8.25mm spine allowed M42 cover artwork 8.25mm spine EDIT 27-10-14_Layout 1 29/10/2014 15:10 Page 1 T h e

C Geological Society Memoir No. 42 a m b r i a n o f

The Cambrian of SW S W W a l e

Towards a United Avalonian Stratigraphy s The Cambrian of SW Wales T o w a r d s

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A. J. Rees, A. T. Thomas, M. Lewis, H. E. Hughes and P. Turner Towards a United Avalonian Stratigraphy U n i t e d

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This Memoir reviews, revises and interprets the biostratigraphy, lithostratigraphy, process sedi- a l o A. J. Rees, A. T. Thomas, M. Lewis, n i a

mentology, palaeoenvironments and sequence stratigraphy of the classic Cambrian succession of n

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r H. E. Hughes and P. Turner a

SW Wales. This 1250 m thick clastic sedimentary succession, of Terreneuvian to Furongian age, t i g r a

was deposited in environments ranging from alluvial fan to mid–outer shelf. Two litho- p h y stratigraphical groups are distinguished, each comprising six formations; where possible and G e o

appropriate, original and well-known names are retained in harmony with current usage. High- l o g i c resolution sequence-stratigraphical techniques, constrained by biostratigraphical data whenever a l

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possible, allow the revised lithostratigraphy to be integrated with the western Avalonian chrono- i e t y

stratigraphy developed in maritime Canada. A twofold subdivision is recognized. Megasequence 1 M e m o

(sequences 3–7) spans the Terreneuvian, C2 and much of C3, with deposition occurring in an i r

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extensional rift-like regime on an epeirogenically active platform. Sequence 8 lies at the base of 4 2

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Megasequence 2, which extends from late C3 into the early Ordovician, and represents passive A . J . R

margin sedimentation. e e s ,

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P . T u r Cover illustration: n e r View westwards across Porth-y-rhaw (Fig. 1.1e) showing the Menevia Formation dipping steeply southwards at its type locality. Ordovician T

volcanic rocks form the hills on Ramsay Island, some 9 km distant. h e

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Photograph: Alan Thomas, September 2006. o l o g i c a l

S Published by the Geological Society o c i e t y

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It is recommended that reference to all or part of this book should be made in one of the following ways: Rees, A. J., Thomas, A. T., Lewis, M., Hughes,H.E.&Turner, P. 2014. The Cambrian of SW Wales: Towards a United Avalonian Stratigraphy. Geological Society, London, Memoirs, 42.

Rees, A. J., Thomas, A. T., Lewis, M., Hughes,H.E.&Turner, P. 2014. Overview and biostratigraphy. The Cambrian of SW Wales: Towards a United Avalonian Stratigraphy. Geological Society, London, Memoirs, 42, 1–31, http://dx.doi.org/10.1144/M42.1

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The Cambrian of SW Wales: Towards a United Avalonian Stratigraphy

BY

A. J. REES Saudi Aramco, Saudi Arabia

A. T. THOMAS University of Birmingham, UK

M. LEWIS Cardiff University, UK

H. E. HUGHES University of Plymouth, UK

and

P. TURNER Brigantia Resources Ltd, UK

2014 Published by The Geological Society London

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Preface vii Lithofacies C3: hummocky cross stratified sandstone 45 Facies analysis and palaeoenvironments 46 Acknowledgements viii Lithofacies Association A: alluvial fan sedimentation 46 Lithofacies Association B: braided stream 48 Chapter 1 Overview and biostratigraphy 1 sedimentation Lithofacies Association C: transgressive marine 48 Opening remarks and objectives 1 sedimentation The Cambrian geology of 3 St Non’s Formation 49 Physiography, nature of exposure and access 3 Caerfai Bay Formation 51 History of research 3 Caer Bwdy Bay Formation 53 Structure 3 Newgale Formation (new) 55 Regional geological context 7 Lithofacies Association NA 55 Definition of the Cambrian and its subdivisions 9 Lithofacies NA1: unstratified, tabular, non-erosive 55 Cambrian biostratigraphy in SW Wales 11 pebbly sandstone Terreneuvian Series 11 Lithofacies NA2: unstratified, lenticular pebbly 55 Unnamed Cambrian Series 2 12 sandstone Unnamed Cambrian Series 3 12 Lithofacies Association NB 58 CS5: oelandicus Biosuperzone 12 Lithofacies NB1: wavy and flaser bedded silty 58 mudstone CS5: gibbus Biozone 12 Lithofacies NB2: unstratified lenticular sandstone 58 Drumian: fissus Biozone 13 Lithofacies NB3: unstratified tabular sandstone 62 Drumian: parvifrons Biozone 13 Lithofacies NB4: muddy diamictite 62 Drumian: punctuosus Biozone 13 Lithofacies NB5: massive to normally graded, crudely 63 Guzhangian Stage: pisiformis Biozone 13 stratified pebbly sandstone Furongian Series 13 Lithofacies NB6: microlaminated, weakly bioturbated 64 List of fossiliferous localities and faunal summaries 13 mudstone and siltstone Trwyncynddeiriog 15 Trwyncynddeiriog Member (new) 65 Point St John–Porth Cadnaw, Ramsay Sound 15 Pen-y-Cyfrwy Member (new) 66 Dwrhyd 15 Cwm Mawr Member (new) 66 Penpleidiau headland, Caerfai Bay 20 Porth Clais Formation (new) 66 Ogof Eynon 26 Lithofacies PC1: massive to normally graded, crudely 68 Solva Harbour 26 stratified pebbly sandstone Porth-y-rhaw 26 Lithofacies PC2: inverse to normal and inversely 68 graded unstratified sandstone References 28 Lithofacies PC3: graded sandstone with 69 hummocky cross stratification and climbing Chapter 2 Lithostratigraphy and palaeoenvironments 33 ripple lamination of the Cambrian in SW Wales Lithofacies PC4: graded, stratified tabular sandstone 70 Caerfai Group (revised) 33 Lithofacies PC5: weakly graded, bioturbated silty 71 Ogof Golchfa Formation (new) 35 mudstone Lithofacies Association A 37 Facies analysis and palaeoenvironments 71 Lithofacies A1: unstratified, tabular, clast-supported 37 Porth-y-rhaw Group (new) 72 conglomerate Whitesands Bay Formation (new) 72 Lithofacies A2: muddy matrix-supported conglomerate 37 Menevia Formation (new) 73 Lithofacies A3: unstratified, lenticular sand-matrix- 37 Lithofacies M1: stratified fine sandstone and siltstone 75 supported conglomerate and pebbly sandstone Lithofacies M2: homogeneous silty mudstone with 75 Lithofacies A4: purple and red silty mudstone 39 discontinuous limestone Lithofacies Association B 40 Lithofacies M3: graded and banded siltstone and black 75 Lithofacies B1: graded pebble conglomerate 40 mudstone Lithofacies B2: stratified pebble conglomerate and 42 Lithofacies M4: laminated black mudstone 77 pebbly sandstone Aber Llong Formation (new) 78 Lithofacies B3: stratified lenticular sandstone and 42 Ogof Velvet Formation (revised) 80 massive silty mudstone Lithofacies Association OVA 80 Lithofacies Association C 42 Lithofacies OVA1: rippled siltstone and sandstone with 80 Lithofacies C1: well-segregated pebble conglomerate 42 interbedded mudstone and coarse sandstone Lithofacies OVA2: siltstone-filled gutter casts 82 Lithofacies C2: trough cross-stratified coarse-grained 44 sandstone Lithofacies OVA3: amalgamated lenticular sandstone 84

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Lithofacies Association OVB 85 Sequence 4 106 Lithofacies OVB1: sheet-like amalgamated sandstone 85 Sequence 5 109 Lithofacies OVB2: thinly-bedded sandstone and shale 85 Sequence 6 109 Lithofacies OVB3: sandstone and mudstone with soft- 86 Sequence 7 112 sediment deformation Sequence 8 113 Lithofacies Association OVC 87 Tectonostratigraphy, provenance and basin analysis 116 Lithofacies OVC1: tabular graded fine–very fine 87 Megasequence 1 (sequences 1–7) 118 sandstone General remarks 118 Lithofacies OVC2: structureless silty mudstone 87 The lower part of the Caerfai Group: 120 Facies analysis and palaeoenvironments 88 tectonosedimentary model and basin margin Treffgarne Bridge Formation 90 evolution References 91 The extensional regime 124 Implications for the late Neoproterozoic– 124 Chapter 3 Sequence stratigraphy, tectonostratigraphy 101 Cambrian tectonostratigraphic evolution of and basin analysis East Avalonia Geological and stratigraphical setting 101 Megasequence 2 (Sequence 8) 125 Sequence stratigraphy 102 References 128 Depositional sequences 106 Sequence 3 106 Index 135

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The classic Cambrian succession of SW Wales comprises some 1250 m Cambrian series are respectively referred to as C2 and C3. Unnamed of clastic rocks that make up a part of the Supergroup, and stages are denoted by CS2, CS3 and so on. which range from Terreneuvian to Furongian in age. The existing Trace fossils low in the Caerfai Group date that part of the succession lithostratigraphical framework owes much to the Nineteenth Century to the Rusophycus avalonensis Ichnozone or the Teichichnus Interval of researchers, with relatively little detailed work having been published the oldest Cambrian series, the Terreneuvian. A little higher up, a high since. We here present a detailed, rationalized and revised lithostratigra- resolution U–Pb date indicates a CS3 (C2) age. Shelly fossils, especially phy for the Cambrian part of the Dyfed Supergroup of the area: where trilobites, become much more common at some levels in the higher parts possible and appropriate, we have retained original and well-known of the Caerfai Group and parts of the Porth-y-rhaw Group, and these names for formations and other units in harmony with current usage. faunas facilitate correlation particularly with the C3 (CS5) to Furongian The practice of recognizing four lithostratigraphical groups is overly (Paibian Stage) parts of the Scandinavian succession. The Baltoparadox- complex and sometimes unworkable however, and a two-fold subdivi- ides oelandicus Biosuperzone and probably the Ptychagnostus gibbus sion is proposed instead. The Caerfai and much of the Lower and Biozone occur in the Caerfai Group. The Porth-y-rhaw Group yields Middle Solva groups of earlier usage are combined into a revised trilobites diagnostic of the Tomagnostus fissus, Hypagnostus parvifrons, Caerfai Group, while the Upper Solva Group, Menevian Group and Ptychagnostus punctuosus, Agnostus pisiformis and Olenus biozones. Lingula Flags comprise the newly erected Porth-y-rhaw Group. Six for- High resolution sequence-stratigraphical techniques, constrained mations, including one divided into three members, are recognized by the biostratigraphical data wherever possible, are applied to the within the revised Caerfai Group, and a further six formations make Dyfed Supergroup across southern Britain to integrate the revised litho- up the Porth-y-rhaw Group. stratigraphy with the Avalonian chronostratigraphy based on the western Deposition of the Dyfed Supergroup took place on the microconti- Avalonian successions of maritime Canada. The relative ease of inte- nent of East Avalonia, one of a number of circum-North-Atlantic ter- gration reflects the shared epeirogenic history, sediment sources and ranes and superterranes that were once part of an accretionary orogen accumulation history of a microcontinent unified early in Cambrian developed on an active Gondwanan margin of the Neoproterozoic super- times. An overall two-fold subdivision into megasequences respectively continent Rodinia and its successor Pannotia or Greater Gondwana. representing tectonically active and passive phases of sedimentation is These peri-Gondwanan terranes are defined by their Neoproterozoic recognized. Megasequence 1 spans the Terreneuvian, C2 and much of evolution on an active continental margin and by the Acadian or Balto- C3. It records transform termination of Avalonian subduction following scandian aspect of their Cambrian-Ordovician overstep successions. oblique convergence, and the accretion of island arcs to the northern The oldest part of the Caerfai Group records a transition from westerly- periphery of Gondwana. Under transpressional regimes, late Precam- derived alluvial fan sediments, through braided stream environments, brian arc-related basins were inverted and recycled into pull-apart suc- into transgressive nearshore marine sandstones. Above lie sedimentary cessor basins which were initially dominated at their margins by rocks deposited under a wide range of conditions ranging from nearshore alluvial fans and coarse-grained fan-deltas built by flood-generated tidally-influenced to shelfal sediment gravity flows. Deposition of the sediment dispersal processes. In the later transform stage of Cado- Porth-y-rhaw Group began with fine-grained turbidites deposited in a mian–Avalonian orogenesis, there was a switch to sediment supplied mid–outer shelf setting. These pass up first into hemipelagites, then from highlands to the east. The Ogof Velvet Formation lies at the base sedimentary deposits formed on a storm-dominated shelf, and finally of Megasequence 2, which extends from late C3 into the early Ordovi- on an extensive, largely shallow subaqueous delta platform. cian, and reflects passive margin sedimentation. The Formation was Cambrian chronostratigraphy is in a state of flux. Instead of the tra- deposited as part of a subaqueous delta platform at the mouth of a ditional tripartite subdivision, four series are now recognized, which huge fine-grained river system that drained part of West Gondwana; are further divided into ten stages. Not all these divisions have been its depositional setting and characteristics are best understood in terms named and ratified, and the bases of some of them have yet to be inter- of nearshore sediment bypass that facilitated rapid progradation under nationally agreed. In the account below, the Unnamed Second and Third high sediment input regimes.

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We are especially grateful to Dr A. W. A. Rushton for his cheerful advice work: Professors P. W. C. Arnott, A. Nielsen, M. P. Smith and the late and enthusiastic discussion over many years, and for freely sharing his W. T. Dean, the late Dr K. Thomson and Dr N. J. P. Smith. Rees acknowl- deep knowledge of all matters relating to the Cambrian. Rees particu- edges a School of Earth Sciences studentship from the University of larly thanks Mr Joe Crossley, who provided early mentorship in all Birmingham; Lewis acknowledges an N.E.R.C. studentship. Mrs A. R. matters sedimentological, and who introduced him to the joys and com- Ankorn and Mr K. Burkhill skilfully drafted many of the illustrations. plexities of Welsh Cambrian geology. Dr N. H. Woodcock construc- Drs M. P. Howe (British Geological Survey), R. M. Owens (National tively criticized earlier versions of the whole manuscript, and Prof. M. Museum of Wales) and Messrs D. Pemberton and M. Riley (Sedgwick Williams, Dr S. G. Molyneux and Dr A. R. A. Thomas individual sec- Museum) kindly loaned specimens in their care. Dr J. Williams-Davies tions of it: all made useful suggestions for its improvement. We thank helpfully advised on the meaning and proper form of some Welsh place- the following for useful discussion concerning various aspects of the names. Rees publishes with the permission of Saudi Aramco.

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