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Early Precambrian stratigraphy of central and southern Rajasthan, India — a reply
203
EARLY PRECAMBRIAN STRATIGRAPHY OF CENTRAL AND SOUTHERN R A J A S T H A N , INDIA -- A R E P L Y
K. NAHA Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, West Bengal (India)
(Received June 12, 1975)
Besides r e s t a t i n g t h e i r views with assertion a n d u t t e r i n g s o m e p l a t i t u d e s , Basu a n d Arora, in t h e i r discussion o f t h e p a p e r b y N a h a a n d H a l y b u r t o n , have n o t c i t e d a single piece o f e v i d e n c e t h a t necessitates m o d i f y i n g o u r conclusions. (A p o i n t t h a t e s c a p e d us, h o w e v e r , is t h a t t h e c o n c l u s i o n s in Raja R a o et a1.(1971) o n w h i c h we d i f f e r e d are e n t i r e l y d u e t o Basu a n d Arora.) They seem to have a notion that renaming a formation whose stratigraphic r e l a t i o n is r a t h e r o b s c u r e solves t h e s t r a t i g r a p h i c p r o b l e m . I t is, t h e r e f o r e , o f little use q u o t i n g o u r p o i n t s o f v i e w ad nauseam. F u r t h e r , it is futile t o t r y t o score d e b a t i n g p o i n t s against p e r s o n s w h o c a n t a l k glibly a b o u t dip o f b e d s a n d t h i c k n e s s o f f o r m a t i o n s ( ' w i d t h ' in t h e i r p a r l a n c e ) in a t e r r a i n w h e r e isoclinal folds w i t h c o a l e s c e d l i m b s a n d with e v i d e n c e o f p o s t - b u c k l i n g h o m o g e n e o u s strain are legion*, a n d w h o c a n t h i n k a b o u t p h y s i c a l c o n t i n u i t y a l o n g ' s t r i k e ' o f a b e d w h o s e t e r m i n a t i o n cm~ be p r o v e d t o be d u e t o a fold-hinge. T h e y d o n o t s e e m t o a p p r e c i a t e t h a t t h e s a m e b e d m a y be d e t a c h e d in t h e p r e s e n t t o p o g r a p h i c surface b e c a u s e o f s u p e r p o s e d folding. I t is also pointless to join issue o n scientific ethics w i t h w o r k e r s w h o c a n c o m p l a i n a b o u t t h e i r 1 9 6 8 a b s t r a c t n o t having b e e n cited, b u t can p e r s i s t e n t l y refrain f r o m referring (see Raja R a o et al. ( 1 9 7 1 ) a n d t h e i r discussion) t o t h e first d e t a i l e d p a p e r on a n y p a r t o f t h e c e n t r a l R a j a s t h a n P r e c a m b r i a n t e r r a i n a f t e r H e r o n ' s 1 9 5 3 m e m o i r , b y Naha, C h a u d h u r i a n d B h a t t a c h a r y y a in 1966. My b r i e f r e p l y is m e a n t m a i n l y f o r readers w h o m i g h t be misled b y s t a t e m e n t s w r o n g l y attribu t e d t o us or b y m i s r e p r e s e n t a t i o n o f s o m e c o n c l u s i o n s o f H e r o n . (1) " N a h a et al . . . . finally c o n c l u d e d t h a t B.G.C. d o e s n o t exist t h e r e ! " No; w h a t we c o n c l u d e d was t h a t t h e B a n d e d Gneissic C o m p l e x in a considerable p a r t o f c e n t r a l R a j a s t h a n is n o t pre-Aravalli in age as H e r o n p r e s u m e d , b u t is a m i g m a t i z e d p r o d u c t o f t h e A r a v a l l i - - R a i a l o m e t a s e d i m e n t s as C r o o k s h a n k c o n t e n d e d . Basu a n d A r o r a s h o u l d d o well n o t t o s t o p at t h e p r e f a t o r y summ a r y in H e r o n ' s m e m o i r ; t h e y s h o u l d r e a d t h r o u g h pp. 5 4 - - 9 4 in o r d e r t o * In this terrain, to talk in terms of "stratigraphically above" and "bottom most Aravalli schist" before proving the sequence of beds from primary structures is to beg the question. It should be chastening for Basu and Arora to note that as early as in 1953 Heron could write about a sector showing low-grade metamorphism in this region: " . . . the two branches are isoclines with their axial planes dipping in opposite directions...".
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find out where, according to Heron, the pre-Aravalli Banded Gneissic Complex crops out. (2) "Naha et al. objected to our interpretation of domal structure" in the Kankroli area. No ; all t h a t we inferred from our work was that the Banded Gneissic Complex of the Kankroll area " c a n n o t . . . represent a mantle gneiss dome during the first folding which affected the sediments after they were deposited" -- a different propostion. The swerving of the formational boundaries around the Banded Gneissic Complex in this area is attendant with changes in orientation of axial planes and axes of the isoclinal early folds (matched by structures within the Banded Gneissic Complex), the cause of which is discussed in our paper. As mentioned in the f o o t n o t e of p.66 of our paper, " t h e gneissic body could well have behaved as a resistant mass during the upright later folding". Basu and Arora seem to have missed the significance of structures of the first generation within the banded gneisses which can be proved to be migmatized metasediments, and the importance of the irregular basin-and-dome structures and other interference patterns within as well as outside the gneissic body in this area. (3) "We may point out that the four geological events proposed for central and southern Rajasthan by Naha et al. in their conclusion differ little from Heron's scheme." There is no harm done if new data corroborate Heron's conclusions in their entirety. But as it so happens, Heron considered the Aravalli--Raialo rocks between Sembal and Amet to be later than the Banded Gneissic Complex there; we demonstrated that this is not so. By contrast, the gneissic and granitic rocks south of Udaipur are pre-Aravalli in age as suggested by Heron and confirmed by Poddar on the strength of the conglomerate horizon at the base of the Aravalli System surrounding the Sarara granite body. (I fail to understand why Basu and Arora should again refer to this conglomerate horizon which was never in doubt.) This means that all the outcrops of the Banded Gneissic Complex considered by Heron to be pre-Aravalli are n o t time-equivalent; some are pre-Aravalli and some post-Aravalli in age. Secondly, Heron took the Raialo Series between Sembal and Amet to be unconformable with the Aravalli System. We consider that there is no stratigraphic break between the two groups. (4) Our remark that " a n angular u n c o n f o r m i t y or an overlap, even if it were present, would be impossible to identify" was made particularly keeping in mind the type of basement--cover relation that has been described from Scotland in recent years. It is, therefore, amusing to find Basu and Arora throwing this as an argument in our teeth. The point they seem to have missed is that the onus lies with them to prove that there is indeed an u n c o n f o r m i t y in spite of an apparently conformable relation. Finally, before Basu and Arora write their next paper on Rajasthan geology, it would be better for them to remember that in folded and metamorphosed terrains structure and stratigraphy have to be solved simultaneously, and that rocks with less metamorphism are n o t always less deformed. They should try to find an answer to the question whether the dip and strike of a bed at its
205 p o i n t o f t e r m i n a t i o n are ' s t r u c t u r a l ' or ' s t r a t i g r a p h i c ' data. S o m e h o m e w o r k on their p a r t as t o h o w t o d i f f e r e n t i a t e b e t w e e n a c h a n g e in facies and infolding w o u l d p e r h a p s be helpful. Also, b e f o r e casting d o u b t s on the coverage o f o u r s t u d y , it w o u l d be a h e a l t h y exercise for Basu and A r o r a t o stop republishing H e r o n ' s m a p in smaller a n d smaller scales until it is unintelligible, and start publishing their o w n maps f r o m k e y sectors giving details, so t h a t their observations can be c h e c k e d in the field.* T h a t w o u l d p a r t l y a t o n e for their m i s c o n t r u i n g o u r views, f o r their insinuating t h a t o u r s t u d y was r a t h e r c u r s o r y , and for their v e n t u r i n g t o suggest t h a t we s h o u l d have t a k e n n o t e o f an a b s t r a c t w i t h o u t a n y detail p u b l i s h e d in 1 9 6 8 , whereas t h e y c o u l d aff o r d t o be blissfully oblivious o f a detailed p a p e r published in 1 9 6 6 . REFERENCES Heron, A.M., 1953. The geology of central Rajputana. Mem. Geol. Surv. India, 79: 1--389. Naha, K. and Halyburton, R.V., 1974. Early Precambrian stratigraphy of central and southern Rajasthan, India. Precambrian Res., 1:55--73. Naha, K., Chaudhuri, A.K. and Bhattacharyya, A.C., 1966. Superposed folding in the older Precambrian rocks around Sangat, central Rajasthan, India. Neues Jahrb. Geol. Palaeontol., Abh., 126: 205--231. Raja Rao, C.S., Poddar, B.C., Basu, K.K. and Dutta, A.K., 1971. Precambrian stratigraphy of Rajasthan--a review. Rec. Geol. Surv. India, 101: 60--79.
*It would then be interesting to find out how intensive and extensive is their work, and how it compares in detail with the earliest study by Naha and his associates in the central Rajasthan Precambrian terrain (Naha et al., 1966).
EARLY PRECAMBRIAN STRATIGRAPHY OF CENTRAL AND SOUTHERN R A J A S T H A N , INDIA -- A R E P L Y
K. NAHA Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, West Bengal (India)
(Received June 12, 1975)
Besides r e s t a t i n g t h e i r views with assertion a n d u t t e r i n g s o m e p l a t i t u d e s , Basu a n d Arora, in t h e i r discussion o f t h e p a p e r b y N a h a a n d H a l y b u r t o n , have n o t c i t e d a single piece o f e v i d e n c e t h a t necessitates m o d i f y i n g o u r conclusions. (A p o i n t t h a t e s c a p e d us, h o w e v e r , is t h a t t h e c o n c l u s i o n s in Raja R a o et a1.(1971) o n w h i c h we d i f f e r e d are e n t i r e l y d u e t o Basu a n d Arora.) They seem to have a notion that renaming a formation whose stratigraphic r e l a t i o n is r a t h e r o b s c u r e solves t h e s t r a t i g r a p h i c p r o b l e m . I t is, t h e r e f o r e , o f little use q u o t i n g o u r p o i n t s o f v i e w ad nauseam. F u r t h e r , it is futile t o t r y t o score d e b a t i n g p o i n t s against p e r s o n s w h o c a n t a l k glibly a b o u t dip o f b e d s a n d t h i c k n e s s o f f o r m a t i o n s ( ' w i d t h ' in t h e i r p a r l a n c e ) in a t e r r a i n w h e r e isoclinal folds w i t h c o a l e s c e d l i m b s a n d with e v i d e n c e o f p o s t - b u c k l i n g h o m o g e n e o u s strain are legion*, a n d w h o c a n t h i n k a b o u t p h y s i c a l c o n t i n u i t y a l o n g ' s t r i k e ' o f a b e d w h o s e t e r m i n a t i o n cm~ be p r o v e d t o be d u e t o a fold-hinge. T h e y d o n o t s e e m t o a p p r e c i a t e t h a t t h e s a m e b e d m a y be d e t a c h e d in t h e p r e s e n t t o p o g r a p h i c surface b e c a u s e o f s u p e r p o s e d folding. I t is also pointless to join issue o n scientific ethics w i t h w o r k e r s w h o c a n c o m p l a i n a b o u t t h e i r 1 9 6 8 a b s t r a c t n o t having b e e n cited, b u t can p e r s i s t e n t l y refrain f r o m referring (see Raja R a o et al. ( 1 9 7 1 ) a n d t h e i r discussion) t o t h e first d e t a i l e d p a p e r on a n y p a r t o f t h e c e n t r a l R a j a s t h a n P r e c a m b r i a n t e r r a i n a f t e r H e r o n ' s 1 9 5 3 m e m o i r , b y Naha, C h a u d h u r i a n d B h a t t a c h a r y y a in 1966. My b r i e f r e p l y is m e a n t m a i n l y f o r readers w h o m i g h t be misled b y s t a t e m e n t s w r o n g l y attribu t e d t o us or b y m i s r e p r e s e n t a t i o n o f s o m e c o n c l u s i o n s o f H e r o n . (1) " N a h a et al . . . . finally c o n c l u d e d t h a t B.G.C. d o e s n o t exist t h e r e ! " No; w h a t we c o n c l u d e d was t h a t t h e B a n d e d Gneissic C o m p l e x in a considerable p a r t o f c e n t r a l R a j a s t h a n is n o t pre-Aravalli in age as H e r o n p r e s u m e d , b u t is a m i g m a t i z e d p r o d u c t o f t h e A r a v a l l i - - R a i a l o m e t a s e d i m e n t s as C r o o k s h a n k c o n t e n d e d . Basu a n d A r o r a s h o u l d d o well n o t t o s t o p at t h e p r e f a t o r y summ a r y in H e r o n ' s m e m o i r ; t h e y s h o u l d r e a d t h r o u g h pp. 5 4 - - 9 4 in o r d e r t o * In this terrain, to talk in terms of "stratigraphically above" and "bottom most Aravalli schist" before proving the sequence of beds from primary structures is to beg the question. It should be chastening for Basu and Arora to note that as early as in 1953 Heron could write about a sector showing low-grade metamorphism in this region: " . . . the two branches are isoclines with their axial planes dipping in opposite directions...".
204
find out where, according to Heron, the pre-Aravalli Banded Gneissic Complex crops out. (2) "Naha et al. objected to our interpretation of domal structure" in the Kankroli area. No ; all t h a t we inferred from our work was that the Banded Gneissic Complex of the Kankroll area " c a n n o t . . . represent a mantle gneiss dome during the first folding which affected the sediments after they were deposited" -- a different propostion. The swerving of the formational boundaries around the Banded Gneissic Complex in this area is attendant with changes in orientation of axial planes and axes of the isoclinal early folds (matched by structures within the Banded Gneissic Complex), the cause of which is discussed in our paper. As mentioned in the f o o t n o t e of p.66 of our paper, " t h e gneissic body could well have behaved as a resistant mass during the upright later folding". Basu and Arora seem to have missed the significance of structures of the first generation within the banded gneisses which can be proved to be migmatized metasediments, and the importance of the irregular basin-and-dome structures and other interference patterns within as well as outside the gneissic body in this area. (3) "We may point out that the four geological events proposed for central and southern Rajasthan by Naha et al. in their conclusion differ little from Heron's scheme." There is no harm done if new data corroborate Heron's conclusions in their entirety. But as it so happens, Heron considered the Aravalli--Raialo rocks between Sembal and Amet to be later than the Banded Gneissic Complex there; we demonstrated that this is not so. By contrast, the gneissic and granitic rocks south of Udaipur are pre-Aravalli in age as suggested by Heron and confirmed by Poddar on the strength of the conglomerate horizon at the base of the Aravalli System surrounding the Sarara granite body. (I fail to understand why Basu and Arora should again refer to this conglomerate horizon which was never in doubt.) This means that all the outcrops of the Banded Gneissic Complex considered by Heron to be pre-Aravalli are n o t time-equivalent; some are pre-Aravalli and some post-Aravalli in age. Secondly, Heron took the Raialo Series between Sembal and Amet to be unconformable with the Aravalli System. We consider that there is no stratigraphic break between the two groups. (4) Our remark that " a n angular u n c o n f o r m i t y or an overlap, even if it were present, would be impossible to identify" was made particularly keeping in mind the type of basement--cover relation that has been described from Scotland in recent years. It is, therefore, amusing to find Basu and Arora throwing this as an argument in our teeth. The point they seem to have missed is that the onus lies with them to prove that there is indeed an u n c o n f o r m i t y in spite of an apparently conformable relation. Finally, before Basu and Arora write their next paper on Rajasthan geology, it would be better for them to remember that in folded and metamorphosed terrains structure and stratigraphy have to be solved simultaneously, and that rocks with less metamorphism are n o t always less deformed. They should try to find an answer to the question whether the dip and strike of a bed at its
205 p o i n t o f t e r m i n a t i o n are ' s t r u c t u r a l ' or ' s t r a t i g r a p h i c ' data. S o m e h o m e w o r k on their p a r t as t o h o w t o d i f f e r e n t i a t e b e t w e e n a c h a n g e in facies and infolding w o u l d p e r h a p s be helpful. Also, b e f o r e casting d o u b t s on the coverage o f o u r s t u d y , it w o u l d be a h e a l t h y exercise for Basu and A r o r a t o stop republishing H e r o n ' s m a p in smaller a n d smaller scales until it is unintelligible, and start publishing their o w n maps f r o m k e y sectors giving details, so t h a t their observations can be c h e c k e d in the field.* T h a t w o u l d p a r t l y a t o n e for their m i s c o n t r u i n g o u r views, f o r their insinuating t h a t o u r s t u d y was r a t h e r c u r s o r y , and for their v e n t u r i n g t o suggest t h a t we s h o u l d have t a k e n n o t e o f an a b s t r a c t w i t h o u t a n y detail p u b l i s h e d in 1 9 6 8 , whereas t h e y c o u l d aff o r d t o be blissfully oblivious o f a detailed p a p e r published in 1 9 6 6 . REFERENCES Heron, A.M., 1953. The geology of central Rajputana. Mem. Geol. Surv. India, 79: 1--389. Naha, K. and Halyburton, R.V., 1974. Early Precambrian stratigraphy of central and southern Rajasthan, India. Precambrian Res., 1:55--73. Naha, K., Chaudhuri, A.K. and Bhattacharyya, A.C., 1966. Superposed folding in the older Precambrian rocks around Sangat, central Rajasthan, India. Neues Jahrb. Geol. Palaeontol., Abh., 126: 205--231. Raja Rao, C.S., Poddar, B.C., Basu, K.K. and Dutta, A.K., 1971. Precambrian stratigraphy of Rajasthan--a review. Rec. Geol. Surv. India, 101: 60--79.
*It would then be interesting to find out how intensive and extensive is their work, and how it compares in detail with the earliest study by Naha and his associates in the central Rajasthan Precambrian terrain (Naha et al., 1966).