RESEARCH COMMUNICATIONS. R. J. Azmi* and S. K. Paul

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1 The highly sinuous and deeply incised Vishwamitri river exhibits a compressed meandering morphology. Lithological control in the formation of meanders in the Vishwamitri river is ruled out as it flows through an alluvial plain. Uplift and/or tilting of the land surface could be the major factors influencing rivers in which lithological control is negligible. The deeply incised meander valleys are found in rivers where there is uplift upstream 18. In the Vishwamitri basin a similar situation occurs, where the river descends from the tectonically uplifted area resulting in incision, as suggested by Alexander and Leeder 14. The meanders of the Vishwamitri river could be called confined meanders 21 as they are confined within high valley banks. The distortions in the meanders are controlled by the subsurface structures. The course of the Vishwamitri river in the alluvial plain does not follow the SW regional slope and instead follows NNE SSW trending course. The asymmetry of the drainage basin, high values of sinuosity and entrenched nature of meanders suggest tectonics as the major influence on the channel morphology of the river. The orientations of the meander loops point to the general direction of the shift of the river channel towards the east. Presence of palaeodrainage in the western side of the present-day channel substantiates this fact. A comparison of the river channels between 1969 and 2003 also corroborates this fact. In view of the structural set-up and neo-tectonic activity 8 we infer that eastward tilting of the area is responsible for the eastward migration of the Vishwamitri river. 1. Nanson, G. C. and Knighton, A. D., Anabranching rivers: their cause, character and classification. Earth Surf. Processes Landforms, 1996, 21, Leopold, L. B. and Wolman, M. G., River channel patterns: braided, meandering and straight. U.S. Geol. Surv. Prof. Pap., 1957, 282, Burnett, A. and Schumm, S. A., Neotectonics and alluvial river response. Science, 1983, 222, Schumm, S. A., Khan, H. R., Winkley, B. R. and Robbins, L. G., Variability of river patterns. Nature, 1972, 237, Schumm, S. A., Dumont, J. F. and Holbrook, J. M., Active Tectonics and Alluvial Rivers, Cambridge University Press, Cambridge, 2000, p Merh, S. S., Geology of Gujarat, Geological Society of India, Bangalore, 1995, p Mukherjee, M. K., The Quaternary landforms of Broach block of Cambay basin Importance of these in petroleum exploration. Paper presented in National Seminar on Quaternary Environments with Special Reference to Western India, M.S. University, Baroda, Maurya, D. M., Rachna Raj and Chamyal, L. S., History of tectonic evolution of Gujarat alluvial plains, western India during Quaternary: a review. J. Geol. Soc. India, 2000, 55, Chamyal, L. S., Maurya, D. M., Bhandari, S. and Rachna Raj, Late Quaternary geomorphic evolution of the lower Narmada valley, western India: implications for neotectonic activity along the Narmada Son Fault. J. Geomorphol., 2002, 46, Chamyal, L. S., Maurya, D. M. and Rachna Raj, Fluvial systems of the drylands of western India: a synthesis of Late Quaternary environmental and tectonic changes. J. Quat. Int., 2003, 104, Juyal, N., Rachna Raj, Maurya, D. M., Chamyal, L. S. and Singhvi, A. K., Chronology of Late Pleistocene environmental changes in the lower Mahi basin, western India. J. Quat. Sci., 2000, 15, Kusumgar, S., Rachna Raj, Chamyal, L. S. and Yadav, M. G., Holocene palaeoenvironmental changes in the lower Mahi basin, western India. Radiocarbon, 1998, 40, Cox, R. T., Van Arsdale, R. B. and Harris, J. B., Identification of possible Quaternary deformation in the northwestern Mississippi Embayment using quantitative geomorphic analysis of drainage basin asymmetry. Bull. Geol. Soc. Am., 2001, 113, Alexander, J. and Leeder, M. K., Active tectonic control of alluvial architecture. In Recent Developments in Fluvial Sedimentology (eds Ethridge, F. G., Flores, R. M. and Harvey, M. D.), SEPM, Spl. Publ., 1987, vol. 39, pp Nanson, G. C., A regional trend to meander migration. J. Geol., 1980, 88, Langbein, W. B. and Leopold, L. B., River meanders Theory of the minimum variance. U.S. Geol. Surv. Prof. Pap., 1966, 422-H, H1 H Brice, J., Meander pattern of the White River in Indiana An analysis. In Fluvial Geomorphology (ed. Morisawa, M.), 1981, pp Hunt, C. B., Geological history of the Colorado river. U.S. Geol. Surv. Prof. Pap., 1969, 669, Gardner, T. W., The history of part of the Colorado river and its tributaries: An experimental study. Four Corners Geological Society Guidebook, Field Conference (Canyonlands), 1975, vol. 9, pp Schumm, S. A., The Fluvial System, Wiley, New York, 1977, p Lewin, J. and Brindle, B. J., Confined meanders. In River Channel Changes (ed. Gregory, K. J.), Wiley, 1977, pp ACKNOWLEDGEMENT. Research grant to R. R. received from Department of Science and Technology (SR/FTP/ES-11/2000) is gratefully acknowledged. Received 5 June 2003; revised accepted 14 January 2004 Discovery of Precambrian Cambrian boundary protoconodonts from the Gangolihat Dolomite of Inner Kumaun Lesser Himalaya: Implication on age and correlation R. J. Azmi* and S. K. Paul Wadia Institute of Himalayan Geology, 33, General Mahadeo Singh Road, Dehra Dun , India The Gangolihat Dolomite (Deoban Formation) of the Calc Zone of Pithoragarh in the Inner Kumaun Lesser Himalaya, hitherto regarded of Mesoproterozoic age (~ Ma) on the basis of the so-called Riphean stromatolites, has yielded numerous earliest Cambrian protoconodont sclerites characterizing the Precam- *For correspondence. ( azmirj@rediffmail.com) CURRENT SCIENCE, VOL. 86, NO. 12, 25 JUNE

2 brian Cambrian boundary (~544 Ma). This fossil discovery fosters a firm chronostratigraphic correlation between the Inner Carbonate Belt and the Krol Belt (Outer Carbonate Belt), now both of Vendian Early Cambrian age. The present contribution thus resolves an important stratigraphic correlation problem of the Lesser Himalayan geology. And it has a major implication on the tectonic interpretation of the Lesser Himalaya, which is crucial for its palinspastic reconstruction. PROTOCONODONTS are small (~0.5 1 mm) organo-phosphatic grasping spines with deep inner cavity and are regarded as the fossil remains of the oldest chaetognaths 1 3 (arrow worms). Protoconodont sclerites are widely distributed in the basal Cambrian as manifestation of the Cambrian explosion and are recorded up to the Early Ordovician 2. The grasping apparatus of protoconodonts is among the first evidence of biomineralization in metazoa globally at or near the Precambrian Cambrian (Pª ª) boundary (e.g. India, China, Mongolia, Siberia, Kazakhstan, Iran, Australia, Canada, etc.), following the Ediacaran soft-bodied animals of the latest Precambrian. The protoconodont genus Protohertzina Missarzhevsky is a significant taxon of the oldest small shelly fossil (SSF) assemblage zone (Anabarites Circotheca Protohertzina Assemblage Zone), which defines the base of the Cambrian or the Pª ª boundary 4, particularly in the phosphorite carbonate facies where the designated global stratotype boundary marker trace fossil Trichophycus pedum (Seilacher) is invariably absent. Here we report the discovery of abundant protoconodont sclerites from the Gangolihat Dolomite of the Inner Carbonate Belt of the Kumaun Lesser Himalaya (Figure 1 a), which is of vital age significance because the formation is commonly regarded as Mesoproterozoic (~ Ma) due to stromatolites comparable to Riphean forms 5. The consequences of the discovery in the prevailing Lesser Himalayan stratigraphy, age and correlation are discussed. As an initial announcement, the discovery was put on record in the WIHG Annual Report 6. The Gangolihat Dolomite 7 constitutes the lower part of the Tejam Group of the autochthonous Calc Zone of Pithoragarh in the Inner Kumaun Lesser Himalaya. The formation embodies the characteristic development of coarsely crystalline magnesite deposits along with some occurrences of base metal mineralization. The extension of the Gangolihat Dolomite in Chakrata Hills of western Garhwal is called Deoban Formation, which also has spectacular development of stromatolites but lacks magnesite deposits 7. The lithostratigraphic classification of the Tejam Group 7 showing the position of the protoconodont-yielding level is given in Table 1. Protoconodont-yielding samples were collected by the authors in September 2001 from the Jhiroli Magnesite Mine of Almora Magnesite Limited, Bageshwar District 1654 Uttaranchal (Figure 1 b). In Pit 1A of the open cast mine (Figure 2) at 1600 MRL (GPS location: N ; E ), ~4.5 m thick grey concretionary dolomite of the footwall, immediately below the lensoidal magnesite body, was sampled at 1 m interval for reconnaissance micropalaeontological study. Interestingly, all three concretionary dolomite samples (JH1 JH3) weighing 3 kg, were found highly productive (about 200 specimens) with 15% glacial acetic acid dissolution technique commonly used to isolate conodonts. Detailed stratigraphic position of the samples is shown in Figures 1 c, d and 2. The fossiliferous zone forms the lower part of the Chandak Member of the Gangolihat Dolomite (=Deoban Formation). The concretionary dolomite of the footwall in contact with the magnesite is consistent all along its 1.5 km strike continuity in the Jhiroli mine area. The stratigraphic consistency of the concretionary dolomite with respect to the magnesite has also been observed in the neighbouring Bauri area and far beyond, at least up to 35 to 40 km in the southeasterly direction in Bans and Chandak areas of Pithoragarh (Figure 1 a). The protoconodont assemblage (Figure 3) recovered from the Jhiroli Magnesite Mine comprises Protohertzina anabarica Missarzhevsky, P. robusta Qian (natural clusters and isolated individuals), P. siciformis Missarzhevsky, P. unguliformis Missarzhevsky, Protohertzina sp. and a Prooneotodus tenuis (Müller) natural cluster. Specimens are grayish-black in colour, suggesting the effect of thermal alteration. A low degree recrystallization is evident in all specimens. The recovered protoconodont assemblage is typical of the earliest Cambrian Meishucunian Zone I of China and the Nemakit Daldynian horizons of Siberian Platform, Kazakhstan, Inner Mongolia, Northwest Canada, and elsewhere that defines the base of the Cambrian or the Pª ª boundary 4,8. In India, the assemblage is characterized by a fairly good population of Protohertzina spp. in the Lower Tal Mussoorie Phosphorites of the Krol Belt, where it is predominantly present in the Pª ª boundary marker small shelly fossil assemblage 9,10. Rare occurrence of this genus was also recorded from the uppermost part of the dolomitic limestone of Krol D in Mussoorie Syncline 11. The present discovery of abundant Protohertzina, therefore, clearly indicates that the Pª ª boundary is certainly present in the Gangolihat Dolomite of the Calc Zone of Pithoragarh. Broadly, the boundary would lie in the lower part of the Chandak Member of the Gangolihat Dolomite, and at least 4.5 m below the magnesite zone in the Jhiroli mine section. The level yielding Protohertzina in the Gangolihat Dolomite thus can satisfactorily be correlated with the Krol Tal transition beds of the Krol Belt in Outer Lesser Himalaya. It appears that the Lower Tal Chert Phosphorite Member is broadly equivalent to the magnesite-rich zone of the Chandak Member, since both are of early Early Cambrian age. It is also of great stratigraphic significance that the Gangolihat magnesites and biohermal dolomites are phosphatic 12, CURRENT SCIENCE, VOL. 86, NO. 12, 25 JUNE 2004

3 Figure 1. a, Part of the geological map of northeastern Inner Kumaun Lesser Himalaya (after Valdiya 7 ). b, Geological detail around Jhiroli Village 40, Bageshwar District, Uttaranchal. A A is the section line of the geological cross-section. c, Geological cross-section passing through 1600 MRL of Pit 1A, showing the position of protoconodont-yielding concretionary dolomite horizon, immediately below the magnesite body. Rectangle shows the approximate area photographed in Figure 2. d, Litholog with sample positions at the protoconodontyielding level indicated in Figures 1 c and 2. which corresponds with the global phosphogenesis near the Pª ª boundary 13. The age and correlation of the Inner Lesser Himalayan carbonates have been conjectural since nearly the beginning of the last century. Initially, these carbonates were correlated with Krol carbonates, and later accepted as that of late Paleozoic Mesozoic age But with the recognition of the stromatolites construed to be of Riphean age, first in the Pithoragarh area by Valdiya 18 and subsequently by other workers (refs 5, 19 and references therein) throughout in the Inner Lesser Himalayan carbonates (carbonates of Sirban, Jammu, Shali, Deoban, Gangolihat, Dhading and Buxa), contiguously extending from the Hazara Jammu region in the west through Himachal Garhwal Kumaun CURRENT SCIENCE, VOL. 86, NO. 12, 25 JUNE

4 Table 1. Lithostratigraphic classification of the Tejam Group, Inner Kumaun Lesser Himalaya (after Valdiya 7 ) Mandhali Formation Thalkedar Limestone Blue-grey banded limestone, often with chert laminae and nodules and argillaceous limestone alternating with calcareous grey phyllite Sor Slate Light green and grey-green sandstone Deoban Formation Dhari Member Blue-grey limestone with calc slate and marlite (Gangolihat Dolomite) Chandak Member* Dolomite limestone characterized by spectacular development of stromatolites. Pockets of flat pebbles intraformational conglomerates, conspicuous chain of lentiform deposits of magnesites Hiunpani Member Fine-grained cherty dolomite of pink and white colours alternating with chert laminae Chhera Member Pink, violet and maroon slate-phyllite interbedded with subordinate pink, green and white marble, often sandy *Fossil-yielding. Figure 2. Panoramic view of Jhiroli Magnesite Mine showing the magnesite quarries under operation in Pit 1A and the underlying northeasterly dipping dolomite strata of the footwall. Arrow shows the position of protoconodont-bearing concretionary dolomite beds. Nepal Sikkim Bhutan to Arunachal in the east, it was proposed that these carbonates are much older than the Krol carbonates. Thus, the notion that the stromatolitic Inner Lesser Himalayan carbonates are older, of Riphean age, whereas the Krol carbonates are younger, of Late Paleozoic Mesozoic age, got entrenched in the Lesser Himalayan geology 20,21. However, this notion was substantially modified during 1980s with revolutionary fossil discoveries (e.g. conodonts, small shelly fossils, trilobites, inarticulate brachiopods, sponge spicules, acritarchs, algae, trace fossils, etc.) from the Blaini Krol Tal (Mussoorie Group 7 ) succession of the Krol Belt, initiated by Azmi and his associates 22 and subsequently by a large number of workers 3,9 11,23. These biostratigraphic contributions firmly established that the Blaini Krol Tal (B K T) succession is of Vendian to Early Cambrian age 24 and not of Late Paleozoic Mesozoic age as it was believed to be for more than a century. This major chronostratigraphic revision thus once again indicated a close temporal equivalence of the B K T with the Inner Carbonate Belt succession (Deoban Mandhali and their equivalents), though this time in a much older time bracket (Riphean Vendian Cambrian). A rational consequence to 1656 this temporal proximity was the realization that the Deoban and Krol carbonates could have a time-transgressive relationship 25. But since the Deoban carbonates were generally considered to lie stratigraphically below the Jaunsar Group and the Krol above it 20,21 (Table 2), and this practice has been consistently followed in the Lesser Himalayan stratigraphy by majority of the workers, the enigma of equating Deoban with Krol has been sustained. Recently, keeping the time proximity of Deoban and Krol, Valdiya 26 took a comprehensive view of the Lesser Himalayan geology, dismantled the commonly accepted tectonostratigraphic set-up (Table 2), and proposed a drastically revised litho-stratigraphic correlation scheme for the Outer and Inner Lesser Himalayan formations. Our palaeontological data herein reported corroborate his new correlation scheme, but with a substantial chronostratigraphic revision as proposed in Figure 4. To elaborate, in correlating Deoban Mandhali and their equivalent successions of the Inner Lesser Himalaya with the Krol Tal succession of the Krol Belt of the Outer Lesser Himalaya, Valdiya 26 placed both sequences in Proterozoic (Riphean Vendian) time frame (compare table 2 and figure 4 in Valdiya 26 ), instead of Vendian to Early Cambrian currently the well-constrained age for the B K T succession 23,24. This existing anomaly in the chronostratigraphic correlation is mainly because, while the age of the Deoban Mandhali succession is largely based on the stromatolites regarded to be of Riphean Vendian age (refs 5, 7 and references therein), the biochronology of the B K T succession is primarily due to consistent Vendian Cambrian evolutionary biotic records 23,24 supported by chemostratigraphy 27. However, this discrepancy in age of Deoban Mandhali and its consequent correlation with B K T is now being resolved with our discovery of typical earliest Cambrian protoconodonts from the Gangolihat Dolomite (=Deoban Formation). Our discovery is the latest in line of an earlier record of Lower Cambrian palaeobasidiospores from the magnesite bed of stromatolitic Dhading Dolomite of Upper Nawakot Group in northeastern Nepal 28 (an eastward extension of the Calc Zone of Pithoragarh; Figure 4), and also of a very recent record of sponge spicules 29, suggesting Early Vendian age to the oldest unit (Chhera CURRENT SCIENCE, VOL. 86, NO. 12, 25 JUNE 2004

5 Figure 3. SEM photomicrographs of Precambrian Cambrian boundary marker protoconodonts from the Gangolihat Dolomite of Jhirauli Magnesite Mine, Inner Kumaun Lesser Himalaya. All illustrated specimens are deposited in the Repository of Wadia Institute of Himalayan Geology, Dehra Dun, under repository numbers WIMF/A 254 WIMF/A 260. a c and k l, Protohertzina robusta Qian (a c, natural cluster, WIMF/A 254; k l, WIMF/A 255); d, e, P. unguliformis Missarzhevsky (WIMF/A 256); f, P. anabarica Missarzhevsky (WIMF/A 257); g, Prooneotodus tenuis (Müller) a natural cluster (WIMF/A258); h, Protohertzina sp. (WIMF/A 259), note the recurved nature of apex; i, j, P. siciformis Missarzhevsky (WIMF/A 260). Note the deep inner cavity extending up to the apical parts in a, c, d, e, f and k. Member) of the Gangolihat Dolomite (Table 1). These biostratigraphic evidences thus precisely place the lower two units of the Gangolihat Dolomite (Chhera and Hiunpani Members) in the Vendian, while the upper two units (Chandak and Dhari Members) in the Pª ª transition interval. A few other biostratigraphic records in the recent past too give indications of Vendian Cambrian age for the Inner Carbonate Belt 30. But the Pª ª boundary protoconodont assemblage from the Gangolihat Dolomite is a robust evidence for equating the Inner Carbonate Belt with the Krol Belt of the Outer Lesser Himalaya, and both the carbonate belts are of Vendian Early Cambrian age. In view of the above, Riphean age for the Lesser Himalayan Inner Carbonate Belt based on stromatolites, comparable with those of the Russian Platform, is in need of in-depth re-evaluation. The changed perspective regarding significance of stromatolites is reflected in Walter s opinion 31. He states, Even if stromatolite biostratigraphy eventually proves to be impossible, we will have accumulated abundant data for making much more precise CURRENT SCIENCE, VOL. 86, NO. 12, 25 JUNE

6 Figure 4. Proposed chronostratigraphic revision of the lithostratigraphic correlation scheme of Valdiya 26 for the Lesser Himalayan formations. Note the stratigraphic levels in Gangolihat Dolomite (=Deoban Formation) yielding Pª ª boundary protoconodonts (this work) and the recently discovered Early Vendian sponge spicules 29 as also the record of Early Cambrian palaeobasidiospores in Dhading Dolomite of Nepal 28. Table 2. Commonly used lithostratigraphic scheme of the Lesser Himalayan formations in Kumaun Garhwal region (after Valdiya 20 ) palaeoenvironmental and palaeobiological interpretations than are presently possible. The record of the domal stromatolites along with SSFs near the Pª ª boundary beds in the Gangolihat Dolomite is not unusual because they do occur at this level elsewhere too (e.g. Middle Deep Spring Formation, Nevada 32 ) The view that the Deoban and Krol carbonates are equivalent rocks had already existed in the past, but could not be sustained as it was primarily based on lithology and stromatolite similarities rather than any definite biostratigraphic resolution 14 17,33. The palaeontological evidence put forth here strongly corroborates the pioneering view of West 14, according to which the Deoban and Mandhali rocks were actually deposited over the Jaunsar Group rocks, but their apparent disposition below the Jaunsar is due to thrusting. This view thus further leads to an important stratigraphic conclusion that the Damtha and the Jaunsar low-grade metamorphics representing respective basements for the Tejam and Mussoorie Groups in the Inner and Outer Lesser Himalaya, are also coevals 14,26 (see Figure 4). Keeping in view the correlation scheme for the Lesser Himalayan formations discussed above (Figure 4), it becomes imperative that the Vendian unconformity (~600 Ma, represented by the Blaini Boulder Bed as an evidence of Varangerian glaciation) separating the Jaunsar low-grade metamorphics from the overlying B K T sediments in the Outer Lesser Himalaya, should also be present in the Inner Lesser Himalaya separating the Damtha low-grade metamorphics from the overlying Deoban Mandhali sediments. And, in fact, a profound angular unconformity has already been noted by some previous workers between the Damtha Group (=Sundarnagar Group) and the overlying Deoban/Shali Formation 21,34. CURRENT SCIENCE, VOL. 86, NO. 12, 25 JUNE 2004

7 The Vendian datum, therefore, is manifested as the regional unconformity that separates the pre-vendian low-grade metamorphics from the overlying Vendian Cambrian sedimentaries (largely unmetamorphosed) in the Lesser Himalayan stratigraphy. The time elapsed at this regional unconformity appears to be enormous (of the order of ~1000 million years), because the underlying basic metavolcanics associated with the Rampur and Sundarnagar Groups (=Damtha and Jaunsar Groups) have given ages of about 1800 Ma 35 and 1500 Ma 36. This interpretation thereby also indicates that the Tejam and Mussoorie Group sediments must have been deposited on a highly peneplained and mature topography of low-grade metamorphics. The other interesting point that is corroborated from our study is that all coarsely crystalline magnesite deposits in the Lesser Himalayan domain are time-controlled 37. For example, the magnesite deposits within the dolomites of Kumaun as well as Nepal, and the Magnesian Sandstone (a genuine dolomite) 38 of Salt Range, are typical of Early Cambrian age. The other magnesite deposits such as in the Pipalkoti Formation of Garhwal, the Jammu Limestone, and in the Shali Limestone of Himachal are most likely to fall in the Early Cambrian chain of magnesite deposits. Significantly, the occurrence of magnesite, though rare, was also noted in the Upper Krol Dolomite (Krol D ) of Mussoorie Syncline 39. In conclusion, the biostratigraphic evidences from the Gangolihat Dolomite (Deoban and equivalent formations), while strongly supporting the new correlation scheme of Valdiya 26 for the Inner and Outer Lesser Himalayan formations, also suggest, however, a substantial revision in the currently accepted age for the Tejam Group (Deoban Mandhali) and equivalent successions of the Inner Lesser Himalaya. Instead of the widely held Meso Neoproterozoic (Riphean Vendian) age for the Tejam Group (Deoban Mandhali), its age should now be regarded as Vendian Early Cambrian, similar to that of the Mussoorie Group (B K T) of the Krol Belt. This much simpler stratigraphic scheme (Figure 4) can be of pivotal utility for the palinspastic reconstruction of the Lesser Himalayan terrain. Besides, this work impels to explore the Vendian Cambrian palaeobiology for integrated biostratigraphy and chemostratigraphy to establish the Pª ª boundary datum (~544 Ma) in the extensive carbonate domain of the Lesser Himalaya. 1. Bengtson, S., Lethaia, 1976, 9, ; Fossil Strata, 1983, 15, Szaniawski, H., J. Paleontol., 1982, 56, ; Acta Palaeontol., 2002, 47, Azmi, R. J., Contrib. XV Indian Colloquium on Micropalaeontology and Stratigraphy, Dehra Dun, 1996, pp Brasier, M. D., In The Precambrian Cambrian Boundary (eds Cowie, J. W. and Brasier, M. D.), 1989, pp ; Valdiya, K. S. (ed.), In Proceedings of the Indo Soviet symposium on stromatolite and stromatolitic deposits. Himalayan Geol., 1989, 13, WIHG Annual Report, Wadia Institute of Himalayan Geology, , p Valdiya, K. S., Geology of the Kumaun Lesser Himalaya, Himachal Times Press, Dehra Dun, 1980, p Qian, Yi and Bengtson, S., Fossil Strata, 1989, 24, Azmi, R. J., Himalayan Geol., 1983, 11, Brasier, M. D. and Singh, P., Geol. Mag., 1987, 124, Azmi, R. J. and Pancholi, V. P., Himalayan Geol., 1983, 11, Valdiya, K. S., Sedimentology, 1972, 19, Donnelly, T. H., Shergold, J. H., Southgate, P. N. and Barnes, C. J., In Phosphrite Research and Development (eds Notholt, A. J. G. and Jarvis, I.), Geol. Soc. Spec. Publ., 1990, vol. 52, pp West, W. D., Rec. Geol. Surv. India, 1932, 66, ; 1939, 74, Auden, J. B., Rec. Geol. Surv. India, 1934, 67, Heim, A. and Gansser, A., Mem. Soc. Helv. Sci. Nat., 1939, 73, Fuchs, G., Osterr. Akad. Wiss., Math. Nat. Kl. Denkschr., 1967, 113, Misra, R. C. and Valdiya, K. S., J. Geol. Soc. India, 1961, 2, Raha, P. K. and Das, D. P., In Proceedings of the Indo Soviet symposium on stromatolite and stromatolitic deposits (ed. Valdiya, K. S.), Himalayan Geol., 1989, 13, Valdiya, K. S., In Stratigraphy and Correlation of Lesser Himalayan Formations (eds Valdiya, K. S. and Bhatia, S. B.), Hindustan Publishing Corp. (India), Delhi, 1980, pp. 7 48; Srikantia, S. V. and Bhargava, O. N., In Recent Researches in Geology (ed. Gupta, V. J.), Hindustan Publishing Corp. (India), Delhi, 1982, pp Azmi, R. J., Joshi, M. N. and Juyal, K. P., In Contemporary Geoscientific Researches in Himalaya (ed. Sinha, A. K.), Bishen Singh Mahendrapal Singh, Dehra Dun, India, 1981, vol. 1, pp Bhatt, D. K., Mamgain, V. D., Misra, R. S. and Srivastava, J. P., Geophytology, 1983, 13, ; Bhatt, D. K., Mamgain, V. D. and Misra, R. S., J. Paleontol. Soc. India, 1985, 30, ; Singh, I. B. and Rai, V., J. Paleontol. Soc. India, 1983, 28, 67 90; Rai, V. and Singh, I. B., J. Paleontol. Soc. India, 1983, 28, ; Kumar, G., Raina, B. K., Bhatt, D. K. and Jangpangi, B. S., J. Paleontol. Soc. India, 1983, 28, 69 92; Kumar, G., Joshi, A. and Mathur, V. K., Curr. Sci., 1987, 56, ; Kumar, G., Bhatt, D. K. and Raina, B. K., Geol. Mag., 1987, 124, ; Tripathi, C., Jangpangi, B. S., Bhatt, D. K., Kumar, G. and Raina, B. K., Geophytology, 1984, 14, ; Venkatachala, B. S., Shukla, M., Bansal, R. and Acaryya, S., Palaeobotanist, 1990, 38, 29 38; Tiwari, M. and Azmi, R. J., Palaeobotanist, 1992, 14, ; Tiwari, M. and Knoll, A. H., J. Himalayan Geol., 1994, 5, ; Tiwari, M., Precambrian Res., , ; Kumar, S. and Rai, V., J. Geol. Soc. India, 1992, 39, ; Mazumdar, A. and Banerjee, D. M., Geology, 1998, 26, Tandon, S. K., Thakur, V. C., Nanda, A. C., Azmi, R. J., Bagati, T. N., Tewari, V. C. and Kumar, R., Excursion Guide: Himalayan Sequence of Dehra Dun Mussoorie Sector, Geol. Soc. of India, p Azmi, R. J. and Joshi, M. N., Himalayan Geol., 1983, 11, Valdiya, K. S., Precambrian Res., 1995, 74, Aharon, P., Schidlowsky, M. and Singh, I. B., Nature, 1987, 327, ; Banerjee, D. M., Schindlowsky, M., Siebert, F. and Brasier, M. D., Palaeogeogr., Palaeoclimatol., Palaeoecol., 1997, 132, ; Jiang, G., Christie-Blick, N., Kaufman, A. J., Banerjee, D. M. and Rai, V., J. Sediment. Res., 2002, 72, Brunel, M., d Albissin, M. Chayé and Locquin, M., J. Geol. Soc. India, 1985, 26, CURRENT SCIENCE, VOL. 86, NO. 12, 25 JUNE

8 29. Tiwari, M., Curr. Sci., 2000, 79, Sinha, A. K. and Raaben, M. A., Himalayan Geol., 1981, 9, ; Venkatachala, B. and Kumar, A., Contrib. XV Indian Colloquim on Micropaleontology and Stratigraphy, Dehra Dun, 1996, pp ; Kumar, A. and Venkatachala, B. S., Curr. Sci., 1998, 75, ; Srivastava, P. and Kumar, S., Curr. Sci., 1997, 72, ; Rai, V., Singh, A. K., Kumar, M. and Gautam, R., J. Paleontol. Soc. India, 1997, 42, Walter, M. R. (ed.), Stromatolites: Developments in Sedimentology 20, Elsevier, Amsterdam, 1976, pp Corsetti, F. A. and Hagadorn, J. W., Sediment. Rec., 2003, 1, Singh, I. B. and Rai, V., Curr. Sci., 1977, 46, Srikantia, S. V. and Bhargava, O. N., Jaunsar Group Discussion. Himalayan Geol., 1976, 6, Miller, C., Klötzli, U., Frank, W., Thöni, M. and Grasemann, B., Precambrian Res., 2000, 103, Kakar, R., Bull. Indian Geol. Assoc., 1986, 19, Valdiya, K. S., Bull. Geol. Soc. India, 1967, 4, Gansser, A., Geology of the Himalayas, Interscience Publishers, London, 1964, p Rao, C. G., Bull. Oil Nat. Gas Comm., 1972, 9, Sengupta, H. P. and Yadav, R. N., J. Geol. Soc. India, 1998, 41, 1 6; Joshi, M. N., Bhattacharya, A. K. and Anantharaman, M. S., Miner. Deposita, 1993, 28, ACKNOWLEDGEMENTS. We thank Dr B. R. Arora, Director, Wadia Institute of Himalayan Geology, Dehra Dun for providing the necessary facilities and Dr R. C. Upadhyay, Jhirauli Magnesite Mine for cooperation during sampling. We have been benefited by the vast experience of Dr M. N. Joshi, D.B.S. College, Dehra Dun on Almora Magnesite, who has critically gone through the manuscript. Discussions with colleagues Dr B. N. Tiwari and Dr N. R. Phadtare have been useful. We appreciate the meticulous microfossil picking by Sanjeev Dabral and SEM photomicrography by Naresh Juyal. Received 1 September 2003; revised accepted 26 February 2004 CURRENT SCIENCE Display Advertisement Rates India Tariff (rupees) Inside pages Inside cover pages Back cover page No. of insertions Size B&W Colour B&W Colour B&W Colour 1 Full page 10,000 20,000 15,000 25,000 20,000 30,000 Half page 6,000 12,000 6 Full page 50,000 1,00,000 75,000 1,25,000 1,00,000 1,50,000 Half page 30,000 60, Full page 1,00,000 2,00,000 1,50,000 2,50,000 2,00,000 3,00,000 Half page 60,000 1,20,000 Foreign Tariff (US $) Inside pages Inside cover pages Back cover page No. of insertions Size B&W Colour B&W Colour B&W Colour 1 Full page Half page Full page Half page Note: For payments towards the advertisement charges, Cheques (local) or Demand Drafts may be drawn in favour of Current Science Association, Bangalore CURRENT SCIENCE, VOL. 86, NO. 12, 25 JUNE 2004