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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. There it is also possible to examine the 47 48 earliest tectonic structures in their pristine state. In over 70% 49 of the GCR sites included in this paper, the rocks have been 50 affected by only a single major ductile tectonic event (D1), 51 followed by weak, late-stage deformations. The Dalradian rocks at 52 two of these GCR sites (Caol Isla and Craignish Point) appear in 53 the field to be almost undeformed, and only two of the remaining 54 sites show the full effects of polyphase deformation (Black Mill 55 Bay and Port Cill Maluaig). The metamorphic grade throughout the 56 area is generally of greenschist facies, with some garnet-bearing 57 epidote-amphibolite-facies rocks occurring along the south-east 58 margin. 59 In this paper, the GCR site reports are arranged in 60 stratigraphical sequence with the oldest first, but there are some 61 62 63 64 65 1 2 3 4 anomalies where either subject matter or geographical location 5 takes precedence. Of the 21 site reports, three are concerned with 6 contemporaneous igneous activity (dykes and sills); two record the 7 occurrence of minerals not normally found in Dalradian rocks that 8 are unusual for their form (gypsum), or are in some way unique in 9 their particular setting (kyanite and stilpnomelane); two deal 10 largely with water-escape structures and clastic dykes, 11 12 respectively; and the remaining 14 are focussed on stratigraphical 13 and structural aspects. Examples of innovatory studies in the 14 region include the first uses in the United Kingdom of sedimentary 15 cross-bedding and graded bedding in metasedimentary rocks to 16 determine the younging direction (see the Kilmory Bay GCR site 17 report). 18 19 1.1 The pre-Dalradian basement and units of uncertain 20 stratigraphical affinity 21 22 The Dalradian Supergroup is not seen in contact with its basement 23 anywhere in the South-west Grampian Highlands but the 24 Palaeoproterozoic Rhinns Complex on Islay almost certainly 25 represents at least the local basement. Intervening structurally 26 between the Rhinns Complex and rocks of undoubted Dalradian 27 affinity on Islay are two groups of Dalradian-like rocks, the 28 Colonsay Group and the Bowmore Sandstone Group. Both are rather 29 30 monotonous sequences of metasandstone and lack any specific 31 characteristics that would help to confirm their Dalradian 32 identity. The Bowmore Sandstone Group, described in the Bun-an- 33 uilt, Islay CGR site report, is separated from the Dalradian proper 34 by the Loch Skerrols Thrust and might be equivalent to the Crinan 35 Grit Formation, described below (Fitches and Maltman, 1984). The 36 Colonsay Group has a tectonized, unconformable contact with the 37 Rhinns Complex, exposed in the Kilchiaran to Ardnave Point GCR 38 site, but it cannot be correlated directly with the Dalradian 39 sequence; it has been compared to the Grampian Group and possibly 40 the lower part of the Appin Group (Stephenson and Gould, 1995). U- 41 Pb ages on detrital zircons from the Colonsay Group strongly 42 support its correlation with the Grampian Group, and help to 43 confirm that the Rhinns Complex is part of the basement to the 44 Dalradian (McAteer, et al., 2010). See Stephenson et al. (2013a) 45 for more detailed discussions of these units and their possible 46 affinities. 47 48 49 1.2 Dalradian stratigraphy 50 51 Appin Group rocks are only exposed in this region on the Isle of 52 Islay, within the core of the Islay Anticline (Rast and Litherland, 53 1970; Basahel, 1971; Wright, 1988; see also BGS 1:50 000 sheets 19, 54 South Islay, 1998 and 27, North Islay, 1994). Strata of all three 55 subgroups are present, and show a remarkable similarity to the 56 mainland succession, some 80 km to the north-east along strike (see 57 Treagus et al., 2013). However, they are poorly exposed and are 58 not represented by any GCR sites. 59 The Lochaber Subgroup on Islay is divided into two units. The 60 lower, Maol an Fhithich Quartzite Formation, consists of massive, 61 62 63 64 65 1 2 3 4 cross-bedded quartzites with phyllitic metamudstones and pebble 5 beds containing extrabasinal granite clasts. The overlying Glen 6 Egedale Slate Formation is composed of striped greenish, phyllitic 7 or slaty metasiltstones that become more calcareous upwards. The 8 Ballachulish Subgroup consists of the Kintra Dolostone Formation, 9 Mulindry Bridge Slate Formation, Cnoc Donn Quartzite Formation and 10 the Neriby Formation, and can be matched confidently with the type 11 12 succession of the Appin–Loch Leven area, with little variation in 13 facies. In the Blair Atholl Subgroup, the Ballygrant Formation, 14 consisting of dark grey, slaty and phyllitic graphitic 15 metamudstones, followed by a bluish grey metalimestone, can be 16 equated confidently with the Cuil Bay Slate and Lismore Limestone. 17 Owing to the south-westerly plunge of the major folds in the Appin– 18 Lismore area, any higher beds of the Blair Atholl Subgroup that may 19 have been deposited there lie beneath the Firth of Lorn, but an 20 extended sequence is present on Islay. There, the Ballygrant 21 Formation is overlain by dark grey phyllitic metamudstones with 22 graded metasandstone or calcareous beds, the Mullach Dubh Phyllite 23 Formation, and a distinctive banded unit containing partly ooidal 24 and stromatolitic, thin-bedded metalimestones, the Lossit Limestone 25 Formation (formerly the Islay Limestone, Spencer, 1971), which is 26 overlain unconformably by the Port Askaig Tillite Formation. 27 The stratigraphical sequence covered by the GCR sites in this 28 paper begins with the at the base of 29 Port Askaig Tillite Formation 30 the Argyll Group, which is magnificently exposed in the Garvellach 31 Isles (Spencer, 1971) (Figure 2a). This GCR site is of 32 international importance as it is the best-preserved example of a 33 Precambrian tillite in the British Isles. The tillite is also 34 instrumental, because of its distinctive character and association 35 with a thick quartzite unit above, and a dolomitic unit below, in 36 forging a stratigraphical correlation between the Dalradian rocks 37 of Scotland and those of Connemara and Donegal (Kilburn et al., 38 1965). Currently, it plays a major role in the search for a 39 continuation of the Dalradian tract prior to the break-up of 40 Rodinia. This is despite the fact that the origin of the formation 41 is still in dispute, the precise age of deposition is not known, 42 and a debate as to whether it represents a ‗Snowball Earth‘ 43 situation has ensued (Dempster et al., 2002). 44 The important features of tillite formation include the ‗Great 45 Breccia‘ and the neighbouring ‗Disturbed Beds‘, which occur in a 46 well-documented sequence of 47 metadiamictites. Within the 47 48 metadiamictites there is a change from locally derived stones to an 49 incoming of foreign exotic stones at bed 12. The top of the 50 sequence can be examined north of Port Askaig on Islay, where a 51 transition to the overlying Bonahaven Dolomite Formation can be 52 seen at the Caol Isla GCR site. The current interpretation for its 53 origin is that the tillite was deposited sub-aqueously from 54 icebergs, and was then partly reworked by tidal currents. 55 The Bonahaven Dolomite Formation (Spencer and Spencer, 1972) was 56 interpreted by Fairchild (1993) to be the ‗cap carbonate‘ to the 57 Portaskaig Tillite and, apart from the lowest beds, consists almost 58 entirely of orange-yellow, Fe-rich dolomitic rocks. It is divided 59 into four members, the lowest of which is subdivided into five 60 units. The lowermost unit contains isolated boulders and has an 61 62 63 64 65 1 2 3 4 affinity with the tillite, and unit 2 consists of well-bedded 5 metasandstones with channel deposits. The metasandstone beds are 6 almost undeformed and contain mud cracks. Sedimentary dykes result 7 from the injection of water-saturated, over-pressured sand and silt 8 as dyke-like bodies into the adjoining sediments. Those at Caol 9 Isla have recently been interpreted as being the result of 10 interstratal dewatering, probably caused by earthquake activity 11 12 associated with the synsedimentary Bolsa Fault (Tanner, 1998a). The 13 Caol Isla GCR site also became famous, briefly, for the reported 14 presence of Neoproterozoic trace fossils by Brasier and McIlroy 15 (1998). However, this identification was withdrawn subsequently by 16 Brasier and Shields (2000). 17 The upper part of the Bonahaven Dolomite Formation is best exposed 18 along the north coast of Islay at the Rubha a’ Mhail GCR site. 19 There, distinctive stratigraphically controlled beds contain 20 stromatolites, which have either spherical or elliptical shapes up 21 to 3 m across, or occur as layer-form sheets. The rocks at this 22 GCR site are more deformed than those at Caol Isla but fine-grained 23 siliciclastic beds still preserve water-escape structures and small 24 clastic dykelets. In addition, there is possible evidence for the 25 former presence of anhydrite, a mineral that, like gypsum, is 26 characteristic of evaporite deposits. 27 The Jura Quartzite Formation is not represented by a GCR site, as 28 this unit crops out over a large area and is particularly well 29 30 exposed. It is a clean, locally feldspathic, cross-bedded and 31 cross-laminated quartzite deposited in a wave- and storm-dominated 32 environment (Anderton, 1976). Anderton (1979) suggested that 33 deposition took place in a series of fault-controlled basins as the 34 formation shows extreme lateral thickness changes from 5 km on Jura 35 to approximately 1000 km on Islay to the south-west, and thinning 36 to c. 100 m on Lunga to the north-east. 37 Over a distance of 110 km between Port Ellen, on Islay and 38 Benderloch to the north-east, the contact between the Jura 39 Quartzite and the overlying Scarba Conglomerate Formation at the 40 base of the Easdale Subgroup appears to be conformable, with no 41 evidence of an orogenic unconformity at this level in the Dalradian 42 succession (see the Camas Nathais CGR site report for further 43 discussion in Treagus et al., 2013). However, marked lateral 44 changes occur within the rock units immediately above this boundary 45 (Figure 2a). For example, the Jura Slate Member is 60 m thick in 46 the south-west at the Kilnaughton Bay GCR site on Islay, reaches a 47 48 maximum thickness of over 200 m a the Lussa Bay site on Jura 49 (Tanner 2005), and is absent from the north-east of Jura at the 50 Kinuachdrachd GCR site. The overlying Pebbly Sandstone Member is 51 not as well developed on Jura as in the type area on the Isle of 52 Scarba where individual blocks are on a metre scale (Anderton, 53 1979). At the Kinuachdrachd and Lussa Bay GCR sites on Jura the 54 clasts seldom exceed one to two centimetres across and are commonly 55 accompanied by rip-up clasts of mudstone. Individual beds commonly 56 show normal grading in units 1–2 m thick. Cross-lamination is 57 found in the tops of many beds, and slump structures, channelling, 58 bottom structures and erosional bases to beds are common. 59 The Easdale Slate Formation consists mainly of dark grey to black 60 graphitic slaty metamudstone, generally pyrite-rich, with pods and 61 62 63 64 65 1 2 3 4 bands of orange-brown-weathering dolostone, and some layers of 5 metasiltstone and metasandstone. The formation maintains the same 6 monotonous lithology from the island of Kerrera, near Oban, 7 southwards across the whole outcrop. The Black Mill Bay GCR site 8 displays examples of all of the main lithologies. In addition, at 9 this site there are black gritty metasandstones and a 4 m-thick 10 debris flow containing strongly elongated sandstone clasts. This 11 12 latter deposit is of the same type as that found at the Port Selma 13 GCR site (Treagus et al., 2013). A clastic dyke sourced by a 14 sandstone bed below the debrite flow was folded and cleaved during 15 D1. 16 On Islay and Jura, the Kilbride Limestone Member of the Port Ellen 17 Phyllite Formation that occurs above the Easdale Slates (Figure 18 2a), is equated with the Degnish Limestone Formation and its 19 equivalent, the Shuna Limestone (Figure 2b), on the mainland. 20 These metalimestones are correlated with the Cranford Limestone in 21 Donegal (Pitcher and Berger, 1976), which has recently been taken 22 as marking the plane of an orogenic unconformity within the 23 Dalradian block (Hutton and Alsop, 2004). To date, no evidence has 24 been reported from the South-west Grampian Highlands to support 25 this contention (Tanner, 2005). 26 The outcrop of the Port Ellen Phyllite Formation on the islands of 27 Islay and Jura lies in strike continuity, and is correlated with 28 the on the mainland (Figure 2b) (Hill, 29 Craignish Phyllite Formation 30 1879). The latter formation is well exposed at the Craignish Point 31 and Fearnach Bay GCR sites and is characterized by grey-green 32 calcareous phyllites with 1—2 m-thick beds of fine-grained 33 quartzite, and thinner bands of orange-brown-weathering 34 metacarbonate rock. Minor components include foliated sheets of 35 basic meta-igneous rock, generally sills, whose ‗volcanic‘ 36 association was first recognized by Peach (1903). Where they are 37 protected from subsequent deformation by thick basic sills, the 38 metasedimentary rocks preserve a unique twinned form of gypsum, now 39 pseudomorphed by calcite (Craignish Point GCR site), whose precise 40 palaeoenvironmental significance is not clear at present. 41 On Islay, the Laphroaig Quartzite Formation (Figure 2a) (see the 42 (Surnaig Farm GCR site report) marks the top of the Easdale 43 Subgroup. The Surnaig Farm GCR site is also an exceptional 44 locality for studying deformed clastic dykes. Over 30 dykes are 45 exposed in a small area; they were originally described by 46 Borradaile (1976) as Neptunian dykes but are re-interpreted here as 47 48 injected clastic material and could be of interstratal origin. 49 They were both folded and cleaved during D1. 50 On the east limb of the Loch Awe Syncline, the Ardrishaig Phyllite 51 Formation is well exposed in the Port Cill Maluaig and Strone Point 52 GCR sites. Despite the higher degree of metamorphism and 53 deformation, the Ardrishaig Phyllites show the same field 54 appearance, and maintain the same range in lithologies, as the 55 Craignish Phyllites at the Loch Fearnach GCR site. The Crinan Grit 56 Formation is best exposed in the Kilmory Bay GCR site where it has 57 a conglomeratic unit, the Ardnoe Member at the base. The Ardmore 58 Formation represents the base of the Crinan Subgroup on the 59 islands, and is correlated with the Ardnoe Member on the mainland. 60 A local calcareous unit, the Shira Limestone and Slate Formation 61 62 63 64 65 1 2 3 4 occurs at the junction between the Easdale and Crinan subgroups 5 (Figure 2a) and is seen on the mainland as a dolomitic breccia at 6 the base of the Ardnoe Member. The top part of the Crinan Subgroup 7 is best exposed in the South Bay, Barmore Island GCR site were it 8 is represented by the Stonefield Schist Formation, which has a 9 conformable and transitional contact with the Loch Tay Limestone 10 Formation. The latter is 75 m thick in Kintyre at the South Bay, 11 12 Barmore Island GCR site but is represented in the core of the Loch 13 Awe Syncline by the Tayvallich Slate and Limestone Formation and 14 the Tayvallich Volcanic Formation, which have a combined thickness 15 of 3200 m (of which the metalimestone accounts for c. 100 m). At 16 the top of the Tayvallich Slate and Limestone Formation is the 17 Kilchrenan Conglomerate Member, described in the Kilchrenan Burn 18 and Shore GCR site report. 19 The Loch Na Cille Boulder Bed found near the top of the Tayvallich 20 Subgroup (see the West Tayvallich Peninsula GCR site report) has 21 been variously interpreted as a tectonic breccia (Peach et al., 22 1911), a volcaniclastic debris flow (Pickett et al., 2006) or a 23 glacial deposit (Elles, 1935; Prave, 1999). In the core of the 24 Loch Awe Syncline the Tayvallich Subgroup passes up into the Loch 25 Avich Grit Formation and Loch Avich Lavas Formation, belonging to 26 the Southern Highland Group (Figure 2b). 27 28 29 1.3 Pre-tectonic igneous dykes and sills 30 31 Basic sills that were emplaced before the regional deformation and 32 metamorphism are common in the upper part of the Argyll Group in 33 the South-west Grampian Highlands, and in places they make up more 34 than 50 per cent of the succession. Contact metamorphism adjacent 35 to the thicker sills causes local baking and hardening of the 36 country rocks. As a result these beds are protected from much of 37 the subsequent deformation, so preserving features such as 38 pseudomorphs after gypsum (Craignish Point GCR site), and 39 sedimentary structures (Kilmory Bay GCR site). Massive sills of 40 this type are reported from the Ardilistry Bay, Ardbeg, Craignish 41 Point and Kilmory Bay GCR sites. Those at Ardbeg and Ardilistry 42 Bay are considered to be representative. 43 Igneous dykes and sills either of unusual composition (i.e. more 44 mafic than normal) or containing unusual minerals, are found at 45 three localities in South Islay, close to the Port Ellen Phyllite– 46 Laphroaig Quartzite contact. At Ardbeg, there is a folded 70 m- 47 48 thick sill that is unusual for containing large crystals (up to 1 49 mm long) of the brittle mica, stilpnomelane. At Ardilistry Bay, 50 there is a unique, 12–14 m-thick basic sill containing at the base 51 a 3 m-thick layer of metapyroxenite and a 1 m-thick meta- 52 anorthosite. The clinopyroxene has been altered to actinolite, and 53 the anorthosite to an assemblage containing albite and epidote. At 54 the Surnaig Farm GCR site, there is a unique occurrence of a 55 cleaved dyke of metamafic rock. 56 57 1.4 Structure 58 59 The area has a simple structural geometry controlled by a series of 60 upward-facing F1 major folds. These are (from south-east to north- 61 62 63 64 65 1 2 3 4 west) the Ardrishaig Anticline (equivalent to the Tay Nappe), the 5 compound Loch Awe Syncline, and the Islay Anticline (Figures 1, 3). 6 The slaty cleavage shared by these structures changes orientation 7 from vertical in the south-east, to dipping south-east at a 8 moderate angle on Islay. The folds are gently curvilinear and the 9 stretching lineation has a regional down-dip orientation. No major 10 tectonic breaks have been recognized. 11 12 The Tay Nappe and its relationship with the Ardrishaig Anticline 13 are fully described and discussed by Tanner et al. (2013b). The 14 Loch Awe Syncline, being a more-obvious, open structure, has been 15 known about for much longer and MacCulloch reported a ‗fan 16 structure‘ in this region in 1819. Hill (1899) recognized that it 17 is a major fold and he was the first person to correlate the 18 Craignish Phyllites on the north-west limb with the Ardrishaig 19 Phyllites on the south-east limb. Roberts (1974) identified it as 20 a major F1 structure, correcting Bailey‘s (1913) interpretation of 21 it as a secondary fold (see the Kilmory Bay GCR site report). The 22 Loch Awe Syncline has a tripartite hinge comprising the Tayvallich 23 Syncline, the Loch Sween Anticline and the Kilmory Bay Syncline. 24 The Kilmory Bay Syncline is the more complex of these three 25 structures and its hinge-zone consists of a bundle of at least five 26 mesoscopic fold closures (see the Kilmory Bay GCR site report). 27 The Islay Anticline was first recognized by Peach and Wilkinson 28 (1909) but both the stratigraphy and the structure were completely 29 30 re-interpreted by Bailey (1917). Part of his map is reproduced 31 here (Figure 4), as it is a fine example of the style of map 32 produced during this period, and is accompanied by a dynamic cross- 33 section, which has not been bettered. There was an early dispute 34 over whether this fold is a syncline or an anticline, and this was 35 not resolved conclusively until the work of Allison (1933). The 36 Islay Anticline is poorly exposed; for such a large structure, 37 there is surprisingly little published information on its geometry, 38 and few structural symbols on the BGS 1:50 000 sheets for the area. 39 The fold trace appears to consist of en-echelon segments displaced 40 by faulting (Figure 1). The fold axis plunges gently to the north- 41 east for most of its outcrop on Islay, except at the far north end 42 of the island where it plunges to the south-west (see the Rubha a’ 43 Mhail GCR site report). 44 The area also lies on the north-west limb of the F4 Cowal 45 Antiform, a major arch-like structure that folds the early fabrics 46 and the Ardrishaig Anticline/Tay Nappe (see Tanner et al., 2013b). 47 48 It has a broad hinge-zone represented by the Cowal Flat Belt, the 49 north-west limit of which was designated the Tarbert Monoform by 50 Roberts (1977c). However, although F4 box folds occur locally in 51 the position of the proposed monoform, examination of the 52 Geological Survey map sheets 29 and 37 suggests that there is a 53 gradual change in overall curvature of the strata across this zone, 54 and no evidence of an abrupt increase in regional dip. 55 In the context of these major structures, the Rubha a’ Mhail GCR 56 site lies in the core of the Islay Anticline; the West Tayvallich 57 Peninsula GCR site contains the hinge-zone of the Tayvallich 58 Syncline; the Kilmory Bay GCR site straddles the complex hinge-zone 59 of the Kilmory Bay Syncline; the Loch Avich GCR site lies in the 60 core of the Kilchrenan Syncline, a component of the Loch Awe 61 62 63 64 65 1 2 3 4 Syncline; and the Strone Point GCR site is located a short distance 5 south-east of the hinge of the Ardrishaig Anticline where there is 6 a analogous pair of congruous mesoscopic F1 folds. 7 Of the remaining sites, 14 lie on the common limb between the 8 Islay Anticline and the Loch Awe Syncline. Bedding dips to the 9 south-east at a moderate angle at all of these sites, except at the 10 Caol Isla GCR site, which is affected by its proximity to the 11 12 hinge-zone of the Islay Anticline. It is everywhere cut by the S1 13 penetrative to spaced cleavage, which dips more steeply in the same 14 direction. F1 fold hinges generally plunge to the north-east or 15 south-west but are locally strongly curvilinear. There is a weak 16 down-dip L1 stretching lineation at most of these localities. A 17 gently dipping to horizontal crenulation cleavage is sporadically 18 developed throughout this fold limb, and is only associated with 19 mesoscopic folds at the Black Mill Bay GCR site. This fabric 20 represents a late deformation, which is of post-D2 age. The 21 effects of the D2 deformation, seen as an intense planar fabric 22 that overprints and almost completely reworks S1, is seen only at 23 the Port Cill Malluaig GCR site. 24 25 1.5 Regional metamorphism 26 27 Metamorphic grade is largely of greenschist facies, except along 28 the south-east margin of the region, where it reaches the garnet 29 30 zone of the epidote-amphibolite facies at a temperature of over 31 500ºC and a pressure estimated at 10 kbar (Graham et al., 1983). A 32 number of studies have been carried out on the regional 33 metamorphism of the concordant basic meta-igneous sheets, and 34 models have been formulated for the regional circulation of 35 metamorphic fluids in these rocks (i.e. Graham et al., 1985; 36 Skelton et al., 1995), with the major antiformal fold closures 37 acting as conduits for the fluids. Apparently anomalous high 38 pressures, calculated using the phengite geobarometer (Graham et 39 al., 1983; Dymoke, 1989), are partly substantiated by the local 40 presence of regional metamorphic kyanite in greenschist-facies 41 rocks (see the Kilnaughton Bay GCR site report). 42 43 1.6 Features of specialized interest 44 45 Topics of particular interest covered in the GCR reports in this 46 paper include: 47 48 • Origin of a Neoproterozoic glacial deposit: the Port Askaig 49 Tillite (Garvellachs Isles GCR site) 50 • Extremely well-preserved Neoproterozoic stromatolites (Rubha 51 a’Mhail GCR site) 52 • Water-escape structures and sedimentary or clastic dykes (Coal 53 Isla and Surnaig Farm GCR sites) 54 • The unique form of gypsum, and its environmental implications 55 (Craignish Point GCR site) 56 • Pre-tectonic igneous dykes and sills, and some unusual minerals 57 • The geometry of cylindrical and curvilinear folds (Strone Point, 58 Fearnach Bay and Port Cill Maluaig GCR sites) 59 • The development of kyanite in greenschist-facies regional 60 metamorphic rocks (Kilnaughton Bay GCR site) 61 62 63 64 65 1 2 3 4 • A putative orogenic unconformity in the Dalradian (Kilnaughton 5 Bay, Lussa Bay and Kinuachdrachd GCR sites 6 • Nature of the Dalradian basement (Kilchiaran to Ardnave Point GCR 7 site). 8 9 2 GARVELLACH ISLES 10 11 (NM 633 088–NM 683 128) 12 13 P.W.G. Tanner 14 15 16 2.1 Introduction 17 18 The Garvellachs (‘Isles of the Sea’) are an isolated group of 19 uninhabited islands in the Firth of Lorn, which preserve the best- 20 exposed section through a Precambrian glacial deposit to be found 21 in the United Kingdom. This unit, the Port Askaig Tillite 22 Formation, consists of a succession of ‗boulder beds‘ 23 (metadiamictites) and interbedded metasedimentary rocks that, 24 although gently tilted, is largely unaffected by tectonic 25 distortion or faulting. Its relationship with the underlying 26 Lossit (formerly Islay) Limestone Formation may be examined in the 27 Garvellachs, and the top of the tillite sequence can be seen on 28 Islay, 50 km to the south-west, where it passes conformably upwards 29 30 into rocks of the Bonahaven Dolomite Formation (see the Caol Isla 31 GCR site report) (Figure 1). The Port Askaig Tillite marks the 32 base of the Argyll Group (Figure 2) and has been used as a key 33 lithostratigraphical marker for correlating the Scottish and Irish 34 Dalradian successions (Kilburn et al., 1965). 35 The architecture and internal morphology of the tillite formation 36 are magnificently exposed on the 7.6 m (25 ft) raised rock platform 37 around the islands, and individual beds can be reliably traced 38 across each island due to the excellent exposures. Here the beds 39 preserve a wealth of sedimentological features, which have been 40 largely destroyed in rocks of similar age elsewhere in Scotland and 41 Ireland due to the effects of later intense deformation and 42 metamorphism. The tillite sequence on the Garvellachs consists of 43 numerous boulder beds, which together with the interstratified 44 sedimentary rocks, has an exposed thickness of 578 m (Spencer, 45 1971, 1981). Together with a further 172 m of beds at the top of 46 the sequence, seen on Islay but not exposed on the Garvellachs, it 47 48 represents the thickest known development of the Port Askaig 49 Tillite. 50 Following regional mapping by the Geological Survey (Peach et al., 51 1907), the first detailed account was by Kilburn et al. (1965) who 52 prepared a measured section across the boulder beds as part of a 53 survey of glacial deposits in the Dalradian successions of Scotland 54 and Ireland. Spencer (1971) then published the definitive 55 monograph on the Port Askaig Tillite, which has proved a vade- 56 mechum for all future work and interpretation. However, despite 57 the benefit of its excellent state of preservation, the age, 58 origin, and source of the tillite have been hotly disputed. 59 Hypotheses for its formation have ranged from non-glacial causes, 60 such as subaqueous mud or debris flows (Schermerhorn, 1974, 1975), 61 62 63 64 65 1 2 3 4 to a glacial origin, either by deposition from grounded glacier ice 5 (Kilburn et al., 1965; Spencer, 1971, 1985), or from floating 6 icebergs supplying exotic materials to a tidally-influenced marine 7 basin (Eyles and Eyles, 1983; Eyles and Clark, 1985; Eyles, 1988). 8 Source terrains for the exotic stones have been sought in Sweden 9 and Finland (Spencer, 1981); Labrador and Greenland (Evans et al., 10 1998); and even South America (Dalziel, 1994). An additional 11 12 problem is its age of deposition. For many years the tillite was 13 assumed to be equivalent to the Varanger Tillites of Scandinavia, 14 Greenland and Svalbard (Hambrey, 1983; Hambrey and Harland, 1985). 15 However, the long-accepted, though imprecise, c. 653 Ma age of 16 these tillites has now been revised to 620–590 Ma, which is more 17 comparable with the top of the Argyll Group than the bottom. Hence 18 various alternative suggestions have been made, which correlate the 19 Port Askaig Tillite with either the Marinoan (c. 635 Ma) or 20 Sturtian (c. 723 Ma) global glacial events (see Stephenson et al., 21 2013a). 22 There has been a recent resurgence of interest in the number, age, 23 and causes of the late-Precambrian glaciations and their temporal 24 relationship to the evolution of metazoan life forms in the 25 Cambrian, and the Garvellach Isles GCR site has a key role to play 26 in this work. 27 28 29 2.2 Description 30 31 The sequence on the Garvellachs begins with the dolomitized, 70 m- 32 thick, upper part of the Lossit Limestone Formation, which is 33 exposed on the most westerly and accessible of the islands, Garbh 34 Eileach (Figure 5). An interesting feature of the metalimestone is 35 that locally it contains arrays of radiating, stellate pseudomorphs 36 a few centimetres long (Spencer, 1971, plate 6), which consist of a 37 quartz-dolomite intergrowth. These pseudomorphs can be examined at 38 (NM 676 127) and are of disputed parentage. 39 The Lossit Limestone is overlain conformably by the Port Askaig 40 Tillite Formation (‗Boulder Bed‘ or ‗Conglomerate‘ of the early 41 authors). Although the type area of the tillite is at Port Askaig 42 on Islay, it is best seen on the Garvellachs where it consists of 43 38 individual metadiamictites, from less than 1 m to over 20 m 44 thick, with boulders to 2 m across. The upper dolomitized part of 45 the Lossit Limestone together with the first 12 metadiamictite 46 units are seen only on the Garvellachs, being absent on Islay where 47 48 Unit 13 lies unconformably upon the metalimestone. 49 The boulder beds were called ‗mixtites‘ by Spencer (1971), but 50 modern workers prefer the non-genetic term ‗diamictite‘, to 51 describe a rock consisting of a poorly sorted aggregate of mud-, 52 sand-, and gravel-sized detritus. The tillite sequence, which 53 includes considerable thicknesses of tabular bedded sedimentary 54 rocks and breccias, was divided into 5 units by Spencer (1971), and 55 includes two distinctive members, the Great Breccia and the 56 Disrupted Beds. The uppermost part of the tillite is not preserved 57 in the Garvallachs, but is seen in the Port Askaig area of Islay. 58 The tillite is excellently exposed on the rock platform around the 59 north-east end of Garbh Eilach and is accessible from 200 m north- 60 west of Belach Buidhe (NM 676 127) to Sloc a‘ Cheatharnaich (NM 675 61 62 63 64 65 1 2 3 4 119). The field relationships have been described in detail by 5 Fairchild (1991). The sequence begins with metadiamictites 1–12, 6 containing mainly dolostone clasts, passes upwards into the Great 7 Breccia and the Disrupted Beds, and ends with the upper 8 metadiamictites (18–32) which are characterized by their high 9 content of ‗exotic‘, usually granitic, boulders (Figure 5). Most 10 features of the succession are well displayed in this section, with 11 12 the upper metadiamictites being best seen between NM 675 119 and NM 13 670 118, close to the landing stage. These include dropstones, 14 erosion surfaces, sandstone wedges, load structures, slumped 15 horizons, wave-ripples, and varve-like sequences (Kilburn et al., 16 1965; Spencer, 1971; Eyles and Eyles, 1983; Eyles and Clark, 1985). 17 The metadiamictites decrease in thickness and frequency upwards, 18 and the whole formation is an upwards-fining sequence that shows a 19 transitional passage into the overlying Bonahaven Dolomite 20 Formation (see the Caol Isla GCR site report). The associated 21 interbeds, which occur in units up to 11 m thick, include rippled 22 and cross-bedded white and brown metasandstones, orange-brown 23 metadolostones, and metaconglomerates, are rich in dolomite at the 24 base of the sequence and become less dolomitic and more feldspathic 25 upwards. Most of the metadolostones have been shown to be detrital 26 in origin and derived from the underlying Lossit (formerly Islay) 27 Limestone (Fairchild, 1983, 1985), and therefore have no 28 palaeoclimatical significance. 29 30 The clasts in the metadiamictite horizons can be divided into 31 intrabasinal stones derived from the local substrate, and 32 extrabasinal, or ‗exotic,‘ stones of unknown provenance. The 33 intrabasinal stones consist of abundant, angular, metadolostone 34 fragments, with dolomitic metasiltstone, metaconglomerate and very 35 rare metalimestones; the exotic ones are mainly of pink granite 36 (commonly referred to as being of Rapakivi-type or nordmakite e.g. 37 Spencer, 1971), with some gneiss, schist and quartz clasts. The 38 extrabasinal stones are subrounded and reach 1.5–2.0 m in diameter. 39 The lowest metadiamictites contain locally-derived stones but 40 exotic stones appear above Unit 12 and become common in the upper 41 beds in which a feldspar-rich matrix is also developed. This 42 upward change in provenance is reflected in the whole-rock 43 geochemistry (Panahi and Young, 1997), which distinguishes lower 44 metadiamictites derived by the erosion of sediments, which had 45 already undergone a previous cycle of post-Archaean weathering, 46 from upper metadiamictites derived from a mixed granitic and post- 47 48 Achaean sedimentary source. 49 Two distinctive members are thicker and coarser grained than on 50 Islay, and are not seen elsewhere in the tillite outcrop. The 51 Great Breccia is 40 m thick and contains enormous intrabasinal 52 clasts of metadolostone, the largest, occurring at NM 639 099 on 53 Eileach an Naoimh, being over 70 m long and folded into an 54 antiform. The overlying Disrupted Beds is a unit 29–40 m thick, 55 well exposed at NM 666 122 on Garbh Eilach, which consists of 56 semicontinuous beds of metadolostone and cross-bedded metasandstone 57 that have been partially boudinaged and pulled-apart. 58 Special features of the metadiamictites that have provided clues 59 as to their glacial origin, or are of particular interest, include: 60 numerous dropstones in finely bedded or laminated sequences; large 61 62 63 64 65 1 2 3 4 polygonal structures on bedding surfaces, illustrated and described 5 in detail by Kilburn et al. (1965), Spencer (1971) and Eyles and 6 Clark (1985) and seen at NM 646 101; millimetre-thick, varve-like, 7 laminations in metasiltstone beds, some with over 2500 laminae 8 (Spencer, 1981) (NM 675 119); sedimentary dykes up to 30 cm thick 9 and traceable for several hundred metres (NM 667 124); and 10 sandstone downfold structures above metadiamictites (Spencer, 1971; 11 12 Eyles and Clark, 1985), which can be seen at a number of places. 13 The Port Askaig Tillite on the Garvellachs has a simple structural 14 setting: the entire sequence of beds dips to the south or south- 15 east at around 35°, and forms part of the north-western limb of the 16 Loch Awe Syncline (F1). A single main penetrative cleavage is 17 developed locally, which dips to the south-east more steeply than 18 bedding and appears to belong to the same generation as the major 19 syncline; it can be seen as a spaced cleavage in some of the 20 metasandstones. The early cleavage is crenulated locally, and late 21 kink bands are also seen (Spencer, 1971). The beds are affected by 22 some internal strain, as shown by the deformation of pebbles and by 23 the slight distortion of the polygonal patterns seen on bedding 24 surfaces. 25 26 2.3 Interpretation 27 28 The mode of origin and source of the tillite have been the subject 29 30 of lively geological interest since MacCulloch, one of the earliest 31 geological travellers in the region, reported the presence of a 32 conglomerate on the Garvellachs in 1819, and suggested that it 33 might be correlated with similar deposits at Schihallion, and on 34 Islay. However it was Thomson (1877, p. 211) who first suggested a 35 glacial origin for this boulder bed and anticipated the modern, 36 most widely accepted, interpretation for the deposit, writing that 37 ‘the entire absence of stratification in one part of the section, 38 which in another shows signs of regular deposition, and the 39 occurrence of far transported rocks of the character already 40 stated, indicate that the mass had been transported and dropped 41 from melting ice in a shallow, tranquil sea, the bottom consisting 42 of mud and sand‘. 43 Spencer (1971) concluded that the tillite was deposited from a 44 grounded ice sheet, an interpretation challenged by Schermerhorn 45 (1974) who interpreted it as a mass-flow deposit, and by Eyles and 46 Eyles (1983) who argued for glacimarine deposition beneath floating 47 48 icebergs, with much reworking of the sediments by currents. These 49 interpretations are dependent upon understanding not only the 50 overall architecture of the deposit, but the depositional 51 environment of the sedimentary interbeds, and the degree to which 52 the diamictites have been reworked by currents. The interpretation 53 of minor features diagnostic of particular environments, such as 54 polygonal sets of sediment-filled cracks, varves, pseudomorphs etc. 55 also plays a vital role in this work and these features have been 56 much discussed in recent years. For example, large polygonal 57 networks seen on bedding surfaces at NM 646 101 on Eileach an 58 Naoimh were interpreted as ice-wedge polygons (indicating subareal 59 exposure in a cold climate) by Spencer (1971, plate 8), but as 60 subaqueous soft-sediment dykes whose formation is not dependent 61 62 63 64 65 1 2 3 4 upon the presence of ice, by Eyles and Clark (1985). Likewise, the 5 pseudomorphs seen in the Lossit Limestone, which Spencer (1971) 6 speculated may be of glendonite (and hence derived from ikaite, a 7 mineral stable only below 4°C (Shearman and Smith, 1985), and 8 diagnostic of a cold climate), could equally well be secondary 9 after gypsum, and indicative of warm, arid conditions. 10 On a continental scale, glacial deposits have been used to 11 12 reconstruct past plate configurations, by determining the direction 13 of transport of the tillite and tracing the origin of the boulders 14 to their source region. U-Pb dating of zircon from two granitic 15 clasts from the Port Askaig Tillite on the Garvellachs and Islay 16 has yielded ages of c. 1800 Ma (Evans et al., 1998). These dates, 17 together with Nd model ages (Fitches et al., 1996), show that the 18 material was derived from a Palaeoproterozoic source, with no 19 involvement of Archaean crust. The exotic clasts might have been 20 derived from Palaeoproterozoic (c. 1800 Ma) terranes in Labrador, 21 South Greenland or Scandinavia. A Laurentian, rather than 22 Gondwanian, source is favoured. It is unfortunate in this respect 23 that a palaeomagnetic study of the Port Askaig Tillite on Garbh 24 Eileach has shown that the rocks were remagnetized during the Early 25 Ordovician (Stuparsky et al., 1982), and hence this technique 26 cannot be used to determine the palaeolatitude at which the tillite 27 was deposited, as had been suggested previously by Tarling (1974). 28 Physical evidence for the direction of movement of the glacial 29 30 material is also ambiguous. The Great Breccia has been interpreted 31 as either a debris flow (Eyles and Eyles, 1983) or as a grounded- 32 ice till (Fairchild, 1985, 1991). The large fold in the Great 33 Breccia (first figured by Peach et al., 1909) has been much 34 discussed in the literature as it is considered by some workers to 35 be a glacial-push fold (Spencer, 1971), with movement from the 36 south-east, or a soft-sediment slump fold (Eyles and Eyles, 1983), 37 with movement in the opposite direction. 38 39 2.4 Conclusions 40 41 The Garvellach Isles GCR site preserves internationally important 42 exposures of a Precambrian glacial deposit, the Port Askaig 43 Tillite, which is unique in the UK for its excellent state of 44 preservation and wealth of small-scale sedimentary features. This 45 stratigraphical unit has been used as a marker horizon for 46 correlating Dalradian sequences in Scotland and Ireland, and has 47 48 been proposed as a link for use in global-scale tectonic plate 49 reconstructions. Although its precise environment of deposition is 50 still being debated, it is generally agreed to be of glacial 51 origin, as evidenced by the presence of numerous, large, ‘exotic’, 52 far-travelled dropstones in finely laminated metasedimentary rock. 53 It was probably formed largely of material dropped from floating 54 icebergs and reworked by tidal currents, and is the thickest 55 deposit of this type in the Dalradian Supergroup. The Port Askaig 56 Tillite is evidence for a major glacial episode having occurred in 57 late-Precambrian times. 58 This GCR site continues to provide a classical testing ground in 59 which to distinguish between different models for Precambrian 60 tillite formation, and for examining the morphology of a wealth of 61 62 63 64 65 1 2 3 4 controversial features such as fossil ice-wedge polygons, soft- 5 sediment deformation structures, varves, exotic boulders of 6 problematical origin, and unusual pseudomorphs. The interpretation 7 of these features is important in understanding the climatic 8 conditions, and hence the latitude, at which these rocks were 9 deposited on the Earth‘s surface prior to the subsequent break-up 10 and dispersal of the continental blocks in the North Atlantic 11 12 region. The value of this site is considerably enhanced by the 13 current interest worldwide in the timing, correlation, and causes 14 of Neoproterozoic glaciations. 15 16 3 CAOL ISLA, ISLAY 17 (NR 429 701–NR 429 710) 18 19 P.W.G. Tanner 20 21 22 3.1 Introduction 23 24 The coastal rock platform and low cliffs north of the Caol Isla 25 Distillery, on the east coast of the Isle of Islay (Figure 1), 26 provide excellent exposures of rocks belonging to the uppermost 27 part of the Port Askaig Tillite Formation and the lower part of the 28 overlying Bonahaven Dolomite Formation, interpreted by some as a 29 30 cap carbonate. The gently dipping metasedimentary rocks are on the 31 east limb of the F1 Islay Anticline. The Dalradian rocks in this 32 part of Islay show little evidence of deformation, and have been 33 affected by only low-grade (greenschist-facies) metamorphism. 34 The main Dalradian lithological units on Islay such as the 35 ‗Dolomitic Group‘ at Caol Isla were recognized by the Geological 36 Survey (Wilkinson, 1907), but were not placed into their correct 37 stratigraphical sequence until the work of Bailey (1917). In one 38 of the first studies to use sedimentary ‗way-up‘ structures in 39 metamorphic rocks in the British Isles, Allison (1933) confirmed 40 Bailey‘s interpretation that the ‗Dolomitic Group‘ lies 41 stratigraphically between the older, Port Askaig Tillite, and the 42 Jura Quartzite above. Spencer and Spencer (1972) later combined 43 groups 3–5 of Wilkinson (1907) to erect the Bonahaven Dolomite 44 Formation, which is made up of four members and has a maximum total 45 thickness of some 350 m (Fairchild, 1991, fig. 14). Wilkinson 46 (1907) included a cross-section, and the rocks were mapped in 47 48 detail by Spencer (1971). An excellent field guide is provided by 49 Fairchild (1991). 50 The GCR site encompasses the best-exposed section through members 51 1 and 2 of the Bonahaven Dolomite Formation, the main focus being 52 on the five units that make up Member 1. Members 3 and 4 of this 53 formation are much better exposed on the north coast of Islay and 54 are the subject of the Rubha a’Mhail GCR site report. The rocks at 55 Caol Isla preserve a great variety of sedimentary structures 56 characteristic of a tidal-dominated shelf environment (Klein, 1970; 57 Kessler and Gollop, 1988). Of particular note are sand-filled 58 cracks in alternating sandstone–mudstone sequences that give rise 59 to well-organized polygonal patterns on bedding surfaces. These 60 patterns reach over 10 cm in diameter, and have been variously 61 62 63 64 65 1 2 3 4 interpreted as ‗sun-cracks‘ indicative of subaerial exposure and 5 drying out of mud layers; synaeresis structures formed at the 6 water–sediment interface; or sections through dykelets resulting 7 from dewatering of the sediment pile during burial. Considerable 8 interest in these rocks was aroused briefly by the report by 9 Brasier and McIlroy (1998) of fossil faecal pellets, of a type 10 produced by the earliest organisms to have developed a gut capable 11 12 of expelling such material. This identification has been 13 subsequently retracted (Brasier and Shields, 2000). 14 15 3.2 Description 16 17 The rocks in the section at Caol Isla dip at 22–28° to the north, 18 are right-way-up, and are unaffected by any major faults. The 19 succession starts at the top of the Con Tom Member of the Port 20 Askaig Tillite Formation (Member 5 of Spencer, 1971), and includes 21 members 1–3 of the Bonahaven Dolomite Formation (Figure 7). There 22 is a transitional contact between the Port Askaig Tillite and the 23 Bonahaven Dolomite. In the vicinity of Caol Isla, Member 1 is seen 24 only on the coastal section due to a local erosion surface having 25 developed at this level farther inland, towards the inferred edge 26 of the original sedimentary basin (Fairchild, 1991). This member 27 is divided into 5 units, for which sedimentary logs were presented 28 by Fairchild (1991, fig. 17). 29 30 Before describing the rocks from this site it is necessary first 31 to clear up confusion over the previous stratigraphical 32 nomenclature. Bailey (1917) introduced the term ‗Lower Fine- 33 grained Quartzite‘ to describe a thick quartzite, which occurs 34 between the Port Askaig Tillite and the ‗Dolomitic Group‘, to 35 distinguish it from the Jura Quartzite (his ‗Upper Fine-grained 36 Quartzite‘), which lies above the ‗Dolomitic Group‘. Klein (1970) 37 misused this term (Spencer, 1971; Klein, 1971) to include part of 38 the ‗Dolomitic Group‘, and both Kessler and Gollop (1988) and 39 Tanner (1998a) perpetuated this misuse for historical reasons. The 40 correct reading of the stratigraphy of this section (as followed 41 here) is that the Con Tom Member of the Port Askaig Tillite 42 (Bailey‘s ‗Lower Fine-grained Quartzite‘) is succeeded by Member 1 43 of the Bonahaven Dolomite Formation. Most of the rocks described 44 by Klein (1970), and all of those studied by Kessler and Gollop 45 (1988), and Tanner (1998a), occur within the Bonahaven Dolomite 46 Formation. 47 48 49 3.2.1 Member 1. (Carraig Artair Member, BGS, 1997) 50 51 Unit 1 (Figure 7) consists of metamudstones and appears to be 52 conformable with the underlying Port Askaig Tillite. 53 Exceptionally, a slaty cleavage cut by a crenulation cleavage 54 affects the metamudstone by the shelter at Carraig Artair (NR 4292 55 7020) and even more unexpectedly, for a sequence of rocks which 56 appear to be so little deformed and of very low metamorphic grade, 57 metamorphic biotite is developed (Fairchild, 1985, fig. 3e). 58 Unit 2 consists of up to 18 m of flaser-bedded and rippled 59 metasandstones. 60 61 62 63 64 65 1 2 3 4 Unit 3 is highly significant in that the cross-bedded 5 metasandstones that make up this unit include a metaconglomerate at 6 the base (at NR 4294 7029), which contains ‗exotic‘ granitic clasts 7 similar to those found throughout the upper part of the Port Askaig 8 Tillite. This metaconglomerate is the youngest bed in the 9 Bonahaven Dolomitic Formation to preserve glacially transported 10 material, albeit reworked by current action, and this occurrence 11 12 provides a link between this formation and the underlying Port 13 Askaig Tillite. 14 Unit 4, comprising interbedded metasandstones, metasiltstones, and 15 metamudstones, is well exposed in a small cliff and rocky 16 promontory at Leac Thiolastaraidh (NR 4297 7037), 370 m north of 17 the pier at Caol Isla Distillery, and its sedimentary structures 18 and lithofacies have been closely studied by a number of workers. 19 Klein (1970, figs 2–4) illustrated cross-lamination, flaser 20 bedding, and two intersecting sets of ripple marks from this 21 locality. Kessler and Gollop (1988) visited the cliff face at Leac 22 Thiolastaraidh and published a measured 14 m section, from which 23 they recorded the possible presence of desiccated algal mats at a 24 number of levels; channels (including ‗gutter casts‘) containing 25 cross-bedded strata; possible hummocky and swaley cross- 26 stratification; ripples; and cross-sets with irregularly-spaced 27 foreset laminae of tidal origin (the ‗tidal bundles‘ of Boersma, 28 1969). 29 30 ‗Sun-cracks‘ were first reported from these rocks (together with 31 ripple marks) by Thomson (1877). They occur as sand-filled cracks 32 in alternating sandstone–mudstone sequences and form polygonal 33 patterns on bedding surfaces (Figure 8a). The polygons commonly 34 form orthogonal patterns (in which the majority of the cracks meet 35 at right angles) and vary from centimetre-scale to larger 36 structures over 20 cm across. Within the larger structures, a 37 basic framework of cracks over 1 cm wide divide the surface into 38 crude polygonal shapes, within which sets of thinner cracks define 39 a second-order pattern. Tanner (1998a) analysed their 3-D 40 morphology in detail, based on slabbed and sectioned material, and 41 found that many of the cracks could be traced through a number of 42 different beds. 43 These rocks also contain possible fossil remains; Fairchild (1977) 44 recorded the presence of numerous phengite spherules, 50–150 mm 45 across, which he considered to be possible glauconitized 46 microfossils, from a metamudstone at the top of Member 1 in an 47 48 equivalent section farther north at Bonahaven (not exposed in the 49 Caol Islay section). 50 Unit 5 consists of a poorly exposed, honeycomb-weathered, 51 dolomitic metasandstone. 52 53 3.2.2 Member 2. (Giur Bheinn Member, BGS, 1997) 54 55 This member is a cross-bedded meta-quartz-arenite, some 30 m thick, 56 exposed in a small cliff section at NR 4293 7044 (Figure 7). It is 57 of historical interest, for this horizon was named the ‘Pipe Rock’ by 58 Peach (in Wilkinson, 1907) and, together with the other weakly 59 deformed and metamorphosed mudstones, quartzites, and carbonate 60 rocks on Islay, was inferred to be of Cambro-Ordovician age, and 61 62 63 64 65 1 2 3 4 incorrectly correlated with the foreland sequence in the North-west 5 Highlands. 6 7 3.2.3 Member 3. (Margadale Member, BGS, 1997) 8 9 In-situ dolomitic metasandstones belonging to this member are 10 uncommon but there are many large tumbled blocks along the tide 11 12 line. It was correlated with the ‘Fucoid Beds’ by Peach (in 13 Wilkinson, 1907) and Peach and Horne (1930), but neither Bailey 14 (1917) nor any subsequent workers (e.g. Fairchild, 1977) have been 15 able to confirm the presence of ‘small pipes’, ‘ordinary pipes’ and 16 ‘trumpet pipes’ in the ‘Pipe Rock’ (Peach and Horne, 1930), or ‘worm- 17 casts’ in this member. These structures are artifacts associated 18 with the patterns of filled cracks seen on bedding surfaces and in 19 cross-section, in both metasandstones and metacarbonate rocks of 20 the Bonahaven Formation. Some of these filled cracks have been 21 strongly affected by bedding-normal compaction and the small buckle 22 folds appear vermiform in cross-section. 23 24 3.3 Interpretation 25 26 In one of the first sedimentological studies of Dalradian rocks, 27 Klein (1970) identified a wide range of sedimentary structures 28 including cross-bedding, ripple marks, ‘tidal bedding’, and flaser 29 30 bedding, in the section north of Caol Isla Distillery. He 31 concluded that the sequence had been deposited in a tide-dominated, 32 shallow water environment. Unfortunately, as pointed out by 33 Spencer (1972), this study suffered from a lack of stratigraphical 34 control and appears to encompass both the metasandstones 35 immediately north of the distillery, which belong to the top of the 36 Port Askaig Tillite, and members 1 and 2 of the Bonahaven 37 Formation. 38 From their detailed study of part of Unit 4 in Member 1, Kessler 39 and Gollop (1988) concluded that the depositional environment was 40 transitional between that of a shallow shelf or shoreface and an 41 intertidal setting. However recognition of the ‗intertidal‘ 42 setting relies in part upon their interpretation of the polygonal 43 sets of sand and silt-filled cracks having formed by the subaerial 44 desiccation of ‗algal mats‘. Thin sections of these rocks show 45 micro-cross-laminated silty layers alternating with homogeneous, 46 non-laminated muddy layers (Tanner, 1998a, fig. 9), with no 47 48 suggestion of the mediation of desiccated microbial mats in the 49 formation of the filled cracks. From a study of the complete 50 section, Fairchild (1991) concluded that units 1 and 2 were 51 deposited in a nearshore, wave-dominated situation, which became 52 landward of a barrier island during the time that units 4 and 5 53 were laid down, followed by an eventual drowning of the barrier 54 system (Member 2). Fairchild (1980a) also studied the origin of 55 the dolomite in these rocks and concluded that it formed in two 56 stages; penecontemporaneous with sedimentation, and during burial 57 diagenesis. 58 Following Thomson (1877), most workers have interpreted the 59 polygonal patterns seen on the bedding surfaces as ‗sun cracks‘ 60 (Knill, 1963) or mud cracks (Klein, 1970; Fairchild, 1991) which 61 62 63 64 65 1 2 3 4 had formed as a result of desiccation during subaerial exposure, 5 and had been infilled subsequently with sand. From a detailed 6 study of the filled cracks, Tanner (1998a) concluded that they are 7 sections through 3-D patterns of dykelets resulting from 8 interstratal dewatering of the sediments, possibly soon after their 9 deposition, with no evidence of subaerial exposure (Figure 8b). 10 However, it is not clear what caused the homogeneous, layer- 11 12 parallel contraction in the mudstone layers. There are two 13 possibilities: it could have been triggered by earth tremors 14 associated with movements on the synsedimentary Bolsa Fault (Figure 15 1) (see the Rubha a’Mhail GCR site report) or a related structure 16 (Tanner, 1998a), or simply be the result of uniform contraction of 17 material held together by microbial slime, as the latter decayed. 18 The second explanation is the less likely, for, apart from the lack 19 of evidence for its presence, such algal growths usually develop in 20 calcareous not muddy rocks. 21 The rocks show little evidence of having been deformed, apart from 22 the sporadic development of a slaty cleavage (generally only 23 visible in thin section), and the slight distortion of some of the 24 polygonal patterns seen on the bedding surfaces. The local 25 presence of metamorphic biotite is consistent with greenschist- 26 facies regional metamorphism. It is of historical interest to note 27 that, in an attempt to date this metamorphic event in these low- 28 grade Dalradian rocks, Leggo et al. (1966) obtained a Rb-Sr 29 30 isochron apparent age of 572 ± 20 Ma from samples of metasandstone 31 collected from the small cliff at Leac Thiolastaraidh. The result 32 remains enigmatic. 33 Late-Precambrian tillites are characterized by having a ‗cap‘ of 34 carbonate rocks, whose significance is the subject of current 35 worldwide research interest, and Fairchild (1993) has suggested 36 that the Bonahaven Dolomite is the ‗cap carbonate‘ to the Port 37 Askaig Tillite, though this has been disputed by others. The 38 paradox presented by these carbonate rocks is that they are usually 39 considered to have formed in a warm or temperate climate, so their 40 presence could require a rapid, unexplained, change in climate 41 immediately following the glacial event. Alternatively, deposition 42 of carbonate rocks both before and after the glacial event could 43 have taken place in cold water conditions. It is therefore 44 important to determine the precise climatic conditions under which 45 the cap carbonate was laid down. 46 47 48 3.4 Conclusions 49 50 The Caol Isla GCR site is of international importance for 51 determining the environment of deposition of the rocks immediately 52 overlying a late-Precambrian glacial deposit, the Port Askaig 53 Tillite. The sequence is crucial in this respect for it forms a 54 transition between the tillite and its possible ‘cap carbonate’, the 55 Bonahaven Dolomite Formation (the upper part of which is described 56 in the Rubha a’Mhail GCR site report). 57 In addition, the well-exposed coastal section provides a 58 representative section for the lower part of the Bonahaven Dolomite 59 Formation of the Argyll Group, and enables a bed-by-bed study to be 60 made of the sedimentary structures in this tide-dominated, shallow 61 62 63 64 65 1 2 3 4 water sequence. Features of particular interest include sand- 5 filled cracks, which form polygonal patterns on the bedding 6 surfaces. Their 3-dimensional morphology suggests that they 7 originated by loss of water from the sediments after burial, 8 possibly triggered by movements on a fault that was active during 9 development of the sedimentary basin (see the Rubha a’Mhail GCR 10 site report). The site is made more valuable by the fact that the 11 12 rocks are almost unaffected by tectonic strain, an unusual 13 situation in the Dalradian. 14 15 4 RUBHA A’ MHAIL, ISLAY 16 (NR 377 781–NR 426 793) 17 18 P.W.G. Tanner 19 20 21 4.1 Introduction 22 23 This rocky, isolated stretch of coastline at the northern tip of 24 Islay (Figure 1), with its raised beaches, sea cliffs and caves, 25 presents a number of magnificent sections through members 3 and 4 26 of the Bonahaven Dolomite Formation. The stratigraphical 27 succession is a continuation of that seen at the Caol Isla GCR 28 site, in which the lower two members of the Bonahaven Dolomite are 29 30 best preserved. Although the outcrop of Member 3 is displaced a 31 number of times by faulting, marker horizons in the sequence can be 32 correlated for up to 3 km along strike. An abrupt change in 33 thickness of this member in the west of the section is interpreted 34 as being the result of movement on an important syndepositional 35 listric discontinuity, the Bolsa Fault. At the top of the sequence 36 there is a transitional contact between Member 4 and the younger 37 Jura Quartzite Formation. 38 This GCR site is renowned for its outstanding 3-dimensional 39 exposures and natural sections through both isolated, rounded 40 stromatolite bodies (bioherms), and stratiform, continuous 41 stromatolite beds (biostromes). Stromatolites are algal bodies 42 that originated as thin, slimy surface films, called microbial 43 mats, which trapped fine-grained carbonate sediment as they grew 44 and progressively accreted into symmetrical masses or layers up to 45 several metres across. The algal filaments are no longer 46 preserved, but evidence of microbial growth is preserved by 47 48 numerous, parallel, millimetre-scale laminations, now largely of 49 dolomite. This laminar texture can be seen clearly where erosion 50 has exposed the interior of these ball-like and sheet-like 51 structures. 52 The locality lies astride the complex hinge-zone of the F1 Islay 53 Anticline (Bailey, 1917), and the rocks are more highly strained 54 than at the Caol Isla GCR site, with the development of large open 55 folds, a penetrative cleavage oblique to bedding, and an evident 56 distortion of angular relationships between sediment-filled cracks 57 and the bedding planes (Borradaile and Johnson, 1973). The rocks 58 belong to the greenschist facies, with the local development of 59 phlogopite in metacarbonate rocks suggesting that temperatures of 60 61 62 63 64 65 1 2 3 4 around 400°C were reached during the regional metamorphism 5 (Fairchild, 1980c). 6 This part of Islay appears not to have been visited by the early 7 geological travellers in the Western Isles such as MacCulloch and 8 Thomson, and the earliest work was carried out by the Geological 9 Survey (Wilkinson, 1907; Bailey, 1917) and Green (1924). Allison 10 (1933) confirmed Bailey‘s (1917) stratigraphical sequence by the 11 12 use of way-up structures, and published the first detailed sketch 13 map of part of the site. Spencer (1972) established the current 14 informal subdivision of the Bonahaven Dolomite into 4 units and 15 prepared a detailed map of the whole section. However, it is 16 Fairchild to whom we largely owe our current understanding of these 17 rocks. With rare perseverance he has investigated in minute detail 18 the stratigraphy, structure, petrography, chemistry, and other 19 aspects of the formation (Fairchild, 1977, 1980a, 1980b, 1980c, 20 1985, 1989), and the description of the rocks given here relies 21 heavily on his field guide to the section (Fairchild, 1991). 22 23 4.2 Description 24 25 The eastern end of the GCR site around Rhuvaal Lighthouse (1 km 26 east of the east end of the section shown in Figure 9) is in the 27 Jura Quartzite, but the main interest lies in the folded and 28 faulted outcrops of members 3 and 4 of the Bonahaven Dolomite 29 30 Formation. The Dalradian rocks are cut by numerous NW-trending 31 dolerite dykes of Palaeogene age, which act as convenient waymarks 32 in the section. 33 Member 3 is up to 200 m thick and forms two main outcrops west of 34 Bagh an Da Dhoruis (Figure 9). The base is seen at only one 35 locality, immediately west of the Bolsa Fault at NR 385 780. The 36 member consists almost entirely of fine-grained dolomitic rocks, 37 with abundant sedimentary structures, which may be divided into 38 three facies (Fairchild, 1980b): 39 (1) rocks consisting of one-centimetre-scale alternations of 40 dolomitic metasandstone and metamudstone; 41 (2) cross-stratified dolomitic metasandstones (to 3 m thick) and 42 lesser pure metadolostone interbeds; and 43 (3) stromatolitic beds, of which ten continuous stromatolitic units 44 (biostromes) are recognized. 45 Stromatolites showing columnar and domal forms are seen at NR 410 46 788; outstanding exposures showing clusters of ellipsoidal and bun- 47 48 shaped stromatolites of different sizes are found at NR 407 789 49 (Figure 10a); and a number of large bioherms, several metres 50 across, are seen at NR 387 782 (Figure 10b), where they lie in beds 51 folded by F1 folds with axial planes dipping eastwards. 52 Special features of these rocks, which give a clue as to their 53 environment of deposition, include shrinkage cracks, stromatolite 54 flake breccias, and quartz-calcite nodules. They are accompanied 55 by abundant cross-lamination, ripple marks on a variety of scales 56 (up to 0.22 m in wavelength), graded bedding, and some striking 57 examples of load structures (Fairchild, 1991). The shrinkage 58 cracks are filled with fine-grained dolomitic material, and are 59 seen as pale-coloured dykelets, up to 0.5 cm wide and 3 cm 60 (exceptionally 40 cm) long, which in cross-section are seen to 61 62 63 64 65 1 2 3 4 descend from the bases of beds (Spencer and Spencer, 1972; 5 Borradaile and Johnson, 1973; Fairchild, 1980b). They are commonly 6 ptygmatically folded, and form an irregular or incomplete polygonal 7 pattern in plan view on the bedding surface. The shrinkage cracks 8 can be seen at NR 408 709 and are particularly well exposed at NR 9 390 783, where they can be examined in three dimensions on small 10 sea stacks scoured clean by the sea. 11 12 The stromatolite flake breccias are seen at NR 387 782. They are 13 intriguing structures, which occur as randomly arranged aggregates 14 of broken stromatolite laminae that occupy either shallow hollows 15 on the surface of a bed or, less likely, partially eroded and then 16 filled, desiccation cracks. The quartz-calcite nodules are clearly 17 seen in cross-section on wave-scoured surfaces on an old sea stack 18 near NR 390 783 and are deformed within a near-vertical cleavage. 19 Member 4 is 40–62 m thick and consists of slaty metasiltstones and 20 fine-grained metasandstones. The base is well exposed in Bagh an 21 Da Dhoruis near NR 410 788 (Figure 9), and the member includes a 10 22 m-thick massive metadolostone, exposed at NR 400 784. 23 The Jura Quartzite forms a large outcrop at the east end of the 24 GCR site on Rubha a‘Mhail, and a transitional contact with the 25 underlying Bonahaven Dolomite is seen in the cliffs at NR 412 787. 26 The quartzite is repeated by folding in the middle of the section, 27 between NR 391 781 and NR 399 783, and crops out within and west of 28 the Bolsa Fault (Figure 9b). It is a well-bedded, white- 29 30 weathering, meta-quartz-arenite with some cross-bedding. 31 Stromatolites were first recognized by Wilkinson (in Peach and 32 Horne, 1930), and were figured from the Rubha a‘Mhail section at 33 Bagh an Da Dhoruis by Anderson (1951, fig. 7), followed by Spencer 34 and Spencer (1972). The biostromes form stratiform layers 0.1–4 m 35 thick (generally 1–2 m), which consist of laminated yellow-brown 36 dolomite with a characteristic ‗elephant-skin‘ texture on the 37 weathered surface. The laminae are 0.5–2 mm thick, and consist of 38 alternating dark (finer grained) and lighter (coarser grained) 39 dolomitic layers whose microstructure has been studied in detail by 40 Fairchild (1980b). They generally form regular layers and rarely 41 develop the columnar forms seen in some stromatolites. These 42 horizons are accompanied by bioherms consisting of families of 43 extraordinary-looking and beautifully exposed spheroidal to 44 ellipsoidal stromatolite bodies, which can be examined in their 45 position of growth on the bedding planes. These bodies are from 10 46 cm to several metres in diameter. 47 48 The major anticlinal closure seen in the eastern outcrop of Member 49 3 in this section plunges southwards and is accompanied by a number 50 of minor folds (Figure 9c). The latter are up to tens of metres in 51 wavelength and, together with a steeply dipping slaty cleavage, are 52 generally congruous to the major structure (Fairchild, 1977). The 53 western limb of this fold has been affected by two major faults, 54 which throw down to the east and cause a double repetition of 55 members 3 and 4 (Figure 9b). The complex fault-zone of one of 56 these dislocations, the Bolsa Fault, can be examined at NR 386 780. 57 Borradaile and Johnson (1973) estimated the strain that has 58 affected Member 3, by measuring the angular relationships between 59 the penetrative cleavage, the filled dykelets and the bedding 60 planes in these rocks. They assumed that, regardless of their 61 62 63 64 65 1 2 3 4 origin, the dykelets would have formed initially at right angles to 5 the bedding surfaces, and used the present angles made with bedding 6 and cleavage to calculate the amount of strain at 5 localities 7 within this GCR site. The measured strains indicate that the 8 measured stratigraphical thickness is only 50–70 % of the original 9 thickness. 10 11 12 4.3 Interpretation 13 14 Minor features having a direct bearing on the environmental setting 15 of these rocks are discussed first. The precise origin of the 16 sediment-filled cracks in the dolomitic metasandstone–metamudstone 17 sequence is uncertain, but they must have formed by some form of 18 contraction in the plane of the bedding, either by subaerial 19 desiccation, interstratal dewatering, or synaeresis (Spencer and 20 Spencer, 1972; Borradaile and Johnson, 1973; Fairchild, 1980b). 21 The latter origin was favoured by all of these authors, but the 22 actual mechanism by which this process takes place is not clear, 23 and no unambiguous natural examples of synaeresis cracks have been 24 reported (see Tanner, 1998a for discussion). The cracks closely 25 resemble those from the Devonian rocks of Caithness, which were 26 interpreted as synaeresis cracks by Donovan and Foster (1972), but 27 as desiccation cracks by Astin and Rogers (1991). 28 The stromatolite flake breccias are very distinctive and are 29 30 derived from the local breakdown or erosion of stromatolite bodies. 31 This process does not involve subaerial exposure of the sediment 32 surface, and the fragments could have been swept by currents into 33 depressions or even into synaeresis cracks at the sediment–water 34 interface. Fairchild (1985) considered the quartz-calcite nodules 35 to be secondary after anhydrite, despite earlier doubts (Fairchild, 36 1980b). 37 Sedimentological analysis has shown that Member 3 was deposited in 38 a shallow-water environment (Fairchild, 1980b, 1989). The layered 39 facies represents a broad lagoonal environment, with local 40 emergence that was affected by infrequent storms; the sandstone 41 facies was deposited in lower intertidal sandflats; and the 42 stromatolitic facies was mainly subtidal, with some intertidal 43 development. Overall, the finely crystalline carbonate rocks, 44 ooids, and pseudomorphs after (?)anhydrite, indicate semi-arid 45 conditions and a warm climate. The petrography and carbonate 46 chemistry of the dolomitic rocks were described by Fairchild 47 48 (1980b, 1985). The dolomites are rich in Fe and Mn, and 49 dolomitization occurred before burial and penecontemporaneous with 50 sedimentation. Unfortunately the stromatolites are not suitable 51 for taxonomic study, do not yield any useful chronostratigraphical 52 information and could have grown in either warm or cold water 53 conditions. 54 Five independent lines of evidence, including wave-ripple 55 geometries and lateral facies relationships, indicate that Member 3 56 was derived from a landmass to the north-west (Fairchild, 1989). 57 Current directions from cross-strata are weakly bimodal or show no 58 preferred direction (Fairchild, 1980b). 59 Member 4 is also of shallow-water origin, with the thick 60 metadolostone having been deposited in a supratidal-flat 61 62 63 64 65 1 2 3 4 environment (Fairchild, 1991). It is succeeded, by the tidal-shelf 5 Jura Quartzite. 6 The Islay Anticline was first recognized by Bailey (1917), who 7 considered it to be a ‗secondary‘ structure. Later workers 8 (Roberts, 1974; Fairchild, 1977) concluded that it is a major 9 primary (D1) structure but it has a number of anomalous features. 10 For example, the structure of its closure at this GCR site, where 11 12 it is best exposed, is not fully understood. Over most of Islay, 13 the anticline plunges and closes to the north, but at this GCR site 14 the closure plunges south, suggesting that the Bonahaven Dolomite 15 Formation has been brought back to ground level by a plunge 16 culmination in the major structure. However, there is an area 17 (between NR 398 787 and NR 410 788) which has an ‗anomalous‘ 18 cleavage orientation (Fairchild, 1977), suggesting that there may 19 be a cleavage that pre-dates the main cleavage associated with the 20 Islay Anticline. These structural problems clearly warrant further 21 investigation. Another interesting structural aspect of the area 22 is that the Bolsa Fault is inferred, from an abrupt change in 23 thickness of Member 3 across the fault-zone, to have been active 24 during sedimentation (Fairchild, 1980c; Anderton, 1985). 25 26 4.4 Conclusions 27 28 The Rubha a’Mhail GCR site is of international importance for the 29 30 excellent state of preservation, in three dimensions, and in their 31 position of growth, of fossil algal bodies (stromatolites). These 32 are amongst the most primitive fossil forms to be preserved in the 33 geological record, and pre-date the evolution of more advanced 34 organisms, the metazoans, which used them as a food source. 35 Stromatolites originated as a microbial slime, which coated the sea 36 floor and, by trapping grains of sediment, enabled a variety of 37 different forms to develop. At this locality, these range from 38 continuous layers to a variety of intriguing spheroidal and 39 elliptical bodies up to 3 m across. The range in morphological 40 types, and excellent state of preservation of these stromatolites 41 is unique in the Precambrian rocks of the British Isles, and they 42 are of value for future study. 43 Sedimentary structures at this GCR site show that the 44 stromatolites grew within a sequence of rocks that was deposited in 45 a shallow-water, subtidal to intertidal environment. Due to the 46 low degree of metamorphism and deformation, the mode of formation 47 48 of these organisms, and their relationship to bedding and other 49 sedimentary structures can be examined in detail. Some 50 problematical small-scale structures found only at this site could 51 shed more light on the environment in which the stromatolites 52 thrived. These include possible desiccation cracks and 53 pseudomorphs after anhydrite (calcium sulphate, normally formed by 54 the evaporation of seawater and hence indicating a warm climate). 55 The dolomitic rocks described from here and the Caol Isla GCR site 56 overlie the Port Askaig Tillite, and this site has provided a type 57 section for a detailed comparison with dolostones associated with 58 other late-Precambrian tillites, in particular those in East 59 Greenland (Fairchild, 1989). 60 61 62 63 64 65 1 2 3 4 5 KILNAUGHTON BAY, ISLAY 5 (NR 346441–NR 345450) 6 7 C.A. Bendall 8 9 10 11 5.1 Introduction 12 13 The transition from the Islay Subgroup to the Easdale Subgroup is 14 important in terms of the sedimentary evolution of the Dalradian 15 Supergroup in the South-west Grampian Highlands. It marks the 16 change from shallow-water marine sands to deeper water muds and 17 gravity-flow deposits. An almost complete succession through this 18 transition is exposed on the south-west side of Kilnaughton Bay 19 near Port Ellen in south Islay (Figure 11). Approximately 550 m of 20 the succession is exposed along the foreshore and in the low cliffs 21 20–50 m inland. The succession here comprises the Jura Quartzite 22 Formation (Islay Subgroup) and the overlying Scarba Conglomerate 23 Formation (Easdale Subgroup). The latter formation includes the 24 Jura Slate Member at the base and the upper part is referred to 25 here as the Pebble Beds for ease of description. These correlate 26 well with the succession found on Jura (see the Lussa Bay and 27 Kinuachdrachd GCR site reports). The Pebble Beds are of particular 28 interest here. The beds are generally about a metre thick and 29 30 consist of rounded pebbles of quartzite between 1 and 2 cm in size. 31 These beds have a well-developed pressure-solution cleavage and the 32 pebbles, although rounded, are distinctively either flattened or 33 ellipsoidal and therefore are potentially good strain indicators. 34 Hence they would enable quantification of the strain associated 35 with the Islay Anticline. 36 Some white bladed porphyroblasts of highly altered kyanite occur 37 in white-mica-rich rocks belonging to the Jura Quartzite Formation 38 (Figure 13). Much of the original kyanite has been replaced by 39 fine-grained pyrophyllite and kaolinite (Burgess et al., 1981). 40 The rocks at this locality, which also include chloritoid-bearing 41 assemblages, have undergone greenschist-facies metamorphism and lie 42 within the biotite zone. In the Dalradian it is most unusual to 43 find kyanite in rocks of biotite grade, as kyanite is usually 44 associated with much higher grade (amphibolite-facies) rocks, 45 according to the ideal Barrovian mineral-zone sequence. 46 47 48 5.2 Description 49 50 Some 300 m thickness of the Jura Quartzite Formation is exposed 51 along the foreshore (Figure 11). At its maximum thickness on Jura, 52 the quartzite is of the order of 5000 m thick, whereas on Islay the 53 thickness varies from c. 3000 m in the north to 1000 m around 54 Kilnaughton Bay (Anderton, 1985). At Kilnaughton Bay the rocks are 55 predominantly coarse-grained metasandstones, with scattered pebbles 56 ranging up to about 5 mm in size. The thickness of the 57 metasandstone beds ranges from a few centimetres to massive beds 58 some 2.5 m thick. Thin partings of metamudstone are common and 59 there are interbedded metasiltstone–metamudstone units several 60 metres thick. Beds of fine- to medium-grained metasandstone are 61 62 63 64 65 1 2 3 4 also quite common. Cross-bedding is ubiquitous throughout the 5 succession and is generally planar. The rocks do not appear to be 6 well sorted and thin-section analysis reveals a bimodal grain-size 7 distribution with the coarser grains supported in a matrix of finer 8 sand grade. Most grains appear to be subangular to subrounded, 9 although the grain shape may have been modified by quartz 10 overgrowth. Although white feldspar can be found, the sandstones 11 12 are not generally feldspathic. 13 Conformably overlying this formation are approximately 60 m of the 14 Jura Slate Member, which is not fully exposed on the foreshore but 15 can be seen in a disused quarry just inland (NR 3470 4435). These 16 are fine-grained rocks with a prominent slaty cleavage. Bedding is 17 defined by thin partings of coarser material, and shows that these 18 rocks are folded by minor folds with wavelengths generally less 19 than 1 m. 20 Conformable above this are the Pebble Beds. This unit consists of 21 a series of pebbly beds that occur within a succession of finer 22 metasandstones and metamudstones whose top is not exposed (see 23 Figure 12). Individual pebbly beds are usually no more than about 24 a metre thick and fine upwards into metasandstone units; they have 25 sharp erosional bases. The pebbles are of rounded quartzite and 26 range from 1–5 cm in size. The pebbly beds are generally matrix 27 supported, with the matrix being of coarse sand grade. Up to 13 28 pebbly beds occur between the top of the Jura Slate Member and the 29 30 lighthouse (NR 3479 4440–NR 3495 4464). The top of the Scarba 31 Conglomerate Formation is not exposed but it grades up into the 32 pelitic Port Ellen Phyllite Formation. 33 These rocks lie on the south-eastern limb of the Islay Anticline 34 (Figures 1 and 3) (Bailey, 1917). The quartzites have an 35 anastamosing spaced cleavage defined by cleavage domains of white 36 micas, which appears to be a primary cleavage as no earlier fabric 37 can be seen in the quartz microlithons. The vergence of cleavage 38 on bedding is to the north-west, which is consistent with the 39 cleavage having formed at the same time as the Islay Anticline. 40 The Islay Anticline is believed to have formed during the first 41 phase (D1) of Dalradian regional deformation (Roberts and Treagus, 42 1977c). The cleavage in the Jura Slate is a primary slaty 43 cleavage. At least one crenulation cleavage is also found in the 44 slaty rocks. It dips between 25 and 50º to the south and gives a 45 sense of vergence (on bedding and the primary cleavage) to the 46 south-east. This cleavage, in places, consists of very closely 47 48 spaced (less than 0.5 mm) planar micaceous surfaces and looks 49 superficially like a slaty cleavage. 50 There is also a spaced cleavage in the pebbly beds that appears to 51 be primary, and contemporaneous with the formation of the Islay 52 Anticline. The pebbles are generally elliptical with aspect ratios 53 (long axis: short axis) of between 1.5:1 and 3:1, and appear to 54 show a consistent alignment, clearly demonstrating that the rocks 55 have been strained. 56 At the top of the Jura Quartzite Formation there is a fine-grained 57 quartzite that contains planar cleavage planes. These cleavage 58 planes are formed by thin partings of white-mica-rich rock, and 59 hence these rocks are likely to be aluminium rich. Lying on the 60 cleavage planes are white bladed grains up to about 1 cm long 61 62 63 64 65 1 2 3 4 (Figure 13). These blades have been interpreted as porphyroblasts 5 of kyanite that have subsequently been retrogressed to a mixture of 6 kaolinite and, more rarely, pyrophyllite (Burgess et al., 1981). 7 The blades lie within the cleavage plane but are randomly 8 orientated within that plane. 9 Other rocks cropping out on the site reveal little about the grade 10 of metamorphism. However, elsewhere in south-east Islay, 11 12 intrusions of mafic meta-igneous rocks indicate that the grade of 13 metamorphism reached greenschist facies, and pelitic rock 14 assemblages usually contain biotite as the highest grade index 15 mineral. The rocks therefore lie within the Barrovian biotite zone 16 and the presence of kyanite is anomalous. 17 18 5.3 Interpretation 19 20 The rocks at this site, together with the GCR sites on Jura, 21 preserve an important episode in Dalradian sedimentation; namely 22 the change from the shallow-marine sandstones of the Jura Quartzite 23 Formation, to the deeper water slump deposits of the Scarba 24 Conglomerate Formation. According to Anderton (1979) this 25 represents a change from a stable tectonic environment to an 26 unstable environment, with rapid subsidence of a basin taking place 27 along syndepositional faults. 28 At this GCR site the cross-bedded metasandstones, along with the 29 30 interbedded metasandstones and metamudstones, are typical of 31 shallow tidal-shelf deposits (Anderton, 1976). The Jura Slate 32 Member was originally laid down as mud with thin sand beds, 33 indicating a rapid change in depositional environment to deeper 34 water conditions. The pebbly beds are slump deposits or debris- 35 flow deposits, which were laid down on a deep-water marine slope in 36 a similar fashion to those described from the Scarba Conglomerate 37 on Jura (e.g. Anderton, 1979). This apparent rapid change in 38 depositional environment indicates tectonic instability and rapid 39 subsidence along basin-bounding faults. 40 The main phase of deformation in the eastern part of Islay was the 41 first, D1 and this resulted in the formation of the Islay Anticline 42 (Roberts and Treagus, 1977c). The predominant cleavage in the 43 Dalradian rocks is associated with this phase, while later stages 44 of deformation have been responsible for the development of 45 crenulation cleavages and minor folding. There are no major fold 46 structures associated with these later events, and it is likely 47 48 that the bulk of the strain that the rocks have experienced 49 occurred during the formation of the Islay Anticline. Pebble beds 50 are useful strain indicators; in this case the pebbles have been 51 flattened rather than stretched, with X:Z ratios (length:height) 52 varying between about 1.5:1 to 3:1. It is possible to discern a 53 stretching direction in these rocks, which appears to plunge 54 approximately east at c. 20°. However, more detailed studies are 55 necessary to quantify the strain. 56 Establishing the temperatures and pressures of metamorphism using 57 geothermometers and geobarometers based on mineral compositions is 58 not straightforward in greenschist-facies rocks, as the rocks do 59 not always attain thermodynamic equilibrium between the constituent 60 61 62 63 64 65 1 2 3 4 mineral phases. Hence it is important to consider other lines of 5 evidence. 6 Kyanite is very rarely found in low-grade Dalradian rocks, as it 7 can only form in highly aluminous pelitic rocks at this grade. The 8 majority of Dalradian pelites are relatively aluminium poor and are 9 correspondingly richer in iron and magnesium; hence the consistency 10 of the Barrovian Zones across the central Grampian Highlands. This 11 12 GCR site has the only reported occurrence of kyanite in the 13 greenschist-facies rocks of the Dalradian of the South-west 14 Grampian Highlands. Using the occurrence of kyanite, and its 15 growth at the expense of pyrophyllite, Skelton et al. (1995) have 16 proposed that the peak metamorphic temperature for this area of 17 Islay was in excess of 430 °C. Indeed, this is one of the critical 18 localities for establishing the grade of metamorphism in the South- 19 west Grampian Highlands. 20 21 5.4 Conclusions 22 23 The Dalradian of the South-west Grampian Highlands has undergone a 24 long convoluted history in terms of sedimentation, deformation, and 25 metamorphism, and the Kilnaughton Bay GCR site provides information 26 on all three of these aspects. The metasedimentary rocks here and 27 in the Lussa Bay and Kinuachdrachd GCR sites record an episode of 28 rapid sea-level change, which was probably caused by movements 29 30 along major basin-bounding faults. All three sites lie on the 31 south-east limb of the Islay Anticline but the rocks at Kilnaughton 32 Bay have undergone greater deformation, which can be quantified by 33 measurements of pebbles within the Scarba Conglomerate Formation. 34 An unusual occurrence of kyanite within the Jura Quartzite 35 indicates that these rocks were heated up to over 430°C during the 36 metamorphism that accompanied the deformation. Kyanite is normally 37 found in medium- to high-pressure, upper amphibolite-facies 38 assemblages its occurrence here in rocks of lower metamorphic grade 39 is thought to be due to an unusually high aluminium content in the 40 sediments that the rocks were derived from. This might be the only 41 known example of kyanite-bearing low-grade metamorphic rocks in 42 Britain. 43 44 6 LUSSA BAY, JURA 45 (NR 637 865–NR 648 870) 46 47 48 C.A. Bendall 49 50 51 6.1 Introduction 52 53 Much of the island of Jura consists of rugged hills, which are 54 composed almost entirely of monotonous Jura Quartzite, but along 55 the south-east coast there are excellent exposures of spectacular 56 rocks that reflect the change in the depositional environment from 57 the shallow-water Jura Quartzite Formation to the deeper water 58 Scarba Conglomerate Formation. The transition between these 59 formations is well exposed at Lussa Bay, which is located about 60 half way along the south-east coast of the island. Sedimentary 61 62 63 64 65 1 2 3 4 structures are particularly well preserved in the coarser-grained 5 metasedimentary rocks of Jura, and this is superbly demonstrated 6 around Lussa Bay. 7 The Dalradian rocks of Jura were first described by Peach et al. 8 (1911) and later by Allison (1933). More recent work includes that 9 of Anderton (1976, 1977, 1979, 1980), which is mainly concerned 10 with the sedimentology of the Islay and Easdale subgroups of the 11 12 Argyll Group. A revised edition of the BGS 1:50 000 Sheet 28W 13 (South Jura) was published in 1996. 14 15 6.2 Description 16 17 The coastal section at Lussa Bay is described below from west to 18 east (Figure 14). 19 Cross-bedding may be observed in the metasandstones of the Jura 20 Quartzite Formation (e.g. at NR 6383 8662), where the overall 21 bedding dips moderately steeply to the south-east. The Jura 22 Quartzite is overlain conformably by a thin unit of slaty 23 metamudstone with subordinate metasandstones that almost certainly 24 correlates with the Jura Slate Member that forms the base of the 25 Scarba Conglomerate Formation elsewhere on Jura. This slaty unit 26 is around 20 metres thick and is overlain conformably by fine- 27 grained, grey metasandstone. Beds in this metasandstone unit are 28 around half a metre thick and they exhibit small-scale cross- 29 30 bedding, load structures and abundant graded bedding (Allison, 31 1933). 32 At least one conglomeratic bed occurs in this succession. This 33 consists of a pebbly bed with subangular quartzite clasts up to 1.5 34 cm across. It varies in thickness, with a maximum of about 1 m. 35 The succession of metasandstones and rare metaconglomerates is 36 approximately 150 m thick and is overlain by another metamudstone 37 unit. This metamudstone crops out along both the north-west and 38 south-east shores of Lussa Bay itself (Figure 14), and erosion of 39 this unit is probably responsible for the development of the bay. 40 On both shores the metamudstone has a well-developed slaty 41 cleavage. Along the south-east shore there is an abrupt facies 42 change from the metamudstone to a superb section of 43 metaconglomerates. Approximately 200 m of this succession are 44 exposed along the coastline from Lussa Bay around Lussa Point and 45 then north-east along the coast. The massive beds vary from a few 46 tens of centimetres to a metre or more in thickness. Most fine 47 48 upwards from coarse pebbly bases (clast sizes up to 10 cm and 49 generally matrix-supported) to coarse metasandstone. Individual 50 beds have sharp bases, and erosional features such as rip-up clasts 51 of mudstone and sandstone are quite common (Figure 15). The 52 phyllitic mudstone rip-up clasts are irregularly shaped lenses 53 varying in size from a few centimetres to a few tens of 54 centimetres. They are generally elongate and are aligned 55 approximately parallel to bedding. Sandstone clasts also form 56 lenses but generally tend to be flatter and more elongate. They 57 vary from a few tens of centimetres to around 1 m in length, and 58 are also aligned approximately parallel to bedding. There are rare 59 beds of fine-grained material (metamudstone and metasiltsone) 60 interbedded with the metaconglomerates. Mass-flow deposits of 61 62 63 64 65 1 2 3 4 incompletely mixed mud, sand, and gravel with large partially 5 disintegrated boulders of sand and mudrock also occur (Anderton, 6 1977). Towards the top of the succession the beds take on a more- 7 recognizable turbiditic nature; they are coarse grained but the 8 grading is more pronounced. 9 Structurally this site lies on the south-east limb of the Islay 10 Anticline (Bailey, 1917; Roberts and Treagus, 1977c). Bedding 11 12 strikes approximately north-east and dips between 35 and 50º to the 13 south-east. The slaty cleavage has approximately the same strike 14 and dip direction but is steeper, giving a sense of vergence 15 towards the Islay Anticline to the north-west. Tectonic structures 16 appear to be restricted to the finer grained lithologies, and are 17 expressed as minor folds and a slaty cleavage. The coarser grained 18 lithologies are only weakly deformed and show little evidence of 19 tectonic strain. The majority of the clasts in the 20 metaconglomerate appear to be undeformed. Hence, because of the low 21 strain in the coarse-grained rocks, they retain their original 22 sedimentary characteristics. 23 The rocks here have been subjected to greenschist-facies 24 metamorphism although, apart from the slaty cleavage in the finer 25 grained horizons, there is little direct evidence of metamorphism. 26 The deformation and the metamorphism occurred during the Grampian 27 Event. 28 29 30 6.3 Interpretation 31 32 The transition from the Islay Subgroup into the Easdale Subgroup at 33 Lussa Bay and elsewhere on Islay and Jura (see the Kilnaughton Bay 34 and Kinuachdrachd GCR site reports) demonstrates a distinct change 35 in sedimentary environment from shallow-marine shelf to a deeper 36 marine slope. This is clearly shown by the metasedimentary rocks 37 that crop out at Lussa Bay. The metasandstones of the Jura 38 Quartzite were interpreted by Anderton (1976) as shallow-marine 39 tidal deposits, as is indicated by sedimentary structures such as 40 cross-bedding. The metamudstones indicate basin deepening, and the 41 metaconglomerates were interpreted by Anderton (1979) as marine- 42 slope slump deposits. These are deposits that formed when clastic 43 material building up on the upper parts of a marine slope became 44 unstable and started to flow down slope. This process tends to 45 disrupt the material, and results in the deposition of coarse- 46 grained, poorly sorted massive beds. The slumping commonly ceases 47 48 before the material can mix with water and develop into turbidity 49 flows. However, some mixing probably took place and this resulted 50 in the observed grading. Towards the top of the succession, where 51 the grading is better developed, it is likely that this water– 52 sediment mixing proceeded further, producing the coarse-grained 53 turbidites. Most of the fine-grained material would have been 54 winnowed out and removed as suspended load, which was then 55 deposited farther out into the basin. 56 The rapid deepening event, from the shallow-water Jura Quartzite 57 to the deeper water Scarba Conglomerate, indicates tectonic 58 instability and is good evidence for fault-controlled rifting, 59 which probably had a strong control on sedimentation during the 60 61 62 63 64 65 1 2 3 4 deposition of much of the Easdale, Crinan and Tayvallich subgroups 5 of the Argyll Group. 6 7 6.4 Conclusions 8 9 Along with exposures at other GCR sites along strike at Kilnaughton 10 Bay on Islay and Kinuachdrachd on Jura, the Lussa Bay GCR site 11 12 provides vital clues for tracing the evolution of the Dalradian 13 basin in early Argyll Group time. This GCR site exposes in 14 particular an almost complete representative section through the 15 Scarba Conglomerate Formation, which occurs here in a region of 16 relatively low tectonic strain and hence has spectacular exposures 17 of some of the best-preserved sedimentary features seen in 18 Dalradian metaconglomerates. On the islands of Scarba and Jura, 19 these rocks are particularly coarse grained and formed by material 20 ‘slumping’ down a deep marine slope. Such was the energy of this 21 slumping that material was ripped up from the sea floor and 22 incorporated into the slump deposit as large ‘rip up clasts’. 23 24 25 7 KINUACHDRACHD, JURA 26 (NR 694 953–NR 708 974, NR 705 985–NM 700 012) 27 28 29 C.A. Bendall 30 31 32 7.1 Introduction 33 34 The Kinuachdrachd GCR site occupies a coastal strip in the remote 35 north-east of the Island of Jura (Figure 16). The strata at this 36 site are from the same stratigraphical interval as those described 37 at the Kilnaughton Bay GCR site on Islay and the Lussa Bay GCR site 38 on Jura. To the west of the site is the Jura Quartzite, which 39 forms the spine of the island, but along the eastern coast are 40 rocks of the overlying Scarba Conglomerate Formation. The rocks of 41 Jura have been described by Peach et al. (1909, 1911) and more 42 recently by Anderton (1976, 1977, 1979, and 1980). Anderton 43 interpreted the succession as representing a change from shallow- 44 water shelf sandstones (Jura Quartzite) to deeper water muds, slump 45 deposits and turbidites (Jura Slate and Scarba Conglomerate). 46 Exposed intermittently inland, and unique in the Scottish 47 48 Dalradian, is a possible fossil slump scar (a result of erosion 49 caused by material slumping down a marine slope) that forms the 50 boundary between the Jura Quartzite and the Scarba Conglomerate 51 (Anderton 1977, 1979). 52 Also cropping out at this site are dykes of metamafic rock. 53 Whereas mafic sills are common throughout the Dalradian of the 54 South-west Grampian Highlands, recognizable dykes are relatively 55 rare. 56 57 7.2 Description 58 59 The Jura Quartzite Formation, which crops out inland from this 60 coastal GCR site, has been described in detail by Anderton (1976). 61 62 63 64 65 1 2 3 4 Sedimentary structures, such as cross-bedding, scours, possible 5 synaeresis cracks, and sandstone dykes, are well preserved. At 6 this north-eastern end of Jura, unlike at Lussa Bay or at 7 Kilnaughton Bay on Islay, the Jura Slate Member is absent, and the 8 Jura Quartzite is overlain directly by the conglomeratic facies of 9 the Scarba Conglomerate Formation. The nature of the Scarba 10 Conglomerate here was described by Anderton (1977). South of 11 12 Barnhill (NR 7050 9705), the lowest part of the Scarba Conglomerate 13 consists of pebbly beds, which pass upwards into fining-up 14 sequences (metasandstone–metamudstone). These then pass upwards 15 into a black metamudstone with a well-developed slaty cleavage, 16 which is probably equivalent to the Easdale Slate Formation 17 elsewhere in the South-west Grampian Highlands. Evidence of 18 slumping may be found in the lower part of the Scarba Conglomerate 19 Formation at (NR 702 966) where the succession is a chaotic mass of 20 boulders, with slump folds and mass flows (Anderton, 1977). 21 North of Barnhill, the boundary between the Jura Quartzite and the 22 Scarba Conglomerate has been interpreted by Anderton (1977, 1979) 23 as a possible fossilized slump scar. This slump scar truncates 24 both the Jura Quartzite and the lowermost Scarba Conglomerate 25 (Figure 16). The debris flows found above the slump scar range 26 from 1 m to 15 m in thickness and contain intraformational boulders 27 up to 6 m in size. The matrix is a poorly sorted mixture of sand 28 and mud. Soft-sediment deformation structures are common as are 29 30 other sedimentary structures such as sole marks, rip-up clasts and 31 small channel deposits, all indicative of high-energy dynamic 32 sedimentary environments. 33 Structurally this site lies on the south-east limb of the Islay 34 Anticline (Bailey, 1917; Roberts and Treagus, 1977c). Sedimentary 35 structures tend to dominate over tectonic structures, the most 36 obvious tectonic structures being the slaty cleavage found in the 37 finer-grained lithologies. A spaced cleavage developed in the 38 metasandstones shows marked cleavage refraction across beds that 39 preserve compositional grading (Figure 17). The tectonic strain 40 here is low, hence the retention of some rather subtle sedimentary 41 structures. 42 Also exposed here are dykes of metamafic rock. These dykes are 43 the predominant type of meta-igneous intrusion in north-east Jura, 44 unlike in much of the younger Dalradian succession in the South- 45 west Grampian Highlands where sills are ubiquitous. In northern 46 Jura the dykes have been examined in detail by Graham and 47 48 Borradaile (1984). They described the dykes as having a typical 49 greenschist-facies assemblage of albite, epidote, actinolite and 50 chlorite. Thinner dykes tend to be schistose, but thicker ones 51 retain a relict ophitic texture in their centres. The dykes have a 52 less-evolved chemical composition than the sills and metavolcanic 53 rocks of the South-west Grampian Highlands, and Graham and 54 Borradaile (1984) concluded that the dykes are likely to have been 55 feeders to the more-evolved rocks. These authors also estimated 56 that the pre-tectonic orientation of the dykes was north-west, and 57 that they were intruded perpendicular to bedding. 58 59 60 61 62 63 64 65 1 2 3 4 7.3 Interpretation 5 6 The transition from the Islay Subgroup into the Easdale Subgroup on 7 Jura demonstrates a distinct change in sedimentary environment from 8 shallow-marine shelf to a deeper marine slope. This is clearly 9 shown by the metasedimentary rocks that crop out at Kinuachdrachd. 10 The rocks described above are probably contemporaneous with those 11 12 described at the Lussa Bay and Kilnaughton Bay GCR sites and 13 therefore demonstrate that there are lateral facies changes along 14 strike. All of these three GCR sites provide evidence for rapid 15 deepening of the basin from a shallow-marine tidal environment, 16 where sedimentation was in approximate equilibrium with subsidence, 17 to a deep-water marine slope (Anderton, 1977, 1979). Therefore, 18 fault-controlled rifting probably had strong control on 19 sedimentation during the deposition of much of the Easdale, Crinan 20 and Tayvallich subgroups of the Argyll Group. 21 This rifting may have resulted in partial melting of mantle rocks, 22 giving rise to the ubiquitous basic igneous intrusive and extrusive 23 rocks evident in the Dalradian rocks of the South-west Grampian 24 Highlands. For this igneous activity to have occurred, the 25 extension of the Dalradian basin must have been in the order of a 26 factor of 2; that is the basin was twice as wide as it was before 27 rifting commenced (e.g. McKenzie and Bickle, 1988). The meta- 28 igneous dykes on Jura probably represent the conduits through which 29 30 the magma travelled upwards to higher crustal levels. From the 31 pre-tectonic trend of these dykes (north-west), Graham and 32 Borradaile (1984) proposed that the extension direction of the 33 Dalradian basin was north-east–south-west at the time of intrusion. 34 35 7.4 Conclusions 36 37 Like the GCR sites at Kilnaughton Bay on Islay and Lussa Bay in 38 central Jura, strata at the Kinuachdrachd GCR site in the north- 39 east of Jura demonstrate the abrupt change in sedimentary 40 depositional environment from shallow-marine shelf to deep-marine 41 slope on the Laurentian margin in early Argyll Group time. Here, a 42 putative fossil slump scar, which is an erosional feature formed by 43 rapid erosion of sea-floor sediments by material slumping down a 44 marine slope, forms the boundary between the Jura Quartzite and the 45 Scarba Conglomerate. Evidence of sediment sliding down the marine 46 slope has been preserved in the form of spectacular slump beds, 47 48 which contain boulders up to 6 m in size. Other dramatic evidence 49 of such high-energy sedimentation is provided by scours, and by 50 material that has been ripped up from the sea floor and 51 incorporated into the slump deposits. 52 In addition, this GCR site includes metamorphosed basaltic dykes, 53 which were probably emplaced as a result of stretching of the crust 54 and the upper mantle. They are thought to have been feeders to the 55 sills that are ubiquitous in upper Argyll Group rocks of the South- 56 west Grampian Highlands (e.g. at the Ardbeg and Ardilistry Bay GCR 57 sites), and in the overlying Tayvallich Volcanic Formation (see the 58 West Tayvallich Peninsula GCR site). The orientation of the dykes 59 gives an indication of the direction in which the stretching took 60 61 62 63 64 65 1 2 3 4 place (north-east–south-west) and suggests that by late Argyll 5 Group time the Dalradian crust had stretched by a factor of two. 6 7 8 SURNAIG FARM, ISLAY 8 (NR 396 451–NR 403 453) 9 10 11 C.A. Bendall 12 13 14 8.1 Introduction 15 16 Much of the detailed sedimentological history of the Dalradian 17 Supergroup has been deciphered from rocks in the South-west 18 Grampian Highlands (e.g. Anderton, 1985). This is because the 19 tectonic strain and metamorphic grade are relatively low, and hence 20 the original sedimentary structures are preserved. This is 21 superbly illustrated near Surnaig Farm, to the south-east of 22 Lagavulin Bay on the south-east coast of Islay (Figure 18), where 23 spectacular sandstone dykes are exposed on the rocky foreshore. 24 The dykes occur in rocks that are assigned to the Laphroaig 25 Quartzite Formation (Easdale Subgroup), which at this locality is 26 at least 260 m thick. Its boundary with the underlying Port Ellen 27 Phyllite Formation is gradational, representing a change from a 28 succession dominated by metamudstones to one dominated by 29 30 metasandstones. Poor exposure makes the positioning of this 31 boundary somewhat arbitrary. Elsewhere on Islay, the Laphroaig 32 Quartzite is overlain by the conglomeratic basal member of the 33 Ardmore Formation (Crinan Subgroup). 34 Other rocks that crop out at this locality include sills of 35 metamafic rock. The basal contact of one of these sills is exposed 36 at the west end of Lagavulin Bay (NR 4025 4536). Here the country 37 rocks are contact metamorphosed (Wilkinson, 1907) but have been 38 little affected by the subsequent regional metamorphism associated 39 with the Grampian Event. There is also a good exposure of a dyke 40 of metamafic rock on the foreshore at NR 3927 4523. 41 The Laphroaig Quartzite Formation was first described by Wilkinson 42 (1907). However, the sequence he proposed was upside-down as was 43 later demonstrated by Bailey (1917) and Allison (1933), using 44 sedimentary way-up indicators. The sandstone dykes were first 45 described by Borradaile (1974) who used the angular relationships 46 between the dykes, the cleavage and the bedding to estimate the 47 48 amount of strain associated with the formation of the Islay 49 Anticline. 50 51 8.2 Description 52 53 Sandstone dykes are not uncommon in Dalradian metasedimentary rocks 54 (e.g. Smith and Rast, 1958) and have been described from several 55 localities on Islay (Borradaile and Johnson, 1973; Borradaile, 56 1974). Most are small, being only a few centimetres or so wide and 57 a few tens of centimetres long. However, much larger sedimentary 58 dykes occur within the Laphroaig Quartzite Formation along the 59 south-east coast of Islay. The best exposed and most impressive of 60 these occur at this GCR site in a rocky bay 400 m west-south-west 61 62 63 64 65 1 2 3 4 of Lagavulin Bay (Figure 18). Here, the largest sedimentary dyke 5 is 0.5 m wide and penetrates 16 m beneath the source bed (outcrop 3 6 of Borradaile, 1974). 7 The dykes occur in the middle of the formation and are intruded 8 into beds of metamudstone, which are 2–3 m thick (Figure 19). These 9 metamudstone beds are interbedded with metasandstone beds, which 10 vary in thickness from 1 m to a few centimetres and form the source 11 12 rocks for the sedimentary dykes. However, it is not always clear 13 which metasandstone bodies represent true bedding and which are 14 cross-cutting sedimentary dykes. Some of the metasandstones are 15 original orthoquartzites, whilst others are calcareous. There are 16 also rare metalimestones, but none are more than 2–3 cm thick. This 17 part of the succession is c. 30 m thick. Above this, metamudstone 18 beds are much thinner (generally less than 10 cm thick), and the 19 succession is dominated by massive metasandstone beds, which vary 20 between 0.5 and 2 m in thickness. The strike of bedding here is c. 21 280° and the dip is about 30° to the south. 22 Upwards of thirty sedimentary dykes may be observed here, the 23 majority of which are only a few centimetres thick and 1–5 m long. 24 All lie at an acute angle to the bedding and are both folded and 25 planar. The planar dykes make angles with bedding of around 20° 26 and the long limbs of the folded dykes make higher angles, around 27 44°, with the bedding (Borradaile, 1974). The largest dyke 28 (mentioned above) is spectacularly folded (Figure 19). The folds 29 30 are close to tight and the cleavage in the host rock metamudstone 31 is axial planar to them. Some of the smaller dykes also show a 32 similar style of folding. There is one clearly exposed example of 33 two planar dykes cross-cutting each other and other examples of 34 dykes forming offshoots from other dykes. All the examples of 35 sedimentary dykes described above occur in a relatively small 36 exposure at NR 3978 4523. 37 Locally the sills of metamafic rock, which dip at 20 to 30° to the 38 east-south-east, dominate the landscape as they form prominent 39 ridges. One forms the headland on the western side of Lagavulin 40 Bay. This sill is c. 15–20 m thick and retains a relict ophitic 41 igneous texture. Also preserved at the base of the sill, at NR 42 4020 4535, are metasedimentary rocks that have been subjected to 43 contact metamorphism and, because of this, have been little 44 affected by the later regional metamorphism. Whereas sills are 45 common, dykes of metamafic rock are rare. There is an unequivocal 46 example of a highly schistose dyke on the foreshore 50 m to the 47 48 east of the sedimentary dyke locality. It is intruded into rocks 49 that are predominantly quartzitic metasandstones, is 2 m wide, 50 vertical and trends north–south, clearly cross-cutting the 51 metasedimentary rocks. 52 The rocks here were deformed and metamorphosed up to the 53 greenschist facies during the Grampian Event and the metamudstones 54 have been recrystallized into phyllites. The dominant cleavage is 55 an S1 penetrative cleavage, and it is this cleavage that is axial 56 planar to the folds of the sedimentary dyke described above. The 57 cleavage observed in the meta-igneous dyke is also S1. 58 59 60 61 62 63 64 65 1 2 3 4 8.3 Interpretation 5 6 Borradaile (1974) interpreted the metasedimentary dykes as 7 neptunian, that is the infilling with sand of fractures in already 8 consolidated sediment. He used the angular relationships between 9 bedding, cleavage and the dykes to estimate the amount of strain 10 that these rocks were subjected to during the D1 phase of 11 12 deformation that produced the major Islay Anticline (Roberts and 13 Treagus, 1977c). He concluded that there was 70% shortening normal 14 to the slaty cleavage and extensions of 63 % and 109 % within the 15 plane of the cleavage in the phyllitic rocks. More recently, many 16 sedimentary dykes elsewhere have been interpreted as due to the 17 forceful injection of sand from an unconsolidated source bed into 18 adjacent consolidated beds (Collinson, 1994); and, furthermore, the 19 formation of large sedimentary dykes due to passive infilling of 20 cracks requires the presence nearby of an unconformity or at least 21 the formation of an erosion surface in cohesive sediments. Hence 22 it may be that none of the dykes described here is neptunian in 23 origin. 24 Neptunian dykes indicate a break in sedimentation and dessication 25 of surface sediments. However, if the dykes resulted from forceful 26 injection, then they might have formed in buried sediments in which 27 liquefied sand was injected into a cohesive host, in this case 28 clay-rich sediment. This process usually requires overpressured 29 30 pore fluid, which commonly arises from rapid burial of the host 31 sediments, with the formation of sand-filled dykes possibly being 32 triggered by earthquakes (Collinson, 1994). 33 Dykes of metamafic rock up to 30 m wide have been described from 34 Jura (Graham and Borradaile, 1984) but they are rare on Islay and 35 on the mainland, where sills predominate. Graham and Borradaile 36 made the significant point that on Jura the host rocks are 37 stratigraphically older than those on Islay and the mainland. This 38 suggests that the igneous ‘plumbing system’ here may have followed 39 the model of Francis (1982), where the level of intrusion of sills 40 is constrained by the lithostatic pressure in the country rock and 41 the hydrostatic pressure in the feeder dyke. Intrusion of sills 42 will only occur where the hydrostatic pressure of the magma in the 43 feeder dyke is greater than the lithostatic pressure of the country 44 rocks; the lithostatic pressure directly relates to depth of 45 burial. At the time of intrusion of the sills and dykes, the 46 stratigraphically older rocks on Jura will have been at a greater 47 48 depth than the Port Ellen Phyllite and Laphroaig Quartzite 49 formations on Islay and, critically, too deep for the intrusion of 50 sills. It is probable that the dyke described above was a feeder 51 dyke for the nearby or higher sills. 52 The igneous dykes are important indicators of the extensional 53 stress regime associated with the Dalradian sedimentary basin. The 54 orientation of this dyke is similar to the majority found on Jura 55 and indicates an extension direction approximately east–west. 56 57 8.4 Conclusions 58 59 Although sedimentary dykes are not uncommon in the Scottish 60 Dalradian, none of the other reported examples are as spectacular 61 62 63 64 65 1 2 3 4 as those found at the Surnaig Farm GCR site. In fact these are 5 some of the best examples of sedimentary dykes in Britain and are 6 certainly the best found in deformed rocks. They are most likely to 7 have resulted from interstratal dewatering, as has been proposed 8 here for sandstone dykes at the Caol Isla GCR site, but some of the 9 diagnostic features have been obscured or destroyed at Surnaig Farm 10 by subsequent deformation. 11 12 The shoreline here also reveals several other intriguing aspects 13 of Dalradian geology, including a fine example of a sill of 14 metamafic rock and, rare for Islay, an unequivocal example of a 15 metamafic-rock dyke. The GCR site, therefore, provides evocative 16 snapshots of the evolution of the Argyll Group, from the 17 sedimentation and igneous activity associated with its early 18 depositional history, to the deformation and metamorphism of these 19 rocks in the subsequent mountain building episode now referred to 20 as the Grampian Event. 21 22 9 ARDBEG, ISLAY 23 (NR 413 459–NR 422 464) 24 25 C.A. Bendall 26 27 28 29 9.1 Introduction 30 31 The rocks on the foreshore near the Ardbeg Distillery on the south- 32 east coast of Islay (Figure 20) include a tightly folded and 33 metamorphosed doleritic sill, first described by Wilkinson (1907). 34 Towards the top of this sill, there are lenses containing the 35 mineral stilpnomelane. This rather unusual mineral (a sheet 36 silicate) is found scattered sporadically through low-grade 37 metamorphic rocks in the British Isles, but it is comparatively 38 rare. It is unusual to find it in such high concentrations and as 39 coarsely grained as it is at this locality. 40 The sill is one of a suite of pre-tectonic basic intrusions that 41 are ubiquitous throughout the Dalradian of the South-west Grampian 42 Highlands. However, this sheet has clearly been folded which is 43 rarely seen on Islay, although folded sills are commonly observed 44 on the mainland, for example at Tayvallich (Wilson and Leake, 1972) 45 and at the Point of Knap (Roberts, 1969). The mineral paragenesis 46 of the sill gives an indication of the grade of regional 47 48 metamorphism that the rocks have experienced. 49 The host rocks are metasandstones, metasiltstones and 50 metamudstones belonging to the Port Ellen Phyllite and Laphroaig 51 Quartzite formations of the Easdale Subgroup, and are at 52 approximately the same stratigraphical level as the rocks at the 53 Surnaig Farm and Ardilistry Bay GCR sites. These rocks demonstrate 54 a variety of sedimentary features, but are also quite important in 55 unravelling the geological history of the area, as they preserve 56 evidence, in the form of tectonic cleavages, for at least three 57 deformational events. Hence at this locality the growth of the 58 stilpnomelane may be considered in both a structural and 59 metamorphic context. 60 61 62 63 64 65 1 2 3 4 9.2 Description 5 6 The metasedimentary rocks at this GCR site comprise the upper part 7 of the Port Ellen Phyllite Formation and the lower part of the 8 Laphroaig Quartzite Formation. North of the site, the Port Ellen 9 Phyllite consists mostly of metamudstone but towards the top of the 10 formation (within the GCR site), metasandstone beds become more 11 12 prevalent. The boundary with the overlying Laphroaig Quartzite is 13 gradational, passing up into interbedded metasandstones and 14 metamudstones with rare metacarbonate rocks. The metasandstone 15 beds are dominant and vary in thickness between c. 0.5 and 2 m. A 16 wide range of sedimentary structures may be observed in this 17 formation, such as cross-bedding, dewatering structures and scours, 18 and sandstone dykes may be seen cross-cutting the bedding in the 19 metamudstones (Borradaile, 1974). 20 Subsequent basic magmatism resulted in the intrusion of a series 21 of doleritic sills. The sills are conspicuous in this part of the 22 island, as the intervening metasedimentary rocks have been 23 preferentially eroded leaving prominent ridges of metadolerite. It 24 is highly likely that they are genetically and spatially related to 25 the Tayvallich lavas found on the mainland, which have been dated 26 at c. 600 Ma using U-Pb dating techniques on zircons (Halliday et 27 al., 1989; Dempster et al., 2002). 28 The major structure in the Dalradian of Islay is the upward- and 29 30 north-west-facing Islay Anticline that formed during the first 31 phase of deformation of the Grampian Event (Bailey, 1917; Roberts 32 and Treagus, 1977c). This GCR site lies on the south-east limb of 33 the anticline, so the beds generally have moderate (40–60°) dips and 34 young to the south-east. The syncline–anticline fold pair that 35 folds the stilpnomelane-bearing metadolerite sill is parasitic to 36 the major Islay Anticline (as indicated by their north-west sense 37 of vergence). Associated with this folding is a penetrative 38 cleavage, which is best developed in the finer grained rocks such 39 as those found in the Port Ellen Phyllite Formation, where it is 40 continuous and slaty in some beds. This cleavage dips steeply to 41 the south-east and is axial planar to minor F1 folds (e.g. at NR 42 4170 4619). 43 At least two later stages of deformation can be recognized on the 44 foreshore beneath the distillery; these take the form of 45 crenulation cleavages and some minor open folding. One of the 46 crenulation cleavages dips steeply (c. 80°) to the north and in 47 48 places is the dominant fabric in the rock. The other crenulation 49 cleavage is only developed sporadically, and dips at a shallow 50 angle to the east. It is not clear which of these later cleavages 51 pre-dates the other, and there is no record of any major folds on 52 Islay associated with either cleavage. 53 Stilpnomelane is found in irregular lenses towards the top of the 54 70 m-thick folded metadolerite sill that crops out on the foreshore 55 just to the west of the distillery at NR 4185 4625 (Figure 20). It 56 is a bronze-coloured mineral with a metallic lustre and forms 57 radiating clusters, with individual grains up to 1 mm in length. 58 In thin section it resembles biotite but has a more reddish brown 59 colour and lacks the perfect mica cleavage (Figure 21). The colour 60 indicates that it is probably ferri-stilpnomelane. The 61 62 63 64 65 1 2 3 4 metadolerite host rock has a relict ophitic texture that is obvious 5 in hand specimen but less well defined in thin section. The 6 stilpnomelane-bearing lenses also contain actinolite, epidote, 7 albite, quartz, chlorite, calcite (rare) and leucoxene. The 8 actinolite is distinctly green and pleochroic; microprobe analysis 9 shows it to be iron-rich (Bendall, 1995) and recalculation of 10 microprobe data, based on the procedure outlined in Droop (1987), 11 12 implies a high ferric iron component. There does not appear to be 13 a tectonic fabric in the rock and the stilpnomelane clearly cross- 14 cuts all the other mineral phases; it is randomly orientated, 15 commonly forming radiating clusters (Figure 21). 16 17 9.3 Interpretation 18 19 Stilpnomelanes have the general formula: 2+ 3+ 20 (K,Na,Ca)0.6(Mg,Fe ,Fe )6Si8Al(O,OH)27.2–4H2O (Deer et al., 1992). 21 They tend to be iron-rich but the composition can vary between the 22 ferric end-member ferri-stilpnomelane and the ferrous end-member, 23 ferro-stilpnomelane. Consequently stilpnomelanes are most likely 24 to occur in iron-rich rocks, such as metamorphosed ironstones and 25 iron-rich meta-igneous rocks, but they are generally restricted to 26 lower- to middle-greenschist-facies metamorphic rocks. At Ardbeg, 27 the ferri-stilpnomelane is associated with actinolite that has high 28 ferrous and ferric iron concentrations with respect to its Mg 29 30 concentrations, supporting the association of stilpnomelane with 31 Fe-rich rocks. 32 Metamorphic mineral assemblages in the finer grained 33 metasedimentary rocks at this GCR site are indicative of the 34 biotite zone, and metadolerite assemblages are typical of 35 greenschist-facies metamorphism. The peak of metamorphism here was 36 probably associated with the D1 phase of deformation that was 37 responsible for the development of the Islay Anticline (Skelton et 38 al., 1995). The stilpnomelane appears to have grown later than the 39 other minerals that occur with it. However, it does not appear to 40 be retrogressive and therefore probably formed around the peak of 41 metamorphism, which was somewhere around 470 °C, according to 42 Skelton et al. (1995). The pressure estimates of 10 kbar assumed 43 in that study were all derived from sources that utilise phengite 44 equilibria (Powell and Evans, 1983). The authors conceded that they 45 are rather on the high side and expressed reservations as to the 46 reliability of such geothermometers. Pressures of around 5 kbar, 47 48 which imply burial depths of between 15 and 20 km, are more typical 49 of greenschist-facies metamorphism. 50 51 9.4 Conclusions 52 53 The Ardbeg GCR site is notable and of some international importance 54 for the occurrence of the metamorphic mineral stilpnomelane within 55 a metadolerite sill. Although stilpnomelane occurs sporadically 56 elsewhere in the Scottish Dalradian, here it is relatively abundant 57 and the fresh crystals are up to 1 mm in size, which is quite large 58 for stilpnomelane. This poorly understood mineral is preserved 59 here in a host rock that has not been significantly retrogressed. 60 Its growth is reasonably well constrained with respect to the 61 62 63 64 65 1 2 3 4 regional deformation and metamorphism, and the overall mineral 5 assemblage can be used to quantify the temperature and pressure of 6 metamorphism under which this particular stilpnomelane formed. An 7 important constraint on the formation of stilpnomelane is the iron- 8 rich chemical composition of the host rock, which can be reliably 9 established at this site as the rock is relatively fresh. 10 This small site also exhibits good examples of sedimentary 11 12 structures such as cross-bedding, dewatering structures and scours, 13 which are found within the Laphroaig Quartzite. Three different 14 small-scale tectonic fabrics are easily distinguished here and 15 these could prove important in establishing tectonic relationships 16 in south-east Islay. 17 18 10 ARDILISTRY BAY, ISLAY 19 (NR 443 485–NR 447 483) 20 21 C.A. Bendall 22 23 24 10.1 Introduction 25 26 Along the coastal sections at Ardilistry Bay, 8 km east of Port 27 Ellen in south-east Islay, metamorphosed basic sills account for 28 over half the succession. One of the sills, exposed along the 29 30 south-east shoreline of the bay (NR 4415 4816–NR 4441 4837), is 31 almost certainly unique in the British Isles. This sill is around 32 12–14 m thick and, towards the base, there is a 3 m-thick layer that 33 consists almost entirely of the amphibole, actinolite. The 34 protolith of this rock was almost certainly a pyroxene-cumulate and 35 the pyroxene has been replaced by actinolite during greenschist- 36 facies metamorphism. Although the mineralogy has changed, the 37 original cumulate texture is retained. The metapyroxenite is 38 overlain by a metamorphosed plagioclase layer approximately 1 m 39 thick, in which albite and epidote have replaced the original 40 plagioclase. 41 The country rocks around the bay are Dalradian metasedimentary 42 rocks belonging to the Easdale Subgroup of the Argyll Group. These 43 rocks were first described by Wilkinson (1907), and have received 44 relatively little attention since. Wilkinson also provided the 45 most comprehensive description of the Islay sills, although this 46 was rather general and he made no mention of this particular sill. 47 48 The area was resurveyed by Basahel (1971) and much of the revised 49 1998 edition of the BGS 1: 50,000 Sheet 19 (South Islay) is based 50 upon his work. 51 52 10.2 Description 53 54 Two formations crop out at Ardilistry Bay; the Port Ellen Phyllite 55 is poorly exposed, especially inland, but the younger Laphroaig 56 Quartzite is exposed intermittently around the coastline (Figure 57 22). The Port Ellen Phyllite Formation consists mainly of 58 metamudstones with subordinate metasandstones and impure 59 metasandstones. The metasandstones become more common towards the 60 top of the formation, where there is a gradation into the thickly 61 62 63 64 65 1 2 3 4 bedded metasandstones with subordinate metamudstones of the 5 Laphroaig Quartzite Formation. This low-lying coastal region is 6 dominated by a series of ridges, parallel to strike, that are 7 formed by sills of resistant metamafic rock (Figure 23). Thinner 8 sills are generally schistose but, while their margins may be 9 schistose, many of the thicker sills retain relict igneous textures 10 that are commonly ophitic. The mineral assemblage of the sills is 11 12 typical of the greenschist facies and consists of chlorite, 13 actinolite, albite, epidote, calcite, quartz and leucoxene. 14 Among several curious features to be found in these sills, are 15 pods rich in the yellow-green mineral epidote. They are quite 16 conspicuous in some of the thicker sills along the north-east shore 17 of the bay. 18 The metapyroxenite-bearing sill is found at NR 4431 4831. There 19 is good exposure in low cliffs along the shoreline, but it is 20 poorly exposed when traced inland. A schematic section of this 21 sill is presented in Figure 24. The basal part is schistose; 22 little remains of the original igneous texture except for relict 23 phenocrysts, which were probably once calcium-rich plagioclase. 24 These have been pseudomorphed by albite, epidote and calcite during 25 greenschist-facies metamorphism and have been flattened during the 26 deformation that produced the schistosity. Although the base of 27 this schistose unit cannot be observed directly, the unit appears 28 to be no more than about a metre thick, and could represent the 29 30 original fine-grained basal margin to the sill. Immediately above 31 this is the actinolite-rich layer, which is approximately 3 m thick 32 and consists almost entirely of actinolite pseudomorphs after 33 clinopyroxene. These are mostly euhedral to subhedral, dark olive 34 green crystals some 2–3 mm in size, and they are randomly 35 orientated. In thin section the actinolite is almost colourless, 36 indicating a high Mg/Fe ratio, which has been substantiated by 37 electron-microprobe analyses (Bendall, 1995). Interstitial to the 38 large actinolites is a fine-grained groundmass of epidote, albite, 39 calcite, leucoxene, rare quartz and acicular actinolite. This 40 layer appears to be fairly homogenous in texture and composition. 41 Above this layer there is a transition zone that is about a metre 42 thick. Pseudomorphs of albite-epidote after plagioclase occur, and 43 increase in abundance upwards at the expense of the actinolite, 44 until they account for more than 90% of the rock, resulting in what 45 is in essence a meta-anorthosite. The pseudomorphs after 46 plagioclase are larger than the actinolites and are generally 47 48 between 1 and 2 cm in size. Most appear to be subhedral and are 49 quite rounded. The meta-anorthosite layer is approximately 1 metre 50 thick and grades upwards into a layer of metamafic rock with relict 51 ophitic texture, which is c. 2 m thick. About 3.8 m of finer 52 grained schistose metamafic rock makes up the uppermost layer of 53 the sill, and the schistosity increases in intensity upwards; the 54 top 1.5 m is highly schistose. Whereas albite-epidote pseudomorphs 55 still occur above the meta-anorthosite layer, they are less 56 abundant (5–10% of the rock) and smaller (generally less than 1 cm) 57 than at lower levels. 58 Structurally, these rocks lie on the south-eastern limb of the 59 Islay Anticline and the beds dip and young to the south-east. The 60 metapyroxenite-bearing sill is concordant with bedding, and its top 61 62 63 64 65 1 2 3 4 dips at 45° to the south-east. The schistosity also dips to the 5 south-east, but is somewhat steeper, giving a sense of vergence 6 towards the north-west. 7 8 10.3 Interpretation 9 10 The relict texture defined by the actinolites in the metapyroxenite 11 12 layer suggests that the igneous protolith to this rock was a 13 clinopyroxene-cumulate. The presence of albite, epidote and 14 calcite, interstitial to the actinolites, suggest that plagioclase 15 was an intercumulus phase. Whole-rock analyses are low in silica 16 (47%) and rich in MgO (19%) (Bendall, 1995). Hence, it is possible 17 that there was some intercumulus olivine, as well as plagioclase. 18 This layer was most likely formed by the early crystallizing phase, 19 clinopyroxene, settling out under the influence of gravity. 20 Once the clinopyroxene had settled out, it appears that 21 plagioclase was then the main crystallizing phase. The plagioclase 22 too may have settled out to form a cumulate anorthosite layer. 23 However, as plagioclase has a relatively low density, and may not 24 settle out as easily as pyroxene, this layer could have formed 25 through crystallization of plagioclase, without any subsequent 26 movement through the magma, making the protolith to the meta- 27 anorthosite a plagioclase-adcumulate. 28 The sill is probably associated with the Tayvallich volcanic 29 30 rocks, which were extruded at around 600 Ma ago (Halliday et al., 31 1989; Dempster et al., 2000). During the mid Ordovician Grampian 32 Event, the sill underwent deformation and greenschist-facies 33 metamorphism along with the country rocks. The sense of vergence 34 to the north-west, shown by the schistosity on bedding, is 35 consistent with it forming during the same (D1) deformation phase 36 that formed the Islay Anticline (Roberts and Treagus, 1977c). 37 Only the margins of the sill are schistose and the inner part 38 retains the original igneous textures. Work by Skelton et al. 39 (1995, 1997) has described the effect of carbonation of 40 greenschist-facies metamafic-rock sills by infiltration of a CO2- 41 bearing hydrous fluid. This has produced a distinctive zoning 42 pattern in the sills, in which the primary amphibole-epidote 43 assemblage is preserved in the cores, whereas the schistose margins 44 have been altered to calcite, chlorite and quartz. These authors 45 also observed that there is an asymmetry in the width of the zones 46 across the sill, such that one altered margin is much wider than 47 48 the other. They concluded that this asymmetry was controlled by 49 the orientation of the sill with respect to the direction of fluid 50 flow and the partitioning of flow along and across the sill. The 51 contrast between a narrow altered margin at the base of the sill at 52 Ardilistry Bay (Figure 24), and a thicker one at the top, might 53 have been controlled in this manner, and it could be significant 54 that the metapyroxenite layer occurs immediately above the 55 narrower, least altered margin. 56 57 10.4 Conclusions 58 59 Metapyroxenite rocks are relatively rare in the British Isles. 60 They are found in the Lewisian Gneiss Complex of north-west 61 62 63 64 65 1 2 3 4 Scotland, which are generally at a high metamorphic grade 5 (amphibolite- to granulite-facies), and occur as very low-grade 6 rocks, such as in the Shetland and the Ballantrae Ophiolite- 7 complexes. However, it is possible that the metapyroxenite at 8 Ardilistry Bay might be the only greenschist-facies metapyroxenite 9 preserved and exposed in the British Isles. 10 The metapyroxenite layer is up to 3 m thick and consists almost 11 12 entirely of coarse-grained actinolite, a Mg-rich calcic amphibole 13 that has replaced original clinopyroxene during metamorphism. It 14 occurs in a sill that also has a 1 m-thick layer of meta- 15 anorthosite, representing an original plagioclase-rich layer. The 16 sill, therefore, is an excellent example of a layered basic igneous 17 intrusion that has been metamorphosed to the greenschist facies. 18 Whereas deformation and fluids associated with the metamorphism 19 have altered the margins of the sill, which now has a schistose 20 fabric, the inner part has retained the original igneous textures, 21 particularly in the metapyroxenite and meta-anorthosite layers. 22 The good exposure, and the very distinctive appearance of this 23 rock, enhances the geological attractiveness of an already 24 geologically fascinating small corner of Islay. 25 26 11 BLACK MILL BAY, LUING 27 (NM 733 087–NM 729 092) 28 29 30 P.W.G. Tanner 31 32 33 11.1 Introduction 34 35 Black Mill Bay is situated on the exposed west-facing coastline of 36 the island of Luing in the Firth of Lorn, 4 km north of the 37 southern tip of the island. The GCR site lies on the north shore 38 of the bay and exposes the Easdale Slate Formation, which forms the 39 upper part of the Easdale Subgroup (Figure 25). Here, the 40 formation shows greater lithological diversity and structural 41 complexity than is generally seen elsewhere in its outcrop, and 42 includes a strongly deformed debris flow deposit (or debrite). The 43 eastern margin of the site is marked by the Cobblers of Lorn, a 44 prominent group of rounded, pale-coloured knolls of felsic igneous 45 rock that provide a readily identifiable mark for mariners. 46 The geology of Black Mill Bay has previously attracted little 47 48 attention, being mentioned only briefly by Peach et al. (1909) and 49 summarized in a single paragraph of a field guide by Baldwin and 50 Johnson (1977). The Easdale Slate Formation consists of black, 51 pyrite-rich, slaty metamudstone with subordinate metasiltstone and 52 metasandstone, in which there are a few beds of calcareous 53 metasandstone, centimetre-thick black pebbly metasandstone, and 54 dark grey, brown-weathering, pods and lenses of ferroan 55 metadolostone. Sedimentary features include the debris flow, a 56 (now cleaved) clastic sedimentary dyke and a few graded beds of 57 metasandstone. 58 The original mudstone–sandstone sequence has been deformed twice, 59 the second event all but obliterating local evidence of the steep 60 to vertical first cleavage (S1). The second event (D2) resulted in 61 62 63 64 65 1 2 3 4 the formation of mesoscopic folds of bedding and slaty cleavage, 5 accompanied by a widespread, gently dipping, crenulation cleavage. 6 7 11.2 Description 8 9 At Black Mill Bay, the metasedimentary sequence consists of black 10 slaty metamudstone, with pyrite cubes to 0.5 cm across, and 11 12 includes several horizons of dark grey, brown-weathering 13 metacarbonate rock of the type that has been reported elsewhere to 14 be of ferruginous ‘metadolomite’ (Anderton, 1979). Large bodies of 15 metadolostone give rise to characteristic whale-back forms. Many 16 of these are arranged in sets, or linear arrays, of doubly-plunging 17 early folds with a consistent sense of vergence; others appear to 18 be strings of boudins, or even primary sedimentary concretions. 19 Also present are calcareous metasandstones, metasiltstones, and 20 beds of black metasandstone that show graded bedding. Although it 21 is folded locally by pairs of tight to isoclinal folds, the 22 succession youngs overall to the east, with most beds being right- 23 way-up. 24 Of greatest interest in this section is the presence of a 4 m- 25 thick debris flow deposit (debrite) at locality A (Figure 26, a and 26 b) (NM 7311 0875). The rocks below the debrite consist of thin, 27 commonly gritty, metasandstones (less than 50 cm thick), that 28 contain clasts of carbonate material and young east on graded 29 30 bedding (with grains up to 2 mm across at their base). These rocks 31 are followed by brown laminated metasandstones and a black slaty 32 metamudstone that contains thin beds of metacarbonate rock. 33 The base of the debrite is marked by three or four beds of black, 34 gritty metasandstone containing carbonate clasts. They vary in 35 thickness laterally from 5–25 cm, and form a 40–57 cm-thick unit 36 that has a channel-fill geometry and an irregular, erosive contact 37 with the underlying metacarbonate rock. Both rock types are cut by 38 numerous quartz-carbonate veins. The metasandstone unit appears to 39 have provided the sediment for a clastic dyke that penetrates into 40 the overlying debrite for a distance of approximately 1.5 m. The 41 overlying 130 cm of the debrite consists of finely banded, silty 42 metamudstone, which carries the S1 spaced cleavage and contains a 43 matrix-supported population of spindle-shaped sandstone clasts 44 ranging in length from 1–20 cm. This mudstone-supported 45 metaconglomerate is overlain by a 90 cm-thick unit of black slaty 46 metamudstone containing a population of smaller, more widely 47 48 dispersed clasts. The metamudstone occupies a gully and, in 49 contrast to the units on either side, has a structure dominated by 50 an intense flat-lying crenulation cleavage (Figure 26c). It is 51 succeeded by 186 cm of near-vertical, brown-weathering 52 metasiltstone, which grades into metacarbonate rock in the top few 53 tens of centimetres and is affected by a strongly developed 54 pressure-solution cleavage dipping at over 80°. This unit contains 55 matrix-supported sandstone clasts, which are generally larger and 56 more abundant than in the metamudstone below. The top of the 57 debrite is not seen clearly and is arbitrarily taken at the point 58 where the last clast is seen, before the transition from 59 metamudstone to metacarbonate rock takes place. 60 61 62 63 64 65 1 2 3 4 The F1 folds seen at the GCR site have steep to vertical axial 5 planes and curvilinear hinges, are upward facing, and approach 6 sheath-fold geometry in metamudstone. At the south end of the 7 section, the S1 cleavage is near vertical and is the dominant 8 planar fabric in metacarbonate rock and metasiltstone, where it is 9 seen as a millimetre- to centimetre-spaced cleavage. The 10 corresponding S1 slaty cleavage in metamudstone is associated with 11 12 a steeply plunging stretching lineation and curvilinear bedding- 13 cleavage intersection lineations. A good example of the magnitude 14 of the D1 strain is seen in the debris flow deposit, where the 15 clasts are elongated parallel to the L1 extension lineation and 16 flattened in the plane of the spaced cleavage (Figure 26, a and b). 17 The clasts have a maximum dimension (Y) in cross-section of 2–7 cm, 18 and exceptionally up to 9 cm. They are up to 20 cm in length and 19 plunge, on average, at 80° towards 197°. 20 In many places, the early structures are strongly modified by a 21 near-horizontal to gently dipping crenulation cleavage (S2), which 22 masks their geometry, and also locally by mesoscopic, recumbent F2 23 folds (Figure 26d). For example, at location B, an isoclinal F1 24 fold, showing strong refraction of the early cleavage between beds 25 and plunging at 40–50° to 001°, is cut by the intense near- 26 horizontal S2 crenulation cleavage and is effectively disguised. 27 The mean orientation of the S2 crenulation cleavage is dip 7° east, 28 strike 339° (N=7), with the π-axis for slaty cleavage and bedding 29 30 plunging at 7° to 010° (N=16) (inset on Figure 25). The local 31 dominance of one cleavage over the other, depending on the rock 32 type, is graphically displayed by the metasiltstone and 33 metamudstone units in the debrite at locality A (Figure 25). This 34 has resulted in a striking contrast between metacarbonate and 35 metasiltstone beds displaying a steeply dipping to vertical, spaced 36 S1 cleavage, and adjacent units of black metamudstone characterized 37 by a horizontal crenulation cleavage that has all but destroyed the 38 earlier slaty cleavage (Figure 26c). The early spaced cleavage is 39 axial planar (mean of fanned cleavage) to a south-plunging F1 40 anticline in the strongly cleaved metacarbonate bed immediately to 41 the east of the debrite. 42 The debrite is crossed in part by a 15 cm-thick, folded, clastic 43 dyke that has been affected by a steeply dipping S1 spaced cleavage 44 continuous with that in the host rock. The fold plunges at 80–85°. 45 The sides of the sandstone clasts in the debrite carry a horizontal 46 set of corrugations that represent the intersection between S1 and 47 48 the flat-lying crenulation cleavage (S2) (Figure 26a). 49 Apart from late, brittle structures, post-D2 structures are 50 uncommon at this GCR site. However, in the deformed metamudstone 51 at locality C (Figure 25), two later generations of crenulation 52 cleavage are seen superimposed upon the S2 crenulation cleavage. 53 The earlier of these has a dip of 50–60° south, strike 070°; and 54 the later one dips at 30° north-east, strike 330°. 55 56 11.3 Interpretation 57 58 It could be argued, that graphitic and pyritous slaty 59 metamudstones, and other lithologies more representative of the 60 Easdale Slate Formation, are best seen in the less intensively 61 62 63 64 65 1 2 3 4 deformed section at Cuan Point on Luing, 5.5 km north of Black Mill 5 Bay. However, the sequence at the Black Mill Bay GCR site includes 6 a debris flow deposit (debrite), which may be correlated with slump 7 and slide deposits found a few kilometres to the west and south- 8 west that constitute the Scarba Conglomerate, described by Anderton 9 (1979) (see the Lussa Bay and Kinuachdrachd GCR site reports). 10 That conglomerate formed because of the depositional basin margin 11 12 becoming unstable during Easdale Slate times, and the debrite at 13 Black Mill Bay, together with a few turbidite deposits, represents 14 the more distant effects of that disturbance. 15 The rocks are affected by an early generation of strongly 16 curvilinear, isoclinal folds (F1), linked with the development of 17 slaty cleavage in mudstones and spaced cleavages in siliciclastic 18 and carbonate rocks. These structures are temporally and 19 geometrically related to the development of the F1 Loch Awe 20 Syncline to the south-east, but fold vergence is difficult to 21 demonstrate at Black Mill Bay due to the cleavage being at a small 22 angle to, or parallel with, the bedding. In addition, a low-angle 23 crenulation cleavage (S2) is strongly developed locally, to the 24 extent that it overprints and reworks the slaty cleavage. This 25 crenulation cleavage has a similar orientation to a much weaker 26 fabric seen at Cuan Point, and that seen at an early stage of 27 development at the Fearnach Bay GCR site. It is likely that it may 28 also be correlated with the low-angle crenulation cleavage that 29 30 commonly affects the slaty cleavage in metamudstones, and similar 31 rocks found in the Easdale Subgroup on Jura, and reworks the 32 earliest penetrative fabric in older strata at the Camas Nathais 33 GCR site to the north (Treagus et al., 2013). 34 35 11.4 Conclusions 36 37 The Black Mill Bay GCR site provides a well-exposed coastal section 38 across part of the Easdale Slate succession, which comprises black 39 slaty metamudstone, making up the major part of the sequence, with 40 units of metasiltstone and metasandstone, and horizons of 41 metacarbonate lenses. Structurally, a wealth of minor structures 42 and fabrics, resulting from varying degrees of interaction between 43 two separate deformational episodes, are preserved within a 44 relatively small area. 45 The feature that makes this site of wider interest is the 46 preservation of a 4 m-thick debris flow deposit. A typical deposit 47 48 of this type consists of a stratiform unit of matrix-supported 49 clasts that vary greatly in size; these are enclosed in a muddy or 50 silty matrix and are commonly accompanied by rafts and large blocks 51 of sedimentary rock. Such flows originate at the basin margin and 52 are capable of transporting detritus for long distances out into 53 the basin. 54 The debris flow deposit at Black Mill Bay was deformed pervasively 55 during the earliest deformation, which caused the formation of a 56 slaty cleavage. As a result, pebbles and cobbles of sandstone, 57 which were originally probably equidimensional or slightly 58 elliptical in shape, have been deformed into a series of parallel 59 rod-like shapes. This is in strong contrast to the debris flow 60 deposit at the Port Selma GCR site (Treagus et al., 2013) where the 61 62 63 64 65 1 2 3 4 boulders are little deformed and delicate sedimentary structures 5 are preserved within the deposit. 6 The main value of this GCR site is to establish a link between 7 other sites that represent the Easdale Subgroup, and to provide a 8 benchmark for comparing the Easdale Slate succession on the 9 mainland with rocks thought to be of equivalent age on Islay and 10 Jura. Likewise, there are significant similarities between the 11 12 geometries of the two sets of structures at Black Mill Bay, and 13 those at the Camas Nathais, Port Selma and Fearnach Bay GCR sites. 14 15 12 CRAIGNISH POINT 16 (NR 759 999–NM 765 005) 17 18 P.W.G. Tanner 19 20 21 12.1 Introduction 22 23 This GCR site is situated near the southern end of the Craignish 24 peninsula, west of Aird, and consists of a narrow coastal outcrop 25 of the Craignish Phyllite Formation (Easdale Subgroup), intruded by 26 a thick metadolerite sill (Figure 27). These exposures are of 27 national, verging on international, importance because of the 28 presence of calcite pseudomorphs after gypsum that are not found 29 30 elsewhere in the Dalradian Supergroup in such an excellent state of 31 preservation, or exhibiting the same wide range of morphological 32 types. The original gypsum crystals had twinned forms ranging in 33 shape from butterflies to bow-ties (Figure 28) that are unique, and 34 have not been reported previously from naturally occurring rocks or 35 sediments worldwide. This occurrence is in contrast with the 36 outcrop of the Craignish Phyllites elsewhere, where such features, 37 if once present, are no longer preserved due to later deformation, 38 metamorphism, and fluid flow (see the Kilmory Bay GCR site report). 39 Following early work by MacCulloch (1819), the ‗Craignish 40 Phyllites‘ were first mapped and named by Hill (1899) and Peach and 41 Horne (1909). The Craignish peninsula was first mapped for the 42 Geological Survey by H.B. Maufe in 1901, but it was Bailey (1913) 43 who established the stratigraphical succession, which was 44 subsequently confirmed by Allison (1941). The peninsula was later 45 the subject of detailed sedimentological studies (Knill, 1959; 46 Anderton, 1976) and structural studies (Knill, 1960). Lath-like 47 48 pseudomorphs after gypsum were first reported from the country 49 rocks beneath a metadolerite sill on Craignish Point by Anderton 50 (1975). However, it was not until 1995 that the exceptional forms 51 taken by the original twinned gypsum crystals were recognized by 52 the author of this site report. 53 The sill-like nature of the basic meta-igneous bodies that are 54 found in abundance throughout the Craignish Phyllites was first 55 recognized by Jamieson (1860). Bailey (1913) noted that, in some 56 cases, they are slightly transgressive, and that they are 57 associated with ‗remarkable contact alteration‘. The sills, which 58 were emplaced prior to the deformation and regional metamorphism of 59 the country rocks, can be many tens of metres thick. They 60 generally have highly sheared and altered margins and less deformed 61 62 63 64 65 1 2 3 4 and metamorphosed interiors (Graham, 1976). Recent research has 5 shown that the alteration is due to the infiltration of a CO2-rich 6 fluid from the country rocks during the regional metamorphism 7 (Graham et al., 1985; Skelton et al., 1995). 8 The Craignish peninsula lies on the north-west limb of the Loch 9 Awe Syncline and the rocks were mildly deformed and metamorphosed 10 to the greenschist facies during the Grampian Event. This part of 11 12 the peninsula has almost entirely escaped any later deformations, 13 including the development of conjugate sets of kink bands for which 14 the surrounding area is well known. It is cut by Palaeogene dykes 15 belonging to the Mull Swarm. 16 17 12.2 Description 18 19 The Craignish Phyllites are clearly exposed on a narrow rock 20 platform backed by small cliffs, with areas of clean, tidally 21 scoured rock. They are overlain conformably by a metadolerite sill 22 that runs parallel to the coast and forms a positive topographic 23 feature. The phyllites consist of a well-bedded sequence of grey- 24 green phyllitic metasandstone and metasiltstone, with 25 characteristic orange-brown-weathering layers of metacarbonate rock 26 of varying thickness. Sedimentary structures such as parallel 27 lamination, cross-lamination, graded bedding, convolute folds, and 28 water-escape structures are common and well preserved. The rocks 29 30 dip at 50–55° to the south-east, and at low Spring tide a maximum 31 thickness of 20 m of metasedimentary rock is exposed at any point 32 along the coast, within the GCR site. Sedimentary logs of the 33 sequence at four localities were given by Anderton (1975, figure 2, 34 locations B–E). 35 Anderton (1975) carried out the first detailed sedimentological 36 study of these rocks and identified four facies associations, 37 namely: 38 39 1. Laminated silt/sand 40 2. Tabular sand facies. 41 3. Sheet sand facies. 42 4. Channel facies. 43 44 Facies 1 consists of finely laminated (0.1–2 cm) alternations of 45 metasandstone and metasiltstone, and is host to most of the 46 pseudomorphs. Facies 2, commonly found interbedded with Facies 1, 47 48 is characterized by 5–40 cm-thick metasandstone beds, which are 49 parallel-sided, have erosional bases, and commonly show sole 50 markings, including flute marks. Parallel lamination is well 51 developed in the metasandstones, and shows a transition to climbing 52 ripples in some beds. Facies 3 consists of structureless 53 metasandstones, over 5 cm thick, that lack the features of the 54 Facies 2 beds; with a decrease in bed thickness it grades into 55 Facies 1. In Facies 4, the metasandstones are coarser in grain 56 size than those of the other facies, and have strongly discordant 57 bases and an irregular cross-sectional geometry. 58 The pseudomorphs after gypsum were reported first from the 59 Craignish Phyllites by Anderton (1975 plate 2, b and c) from his 60 locality T2 (NM 761 002), which is within the area of this GCR 61 62 63 64 65 1 2 3 4 site. The host rocks are grey-green phyllitic metasiltstones with 5 thin, 1–2 cm-thick, orange-weathering carbonate bands and nodules, 6 phyllitic calcsilicate rocks, laminated phyllitic metasandstone– 7 metasiltstone units, and thicker metasandstone beds to 27 cm thick. 8 The pseudomorphs are blade- and lath-shaped bodies 0.1–4.0 cm long, 9 which are concentrated along certain bedding-parallel horizons that 10 can be traced laterally for several metres. These horizons 11 12 commonly occur at the interface between porous, coarse-grained 13 metasandstone and an underlying finer grained, less permeable bed. 14 Other localities with less well-preserved calcite pseudomorphs were 15 reported by Anderton (1975). 16 Anderton likened the pseudomorphs to the ‗desert roses‘ and 17 isolated gypsum crystals found at the present day in tidal-flat 18 environments in many places in the world, but failed to recognize 19 the true geometry of the twinned crystals found, in particular, 20 within the GCR site. There the pseudomorphs are preserved as 21 cavities or impressions on the weathered rock surface. They vary 22 from irregularly shaped pits 1–3 mm across, to randomly orientated 23 acicular impressions from a few millimetres to over 3 cm long, to 24 millimetre-sized cavities with a distinct bow-tie shape (Figure 25 28a), and centimetre-sized ‗butterfly‘ forms (Figure 28b). All 26 types of pseudomorph are most common on the bases of cross- 27 laminated metasandstone beds that vary in thickness from 1–27 cm, 28 and rarely contain nodular calcareous patches, and rare sediment 29 30 rafts. It is particularly noticeable that different beds and 31 bedding surfaces within the laminated phyllitic rocks are 32 characterized by a particular size and form of pseudomorph. Thus, 33 a horizon on which minute sub-millimetre-sized bow-ties are 34 preserved is found adjacent to one on which only centimetre-sized 35 butterflies are seen. 36 The precursor mineral to the pseudomorphs grew across and 37 preserved the original sedimentary fabric of the rock, and this 38 internal texture was inherited by the optically continuous calcite 39 crystal that replaced it. In some cases, the pseudomorph displays 40 areas, especially rims, that are clear of inclusions but it is not 41 clear whether these inclusion-free areas formed during gypsum 42 growth or during its replacement by calcite. Anderton (1976, 43 figure 3) illustrated the situation where calcite pseudomorphs that 44 grew along the interface between coarse silt and underlying silty 45 clay, were all truncated at the base of the overlying bed. He 46 concluded that, following the growth of the gypsum crystals across 47 48 the bedding interface, less-saline fluid flowing through the more 49 porous upper bed had caused dissolution of any parts of the gypsum 50 crystals that had penetrated the upper bed, resulting in the 51 observed truncation. 52 All of the metasedimentary rocks have been affected by orogenic 53 deformation to some extent, as expressed locally in the field by 54 the development of a penetrative, slaty cleavage in the thin 55 metamudstone seams. This cleavage dips more steeply south-east 56 than the bedding (Figure 27), in keeping with the location of the 57 GCR site on the north-west limb of the Loch Awe Syncline. 58 59 60 61 62 63 64 65 1 2 3 4 12.3 Interpretation 5 6 In 1975, Anderton published the results of a detailed 7 sedimentological study of the coastal strip that includes this GCR 8 site, and concluded that the Craignish Phyllites in this part of 9 the outcrop were deposited in a tidal-flat to shallow-marine 10 environment. This interpretation, though prompted by the finely 11 12 laminated, thinly bedded and fine-grained nature of the beds, was 13 based largely upon the identification of gypsum in the rocks at 14 Craignish Point, and thence by comparison with present day examples 15 of similar gypsum-bearing sequences. Evidence or reasons to 16 support the inference that the calcite pseudomorphs were derived 17 specifically from the alteration of gypsum were not given, apart 18 from there being a morphological similarity between the lath-like 19 pseudomorphs and prismatic gypsum. Recent work by the author has 20 confirmed that the precursor mineral could only have been gypsum, a 21 finding which supports the environmental interpretation placed upon 22 the four sedimentary facies by Anderton. 23 From the sedimentary logs and other field data, Anderton (1975) 24 concluded that the tidal-flat sediments of the laminated Facies 1 25 were cut by meandering channels filled with the coarser sediments 26 belonging to the channel Facies 4. The sheet (3) and tabular-sand 27 (2) facies were more rapidly deposited, and Anderton suggested that 28 the latter were flood-tide storm sediments deposited in a subtidal 29 30 to low intertidal setting. 31 All of the features shown by the pseudomorphs indicate that the 32 gypsum grew synchronous with, or shortly after, the sedimentation. 33 This conclusion is supported by the observation that some of the 34 pseudomorphs have been affected by pressure-solution corrosion 35 associated with the development of the earliest cleavage in these 36 rocks. 37 The excellent state of preservation of the pseudomorphs may be due 38 to the shielding effect of (and contact metamorphism by?) the thick 39 metadolerite sill that runs along the GCR site. The sill was 40 emplaced before the regional deformation and metamorphism of the 41 Dalradian rocks, and could have protected the gypsum-bearing rocks 42 locally from the main effects of the Grampian deformation. An 43 analogous situation is found locally at the Kilmory Bay GCR site, 44 where sedimentary structures are well preserved in a narrow zone of 45 slightly hornfelsed rock adjacent to a thick metadolerite sill (see 46 Roberts, 1977c, locality 10). 47 48 49 12.4 Conclusions 50 51 The Craignish Point GCR site is one of those exceptional places 52 where, despite the many pairs of eyes that have looked at the rocks 53 since Jamieson first described them in l860, there still remain new 54 features to discover and interpret. 55 The main feature of the site is that it preserves valuable 56 evidence that crystals of gypsum (hydrous calcium sulphate) grew in 57 the muds and fine sands soon after they had been laid down in 58 Neoproterozoic times, many millions of years before the rocks were 59 intruded by an igneous body and involved in the Caledonian Orogeny. 60 The fact that this mineral was able to crystallize from seawater 61 62 63 64 65 1 2 3 4 signifies that there was a hot climate at that time. Combined with 5 other field evidence, this suggests that the environmental setting 6 was probably analogous to that found at the present day on tidal 7 flats in regions such as the Persian/Arabian Gulf. 8 A 34 m-thick dolerite sill, which now overlies these rocks, 9 protected them from much of the deformation that has affected the 10 Craignish Phyllites throughout the rest of the outcrop (see the 11 12 Fearnach Bay GCR site), so preserving the sedimentary structures, 13 as well as the pseudomorphs. 14 The aspect that gives this site a possible international, status 15 is the extraordinary and unique forms shown by the former gypsum 16 crystals (now replaced atom-for-atom by calcite). They display 17 bow-tie and butterfly shapes, which reflect their internal 18 structure, that of two crystals which have grown simultaneously to 19 form an asymmetrical cross. They are aesthetically pleasing forms, 20 but fragile and easily destroyed. Current research is aimed at 21 determining whether they could be used as a more precise guide to 22 the climatic and other physical conditions that prevailed at the 23 time of their growth. 24 This GCR site also provides a representative section for a part of 25 the Craignish Phyllite Formation, which in combination with other 26 data, including those from the Black Mill Bay and Fearnach Bay GCR 27 sites, can be used to model the original sedimentary architecture 28 of the Easdale Subgroup. 29 30 31 13 FEARNACH BAY 32 (NM 838 130–NM 832 141) 33 34 P.W.G. Tanner 35 36 37 13.1 Introduction 38 39 Fearnach Bay is located at the head of Loch Melfort, some 11 km 40 east-north-east of the Black Mill Bay GCR site on the Isle of 41 Luing. The GCR site consists of a narrow strip of coastal 42 exposures, divided into two parts by the estuary of the River Oude 43 (Figure 29). The rocks belong to the Craignish Phyllite Formation 44 (Easdale Subgroup), and consist mainly of grey-green, generally 45 calcareous, phyllitic metamudstones and metasiltstones, with thin 46 quartzite and metacarbonate beds. The strata are largely right-way 47 48 up, dip at a moderate angle to the east, and were strongly folded 49 on all scales, from microscopic to mesoscopic, during the main 50 phase of deformation (D1). The effects of later deformation on 51 these structures are minimal. 52 The section provides a well-exposed sequence of rock types that 53 characterize the Craignish Phyllite, and displays in splendid 54 detail the 3-dimensional geometry of the first generation of minor 55 folds and cleavages. Minor fold vergence, cleavage-bedding 56 relationships, and way-up structures all indicate that the strata 57 lie on the north-western limb of the Loch Awe Syncline. This GCR 58 site also presents a good example of cylindroidal fold geometry on 59 a mesoscopic scale (Figure 29, inset), together with excellent 60 examples of structural features such as cleavage refraction and 61 62 63 64 65 1 2 3 4 cleavage fans, and sedimentary structures such as pseudo-ripple 5 marks and ripple-drift lamination. 6 There is very little published work on the geology of this GCR 7 site, apart from a comment in the original Geological Survey memoir 8 (Peach et al., 1909); strain measurements made on a deformed basic 9 sill (Borradaile, 1972b); and a brief mention in a field guide 10 (Borradaile, 1977). The rock types closely resemble those at the 11 12 Port Cill Maluaig and Strone Point GCR sites, but the structural 13 styles at the three localities are completely different. 14 15 13.2 Description 16 17 The Craignish Phyllite Formation at Fearnach Bay consists largely 18 of finely banded grey-green phyllitic metamudstone, with units of 19 more-siliceous phyllitic metasiltstone up to 20 cm thick. The 20 metamudstones and metasiltstones are commonly interbedded with thin 21 beds of orange-brown-weathering metacarbonate rock, which vary from 22 2–6 cm in thickness, and have a characteristic etched appearance. 23 In places, particularly in the north of the site around the pier, 24 they are accompanied by beds of massive metacarbonate rock up to 20 25 cm thick, which show a gradational contact with the siliciclastic 26 host rock. Beds of fine-grained, white quartzite from 15–50 cm 27 thick occur throughout the sequence. South of locality A (NM 8350 28 1342) (Figure 29), the Craignish Phyllite becomes lithologically 29 30 more monotonous; it is more siliceous and tougher, less well 31 bedded, and with fewer metacarbonate and quartzite beds than 32 farther north. Quartz-carbonate veins occur throughout the 33 sequence. 34 Taken as a whole, the sequence becomes younger to the east. 35 Ripple-drift cross-lamination is the main way-up indicator and 36 occurs in units up to 3 cm thick at the base of thick, orange-brown 37 metacarbonate beds (Figure 30a), and also in alternating sequences 38 of ripple-drift structures and 1–3 cm-thick metacarbonate beds, 39 forming stacks up to 0.5 m thick. 40 The structure consists of a number of asymmetrical early (F1) 41 folds, usually in pairs, which have N–S-trending axial traces. 42 They are accompanied by a penetrative slaty cleavage (Figure 30b) 43 that is associated with a poorly developed stretching lineation, 44 plunging to the east-north-east at around 30°. The cleavage shows 45 marked refraction across the more-competent layers in the slaty 46 metamudstone, and slight normal fanning in fold closures. Metre- 47 48 scale mesoscopic folds are found throughout the section; in the 49 north, in exposures around the pier, there is a set of at least 50 five folds with overall vergence indicating a syncline to the east. 51 Bedding in the middle limbs of these structures dips at 80°, and is 52 overturned locally. The associated slaty cleavage dips at c. 31°, 53 and the fold hinges plunge at c. 10° towards 002°. There is 54 evidence for a preceding episode of layer-parallel extension, or 55 boudinage. 56 Many of the beds east of the pier display patterns of lenticular, 57 discontinuous, en-echelon, regularly spaced, pod-like structures on 58 their top surfaces, which are 5–10 cm long and 4 cm deep. Bottom 59 structures have been reported from this GCR site by Borradaile 60 (1977), but those examples are pseudo- or tectonic ripples 61 62 63 64 65 1 2 3 4 resulting from the development of small, doubly plunging, en- 5 echelon, buckle folds on the surface of the bed. 6 South of the River Oude, the coastal section as far as locality A, 7 runs parallel to the strike of both bedding and cleavage, but 8 several natural cross-sections are seen. In two of these sections 9 a deformed basic intrusion is seen to be concordant with bedding in 10 the metasedimentary rocks, and shows some internal compositional 11 12 layering. A 15 cm-thick bed of quartzite, with what appears to be 13 a muddy top, occurs between it and the more-typical phyllitic 14 country-rock. The sill can be traced around several fold closures 15 at this locality, and contains a penetrative fabric, defined by 16 flattened amygdales. This fabric is continuous with the slightly 17 fanned, slaty cleavage in the adjoining rocks. Measurement of the 18 strain, represented by deformed amygdales, carried out at two 19 localities by Borradaile (1972b, locations 13 and 14), gave X/Y 20 ratios of 2.87–2.91. Buckle folds, affecting beds that vary 21 considerably in thickness and competence, give rise to a plethora 22 of minor folds whose wavelengths show a marked positive correlation 23 with the bed thickness. 24 The geometry of the tectonic structures at this GCR site is 25 summarized by the stereographic projections in Figure 29. Because 26 the cleavage in the northern section has a more-shallow dip than in 27 the southern section, there is quite a large spread in the 28 orientation of poles to the S1 slaty cleavage. The computed π-axis 29 30 derived from a plot of poles to bedding for the whole site is 31 [plunge 05°NE; trend, 020°; N (number of readings)=20]. This is 32 almost coincident with the mean orientation for the calculated 33 best-fit line of intersection of the slaty cleavage planes [plunge 34 04°NE; trend, 024°; N=10] and the mean orientation of bedding- 35 cleavage intersections and minor fold hinges [plunge 06°NE; trend, 36 021°; N=9], and shows that on a major scale, the structures are 37 almost perfectly cylindroidal. It also demonstrates the inherent 38 symmetry of structures in rock. 39 A second tectonic fabric (S2) is present locally as a millimetre- 40 spaced crenulation cleavage in slaty metamudstone layers, and 41 generally dips at less than five degrees. It is associated with 42 rare, centimetre-scale, late folds, which refold tight to isoclinal 43 F1 folds and plunge at a low-angle to the north-north-east. 44 45 13.3 Interpretation 46 47 48 The Craignish Phyllite was laid down in a shallow-water 49 environment, as witnessed at this GCR site by the preservation of 50 abundant ripple-drift bedding at different levels in the sequence. 51 The depositional environment was probably similar to the near-shore 52 to intertidal settings in which gypsum-bearing sediments, preserved 53 at the Craignish Point GCR site, were being deposited at the same 54 time (Anderton, 1976). However, in the area of Fearnach Bay, the 55 Craignish Phyllite originally consisted of somewhat calcareous muds 56 and silts (now metamorphosed to chlorite-white mica-carbonate- 57 bearing phyllitic metamudstones and metasiltstones) and this 58 contrasts with its lateral equivalent at Craignish Point, 15 km to 59 the south-west, where the formation was formed from interbedded 60 sands and silts with subordinate muddy layers. 61 62 63 64 65 1 2 3 4 The Craignish Phyllite is correlated across the Loch Awe Syncline 5 with the Ardrishaig Phyllite. Indeed, the Craignish Phyllite at 6 Fearnach Bay has much more in common with the Ardrishaig Phyllite, 7 as seen at the Strone Point and Port Cill Maluaig GCR sites, than 8 with rocks of the same formation farther west (i.e. at the 9 Craignish Point GCR site). This is because the rocks at the three 10 easterly sites are at a higher metamorphic grade, and have been 11 12 more pervasively deformed, than the same lithostratigraphical 13 sequence farther west. 14 The fold structures at Fearnach Bay are of primary, D1, age and 15 appear to be virtually unaffected by any later deformation. They 16 may be correlated with D1 structures at the Black Mill Bay GCR 17 site, with which they share a common geometry (compare the inset on 18 Figure 29 of this report with the inset on Figure 25 of the Black 19 Mill Bay GCR site report). 20 21 13.4 Conclusions 22 23 The Fearnach Bay GCR site occupies a crucial position on the north- 24 western limb of one of the most fundamental structures of the 25 South-west Grampian Highlands, the near-upright F1 Loch Awe 26 Syncline. A train of metre-scale folds, accompanied by a 27 penetrative cleavage, is virtually unaffected by later deformation 28 making this a site of major national importance. The geometry of 29 30 the minor structures, which include folds, cleavage, and 31 lineations, is that of a perfectly cylindroidal structure plunging 32 at c. 5° to the north-north-east. Cleavage dips more steeply than 33 bedding on the long limbs of these folds, consistent with their 34 position on the north-western limb of the regional-scale fold. The 35 inspiration to be gained from seeing such beautifully preserved 36 folds and related cleavages in three dimensions, in all of their 37 intricate detail, enhances the value of the site considerably. 38 The Dalradian rocks at this GCR site were formed from muddy and 39 calcareous sediments, laid down in a shallow-water environment, and 40 represent a more highly metamorphosed and deformed part of the 41 Craignish Phyllite than is seen at the Craignish Point GCR site. 42 Hence, this site provides a lithostratigraphical link, between the 43 Craignish Phyllite and its generally higher grade equivalent on the 44 other limb of the Loch Awe Syncline, the Ardrishaig Phyllite. 45 Regional metamorphism (to lower greenschist facies) altered the 46 mud-rich sedimentary rocks to chlorite-white mica-rich phyllitic 47 48 rocks, and caused recrystallization of the less reactive rocks such 49 as limestone and quartzite. After lithification, the sedimentary 50 rocks were intruded by basic magma that crystallized to form 51 several thin sheets, concordant with the bedding. 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 14 KILMORY BAY 5 (NR 698 756–NR 704 725) 6 7 J.L. Roberts and P.W.G. Tanner 8 9 10 11 14.1 Introduction 12 13 The coastline between Kilmory Bay and the Point of Knap, in the 14 Knapdale area of Kintyre, provides a 4 km-long section through the 15 rocks of the Ardrishaig Phyllite Formation and Crinan Grit 16 Formation of the Argyll Group, preserved in the core of the Loch 17 Awe Syncline. These rocks are extremely well exposed along the 18 seaward edges of the raised beaches, which are a characteristic 19 feature of this coastline. The only significant gap in exposure is 20 caused by the Quaternary deposits around the head of Kilmory Bay. 21 The Kilmory Bay GCR site is of national importance, for providing 22 excellent examples of the geometrical and kinematic relationships 23 between major and minor structures in folded rocks (Roberts, 1959). 24 Mesoscopic fold closures are exposed in three dimensions, and are 25 accompanied by a great diversity of spaced and penetrative 26 cleavages that form pronounced cleavage fans. The 27 interrelationships between these structures remain to be fully 28 explored. In addition, recent research has focussed upon the 29 30 origin of the sedimentary dykes that are reasonably common in the 31 Ardrishaig Phyllites at this site (Phillips and Alsop, 2003), and 32 upon the effects of fluid flow during the regional metamorphism of 33 the basic sills (Graham et al., 1983; Skelton et al., 1995) 34 The stratigraphy and structure of the area were established by 35 J.S.G. Wilson of the Geological Survey, as reported by Peach et al. 36 (1911). He used graded bedding in the Crinan Grits to show that 37 this formation is younger than the Ardrishaig Phyllites, and 38 recognized that, as the minor folds affecting the Crinan Grits 39 plunge to the north-north-east, this formation must lie 40 structurally above the Ardrishaig Phyllites (Figure 31) (see also 41 Bailey, 1913). 42 Three major folds comprise the compound Loch Awe Syncline in 43 Knapdale (see the Port Cill Maluaig GCR report, Figure 34). The 44 most south-easterly of these folds, the Kilmory Bay Syncline is 45 seen in its type area in Kilmory Bay as a plexus of 5 or more major 46 closures (Figure 31). Bailey (1922) interpreted the Kilmory Bay 47 48 Syncline as a secondary structure that had affected a stack of 49 recumbent folds and intervening slides, formed during the primary 50 deformation. However, more-recent work by Roberts (1959, 1974) has 51 shown that at Kilmory Bay the rocks are affected by only one major 52 deformation (D1), and that the Kilmory Bay Syncline is an early 53 structure (F1), associated with the deformation that elsewhere 54 produced early nappe-like structures. Roberts (1966a), provided a 55 brief account of the sedimentological features of these rocks, and 56 Roberts (1959), and Roberts and Sanderson (1974) discussed the 57 origin of minor F1 folds with curved hinges. 58 59 60 61 62 63 64 65 1 2 3 4 14.2 Description 5 6 7 14.2.1 Stratigraphy 8 9 The Ardrishaig Phyllite Formation (informally referred to as the 10 ‘Ardrishaig Phyllites’) is the oldest stratigraphical unit in the 11 12 area. It lies on the south-east limb of the Kilmory Bay Syncline, 13 and is exposed almost continuously along the seaward edge of the 14 raised beach, which forms the coastline between Port Ban (NR 700 15 740) and the Point of Knap (NR 697 721). It consists typically of 16 greenish-grey phyllitic metamudstones and metasiltstones, 17 interbedded with beds of fine-grained quartzite and rare 18 metalimestone, dipping steeply to the north-west (Figure 31). Beds 19 of fine-grained quartzite are common locally on the coastline 20 facing Glas Eilean. Ripple-drift bedding is developed in some of 21 the more-silty layers, and sedimentary dykes, up to 25 cm thick, of 22 fine-grained but rather impure quartzite, are found at a number of 23 localities, including NR 696 731, where they trend east-north-east 24 and cross-cut the bedding in the Ardrishaig Phyllites (Figure 32). 25 Those at NR 700 743 have been described by Phillips and Alsop 26 (2003), who presented evidence for some of these dykes being of 27 post-D1 age. However, recent work by P.W.G. Tanner has identified 28 features, which clearly indicate that they are all part of a linked 29 30 system of pre-tectonic injections of mobilized water-saturated 31 sandstone. 32 Groove and flute casts occur on the soles of fine-grained 33 quartzite beds found immediately to the south-east of NR 696 731, 34 and indicate that they were deposited by turbidity currents flowing 35 from the north-west. Farther south, a sedimentary breccia, up to 2 36 m thick, is exposed in the cliffs backing the raised beach at NR 37 698 726. Lying in a matrix of sandy metalimestone, the fragments 38 consist mostly of pale-coloured limestone, flattened parallel to 39 the slaty cleavage, along with less-deformed pebbles of blue 40 quartz, phyllitic metamudstone, and fine-grained quartzite (Peach 41 et al., 1911). 42 The Ardnoe Member (formerly ‗group‘) is the lowest division of the 43 Crinan Grit Formation at Kilmory Bay. Its base is marked by a 44 thick bed of massive fine-grained quartzite, which is exposed along 45 the north-west side of Port Ban (NR 700 740). A dolomitic breccia 46 occurs locally at the contact with the underlying Ardrishaig 47 48 Phyllites, and is at the same stratigraphical level as the Shira 49 Limestone (Figure 2). This bed is overlain by a sequence of 50 schistose pebbly quartzites and fine-grained metaconglomerates, 51 which make up the lowest division of the member. At Port Ban, the 52 pebbly quartzites are coarser grained, and the bases of individual 53 beds are commonly conglomeratic. Graded bedding, together with 54 metre-scale cross-bedding in the upper parts of these beds, where 55 they are exposed on the low headland at NR 699 741, show these 56 rocks to be younger than the underlying Ardrishaig Phyllites (Peach 57 et al., 1911). The upper part of the member consists mostly of 58 slaty metamudstones and fine-grained metalimestones, interbedded 59 with fine-grained quartzites. 60 61 62 63 64 65 1 2 3 4 The main part of the Crinan Grit Formation (the ‗Crinan Grits‘) 5 crops out north of Kilmory Bay in the core and along the north-west 6 limb of the Kilmory Bay Syncline (Figure 31). The base of the 7 ‗Crinan Grits‘ is marked by a massive bed of pebbly 8 metaconglomerate, which is exposed on the coast opposite Eilean a‘ 9 Chapuill at NR 696 747. Traced inland towards the east-north-east, 10 this bed contains pebbles of blue and white quartz, feldspar, and 11 12 dolomite, said to be the size of ‗pigeons‘ eggs‘ (Peach et al., 13 1911), and its base is markedly lobate, either as the result of 14 load-casting, or syn-depositional erosion. This bed is overlain by 15 massive beds of gritty meta-arenite, which were originally pebbly 16 conglomerates and coarse-grained feldspathic sandstones. The 17 pebbles consist of white and blue quartz, microcline, orthoclase, 18 perthite, and oligoclase, listed in order of decreasing abundance. 19 The pebbly metaquartzites are locally interbedded with thin layers 20 of dark slaty metamudstone, and commonly show graded bedding, thus 21 enabling the stratigraphical sequence to be determined. Cross- 22 bedding is developed locally towards the tops of these graded beds, 23 which represent attenuated Bouma sequences. These features are 24 particularly well exposed along the rocky coast south of Port Liath 25 (NR 698 757), where the beds are vertical and strike north-north- 26 east on the north-west limb of the Kilmory Bay Syncline (Figure 27 31). 28 Both the Crinan Grits and the Ardrishaig Phyllites are intruded by 29 30 sill-like bodies of basic meta-igneous rock that were folded and 31 deformed, along with their Dalradian host rocks, during D1. The 32 interiors of these basic sills commonly preserve relict igneous 33 textures, whereas their margins are generally highly schistose. 34 The original character of the metasedimentary rocks is best seen 35 where they are protected from the effects of subsequent 36 deformation, having been indurated (‗baked‘) by contact 37 metamorphism adjacent to the thicker sills. All of these rocks are 38 affected by greenschist-facies metamorphism at chlorite grade, 39 contemporaneous with the formation of the S1 slaty cleavage in the 40 more-pelitic rocks. 41 42 14.2.2 Structure 43 44 The closure of the north-westerly syncline of the compound Kilmory 45 Bay Syncline is seen to the north-west of Kilmory Bay at NR 696 46 747. There, the Crinan Grits dip moderately towards the north- 47 48 north-east, and away from the Ardnoe Member that forms the 49 headland. Traced north along the coast towards Port Liath (NR 689 50 757), these pebbly quartzites become vertical and trend north- 51 north-east on the north-west limb of this syncline. The outcrop of 52 the conglomeratic bed, where exposed on the coast opposite Eilean 53 a’Chapuill (NR 697 749), defines the closure of the next anticline 54 to the south-east, and part of the succeeding syncline. However, 55 the south-east limb of this major syncline, and the closure of the 56 major anticline to its south-east, are obscured by superficial 57 deposits in Kilmory Bay. The closure of the following syncline is 58 exposed to the south-east, between Kilmory Bay and Port Ban (NR 700 59 740), where beds of the Ardnoe Member lie in its core. All of 60 these major F1 folds plunge at a moderately steep angle towards the 61 62 63 64 65 1 2 3 4 north-east within axial planes that dip very steeply to the north- 5 west. 6 The slaty cleavage (S1) developed in the metamudstone beds, 7 strikes consistently north-east and dips at 60–80º to the north- 8 west throughout the area, and is statistically axial planar to both 9 the mesoscopic and major folds. The Ardrishaig Phyllites, with 10 their extreme contrast in lithology, between bands of gritty 11 12 quartzite up to 2 m thick, and interbedded units of phyllitic 13 metasiltstone, give rise to an extraordinary range of cleavage 14 refraction geometries. Slaty cleavage is represented by a shape- 15 or spaced-cleavage in the coarser grained rocks. It typically 16 shows cleavage refraction as it passes into the coarser grained 17 beds, producing a strongly divergent cleavage fan about the fold 18 hinges. Typically, a fibrous mineral lineation, which Clough (in 19 Gunn et al., 1897) termed the stretching direction, is developed on 20 S1, and pitches steeply down-dip. The stretching lineation is 21 commonly revealed by elongated rusty pyrite blebs. 22 The minor F1 folds in the Ardnoe Member are co-axial with the 23 major folds that affect the overlying Crinan Grits; most of them 24 plunge at moderate angles to the north-east (although some of them 25 are curvilinear) within axial planes that dip very steeply towards 26 the north-west. The exposures around NR 700 744, to the south of 27 Kilmory Bay, show a spectacular series of very tight F1 folds 28 affecting three beds of fine-grained quartzite. The anticlinal 29 30 fold hinges are stripped bare by erosion to form a series of 31 truncated whale-backs in the quartzite beds, plunging gently 32 towards the north-east (Figure 33). At low Spring tides, the 33 lowermost bed of fine-grained quartzite can be traced with scarcely 34 a break around a complex series of minor F1 folds. The quartzite 35 beds maintain approximately the same thickness normal to the 36 bedding around the fold-hinges (Class 1C folds of Ramsay, 1967), 37 whereas the intervening layers of less competent rock thicken into 38 the fold cores to form Class 3 folds. 39 The minor F1 folds in the Ardrishaig Phyllites are coplanar with 40 major and minor folds in the Crinan Grits. Their axial planes dip 41 very steeply towards the north-west, parallel to the slaty cleavage 42 in the metamudstones. However, their fold hinges are strongly 43 curved within a fairly constant axial-plane orientation, giving 44 rise to minor F1 folds plunging to either the north-east or south- 45 west at moderately steep angles. Locally, the hinges of F1 folds 46 change pitch through more than 90° within a short distance, forming 47 48 curvilinear folds. Wherever graded bedding or ripple-drift bedding 49 allows the stratigraphical order of the sedimentary beds to be 50 determined, its relationship with the slaty cleavage, or its 51 equivalent as a plane of grain flattening, shows that these D1 52 structures are all upward-facing. No evidence of an earlier, pre- 53 D1, fabric has been noted. 54 55 14.3 Interpretation 56 57 The sedimentological features of the Ardrishaig Phyllites and the 58 Crinan Grits have not been described in any detail from Kilmory 59 Bay, and warrant further study. The Ardrishaig Phyllites probably 60 represent tidal-flat deposits, like the Craignish Phyllites, their 61 62 63 64 65 1 2 3 4 stratigraphical equivalent to the north-west of the Loch Awe 5 Syncline (Anderton, 1975), whereas the graded beds of the Crinan 6 Grits were evidently deposited by turbidity currents in deeper 7 water. 8 Following the injection of sedimentary dykes, and the intrusion of 9 dolerite sills, all of these rocks were affected by the first phase 10 of the regional deformation (D1) under conditions of chlorite-grade 11 12 metamorphism. The resultant slaty cleavage (S1) is axial-planar to 13 a series of upward-facing major F1 folds with a north-east trend. 14 This geometry is incompatible with the interpretation by Bailey 15 (1922) that the Kilmory Bay Syncline is a later structure, 16 superimposed upon a primary nappe-complex. Although the major F1 17 folds in the Crinan Grits generally plunge at a moderately steep 18 angle, towards the north-east, the minor F1 folds in the Ardrishaig 19 Phyllites typically have curved hinges, plunging to the south-west, 20 as well as to the north-east. The curvilinear nature of these 21 folds reflects the non-cylindroidal nature of the F1 fold-buckles 22 as they formed, subsequently accentuated by the deformation 23 (Roberts and Sanderson, 1974; cf. Roberts, 1959). This has caused 24 the individual fold-hinges to rotate away from the position in 25 which they were formed, at a high angle to the stretching 26 direction, while undergoing a relative increase in their length. 27 Typically, this gives rise to individual folds with curved hinges, 28 pitching away from one another in opposite directions within a 29 30 common axial plane. 31 32 14.4 Conclusions 33 34 The Kilmory Bay GCR site provides a representative cross-section 35 through the upper part of the Argyll Group, from the Ardrishaig 36 Phyllites (for which the complete sequence is exposed) into the 37 overlying Crinan Grits. Graded bedding in the Crinan Grits can be 38 used to determine that this formation is younger than the 39 Ardrishaig Phyllites, as was first demonstrated by the Geological 40 Survey in 1911, making this site one of historical interest. These 41 two formations are folded by the compound Kilmory Bay Syncline, a 42 major early fold, which is one of the three major folds that 43 comprise the regionally important Loch Awe Syncline. 44 The rocks display a great variety of sedimentary and structural 45 features, some of which are seldom seen with such clarity in the 46 Dalradian, making this site of national interest. Graded bedding, 47 48 accompanied by channelling and lateral facies changes, together 49 with cross-bedding and cross-lamination, is found throughout the 50 rock succession. Strain in the hinge-zones of the major folds is, 51 in places, sufficiently low that the evidence for the origin of 52 certain rather enigmatic sedimentary structures, such as 53 sedimentary injections and dykes, is preserved in the more 54 competent beds. 55 In addition, the clean, wave-washed exposures and 3-D nature of 56 some parts of the coastal section, allow the relationships between 57 major and minor fold structures, and a great variety of cleavage 58 types, to be studied with exceptional precision. In the Ardrishaig 59 Phyllites, the extreme contrast in lithology between beds of gritty 60 quartzite, up to 2 m thick, and interbedded units of what was 61 62 63 64 65 1 2 3 4 originally a somewhat silty mudrock, gives rise to an extraordinary 5 range of cleavage refraction geometries. Superb examples of minor 6 F1 folds are seen in the Ardnoe Member at the stratigraphical base 7 of the Crinan Grits, with equally good examples of minor F1 folds 8 with curved hinges being found within the Ardrishaig Phyllites. 9 10 11 15 PORT CILL MALUAIG 12 (NR 722 700–NR 714 690) 13 14 J.L. Roberts and P.W.G. Tanner 15 16 17 15.1 Introduction 18 19 This relatively small GCR site is located along the seaward edge of 20 the raised beach to the south-west of Port Cill Maluaig, on the 21 east coast of Loch Caolisport in Knapdale (Figure 34). Minor 22 geological structures are the main focus of attention, as the rocks 23 all belong to the Ardrishaig Phyllite Formation and show very 24 little overall variation in lithology. The ‘Ardrishaig Phyllites’, 25 as previously known, are the uppermost formation of the Easdale 26 Subgroup. They have been intruded by basaltic sills, and all of 27 the rocks have undergone greenschist-facies regional metamorphism 28 within the biotite zone. 29 30 The GCR site lies on the south-east limb of the F1 Ardrishaig 31 Anticline (Figure 34), which is correlated with the Aberfoyle 32 Anticline on the south-east side of the F4 Cowal Antiform to form 33 part of an overall F1 structure that is generally referred to as 34 the Tay Nappe. The site is located within the Knapdale Steep Belt 35 (Roberts, 1974) and, in contrast to the other GCR sites in 36 Knapdale, the rocks have been affected by both the D1 and D2 phases 37 of the Grampian Event. These two phases of early deformation are 38 closely related to one another: D1 resulted in the development of 39 the slaty cleavage (S1), which was overprinted and virtually 40 destroyed by the development of the later, closely spaced S2 41 fabric. Bedding in these rocks is inverted and, together with S1, 42 dips steeply towards the north-west; it is cut by a penetrative S2 43 fabric which dips at a lower angle in the same direction (inset on 44 Figure 34). 45 This site provides exceptional examples of minor F2 folds with 46 strongly curvilinear fold hinges, which form miniature elongated 47 48 basins and domes locally, where they affect thin beds of fine- 49 grained quartzite. It also provides a lithological and structural 50 contrast with the Ardrishaig Phyllites within the Kilmory Bay GCR 51 site, which lie on the opposing, north-west limb of the Ardrishaig 52 Anticline, and are dominated by early (D1) minor structures. 53 There is no published map that gives details of the geology of the 54 area around, and including, the GCR site and the only description 55 is in a field guide by Roberts (1977c). However, a 13.4 m-thick 56 basic meta-igneous sill, exposed at the north end of the section, 57 figured prominently in a study of variations in fluid flow during 58 regional metamorphism across Knapdale (Skelton et al., 1995). 59 60 61 62 63 64 65 1 2 3 4 15.2 Description 5 6 The Ardrishaig Phyllites at this site consist of grey-green 7 phyllitic metamudstone and metasiltstone, interbedded with fine- 8 grained quartzite and uncommon metalimestone. The quartzite beds 9 vary in thickness along their length and some are seen to die out 10 within a few metres. A number of the thicker quartzite beds 11 12 contain discontinuous, calcite-rich, layers or lenses. 13 Minor F2 folds are common. They are identified from the fact that 14 they fold the following: (a) pre-existing slaty cleavage in 15 metamudstone, or finely-spaced cleavage in metasandstone; (b) a 16 penetrative fabric in the basic meta-igneous bodies; (c) a pre- 17 existing striation lineation; and (d) in a few instances, early 18 isoclinal folds. The minor F2 folds verge overall towards the 19 north-west, and are associated, in the phyllitic rocks, with a 20 penetrative, finely-spaced crenulation cleavage developed parallel 21 to their axial surfaces. This cleavage forms the dominant fabric 22 in the rock, dipping north-west at a moderately steep angle (30– 23 60°), consistently shallower than the bedding (60–80°). A fibrous 24 mineral lineation, plunging at 25–45° to the west-north-west, 25 defines the D2 stretching direction. 26 The mesoscopic and minor F2 folds occur as Class 1C folds (Ramsay, 27 1967) where they affect the more-competent layers of fine-grained 28 quartzite, rather than Class 3 folds, which are developed in the 29 30 less-competent layers formed by the phyllitic rocks. Thus, the 31 more-competent layers tend to maintain the same thickness, as they 32 are traced around the fold hinges, while the less-competent layers 33 thicken into the fold hinges, so that overall the folding is 34 similar in style. 35 The F2 fold hinges mostly plunge north-east at a moderately steep 36 angle. However, some minor F2 folds have hinges that are strongly 37 curved within their axial planes, so that their plunge passes from 38 north-east through the horizontal to south-west within a short 39 distance. Where two adjacent curvilinear minor folds run parallel 40 to one another, the two fold trains are commonly ‗out of phase‘, 41 with plunge culminations in one, being adjacent to plunge 42 depressions in the other, and nearest neighbours in the two fold 43 trains plunging in opposing directions. Alternatively, adjacent 44 minor folds affecting the same bed may plunge, sometimes steeply, 45 in opposite directions (Figure 35). Excellent examples of 46 curvilinear F2 fold-hinges are seen to the south-west of a thin 47 48 Palaeogene basalt dyke at NR 717 697, giving rise to hump-backed 49 exposures of fine-grained quartzite, all closely packed together. 50 The D2 deformation strongly affects, and overprints, the earlier 51 S1 cleavage, which lies close to the bedding and is difficult to 52 distinguish from the microcrenulation form of S2 in the field. 53 Minor F1 folds are rarely seen in these rocks, probably due to the 54 intensity of the D2 deformation. However, an area of reddened 55 rocks, exposed on the raised beach 50 m to the north-east of the 56 small beach at Port Mhoirich (NR 716 696), displays a series of 57 minor, possibly F1 folds, affected by the F2 folding. 58 A gently dipping to horizontal crenulation cleavage locally 59 affects the D2 structures and represents the late-stage deformation 60 that is seen generally throughout Knapdale. 61 62 63 64 65 1 2 3 4 15.3 Interpretation 5 6 The Ardrishaig Anticline is a major fold structure in the South- 7 west Grampian Highlands that can only be recognized on 8 stratigraphical grounds, from the correlation of the Ardrishaig 9 Phyllite Formation and Erins Quartzite Formation, within the Cowal 10 Antiform to the south-east, with the equivalent Ardrishaig Phyllite 11 12 Formation and Crinan Grit Formation to the north-west (Figure 34). 13 The relative structural age of the Ardrishaig Anticline is a matter 14 of inference, since the nature of the early (D1) structures within 15 the Cowal Antiform, has been obscured by the intensity of the D2 16 deformation. Recognition of the age of the Ardrishaig Anticline, 17 as an F1 fold, relies upon evidence from the equivalent Aberfoyle 18 Anticline in the Highland Border region (see the Ardscalpsie Point 19 and Cove Bay to Kilcreggan GCR site reports in Tanner et al., 20 2013b). Having concluded that the Ardrishaig Anticline is a D1 21 structure, it is then clear, that the north-westerly vergence 22 displayed by the F2 folds on the lower limb of this fold at Port 23 Cill Maluaig, corresponds to the same vergence shown by F2 folds on 24 the lower limb of the Tay Nappe in the Aberfoyle Anticline on the 25 south-east limb of the Cowal Antiform (for example at the Cove Bay 26 to Kilcreggan GCR site). 27 The curved nature of the F2 fold hinges at Port Cill Maluaig 28 reflects the non-cylindroidal character of the initial F2 fold- 29 30 buckles, which can be interpreted as having been accentuated by the 31 intensity of the D2 deformation (Roberts and Sanderson, 1974). 32 Individual fold hinges change their direction of plunge in a 33 regular manner, forming sinuous lines that lie in a plane. The 34 well-developed stretching lineation lies in the same plane, 35 bisecting the angle between opposing plunge directions (the ‗apical 36 angle‘). This provides the clue as to how the originally horizontal 37 bedding surfaces have been distorted to form basins and domes on a 38 metre scale, by folding followed by progressive rotation of 39 individual fold hinges towards the stretching direction. The 40 reasons for this are discussed in the Glen Orchy GCR site report in 41 Treagus et al. (2013), and examined in full in the Strone Point GCR 42 site report in this paper. 43 44 15.4 Conclusions 45 46 The coastline south of Port Cill Maluaig exposes a representative 47 48 cross-section through the Ardrishaig Phyllites on the south-east 49 limb of the Ardrishaig Anticline, one of the major early (F1) folds 50 in the South-west Grampian Highlands. The rocks consist of grey- 51 green phyllitic metasiltstones and metamudstones, with beds of 52 fine-grained quartzite and two thick sills of dark green, 53 amphibolitic meta-igneous rock. The rocks are affected by two 54 phases of regional deformation: the D1 phase produced an early 55 cleavage lying very close to the bedding, presumably on the limbs 56 of tight to isoclinal F1 folds, whilst the D2 phase resulted in 57 development of a penetrative fabric, which virtually obliterated 58 the evidence for the sedimentary and early structural history of 59 these rocks. 60 61 62 63 64 65 1 2 3 4 The GCR site displays superb examples of minor F2 folds with 5 strongly curved fold hinges. They are displayed at Port Cill 6 Maluaig in three-dimensions with a perfection that is difficult to 7 match from elsewhere in the Dalradian outcrop. 8 9 16 STRONE POINT 10 11 (NN 113 088–NN 121 089) 12 13 P.W.G. Tanner 14 15 16 16.1 Introduction 17 18 One of the more-important major structures in the South-west 19 Grampian Highlands is the Ardrishaig Anticline, an early fold whose 20 axial trace passes just to the north of Inverary and continues to 21 the south-west, parallel to the west coast of Loch Fyne. The 22 significance of the Ardrishaig Anticline lies in the fact that it 23 is folded south-eastwards over the Cowal Antiform to correlate with 24 the Tay Nappe (Aberfoyle Anticline) in the Highland Border region. 25 The actual closure of the Ardrishaig Anticline is not well exposed 26 but its geometry is almost certainly mirrored by that of one of its 27 satellite folds, the Strone Point Anticline, which is seen at 28 Strone Point, 2 km east of Inverary (Figure 36). The hinge-zone 29 30 and north-west limb of the Strone Point Anticline may be examined 31 readily at this GCR site. 32 This GCR site is also valuable for providing an accessible section 33 in which to examine the range in rock types, and structures, seen 34 in the Ardrishaig Phyllite Formation south-east of the Loch Awe 35 Syncline, as compared to those found in the equivalent Craignish 36 Phyllite Formation on the north-west limb of the syncline, for 37 example, at the Fearnach Bay GCR site. 38 The Strone Point section was first described for the Geological 39 Survey by Hill (1905), who considered that the rocks there are 40 representative of the facies shown by the Ardrishaig Phyllite 41 ‗Group‘ in the surrounding part of Cowal. Apart from reporting the 42 presence of ‗isoclinal folding‘, Hill (1905) gave no specific 43 details of the structure. The only published account of the 44 structural geometry is by Borradaile (1972b), who analysed in 45 detail the mode of formation of the highly curvilinear minor folds 46 that occur there. This GCR site provides an ideal opportunity to 47 48 examine the geometry of these minor folds and their relationship to 49 the regional stretching lineation. 50 51 16.2 Description 52 53 The Ardrishaig Phyllites are admirably exposed in the almost 54 continuous rock exposure found on the narrow rock platform, backed 55 by low cliffs a few metres high. The phyllitic metamudstones and 56 metasiltstones that characterize the formation, are grey-green in 57 colour, with a silvery or silky sheen, and contain beds of 58 quartzite and metacarbonate rock. The quartzite beds are 59 prominent, but few in number, and consist of fine-grained quartzite 60 in uniform, non-graded units up to 2 m thick. The phyllitic rocks 61 62 63 64 65 1 2 3 4 contain orange-weathering bands of metacarbonate rock, probably 5 dolomitic, that are 20–50 cm thick, and are particularly common in 6 the hinge-zone of the major anticline. There is also one small 7 body of metadolerite, of the type commonly found in the Ardrishaig 8 Phyllites elsewhere in the area. 9 The coastline east-north-east from Strone Point provides an 10 excellent, slightly oblique, section through the north-west limb of 11 12 the Strone Point Anticline (which includes a mesoscopic fold pair 13 related to the major structure), and the hinge-zone, but the south- 14 east limb lies beneath the loch. The closure of this fold, 15 identified and named by Borradaile (1972b), is well exposed at NN 16 1180 0880, where it is marked by symmetrical folds of metre-scale 17 wavelength, which affect interbanded phyllitic rocks and orange- 18 coloured metacarbonate rock. Although no way-up evidence has been 19 recorded from these rocks, the Strone Point fold is an upward- 20 facing anticline based upon evidence of inverted graded in the 21 adjoining Ben Lui Schists (Borradaile, 1972b). 22 An intensely developed, penetrative cleavage, seen as a spaced 23 cleavage in the metacarbonate rocks, and a slaty cleavage in the 24 metamudstones, is axial planar to the major fold. In the hinge- 25 zone, it is associated with a zig-zag interdigitation of the two 26 lithologies, resulting in a blurring of the original boundaries 27 between the layers (Figure 37). The cleavage is very consistent in 28 orientation (Figure 36, inset; Figure 38 c), and on the north-west 29 30 limb of the fold it dips consistently more steeply to the north- 31 west than the bedding. This is in agreement with the presence of 32 the anticlinal hinge to the south-east, but appears at first sight 33 to be in conflict with the observation that both S- and Z-shaped, 34 congruous, tight to isoclinal minor folds occur on the same fold 35 limb. However, these folds give a consistent sense of vergence to 36 the south, when viewed in the vertical plane, with minor axial 37 planes consistently dipping at a steeper angle to the north-west, 38 than bedding. The change in the down-plunge minor fold pattern, is 39 due to a randomly distributed variation about the horizontal in the 40 plunge direction of the minor folds from north-east to south-west. 41 Indeed, individual minor folds are strongly curvilinear, and change 42 their plunge direction in a single exposure (Borradaile, 1972b, 43 plate 2). This is particularly well seen in the quartzite beds, as 44 illustrated by Voll (1960, figures 19 a and b) from just outwith 45 the GCR site (Figure 38 a and b). Quartzite beds are thickened 46 into bulbous shapes where they pass around minor fold closures, but 47 48 are considerably thinned on the fold limbs. 49 An important feature of these rocks is the development of a down- 50 dip stretching lineation on the slaty cleavage planes (Borradaile, 51 1972b), which is marked by a fine, silky striation lineation, first 52 described by Clough (in Gunn et al., 1897) from the Cowal 53 peninsula. When plotted on the same stereographic projection as 54 the poles to cleavage, the D1 fold hinges and bedding/cleavage 55 intersection lineation lie on a great circle that is only at an 56 angle of 04° to the mean orientation of the penetrative cleavage, 57 and contains the stretching lineation (Figure 38 c). From a thin- 58 section study of 40 deformed quartz grains in slaty metamudstone, 59 Borradaile (1972b) confirmed that the lineation is parallel to the 60 long axes of the grains, and hence represents the X direction. 61 62 63 64 65 1 2 3 4 Pyrite crystals that grew in the mudstone before deformation are 5 commonly streaked-out parallel to the stretching lineation, a 6 feature accentuated by their subsequent oxidation to form rusty- 7 looking, commonly elliptical streaks on the cleavage plane. It is 8 of historical interest that the Duke of Argyll published a paper in 9 1889 interpreting these artefacts as fossil annelid worm tubes. 10 11 12 16.3 Interpretation 13 14 The Ardrishaig Phyllites have been affected by a single ductile 15 deformation phase (D1) that resulted in the formation of the Strone 16 Point Anticline and its associated minor folds, spaced and slaty 17 cleavages, and stretching lineation. The only evidence of later 18 ductile deformation is given by open warps that locally affect the 19 early cleavage in the hinge-zone of the major structure, and 20 centimetre-spaced kink bands, which are common from Strone Point 21 northwards along the west side of the headland. 22 The Strone Point Anticline is an upward-facing F1 structure, whose 23 axial surface is inferred to dip north-west at 29°, parallel to the 24 mean orientation of the associated penetrative cleavage. In the 25 absence of bedding readings from the south-east limb, and with 26 minor folds and bedding cleavage intersection lineations having 27 orientations that vary in trend by 180° or more (Figure 38 c), 28 there is a problem with determining its axis orientation. Within 29 30 the hinge-zone of the major fold, a strong ?axial lineation plunges 31 at 10–26° to the north, which would make the fold a sideways- 32 closing structure. However, if the major fold mimics the behaviour 33 of its minor satellites, and is strongly curvilinear, the 34 orientation seen in one slice through the structure at Strone 35 Point, is but a single snapshot of a structure that has an overall 36 near-horizontal axis that trends north-east–south-west, and hence 37 faces up to the south-east. 38 The Strone Point Anticline lies to the south-east of a major F1 39 fold of the same age, the Ardrishaig Anticline, which is considered 40 to represent the Tay Nappe to the north of the later Cowal Antiform 41 (Bailey, 1938; Roberts, 1966a). The presence of other major folds 42 of the same age as the Ardrishaig Anticline in the Ardrishaig 43 Phyllites is consistent with the recent interpretation of the Tay 44 Nappe as a plexus of large fold closures, rather than a single 45 major fold (Krabbendam et al., 1997). 46 An S1 slaty, to spaced cleavage, is dominant at this locality and 47 48 is related to the formation of the Strone Point Anticline. 49 Although Borradaile (1972b) stated confidently that the main 50 cleavage was the first to form in these rocks, and is of D1 age, he 51 also recorded the presence of a coplanar crenulation cleavage in a 52 few instances. Further fabric analysis is needed to clarify this 53 situation. 54 The significance of the curvilinear minor folds, and their 55 relationship to the stretching lineation, is discussed in the Glen 56 Orchy GCR site report in Treagus et al. (2013). The wide range in 57 minor fold axis orientations at the Strone Point GCR site has 58 resulted from the rotation of the originally horizontal, or 59 slightly sinusoidal minor fold hinges, towards the orientation of 60 the NNW-trending stretching lineation (the X direction of the 61 62 63 64 65 1 2 3 4 strain ellipsoid), with increasing, but locally variable, amounts 5 of strain. Only a single section through the major fold is 6 available for study, so it is not known whether the major fold in 7 this case is also curvilinear. 8 9 16.4 Conclusions 10 11 12 Strone Point is an excellent GCR site for demonstrating the 13 techniques used for unravelling the geometry and evolution of a 14 large fold structure, in this instance the Strone Point Anticline. 15 It also provides a well-exposed and readily accessible 16 representative section of the Ardrishaig Phyllite Formation to the 17 south-east of the Loch Awe Syncline, which is stratigraphically 18 equivalent to the Craignish Phyllite Formation to the north-west 19 (see the Fearnach Bay and Craignish Point GCR site reports). 20 The main value of this locality lies in the fact that the Strone 21 Point Anticline is of the same age as, and can act as a proxy for, 22 the neighbouring but poorly exposed Ardrishaig Anticline. The 23 axial trace of the latter major structure runs in a north-easterly 24 direction a few kilometres to the north-west of Strone Point. This 25 surrogate role is important, as the Ardrishaig Anticline is the 26 north-western equivalent of the Tay Nappe, one of the largest fold 27 structures in the South-west Grampian Highlands. Thus, the 28 structures described from this GCR site may be compared in detail 29 30 with the fabrics (such as cleavages) and minor fold structures 31 found in the core of the Tay Nappe, for example at the Cove Bay to 32 Kilcreggan GCR site, described in Tanner et al. (2013b). 33 This is also a valuable location for studying the mode of 34 development of folds with highly curved hinges, and complements a 35 similar study, in rocks of the Beinn Udlaidh Syncline at the Glen 36 Orchy GCR site (Treagus et al., 2013), which were at the time much 37 more deeply buried in the Earth‘s crust, and hence hotter. 38 39 17 KILCHRENAN BURN AND SHORE 40 (NN 035215, NN 034227) 41 42 J.E. Treagus 43 44 45 17.1 Introduction 46 47 48 This GCR site, on the north-west side of Loch Awe, provides the 49 best exposures of the Kilchrenan Conglomerate Member (formerly the 50 Kilchrenan Boulder Bed), an important unit, which occurs near the 51 top of the Argyll Group. More precisely, it is found near or at 52 the top of the Tayvallich Slate and Limestone Formation and below 53 the Tayvallich Volcanic Formation and Loch na Cille ‘Boulder Bed’ 54 (see the West Tayvallich Peninsula GCR site report). The member is 55 important for the light that it throws on the sedimentary 56 environment of the Tayvallich Subgroup, but it is particularly 57 important for information that it yields on the strain experienced 58 by these rocks in the first phase of deformation. Its 59 characteristics have been described briefly by Borradaile (1973, 60 1977). 61 62 63 64 65 1 2 3 4 The Kilchrenan Conglomerate Member is a thin, but locally 5 continuous unit, confined to some 4 km outcrop length in the hinge- 6 zone of the F1 Loch Awe Syncline. The metaconglomerate has only 7 undergone weak deformation after D1, and comprises clasts of one 8 dominant lithological type in a homogeneous matrix of another; 9 these facts are important in its use for strain measurements. The 10 exposures are around the village of Kilchrenan, to its south on the 11 12 shore of Loch Awe and to its west in the Kilchrenan Burn. The only 13 other outcrops of a metaconglomerate occupying a similar 14 stratigraphical position are found east of Loch Awe, but those have 15 not been described formally. 16 17 17.2 Description 18 19 This GCR site incorporates two localities for the Kilchrenan 20 Conglomerate Member, a single bed, at most 30 m thick, which is 21 otherwise poorly exposed. The Kilchrenan Burn exposes the member 22 intermittently from NN 0335 2289 to NN 0342 2276, but the best 23 exposures are seen in weathered rocks on the west bank near the 24 former grid reference (Figure 39, locality A). The member here 25 consists of a matrix of unbedded slaty metamudstone (once a 26 carbonaceous silty mud) supporting elliptical clasts of gritty 27 quartzite. In some beds the clasts are tightly-packed oblate 28 spheroids (Figure 40), which typically have maximum lengths of some 29 30 20 cm and minimum lengths of some 5 cm, the latter being 31 perpendicular to the slaty cleavage. The cleavage, which dips at 32 low angles to the north-west (e.g. 030/16), dominates the matrix. 33 Bedding is rarely evident, but appears to be slightly less steeply 34 dipping to the west, than the cleavage. 35 The second locality consists of exposures on the shore of Loch Awe 36 (Figure 39, locality B), which are best seen from NN 0351 2157 to 37 NN 0340 2148, south-west of Struan, particularly in the low, moss- 38 covered, crags away from the shoreline. The description of the 39 member given above generally applies, but here angular black 40 mudstone and rounded limestone clasts have also been incorporated 41 into the conglomerate, and there is a range in clast sizes (long 42 axes), from 20 cm down to 0.5 cm. There are rare clasts of granite, 43 first noted by Kynaston and Hill (1908). Another feature here is 44 the presence of a crenulation cleavage that post-dates the slaty 45 cleavage and slightly deforms the clasts. Although the deformation 46 has resulted in generally oblate-shaped clasts, they do show a 47 direction of stretching, down-dip within the NW-dipping (e.g. 48 o o 49 030 /27 ) cleavage. 50 51 17.3 Interpretation 52 53 Very little comment has been made in the literature regarding the 54 sedimentary character of the Kilchrenan Conglomerate or of its 55 significance in terms of the depositional environment of the 56 Tayvallich Subgroup. Kilburn et al. (1965) interpreted it as a 57 slump conglomerate, and its position between slump breccias and 58 conglomerates in the Tayvallich Slate and Limestone Formation 59 below, and the breccias of the Loch na Cille ‘Boulder Bed’ in the 60 Tayvallich Volcanic Formation above (see the West Tayvallich 61 62 63 64 65 1 2 3 4 Peninsula GCR site report), would support the interpretation of the 5 deposition of the subgroup on an unstable shelf. The strong 6 sorting of the clasts seen in the Kilchrenan Burn locality and 7 their assumed high original sphericity, suggest a shallow water 8 deposit, which has been transported to, and preserved in, deeper 9 waters. Subsequent volcanicity and faulting supports the concept 10 of increasing instability of the shelf (Anderton, 1985). The 11 12 presence of rare granite clasts has important implications for the 13 palaeogeography of the region and warrants further research. 14 Structurally, both localities are situated on the gentle west- to 15 NW-dipping, south-eastern limb of the Loch Awe Syncline. Although 16 bedding is not easily distinguished in these rocks, this structural 17 context is supported by the observation of cleavage dipping more 18 steeply than bedding, as is seen in the Kilchrenan Burn locality. 19 However, according to the mapping of Borradaile (1973), the 20 exposures on the shore of Loch Awe lie on the short limb of a 21 parasitic fold, with cleavage dipping shallower than the overturned 22 bedding (Figure 39). The cleavage in which the clasts are deformed 23 is clearly the first slaty cleavage. Measurement of the clast 24 shape should therefore give an indication of the strain experienced 25 during D1, with the usual assumptions that the clasts were 26 originally subspherical and that they will give a minimum value of 27 the strain for the whole rock. Provisional strain ratios for the 28 clasts give X:Y:Z average ratios of 1.5:1.23:0.54 for the quartzite 29 30 clasts of the Loch Awe locality and up to 1.8:1.8:0.3 for the 31 oblate shapes in the Kilchrenan Burn (Figure 40). 32 33 17.4 Conclusions 34 35 The Kilchrenan Burn and Shore GCR site is primarily of national 36 importance for the study of pebbles in the Kilchrenan Conglomerate 37 Member of the Tayvallich Slate and Limestone Formation. 38 Conglomerates in which original rounded pebbles of one rock-type 39 are not in contact with one another and sit in a uniform matrix, 40 are extremely unusual and are of great importance to structural 41 studies. The pebbles were deformed in the earliest deformation 42 experienced by the Dalradian rocks of the South-west Grampian 43 Highlands (D1) and the dimensions and orientation of the pebbles 44 indicate the direction and strength of the forces that formed the 45 mountain belt. 46 Of almost equal importance is the light that this unique 47 48 sedimentary deposit can throw on the nature of the sedimentary 49 basin in late Argyll Group time, particularly its depth, slope and 50 stability. From the initial roundness of the pebbles and the very 51 localized nature of the deposit, it has been suggested that the 52 basin was on a shallow shelf, which became increasingly unstable 53 with time, eventually resulting in the volcanicity and 54 contemporaneous faulting seen in the overlying rocks. 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 18 WEST TAYVALLICH PENINSULA 5 (NR 732 878–NR 690 803) 6 7 E.A. Pickett 8 9 10 11 18.1 Introduction 12 13 This large GCR site is on the west coast of the Tayvallich 14 peninsula, which lies on the Argyll mainland, east of the Isle of 15 Jura. It provides some of the best exposures of Neoproterozoic 16 submarine extrusive rocks and associated reworked volcanic rocks in 17 the British Isles, including spectacular pillow lavas and 18 hyaloclastites. The site is of stratigraphical importance as it 19 provides a well-exposed section through the Crinan Grit Formation, 20 the Tayvallich Slate and Limestone Formation and the Tayvallich 21 Volcanic Formation. The lower boundary of the Tayvallich Volcanic 22 Formation is particularly well exposed. The southern end of the 23 peninsula also displays the controversial Loch na Cille Boulder Bed 24 which lies within the volcanic sequence. 25 26 This area has been a classical geological site since the early 27 20th century when B.N. Peach discovered the pillow lavas and so 28 established the existence of extrusive igneous rocks in the 29 30 Dalradian of the South-west Grampian Highlands (Peach, 1904; Peach 31 et al., 1911). His observations and interpretations of pipe 32 amygdales and lava flow morphology were essential in establishing 33 the stratigraphical sequence of the area. Elles (1935), in her 34 study of the Loch na Cille Boulder Bed, disputed Peach‘s proposed 35 stratigraphy but Allison (1941) conclusively vindicated the 36 original interpretation. Wood (1964) described in detail 37 structures observed in pillow lavas on the peninsula. Wilson and 38 Leake (1972) carried out the first major geochemical study of a 39 wide range of rocks from the Tayvallich peninsula (including lavas, 40 intrusive rocks and sedimentary rocks) and Graham (1976) made 41 further analyses of the meta-igneous rocks rocks as part of a wider 42 study of metabasalts in the South-west Grampian Highlands. Recent 43 geochemical studies of the Tayvallich lavas were carried out by 44 Hyslop and Pickett (1999), Pickett et al. (2006) and Fettes et al. 45 2011 as part of investigations of Dalradian basic meta-igneous 46 rocks and their tectonic significance. The most recent detailed 47 48 study on the entire sequence was carried out by Gower (1977) who 49 described a series of localities along the west coast of the 50 peninsula. Much of the following description is based on his 51 observations. 52 A feature of great importance within the site is a trachytic 53 (‗quartz keratophyre‘) intrusion that is believed to be 54 contemporaneous with the extrusion and deposition of the volcanic 55 and volcaniclastic rocks. This intrusion was dated by Halliday et 56 al. (1989) who obtained a conventional multi-grain U-Pb zircon age 57 of 595 ± 4 Ma, thus providing an age for the Tayvallich Subgroup 58 and indicating that most of the Dalradian Supergroup is of 59 Neoproterozoic age. Dempster et al. (2002) subsequently reported a 60 U-Pb TIMS (Thermal Ionisation Mass Spectrometry) zircon age of 601 61 62 63 64 65 1 2 3 4 ± 4 Ma from a tuff on the west coast of the Tayvallich peninsula at 5 Port Bhuailteir (NR 688 810) (Figure 41). This date is within 6 error of the earlier determination, and confirmed the age of the 7 Tayvallich Volcanic rocks. 8 The structure of the area is dominated by the F1 Tayvallich 9 Syncline, which plunges at 20 - 50° to the south-south-west and has 10 an axial plane which dips to the east-south-east at 70 - 80°. This 11 12 fold is an important component of the major Loch Awe Syncline. 13 There is a strong axial planar slaty cleavage associated with the 14 fold although subsequent deformation episodes have overprinted this 15 cleavage with a crenulation cleavage locally. A stretching 16 lineation plunges down the dip on the slaty cleavage surfaces. 17 18 18.2 Description 19 20 This large site is best considered as a number of separate sections 21 (Figure 41): 22 (1) the axial zone of the syncline at Loch na Cille, (2) Rubha 23 Riabhag to An Aird, (3) An Aird to Port an Sgadain, (4) Port an 24 Sgadain to Port Bealach nan Gall, 5) the closure of the Tayvallich 25 Syncline around the northern slopes of Barr na h-Iolaire. 26 27 18.2.1 Area 1: Loch Na Cille 28 29 30 This area is in the axial zone of the Tayvallich Syncline and 31 contains the youngest rocks of the area, vertically dipping 32 feldspathic metasandstones, which are exposed on the south-east 33 limb of the syncline at NR 698 806. Mineralogically they are 34 distinct from the older Crinan Grits in that they contain detrital 35 epidote and more feldspar and mica. Gower (1973) proposed the name 36 Kells Grit for these metasandstones, which have been correlated 37 with the Loch Avich Grit of the Southern Highland Group farther 38 north-east (Borradaile, 1973) (see the Loch Avich GCR site report). 39 The Kells Grit is underlain by a sequence of metalimestones and 40 metabreccias containing clasts of igneous rock within a calcareous 41 matrix. One of these metabreccias is the Loch na Cille Boulder 42 Bed, which lies at the base of the metalimestone succession. It 43 crops out down the centre of the peninsula of Rubha na Cille and is 44 best exposed at the northern end (NR 688 803). It 45 stratigraphically overlies pillow lavas that are exposed on the 46 west coast and is in turn overlain by grey metalimestone containing 47 48 lava fragments. The Loch na Cille Boulder Bed is particularly well 49 exposed on the east shore of Rubha Fitheach, where fragments of 50 pillow and vesicular lava are deformed and flattened within the 51 main cleavage. The clasts are predominantly fragments of mafic 52 meta-igneous rock and pebbles of pale-coloured felsic metavolcanic 53 rock in a chloritic, calcareous or quartzose matrix. However, 54 Elles (1935) also recorded the presence of quartzite, gneiss and 55 schist and noted that many of the boulders are rounded. In a 56 recent re-examination of the metabreccias, Prave (1999) also 57 observed a range of clasts in beds towards the top of the unit, 58 which he termed ‗extrabasinal‘, including granitic rocks, schists, 59 felsic volcanic rocks and quartzite. 60 61 62 63 64 65 1 2 3 4 18.2.2 Area 2: Rubha Riabhag to an Aird 5 6 North-eastwards along the coast from Rubha na Cille progressively 7 older members of the Tayvallich Volcanic Formation are exposed. 8 The lithologies include massive lava, excellently preserved pillow 9 lava, pillow breccia and stratified and reworked hyaloclastite and 10 volcaniclastic rocks. The hyaloclastites were first recognized by 11 12 Gower (1977). The pillow lavas are particularly well exposed at NR 13 693 817 with pillows up to 3 m by 1 m in size (Figure 42). 14 Stratigraphical way-up is well defined by the pillow morphology and 15 by concentrations of feldspar crystals (up to 1 cm long) at the 16 pillow bases. Many of the pillows also display concentric bands of 17 vesicles towards their tops. Recent geochemical studies have shown 18 that the pillow lavas have high-Fe-Ti tholeiitic compositions and 19 were derived from a relatively enriched mantle source (Fettes et 20 al., 2011). 21 Within the volcanic sequence is a distinctive 8 m-thick bed of 22 ‗porphyry breccia‘, described by Peach et al. (1911), which is 23 composed of pink-orange trachytic boulders (up to 1 m across) 24 within a tuffaceous matrix (NR 690 813). These boulders are 25 lithologically similar to a trachytic ‗feldspar porphyry‘ or 26 ‗quartz keratophyre‘ intrusion (over 16 m thick), which is exposed 27 in the intertidal zone nearby (NR 695 822). The mineralogy of this 28 intrusion was studied in detail by Peach et al. (1911). It 29 30 contains tabular albite crystals up to 3 mm in length displaying 31 chess-board twinning within a finer grained albite-rich groundmass 32 containing quartz, epidote, white mica, chlorite and opaque 33 minerals. Parts of the intrusion show a poorly developed mineral 34 banding of alternating mafic and felsic layers parallel to the 35 upper contact. This contact is concordant with the surrounding 36 bedded volcanic rocks and a chilled margin is present locally 37 (Gower, 1977). It is from this intrusion that zircons have 38 provided a U-Pb age of 595 ± 4 Ma, one of only two reliable 39 radiometric dates from the upper part of the Dalradian Supergroup 40 (Halliday et al., 1989). 41 42 18.2.3 Area 3: An Aird to Port An Sgadain 43 44 The small peninsula of An Aird is composed largely of a 45 metabasaltic laccolith, which might have acted as a feeder to some 46 of the overlying pillow lavas. The western side of the peninsula 47 48 is composed of pillow lavas, overlain to the east by pillow 49 breccias and hyaloclastites. The pillow lavas at this locality are 50 excellently exposed and are the ones originally described by Peach 51 (1904) and Peach et al. (1911). Graham (1976) described their 52 petrology, noting a range of features including microphenocrysts of 53 twinned plagioclase (now albite), ‘trachytic’ textures and amygdales. 54 Northwards from An Aird, are pillow breccias and hyaloclastites 55 (Gower, 1977). Thin rusty-weathering metalimestone lenses are also 56 present and a distinctive dolomitic breccia occurs directly south- 57 west of Port an Sgadain. 58 59 60 61 62 63 64 65 1 2 3 4 18.2.4 Area 4: Port An Sgadain to Port Bealach Nan 5 Gall 6 7 At the southern end of this section, the base of the Tayvallich 8 Volcanic Formation and its contact with the underlying Tayvallich 9 Slate and Limestone Formation are very well preserved. Here the 10 lava flows have pipe amygdales at their base and scoriaceous tops, 11 12 commonly with rusty-weathering carbonate veins penetrating between 13 the blocks of lava (Gower, 1977). Peach et al. (1911) used these 14 exposures to demonstrate that the sequence here is the right way up 15 and dips at 40° to the east. During its extrusion the lowest lava 16 produced load structures and push folds in the underlying soft 17 sediment. In places bulbous masses of lava became detached and 18 isolated within the sediment (Wood, 1964). The Tayvallich Slate 19 and Limestone Formation is very variable and includes dolomitic 20 metalimestones, phyllitic rocks, thin-bedded and massive 21 metalimestones and conglomeratic beds. The metalimestone beds 22 contain numerous quartz clasts (up to 1.5 cm across). The 23 formation exhibits a variety of sedimentary structures including 24 grading, cross-bedding, channels and flute casts, possibly 25 reflecting a turbiditic origin. At Port Bealach nan Gall, the 26 quartzitic Crinan Grits are exposed beneath the metalimestones. At 27 this locality the Crinan Grits comprise graded, very coarse- 28 grained, locally conglomeratic metasandstone containing excellent 29 30 examples of flute casts (50–60 cm long). Beds of metamudstone, one 31 of which contains large carbonate nodules (up to 2 m in diameter), 32 are also exposed. 33 34 18.2.5 Area 5: Barr Na H-Iolaire 35 36 The closure of the Tayvallich Syncline can be traced out by 37 following the scarp that represents the base of the lavas around 38 the northern slopes of Barr na h-Iolaire. Although the underlying 39 metalimestone is largely obscured, the base of the Tayvallich 40 Volcanic Formation can be traced around the minor folds associated 41 with the closure of the syncline. In their detailed map of this 42 area, Wilson and Leake (1972) showed the distribution of beds of 43 meta-agglomerate within the Tayvallich Volcanic Formation. They 44 described the distinctive geochemical composition of one meta- 45 agglomerate, the ‘Barbreack Agglomerate’, which they found to be much 46 more siliceous than the pillow lavas and to contain very high 47 48 levels of Nb, Zr, U, Th, La and Ce. 49 50 18.3 Interpretation 51 52 The sequence preserved along the west coast of the Tayvallich 53 peninsula is interpreted as recording a period of rapid subsidence 54 and basin deepening associated with active rifting along a 55 continental margin during late-Dalradian times (Anderton, 1985). 56 The Crinan Grit, which is the oldest formation at this GCR site, 57 was interpreted by Anderton as the result of turbidite deposition 58 in submarine fan channels. 59 The overlying Tayvallich Slate and Limestone Formation is also 60 largely turbiditic. The orientation of cross-bedding and flute 61 62 63 64 65 1 2 3 4 casts suggests current directions from the south and south-east 5 (Gower, 1977). Anderton (1985) interpreted the abrupt change from 6 clastic to carbonate sedimentation as the result of a relatively 7 sudden re-organization of the palaeogeography. This stopped or 8 diverted the clastic input and the sediment-starved shelves began 9 to accumulate carbonate-rich deposits. Erosion and reworking of 10 the carbonates generated turbiditic deposits in adjacent basins. 11 12 The overlying Tayvallich Volcanic Formation indicates a period of 13 magmatism caused by eventual rupturing of the continental crust 14 within the widening rift (Anderton, 1985). Wood (1964) used the 15 orientation of push folds in underlying metasedimentary rocks to 16 determine that lavas at the base of the section flowed northwards 17 into soft sediments. Extrusion of the volcanic pile was largely 18 submarine, as indicated by the pillow lavas and hyaloclastites, and 19 many of the lavas are extensively spilitized, probably reflecting 20 sea-floor hydrothermal processes that occurred soon after 21 extrusion. Graham (1976) interpreted the geochemistry of the lavas 22 and associated intrusions as indicating initial continental rifting 23 preceding ocean-floor spreading. Recent work has confirmed that 24 the lavas have enriched characteristics, typical of within-plate 25 ocean-island basalts (OIB) but also found on extensional plate 26 margins where thinning of the continental lithosphere prior to 27 rupturing results in basin formation and relatively low degrees of 28 decompression melting of the underlying mantle (Fettes et al., 29 30 2011). Geochemical evidence of crustal contamination has been 31 found in some of the Tasyvallich lavas, though not in those from 32 the Tayvallich peninsula. 33 The trachytic intrusion exposed at the GCR site is geochemically 34 related to the extrusive rocks and probably represents part of the 35 subvolcanic feeder system (Graham, 1976). Lithologically similar 36 trachytic blocks occur as clasts in overlying volcanic breccias, 37 supporting geochemical evidence that the intrusion is cogenetic 38 with the volcanic rocks and hence is a suitable subject for 39 radiometric dating of the volcanism. 40 Reworking of volcanic material by sedimentary processes produced a 41 variety of volcaniclastic rocks, which are intercalated with the 42 extrusive rocks. The Loch na Cille Boulder Bed towards the top of 43 the Tayvallich sequence was originally interpreted by Peach et al. 44 (1911) as a tectonic breccia or ‗crush-conglomerate‘. Various 45 other origins were later proposed, including a glacial origin 46 (Elles, 1935), a porphyritic lava flow including xenoliths of 47 48 ‗acidic‘ boulders (Allison, 1941) and a hyaloclastite resulting 49 from a submarine fissure eruption (Gower, 1977). Hyslop and Pickett 50 (1999) interpreted the unit as being deposited from debris flows 51 and forming part of the overall Tayvallich volcanic-volcaniclastic 52 sequence. Conversely, Prave (1999) revived the glaciogenic 53 interpretation of Elles (1934), citing the presence of extrabasinal 54 clasts, many of them rounded, of granite, schist, felsic volcanic 55 rock and quartzite in beds towards the top of the unit. These 56 observations, coupled with the stratigraphical proximity of the 57 Loch na Cille Boulder Bed to the c. 595 Ma trachytic intrusion, led 58 Prave to suggest that the boulder bed is evidence for a Varangerian 59 glaciation (c. 620–590 Ma) in the Dalradian of Scotland (see 60 Stephenson et al., 2013a). On balance it would seem that there is 61 62 63 64 65 1 2 3 4 good evidence for some form of volcaniclastic deposition, but that 5 the higher beds with the extrabasinal clasts merit further 6 investigation. 7 8 18.4 Conclusions 9 10 The West Tayvallich Peninsula GCR site preserves an excellently 11 12 exposed succession that passes stratigraphically up from the Crinan 13 Grit Formation, through the Tayvallich Slate and Limestone 14 Formation and the Tayvallich Volcanic Formation to the Loch na 15 Cille Boulder Bed and, at the top of the sequence, the Kells Grit. 16 These formations comprise some of the most distinctive and 17 persistent stratigraphical markers of the Dalradian terrane and 18 hence are valuable for correlation purposes. In addition to its 19 stratigraphical significance, the site is of national importance 20 for its wide range of extrusive volcanic rocks and associated 21 volcaniclastic sedimentary rocks, formed during rifting of 22 continental crust in the Neoproterozoic. A variety of igneous and 23 sedimentary processes can be demonstrated and are invaluable in 24 current studies involving reconstructing the tectonic regime and 25 environments in which the higher parts of the Dalradian succession 26 were deposited. The site exhibits minor structures associated with 27 the Tayvallich Syncline, an important component of the major F1 28 Loch Awe Syncline that controls outcrop patterns over a wide area 29 30 of the South-west Grampian Highlands. 31 A trachytic intrusion and a stratiform tuff from this site have 32 yielded concordant U-Pb (zircon) ages of c. 600 Ma. These are the 33 only reliable radiometric dates from the upper part of the 34 Dalradian and are therefore critical in establishing a 35 chronostratigraphical framework for the Dalradian Supergroup as a 36 whole. 37 The site has been a classic locality for teaching and research, 38 since at least 1911, and it was here that pillow lavas, erupted 39 underwater, were first recognized and described in the Dalradian of 40 Scotland. The study of these exceptional volcanic rocks in terms 41 of their geochemistry and depositional setting continues today. 42 43 19 SOUTH BAY, BARMORE ISLAND 44 (NR 868 714–NR 872 702) 45 46 47 J.L. Roberts and P.W.G. Tanner 48 49 50 19.1 Introduction 51 52 Barmore Island is situated a few kilometres north of Tarbert, on 53 the east side of the Kintyre peninsula. It is connected to the 54 mainland by a narrow isthmus (Figure 43). The coast immediately 55 south of the isthmus provides a well-exposed and unique cross- 56 section through the Dalradian rocks of Knapdale and North Kintyre, 57 which includes the critical contact between the Argyll and Southern 58 Highland groups. This rugged coastline also displays the effects 59 of glacial scouring by Pleistocene ice-sheets, flowing southwards 60 along Loch Fyne. 61 62 63 64 65 1 2 3 4 The sequence begins with the Stonefield Schist Formation that lies 5 at the top of the Crinan Subgroup and appears to be conformable 6 with the overlying Loch Tay Limestone Formation, which here 7 represents the Tayvallich Subgroup. Towards the south-east, these 8 rocks pass stratigraphically upwards, with no discernable break, 9 into the Southern Highland Group, which comprises the Glen Sluan 10 Schist Formation, the Green Beds Formation, and the Beinn Bheula 11 12 Schist Formation. The Loch Tay Limestone forms one of the most 13 important marker horizons for lithostratigraphical correlation 14 within the Dalradian Supergroup. It is equivalent to the Tayvallich 15 Slate and Limestone Formation, which lies in the core of the Loch 16 Awe Syncline farther to the north-west. 17 This GCR site lies adjacent to the major F4 Tarbert Monoform, 18 which separates the Knapdale Steep Belt to the north-west from the 19 inverted rocks of the Cowal Antiform to the south-east. This 20 setting is comparable with that of the Portincaple GCR site (Tanner 21 et al., 2013b), which is located on the closure of the Highland 22 Border Downbend (F4) between the Cowal Antiform and the Highland 23 Border Steep Belt. The regional metamorphic grade reached by the 24 rocks in the two areas is however different: those at Portincaple 25 are barely in the biotite zone, whereas those at Barmore Island are 26 in the garnet zone. A detailed structural analysis has not been 27 made of the rocks of this site and they warrant further study. 28 Following descriptions of the general geology by the Geological 29 30 Survey (Peach et al., 1911) and by McCallien (1925), some of the 31 structural features were described by Roberts (1966b, 1974, 1977c). 32 A notable early study of the origin of the ‗green beds‘ was carried 33 out by Phillips (1930), and the petrography of the Loch Tay 34 Limestone has been investigated in considerable detail by Gower 35 (1973) and Graham et al. (1983). 36 37 19.2 Description 38 39 19.2.1 Stratigraphy 40 41 The Stonefield Schists occur at NR 806 714 as garnetiferous mica 42 schists, interbedded with quartz-mica schist and schistose gritty 43 metasandstone. The bedding is approximately vertical and strikes 44 north-east, with graded bedding in one exposure showing that the 45 sequence youngs to the south-east. A transition zone some 6 m wide 46 separates the Stonefield Schists from the Loch Tay Limestone. 47 48 Within this transition zone, thin, weathered, calcareous bands 49 appear, become thicker up-sequence and are replaced progressively 50 by beds of dark-grey sugary metalimestone (up to 7 cm thick) 51 separated by thin beds of dark metametamudstone. 52 The Loch Tay Limestone is about 75 m thick in this section, and 53 occurs typically as a grey crystalline metalimestone (containing 54 some dolomite) with a granular texture, interbedded with minor 55 amounts of dark schistose metamudstone. It has a banded appearance 56 due to differential weathering of the centimetre-scale layers, some 57 of which are slightly more micaceous, whereas others are more 58 quartzose. Graded bedding in the more-quartzose layers is revealed 59 by the transition from a quartz-rich metalimestone at the base of 60 the bed to an increasingly carbonate-rich rock at the top. Such 61 62 63 64 65 1 2 3 4 grading shows younging to the south-east, so the beds are inverted. 5 A carbonate metabreccia, in which highly deformed fragments of 6 metalimestone are flattened in a plane parallel to the steep 7 bedding and stretched down dip, is developed close to the south- 8 east margin of the formation. Pinch-and-swell structure commonly 9 affects the more-quartzose layers, as well as quartz-plagioclase 10 veins (Figure 44). The metalimestone is affected by a system of 11 12 linked, listric faults associated with thick bodies of haematite- 13 stained fault breccia. Two thick sills of basic meta-igneous rock, 14 now garnet-biotite amphibolite, occur within the outcrop of the 15 Loch Tay Limestone. 16 The Glen Sluan Schist Formation is exposed immediately to the 17 south-east of the Loch Tay Limestone at NR 868 710. There is a 18 sharp contact between orange-weathering metacarbonate rock and the 19 stratigraphically younger micaceous schist, which contains numerous 20 thick quartz lenses. The Glen Sluan Schists consist largely of 21 quartzose mica schist, which is interbedded with minor amounts of 22 schistose pebbly metasandstone towards its contact with the Green 23 Beds Formation. Albite is commonly developed as conspicuous 24 porphyroblasts in the more-pelitic layers, and can be distinguished 25 from detrital feldspar in the more-psammitic layers by the fact 26 that the porphyroblasts appear undeformed in hand specimen. 27 Individual ‗green beds‘ occur within the succession from the Glen 28 Sluan Schist Formation at NR 869 710 to the Beinn Bheula Schist 29 30 Formation south of Sgeir Port a‘ Ghuail (Figure 43). Where they 31 form a dominant proportion of the succession, they define the Green 32 Beds Formation. They occur typically as massive chlorite-epidote- 33 biotite schists, interbedded with epidotic metaconglomerates, 34 quartz-mica schists, and albite schists. The bedding in the ‗green 35 beds‘ is defined by very regular, parallel-sided, alternating fine- 36 and coarse-grained layers, which vary in colour from green to dark 37 greenish grey, commonly with a speckled appearance (possibly due to 38 biotite). The layers vary in thickness from 1–20 cm and their 39 distinctive greenish colour is due to the presence of chlorite and 40 epidote. Locally, individual beds pass downwards into 41 metaconglomerate (schistose pebbly grit) with detrital grains of 42 quartz and feldspar. Graded bedding and small-scale channelling 43 are well developed locally in schistose gritty metasandstones 44 interbedded with the green-coloured beds. They show overall 45 younging towards the south-east, but some beds clearly young in the 46 opposite direction, suggesting that early (F1) folds are present in 47 48 the section. 49 The Green Beds Formation passes upwards into the Beinn Bheula 50 Schist Formation, which is exposed at the south-east end of the 51 section. The latter consists of a very thick sequence of 52 garnetiferous mica schists, biotite schists, quartz-mica schists, 53 albite schists, and schistose gritty metasandstones containing 54 detrital grains of quartz, often of a bluish hue, together with 55 pink or white oligoclase feldspar. 56 57 19.2.2 Structure 58 59 The rocks at this GCR site were deformed during the D1–D4 phases of 60 Caledonian deformation. No F1 folds have been observed but there 61 62 63 64 65 1 2 3 4 is an early schistosity (S1), lying subparallel to bedding. 5 Locally, both bedding and S1 are deformed by tight F2 folds, giving 6 rise to ‘strain-bands’ (Clough, in Gunn et al., 1897) and minor folds 7 that plunge steeply north-east and verge towards the north-west. 8 Typically, the F2 folds have an intense cleavage developed parallel 9 to their axial planes, which affects the earlier S1 schistosity. 10 The ‗strain-bands‘, or S2 spaced cleavage domains as they are now 11 12 known, are typically developed in the schistose gritty 13 metasandstones. 14 L3 linear structures trend north-east, and are generally 15 horizontal, but there is a 500 m-wide zone between NR 871 705 and 16 NR 872 700 where these F3 fold hinges plunge more steeply towards 17 the north-north-east. F4 folding affects the rocks to the south- 18 east of the Loch Tay Limestone, giving rise to alternating zones of 19 steeply dipping and flat-lying rocks. Typically the F4 folds have 20 axial planes dipping at a moderately steep angle towards the south- 21 east, parallel to a crenulation cleavage, while their fold hinges 22 plunge at less than 10º to the north-east. 23 24 19.3 Interpretation 25 26 Stratigraphically, the Loch Tay Limestone can be correlated with 27 the Tayvallich Slate and Limestone Formation, which crops out in 28 the core of the Loch Awe Syncline, farther to the north-west (see 29 30 the West Tayvallich Peninsula GCR site report; Figure 34). These 31 formations mark a change in sedimentation from the pebbly 32 quartzites and pebble metaconglomerates, typical of the upper parts 33 of the Argyll Group, into the pebbly metagreywackes of the 34 overlying Southern Highland Group. Such a change in lithology is 35 not accompanied by any corresponding change in the mode of 36 deposition, since graded bedding is a characteristic feature of 37 this part of the Dalradian sequence, both above and below the Loch 38 Tay Limestone. Thus, there is no sedimentological evidence for a 39 profound break in the succession, or orogenic unconformity, between 40 the Argyll Group and the Southern Highland Group. 41 The Green Beds Formation occurs at the same stratigraphical level 42 as the Loch Avich Lavas Formation, which crops out farther to the 43 north-west within the Loch Awe Syncline (see the Loch Avich GCR 44 site report). The ‗green beds‘ occur as finely laminated rocks, 45 passing imperceptibly into graded beds of schistose gritty 46 metasandstone, suggesting that they were deposited by turbidity 47 48 currents. Their composition suggests a volcanic origin and J.B. 49 Hill (in Peach et al., 1911) suggested that they were derived from 50 the weathering of basic igneous rocks. Their volcaniclastic origin 51 has been confirmed by the regional petrographical and geochemical 52 study of Pickett et al. (2006). 53 The GCR site lies on the steep, NW-dipping, overturned, south- 54 eastern limb of the major, upward-facing F1 Ardrishaig Anticline 55 and forms part of the Knapdale Steep Belt. The S2 spaced cleavage 56 and minor folds verge to the north-west as a ‗symmetry-constant 57 continuation‘ of the earlier D1 deformation, here represented only 58 by an S1 schistosity (Voll, 1960; Roberts, 1974). In a short 59 distance to the south-east, the inverted beds become less steeply 60 inclined as the Ardrishaig Anticline is folded over the broad F4 61 62 63 64 65 1 2 3 4 Cowal Antiform, and the minor F4 folds and SE-dipping S4 5 crenulation cleavage seen at this site are related to the Tarbert 6 Monoform, a major F4 structure developed on the north-western limb 7 of the Cowal Antiform. 8 9 19.4 Conclusions 10 11 12 The South Bay, Barmore Island GCR site provides a representative 13 cross-section through the Dalradian rocks of the uppermost Argyll 14 Group and the Southern Highland Group in Knapdale and North 15 Kintyre. The exposures show clearly that the contact between the 16 two groups is conformable, with no evidence of a structural break, 17 or orogenic unconformity. 18 The Loch Tay Limestone Formation is exposed over the full width of 19 its outcrop, allowing it to be compared with its stratigraphical 20 equivalent to the north-west, namely the Tayvallich Slate and 21 Limestone Formation. The site also provides an excellent section 22 through the Green Beds, which are otherwise poorly exposed in the 23 South-west Grampian Highlands. Their composition and finely- 24 laminated nature suggests that they were originally volcaniclastic 25 deposits, which were subsequently reworked by turbidity currents. 26 They may be correlated with pillow lavas at Loch Avich, to the 27 north-west. This GCR site also provides typical examples of the 28 structural features that developed in response to three distinct 29 30 phases (D1, D2 and D4) in the deformation history of the Dalradian 31 rocks in the South-west Grampian Highlands. 32 33 20 LOCH AVICH 34 (NM 957174–NM 952155) 35 36 E.A. Pickett 37 38 39 20.1 Introduction 40 41 This GCR site lies at the north-eastern end of Loch Avich, west of 42 Loch Awe. It contains the type locality for the Loch Avich Lavas 43 Formation and shows a complete section through the underlying 44 gritty metasandstones and metamudstones of the Loch Avich Grit 45 Formation. The uppermost part of the Tayvallich Volcanic 46 Formation, which underlies the Loch Avich Grit Formation, is also 47 48 present in the northern part of the site. The Loch Avich Grit and 49 the Loch Avich Lavas are the youngest observed Dalradian rocks in 50 the core of the Loch Awe Syncline and form the lower part of the 51 Southern Highland Group. Hence the lavas are of great significance 52 in the tectono-magmatic evolution of the Dalradian basin(s). 53 The Loch Avich Grit Formation has long been recognized and was 54 first described by Hill (1905) and Bailey (1913). The outcrop of 55 the Loch Avich Lavas Formation was originally classed as 56 ‗epidiorite‘ (Hill, 1905) and lavas (Bailey, 1913) and was not 57 distinguished from the older Tayvallich Volcanic Formation. It has 58 only relatively recently been recognized and described as a 59 separate formation lying above the Loch Avich Grit Formation 60 (Borradaile, 1972a). This discovery was especially important as 61 62 63 64 65 1 2 3 4 the lavas represent the only known extrusive volcanic rocks in the 5 Southern Highland Group. Several samples from the Loch Avich Lavas 6 Formation were analysed geochemically and compared with lavas from 7 the Tayvallich Volcanic Formation by Borradaile (1973) as part of 8 his investigation of the structure and stratigraphy of the northern 9 Loch Awe district. The Loch Avich lavas were the subject of recent 10 research by Pickett et al. (2006) into the origin of the 11 12 volcaniclastic Green Beds, with which they have been correlated 13 stratigraphically, and were also included in the regional 14 geochemical synthesis of Dalradian volcanism by Fettes et al. 15 (2011). 16 17 20.2 Description 18 19 Around Loch Avich, the Tayvallich Volcanic Formation and the 20 overlying Loch Avich Grit Formation lie within the core of the 21 Kilchrenan Syncline (Figure 45), which is part of the major F1 Loch 22 Awe Syncline. A complete section through the Loch Avich Grit 23 Formation is preserved in the Allt Mór burn, north-west of the hill 24 of An Cnap, where its observed thickness has been calculated as 650 25 m (Borradaile, 1973). Elsewhere in the Loch Awe area it attains a 26 thickness of 1100 m. The formation typically comprises chloritic 27 and feldspathic gritty metasandstones, which show graded bedding, 28 together with green and green-grey metamudstones (Borradaile, 29 30 1973). The metasandstones are laterally discontinuous on a large 31 scale and exhibit evidence of channelling. They contain angular 32 grains of K-feldspar, some plagioclase, quartz, detrital epidote 33 and opaque minerals in a chloritic matrix. Black slaty 34 metamudstones, black calcareous metamudstones and detrital volcanic 35 material occur as lenses within the metasandstones. Good exposures 36 of typical lithologies can be observed near the burn junction at NM 37 954 165, north of An Cnap. Slightly calcareous black metamudstones 38 occur at the top of the formation and can be observed by the 39 roadside at the east end of Loch Avich in the southern part of the 40 GCR site (NM 951 155). These rocks display NW-dipping bedding as 41 well as slaty and crenulation cleavages. 42 The overlying Loch Avich Lavas Formation is represented only by a 43 small outcrop preserved in the core of the Kilchrenan Syncline, on 44 the hill of An Cnap, north of Loch Avich, at NM 954 158 (Figure 45 45). The formation consists of an observed thickness of 300–500 m 46 of greenschist-facies pillow lavas with no significant sedimentary 47 48 intercalations (Borradaile, 1973). Recent geochemical studies have 49 indicated that the Loch Avich lavas are tholeiitic and have 50 basaltic andesite to andesitic compositions (Pickett et al., 2006; 51 Fettes et al., 2011). The south-western end of the lava outcrop 52 terminates within the synclinal fold closure, where the lavas 53 directly succeed black calcareous metamudstones at the top of the 54 Loch Avich Grit Formation. 55 The following description of the lavas is based on field 56 observations made by Pickett (1997) and Hyslop and Pickett (1999). 57 Well-preserved pillow lavas are present in the central part of the 58 outcrop, whereas more homogeneous lavas are exposed along the 59 south-eastern margin. The pillows are 1–1.5 m wide and are 60 separated by 5 cm-thick epidotic rims of formerly glassy pillow 61 62 63 64 65 1 2 3 4 rinds. The asymmetrical pillow shapes indicate that they are the 5 right way up (Figure 46). The pillow cores are generally 6 porphyritic, whilst the margins are fine grained and display flow 7 structures. Mineralized and chloritized vesicles are common. 8 Several pillows display concentric rims of vesicles, which have 9 been infilled by quartz in places, producing a ‗gritty‘ appearance. 10 The lavas are generally fine grained and are altered to a pale buff 11 12 to light greenish grey colour. Many display a foliation, especially 13 away from the central part of the outcrop. Small pockets of green 14 slaty metamudstone are intercalated with the pillows locally. 15 The structure of the area is dominated by the Kilchrenan Syncline 16 whose axial trace trends north-east–south-west through the southern 17 part of the GCR site, across the hill of An Cnap (Borradaile, 18 1973). The north-west limb of the syncline, on which most of the 19 site lies, is overturned, with bedding dipping at c. 40–75° to the 20 north-west. On this limb, the south-west-striking S1 slaty 21 cleavage dips at c. 40–50° to the north-west. 22 23 20.3 Interpretation 24 25 The Loch Avich Grit Formation was interpreted by Borradaile (1973) 26 as a sequence of turbiditic deposits that, together with the 27 overlying Loch Avich Lavas Formation, was generated in a subsiding 28 deep marine basin during rifting of a continental margin during 29 30 late-Dalradian times (Anderton, 1985). The Loch Avich lavas are 31 interpreted as representing the final phase of a period of 32 extensive volcanism (that also generated the Tayvallich Volcanic 33 Formation; see the West Tayvallich Peninsula GCR site report), 34 which accompanied the lithospheric stretching and rifting. They 35 constitute the youngest expression of volcanism in the Dalradian 36 and are generally regarded as a source of the volcaniclastic 37 detritus in the green beds that occur at a comparable 38 stratigraphical level throughout much of the Southern Highland 39 Group outcrop (Pickett et al., 2006). 40 Borradaile (1973) suggested that the Loch Avich lavas were fed by 41 some of the sills that occur below the lavas of the Tayvallich 42 Volcanic Formation, although there is no direct field evidence to 43 corroborate this. He interpreted the Loch Avich lavas as being 44 comagmatic with the Tayvallich volcanic rocks and its sill-feeders, 45 the Loch Avich lavas representing a new phase of submarine 46 volcanism. However, the geochemistry of the Loch Avich lavas does 47 48 exhibit some suble differences from that of the underlying 49 Tayvallich volcanic rocks (Hyslop and Pickett, 1999; Pickett et 50 al., 2006). Both show evidence of mixing of enriched and depleted 51 components of their mid-ocean-ridge type mantle source and the Loch 52 Avich Lavas in particular show signs of crustal contamination 53 (Fettes et al., 2011). 54 55 20.4 Conclusions 56 57 The Loch Avich GCR site is the type locality for the Loch Avich 58 Lavas Formation, the only known extrusive volcanic rocks in the 59 Southern Highland Group and the highest in the Dalradian 60 succession. The complete section extends from the top of the 61 62 63 64 65 1 2 3 4 Tayvallich Volcanic Formation, through the Loch Avich Grit 5 Formation to the base of the Loch Avich Lavas Formation. 6 The Loch Avich Grit Formation exhibits a range of metasedimentary 7 rocks, which are valuable for the interpretation of the 8 depositional environments in latest Neoproterozoic time. The Loch 9 Avich Lavas Formation is well exposed and displays good pillow 10 forms with many relict igneous features. These lavas are the 11 12 subject of current geochemical studies associated with research 13 into the origin of the widespread volcaniclastic Green Beds with 14 which they have been correlated stratigraphically. The section is 15 critical for studies of Dalradian stratigraphy and in 16 reconstructing the depositional environment and tectono-magmatic 17 setting of the uppermost part of the Dalradian Supergroup. 18 19 21 BUN-AN-UILT, ISLAY 20 (NR 295 692–NR 29 694) 21 22 C.A. Bendall 23 24 25 21.1 Introduction 26 27 The rocks of Islay may be separated into four major divisions, 28 namely the Rhinns Complex, Colonsay Group, Bowmore Sandstone Group, 29 30 and Dalradian Supergroup. The boundaries between these units are 31 tectonic and poorly exposed, and partly because of this, 32 correlation of the Rhinns Complex, the Colonsay Group and the 33 Bowmore Sandstone Group with rocks elsewhere in Scotland has proved 34 equivocal. Of interest here is the Bowmore Sandstone Group and its 35 relationship to the Dalradian rocks. The boundary between these 36 two units is a tectonic break known as the Loch Skerrols Thrust. 37 This structure was first recognized and described by Wilkinson 38 (1907) who noted the existence of mylonitic rocks at the junction 39 between the two units. Bailey (1917) alluded to the presence of a 40 tectonic break on stratigraphical grounds, because the Bowmore 41 Sandstone rocks appear to lie within the western limb of the Islay 42 Anticline and are not repeated on the eastern limb. 43 For much of its length, the thrust is not exposed and its presence 44 is inferred from a change in lithology and by the local strain 45 increase apparent in nearby exposures. The only locality where the 46 thrust is actually exposed is at Bun-an-uillt on the east side of 47 48 Loch Gruinart. Here the thrust is manifested by a shear-zone with 49 intensely foliated rocks (mylonites) that separates the Jura 50 Quartzite of the Dalradian Argyll Group to the south from the 51 Bowmore Sandstone to the north. 52 The interpretation and significance of the Loch Skerrols Thrust is 53 by no means resolved and, consequently, nor is the stratigraphical 54 status of the Bowmore Sandstone. The latter, which shows no 55 significant signs of metamorphism, has been variously correlated 56 with the Torridonian (Wilkinson, 1907; Green, 1924; Peach and 57 Horne, 1930; Stewart, 1969), the Moine (Roberts, 1974), or the 58 Dalradian Crinan Grit Formation (Fitches and Maltman, 1984). 59 60 61 62 63 64 65 1 2 3 4 21.2 Description 5 6 Whereas the stratigraphy and structure of the Dalradian rocks of 7 Islay are reasonably well understood, the same cannot be said for 8 the Bowmore Sandstone Group. It is generally poorly exposed, but 9 is assumed to be bounded by the Loch Skerrols Thrust to the east 10 and the Loch Gruinart Fault to the west. It has been described by 11 12 Stewart (1969) and Fitches and Maltman (1984) as consisting of 13 monotonous grey-brown feldspathic sandstones, with indistinct 14 bedding and poorly developed tectonic fabrics. The group was 15 divided into two by Amos (1960): the Laggan Sandstone Formation 16 overlain by the Blackrock Grit Formation. The Laggan Sandstone 17 Formation consists of fine- to medium-grained sandstones with thin 18 siltstones and mudstones, and the Blackrock Grit Formation consists 19 mainly of coarse-grained sandstones with pebbly bands. 20 The area around Loch Gruinart is low lying and exposure inland is 21 very poor. Exposure of the thrust is limited to a thin strip along 22 the eastern shoreline of the loch (NR 2950 6930, Figure 47), where 23 it separates the Jura Quartzite Formation from the Blackrock Grit 24 Formation. There are two outcrops of the Jura Quartzite along the 25 shoreline. This is due to the shallow dip of the thrust (see 26 below) and later faulting. South of the thrust at NR 2947 6919, 27 the Jura Quartzite rocks are white, medium-grained quartzites, 28 which have a somewhat shattered appearance. Bedding is difficult 29 30 to identify and the planar surfaces in the rock are most likely to 31 be tectonic foliation surfaces, as they contain a stretching 32 lineation. The Loch Skerrols Thrust is not a simple planar 33 boundary but a zone of intensely deformed and recrystallized rocks 34 (Figure 48), which Fitches and Maltman (1984) referred to as the 35 Loch Skerrols Shear-zone. The intensity of the foliation increases 36 in the Jura Quartzite as the thrust (shear-zone) is approached, as 37 indicated by its closer spacing. The foliation surfaces generally 38 have shallow dips and a well-developed stretching lineation, which 39 generally plunges towards 110° but with a variable amount of plunge 40 due to later folding. The later folds are upright open structures 41 that have hinges trending approximately east–west. However, these 42 are only minor folds with wavelengths of a few tens of centimetres. 43 Immediately north of the shear-zone are greenish weathering impure 44 feldspathic sandstones, which belong to the Blackrock Grit 45 Formation. They also have a fabric, but this fabric becomes less 46 intense northwards away from the shear-zone. The foliation 47 48 surfaces are more micaceous than those in the Jura Quartzite and 49 the rocks have an upright crenulation cleavage striking about 200°. 50 Unlike the quartzites they do not show an obvious stretching 51 lineation. The exposure ends a few tens of metres north of the 52 shear-zone. About a hundred metres farther north, just west of 53 Bun-an-uillt house, there is prominent headland that consists of 54 shattered white quartzite. This appears to be Dalradian and was 55 interpreted by Fitches and Maltman (1984) as lying adjacent to a 56 later fault, not the Loch Skerrols Thrust. The Loch Skerrols 57 Thrust is cut by several later faults such as this one, and these 58 faults, along with later folding, probably explain the outcrop 59 pattern of the thrust at the GCR site. 60 61 62 63 64 65 1 2 3 4 21.3 Interpretation 5 6 As emphasised by Fitches and Maltman (1984), the interpretation of 7 the Loch Skerrols Thrust is vital to any consideration of the 8 stratigraphical affinity of the Bowmore Sandstone. Early workers 9 (see introduction) had considered that the Bowmore Sandstone could 10 be correlated with either the Torridonian or the Moine rocks of the 11 12 Northern Highlands, which presumably would imply that the Loch 13 Skerrols Thrust is a structure analogous to the Moine Thrust. 14 Fitches and Maltman concluded, however, that the thrust is a re- 15 activated normal fault that developed into a shear-zone during the 16 same deformation event that produced the Islay Anticline. Because 17 the strain associated with the thrust diminishes considerably 18 towards Tallant (NR 330 590), some 11 km south-south-east of this 19 GCR site, and Laggan Bay, they also concluded that there might be 20 stratigraphical continuity between Dalradian rocks on the north- 21 western limb of the Islay Anticline, and the Bowmore Sandstone. 22 The Bowmore Sandstone was thought to be ‘laterally equivalent to the 23 Crinan Grits’, being the shallow-water correlative of the deeper 24 water grits. 25 A provenance study of clasts in the Bowmore Sandstone has revealed 26 various gneisses and pegmatites of Lewisian type (Saha, 1985). In 27 particular, distinctive blue quartz that is characteristic of 28 Scourian granulite-facies gneisses is present, as it is in the 29 30 Colonsay Group and the Dalradian rocks of north-east Islay. The 31 Blackrock Grit Formation also contains pebbles of chert, jasper and 32 ferruginous sandstone indicative of non-metamorphic or low-grade 33 sedimentary rocks (e.g. Torridonian) in the source area. 34 Saha (1989) also studied the variation in strain associated with 35 the Loch Skerrols Thrust, and concluded that the thrust is a break- 36 thrust that developed on the overturned north-western limb of the 37 Islay Anticline. He drew no conclusions as to the stratigraphical 38 affinity of the Bowmore Sandstone. However, it is implicit in his 39 structural model that it must be Dalradian and it would be 40 reasonable to reach the same conclusion as Fitches and Maltman 41 (1984), namely that it correlates with the Crinan Grit Formation. 42 In this case the older Jura Quartzite would have been thrust over 43 the younger Bowmore Sandstone (=Crinan Grits). Finally, it should 44 be stressed that there is as yet no consensus as to the precise 45 significance of the Loch Skerrols Thrust as a major tectonic 46 structure, nor to the stratigraphical affinity of the Bowmore 47 48 Sandstone, and it has even been argued that the lithologies could 49 be consistent with the Grampian Group (Stephenson and Gould, 1995). 50 51 21.4 Conclusions 52 53 The Bun-an-uilt GCR site includes the only exposure of the Loch 54 Skerrols Thrust, one of the more enigmatic structures of the 55 Scottish Caledonides, and hence is of great national importance. 56 However, due to poor exposure its tectonic significance and the 57 relationship between the rocks that lie above and below it are 58 poorly understood. The Jura Quartzite lies above the thrust, and 59 the Blackrock Grit Formation of the Bowmore Sandstone Group lies 60 below. The tectonic significance of the thrust is therefore 61 62 63 64 65 1 2 3 4 important in defining the stratigraphical status of the Bowmore 5 Sandstone Group, which has been variously correlated with the 6 Torridonian, the Moine or the Dalradian. The most recent 7 interpretations have suggested that the thrust is not a major 8 tectonic boundary (such as the Moine Thrust) and, therefore, that 9 the Bowmore Sandstone may be the lateral equivalent of rocks 10 belonging to the Upper Argyll Group of the Dalradian. 11 12 13 22 KILCHIARAN TO ARDNAVE POINT, ISLAY 14 (NR 185 587–NR 298 740) 15 16 C.A. Bendall 17 18 19 22.1 Introduction 20 21 Isolated from the rest of the South-west Grampian Highlands by the 22 Loch Gruinart Fault, rocks belonging to the Colonsay Group crop out 23 in western Islay and on the islands of Oronsay and Colonsay. On 24 Islay they form a continuous outcrop occupying the northern half of 25 the Rhinns of Islay (Figure 49). This GCR site includes an almost 26 continuous section along the west coast of the Rhinns, from 27 Kilchiaran to Ardnave Point. Also included within this site is a 28 coastal exposure of the meta-igneous basement rocks that form the 29 30 southern part of the Rhinns of Islay. Their contact with the 31 Colonsay Group is a zone of high strain. 32 The Colonsay Group was first described by Wilkinson (1907), and 33 more recently by Stewart (1969), Stewart and Hackman (1973), 34 Fitches and Maltman (1984), Bentley (1988) and Muir et al. (1995). 35 The group is 5.5 to 6 km thick. On Islay, the lower part consists 36 of a series of low-grade metasedimentary rocks, which are 37 predominantly gritty metasandstones and metamudstones. These rocks 38 have been subjected to several phases of deformation, which have 39 resulted in a series of major upright folds trending north-east– 40 south-west. The upper part of the group is exposed only on Oronsay 41 and Colonsay. Stewart and Hackman (1973) proposed a tentative 42 correlation between the upper part of the Islay succession and the 43 lower part on Oronsay, but Bentley (1988) thought it possible that 44 up to a kilometre of intervening strata is covered by sea between 45 Islay and Oronsay. 46 None of the above authors drew any firm conclusions as to the 47 48 stratigraphical correlation of the Colonsay Group with other major 49 units in the Scottish Highlands. Stewart (1969) concluded that it 50 could not be correlated reliably with any other stratigraphical 51 unit, but Stewart and Hackman (1973) tentatively suggested a 52 correlation with the Appin Group (Dalradian) of the Lochaber area. 53 Fitches and Maltman (1984) did not rule out its correlation with 54 the Dalradian, but they did have reservations on structural 55 criteria. Bentley (1988) suggested that the Colonsay Group–Appin 56 Group correlation is unlikely on geochronological grounds (see 57 below). He also ruled out a correlation with the Torridonian on 58 structural and tectonic criteria, and with the Moine Supergroup on 59 geochronological grounds, but proposed a tentative correlation with 60 the Iona Group. 61 62 63 64 65 1 2 3 4 The basement rocks of the Rhinns Complex (Muir, 1990; Muir et al., 5 1994a), are now known to be younger than was thought by early 6 workers such as Wilkinson (1907), who assumed that this series of 7 amphibolite-facies metagabbros and metasyenites were part of the 8 Lewisian Gneiss Complex. Subsequent investigations have yielded an 9 age of 1782 +/- 5 Ma (Marcantonio et al., 1988), and although this 10 correlates well with the tectonothermal reworking of the Lewisian 11 12 during the Laxfordian Event (Mendum et al., 2009), stable isotopes 13 indicate that the gneisses of the Rhinns Complex are derived 14 dominantly from juvenile mantle material, which is not known to be 15 associated with the Laxfordian Event. Consequently they are now 16 believed to be part of an extensive Palaeoproterozoic orogenic 17 province in the North Atlantic region (Dickin, 1992; Muir et al., 18 1992). They cannot therefore be correlated with any other rock 19 units in Scotland. They are too old to be Moine or metamorphosed 20 Torridonian and they are too young to be Lewisian. 21 22 22.2 Description 23 24 The basement rocks of the Rhinns Complex are not particularly well 25 exposed, and the best exposures occur on the west coast of the 26 Rhinns. They consist predominantly of metagabbros and 27 metasyenites, interpreted as an alkaline igneous complex that has 28 been subjected to multiphase deformation and amphibolite-facies 29 30 metamorphism (Muir et al., 1992). The metasyenite is well exposed 31 along the coast where it appears as a pink feldspathic foliated 32 rock, with thin schistose amphibolite sheets; good examples of 33 these may be found about 1.5 km south-west of Kilchiaran at NR 185 34 591. Towards the boundary with the overlying Colonsay Group the 35 fabric intensifies, becoming mylonitic at the boundary (the 36 Kilchiaran Shear-zone or Bruichladdich Slide). The position of the 37 boundary is easily identified by the distinct change in lithology 38 from the sheared feldspathic gneiss to a gritty metasandstone (the 39 Eilean Liath Grit), with an intense mylonitic fabric. For the most 40 part, the actual contact between the two lithologies is not exposed 41 but lies within a deeply eroded gully. However, at NR 1878 5933, 1 42 km south-west of Kilchiaran, the contact is exposed at low tide 43 (Figure 50). At this locality the shear-zone strikes at c. 040°, 44 but the strike varies inland due to folding (see Figure 49). 45 The Colonsay Group on Islay was mapped by Stewart and Hackman 46 (1973), who identified ten formations that are essentially a series 47 48 of gritty metasandstones, metamudstones and metagreywackes (Table 49 1). They described the Eilean Liath Grit, the Rubha Gàidhealach 50 Grit and the Crosprig Grit as coarse feldspathic sandstones. 51 Cross-bedding was identified in both the Rubha Gàidhealach Grit and 52 the Crosprig Grit, and conglomeratic facies with pebbles up to 4 cm 53 in size were recognized in the Crosprig Grit. 54 There is good exposure of the Eilean Liath Grit along the coast 55 south-west of Kilchiaran and the overlying Kilchiaran Phyllite is 56 well exposed at Kilchiaran Bay (NR 201 599). The phyllite is a 57 mudstone with thin silty bands, which has been deformed and 58 metamorphosed and now has a strong slaty cleavage. A thin bed of 59 possible volcaniclastic origin has been identified near the top of 60 the formation by Batchelor (2011). The overlying formation is the 61 62 63 64 65 1 2 3 4 Rubha Gàidhealach Grit, which is exposed on the headland at NR 198 5 601. The overlying Rubha na h-Àirde Mòire Phyllite is fine grained 6 at the base where the cleaved metamudstone has been quarried for 7 slate; however, it grades upwards into flaggy fine-grained 8 metasandstones. These are exposed around the headland of Rubha na 9 h-Áirde Mòire (Figure 49) where they occupy the core of a large, 10 kilometre-scale syncline. Above the Rubha na h- irde M ire 11 À ò 12 Phyllite are the Cosprig Grit and the Kilchoman Phyllite. The 13 Kilchoman Phyllite consists of grey metamudstones with calcareous 14 bands; graded beds have been observed in this unit (Stewart and 15 Hackman, 1973). Muir et al. (1995) re-examined these rocks and 16 concluded that the Crosprig Grit and the Kilchoman Phyllite are not 17 separate formations but are simply the Rubha Gàidhealach Grit and 18 the Kilchiaran Phyllite repeated by upright folding (Table 1). 19 They did not, however, re-examine the upper four formations; these 20 were described by Stewart and Hackman only. The Smaull Greywacke 21 consists of a sequence of graded metagreywacke units, some of which 22 are be quite coarse grained, with grain sizes of up to 2–3 mm. This 23 grades upwards into the Sanaigmore Phyllite, which is a dark-grey 24 metamudstone with silty calcareous bands. Above this, is the 25 Sanaig Greywacke, which consists of graded metagreywacke. The 26 youngest Colonsay Group formation on Islay is the Ardnave 27 Formation, which consists of a thick sequence of metamudstones and 28 fine-grained laminated metagreywackes. 29 30 The structure of the Colonsay Group was described in detail by 31 Fitches and Maltman (1984). They identified four stages of 32 deformation on both Islay and Colonsay, which they numbered D1 to 33 D4; these numbers relate to the Colonsay Group only. The first 34 phase, D1, is a grain-alignment fabric, which is sub-parallel to 35 bedding. D2 is expressed by a strong stretching lineation 36 developed in metasandstones, and by close to isoclinal, recumbent 37 to reclined minor sheath folds. These folds have axial planar 38 crenulation and pressure-solution cleavages. D3 is the phase of 39 deformation that produced most of the mapped folds. These are 40 fairly upright, gently NE-plunging folds with wavelengths up to 41 several hundred metres, and axial planar crenulation cleavages. D4 42 is represented by chevron folds and kink bands, and associated 43 crenulation cleavages. The axial planes to D4 folds are upright 44 and strike east–west. 45 On Islay the dominant phase of deformation is D3; all the folds 46 shown on Figure 49 are F3 folds. Minor structures of more than one 47 48 generation can be observed in most exposures of phyllitic rock, for 49 example, two crenulation cleavages, one steep and the other flat 50 lying, are present in the Kilchiaran Phyllite near Kilchiaran. 51 Minor folds are ubiquitous in the coarser grained horizons. 52 Two thin lamprophyre dykes intrude the Kilchiaran Phyllite at 53 Kilchiaran Bay; these have a tectonic fabric and consequently they 54 were intruded at least prior to some of the deformation. 55 Associated with this deformation is low-grade metamorphism. 56 Chlorite, white mica, and rare biotite have been identified in 57 Colonsay Group rocks (Fitches and Maltman, 1984), indicating that 58 the rocks were subjected to greenschist-facies metamorphism. 59 60 61 62 63 64 65 1 2 3 4 22.3 Interpretation 5 6 It is now generally accepted that the basement to the Colonsay 7 Group, namely the Rhinns Complex, is not Lewisian but is 8 Palaeoproterozoic. There are no rocks of comparable age in 9 Scotland, but rocks similar in age and lithology are found on 10 Inishtrahull off the north coast of Ireland (Muir et al., 1992). 11 12 It is possible that the Rhinns Complex is a unique terrane in the 13 British Isles, and may possibly be linked to the Ketilidian Belt of 14 south Greenland (Stone et al., 1999). Alternatively, it is 15 possible that it forms the basement to the Dalradian Supergroup 16 over much of its outcrop. Much, therefore, depends on the tectonic 17 significance of the Loch Gruinart Fault, which is not exposed. 18 Although the nature of the contact between the Rhinns Complex and 19 the Colonsay Group is tectonic (the Kilchiaran Shear-zone or 20 Bruichladdich Slide), the Colonsay Group almost certainly forms the 21 cover to the Rhinns Complex basement. The contact shear-zone was 22 interpreted as a sheared unconformity by Wilkinson (1907) and 23 subsequently by Bentley (1988), on account of the presence of a 24 ‗basal conglomerate‘ close to the boundary. However, Stewart and 25 Hackman (1973) disputed this, as they did not detect any facies 26 changes close to the shear-zone, and observed that the five lowest 27 units of the Colonsay Group are truncated against the basement. 28 They therefore interpreted the contact as a zone of high strain and 29 30 a tectonic break, which they referred to as the Bruichladdich 31 Slide. Muir et al. (1995) suggested that the contact is somewhat 32 more complex than the term ‗slide‘ implies, involving tectonic 33 interleaving of Colonsay Group and basement rocks, and renamed it 34 the Kilchiaran Shear-zone. The Kilchiaran Shear-zone/Bruichladdich 35 Slide is folded around F3 folds and hence is either a D1 or D2 36 structure. 37 The lowest 800 m of the Colonsay Group succession on Islay has 38 been interpreted as representing delta-top sheet sands and 39 interdistributary muds, whereas the upper part suggests deeper 40 water, delta-slope turbidites. The sediments were derived from the 41 west, from a hinterland of deformed high-grade gneisses with a 42 sedimentary cover (Stewart and Hackman, 1973). Saha (1985) pointed 43 out that the source area could not have been very distant, as the 44 feldspar clasts are angular and fresh. The presence of clasts of 45 blue quartz has been taken to suggest granulite-facies rocks, 46 although the local Rhinns Complex basement is neither granulite 47 48 facies nor contains blue quartz; a more likely source would be the 49 Lewisian Gneiss Complex of the Hebridean Terrane. 50 Correlation of the Colonsay Group with other Highland rocks would 51 greatly aid interpretation of the significance of the Loch Gruinart 52 Fault, and the relationship of this west Islay terrane with other 53 Highland terranes. For example, if the Colonsay Group should prove 54 to be an integral part of the Dalradian Supergroup, then the Loch 55 Gruinart Fault would not be a terrane boundary, and the Rhinns 56 Complex could form the basement to the Dalradian elsewhere. 57 However, the earlier, D1 and D2 structures in the Colonsay Group, 58 which are also present in the Rhinns Complex, have no obvious 59 counterparts in the Dalradian of eastern Islay or in the South-west 60 Grampian Highlands; it is the later D3 and D4 structures that can 61 62 63 64 65 1 2 3 4 be correlated most readily with those of the Islay Anticline 5 (Fitches and Maltman, 1984; Bentley, 1988). Hence, the Colonsay 6 Group does not appear to have the same tectonic history as the 7 Dalradian. This would imply that the west Islay terrane is 8 unrelated to other terranes in Scotland. Indeed Rogers et al. 9 (1989) suggested that it was the docking of this terrane with the 10 Grampian Terrane that initiated the tectonic activity in the 11 12 Dalradian rocks that is generally referred to as the Grampian 13 Event. 14 Establishing an age for the Colonsay Group has proved elusive. 15 Bentley (1988) dated some appinitic intrusions on Colonsay, which 16 he interpreted as post-dating the early deformation and pre-dating 17 the late deformation. From 40Ar/39Ar stepwise heating methods on 18 hornblende, he suggested that the best estimate of the age of these 19 intrusions is c. 600 Ma. This implied that the Colonsay Group is 20 older than 600 Ma, as is some of the deformation that has affected 21 it. However, this date is now believed to be a result of excess 22 argon, and a U-Pb date of 439 ± 9 Ma, derived from zircons in one of 23 the intrusions by ion-microprobe (SHRIMP) is currently accepted as 24 the crystallization age (Muir et al., 1997). Furthermore, the 25 intrusion is now considered to have been emplaced after all main 26 phases of deformation and hence the Colonsay Group and its 27 deformation pre-date c. 440 Ma. This does strengthen the case for 28 correlating the Colonsay Group with part of the Dalradian, although 29 30 the difficulties in matching their detailed structural histories 31 remains a problem. 32 The Colonsay Group is currently interpreted as a sequence of low- 33 grade metasedimentary rocks that were deposited sometime in the 34 Neoproterozoic. These rocks were laid unconformably on a 35 Palaeoproterozoic basement in an intracratonic basin setting, and 36 were possibly deformed during a Neoproterozoic orogenic event(s) 37 that is not recognized in Dalradian rocks. However, recent 38 research suggests that tectonism could have affected at least part 39 of the Dalradian prior to the Grampian Event (Tanner and Bluck, 40 1999). Hence it may yet be shown that the Colonsay Group does 41 indeed have structural and stratigraphical affinities with the 42 Dalradian rocks of the central Grampian Highlands and this 43 interpretation has been strengthened by U-Pb ages of detrital 44 zircons, which are comparable with the Grampian Group (McAteer et 45 al., 2010). 46 47 48 22.4 Conclusions 49 50 The continuous section from Kilchiaran to Ardnave Point, on the 51 north-western coast of the Rhinns of Islay, provides excellent 52 exposures of the lower part of the Colonsay Group, arguably the 53 most enigmatic sequence of metasedimentary rocks in the Grampian 54 Terrane. Its stratigraphical affinities are uncertain; the most 55 obvious lithological correlations are with the Grampian Group and 56 lowest Appin Group of the Dalradian. However, rocks of the 57 Colonsay Group record four distinct phases of deformation, 58 including an early event that cannot be recognized in undoubted 59 Dalradian rocks nearby, suggesting that they might belong to a 60 61 62 63 64 65 1 2 3 4 separate west Islay terrane, having a different tectonic history to 5 the main Dalradian outcrop. 6 The contact of the group with meta-igneous basement rocks of 7 Palaeoproterozoic age is also exposed at this GCR site. The 8 contact is sheared and mylonitic in places and the original 9 relationship between the Colonsay Group and the Rhinns Complex, 10 which is like no other basement in Scotland, has been a matter of 11 12 some debate. 13 The age of the Colonsay Group and its relationship to its 14 Palaeoproterozoic basement have profound implications for the 15 identification of the diverse geological terranes that came 16 together during the Caledonian Orogeny to form the Scottish 17 Highlands. Hence, they might provide a vital link in understanding 18 the complex inter-relationships between the Neoproterozoic rocks of 19 North America, Greenland and north-west Europe, and are potentially 20 of international importance. 21 22 23 ACKNOWLEDGEMENTS 24 25 This regional paper is the combined work of 6 authors, many of 26 whom, in addition to their own site descriptions, have made 27 valuable comments on other aspects of the work. Sadly, John 28 Roberts died during the early stages of preparation and his 29 30 contributions have been completed by P.W.G. Tanner. John made 31 major contributions to his own areas of the Dalradian outcrop and 32 to our overall understanding of Grampian Highland geology. 33 The paper has been compiled by P.W.G. Tanner and has been edited 34 by D. Stephenson. The GCR editor was P.H. Banham and the referee 35 was M.R.W. Johnson, who also provided valuable editorial 36 suggestions. The project was cofunded by the Joint Nature 37 Conservation Committee (JNCC) and the British Geological Survey 38 (BGS) and has been managed by N.V. Ellis for JNCC and D.J. Fettes 39 and M. Smith for BGS. 40 The initial site selection and site documentation for the 41 Dalradian block of the Geological Conservation Review was by S.J. 42 Moorhouse. Since then, much new mapping and refined interpretation 43 has taken place, and the site list has been revised, firstly by 44 J.E. Treagus and subsequently through a panel consisting of R. 45 Anderton, A.L. Harris, J.R. Mendum, J.L. Roberts, P.W.G. Tanner, R. 46 Threadgould and J.E. Treagus. The necessary amendments to the GCR 47 48 documentation were greatly facilitated by R. Wignall (for Scottish 49 Natural Heritage). 50 K.M. Goodenough and S.W. Horsburgh assisted with preliminary 51 drafts of key maps and all diagrams were drafted for publication by 52 S.C. White (JS Publications, Newmarket) and K.J. Henderson (BGS, 53 Edinburgh). Photographs were scanned and prepared by B.M. McIntyre 54 (BGS, Edinburgh). Photographs from the BGS collection are 55 reproduced by kind permission of the Director, BGS © Natural 56 Environment Research Council; all rights reserved (PR/23–27). 57 The first complete draft of the Dalradian GCR was submitted to the 58 JNCC in June 2009. In 2010, the JNCC terminated its involvement in 59 Earth Science conservation and abandoned its contractual agreements 60 to publish the remaining GCR volumes. So, the authors are greatly 61 62 63 64 65 1 2 3 4 indebted to Diarmad Campbell, Chief Geologist Scotland for the BGS, 5 for funding the drafting of remaining figures and to the 6 Geologists‘ Association and Elsevier, for ensuring that this volume 7 is published as a Special Issue of their Proceedings. We are 8 particularly grateful to Neil Ellis of the JNCC for his efforts to 9 secure a new publisher and to Professor James Rose, Editor in Chief 10 of the PGA, for making it all happen. 11 12 Finally, on behalf of all of the site authors, we would like to 13 record our thanks to the owners and managers of land and quarries 14 who have allowed access to the sites, either during previous work 15 or specifically for the GCR exercise. 16 17 REFERENCES 18 19 Agrell, S. O. (1942) 1. A petrological investigation of the 20 adinoles at Dinas Head, Cornwall. 2. A petrofabric study of the 21 Ben Vuroch granite and the adjacent schists in Perthshire. 22 Unpublished PhD thesis, University of Cambridge. 23 Ahmed-Said Y. and Tanner P. W. G. (2000) P-T conditions during 24 emplacement, and D2 regional metamorphism, of the Ben Vuirich 25 Granite, Perthshire, Scotland. Mineralogical Magazine, 64, 737–53. 26 Ague, J.J. & Baxter, E.F., 2007. 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Mineralogical Magazine, 68, 769-786. 56 57 58 Figure 1 Map of the South-west Grampian Highlands showing 59 subgroups of the Dalradian Supergroup, the axial plane traces of 60 61 62 63 64 65 1 2 3 4 major folds, the line of section A–B on Figure 3 and the locations 5 of the GCR sites included in this paper. Only areas described in 6 this regional paper are ornamented. 7 GCR sites: 1 Garvellach Isles, 2 Caol Isla, Islay, 3 Rubha 8 a’Mhail, Islay, 4 Kilnaughton Bay, Islay, 5 Lussa Bay, Jura, 6 9 Kinuachdrach, Jura, 7 Surnaig Farm, Islay, 8 Ardbeg, Islay, 9 10 Ardilistry Bay, Islay, 10 Black Mill Bay, Luing, 11 Craignish 11 Point, 12 Fearnach Bay, 13 Kilmory Bay, 14 Port Cill Maluaig, 15 12 Strone Point, 16 Kilchrenan burn and shore, 17 West Tayvallich 13 peninsula, 18 South Bay, Barmore Island, 19 Loch Avich, 20 14 Bun-an-Uillt, Islay, 21 Kilchiaran to Ardnave Point, Islay. 15 Abbreviations: AA Ardrishaig Anticline, BF Bolsa Fault, IA 16 Islay Anticline, KBS Kilmory Bay Syncline, KSZ Kilchiaran Shear- 17 zone, LAS Loch Awe Syncline, LGF Loch Gruinart Fault, LST Loch 18 Skerrols Thrust, PBF Pass of Brander Fault, TF Tyndrum Fault, TS 19 Tayvallich Syncline. 20 21 Figure 2 Stratigraphical columns (not to scale) showing lateral 22 correlations between members and formations of the Dalradian 23 Supergroup in the South-west Grampian Highlands. A the islands of 24 Islay, Jura and the Garvellachs, B the Loch Awe Syncline, C the 25 Ardrishaig Anticline, core and south-east limb, D and E rocks of 26 uncertain affinity on Islay and Colonsay, and those forming the 27 basement to the Dalradian Supergroup. 28 GB Great Breccia, DB Disrupted Beds. 29 30 Figure 3 True-scale cross-section showing the location of GCR 31 sites included in this paper (numbered as on Figure 1), the 32 correlation between stratigraphical sequences, and major 33 structures, across the South-west Grampian Highlands. Line of 34 section A–B is shown on Figure 1. (Cross-section: P.W.G. Tanner.) 35 36 Figure 4 Facsimile copy of part of the geological map of Islay 37 published by E.B. Bailey (1917). 38 39 Figure 5 Map of the Garvellach Isles, Firth of Lorn, after 40 Spencer, (1971). 41 42 Figure 6 Tillite bed containing large exotic clasts of pink 43 granite, from the north end of Garbh Eileach, Garvellach Isles. 44 Hammer shaft is 47 cm long. (Photo: P.W.G. Tanner.) 45 46 Figure 7 Map of the shore section north of Caol Isla distillery, 47 Isle of Islay. Adapted from Fairchild, (1991). 48 49 Figure 8 50 (a) Casts of sand-filled dewatering cracks seen in relief on the 51 base of a metasandstone bed from Leac Thiolastaraidh, north of Caol 52 Isla, Isle of Islay (NR 4297 7037). 53 (b) Negative print taken from an acetate peel of a typical 54 dewatering crack from Leac Thiolastaraidh. The illustration shows 55 a sandstone bed (s) overlying a layer of mudrock (m), both units 56 having been cut by the upward injection of a sand-filled dewatering 57 crack. Following exposure to erosion at the Earth’s surface, the 58 mudrock layer lying beneath the sandstone might be eroded away, 59 leaving the sandstone infill (c) as a cast protruding from the base 60 of the bed, as in Figure 8a. 61 62 63 64 65 1 2 3 4 (Photos: P.W.G. Tanner.) 5 6 Figure 9 Map of the coastal section at the Rubha a’Mhail, Isle 7 of Islay, after Fairchild, (1991). The outline map (a) shows the 8 locations of detailed sections (b) and (c). 9 10 Figure 10 Stromatolite bodies from Member 3 of the Bonahaven 11 Dolomite Formation, north-east of Port a’Chotain, Rubha a’Mhail GCR 12 site, Isle of Islay. 13 (a) Domal stromatolite bioherms at Bagh an da Dhorius, NR 410 14 788. Hammer shaft (arrowed) is 47cm long. 15 (b) Large bioherm at NR 407 789. Hammer shaft is 47 cm long. 16 (Photos: P.W.G. Tanner.) 17 18 Figure 11 Map of the area around the Kilnaughton Bay, Islay GCR 19 site, south-east Islay. 20 21 Figure 12 Stratigraphical log of the pebble beds in the upper 22 part of the Scarba Conglomerate Formation from NR 3479 4440–3492 23 4464, on the north side of Carraig Fhada, Kilnaughton Bay, Isle of 24 Islay. 25 26 Figure 13 Rosettes and blades of kyanite, largely pseudomorphed 27 by kaolinite and pyrophyllite (Burgess et al., 1981), lying on a 28 bedding plane at a low angle to the S1 cleavage in the Jura 29 Quartzite at Kilnaughton Bay, Isle of Islay. Scale is in cm/mm. 30 (Photo: P.W.G. Tanner.) 31 32 Figure 14 Map of the area around the Lussa Bay GCR site, Isle of 33 Jura. 34 35 Figure 15 A representative, 55 cm-thick, pebbly unit from the 36 Scarba Conglomerate Formation at Lussa Bay, Isle of Jura, showing 37 poorly developed graded bedding with rip-up clasts of metamudstone. 38 Beds young to the right of the photo (south-east). Spirit level 39 (top left) is 5 cm long. (Photo: P.W.G. Tanner.) 40 41 Figure 16 Map of the area around the Kinuachdrachd GCR site, 42 Isle of Jura (after BGS 1:50 000 Sheet 36, Kilmartin, 2003 and 43 Anderton, 1977). 44 45 Figure 17 Refracted S1 cleavage cutting near-horizontal bedding 46 close to the hinge-zone of a mesoscopic F1 fold, in metasandstone 47 beds, viewed to the north-east, Kinuachdrachd, Isle of Jura. 48 Spirit level is 5 cm long. (Photo: P.W.G. Tanner.) 49 50 Figure 18 Map of the area around the Surnaig Farm GCR site, south- 51 east Islay, showing the ‗sandstone dykes‘ locality. 52 53 Figure 19 A folded sedimentary dyke (viewed to the west), within the 54 Laphroaig Quartzite Formation, 300 m south-west of Surnaig Farm, south- 55 east Islay (NR 3982 4525). The dyke is between 40 cm and 50 cm wide and 56 the spirit level (centre) is 5 cm long. (Photo: P.W.G. Tanner.) 57 58 Figure 20 Map of the area around the Ardbeg GCR site, south-east 59 Islay, showing the stilpnomelane locality. 60 61 62 63 64 65 1 2 3 4 Figure 21 Photomicrograph (in plane-polarized light) of the 5 stilpnomelane-bearing metamafic sill on the foreshore 400 m east of 6 Ardbeg Distillery at NR 4185 4625. Radiating clusters of 7 stilpnomelane can be seen overgrowing actinolite, albite, quartz, 8 and epidote. The field of view is 6 mm. (Photo: A. Condron.) 9 10 Figure 22 Map of the area around the Ardilistry Bay GCR site, south- 11 east Islay. 12 13 Figure 23 Topographic expression of sills of metamafic rock at 14 Ardilistry Bay, south-east Islay, viewed looking south-east from 15 Locality X on Figure 22. (Photo: P.W.G. Tanner.) 16 17 Figure 24 Schematic vertical section through the metapyroxenite- 18 bearing sill at Ardilistry Bay, south-east Islay. 19 20 Figure 25 Map of the area around the Black Mill Bay GCR site. 21 A–D, localities mentioned in the text. Inset: An equal-area 22 stereographic projection of poles to bedding, slaty cleavage (S1), 23 and crenulation cleavage (S2), together with the best-fit line (π- 24 girdle) containing the poles to bedding and cleavage. The nearly 25 horizontal π-axis, gives the mean orientation of the related major 26 fold axis. 27 28 Figure 26 A gravity-flow deposit, or debrite, at locality A on 29 Figure 25 (NM 7311 0875), Black Mill Bay GCR site. The scale is 5 30 cm long. 31 (a) A near-vertical face approximately parallel to S1, showing 32 deformed clasts of sandstone in a silty matrix. 33 (b) A horizontal, plan-view section through the deformed clasts 34 shown in (a). 35 (c) An illustration of lithological control on cleavage 36 development at locality A. The near-vertical S1 cleavage dominates 37 in metacarbonate rock (M) and in the debrite (D), whereas the S2 38 crenulation cleavage is more strongly developed in the intervening 39 black pelite (P) and gives rise to a near-horizontal parting in 40 this lithology. A sedimentary dyke (S), now folded and cleaved, 41 cuts the debrite and the pelite. 42 (Photos: P.W.G. Tanner.) 43 44 Figure 27 Map showing the outline geology of the Craignish Point 45 GCR site. 46 47 Figure 28 48 (a) Weathered-out cavities in metasandstone, previously occupied 49 by calcite pseudomorphs of bow-tie gypsum, Craignish Phyllite 50 Formation, Craignish Point. The bow-ties are 3–6 mm across. 51 (b) Pseudomorphs of butterfly-twinned gypsum lying on a bedding 52 plane in the Craignish Phyllite Formation, Craignish Point. The 53 pseudomorphs are up to 5 cm across. 54 (Photos: P.W.G. Tanner.) 55 56 Figure 29 Map of the area around the Fearnach Bay GCR site, Loch 57 Melfort. A, locality mentioned in the text. 58 Inset: Equal-area stereographic projections of poles to bedding, 59 together with minor D1 fold hinges, and slaty cleavage (S1) (see 60 text for explanation). 61 62 63 64 65 1 2 3 4 5 Figure 30 6 (a) Cross-lamination at the base of a massive bed of 7 metacarbonate rock in the Craignish Phyllite Formation, NM 8336 8 1406, Fearnach Bay, Loch Melfort. Scale is 5 cm long. 9 (b) The contact between a basic sill (B) and phyllitic 10 semipelite (P) folded by an upright F1 syncline at locality A (NM 11 8350 1342), Fearnach Bay, Loch Melfort (Figure 29). The fanned 12 cleavage in the phyllite contrasts with the axial planar cleavage 13 in the metadolerite sill (M). A thin band of quartzite (Q) occurs 14 at the contact. Hammer shaft is 60 cm long. 15 (c) Intense folding of sandy metacarbonate layers, and cleavage 16 refraction in the intervening metapelite, within the Craignish 17 Phyllite Formation, a short distance north of locality A (NM 8350 18 1342), Fearnach Bay, Loch Melfort (Figure 29). The bar scale is 19 5cm long. 20 (Photos: P.W.G. Tanner.) 21 22 Figure 31 Preliminary map of the area around the Kilmory Bay GCR 23 site with geological boundaries taken from Roberts (1977c). 24 25 Figure 32 Panoramic view of a train of F1 folds with fanned 26 spaced cleavages, plunging north-north-east, Crinan Grit Formation 27 (Ardnoe Member), Kilmory Bay (NR 700 743). (Photo: P.W.G. Tanner.) 28 29 Figure 33 En-echelon segmented sedimentary dyke (centre) 30 parallel to spaced axial planar cleavage, Ardrishaig Phyllite, 31 Kilmory Bay (NR 697 728). (Photo: P.W.G. Tanner.) 32 33 Figure 34 Map of Knapdale and north Kintyre (based on Roberts, 34 1977c), showing outcrops of the main Dalradian units and locations 35 of GCR sites: 1 West Tayvallich Peninsula, 2 Kilmory Bay, 3 Port 36 Cill Maluaig, 4 South Bay, Barmore Island. AA Ardrishaig 37 Anticline, KBS Kilmory Bay Syncline, TS Tayvallich Syncline. Inset 38 shows the geology of the Port Cill Maluaig GCR site (3). 39 40 Figure 35 41 (a) Minor F2 folds showing strongly curved fold-hinges, which 42 plunge in opposing directions, Port Cill Maluaig. Hammer shaft is 43 47 cm long. 44 (b) Mesoscopic and minor F2 folds from Port Mhoirich (Port Cill 45 Maluaig GCR site), viewed to the north-east. Note the control of 46 bed thickness on the wavelength of the folds. Hammer shaft is 47 47 cm long. 48 (Photos: P.W.G. Tanner.) 49 50 Figure 36 Map of the area around the Strone Point GCR site, 51 showing the position of the Ardrishaig and Strone Point anticlines, 52 linked by an inferred syncline (modified from Borradaile, 1970). 53 The inset map shows the detailed geology of the Strone Point GCR 54 site. 55 56 Figure 37 Strongly folded and cleaved layers of orange-brown 57 metacarbonate rock (C) and grey-green phyllitic metamudstones (P) 58 define rather open S-folds on the lower part of the hinge-zone of 59 the Strone Point Anticline at Strone Point (NN 1183 0884). The 60 penetrative S1 cleavage dips at a shallow angle to the north-west, 61 62 63 64 65 1 2 3 4 the direction of view being to the south-west. Solid lines 5 highlight the trace of the bedding. Hammer shaft is 47 cm long. 6 (Photo: P.W.G. Tanner.) 7 8 Figure 38 Sketch of typical curvilinear folds at Strone Point, 9 based on part of figure 19 of Voll (1960). The geometry of the 10 folds is illustrated by the equal-area stereographic projection, 11 reproduced from figure 2a of Borradaile (1970), augmented by data 12 collected by P.W.G. Tanner. 13 14 Figure 39 Map of the hinge-zone of the F1 Loch Awe Syncline at 15 Kilchrenan, showing the principal exposures of the Kilchrenan 16 Conglomerate Member of the Tayvallich Slate and Limestone Formation 17 (after Borradaile 1973, 1977). 18 19 Figure 40 The Kilchrenan Conglomerate in exposures on the bank 20 of the Kilchrenan Burn. The clasts are of quartzite in a gritty, 21 muddy matrix. Coin is 2 cm diameter. (Photo: J.E. Treagus.) 22 23 Figure 41 Simplified map of the West Tayvallich peninsula. 24 Numbers refer to sections described in the text. Modified after 25 BGS 1:50 000 Sheet 28E (Knapdale, 1996). Additional information 26 from Elles (1935), Wilson and Leake (1972) and Gower (1977). 27 28 Figure 42 Basaltic pillow lavas in the Tayvallich Volcanic 29 Formation showing concentric bands of vesicles, coast of An Aird 30 (NR 7020 8370). Hammer shaft is 37 cm long. (Photo: BGS No. P 31 219459, reproduced with the permission of the Director, British 32 Geological Survey, © NERC.) 33 34 Figure 43 Map of the coastal section between Barmore Island, 35 Loch Fyne and East Loch Tarbert; outcrops of basic meta-igneous 36 rock omitted (after Roberts, 1977c, with additional data). 37 38 Figure 44 Typical Loch Tay Limestone lithology of thinly bedded 39 metalimestone interbedded with dark grey metamudstone. A strongly 40 boudinaged quartzofeldspathic vein occupies the centre of the 41 photograph. NR 8683 7123, South Bay, Barmore Island. Hammer shaft 42 is 60 cm long. (Photo: P.W.G. Tanner.) 43 44 Figure 45 Map of the area north-east of Loch Avich that includes 45 the outcrop of the Loch Avich Lavas Formation (after Borradaile, 46 1977). 47 48 Figure 46 Basaltic andesite pillow lavas at An Cnap, Loch Avich, 49 viewed to the south-east (NM 9544 1586). Hammer shaft is 35 cm 50 long. (Photo: E.K. Hyslop, BGS No. P 726591.) 51 52 Figure 47 Map of the Loch Skerrols Thrust on the east side of 53 Loch Gruinart, Isle of Islay showing the Bun-an-Uillt GCR site 54 (after Fitches and Maltman, 1984). 55 56 Figure 48 57 (a) The main exposure of cataclastic rocks within the Blackrock 58 Grit Formation, below the Loch Skerrols Thrust, NR 2950 6930, c. 59 300 m south-west of Bun-an-Uilt, Islay. Scale is 5 cm long. 60 61 62 63 64 65 1 2 3 4 (b) Possible stretching lineation in the Jura Quartzite, above the 5 Loch Skerrols Thrust (NR 2947 6919). Scale is 5 cm long. 6 (Photos: P.W.G. Tanner.) 7 8 Figure 49 Map of north-western Islay showing outcrops of 9 the Colonsay Group and its basement of Rhinns Complex, modified 10 after Stewart and Hackman (1973) and Muir et al. (1995). 11 12 Figure 50 Topographical expression of basement–cover contact c. 13 1 km south-west of Kilchiaran Bay, north-west Islay. Gritty 14 metasandstones of the Eilean Liath Grit Formation of the Colonsay 15 Group to the right of the inlet are separated from highly sheared 16 feldspathic gneisses of the Rhinns Complex to the left by the 17 Kilchiaran Shear-zone, which controls the line of the inlet. Inset 18 shows the strongly sheared rocks in the contact zone; spirit level 19 is 5 cm long. (Photos: P.W.G. Tanner.) 20 21 22 Table 1 Stratigraphical sequences for the lower part of the 23 Colonsay Group according to Stewart and Hackman (1973) and Muir et 24 al. (1995). 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 Table 2.1

Stewart and Hackman (1973) Muir et al. (1995)

Kilchoman Phyllite (Rubha na h-Airde Móire Phyllite

-repeated through folding) Crosprig Grit (Rubha Gaidhealach Grit- repeated

through folding) Rubha na h-Airde Móire Phyllite Rubha na h-Airde Móire Phyllite Rubha Gaidhealach Grit Rubha Gaidhealach Grit Kilchiaran Phyllite Kilchiaran Phyllite Eilean Liath Grit Eilean Liath Grit

Table 2.1 Stratigraphical sequences for the lower part of the

Colonsay Group according to Stewart and Hackman (1973) and Muir

et al. (1995).

1 Figure 2.1 Loch Etive Isle of Mull Kerrera PBF Oban Dalmally

16 Ben Lui

TF 19 The Luing Garvellach 12 A Isles 1 10 15 Lunga Loch Awe Scarba

700 11 Colonsay small outcrop 6 AA of Rhinns Complex

5 Great Glen Fault 17 Cowal IA KBS BF 3 LST Loch Fyne TS

Jura LAS 13 B 2 18 Sound of Jura 14 21 20

KSZ So Bute un Is d o lay f

Gigha 6 Islay 50 IA 9 8 4 7 Port Ellen

N Arran

0 kilometres 10

150 Campbeltown 200

Old Red Sandstone Supergroup Dalradian Supergroup 7 GCR site in Chapter 2 Lorn Plateau Volcanic Blair Atholl Subgroup Formation and associated fault sedimentary rocks Appin Ballachulish Subgroup Group thrust or shear-zone Lochaber Subgroup Dalradian Supergroup axial plane trace of major anticline Southern Highland Group Bowmore Sandstone axial plane trace of Group Neoproterozoic – major syncline Tayvallich Subgroup stratigraphical Colonsay Group affinities uncertain Crinan Subgroup Argyll Rhinns Complex Palaeoproterozoic Group Easdale Subgroup Islay Subgroup (Port Askaig Tillite at base) Figure 2.2

(C) Southern (B) Highland Beinn Bheula Schist Formation Group Loch Avich Lavas Formation Green Beds Formation Loch Avich Grit Formation Glen Sluan Schist Formation Loch na Cille Boulder Bed

Tayvallich Volcanic Formation

Tayvallich Loch Tay Limestone Formation Kilchrenan Conglomerate Formation

Tayvallich Slate and Limestone Formation

Crinan Grit Formation Crinan Stonefield Schist Formation (A)

Ardmore Formation Argyll Ardnoe Member Upper Erins Quartzite Formation Group Laphroiag Quartzite Formation Shira Limestone and Slate Formation Stornoway Phyllite Formation

Port Ellen Phyllite Formation Craignish Phyllite Formation Lower Erins Quartzite Formation Ardrishaig Phyllite Formation Degnish Limestone Formation Kilbride Limestone Member (Shuna Limestone) Easdale Easdale Slate Formation Easdale Slate Formation Scarba Conglomerate Formation Selma Breccia Jura Slate Member Selma Black Slates (E) Jura Quartzite Formation (D) Loch Skerrols Thrust Colonsay Group Islay ?Dalradian sst Bonahaven Dolomite Formation relationship?Dalradian uncertain Bowmore Loch Gruinart Fault q Sandstone DB contact not seen Group sheared unconformity GB Port Askaig Tillite Formation Rhinns Complex Rhinns Complex Appin Lossit Limestone Formation (Palaeoproterozoic) Group Blair Atholl Figure 2.3

NW Firth of Isle of Sound Loch Loch SE Lorn Jura of Jura Sween Fyne A B

Loch Awe Syncline (F1)

Loch Sween Loch Gruinart Anticline Fault Tayvallich Kilmory Bay Syncline Syncline

16 e app 10 ay N 12 T 19 15 /2) e (F1 11 clin nti 6 A aig 2 sh ri 17 rd 3 A 1 present ground surface 13 18 21 20 5 14 Lo Islay Anticline (F1) ch Ske rro ls 9 Th ru st 8 7 4

Southern Highland Group Appin Group thrust or shear-zone undifferentiated Blair Atholl Subgroup: undifferentiated Glen Sluan Schist Formation fault Argyll Group Dalradian? Tayvallich Subgroup: undifferentiated Bowmore Sandstone Group axial plane trace of F1/2 folds

Ben Lui Schist Formation Crinan Colonsay Group axial plane trace of F4 folds Upper Erins Quartzite Formation Subgroup Palaeoproterozoic Easdale Subgroup: undifferentiated 7 GCR site in Chapter 2 Rhinns Complex undifferentiated Islay 0 kilometres 5 Port Askaig Tillite Formation Subgroup Figure 2.5

13 fault inclined bedding, 35 dip in degrees 31 Dun Chonnuil 29

12 Garbh Eileach 35

32 40

43

34 Member 4 11 Firth of Lorn A’ Chuli 34 Member 3

35

Eileach Port Askaig an Naoimh 10 Tillite Garvellach Isles Member 2 Formation 37

Member 1 40 36 Disrupted Beds (DB) N Great Breccia (GB) 09 column not 0 metres 500 to scale Lossit Limestone Formation

64 65 66 67 68 Figure 2.7 cliffline 0 metres 200 N

LWM stream

21 Leac Thiolastaraidh unit 4 unit 3

unit 2 base of unit 1 Bonahaven Formation 27

distillery 70

Caol Ila Port village Askaig 43

Bonahaven Dolomite Formation Member 4 (see Rubha a’ Mhail GCR site)

Member 3

Member 2

Member 1 (units 1–4)

Port Askaig Tillite Formation Con Tom Member (unit 5)

fault

inclined bedding, dip in degrees Figure 2.9

3 3 (a) 39 40 41 42 43 (b) 85 Bolsa Fault 90 0 metres 200 Rubha Zone a’Mhail N 79 60 20 N inset (b) 40 35 inset (c) 78 Port an Isl a y 0 kilometres 1 t-Struthain cliffline 780 (c) 400 405 410 415 12 790 18 Bàgh an da Dhorius 30

20 17 18 dolerite dyke cliffline (Palaeogene) Jura Quartzite Formation ?

Bonahaven Dolomite Formation N 55 outcrop of 785 Member 4 stromatolite horizon Port a’Chotain Member 3 fault 0 metres 200 25 inclined bedding, Member 2 dip in degrees Figure 2.11 35

45

35 Kilnaughton Bay

track

55 log Carraig 44 Fhada lighthouse

N

0 metres 200

Easdale Subgroup

Pebble Beds Scarba Conglomerate Jura Slate Member Formation

Islay Subgroup Jura Quartzite Formation

fault

inclined bedding, dip in degrees

direction of younging Figure 2.12 NR 3492 4436

conglomerate

sandstone

mudstone

5

metres

0

silt mud silt fine medium coarse NR 3479 4441 pebbles

sandstone grain size Figure 2.14

Easdale Subgroup metaconglomerate

metamudstone Scarba Conglomerate metasandstone and Formation metaconglomerate

87 Jura Slate Member Islay Subgroup Inverlussa

Lussagiven Jura Quartzite Formation

inclined bedding, dip in degrees 35 direction of younging Lussa Bay 50 Lussa Point Eilean N an Rubha Sound of Jura

0 metres 200

64 65 Figure 2.16 Scarba Conglomerate Formation Gulf of coarse-grained quartzite and quartz-rich Corryvreckan metaconglomerate with slaty pelitic partings Jura Slate Member: graphitic metamudstone with thin beds of pebbly quartzite unsorted metaconglomerate with rip-up clasts of sandstone conglomerate and graphitic mudstone Jura Quartzite Formation 01 quartzite, coarse-grained, cross-bedded, with pebbly beds 70 quartz-dolerite dyke (late Carboniferous)

metamafic-rock dyke (Neoproterozoic)

fault

inclined bedding, dip in degrees

00 SlumpScar

Kinuachdrachd Fault

99

Kinuachdrachd

Kinuachdrachd Harbour

98 Sound of Jura

Barnhill 97

track

N

96

0 metres 500

60 70 71 Figure 2.18

Easdale Subgroup Laphroaig Quartzite Formation 25 Port Ellen Phyllite Formation A846 Lagavulin Surnaig Bay metamafic-rock intrusion Farm (Neoproterozoic) Palaeogene dyke 20 fault 34 inclined bedding, sandstone dip in degrees dykes N

0 metres 200 45 40 Figure 2.20

52 89

59

S Ardberg 41 Distillery 42

Easdale Subgroup

A846 Laphroaig Quartzite 39 Formation 46 Port Ellen Phyllite Formation metamafic-rock intrusion (Neoproterozoic) Palaeogene basaltic dyke

fault inclined bedding, dip in degrees N axial plane trace of anticline

axial plane trace of syncline 0 metres 200 S stilpnomelane locality 41 Figure 2.22

Easdale Subgroup Laphroaig Quartzite Formation Port Ellen Phyllite Cnoc Formation Rhaonastil

Ardilistry metamafic-rock sill Bay Cnoc Rhaonastil: Palaeogene composite intrusion metapyroxenite X Palaeogene dyke

fault 48 inclined bedding, N dip in degrees 48

0 metres 200 44 Figure 2.24top

schistose metamafic rock with schistosity increasing upwards in intensity

1

metres

0

metamafic rock with relict ophitic texture and albite/epidote pseudomorphs after plagioclase

meta-anorthosite layer

transition layer

metapyroxenite layer

schistose metamafic rock with albite/epidote pseudomorphs after plagioclase

base Figure 2.25 dolerite HWM

felsic igneous sheet Easdale Slate Formation black slaty metamudstone with bands of metasiltstone, metsandstone and gritty Cnoc metasandstone Mór debrite metacarbonate rock beds (mainly dolomitic)

fault LWM inclined bedding, direction of dip in degrees younging inclined slaty cleavage (S1), dip in degrees vertical slaty cleavage (S1) inclined crenulation 22 cleavage (S2), dip in degrees isoclinal F1 fold 27 N

Cobblers of Lorn B ? ?

09 78

73

5 ??

32 old sea cliff pole to bedding pole to slaty cleavage (S1) pole to crenulation cleavage (S2) C π-axis intense local raised beach development of S3 and S4 crenulation cleavages folded and boudinaged 84 55 dolomitic beds fence D 9 recumbent F2 76 34 folds 85 HWM 14 65 2 m-thick graded 83 metasandstone bed LWM N debrite A

0 metres 100 Black Mill Bay

73 Figure 2.27

dolerite dyke (Palaeogene) LWM metadolerite sheet (dots at base) HWM Craignish Phyllite Formation track 55 76 53 69 pier A i r d N

Sound of Jura 54 0 metres 200 77 fault 58 inclined bedding, 60 dip in degrees inclined cleavage (S1), 00 Craignish Point, 1 km dip in degrees Figure 2.29

83 84

HWM 58 22 n o 9 ex LWM po 31 su 2 79 re Melfort pier 50 39 Home Farm

14 Fearnach N (a) N B a y

0 metres 100

F

53 N (b) 80

44 14

Minor intrusions dolerite dyke (Palaeogene) F felsite (Caledonian) 63 39 60 M mafite (Caledonian) A 78 15 pole to bedding, with best-fit great circle, metadolerite (Neoproterozoic) 37 and its pole, the π-axis ( ) 7 minor F1 fold hinge Dalradian Supergroup pole to slaty cleavage (S1), with best-fit great circle, and its pole ( ) Craignish Phyllite Formation

F 40 fault 31

direction of younging 27 F 57 inclined bedding, dip in degrees F F F inclined S1 slaty cleavage, houses dip in degrees 21 36 F axis of minor fold (F1), 13 Sgeir na plunge in degrees Ceardaich M Figure 2.31 0 metres 500 Port N Liath

86

30

79 Kilmory Bay Kilmory Loch Sween 69 Syncline

75 82

84 track

57 83 70 Eilean 71 a’Chapuill 76 42 72 65 17 23 63

Kilmory 58 B a y 66 74 Port Balimore Bàn 60

69 78

72

73

78 37 73

70 29 Glas 79 Eilean 51 60 45 Loch Caolisport

Point of 72 70 Knap 71

sills of basic meta-igneous rock

Crinan Grit Formation Ardrishaig Phyllite Formation with beds of quartzite direction of axis of F1 fold, younging plunge in degrees inclined bedding, axial plane trace of dip in degrees major anticline inclined S1 cleavage, axial plane trace of dip in degrees major syncline Figure 2.34

72

N 0 kilometres 2 Port Cill LWM Maluaig C 70 46 HWM TS

85 55 Rubha Cill Maluaig

Sound of Jura

Tayvallich C Port 53 KBS Mhoirich 30 1 Loch Sween 75 80 B8024 47 AA N

0 metres 200 69

75

E

S 2 Loch Caolisport E Point of Knap

70 3 4

S 70 75S 80 85 90

Southern Highland Group Argyll Group fault Beinn Bheula Schist Formation Tayvallich Volcanic Formation axial plane trace of major anticline Green Beds Loch Tay Limestone and Tayvallich Slate and Limestone formations axial plane trace of major syncline Glen Sluan Schist Formation Stonefield Schist Formation inclined bedding, Erins Quartzite Formation (E) and dip in degrees Crinan Grit Formation (C), with inclined S2 cleavage, Stronachullin Phyllite Member (S) dip in degrees Ardrishaig Phyllite Formation Figure 2.36

A83 09 0 kilometres 2 N HWM LWM 80 25 N 28 38 11 Strone 30 38 27 Point 28 0 metres 200 Strone Point 20 Loch Fyne Anticline 12

Loch Awe 00

dolerite (? late Carboniferous)

metadolerite (Neoproterozoic)

Crinan Grit Formation/ Ardrishaig Anticline Ben Lui Schist Formation

Ardrishaig Phyllite Formation axial plane trace of major anticline Strone Point Anticline 10 axial plane trace of Strone inset Loch Fyne major syncline Inverary Point map inclined bedding, dip in degrees inclined S1 cleavage, dip in degrees hinge-parallel lineation, plunge in degrees 10 Figure 2.38

X (stretching (a) direction)

(b)

25 268 305 15 20 cm

S2 fold hinge

(c) 30 cm

N great circle representing the mean orientation of S1

stretching lineation (x) bedding/S1 cleavage intersection lineations and minor F1 fold hinges field occupied by poles to S1 cleavage Figure 2.39 03 N

0 metres 200 Loch Awe Syncline

B845

23

Kilchrenan 8 A

16

K il ch ren an

B metadolerite sills u r (Neoproterozoic) n Tayvallich Volcanic Formation Tayvallich Slate and Limestone Formation with: Kilchrenan 22 Conglomerate Member Kilchrenan Grit Member axial plane trace of syncline 27 Struan inclined bedding, 46 dip in degrees B inclined S1 cleavage, dip in degrees A site locality Loch Awe Figure 2.4168 69 70 71 72 73 74 75 76

N 0 kilometres 1 Tayvallich

87

86

Port Bealach Barr na nan Gall h-Iolaire 85 4 80 Port an Sgadain 86

3 84

P Tayvallich Syncline An Aird

Rubha na h-Airde 83

Sound of Jura 2 I 86 82 P

axial plane trace of major syncline

81 T inclined dominant foliation, dip in degrees Rubha Riabhag Loch Sween vertical dominant foliation

Rubha nam direction of younging 80 Fitheach 1 locations of dated trachytic intrusion (I) and tuff (T) Rubha 85 Island of na Cille Danna P good pillow lava localities Loch na Cille

Southern Highland Group Neoproterozoic metabasaltic rocks, Kells Grit: feldspathic metasandstone intrusive and extrusive, with volcaniclastic metasedimentary rocks Argyll Group Tayvallich Volcanic Formation: alkali basalt lavas, some pillowed, Note: Palaeogene dolerite dykes omitted with volcaniclastic rocks, hyaloclastic rocks, igneous breccias and metalimestone metalimestone containing lava fragments; Loch na Cille Boulder Bed at base Tayvallich Subgroup igneous breccia Tayvallich Slate and Limestone Formation: fine-grained laminated metalimestone with black graphitic metamudstone and pebbly quartzite

Crinan Grit Formation: fine- to medium-grained quartzite, locally Crinan pebbly, with subordinate slaty metamudstone Subgroup Figure 2.43 86 87 0 metres 500 N A83 North Bay Barmore Island

South 70 Bay

80 71

e n y F h c o L

lodge

76

70 Sgeir Port a ’Ghuail

A83

East Loch Tarbert 69

Southern Highland Group Beinn Bheula Schist Formation

Green Beds Formation

Glen Sluan Schist Formation Argyll Group Loch Tay Limestone Formation

Stonefield Schist Formation notable outcrops of ‘green beds’ in various formations inclined bedding, dip in degrees

direction of younging Figure0 metres2.45 500 Lochan nan Caorach

65

Am Bodach 17

75

Craignamoraig

70 path 40

75 205 m 16 An Cnap P Allt Mór

Kilchrenan Drissaig Syncline

N

L o c h Av i ch 95to Loch Awe 96 Lower Old Red Sandstone Lorn Plateau Volcanic Formation: andesitic lavas and conglomerates Dalradian Supergroup Loch Avich Lavas Formation: basaltic andesite lavas; P, pillowed Loch Avich Grit Formation: chloritic Southern and feldspathic gritty metasandstones Highland with green metamudstones; calcareous Group black metamudstones occur at top of formation

Tayvallich Volcanic Formation: basaltic lavas and volcaniclastic Argyll sedimentary rocks Group metamudstones

fault, with sense of main movement

direction of younging

inclined bedding, dip in degrees

axial plane trace of major syncline

axial plane trace of major anticline Figure 2.47 60 Loch Gruinart 10 50 Bun-an-Uillt Loch 22 Skerrols Thrust 69 30

N 40

0 metres 500 68 30

20 15 30 20

45 Craigens 30 31 Argyll Group Jura Quartzite Formation

Bowmore Sandstone Group

Blackrock Grit Formation

fault

thrust

inclined foliation, dip in degrees

lineation, plunge in degrees Figure 2.49

20 Colonsay Group N 0 kilometres 2 Ardnave Point Ardnave Formation

Sanaig Greywacke Formation

Sanaigmore Phyllite Formation Sanaigmore Bay Smaull Greywacke Formation

Kilchoman Phyllite Formation

Crosprig Grit Formation 70 Rubha-na-h-Airde Móire Phyllite Formation

Loch Gruinart Rubha Gaidhealach Grit Formation

Kilchiaran Phyllite Formation poor exposure Eilean Liath Grit Formation Saligo Bay Loch Rhinns Complex Gorm

B8018

B8017 fault

Loch Gruinart Fault shear-zone (Bruichladdich Slide/ Kilchiaran Shear-zone Machir Bay axial plane trace of major anticline

Rubha na axial plane trace of major syncline h-Airde Bruichladdich Móire

60 Kilchiaran small outcrops of Eilean Liath Grit Formation Loch Indaal Figure 2.4 colour Click here to download high resolution image Figure 2.6 colour Click here to download high resolution image Figure 2.8a Click here to download high resolution image Figure 2.8b Click here to download high resolution image Figure 2.10a Click here to download high resolution image Figure 2.10b colour Click here to download high resolution image Figure 2.13 colour Click here to download high resolution image Figure 2.15 colour Click here to download high resolution image Figure 2.17 colour Click here to download high resolution image Figure 2.19 colour Click here to download high resolution image Figure 2.21 Click here to download high resolution image Figure 2.23 colour Click here to download high resolution image Figure 2.26a colour Click here to download high resolution image Figure 2.26b colour Click here to download high resolution image Figure 2.26c colour Click here to download high resolution image Figure 2.28a colour Click here to download high resolution image Figure 2.28b colour Click here to download high resolution image Figure 2.30a colour Click here to download high resolution image Figure 2.30b colour Click here to download high resolution image Figure 2.30c colour Click here to download high resolution image Figure 2.32 colour Click here to download high resolution image Figure 2.33 colour Click here to download high resolution image Figure 2.35a colour Click here to download high resolution image Figure 2.35b colour Click here to download high resolution image Figure 2.37 colour Click here to download high resolution image Figure 2.40 colour Click here to download high resolution image Figure 2.42 colour Click here to download high resolution image Figure 2.44 colour Click here to download high resolution image Figure 2.46 colour Click here to download high resolution image Figure 2.48a + b colour Click here to download high resolution image Figure 2.50 colour Click here to download high resolution image Figure 2.4 B&W Click here to download high resolution image Figure 2.6 B&W Click here to download high resolution image Figure 2.10b B&W Click here to download high resolution image Figure 2.13 B&W Click here to download high resolution image Figure 2.15 B&W Click here to download high resolution image Figure 2.17 B&W Click here to download high resolution image Figure 2.19 B&W Click here to download high resolution image Figure 2.23 B&W Click here to download high resolution image Figure 2.26a B&W Click here to download high resolution image Figure 2.26b B&W Click here to download high resolution image Figure 2.26c B&W Click here to download high resolution image Figure 2.28a B&W Click here to download high resolution image Figure 2.28b B&W Click here to download high resolution image Figure 2.30a B&W Click here to download high resolution image Figure 2.30b B&W Click here to download high resolution image Figure 2.30c B&W Click here to download high resolution image Figure 2.32 B&W Click here to download high resolution image Figure 2.33 B&W Click here to download high resolution image Figure 2.35a B&W Click here to download high resolution image Figure 2.35b B&W Click here to download high resolution image Figure 2.37 B&W Click here to download high resolution image Figure 2.40 B&W Click here to download high resolution image Figure 2.42 B&W Click here to download high resolution image Figure 2.44 B&W Click here to download high resolution image Figure 2.46 B&W Click here to download high resolution image Figure 2.48a + b B&W Click here to download high resolution image Figure 2.50 B&W Click here to download high resolution image