- Email: [email protected]
Structural Framework of Deolapar Area, Central India and its Implications for Proterozoic Nappe Tectonics: Comment*
934
Moore, G.T., Hayashida, D.N., Ross, C.A. and Jacobson, S.R., (1992) Paleoclimate of the Kimmeridgian/Tithonian (late Jurassic) world: results using a general circulation model. Palaeogeogr. Palaeoclimatol. Palaeoecol., v. 93, pp. 47-72. Nesterov, 1.1. and Ushatinsky, I.N. (1991) Black shales strata of Western Siberia. Int. Symp. Black shale basins and related mineral deposits, IGCP Project 254, Novosibirsk, pp. 298-300. Nesterov, I.I., Salvamanov, EK., Kontorovich, A.E., Kulakhmetov, N.K., Surkov, V.S., Trofimuk, A.A.and Shpilman, V.I. (1990) West Siberian oil and gas superprovince. In: J. Brooks (Ed.), Classic petroleum provinces. Geol. SOC.Spl. Publ., v. 50, pp. 491-502.
Parrish, J.T. (1993) Climate of the supercontinent pangea. J. Geol., v. 101, pp. 215-233. Schidlowski, M., (1987) Application of stable carbon isotopes to early biochemical evolution on earth. Amer. Rev. Earth Planet. Sci., v. 15, pp. 47-72. Singh, N.P., (1996) Mesozoic-tertiary biostratigraphy and biogeochronological datum planes in Jaisalmer Basin, Rajasthan. Cont. XV Ind. Colloq. Micropal. Strat. Dehradun, 63-89 KDMIPE and WIHG publ. Weissert, H. (1989) C-isotope stratigraphy, a monitor of paleoenvironmental change: a case study from the early Cretaceous. Surv. Geophys., v. 10, pp. 1-61.
Gondwana Research (Gondwana Newsletter Section) V 6, No. 4, pp. 934-935. 02003 lnternational Association for Gondwana Research, Japan.
GNL
DISCUSSION
Structural Framework of Deolapar Area, Central India and its Implications for Proterozoic Nappe Tectonics: Comment‘ S. Mohanty Department of Applied Geology, Indian School of Mines, Dhanbad - 826004, India, E-mail: [email protected] ~
Deolapar area has attracted attention of Precambrian geologists of India for report of a nappe structure by West (1936), mainly on the basis of stratigraphic omissions and inversions in the Sausar Group. In a recently published paper, Chattopadhyay et al. (2003) have tried to solve the problem of nappe controversy by remapping the area around Deolapar. However, when we compare the map published by West (1936) on a scale larger than that of Chattopadhyay et al. (2003), it becomes clear that some of the metasedimentary units (Chorbaoli Formation and Junewani Formation) mapped by West (1936) show westward closures between Kadbikhera and Khapa, but t h e s e have b e e n regrouped as Tirodi Gneiss by Chattopadhyay et al. (2003) without any closures. No explanation is available for this change. In addition, the paper by Chattopadhyay et al. (2003) has the following lacunae, which require clarification.
Evidence of Thrusting The authors have stated that the basement block shows evidence of shearing (p. 109, col. 2, Sth line from the bottom), but neither attempt was made to decipher the
sense of shearing from the foliated fabric nor the orientation of any lineation related to shearing is reported. Subsequently the authors have stated that typical fault shear zone rocks are absent in the area (p. 115, col. 2, line 1-4). The only evidence of thrusting is mentioned to be a ‘fibrolite-biotite schist’,mapped at the contact of the Tirodi Gneiss (basement block) and the adjacent supracrustal rocks (Sausar Group). The authors considered that the granitic rocks of the basement were considerably modified by fluid flow during thrusting to give rise to aluminum-rich protoliths, which were modified to give the fibrolite biotite schists. However, alternative explanation (cf. Mohanty, 1993) for the development of such aluminum-rich protoliths by weathering and leaching of the granitic basement rocks (development of palaeosol) before the deposition of the Sausar Group is possible. This alternative explanation would explain the absence of any shear zone rocks along the contact of the basement and cover rocks. The authors have not given any reason against such a possibility.
Evidence for Unconformable Relation Chattopadhyay et al. (2003a) state that “Typical
’*SeeAnupam Chattopadhyay e t al. (2003) Gondwana Research, v. 6, pp. 207-1 27
unconformable b a s e m e n t cover relation ... is not observed”. Unpublished work by me and my co-worker in t h e s a m e a r e a h a s s h o w n t h a t a polymictic conglomerate horizon does exist at the contact of the Tirodi Gneiss and the Sausar Group in the area around Deolapar. It may be pointed out that Chattopadhyay and his co-workers (Chattopadhyay et al., 2003b) also failed to identify similar polymictic conglomerate horizon from the southern part of the Sausar belt (cf. Mohanty, 1993; Mohanty, 2003).
Folded Thrust The curved nature of the supposed thrust outcrop led the authors to suggest that “the thrust has been folded by later (F,) upright folds”. However, it is well known in thrust tectonics that most of the thrust surfaces are curviplanar in geometry and produce antiforms and synforms over ramps and flats. No evidence is provided to prove that the supposed thrust is indeed folded and not curviplanar in geometry.
Interpretation of the Map It is interesting to note that Chattopadhyay et al. (2003a) have shown a thin belt of Tirodi Gneiss inside t h e dolomitic marble of Bichua Formation (Fig. 2). This boundary is structurally higher up than the supposed thrust boundary. No explanation for such a map pattern is given by the authors. Again, in the area around Manegaon, circular, elliptical and crescent shaped outcrops of marble surrounded in all sides by the Tirodi Gneiss are shown. These outcrops are also away from the supposed thrust contact. The authors consider that the supposed t h r u s t i n g developed d u r i n g t h e first deformation. As the authors consider that no regional F, folds are present in the area, the crescent shaped outcrops in this part should be explained by the authors.
Sausar Belt as Mesoproterozoic Fold Belt Sausar Fold Belt occurs to the south of the Vindhyan Mountains. The Vindhyan Supergroup is considered to be deposited after Satpura (Sausar) orogeny (cf. Ram et al., 1996; Raza and Casshyap, 1996; Chakraborty and Bhattacharyya, 1996). The age of the basal succession of the Vindhyan Supergroup is found to be 1600 m.y. (Rasniussen e t al., 2002; Ray et al., 2002). Therefore, the Sausar fold belt can not be Mesoproterozoic (as suggested by Chattopadhyay et al., 2003) but has to be
Gondwnnn Research, K 6,No. 4, 2003
Palaeoproterozoic age. Constraining thrust emplacement at 900-950 m.y. and correlation of Sausar (Satpura) with Grenville orogeny by Chattopadhyay et al. (2003a) need justification. The metadolerite dykes of Tirodi Gneiss, terminating at the contact of Sausar Group, are reported to be 1000 m.y. old, and the Sausar metamorphism is thought to be 850 m.y. (Chattopadhyay et al., 2003a). These dates for Tirodi Gneiss a n d Sausar Group would p u t t h e m stratigraphically above the Lower Vindhyan Supergroup against the accepted status of these units in Indian geology. These radiometric ages quoted by the authors reflect only the last thermal event and need to be re examined carefully.
References Chakraborty, C. and Bhattacharyya, A. (1996) Fan-delta sedimentation in a foreland moat: Deoland Formation, Vindhyan Supergroup, Son Valley. Geol. SOC.India, Mem. NO. 36, pp. 27-48. Chakraborty, C. and Bhattacharyya, A. (1996) The Vindhyan Basin: an overview in the light of current perspective. Geol. SOC.India Mem. No. 36, pp. 301-312. Chattopadhyay,A., Huin, A.K. and Khan, A S . (2003a) Structural framework of Deolapar Area, Central India and its Implications for Proterozoic Nappe Tectonics. Gondwana Res., v. 6, pp. 107-117. Chattopadhyay, A,, Khan, A.S., H u h , A.K. and Bandyopadhyay, B.K. (2003b) Reinterpretation of Stratigraphy and Structure of Sausar Group in Ramtek-Mansar-KandriArea, Maharashtra, Central India. J. Geol. SOC.India, v. 61, pp. 75-89. Mohanty, S. (1993) Stratigraphic position of the Tirodi Gneiss in the Precambrian Terrane of Central India: evidence from the Mansar area, Nagpur district, Maharashtra. J. Geol. SOC. India, v. 42, pp. 55-60. Mohanty, S. (2003) Reinterpretation of Stratigraphy and Structure of Sausar Group in Ramtek-Mansar-KandriArea, Maharashtra, Central India: discussion. J. Geol. SOC.India (in press). Ram, J., Shukla, S.N., Pramanik, A.G., Varma, B.K., Chandra, G. and Murty, M.S.N. (1996) Recent investigations in the Vindhyan Basin: implications for the basin tectonics. Geol. SOC.India, Mem. No. 36, pp. 267-286. Rasmussen, B., Bose, PK., Sarkar, S., Banerjee, S., Fletcher, I.R. and McNaughton, N.J. (2002) 1.6 Ga U-Pb zircon age for the Chorhat Sandstone, lower Vidhyan, India: possible implications for early evolution of animals. Geology, v. 30, pp. 103-106. Ray, J.S., Martin, M.W., Veizer, J. and Bowring, S.A. (2002) U-Pb zircon dating and Sr isotope systematics of the Vindhyan Supergroup, India. Geology, v. 30, pp. 131-134. Raza, J. and Casshyap, S.M. (1996) A tectono-sedimentary model of evolution of Middle Proterozoic Vindhyan Basin. Geol. SOC.India, Mem. No. 36, pp. 287-300. West, W.D. (1936) Nappe structure in the Archaean rocks of the Nagpur district. Trans. Nat. Inst. Sci. India, v. I,pp. 93-102.
Moore, G.T., Hayashida, D.N., Ross, C.A. and Jacobson, S.R., (1992) Paleoclimate of the Kimmeridgian/Tithonian (late Jurassic) world: results using a general circulation model. Palaeogeogr. Palaeoclimatol. Palaeoecol., v. 93, pp. 47-72. Nesterov, 1.1. and Ushatinsky, I.N. (1991) Black shales strata of Western Siberia. Int. Symp. Black shale basins and related mineral deposits, IGCP Project 254, Novosibirsk, pp. 298-300. Nesterov, I.I., Salvamanov, EK., Kontorovich, A.E., Kulakhmetov, N.K., Surkov, V.S., Trofimuk, A.A.and Shpilman, V.I. (1990) West Siberian oil and gas superprovince. In: J. Brooks (Ed.), Classic petroleum provinces. Geol. SOC.Spl. Publ., v. 50, pp. 491-502.
Parrish, J.T. (1993) Climate of the supercontinent pangea. J. Geol., v. 101, pp. 215-233. Schidlowski, M., (1987) Application of stable carbon isotopes to early biochemical evolution on earth. Amer. Rev. Earth Planet. Sci., v. 15, pp. 47-72. Singh, N.P., (1996) Mesozoic-tertiary biostratigraphy and biogeochronological datum planes in Jaisalmer Basin, Rajasthan. Cont. XV Ind. Colloq. Micropal. Strat. Dehradun, 63-89 KDMIPE and WIHG publ. Weissert, H. (1989) C-isotope stratigraphy, a monitor of paleoenvironmental change: a case study from the early Cretaceous. Surv. Geophys., v. 10, pp. 1-61.
Gondwana Research (Gondwana Newsletter Section) V 6, No. 4, pp. 934-935. 02003 lnternational Association for Gondwana Research, Japan.
GNL
DISCUSSION
Structural Framework of Deolapar Area, Central India and its Implications for Proterozoic Nappe Tectonics: Comment‘ S. Mohanty Department of Applied Geology, Indian School of Mines, Dhanbad - 826004, India, E-mail: [email protected] ~
Deolapar area has attracted attention of Precambrian geologists of India for report of a nappe structure by West (1936), mainly on the basis of stratigraphic omissions and inversions in the Sausar Group. In a recently published paper, Chattopadhyay et al. (2003) have tried to solve the problem of nappe controversy by remapping the area around Deolapar. However, when we compare the map published by West (1936) on a scale larger than that of Chattopadhyay et al. (2003), it becomes clear that some of the metasedimentary units (Chorbaoli Formation and Junewani Formation) mapped by West (1936) show westward closures between Kadbikhera and Khapa, but t h e s e have b e e n regrouped as Tirodi Gneiss by Chattopadhyay et al. (2003) without any closures. No explanation is available for this change. In addition, the paper by Chattopadhyay et al. (2003) has the following lacunae, which require clarification.
Evidence of Thrusting The authors have stated that the basement block shows evidence of shearing (p. 109, col. 2, Sth line from the bottom), but neither attempt was made to decipher the
sense of shearing from the foliated fabric nor the orientation of any lineation related to shearing is reported. Subsequently the authors have stated that typical fault shear zone rocks are absent in the area (p. 115, col. 2, line 1-4). The only evidence of thrusting is mentioned to be a ‘fibrolite-biotite schist’,mapped at the contact of the Tirodi Gneiss (basement block) and the adjacent supracrustal rocks (Sausar Group). The authors considered that the granitic rocks of the basement were considerably modified by fluid flow during thrusting to give rise to aluminum-rich protoliths, which were modified to give the fibrolite biotite schists. However, alternative explanation (cf. Mohanty, 1993) for the development of such aluminum-rich protoliths by weathering and leaching of the granitic basement rocks (development of palaeosol) before the deposition of the Sausar Group is possible. This alternative explanation would explain the absence of any shear zone rocks along the contact of the basement and cover rocks. The authors have not given any reason against such a possibility.
Evidence for Unconformable Relation Chattopadhyay et al. (2003a) state that “Typical
’*SeeAnupam Chattopadhyay e t al. (2003) Gondwana Research, v. 6, pp. 207-1 27
unconformable b a s e m e n t cover relation ... is not observed”. Unpublished work by me and my co-worker in t h e s a m e a r e a h a s s h o w n t h a t a polymictic conglomerate horizon does exist at the contact of the Tirodi Gneiss and the Sausar Group in the area around Deolapar. It may be pointed out that Chattopadhyay and his co-workers (Chattopadhyay et al., 2003b) also failed to identify similar polymictic conglomerate horizon from the southern part of the Sausar belt (cf. Mohanty, 1993; Mohanty, 2003).
Folded Thrust The curved nature of the supposed thrust outcrop led the authors to suggest that “the thrust has been folded by later (F,) upright folds”. However, it is well known in thrust tectonics that most of the thrust surfaces are curviplanar in geometry and produce antiforms and synforms over ramps and flats. No evidence is provided to prove that the supposed thrust is indeed folded and not curviplanar in geometry.
Interpretation of the Map It is interesting to note that Chattopadhyay et al. (2003a) have shown a thin belt of Tirodi Gneiss inside t h e dolomitic marble of Bichua Formation (Fig. 2). This boundary is structurally higher up than the supposed thrust boundary. No explanation for such a map pattern is given by the authors. Again, in the area around Manegaon, circular, elliptical and crescent shaped outcrops of marble surrounded in all sides by the Tirodi Gneiss are shown. These outcrops are also away from the supposed thrust contact. The authors consider that the supposed t h r u s t i n g developed d u r i n g t h e first deformation. As the authors consider that no regional F, folds are present in the area, the crescent shaped outcrops in this part should be explained by the authors.
Sausar Belt as Mesoproterozoic Fold Belt Sausar Fold Belt occurs to the south of the Vindhyan Mountains. The Vindhyan Supergroup is considered to be deposited after Satpura (Sausar) orogeny (cf. Ram et al., 1996; Raza and Casshyap, 1996; Chakraborty and Bhattacharyya, 1996). The age of the basal succession of the Vindhyan Supergroup is found to be 1600 m.y. (Rasniussen e t al., 2002; Ray et al., 2002). Therefore, the Sausar fold belt can not be Mesoproterozoic (as suggested by Chattopadhyay et al., 2003) but has to be
Gondwnnn Research, K 6,No. 4, 2003
Palaeoproterozoic age. Constraining thrust emplacement at 900-950 m.y. and correlation of Sausar (Satpura) with Grenville orogeny by Chattopadhyay et al. (2003a) need justification. The metadolerite dykes of Tirodi Gneiss, terminating at the contact of Sausar Group, are reported to be 1000 m.y. old, and the Sausar metamorphism is thought to be 850 m.y. (Chattopadhyay et al., 2003a). These dates for Tirodi Gneiss a n d Sausar Group would p u t t h e m stratigraphically above the Lower Vindhyan Supergroup against the accepted status of these units in Indian geology. These radiometric ages quoted by the authors reflect only the last thermal event and need to be re examined carefully.
References Chakraborty, C. and Bhattacharyya, A. (1996) Fan-delta sedimentation in a foreland moat: Deoland Formation, Vindhyan Supergroup, Son Valley. Geol. SOC.India, Mem. NO. 36, pp. 27-48. Chakraborty, C. and Bhattacharyya, A. (1996) The Vindhyan Basin: an overview in the light of current perspective. Geol. SOC.India Mem. No. 36, pp. 301-312. Chattopadhyay,A., Huin, A.K. and Khan, A S . (2003a) Structural framework of Deolapar Area, Central India and its Implications for Proterozoic Nappe Tectonics. Gondwana Res., v. 6, pp. 107-117. Chattopadhyay, A,, Khan, A.S., H u h , A.K. and Bandyopadhyay, B.K. (2003b) Reinterpretation of Stratigraphy and Structure of Sausar Group in Ramtek-Mansar-KandriArea, Maharashtra, Central India. J. Geol. SOC.India, v. 61, pp. 75-89. Mohanty, S. (1993) Stratigraphic position of the Tirodi Gneiss in the Precambrian Terrane of Central India: evidence from the Mansar area, Nagpur district, Maharashtra. J. Geol. SOC. India, v. 42, pp. 55-60. Mohanty, S. (2003) Reinterpretation of Stratigraphy and Structure of Sausar Group in Ramtek-Mansar-KandriArea, Maharashtra, Central India: discussion. J. Geol. SOC.India (in press). Ram, J., Shukla, S.N., Pramanik, A.G., Varma, B.K., Chandra, G. and Murty, M.S.N. (1996) Recent investigations in the Vindhyan Basin: implications for the basin tectonics. Geol. SOC.India, Mem. No. 36, pp. 267-286. Rasmussen, B., Bose, PK., Sarkar, S., Banerjee, S., Fletcher, I.R. and McNaughton, N.J. (2002) 1.6 Ga U-Pb zircon age for the Chorhat Sandstone, lower Vidhyan, India: possible implications for early evolution of animals. Geology, v. 30, pp. 103-106. Ray, J.S., Martin, M.W., Veizer, J. and Bowring, S.A. (2002) U-Pb zircon dating and Sr isotope systematics of the Vindhyan Supergroup, India. Geology, v. 30, pp. 131-134. Raza, J. and Casshyap, S.M. (1996) A tectono-sedimentary model of evolution of Middle Proterozoic Vindhyan Basin. Geol. SOC.India, Mem. No. 36, pp. 287-300. West, W.D. (1936) Nappe structure in the Archaean rocks of the Nagpur district. Trans. Nat. Inst. Sci. India, v. I,pp. 93-102.