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Structural history of Abu Roash district, Western Desert, Egypt
Journal o.¢ African Earth 5cwnces, Vol 9, No 3/4, pp 435-443. 1989
I)899 4362/89 $3 00 + 0 00 Pergamon Press plc
Printed m Great Britain
Structural history of Abu Roash district, Western Desert, Egypt *M. L. ABDELKHALEK,*M. A. EL SHARKAWl, *M. DARWISH,**M. HAGRASand *A. S E H I M *Geology Dept., Faculty of Science, Cairo University, Glza **Bapetco Oll Company, Egypt Abstract - The Abu Roash district is located 10 km to the southwest of Cairo, and is geoloDcally significant because of its surface exposure of Upper Cretaceous rocks. It can be used as a stratigraphic and structural model for the Western Desert. The exposed Upper Cretaceous section is differentiated into two formations, these being the Khoman and Abu Roash Formations seen in the Wes tern Desert penetrations. The Abu Roash Formation has been studied in detail at outcrop and can be dtrecfly correlated, both lithologlcally and paleontologically, with the "A" to "G" members differentiated in the subsurface The structure of the Abu Roash area is complex with frequent folding and faulting. The faults dissect different rock umts of different ages which range from Upper Cenomanian to the Quaternary. They are classified into three categories, normal, reverse and strike-shp faults The Abu Roash Massif is also characterized by heterogeneous fold styles with different directions. The folds are plunging anticlines and synclmes oriented In a NE-SW dtrectaon with the excepuon of the Tel E1Mabsuta folds which plunge to the WNW. The nor theast trending folds of the area resulted from the combination of compressional stresses initiated from wrenctung in addition to arclung of the basement. These folds are believed to have developed during the Late Cretaceous - Early Eocene time
The p u r p o s e of this p a p e r is to a n a l y s e the s t r u c t u r a l p a t t e r n of t h e district b y u s i n g t h e n e w r o c k u n i t s in o r d e r to predict t h e s t r e s s m o d e l of the area.
INTRODUCTION
The N o r t h e r n p a r t of t h e W e s t e r n D e s e r t f o r m s a n a l m o s t f e a t u r e l e s s plain which, with t h e exception of t h e A b u R o a s h area, 10 k m to t h e s o u t h w e s t of Cairo (Fig. l), offers few geological f e a t u r e s t h a t w o u l d reflect its intricate geological history. The b e s t c o m p l e t e U p p e r C r e t a c e o u s s e c t i o n is e n c o u n t e r e d in t h e s u b s u r f a c e , a n d the only s u r f a c e e x p o s u r e in t h e N o r t h e r n W e s t e r n D e s e r t is r e c o r d e d in A b u R o a s h area. The s t u d i e d s u r f a c e a n d s u b s u r f a c e U p p e r C r e t a c e o u s s u c c e s s i o n s were t h e s u b j e c t of v a r i o u s classifications a n d n o m e n c l a t u r e s . The stratigraphic aim of t h e p r e s e n t w o r k is to correlate a n d to m a t c h t h e s u b s u r f a c e stratigraphic r o c k u n i t s with their s u r f a c e c o u n t e r p a r t s o n t h e b a s i s of t h e Code of S t r a t i g r a p h i c N o m e n c l a t u r e (Hedberg, 1961, a n d 1970). Accordingly, the s u r f a c e s e c t i o n is d i f f e r e n t i a t e d into two f o r m a t i o n s of w h i c h t h e lower one w a s differentiated into s e v e n m e m b e r s (Fig. 2). E n e c h e l o n folds are visually t h e m o s t s p e c t a c u lar f e a t u r e s of A b u R o a s h s t r u c t u r e s . The folds vary in size, form a n d m a g n i t u d e . F u r t h e r m o r e , t h e y a s s u m e different orientations a n d configurations. The folds are d i s s e c t e d b y all t y p e s of en e c h e l o n f a u l t s with different trends.
CRETACEOUS STRATIGRAPHY OF ABU ROASH DISTRICT
The U p p e r C r e t a c e o u s s e c t i o n r e c o r d e d at A b u R o a s h a r e a is d i f f e r e n t i a t e d into K h o m a n F o r m a t i o n on the top a n d A b u R o a s h F o r m a t i o n at t h e b a s e (Fig. 2). The t h i n n i n g of t h e f o r m a t i o n s at the A b u R o a s h o u t c r o p is m o s t p r o b a b l y d u e to the location of the a r e a on the f l a n k s of the N a t r u n d e p o c e n t e r during the J u r a s s i c - C r e t a c e o u s time. K h o m a n F o r m a t i o n (Maestriehtian-Campanian) This f o r m a t i o n w a s firstly i n t r o d u c e d b y Norton (1967). The d e s i g n a t e d t y p e s e c t i o n is located at /kin K h o m a n , s o u t h w e s t of B a h a r l y a Oasis. In ~ts type locality, t h e f o r m a t i o n c o n s i s t s of m o r e t h a n 50 m e t e r s of highly fossflfferous, f r a c t u r e d c h a l k y Imaestone. The K h o m a n F o r m a t i o n w a s s u b divided b y A a d l a n d a n d H a s s a n (1972), a s u p p e r K h o m a n "A" a n d lower K h o m a n "B" m e m b e r s . The u p p e r one is m a d e u p of c h a l k y limestone while t h e lower one c o n s i s t s of s h a l e a n d calcar e o u s siltstone. According to B a r a k a t e t al. (1984), Abdel H a m i d (1985) a n d G h a n e m (1985), the
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Abu Roash F o r m a t i o n (Lower Senonian-Upper Cenomanian} The n a m e is t a k e n from Abu Roash area as a type locality (Norton, 1967). He gave neither details nor did he describe a type section. Aadland and H a s s a n (1972) divided Abu Roash Formation into seven m e m b e r s designated from top to base: "A, B, C, D, E, F and G'. The formation at the outcrop c a n be directly correlated with the "A" to "G" m e m b e r s differentiated in the subsurface. The h o m o g e n e o u s chalky limestone of the Khoman Formation c o n t r a s t s with the interbedded limestone and shale of the Abu Roash Formation, but the contact between the two formations is hidden by the advanced erosion of the soft lithology of unit 'Sl~'.
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K h o m a n "A" m e m b e r is raised to the formation r a n k a n d the n a m e K h o m a n is retained for the Chalk, while the Khoman "B" m e m b e r merges lithologically into the underlying Abu Roash Formation (i.e. "A" member). In the present work, the classification cited by those a u t h o r s was used a n d applied. The K h o m a n Formation at Abu Raosh area consists of chalky limestone with c o m m o n chert b a n d s at its u p p e r part. The chalk is snow white to light greyish white a n d dolomitized, especially along the joints a n d fault traces. Petrographically, this chalk is homogeneous, microcrystalline to cryptocrystalline with foraminiferal tests. These indicate deposition u n d e r comparatively quiet open marine environment.
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moderately h a r d in some cases. The limestone is highly argillaceous, glauconitic and s a n d y in the lower parts, being dolomitized partially. The occurrence of the bryozoan a c c u m u l a t i o n s a n d the predominance of the pellets, intraclasts a n d terrigenous s a n d grains within the carbonates indicates a shallow shelf marine conditions. Abu Roash "B" m e m b e r (Upper Tu~onian). In Abu Roash, this m e m b e r attains about 35 m thick and is considered as a good m a r k e r unit. The unit consists of thick limestone with alternating thin flint b a n d s at the upper part. The limestone is white, light grey, greyish white, crystalline to microcrystalline, chalky and argillaceous in some parts. The lower part is highly argillaceous, s a n d y with pellets a n d intraclasts being gypsiferous limestone. This aspect indicates a shallow shelf marine environment. Abu Roash "C" m e m b e r (Turonian). This unit attains about 60 m thick in the area where it
Structural history of Abu Roash district, Western Desert, Egypt consists of shale, limestone and a porous sandstone. The shale is soft, green, olive green,, flaky a n d calcareous in some parts. The limestone is highly fossiliferous, white, yellowish white, a n d sandy. At the middle part of this unit, it b e c o m e s cherty. The predominance of the bryozoan remains and the presence of the calcarenltes indicates a shallow shelf marine (sublittoral) environment. Also, the Durania biostrome at El H a s s a n a , is digitized by a red silt b e d s denoting a very shallow water conditions. Abu R o a s h "D" m e m b e r (Turonlan). This m e m b e r forms the high cliffs and ridges at Abu Roash area. The unit consists of a thick limestone s u c c e s s i o n with thin s a n d and claystone streaks at the top. The limestone is white, tan, brownish, massive, hard, highly Jointed, m e d i u m to fine crystalline, dolomitized and cherty at its base. The m a c r o f a u n a l c o n t e n t (e.g. Acteonella salamonis, Nerinea requieniona, etc.) with its lithofacies associations indicate a shallow shelf conditions with c o n t i n u o u s supply of terrigenous clastics at the end of deposition. Also, the presence of the interbedded dolomites could indicate lagoonal conditions. Abu R o a s h "E" m e m b e r (Turonian). This unit attains 28.5 m t h i c k n e s s and is composed of reddish b r o w n s a n d s t o n e followed b y yellowish green argillaceous limestone, white limestone, yellowish b r o w n clay, white limestone and yellowish brown limestone at the b a s e which is sandy, saliferous and gypsiferous. The formation w a s deposited u n d e r a shallow shelf marine environment which Is semirestricted at the begining of deposition. Abu R o a s h "F" m e m b e r (Lower TuronlanUpper C e n o m a n i a n ) . This unit is composed of 41.5 m thick of b e d d e d limestone, white, tan, brownish, fine crystalline, hard, soft, highly argillaceous in the lower and middle parts and cherty at the u p p e r part. There are thin streaks of claystone interbedded within the thick limestone at the u p p e r part. The presence of foraminiferid tests at the lower part indicates the deposition u n d e r a quiet open marine environment and the presence of the Oyster b e d and the predominance of pellets, intraclasts and the glauconite within the c a r b o n a t e s of the u p p e r part could suggest shallowing upwards. Also, the occurrence of detrital limestone indicates a very shallow shelf marine conditions for this u p p e r part. The age ofAbu Roash "F" m e m b e r is controversial and the faunal assemblage h a s a transitional character. Even the Cyphosoma abatei and Ostrea flabellata are not proper Cenomanian fossils, in other regions, it is t a k e n as a m a r k e r of the b a s e
437
Turonian (Said, 1962). According to F a h m y (P.C.), the facies of the lower beds, is quite correlatable with the s u b s u r f a c e C e n o m a n i a n section at Abu Gharadig basin. Abu R o a s h "G" m e m b e r (Upper C e n o m a n i a n ) . The surface section c a n be c o m p a r e d with the upper part of the s u b s u r f a c e unit "G" and It is c o m p o s e d of reddish b r o w n sandstone, and white, t a n brownish, sandy, argillaceous, occasionally dolomitic limestone, Interbedded with g y p s e o u s greyish green shale, green siltstone and gypseous, green, brownish, s a n d y claystone. Generally, the clastic ratio increases d o w n w a r d s as encountered in the subsurface. The predominance of the bioturbated beds, the Pelecypods (Hipurites sp. and Duraniasp.), and the glauconltic sandstone denote a deposition u n d e r a shallow marine to lagoonal environment. STRUCTURAL PATTERN OF THE ABU ROASH UPLIFT The Abu Roash folded and faulted district reflects the intricate structural p a t t e r n s of the Northern W e s t e m Desert that are concealed u n d e r the younger sediments. Folds are visually the m o s t spectacular of the Abu Roash s t r u c t u r e s and display the strain effect. Glran E1 Ful-Gebel E1 Higaf a n t i c l i n e This anticline is located in the s o u t h e a s t part of the district (Fig. 3). The fold is a gentle, asymmetric anticline with a s o u t h w e s t plunge and the axial plane dips to the northwest direction. The s o u t h e r n limb is represented by Giran El Ful and the n o r t h e m limb is represented b y Gebel El Hiqaf. Wadi El Talun r u n s along the deeply d e n u d e d core of the anticline which forms a V-Shape depression. The fold limbs are b r e a c h e d b y several transverse normal faults of almost northwest direction, b u t with ENE direction at the s o u t h w e s t e r n part of Gebel El Hiqaf. The faults form a northwest large graben in Giran El Ful limb, perpendicular to the fold axis. The transverse faults are responsible for the missing o f A b u Roash "G, F, and E" m e m b e r s in the n o r t h e a s t part of Gebel El Hiqaf. Also, the NW and NNW transverse faults in the n o r t h e a s t e r n part of Gebel El Hiqaf form at least three major northwest trending warped folds by dragging mechanism. There is another p r o n o u n c e d small double plunging syncline, a b o u t 150 m in diameter, b o u n d e d b y two NNW oriented normal faults. The anticline is affected by a spectacular NE dextral strike-slip fault which r u n s subparallel to the fold axls. The fault is characterized b y a vertical relief and the dip-slip c o m p o n e n t vanished toward the closure of the fold.
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Fig. 3. Stratigraphlc and structural map ofAbu Roash district. Gebel El Hiqaf, w h i c h f a c e s Wadi El T a l u n in t h e s o u t h w e s t e r n p a r t along t h e low reliefed foot-hills for t h e m o s t part, is formed of U p p e r C e n o m a n i a n rocks. The m o s t s p e c t a c u l a r of t h e s e hillocks is a n antlform with s h a l e core a n d n o r t h w e s t axas (Fig. 3). The a r e a r e p r e s e n t e d b y the antfform is d i s t u r b e d b y several t r a n s v e r s e faults, s o m e of t h e m are h i d d e n b y superficial R e c e n t deposits. T h e s e f a u l t s c o u l d b e r e s p o n s i b l e for t h e mobilization of t h e I n c o m p e t e n t clastic s e d i m e n t s to migrate t o w a r d s t h e region of less p r e s s u r e , i.e. into t h e core of t h e m a j o r anticline. Sidr El K h a m i s s y n e l i n e The syncline is located to t h e WNW of Gebel El Hiqaf (Fig. 3) The fold is a gentle syncline with a s o u t h w e s t p l u n g e direction. The syncline is a s y m metric with d i s p l a c e d a n d w a r p e d axis a n d is b r e a c h e d b y s o m e longitudinal a n d t r a n s v e r s e n o r m a l faults. The fold axis is d i s p l a c e d a n d highly r o t a t e d b y two dextral strike-slip faults intersecting at a b o u t 3 0 °, a n d f o r m s synclinal fold with a NE plunge. The syncline h a s b e e n r e s u l t e d from t h e horizontal c o m p r e s s i o n along the two intersecttng strlke-slip faults. El Ghigiga a n t i c l i n e a n d t h e related en e c h e l o n folds It is located to t h e n o r t h of Sidr El Khamis. El
Ghigiga s t r u c t u r e is a p a r t of a m a j o r anticline dislocated a n d r o t a t e d b y en e c h e l o n left-stepping dextral strike-slip faults (Fig. 3). There are two a n t i c h n e s e x p o s e d on the s u r f a c e b e s i d e El Ghigiga anticline. El Ghlglga f o r m s a big circular arc with a cons p i c o u s c u l m i n a t e d fiat-topped surface. The anticline Is a s y m m e t r i c with a SW p l u n g e a n d displaced fold axis. The middle p a r t of the fold is dist u r b e d b y longitudinal radial n o r m a l g r o u p of faults forming a graben, h o r s t a n d s t e p s y s t e m s . The s t r u c t u r e of El Ghigiga is b r e a c h e d in the s o u t h w e s t e r n p a r t b y a t r a n s v e r s e overlaped right lateral strike-slip faults of NW direction c a u s i n g the dislocation of t h e fold axis in a dextral m a n n e r with rotation of the a d j a c e n t b e d s . There are two d e c k - c a r d s t r u c t u r e s located on t h e d o w n t h r o w n side of NNE fault. The anticline is d i s t u r b e d b y a m a j o r high angle strlke-slip fault r u n n i n g in a WNW direction. The fault h a s a s t e e p s c a r p t o w a r d Wadi El Q a r n a n d along w h i c h t h e rest of El Ghigiga is dislocated a b o u t 2.7 k m in a dextral m a n n e r . The dislocated anticline is b r e a c h e d b y several en echelon dextral strike-slip f a u l t s of a l m o s t WNW a n d E-W t r e n d with f r e q u e n t folding of the dist u r b e d beds. The e a s t e r n p a r t 6f t h e fold is elimin a t e d b y a NW maj or fault. The n o r t h e r n p a r t of the fold is facing El H a s s a n a s t r u c t u r e with a steep
Sta-uctural history of Abu Roash distract, Western Desert, Egypt relief controlled b y dextral strike-slip fault along w h i c h t h e r e s t of t h e fold is d i s l o c a t e d a b o u t 2.2 kin to t h e east. The third anticline r e s e m b l e s exactly the style of t h e last one. The e a s t e r n p a r t of t h e fold is m i s s e d b y a NE fault a n d t h e n o r t h e r n r e m n a n t of t h e fold is d i s l o c a t e d right laterally a n d h i d d e n u n d e r t h e Quaternary sediments. Tel E1 M a b s u t a f o l d s The a r e a b e t w e e n El Ghiglga a n d Tel El M a b s u t a is a c c o m p a n i e d b y t h r e e anticlines, developed b y the c o m p r e s s i o n a s s o c i a t e d with the El GhigigaTel El M a b s u t a strike-slip fault (Fig. 3). Tel El M a b s u t a folds are t h e only folds, developed in t h e Late E o c e n e tIme. The s t r u c t u r e is a gentle anticline a n d a syncline to t h e s o u t h . Both folds t r e n d in a WNW direction w h i c h is t h e only different t r e n d of folds in the district. Tel El M a b s u t a s t r u c t u r e is b o u n d e d from t h e n o r t h e m p a r t b y t h e m a j o r right lateral strike-slip fault w h i c h d i s t u r b El Ghigiga anticline. There is a n o t h e r o b v i o u s s e t of parallel strike-slip faults wlth dextral c o m p o n e n t . T h e s e faults c u t along the ductile r o c k s o f A b u R o a s h "A" m e m b e r exhibiting Z - s h a p e d a s y m m e t r i c a l drag folds indicating right h a n d e d simple shear. The origin of Tel El M a b s u t a folds is believed to b e s e c o n d o r d e r drag folds initiated from t h e m a j o r WNW dextral strike-slip faults t o g e t h e r with clockwise internal rotation a n d sigmoidal dragging along c l o s e - s p a c e d dextral strike-slip faults of t h e area. Wadl E1 Qarn a n t i c l i n e It o c c u p i e s a wide valley extending a b o u t 5 k m from n o r t h e a s t to s o u t h w e s t . There are two promin e n t small n e i g h b o u r i n g conical d a r k ' d o l o m i t e hillocks n a m e d El Qulei developed at t h e inters e c t i o n of two o b v i o u s strike-slip faults (Fig. 3). Wadi El Q a r n is a gentle, slightly a s y m m e t r i c anticline with NE p l u n g e direction. The m a i n shell of t h e s t r u c t u r e is t h e K h o m a n C h a l k a n d A b u R o a s h "A" m e m b e r w h i c h is e x p o s e d b y a sinistral fault at t h e c l o s u r e of t h e fold. In s o m e places, K h o m a n C h a l k is c u t b y different calcite veins a n d dolomite ridgelets. T h e s e f e a t u r e s m a r k t h e position ofj oint z o n e s a n d fault lines in m o s t cases. The m o s t I m p o r t a n t g r o u p of faults are a NE en echelon strike-slip faults. There are two distinctive drag folds a s s o c i a t e d with one strike-slip fault of NE direction. The folds are a s y m m e t r i c anticline a n d syncline with SW plunge a n d the fold axes m a k e a c u t e angle with t h e fault line (Fig. 3). T h e s e folds m a y b e originated b y the c o m p r e s s i o n a l s t r e s s e s a s s o c i a t e d with wrenching. Gebel A b u R o a s h a n t i c l i n e The structure is a s y m m e t r i c ,
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elongated, d o u b l e plunging anticline. There ls a n a r r o w asyrnrnetrica] syncline b e t w e e n Wadl E1 Q a r n a n d Gebel A b u Roash. Tear f a u l t s are responsible for t h e w a r p i n g of t h e fold axis a n d t h e anticline suffered from horizontal dislocations along t h e s e faults. The middle p a r t of t h e anticline is b r e a c h e d b y a curvilinear dextral strike-slip fault w h i c h e x t e n d s till the e a s t e r n p o r t i o n of E1 Mldauwara. The m o s t distinctive strike-sllp fault is t h e fault of NW t r e n d w h i c h c u t s a c r o s s the s o u t h w e s t e m n o s e of t h e fold. The fold axis is dislocated a n d r o t a t e d along this fault in a dextral fashion. The fault line s e p a r a t e s El H a s s a n a , to the west, from the m a i n anticline (Fig. 3). The c o n s t r a i n e d area b e t w e e n El H a s s a n a a n d t h e two related en echelon anticlines of El Ghigiga displays a syncline of SW plunge. It is s u g g e s t e d t h a t t h e syncline c o u l d b e g e n e r a t e d b y local reorientation of t h e s h e a r s t r e s s field t h a t accomp a n l e d the strike-slip f a u l t s in the area. E1 M l d a u w a r a a n t l c U n e It is a highly b r e a c h e d a s y m m e t r i c anticline with SW plunge. There is no visible s y n f o r m b e t w e e n Gebel A b u R o a s h anticline a n d El M i d a u w a r a anticline. Also, t h e area o c c u p i e d b y the two anticlines is c h a r a c t e r i z e d b y i n c o n s i s t e n t stratigraphic p a t t e r n (Fig. 3). The a u t h o r s s u g g e s t t h a t a m a j o r b u r i e d fault o c c u r s b e t w e e n t h e two anticlines parallel to El G a a s c a r p a n d is r e s p o n s i b l e for c o n s u m i n g t h e s y n f o r m b e t w e e n t h e two anticlines (Fig. 4). There is o b v i o u s t r e n c h trace s c a r p s in the area a r o u n d the fault. The core of t h e m a i n anticline h a s a n o b v i o u s group of small anticlines a n d s y n c l i n e s wlth their long axes parallel to t h e m a i n fold axis. The origin of t h e s e folds is related to t h e m e c h a n i s m of the m a i n fold. The i n d u c e d s t r e s s t h r o u g h the b e d d i n g p l a n e s w a s t h e m a i n c a u s e for t h e formation of t h e s e m e s o s c o p i c folds. -, '
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El M i d a u w a r a anticline is d e n s e l y t r a v e r s e d b y all t y p e s of faults. The m o s t significant is a t r a n s v e r s e o p e n c u r v e d dextral strike-sllp fault along w h i c h the rest of t h e anticline w a s shifted b e s i d e t h e village. The d i s l o c a t e d p a r t of t h e fold is c h a r a c t e r ized b y a reverse a n d t h r u s t f a u l t s parallel a n d s u b p a r a l l e l to t h e fold axis. The reverse faulting a n d t h r u s t i n g J u s t locate at the convex p a r t of t h e dextral strlke-slip fault. At this area, the fault b l o c k s m o v e obliquely, a n d this being a convergent w r e n c h i n g w h i c h t e n d s to e n h a n c e t h e c o m p r e s sive reverse a n d t h r u s t faults. S T R E S S MODEL OF ABU ROASH DISTRICT
The A b u R o a s h a r e a is aligned on t h e m a s t e r s h e a r tectonic belt cited b y Neev a n d Hall {1982) as t h e , P e l u s i u m M e g a s h e a r Line,. The s u r f a c e a n d s u b s u r f a c e d a t a s h o w s the a r e a a s a highly culmin a t e d s t r u c t u r e b o u n d e d b y two regional dextral strike-slip f a u l t s r u n n i n g in t h e NE directlon (Fig. 5). The m a c r o s t r u c t u r e s d e s c r i b e d in the area are in the form of NE a n d WNW en e c h e l o n folds dis-
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s e c t e d b y NE, NW a n d WNW n o r m a l faults, NE reverse a n d t h r u s t faults, a n d NE, NW a n d WNW c o n j u g a t e strike-slip faults. The a r r a n g e m e n t of t h e en echlelon folds a n d faults in the area s u g g e s t s t h a t t h e y are a w r e n c h zone r e s u l t e d b y shear stresses rather than the pure compressional forces. B a s e m e n t is involved in t h e d e f o r m a t i o n as indicated from the b a s e m e n t relief m a p a n d t h e seismic profiles of the area. The m a i n s t r u c t u r e s developed on t h e w r e n c h i n g physical m o d e l s are a s y s t e m o f e n e c h e l o n faults, t e r m e d "Riedle {R) shears", a n d their c o n j u g a t e s , the "Conjugate Riedle (R') s h e a r s " (Fig. 6). The R s h e a r intersects t h e trace of the m a s t e r w r e n c h fault at a b o u t 15 ° a n d the R' at a p p r o x i m a t e l y 75 °. The principal t r e n d ,Y s h e a r , f o r m s parallel to the m a s t e r fault, a n d a f o u r t h f u n d a m e n t a l s h e a r ,P shear,, s y m m e t r i c to t h e R s h e a r a c r o s s t h e m a s t e r fault is s o m e t i m e s o b s e r v e d (Tchalenko, 1970; Bartlett, et al., 1981 a n d J a m l s o n , 1983). The P s h e a r serves to c o n n e c t the e n d s of R shear, t h u s forming a c o n t i n u o u s irregular fault or forms Y S h a p e with the R shear. The R, P a n d Y s h e a r s exhibiting t h e s a m e s e n s e of d i s p l a c e m e n t a n d t h e
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Structural history of Abu Roash district, Western Desert, Egypt
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R' s h e a r is t h e only one w h i c h h a s the opposite s e n s e of dislocation. The en e c h e l o n folds are developed o n t h e s u r f a c e of t h e anisotropic physical m o d e l s ( c o m p o s e d of layers of highly c o n t r a s t i n g m e c h a n i c a l behaviour). The fold axes m a k e 30 ° 45 ° with t h e m a s t e r Y s h e a r trend. In t h e w r e n c h zones, folds form early in the d e f o r m a t i o n a n d are a c c o m p a n i e d or followed b y c o n j u g a t e strike-slip, reverse, a n d / or n o r m a l faults a n d t h e d e v e l o p m e n t of t h e m a i n strike-slip fault is t h e last stage in t h e w r e n c h zone d e f o r m a t i o n (Wilcox et al., 1973). T h e s e m a i n s t r u c t u r e s a s s o c i a t e d with the w r e n c h i n g of t h e p h y s i c a l m o d e l s are applied in natural deformation by Tckalenko and A m b r a s e y s , 1970 a n d b y J a m i s o n , 1983. The four fault s y s t e m s a n d t h e folds are well developed a n d d e m o n s t r a t e d at A b u R o a s h district {Fig. 6). The R s h e a r is t h e m o s t diagnostic s y s t e m i n A b u R o a s h a r e a a n d r e p r e s e n t e d b y a series of WNW long p e r s i s t e n t dextral e n echelon faults (Fig 6). The R' s h e a r a p p e a r s a s a small NW to NNW sinistral f a u l t s at t h e n o r t h e r n p a r t of t h e s t u d y area. The P s h e a r f o r m s a Y - s h a p e intersection p a t t e r n with t h e R s h e a r at El Qulei a n d ties a n o t h e r two R s h e a r f a u l t s forming one irregular fault (El Ghigiga - Tel El M a b s u t a fault). The m a s t e r Y s h e a r t r e n d is p r e s e n t e d b y two regional dextral faults b o u n d i n g the a r e a a n d oriented in a NE direction. Two Y s h e a r f a u l t s are also developed at Wadi El Q a r u n . The en e c h e l o n folds are visually very well d e m o n s t r a t e d at A b u R o a s h o u t c r o p a n d are s p e c t a c u l a r l y developed in all e x p o s u r e s . The a u t h o r s s u g g e s t that, in the early stage of t h e d i s p l a c e m e n t on t h e w r e n c h zone, the en echelon folds were initiated a n d e n h a n c e d in the Late C r e t a c e o u s time. As d e f o r m a t i o n proceeds, t h e NW a n d WNW strlke-sllp faults b r o k e the folds c o n c u r rent with the initiation of t h e NE dextral strike-slip faults. At a later stage, the folds have b e e n dislocated a n d r o t a t e d along t h e dextral NW a n d WNW strike-slip faults. The c o n j u g a t e strike-slip faults c o u l d be r o t a t e d in a clockwise a n d / o r anticlockwise with the c o n t i n u a t i o n of t h e lateral slip until the Q u a t e r n a r y time as evident from the slightly folding of t h e E o c e n e b e d s a n d t h e displaced Q u a t e r n a r y clastics. The dislocation along t h e two deeply s e a t e d dextral strike-slip z o n e s b o u n d i n g the district is i n d i c a t e d b y t h e d i s p l a c e m e n t of the Q u a t e r n a r y clastics (Fig. 5). The WNW folds of Tel El M a b s u t a have b e e n developed in t h e Late Eocene as a r e s u l t of t h e horizontal d i s p l a c e m e n t along the WNW dextral strike-slip faults. SUMMARY AND CONLUSION
The e x p o s e d U p p e r C r e t a c e o u s section at A b u
R o a s h exhibits t h e m a i n lithology of t h e equivalent s u b s u r f a c e section. Accordingly, t h e s u r f a c e s e c t i o n is differentiated into two f o r m a t i o n s a n d seven members. The s t u d y area is d i s t u r b e d b y NE, NW a n d WNW n o r m a l faults, NE reverse a n d t h r u s t faults, and NE, NW a n d WNW strike-slip faults. The faults dissect t h e rock u n i t s of different age w h i c h range from U p p e r C r e t a c e o u s to t h e Q u a t e r n a r y . The n o r m a l f a u l t s generally m a k e a c u t e angle with the direction of d i s p l a c e m e n t along the strikeslip faults a n d p e r p e n d i c u l a r to t h e fold axes (Fig. 6). The n o r m a l faults are originated b y tensile s t r e s s e s a s s o c i a t e d with wrenching. The reverse a n d t h r u s t faults are developed b y convergent w r e n c h i n g at t h e convex side of a strike-slip fault beside t h e village. The strike-slip faults are the s u r f a c e echo of t h e m a j o r NE dextral w r e n c h faults b o u n d i n g the s t u d y area (Fig. 6). The high order drag folds a s s o c i a t e d with strike-slip faults are a c o m m o n f e a t u r e on t h e all scales a n d s h o u l d be developed b y t h e horizontal c o m p r e s s i o n associated with s h e a r i n g along t h e faults. Generally, t h e fold axes m a k e divergent angle with the direction of dislocation along t h e a s s o c i a t e d m a s t e r strikeslip faults, a s in Wadi El Qarn; or are perpendicular to t h e m as t h e c a s e of Sidr El K h a m i s (Fig. 6}. The A b u R o a s h area is c h a r a c t e r i z e d b y a heterog e n e o u s fold style with NE a n d WNW directions. The origin of Tel El M a b s u t a folds is believed to be s e c o n d order drag folds initiated from the WNW dextral strike-slip faults a s s i s t e d b y t h e clockwise internal rotation a n d sigmoidal dragging along t h e s e faults. The NE t r e n d i n g en e c h e l o n folds of the s t u d y area have r e s u l t e d from the horizontal c o m p r e s sional s t r e s s e s a s s o c i a t e d with wrenching, a n d a s s i s t e d b y vertical radial s t r e s s e s initiated from the arching of t h e b a s e m e n t a n d t h e a s s o c i a t e d block faulting. REFERENCES
Aadland, A. J. and Hassan, A. A. 1972. Hydrocarbon PotenUal of the Abu Gharadig basin in the Western Desert, Egypt. 8th Arab Petrol. Cong., Algiers, No. 81 (B-3). Abdel Hamid, M. L. 1985. Contnbutaon to the Geology of Upper Cretaceous wlth special emphasis on TuronianSenonlan sedimentation patterns and hydrocarbon potentials in the Abu Gharagig Area, North Western Desert. Ph.D. Thesis, Carlo Univ., 189 pp. Barakat, M. G., Darwish, M. and Abdel Hamid, M. L. 1984. Detection and Delineation of Reservoir Potential within Abu Roash Formation, East Abu Gharadig Area, North Western Desert, Egypt. Abstracts of papers presented at the twenty-second Ann.,Meet., Geol. Soc. of Egypt, pp. I.
Structural history of Abu Roash district, Western Desert, Egypt Bartlett, W. L., Friedman, M. a n d Logan, J : M. 1981. Wrench faults in limestone layers. Tectonophysics, 70, 255- 277. G h a n e m , M. F. 1985. S u b s u r f a c e Geology of the C r e t a c e o u s s e d i m e n t s in the North Western Desert of Egypt a n d its h y d r o c a r b o n potentialities. Ph. D. Thesis, Fac. of Sci., Cairo Univ., 131 pp. Hedberg, H. D. 1961. Stratigraphic classification and t e r m i n o l o g y . 21st. Inter. Geol. Cong. Norden, Compenhagen, pt. 2, 7-38. Hedberg, H. D. 1970. Preliminary report on lithostratigraphic units. Inter. Geol. Cong.,Montreal, Canada. J a m i s o n , W. R., 1983. Wrench Tectonics: comp. ofstr. styles, p. 4, Amoco Production c o m p , Tulsa, Oklahoma.
443
Neev, D. a n d Hall, J. K. 1982. The p e l u s l u m Megashear s y s t e m across Africa a n d associated lineament swarms. Jour. Geoph. Res., 87 (B2), 1015-1030. Norton, P. 1967. Rock stratigraphic n o m e n c l a t u r e of the Western Desert, Egypt. G u p c o int. rept. 18 pp. Said, R. 1962. The GeologyofEgypt. NewYork, Elsevier, 377 pp. Tckalenko, T. S. 1970. Similarities between s h e a r zones of different magnitudes. GSA Bull, pp. 1625-1639. Tckalenko, T. S a n d Ambraseys, N. N. 1970. S t r u c t u r a l analysis of the D a s h t - e Bayaz (Iran) e a r t h q u a k e fractures. GSA BulL, 81, 41-60. Wilcox, R. E, Harding, T. P. andWeely, D. R. 1973. Basic Wrench Tectonics. AAPG. 57, 74-96, 16 figs.
I)899 4362/89 $3 00 + 0 00 Pergamon Press plc
Printed m Great Britain
Structural history of Abu Roash district, Western Desert, Egypt *M. L. ABDELKHALEK,*M. A. EL SHARKAWl, *M. DARWISH,**M. HAGRASand *A. S E H I M *Geology Dept., Faculty of Science, Cairo University, Glza **Bapetco Oll Company, Egypt Abstract - The Abu Roash district is located 10 km to the southwest of Cairo, and is geoloDcally significant because of its surface exposure of Upper Cretaceous rocks. It can be used as a stratigraphic and structural model for the Western Desert. The exposed Upper Cretaceous section is differentiated into two formations, these being the Khoman and Abu Roash Formations seen in the Wes tern Desert penetrations. The Abu Roash Formation has been studied in detail at outcrop and can be dtrecfly correlated, both lithologlcally and paleontologically, with the "A" to "G" members differentiated in the subsurface The structure of the Abu Roash area is complex with frequent folding and faulting. The faults dissect different rock umts of different ages which range from Upper Cenomanian to the Quaternary. They are classified into three categories, normal, reverse and strike-shp faults The Abu Roash Massif is also characterized by heterogeneous fold styles with different directions. The folds are plunging anticlines and synclmes oriented In a NE-SW dtrectaon with the excepuon of the Tel E1Mabsuta folds which plunge to the WNW. The nor theast trending folds of the area resulted from the combination of compressional stresses initiated from wrenctung in addition to arclung of the basement. These folds are believed to have developed during the Late Cretaceous - Early Eocene time
The p u r p o s e of this p a p e r is to a n a l y s e the s t r u c t u r a l p a t t e r n of t h e district b y u s i n g t h e n e w r o c k u n i t s in o r d e r to predict t h e s t r e s s m o d e l of the area.
INTRODUCTION
The N o r t h e r n p a r t of t h e W e s t e r n D e s e r t f o r m s a n a l m o s t f e a t u r e l e s s plain which, with t h e exception of t h e A b u R o a s h area, 10 k m to t h e s o u t h w e s t of Cairo (Fig. l), offers few geological f e a t u r e s t h a t w o u l d reflect its intricate geological history. The b e s t c o m p l e t e U p p e r C r e t a c e o u s s e c t i o n is e n c o u n t e r e d in t h e s u b s u r f a c e , a n d the only s u r f a c e e x p o s u r e in t h e N o r t h e r n W e s t e r n D e s e r t is r e c o r d e d in A b u R o a s h area. The s t u d i e d s u r f a c e a n d s u b s u r f a c e U p p e r C r e t a c e o u s s u c c e s s i o n s were t h e s u b j e c t of v a r i o u s classifications a n d n o m e n c l a t u r e s . The stratigraphic aim of t h e p r e s e n t w o r k is to correlate a n d to m a t c h t h e s u b s u r f a c e stratigraphic r o c k u n i t s with their s u r f a c e c o u n t e r p a r t s o n t h e b a s i s of t h e Code of S t r a t i g r a p h i c N o m e n c l a t u r e (Hedberg, 1961, a n d 1970). Accordingly, the s u r f a c e s e c t i o n is d i f f e r e n t i a t e d into two f o r m a t i o n s of w h i c h t h e lower one w a s differentiated into s e v e n m e m b e r s (Fig. 2). E n e c h e l o n folds are visually t h e m o s t s p e c t a c u lar f e a t u r e s of A b u R o a s h s t r u c t u r e s . The folds vary in size, form a n d m a g n i t u d e . F u r t h e r m o r e , t h e y a s s u m e different orientations a n d configurations. The folds are d i s s e c t e d b y all t y p e s of en e c h e l o n f a u l t s with different trends.
CRETACEOUS STRATIGRAPHY OF ABU ROASH DISTRICT
The U p p e r C r e t a c e o u s s e c t i o n r e c o r d e d at A b u R o a s h a r e a is d i f f e r e n t i a t e d into K h o m a n F o r m a t i o n on the top a n d A b u R o a s h F o r m a t i o n at t h e b a s e (Fig. 2). The t h i n n i n g of t h e f o r m a t i o n s at the A b u R o a s h o u t c r o p is m o s t p r o b a b l y d u e to the location of the a r e a on the f l a n k s of the N a t r u n d e p o c e n t e r during the J u r a s s i c - C r e t a c e o u s time. K h o m a n F o r m a t i o n (Maestriehtian-Campanian) This f o r m a t i o n w a s firstly i n t r o d u c e d b y Norton (1967). The d e s i g n a t e d t y p e s e c t i o n is located at /kin K h o m a n , s o u t h w e s t of B a h a r l y a Oasis. In ~ts type locality, t h e f o r m a t i o n c o n s i s t s of m o r e t h a n 50 m e t e r s of highly fossflfferous, f r a c t u r e d c h a l k y Imaestone. The K h o m a n F o r m a t i o n w a s s u b divided b y A a d l a n d a n d H a s s a n (1972), a s u p p e r K h o m a n "A" a n d lower K h o m a n "B" m e m b e r s . The u p p e r one is m a d e u p of c h a l k y limestone while t h e lower one c o n s i s t s of s h a l e a n d calcar e o u s siltstone. According to B a r a k a t e t al. (1984), Abdel H a m i d (1985) a n d G h a n e m (1985), the
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Abu Roash F o r m a t i o n (Lower Senonian-Upper Cenomanian} The n a m e is t a k e n from Abu Roash area as a type locality (Norton, 1967). He gave neither details nor did he describe a type section. Aadland and H a s s a n (1972) divided Abu Roash Formation into seven m e m b e r s designated from top to base: "A, B, C, D, E, F and G'. The formation at the outcrop c a n be directly correlated with the "A" to "G" m e m b e r s differentiated in the subsurface. The h o m o g e n e o u s chalky limestone of the Khoman Formation c o n t r a s t s with the interbedded limestone and shale of the Abu Roash Formation, but the contact between the two formations is hidden by the advanced erosion of the soft lithology of unit 'Sl~'.
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K h o m a n "A" m e m b e r is raised to the formation r a n k a n d the n a m e K h o m a n is retained for the Chalk, while the Khoman "B" m e m b e r merges lithologically into the underlying Abu Roash Formation (i.e. "A" member). In the present work, the classification cited by those a u t h o r s was used a n d applied. The K h o m a n Formation at Abu Raosh area consists of chalky limestone with c o m m o n chert b a n d s at its u p p e r part. The chalk is snow white to light greyish white a n d dolomitized, especially along the joints a n d fault traces. Petrographically, this chalk is homogeneous, microcrystalline to cryptocrystalline with foraminiferal tests. These indicate deposition u n d e r comparatively quiet open marine environment.
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moderately h a r d in some cases. The limestone is highly argillaceous, glauconitic and s a n d y in the lower parts, being dolomitized partially. The occurrence of the bryozoan a c c u m u l a t i o n s a n d the predominance of the pellets, intraclasts a n d terrigenous s a n d grains within the carbonates indicates a shallow shelf marine conditions. Abu Roash "B" m e m b e r (Upper Tu~onian). In Abu Roash, this m e m b e r attains about 35 m thick and is considered as a good m a r k e r unit. The unit consists of thick limestone with alternating thin flint b a n d s at the upper part. The limestone is white, light grey, greyish white, crystalline to microcrystalline, chalky and argillaceous in some parts. The lower part is highly argillaceous, s a n d y with pellets a n d intraclasts being gypsiferous limestone. This aspect indicates a shallow shelf marine environment. Abu Roash "C" m e m b e r (Turonian). This unit attains about 60 m thick in the area where it
Structural history of Abu Roash district, Western Desert, Egypt consists of shale, limestone and a porous sandstone. The shale is soft, green, olive green,, flaky a n d calcareous in some parts. The limestone is highly fossiliferous, white, yellowish white, a n d sandy. At the middle part of this unit, it b e c o m e s cherty. The predominance of the bryozoan remains and the presence of the calcarenltes indicates a shallow shelf marine (sublittoral) environment. Also, the Durania biostrome at El H a s s a n a , is digitized by a red silt b e d s denoting a very shallow water conditions. Abu R o a s h "D" m e m b e r (Turonlan). This m e m b e r forms the high cliffs and ridges at Abu Roash area. The unit consists of a thick limestone s u c c e s s i o n with thin s a n d and claystone streaks at the top. The limestone is white, tan, brownish, massive, hard, highly Jointed, m e d i u m to fine crystalline, dolomitized and cherty at its base. The m a c r o f a u n a l c o n t e n t (e.g. Acteonella salamonis, Nerinea requieniona, etc.) with its lithofacies associations indicate a shallow shelf conditions with c o n t i n u o u s supply of terrigenous clastics at the end of deposition. Also, the presence of the interbedded dolomites could indicate lagoonal conditions. Abu R o a s h "E" m e m b e r (Turonian). This unit attains 28.5 m t h i c k n e s s and is composed of reddish b r o w n s a n d s t o n e followed b y yellowish green argillaceous limestone, white limestone, yellowish b r o w n clay, white limestone and yellowish brown limestone at the b a s e which is sandy, saliferous and gypsiferous. The formation w a s deposited u n d e r a shallow shelf marine environment which Is semirestricted at the begining of deposition. Abu R o a s h "F" m e m b e r (Lower TuronlanUpper C e n o m a n i a n ) . This unit is composed of 41.5 m thick of b e d d e d limestone, white, tan, brownish, fine crystalline, hard, soft, highly argillaceous in the lower and middle parts and cherty at the u p p e r part. There are thin streaks of claystone interbedded within the thick limestone at the u p p e r part. The presence of foraminiferid tests at the lower part indicates the deposition u n d e r a quiet open marine environment and the presence of the Oyster b e d and the predominance of pellets, intraclasts and the glauconite within the c a r b o n a t e s of the u p p e r part could suggest shallowing upwards. Also, the occurrence of detrital limestone indicates a very shallow shelf marine conditions for this u p p e r part. The age ofAbu Roash "F" m e m b e r is controversial and the faunal assemblage h a s a transitional character. Even the Cyphosoma abatei and Ostrea flabellata are not proper Cenomanian fossils, in other regions, it is t a k e n as a m a r k e r of the b a s e
437
Turonian (Said, 1962). According to F a h m y (P.C.), the facies of the lower beds, is quite correlatable with the s u b s u r f a c e C e n o m a n i a n section at Abu Gharadig basin. Abu R o a s h "G" m e m b e r (Upper C e n o m a n i a n ) . The surface section c a n be c o m p a r e d with the upper part of the s u b s u r f a c e unit "G" and It is c o m p o s e d of reddish b r o w n sandstone, and white, t a n brownish, sandy, argillaceous, occasionally dolomitic limestone, Interbedded with g y p s e o u s greyish green shale, green siltstone and gypseous, green, brownish, s a n d y claystone. Generally, the clastic ratio increases d o w n w a r d s as encountered in the subsurface. The predominance of the bioturbated beds, the Pelecypods (Hipurites sp. and Duraniasp.), and the glauconltic sandstone denote a deposition u n d e r a shallow marine to lagoonal environment. STRUCTURAL PATTERN OF THE ABU ROASH UPLIFT The Abu Roash folded and faulted district reflects the intricate structural p a t t e r n s of the Northern W e s t e m Desert that are concealed u n d e r the younger sediments. Folds are visually the m o s t spectacular of the Abu Roash s t r u c t u r e s and display the strain effect. Glran E1 Ful-Gebel E1 Higaf a n t i c l i n e This anticline is located in the s o u t h e a s t part of the district (Fig. 3). The fold is a gentle, asymmetric anticline with a s o u t h w e s t plunge and the axial plane dips to the northwest direction. The s o u t h e r n limb is represented by Giran El Ful and the n o r t h e m limb is represented b y Gebel El Hiqaf. Wadi El Talun r u n s along the deeply d e n u d e d core of the anticline which forms a V-Shape depression. The fold limbs are b r e a c h e d b y several transverse normal faults of almost northwest direction, b u t with ENE direction at the s o u t h w e s t e r n part of Gebel El Hiqaf. The faults form a northwest large graben in Giran El Ful limb, perpendicular to the fold axis. The transverse faults are responsible for the missing o f A b u Roash "G, F, and E" m e m b e r s in the n o r t h e a s t part of Gebel El Hiqaf. Also, the NW and NNW transverse faults in the n o r t h e a s t e r n part of Gebel El Hiqaf form at least three major northwest trending warped folds by dragging mechanism. There is another p r o n o u n c e d small double plunging syncline, a b o u t 150 m in diameter, b o u n d e d b y two NNW oriented normal faults. The anticline is affected by a spectacular NE dextral strike-slip fault which r u n s subparallel to the fold axls. The fault is characterized b y a vertical relief and the dip-slip c o m p o n e n t vanished toward the closure of the fold.
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Fig. 3. Stratigraphlc and structural map ofAbu Roash district. Gebel El Hiqaf, w h i c h f a c e s Wadi El T a l u n in t h e s o u t h w e s t e r n p a r t along t h e low reliefed foot-hills for t h e m o s t part, is formed of U p p e r C e n o m a n i a n rocks. The m o s t s p e c t a c u l a r of t h e s e hillocks is a n antlform with s h a l e core a n d n o r t h w e s t axas (Fig. 3). The a r e a r e p r e s e n t e d b y the antfform is d i s t u r b e d b y several t r a n s v e r s e faults, s o m e of t h e m are h i d d e n b y superficial R e c e n t deposits. T h e s e f a u l t s c o u l d b e r e s p o n s i b l e for t h e mobilization of t h e I n c o m p e t e n t clastic s e d i m e n t s to migrate t o w a r d s t h e region of less p r e s s u r e , i.e. into t h e core of t h e m a j o r anticline. Sidr El K h a m i s s y n e l i n e The syncline is located to t h e WNW of Gebel El Hiqaf (Fig. 3) The fold is a gentle syncline with a s o u t h w e s t p l u n g e direction. The syncline is a s y m metric with d i s p l a c e d a n d w a r p e d axis a n d is b r e a c h e d b y s o m e longitudinal a n d t r a n s v e r s e n o r m a l faults. The fold axis is d i s p l a c e d a n d highly r o t a t e d b y two dextral strike-slip faults intersecting at a b o u t 3 0 °, a n d f o r m s synclinal fold with a NE plunge. The syncline h a s b e e n r e s u l t e d from t h e horizontal c o m p r e s s i o n along the two intersecttng strlke-slip faults. El Ghigiga a n t i c l i n e a n d t h e related en e c h e l o n folds It is located to t h e n o r t h of Sidr El Khamis. El
Ghigiga s t r u c t u r e is a p a r t of a m a j o r anticline dislocated a n d r o t a t e d b y en e c h e l o n left-stepping dextral strike-slip faults (Fig. 3). There are two a n t i c h n e s e x p o s e d on the s u r f a c e b e s i d e El Ghigiga anticline. El Ghlglga f o r m s a big circular arc with a cons p i c o u s c u l m i n a t e d fiat-topped surface. The anticline Is a s y m m e t r i c with a SW p l u n g e a n d displaced fold axis. The middle p a r t of the fold is dist u r b e d b y longitudinal radial n o r m a l g r o u p of faults forming a graben, h o r s t a n d s t e p s y s t e m s . The s t r u c t u r e of El Ghigiga is b r e a c h e d in the s o u t h w e s t e r n p a r t b y a t r a n s v e r s e overlaped right lateral strike-slip faults of NW direction c a u s i n g the dislocation of t h e fold axis in a dextral m a n n e r with rotation of the a d j a c e n t b e d s . There are two d e c k - c a r d s t r u c t u r e s located on t h e d o w n t h r o w n side of NNE fault. The anticline is d i s t u r b e d b y a m a j o r high angle strlke-slip fault r u n n i n g in a WNW direction. The fault h a s a s t e e p s c a r p t o w a r d Wadi El Q a r n a n d along w h i c h t h e rest of El Ghigiga is dislocated a b o u t 2.7 k m in a dextral m a n n e r . The dislocated anticline is b r e a c h e d b y several en echelon dextral strike-slip f a u l t s of a l m o s t WNW a n d E-W t r e n d with f r e q u e n t folding of the dist u r b e d beds. The e a s t e r n p a r t 6f t h e fold is elimin a t e d b y a NW maj or fault. The n o r t h e r n p a r t of the fold is facing El H a s s a n a s t r u c t u r e with a steep
Sta-uctural history of Abu Roash distract, Western Desert, Egypt relief controlled b y dextral strike-slip fault along w h i c h t h e r e s t of t h e fold is d i s l o c a t e d a b o u t 2.2 kin to t h e east. The third anticline r e s e m b l e s exactly the style of t h e last one. The e a s t e r n p a r t of t h e fold is m i s s e d b y a NE fault a n d t h e n o r t h e r n r e m n a n t of t h e fold is d i s l o c a t e d right laterally a n d h i d d e n u n d e r t h e Quaternary sediments. Tel E1 M a b s u t a f o l d s The a r e a b e t w e e n El Ghiglga a n d Tel El M a b s u t a is a c c o m p a n i e d b y t h r e e anticlines, developed b y the c o m p r e s s i o n a s s o c i a t e d with the El GhigigaTel El M a b s u t a strike-slip fault (Fig. 3). Tel El M a b s u t a folds are t h e only folds, developed in t h e Late E o c e n e tIme. The s t r u c t u r e is a gentle anticline a n d a syncline to t h e s o u t h . Both folds t r e n d in a WNW direction w h i c h is t h e only different t r e n d of folds in the district. Tel El M a b s u t a s t r u c t u r e is b o u n d e d from t h e n o r t h e m p a r t b y t h e m a j o r right lateral strike-slip fault w h i c h d i s t u r b El Ghigiga anticline. There is a n o t h e r o b v i o u s s e t of parallel strike-slip faults wlth dextral c o m p o n e n t . T h e s e faults c u t along the ductile r o c k s o f A b u R o a s h "A" m e m b e r exhibiting Z - s h a p e d a s y m m e t r i c a l drag folds indicating right h a n d e d simple shear. The origin of Tel El M a b s u t a folds is believed to b e s e c o n d o r d e r drag folds initiated from t h e m a j o r WNW dextral strike-slip faults t o g e t h e r with clockwise internal rotation a n d sigmoidal dragging along c l o s e - s p a c e d dextral strike-slip faults of t h e area. Wadl E1 Qarn a n t i c l i n e It o c c u p i e s a wide valley extending a b o u t 5 k m from n o r t h e a s t to s o u t h w e s t . There are two promin e n t small n e i g h b o u r i n g conical d a r k ' d o l o m i t e hillocks n a m e d El Qulei developed at t h e inters e c t i o n of two o b v i o u s strike-slip faults (Fig. 3). Wadi El Q a r n is a gentle, slightly a s y m m e t r i c anticline with NE p l u n g e direction. The m a i n shell of t h e s t r u c t u r e is t h e K h o m a n C h a l k a n d A b u R o a s h "A" m e m b e r w h i c h is e x p o s e d b y a sinistral fault at t h e c l o s u r e of t h e fold. In s o m e places, K h o m a n C h a l k is c u t b y different calcite veins a n d dolomite ridgelets. T h e s e f e a t u r e s m a r k t h e position ofj oint z o n e s a n d fault lines in m o s t cases. The m o s t I m p o r t a n t g r o u p of faults are a NE en echelon strike-slip faults. There are two distinctive drag folds a s s o c i a t e d with one strike-slip fault of NE direction. The folds are a s y m m e t r i c anticline a n d syncline with SW plunge a n d the fold axes m a k e a c u t e angle with t h e fault line (Fig. 3). T h e s e folds m a y b e originated b y the c o m p r e s s i o n a l s t r e s s e s a s s o c i a t e d with wrenching. Gebel A b u R o a s h a n t i c l i n e The structure is a s y m m e t r i c ,
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439
elongated, d o u b l e plunging anticline. There ls a n a r r o w asyrnrnetrica] syncline b e t w e e n Wadl E1 Q a r n a n d Gebel A b u Roash. Tear f a u l t s are responsible for t h e w a r p i n g of t h e fold axis a n d t h e anticline suffered from horizontal dislocations along t h e s e faults. The middle p a r t of t h e anticline is b r e a c h e d b y a curvilinear dextral strike-slip fault w h i c h e x t e n d s till the e a s t e r n p o r t i o n of E1 Mldauwara. The m o s t distinctive strike-sllp fault is t h e fault of NW t r e n d w h i c h c u t s a c r o s s the s o u t h w e s t e m n o s e of t h e fold. The fold axis is dislocated a n d r o t a t e d along this fault in a dextral fashion. The fault line s e p a r a t e s El H a s s a n a , to the west, from the m a i n anticline (Fig. 3). The c o n s t r a i n e d area b e t w e e n El H a s s a n a a n d t h e two related en echelon anticlines of El Ghigiga displays a syncline of SW plunge. It is s u g g e s t e d t h a t t h e syncline c o u l d b e g e n e r a t e d b y local reorientation of t h e s h e a r s t r e s s field t h a t accomp a n l e d the strike-slip f a u l t s in the area. E1 M l d a u w a r a a n t l c U n e It is a highly b r e a c h e d a s y m m e t r i c anticline with SW plunge. There is no visible s y n f o r m b e t w e e n Gebel A b u R o a s h anticline a n d El M i d a u w a r a anticline. Also, t h e area o c c u p i e d b y the two anticlines is c h a r a c t e r i z e d b y i n c o n s i s t e n t stratigraphic p a t t e r n (Fig. 3). The a u t h o r s s u g g e s t t h a t a m a j o r b u r i e d fault o c c u r s b e t w e e n t h e two anticlines parallel to El G a a s c a r p a n d is r e s p o n s i b l e for c o n s u m i n g t h e s y n f o r m b e t w e e n t h e two anticlines (Fig. 4). There is o b v i o u s t r e n c h trace s c a r p s in the area a r o u n d the fault. The core of t h e m a i n anticline h a s a n o b v i o u s group of small anticlines a n d s y n c l i n e s wlth their long axes parallel to t h e m a i n fold axis. The origin of t h e s e folds is related to t h e m e c h a n i s m of the m a i n fold. The i n d u c e d s t r e s s t h r o u g h the b e d d i n g p l a n e s w a s t h e m a i n c a u s e for t h e formation of t h e s e m e s o s c o p i c folds. -, '
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El M i d a u w a r a anticline is d e n s e l y t r a v e r s e d b y all t y p e s of faults. The m o s t significant is a t r a n s v e r s e o p e n c u r v e d dextral strike-sllp fault along w h i c h the rest of t h e anticline w a s shifted b e s i d e t h e village. The d i s l o c a t e d p a r t of t h e fold is c h a r a c t e r ized b y a reverse a n d t h r u s t f a u l t s parallel a n d s u b p a r a l l e l to t h e fold axis. The reverse faulting a n d t h r u s t i n g J u s t locate at the convex p a r t of t h e dextral strlke-slip fault. At this area, the fault b l o c k s m o v e obliquely, a n d this being a convergent w r e n c h i n g w h i c h t e n d s to e n h a n c e t h e c o m p r e s sive reverse a n d t h r u s t faults. S T R E S S MODEL OF ABU ROASH DISTRICT
The A b u R o a s h a r e a is aligned on t h e m a s t e r s h e a r tectonic belt cited b y Neev a n d Hall {1982) as t h e , P e l u s i u m M e g a s h e a r Line,. The s u r f a c e a n d s u b s u r f a c e d a t a s h o w s the a r e a a s a highly culmin a t e d s t r u c t u r e b o u n d e d b y two regional dextral strike-slip f a u l t s r u n n i n g in t h e NE directlon (Fig. 5). The m a c r o s t r u c t u r e s d e s c r i b e d in the area are in the form of NE a n d WNW en e c h e l o n folds dis-
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s e c t e d b y NE, NW a n d WNW n o r m a l faults, NE reverse a n d t h r u s t faults, a n d NE, NW a n d WNW c o n j u g a t e strike-slip faults. The a r r a n g e m e n t of t h e en echlelon folds a n d faults in the area s u g g e s t s t h a t t h e y are a w r e n c h zone r e s u l t e d b y shear stresses rather than the pure compressional forces. B a s e m e n t is involved in t h e d e f o r m a t i o n as indicated from the b a s e m e n t relief m a p a n d t h e seismic profiles of the area. The m a i n s t r u c t u r e s developed on t h e w r e n c h i n g physical m o d e l s are a s y s t e m o f e n e c h e l o n faults, t e r m e d "Riedle {R) shears", a n d their c o n j u g a t e s , the "Conjugate Riedle (R') s h e a r s " (Fig. 6). The R s h e a r intersects t h e trace of the m a s t e r w r e n c h fault at a b o u t 15 ° a n d the R' at a p p r o x i m a t e l y 75 °. The principal t r e n d ,Y s h e a r , f o r m s parallel to the m a s t e r fault, a n d a f o u r t h f u n d a m e n t a l s h e a r ,P shear,, s y m m e t r i c to t h e R s h e a r a c r o s s t h e m a s t e r fault is s o m e t i m e s o b s e r v e d (Tchalenko, 1970; Bartlett, et al., 1981 a n d J a m l s o n , 1983). The P s h e a r serves to c o n n e c t the e n d s of R shear, t h u s forming a c o n t i n u o u s irregular fault or forms Y S h a p e with the R shear. The R, P a n d Y s h e a r s exhibiting t h e s a m e s e n s e of d i s p l a c e m e n t a n d t h e
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442
M. L. ABDELKHALEK,M. A. EL SHARKAWI,M. DARWISH,M. HAORASand A. SEHIM
R' s h e a r is t h e only one w h i c h h a s the opposite s e n s e of dislocation. The en e c h e l o n folds are developed o n t h e s u r f a c e of t h e anisotropic physical m o d e l s ( c o m p o s e d of layers of highly c o n t r a s t i n g m e c h a n i c a l behaviour). The fold axes m a k e 30 ° 45 ° with t h e m a s t e r Y s h e a r trend. In t h e w r e n c h zones, folds form early in the d e f o r m a t i o n a n d are a c c o m p a n i e d or followed b y c o n j u g a t e strike-slip, reverse, a n d / or n o r m a l faults a n d t h e d e v e l o p m e n t of t h e m a i n strike-slip fault is t h e last stage in t h e w r e n c h zone d e f o r m a t i o n (Wilcox et al., 1973). T h e s e m a i n s t r u c t u r e s a s s o c i a t e d with the w r e n c h i n g of t h e p h y s i c a l m o d e l s are applied in natural deformation by Tckalenko and A m b r a s e y s , 1970 a n d b y J a m i s o n , 1983. The four fault s y s t e m s a n d t h e folds are well developed a n d d e m o n s t r a t e d at A b u R o a s h district {Fig. 6). The R s h e a r is t h e m o s t diagnostic s y s t e m i n A b u R o a s h a r e a a n d r e p r e s e n t e d b y a series of WNW long p e r s i s t e n t dextral e n echelon faults (Fig 6). The R' s h e a r a p p e a r s a s a small NW to NNW sinistral f a u l t s at t h e n o r t h e r n p a r t of t h e s t u d y area. The P s h e a r f o r m s a Y - s h a p e intersection p a t t e r n with t h e R s h e a r at El Qulei a n d ties a n o t h e r two R s h e a r f a u l t s forming one irregular fault (El Ghigiga - Tel El M a b s u t a fault). The m a s t e r Y s h e a r t r e n d is p r e s e n t e d b y two regional dextral faults b o u n d i n g the a r e a a n d oriented in a NE direction. Two Y s h e a r f a u l t s are also developed at Wadi El Q a r u n . The en e c h e l o n folds are visually very well d e m o n s t r a t e d at A b u R o a s h o u t c r o p a n d are s p e c t a c u l a r l y developed in all e x p o s u r e s . The a u t h o r s s u g g e s t that, in the early stage of t h e d i s p l a c e m e n t on t h e w r e n c h zone, the en echelon folds were initiated a n d e n h a n c e d in the Late C r e t a c e o u s time. As d e f o r m a t i o n proceeds, t h e NW a n d WNW strlke-sllp faults b r o k e the folds c o n c u r rent with the initiation of t h e NE dextral strike-slip faults. At a later stage, the folds have b e e n dislocated a n d r o t a t e d along t h e dextral NW a n d WNW strike-slip faults. The c o n j u g a t e strike-slip faults c o u l d be r o t a t e d in a clockwise a n d / o r anticlockwise with the c o n t i n u a t i o n of t h e lateral slip until the Q u a t e r n a r y time as evident from the slightly folding of t h e E o c e n e b e d s a n d t h e displaced Q u a t e r n a r y clastics. The dislocation along t h e two deeply s e a t e d dextral strike-slip z o n e s b o u n d i n g the district is i n d i c a t e d b y t h e d i s p l a c e m e n t of the Q u a t e r n a r y clastics (Fig. 5). The WNW folds of Tel El M a b s u t a have b e e n developed in t h e Late Eocene as a r e s u l t of t h e horizontal d i s p l a c e m e n t along the WNW dextral strike-slip faults. SUMMARY AND CONLUSION
The e x p o s e d U p p e r C r e t a c e o u s section at A b u
R o a s h exhibits t h e m a i n lithology of t h e equivalent s u b s u r f a c e section. Accordingly, t h e s u r f a c e s e c t i o n is differentiated into two f o r m a t i o n s a n d seven members. The s t u d y area is d i s t u r b e d b y NE, NW a n d WNW n o r m a l faults, NE reverse a n d t h r u s t faults, and NE, NW a n d WNW strike-slip faults. The faults dissect t h e rock u n i t s of different age w h i c h range from U p p e r C r e t a c e o u s to t h e Q u a t e r n a r y . The n o r m a l f a u l t s generally m a k e a c u t e angle with the direction of d i s p l a c e m e n t along the strikeslip faults a n d p e r p e n d i c u l a r to t h e fold axes (Fig. 6). The n o r m a l faults are originated b y tensile s t r e s s e s a s s o c i a t e d with wrenching. The reverse a n d t h r u s t faults are developed b y convergent w r e n c h i n g at t h e convex side of a strike-slip fault beside t h e village. The strike-slip faults are the s u r f a c e echo of t h e m a j o r NE dextral w r e n c h faults b o u n d i n g the s t u d y area (Fig. 6). The high order drag folds a s s o c i a t e d with strike-slip faults are a c o m m o n f e a t u r e on t h e all scales a n d s h o u l d be developed b y t h e horizontal c o m p r e s s i o n associated with s h e a r i n g along t h e faults. Generally, t h e fold axes m a k e divergent angle with the direction of dislocation along t h e a s s o c i a t e d m a s t e r strikeslip faults, a s in Wadi El Qarn; or are perpendicular to t h e m as t h e c a s e of Sidr El K h a m i s (Fig. 6}. The A b u R o a s h area is c h a r a c t e r i z e d b y a heterog e n e o u s fold style with NE a n d WNW directions. The origin of Tel El M a b s u t a folds is believed to be s e c o n d order drag folds initiated from the WNW dextral strike-slip faults a s s i s t e d b y t h e clockwise internal rotation a n d sigmoidal dragging along t h e s e faults. The NE t r e n d i n g en e c h e l o n folds of the s t u d y area have r e s u l t e d from the horizontal c o m p r e s sional s t r e s s e s a s s o c i a t e d with wrenching, a n d a s s i s t e d b y vertical radial s t r e s s e s initiated from the arching of t h e b a s e m e n t a n d t h e a s s o c i a t e d block faulting. REFERENCES
Aadland, A. J. and Hassan, A. A. 1972. Hydrocarbon PotenUal of the Abu Gharadig basin in the Western Desert, Egypt. 8th Arab Petrol. Cong., Algiers, No. 81 (B-3). Abdel Hamid, M. L. 1985. Contnbutaon to the Geology of Upper Cretaceous wlth special emphasis on TuronianSenonlan sedimentation patterns and hydrocarbon potentials in the Abu Gharagig Area, North Western Desert. Ph.D. Thesis, Carlo Univ., 189 pp. Barakat, M. G., Darwish, M. and Abdel Hamid, M. L. 1984. Detection and Delineation of Reservoir Potential within Abu Roash Formation, East Abu Gharadig Area, North Western Desert, Egypt. Abstracts of papers presented at the twenty-second Ann.,Meet., Geol. Soc. of Egypt, pp. I.
Structural history of Abu Roash district, Western Desert, Egypt Bartlett, W. L., Friedman, M. a n d Logan, J : M. 1981. Wrench faults in limestone layers. Tectonophysics, 70, 255- 277. G h a n e m , M. F. 1985. S u b s u r f a c e Geology of the C r e t a c e o u s s e d i m e n t s in the North Western Desert of Egypt a n d its h y d r o c a r b o n potentialities. Ph. D. Thesis, Fac. of Sci., Cairo Univ., 131 pp. Hedberg, H. D. 1961. Stratigraphic classification and t e r m i n o l o g y . 21st. Inter. Geol. Cong. Norden, Compenhagen, pt. 2, 7-38. Hedberg, H. D. 1970. Preliminary report on lithostratigraphic units. Inter. Geol. Cong.,Montreal, Canada. J a m i s o n , W. R., 1983. Wrench Tectonics: comp. ofstr. styles, p. 4, Amoco Production c o m p , Tulsa, Oklahoma.
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Neev, D. a n d Hall, J. K. 1982. The p e l u s l u m Megashear s y s t e m across Africa a n d associated lineament swarms. Jour. Geoph. Res., 87 (B2), 1015-1030. Norton, P. 1967. Rock stratigraphic n o m e n c l a t u r e of the Western Desert, Egypt. G u p c o int. rept. 18 pp. Said, R. 1962. The GeologyofEgypt. NewYork, Elsevier, 377 pp. Tckalenko, T. S. 1970. Similarities between s h e a r zones of different magnitudes. GSA Bull, pp. 1625-1639. Tckalenko, T. S a n d Ambraseys, N. N. 1970. S t r u c t u r a l analysis of the D a s h t - e Bayaz (Iran) e a r t h q u a k e fractures. GSA BulL, 81, 41-60. Wilcox, R. E, Harding, T. P. andWeely, D. R. 1973. Basic Wrench Tectonics. AAPG. 57, 74-96, 16 figs.