Modes of structural evolution of Abu Gharadig Basin, Western desert of Egypt as deduced from seismic data

Journal of African Earth Sciences, Vol. 9, No. 2, pp. 273-287, 1989

0899-5362/89 $3.00 + 0.00 © 1990 Pergamon Press plc

Printed in Great Britain

Modes of structural evolution of Abu Gharadig Basin, Western Desert of Egypt as deduced from seismic data A. I. BAYOUMI*and H. I. LOTFV** *Department of Geophysics, Cairo University,Egypt ** GeologyDepartment, E1MinyaUniversity,Egypt Abstract - The present study,whichis mainlydevotedto the criticalanalysisof all availabledrill hole data and seismicreflectionrecords in the Abu GharadigBasinareawhichliesprincipallyto the east of the Qattara Depression in the central portion of the Northern Western Desert of Egypt, indicates that the Abu Gharadig Basin is an elliptical, multicyclic,E-W trending smacturallycontrolled basin developed originally as a result of the deep crustal extensional tectonics that affected Northern Egypt during the Mesozoic Era. The basin seems to have been initiatedby a localizedmantlebulge that led to the restricteddeep crustalextensionin the areaunder studyduring the earlyphases of the Alpinemovementswhichtookplace in LateJurassic.The basin was brought to its active maturation phase during the Cretaceous as a result of the large-scale extensional tectonics along two major bounding E-W trending listric growth faults.Tilting of the basin, took place duringthe UppermostCretaceous/EarlyTertiaryas a result of the compressivestresses (NWSE) that were associatedwith the Laramidephase during these times.

SUBSURFACE STRATIGRAPHY AND MORPHOLOGICAL EVOLUTION OF ABU GHARADIG BASIN

INTRODUCTION

The C r e t a c e o u s m a r i n e r o c k s of Abu G h a r a d i g Basin, w h i c h 11es principally to t h e e~st of t h e Q a t t a r a D e p r e s s i o n b e t w e e n l a t i t u d e s 29°00'N 0 ° 15'N a n d l o n g i t u d e s 27 ° 15'E- 9 ° 15'E (Fig. 1), are generally c o n s i d e r e d one of t h e m o s t i m p o r t a n t s e d i m e n t a r y s e q u e n c e s of the W e s t e r n Desert of Egypt in t e r m s of p e t r o l e u m potentials. The a r e a is generally c h a r a c t e r i z e d b y two m a j o r s u r f a c e morphologic features. According to Abu Al Izz (1971), t h e s e are (a) t h e r o c k y S o u t h e r n Miocene P l a t e a u with its n o r t h e r n portion m a d e up of Middle M i o c e n e - M a r m a r i c a "limestones" (Tm) a n d its s o u t h e r n p o r t i o n s c o n s i s t i n g of the Lower Miocene M o g h r a " s a n d s t o n e s a n d gravels" (Tm), a n d (b) t h e Q a t t a r a D e p r e s s i o n itself w h i c h occupies t h e w e s t e r n p o r t i o n of t h e b a s i n a r e a with its floor m a d e u p of I ~ w e r Miocene Moghra "clastics" covered b y p l a y a s a n d shallow saline w a t e r p o n d s in its d e e p e s t portions a n d enc r u s t a t i o n s , silt, loose s a n d s a n d S a n d d u n e s as well a s s a b k h a s in its shallow p o r t i o n s (Fig. 2). The Eocene , l i m e s t o n e s , (Te) a n d t h e Oligocene "clastics" (To), w h i c h are t h e oldest exposed r o c k u n i t s in t h e area, are f o u n d to crop o u t in t h e s o u t h e r n p o r t i o n s of t h e Miocene P l a t e a u a n d the Q a t t a r a Depression.

The s t r a t i g r a p h y of t h e N o r t h e m W e s t e r n Desert w a s extensively s t u d i e d b y so m a n y w o r k e r s , a m o n g t h o s e are Said (1962), Norton (1967), S o l i m a n a n d E l b a d r y (1970), Gezeery a n d T a h a (1971), Norton et al ( 1971), Elowi a n d Abdine (1972), Abdine a n d Deibis (1972), Awad (1984) a n d E l z a r k a (1984). However, in a previous s t u d y , B a y o u m i a n d Lofty (1985) u s e d t h e s u b s u r f a c e i n f o r m a t i o n g a t h e r e d from m o r e t h a n 35 wells (Fig. 6) drilled in the Abu G h a r a d i g B a s i n a r e a to t r a c e - o u t t h e m o r p h o logical evolution of t h e b a s i n w h e r e t h e y i n d i c a t e d that: 1) t h e s e d i m e n t a r y section overlying t h e P r e c a m b r i a n b a s e m e n t c o m p l e x varies considerably b o t h in t h i c k n e s s a n d lithology in t h e Abu G h a r a d i g B a s i n area, being t h i n c o n t i n e n t a l a t t h e n o r t h e m a n d s o u t h e m portions, a n d t h i c k m a r i n e in t h e c e n t r a l portion; (figure 3 is a generalized s t r a t i g r a p h i c section s h o w i n g t h e m a j o r stratigraphic u n i t s in t h e a r e a a n d t h e i r c h a r a c t e r i s t i c features); 2) t h e Abu G h a r a d i g B a s i n is a n elliptical multi-cyclic E a s t - W e s t t r e n d i n g b a s i n , developed w i t h i n t h e N o r t h e r n E g y p t i a n c o n t i n e n t a l shelf a n d being s t r u c t u r a l l y controlled from t h e n o r t h b y

273

274

A.I. BAYOUMIand H.I. LOTFY

a major East-West trending normal fault separating the m a i n s u b s i d i n g basinal block from a relatively stable northern platform; 3) the initial p h a s e of formation of Abu Gharadig Basin appears to have started with the beginning of the first Mesozoic sedimentary cycle. A slow a n d restricted s o u t h w a r d s transgression of the Tethyan Sea, reached A b u Gharadig B a s i n area during the Middle J u r a s s i c , a n d formed an East-West trending "local e m b a y m e n t . Deposition of the Middle J u r a s s i c Khatatba " s a n d s t o n e s a n d s h a l e s , a n d Upper J u r a s s i c Masajid "limestones" followed (Fig. 4A);

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Modes of structural evolution of Abu Gharadig Basin,western desert of Egypt as deduced from seismic data.

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Fig. 4. Morphologic evolution ofAbu Gharadig Basin deduced from drill holes. of the basin (Fig. 4B). During the Upper Cretaceous three thick m a r i n e formations of total thickness of more t h a n 2000 m e t r e s were deposited in Abu Gharadig Basin as shallow to fluvio-marine Bahariya "sandstones and shales", shallow to open marine Abu Roash "carbonates, shales and sandstones a n d open m a r i n e Khoman "chalky limestones", suggesting a rapidly subsiding basin floor along the controlling fault FI a n d emphasizing the active m a t u r a t i o n p h a s e of the basin (Fig. 4 C); 5) tilting of the basin is interpreted to have t a k e n place as a result of the compressional stresses t h a t h a d affected Northern Egypt during the Uppermost Cretaceous-Early Tertiary. These stresses h a d ceased the subsidence of the basin along the bounding fault and raised the Syrian Arc System of folds allover Northern Egypt. Deposition of the Paleocene/Eocene Apollonia shallow marine limestones occurred in the newly developed Tertiary Basin to the east of Abu Gharadig (Fig. 4D). A supporting i n d e p e n d e n t piece of evidence as to the morphological configuration of the basin previously indicated c a n be readily visualized from the inspection of both the Bouguer and aeromagnetic anomaly m a p s in the area u n d e r s t u d y (Figs. 5 A and 5 B) which emphasizes the existence of the m a j o r E-W trending fault FI controlling the

basin from the north a n d r u n n i n g along with the steep gravity gradient separating the East-West trending high anomaly belt occupying the northern platform from the major gravity m i n i m u m occupying the m a i n basinal block in the central portion. However, the trace of s u c h a bounding fault (F~) appears to be camouflaged on the magnetic map by the effect of the basaltic and doleritic intrusions recorded in drill holes within the thickness range of the Paleozoic section in the locality where s u c h fault exists and seem to be genetically associated with it. Also, the inspection of both the gravity and magnetic anomalies in the area u n d e r s t u d y indicates that a n o t h e r E-W trending fault (F2) exists as dissected segments, separating the upthrown block of the s o u t h e r n platform from the downthrown block of Abu Gharadig basin, a m a t t e r which emphasizes t h a t the basin is actually a structurally-controlled one, b o u n d e d from the north and south by two major normal faults. As for the basin block itself, it could be considered to be m a d e up of two sub-basins; the first occupying the area a r o u n d Abu S e n n a n a n d the second occupying the central portion of the map area, with the major EI-Kheit gravity hlgh (A), delimiting Abu Gharadig basin from the west, see Fig. 1. Moreover, most of the local gravity and

276

A.I. BAYOUMIand H.I. LOTFY

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Modes of structural evolution of Abu Gharadig Basin,western desert of Egypt as deduced from seismic data

277

magnetic anomalies observed in the area are found A) Structural Configuration based on Northto have their origin either with the granitic/ South trending seismic reflection profiles Six seismic profiles were studied a) line Q-14 (NgranodioriUc b a s e m e n t complex below the basin floor or with the basic and ultrabasic intrusions S) on the n o r t h e r n platform area (Fig. 7A), b) line within the Paleozoic section, such as those en- WFD (N-S) which crosses both the basinal block countered at Sheiba l-X, Sharil I-X a n d Agnes I-X and the n o r t h e r n platform (Fig. 7A), c) lines WEL or both. (NNW-SSE) a n d WKZ (N-S) within the basinal block (F ig. 713) and d) lines 3BR- 15 (NNW-SSE) and SEISMO-STRUCTURAL CONFIGURATION 3BR-8 (NNW-SSE) on the s o u t h e m portion of the OF THE BASIN map area (Fig.7C). The interpretation of s u c h diplines clearly emphasizes the points which follow. The subsurface structural configuration of Abu I) The n o r t h e m platform is generally characterG h a r a d i g B a s i n w a s d e d u c e d f r o m t h e ized by a thin, partially eroded sedimentary interpretation of more t h a n o n e - h u n d r e d seismic sequence as shown in Figure (7A). Among the lines shot by three major petroleum companies conspicuous features is the m a s t e r listric fault namely, WEPCO, AMOCO a n d BRASPETRO and previously pointed out as the normal fault Fs covering more t h a n 30 000 k m 2 of Abu Gharadig affecting mainly the Cretaceous sequence and Basin area. For the sake of brevity, figure 6 throwing more t h a n 2000 metres to the south. This illustrates the location of only ten of s u c h seismic growth fault F v which dies-out u p w a r d s is generallines which show the relationship between t h e A b u ly characterized by a large rollover anticlinal bend Gharadig basinal block and the bounding north- within the Cretaceous section indicating the ern and s o u t h e r n platforms as well as the major stratal rotation associated with the pulling-apart basin-bounding faults. Also, it is indicated here of crustal blocks on both sides. F~ is important in that all our s t r u c t u r a l conclusions cited in the that it represents the n o r t h e m basin-bounding following section represent, in fact, the resultant of fault separating the s o u t h e r n downthrown basinal all interpreted s t r u c t u r a l elements visualized on block characterized by a thick m a r i n e northerly the whole net of seismic lines tied together allover dipping Cretaceous sequence from the n o r t h e r n the area with 35 drill holes. u p t h r o w n stable platform defined by a thin eroded Cretaceous section.

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A.I. BAYOUW~and H.I. LOTFY

278

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Fig. 7A. Major structural features characterizing both the northern platform and the basinal block along North-South trending seismic lines perpendicular to the regional strike of Abu Gharadig basin. It could be m e n t i o n e d here that the c o m p l e x pattern of faulting s y s t e m displayed in the s o u t h e m portion of line Q-14 is referred to the land which collapses towards the basin-controlling fault F~w h i c h lies s o m e distance to the s o u t h of this line (Q- 14) a s showri in Fig. 6. 2) On the other hand, the Abu Gharadig basinal block, a s displayed o n lines WKX a n d W E L (Fig. 7B), is characterized by a thick northerly dipping, c o n f o r m a b l e , s e d i m e n t a r y , s e c t i o n

(Cretaceous s e q u e n c e is more t h a n 3 5 0 0 metres thick), cut by normal faulting. Here, the fault pattern is simple. The m o s t important of all is the intra-basin listric n o r m a l fault, (F) complicated in places by a set of m i n o r antithetic faults within the Cretaceous formations. However, the u p a r c h i n g recorded on the d o w n t h r o w n side of (F) on line WKZ s e e m s to have b e e n initiated by c o m p r e s s i o n s t r e s s e s at d e p t h d u r i n g t h e U p p e r m o s t Cretaceous times. It s e e m s also possible to

Modes of structural evolution of Abu Gharadig Basin,western desert of Egypt as deduced from seismic data

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U~E TRENt;LOC ~EFLE(:.0NRECOGNIZED! ~K~A'rrl'r,.r,E REFLECTORS

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Fig. 7B. Major structural features characterizing the basinal block along North-South trending seismic lines perpendicular to the regional strike ofAbu Gharadig Basin. (marking the periodic re-activation of sliding consider s u c h a fault (F) as being the s o u t h e r n movement along it during the Cretaceous times); basin-bounding fault during the Lower Cretaceous in fact (F2) generally represents the maJ or s o u t h e r n since it separates a thick Lower Cretaceous basin-bounding fault, separating the subsiding sequence on its downthrown side to the north from downthrown basinal block to the north from the a relatively thin sequence in its u p t h r o w n side as relatively stable upthrown s o u t h e r n platform block. shown in line WEL. The Top Abu Roash is t r u n c a t e d along the Base 3) The s o u t h e r n platform as it is displayed on Tertiary in the s o u t h e m portions of both seismic lines 3BR-8 and 3BR-15 (Fig. 7C) is characterized lines (3BR-15 and 3BR-8). by northward dipping strata accompanied by northward thickening of the sedimentary section. B) Structural Configuration Parallel to the Few of these normal faults are extended upwards Regional Strike of the Basin (East-West to affect the Tertiary sequence w h e r e a s m a n y of trending profiles) t h e m die upward at the Base Tertiary level. The Here four seismic lines were interpreted, a) line fault F2, (indicated on the gravity and magnetic 26 (E-W) which lies along the n o r t h e r n platform map as dissected segments), is the m o s t conspi- area, b) line WKH (WNW-ESE) a n d line WKX (E-W) c u o u s of all faults displayed on both lines. It is a within the expected basin b o u n d a r i e s and c) large listric normal growth fault down to the north, line 3BR-11 (ENE-WSW) along the s o u t h e m associated in places with minor antithetic faults platform.

28()

A.I. BAYOUMIand H.I. L()'r~ FAULT I~COGNIZED ~FLEC TORS PAT T ERN - Top A p o l l o n i ~

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ICeu I t .

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F 1 d ~ ' [ i n r ' ~ lhr~ ~ o l l t h ~ r n Ihr

s,.hsidt. R hri~i,mI Ur e l ac~-nll~ . . F,~rw~l, I ~ q)lhr~wI.R

n~,r ! h blc~rk

s * c t t~m.

e x l,ql s

of

~lnhl~

Ahll

downt hrowI hill

P,hAr~dlIR

.hi Ithe~h

, ~. I ul" i o , ~ h t p.~ } .

inr! lal I T er~led

h~sln-hotlnd|n!ll

thP

plel[ol'l ~-qtl~.nce

To

I he

block with

(thirk

ftqul tn

41hd

Rm|th

t he

@xlst.q

(thin

.nccm[oriitle~i).

* Frequent Unconforml t Ie~ ® o i

Fig. 7C. Major structural features characterizing the southern platform along North-South trending selsmlc lines perpendicular to the regional strike ofAbu Gharadig Basin. I) The n o r t h e m platform, a s it is displayed on line 26 (Fig. 8A), s e e m s to have b e e n characterized b y a t h i n partially eroded s e d i m e n t a r y s e q u e n c e striking E a s t - W e s t c u t b y n u m e r o u s n o r m a l faults (with or w i t h o u t antithetic faults) affecting m a i n l y t h e C r e t a c e o u s a n d older strata. It is indicated t h a t the Top A b u R o a s h c o u l d only be t r a c e d o u t in the w e s t e r n portion of the section d u e to erosion f u r t h e r e a s t in t h e u p t h r o w n b l o c k of t h e n o r m a l fault (Fu). This is confirmed b y d a t a o b t a i n e d from Sheiba-lX well. At this well location, erosion or n o n - d e p o s i t i o n of t h e Aptian Alamein "dolomites" r e s u l t s in t h e Top Masajid (Top J u r a s s i c ) to be the first r e c o g n i z e d r e f l e c t o r b e l o w t h e E a r l y C e n o m a n i a n B a h a r i y a reflector. 2) The A b u G h a r a d i g b a s i n a l b l o c k itself, a s displayed on lines WKH a n d WKX (Fig. 8B) that lie within the e x p e c t e d b a s i n b o u n d a r i e s , is generally characterized b y a n E a s t - W e s t striking thick

conformable s e d i m e n t a r y section c o m p l i c a t e d b y normal faulting affecting m a i n l y the C r e t a c e o u s a n d older rocks. Both lines also d i s p l a y t h a t t h e B a s e Tertiary a s well a s o t h e r older horizons lie at relatively d e e p e r levels t h a n t h o s e r e c o r d e d on lines 26 a n d 3BR- i I {Fig. 811) w i t h o u t a n y m a r k e d unconformity. 3) The s o u t h e r n platibrm, o n t h e o t h e r h a n d , a s it is displayed on line 3B3R- 11 (Fig. 8A), like line 26, is generally c h a r a c t e r i z e d b y E a s t - W e s t s t r i k i n g s t r a t a with t h i n p a r t i a l l y e r o d e d Cretaceous sequence cut by numerous normal faults a s s o c i a t e d with several antithetic faults. The u n c o n f o r m i t y b e t w e e n the B a s e Tertiary a n d the Top A b u R o a s h is c h a r a c t e r i z e d b y t h e partial or complete e r o s i o n for n o n - d e p o s i t i o n of the C a m p a n i a n K h o m a n ,chalk, a s is s h o w n b y the t r u n c a t i o n of the Top A b u Roash. The Early Tertiary s e c t i o n o n t a p s at t h e w e s t e r n e n d of line 3BR- I I.

Modes of structural evolution of Abu Gharadig Basin,western desert of Egypt as deduced from seismic dam

eel tREND UOCM~

RECOGNIZED FAU LT REF LECYORS PAT T E R N

,iT i ITUDE

~lpllc|lld - lop lbu lonlh

-_Lop..l/N~ UP N

-

e

r~ I RUCT" U RA L

"l'ypicll

E~abl~

m t i e r n o[ nnrml[ ~ l s t - i o t i fl~r-/yil,¢ [aSit S y i t ~ . reuited C i l l i c . o u s

,a

ii

__.

CON F IGU R ^ T I O k l

plstlorm

~{rai# v i t h secJ:h,.

diililyinll

fSatVi~¢

tfii. ~1;

llirli/ll/

~m.Jl

a5

#.i~llt~ ~re~melt~

Ilnconf0rmitips.

T.17-F.llo~oi~

_c .Z" !

• Frequent IksrMf~rll~tld

° L ~

STRUCTURAL

-_h- T-,.. v

7a811 vort tool d t Iplacement I

- Top ~

i81o~ 8 n m r o u 8

Z

- n t I

~FtJRV0m AT]'BUm[

~E w I

I:hJn p 0 r t i a l l y

Top Ileher lyu

f e u l t n essocisted

Top A l k a l i n

v i t h minor

q~

I

4 1 zLU

loush

CONFIGURATION

TTpical stllble p181~form features showJnR f [ & t . | y i n l l ! # r l y }w)t'tzemtlJl straits ~no r o t l t i o n ) rhsrscior|zed by eroded fllulted Crelireou,q R~quPnt'p with

~eq.ent UneOnrormt ¢iffs.

ant i t h e t ic remits,

._~ I

r~

•

Prequeat Ilvuc~ forml t

i•

SOUTHERN

Fig. 8A. Major structural features characterizing the stable platform areas along East-West trending seismic lines parallel to the regional strike of the basin. C) D e p t h S e c t i o n s It is indicated here that most of the major structural elements of Abu Gharadig Basin that are clearly outlined on the seismic lines, particularly those trending North-South, can be clearly observed on the two structural cross sections (Fig. 9) which have been prepared using the constructed set of average velocity contour maps shown in figure 10, and by making use of data obtained from more than 35 drill holes (Fig. 6). A£S 9:2-F

MAJOR STRUCTURAL/MORPHOLOGIC M ~ OF ABU GHARADIG BASIN According toAnderson e t a/. (1983), structurallycontrolled sedimentary basins evolve generally either as: a) sagged faulted basins, associated with narrow zones of deep plastic extensions located directly beneath the deepest part of the sag and separated from one another by 20 to 30 km, b) large homoclinally rotated basins associated with zones of deep plastic extension that are laterally

281

282

A.I. BAYOUMIand H.I. Lo'n-'Y

LINE

FAULT }TRENDLOCAIION REFLECIIONRECOGNIZ ED AIIIIU~E RE FLECT(~c~, PA T T ~RN

I t~ UJ "0 0

e ~q <(

X

LU

Z

-

Top Dabaa

Generally

-

Top A p o l l o n i a

with smal 1

Base ~ertiary

faults

Top Abu R o a s h

have

-

•~ .

F-

_~

{=x T ~ m <[

~0 0

--~

-'Fop

F~ahariw

-- rI'Op glamein

'lyp](al east-west with

l i t t I~,

CON FIGURATION

I

simple

that

influence basin

ST RU CI U RAL

thick

(:orlf()rmable

hasina)

strikln~

block

f}at-lvinR

(if el a c e o u ~ reht

ionshi

features

horizontal

sF,(l uellc e

as

showin R

strata we]]

as

p.

in the

control.

G e n e r a l Iy Con f o r m a b l e .

C~ 0 --..

®

Fig. 8B. Major structural features characterizing the basinal black along East-West trending seismic lines parallel to the regional strike ofAbu Gharadig Basin. displaced from the surface trace of the master either planar (steep or gentle) or listric flatened-out listric faults by distance depending on the radius with depth. Most of these master faults are of curvature of the major controlling fault, or c) generally associated either with a conspicuous complex basins which are assemblages of pattern of down bending or flexing of the basin-fill complexely deformed s u b - b a s i n s (mosaics) sediments against the fault plane (the so-called associated with sharply curved shallow listric faults rollover anticlinal bends) a n d / o r antithetic faultmerging downwards into a large planar fault ing representing the gravitational collapses of the representing a detachment surface as shown in platform inside the basin as it evolves to figure 1 1. accomodate the space from pull-apart of crustal As far as the nature and geometry of the basin- blocks associated with the deep plastic extensional controlling faults are concemed, they are generally deformation as shown in figure 12.

Modes of structural evolution of Abu Gharadig Basin,western desert of Egypt as deduced from seismic data,

283

SOUI~

NORTH Line

WF D

L i n e WKZ

Line

L ~

3 BR_IS

,I

W O.]L|

® OASIC llJA~I4FM! CONI~I°I.EIt

Pt |RMeosolc I

HORTH

i

Line Off, ~ I

~__~

.~OtrTH Li.e

~

W E L

r

~

~

¢1

W'

_i

Li.e

3BI].

~--

J . . . .

8

" --=

N

bJ

•

re

•

,~, "

IC

i

-"

:

| PLA I F ? R M I

Fig. 9. Structural configuration ofAbu Gharadlg Basin along the North-South trend, interpreted from seismic data, using average velocity contour maps (Fig. I0). and drilling data (vertical exaggeration 10X). In view of the above-mentioned considerations, it is interpreted here that: I) The Abu Gharadig Basin is a homoclinally rotated basin developed within the n o r t h e r n portion of the African plate and controlled by two major East-West trending listric growth faults (F~) a n d (F2) with F~being the n o r t h e r n basin bounding fault separating the downthrown basinal block from the n o r t h e r n stable platform and F 2 the s o u t h e r n b a s i n bounding fault separating the subsiding basinal block from the s o u t h e r n relatively stable platform. 2) The Abu Gharadig basinal block, which occupies the central portion of the area u n d e r

study, is characterized by a thick marine Cretaceous sequence dipping n o r t h w a r d s and including within its n o r t h e r n portion, a large tensional rollover anticlinal flexure. Within the s o u t h e r n portion of the basinal block, the basin floor rises gently s o u t h w a r d s toward the s o u t h e m platform. The Cretaceous sediments are fractured by several m i n o r antithetic faults on the downthrown side of F 2 m a r k i n g the periodic subsidence of the basinal block with respect to the s o u t h e r n platform along F 2 as a response of the continuous sliding of the b a s i n along F~ to the north (since J u r a s s i c times). Within the basinal block, in general, t h e U p p e r C r e t a c e o u s

284

A.I. BAYOUMIand H.I. LOTFV

ze~ __~

~o--~i

Z

z~

•

2%w :

©

////

]TOP ALAMEIN

~

.

~

i

~

7¢,0¢T

//A

..........

"\

m,

Fig. I0. Average velocity contour maps on the major reflectors within Abu Gharadig Basin area (Lofty, 1984).

l~de

~ed-

l~oee c B )J

(A)J

faulted

~on,p,lex toas*n ( Mosaic ),i

has,4

r

'

r

, i

Basin

•1(~ m

lOKm

:

20Kin 3C ~-,. Eestricted plastic

crustal

" ~ l . . . . . . . . C4 Moho

plastic-extension

Deep

extension due to a l o c a l i z e d

aSSOCiated

anntle

ductile

bulge

continuous

in

the quasi-

zone

of p l a s t i c

with

decouplin 8

transition

msitioned

some

Mcl'~o

~JO~rr Regional

crustal

extensions

at

great

distances away from the basins axes.

zones distance

e x t e n s i o n r e s u l t i n g ane2astic

from the b a s i n axis in t h t n n i n B downdip of the =mjor l l s t r l c of the c r u s t a l slob. [Stt~m't f a u l t . stretching 1971,

and

and

Wallace,

1979].

i

Sassed

basin

ductile Ares Q:

stretchin 8

will

8ctlve

above

be

with

the zone.

aetmmically

periodiclurfsce

ruptures serked by a n t i t h e t ~ with

faulting of

sliding

fault

end

the basin.

lmrBe k e m c l i n t l of basin the

floor

ImJor

rotation

blocks along

lintric

fault.

A set faults

ser8tn 8

a

set

extensional of

downward

master into

a

basins listric plsnnr

:

A- Projects t o d e c o u p l i n 8 zones r e p r e s e n t i n g d u c t i l e t r a n s i t i o n zones in the c r u s t . [Eaton, Ig79 and l e Pichon and S i b u e t ,

Imster

deepening

thin-skinned along

d e t n c h l e n t f a u l t which e i t h e r

resctivstlon

along t h e

of

subsiding

of

198] ].

or

B- Extends as a p l a n a r i

rXs~d c r u l t a l

fault

a l o n g which

,slabsi dtverJei~i,,[ ~ h J c k e , 198tj

Fig. 1 1. Modes of structural evolution of sedimentary basins (Modified after Anderson et aL, 1983).

Modes of structural evolution of Abu Gharadig Basin,western desert of Egypt as deduced from seismic data.

285

1) the left lateral drift of the African plate relative to t h e E u r o p e a n one, which h a p p e n e d in the Middle J u r a s s i c a n d during t h e early p h a s e s of the Alpine Orogeny as a result of the spreading of the central portion of t h e Atlantic Ocean, w a s according to Wood (1984), generally responsible for the opening of t h e Tethys a n d for t h e initiation of Abu Gharadig b a s i n t h r o u g h a localized m a n t l e bulge in t h e Qattara Depression area, this indicated by the late J u r a s s i c basaltic i n t r u s i o n s prevailing Case B Northern Egypt in this particular locality. The master listric fault is characteriz~ i~ rollover (2) During the Cretaceous, t h e tensile stresses anticline ma~ir'~ tl'~ stratal rotation prevailing Northern Egypt d u r i n g t h e Alpine movement, as a result of s u c h sinistral drift of Africa, which r e a c h e d its acme during the Upper Cretaceous, m u s t have b e e n so active to the degree t h a t the floor of the Abu Gharadig Basin started a regional homocllnal rotation along the m a j o r EastWest trending listric fault (F~), leading to the F f deepening a n d b r o a d e n i n g of the b a s i n a n d bring....... ing it u p to a w e l l - p r o n o u n c e d m a t u r a t i o n p h a s e especially d u r i n g t h e Turonian. (3) F r o m T u r o n i a n times onward, the c o n t i n u e d opening of the Central Atlantic Ocean w a s eclipsed by a more rapid opening of the North Atlantic Ocean resulting in a reversal of t h e relative m o t i o n Fig. 12. Cross sectional sketches showing strata] response between North Africa a n d E u r o p e to dextral a n d to accomodate the void space arised from pulling-aparat of leading to t h e initial stage of closing the Tethys blocks and the subsequent subsidence of the basinal block along the listrlc fault (Modified after Anderson et al., 1983). (Wood, 1984). This P o s t - T u r o n i a n right-lateral t r a n s c u r r e n t m o v e m e n t along the n o r t h e m formations are frequently folded, having the b o u n d a r y of the African Plate resulted in the general t r e n d p a t t e r n of t h e Syrian Arc s y s t e m of compressive stresses t h a t were responsible for the folds. death a n d declination o f A b u Gharadig as a basin 3) To t h e n o r t h of the n o r t h e r n b a s i n b o u n d i n g a n d beginning a tilting p h a s e associated with the fault F I t h e n o r t h e r n platform includes generally Laramide p h a s e of t h e Alpine Orogeny. The t h i n partially eroded continental Cretaceous sedi- elevation of the Syrian Arc S y s t e m of folds over m e n t a r y sequence, with frequent unconformities, Northern Egypt including Abu Gharadig Basin (the C a m p a n i a n K h o m a n "chalk" a n d t h e Aptian itself is m o s t probably related to s u c h T u r o n i a n Alamein ~dolomites, are thin, partially eroded a n d / dextral drift of Africa relative to Europe. or non-deposited) a n d the s e d i m e n t a r y u n i t s generally dip n o r t h w a r d s towards M a t r u h Basin. CONCLUSIONS 4) To t h e s o u t h of t h e s o u t h e r n b a s i n - b o u n d i n g fault F 2, t h e s o u t h e m platform also shows, t h i n II Critical analysis of all available exploratory partially eroded continental sequence dipping gently data, including drill hole, gravity magnetic a n d n o r t h w a r d s toward t h e Abu Gharadig depocenter. seismic reflection d a t a indicates t h a t the Abu Gharadig Basin, which h a s proven to c o n t a i n large MODES OF STRUCTURAL EVOLUTION reserves of hydrocarbons, within its t h i c k m a r i n e OF THE BASIN Cretaceous sequence, is a n extensional, elliptical, multicyclic, E-W t r e n d i n g structurally-controlled The s t r u c t u r a l evolution ofAbu Gharadig Basin, s e d i m e n t a r y b a s i n developed within the n o r t h e r n within t h e n o r t h e m portion of the African plate, portion of t h e African plate as a c o n s e q u e n c e of the which developed as a result of t h e deep a n d left-lateral drift of N o r t h e m Africa relative to complex extensional tectonics prevailing across E u r o p e w h i c h s t a r t e d in Middle J u r a s s i c Northern Egypt d u r i n g the time s p a n of t h e Alpine times. Orogeny t h a t h a d resulted in the opening a n d t h e n 21 The Abu Gharadig Basin exhibits on typical the s u b s e q u e n t closing of t h e T e t h y a n Sea along b a s i n / p l a t f o r m s t r u c t u r e a subsiding b a s i n a l b l o c k the n o r t h e r n m a r g i n of the African plate, is i m p l y i n g e n o r m o u s l y t h i c k c o n f o r m a b l e interpreted in figure 13 where Cretaceous m a r i n e s e q u e n c e b o u n d e d n o r t h w a r d s Case A Themaster listric fault is characterized by minor ontithetic faults markircj gravilational cOItapses.

286

A.I. BAYOUMIand H.I. LoTvY AGE MIOCENE

TECTONIC I NT

II2NelRY

PREVAILED STRESSES STRUCTU~,AL RESPONRE~. was ,~ound end e f f e c t i v e in the Gullr where s Miocene besin ~ v e d malnl

RIFTING

~

EOCENE

~]-evetion nf

of Suez b

rifti

reRi-n ] -

the Syrian

arc

stresses prevailed dt, r i n l ~ W l t h s general re e . . . . . . . . . . . . . i the ~ plm.qe of the I | [rei~vpnntl~,n o[ mnvement~ klptne'OroReny III old f,~,,,,s.

~j,,,g

(.D 0 rr"

A NW-SSE tenJJ~e s t r e s s ~ L e r g e scale deep crnstg] that prevailed the ~ e~tenslon associated with North Western Desert ]srge homocI|n~| rotation during the CretaCeous aJon 8 the major ]istrlc [aults sad reached i t s scene tn ]ceding to the deepening and the TuronJan broadening of Ahu GharadlR basin

I (_9 localized dolerltlc ~ x t e n s i o n accompanied by Ind bsssJtlc mgntle h~illlOwmild suh.qidence along m|ee intruded the ~he old major east-~e~t

J URASS IC

hit tsru

depressinn arPa

f r 8ctures

TRIASSIC

. rEpeJrosenic LATE

HERCYNIJ~J~Ji°~-~--']ll~wldespread

J

l .... itin,

In

nature ceuswd a genPra[ u p l i [ L and 1 eJJ ov~r Ihe North Wester,, [~czrr,

leRression ,. . . .

depo~i, i. . . . . ,

P.rmi . . . . . d

Tri .... i.

J

EARLY

Fig. 13. Speculations on the mode of structural evolution of Abu Gharadig Basin. a n d s o u t h w a r d s b y two relatively stable platforms 5) The Post T u r o n i a n dextral drift of Africa showing thin partially e r o d e d C r e t a c e o u s s u c c e s s - relative to E u r o p e c r e a t e d c o m p r e s s i v e s t r e s s e s in ion. Two m a j o r listric growth faults (FI a n d F 2) the N o r t h e m portion of t h e African Plate a n d b o u n d t h e m a i n b a s i n a l b l o c k a n d control its r e s u l t e d in uplift of the A b u G h a r a d i g B a s i n d u r i n g s u b s i d e n c e with r e s p e c t to t h e n o r t h e r n a n d the U p p e r m o s t C r e t a c e o u s / E a r l y Tertiary. s o u t h e r n platforms. Starting from t h e s o u t h e r n platform a n d going b a s i n w a r d , the s e d i m e n t a r y Aeknmuledj/ements - The authors wish to thank the s e q u e n c e is dipping gently n o r t h w a r d s a n d also authorities of the Egyptian General Petroleum thickening gradually b a s i n w a r d t o w a r d s the basin- Corporation (E,G.P.C.) for suppl-~ng the exploratory b o u n d i n g fault F i. data used to conduct the present study which are 3) Initiation of t h e A b u G h a r a d i g B a s i n s t a r t e d confined to uninterpreted one hundred seismic lines, during the Late J u r a s s i c a s a result of t h e i n t r u s i o n gravity, magnetic and weU log data obtained from more of a d e e p - s e a t e d basaltic a n d doleritic m a n t l e than 35 drill holes; interpretation of such exploratory bulge, related to t h e early p h a s e of t h e Alpine data in terms of subsurface geologic conditions was the Orogeny. This r e s u l t e d in a restricted c r u s t a l main responsibility of the authors. extension, a c c o m p a n i e d b y a slow mild s u b s i d e n c e REIr~RENCF_~ of the b a s i n a l block, along a n E a s t - W e s t trending fracture (F:) in t h e N o r t h e r n W e s t e r n Desert of Egypt. Abdine, A.S. and Deibis, S. 1972. Lower Cretaceous 4) Major s t r u c t u r i n g in the b a s i n s t a r t e d in the Apt|an sediments and their oil prospects in the Lower C r e t a c e o u s a n d c o n t i n u e d t h r o u g h o u t t h e Northern Western Desert, Egypt. Eighth Arab Petroleum Congress, Algeria. C r e t a c e o u s epoch a s a r e s u l t of the regional tensile s t r e s s e s w h i c h h a d affected t h e n o r t h e r n portion of AbuAl Izz, M.S. 1971. Landforms of Egypt, translated by Y.A. Fayld, the American University in Cairo Press, t h e African plate d u r i n g t h a t time a n d r e a c h e d its Cairo, Egypt. a c m e in the Turonian. A large scale c r u s t a l extenAnderson, R.E. et al. 1983. ImplicaUons of selected subsion o c c u r r e d a s s o c i a t e d with regional homoclinal surface data on the structural form and evolution of rotation of t h e sliding b a s i n a l b l o c k of A b u some basins in the northern Basin and Range proGharadig along t h e n o r t h e r n b a s i n b o u n d i n g vince, Nevada and Utah: Geological Society of Amerlca listric fault (FI). This lead to t h e deepening a n d Bull., 94, 1055-1072. b r o a d e n i n g of t h e basin.

Modes of structural evolution of Abu Gharadig Basin,western desert of Egypt as deduced from seismic data. Awad, G.M. 1984. Habitat ofoil in Abu Gharadig and Fayum Basins, Western Desert, Egypt, ~ Bu//., 88, No. 5, 564-573. Eaton, G.P. 1979. Regional geophysics, Cenozoic and geologic resources of the Basin and Range province and adjoining regions. In:. Basin and Range Symlx~lum Rocky Mountain Association of Geologists and Utah Geological Association. (Edited by Newman, G.W., and Geode, H.D.), 11-39 p.. Elowi M. and Abdine, S. 1972. Rock units correlation chart of the Northern Western Desert, Egypt, Wepco Petroleum Company. El Zarka, M.H.1984. The application of facies parameters and mapping techniques to exploration of the subsurface Lower Cretaceous of the Qattara Depression, Western Desert, Egypt, Journal of Petroleum Geology 7, 277-302. Gezeery, N. and Taha, I. 1971. Contribution to the stratigraphy, tectonics and oil shows in Abu Gharadig Basin, Western Desert, Ninth Annual Meeting Geological Society of Egypt, Cairo. Le Pichon, X., and Sibuet, J. 1981. Passive margine: A model of formation: Journal of Geophysical Research, 88, 3708-3720. Lofty, H.I. 1984. A geophysical study on the Qattara Depression Area, Western Desert, Egypt, M.Sc. thesis, El Minya University, Egypt.

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Norton, P. 1967. Rock stratigraphic nomenclature of the Western Desert, Exploration Department report No. 4, Pan American Oil Company, Cairo, Egypt. Norton, P.,Hagras, M.andAbdallah, A.M. 1971. Progress report on rock stratlgraphic units in the central part of the Western Desert. Ninth Annual meeting, Geological

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