Stratigraphic Evolution and Geochemistry of the Neogene Surma Group, Surma Basin, Sylhet, Bangladesh

STRATIGRAPHIC EVOLUTION ABDUL AND GEOCHEMISTRY OF THE MANNAN NEOGENE SURMA GROUP, Department of Geology, SURMA BASIN, SYLHET, University of Oulu BANGLADESH

OULU 2002 ABDUL MANNAN

STRATIGRAPHIC EVOLUTION AND GEOCHEMISTRY OF THE NEOGENE SURMA GROUP, SURMA BASIN, SYLHET, BANGLADESH

Academic Dissertation to be presented with the assent of the Faculty of Science, University of Oulu, for public discussion in Auditorium GO 101, Linnanmaa, on June 15th, 2002, at 12 noon.

Reviewed by Docent Kari Strand Doctor Kalle Kirsimäe

ISBN 951-42-6711-7 (URL: http://herkules.oulu.fi/isbn9514267117/)

ALSO AVAILABLE IN PRINTED FORMAT Acta Univ. Oul. A 383, 2002 ISBN 951-42-6710-9

ISSN 0355-3191 (URL: http://herkules.oulu.fi/issn03553191/)

OULU UNIVERSITY PRESS OULU 2002 Mannan, Abdul, Stratigraphic evolution and geochemistry of the Neogene Surma Group, Surma Basin, Sylhet, Bangladesh Department of Geology, University of Oulu, P.O.Box 3000, FIN-90014 University of Oulu, Finland Oulu, Finland 2002

Abstract The Surma basin is a part of the Bengal Basin situated in northeastern Bangladesh. The presence of eight gas fields and one oil field makes this an area that is interesting both economically and geologically. In spite of detailed geological and geophysical investigations, information available on palynostratigraphy and geochemistry for the area is scanty. The aim of the present work was to investigate the palynological assemblages, mineralogy and geochemistry of the Surma Group (SG) sequences in Surma Basin, Bangladesh. Core samples (n = 188) were gathered from the wells following: Patharia well-5, Rashidpur well-1, Atgram well- IX, Habiganj well-1, Kailastila well-1 and Fenchuganj well-2. They were provided by BAPEX (Bangladesh Petroleum Exploration Company). X-ray Fluorescence (XRF), Atomic Absorption Spectrometry (AAS), Loss of Ignition (LOI), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) were used for geochemical and mineralogical study of shale samples. In the palynological study, the distributions of pollens and spores were determined. For data analysis, SPSS computer programme was used. Palynological assemblages of the Surma Group of sedimentary sequence of Bangladesh include taxa range in age from the lower Miocene to the Upper Miocene which can be potentially used in dating and correlation. The Lower Miocene interval is correlated with the Simsang Palynological Zone IV of Meghalaya, India and the Bengal Palynological Zone (BPZ) V. The Upper Miocene is correlated with the Simsang Palynological Zone IV of Meghalaya, India and the BPZ Zone V of Bengal. They were deposited in two types of paleoenvironments ranging from the brackish type to shallow marine to brackish. The sequence contains reworked palynomorphs of BPZ IV and III namely Meyeripollies naharkotensis, Polypodiesporites Oligocenecus, Palmepollenities Eocencus and ornamented Tricolpate pollen of the Eocene-Oligocene age which are mainly encountered in the lower Miocene sediments indicative of increased tectonic activity in the area. Geochemical ratios (SiO2/Al2O3, Cu/Zn, Maturity = K2O+ Al2O3/Na2O+MgO, Rb/K2O, K2O/Na2O, Cr/Rb, Zr/Rb, V/ Rb, Th/U etc.) were useful for determining grain size, maturity, tectonics and environment of deposition. High Ba enrichment was detected in the Patharia well-5 and showed high surface water productivity and diagenetic mobilisation. Tectonic descrimination was achieved using SiO2 and K2O/ Na2O ratio. XRD analysis revealed the minerals kaolinite, illite, chlorite, illite/smectite (I/S) and kaolinite/smectite (K/S) mixed layers. Kaolinite/Smectite here reported for the first time in Bangladesh. Clay mineral analyses provided evidence for diagenesis. Smectite diagenesis and dehydration have contributed to the generation of overpressure in the Bhuban Formation in the Patharia well-5. Geochemical ratios of the present study from the Surma Basin is undoubtedly a powerful technique and can be applied to any sedimentary basin analysis to infer the palaeoenvironment, palaeoclimate and palaeotectonics.

Keywords: palynostratigraphy, geochemistry, Surma Basin, Kaolinite-Smectite, Illite- Smectite, diagenesis, geochemical ratios, Bangladesh In the name of Allah, the Most Beneficent, the Most Merciful

Dedicated to: my parents and my wife

“And it is He Who spread out the earth, and set thereon mountains standing firm, and flowing rivers: and fruit of every kind He made in pairs, two and two he draweth the night as a veil over the day. Behold, verily in these things there are signs for those who consider” Holy Quran (13:3). “The world is sweet and verdant green, and Allah appoints you to be His regents in it, and will see how you acquit yourselves…” one of the sayings of Prophet Muhammad (peace be upon him).

Acknowledgements

Thanks to God and may His peace and blessings be upon all his prophets for granting me the chance and the ability to successfully complete this study. I wish to express my deepest gratitude to my supervisors Prof. Risto Aario and Prof. Vesa Peuraniemi for their valuable advice and guidance of this work. Special thanks and gratitude to Prof. Vesa Peuraniemi for his genuine support, valuable advice and sincere comments which helped me a lot to finish this study. The Institute of Geosciences and Astronomy, at the University of Oulu, Finland provided support, including field, laboratory and office work as well as analyses of samples. This study was partially financed by the University of Oulu through a post-graduate grant. I am very grateful to the authority of the University of Oulu. I am also grateful to the authority of the University of Rajshahi, Bangldesh for allowing me to undertake the present work. I also want to express my gratitude to the official referees of my dissertation work Dr. Kari Strand (University of Oulu) and Dr. Kalle Kirsimäe (University of Tartu, Estonia) whose comments and criticism were helpful in refining the draft version of the dissertation into its final form. I also want to express my thanks to Dr. Seppo Gehör (University of Oulu) for checking a major portion of the thesis and for his valuable comments and criticism during the preparation of the manuscript. I wish to thank Prof. Kauko Laajoki, who regularly gave his time to encourage me, especially during late night research sessions when he was the only companion during coffee breaks at the department. I also wish to thank Prof. Tuomo Alapieti, Director of the Institute of Geosciences and Astronomy, University of Oulu for providing scientific and other facilities of the department to my disposal. My sincere thanks are due to the opponent of this thesis, Prof. Raimo Uusinoka (University of Tampere), for giving his valuable time, despite of his tight schedules. I am grateful to the authority of the Bangladesh Petroleum Exploration Corporation (BAPEX) for providing the core samples for this study. I am grateful to Mr. Lutfor Rahman Chowdhury, General Manager, BAPEX for his valuable collaboration and practical assistance in palynological laboratory analysis. My sincere thanks to all my friends and collegues in BAPEX who co-operated nicely during the collection of core- samples and laboratory work. My sincere thanks to my friend Prof. Sifatul Qader Chowdhury, Department of Geology, University of Dhaka, Bangladesh for his nice suggestions towards the planning of this research work. I am also thankful to the numerous individuals who have directly or indirectly contributed to the completion of this work. Out of them at least two names should be mentioned here whom are my beloved students Prof. Badrul Islam, department of geology and mining, University of Rajshahi, Bangladesh and Dr. M. Riajul Islam, University of Idaho, Moscow, Id. USA. I am deeply indebted to my friend Dr. Nuruddin Ashammakhi, Docent, Faculty of Medicine, University of Oulu, whose assistance and encouragement made this work possible towards the end. I cannot forget the sleepless nights he spent before the submission of the thesis and was helping me like a guide. I am also indebted to Prof. Mohammad Hassan, Head of the department of Geochemistry, Al-Azhar University, Egypt for giving suggestions and co-operation in the geochemical and mineralogical part of the thesis. Special thanks are due to the staff of the Institute of Electron Optics at the University of Oulu, and particularly to Mr. Olavi Taikina-Aho for their assistance with the microanalyses. I wish to express my gratitude to Mr. Brayan Dopp, Language Center, University of Oulu, for revising the English language of the manuscript. Also, I give my deep felt regards to Mrs. Kristiina Karjalainen, who has drawn many of the figures and has taken care of the electronic forms of them, Mrs. Ulla Paakkola, for preparing most of the thin sections and Mrs. Riitta Kontio, who co-operated in the geochemical laboratory works for this study. My heartful thanks to all of them. My sincere thanks to all of those who have co-operated both in Finland and in Bangladesh. Finally, I am particularly grateful to my wife, Syeda Wahida Akter, for helping and assisting me in all the stages of this work. Without her help this study would never have been possible. Her constant and continuous co-operation starting from laboratory analyses (cutting, crushing, seiving etc.) to the end of the work – a long way’s journey, proves her love and support during the whole course of this work. Special thanks to my daughter, Noor-E Sadia, for typing the major portion of this thesis and to all my other children for their patience and all kinds of support during all of my studies.

Oulu dated the 29th April, 2002 Abdul Mannan Contents

Abstract Acknowledgements Contents 1 Introduction ...... 11 2 Previous studies ...... 14 2.1 Palynology ...... 14 2.2 Geochemistry ...... 18 3 Aims of this study ...... 19 4 Study Area ...... 20 4.1 Stratigraphy ...... 20 4.2 Structure and tectonics ...... 27 4.3 Palaeogeography and Palaeotectonics ...... 29 4.4 Surma Basin (SB), Sylhet (North East Bangladesh) ...... 30 4.4.1 Wells studied ...... 30 4.4.2 Regional geologic setting ...... 31 5 Analytical methods ...... 35 5.1 Palynological slide preparation ...... 35 5.2 Geochemical analysis ...... 35 5.2.1 X-ray Fluorescence (XRF) ...... 36 5.2.2 Atomic Absorption Spectrometry (AAS) ...... 36 5.2.3 Loss of Ignition (LOI) ...... 36 5.2.4 Accuracy of analyses ...... 36 5.3 Mineralogical analysis ...... 38 5.3.1 X – Ray Diffraction (XRD) ...... 38 5.3.2 Transmission Electron Microscopy (TEM) ...... 38 5.3.3 Scanning Electron Microscopy (SEM) ...... 38 5.3.4 Petrographic microscopy (optical) ...... 38 5.4 Statistical analysis and ratios ...... 39 6 Stratigraphy ...... 40 6.1 Lithofacies ...... 40 6.2 Palynological study ...... 41 6.3 Pollen data and pollen assemblage zone ...... 41 6.4 Palynostratigraphic zonation ...... 41 6.4.1 Palynostratigraphic zonation of Fenchuganj well – 2 ...... 42 6.4.2 Atgram well – IX ...... 44 6.4.3 Habiganj well – 1 ...... 44 6.4.4 KailasTila well – 1 ...... 45 6.4.5 Patharia well – 5 ...... 45 6.4.6 Rashidpur well – 1 ...... 45 6.4.7 Comparison with surrounding areas (from India) ...... 46 6.4.8 Comparison with other Miocene Assemblages ...... 46 6.4.8.1 Assam and Meghalaya sequences ...... 46 6.4.8.2 Bengal Basin ...... 47 6.5 Palaeoenvironment and Palaeoclimate ...... 49 6.6 Age ...... 50 6.7 Maturity ...... 50 6.8 A list of palynomorph recovery from the Fenchuganj well-2 with their possible botanical affinity ...... 52 7 Geochemical results ...... 59 7.1 Major elements (XRF & AAS) ...... 59 7.1.1 Major elements ...... 59 7.2 Trace elements ...... 89 7.2.1 Barium enrichment ...... 100 7.2.2 Total Rare Earth Elements (∑REE) ...... 108 8 Mineralogical Results ...... 115 8.1 XRD ...... 115 8.1.1 Non Clay Minerals ...... 116 8.1.2 Clay minerals ...... 117 8.1.2.1 Diagenetic model of Surma Basin ...... 119 8.1.2.2 Implication of Smectite diagenesis and dehydration...... 120 8.1.2.3 Clay minerals of SG and its implication in petroleum geology. 121 8.1.2.4 Kaolinite - Smectite (K/S) mixed layer clay: a new mineral in Bangladesh ...... 122 8.2 TEM and SEM ...... 129 8.3 Petrography ...... 130 9 Discussion ...... 137 10 Conclusions ...... 146 11 References ...... 149 Appendices 1 Introduction

This thesis is fundamentally concerned with the stratigraphy (based on pollen analysis) and geochemistry of the Neogene Surma Group (SG) sediments from core samples of the Surma Basin (SB), Sylhet, Bangladesh. The SB is a sub-basin of the Bengal Basin situated in the northeastern part of Bangladesh. The basin is bounded on the north by the Shillong plateau, on the east and southeast by the Chittagong-Tripura fold belt of the Indo-Burman ranges and on the west by the Indian Shield platform, to the south and southwest it is open to the main part of the Bengal Basin. The thickness of the late Mesozoic and Cenozoic strata in the SB range is from about 13 to 17 km (Evans 1964, Hiller & Elahi 1984), and much of this strata is Neogene in age. The Bouger anomaly map shows gradually higher values (negative) towards the center of the basin (Alam et al. 1990). An Aeromagnetic interpretation map by Hunting (1980) indicates a gradual deepening of the basement towards the center of the basin and also reveal subsurface synclinal features and faults within the basin. Its topography is predominantly flat with some north-south trending ridges of twenty to several hundred meters elevation present on the north – eastern border. It is actively subsiding (Johnson & Alam 1991). The geology and the hydrocarbon potential of the SB has been investigated by many workers (Holtrop & Keizer 1970, Lietz & Kabir 1982, Hiller & Elahi 1984, Khan et al. 1988, Johnson & Alam 1991), detailed geochemical and palynological studies of SB sediments are lacking. A number of wells have been drilled in the SB with the discovery of eight gas fields and the recent discovery of commercial quantities of oil in the Sylhet – 7 well make this area more interesting economically and geologically. Core samples (n = 188) of the wells of Patharia well – 5, Rashidpur well – 1, Atgram well – IX, Habiganj well – 1, Kailastita well – 1 and Fenchuganj well – 2 were provided by the Bangladesh Petroleum Exploration Company (BAPEX) for the present study. A Palynological study was done from 74 samples. The samples studied are from 959 m down to 4735 m in depth. The study is aimed to determine the environmental conditions prevailing when the sediments were deposited. The study also included details of the geochemistry of the basin for understanding the tectono-environmental condition of the deposition of the sediments, as well as the diagenetic changes of the sediments during burial. 12

The SG is diachronous unit consisting of a succession of alternating shales, sandstones, siltstones and sandy shales with occasional thin conglomerates, indicative of repetitive deposition from pro – delta, delta front and paralic facies with intermittent, wholly marine facies (Holtrop & Keizer 1970). The group is divided into the Bhuban and the Bokabil Formations, based on differences in their gross lithologies (Mathur & Evans 1964). Tertiary palynostratigraphy is a notoriously difficult discipline and is confronted with various problems. For instance, the mean species duration is relatively high in angiosperms which form the dominant taxonomic group in most Tertiary vegetations. In addition, the occurrence of a plant species in a certain area is largely controlled by ecological, climatic and biogeographic factors. Hence, the first and last appearances of Tertiary palynomorphs in a given section are generally not considered to be very reliable stratigraphic indicators. Correspondingly, climate stratigraphy and biostratigraphy are now widely used in the Tertiary although there is no well – defined methodology available and there is always the problem of distinguishing autochthonous or parautochthonous from allochthonous palynomorphs (Ashraf et al. 1997). Palaeoclimate reconstructions in the Neogene are difficult because all methodological approaches used so far, i.e. the geological – palaentological approaches and the climate modeling approaches, suffer from considerable uncertainties (Utsechar et al. 1997). Detailed palynological studies of this area are lacking because of very poor preservation of the palynomorphs. Palynomorph recovery from the cores of the wells drilled in the area was low. Some parts, the wells were almost barren. The Palynomorph content in the prepared palynological slides for palynostratigraphic analysis was extremely poor. No palynomorphs (other than fungal spores) were detected in 38 slides out of 74 slides. Due to this, the idea of doing a quantitative analysis of the palynomorphs was abandoned except for the Fenchuganj well-2, the results of this well allowed for some quantitative considerations. The analysis have furnished definite evidence of palynological assemblages of the SG of the sedimentary sequence of Bangladesh, including taxa range in age from the lower Miocene to Upper Miocene, which can be potentially used in dating and correlation. The Lower Miocene interval is correlated with the Simsang Palynological Zone IV of Meghalaya, India (Baksi 1965) and the Bengal Palynological Zone (BPZ) V (Baksi 1971). The Upper Miocene is correlated with the Simsang palynolgical Zone IV of Meghalaya, India (Baksi 1965) and BPZ zone V of Bengal (Baksi 1971). No palynological research paper has been published from this region yet. Identification and comparison of the palynomorphs were made on the basis of the published literatures on Tertiary Palynology of the Assam and Bengal Basin of India (Neighbouring country). The works include Ghosh (1941), Sahni and coworkers (1947), Sen (1948), Lakhanpal (1955), Meyer (1958), Baksi (1962, 1965, 1971), Biswas (1962, 1965), Banerjee (1964), Sah and Datta (1966, 1968, 1974), Ghosh (1969), Venkatachala and Kar (1969), Datta and Sah (1970, 1974), Deb (1970), Banerjee and Misra (1972), Salujha and coworkers (1969, 1971, 1972, 1973 and 1974), Kar, Sing and Sah (1972), Banerjee and coworkers (1973), Sah and Sing (1974), Singh and Singh (1978), Singh and Tiwary (1979), Jain and Kar (1979), Dutta and Singh (1980), Mehrotra (1981, 1983), Reiman and Thanug (1981), Ramanujan (1982, 1987 & 1988), Handique and Dutta (1981), Varma and Patil (1985), Saxena and coworkers (1986a, 1986b), Singh and coworkers (1986a 1986b), Varma and 13 coworkers (1986), Singh (1977a,b), Singh and Rao (1990) and Kar (1990). Special attention was given to those works from adjacent areas of the present study area – the SB. The geochemistry of the SG sediments was carried out on the basis of 188 core samples of the wells following: Patharia well – 5, Rashipur well – 1, Atgram well – IX, Habigang well – 1, KailasTila well – 1 and Fenchuganj well – 2. Detailed inorganic geochemistry of the study area is presented for the first time. Many works have been done only for organic geochemistry (Shamsuddin 1989, Ahmed et al. 1991, Manzur et al. 1991, Rafiqul et al. 1993). Very few works were under taken for the inorganic geochemistry (Imam & Shaw 1985, Imam 1987, 1989, 1993, 1994 and Islam 1996) of the study area. The inorganic geochemistry of the SB was prepared on the basis of the chemical and minerological analysis. The chemical analysis includes X-Ray Fluorescence (XRF), Loss of Ignition (LOI) and Atomic Absorption Spectrometry (AAS). The mineralogical analyses were done by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and petrographic microscopy. The major elements like SiO2, Al2O3, MgO, CaO, Na2O, K2O, Fe2O3, TiO2, MnO and P2O5 were analyzed. Most of the important trace elements were also measured. Some statistical analyses were carried out such as Cu/Zn ratio in order to ascertain redox parameter, SiO2/Al2O3 for grain size, M = K2O + Al2O3/K2O + Na2O, Rb/K2O for maturity. Other ratios of Cr/Rb, Zr/Rb, V/Rb, Th/U and Cr/Th are sensitive indexes for provenence (Taylor & McLennan 1985, McLennan 1989, Condie and Wronkiewiez 1990, Götze 1998, Hassan et al. 1999) and were performed. Correlations between Cr, Rb, Zr and K2O%, TiO2% and Al2O3 (Grömet et al. 1984, Bellanca et al. 1999) were performed. A Tectonic classification graph, by using cross plots of SiO2 vs. K2O/ Na2O (Roser and Krosch 1986) was prepared to discriminate the tectonic setting of the area. A study of the clay mineralogy of shale was undertaken in order to find out the diagenetic changes that have taken place in the subsurface, to find out the environmental conditions and to discuss the implications of clay mineral diagenesis on major geologic processes such as overpressure generation and structural developments. The study also focuses on the evolution of climate variabilities during the Neogene by using the clay mineralogy analysis of Neogene shale samples of the Surma Basin. An attempt was made to ascertain the semi-quantitative estimate of the relative abundances of clay minerals of SB and also to ascertain the petroleum generation capability of sediments under study with the knowledge of clay mineral diagenesis. 2 Previous studies

2.1 Palynology

So far, no palynological research paper has been published from this region. Identification and comparison were made for this work on the basis of published literature on the Tertiary Palynology of the Assam and Bengal Basin of the neighbouring country, India. In India, the known records of Tertiary plants go back to the later part of the eighteenth century (Sonnerat 1782) which remained practically uninvestigated until the first quarter of the present century (Sahni 1921). Later, the first record of a fossil pollen from the Tertiary strata in India by palynological means was on the oil-bearing strata of Assam (Sahni et al. 1947). Since then a number of palynological studies on the Tertiary of India have been carried out (Sen 1948, Lakhanpal 1955, Meyer 1958, Baksi 1962, 1965, Biswas 1965, Banerjee 1964, 1967, Ghosh 1964, 1969, Sah & Datta 1966, 1968, Sah et al. 1966, 1968 and 1974, Datta & Sah 1967, Venkatachala & Kar 1969, Salujha et al. 1972a,b, Banerjee et al. 1973, Singh et al. 1978, 1986a,b, Singh 1977a,b, 1981, Singh & Tiwary 1979, Mehrotra 1981, 1983, Ramanujan 1982, 1987, 1988 and the references therein). In the Tertiary succession of India, the Jaintia Group (Paleocene-Eocene), Barail Group (Oligocene) and the Surma Group (Miocene-Pliocene) has been studied well. The Tertiary Palynology of Punjab Basin, Cambay Basin, Cauvery Basin, Bengal Basin and Assam Basin has given a good account of the palynology of the area and specially the Surma Group of Bengal Basin and Assam Basin were the guideline for the present study as a neighbouring area since no published paper was available till date. The main features (common assemblages and restricted occurrences) of the Tertiary palynology of India are presented in Table 1 and in Table 2. 15

Table 1. Tertiary Palynostratigraphy of India based on published work.

Age Name of the Common assemblages Restricted forms Reference basin Jaintia Punjab Todisporites, Dandotiaspora, Lycopodiacidites, Mathur 1964, Group Basin Osmundacidites, Couperipollis, Palypodiaceaesporites, Salujha et al. (Pal – Palmepollenites, Granodiporites, Triorites, 1969, Singh & Eocene) Cordosphaeridium, Nyssapollenites, Polycolpites, Khanna 1980 and Homotryblium, Gonyaulacysta, Microhystridium, Khanna & Sing Lygodiumsporites, Todisporites, Cannospharopsis, 1981, Sing et al. Cyathidites, Proxapertites, Hystrichospheridium, 1978. Couperipollis, Tricolpites and Oligosphaeridium, Palmidites Cleitosphaeridium, Cyclonephelium, Thalassiphora, Subathua, Araneosphara, Achilidinium, Verrutrucolpites and Podocarpidites Cambay Lygodiumsporites, Biretisporites, Polypodiaceaesporites, Varma & Basin Verrucatosporites, Arecipites, Maurisites, Dangwal 1964, Scizoaeoisporites, Spinizonocolpites, Venkatachala & Palmaepollenites, Proxaperites, Psilodiporites, Retitricolpits Choudhury 1977, Tricolpites, Nympheaceaepites, Margocolporites, Rhoipites, Rawat & Proteacidites, Cupuliferoipolenites, Venkatachala Palaeocaesalpiniaceaepites, Marginipollis, Cyathidites, 1977 and Mathur Cicatricosisporites etc. Intrapunctiporis, Couperipollis, et al. 1977. Assamialetes, Tricolporopollis, Todisporites, Palmidites ans Lakiapollis. Couvery Lygodiumsporites, Laevigatosporites, Venkatachala & Basin Scizaeosporites, Proxapertites, Spinainaperturites, Rawat 1973 and Couperipollis, Palmaepollenites, Psilodiporites, Marginipollis, Sastri et al. 1977. Lilicidites, Tricolpites and Margocolporites, Rhoipites, Myricipites. Caprifoliipites etc. Bengal Assamialetes, emendatus, Lycopodiales, Granutusporites, Baksi 1971. Basin (Zone Palmeopollenites and Ornamented tricolpate, – II and Couperpollis. tricolporate, Caesalpiniaceae, Zone – III) polycolpate, polycolporate (Hexacolpites) Couvery Lygodiumsporites, Laevigatosporites, Venkatachala & Basin Scizaeosporites, Proxapertites, Spinainaperturites, Rawat 1973 and Couperipollis, Palmaepollenites, Psilodiporites, Marginipollis, Sastri et al. 1977. Lilicidites, Tricolpites and Margocolporites, Rhoipites, Myricipites. Caprifoliipites etc. Couvery Lygodiumsporites, Laevigatosporites, Venkatachala & Basin Scizaeosporites, Proxapertites, Spinainaperturites, Rawat 1973 and Couperipollis, Palmaepollenites, Psilodiporites, Marginipollis, Sastri et al. 1977. Lilicidites, Tricolpites and Margocolporites, Rhoipites, Myricipites. Caprifoliipites etc. Couvery Lygodiumsporites, Laevigatosporites, Venkatachala & Basin Scizaeosporites, Proxapertites, Spinainaperturites, Rawat 1973 and Couperipollis, Palmaepollenites, Psilodiporites, Marginipollis, Sastri et al. 1977. Lilicidites, Tricolpites and Margocolporites, Rhoipites, Myricipites. Caprifoliipites etc. 16

Table 1 continued Age Name of the Common assemblages Restricted forms Reference basin Bengal Assamialetes, emendatus, Lycopodiales, Baksi 1971. Basin (Zone Palmeopollenites and Granutusporites, – II and Couperpollis. Ornamented tricolpate, Zone – III) tricolporate, Caesalpiniaceae, polycolpate, polycolporate (Hexacolpites) Jaintia Assam Basin Cyathidites, Lygodiumsporites eocenius, Verrutricolporites, Biswas 1962, Group Dandotiaspora Dilata, Polypodiisporites, Bacutricolporites sp. Sah & Datta Monolitesmawkmaensis, A, Onagraceae and 1966, 1968, Assamialetesemendatus, Couperipollis Polygalaceae 1974, Dutta & brevipinosus, Liliacisites Sah 1970, microreticulatus, Palmepollenites Salujha et al. sommunis, Tricolpites, Tricolporopollis 1971 Sein & Sah Schizoporis, Retialetes, Triorites, 1974, Sing Schizaeosporites, Palaeocaesaceaepites, 1977a,b, Sing & Nymphea, Caesalpinia etc Tewari 1979, Dutta & Jain 1980 and Mehrotra 1981. Barail Assam Basin Foldexina inaperturata, Meyeripollis, Schizaeoisporites and Biswas 1962, Group Cyathidites Couper, Lygodiumsporites Nympheaceaepites Baksi 1962, (Oligoce Potonie´, Todisporites Couper, Strialetes, 1965, Sah & ne) Magnastriatites Muller, Monolites Dutta 1966, Potonie´, Assamialetes, Jaintiapollenites, 1968, 1974, Caryophyllaceaepites, Triorites Mehrotra 1981, (Erdman) Couper, Pinuspollenites, 1983, Kar 1990 . podocarpidites, Striatites, Todisporites etc. Bengal Lygodium, Polypodium, Coniferae, Graminae Biswas 1963. Basin Cicatricosisporites, Bauhinia, and Cyperacea Barringtonia etc. Couvery Lacrimapollis pilosus, Verrucatosporites Magnastriatites Venkatachala & Basin bullatus, Malavacearumpollis cauveriensis Rawat 1973. paucibaculatus Surma Bengal Cicatricosisporites, Bauhinia, Palmaepollenites, Biswas 1963, Group Basin Rhizophoraceae, Hystrichospheridium, Nympheaeceapites and Deb 1970 and (Mio – Coniferaepites, , Graminae Barringtonia, histrichosphaerids Baksi 1962, Pliocene) Polygonaceapites zonoides Baksi, 1971. Tricolpate – tricolporate, triporate, monocolpate, dinoflagellate (BPZ – V, Baksi) Bisaccate conifer belonging to Cedrus, Baksi 1971. Pinus, Abies, Picea, Tsuga, Schizaeceae and Parkeriacea (BPZ – VI, Baksi) Couvery Verrucatosporites bullatus, Talisiipites retipilatus, Venkatachala & Basin Polypodiisporites ornatus, Costatipollenites pau- Rawat 1973. Lygodiumsporites sp., striatopollis and ciornatus, Tricolpites hystrichospherids. longicolpatus, Striatri- porites sp., Tiliaepolen- ites sp. etc. 17

Table 2. Published stratigraphic ranges of selected Palynomorphs in the Tertiary sequences in India (Neighbour country of Bangladesh).

No. Selected Palynomorphs Eocene Oligocene Miocene Pliocene LMU 1 Echitricolporites sp. 2 Graminidites sp. 3 Polypodiaceaesporites sp. 4 Proteacidites symphonemoides 5 Podocarpidites (Coniferipites) chattachari 6 Lirasporis intergranifer 7 Bombacacidites assamicus 8 Spinosopites acolporata 9 Cicatricosisporites (Ceratopteris) sp. and Parkeriaceaesporites sp. 10 Corrugatisporites terminalis 11 Palmaecolpollenites sp. cf. Monocolpopites broadcolpusi 12 Palmaepollenites eocenicus 13 Palmaepollenites subtilis 14 Eximispora tuberculata 15 Monosulcites brevispinosus 16 Monosulcites rarispinosis 17 Meyeripollis naharkotensis

In the Miocene, the climate changed to cool, sub-tropical to temperate and the flora was rich. The sediments were mostly fresh-water continental deposits, brackish lagoonal conditions existing only in parts of the Sub-Himalayan region, Cauvery and Bengal Basins and the southern part of Assam. Drier conditions existed in the Pliocene, as may be reasoned by the common occurence of Graminidites etc. Coastal or Lagoonal conditions existed in the Bengal and Kutch basins in the Pliocene. Most of the angiospermous genera known from the Paleogene and Early Neogene are derived from woody plants. In the Late Neogene, herbaceous angiospermous elements like grasses, chenopods, amaranths etc. are conspicuous. In summary, it can be stated that the Tertiary sediments in India can be distinguished from the Cretaceous by the predominance of angiospermous elements and the absence of such taxa as Trilobosporites, Appendicisporites etc. in the former. Statistical variations in the assemblage along with the disappearence of taxa like Nothofagidites seem to be more convenient for making the boundery between Eocene and Oligocene, at present. The boundary between the Oligocene and the Miocene is marked by the appearence of temperate to cold climate elements in the latter, along with the disappearence of, or the rare occurence of, Paleogene forms like Numphaeceapites, Triorites etc. In the Pliocene, herbaceous elements and grains of such advanced families as Gramiae, Compositae etc. are conspicuous in the assemblage. 18 2.2 Geochemistry

An extensive geological and geophysical (mostly seismic) survey has been carried out in order to describe the stratigraphy, regional setting, structural evolution, condition of deposition of sediments and the paleogeography of the basin. Despite the study of the geology and petroleum prospects (Bakhtine 1966, Holtrop & Keizer 1970, Alam 1972, Brunnschweiler 1966, Lietz & Kabir 1978, Khan et al. 1988, Imam 1987, 1989, 1993, 1994, Johnson & Alam 1991, Raghava & Mustaque 1994), relatively few attempts have been made to study the geochemistry of the area under study. Most of the geochemical works deal with the organic geochemistry of the sediments (Khan et al. 1988, Pairazian et al. 1985, Shamsuddin 1989, Manzur et al. 1991). Only the works of Imam (1987, 1989, 1993, 1994) are dealing with inorganic geochemistry. But his work was mostly with sandstones. Besides this, a few attempts were made on the mineralogy of the sediments from Bengal Basin as well as from the study area (Datta & Subramanian 1997, Datta et al. 1999, Islam & Lotse 1986, Biswas & Chowdhury 1995, Chowdhury et al. 1987). The work from this same Department of Geology, University of Oulu, Finland by Islam (1996) dealing with the weathering crust in Bangladesh, is a good addition to the account of inorganic geochemistry of the sediments of Bangladesh. Despite the works mentioned, no attempt was made on a detailed geochemical investigation of the area under study. 3 Aims of this study

1. To find out the ageframe of the Surma Group (SG). 2. To find out the environmental conditon prevailing when SG sediments were being deposited. 3. To find out paleoecological and paleogeographical conditons of the SG with the help of diagnostic and common palynomorphs. 4. To find out the geochemistry of the SG sediments and their diagenetic changes. 5. To correlate SG with Bengal palynological zonations, Assam palynological zonations, and the Garo hills of India. 6. To ascertain the botanical affinities of the most important spore – pollen encountered. 7. To provide some guidelines for the geochemical exploration in the area. 4 Study Area

Bangladesh extends from Latitude 20°43´ to 26°36´N and Longitude 88°3´ to 92°40´E. Bangladesh occupies the greater part of the Bengal Basin, covers part of the Himalayan piedmont plain and covers the eastern and southeastern hill ranges of the Sylhet, Chittagong and Chittagong Hill tracts (Paul & Lian 1975). The northeastern part of Sylhet (Jaintapur) is characterized by low rounded hillocks with cliffs and scarps. Fig. 1 shows the location and geology of the study area.

4.1 Stratigraphy

The stratigraphy of Bangladesh is somewhat problematic because the greater part of the country is covered by thick alluvium and almost all the strata are devoid of faunal fossils (Khan & Mominullah 1980). The works leading to the establishment of stratigraphy in Bangladesh are mainly based on lithologic interpretation. Figs 2 and 3 shows the details of the physiography and geology of Bangladesh. The lithostratigraphic units are defined and described in terms of their lithological composition and geographical location only (Fig. 4). 21

Fig. 1. Location map of the study area (Surma Basin, Sylhet, Bangladesh). (Map from Alam et al. 1990, permitted from Geol Surv Bangladesh.) 22

Fig. 2. Physiographic map of Bangladesh. (Map from Alam et al. 1990, permitted from Geol Surv Bangladesh.) 23

Fig. 3. Geologic map of Bangladesh. (Map from Alam et al. 1990, permitted from Geol Surv Bangladesh.)

Bangladesh occupies most of the Bengal Basin - a major geotectonic element of the Assam -Himalayan region and is considered apparently the largest depositional feature in the world today (Graham et al. 1975, Salt et al. 1986, Kuchl et al. 1989). The Bengal Basin is the site of the worlds largest delta (about 60,000 km2) formed by rivers (Ganges, Brahmaputra/Jamuna, Padma, Meghna) that drain a large proportion of the Himalayas (Johnson & Alam 1991). Bangladesh has a thick stratigraphic succession of mostly Tertiary sediments (Table 3). The thickness of practically all units increase in a southerly direction and the strata that are deltaic or shallow marine in the north become progressively more marine to the south (Alam 1989). 24

JAINTIA GROUP

Fig. 4. Cross-section of Bengal Basin from west to east ( Modified from Paul & Lian 1975). 25

Table 3. Stratigraphic Sequence in Bangladesh.

Geologic age Stable shelf Bengal foredeep Lithology Group Formation Group Formation Holocene Alluvium Alluvium Silt, sand, gravel and clay Pleistocene late Pliocene Dihing Modhupur Clay Pebbly sandstone, sticky clay Madhupur Maduhpur Mid-Pliocene E. Sandstone, coarse quartz pebbles, Pliocene Dupi Tila Dupi Tila petrified wood

Girujan clay Claystone with siltstone and Tipam sandstone Sandstone, coarse-grained cross- bedded pebbles of granite, Tipum quartzite, shale and lignite. Clay mostly at base. Miocene Surma Jamalgong Boka Bill Marine shale, pyritic gray marine fossils Surma Bhuban Sandy shale, sandstone, breccia interbeds Oligocene Barail Bogra Barail Jenam Siltstone, fine-grained sandstone, carbonaceous shale Late Eocene Kopili Sandstone, locally glauconitic: shale, highly fossiliferous: thin Middle Eocene Sylhet calcareous beds limestone, num- Limestone ? ? mulitic, sandstone interbeds Early Eocene Jaintia Tura sandstone, coal and shale Sandstone ? ? Paleo-cene Late-Middle Upper Sibganj Sandstone, coarse yellow-brown, Cretaceous Gondo- Trapwash ? ? clay, white, volcanic ash wana Early Cretaceous Rajmahal Basalt, amygdaloidal, andesite, Jurassic Traps ? ? surpentinized, shale, agglomerate Late Permian Lower Sandstone, felspathic grey- Gondo-wana Paharpur ??wacke, coal, shale, sandstone, Early Permian coarse grained, shale, coal, thick Kuchma seams Pre-Cambrian Basement Gneiss and schist complex Sources: Based on Khan (1980) and Zaher & Rahman (1980). ? = not classified

Calcutta - Mymensing hinge zone of Eocene subdivides the Bengal Basin tectonically into two major subdivisions - shelf and geosynclinal area having different stratigraphic history. After the Pre-Cambrian era, the history of the basement complex was one of the peneplanation until Permo-Carboniferous time when Permian, Mesozoic and Gondowana sediments with coal accumulated in the western side of the basin. The break up of 26

Gondowanaland led to the eventual separation of peninsular India from the southern continents, permitting a Cretaceous marine transgression (Alam 1989). The Bengal Basin has been infilled with sediments from the north, east and west. During this process, the basin has generally deepened and the sea level has varied considerably from its present position. During the Cretaceous Period, the sea transgressed northwards towards the southern edge of the Shillong plateau and subsequently regressed far south into the Bengal Basin, causing at least four major transgressions and regressions. Argillaceous and arenaceous deposits accumulated on the stable shelf zone in freshwater to littoral facies. The sedimentation at the same time in the fore deep and mobile belt was marine, at least during the late Cretaceous (Alam 1989). From the Paleocene to the early Eocene, the shelf was subjected to repeated submergence and emergence marked by the Tura Sandstone (240 m). Extensive marine transgression took place in the Middle Eocene and the hinge-line was initiated due to a deeply seated basement fault between the stable shelf to the north – west and a geosynclinal trough to the south – east (Raju 1968). The Nummulitic Sylhet Limestone was deposited over most of the shelf area (about 245 m) in a shallow clear water and open marine shelf environment in a warm climate. During the Late Eocene Period, the Kopili Formation (238 m) consisting of carbonaceous pyritic shale and glauconitic sandstone (Ahmed & Zaher 1965) was deposited in a brackish to marine environment. The formation contains microforaminiferal assemblages of Globorotalia cocoensis biozone (Khan & Mominullah 1980). During the Paleocene to Eocene period, the Jaintia Group consists of three formations: Tura Sandstone, Sylhet Limestone and the Kopili Formation, which were deposited on the shelf (Total thickness is 725 m) in a shallow marine and marine environment. The upliftment of the Arakan – Yoma – Chin geanticline and basin – wide movement took place in Early Oligocene. The sea regressed from the Shillong Plateau area and fluviomarine Barail sediments were deposited along the southern rim of the Shillong Plateau; at the same time the area extending from the SB to the Chittagong Hill Tracts subsided and was filled with fine grained marine Barail shales and siltstones (Holtrop & Keizer 1969). The thickness of the Barail Group generally decreases towards the shelf. The deposition of the Barail Group in the fore deep basin and the mobile belt varies from 800 – 1000 m whereas on the shelf it is only 163 m and is represented by the Bogra Formation (Ahmed & Zaher 1965). The thickness of formations here is approximate and varies considerably from well to well. During the Miocene, a major uplift began in the Himalayas subjecting the Bengal Basin to related tectonic movements (Fairbridge 1983). The deep basin featured conspicuous subsidence and marine transgressions through much of the Miocene. The SG (5000 m) and the Tipam Group (2270 m) were then deposited in deltaic to shallow marine and continental environments, prograding to the southeast with depositional conditions changing to marine (Alam 1989). The SG has been divided into Bhubon and Boka Bil Formation in the geosynclinal facies of Bangladesh. The Bhubon Formation was deposited in an environment ranging from a shallow inner neritic to a lower deltaic plain, and that for the BokaBil Formation was in a range from marine at the bottom to transitional marine at the top. The top of the BokaBil Formation is also known as Upper marine shale (UMS) representing the last 27 widespread marine transgression over the Bengal Fore deep. During this period, a large delta complex started to build on the northeast side of the Bengal Basin. Smaller deltas were possibly building on the eastern side of the basin, presently occupied by Tertiary hills of Chittagong and Chittagong Hill Tracts (Alam 1989). During the Late Miocene Period, the Himalayan movements continued. The global eustatic regression in this period produced an important unconformity that is also seen in seismic reflections recorded for offshore in the Bay of Bengal (Curray & Moore 1979). The DupiTila and Dihing Formations were deposited by Pliocene marine transgressions and are represented by a thick sequence of about 2500m of fluvial and deltaic facies.The Quaternary was marked by the usual glacio-eustatic oscillations, superimposed on a general regression that has left widespread traces in the Bengal Basin (Morgan & McIntire 1959, Mallick 1971, and Bakr 1977). The Quaternary is represented in Bangladesh by the Tippera surface (Lalmai terrace and Chandina deltaic plain), Modhupur tract, Barind tract and Meghna flood plain (Bakr 1977), St. Martins limestone (the former is deltaic and the later represent a littoral facies) and Matamuhuri flood -plain of Maiskhal Island (Mannan 1993). Formation Modhupur clay is mottled and red, which is considered to be Pleistocene. It is followed by the Recent Alluvium consisting of loose gravel, sand, silt and clay with occasional pebbles and boulders. Peat deposits also occur locally (Khan & Mominullah 1980). The thickness of Alluvium ranges from zero to some tens of meter and increases toward the south.

4.2 Structure and tectonics

The structure and tectonics of Bangladesh and adjoining areas have been studied by a number of investigators including Bakhtine (1966), Sengupta (1966), Raju (1968), Holtrop & Keizer (1970), Alam (1972), Desikachar (1974), Graham et al. (1975), Guha (1978), Khan (1980), Matin et al. (1983), Banerjee (1984), Le Dain et al. (1984), Salt et al. (1986), Alam (1989), Rahman et al. (1990). The overall structure and tectonics of the Bengal Basin are briefly discussed below on the basis of the results of these investigations. Fig. 3 shows the generalized tectonic map of Bangladesh and adjoining areas. The Bengal Basin is bordered on the north by the Pre-Cambrian Shillong Plateau and to the west by the Indian Platform. To the east rises the Arakan-Yoma-Naga folded system, and to the south it plunges into the Bay of Bengal. The Bengal Basin is an exogeosyncline – that is, one in which thick detrital sediments within the craton were derived from uplift beyond the margin of the craton. The Bengal foredeep is a part of the exogeosyncline. The Bengal exogeosyncline is one of the worlds largest, and is part of the Bengal Geosyncline. The latter includes the Bengal Basin and the Bay of Bengal (Alam 1989). The major structures described below are: 1) shelf zone, 2) hinge zone, 3) Bengal foredeep, 4) mobile belt, and 5) Sub-Himalayan Fore deep. 1) Shelf zone is a major tectonic element of Bangladesh lying in the western and northwestern portion of it. The margin has a northeast-southwest trend along which the basement complex slopes steeply downward to form a hinge zone. The thickness of the 28 sediments over the shelf is about 3000m and they are marked by several unconformities (Alam 1989). The northern portion is known as the Rangpur platform and the southern is the Bogra shelf. The Indian shield and Shillong massif are connected by the Rangpur platform. The width of the platform is 100 km. Here, the slope is fairly smooth according to the seismic data. The sedimentary deposits of this area form monoclinal beds with dips of 1–2°. Towards the northern portion of the platform the plunge of the basement is about 3–4° and the depth of the basement is over 2000 m. Southern slope of the Rangpur platform is gently plunging towards the southeast and extends to the Calcutta-Mymensing hinge zone. The thickness of sedimentary rocks is increasing towards the southeast. The thickness of the sediments over the shelf is about 8000 m and they are marked by several unconformities. The basement complex near the western margin of the shelf is marked by a series of buried ridges and normal gravity faults. The east-west trending Dauki fault separates the stable shelf and the Shillong massif (Fig. 3). The shelf experienced the first marine transgression during the Late Cretaceous. The second major one was in the Miocene generated by the uplift of the Himalayan and Burman ranges. 2) Hinge zone is a narrow zone of 25 km in width. Here, the monoclinal dip is 5–6°. The bed dips over 20° in the hinge - line (Guha 1978). The hinge zone in the northeast seems to be connected with the Dauki fault by a series of east-west trending faults. It is also marked by deep basement faults probably started with the breakup of Gondowanaland. Parallel to the hinge zone is the Bengal foredeep, which consist of several smaller troughs and structural highs. 3) The Bengal foredeep, which is a large elongated trough, occupies the vast area between the hinge-line and Arakan-Yoma-Naga folded system. This is the deeper part of the Bengal Basin where the basement is deeply subsided here and the subsidence is directly related with the uplift of Himalayas-Burmese mountain chain. It is about 450 km wide in the south of Bangladesh and narrowing towards the northeast. The Basement is probably 12–15 km deep. The folded belts of the Indo - Burman ranges mark the eastern boundary of the Bengal foredeep. The total thickness of the sediments here is high which exceeds 12,000 m. According to gravity surveys and drilling data reported by Bakhtine (1966), Guha (1978), Khan (1980), Matin et al. (1983), the Bengal foredeep can be further subdivided into five sub -zones: 1) Faridpur trough, 2) Barisal gravity high, 3) Hatia trough, 4) Sylhet trough, and 5) South Shillong shelf zone. 4) Mobile belt: The eastern side of the Bengal Basin is bordered by a mobile belt known as Tripura - Chittagong fold belt, which extends north south as part of the Indo - Burmese mobile belt. In Bangladesh, this belt is represented mainly by the hills of the Chittagong Hill tracts, Chittagong and Sylhet, which appear to be analogous to the Sub - Himalayan or Siwalik ranges. They are characterized by the presence of long narrow folds composed of thick sandy shales of the Neogene age, which are 4000–8000 m thick (Alam 1989). The structure of this belt is of three categories: 1) On the west, they show box like forms, 2) the hills of the middle portion are of disturbed asymmetric structures, and 3) those on the eastside have more highly disturbed and complicated structures. 5) The Sub Himalayan fore deep is a continuous east - west Indo - Gangetic geosynclinal belt extending along the south foot of the Himalayas. Part of it cuts into 29

Bangladesh in the northwest corner (Fig. 3). The sediments of this unit include coarse to fine clastics that are derived directly from the Himalayan uplift and are essentially of fluvial mollasse in character. The north margin of this fore deep is strongly folded and faulted (Alam 1989).

4.3 Palaeogeography and Palaeotectonics

The paleogeography and paleotectonics of the Bengal Basin and surrounding areas have been studied by Dietz and Holden (1970), Curray and Moore (1971) Desikachar (1974), Graham et al. (1975), Banerjee (1981,1984), Curray et al. (1983), Gansser (1983), Le Dain et al. (1984), Salt et al. (1986), Alam (1989), Molnar (1984) and Klootwijk et al. (1992) gave a good account of the matter. Some of the important implications of these reconstructions are: 1. Basin development began in the Early Cretaceous epoch (ca. 127 Ma) when the Indian plate rifted away from Antartica. After a plate reorganization ca.90 Ma, the Indian plate migrated rapidly northward and collided with Asia between ca. 55 and 40 Ma (Curray et al. 1983, Molnar 1984). 2. Sedimentation in the Bengal Basin has been controlled by the movement of the Indian plate, by the collision pattern of the Indian plate with the Burmese and Tibetan plates and by the uplift and erosion of the Himalayas and Indo-Burmese mountain ranges (Alam 1989). The sedimentation in the Basin has been almost continuous since the Cretaceous except for some localised discontinuities. The sedimentary sequences overlying the crystalline basement are invariably concealed under a thick cover of aluminium in Assam, West Bengal and Bangladesh (Banerjee 1984). 3. Folding in response to eastwards directed subduction beneath the western Indo- Burman Ranges has been responsible for the growth of the series of elongate N-S asymmetrical anticlinal structures of eastern Bangladesh. These structures form the attenuated frontal fold belt of the Chittagong Hill tracts and eastern margin of the SB (Salt et al. 1986). 4. The basin was formed as a result of unequal subsidence along certain specific trends in the northeastern portions of the Indian shield since the Cretaceous. The basin thus differentiated into several depressions and ridges, of which prominent were the Jessore depression and the East Bengal Ridge. During the first phase of evolution the basin remained under the influence of the marine transgression till the end of Eocene when a major regressive phase took over most of its part (Banerjee 1984). The Eocene period was the period of maximum marine transgression. 5. In the Oligocene, large-scale uplift and erosion resulted in the marine retreat and progressive development of prograding deltaic conditions in the greater part of the basin. By the Miocene period, the impact of the collision of the Indian plate with the Tibetan and Burmese plates was severely felt in the basin, resulting in a large influx of clastic sediment both from the west and the east. The effect was a switch from flysch sedimentation to molasse sedimentation in most of the basin (Alam 1989). In the deeper part of the basin, the sedimentation was controlled by turbidities. The proto Bengal fan, similar to the present day Bengal deep sea fan (Curray and Moore 1974, 30

Graham et al. 1975) possibly played an important role in the sedimentation of the Bengal fore deep. Since Miocene times, the proto Bengal fan has been deformed by the compression of the Indian plate with the Burmese plate and was influenced by the tectonic activities of the Himalayas (Alam 1989). 6. During the Late Tertiary, the area experienced tectonic upheavals in the northeastern portion and the continuation of the fluvial delta complex co-existed in the lower plains since Plio-Pleistocene times. 7. Ninety East Ridge, lying on the Indian plate has been the site of a great "Megashear" or a gigantic "transform fault" along which the Indian plate glided a long distance towards the north without disrupting surrounding crustal plates (Dietz & Holden 1970). 8. Tectonic forces generated by the overridden Burmese plate from the east may have been responsible for east-west horizontal compression and differential vertical movement of the basinal materials to develop the folded belt of Chittagong and Chittagong Hill Tracts (Hossain 1985).

4.4 Surma Basin (SB), Sylhet (North East Bangladesh)

The Surma Basin is a sub-basin of the Bengal Basin situated in the northeastern part of Bangladesh. The basin is bounded on the north by the Shillong plateau, east and southeast by the Chittagong-Tripura fold belt of the Indo-Burman ranges, and west by The Indian Shield platform. To the south and southwest it is open to the main part of the Bengal Basin. The published Bouger anomaly map show gradual higher values (negative) towards the center of the basin. The Aeromagnetic interpretation map by Hunting (1980) indicates a gradual deepening of basement towards the center of the basin and also reveals subsurface synclinal features and faults within the basin. Its topography is predominantly flat with some north-south trending ridges of twenty to several hundred meters elevation present in the north-eastern border. It is actively subsiding (Johnson & Alam, 1991). The thickness of late Mesozoic and Cenozoic strata in the Sylhet Basin ranges from about 13 to 17 km has been estimated by some authors (Evans 1964, Hiller & Elahi 1984). Much of these strata are Neogene in age (Johnson and Alam 1991). The geology and hydrocarbon potential of the SB have been investigated by many workers (Holtrop & Keizer 1970, Lietz & Kabir 1982, Hiller & Elahi 1984, Khan et al. 1988, Chowdhury et al. 1987) but palynological studies are lacking. A number of wells have been drilled in the SB with the discovery of eight gas fields and the recent discovery of commercial quantities of oil in Sylhet–7 make this area more interesting to the geologists.

4.4.1 Wells studied

Wells studied for the present work include Atgram well – IX, Fenchuganj well – 2, Habiganj well – 1, Kailastila well – 1, Patharia well – 5 and Rashidpur well – 1 (Fig. 5). The necessary information of the wells is given in Table 4 as follows. 31

Table 4. Wells studied.

Well Name Location Drilled by Drilling time Total depth Atgram – IX 25°00´ 20´´N Parker drilling Co. 25.06.1981 – 4967,8m 92°24´ 30´´ E 10.06.1982 ( 16,288 ft ) Fenchuganj – 2 24°36´46´´ N PetroBangla* May 1980 3779m 91°57´23´´ E Habiganj – 1 24°13´55´´ N PSOC** 24.03.1963 – 3507,5m 91°22´47´´ E 22.05.1963 ( 11,500 ft ) Kailastila – 1 24°51´13´´ N PSOC** 31.08.1961 – 4140,9m 92°22´47´´ E 22.03.1968 ( 13,577 ft ) Patharia – 5 24°34´ N PetroBangla* 1989– 3436m 92°13´ E Rashidpur – 1 24°18´32´´ N PSOC** 22.02.1960 – 12,663ft 91°36´26´´ E 20.7.1960 * PetroBangla= National Petroleum exploration company of Bangladesh ** PSOC=Pakistan Shell Oil Company

4.4.2 Regional geologic setting

The Surma Basin is believed to come into existence in the Late to Post-geosynclinal phase. Partly a fault bounded trough, subsiding from the Oligocene or earlier Pliocene (Holtrop & Keizer 1970). The basin covers an area of roughly 10 000 km2 and is bounded in the north by the Pre-Cambrian Basement Complex (Wadia 1975) of the Shillong Massif and the Barail Ranges, by the Barail - Imphal Ridges in the east towards Assam and in the south by the Tripura High. In the west, The SB gradually ascends towards the Eocene Hinge Zone, while passing into The Bengal Foredeep (Khan et al. 1988). 32 e wells locations. with the sample Fig. 5. Showing the lithology of th of lithology Fig. 5. Showing the 33

The SB is a sub-basin of the Bengal Basin, the development of which began in the Early Cretaceous epoch (ca. 127 Ma) when the Indian plate rifted away from Antarctica (Johnson & Alam 1991). After a plate reorganization ca. 90 Ma, the Indian plate migrated rapidly northward and collided with Asia between ca. 55 and 40 Ma (Curray et al. 1983, Molnar 1984). The basin has been characterized by deltaic sedimentation since The Oligocene epoch. Today, the onshore part of the Bengal Basin is the site of the world's largest delta (about 60 000 km2) formed by rivers (Ganges, Brahmaputra/Jamuna, Padma, Meghna) that drain a large portion of the Himalayas (Johnson & Alam 1991). This subaerial delta feeds the world's largest submarine fan (Bengal Fan), which extends more than 3 000 km south into the Bay of Bengal (Curray & Moore 1974). The Bengal Basin gradually is being encroached on by the Indo-Burman ranges, an ~ 230-km-wide, active orogeine belt associated with eastward subduction of The Indian plate below Myanmar (Burma) (Brunnschweiler 1966, LeDain et al. 1984, Sengupta et al. 1990). In The Early Miocene, as the collision between the Indian and the Eurasian plates continued, there were further major phases of uplift in the Himalayas. Consequently, a large volume of clastic sediments was supplied to and began progressively to fill the Basin (Imam & Shaw 1985). SB is characterized by a large, closed, negative gravity anomaly (as low as 84 milligals), has minimal topography (elevations of about 5 to 20 m) and numerous lakes and swamps, and is actively subsiding (Johnson & Alam 1991). On the basis of seismic data, The SB cumulatively comprises an approximately 17 km thick (Hiller & Elahi 1984) sedimentary column from Post - Eocene Sylhet Limestone to Recent clastics. SB was structurally evolved by the contemporaneous interference of two major tectonic movements, i.e. the emerging of the Shillong Massif in the north and the west prograding mobile Indo-Burman Fold Belt (Hiller & Elahi 1984). The northern and eastern parts of the basin are far more complicated than the southern and western portions. The relief and complexicity increases towards the east (Haque 1982). The anticlines are commonly faulted and many produce gas (Johnson & Alam 1991). Structural relief between paired anticlinal crests and adjacent synclinal troughs may be as much as 7 000 m (Hiller & Elahi 1984), and the synclines have acted as major late Neogene and Quaternary depocenters. The folds decrease in amplitude westward, and are not present west of about 91° (Lietz & Kabir 1982), where the Sylhet trough merges with the main part of the Bengal Basin. The SG (Early Miocene - Quaternary) is a diachronous unit consisting of a succession of alternating shales, sandstone, siltstones and sandy shales with occasional thin conglomerates, indicative of repetitive deposition from pro-delta, deltafront, and paralic facies with intermittent, wholly marine facies (Holtrop & Keizer 1970). The group is divided into the Bhuban and the Bokabil Formations, based on differences in their gross lithologies (Mathur & Evans 1964) (Table 3). 34

Table 5. A chart showing the stratigraphy of the Surma Basin (SB) and surrounding areas including the Surma Group (SG) (*). Ages of nonmarine units are based on palynology. Data is from Baksi (1965), Chkaraborty (1972), Holtrop & Keizer (1970), Gupta (1976), Murthy et al. (1976), Banerji (1981,1984), and Murty (1983). Note that the time scale is not linear. Wavy lines show unconformities. The Time scale was adopted from Palmer (1983). 5 Analytical methods

5.1 Palynological slide preparation

Standard palynological techniques were utilized to isolate the palynomorphs. Palynomorphs mainly separated from shaly sequences. Samples were analyzed using conventional method for macrelation of spore/pollen from sediments. The subsequent acetolization has been carried out with the methods of Erdtman (1960). The samples were treated with 10 % HCl acid to dissolve carbonates and later with 10 % KOH to separate humas materials from taken sediment of 5–10 grams. After neutralization the residue was treated with concentrated HF acid to remove silicates. Palynomorphs were collected by filtration using 200 mm, 33 mm accordingly and in need, 10 mm polypropylene sieves were also used. After dehydrating, the residue was acetolized again. The residue was mainly preserved in glycerine jelly and glass slides were prepared on glycerine gelatine for microscope study.

5.2 Geochemical analysis

The core samples were a total of 188 in number and from 6 exploratory wells of Sylhet, Bangladesh: Habiganj well – 1, KailasTila well – 1, Rashidpur well – 1, Atgram well – IX well, Fenchuganj well – 2 and Patharia well – 5 (Sample analyzed for the study were 168 in number). The details of the samples are presented in the appendix-1 and 2. The present study includes major, trace and total REE (rare earth element) analysis of SG sediments (mainly shale) from the SB, Sylhet, Bangladesh. Sample preparation. The core samples were cut into pieces by using a rock cutter and then were crushed by a crushing machine to reduce the rock aggregate to monomineralic particles. These samples were divided into seven parts by a mechanical divider. One part was crushed by Vibrating Disk Mills (Herzog, type: Hsm 100 A) and six parts were sieved to get size fraction <0.06 mm and in some cases <0.125 mm. Both crushed and sieved fractions were analyzed. For clay mineralogical studies, clay size fractions were 36 separated by centrifugation. Thin sections were prepared both from original core and mixed powdered samples.

5.2.1 X-ray Fluorescence (XRF)

The crushed samples were used to determine the major element composition by XRF. Analyses were carried out with a Siemens SRS-X-Ray 303 AS XRF spectrometer with standard curves based on International Rock Standards at the Institute of Electron Optics, University of Oulu, Finland. Analysed major elements were SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O and P2O5 and trace elements were As, Ba, Bi, Ce, Cl, Cr, Cs, Er, Gd, La, Nb, Nd, Pb, Pr, Rb, Sc, Sr, Th, U, V, and Zr.

5.2.2 Atomic Absorption Spectrometry (AAS)

Two hundred mg of sample was taken into a Teflon crucible. Then one ml HClO4 and two ml HF were added and placed in the sand bath for heating. Afterwards, two ml conc. HCl was added to dissolve the sample and allowed to evaporate until there was no solution left. Finally, diluted fraction of the deposit was analysed for trace elements using a spectrum of AA300 with an acetylene-air flame. Organic maturity (OM) rich elements Co, Cr, Cu, Ni, Pb and Zn were selected for the analysis.

5.2.3 Loss of Ignition (LOI)

First the samples were dried at 110–120°C. Then dried samples were heated for two hours at 950°C and LOI was determined.

5.2.4 Accuracy of analyses

The accuracy determinations by both AAS and XRF was checked using certified reference materials. A correlation between the data of Cu, Zn, Ni and Pb (Patharia well-5 and Rashidpur well-1) measured by AAS and XRF was done and presented in figure 6. They are all in positive correlation. For the well, Rashidpur-1 three elements (Zn, Cr & Ni) are correlated strongly and Pb shows a positive which indicate that two results (AAS & XRF) are in good agreement. The error was found to be 3–4%. Nesbitt (1992) mentioned that for XRF analyses, the error in major oxides is about 1–2%. Wronkrewicz and Condie (1987) mentioned that the precision and accuracy are within 5% for most major elements and for minor and trace elements are 5–10% for the samples measured by XRF and INA 37

(Instrumental neutron activation) methods. However present study analyses bears 95 % confidence level.

70 140

60 120

50 100

40 80

30 60

20 40

Cu ( XRF ) ( XRF Cu 20 Zn ( XRF ) ( XRF Zn 10 20 30 40 50 60 70 20 40 60 80 100 120 140

Cu ( AAS) Zn ( AAS )

50 120 ppm

100 40

20 40

20 ( XRF ) Pb Ni ( XRF ) 20 40 60 80 100 120 0 10 20 30 40 Ni ( AAS ) A Pb ( AAS )

120 50

110

100 40

80 30

60 20 Zn ( AAS) Cu ( AAS) 60 70 80 90 100 110 0 10 20 30 40 50

Zn ( XRF) Cu ( XRF)

50 100 ppm 90

40 80

60 30

40 ( AAS) Pb Ni ( AAS) Ni ( 30 40 50 60 70 80 90 10 20 30

Ni ( XRF) B Pb ( XRF) ppm ppm

Fig. 6. A. Figure showing the correlation of the data (in ppm) of Cu, Zn, Ni and Pb for Patharia well – 5. All of them are in good positive correlation. B. Figure showing the correlation of the data of Cu, Zn, Ni and Pb for Rashidpur well-1. Three of them (Zn, Cu & Ni) are correlated strongly in positive and Pb shows also positive correlation means two results are in good agreement. 38 5.3 Mineralogical analysis

5.3.1 X – Ray Diffraction (XRD)

Crushed and clay fraction were examined by a SIEMENS D 5000 X-Ray diffractometer with Ni filtered CuKα radiation using 40 kV-40 mA. Samples were X-rayed using Ni- filter Cukα radiation with a scan range from 4 to 40º 2θ, a step size of 0.2º and a dwell time of 1s per step. The clay fraction (<2 µm) was separated out from the shale by disaggregating and despersing the sample in distilled water and immediately washed by centifugation. The fraction of <2 µm was isolated by centrifugation and suspension was dried on glass slides. The clay sample in oriented mounts were run under three separate conditions: i) air dry state. ii) after ethylene glycol treatment and iii) after heating to 550º C for 1 hour.

5.3.2 Transmission Electron Microscopy (TEM)

Clay-size fractions were prepared for a TEM study by dispersing the material in alcohol. The samples were placed on a formvar - coated TEM grid (150 mesh) and examined with JEOL, JEM-100 CX fitted with link AN10-25S and a Jeol JEM 1200 Leo 912 OMEGA equipped with the same microanalysis electron microscope. Various magnifications were used to obtain suitable micrographs of clay minerals. Electron microscopy is the only method capable of measuring the size of individual single particles. The method allows for direct measurement of the several dimensions of the particles and thus also the shape of the particles is to be taken into account (Bates 1971).

5.3.3 Scanning Electron Microscopy (SEM)

A few thin sections of clay-size fractions coated with an Au-Pd conductor (Polaron SEM coating unit E 150) were examined morphologically under JEOL JSM 6400 (with Link EXL, EDS) Scanning Electron Microscopy (SEM). The thin sections of representative samples were analysed for semiquantitative determination of primary and secondary minerals especially for Ba detection.

5.3.4 Petrographic microscopy (optical)

Thin sections were made both from the core samples for mineralogical analyses by petrographic microscope. 39 5.4 Statistical analysis and ratios

Data acquired from XRF and AAS was used for the analyses of major oxides, ΣREE and certain trace elements. Table 6 illustrates statistical analyses and ratios used in this study. Correlation of data between the present study and the published data was done whenever possible.

Table 6. Statistical analysis and ratios.

Subject Ratio / Crossplots Reference Grain size SiO2/ Al2O3 Björlykke 1974 Maturity (M) 1) K2O+Al2O3/ Björlykke 1974 Na2O + MgO 2) K2O/ Na2O; Björlykke 1974 Rb/ K2O Wedephol 1969 4) Cu/ Zn Hallsberg 1976 Major element % Major element Harker type ΣREE diagram ΣREE Bhatia 1985 Provenance 1) Cr/ Rb, Zr/ Rb, V/ Rb and Bellanca et al. 1999, Ba/Rb; 2) Cr/ Ni Garver & Scott 1995 Environment of deposition Cr/ V and Ni Dypvik 1979 Tectonic setting SiO2 vs. K2O/ Na2O Roser & Krosch 1986 Mineralogy 1) Crossplots of Cr, Rb, Zr, K2O% and Grömet et al. 1984 (Weathering & Diagenesis) TiO2 versus Al2O3 2) Crossplots of Cr+Ni, V, MnO% and Fe2O3 McLennan et al. 1983 versus MgO 3) Crossplots of U, Th, Ba, Al2O3 versus McLennan et al. 1983 K2O% 6 Stratigraphy

The stratigraphy of the Surma Basin (SB) and surrounding areas is summarized in Table- 2. The stratigraphy of the Neogene Surma Group sediments of SB is presented on the basis of core sample studies (n=188) and of palynological studies (74) of six exploratory wells: Atgram–IX, Fenchuganj–2, Habiganj–1, Kailastila–1, Patharia–5 and Rashidpur–1. The studied samples range in depth from 959m to 4735m. Detailed description of the various lithologies composing the Surma Group are provided in the measured cores of the wells (appendix-1) and also presented graphically (Fig. 5). The Surma Group (SG) is a thick sequence of clastic sediments consisting of an alternation of sandstone, shale and siltstone that infilled the vast basinal area of the Bengal Basin during Miocene-Pliocene time. In the subsurface, the unit is represented by thick sand-shale sequences in all the wells drilled in the area. The Surma Group unconformably overlies the Barail Group of the Oligocene age and is overlain by the sandstone dominating the Tipam Group of the Pliocene age (Holtrop and Keizer 1970). The SG is divided into a lower Bhuban formation and an upper Bokabil formation based on gross lithology.

6.1 Lithofacies

Two major lithofacies were identified in the SG unit: sandstone lithofacies A and combined facies B consisting of claystone, mudstone and shale. Facies B is the most abundant, whereas facies A is less common. These lithofacies generally are defined on the basis of grain size, clay content and depositional bedding characteristics. Facies A consist of massive, thinly inter-bedded and inter-laminated, fine to medium-grained sandstone. Facies B consists of laminated bluish, bluish gray and gray to black shale from gray to yellowish-gray siltstone to very fine grained sandstone. Lithofacies A may grade vertically into the combined lithofacies and be interbedded with the combined lithofacies. Facies B shows two types of lithofacies in the shaly layers. They are: 41

a) homogeneous shale and b) shale with sand or sand partings. The first type shares the most abundant lithofacies. Lithofacies B is composed of siltstone and sandstone lamine, layers are generally 1- 5mm thick. Thick intervals as much as 30-40cm are common in the lithofacies B.

6.2 Palynological study

Palynological studies based on 74 selected core samples from the wells of Atgram – IX, Fenchuganj – 2, Habiganj – 1, Kailastila – 1, Patharia – 5 and Rashidpur – 1. Details of the samples are given in appendix 1 and 2 (* marked). The samples were selected from all the cores of the wells representing the various lithostratigraphic positions. From individual cores, the samples were selected so that the top, bottom and the middle of the cores are represented. The study was aimed to establish the age of the SG, palynostratigraphy in order to better constrain reconstruction of palaeoenvironmental and palaeoclimatic variations in Bangladesh during the Neogene.

6.3 Pollen data and pollen assemblage zone

Pollen analysis has been performed on selected samples. In order to make sense of the considerable amount of data shown in a typical pollen diagram it is necessary to divide the diagram into pollen-stratigraphic units characterized by distinctive groups of pollen types. In this way, pollen zones were constructed from the samples of Fenchuganj-2 and are presented in Figs 7 and 8. Detailed palynological studies of this area is lacking because of poor preservation of the palynomorphs. Palynomorph recovery from the cores of the wells drilled in the area was few. In some parts the wells were almost void of specimen. Palynomorph content in the prepared palynological slides for palynostratigraphic analysis were extremely poor for the present study. No palynomorphs (other than fungal spores) were detected in 38 out of the 74 slides. Due to this, the idea of a quantitative analysis of palynomorphs was abandoned except for the Fenchuganj well – 2. The results of this well allowed us to make some quantitative analysis and a qualitative analysis was attempted for the remaining wells. No palynological research paper has been published from this region yet. Identification and comparison were made for the present study on the basis of published literature on the Tertiary Palynology of the Assam and the Bengal Basin of India (Table 1 and Table 2).

6.4 Palynostratigraphic zonation

Among the wells studied, palynomorph recovery was predominantly good only for the Fenchuganj well – 2. Both the qualitative and quantitative analysis of the palynoflora was 42 possible only for this well. A total of 35 samples were selected from 12 cores of this well for palynological study. The depth of the samples ranged from 957.6 m to 4095 m. The palynoassemblages from sediments recovered from SG (Miocene) are rich in pteridophytic spores and angiospermous pollen grains whereas the gymnospermous pollen grains and fungal remains are comparatively less represented. The assemblage consists of 63 genera and 95 species of palynomorphs. The SG sediments dominantly consist of shale with some sandstones. On the basis of the qualitative and quantitative analyses of the palynoflora, the SG sequence of this well has been divided into three biostratigraphic zonations. The following parameters have been taken into consideration to establish and recognize these zones: a) Maximum development of various palynotaxa, b) the first and last appearance of them, and c) decline, restricted occurrence and absence of certain palynotaxa. Then comparisons with other similar palynoassemblages of surrounding areas (India) were made along with interpretations regarding paleoclimate, environment of deposition and age.

6.4.1 Palynostratigraphic zonation of Fenchuganj well – 2

The three local palynostratigraphic zones in the SG sediments sequence of Fenchuganj well – 2 are as follows in ascending order of stratigraphy. iii) Disaccate Zone, ii) Tricolpate-trilete zone, and i) Palmepollenite zone. i) Palmepollenite zone. This zone constitutes the lower biostratigraphic unit of the SG. The zone is characterized by Palmepollenites which are abundant. Species restricted to this zone are lygodiumsporites, Trilete Cing 1a, Polypodiisporites Oligocenisus, Monolites mawkmaensis, Texodium, Tsuga, Inaparturate L1a, Inapeturate reticulate, Retipilonapites, Cl-grain 1a, Diporopollenites, Polycolpites sp, Monaporites anulatus, Polyporina, Chamopodiacea, Tricolporates (C3P3)-Pet 2a and Fusiformisporites. The following characteristic palynotaxa have been identified - Cyathidites minor, Lycopodium sporites, Cicatricosisporites macrocostatus, Schizacea, Trilete-L1a, Trilete- ret 1a, Trilete-granu 1a, Trilete-verru 1a, Trilete sig+rug 1a, Iaevigatosporites 1a, Verrucatosporites, Emparitus dissaccate, Inaperturate L1a, Couperipollis sp, Dicolpites Tricolpats C3-4a, C3-L2a, C3-ret 1a, C3-ret 2a, C3-verru 1a, Marginipolis sp, Meyeripollis naharilotensis, Carya, Betula, Triporopollenits sp, P3 L2a, P3 ret 2a, P3 grain 1a, Florschuetzia levipolli, Rhizophora, C3P3- L1a, C3P3 -L2a, C3P3-Pet 1a, Hystriokiospheridum and Fungus remains. Comments. The significant feature of this zone is that the Palmepollenite constitutes 31 % of the sequence. The dominance of this taxon, is, therefore, important and helps us in distinguishing from the overlying Tricolpate-trilete zone. Pteridophytic spores contains 48 % in this zone. Angiouspermous pollen grains represent 50% while gymnospermous pollen grains are insignificantly represented by 3.8 %. Among the pteridophytic spores, triletes are very abundant. Trilete spores are represented by Cicatricosisporites macrocostatus, Trilete levigates, Trilete reticulate, 43

Trilete granulate and Trilete verrulate. Monolete forms are mainly Laevigatosporites Type-1. Pollens of Graminidites have not been observed in this zone. Presence of Rhizophora pollen provides a basis to interpret the paleoenvironment of the drilled sequence. Last appearance: Monolites mawkamaensis, Monoporites anulatus, Tsuga, Texodium and Inaperturates L1a. ii) Tricolpate - trilete zone. Tricolpates (C3) and Triletes (T) are the most common forms of palynoforms. Some of the forms were restricted to this zone only. Species restricted to this zone are Tricopates rug1a type, Carpinus, Triporopollenits Scab 1a, Polygonacidites sp, Syncolporats sp, Trilete-rug 1a, Dandotriasporits sp. and Eximispera. Characteristics palynotaxa are Cyathidites minor, Triletes L1a type, Triletes ret 1a, Triletes grain 1a, Triletes-verru 1a, Disaccate striat, Palmepollenites, Couperipollis sp, Tricolpates (C3)-L1a type, C3-rat 2a, C3-scab 2a, C3-apl 1a, Marginipollis sp, Triporopollenites ret 1a, Rhizophora, Tricolporates (C3P3) Pet 1a type, C3P3 -Fov 1a, C3P3 - Scab 1a, Hystrichospheridum. Comments. This zone is characterized by very high frequency of Tricolpate-Triletes which is about 50% of the palynofossils. For this reason the zone is named after these taxon. Graminitides is conspicuous by its complete absence in this zone. Gymnospermous pollen grains are about 4%, the presence of Rhizopora pollen is important for paleoenvironmental interpretation. iii) Disaccate zone. This zone shows a clear dominance of Disaccate pollen. Ninety eight forms of disaccate were present in the zone. Species restricted to this zone are Sphagnum, Undulatisporites, Polypodiaceosprites, Trilete-fov 1a type, trilete-apl 1a type, Striatopollis bellus, Florschuetzia levipoli, Tricolporates granulates 1a, Tetracolporates L1a, Tetred and Alnipollenites. Characteristic palynotaxa identified in this zone are Cyathidites minor, Lycopodium sporites, Cicatricosisporites macrocostatus, Schizaccae, Trilete L1a, T–ret 1a, T–gran 1a, T -verru 1a, Laevigatosporites, Verrucatosporites, Emparitus, Monolet ret 1a, Palmepollenites, Couperipollis sp., Tricolpate L1, C3 - L2a, C3 - ret 1a, C3 - ret 2a, C3 - verru 1a, C3 - scab 2a, C3 - apl 1a, Marginipollis sp., Meyeripollis naharikotensis, Carya, Betula, Triporopollenites, P3 - L2a, P3 - ret 1a, P3 - ret 2a, P3 - grain 1a, Triolites L1a, Florschuezia meridonalis, Rhizophora, Nyssa pollenites sp., Triporopollenites - L1a, C3P3–L2a, C3P3–scab 1a, Hystrioispheridum. Comments. Disaccate is the dominant pollen of this zone. Pteridophytes are usually represented by the monolete forms such as Levigatosporites, and Verrucatosporites, as well as by a few trilete spores like Cyathidites minor. Monocolpate pollen occur in low frequencies and are represented by the genus palmepollenites. The microfloral association of the Palynological zone I can be compared with the palynological assemblage of the Simsang Palynological Zone IV of Meghalaya, India (Baksi 1965) and the Bengal Palynological Zone (BPZ) V (Baksi 1971) and indicate Lower to Middle Miocene age. The presence of Rhizophora pollen and the presence of dynoflagillates provide a basis to interpret the paleoenvironment of the drilled sequence as brackish to shallow marine deposits. Representative forms of Foraminifera also indicate that sediments may be deposited in shallow marine condition. 44

The microflora of the Palynological zone II can also be compared with those of the Simsang Palynological Zone IV of Meghalaya, India and BPZ V (Baksi 1971). Based on these comparisons the Palynological Zone II is presumed to be of Middle to Upper Miocene. It is interesting that the floral change from monolet pollen Palmepollines to Tricolpate – trilete could be recognized well by an increase in frequency by 50%. The presence of mangrove pollen Rhizophora indicates the brackish environmental deposition of this drilled sequence. The microfloral assemblage of the Palynological Zone III may be compared with Simsang Palynological Zone IV of Meghalaya, India (Baksi 1965) and BPZ Zone V of Bengal (Baksi 1971). According to these comparisons, the age of Palynological Zone-III is presumed to be Upper Miocene. A significant floral change has been observed in this zone by the abundance of dissacate pollen and a decline in the pteridophytic spores, particularly triletes. These pollens might have migrated from the extra-peninsular region in the north, which would have been sufficiently high during Lower Miocene.

6.4.2 Atgram well – IX

In this well, 15 samples were taken from a depth ranging from 3 638 m to 4 735 m, but the recovery of palynomorphs were from only four samples representing three different cores. The qualitative analysis of palynoassemblage reveals the presence of following taxa: Bisaccate, Laevigatosporites, Rhizophora, Disaccate, Gymnosperm, Cyathidites minor, Verrutriletes, Cicatricosisporites, Palmepollenites, Couperipolies, Polypo- diaexoisporites, Verrucatosporites, Simsangia, Meyerripollies Naharkotensis, Polypodeace, and Florschuetzia trilobata. Among Foraminifera, Hystrichosphaeridium and Veryhachium were present in the area. Some dinoflagillete cysts were also identified in the sequence. The age of the sequence has been identified as Miocene Foraminifera with Hystrichospheridium indicates a marine- brackish environment. Hystrichospherida is a widely distributed facies indicator. The presence of reworked microfauna elements and conifer pollens, together with the transgression indicators, indicate an increased tectonic activity in the Surma Basin during this period and they are related to the burial of the basin and the uplift of the Himalayas.

6.4.3 Habiganj well – 1

In this well, only three samples were productive out of 19 samples ranging from 1250 m to 3125 m. The three samples were representing three different cores. The qualitative analysis of palynoassemblage reveals the presence of the following taxa: Disaccate pollen, Pteridophyte Monoletes, Cicatrieosisporites, Laevigatosporites, Tetracolpates, Alnipollenites, Triporopollenites, Simsangia, Monocolpate, Triporate, Tricolpites, 45

Tricolporopollenites, Varrucosisporites sp., and Polypodiisporites. The samples were identified as Pliocene to Miocene in age.

6.4.4 KailasTila well – 1

In this well, ten samples ranging from 1 175 m to 3 969 m were taken for the study but only four samples were productive. The following palynotaxa were identified: Disaccate pollen, Pteridophyte Monoletes, Cicatricosisporites, Laevigatosporites, Tetracolpate, Alnipollenites, Triporopollenites, Simsangia, Monocolpate, Triporates, Bissaccates, Graminidites sp., Cyathidites, Polypodisporites, Trilete, Verrucosisporites, Polypodiacoispoiretes, and Lycodiumsporites. Some dinocysts (Indeterminate) were also observed. The samples were identified as Pliocene to Lower Miocene in age.

6.4.5 Patharia well – 5

The qualitative analysis of palynomorphs of the Patharia well-5 has been done on the basis of 12 core samples ranging from 956,1 m to 2 833 m taken from 5 cores. Summary of Palynostratigraphy of Patharia well-5 is given in Table 6. The samples were identified as Lower Miocene in age (Lower Bhuban). Characteristic palynomorphs are: Disaccate, Verrucatosporites, Simsangia, Cicatricosisporites, Polypodiaceoisporites, Palmaepollenites, Couperipollis, Gymnosperm, Triorite, Verrutriletes, Monocolpate, Meyernipollies Naharkotensis, and Florschuetzia. Forms of Foraminifera that were encountered in the core no. 3 and 4 are: Globogerine bulloids, Globoretalia, Globigerinoides, and Haplophragioides. The environment of deposition was brackish to shallow marine condition for this well. The samples studied were identified as Lower Miocene age (Lower Bhuban).

6.4.6 Rashidpur well – 1

Only few palynomorphs were recovered from the samples in this well. Five samples were productive out of 10 samples ranging from 1 081 m to 2 477 m. The palynomorphs encountered in this well were: Disaccate pollen, Pteridophyte monoletes, Cicatricosisporites, Laevigatosporites, Tetracolpate, Alnipollenites, Triporopollenites, Simsangia and Monocolpate. Some dinocyst (Indeterminate) were observed also. 46 6.4.7 Comparison with surrounding areas (from India)

The microfloral association of the Palynological zone I can be compared with the palynological assemblage of the Simsang Palynological Zone IV of Meghalaya, India (Baksi 1965) and the Bengal Palynological Zone (BPZ) V (Baksi 1971) and indicate a Lower to Middle Miocene age. The presence of Rhizophora pollen and the presence of dynoflagillates provides a basis for interpreting the palaeoenvironment of the drilled sequence as brackish to shallow marine deposits. Representative forms of Foraminifera also indicates that the sediments may be deposited in a shallow marine condition. The microflora of the Palynological zone II can also be compared with those of the Simsang Palynological Zone IV of Meghalaya, India and BPZ V (Baksi 1971). Based on these comparisons, the Palynological Zone II is presumed to be of Middle to Upper Miocene. It is interesting that the floral change from monocolpate pollen Palmepollines to Tricolpate - trilete could be recognized well by an increase in frequency by 50 %. The presence of mangrove pollen Rhizophora indicates the brackish environmental deposition of this drilled sequence. The microfloral assemblage of the Palynological Zone III may be compared with the Simsang Palynological Zone IV of Meghalaya, India (Baksi 1965) and BPZ Zone V of Bengal (Baksi 1971). According to these comparisons, the age of Palynological Zone-III is presumed to be Upper Miocene. A significant floral change has been observed in this zone by the abundance of dissacate pollen and a decline in the pteridophytic spores, particularly triletes. These pollens might have migrated from the extra-peninsular region in the north which would have been sufficiently high during the Lower Miocene.

6.4.8 Comparison with other Miocene Assemblages

6.4.8.1 Assam and Meghalaya sequences

Baksi (1965) reported Simsang Palynological Zone IV from the southern Shillong Plateau, Assam, now Meghalaya. That assemblage contained a few marine hystrichosphaerids and microforaminifers in the lowermost part. Marine micro-organism disappeared in the middle part of the zone. There are some common elements between the Miocene assemblages of Simsang Palynological Zone IV and the well studied. The abundance of bisaccate coniferous pollen belonging to Coniferipites Baksi and a restricted occurrence of Coniferipites chattacharai are the characteristic features of Simsang Zone IV assemblage. In the present study, bisaccates occur sporadically in the Miocene assemblages but Podocarpidites pollen resembling Conferipites chattacharai is restricted to the Miocene, as it is in Simsang Zone IV assemblage. Also, Baksi’s Schizaeaceaesporites and Parkeriaceaesporites appear to be equivalent to Magnastriatites howardi, but they are more abundant in the Simsang Zone IV (Sah and Dutta 1967), as they are in the Miocene of the studied well. 47

In addition, the long-range taxa Spinosopites acolporata, and Polypodiaceaesporites sp. of Baksi (= Verrucatosporites usmensis) and various palm pollen occur in both assemblages. Sah & Dutta (1967) described the stratigraphic succession of the Tertiary palynomorphs in Assam. They reported high abundances of Cicatricosisporites macrocostatus (Baksi et al. 1967) in the Miocene.

6.4.8.2 Bengal Basin

In describing the Bengal basin Zone V and VI, Baksi (1971) points out that they indicate a major shift of flora elements due to well recognized tectonic events related to the Himalayan orogeny, involving the uplift of the South Shillong Front and the associated development of progressively colder climates in the surrounding areas of the Bengal Basin (including the Surma Basin). Baksi also states that the Simsang Palynological Zone IV can be ”confidently” correlated with the Bengal Palynological Zone V by the index elements designated by him as ”Coniferipites – Cicatricosisporites Assemblage Zone”. This assemblage appears to be equivalent to Zone - III. Meyeripollis naharkotensis occurs sporadically in the Simsang palynological Zone IV (Miocene) of Meghalaya (Baksi 1965). In this respect the study area bears a significant resemblance to the Simsang Zone IV. Other resemblances are the presence of Schizaeaceaesporites sp. of Baksi, 1962 (= Magnastriatites howardi), small tricolporate pollen, Parkeriaceaesporites of Baksi, 1962 (= Verrucatosporites usmensis). Comment. Palynomorph recovery was very poor. Only Fenchuganj well-2 slides allow one to make quantitative analysis. 48

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Reconstruction of palaeoenvironment and palaeoclimatic variation in the Surma Basin of Bangladesh during the Neogene is based on complex stratigraphic sequences which include a variety of sediment facies and a variety of palynomorphs, indicating a range of depositional environments. The most frequent and qualitatively most important of these are illustrated in Figs 7 and 8. The most gross environments of deposition for the SG sediments are shallow marine to brackish interdeltaic environments. This interpretation is based on the predominance of lithofacies B and the distribution of pollen, spores, microplankton and dinoflagellets. The pollen flora with dominating angiosperm species suggest tropical to subtropical areas. The occurrence of aquatic ferns (Cicatricosisporites) indicate brackish conditions of sedimentation. The proximity of the sea is indicated by the presence of marine microplankton. Recording of microfauna such as Bolivia, Bulimina, Globigerina and Rotalia from Rashidpur well (Holtrop & Keizer 1969) is indicative of more open marine sedimentation. Foraminifera with Hystrichosphaeridium indicates a marine-brackish environment. Hystrichosphaerida is a widely-distributed facies indicator. The presence of reworked microflora elements and conifer pollens, together with the transgression indicators, indicates an increased tectonic activity in the SB during the Neogene and they are related to the Himalayas. The SB area were progressively coming under the tectonic control to the great Himalayan orogeny and the crustal shortening due to the collision of the Indian and Asian plates and have resulted in extensive uplifts and thrusting of the older rocks (Banerjee 1984). Throughout the Miocene, SB has witnessed a conspicuous subsidence and marine transgression. The transgression of the Miocene certainly affected the coastline. The above mentioned palaeo-environmental circumstances must at some stage have contributed to the retrogressive succession of the vegetation in the area. In this connection, it is pertinent to refer to some ecological aspects of palms, as they feature so prominently in the pollen sequence. At present, palms are considered to be very important in the evolution of tropical forest ecosystems and must have been even more so in the past (Moore 1973). The pteridophytic-rich assemblage may suggest the existence of an inland swamp flora during the time of deposition of these sediments. The presence of Foraminifera in association with a palynomorph assemblage containing mangrove forms and dinoflagellate cysts suggest a shallow marine environment. The sandstones with siltstone and shales were deposited in a shallow marine environment throughout the Miocene. The shallow marine and brackish environment reflects a marine transgression. The pollen spectra of SB contain a high percentage of regionally produced pollen of mixed sub - tropical vegetation. It reflects a mixture of palm and coniferous forest in which palmepollinite is dominant. Palynostratigraphic zones suggest a progressive improverishment of the mixed type of subtropical to tropical forest types existing there and the development of dominant palm vegetation. Grasses, according to the pollen data, were already established in this area during the early to Mid. Miocene and experiencing summer rainfall similar to the present day climate. 50

As far as known, the Rhizophora pollen type is unique and cannot be confused with pollen from other taxa (Germeraad et al. 1968, Muller 1964). The presence of Rhizophora in the Miocene sequence suggest the Neogene sediments of marine origin. The Surma Basin has undergone two successive phases of evolution. The marine transgressive phase, followed by a regressive phase resulting in a series of continental fluvio-deltaic to marginal marine sedimentation during the Neogene. The Great Himalayan Orogeny and associated tectonics dominated the SB during Miocene-Pliocene times. Major changes in sea level for Neogene are suggested based upon trangressive- regressive phenomena. A marked rise in sea level would have caused a marine transgression (Bandy 1968). The proximity of the sea is indicated by the presence of marine microplankton. The overall evidence thus suggests that vegetation was establishing on a marshy area close to the sea. It also suggests a subtropical humid climate (Srivastava 1970). Similar marine deposition and its environment were also identified for West Bengal, Meghalaya and Tripura geo-provinces of India (Banerjee 1984).

6.6 Age

Studies of pollen samples from the SG sediments of Surma Basin have indicated a Neogene age for the unit. On the basis of a regional study of dinoflagellates, SG can be assigned as early to late Miocene in age. This conclusion is based upon the following reasons: 1. The stratigraphic position of the SG unit 2. The absence of typical Oligocene assemblage and 3. A general resemblance of the microflora to the Miocene assemblages in Assam, Meghalaya and Bengal, India – the neighbouring country. In Assam sequence, which is very close to the Surma Basin, Magnastriatites howardi {= Cicatricosisporites macrocostatus (Baksi 1962), Sah & Dutta (1968) is restricted to the Miocene}. 4. An abundance of Conifer pollen and the presence of taxa Florschuetzia trilobata, F. levipol.

6.7 Maturity

Maturity. The maturity of palynomorphs is refered to color indices of thermal alteration index which attain during and after sedimentation. Fenchuganj well-2 samples were marked pale-yellow to brownish colour of polynomorphs indicating the mature stage from the depth below 3615 m (Core No. 10). From core 3 to 9, low maturity was assigned. According to the maturity study, the sediments within the depth range of 3615 – 4095m are to be considered as an organically matured stage. 51 ganj well – 2 of Surma Basin, Bangladesh. Surma of 2 well – ganj ing % variations of main taxa from Fenchu from taxa main variations of % ing Fig. 7. Pollem diagram show Fig. 7. Pollem diagram 52

Fig. 8. Pollen and spores from study areas (SB).

6.8 A list of palynomorph recovery from the Fenchuganj well-2 with their possible botanical affinity

Palynoflora taxa present in the well Fenchuganj –2. Botanical affinity 1. Cyathidites minor Cyatheaceae 2. Lycopodium sporites Lycopodiaceae 3. Sphagnum Sphagnaceae 4. Eximispra Unknown 5. Cicatricosisporites Schizaeacea 6. Undulatis porites Unknown 7. Dandotriasporites Unknown 53

8. Scizaoisporite Schizaeceae 9. Polypodiaceoisporite Polypodiaceae 10. Lygodiumsporites Schizaeceae 11. Trilete(T)-L1a Sphagnum 12. T – reticulate Sphagnum 13. T – granulate 1a Sphagnum 14. T – Verrulate 1a Unknown 15. T – rugosa 1a Unknown 16. T – Cing 1a Unknown 17. T – fovulate 1a Unknown 18. T – Sig + rug 1a Unknown 19. T – aperturate Unknown 20. Leavigatosporites 1a Polypodiaceae 21. Polypodiisporites sp. Polypodiaceae 22. Verrucatosporites Polypodiaceae 23. Emparitus Unknown 24. Polypodiisporites Oligocenicus Polypodiaceae 25. Monolites Mawkmaensis Polypodiaceae 26. Monolites reticulate 1a Polypodiaceae 27. Texodium Unknown 28. Disaccate Podocarpidites 29. Tsuga Pinaceae 30. Disaccate striat Podocarpidites 31. Inaperturate L1a Unknown 32. Inaperturate – reticulate Unknown 33. Retipilonapites Unknown 34. Palmaepollenites Arecaceae 35. Couperipollis sp. Palmae 36. Monocolpate (C1) – grain 1a Unknown 37. C1 – Echniiz 1a Unknown 38. C1 – L2a Unknown 39. Dicolpatis sp. Unknown 40. Liliacidites Liliaceae 41. Tricolpate (C3) – L1a Acer 42. C3 – L2a Acer 43. C3 – reticulate 1a Unknown 44. C3 – reticulate 2a Unknown 45. C3 – grain 1a Unknown 46. C3 – Verrulate Unknown 47. C3 – regose 1a Unknown 48. C3 – scab 2a Unknown 49. C3 – apl 1a Unknown 50. C3 – favulate 1a Unknown 51. Diporopollenites Unknown 52. Striatopollis Unknown 53. Marginipollis Unknown 54

54. Meyeripollis naharilotensis Unknown 55. Polycolpites sp. Lamiaceae 56. Tetracolpate (C4) – L1a Unknown 57. Monoporites anulatus Unknown 58. Carya Caryaceae 59. Carpinus Corylaceae 60. Betula Betulaceae 61. Triporopollenites Moraceae 62. Triporate (P3) – L1a Ovoidites 63. P3 – L2a Ovoidites 64. P3 – reticulate 1a Ovoidites 65. P3 – ret 2a Ovoidites 66. P3 – grain 1a Unknown 67. P3 – scab 1a Unknown 68. Periporate – reticulate 2a Unknown 69. Polyporina Malvaceae 70. Periporate – L1a Unknown 71. Chanopodiaeea Unknown 72. Polygonacidites sp. Dorseraceae 73. Florsechetzia meridonalis Unknown 74. Florsechetzia Levipol Unknown 75. Florsechetzia trilobata Unknown 76. Rhizophora Rhizophoraceae 77. Illex pollenites Aquifoliaceae 78. Rhoipites niditus Unknown 79. Nyssa pollenites Nyssa 80. Tricolporate (C3P3, sp.) – L1a Quercus 81. C3P3 – L2a Quercus 82. C3P3 – Pet 1a Unknown 83. C3P3 – Pet 2a Unknown 84. C3P3 – Fovulate 1a Unknown 85. C3P3 – Scab 1a Unknown 86. Tetracoliporites similes Unknown 87. Tetracolporates (C4P4) – L1a Unknown 88. Syncolporate sp. Unknown 89. Tetred Unknown 90. Alnipollenites Betulaceae 91. Hystrioispheridum Algae 92. Rusiformisporite Fungal spore 93. Fungus Fungi 94. Dinoflagellates 95. Foraminifera 55

Palmaepollenites Polypodissporities Polypodiaceoisporite

Verrucatosporites Verrucatosporites Dissacate alnipollenite

Dissacate pollen Monocolpate pollen Cicatricosisporite

Verrucatosporite Verrutrilete spore Palmepollenite

Plate 1. (All figures X500 magnification). 56

Verrutrilet Tricolporate reticulate Tricolpate verrucates

Tricolporate Tricolporate Tricolporate

Tricolporate levigate Mayeripollis naharkotensis Mayeripollis

Trilete spore Dicolpate pollen Palmaepollenites

Plate 2. (All figures X500 magnification). 57

Dynoflagelates Cicatricosisporites

Polypodiacedisporites Hystrichospheridium

Verrucatosporites Grami Pollenites

Plate 3. (All figures X500 magnification). 58

Meyeripollis Palmepollenite pollen Disaccate pollen naharkotensis

Cicatricosisporites Marco Hystrichospheridium Trilete Granulate cocostatus

Trilete spore Hystrichospheridium Florsechetzia trilobata

Alnipollenite Fungal pore Disaccate pollen Couperipollis spinson (Monocolpate)

Plate 4. (All figures X500 magnification). 7 Geochemical results

7.1 Major elements (XRF & AAS)

Major, trace and REE geochemistry of the core samples of SG sediments of the SB were studied in detail. The detail XRF and AAS analysis of the samples is presented in Appen- dix 2. The abundance of major elements analyzed in the samples are presented graphically (Figs 9–14) for all the wells.

7.1.1 Major elements

The major element composition of the SG sediments of SB determined in this study is compared to the composites (Average shale, Pettijohn 1975, The North American Shale Composite, Grömet et al. 1984, AGV-1, Flanagan 1973, Bhuban shale of Jaintiapur, Syl- het, Bangladesh, Islam 1996, and well data from the present study) and presented in Table 8. In general, the bulk composition of the Neogene shale (NS) of the present study com- pares quite closely with these previously published estimates of average shale compositions. The ratio of SiO2/Al2O3 is centered within the range established for shales.

Table 8. Comparison of Chemical composition of shales. Wt% NASC* AGV-1 Bhuban Shale** Average Shale Present study (Grömet (Flanagan 1973) (R. Islam 1996) (Pettijohn 1975) Habiganj well-1 et al. 1984) (A. Mannan 2002) SiO2 64.82 60.41 68.65 58.10 61.63 TiO2 6.80 1.06 0.74 - 0.87 Al2O3 17.05 17.66 14.96 15.40 16.34 Fe2O5.706.264.554.026.95 MnO0.250.100.03-0.09 MgO2.831.571.032.443.01 CaO 3.51 5.02 0.05 3.11 1.93 Na2O1.314.360.771.301.57 K2O3.972.932.913.243.25 P2O5 0.15 0.50 0.09 - 0.16 * NASC = North American Shale Composite. ** Bhuban Shale = Sylhet, Bangladesh. These chemical results suggest that the present study data has the qualities of an average of averages. 60 4,0 5 3,5 4 3,0 ) ) 3 2,5 wt % wt wt % ( ( 2,0 O 2 2 K CaO CaO 1,5 ndix – 1 and 2. Dash 1 1,0 ,5

0 Depth

Depth (m) 3633 3634 3635 3636 3637 3638 3639 3640 4729 4733 4735 m 3633 3634 3635 3636 3637 3638 3639 3640 4729 4733 4735 8 1,4 7 1,2 le interval see Appe 6 1,0 5 (wt%) 3 O (wt %) O (wt ,8 2 O a 2 4 N Fe ,6 3 ,4 2 Depth Depth (m) 3633 3634 3635 3636 3637 3638 3639 3640 4729 4733 4735 m 3633 3634 3635 3636 3637 3638 3639 3640 4729 4733 4735 ,2 20 18 16 14 ,1 the well Atgram – IX. For details of samp details of IX. For well Atgram – the 12 (wt%) 3 O 2 10 MnO (wt %) (wt MnO Al 8 6 0,0

4 Depth

Depth m 3633 3634 3635 3636 3637 3638 3639 3640 4729 4733 4735 m 3633 3634 3635 3636 3637 3638 3639 3640 4729 4733 4735 k of sample intervals. k of 3,5 100 3, 90 2,5 80 2,0 70 1,5 (wt%) 2 MgO (wt %) (wt MgO 60 SiO 1,0 ,5 50 lines indicate the brea Fig 9. Major element variation curves for 0,0

40 Depth Depth m 3633 3634 3635 3636 3637 3638 3639 3640 4729 4733 4735 m 3633 3634 3635 3636 3637 3638 3639 3640 4729 4733 4735 61 4,0 16 14 3,5 12 10 s indicate 3,0 8 O (wt%) 2,5 2 6 CaO (wt%) CaO K 4 2,0 2 1,5 0 Depth Depth 200 3141 3268 3730 3766 4094 3137- 3624- 3742- 3141 3268 3730 3766 4094 2190- 3259- 3265- 4086- (m) 2200 3137- 3624- 3742- 2190- 3259- 3265- 4086- (m) 2 9 1,7 tails of the sample. Dash line 1,6 8 1,5 7 1,4 (wt %) 3 O (wt %) O (wt 2 O 2 6 Na Fe 1,3 5 1,2 1,1 4 Depth Depth 200 3141 3268 3730 3766 4094 200 3137- 3624- 3742- 2190- 3259- 3265- 4086- (m) 2 3141 3268 3730 3766 4094 3137- 3624- 3742- 3259- 3265- 4086- 2190- (m) 2 ,8 20 – 2. SeeAppendix-1 and 2 for de ,7 18 ,6 16 ,5 ,4 14 (wt%) 3 O 2 ,3 MnO (wtMnO %) Al 12 ,2 10 ,1 0,0 8 Depth Depth 3141 3268 3730 3766 4094 3137- 3624- 3742- 2190- 3259- 3265- 4086- (m) 2200 3141 3268 3730 3766 4094 3137- 3624- 3742- 3259- 3265- 4086- 2190- (m) 2200 7 80 6 70 ) 5 wt % ( 4 (wt %) (wt O 2 60 g M SiO 3 50 2 1 40 Depth Depth 200 3141 3268 3730 3766 4094 3137- 3624- 3742- 3259- 3265- 4086- 2190- (m) 2 3141 3268 3730 3766 4094 3137- 3624- 3742- 2190- 3259- 3265- 4086- (m) 2200 Fig 10. Major element variation curves for the well Fenchuganj Fenchuganj well the for curves variation element Major 10. Fig the break of sample intervals. 62 ,0 4 16 14 1 3,5 ) 10 3,0 8 CaO /wt % /wt CaO O (wt%) 6 2 K 2,5 4 2 ,0 2

0 Depth Depth (m)

(m) 1255- 1256 1257- 1258 1259 1849- 1850 1851- 1852 3110- 3111 1255- 1256 1257- 1258 1259 1849- 1850 1851- 1852 3110- 3111 1,6 8,5 8,0 1,5 2 for details of the sample interval. Dash interval. 2 for sample details of the 7,5 7,0 1,4 6,5 (wt %) (wt O (wt %) O (wt 3 2 O 6,0 2 Na Fe 1,3 5,5 5,0 1,2

4,5 Depth Depth (m)

(m) 1255- 1256 1257- 1258 1259 1849- 1850 1851- 1852 3110- 3111 1255- 1256 1257- 1258 1259 1849- 1850 1851- 1852 3110- 3111 1,0 20 18 ,8 16 ,6 14 MnO (wt %) (wt MnO 4 , (wt%) 3 O 12 2 the well Habiganj – 1. See Appendix 1 and Al ,2 10 0,0 8 Depth Depth (m)

(m) 1255- 1256 1257- 1258 1259 1849- 1850 1851- 1852 3110- 3111 1255- 1256 1257- 1258 1259 1849- 1850 1851- 1852 3110- 3111 3,6 70 3,4 68 3,2 66 3,0 64 2,8 62 2,6 (wt %) (wt 2 60 2,4 SiO MgO (wt %) (wt MgO 58 2,2 2,0 56 1,8 54 Depth Depth (m) lines indicate the break of sample intervals. of sample break the indicate lines (m) variation curves for Fig 11. Major element 1255- 1256 1257- 1258 1259 1849- 1850 1851- 1852 3110- 3111 1255- 1256 1257- 1258 1259 1849- 1850 1851- 1852 3110- 3111 63 6 4,0 sh 5 3,5 4 3,0 3 O (wt %) O (wt 2 K CaO (wt %) CaO (wt 2 2,5 1 2,0 0 Depth Depth 1175 1176 1177 2272 2973 2974 3731 3968 3730 1175 1176 1177 2272 2973 2974 3731 3968 (m) 3730 (m) 1,6 9 1,4 8 1,2 7 1,0 (wt %) (wt 3 O 0 (wt %) 0 (wt 2 2 6 ,8 Fe Na ,6 5 ,4

4 Depth 1175 1176 1177 2272 2973 2974 3731 3968 Depth (m) 3730 1175 1176 1177 2272 2973 2974 3731 3968 (m) 3730 1. See Appendix 1 and 2 for details of the sample interval. Da ,16 20 ,14 18 ,12 16 (wt %) (wt 3 ,10 O 2 l A 14 MnO (wt %) (wt MnO ,08 12 ,06 10 ,04 Depth Depth 1175 1176 1177 2272 2973 2974 3731 3730 3968

(m) (m) 1175 1176 1177 2272 2973 2974 3730 3731 3968 70 1,6 68 1,4 66 1,2 64 (wt %) (wt 1,0 2 0 (wt %) 0 (wt 2 62 SiO Na ,8 60 ,6 58 ,4 56 Depth Depth 1175 1176 1177 2272 2973 2974 3731 3968 3730 1175 1176 1177 2272 2973 2974 3731 3968 (m) 3730 (m) Fig 12. Major element variation curves for the well Kailsatila – Kailsatila well the for curves variation element Major 12. Fig lines indicatesample intervals. the break of 64 4,0 20 3,5 lines 3,0 ) 2,5 10 wt % ( 2,0 O (wt %)O (wt 2 CaO CaO 1,5 K 1,0 ,5 0 Depth Depth Dash interval. sample (m)

(m) 959- 964 1450- 1457 1837 2290- 2307 2828- 2834 3160- 3168 959- 964 1450- 1457 1837 2290- 2307 2828- 2834 3160- 3168 9 2,0 8 1,8 7 2 for details of the 2 for details 1,6 6 (wt %) (wt 3 5 O (wt%) 2 O 1,4 2 a N Fe 4 1,2 3 2 Depth 1,0 Depth (m)

959- 964 1450- 1457 1837 2290- 2307 2828- 2834 3160- 3168 (m) 959- 964 1450- 1457 1837 2290- 2307 2828- 2834 3160- 3168 ,7 22 20 ,6 18 ,5 16 ,4 14 (wt %) (wt 3 12 ,3 O 2 10 Al MnO (wt %) MnO ,2 8 the well Patharia – 5. See Appendix 1 and – 5. See Appendix 1 Patharia the well ,1 6 0,0 4 Depth Depth (m)

(m) 959- 964 1450- 1457 1837 2290- 2307 2828- 2834 3160- 3168 959- 964 1450- 1457 1837 2290- 2307 2828- 2834 3160- 3168 80 3,5 3,0 2,5 70 2,0 (wt %) (wt 2 MgO (wt (wt %) MgO 1,5 60 SiO 1,0 ,5 50 Depth Depth (m)

(m) 959- 964 1450- 1457 1837 2290- 2307 2828- 2834 3160- 3168 959- 964 1450- 1457 1837 2290- 2307 2828- 2834 3160- 3168 Fig 13. Major element variation curves for for curves variation element Major 13. Fig intervals. sample of break the indicate 65 6 3,8 h 3,6 5 3,4 4 3,2 3 O (wt %) O (wt 2 K CaO (wt %) (wt CaO 3,0 2 2,8 1 e sample interval. Das interval. e sample 2,6

0 Depth Depth (m)

(m) 1081- 1087 1248- 1254 1827- 1832 2134- 2135 2472- 2473 1081- 1087 1248- 1254 1827- 1832 2134- 2135 2472- 2473 1,8 8,5 1,7 8,0 1,6 7,5 and 2 for details of th 1,5 7,0 (wt %) 3 O (wt%) O 2 2 a 1,4 6,5 Fe N 1,3 6,0 1,2 Depth 5,5 Depth (m) (m) 1081- 1087 1248- 1254 1827- 1832 2134- 2135 2472- 2473 1081- 1087 1248- 1254 1827- 1832 2134- 2135 2472- 2473 ,18 19 ,16 18 17 ,14 16 ,12 (wt%) 3 O 2 15 l MnO (wt %) (wt MnO ,10 A 14 ,08 13 ,06 Depth 12 Depth (m) 1 for Appendix Rashidpur – 1. the rves See well (m) 1081- 1087 1248- 1254 1827- 1832 2134- 2135 2472- 2473 1081- 1087 1248- 1254 1827- 1832 2134- 2135 2472- 2473 12 68 10 66 k of sample intervals. 8 64 (wt %) (wt 2 6 62 MgO (wt %) (wt MgO SiO 4 60 2

58 Depth Depth (m) 1081- 1087 1248- 1254 1827- 1832 2134- 2135 2472- 2473 Fig 14. Major element variation cu (m) brea lines indicate the 1081- 1087 1248- 1254 1827- 1832 2134- 2135 2472- 2473 66

Gulf coast shales have K2O contents that increase systematically with depth from ~2 to 4 wt% in the Paleocene-Eocene Wilcox Formation and from 2 to 5 wt% in the Oligocene - Miocene Frio-Anahnac succession (Bloch et al. 1998). For the present study all the welldata of K2O content varies between 2-3.5 wt%. Potter et al. (1980) has shown the geochemical properties of shales change with time. For example, several workers have reported significantly higher K2O in early Paleozoic shales ,than in younger shales. Comparison between the histogram of modern sediments of the Sylhet area (Islam 1996) and the SG sediments (Miocene) of Rashidpur well-1 shows a marked decrease of SiO2 content(Fig.15). The mean value of SiO2 varies from 79.0 to 62.65 (Fig.16a). The average value of Al2O3, K2O and TiO2 has enriched markedly from 9.7 (mean value) to 15.85, 1.75 to 3.12 and 0.63 to 0.86 respectively (also Fig.16b). SiO2. Most samples from the wells showed SiO2 contents between 60 and 70 wt%. The highest value was 89.40% in the Atgram well-IX well (Sample No. 83) which is a shale (this is an exceptional sample with very high content of SiO2). The highest value in general was 75.51% (Atgram well-IX) and 74.51% (Patharia well – 5), both the samples were sandy shale. Variation was least in Rashidpur well – 1 (Standard deviation = 1.54) suggestive of a uniform source composition during that interval. Sample interval (Depth) interval Sample Sample

Fig 15. Comparison between the silicon dioxide of modern sediments of Sylhet area and the SG sediments (Miocene) of Rashidpur well – 1 shows a marked decrease of SiO2 content. Dash line indicate the average line. For younger sediments it is 78 % and 63 % for the present study. Younger sediment data from Islam 1966. Samples are from Jaintiapur, Sylhet. (exposed rock) Figure 14 shows the actual depth of the samples of Rashidpur well – 1. 67

Al2O3 % SiO2 %

TiO2 % K2O %

Fig 16. A Histogram showing major oxides of Rashidpur well – 1. Percentages are in wt %. 68

Al2O3 % SiO2 %

TiO2 % K2O %

Fig 16. B. Histogram showing major oxides of modern sediments (Data from Islam 1996). Note the differences of oxides between the samples. Percentages are in wt %.

The SiO2/Al2O3 ratio (wt % SiO2/ wt % Al2O3) is also often used as a grain-size indicator (Dypvik 1979). A plot of this ratio prepared for all the wells has been presented. The grain size parameters studied show little fluctuations in fine grained sequence. The wells Atgram well-IX, Rashipur well-1, Habiganj well-1 and KailasTila well-1 have downward coarsening tendency and the Fenchuganj well-2 well show relatively coarse grained upward. The overall sequence of the SiO2/Al2O3 ratio shows the dominance of the fining with a coarsening tendency at the bottom for the wells mentioned. The SG sediments (Neogene shale) reflects a shallowing of the depositional basin and the deposition of more altered material. Palynological and geochemical studies match well the shallowing conditions shown. The SiO2 content in all the wells studied seems to decrease with increasing Al2O3. As large amounts of quartz are found in the residual material, it is reasonable to assume that desilicification took place mainly by a destruction of aluminosilicates. A plot of Al2O3 against the SiO2%/Al2O3% ratio of samples from Patharia well-5 shows a strong negative correlation. The Fe2O3/ Al2O3, Na2O/Al2O3 and K2O/Al2O3 ratios in the samples vary 69 against the Al2O3 concentrations (Fig. 17). The results indicate the presence of clay minerals.

K2O/Al2O3 SiO2/Al2O3 Al Al 2 2 O O 3 3 (wt %) (wt %) (wt

Fe2O/Al2O3 Na2O3/Al2O3 Al Al 2 2 O O 3 3 (wt %) (wt (wt %) (wt

Fig 17. Crossplots of Al2O3 versus K2O/Al2O3, SiO2/Al2O3, Fe2O/Al2O3 and Na2O3/Al2O3 for Patharia well – 5 show negative correlation indicating the presence of clay minerals. Percentages are in wt %.

The percentage of selected major elements in Harker-type (%) variation diagrams for the rocks of the study area has been prepared (Fig. 18–22). In case of SiO2, Al2O3, Fe2O3, TiO2 and K2O, normal source variation and grain size variation give rise to similar trends. Except KailasTila well-1 and Rashidpur well-1, MgO also have similar trends. In the case of CaO, Na2O and MnO, the vectors were different. The results illustrate a similar elements composition from all the samples, reflecting homogeneity of the sediment suite. Variation in the chemical composition reflects changes in the mineralogical composition of the sediments due to the effects of weathering, marine sedimentation and early diagenetic processes (Shaw & Weaver 1965, Drever 1971, Nesbitt & Young 1984,1989). The 70 abundance of Al, Si, K and Ti in shales may be perturbed from parent material by weathering, transport and depositional processes (Nesbitt & Markovies 1996).

SiO (wt %) SiO2 (wt %) 2 Fig 18. Harker type major element (%) variation diagrams for Atgram well – IX. 71 2 TiO CaO

SiO2 SiO2 CaO O 3 2 O 2 Na Al

SiO2 SiO2 O 2 MnO K

SiO 2 SiO2 3 O 2 MgO Fa

SiO2 (wt %) SiO2 (wt %)

Fig 19. Harker type major element percentage variation diagram for Fenchuganj well – 2. 72

SiO2 (wt %) SiO2 (wt %)

Fig 20. Harker type major element percentage variation diagrams for Habiganj well – 1. 73 2 TiO MgO

SiO2 (wt %) SiO2 (wt %) O 3 2 O 2 Na Al

SiO2 (wt %) SiO2 (wt %) MgO CaO SiO (wt %) 2 SiO2 (wt %) 3 O O 2 2 K Fe

SiO2 (wt %) SiO2 (wt %) Fig 21. Harker type major element percentage variation diagrams for Kailastila well – 1. 74

SiO2 (wt %) SiO2 (wt %) Fig 22. Harker type major element percentage variation diagrams for Patharia well – 5. . 75

The different maturity parameters studied display complicated development with an increasing maturity having fluctuations in different depth interval. The maturity index (M) used in this study was defined by Björlykke as M = Al2O3 + K2O / MgO + Na2O (Björlykke 1974) and the parameter is stratigraphically controlled, showing the same trend in different districts. The index is also controlled by the clay mineral. The other two maturity parameters used for the present study were K2O/Na2O and Rb/K2O ratios (Björ- lykke 1971, Dypvik 1977, 1979). Dypvik has shown that high K2O/Na2O and Rb/K2O ratios are indicative of a Kaolinization process and typically mature sediment. All the maturity parameters used in this study show an apparently fluctuating maturity for all wells. In general, the lower portions showed increasing maturity. All the three maturity parameters showed a similar trend for the wells studied (Fig. 23–28). It is possible to demarcate the wells stratigraphically by mature and immature zones. The maturity increases with depth, indicating renewed deposition, new weathering conditions and particularly source area variations. The study also showed that the maturity increases with the decrease of the silica content and grain size. The trend of K2O/Na2O ratios in Fig. 23–28 is interesting and is consistent with the grain size analysis. Na and K in the studied Neogene shales are mostly confined in the detrital illites. Deer et al. (1963) noted that fresh muscovite contains more K and Na than average illite. Micas are often degraded to illite during weathering by loss of K and Na, uptake of H2O and partial opening of the lattice. Coarser fractions of the sediment will tend to include fresher micas, alternatively, finer fractions carry more degraded illites and more smectites (Pearson 1979). An increase in sodium and potassium with coarseness of the sediment may then arise naturally. The ratio of K2O/Na2O shows a fluctuating scenario for all wells studied which are between 0.70 and 4.26. These differences obviously reflect in part the different original compositions of the source rocks. 76