CHAPTER I

GEOLOGY

1. INTRODUCTION

The study area lies between longitudes 25 degree to 31 degree E (about 600

km.), and latitude 28 degree to 31 degree N (about 300 km.). The area

covers about 180,000 Sq. km. The study area is bounded to the north by the

Mediterranean Sea, to the south by the Bahariya Oasis, to the east by the

River Nile and to the west by the Libyan border.

This regional geologic study was undertaken to evaluate the stratigraphic

and the structural history of the area, in order to outline the different

geologic provinces.

2. STRATIGRAPHY

2.1 Paleozoic (Early Cambrian – Late Permian)

The Paleozoic sediments overlain unconformably by the Jurassic or younger

sediments (Fig. 1.1). It is dominated by sandstones and siltstones with an

abundance of limestone and shales in the Upper part of the section.

Figure 1.1: Western Desert Geologic Column.

In 1988 paleoservice examined cuttings form 24 Western Desert wells and

they found Acritarchs and Miospores which suggested that shallow marine

to delta complex conditions prevailed at the time of Paleozoic deposition in

Egypt. The Paleozoic in north Egypt ranges in age form early Cambrian to

late Permian and it non-conformably overlies the basement. Only the

Cambro-Ordovician section was found covering most of the Western Desert

While amore complete Paleozoic covered only the northern belt of Egypt

(Mediterranean coast) and the Ghiarabub Siwa area (Fig.1.2) The subsurface

Paleozoic succession is defined by Paleoservice (1986) into the Siwa and

Faghur groups.

Figure 1.2: Paleozoic Sub-crop Map.

The Siwa group is the lower of the Paleozoic section and it overlies

nonconformably the Pre-Cambrian crystalline basement (Fig. 1.1). It ranges

in age form Cambrian to late Silurian and is subdivided into three formations

which are "Shifah, Kohla and Basur Formations."

2.2 Mesozoic

2.2.1 Jurassic

The Jurassic sediments in the Western Desert represent the first main

transgression which invaded the area after the Hercynian orogeny. The

Jurassic section is divided into four formations, which are:"The Baharein,

Wadi Natrun, Khatatba and Masajid formations" (Fig.1.1). The

environments of these sediments range from a deep marine (Kattaniya,

North Kattaniya and abu Gharadig areas) to a shallow marine environment

(Matruh – Sidi Barrani and the southern part of the Western Desert).

The Jurassic overlies conformably the Paleozoic and in some cases,

unconformably throughout the study area. The Jurassic strata overlie

nonconformably the basement in the eastern part of the Western Desert in

the Kattaniya area. It is apparently overlain unconformably throughout most

of the study area by the lower part of the lower Cretaceous (Barremian –

Aptian) while in the Sharib-Sheiba high and southern platform areas it is

overlain uncoformably by the upper section of the Lower Cretaceous

(Kharita-Bahariya Formation).

The Jurassic exceeds 8156 feet in the Natrun T 57-1 well (drilled thickness,

base not reached) which is near the center of the Jurassic depocenter

(Kattaniya Area) (Figs. 1.3& 1.4).

Figure 1.3: Khatatba Formation Isopach Map.

Figure 1.4: Masajid Formation Isopach Map

It is thin, mostly less than 845 feet (Bahariya-1 well), along the Southern

platform. The Jurassic strata become more continental to the south and west

(Bahrien Formation). It is absent along the crest of the uplifted axis of the

Ghiarabub-Siwa Paleozoic basin.

2.2.2 Cretaceous

Cretaceous sediments cover the whole study area, and represent the Western

Desert’s second major transgressive Mesozoic cycle which occurred after a

period of up-lift and erosion that prevailed in the Late Jurassic and Early

Cretaceous. The Lower Cretaceous is divided into three main Formations,

Alam El Bueib, Alamein and Kharita which are mainly of continental

environment; while the upper Cretaceous is also divided into three

Formations: Bahariya, Abu Roash and Khoman, which are mainly of

shallow marine to open marine environment. An unconformity is present

between the Lower and Upper Cretaceous (Fig. 1.1).

Cyclic sedimentation prevailed during the Cretaceous where it can be seen

that the base of the section was continental to shallow marine (Alam El

Bueib Formation), then became shallow marine (Alamein Formation) during

the Aptian, then continental to shallow marine (Kharita Formation) during

the Albian and become shallow marine to open marine (Bahariya, Abu

Roash and Khoman Formations) during the Cenomanian, Turonian and

Senonian.

The lithology of the Lower Cretaceous shows rapid facies variations in the

Northwestern portion of the study area where the sandstones in the Aptian

(Alam El Bueib Formation) either laterally change into the carbonates of the

Sidi Barrani Member or into a shale known as the Matruh shale. The top of

the Aptian (Alamein Formation) is mainly carbonate, with an overlying

shale member (Dahab member). The Albian is mainly sand and sandstone,

while the base of the Upper Cretaceous (Baharyia and Abu Roash

Formations) is mainly sandstone, shale and limestone. The Senonian, which

is the top upper part of the Cretaceous, (Khoman Formation) is mainly

chalky limestone.

2.3 Tertiary

Apollonia Formation (Paleocene – Eocene)

The Apollonia Formation ranges in age from Paleocene in some area to Late

Eocene, and consists of an open marine sequence of limestones and some

shales. The Formation is divided lithologically and partly paleontologically

into four units, arranged from top to bottom as:-

Apollonia “A” (Middle to Late Eocene) Massive chalky limestone.

Apollonia “B” (Middle Eocene) Mainly shale.

Apollonia “C” (Middle Eocene) glauconitic limestone.

Apollonia “D” (Early Eocene to Paleocene) limestone with shale

intercalations.

The Apollonia Formation mostly overlies unconformably the Khoman

Formation, (Cretaceous age) where the Paleocene and Lower Eocene is

missing. In areas which were structurally low during deposition, the

Paleocene is present in some wells in the Abu Gharadig province, on the

flanks of the Kattaniya horst and to the northwest. The Apollonia is overlain

unconformably by the Daba Formation, where the top part of the Apollonia

is missing, while it is overlain conformably by the Daba in the Eocene

depocenter areas (Fig.1.1).

The thickest Apollonia section was recorded in the El Gindi-1 well, where it

reaches 5868 feet. It is well developed in the El Gindi, Natrun, and Abu

Gharadig basins. The Apollonia is thick over the invested Ghiarabub – Siwa

Paleozoic basin and thickens westward toward Libya where it becomes very

thick in the Sirte basin. The Apollonia is thin, less than 500 feet, in the

Meleiha – Khalda – Razzak – Alamein complex, while it is absent along the

coastal ridge and in the Kattaniya high (Fig. 1.5).

Figure 1.5: Apollonia Formation Isopach Map.

The Apollonia Formation consists of white, light grey, brownish grey, hard

dense, cherty limestone occasionally chalky, dolomitic to the northwest,

calcarenitic in many places, nummulitic, with some shaly zones , where the

Formation is thickest. An increase in sand and clay percentage occurs on the

flanks of the Kattaniya horst.

Daba Formation (Upper Eocene – Oligocene)

The Daba Formation consists of Late Eocene to Oligocene age shale with

thin beds of limestone. The limestone content is great in the Zebeida-1 and

Um Barka-2 wells. In other wells as in T 65-1 well, thin beds of dolomite

and sandstone occur.

The Daba generally overlies unconformably the Apollonia Formation; but in

the coastal ridge, WD 19 areas, Sidi Barrani-1, Mamura-1 and Alamein-1

wells it overlies the Khoman Formation. The Daba Formation is overlain

conformably by the Moghra Formation (Fig.1.1).

The thicknest Daba section was recorded in the WD 7-1 well, where it

reaches 2712 feet. It is well developed in the Abu Gharadig, Natrun and

Burg el Arab areas, while it is absent along the Kattaniya high, Matruh area

and the Ghiarabub – Siwa Paleozoic basin (Fig. 1.6).

Figure 1.6: Daba Formation Isopach Map.

The Daba Formation consists mainly of shale, which is grey green, greenish

grey, calcareous, glauconitic and pyrite in character.

Moghra Formation (Upper Oligocene – Lower Miocene)

The moghra Formation consists of shallow marine sandstone, siltstone and

shale with thin beds of limestone. Where the carbonate sediments increase

within the Moghra Formation, it is named the Mamura Formation. In the

eastern part of the study area the Moghra section includes a basalt extrusion

known as the Abu Zabal basalt of Oligocene age.

It overlies conformably the Daba Formation and is overlain conformably by

the Marmarica Formation if it is present. The thickest Moghra section is

3163 feet in Dahad-1 well.

Marmarica Formation (Middle Miocene – Late Neogene)

The Marmarica Formation is of Middle Miocene to late Neogene age,

shallow to marginal marine limestone with some shale intercalations in the

lower part of the section.

The Marmarica is present in the northern part of the Western Desert, while it

is absent in the south.

3. SEDIMENTARY POROVINCES

Nine sedimentary provinces could be defined in this study. These provinces

are structurally distinct. All the provinces generally trend east- west except

the Paleozoic basin along the Libyan – Egyptian border (Fig.1.7).

Figure 1.7: Sedimentary Provinces Occurrence Map.

3.1 Coastal Ridge Province

This province lies along the Mediterranean coast line and includes the Sidi

Barrani-1, Mamura-1, and Mersa Matruh-1, wells. The coastal Ridge

province is characterized by the absence of the Eocene sediments and is the

site of a Carboniferous and Mesozoic carbonate platform, except for the

Cretaceous-age Matruh shale tongue which is limited to the Mersa Mtruh

area (Fig.1.8). The province is characterized by the thickening of Jurassic

and Cretaceous sediments north of a hinge line extending to the east from

Libya. The area contains open marine sediments deposited since

Carboniferous and until the Late Cretaceous, time. Later it was uplifted,

forming the Coastal Ridge where the Apollonia Formation was not deposited

and where the Daba Formation unconformably overlies a thin Khoman

Formation.

Figure 1.8: Aptian-Neocomian Facies Occurrence Map.

3.2 Northern Province

This province extends from the Barakat, Bir Khamsa and East Faghur-1

wells in the west, to the Shaltut and Burg El Arab wells in the east. The

province is characterized by a thick Lower Cretaceous section and relatively

thick Jurassic sediments (Figs. 1.7 & 1.9). The province is divided into two

sub-basins which are: the Northern Meleiha – Alamein sub-basins and the

southern Meleiha – Alamein sub-basins. Details of each will be discusses

below:

Figure 1.9: North-South Structural Cross Section (B-B`).

3.2.1 Northern Meleiha – Alamein sub-basins

This area is characterized by the presence of the intra-Turonian

unconformity and, in addition, the presence of both the Alamein and Um

Barka carbonates (Fig. 1.8). These features indicate that this sub-basin was a

basinal low during the Lower Cretaceous and was partially uplifted during

Turonian time (intra- Turonian unconformity).

3.2.2 Southern Meleiha – Alamein sub-basins

This area is characterized by a complete and relatively thick Turonian

section and by the presence of only the Alamein Carbonate while the Um

Barka Carbonate is absent (Fig. 1.8). This sub-basins was considered as the

southern, near basin edge portion of the early Cretaceous-age Alamein –

Meleiha basins (Fig. 1.7).

3.3 Sharib – Sheiba High

The Sharib-Sheiba high, which occupies a large area in the central Western

Desert was at least partly tectonically active since Paleozoic time, because

there is a thick middle Jurassic and Carboniferous limestones south of that

high in the Abu Gharadig Basin margin (Sheiba 42-1 well), while there is a

thin middle Jurassic and no Carboniferous in the Sharib-Sheiba high itself

(Fig. 1.2) the Sharib – Sheiba high is characterized by the absence of the Um

Barka and Alamein Carbonates (Fig. 1.8 & 1.7), the presence of well

developed, severe unconformities within the Mesozoic sections which leaves

the Mesozoic very thin in comparison to the basins to the north and south

( Fig. 1.9) and general absence of the Khoman Formation (Fig. 1.10).

Figure 1.10: Apollonia Formation Isopach Map.

3.4 Abu Gharadig Basin province

The Abu Gharadig basin was actively subsiding since the Paleozoic which is

reflected by the presence of thick Middle Jurassic and

Carboniferous sediments in the basin margin to the north ( in Sheiba 42-1

well) ( Fig. 1.2 & 1.9). Most of the wells in the basin were bottomed in the

Kharita sandstone, except the Sheiba 42-1 well, which penetrated the

Carboniferous. Thus the basin down warp was initiated since Carboniferous

time and contained throughout the Jurassic and Cretaceous. The strongest

down warp was during the Maastrichtian when a thick Khoman section was

deposited (Fig. 1.7 &1.9). This province is characterized by a very thick

Upper Cretaceous (Khoman, Abu Rawash and Baharyia sections) and

relatively thick Eocene and Jurassic sediments. Lower Cretaceous Alamein

Carbonate and Carboniferous are also present.

3.5 Southern Platform Province

This Province occupies mostly the southern part of the study area where

most of the Mesozoic section pinches out and/or becomes very thin (Figs.1.7

& 1.11). Nashfa-1 well penetrated this section. The platform also is

characterized by the presence of severe unconformities within Mesozoic

sections.

Figure 1.11: North-South Structural Cross Section (C-C`).

3.6 Kattaniya High Province

This Province is characterized by the presence of very thick Jurassic

sediments (the thickest Jurassic section in the Western Desert) and also by

the presence of thick lower Cretaceous sediments (Fig. 1.11 &1.12). The

absence of the Khoman and Apollonia sections are one of the characteristic

features of the area (Fig. 1.7). The Jurassic basin depocenter was mainly

developed in the same area as the present-day Kattaniya Horst which was

structurally inverted during the latest Cretaceous/early Tertiary time.

3.7 Natrun Basin Province

The Natrun Basin is characterized by the presence of a very thick Eocene

section (Apollonia Formation) as well as a relatively thick lower Cretaceous

(Fig. 1.11 & 1.12). It was structurally down warped at the same time the

Kattaniya Horst was uplifted and is considered the northern Eocene flank of

that horst (Fig. 1.7).

3.8 Gindi Basin Province

This basin is characterized also by the presence of a very thick Eocene

section (Apollonia Formation) ( Fig. 1.11). It is considered as an Eocene

basin along the southern flank of the Kattaniya Horst (Fig. 1.7).

Figure 1.12: East-West Structural Cross Section (D-D`).

3.9 Ghirabub – Siwa Basin Province

This province is a north – south oriented sedimentary basin which was a

Paleozoic depocenter that has been inverted during the early Mesozoic. The

Mesozoic section on laps the Paleozoic basin whose present-day NW-SE

trending structural axis is near the Libyan border (Fig. 1.13). The basin is

characterized by the absence of the Daba section, the presence of relatively

thick Apollonia section and an on lapping of thin Upper Cretaceous section.

The lower Cretaceous Alamein and Um Barka carbonate markers are absent.

The Jurassic sediments are also absent. The Paleozoic is relatively complete

and thick (Figs. 1.7, 1.2 & 1.13).

Figure 1.13: East-West Structural Cross Section (A-A`)

CHAPTER II

STRUCTURE

The structures in the Western Desert are related to the plate tectonic motions

between Africa and Eurasia. There were three main stage of motion which

was reflected the significant structural features in the Western Desert these

three stages are;

1. Jurassic – pre – Turonian sinistral motion which is oriented mainly

northwest.

2. East – West post – Turonian dextral motion.

3. Middle – Late Tertiary compression related to the Alpine tectonism.

The first two stages were mainly extensional forces (wrenching).

In addition, it might be expected that Tertiary extensional features could

have occurred in relation to the opening of Red Sea/Gulf of Suez rift system.

The structural trends in the heterogeneous crystalline basement beneath the

Western Desert no doubt also affected the structural picture of the Western

Desert.

The structure in the Western Desert will be discussed under two main

subjects which are;

1. STRUCTURAL BASINAL DEVELOPMENT

A series of isopach maps which show syndepositional tectonic elements

were compiled for the Western Desert. These cover the geological

succession from Middle Jurassic to Oligocene. The maps show the basinal

history in the area and highlight areas or trends which were relatively

positive during given periods of deposition.

1.1 Bahrein / Wadi Natrun Formations

They include older continental strata and contain sandstones with minor

shales and anhydrites; these are derived as a result of weathering from

underlying Paleozoic and Basement, exposed during the Hercynian

movement. It laterally changed to a marine carbonate interbedded with

shallow marine shales and siltstone sediments in the Eastern part of Egypt

(restricted in the area of Wadi Natrun, Kattaniya and El Gindi provinces).

These carbonate sediments are known as Wadi Natrun Formation.

1.2 Khatatba Isopach and Facies Maps

From these maps, northeast trending structural features can clearly be

observed throughout the study area (Fig. 1.3). The rhombic pattern

throughout the area suggests the wrenching mechanism that prevailed during

Jurassic time (right lateral motion of Gondawana land with Eurasia that lead

to the opening of the Tethyan Sea). These rhombic features form basins and

highs, with the basins being named the Abu Gharadig, Kattaniya, East

Matruh, Sidi Barrani and East Alamein basins, while the high areas are

Sharib- Sheiba, Burg El Arab and North Dada highs (Fig. 1.3).

The dominant lithology of the Khatatba Formation is shale. It changes

laterally to sand toward the southwest and west and carbonates to the

northwest. In the far northwest corner of Egypt it changes to massive

carbonates ranging in age from Middle Jurassic to Lower Cretaceous (Sidi

Barrani Formation) (Figs. 2.1 & 1.8).

Figure 2.1: Khatatba Formation Facies Summary Map

1.3 Masajid Formation Isopach Map

The same Khatatba can be seen to extend the masajid time, but the structural

orientations become closer to the Cretaceous trends which are East-West

(Fig. 1.4). In general, the middle Jurassic (Katatba and Masajid) was a

period of a throughgoing fault trends generally northeast. These

throughgoing faults, known as transcurrent African faults, were initiated in

the middle Carboniferous and are highlighted by the associated isopach

trends observed in middle Jurassic time (Fig. 1.2). Some of these faults are:

the Abu Gharadig main faults; the Kattaniya graben faults; the Qattara,

Razzag and Alamein faults; complexes; and the GPT, GPY, GPZ and WD33

fields complex faults within the Abu Gharadig province (Fig. 1.7 & 2.2).

The Masajid Formation consists of Middle to Late Jurassic-age shallow

marine carbonates with shales interbeds.

Figure 2.2: Total Paleozoic Isopach Map.

Masajid partly change into study facies to the south and west, while it

become a part of Sidi Barrani sediments to the far north west of Egypt as

mentioned before (Fig. 1.8).

1.4 Alam El Bueib Isopach and Facies Maps

All the structural features observed from the isopach map ( highs and

troughs ) trend mostly east – west and include the following basins and

troughs: Shoushan, Sidi Barrani – Matruh, Alamein, Burg El Arab and Abu

Gharadig basins while the high areas include the well established Sharib-

Sheiba high (extending from kadam and Ras Qattara in the west to the Tiba

wells in the east) (Figs. 2.3 & 2.4). The basins still retain a rhombic shape,

but tend to trend east-west.

Figure 2.3: Alam El Bueib Formation Isopach Map.

Figure 2.4: Alamein Carbonate Fault Shape Map.

It represents the first continental to shallow marine deposits which came

after the post Jurassic uplift. It is mainly sandstone and shale with thin

carbonate beds. The sediments change laterally to the northwest and

northeast either to carbonate (Sidi Barrani formation) or to mostly shale

body which is known as Matuh shale (restricted in Matruh and north

Alamein areas).The Upper part of the Alam El Bueib is characterized by the

presence of two carbonate bodies (Alamein and Umbarka beds). The

occurrence of these two bodies is shown in (Fig. 1.8), the lower most part of

Alam El Bueib is known as Betty member where it is recognized by its red,

kaolinitic sandstone (Fig. 2.5).

Figure 2.5: Alam El Bueib Formation Facies Map.

1.5 Kharita Formation Isopach and Facies Maps

The same Alam El Bueib structural features extend to the Kharita and can be

observed from the Alam El Bueib Isopach map (Fig. 2.6). The significant

features of the map are as follows:

Figure 2.6: Kharita Formation Isopach Map.

1. The Abu Gharadig Basin is well established and extends to the east to

include what is called the Mubarak low (Fig. 1.12).

2. A new, large east-west trough is developed in the south which

includes the Bahrein and Bahariya wells (Fig. 2.6).

3. The Kattaniya-Gindi trough is established with a strongly developed

north-east trend.

4. The Sidi Barrani-Matruh, Alamein and west Delta basins complex are

established.

The Kharita Formation consists mainly of sandstones with shale interbeds.

1.6 Bahariya Formation Isopach and Facies Maps

The same structural features present during Kharita time are observed in the

Bahariya map with some deviation of the structural trends from east-west to

the north-west in the Sidi Barrani and Shoushan areas (Fig. 2.7).

Figure 2.7: Bahariya Formation Isopach Map.

Bahariya Formation consists of shallow marine sandstone with shale and

thin carbonate beds. It is characterized by the presence of limestone marker

at the basin the Northern Province. Generally the Bahariya sediment is

mainly sandstones which become more calcareous to the north and east,

while it becomes mainly shaly in part to the west (Fig. 2.8). The lower part

of Kharita changes partly to a green sandstones (highly glauconitic) bed

which is known as Varaconian sandstone (Razzak Member) and is restricted

to the Alamein and WD 19 areas (Fig. 2.9).

Figure 2.8: Bahariya Formation Facies Summary Map.

Figure 2.9: Kharita Formation Facies Summary Map.

1.7 Abu Roash ˝G˝ Isopach and Facies Maps

Most of the structural features appear to trend east-west, (Fig. 2.7) and

include the followings:

1. Abu Subeiha, Daba and Burg El Arab troughs.

2. Abu Gharadig, Mubarak and southern Kattaniya troughs.

3. UmBarka, Barakat, Meleiha and Khalda hings.

4. Razzak-Alamein high.

The Abu Roash ˝G˝ sediment consists of marine shale, limestone and

sandstone. It is heterogeneous both vertically and laterally. It has more

carbonate to the north (Sharib-Sheiba, Coastal ridge, and northern provinces)

while it becomes more sandy throughout Abu Gharadig, Mubarak and

southern platform provinces (Fig. 2.10).

Figure 2.10: Abu Roash "G" Member Facies Summary Map.

1.8 Turonian Isopach and Facies Maps

On this map it can be seen that most of the features trend strongly east-west.

All the earlier Cretaceous structural features noted above extend to the

Turonian and become even more pronounced (Fig. 2.11).

Figure 2.11: Abu Roash "G" Member Isopach Map.

The Turonian sediments consist of marine limestones, shales and sandstones

with an anhydrite marker within Abu Roash ˝E˝ member. Turonian sediment

changes mostly to carbonate north of Sharib-Sheiba province while it

becomes heterogeneous to mainly sandy to the south of Northern Province

(Fig. 2.12).

Figure 2.12: Turonian Facies Summary Map.

1.9 Khoman Isopach Map

The Khoman Isopach map shows strongly all the major structural features

known in the Western Desert, and includes the Abu Gharadig Basin,

Mubarak Basin, Kattaniya high, Natrun and Gindi Basins. All Kattaniya

high Natrun and Gindi Basins which are trending northeast (Fig. 1.10).

The Khoman sediment consists mainly of deep marine chalky limestones.

The facies variation is limited.

1.10 Apollonia Isopach Map

This map shows very strongly the northeast trending Kataniya high and its

southern and northern flanks of El Gindi and Natrun basins respectively.

Also it shows the north to north-northwest low trend over the old Paleozoic

basin to the west of the study area (Fig. 1.5). All the other previously

described structural features are still strongly observed.

The Apollonia sediments consist of an open marine sequenceof limestone

and some shales. An increase in the sand and shale percentage occurs on the

flanks of the Kattaniya high.

The Esna shale is equivalent to the Paleocene and lower Eocene (Part of the

Apollonia), where there are shale beds. The Thebes Formation is equivalent

to the lower Eocene while the Mokattam Formation is equivalent to the

middle and upper Eocene (part of the Apollonia Formation).

1.11 Daba Isopach Map

This map shows north-northwest trending structural features which includes

the area lying to the east of the old Paleozoic Ghiarabub – Siwa Basin (Fig.

1.6). The Paleozoic Ghiarabub – Siwa Basin during Daba time was a high

structural trend which developed over the old Eocene low in the area.

The Daba sediment consists of shale with thin beds of Limestone. The

Carbonate content increases in Zebeida and Umbarka areas.

2. STRUCTURAL PROVINCES

There are nine geological provinces in the Western Desert which have been

observed and were discussed above in the ˝Sedimentary Provinces ˝ (Fig.

1.7). These provinces will be analyzed and described here from the structural

point of view by using the Alamein Fault shape map, as follows;

2.1 Abu Gharadig Province

The Abu Gharadig basin is an east-west rhombic basin complex composed

of two main areas; which are the basin margin area to the north and the main

basin area (Fig. 2.4). This province is separated from the Sharib-Sheiba high

by a series of en-echelon faults which trend east-west and partly west-

northwest. The basin margin area lies between two sets of these en-echelon

faults and has a relatively thin sedimentary section compared to that in the

main basin area.

From the geological history and ispoach map discussion it is seen that the

Abu Gharadig basin is a generally complex structural low composed of half

grabens (pullapart grabens). Along the areas of ˝master ˝ fault bends

structural, ˝pop-ups ˝ occur, such as northeast Abu Gharadig field, structure,

especially along the northern edge of the basin these ˝pop-up˝ structures also

are observed near the ends of the en-echelon faults. These features of

both ˝pop-up˝ highs and pull-apart grabens are probably the up-ward

expression of a deep-seated, pattern of pre-existing faults (Jurassic northeast

and throughgoing faults). In addition to these structural features, there are

also buckle folds, which trend in a north-east direction (example: the Abu

Gharadig fold and the GPC structures). All these features of Abu Gharadig

basin were the result of extension during Turonian time imposed on major

pre-existing east-west fault trends (Jurassic time) (Fig. 2.4 &1.9).

2.2 Northern Province

This province is a basin complex extending from the Umbarka and Barakat

areas in the west to the western edge of the Delta province in the east. The

dominant structural features within this province are the large numbers of an

en-echelon faults which trend east-west or north-west. These faults generally

have less individual length than the main en-echelon faults of the Abu

Gharadig province. The northwest trending faults are associated with a series

of northeast trending folds (one of these fold series is the Alamein-Razzak

field complex). The northwestern portion of the basin complex is

characterized by the presence of north-south trending faults which form a set

of north-south horsts and grabens (Fig. 2.4).

As in the Abu Gharadig basin, there are some pull-apart grabens at the ends

of the en-echelon faults; examples of these pull-apart grabens are the east

Alamein basin and Shushan basin. Near the eastern portion of Northern

province, in the area north of Natrun basin, the en-echelon faults having a

more east-west trend, were throughgoing, and stronger. No major

compressional phase of the Alpine orogeny (mddle-late Miocene), but it may

have led to a north-south tightening of the structures seen in the northeast

portion of this province.

2.3 Sharib-Sheiba province

The Sharib-Sheiba province was a relatively high area since the Paleozoic as

noted in the stratigraphy and geological history. This high area separates the

two major Western Desert basin complexes (the Northern and Abu Gharadig

Basins complex). Because the area was more, or less stable since the

Paleozoic, no observed wrench – type structures are present. Some east-west

closures have been noted along the up-thrown sides of the main en-echelon

faults of the Abu Gharadig basin margin (Figs. 2.4 & 1.9).

2.4 El Gindi Province

This province is basically a large Eocene basin. From the structural point of

view, the basin is a big pull-apart graben due to the strong bend of the main

north-east trending fault which bounds this graben to the north one, or

possibly two, pull-apart, minor grabens can be observed in the province (Fig.

2.4 & 1.11).

2.5 Natrun Province

This area is similar structurally to the Northern Province, where there are

northwest to east-west trending en-echelon faults associated with a series of

pull-apart grabens and ˝pop-up˝ structures. (Figs. 2.4, 1.11 & 1.12).

2.6 Kattaniya Province

The Kattaniya province is a big high which separates the Natrun Eocene

basin to the north, and El Gindi Eocene basin to the south (Fig.1.7). The

only observed discrete structure on the high is a four way anticline. The

Kattaniya high is considered to be a large scale inverted structure due to a

combination of major north-south Eocene compression and wrench faulting

along its flanks (Fig. 2.4).

2.7 Coastal Ridge Province

This province is an inverted basin, most of the structural features observed

are northwest trending folds with en-echelon northwest faults (Fig. 2.4). The

north-west anticlinal folds seen in Egypt are also present on the northern

Cyrenaica platform in Libya. These folds are also considered as ˝pop-

ups˝ due to possible wrench faulting and the north-south compressional

force which prevailed during the Eocene to late Miocene i.e. a small scale

equivalent of the Kattaniya horst.

2.8 Ghiarabub-Siwa Province

This province is a complex of two structural features. The first one

comprises en-echelon, northwest trending faults with pull-apart grabens and

some northeast trending buckle folds. The second structural feature, which

prevails in the northern area, is the northwest structural folds. There may be

also a north-south fault trends at the eastern edge of this province. The area

also includes the eastern edge of the Cyrenaica platform and a deep-seated

Paleozoic structural axis (Fig. 1.13).

2.9 Southern Platform Province

This province is a high and tilted since Jurassic time (Figs. 2.4, 1.9 &1.11).

Most of the faults seen in this area are throughgoing northeast trending

faults. No observed closures can be seen on the Alamein fault shape map.

There is a possibility of structural highs and, rarely, pull-apart grabens at the

unmapped Paleozoic and Jurassic levels

CHAPTER III

ACTIVE FAULTING AND SEISMIC HAZARD

ASSESSMENT

1. INTRODUCTION

Earthquakes are the most important phenomena of natural hazard. They

affect human life and man-made constructions. The evaluation of ground

motion of moderate to strong events, as the reference or controlling

earthquakes, expected by using the deterministic approach is of importance

for designing the earthquake resistant structures. The geophysical data

provide information on the geometry of large-scale structures at depth,

which add the critical third spatial dimension to our observation. So, the

Bouguer gravity anomaly map of the Western Desert province was used to

interpret and to evaluate the deep-seated structures and the tectonic setting of

the region, which are the principle contributors to its geologic hazards.

The basic GIS data set layers are the geologic, interpreted subsurface

structures derived from geophysical data and the catalogue of earthquake

databank. The accumulated data provide a central core of information that

needs only to be updated. Currently, the available personal computer is

sufficiently powerful to fulfil this goal and it has enough strong for the

archiving, displaying and analyses of the natural hazard data of the site of

interest using a relatively simple database concept. This database concept

makes possible the utilization of valuable data and information for disaster

mitigation, prevention and preparedness.

2. GEOMORPHOLOGY

The study area is the north Western Desert, which extends from the Nile

valley to the Egyptian-Libyan border and from latitude 27˚ N northward to

the Mediterranean Sea coast (Fig. 3.1). Geomorphologically, it is a plateau

of stone desert with numerous large and deep closed-in topographic

depressions. An outstanding characteristic of northwestern Desert is the

almost complete absence of drainage system and the paucity of water in

general. Another particular feature is represented by the NNW-SSE trending

high longitudinal sand dunes. In the deep offshore area, northwest of the

Nile Delta, referred to as the Herodotus Basin, the sedimentary fill is likely

to exceed 13 km in thickness. West of Alexandria, the shelf narrows

considerably to widen again in the Gulf of Salum near the Libyan border

(Fig. 3.2) (Schlumberger 1984).

Figure 3.1: Location Map of the Study Area.

Figure 3.2: Main Structural Features of the Northern Western Desert and

Mediterranean Sea (Eyal et al. 1981; Schlumberger 1984, Wood World-

Clyde Consultants 1985 and Meshref 1990)

3. GEOLOGIC SETTING

Egypt is a part of the North African Craton, which during its geologic

history underwent periodic transgressions from the ancient Tethys situated to

the north and northeast of the country. The Egyptian Western Desert

platform may be subdivided from south to north into major geological

portions. These are the well-defined Nubian-Arabian Shield or massif and

the surrounding shelf areas, of which structural unit boundaries cannot be

traced with any great precision. Thus, the area is subdivided according to its

salient features into the Mediterranean Sea, the hinge zone and the unstable

shelf, (Fig. 3.2) (Schlumberger 1984).

1. Mediterranean Sea

Presently submerged and partially buried under thick Plio-Pleistocene

deposits related to the Nile Delta. During the Paleozoic time, at least two

phases of major deformations produced a N to NW trending system of block

faulting and gentle folding with marked unconformities within the Paleozoic

section. Although these movements were rejuvenated, a new trend of

structures was superimposed and became predominant in the modeling of

basement and the overlying sedimentary rocks. With a general easterly trend

(ENE to ESE), this new grain resulted from the Alpine orogenic phases,

which followed one another throughout the Mesozoic time and had their

climax during the Early Tertiary.

The structures consist mainly of parallel and elongated tilted fault blocks

that are horst and half-graben structures with associated erosion of the

upthrown blocks. Concurrently, from Late Jurassic onwards, differential

depocenters developed with correspondingly strong changes in thickness and

facies, especially during the late Cretaceous. Structures result primarily from

vertical movements of basement blocks and consist of draped layers over

and/or faulted anticlinal features. Compressional anticlines are subordinate

and probably derived from drag folding, related to lateral movements along

basement faults.

2. Hinge Zone

It is located between the mobile shelf, which has normal faults, their

down thrown sides are in the northern direction, and the Mediterranean basin

area. It causes a raped basinwards thickening of Oligocene to Pliocene

sediments, coinciding with the present Mediterranean coastal area.

3. Unstable Shelf

This is situated north of the stable shelf with the transition between the

two structural depositional units following a line approximately set from the

Siwa Oasis through Farafra Oasis and Suez into Central Sinai. The

sedimentary sequence of the unstable shelf is relatively thick with the lower

part of the section composed mainly of clastic sediments, followed up by a

section of middle calcareous series and topped by a blanket of biogenic

carbonates (Fig. 3.3). The formations are gently folded and show signs of

lateral stress. Overthrusts are reported from the northern structures. These

structural deformations are related to the Laramide phase of the Alpine

Orogeny. The trend of these fold bundles is slightly arcuate to the NE and

referred to as the Syrian arc system. The structural setting of the

Mediterranean Sea is affected by the structures of the north Western Desert.

Figure 3.3: Geologic Map of the Study area (modified after EGSMA 1981)

4. REGIONAL PLATE TECTONIC SETTING

The primary tectonic features in the vicinity of Egypt are three major plate

boundaries. The African-Eurasian plate margin, Aqaba-Dead Sea transform

fault and the Sea-floor spreading of the Red Sea. These separate the African,

Eurasian and Arabian plates, in which the African and Eurasian plates are

converging across a wide zone in the northern Mediterranean Sea. This zone

is characterized by folding within the Mediterranean sea-floor and

subduction of the northeastern African plate beneath Cyprus and Crete

(Mckenzie et al. 1970 and Maamoun et al. 1980). It is believed that, there is

a zone of convergence (folding and reverse faulting) and strike-slip faulting

to the north of the margin. The effects of the plate interactions are mainly

northward and remote from the Egyptian coastal margin. Some secondary

deformations appear to be occurring along the northern Egyptian coast, as

represented by earthquakes and tectonic activities, such as the subsidence of

the Nile River Delta, the vertical movements in the Qattara and Faiyum

areas and the rotation of Sinai block (Maamoun et al. 1980).

The presence of three plates is postulated for the northeast part of Africa.

The relative motion of these plates has led to the opening of the Red Sea,

Gulf of Aqaba and in part, the Gulf of Suez. The spreading of the Red Sea is,

without doubt, due to the northwestward motion of the Arabian plate along a

transform fault. Left-lateral movement of the Arabian plate along the Gulf of

Aqaba has also been established. The lateral displacement (Freund et al.

1970) amounts to about 110 km along the Dead Sea rift.

The opening of the Red Sea near Aqaba and Suez Gulfs junction is far larger

than the lateral displacement along the Aqaba Gulf. The width of the Red

Sea in its northern part is about 190 km. The difference may have been

absorbed by differential plate movements between Sinai and the Arabian

plates and although plates are considered rigid units, the Sinai plate might

have undergone deformations to some degree with for-shortening, that led to

the partial opening of the Gulf of Suez. A clockwise movement of the

Nubian plate away from the Arabian and Sinai plates could be also taken

into consideration. It is interesting to note however that, the Gulf of Suez

took part to a certain extent in the spreading only and it represents the

aborted arm of the triple junction of Red Sea and Gulf of Aqaba. Pelusium

line of ENE to WSW trending lineation passing just to the north of Cairo is

postulated to be a boundary between the continental and oceanic crusts. The

eastern extension of the Pelusium line cuts ENE across the continental shelf

of Northern Sinai before bending northwards forming a series of NNE to

SSW striking faults, that follow the continental slope of Palestine. In

Northern Sinai, it separates a belt of contrasting structural orientations.

Left lateral transcurrent movements along the Pelusium fault zone has

created a compressional stress field for halokinesis in the diaper belt and

Levant platform. Structures in the South Delta province affect essentially the

Eocene to Triassic succession, similar to that of the Western Desert and

Northern Sinai. The NE-SW folds of Northern Sinai cross the southern part

of the Nile Delta in apparent continuity with a zone of linear uplifts

southeast of Khatatba. The northern boundary of the area is marked by the

faulted flexture, where a rapid northward increase in the slope of pre-

Oligocene sequence is occurred. In the south Delta province, there is a

contrast between the structures in the south east part from Abu Sultan to Abu

Roash-Khatatba, which have NE-SW and ENE-WSW trends and the

northwestern and central Delta parts, where deep structures have E-W and

ENE-WSW trends. Both types of structures were uplifted essentially in the

Late Cretaceous to Eocene and suffered considerable erosion. On and under

the continental slope, the surface sediments are affected by extensive

superficial faulting, large-scale slumping and diapiric phenomena. The

northeastern margin of the Nile Delta and the southwestern margin of the

Nile cone are marked by the Bardawil escarpment, which delineates a major

WNW-ESE fault zone comprised of several steep NNE facing fractures and

southwestward tilting of sediments.

Two areas in northeastern Africa showed strong subsidence during the

Paleozoic, the Kufra basin and Dakhla basin, more than 2.5 km of Paleozoic

sediments in Kufra basin and 3.0 km of Early Paleozoic to Carboniferous

rocks in Dakhla basin. Subsidence initiated in the closing stage of Pan

African crustal accretion. The Kufra basin is related to the transcontinental

shear, the Pelusium line, which stretches from Anatolia down to the Nile

Delta, SW across Africa to Niger Delta and into the Atlantic Ocean.

The sedimentary cover of the north Western Desert is a part of the foreland

deposits, which fringe the northern continental margin of the Afro-Arabian

Shield. From the southern most part of the Western Desert, the exposed Pre-

Paleozoic basement shows a regional northward slope, with corresponding

increasing thickening of the unconformable sedimentary cover, made up of

Paleozoic, Mesozoic and Tertiary to Recent rock units. In the Mediterranean

Coastal area, the basement lies 5 km below the surface.

5. VOLCANIC ACTIVITIES

Volcanic activities in Egypt have occurred during most of its geological

history. Paleozoic volcanic (Rabat basalts) are reported from the exposed

Devonian formations in the Siwa Oasis in the north Western Desert. During

Mesozoic time, frequent volcanic activities are recorded in the Late

Cretaceous formations and related to the early phases of crustal disturbance

during the laramide orogenic phase. Triassic intrusives (Camel Pass basalt)

are present in the northern Western Desert. Tertiary volcanic in the form of

basalt flows, sheets and dykes ((Abu Zabaal basalts) occurred in the north

Western Desert during the Oligo-Miocene, in the Nile Valley and Nile Delta

(Schlumberger 1984).

6. SUBSRFACE STRUCTURES DEDUCED FROM

GRAVITY DATA

The Bouguer Gravity Anomaly Map of Egypt (Fig. 3.4) was established by

the General Petroleum Company of Egypt in 1985 on scale 1: 2000000 and

contour interval 5 mgal. The present study area is characterized by the

dominance of negative anomaly at the southwestern corner, which has – 60

mgal, while at the southern corner, it has -30 mgal. The central part of the

area, which has -40 mgal, is characterized by the occurrence of negative

anomalies. These negative anomalies may be due to the presence of basins.

Closed anomalies of different shapes, trends and amplitudes are present.

These anomalies can be interpreted in terms of structures dissecting the

basement surface and also the sedimentary cover. The northwestern part of

the area is characterized by higher positive gravity anomaly, that has 50

mgal, which suggests denser rock materials rather than shallower anomaly

sources. The major and minor anomalies of varying trends, extensions and

gradients are probably due to faults separating the different basement blocks.

The considered Bouguer gravity anomaly map is digitized and converted to

digital format file of x, y & z points.

Figure 3.4: Bouguer Gravity Anomaly Map of the Study area (after

GPC1985)

1. Euler Deconvolution For Subsurface Faults Detection

It is used for the rapid interpretation of gravity field data. It is good for

delineating contacts and it is utilized to determine the subsurface geologic

positions of structures (Rrid et al. 1990). The quality of such depth

estimation depends on the choice of structural index and adequate sampling

of data. The potential advantage of Euler depth is that, the method does not

assume particular geologic model. Thus, it can be applied and interpreted

even when the geology can not be represented. In the Euler deconvolution

technique, the field and its three orthogonal gradients (two horizontals and

vertical) are used compute the anomaly source locations (Thompson 1982).

It is applied to grided data, which measure the gradients, locate the square

windows within the grids of gradient values and field, and locate the

structural windows.

The Euler plots using the Structure Index of 0.0 for gravity thick fault steps

by Geosoft program, version (1994) were calculated (Fig. 3.5). The Euler

anomalies at the northeastern and northwestern parts of the area, as well as

the coastal plain (hinge zone) have shallow depths of the faults. South of the

Qattara Depression and Siwa Oasis have moderate to deep fault depths. The

linear clustering of plots shows the extension of the expected linear steps of

distinct density contrasts due to faulting at varying depths. The area is

dissected by several regional faults trending NW-SE and NE-SW, which

control the structures of the area and also the path of River Nile. The fault

system of N50˚ - 60˚E trend defined the Pelusium megashear system, as

extending from the border zones of Anatolia, along the eastern

Mediterranean Sea region and across Africa from the Nile Delta to the Delta

of Niger into the Atlantic Ocean (Neev et al. 1982). This is a major deep

seated tectonic zone of strike slip faulting, it is penetrating the earth's crust

into the upper mantle, that is distinguished at the surface by the linear

fractures extending hundreds of kilometers and reflect their orientation

during the geologic time. It distinguishes the second structural deformation

of N70˚ – 80˚ W trend and N54˚W (Wadi Hodin Wadi Kharit trend), which

cuts with the Pelusium line (Fig. 3.6).

Figure 3.5: Interpreted subsurface faults derived from gravity data

using Euler deconvolution technique.

Figure 3.6: Interpreted traced subsurface faults derived

from gravity data.

Tertiary volcanic intrusions in the form of basalt flows, sheets and dykes

occur in the north Western Desert (Sigaev 1959) (Fig. 3.7), at the

intersection of the two tectonic trends: the NE-SW (Pelusium Megashear

and rejuvenation of Syrian arc faults, which dated to Cretaceous) and the

NW-SE, where the basalt flows on the extension of Pelusium line in Gabal

Qatrani (North Qaron Lake), Gabal Abu Roash (West Giza City), Bahariya

(uplift line) Oasis and south Wadi El-Rayan.

Figure 3.7: Comparison between the interpreted subsurface faults derived

from gravity data, surface faults after EGSMA (1981) and extruded basalts

The surface faults (after EGSMA 1981) at the southern part of the area, near

the Nile Valley, have NW-SE trend, which are parallel to the differentiated

subsurface faults, that are detected from the gravity method and basalt

extruded flow through these fault (Fig. 3.7).

The surface faults at the Bahariya and Farafra Oasis have NE-SW direction,

which are parallel to the subsurface faults (Pelusium line, in apparent

continuity with a zone of linear uplifts) and it cuts with a fault, that have

NW-SE trend, in which the area of intersection has basaltic intrusion and

lava flow along the fault trend.

The surface faults at Wadi El-Natrun and Wadi El-Rayan have NW-SE

direction, which are parallel to the subsurface faults. The surface faults near

the coastal plain are parallel to the normal faults of the hinge zone. The

Bahariya and Farafra Oases are uplifted, where the fault the extension of the

sinistral strike-slip fault of Wadi Kharit and Wadi Hodein, that trend N54.W

in the basement rocks (Greiling et al. 1996) is observed in the northern

Western Desert with the same direction, but it cuts with the Pelusium

megashear line at Bahariya Oasis and where basalts are intruded at the

intersection area. Also, the basalts intruded along the zone of NW-SE strike-

slip fault at southeastern Bahariya Oasis. The continuity of this strike-slip

fault north of Bahariya Oasis to the Qattara Depression is cut by the Qattara

Euratothens line, which is parallel to the Pelusium line.

2. Structural Analysis

The structural analysis is the interpreted subsurface fault trends are shown in

Fig. (3.6), in which the lengths of fault segments are measured, counted and

tabulated in table. (3.1). Also, the azimuths of all major and minor

subsurface faults are measured, clustered in 10 classes and plotted in the

form of rose diagram (Fig. 3.8).

Table 3.1: Structural analysis for the subsurface faults characterizing the

study area.

Figure 3.8: Rose diagram showing the trends of fractures in the study area.

The rose diagram shows that, the major subsurface fault lengths are oriented

N50˚- 70˚E and N70˚ - 80˚W. Also, the minor subsurface fault lengths are

oriented N40˚ - 50˚E and N60˚ - 70˚E. Moreover, the rose diagram exhibits

that , the major subsurface fault numbers are oriented N50˚ - 60˚E and

NN60˚ - 80˚W. Also the minor subsurface fault lengths are oriented N40˚ -

50˚E, N60˚ - 70˚E, N40˚ - 60˚W added to N20˚ - 30˚W. It is observed that,

the subsurface fault numbers in the NE trend is mostly twice those extending

in the NW direction and the subsurface fault lengths in the NE trend is (2/3)

those in the NW direction.

7. SEISMIC HAZARDS ASSESSMENT

Seismic hazard describes the potential for danger from earthquakes such as

ground shaking. The earthquakes cause damages to human life and man-

made structures. The decision-marker for urban planning should have

knowledge about the probable characteristics of earthquakes to be expected

in future. The results of seismic hazard analyses are essential by the peak

ground acceleration of ground motion, which is used to construct the seismic

safety and find out the necessary steps for prevention or mitigation of

damage caused by earthquakes. Then, the seismic hazard assessment

information has to be applied for better design of earthquake-resistance

structures for urbanization and construction of a land use map for the future

planning of interested area. So, the seismic measures should taken in

construction work in the sustainable development of the Western Desert

region.

1. Seismic Background

The assessment of earthquake damage and losses must allow for two

parameters in particular. These are the seismicity of a region and the

vulnerability of the elements at risk. Most of the concerned region is seismic

and some zones in the Eastern Mediterranean are a seismic.

If not only instrumental data are considered, but also historical records as

well, there are dangerous seismic "hot spots" in particular in some parts of

the region. Greece and Greek islands in the Aegean Sea, Create, the western

coast of Turkey, Cyprus, the southwest of the East Analotion fault and the

eastern coast of the Mediterranean Sea from Aleppo to the Gulf of Aqaba.

On the southern side of the Mediterranean particularly in the Nile Delta,

although the problem is not so much high seismicity, but dangerous sub-soil

and high vulnerability. Soft sub-soil increase damage considerably in some

important places, where soft alluvial deposits are found. There are the Nile

Delta and the low-lying areas along the Nile including the part west of it,

especially in El Fayum and many other new settlements. Other important

parameters are the shear strength and stiffness of the buildings. The high

quality code protects modern buildings and there is no guarantee against the

use of unsuitable material.

The modern buildings are very vulnerable, because of their inadequate

strength. Old buildings of brick or stone masonry and in particular rural

buildings are at any rate likely to suffer sever damage. Particularly the latter

category often incorporates adobe, which can be called a deadly construction

material, because it either crushes or suffocates the inhabitants. The record

of earthquake activity in Egypt is of moderate to low level in comparison

with the surrounding areas. Historical and recent expense showed that, their

hazards effect is serious.

These epicenters are dangerous in the vicinity of the over populated cities,

soil characteristics of areas where Egypt ̀s big cities are considered and the

absence of earthquake engineering codes and construction control.

The distribution of recent seismicity (macro and micro) shows the presence

of different seismogenic zones with different behaviors of mechanism and

level of activity. They are associated with pre-existing faults, that are

presently active. Mainly strike-slip faults in the solution of earthquakes (Gilf

El Kibir 1978; Aswan 1981; Dahshour 1992 and Aqaba 1993 and 1995) are

mostly active. Dead Sea-Aqaba transform represents a plate boundary, that

strikes in N25˚E over a total length of about 110 km. Many geological and

geophysical evidence presented multi-stage sinistral shears of about 107 km

along the transform, which related to the Red Sea opening. Egypt is in a

vulnerable location for earthquakes, surrounded by three plate boundaries

Africa, Arabia and Red Sea.

Africa is moving closer to Europe at a rate of up to 2 cm per year and sliding

below it (subduction). Arabian plate is moving away from the Africa plate,

including the Sinai Peninsula in a counter clockwise direction, closing the

Arabian Gulf. The sea-floor spreading of the Red Sea is a common source of

earthquakes. Egypt is located in a region, where there is a great deal of plate

movements. An earthquake in Greece or Cyprus or Turkey may affect all

countries in the Eastern Mediterranean region, because the shifting leaves

sediments 2 to 3 km into the earth's crust making formations unstable and

likely to amplify the movement from any subsequent nearby seismic

displacement. Recent sediments enlarge the initial motion that comes from

seismic waves. Egypt lies in a shallow hypocenteral earthquake zone, where

the source of the earth's shifting is not more than 60 km deep. The most

dangerous and destructive earthquakes are the shallow focal earthquakes.

The mean depth of earthquake sources in Egypt is about 20 km below the

earth's surface (Mckenzie 1970; Maamoun et al. 1980 and Schlumberger

1984).

Mckenzie et al. (1970) studied the seismicity of the Mediterranean Sea

region and supposed that Sinai sub-plate is located between the African plate

and Eurasian plate. The relative motion between the African and Eurasian

plates were determined from the mechanisms of some earthquakes. A

detailed seismicity map of Egypt and the surrounding areas was established

by Maamoun et al. (1980) and correlated to the geological structures of the

area. The seismicity map contains historical and recent events. Maamoun et

al. (1980) divided the area into different seismic zones according to the

earthquakes distributions and structures of the area. They illustrated that:

1. Egypt's Mediterranean coastal area seismic zone: The earthquakes in

this zone have magnitudes range between 5 and 6. This zone occurs in

relation to the continental shelf (canyon off Alexandria) and the

continental slope. The focal mechanism of the earthquake observed

near the canyon off Alexandria shows a dextral strike-slip movement.

2. Gaghbub-Rayan seismic zone: Its structural block is nearly oriented

E-W of Gaghbub, Siwa, Qattara and Rayan. IT is historical

earthquakes.

3. Gilf El Kibir seismic zone: It is recently marked by the Gilf El Kibir

earthquake (5.8 magnitude). The fault plane solution of this event

shows a dextral strike-slip movement along a fault plane striking NE.

The zone may contain the Gabal Oweinat area.

Makris et al. (1980) established the crustal structure by using deep seismic

sounding profiles in the north Western Desert. The profiles extend fro Sidi

Barrani to El Alamein, from Sidi Barrani to Siwa Oasis and from Cairo to

Bahariya Oasis. The results obtained in this work indicated that: parallel to

Mediterranean Sea coast, the Moho is found at a depth range between 28 and

30 km. The sedimentary cover in average is about 6 km. This area is

considered a continental margin and it is characterized by a decrease of the

crustal thickness towards the sea. The boundaries of this block coincide with

those of Matruh-Cyrenacia plate (Maamoun et al. 1980). The Cairo-Bahariya

section is positioned on a basement high with a thin Eocene cover, whose

thickness does not exceed 1.5 km. Here, the crustal thickness was found to

be about 32 to 34 km. Then, the igneous crust is thicker along the coastal

part of Northern Egypt, since the sediments are thin and the remaining

igneous portion of the crust is approximately 4 to 5 km thicker than along

the Mediterranean Sea coast.

2. Deterministic Approach for Seismic Hazard Analysis

It aims at estimating the horizontal peak ground acceleration from the

reference or controlling strong motion events. The deterministic analyses

make use of discrete and single valued events or models to arrive at the

required description of earthquake hazard. The analysis requires the

specification of three basic elements; the earthquake sources, the controlling

earthquake of specified size and a mean for determining the hazard. In this

case, the horizontal peak ground acceleration (PGA) at the specified distance

to the site of interest can be determined.

1. Data

The earthquake data used for the current study were collected from the

published catalogue of the International Seismology Center (ISC)

(http://www.iris.edu). The catalogue of earthquakes covers a time span from

1964 to 2003. The epicenters map shows the epicenters of all the reported

earthquakes with magnitude greater than or equal to 3.5 (ms) (Fig. 3.9).

The primary parameters of earthquake catalogue are the data, time of an

event, the location, focal depth and a parameter classifying the strength

(magnitude Ms). The correlation among the epicenteral locations and

tectonic regimes (normal, thrust or strike-slip faulting) with the geotectonic

fault activities and the epicenters of earthquake occurrences of various

magnitudes and the plots of epicenters provide the primary basis for the

recognition and delineation of hazardous regions or seismic zones.

Significant earthquakes tend to occur repeatedly in certain regions, whereas

other regions have experienced few or no events during long historical

periods.

Figure 3.9: Epicenter distribution map in the period 1969-2003 (data

retrieved from ISC)

2. Seismic Sources Description

The first object in a seismic hazard is the definition of the sources of

earthquakes, that could affect the particular location at which the hazard is

being estimated. Many researchers studied the seismic source zones of

Egypt. The seismic source zones in Northern Egypt are established

according to the structures and seismicities (Fig. 3.10), and can be

summarized as follows:

Figure 3.10: Seismic source zones in the Western Desert district.

3. Gulf of Aqaba Seismic Source

The Gulf of Aqaba is about 180 km long, 16-17 km wide in its northern part

and 23-25 km wide in the southern part. Its maximum depth reaches 1850 m.

The Gulf of Aqaba occupies the southern segment of the Dead Sea rift. The

interior of the Gulf of Aqaba is occupied by three deep and elongated basins,

striking N 20˚ - 25˚E, which are arranged en-echelon and the basins are

separated by low sills. The seismicity along this part of the plate boundary is

relatively high.

The axial trough was suggested to be the youngest part of the Red Sea. The

Aqaba rift has been active since the Late Mesozoic time (Meshref 1990).

This rift was formed in the Cenozoic by break-up of the once continuous

Arabian-African platform, which had been a tectonically stable area since

the end of the Precambrian. The Dead Sea rift is a plate boundary of the

transform type. The transform fault of the Gulf of Aqaba is displaced with a

rate of about 0.65 cm/year (Wood World-Clyde consultants 1985), which

connects the Red Sea, where sea-floor spreading occurs, with Zagros zone of

continental collision. The rifting process of Gulf of Aqaba, as a strike-slip

motion on faults along the Dead Sea rift, led to a total left-lateral

displacement of about 110 km (Freund et al. 1970). The major rift faults are

arranged en-echelon and produce between them rhomb-shaped depressions.

The rhomb-shaped depressions are in the form of grabens, in which slices of

the sedimentary cover were down-dropped.

1. Gulf of Suez Seismic Source

The Gulf of Suez is an area of subsidence within the stable shelf of the

northern part of the Nubian-Arabian shield. It was formed originally during

the Early Paleozoic time, as a narrow embayment of the Tethys and

intensively rejuvenated during the rifting phase of the great East African Rift

system in the Early to Middle Tertiary time.

The Gulf of Suez is an intensely faulted area. The present shape of the Gulf

of Suez has been determined by the fracture systems, which were and

possibly still, due to the tectonic events caused by movements of the Nubian,

Arabian and Sinai plates and the resulting East African Rift system.

The Gulf of Suez is subdivided into three provinces: northern, middle and

southern. The provinces are separated from each other by faults following

the Aqaba trend. Each province is characterized by uplifted and subsided

block on each shoulder.

The Gulf of Suez was considered as the most active seismic zone in Egypt,

in which one of the largest earthquakes (Shedwan March 31, 1969) with

magnitude 6.9 occurred in this region. Ben Menahem and Abodi (1971)

reported that, the fault motions along the rift combine extensional opening of

the rift, which is about 10 percent of the rate on the Red Sea (about 1cm per

year, or 1mm per year on the Suez rift), with1 to 2mm per year of the left-

slip (Wood World-Clyde consultants 1985).

2. Northern Red Sea Seismic Source

The Red Sea has a complex tectonic history. Initial doming and uplifting

events in the proto-Red Sea began in the Late Oligocene or Early Miocene.

Continental rifting or faulting then occurred and was followed by sea-floor

spreading. This spreading resulted in the emplacement of a new oceanic

crust in the southern Red Sea and possibly in the Northern Red Sea. The Red

Sea spreading is believed to be at tha rate of 0.5 cm/year, which may

increase to 0.6 cm/year or more in the extreme northern part of the Red Sea

at its Junction with the Gulf of Suez (Meshref 1990). The northern Red Sea

rift is associated to the central axial rift of the Red Sea. North faults

predominate in the region and generally form a series of eastward facing

steps from the Red Sea Mountains to the center of the Red Sea.

3. Nile Delta Cone Zone

This zone contains sets of events, that strike the structures of the area, where

it consists of anticline trends E-W and NW-SE and also the normal faults at

the hinge zone to the south.

4. Levantine Basin Zone

It is the eastern Mediterranean Sea coast, that has normal Faulting of the

hinge zone, where sets of events strike this zone.

5. High Aegean Arc and Herodotus Basin Zone

It includes the pattern of seismicity at the off-coast of the Egyptian-Libyan

border. The seismicity is considered as the reflection of the subduction,

which exists further to the north beneath the southern Europen coast line.

In addition to the fore-mentioned seismic zones, several portions in the

vicinity of the area of study show some seismic activities in the recent years.

These areas are:

i. Western Desert Seismic Activities

This area is considered to include the potentially significant faults near West

Sohag district. The main trend of this fault system is NW-SE (the extension

of Wadi Kharit-Wadi Hodein sinistral strike-slip fault). Some of them are

accompanied by clusters of seismic activities near or associated with the

main fault trend.

Criteria for recognizing active faults using the data source of the Western

Desert are:

1. Geologic as the enclosed depression of Bahariya Oasis, side hill

ridges, warping of young deposits and stratigraphic off-set Quaternary

deposits by faulting.

2. Seismological earthquakes, when well located instrumentally, indicate

an active fault. The earthquake epicenters are assigned to individual

faults with a high degree of confidence. Also, potential active, where

the alignment of some earthquake epicenters along a fault trace, but

the locations have a low degree of confidence. Table (3.2) represents

the criteria of the northern Western Desert active faults.

Table 3.2: Criteria for recognizing the active faults in the north Western

Desert.

ii. Abu Dabbab Seismic Activities

The area is characterized by high levels of microseismic activity.

Hypocenter depth ranges from 6 to 14 km. No surface tectonic features have

been mapped in this area, that appear to be directly related to the micro-

earthquake activity, although the area is close to one of the zones that is

perpendicular to the Red Sea, where the active region is controlled with the

NW-SE direction. One possible explanation of the high level of seismic

activity of Abu Dabbab is that, it may be related to the fracturing of crust,

associated with the Red Sea opening (Daggett et al. 1980). Also, the

mechanism for the 1955 Abu Dabbab event (5.3 Ms), located near the Red

Sea coast suggests normal oblique faulting. Some of the fault planes

appropriate for the first motion.

Data are consistent with displacement of faults parallel to the Red Sea rift

faults (Wood World-Clyde consultants 1985).

iii. Aswan Seismic Activities

The occurrence of November 14th, 1981, Aswan earthquake (6 Ms), that lies

approximately 55km south of the High and Aswan Dams near Marawa,

indicates the available mechanisms for larger earthquakes within and near to

Aswan area. The best determined mechanism for the 1981 Aswan event is of

strike-slip type. These mechanisms agree with the geologic evidence,

indicating the presence of these fault types near the earthquake location.

Within the Aswan area, Late Cenozoic faulting has been observed in two

areas. Predominantly normal faulting on the Red Sea fault system east of the

crest of the Red Sea Mountain and strike-slip faulting on the faults of the

Western Desert fault system on the Nubia plain and sinn El-Kaddab plateau.

The Western Desert fault system consists of E-W faults that exhibit right-

lateral slip displacement and asset of N-S faults, that show left-lateral

displacement. The E-W faults dominate in the concerned area have had

greater degrees of activity in the Quaternary and have larger total

displacements than the N-S faults. The E-W faults slip rates are about 0.03

mm per year, whereas the N-S faults have lower slip rates, inferred to be

0.01 to 0.02 mm per year. The N-S faults are likely produced by slight block

rotations due to the effects of the E-W slip. The geometry needed to produce

such clockwise rotations is that, the Western Desert fault system apparently

represents adjustments within the African plate to differential spreading

within the Red Sea rift, where the plate boundary tectonic effects related to

the anomalies of the Red Sea rift (Wood World-Clyde consultants 1985).

4. Seismic Hazard Results

Values of peak ground acceleration have been determined through the

equation after Deif and Khalil (2003) for the evaluation of the earthquake

effects. The earthquake hazard at a concerned site is the horizontal peak

ground acceleration PGA (gal) resulting from the occurrence of earthquake

magnitude (Ms), on a fault at a specified distance R (km), between the site

and the earthquake. We calculated the hazard at the area of interest from the

large controlling earthquakes. As shown in Table (3.3).

Table 3.3: The large controlling earthquakes around the study area.

Despite the fact that Egypt is characterized by a relatively low seismicity,

the hazard map which is calculated from all the reference destructive

earthquakes, that occurred around the Western Desert (Fig. 3.11). It

represents the maximum ground acceleration in the investigated area. It

shows that, the relatively low PGA level particularly at El Salum to Marsa

Matruh area, that is expected to be about 20 Gal. On the other hand, the most

densely populated areas of Egypt in the Nile Delta are the greater Cairo and

Faiyum areas, the maximum PGA of approximately 50 Gal has been

calculated. Also, the PGA level of West Sohag area is about 20 Gal and

parallel to the northern coastal plane is about 10 Gal. Then, this map

indicates that, the evaluated seismic hazard increases in the Salum Marsa

Matruh area, south of the Nile Delta, Cairo area and West Sohag area; and

decreases outside it. The PGA is significant to engineers for the earthquake

resistant building construction code.

Figure 3.11 Distribution of peak ground acceleration for the Western Desert,

using reference moderate to strong earthquakes through the deterministic

method.

CHAPTER IV

EVALUATION OF SUBSURFACE STRUCTURES OF

SIWA OASIS USING 3D MAGNETIC MODELING

1. INTRODUCTION

Siwa Oasis is located at the extreme Western part of the Western Desert of

Egypt, some 260 km to the south of Matruh (Fig. 4.1). This area is located

between latitudes 29˚ & 29˚ 25` N and longitudes 25˚ 30` & 26˚E, covering

an area of about 400 km2. The area was subjected to intensive land magnetic

survey using two magnetic protons of Geometrics type of accuracy 1 nT.

The qualitative and quantitative analysis was performed for the obtained

land magnetic map as well as the RTP aeromagnetic map of the studied area

(EGPC 1979). The results of the interpretation allow establishing the

geometry of the basement surface and the predominant subsurface structures

in the studied area.

In order to detect the subsurface structures and their continuation in the

sedimentary rocks, a Bouguer anomaly map was interpreted for the studied

area. Furthermore the correlation between the structural trends deduced from

interpretation of the potential field data, and the surface geologic lineaments

(Fig. 4.1) was performed.

Moreover, three dimensional magnetic model (3D) was performed for the

land survey area. The direct comparing and contrasting of the quantitative

interpretation based on various methods; gravity, magnetic, and surface

geologic data, finally lead to an integrated final interpretation.

Figure 4.1: Geologic map of Siwa area, (after CONOCO 1987).

2. GEOLOGY OF THE AREA

The Siwa depression is formed along the same structural trends that are

geologically related to lines of extensive joining or faulting.

It is obvious, from the geologic map (EGPC 1979), that the studied area is a

depression formed in the Marmarica Plateau of the northern Western Desert

(Fig. 4.1). This plateau occupies the northern part of the studied area and

consists of shallow marine limestone with a few marly intercalations. The

Moghra Formation occurs in the southern part of the area and consists of

shallow marine calstics of shale and white bandy carbonates. The Mokattam

Formation consists of mainly limestone and it is found in the southeastern

part of the area. The sand dunes cover the southern part and a wide area of

the western parts of the study area.

According to Said (1990) the subsurface geology of the Western Desert is

composed of three major lithological divisions. These, as proposed by

Barakat (1982) are from base to top (i) Lower Clastic Division (ii) Middle

Calcareous Division and (iii) Upper Clastic Division.

3. DATA ACQUICISITION AND INTERPRETATIONS

3.1 Land Magnetic Survey and Interpretation of RTP Magnetic Map:

A detailed land magnetic survey was performed for the study area using two

proton magnetometers of the geometrics type. One of them was fixed at the

selected base station in a quiet magnetic area. The second portion

magnetometer was used in the survey along a mesh like profiles covering the

study area (Fig. 4.2). The diurnal variations of the magnetic field were

corrected for and also, the latitude variations (gradient) were corrected. The

corrected data of the observed magnetic field were contoured and the total

magnetic intensity map was obtained.

Figure 4.2: Total magnetic intensity map of Siwa area (land survey).

The locations of the observed stations were determined using the Global

Positioning System (GPS) with an accuracy of about 1 meter.

Due to the declination and inclination angles of the magnetic vector, there

will be a shift in the location of the magnetic anomalies of the subsurface

source on the map from its true location. Therefore, reduction to the North

Pole technique (RTP) (Mendonca and Silva 1993) was applied to the total

magnetic intensity map and the RTP magnetic map was produced (Fig. 4.3).

The RTP map was used for the quantitative interpretation.

Figure 4.3: RTP magnetic map of Siwa area.

Close study of the RTP magnetic map, reveals that most of the anomaly

have NE-SW, NW- SE and E-W trends. The presence of negative magnetic

anomaly in the middle of the studied area may be due to the thick

sedimentary cover, whereas the positive magnetic anomalies may be related

to thin sedimentary cover (uplift of basement) at that area. Also, the high

frequencies (short wave lengths) magnetic anomalies present in the

northeastern part of the map may be due to shallow subsurface magnetic

sources, and the low frequencies anomalies are present at the western part of

the map.

3.2. Interpretation of the RTP Aeromagnetic Map:

The aeromagnetic survey (EGPC 1979) was on a flight elevation of 1 km

(Fig. 4.4). On looking to on the map, it can be stated that most of the

anomalies trend in E-W, and NE-SW directions. The high frequency

anomalies are present in the southern and eastern parts of the map and it

could be related to shallow subsurface magnetic sources. The low

frequencies anomalies are found in the northern part of the map. This map

represents the regional trends of the studied area.

Figure 4.4: RTP aeromagnetic anomaly map of Siwa area after (EGPC 1979)

3.3. Interpretation of the Bouguer Anomaly Map:

The Bouguer anomaly map (EGPC 1979) of Siwa area (Fig. 4.5) shows that

the main trends of the anomalies are NE-SW, (Syrian Arc system tectonic

trend), E-W (Mediterranean trend) and NW-SE (Gulf of Suez tectonics). The

thick sedimentary sequences are found at the western and southern parts of

the map and are indicated by negative Bouguer anomalies.

Figure 4.5: Bouguer anomaly map of Siwa (EGPC 1979)

4. APLICATION OF THE TREND ANALYSIS

TECHNIQUE

The trend analysis method was applied to the RTP land magnetic map and

the obtained deduced structure map is shown in (Fig. 4.6). Furthermore, the

regionality of these structures was obtained by applying this technique to the

RTP aeromagnetic and Bouguer anomaly maps covering a large-scale area

to produce a structural map (Fig. 4.7).

Figure 4.6: The structures deduced from the RTP land magnetic survey map.

Figure 4.7: Structural map deduced from the aeromagnetic and Bouguer

anomaly map of the study area.

The deduced structures were correlated with the surface geologic structures

published by CONOCO and Egyptian General Petroleum Cooperation, 1987

(Fig. 4.1). The structure lineaments for each map are grouped into 10˚

degrees according to their lengths. The results of the statistical method were

represented by rose diagrams (Fig. 4.8). These diagrams represent the main

structural trends predominant in the area.

The results indicate that the predominant structures in the study area are E-

W, N35˚- 45˚W and N45˚- 65˚E trends. This result agrees with the result

obtained by Meshref, in Said (1990).

Figure 4.8: Rose diagram to represent the main structures prevailing in the

study area at different levels.

5. CALCULATION OF THE FAULT PARAMETERS

USING HILBERT TRANSFORM.

According to Nabighian (1974), this method is used to calculate the

parameters of the buried causative bodies. The Helbert method was applied

to a set of profiles crossing the anomalies of magnetic and Bouguer anomaly

maps to detect the parameters of the deduced subsurface structures.

The results (Fig. 4.9) indicate that the mean depth to the basement ranges

between 3.1 and 3.8 km. The depth to the upper surface of the deduced faults

ranges between 0.5 km to 1 km whereas the depth to the lower surface

ranges between 3.3 km to 3.9 km respectively. The dip angels of these faults

are between 25˚ to 45˚.

6. APPLICATION OF WERNER DECONVOLUTION

METHOD

The Werner method (1953) is used to analyze the potential field data of

arbitrary bodies. The simplifications seem to be a limitation; the method

does provide a wide range of applications (Klingele et al. 1991). Therefore

the full derivation method of the interpretation was applied to the potential

field data using Cerovsky and Pasteka technique (2003). This method

depends on n-point operator and it means that we are solving n equations of

either n or less unknowns. Furthermore the clustering algorithm was applied

to the Werner deconvolution data. These clusters were indicated by

removing the interfered noises signals and produced the most convenient

solutions.

Figure 4.9: Evaluation of fault parameters along Ah1-Ah1` of aeromagnetic

map.

Where: (a) the magnetic profile (b) amplitude, horizontal and vertical

gradient of the anomaly (c) the analytical signal and (d) the fault model.

The results, Figs. (4.10 And 4.11), indicate that the mean depth to the

basement ranges between 2.5 km and 3.4 km. Also, it shows more accurate

results that correlate well with the drilling data, especially the results

produced from the land magnetic data. This may be due to the detailed and

condensed land magnetic survey.

Figure 4.10: Results of Werner deconvolution method along profile L3-L3`

of the land magnetic map

Figure 4.11: Results of Werner deconvolution method along profile A3-A3`

of the aeromagnetic map.

7. APPLICATION OF THE THREE DIMENSIONS

MAGNETIC MODEL TO THE LAND MAGNETIC

DATA

The magnetic field was computed using formulae for polyhedral bodies

derived by Pohanka (1998) and Singh and Guptasarma (2001). This was

evaluated for x, y, z and total field components. Moreover, all components

of the field derivatives tensor were computed. Magnetic field is computed in

user-defined sensor height over the magnetic relief.

The 3D model program is implemented in C++ (Pohanka 1998; and Singh

and Guptasarma 2001) was applied to the land magnetic data in order to

create compact models and to simplify defining the 3D geometry of

anomalous bodies. The algorithms used in 3D model program (Cerovsky et

al., 2004) are for general case, arbitrary measuring point r, and polyhedron

vertices vi. The computation also includes transformation of coordinates

from degrees to meters.

The model "observation area" is a rectangular area defined on the Earth`s

surface in rectangular coordinates. Input and output files for relief and

measured, modeled or difference field are in the form of grid files. Three

main views are provided on the modeled half-space the map view, profile

view and 3D view.

The map view (Fig. 4.12) provides an overview on the model geometry. It

shows that the light gray color (-ve anomalies) is found in the central and

southwestern parts of the map. These types of anomalies may be considered

as indication for the presence of thick sedimentary sequences or deeper

basement surface. Whereas the dark black color (+ve anomalies) is found in

the northwestern of the map. This may be due to the presence of shallower

basement rocks. The correlation between this map and the modeled map

(Fig. 4.13). Shows that the –ve anomalies are correlated with deeper parts in

the basement rocks and the +ve anomalies are correlated with shallower

parts. For further confirmation, two profiles cross sections were performed

along the modeled area.

Col 4 and Col 9 are the locations of the two modeled cross section profiles

Figure 4.12: Land magnetic map view in nT of the studied area after the

coordinates transformation.

Figure 4.13: The 3D modeled land magnetic map of the study area.

On looking to the deduced cross section, it is noted that the –ve anomalies

are represented by deeper depths to the basement rocks e.g. in the south part

of the profiles, followed by intruded basement heights that are represented

by +ve anomalies along the two profiles. Furthermore the profiles illustrate

that the depth to the basement ranges from 3 km to 3.6 km and its dipping

increases toward north (Figs. 4.14 & 4.15).

Mt: modeled field, d Mt; difference field and m Mt; measured field

Figure 4.14: Cross0section along column 4 of the modeled

area. 

Mt: modeled field, d Mt: difference field and m Mt: measured field

Figure 4.15: Cross-Section along column 9 of the modeled area.

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