The Dalradian Rocks of the South-West Grampian Highlands of Scotland
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Revised Manuscript 8/7/2012 Click here to view linked References 1 2 3 4 5 The Dalradian rocks of the south-west Grampian 6 7 Highlands of Scotland 8 9 P.W.G. Tanner, C.A. Bendall, E.A. Pickett, J.L. Roberts, J.E. 10 11 Treagus and D. Stephenson 12 13 P.W. Geoff Tanner Department of Geographical and Earth Sciences, 14 University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow 15 G12 8QQ. 16 Charles A. Bendall Institute of Geography and Earth Science, 17 Aberystwyth University, Aberystwyth SY23 3DB. 18 Elizabeth A. Pickett North Pennines AONB Partnership, Weardale 19 Business Centre, The Old Co-op Building, 1 Martin Street, Stanhope, 20 Weardale, Co. Durham DL13 2UY; formerly British Geological Survey, 21 Edinburgh. 22 John L. Roberts (deceased) formerly Department of Geology, 23 University of Newcastle-upon-Tyne. 24 Jack E. Treagus 15 Raynham Avenue, Didsbury, Manchester M20 6BW; 25 formerly Department of Earth Sciences, University of Manchester. 26 27 * David Stephenson British Geological Survey, Murchison House, 28 West Mains Road, Edinburgh EH9 3LA. 29 [email protected] 30 0131 650 0323 31 32 * Corresponding author 33 34 Keywords: 35 Geological Conservation Review 36 South-west Grampian Highlands 37 Dalradian Supergroup 38 Lithostratigraphy 39 Structural geology 40 Metamorphism 41 42 43 ABSTRACT 44 45 The South-west Grampian Highlands, as defined here, include the 46 Inner Hebridean islands of Islay and Jura, and extend north-east as 47 far as Dalmally at the northern tip of Loch Awe. Due to a 48 favourable combination of excellent coastal exposures and low 49 tectonic strain, the late-Neoproterozoic rocks of the Dalradian 50 Supergroup in this region are ideal for studying sedimentary 51 structures. In addition, the diversity in protolith lithology from 52 carbonate rocks to siliciclastic rocks of all grain sizes and 53 volcanic rocks makes it possible to establish a very detailed 54 lithostratigraphical succession and to recognize lateral facies 55 changes. The stratigraphical range extends from the base of the 56 Appin Group to the base of the Southern Highland Group and the area 57 provides type localities for many regionally extensive formations 58 of the Argyll Group. Rocks forming part of the basement to the 59 Dalradian basins, the Rhinns Complex, are seen on Islay, where they 60 61 62 63 64 65 1 2 3 4 are overlain by the Colonsay Group, a thick metasedimentary 5 siliciclastic sequence of uncertain stratigraphical affinity. 6 The structure of the Dalradian rocks in the South-west Grampian 7 Highlands is controlled by early (D1) major folds (Islay Anticline, 8 Loch Awe Syncline, and Ardrishaig Anticline), associated with a 9 ubiquitous, penetrative, slaty or spaced cleavage. Most of the 10 Dalradian rocks have been regionally metamorphosed under 11 greenschist-facies conditions and amphibolite-facies (garnet zone) 12 assemblages occur only in a narrow central zone, strongly affected 13 by the D2 deformation. 14 The area provides GCR sites of international importance for 15 studying Neoproterozoic glacial deposits, splendidly preserved 16 stromatolite bioherms and calcite pseudomorphs after gypsum. 17 Deformed and undeformed sandstone dykes and interstratal dewatering 18 structures are well displayed at several sites. Other features 19 include thick sills of basic meta-igneous rock with unusual 20 minerals such as stilpnomelane, and greenschist-facies rocks 21 containing regional metamorphic kyanite. The area is of historical 22 interest for the first recognition in Scotland, prior to 1910, of 23 sedimentary way-up structures and pillow lavas in regionally 24 deformed and metamorphosed rocks. 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 5 1 INTRODUCTION 6 7 P.W.G. Tanner 8 9 10 The South-west Grampian Highlands region, as defined in this paper, 11 12 includes the islands of Islay, Jura, and the Garvellach Isles in 13 the north-west. It is bounded to the south-east by the base of the 14 Southern Highland Group, which runs from Campbeltown along the 15 Kintyre Peninsula and the south-east side of Loch Fyne, to Ben Lui 16 (Figure 1). The north-eastern boundary follows the lower part of 17 Loch Etive and the A85 road to Dalmally and Ben Lui. The region 18 has an extremely long, indented coastline that faces into the 19 prevailing south-west wind, which results in many kilometres of 20 clean, well-scoured, coastal rock exposures being available for 21 detailed study. Thus, 19 out of 21 of the GCR sites reported here 22 are on coastal exposures. 23 The primary survey of the South-west Grampian Highlands was begun 24 in 1880 and culminated in the publication of ‗The Geology of the 25 Seaboard of Mid-Argyll‘ (Peach et al., 1909). The Geological 26 Survey memoirs and accompanying geological maps produced during 27 this period remain the sole source of reference to the distribution 28 of rock types, and their petrography, for a considerable part of 29 30 the region. This work was carried out whilst major advances were 31 being made in structural geology and sedimentology worldwide, but 32 much of this work came too late to help define and resolve some of 33 the more fundamental problems in Highland geology. For example, 34 the stratigraphical sequences established by the early workers such 35 as Green (1924), Hill (1899, 1909) and Wilkinson (1907) were later 36 shown to be wrong, as these geologists did not have the tools to 37 identify way-up, and based their interpretations on Uniformitarian 38 principles, such as the Law of Superposition, and the dip 39 direction. This approach was successful in areas with simple 40 upright, open folds but obviously failed in situations where the 41 rocks had, for example, already been inverted by regional-scale 42 folding. As a result, some parts of the stratigraphical sequence 43 had to be revised when way-up techniques were first applied (Vogt, 44 1930; Bailey, 1930; Allison, 1933). 45 Despite the progress made by Pumpelly et al. (1894) in 46 interpreting bedding/cleavage relationships in the USA, and by 47 48 Clough (in Gunn et al., 1897) in recognizing the effects of 49 polyphase deformation in the Cowal peninsula, there is no 50 indication from the work published prior to 1909 that these 51 techniques were used in the areas described in this paper. The 52 overall structure of this region was finally established by Bailey 53 (1922); this framework has not been superseded but was 54 progressively modified as modern techniques of structural geology 55 were applied from the late 1950s onwards (i.e. Shackleton, 1958; 56 Knill, 1960; Rast, 1963; Borradaile, 1970, 1973; Roberts, 1974). 57 The Dalradian rocks of this region, with their great diversity in 58 lithology, relatively simple structure and low-grade regional 59 metamorphism, have played a very important part in establishing the 60 overall stratigraphical sequence in the Grampian Highlands as a 61 62 63 64 65 1 2 3 4 whole. All subgroups of the Argyll Group are named after 5 localities in this region, and these encompass a wide range of 6 metamorphosed rock types from tillites to carbonate rocks and basic 7 igneous rocks, and mudrocks to conglomerates. The Argyll Group 8 includes two important marker horizons: the Port Askaig Tillite 9 Formation at the base, and the Loch Tay Limestone Formation at the 10 top (see Stephenson et al., 2013a for discussion) (Figure 2). 11 12 The environment in which the Argyll Group was deposited shows 13 increasing tectonic instability with time (Anderton, 1985). 14 Syndepositional basin-bounding faults became increasingly active 15 throughout this period, as witnessed by marked lateral variations 16 in the thickness and facies of both members and formations, 17 together with the incoming and increasing frequency of debris flows 18 and coarse-grained turbidite-facies rocks. Sediments of the Islay 19 Subgroup were deposited in shallow water, some even in the 20 intertidal zone, as indicated by storm deposits in the Jura 21 Quartzite Formation, and pseudomorphs after gypsum in the Craignish 22 Phyllite Formation. Deepening of the basin in Easdale Subgroup 23 times is indicated by the deposition of a considerable thickness of 24 black, euxinic mudrock, which was followed by a thick sequence of 25 coarse-grained quartzofeldspathic turbidites in Crinan Subgroup 26 times. Volcanicity reached a peak during Tayvallich Subgroup times 27 as the now sediment-starved basin subsided further and the 28 underlying lithosphere thinned and finally ruptured. 29 30 In view of the possibility that one or more orogenic 31 unconformities is present in the Dalradian rocks of the South-west 32 Grampian Highlands (Dempster et al., 2002; Hutton and Alsop 2004; 33 but see Tanner, 2005), emphasis in this paper is placed upon 34 relationships between the various stratigraphical units, and 35 especially on the nature of the contacts between subgroups, 36 including the critical junction between the Jura Quartzite and the 37 Easdale Slate. 38 The overall structure of the South-west Grampian Highlands is 39 controlled by two upward-facing major folds, the Islay Anticline 40 and the Loch Awe Syncline, both of D1 age. The axial plane of the 41 Islay Anticline dips to the south-east, and the fold faces up to 42 the north-west, whereas the Loch Awe Syncline is an upright, 43 symmetrical structure. The Dalradian rocks are least deformed 44 along the western seaboard and, in areas of particularly low strain 45 such as on Islay and Jura, many of the original sedimentary 46 features are preserved.