The Geology and Tectonics of the Oman Region, The Geological Society UK

17
The Geology and Tectonics of the Oman Region

description

The Geology and Tectonics of the Oman Region, The Geological Society UK; Explains Oman Ophiolite geology and mountain formation

Transcript of The Geology and Tectonics of the Oman Region, The Geological Society UK

Page 1: The Geology and Tectonics of the Oman Region, The Geological Society UK

The Geology and Tectonics

of the Oman Region

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Geological Society Special Publications Series Editor K. COE

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G E O L O G I C A L S O C I E T Y S P E C I A L P U B L I C A T I O N N O 49

The Geology and Tectonics of the Oman Region

EDITED BY

A. H. F. R O B E R T S O N Department of Geology and Geophysics

Grant Institute University of Edinburgh, UK

M. P. S E A R L E Department of Geology

University of Leicester, UK

A. C. R I E S Earth Sciences and Resources Institute

University of Reading, UK

1990

Published by

The Geological Society

London

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THE GEOLOGICAL SOCIETY

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Published by the Geological Society from: The Geological Society Publishing House Unit 7 Brassmill Enterprise Centre Brassmill Lane Bath Avon BA1 3JN UK (Orders: Tel. 0225 445046)

First published 1990 Reprinted 1992

�9 The Geological Society 1990. All rights reserved. No reproduction, copy or transmission of this publication may be made without written permission. No paragraph of this publication may be reproduced, copied or transmitted save with the written permission or in accordance with the provisions of the Copyright Act 1956 (as amended) or under the terms of any licence permitting limited copying issued by the Copyright Licensing Agency, 33-34 Alfred Place, London WC1E 7DP. Users registered with Copyright Clearance Center: this publication is registered with CCC, 27 Congress St., Salem, MA 01970, USA. 0305-8719/90 $03.00.

British Library Cataloguing in Publication Data Robertson, Alastair, 1949-

The geology and tectonics of the Oman region. 1. Oman. Geological features. Geological I. Title lI. Searle, Michael P. Ill. Ries, Alison C. llIl. Series 555.3'53

ISBN 0 - 9 0 3 3 1 7 - 4 6 - X

Printed in Great Britain at The Alden Press, Oxford

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Contents

Acknowledgements ix

Preface x

ROBERTSON, A. H. F., SEARLE, M. P. & RIES, A. C. Introduction xi

Evolution of the Oman Tethyan Continental Margin

ROBERTSON, A. H. F. • SEARLE, M. P. The northern Oman Tethyan continental margin: stratigraphy, structure, concepts and controversies 3

BLEND1NGER, W., VAN VL1ET, A. & HUGHES CLARKE, M. W. Updoming, rifting and con- tinental margin development during the Late Palaeozoic in northern Oman 27

LEE, C. W. A review of platform sedimentation in the Early and Late Permian of Oman, with particular reference to the Oman Mountains 39

RABU, D., LE METOUR, J., BECHENNEC, F., BEURRIER, M., V1LLEY, M. & BOURDILLON-DE- GRISSAC, CH. Sedimentary aspects of the Co-Alp ine cycle on the northeast edge of the Arabian Platform (Oman Mountains) 49

PRATT, B. R. & SMEWlNG, J. D. Jurassic and Early Cretaceous platform margin configur- ation and evolution, central Oman Mountains 69

SCOTT, R. W. Chronostratigraphy of the Cretaceous carbonate shelf, southeastern Arabia 89

HAAN, E. A., CORB1N, S. G., HUGHES CLARKE, M. W. & MABILLARD, J. E. The Lower Kahmah Group of Oman: the carbonate fill of a marginal shelf basin 109

WAGNER, P. D. Geochemical stratigraphy and porosity controls in Cretaceous carbonates near the Oman Mountains 127

WATTS, K. F. Mesozoic carbonate slope facies marking the Arabian platform margin in Oman: depositional history, morphology and palaeogeography 139

COOPER, D. J. W. Sedimentary evolution and palaeogeographical reconstruction of the Mesozoic continental rise in Oman: evidence from the Hamrat Duru Group 161

BERNOULLI, D., WE1SSERT, H. & BLOME, C. D. Evolution of the Triassic Hawasina Basin, Central Oman Mountains 189

TOZER, E. T. & CALON, T. J. Triassic ammonoids from Jabal Safra and Wadi Alwa, Oman, and their significance 203

BECHENNEC, F., LE METOUR, J., RABU, D., BOURDILLON-DE-GR1SSAC, CH., DE WEVER, P., BEURRIER, M. & V1LLEY, M. The Hawasina Nappes: stratigraphy, palaeogeography and structural evolution of a fragment of the south-Tethyan passive continental margin 213

DE WEVER, P., BOURDILLON-DE-GRISSAC, CH. & BECHENNEC, F. Permian to Cretaceous radiolarian biostratigraphic data from the Hawasina Complex, Oman Mountains 225

K1CKMAIER, W. & PETERS, Tj. Manganese occurrences in the A1 Hammah Range -- Wahrah Formation, Oman Mountains 239

ROBERTSON, A. H. F., BLOME, C. D., COOPER, D. W. J., KEMP, A. E. S., & SEARLE, M. P. Evolution of the Arabian continental margin in the Dibba Zone, Northern Oman Mountains 251

ROBERTSON, A. H. F., KEMP, A. E. S., REX, D. C. & BLOME, C. D. Sedimentary and structural evolution of a continental margin transform lineament: the Hatta Zone, Northern Oman Mountains 285

MANN, A. & HANNA, S. S. The tectonic evolution of pre-Permian rocks, Central and Southeastern Oman Mountains 307

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vi CONTENTS

LE M~TOUR, J., RABU, D., TEGYEY, M., BECHENNEC, F., BEURRIER, M. & VILLEY, M. Subduction and obduction: two stages in the Eo-Alp ine tectonometamorphic evolution of the Oman Mountains 327

HANNA, S. S. The Alpine deformation of the Central Oman Mountains 341

SEARLE, M. P., COOPER, D. W. J. & WATTS, K. F. Structure of the Jebel Sumeini-Jebel Ghawil area, Northern Oman 361

DUNNE, L. A., MANOOGIAN, P. R. ~: PIERINI, D. F. Structural style and domains of the Northern Oman Mountains (Oman and United Arab Emirates) 375

MICHAELlS, P. L. & PAUKEN, R. J. Seismic interpretation of the structure and stratigraphy of the Strait of Hormuz 387

BoovE, D. R. D., Mou, D. & WAITE, R. I. Structural evolution of the Suneinah Foreland, Central Oman Mountains 397

WARBURTON, J., BURNHILL, T. J., GRAHAM, R. H. & ISAAC, K. P. The evolution of the Oman Mountains Foreland Basin 419

CAWOOD, P. A., GREEN, F. K. & CALON, T. J. Origin of culminations within the Southeast Oman Mountains at Jebel Ma Jhool and Ibra Dome 429

COFFIELD, D. Q. Structures associated with nappe emplacement and culmination collapse in the Central Oman Mountains 447

SUELTON, A. W. The interpretation of gravity data in Oman: constraints on the ophiolite emplacement mechanism 459

EL-SHAZLY, A. K. & COLEMAN, R. G. Metamorphism in the Oman Mountains in relation to the Semail ophiolite emplacement 473

NOLAN, S. C., SKELTON, P. W., CLISSOLD, B. P. & SMEWING, J. D. Maastrichtian to early Tertiary stratigraphy and palaeogeography of the Central and Northern Oman Mountains 495

SKELTON, P. W., NOLAN, S. C. & SCOTT, R. W. The Maastrichtian transgression onto the northwestern flank of the Proto-Oman Mountains: sequences of rudist-bearing beach to open shelf facies 521

MANN, A., HANNA, S. S. ~: NOLAN, S. C. The post-Campanian tectonic evolution of the Central Oman Mountains: Tertiary extension of the Eastern Arabian Margin 549

MAIZELS, J. & MCBEAN, C. Cenozoic alluvial fan systems of interior Oman: palaeoenviron- mental reconstruction based on discrimination of palaeochannels using remotely sensed data 565

Geology and Tectonics of South Oman

GASS, I. G., RIES, A. C., SHACKLETON, R. M. • SMEWING, J. D. Tectonics, geochronology and geochemistry of the Precambrian rocks of Oman 585

WRIGHT, V. P., RIES, A. C. & MUNN, S. G. lntraplatformal basin-fill deposits from the Infracambrian Huqf Group, east Central Oman 601

MATTES, B. W. & CONWAY MORRIS, S. Carbonate/evaporite deposition in the Late Pre- cambian-Early Cambrian Ara Formation of Southern Oman 617

HEWARD, A. P. Salt removal and sedimentation in Southern Oman 637

RIES, A. C. & SHACKLETON, R. M. Structures in the Huqf-Haushi Uplift, east Central Oman 653

MOSELEY, F. The structure of Masirah Island, Oman 665

SHACKLETON, R. M., RIES, A. C., BIRD, P. R., FILBRANDT, J. B., LEE, C. W., & CUNNINGHAM, G. C. The Batain Melange of NE Oman 673

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CONTENTS vii

FILBRANDT, J. B., NOLAN, S. C. d~; RIES, A. C. Late Cretaceous and early Tertiary evolution of Jebel Ja' alan and adjacent areas, NE Oman 697

SHACKLETON, R. M. & RIES, A. C. Tectonics of the Masirah Fault Zone and eastern Oman 715

MOUNTAIN, G. S. & PRELL, W. L. A multiphase plate tectonic history of the southeast continental margin of Oman 725

PRELL, W. L. and the shipboard scientific party of ODP Leg 117. Neogene tectonics and sedimentation of the SE Oman continental margin: results from ODP Leg 117 745

SHIMMIELD, G. B., PRICE, N. B. & PEDERSON, T. F. The influence of hydrography, bathymetry and productivity on sediment type and composition on the Oman Margin and in the Northwest Arabian Sea. 759

Regional Tectonic Setting GLENN1E, K. W., HUGHES CLARKE, M. W., BOEUF, M. G. A., PILAAR,W. F. H. & REINHARDT, B. M. Inter-relationship of the Makran-Oman Mountains belts of convergence 773

STONELEY, R. The Arabian continental margin in Iran during the Late Cretaceous 787

SENGOR, A. M. C. A new model for the late Palaeozoic-Mesozoic tectonic evolution of lran and implications for Oman 797

Index 835

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Acknowledgements

This volume could not have been completed without the help of a number of individuals and institutions. We are indebted to the Ministry of Petroleum and Minerals, Sultanate of Oman and the Governments of the United Arab Emirates for their support. Twenty- six of the papers in this volume derive from work funded by Amoco Oman Petroleum Company, through an operating grant awarded to Professor W. H. Kanes on behalf of the Earth Sciences and Resources Institute based at the University of South Carolina. Those geologists involved sincerely thank AMOCO and ESRI for giving them the opportunity to work in the Oman region; they are also particularly grateful to Dr J. D. Smewing of the Earth Resources Institute, Swansea for his invaluable scientific support and assistance with logistics.

The International Discussion Meeting was held at the Royal Society of Edinburgh from March 29-31st, 1988 and was sponsored by the Geological Society of London and Amoco Production Company (International), with additional financial support from Petroleum Development, Oman; British Petroleum, London; the Earth Sciences and Resources Institute, Reading and the Earth Resources Institute, Swansea.

The conveners of the International Discussion Meeting gratefully acknowledge assist- ance from the City of Edinburgh's Director of Finance and his staff, and the District of Council's Department of Public Relations and Tourism. We also thank the Royal Bank of Scotland for providing conference materials. The Lord Provost of the City of Edin- burgh and the Principal of the University of Edinburgh kindly provided hospitality for the conference participants.

A. H. F. Robertson thanks the Head of the Department of Geology and Geophysics, Professor G. S. Boulton, for facilities to arrange the conference and then edit the volume; and also sincerely thanks Mrs Marcia Wright for her invaluable secretarial assistance with organizing the conference. Dr J. E. Dixon is thanked for producing the design which appears on the front cover of this volume.

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Preface

Recent years have seen an extraordinary upsurge of international interest in the Geology and Tectonics of the Oman Region. A notable innovation is extensive collaborative work involving a large number of academic and oil company geologists from different countries. Indeed, a substantial part of the recent work has been sponsored by Amoco Oman Petroleum Company, with the support of the Oman Government's Ministry of Petroleum and Minerals. Publication of this volume demonstrates the effectiveness of collaboration between universities, government institutions and oil companies and shows how this can lead to presentation and publication of much new data on geologically and economically important areas. The Shell-supported Oman Memoir (Glennie et al. 1974) laid the foundations for all future studies of the Oman Mountain area. Publication of this sequel will reinforce interest in this now classic area.

Many of the studies reported here focus on the sedimentary and structural evolution of the Late Palaeozoic-Mesozoic-Tert iary continental margin and its relevance to exploit- ing the area's natural resources, notably the hydrocarbon fields of the central and northern Oman Mountain front and also in South Oman.

The Oman Mountain area is the world's best exposed deformed passive margin and as such is an ideal area to study fundamental geological processes, including rifting, passive margin development, thrusting, foreland basin genesis with the integration of surface geology and subsurface seismic and well data.

The approach adopted here is multidisciplinary and includes aspects of sedimentation, stratigraphy, palaeontology, structural geology, geophysics, geochemistry and marine geology. The area covered is the Oman Mountains, Southern Oman, offshore areas, and the wider tectonic context of Iran and the Indian ocean.

The volume follows an International Discussion Meeting held at the Royal Society of Edinburgh, from March 29-31, 1988, convened by A. H. F. Robertson, M. P. Searle, J. D. Smewing and A. C. Ries.

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Introduction

A. H. F. R O B E R T S O N , M.

The Oman Mountains which occupy the south- eastern corner of the Arabian Peninsula form an arcuate chain, approximately 700 km long and up to 120 km wide, stretching from the Arabian Gulf and Straits of Hormuz in the northwest to the Arabian Sea in the southeast. The Range forms the southeastern margin of the Arabian continental plate, which is bounded by the Red Sea spreading centre in the west, the dominantly transform fault-bounded Arabian Sea margin along the southeast coast and the major Tethyan suture zones along the northern and eastern margins, exposed in Iran, Pakistan and Turkey. The Gulf of Oman is floored by Cretaceous oceanic crust which is currently sub- ducting northwards beneath the Makran active continental margin (White & Ross 1979). Thus the Oman Mountains define a continent-ocean collision boundary of dominantly Late Cretace- ous age, but affected by further compression and uplift during the Tertiary.

The Oman Mountains are well known for the world's largest intact and best exposed obducted ophiolite complex, the Semail ophiolite. Under- lying the ophiolite is a complicated assemblage of thrust sheets of proximal to distal deep-sea sediments, volcanic and melange units, termed the Hawasina Complex and the Haybi Com- plex. These thrust units, are in turn structurally underlain by carbonate slope deposits (Sumeini Group), which have been thrust onto the Ara- bian platform. The shelf carbonate successions are part of the giant Middle Eastern hydro- carbon province and the Oman foreland has been subjected to extensive subsurface seismic exploration and drilling operations. The Oman Mountains are also exceptionally well exposed throughout and form one of the most important field geological laboratories.

History of research

The earliest geological exploration was made during the reconnaissance visits of Blanford (1872), Pilgrim (1908) and Lees (1928). In the late 1950s geologists of the Iraq Petroleum Company were active in the field particularly in the northern mountain area. The first major regional surveys began in the 1960s by geologists of Shell Petroleum Development (Oman) Ltd. (Wilson 1969; Glennie et al. 1973, 1974).

The map (scale 1:500 000) and memoir pro- duced by the Shell team (Glennie et al. 1974) has become the classic study of the Oman Moun-

P. S E A R L E & A. C. R I E S

tains and formed the basis for all subsequent research. The basic conclusion that the Oman Mountains preserves a tectonically emplaced Late Palaeozoic and Mesozoic continental mar- gin and Tethyan basin sequence, together with a huge slab of Cretaceous oceanic crust and mantle has never since been seriously disputed.

Subsequent research during the 1970s and early 1980s concentrated on more detailed map- ping, with emphasis on the Semail ophiolite. The work of a British-funded group, the Open University Oman Ophiolite Project (U.K. 1975-1986) resulted in publication of four maps (scale 1:100000) of the northern part of the Oman territory, eight PhD theses and a sub- stantial number of journal articles, the results being summarised in a Memoir of the Geo- logical Society (Lippard et al. 1986).

During the earlier part of this period an American-funded group from the United States Geological Survey and the University of California mapped a strip across the south- eastern Oman Mountains from Muscat south- wards. A detailed map was produced (scale: 1:100 000) and the results, mainly detailing the Semail ophiolite, were published in a special issue of the Journal o f Geophysical Research (86, B4, 1981).

In 1981 Amoco Petroleum Company (Inter- national) acquired the concession for much of the Oman Mountain area and during the sub- sequent six years obtained an improved seismic coverage of the foreland area adjacent to the Oman Mountains. They also funded a major field-based research programme through the Earth Sciences and Resources Institute. Twenty-six papers in this volume are based on the Amoco-funded work.

During 1982-1984 geologists of the French Bureau de Rrcherches Grologiques et Minirres (BRGM) mapped a large area of the central and southern mountains, resulting in publication of 13 maps at 1:100000 scale, several theses and related journal articles. French University Groups simultaneously focussed on the Semail ophiolite. The results of the various projects were published in a special volume of Tectono- physics 51, 1988).

Evolution of the Oman passive margin

Key issues addressed in this volume are the rift, drift and emplacement history of the Oman Tethyan continental margin. How and when did

xi

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xii INTRODUCTION

5'6 ~

A R A B I A N GULF

o

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INTRODUCTION XUl

rifting take place? When did ocean floor spread- ing begin? What was the nature of the continen- tal boundary? How did the platform and slope develop through time, compared with in situ passive margins (e.g. Atlantic). What was the deep-sea basin like? How did the huge mountain-sized volcanic and limestone exotics form? How did Late Cretaceous thrusting affect the margin? How was platform deposition re- established after thrust emplacement? What was the nature of Tertiary deformation -- tensional or compressive?

In an introductory paper Robertson & Searle review alternative stratigraphic and structural schemes proposed for the Oman Mountains. They propose a compromise stratigraphical nomenclature that as far as possible follows currently accepted international convention. The authors then discuss alternative reconstruc- tions of the Oman passive margin and summar- ize the structural and emplacement history.

The following papers cover a number of topics, as follows.

( i ) R i f t i n g a n d p a s s i v e m a r g i n e v o l u t i o n

Blendinger et al. discuss sub-surface evidence for the timing and mechanism of rifting. Many

Fig. 1. Outline tectonic map of Oman showing the locations of the areas studied. The numbers are in the order in which they occur in the book, as follows: 1, Robertson & Searle (whole area); 2, Blendiger et al. (box); 3, Lee (box); 4, Rabu et al. (whole of exposed platform); 5, Pratt & Smewing (mainly in large box); 6, Scott (whole of Cretaceous shelf); 7, Haan et al. (box); 8, Wagner (whole shelf); 9, Watts (whole area of platform slope units); 10, Cooper (whole area of basinal units); 11, Bernoulli et al. (S part of large box); 12, Tozer & Calon (box); 13, Bechennec et al. (large box); 14, De Wever et al. (large box); 15, Kickmaier & Peters (box); 16, Robertson et al. (a) (box); 17, Robertson et al. (b) (box); 18, Mann & Hanna (box); 19, Le Metour et al. (large box); 20, Hanna (large box); 21, Searle et al. (box); 22, Dunne et al. (box); 23, Michaelis & Pauken (Hormuz area); 24, Boote (box); 25, Warburton et al. (box); 26, Cawood et al. (box); 27, Coffield (box); 28, Shelton (Batinah plain area); 29, EI-Shazly & Coleman (large box, mainly E part); 30, Nolan et al. (area of Tertiary platform); 31, Skelton et al. (circum N mountain area); 32, Mann et al. (whole of central mountain flanks); 33, Maizels & McBean (large box); 34, Gass et al. (box); 35, Wright et al. (box); 36, Mattes & Conway Morris (box); 37, Heward (box); 38, Pies & Shackleton (box); 39, Moseley (box); 40, Shackleton et al. (box); 41, Filbrandt et al. (box); 42, Shackleton & Pies (box); 43, Mountain & Prell SE continental margin of Oman; 44, Prell (box); 45, Shimmield & Price (box); 46, Glennie et al. (whole of Oman- Makran area); 47, Stoneley (S Iran); 48, Sengrr (whole of Iran and Oman).

authors favour Middle to Late Triassic con- tinental break-up (e.g. Lippard et al. 1986), but Blendinger et al. suggest seafloor spreading was underway by early Late Permian. They favour 'a low-angle normal faulting' model rather than the 'classical (thermal) rift' model. The authors also report new fossil and lithology data for the Permian of the southeastern mountains, and argue it was part of the adjacent Saih Hatat carbonate platform, not a distal Oman Exotic (cf. Bechennec et a l . ) .

The Permian of the Oman Mountains is criti- cal to the debate on rift history. Lee summarizes the Early and Late Permian platform suc- cessions throughout Oman, with emphasis on South Oman. Early Permian lithofacies reflect the Dwyka glaciation of Gondwana, while Late Permian times saw carbonate platform devel- opment along the margins of the developing Tethyan basin. Blendinger et al. argu~ for an asymmetrical simple shear rift model in contrast with Glennie et a l . ' s (1974) classical symmetri- cal rift. Late Permian deep-water sedimentary and volcanic rocks have been confirmed in the Hawasina Complex, as summarized by Bechen- nec et al. and Blendinger et al.

Rabu et al. synthesize the Permian to Late Cretaceous evolution of the platform suc- cessions. They recognize five main depositional cycles. Important controls of platform depo- sition include Late Permian, and Late Jurassic (Late Tithonian) crustal extension, and Late Cretaceous foreland basin development (Muti Formation). COntrasting stable and unstable shelf areas can be recognized in the Saih Hatat and Jebel Akhdar. Pratt & Sinewing present new data on the sedimentology and palaeo- environments of the Mesozoic platform suc- cessions. They reconstruct a stepped platform edge and argue that lateral structures, princi- pally the Semail gap, exerted a strong influence on Mesozoic carbonate platform sedimentation.

Accurate chronostratigraphic correlation is clearly essential. Scott utilizes a quantitative correlation technique that relates taxa in indi- vidual successions to a composite standard and to absolute age. Using this method he deter- mines durations of specific depositional cycles and erosive events in relation to regional tec- tonics and inferred eustatic sealevel changes.

The evolution of the carbonate platform is also considered by ltaan et al. who discuss the Early Cretaceous Kahmah Group (Rayda, Salil and Habshan Fms.). Information from surface exposures and seismic data provide an inte- grated picture of facies geometry and distri- bution. Seismic clinoforms suggest a period of highly diachronous deposition, possibly con- trolled by salt movement in the basement.

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xiv INTRODUCTION

A major goal of industry studies of the plat- form successions has been to define the nature and extent of reservoir carbonates. Based on a five-well transect, Wagner shows that isotope stratigraphy (613 C, 618 O) and elemental com- position (Sr) of carbonates can be used to recognize and correlate diagenetic differences and topographic relationships. Semi-enclosed marine-like conditions with local anoxic events are implied for Early-Middle Cretaceous car- bonates from wells in central Oman. Remaining porosity was destroyed below the thrust sheets, but excellent porosity survived beyond the thrust front. Lithofacies is apparently not im- portant in defining porosity distribution.

A priority of recent research has been to determine the depositional environment, and distribution of slope and basinal lithofacies in the Oman Mountains. Watts reconstructs the platform edge and slope geometry through time (Sumeini Group). Facies models are important since modern slopes are still imperfectly known. The main controls of slope development were Permian?, Triassic and Jurassic rift events and tectonic oversteepening prior to Late Cre- taceous thrust emplacement, modified by eu- static sealevel change and several other factors.

Cooper shows that two contrasting types of succession are present regionally: the shale-rich 'A1 Ayn sub-basin' and the more carbonate-rich 'Duru sub-basin'. The shale-rich facies are correlated with base-of-slope carbonate con- glomerates in the Hawasina window and are therefore relatively proximal. Palaeocurrent data show that the main source of Jurassic oolitic calciturbidites (Guweyza Limestone Formation) was to the southeast, not the ad- jacent platform as might be expected. In ad- dition, sediment distribution was apparently influenced by inferred southeasterly trade winds.

Bernoulli et al. recognize four units in the Late Triassic deep-water successions (Zulla Fm.). Detrital components reflect changing margin palaeoenvironments, whereas pelagic sedimentation shows basin-wide oceanographic influences. Chief amongst these are periodic low-oxygen conditions, resulting in laminated dolomite and increased radiolarian abundance, possibly related to changes in circulation and surface-water productivity. The Oman Triassic deep-water sediments have to be seen as part of an elongate South Tethyan seaway stretching as far as the central Mediterranean area.

Also, concerning the deep-water Hawasina Complex, Tozer & Calon discuss the detailed biostratigraphy and significance of Triassic am- monoids from detached blocks of condensed

pelagic limestones. These are similar to the Ammonitico Rosso and the Hallstatt facies else- where in the Tethyan region, where they often signify condensed deposition on seamounts.

Based on the BRGM mapping of the central and southern mountains, Bechennee et al. sum- marize their conception of the Late Permian to Late Cretaceous evolution of the 'Hamrat Duru Basin'. Rifting took place in Late Permian and spreading in Middle to Late Triassic time. The deep-water fades accumulated in a subsided intracontinental basin. A large platform (Baid Platform) was sited northeast of the Arabian platform and then disintegrated into smaller units (Misfah and Baid Horsts) in Middle-Late Triassic time.

In recent years it has become possible rou- tinely to extract well preserved radiolaria from ribbon cherts, and these microfossils have be- come a powerful stratigraphic tool. A prodigious number of radiolarian chert samples was pro- cessed from every major chert horizon in the BRGM mapping area and the main results are reported by De Wever et al. Additional radio- larian results obtained by C. Blome (USGS) from the northern mountains (USGS) are re- ported in Robertson et al. a, b; Cooper; Watts; Searle et al. and Bernoulli et al. Workers on radiolaria do not yet agree on the age ranges of all taxa, and thus it is important to document species lists in full.

Further insight into pelagic depositional pro- cesses come from sedimentary geochemistry. Kickmaier & Peters discuss the field relations and structural setting of manganiferous deposits in deep-water distal successions (Wahrah For- mation) of Late Jurassic-Early Cretaceous age. Stratiform manganese enrichment is attributed to both sedimentary and tectonic processes. High Mn/Fe ratios and low trace metal abun- dances favour an origin by settling of widely dispersed hydrothermal precipitates. In contrast to other authors, Kickmaier & Peters apparently envisage ocean floor spreading in the Oman area only in the Late Cretaceous (Semail ophiolite) and suggest the Owen Fracture Zone as a possible source area for the manganese.

(ii) I n t egra t ed s tudies

The development of the Oman continental mar- gin can only be fully understood when the passive margin and the emplacement histories are considered together. Structure has to be properly unravelled if the margin geometry is to be correctly restored and palaeogeography has clearly exerted an important control on thrusting.

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INTRODUCTION xv

Robertson et al. a discuss the Dibba Zone, northern mountains. They summarize the plat- form and slope deposition and present new data on deep-water sediments (Hamat Duru Group), major Oman Exotics (Jebel Qamar), melange (Kub Melange) and Late Cretaceous volcanic rocks. The Dibba Zone is interpreted as an oblique rifted (transtensional) margin segment that became largely inactive after continental break-up.

The N W - S E trending Hatta Zone in the northern mountains to the south of the Dibba Zone is interpreted by Robertson et al. b as a right-lateral transform fault, that influenced slope and basinal sedimentation, volcanism and thrust emplacement. The structure is dominated by a major transverse culmination (Jebel Qui- mah) and a lateral ramp (Jebel Raudha-Masfut ridge). Triassic to Early Cretaceous platform slope and proximal base-of-slope lithofacies pre- dominate. Basement lithologies, not reported elsewhere were transported through northward?- facing channels. Structurally higher, more distal units are dominated by Late Cretaceous basic within-plate, seamount-type volcanic rocks.

(iii) S t ruc ture a n d e m p l a c e m e n t h i s tory

In the cores of the Jebel Akhdar and Saih Hatat large-scale anticlines, pre-Permian basement rocks reveal a deformation fabric believed to have developed either during regional Late Palaeozoic deformation and metamorphism (Mann & Hanna), or are instead (partly or wholly) related to Late Cretaceous ophiolite obduction (Le Metour. et al.). Mann & Hanna focus on major basement-cover duplexes in the Saih Hatat and favour an important phase of Late Palaeozoic deformation and low grade metamorphism. In this area the Mesozoic plat- form succession has clearly been involved in the Late Cretaceous thrusting. Le Metour et al. report several phases of deformation includ- ing NE-directed shearing and blueschist-eclogite facies metamorphism and conclude that these events mainly relate to the Late Cretaceous emplacement.

The Late Cretaceous structural style of the Oman Mountains is discussed by a number of authors, particularly with regard to the sequence of thrusting and the relative age of thrust cul- minations. Hanna argues that in the Jebel Akhdar-Saih Hatat areas all thrust-related struc- tures are Late Cretaceous in age. In contrast, the Tertiary was dominated by extensional tec- tonics, as discussed in detail by Mann et al. for the first time.

In the northern Oman Mountains Searle et al.

show that thrust culmination of slope carbonates in the Jebel Sumeini area was primarily a Late Cretaceous event with Maastrichtian limestones unconformably overlying Cretaceous structures, but that a second phase of shortening involving folding and short-distance thrusting occurred during the Early Tertiary.

Dunne et al. present seven structural sections across the foreland west of the mountain front in the northern Oman Mountains based on ex- tensive seismic data. They show that the shelf carbonates are indeed affected by thrusting simi- lar to that observed in the Musandam moun- tains. The thrusting is a direct consequence of the emplacement of the Semail ophiolite during the Late Cretaceous and subsequent reacti- vation during the Tertiary.

Miehaelis & Pauken show two seismic pro- files across the northward extension of the Musandam Peninsula in the Strait of Hormuz. Their data support earlier suggestions of two phases of compression in this area, one during the Late Cretaceous, the other during Oligocene- Miocene time.

Boote et al. present an evolutionary history of the Sumeinah foreland basin in the central Oman Mountains southwest of the mountain front and northeast of the Lekhwair peripheral bulge. Through seismic and well data they document the Turonian uplift and erosion of the peripheral bulge and the subsequent for- mation and infilling of the Suneinah foredeep caused by loading of the crust by the allochthon. A second foredeep was developed during the Tertiary as a result of compression from the north. Boote et al. interpret the seismic data in terms of regional transpressional deformation, in contrast to the more orthogonai compression favoured by most other authors.

In several contributions there is discussion of the thrusting in specific areas of the foreland fold and thrust belt, particularly involving the Hawasina basinal rocks. Warburton et al. de- scribe and interpret the development of the thrust front and associated foreland basin in the Hamrat ad Duru near the front of the central Oman Mountains. They point out that the flex- ural bulge appears to have remained relative static near the shelf edge, followed by collapse associated with loading during emplacement of the Semail ophiolite.

Cawood et al. present data on the Ibra dome and Jebel Ma-jhool culminations in the south- eastern Oman Mountains. They relate these structures to an important phase of gravity con- trolled re-thrusting, in line with earlier sugges- tions for adjacent areas. Coflield describes high-angle normal faults around the eastern

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xvi INTRODUCTION

Jebel Akhdar and the western Saih Hatat culmi- nations which are related to gravity-driven cul- mination collapse. These faults have a similar origin to surge zones described elsewhere. The age of the structures is debatable, with sedi- mentological evidence of emergence during the Late Cretaceous and reactivation during mid- Tertiary.

Shelton uses potential field data to investigate the location of the Semail ophiolite at depth below the Batinah Plain. He discusses several alternative models and draws comparisons with other passive margins. He concludes that the Semail ophiolite is rootless and that it pinches out to the east near the present coastline. He also discusses possibilities for the configuration of the mountain front. Shelton favours a gravity sliding emplacement mechanism for the Semail ophiolite, similar to that advocated by Glennie et al.

(iv) Tertiary margin d e v e l o p m e n t

The sedimentary and structural development of the beautifully exposed Tertiary successions are documented here in some detail for the first time.

Nolan et al. present a formally defined stra- tigraphy for these successions. Whereas in the past the Tertiary platform was assumed to have been relatively tectonically stable, the authors demonstrate a complex and variable pattern of transgression over the emplaced units, of dif- ferential subsidence, and the accumulation of slope deposits, including fan deltas, debris flows and turbidites within an unstable shelf.

The history of the initial transgression over the northeastern flank of the Late Cretaceous emplaced thrust sheets is documented in detail by Skelton et al. They reconstruct a pattern of rudist-bearing beach deposits that passed later- ally and vertically into open shelf facies that are in turn truncated and overlain by Palaeogene slope deposits. The rudist assemblages are simi- lar to other open southern Tethyan shelves in the Mediterranean and Middle East.

Mann et al. show that the Tertiary successions were later deformed, dominantly by extensional tectonics, with evidence of large-scale faults that step down towards the coast, localized block faulting and roll-over folds. In contrast to the Musandam in the north, no definite evidence of mid-Tertiary compressional structures was found in the central and southern Oman Moun- tains, thus contradicting earlier suggestions of important cross-over folding in this area.

The flanks of the Oman Mountains are mantled by huge volumes of Quaternary alluv-

ium. Their depositional history can best be documented by remote sensing techniques that descriminate on the basis of contrasts in the reflectance and radiance of surface materials. Maizels & MeBean use Landsat and radar imagery and show that palaeochannels can be recognized within a complete alluvial fan sys- tem. At least 20 successive generations of exhumed multistorey palaeodrainage systems are detectable.

One currently enigmatic and controversial aspect is the occurrence of high-pressure/ low-temperature blueschist rocks in the Saih Hatat platform unit. How did these rocks come to be metamorphosed at great depths and then largely exhumed all within Cretaceous time? After briefly summarising the occurrence of the metamorphic sole at the base of the Semail ophiolite, EI-Shazly & Coleman discuss the high-pressure rocks and attribute them to the attempted subduction of the Oman continental margin, or a microcontinent following a change in the plate motion of Africa relative to Eurasia. In contrast to earlier workers, they argue that the deeply buried continental margin rocks followed 'clockwise' P - T paths.

South O m a n

For the purposes of this book, South Oman refers to that area of Oman which lies outside the Oman Mountains and consists of basement to Tertiary sediments that form the foreland to the Oman Mountains Thrust Belt. Outcrop is restricted to NE Oman, between the Batain Coast and Wahiba Sands, the Huqf-Haushi area (onshore from Masirah Bay) and the Mirbat area of southern Oman, in addition to Masirah Island and the Kuria Muria group of islands.

The papers on South Oman begin with a comparative account by Gass et aL of the struc- ture, metamorphism, geochronology and geo- chemistry of the Precambrian basement rocks of Oman, which are only exposed at Jebel Ja'alan and Qalhat in NE Oman, the Kuria Muria Islands and the area E of Mirbat in S Oman. These rocks have a similar age, geo- chemistry and tectonic history to those Pre- cambrian rocks exposed in other parts of the Afro-Arabian Shield and hence the Pan-African domain can now be extended farther to the E.

The oldest non-metamorphic rocks exposed in South Oman are the Infracambrian Huqf Group which is exposed in East Central Oman onshore from Masirah Bay. This area, often known as the Huqf-Haushi area, is a large inlier affording the only exposures of the Palaeo- zoic sequence in S Oman. The Huqf Group

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INTRODUCTION xvii

sequence of carbonates and siliciclastics in the Huqf-Haushi area are interpreted from detailed sedimentological studies by Wright et al. to be intraplatformal basin deposits, infilling a depression on the Arabian-Nubian shield.

South Oman is becoming increasingly import- ant as a petroleum province, with the main fields occurring on the Eastern flank of the South Oman Salt Basin. Mattes & Conway Morris give a detailed account of the sub-surface Infra Cambrian Ara Formation which is an unusual example of a hydrocarbon habitat with trap, seals and source rocks all within a single forma- tion. The authors discuss the facies, palaeo- environments and palaeontology of five main depositional cycles, including evaporite and red bed accumulation within a major subsiding basin system.

Subsurface data presented by Heward indi- cate that periodic withdrawal and dissolution of the Infracambrian Ara Salt Formation from In- fracambrian to the present day has been the main process responsible for the development of the different types of hydrocarbon traps.

The structures seen in the Infracambrian to Cretaceous rocks exposed in the Huqf -Haush i area are documented by Ries & Shackleton. The oldest structure is the Huqf -Haush i Uplift, which runs parallel to the SE continental margin of Oman and records a history of intermittent uplift throughout the Phanerozoic, local open folds of probable pre-Ordovician age and two sets of faults trending N N E - S S W and N - S . The latter, of Late Cretaceous age, is related to the Masirah Transform Fault.

The geology and tectonic history of the Masi- rah ophiolitic complex is reviewed by Moseley. This author uses unpublished Tithonian ages from cherts associated with the ophiolite to argue that the Masirah Ophio!ite is older and therefore unrelated to the Semail Ophiolite. The origin of the Masirah Melange is attributed to the Masirah Fault and regarded as post mid- Cretaceous. Moseley suggests that the geochem- istry of granitic rocks incorporated within the ophiolitic complex indicates derivation from the Arabian continental crust, incorporated dur- ing ophiolitic obduction in the Late Cretaceous. The ophiolite is overlain by early Tertiary lime- stones which are gently folded and faulted.

The previously geologically unknown area of NE Oman, which lies between the Batain coast and the E of Jebel Ja'alan basement high and the Wahiba Sands, has been shown by Shaekleton et al. to be mostly occupied by a melange, named the Batain Melange, and con- tains clasts which range in age from Permian to Late Cretaceous. This Melange is argued to be

tectonic in origin and is similar to the Hawasina Melange in the Oman Mountains. The dominant structures affecting these rocks are WNW- vergent thrusts and folds, forming the Batain fold and thrust belt, which, from stratigra- phic relationships are pre-early Tertiary in age. These structures must be related to compres- sional motion on the Masirah Fault.

The sedimentology and tectonic history of the Maastrichtian and early Tertiary sequences in the Jebel Ja'alan area are described by Filbrandt et al. Maastrichtian fluvial conglome- rates, which rest directly on the Precambrian basement, and overlie shallow-water lime- stones, are overlain by sequences documenting syn-sedimentary deformation associated with the uplift of Jebel Ja'alan. The youngest sedi- ments of middle Eocene age record a sequence of tectonic events associated with the uplift of the Precambrian basement of Jebel Ja'alan. This Tertiary deformation records further trans- pressional motion on the Masirah Fault.

Shaekleton & Ries argue that the main tec- tonic features of the SE continental margin of Oman, the Masirah Fault, the ophiolitic rocks on Masirah Island, Ras Madrakah and Ras Jibsch, the radiolarites on the Hikman Penin- sula, the Huqf -Haush i Uplift and the Batain Melange, all have to be accounted for in any tectonic model developed for the tectonic evol- ution of this margin. These authors propose Jurassic/Early Cretaceous left-lateral motion along the Masirah Fault and northward displace- ment of the block once adjacent to the SE Oman margin together with upthrusting of the Masirah Ophiolite, followed by a Late Cre- taceous reversal of motion along the Masirah Fault, associated with oblique obduction of the Masirah Ophiolite Zone onto the continental margin and the deformation of the Batain Melange.

Marine geophysical data presented by Moun- tain & Prell indicate that the Owen Basin ocean floor is Late Cretaceous or younger and that the ophiolitic rocks represent fragments of this ocean floor emplaced as a result of trans- pression along the Masirah Fault. Reorganiz- ation of the plate boundaries during the opening of the Gulf of Aden in the Neogene led to further deformation of the SE margin of Oman.

The final two papers in this section deal with the oceanography of the present day passive continental margin of South Oman. Prell and others summarize the preliminary results of Ocean Drilling Project Leg 117. The chief objec- tives of the Leg were to test ideas about the past influence of the Asian summer monsoon on deposition in the northwestern Indian Ocean

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xviii INTRODUCTION

and to try resolve questions about the tectonic development of the Oman margin and the Owen Ridge. The cores reveal a complex interaction between siliceous biogenic productivity and the tectonic subsidence of slope basins of varied morphology. Up to 1000 m of subsidence has taken place during the late Neogene. Upwelling leading to high organic productivity appears to have begun in early Middle Miocene.

Based on a recent cruise of the R / V Charles Darwin, Shimmield et al. report evidence of a pronounced oxygen minimum zone between 200 and 1500 m throughout the northwestern Arabian Sea. The bathymetry of the margin is complex and appears to relate to tectonic move- ments along the Masirah Line and this also favoured the development of small anoxic basins south of Masirah. Otherwise, upwelling was the main control of calcareous and siliceous depo- sition on the South Oman margin, as also dis- cussed by Prell et al.

Regional tectonic setting

In this concluding section, Oman geological development is placed in the regional setting of Iran and the Indian Ocean.

Glennie et al. draw on a large body of pre- viously unpublished oil company information on Southern Oman and present a unified tec- tonic model for the evolution of the Oman margin in relation to Makran. They relate the emplacement of the Semail ophiolite to collision of the Arabian passive margin with a subducting trench, as in some recent models. Crustal re- laxation followed collision and this led to uplift and gravity spreading of allochthonous units further onto the Arabian shelf. Continuing con- vergence was transferred to the Makran margin, giving rise to a long history of subduction and accretion that continues to this day.

Stoneley focusses on apparent differences be- tween the Cretaceous development of the southern Tethyan margin in Iran and Oman. He develops his long-held view that the Iranian ophiolites developed in a setting of Late Cre- taceous rifting, followed by gravity gliding over an intra-shelf basin. He suggests that the Iranian and Oman Tethyan margin segments were offset by a major transform fault and thus the genesis

and emplacement of the ophiolites in these two areas took place in different tectonic settings.

Finally, in a wide ranging synthesis, SengiJr reviews available data for Iran that is relevant to the tectonic setting of Oman. He presents a new tectonic model for the Late Palaeozoic- end Mesozoic evolution of the entire Middle Eastern Tethys area. His main thesis is that the Oman Tethys developed, not as a Red Sea-type small ocean basin as in many conventional models, but instead represents a back-arc (retro- arc) basin generated by spreading behind a major Late Palaeozoic magmatic arc (Podatak- sasi arc). SengOr compares the Oman Tethys to modern marginal basin settings, including the Tyrrhenian Sea and the Sea of Japan. In his view many aspects of Oman geology are consist- ent with a similar setting.

References

BLANFORD, W. T. 1872. Notes on Muskat and Musandam on the coast of Arabia. Records of the Geological Survey of India, 5, 75-77.

GLENNIE, K. W., BOEUF, M. G. A., HUGHES CLARKE, M. W., MOODY-STUART, M., PILAAR, W. F. H. & REINHARDT, B. M. 1973. Late Cretaceous nappes in the Oman Mountains and their geologic signifi- cance. American Association of Petroleum Geo- logists Bulletin, 57, 5-27.

GLENNIE, K. W., BOEUF, M. G. A., HUGHES CLARKE, M. W., MOODY-STUART, M., PILAAR, W. F. H. REINHARDT, B. M. 1974. The Geology of the Oman Mountains. Verhandelingen van het Koninklijk Nederlands geologisch minjbouw- kundig Genootschap.

LEES, G. M. 1928. The geology and tectonics of Oman and parts of south-eastern Arabia. Quar- terly Journal of the Geological Society of London, 84, 585-670.

LIPPARD, S. J., SHELTON, A. W. & GASS, I. G. 1986. The ophiolite of Northern Oman. Geological Society, London, Memoir, 11.

PILGRIM, G. 1908. Geology of the Persian Gulf and adjoining portions of Persia and Arabia. Memoir of the Indian Geological Survey, 34, 1-77.

WHITE, R. S. & Ross, D. A. 1979. Tectonics of the Western Gulf of Oman. Journal of Geophysical Research, 84, 3479-89.

WILSON, H. H. 1969. Late Cretaceous eugeosynclinal sedimentation, gravity tectonics and ophiolite emplacement in the Oman Mountains, southeast Arabia. American Association of Petroleum Geo- logists Bulletin, 53, 626-71.