Geology of the Country Around Pwllheli
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BRITISH GEOLOGICAL SURVEY Geology of the country around Pwllheli Memoir for 1:50 000 Geological Sheet 134 (England and Wales) This memoir and the .1:50 000 Series geological map that it describes are the products of a mapping contract between the Natural Environment Research Council and Cardiff University. The interpretations presented are those of the authors. London: The Stationery Office 2002 CONTENTS One Introduction 1 Five Igneous intrusions 58 Previous research 2 Basic intrusions of Llanvirn age 58 Geological history 3 Basic intrusions of Caradoc age 59 Two Precambrian to ? Cambrian 7 Acid and intermediate intrusions of Caradoc age 63 Sam Complex 7 Six Igneous geochemistry 67 Monian Supergroup 9 Magmatism of Llanvirn age: the basic intrusions and acid Gwna Melange 9 tuff of Mynydd Rhiw and Sam 67 Three Cambrian 10 Magmatism of Caradoc age 67 Harlech Grits Group 11 Pen-y-chain Rhyolitic Complex, including the Pitts Head Hell's Mouth Formation 11 Tuff Formation 67 Trwyn y Fulfran Formation 13 Allt Fawr Rhyolitic Tuff Formation 69 Cilan Formation 13 Llanbedrog Volcanic Group 70 Mawddach Group 15 Details 75 Ceiriad Formation 15 Nod Glas Formation basaltic volcanism and associated Nant-y-big Formation 16 intrusions 80 Maentwrog Formation 18 Major basic intrusions 83 Ffestiniog Flags Formation 20 Seven Quaternary 95 Four Ordovician 21 Previous work 95 Mawddach Group 21 Preglacial landscape evolution 97 Dol-cyn-afon Formation 21 Pleistocene glaciation 97 Ogwen Group 23 Erosional evidence 97 Nant Ffrancon Subgroup 23 Glacigenic deposits 98 Wig Bach Formation 25 Postglacial deposits 114 Bryncroes Formation 26 Quaternary history 115 St Tudwal's Formation -26 Eight Structure and metamorphism 122 Pared-mawr Member 29 Gwna Melange 122 Trwyn-yr-Wylfa Member 29 Lower Palaeozoic succession 123 Penrhyn Du Member 30 Folds 124 Machroes Member 31 Faults 126 Llanengan Formation 31 Cleavage 128 Trygarn Formation 31 Metamorphism 129 Nant Ffrancon Subgroup undivided 32 Metabasite secondary mineralogy 129 Hen-dy-capel Ironstone 32 White mica crystallinity index values 130 Arenig to Llanvirn biostratigraphy 33 Ogwen Group: middle part 34 Nine Economic geology 131 Cwm Eigiau Formation 34 Iron 131 Pen-y-chain Rhyolitic Complex and Pitts Head Tuff Manganese 131 Formation 39 Vein mineralisation 133 Upper Lodge Formation 40 Details of mines 134 Allt Fawr Rhyolitic Tuff Formation 40 Building stone and aggregate 136 Dwyfach Formation 41 Sand and gravel 136 Bodwrog Member 41 Information sources 137 Plastirion Member 41 Llanbedrog Volcanic Group 43 References 139 Penmaen Formation 43 Fossil inventory 146 Foel Ddu Rhyodacite Formation 47 Index 147 Carneddol Rhyolitic Tuff Formation 48 Yoke House Formation 50 Ogwen Group: upper part 53 FIGURES Nod Glas Formation 53 Strata above the Ogwen Group 55 1 Geology of the Pwllheli district x Crugan Mudstone Formation 55 2 Topographical map of the district vi CONTENTS 3 Stratigraphy of the Lower Palaeozoic rocks of the 38 'Immobile' elements Y, Nb, Zr and LaN/YbN, SREE and district 4 TiO2 against Zr/TiOg, Llanbedrog Volcanic Group and 4 Cambrian of St Tudwal's Peninsula 10 associated intrusions 79 5 Cambrian succession of St Tudwal's Peninsula 12 39 Zr/TiO2 against SiOg, and Zr/TiOg against SiOg, 6 Sections through the lower part of the Harlech Grits Llanbedrog Volcanic Group and associated Group 13 intrusions 80 7 Upper Ceiriad Formation and lower Nant-y-big 40 Chrondrite-normalised REE plots, Llanbedrog Formation, Portfi Ceiriad 16 Volcanic Group and associated intrusions 81 8 Nant-y-big and Maentwrog formations as exposed in 41 Geochemical characteristics of the magmatism Porth Ceiriad 19 associated with the Nod Glas Formation and of the 9 Ffestiniog Flags Formation as exposed on St Tudwal's major basic intrusions 91 Island East 20 42 Glacial stages in north-west Wales according to 10 Ordovician strata of the district 22 Whitton and Ball (1970) 96 11 Penbennar and graphic log of part of the Dol-cyn- 43 Erosional evidence of ice movement direction 98 afon Formation 24 44 Quaternary deposits exposed on the coast at 12 Graphic logs of the St Tudwal's Formation 27 Glanllynnau 100 13 Palaeocurrent directions from the St Tudwal's 45 Cartoon showing deglaciation phase, present day Formation 28 coastal exposure and inland topography at 14 Graphic logs of the base of the Pared-mawr Member Glanllynnau 102 at Trwyn Llech-y-doll 30 46 Kettle hole deposits at Glanllynnau, with inferred 15 Graptolites from the Nant Ffrancon Subgroup, north changes in environment and climate 104 of Sam Meyllteyrn 34 47 Quaternary sediments exposed at the eastern end of 16 Fence diagram showing lateral variation in the Porth Neigwl (Hell's Mouth) 105 Caradoc succession of the district 35 48 Seismic refraction studies from the thick drift behind 17 Time-correlation diagram for the Caradoc succession Porth Neigwl 106 of the district 36 49 High resolution shallow seismic reflection surveys, at 18 Evolution of volcanic centres in north-west Wales Porth Neigwl 107 during the Caradoc 37 50 Drift deposits of the area around Cors Geirch 113 19 Pen-y-chain Rhyolitic Complex 38 51 Sequence of events during deglaciation 117 20 History of interpretations of the Llanbedrog 52 Major structures of north-west Wales 123 area 46 53 Structural geology of the Lower Palaeozoic rocks of 21 Llanbedrog Volcanic Group: isopachs 48 the district 125 22 Geological map of the'Carreg-y-Defaid area 52 54 Variation in white mica crystallinity values in the 23 Graptolites from the Nod Glas Formation 54 district and adjoining areas 129 24 Map of the basic sill complex in western Llyn 59 55 Hen-dy-capel Ironstone 132 25 Basic intrusions of Caradoc age and volcanism 56 Map of the St Tudwal's orefield 134 associated with the Nod Glas Formation 60 57 Cross-section through St Tudwal's orefield 135 26 Efailnewydd Fault and adjacent igneous intrusions 62 27 Llanbedrog Volcanic Group intrusions 64 28 Geochemical features of the Llanvirn magmatism 69 TABLES 29 Zr/TiOg v Nb/Y diagram, Ordovician magmatism in the Llyn compared with the Snowdon centre 70 1 Summary of the geological succession inside front 30 Total alkali-silica (TAS) diagram for the Ordovician cover igneous rocks of the district 71 2 Analyses of rocks from the Sarn Complex 8 31 Ordovician magmatic rocks from the Llyn plotted on 3 Analyses of rocks from the Llanvirn basic sills of an AFM diagram 72 Mynydd Rhiw and Sam 68 32 Zr/TiO2 v Nb/Y diagram, Pen-y-chain Rhyolitic 4 Analyses of rocks from the Mynydd Rhiw Rhyolitic Complex 74 Tuff 70 33 Y-Zr-Nb ternary diagram, Pen-y-chain Rhyolitic 5 Analyses of rocks from the Pen-y-chain Rhyolitic Complex, Allt Fawr Rhyolitic Tuff Formation and Complex 73 Cameddol Rhyolitic Tuff Formation 74 6 Analyses of rocks from the Upper Lodge Volcanic 34 Binary diagrams, Pen-y-chain Rhyolitic Complex, Group 76 Pitts Head Tuff Formation, Allt Fawr Rhyolitic Tuff 7 Analyses of basic intrusions associated with the Formation and Cameddol Rhyolitic Tuff Llanbedrog Volcanic Group 81 Formation 75 8 Analyses of rocks from the Penmaen Formation 82 35 Zr/TiO2 v Nb/Y diagram, Upper Lodge Volcanic 9 Analyses of rocks from the Bryn Crin Basaltic Group and associated intrusions 76 Trachyandesite 83 36 Zr/TiOg v Nb/Y diagrams, Llanbedrog Volcanic 10 Analyses of rocks from the Foel Ddu Rhyodacite Group and from associated intrusions 77 Formation 84 37 Log Nb v Log Zr for analyses from the Llanbedrog 11 Analyses of rocks from the Carn Fadryn Suite of Volcanic Group and associated intrusions 78 alkaline intrusions 85 CONTENTS vii 12 Analysis of a sample from the Wyddgrug 7 Contact between the Nant-y-big and overlying Microgranite 86 Maentwrog formations in Porth Ceiriad 18 13 Analyses of rocks from the Cameddol Rhyolitic Tuff 8 Quartzose sandstones at the base of the Pared-mawr Formation 87 Member, Trwyn Llech-y-doll 29 14 Analyses of rocks from the Mynydd Tir-y-cwmwd 9 Rhyolitic flow banding and breccias, Pen-y-chain Porphyritic Granophyric Microgranite 88 Rhyolitic Complex 39 15 Analyses of rocks from the Nanhoron Suite of 10 Trilobites of the Dwyfach Formation 42 peralkaline intrusions 89 11 Brachiopods and ostracods of the Cwm Eigiau and 16 Analyses of igneous rocks from the Nod Glas Dwyfach formations 44 Formation and associated high-level intrusions 90 12 Autobrecciated basaltic trachyandesite, Penmaen 17 Analyses of rocks from the Pen-yr-Orsedd dykes 92 Formation 47 18 Analyses of rocks from the Carreg-yr-Imbill 13 Welded tuff, Nant-y-Gledrydd Member, Cameddol Intrusion 93 Rhyolitic Tuff Formation 50 19 Analyses of rocks from the Penarfynydd Layered 14 Shardic ashflow tuff, Bodgadle Member, Cameddol Intrusion 94 Rhyolitic Tuff Formation 50 15 Shardic bubbles in crystal tuff near the base of the Bodgadle Member, Cameddol Rhyolitic Tuff PLATES Formation 51 16 Fauna of the Crugan Mudstone Formation 56 Cover photograph View looking east towards Trwyn 17 Gabbroic pegmatite patch within the Carreg-yr-Imbill Llech-y-doll, St Tudwal's Peninsula dolerite 61 18 Andesine and clinopyroxene within the Bryn Crin Frontispiece Mineral specimens from Carreg-yr-Imbill ii Basaltic Trachyandesite 65 1 View south-west behind Porth Neigwl (Hell's 19 Granophyric texture, Mynydd Tir-y-cwmwd Mouth) 1 Porphyritic Granophyric Microgranite 66 2 Gwna Melange overlain by Quaternary till south of 20 Cam Fadryn and surrounding drift-covered Porth Towyn 9 lowlands 97 3 Gwna metasediments on south side of Porth 21 Glanllynnau section 101 Towyn 9 22 Sands and gravels of Porth Neigwl 108 4 Microflora and macrofauna of the Cambrian rocks of 23 Drift deposits of Porth Ceiriad 110 St Tudwal's Peninsula 14 24 F3 chevron folding 124 5 Bentonites, Nant-y-big Formation, Porth Ceiriad 17 25 Partially phosphatised chamosite pisoid and ooids in 6 Nant-y-big Formation, Porth Ceiriad 17 Hen-dy-capel Ironstone 133.