalt occurs either as large flows only slightly affected by erosion or as
erosional remnants perched on top of prominent buttes and comprises the
southeast termination of Harrat Khayber, the northern termination of Harrat
Rahat and the small Harrat Hirmah between them.

2.2.4.1 Harrat Rahat
The Harrat Rahat area is situated between latitudes 24 o 36`N 21o 37`
and, and longitudes 38 o 45` and 40 o 48`E. It extends from Al- Madinah Al
Munawarah in the north to the northern outskirts of Jeddah in the south. It

21
is an area of about 52,600 km2 of which approximately one third of exposed
area which is composed of Precambrian Arabian Shield rocks. All the
designated Cenozoic units overlie the Precambrian basement with marked
unconformity (Camp and Roobol, 1991). The topographic crest has an
average elevation of about 1300 m above sea level (a.s.l) but the elevation
of the study area in the northern part of Harrat Rahat varies between 680 m
and 780m a.s.l.
Harrat Rahat has been divided into three major stratigraphic units
from oldest to youngest and these are Shawahit ,Hammah and Madinah
basalt (Camp and Roobol, 1991). These are separated by two prominent
disconformities with diagnositic erosional patterns and mature lateritic
surfaces. The three stratigraphic units correspond to changes in the style of
volcanic activity and the nature of the erupted products.The Shawahit lavas
are composed of coarse dictytaxitic olivine transitional basalt (OTB) and
minor (about 10%) alkali olivine basalt (AOB) which covers more
extensive areas (an E-W distance 100 km on Harrat ar Rukhq) than the
Hammah basalt.The Madinah basalt is composed of OTB,AOB and
hawaiite,and its lavas are finer grained less extensive even than those of the
Hammah basalt.The surface of lava flow is very rough with many loose
slabs of tilted basalt (Fig 2.3 and 2.4).
The Harrat evolved during the past 10 million years from basaltic
eruptions that migrated to the north along northerly trending linear vent

40o 00`
24o 32.5`

40o 54`E
o
24 32.5`N

24o 21`N
40 00`E

24o 21`
40o 54`

o

Urayfi Formation-Epiclastic Sanstonand Breccia,
Tuff, Reworked Tuff, Mafic to Silicic Lava
Rhyolite-Ignimbrite Member (ur)-Rhyolite,
Iignimbrite, Rhyolitic Breccia and Tuff.
Qidirah Formation-Andesite, Basalt,
Volcanic Breccia and Tuff.
Monzonite, Diorite and Gabbro.

,

and

Tertiary Volcanic
Rocks

Quaternary Basalts
Sand, Gravel and Silt
Deposits in Wadis.
Volcanic Cone.

Fig. 2.3: Geologic Map of the Study Area (Pellaton,1981)

Scale 1:250,000

22

23

Fig.2.4: Rough Basaltic Surface in the Study Area.

24
system. The youngest flows are those of 11 “Post-Neolithic”
eruptions (< 4,500 B.P.) and 2 historic eruptions (641 AD. and 1256 A.D.).
These are the last known eruption which occurred near Al-Madinah in
Saudi Arabia. Fire fountaining produced, during this eruption, a linear
system of six scoria cones aligned along a 2.25 km long fissure oriented
343o.There are two sources that show remarkably good agreement in detail
and dates which together with our field observations,one Wafa al Wafa was
written by Al Samhoudi who died 911 A.H. and reprinted by Abdulhameed
(1984) and the other ,Jazb Al Kulub,was recorded by an early European
traveler in Burkharadt,and published as a footnote in Burton (1893) and
Camp et al.(1987).

25
Extracts from Wafa Al Wafa
The earthquake started in Al Madinah at the first day of Jumada Al
Akhira, 654 A.H.(Monday 1 June 1256 A.D.), but was so mild light that
some people did not feel it thought it recurred after that. According to Al
Qutub Al Qastalani, it was very strong on Tuesday (the second day of
Jumada Al Akhira, 654 A.H ) and every body feel it. In the third part of the
night of 3rd or 4th day of the month a strong earthquake hit Al Madinah,
frightened the people and continued until Friday (5 Jumada Al Akhira, 654
A.H.),a major event occurred when the ground and ceiling of the houses
were shaken. Eighteen earthquakes ware felt in Al Madinah at mid-day and
fire appeared associated with black smoke clouds which accumulated in
the atmosphere. The greatest fire covered the horizon to the east of Al
Madinah.
Historians said that the beginning of lava flow appearance was in up
stream of wadi (valley) Al-Ehileene. The wadi Al Ehileene was located with
eastern part of Al Madinah at Al Swairgiah road, a distance half day
walking. Historians stated that lava flow burnt stone and mountains while
it flowing rapidly on a valley of four farsakh long, four miles wide and
approximately a height and half of a man depth (According to Qalaji and
Qunaibi(1404), (farsakh = 3 mils = 5544 m, ).
Al-Qutub al-Qastalani stated that:

26
The lava appeared at eastern side in qari Al-Hyla locus near
Quraidha quarter east of Quba not far from Madinah. Qari Al-Hyla
located between Quraidha and a locus called Al-Ehileene. It was flamed up
in this bottom, then extended east-ward up to near A-Ehileene, then the
lava flow extends towards Al-sham to reach position called Qarein AlArnab near Uhod Mountain, where lava flow consolidated. Lava flow
running on valley surface while rocks melting to be as lead it rocks became
black instead of red as soon as lava flow not off those dissolved stones
accumulated at the end of the valley which ended with heights (Harrat).
Lava flow diversed from Shadhaat valley to Wa’erah Mountain. Such flow
and stones accumulation tidily closed the above mentioned valley with a
hug dam of solid stone resulting form lava. Such Formation is so difficult
for described to describe. There was no passage for human or animals. I
said that was one of the lava flow advantage.

27
The crest forms a regional watershed separating the Najd plain to the
east from the mountainous Al-Hijaz region and Tihamat coastal plain to the
west. A major escarpment formed as a result of uplift and tilting associated
with the development of Red Sea runs approximately parallel to the
western edge of Harrat Rahat in the eroded Precambrian basement.
Most Tertiary sedimentary rocks are covered by thick deposits of
Quaternary alluvium and by reef limestone on the coastal plain. Outcrops
of the Tertiary sedimentary succession are preserved on the coastal plain in
areas of inverted topography beneath resistant basalt remnants of Harrat
Rahat.
Quaternary deposits are mainly unconsolidated and cover most of the
coastal plain as well as large areas on the periphery of Harrat Rahat (Fig
2.3). Minor deposits occur on Harrat Rahat and in the area between the Red
Sea escarpment and the coastal plain. The Quaternary deposits have been
subdivided into five units:
1. Wadi alluvium (poorly sorted unconsolidated sand and gravel,
(Qal)).
2. Eolian sand (small dune field).
3. Terraced, slightly consolidated alluvium of old wadi deposits (fan
gravel and talus).
4. Sabkhah deposits (terrrigenous sand, clay and evaporates).
5. Lithified reef limestone.

28
2.2.5 Structures
The basement complex rocks show sign of very old folding and
faulting, ascribable to various tectonic phase,mostly compressive which
occurred repeatedly in Precambrian times (Italconsult, 1979a; Fig 2.5).
The faults can be divided into:1. Most faults in Al-Madinah quadrangle appear to belong to the Najd
fault system: They are numerous and generally strike N 40o - 50 o W,
The Precambrian Najd fault systems.
2. In the southwestern parts of the quadrangle, the faults follow one
another over 75 km to form the As Saadin graben.
3. In the northern parts, the faults are left lateral with displacement of
up to 6 km and they intersect the Al-Ays Group and the post furayh
granite Group in Jabal Taar, Jabal Murayr and Jabal Qidirah.
4. Several east-west fractures belonging to another fault system
probably later than Najd, occur in the Jabal Al-Arajib granite
batholith and Jabal Al-Bayda alkalic granite.
There are many folds which occur with in tectonic events. The Al
ays group rocks are folded with the axial planes of the folds generally
trending north. A major anticlinal structure occurs in the north where the
Jabal Farshah anticlinorium exposes.
The Urafi Formation is contained in an isoclinal structure with
numerous secondary folds plunging slightly to the east-southeast. To the

29
west the Urafi Formation is contained in the as Suhaylat syncline with a
north-south axis. Three anticlinal structures, created by a major phase of
folding with north-northwest trending axial planes, affected all three
Formations of the Furayh Group. The clearest syncline belonging to this
major phase is that at Al-Aqul area (Fig 2.6), constituting the Furayh basin,
east of Al-Madinah airport, and it has a north-south axis but is complicated
by numerous secondary folds (Pellaton,1981).

30

Fig 2.5: Sketch Map of the Arabian Shield Showing Major Structural Element
(Nehlig et al., 2002).

31

Fig. 2.6: Folding in Tiyyam Mountain (Furayh Group), NE of the Study Area.

32

CHAPTER 3
METHODOLOGY AND FIELD WORK
3.1 Magnetic Method
3.1.1 Introduction
Magnetic methods have a long history behind them. Early studies of the
magnetism of rocks started with the discovery of the “lodestone” (magnetite
rich rock). It is generally believed that the Chinese were the first to make use
of the directional properties of the lodestone as early as the second century
B.C. (Sharma, 1986). The first scientific analysis of the Earth’s magnetic field
and associated phenomena was published by the English physicist William
Gilbert in 1600 in his book De Magnete. Measurements of variations in the
Earth’s magnetic field were made in Sweden to locate iron ore deposits as
early as 1640A.D. (Reynolds, 2000)
Magnetic prospecting, the oldest method of the geophysical exploration,
is used to explore for oil, minerals, and even archaeological artifacts. In
prospecting for oil, it gives information from which one can determine the
depth to basement rocks and thus locate and define the extent of the
sedimentary basin (Dobrin and Savit, 1988). It is sometime employed,

33
although not always, successfully to map topographic features on the
basement surface that might influence the structure of the overlying sediments.
Sedimentary rocks exert such a small magnetic effect compared with
igneous rocks. Most variations in magnetic intensity measurable at the surface,
results from topographic or lithologic changes associated with the basement or
from igneous intrusive rocks.
Both airborne and ground magnetic surveys are the most versatile and
essential geophysical prospecting for many application (Table 3.1). The field
measurements are easily made, cheep and simple compared to most
geophysical techniques.
The magnetic method of the prospecting has great deal in common with
gravitational method. Both make use of potential fields. Both seek anomalies
caused by changes in physical properties of the subsurface rocks. Both have
similar applications in oil exploration. This partly due to the difference
between the dipole magnetic filed dependent on its susceptibility and the polar
gravity field (dependent on density of rocks). The former have magnitude and
variable direction and the later have magnitude and vertical direction only
(where the gravity map shows mainly regional effects, the magnetic maps
appears to be a multitude of residual anomalies (Telford et al., 1984)).

34

Table 3.1: Application of Geomagnetic Surveys (Reynold, 2000)

Locating

Mapping



Pipes, cables and metallic objects.



Archaeological remains



Buried



Concealed basic igneous dykes

(shells,bombs,ect)



Metalliferous mineral lodes

Buried metal drums of contaminated



Geological





military

ordnance

boundaries

between

or toxic waste

magnetically contrasting lithologies,

Concealed mineshafts and adits

including faults


Large-scale geological structures

35
Magnetic measurement at any point on the Earth’s surface comprises
three components form different sources. The source of the first component is
the Earth magnetic field, since the Earth acts as a large bar magnet. The
second component is the rock magnetism. The third component is the
magnetism caused by external sources outside the planet Earth such as
magnetism from the Sun and the outer space.
3.1.2 Magnetic Survey
In the study area, the magnetic survey grid marked perpendicular on the
main geological structure. The distances between lines are 150m to 300m, the
distance between stations along the profiles was 20m, and the direction of
traversing was from north to south. Before carrying out the magnetic survey, a
suitable site inside the grid survey area was chosen for the construction of the
base station, particularly inside the study area (Fig.3.1). A total magnetic
intensity survey was carried out along all the survey profiles. Measurements
were

taken

by

two

proton-procession

magnetometer

ENVI-MAG

manufactured by Scintrex Limited 1994, Canada (Fig 3.2). The ENVI-MAG is
a total field instrument using the proton processing technique to measure the
local magnetic field. The magnetometer has a sensitivity of 0.1 nT and is
supplied by internal memory and microprocessor which allows sorting and
recording the survey


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