A Formational Framework for the Mercia Mudstone Group (Triassic) of England and Wales
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A formational framework for the Mercia Mudstone Group (Triassic) of England and Wales National Geoscience Framework Programme A S Howard, G Warrington, K Ambrose and J G Rees HOW TO NAVIGATE THIS DOCUMENT Bookmarks The main elements of the table of contents are bookmarked enabling direct links to be followed to the principal section headings and sub-headings, figures, plates and tables irrespective of which part of the document the user is viewing. In addition, the report contains links: from the principal section and subsection headings back to the contents page, from each reference to a figure, plate or table directly to the corresponding figure, plate or table, from each figure, plate or table caption to the first place that figure, plate or table is mentioned in the text and from each page number back to the contents page. RETURN TO CONTENTS PAGE British Geological Survey Research Report RR/08/04 BRITISH GEOLOGICAL SURVEY RESEARCH REPOrt RR/08/04 The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty’s Stationery Office. Licence No: 100017897/ 2008. Subject index A formational framework for Geology, stratigraphy, lithostatigraphy, Mercia Mudstone Group, Triassic the Mercia Mudstone Group Front cover (Triassic) of England and Wales Red Mercia Mudstone Group is faulted against Blue Anchor Formation and Rhaetic strata, Blue Anchor, Somerset (P211340). National Geoscience Framework Programme A S Howard, G Warrington, K Ambrose and J G Rees Geographical index England and Wales Bibiographical Reference HOWARD , A S, WARRINGTON , G, AMBRO S E , K, AND REE S , J G. 2008. A formational framework for the Mercia Mudstone Group (Triassic) of England and Wales. British Geological Survey Research Report, RR/08/04. Copyright in materials derived from the British Geological Survey’s work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail [email protected]. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract. Your use of any information provided by the British Geologi- cal Survey (BGS) is at your own risk. Neither BGS nor the Natural Environment Research Council gives any warranty, condition or representation as to the quality, accuracy or completeness of the information or its suitability for any use or purpose. All implied conditions relating to the quality or suitability of the information, and all liabilities arising from the sup- ply of the information (including any liability arising in negligence) are excluded to the fullest extent permitted by law. ISBN 0 85272 577 1 © NERC 2008. All rights reserved Keyworth, Nottingham British Geological Survey 2008 British Geological Survey Research Report RR/08/04 BRITISH GEOLOGICAL SURVEY ACKNOWLEDGEMENTS The full range of our publications is available from BGS shops at This review was commissioned by the BGS Stratigraphy Com- Nottingham, Edinburgh, London and Cardiff (Welsh publications mittee under the Chairmanship of P M Allen. It is based on a syn- only) see contact details below or shop online at www.geologyshop. thesis of new stratigraphical information on the Merca Mudstone com Group acquired since the publication, in 1980, of the Geological Society of London Special Report ‘A correlation of Triassic The London Information Office also maintains a reference rocks in the British Isles’ (Warrington, 1980). During this period, collection of BGS publications, including maps, for consultation. numerous BGS geological survey projects have acquired new We publish an annual catalogue of our maps and other publications; knowledge of the Mercia Mudstone Group succession in England this catalogue is available online or from any of the BGS shops. and Wales, and many BGS colleagues provided us with informa- tion and expertise to assist with this review. Among these, we The British Geological Survey carries out the geological survey of particularly wish to thank H C Ivimey-Cook, D McC Bridge, T J Great Britain and Northern Ireland (the latter as an agency service Charsley, N S Jones, J N Carney, S Holloway and R A Ellison for for the government of Northern Ireland), and of the surrounding information on local Triassic successions, advice on geophysical continental shelf, as well as basic research projects. It also log correlation, and for comments on earlier drafts of the report. undertakes programmes of technical aid in geology in developing The report has been edited by A A Jackson and has benefited countries. from the comments of former and present members of the BGS The British Geological Survey is a component body of the Natural Stratigraphy Committee, notably J H Powell and R W O’B Knox, Environment Research Council. and from external revies by J C W Cope and A Rufell on behalf of the Stratigraphy Commission, Geological Society of London. British Geological Survey offices The Geophysical log of the Wiscombe Park 2 borehole is pub- lished with the kind permission of British Gypsum plc. Cartogra- BGS Central Enquiries Desk phy in this report is by I Longhurst. Tel 0115 936 3143 Fax 0115 936 3276 email [email protected] Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG Tel 0115 936 3241 Fax 0115 936 3488 email [email protected] Murchison House, West Mains Road, Edinburgh EH9 3LA Tel 0131 667 1000 Fax 0131 668 2683 email [email protected] London Information Office at the Natural History Museum (Earth Galleries), Exhibition Road, South Kensington, London SW7 2DE Tel 020 7589 4090 Fax 020 7584 8270 Tel 020 7942 5344/45 email [email protected] Columbus House, Greenmeadow Springs, Tongwynlais, Cardiff CF15 7NE Tel 029 2052 1962 Fax 029 2052 1963 Maclean Building, Crowmarsh Gifford, Wallingford OX10 8BB Tel 01491 838800 Fax 01491 692345 Geological Survey of Northern Ireland, Colby House, Stranmillis Court, Belfast BT9 5BF Tel 028 9038 8462 Fax 028 9038 8461 www.bgs.ac.uk/gsni/ Parent Body Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU Tel 01793 411500 Fax 01793 411501 www.nerc.ac.uk Website www.bgs.ac.uk Shop online at www.geologyshop.com British Geological Survey Research Report RR/08/04 Contents 1 Introduction 1 FIGURES 1.1 Tectonic and depositional setting 1 1.2 Stratigraphical overview 1 1 Outcrop of the Mercia Mudstone Group 23 1.3 Approach to the review 2 2 Summary of current Mercia Mudstone Group Lithostratigraphy 24 2 Review of existing nomenclature 3 3 Geophysical log correlation of the Mercia Mudstone 2.1 Distribution of the Mercia Mudstone Group 3 Group in the Wessex Basin 25 2.2 Stratigraphical framework 3 4 Geophysical log correlation of the upper part of the 2.3 Recommendations for rationalising Mercia Mudstone Group 26 Mercia Mudstone lithostratigraphy 6 5 South-west England lithostratigraphy 27 3 Revised lithostratigraphical framework 7 6 Worcester and Knowle basins lithostratigraphy 28 3.1 Revised formation nomenclature 7 7 Needwood and Stafford basins lithostratigraphy 29 3.2 Regional lithostratigraphy and subdivisions 7 8 East Midlands Shelf lithostratigraphy 30 9 Cheshire Basin lithostratigraphy 31 4 Formal definitions and diagnoses of formations 9 10 West Lancashire lithostratigraphy 32 4.1 Mercia Mudstone Group 9 11 East Irish Sea and North Sea basins 4.2 Tarporley Siltstone Formation 9 lithostratigraphy 33 4.3 Sidmouth Mudstone Formation 11 4.4 Arden Sandstone Formation 13 4.5 Branscombe Mudstone Formation 14 4.6 Blue Anchor Formation 16 References 18 Appendix List of boreholes 22 iii British Geological Survey Research Report RR/08/04 Summary This Research Report presents a reviewed and revised Branscombe Mudstone Formation and Blue Anchor lithostratigraphical nomenclature for the Mercia Mudstone Formation. Group (Triassic) of England and Wales. Irrespective of Local synonyms of these newly defined formations existing nomenclature, five units that were in stratigraphical are abandoned in this revised scheme. In some regions, continuity prior to any subsequent tectonism or erosion these newly defined units may encompass two or more are identified as constituting the group. These units are subdivisions that were formerly defined as formations. formations as defined by international and UK codes of In such cases, the former ‘formations’ are downgraded stratigraphical practice (North American Commission on to member status, with the newly defined formation as Stratigraphic Nomenclature, 1983: 2005; Whittaker et al., their parent stratigraphical unit. Informal nomenclature is 1991; Rawson et al., 2002). Where present, the five units retained for some highly discontinuous, locally distrbuted are consistently mappable at outcrop using conventional units at basin margins, where correlation with adjacent geological surveying techniques, and are recognisable formations is uncertain. in the subsurface in both borehole core and geophysical The report includes formal definitions of the Mercia logs. The various local lithostratigraphical names formerly Mudstone Group and its five component formations. applied to these mappable subdivisions are replaced by a Definitions of members, beds and informal subdivisions single formation name for each unit. These are, in upward within these formations are given in the BGS Lexicon of succession, the Tarporley Siltstone Formation,