stratigraphic evolution and geochemistry of the neogene surma group, surma basin...

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

ABDULMANNAN

Department of Geology,University of Oulu

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 ofthe Faculty of Science, University of Oulu, for publicdiscussion in Auditorium GO 101, Linnanmaa, on June15th, 2002, at 12 noon.

OULUN YLIOPISTO, OULU 2002

Copyright © 2002University of Oulu, 2002

Reviewed byDocent Kari StrandDoctor Kalle Kirsimäe

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

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

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

OULU UNIVERSITY PRESSOULU 2002

Mannan, Abdul, Stratigraphic evolution and geochemistry of the Neogene SurmaGroup, Surma Basin, Sylhet, Bangladesh Department of Geology, University of Oulu, P.O.Box 3000, FIN-90014 University of Oulu, Finland Oulu, Finland2002

Abstract

The Surma basin is a part of the Bengal Basin situated in northeastern Bangladesh. The presence ofeight gas fields and one oil field makes this an area that is interesting both economically andgeologically. In spite of detailed geological and geophysical investigations, information available onpalynostratigraphy and geochemistry for the area is scanty.

The aim of the present work was to investigate the palynological assemblages, mineralogy andgeochemistry 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 AbsorptionSpectrometry (AAS), Loss of Ignition (LOI), X-ray diffraction (XRD), Transmission ElectronMicroscopy (TEM) and Scanning Electron Microscopy (SEM) were used for geochemical andmineralogical study of shale samples. In the palynological study, the distributions of pollens andspores were determined. For data analysis, SPSS computer programme was used.

Palynological assemblages of the Surma Group of sedimentary sequence of Bangladesh includetaxa range in age from the lower Miocene to the Upper Miocene which can be potentially used indating and correlation. The Lower Miocene interval is correlated with the Simsang PalynologicalZone IV of Meghalaya, India and the Bengal Palynological Zone (BPZ) V. The Upper Miocene iscorrelated with the Simsang Palynological Zone IV of Meghalaya, India and the BPZ Zone V ofBengal. They were deposited in two types of paleoenvironments ranging from the brackish type toshallow marine to brackish. The sequence contains reworked palynomorphs of BPZ IV and IIInamely Meyeripollies naharkotensis, Polypodiesporites Oligocenecus, Palmepollenities Eocencusand ornamented Tricolpate pollen of the Eocene-Oligocene age which are mainly encountered in thelower 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 ofdeposition. High Ba enrichment was detected in the Patharia well-5 and showed high surface waterproductivity 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) andkaolinite/smectite (K/S) mixed layers. Kaolinite/Smectite here reported for the first time inBangladesh. Clay mineral analyses provided evidence for diagenesis. Smectite diagenesis anddehydration have contributed to the generation of overpressure in the Bhuban Formation in thePatharia well-5.

Geochemical ratios of the present study from the Surma Basin is undoubtedly a powerfultechnique 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 thereonmountains standing firm, and flowing rivers: and fruit of everykind He made in pairs, two and two he draweth the night as aveil over the day. Behold, verily in these things there are signsfor those who consider”

Holy Quran (13:3).“The world is sweet and verdant green, and Allah appoints you tobe His regents in it, and will see how you acquit yourselves…” oneof 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 methe 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 andgratitude to Prof. Vesa Peuraniemi for his genuine support, valuable advice and sincerecomments which helped me a lot to finish this study. The Institute of Geosciences andAstronomy, at the University of Oulu, Finland provided support, including field,laboratory and office work as well as analyses of samples. This study was partiallyfinanced by the University of Oulu through a post-graduate grant. I am very grateful tothe authority of the University of Oulu. I am also grateful to the authority of theUniversity 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 thedissertation 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 valuablecomments 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 nightresearch sessions when he was the only companion during coffee breaks at thedepartment. I also wish to thank Prof. Tuomo Alapieti, Director of the Institute ofGeosciences and Astronomy, University of Oulu for providing scientific and otherfacilities 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. LutforRahman Chowdhury, General Manager, BAPEX for his valuable collaboration andpractical assistance in palynological laboratory analysis. My sincere thanks to all myfriends 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 QaderChowdhury, Department of Geology, University of Dhaka, Bangladesh for his nice

suggestions towards the planning of this research work. I am also thankful to thenumerous individuals who have directly or indirectly contributed to the completion of thiswork. Out of them at least two names should be mentioned here whom are my belovedstudents 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 ofMedicine, University of Oulu, whose assistance and encouragement made this workpossible towards the end. I cannot forget the sleepless nights he spent before thesubmission 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 partof the thesis.

Special thanks are due to the staff of the Institute of Electron Optics at the Universityof Oulu, and particularly to Mr. Olavi Taikina-Aho for their assistance with themicroanalyses. 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 mydeep felt regards to Mrs. Kristiina Karjalainen, who has drawn many of the figures andhas taken care of the electronic forms of them, Mrs. Ulla Paakkola, for preparing most ofthe thin sections and Mrs. Riitta Kontio, who co-operated in the geochemical laboratoryworks for this study. My heartful thanks to all of them. My sincere thanks to all of thosewho have co-operated both in Finland and in Bangladesh.

Finally, I am particularly grateful to my wife, Syeda Wahida Akter, for helping andassisting me in all the stages of this work. Without her help this study would never havebeen 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 herlove 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 fortheir 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 ................................................................................................................. 112 Previous studies ........................................................................................................... 14

2.1 Palynology .......................................................................................................... 142.2 Geochemistry ..................................................................................................... 18

3 Aims of this study ....................................................................................................... 194 Study Area .................................................................................................................. 20

4.1 Stratigraphy ........................................................................................................ 204.2 Structure and tectonics ....................................................................................... 274.3 Palaeogeography and Palaeotectonics ................................................................ 294.4 Surma Basin (SB), Sylhet (North East Bangladesh) .......................................... 30

4.4.1 Wells studied ............................................................................................. 304.4.2 Regional geologic setting .......................................................................... 31

5 Analytical methods ..................................................................................................... 355.1 Palynological slide preparation .......................................................................... 355.2 Geochemical analysis ......................................................................................... 35

5.2.1 X-ray Fluorescence (XRF) ........................................................................ 365.2.2 Atomic Absorption Spectrometry (AAS) .................................................. 365.2.3 Loss of Ignition (LOI) ............................................................................... 365.2.4 Accuracy of analyses ................................................................................. 36

5.3 Mineralogical analysis ....................................................................................... 385.3.1 X – Ray Diffraction (XRD) ....................................................................... 385.3.2 Transmission Electron Microscopy (TEM) .............................................. 385.3.3 Scanning Electron Microscopy (SEM) ..................................................... 385.3.4 Petrographic microscopy (optical) ............................................................ 38

5.4 Statistical analysis and ratios ............................................................................. 396 Stratigraphy ................................................................................................................. 40

6.1 Lithofacies .......................................................................................................... 406.2 Palynological study ............................................................................................ 416.3 Pollen data and pollen assemblage zone ............................................................ 416.4 Palynostratigraphic zonation .............................................................................. 41

6.4.1 Palynostratigraphic zonation of Fenchuganj well – 2 .............................. 42

6.4.2 Atgram well – IX ...................................................................................... 446.4.3 Habiganj well – 1 ...................................................................................... 446.4.4 KailasTila well – 1 .................................................................................... 456.4.5 Patharia well – 5 ........................................................................................ 456.4.6 Rashidpur well – 1 .................................................................................... 456.4.7 Comparison with surrounding areas (from India) ..................................... 466.4.8 Comparison with other Miocene Assemblages ......................................... 46

6.4.8.1 Assam and Meghalaya sequences ............................................... 466.4.8.2 Bengal Basin ................................................................................ 47

6.5 Palaeoenvironment and Palaeoclimate ............................................................... 496.6 Age ..................................................................................................................... 506.7 Maturity .............................................................................................................. 506.8 A list of palynomorph recovery from the Fenchuganj well-2 with their

possible botanical affinity .................................................................................. 527 Geochemical results .................................................................................................... 59

7.1 Major elements (XRF & AAS) .......................................................................... 597.1.1 Major elements .......................................................................................... 59

7.2 Trace elements ................................................................................................... 897.2.1 Barium enrichment .................................................................................. 1007.2.2 Total Rare Earth Elements (∑REE) ........................................................ 108

8 Mineralogical Results ............................................................................................... 1158.1 XRD ................................................................................................................. 115

8.1.1 Non Clay Minerals .................................................................................. 1168.1.2 Clay minerals .......................................................................................... 117

8.1.2.1 Diagenetic model of Surma Basin ............................................. 1198.1.2.2 Implication of Smectite diagenesis and dehydration. ................ 1208.1.2.3 Clay minerals of SG and its implication in petroleum geology. 1218.1.2.4 Kaolinite - Smectite (K/S) mixed layer clay: a new mineral

in Bangladesh ............................................................................ 1228.2 TEM and SEM ................................................................................................. 1298.3 Petrography ...................................................................................................... 130

9 Discussion ................................................................................................................. 13710 Conclusions ............................................................................................................... 14611 References ................................................................................................................. 149Appendices

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 theSurma Basin (SB), Sylhet, Bangladesh.

The SB is a sub-basin of the Bengal Basin situated in the northeastern part ofBangladesh. The basin is bounded on the north by the Shillong plateau, on the east andsoutheast by the Chittagong-Tripura fold belt of the Indo-Burman ranges and on the westby the Indian Shield platform, to the south and southwest it is open to the main part of theBengal Basin. The thickness of the late Mesozoic and Cenozoic strata in the SB range isfrom about 13 to 17 km (Evans 1964, Hiller & Elahi 1984), and much of this strata isNeogene in age. The Bouger anomaly map shows gradually higher values (negative)towards the center of the basin (Alam et al. 1990). An Aeromagnetic interpretation mapby Hunting (1980) indicates a gradual deepening of the basement towards the center ofthe basin and also reveal subsurface synclinal features and faults within the basin. Itstopography is predominantly flat with some north-south trending ridges of twenty toseveral hundred meters elevation present on the north – eastern border. It is activelysubsiding (Johnson & Alam 1991). The geology and the hydrocarbon potential of the SBhas been investigated by many workers (Holtrop & Keizer 1970, Lietz & Kabir 1982,Hiller & Elahi 1984, Khan et al. 1988, Johnson & Alam 1991), detailed geochemical andpalynological studies of SB sediments are lacking.

A number of wells have been drilled in the SB with the discovery of eight gas fieldsand the recent discovery of commercial quantities of oil in the Sylhet – 7 well make thisarea more interesting economically and geologically.

Core samples (n = 188) of the wells of Patharia well – 5, Rashidpur well – 1, Atgramwell – IX, Habiganj well – 1, Kailastita well – 1 and Fenchuganj well – 2 were providedby the Bangladesh Petroleum Exploration Company (BAPEX) for the present study. APalynological study was done from 74 samples. The samples studied are from 959 mdown to 4735 m in depth. The study is aimed to determine the environmental conditionsprevailing when the sediments were deposited. The study also included details of thegeochemistry of the basin for understanding the tectono-environmental condition of thedeposition of the sediments, as well as the diagenetic changes of the sediments duringburial.

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The SG is diachronous unit consisting of a succession of alternating shales,sandstones, siltstones and sandy shales with occasional thin conglomerates, indicative ofrepetitive deposition from pro – delta, delta front and paralic facies with intermittent,wholly marine facies (Holtrop & Keizer 1970). The group is divided into the Bhuban andthe Bokabil Formations, based on differences in their gross lithologies (Mathur & Evans1964).

Tertiary palynostratigraphy is a notoriously difficult discipline and is confronted withvarious problems. For instance, the mean species duration is relatively high inangiosperms which form the dominant taxonomic group in most Tertiary vegetations. Inaddition, the occurrence of a plant species in a certain area is largely controlled byecological, climatic and biogeographic factors. Hence, the first and last appearances ofTertiary palynomorphs in a given section are generally not considered to be very reliablestratigraphic indicators. Correspondingly, climate stratigraphy and biostratigraphy arenow widely used in the Tertiary although there is no well – defined methodologyavailable and there is always the problem of distinguishing autochthonous orparautochthonous from allochthonous palynomorphs (Ashraf et al. 1997).

Palaeoclimate reconstructions in the Neogene are difficult because all methodologicalapproaches used so far, i.e. the geological – palaentological approaches and the climatemodeling approaches, suffer from considerable uncertainties (Utsechar et al. 1997).

Detailed palynological studies of this area are lacking because of very poorpreservation of the palynomorphs. Palynomorph recovery from the cores of the wellsdrilled in the area was low. Some parts, the wells were almost barren. The Palynomorphcontent in the prepared palynological slides for palynostratigraphic analysis wasextremely poor. No palynomorphs (other than fungal spores) were detected in 38 slidesout of 74 slides. Due to this, the idea of doing a quantitative analysis of the palynomorphswas abandoned except for the Fenchuganj well-2, the results of this well allowed forsome quantitative considerations.

The analysis have furnished definite evidence of palynological assemblages of the SGof the sedimentary sequence of Bangladesh, including taxa range in age from the lowerMiocene to Upper Miocene, which can be potentially used in dating and correlation. TheLower Miocene interval is correlated with the Simsang Palynological Zone IV ofMeghalaya, India (Baksi 1965) and the Bengal Palynological Zone (BPZ) V (Baksi1971). The Upper Miocene is correlated with the Simsang palynolgical Zone IV ofMeghalaya, India (Baksi 1965) and BPZ zone V of Bengal (Baksi 1971). Nopalynological research paper has been published from this region yet. Identification andcomparison of the palynomorphs were made on the basis of the published literatures onTertiary Palynology of the Assam and Bengal Basin of India (Neighbouring country). Theworks include Ghosh (1941), Sahni and coworkers (1947), Sen (1948), Lakhanpal (1955),Meyer (1958), Baksi (1962, 1965, 1971), Biswas (1962, 1965), Banerjee (1964), Sah andDatta (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

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coworkers (1986), Singh (1977a,b), Singh and Rao (1990) and Kar (1990). Specialattention 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 coresamples of the wells following: Patharia well – 5, Rashipur well – 1, Atgram well – IX,Habigang well – 1, KailasTila well – 1 and Fenchuganj well – 2. Detailed inorganicgeochemistry of the study area is presented for the first time. Many works have been doneonly 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 inorganicgeochemistry (Imam & Shaw 1985, Imam 1987, 1989, 1993, 1994 and Islam 1996) ofthe study area. The inorganic geochemistry of the SB was prepared on the basis of thechemical and minerological analysis. The chemical analysis includes X-Ray Fluorescence(XRF), Loss of Ignition (LOI) and Atomic Absorption Spectrometry (AAS). Themineralogical analyses were done by X-Ray Diffraction (XRD), Transmission ElectronMicroscopy (TEM), Scanning Electron Microscopy (SEM) and petrographic microscopy.The major elements like SiO2, Al2O3, MgO, CaO, Na2O, K2O, Fe2O3, TiO2, MnO andP2O5 were analyzed. Most of the important trace elements were also measured. Somestatistical analyses were carried out such as Cu/Zn ratio in order to ascertain redoxparameter, SiO2/Al2O3 for grain size, M = K2O + Al2O3/K2O + Na2O, Rb/K2O formaturity.

Other ratios of Cr/Rb, Zr/Rb, V/Rb, Th/U and Cr/Th are sensitive indexes forprovenence (Taylor & McLennan 1985, McLennan 1989, Condie and Wronkiewiez 1990,Götze 1998, Hassan et al. 1999) and were performed. Correlations between Cr, Rb, Zrand 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 andKrosch 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 thediagenetic changes that have taken place in the subsurface, to find out the environmentalconditions and to discuss the implications of clay mineral diagenesis on major geologicprocesses such as overpressure generation and structural developments.

The study also focuses on the evolution of climate variabilities during the Neogene byusing the clay mineralogy analysis of Neogene shale samples of the Surma Basin. Anattempt was made to ascertain the semi-quantitative estimate of the relative abundances ofclay minerals of SB and also to ascertain the petroleum generation capability of sedimentsunder 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 publishedliterature on the Tertiary Palynology of the Assam and Bengal Basin of the neighbouringcountry, India.

In India, the known records of Tertiary plants go back to the later part of the eighteenthcentury (Sonnerat 1782) which remained practically uninvestigated until the first quarterof the present century (Sahni 1921). Later, the first record of a fossil pollen from theTertiary 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 Indiahave 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 etal. 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 referencestherein).

In the Tertiary succession of India, the Jaintia Group (Paleocene-Eocene), BarailGroup (Oligocene) and the Surma Group (Miocene-Pliocene) has been studied well. TheTertiary Palynology of Punjab Basin, Cambay Basin, Cauvery Basin, Bengal Basin andAssam Basin has given a good account of the palynology of the area and specially theSurma Group of Bengal Basin and Assam Basin were the guideline for the present studyas a neighbouring area since no published paper was available till date.

The main features (common assemblages and restricted occurrences) of the Tertiarypalynology of India are presented in Table 1 and in Table 2.

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

Age Name of the basin

Common assemblages Restricted forms Reference

Jaintia Group (Pal – Eocene)

Punjab Basin

Todisporites, Dandotiaspora, Osmundacidites, Couperipollis, Palmepollenites, Cordosphaeridium, Homotryblium, Gonyaulacysta, Lygodiumsporites, Todisporites, Cyathidites, Proxapertites, Couperipollis, Tricolpites and Palmidites

Lycopodiacidites, Palypodiaceaesporites, Granodiporites, Triorites, Nyssapollenites, Polycolpites, Microhystridium, Cannospharopsis, Hystrichospheridium, Oligosphaeridium, Cleitosphaeridium, Cyclonephelium, Thalassiphora, Subathua, Araneosphara, Achilidinium, Verrutrucolpites and Podocarpidites

Mathur 1964, Salujha et al. 1969, Singh & Khanna 1980 and Khanna & Sing 1981, Sing et al. 1978.

Cambay Basin

Lygodiumsporites, Biretisporites, Verrucatosporites, Scizoaeoisporites, Palmaepollenites, Proxaperites, Tricolpites, Nympheaceaepites, Proteacidites, Palaeocaesalpiniaceaepites, Cicatricosisporites etc.

Polypodiaceaesporites, Arecipites, Maurisites, Spinizonocolpites, Psilodiporites, Retitricolpits Margocolporites, Rhoipites, Cupuliferoipolenites, Marginipollis, Cyathidites, Intrapunctiporis, Couperipollis, Assamialetes, Tricolporopollis, Todisporites, Palmidites ans Lakiapollis.

Varma & Dangwal 1964, Venkatachala & Choudhury 1977, Rawat & Venkatachala 1977 and Mathur et al. 1977.

Couvery Basin

Lygodiumsporites, Scizaeosporites, Proxapertites, Couperipollis, Palmaepollenites, Lilicidites, Tricolpites and Myricipites.

Laevigatosporites, Spinainaperturites, Psilodiporites, Marginipollis, Margocolporites, Rhoipites, Caprifoliipites etc.

Venkatachala & Rawat 1973 and Sastri et al. 1977.

Bengal Basin (Zone – II and Zone – III)

Assamialetes, emendatus, Palmeopollenites andCouperpollis.

Lycopodiales, Granutusporites, Ornamented tricolpate, tricolporate, Caesalpiniaceae, polycolpate, polycolporate (Hexacolpites)

Baksi 1971.

Couvery Basin




Couvery Basin




Couvery Basin




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Table 1 continuedAge Name of the

basin Common assemblages Restricted forms Reference

Bengal Basin (Zone – II and Zone – III)

Assamialetes, emendatus, Palmeopollenites andCouperpollis.

Lycopodiales, Granutusporites, Ornamented tricolpate, tricolporate, Caesalpiniaceae, polycolpate, polycolporate (Hexacolpites)

Baksi 1971.

Jaintia Group

Assam Basin Cyathidites, Lygodiumsporites eocenius, Dandotiaspora Dilata, Polypodiisporites, Monolitesmawkmaensis, Assamialetesemendatus, Couperipollis brevipinosus, Liliacisites microreticulatus, Palmepollenites sommunis, Tricolpites, Tricolporopollis Schizoporis, Retialetes, Triorites, Schizaeosporites, Palaeocaesaceaepites, Nymphea, Caesalpinia etc

Verrutricolporites, Bacutricolporites sp. A, Onagraceae and Polygalaceae

Biswas 1962, Sah & Datta 1966, 1968, 1974, Dutta & Sah 1970, Salujha et al. 1971 Sein & Sah 1974, Sing 1977a,b, Sing & Tewari 1979, Dutta & Jain 1980 and Mehrotra 1981.

Barail Group(Oligocene)

Assam Basin Foldexina inaperturata, Meyeripollis, Cyathidites Couper, Lygodiumsporites Potonie´, Todisporites Couper, Strialetes, Magnastriatites Muller, Monolites Potonie´, Assamialetes, Jaintiapollenites, Caryophyllaceaepites, Triorites (Erdman) Couper, Pinuspollenites, podocarpidites, Striatites, Todisporites etc.

Schizaeoisporites and Nympheaceaepites

Biswas 1962, Baksi 1962, 1965, Sah & Dutta 1966, 1968, 1974, Mehrotra 1981, 1983, Kar 1990 .

Bengal Basin

Lygodium, Polypodium, Cicatricosisporites, Bauhinia, Barringtonia etc.

Coniferae, Graminae and Cyperacea

Biswas 1963.

Couvery Basin

Lacrimapollis pilosus, Verrucatosporites bullatus, Malavacearumpollis paucibaculatus

Magnastriatites cauveriensis

Venkatachala & Rawat 1973.

Surma Group(Mio – Pliocene)

Bengal Basin

Cicatricosisporites, Bauhinia, Rhizophoraceae, Hystrichospheridium, Coniferaepites, , Graminae Barringtonia, Polygonaceapites zonoides Baksi, Tricolpate – tricolporate, triporate, monocolpate, dinoflagellate (BPZ – V, Baksi)

Palmaepollenites, Nympheaeceapites and histrichosphaerids

Biswas 1963, Deb 1970 and Baksi 1962, 1971.

Bisaccate conifer belonging to Cedrus, Pinus, Abies, Picea, Tsuga, Schizaeceae and Parkeriacea (BPZ – VI, Baksi)

Baksi 1971.

Couvery Basin

Verrucatosporites bullatus, Polypodiisporites ornatus, Lygodiumsporites sp., striatopollis and hystrichospherids.

Talisiipites retipilatus, Costatipollenites pau-ciornatus, Tricolpites longicolpatus, Striatri-porites sp., Tiliaepolen-ites sp. etc.

Venkatachala & Rawat 1973.

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

In the Miocene, the climate changed to cool, sub-tropical to temperate and the florawas rich. The sediments were mostly fresh-water continental deposits, brackish lagoonalconditions existing only in parts of the Sub-Himalayan region, Cauvery and BengalBasins and the southern part of Assam. Drier conditions existed in the Pliocene, as maybe reasoned by the common occurence of Graminidites etc. Coastal or Lagoonalconditions existed in the Bengal and Kutch basins in the Pliocene. Most of theangiospermous genera known from the Paleogene and Early Neogene are derived fromwoody 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 distinguishedfrom the Cretaceous by the predominance of angiospermous elements and the absence ofsuch taxa as Trilobosporites, Appendicisporites etc. in the former. Statistical variations inthe assemblage along with the disappearence of taxa like Nothofagidites seem to be moreconvenient for making the boundery between Eocene and Oligocene, at present. Theboundary between the Oligocene and the Miocene is marked by the appearence oftemperate to cold climate elements in the latter, along with the disappearence of, or therare 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.

No. Selected Palynomorphs Eocene Oligocene Miocene PlioceneL M U

1 Echitricolporites sp.2 Graminidites sp.3 Polypodiaceaesporites sp.4 Proteacidites symphonemoides5 Podocarpidites (Coniferipites) chattachari6 Lirasporis intergranifer7 Bombacacidites assamicus8 Spinosopites acolporata9 Cicatricosisporites (Ceratopteris) sp. and

Parkeriaceaesporites sp.10 Corrugatisporites terminalis11 Palmaecolpollenites sp. cf. Monocolpopites

broadcolpusi12 Palmaepollenites eocenicus13 Palmaepollenites subtilis14 Eximispora tuberculata15 Monosulcites brevispinosus16 Monosulcites rarispinosis17 Meyeripollis naharkotensis

18

2.2 Geochemistry

An extensive geological and geophysical (mostly seismic) survey has been carried out inorder to describe the stratigraphy, regional setting, structural evolution, condition ofdeposition of sediments and the paleogeography of the basin. Despite the study of thegeology 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 havebeen 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 theworks of Imam (1987, 1989, 1993, 1994) are dealing with inorganic geochemistry. Buthis work was mostly with sandstones. Besides this, a few attempts were made on themineralogy 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 ofOulu, Finland by Islam (1996) dealing with the weathering crust in Bangladesh, is a goodaddition to the account of inorganic geochemistry of the sediments of Bangladesh.Despite the works mentioned, no attempt was made on a detailed geochemicalinvestigation 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 Himalayanpiedmont plain and covers the eastern and southeastern hill ranges of the Sylhet,Chittagong and Chittagong Hill tracts (Paul & Lian 1975). The northeastern part ofSylhet (Jaintapur) is characterized by low rounded hillocks with cliffs and scarps. Fig. 1shows the location and geology of the study area.

4.1 Stratigraphy

The stratigraphy of Bangladesh is somewhat problematic because the greater part of thecountry 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 inBangladesh are mainly based on lithologic interpretation. Figs 2 and 3 shows the details ofthe physiography and geology of Bangladesh. The lithostratigraphic units are defined anddescribed 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 etal. 1990, permitted from Geol Surv Bangladesh.)

22

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

23

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

Bangladesh occupies most of the Bengal Basin - a major geotectonic element of theAssam -Himalayan region and is considered apparently the largest depositional feature inthe world today (Graham et al. 1975, Salt et al. 1986, Kuchl et al. 1989). The BengalBasin 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 thatare deltaic or shallow marine in the north become progressively more marine to the south(Alam 1989).

24

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

JAINTIAGROUP

25

Table 3. Stratigraphic Sequence in Bangladesh.

Calcutta - Mymensing hinge zone of Eocene subdivides the Bengal Basin tectonicallyinto two major subdivisions - shelf and geosynclinal area having different stratigraphichistory.

After the Pre-Cambrian era, the history of the basement complex was one of thepeneplanation until Permo-Carboniferous time when Permian, Mesozoic and Gondowanasediments with coal accumulated in the western side of the basin. The break up of

Geologic age Stable shelf Bengal foredeep LithologyGroup Formation Group Formation

Holocene Alluvium Alluvium Silt, sand, gravel and clayPleistocene late Pliocene

Madhupur

Dihing

Maduhpur

Modhupur ClayPebbly sandstone, sticky clay

Mid-Pliocene E. Pliocene Dupi Tila Dupi Tila

Sandstone, coarse quartz pebbles, petrified wood

TipamGirujan clay Claystone with siltstone and

sandstone

Tipum

Sandstone, coarse-grained cross-bedded pebbles of granite, quartzite, shale and lignite. Clay mostly at base.

Miocene Surma Jamalgong

Surma

Boka Bill Marine shale, pyritic gray marine fossils

Bhuban Sandy shale, sandstone, breccia interbeds

Oligocene Barail Bogra Barail Jenam Siltstone, fine-grained sand-stone, carbonaceous shale

Late Eocene

Middle Eocene

Early Eocene

Paleo-cene

Jaintia

Kopili

SylhetLimestoneTuraSandstone

? ?

? ?

Sandstone, locally glauconitic: shale, highly fossiliferous: thin calcareous beds limestone, num-mulitic, sandstone interbeds sandstone, coal and shale

Late-Middle Cretaceous

Upper Gondo-wana

Sibganj Trapwash ? ?

Sandstone, coarse yellow-brown, clay, white, volcanic ash

Early Cretaceous Jurassic

RajmahalTraps ? ?

Basalt, amygdaloidal, andesite, surpentinized, shale, agglomerate

Late Permian

Early Permian

Lower Gondo-wana Paharpur

Kuchma

? ?Sandstone, felspathic grey-wacke, coal, shale, sandstone, coarse grained, shale, coal, thick seams

Pre-Cambrian Basementcomplex

Gneiss and schist

Sources: Based on Khan (1980) and Zaher & Rahman (1980). ? = not classified

26

Gondowanaland led to the eventual separation of peninsular India from the southerncontinents, 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 variedconsiderably from its present position. During the Cretaceous Period, the sea transgressednorthwards towards the southern edge of the Shillong plateau and subsequently regressedfar south into the Bengal Basin, causing at least four major transgressions andregressions. Argillaceous and arenaceous deposits accumulated on the stable shelf zone infreshwater to littoral facies. The sedimentation at the same time in the fore deep andmobile belt was marine, at least during the late Cretaceous (Alam 1989).

From the Paleocene to the early Eocene, the shelf was subjected to repeatedsubmergence and emergence marked by the Tura Sandstone (240 m). Extensive marinetransgression took place in the Middle Eocene and the hinge-line was initiated due to adeeply seated basement fault between the stable shelf to the north – west and ageosynclinal trough to the south – east (Raju 1968). The Nummulitic Sylhet Limestonewas deposited over most of the shelf area (about 245 m) in a shallow clear water and openmarine shelf environment in a warm climate.

During the Late Eocene Period, the Kopili Formation (238 m) consisting ofcarbonaceous pyritic shale and glauconitic sandstone (Ahmed & Zaher 1965) wasdeposited in a brackish to marine environment. The formation containsmicroforaminiferal assemblages of Globorotalia cocoensis biozone (Khan & Mominullah1980).

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 theshelf (Total thickness is 725 m) in a shallow marine and marine environment.

The upliftment of the Arakan – Yoma – Chin geanticline and basin – wide movementtook place in Early Oligocene. The sea regressed from the Shillong Plateau area andfluviomarine Barail sediments were deposited along the southern rim of the ShillongPlateau; at the same time the area extending from the SB to the Chittagong Hill Tractssubsided 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 from800 – 1000 m whereas on the shelf it is only 163 m and is represented by the BograFormation (Ahmed & Zaher 1965). The thickness of formations here is approximate andvaries considerably from well to well.

During the Miocene, a major uplift began in the Himalayas subjecting the BengalBasin to related tectonic movements (Fairbridge 1983). The deep basin featuredconspicuous 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 marineand continental environments, prograding to the southeast with depositional conditionschanging to marine (Alam 1989).

The SG has been divided into Bhubon and Boka Bil Formation in the geosynclinalfacies of Bangladesh. The Bhubon Formation was deposited in an environment rangingfrom a shallow inner neritic to a lower deltaic plain, and that for the BokaBil Formationwas in a range from marine at the bottom to transitional marine at the top. The top of theBokaBil 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 largedelta complex started to build on the northeast side of the Bengal Basin. Smaller deltaswere possibly building on the eastern side of the basin, presently occupied by Tertiaryhills of Chittagong and Chittagong Hill Tracts (Alam 1989).

During the Late Miocene Period, the Himalayan movements continued. The globaleustatic regression in this period produced an important unconformity that is also seen inseismic reflections recorded for offshore in the Bay of Bengal (Curray & Moore 1979).The DupiTila and Dihing Formations were deposited by Pliocene marine transgressionsand are represented by a thick sequence of about 2500m of fluvial and deltaic facies.TheQuaternary was marked by the usual glacio-eustatic oscillations, superimposed on ageneral regression that has left widespread traces in the Bengal Basin (Morgan &McIntire 1959, Mallick 1971, and Bakr 1977). The Quaternary is represented inBangladesh 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 -plainof Maiskhal Island (Mannan 1993). Formation Modhupur clay is mottled and red, whichis considered to be Pleistocene. It is followed by the Recent Alluvium consisting of loosegravel, sand, silt and clay with occasional pebbles and boulders. Peat deposits also occurlocally (Khan & Mominullah 1980). The thickness of Alluvium ranges from zero to sometens 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 anumber 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 etal. (1986), Alam (1989), Rahman et al. (1990). The overall structure and tectonics of theBengal Basin are briefly discussed below on the basis of the results of theseinvestigations. Fig. 3 shows the generalized tectonic map of Bangladesh and adjoiningareas.

The Bengal Basin is bordered on the north by the Pre-Cambrian Shillong Plateau andto the west by the Indian Platform. To the east rises the Arakan-Yoma-Naga foldedsystem, and to the south it plunges into the Bay of Bengal. The Bengal Basin is anexogeosyncline – that is, one in which thick detrital sediments within the craton werederived from uplift beyond the margin of the craton. The Bengal foredeep is a part of theexogeosyncline. The Bengal exogeosyncline is one of the worlds largest, and is part of theBengal Geosyncline. The latter includes the Bengal Basin and the Bay of Bengal (Alam1989).

The major structures described below are: 1) shelf zone, 2) hinge zone, 3) Bengalforedeep, 4) mobile belt, and 5) Sub-Himalayan Fore deep.

1) Shelf zone is a major tectonic element of Bangladesh lying in the western andnorthwestern portion of it. The margin has a northeast-southwest trend along which thebasement 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 isthe Bogra shelf. The Indian shield and Shillong massif are connected by the Rangpurplatform. The width of the platform is 100 km. Here, the slope is fairly smooth accordingto the seismic data. The sedimentary deposits of this area form monoclinal beds with dipsof 1–2°. Towards the northern portion of the platform the plunge of the basement is about3–4° and the depth of the basement is over 2000 m.

Southern slope of the Rangpur platform is gently plunging towards the southeast andextends to the Calcutta-Mymensing hinge zone. The thickness of sedimentary rocks isincreasing towards the southeast. The thickness of the sediments over the shelf is about8000 m and they are marked by several unconformities.

The basement complex near the western margin of the shelf is marked by a series ofburied ridges and normal gravity faults. The east-west trending Dauki fault separates thestable shelf and the Shillong massif (Fig. 3). The shelf experienced the first marinetransgression during the Late Cretaceous. The second major one was in the Miocenegenerated 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 northeastseems to be connected with the Dauki fault by a series of east-west trending faults. It isalso marked by deep basement faults probably started with the breakup ofGondowanaland. Parallel to the hinge zone is the Bengal foredeep, which consist ofseveral smaller troughs and structural highs.

3) The Bengal foredeep, which is a large elongated trough, occupies the vast areabetween the hinge-line and Arakan-Yoma-Naga folded system. This is the deeper part ofthe Bengal Basin where the basement is deeply subsided here and the subsidence isdirectly related with the uplift of Himalayas-Burmese mountain chain. It is about 450 kmwide in the south of Bangladesh and narrowing towards the northeast. The Basement isprobably 12–15 km deep. The folded belts of the Indo - Burman ranges mark the easternboundary of the Bengal foredeep. The total thickness of the sediments here is high whichexceeds 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 subdividedinto five sub -zones: 1) Faridpur trough, 2) Barisal gravity high, 3) Hatia trough, 4) Sylhettrough, and 5) South Shillong shelf zone.

4) Mobile belt: The eastern side of the Bengal Basin is bordered by a mobile beltknown 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 theChittagong 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 narrowfolds 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 likeforms, 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 - Gangeticgeosynclinal 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 tofine clastics that are derived directly from the Himalayan uplift and are essentially offluvial mollasse in character. The north margin of this fore deep is strongly folded andfaulted (Alam 1989).

4.3 Palaeogeography and Palaeotectonics

The paleogeography and paleotectonics of the Bengal Basin and surrounding areas havebeen 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), LeDain 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 thesereconstructions 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, theIndian plate migrated rapidly northward and collided with Asia between ca. 55 and 40Ma (Curray et al. 1983, Molnar 1984).

2. Sedimentation in the Bengal Basin has been controlled by the movement of the Indianplate, by the collision pattern of the Indian plate with the Burmese and Tibetan platesand 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 theCretaceous except for some localised discontinuities. The sedimentary sequencesoverlying the crystalline basement are invariably concealed under a thick cover ofaluminium 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-Sasymmetrical anticlinal structures of eastern Bangladesh. These structures form theattenuated 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 trendsin the northeastern portions of the Indian shield since the Cretaceous. The basin thusdifferentiated into several depressions and ridges, of which prominent were theJessore depression and the East Bengal Ridge. During the first phase of evolution thebasin remained under the influence of the marine transgression till the end of Eocenewhen a major regressive phase took over most of its part (Banerjee 1984). TheEocene period was the period of maximum marine transgression.

5. In the Oligocene, large-scale uplift and erosion resulted in the marine retreat andprogressive development of prograding deltaic conditions in the greater part of thebasin. By the Miocene period, the impact of the collision of the Indian plate with theTibetan and Burmese plates was severely felt in the basin, resulting in a large influxof clastic sediment both from the west and the east. The effect was a switch fromflysch sedimentation to molasse sedimentation in most of the basin (Alam 1989). Inthe deeper part of the basin, the sedimentation was controlled by turbidities. The protoBengal 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 theBengal fore deep. Since Miocene times, the proto Bengal fan has been deformed bythe compression of the Indian plate with the Burmese plate and was influenced by thetectonic activities of the Himalayas (Alam 1989).

6. During the Late Tertiary, the area experienced tectonic upheavals in the northeasternportion and the continuation of the fluvial delta complex co-existed in the lowerplains 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 distancetowards the north without disrupting surrounding crustal plates (Dietz & Holden1970).

8. Tectonic forces generated by the overridden Burmese plate from the east may havebeen responsible for east-west horizontal compression and differential verticalmovement of the basinal materials to develop the folded belt of Chittagong andChittagong 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 ofBangladesh. The basin is bounded on the north by the Shillong plateau, east and southeastby the Chittagong-Tripura fold belt of the Indo-Burman ranges, and west by The IndianShield platform. To the south and southwest it is open to the main part of the BengalBasin. 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 alsoreveals subsurface synclinal features and faults within the basin. Its topography ispredominantly flat with some north-south trending ridges of twenty to several hundredmeters 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 Basinranges 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). Thegeology 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 havebeen drilled in the SB with the discovery of eight gas fields and the recent discovery ofcommercial 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.

4.4.2 Regional geologic setting

The Surma Basin is believed to come into existence in the Late to Post-geosynclinalphase. 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 boundedin the north by the Pre-Cambrian Basement Complex (Wadia 1975) of the ShillongMassif and the Barail Ranges, by the Barail - Imphal Ridges in the east towards Assamand in the south by the Tripura High. In the west, The SB gradually ascends towards theEocene Hinge Zone, while passing into The Bengal Foredeep (Khan et al. 1988).

Well Name Location Drilled by Drilling time Total depthAtgram – IX 25°00´ 20´´N

92°24´ 30´´ EParker drilling Co. 25.06.1981 –

10.06.19824967,8m ( 16,288 ft )

Fenchuganj – 2 24°36´46´´ N91°57´23´´ E

PetroBangla* May 1980 3779m

Habiganj – 1 24°13´55´´ N91°22´47´´ E

PSOC** 24.03.1963 – 22.05.1963

3507,5m ( 11,500 ft )

Kailastila – 1 24°51´13´´ N 92°22´47´´ E

PSOC** 31.08.1961 – 22.03.1968

4140,9m ( 13,577 ft )

Patharia – 5 24°34´ N92°13´ E

PetroBangla* 1989– 3436m

Rashidpur – 1 24°18´32´´ N91°36´26´´ E

PSOC** 22.02.1960 – 20.7.1960

12,663ft

* PetroBangla= National Petroleum exploration company of Bangladesh ** PSOC=Pakistan Shell Oil Company

32

Fig.

5. S

how

ing

the

litho

logy

of t

he w

ells

with

the

sam

ple

loca

tions

.

33

The SB is a sub-basin of the Bengal Basin, the development of which began in theEarly 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 migratedrapidly 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 TheOligocene epoch. Today, the onshore part of the Bengal Basin is the site of the world'slargest delta (about 60 000 km2) formed by rivers (Ganges, Brahmaputra/Jamuna, Padma,Meghna) that drain a large portion of the Himalayas (Johnson & Alam 1991). Thissubaerial delta feeds the world's largest submarine fan (Bengal Fan), which extends morethan 3 000 km south into the Bay of Bengal (Curray & Moore 1974). The Bengal Basingradually is being encroached on by the Indo-Burman ranges, an ~ 230-km-wide, activeorogeine belt associated with eastward subduction of The Indian plate below Myanmar(Burma) (Brunnschweiler 1966, LeDain et al. 1984, Sengupta et al. 1990). In The EarlyMiocene, as the collision between the Indian and the Eurasian plates continued, therewere further major phases of uplift in the Himalayas. Consequently, a large volume ofclastic sediments was supplied to and began progressively to fill the Basin (Imam & Shaw1985).

SB is characterized by a large, closed, negative gravity anomaly (as low as 84milligals), has minimal topography (elevations of about 5 to 20 m) and numerous lakesand swamps, and is actively subsiding (Johnson & Alam 1991). On the basis of seismicdata, 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 majortectonic movements, i.e. the emerging of the Shillong Massif in the north and the westprograding mobile Indo-Burman Fold Belt (Hiller & Elahi 1984).

The northern and eastern parts of the basin are far more complicated than the southernand western portions. The relief and complexicity increases towards the east (Haque1982). The anticlines are commonly faulted and many produce gas (Johnson & Alam1991). Structural relief between paired anticlinal crests and adjacent synclinal troughsmay be as much as 7 000 m (Hiller & Elahi 1984), and the synclines have acted as majorlate Neogene and Quaternary depocenters. The folds decrease in amplitude westward, andare not present west of about 91° (Lietz & Kabir 1982), where the Sylhet trough mergeswith the main part of the Bengal Basin.

The SG (Early Miocene - Quaternary) is a diachronous unit consisting of a successionof alternating shales, sandstone, siltstones and sandy shales with occasional thinconglomerates, indicative of repetitive deposition from pro-delta, deltafront, and paralicfacies with intermittent, wholly marine facies (Holtrop & Keizer 1970).

The group is divided into the Bhuban and the Bokabil Formations, based ondifferences in their gross lithologies (Mathur & Evans 1964) (Table 3).

34

Table 5. A chart showing the stratigraphy of the Surma Basin (SB) and surrounding areasincluding 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 isnot 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 usingconventional method for macrelation of spore/pollen from sediments. The subsequentacetolization has been carried out with the methods of Erdtman (1960). The samples weretreated with 10 % HCl acid to dissolve carbonates and later with 10 % KOH to separatehumas materials from taken sediment of 5–10 grams. After neutralization the residue wastreated with concentrated HF acid to remove silicates. Palynomorphs were collected byfiltration using 200 mm, 33 mm accordingly and in need, 10 mm polypropylene sieveswere also used. After dehydrating, the residue was acetolized again. The residue wasmainly preserved in glycerine jelly and glass slides were prepared on glycerine gelatinefor 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 were168 in number). The details of the samples are presented in the appendix-1 and 2. Thepresent study includes major, trace and total REE (rare earth element) analysis of SGsediments (mainly shale) from the SB, Sylhet, Bangladesh.

Sample preparation. The core samples were cut into pieces by using a rock cutter andthen were crushed by a crushing machine to reduce the rock aggregate to monomineralicparticles. These samples were divided into seven parts by a mechanical divider. One partwas crushed by Vibrating Disk Mills (Herzog, type: Hsm 100 A) and six parts weresieved to get size fraction <0.06 mm and in some cases <0.125 mm. Both crushed andsieved fractions were analyzed. For clay mineralogical studies, clay size fractions were

36

separated by centrifugation. Thin sections were prepared both from original core andmixed 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 withstandard 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 andtwo 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 nosolution left. Finally, diluted fraction of the deposit was analysed for trace elements usinga spectrum of AA300 with an acetylene-air flame. Organic maturity (OM) rich elementsCo, 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 twohours at 950°C and LOI was determined.

5.2.4 Accuracy of analyses

The accuracy determinations by both AAS and XRF was checked using certifiedreference materials.

A correlation between the data of Cu, Zn, Ni and Pb (Patharia well-5 and Rashidpurwell-1) measured by AAS and XRF was done and presented in figure 6. They are all inpositive correlation. For the well, Rashidpur-1 three elements (Zn, Cr & Ni) are correlatedstrongly and Pb shows a positive which indicate that two results (AAS & XRF) are ingood agreement. The error was found to be 3–4%. Nesbitt (1992) mentioned that for XRFanalyses, 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 forminor 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.

Fig. 6. A. Figure showing the correlation of the data (in ppm) of Cu, Zn, Ni and Pb for Pathariawell – 5. All of them are in good positive correlation. B. Figure showing the correlation of thedata of Cu, Zn, Ni and Pb for Rashidpur well-1. Three of them (Zn, Cu & Ni) are correlatedstrongly in positive and Pb shows also positive correlation means two results are in goodagreement.

Cu ( AAS)

70605040302010

Cu

( XR

F )

70

60

50

40

30

20

10

Zn ( AAS )

14012010080604020

Zn (

XRF

)

140

120

100

80

60

40

20

Ni ( AAS )

12010080604020

Ni (

XR

F )

120

100

80

60

40

20

Pb ( AAS )

403020100

Pb (

XRF

)

50

40

30

20

10

Zn ( XRF)

11010090807060

Zn (

AAS)

120

110

100

90

80

70

60

Cu ( XRF)

50403020100

Cu

( AAS

)

50

40

30

20

Ni ( XRF)

90807060504030

Ni (

AAS

)

100

90

80

70

60

50

40

Pb ( XRF)

302010

Pb (

AAS)

50

40

30

20

A

B

ppm

ppm

ppm ppm

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 diffractometerwith 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 dwelltime of 1s per step. The clay fraction (<2 µm) was separated out from the shale bydisaggregating and despersing the sample in distilled water and immediately washed bycentifugation. The fraction of <2 µm was isolated by centrifugation and suspension wasdried on glass slides. The clay sample in oriented mounts were run under three separateconditions:

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 withJEOL, JEM-100 CX fitted with link AN10-25S and a Jeol JEM 1200 Leo 912 OMEGAequipped with the same microanalysis electron microscope. Various magnifications wereused to obtain suitable micrographs of clay minerals. Electron microscopy is the onlymethod capable of measuring the size of individual single particles. The method allowsfor direct measurement of the several dimensions of the particles and thus also the shapeof 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 SEMcoating unit E 150) were examined morphologically under JEOL JSM 6400 (with LinkEXL, EDS) Scanning Electron Microscopy (SEM). The thin sections of representativesamples were analysed for semiquantitative determination of primary and secondaryminerals especially for Ba detection.

5.3.4 Petrographic microscopy (optical)

Thin sections were made both from the core samples for mineralogical analyses bypetrographic microscope.

39

5.4 Statistical analysis and ratios

Data acquired from XRF and AAS was used for the analyses of major oxides, ΣREE andcertain 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 wheneverpossible.

Table 6. Statistical analysis and ratios.

Subject Ratio / Crossplots ReferenceGrain size SiO2/ Al2O3 Björlykke 1974Maturity (M) 1) K2O+Al2O3/

Na2O + MgOBjörlykke 1974

2) K2O/ Na2O; Rb/ K2O

Björlykke 1974Wedephol 1969

4) Cu/ Zn Hallsberg 1976Major element % Major element Harker typeΣREE diagram ΣREE Bhatia 1985Provenance 1) Cr/ Rb, Zr/ Rb, V/ Rb and

Ba/Rb; 2) Cr/ NiBellanca et al. 1999, Garver & Scott 1995

Environment of deposition Cr/ V and Ni Dypvik 1979Tectonic setting SiO2 vs. K2O/ Na2O Roser & Krosch 1986Mineralogy(Weathering & Diagenesis)

1) Crossplots of Cr, Rb, Zr, K2O% and TiO2 versus Al2O3

Grömet et al. 1984

2) Crossplots of Cr+Ni, V, MnO% and Fe2O3 versus MgO

McLennan et al. 1983

3) Crossplots of U, Th, Ba, Al2O3 versus K2O%

McLennan et al. 1983

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 thebasis of core sample studies (n=188) and of palynological studies (74) of six exploratorywells: 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 areprovided 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 analternation of sandstone, shale and siltstone that infilled the vast basinal area of theBengal Basin during Miocene-Pliocene time. In the subsurface, the unit is represented bythick sand-shale sequences in all the wells drilled in the area. The Surma Groupunconformably overlies the Barail Group of the Oligocene age and is overlain by thesandstone 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 basedon gross lithology.

6.1 Lithofacies

Two major lithofacies were identified in the SG unit: sandstone lithofacies A andcombined facies B consisting of claystone, mudstone and shale. Facies B is the mostabundant, whereas facies A is less common. These lithofacies generally are defined onthe basis of grain size, clay content and depositional bedding characteristics. Facies Aconsist of massive, thinly inter-bedded and inter-laminated, fine to medium-grainedsandstone.

Facies B consists of laminated bluish, bluish gray and gray to black shale from gray toyellowish-gray siltstone to very fine grained sandstone. Lithofacies A may gradevertically 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 ofthe samples are given in appendix 1 and 2 (* marked). The samples were selected fromall the cores of the wells representing the various lithostratigraphic positions. Fromindividual cores, the samples were selected so that the top, bottom and the middle of thecores are represented. The study was aimed to establish the age of the SG,palynostratigraphy in order to better constrain reconstruction of palaeoenvironmental andpalaeoclimatic 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 theconsiderable amount of data shown in a typical pollen diagram it is necessary to dividethe diagram into pollen-stratigraphic units characterized by distinctive groups of pollentypes. In this way, pollen zones were constructed from the samples of Fenchuganj-2 andare presented in Figs 7 and 8.

Detailed palynological studies of this area is lacking because of poor preservation ofthe palynomorphs. Palynomorph recovery from the cores of the wells drilled in the areawas few. In some parts the wells were almost void of specimen. Palynomorph content inthe prepared palynological slides for palynostratigraphic analysis were extremely poor forthe present study. No palynomorphs (other than fungal spores) were detected in 38 out ofthe 74 slides. Due to this, the idea of a quantitative analysis of palynomorphs wasabandoned except for the Fenchuganj well – 2. The results of this well allowed us to makesome quantitative analysis and a qualitative analysis was attempted for the remainingwells. No palynological research paper has been published from this region yet.Identification and comparison were made for the present study on the basis of publishedliterature on the Tertiary Palynology of the Assam and the Bengal Basin of India (Table 1and Table 2).

6.4 Palynostratigraphic zonation

Among the wells studied, palynomorph recovery was predominantly good only for theFenchuganj 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 wellfor palynological study. The depth of the samples ranged from 957.6 m to 4095 m. Thepalynoassemblages from sediments recovered from SG (Miocene) are rich inpteridophytic spores and angiospermous pollen grains whereas the gymnospermouspollen grains and fungal remains are comparatively less represented. The assemblageconsists of 63 genera and 95 species of palynomorphs. The SG sediments dominantlyconsist of shale with some sandstones.

On the basis of the qualitative and quantitative analyses of the palynoflora, the SGsequence of this well has been divided into three biostratigraphic zonations. Thefollowing parameters have been taken into consideration to establish and recognize thesezones: a) Maximum development of various palynotaxa, b) the first and last appearanceof 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 depositionand age.

6.4.1 Palynostratigraphic zonation of Fenchuganj well – 2

The three local palynostratigraphic zones in the SG sediments sequence of Fenchuganjwell – 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, Inapeturatereticulate, Retipilonapites, Cl-grain 1a, Diporopollenites, Polycolpites sp, Monaporitesanulatus, Polyporina, Chamopodiacea, Tricolporates (C3P3)-Pet 2a andFusiformisporites.

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, DicolpitesTricolpats 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, P3grain 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 constitutes31 % of the sequence. The dominance of this taxon, is, therefore, important and helps usin distinguishing from the overlying Tricolpate-trilete zone.

Pteridophytic spores contains 48 % in this zone. Angiouspermous pollen grainsrepresent 50% while gymnospermous pollen grains are insignificantly represented by 3.8%. Among the pteridophytic spores, triletes are very abundant. Trilete spores arerepresented by Cicatricosisporites macrocostatus, Trilete levigates, Trilete reticulate,

43

Trilete granulate and Trilete verrulate. Monolete forms are mainly LaevigatosporitesType-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. andEximispera.

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-Trileteswhich is about 50% of the palynofossils. For this reason the zone is named after thesetaxon. Graminitides is conspicuous by its complete absence in this zone.

Gymnospermous pollen grains are about 4%, the presence of Rhizopora pollen isimportant for paleoenvironmental interpretation.

iii) Disaccate zone. This zone shows a clear dominance of Disaccate pollen. Ninetyeight 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, Lycopodiumsporites, 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 usuallyrepresented by the monolete forms such as Levigatosporites, and Verrucatosporites, aswell as by a few trilete spores like Cyathidites minor. Monocolpate pollen occur in lowfrequencies and are represented by the genus palmepollenites.

The microfloral association of the Palynological zone I can be compared with thepalynological assemblage of the Simsang Palynological Zone IV of Meghalaya, India(Baksi 1965) and the Bengal Palynological Zone (BPZ) V (Baksi 1971) and indicateLower to Middle Miocene age. The presence of Rhizophora pollen and the presence ofdynoflagillates provide a basis to interpret the paleoenvironment of the drilled sequenceas brackish to shallow marine deposits. Representative forms of Foraminifera alsoindicate that sediments may be deposited in shallow marine condition.

44

The microflora of the Palynological zone II can also be compared with those of theSimsang Palynological Zone IV of Meghalaya, India and BPZ V (Baksi 1971). Based onthese comparisons the Palynological Zone II is presumed to be of Middle to UpperMiocene. It is interesting that the floral change from monolet pollen Palmepollines toTricolpate – trilete could be recognized well by an increase in frequency by 50%. Thepresence of mangrove pollen Rhizophora indicates the brackish environmental depositionof this drilled sequence.

The microfloral assemblage of the Palynological Zone III may be compared withSimsang Palynological Zone IV of Meghalaya, India (Baksi 1965) and BPZ Zone V ofBengal (Baksi 1971). According to these comparisons, the age of Palynological Zone-IIIis presumed to be Upper Miocene.

A significant floral change has been observed in this zone by the abundance ofdissacate pollen and a decline in the pteridophytic spores, particularly triletes. Thesepollens might have migrated from the extra-peninsular region in the north, which wouldhave 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 therecovery 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 thearea. Some dinoflagillete cysts were also identified in the sequence. The age of thesequence has been identified as Miocene Foraminifera with Hystrichospheridiumindicates a marine- brackish environment. Hystrichospherida is a widely distributed faciesindicator. The presence of reworked microfauna elements and conifer pollens, togetherwith the transgression indicators, indicate an increased tectonic activity in the SurmaBasin during this period and they are related to the burial of the basin and the uplift of theHimalayas.

6.4.3 Habiganj well – 1

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

45

Tricolporopollenites, Varrucosisporites sp., and Polypodiisporites. The samples wereidentified 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 butonly four samples were productive. The following palynotaxa were identified: Disaccatepollen, 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 asPliocene 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 thebasis of 12 core samples ranging from 956,1 m to 2 833 m taken from 5 cores. Summaryof 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, andFlorschuetzia.

Forms of Foraminifera that were encountered in the core no. 3 and 4 are: Globogerinebulloids, 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 wereproductive out of 10 samples ranging from 1 081 m to 2 477 m. The palynomorphsencountered 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 thepalynological assemblage of the Simsang Palynological Zone IV of Meghalaya, India(Baksi 1965) and the Bengal Palynological Zone (BPZ) V (Baksi 1971) and indicate aLower to Middle Miocene age. The presence of Rhizophora pollen and the presence ofdynoflagillates provides a basis for interpreting the palaeoenvironment of the drilledsequence as brackish to shallow marine deposits. Representative forms of Foraminiferaalso 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 theSimsang Palynological Zone IV of Meghalaya, India and BPZ V (Baksi 1971). Based onthese comparisons, the Palynological Zone II is presumed to be of Middle to UpperMiocene. It is interesting that the floral change from monocolpate pollen Palmepollines toTricolpate - trilete could be recognized well by an increase in frequency by 50 %. Thepresence of mangrove pollen Rhizophora indicates the brackish environmental depositionof this drilled sequence.

The microfloral assemblage of the Palynological Zone III may be compared with theSimsang Palynological Zone IV of Meghalaya, India (Baksi 1965) and BPZ Zone V ofBengal (Baksi 1971). According to these comparisons, the age of Palynological Zone-IIIis presumed to be Upper Miocene.

A significant floral change has been observed in this zone by the abundance ofdissacate pollen and a decline in the pteridophytic spores, particularly triletes. Thesepollens might have migrated from the extra-peninsular region in the north which wouldhave 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 ShillongPlateau, Assam, now Meghalaya. That assemblage contained a few marinehystrichosphaerids and microforaminifers in the lowermost part. Marine micro-organismdisappeared in the middle part of the zone. There are some common elements betweenthe Miocene assemblages of Simsang Palynological Zone IV and the well studied. Theabundance of bisaccate coniferous pollen belonging to Coniferipites Baksi and arestricted occurrence of Coniferipites chattacharai are the characteristic features ofSimsang Zone IV assemblage. In the present study, bisaccates occur sporadically in theMiocene assemblages but Podocarpidites pollen resembling Conferipites chattacharai isrestricted to the Miocene, as it is in Simsang Zone IV assemblage.

Also, Baksi’s Schizaeaceaesporites and Parkeriaceaesporites appear to be equivalentto Magnastriatites howardi, but they are more abundant in the Simsang Zone IV (Sah andDutta 1967), as they are in the Miocene of the studied well.

47

In addition, the long-range taxa Spinosopites acolporata, and Polypodiaceaesporitessp. of Baksi (= Verrucatosporites usmensis) and various palm pollen occur in bothassemblages. Sah & Dutta (1967) described the stratigraphic succession of the Tertiarypalynomorphs in Assam. They reported high abundances of Cicatricosisporitesmacrocostatus (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 indicatea major shift of flora elements due to well recognized tectonic events related to theHimalayan orogeny, involving the uplift of the South Shillong Front and the associateddevelopment of progressively colder climates in the surrounding areas of the BengalBasin (including the Surma Basin). Baksi also states that the Simsang Palynological ZoneIV can be ”confidently” correlated with the Bengal Palynological Zone V by the indexelements 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 significantresemblance to the Simsang Zone IV. Other resemblances are the presence ofSchizaeaceaesporites sp. of Baksi, 1962 (= Magnastriatites howardi), small tricolporatepollen, Parkeriaceaesporites of Baksi, 1962 (= Verrucatosporites usmensis).

Comment. Palynomorph recovery was very poor. Only Fenchuganj well-2 slides allowone to make quantitative analysis.

Table 7. Summ

ary of palynostratigraphy of Patharia well – 5.

Core

No.

Depth

intervalLithology

Palyno-Assem

plage of Spore/PollenD

epositional environm

entA

geC

omm

ents

1950.10 m

to 964.1 m

Mainly consist of fine clstic

sediments, silty shale occur in

partings

Discrete pollen, pteridophytes and M

onolet laevigatosporites, verrucat-osporities, Trilete spore. A

ngiosperm pollen are Tetracolporites, A

lni-pollenites, Triporopollenites and Sim

sangia also present. Only one

slide contains Meyeripollis naharkotensis (m

arker of Oligocene) as

reworked form

.

Brackish

Lower

Miocene

Correlatable to Fenchuganj

well-2 at interval of 4200 m

to 4400 m

21450.5 m

to 1457.3 m

Mainly of fine clastic sedi-

ment, m

edium to fine grain

sand and alternation of shale/silty shale in the upper part of the core w

ith some conglom

-erate. Pebbles of shale and silty shale.

Disaccate gym

nosperm pollen. A

lso found Cyathidites m

inor, Verrutri-letes, C

icatricosisporites macrocostatus, Polypodiaceoisporites as

major constituent and M

onolet Laevigatosporites, Monocolpate, Tri-

colpate and Palmaepollenites, Striatopollis bellus, Syncolpate pollen,

Undulatisporites, C

orrugatesporites, Couperispollis brevispinosus have

been identified in these cores. Meyeripollis naharkotensis of O

ligocene found as rew

orked form.

Brackish

Lower

Miocene


well-2 at depth of 4300.0 m

to 4500.0 m

31829.0 m

to 1837.0 m

Dom

inantly shale and silty shale

Polypodiaceoisporites, Polypodiosporites, Verrucatosporites, and some

reworked D

isaccate, gymnosperm

pollen. M

eyeripollis naharkotensis is absent in this core.

Shallow m

arineLow

er M

iocene

42299.5 m

to 2307.25m

Mainly shale dark grey col-

our, hard and compact. Top

part of the core contais light grey to clean sand, m

edium to

fine grain and unconsoli-dated.

Verrutriletes, Cicatricosisporites, Polypodisporites, Laevigatosporites,

Monocoplate and Palm

aepollenites are found in low frequencies. M

ey-eripollis naharkotensis in this core.

Shallow m

arine to brackish

Lower

Miocene



to 4500 m

52828 m

to 2833 m

Mainly blackish shale hard

and compact.

Microflora are very low

in frequency mainly are of Triporate, Trilet

Laevigate, Polypodiisporites, Verrutriletes, Palmaepollenites, Verrucat-

osporites, Cicatricosisporites nacrocostatus, Tricolporites, C

yathisites m

inor, Couperipollis rarispinous, Sim

sangia also present.M

eyeripollis nahkotensis ( marker of O

ligocene) and Disaccate gym

-nosperm

pollen found as reworked form

in one slide only.

Brackish

Lower

Miocene



to 4500 m. Som

e older than O

ligocene palynomorph assem

-blage (Spore/pollen) also encountered. M

ost of them

were very badly preserved and

mostly rew

orked. For this reason proper iden-tification could not be done.

48

49

6.5 Palaeoenvironment and Palaeoclimate

Reconstruction of palaeoenvironment and palaeoclimatic variation in the Surma Basin ofBangladesh during the Neogene is based on complex stratigraphic sequences whichinclude a variety of sediment facies and a variety of palynomorphs, indicating a range ofdepositional environments. The most frequent and qualitatively most important of theseare illustrated in Figs 7 and 8.

The most gross environments of deposition for the SG sediments are shallow marine tobrackish interdeltaic environments. This interpretation is based on the predominance oflithofacies B and the distribution of pollen, spores, microplankton and dinoflagellets.

The pollen flora with dominating angiosperm species suggest tropical to subtropicalareas. The occurrence of aquatic ferns (Cicatricosisporites) indicate brackish conditionsof sedimentation. The proximity of the sea is indicated by the presence of marinemicroplankton. Recording of microfauna such as Bolivia, Bulimina, Globigerina andRotalia from Rashidpur well (Holtrop & Keizer 1969) is indicative of more open marinesedimentation. Foraminifera with Hystrichosphaeridium indicates a marine-brackishenvironment. Hystrichosphaerida is a widely-distributed facies indicator. The presence ofreworked microflora elements and conifer pollens, together with the transgressionindicators, indicates an increased tectonic activity in the SB during the Neogene and theyare related to the Himalayas. The SB area were progressively coming under the tectoniccontrol to the great Himalayan orogeny and the crustal shortening due to the collision ofthe Indian and Asian plates and have resulted in extensive uplifts and thrusting of theolder rocks (Banerjee 1984).

Throughout the Miocene, SB has witnessed a conspicuous subsidence and marinetransgression. The transgression of the Miocene certainly affected the coastline. Theabove mentioned palaeo-environmental circumstances must at some stage havecontributed to the retrogressive succession of the vegetation in the area. In thisconnection, it is pertinent to refer to some ecological aspects of palms, as they feature soprominently in the pollen sequence. At present, palms are considered to be very importantin 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 floraduring the time of deposition of these sediments. The presence of Foraminifera inassociation with a palynomorph assemblage containing mangrove forms anddinoflagellate cysts suggest a shallow marine environment. The sandstones with siltstoneand shales were deposited in a shallow marine environment throughout the Miocene. Theshallow marine and brackish environment reflects a marine transgression.

The pollen spectra of SB contain a high percentage of regionally produced pollen ofmixed sub - tropical vegetation. It reflects a mixture of palm and coniferous forest inwhich palmepollinite is dominant. Palynostratigraphic zones suggest a progressiveimproverishment of the mixed type of subtropical to tropical forest types existing thereand 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 experiencingsummer rainfall similar to the present day climate.

50

As far as known, the Rhizophora pollen type is unique and cannot be confused withpollen from other taxa (Germeraad et al. 1968, Muller 1964). The presence of Rhizophorain the Miocene sequence suggest the Neogene sediments of marine origin.

The Surma Basin has undergone two successive phases of evolution. The marinetransgressive phase, followed by a regressive phase resulting in a series of continentalfluvio-deltaic to marginal marine sedimentation during the Neogene. The GreatHimalayan Orogeny and associated tectonics dominated the SB during Miocene-Pliocenetimes.

Major changes in sea level for Neogene are suggested based upon trangressive-regressive phenomena. A marked rise in sea level would have caused a marinetransgression (Bandy 1968). The proximity of the sea is indicated by the presence ofmarine microplankton. The overall evidence thus suggests that vegetation wasestablishing on a marshy area close to the sea. It also suggests a subtropical humidclimate (Srivastava 1970). Similar marine deposition and its environment were alsoidentified for West Bengal, Meghalaya and Tripura geo-provinces of India (Banerjee1984).

6.6 Age

Studies of pollen samples from the SG sediments of Surma Basin have indicated aNeogene age for the unit. On the basis of a regional study of dinoflagellates, SG can beassigned as early to late Miocene in age. This conclusion is based upon the followingreasons:1. The stratigraphic position of the SG unit2. 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 tothe 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 alterationindex which attain during and after sedimentation. Fenchuganj well-2 samples weremarked pale-yellow to brownish colour of polynomorphs indicating the mature stagefrom the depth below 3615 m (Core No. 10). From core 3 to 9, low maturity wasassigned. According to the maturity study, the sediments within the depth range of 3615 –4095m are to be considered as an organically matured stage.

51

Fig.

7. P

olle

m d

iagr

am sh

owin

g %

var

iatio

ns o

f mai

n ta

xa fr

om F

ench

ugan

j wel

l – 2

of S

urm

a B

asin

, Ban

glad

esh.

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 affinity1. Cyathidites minor Cyatheaceae2. Lycopodium sporites Lycopodiaceae3. Sphagnum Sphagnaceae4. Eximispra Unknown5. Cicatricosisporites Schizaeacea6. Undulatis porites Unknown7. Dandotriasporites Unknown

53

8. Scizaoisporite Schizaeceae9. Polypodiaceoisporite Polypodiaceae

10. Lygodiumsporites Schizaeceae11. Trilete(T)-L1a Sphagnum12. T – reticulate Sphagnum13. T – granulate 1a Sphagnum14. T – Verrulate 1a Unknown15. T – rugosa 1a Unknown16. T – Cing 1a Unknown17. T – fovulate 1a Unknown18. T – Sig + rug 1a Unknown19. T – aperturate Unknown20. Leavigatosporites 1a Polypodiaceae 21. Polypodiisporites sp. Polypodiaceae22. Verrucatosporites Polypodiaceae23. Emparitus Unknown24. Polypodiisporites Oligocenicus Polypodiaceae25. Monolites Mawkmaensis Polypodiaceae26. Monolites reticulate 1a Polypodiaceae27. Texodium Unknown28. Disaccate Podocarpidites29. Tsuga Pinaceae30. Disaccate striat Podocarpidites31. Inaperturate L1a Unknown32. Inaperturate – reticulate Unknown33. Retipilonapites Unknown34. Palmaepollenites Arecaceae35. Couperipollis sp. Palmae36. Monocolpate (C1) – grain 1a Unknown37. C1 – Echniiz 1a Unknown38. C1 – L2a Unknown39. Dicolpatis sp. Unknown40. Liliacidites Liliaceae41. Tricolpate (C3) – L1a Acer42. C3 – L2a Acer43. C3 – reticulate 1a Unknown44. C3 – reticulate 2a Unknown45. C3 – grain 1a Unknown46. C3 – Verrulate Unknown47. C3 – regose 1a Unknown48. C3 – scab 2a Unknown49. C3 – apl 1a Unknown50. C3 – favulate 1a Unknown51. Diporopollenites Unknown52. Striatopollis Unknown53. Marginipollis Unknown

54

54. Meyeripollis naharilotensis Unknown55. Polycolpites sp. Lamiaceae56. Tetracolpate (C4) – L1a Unknown57. Monoporites anulatus Unknown58. Carya Caryaceae59. Carpinus Corylaceae60. Betula Betulaceae61. Triporopollenites Moraceae62. Triporate (P3) – L1a Ovoidites63. P3 – L2a Ovoidites64. P3 – reticulate 1a Ovoidites65. P3 – ret 2a Ovoidites66. P3 – grain 1a Unknown67. P3 – scab 1a Unknown68. Periporate – reticulate 2a Unknown69. Polyporina Malvaceae70. Periporate – L1a Unknown71. Chanopodiaeea Unknown72. Polygonacidites sp. Dorseraceae73. Florsechetzia meridonalis Unknown74. Florsechetzia Levipol Unknown75. Florsechetzia trilobata Unknown76. Rhizophora Rhizophoraceae77. Illex pollenites Aquifoliaceae78. Rhoipites niditus Unknown79. Nyssa pollenites Nyssa80. Tricolporate (C3P3, sp.) – L1a Quercus81. C3P3 – L2a Quercus82. C3P3 – Pet 1a Unknown83. C3P3 – Pet 2a Unknown84. C3P3 – Fovulate 1a Unknown85. C3P3 – Scab 1a Unknown86. Tetracoliporites similes Unknown87. Tetracolporates (C4P4) – L1a Unknown88. Syncolporate sp. Unknown89. Tetred Unknown90. Alnipollenites Betulaceae91. Hystrioispheridum Algae92. Rusiformisporite Fungal spore93. Fungus Fungi94. Dinoflagellates95. Foraminifera

55

Plate 1. (All figures X500 magnification).

Palmaepollenites Polypodissporities Polypodiaceoisporite

Verrucatosporites Verrucatosporites Dissacate alnipollenite

Dissacate pollen Monocolpate pollen Cicatricosisporite

Verrucatosporite Verrutrilete spore Palmepollenite

56


Verrutrilet Tricolporate reticulate Tricolpate verrucates

Tricolporate Tricolporate Tricolporate

Tricolporate levigate Mayeripollis naharkotensis Mayeripollis

Trilete spore Dicolpate pollen Palmaepollenites

57


Dynoflagelates Cicatricosisporites

Polypodiacedisporites Hystrichospheridium

Verrucatosporites Grami Pollenites

58


MeyeripollisPalmepollenite pollen Disaccate pollen naharkotensis

Cicatricosisporites Marco Hystrichospheridium Trilete Granulatecocostatus

Trilete spore Hystrichospheridium Florsechetziatrilobata

Alnipollenite Fungal pore Disaccate pollen Couperipollis spinson (Monocolpate)

7 Geochemical results

7.1 Major elements (XRF & AAS)

Major, trace and REE geochemistry of the core samples of SG sediments of the SB werestudied 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 graphical-ly (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 iscompared to the composites (Average shale, Pettijohn 1975, The North American ShaleComposite, 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 Table8. 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 composi-tions. The ratio of SiO2/Al2O3 is centered within the range established for shales.

Table 8. Comparison of Chemical composition of shales.Wt% NASC*

(Grömet et al. 1984)

AGV-1(Flanagan 1973)

Bhuban Shale**(R. Islam 1996)

Average Shale (Pettijohn 1975)

Present studyHabiganj well-1(A. Mannan 2002)

SiO2 64.82 60.41 68.65 58.10 61.63TiO2 6.80 1.06 0.74 - 0.87Al2O3 17.05 17.66 14.96 15.40 16.34Fe2O 5.70 6.26 4.55 4.02 6.95MnO 0.25 0.10 0.03 - 0.09MgO 2.83 1.57 1.03 2.44 3.01CaO 3.51 5.02 0.05 3.11 1.93Na2O 1.31 4.36 0.77 1.30 1.57K2O 3.97 2.93 2.91 3.24 3.25P2O5 0.15 0.50 0.09 - 0.16* NASC = North American Shale Composite. ** Bhuban Shale = Sylhet, Bangladesh. These chemical results sug-gest that the present study data has the qualities of an average of averages.

60

Na 2

O (w

t %)

1,4

1,2

1,0

,8,6

,4

Depth(m) 36

33

3634

3635

3636

3637

3638

3639

3640

4729

4733

4735

Depthm 36

33

3634

3635

3636

3637

3638

3639

3640

4729

4733

4735

CaO

(wt %

)5

43

21

0 Depth(m) 36

33

3634

3635

3636

3637

3638

3639

3640

4729

4733

4735

K2O

(wt %

)4,

03,

53,

02,

52,

01,

51,

0,5

Fe2O

3 (w

t %)

87

65

43

2

Depthm 36

33

3634

3635

3636

3637

3638

3639

3640

4729

4733

4735

Al 2O

3 (w

t %)

2018

1614

1210

86

4

Depthm 36

33

3634

3635

3636

3637

3638

3639

3640

4729

4733

4735

MnO

(wt %

),2

,10,

0

Depthm 36

33

3634

3635

3636

3637

3638

3639

3640

4729

4733

4735

SiO

2 (w

t %)

100

9080

7060

5040

Depthm 36

33

3634

3635

3636

3637

3638

3639

3640

4729

4733

4735

MgO

(wt %

)

3,5

3,2,

52,

01,

51,

0,5

0,0

Depthm 36

33

3634

3635

3636

3637

3638

3639

3640

4729

4733

4735

Fig

9. M

ajor

ele

men

t var

iatio

n cu

rves

for

the

wel

l Atg

ram

– IX

. For

det

ails

of s

ampl

e in

terv

al se

e A

ppen

dix

– 1

and

2. D

ash

lines

indi

cate

the

brea

k of

sam

ple

inte

rval

s.

61

SiO

2

(wt %

)80

7060

5040

Depth(m) 21

90-

2200

3137

-

3141

3259

-

3265

-

3268

3624

-

3730

3742

-

3766

4086

-40

94

CaO

(wt%

)16

1412

108

64

20

Depth(m) 21

90-

2200

3137

-

3141

3259

-

3265

-

3268

3624

-

3730

3742

-

3766

4086

-40

94

K2O

(wt%

)4,

03,

53,

02,

52,

01,

5

Depth(m) 21

90-

2200

3137

-

3141

3259

-

3265

-

3268

3624

-

3730

3742

-

3766

4086

-40

94

Fe2O

3 (w

t %)

98

76

54

Depth(m) 21

90-

2200

3137

-

3141

3259

-

3265

-

3268

3624

-

3730

3742

-

3766

4086

-40

94

MgO

(wt %

)7

65

43

21

Depth(m) 21

90-

2200

3137

-

3141

3259

-

3265

-

3268

3624

-

3730

3742

-

3766

4086

-40

94

Na 2

O (w

t %)

1,7

1,6

1,5

1,4

1,3

1,2

1,1

Depth(m) 21

90-

2200

3137

-

3141

3259

-

3265

-

3268

3624

-

3730

3742

-

3766

4086

-40

94

Al 2O

3 (w

t %)

2018

1614

1210

8

Depth(m) 21

90-

2200

3137

-

3141

3259

-

3265

-

3268

3624

-

3730

3742

-

3766

4086

-40

94

MnO

(wt %

),8

,7,6

,5,4

,3,2

,10,

0

Depth(m) 21

90-

2200

3137

-

3141

3259

-

3265

-

3268

3624

-

3730

3742

-

3766

4086

-40

94

Fig

10. M

ajor

elem

ent

vari

atio

n cu

rves

for t

he w

ell F

ench

ugan

j – 2

. See

App

endi

x -1

and

2 fo

r det

ails

of t

he sa

mpl

e. D

ash

lines

indi

cate

the

brea

k of

sam

ple

inte

rval

s.

62

Depth(m)

Al 2O

3 (w

t %)

2018

1614

1210

812

55-

1256

1257

-

1258

1259

1849

-18

50

1851

-

1852

3110

-

3111

CaO

/wt %

)16

141

108

64

20

Depth(m)

1255

-

1256

1257

-

1258

1259

1849

-18

50

1851

-

1852

3110

-

3111

Fe2O

3 (w

t %)

8,5

8,0

7,5

7,0

6,5

6,0

5,5

5,0

4,5

Depth(m)

1255

-

1256

1257

-

1258

1259

1849

-18

50

1851

-

1852

3110

-

3111

K2O

(wt %

)

4,0

3,5

3,0

2,5

2,0

Depth(m)

1255

-

1256

1257

-

1258

1259

1849

-18

50

1851

-

1852

3110

-

3111

Na 2

O (w

t %)

1,6

1,5

1,4

1,3

1,2

Depth(m)

1255

-

1256

1257

-

1258

1259

1849

-18

50

1851

-

1852

3110

-

3111

MnO

(wt %

)1,

0,8

,6,4

,20,

0

Depth(m)

1255

-

1256

1257

-

1258

1259

1849

-18

50

1851

-

1852

3110

-

3111

MgO

(wt %

)3,

63,

43,

23,

02,

82,

62,

42,

22,

01,

8

Depth(m)

1255

-

1256

1257

-

1258

1259

1849

-18

50

1851

-

1852

3110

-

3111

SiO

2 (w

t %)

7068

6664

6260

5856

54

Depth(m)

1255

-

1256

1257

-

1258

1259

1849

-18

50

1851

-

1852

3110

-

3111 Fi

g 11

. Maj

or e

lem

ent v

aria

tion

curv

es fo

r th

e wel

l Hab

igan

j – 1

. See

App

endi

x 1

and

2 fo

r de

tails

of t

he sa

mpl

e in

terv

al. D

ash

lines

indi

cate

the

brea

k of

sam

ple

inte

rval

s.

63

CaO

(wt %

)6

54

32

10

Depth(m) 39

68

3731

3730

2974

2973

2272

1177

1176

1175

K2O

(wt %

)4,

03,

53,

02,

52,

0

Depth(m) 39

68

3731

3730

2974

2973

2272

1177

1176

1175

Fe2O

3 (w

t %)

98

76

54

Depth(m) 39

68

3731

3730

2974

2973

2272

1177

1176

1175

Na 2

0 (w

t %)

1,6

1,4

1,2

1,0

,8,6

,4

Depth(m) 39

68

3731

3730

2974

2973

2272

1177

1176

1175

Al 2O

3 (w

t %)

2018

1614

1210

Depth(m) 39

68

3731

3730

2974

2973

2272

1177

1176

1175

MnO

(wt %

),1

6,1

4,1

2,1

0,0

8,0

6,0

4

Depth(m) 39

68

3731

3730

2974

2973

2272

1177

1176

1175

SiO

2 (w

t %)

7068

6664

6260

5856

Depth(m) 39

68

3731

3730

2974

2973

2272

1177

1176

1175

Na 2

0 (w

t %)

1,6

1,4

1,2

1,0

,8,6

,4

Depth(m) 39

68

3731

3730

2974

2973

2272

1177

1176

1175

Fig

12. M

ajor

ele

men

t var

iatio

n cu

rves

for

the

wel

l Kai

lsat

ila –

1. S

ee A

ppen

dix

1 an

d 2

for

deta

ils o

f the

sam

ple

inte

rval

. Das

hlin

es in

dica

te th

e br

eak

of sa

mpl

e in

terv

als.

64

CaO

(wt %

)20

100

Depth(m) 95

9-96

414

50-

1457

1837

2290

-23

07

2828

-

2834

3160

-

3168

Depth(m)

K2O

(wt %

)4,

03,

53,

02,

52,

01,

51,

0,5

959-

964

1450

-14

5718

37

2290

-23

07

2828

-

2834

3160

-

3168

Fe2O

3 (w

t %)

98

76

54

32

Depth(m) 95

9-96

414

50-

1457

1837

2290

-23

07

2828

-

2834

3160

-

3168

Na 2

O (w

t %)

2,0

1,8

1,6

1,4

1,2

1,0

Depth(m) 95

9-96

414

50-

1457

1837

2290

-23

07

2828

-

2834

3160

-

3168

MnO

(wt %

)

,7,6

,5,4

,3,2

,10,

0

Depth(m) 95

9-96

414

50-

1457

1837

2290

-23

07

2828

-

2834

3160

-

3168

Al 2O

3 (w

t %)

2220

1816

1412

108

64

Depth(m) 95

9-96

414

50-

1457

1837

2290

-23

07

2828

-

2834

3160

-

3168

SiO

2 (w

t %)

8070

6050

Depth(m) 95

9-96

414

50-

1457

1837

2290

-23

07

2828

-

2834

3160

-

3168

MgO

(wt %

)

3,5

3,0

2,5

2,0

1,5

1,0

,5

Depth(m) 95

9-96

414

50-

1457

1837

2290

-23

07

2828

-

2834

3160

-

3168

Fig

13. M

ajor

ele

men

t var

iatio

n cu

rves

for

the

wel

l Pat

hari

a –

5. S

ee A

ppen

dix

1 an

d 2

for

deta

ils o

f the

sam

ple

inte

rval

. Das

h lin

esin

dica

te th

e br

eak

of sa

mpl

e in

terv

als.

65

CaO

(wt %

)

65

43

21

0

Depth(m) 10

81-

1087

1248

-

1254

1827

-18

3221

34-

2135

2472

-24

73

K2O

(wt %

)

3,8

3,6

3,4

3,2

3,0

2,8

2,6

Depth(m) 10

81-

1087

1248

-

1254

1827

-18

3221

34-

2135

2472

-24

73

Fe2O

3 (w

t %)

8,5

8,0

7,5

7,0

6,5

6,0

5,5

Depth(m) 10

81-

1087

1248

-

1254

1827

-18

3221

34-

2135

2472

-24

73

Na 2

O (w

t %)

1,8

1,7

1,6

1,5

1,4

1,3

1,2

Depth(m) 10

81-

1087

1248

-

1254

1827

-18

3221

34-

2135

2472

-24

73

Al 2O

3 (w

t %)

1918

1716

1514

1312

Depth(m) 10

81-

1087

1248

-

1254

1827

-18

3221

34-

2135

2472

-24

73

MnO

(wt %

),1

8,1

6,1

4,1

2,1

0,0

8,0

6

Depth(m) 10

81-

1087

1248

-

1254

1827

-18

3221

34-

2135

2472

-24

73

SiO

2 (w

t %)

6866

6462

6058

Depth(m) 10

81-

1087

1248

-

1254

1827

-18

3221

34-

2135

2472

-24

73

MgO

(wt %

)

1210

86

42

Depth(m) 10

81-

1087

1248

-

1254

1827

-18

3221

34-

2135

2472

-24

73

Fig

14. M

ajor

ele

men

t var

iatio

n cu

rves

for

the

wel

l Ras

hidp

ur –

1. S

ee A

ppen

dix

1 an

d 2

for

deta

ils o

f the

sam

ple i

nter

val.

Das

hlin

es in

dica

te th

e br

eak

of sa

mpl

e in

terv

als.

66

Gulf coast shales have K2O contents that increase systematically with depth from ~2 to4 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 thewelldata of K2O content varies between 2-3.5 wt%. Potter et al. (1980) has shown thegeochemical properties of shales change with time. For example, several workers havereported significantly higher K2O in early Paleozoic shales ,than in younger shales.

Comparison between the histogram of modern sediments of the Sylhet area (Islam1996) and the SG sediments (Miocene) of Rashidpur well-1 shows a marked decrease ofSiO2 content(Fig.15). The mean value of SiO2 varies from 79.0 to 62.65 (Fig.16a). Theaverage value of Al2O3, K2O and TiO2 has enriched markedly from 9.7 (mean value) to15.85, 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 ashale (this is an exceptional sample with very high content of SiO2). The highest value ingeneral was 75.51% (Atgram well-IX) and 74.51% (Patharia well – 5), both the sampleswere sandy shale. Variation was least in Rashidpur well – 1 (Standard deviation = 1.54)suggestive of a uniform source composition during that interval.

Fig 15. Comparison between the silicon dioxide of modern sediments of Sylhet area and the SGsediments (Miocene) of Rashidpur well – 1 shows a marked decrease of SiO2 content. Dash lineindicate 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.

Sample interval (D

epth)

Sample

67

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

SiO2 %

K2O %TiO2 %

Al2O3 %

68

Fig 16. B. Histogram showing major oxides of modern sediments (Data from Islam 1996). Notethe 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 indi-cator (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. Thewells Atgram well-IX, Rashipur well-1, Habiganj well-1 and KailasTila well-1 havedownward coarsening tendency and the Fenchuganj well-2 well show relatively coarsegrained upward. The overall sequence of the SiO2/Al2O3 ratio shows the dominance ofthe fining with a coarsening tendency at the bottom for the wells mentioned. The SG sedi-ments (Neogene shale) reflects a shallowing of the depositional basin and the depositionof more altered material. Palynological and geochemical studies match well the shallo-wing conditions shown.

The SiO2 content in all the wells studied seems to decrease with increasing Al2O3. Aslarge amounts of quartz are found in the residual material, it is reasonable to assume thatdesilicification took place mainly by a destruction of aluminosilicates. A plot of Al2O3against the SiO2%/Al2O3% ratio of samples from Patharia well-5 shows a strong negativecorrelation. The Fe2O3/ Al2O3, Na2O/Al2O3 and K2O/Al2O3 ratios in the samples vary

SiO2 %

K2O %TiO2 %

Al2O3 %

69

against the Al2O3 concentrations (Fig. 17). The results indicate the presence of clay mine-rals.

Fig 17. Crossplots of Al2O3 versus K2O/Al2O3, SiO2/Al2O3, Fe2O/Al2O3 and Na2O3/Al2O3 forPatharia 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 forthe 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 caseof CaO, Na2O and MnO, the vectors were different. The results illustrate a similar ele-ments composition from all the samples, reflecting homogeneity of the sediment suite.Variation in the chemical composition reflects changes in the mineralogical compositionof the sediments due to the effects of weathering, marine sedimentation and early diage-netic processes (Shaw & Weaver 1965, Drever 1971, Nesbitt & Young 1984,1989). The

K2O/Al2O3 SiO2/Al2O3

Fe2O/Al2O3 Na2O3/Al2O3

Al2 O

3 (wt %

)A

l2 O3 (w

t %)

Al2 O

3 (wt %

)A

l2 O3 (w

t %)

70

abundance of Al, Si, K and Ti in shales may be perturbed from parent material by weathe-ring, transport and depositional processes (Nesbitt & Markovies 1996).

Fig 18. Harker type major element (%) variation diagrams for Atgram well – IX.

SiO2 (wt %) SiO2 (wt %)

71

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

SiO2

CaO

Na 2

O

TiO

2

CaO

Fa2O

3

Al 2

O3

K2OMnO

SiO2

SiO2

SiO2SiO2

SiO2

MgO


72

Fig 20. Harker type major element percentage variation diagrams for Habiganj well – 1.


73

Fig 21. Harker type major element percentage variation diagrams for Kailastila well – 1.


SiO2 (wt %)SiO2 (wt %)


SiO2 (wt %)SiO2 (wt %)

TiO

2

MgO

Al 2

O3

Na 2

OC

aOMgO

K2O

Fe2O

3

74

Fig 22. Harker type major element percentage variation diagrams for Patharia well – 5. .


75

The different maturity parameters studied display complicated development with anincreasing 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 sametrend in different districts. The index is also controlled by the clay mineral. The other twomaturity 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/K2Oratios are indicative of a Kaolinization process and typically mature sediment.

All the maturity parameters used in this study show an apparently fluctuating maturityfor all wells. In general, the lower portions showed increasing maturity. All the threematurity parameters showed a similar trend for the wells studied (Fig. 23–28). It is pos-sible to demarcate the wells stratigraphically by mature and immature zones. The matu-rity increases with depth, indicating renewed deposition, new weathering conditions andparticularly source area variations. The study also showed that the maturity increases withthe decrease of the silica content and grain size.

The trend of K2O/Na2O ratios in Fig. 23–28 is interesting and is consistent with thegrain size analysis. Na and K in the studied Neogene shales are mostly confined in thedetrital illites. Deer et al. (1963) noted that fresh muscovite contains more K and Na thanaverage 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 willtend to include fresher micas, alternatively, finer fractions carry more degraded illites andmore smectites (Pearson 1979). An increase in sodium and potassium with coarseness ofthe sediment may then arise naturally. The ratio of K2O/Na2O shows a fluctuatingscenario for all wells studied which are between 0.70 and 4.26. These differencesobviously reflect in part the different original compositions of the source rocks.

76

Fig 23. Showing the grain size (SiO2/Al2O3) and maturity (M) index curves for the well Atgram– IX. Note that the maturity curves are more or less similar and reverse to the curve of SiO2/Al2O3 indicate that the maturity increases with the decrease of silica content and grain size.Figure 9 shows the actual depth of the samples.

SiO2/Al2O3 K2O/Na2OM = K2O + Al2O3 /Na2O + MgO

Cu/ZnRb/K2O

77

Fig 24. Showing the grain size (SiO2/Al2O3) and maturity (M) index curves for the wellFenchuganj – 2. Note that the maturity curves are more or less similar and reverse to the curveof SiO2/Al2O3 indicate that the maturity increases with the decrease of silica content and grainsize. Figure 10 shows the actual depth of the samples.


Cu/ZnRb/K2O

78

Fig 25. Showing the grain size (SiO2/Al2O3) and maturity (M) index curves for the wellHabiganj – 1. Note that the maturity curves are more or less similar and reverse to the curve ofSiO2/Al2O3 indicate that the maturity increases with the decrease of silica content and grainsize. Figure 11 shows the actual depth of the samples.


Cu/ZnRb/K2O

79

Fig 26. Showing the grain size (SiO2/Al2O3) and maturity (M) index curves for the wellKailastila – 1. Note that the maturity curves are more or less similar and reverse to the curveof SiO2/Al2O3 indicate that the maturity increases with the decrease of silica content and grainsize. Figure 12 shows the actual depth of the samples.


Cu/ZnRb/K2O

80

Fig 27. Showing the grain size (SiO2/Al2O3) and maturity (M) index curves for the wellPatharia – 5. Note that the maturity curves are more or less similar and reverse to the curve ofSiO2/Al2O3 indicate that the maturity increases with the decrease of silica content and grainsize. Figure 13 shows the actual depth of the samples.


Cu/ZnRb/K2O

81

Fig 28. Showing the grain size (SiO2/Al2O3) and maturity (M) index curves for the wellRashidpur – 1. Note that the maturity curves are more or less similar and reverse to the curveof SiO2/Al2O3 indicate that the maturity increases with the decrease of silica content and grainsize. Figure 14 shows the actual depth of the samples.

The enrichment of MgO in all the wells is related to a decrease of CaO in the sequen-ces and the opposite is due to the weathering effect. A high MgO content is often correla-ted with low temperature oxidative diagenesis (Bhat & Ahmed 1990). In such a case, anincrease of MgO in the rock is accompanied by a drop in CaO (Andrews 1980). Mg corre-lates well with Al only if samples with MgO greater than 3 % are omitted (Fig. 29 showsthe correletion of Al2O3 and MgO for 3 wells) thus indicating that this element is origi-nally associated with aluminosilicate phases and assumes minor association with carbona-tes during diagenesis. (Bellanca et al. 1999).

There are clear positive correlations between K content and the abundances of Al, Cs,Ba, Th and U (Fig. 30), suggesting that absolute abundances of these elements are prima-rily controlled by the amount of the dominant original clay mineral (illite).

The higher K content is almost certainly due to the original presence of large quantitiesof illite (McLennan et al. 1983). Decreasing K2O/Na2O ratio in the well indicates thedecreasing maturity of the sediments, reflecting reduced influx of highly Kaolinzed mate-rial. Factors such as increased erosion in relation to weathering, and/or transgressionresulted in Kaolinite - depleted rocks (Dypvik 1979). This interpretation is reasonable

/

Cu/Zn M = K2O + Al2O3/Na2O+MgO

SiO2/Al2O3 K2O/Na2O Rb/K2O

82

when combining the results of other geochemical ratios like Rb/K2O and Cs/K2O andalso with the mineralogical data followed (Fig. 31). K2O show a strong positive correla-tion indicating their association with the aluminosilicate phases (Fig. 31).

The absolute abundances of transition elements are correlated with the concentrationof Mg (Fig. 32). The positive correlation is probably due to the incorporation of these ele-ments in chlorite, the other major clay mineral (McLennan et al. 1983).

The sequences of the wells exhibit depletions of Na and Ca probably reflecting intensechemical weathering of their source rocks. Al2O3, CaO, Na2O and K2O are related withthe chemical index of Alteration. They exhibit variations in the whole sequence as shownin Fig. 9–14, which reflect chiefly variable climatic zones or rates of tectonic uplift insource areas.

It is notable that the peaks of MnO and CaO occur at the same positions. The Mn con-tent of the Neogene shale may be ascribed to concentrations of the element by secondaryoxidation and would indicate that oxygen was present at the seafloor during their deposi-tion (Bellanca et al. 1999).

A correct approach to interpret the chemical results in terms of paleoproductivityreconstruction and of aluminosilicate phase fluxes is to infer the principal elemental asso-ciations (Bellanca et al. 1999).

Because Al concentration is reasonably thought to be a good measure of detrital flux,the excellent positive correlations of K2O, TiO2, Na2O and MgO with Al2O3 indicate thatthese elements are associated entirely with detrital phases as shown in figure 30. and alsosuggesting that they are associated with aluminous clay minerals such as illite or smectite,and indicate that weathering was an important factor in the source area, where K and Mgare fixed in clay minerals and Ca is preferentially leached (Nesbitt et al. 1980). Alternati-vely, the clays may have been more aluminous and later enriched in K to form illite (Fedoet al. 1995, 1996).

83

Fig 29. Showing the crossplots of Al2O3 vs. MgO for (a) Rashidpur well – (b) Fenchuganj well– 2 and (c) Patharia well – 5. All are in positive correlation. Patharia – 5 well correlate strongly.

Al2O3 (wt %)

Al2O3 (wt %)

Al2O3 (wt %)

MgO

(wt %

)M

gO (w

t %)

MgO

(wt %

)

84

Fig 30. Shows the crossplot of K2O, TiO2, Cr, Rb and Zr versus Al2O3 of Patharia well – 5.Except Zr all are in positive correlation.

Al2O3 wt %

20181513108530

K2O

wt %

4,0

3,0

2,0

1,0

0,020181513108530

TiO

2 wt %

1,0

,8

,6

,4

,2

0,0

Al2O3 wt %

20181513108530

Rb

ppm

300

200

100

0

Al2O3 wt %

20181513108530

Cr

ppm

180

160

140

120

100

80

60

40

20

0

Al2O3 wt %

20181513108530

Zr p

pm

1400

1200

1000

800

600

400

200

0

Al2O3 wt %

85

Fig 31. Crossplots of U, Th, Cs, Ba and Al2O3 versus K2O of Patharia well-5. All are in positivecorrelation.

K2O ( Wt. % )4,03,53,02,52,01,51,0,50,0

U (p

pm )

6

4

2

0

-24,03,53,02,52,01,51,0,50,0

Al 2O

3 ( W

t. %

)

20

18

16

14

12

10

8

6

K2O ( Wt. % )

4,03,53,02,52,01,51,0

Th (

ppm

)

40

30

20

10

0

-10

K2O ( Wt. % )

4,03,53,02,52,01,51,0

Cs (

ppm

)

30

20

10

0

K2O ( Wt. % )

4,03,53,02,52,01,51,0

Ba

( ppm

)

4000

3500

3000

2500

2000

1500

1000

500

0

K2O ( Wt. % )

86

Fig 32. Crossplots of (Cr+Ni), Fe2O3, MnO and versus MgO of Patharia well – 5. Except MnOall are in positive correlation.

Tectonic setting determination of SG sediments using SiO2 content and the K2O/Na2Oratio.

Literature analyses of sandstones and shales from ancient sedimentary sequences ofinferred tectonic settings have been used to establish a tectonic classification based onSiO2 content and K2O/Na2O ratio (Roser & Korsch 1986). Three first-order tectonic cate-gories, broadly similar to those of Crook (1974), have been made. Several depositionalsettings are possible under each category.

The categories presented by Roser and Korsch (1986) and those which have beenadopted for the present study are:

1) Passive Continental Margin (PM). Mineralogically mature (quartz-rich) sedimentsdeposited in plate interiors at stable continental margins or intracratonic basins. Repre-sented by Ordovician and Silurian greywackes and shales from Australia that are recycledquartz - rich sediments derived from older adjacent continental terrains (Wyborn & Chap-pell 1983).

2) Active Continental Margin (ACM). Quartz-intermediate sediments derived fromtectonically active continental margins on or adjacent to active plate boundaries. Repre-sented by rocks from: a) the Franciscan Complex, California (Bailey et al. 1964) andKodiak Formation, Alaska (Connelly 1978) the Santa Ynez Mountains, California (Vande Kamp et al. 1976) which were deposited at a complicated continental margin whereboth subduction and strike - slip processes were active. Hence this category includescomplex active margins including material derived from continental margin magmaticarcs (and deposited in a variety of basin settings including trench, forearc, intra-arc andback-arc) and material derived from uplifted areas associated with strike - slip faults anddeposited in pull - apart basins.

M gO ( W t. % )

3,53,02 ,52 ,01 ,51 ,0

V (p

pm )

180

160

140

120

100

80

60

40

3,53,02 ,52 ,01 ,51 ,0

Cr +

Ni (

ppm

)300

200

100

0

M gO ( W t. % ) M gO (W t.% )

3,53 ,02 ,52 ,01 ,51 ,0

Fe2O

3 (W

t. %

)

9

8

7

6

5

4

3

2

3 ,53 ,02 ,52 ,01 ,51 ,0

MnO

( W

t % )

,7

,6

,5

,4

,3

,2

,1

0 ,0

M gO ( W t. % )

87

3) Oceanic Island Arc (ARC). Quartz-poor volcanogenic sediments derived from ocea-nic island arcs. Represented by: a) the Baldwin Formation, Australia (Chappel 1968)which are forearc basin sediments derived from an andesitic island arc source, and b) TheUyak and Cape Current greywackes, Alaska (Connelly 1978), which were derived froman andesitic source and deposited in a trench adjacent to an active volcanic arc. Hence,sediments in this category were derived from an island arc source and were deposited in avariety of settings including forearc, intra-arc and back-arc basins and trenches.

All the categories defined reflect the composition of rocks in the source areas. Reading(1982) defined five types of sedimentary basins related to plate tectonic setting.

A binary plot of K2O/Na2O–SiO2 diagram prepared for all the six wells (Fig. 33). Datafrom the six wells of SB fall into distinct field of ACM which is consistent with the paleo-tectonic history of the Basin. Except 3 samples from Atgram well - IX which fall in PMand 2 samples from Patharia well-5 fall in Oceanic Island arc margin (ARC). The datasuggest that the chemistry of Neogene Shale (NS) can indeed be used for tectonic discri-mination.

Seismotectonic and tectonic reports of the region by Chandra (1978), Chaudhury andSrivastava (1976), Molnar (1984), Nandy and Dasgupta (1991) and Verma (1991) as wellas the report from the study area by Khan (1985) are in good agreement with the presentresult.

88

Fig 33. Tectonic discrimination diagram for shales of the wells 1) Fenchuganj well-2 (N=20), 2)Habiganj well-1 (N=19), 3) Kailastila well-1 (N=9), 4) Atgram well-IX (N=10), 5) Rashidpurwell-1 (N=40) and 6) Patharia well-5 (N=73). PM = Passive margin; ACM = Active continentalmargin; ARC = Oceanic Island arc margin. Most of the samples are in Active Continentalmargin except 3 samples from Atgram well-IX which fall in Passive margin and 2 samples fromPatharia well-5 fall in Oceanic Island Arc margin. (Roser & Korch 1986)

SiO2

SiO2

SiO2

SiO2

SiO2 SiO2

K2O

/Na 2

O

K2O

/Na 2

O

K2O

/Na 2

O

K2O

/Na 2

O

K2O

/Na 2

O

K2O

/Na 2

O

wt % wt %

89

7.2 Trace elements

The trace element concentrations of the wells studied are given in appendix 2. Variousgeochemical ratios calculated for the trace-element geochemistry from the study area aredisplayed in Table 9.

A number of bivariate plots of trace elements were prepared by using an SPSS compu-ter programme and are presented). Trace-element concentrations in sediments result fromthe competing influences of provenance, weathering, diagenesis, sediment sorting, andthe aqueous geochemistry of the individual elements. REE, as well as, Th, Sc and to a les-ser extent Cr and Co are the most useful for provenance characterisation, because they areamong the least soluble trace elements and are relatively immobile. These elements arebelieved to be transported exclusively in the terrigenous component of sediment and the-refore reflect the chemistry of their source (McLennan et al. 1980).

The Cu/Zn ratio as a radox parameter, as put forward by Hallberg (1976), was com-puted for all the wells and is shown in figures 23–28. The Cu/Zn ratio of the wells showsfluctuations for the whole sequence indicating reducing and oxidizing conditions. Theincreasing value of the ratio indicates a reducing, depositional condition while decreasingCu/Zn values suggest increased oxidizing conditions. However, the highest Cu/Zn ratio isseen in different depth levels in the wells studied, which will be discussed later in the dis-cussion chapter.

The abundance of Th and U in the wells are quite variable. The average concentrationof Th and U is 28 and 2 ppm respectively in the sediments and Th/U = 3.80 (min.) and 35(max.) (Figs 34–39). The higher abundance of Th in the rocks reflects the presence of Thbearing minerals. The higher Th/U ratio probably indicates the derivation of these sedi-ments from the recycling of the crust. Recycles of sediments themselves lead to a furtherloss of U and increase in Th/U ratio. The higher value of the Th/U ratio in the core samp-les is because of oxidation.

As originally pointed out by Taylor and McLennan (1985), the Th/U ratio does notchange or record a change in source composition. The present observation suggests thatthe Th/U in NS does not record a change in source composition but is controlled by theweathering erosion - diagenesis cycle. Condie (1993) has shown a similar observationwith a study of shales from upper continental crust, USA.

According to McLennan et al. (1993) the Th/Sc ratio is a sensitive index of the bulkcomposition of the provenance. The abundances of Th in sedimentary rocks can be rela-ted to the abundances in the upper continental crust. Accordingly, the average La/Th andTh/Sc ratios are fairly constant in sedimentary rocks: ~ 2.8 and ~ 1.0 respectively(McLennan 1989). The average La/Th ratio of the samples investigated is 2.6, and corres-ponds closely with these data.

The Rb and Th levels, in turn, suggest an enrichment of a felsic component (Lahtinen1996). Rb and Ba are considered to some extent to be concentrated relative to K and fixedin clays during weathering (Nesbitt et al. 1980). Th is considered to reliably characterisethe source composition (McLennan et al. 1990) Possibly, the main part of Th occurs inferromagnesium minerals with subsequent attachment to clays during weathering.

90

Table 9. Major chemical and lithological characteristics of the wells of Surma Basin,Sylhet, Bangladesh.

Some four elements like V, Ba, Cr and Zr were normalized to Rb and the profiles wereconstructed and presented (Figs 34–39). The reason for normalization to Rb is that thiselement is inert with respect to biogenic processes (Emelyanov & Shimkus 1986).

The stratigraphic distribution of V is clearly expressed in the V/Rb profiles for all thewells (Figs 34–39). Generally for the wells the curve shows higher values suggesting theshales were deposited under prevalently poorly oxygenated to euxinic conditions (Bel-lanca et al. 1999).

Cr/Rb, Zr/Rb and Ba/Rb depth profiles display ratio values higher, on average forNeogene shales, which could be explained by enhanced terrigenous input during theirdeposition. Ba/Rb higher values at different depth could also reflect diagenetic mobiliza-tion of barite from sulphate-poor, strongly anoxic sediments to sulphate-bearing, dysoxicenvironments (Bellanca et al. 1999).

Condie & Wronkiewiez (1990) used the Cr/Th ratio as a provenance indicator. The Cr/Th ratio increases notably in the samples investigated. Many studies have noted anoma-lous concentrations of Cr and Ni in shale and have inferred the presence of ultramafficrocks in the source region. Much of this work has focused on Archeon rocks (Danchin1967, McLennan et al. 1983, Taylor et al. 1986, Wronkiewicz & Condie 1987).

Well N SiO2% SiO2/Al2O3

K2O/Na2O

M* U/K2O Th/K2O Ba/K2O Cr/V Ni/Cu V/Ni Zr/Hf

KailasTila-1 9 59.04 – 67.84

3.14 – 6.09

1.77 – 4.26

2.54 –14.27

0.29 –1.79

6.04 – 16.15

1.29 –216.15

0.87 –1.08

1.45 – 2.57

1.15 –2.22

30.11 –71.00

Atgram-IX 11 46.20 –89.40

2.92 –18.98

1.59 –3.06

4.13 –10.33

0.28 – 4.60

.00– 7.48

150.99 – 431.03

0.16 –1.18

1.15 –2.43

1.67 – 2.95

16.13 –48.20

Habiganj-1 19 55.41 –67.43

3.27 –6.14

1.64 –2.59

3.91 –4.60

0.27 – 2.37

7.64 – 12.32

144.44 –203.11

0.89 –1.14

1.38 –2.70

1.66 – 2.20

36.67 – 229.00

Fenchuganj-2

20 50.57 –71.30

2.98 – 6.34

1.67 –3.15

2.65 –4.69

1.14 –2.49

5.70 –14.39

99.15 –206.97

0.94 –1.27

1.171 –2.40

1.56 –2.30

30.00 –317.00

Rashidpur-1 40 58.76 –66.30

3.31 –5.03

1.81 –2.60

1.65 –4.89

.00 –1.43

7.19 –10.36

140.94 –302.36

.82 –14.00

1.57 –12.00

1.36 –3.08

24.00 –285.00

Patharia-5 69 55.32 –76.22

2.99 –9.04

0.70 –3.05

3.26 –5.56

.00 –3.39

.00 –13.11.

26.07 –286.00

0.27 –2.59

1.13 –3.82

0.92 –4.18

16.14 –1259.00

Well N Zr/Nb Th/U Th/Sc Th/Cr La/Th Cu/Zn Cr/Th Cr/Rb Zr/Rb V/Rb Ba/RbKailastila-1 9 12.39 –

29.584.75 –35.00

1.21 –2.21

0.17 – 0.28

1.17 –2.13

0.11 –0.52

3.60 –5.96

0.55 –1.00

1.08 –2.69

0.52 –1.08

2.02 –4.25

Atgram-IX 11 13.38 –33.50

3.80 –22.00

1.27 –1.75

0.21 –1.00

0.74 –2.58

0.27 –0.51

–5.00

0.10 –0.75

1.06 –4.24

0.60 –1.15

2.68 –11.36

Habiganj-1 19 9.64 –16.65

5.20 –31.00

1.17 –2.89

0.21 –0.35

0.81 –2.00

0.28 –0.42

2.85 –4.73

0.53 –0.63

0.93 –1.94

0.46 –0.66

2.32 –2.77

Fecnhuganj-2

20 12.47 –28.82

7.25 –30.00

1.33 –2.31

0.16 –0.35

1.10 –2.30

0.30 –0.48

2.86 –6.45

0.53 –0.86

1.03 –2.62

0.55 – 0.73

2.13 –3.47

Rashidpur-1 40 7.82 –15.44

6.25 –30.00

1.76 –2.64

0.18 –0.28

1.18 –2.16

0.07 –0.45

3.61 – 5.44

0.53 –0.91

0.93 –2.15

0.60 –0.77

2.32 –5.54

Patharia-5 69 4.56 –61.14

00 –425.00

00 –317.00

00 –0.88

0.58 –13.67

0.17 –4.60

1.14 –6.83

0.17 –2.22

0.54 –7.86

0.61 – 0.86

0.48 – 55.80

M* = maturity = Al2O3 + K2O/Na2O + MgO

91

Fig 34. Atgram Well – IX geochemical ratio graph. Figure 9 shows the actual depth of thesamples.

92

Fig 35. Fenchuganj Well – 2 geochemical ratio graph. Figure 10 shows the actual depth of thesamples.

93

Fig 36. Habiganj Well – 12 geochemical ratio graph. Figure 11 shows the actual depth of thesamples.

94

Fig 37. Kailastila Well – 1 geochemical ratio graph. Figure 12 shows the actual depth of thesamples.

95

Fig 38. Patharia Well – 5 geochemical ratio graph. Figure 13 shows the actual depth of thesamples.

96

Fig 39. Rashipur Well – 1 geochemical ratio graph. Figure 14 shows the actual depth of thesamples.

Most of these studies indicate that the high correlation coefficient between Cr and Ni isa result of their presence in clay minerals and, ultimately, their derivation from ultramaf-fic rocks. Garver & Royce (1993) suggest that where Cr and Ni concentrations are ano-malously high, a Cr/Ni ratio of about 1.2 to 1.6 should be expected if the elements werederived from a source with ultramafic rocks, higher ratios are probably indicative of deri-vation of these elements from mafic volcanic rocks. Therefore, anomalous concentrationsof Cr and Ni can be used to determine if ultramafic rocks were in the source region, andthen inferences as to the tectonic implications of this information can be explored (Garver& Scott 1995). Cr and Ni values are generally high for the samples investigated (Cr ~ 14-20, Ni ~ 17-97 ppm). The average ratio of Cr/Ni is 2.2 with variation among the valuesbetween the wells. The higher concentration indicates that the source region was compo-sed of ultramafic rocks. Cr, V and Sc are all positively correlated with Al2O3 as shown infigure 30, suggesting that they may be bound in clays and concentrated during weathering(Fedo et al. 1996).

Vanadium is thought to be enriched in organic rich, reducing environments while theCr content often is relatively constant in the clastic fraction. Cr/V ratio (ppm Cr/ppmV)

97

may therefore be used as a redox index (Ernst 1970). Ni may be enriched in reducingenvironment.

The Cr/V ratio was computed for all the wells studied (Table 9). Fig. 40 shows thesequence of Cr/V and Ni for the wells of Rashidpur – 1, Fenchuganj – 2 and Patharia – 5.Both the sequences are more or less similar indicating that they were deposited in thesame reducing environment. High Cr/V ratios with Ni indicate that the depositional envi-ronment was well ventilated (Dypvik 1979), oxidizing condition. The Cr/V ratio also dec-reases through the sequence suggesting increasingly reducing syn-depositional condi-tions. Ni contents also indicate increasingly reducing conditions, which probably have apost-depositional origin (Dypvik 1979).

A higher Cr/V ratio probably reflecting a source rock richer in Cr. The analysis alsoshows that Cr increases with the increase of vanadium. Fig.41. shows the cross-plots ofCr and V for 3 wells of SB. They are positively correlated. The analyses also indicate ahigher K2O/Na2O ratio and lower Cr/V ratio. Differences in Cr/V ratios in the wells arethe indicators of differences in the nature/degree of weathering.

A high order positive correlation between V and Sc, low concentration of Sc and widerange of Cr/V ratio (0.8–14.00) indicate that mafic-ultramafic components were a sub-stantiate portion of the provenance, but their unstable minerals have been removed. Theconcentration of Cr and Ni vary between 19–202 ppm and 17–109 ppm respectively.

The positive covariance between Zr–Cr, V–Ni and their concentration demonstrate thatat the time of deposition the source area was subject to intense chemical and physicaldisintegration. According to Wronkiewicz and Condie (1989), chemical weathering of themafic ultramafic source rocks would tend to selectively enrich weathering products in Crand Ni.

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Fig 40. Sequence showing the geochemical ratio Cr/V and Ni for wells Rashipur – 1 (A-1 andA-2), Fenchuganj – 2 (B-1 and B-2) and Patharia – 5 (C-1 and C-2) indicating that they weredeposited in the same environment. Fig. 14, 10 and 13 shows the actual depth of the samples ofthe wells mentioned.

Sequence number (D

epth)

Sequence number (D

epth)

Ni ppm

Cr/V

Cr/V

Ni ppm

Sequence number (D

epth)

Sequence number (D

epth)

Sequence number (D

epth)

Sequence number (D

epth)

Sequence number (D

epth)

Sequence number (D

epth)

Cr/V Ni ppm

99

Fig 41. Crossplot of Cr-V for (a) Patharia well – 5 (b) Rashidpur well – 1 and (c) Fenchuganjwell – 2. All are in positive correlation.

Sr contents of NS vary from 170–178 ppm. The value reflects the "average" intensityof chemical weathering of the shale sources. Studies of modern weathering show that Ca,

100

Na and Sr are rapidly lost during chemical weathering and that the amount of these ele-ments lost in proportional to the degree of weathering (Wronkiewicz & Condie 1987,1989, Condie 1991). The present study shows that NS typically have Ba/Sr and Rb/Srratios considerably higher which may also result from Sr loss during weathering. Intensetectonic activity and burial diagenesis reported in the literature from the study area sug-gest that the average intensity of chemical weathering may have been greater than after-wards. If Pre-Neogene weathering was more intense than in later times, a greater propor-tion of Ca and Sr should have ended up in seawater. Because of their short residencetimes in seawater, Ca and Sr should have been recycled in the Neogene Crust in shallowmarine carbonates.

Among Large-ion lithophile elements (LILE, Rb, Ba, Th, Sr, Cs, U and Pb), Ba and Rbare enriched throughout the sequence. U, Cs and Pb are strongly depleted whereas Srexhibits gradual depletions for some wells and slightly enriched for other wells.

Among High Field Strength Elements (HFSE, Zr, Nb, Hf, Ta, and V)) only Zr enrichedthroughout the sequences. Nb, Hf and Ta however, exhibit gradual depletion.

7.2.1 Barium enrichment

Ba values generally vary between 11 and 300 ppm (Bellanca et al. 1999). For the presentstudy, significantly higher concentrations (up to 6 808 ppm) occur in the middle part ofthe section of Patharia well-5 which is interesting and deserves to be explored well. Adetail study on Ba was undertaken by using XRD, STEM and SEM for some selectedsamples of Patharia well-5. The depth and the Ba concentrations of this well are given inTable 10.

Barite deposits occur in a wide range of deep-sea locations (Torres et al. 1996). Theirdistribution, composition and correlation with sediment type have been discussed by anumber of authors. A variety of mechanisms, including hydrothermal, biogenic anddiagenetic processes, result in the accumulation of barium sulfate in the marine sedimen-tary environment. Barite can be formed by direct precipitation when a barium-enrichedhydrothermal fluid reacts with sea water sulfate (Torres et al. 1996). These deposits arerestricted to the vicinity of hydrothermal activity, as observed, for example, along the EastPacific Rise (Church 1979), the Gorda Ridge, north -eastern Pacific (Koski et al. 1988)and the Guaymas Basin, Gulf of California (Koski et al. 1988).

High concentrations of micro-crystalline barite (up to 30 x 50 µm in size) in sedi-ments from the Eastern Equatorial Pacific, the Indian Ocean (Goldberg & Arrhenius1958) and the Antarctic convergence Zone (Brahms et al. 1992) are thought to result fromthe precipitation of barium sulfate within microenvironments of decaying biologicaldebris in the water column (Dehairs et al. 1980, 1990, Collier & Edmond 1984, Bishop1988).

Some earlier workers like Price and Calvert (1978) and Prevot & Lucas (1980) hasgiven a good account of Ba enrichment. According to them Ba transported to the seabecomes largely separated already during the formation of hydrolyzate sediments. Withreference to its adsorption properties, Ba behaves like potassium, which forms a univalention, while barium forms a bivalent ion. The ionic potential of K+ and Ba2+ is smaller and

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the quantity adsorbed by clays great. Barium is adsorbed so strongly that it has largelybeen removed in the near-shore sediments. Therefore, barium is adsorbed in the hydroly-zates, and the argillaceous sediments are the richest in barium. In some cases marinedeposits contain notable amounts of barium as barite concretions and nodules, which maycarry as much as 82 % BaSO4 and are probably formed by chemical precipitations.

Price and Calvert (1978) as well as Prevot and Lucas (1980) stated that Ba is someti-mes related to clay minerals or to iron oxides.

Some recent workers stated that Ba enrichment in sedimentary deposits can be consi-dered as an indicator of a high flux of biogenic material to the sediments and therefore ofhigh surface-water productivity (Schmitz 1987, Dymond et al. 1992, Van Os et al. 1994).

Several diagenetic models have been proposed for the origin of barites. For example,Goldberg and coworkers (1969) suggested that barite nodules from the coast of Californiaoriginated in a coastal lagoon or a shallow hydrothermal environment. In contrast, Dean& Schreiber (1978) concluded that for the barite at DSDP sites 369 and 370, deposits for-med diagenetically in sediments exposed at the sediment-water interface during long hia-tuses.

SEM studies on some selected samples (with high Ba concentration) revealed that theBa in Patharia well-5 is associated with SiO2 and clay minerals (Figs 42–43) Energy-dis-persive X-ray analysis, in conjunction with the SEM, was used to confirm barium andsulphur as the main constituents of these crystals (Figs 44–45). TEM study was underta-ken to detect barite crystals (Figs 56–58). TEM photographs showed the presence ofbarium crystals nicely. Under the SEM, a sample of barite occurrence revealed large crys-tals (10–38 µm). Large crystals are rounded to subrounded with broken surfaces, sho-wing a very fragile nature. Smaller barite crystals show euhedral shapes. A SEM studyreveals also the presence of small barite crystals 2–8 µm in length in association withsilica and clay minerals.

Dehairs and coworkers (1980), Nuel and Shelton (1986), Breheret and Delemette(1989) and Mills (1971) found a similar type of Ba. In their study, they find that the Ba insuspended particles is dominantly in the form of barite and so conclude that it formed inthe upper water column by the breakdown of organic matter, release of Ba, and formationof barite in a microenvironment.

These beds from the present study area deposited in a shallow marine environment tobe enriched by adsorbed Ba. During diagenesis oxidation conditions prevailed liberatingsulfate ions. Meanwhile, due to compaction and lithification, Ba left clay minerals andcombined with the liberated sulfate ions forming barite grains, existed in certain strati-graphic horizons of shales and shaly beds. In these beds, at depth of 1 451.5 m Ba isdetected up to 6 808 ppm (Table 10). The development of barite fronts requires a labileBarium source which is not likely to be clastic detritus but rather a large "bio-barite" fluxobserved in areas of high biogenic opal productivity (Dehairs et al. 1980, 1990, Collier &Edmond 1984, Bishop 1988, Dymond et al. 1992). The deposition of "bio-barite" microc-rystals on the sea floor provides a source of labile barium, which can be subsequentlyremobilised diagenetically within the sediment column, in the zone of sulphate depletion(Torres et al. 1996).

In the light of the discussions and observations on Barium it can be concluded that itsoccurence is controlled by barite and a diagenetic mechanism. The remobilisation ofbiogenic barium in sulphate depleted zones, and subsequent precipitation, results in the

102

accumulation of authigenic barite in several continental margin settings. Authigenic bari-tes are likely to occur in areas where intensive high productivity results in a large flux ofbiogenic barium to the ocean floor (Von Breymann et al. 1993). Ba enrichment alsoshows a high surface-water productivity. Ba/Rb values locally increasing (0.48–55.80 inPatharia well – 5) in NS could also reflect diagenetic mobilization of barite.

In addition, Ba can be used to determine the provenance of various sediments. Cullerset al. (1988) suggested that Co and Ba might be also suitable for distinguishing silicic andbasic sources of sands. The best provenance discrimination was obtained if the Ba/Sc andBa/Co ratios were used. In the present study, the high Ba/Co and Ba/Sc ratios of thesamples point to granitic rocks as possible source rocks of the SG sediments.

Ba/Rb (0.48–55.80) values locally increasing from NS could also reflect diageneticmobilization of barite from sulfate - poor, strongly anoxic sediments to sulfate bearing,dysoxic environments (Bellance et al. 1999).

Finally, it was also considered whether or not the barium values are due to contamina-tion during drilling operations. The barium data are not due to contamination because ofthe followings. If contamination were occurring one would expect that the coarser (incomparison to clay fraction) sandy layers would be more contaminated. This is not in thecase of the present data. The higher barium values observed are obtained for clay-richshale samples. The sample no 180 of Patharia well-5 having high Ba values (2688 ppm),which is the only sample obtained from the shale with thin layer of sand. But with thesimilar lithology the nearest samples (179 & 181) show low barium concentrations. Thesample nos. 149-160, having lithology of shale with sand, but the barium values are lower(appendix-2).

Such characteristics, not possible if data contamination by drilling fluid was effective.Thus the barium data presented in this study can be useful in studies of provenance andpore-water chemistry.

The high enrichment of Ba has also been encountered in other wells of Bangladesh asfollows:1. Well – Bakhrabad –1 (South of the study area)

Ba – 2,638 ppm (Depth: 973.5 m)Ba – 7,881 ppm (Depth: 978 m)

2. Well Jaldi –1 (Coastal area)Ba – 1938 ppm (Depth:1645 m)

3. Well Cox’s Bazar –1 (Coastal area)Ba – 13,478 ppm (Depth: 2905m)

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Fig 42. SEM photograph showing the colour bands of minerals present in the sample number118 of Patharia well – 5. Band – 1 (red) for BaSO4, Band – 2 (green) for clay minerals and band– 3 (blue) for SiO2. Black band were not classified. Semi quantitative estimation of red colourband is given which proves that the mineral is Barite, BaSO4. Scale bar 500 µm.

104

Fig 43. Backscattered electron image and colour band SEM photograph of sample number 118of Patharia well – 5. Band – 1 (red) for BaSO4, Band – 2 (green) for clay minerals and Band –3 (blue) for SiO2. Black band were not classified. Semi quantitative estimation of red colourband is given which proves that the mineral is Barite BaSO4. Scale bar 200 µm.

105

Fig 44. Backscattered electron image of sample number 180 of Patharia well – 5 with the graph(down) showing the elemental composition of a Barite mineral.

106

Fig 45. Backscattered electron image of sample number 180 of Patharia well – 5 with the graph(down) showing the elemental composition of a Barite grain.

107

Fig 46. A) SEM photograph showing a large Barite grain (White, X4500). B) SEM photographshowing higher concentration of Barium in the sample 118 of Patharia well – 5 (X1600).

A

B

108

Table 10. High Concentrations of Barium in Patharia well – 5.

7.2.2 Total Rare Earth Elements (∑REE)

REE, lanthanum-lutetium are commonly-used indicators of igneous processes. The REEhave a similar charge (3+), but a slight decrease in ionic radii from the light REE (LREE)to the heavy REE (HREE), known as the lanthanide contraction (Garver & Scott 1995).REE distributions in sedimentary rocks have played a central role (McLennan 1989).Sedimentary REE patterns may provide an index for the average provenance composi-tions. REE are particularly useful for studying provenance because of their low solubilityduring weathering and diagenetic processes (Bhatia & Taylor 1981).

Table 11 shows the total of REE, HFSE and LILE elements for the Surma Basin. Aplot of total REE content (∑REE) variation in relation to burial depth for all wells hasbeen presented in Fig. 47. In general, the plot shows the variation in ∑REE content for thewhole sequence of the wells. The average total content of REE is between 150 to 230ppm. The ∑REE generally show systematic enrichment and subsequent depletion throug-hout the sequence. A comparison between the sequence of ∑REE and the maturity para-meters and also the grainsize parameter (SiO2/Al2O3) shows that ∑REE sequence has gota good relationship with these sequences as shown in figures 23–28 and figure 48. Thus,it can conclude that ∑REE pattern is compatible with the maturity and mineralogy of ter-rigenous detritus.

Sample No Depth (mbsf) Concentration (ppm)118 1 451.5 m 6 808120 1 452.5 m 1 420121 1 453.0 m 1 521126 1 454.0 m 1 398172 3 161.0 m 2 091180 3 165.0 m 2 688

109

Fig 47. Showing the ΣREE content variation in relation to burial depth for all wells. (1)Habiganj well – 1, (2) Kailastila – 1, (3) Patharia – 5, (4) Fenchuganj well –2, (5) Rashidpur –1and (6) Atgram – IX. Figures 9 – 14 shows the actual depth of the samples. See appendix. 1 and2 for details of the samples.

A plot of ∑REE parameters against the parameters (K2O/Na2O ratio, SiO2/Al2O3 andAl2O3) shows an increase in ∑REE with an increase in the ratios mentioned. Thus thedominant source rocks from dacites to granite-gneisses and sedimentary rocks is ref-lected in the ∑REE characteristics of SG sediments (Fig. 48). Similar results were alsoobserved by Bhatia (1985). Neogene shale (NS) have a significantly higher K2O andAl2O3 content due to the enrichment of clay minerals and thus have a higher K2O/Na2Oratio. ∑REE increases with an increase in K2O/Na2O and probably a decrease in SiO2/Al2O3, as the dominant source rock changes from dacites to granite-gneisses and sedi-mentary rocks. REE abundance may also reflect its concentration in the clay-size fraction(clay minerals) with reference to mineralogy of terrigenous detritus (See also Bhatia1985, Cullers et al. 1979). Clay minerals are a major carrier of the REE in sediments.Chaudhury and Cullers (1979) have suggested that the REE content in sediments may bediluted by quartz and thus it may be inversely related to the quartz content. In this study,the relationship in the hypothesis by Chaudhury and Cullers (1979) can be graphicallydemonstrated as shown in figuress 23–28 and 48.

The significant variation in The ∑REE sequences pattern noted in the present worksuggests that the variation in Tertiery (Miocene) sedimentary rock is possible and is cont-rolled by their source rocks and tectonic settings.

110

Fig 48. Plots of K2O/Na2O, SiO2/Al2O3 and Al2O3 versus ΣREE for Patharia well-5 shales. Notethe positive correletion indicating increasing maturity (arrow in Fig. a) due to changes ingranite-gneiss and sedimentary source rocks and presence of clay minerals in ΣREE (Fig. c).ΣREE increases with the decrease in SiO2/Al2O3 (arrow in b) indicate that ΣREE content isinversely related to the quartz content.

ΣREE

Al2/O3 (wt %)

K2O/Na2O

SiO2/Al2O3

ΣREE

ΣREE

111

Table 11. Total of ΣREE, HFSE and LILE. Samples 1–19 (Habiganj well – 1), samples 20–29 (KailasTila well – 1), samples 30–69 (Rashidpur well – 1), samples 71–85 (Atgramwell – IX), samples 86–114 (Fenchuganj well – 2), samples 115–152 and 154–188(Patharia well – 5).

Sample No

∑REE(Total in ppm)

∑ HFSE (Total in ppm)

∑LILE (Total in ppm)

1 219,00 290,00 925,002 217,00 294,00 916,003 217,00 304,00 913,004 215,00 293,00 897,005 247,00 301,00 936,006 232,00 290,00 947,007 194,00 290,00 94,008 226,00 302,00 955,009 255,00 301,00 1189,0010 256,00 294,00 1013,0011 186,00 299,00 846,0012 206,00 308,00 954,0013 223,00 373,00 863,0014 238,00 299,00 999,0015 245,00 304,00 1009,0016 262,00 285,00 1014,0017 212,00 377,00 808,0018 230,00 399,00 863,0019 247,00 400,00 836,0020 225,00 328,00 1052,0022 203,00 338,00 800,0023 215,00 294,00 900,0024 276,00 339,00 1038,0025 231,00 284,00 915,0026 186,00 420,00 783,0027 175,00 362,00 758,0028 243,00 330,00 932,0029 217,00 349,00 834,0030 235,00 335,00 971,0031 240,00 385,00 886,0032 257,00 389,00 939,0033 241,00 402,00 847,0034 217,00 374,00 903,0035 176,00 379,00 875,0036 318,00 398,00 1017,0037 217,00 353,00 866,0038 233,00 346,00 840,0039 239,00 386,00 875,0040 252,00 215,00 873,00

112

Table 11 continued

Sample No




41 235,00 343,00 906,0042 192,00 394,00 851,0043 220,00 340,00 1298,0044 201,00 312,00 891,0045 244,00 358,00 887,0046 191,00 310,00 909,0047 196,00 311,00 910,0048 220,00 318,00 896,0049 201,00 326,00 938,0050 204,00 314,00 919,0051 191,00 325,00 901,0052 217,00 345,00 881,0053 218,00 308,00 951,0054 210,00 309,00 940,0055 207,00 337,00 887,0056 216,00 285,00 982,0057 212,00 309,00 933,0058 223,00 337,00 963,0059 198,00 309,00 924,0060 240,00 293,00 974,0061 221,00 298,00 979,0062 207,00 339,00 957,0063 227,00 344,00 925,0064 215,00 396,00 841,0065 187,00 381,00 891,0066 227,00 334,00 893,0067 220,00 356,00 862,0068 211,00 324,00 939,0069 232,00 324,00 1015,0071 186,00 295,00 891,0072 215,00 296,00 898,0073 194,00 303,00 866,0074 237,00 301,00 991,0075 232,00 283,00 932,0076 212,00 292,00 894,0077 214,00 307,00 921,0078 149,00 376,00 592,0079 204,00 377,00 983,0080 257,00 434,00 869,0085 54,00 152,00 452,0086 184,00 332,00 710,0088 194,00 373,00 765,00

113

Table 11 continued

Sample No




90 250,00 317,00 87,0091 210,00 315,00 1096,0092 211,00 340,00 1035,0095 267,00 354,00 1109,0096 266,00 359,00 1021,0097 215,00 380,00 925,0099 238,00 332,00 968,00100 244,00 343,00 1053,00103 219,00 306,00 843,00104 238,00 321,00 991,00105 243,00 333,00 1046,00108 240,00 360,00 986,00109 203,00 289,00 855,00110 186,00 305,00 786,00111 207,00 320,00 795,00112 224,00 325,00 975,00113 221,00 331,00 828,00114 213,00 280,00 886,00115 246,00 307,00 921,00116 250,00 310,00 914,00117 221,00 316,00 1235,00118 73,00 306,00 7205,00119 208,00 344,00 1007,00120 223,00 334,00 1828,00121 292,00 360,00 1935,00122 202,00 274,00 1046,00124 215,00 370,00 908,00125 241,00 337,00 1228,00126 198,00 306,00 1834,00127 161,00 279,00 789,00128 216,00 333,00 903,00129 235,00 290,00 1108,00130 135,00 284,00 1164,00131 137,00 265,00 1236,00132 172,00 311,00 798,00133 232,00 318,00 867,00134 190,00 303,00 786,00135 236,00 322,00 872,00136 251,00 329,00 894,00137 223,00 322,00 878,00138 219,00 316,00 887,00139 218,00 317,00 881,00

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Table 11 continuedSample No




140 232,00 318,00 983,00141 230,00 312,00 910,00142 241,00 287,00 903,00143 216,00 287,00 896,00144 237,00 285,00 903,00145 227,00 211,00 929,00146 249,00 311,00 922,00147 255,00 184,00 1107,00148 246,00 315,00 1020,00149 225,00 257,00 933,00150 229,00 322,00 981,00151 246,00 215,00 971,00152 218,00 210,00 1129,00154 196,00 153,00 754,00155 140,00 269,00 956,00156 234,00 325,00 1070,00157 160,00 318,00 889,00158 165,00 269,00 720,00159 161,00 275,00 196,00160 183,00 251,00 516,00161 254,00 609,00 999,00162 230,00 355,00 847,00163 220,00 291,00 938,00164 225,00 303,00 991,00165 237,00 348,00 925,00166 231,00 305,00 1044,00167 244,00 321,00 1019,00168 204,00 307,00 881,00169 221,00 315,00 934,00170 227,00 323,00 870,00171 272,00 325,00 1084,00172 196,00 361,00 2546,00173 183,00 339,00 1018,00174 221,00 424,00 442,00175 168,00 301,00 897,00177 194,00 336,00 1076,00178 2331,00 347,00 3343,00180 173,00 212,00 3165,00181 214,00 264,00 1001,00183 220,00 314,00 1422,00184 227,00 323,00 1151,00185 194,00 338,00 1409,00186 209,00 350,00 145,00187 266,00 1328,00 1134,00188 209,00 1350,00 1147,00

8 Mineralogical Results

8.1 XRD

X-ray Diffraction Analyses are based on a selected 15 representative core samples fromthree different wells: Atgram well-IX, Fenchuganj well-2, and Patharia well-5. Theanalyses aimed at documenting the gross mineralogy, clay mineralogy and depthdependent clay diagenesis of shales of SG sediments of Surma Basin. It was also aimed atdiscussing the implication of clay mineral diagenesis on major geologic processes likeoverpressure generation and structural developments, with a guideline for defining theoptimum exploration strategy using clay technology as a powerful tool.

The study also concentrates on the evolution of climate variabilities during theNeogene by using the clay mineralogy analysis of Neogene shale (NS) samples of SB. Aclay fraction (< 2 µm) was separated out from the shale by disaggregating and despersingthe sample in distilled water and immediately washed by centifugation. The fraction of< 2 µm was isolated by centrifugation and suspension were dried on glass slides. The claysamples 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.The digital data were interpreted using Diffrac plus software of the Bruker Analytical

X-ray system, which comprises a search-match routine based on a Powder DiffractionFile. Minerals identified in the studied NS samples include quartz, kaolinite, illite,chlorite, illite/smectite, kaolinite/smectite mixed layers. However, minor amount offeldspar is still present in many of the samples, and trace amounts of calcite and dolomiteare also present in many samples.

X-ray diffractograms of separated clay fraction (< 2 µm) of some selected shalesamples are shown in Fig. 50–54.

A semi quantitative XRD analysis was done using a Diffrac plus computer programand the percentages of each of the minerals of NS were calculated and are given in Table13. The program allows one to calculate the major diffraction peak heights. First, itmeasures the left and right angles (2-theta) of the peaks. The measuring of net height, raw

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area and finally net area is followed. The net area of peaks are converted to thepercentages. As an example in the sample number 124 are four peaks of kaolinite-smectite, illite-smectite, Quartz and Feldspar. The peak areas calculated for these peakswere 199, 148, 30 and 15 respectively. The total net area was 392, and thus individualmineral percentages were calculated as 51%, 37%, 8% and 4% respectively which isshown in the following table.

Table 12. Semi – quantitative XRD analysis of sample no. 124.

8.1.1 Non Clay Minerals

Quartz: Quartz forms one of the most abundant minerals in most of the samples. Quartzis identified by its distinctive reflections at 4.26 Å and 3.35 Å. The 3.35 Å peak of quartzwas more intense than the other peaks. There was a coinciding in some samples, with astrong reflection of illite at 3.33 Å which makes this 3.35 Åpeak difficult to use, due to asquartz is abundance. The variation in the samples of Patharia well-5 was high.Fenchuganj well-2 and Atgram well-IX well samples do not show major variation.

Feldspar: Feldspar is the next important non-clay mineral present in most of thesamples but in minor amount. It is identified by distinct reflection in the spacing range of3.8 Å to 3.2 Å No meaningful variation in the abundance of feldspar is evident from theXRD reflections of NS.

Calcite: Calcite is identified by only a weak reflection at 3.01 Å showing its presencein trace amounts.

Dolomite: Dolomite is identified by only a weak reflection at 2.8 Å, indicating a traceamount of the mineral.

Sample No. Left Angle 2-theta

Right Angle 2-theta Net height Cps Raw Area

CpsX2-th Net Area CpsX2-th %

124 5,22 6,18 554 847 199 51% K/S10,8 10,58 109 569 15 4% F11,4 12,68 540 1038 148 37% I/S20,6 21 124 471 30 8% Q

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Table 13. Relative clay & non-clay mineral abundance in clay fraction samples of SurmaBasin, Bangladesh.

8.1.2 Clay minerals

Illite: Illite is the major clay mineral present in all the samples and is identified by aseries of basal reflections at 10.1 Å, 4.98–5.01 Å 3.33 Å, and 2.89–2.92 Å Onglycolation, illite is essentially nonexpanding. On heating to 550o C the (001) peak ofillite may show a slight collapse. Values of less than 10 Å may be due to a K+ deficiencyor the substitution of Fe2+ or Mg2+ for [Al3+]IV (Güven et al. 1980).

Kaolinite: Kaolinite is another major component in all the samples. It is represented bya basal (001) reflection at 7.06-7.14 Å and (002) reflection at 3.53 Å, the collapse ofKaolinite structure to an amorphous material takes place on heating to 550° C and this

Sample No.Depth (m) Illite-Smectite

(%) Illite (%) Kaolinite-Smec-tite (%)

Kaolinite(%) Chlorite (%)

* 77 3639 9 33 31 * 84 4733 64 13* 84 (H) 4733 10 55 26** 91 3137 –3143 21 39 26** 91 (H) 3137 –3143 29 19 38 1**100 3730 –3379 31 38 7***117 1450.5 21 38 5118 (H) 1451.00 4 40 42121 1452.00 14 57 22121 (H) 1452.00 1 29 27124 1454.5 37 51124 (H) 1454.5 8 37 29124 (G) 4 23 59132 1829.5 28 70 1132 (H) 1829.5 4 41 27147 2290.5 29 71147 (H) 2290.5 6 40 18155 2294.5 14 18 68168 2832.7 8 26 66168 (G) 2832.7 13 33 54172 3161.25 26 74172 (H) 3161.25 10 48 42180 3164.7 34 66180 (H) 13 56 31180 (G) 9 39 52183 3166.2 6 37 32 8183 (H) 6 46 19*Atgram well – IX. ** Fenchuganj – 2 well. *** Patharia – 5 well (117–183). (H) = Heated, (G) = Glycolated

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confirms the identification of the mineral. Kaolinite reflections do not show any systemicvariation for the samples studied.

Chlorite: Chlorite is represented by its basal reflections at 14.25 Å, 7 Å, 4.7 Å and 3.5Å respectively. The basal reflection at 14.25 Å could not be used directly for theidentification of chlorite because of an interfluence with an Illite-Smectite mixed layer,and 7.14 Å also could not be used for chlorite identification because of the interferenceand coincidence of kaolinite reflections.

Illite-Smectite (I/S) mixed layer clay. None of the samples show discrete or puresmectite, but many have an illite/smectite layer phase. The amount of interlayering ofillite with smectite in the mixed layer I/S has been found to vary depending upon theposition of the sample in the stratigraphic sequence.

The expandibility or the percentage of smectite could not be calculated due to theabsence of 17 Å reflections. The apparent absence of 17 Å peak intensity in thediffraction profile of the clay fraction of the Neogene shale is possibly due to thetransformation of illite/Smectite to illite during burial diagenesis. Imam (1993) studyingthe Neogene sediments of the Bengal Basin, Bangladesh, noted the disappearence of 17 Åpeak intensity in the stratigraphically deepest samples. He pointed out that the absence ofa 17 Å peak is diagenetic and caused by the illite/smectite becoming orderly interlayeredwith more than 60% illite layer.

According to Srodon (1980). “If a reflection occurrs between 5.3° and 8.7° 2θ in thediffraction pattern of an ethylene glycol-solvated illite/smectite, the interstratification isordered to some degree”. It can be noted that a reflection occurs between 5.3° and 8.7° 2θin the studied samples of Neogene shale, and therefore the samples are ordered to someextent, according to Srodon.

The transformation of smectite to illite through an intermediate mixed-layer illite/smectite (I/S) clay is a widely recognized clay diagenesis reaction in shales withprogressive burial (Weaver 1956, Dunoyer de Segonzac 1970, Perry & Hower 1970,Weaver & Beck 1971a,b, Hower et al. 1976, Boles & Franks 1979), and more recentworks (Srodon 1999, Srodon et al. 1992, Sato et al. 1996) have given a good account ofthis.

Mixed layer illite/smectite (I/S) is dominant in the clay-size fraction of many shalesfrom Tertiary basins. The reaction of smectite to illite in these clays has receivedconsiderable attention because of its potential for: 1) flushing hydrocarbons from theshales (Burst 1969, Bruce 1984), 2) catalyzing hydrocarbon generation (John &Shimoyama 1972), 3) producing high pore - fluid pressures (Powers 1967) and 4)providing cementation agents to sandstones (Towe 1962, Boles & Franks 1979, Lahann1980).

Present study on Neogene shale of SB, Sylhet Bangladesh shows similar results fromI/S interlayers of the Gulf coast. Illite/smectite provides a useful tool for explainingdiagenesis and for reconstructing maximum burial conditions.

Hower (1981) stated that the usual spacing for samples from shales and sandstonesunder air-dried conditions is close to 15 Å. In the present study randomly interstratifiedIllite/Smectite were identified by the reflection at the 14 Åpeak with air-dried, untreatedsamples as well as in heated (550°C) and glycolated samples. Fig. 50 shows the randomlyinterstratified I/S layer in air-dried samples. The same reflections at the 14 Åpeak werealso identified in the samples heated to 550°C (Fig. 51) and glycolated samples (Fig. 53).

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After Weaver's study of mixed-layer clays in sedimentary rocks (Weaver 1956), itbecame evident that the illitization of smectite progressing with depth in sedimentarybasins is a universal phenomenon (Srodon 1999). The process was related to petroleumoccurrences (Burst 1969) that generated the interest of the oil industry.

The most distinctive feature of clay diagenesis in the NS is shown by the systematicchange in illite/smectite mixed layer clay minerals with increasing stratigraphic and burialdepths. Randomly interstratified illite/smectite mixed layer clay shows a progressive lossof smectite content via transition to the illite layer with increasing depth: This is referredto as "illitization" (Imam 1994).

The problem of the smectite illitization mechanism has been vigorously debated fromseveral standpoints over the past 30 years. Because of the abundance of this mineral, theillitization mechanism is helpful for understanding the evolution of pore water chemistryand sandstone cementation during diagenesis (Boles & Franks 1979).

The diagenetic illitization of illite/smectite in shales has also been observed in otherwells studied in the Bengal Basin (Imam 1983, 1987, 1993, Imam & Shaw 1985) and inmany other basins of the world i.e. the Gulf Coast basin, USA (Perry & Hower 1970,Hower et al. 1976 and Berger et al. 1999), North Sea basin, U.K. (Pearson & Small1988), Colorado River Delta basin, USA etc.

8.1.2.1 Diagenetic model of Surma Basin

Mixed layers are created by the process of weathering. The formation of mixed layers I/Sand K/S and their evolution towards illite or chloride by the fixation of potassium,sodium or magnesium have taken place in the Neogene shale of SG sediments from theSurma Basin. Burial diagenesis occurs then on a population of mixed layer mineralswhere all proportions of illite/smectite and kaolinite/smectite are possible.

The present study reveals that the transformation of smectite to illite took placethrough an intermediate mixed layer illite/smectite. It also shows the transformation ofsmectite to illite through mixed layer kaolinite/smectite. This is one path way with theadsorption of potassium and sodium to produce these illites. Another path way is themagnesium path way which produces chlorites due to adsorption of Mg2+. The diagenetictransition of smectite to illite is accompanied by the expulsion of interlayer water fromsmectite to the pore water system, which is referred to smectite dehydration (Powers1967, Burst 1969, Perry and Hower 1970, Imam 1994).

Mixed layers are intermediate stages which occur during aggradation by deepdiagenesis. This aggradation is the result of an incorporation of certain cations taken upfrom interstitial solutions and of a rearrangement within the lattice. Clay minerals subjectto deep burial diagenesis include I/S mixed layers due to the equilibrium between theminerals and interstitial solution under the physical and chemical conditions of deepdiagenesis.

The aggradation of degraded 2:1 clay minerals consists essentially of a loss of water,an adsorption of Na+, K+ and Mg2+ and a rearrangement of ions within the lattice. Theions migrate from the interstitial solution towards the interlayers, from the interlayers

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towards the octahedral layers (Mg, Fe, Al) and finally to the tetrahedral layers accordingto the scheme presented by Millot and coworkers (1966).

The transformation of clay minerals are thus an aggradation leading to illite andchlorite lattices. Kaolinite is unstable in this confined environment and the ions if releasesprovide ions for the aggradation of illite and chlorite.

These transformations during deep diagenesis are irreversible at the depths at whichthey normally occur.

The diagenetic evolution of the Surma basin has been presented as a proposed modelin Fig. 49.

Fig 49. Diagenetic model of Surma Basin.

8.1.2.2 Implication of Smectite diagenesis and dehydration.

In Patharia well-5 subsurface overpressure has been encountered in the SG (BhubanFormation) sediments with the top of overpressure at a depth of about 480 m.Considering the huge thickness of shale in early Neogene SG sequence, the smectite toillite diagenetic transition should have made a significant amount of water available in thesubsurface for migration according to the clay dehydration model of Imam (1994).

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According to Imam, the diagenetic transition of smectite to illite is accompanied by theexpulsion of interlayer water from smectite to the pore water system, which is referred toas smectite dehydration. Powers (1967), Burst (1969) and Perry and Hower (1972)propose that clay dehydration models be related to the various stages of smectite to illitetransition, in which the smectite diagenetic derives water available for migration in thesubsurface. Such water may act as an agent in helping petroleum migration, overpressuregeneration and structural shaping (Potter et al. 1980). It is suggested by Imam (1994) thatsmectite diagenesis and dehydration have contributed to the generation of overpressure inthe Bhuban Formation in Patharia well-5.

Powers (1967) first advocated the idea that the smectite to illite transition may berelated to abnormal fluid pressure generation at depth. Other models have been suggestedfor overpressure generation, i.e. compaction disequillebrium (Chapman 1980) andaquathermal pressuring (Barker 1972), but the smectite to illite diagenesis and consequentdehydration has been considered as one of the most important (Plumely 1980, Bruce1984).

8.1.2.3 Clay minerals of SG and its implication in petroleum geology.

To conclude the subject of SG clay minerals, it is quite expected to mention some wordson its implication in petroleum geology. In this connection a review of studies on thesubject has been made and presented to understand them in a better way.

The clay minerals have significant controls on the porosity and permeability propertiesof sandstones, and this may have implications on reservoir performances during drilling,production and well stimulation operations (Imam 1989). The pore systems ofsedimentary rocks may be lined or filled with a variety of different clay minerals. Theseclays can greatly reduce permeability, increase acid or fresh water sensitivity, totally alterthe electric log response and increase irreducible water saturations. The composition ofthe clays is of great importance because their different compositions will cause them toreact differently to various drilling and completion fluids. As a result, fluids should bedesigned for the specific variety of clay present in the pores (Almon & Davies 1981).

Clay minerals can cause formation damage and production problems during drilling,production and well stimulation operations. Smectite is water sensitive and would swellwith fresh water, resulting in a loss of permeability. Kaolinite may be dislodged, migrateand block a pore throat during production. Chlorite is acid sensitive and would reactduring acid treatment to produce precipitates that could damage reservoir performances.Kaolinite can also choke pores and chlorite and smectite coat grain surfaces, reducingpore throat diameter. Some illite morphology bridges the gaps between grains andseverely damages permeability and so on (Imam 1989).

The clay mineralogy presented in the present study for the Neogene shales of theSurma Basin may provide a guideline for defining optimum exploration strategies andefficient well management.

The engineering problem in Kaolinite-rich sands can be easily resolved through theuse of any of the clay stabilization systems (such as polyhydroxy-aluminium compoundsor cationic polymer systems), as long as the treatment is carried out early in the history of

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a well (Haskin 1976). The production problems caused by the smectite type of clayminerals can be overcome by the use of oil base, potassium or ammonium chloridedrilling, completion and stimulation fluids. If swelling has already occurred within thereservoir, the damage may be corrected by acidizing with a peak mixture of HCl and HF,providing injectivity is not totally lost (Almon and Davies, 1981). The main engineeringproblem posed by illite is that it creates large volumes of microporosity. Illite sometimesgrows in pores as masses of long, hair like crystals, which can considerably reduce thepermeability of sediment (Guven et al. 1980). In the presence of fresh water, these illitesfibres tend to clump together, further reducing permeability. If these illites are notdissolved prior to production, they may break during production and will migrate to thepore-throats and act as a check valve (Almon & Davis 1981).

Illite can be dissolved by using an acid mixture consisting of HCl and HF.Chlorite which is causing reservoir damage by its acid sensitive nature can be dealt

with by using appropriate chemicals (an oxygen scavenger and an iron chelating agent)and care is taken to recover all the acid introduced into the well (Smith et al. 1969).

8.1.2.4 Kaolinite - Smectite (K/S) mixed layer clay: a new mineral in Bangladesh

The occurrence of interstratified Kaolinite/Smectites (K/S) in nature was first reported bySudo & Hayaski (1956) and were subsequently confirmed by Altschuler et al. (1963).These minerals were rediscovered in the Tertiary clays of Yucatan by Schultz et al.(1971) and in hydrothermal deposits of lower Silesia by Wiewiora (1971,1973). Sincethen many occurrences of K/S have been described. Studies of the nature of layersequences of such mixed-layer minerals, however, are few: Sakharov and Drits (1973),Cradwick and Wilson (1978), Kohyama and Shimoda (1974) and Tsuzuki and Sato(1974).

Natural occurrences of K/S are limited. Kaolinite-Smectite in sedimentary rocks, ismost often a detrital component, as shown by Theiry (1981) in his study of Eocene claysfrom the Paris Basin. Most probably, the abundance of K/S is highly underestimatedbecause it is difficult to detect, in particular as a minor component (De'Vaux et al. 1990,Hughes et al. 1993, Cuadros et al. 1994).

For the present study kaolinite-smectite mixed layer clays were found in wellsPatharia-5, Fenchuganj-2 and Atgram-IX at different depths. In the Patharia well, K/Smixed layered were present at depths 1 454 m, 2 290,5 m and 3 161 m respectively. InAtgram-IX well it was at 4 733,8 m and in Fenchuganj-2 well at 3 730 m depth.

The K/S was identified by its clear 7.2 Å peak and 3.5 Åpeak respectively. It shows aweak reflection at 2.5 Å peak. The K/S reflections do not show quantitatively significantdifferences between samples. The K/S reflections are all in similar positions andrelatively sharp and intense (Fig. 50 – 54).

Kaolinite-smectite mixed layer clay has been carefully examined by X-ray diffractionand rather unusual properties were discovered which cannot be attributed to a normalKaolinite.

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Fig

50. X

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124

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125

Fig. 52. Characteristics X-ray diffraction patterns of K/S mixed layer clay fraction. A)diffraction of 10Å of the sample no. 91 of Fenchuganj well-2, H, heated; G, glycolated and U,Untreated. B) diffraction of heated, glycolated and untreated clay fraction of sample no 91 ofFenchuganj well-2, with 3.5Å. C) diffraction of heated, glycolated and untreated clay fractionof sample no 183 of Patharia well-5 with 10Å.

126

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127

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128

The X-ray diffraction pattern of kaolinite-smectite mixed layer (disoriented) does notresemble that of a pure Kaolinite. It differs in the number of reflections, their relativeintensities and the exact angular position of the basal reflection.

If an XRD pattern is similar to that of Kaolinite and if the reflection at about 7 Åshows larger d-values than that of halloysite (7 Å) and expands by treatment withethylene glycol, the specimen can be identified as an interstratified Kaolinite. Fig 53which shows an enlargement of 7 Å reflection after treatment with ethylene glycol,proves the idenfication of Kaolinite-Smectite mixed layer. Discrete Kaolinite in thesample will affect the position of neighbouring kaolinite/smectite peak (Tomita &Takahashi 1986).

For more confirmation the samples were heated to 550° C and may be interpreted withthe aid of the concept of Ross (1968). According to this idea we may explain the structureby assuming that the 2:1 type layers do not dehydroxylate; in this way the diffractioncomposed of 2:1 layers can be separated by X-ray amorphous metakaolinite material. Onheating, the breakdown of kaolinite layers causes the reflection to gradually decrease andnearly to disappear, if further heated.

The chemical formula of the K/S mixed layer was calculated and given by Wiewiora(1971) as follows: 1) Kaolinite layers have an ideal composition,

Al2Si2O5(OH)4.2) Smectite layers have the general formulae, Mx + y [(Al2 – y Mg y)(Si4 – x Al x) O5(OH)2]

where M means Mg, Ca, Na, K. An additional assumption is necessary, that x = y.Srodon (1980) suggested that K/S forms by the dissolution of smectite layers and the

crystallisation of kaolinite layers. Thus, the two pathways of kaolinization of smectitediffer by the site of kaolinite nucleation: within versus outside smectite crystals. In mostof the published cases, it can be shown that kaolinite/smectite forms at the expense ofsmectite (Altschuler et al. 1963, Drits & Sakharov 1976).

Shinoyama et al. (1969) described randomly interstratified kaolinite-smectite fromacid clay in Japan. In this case, 2:1 (two tetrahedral per one octahedral) expanding layersof smectite were distinguished. Wiewiora (1973), Sakharov & Drits (1973) proposecrystallisation of K/S from solution. Most authors writing on the subject say that K/Salways evolves from smectite. K/S originating from Kaolinite has not been reported.Wiewiora (1971) made an attempt to evaluate the proportions of kaolinite and smectitelayers in the clay size fraction and found about 80–90 per cent of Kaolinite.

Wiewiora (1971) observed the K/S peaks have combined diffraction effects indicatingthat kaolinite - nonexpanding and smectite - expanding layers are interlayered. Afterheating for 3 hours at 550° C kaolinite layers fully dehydroxylate.

For the present case, the samples were heated to 550° C for 1 hour and the reflectionsof K/S were decreased to a noticeable mark. Fig. 50 shows the non-heated and fig. 51shows the heated reflections of K/S mixed layer. After treatment with ethylene glycol, thesamples were expanded much proving them as kaolinite-smectite mixed layer. Fig. 53shows nicely the expansion of the reflections. A comparison between fig. 51 (heated) andFig. 53 (glycolated) can provide a very useful basis for the understanding andidentification of the K/S mixed layer. The matching number on Powder Diffraction Filefor K/S mixed layer was 29 – 1490.

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Semi-quantitative analysis shows that K/S mixed layer was present by 74% when itattains maximum abundance in the well Patharia-5 at the depth of 3161.25m.

Kaolinite/smectite mixed layer mineral also played an effective role in the diagenetichistory of Surma Basin. The present study reveals that the transformation of smectite toillite took place through intermediate mixed layers of illite/smectite and kaolinite/smectiteby the adsorption of potassium and sodium. Fig. 49 shows the diagenetic model of SurmaBasin where K/S mixed layers were transformed to produce illite under a K-richenvironment.

The dominance of a kaolinite/smectite mixed layer in the clay mineral assemblagereflects the onshore position and shallow nature of the environment with warm climatesand seasonal rainfall. The abundance of K/S mixed layers also indicates that all sourceareas for the sediments of Surma Basin had highly kaolinitic soil profiles, reflecting theirintense weathering. Thus, the presence of a K/S mixed layer bears the evidence ofclimate, environment and diagenesis as well as a clue to provenance.

8.2 TEM and SEM

Non clay accessery minerals detected by TEM in SB include barite, rutile, Fe-oxides andK-feldspar (Fig. 56–58). TEM micro analyses reveal that the order of abundance of theoctahedral cations is Al > Mg > Fe.

Typical smectite and illite characterisation were done using TEM images. Quantitativeanalyses was undertaken for understanding different element concentrations of illite andsmectite (Fig. 55).

A Scanning electron microscope (SEM) has been used in the present study. The veryhigh resolution obtained in the SEM readily describes the minerals. The analyses alsoreveals their morphology, textural relationship and growth habits. SEM analyses werecarried out with magnifications between 100X and 3500X with Au–Pb coated samples.

The minerals within the Surma Group sediments include quartz, calcite, chlorite,kaolinite, illite, smectite and barite. Quartz forms one of the most important diageneticminerals in Surma Basin shales. The quartz occurs as quartz overgrowth. The quartzovergrowth include the small isolated and incomplete growth of quartz crystal faces ondetrital grain (Fig. 60). Pittman (1972) and Imam (1986) have described such growths.Calcite forms an abundant cement in the SG shales. SEM studies reveal from the mutualtextural relationship of calcite and quartz overgrowth, that the pore filling calcite postdates quartz overgrowth as the calcite is seen to envelope the crystal faces of quartzovergrowth (Fig. 59).

Chlorite is also a common clay mineral in Neogene shale of SB. Chlorite occurs aspore filling materials and as a replacement of detrital grain. Some Common morphologiesof chlorites are clusters of bladed or platy crystals. Illite is one of the most common clayminerals in the Neogene shale of SB. Under SEM, illite appears as sheets or large flackycrystals or as fibres. Kaolinite is also a common clay mineral of SB. It is like plateletscrystal under SEM, usually altered from illite. The weathering of K-feldspar leads to theformation of kaolinite. Barite was also detected in Neogene shales. Some samples fromPatharia well-5 having a very high concentration of Ba were studied under SEM.Quantitative analyses showed the Barite (BaSO4) was dominant (Fig. 59). The study alsorevealed that Ba was associated with SiO2 and clay minerals (Fig. 59).

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Pittman (1979) suggested the source of silica for quartz overgrowth to be the result of:(1) pressure solution, (2) diagenesis of clay minerals, (3) decomposition of feldspar, (4)replacement of silicates by carbonates, (5) precipitation from percolating ground water,(6) dissolution of siliceous organism such as sponges, diatoms, radiolaria etc. and (7)hydration of volcanic glass.

The transformation of smectite to illite during the clay mineral diagenesis of shalesreleases Si+4 ions as a by-product which could migrate to adjacent sandstone beds duringshale dewater and could cause quartz cementation (Towe 1962, Boles & Frank 1979;Lahann 1980 & Boles 1981). The amount of Si+4 ions released into the pore water systemwould depend on the availability of shale containing smectite or illite/smectite mixedlayer clay mineral and the degree of diagenesis. The increasing abundance of quartzovergrowth in deeply buried rock is correlated with an increasing degree of burialdiagenesis of shales (Imam 1986). It is thus suggested that the illite/smectite claydiagenesis is one of the important sources of quartz cement in the Neogene shales.Replacement of quartz and feldspar by calcite and the dissolution of feldspar are bothconsidered to be the potential source of quartz cement, as both would release silica intothe pore water (Pittman 1979). In the present study, the replacement of silicate grains bycalcite cement and the dissolution of feldspar were observed. Smectite to illite diagenesisin the illite/smectite clay in Neogene shales is an important source of quartz cement ofSB. The replacement of silicate by calcite and dissolution of feldspar may also have beena minor source locally (Imam 1986).

8.3 Petrography

Thin sections of the Neogene shales of SB contain mainly quartz with minor amounts ofmuscovite, biotite, calcite and clay minerals like chlorite and illite. The quartz grains aremonocrystalline, a few grains show a polycrystalline structure. The grain size wasmedium to fine quartz with normal extinction and most grains were fractured, subangularto subrounded. They were moderately sorted, at places, quartz were found to be ill sorted.The quartz also shows overgrowth. K-feldspar grains are abundant, their grainsize beingcomparable more with the finer fraction of the quartz. The K-feldspar grains arecomparatively finer and plagioclose is rare (Fig. 60).

The quartz is constituted of subangular to subrounded framework grains havingmedium sphericity with a dominant carbonate cement. Quartz grains are fracturedprovides the evidence of intense chemical weathering. Feldspar with a few mica minerals(biotite, muscovite) were present. Matrix of from less than 5% to over 15% were presentand composed of very fine and fine quartz grains admixed with argillaceous material andchlorite flakes. The size of quartz grains shows a distinct bimodality in which the finerfraction is dominant.

Plagioclase occurs as subhedral to euhedral grains. At places, muscovite is foundinterleaved with biotite. Mica occurs as massive grains and as flakes. They are seenchanging to K-feldspar and show alteration to chlorite. The rocks are in general, verypoor in heavy minerals. The sandstones are mineralogically immature. The finer grainshows more angularity than the coarser grains. Patharia well-5 samples show overgrowthof quartz grains and alternative lamine of quartzwacke and siltstone bands.

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Diagenetic replacement of the feldspar grains by carbonate cement is observed. Theseare indicative of the poor textural as well as mineralogical maturity of the SG sediments.Lithic fragments are composed essentially of shale and chert and calcareous particles.Isolated patches of calcareous material were present as cementing material. Calciticcementing material were also present as remnant particles and as antiperthitic cementbetween calcite and quartz grains. Some samples showed ferruginous cement.

Rashidpur well samples are mostly sandy siltstone, micaceous shale and silty shale asconstituents. Some of the wells contain an appreciable amount of clayey detrital matrixand chlorite flakes are important to mention.

Fig. 55. A) Energy disperse X – ray spectra of typical illite from clay separates of sample no 84of Atgram well – IX. Qualitative analysis shows different concentrations of Al, Si, K, Na, Mgand Fe. B) Energy disperse X – ray spectra of typical smectite from clay separates of sampleno 124 of Patharia well – 5. Qualitative analysis shows different concentrations of Al, Si, K,Na, Mg and Fe. Large crystal is about 1700 nm in width.

Energy (keV)

Energy (keV)

Fe

Fe Fe

500 nm

500 nm

cps

cps

O

Si

Al

MgK

Mg

K

OSi

Al

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Fig. 56. TEM – photographs of clay fraction showing clay minerals, silica and iron oxide.Sample from Patharia well – 5.

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Fig. 57. (Top) TEM photographs of clay fraction showing illite (X14000) from the Patharia well– 5. (Bottom). TEM photographs of clay fraction showing kaolinite (X14000) from the Pathariawell – 5.

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Fig. 58. TEM photographs of clay fraction showing Barite grains (black).

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Fig. 59. (A) SEM micrograph of quartz overgrowth with interlocking texture (calcite cement),Q, Quartz; Ch, Chlorite; (X3500). (B) SEM micrograph of quartz (Q) with interlocking clayminerals (I, Illite and K, Kaolinite). (C) SEM micrograph with a generalized view showingsilica and clay minerals associated with some heavies (Barium and Titanium), I, Illite; Q,Quartz; Ba, Barite (X100). (D) SEM micrograph showing clay minerals (illite, kaolinite andchlorite). (E) SEM micrograph showing higher concentration of Barite (white grain) associatedwith quartz and clay minerals (X500). (F) SEM micrograph showing quartz, clay minerals anddominantly Barite (white grain), (X2000). Samples are from Patharia well – 5.

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Fig. 60. A–D. Showing quartz, orthoclase feldspar and plagioclase feldspar as main constituent.Orthoclase are finer. Feldspar shows the alteration. Quartz and feldpar are of different grainsizes. Mica and calcite are common as accessory minerals. Grain boundaries and fractures onquartz and feldspar bears the evidence of intense weathering. Q, Quartz, F, Feldspar, C,Calcite, M, Mica, B, Biotite.

9 Discussion

Palynological evidence suggests that marked vegetation changes occurred that wererelated to paleoenvironmental changes in the Surma Basin during the Neogene. Thequalitative assessment of the palynoassemblages recovered from the wells reveals a closesimilarity amongst themselves in their fair occurrences of pteridophytic spores, viz.,Cyathidites, Cicatricosisporites, schizoeisporites. Among these, the Cyathideous formsare more frequent. Triporoletes characterize the brayophytic component whilegymnospermous pollen are poorly represented specially in older rocks. A reworking ofOligocene-Eocene pollen taxa namely Meyeripollies naharkotensis, PolypodiesporitesOligocenecus and Palmopollenites Eocenecus has also been recorded. The palynologicalassemblages of the Surma Group of SB sedimentary sequences is dominated by triletebearing pteridophytic spores and angiouspermous pollen. The gymnospermouscomponent are very poor and monolete spores are rare.

The qualitative and quantitative analyses of the palynofloral assemblages and itscomparison with other known equivalent assemblages from India have been discussed.Pteridophytic spores are richly represented in the SG sediment of the Surma Basin. Theircomparison with the extant flora indicates the presence of the Lycopodiaceae,polypodiaceae and Cyatheaceae. Plants of these families are mainly found in tropical andsubtropical areas. Angiouspermous pollen also form a significant group and arerepresented by Palmaepollenites dominantly. The distribution of this pollen type isrestricted to tropical and subtropical regions. Gymnospermous pollen grains arecomparatively less represented in the assemblage than pteridophytic spores. Thethallophytic remains are represented in the assemblage by dinoflagellate cysts, fungi andfungal spores.

The quantitative analysis shows the palynoassemblages are populated by 63 generaand 95 species of angiospermous and gymnospermous pollen grains, pteridophyticspores, dinoflagellates cysts and fungal remains. On the basis of quantitative analysisthree local palynostratigraphic zones were distinguished from the SG sediment sequenceof Fenchuganj well-2. They were i) Palmepollenite zone (Z-1), ii) Tricolpate - trilete zone(Z-2) and iii) Dissacate zone (Z-3) in descending order of stratigraphy. The zone-1constitutes the lower bio-stratigraphic unit of the SG with the dominant representation ofpalmepollenite (31%) taxa. Gymnospermous pollen grains are insignificantly representd

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by 3.8%. Pteridophytic spores constitute a major portion of the assemblage. Among thepteridophytic spores, trilete spores form the dominant element while monolete sporesremain insignificant. Cicatricosisporites macrocostatus is the most dominant throughoutthe zone. The presence of Rhizophora pollen provides a basis for interpreting thepalaeoenvironment of the drilled sequence. The zone-3 shows a clear dominance ofdissaccate pollen. Pteridophytes are being represented by the monolete formslevigatosporites and verrucatosporites. Cyathidites minor was the important constituentof triletes spores.

A comparison of the present assemblages with those of known Miocene assemblagesof Assam and Meghalaya (Baksi 1965, Singh et al. 1986, Banerjee 1964) and BengalBasin (Baksi 1971, Deb 1970) of India have been attempted. Comparative study revealsthat the SG sediments of Surma Basin, Bangladesh is closely comparable with that of theAssam and Maghalaya sequences (Garo Hills). The microfloral association of thePalynological zone I can be compared with the palynological assemblage of SimsangPalynological zone IV of Meghalaya, India (Baksi 1965) and Bengal Palynological zone(BPZ)-V (Baksi 1971) and indicate a Lower to Middle Miocene age. The presence ofForaminifera indicate that the sediments of this zone were deposited in shallow marineconditions. The presence of Rhizophora pollen and dynoflagellates indicate brackish toshallow marine deposits.

The microflora of Palynological zone II can also be compared with those of theSimsang Palynological Zone IV of Meghalaya, India and BPZ V (Baksi 1971). Based onthese comparisons, Palynological Zone II is presumed to be of Middle to Upper Miocene.It is interesting that the floral change from monocolpate pollen Palmepollenites toTricolpate - trilete could be recognized well by an increase in frequency by 50%. Thepresence of mangrove pollen Rhizophora indicates the shallow marine to brackishenvironmental deposition of this drilled sequence.

The microfloral assemblage of Palynological Zone III may be compared with SimsangPalynological Zone IV of Meghalaya, India (Baksi 1965) and BPZ Zone-V is presumed tobe Upper Miocene. The qualitative assessment of the palyno assemblages recovered fromthe six wells of Surma Basin, Bangladesh reveals a close similarity amongst themselves.The common occurrence of pteridophytic spores, viz; Cyathidites, Cicatricosisporites,Schizoeoisporites is remarkable. Among these, the Cyathiceous forms are more frequent.Triporolets characterize the bryophytic component.

The stratigraphy of Neogene SG sediments of the Surma Basin are presented on thebasis of palynofacies and lithofacies of the unit. The SG unit contains sandstonelithofacies A and combined facies B consisting of claystone, mudstone and shale. FaciesB is abundant and facies A is less common. In lithofacies, B shale is predominant incomparison to others consisting of siltstone and sandstone.

The reconstruction of the paleoenvironment and paleoclimate in the Surma Basinduring the Neogene is based on a complex stratigraphic sequence including a variety oflithofacies as well as palynofacies indicating shallow marine to brackish interdeltaicenvironments. This interpretation is based on the predominance of lithofacies B and thedistribution of pollen, spores, microplankton and dinoflagellates. The sandstones withsiltstone and shales were deposited in a shallow marine environment throughout theMiocene.

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The angiosperm dominating pollen flora suggest a tropical to subtropical area. Theoccurrence of Cicatricosisporites, an aquatic fern, indicate a marine brackishenvironment. Palynomorph assemblage containing mangrove forms and dinoflagellatesand dinoflagellates cysts suggest a shallow marine environment. The shallow marine andbrackish environment reflects a marine transgression. The transgression of the Miocenecertainly affected the coastline. Major changes in sea level for the Neogene are suggestedbased upon transgressive-regressive phenomena. A marked rise in the sea level wouldhave caused a marine transgression (Bandy 1968). The transgression of the sea over awide area might have increased humidity and moderated the temperature. There is adefinite increase in the gymnosperm microflora indicative of marshland and warmclimate (Srivasta & Banerjee 1969). A high frequency of Dissaccates indicates tectonicactivity and an environmental change in the source area. The presence of reworkedmicroflora elements and conifer pollen together with trangression inidicators, indicate anincreased tectonic activity in the Surma basin during the Neogene and they are related tothe Himalayas.

The Surma Basin area has been progressively coming under the tectonic control of theGreat Himalayan Orogeny and the crustal shortening due to the collision of the Indian andAsian plates and has resulted in extensive uplifts and thrusting of the older rocks(Benerjee 1984). The above mentioned paleoenvironmental circumstances must at somestages have contributed to the retrogressive succession of the vegetation in this area. Inthis connection, it is pertinent to refer to some ecological aspects of palm, as they featureso prominently in the pollen sequence. At present, palm are considered to be veryimportant in the evolution of tropical forests ecosystems and must have been even moreso in the past (Moore 1973).

The pollen spectra of the Surma Basin contains a high percentage of regionallyproduced pollen of mixed sub-tropical vegetation. It reflects a mixture of palm andconiferous forest in which palmepollenite is dominant.

Palynostratigraphic zones suggest a mixed type of subtropical to tropical forests typesexisting there and the development of dominant palm vegetation. Grass, according to thepollen data, were already established in the area during the early to Middle Miocene, andexperiencing summer rainfall similar to the present day climate. The pollen evidence for amixed tropical - subtropical forests could be consistent with the Miocene riverineenvironment in the area as suggested by the lithological data.

The age of the Surma Group sediments of Surma Basin was assigned to the Neogene.On the basis of pollen investigations, the Surma Group was assigned to the early to lateMiocene in age.

The geochemistry of the shales in the SG sediments of the SB has been studied indetails. The SG sediments are typical continentally-derived. A comparison of the NS ofthe present study with previously published shale compositions suggests that the presentstudy has the qualities of an average of averages.

Major element variation curves shows nicely their distribution and interrelationbetween the oxides present in the sequence. Common pictures are as follows:

i) The increase of Al2O3 was related to the decrease of SiO2ii) Increase of CaO and MnO was related to the decrease of Na2O, MgO and K2Oiii) The fluctuation of Al2O3, Fe2O3 and MgO were more or less similar.

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The decrease of SiO2 content in the wells with the increase of Al2O3 may be related toa desilicification phenomenon which took place mainly by the destruction ofaluminosilicates. This is also associated with the formation of clay minerals and grain sizevariation of the sequence.

The increase of CaO and MnO with the decrease of Na2O, MgO and K2O shows thevariation in the chemical composition, reflecting changes in the mineralogicalcomposition of the sediments due to the effects of weathering, marine sedimentation andearly diagenetic processes (Shaw & Weaver 1965, Drever 1971, Nesbitt & Young 1984,1989). Calcium shows a decreasing trend, except for a few intervals in the sequence ofMiocene time. This decrease in the Calcium content can be attributed to the terrigenousinflux and the low abundance of fossil shell and shell fragments, which contribute a majoramount of the Calcium in them. The average magnesium content is low during lower toMiddle Miocene. It shows a little higher content in the Upper Miocene sequences, whichis due to Mg ions absorbed on clays. Manganese content increases at different intervals inthe wells. Clay mineral content is found to cause a Mn enrichment (Becini and Turi1974).

The high K2O and low CaO contents of many samples are interesting. Manysedimentary processes can severely affect the abundance of these elements, includingweathering (Nesbitt 1980), diagenesis and burial metamorphism (Hower et al. 1976). Thehigh K content is almost certainly due to the original presence of large quantities of illite(McLennan et al. 1983). There are clear, positive correlations between K content and theabundances of Al, Cs, Ba, Th and U (Fig. 31), suggesting that the absolute abundances ofthese elements are primarily controlled by the amount of the dominant original claymineral (illite) (McLennan et al. 1983). Decreasing the K2O/Na2O ratio in the wells (Fig.23–28) indicates the decreasing maturity of the sediments, reflecting a reduced influx ofhighly Kaolinized material. Factors such as increased erosion in relation to weathering,and/or transgression resulted in Kaolinite-depleted rocks (Dypvik 1979). The trend ofK2O/Na2O is interesting and is consistent with the grain size analysis. Na and K in thestudied SG Neogene shale are mostly confined in the detrital illites. The variation of thecurve K2O/Na2O obviously reflects in part the different original compositions of thesource rocks. CaO, Na2O, MgO as well as iron were leached from the profile. K2O wascommonly enriched strongly in profiles and suggested the geochemical evidence pointedto alkaline and reducing ground waters (McLennan et al. 1983).

The sequences of the wells exhibit a depletion of Na and Ca (Fig. 9-14), probablyreflecting intense chemical weathering of their source rocks. The variation of Al2O3,CaO, Na2O and K2O in the whole sequence, reflect chiefly variable climatic zones orrates of tectonic uplift in source areas.

The enrichment of MgO in all the wells is related to the decrease of CaO, which in turnis due to the weathering effect. High MgO content is often correlated with lowtemperature oxidative diagenesis. In such a case, an increase of MgO in the rocks isaccompanied by a drop in CaO (Andrews 1980).

The positive correlation of Al2O3 and MgO (Fig. 29) indicate that Mg is originallyassociated with aluminosilicate phases and assumes a minor association with carbonatesduring diagenesis (Bellanca et al. 1999).

The excellent positive correlations of K2O, TiO2 and MgO with Al2O3 (Fig. 29 & 30)indicate that these elements are associated entirely with detrital phases and also

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suggesting that they are associated with aluminous clay minerals such as illite or smectite.It also indicate that weathering was an important factor in the source area, where K andMg are fixed in clay minerals and Ca is preferentially leached (Nesbitt et al. 1980).

The correlation between FeO and MgO (Fig. 32) appears positive suggesting themineral chlorite controls the distribution of Al2O3, MgO and Fe2O3, indicating theirassociation with the clay minerals. The variation of the oxides in the sequences reflectchiefly variable climatic zones or rates of tectonic uplift in source areas.

The concentration of Fe is due to the formation of Fe-hydroxide and due to thepresence of biotite. The higher concentration of Fe2O3 in the wells is indicative ofoxidation, hydration and leaching processes involved during weathering (Mikkel &Henderson 1983).

The different maturity parameters studied display complicated development with anincreasing maturity of different depth intervals. The maturity index ratios are indicative ofa kaolinization process and typically mature sediment. The maturity increases with thedecrease of the silica content and grain size. Cu/Zn ratio of NS indicates fluctuatingoxidizing-reducing conditions in the environment of deposition.

The maturity index curves of the wells of SB provide a meaningful chemostratigraphicsubdivision of bore holes with mature and immature zones. The study demarcated matureand immature zones of the wells of SB on the basis of various maturity indexes. They aregiven in the following table.

Table 14. Mature and immature zones of the wells by the maturity parameters.

The higher abundance of Th in the rocks reflects the presence of Th bearing minerals.The higher Th/U ratio probably indicates the derivation of these sediments from therecycling of the crust. Recycles of sediments themselves lead to a further loss of U and anincrease in the Th/U ratio. The higher value of the Th/U ratio in the core samples isbecause of oxidation. The increase of Th is also related to clay minerals. Th tends toincrease with a fall in potassium and because a concentration of potassium is directlyrelated to illite composition in sediments. Thorium concentration in sediments varies withillite content. Near shore sediments have degraded illites where concentrations of thorium(or uranium) could be higher and the reverse may be true with distance from the shore.Th/U is controlled by the weathering-erosion-diagenesis cycle. Condie (1993) has shownsimilar observation with the study of shales from the upper continental crust, USA.However, the whole sequence of Th/U ratio represents different stages in oxidation,leaching and deposition under marine conditions.

Well Matured zone Immature zone Atgram well - IX 3647–3990m 3639–3640mFenchuganj – 2 well (1) 3259.96–3269.55m

(2) 3624–3779m3137–3143m

Habiganj well – 1 (1) 1849.22–1849.83m(2) 1851.66–1852.65m

1849.83–1850.74m

Kailastila – 1 well 1175.30–1176.22m 2974.52–3731.24mPatharia well – 5 (1) 2831.00–2834.84m

(2) 3165.25–3166.75m2300–2307.25m

Rashidpur –1 well (1) 1087.40–1248.93m(2) 1250.6–1832.45m

1248,93-1249.68m

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Four elements like V, Ba, Cr and Zr were normalized to Rb and the profiles of thewells were fluctuating at different depths. The reason for normalization to Rb is that thiselement is inert with respect to biogenic procceses (Emelyanov & Shimkus, 1986).Geochemical ratios of Cr/Rb, Zr/Rb and Ba/Rb display higher values on the average forNeogene shale of SB, which could be explained by an enhanced terrigenous input duringtheir deposition. Higher values of Ba/Rb at different depths could also reflect a diageneticmobilization of barite (Bellanca et al. 1999).

Cr and Ni values are generally high for the Neogene shale samples of SB (Cr ~14–20,Ni ~ 17–97ppm). The average ratio of Cr/Ni is 2.2 with variation among the valuesbetween the wells. The higher concentration indicates that the source region wascomposed of ultramaffic rocks. Cr and V are positively correlated with Al2O3 (Fig. 30),suggesting that they may be bound in clays and concentrated during weathering (Fedo etal. 1996). Ni may be enriched in a reducing environment. Vanadium also is thought to beenriched in an organically rich, reducing environment.

The sequences completed for Cr/V and Ni (Fig 40) are more or less similar, indicatingthat they were deposited in the same reducing environment. A higher Cr/V ratio probablyreflecting a source rock richer in Cr. The weathering processes would also have differentformations of clay minerals. Differences in Cr/V ratios in the wells are indicators ofdifferences in nature or degree of weathering.

Tectonic discrimination of SG sediments using SiO2 content and K2O/Na2O ratioindicates that the SB sediments were deposited in an active continental margin (ACM).Seismotectonic and tectonic reports of the region as well as the report from the study areaare in good agreement with the present result.

Mineralogical and geochemical data from the Surma Basin suggest that the detritalinput was especially intence during the Neogene probably because of tectonic forceslinked to the active upliftment of the Himalayas.

The effect of variable degrees of weathering in source areas can be important ininfluencing alkali and alkaline earth element contents of terrigenous sedimentary rocks(Nesbitt et al. 1980, Reimer 1985). The degree of chemical weathering is a functionchiefly of climate and erosion rate, the latter of which varies with the rate of tectonicuplift (Wronkewicz and Condie 1987). Neogene shale of the Surma Basin reflects avariety of climatic conditions and perhaps also different tectonic regimes. The SurmaGroup unit exhibits the greatest depletion in Ca and Mn and appears to reflect the mostintensely weathered source rock. The Neogene shale of the SG also exhibits less depletionof the alkaline element and reflects an active uplift of the Himalayas (Wronkewicz andCondie 1987).

The dominance of shale with sandstone in SG reflects a low-energy depositionalenvironment. The distribution of HFSE (Zr, Nb, Hf, Th, and V), REE, Sc and Co of theNeogene shale of SB generally have relatively short residence times in seawater and arethe least mobile during weathering, transport and diagenesis and thus provides a clue tothe provenance of terrigenous sediments. The data presented in table-11 and in appendix-2, in figures 34–39 and 47 indicate that REE, HFSE tend to increase and fluctuate inabundance with the stratigraphic height. This stratigraphic geochemical trend suggeststhat both granite and basalt sources increase with time during the deposition of SGsediments.

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In terms of the distribution of Th-Hf-Co-Sc, the Neogene shales clearly show a largecontribution of mafic and ultramafic components. The high to very high ratios of Cr/Vand Ni/Co also reflect a significantly greater contribution of mafic-ultramafic componentsin the sources (Taylor & McLennan 1985).

The relative contributions of granite and basaltic sources are reflected in thedistribution of Zr and Cr in Neogene shale of SB, since these two elements are enriched inthe whole sequence (Appendix-2). The enrichment of these elements again indicatesincreased input into the Surma Basin of both granite and basalt. The low Cr/Zr ratio (<2in average) in the Neogene shales also indicates the importance of granite relative tobasalt (Wronkewicz and Condie 1987).

Taylor and McLennan (1985) have shown that Th/Sc and La/Sc ratios reflect theproportion of the mafic and ultramafic component compared to the felsic component. Theabrupt increase of these ratios in Neogene shale of SB (Table 11, Appendix-2) attests tothe importance of granites in the source areas. The best provenance discrimination wasobtained if the Ba/Sc and Ba/Co ratios were used (Cullers et al. 1988). In the presentstudy, the high Ba/Co and Ba/Sc ratios of the samples point to granitic rocks as possiblesource rocks.

The high proportion of granitic source material entering the Surma Basin, as impliedby the geochemistry of the Neogene shales may be derived from the Himalaya locatedtowards the northern boundary of the Surma Basin, a model more consistent withprevious source area studies of the Surma Basin (Ray 1982, Molnar 1984, Johnson andAlam 1991).

The diagenetic history of the NS sequences has been studied in details. Illite, kaolinite,chlorite, I/S and K/S mixed layer are the main clay minerals in the SB. Thetransformation of smectite to illite through an intermediate mixed-layer I/S clay is awidely recognized clay diagenesis reaction in shales with progressive basal depth. Mixedlayer kaolinite-smectite has been reported for the first time and is thus a new mineral inBangladesh.

Progressive illitization of illite/smectite mixed layer clay in SG shales is a depth -temperature related diagenetic change which means that I/S mixed layer clay getsprogressively wider in illite at the expense of the smectite layer with increasing burialdepth. The present study also reveals that smectite diagenesis and dehydration havecontributed to the generation of overpressure in the Bhuban Formation in Patharia well-5.

The dominance of detrital kaolinite is a clay mineral assemblage which also indicatesillite and I/S, K/S, reflecting the onshore position and shallow nature of the environment.Since kaolinite is commonly deposited close to the source as a result of differentialflocculation in fresh or brackish water and sedimentation. Kaolinite contents of NSmainly rely on climate and suggests that warmer conditions must have been associatedwith increased humidity on shore.

The Surma Basin clay mineral assemblage also reflects physical and chemicalweathering in the source area. Kaolinite reflects the composition of the soil under humidtropical conditions where chemical weathering predominates. Intensive weathering,involving warm temperatures and high rates of water percolation is most conducive forincreased kaolinite formation in source areas (Gibson et al. 2000). Robert and Chamley(1987) proposed that higher rates of percolation could result from increased topographicrelief and/or by increased amounts of rainfall.

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An increase in topographic relief could result either from a significant relativelowering of sea level or by the relative tectonic uplift of the source area. The kaoliniteincrease could reflect a general Miocene temperature increase and/or the beginnings ofincreased precipitation and percolation. The high levels of kaolinite also indicate that allsource areas for sediments of Surma Basin had highly kaolinite soil profiles reflecting thesame intense weathering. The occurrence of kaolinite, which is not easily transported overlarge distances, rather suggest a deposition close to the shore where exposed land massesconstitute a potential source of the mineral which is also in agreement with thepalynological report of this study.

The large abundance of illite and specially chlorite suggests an active erosion ofexposed rocks. The clay mineral assemblage of the SB is strongly dominated by Illite/Smectite and Kaolinite/Smectite content. This suggest warm climates with seasonalrainfall through much of the Miocene. The increased amount of illite also suggests astrong renewal of active erosional processes on land. The cause of such renewal could betectonic (Chamley et al. 1997). The tectonic instability expressed by the clay mineralfrom open marine to a more restricted continental environment which has been inferredfrom the palynological analysis.

Vegetational changes in the Surma Basin area also suggests an increase both intemperature and precipitation (Gibson et al. 2000). Bouquillon et al. (1990) noted that theclay mineral assemblage of illite and chlorite represent the characteristic signature ofsediments derived from the Himalayas.

Chlorite is a common product of the erosion of low grade metamorphic rocks and illiteis obtained from the weathering of siliceous, igneous and high grade metamorphic rocks.Thus the clay mineral assemblages of the Surma Basin represents the sediments derivedfrom the Himalayas.

SEM revealed the quartz outgrowth in the Neogene SG sediments of the SB. Thegeochemical results are suggestive of a felsic continental source material mainly for thesesediments. It is useful, however, to evaluate the data in relation to the tectonism of thearea. Trace element geochemistry of the area has proved that it is a useful tool as apalaeoenvironment marker.

The results presented show that the approach used can provide a meaningfulchemostratigraphic subdivision of a bore hole and can also highlight geochemicallyanomalous zones. Statistically derived chemostratigraphic zonal sequences are in goodagreement with the litho stratigraphy of the area.

Regarding paleoenvironments, trace-element geochemistry indicates that the surfacewaters must have had intense primary productivity (Ba enrichment) and that the bottomconditions must have been anoxic and sulphidic. This is in good agreement with theobservation of the faunal content of the sediment, reported by pollen analysis. Thepresence of micro plankton foraminifera, thought to live at greater depth stresses, thatenvironmental conditions, must have allowed the presence of planktic fauna only insurface waters.

The present study confirms that depositional conditions must have been reducing. Theoxygen-minimum zone must have impinged on the seafloor for a sufficiently long time toallow sulphate reduction to occur at the base of the water column. Geochemicalsignatures recorded by the present data indicate a brackish-marine nature for thepaleoenvironment. The occurrence of kaolinite, which is not easily transported over large

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distances, suggest rather a deposition close to shore which is also in good agreement withthe palynological report of this study.

The tectonic instability expressed by the clay mineral from an open marine to a morerestricted continental environment has also been inferred from the palynological analysis.Palynostratigraphic zones suggest a mixed type of subtropical and tropical forest types.

The presence of foraminifera in association with a palynomorph assemblagecontaining mangrove forms and dinoflagellate cysts suggest a marine environment. Thesandstone with siltstone and shale were deposited in a shallow marine environmentthroughout the Miocene. The shallow marine and brackish environment reflects a marinetransgression. During the Miocene SB has witnessed a conspicuous subsidence andmarine transgression. Major changes in sea level for the later Neogene are suggestedbased upon transgressive-regressive phenomena. A marked rise in the sea level wouldhave caused a marine transgression (Bandy 1968). Undoubtedly, sea level and climatechanges affected the deposition of the SG sediments of the Surma Basin.

10 Conclusions

An analysis of stratigraphic, sedimentologic, palynologic, geochemic and mineralogicdata have been undertaken during the present investigation with the aim of providing acomprehensive and detailed analysis of Neogene Surma Group sediments from SurmaBasin, Sylhet, Bangladesh. Particularly, the study was aimed at obtaining the age frame ofthe Surma Group and to review the stratigraphical data based on pollen analysis in orderto better direct a reconstruction of palaeoenvironmental and palaeoclimatic variations inBangladesh during the Neogene.

In the present study, palynological and petrographic techniques were combined withmajor and trace-element analysis in order to gain more information on the provenance,sedimentary history and geochemistry of the Neogene Surma Group sediments of theSurma Basin.

Palynological data indicate that the age of the Surma Group is Miocene. Thesedimentary, palynological and geochemical study suggest that the deposition took placeunder shallow marine to brackish environmental condition.

The paleoecological and paleogeographical conditions of the Surma Group includemarine transgression with subsidence during the Miocene. During the late Miocene, asignificant shift in the flora occurred by the growth of conifer forest. A strong marineinfluence in the depositional environment indicated by the presence of micro-foraminifera. The presence of hystrichospherids suggest a more open marineenvironment. Combined lithofacies B, accounts for more than two-thirds of the totalformation contains plant and marine animal fossils.

Palynological assemblages of the Surma Group of sedimentary sequence ofBangladesh include taxa range in age from the Lower Miocene to the Upper Miocenewhich can be potentially used in dating and correlation.

Three palynological zones have been recognized from the well Fenchuganj -2. Theyare demarcated as zone -1 (Palmepollenite zone), zone -2 (Tricolpate – trilete zone) andzone - 3 (Dissacate zone).

Surma Group sediments are correlated with Simsang Palynological Zone IV ofMeghalaya, India and Bengal Palynological Zone (BPZ) V of India.

Geochemical and mineralogical study of the Surma Group reveals the origin, conditionof deposition, sedimentation, tectonics and diagenetic history of the formation.

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Geochemical ratios were useful for determining grain size, maturity, tectonics and theenvironment of deposition. Trace element analysis indicates that Surma Group sedimentcontains elements which are environmentally sensitive.

Trace-element geochemistry indicates that the surface waters must have had intenceprimary productivity (Ba enrichment) and that the bottom condition must have beenanoxic and sulphidic. The higher values of Th/U ratio in the core samples is because ofoxidation.

Geochemical ratios of Cr/Rb, Zr/Rb and Ba/Rb show higher values due to enhancedterrigenous input during their deposition. Higher values for Ba/Rb reflect a diageneticmobilization of barite. The enrichment of Ni and V was due to reducing environment.

Mineralogical and geochemical data from the Surma Basin suggest that the detritalinput was specially intense during the Neogene probably because of tectonic uplift linkedto the upheaval of the Himalayas. The result of the integrated petrographic techniqueswith major and trace-element analyses suggest that the source area of the detritusconsisted of the granites - major constituents of Himalayan rocks.

Clay mineral analysis indicates that major components of the rock studied are quartzand clay minerals together with small amounts of feldspar. The principal detrital clayminerals are illite, kaolinite, I/S, K/S mixed layers and chlorite. Clay mineral distributionprovides the evidence on diagenesis and clue to the provenance. A clay mineral suite withabundant illite, I/S, K/S, chlorite, quartz and feldspar reflects physical weathering in thesource area. Kaolinite reflects the composition of the soil under humid tropical conditionswhere chemical weathering predominates. The absence of smectite is remarkable, whichis due to its diagenetic convertion to illite/smectite and kaolinite/smectite mixed layers.The existence in noticeable amounts of mixed layers I/S and K/S is attributed to thedegree of burial diagenesis.

TEM and SEM study shows different element concentration of illite and smectite andquartz overgrowth. Quantitative analysis also showed the barite was dominant from theBa grains. SEM study also provides that Ba was associated with SiO2 and clay minerals Asummary conclusion is given in the follwing table.

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Table 15. Summary conclusion.

The methodology adopted in the present study by using geochemical ratios can beapplicable to any sedimentary basin of the world to infer palaeoclimate, palaeotectonicsand palaeoenvironment.

Target/Method Results Comments1) Age Miocene On the basis of pollen analysis.2) Depositional environment

Shallow marine to brackish By sedimentary, palynological and geochemical studies.

3) Paleoecology and paleogeography

Marine transgression with subsidence in Miocene. Changes in sea level.

By palynological and geochemical studies.

4) Palynological zones Three zones demarcated.Z-1 (Palmepollenite), Z-2 (Tricolpate - trilete)Z-3 (Dissaccate)

On the basis of pollen analysis from the samples of Fenchuganj well – 2.

5) SiO2/Al2O3 Dominance of the fining with a coarsening tendency at the bottom.

Grain size indicator.

6) Maturity index M=Al2O3+K2O/MgO+Na2O,K2O/Na2O andRb/K2O

Increasing maturity with fluctuations at different depth intervals.

Different matured and immatured zones of the 6 wells were demarcated. Maturity increases with the decrease of silica content and grain size.

7) Tectonic discrimination by using SiO2 and K2O/Na2O

Data of six wells fall into the ACM (Active Continental Margin) category.

The ratios were useful for tectonic discrimination

8) Th/U The higher values were fluctuating in all the six well sequences.

The higher value of Th/U ratio is because of oxidation. The sequence represent different stages in oxidation, leaching and deposition under marine conditions.

9) Ba Barium enrichment High surface water productivity and diagenesis mobilizations.

10) XRD Major components are quartz and clay mineral together with feldspar. Clay minerals are illite, kaolinite, I/S, K/S mixed layer and chlorite.

Burial diagenesis.

11) SEM Quartz overgrowth Dominance of barite and its associations with silica and clay minerals.

DiagenesisAnoxic and sulphidic bottom conditions.

12) Provenance Continental origin, Granitic rocks of Himalaya.

On the basis of mineralogical and geochemical studies.

11 References

Ahmed M, Khan SI & Sattar MA (1991) Geochemical characterization of oils and condensates in theBengal Foredeep. Bang Jour SE Asian Earth Sciences 5: 391–399.

Ahmed W & Zaher MA (1965) Paharpur Gondowana coal field and subsurface Geology of RajshahiDivision. Report of Geol Surv Bangladesh.

Alam M (1972) Tectonic classification of Bengal Basin. Geol Soc Am Bull 83: 519–522.Alam M (1989) Geology and depositional history of Cenozoic sediments of the Bengal Basin of

Bangladesh. Palaeogeog Palaeoclim Palaeoecol 69: 125–139.Alam MK, Hasan AKM, Khan MR & Whitney JW (1990) Geological map of Bangladesh. Published

by Ministry of Energy and Mineral Resources, Geol Surv of Bangladesh with cooperation of U SGeol Surv.

Almon WR & Davies DK (1981) Formation damage and crystal chemistry of clays. In: LongstaffeFJ (ed) “Short courses in clays and the Resource Geologist”, Miner Assoc of Canada 7: 81–102,The CO-OP Press, Edmonton.

Altschuler ZS, Dwornik EJ & Kramer H (1963) Transformation of montmorillonite to kaolinite dur-ing weathering. Science 141: 148–152.

Andrews, AJ (1980) Saponite and caladonite in layer 2 Basalts, DSDP Log 37. Contrib Min Petrol73: 223–340.

Ashraf AR, Utescher T & Mosbrugger (1997) Proceeding 4th EPPC: Palaentological-meteorologicalpalaeoclimate reconstructions Neogene Lower Rhine Embayment: 263–269.

Bailey EH, Irwin WP & Jones DL (1964) Franciscan and related rocks, and their significance in thegeology of western California: California Div Mines Geol Bull 183: 177 p.

Bakhtine MI (1966) Major tectonic features of Pakistan Part II. East Prov Sci Ind 4: 89–100.Bakr MA (1977) Quaternary geomorphic evolution of the Brahmanbaria-Noakhali area, Bangladesh.

Bangla Geol Surv Rec 1: 48.Baksi SK (1962) Palynological investigation of Simsang River Tertiaries, South Shillong Front, As-

sam. Bull Geol Min Metall Soc India. 26: 1–22.Baksi SK (1965) Stratigraphy of Barail Series in southern part of Shillong Plateau India. Bull Amer

Assoc Petro Geol 49: 2282–2288.Baksi SK (1971) On the palynological biostratigraphy of Bengal Basin. Seminar on Paleopalynol and

Ind stratig, Calcutta Univ, Botany Dept, p 188–206.Bakr MA (1977) Quaternary geomorphic evolution of the Brahmanbaria-Noakhali area Bangladesh.

Bangl Geol Surv Rec 1: 48.Bandy OL (1968) Cycles in Neogene paleoceanography and eustatic changes: Palaeogeog Palaeocli-

mat Palaeoecol 5: 63–76.

150

Banerjee D (1964) A note on the microflora from Surma (Miocene) of Garo Hills Assam Bull GeolMin Metall Soc Ind 29: 1–8.

Banerjee RK (1981) Cretaceous–Eocene sedimentation, tectonism and biofacies in the Bangal BasinIndia: Paleogeog Palaeoclimat Palaeoecol 34: 57–73.

Banerjee RK (1984) Post-Eocene biofacies, paleoenvironments and paleogeography of the BengalBasin India: Palaeogeog Palaeoclimat Palaeoecol 45: 49–73.

Banerjee D & Misra CM (1972) Hystrichosphaerids in the Tertiary formations of Assam & Tripura.Proc Sen Palaeopalynol Ind Stratig Calcutta 1971: 206–211.

Banerjee D, Misra CM & Koshal VN (1973) Playnology of the Tertiary subcrpos of Upper Assam.Paleobot Vol 20: 1–6.

Barker C (1972) Aquathermal pressuring-role of temperature in development of abnormal pressurezone. Amer Assoc Petro Geol Bull 56: 2068–2071.

Bates TF (1971) The Kaolin Minerals. In: Gard JA (ed) Mieralogical Society Monograph 3; The Elec-tron Optical Investigation of Clays Aldren & Macaulay Ltd Oxford.

Becini A & Turi A (1974) Mn distribution in the Mesozoic carbonate rocks from Lima. Valley, North-ern Apennines. J Sed Petrol 44: 774–782.

Bellanca A, Massetti D, Neri R & Venezia F (1999) Geochemical and Sedimentological evidence ofproductivity cycles recorded in Toarcian Black shales from the Belluno Basin, Southern Alps,Nothern Italy. J Sedimet Research 69: 466–476.

Berger G, Velde B & Aigouy T (1999) Potassium sources and illitization in Texas gulf coast shalediagenesis. J Sed Research 69: 151–157.

Bhatia MR (1985) Rare earth element geochemistry of Australian Palaeozoic graywackes andmudrocks: Provence and tectonical control. Sediment Geol 45: 97–113.

Bhatia MR & Taylor SR (1981) Trace-element geochemistry and sedimentary provinces: a studyfrom Tasman Geosyncline Australia. Chem Geol 33: 115–125.

Bishop JKB (1988) The barite-opal-organic carbon association in oceanic particulate matter. Nature311: 341–343.

Biswas B (1962) Stratigraphy of the Mahadev, Langpar, Cherra and Tura formations, Assam, India.Bull Geol Min Metall Soc India 25: 1–48.

Biswas B (1963) Results of exploration for petroleum in the western part of the Bengal basin, India.Proc Symp Dev of Petrol Resources of Asia & the Far East: EGAFE Miner Resour Dev Sr No 18:241–250.

Biswas SK (1965) A new classification of the Tertiary wells of Kutch, W. India. Bull Geol Min MetSoc India 35: 1–6.

Björlykke K (1971) Petrology of Ordovician sediments from Wales. Norsk geol Tidsskr 51: 123–139.Björlykke K (1974a) Geochemical and mineralogical influence of Ordovician Island Arcs on epicon-

tinental clastic sedimentation. A study of Lower Palaeozoic sedimentation in the Oslo RegionNorway. Sedimentol 21: 251–272.

Björlykke K (1974b) Depositional history and geochemical composition of lower Paleozoic epicon-tinental sediments from the Oslo region. Norges Geologiske Undersokelse 305: 1–81.

Bloch J, Hutcheon IE & Caritat PD (1998) Tertiary volcanic rocks and the potassium content of GulfCoast shales-The smoking gun. Geol 26: 527–530.

Boles JR & Franks SG (1979) Clay diagenesis in Wilcox sandstones of southwest Texas: implicationsof smectite diagenesis on sandstone cementation. J Sed Petrol 49: 55–70.

Boles JR (1981) Clay diagenesis and effects on sandstone cementation (case histories from the GulfCoast Tertiary) In: Longstaffe FJ (ed) “Short courses in clays and the Resource Geologist”, MinerAssoc Canada p 148–168.

Brahms CC, Bohrmann G, Schlüter M, Rutgers VD & Loeff M (1992) Biochemical cycle of bariumin the South Atlantic.Forchungszent Mar Geowiss Kiel GEOMAR Rep 15: 76.

151

Brass GW & Raman CV (1990) Clay mineralogy of sediments from the Bengal Fan. In: Proc ODP(Scientific result) 116: 35–41.

Breheret JG & Delamette M (1989) The mid-Cretaceous barite nodules of the Vocontian domain (SEFrance) as markers of basinal discontinuities within marly deposits. C R Acad Sci. Paris Sér II.308: 1369–1374.

Bruce CH (1984) Smectite dehydration – its relations to structural development and hydrocarbon ac-cumulation in northern Gulf of Mexico Basin. Amer Assoc Petro Geol Bull 68: 673–683.

Brunnschweiler RO (1966) On the geology of the Indo-Burman ranges (Arakan coast and Yoma, Chi-na Hills, Naga Hills): Geol Soc Austral Jour 13: 137–194.

Burst JF (1969) Diagenesis of Gulf Coast clayey sediments and its possible relation to petroleum mi-gration. Bull Amer Ass Petrol Geol 53: 73–93.

Chakraborty A (1972) On the rock stratigraphy and the tectonics of the sedimentary belt in the south-west of the Shillong Plateau, Meghalaya: Oil and Nat Gas Commn (India) Bull 9: 113–141.

Chandra U (1978) Seismicity, earthquake mechanisms and tectonics along the Himalayan mountainrange and vicinity. Phy Earth and Planet Int 16: 109–131.

Chapman RE (1980) Mechanical versus thermal cause of abnormally high pore pressure in shales.Amer Assoc Petro Geol Bull 64: 2179–2283.

Chappell BW (1986) Volcanic greywackes from the Upper Devonian Baldwin Formation, Tam-worth–Barraba district, New South Wales. J Geol Soc Australia 15: 87–102.

Chaudhury HM & Srivastava HN (1976) Seismicity and focal mechanism of some recent earthquakesin northeast India. Annal di Geophys 29: 41 – 56.

Chaudhury S & Cullers RL (1979) The distribution of rare-earth elements in deeply buried gulf coastsediments. Chem geol 24:327–338.

Chowdhury LR, Begum S & Mutalib MA (1987) Palynological investigation of Cores 1–16, fromFenchuganj well – 2, Sylhet Report of BOGMC Res Lab Div Dhaka Bangladesh.

Church TM (1979) Marine barite. In: Burns RG (ed) Marine Minerals. Min Soc Am Rev Min 6: 170–210.

Collier RW & Edmond JM (1984) The trace element geochemistry of marine biogenic particulatematter. Prog Oceanorg 13: 113–119.

Condie KC (1991) Another look at rare earth elements in shales. Geochim Cosmochim Acta 55:2527–2531.

Condie KC (1993) Chemical composition and evolution of the upper continental crust: constrastingresults from surface samples and shales. Chem Geol 104: 1–37.

Condie KC & Wronkiewicz DJ (1990) The Cr/Th ratio in Precambrain pelites from the Kaapvaal Cra-ton as an index of craton evolution: Earth Planet Sc Letters 97: 256–267.

Connelly W (1978) Uyak complex, Kodiak Islands, Alaska: a Cretaceous subduction complex: GeolSoc Amer Bull 89: 755–769.

Cradwick PD & Wilson MJ (1978) Calculated X-ray diffraction curves for the interpretation of athree-component interstratified system. Clay Miner 13: 53–65.

Crook KAW (1974) Lithogenesis and geotectonics: the significance of composition of compositionalvariations in flysch arenites (graywackes). In: Dott RH & Shaver RH (eds) Modern and ancientgeosynclinal sedimentation: SEPM Spec Pub 19: 304–310.

Cuadros J, Delgado A, Cardenete A, Reyes E & Linares J (1994) Kaolinite/montmorillonite resem-bles beidellite. Clays Clay Miner 42: 643–651.

Culler RL, Basu A, Suttner LJ (1988) Geochemical signature of provenance in sand-size material insoils and stream sediments near the Tobacco Root batholith, Mt, USA Chem Geol 70: 335–348.

Culler RL, Chaudhuri S, Kilbane N & Koch R (1979) Rare earths in size fractions and sedimentaryrocks of Pennsylvanian-Permian age from the mid-continent of the USA Geochim CosmochimActa 43: 1285–1302.

152

Curray IR, Emmel FJ, Moore DG & Raitt RW (1983) Structure, tectonics and geological history ofthe northeastern Indian Ocean. In: Nairn AEM & Stelhi F (eds) The Ocean basins and margins.Vol 6 The Indian Ocean: New York Plenum, p 399–449.

Curray JR & Moore DG (1971) Growth of the Bengal Deep – Sea Fan and Denudation in the Hima-layas. Geol Soc Am Bull 82: 563–572.

Curray IR & Moore DG (1979) Sedimentary and tectonic processes in the Bengal Deep sea fan andgeosyncline. In: Burke CA & Drake CL (eds) The geology of Continental Margins: New YorkSpringer-Verlag, p 617–628.

Danchin RV (1967) Choromium and nickel in the Fig Tree shale from South Africa Science 158:261–262.

Datta DK & Subramanian V (1997) Texture and mineralogy of sediments from the Ganges-Brah-maputra-Meghna river system in the Bengal Basin, Bangladesh and their environmental implica-tions. Env Geol 30: 181–188.

Datta DK, Gupta LP & Subramanian V (1999) Dissolved fluoride in the lower Ganges-Brahmaputra-Meghna River system in the Bengal Basin, Bangladesh. Env Geol 39: 1163–1168.

Deb U (1970) Palynological Investigations of Tertiary Sediments of Bengal Basin, South of Calcutta.Bull Geol Min Metall Soc India 42: 127–140.

Deer WA & Howie RA (1963) Rock forming minerals. 4. Framework silicates, Longmans, Green andCo, London, 435 p.

Dehairs F, Chesselet R, Jedwab & J (1980) Discrete suspended particles of barite and the barium cy-cle in the ocean. Earth Planet Sci Lett 49: 528–550.

Dehairs F, Goeyens L, Stroobants N, Bernard P, Goyet C, Poisson A & Chesselet R., (1990) On sus-pended barite and the oxygen minimum in the Southern Ocean. Glob Biogeoch Cycles 4: 85–102.

Delvaux B, Herbillon AJ, Vielvoye L & Mestdagh MM (1990) Surface properties and clay mineral-ogy of hydrated halloysitic soil clays. II. Evidence for the presence of halloysite/smectite (H/Sm)mixed-layer clays. Clay Miner 25: 141–160.

Desikachar SV (1974) A review of the tectonic and Geological history of Eastern India in terms of`Plate Tectonics´ theory. J Geol Soc India 15: 137–149.

Dietz RS, Holden JC (1970) Reconstruction of Pangea. J Geophys Res 75: 4939–4956.Drever JI (1971) Early diagenesis of clay minerals, Rio Ameca Basin, Mexico: J Sed Petrol 41: 982–

994.Drits VA & Sakharov BA (1976) X-ray structural analysis of mixed-layer minerals. Transact. AS

USSR 295. Moskow: 256 p (In Russia).Dunoyer de Segonzac G (1970) The transformation of clay minerals during diagenesis and low grade

metamorphism. A review: Sedimentol 15: 281–346.Dutta SK & Jain KP (1980) Geology and palynology of the area around Lumshnong, Jaintia hills,

Meghalaya, India. Biol Mem 5: 56–81.Dutta SK & Sah SCD (1970) Palynostratigraphy of the Tertiary sedimentary formations of Assam 5.

Stratigraphy and palynology of South Shillong Plateau Palaentograph 131 B: 1–72.Dutta SK & Sah SCD (1974) Palynostratigraphy of the sedimentary formations of Assam, India. Age

of the Laitryngew-Mawkma coal bearing sandstones and their relationship with the Cherra For-mation. Paleobot 21: 48–51.

Dutta SK & Singh HP (1980) Palynostratigraphy of sedimentary formations in Arunachal Pradesh.Palynology of Siwalik rocks of the Lesser Himalayas, Kameng District, Arunachal Pradesh. ProcIV Int palynol Conf Lucknow (1976–77) India 2: 617–626.

Dymond J, Suess E & Lyle M (1992) Barium in deep sea sediment: a geochemical proxy for palaeo-productivity: Paleoceonograph 7: 163–181.

Dypvik H (1977) Mineralogical and geochemical studies of Lower Paleozoic rocks from the Trond-heim and Oslo Regions, Norway Norsk Geologisk Tidsskrift 57: 205–241.

153

Dypvik H (1979) Mineralogy and geochemistry of the Mesozoic sediments of Andya, Northern Nor-way, Sediment Geol 24: 45–67.

Emelyanov EM & Shimkus KM (1986) Geochemistry and sedimentology of the Mediterranean Sea:Dordrecht The Netherlands Reidel, p 553.

Erdtman G (1960) The acetolysis method. Svensk Bot Tidskr 54: 561–564.Ernest W (1970) Geochemical Facies Analysis. Elsevier, Amsterdam, p 152Evans P (1964) The tectonic framework of Assam. J Geol Soc Ind 5: 80–96.Fairbridge RW (1983) Himalayan uplift and the Growth of Ganges – Brahmaputra delta. Geol Soc

Am Abstr 15: 569.Fedo CM, Eriksson KA & Krogstad EJ (1996) Geochemistry of shales from the Archean (~3.0 Ga)

Buhwa Greenstone Belt, Zimbabwe: implications for provenance and source area weathering. Ge-ochim Cosmochim Acta 60: 1751–1763.

Fedo CM, Nesbitt HW & Young GM (1995) Unraveling the effects of potassium metasomatism insedimentary rocks and paleosols, with implications for paleoweathering and provenance. Geology23: 921–924.

Flanagan FJ (1973) Values for international geochemical reference samples. Geochim CosmochimActa 37: 1189–1200.

Gansser A (1983) Geology of the Bhutan Himalaya, Birkhanser, Basel, p181Garver JI & Royce PR (1993) Chromium and nickel in shale of the foreland deposits of the Ordovi-

cian Taconic Orogeny: Using shale as a provenance indicator for ultramafic rocks: Geol SocAmer Abstracts with programs, 25: 17.

Garver JI & Scott JT (1995) Trace elements in shale as indicators of crustal provenance and terreneeccretion in the southern Canadian Cordillera GSA Bulletin 107: 440–453.

Germeraad JH, Hopping CA & Muller J (1968) Palynology of Tertiary sediments from tropical areas.Rev of Palaeobot and palynology 6: 189–348.

Ghosh AK (1941) Fossil pollen in Tertiary rocks of Assam. Sci Cult 6: 679.Ghosh AMN (1964) The Tura Sandstone Stage of the Garo Hills, its possible stratigraphical position.

Record Geol Surv India 83: 423–444. Ghosh TK (1969) Early Tertiary plant microfossils from the Garo Hills, Assam India. J Sen Mem Cal-

cutta India, p 123–138.Gibson TG, Bybell LM & Mason DB (2000) Stratigraphic and climatic implications of clay mineral

changes around the Paleocene/ Eocene boundary of the northeastern US margin. Sed Geol 134:65–92.

Graham SA, Dickinson WR & Ingersoll RV (1975) Himalayan – Bengal Model for Flysh Dispersalin the Appalachian – Ouachita System. Geol Soc Am Bull 86: 273–286.

Goldberg ED & Arrhenius GOS (1958) Chemistry of Pacific pelagic sediments. Geochim Cosmo-chim Acta 13: 153–212.

Goldberg ED, Somayajulu LK, GalowayJ, Kaplan IR & Faure G (1969) Differences between baritesof marine and continental origins. Geochim Cosmochim Acta 33: 287–289.

Grömet LP, Dymek RF, Haskin LA & Korotev RL (1984) The ”North American Shale Composite”:Its compilation, major and trace element characteristics: Geochem et Cosmochim Acta 48: 2469–2482.

Guha DK (1978) Tectonic framework and oil & gas prospects of Bangladesh. Proc 4 th Annl ConfBangladesh Geol Soc, p 65–75.

Gupta VJ (1976) Indian Cenozoic stratigraphy: Delhi, India. Hindustan Publishiry Corporation, p344.

Guven N, Hower WF & Davies DK (1980) Nature of authigenic illites in sandstone reservoir. J SedPetrol 50: 761–766.

Götze J (1998) Geochemistry and provenance of the Altendorf feldspathic sandstone in the MiddleBunter of the Thuringian basin (Germany). Chem Geol 150: 43–61.

154

Hallberg RO (1976) A geochemical method for investigation of paleoredox conditions in sediments.Ambio Spec report 4: 139–147.

Handique GK & Dutta SK (1981) A study of the Surma- Tipam groups of upper Assam Valley, Southof Brahmaputra. J Palaeont Soc India 25: 42–52.

Haque M (1982) Tectonic set up of Bangladesh and its relation to hydro-carbon accumulation phase-1: Center for Policy Reseach (DU) and Universities Field Staff International (UFSI) USA.

Haskin CA (1976) A review of hydroxy-aluminum treatments. Paper SPE 5692 presented at the Sympon Formation Damage Control Houston Texas.

Hassan S, Ishiga H, Dozen K & Naka T (1999) Geochemistry of Permian-Triassic shales in the SaltRange, Pakistan: implications for provenance and tectonism at the Gondowana margin. ChemGeol 158: 293–314.

Hiller K & Elahi M (1984) Structural development and hydrocarbon entrapment in the Surma Basin,Bangladesh (northwest Indo-Burman fold belt): Singapore Fifth Offshore Southwest Conference656–663.

Holtrop IF & Keizer I (1970) Some aspects of the stratigraphy and correlation of the Surma Basinwells, East Pakistan: ESCAFE Min Resour Dev Series 36: 143–154.

Hossain KM (1985) Plate tectonic theory and the development of the folds of Chittagong and Chit-tagong Hill Tracts. Bangl Jour Geol 4: 57–67.

Hower J (1981) X-ray diffraction identifications of mixed layer clay minerals. In: Longstaffe FJ (ed)“short courses in clays and the Resource Geologist” Min Assoc Canada 7: 39–59 The CO-OPPress Edmonton.

Hower J, Eslinger E, Hower M & Perry E (1976) Mechanism of burial metamorphism of argillaceoussediment. 1. Mineralogical and chemical evidence. Geol Soc Am Bull 87: 725–37.

Hughes RE, Moore DM & Reynolds REJ (1993) The nature, detection, occurence and origin of kao-linite/smectite. In: Murray H, Bundy W & Harvey C (eds) Kaolin Genesis and Utilization 291–323. Boulder CO: Clay Mineral Soc p 291–323.

Hunting (1980) Aeromagnetic survey of Bangladesh. Geology and Geophysics Ltd.Imam MB (1987) Implication of shale diagenesis on cementation of reservoir sandstonein the Neo-

gene Surma Group of Bengal Basin. Bangl Jour Geol Soc India 30: 447–492.Imam MB (1989) Comparison of Burial Diagenesis in some Deltaic to Shallow Marine Reservoir

Sandstones from different Basins. J Geol Soc India 33: 524–537.Imam MB (1993) Mineralogy and clay diagenesis of Neogene shales from Patharia-5 and Sitakund-

1 wells, eastern folded belt of Bangladesh. Report of Bangladesh Petrol Inst under Institutionalcooperation project NORAD BGD-023, p 97.

Imam MB (1994) X-Ray Diffraction Study on Noegene Shales from Patharia Anticline, EasternBangladesh, with emphasis on Smectite clay Dehydration and Implications. J Geol Soc India 44:547–561.

Imam MB & Shaw HF (1985) The diagenesis of Noegene clastic sediments in the Surma Basin. BanglJour Sed Petrol 55: 665–691.

Islam AKME & Lotse EG (1986) Quantitative mineralogical analysis of some Bangladesh soils withX-ray, ion exchange and selective dissolution techniques. The min Soc 21: 31–42.

Islam MR (1996) The ancient weathering crust in finnish Lapland and the recent weathering crust inBangladesh – a comparison. Institute of Geosciences and Astronomy Dept of Geol Univ Oul.

Islam MR, Lahermo P, Salminen, Rojstaczer S & Peuraniemi V (2000) Lake and reservoir water qual-ity affected by metals leaching from tropical soils, Bangladesh. Env Geol 39: 1083–1089.

Jain KP & Kar RK (1979) Palynolgy of Neogene sediments around Quilon and Varkala. KeralaCoast, south India-1. Fungal remains. Paleobot 26: 105–118.

Johns WD & Shimoyama A (1972) Clay minerals and petroleum-forming reaction during burial anddiagenesis. Bull Amer Ass Petrol Geol 56: 2160–2167.

155

Johnson SY & Alam AMN (1991) Sedimentation and tectonics of the Sylhet trough, Bangladesh.Geol Soc Am Bull 103: 1513–1527.

Kar RK (1990) Palynology of Miocene and Mio-Pliocene sediments of north-east India. J Palynol 91:171–217.

Kar RK, Singh RY & Sah SCD (1972) On some algal and fungal remains from Tura Formation ofGaro Hills, Assam. Paleobot 19: 146–154.

Khan AA (1989) Geodynamics of the Eastern Folded Belt of the Bengal Geosyncline with referenceto Burma. Bangl Jour Geol 8: 13–22.

Khan MAM (1978) Geology of the Eastern and the Northeastern part of Sadar Subdivision, SylhetDistrict, Bangladesh, Records of the Geol Surv Bangladesh, p 20.

Khan MAM (1980) A brief account of the geology and hydrocarbon exploration in Bangladesh. In:Offshore SouthEast Asia Conf Feb 1980. SEAPEX Session, p 6.

Khan MAM, Ismail M & Ahmad M (1988) Geology and hydrocarbon prospects of The Surma Basin,Bangladesh: Seventh Offshore Southeast Asian Conf. Singapore, p 364–378.

Khan MR & Mominullah M (1980) Stratigraphy of Bangladesh. In: Petrol and Min Resour Bangla-desh. Seminar and Exhibition Dhaka 1980: 35–40.

Khanna AK & Singh HP (1981) Palynological evidences in determination of age and environment ofdeposition of the Subathu Formation, Simla Hills. Himal Geol 9: 292–303.

Klootwijk CT, Gee JS, Peirce JW & Smith GM (1992) Neogene evolution of the Himalayan-Tibetanregion: constraints from ODP Site 758, northern Ninety east ridge; bearing on climatic change.Palaeogeog Palaeoclim Palaeoecol 95: 95–110.

Kohyama N & Shimoda S (1974) On a few specimens called called mixed layer Kaolinite-montmo-rillonite: comparisons of observed and calculated X-ray diffraction pattern: J Min Soc Japan 11:99–106 (In Japanese).

Koski RA, Shanks WC, Bohrson WA & Oscarson RL (1988) The composition of massive sulfide de-posits from the sediment covered floor of Escanaba Trough, Gorda Rodge: Implications for dep-ositional prosesses. Can Mineral 26: 655–673.

Kuehl SA, Harju TM & Moore WS (1989) Shelf sedimentation of the Ganges-Brahmaputra river sys-tem: Evidence for sediment bypassing to the Bengal fan. Geol 17: 1132–1135.

Lakhanpal RN (1955) Recognizable species of Tertiary plants from Damalgiri in the Garo Hills, As-sam. Palaeobot 3: 27–31.

Lahann RW (1980) Smectite diagenesis and sandstone cementation: the effect of reactions tempera-ture. J Sed Petrol 50: 755–760.

Lahtinen R (1996) Geochemistry of Palaeoproterozoic supracrustal and plutonic rocks in the Tam-pere-Hameenlinna area, southern Finland. Geol Surv Finland Bulletin 389: 113.

Le Dain AY, Tapponier P & Molnar P (1984) Active faulting and tectonics of Burma and surroundingregions: J Geophys Research 89: 453–472.

Lietz JK & Kabir J (1982) Prospects and constraints of oil exploration in Bangladesh: Singapore,Fourth Offshore Southeast Asia Conference Singapore, p 1–6.

Mallick S (1971) Geomorphology and Quaternary geology of the area around Burdwan, West BengalIndia Geos 8: 31–46.

Mannan A (1993) Stratigraphy and structure of the Noegene of Maiskhal Island, Cox’s Bazar, Bang-ladesh with special reference to its Morphotectonic evolution. Thesis Philosophy of Licentiate(Ph. L) Oulu Univ, Depart Geol , Finland.

Mathur K (Mrs) (1963) Occurrence of Pediastrum in Subathu Formation (Eocene), HimachalPradesh. Sci Cult. 29: 250.

Mathur YK (1966) On the microflora in the Supra Trappeans of Western Kutch, India Q Jl Geol MinMetall Soc India 38: 33–51.

Mathur K (1973) Studies in the paleoflora of the Himalayan foot hills-2. On the palynoflora in theLower Siwalik sediments of Nepal. J Palynology 8: 54–62.

156

Mathur LP & Evans P (1964) Oil in India: Special Brochure International Geol Congress 2nd SessionNew Delhi India, p 22.

Mathur YK, Jugal NP & Chopra AS (1977) Ecologic implications in palynostratigraphy with specialreference to the Kalol Formation, Cambay Basin Abs Proc 4th Colloq Indian Micropaleont StratigrDehradun, India. p 90.

Matin MA, Khan MAM, Fariduddin M, Boul MA, Taolad HMM & Kononov AI (1983) Tectonic mapof Bangladesh – past and present. Bang J Geol 2: 29–36.

Manzur A, Khan SI & Sattar MA (1991) Origin of Sylhet-7 (Haripur) oil: A study of normal alkane,regural isoprenoid and pentacyclic triterpane distribution patterns. J Bangl Chem Soc 4: 231–241.

McLennan SM (1989) Rare earth elements in sedimentary rocks: influence of provenance and sedi-mentary processes. Mineral 21: 169–200.

McLennan SM, Hemming S, McDaniel DK & Hanson GN (1993) Geochemical approaches to sedi-menatation, provenance and tectonics Geol Soc Am Spec Paper 285: 21–40.

McLennan SM, Taylor SR & Eriksson KA (1983) Geochemistry of Archean shales from the PhilbaraSupergroup, Western Australia. Geochem Cosmochim Acta 47: 1211–1222.

McLennan SM, Taylor SR, McCulloch MT & Maynard JB (1990) Geochemical and Nd-Sr isotopiccomposition of deep sea turbidites: Crustal evolution and plate tectonic associations. GeochemCosmochim Acta 47: 1211–1222.

McLennan SM & Taylor SR (1991) Sedimentary rock and crustal evolution: tectonic setting and sec-ular trends. J Geology 99: 1–21.

Mehrotra NG (1981) Palynological correlation of Mikir Formation with Lower Paleogene sedimentsof Shillong Plateau. Geophytol 11: 133–142.

Mehrotra NG (1983) Palynology of Mikir Formation in the type area. Geosc Jour 4: 1–34.Meyer BL (1958) Palynological investigations of some samples from Nahor, Assam. J Palaeont Soc

India 5: 156–157.Mikkel S & Hendersson JB (1983) Archean chemical weathering at three localities on the Canadian

shield. Precam Research 20: 189–224.Millot G (1970) Geology of Clays. Springer-Verlag, Heidelberg.Millot G, Lucas J & Paquet H (1966) Evolution chimique par dégradation et agradation des minéraux

argileux dans l’hydrosphere. Geol Rundschaw 55: 1–20. Mills JW (1971) Bedded barite deposits of Stevens county, Washington. Econ Geol 66: 1157–1163.Molnar G (1984) Structure and Tectonics of the Himalaya-Constraints and implications of geophysi-

cal data: Earth and planetary Sciences Annl Rev 12: 489–518.Moore HE (1973) Palms in the tropical forest ecosystems of Africa and South America. In: BJ Meg-

gers BJ, ES Ayensu ES & WD Duckworth WD (eds) Tropical Forest Ecosystems in Africa andSouth America: A comparative review. Smithsonian Inst. Press Washington DC, p 63–88.

Morgan JP & McIntire WG (1959) Quaternary Geology of Bengal Basin. Geol Soc Am Bull 70: 319–341.

Muller J (1964) A palynological contribution to the history of the mangrove vegetation in Borneo. In:Cranwell LM(ed), Ancient Pacific Floras Univ Hawaii Press, Honolulu, p 33–42.

Murty KN (1983) Geology and hydrocarbon prospects of Assam shelf- Recent advances and presentstatus: Petrol Asia Jour 6: 1–14.

Murty MVN, Chakrabarti C & Talukdar SE (1976). Stratigraphic reversion of the Cretaceous-Terti-ary sediments of the Shillong Plateau. Geol Surv India Records 107: 80–90.

Mustaque ASM & Raghava MS (1985) The Bouger Gravity Map of Bangladesh and its Tectono–Ge-ologic Implications. Bangl J Geol 4: 43–56.

Nandy DR & Dasgupta S (1991) Seismotectonic domains of northeastern India and adjacent areas.In: Sharma KK (ed) Geology and Geodynamic evolution of the Himalayan collision zone Part II:371–384.

157

Nesbitt HW (1980) Characterisation of mineral-formation water intractions in Carbonaferous sand-stones and shales of the illinois Sedimentary Basin. Am Jour Sci 280: 607–611.

Nesbitt HW (1992) Diagenesis and metasomatism of weathering profiles, with emphasis on Precam-brian paleosols. In: Martini IP & Chesworth W (eds) Weathering, soils and paleosols. Elsevier,Amsterdam.

Nesbitt HW & Markovics G (1996) Weathering of granodioritic crust, long term storage of elementsin weathering profiles and petrogenesis of silisiclastic sediments. Geochim Cosmochim Acta 61:1653–1670.

Nesbitt HW, Markovis G & Price RC (1980) Chemical processes affecting alkalis earth during chem-ical weathering. Geochim Cosmochim Acta 44: 1659–1666.

Nesbitt HW & Young GS (1982) Early Proterozoic climates and motions inferred from major elementchemistry of lutites. Nature 299: 715–717.

Nesbitt HW & Young GM (1984) Prediction of some weathering trends of plutonic and volcanicrocks based on thermodynamic and kinetic considerations. Geochim Cosmochim Acta 48: 1523–1534.

Nesbitt HW & Young GM (1989) Formation and diagenesis of weathering profiles. J Geol 97: 129–346.

Nuelle LM & Shelton KL (1986) Geologic and geochemical evidence of possible bedded barite de-posits in Devonian rocks of the Valley and Ridge province, Appalachian mountains. Econ Geol81: 1408–1430.

Pairazian VV, Ismail M, Khan SI & Ahmed M (1985) Geochemical Aspects of gas and CondensateFormation in the Folded Beld of the Bengal Basin, Bangladesh. Bang Jour Geol 4: 1–10.

Palmer AR (1983) The Decade of North American Geology. 1983 Geologic time scale. Geol 11: 503–504.

Paul DD & Lian HM (1975) Offshore Tertiary Basins of Southeast Asia: Bay of Bengal to South Chi-na sea: 9 th World Petrol Cong Proc 3: 107–121.

Pearson MJ (1979) Geochemistry of the Hepworth Carboniferous sediment sequence and origin ofthe diagenetic iron minerals and concretions. Geochim Cosmochim Acta 43: 927–941.

Pearson MJ & Small JS (1988) Illite smectite diagenesis and paleotemperature in Northern North SeaQuaternary to Mesozoic shale sequence. Clay Minerals 23: 109–112.

Perry E & Hower J (1970) Burial diagenesis in Gulf Coast pelitic sediments. Clays Clay Miner 18:165–77.

Pettijhon FJ (1975) Sedimentary rocks. 3rd ed. Harper and Row, New York, p 682. Pittman ED (1972) Diagenesis of quartz in sandstone as revealed by scanning electron microscopy. J

Sed Petrol 42: 507–519. Pittman ED (1979) Recent advances in sandstone diagenesis. Annl Rev Earth Planet Sciences 7: 39–

62.Plumley WJ (1980) Abnormally high fluid pressure; survey of some basic principles. Amer Assoc

Petrol Geol Bull 64: 414–430.Potter PE Maynard JB & Pryor WA (1980) Sedimentology of shale. Springer-Verlag, New York, p

303.Powers MC (1967) Fluid release mechanism in compacting marine mud rocks, their importance in oil

exploration. Amer Assoc Petrol Geol Bull 51: 1240–1254. Prevot L & Lucas J (1980a) Behaviour of some trace elements in phosporite from the Namibian shelf.

Chem Geol 23: 151–170.Prevot L & Lucas J (1980 b) Behavior of some trace elements in phosphatic sedimentary formations;

SEPM spec publ 29: 31–39. Tulsa.Price NB & Calvert SE (1978) The geochemistry of phosphorite from the Namibian shelf. Chem Geol

23: 151–170.

158

Rafiqul I, Mustafa AI, Huq D, Kabir AZ & Manzur A (1993) A study on KailasTila -2 crude oil. Dha-ka Univ Studies B 41: 1–6.

Raghava MSV & Mustaque A (1994) Gravity Modeling of Crustal Structure and the Possible Tec-tonic History of the Bengal Region. Bangl Jour Geol 13: 1–14.

Rahman MA, Mannan MA, Blank HR, Jr., Kieinkoph MD & Kneks RP (1990) Bouger gravity anom-aly map of Bangladesh: Geol Surv Bangl Scale 1:1000,000. Published by Geol Surv Bangladesh.

Raju ATR (1968) Geological evolution of Assam and Cambay Tertiary basins of India. Am AssocPet Geol Bull 52: 2422–2431.

Ramanujan CGK (1982) Tertiary palynology and palynostratigraphy of southern India. J Paleont SocIndia Spl Publ 1: 57–64.

Ramanujan CGK (1987) Palynology of the Noegene Warkalli beds of Kerala State in south India. Jpaleont Soc India 32: 26–46.

Ramanujan CGK (1988) Palynology of the Neogene Warkalli beds of Kerala state in south India. JPaleon Soc India 32: 26–46.

Rawat MS, Mukherjee J & Venkatachala BS (1977) Palynology of the kadi Formation, Cambay Ba-sin, India. Proc. 4th Colloq Ind Micropal Stratigr Dehradun: 179–192.

Ray DK (1982) Tectonic map of south and east Asia: Hyderabad (Geological Survey of India) Com-mission for the geological map of the world – Subcomission for south and east Asia, 7sheets, scale1:5000 000. Published by Geol Surv Ind.

Reading HG (1982) Sedimentary basins and global tectonics: Proc Geologists Assoc 93: 321–350. Reimann KU & Thaung A (1981) Results of paynostratigraphical investigations of the Tertiary se-

quence in the Chindwin Basin/Northwestern Burma. VI Int Palyn Conf Lucknow (1976–77) India3: 380–395.

Reimer TO (1985) Volcanic rocks and weathering in the early Protozoic Witwatersrand. Supergroup,South Africa. Geol Surv Finl Bull 331: 33–48.

Rettke RC (1981) Probable Burial diagenetic and provenance effects on Dakuta Group clay mineral-ogy Denver Basin. J Sed Petrol 51: 541–551.

Reynolds RC Jr. & Hower J (1970) The nature of interlayering in mixed-layer illite-montmorillonites.Clays Clay Miner 18: 25–36.

Robert C & Chamley H (1987) Cenozoic evolution of continental humidity and paleoenvironment,deduced from the kaolinite content of oceanic sediments. Palaeogeog Palaeoclimat Palaeoecol 60:171–187.

Roser BP & Korsch RJ (1986) Determination of tectonic setting of sandstone -mudstone suites usingSiO2 content and K2O/Na2O ratio. J Geology 94: 635–650.

Roser BP & Korsch RJ (1988) Provenance signatures of sandstone suites determined using discrimi-nant function analysis of major element data. Chem Geol 67: 119–139.

Ross M (1968) X-ray diffraction effects by non-ideal crystals of biofite, muscorite, montmorillonite,mixed layers clays, graphite and priclase: Z Kristall 126: 80–97.

Sahni B Sitholey RV & Puri GS (1947) Assam progress report for 1944–46, Correlation of the Ter-tiary succession in Assam by means of microfossil. Suppl J Ind Bot Soc 26: 262–263.

Sah SCD & Dutta SK (1966) Palynostratigraphy of the sedimentary formations of Assam: 1. Strati-graphical position of the Cherra formation. Palaeobot 15: 72–86.

Sah SCD & Dutta SK (1968) Palynostatigraphy of the Tertiary sedimentary formations of Assam: 2.Stratigraphic significanse of spores and pollen in the Tertiary succession of Assam. Palaeobot 16:177–195.

Sah SCD & Dutta SK (1974) Palynostratigraphy of the sedimentary formations of Assam – 3. Bios-tratigraphic zonation of the Cherra Formation of South Shillon Plateau. Paleobot 21: 42–47.

Sah SCD & Singh RY (1974) Palynological biostratigraphy of the Tura Formation in the type area.Symp Strat Palynol Spl Publ Birbal Sahni Institute of Paleobotany Lucknow, p 79–98.

159

Sahni B (1921) The present position of Indian Palaeobotany. Proc Asiat Soc Beng. (N.S.) 17: CLII–CLXXV.

Sakharov BA & Drits VA (1973) Mixed-layer kaolinite-montmorillonite: a comparison of observedand calculated diffraction patterns. Clays Clay Miner 21: 15–17.

Salt CA, Alam MM & Hossain MM (1986) Bengal Basin – Current exploration of the hinge zone ofsouthwestern Bangladesh, 6th offshore Southeast Asia Conference, Singapore, p 55–67.

Salujha HP, Khanna AK & Sah SCD (1973) Problems and prospects of Tertiary palynology in north-ern India. Bull. Ind Geol. Assoc 6: 71–77.

Salujha SK, Srivastana NC & Rawat MS (1969) Microflora assemblage from Subathu sediments ofSimla Hills. J palaeont Soc India 12: 25–40.

Salujha SK, Kindra GS & Rehman K (1972a) Palynology of Tertiary sediments in Assam. I. The Pal-aeogene of Garo Hills Abstr Em Palaeopalynol Ind Strat Calcutta: 37.

Salujha SK, Kindra GS & Rehman K (1972b) Palynology of the South Shillong Front part-1. ThePaleogene of Garo Hills. Proc Sem Paleopalynol Ind Stratigr: 265–291.

Salujha SK, Kindra GS & Rehman K (1974) Palynology of the South Shillong part II-The Paleogenesof Khasi and Jaintia Hills. Paleobot 21(3): 267–284.

Sasrti VV, Raju ATR, Venkatachala BS & Banerji RK (1977) Biostratigraphy and evolution of theCauvery Basin. India. J Geol Soc India 18: 355–377.

Sato T, Murakami T & Watanabi T (1996) Change in layer charge of smectites and smectite layers inillite/smectite during diagenetic alternation. Clay Clay Miner 44: 460–469.

Saxena RK, Rao MR & Singh HP (1986a) Palynology of the Barail (Oligocene) and Surma (LowerMiocene) sediments exposed along Sonapur-Badarpur Road Section, Jaintia Hills (Meghalaya)and Cachar (Assam). Part-VI. Palynostratigraphic zonation. Paleobot 35: 150–158.

Saxena RK, Rao MR & Singh HP (1986b) Palynology of the Barail (Oligocene) and Surma (LowerMiocene) sediments exposed along Sonapur-Badapur Road Section, Jaintia Hills (Meghalaya)and Cachar (Assam). Part I. Dinoflagellate cysts. J Palaeont Soc India 29: 52–62.

Schmitz B (1987) Barium, equatorial high productivity, and northward wandering of the Indian Con-tinent: Paleoceanograph 2: 63–77.

Schultz LG, Shephard AO, Blackmon PD & Strakey HC (1971) Mixed-layer kaolinite-montmorillo-nite from the Yucatan Peninsula, Mexico. Clays Clay Miner 19: 137–150.

Sein MK & Sah SCD (1974) Palynological demarcation of the Eocene – Oligocene sediments in theJowai – Badarpur road section, Assam. Symp Start Palynol B S I P Spl Publ 3: 99–105.

Sen J (1948) Microfossils of Assam coal fields 1. The coal seam at Laitryngew and the age of theCherra Sandstone. Bull Bot Soc Beng 2: 1–11.

Sengupta S (1966) Geological and Geophysical studies in western part of Bengal Basin, India. AmAssoc Pet Geol Bull 50(5): 1001–1018.

Sengupta S, Ray KK, Achrarya SK & De Smeth JB (1990) Nature of ophiolite occurrences along theeastern margin of the Indian plate and their tectonic significance. Geology 18: 439–442.

Shamsuddin AHM (1989) Organic Geochemistry of Oligocene-Miocene Deposits of the BengalForedeep, Bangladesh. J Geol Soc India 33: 332–352.

Shaw DB & Weaver FM (1965) The mineralogical composition of shales: J Sed Petrol 35: 213–222.Shinoyama A, Jhons WD & Sudo T (1969) Montmorillonite-kaolinite clay in acid clay deposits from

Japan. Proc Int Clay Conf Tokyo 1969 1: 225–233.Singh HP (1981) Paleogene palynostratigraphy of Simla Hills. Paleobot 28–: 398–401.Singh HP & Khanna AK (1980) Palynology of the Paleogene marginal sediments of Himachal

Pradesh, India. Proc IV Int palynol Conf Lucknow (1976–77) 2: 462–471.Singh HP & Rao MR (1990) Tertiary palynology of Kerala Basin-An overview. Paleobot 38: 256–

262.

160

Singh HP, Rao MR & Saxena RK (1986a). Palynology of the Barail (Oligocene) and Surma (LowerMiocene) sediments exposed along Sonapur-Badarpur Road Section, Jaintia Hills (Meghalaya)and Cachar (Assam) Part VII Discussion. Paleobot 35: 331–341.

Singh HP, Rao MR & Saxena RK (1986b) Palynology of the Barail (Oligocene) and Surma (LowerMiocene) sediments exposed along Sonapur-Badarpur Road Section, Jaintia Hills (Meghalaya)and Cachar (Assam) Part II Fungal remains. Paleobot 35: 93–105.

Singh HP, Khanna AK & Sah SCD (1978) Palynological zonation of Subathu Formation in the KalkaSimla area, Himachal Pradesh Himalya Geol 8: 33–46.

Singh RY (1977a) Stratigraphy and Palynology of the Tura Formation in the type area (Part II). De-scrip palynol Paleobot 23: 189–205.

Singh RY (1977b) Stratigraphy and Palynology of the Tura Formation in the type area (Part III) Dis-cussion. Paleobot 24: 1–12.

Singh RY & Singh HP (1978) Morphological observations on some spores and pollen grains from thePaleocene subsurface assemblages of Garo Hills, Meghaylaya. Paleobot 25: 475–480.

Singh RY & Tewari BS (1979) A note on the Palynostratigraphy of the Tertiary sediments of upperAssam Bull Indian Geol Assoc 12: 151–159.

Singh, RY, Dogra NN & Vimal KP (1985) Palynology of the Barail sediments. J Palynology 21: 28–55.

Smith CF, Crowe CW & Nolan TJ III. (1969) Secondary deposition of iron compounds followingacidizing treatments. J Petrol Tech Sept 1969: 1121–1129.

Sonnerat M (1782) Voyage aux Indes Orientales et la Chine, fait par ordre du Roi, depuis 1774 jusqu’en 1781, 2 Vols. Paris.

Srivastava NC & Banerjee D (1969) Hystrichosphaerids from Tertiary subcrops of Assam, India. Jsen mem: 101–108.

Srodon J (1980) Synthesis of mixed-layer kaolinite/smectite. Clays Clay Miner 28: 419–24.Srodon J (1981) X-Ray identification of randomly interstratified illite/smectite in mixture with dis-

crete illite, Clay Min 16: 297–304.Srodon J (1999) Nature of mixed-layer clays and mechanism of their formations and alternation. Annl

Rev Earth & Planet Sciences 27: 19–53.Srodon J, Elsass F, McHardy WJ & Morhgan DJ (1992) Chemistry of illite-smectite inferred from

TEM measurements of fundamental particles. Clay Miner 27: 137–58.Sudo T & Hayashi H (1956) A randomly interstratified kaolin-montmorillonite in acid clay deposits

in Japan. Nature 178: 1115–1116.Taylor SR & McLennan SM (1985) The Continental Crust: Its composition and Evolution. Oxford,

Balckwell Scientific, p 312. Thiery M (1981) Sédimentation continentale et altérations associées: calcitisations, ferruginisations,

et silifications. Les argiles plastiques du Sparnacien du Basin de Paris. Sci Geol Mem 64: 173.Tomita K & Takahashi H (1986) Quantification curves for the X-ray powder diffraction analysis of

mixed layer Kaolinite/Smectite. Clays and clay Minerals 34: 323–329.Torres ME, Brumsack HJ, Borhmann G & Emeis KC (1996) Barite fronts continental margin sedi-

ments: A new look at barium remobilization in the zone of sulfate reduction and formation ofheavy barites in diagenetic fronts. Chem Geol 127: 125–139.

Towe KM (1962) Clay mineral diagenesis as a possible source of silica cement in sedimentary rocks.J Sed Petrol 32: 26–28.

Tsuzuki Y & Sato M (1974) Some structure models of kaolin/montmorillonite random mixed-layerminerals. J Min Soc Japan 11: 106–110.

Utescher T, Gebka M, Mosbrugger V, Schilling H-D & Ashraf AF (1997) Regional palaentological-meteorological palaeoclimate reconstruction of the Neogene Lower Rhine Embayment. In: Pro-ceeding 4th EPPC: Palaentological-meteorological palaeoclimate reconstructions Neogene LowerRhine Embayment: 263–269.

161

Van De Kamp PC, Leake BE & Senior A (1976) The petrography and geochemistry of some Califor-nian arkoses with application to identifying gneisses of metasedimentary origin: J Geol 84: 195–212.

Varma CP & Dangwal AK (1964) Tertiary hystrichosphaerids from India. Micropaleont 10: 63–71.Varma YNR & Patil RS (1985) Fungal remains from the Tertiary carbonaceous clays of Tonakkal

area. Kerala. Geophytol 15: 151–158.Varma YNR, Ramanujan CGK & Patil RS (1986) Palynoflora of Tertiary sediments of Tonakkal ar-

ea, Kerala. J Palynol 22: 39–53.Verma RK (1991) Seismicity of the Himalaya and the northeast India, and nature of continent-conti-

nent collision. In: Sharma KK (ed) Geology and Geodynamic evolution of the Himalayan colli-sion zone Part II: 345–370.

Venkatachala BS & Choudhury HM (1977) Paleoecology of Kadi Formation, Cambay Basin, India.Proc 4th Colloq Ind Micropal Stratgr, Dehradun India, p 259– 277.

Venkatachala BS & Kar RK (1969) – Palynology of the Tertiary sediments of Kutch. 1. Spores andpollen from borehole No. 14. Palaeobot 17: 157–178.

Venkatachala BS & Rawat MS (1973) Playnology of the Tertiary sediments in the Cauvery Basin. 2.Oligocene-Miocene Palynoflora from the Subsurface. Paleobot 20: 238–263.

Von Breymann MT, Eimeis KC & Suess E (1993) Water-depth and diagenetic constraints in the useof barium as a paleoproductivity indicator. In: Summerhayes CP, Prell W & Emeis KC (eds) Ev-olution of Upwelling Systems since the Early Miocene. Geol Soc London Spec Publ 64: 273–284.

Wadia DN (1975) Geology of India. Macmillan. ELBS edition.Weaver CE (1956) Distribution and identification of mixed-layer clays in sedimentary rocks. Am

Mineral 41: 202–221.Weaver CE & Beck KE (1971a) A discussion of the origin of clay minerals in sedimentary rocks.

Clays Clay Min 5: 159–173.Weaver CE & Beck KE (1971b) Clay water diagenesis during burial: how mud becomes grains. Geol

Soc Amer Spec Paper 134: 96 Wedephol KH (1969) Handbook of Geochemistry. Vol 1 SpringerVerlag, Berlin, p 422.Wiewiora A (1971) A mixed layer kaolinite-smectite from Lower Silesia, Poland: Clays and Clay

Min 34: 323–329.Wiewiora A (1973) Mixed-layer kaolinite-smectite from lower Silesia, Poland: Final report. Proc Int

Clay Conf Madrid 1972, p 75–88. Wronkiewicz DJ & Condie KC (1987) Geochemistry of Archean shales from the Witwatersrand Su-

pergroup, South Africa: Source-area weathering and provenance. Geochim. Cosmochim. Acta 51:2401–2416.

Wyborn LAI & Chappell BW (1983) Chemistry of the Ordovician and Silurian greywackes of theSnowy Mountains, southeastern Australia: an example of chemical evolution of sediments withtime: Chem Geol 39: 81–89.

Zaher MA & Rahman A (1980) Prospects and investigations for minerals in the northern part ofBangladesh. In: Petroleum and Mineral Resources of Bangladesh, Seminar and Exhibition, DhakaOct 1980. Published by the Ministry of Energy & Mineral resources Govt. of Bangladesh, p 9–18.

Appendices

Appendix 1 Depth and lithology of studied samples.

Habiganj well – 1

KailasTila well – 1

Sample No Core Depth (m) Lithology*1 1 1255.47 (Top) Light colour, bluish grey, shale, hard and

compact, fine grained2 1 1256.38 (Bottom) '' (thinly laminated)3 1 1256.38 (Top) '' (fragile)*4 1 1257.3 (Bottom) ''5 1 1257.3 (Top) ''6 1 1257.21 (Bottom) ''7 1 1258.21 (Top) ''*8 1 1259.12 (Bottom) ''9 2 1849.22 (Top) ''*10 2 1849.83 (Bottom) Bluish grey shale, hard and compact, thiny

laminated11 2 1849.83 (Top) Bluish grey, sandy shale, micro-cross lami-

nation present, light coloured*12 2 1850.74 (Top) Bluish grey shale with silt partings13 2 1851.66 (Bottom) Dark colour, bluish grey shale (sandy).

Hard and compact *14 2 1851.66 (Top) Hard and compact, bluish grey shale (dark in

colour)15 2 1852.65 (Bottom) ''*16 3 3110.64 (Top) Dark grey sandy shale17 3 3110.64 (Bottom) ''18 3 3111.55 (Top) ''* marked samples were used for pollen analysis.

Sample No Core Depth (m) Lithology*20 2 1175.30 (Bottom) Light coloured, grey shale, fragile*21 2 1176.22 (Top) Light grey shale with massif sand22 2 1177.80 (Bottom) Light grey sandy shale*23 5 2272.89 (Bottom) Dark colour, bluish shale hard and compact*24 6 2973.82 (Top) '' (with sand partings, light colour)*25 6 2974.52 (Bottom) ''*26 7 3730.14 (Top) Hard and compact sandy shale, light grey

coloured*27 7 3731.24 (Bottom) ''*28 8 3967.89 (Top) Bluish grey, hard and compact, sandy shale*29 9 3968.54 (Bottom) Dark coloured, hard and compact, sandy

shale*30 2 1081.73 (Top) Bluish grey shale31 2 1082.64 (Bottom) ''*32 2 1082.64 (Top) ''* marked samples were used for pollen analysis.

Rashidpur well – 1 (depth and lithology of studied samples)

Sample No Core Depth (m) Lithology*33 2 1083.64 (Bottom) '' with sand partings34 2 1083.64 (Top) ''35 2 1084.62 (Bottom) ''*36 2 1084.62 (Top) Light grey sandy shale37 2 1085.64 (Bottom) ''38 2 1085.64 (Top) ''*39 2 1086.30 (Bottom) ''40 2 1086.30 (Top) ''41 2 1087.40 (Bottom) ''*42 2 1087.40 (Top) ''43 2 1087.63 (Bottom) ''*44 3 1248.16 (Top) ''45 3 1248.93 (Bottom) Bluish grey shale46 3 1248.93 (Top) ''*47 3 1249.68 (Bottom) Light grey shale48 3 1250.6 (Bottom) Light colour shale*49 3 1250.6 (Top)51 3 1251.6 (Bottom) ''52 3 1251.6 (Top) ''*53 3 1252.42 (Bottom) '' (moderately hard)54 3 1252.42 (Top) ''55 3 1253.22 (Bottom) ''56 3 1253.22 (Top) ''57 3 1254.25 (Bottom) ''58 4 1827.89 (Top) Light coloured sandy shale*59 4 1828.69 (Bottom) ''60 4 1828.60 (Top) Light coloured bluish grey sandy shale61 4 1829.22 (Bottom) ''62 4 1830.42 (Top) Light coloured bluish grey sandy shale*63 4 1832.45 (Bottom) Light coloured bluish grey shale64 5 2134.5 (Top) Light coloured sandy shale*65 5 2135.54 (Bottom) ''*66 6 2472.23 (Top) Dark coloured sandy shale (hard and com-

pact)67 6 2473.42 (Bottom) ''68 6 2473.76 (Bottom) ''*69 7 2677.97 (Bottom) Bluish grey sandy shale*70 3 4268.72 (Well-2) ''* marked samples were used for pollen analysis.

Atgram well – IX (depth and lithology of studied samples)

Sample No Core Depth (m) Lithology*71 1 3633.52 (Top) Hard and compact, dark col-

oured shale72 1 3633.95 (Bottom) ''73 1 3635.35 (Top) ''74 1 3636.22 (Bottom) ''75 1 3637.42 (Top) ''76 1 3638.00 (Bottom) ''77 1 3639.20 (Top) ''*78 1 3640.10 (Bottom) ''79 2 3637.42 (Top) Hard and compact, dark col-

oured sandy shale*80 2 3638.02 (Bottom) Light coloured sandy shale,

hard and compact*81 3 3990.74 (Top) Medium grained sandstone*82 4 4729.15 (Top) Dark coloured, hard and

compact shale83 4 4733.85 (Bottom) ''84 4 4733.85 (Top) ''*85 4 4735.41 (Bottom) ''* marked samples were used for pollen analysis.

Fenchuganj well – 2 (depth and lithology of studied samples)

Patharia well – 5 (depth and lithology of studied samples)

Sample No Core Depth interval (m) Lithology*86 4 2190-2200 (Top) Bluish grey shale with sand partings87 4 2130-2200 (Bottom) ''88 4 2190-2200 (Top) Bluish grey shale*89 4 2190-2200 (Bottom) Bluish grey sandy shale*90 7 3137-3143 (Top) Bluish grey shale91 7 3137-3143 (Bottom) ''92 7 3137-3143 (Top) ''*93 7 3137-3143 (Bottom) ''*94 8 3259.96–3269,55 (Top) ''95 8 '' (Bottom) ''96 8 '' (Top) ''*97 8 '' (Bottom) ''*98 10 3624-3615 (Top) Bluish grey sandy shale*99 10 3624-3615 (Bottom) Bluish grey, hard and compact shale100 11 3730-3379 (Top) ''101 11 '' (Bottom) '' (with sand partings)*102 11 3730-3779 (Top) Bluish grey shale, hard and compact103 11 ''(Bottom) '' (with sand partings)*104 11 3770-3779 (Bottom) ''105 11 '' (Top) ''106 11 3770-3779 (Bottom) Bluish grey shale, hard and compact*107 12 4086-4095 (Top) ''108 12 '' (Bottom) '' (with sand partings)*109 12 4089-4095.5 (Top) Bluish grey shale, hard and compact110 12 '' (Bottom) ''111 12 4086-4095.5 (Top) ''*112 12 '' (Bottom) ''113 12 4086-4095.5 (Top) ''*114 12 '' (Bottom) ''* marked samples were used for pollen analysis.

Sample No Core Depth (m) Lithology*115 1 959 – 964 Hard and compact, dark grey shale116 1 '' Hard and compact, bluish grey shale*117 2 1450.5 – 1457.3 Bluish grey compact shale118 2 '' Light grey shale119 2 '' Bluish grey shale, hard andcompact120 2 '' ''*121 2 '' Bluish grey shale with sand partings122 2 '' Bluish grey shale123 2 '' Light grey shale with intercalation of sandstone124 2 '' ''*125 2 '' Bluish grey shale* marked samples were used for pollen analysis.

Sample No Core Depth interval (m) Lithology126 2 '' Bluish grey shale with sand partings, hard lami-

nated127 2 '' Light grey sandy shale, hard and compact*128 2 '' Light grey shale129 2 '' Light grey sandy shale, hard and compact130 2 '' ''131 3 1829–1837 Bluish grey shale, hard and compact 132 3 '' ''*133 3 '' Black shale, hard and compact134 3 '' ''135 3 '' ''136 3 '' ''137 3 '' ''138 3 '' ''139 3 '' ''*140 3 '' ''141 3 '' '' 142 3 1829–1837 Black shale, hard and compact143 3 '' ''144 3 '' ''145 3 '' ''*146 3 '' ''147 4 2290.5–2307.25 Bluish grey shale, hard and compact*148 4 '' ''149 4 '' Bluish grey shale, thin partings of sandstone,

hard and laminated150 4 '' ''151 4 '' ''152 4 '' ''*153 4 '' ''154 4 '' ''*155 4 '' Bluish grey shale, thin partings of sandstone,

hard and laminated156 4 '' ''157 4 '' Bluish grey massive shale with and,*158 4 '' ''159 4 '' Light grey shale with sand, hard and compact*160 4 '' Black shale with thin partings of sand*161 5 2828.75–2834.84 Blackish grey shale, very hard and compact*162 5 '' ''163 5 '' Bluish grey shale with silt partings, hard and

compact164 5 '' ''165 5 '' ''*166 5 '' ''167 5 '' Black shale, hard and compact168 5 '' ''169 5 '' ''*170 5 '' ''

Sample No Core Depth interval (m) Lithology171 6 3160.7–3168.25 Dark black shale, very hard and compact172 6 '' ''173 6 '' ''174 6 '' ''175 6 '' ''176 6 '' ''*177 6 '' ''178 6 '' ''*179 6 '' Bluish grey shale, thin layer of sand, hard and

compact180 6 '' ''181 6 '' '' 182 6 '' Bluish grey shale, thin layer of sand, hard and

compact*183 6 '' Black shale, hard and compact184 6 '' ''185 6 '' ''186 6 '' ''187 6 '' ''*188 6 '' ''* marked samples were used for pollen analysis.

Appendix 2 Major and minor trace elements of the core samples

1 2 3 4 5 6 7 8wt%SiO2 61.63 61.86 61.76 61.52 61.22 60.94 61.20 0.90TiO2 0.87 0.86 0.86 0.85 0.85 0.86 0.86 0.85A12O3 16.34 16.25 16.05 15.80 16.30 16.74 16.81 16.23Fe2O3 6.95 7.11 7.14 7.24 7.17 7.19 7.11 7.50MnO 0.09 0.10 0.10 0.11 0.10 0.10 0.09 0.11MgO 3.01 3.41 3.02 3.01 3.04 3.05 3.04 3.06CaO 1.93 2.02 2.10 2.07 1.98 1.93 1.83 2.03Na2O 1.57 1.58 1.56 1.54 1.54 1.56 1.58 1.54K2O 3.25 3.24 3.22 3.16 3.26 3.35 3.35 3.25P2O5 0.16 0.16 0.16 0.16 0.16 0.15 0.15 0.17S 0.15 0.19 0.16 0.17 0.18 0.16 0.22 0.28F 0.07 0.07 0.06 0.07 0.07 0.07 0.07 0.07LOI* 2.73 5.59 5.67 5.62 5.64 5.69 5.49 5.59Total 98.96 102.27 102.09 101.54 101.76 102.04 101.87 101.82ppmBa 480 468 476 472 485 488 487 497Ce 82 87 85 87 95 94 84 84Cl 197 228 210 210 216 247 241 240Co* 24 25 25 22 23 30 22 23Cr * 119 120 122 120 121 125 120 123Cu* 27 27 26 26 27 29 30 28Dy 1 3 4 2 4 5 12 1Er 12 6 2 9 13 10 7 7Gd 6 4 8 2 6 3 -2 12Hf 4 3 4 0 4 3 6 3La 37 41 38 38 36 39 42 45Nb 21 19 19 19 21 21 22 21Nd 41 40 39 38 45 14 42 41Ni* 64 62 60 60 61 65 66 61Pb* 25 28 23 23 21 26 27 26Pr 40 36 41 39 48 36 37 35Rb 199 197 198 190 204 210 210 203Sc 14 15 14 12 16 15 14 15Sr 170 173 174 170 176 176 176 178Th 31 28 28 28 29 28 29 30U 2 4 0 1 2 1 3 2V 127 129 128 123 129 129 130 129Zn* 96 95 93 94 94 98 97 96Zr 224 227 233 228 233 221 220 232* Marked elements have been measured by AAS rest of the elements by XRF#Sample1–8:Habiganjwell-1

9 10 11 12 13 14 15 16wt%SiO2 59.73 58.56 55.41 61.51 65.26 58.96 59.55 58.73TiO2 0.91 0.88 0.55 0.88 0.85 0.90 0.94 0.90A12O3 17.61 17.93 9.02 16.73 15.40 17.61 17.99 18.15Fe2O3 7.47 7.58 4.78 7.31 6.56 7.87 7.62 7.82MnO 0.11 0.11 0.82 0.12 0.07 0.13 0.13 0.13MgO 3.23 3.31 2.01 3.15 2.84 3.33 3.24 3.38CaO 1.90 2.29 14.79 1.86 1.01 2.06 1.57 1.81Na2O 1.46 1.42 1.29 1.53 1.47 1.43 1.45 1.44K2O 3.54 3.68 2.11 3.35 3.13 3.55 3.57 3.66P2O5 0.15 0.15 0.16 0.16 0.13 0.16 0.14 0.15S 0.07 0.03 0.01 0.03 0.30 0.02 0.02 0.01F 0.07 0.07 0.08 0.07 0.07 0.07 0.07 0.07LOI* 5.94 6.42 12.78 15.70 4.12 6.22 5.85 6.14Total 102.47 101.69 103.96 112.64 101.40 102.56 102.38 102.58ppmBa 719 541 388 519 472 530 543 537Ce 102 96 60 90 93 94 102 91Cl 406 404 149 397 96 348 368 415Co* 28 27 28 27 27 36 28 24Cr * 128 127 74 117 118 125 127 128Cu* 41 40 16 39 25 40 47 42Dy 10 6 2 -0 -1 5 2 5Er 8 12 10 6 6 8 14 4Gd 6 10 4 3 9 4 6 7Hf 1 4 26 26 -0 5 5 3La 44 42 6 10 21 46 45 43Nb 22 22 1 4 15 21 21 21Nd 45 42 44 42 27 41 45 39Ni* 69 72 22 22 35 65 64 68Pb* 32 30 45 42 15 30 23 32Pr 44 45 69 72 47 34 30 39Rb 216 229 140 198 186 218 217 225Sc 16 15 9 15 13 14 17 17Sr 171 170 271 162 143 169 165 167Th 32 31 26 29 25 33 29 30U 3 1 -5 2 3 2 3 3V 140 142 65 131 115 139 143 142Zn* 199 192 57 96 89 102 106 101Zr 229 212 245 236 304 226 230 213* Marked elements have been measured by AAS rest of the elements by XRF.# Sample 9–16: Habiganj well-1

17 18 19 20 22 23 24 25wt%SiO2 67.43 64.62 66.49 60.00 67.84 60.60 58.18 63.89TiO2 0.78 0.83 0.81 0.92 0.76 0.92 0.95 0.85A12O3 14.14 14.88 14.67 19.09 16.21 17.51 17.34 15.71Fe2O3 5.96 6.39 6.22 7.89 5.79 7.92 8.32 7.09MnO 0.08 0.12 0.07 0.14 0.06 0.08 0.12 0.09MgO 2.58 2.74 2.64 1.91 1.99 3.28 3.36 3.31CaO 1.27 2.07 1.00 0.93 0.78 2.44 1.89 1.44Na2O 1.45 1.44 1.44 0.61 0.89 1.33 1.23 1.35K2O 2.88 3.05 2.98 2.60 2.23 3.81 3.48 3.16P2O5 0.13 0.13 0.13 0.05 0.04 0.16 0.17 0.15S 0.28 0.27 0.30 0.02 0.08 0.11 0.04 0.25F 0.06 0.07 0.06 0.05 0.05 0.07 0.08 0.07LOI* 3.97 4.83 3.89 6.82 4.98 6.00 5.67 5.06Total 101.18 101.64 100.88 101.27 101.89 103.51 100.66 102.42ppmBa 440 466 462 562 440 495 554 513Ce 88 99 97 92 90 89 115 97Cl 51 73 64 89 108 43 117 75Co* 30 24 26 25 52 20 20 40Cr * 104 113 109 151 106 123 147 117Cu* 22 23 23 28 46 44 38 35Dy 1 4 7 6 4 4 15 3Er 1 3 7 18 14 12 2 14Gd 7 2 5 0 1 7 8 5Hf 6 7 6 5 9 7 4 7La 44 47 50 49 39 45 52 49Nb 19 21 20 18 14 18 20 16Nd 43 45 41 39 36 34 41 36Ni* 56 60 62 72 101 64 80 74Pb* 24 23 22 43 24 22 24 24Pr 28 30 40 21 19 24 43 27Rb 164 183 172 222 148 179 231 186Sc 12 12 10 19 14 17 19 15Sr 146 161 141 161 143 176 182 154Th 22 25 26 42 28 23 35 23U 3 1 1 3 4 2 1 2V 98 111 103 140 116 142 156 125Zn* 82 78 80 261 88 92 96 99Zr 310 322 333 256 271 223 249 215* Marked elements have been measured by AAS rest of the elements by XRF# Sample 17-19: Habiganj well-1, sample 20–25: Kailas Tila-1 well

26 27 28 29 30 31 32 33wt%SiO2 64.40 67.25 59.04 63.14 62.83 62.81 62.42 63.53TiO2 0.61 0.64 1.00 0.93 0.87 0.87 0.87 0.86A12O3 10.94 11.04 18.47 16.49 15.57 14.85 15.66 14.62Fe2O3 4.47 4.81 8.08 7.36 6.77 6.97 6.67 7.06MnO 0.08 0.07 0.09 0.09 0.11 0.11 0.11 0.12MgO 3.92 3.89 3.16 2.89 3.17 3.34 3.13 3.11CaO 5.48 4.20 0.67 0.82 2.57 2.57 2.46 2.84Na2O 1.38 1.46 1.41 1.52 1.29 1.24 1.27 1.34K2O 2.51 2.58 3.57 3.15 3.02 2.94 2.98 2.75P2O5 0.14 0.13 0.14 0.16 0.16 0.15 0.14 0.18S 0.01 0.02 0.03 0.05 0.10 0.35 0.13 0.09F 0.06 0.07 0.07 0.06 0.06 0.05 0.06 0.06LOI* 7.12 6.04 5.15 4.44 5.76 5.22 6.02 6.05Total 101.22 102.37 100.14 101.32 102.48 101.66 102.14 102.81ppmBa 423 439 464 501 552 474 513 450Ce 82 78 103 99 101 96 108 99Cl 152 83 73 40 129 112 71 72Co* 27 52 35 22 29 27 27 28Cr * 72 75 137 118 136 129 137 138Cu* 17 24 44 36 37 38 38 38Dy 3 1 -0 5 1 7 6 4Er 12 -2 16 10 7 9 4 5Gd 1 10 3 1 1 0 5 5Hf 5 7 8 6 5 8 4 9La 35 35 48 47 43 36 42 48Nb 12 12 19 18 22 21 22 21Nd 33 33 41 38 42 46 48 42Ni* 35 41 89 65 84 74 82 84Pb* 15 16 27 25 28 23 24 22Pr 20 20 32 17 40 46 44 38Rb 132 129 230 118 169 161 171 151Sc 10 11 19 14 13 14 14 15.9Sr 188 140 172 159 175 167 182 183Th 18 19 23 24 25 29 30 28U 2 4 1 3 2 2 0 1V 69 74 156 127 122 116 122 114Zn 54 103 103 92 88 87 91 87Zr 355 307 251 276 286 311 319 324* Marked elements have been measured by AAS rest of the elements by XRF# Sample 26 – 29 Kailas Tila-1 well, Sample 30 – 33 Rashidpur well-1

34 35 36 37 38 39 40 41wt%SiO2 61.38 61.65 58.76 64.11 62.63 63.53 63.88 62.62TiO2 0.86 0.87 0.76 0.88 0.87 0.87 0.89 0.87A12O3 15.57 15.26 14.92 15.18 15.22 15.47 15.01 16.28Fe2O3 6.65 7.09 6.97 6.91 6.94 6.39 6.77 7.03MnO 0.12 0.10 0.15 0.10 0.11 0.09 0.10 0.10MgO 3.23 3.09 3.08 3.11 3.45 3.08 3.13 3.30CaO 3.47 2.42 5.15 2.08 2.40 1.96 2.39 1.83Na2O 1.31 1.27 1.29 1.47 1.31 1.36 1.37 1.31K2O 2.91 2.86 2.80 3.30 2.86 2.94 2.80 3.04P2O5 0.15 0.15 0.14 0.14 0.14 0.14 0.15 0.14S 0.08 0.48 0.32 0.21 0.10 0.09 0.11 0.08F 0.06 0.05 0.12 0.06 0.06 0.05 0.06 0.048LOI* 6.73 5.54 6.53 5.09 5.98 5.46 5.66 5.73Total 102.74 102.04 102.23 102.53 102.33 102.19 102.55 102.59ppmBa 481 479 504 490 462 482 484 506Ce 91 47 119 95 93 97 100 94Cl 79 90 77 88 80 77 77 74Co* 33 23 35 28 29 26 31 31Cr * 138 128 119 120 140 133 131 136Cu* 40 38 34 40 4 39 39 40Dy 2 2 19 6 2 0 4 8Er 0 1 9 4 5 7 13 7Gd 0 6 12 -6 8 6 6 12Hf 6 3 8 8 1 6 6 5La 41 43 58 42 41 46 44 40Nb 22 21 20 20 -17 21 21 22Nd 43 43 53 40 21 41 45 40Ni* 88 73 69 73 44 75 76 81Pb* 23 24 21 22 48 24 24 26Pr 40 34 48 36 40 12 42 40Rb 165 158 159 162 154 162 155 168Sc 11 13 16 12 14 15 14 14Sr 190 174 288 155 173 167 172 167Th 29 29 29 26 -2 27 25 25U 0 3 4 4 1 26 2 4V 121 119 118 111 119 120 116 130Zn* 90 85 81 90 61 92 88 92Zr 299 310 275 284 285 315 144 271* Marked elements have been measured by AAS rest of the elements by XRF# Sample 34–41: Rashidpur well-1

42 43 44 45 46 47 48 49wt%SiO2 63.22 62.45 63.97 62.81 63.66 63.95 63.30 62.54TiO2 0.83 0.83 0.84 0.81 0.85 0.82 0.84 0.86A12O3 14.30 15.47 15.82 14.71 15.75 15.70 15.66 16.08Fe2O3 6.74 6.88 6.54 8.15 6.75 6.47 6.91 7.12MnO 0.11 0.10 0.10 0.13 0.10 0.09 0.10 0.11MgO 3.47 3.28 2.80 2.84 9.81 2.75 2.45 2.89CaO 3.19 2.46 1.91 2.12 1.88 1.86 1.97 1.93Na2O 1.36 1.36 1.241 1.57 1.63 1.54 1.58 1.58K2O 2.91 2.97 3.16 2.98 3.12 3.17 3.12 3.18P2O5 0.15 0.16 0.17 0.21 0.17 0.16 0.18 0.18S 0.125 0.09 0.122 0.11 0.13 0.12 0.14 0.13F 0.063 0.07 0.07 0.06 0.06 0.06 0.07 0.06LOI* 6.26 5.97 4.99 5.76 5.08 4.96 5.21 5.43Total 102.92 102.33 102.32 102.17 101.94 102.13 102.29 101.85ppmBa 468 898 469 466 472 476 466 484Ce 93 85 81 96 86 87 91 91Cl 82 91 84 90 105 83 83 98Co* 57 34 26 41 25 16 36 43Cr * 124 126 103 106 103 102 110 111Cu* 35 36 28 25 29 7 27 30Dy 2 3 3 5 7 9 3 6Er 8 2 3 6 8 6 12 0Gd 2 14 6 -1 3 12 10 1Hf 4 5 6 4 4 3 6 2La 40 41 43 48 40 40 43 42Nb 39 21 21 20 21 21 21 22Nd 20 41 37 44 40 40 40 43Ni* 78 77 51 48 52 36 55 55Pb* 23 23 26 22 27 27 26 27Pr 30 34 28 46 26 31 40 39Rb 158 162 180 177 182 185 187 190Sc 13 14 12 14 12 11 12 13Sr 169 176 175 181 182 178 176 184Th 30 25 25 26 26 24 27 27U 1 1 4 3 2 2 1 1V 110 120 112 112 119 111 117 128Z* 83 87 91 84 91 92 94 93Zr 305 273 244 288 244 245 248 258* Marked elements have been measured by AAS rest of the elements by XRF# Sample 42–49: Rashidpur well-1

50 51 52 53 54 55 56 57wt%SiO2 63.39 62.97 63.48 62.078 63.21 63.58 61.34 62.45TiO2 0.84 0.83 0.84 0.85 0.83 0.85 0.87 0.85A12O3 15.91 15.35 15.52 16.33 16.67 15.33 17.72 16.19Fe2O3 6.45 7.17 6.63 6.95 6.28 6.79 7.13 7.06MnO 0.09 0.11 0.10 0.10 0.09 0.10 0.11 0.11MgO 2.80 2.85 2.87 2.91 2.76 2.81 3.02 2.88CaO 1.91 2.09 2.18 1.90 1.71 2.14 1.60 1.85Na2O 1.60 1.56 1.56 1.56 1.69 1.56 1.60 1.57K2O 3.19 3.07 3.13 3.29 3.32 3.03 3.59 3.26P2O5 0.16 0.18 0.17 0.17 0.15 0.18 0.16 0.17S 0.12 0.12 0.13 0.12 0.07 0.16 0.08 0.09F 0.07 0.06 0.08 0.07 0.06 0.06 0.07 0.07LOI* 5.13 5.55 5.35 5.43 5.08 5.24 5.51 5.37Total 101.85 102.40 102.50 101.95 102.14 102.01 103 102.14ppmBa 479 475 463 496 498 471 515 484Ce 87 88 103 93 97 93 82 90Cl 51 46 65 70 74 67 77 84Co* 31 24 24 21 64 36 24 24Cr * 107 110 110 109 101 109 108 109Cu* 24 25 25 28 32 28 35 30Dy 4 0 3 0 2 4 3 3Er 10 -1 3 9 8 10 13 7Gd 2 5 0 4 8 2 9 6Hf 3 6 7 4 4 4 2 1La 43 40 45 42 33 36 52 44Nb -17 -18 -19 -18 21 21 22 21Nd 21 20 21 21 40 37 38 42Ni* 38 43 44 43 52 54 55 54Pb* 52 54 55 54 21 18 24 20Pr 22 20 23 21 41 41 35 41Rb 191 182 184 202 192 179 213 194Sc 13 14 14 13 13 11 15 12Sr 180 178 171 181 183 179 179 183Th 30 28 28 29 28 26 28 29U 3 4 2 2 1 2 1 4V 121 117 117 126 122 119 131 123Zn* 91 88 91 94 96 89 99 94Zr 248 258 273 242 243 268 208 245* Marked elements have been measured by AAS, rest of the elements by XRF# Sample 50–57: Rashidpur well-1

58 59 60 61 62 63 64 65wt%SiO2 60.82 62.56 60.02 59.85 61.57 62.33 65.44 66.30TiO2 0.89 0.88 0.91 0.91 0.92 0.88 0.82 0.79A12O3 17.03 16.71 18.12 18.01 16.84 16.73 13.84 13.19Fe2O3 7.77 6.90 7.82 7.85 7.61 7.31 6.05 5.66MnO 0.12 0.10 0.12 0.12 0.12 0.11 0.07 0.07MgO 3.13 2.99 3.26 3.26 3.02 3.01 2.83 2.74CaO 1.48 1.45 1.28 1.29 1.50 1.37 2.71 2.76Na2O 1.46 1.52 1.45 1.45 1.49 1.50 1.54 1.54K2O 3.31 3.23 3.50 3.48 3.26 3.25 2.92 2.78P2O5 0.17 0.15 0.17 0.15 0.16 0.15 0.12 0.12S 0.04 0.06 0.02 0.05 0.03 0.02 0.15 0.05F 0.59 0.06 0.07 0.06 0.06 0.06 0.06 0.07LOI* 5.62 5.23 5.77 5.82 5.34 5.29 5.33 5.20Total 102.14 102.04 102.7 102.19 102.21 102.18 102.06 101.44 ppmBa 511 495 518 517 513 495 456 551Ce 99 90 99 94 93 96 92 95Cl 78 65 71 53 83 64 78 73Co* 35 26 22 27 38 31 113 41Cr * 133 128 137 136 133 131 109 104Cu* 36 36 40 40 36 34 26 25Dy 6 1 19 4 0 4 6 -4Er 4 6 12 12 1 5 13 1Gd 2 16 8 3 5 8 7 -0Hf 4 4 4 2 2 3 6 6La 46 45 41 41 44 54 37 38Nb 21 16 22 22 22 21 21 19Nd 41 44 45 45 43 33 40 42Ni* 70 68 71 74 64 66 53 52Pb* 18 19 21 22 21 21 16 17Pr 45 24 39 45 42 39 39 38Rb 204 190 214 216 199 194 171 148Sc 14 15 17 16 16 14 12 10Sr 181 174 178 178 174 173 154 138Th 31 30 30 28 28 25 28 20U 3 1 2 0 1 1 3 1V 133 127 143 142 127 132 105 94Zn* 99 95 103 102 94 93 80 78Zr 264 247 222 226 269 275 329 324* Marked elements have been measured by AAS rest of the elements by XRF# Sample 58 – 65: Rashidpur well-1

66 67 68 69 71 72 74 75wt%SiO2 62.05 65.41 61.69 59.43 65.61 65.05 66.69 62.04TiO2 0.91 0.86 0.93 0.93 0.79 0.79 0.78 0.84A12O3 16.95 15.51 17.54 17.34 16.12 16.23 15.61 17.38Fe2O3 7.11 6.56 7.39 7.34 6.71 6.95 6.44 7.68MnO 0.09 0.09 0.09 0.17 0.10 0.10 0.09 0.12MgO 2.85 2.62 2.96 2.90 2.72 2.77 2.66 2.93CaO 1.16 1.22 1.17 2.74 1.02 1.01 1.01 0.93Na2O 1.41 1.44 1.41 1.39 1.29 1.27 1.29 1.25K2O 3.29 3.07 3.42 3.62 3.47 3.53 3.39 3.70P2O5 0.13 0.14 0.13 0.14 0.15 0.15 0.14 0.16S 0.02 0.02 0.02 0.02 0.02 0.05 0.02 0.03F 0.06 0.06 0.06 0.07 0.07 0.07 0.07 0.07LOI* 5.22 4.68 5.38 6.36 4.58 4.61 4.40 5.10Total 101.46 101.878 102.21 102.69 102.84 102.78 102.76 102.44ppmBa 481 462 482 527 534 533 519 582Ce 103 101 95 103 95 89 76 99Cl 49 55 55 52 29 46 32 33Co* 25 52 27 27 34 28 46 55Cr * 130 120 137 130 19 97 89 111Cu* 33 27 34 37 36 36 33 49Dy 4 8 2 1 9 6 7 5Er 1 11 9 12 7 7 9 16Gd 5 -3 -1 5 6 9 5 5Hf 4 4 1 4 5 5 6 7La 48 42 45 43 14 43 49 47Nb 22 21 21 23 15 16 15 16Nd 46 39 44 43 38 36 36 43Ni* 70 64 74 61 51 55 38 60Pb* 27 24 25 29 25 26 26 30Pr 44 40 40 45 17 25 12 22Rb 187 176 208 227 189 192 183 217Sc 14 14 15 16 14 17 15 19Sr 154 157 172 182 108 108 105 122Th 26 27 29 31 19 22 19 26U 1 1 2 1 5 5 4 4V 133 119 140 137 116 122 112 135Z* 94 86 97 98 86 91 86 97Zr 268 292 254 248 232 229 235 230*Marked elements have been measured by AAS rest of the elements by XRF.# Sample 66 – 69: Rashipur well - 1, Sample 71 – 75: Atgram well-IX

76 77 78 79 83 84 85 86wt%SiO2 64.20 46.20 64.32 75.51 72.02 70.22 89.40 71.30TiO2 0.82 0.80 0.80 0.55 0.78 0.89 0.28 0.77A12O3 16.37 16.47 16.68 7.76 12.87 13.77 4.71 13.28MnO 6.96 6.86 6.80 4.72 5.94 6.13 2.56 0.07MgO 0.11 0.10 0.09 0.17 0.09 0.06 0.02 2.42CaO 2.78 2.71 2.73 1.21 1.47 1.57 0.50 0.64Na2O 1.04 1.08 1.04 4.06 0.33 0.26 0.38 1.58K2O 1.27 1.26 1.24 1.09 0.93 0.96 0.49 2.66P2O5 3.53 3.48 3.54 1.73 2.56 2.94 0.87 0.13S 0.14 0.15 0.14 0.09 0.17 0.13 0.07 0.03F 0.05 0.03 0.02 0.33 0.35 0.25 0.88 0.05LOI* 4.63 4.61 4.60 4.25 4.81 5.06 2.26 3.44Total 102.17 102.00 102.28 101.65 102.57 102.83 102.49 101.96ppmBa 555 538 547 330 685 516 375 416Ce 92 86 86 63 90 113 25 79Cl 47 34 35 35 73 52 24 71Co* 27 36 39 46 51 49 318 84Cr * 101 95 95 59 63 81 14 95Cu* 36 36 36 21 30 25 7 23Dy 11 5 2 2 3 9 -3 7Er 9 13 7 0 -0 13 -0 7Gd 12 2 11 17 14 7 8 1Hf 6 7 5 9 8 10 8 9La 47 45 41 27 41 49 16 41Nb 16 15 16 10 16 19 5 14Nd 38 38 35 28 39 44 16 35Ni* 53 52 49 30 36 34 17 50Pb* 28 26 26 11 18 14 3 13Pr 23 23 32 12 17 22 -8 14Rb 201 186 196 79 127 156 33 137Sc 14 15 15 8 11 13 3 13Sr 114 108 111 147 128 147 37 114Th 22 19 24 14 18 22 -0 18U 1 4 5 3 3 2 4 5V 124 117 118 50 88 113 38 89Zn* 100 96 89 45 96 74 26 71Zr 214 229 241 335 307 352 129 280* Marked elements have been measured by AAS rest of the elements by XRF# Sample 76 – 85: Atgram well - IX, Sample 86: Fenchuganj well-2

88 90 91 92 95 96 97 99wt%SiO2 57.80 59.82 59.82 62.44 51.73 50.57 61.86 63.01TiO2 0.58 0.89 0.89 0.83 0.90 0.94 0.71 0.93A12O3 9.11 16.38 17.15 15.17 16.68 16.96 12.66 15.96MnO 0.67 0.08 0.08 0.07 0.07 0.08 0.06 0.08MgO 1.73 3.5 3.86 3.65 5.63 5.84 4.51 2.89CaO 13.56 1.87 1.91 2.13 4.59 4.93 4.20 0.50Na2O 1.18 1.25 1.24 1.35 1.18 1.16 1.35 1.63K2O 2.01 3.52 3.59 3.32 3.66 3.65 2.85 3.32P2O5 0.13 0.12 0.13 0.13 0.12 0.05 0.12 0.13S 0.23 0.08 0.06 0.04 0.04 0.05 0.12 0.03F 0.08 0.08 0.09 0.09 0.09 0.06 0.07 0.07LOI* 11.43 6.26 6.32 6.20 9.89 10.33 7.91 3.97Total 103.30 101.58 102.65 102.01 102.56 102.86 102.26 100.12

ppmBa 349 529 669 633 634 525 561 558Ce 73 108 101 93 101 111 88 101Cl 10 38 49 17 77 121 107 50Co* 21 26 26 52 27 24 42 42Cr * 83 126 128 109 138 172 99 126Cu* 15 36 38 28 37 41 25 25Dy 1 4 3 0 10 3 4 10Er 8 13 0 2 9 11 9 -0Gd 20 1 1 0 6 6 7 8Hf 1 5 4 5 5 5 6 6La 33 47 49 48 49 45 47 52Nb 11 19 17 17 20 20 14 18Nd 27 48 36 43 49 46 40 40Ni* 36 67 65 52 71 75 54 54Pb* 9 20 21 17 20 18 17 17Pr 32 29 22 25 43 44 28 27Rb 121 229 226 204 244 247 164 202Sc 13 18 18 16 19 20 13 17Sr 255 149 138 131 167 171 144 155Th 29 33 32 27 41 41 30 27U -4 3 4 3 -0 -0 1 1V 69 126 131 112 140 144 91 124Zn* 49 93 99 82 88 90 83 83Zr 317 237 240 271 274 281 313 261* Marked elements have been measured by AAS rest of the elements by XRF# Sample 88–99: Fenchuganj well-2

100 103 104 105 108 109 110 111wt%SiO2 57.81 67.44 59.73 58.57 60.49 65.50 70.29 67.92TiO2 1.01 0.84 1.02 1.03 0.89 0.78 0.66 0.74A12O3 17.90 14.44 17.92 18.07 16.74 14.70 12.69 12.69MnO 0.08 .13 0.12 0.10 0.10 0.09 0.09 0.13MgO 3.21 2.59 3.24 3.21 3.19 2.89 2.25 2.63CaO 0.46 0.92 0.55 0.49 1.38 1.49 2.16 1.99Na2O 1.34 1.59 1.42 1.36 1.39 1.43 1.41 1.46K2O 3.38 2.66 3.31 3.37 3.44 3.02 2.61 2.65P2O5 0.14 0.18 0.16 0.14 0.14 0.14 0.12 0.16S 0.08 0.03 0.04 0.04 0.07 0.03 0.01 0.04F 0.07 0.06 0.07 0.06 0.07 0.07 0.04 0.06LOI* 5.08 3.98 5.05 5.13 5.48 4.94 4.50 4.92Total 100.26 101.26 101.19 100.24 100.78 101.54 102.00 102.47ppmBa 539 477 522 545 535 490 427 446Ce 100 90 107 95 101 90 80 85Cl 12 33 18 26 50 51 30 24Co* 33 28 32 26 28 25 31 27Cr * 198 129 152 202 130 112 89 107Cu* 50 33 49 52 38 3 26 28Dy 11 3 1 8 2 4 3 3Er 4 3 7 7 11 4 10 7Gd -4 7 6 10 3 6 7 6Hf 4 8 8 8 5 5 8 6La 48 44 49 50 51 45 35 46Nb 20 14 19 20 18 15 12 14Nd 46 40 42 43 42 39 40 39Ni* 97 66 88 90 76 60 49 59Pb* 23 15 24 26 23 16 23 21Pr 38 32 26 30 30 15 11 21Rb 246 153 223 236 213 170 139 141Sc 19 15 19 19 17 15 11 15Sr 194 164 170 181 161 136 172 155Th 36 20 30 35 31 25 21 20U -0 3 3 3 3 3 2 4V 156 111 151 157 134 107 88 92Zn* 108 90 106 108 119 87 86 86Zr 263 240 243 253 284 225 249 257* Marked elements have been measured by AAS rest of the elements by XRF# Sample 100 – 111: Fenchuganj well-2

112 113 114 115 116 117 118 119wt%SiO2 60.65 65.86 66.12 59.86 60.50 63.60 69.31 62.14TiO2 0.88 0.78 0.78 0.92 0.91 0.85 0.74 0.89A12O3 17.02 14.41 14.99 18.10 17.70 15.98 12.16 16.97MnO 0.08 2.90 2.77 0.12 0.12 0.06 0.09 0.06MgO 3.32 1.85 1.29 3.29 3.23 2.87 2.16 3.08CaO 1.43 1.45 1.41 1.03 1.24 1.06 2.53 1.07Na2O 1.35 2.96 2.96 1.45 1.47 1.55 1.69 1.60K2O 3.51 0.14 0.13 3.42 3.34 3.03 2.38 3.26P2O5 0.13 0.03 0.04 0.15 0.14 0.11 0.12 0.11S 0.04 0.05 0.04 0.04 0.04 0.04 0.16 0.03F 0.07 5.02 4.67 0.06 0.07 0.06 0.04 0.07LOI* 5.55 5.51 4.53 4.39 4.82 5.30 5.41 4.94Total 101.42 102.05 102.21 100.68 101.70 101.75 101.99 101.43ppmBa 540 467 535 503 500 918 6808 614Ce 93 99 91 95 95 94 18 86Cl 34 89 42 111 81 85 166 111Co* 125 104 115 25 55 43 36 32Cr * 8 7 18 150 141 124 85 130Cu* 42 31 34 37 16 31 23 30Dy 6 6 -0 6 5 - 1 5Er 2 10 -0 5 7 1 6 9Gd 1 14 18 9 10 6 6 3Hf 7 5 7 2 4 5 2 6La 49 41 42 45 44 40 33 40Nb 17 15 14 22 21 20 18 22Nd 41 33 36 41 43 37 10 37Ni* 73 57 65 66 33 56 32 57Pb* 28 22 26 18 12 19 7 14Pr 32 27 26 45 46 43 - 28Rb 25 17 12 212 204 149 122 192Sc 17 13 12 16 16 12 11 13Sr 153 140 130 146 151 118 238 138Th 30 23 20 28 29 17 17 27U 2 3 3 2 4 4 2 3V 133 104 112 141 138 116 76 125ZN* 109 86 98 92 56 91 78 80Zr 251 266 228 236 240 258 249 277* Marked elements have been measured by AAS rest of the elements by XRF# Samples 112 – 114: Fenchuganj well-2; Samples 115 – 119: Patharia well-5.

121 122 124 125 126 127 128 129wt%SiO2 70.75 61.01 66.26 61.46 59.69 74.51 65.96 66.63TiO2 0.57 0.85 0.83 0.83 0.94 0.54 0.054 0.76A12O3 9.51 16.82 15.12 15.51 17.75 9.88 10.12 14.12Fe2O3 5.04 7.01 6.41 8.41 7.90 4.44 5.83 6.01MnO 0.13 0.06 0.05 0.14 0.10 0.10 0.22 0.06MgO 1.99 2.99 2.71 2.99 3.05 1.59 2.11 2.39CaO 3.99 1.00 1.21 1.93 0.83 2.50 6.06 1.22Na2O 1.33 1.54 1.72 1.51 1.65 1.49 1.34 1.57K2O 1.91 3.15 2.88 2.99 3.36 2.02 2.10 2.73P2O5 0.27 0.11 0.1 0.25 0.15 0.21 0.54 0.10S 0.13 0.04 0.16 0.87 0.06 0.13 0.04 0.02F 0.60 0.08 0.40 0.07 0.07 0.04 0.08 0.05LOI* 5.41 4.94 1.02 4.63 4.53 3.74 3.85 4.11Total 101.43 99.83 98.54 102.10 101.40 101.40 99.04 100.06ppmBa 1521 720 570 812 1398 486 455 753Ce 132 89 88 106 84 68 85 95Cl 197 121 111 90 303 92 103 125Co* 43 27 63 27 25 67 74 109Cr * 74 131 112 124 129 11 10 11Cu* 16 28 24 33 28 34 20 24Dy 2 11 2 - 0 - 11 2Er 2 6 10 - 8 - 9 2Gd 3 9 4 5 1 5 5 5Hf 60 37 46 35 40 4 4 5Nb 16 20 20 20 22 14 15 20Nd 66 38 40 51 36 32 38 41Ni* 30 55 40 53 56 55 32 39Pb* 11 25 13 11 20 17 11 12Pr 27 46 25 44 29 21 31 49Rb 96 155 155 178 202 18 30 25Sc 10 16 12 14 16 8 10 13Sr 276 110 128 180 168 160 288 142Th 21 20 27 27 25 16 24 29U 2 4 4 2 3 3 - 1V 67 123 107 125 137 61 76 104Zn* 44 26 38 55 45 79 56 73Zr 295 215 306 255 238 229 249 229* Marked elements have been measured by AAS rest of the elements by XRF# Samples 121 – 129: Patharia well – 5.

130 131 132 133 134 135 136 137wt%SiO2 73.69 71.55 65.60 62.61 67.02 62.28 62.26 63.44TiO2 0.51 0.65 0.79 0.87 0.75 0.86 0.87 0.84A12O3 9.44 12.42 15.08 16.83 14.24 16.60 16.85 16.07Fe2O3 3.80 5.00 6.47 6.79 6.05 7.09 6.78 6.92MnO 0.11 0.07 0.07 0.07 0.06 0.08 0.07 0.08MgO 1.44 2.29 2.80 3.04 2.64 3.13 3.04 3.03CaO 3.81 1.30 1.33 1.33 1.33 1.41 1.36 1.52Na2O 1.55 1.77 1.77 1.68 1.76 1.66 1.68 1.67K2O 1.99 2.53 2.94 3.27 2.84 3.26 3.27 3.17P2O5 0.89 0.11 0.13 0.13 0.12 0.12 0.13 0.14S 0.01 0.00 0.00 0.00 0.01 0.02 0.00 0.07F 0.05 0.06 0.06 0.00 0.06 0.07 0.07 0.07LOI* 4.45 3.69 4.38 4.85 4.13 4.93 5.81 4.87Total 101.11 101.66 101.25 101.92 101.20 101.71 101.75 102.03ppmBa 843 964 465 479 466 478 481 496Ce 59 50 68 92 76 95 100 96Cl 106 59 122 68 153 68 64 54Co* 144 95 33 28 32 26 39 41Cr * 60 78 105 119 25 117 114 113Cu* 12 21 28 39 31 38 40 36Dy 1 4 7 3 0 0 13 4Er 1 5 2 6 7 9 7 4Gd 0 3 8 4 3 3 5 3Hf 7 7 5 5 2 6 4 6La 28 34 34 48 36 42 43 20Nb 14 17 20 21 19 21 21 42Nd 29 23 33 41 36 45 42 64Ni* 9 12 13 18 16 20 19 19Pr 17 18 20 38 32 42 41 32Rb 96 123 158 188 149 191 193 24Sc 7 8 11 12 11 13 14 14Sr 192 113 126 144 124 148 150 145Th 19 13 20 26 22 27 29 26U -1 2 3 3 3 4 3 3V 56 83 100 120 93 120 121 116Zn* 50 72 80 98 86 102 100 94Zr 238 217 246 248 245 250 258 253* Marked elements have been measured by AAS rest of the elements by XRF# Sample 130 – 137: Patharia well-5

138 139 140 141 142 143 144 145wt%SiO2 62.76 62.37 60.75 62.53 61.05 61.77 61.42 60.75TiO2 0.84 0.87 0.87 0.85 0.87 0.85 0.07 0.87A12O3 16.23 16.74 16.87 16.54 17.06 16.52 17.87 17.11Fe2O3 6.95 6.87 7.41 6.55 7.30 6.98 7.36 7.31MnO 0.08 0.08 0.09 0.09 0.08 0.10 0.09 0.10MgO 3.05 3.06 3.13 3.00 3.16 3.08 3.19 3.24CaO 1.38 1.44 1.47 1.51 1.42 1.65 1.41 1.52Na2O 1.67 1.66 1.59 1.6 1.61 1.62 1.62 1.56K2O 3.19 3.31 3.34 3.26 3.35 3.28 3.38 3.45P2O5 0.14 0.14 0.16 .0.14 0.14 0.15 0.15 0.15S 0.02 0.01 0.04 0.01 0.02 0.03 0.02 0.04F 0.06 0.07 0.06 0.06 0.07 0.06 0.07 0.06LOI* 4.76 4.98 5.19 5.05 5.09 5.04 5.11 5.26Total 102.33 103.79 101.18 101.43 102.22 101.29 101.99 101.62ppmBa 488 473 554 521 493 487 488 506Ce 96 85 95 95 91 96 93 97Cl 61 60 58 51 49 52 53 46Co* 30 49 31 28 34 35 26 29Cr * 114 118 126 114 124 118 121 126Cu* 41 41 42 39 61 46 42 43Dy 2 9 8 1 10 -3 8 1Er 0 5 1 0 10 3 7 4Gd 1 5 -0 1 4 4 8 -Hf 3 4 3 4 4 2 1 4La 43 41 42 41 48 38 44 46Nb 21 21 22 22 - - - 22Nd 41 40 39 46 41 43 40 44Ni* 65 65 68 63 69 67 68 70Pb* 18 18 23 18 22 18 21 18Pr 36 33 47 46 37 35 37 35Rb 186 195 204 184 197 190 199 207Sc 14 14 12 13 16 16 13 14Sr 151 151 155 147 150 151 150 152Th 29 27 30 26 28 27 27 30U 4 2 3 1 1 3 3 3V 116 120 127 120 124 121 124 128Zn* 100 100 99 94 97 98 102 101Zr 261 248 252 245 239 242 238 138* Marked elements have been measured by AAS rest of the elements by XRF# Samples 138 – 145: Patharia well–5

146 147 148 149 150 151 152 154wt%SiO2 61.01 58.84 57.93 59.90 60.63 61.00 61.56 76.22TiO2 0.86 0.98 0.99 0.96 0.99 0.94 0.77 0.60A12O3 16.61 18.53 19.38 17.00 18.17 17.74 17.24 10.22Fe2O3 7.11 8.21 8.47 7.95 7.90 8.05 7.47 3.93MnO 0.10 0.11 0.11 0.11 0.10 0.09 0.09 0.05MgO 3.10 3.18 3.28 3.11 3.10 3.20 2.94 1.47CaO 1.57 0.51 0.47 0.53 0.55 0.52 0.54 0.70Na2O 1.60 1.51 1.46 1.56 1.57 1.58 1.61 1.73K2O 3.33 3.36 3.54 3.20 3.29 3.23 1.13 1.95P2O5 0.15 0.13 0.13 0.14 0.14 0.14 0.13 0.12S 0.04 0.03 0.02 0.03 0.04 0.06 0.08 0.04F 0.07 0.06 0.05 0.06 0.07 0.05 0.06 0.04LOI* 5.09 5.06 5.39 5.03 4.89 4.75 4.56 2.40Total 100.85 100.76 101.45 100.58 101.63 101.63 100.64 100.05ppmBa 506 559 536 522 560 564 706 462Ce 102 102 99 93 87 94 88 84Cl 88 147 150 130 143 208 176 228Co* 30 30 25 34 36 39 37 36Cr * 128 164 171 157 139 159 154 119Cu* 43 50 49 51 48 50 21 18Dy 6 6 5 -2 6 0 2 0Er 5 7 5 0.2 13 12 8 4Gd 1.6 3.4 2 5 4 9 3 6Hf 2 5 4 1 6 7 3 5La 46 42 42 40 42 46 41 33Nb 22 22 22 22 22 22 22 18Nd 42 47 44 42 42 43 41 39Ni* 52 104 96 109 106 106 72 64Pb* 17 20 28 31 26 22 27 18Pr 47 48 49 47 35 421 35 30Rb 200 213 231 185 204 199 199 125Sc 14 17 18 16 18 17 14 11Sr 154 171 176 156 170 167 173 -31Th 29 29 31 23 -1 -0.2 3 138U 2 2 3 1 2 2 0 3V 122 150 164 142 146 143 136 97Zn* 102 113 129 133 120 118 127 103Zr 244 115 242 201 251 113 113 82* Marked elements have been measured by AAS rest of the elements by XRF# Sample 146 – 154: Patharia well–5

155 156 157 158 159 160 161 162wt%SiO2 60.38 71.77 69.07 74.55 55.32 63.01 67.14 64.86TiO2 0.94 0.74 0.68 0.62 0.59 0.93 0.78 0.85A12O3 17.52 12.38 13.10 11.24 6.12 15.48 13.24 15.50Fe2O3 8.19 5.51 5.91 4.89 3.03 8.29 6.11 6.60MnO 0.06 0.06 0.05 0.04 0.64 0.06 0.18 0.07MgO 3.17 2.18 2.16 1.73 1.10 3.10 2.22 2.75CaO 0.47 0.76 0.61 0.65 18.73 0.80 2.54 1.17Na2O 1.65 1.78 1.79 1.81 1.14 1.64 1.78 1.59K2O 3.22 2.30 2.39 2.07 1.18 2.90 2.44 2.10P2O5 0.11 0.14 0.11 0.11 0.09 0.12 0.18 0.13S 0.25 0.13 0.41 0.53 0.01 0.48 0.04 0.03F 4.70 3.22 3.28 2.87 14.88 4.41 5.21 0.06Total 100.96 101.19 99.84 101.38 103.07 101.62 102.07 101.87ppmBa 693 641 601 412 836 275 624 476Ce 54 94 67 61 69 63 103 97Cl 150 313 235 229 261 119 273 106Co* 68 31 120 40 121 122 48 34Cr * 83 158 104 111 85 140 159 117Cu* 11 34 22 27 20 11 34 42Dy 3 1 6 4 1 2 0 0Er 1 12 3 4 1 9 7 11Gd 5 2 4 5 5 15 4 4Hf 5 3 7 5 6 4 10 3La 32 44 36 33 33 22 48 40Nb 15 22 17 17 15 15 21 21Nd 30 43 35 32 30 30 49 39Ni* 42 101 74 75 71 28 75 83Pb* 16 23 15 13 15 4 17 26Pr 17 42 21 34 24 46 43 39Rb 87 199 113 122 92 63 161 171Sc 6 17 9 10 8 12 13 13Sr 135 162 133 145 128 338 146 135Th 14 26 16 18 13 38 32 26U 3 0 3 1 4 -6 4 2V 63 142 85 87 65 54 126 115Zn* 64 116 85 84 95 36 102 102Zr 227 258 265 229 229 495 538 296* Marked elements have been measured by AAS rest of the elements by XRF# Sample 155 – 162: Patharia well–5

163 164 165 166 167 169 169 170wt%SiO2 61.46 59.39 64.10 60.86 59.19 62.63 62.17 64.57TiO2 0.90 0.98 0.86 0.91 0.99 0.85 0.90 0.87A12O3 17.52 18.48 15.89 17.79 18.55 15.49 16.90 15.52Fe2O3 7.54 8.29 7.03 7.76 8.34 7.05 7.40 7.09MnO 0.10 0.11 0.12 0.10 0.10 0.19 0.09 0.12MgO 2.94 3.21 2.68 2.10 3.21 2.65 2.89 2.84CaO 0.80 0.78 1.31 0.80 0.89 2.08 0.87 1.21Na2O 1.72 1.66 1.73 1.71 1.63 1.73 1.71 1.77K2O 3.33 3.56 3.01 3.39 3.57 2.95 3.18 2.92P2O5 0.16 0.15 0.17 0.16 0.21 0.18 0.16 0.18S 0.03 0.02 0.02 0.02 0.04 0.03 0.04 0.06F 0.07 0.07 0.06 0.07 0.07 0.07 0.06 0.06LOI* 4.68 5.12 4.67 4.78 4.99 5.03 4.53 4.43Total 101.43 102.04 101.87 101.52 101.85 101.11 101.1 101.59ppmBa 523 535 521 605 563 493 521 494Ce 85 94 94 98 100 80 94 92Cl 102 184 141 128 139 94 106 143Co* 20 39 42 36 36 40 40 42Cr * 125 135 117 126 136 117 124 114Cu* 38 44 35 39 53 38 43 28Dy 4 5 2 3 6 1 2 -1Er 2 6 12 11 9 0 6 5Gd 2 4 4 7 4 1 6 16La 44 40 42 43 44 43 38 41Nb 21 23 21 22 24 20 22 21Nd 42 42 44 38 42 40 40 40Ni* 74 78 70 73 91 73 81 85Pb* 27 30 26 26 32 20 20 20Pr 38 43 36 37 38 34 37 34Rb 194 220 174 208 222 164 188 167Sc 16 17 13 17 15 13 14 13Sr 148 156 161 156 158 158 169 153Th 26 29 27 29 29 23 25 24U 3 2 2 3 5 4 2 3V 130 142 115 134 146 113 126 114ZN* 93 97 97 93 121 107 112 123Zr 230 231 276 230 247 245 244 251* Marked elements have been measured by AAS rest of the elements by XRF# Sample 163 – 170: Patharia well – 5

171 172 173 174 175 177 178 180wt%SiO2 59.45 61.51 65.62 66.75 72.38 65.46 72.62 59.28TiO2 0.94 0.94 0.72 0.89 0.67 0.85 0.71 0.92A12O3 18.21 17.42 13.06 14.92 12.13 15.81 11.51 18.35Fe2O3 7.47 7.36 5.43 6.22 5.13 6.63 4.71 7.76MnO 0.07 0.08 0.27 0.07 0.06 0.07 0.05 0.08MgO 3.16 3.10 2.18 2.59 2.08 2.79 1.94 3.35CaO 0.89 1.01 4.32 0.98 0.92 0.84 1.72 0.81Na2O 1.63 1.74 1.72 1.79 1.85 1.81 1.87 1.64K2O 3.58 3.44 2.45 2.80 2.15 3.08 2.06 3.14P2O5 0.24 0.12 0.12 0.13 0.12 0.13 0.13 0.12S 0.07 0.06 0.06 0.07 0.06 0.05 0.08 0.10F 0.08 0.06 0.06 0.06 0.06 0.05 0.08 0.07LOI* 4.63 5.83 1.79 3.80 2.83 3.77 3.36 1.79Total 00.65 103.08 98.00 101.52 100.69 101.58 101.30 98.81ppmBa 647 2091 591 713 610 695 3026 2688Ce 104 80 71 95 65 80 48 68Cl 119 402 114 222 185 210 311 447Co* 36 29 59 75 52 96 55 45Cr * 142 136 97 127 91 127 89 139Cu* 43 34 23 32 21 31 19 46Dy 2 1 2 7 0 7 1 -1Gd 17 4 8 2 6 0 11 3Hf 4 8 7 6 7 5 2 2La 48 43 34 39 34 39 35 46Nb 22 22 18 20 17 18 15 22Nd 48 30 38 42 33 35 23 27Ni* 86 76 59 72 55 68 46 88Pb* 19 23 17 22 15 23 11 29Pr 48 30 33 35 27 31 5 18Rb 209 200 139 153 122 171 99 220Sc 17 16 12 11 8 11 9 18Sr 165 181 239 152 127 150 181 184Th 27 26 23 24 14 21 16 24U 3 3 1 3 2 1 3 2V 139 131 92 113 77 114 75 140Zn* 118 94 90 81 70 88 67 104Zr 254 290 278 347 248 279 296 245* Marked elements have been measured by AAS rest of the elements by XRF# Sample 171 – 180: Patharia well – 5

181 183 184 185 186 187 188wt%SiO2 66.27 60.81 62.17 68.44 63.55 58.73 64.64TiO2 0.84 0.91 0.88 0.77 0.88 0.95 0.82A12O3 15.21 17.90 17.18 13.71 16.44 18.51 15.56Fe2O3 6.67 7.50 7.22 5.77 7.08 8.46 6.72MnO 0.06 0.07 0.07 0.06 0.06 0.07 0.07MgO 2.78 3.19 3.06 2.37 2.98 3.34 2.82CaO 0.86 0.75 0.79 0.91 0.86 0.64 0.99Na2O 1.82 1.68 1.11 1.78 1.74 1.65 1.77K2O 2.92 3.56 3.39 2.66 3.24 3.68 3.01P2O5 0.28 0.12 0.12 0.12 0.13 0.12 0.14S 0.03 0.07 0.06 0.06 0.06 0.10 0.06F 0.05 0.07 0.08 0.04 0.07 0.06 0.07LOI* - 4.70 4.48 3.43 4.12 4.60 4.03Total 97.86 101.60 101.44 100.38 101.46 101.17 101.37ppmBa 647 987 731 1068 747 681 747Ce 83 91 91 78 84 112 84Cl 240 196 157 269 221 360 221Co* - 37 40 54 33 31 26Cr * 121 137 132 108 126 145 126Cu* - 39 37 26 34 46 33Dy 7 7 3 6 7 -0 7Er 10 9 8 12 6 7 6Gd 5 -1 -5 4 4 -0 4Hf 8 4 4 5 4 1 4La 43 44 48 37 45 59 45Nb 20 22 21 18 21 22 21Nd 38 40 40 31 36 44 36Ni* - 84 79 58 72 79 68Pb* - 21 24 20 21 19 23Pr 28 30 42 26 27 44 27Rb 161 211 198 144 186 226 186Sc 13 15 15 10 15 18 15Sr 1143 160 154 146 151 160 151Th 222 27 28 21 24 27 24U 3 2 2 3 5 3 5V 114 135 133 100 125 158 125Zn* - 91 89 73 90 102 93Zr 199 249 257 284 284 1259 1284* Marked elements have been measured by AAS rest of the elements by XRF# Sample 181 – 188: Patharia well – 5

stratigraphic evolution and geochemistry of the neogene surma group, surma basin...

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