The Durness Group of Nw Scotland: a Stratigraphical and Sedimentological Study of a Cambro-Ordovician Passive Margin Succession
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THE DURNESS GROUP OF NW SCOTLAND: A STRATIGRAPHICAL AND SEDIMENTOLOGICAL STUDY OF A CAMBRO-ORDOVICIAN PASSIVE MARGIN SUCCESSION by ROBERT JAMES RAINE A thesis submitted to The University of Birmingham for the degree of DOCTOR OF PHILOSOPHY School of Geography, Earth and Envrionmental Sciences The University of Birmingham June 2009 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. ABSTRACT The Cambrian to Ordovician Durness Group was deposited on the Scottish sector of the passively-subsiding, continental margin of the Laurentian craton, and now forms part of the Hebridean terrane, lying to the west of the Moine Thrust zone. It represents c. 920 m of shallow marine, peritidal carbonates with minor siliciclastic and evaporitic strata. Facies analysis shows that the carbonates represent deposition within coastal sabkha, intertidal and shallow subtidal to shelfal environments and sedimentary logging of all available sections has revised the thicknesses of the lithostratigraphic formations within the Durness Group. A diverse array of microbialites is documented, and their application for interpreting the sea- level and palaeoenvironmental history is discussed. The enigmatic ‘leopard rock’ texture is here concluded to represent a thrombolite, thus significantly increasing the abundance of microbial facies within the section. A revised conodont biostratigraphy for the Ordovician upper five formations of the Durness Group allows more precise correlation with the once contiguous sections in western Newfoundland and Greenland and dating of the lithostratigraphical and sequence stratigraphical subdivisions for the first time. Based upon the new conodont biostratigraphy, a sequence stratigraphical model for the Cambro- Ordovician strata in Scotland is proposed, comprising four depositional megasequences, which correlate well with the Sauk sea level sequence recognised across Laurentia. This study allows for further correlation with Laurentian margin sections and the global sea-level record. i ACKNOWLEDGEMENTS I would like to thank all the help and support from my supervisors Paul Smith (Birmingham) and Maarten Krabbendam (BGS). Paul is particularly thanked for tireless reading of paginated ramblings, trying to explain the finer points of en- and em-dash usage to deaf ears and for much assistance, culinary delights and enthusiasm in the field. John Repetski, Dave Evans and Tim Palmer have been good company in the field and always showed interested in my project. Thanks are also due to Rosie, Lauren, Kate and Liam, Neil (for camping barn trips and painting a 6’ chaffinch on my wall), Tom Challands, Giles Miller, Nick, Luis (BGS), Mark Dean (BGS), Ross, Jenny, Chris, Tim, Andy, Helen, Carl, Ruth, Karen, Aruna, Pete Turner, Beth, Leyla, Jason, Paul Hands, Donald the Highland Ranger, and any others who deserve a mention. Lil and Helen are thanked greatly for their friendship, encouragement and joint enthusiasm for curry. Russel is thanked for breaking the boredom with cider making and drinking. My parents and sisters deserve special mention for their support and financial assistance and whose recurring greeting over the past few years has been ‘have you finished your thesis yet?’ For the many housemates and Geology vagrants at who have been both a help and a welcome distraction at times and who now have jobs in the real world. Dr Liam Herringshaw, Emma Nichols, Dr Simon Holford and Gui the French guy. The work was funded primarily by the School of Geography, Earth and Environmental Sciences. and the British Geological Survey (BUFI Grant E2152S60). The Palaeontological Association (Sylvester Bradley Award) and the BSRG (Farrell Fund) are acknowledged for financial assistance with fieldwork and conference attendance. The school funding has involved the opportunity to work as assistant curator (and for a brief stretch conservator) within the Lapworth Museum, whilst spinning out my PhD. The main benefit of which has been working with Jon Clatworthy, who is perhaps one of the most helpful and kind persons around. I have to thank him for never putting fine specimens of dolomite in the skip. Finally thanks to all at Ichron Ltd., who always assumed that I slept in the office whilst writing. ii ‘It is not that I object to the work, mind you; I like work: it fascinates me. I can sit and look at it for hours.’ Jerome K. Jerome (1859–1927): Three Men in a Boat (To Say Nothing of the Dog) 1889 ‘Stands Scotland where it did?’ William Shakespeare (1564–1616): Macbeth 1606 iii TABLE OF CONTENTS Page Abstract i Acknowledgements ii Chapter 1. INTRODUCTION AND STRATIGRAPHICAL REVISION OF THE DURNESS GROUP, NW SCOTLAND 1.1 Geological setting of the Durness Group 1 1.2 Study areas 2 1.2.1 An t-Sròn, Loch Eriboll 2 1.2.2 Smoo Inlet and Leirinmore 4 1.2.3 Balnakeil Bay 4 1.2.4 Inland exposures between Loch Borralie and Loch Caladail 6 1.2.5 The Kyle of Durness 8 1.2.6 Grudie 8 1.2.7 Ardvreck Castle and Inchnadamph, Loch Assynt 9 1.2.8 Ord, Skye 9 1.2.9 Strath Suardal and Loch Lonachan, Skye 9 1.3 Previous research on the Cambro-Ordovician of northwest Scotland 10 1.3.1 Sedimentology 11 1.3.2 Sequence stratigraphy and correlation 11 1.3.3 Palaeontology and biostratigraphy 11 1.3.4 Chemostratigraphy 14 1.4 Lithostratigraphical revision of the Durness Group 15 1.4.1 Durness Group 16 1.4.2 Ghrudaidh Formation 17 1.4.3 Eilean Dubh Formation 17 1.4.4 Sailmhor Formation 18 1.4.5 Sangomore Formation 20 1.4.6 Balnakeil Formation 20 1.4.7 Croisaphuill Formation 21 1.4.8 Durine Formation 22 1.5 Recent developments in Cambrian and Ordovician chronostratigraphy 22 1.5.1 Current Cambrian stratigraphy 22 1.5.2 The Cambrian–Ordovician boundary 26 1.5.3 Current Ordovician stratigraphy 26 Chapter 2. FACIES ANALYSIS OF THE IAPETUS PASSIVE MARGIN SEDIMENTS IN NW SCOTLAND: THE CAMBRO-ORDOVICIAN DURNESS GROUP 2.1 Introduction 27 iv 2.2 Lithofacies 27 2.2.1 Ooidal grainstone/wackestone 30 2.2.2 Oncoidal grainstone/packstone 33 2.2.3 Burrowed mudstone/wackestone 35 2.2.4 Ribbon carbonate 37 2.2.5 Bioclastic wackestone/packstone 39 2.2.6 Stromatactoid carbonate 40 2.2.7 Peloidal grainstone 42 2.2.8 Thrombolite boundstone 44 2.2.9 Columnar stromatolite boundstone 46 2.2.10 Hemispheroidal stromatolite boundstone 46 2.2.11 Intraclast rudstone 47 2.2.12 Parallel laminated carbonate 50 2.2.13 Ripple laminated carbonate 53 2.2.14 Quartz arenaceous carbonate 55 2.2.15 Fenestral carbonate 57 2.2.16 Crinkly laminated and planar stromatolite boundstone 58 2.3 Facies associations 59 2.3.1 Shallow subtidal facies association 59 2.3.2 Intertidal flat facies association 60 2.3.3 Sabkha/supratidal facies association 60 2.4 Stratigraphic trends 76 2.5 Conclusions and environmental interpretations 85 Chapter 3. TEMPORAL TRENDS OF CAMBRO-ORDOVICIAN MICROBIALITES: A CASE STUDY FROM THE DURNESS GROUP OF NW SCOTLAND 3.1 Introduction 87 3.2 Development of terminology 88 3.3 Histrory of research 91 3.3.1 Stromatolites 91 3.3.2 Thrombolites 92 3.3.3 Dendrolites and leiolites 93 3.4 Methods 94 3.5 Stromatolites from the Durness Group 94 3.5.1 Stratiform stromatolites 94 3.5.2 Encrusting stromatolites 97 3.5.3 Hemispheroidal stromatolites 97 3.5.4 Columnar stromatolites 98 3.5.5 Oncolites 102 3.6 Leiolites from the Durness Group 104 3.7 Thrombolites from the Durness Group 104 3.7.1 Reinterpretation of the ‘leopard rock’ 105 v 3.7.2 Domal and columnar thrombolite bioherms 109 3.7.3 Stratiform thrombolite biostromes 109 3.8 Microbialites as environmental indicators 111 3.8.1 Stromatolites 111 3.8.2 Thrombolites 113 3.9 Durness Group microbialies and palaeoenvironment 114 3.10 Temporal trends of palaeoenvironmental change 116 3.10.1 Short term temporal trends 116 3.10.2 Long term temporal trends 118 3.11 The rise and demise of Cambro-Ordovician microbialites 121 3.12 Conclusions 125 Chapter 4. CONODONT BIOSTRATIGRAPHY OF THE DURNESS GROUP 4.1 Introduction 127 4.2 Sampling and processing methods 127 4.3 Previous research 130 4.3.1 Early studies concerning the upper Durness Group 130 4.3.2 The Cambrian–Ordovician boundary interval 131 4.4 Conodont zonation 133 4.4.1 North American midcontinent zones 133 4.5 Conodont biostratigraphy of the Durness Group 137 4.5.1 Rossodus manitouensis Zone 137 4.5.2 ‘Low diversity interval’ 139 4.5.3 Macerodus dianae Zone 139 4.5.4 Acodus deltatus –Oneotodus costatus Zone 140 4.5.5 Oepikodus communis Zone 141 4.5.6 ?Reutterodus andinus Zone 145 4.5.7 Tripodus laevis Zone 146 4.5.8 Histiodella altifrons Zone 147 4.6 Palaeobiogeography 150 4.6.1 Extent of the Tropical Domain 150 4.6.2 A multivariate statistical analysis of Laurentian faunas at the time 152 of maximum flooding 4.7 Conclusions 157 Chapter 5. SEQUENCE STRATIGRAPHY OF THE SCOTTISH LAURENTIAN MARGIN AND RECOGNITION OF THE SAUK MEGASEQUENCE 5.1 Introduction 158 5.2 Terminology 162 5.3 Metre-scale, shallowing upward parasequences within the Durness Group 163 5.3.1 Subtidally dominated parasequences 165 5.3.2 Peritidally dominated parasequences 165 vi 5.3.3 Origin of metre-scale parasequences 167 5.3.4 Allocyclic model 168 5.3.5 Autocyclic model 169 5.3.6 Parasequence sets and accommodation curve 171 5.4 SLM I Supersequence 175 5.5 SLM I–II boundary 177 5.6 SLM II Supersequence 178 5.6.1 SLM IIa Sequence 179 5.6.2 SLM IIb Sequence 180 5.7 SLM II–III boundary 181 5.8 Cambrian Grand Cycles and correlation with the Durness Group 184 (SLM IIa, IIb & IIIa) 5.9 SLM III Supersequence 185 5.9.1 SLM IIIa Sequence 186 5.9.2 SLM IIIb Sequence 186 5.9.3 SLM IIIc Sequence 189 5.10 SLM IV Supersequence 194 5.11 The end of the SLM Megasequence in Scotland 198 5.12 Conclusions and relative sea-level history 201 Chapter 6.