CORRESPONDENCE Response Organized opposition to infrastructure projects in India does not seem to suffer from shortage of funds. Therefore, it is a valid question to ask whether environmental activism, at least some of it, is driven by non-environmental considerations. Still, if Rauf Ali thinks all this is only a ‘state of mind’, then it can be easily cured by disclosing who pays for obstructing infrastructure projects in India. Why is that such a closely guarded secret? Just as Ali is anguished that, in the context of saving tigers, the opinion of ‘a group of non-scientists’ has prevailed, likewise we too felt aggrieved that the group which met in Bangalore and went
public with some theories about ILR, did not include a single water-resources engineer. Ali’s comment that ‘… at least one economic analysis shows that the costs of pumping the water uphill will make the project unviable’, is based on a paper N. Pelkey, a professor of environmental sciences and information technology in Pennsylvania. He is not a known authority on strategic planning for food, water and energy security for India, wrote his paper before the feasibility reports were made public, thus perhaps without reading them. But Ali seems to think that such a paper by a foreigner from whatever discipline is sufficient to trash 25 years of work by
a team of more than a hundred Indian water-resources engineers in the NWDA, CWC and other specialized institutions of the Government of India – say 2500 engineer-years of work. In that case, since food and energy security has strong strategic implications, whenever India plans major infrastructure projects, one can find papers, and rather easily, that will seek to trash the projects. CHETAN PANDIT Indian Water Resources Society – Delhi Center, C/o Upper Yamuna River Board e-mail:
[email protected]
Nannofossil assemblage in Kutch Jyotsana Rai’s1 report on the occurrence of nannofossils of Albian age from a plant bed of the Bhuj Formation is interesting and significant. It is an accidental but important discovery. She has rightly stressed its importance on the age and environment of deposition of the Bhuj Formation. However, conclusions drawn by her on these two aspects raise controversies and need to be discussed. I had reviewed this paper. Considering the limitations of the study, I suggested modifications in order to avoid contradictions with the existing field data and proven facts. However, it appears that my comments and suggestions were not taken into account while revising the manuscript. For the benefit of the researchers I feel it is necessary to explain here the anomalies created by rash conclusions drawn on limited data. Two important conclusions drawn are: (i) The nannofossil assemblage indicates early Middle Albian age of the Bhuj Formation (referred as Bhuj ‘Member’ in the text by Rai); (ii) The presence of nannofossils confirms the marine environment of the Bhuj Formation supporting ‘an uninterrupted marine succession from at least Late Bajocian to early Middle Albian in Kutch basin’. The following points need to be noted for discussion: 1. Occurrence of nannofossils is limited, only one sample out of two collected from a shale bed in the Bhuj Formation yielded nannoforms.
2. Middle Albian age of the Bhuj Formation has been determined on the basis of one sample only from the Lower Member of the Bhuj Formation in Central Mainland, which is equivalent to the Neocomian Ghuneri Member in Western Mainland, which occurs below the Aptian Ukra Member of the formation. 3. The sample comes from a fossiliferous horizon, which is rich in well-preserved terrestrial plant fossils. The excellent state of preservation of the leaves speaks of provenance proximity and thereby the environment. 4. Association of terrestrial plant fossils and marine nannofossils together in a bed is baffling and needs to be explained. 5. The horizon from where the nannofossil-bearing sample was collected is overlain by an intensely bioturbated zone which is devoid of nannofossils as also the barren shales below it. 6. The sandstone-dominated Bhuj Formation, which is interpreted as marine deposit, is barren of fossil fauna but rich in fossil flora occurring in shale beds. Age of Bhuj Formation: In the type area around Bhuj the formation is 400 m (+) thick and divided informally into two members, lower and upper2,3. The formation thickens enormously towards the west and in Gadhuli–Ghuneri area attains a thickness of over 900 m. In this area the formation comprises three members –
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Ghuneri, Ukra and Upper in ascending order. The palyno-assemblage indicates Neocomian and Albian to (?)Santonian ages for the Ghuneri and Upper members respectively, whereas the ammonite index and absolute dating determined the Aptian age of the Ukra Member. The Neocomian age of the Ghuneri Member is also supported by the ammonite index4. The Ghuneri and Upper members have the same lithofacies association, distinguished only by the local occurrence of Ukra Member between them. As the green, glauconitic shales and marl beds of Ukra Member pinch out, it is difficult to distinguish the two members. Both merge into one formation that continues eastward in the rest of the Mainland as the Bhuj Formation2. This formation comprises more than half of the total thickness of the Mesozoic succession. Detailed mapping by tracing of the marker-defined litho-units (see figure 10 in Biswas 3) established that the Lower Member of the Bhuj Formation of the type area changes laterally into the facies of the Ghuneri Member as the formation thickens westward. Several dark grey, carbonaceous shales with well-preserved fossil-leaf impressions and carbonized plant remains, occur at different levels within the formation. The megaflora and palynomorph (the formation is rich in microflora also) indicate Neocomian age for the Bhuj Formation5 (mainly Lower Member in the type area), which agrees with the stratigraphic position explained above. The reported occurrence of the 571
CORRESPONDENCE nannofossil is from one of the plant beds of the Lower Member exposed near Jakh temple, 25 km west of Bhuj. Apparently, Rai had not taken note of the precise stratigraphic location of the sampled bed since she followed a classification4,6, which is more concerned with the nomenclature priority than the ground reality described above. This misled Rai to believe that Albian ‘Bhuj Member’ (Upper Member2) continues eastward in the Bhuj area, and the sample was collected from the lower part of the member, whereas in reality the sample was collected from the Neocomian Lower Member (= Ghuneri Member). Thus, the reported occurrence of MidAlbian nannofossils in rocks below the Aptian Ukra beds created a stratigraphic anomaly. I would, therefore, suggest that a definite conclusion regarding the Albian age of the Bhuj Formation in the type area should be postponed till all the plant and other shale beds are examined for the nannofossils. Depositional Environment of Bhuj Formation: Presence of marine fossil in sediments does not necessarily mean that the deposit is holomarine. There are reports of occurrence of micro-fauna in aeolian and fluvial deposits 7,8. It is difficult to accept that the Bhuj Formation with well-preserved Upper Gondwana floral assemblage but barren of fossil fauna is holomarine deposit as interpreted by some workers 9, whose views Rai has tried to validate by the reported single occurrence of marine nannofossils. She does not discuss the contradictory evidence presented by the well-preserved plant fossils and marine nannofossils in the same bed. Proponents of marine deposition4,9 based their opinion mainly on the repeated occurrence of trace fossils and the bioturbated ferruginous beds in the formation. They tried to explain the absence of hardbodied fossils by desolution processes, but do not mention the absence of microfauna and preservation of fragile terrestrial plant fossils in the so-called marine sediments. Mere presence of bioturbated zones or trace fossils does not evince a marine origin for the host sediments. Trace fossils represent behavioural traits of organisms and it is an established fact that like behaviour can be seen in all types of environments 10. Detailed study of the trace fossils reveals that they are typically restricted occurrence of ichno assemblage in transitional environments (K. G. Kulkarni, pers. commun.). 572
The Mesozoic sequence typically represents a transgressive-regressive megacycle11,12. The early Middle Jurassic transgressive sequence is characterized by highly fossiliferous shale–limestone– sandstone litho-association. The upper Late Jurassic–Early Cretaceous, thick regressive sequence (Bhuj Formation) is predominantly sandy and barren of fossil fauna, but rich in fossil flora and ichnofossils. Based on detailed studies and extensive mapping, the Bhuj Formation has been interpreted as a wave-dominated estuarine palaeo-delta with well-developed aggradational/progradational sequences during normal regression of the sea 11,12. The delta prograded westward progressively shifting the wavefront, which left the marine (tidal) signatures like bioturbated sediments and occasional mollusk shells (poorly preserved) across the basin. In the delta front zone in Western Mainland, the fossiliferous Ukra Member represents a short transgressive break in the delta progradation during a high stand. In the east, thick sequences characterized by multistoried stacks of current-bedded sandstones with frequent channel cut and fills represent the proximal fluvial facies of the formation11. Therefore, the conclusion by Rai that ‘an uninterrupted marine succession from Late Bajocian to Middle Albian occurs in Kutch Basin’, is only partially true for the western end of the basin where the transitional facies of the Lower Member grades into the coastal facies of the Ghuneri Member in the delta front. Once it is understood that the host rocks are deposits of transitional environment, it is not difficult to explain the apparently contradictory occurrence of plant and marine fossils together in a carbonaceous shale bed. In estuarine delta environment tidal currents penetrate deep into the hinterland during high tides. Further, penetration of tidal current is deeper over the prograding delta lobes during sea-level highstands in fluctuating conditions. In the present case, tidal current during high tides carried the planktonic nannofossils towards the hinterland over the swampy lower delta plain, where these tiny fossils were trapped with the leaves and other plant remains in lakes and local pools. In fact, such occurrence is expected in tide-dominated prograding delta front and provides a supporting evidence for deltaic environment of deposition11,12. 1. Rai, Jyotsana, Curr. Sci., 2006, 91, 519– 526.
2. Biswas, S. K., Geology of Kutch, vols I and II, Spl. Publ. KDMIPE, ONGC, Dehradun, 1993, p. 415. 3. Biswas, S. K., Q. J. Geol., Min. Metall. Soc. India, 1977, 49, 1–52. 4. Krishna, J., Newsl. Stratigr., 1991, 23, 141–150. 5. Venkatachala, B. S., Palaeobotanists, 1969, 18, 75–86. 6. Krishna, J., Singh, I. B., Howard, J. D. and Jafar, S. A., 1983, 305, 790–792. 7. Biswas, S. K., Sediment. Geol., 1971, 5, 147–164. 8. Raj, Rachna and Chamyal, L. S., J. Palaeontol. Soc. India, 1998, 43, 55–67. 9. Howard, J. D. and Singh, I. B., Palaeogeogr., Palaeoclimatol., Palaeoecol., 1985, 52, 99–122. 10. Ekdale, A. A., Bromley, R. G. and Pemberton, S. G., Short Course No. 15, Society of Economic Paleontologists and Mineralogists, Tulsa, OK, 1984, p. 317. 11. Biswas, S. K., In Sedimentary Basins of India, Tectonic Context (eds Tandon, S. K., Pant, C. C. and Casshyap, S. M.), Gyanodaya Prakashan, Nainital, 1991, pp. 74–103. 12. Biswas, S. K., Q. J. Geol., Min. Metall. Soc. India, 1981, 53, 56–85.
S. K. BISWAS 201/C, ISM House, Thakur Village, Kandivali (East), Mumbai 400 101, India e-mail:
[email protected]
Response: S. K. Biswas, a name synonymous with Kutch stratigraphy has always been a source of inspiration throughout my research career in the Kutch basin and his comments on my paper are welcome. I wish to add here that the suggestions and corrections by the two referees for the revision of the manuscript were contrasting. I modified the manuscript based on these comments. However, many of the comments were not valid and hence not incorporated. It may be added here that lithostratigraphic mapping of Kutch was done by Biswas1. Later work on palaeobiology and depositional facies has provided a more precise interpretation on depositional environment 2. The queries raised by Biswas are addressed pointwise below. The present rare but important finding throws light on the precise age and environment of part of Umia Formation exposed in this part of the succession. 1. The nannofossil assemblage recorded in my study, although only from one
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2.
3.
4.
5.
6.
sample, gives a precise age for Bhuj sediments in the eastern part of Kutch. Earlier ages were only presumptions without any data. The early Middle Albian age for Bhuj Member given by me is based on precise global nannoplankton markers. The age of Ukra Member in the Western Mainland based on ammonoids is Aptian–Albian2 and not Aptian only as claimed by Biswas. The lithological boundaries are not time boundaries, hence age of the central mainland succession cannot be determined with precision from a correlation with western Kutch, as suggested by Biswas. The nannofossil-yielding samples of my study come from a carbonaceous shale unit. This lithofacies are interpreted as deposits of a coastal lagoon3. Coastal lagoons mostly contain high content of plant debris along with marine fauna. There is no ambiguity in interpretation. Occurrence of nannofossil along with plant debris supports coastal lagoon depositional environment. Biswas argues that plant fossils are well preserved. My study clearly indicates that the plant debris is poorly preserved (impression and not compression) and shows evidence of high bacterial decay. As mentioned above marine nannofossils and other marine fossils can occur together with plant fossils in a coastal lagoon and near-shore environments, and there is nothing baffling about it. The bioturbated zone and barren shales below are highly oxidized and non-calcareous. Such lithologies do not preserve nannofossils. World over, there are thick coastal sandy sequences deposited in a marine system which do not preserve marine fauna (I. B. Singh, pers. commun.). Preservation of plant fossils within Bhuj Member is in lagoonal deposit. Bhuj sandstone has also yielded marine bivalve Indotrigonia4.
Age of Umia (Bhuj) Formation: I have followed the traditional names (Patcham, Chari, Katrol and Umia formations) 5. Later classification (Jhurio, Jumara, Juran and Bhuj formations) 1 has inherent problems as discussed by others. The purpose of the present communication was not to discuss the merits of various classification schemes. Bhuj Formation has been divided into three members in the western part of
Kutch (Ghuneri, Ukra and Upper) 1. The Ghuneri and Upper members are similar in character. The Ukra Member contains ammonoids of Aptian–Albian2,6. In the central part of Kutch two informal Lower and Upper members of the Bhuj Formation (= Umia Formation) are erected1 without a boundary between them. For the sake of convenience and lithological similarity, the Ghuneri and Upper members of western Kutch are correlated with the Lower and Upper members of central Kutch with no age control 1. Palynotaxa can be used for palaeovegetation reconstruction and not for precise age determination. Lithostratigraphic mapping of lithounits and marker beds cannot be taken as time lines. Lithological boundaries are mostly time-transgressive, especially when traced over long distances. Thus lithological correlation between western and central Kutch cannot be taken as time correlation. In this context Ukra Member (Aptian–Albian) now can be considered as time equivalent to nannofossil-yielding horizon (Albian) in central Kutch, as it is based on precise age determination by index fossils. It is now possible to correlate the central and western parts of Bhuj Formation of Kutch using time-marker fossils. Even if the present study is based on one sample, it can be more precisely used than lithostratigraphic and palynological correlation. I can understand the concern of Biswas, as it strongly changes his correlation scheme of the Bhuj Formation between western and central Kutch, but this is the way science progresses. Depositional environment of Umia (= Bhuj) Formation: If we try to see things with a pre-conceived notion, then it would be difficult to accept the marine nature of the Umia Formation. Since the Ukra Member contains ammonoids, there is no problem in accepting its age and marine nature. It is only the lower Ghuneri and upper Bhuj members, which are normally devoid of marine macro- or micro-fauna, that are considered as fluvial deposits1. Interpretation of coastal marine environment of Bhuj Formation is based on detailed facies analysis of the succession, emphasizing facies sequences and other sedimentary structures3 (U. K. Shukla, unpublished). Coastal lagoon, estuarine channel, tidal flat and shelf sheet sand depositional domains have been identified and they occur in a predictive cyclic vertical succession3. Biswas uses a term holomarine. It is not used in depositional
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environmental studies. In the interpretation of depositional environment all the sedimentary features along with flora and fauna are considered. It is true that in the coastal zone marine microfossils are transported and deposited in coastal aeolian domes, but the nature of sedimentary structures helps identify deposits as Aeolian. The bioturbated horizons identified7 in the Bhuj Formation were earlier interpreted as laterite horizons1. Now there seems a view that these laterite beds are bioturbated horizons. The Umia Formation (= Bhuj Formation) has been interpreted as deposits in embayment with many small estuarine channels formed in a tide-dominated shallow sea using detailed facies analysis (U. K. Shukla, unpublished). The need of the hour is to search all the plant beds of Umia Formation for presence of nannofossil, with the hope to find nannofossils and other marine fauna from several horizons. Biswas negates the use of trace fossils as indicators of depositional environment7,8, but quotes unpublished study supporting use of trace fossils as environmental indicators. Again the term transitional environment is vague. Detailed facies analysis of the Bhuj Formation3 is available, which Biswas seems to have ignored. Biswas tries to interpret the Bhuj Formation as deposits of delta system with influence of tides and waves. A delta is built mostly by marine processes in a sea and hence is part of a marine depositional system where different coastal facies dominate. This has been already documented3,7. 1. Biswas, S. K., Q. J. Geol. Min. Metall. Soc. India, 1977, 49, 1–52. 2. JaiKrishna, Singh, I. B., Howard, J. D. and Jafar, S. A., Nature, 1983, 305, 790–792. 3. Shukla, U. K. and Singh, I. B., J. Palaeontol. Soc. India, 1990, 35, 189–196. 4. Shukla, U. K. and Singh, I. B., Curr. Sci., 1993, 65, 171–174. 5. Waagen, W., Palaeontol. Indica, 1875, 9.1. 6. JaiKrishna, Newsl. Stratigr., 1991, 23, 141–150. 7. Singh, I. B. and Shukla, U. K., J. Palaeontol. Soc. India, 1991, 36, 121–126. 8. Howard, J. D. and Singh, I. B., Palaeogeogr., Palaeoclimatol., Palaeoecol., 1985, 52, 99–122.
J YOTSANA RAI Birbal Sahni Institute of Palaeobotany, Lucknow 226 007, India e-mail:
[email protected] 573
