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Palynoflora from subsurface Lower Cretaceous Intertrappean beds in Domra Sub-basin of the Raniganj Gondwana Basin, West Bengal, India
Cretaceous Research (1997) 18, 37 – 50
Palynoflora from subsurface Lower Cretaceous Intertrappean beds in Domra Sub-basin of the Raniganj Gondwana Basin, West Bengal, India Vijaya Birbal Sahni Institute of Palaeobotany , 53 University Road , Lucknow 226 007 , India Revised manuscript accepted 17 June 1996
A new sub-basin, Domra, has been located recently at the extreme eastern fringe of the Raniganj Gondwana Basin, West Bengal, India. In Domra Borehole PGD-2, the Mesozoic sequence, which is represented by five Intertrappean beds, was analysed for spores and pollen. Palynomorphs were recovered from only Intertrappean Beds 1 and 2 (263 – 248 m). Major elements of the assemblage are Callialasporites , Araucariacites , Podocarpidites and several pteridophytic trilete spores. The overall composition suggests an Early Cretaceous age. The absence of Aequitriradites , Cooksonites , Coptospora , and Triporoletes indicates that the palynoflora is older (Valanginian – Hauterivian) than that of the Cyclosporites hughesii Zone and closely comparable to that of the Foraminisporis wonthaggiensis Zone in Australia. ÷ 1997 Academic Press Limited KEY WORDS: palynology; Early Cretaceous; Rajmahal Volcanic Formation, Domra Sub-basin; India.
1. Introduction Late Jurassic and Early Cretaceous age determinations still remain the subject of discussion in continental sequences on peninsular India. The famous Rajmahal Flora from Intertrappean beds was originally assigned a Late Jurassic age (Sahni & Rao, 1933; Sahni, 1935). However, subsurface palynological data from these beds and radiometric dates of lava flows in the Rajmahal Basin suggest an Early Cretaceous age for the sequence (Tiwari & Tripathi, 1995). During exploration in 1984 – 85 of coal reserves in the easternmost part of the Damodar Basin in West Bengal, the Geological Survey of India located lava flows equivalent to those of the Trap – Intertrap sequence in the Rajmahal Basin (Figure 1). The two areas are separated from each other by metamorphic rocks, and the basinal structure in the Domra area is named the Domra Sub-basin. The two sub-basinal structures, i.e., the Domra and Panagarh Sub-basins, are on the eastern fringe of Raniganj Gondwana Basin, West Bengal. The entire area is covered by thick deposits of Recent and Quaternary sediments; underlying this are Tertiary and the Upper Gondwana sediments. In the Domra area the subsurface Mesozoic sequence is represented by lava flows (two to five in number) with Intertrappean / Infratrappean beds containing fossil leaf impressions (Bhattacharjee, 1990). The present palynological study deals with the Intertrappean Beds 1 and 2 from the top of the Mesozoic sequence encountered in the Borehole PGD-2 of the Domra Sub-basin (Figure 1). Here a pile of lava flows with Intertrappean and Infratrappean beds (Rajmahal Volcanic Formation) of the Upper Gondwana sequence occurs beneath the Tertiary sequence (Figure 2, Table 1). 0195 – 6671 / 97 / 010037 1 14 $25.00 / 0 / cr960048
÷ 1997 Academic Press Limited
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Figure 1. Map showing location of Domra Sub-Basin and Rajmahal Basin, and the drill site of Borehole PGD-2 in Raniganj Gondwana Basin, West Bengal.
2. Observations Borehole PGD-2 was drilled to a depth of 1000.00 m. It penetrated Tertiary, Mesozoic (Intertrappean, Infratrappean and Panchet), and Late Permian (Raniganj) sediments. Samples from the Mesozoic (Rajmahal Formation) occurring between 248.00 and 263.00 m have been palynologically examined (Figure 2). Thirty-seven samples from the Intertrappean deposits were processed for their organic contents; 17 proved to be productive, but identifiable spores and pollen were recorded from only 14 (Figure 2). The slides with figured specimens are stored in the Repository of the Birbal Sahni Institute of Palaeobotany, Lucknow, India. The objectives of the study were to date the Mesozoic strata, and determine the
Table 1. Geological succession in Borehole PGD-2 Period
Formation
Lithology
Alluvium Tertiary ààààààààààààààààààààààààààààààààààààààààààààà Unconformity Early Rajmahal Infratrappean horizon: grey shale, carbonacCretaceous Volcanic eous shale and sandstone. Formation Traps: Vesicular, greenish-grey basalt. Intertrappean beds: 1 – 5 horizons. ààààààààààààààààààààààààààààààààààààààààààààà Unconformity Early Panchet Pale green, medium- to Triassic Formation coarse-grained shales. Late Raniganj Off-white, fine to medium, coarse-grained shales Permian Formation containing major coal seams. Unconformity
Lower Cretaceous palynoflora
39
Figure 2. Lithological column in Borehole PGD-2 showing position of the Traps, Intertrappean and Infratrappean sediments, the depth in metres of the samples studied, and the occurrence of characteristic species recovered from samples in between Traps 1 and 2.
relationship between Intertrappean palynoassemblages of the Domra Sub-basin and the Rajmahal Basin (Figure 1). Since this is the first palynological report on the Intertrappean beds of the Mesozoic sequence in the Domra Sub-basin, a comprehensive list of spore-pollen species is presented (Table 2) which provides the basic data for their age determination. A comparison with the Lower Cretaceous palynological sequence of Australia is made. Based on qualitative assessment of marker species, palynofloras of Borehole PGD-2 and those of the Rajmahal Basin have been also compared (Table 3). The palynoassemblages from the sediments between 263.00 to 248.00 m represent the lower part of the First Intertrappean Bed (IT-1) and the upper part of the Second Intertrappean Bed (IT-2). Their spore / pollen composition is broadly similar (Figures 3 – 5), but the first appearance datum (FAD) of Crybelosporites stylosus , Foraminisporis wonthaggiensis and Januasporites spiniferus at 260.00 m enables palynostratigraphic subdivision into two zones, here designated as palynozones A and B. The assemblage at 260.00 m includes older as well as younger characteristic species.
2.1. Palynozone A: 263.00 – 260.00 m (Tables 2 , 4 ; Figure 2 ) The upper part of Intertrappean Bed 2 (IT-2) yielded a rich palynoflora. Gymnospermous pollen grains are dominant, but pteridophytic spores are frequent and diverse. The most significant feature of this zone is the occurrence of Obtusisporis yarragadensis , Polycingulatisporites crenulatus and Taurocusporites segmentatus —species of Tithonian aspect—in association with Cicatricosisporites australiensis , Contignisporites cooksoniae , Murospora florida and Retitriletes circolumenus.
Table 2. Distribution of spore / pollen species in Intertrappean beds in Borehole PGD-2, Domra Sub-basin, West Bengal
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Lower Cretaceous palynoflora
41
Spore-pollen species
Domra Sub-basin palynozones II IV O I III V
Rajmahal Basin palynological levels
Australia
Table 3. Comparison of distribution of significant species in palynozones of Borehole PGD-2 with that identified in the Early Cretaceous palynoflora of the Rajmahal Basin (after Tiwari & Tripathi, 1995) and in the Cicatricosisporites australiensis —Foraminisporis wonthaggiensis Zone in Australia (after Dettmann, 1963; Burger, 1990b). Species of non-striate bisaccate pollen are not included. Species marked with an asterisk (p) have been recorded in Domra Sub-basin palynozone ‘‘A’’ and are the characteristic elements from the known Tithonian palynozones of Australia (see Filatoff, 1975)
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Figure 3. All photomicrographs are 3500, unless otherwise stated. Birbal Sahni Institute of Palaeobotany Slide Numbers are provided. 1, Matonisporites cooksoniae , 11569. 2, Triletes tuberculiformis , 11570. 3 , Impardecispora apiverrucata , 3750, 11571. 4, Osmundacidites singhii , 11572. 5, Concavissimisporites penolaensis , 3750, 11579. 6, Dictyophyllidites harrisii , 11573. 7, Santhalisporites baskoensis , 11574. 8, Retitriletes circolumenus , 11570. 9, Foveotriletes parviretus , 11570. 10, 11, Crybelosporites stylosus , 11571. 12, Todisporites minor , 11574. 13, Foraminisporis wonthaggiensis , 11570. 14, Gleicheniidites senonicus , 11574. 15, Januasporites spiniferus , 11572. 16, Leptolepidites major , 11570. 17 , Santhalisporites imperfectus , 11572. 18, Ceratosporites equalis , 11570. 19, Staplinisporites caminus , 11570. 20, Klukisporites venkatachalae , 11572.
Figure 4. All photomicrographs are 3500. 1, Cicatricosisporites ludbrookae , 11575. 2 , Contignisporites dettmanniae , 11569. 3 , Densoisporites velatus , 11573. 4 , Contignisporites glebulentus , 11569. 5 , Klukisporites variegatus , 11569. 6 , Cicatricosisporites jansonii , 11569. 7 , Lycopodiumsporites circolumenus , 11573. 8 , Contignisporites fornicatus , 11576. 9 , Biformaesporites baculosus , 11578. 10 , Baculatisporites comaumensis , 11572. 11 , 17 , Murospora florida , 11569. 12 , Foveosporites sp., 11577. 13, Lycopodiacidites dettmanniae , 11572. 14 , Crassimonoletes singhei , 11578. 15, Dettmannites attenuarus , 11570. 16 , Equisetosporites virginiaensis , 11569.
Figure 5. All photomicrographs are 3500, unless otherwise stated. 1, Osmundacidites wellmanii , 11574. 2, Stereisporites pocockii , 11578. 3, Obtusisporites yarragadensis , 11579. 4, Microcachryidites antarcticus , 11580. 5 , Podocarpidites alatireticulatus , 11572. 6, Callialasporites dampieri , 11580. 7, Taurocusporites segmentatus , 11580. 8, Polycingulatisporites crenulatus , 11580. 9 , Callialasporites triletes , 3750, 11580. 10, Callialasporites monoalasporus , 11574. 11, Podocarpidites grandis , 11580. 12, Pinuspollenites globosaccus , 11580. 13, Callialasporites turbatus , 11572. 14, Callialasporites lametaensis , 11578. 15, Podosporites tripakshi , 11580. 16, Cycadopites sakarigaliensis , 11574.
Genera
248.00
IT 1 251.00
251.50
252.00
255.00
255.50
256.00
259.10
Intertrappean bed 2 (Depth in m) 260.00
260.80
262.80
263.00
Table 4. Percentage frequencies of palynomorph genera in productive samples of Borehole PGD-2, Domra Sub-basin, West-Bengal, India (1 5 presence recorded)
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Lower Cretaceous palynoflora
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2.2. Palynozone B : 260.00 – 248.00 m (Tables 2 , 4 ; Figure 2) In the upper part of Intertrappean Bed 2, between 251.00 m and 260.00 m, several taxa have stratigraphic first appearances including Cicatricosisporites hughesii , C. ludbrookiae , Crybelosporites stylosus , Foraminisporis wonthaggiensis and Januasporites spiniferus. The hilate spore genera Aequitriradites , Cooksonites , Coptospora and Triporoletes are the key taxa in Hauterivian to Aptian palynofloras of the Indian peninsula and Australia. However, they were not recorded from Borehole PGD-2. Along with the presence of the above-mentioned Tithonian palynotaxa this suggests a Valanginian age for the palynoflora (Dettmann, 1963; Singh et al. , 1964; Venkatachala et al. , 1972; Dettmann & Thomson, 1987; Helby et al. , 1987; Burger, 1988; Singh & Venkatachala, 1988; Tiwari & Tripathi, 1995). The recovery of spores and pollen from 248.00 m, at the base of Intertrappean Bed I (IT-I) just above the Trap II (T-II), was low. In general, the impact of flow caused a darkening of colour to blackish-brown but some translucent palynomorphs are also present in the assemblage, a condition that seems to have been the result of oxidizing conditions at the time of deposition. The assemblage at this depth contains 60% pteridophytic spores (Cicatricosisporites , Concavissimisporites , Contignisporites , Dictyophyllidites , Foveotriletes , Impardecispora , Klukisporites , Leptolepidites , Murospora ) and about 20% Araucariacites and Callialasporites (Table 4). Specimens of Murospora and Concavissimisporites are quite frequent, and their surfaces are highly corrugated, suggesting that they have been reworked from older sediments. Permian and Triassic palynotaxa occur in the sequence 263.00 to 248.00 m. These include Arcuatipollenites , Crescentipollenites , Densipollenites , Falcisporites , Faunipollenites , Krempipollenites , Satsangisaccites and Striatopodocarpites . They are most common at 256.00, 255.50 and 255.00 m, and their highly degraded state of preservation confirms reworking from older horizons. At depths 248.00, 252.30, 261.50, 262.20 and 263.00 m the yield of spores and pollen was poor; all are dark in colour (brown to brownish-black) and opaque. 3. Comparison with Rajmahal and Australian Early Cretaceous palynozones Lava flows of Cretaceous age (Rajmahal Formation) have been reported previously from the Rajmahal Basin, Bihar (Rajarao, 1987), but this is the first documentation of their occurrence in the Domra Sub-basin of the Raniganj Gondwana Basin. They vary from two to five in number. Within the Intertrappean beds occurrences of leaf impressions of Ptilophyllum and Otozamites have indicated previously that these beds may be correlated with Upper Gondwana Mesozoic sediments (Bhattacharjee, 1990). As indicated in Figure 1, the Rajmahal Basin and Domra Sub-basin are in close proximity, and the Gondwana sediments of both are capped by lava flows. However, they are separated from each other by metamorphic rocks, and the present palynological study indicates that the Trap – Intertrap sequence of the Domra Sub-basin is older to that of the Rajmahal Basin. A comparison between the palynoflora recovered from Intertrappean beds of Domra Sub-basin (Borehole PGD 2) with that of the Rajmahal Basin (Borehole RJNE 32; Tiwari & Tripathi, 1995) is presented below (see also Table 3).
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As is evident from the published palynological data, Cicatricosisporites australi ensis marks the base of Early Cretaceous palynofloras in Australia and India (Dettmann, 1963; Helby et al. , 1987; Singh & Venkatachala, 1988; Burger, 1990a). The records of Cicatricosisporites australiensis , C. hughesii and C. ludbrookiae at 262.80 m and upwards in Borehole PGD-2 indicate comparability with the Early Cretaceous palynozones of Australia and India. A number of species with ranges that extend from Valanginian to Hauterivian occur widely in India, and are present in the Intertrappean beds of both Borehole PGD-2 and the Rajmahal Basin. These include Cicatricosisporites hughesii , C. ludbrookiae , Contignisporites glebulentus , Microcachryidites antarcticus and Santhalisporites imperfectus (Table 3). But the lava flows of the Rajmahal Basin have been dated as c. 118 to c. 116 Ma (Aptian) and contain early angiospermid pollen in the upper part of IT-I (c. 117.5 Ma: Tiwari & Tripathi, 1995). Moreover, the presence of Aequitriradites , Cooksonites , Coptospora and Triporoletes in the Rajmahal palynoflora indicates a younger aspect, closer to that of the Cyclosporites hughesii Zone than to the Borehole PGD-2 assemblage (Tripathi et al. , 1990; Tiwari & Tripathi, 1995). In the Australian Mesozoic sequence (Helby et al. , 1987; Burger, 1990a,b), a precise stratigraphic range of marker species, viz. , an association of Aequitriradites hispidus , Callialasporites spp, Cicatricosisporites australiensis , Crybelosporites stylosus , and Dictyotosporites speciosus , has been recorded from the transition of Tithonian to Valanginian (cf. time scale of Harland et al. , 1990). From the top of the Tithonian palynozone these species continue to occur in the younger palynozones of the Lower Cretaceous (Figure 6). The ranges of Aequitriradites hispidus and Crybelosporites stylosus are most significant because these extend only into the lower part of the Cyclosporites hughesii Zone. The first appearance of Foraminisporis wonthaggiensis at the top of the Cicatricosisporites australiensis (Crybelosporites stylosus ) Zone is also noteworthy (Dettmann, 1963; Dettmann & Playford, 1969; Burger, 1990b). It is the occurrences of such stratigraphically significant species along with the first appearance of Foraminisporis wonthaggiensis and Januasporites spiniferous in Borehole PGD-2 which suggests that the assemblage is older than the Rajmahal flora. And the continuity of certain forms such as Equisetosporites concinnus , Polycingulatisporites crenulatus and Taurocusporites segmentatus , which characterize the Tithonian palynozone (Filatoff, 1975; Helby et al. , 1987), is important in connection with determining the age of the present assemblage (Figure 6; Table 3). 4. Conclusions In view of the ranges of marker species (Figure 6) in the non-marine and marine Early Cretaceous sequences of Australia, it is concluded that the assemblage in Borehole PGD-2, which contains a fair representation of Crybelosporites stylosus , along with other characteristic forms including Callialasporites dampieri , Cicatricosisporites spp., Contignisporites spp., Foraminisporis wonthaggiensis , Foveosporites canalis , Foveotriletes parviretus , Januasporites spiniferus , Microcachryidites antarcticus , Murospora florida and Osmundacidites spp., is closely similar to that of the Foraminisporis wonthaggiensis palynozone of Australia (Burger, 1990b). Together with the absence of Appendicisporites spp., Cooksonites variabilis , Crybelosporites striatus , Cyclosporites hughesii , Dictyotosporites speciosus and
Figure 6. Distribution of marker spore-pollen species in—Late Jurassic-Early Cretaceous palynozones in Australia (after Dettmann, 1963; Burger, 1990a, b) and in Borehole PGD-2 palynozone. Solid lines 5 range of species. 1 5 Polycingulatisporites crenulatus ; 2 5 Retitriletes circolumenus ; 3 5 Microcachryidites antarcticus ; 4 5 Contignisporites cooksoniae ; 5 5 Araucariacites fiscus ; 6 5 Murospora florida ; 7 5 Aequitriradites hispidus ; 8 5 Cicatricosisporites australiensis ; 9 5 Crybelosporites stylosus ; 10 5 Cicatricosisporites ludbrookaei ; 11 5 Cicatricosisporites hughesii ; 12 5 Foraminisporis wonthaggiensis ; 13 5 Januasporites spiniferus ; 14 5 Callialasporites reticulatus ; 15 5 Equisetosporites virginiaensis. p indicates the level of comparison of Domra Sub-basin palynoflora with Foraminisporis wonthaggiensis palynozone of Australia; age Valangian.
Lower Cretaceous palynoflora
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Pilosisporites notensis , this suggests that the Borehole PGD-2 assemblage may be correlated with the Foraminisporis wonthaggiensis Zone, which is Valanginian to Hauterivian in age. Acknowledgements Personnel of Geological Survey of India located the extension of subsurface Gondwana Sequence in the east of the Raniganj Basin. Their kind help in obtaining samples from this area is gratefully acknowledged. The author is grateful to Dr N. D. Mitra, former Senior Deputy Director General of Geological Survey of India, and to Mr T. K. Bhattacharjee, Senior Geologist, Coal zone I, Geological Survey of India, for their continuing support and kind permission in the provision of samples and field data. Sincere thanks are extended to Dr R. S. Tiwari, ex Director, Birbal Sahni Institute of Palaeobotany, Lucknow, for his constructive suggestions during the preparation of the manuscript. References Bhattacharjee, T. K. 1990. Status report on regional exploratory drilling of Lower Gondwana Sequence in Domra -Panagarh area , along the eastern margin of the Raniganj Coalfield , concealed below alluvium and other younger deposits in Bardhaman District , West Bengal. Unpublished Report, Geological Survey of India, 100 pp. Burger, D. 1988. Early Cretaceous environments in the Eromanga Basin; palynological evidence from GSQ Wyandra-1 core hole. Memoirs of the Association of Australasian Palaeontologists 5, 173 – 186. Burger, D. 1990a. Australian Phanerozoic timescales: 8. Jurassic biostratigraphic charts and explanatory notes. Bureau of Mineral Resources of Australia , Record 1989(38), 38 pp. Burger, D. 1990b. Australian Phanerozoic timescales: 9. Cretaceous biostratigraphic charts and explanatory notes. Bureau of Mineral Resources of Australia , Record 1989(39), 36 pp. Dettmann, M. E. 1963. Upper Mesozoic microfloras from south-eastern Australia. Proceedings of the Royal Society of Victoria 77, 148 pp. Dettmann, M. E. & Playford, G. 1969. Palynology of the Australian Cretaceous: a review. In Stratigraphy and palaeontology: essays in honour of Dorothy Hill (ed. Campbell, K. S. W.), pp. 174 – 210 (Australian National University Press, Canberra). Dettmann, M. E. & Thomson, M. R. A. 1987. Cretaceous palynomorphs from the James Ross Island area, Antarctica—a pilot study. British Antarctic Survey , Bulletin 77, 13 – 59. Filatoff, J. 1975. Jurassic palynology of the Perth Basin, Western Australia. Palaeontographica B 154, 1 – 111. Harland, W. B., Armstrong, R. L., Cox, A. V., Craig, L. E., Smith, A. G. & Smith, D. G. 1990. A geological timescale 1989 , xvi 1 263 pp. (Cambridge University Press, Cambridge). Helby, R. J., Morgan, R. & Partridge, A. D. 1987. A palynological zonation of the Australian Mesozoic. Memoirs of the Association of Australasian Palaeontologists 4, 1 – 94. Rajarao, C. S. 1987. Coal resources of Bihar (excluding Dhanbad District). Bulletin of the Geological Survey of India. Series A 45, IV(1), 336 pp. Sahni, B. 1935. Recent discoveries in Rajmahal Flora. Proceedings of the 6th International Botanical Congress , pp. 248 – 249 (Amsterdam). Sahni, B. & Rao, A. R. 1933. On some Jurassic plants from the Rajmahal Hills. Journal of Asiatic Society of Bengal , New Series 27, 183 – 208. Singh, H. P. & Venkatachala, B. S. 1988. Upper Jurassic – Lower Cretaceous spore-pollen assemblages in the Peninsular India. Palaeobotanist 36, 168 – 176. Singh, H. P., Srivastava, S. K. & Roy, S. K. 1964. Studies on the Upper Gondwana of Cutch. 1—Mio- and macrospores. Palaeobotanist 12, 282 – 306. Tiwari, R. S. & Tripathi, A. 1995. Palynological assemblages and absolute age relationship of Intertrappean beds in the Rajmahal Basin, India. Cretaceous Research 16, 53 – 72. Tripathi, A., Tiwari, R. S. & Kumar, P. 1990. Palynology of the subsurface Mesozoic sediments in Rajmahal Basin, Bihar. Palaeobotanist 37, 367 – 388. Venkatachala, B. S., Sharma, K. D. & Jain, A. K. 1972. Palynological zonation of Jurassic – Lower Cretaceous sediments in the subsurface of Cauvery Basin. In Proceedings of seminar on palaeopalynology and Indian stratigraphy , Calcutta , 1971 (eds Ghosh, A. K., Chanda, S., Ghosh, T. K., Baksi, S. K. & Banerjee, M.), pp. 172 – 187 (University Grants Commission and Botany Department, University of Calcutta).
Palynoflora from subsurface Lower Cretaceous Intertrappean beds in Domra Sub-basin of the Raniganj Gondwana Basin, West Bengal, India Vijaya Birbal Sahni Institute of Palaeobotany , 53 University Road , Lucknow 226 007 , India Revised manuscript accepted 17 June 1996
A new sub-basin, Domra, has been located recently at the extreme eastern fringe of the Raniganj Gondwana Basin, West Bengal, India. In Domra Borehole PGD-2, the Mesozoic sequence, which is represented by five Intertrappean beds, was analysed for spores and pollen. Palynomorphs were recovered from only Intertrappean Beds 1 and 2 (263 – 248 m). Major elements of the assemblage are Callialasporites , Araucariacites , Podocarpidites and several pteridophytic trilete spores. The overall composition suggests an Early Cretaceous age. The absence of Aequitriradites , Cooksonites , Coptospora , and Triporoletes indicates that the palynoflora is older (Valanginian – Hauterivian) than that of the Cyclosporites hughesii Zone and closely comparable to that of the Foraminisporis wonthaggiensis Zone in Australia. ÷ 1997 Academic Press Limited KEY WORDS: palynology; Early Cretaceous; Rajmahal Volcanic Formation, Domra Sub-basin; India.
1. Introduction Late Jurassic and Early Cretaceous age determinations still remain the subject of discussion in continental sequences on peninsular India. The famous Rajmahal Flora from Intertrappean beds was originally assigned a Late Jurassic age (Sahni & Rao, 1933; Sahni, 1935). However, subsurface palynological data from these beds and radiometric dates of lava flows in the Rajmahal Basin suggest an Early Cretaceous age for the sequence (Tiwari & Tripathi, 1995). During exploration in 1984 – 85 of coal reserves in the easternmost part of the Damodar Basin in West Bengal, the Geological Survey of India located lava flows equivalent to those of the Trap – Intertrap sequence in the Rajmahal Basin (Figure 1). The two areas are separated from each other by metamorphic rocks, and the basinal structure in the Domra area is named the Domra Sub-basin. The two sub-basinal structures, i.e., the Domra and Panagarh Sub-basins, are on the eastern fringe of Raniganj Gondwana Basin, West Bengal. The entire area is covered by thick deposits of Recent and Quaternary sediments; underlying this are Tertiary and the Upper Gondwana sediments. In the Domra area the subsurface Mesozoic sequence is represented by lava flows (two to five in number) with Intertrappean / Infratrappean beds containing fossil leaf impressions (Bhattacharjee, 1990). The present palynological study deals with the Intertrappean Beds 1 and 2 from the top of the Mesozoic sequence encountered in the Borehole PGD-2 of the Domra Sub-basin (Figure 1). Here a pile of lava flows with Intertrappean and Infratrappean beds (Rajmahal Volcanic Formation) of the Upper Gondwana sequence occurs beneath the Tertiary sequence (Figure 2, Table 1). 0195 – 6671 / 97 / 010037 1 14 $25.00 / 0 / cr960048
÷ 1997 Academic Press Limited
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Figure 1. Map showing location of Domra Sub-Basin and Rajmahal Basin, and the drill site of Borehole PGD-2 in Raniganj Gondwana Basin, West Bengal.
2. Observations Borehole PGD-2 was drilled to a depth of 1000.00 m. It penetrated Tertiary, Mesozoic (Intertrappean, Infratrappean and Panchet), and Late Permian (Raniganj) sediments. Samples from the Mesozoic (Rajmahal Formation) occurring between 248.00 and 263.00 m have been palynologically examined (Figure 2). Thirty-seven samples from the Intertrappean deposits were processed for their organic contents; 17 proved to be productive, but identifiable spores and pollen were recorded from only 14 (Figure 2). The slides with figured specimens are stored in the Repository of the Birbal Sahni Institute of Palaeobotany, Lucknow, India. The objectives of the study were to date the Mesozoic strata, and determine the
Table 1. Geological succession in Borehole PGD-2 Period
Formation
Lithology
Alluvium Tertiary ààààààààààààààààààààààààààààààààààààààààààààà Unconformity Early Rajmahal Infratrappean horizon: grey shale, carbonacCretaceous Volcanic eous shale and sandstone. Formation Traps: Vesicular, greenish-grey basalt. Intertrappean beds: 1 – 5 horizons. ààààààààààààààààààààààààààààààààààààààààààààà Unconformity Early Panchet Pale green, medium- to Triassic Formation coarse-grained shales. Late Raniganj Off-white, fine to medium, coarse-grained shales Permian Formation containing major coal seams. Unconformity
Lower Cretaceous palynoflora
39
Figure 2. Lithological column in Borehole PGD-2 showing position of the Traps, Intertrappean and Infratrappean sediments, the depth in metres of the samples studied, and the occurrence of characteristic species recovered from samples in between Traps 1 and 2.
relationship between Intertrappean palynoassemblages of the Domra Sub-basin and the Rajmahal Basin (Figure 1). Since this is the first palynological report on the Intertrappean beds of the Mesozoic sequence in the Domra Sub-basin, a comprehensive list of spore-pollen species is presented (Table 2) which provides the basic data for their age determination. A comparison with the Lower Cretaceous palynological sequence of Australia is made. Based on qualitative assessment of marker species, palynofloras of Borehole PGD-2 and those of the Rajmahal Basin have been also compared (Table 3). The palynoassemblages from the sediments between 263.00 to 248.00 m represent the lower part of the First Intertrappean Bed (IT-1) and the upper part of the Second Intertrappean Bed (IT-2). Their spore / pollen composition is broadly similar (Figures 3 – 5), but the first appearance datum (FAD) of Crybelosporites stylosus , Foraminisporis wonthaggiensis and Januasporites spiniferus at 260.00 m enables palynostratigraphic subdivision into two zones, here designated as palynozones A and B. The assemblage at 260.00 m includes older as well as younger characteristic species.
2.1. Palynozone A: 263.00 – 260.00 m (Tables 2 , 4 ; Figure 2 ) The upper part of Intertrappean Bed 2 (IT-2) yielded a rich palynoflora. Gymnospermous pollen grains are dominant, but pteridophytic spores are frequent and diverse. The most significant feature of this zone is the occurrence of Obtusisporis yarragadensis , Polycingulatisporites crenulatus and Taurocusporites segmentatus —species of Tithonian aspect—in association with Cicatricosisporites australiensis , Contignisporites cooksoniae , Murospora florida and Retitriletes circolumenus.
Table 2. Distribution of spore / pollen species in Intertrappean beds in Borehole PGD-2, Domra Sub-basin, West Bengal
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Lower Cretaceous palynoflora
41
Spore-pollen species
Domra Sub-basin palynozones II IV O I III V
Rajmahal Basin palynological levels
Australia
Table 3. Comparison of distribution of significant species in palynozones of Borehole PGD-2 with that identified in the Early Cretaceous palynoflora of the Rajmahal Basin (after Tiwari & Tripathi, 1995) and in the Cicatricosisporites australiensis —Foraminisporis wonthaggiensis Zone in Australia (after Dettmann, 1963; Burger, 1990b). Species of non-striate bisaccate pollen are not included. Species marked with an asterisk (p) have been recorded in Domra Sub-basin palynozone ‘‘A’’ and are the characteristic elements from the known Tithonian palynozones of Australia (see Filatoff, 1975)
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Figure 3. All photomicrographs are 3500, unless otherwise stated. Birbal Sahni Institute of Palaeobotany Slide Numbers are provided. 1, Matonisporites cooksoniae , 11569. 2, Triletes tuberculiformis , 11570. 3 , Impardecispora apiverrucata , 3750, 11571. 4, Osmundacidites singhii , 11572. 5, Concavissimisporites penolaensis , 3750, 11579. 6, Dictyophyllidites harrisii , 11573. 7, Santhalisporites baskoensis , 11574. 8, Retitriletes circolumenus , 11570. 9, Foveotriletes parviretus , 11570. 10, 11, Crybelosporites stylosus , 11571. 12, Todisporites minor , 11574. 13, Foraminisporis wonthaggiensis , 11570. 14, Gleicheniidites senonicus , 11574. 15, Januasporites spiniferus , 11572. 16, Leptolepidites major , 11570. 17 , Santhalisporites imperfectus , 11572. 18, Ceratosporites equalis , 11570. 19, Staplinisporites caminus , 11570. 20, Klukisporites venkatachalae , 11572.
Figure 4. All photomicrographs are 3500. 1, Cicatricosisporites ludbrookae , 11575. 2 , Contignisporites dettmanniae , 11569. 3 , Densoisporites velatus , 11573. 4 , Contignisporites glebulentus , 11569. 5 , Klukisporites variegatus , 11569. 6 , Cicatricosisporites jansonii , 11569. 7 , Lycopodiumsporites circolumenus , 11573. 8 , Contignisporites fornicatus , 11576. 9 , Biformaesporites baculosus , 11578. 10 , Baculatisporites comaumensis , 11572. 11 , 17 , Murospora florida , 11569. 12 , Foveosporites sp., 11577. 13, Lycopodiacidites dettmanniae , 11572. 14 , Crassimonoletes singhei , 11578. 15, Dettmannites attenuarus , 11570. 16 , Equisetosporites virginiaensis , 11569.
Figure 5. All photomicrographs are 3500, unless otherwise stated. 1, Osmundacidites wellmanii , 11574. 2, Stereisporites pocockii , 11578. 3, Obtusisporites yarragadensis , 11579. 4, Microcachryidites antarcticus , 11580. 5 , Podocarpidites alatireticulatus , 11572. 6, Callialasporites dampieri , 11580. 7, Taurocusporites segmentatus , 11580. 8, Polycingulatisporites crenulatus , 11580. 9 , Callialasporites triletes , 3750, 11580. 10, Callialasporites monoalasporus , 11574. 11, Podocarpidites grandis , 11580. 12, Pinuspollenites globosaccus , 11580. 13, Callialasporites turbatus , 11572. 14, Callialasporites lametaensis , 11578. 15, Podosporites tripakshi , 11580. 16, Cycadopites sakarigaliensis , 11574.
Genera
248.00
IT 1 251.00
251.50
252.00
255.00
255.50
256.00
259.10
Intertrappean bed 2 (Depth in m) 260.00
260.80
262.80
263.00
Table 4. Percentage frequencies of palynomorph genera in productive samples of Borehole PGD-2, Domra Sub-basin, West-Bengal, India (1 5 presence recorded)
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Lower Cretaceous palynoflora
47
2.2. Palynozone B : 260.00 – 248.00 m (Tables 2 , 4 ; Figure 2) In the upper part of Intertrappean Bed 2, between 251.00 m and 260.00 m, several taxa have stratigraphic first appearances including Cicatricosisporites hughesii , C. ludbrookiae , Crybelosporites stylosus , Foraminisporis wonthaggiensis and Januasporites spiniferus. The hilate spore genera Aequitriradites , Cooksonites , Coptospora and Triporoletes are the key taxa in Hauterivian to Aptian palynofloras of the Indian peninsula and Australia. However, they were not recorded from Borehole PGD-2. Along with the presence of the above-mentioned Tithonian palynotaxa this suggests a Valanginian age for the palynoflora (Dettmann, 1963; Singh et al. , 1964; Venkatachala et al. , 1972; Dettmann & Thomson, 1987; Helby et al. , 1987; Burger, 1988; Singh & Venkatachala, 1988; Tiwari & Tripathi, 1995). The recovery of spores and pollen from 248.00 m, at the base of Intertrappean Bed I (IT-I) just above the Trap II (T-II), was low. In general, the impact of flow caused a darkening of colour to blackish-brown but some translucent palynomorphs are also present in the assemblage, a condition that seems to have been the result of oxidizing conditions at the time of deposition. The assemblage at this depth contains 60% pteridophytic spores (Cicatricosisporites , Concavissimisporites , Contignisporites , Dictyophyllidites , Foveotriletes , Impardecispora , Klukisporites , Leptolepidites , Murospora ) and about 20% Araucariacites and Callialasporites (Table 4). Specimens of Murospora and Concavissimisporites are quite frequent, and their surfaces are highly corrugated, suggesting that they have been reworked from older sediments. Permian and Triassic palynotaxa occur in the sequence 263.00 to 248.00 m. These include Arcuatipollenites , Crescentipollenites , Densipollenites , Falcisporites , Faunipollenites , Krempipollenites , Satsangisaccites and Striatopodocarpites . They are most common at 256.00, 255.50 and 255.00 m, and their highly degraded state of preservation confirms reworking from older horizons. At depths 248.00, 252.30, 261.50, 262.20 and 263.00 m the yield of spores and pollen was poor; all are dark in colour (brown to brownish-black) and opaque. 3. Comparison with Rajmahal and Australian Early Cretaceous palynozones Lava flows of Cretaceous age (Rajmahal Formation) have been reported previously from the Rajmahal Basin, Bihar (Rajarao, 1987), but this is the first documentation of their occurrence in the Domra Sub-basin of the Raniganj Gondwana Basin. They vary from two to five in number. Within the Intertrappean beds occurrences of leaf impressions of Ptilophyllum and Otozamites have indicated previously that these beds may be correlated with Upper Gondwana Mesozoic sediments (Bhattacharjee, 1990). As indicated in Figure 1, the Rajmahal Basin and Domra Sub-basin are in close proximity, and the Gondwana sediments of both are capped by lava flows. However, they are separated from each other by metamorphic rocks, and the present palynological study indicates that the Trap – Intertrap sequence of the Domra Sub-basin is older to that of the Rajmahal Basin. A comparison between the palynoflora recovered from Intertrappean beds of Domra Sub-basin (Borehole PGD 2) with that of the Rajmahal Basin (Borehole RJNE 32; Tiwari & Tripathi, 1995) is presented below (see also Table 3).
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Vijaya
As is evident from the published palynological data, Cicatricosisporites australi ensis marks the base of Early Cretaceous palynofloras in Australia and India (Dettmann, 1963; Helby et al. , 1987; Singh & Venkatachala, 1988; Burger, 1990a). The records of Cicatricosisporites australiensis , C. hughesii and C. ludbrookiae at 262.80 m and upwards in Borehole PGD-2 indicate comparability with the Early Cretaceous palynozones of Australia and India. A number of species with ranges that extend from Valanginian to Hauterivian occur widely in India, and are present in the Intertrappean beds of both Borehole PGD-2 and the Rajmahal Basin. These include Cicatricosisporites hughesii , C. ludbrookiae , Contignisporites glebulentus , Microcachryidites antarcticus and Santhalisporites imperfectus (Table 3). But the lava flows of the Rajmahal Basin have been dated as c. 118 to c. 116 Ma (Aptian) and contain early angiospermid pollen in the upper part of IT-I (c. 117.5 Ma: Tiwari & Tripathi, 1995). Moreover, the presence of Aequitriradites , Cooksonites , Coptospora and Triporoletes in the Rajmahal palynoflora indicates a younger aspect, closer to that of the Cyclosporites hughesii Zone than to the Borehole PGD-2 assemblage (Tripathi et al. , 1990; Tiwari & Tripathi, 1995). In the Australian Mesozoic sequence (Helby et al. , 1987; Burger, 1990a,b), a precise stratigraphic range of marker species, viz. , an association of Aequitriradites hispidus , Callialasporites spp, Cicatricosisporites australiensis , Crybelosporites stylosus , and Dictyotosporites speciosus , has been recorded from the transition of Tithonian to Valanginian (cf. time scale of Harland et al. , 1990). From the top of the Tithonian palynozone these species continue to occur in the younger palynozones of the Lower Cretaceous (Figure 6). The ranges of Aequitriradites hispidus and Crybelosporites stylosus are most significant because these extend only into the lower part of the Cyclosporites hughesii Zone. The first appearance of Foraminisporis wonthaggiensis at the top of the Cicatricosisporites australiensis (Crybelosporites stylosus ) Zone is also noteworthy (Dettmann, 1963; Dettmann & Playford, 1969; Burger, 1990b). It is the occurrences of such stratigraphically significant species along with the first appearance of Foraminisporis wonthaggiensis and Januasporites spiniferous in Borehole PGD-2 which suggests that the assemblage is older than the Rajmahal flora. And the continuity of certain forms such as Equisetosporites concinnus , Polycingulatisporites crenulatus and Taurocusporites segmentatus , which characterize the Tithonian palynozone (Filatoff, 1975; Helby et al. , 1987), is important in connection with determining the age of the present assemblage (Figure 6; Table 3). 4. Conclusions In view of the ranges of marker species (Figure 6) in the non-marine and marine Early Cretaceous sequences of Australia, it is concluded that the assemblage in Borehole PGD-2, which contains a fair representation of Crybelosporites stylosus , along with other characteristic forms including Callialasporites dampieri , Cicatricosisporites spp., Contignisporites spp., Foraminisporis wonthaggiensis , Foveosporites canalis , Foveotriletes parviretus , Januasporites spiniferus , Microcachryidites antarcticus , Murospora florida and Osmundacidites spp., is closely similar to that of the Foraminisporis wonthaggiensis palynozone of Australia (Burger, 1990b). Together with the absence of Appendicisporites spp., Cooksonites variabilis , Crybelosporites striatus , Cyclosporites hughesii , Dictyotosporites speciosus and
Figure 6. Distribution of marker spore-pollen species in—Late Jurassic-Early Cretaceous palynozones in Australia (after Dettmann, 1963; Burger, 1990a, b) and in Borehole PGD-2 palynozone. Solid lines 5 range of species. 1 5 Polycingulatisporites crenulatus ; 2 5 Retitriletes circolumenus ; 3 5 Microcachryidites antarcticus ; 4 5 Contignisporites cooksoniae ; 5 5 Araucariacites fiscus ; 6 5 Murospora florida ; 7 5 Aequitriradites hispidus ; 8 5 Cicatricosisporites australiensis ; 9 5 Crybelosporites stylosus ; 10 5 Cicatricosisporites ludbrookaei ; 11 5 Cicatricosisporites hughesii ; 12 5 Foraminisporis wonthaggiensis ; 13 5 Januasporites spiniferus ; 14 5 Callialasporites reticulatus ; 15 5 Equisetosporites virginiaensis. p indicates the level of comparison of Domra Sub-basin palynoflora with Foraminisporis wonthaggiensis palynozone of Australia; age Valangian.
Lower Cretaceous palynoflora
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Pilosisporites notensis , this suggests that the Borehole PGD-2 assemblage may be correlated with the Foraminisporis wonthaggiensis Zone, which is Valanginian to Hauterivian in age. Acknowledgements Personnel of Geological Survey of India located the extension of subsurface Gondwana Sequence in the east of the Raniganj Basin. Their kind help in obtaining samples from this area is gratefully acknowledged. The author is grateful to Dr N. D. Mitra, former Senior Deputy Director General of Geological Survey of India, and to Mr T. K. Bhattacharjee, Senior Geologist, Coal zone I, Geological Survey of India, for their continuing support and kind permission in the provision of samples and field data. Sincere thanks are extended to Dr R. S. Tiwari, ex Director, Birbal Sahni Institute of Palaeobotany, Lucknow, for his constructive suggestions during the preparation of the manuscript. References Bhattacharjee, T. K. 1990. Status report on regional exploratory drilling of Lower Gondwana Sequence in Domra -Panagarh area , along the eastern margin of the Raniganj Coalfield , concealed below alluvium and other younger deposits in Bardhaman District , West Bengal. Unpublished Report, Geological Survey of India, 100 pp. Burger, D. 1988. Early Cretaceous environments in the Eromanga Basin; palynological evidence from GSQ Wyandra-1 core hole. Memoirs of the Association of Australasian Palaeontologists 5, 173 – 186. Burger, D. 1990a. Australian Phanerozoic timescales: 8. Jurassic biostratigraphic charts and explanatory notes. Bureau of Mineral Resources of Australia , Record 1989(38), 38 pp. Burger, D. 1990b. Australian Phanerozoic timescales: 9. Cretaceous biostratigraphic charts and explanatory notes. Bureau of Mineral Resources of Australia , Record 1989(39), 36 pp. Dettmann, M. E. 1963. Upper Mesozoic microfloras from south-eastern Australia. Proceedings of the Royal Society of Victoria 77, 148 pp. Dettmann, M. E. & Playford, G. 1969. Palynology of the Australian Cretaceous: a review. In Stratigraphy and palaeontology: essays in honour of Dorothy Hill (ed. Campbell, K. S. W.), pp. 174 – 210 (Australian National University Press, Canberra). Dettmann, M. E. & Thomson, M. R. A. 1987. Cretaceous palynomorphs from the James Ross Island area, Antarctica—a pilot study. British Antarctic Survey , Bulletin 77, 13 – 59. Filatoff, J. 1975. Jurassic palynology of the Perth Basin, Western Australia. Palaeontographica B 154, 1 – 111. Harland, W. B., Armstrong, R. L., Cox, A. V., Craig, L. E., Smith, A. G. & Smith, D. G. 1990. A geological timescale 1989 , xvi 1 263 pp. (Cambridge University Press, Cambridge). Helby, R. J., Morgan, R. & Partridge, A. D. 1987. A palynological zonation of the Australian Mesozoic. Memoirs of the Association of Australasian Palaeontologists 4, 1 – 94. Rajarao, C. S. 1987. Coal resources of Bihar (excluding Dhanbad District). Bulletin of the Geological Survey of India. Series A 45, IV(1), 336 pp. Sahni, B. 1935. Recent discoveries in Rajmahal Flora. Proceedings of the 6th International Botanical Congress , pp. 248 – 249 (Amsterdam). Sahni, B. & Rao, A. R. 1933. On some Jurassic plants from the Rajmahal Hills. Journal of Asiatic Society of Bengal , New Series 27, 183 – 208. Singh, H. P. & Venkatachala, B. S. 1988. Upper Jurassic – Lower Cretaceous spore-pollen assemblages in the Peninsular India. Palaeobotanist 36, 168 – 176. Singh, H. P., Srivastava, S. K. & Roy, S. K. 1964. Studies on the Upper Gondwana of Cutch. 1—Mio- and macrospores. Palaeobotanist 12, 282 – 306. Tiwari, R. S. & Tripathi, A. 1995. Palynological assemblages and absolute age relationship of Intertrappean beds in the Rajmahal Basin, India. Cretaceous Research 16, 53 – 72. Tripathi, A., Tiwari, R. S. & Kumar, P. 1990. Palynology of the subsurface Mesozoic sediments in Rajmahal Basin, Bihar. Palaeobotanist 37, 367 – 388. Venkatachala, B. S., Sharma, K. D. & Jain, A. K. 1972. Palynological zonation of Jurassic – Lower Cretaceous sediments in the subsurface of Cauvery Basin. In Proceedings of seminar on palaeopalynology and Indian stratigraphy , Calcutta , 1971 (eds Ghosh, A. K., Chanda, S., Ghosh, T. K., Baksi, S. K. & Banerjee, M.), pp. 172 – 187 (University Grants Commission and Botany Department, University of Calcutta).