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Developing carbonate platforms: Southern Gulf of Suez, northern Red Sea — Comment
Marine Geology, 69 (1986) 363--366
363
Elsevier Scientific Publishers B.V., Amsterdam -- Printed in The Netherlands
Discussion DEVELOPING CARBONATE PLATFORMS: SOUTHERN GULF OF SUEZ, NORTHERN RED SEA -- COMMENT
ANDRl~ GUILCHER Universit~ de Bretagne Occidentale, B.P. 814, F-29285 Brest (France) (Accepted for publication June 7, 1985) In their recent contribution on the Red Sea coral reefs, Roberts and Murray (1984) state that, if the origin of the elongated reefs running in a northwest and southeast direction at the southern entrance of the Gulf of Suez is "uncertain", they owe their shape t o the " d y n a m i c marine setting in which they are developing", that is, to the "tidal flow between the Red Sea and the Gulf resulting in strong rectilinear currents". In addition, " t h e unidirectional nature of the regional wind field" reinforces the action of the tidal currents. This statement does not take into account the regional structural network that frames the reefs in the vicinity of the southern part of the Sinai Peninsula as well as on the Farsan Bank off the west coast of Saudi Arabia, and has been described and defined in other papers (Guilcher, 1955, 1979, 1982)o The authors admit only (op. cir., p.166) that "some of these features, such as the Shoab Ali Complex . . . have obviously accreted over a controlling subsurface structure". As a matter of fact, I consider that all these coral structures running in a northwest and southeast direction are essentially structurally controlled (although the coral life continues, of course, to thrive over their flanks and on their tops when depth permits). The evidence is m a x i m u m at the triple junction of the southernmost tip of the Sinai Peninsula (Ras Muhammad and surrounding shoals), where quite recent reefs are certainly faulted in the above-mentioned direction (Guilcher, 1979, photo 3 and fig.7), as Roberts and Murray accept for Shoab Ali a little more to the northwest. On the Farsan Bank, huge northwest--southeast structures, rising abruptly to the surface from 700 m below sea level, seem also to be made of reefs that have grown over subsiding tectonic ridges. Tidal currents can in any case be considered as a main factor on the Farsan Bank, where the tidal range is very small. Dr. E1 Rabaa, Sudan, has also expressed, during a meeting held in Jeddah in 1981, the opinion that similar features found along the coast of Sudan, and equally reported there by Braithwaite (1982), also result from a slow subsidence of horsts. I suggested in 1955 to give the name of shabs to such an elongated reef, from a terminology in use in the Red Sea countries. It appeared, however, at the Jeddah meeting, that shab is actually used in Arabic for many types of reefs so that, after a discussion with Drs. Bird, Australia, and Schroeder, F.R.G., also marine geologists, it was found that ridge r e e f in English, rdcif
364 en cr~te in F r e n c h , should be t h e m o s t suitable t e r m . A n y h o w , it is necessary
t o c o n s i d e r t h e m as a special t y p e o f reef: such an o p i n i o n was e x p r e s s e d to m e b y t h e late Ph. K u e n e n in 1 9 6 5 a f t e r he r e a d t h e 1955 p a p e r on the Farsan Bank. This d o e s n o t m e a n at all t h a t tidal c u r r e n t s h a v e no i n f l u e n c e on t h e g r o w t h o f t h e reefs at the s o u t h e r n e n t r a n c e o f the G u l f o f Suez. This o n l y m e a n s t h a t , in m y o p i n i o n a n d in t h e R e d Sea c o n t e x t , t h e ridge reefs o w e essentially t h e i r s h a p e t o s t r u c t u r a l t r e n d s t h a t have an o v e r w h e l m i n g influence in t h e R e d Sea as a whole. REFERENCES Braithwaite, C.J.R., 1982. Patterns of accretion of reefs in the Sudanese Red Sea. Mar. Geol., 46: 297--325. Guilcher, A., 1955. G~omorphologie de t'extr~mit~ septentrionale du Banc Farsan (Met Rouge). Ann. Inst. Oc4anogr., 30: 55--100. Guilcher, A., 1979. Les r~cifs coralliens de l'est et du sud de la Presqu'ile du Sinai'. Ann. G~ogr., 88: 393---418. Guilcher, A., 1982. Physiography of the Red Sea coast and coastal waters: a summary of the present state of knowledge with suggestions for further research. J. Fac. Mar. Sci., Jeddah, 2: 27--36. Roberts, H.H. and Murray, S.P., 1984. Developing carbonate platforms: southern Gulf of Suez, northern Red Sea. Mar. Geol., 59: 165--185.
DEVELOPING CARBONATE PLATFORMS: SOUTHERN GULF OF SUEZ, NORTHERN RED SEA -- REPLY
HARRY H. ROBERTS and STEPHEN P. MURRAY Coastal Studies Institute, School o f Geoscience, Louisiana State University, Baton Rouge, LA 70803 (U.S.A.)
(Accepted for publication October 1, 1985)
In r e s p o n s e to Dr. G u i l c h e r ' s c o m m e n t s c o n c e r n i n g o u r r e c e n t p a p e r o n t h e small, elongate c a r b o n a t e p l a t f o r m s in t h e s o u t h e r n G u l f o f Suez, we w o u l d like t o clarify o u r o b s e r v a t i o n s w i t h regard t o t h e origin, as o p p o s e d t o shapes a n d o r i e n t a t i o n s , o f t h e s e features. We t h i n k t h a t o u r p a p e r is n o t in c o n f l i c t w i t h Dr. G u f l c h e r ' s c o m m e n t s a n d t h e p r o b l e m is o n e o f clarity in t h e t e x t . It c e r t a i n l y was n o t o u r i n t e n t t o m i n i m i z e t h e i m p o r t a n c e o f s t r u c t u r e in t h e origin a n d basic o r i e n t a t i o n o f s u b s t r a t e s f o r t h e s e features. On pp. 172 a n d 173 we s t a t e t h a t b o t h e l o n g a t e salt swells a n d Gulf-parallel faulting m a y be r e s p o n s i b l e f o r relief o n w h i c h t h e c a r b o n a t e s s t a r t e d acc u m u l a t i n g : " A l t h o u g h t h e excessive t h i c k n e s s o f M i o c e n e a n d y o u n g e r salt has caused e x p l o r a t i o n p r o b l e m s (Kanes a n d A b d i n e , 1983), m o b i l i z a t i o n of t h e s e s e d i m e n t s has p r o d u c e d e l o n g a t e Gulf-parallel s t r u c t u r e s w i t h
363
Elsevier Scientific Publishers B.V., Amsterdam -- Printed in The Netherlands
Discussion DEVELOPING CARBONATE PLATFORMS: SOUTHERN GULF OF SUEZ, NORTHERN RED SEA -- COMMENT
ANDRl~ GUILCHER Universit~ de Bretagne Occidentale, B.P. 814, F-29285 Brest (France) (Accepted for publication June 7, 1985) In their recent contribution on the Red Sea coral reefs, Roberts and Murray (1984) state that, if the origin of the elongated reefs running in a northwest and southeast direction at the southern entrance of the Gulf of Suez is "uncertain", they owe their shape t o the " d y n a m i c marine setting in which they are developing", that is, to the "tidal flow between the Red Sea and the Gulf resulting in strong rectilinear currents". In addition, " t h e unidirectional nature of the regional wind field" reinforces the action of the tidal currents. This statement does not take into account the regional structural network that frames the reefs in the vicinity of the southern part of the Sinai Peninsula as well as on the Farsan Bank off the west coast of Saudi Arabia, and has been described and defined in other papers (Guilcher, 1955, 1979, 1982)o The authors admit only (op. cir., p.166) that "some of these features, such as the Shoab Ali Complex . . . have obviously accreted over a controlling subsurface structure". As a matter of fact, I consider that all these coral structures running in a northwest and southeast direction are essentially structurally controlled (although the coral life continues, of course, to thrive over their flanks and on their tops when depth permits). The evidence is m a x i m u m at the triple junction of the southernmost tip of the Sinai Peninsula (Ras Muhammad and surrounding shoals), where quite recent reefs are certainly faulted in the above-mentioned direction (Guilcher, 1979, photo 3 and fig.7), as Roberts and Murray accept for Shoab Ali a little more to the northwest. On the Farsan Bank, huge northwest--southeast structures, rising abruptly to the surface from 700 m below sea level, seem also to be made of reefs that have grown over subsiding tectonic ridges. Tidal currents can in any case be considered as a main factor on the Farsan Bank, where the tidal range is very small. Dr. E1 Rabaa, Sudan, has also expressed, during a meeting held in Jeddah in 1981, the opinion that similar features found along the coast of Sudan, and equally reported there by Braithwaite (1982), also result from a slow subsidence of horsts. I suggested in 1955 to give the name of shabs to such an elongated reef, from a terminology in use in the Red Sea countries. It appeared, however, at the Jeddah meeting, that shab is actually used in Arabic for many types of reefs so that, after a discussion with Drs. Bird, Australia, and Schroeder, F.R.G., also marine geologists, it was found that ridge r e e f in English, rdcif
364 en cr~te in F r e n c h , should be t h e m o s t suitable t e r m . A n y h o w , it is necessary
t o c o n s i d e r t h e m as a special t y p e o f reef: such an o p i n i o n was e x p r e s s e d to m e b y t h e late Ph. K u e n e n in 1 9 6 5 a f t e r he r e a d t h e 1955 p a p e r on the Farsan Bank. This d o e s n o t m e a n at all t h a t tidal c u r r e n t s h a v e no i n f l u e n c e on t h e g r o w t h o f t h e reefs at the s o u t h e r n e n t r a n c e o f the G u l f o f Suez. This o n l y m e a n s t h a t , in m y o p i n i o n a n d in t h e R e d Sea c o n t e x t , t h e ridge reefs o w e essentially t h e i r s h a p e t o s t r u c t u r a l t r e n d s t h a t have an o v e r w h e l m i n g influence in t h e R e d Sea as a whole. REFERENCES Braithwaite, C.J.R., 1982. Patterns of accretion of reefs in the Sudanese Red Sea. Mar. Geol., 46: 297--325. Guilcher, A., 1955. G~omorphologie de t'extr~mit~ septentrionale du Banc Farsan (Met Rouge). Ann. Inst. Oc4anogr., 30: 55--100. Guilcher, A., 1979. Les r~cifs coralliens de l'est et du sud de la Presqu'ile du Sinai'. Ann. G~ogr., 88: 393---418. Guilcher, A., 1982. Physiography of the Red Sea coast and coastal waters: a summary of the present state of knowledge with suggestions for further research. J. Fac. Mar. Sci., Jeddah, 2: 27--36. Roberts, H.H. and Murray, S.P., 1984. Developing carbonate platforms: southern Gulf of Suez, northern Red Sea. Mar. Geol., 59: 165--185.
DEVELOPING CARBONATE PLATFORMS: SOUTHERN GULF OF SUEZ, NORTHERN RED SEA -- REPLY
HARRY H. ROBERTS and STEPHEN P. MURRAY Coastal Studies Institute, School o f Geoscience, Louisiana State University, Baton Rouge, LA 70803 (U.S.A.)
(Accepted for publication October 1, 1985)
In r e s p o n s e to Dr. G u i l c h e r ' s c o m m e n t s c o n c e r n i n g o u r r e c e n t p a p e r o n t h e small, elongate c a r b o n a t e p l a t f o r m s in t h e s o u t h e r n G u l f o f Suez, we w o u l d like t o clarify o u r o b s e r v a t i o n s w i t h regard t o t h e origin, as o p p o s e d t o shapes a n d o r i e n t a t i o n s , o f t h e s e features. We t h i n k t h a t o u r p a p e r is n o t in c o n f l i c t w i t h Dr. G u f l c h e r ' s c o m m e n t s a n d t h e p r o b l e m is o n e o f clarity in t h e t e x t . It c e r t a i n l y was n o t o u r i n t e n t t o m i n i m i z e t h e i m p o r t a n c e o f s t r u c t u r e in t h e origin a n d basic o r i e n t a t i o n o f s u b s t r a t e s f o r t h e s e features. On pp. 172 a n d 173 we s t a t e t h a t b o t h e l o n g a t e salt swells a n d Gulf-parallel faulting m a y be r e s p o n s i b l e f o r relief o n w h i c h t h e c a r b o n a t e s s t a r t e d acc u m u l a t i n g : " A l t h o u g h t h e excessive t h i c k n e s s o f M i o c e n e a n d y o u n g e r salt has caused e x p l o r a t i o n p r o b l e m s (Kanes a n d A b d i n e , 1983), m o b i l i z a t i o n of t h e s e s e d i m e n t s has p r o d u c e d e l o n g a t e Gulf-parallel s t r u c t u r e s w i t h