Ordovician–Silurian acritarch biostratigraphy of the Tethyan Garhwal Himalaya, India

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Review of Palaeobotany and Palynology 103 (1998) 167–199

Ordovician–Silurian acritarch biostratigraphy of the Tethyan Garhwal Himalaya, India H.N. Sinha a,Ł , Bijai Prasad b , S.S. Srivastava c a

Deparment of Geology, P.K. Roy Memorial College, Dhanbad-826004, India KDM Institute of Petroleum Exploration, ONGC, Dehradun-248195, India c Department of Earth Sciences, University of Roorkee, Roorkee-247667, India b

Received 6 January 1997; revised version received 15 December 1997; accepted 2 March 1998

Abstract The Ordovician–Silurian succession of the Shiala and Yong Limestone formations, Tethyan Garhwal Himalaya (Uttar Pradesh), reveals a rich assemblage of acritarchs. The assemblage includes sphaeromorphs, acanthomorphs, netromorphs, polygonomorphs, diacromorphs and herkomorphs. Seventy species belonging to thirty-seven acritarch genera have been identified. The new forms are described. Four distinct local acritarch assemblage zones are proposed. They are: (IV) Leiofusa algerensis–Multiplicisphaeridium osgoodence; (III) Tetradinium minutum–Dactylofusa sp. cf. D. oblancae; (II) Domasia trispinosa–Deunffia monospinosa; (I) Baltisphaeridium longispinosum var. longispinosum–Multiplicisphaeridium ornatum. The palynological assemblages identified in the Garhwal Himalaya correlate with Ordovician–Silurian assemblages of the United Kingdom, Belgium, Russia, North Africa, Saudi Arabia and China, etc. The presence of biostratigraphic index forms suggest a Late Ordovician to Middle Silurian age for the Shiala Formation and a Middle to Late Silurian age for the Yong Limestone. Thus, the Ordovician–Silurian boundary lies within the Shiala Formation and negates an earlier view that this boundary lies at the base of the Yong Limestone Formation. In the systematic part, five new species of acritarchs, Neoveryhachium distinctum Sinha, Prasad and Srivastava, sp. nov., Tetradinium minutum Sinha, Prasad and Srivastava, sp. nov., Triangulina densus Sinha, Prasad and Srivastava, sp. nov., Geron indicus Sinha, Prasad and Srivastava, sp. nov., and Leiofusa shialensis Sinha, Prasad and Srivastava, sp. nov., are described and illustrated.  1998 Elsevier Science B.V. All rights reserved. Keywords: Ordovician–Silurian; Tethyan Garhwal Himalaya; acritarchs; Shiala Formation; Yong Limestone Formation

1. Introduction The term ‘Tethys’ was used by Suess (1885) as a great Mesozoic sea (Sto¨cklin, 1984) which separated the two old large land masses of Laurasia to the north and Gondwanaland to the south. Gondwanaland was made up of South America, Africa, India, Australia Ł Corresponding

author.

and Antarctica. Auden (1935, 1937) recognised the ‘Tethys Himalayan facies’ or ‘Tethys facies’ north of the central crystallines. This facies is resting on the basement of the Central Crystallines of the Himalaya (Sinha, 1989). In the Extrapeninsular India, Lower Paleozoic marine sediments are well exposed in the Tethyan Zone of Kashmir, Spiti and Kumaon–Garhwal Himalaya. However, the Tethyan Basin of Kumaon–

c 1998 Elsevier Science B.V. All rights reserved. 0034-6667/98/$19.00 PII: S 0 0 3 4 - 6 6 6 7 ( 9 8 ) 0 0 0 3 1 - 1

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Garhwal Himalaya is not continuous with the Spiti Basin in the northwest, or the Nepal Basin in the southeast. The depositional basin has been closed by the axially uplifted crystalline rocks on either side (Sinha, 1989). The Tethyan Himalaya is characterised by a large number of various groups of rocks which are less in thickness in contrast to the southern Himalaya. Also, they comprise a rich fossiliferous Precambrian–Paleozoic–Mesozoic sedimentary succession. These fauna are typical marine, neritic and sublittoral in nature. The Tethys Himalaya has been bound by real geosynclines and deep-seated faults in the north and south (Sinha, 1989). However, Kumar et al. (1977) carried out sedimentological study for paleoenvironmental deduction in this region and concluded that the deposition of sediments started in a single basin

of sedimentation which existed right from the Late Precambrian to (?) Early Eocene. The rate of sedimentation kept pace with the sinking of the basin of deposition (Kumar et al., 1977). The first study of Ordovician–Silurian acritarch assemblages has been carried out by Sinha et al. (1996a) from the Indian subcontinent, in general, and from the Shiala and Yong formations of the Tethyan Garhwal Himalaya, in particular. The formations in the Garhwal Himalaya are well exposed in and around the village of Sumna (30º400 N: 80º500 E) of the Chamoli district, Uttar Pradesh (Fig. 1). The international border between India and China (Tibet) is only 16 km (approximately) north of Sumna. This area has been mapped by Shah and Sinha (1974) and Sinha (1989), see Fig. 1. Seventy-one rock samples were systematically

Fig. 1. Geological map of the Sumna–Rimkhim area of the Tethyan Zone of the higher Garhwal Himalaya, India (Sinha, 1989). 1 D Martoli Group; 2 D Ralam Formation; 3 D Garbyang Formation; 4 D Shiala Formation; 5 D Yong Limestone Formation; 6 D Variegated Formation; 7 D Muth Quartzite Formation; 8 D Kuling Shale–Fenestella Shale; 9 D Kalapani Limestone Member=Kuti Formation; 10 D Kioto Formation; 11 D Lapthal Formation; 12 D Spiti Shale; 13 D Giumal Sandstone; 14 D Alluvial.

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collected along the Sumna–Rimkhim section which lies on the bank of the Yong river (Figs. 1 and 3). This section comprises the Garbyang, Shiala, Yong Limestone and Variegated formations (Figs. 2 and 3). An integrated strato-litho-petrographic column for this section is shown in Fig. 2. The rock types include clastics and non-clastic sediments. Majority of samples are fine-grained calcareous-quartz-arenite, siltyshale and wackstone (bioclast) (Fig. 2). Abundant acritarchs together with infrequent to rare chitinozoans and melanosclerites were recovered from greenish siltyshale of the Shiala Formation, whereas, some fine-grained quartz-arenites yielded rare acritarchs. The Yong Limestone is mainly greenish black in colour and yields abundant melanosclerites, rare chitinozoans and rare to abundant acritarchs (Sinha et al., 1996b). The sediments of the Tethyan sequence are affected by postdepositional tectonic activities. However, the preservation of acritarchs is largely good and indicating high-thermal maturity (colour of the cyst ranging from brown to dark brown and in some cases it is black), whereas the chitinozoan preservation is fairly moderate (colour of the collarette is black). Few specimens are poorly preserved, broken and corroded by pyrite. In such specimens, the colour of the exine becomes black. Similar observations have been recorded earlier by Tiwari et al. (1996) from the Paleozoic–Mesozoic Tethyan sequence in the Niti area. They have suggested the dark brown to black colour of exine may be due to partial oxidation during deposition, syndepositional chemical and physical stresses, post-depositional tectonics and diagenesis of a higher degree. The acritarch assemblages are very diverse (from a few individuals up to about 1000 specimens per 10 g of rock) with many previously described forms (Plates I–VIII), known from Ordovician–Silurian rocks in other parts of the world and variation in specific diversity (from monospecific assemblage to a maximum of fifty

Fig. 2. Integrated strato-litho-petrographic column of the Sumna– Rimkhim section along the right bank of Yong gad valley showing sample positions. 1 D siltstone; 2 D alternation of siltstone and siltyshale; 3 D limestone (packstone); 4 D sandstone (calcareous quartz arenite); 5 D limestone (boundstone, algal); 6 D siltyshale; 7 D dolostone (mudstone, dolomite); 8 D lime-

stone, sandy (mudstone, arenaceous); 9 D limestone (mudstone); 10 D limestone; 11 D limestone (wackstone, bioclast); 12 D limestone (mudstone, argillaceous). Petrographic units described under bracket.

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PLATE I

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The text figures of some dark forms have been prepared and illustrated here in order to delineate the morphological details (Fig. 6). The basic objective of this paper is to enlist (Table 3) and document the Ordovician–Silurian acritarchs from the Himalaya, with a view to establish the Lower Paleozoic acritarch biostratigraphy and their importance in age determination of the samples examined. For the first time four Ordovician–Silurian acritarch biostratigraphic assemblage zones (assemblage palynozones I, II, III and IV in stratigraphic order) have been established on the basis of first and last occurrences (FAD and LAD) of stratigraphically significant forms. The recognised acritarch biozones described herein are considered as local ones because their lateral extension needs further study and deeper probe.

2. Previous work

Fig. 3. The Sumna–Rimkhim mule track section showing location of samples.

species per sample) are noted. Apart from these the assemblages also contain a good number of new forms which are described and illustrated herein.

A number of contributions with regard to paleontology are available from the Paleozoic sediments of the Tethyan Garhwal Himalaya (Strachey, 1857; Diener, 1897, 1898; Reed, 1912; Gansser, 1964; Shah and Sinha, 1974; Mehrotra and Sinha, 1978; Goel et al., 1987; Srivastava and Kumar, 1992; etc.). Though paleontological data have been available for the last four or five decades, no detailed palynological work has been attempted until now, excepting that by Khanna et al. (1985) and Sinha et al. (1996a,b).

PLATE I All magnifications ð1900 approximately, unless otherwise mentioned. 1. Multiplicisphaeridium osgoodense (Cramer and Diez) Eisenack et al., 1973. Slide No. R-47(2); coord. 92 ð 37:8. ð760. 2. Oppilatala? indianae (Cramer and Diez) Priewalder, 1987. Slide No. R-47(2); coord. 94 ð 30:9. ð760. 3. Solisphaeridium sp. A. Slide No. R-8(0); coord. 103:7 ð 66:1. 4. Micrhystridium shinetonense Downie, 1958. Slide No. R-8(0); coord. 107:2 ð 59. 5. cf. Diexallophasis sp. A. Slide No. R-47(1); coord. 107:3 ð 64:3. ð760. 6, 15. Micrhystridium parinconspicuum Deflandre, 1945. 6 D Slide No. R-8(0); coord. 106:4 ð 68:5; 15 D Slide No. R-8(0); coord. 104:9 ð 63:3. 7. Solisphaeridium nanum (Deflandre) Turner, 1984. Slide No. R-21(5); coord. 93:4 ð 66:2. 8. Cheleutochroa diaphorosa Turner, 1984. Slide No. R-15(1); coord. 106 ð 50:8. ð760. 9. Micrhystridium sp. A. Slide No. R-8(0); coord. 99:1 ð 63:6. 10. Multiplicisphaeridium cladum (Downie) Lister, 1970a. Slide No. R-8(0); coord. 104:6 ð 52:3. 11. Buedingiisphaeridium tremadocum Rasul, 1979. Slide No. R-8(0); coord. 105:3 ð 50:5. 12. Micrhystridium sp. cf. M. Shinetonense Downie, 1958. Slide No. R-8(0); coord. 103:2 ð 60:4. 13. Multiplicisphaeridium thusui (Thusu) Eisenack, 1976. Slide No. R-8(0); coord. 104:1 ð 63:4. 14. Micrhystridium sp. B. Slide No. R-21(5); coord. 103:6 ð 44:5. 16. Filisphaeridium henryi (Paris and Deunff, 1970) Sarjeant and Stancliffe, 1994. Slide No. R-8(0); coord. 103:3 ð 36:2.

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PLATE II

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Khanna et al. (1985) reported the occurrence of chitinozoa, scolecodonts, melanosclerites, and one genera of Baltisphaeridium from the Yong Limestone. However, Sinha et al. (1996a,b) reported prolific, well preserved and diverse species of acritarchs together with chitinozoa and melanosclerites from the Shiala and Yong formations. They delineated an Ordovician–Silurian boundary in the Shiala Formation based on stratigraphically significant acritarch forms which were earlier known from the base of the Yong Limestone Formation (Sinha et al., 1996a). Sinha et al. (1996c) have also attempted to deduce paleoenvironment based on the Veryhachium Complex with associated microflora of this section. However, Kumar et al. (1977) inferred environment of deposition of this area based on sedimentological features and postulated a southern source for the sediments of these formations. They divided the Garbyang and Shiala formations into seven divisions (A to G) based on distinct phases of sedimentation history marked by variation in lithology, sedimentary structures and paleocurrent patterns (Table 1) and inferred the varying conditions of depositional environment. Paleoenvironmental control on acritarch distribution has been discussed by many such as Staplin (1961), Wall (1965), Smith and Saunders (1970), Gray and Boucot (1972), Dorning (1981a,b), AlAmeri (1983), Wicander and Playford (1985), Hill and Molyneux (1988), Wood and Stephenson (1989), and Richardson and Rasul (1990). All of them suggested more shallow–intermediate to open marine

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environments. Cramer (1968, 1969, 1970a,b,c, 1973) and Cramer and Diez (1972, 1974a,b) recorded a series of Silurian acritarch biofacies paralleling palaeolatudinal temperature gradients. Vavrdova (1974, 1982), Nautiyal (1976, 1977), Hill and Molyneux (1988), and Colbath (1990) studied on the climatic zonation of acritarchs. Jacobson (1979) recognised three morphological classes of acritarchs indicating environment of deposition. The classes are: (1) the Leiosphaerid class representing near shore, shallow water environment; (2) the Pateinosphaerid– Dicommopalla class indicating shoal habitat; and (3) the Baltisphaerid–Veryhachid–Polygonium class characteristic of open sea environment. The Shiala and Yong formations yielded well preserved and diverse species of acritarchs together with incertae sedis and total absence of palynomorphs from the Variegated Formation (Fig. 4). We consider that: (1) Flora might have existed but they got oxidised due to prevalence of oxidising environment in the basin of deposition. This feature manifests itself by the presence of typical chocolate coloured siltyshale as a dominant lithology of this formation. (2) Whatever little quantity of palynomorphs escaped oxidation, might have possibly been eaten by macro-invertebrates (Tappan, 1980). It is observed, at times, that some shallow water Ordovician–Silurian acritarch species of the Tethyan Garhwal Himalaya are found to occur in deep water sediments. Likewise, some deep water species are found to occur in shallow water sediments. Both

PLATE II All magnifications ð760, unless otherwise mentioned. 1. Veryhachium sp. A. Slide No. R-8(0); coord. 107:5 ð 37:7. ð1900. 2. aff. Lophosphaeridium. Slide No. R-5(1); coord. 101 ð 51. 3. Dorsennidium rhomboidium (Downie) Sarjeant and Stancliffe, 1994. Slide No. R-8(0); coord. 107:7 ð 31:2. ð1900. 4. Helosphaeridium citrinipeltatum (Cramer and Diez) Dorning, 1981a. Slide No. R-21(5); coord. 92:6 ð 64. ð1900. 5. Veryhachium downiei Stockmans and Williere, 1962. Slide No. R-8(0); coord. 103 ð 63:1. ð1900. 6. Baltisphaeridium accinctum Loeblich and Tappan, 1978. Slide No. R-8(1); coord. 100:5 ð 27:5. 7, 8. Baltisphaeridium citrinum (Downie) Stockmans and Williere, 1974. 7 D Slide No. R-21 (5); coord. 93:1 ð 31:5. ð1900; 8 D Slide No. R-21 (5); coord. 106:4 ð 65:6. ð1900. 9, 14. Ordovicidium nudum (Eisenack) Loeblich and Tappan, 1978. 9 D Slide No. R-11(2); coord. 103:6 ð 59; 14 D Slide No. R-17(2); coord. 95:5 ð 41:3. 10, 11. Vulcanisphaera imparilis Rasul, 1976. 10 D Slide No. R-11 (2); coord. 105:5 ð 35:6; 11 D Slide No. R-11 (3); coord. 92:5 ð 42:5. 12. Peteinosphaeridium sp. A. Slide No. R-8(2); coord. 92 ð 65:5. 13. Baltisphaerosum bystrentos (Loeblich and Tappan) Turner, 1984. Slide No. R-8(2); coord. 95 ð 31:3.

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PLATE III

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Table 1 Lithostratigraphic subdivisions and environment of deposition of the Tethyan succession of Garhwal Kumaon Himalaya (Kumar et al., 1977) Formation

Member Lithology

Characteristic sedimentary structure

Environment of deposition

Variegated

C

Shale and Limestone

Variegated

B

Low energy shallow water carbonates do

Yong Limestone (Variegated Member A of Kumar et al., 1977)

–

No wave or current features, imperceptible bedding features

Zero-energy carbonate flat

Shiala 75 m

G

Shiala 70 m

F

Shiala 180 m

E

Small ripples, shale layers burrowed Cross-bedding, parallel bedding and ripple bedding Cross-bedding

Shiala 180 m

D

Limestone shale and siltstone Greyish green biohermal limestone, calcareous mud, thin shale bands merging into a biostromal limestone Sandy shell limestone and shale Shell limestone, shale and sandstone Shell limestone and calcareous shale Orthoquartzite and shale

No primary sedimentary structure observed do

Medium energy coastal sand area Medium to high energy coastal area Medium to high energy coastal area Medium energy sandy tidal flat=shoal complex

Shiala 165 m

C

Shiala 40 m

B

Shiala 140 m

A

Garbyang 140 m

G

Shale limestone intraformational conglomerate and shale Shale and limestone Shales, sandstone with shell limestone Silty sandstone and shale

of these features are possible due to patterns of current circulation operating in the marine ecosystem effecting the watermass and hence the biomass. It has been described in the preceding paragraphs that few phytoplanktonic microfossils are black in

Current and interference ripples (tadpole type); Wrinkle marks and spring pits Convolute bedding, small ripple bedding Sand layers show channeling, ripples of wave origin Graded sand layers with ripple bedding Graded rhythmites, graded ripple bedding, extensive bioturbation with vertical burrows

Subtidal to intertidal zone

Intertidal–subtidal zone Transition (subtidal) zone Transition zone shelf mud area

colour. These specimens might have deposited in shallower water conditions and hence were more oxidised prior to final burial (Jacobson and Achab, 1985).

PLATE III All magnifications ð760 approximately, unless otherwise mentioned. 1. Dasydiacrodium sp. A. Slide No. R-47(0); coord. 92:8 ð 38. ð1900. 2, 3, 10. Micrhystridium? diornamentum Rasul, 1979. 2 D Slide No. R-8 (O); coord. 103:5 ð 40:8. ð1900; 3 D Slide No. R-8 (O); coord. 102:6 ð 40. ð1900; 10 D Slide No. R-8 (O); coord. 95:8 ð 38:5. ð1900. 4. Lophosphaeridium sp. B. Slide No. R-12(1); coord. 99:8 ð 61:6. 5. Baltisphaeridium archaicum Cramer and Diez, 1972. Slide No. R-8(2); coord. 94:1 ð 36. 6. Orthosphaeridium sp. cf. O. quadrinatum (Burmann, 1970) Eisenack, Cramer and Diez, 1976. Slide No. R-8(1); coord. 105:5 ð 28:7. 7. Polygonium delicatum Rasul, 1979. Slide No. R-15(1); coord. 102 ð 58. 8a, b. Multiplicisphaeridium variabile (Lister) Dorning, 1981a. Slide No. R-47 (1); coord. 94:6 ð 60. ð950. 9. Goniosphaeridium splendens (Eisenack) Turner, 1984. Slide No. R-21 (5); coord. 93:2 ð 50:8. ð1900. 11. Micrhystridium robustum Downie, 1958. Slide No. R-21 (5); coord. 94 ð 54:8. ð1900. 12. Baltisphaeridium sp. A. Slide No. R-8(1); coord. 91:2 ð 34.

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3. Stratigraphy The detailed geological account and generalised lithostratigraphic framework of the Yong Gad section near Sumna as given by Sinha (1989) is illustrated in Table 2 along with the brachiopod zones. The Tethyan sediments range in age from the Precambrian to Cretaceous or (?) Eocene (Srivastava and Kumar, 1992) comprising the Vaikrita Group to the Sangchamalla Formation. However, our study has been confined to the Garbyang, Shiala, Yong and Variegated formations. The Garbyang Formation was first studied by Heim and Gansser (1939) in the Kali river valley and they designated ‘Garbyang Series’. Gansser (1964) redesignated it as ‘Garbyang Formation’. The best exposurers of the Garbyang Formation are located in the Girthi Ganga gorge near Sumna. Shah and Sinha (1974) and Sinha (1989) gave a detailed account of various lithological units of this formation. They reported the occurrence of tiny remnants of trilobite, some minute lingulid brachiopods, flat gastropod Eccliopteris kushanensis Grabau and assigned Cambrian to Ordovician age to the Garbyang Formation. However, Kumar et al. (1977) made an attempt to subdivide the Tethyan succession of Garhwal–Kumaon Himalaya on the basis of lithology and sedimentary structure (Table 1). The Shiala Formation is 400 to 500 m thick shale with intercalations of sandy limestone or crinoid breccia of brown weathering overlying the typical Garbyang sequence of the Shiala pass (Gansser, 1964). It is distinguished from the Garbyang by the presence of biostromal band in it. Shah Table 2 Generalised lithostratigraphic framework of the area (Sinha, 1989) Time unit

Litho unit

Silurian

Variegated Formation Strophonella zone Yong Limestone Formation Calostylis zone

Ordovician Shiala Formation

Garbyang Formation

and Sinha (1974) demarcated the lower boundary of the Shiala Formation by the presence of a regular development of fossiliferous crinoid bearing limestone horizons sedimentary sequence. They assigned a Middle to Late Ordovician age. On the basis of the occurrence of typical conodont species Amorphognatus tvaerensis Goel et al. (1987) assigned a Middle Ordovician age. The Yong Limestone as named by Shah and Sinha (1974) is conformably overlying the Shiala Formation and was previously referred to as ‘Nodular Limestone’ (Heim and Gansser, 1939) and Somna Series (Dave and Rawat, 1968). This formation is rich in fossils with poor preservation and has been assigned a Late Ordovician to Early Silurian (Caradocian to Landoverian) age on the basis of faunal and floral assemblage by Shah and Sinha (1974). They have also erected a biostratigraphic zone corresponding to this formation based on the coral form Calostylis. Shah and Sinha (1974) redesignated the ‘Variegated Silurian’ of Heim and Gansser (1939) as ‘Variegated Formation’. However, Griesbach (1891) mentioned it as ‘Red Crinoidal Limestone’. Overlying the Yong Limestone the lower part of this formation is represented by purplish algal limestone and phyllite, sometimes arenaceous followed by upper part which is mainly composed of purplish and green calcareous quartzite. The colour contrast with the underlying green Yong Limestone and overlying white Muth Quartzite is a characteristic feature of this formation. Shah and Sinha (1974) assigned a Silurian age to this formation on the basis of the (brachiopod) Strophonella zone.

4. Structural setting

Assemblage zones

Rafinesquina alternate zone Monotrypa zone Rafinesquina arenea zone Orthis testudinaria zone Eccliopteris zone

The rocks of the Tethyan Zone display an entirely different structural set up as compared to the rocks of the Lesser Himalaya. This manifests in the form of the absence of isoclinal folds and prevalence of the Jura type of folding (Shah and Sinha, 1974). The regional fold axis of the Paleozoic sedimentary sequence largely is northwest–southeast. It gradually changes to north–south in the Mesozoic sequence. In the lower part of the Garbyang Formation, there is a predominance of gravity structure

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Fig. 4. Stratigraphic distribution of selected acritarch species in Ordovician–Silurian sediments of the Sumna–Rimkhim section, Tethyan Garhwal Himalaya, India.

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producing ‘tooth-paste’ like feature (Shah and Sinha, 1974). However, various types of folds are recognised in the Garhwal Tethyan Zone which are disharmonic folds, en echelon structure and cross-puckers. No large scale thrusting has been seen in the study area. Shah and Sinha (1974) recognised two major types of faults: (1) essentially strike faults turning to strike slip faults occasionally (NW–SE to NNW–SSE trending); (2) transverse NE–SW- to E–W-trending faults. The fault along the Sumna–Rimkhim Section (Fig. 1) is a strike fault. In the Yong Limestone exposures, the base is green in colour and limestone is at the top which is separated by pink shale and limestone. The pink shale and limestone belong to the overlying ‘Variegated’ Formation, exposures of which are found repeating at Yong, possibly because of faulting (Shah and Sinha, 1974).

5. Laboratory work The standard palynological techniques have been applied for the recovery of palynomorphs. Fifty grams of pea size rock material from each sample was dissolved first in hydrochloric acid and then in hydrofluoric acid and finally in nitric acid. The macerated and decomposed material was washed four times with distilled water by centrifuging. Residue thus obtained was treated with heavy liquid by using KI, CDI2 and Znl. After obtaining the residue through heavy liquid analysis of each rock sample, four slides were prepared from each fraction, but this varied with the volume of residue available. All slides are labelled with relevant rock sample numbers in following manner: R-8(0) is slide number ‘one’, R-8(1) is slide number ‘two’, prepared from sample R-8; R and M stands for rock samples taken from the Sumna–Rimkhim section. A Leitz photo microscope (Orthoplan No. 042230, German make) was used for palynological studies and photography of the palynomorphs. Samples M-1 to M-5 and overlying samples R-1 and R-2 contain no acritarchs. Also, samples R-52 to R-66 from the uppermost part of the column were barren. However, sample R-3 to R-51 does yield some fossil palynomorphs (melanosclerites, chitino-

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zoans and acritarchs, Fig. 4). To facilitate clarity in the understanding of Fig. 4, informal names like ‘Unfossiliferous sequence — B’ towards the uppermost part of the lithological column have been given. The ‘Unfossiliferous sequence ‘A’ and ‘B’ means that the aforesaid segment in the stratigraphic column is devoid of acritarchs. It is important to note here that the Tethyan acritarch assemblages are not reworked because the thin processes of acritarchs are well preserved and overall the palynomorphs are not abraded and fragmentary. Moreover, the maturity of organic matter particulate in the samples and the acritarchs is the same. It has been observed that the majority of Ordovician–Silurian acritarch species of the Tethyan Garhwal Himalaya vary in process length and vesicle diameter. The vesicle diameter is relatively shorter in size range compared to that of the holotype described elsewhere. This observation is thought provoking for taxonomists in future studies. However, the size of the recovered acritarch species in the Ordovician– Silurian assemblages of this region could possibly be due to the influence of external physical factors like change in water salinity, eH, pH and temperature conditions. The material described in this paper is in the collections of the Department of Earth Sciences, University of Roorkee, India. Samples are registered as R-1 to R-51. The species list of recovered acritarchs of the Shiala Formation and Yong Limestone is given in Table 3.

6. Systematic description of new species Group ACRITARCHA Evitt, 1963 Subgroup SPHAEROMORPHITAE Downie et al., 1963 Genus Neoveryhachium (Cramer) Sarjeant and Stancliffe, 1994 Type: Neoveryhachium carminae Cramer, 1969. Neoveryhachium distinctum Sinha, Prasad and Srivastava, sp. nov. (Plate V, 4–7) Holotype: Plate V, 7 (slide R-8(0), coord. 98:7 ð 37:6; Shiala Formation, Sumna–Rimkhim mule track section at 2.320 km pt.

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Table 3 Selected acritarch species list recovered from the Shiala Formation and Yong Limestone Formation Subgroup SPHAEROMORPHITAE Downie et al., 1963 Lophosphaeridium sp. A, B, C 1. Lophosphaeridium parvum Stockmans et Williere, 1963 cf. Leiosphaeridia sp. A. Subgroup ACANTHOMORPHITAE Downie et al., 1963 2. Aremoricanium sp. cf. A. simplex Loeblich et MacAdam, 1971 3. Baltisphaeridium accinctum Loeblich et Tappan, 1978 B. archaicum Cramer et Diez, 1972 B. citrinum (Downie) Stockmans et Williere, 1974 B. longispinosum subsp. delicatum Turner, 1984 B. longispinosum var. longspinosum Eisenack, 1959 4. Baltisphaerosum bystrentos (Loeblich et Tappan) Turner, 1984 5. Buedingiisphaeridium tremadocum Rasul, 1979 6. Cheleutochroa diaphorosa Turner, 1984 7. Filisphaeridium henryi (Paris et Deunff) Sarjeant et Stancliffe, 1994 8. Holosphaeridium citrinipeltatum (Cramer et Diez) Dorning, 1981a H. sp. cf. H. clavispinulosum Lister, 1970a 9. Micrhystridium? diornamentum Rasul, 1979 M. equispinosum Turner, 1984 M. exiguum Rasul, 1979 M. parinconspicuum Deflandre, 1945 M. robustum Downie, 1958 M. shinetonense Downie, 1958 10. Multiplicisphaeridium cladum (Downie) Lister, 1970a M. ornatum Pothe de Baldis, 1971 M. thusui (Thusu) Eisenack, 1976 M. variabile (Lister) Dorning, 1981a 11. Oppilatala? indianae (Cramer et Diez) Priewalder, 1987 12. Ordovicidium nudum (Eisenack) Loeblich et Tappan, 1978 13. Orthosphaeridium sp. cf. O. quadrinatum (Burmann, 1970) Eisenack et al., 1976 14. Peteinosphaeridium sp. A 15. Solisphaeridium nanum (Deflandre) Turner, 1984 16. Stelliferidium redonense (Paris et Deunff) Deunff et al., 1974 17. Vulcanisphaera imparilis Rasul, 1976 Subgroup POLYGONOMORPHITAE Downie et al., 1963 18. Coryphidium miladum Cramer et Diez, 1976a 19. Dorsennidium rhomboidium (Downie) Sarjeant et Stancliffe, 1994 D. minutum (Downie) Stancliffe et Sarjeant, 1996 D. europaeum (Stockmans et Williere) Sarjeant et Stancliffe, 1994 20. Goniosphaeridium splendens (Eisenack) Turner, 1984 21. Impluviculus sp. cf I. stellum Rasul, 1979 22. Neoveryhachium distinctum Sinha, Prasad et Srivastava, sp. nov. 23. Polygonium delicatum Rasul, 1979 P. gracile (Vavrdova) Jacobson et Achab, 1985

Table 3 (continued) 24. 25. 26. 27.

Stellechinatum celestum (Martin) Turner, 1984 Tetradinium minutum Sinha, Prasad et Srivastava, sp. nov. Triangulina densus Sinha, Prasad et Srivastava, sp. nov. Veryhachium calandrae Cramer, 1970 V. downiei Stockmans et Williere, 1962 V. lairdi (Deflandre) Deunff ex. Downie, 1959 V. longispinosum Jardine et al., 1974 V. oklahomense Loeblich, 1970 V. reductum (Deunff) Downie et Sarjeant, 1965 V. valiente Cramer, 1964a V. sp. cf. V. wenlockium (Downie) Downie et Sarjeant, 1964 28. Villosacapsula sp. A Subgroup HERKOMORPHITAE Downie et al., 1963 29. Cymatiosphaera sp. A Cymatiosphaera sp. cf. C. celtica Deunff, 1958 30. Cymatiogalea sp. A Subgroup DIACROMORPHITAE Downie et al., 1963 31. Acanthodiacrodium rotundatum Gorka, 1967 A. simplex Combaz, 1967 32. Dasydiacrodium sp. cf. D. longicornutum Gorka, 1967 Subgroup NETROMORPHITAE Downie et al., 1963 33. Dactylofusa sp. cf. D. oblancae (Cramer et Diez) Cramer, 1970 34. Deunffia brevispinosa Downie, 1960 D. monacantha Deunff, 1951 D. monospinosa Downie, 1960 35. Domasia algerensis (Cramer) Hill, 1974 D. sp. cf.D. limaciforme (Stockmans et Williere) Cramer, 1970 D. trispinosa Downie, 1960 36. Geron sp. cf.G. amabilis Cramer, 1969 G. indicus Sinha, Prasad et Srivastava, sp. nov. 37. Leiofusa algerensis Cramer, 1970 L. bernesgae Cramer, 1964b L. elenae Cramer, 1964b L. parvitatis Loeblich, 1970 L. shialensis Sinha, Prasad et Srivastava, sp. nov.

Paratypes: Plate V, 4 (slide R-47(0), coord. 93:8 ð 52:8); Yong Limestone, Sumna-Rimkhim mule track section at 4.150 km pt: Plate V, 5 (slide R-8(0), coord. 98:7 ð 37:8) and Plate V, 6 (slide R-8(0), coord. 105:2 ð 28:7); Shiala Formation, Sumna-Rimkhim mule track section at 2.320 km pt. Etymology: The specific name is given distinctum as it has a distinct pylome. Diagnosis: Vesicle quadrangular, sides straight to convex, measures 12–15 µm at maximum length; eilyma two layered, endeilyma distinct, very close in contact to the perieilyma, endeilyma protrude

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into the spines present at the corners formed by perieilyma. Perieilyma finely granulate to scabrate; spines arise from the angles of the vesicle, one to two additional spines also present at the central region of the vesicle on both the sides. Spines simple, short and hollow, with sharp acuminating tips, height ranging from 2 to 5 µm, pylome distinct having quadrangular outline. Dimensions: Vesicle 12–15 µm long, 9–15 µm. wide; spines 2–5 µm. long (4 specimens measured). Comparison: The new species of Neoveryhachium (N. distinctum) differs from other known species in having straight to convex vesicle outline, granulate eilyma and being smaller in size range. N. carminae Cramer, 1964a differs from N. distinctum sp. nov.in having concave vesicle outline and psilate to finely granulate eilyma. N. neocarminae Cramer et Diez, 1972 is distinct in having folds on the vesicle in the form of muri. N. mayhillense Dorning, 1981a has laevigate vesicle with striae parallel to the sides and no excystment structure. Occurrence: Rare in D. trispinosa–D. monospinosa Assemblage Zone, abundant in the lower part of the L. algerenis–M. osgoodence Assemblage Zone. Genus Tetradinium Vavrdova, 1973 Type: Tetradinium arenigum Vavrdova, 1973. Tetradinium minutum Sinha, Prasad and Srivastava, sp. nov. (Plate IV, 10–13) Holotype: Plate IV, 11 (Slide R-21(5), coord. 96:6 ð 26:7); Shiala Formation, Sumna–Rimkhim mule track section at 2.345 km pt. Paratypes: Plate IV, 12 (slide R-21(5), coord. 93 ð 31:3); Shiala Formation, Sumna–Rimkhim mule track section at 2.345 km pt; Plate IV, 13 (slide R-47(0), coord. 99:7 ð 59:3) Yong Limestone, Sumna–Rimkhim mule track section at 4.150 km pt. Etymology: minutum for extremely small size range. Diagnosis: Vesicle rhombic to subquadrangular in outline with straight sides and pointed corners; vesicles measure 6 to 10 µm at maximum diameter, wall psilate; two distinct processes essentially emerge from each angles of the vesicle, processes simple, homomorphic, distally pointed, hollow and often variable in heights, measure up to 2–5 µm, freely communicating with vesicles interior; pylome indiscernible.

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Dimensions: Vesicle 6–10 µm long, 5–9.5 µm wide, spines 2–5 µm long (4 specimens measured). Comparison: The new species morphologically compares with Tetradinium by its rhombic shape and having more than one process at each corners. The new form is distinguishable from other known species of the genus in having essentially two short processes at each corners and being smaller size range. Type species of the genus T. arenigum Vavrdova, 1973 is quite distinct from present one in having 4–10 processes at each corners and being larger in size range. Occurrence: Abundant in the upper part of the D. trispinosa–D. monospinosa, rare in T. minutum–D. sp. cf. D. oblancae and abundant in the lower part of the L. algerensis–M. osgoodence Assemblage Zones. Triangulina Cramer, 1964a Type: Triangulina alargada Cramer, 1964a. Triangulina densus Sinha, Prasad and Srivastava, sp. nov. (Plate IV, 3 and 4) Holotype: Plate IV, 4 (slide R-8(0), coord. 103:3 ð 40:1); Shiala Formation, Sumna–Rimkhim mule track section at 2.320 km pt. Paratype: Plate IV, 3 (slide R-8(0), coord. 103:5 ð 68:4); Shiala Formation, Sumna–Rimkhim mule track section at 2.320 km pt. Etymology: The inner body (endeilyma) being dense, hence the name densus. Diagnosis: Vesicle triangular, two layered; inner body (endeilyma) dense, triangular in shape, smooth, measuring 7–10 µm, outer body (perieilyma) distinctly separated from endeilyma, endeilyma never protrude into the spines at corners, perieilyma thin, psilate of a broad, thick and stout cylindrical process with closed to pointed distal ends, measuring 3–7 µm in height, processes never communicate to the inner body (endeilyma) and distinctly separated from it; pylome indiscernible. Comparison: Triangulina densus sp. nov. is morphologically comparable with the generic circumscription of Triangulina and differ from known species of the genus in having dense endeilyma and being smaller size range. Occurrence: Abundant in the lower part of the D. trispinosa–D. monospinosa Assemblage zone.

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PLATE IV

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Geron Cramer, 1967 Type: Geron guerillerus Cramer, 1966. Geron Indicus Sinha, Prasad and Srivastava, sp. nov. (Plate VI, 6 and 11) Holotype: Plate VI, 6 (slide R-8(0), coord. 106 ð 67:7); Shiala Formation, Sumna–Rimkhim mule track section at 2.320 km pt. Paratype: Plate VI, 11 (slide R-8(0). coord. 100:1 ð 34:5); Shiala Formation, Sumna–Rimkhim mule track section at 2.320 km pt. Etymology: We keep the name indicus as it is the first record from India. Diagnosis: Vesicle oval to sub rectangular, measuring 10–12 µm at maximum length, vesicle wall two layered, endeilyma thick, comparatively dark, ornamented with closely spaced numerous microspines, perieilyma thin, light brown membraneous, structured with sparsely distributed fine grana, two stoutrod shaped processes drawn out from perieilyma at basal pole, processes stiff, cylindrical, distally closed, length varies from 3 to 8 µm; pylome indiscernible. Comparison: The new species of Geron differs from

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known species of the genus in having dense endeilyma and closely spaced microspines on it with two processes at basal pole. Occurrence: Common in the D. trispinosa–D. monospinosa Assemblage Zone. Genus Leiofusa (Eisenack) Cramer, 1970a Type: Leiofusa fusiformis Eisenack, 1934. Leiofusa shialensis Sinha, Prasad and Srivastava, sp. nov. (Plate VI, 13) Holotype: Plate VI, 13 (Slide R-8(0), Coord. 105 ð 50.2); Shiala Formation, Sumna–Rimkhim mule track section at 2.320 km pt. Etymology: First report from the Shiala Formation of the Tethyan Garhwal Himalaya, India, hence the name shialensis. Diagnosis: Vesicle fusiform, elongate, measuring 7 µm at its longitudinal axis, cyst wall psilate, with thin exinal folds, processes simple, processes two in number, emerging from opposite poles, hollow, acuminating tips, freely communicating with vesicle, measuring 3 µm, few small processes emerge from the vesicle surface; pylome indiscernible.

PLATE IV All magnifications ð1900 approximately, unless otherwise mentioned. 1. Lophosphaeridium sp. cf. L. parvum Stockmans and Williere, 1963. Slide No. R-22(1); coord. 94 ð 59. ð760. 2. Micrhystridium exiguum Rasul, 1979. Slide No. R-8(0); coord. 95 ð 35:2. 3, 4. Triangulina densus Sinha, Prasad and Srivastava, sp. nov. 3 D Paratype, Slide No. R-8(0); coord. 103:5 ð 68:4; 4 D Holotype, Slide No. R-8(0); coord. 103:3 ð 40:1. 5, 6. Coryphidium miladum Cramer and Diez, 1976. 5 D Slide No. R-17(2); coord. 97 ð 50:7. ð760; 6 D Slide No. R-21(1); coord. 104:8 ð 27:9. ð760. 7. Veryhachium reductum(Denuff) Downie and Sarjeant, 1965. Slide No. R-8(0); coord. 102:4 ð 69. 8. Veryhachium sp. cf. V. wenlockium (Downie) Downie and Sarjeant, 1965. Slide No. R-8(0); coord. 103:4 ð 69:6. 9. Multiplicisphaeridium ornatum Pothe de Baldis, 1971. Slide No. R-3(1); coord. 107:6 ð 26:3. ð760. 10–13. Tetradinium minutum Sinha, Prasad and Srivastava, sp. nov. 10 D Slide No. R-21(0); coord. 108 ð 52:5; 11 D Holotype, Slide No. R-21(5); coord. 96:6 ð 26:7; 12 D Paratype, Slide No. R-21(5); coord. 93 ð 31:3; 13 D Paratype, Slide No. R-47(0); coord. 99:7 ð 59:3. 14. Micrhystridium equispinosum Turner, 1984. Slide No. R-8(0); coord. 103:5 ð 41. 15–17, 20. Polygonium gracile (Vavrdova) Jacobson and Achab, 1985. 15 D Slide No. R-47(1); coord. 91:5 ð 37:2. ð760; 16 D Slide No. R-8 (1); coord. 92:4 ð 42:5. ð760; 17 D Slide No. R-12 (1); coord. 96 ð 46:5. ð760; 20 D Slide No. R-8(0); coord. 102:5 ð 51:4. 18. Dasydiacrodium sp. cf. D. longicornutum Gorka, 1967. Slide No. R-8(0); coord. 105:1 ð 38. 19. Acanthodiacrodium retundatum Gorka, 1967. Slide No. R-21(5); coord. 93 ð 48:3. 21. Cymatiosphaera sp. cf. C. celtica Deunff, 1958. Slide No. R-8(0); coord. 104:4 ð 59:4. 22. Leiofusa bernesgae Cramer, 1964. Slide No. R-8(0); coord. 104:3 ð 70:6. 23. Polygonium sp. A. Slide No. R-43(2); coord. 101:7 ð 32:1. ð760. 24. Stelliferidium redonense (Paris and Deunff, 1970) Deunff et al., 1974. Slide No. R-10(1); coord. 105 ð 39:4. ð760.

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PLATE V

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Comparison: Leiofusa shialensis sp. nov. is morphologically comparable with generic circumscription of Leiofusa and differ from known species of the genus in having extremely smaller size range of the vesicle. However, this form is well preserved, rare and only one specimen has been recovered from the sample processed. In spite of monospecific assemblage recovered, it has been erected as a new species due to its typical morphological character described above. Occurrence: Rare in the lower part of D. trispinosa– D. monospinosa Assemblage zone.

7. Acritarch assemblages and zone Silurian acritarchs from the Northern Gondwana domain are poorly known, compared to Laurentia and Baltica (Le He´risse´ et al., 1995). However, the record from India is almost negligible due to rare occurrence of marine Paleozoic sediments in Peninsular India and patchy exposure in Extrapeninsular India (Sinha et al., 1996a). This is the first detailed work on the Group Acritarcha from the Himalaya, in general, and from the Tethyan Garhwal Himalaya, in particular. Stratigraphic distributions of selected acritarch species recovered from Ordovician–Silurian successions representing the Shiala and Yong formations of the Sumna– Rimkhim section in the Tethyan Garhwal Himalaya are shown in Fig. 4. The distribution of incertae sedis (melanosclerite and chitinozoans) recovered in asso-

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ciation with acritarchs have also been shown in this diagram (Fig. 4). These phytoplanktonic microfossils are potentially good for determining precise age (Le He´risse´ et al., 1995). The stratigraphic distributions of these forms indicate that some biostratigraphic index forms show their first and last occurrence at a certain stratigraphic level. Thus, the first and last occurrence levels (FADs and LADs) of biostratigraphically significant forms, and their quantitative distributions enable in establishing four distinct acritarch assemblage zones in the exposed Ordovician–Silurian succession of the Sumna–Rimkhim mule track section (Fig. 4). The base and the top of these assemblage zones are marked at the first occurrence and=or last occurrence level of age diagnostic species. The acritarch assemblage zones of the Tethyan Garhwal Himalaya have been shown and correlated with other areas around the world in the biostratigraphic map of Briggs (1995) (Fig. 5). The biostratigraphic correlation has been attempted on the basis of identical floral assemblages occurring at various overseas succession ranging in age from Ordovician to Silurian. The acritarch zones delineated in Ordovician– Silurian section of the study area are as follows in descending stratigraphic order: – Zone IV: Leiofusa algerensis–Multiplicisphaeridium osgoodence Assemblage Zone. – Zone III: Tetradinium minutum–Dactylofusa sp. cf. D. oblancae Assemblage Zone.

PLATE V All magnifications ð1900 approximately, unless otherwise mentioned. 1, 2, 9, 10. Dorsennidium minutum (Downie) Stancliffe and Sarjeant, 1996. 1 D Slide No. R-8(0); coord. 102:9 ð 62:3; 2 D Slide No. R-8(0). coord. 103:7 ð 65; 9 D Slide No. R-8(0); coord. 104:3 ð 69:7; 10 D Slide No. R-8(0); coord. 104:8 ð 50:4. 3. Veryhachium oklahomense Loeblich, 1970. Slide No. R-8(0); coord. 106 ð 30:5. 4–7. Neoveryhachium distinctum Sinha, Prasad and Srivastava, sp. nov. 4 D Paratype, Slide No. R-47(0); coord. 93:8 ð 52:8; 5 D Paratype, Slide No. R-8(0); coord. 98:7 ð 37:8; 6 D Paratype, Slide No. R-8(0); coord. 105:2 ð 28:7; 7 D Holotype, Slide No. R-8(0); coord. 98:7 ð 37:6. 8. Dorsennidium europaeum (Stockmans and Williere) Sarjeant and Stancliffe, 1994. Slide No. R-8(0); coord. 103:8 ð 66:8. 11. Dorsennidium sp. (Wicander, 1974) Sarjeant and Stancliffe, 1994. Slide No. R-8(0); coord. 106:1 ð 29:2. 12. Veryhachium sp. B. Slide No. R-8(0); coord. 109:8 ð 62:2. 13. Stellechinatum celestum (Martin) Turner, 1984. Slide No. R-21(5); coord. 111 ð 33:7. 14. Veryhachium valiente Cramer, 1964. Slide No. R-8(0); coord. 105:2 ð 28:7. 15. Veryhachium longispinosum Jardine et al., 1974. Slide No. R-8(0); coord. 105:1 ð 37:2. 16. Veryhachium calandrae Cramer, 1970. Slide No. R-50(1); coord. 111:6 ð 63:3. 17. Veryhachium lairdi (Deflandre) Deunff ex. Downie, 1959. Slide No. R-21(5); coord. 98:3 ð 29.

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PLATE VI

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The salient features of each assemblage zone are briefly described below in the ascending stratigraphic order with discussion on their age. 7.1. Zone I: Baltisphaeridium longispinosum var. longispinosum–Multiplicisphaeridium ornatum Assemblage Zone

Fig. 5. Acritarch correlation with the study area to that of other regions of the world in the biogeographical map of Briggs (1995) of mid-Silurian times.

– Zone II: Domasia trispinosa–Deunffia monospinosa Assemblage Zone. – Zone I: Baltisphaeridium longispinosum var. longispinosum–Multiplicisphaeridium ornatum Assemblage Zone.

Reference section: Shiala Formation between 2.310 and 2.320 km point of the Sumna–Rimkhim section, Tethyan Garhwal Himalaya. Definition: Common to rare occurrence of Baltisphaeridium longispinosum var. longispinosum, B. longispinosum subsp. delicatum, B. archaicum, Filisphaeridium henryi and Multiplicisphaeridium ornatum. Base: The base of the zone could not be identified precisely as the rocks below the stratigraphic level representing the sample number R-2 are mainly unfossiliferous (Fig. 4). Thus, the base may be little below this level within the Shiala Formation. Top: The last occurrence of Baltisphaeridium longispinosum var. longispinosum, B. archaicum and=or first occurrence of Geron sp. cf. G. amabilis, G. indicus sp. nov. Significant accessory forms: Deunffia monacantha, Baltisphaeridium longispinosum subsp. delicatum, Coryphidium miladum, Ordovicidium nudum, Baltisphaerosum bystrentos, Polygonium gracile, Fil-

PLATE VI All magnifications ð1900, unless otherwise mentioned. 1–5. Deunffia monospinosa Downie, 1960. 1 D Slide No. R-8(0); coord. 99:9 ð 63:9; 2 D Slide No. R-8(0). coord. 99:4 ð 63:1; 3 D Slide No. R-8(0); coord. 102:6 ð 69:4; 4 D Slide No. R-8(0); coord. 94:7 ð 47:9; 5 D Slide No. R-8(0); coord. 106:4 ð 48:7. 6, 11. Geron indicus Sinha, Prasad and Srivastava, sp. nov. 6 D Holotype, Slide No. R-8(0); coord. 106 ð 67:7; 11 D Paratype, Slide No. R-8(0); coord. 100:1 ð 34:5. 7. Deunffia brevispinosa Downie, 1960. Slide No. R-8(1); coord. 100:5 ð 59:9. ð760. 8. Dactylofusa sp. cf. D. oblancae (Cramer and Diez) Cramer, 1970. Slide No. R-47(0); coord. 93:6 ð 51:2. 9, 17. Leiofusa parvitatis Loeblich, 1970. 9 D Slide No. R-8(0); coord. 106:7 ð 32:6; 17 D Slide No. R-47(0); coord. 94:8 ð 47:8. 10, 12. Geron sp. cf. G. amabilis Cramer, 1969. 10 D Slide No. R-8(0); coord. 97 ð 40; 12 D Slide No. R-8(0); coord. 107:4 ð 49:6. 13. Holotype, Leiofusa shialensis Sinha, Prasad and Srivastava, sp. nov. Slide No. R-8(0); coord. 105 ð 50:2. 14. Domasia sp. cf. D. limaciforme (Stockmans and Williere) Cramer, 1970. Slide No. R-39(0); coord. 107:2 ð 69:9. 15. Eupoikilofusa sp. A. Slide No. R-8(0); coord. 104:8 ð 36:1. 16. Leiofusa algerensis Cramer, 1970. Slide No. R-8(0); coord. 105:5 ð 37:6. 18. Acanthodiacrodium simplex Combaz, 1967. Slide No. R-8(0); coord. 102:4 ð 39:6. 19. Deunffia monacantha Deunff, 1951. Slide No. R-8(2); coord. 92:2 ð 64:3. ð760. 20, 21. Domasia trispinosa Downie, 1960. 20 D Slide No. R-8(0); coord. 97 ð 27:6; 21 D Slide No. R-8(0); coord. 97 ð 27:6. ð2660. 22. Domasia algerensis (Cramer) Hill, 1974. Slide No. R-8(0); coord. 100:6 ð 34:3. 23. Domasia sp. A. Slide No. R-47(0); coord. 94:7 ð 40:4. 24, 25. Leiofusa bernesgae Cramer, 1964b. 24 D Slide No. R-8(0); coord. 94:1 ð 55:5; 25 D Slide No. R-8(0); coord. 104:7 ð 65:5.

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PLATE VII

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isphaeridium henryi, Cymatiosphaera sp. A, and Lophosphaeridium parvum. Correlation: The most important species of stratigraphic value of this assemblage zone are Baltisphaerosum bystrentos, Baltisphaeridium archaicum, B. longispinosum subsp. delicatum, Deunffia monacantha and Baltisphaeridium longispinosum var. longispinosum. These biostratigraphic index forms have been already described by Deunff (1951, 1955, 1958), Eisenack (1951, 1959, 1962), Timofeev (1959), Staplin et al. (1965), Gorka (1969), Henry (1969), Henry and Thadeu (1971), Cramer and Diez (1972), Loeblich and Tappan (1978), Turner (1984) and others from the Lower Paleozoic sequence of different parts of the globe. Age: Caradocian to early Llandoverian. 7.2. Zone II: Domasia trispinosa–Deunffia monospinosa Assemblage Zone Reference section: Shiala Formation between 2.320 and 2.355 km point of the Sumna–Rimkhim section. Tethyan Garhwal Himalaya. Definition: Dominant occurrence of Domasia trispinosa, subdominant presence of Deunffia brevispinosa, Domasia algerensis and Deunffia monospinosa together with common occurrence of Solisphaeridium nanum, Geron sp. cf. G. amabilis, G. indicus sp. nov. and Baltisphaeridium citrinum. Base: The first occurrence of Geron sp. cf. G. amabilis, G. indicus sp. nov. and=or last occurrence of Baltisphaeridium longispinosum var. longispinosum and B. archaicum. Top: The last occurrence of Deunffia brevispinosa, and=or first occurrence of Baltisphaeridium citrinum. Significant accessory forms: Common occurrence of Neoveryhachium distinctum sp. nov., Micrhystridium robustum, Baltisphaeridium citrinum, Filisphaeri-

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dium henryi, Leiofusa bernesgae, Micrhystridium parinconspicuum and Geron indicus sp. nov.. However, Filisphaeridium henryi, Acanthodiacrodium simplex, Ordovicidium nudum, Veryhachium longispinosum and Coryphidium miladum have been also recorded from this zone. Restricted forms: Geron sp. cf. G. amabilis, G. indicus sp. nov., Leiofusa shialensis sp. nov., Stelliferidium redonse, Domasia trispinosa, D. algerenis, Deunffia brevispinosa, Multiplicisphaeridium cladum, M. thusui, and Triangulina densus sp. nov., etc. Correlation: The acritarch assemblage of this zone is mainly dominated by the polygonomorphs (Veryhachium, Neoveryhachium and Dorsinnidium), netromorphs (Domasia and Deunffia) and acanthomorphs (Multiplicisphaeridium and Micrhystridium). This biozone is characterised by abundant acritarchs which are gradually increasing in abundance and diversity along with few chitinozoans. Also, several new species appear approximately in the lower and upper part of the zone. Few of them ranges up to Zone IV. Among the recovered acritarchs are Domasia trispinosa, Deunffia monospinosa, D. brevispinosa, Geron sp. cf. G. amabilis, Leiofusa parvitatis, which collectively indicate a Llandoverian to Wenlockian age. These biostratigraphic index species are characteristic of Early Silurian assemblages in the eastern as well as in western Europe (Downie, 1960, 1963; Hill, 1974; Cramer and Diez, 1972, 1979; Cramer et al., 1979; Martin, 1969; Lister, 1970a,b; Lister and Downie, 1974; Sheshegova, 1975, Thusu, 1973; Cramer, 1969, 1970a,b,c; Loeblich, 1970). The restricted occurrence of Geron in the Tethyan assemblage further suggests a Llandoverian to Wenlockian age. Age: Llandoverian to Wenlockian.

PLATE VII All magnifications ð1900, unless otherwise mentioned. 1, 5. Baltisphaeridium longispinosum var. longispinosum Eisenack, 1959. 1 D Slide No. R-5(2); coord. 103 ð 33:5; 5 D Slide No. R-5(1); coord. 99:7 ð 27:8. 2. Leiofusa elenae Cramer, 1964b. Slide No. R-15(1); coord. 100:7 ð 49:8. 3. aff. Orthosphaeridium sp. A. Slide No. R-50(1); coord. 111 ð 57:3. 4. Stellechinatum sp. A. Slide No. R-47(3); coord. 108 ð 49:4. ð760. 6. Baltisphaeridium longispinosum subsp. delicatum Turner, 1984. Slide No. R-5(2); coord. 100:5 ð 33:5.

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PLATE VIII

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7.3. Zone III: Tetradinium minutum–Dactylofusa sp. cf. D. oblancae Assemblage Zone Reference section: Shiala and Yong Limestone Formation; between 2.355 and 4.155 km point of the Sumna–Rimkhim section, Tethyan Garhwal Himalaya. Definition: Common occurrence of Domasia sp. cf. D. limaciforme and Dactylofusa sp. cf. D. oblancae; and dominant presence of Leiofusa algerensis, L. parvitatis and Stellichinatum celestum. Base: The last occurrence of Deunffia brevispinosa and=or first occurrence of Biltisphaeridium citrinum. Top: The last occurrence of Leiofusa algerensis, Veryhachium sp. cf. V. wenlockium and Stellichinatum celestum. Significant accessory forms: Deunffia monospinosa., Tetradinium minutum sp. nov., Domasia sp. cf. D. limaciforme and Dactylofusa sp. cf. D. oblancae. Restricted forms: Deunffia monospinosa, Domasia sp. cf. D. limaciforme and Stellichinatum celestum. Correlation: The characteristic forms of this assemblage zone are Stellichinatum celestum, Veryhachium sp. cf. V. wenlockium and Leiofusa algerensis, reported from Lower Paleozoic successions in Belgium (Martin, 1969), Bohemia (Burmann, 1970), Algeria (Jardine et al., 1974), Indiana (Colbath, 1979), England (Downie, 1959, 1963; Lister and Downie, 1967), Canada and USA (Thusu, 1973), Jordon (Basha, 1992) and others. Thus, the presence of species of Domasia and De-

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unffia in this assemblage zone suggests Wenlockian age, since, the species of these two forms do not extend beyond the Wenlockian (Cramer and Diez, 1979). Remarks: The acritarch contents of T. minutum– Dactylofusa sp. cf. D. oblancae Assemblage Zone is poor to moderate. However, it contains a rich assemblage of chitinozoans and melanosclerities. The chitinozoan assemblage includes forms such as Desmochitina sp., Ancyrochitina sp., and Conochitina sp. which are very common in the Silurian sediment. The specific identification could not be made, as it is beyond the scope of the present work. Age: Wenlockian to early Ludlovian. 7.4. Zone IV: Leiofusa algerensis– Multiplicisphaeridium osgoodense Assemblage zone Reference section: Yong Limestone Formation; between 4.155 and 4.175 km point of the Sumna– Rimkhim section, Tethyan Garhwal Himalaya. Definition: Dominance of species nominated together with common occurrence of Oppilatala? indianae, Stellichinatum celestum, Veryhachium sp. cf. V. wenlockium and Multiplicisphaeridium variabile. Base: The last occurrence of Leiofusa algerensis, Veryhachium sp. cf. V. wenlockium, Stellichinatum celestum and Neoveryhachium distinctum sp. nov. Top: The top of this zone could not be identified as the strata above sample no. R-51 are devoid of acritarchs.

PLATE VIII All magnifications ð1900, unless otherwise mentioned. 1, 2. Impluviculus sp. cf. I. stellum Rasul, 1979. 1 D Slide No. R-8(0); coord. 104:4 ð 66:2; 2 D Slide No. R-8(0); coord. 104 ð 59:3. 3. Villosacapsula sp. A. Slide No. R-8(0); coord. 103:3 ð 69:3. 4. Villosacapsula sp. B. Slide No. R-8(0); coord. 103:3 ð 30. 5. Cymatiosphaera sp. A. Slide No. R-8(1); coord. 94:2 ð 44:4. 6. cf. Leiosphaeridia sp. A. Slide No. R-8(2); coord. 104:7 ð 46:5. 7, 11. Ancyrochitina sp. A. 7 D Slide No. R-20(1); coord. 93:9 ð 51. ð760; 11 D Slide No. R-20(1); coord. 93:9 ð 51. ð950. 8. Desmochitina sp. A. Slide No. R-40(1); coord. 95:1 ð 35:8. 9. Aremoricanium sp. cf. A. simplex Loeblich and MacAdam, 1971. Slide No. R-8(1); coord. 94:7 ð 23:8. 10. Multiplicisphaeridium sp. A. Slide No. R-8(1); coord. 99:6 ð 42. 12. Conochitina sp. A. Slide No. R-21(5); coord. 94:7 ð 27:7. ð760. 13. Lophosphaeridium sp. B. Slide No. R-12(1); coord. 92:1 ð 65. 14. Helosphaeridium sp. cf. H. clavispinulosum Lister, 1970a. Slide No. R-21 (5); coord. 94:1 ð 73. 15. Lophosphaeridium sp. C. Slide No. R-8(2); coord. 106:8 ð 29:5. 16. Cymatiogalea sp. A. Slide No. R-21(5); coord. 95:2 ð 36:5. 17. Lophosphaeridium parvum Stockmans and Williere, 1963. Slide No. R-21 (1); coord. 108:5 ð 41:7.

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Fig. 6. Some sketches of dark species of acritarchs for delineating the morphological details. Not to scale. (a) Baltisphaeridium citrinum (Downie) Stockmans and Williere, 1974. (b-1, 2) Leiofusa bernesgae Cramer, 1964b. (c) Ordovicidium nudum (Eisenack) Loeblich and Tappan, 1978. (d) Multiplicisphaeridium ornatum Pothe de Baldis, 1971. (e) Leiofusa elenae Cramer, 1964b. (f) Dactylofusa sp. cf. D. oblancae (Cramer and Diez, 1968) Cramer, 1970a (g-1, 2) Vulcanisphaera imparilis Rasul, 1976. (h-1) Baltisphaeridium longispinosum var. longispinosum Eisenack, 1959 (Rupture possibly along pylome). (h-2) B. longispinosum var. longispinosum Eisenack, 1959. (i) B. longispinosum subsp. delicatum Turner, 1984. (j) Geron sp. cf. G. amabilis Cramer, 1969. (k) Aremoricanium sp. cf. A. simplex Loeblich and MacAdam, 1971 (vesicle corroded by pyrite).

Significant accessory forms: Leiofusa algerensis, L. bernesgae, L. parvitatis, Veryhachium sp. cf. V. wenlockium, Multiplicisphaeridium osgoodense, Oppilatala? indianae and Multiplicisphaeridium variabile. Correlation: The age marker acritarch species of this zone, such as Oppilatala? indianae, Multiplicisphaeridium variabile and Leiofusa bernesgae are

reported globally in the Late Silurian rocks. (Fisher, 1953; Cramer, 1964a,b; Lister and Downie, 1967; Jardine and Yapaudjian, 1968; Lister, 1970a; Pothe de Baldis, 1971, 1975; Cramer and Diez, 1972, 1979; Cramer et al., 1974). Age: Early Ludlovian. The sediments overlying the Yong Limestone Formation belong to the Variegated Formation and the

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Fig. 6 (continued).

samples from this formation are observed to be devoid of acritarchs. However, the Muth Quartzite section which overlies the Variegated Formation has yielded rich assemblages of pentamerid brachiopods (Pentamerifera sp.) of Late Silurian age and some corals (Goel et al., 1987). The record of Late Silurian pentamerid brachiopods in the top horizon of the Muth Quartzite sequence suggests Ludlovian age for the Variegated Formation.

8. Acritarch biostratigraphy vs. lithostratigraphy The acritarch studies of the Sumna–Rimkhim section of the Tethyan Himalaya in the Garhwal region have provided the new findings to the stratigraphy of this region and revised the earlier date assigned to the Shiala Formation. The recognition of acritarch assemblage zones and stratigraphically potential species suggest a Sil-

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urian age for the Shiala Formation which was earlier dated as Ordovician. This shows that the lithostratigraphic boundary may not coincide with the biostratigraphic boundary (Sinha et al., 1996a). Thus, the lower Paleozoic sediments of the Sumna–Rimkhim section of the Tethyan Garhwal Himalaya ranges in age from Ordovician to Late Silurian (Caradocian to Ludlovian).

Acknowledgements The authors gratefully acknowledge Dr. Stephen R. Jacobson (Chevron U.S.A., Colorado) and an anonymous reviewer for reading the manuscript and offering constructive comments and English improvement. We thank Dr. Arun Kumar of Carleton University, Canada for providing valuable reprints on acritarchs from his own library. We also thank Professor S.K. Upadhyay, Professor A.K. Jain, Professor A.K. Awasthi and Roopam Mukherjee (R=S) of Earth Sciences department of Roorkee University, Mr. Kuldeep Chandra, Dr. Jagdish Pandey, Dr. Alok Dave and Mr.P.N. Kapoor of ONGC for assistance at the KDM Institute of Petroleum Exploration, Dehradun. The Defence Ministry (Government of India) is acknowledged for allowing the first author for field work and field photographs. Mr. Rajmukta Sundaram and Mr. Gangadhar Prasad accompanied the first author in 1992 and 1993 expeditions for field work at higher altitudes of the Tethys Himalaya, respectively. Financial support was received from the UGC, Government of India, to the first author in the form of fellowship during his doctoral programme. We thank Shri S.K. Mitra of ISM for neat drafting. Finally the first author honour the affectionate encouragement of his beloved wife Dr. Laxmi Shiveshwari Shivi in bringing out this paper.

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