Tectonic evolution of the northeastern part of the African continental margin, Egypt

Pergamon hurna, of Ah/can Earth Sciences. Vol 33. No. 1, pp. 49-68. 2001

o 2001 Elsev~cr Science Ltd

Pll:SO899-5362(00)00067-7 All rights reserved PrInted I” Great Britain 0899.5362/01 S- see front matter

Tectonic evolution of the northeastern part of the African continental margin, Egypt

I.M. HUSSEIN’ and A.M.A. ABD-ALLAH2,*

‘Egyptian General Petroleum Corperation (EGPC), Egypt

*Faculty of Science, Ain Shams University, Egypt

ABSTRACT-The area between Manzalah Lake and the southern Galala Plateau in northeast

Egypt constitutes the Galalas, Cairo-Suez, southern Nile Delta and northern Nile Delta structural

provinces. The northern Galala Fault separates the Galalas Province from the Cairo-Suez Province

and is considered to be the westward extension of the Themed Fault in central Sinai. The pre-

Eocene rocks are affected by northeast to east-northeast-orientated folds and reverse faults, as

well as east-west-orientated oblique-slip faults with dextral and normal components. Some folds

and reverse faults are interpreted to have been formed by northwest to north-northwest-orientated

compression related to the Syrian Arc movement, whereas the others by the secondary northwest

orientated shortening, which accompanied dextral strike-slip component along the planes of the

east-west-orientated faults. The east-west-orientated faults were initially formed during the Late

Triassic/Early Jurassic extension related to the drifting of the African/Arabian Plate away from the

Eurasian Plate as a result of opening of the Neotethyan Sea. The Neotethyan began to close due

to convergence between the two plates, leading to the Syrian Arc deformation. This deformation

mildly started in Late Cenomanian and followed by a more intensive phase in ConacianSantonian.

It mildly continued in the Maastrichtian, Early Palseocene and Late Palaaocene/Early Eocene. The

southward thinning of the pre-Eocene rocks controlled the intensity and style of deformation.

Two deformational mechanisms are proposed for the Nile Delta hinge zone. The first is related to

Late Oligocene-Early Miocene north-northwest-orientated Alpine compression. The second is

related to northward gravitational sliding of the post-Oligocene shale and sandstone over Cretaceous-

Eocene carbonates. “ 2001 Elsevier Science Limited. All rights reserved.

(Received 212100: accepted 2713101)

INTRODUCTION

The northern continental passive margin of Africa 1962). This study area is located in the unstable shelf,

occupies the southern Mediterranean Basin. This part which witnessed several tectonic events.

of the margin in Egypt was studied by Sadek (I 9281, In the southern part of the study area, Palzeozoic-

Said (1962, 19901, Salem (I 9761, Meshref (I 9821, Miocene rocks are exposed on the western side of

Owrig (I 982) and others. This study focuses on a the Gulf of Suez. In the north, these rocks are down-

part of the north Eastern Desert, which extends from faulted underneath the surficial Pliocene-Recent

the southern Galala Plateau to the Mediterranean Sea sediments. Subsurface structural studies (Bayoumi

(Fig. 1). Egypt can be divided into two shelves: the and Shenouda, 1971; Sestini, 1984; Harms and Wray,

northern unstable shelf is highly deformed; whereas 1990; Sarhan and Hemdan, 1994; Abdel-Aal et a/. , the southern stable one has nearly horizontal 1994; Abd-Allah and Bakry, 1994; Sarhan et a/, , 1996

sedimentary beds overlying basement rocks (Said, and others) indicate that the northern part of the study

*Corresponding author

[email protected]

Journal of African Earth Sciences 49

I.M. HUSSE1NandA.M.A. ABD-ALLAH

1 \ Nile

Seismic lines of present study

.._._ Seismic lines of Abd-Allah

and Bakry (I 994)

- - Subsurface fault

J ’ ’ scarp 0 Well

WESTERN DESERT /

I I EASTERN DESERT I[

Figure 7. IA1 Locat,on map of rhe sludy area. (Bl Ma/or struciural elements and provinces, wtth seismic

lmes and deep wells ,ndxared. X-X’ indicates the sectton line of Fig. 4.

50 Journalof African Earth Soences

Tectonic evolution of the northeastern part of the African continentalmargin, Egypt

area is dissected by east-west to west-northwest-

orientated faults. Moustafa et al. (1985), Moustafa

and Abd-Allah (1991, 1992), Moustafa and Khalil

(1995) and Youssef and Abd-Allah (in press) investigated the surface structures affecting the

southern part of the study area.

The African margin evolved by the break up of

Pangaaa in the Late Triassic/Early Jurassic. Rifting

and drifting of the African/Arabian Plate away from

the Eurasian Plate opened the Neotethys (Smith,

1971; Sengor, 1979; Sengor et al., 1984; Dercourt

et a/. , 1986) and caused the formation of a passive

continental margin in northern Egypt (Dixon and

Robertson, 1984; Moustafa and Khalil 1989, 1994;

May, 1991). Neotethyan Sea attained its maximum

extension in Late Cretaceous (Sengor and Yilmaz,

1981). It seems that the Early Mesozoic continental

margin of the African/Arabian Plate was irregular and

a promontory had existed in the area of northern

Arabia now occupied by Syria, Lebanon and northern

Iraq (Moustafa and Khalil, 1994). In Late Cretaceous,

the Neotethys began to close due to northwest to

north-northwest-orientated oblique convergence of

the African/Arabian Plate and the Eurasian Plate

(Smith, 1971), resulting in the formation of the

Syrian Arc fold belt (Syrian Arc System of Krenkel,

1925) that extends from Turkey to the northern

unstable shelf in Egypt. During the Eocene-Recent,

the north African continental margin was subjected

to northwest to north-northwest-orientated com-

pression associated with the Alpine Orogeny, which

was the product of the northerly motion of Africa

toward Eurasia (Youssef, 1968; Dietz and Hoden,

1970).

The objectives of this study include understanding

the superposition of different tectonic events, the

style of tectonic deformation in the Nile Delta hinge

zone and the impact of structures on sedimentation.

STRATIGRAPHIC FRAMEWORK

Rock units in the study area range in age from

Precambrian to Recent (Fig. 2). The Precambrian

crystalline rocks are encountered in the Hg69-1 well

(Fig. 1) at a depth of 2530 m. The Precambrian rocks

are exposed in the area south of the southern Galala

Plateau. The depth to Precambrian rocks increases

northward. Paleozoic rock units are exposed in the

core of the Wadi Araba Anticline and in the eastern

scarp of northern Galala Plateau (Fig. 3). These rock

units are dominantly sandstone with subordinate

carbonate intervals. The same facies was found

overlying Precambrian rocks in the Hg69-1 well. In

most of the study area, thick shale, sandstone and

limestone represent the Jurassic rocks, whereas a

thin Jurassic sandstone section is exposed in the

northern Galala Plateau.

The Lower Cretaceous Malha Formatoin forms a

major sandstone lens in the central part of the study

area. It has a maximum thickness of 850 m in the

Abu Hammad-l well and decreases in thickness to

320 m northward at the Monaga-1 well and to 50 m

southward at the northern Galala Plateau (see Fig. 1

for locations). A thick carbonate-marl section of

Cenomanian-Turonian age is overlain by an uncon-

formity that extends throughout the study area. This

section is covered by Campanian-Maaszrichtian chalk

in the southern Galala Plateau and the Hg69-1 well.

Middle Palaaocene-Lower Eocene carbonates overly

the Mesozoic rocks in the Galalas Plateaux.

Middle Eocene carbonate and Upper Eocene marl

and shale are well developed in the southern half of

the study area. An unconformity separates the Eocene

rocks and Oligocene rocks. The 110 m thick Oligo-

cene sand and gravel of the Cairo-Suez district abruptly

change northward to 1100 m thick shale in the

subsurface. In the central part of the study area, the

Oligocene rocks are overlain by Oligo-Miocene basaltic

flows.

The Miocene sequence varies northward in facies

and thickness. Sadek (1926) divided the Miocene

rocks in the Cairo-Suez district into a Lower/Middle

Miocene marine unit and an Upper Miocene non-

marine unit. The marl and shale of the marine unit

change into a thick shale unit to the north. Continental

sandstone of the non-marine unit also changes into

marine shale and sandstone in the north. Pliocene

rocks, east of the Nile Delta, consist of soft clay with

sandstone at the top.

STRUCTURAL CHARACTERISCS

AND ANALYSIS

Detailed surface and subsurface structural mapping

allowed the division of the study area into four structural

provinces. These are referred to as the Galalas, Cairo-

Suez, southern Nile Delta and northern Nile Delta and

are separated by east-west-orientated faults (Fig. 1).

Galalas structural province

The Galalas structural province consists of the

northern Galala Plateau, the southern Galala Plateau

and the intervening Wadi Araba fold (Figs 3 and 4).

The northern Galala Fault separates this province from

the Cairo-Suez Province (Fig. 1).

Northern Galala Fault

The northern Galala Fault consists of east-west-

orientated segments of listric normal surfaces. These

segments are linked by northwest-orientated

Journal of African Earth Sciences 5 1


Tectonic Evolution

Closing of Neo-Tethys by compression

E Third phase of Syrian arc deformation

( main severe phase)

2 * Formation of lsmalia horst 2. * Reactivation of North Galala fault

G * Formation of South Qantara high area

Formation of most E-W oriented faults Opening of Neo-Tethys by extension

Figure 2. Composite stratigraphic section of the study area showing the aenal distribution of the lithofacies of

each geological time unit, as well as the tectonic movements that affected the study area.

52 .Joumal of Afrfcan Earth Scier~es


Q: Quaternary Tm: Miocene

*TV. Tertiary volcanics To: Oligocene Te: Eocene K: Cretaceous J, Jurassic Pz: Paleozoic B’ Precambrian

Te

Figure 3. Geologic map of the Galalas siructural province. Palazozoic outcrops in Wadi Araba

area were compilied from the Ggeologic Map of Egypt (1981).

synthetic transfer faults (Fig. 5). The upper parts of

the fault surfaces have dips ranging between 67Oand

83O and dextral and normal-slip movements (Youssef

and Abd-Allah, inpress). However, the seismic lines

on the hanging wall block of this fault show that the

lower parts of the fault surfaces have gentler dips.

The Paleozoic-Cretaceous rocks forming this block

are folded by northeast to east-northeast-orientated

hinge lines of right-stepped en &he/on doubly

plunging folds of the Wadi Ghoweiba fold belt (Fig.

6). These folds are similar to those described around

the Themed Fault by Moustafa and Khalil (I 994,

1995). The Themed Fault (Hinge Belt of Shata,

1959) has an east-west orientation and was con-

sidered by Moustafa and Khalil (1994) as the

southernmost border of Early Mesozoic passive

continental margin of the north Sinai Peninsula. This

fault has a predominant dextral strike-slip move-

ment. It formed in Early Mesozoic times and reju-

venated in the Eocene/Miocene age (Youssef, 1968;

Moustafa and Khalil, 1994). Moreover, the northern

Galala Fault was originally formed in Late Triassic/

Early Jurassic time. It was reactivated during Early

Oligocene, Late Oligocene-Early Miocene, Early Late

Miocene and Post-Miocene (Youssef and Abd-Allah,

in press). The Themed Fault separates a tectonically

unstable part occupied by Syrian Arc structures in

northern Sinai from a tectonically stable part with

flat-lying beds in central and southern Sinai.

Therefore, the northern Galala and the Themed

Faults have the same structural characteristics and

origin. The authors propose that the northern Galala

Fault represents the westward continuation of the

Themed Fault because both faults:

il have east-west orientation and northward inclined

planes;

iii have dextral strike-slip movement;

iiil initially formed in Early Mesozoic time and

reactivated in Tertiary time; and

iv) have right-stepped en .&he/on folds affecting the

Cretaceous rocks and obliquely orientated to fault

plane.


I.M. HUSSEIN andA.M.A. ABD-ALLAH

Cauo-Suez I Galalas Structural Province

Structural Province1 Northern Galala Plateau 1 Wadi Araba A n t i c I i n e 1 Southern Galala Plateau

1200 - X X‘ - 1200

m m

800 -

m Lower-Mlddle Eocene m Cenomaman-Turonian

South Galala Formation (Middle-Late Paleocene) El Malha Formation (Lower Cretaceous) -I 2

4Km

a Sant Antony Formation (MaastrichtIan) m Jurassic

Sant Paul Formation (Campanian) Paleozoic

- 800

Figure 4. Geologic cross_secfIor? of the Galalas structural province. See Fig. 7 for location.

The Themed Fault probably extends underneath the

horizontal Eocene beds of El-Raha Plateau and the

Eocene-Miocene rocks in the Gulf of Suez graben into

the northern Galala Fault west of the Suez Rift (Fig.

7). The Tertiary dextral divergent wrenching move-

ments (Moustafa et al., 1985) in the Cairo-Suez

Province rejuvenated the northern Galala Fault to

affect the Eocene-Miocene rocks.

Wadi Araba overturned an ticline

Moustafa and Khalil (1995) interpreted the structure

of the northeastern part of Wadi Araba as a northeast-

plunging breached anticline. They agree with Sultan

and Schutz (1986) that the Wadi Araba Anticline

represents the southernmost limit of the Late Creta-

ceous Syrian Arc folding in the Eastern Desert.

The vertical to overturned Cenomanian-Turonian

beds in the southeastern limb of Wadi Araba Anticline

are overlain by gently dipping (5’-12O) Campanian-

Maastrichtian chalk. This can be explained as a

northwest-dipping reverse fault separating the

southeastern limb of this fold from the northern scarp

of the southern Galala Plateau (Figs 3, 4 and 7).

Another southeast-dipping reverse fault may separate

the northwest-dipping (40°-70°) Cenomanian-

Turonian beds, which occupy the northwestern limb

of Wadi Araba Anticline from the southern scarp of

the northern Galala Plateau (Figs 3, 4 and 7).

54 .lournal of Afrtcan Earth Soerrces

Therefore, the Wadi Araba doubly-plunging fold

represents a large overturned anticline on the hanging

walls of both thrust faults (Figs 3, 4 and 7).

An angular unconformity is observed between the

Cenomanian-Turonian and the Campanian-Maastrich-

tian in the southern Galala Plateau and between the

Cenomanian-Turonian and the Middle/Late Palaaocene

in the northern Galala Plateau. This unconformity is

contemporaneous with the severe phase of folding

and reverse faulting and is affected by them. Folding

related to the Syrian Arc movement continued in the

Early Palaaocene before the deposition of the Middle/

Late Palaeocene rocks. The Wadi Araba Anticline was

eroded during the Campanian-Maastrichtiaan. During

Oligo-Miocene, the eastern part of this fold was

breached by two Clysmic-trending faults, which led

to the downthrow of the eastern part of the anticline

beneath Tertiary rocks in the Gulf of Suez (Fig. 7).

Northern and southern Galalas Plateaux

The Middle Jurassic rocks are exposed in the north-

eastern part of the northern Galala Plateau (Figs 3

and 4). The southward disappearance of these rocks

indicates that the southern part of the northern Galala

Plateau, as well as the Wadi Araba-southern Galala

area, were structurally high during Jurassic times. The

northern Galala Plateau was under the Early Creta-

ceous Sea where the Early Cretaceous Malha Formation

Tectonic evolution of the northeastern part of the African continentalmargin, Egypt 27’

310130

Nile Delta

Ismalia Horst

/

\

20 Km

- Surface Fault - Subsurface Fault

Figure 5. Structural map of the top of the Oligocene rocks. Y-Y’ indicates the section line

of Fig. 8.

is exposed in its northern part. The Campanian-

Maastrichtian rocks were exposed in the southern

Galala Plateau (Fig. 4). The Maastrichtian chalks of

the Sant Antony Formation are deformed by

northward-slumping and sliding structures that indicate

syn-sedimentationally repeated tectonic movements.

The distribution of the Campanian-Maastrichtian rocks

(Fig. 7) indicates that the Campanian-Maastrichtian

Sea most probably invaded the southern Galala area

from the south and/or southwest, whereas the

northern Galala-Wadi Araba area remained above sea

level. A final phase of Syrian Arc folding in this

structural province took place in Early Eocene. This is

evidenced by the gentle southeast dipping of the Lower

Eocene beds in the southern Galala Plateau (Fig. 4)

and the diachronic stratigraphic boundary between

the Lower Eocene beds and the underlying Middle /

Late Palaeocene beds of the South Galala Formation.


/.&I. HUSSElNandA.M.A. ABD-ALLAH

Ismalia

at Bitter Lake Graben

I Fold Belt

Wadi G~ow~iba Fold B

Northern Galala Plateau

??& Area with eroded CenoR~anian-Upper Cretaceous rocks

A Reverse fault * Subsurface fault

* - Dextral oblique-slip fault

t- Anticline + Syncline

Figure 6. Structural map of the top of the Cenomanmn-Upper Cretaceous rocks.

The southern Galala Plateau is bounded from the acted as a synthetic zone between two right-stepped

south by a northeast-orientated oblique-slip fault en &he/on northeast dipping Clysmic-trending faults

with s~n~stra~ and Norman-slip components. This fault (Figs 3 and 71.

56 .Journalof African Earth Sciences


Suez Road 1 32”

Cairo-Suez Province.

u Reverse fault - Surface fault

- Rift-bounding fault 4 Exposed anticline

/J Subsurface anticline

I : Monocline (after Moustafa and Khalil, 1995)

rsoo Isopach contours in feet of Campanian-Maastrichtian rocks (after Tewfik, 1988)

\mj Area with no Campanian-Maastrichtian rocks

v///l Area in which Campanian-Maastrichtian rocks present

Figure 7. Structural map of both sides of the northern Gulf of Suer Raft. The Themed Fault extends

westward in the Suez Rift area to join with the northern Galala Fault.

The Cairo-Suez structuralprovince

The exposed Tertiary rocks in the Cairo-Suez Province

are deformed by east-west and northwest-orientated

normal faults. The east-west and some of northwest-

orientated faults form 14 belts, each with right-

stepping en &he/on normal faults. These belts were

studied by Moustafa eta/. (19851, Moustafa and Abd-

Allah (1991, 1992) and Youssef and Abd-Allah (in

press) and were interpreted to have been formed by

dextral divergent wrenching along east-west-

orientated deep-seated faults. Detailed subsurface

mapping indicates that the east-west-trending, deep-

seated faults deform Jurassic-Cretaceous rocks (Fig.

6) and have the same north and south dip directions

of the en &he/on normal faults on the surface. The

en &he/on normal faults affect the Tertiary rocks and

have dip amounts between 65’ and 90”. These short

en &he/on faults are linked together at a 650-I 300

m depth interval to from several long east-west-

orientated faults affecting the Jurassic-Cretaceous

rocks. This supports the conclusion of Segall and

Pollard (1980) that the en &he/on faults at the

surface, such as those affecting the Tertiary rocks

in the Cairo-Suez Province coalesce at depth into a

single fault like those affecting the Jurassic-

Cretaceous rocks in the same province. The east-

west elongated en &he/on fault belts form four

alternating fault blocks of graben-horst structural


I.M. HUSSEIN andA. M.A. ABD-ALLAH

styles. These blocks are terminated from the east and

west by Clysmic-trending faults. The interior parts of

the horst blocks are deformed by northwest to north-

northwest-orientated faults, which are terminated at

the faults bounding each horst block (Fig. 5). In the

subsurface, the same structural styles are expressed

in Jurassic-Cretaceous rocks but with less deformed

interior parts (Fig. 6).

Throughout the study area, the deep-seated faults

that deform the Jurassic-Cretaceous rocks are in

the form of east-west, east-northeast and west-

northwest trends (Fig. 6). These fault trends are

simply referred as east-west trends. These faults

are linked together to form consistent zigzag fault

systems similar to those dominant in northern Egypt.

Most of these faults were initially formed by north-

south to north-northwest-south-southeast-directed

extension related to Late Triassic/Early Jurassic

divergence between African/Arabian and Eurasian

Plates to form the passive continental margin of the

African/eastern Mediterranean basin in northeast

Egypt. The majority of these faults accommodated

younger dextral strike-slip movements due to reac-

tivation by northwest-southeast to north-northwest-

south-southeast-directed compression that accom-

panied the Syrian Arc movement during the Late

Cretaceous. The east-west-orientated faults bound

several east-west elongated blocks (Great Bitter Lake

and Wadi Ghoweiba grabens). The internal parts of

these blocks are deformed by northeast- to east-

northeast-orientated small folds and reverse faults,

which are terminated against the bounding faults (Fig.

6). These folds and reverse faults indicate northwest-

southeast to north-northwest-south-southeast-

trending shortening, which resulted from dextral strike-

slip movements along east-west-orientated fault

planes bounding the elongated blocks. The large folds

that are not controlled by these blocks are interpreted

as being formed by northwest-southeast to north-

northwest-south-southeast-directed compression

during Late Cretaceous. Some east-west-trending

faults dextrally offset the large folds (Fig. 6). Detailed

subsurface structural analysis revealed that the

northwest-southeast to north-northwest-south-

southeast-directed compression started after the

deposition of the Cenomanian rocks and continued

during Conacian Santonian times to produce more

intensive deformation.

The eastern part of the Cairo-Suez structural pro-

vince, in the western side of Suez Rift, is dominated

by Clysmic-trending faults (Figs 5 and 6). These faults

bound southwest-tilted fault blocks characterising the

northern dip province of the Suez Rift. The Clysmic

faults extend northward into the area west of the

Bitter Lakes and transfer their displacements into

east-west-orientated faults (Figs 5 and 6). The Clysmic

faults were formed during different stages of Suez

rifting, which started in the Oligocene (Robson, 19711,

Olig-Miocene (Garfunkel and Bartov, 1977) or Early

Miocene (Moustafa, 1993). The east-west-orientated

faults in the Cairo-Suez structural province were,

therefore, formed in Late Triassic/Early Jurassic times,

whereas the northwest-orientated faults were formed

in Oligo-Miocene times. Both fault sets were activated

by the extension that resulted in the opening of the

Suez Rift.

Southern Nile Delta structuralprovince

The southern Nile Delta Province (south Delta block

of Said, 1981) extends northward from the east-

west-orientated faults bounding the northern part

of the Cairo-Suez Province to the Nile Delta hinge

zone (Fig. I). This structural province is completely

covered by Pliocene-Quaternary sediments. Struc-

turally, the province is deformed by a number of

east-west-orientated faults that divide it from

south to north into the Bitter Lake graben and the

lsmalia horst (Figs 6 and 8). Like the east-west-

orientated faults affecting the latter province, the

east-west-orientated faults in this province were

originally formed in Late Triassic/Early Jurassic,

during the rifting of the African/Arabian Plate away

from the Eurasian Plate. These faults reactivated

to deform the Upper Cretaceous rocks and con-

tinued to affect the overlying Tertiary rocks. The

Jurassic-Cretaceous rocks of this province are de-

formed by northeast-orientated, right-stepping en

&he/on folds forming the Great Bitter Lake fold

belt. Folds of this belt are terminated against the

east-west-orientated faults that bound the Great

Bitter Lake graben. In addition, the Jurassic-

Cretaceous rocks are deformed by several east-

west-orientated oblique-slip faults with dextral and

normal-slip components. On the other hand, the

overlaying Oligocene-Miocene rocks are less de-

formed. Gebel Shabrawet overturned anticline is

the easternmost one in this belt (Fig. 6). It resulted

from the displacement of the hanging wall of an

east-northeast-orientated thrust fault. The area east

of Great Bitter Lake graben is also deformed by

Clysmic-trending faults. The lsmalia horst is inter-

nally affected by northeast-orientated folds that

dextrally offset by east-west-orientated faults.

Northern Nile Delta strut turalprovince

The northern Nile Delta structural province lies north

of the Nile Delta hinge zone and extends north of

the study area. This province is dissected by several

east-west-trending listric normal faults that bound

southward tilted fault blocks of half-grabens. The


Oligocene and Miocene rocks filled the low areas

between rotated blocks, forming several east-west

elongated wedge-shaped basins (Figs 8 and 9).

In the northernmost part of the study area, the

Oligocene and Miocene rocks are affected by north-

south to north-northwest, northwest and northeast

to north-northeast-orientated faults. These are

referred to as the Baltim, Temsah and Rossetta trends,

respectively, by Abdel-Aal et al. (1994) and shown in

Fig. 1. Some of these faults extended down to deform

Jurassic and Cretaceous rocks (Fig. 8). The faults of

the Rossetta, Temsah and east-west trends appear

to dissect basement rocks, as shown by Owrig

(1982). Analysis of the seismic data indicate that

the Temsah, Rossetta and east-west-orientated faults

were formed during Early Oligocene and were

reactivated during Early Tortonian, Early Pliocene and

Pleistocene times. The structural style and defor-

mation of this province are tightly similar to those in

the following Nile Delta hinge zone.

DEFORMATION MECHANISMS OF THE NILE

DELTA HINGE ZONE

The Nile Delta hinge zone is the most significant shear

zone in the tectonic evolution of the Nile Delta. It had

influenced the overall facies variations of the African

continental margin in the study area. It consists of

several northward-dipping listric normal faults. These

faults are the longest and have the largest amounts

of throws to the north (Figs 5, 6, 8 and 9). The

northward subsidence and sinking of the African

margin in the study area are mainly related to

movements on the planes of these faults.

The east-west-orientated faults forming the Nile

Delta hinge zone change their trend to north-south in

the west (Figs 5 and 6). The estimated depths of

detachment determined by the methods of Williams

and Vann (1987) and Dula (I 991) increase northward

from 2.8 km to 10.4 km. Some of these faults deform

the Oligocene-Pliocene rocks over the pre-Eocene

rocks, whereas others deform the Eocene rocks (Figs

8 and 9). The steeper segments of these faults affect

the Miocene and Pliocene rocks, whereas the gentle

segments deform the underlying Oligocene and Eocene

rocks. The northward dip of each listric normal fault

forms the southward tilted fault block. The rotation

on the surfaces of these faults started in the Late

Oligocene/Early Miocene and continued in Late

Miocene/Early Pliocene and Pliocene times. The

contact between the Upper Miocene Abu Madi

Formation and the Pliocene rocks represents a regional

angular unconformity with a salient southward strati-

graphic onlap. This indicates that tectonic subsidence

accelerated during Early Pliocene.

Two mechanisms are suggested for the deformation

of the Nile Delta hinge zone. The first is related to the

Late Oligocene-Early Miocene northwest-southeast

to north-northwest-south-southeast directed com-

pression that accompanied the Alpine Orogeny since

the end of the Eocene. This compression rejuvenated

the pre-existed east-west-orientated desp-seated

Mesozoic faults. This is evident by the upward

extension of some fault planes, which affect Eocene

and older rocks to dissect Oligocene-Pliocene rocks

(Figs 8 and 9).

The second mechanism is related to the northward

gravitational sliding of the Oligocene -Pliocene shale

and sandstone by the action of its own weight over

the top of the Eocene carbonate rocks. This top is

considered to be the basal-slip plane which separates

brittle Cretaceous-Eocene carbonates at the base from

overlying overpressured ductile Oligocene and Miocene

shale. The northward deepening of the basal-slip plane

in the study area is mainly related to continuous

tectonic subsidence of the outer parts of the African

margin since Late Cretaceous times. The sliding of

the Oligocene-Pliocene rocks took place during and

after sedimentation across an east-west set of growth

normal faults perpendicular to the slide direction. The

growth normal faults of the Nile Delta hinge zone form

the open-ended slide structure of Crans et al. (I 980).

The initial phase of this sliding took place after the

deposition of Oligocene rocks and continued periodically

after the deposition of the Miocene and the Pliocene

rocks. The thickness of each slide sheet increases

northward. Harms and Wray (I 990) showed that the

rate of the sedimen-tation of each of the Miocene

and Pliocene rocks increased basinward. The spacing

of the growth fault is inversely related to the slide

sheet thickness and to the rate of sedimentation. The

fault spacing decreases northward from IO km to

2 km. The disturbances in the fault spacing in some

parts of the Nile Delta hinge zone are mostly attributed

to reactivation of old faults and to irregularities of

facies thickness and type. The south-dipping antithetic

faults were developed in the hinge zone to compensate

for space problems.

The two proposed mechanisms acted together

during the deformation of the Nile Delta hinge zone.

RELATIONSHIP BETWEEN THE EASTERN

AND WESTERN DESERTS

The study area represents the northern part of the

Eastern Desert, where deformation of the Jurassic-

Cretaceous rocks increases southward (Fig. 6). This

may be related to southward thinning of these

rocks. The thick sequence in the north could have

accommodated the stresses by inter- and intra-

.Joumal of African Earth Sciences 59

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granular deformations. The only intensively deformed

zone in the north is related to the Nile Delta hinge

zone, which was been described above.

The Jurassic-Cretaceous rocks in the northern

Western Desert are deformed by southward-dipping

east-west-orientated faults (Moustafa et al., 1998).

Contrary to these faults, the northern parts of the

Eastern Desert and Sinai are affected by north-

northwest to north-dipping east-west to east-

northeast-orientated faults. The accommodation zone

between these two sets of faults, which are dipping

opposite to each other, is expressed by several

northwest-elongated horst and graben blocks in the

Nile Valley area (Fig. IO). Individual faults within the

accommodation zone are northwest-trending and dip

in the same direction as in the Western and Eastern

Deserts. Faults of east-west set are abundant in nor-

thern Egypt and have dextral diagonal-slip movement

(Youssef, 1968; Moustafa et al,, 1985, 1998;

Moustafa and Khalil, 1994). The northeast elongated

folds of the Syrian Arc deformations in the study area

(Figs 3, 6 and 7) extend due east in northern Sinai and

due west in northern Western Desert (Fig. IO). Similar

fold trends were traced by Garfunkel and Goudarz

(1980) in eastern Libya and by Neev (1975) in Turkey

and Syria.

TECTONIC MOVEMENTS AND SEDIMENTATION

An interplay of sedimentation and tectonic move-

ments plays an important role in the geological evo-

lution of the study area. The influence of the tectonic

movements on sedimentation in this part of the

African continental margin is described below.

Jurassic In northern Egypt, tectonic depocenters and positive

structures controlled the sedimentation. These tec-

tonic elements continued to be active from Late

Palaaozoic to Late Mesozoic (Sestini, 1984). Most of

these elements trend roughly east-west or east-

northeast with similar trends in southern Egypt

(Klitzsch, 1984, 1990). The Mesozoic Sea north of

Egypt has been considered as the southern part of

Neo-tethyan Sea (Sengor et al., 1984). The pre-

Jurassic rocks in the study area were affected by

north-south to north-northwest-south-southeast

directed extension, which accompanied the opening

of the Neo-tethyan Sea and resulted in the develop-

ment of the east-west to east-northeast-orientated

faults. These faults parallel to a set of other faults in

north Sinai. The majority of the east-west to east-

northeast-orientated faults dip northward in the

Eastern Desert, making a regional northerly sloping

platform (Fig. 8). This platform is considered the

southern part of the Early Mesozoic passive continental

margin of the eastern Mediterranean Basin in northern

Egypt. This basin originated from Early Mesozoic rifting

(Bein and Gvirtzman, 1977; Garfunkel and Derin,

1984) and associated with strike-silp movement

between Africa and Arabia (Neev, 1975). The

platform was onlapped by the sediments of the

Neotethyan Sea at the beginning of the Jurassic. The

Jurassic facies were, therefore, marine in the north

and represented by a 2320 m thick carbonate and

shale section and by shallow marine thin elastic to the

south (Fig. 1 1 a).

The northern Galala Fault separates two distinct

Jurassic facies. The first is due north, occupies its

downthrown block and is dominantly shale; whereas

the second facies is due south, occupies its upthrown

block and constitutes near-shore sandstone. To the

north, the southern bounding fault of the Great Bitter

Lake graben separates the Cairo-Suez and the south-

ern Nile Delta structural provinces. At this fault, the

thin section of Jurassic shale in the Cairo-Suez

Provrnce is abruptly changed into thick limestone, shale

and dolomite in the two Nile Delta structural provinces

(Fig. 1 la). These observations indicate that the

northern Galala Fault and the southern bounding fault

of the Great Bitter Lake graben were initially formed

during Late Triassic/Early Jurassic extension. This

result is in full agreement with Sestini (1984), who

stated that the faulted blocks of the African margin

formed during Triassic and Early Jurassic times.

Therefore, most of the east-west-orientated faults

in the study area and in other parts of northern Egypt

formed in Late Triassic/Early Jurassic.

Lower Cretaceous-Cenomanian

During most of the Cretaceous and Tertiary, the

rate of subsidence in northern Egypt was greater

than that in the south (Kuss and Lepping, 1989).

This subsidence took place by the movemerrts on

the planes of the east-west-orientated faults that

bound north-ward step-faulted blocks. The lower

Cretaceous-Cenomanian limestone, shale and

sandstone gradually decrease in thickness south-

ward, toward the Cairo-Suez structural province.

A salient pervasive high area is observed in the

area of the South Qantara well, in the northern part

of the southern Nile Delta Province, as indicated

by a thin section of Cenomanian sediments (Fig.

11 b, cl. The structural style of this high area is not

clear, whether it is fault ridge or horst block.

However, it was formed in the Early Cretaceous

and reactivated during the deposition of the

Cenomanian rocks. The absence of the Jurassic-

lower Cretaceous rocks in the southern Galala

Plateau (Fig. 4) indicates that its northern bounding


- WESTERN

- - Dextral oblique-slip fault

- Reverse fault

- Normal fault

I 4 Plunging anticline

Figure 10. Cretaceous-Early Tertiary structures m northern Egypt. Structures in Sinai and the Western Desert are compiled

from Moustafa et al. (1998). The accommodation zone between the northward-dipping faults in the Eastern Desert and the

southward-dipping faults in the Western Desert IS represenied by the northwest elongated horsi and graben tlocks in the Nile

Valte y area.

fault was originally formed as a normal fault in Late

Triassic/Early Jurassic and uplifted the plateau area

during Jurassic-Early Cretaceous times. The northern

Galala Fault was reactivated during the Early Ceno-

manian, as evidenced by the abrupt change in the

thickness of the Cenomanian rocks across the fault.

Turonian The Turonian rocks show a southward facies change.

The Abu Hammad-l well on the top of lsmalia horst

separates a thin carbonate facies due north, which

occupies the Nile Delta structural provinces from a

thick carbonate/shale facies to the south in the Cairo-

Suez Province (Fig. 1 Ic). This suggests that the

bounding faults of the lsmalia horst (Figs 5 and 6)

were formed or reactivated in CenomanianiTuronian

times. Another movement on the northern Galala Fault

took place in Cenomanian/Turonian times, where its

upthrown side has thin Turonian rocks relative to its

downthrown side. The Late Cenomanian deformations

represent the first less deformed phase of Syrian Arc

movement.

During ConacianiSantonian times, the Palaaozoic-

Turonian rocks in the study area were affected by

the main severe second phase of the Syrian Arc

movement. Uplifting, marine regression and intensive

erosion, forming a major unconformity throughout the

study area, accompanied the second Syrian Arc

phase. Also, during this phase, the normal slip of the

northern bounding fault of the southern Galala Plateau

might have been inverted into a reverse fault, as well

as reactivating the other east-west-orientated faults

in the study area. Uplifting of the area north of the

Eastern Desert and Sinai took place during Conacian

times and is related to the initiation of Syrian Arc

movement (Kerdany and Cherif, 1990). The second

Syrian Arc phase was reported in several parts of

Egypt (Moustafa, 1988; Moustafa and Khalil, 1994,

1995; and others). This phase formed the fold/thrust

belt of the Syrian Arc, which extended from Turkey

to northern Egypt and Libya.

Campanian-Maastrichtian

The scattered Campanian-Maastrichtian rocks might

have been deposited in low areas flanking uplifted

blocks and may have resulted from folds and faults

in the northeastern part of Egypt (Fig. 7). T!ie chalks

of the Campanian Sant Paul Formation and the

Maastrichtian Sant Antony Formation are only found

in the southern Galala Plateau (Figs 4 and 7) and in

the Hg69-1 well. These chalks were deposited in the

deeper shelf of an open marine environment (Bandel

and Kuss, 1987; Kuss and Lepping, 1989; Kuss,

1992). The Syrian Arc renewed tectonic activity

resulted in the lowering of the northern outskirts of

the mid-Cretaceous shelf, where deeper shelf chalks

and marl were deposited (Kuss, 1992). The Palaao-

geographical reconstruction of the study area indicates

./ournal of African Earth Sciences 63

r

500 -

o-

-sg-

500 -

o-

500- m

500-

0=-

-500 - m

TOO-

o-

-500 -

-1ooo- m

500 -

o-

-500 -

-lOOO-

-lSOO- m

End of UDDer Cretac :eous Ku Kc

KU End of Eocene

To

.u ‘: m

.P 2 r: I

N

End of Jurassic

z? v! 0 L

20 Km

End of Cenomanian

Location of wells

lz5l Dolomite

E&I Limestones

cl -_- Shales --

I3 Marls

m Sandstones

End of Oligocene

Upper Miocene

End of Miocene

Figure 17. North-south-orieniated stratigraphical cross-sections reconstructed at different geological times, showing

the lateral and vertical lithofacies distribut,on at each time unit.


that the southern Galala Plateau and the downthrown

block of the northern Galala Fault were the only

depositional sites for the Campanian-Maastrichtian

chalks (Fig. 7). These chalks show northward-slumping

structures related to the third phase of the Syrian

Arc movement. This phase was reported from several

parts of north Africa (Barr, 1972; Eliagoubi and Powell,

1980; Dixon and Robertson, 1984; Sestini, 1984;

Moustafa and Khalil, 1989, 1995; Camion, 1991;

Moustafa et a/., 1998). Toward the end of the

Cretaceous, the Syrian Arc tectonic movements were

accompanied by magmatic activity (Hashad, 1980).

Pakeocene-Lower Eocene During the Middle Palaeocene-Middle Eocene, an

extensive transgression flooded broad areas of north

Africa (Kuss, 1992). The Middle /Upper Palaaocene

marl and shale of the South Galala Formation

unconformably overlie the Maastrichtian chalks in the

southern Galala Plateau (Fig. 4) and in the Hg69-1

well area. They also overlie the Turonian marl and

shale in the northern Galala Plateau (Fig. 4) and Gebel

Ataqa. Bandel and Kuss (1987) demonstrated that

the deposition of the South Galala Formation in a

southward ramp basin extended parallel to the

shoreline of the Gulf of Suez between Gebel Ataqa in

the north (tidal flat) and the southern Galala Plateau

in the south (deeper shelf). A fourth phase of Syrian

Arc activity is documented to have taken place in

Early Palaaocene.

The Lower Eocene carbonates are exposed in the

northern and southern Galala Plateaux and penetrated

the Hg69-1 well. The absence of the Lower Eocene

rocks over the uplifted folded and faulted parts of the

study area indicates that the fifth phase of Syrian

Arc activity started in Late PalaaoceneiEarly Eocene

and continued during the Early Eocene. The Cretaceous

rocks are unconformably overlain by the Middle

Eocene rocks in these uplifted regions. Strougo (1986)

reports a Late Palaeocene tectonic event that is

marked by changes of depositional setting in several

parts of Egypt.

The development of the Syrian Arc structures in

the study area occurred in five tectonic phases from

Late Cenomanian to Early Eocene. But Sestini (1984)

indicated that the Syrian Arc deformations continued

up to Middle Eocene in other northern parts of Egypt.

These tectonic phases are contemporaneous with the

development stages of the Neotethyan Sea in the

study area and other parts of northern Egypt.

Middle-Upper Eocene

There was a general transition in tectonic style from

the Syrian Arc compression to the Red Sea/Gulf of

Suez extension all over Egypt (Morgan, 1990). The

Galalas and the Cairo-Suez structural provinces are

capped by Middle Eocene carbonate, whereas the

Upper Eocene marls and shales occupy several down

faulted areas. Strougo et a/. (1992) presented an

environmental model for Middle Eocene rocks in the

Cairo-Suez Province and the Nile Valley. Lagoonal

limestone covers the eastern half of the Cairo-Suez

structural province and graded westward to be a

deeper marine depositional environment. Upper Eocene

rocks show the same eastward shallowing (Abul-Nasr

et a/. , 1993). These Middle-Upper Eocene facies

changes might be related to uplifting of the deep-

seated Syrian Arc structures in the eastern half of

the Cairo-Suez Province.

The Eocene rocks are completely absent over the

lsmalia horst, where the Turonian rocks are directly

overlain by the Oligocene rocks (Fig. 1 Id, e). This

indicates that east-west bounding faults of the lsmalia

horst were reactivated during Eocene times where

the horst separates two basins. The first basin lies

to the north and comprises a thin section of Middle/

Upper Eocene shales in the Nile Delta structural

provinces. The second basin occupies the Cairo-Suez

and Galalas Provinces to the south and is dominated

by carbonate (Fig. 1 Id). It seems that the Middle-

Upper Eocene tectonic deformations in the present

study area are synchronous with the D2 deformational

phase of the Themed Fault in Sinai. Moustafa and

Khalil (1994) indicated that this phase of deformation

is synchronous with a similar tectonic event in Bitlis

suture zone, southeastern Turkey (Hempton, 1985)

and with one of the events in the Palmyra fold belt

(Chaimov et al., 1992).

Oligocene

In the Early Oligocene, the majority of the east-west-

orientated deep-seated faults were reactivated to

form elevated lands in the southern half of the study

area and low lands in the north. This was accompanied

by the extrusion of basaltic flows. The 1 100 m thick

marine (Said, 1990) Oligocene shales in the area north

of the Monaga-1 well decrease in thickness and

change into shale and sandstone in the area of the

lsmalia horst and into continental and fluviatile (Shukri,

1953) loose sands and gravels exposed in the Cairo-

Suez Province (Fig. 1 1 e). This northward basinal facies

change is affected by several east-west-orientated

listric normal faults that bound southward-rotated

blocks in the northern Nile Delta structural province

(Figs 8 and 9). These blocks form east-west-elongated

wedge-shaped basins. The dip of the Oligocene beds

in these basins is greater than those of the overlying

Miocene beds (Fig. 9). The thickness of Oligocene

and Miocene rocks in the downthrown side of the

east-west-orientated fault is much greater than on its

I. M. HUSSEIN and A.M.A. ABD-ALLAH

upthrown side. These two observations suggest that

the east-west-orientated faults in the northern part

of the study area were reactivated as growth normal

faults during the deposition of Oligocene and Miocene

rocks.

Miocene The Miocene rocks unconformably overlie the

Oligocene and older rocks in the study area. South-

ward invasion of the Miocene Sea led to develop-

ment of northward basinal facies. Shallow marine

marls and shales of the Lower/Middle Miocene

are expoc ad in the Cairo-Suez Province and change

into thick deeper marine shale facies (North Delta

embayment of Said, 1981) in the area just north

of the Monaga-1 well, Nile Delta structural pro-

vinces (Fig. 11 f). The North Delta Miocene rocks

extend along the coastline in the study area toward

the area north of Sinai and the Nile delta. lsmalia

horst played an important role in this facies dis-

tribution, where its bounding faults were reactivated

in Early Miocene and Late Miocene. This horst

separated a continental Upper Miocene facies in

the Cairo-Suez Province from a marine facies to

the north. During the Miocene, the structural

deformations in the study area are mainly related

to the Suez rifting phases in its southern part and

to the formation of the Nile Delta hinge zone in

its northern part.

SUMMARY AND CONCLUSIONS

Northeast Egypt can be divided into the Galalas,

Cairo-Suez, southern Nile Delta and northern Nile

Delta structural provinces. The northern Galala Fault

consists of several east-west-orientated listric nor-

mal faults that are linked by northwest-orientated

synthetic transfer faults. Jurassic-Cretaceous rocks

in the downthrown block of this fault are folded by

right-stepping en &he/on of doubly-plunging folds

orientated northeast to east-northeast. The northern

Galala and Themed Faults have the same structural

characteristics and origin; the former might repre-

sent the westward continuation of the latter. Most

of the east-west-orientated faults were initially

formed by north-south to north-northwest-south-

southeast directed extension related to the Late

Triassic/Early Jurassic divergence between Africa

and Eurasia. The majority of these faults had dex-

tral strike-slip movements and are reactivated by


southeast-directed compression in the Late

Cretaceous due to Syrian Arc deformation. The

east-west to east-northeast-west-northwest-orien-

tated faults bound east-west elongated blocks. The

interior of these blocks are deformed by northeast

to east-northeast-trending small folds and reverse

faults, which are terminated against the bounding

faults. These folds and reverse faults are inter-

preted as due to northwest-southeast to north-

northwest-south-southeast-directed shortening

resulting from the dextral strike-slip movements along

the bounding faults. Large folds, which are not

controlled by these blocks, might have formed by


southeast directed Late Cretaceous compression.

The Syrian Arc deformation mildly started in Late

Cenomanian and was followed by a main intensive

phase in the ConacianiSantonian. It mildly continued

in the Maastrichtian, Early Palaaocene and Late

Palaaocene-Early Eocene.

The Nile Delta hinge zone consists of several

southward half-grabens. These grabens are bounded

by east-west-orientated northward-dipping listric

faults. Two mechanisms are suggested for the

deformation of the Nile Delta hinge zone. The first is

related to the Late Oligocene-Early Miocene,


southeast directed compression that resulted from

the Alpine Orogeny since the end of the Eocene. This

compression reactivated the east-west-orientated

deep-seated Mesozoic faults. The second mechanism

is related to northward gravitational sliding of Oli-

gocene-Pliocene shale and sandstone over the pre-

Eocene carbonate rocks. Both mechanisms acted

together during the deformation of the Nile Delta hinge

zone.

The Jurassic-Cretaceous rocks in the north Western

Desert are deformed by southward-dipping east-west-

orientated faults. Contrary to these, the northern

parts of the Eastern Desert and Sinai are affected by

north-northwest to northward-dipping east-northeast

to east-west-orientated faults. The accommodation

zone between these faults is in the form of several

northwest elongated horst and graben blocks in the

Nile Valley area.

ACKNOWLEDGEMENTS

The authors wish to express their cordial appre-

ciation and gratitude to Professor M.H. Abdel-Aal

(Ain Shams University) for his help during the inter-

pretations of the seismic lines. They are indebted

to Professor A.R. Moustafa (Ain Shams University)

for reviewing the manuscript. Special thanks to

EGPC who provided the subsurface data and per-

mission to publish this paper and also to two anony-

mous referees who reviewed the manuscript and

provided useful comments.

Editorial handling - P. Bo wden


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Documents

Tectonic evolution of the northeastern part of the African continental margin, Egypt