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THE BHUTAN HIMALAYA : A GEOLOGICAL ACCOUNT GEOLOGICAL SURVEY OF INDIA SPECIAL PUBLICATION 39 Edited by 0. N. BHARGA * -I'CiSI -. -2 -0- 0- - . 1000- 1995 (DSK I1 ) THE BHUTAN HIMALAYA : A GEOLOGICAL ACCOUNT SPECIAL PUBLICATION 39 GEOLOGICAL SURVEY OF INDIA Sit. w. w-h TT4rm: Edited by : 0. N. BHARGAVA O r n a ~ ~ ~ W T K & f ; 9 ~ ~ ~ ~ PUBLISHED BY ORDER OF THE GOVERNMENT OF INDIA & THE ROYAL COVT. OF BHUTAN 1995 O INDIA, GEOLOGICAL SURVEY (1995) Published by Director General Geological Survey of India 27, Jawaharlal Nehru Road Calcutta-700 01 6 Processing for Printing S. Sanyal Samiran Banerjee under the guidance of Director Publication and Information Division Geological Survey of India 29, Jawaharlal Nehru Road Calcutta-700 016 Printed at Nabarnudran Private Limited 107A. Acharya Prafulla Chandra Road Calcutta-700 004 India I I PRICE : Rs. 660.00 f 77.00 US $238.00 PREFACE This Special Publication presents a synoptic and critical review o/ the existing geoscientific knowledge ofthe Bhutan Himalaya. The section of Tectonostratigraphy brings out the regional description (?/allt he formations exposed in Bhutan. An interesting feature of this contribution is the detailed description of Quaternary deposits of Bhutan. Though no detailed work was carried out during last few years in the Siwalik, Damuda, Setikhola and Diuri segments, the information contained in these chapters are based on previous unpublished work, with marginal additions. On the other hand, the Baxa, the Shumar, the Thimpu and the Tethyan chapters present new data. The present work necessitated defining af ew newf ormations, viz. the Jaishidanda, the Singhi, the Deshichiling and the Quartzite. Thef irst occurs as a tectonic scale below the Thimphu thrust sheet and the latter three form parts of the Tethyan succession developed over the Thimphu crystallines. The chapter on the Granitic Roch provides new petrochemical and geochronological data, which are very useful in regional correlation. The Synthesis chapter presents the stratigraphic column, attempts correlation of various formations of the Bhutan Himalaya with the better knownformations of the other parts of the Himalaya and provides a broad structural and metamorphic set-up. The chapter on Geological History presents an evolutionary model. In such an attempt certain generalisations become unavoidable. Chapter on Mineral Resources briefly gives thegrade and reserves of important mineral deposits of Bhutan. It is expected that this write-up willfill the voids in the knowledge of the eastern Himalaya. It is hoped that the volume will provide useful geoscientific information to the future workers. (D. B. Dimri) Director General Geological Survey of India ACKNOWLEDGEMENTS The idea of compiling the geology of A. Mondal in Chemical Laboratories, Cenual Bhutan was conceived by B.D. Dungrakoti. Headquarters and Eastern Region were kind However, before it could materialise, he was enough to analyse various samples on priority. transferred and the mantle fell on me. At that time several contradictions and fallacies existed in Colleagues in the Royal Government of the geology of Bhutan. There were three different Bhutan gave ample and unstinted support to the geological maps, published by (i) Geological Geological Survey of India field parties and Survey of India, (ii) Prof. A. Gansser and (iii) arranged for various facilities. A special mention ESCAP (UN). Besides many areas lacked cogent may be made of Do ji Wangda and m a yD oji, map pattern. the latter was also associated with the mapping in Ha district. Dr. T.C.A. Raghavan, the then Fit Thus mapping and detailed traverses were Secretary, Indian Embassy, Thimphu evinced organised in all the crucial sections, the result of keen interest in the present work. which is the present new and revised version of the Bhutan Geology. This revision with field First draft of the manuscript was typed by work restricted to critical sections during 1993 Dr. J.U. Rao. Fmal typing was ably and and 1994 has been possible only due to sound painstakingly performed by S. Mukhopadhyay. edifice provided by previous workers. whose Chandra Shekhar and Prasanta Chalaaborty drew efforts were ably guided by S.P. Nautiyal (1960- final drawings and maps. S. Singanenjam and 63), B. Laskar (1963-69), P. Kar (1969-74). A.R. T.K. Pyne meticulously perused the final Gokul (1974-80). C.P. Vohra (1980-85), B.S. manuscript Jangpangi (1985-87) and B.D. Dungrakoti (1988- 1992). This write-up has been possible due to a great sacrifice on part of my wife Malti. Despite Dr. D.K. Paul and Dr. M. Ram&shnan her serious illness at Chandigarh, she kept on took keen interest in this write-up, the former encouraging and inspiring me to stay back at arranged for quick geochemical and isotopic Samtse and complete this book. studies. Discussions with Dr. S.K. Acharyya. K.K. Ray, SiWas Ghosh and Dr. S.K. Ray were This book is published with the kind always educative, refreshing and inspiring. permission of the Royal Government of Bhutan and the Director General, Geological Survey of B.P. Rishi in NAA Lab, Pune and I.B. India Ghosh, S. Singh, A.K. Sanyal, S.K. Bhaduri and THE BHUTAN HIMALAYA : A GEOLOGICAL ACCOUNT CONTENTS PREFACE ACKNOWLEDGEMENTS 1. INTRODUCTION 0. N. Bhargava - 2. GEOLOGY OF BHUTAN A SYNOPTIC VIEW 0.N . Bhargava 3. TECTONOSTRATIGRAPHY 3.1 Quaternary Sediments T. Laskar 3.2 Siwalik Group G. Lakshrninarayana and Bhagwan Singh 3.3 Damuda Subgroup G. Lakshrninarayana 3.4 Setikhola Formation Ashutosh Joshi 3.5 Baxa Group S. K. Tangri 3.6 Diuri Formation S. K. Tangri 3.7 Shurnar Formation S. Dasgupta 3.8 Jaishidanda Formation S. Dasgupta 3.9 Thirnphu Group P. R. Golani 3.10 Tethyan Sequence S. K. Tangri and A. C. Pande 4. THE GRANITIC ROCKS A. Sarkar and S. Dasgupta 5. SYNTHESIS 5.1 Stratigraphy 0. N. Bhargava 5.2 Correlation 0.N . Bhargava 5.3 Structure and Metamorphism 0. N. Bhargava and S. Dasgupta 5.4 Geological History 0. N. Bhargava - 6. MINERAL RESOURCES A COMPILATION S. A. Chore Plate I : Geological Map of the Bhutan Himalaya (in pouch).