Revision of the solid geology shown on the ‘Assynt District’ special geological map. A report on the 2002 fieldwork Integrated Geoscience Surveys (North) Internal Report IR/03/058 BRITISH GEOLOGICAL SURVEY INTERNAL REPORT IR/03/058 Revision of the solid geology shown on the ‘Assynt District’ special geological map. A report on the 2002 fieldwork M Krabbendam, K M Goodenough, R M Key, A G Leslie, S C Loughlin The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty’s Stationery Office. Ordnance Survey licence number GD 272191/2003 Key words Moine Thrust, thrust, gneiss, Lewisian Gneiss. Bibliographical reference KRABBENDAM, M, GOODENOUGH, K M, KEY, R M, LESLIE, A G AND LOUGHLIN, S. C . 2003. Revision of the solid geology on the ‘Assynt District’ special geological map. A report on the 2002 fieldwork. British Geological Survey Internal Report,IR/03/058. 37pp. © NERC 2003 Keyworth, Nottingham British Geological Survey 2003 IR/03/058:Version 0.2 Last modified: 31/03/2003 13:00 BRITISH GEOLOGICAL SURVEY The full range of Survey publications is available from the BGS Keyworth, Nottingham NG12 5GG Sales Desks at Nottingham and Edinburgh; see contact details 0115-936 3241 Fax 0115-936 3488 below or shop online at www.thebgs.co.uk e-mail: [email protected] The London Information Office maintains a reference collection www.bgs.ac.uk of BGS publications including maps for consultation. 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Forde House, Park Five Business Centre, Harrier Way, Sowton, Exeter, Devon EX2 7HU 01392-445271 Fax 01392-445371 Geological Survey of Northern Ireland, 20 College Gardens, Belfast BT9 6BS 028-9066 6595 Fax 028-9066 2835 Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB 01491-838800 Fax 01491-692345 Parent Body Natural Environment Research Council, Polaris House, North Star Avenue, Swindon, Wiltshire SN2 1EU 01793-411500 Fax 01793-411501 www.nerc.ac.uk 3131iiii ii IR/03/058:Version 0.2 Last modified: 31/03/2003 13:00 Foreword This report succinctly describes the main conclusions regarding the solid geology of the Assynt District based on fieldwork carried out for the Moine Thrust Project in the summer of 2002. The aim of the fieldwork was to revise, rather than resurvey, the solid geology of the Assynt Special Sheet (which includes parts of Sheets 107E, 108W, 101E, 102W). Reports concerning the survey of the drift, and the minor intrusions within the Assynt half-window, will be issued separately. Contents Foreword ...................................................................................................................................... iii Contents........................................................................................................................................ iii Summary .......................................................................................................................................vi 1 Introduction: aims and strategy of fieldwork ......................................................................1 1.1 Previous Work ................................................................................................................1 1.2 Lithology and Stratigraphy .............................................................................................1 1.3 Area covered during Summer 2002 Survey....................................................................2 2 Mapping in the Assynt half-window.....................................................................................4 2.1 Course of Ben More Thrust ............................................................................................4 2.2 Internal structure of the Ben more thrust sheet...............................................................6 2.2.1 Coire a’ Mhadhaidh Detachment.................................................................................6 2.2.2 Imbricates in the Loch an Eircill–Loch nan Caorach area [NC 29 27–NC 30 26]......7 2.2.3 Cnoc Chaornaidh [NC 30 08]......................................................................................7 2.2.4 Glencoul [NC 29 28] ...................................................................................................7 2.3 The southern termination of the Glencoul Thrust [NC 26 22-NC 28 21].......................8 2.4 Various areas within THE Glencoul and Sole Thrust sheets..........................................9 2.4.1 Glas Bheinn area..........................................................................................................9 2.4.2 Cnoc an Droighinn [~NC 265 235] .............................................................................9 2.4.3 Beinn an Fhurain (north) [~NC 28 21]........................................................................9 2.4.4 Cnoc nan Uamh Thrust [~NC 27 20] ........................................................................10 2.4.5 Beinn an Fhuarain (south); south of Allt nam Uamh [~NC 26 15]...........................10 3 Minor Intrusions...................................................................................................................10 4 Moine .....................................................................................................................................11 4.1 Coire Cean Loch–Corriekinloch–Loch Merkland ........................................................11 5 Lewisian Gneiss Complex ....................................................................................................12 3131iiiiii iii IR/03/058: Version 0.3 Last modified: 15/07/2003 11.00 5.1 Introduction...................................................................................................................12 5.2 Lithologies noted during the 2002 mapping .................................................................12 5.3 Deformation and metamorphism ..................................................................................16 5.3.1 Introduction ...............................................................................................................16 5.3.2 Our observations........................................................................................................16 5.3.3 Canisp Shear Zone and Lochinver Monocline ..........................................................17 5.3.4 Brittle deformation ....................................................................................................17 5.4 Overview of the evolution of the Lewisian Gneiss complex in Assynt........................17 5.5 Lewisian inliers in the ben more and glencoul Thrust Sheets ......................................19 5.5.1 Present work ..............................................................................................................19 6 Torridonian...........................................................................................................................20 7 References .............................................................................................................................20 Figures 1.1: Overview topographical map of the Assynt area. 1.2: Areas resurveyed during the 2002 field season. 1.3: Overview of geology of the Assynt District special geological map. 2.1: Geological map showing different interpretations of the trace of the Ben More Thrust. 2.2: Field slips of the area west and north of Ben Uidhe, showing newly mapped course of the Ben More Thrust. 2.3: View along ‘Glen Beag’ (unnamed glen to the south of the Stack of Glencoul) to Beinn Uidhe showing folded quartzite in background and truncation of quartzite syncline against fault in foreground. 2.4: View of east face of Na Tuadhan, with position of Coire a’ Mhadaidh Detachment. 2.5: Sketch cross-sections showing internal structure of the Ben More Thrust Sheet and the relationship of the Ben More Thrust and the Coire a’ Mhadaidh Detachment. 2.6: Contact between Lewisian gneisses and Basal Quartzite within the Ben More Thrust Sheet. 2.7: Schematic north–south, down-plunge section from the Stack of Glencoul to Beinn Uidhe, taken approximately perpendicular to the transport direction. 2.8: View southwards into Coire Dearg on northern side of Glas Bheinn. 5.1: Lewisian migmatites exposed in a road cut on the north side of Loch Assynt (P517131). 5.2: Lewisian migmatites (agmatites) exposed on rock pavements on the north side of Loch Assynt (P517134). 5.3: Lewisian migmatites exposed on rock pavements on the north side of Loch Assynt (P517135). 5.4: Granitic vein network in Lewisian hornblende-gneiss west of Quinag