This work is protected by copyright and other intellectual property rights and duplication or sale of all or part is not permitted, except that material may be duplicated by you for research, private study, criticism/review or educational purposes. Electronic or print copies are for your own personal, non- commercial use and shall not be passed to any other individual. No quotation may be published without proper acknowledgement. For any other use, or to quote extensively from the work, permission must be obtained from the copyright holder/s. "STRATICRAPHY, SEDIMENTOLOGY AND PALAEONTOLOGY OF THE LOWER CARBONIFEROUS OF ANGLESEY" VOLUME 1 by Jeremy Robin DAVIES Submitted for the degree of Doctor of Philosophy University of Keele 1982 CONTENTS Page Abs tract CHAPTER ONE - GENERAL INTRODUCTION 1.1 AIMS AND APPROACH 1 1.2 LOCATION OF OUTCROPS AND EXTENT OF EXPOSURE 2 1.3 DINANTIAN CORRELATION 3 (a) Biostratigraphy 3 (b) Chronostratigraphy 4 6 (c) Eustacy v. Tectonism 1.4 GEOLOGICAL SETTING 8 (a) Gross Palaeogeography 8 (b) Dinantian of North Wales 9 11 (c) Dinantian of Anglesey 1.5 METHODOLOGY 12 (a) Field Work 12 (b) Laboratory Work 13 CHAPTER TWO - STRATIGRAPHIC FRAMEWORK 2.1 INTRODUCTION 15 2.2 PRE-EXISTING CLASSIFICATIONS 16 2.3 CYCLIC STRATIGRAPHY 20 2.4 STRATIGRAPHIC NOMENCLATURE 24 (a) Basal Formations 25 (b) Careg-onen Limestone Formation 26 (1) Penmon Area 26 (ii) Principal Area 29 (iii) Age of the Careg-onen Limestone Formation 30 (c) Flagstaff Limestone Formation 31 (i) Penmon Area 31 (ii) Priticipal Area 35 (iii) Age.of the Flagstaff Limestone Formation 37 (d) Moelfre Limestone Formation 37 (i) Principal Area 38 (ii) Penmor Area 41 (iii) Straitside Area 43 (iv) Age of the Moelfre Limestone Formation 43 (e) Traeth Bychan Limestone Formation 44 (i) Principal Area 44 (ii) Penmon Area 50 (iii) Straitside Area 53 (iv) Age of the Traeth Bychan Limestone Formation 54 (f) Red Wharf Cherty Limestone Formation 5 5• (i) Principal Area 55 (ii) Age of the Red Wharf Che.rty Limestone Format ion 57 2.5 REGIONAL CORRELATION 57 Page CHAPTER THREE - PALAEOKARSTIC SURFACES AND ASSOCIATED PHENOMENA 3.1 INTRODUCTION 61 3.2 MORPHOLOGY OF PALAEOKARSTIC SURFACES 62 (a) Description 62 (b) Interpretation 64 (c) Channels 68 (i) Description 68 (ii) Interpretation 69 (d) Sandstone pipe horizons 71 (i) General Description 71 (ii) Locality Details 72 80 (iii) Interpretation 3.3 BENTONITIC PALAEOSOLS 83 (a) Description 83 (b) Palaeosol Geochemistry 84 85 (c) Interpretation 3.4 ALTERATION PHENOMENA OF THE HOST LIMESTONES 88 (a) Introduction 88 (b) Palaeokarstic profiles 90 (c) Laminated crusts 91 (i) Description 91 (ii) Interpretation 94 (d) Rhizoliths 99 (i) Description 99 (ii) Interpretation 101 (e) Caicrete Ooids 105 (i) Description 105 (ii) Interpretation 106 (f) Brecciation textures 108 (i) Description 108 (ii) Interpretation 109 (g) Recrystallisation effects 111 (1) Growths of micro- and pseudospar description ill (ii) Intpretation 113 (iii) Needle crystal spherulites description 117 (iv) Interpretation 118 (h) Early cementation 121 3.5 OTHER MISCELLANEOUS FEATURES OF PALAEOKARSTIC HORIZONS 123 (a) Collapse Structures 123 (b) Borings 125 CHAPTER FOUR - SANDSTONES 4.1 INTRODUCTION 126 4.2 DISTRIBUTION 127 4.3 SETTING 127 4.4 LITHOFACIES 127 (a) Lithofacies A 128 (i) Description 128 (ii) Interpretation 130 (b) Lithofacies B 131 (1) Description 131 (ii) Interpretation 133 Page (c) Lithofacies C 134 (1) Description 134 (ii) Interpretation 136 4.5 CHANNEL SANDSTONE S 137 (a) Benilech Sandstone 137 Ci) Section Details 137 (ii) Environmental Interpretation Lithofacies A 143 (iii) Environmental Interpretation Lithofacies B 145 (iv) Environmental Interpretation Lithofacies C 152 (b) Helaeth Sandstone 154 (i) Description of Western Exposures Conglomerates and Related Lithofacies 154 (ii) Environmental Interpretation for the Western Exposures of the Helaeth Sandstone 157 (iii) Description of Eastern Cliffs : Calcareous Sandstones and Related Lithofacies 160 (iv) Environmental Interpretation for Eastern Cliffs Section 164 Cc) Other Channel Sandstones 173 Ci) Pedolau Sandstone 173 (ii) Aber Sandstone 173 4.6 SHEET SANDSTONES 173 (a) Description 173 Environmental Interpretation (b) 174 4.7 BASAL SANDSTONES 174 (a) Lligwy Sandstone Formation and other Basal Sandbodies of the Principal Area 175 Ci) Description 175 (ii) Environmental Interpretation 178 (b) Lligwy BaySandstone 180 (i) Descjiption 180 (ii) Environmental Interpretation 183 4.8 SILICLASTIC DEPOSITION DURING THE ANGLESEY DINANTIAN 185 CHAPTER FIVE - FACIES ANALYSIS OF THE LIMESTONE SEQUENCE 5.1 INTRODUCTION 187 5.2 LIMESTONE CLASSIFICATION 187 5.3 GRAIN TYPES 188 (a) Skeletal Grains 189 (b) Peloids 190 (c) Ooids 191 Cd) Oncoids 191 Ce) Intraclasts 191 (f) Compound Grains 191 5.4 DIAGENESIS 192 (a) Pressure Solution 192 (b) Recrystallisation 193 Cc) Cementation 194 Cd) Dolomitisation 194 Ce) Silicification 195 Page 5.5 CARBONATE LITHOFACIES 196 (a) Lithofacies I : Argillaceous Skeletal Wackestone/ Packstones and Intercalated Shales 197 (1) Description 197 (ii) Subfacies Ia : Shales with Limestone Nodules 199 (iii) Interpretation 199 (b) Lithofacies II Skeletal Packstone/Grajnstones 202 (i) Description 202 (ii) Interpretation 205 (iii) Subfacies ha : Intraclast Skeletal Packstone/Crainstones 207 (iv) Subfacies ha : Comparison with Modern Analogues 208 (c) Lithofacies III : Skeletal Peloid Grainstones 209 (i) Description 209 (ii) Interpretation 211 (iii) Subfacies lila : Thinly Bedded Skeletal Crainstones with Shaly Partings 213 (iv) Interpretation 215 (d) Lithofacies IV : Ooid, Skeletal Grainstones 216 (1) Description 216 (ii) Ooid Diagenesis : A Review 218 (iii) Implications for the Chemistry of Ancient Seas 221 (iv) Environmental Significance of Ooids and Oolitic Deposits 222 (e) Lithofacies V : Calcite Mudstones 224 (i) Description 224 (ii) Interpretations 226 5.6 FACIES ANALYSIS 227 (a) Careg-onen Formation 228 (b) Fades Analysis of Minor Cycles 231 (c) Flagstaff Formation 235 (i) Lower Groups of Cycles (Fl to F3, Lligwy to,Forllwyd Beds) 235 (ii) Cycle F4 (Moryn Beds) 237 (iii) Higher Group of Cycles (F5, F6; Pedolau and Royal Charter Beds) 238 (d) Moelfre Formation 239 (e) Traeth Bychan Formation 242 (f) Red Wharf Formation 245 CHAPTER SIX - SUMMARY AND CONCLUSIONS 6.1 SUMMARY 247 6.2 GROSS TRENDS 248 6.3 CAUSATIVE MECHANISMS 251 Acknowledgements 255 References 257 ABSTRA T The Anglesey Dinantian succession records the establishment and growth of a land-attached carbonate platform. Onlap of underlying basement terrain and marginal terrigenoclastic accumulations is readily demonstrated. Carbonate deposition however was repeatedly interrupted, palaeokarstic surfaces recording the periodic lowering of sea level and the emergence of extensive areas of the limestone shelf. The sequence is constructed therefore of numerous transgressive/ regressive minor cycles. These have been grouped together into five broader, lithostratigraphically based formations. During periods of raised sea level active carbonate production was achieved in a mosaic of fades of variable but not pronounced water depth. During periods of marine regression and emergence the carbonate platform was subject to the effects of subaerial weathering. Wind-blown volcanic ash accumulated on the exposed limestone surfaces and was colonised and stabilised by vegetation. Complex pedogenic alteration effects were promoted in the underlying carbonate sediment whilst dissolution beneath such soil covers led to the distinctive huninocky topography of palaeokarstic surfaces. The lowering of erosive base level during regressive periods also resulted in the rejuvination of siliciclastic source areas within the adjacent hinterland of older rocks. Marginal alluvial fans prograded onto the emergent shelf and rivers incised complex channel systems. Beyond the marginal fans transportation and deposition of terrigenous sediment appears to have been largely confined to such channels. Marine transgressions saw the drowning of the channel complexes and the shutting down of terrigenous supply, and culminated in the inundation of palaeokarstic levels and the re-establishment of carbonate facies mosaics on the shelf. 1 CHAPTER ONE GENERAL INTRODUCTION 1.1 AIMS AND APPROACH The Anglesey Lower Carboniferous (Dinantian) is a mixed sedimentary sequence of carbonate and siliciclastic rocks attaining a maximum thick- ness of 280 m. Hitherto work on these strata has been undertaken from a largely stratigraphic point of view trying to place the succession within broad zonal schemes and to suggest correlations with Lower Carboniferous sequences outcropping on the North Wales mainland. These previous studies (Morton, 1901; Greenly, 1919; Nichols, 1962 and Mitchell, 1964) were made without a detailed understanding of recent sedimentary environments particularly in the carbonate field. Consequently ideas on the depositional environments in which these rocks were formed are restricted to general comments concerning depth of water, energy conditions, proximity to shoreline and climate. The present study initially steered away from the wider stratigraphic aspects and applied a more sedimentological approach. Taking advantage of the great volume of recent literature detailing recent carbonate and siliciclastic deposits, this provided an opportunity to present a more detailed environmental interpretation for the sedimentary sequences encountered. A complimentary aim was to assess sediment/fauna relationships for the various lithofacies within the succession. The project