Annual Report 2011-2012 - Birbal Sahni Institute of Palaeobotany

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EBIRBAL SAHNI INSTITUT 1946 OF PALAEOBOTANY Annual Report 2011-2012 varied types of sub-environment ranging from inner neritic, tidal flat, freshwater. (jointly with I.B. Singh) Rahul Garg (superannuated w.e.f. 30.11.2010), Vandana Prasad, Biswajeet Thakur & Khowaja Ateequzzaman (superannuated w.e.f. 31.12.2009) Multidisciplinary integrative sedimentological, mineralogical, geochemical, biostratigraphic and palynofacies studies of the Um Sohryngkew Cretaceous- Tertiary (K-T) transition in the Khasi Hills of Meghalaya, reveal biotic and environmental changes about 800 km from the Deccan volcanic province (DVP). Upper Cretaceous sedimentary succession indicates deposition in a shallow marine environment with high detrital influx from nearby continental terrains. High kaolinite and illite in clay mineral assemblages indicate high humidity and high runoff. In the Danian, sandy shale is replaced by marly limestone, indicating decreased detrital influx and a rising sea level. Microfossil assemblages (foraminifera, dinocysts & nannofossils) define the K-T boundary and suggest stressful eutrophic conditions and sea level fall, followed by a rise as indicated by increased diversity. Marly limestone deposition and the first diverse nannofossils, dinoflagellates and planktic foraminiferal assemblages occur in zone P1c and mark the biotic recovery in the Danian correlative with the recovery after the last Deccan volcanic pulse in C29N. Biotic turnover (blooms of Guembelitria cretacea) likely due to mesotrophic to eutrophic conditions suggests extended periods of environmental stress during Late Maastrichtian yielding critical information related to the main phase-2 of Deccan volcanism during C29r. Kaolinite increases beginning about 40 cm below the K-T boundary and is dominant in the Danian, suggesting humid climatic conditions in the Meghalaya area. In contrast, semi-arid climate conditions prevailed in the cotemporaneous Deccan Traps province, which appears to be linked to “mock aridity”. These results clearly link the K-T mass extinction, high pre-K-T biotic stress, delayed biotic recovery, and anomalous PGE concentrations to Deccan volcanic activity. (jointly with Gerta Keller & co-workers, Princeton University, USA) Rahul Garg (superannuated w.e.f. 30.11.2010) & Vandana Prasad Project 6.2: Mesozoic nannofossils from western Indian continental shelves and its palaeobiogeographic significance Kachchh Basin (Wagad Island)— The oldest marine sedimentary rock unit in the Patcham Island, the Kaladongar Formation is exposed in Kuar Bet Islet situated about one km, northwest of Patcham. The Kaladongar Formation is divided into two members, lower Dingi Hill and upper Kaladongar Sandstone. The Dingi Hill Member, exposed in the Patcham Island, shows thinly bedded alternations of khaki coloured and red siltstones with hard calcareous sandstones and abundant ichnotaxa. At Point 16 in the Kuar Bet (also called Mori Bet), exposures are seen in a hillock with profuse pelycepods with rare gastropod shells near the top. The lower part of the section shows current and flaser bedding, suggesting high energy conditions. Within in this current bedding unit, a sample has yielded calcareous nannofossils (GPS location 23°59’40"N: 69°42’28"E). The recovered nannotaxa include are Axopodorhabdus cylindratus, Biscutum finchii, Biscutum sp., Bussonius prinsii, Crepidolithus crassus, C. pliensbachensis, Crucirhabdus primulus, Diazmatolithus lehmanii, Discorhabdus criotus, Discorhabdus striatus, Ethmorhabdus gallicus, Lotharingius contractus, Micula staurophora, Mitrolithus elegans, Octopodorhabdus sp., Parhabdolithus liassicus constrictus, Schizosphaerella sp., Triscutum sullivanii., Tubirhabdus patulus, Watznaueria barnesae, and W. fossacincta. 20 Presence of Biscutum finchii (FAD NJ5-LAD NJ6), Bussonius prinsii (NJ5B), Crucirhabdus primulus (NJ5B) and Discorhabdus criotus (FAD NJ7) suggests the placement of this assemblage between NJ5 to NJ7 nannofossil Zones straddling the Pleinsbachian- Torcian boundary. NJ5 represents upper Pleinsbachian whereas NJ6-7 indicates Early Toarcian. This find has wide palaeogeographical implications as it indicates that after faulting, the first transgressive event for the Kutch basin might have occurred during the Pleinsbachian- Toarcian boundary interval, some 15 million years earlier than the much accepted Late Bajocian (ammonite: Leptosphinctes sp. and coral: Isastrea bernardiana records). Record of Late Pleinsbachian age nannofossils from Masirah Island from the Sultanat of Oman, Arabia also strengthens this Kachchh finding. Additionally, and interestingly, Pleinsbachian- Aalenian interval age reworked nannofossils have previously been recorded from the Callovian sediments of the Jara Dome, the easternmost extremity of the Kachchh Mainland (Rai, 2003). It is proposed that a global eustatic rise during the Pleinsbachian-Aalenian boundary interval coupled with local tectonics could be twin reasons for this early record. Jyotsana Rai www.bsip.res.in
EBIRBAL SAHNI INSTITUT Annual Report 2011-2012 1946 OF PALAEOBOTANY Jaisalmer Basin— Thesis entitled ‘Subsurface Cretaceous age calcareous nannofossils biostratigraphy from Tanot well-1, Jaisalmer basin, Rajasthan’ has been documented. The highlights of Thesis are– i) in all 222 nannofossils species were recovered from 114 bore-well samples which are delineated in 17 alpha-neumeric biozones and 5 subzones on the basis of first occurrences of marker taxa, ii) 23 light-microscopic plates containing family-wise nannotaxa and 4 scanning electron microscopic plates are prepared, iii) the recovered nannotaxa have been plotted with respect to stratigraphic distribution, preservation factor, and r & k-strategist mode related frequency distribution, iv) on the basis of recorded global marker taxa A. octoradiata, R. levis, T. orionatus, E. eximius S. primitivum Z. biperforatus Z. noeliae Z. kerguelenesis R. planus E. rarus H. chiastia A. albianus C. kennedyi S. gausorhethium Z. xenotus B. africana E. turriseiffelii and B. stenorhetha the age assigned for these sediments is from Albian to Lower Maastrichtian, v) record of high latitude cold water forms, viz. S. primitivum, R. parvidentatum, B. dissimilis, A. octoradiata, Z. kergulensis and Nephrolithus spp. with warm water taxa W. barnesae and nannoliths, viz. Braarudospherids and nannoconids indicate mixing of warm and cold water currents in Jaisalmer Basin during Late Cretaceous time situated at mid latitudes (ca. 30° S), and vi) the recorded nannofossil assemblage broadly indicates shelf deposit. Pariwar Formation in the Jaisalmer Basin, Rajasthan has been precisely dated for the first time as early Middle Albian on the basis of presence of a well diversified, moderately preserved calcareous nannofossil assemblage of the upper part of Chiastozygus litterarius Zone CC7b/ Prediscosphaera columnata Zone CC8 of Sissingh 1978 corresponding with NC8/9 zones of Bown et al. (1998). Jyotsana Rai & Abha Project 6.3: Integrated diatom stratigraphy and palynofacies analysis of Tertiary sediments of Andaman-Nicobar Group of Islands: Implication to palaeoclimate and basin evolution Over 100 rock samples from 4 measured stratigraphic sections, viz. Arong road, Mus Jetty, Kakana beach Cliff, and Sawai Bay sections of Car Nicobar Island have been chemically processed for palynofossils. The productive samples of all the sections yielded rich palynofloral assemblages mostly dominated by diatoms. Important diatom taxa recorded are: Actinocyclus ellipticus, Coscinodiscus excentricus, C. marginatus, C. rothii, Gramatophora undulata and Arachnoidiscus ehrenbergii; sillicoflagellates viz., Dictyocha brevispina, D. fibula and Mesocene circulus are occasionally found in some of the samples from Mus Jetty and Kakana beach Cliff sections. Morphotaxonomic study of the recorded palynofossils is completed. SEM photography of selected stratigraphically important taxa is done for better understanding of taxa. A Late Pliocene age is assigned to the palynofloral assemblages recorded from Car Nicobar Island. Two papers on palynoflora of Car Nicobar Island are finalized. Rich palynofloral assemblages have been recovered from 3 measured stratigraphic sections of Hut Bay Island, Little Andaman. Morphotaxonomic study and photo-documentation of palynofossils recovered from www.bsip.res.in 21
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