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1 2 3 4 5 The rocks of the central Grampian 6 7 Highlands of 8 9 10 J.E. Treagus, P.W.G. Tanner, P.R. Thomas, R.A. Scott and D. 11 12 Stephenson 13 14 15 Jack E. Treagus 15 Raynham Avenue, Didsbury, Manchester M20 6BW; 16 formerly Department of Earth Sciences, University of Manchester. 17 P.W. Geoff Tanner Department of Geographical and Earth Sciences, 18 University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow 19 G12 8QQ. 20 Peter R. Thomas (deceased) formerly Department of Civil 21 Engineering, Paisley College of Technology. 22 Robert A. Scott Cambridge Arctic Shelf Programme, West Building, 23 181a Huntingdon Road, Cambridge CB3 0DH. 24 25 * David Stephenson British Geological Survey, Murchison House, 26 West Mains Road, EH9 3LA. 27 [email protected] 28 0131 650 0323 29 30 * Corresponding author 31 32 Keywords: 33 Geological Conservation Review 34 Central Grampian Highlands 35 Dalradian Supergroup 36 37 Structural Geology 38 39 40 41 42 ABSTRACT 43 44 The Central Grampian Highlands, as defined here, are bounded to the 45 north-west by the , to the south-west by Etive 46 and the Pass of Brander Fault and to the south-east by the main 47 outcrop of the Formation. The more arbitrary 48 northern boundary runs north-west along the and westwards 49 to Fort William. The detailed stratigraphy of the Dalradian 50 Supergroup ranges from the uppermost through to the 51 top of the Group, most notably seen in the two classic areas 52 of - and –Loch Tay; Southern 53 Group strata are preserved only in a small structural inlier south 54 of . 55 Major F1 and F2 folds are complicated by co-axial northeast- 56 trending F3 and F4 folding, as well as by locally important north- 57 or NW-trending folds. In the Loch Leven area, nappe-like F1 folds 58 verge to the north-west, whereas to the south-east the major 59 recumbent F1/F2 Tay Nappe verges to the south-east. The trace of 60 the upright Loch Awe Syncline lies between the opposing nappes, but 61 62 63 64 65 1 2 3 4 in this region a large mass of late-Caledonian granitic rocks 5 obscures their mutual relationship. Three tectonic ‘slides‘ are 6 identified that are certainly zones of high strain but which in 7 part could be obscuring stratigraphical variations. 8 The regional metamorphism ranges from greenschist facies on the 9 western seaboard of Argyll to facies in most of the 10 remainder of the region. The study of garnets, together with 11 kyanite and staurolite in the Schiehallion area, has enabled a 12 detailed history of the metamorphism and structure to be 13 unravelled. 14 Stratabound mineralization occurs in the Easdale Subgroup, where 15 there is also evidence of changes of sedimentary environment 16 associated with volcanicity and lithospheric stretching. The 17 region is dissected by a series of NE-trending, dominantly left- 18 lateral, faults, subparallel to the Great Glen Fault, whose 19 movement history is illustrated here by that of the Fault. 20 21 22 23 24 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 8 J.E. Treagus 9 10 The Central Grampian Highlands, as defined in this special issue, 11 12 are bounded to the north-west by the Great Glen Fault between 13 Lismore and Fort William, and to the south-east by the main outcrop 14 of the Loch Tay Limestone Formation between the Tyndrum Fault and 15 (Figure 1). The south-western and north-eastern 16 boundaries are essentially geographical, rather than geological, 17 but they have been chosen to reflect to a certain extent areas 18 studied by a distinct group of workers. The sites in this region 19 have been selected to illustrate both the stratigraphy and the 20 structure of the Dalradian rocks and, to a lesser extent, aspects 21 of their mineralization and metamorphism. 22 23 1.1 Lithostratigraphy and Sedimentary Environments 24 25 Most of the formations of the Appin and Argyll groups are 26 represented, many in their type areas, but only the uppermost of 27 the poorly-correlated Grampian Group formations are represented. 28 Although many of the formations of the Appin and Argyll groups show 29 30 remarkable similarities in facies across the overall outcrop of the 31 Dalradian, there are significant variations in the lowermost 32 formations of both groups within the Central Grampian Highlands 33 (Figure 2). 34 The uppermost formations of the Grampian Group are well 35 represented in the GCR site selection, as is the transition up into 36 the Subgroup of the Appin Group. Near the north-western 37 section of the boundary with the Northern Grampian Highlands, the 38 contact between the Grampian Group and the Lochaber Subgroup, 39 previously interpreted as the tectonic Fort William Slide, has been 40 re-interpreted by Glover (1993) as a local unconformity. Although 41 that area is not represented in the GCR site selection, in the 42 River Leven GCR site, to the east of Loch Leven, this contact can 43 be shown to be a sedimentary transition, contrary to other 44 suggestions (e.g. Lambert, 1975); the passage from the Eilde Flags 45 into the Eilde is continuous, both sedimentologically and 46 structurally (Treagus, 1974). The thick sequence of psammites and 47 48 semipelites of the Lochaber Subgroup in the type area is described 49 in detail in the reports of several GCR sites around the east and 50 north sides of Loch Leven (River Leven, Nathrach, Rubha Cladaidh, 51 Tom Meadhoin and Doire Ban, Stob Ban), which provide abundant 52 evidence from sedimentary structures, especially cross-bedding, for 53 the shallow-water environment. The lens-like bodies of coarse 54 feldspathic metasandstones of the Eilde and , 55 seen in the River Leven and Rudbha Cladaich GCR sites respectively, 56 are entirely local to the Loch Leven area. On the other hand, the 57 clean metasandstones of Binnein-type quartzite, seen in the River 58 Leven, Nathrach, Tom Meadhoin and Doire Ban and Rudbha Cladaich GCR 59 sites, are widespread; they extend both along strike into the 60 South-west Grampian Highlands as the Maol an Fhithich Quartzite of 61 62 63 64 65 1 2 3 4 and across strike as the thin quartzites seen immediately 5 above the Grampian Group psammites in the , Allt Druidhe 6 and Strath Fionan GCR sites. Along strike to the north-east, these 7 quartzites are regarded as members of the more dominantly 8 semipelitic and Quartzite Formation of the Glen 9 Spean area (Key et al., 1997) (see Leslie et al., 2013). 10 The classic localities of the succeeding Subgroup are 11 12 described in the St. John’s Church, and Ardsheal Peninsula 13 GCR site reports and those of the Blair Subgroup in the 14 Ardsheal Peninsula and Lismore Island GCR site reports. The type 15 and sedimentary structures of the shallow-water Appin 16 Group are well illustrated at these GCR sites. Most of these 17 formations can be correlated lithologically with those of the Isle 18 of Islay in the South-western Grampian Highlands (Tanner et al., 19 2013a), and can be matched virtually formation by formation, with 20 those seen at the Strath Fionan GCR site, in the east of the 21 Central Grampian Highlands, discussed below. 22 There has been controversy concerning the identity of certain 23 formations within the Appin Group in the Loch Leven area, which has 24 considerable implications for the structural interpretation (e.g. 25 Treagus, 1974; Roberts, 1976; Hickman, 1978). Therefore particular 26 attention has been paid in that area to the nature and ‗way-up‘ 27 evidence at the transitional junctions, in order to test the 28 stratigraphical succession established by Bailey (1960). At the 29 30 Tom Meadhoin and Doire Ban GCR site, in particular, the description 31 here supports the contention of Roberts (1976) that the quartzite 32 in the core of the Anticline is the Binnein Quartzite 33 and not the Glen Coe Quartzite as maintained by Bailey (1960) and 34 Hickman (1975). This has considerable implications for the local 35 stratigraphical and structural interpretation. 36 To the south-west of Loch Leven, in the – 37 area, there has also been dispute concerning Bailey‘s original 38 (1922) stratigraphical attributions (Voll, 1960; Litherland, 1980, 39 1982). Here the Benderloch Slide, the correlative of the 40 Ballachulish Slide seen within several GCR sites in the Loch Leven 41 area (see below), is seen in the Camas Nathais GCR site (Tanner et 42 al., 2013a); it excises most of the Subgroup and is 43 now considered to be have had its origins in synsedimentary 44 processes. To the south-east of the slide are formations of the 45 Islay and lower Easdale subgroups of the . According 46 to Litherland (1980), the latter rocks, some of which were 47 48 attributed to the Appin Group by Bailey (1922) and Voll (1960), as 49 well as those of the Appin Group to their east, show dramatic 50 changes in thickness and in the Loch Creran area. Only 51 limited representatives of these strata occur at the Camas Nathais 52 and Port Selma GCR sites, but the latter includes a spectacular 53 limestone metabreccia, containing microfossils (oncolites, 54 catagraphs and possible bryozoans). This facies, part of a 55 submarine slide deposit, is equivalent to the 56 in the Easdale Subgroup seen on the Isle of Jura (see Tanner et 57 al., 2013a), but is not seen elsewhere in the Central Grampian 58 Highlands. 59 To the south-east of the Loch Leven area, the Grampian Group rocks 60 are poorly known and much of the ground is occupied by igneous 61 62 63 64 65 1 2 3 4 intrusions. In the area, the Grampian Group appears to be 5 transitional up into the Appin Group, which is represented only by 6 very abbreviated sequences of part of the Lochaber and Ballachulish 7 subgroups, some of which are seen in the River Orchy GCR site. 8 Here, the Boundary Slide, like the Benderloch Slide, brings the 9 abbreviated Ballachulish Subgroup into contact with the pebbly, 10 graphitic facies of the overlying Argyll Group; the Blair Atholl 11 12 and Islay subgroups are entirely absent. These same relationships 13 extend farther east to the west side of the Schiehallion area 14 (where they are seen in the Allt Druidhe GCR site) and continue 15 north-eastwards, but with the gradual appearance and thickening of 16 the missing formations between the Ballachulish Subgroup and the 17 Carn Mairg Quartzite of the Easdale Subgroup (Figure 4a). Thus in 18 the east of the Schiehallion area a complete sequence of the Appin 19 Group is restored, although still abbreviated compared with the 20 type area (Figure 2). The total thickness of the group here is 21 about 1 km compared with 5 km at Loch Leven (Treagus, 2000); some 22 of the reduction might be attributed to the higher overall strain 23 in the Schiehallion area, although delicate cross-bedding is still 24 preserved in places. 25 The Schiehallion area provides transitions from the Appin Group, 26 both down into the Grampian Group (at the Strath Fionan GCR site) 27 and upwards (via the Tempar Burn GCR site) into a complete Argyll 28 Group succession. To the north of the Schiehallion area the 29 30 quartzites and psammites of the Glen Spean Subgroup, at the top of 31 the Grampian Group, are well represented in the A9 and River Garry 32 and Creag nan Caisean–Meall Reamhar GCR sites but correlation with 33 other sequences of the Grampian Group cannot yet be made. The A9 34 and River Garry GCR site provides an unrivalled wealth of 35 sedimentary structures. The Tempar Burn GCR site contains the 36 classic section for the famous glacigenic Port Askaig Tillite 37 (locally known as the Schiehallion Boulder Bed), here much reduced 38 in thickness compared with the type locality at Port Askaig on the 39 Isle of Islay and the Garvellach Isles GCR site (Tanner et al., 40 2013a). Immediately to the east of the Strath Fionan GCR site and 41 in the area, towards the boundary with the North-east 42 Grampian Highlands, the Boundary Slide re-asserts itself. It is 43 not clear to what extent the hiatus attributed to the Boundary 44 Slide is due to sedimentological and/or tectonic factors, although 45 the attenuation or absence of sedimentary structures in formations 46 in which they are normally clear and abundant, adjacent to the 47 48 slide, indicates the importance of high strains. 49 The remainder of the GCR sites in the central region provide 50 sections through most of the formations of the Argyll Group, many 51 in type localities. From the GCR site near Tyndrum, 52 through the Ben , Slatich and Strath Fionan GCR sites, 53 eastwards to the Creag an Chanaich to Frenich Burn GCR site near 54 Aberfeldy, all the Argyll Group formations are represented. 55 Although many of the formations can be readily correlated along 56 strike with those in the South-west and North-east Grampian 57 Highlands, significant variations in the stratigraphy occur in the 58 Easdale Subgroup. Facies and thickness variations are particularly 59 noticeable in the turbiditic pebbly sandstones and graphitic 60 mudstones within the Schist and Carn Mairg Quartzite 61 62 63 64 65 1 2 3 4 formations. The rare exposures of the Farragon Volcanic Formation, 5 as seen in the Slatich GCR site, provide the earliest evidence of 6 substantial volcanic activity in this sector of the Grampian 7 highland Terrane which, together with the turbidites, heralds a 8 change to a more unstable sedimentary environment. 9 Two GCR sites have been chosen to illustrate the stratabound 10 mineralization in the Easdale Subgroup, and also provide evidence 11 12 of changes of sedimentary environment associated with volcanicity 13 and lithospheric stretching. The GCR site at Creag an Chanaich to 14 Frenich Burn contains most of the outcrop of the celebrated 15 ‗Aberfeldy‘ baryte/sulphide mineralization at the upper margin of 16 the Ben Eagach Schist Formation. The GCR site at Auchtertyre, in 17 the Tyndrum district, contains the rare sulphide mineralization 18 that occurs locally at the upper margin of the Schist 19 Formation. 20 The Tayvallich Subgroup is represented in the Central Grampian 21 Highlands by the Ben Lawers GCR site. There, it consists 22 dominantly of crystalline metalimestone with only thin 23 representing the more substantial volcanic component seen in the 24 type area around Loch Awe in the South-west Grampian Highlands. 25 26 1.2 Structure 27 28 The GCR sites of the Central Grampian Highlands exemplify the 29 30 classic interpretation of the overall structure of the Grampian 31 Terrane, as described by Stephenson et al. (2013a). The refolded, 32 recumbent NW-facing major folds of the Appin–Loch Leven area are 33 particularly well represented in the Schiehallion–Ben Lawers area. 34 Relationships in the area between these NW-facing folds and the SE- 35 facing Tay Nappe are unclear, not least because of large intrusions 36 of late-Caledonian granitic rocks in the Loch Etive–Moor of Rannoch 37 area. In the South-west Grampian Highlands (Tanner et al., 2013a), 38 the upright Loch Awe Syncline fulfils this central role between the 39 SE-facing Tay Nappe and the NW-facing Islay Anticline; Roberts and 40 Treagus (1977c) have attempted to project this fold into the 41 Central Grampian Highlands. Several GCR sites specifically were 42 chosen to examine three major slides of the region, the Benderloch, 43 Ballachulish and Boundary slides. These slides are of particular 44 interest for the debate as to whether they are, wholly or in part, 45 of sedimentological or tectonic origin. 46 The structural significance of each of the GCR sites is described 47 48 below, as far as possible in the context of the two classic areas 49 of Appin–Loch Leven and Schiehallion–Ben Lawers. 50 In the Appin–Loch Leven area, eight GCR sites illustrate the major 51 folds of the two dominant deformation phases (Figures 3a, b); the 52 descriptions in the text progress essentially from east to west, 53 into areas of decreasing complexity. The descriptions and 54 interpretations in the site reports follow the work of J.L. Roberts 55 and J.E. Treagus, as summarized by Roberts and Treagus (1977a, 56 1977b, 1977c). This work largely supported the original 57 interpretation of Bailey (1960), but was based primarily on 58 observations of minor structures that were largely ignored by 59 Bailey. These views were challenged in part by Hickman (1978), but 60 Roberts and Treagus (1980) defended their position. 61 62 63 64 65 1 2 3 4 The three major first-phase folds are the Appin Syncline, the 5 Kinlochleven Anticline and the Ballachulish Syncline (Figure 3b); 6 their D1 age is justified from the evidence of minor structures in 7 the individual GCR site reports. However, the Appin Syncline, as 8 seen at the Ardsheal Peninsula and Onich GCR sites, was interpreted 9 as a D2 structure by Hickman (1978) as was the antiform at the Tom 10 Meadhoin and Doire Ban GCR site and the synform between the Rubha 11 12 Cladaich and Nathrach GCR sites (the upward- and downward-facing 13 nose of the Kinlochleven Anticline respectively, according to the 14 Roberts and Treagus model preferred here). Hickman also disputed 15 the existence of the Ballachulish Syncline at the Stob Ban GCR 16 site, on stratigraphical grounds. The two major second-phase folds 17 are the Stob Ban Synform and the Kinlochleven Antiform (not to be 18 confused with the F1 Kinlochleven Anticline) (Figure 3b); evidence 19 for their existence and relative age, which has not been disputed, 20 is examined in a number of GCR site reports. 21 The F1 Appin Syncline, which faces steeply up to the north-west, 22 epitomizes the structural style of the westernmost part of the 23 Central Grampian Highlands. The north-west limb is well 24 represented in the Ardsheal Peninsula GCR site and the core and 25 south-east limb in the Onich GCR sites. Relationships of the F1 26 minor folds and cleavage to the major fold are exceptionally well 27 seen, as is the growth of chlorite (at Ardsheal) and of biotite (at 28 Onich) in the stretching direction on the S1 cleavage planes. 29 30 These observations support those of Bailey (1960) and Treagus and 31 Treagus (1971) but contradict both those of Hickman (1978) and 32 those of Bowes and Wright (1967) who suggested that the fold is a 33 product of a third generation of deformation, associated with 34 retrograde metamorphism. These GCR sites also provide evidence of 35 the beginning of the D2 deformation, which increases in intensity 36 eastwards towards the major F2 Stob Ban Synform. The Lismore 37 Island GCR site, at a higher level in the Appin Syncline, although 38 less well known with respect to the D1 history, illustrates the 39 style of D2 particularly well, with large-scale folds that verge in 40 sympathy with the Stob Ban Synform (Hickman, 1978). 41 The Tom Meadhoin and Doire Ban GCR site gives an unrivalled view 42 of the upward-facing core of the F1 Kinlochleven Anticline, with 43 associated minor structures. In its northern part, it illustrates 44 relatively large-scale F2 folds on the north-west limb of the most 45 significant major fold of that age, the Stob Ban Synform, which is 46 fully exposed to the north-east at the Stob Ban GCR site. The 47 48 Rubha Cladaich and Nathrach GCR sites illustrate the downward 49 facing of the D1 structures on the north-west and south-east limbs 50 respectively of the F1 major fold, the Kinlochleven Anticline. 51 Lying on the south-east limb of the Stob Ban Synform, these two 52 sites display D2 minor structures sympathetic to the major fold and 53 syn-D2 growth of garnet is well seen. The south-east limb of the 54 complementary F2 Kinlochleven Antiform, to the east of the Stob Ban 55 Synform, is represented by the River Leven GCR site, although the 56 D1 facing here is not well documented. 57 The St John’s Church GCR site has been chosen to illustrate the 58 Ballachulish Slide, a major dislocation, also seen in the Tom 59 Meadhoin and Doire Ban GCR site, which removes part of the 60 succession on the common limb between the Kinlochleven Anticline 61 62 63 64 65 1 2 3 4 and the next highest major F1 fold, the Ballachulish Syncline. 5 This structure was originally a low-angle syn-D1 normal fault 6 according to Bailey (1960), but it has been suggested by Soper and 7 Anderton (1984) that it had a syn-sedimentation origin. According 8 to Roberts and Treagus (1977a, 1977c), the Ballachulish Syncline is 9 the correlative of the Loch Awe Syncline of the South-west Grampian 10 Highlands. The documented stratigraphical evidence for the 11 12 existence of the Ballachulish Syncline (Roberts, 1976) is given in 13 the Stob Ban GCR site report; however, this evidence and evidence 14 for the age of the Ballachulish Slide at that site clearly need re- 15 examination. The Rubha Cladaich and Nathrach GCR sites also 16 provide evidence for a set of structures that, according to Treagus 17 (1974), are associated with the major post-D2 swing in strike of 18 the earlier structures across Loch Leven (Figure 3a). 19 In the Schiehallion–Loch Tay area (Figure 4a, b), the site 20 descriptions and interpretations follow the work of Nell (1984) and 21 Treagus (1987, 2000), but differ substantially from the earlier 22 views of Bailey and McCallien (1937) and of Rast (1958). Three GCR 23 sites (Creag nan Caisean–Meall Reamhar, A9 and River Garry and 24 Strath Fionan,) illustrate aspects of the structure of the 25 Schiehallion district, where major F1 folds on the lower limb of 26 the SE-facing Tay Nappe are folded by major F2 folds. The Allt 27 Druidhe GCR site has been chosen to illustrate the high strains 28 associated with the Boundary Slide in this area and that at Meall 29 30 Dail-chealach demonstrates the style of major late kink folds. To 31 the south of Schiehallion, minor structures at the Slatich GCR site 32 in Glen Lyon can be used to demonstrate the D2 age of a major fold, 33 the Ruskich Antiform, which can be traced laterally to the south- 34 west into the Fold; these F2 folds are major components of 35 the architecture of the Grampian Terrane, at the north-western 36 margin of the Tay Nappe. The Slatich and Ben Lawers GCR sites also 37 exemplify D3 minor structures superimposed on those of the D1 and 38 D2 phases. These sites also provide a structural link between the 39 Schiehallion district and the Flat Belt, part of the inverted limb 40 of the Tay Nappe, which dominates the structure of the south- 41 eastern Dalradian. The Ben Lawers GCR site in particular contains 42 the hinge-zone of one of the major post-D2 folds that affect the 43 Flat Belt, the F4 Ben Lawers Synform. 44 In the Tyndrum area the River Orchy GCR site, in a comparable 45 position and along strike to the south-west of the A9 and River 46 Garry GCR site, contains the hinge-zone of a major SE-facing early 47 48 fold, the presumed D1 age of which has recently been shown to be D2 49 (Tanner and Thomas, 2010). The nearby Ben Oss GCR site, uniquely 50 in this special issue, has been chosen to illustrate the history of 51 movement and mineralization on a major fault, the NE-trending 52 Tyndrum Fault, which is one of several that cause major 53 displacements within the Grampian Terrane. 54 55 1.3 Metamorphism 56 57 The selected GCR sites encompass rocks in both the greenschist and 58 amphibolite facies. The greenschist-facies rocks are 59 characteristic of the western seaboard of Argyll and of the 60 Highland Border region; the remainder of the region is occupied by 61 62 63 64 65 1 2 3 4 amphibolite-facies rocks. The Ardsheal Peninsula and Onich GCR 5 sites exemplify, particularly well, low-grade slaty and phyllitic 6 rocks. Chlorite-muscovite stacks at the former site and biotite 7 crystals, up to 10 mm long at the latter site, are visibly 8 elongated in the stretching direction on the S1 cleavage surfaces. 9 To the east of these two GCR sites, the rocks pass into the lower 10 amphibolite facies, coincident with the onset of the D2 11 12 deformation. Garnet and K-feldspar porphyroblasts are especially 13 prominent in the Binnein Schist at the Rubha Cladaich GCR site; 14 there it can be seen that these porphroblasts are wrapped by the 15 dominant S2 schistosity and include the S1 fabric. 16 Garnet is prominently displayed in schistose rocks across the 17 central part of the area, and is well seen at the River Orchy, 18 Strath Fionan and Tempar Burn GCR sites, here too with clear 19 wrapping by the dominant S2 fabric; curved helecitic inclusion 20 trails of S1 can also be seen in the field within the 21 porphyroblasts. Amphibole (syn- to post-D2) is well developed in 22 the Farragon Volcanic Formation at the Slatich and Ben Lawers GCR 23 sites, and in the concordant amphibolites at the Strath Fionan GCR 24 site. The Ben Lawers Schist Formation at the Creag an Chanaich to 25 Frenich Burn GCR site displays coarse crystals of amphibole but 26 within the Ben Lawers Synform (in the Ben Lawers GCR site) the 27 formation is in the greenschist facies. Both staurolite (syn-D2) 28 and kyanite (cross-cutting S2) are seen at the Strath Fionan GCR 29 30 site. 31 32 2 RIVER LEVEN SECTION 33 (NN 191 619–NN 205 189) 34 35 J.E. Treagus 36 37 38 2.1 Introduction 39 40 The River Leven, east of Kinlochleven, provides a virtually 41 continuous section, from the uppermost formation of the Grampian 42 Group (the Eilde Flag Formation) through the three lowest members 43 of the Loch Treig Schist and Quartzite Formation at the base of the 44 Appin Group (the Eilde Quartzite, the Eilde , and the 45 Binnein Quartzite) (Figure 5). Its importance lies in the fact 46 that a sedimentary transition between the Grampian and Appin groups 47 48 can be clearly demonstrated. In addition, structural information 49 from these rocks, in conjunction with that from other GCR sites 50 around Loch Leven to the west, enables the large-scale tectonic 51 picture of the western Grampian fold-belt to be understood. The 52 exposures in the present-day bed and banks of the river are 53 complemented in the east by those in an old course of the river and 54 in the west by exposures adjacent to a pipeline. 55 The stratigraphical succession and the broad structural 56 relationships of the Kinlochleven area have been described by 57 Bailey (1960), with detailed structural data relating to the two 58 regional phases of deformation being provided by Treagus (1974). A 59 description of the GCR site was included in a guide to the Loch 60 Leven area by Roberts and Treagus (1977b). A different 61 62 63 64 65 1 2 3 4 interpretation of the structure was proposed by Hickman (1978), but 5 this was refuted by Roberts and Treagus (1980). 6 7 2.2 Description 8 9 The GCR site extends from the youngest rocks at 1910 6186, just 10 west of the old bridge across the River Leven at Kinlochleven, east 11 12 to the oldest at 2047 6088, some 1.7 km upstream (Figure 5). At 13 the former location, and for some 400 m upstream, are excellent 14 exposures of the well-bedded, pure white, Binnein Quartzite with 15 plentiful cross-bedding that youngs consistently to the west in 16 vertical rocks (Figure 6). Characteristic rusty spots, a few 17 millimetres in length are seen elongated parallel to a steep west- 18 dipping cleavage in interbedded . The transitional junction 19 with the Eilde Schists is exposed in the river at about 1950 6173; 20 younging in quartzite ribs immediately east of the junction is to 21 the west. 22 The Eilde Schists are dark-grey pelites and semipelites with ribs 23 of quartzite and feldspathic psammite, particularly near the top 24 and bottom transitional beds, where carbonaceous and calcareous 25 beds are also more common. Much of the accessible outcrop is in 26 the flat limbs of folds, which are knee-shaped and tens of metres 27 in wavelength; these folds verge north-west with an axial-planar, 28 steep NW-dipping, intensely-developed, crenulation cleavage. The 29 30 junction of the Eilde Schists with the Eilde Quartzite is exposed 31 at about 2006 6150, on the vertical limb (some 50 m wide and o 32 striking 037 ) of a major fold; to the east a flat limb, some 100 m 33 wide is exposed at waterfalls, where inverted cross-bedding and 34 slump-folds are excellently displayed in the feldspathic 35 quartzites. The dip then gradually increases to the east across a 36 major fold hinge, to return to vertical at about 2025 6606; some 37 good exposures are seen in the old river course which runs north- 38 west from here. Associated with this major fold are small-scale 39 folds in thin quartzite beds and a steep axial-planar crenulation 40 cleavage in the pelites. 41 The junction with the Eilde Flags at the top of the Grampian Group 42 is easily identified at the point where an overflow channel from 43 the pipeline to the south joins the river at 2026 6598. Thin- 44 ribbed psammites and semipelites occur to the east of the reddened 45 quartzites of the Eilde Quartzite, with typical Grampian Group 46 thicker-bedded feldspathic psammites coming in immediately east of 47 48 a porphyritic microdiorite dyke. Westward-younging cross-beds and 49 slump-folds in the steep east-dipping rocks are well-seen 50 throughout, but one 5 m-wide unit, between two waterfalls on the 51 north bank, displays 10 cm- to 1 m-thick beds of slump folds 52 particularly well, together with 50 cm-thick beds of unusually 53 pebbly psammites. Pebbles in the latter are strongly elongated, up 54 to a length of 4 cm, in the plane of the dominant, steep west- 55 dipping, crenulation cleavage. Cleavage/bedding intersections are 56 sub-horizontal (e.g. 5o to 062o). 57 58 59 60 61 62 63 64 65 1 2 3 4 2.3 Interpretation 5 6 7 The section in the River Leven is so unbroken and the way-up 8 indicators are so unambiguous, that there can be no doubt that it 9 provides a continuous stratigraphical section from the Eilde Flags 10 (oldest), at the top of the Grampian Group, through the Eilde 11 12 Quartzite and the Eilde Schists into the Binnein Quartzite, the 13 lowest members of the Loch Treig Schist and Quartzite Formation at 14 the base of the Lochaber Subgroup in the Appin Group. Most 15 importantly, the attitude and style of the minor structures is 16 consistent throughout the section and there is no evidence for a 17 tectonic or metamorphic break at the junction between the Grampian 18 and Appin group rocks. This observation became important with the 19 assertion by Piasecki and van Breeman (1983) that rocks near 20 Aviemore, which they assigned to the lower part of the Grampian 21 Group, share a structural and tectonic event, then dated at 750 Ma, 22 with underlying migmatitic rocks. Since there is no evidence in 23 the Appin Group of events of this age, it appeared at that time 24 that there must be a tectonic and/or metamorphic break within, or 25 at the top of, the Grampian Group. However, since all of the rocks 26 with evidence of a 750 Ma or older event (the date has subsequently 27 been revised to c. 800 Ma) are now regarded as part of the pre- 28 Grampian Group Group (see Leslie et al., 2013), such an 29 30 interpretation is no longer necessary. 31 The Grampian–Appin group junction exposed in the River Leven had 32 been proposed as a possible site of a major tectonic boundary on 33 the basis of the change in sedimentary environment (Lambert, 1975). 34 However, the change in the nature of the metasedimentary 35 lithologies at this junction does not mark a dramatic alteration in 36 the original sedimentary environment. The major change is from the 37 more-feldspathic (K-feldspar rich in thin section) Eilde Flags and 38 their unusually coarse pebble beds into the cleaner Eilde Quartzite 39 (plagioclase rich in thin section); however, psammites with 40 mixtures of both feldspars occur in both the Eilde Quartzite and 41 Eilde Schists, as well as in the Glen Coe Quartzite to the west of 42 the GCR site, and the ‗background‘ semipelite and in all 43 units remains of a consistent nature. The scale and nature of the 44 cross-bedding and slump-folds also remains consistent across the 45 junction. 46 Throughout the section, the minor- and intermediate-scale folds, 47 48 with their associated cleavage, consistently verge north-west in a 49 series of step-like folds. The cleavage in the pelites is very 50 intensely developed and might be mistaken for a first cleavage, but 51 in thin section it is clearly a crenulation of an earlier 52 penetrative cleavage (Treagus, 1974). In fact in some locations it 53 is possible to see an angle between bedding and this earlier 54 penetrative cleavage, but the plunge of their apparent intersection 55 and their vergence relationship is so variable as to render these 56 observations of no use in the determination of facing of the first 57 structures. The folds and dominant cleavage belong to the regional 58 D2 phase of Treagus (1974); the intermediate-scale folds are on the 59 eastern limb of the major F2 Kinlochleven Anitiform (which can be 60 seen clearly from the western part of the GCR site in the hills to 61 62 63 64 65 1 2 3 4 the north of Kinlochleven). Minor F2 folds and cleavage on the 5 western limb of this fold are seen in the Nathrach GCR site. At 6 Nathrach, D1 structures face downwards and, according to the 7 regional structural interpretation of Treagus (1974), the 8 Kinlochleven Antiform would be responsible for a change in the 9 facing of D1 structures to steeply upward-facing on its eastern 10 limb; this facing has not been confirmed in the rocks exposed in 11 12 the River Leven GCR site, but should be a subject of future 13 research. Bailey (1960), by contrast, considered that the rocks of 14 this GCR site lie in the hinge area of a major recumbent F1 15 syncline, facing towards the west, a view that was shared by 16 Hickman (1978). In the latter view, minor F1 folds and their axial 17 planar cleavage would be recumbent, but the minor structure 18 evidence presented here does not conform with that interpretation. 19 20 2.4 Conclusions 21 22 The River Leven Section GCR site is of national importance in 23 demonstrating the continuity of sedimentation and the similarity of 24 structural features across the boundary between the Grampian and 25 Appin groups of the Dalradian succession. It is therefore not a 26 candidate for the location of a major break in the history of 27 geological events in the , as had once been 28 suggested. The site provides a unique continuous section across 29 30 the topmost formation of the Grampian Group into the loweest 31 formation of the Appin Group and is rich in structures that provide 32 evidence both for the sedimentary environment of these rocks and 33 for their deformation history. The small-scale deformational 34 structures (folds and cleavages) are important in the 35 reconstruction of the largest-scale folds, many kilometres in 36 length, that make up this western part of the Grampian Mountain 37 Belt; the site will be important for further research in this 38 respect. 39 40 3 NATHRACH 41 (NN 164 624) 42 43 J.E. Treagus 44 45 46 47 3.1 Introduction 48 49 The Nathrach GCR site, which is on a well-exposed hillside above 50 the main road on the north side of Loch Leven west of Kinlochleven, 51 has both stratigraphical and structural importance. Firstly, it 52 contains fine exposures of the Binnein Schist and Binnein Quartzite 53 members of the Loch Treig Schist and Quartzite Formation (Lochaber 54 Subgroup) and of their transitional junction (Figure 7). The 55 exposures of quartzite are rich in sedimentary structures. 56 Secondly, the interbedded thin quartzites and pelites of the 57 transitional beds contain minor structures, unrivalled for their 58 clarity (Figure 8), which give important information on both the 59 geometry and position of major folds of two deformation phases. 60 The locality is therefore critical, together with other GCR sites 61 62 63 64 65 1 2 3 4 (River Leven, Rubha Cladaich, St John’s Church, Tom Meadhoin and 5 Doire Ban and Stob Ban) to the interpretation of the large-scale 6 recumbent refolded folds which characterize the Loch Leven– 7 Ballachulish area. 8 The stratigraphy and the structure of the GCR site have been 9 described by Bailey (1960) and Treagus (1974). A description of 10 the site is also included in a geological field guide to the Loch 11 12 Leven area by Roberts and Treagus (1977b). 13 14 3.2 Description 15 16 There are three areas of interest in this GCR site. The principal 17 interest is in the structure of the Binnein Schists, as displayed 18 in a series of prominent NW-facing exposures about 50 m long at 19 around 1656 6235 (Figure 7, locality A). The member here comprises 20 finely bedded pelite and semipelite with some interbedded 21 centimetre- to 0.5 metre-thick white quartzite beds. Cross-bedding 22 in these pure quartzites, which are of a similar facies to the 23 Binnein Quartzite, youngs to the north-west in the near-vertical 24 NE-striking beds. The exposures provide numerous examples of tight 25 folds (F2 in the interpretation below) with centimetre- to metre- 26 scale amplitude and wavelength (Figure 8). At the south-western 27 end of the crags, the plunge of these folds is at steep angles (70– 28 80o) to the south-west, but towards the north-eastern end they 29 30 plunge steeply to the north-east. The penetrative fabric of the 31 pelite, which is subparallel to the bedding, has been affected by a 32 tight crenulation cleavage (S2), which is near vertical, NE- 33 striking and axial planar to the folds; the penetrative fabric in 34 the semipelite is tightly folded together with the bedding. The 35 vergence geometry of the folds and of the bedding in relation to 36 the crenulation cleavage is of an ‘S’ type (assuming an average 37 vertical plunge). In certain exposures it is possible to see that 38 earlier folds (F1) are deformed by this main set; neither the 39 vergence nor the plunge of these early folds is easily measured, 40 but the nature of the interference patterns shows that they are not 41 precisely co-axial with the later set. The earlier folds have the 42 penetrative cleavage (S1) in the semipelite parallel to their axial 43 surfaces. A weaker, NNE-trending, crenulation cleavage associated 44 with more-open folds is developed locally; this is the third set of 45 structures more fully described at locality C, below. 46 The second area of interest is the structure of the Binnein 47 48 Quartzite, best seen in prominent crags centred about 1640 6237, 49 some 10–50 m west of a sharp change of slope which marks the 50 Binnein Quartzite–Binnein Schist junction (Figure 7, locality B). 51 Here, the member consists of white quartzite, well bedded (10–90 52 cm-thick beds) with rare semipelite and pelite units. Cross- 53 bedding, not easily seen near the junction with the Binnein 54 Schists, is seen to young consistently to the south-east in the 55 near-vertical NE-striking beds. Of particular interest are thin 56 (10 cm) beds of quartzite, interbedded with the pelite and 57 semipelite, which commonly display well-developed tight folds. 58 These asymmetric folds, with a short limb length of some 10–20 cm, 59 plunge at 30–50o to 070o, have a ‗Z‘ geometry and have a penetrative 60 schistosity in the semipelite parallel to their axial-surfaces, as 61 62 63 64 65 1 2 3 4 noted in the F1 folds at locality A (Figure 9). In the adjacent 5 pelites, a crenulation cleavage (of the type noted above related to 6 the later F2 folds) is developed, which transects the F1 folds; 7 folds related to this cleavage are not developed here, although 8 they are seen, with the characteristic ‗S‘ geometry, in more 9 schistose lithologies nearer to the junction with the Binnein 10 Schists. Similar relationships can be observed elsewhere near this 11 12 junction (Figure 7). 13 The third area is a series of crags at the top of a small hill 14 near the road at 1630 6205 (Figure 7, locality C), which exposes 15 Binnein Schists of the type described at locality A, and in a 16 similar structural position. Metre-thick quartzites, north 17 striking and near vertical, display excellent cross-sets younging 18 to the west. The quartzites are affected by open folds, and the 19 interbedded pelites display the development of an axial-planar, 20 wide-spaced (1 cm), crenulation affecting the two earlier cleavages 21 described above. The axial surfaces of these crenulations strike 22 east-north-east and are near vertical; the plunge of this third set 23 of folds is approximately vertical. 24 25 3.3 Interpretation 26 27 The Nathrach GCR site is located across the transitional junction 28 between the Binnein Quartzite and the Binnein Schists and, although 29 30 a totally continuous section is not exposed, the abundant easterly- 31 younging cross-bedding in the quartzites at locality B leaves no 32 doubt that the schists are the younger member. In a regional 33 context, this junction is on the western, sub-vertical, limb of a 34 tight, upright, antiform which closes to the north-east on Sgor an 35 Fharain (180 640) (Figure 3a). The same transitional junction on 36 the eastern limb of the fold can be examined east of the Nathrach 37 Burn around 170 627 where cross-bedding youngs to the west (Figure 38 7). Bailey (1960), from stratigraphical considerations, proposed 39 that this fold (named the Mamore Syncline by Treagus, 1974) is a 40 downward-facing D1 structure, a major subsidary fold on the eastern 41 limb of the regional Kinlochleven Anticline. This proposal is 42 consistent with the minor structures observed in the western area 43 of the GCR site at locality B, which show that the rocks face 44 downwards on the first penetrative cleavage, related to minor folds 45 that must lie on the western limb of a major syncline (see also 46 Treagus, 1974). These observations are complemented by those at 47 48 the Rubha Cladaich GCR site, which lie on the western limb of the 49 downward-facing F1 Kinlochleven Anticline. 50 To the east of the junction of the Binnein Quartzite and Binnein 51 Schists, at locality A, interbedded quartzites within the Binnein 52 Schists young towards the west. Since the crenulation cleavage and 53 related minor folds (D2 of the regional structure) have the same 54 vergence here as in locality B, the fold between the two localities 55 must either be a subsidiary F1 fold related to the Mamore Syncline 56 or the major fold itself. The importance of these observations is 57 to show that the major reversals of younging in the area are the 58 product of the downward-facing D1 folding and not of the D2 phase. 59 Similarly, the consistent vergence of the D2 structures here and in 60 the Rubha Cladaich GCR site, on the opposing limb of the 61 62 63 64 65 1 2 3

4 Kinlochleven Anticline, can be used to demonstrate the D1 age of 5 that fold. 6 Locality C exhibits examples of a third generation of minor folds 7 (F3), and a related E–W-trending crenulation cleavage (S3). Both 8 are geometrically associated with the major swing in strike, to a 9 south-east direction, of all earlier structures across Loch Leven 10 (Figure 3,3a). Treagus (1974) suggested that this change in strike 11 12 (the Loch Leven Fold) on the south side of the loch might be the 13 result of the intrusions of the Glen Coe Caldera-volcano Complex of 14 late-Silurian age. The re-interpretation of this volcanism in a 15 NW-orientated graben, resulting from regional NE-directed tension 16 or from a transtensional scenario (Moore and Kokelaar, 1997), might 17 account for such a deflection in the adjacent Dalradian rocks. 18 19 3.4 Conclusions 20 21 The importance of the Nathrach GCR site lies in its wealth of 22 sedimentary structures across the junction between two members of 23 the Loch Treig Schist and Quartzite Formation (Lochaber Subgroup) 24 and in its abundant minor tectonic structures (folds and cleavages) 25 of unrivalled clarity. Taken together, these structures 26 demonstrate that the earliest major folds, of many kilometres 27 amplitude and originally probably flat-lying, have been bent by 28 later forces to assume remarkable, totally downward-facing (i.e. 29 upside-down) attitudes. Even later forces have twisted the earlier 30 o 31 folds yet again through 90 to change the whole grain of the country 32 rock for several tens of kilometres, which may be related to the 33 tensional opening of the crust associated with development of the 34 Glen Coe Caldera-volcano to the south. 35 36 4 RUBHA CLADAICH 37 (NN 120 610–NN 133 613) 38 39 J.E. Treagus 40 41 42 4.1 Introduction 43 44 The Rubha Cladaich GCR site, adjacent to the main road on the north 45 shore of Loch Leven, provides important sedimentological and 46 structural information in a section across three members of the 47 48 Loch Treig Schist and Quartzite Formation in the Ballachulish 49 Subgroup of the Appin Group. The sedimentary make-up of the three 50 members (Glen Coe Quartzite, Binnein Schist and Binnein Quarzite), 51 their transitional facies and their sedimentary structures can be 52 seen clearly on clean glaciated surfaces. In addition, the 53 evidence from minor structures supports the relative ages and 54 geometry of the three principal phases of deformation in the 55 region. Together with complementary information from the Nathrach 56 GCR site, these structures can be used to demonstrate the D1 age of 57 the dominant major fold in the area, the Kinlochleven Anticline. 58 The rocks at this site are exceptionally rich in porphyroblasts 59 both of garnet and of plagioclase, which can be related to the 60 structural history. 61 62 63 64 65 1 2 3 4 The principal features of the GCR site have been described by 5 Bailey (1960), Treagus (1974), and in a field guide to the Loch 6 Leven area by Roberts and Treagus (1977b). 7 8 4.2 Description 9 10 This GCR site provides both sedimentological and structural 11 12 information from the base of the Glen Coe Quartzite, down through 13 the Binnein Schists into the top of the Binnein Quartzite in a 14 series of shoreline exposures, complemented by road-cuttings 15 (Figure 10). At the western margin of these exposures (NN 1216 16 6098 to 1228 6098), the peninsula of Rubha Cladaich exposes 17 glaciated surfaces of the thick-bedded (0.5–2 m) feldspathic Glen 18 Coe Quartzite. The western edge of the exposure is part of a steep 19 N-dipping ‘short’ limb of a major steeply-plunging fold, the long 20 limb of which, dipping at 48–72o to the west-north-west, occupies 21 the remainder of the exposure to the east. Cross-bedding, in 30 22 cm-thick sets younging to the south, is particularly well seen on 23 the short limb, by the mouth of a small burn. Other sedimentary 24 features that are seen in the quartzites on both limbs include 25 ripple cross-lamination, slump folds and mud-flakes that are 26 associated with thin pelitic beds (centimetres thick). The latter 27 contain thin string-like concentrations of feldspar clasts up to 2 28 cm long. 29 30 The hinge area of the major fold, which is exposed on the main 31 promontory to the east of the burn, has a vertical penetrative o 32 axial plane schistosity that strikes 072 . Calculated plunges of o 33 minor folds within the hinge are variable from steep (e.g. 80 to 34 the south-west), through vertical, to 44o to 020o; the latter are 35 presumed to be more typical of the regional plunge of the major 36 folds (Treagus, 1976). The NNE-striking long limb of the fold is 37 corrugated by metre-wavelength, open folds with axial-surfaces 38 trending east-north-east that are locally associated with a widely 39 spaced crenulation cleavage. 40 To the east of the main promontory the next exposures, around 1233 41 6098, contain the Glen Coe Quartzite–Binnein Schist junction, 42 showing a transition from metre-thick quartzites in semipelite into 43 garnetiferous pelites with only thin (10 cm) beds of quartzite. 44 The latter quartzites exhibit cross-bedding and channel structures 45 that indicate younging to the west. In the Binnein Schists, 46 porphyroblasts both of plagioclase feldspar and of garnet are 47 common in the pelites. Both are wrapped by a dominant, near- 48 o o 49 vertical, crenulation cleavage (e.g. 070 /86 NW) that crosses the o o 50 bedding (e.g. 020 /70 W) in a clockwise sense; minor folds related 51 to the cleavage show consistent ‗S‘ vergence and plunge to the 52 south-west (e.g. 40o to 335o). Of particular importance, seen in 53 the middle of this exposure, are several 10–20 cm-wavelength 54 isoclinal folds (F1 in the interpretation below) that are clearly 55 folded or are cross-cut by the dominant crenulation cleavage (S2) 56 and its associated minor folds (F2) (Figure 11). These earlier 57 folds, which have a penetrative schistosity (S1) parallel to their 58 axial-surfaces, plunge steeply to the north-east and have an ‗S‘ 59 vergence. Particularly clear patterns produced by the interference 60 of the two sets of folds are seen at the extreme edge of this 61 62 63 64 65 1 2 3 4 exposure at about NN 1249 6101. Similar relationships can be seen 5 on a steep glaciated surface farther east at NN 1262 6004. 6 Locally, open folds (F3) with E-W-trending axial surfaces and a 7 coarse crenulation cleavage, deform the earlier cleavages. 8 The junction of the Binnein Schists with the Binnein Quartzite is 9 exposed in a road-cutting at NN 1267 6017, where there is a 10 transition over a distance of a metre from the typical Binnein 11 Schist lithology, described above, into thinly-bedded (centimetre- 12 o 13 thick) pure quartzite. The quartzite, which dips at about 50 to 14 the west, becomes thicker-bedded eastwards, with 30 cm-thick units 15 and clear cross-sets younging to the west that are best displayed 16 near the end of the roadcut (NN 1284 6027), but also seen on the 17 foreshore at NN 1279 6020. Also seen are folds of a metre 18 wavelength, with a similar geometry to the dominant F2 set in the 19 previous exposures, which plunge moderately to steeply to the 20 south-west with an ‘S‘ vergence. 21 22 4.3 Interpretation 23 24 The almost continuous exposure across the two boundaries between 25 the three members gives an exceptionally compact section in which 26 the nature of this sedimentary succession, the detail of its 27 sedimentary structures and its way-up can be studied. 28 Structurally, the importance of the site lies in the clear 29 30 superimposition of the three regional phases of deformation. The 31 folds and cleavage in the Glen Coe Quartzite are identifed as D1, 32 as no earlier structure can be observed; their plunge and vergence 33 are compatible with their position on the upper limb of the major 34 downward-facing F1 Kinlochleven Anticline, which lies in the 35 outcrops of Eilde Quartzite and Eilde Schist between here and the 36 Nathrach GCR site to the east. In the adjacent Binnein Schists, 37 the S1 schistosity is intensely crenulated by the dominant S2 38 cleavage, but clear examples of F1 folds are also preserved (Figure 39 11). The D2 minor folds complement those seen at the same 40 stratigraphical boundary in the Nathrach GCR site. Their plunge 41 and vergence demonstrate that the Kinlochleven Anticline is cross- 42 cut by the D2 structures and also support the interpretation of the 43 geometry and position of the major F2 Stob Ban Synform to the west 44 (see also the Stob Ban and Tom Meadhoin and Doire Ban GCR site 45 reports). The growth of the garnet and plagioclase porphyroblasts 46 can be seen to be associated with the development of the S2 47 48 cleavage that wraps them, both in thin section and in hand specimen 49 (Treagus, 1974). 50 The third generation of minor structures is seen in the open folds 51 of the Glen Coe Quartzite exposures and as the coarse ENE-striking 52 crenulation cleavage in the Binnein Schists. These structures are 53 related to the regional F3 Loch Leven Antiform, which is 54 responsible for the deflection of the regional north-east strike 55 into the south-east strike on the S side of Loch Leven (Treagus, 56 1974; Figure 3a; see also the Nathrach GCR site report). 57 The interpretation of the D1 and D3 major structures given here 58 agrees with the interpretation of Bailey (1960), which was 59 supported by the minor structural data given by Treagus (1974). 60 The latter author used the minor structures to propose the present 61 62 63 64 65 1 2 3 4 three-phase history. However, Hickman (1976) refuted that the 5 downward-closing fold between the present site and that at Nathrach 6 is the closure of the F1 Kinlochleven Anticline, attributing it to 7 a secondary phase, although he did not present any specific data to 8 support this interpretation. His main argument, from the NW- 9 striking exposures on the south side of Loch Leven, appeared to be 10 that minor F1 folds plunge to the south-east on the south-west limb 11 12 and to the north-west on the north-east limb and thus must be 13 folded around the major closure. However, the data presented here 14 and in the Nathrach GCR site report show that F1 minor folds 15 exhibit a considerable variation in plunge from steep to the south- 16 west, through vertical to gentle NE-plunging attitudes. Allowing 17 for the late swing of strike across Loch Leven, this variation in 18 fold geometry is similar to that recorded by Hickman, but here it 19 can be seen within a single F1 major fold limb. Furthermore, there 20 is no evidence from post-D1 minor structures to support the 21 existence of a major secondary synform to the east of the Rubha 22 Cladaich GCR site. 23 24 4.4 Conclusions 25 26 The Rubha Cladaich GCR site provides both essential 27 sedimentological and structural information for the interpretation 28 of the Dalradian of the western side of the Grampian mountain belt. 29 30 Firstly, it allows the establishment of the stratigraphical 31 succession of the lower part of the Appin Group (the Glen Coe 32 Quartzite, Binnein Schist and Binnein Quartzite members of the Loch 33 Treig Schist and Quartzite Formation) in an unusually compact and 34 continuous shore section. Sedimentary structures, which give 35 information concerning the conditions of sedimentation, as well as 36 the way-up of the succession, are particularly well preserved on 37 clean glaciated surfaces. Similarly, these surfaces provide clear 38 evidence of minor tectonic structures (folds and cleavages) of 39 three generations. Study of these structures, particularly in the 40 context of adjacent GCR sites allows the geometry and relative age 41 of major folds of the three generations to be understood. 42 This site complements the Nathrach GCR site in demonstrating the 43 dramatic large-scale closure of the Kinlochleven Anticline, the 44 most important of the early regional folds in the area, which is 45 here totally inverted by the second phase of deformation. It also 46 exhibits still later folds, with their associated cleavage, which 47 are essential to the understanding of the regional swing in the 48 o 49 trend of the beds and earlier structures through 90 across the 50 eastern end of Loch Leven. 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 5 TOM MHEADHOIN AND DOIRE BAN 5 (NN 071 610–NN 091 631 AND NN 097 634–NN 089 645) 6 7 J.E. Treagus 8 9 10 11 5.1 Introduction 12 13 The GCR site at Tom Meadhoin and Doire Ban occupies a summit ridge 14 between Loch Leven and Gleann Righ to the north (Figure 12). It is 15 important for the placing of major folds into the regional 16 deformational picture, establishing in particular the Tom Meadhoin 17 Antiform as the closure of the major F1 Kinlochleven Anticline, 18 which is here upward facing as a result of refolding by D2 19 structures. Information from this site has also allowed a revision 20 of the stratigraphical identity of lithological units on the north 21 side of Loch Leven, which in turn has lead to a refinement of the 22 structural interpretation. One of the major dislocations of the 23 area, the Ballachulish Slide, which separates the Kinlochleven 24 Anticline from the structurally overlying Ballachulish Syncline, is 25 also exposed. 26 The rocks were described in some detail by Bailey (1960), who 27 identified the Tom Meadhoin Antiform as an ‗early‘ structure, but a 28 stratigraphical revision was made by Roberts (1976) who also 29 30 provided the structural detail reported here. 31 32 5.2 Description 33 34 The stratigraphical succession of this site (Figure 12) is in the 35 Appin Group. It progresses from the Binnein Quartzite (oldest) 36 through the Binnein Schists and the Glen Coe Quartzite into the 37 Leven Schists (all Lochaber Subgroup) and then across the 38 Ballachulish Slide into the abbreviated Ballachulish Limestone and 39 Ballachulish of the Ballachulish Subgroup. The Binnein 40 Quartzite is the best-seen unit, being prominently exposed on the 41 crags of Tom Mheadhoin, where it forms the core of a doubly- 42 plunging antiform, the Tom Meadhoin Antiiform (Bailey, 1960) The 43 rock is a well-bedded, pure-white quartzite in which the dip can be 44 readily measured to demonstrate the north-easterly and south- 45 westerly plunges at its north-eastern and south-western ends 46 respectively. The craggy outcrops between NN 099 629 and NN 084 47 48 620 exhibit many examples of cross-bedding, unambiguously younging 49 up towards the boundary with the surrounding schists. 50 The transitional facies from the Binnein Quartzite to the Binnein 51 Schists consists of impure quartzites (commonly cross-bedded) and 52 dark pelite and semipelite (commonly graded), as seen in the Rubha 53 Cladaich GCR site. These lithologies are well seen at the north- 54 eastern end of the antiform (NN 095 637), and on both limbs of its 55 south-western closure at NN 620 088 and NN 083 620. At these three 56 exposures the angular relations between the steep NW-dipping 57 penetrative cleavage and the shallower dipping bedding clearly 58 shows that the cleavage is axial-planar to the major fold. The 59 cleavage/bedding intersections plunge at about 35o to the north-east 60 61 62 63 64 65 1 2 3 4 at the north-eastern closure of the antiform and at less than 20o to 5 the south-west at its south-western end. 6 A thin feldspathic quartzite can be traced from the shore at Loch 7 Leven, north-eastwards along the south-east side of the outcrop of 8 the Binnein Quartzite and Binnein Schists on Tom Mheadhoin (Figure 9 12). This outcrop of the Glen Coe Quartzite is also well exposed 10 to the south-east of Doire Ban, from about NN 097 636 to NN 092 11 12 641, where it defines a synform and an antiform. These folds 13 plunge to the north-east, as is seen from the changing dip of 14 bedding and the plunge of the intersection of a crenulation 15 cleavage with the bedding. The crenulation cleavage is axial- 16 planar to the major fold-pair. 17 The Leven Schists and younger formations to the east and north of 18 the succession described above are not well exposed in the area of 19 this GCR site. However, the Ballachulish Slide, which brings the 20 younger formations (the Ballachulish Limestone, the Ballachulish 21 Slates and the Appin Quartzite) against the Leven Schists can be 22 located within the area. In a gully of a burn draining east from 23 Doire Ban, about NN 096 645, the Leven Schists are in contact with 24 the Ballachulish Slates, with an intervening thin remnant of 25 Ballachulish Limestone in places. The location of the slide can be 26 followed south-west around the antiform/synform pair discussed 27 above. To the north-east of Tom Meadhoin the slide can also be 28 located at the head of a burn at NN 091 625, where the Ballachulish 29 30 Limestone is missing and the Ballachulish Slates are brought 31 against the Leven Schists. The flagginess of the rocks at these 32 exposures of the Ballachulish Slide appears to be parallel to the 33 penetrative cleavage in the adjacent rocks. 34 35 5.3 Interpretation 36 37 Perhaps the most important aspects of this GCR site are the re- 38 interpretation of the quartzite of Tom Meadhoin as the Binnein 39 Quartzite, surrounded by the Binnein Schists, and the 40 identification of the dominant structure as an F1 fold core by 41 Roberts (1976). Bailey (1960) had interpreted both this outcrop of 42 quartzite and the quartzite outcrop to its south-east as the Glen 43 Coe Quartzite, occupying two antiformal cores, with a synform in 44 the intervening schists (his Leven Schists). He interpreted the 45 three folds as comprising the upward-facing hinge of the 46 Kinlochleven Anticline, one of the major early recumbent nappes 47 48 folded into an upright position by the secondary Stob Ban Synform 49 (see the Stob Ban GCR site report). The relations of the S1 50 penetrative cleavage to bedding seen at this GCR site confirm that 51 the Tom Meadhoin Antiform is indeed a D1 structure and a true 52 anticline (Roberts, 1976). However, the quartzite in the fold core 53 on Tom Meadhoin is undoubtedly the Binnein Quartzite, from its 54 clean, white, well-bedded character and its characteristic 55 transition to the Binnein Schists. This revision of the 56 stratigraphy removes the need for the two folds to the south-east 57 of Tom Meadhoin. It also confirms the progressive thinning of the 58 Glen Coe Quartzite from 2 km at its type locality on the south side 59 of Loch Leven, to a few hundred metres north of Mam Gualainn (118 60 638) (Figure 12), as described by Treagus (1974), to a few tens of 61 62 63 64 65 1 2 3 4 metres at Doire Ban. Furthermore, to the north-west of this GCR 5 site, the expanse of pelite identified as Leven Schists by Bailey 6 (1960) must represent a direct passage from the Binnein Schists 7 into the Leven Schists, in which the Glen Coe Quartzite is not 8 represented at all. 9 A second important structural observation is that the two NE- 10 plunging folds of the outcrop of the Glen Coe Quartzite on Doire 11 12 Ban are associated with a crenulation cleavage and are therefore of 13 D2 age in the regional deformation sequence. These folds are 14 subsidary folds to the regional SW-plunging F2 Stob Ban Synform, 15 which crops out to the south-east (see Figure 12 and the Stob Ban 16 GCR site report). This F2 fold is responsible for the folding of 17 the once-recumbent Kinlochleven Anticline to its upward-facing 18 position at Tom Meadhoin and its downward-facing attitude farther 19 east (see the Rubha Cladaich and Nathrach GCR site reports). 20 As Bailey (1960) pointed out, the Ballachulish Slide appears to be 21 contemporary with the early (D1) deformation in this area, which is 22 confirmed by the observations at this GCR site. However, as Bailey 23 also pointed out, the slide must be an original low-angle normal 24 fault rather than a thrust, as it thins the upper limb of a major 25 anticline, the Kinlochleven Anticline. Soper and Anderton (1984) 26 have suggested that such movements may have taken place during 27 sedimentation. Further research at the site might help to shed 28 more light on this problem. 29 30 31 5.4 Conclusions 32 33 The Tom Meadhoin and Doire Ban GCR site is of critical importance 34 for the light that it sheds on both stratigraphical and structural 35 arguments in the Dalradian on the west side of the Grampian fold- 36 belt. As well as clarifying the local stratigraphical succession, 37 it enables the dramatic thinning of one formation, the Glen Coe 38 Quartzite, to be reconstructed. It also provides an unusually 39 complete view of the hinge area of one of the major folds of the 40 region, the F1 Kinlochleven Anticline. This fold, which was 41 originally flat-lying, extends over a distance of at least 15 km 42 from Tom Meadhoin eastwards to the River Leven GCR site. A further 43 period of deformation was responsible for the bending of this fold 44 from its original flat-lying attitude into its present upright 45 position and the superimposed structures associated with this D2 46 event can be well demonstrated. One of the major structural 47 48 dislocations of the region, the Ballachulish Slide, is contemporary 49 with the earlier folding and is also well seen. This GCR site 50 promises to be important for future investigations of the structure 51 of the region. 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 6 STOB BAN 5 (NN 138 658–NN 149 668) 6 7 J.E. Treagus 8 9 10 11 6.1 Introduction 12 13 This GCR site, on the northern side of Stob Ban, one of the highest 14 mountains in the Mamore range, contains the magnificently exposed 15 core of one of the major F2 folds of the Loch Leven area, the Stob 16 Ban Synform. This upright, SW-plunging fold is responsible for the 17 change from the downward-facing D1 structures of the Nathrach and 18 Rubha Cladaich GCR sites, to the south-east of this site, to the 19 upward-facing D1 structures of the Tom Meadhoin and Doire Ban and 20 Onich GCR sites to the south-west. At the Stob Ban GCR site, the 21 Stob Ban Synform refolds the Ballachulish Syncline, which, along 22 with the underlying Kinlochleven Anticline (seen in the Tom 23 Meadhoin and Doire Ban GCR site) is one of the major, originally 24 flat-lying, F1 nappe structures of the western part of the Grampian 25 fold-belt. The interpretation of the structure is dependant upon 26 Bailey’s (1960) interpretation of the stratigraphical succession, 27 which in this area is very abbreviated and is also hornfelsed by a 28 granitic pluton. This interpretation was supported by Roberts 29 30 (1976) but doubted by Hickman (1978). It is clear from Bailey’s 31 description (in Bailey, 1960) that the realization of the existence 32 of this refolding was central to his interpretation of the whole 33 regional structure. 34 35 6.2 Description 36 37 This remote and poorly known GCR site extends from Coire an 38 Lochain, the north-western coire of Stob Ban, across the mountain’s 39 north ridge and down the eastern flank into Coire a’ Mhusgain. 40 According to Bailey (1960, pp. 47–49), and as recorded by him on 41 the Geological Survey’s 1” Sheet 53 (1948), the GCR site contains the 42 isoclinal core of the recumbent Ballachulish Syncline, folded by 43 the upright secondary Callert Synform (now known as the Stob Ban 44 Synform). As Figure 13 shows, the right-way-up sequence of thick 45 Leven Schists and 30 m of Ballachulish Limestone on the lower limb 46 of the syncline (equivalent to the upper limb of the Tom Meadhoin 47 48 Anticline, as seen in the Tom Meadhoin and Doire Ban GCR site) is 49 followed upwards by 30 m of the Ballachulish Slates in the core and 50 then the reverse sequence of Ballachulish Limestone–Leven Schists 51 on the upper limb. The two limbs of this once-recumbent F1 fold 52 have subsequently been folded into the tight upright SW-plunging F2 53 Stob Ban Synform. All of the lithologies have been hornfelsed by 54 the granitic Mullach nan Coirean Pluton. 55 Further details have been given by J.L. Roberts (1976 and in 56 Roberts and Treagus, 1977b). The first penetrative cleavage (S1) 57 is everywhere parallel to bedding; it has steep south-east dips of 58 75o to the south-east on the north-west limb of the Stob Ban Synform 59 and near vertical dips on the south-east limb. The closure of the 60 F2 synform is well exposed on the steep eastern slopes of the north 61 62 63 64 65 1 2 3 4 ridge of Stob Ban (at about NN 146 666). The outcrop of the Leven 5 Schists in the core terminates some 20 m below the crest of the 6 ridge and the underlying thin Ballachulish Limestone can be traced 7 around the hinge. Below this, the Ballachulish Slates in the core 8 of the early F1 syncline and the lower, repeated Ballachulish 9 Limestone and Leven Schist sequence, now right-way-up, can be 10 traced from the hinge both to the north-west and south-east along 11 12 the ridge (between about NN 147 663 and NN 144 668). Minor F2 folds 13 are symmetrical in the hinge-zone and become increasingly north- 14 west vergent to the north-west and increasingly south-east vergent 15 to the south-east, thus demonstrating the D2 age of the major fold. 16 Another well-exposed section is along the Allt Coire an Lochain 17 which drains north-west from the lochan at NN 141 662 just south- 18 east of the synformal fold core. The two outcrops of Ballachulish 19 Limestone (now calcsilicate hornfels) and the intervening 20 Ballachulish Slates are well seen in the gorge (at about NN 140 21 663), with the upper part of the Leven Schists (now pelitic 22 hornfels) seen at the lip of the corrie (NN 1407 6624). Throughout 23 this section F2 folds, with steeply dipping axial planes, are seen 24 plunging to the south-west and verging to the north-west on the 25 north-west limb of the major F2 synform. The Ballachulish 26 Limestone can be seen on the south-east limb of the synform midway 27 along the north-east shore of the lochan, followed uphill to the 28 east (to about NN 143 661) by the Ballachulish –Ballachulish 29 30 Limestone–Leven Schist sequence, in which the minor folds verge 31 south-east on the south-east limb of the major F2 synform. 32 33 6.3 Interpretation 34 35 The existence and identity of the Ballachulish Syncline was deduced 36 by Bailey (1960) from the apparent repetition of the Leven Schists 37 and Ballachulish Limestone about the core of Ballachulish Slates. 38 The identities of these formations is made difficult both by their 39 great abbreviation from their thicknesses at their type localities 40 and from their severe hornfelsing by the adjacent granite. Roberts 41 (1976) confirmed the folding of the regional S1 cleavage, parallel 42 to bedding, around the major F2 synform, but he regarded Bailey’s 43 interpretation of the stratigraphy and D1 structure to be a working 44 hypothesis, being unable to provide any way-up evidence or D1 minor 45 structure evidence to add to Bailey’s observations. Hickman (1978) 46 suggested that the supposed Leven Schist and Ballachulish Limestone 47 48 sequences are local facies variations within the Ballachulish 49 Slates, although such variations have not been reported elsewhere. 50 In fact, the reality of the stratigraphy of the inverted limb of 51 the major F1 syncline is better demonstrated to the south of the 52 Stob Ban GCR site, between Tom Meadhoin and Callert House (Figure 53 12), where the Ballachulish Slates are succeeded upwards by a large 54 expanse of Ballachulish Limestone and Appin Quartzite occurs in the 55 fold core. 56 The sympathetic relations of the S2 crenulation cleavage and F2 57 minor folds to the major Stob Ban Synform in the area of the GCR 58 site, as reported by Roberts (1976), support Bailey‘s (1960) 59 identification of that structure as a major component of the 60 secondary deformation in the Loch Leven area. 61 62 63 64 65 1 2 3 4 6.4 Conclusions 5 6 The Stob Ban GCR site not only provides sections across one of the 7 major early folds seen in the western Grampian fold-belt (the F1 8 Ballachulish Syncline), but also contains the hinge of one of the 9 major later folds (the F2 Stob Ban Synform). Later folds such as 10 this are responsible for major variations in attitude of the early 11 12 folds throughout the Loch Leven area. Despite considerable 13 reductions in thickness of some of the lithological units, relative 14 to their type areas and despite contact metamorphism in the aureole 15 of an adjacent granite, the stratigraphy within the GCR site is 16 clear enough for the repetition of formations on the two limbs of 17 the early fold to be unusually well seen. Moreover, the striking 18 manner of the changing geometry of minor folds across the site can 19 be used to demonstrate perfectly the existence and age of the 20 later, F2 fold. The detailed structural relationships at this GCR 21 site are only poorly known and there is considerable scope for 22 further study. 23 24 7 ST JOHN’S CHURCH, LOCH LEVEN 25 (NN 065 587) 26 27 J.E. Treagus 28 29 30 31 7.1 Introduction 32 33 The St John’s Church GCR site, on the south shore of Loch Leven 1.5 34 km south-east of the Ballachulish Bridge, provides a rare section 35 across one of the ‘slides’ (synmetamorphic low-angled faults) that 36 are a major feature of the Dalradian in the western part of the 37 Central Grampian Highlands. An Appin Group succession from the 38 lower part of the Leven Schists up to the base of the Appin 39 Quartzite, is exposed across the core of one of the major recumbent 40 nappes of the area, the Ballachulish Syncline; the Ballachulish 41 Slide occurs on the lower limb of this fold. Here the fold core 42 and the slide are turned into a steeply dipping attitude as a 43 result of later folding. 44 The exposures were described and the structure was illustrated by 45 Bailey (1960, pp. 58–59, figure 7G); Roberts (1976) and Roberts and 46 Treagus (1977b) have described the general context. 47 48 49 7.2 Description 50 51 The exposures on the eastern side of the GCR site provide a section 52 from the Ballachulish Slates through transitional beds (the Appin 53 Transition Formation) at the base of the Appin Quartzite, which 54 lies on what was originally the upper limb of the Ballachulish 55 Syncline. To the west, two tectonic junctions bring in 56 respectively, a thin slice of Ballachulish Limestone and the basal 57 facies of the Leven Schists (Figure 15). The former lies in the 58 core of the syncline; the latter lies on the original lower limb. 59 The bedding and a penetrative schistosity dominantly dip steeply to 60 the north-west, an attitude acquired during later folding. The 61 62 63 64 65 1 2 3 4 section is described from south-east to north-west from the shore 5 opposite St John’s church. 6 A group of exposures on the first peninsula on the south-east side 7 of the GCR site (Figure 15, locality A; NN 0669 5869) expose 8 phyllitic graphitic pelites with thin semipelite beds, which are 9 attributed to the upper part of the Ballachulish Slate Formation. 10 Thin beds of gritty quartzite, typical of the transition into the 11 12 Appin Quartzite Formation, occur at the north-western end of this 13 peninsula. Strongly deformed ripple-drift lamination can be 14 discerned, but way-up is not easily determined. The bedding, 15 together with the penetrative schistosity, strike north-east and 16 dip steeply north-westwards at 80–85o; rarely, the two planar 17 surfaces can be seen at a narrow angle to one another and to be 18 axial planar to tight folds plunging steeply to the south-west. A 19 strongly developed stretching lineation (pyrite blebs and ) and 20 possibly the intersection lineation pitch down the dip of the 21 schistosity. The schistosity is folded by minor tight folds, which 22 plunge subvertically and dominantly have a ‗Z‘ geometry and an 23 axial-planar crenulation cleavage. 24 After a gap of some 50 m, the rocks at the south-eastern end of 25 the second peninsula (Figure 15, locality B; NN 0665 5875) are 26 black phyllitic pelites, with a more variable strike than in the 27 previously described exposures. They become more-dominated by beds 28 of gritty quartzite, up to 70 cm thick, towards the north-west. 29 30 Here too, the penetrative schistosity is very close to the bedding 31 and their cross-cutting relationships and the direction of their 32 intersection are difficult to determine. The outcrop of these 33 transitional beds ends at a NE-trending, 4 m-thick microdiorite 34 dyke. Between this dyke and a second 5 m-thick dyke, occurs a 10 35 m-thick unit of yellow-weathered, grey metacarbonate rock, which is 36 interbanded with millimetre- to centimetre-thick beds of dark 37 semipelite, a lithological association typical of the Ballachulish 38 Limestone Formation. These NE-striking beds are locally strongly 39 folded by steeply plunging minor folds exhibiting both ‗S‘ and ‗Z‘ 40 geometries. A critical locality (NN 0663 5882) occurs at the 41 junction of the metalimestone with the western dyke where, over a 42 distance of about one metre, a few centimetres of platy quartzose 43 schist are seen (Figure 16). This schist is interpreted as a 44 slither of the basal facies of the Leven Schists, which crop out 45 over the remainder of the peninsula to the west of the dyke. The 46 Leven Schists here consist of quartz-rich psammite interbedded on a 47 centimetre scale with ribs of semipelite; no way-up criteria have 48 o 49 been established. One well-exposed minor fold has a plunge of 40 50 to the north-east and an ‘S‘ geometry, but otherwise there are no 51 well-developed minor structures. 52 53 7.3 Interpretation 54 55 According to Bailey (1960, pp.55–59) the succession of Ballachulish 56 Slates and the Appin Transition Formation, described above, is 57 corrugated by intermediate-scale folds subsidiary to the 58 Ballachulish Syncline (an F1 fold in modern nomenclature). The core 59 of this syncline lies close to the junction of the transitional 60 beds with the Ballachulish Limestone; the Ballachulish Limestone 61 62 63 64 65 1 2 3 4 and Leven Schist exposures to the west belong to the lower limb. 5 The minor folds and cleavage/bedding relationships, seen in the 6 section of Ballachulish Slates and transitional beds, certainly 7 show varying vergence directions, suggesting such intermediate- 8 scale folds of perhaps several tens of metres wavelength. S0/S1 9 intersections appear to be parallel to the steeply pitching 10 stretching lineation. However, the D1 age of these structures has 11 12 not been confirmed in thin section in any of the more-recent 13 studies. In fact the existence of the syncline has not been 14 confirmed in current studies, either from minor folds and 15 cleavage/bedding relationships, or from sedimentary way-up 16 structures, (although the synformal structure is clear from the 17 regional stratigraphical context (Figure 15). 18 According to Bailey (1960) the Ballachulish Syncline, which was 19 originally recumbent, has been rotated into its present steep, NW- 20 dipping, upward- and SE-facing, attitude by the secondary folding 21 of the region. Some of the folds seen in the section described 22 above are associated with a crenulation cleavage, and their south- 23 westerly plunge and ‗Z‘ geometry agree with them being F2 folds on 24 the western limb of the major F2 Stob Ban Synform, similar to those 25 described in the Tom Meadhoin and Doire Ban GCR site to the north- 26 east. 27 Although the junction between the Appin Transition Formation and 28 the Ballachulish Limestone to its west is obscured by a 29 30 microdiorite dyke, the increase in quartzite content towards the 31 junction certainly supports the concept that this junction is a 32 ‗slide‘ (the regional Sgorr a‘Choise Slide, a minor branch of the 33 Ballachulish Slide according to Bailey (1960). The axial trace of 34 the Ballachulish Syncline, according to Bailey, lies within the 35 outcrop of the Ballachulish Limestone. Minor folds in this outcrop 36 show reversals of vergence that would be expected in D1 structures. 37 The junction to the west between the limestone and the few 38 centimetres of quartzitic Leven Schists, does not exhibit the usual 39 transition from metalimestone into the pelitic top of the Leven 40 Schists and according to Bailey represents the Ballachulish Slide 41 on the western limb of the syncline. The exposure of the ‗slide‘, 42 although only one metre in length, shows a slight discordance of 43 bedding orientation across it, between the limestone and the flaggy 44 schist (Figure 16). There is no evidence that it is a much later 45 dislocation that post-dates all the folding, but thin sections 46 would be needed to establish the exact age of movements. On the 47 48 west side of the western dyke, which obscures the remainder of the 49 outcrop of the ‗slide‘, schistose psammites are typical of the 50 basal part of the Leven Schists; no ‗way-up‘ evidence has been 51 found. Thin-section investigation of the folds and cleavage here 52 would be particularly useful in the delineation of the structural 53 relationships. 54 Since the Ballachulish Slide occurs on the lower limb of a once 55 recumbent syncline and involves no repetition of strata, Bailey 56 (1960) noted that it would have to have originated as a low-angle 57 normal fault (i.e. a lag) and not as a thrust as would have been 58 associated traditionally with nappe structures. This is a common 59 occurrence in the nappes of the Central Grampian Highlands and led 60 Soper and Anderton (1984) to suggest that such ‗slides‘ might have 61 62 63 64 65 1 2 3 4 originated as synsedimentary extensional faults. Further research 5 at this locality might help to resolve this debate. 6 7 7.4 Conclusions 8 9 The St John’s Church, Loch Leven GCR site contains one of the few 10 exposures across a major dislocation (the Ballachulish Slide) of 11 12 the type that has disrupted many of the major early folds of the 13 Grampian Fold-belt. These ‘slides’ are of great interest since the 14 faults might have been initiated at the time of sedimentation and 15 developed further during the onset of folding, when they translated 16 the rocks above for many kilometres. The section is well exposed 17 in coastal outcrops that are much visited by student and 18 professional geologists and would benefit from further research. 19 20 8 ONICH DRY RIVER GORGE AND ONICH SHORE SECTION 21 (NN 025 617–NN 031 626 AND NN 030 614–NN 052 612) 22 23 J.E. Treagus 24 25 26 8.1 Introduction 27 28 The two adjacent GCR sites at Onich (Figure 17), especially the 29 30 almost continuously exposed section on the north shore of Loch 31 Leven, are important for two reasons. Firstly, the sites expose 32 four formations of the Ballachulish Subgroup (Appin Group) in 33 unrivalled sedimentary detail; exposures along the hill to the west 34 of Dubh ghlac, a glacial meltwater channel north-west of Onich 35 village, exhibit particularly striking sedimentary structures in 36 the Appin Quartzite. Secondly, clear minor structure observations 37 can be made across a major syncline that has not been complicated 38 by later deformation. The syncline is the regionally important 39 Appin Syncline, which can be traced for 50 km down the west coast 40 of Argyll and within which the Ardsheal Peninsula and Lismore 41 Island GCR sites also lie. 42 The shore section has been described in detail by Bailey (1960), 43 Roberts (1976), Roberts and Treagus (1977b) and Treagus (1991). It 44 is possibly the most visited section in the whole of the Grampian 45 Terrane, attracting professional geologists, student parties and 46 interested amateurs. It provides an example of a key part of the 47 48 Dalradian succession, and is used as a mapping exercise and for 49 demonstrating the use of minor structures to deduce the geometry of 50 major structures. 51 52 8.2 Description 53 54 The Onich shore section provides excellent sections across all 55 formations of the Ballachulish Subgroup, except for the 56 Ballachulish Limestone. It is described here from east to west. 57 The oldest formation, the Ballachulish Slates, is well seen in the 58 steep, SE-dipping beds of a small quarry at NN 0494 6109 to the 59 north of the main road, as well as in shore exposures between NN 60 0495 6102 and NN 0465 6103. This formation is typically a 61 62 63 64 65 1 2 3 4 pyritiferous black slaty pelite with thin semipelite beds that 5 exhibit small-scale graded bedding and cross-lamination indicating 6 younging both to the south-east and to the north-west (for 7 explanation, see structural detail below). Cubes of pyrite, up to 8 1 cm across, make the slate very distinctive and roofing slates 9 from here, but mainly from the quarries at South Ballachulish, can 10 be seen throughout Britain. In the Onich area, most of the pyrite 11 12 has been altered to red-brown pyrrhotite, due to contact 13 metamorphism by the Ballachulish granitic pluton, which crops out 14 1.5 km to the south. 15 The Appin Quartzite is particularly well exposed on the foreshore 16 at NN 0437 6105, where steep, overturned, SE-dipping beds show many 17 examples of trough cross-bedding younging to the north-west (Figure 18 19). Unfortunately, the junction with the Ballachulish Slates to 19 the east is not exposed and the Appin Transition Formation is not 20 represented. Here the Appin Quartzite is white and gritty, 21 containing pebbles of pink feldspar and creamy quartz in various 22 concentrations. Individual clasts, only slightly deformed, are up 23 to 2 cm in longest dimension. The shore exposures, on the east 24 limb of the Appin Syncline, are complemented by those on the west 25 limb in the Onich Dry River Gorge GCR site. The latter occur along 26 the track in the valley bottom (Dubh ghlac) and on the hill to its 27 north-west (Druim nan Sleibhean), from NN 0242 6178 to NN 0314 28 6254. Numerous small exposures occur, particularly in a drainage 29 30 ditch to the north-west of the track, exhibiting ripple marks of 31 various dimensions and orientations on NE-striking bedding surfaces o 32 that dip at 50–75 to the south-east. The ripple marks commonly 33 have a wavelength of some 30 cm but smaller (2–3 cm scale) 34 structures are also seen. Most ripples have a sub-horizontal 35 orientation, but others with a down-dip orientation or a steep 36 pitch to the south-west or north-east can be observed locally and 37 produce interference patterns with the former set. Most 38 importantly, 20–30 cm-high cross-sets can be seen (e.g. by the 39 track at NN 0250 6180 and on the hill at NN 0265 6200), which 40 clearly demonstrate that the beds are right-way-up and young to the 41 south-east. The overall width of the quartzite outcrop suggests 42 that an intermediate-scale fold must occur in the unexposed ground 43 to the east of the track, as shown in Figure 18. 44 The Appin Limestone, which occupies most of the shore section from 45 its faulted junction with the Appin Quartzite to NN 0390 6126 46 (below the school), is a very varied member, which in this area 47 48 comprises the lower part of the Appin and Limestone 49 Formation. For the first 150 m west of the Appin Quartite it 50 contains much platy pink quartzite, probably repeated in folds, but 51 typically it consists of thin beds of cream-coloured metadolostone 52 (tens of centimetres thick) and lesser white metalimestone with 53 local interbeds of phyllitic semipelite, pink quartzite and 54 psammite. Small-scale graded bedding and cross-lamination in the 55 steep SE-dipping beds are well seen near the faulted junction with 56 the Appin Quartzite and in psammite interbeds 150 m to the west. 57 These sedimentary structures mostly indicate younging to the south- 58 east, so that the beds are right-way-up (for explanation, see 59 structural detail below). A pink microdiorite dyke at NN 0395 6125 60 61 62 63 64 65 1 2 3 4 marks the core of an anticline in metalimestone, exhibiting south- 5 east younging on its south-east limb. 6 For a further 200 m, on the north-west limb of the anticline, thin 7 metalimestones are seen in gradational contact with the upper part 8 of the Appin Phyllite and Limestone Formation, here called the 9 Appin Phyllite. The latter consists of grey, phyllitic semipelites 10 containing numerous intercalations of schistose calcsilicate rock 11 12 and purer metalimestone as well as flaggy quartzite. The Appin 13 Phyllite to the west is exposed continuously between NN 0371 6127 14 and NN 0318 6129 (below the Onich Hotel). Centimetre-scale graded 15 bedding and cross-laminations are common and these structures show 16 consistent younging to the north-west in the overturned, steeply 17 SE-dipping beds. SE-verging minor folds of thin metalimestones are 18 seen from NN 0371 6127 to the centre of the main synclinal fold, 19 which is marked by some 50 m of younger dolomitic metalimestone at 20 NN 0342 6131 (Figures 17, 18). The excellent exposures at the far 21 west end of the section, below the hotel, show the same sedimentary 22 structures, including slump-folds, younging to the east in 23 shallower SE-dipping beds on the west limb of the syncline. 24 Isolated exposures of Appin Phyllite, Appin Limestone and Appin 25 Quartzite are seen on the west limb, at NN 0302 6140 (south of the 26 road), NN 0259 6146 and NN 02416153 (north of the road), 27 respectively. 28 As demonstrated above, the gross stratigraphical repetition across 29 30 the site, together with change in the dip of bedding and the 31 abundant sedimentary way-up indicators, clearly reveals the 32 presence of a major tight syncline, with an overturned south-east 33 limb. This is interrupted by one conspicuous intermediate-scale 34 anticline at NN 0393 6125 and a faulted-out syncline as described 35 above. In addition, the shore section presents an unusual wealth 36 of first phase (D1) minor structures (folds and cleavage/bedding 37 relationships) that can be used to demonstrate the presence and 38 geometry of the major syncline, the anticline and several other 39 smaller-scale folds, which are described below from east to west 40 (Figures 17, 18). 41 The outcrops of the Ballachulish Slates at the east end of the 42 shore section are too isolated to be useful in the interpretation 43 of the major first fold geometry. However, these outcrops, do 44 contain reversals of younging and of cleavage/bedding relationships 45 that demonstrate the presence of intermediate-scale F1 folds. The 46 exposures also contain a later crenulation cleavage and minor 47 48 folds, which indicate that a major synform is to be expected to the 49 east of the GCR site. 50 Proceeding westwards, the Appin Quartzite outcrop at the eastern 51 end of the shore section is isolated by a fault or slide along its 52 western margin with the Appin Limestone, marked by a few 53 centimetres of platy schists with lens-shaped quartz-veins. The 54 dolomitic metalimestone a few metres west of the fault contains 55 both E- and W-verging, vertically-plunging, isoclinal folds; W- 56 verging minor structures in the E-younging rocks for 150 m to the 57 west show that a major- or intermediate-scale syncline must be 58 replaced by the fault, as illustrated in the cross-section of 59 Figure 18. 60 61 62 63 64 65 1 2 3 4 On the hill to the north-east of the shore section, at NN 4096 5 6175, a 10 m-thick psammitic bed within the Appin Limestone closes 6 in the probable core of this syncline (J.L. Roberts, personal 7 communication, 1977). The intermediate-scale anticline at NN 0393 8 6125 can be identified both from exposure of its hinge and from 9 changes in minor fold and cleavage vergence in the Appin Limestone 10 for 100 m around the hinge area. To the west, at NN 0375 6128, 11 12 another fold-pair with a 20 m wavelength can be identified from 13 vergence reversals in beds transitional into the Appin Phyllite. 14 On the foreshore, from NN 037 613 westwards, well-developed minor 15 structures in the Appin Phyllite consistently verge to the east, 16 indicating that a major syncline lies to the west (Figure 18); the 17 hinge of this fold can be identified in metalimestones at NN 0338 18 6130. Excellent minor structures can then be seen, verging west, 19 away from the hinge, in the remaining 150 m of exposure. Minor 20 folds throughout the shore section plunge on average at 10–30o to 21 the south-west, although locally they plunge at up to 50o to the 22 south-west and at low angles to the north-east. A penetrative 23 phyllitic cleavage, axial planar to the minor folds, dips at 60–80o 24 to the south-east. A down-dip stretching lineation is well seen on 25 the cleavage planes in the phyllitic rocks, emphasized by the 26 orientation of millimetre-long biotite crystals. 27 28 29 8.3 Interpretation 30 31 The Onich GCR sites provide good type examples of the lithologies 32 and of sedimentary structures within the Ballachulish Slate and 33 Appin Quartzite formations. The exposures of the Appin Limestone 34 and the Appin Phyllite and their transitional junction are also 35 unrivalled for their clarity and detail. In contrast to exposures 36 of these two stratigraphical units farther to the south-west (e.g. 37 at the Arsheal Peninsula GCR site), at Onich they exhibit 38 considerable interleaving of phyllitic rocks, metadolostone and 39 grey metacarbonate rock as well as of local psammites. Some of 40 this interleaving can be attributed to tight interfolding (see 41 below) but it did lead Bailey (1960, pp. 37–38) to abandon the 42 distinction of the two units and in most areas they are now 43 combined formally as the Appin Phyllite and Limestone Formation. A 44 separate Appin Limestone Member is recognized only where it can be 45 clearly distinguished. 46 Structurally, there can be no doubt that the minor folds and 47 48 cleavage are of the first regional generation (D1) and they clearly 49 demonstrate that the major syncline, the Appin Syncline of Bailey 50 (1960) is of this age. Although strictly this D1 age can only be 51 demonstrated from thin section examination (Roberts, 1974; Treagus, 52 1991) the cleavage as viewed in the field is penetrative and the 53 folds do not refold any earlier cleavages, lineations or folds. 54 It should be noted that Hickman (1978) regarded the Appin Syncline 55 as a D2 structure, partly on regional considerations and partly 56 from comparisons with observations on the structure farther to the 57 south-west (see the Ardsheal Peninsula and Lismore Island GCR site 58 reports). His explanation for the penetrative nature of his ‗S2‘ 59 slaty cleavage was that the regional first cleavage, seen in the 60 61 62 63 64 65 1 2 3 4 schists to the east (see the Rubha Cladaich and Nathrach GCR site 5 reports) has been overprinted to become unrecognizable at Onich. 6 7 8.4 Conclusions 8 9 The two GCR sites at Onich are important for two reasons. Firstly, 10 they contain unrivalled sections of four stratigraphical units of 11 12 the Ballachulish Subgroup–the Ballachulish Slates, the Appin 13 Quartzite, the Appin Limestone and the Appin Phyllite. These are 14 rich in detail revealing the nature of the original sediments, 15 particularly structures showing the original order of deposition. 16 Secondly, they contain a wealth of minor structures (folds and 17 cleavages) resulting from the deformation of the sedimentary rocks. 18 The latter structures can be used to demonstrate the D1 relative 19 age, position and shape of a major fold having an outcrop width of 20 several kilometres. This Appin Syncline is a key component of the 21 overall structure of the Grampian mountain belt. 22 23 9 ARDSHEAL PENINSULA 24 (NM 961 555–NN 007 579) 25 26 J.E. Treagus 27 28 29 30 9.1 Introduction 31 32 The Ardsheal Peninsula GCR site, on the south-east side of Loch 33 Linnhe, gives a superbly-exposed section across five 34 stratigraphical units in the Appin Group, from the Appin Transition 35 Formation of the Ballachulish Subgroup, up into the Cuil Bay Slates 36 of the Blair Atholl Subgroup. Although the whole outcrop width of 37 the F1 Appin Syncline is represented on the peninsula, the almost 38 continuous clean coast section described here is the on the north- 39 west limb. Minor structures related to the major F1 fold are 40 exceptionally well exposed, and minor structures belonging to the 41 regional D2 phase are superimposed on the early structures in the 42 eastern part of the GCR site. 43 The area of the GCR site was described in general by Bailey 44 (1960), who was the first to recognize the major structure as an 45 upward-, NW-facing primary syncline. It was subsequently described 46 in detail by Treagus and Treagus (1971), after Bowes and Wright 47 48 (1967) had given a very different account of the structure in their 49 examination of the setting of the igneous rocks of the area. The 50 latter are discussed in site reports in the Caledonian igneous 51 rocks of Great Britain GCR volume (Stephenson et al., 1999). 52 53 9.2 Description 54 55 The five most instructive areas of exposed Dalradian rocks in this 56 GCR site are situated along the south-west and north-west coasts of 57 the peninsula (Figure 20, localities 1–5). The youngest formation, 58 the Cuil Bay Slates, is exposed at locality 1, the type locality 59 for this formation on the north-west side of Cuil Bay, between NM 60 9763 5539 and NM 9724 5544. At the east end of these exposures, on 61 62 63 64 65 1 2 3 4 a small peninsula below a fishing , centimetre-thick graded 5 beds of grey silt to black mud (now slaty semipelite and pelite) 6 are particularly well seen. These beds are folded into 3 m- 7 wavelength, tight to open, upright folds, which plunge to the 8 south-west, face upwards and verge to the north-west (Figure 21b). 9 They have an axial-planar slaty cleavage dipping at 50–60o to the 10 south-east and a stretching lineation of elongate chlorite- 11 12 muscovite stacks, which pitches steeply to the north-east on the 13 cleavage. Elsewhere, the slaty cleavage is strongly affected by a 14 second generation of open folds, particularly well seen on the 15 south-east side of this small peninsula, Rubha Beag. These folds 16 also plunge to the south-west and have a north-west vergence, but 17 they are related to a crenulation cleavage, which dips at 60–70o to 18 the north-west and cross-cuts the earlier folds. The effect of 19 this folding is to rotate the earlier slaty cleavage into flat- 20 lying attitudes and, locally, to refold the earlier fold-set. The 21 first cleavage surfaces are strongly crenulated and kinked; some 22 kinks are due to original refraction through the semipelite and 23 pelite beds, others result from the second deformation and yet 24 others are related to later deformation associated with the 25 intrusion of dykes. 26 The junction with the next oldest stratigraphical unit, the Appin 27 Phyllite, is not exposed; but this unit occupies the whole of the 28 next small peninsula to the west, Rubha Meadhonach (locality 2). 29 30 Bedding, which is less easy to see in these rather uniform grey 31 phyllitic semipelites, dips steeply to the south-east and locally 32 exhibits upward-younging graded units and ripple-drift cross- 33 bedding, particularly on the north-west side of the outcrop. The 34 first cleavage dips steeper to the south-east than the bedding, and 35 intersections plunge gently to the south-west. The second cleavage 36 and folds are not strongly developed, but the intersection of the 37 second cleavage with the first plunges at varying angles to the 38 north-east. Kink bands, commonly conjugate, are well developed. 39 The westernmost subsidiary peninsula of the GCR site, Rubha Mor, 40 exposes a further three stratigraphical units, the Appin Limestone 41 and the Appin Quartzite, as well as the distinctive transitional 42 beds to the Ballachulish Slates (the Appin Transition Formation). 43 The Appin Limestone and its steep SE-dipping junction with the 44 Appin Quartzite are exceptionally well displayed on the south-east 45 coast of Rubha Mor between NM 966 557 and NM 963 556 (locality 3). 46 The youngest beds, on the seaward side, are mixed metacarbonate 47 48 rocks, quartzites, phyllitic semipelites and psammites (Figure 49 21b), followed downwards by a clear sequence of white quartzite, 50 grey metacarbonate rock, cream dolomitic metalimestone and 51 phyllitic semipelite, each a metre or two thick, before the well- 52 exposed junction with the Appin Quartzite is reached high on the 53 cliff. These latter beds are seen in a spectacular fold-pair in a 54 15 m-long section at the south-west end of their outcrop. Here, 55 disharmonic minor folds and refracting penetrative axial-planar 56 cleavage are beautifully displayed; the vergence of minor folds and 57 cleavage/bedding is to the north-west and the plunge is 20–40o to 58 the south-west (Figure 21b). 59 The Appin Quartzite consists of thick-bedded (up to 2 m) white 60 quartzite with quartz and feldspar pebbles up to 20 mm across in a 61 62 63 64 65 1 2 3 4 gritty matrix; the clasts are only weakly elongated in a poorly 5 developed cleavage in the finer grained beds. The quartzite 6 commonly displays cross-beds younging to the south-east, which are 7 especially well-seen near the top of the formation at NM 9640 5565 8 (locality 3) and near the base on the north-west side of the 9 promontory at about NM 968 557 (locality 4). The Appin Transition 10 Formation is seen on the shore of from about NM 963 558 11 12 to NM 971 565 (locality 5). These steeply SE-dipping beds of 13 striped psammites, semipelites and graphitic pelites display a 14 variety of SE-younging sedimentary structures including small-scale 15 grading, ripple-drift cross-bedding, load structures, slump-folds 16 and sedimentary dykes. Cleavage in the pelitic beds is 17 penetrative. The vergence of minor folds and cleavage/bedding 18 intersection is to the north-west and the plunge is 10–30o to the 19 south-west. 20 21 9.3 Interpretation 22 23 All of the stratigraphical units that crop out at localities 1–5 24 exhibit the same north-west vergence of both the earliest folds of 25 bedding and of the cleavage/bedding intersection related to those 26 folds. According to Bailey (1960), Voll (1964) and Treagus and 27 Treagus (1971), they lie on the north-west limb of the regional 28 fold, the Appin Syncline, which all these authors agree is one of 29 30 the early (D1) fold-set. The hinge-zone and south-east limb of 31 this fold, affecting the Appin Phyllite and Limestone Formation, is 32 best seen, up plunge to the north-east, in the Onich GCR sites. A 33 generalized structural profile of the fold as it affects the rocks 34 of the Ardsheal and Onich GCR sites is illustrated in Figure 18, 35 which shows the five locations discussed above. The later folds 36 and associated crenulation cleavage, which are seen at Ardsheal on 37 the north-west limb of the Appin Syncline, but also on its south- 38 east limb at Onich, show a consistent north-west vergence on both 39 limbs. These folds belong to the regional D2 phase of Treagus and 40 Treagus (1971) and are subsidiary to the major Stob Ban Synform, 41 seen to the east (see the Tom Meadhoin and Doire Ban and Stob Ban 42 GCR site reports). 43 The paper by Treagus and Treagus (1971) contradicted the earlier 44 view of Bowes and Wright (1967) that the syncline is a third-phase 45 structure associated with the retrograde metamorphism in the area. 46 Bowes and Wright proposed that ‗elongate compositional blebs‘ and 47 48 small isoclinal fold noses in semipelites within the Appin Phyllite 49 are evidence of an earlier D1 phase. Mica flakes that cut across 50 these earlier features were said to be attributable to a D2 phase. 51 The lineation resulting from the mica flakes was said to be folded 52 around the Appin Synform, which was therefore interpreted as a D3 53 phase. No locations nor detailed measurements of these structures 54 were given and no evidence was found by Treagus and Treagus (1971), 55 or has been found since, to support these views. Importantly, 56 thin-section examination of the pelites from the Ardsheal Peninsula 57 GCR site has shown that the penetrative cleavage that is axial 58 planar to the dominant minor folds and to the major syncline was 59 developed as part of the regional, progressive, chlorite-muscovite 60 metamorphism. 61 62 63 64 65 1 2 3 4 9.4 Conclusions 5 6 At the Ardsheal Peninsula GCR site it can be demonstrated that the 7 Appin Syncline, one of the major folds in the western part of the 8 Grampian Fold-belt, was produced as part of the earliest 9 deformation of these rocks. The site provides superlative examples 10 of the minor folds produced by this deformation as well as evidence 11 12 of the effects of a later period of deformation, superimposed on 13 the earlier one. The site also provides excellent clean exposures 14 of five stratigraphical units in the upper part of the Appin Group 15 with clear evidence of the nature of the original sediments; it 16 contains the type locality of one of them–the Cuil Bay Slates. The 17 site is a necessary complement to the GCR sites at Onich to give a 18 complete cross-section of the Appin Syncline. The site is of 19 exceptional importance educationally and is used extensively by 20 undergraduate parties and students undertaking mapping projects. 21 22 10 SOUTH COAST, LISMORE ISLAND 23 (NM 798 386–NM 784 366–NM 813 383) 24 25 J.E. Treagus 26 27 28 29 10.1 Introduction 30 31 The South Coast, Lismore Island GCR site comprises the coastal 32 exposures and much of the inland outcrop at the south-western end 33 of the elongate island of Lismore to the north of Oban (Figure 22). 34 It is important primarily for its unique exposures of the Lismore 35 Limestone, one of the formations of the Blair Atholl Subgroup of 36 the Appin Group. The substantial middle and upper divisions of the 37 formation are represented; these comprise dominant metalimestone 38 with lesser slaty pelite and transitional facies. Structurally, 39 the site lies in the core of the F1 Appin Syncline, a major 40 structural feature of the western side of the Grampian Fold-belt, 41 which is also the focus of interest of the Ardsheal Peninsula and 42 Onich GCR sites, at progressively lower structural levels. On 43 Lismore, the D1 structure is strongly modified by D2 structures. 44 The geology of the island was described briefly in the Geological 45 Survey memoir by Lee and Bailey (1925) and both the stratigraphical 46 and structural features have been described subsequently in great 47 48 detail by Hickman (1975, 1978). The following descriptions are 49 largely based upon the work of Hickman, with additional 50 observations by the author. 51 52 10.2 Description 53 54 The Lismore Limestone Formation is estimated to be about 1 km thick 55 and the sequence of blue-grey metalimestone and minor black slaty 56 pelite has been divided into fifteen members by Hickman (1975). 57 These members can be grouped into three distinct lithological 58 ‘units’. Of these only the middle and upper units are exposed within 59 the area of the GCR site (Figure 22). The middle unit (about 450 m 60 thick) comprises three metalimestone-pelite cycles, each containing 61 62 63 64 65 1 2 3 4 a lower metalimestone member (50–200 m thick), a transitional banded 5 argillaceous metalimestone member (20 m thick) and an upper pelite 6 member (10–20 m thick). The metalimestone is blue-grey, 7 recrystallized and to varying degrees graphitic and pyritiferous, 8 with partings at 5–20 cm intervals. The transitional beds have 9 individual beds, typically 3–10 cm thick, which are graded. The 10 pelite members are grey, highly carbonaceous and pyritiferous with 11 12 a low carbonate content. The upper unit has a lower metalimestone 13 member (300 m) and an upper banded argillaceous metalimestone 14 member (50 m), which rarely preserves festoon structures and load 15 casts. Cross-stratification is rarely preserved in the 16 metalimestones and isoclinal folds in the pelites and argillaceous 17 metalimestones might be slump structures. 18 The whole sequence across the upper unit and the upper two cycles 19 of the middle unit may be seen within the GCR site in a traverse of 20 the craggy inland outcrops from Bagh Clach an Dobhrain (NM 799 377) 21 to Miller’s Port (NM 812 372). Excellent clean exposures of 22 metalimestone are seen all around the coast on the wave-cut 23 platforms, those of the upper unit on the north-west side of the 24 island (e.g. at NM 789 364) and those of the middle unit on the 25 south-east side (e.g. south-west of Miller’s Port at NM 812 372). 26 The banded argillaceous metalimestone of the upper unit is well 27 seen at Bagh Clach an Dobhrain (at NM 799 375) and argillaceous 28 metalimestones and pelites of the middle unit are seen inland on 29 30 the ridge of Druim na Curra, near Fiart Farm (at NM 802369 and NM 31 803 370 respectively). The continuous ‘slate belt’ that passes 32 through the Loch Fiart area, identified on both the original (1923) 33 and the more-recent (1992) Geological Survey maps, was originally 34 regarded as the eroded crest of an anticline, with the pelite 35 underlying metalimestone everywhere on the island (Lee and Bailey, 36 1925). However, Hickman (1978) demonstrated that the pelite is an 37 interbedded part of the stratigraphical succession. 38 The structure of the metalimestones within the GCR site is 39 dominated by upright asymmetric tight to open folds with a 40 wavelength of 0.5–20 m (Figure 23). The folds typically plunge at 41 20–50o to the south-west and have a strongly developed, steeply 42 dipping, axial-planar crenulation cleavage in the slightly 43 argillaceous metalimestone beds. Stretched pyrite blebs pitch 44 steeply north-east on the cleavage. This cleavage is axial planar 45 to folds of a bedding-parallel phyllitic cleavage. Hickman (1978) 46 mapped a major antiform-synform fold-pair (A and B on Figure 22) 47 48 related to a change in vergence of minor folds associated with the 49 dominant later deformation. Hickman (1978) also drew the trace of 50 a tight syncline through the site area (fold C on Figure 22), 51 related to the earlier deformation. The evidence for the early 52 syncline lies in a closure of the stratigraphical units in the area 53 to the north-east of the GCR site, where it is cross-cut by the 54 later folds and cleavage (see below). 55 56 10.3 Interpretation 57 58 The detailed stratigraphical succession within the Lismore 59 Limestone Formation contains variations between pure metalimestone 60 and pelite very similar to those of the Blair Atholl Dark Limestone 61 62 63 64 65 1 2 3 4 and Dark Schist Formation of the Strath Fionan GCR site, with which 5 it can be correlated confidently (Lee and Bailey, 1925). The 6 obvious differences between the two GCR sites are the stronger 7 representation of transitional facies between pure metalimestone 8 and pelite in the former and of thicker pelite members in the 9 latter. Unsurprisingly, the Lismore GCR site, with its lower grade 10 of metamorphism, reveals more sedimentary structures, such as 11 12 festoon bedding, cross-bedding and load casts. The common graded 13 bedding however, which might be expected to survive the higher 14 grade metamorphism, has not been recorded in the Schiehallion area. 15 The main feature of structural interest on Lismore is the NE- 16 trending syncline that crops out through the centre of the GCR site 17 and the Loch Fiart area (Figure 22, fold C). This fold was 18 assigned to D1 by Hickman (1978) on the basis that the dominant D2 19 minor structures are superimposed across the stratigraphical 20 repetition that marks the position of the D1 fold. Hickman did not 21 provide vergence data to support the position or age of the 22 syncline within the GCR site area. However, in the area that lies 23 on the south-east limb of this fold, the phyllitic cleavage is seen 24 (though rarely) at a small angle to bedding and axial-planar to 25 small, 10 cm-amplitude isoclines (e.g. at Miller‘s Port, 812372). 26 The paucity of vergence information on these folds and cleavage 27 across the trace of the syncline does not allow confirmation of the 28 position or age of the syncline. However, the vergence of the 29 30 later folds is consistently towards the north-west across the trace 31 of the fold (see the cross-section on Figure 22) and the steeply 32 SE-dipping rocks on its south-east limb are consistently inverted 33 and face down on the steeper dipping later cleavage. 34 The trace of syncline C in the north of the island, as drawn by 35 Hickman (1978), can be extrapolated to the north-east, through the 36 Island of Shuna directly into that of the F1 Appin Syncline at Cuil 37 Bay and Onich (see the Ardsheal Peninsula and Onich GCR site 38 reports). The vergence and plunge of the D2 structures is 39 consistent with observations of their relations to the Appin 40 Syncline at the above sites, and to the major F2 Stob Ban Synform 41 to the east (see also the Tom Meadhoin and Doire Ban and Rubha 42 Cladaich GCR site reports). Hickman (1978), however, considered 43 that the Appin Syncline, which he regarded as a D2 structure, 44 should be correlated with the major F2 synform A on Figure 22. 45 However, the trace of this fold, as shown by Hickman (1975) in the 46 north of Lismore, does not extrapolate well with the projected 47 48 trace of the Appin Syncline from Ardsheal through the island of 49 Shuna. 50 51 10.4 Conclusions 52 53 The South coast, Lismore Island GCR site is the type locality for 54 the Lismore Limestone Formation of the Blair Atholl Subgroup, which 55 can be examined in detail in superb coastal and inland exposures. 56 The information gained from the cycles of limy and muddy 57 sedimentation that made up the formation is essential to the 58 understanding of the development of the shallow shelf on which the 59 early Dalradian sequence was deposited, particularly in comparison 60 with other sites in the area and elsewhere in the Central Grampian 61 62 63 64 65 1 2 3 4 Highlands. The site also provides, from its two sets of minor 5 structures, valuable evidence for the location, identity and 6 geometry of major F1 and F2 folds in the region, which play a 7 fundamental role in the development of the Grampian Mountain-belt. 8 The Appin Syncline, the most important major F1 fold on the north- 9 west side of the mountain-belt, can be traced through this GCR site 10 and its geometry can be established. The later D2 folds and 11 12 cleavage are particularly well displayed and demonstrate a 13 consistent geometry down this side of the mountain-belt from the 14 GCR sites at Onich and the Ardsheal peninsula into the Lismore 15 Island GCR site. The site has great potential for future 16 stratigraphical as well as structural research. 17 18 11 CAMAS NATHAIS 19 (NM 874 370–NM 875 382) 20 21 P.W.G. Tanner 22 23 24 11.1 Introduction 25 26 This GCR site lies 7 km north of Oban in the Benderloch area and is 27 located on the raised beach platform around the head and south-east 28 side of a narrow inlet of the sea, Camas Nathais. Well-scoured 29 30 rock exposures display an excellent example of a ductile fault, the 31 Benderloch Slide (Litherland, 1980). The slide contact, which is 32 the main feature of interest at this locality, is well exposed for 33 a strike distance of over 100 m and both the plane of movement, and 34 the structures in the rocks on either side of it, can be examined 35 in three dimensions. Such contacts are rarely seen in the 36 Highlands, and this example is exceptional for its clarity, and for 37 the quality of the rock exposure. 38 For many years the geology of this area was the focus for a 39 fundamental geological debate between the followers of E.B. Bailey 40 (1922, 1953), who interpreted the abrupt east–west changes in rock 41 type seen on a regional scale in terms of nappe tectonics, and 42 those who supported G. Voll (1964), who invoked extreme local 43 facies changes. Bailey considered that radically different 44 stratigraphical successions had been brought together by 45 syntectonic faults called slides, the slide at Camas Nathais being 46 correleated with a major tectonostratigraphical dislocation in the 47 48 south-west and Central Grampian Highlands, later named the ‗Iltay 49 Boundary Slide‘ (Stephenson et al., 2013a). On the other hand, 50 Voll concluded that the lithological differences seen across the 51 area are of sedimentary origin, and that the rocks constitute a 52 single, unbroken succession. The situation was resolved by 53 Litherland (1980, 1982) who, by means of detailed field mapping, 54 demonstrated that the so-called ‗Banded Leven Schists‘ of Bailey 55 (1922) could be subdivided into the ‗true‘ Leven Schists of the 56 lower Appin Group to the west, and rocks belonging to the lower 57 part of the Argyll Group to the east. However, he confirmed that 58 these two parts of the Dalradian succession had been brought 59 together by a major tectonic break, which he named the Benderloch 60 Slide. 61 62 63 64 65 1 2 3 4 At the head of Camas Nathais, the Benderloch Slide appears to 5 place part of the Ballachulish Limestone (Appin Group) against the 6 base of the Argyll Group, so excising a significant part of the 7 Appin sequence. The plane of movement is marked both by a strong 8 lithological contrast between black graphitic slaty pelites on one 9 side and orange-yellow metadolostone and pale grey quartzite on the 10 other, and by a zone many metres thick of sheared rocks that 11 12 display a pronounced stretching lineation. The tectonic break is 13 inferred to have developed early in the tectonic history of the 14 area, with both the fault plane and the adjacent rocks having been 15 subjected to several phases of later movement. The Benderloch 16 Slide is no longer regarded as part of an intraregional ‗Boundary 17 Slide‘, but it can be correlated with the Ballachulish Slide found 18 farther north (Rast and Litherland, 1970). The GCR site reports 19 for St John’s Church and for Tom Meadhoin and Doire Ban also 20 discuss the possibility that the slide might be of synsedimentary 21 origin and was only re-activated during later ‗tectonic‘ events. 22 23 11.2 Description 24 25 The geological setting of this site is shown on Figure 24, with the 26 Benderloch Slide being strikingly exposed at (NM 87626 37938), near 27 the head of the bay (Figure 25). The structural break was first 28 recognized by Bailey, who stated that ‗...on the western side of 29 30 Garbh Ard the margin of the quartzite, where exposed upon the 31 foreshore, is separated from the Appin Limestone by a few feet of 32 black slates and limestone but, unfortunately, there are signs of 33 considerable movement having taken place near the junction line’ (in 34 Kynaston and Hill, 1908, p.36). Bailey later re-assigned the rocks 35 described above as ‘Appin Limestone’, to the Ballachulish Limestone. 36 The slide plane dips at 42–46° to the north-west and lies within a 37 sequence of rocks that all dip steeply (50–75°) in a north-westerly 38 direction. The junction between a 0.5–1.0 m-thick band of 39 boudinaged, discontinuous orange-weathering metadolostone below, 40 and a black pelite-metalimestone unit above, marks the actual 41 dislocation (Figure 26). When the slide plane is viewed at beach 42 level from the north-west, flat surfaces of intensely lineated 43 quartzite are seen in erosional windows carved out of this 44 metadolostone sheet. 45 On the Fionn Ard peninsula, north-west of Camas Nathais, there is 46 a wide outcrop of dark, finely banded semipelitic and pelitic rocks 47 48 belonging to the Leven Schists. At the head of the bay, units of 49 grey-green phyllitic semipelite and cream-coloured, finely 50 laminated metadolostone structurally underlie these rocks, but the 51 contact is not exposed (Figure 24). The metadolostone weathers 52 orange-brown and forms thick units, one of which was formerly 53 quarried at the site, but also occurs interbedded on a metre-scale 54 with semipelite. The lowermost 10 m or so of rock above the slide 55 consists of dark grey to black phyllitic rocks, which contain a 56 band of black metalimestone less than a metre thick (L on Figure 57 25). The metadolostone-‗phyllite‘ sequence, and the dark 58 ‗phyllite‘ and metalimestone, have been correlated, respectively, 59 with the lower and upper parts of the Ballachulish Limestone 60 Formation (Bailey, 1922; Litherland, 1980), but Litherland (1982) 61 62 63 64 65 1 2 3 4 subsequently correlated the dark phyllitic unit with the much 5 younger Lismore Limestone. 6 The rocks below and south-east of the slide plane consist largely 7 of quartzite and were subdivided into the Baravullin Quartzite, 8 Castle Dolomitic Flags, and Tralee Bay Quartzite by Litherland 9 (1980). The Baravullin Quartzite is pale grey to white in colour 10 and occurs in the footwall immediately below the slide. It has 11 12 been intensely brecciated, but evidence of crudely defined bedding 13 planes can still be detected, defining units up to 40 cm thick 14 separated by thin phyllitic layers or films. The quartzite is 15 underlain structurally by the Castle Dolomitic Flags, which form 16 the spine of the Garbh Ard peninsula and consist of a banded 17 alternation of buff-coloured quartzite ribs with dolomitic 18 semipelite and greenish phyllitic rock. 19 A stretching lineation is strongly developed in rocks adjacent to 20 the slide contact (inset to Figure 26), and dominates the fabric 21 for a distance of some 10 m into the hanging-wall semipelites and 22 pelites. It has a consistent orientation and plunges at an average 23 of 34° to 352°. The orange-weathering metadolostone that lies 24 immediately below the slide plane has been boudinaged, with boudin 25 necks aligned normal to the stretching lineation. This lineation 26 is deformed by a crenulation cleavage that affects rocks on both 27 sides of the slide. Evidence for later movement on the fault plane 28 is given by a veneer of silicified fault breccia, up to 15 cm 29 30 thick, found in contact with the metadolostone (Litherland, 1982). 31 It contains fragments of randomly orientated, crenulated slaty 32 rocks similar to those found adjacent to the slide. 33 In the immediate footwall to the slide, beneath the metadolostone 34 bed, there is a 1 m-thick zone of highly fractured quartzite, with 35 numerous quartz veins, underlain by a considerable thickness of 36 pervasively fractured rock. Throughout this fractured zone the 37 quartzite consists of interlocking fragments 1 to 2 cm across, 38 separated by deeply weathered seams and fissures, which are of 39 possible pressure-solution origin. In some beds the fragments are 40 equidimensional, but in general they are crudely aligned at a 41 moderate angle to bedding. In three dimensions they have extremely 42 flattened and elongated pancake shapes, and have a veneer of white 43 mica on their surfaces. Where minor folds are developed in these 44 rocks, the fragments are aligned roughly parallel to the axial 45 planes of the folds and to the penetrative cleavage in the more- 46 micaceous bands, so that they appear to define a rough cleavage. 47 48 This relationship is seen to within 2 m of the slide plane. 49 Deeper into the footwall, bedding in the Baravullin Quartzite and 50 Castle Dolomitic Flags is cut by a less-steep penetrative main 51 cleavage dipping to the south-west, which is in turn cut and 52 reworked by a crenulation cleavage that dips at 20–35° to the 53 north-west, and is associated with minor folds up to a few tens of 54 metres in wavelength. 55 In the hanging wall, metre-scale upright folds are associated with 56 a penetrative axial planar cleavage in the more-micaceous bands, 57 which dips steeply to the north-west. In places there is clear 58 evidence that a generation of minor folds, with an axial-planar 59 penetrative cleavage, pre-dates the main cleavage. These early 60 folds are clearly seen in the laminated metacarbonate rocks, which 61 62 63 64 65 1 2 3 4 develop thin axial-planar quartz veins. As in the footwall rocks, 5 the main cleavage is cut by a spaced crenulation cleavage, which 6 dips at a low angle (30–40°) in the same direction and is 7 accompanied by minor folds plunging to the south-west. Calcareous 8 bands in the phyllitic rocks preserve the main cleavage but are 9 commonly reduced to a series of sigmoidally shaped lenses by the 10 development of a cross-cutting, later, spaced pressure-solution 11 12 cleavage having the same geometry as the crenulation cleavage in 13 the phyllitic rocks. 14 15 11.3 Interpretation 16 17 The pelites, semipelites, psammites and dolomitic rocks in the 18 hanging wall to the slide at Camas Nathais (units C to F, Figure 19 25) have been correlated with the lower part of the Ballachulish 20 Limestone by previous workers, and the dark phyllitic pelite and 21 black metalimestone beneath them (unit D, Figure 25) with either 22 the upper part of the Ballachulish Limestone (Bailey, 1922; 23 Litherland, 1980), or with the Lismore Limestone (Litherland, 24 1982). 25 South-east of the Benderloch Slide, the Barravullin Quartzite, 26 Castle Dolomitic Flags and Tralee Bay Quartzite can be correlated 27 with elements of the Islay Subgroup on Islay and Jura. Farther to 28 the north-east, the Tralee Bay Quartzite is overlain by the Selma 29 30 Black Slates and the Selma Breccia (Figure 24), rocks equated 31 respectively with the Jura Slate (see the Kilnaughton Bay GCR site 32 report) and Scarba Conglomerate (see the Lussa Bay GCR site 33 report). Crucial to this assignment of rocks south-east of the 34 slide to the Argyll Group is the correlation of the Baravullin 35 Boulder Bed with the Port Askaig Tillite (see the Garvellach Isles 36 GCR site report). The Baravullin Boulder Bed is not exposed at 37 Camas Nathais but, according to Litherland (1980), it crops out 38 inland 3 km to the north-east of the GCR site, and on the south- 39 east side of the projected line of the slide (Figure 24). A thin 40 sliver of the Islay Limestone (locally called the Baravullin 41 Dolomite) was also reported from this locality. These 42 stratigraphical correlations suggest that (1) the Benderloch Slide 43 lies at, or close to, the base of the Argyll Group, and (2) 44 following Litherland (1980), most of the Ballachulish Subgroup, and 45 all of the Blair Atholl Subgroup, are cut out at the line of the 46 slide. 47 48 Evidence from the area around Loch Creran, well to the north of 49 Camas Nathais, shows that the main movement on the Benderloch Slide 50 occurred before the first deformation. The latter affected both 51 the footwall and hangingwall, and was responsible for the formation 52 of the early penetrative cleavage. In the Loch Creran area the S1 53 slaty cleavage cuts across the plane of the slide (Voll, 1964) and 54 the slide is locally folded by map-scale F1 folds (Litherland, 55 1982). 56 The Dalradian sediments preserved in this area were laid down in 57 an unstable environment at the margin of a large basin and the 58 slide could represent mass movement of part of the bedded sequence 59 down the submarine slope, into the developing basin. Evidence for 60 basin-margin instability and slumping of material at a slightly 61 62 63 64 65 1 2 3 4 later time is clearly preserved in the form of the Selma Breccia 5 described in the Port Selma GCR site report. The evidence that 6 brecciation of the quartzite in the footwall to the slide at Camas 7 Nathais occurred before an early phase of deformation is compatible 8 with these conclusions, although the relationship of the earliest 9 penetrative cleavage to the slide cannot be demonstrated directly. 10 However, the stretching lineations associated with the slide 11 12 movement are deformed by the low-angle, NW-dipping crenulation 13 cleavage that affects both the footwall and hangingwall rocks at 14 this site, so placing a minimum age on the main slide movement. 15 Some rejuvenation of the faulting then occurred after the 16 crenulation event, as is witnessed by disorientated fragments of 17 already crenulated phyllitic pelite and semipelite in the late 18 fault breccia. 19 New fieldwork carried out to date suggests that the rocks on both 20 sides of the Benderloch Slide have been affected by the same 21 sequence of structural events. Correlation of the early penetrative 22 cleavage and the crenulation cleavage between footwall and hanging 23 wall, and with the D1–D4 deformation phases documented in the wider 24 area by Litherland (1982) is premature. It needs confirmation by a 25 more-complete study of the geometry of the structures, and of the 26 tectonic fabrics in thin section. The stratigraphical correlation 27 of the hanging wall sequence with the known Dalradian succession in 28 the area, the nature of the fault rocks, and the shear sense on the 29 30 slide plane, also remain to be determined. 31 32 11.4 Conclusions 33 34 At Camas Nathais a large fault, termed the Benderloch Slide, has 35 juxtaposed two different parts of the Dalradian sequence. As the 36 rocks within such fault-zones are generally physically weaker than 37 the country rocks, they are easily removed by erosion when they 38 become exposed at the Earth’s surface. In this way, crucial 39 evidence is destroyed that could have been used to determine the 40 nature and amount of movement that had taken place on the fault, 41 and the relative timing of the event. Hence, it is only in 42 exceptional cases, such as are seen at this GCR site, that the zone 43 of crushed and distorted rocks that forms at the contact between 44 the two fault blocks is preserved. This site is also of historical 45 importance as there has been a dispute over the nature and 46 significance of the junction between the two groups of rocks at 47 48 this locality, since E.B. Bailey first identified it in 1908. 49 It is important to know whether the fault movement occurred early 50 in the history of deformation, when the fault would be subjected to 51 later folding and overprinted by the main cleavage, or later, when 52 it would cut and displace the fold structures themselves. Field 53 evidence from outwith the GCR site, as summarised above, indicates 54 that the first movement on the fault took place before the main 55 deformation had begun, probably at about the time that the 56 sediments were being laid down. This plane of weakness could have 57 provided a focus for later, repeated movement, as found at Camas 58 Nathais, and its origin is in stark contrast to that of the Selma 59 Breccia, described in the Port Selma GCR site report. 60 61 62 63 64 65 1 2 3 4 This site has considerable potential for further research. The 5 fault rocks are well preserved, despite later folding and 6 metamorphism, and by studying the structural fabrics under the 7 microscope it should be possible to determine the relative 8 direction of movement of the two blocks. Many outstanding 9 questions still remain to be resolved before we have a clear idea 10 of the geometry, timing, and causes, of the displacements on this 11 12 major slide. 13 14 12 PORT SELMA, ARDMUCKNISH 15 (NM 902 381) 16 17 P.W.G. Tanner 18 19 20 12.1 Introduction 21 22 The Port Selma GCR site consists of a small rocky headland, and the 23 ridge inland from it, crowned by the ancient vitrified hill fort of 24 Dun MacSniachan. It lies south of Port Selma, near Benderloch 25 (Figure 27), and is well known for its excellent coastal exposures 26 of the Selma Breccia. Port Selma is one of a handful of localities 27 in the Dalradian Supergroup from which possible microfossils have 28 been reported. 29 30 The Selma Breccia belongs to the Easdale Subgroup and has been 31 correlated with the Scarba Conglomerate on Jura (see the Lussa Bay 32 and Kinuachdrach GCR site reports). It consists of five 33 metabreccia units, the uppermost of which contains numerous blocks 34 of limestone of a type that is exotic to the local area. It was 35 from this unit, and the interbreccia bed beneath it, that 36 Litherland (1975) recorded the presence of the fossil remains for 37 which the locality is best known. These microfossils, called 38 oncolites and catagraphs, are of uncertain origin but could be 39 fossil algae or bacteria. 40 This GCR site provides an important insight into the nature of the 41 depositional environment found at the margin of the Dalradian basin 42 during early Argyll Group times, shortly after a major late 43 Neoproterozoic (Port Askaig) glacial episode. It also serves as an 44 excellent case study for establishing the criteria that may be used 45 for distinguishing between breccias of sedimentary and tectonic 46 origin, a topic of considerable debate wherever such rocks occur 47 48 worldwide. The present case is of particular significance and 49 interest because the breccia has been deformed and metamorphosed 50 subsequently during an orogenic event, so obscuring some of its 51 original sedimentary features, and making diagnosis more difficult. 52 It was first described by Flett (in Kynaston and Hill, 1908) who 53 interpreted it as a ‗crush breccia‘, a rock formed by the in-situ 54 brecciation of originally intact rock layers. A detailed field and 55 microscopic study of the formation was made subsequently by 56 Litherland (1970, 1975) who concluded that it had formed by 57 submarine slumping of sediment. 58 59 60 61 62 63 64 65 1 2 3 4 12.2 Description 5 6 The rocks at this GCR site consist of the Selma Black Slates 7 overlain by the stratigraphically younger Selma Breccia (Litherland 8 1970, 1975); the regional geological setting is shown on Figure 24. 9 The rocks are affected by several upright folds of bedding, but 10 overall the succession dips and youngs to the south-east. 11 12 The Selma Black Slates, make up the northern part of the site 13 (Figure 27), and consist largely of slaty pelites that contain thin 14 layers and lenses of metamorphosed siltstone, sandstone, calcareous 15 gritty sandstone and limestone, with some of the carbonate lenses 16 being of possible diagenetic origin. The bedded metasediments 17 commonly show the effects of slump folding, with folds reaching 18 over a metre in amplitude; in places such movements culminated in 19 the formation of ‗balled-up‘ masses. The synsedimentary origin of 20 these structures is, in some cases, confirmed by the observation 21 that the base of an overlying bed cuts across the eroded tops of 22 the slump structures. In addition, their pretectonic nature is 23 revealed by the fact that both limbs of such folds are seen in thin 24 section to be transected by the early (S1) cleavage. 25 A package of gently dipping gritty metasandstone beds, each up to 26 30 cm thick and showing clear, right-way-up graded bedding is found 27 at NM 902 381. The dark slaty metamudstones making up the lower 28 part of this member pass up into a thick unit of pebbly 29 30 metamudstone (Figure 27). The metamudstone contains fine, barely 31 visible, silty laminations and some lenses of gritty metasandstone, 32 as well as isolated boulders up to 24 cm across. 33 The succeeding Selma Breccia was subdivided by Litherland (1970, 34 1975) into 5 metabreccia units separated by 4 interbreccia units, 35 which have a total thickness of up to 100 m. They are right way 36 up, and dip to the south-east at 33–64° (Figure 27, units 1–5). 37 The lowest metabreccia has a conformable contact with the 38 underlying pebbly metamudstone. The clasts in the three 39 metabreccia beds are angular to rounded in shape, and vary from 40 microscopic in size to over 3 m across (Figure 28). They range in 41 original composition from homogeneous or laminated slaty mudstone, 42 some with sandy layers, to finely banded siltstone, sandstone, 43 quartzite, and homogeneous pale- to dark-grey limestone. The 44 relative proportions of the different clast types varies upwards in 45 the sequence from 60% of fine-grained quartzite clasts and 3% 46 limestone in the lowest metabreccia, to 15% of fine-grained 47 48 quartzite clasts and 60% limestone in the highest metabreccia; the 49 other constituents making up the remaining 25–37% of the population 50 are relatively constant in amount (Litherland, 1970). Thus the 51 proportion of limestone clasts increases markedly up-sequence as 52 the quartzite clasts diminish in number. Many of the clast types 53 may be matched lithologically with underlying units such as the 54 Selma Black Slates and Selma Quartzite, and the clasts of pale-grey 55 limestone resemble metalimestones from the Appin Group, which crop 56 out on Islay and on the Appin Peninsula. No exotic clasts of 57 igneous or high-grade metamorphic origin have been identified. 58 The problematical microfossils occur in angular dark-grey 59 limestone clasts (to 60 cm across) in the topmost metabreccia (B5 60 of Figure 27), and in the underlying interbreccia unit, IB4, 61 62 63 64 65 1 2 3 4 exposed inland. Oncolites are seen in hand specimen as rusty- 5 weathering spots 1–4 mm across, and are found in thin section to 6 have a concentric internal structure; their morphology has been 7 described in detail by Litherland (1975). The acccompanying 8 catagraphs are up to 7 mm across and have a more disordered 9 internal structure. Bryozoan-like fossils were also reported by 10 Litherland. Of particular interest are 0.05–0.2 mm-diameter tubes 11 12 that penetrate the clasts and, although they are now filled with 13 sparry calcite, could represent burrows formed by parasitic algae 14 (Litherland, 1975). 15 Bedding in these rocks has a north-easterly strike and the Selma 16 Black Slates have been affected by a series of upright folds with 17 axes plunging gently to either the north-east or the south-west. 18 These folds are associated with the development of a steeply 19 dipping to vertical slaty cleavage that trends north-east, dips to 20 the south-east, and is accompanied by a gently curvilinear bedding- 21 cleavage intersection lineation. This cleavage also affects the 22 matrix, together with the more-argillaceous clasts in the Selma 23 Breccia. The slaty cleavage in the Selma Black Slates and locally 24 in the metabreccia is cut, and in places strongly overprinted by, a 25 spaced crenulation cleavage, which dips to the north-west at a low 26 angle. 27 The slaty cleavage is evident in the argillaceous matrix of the 28 metabreccia beds, and the clasts have been tectonically deformed to 29 30 varying degrees depending upon their lithology. Slaty fragments 31 and mud wisps have been moderately flattened in the cleavage, with 32 quartzite clasts being least deformed, as would be expected. The 33 slaty cleavage is refracted around some of the more-competent 34 boulders and pebbles, and in some cases faint ‗strain shadows‘ are 35 developed. Minor folds and corrugations of the internal bedding 36 laminations are noted in a number of clasts. Where seen on the 37 flat, clean exposures of the metabreccia in the intertidal zone, 38 these folds have axial traces that are parallel to the 055–065° 39 trend of the penetrative cleavage that affects the enclosing matrix 40 (Figure 28), and also to the crude alignment shown by the longest 41 dimensions of the clasts. Contractional deformation has resulted 42 in the formation of internal folds in thosee clasts in which the 43 lithological layering was initially at a high angle to the trace of 44 the slaty cleavage. However, where such bedding laminations were 45 originally at a small angle to, or parallel with, the orientation 46 of the slaty cleavage, the laminations are stretched, disrupted and 47 48 boudinaged but never folded. These relationships are a 49 demonstration of the fact that the folds preserved in the clasts 50 developed after the breccia was deposited. An analogous situation 51 holds for the folds in the country rock xenoliths enclosed in the 52 Ben Vuirich Granite (see the Ben Vuirich GCR site report in 53 Stephenson et al., 2013b). 54 A major NE-trending fault cuts the Selma Black Slates and causes a 55 displacement of the metabreccia outcrops (Figure 27). 56 57 12.3 Interpretation 58 59 Minor sedimentary structures show clearly that the Selma Black 60 Slates and Selma Breccia form a single, conformable sequence. 61 62 63 64 65 1 2 3 4 These units are part of the Easdale Subgroup and are correlated 5 with the Jura Slate Member (see the Kilnaughton Bay GCR site 6 report) and the Scarba Conglomerate Formation (see the Lussa Bay 7 and Kinuachdrach GCR site reports), respectively. There is 8 evidence throughout this succession for increasing instability of 9 the basin margin, as witnessed by the occurrence of slump folds, 10 large isolated clasts and rafts of disorientated 11 12 enclosed in mudrock. Coarse-grained, graded beds are found in the 13 Selma Black Slates and breccia units up to 6 m thick occur in the 14 overlying Selma Breccia. 15 Early workers identified the Selma Breccia and interpreted it as a 16 ‗crush conglomerate‘ (Flett in Kynaston and Hill, 1908), but 17 subsequent workers are agreed that it is of sedimentary origin 18 (Kilburn et al., 1965). Litherland (1970, 1975) referred to it as 19 a ‗sedimentary slide tilloid‘ but did not discuss the reasons for 20 this interpretation. 21 The critical evidence for the origin of the Selma Breccia may be 22 summarized as follows: 23 24 (i) The underlying sequence shows evidence of soft-sediment 25 deformation and mass-flow deposits, forerunners of the more- 26 extensive and distinctive breccias above. 27 (ii) The metabreccias form a bedded sequence consisting of 9 28 separate units, which are laterally continuous. The interbreccia 29 30 units contain large isolated boulders of original sedimentary rock. 31 (iii) The first metabreccia unit shows a normal sedimentary contact 32 with the underlying bed, with large boulders at the base preserved 33 in the act of sinking into the underlying mudrock. 34 (iv) The metabreccia beds consist of a varied assemblage of 35 angular original sedimentary clasts set in an originally muddy to 36 sandy matrix. The clasts appear to have been lithified, and in the 37 case of some of the limestone blocks, bored by organisms, before 38 being transported to their final resting place. 39 (v) Although there is a random admixture of clast types in all of 40 the metabreccia units, blocks of limestone only appear in the upper 41 beds and are dominant in the top metabreccia unit. Some blocks 42 consist of breccia, suggesting that earlier brecciated material has 43 been reworked in the flows. 44 (vi) There is evidence of local fracturing and brecciation of 45 clasts in situ. 46 (vii) All of the clasts are deformed in the S1 slaty cleavage but 47 48 there is no evidence that the bedding in the source rocks was 49 folded prior to their disruption, and incorporation into, the Selma 50 Breccia. 51 (viii) There is a lack of cataclastic or mylonitic rocks, or 52 evidence for intense shearing, within or between the metabreccia 53 units. 54 55 From the above features it is clear that the deposit originated as 56 a synsedimentary slump deposit, and did not form by the in-situ 57 brecciation, shearing or cataclasis of lithified, bedded strata. 58 Bedding in the clasts was not folded at the time of deposition. 59 Another important conclusion that can be drawn from this study is 60 61 62 63 64 65 1 2 3 4 that there is no evidence for either a major stratigraphical or 5 structural break at this level in the Dalradian succession. 6 Although possible microfossils such as oncolites and catagraphs 7 found in the metabreccias resemble forms that have been reported by 8 Russian workers from the Ediacaran-age strata of Spitzbergen 9 (Litherland, 1975), none of these forms provide reliable evidence 10 for the stratigraphical age of the rocks. However, the tube-like 11 12 structures that penetrate the oncolites and their binding matrix, 13 were interpreted by Litherland as the burrows of parasitic algae. 14 If this is correct then it indicates that the limestone clasts were 15 lithified and colonized by algae whilst they were accumulating on 16 the basin margin, prior to their transportation into the 17 sedimentary basin by slumping. Litherland (1975) speculated that 18 the source for these exotic limestone clasts might be the Cambro- 19 Ordovician Group on the North-west foreland of Scotland, 20 but more-recent work (see Rushton et al., 2000) on the age of the 21 Durness Group shows that it is much too young to have provided 22 sediment to the Dalradian basin. 23 24 12.4 Conclusions 25 26 The Selma Breccia, which is magnificently exposed at this site, is 27 a classic example of a rock formed by the large-scale slumping of 28 poorly sorted and very coarse-grained sediment into a marine basin. 29 30 Sedimentary units formed in this way are not easy to identify with 31 certainty, and their interpretation is commonly controversial. 32 This is especially so where the deposit has been deformed and 33 folded subsequently. The low grade of metamorphism and relatively 34 low strain at Port Selma make this an exceptional locality for 35 studying and interpreting such a metasedimentary deposit, and for 36 this reason alone it warrants consideration as a site of national 37 importance. 38 The metabreccias consist of angular blocks of rock, ranging in 39 size from pebbles to boulders up to 3 m across, enclosed in what 40 was originally a muddy or sandy matrix. A variety of different 41 rock types are seen in the metabreccia, some of which can be 42 matched in the local, or neighbouring, Dalradian sequences of 43 slightly older age. 44 Port Selma is also well known for the minute fossil remains, a few 45 millimetres across, which are found in the limestone blocks at the 46 top of the sequence. Unfortunately these fossils are not 47 48 sufficiently diagnostic to tell us the age of the rock that 49 provided much of the material for the original sedimentary slide 50 breccia, and hence place a maximum age on the metabreccia itself, 51 but they do provide some potential clues as to the likely source 52 area. 53 54 55 56 57 58 59 60 61 62 63 64 65 1 2 3 4 13 RIVER ORCHY 5 (NN 242 318–NN 247 331) 6 7 P.W.G. Tanner 8 9 10 11 13.1 Introduction 12 13 This GCR site is located in the wooded valley of the River Orchy, 14 10 km north-east of Dalmally. It is notable for the wealth of 15 minor structural features that it displays, most of which can be 16 related to the closure of an early major fold, the F2 Beinn Udlaidh 17 Syncline. This structure folds the important sedimentary 18 transition between the Grampian Group and the younger Appin Group 19 (Figure 29). The Grampian Group is represented by psammites and 20 semipelites, which can be shown to be overlain stratigraphically by 21 the Beinn Udlaidh Quartzite and the Leven Schists, both of which 22 belong to the lowest part of the Appin Group. 23 A major feature that makes this section invaluable for teaching 24 and demonstration purposes is that most of the minor structures 25 that can be examined in the field formed during the same phase of 26 deformation, and can be related to a single large F2 fold (Figures 27 30, 31). The structures are particularly well seen, when water 28 levels are low, on the well-scoured rock surfaces in the banks and 29 30 bed of this spate river. The best localities are in the vicinity 31 of the dramatic waterfall and rocky gorge at Eas Urchaidh (the 32 ‗Falls of Orchy‘, Figure 29), and along its tributary, the Allt 33 Broileachan. This site is excellent for examining the three- 34 dimensional form of plunging minor folds on the metre scale, and 35 for demonstrating their relationship to the cleavages, lineations, 36 and quartz veins found in the different rock types. Minor folds 37 are best seen in relief in the quartzite, and in potholes in the 38 pelite north-north-east of the Iron Bridge at 243 321; they clearly 39 change in vergence northwards as the river section passes from the 40 upper limb of the major fold, through the hinge-zone, to the lower 41 limb. The curvilinear nature of the major syncline axis is 42 revealed by the progressive change in the trend of the minor fold 43 hinges by over 90o in less than a kilometre (Figure 29). 44 45 The general geology of was established during the 46 primary mapping by the Geological Survey for sheets 45 and 46 47 48 (Kynaston and Hill, 1908). However, it was Bailey and Macgregor 49 (1912) who, recognizing the importance of the area, made the first 50 comprehensive structural and stratigraphical interpretation. They 51 recognized that the distribution of the three main rock types is 52 controlled by a flat-lying, isoclinal nappe, the Beinn Udlaidh 53 fold. These workers also found that this fold, which they 54 considered to be as well exposed as any of the small-scale nappes 55 in the European Alps, had been bent around a later upright fold, 56 the Glen Orchy ‗Anticline‘ (now referred to as an ‗antiform‘ or 57 ‗dome‘). Cummins and Shackleton (1955, figure 7) first identified 58 way-up structures in Glen Orchy, some tens of metres above the 59 waterfall Eas à Chataidh at 248 331, and confirmed the order of 60 succession established by Bailey and Macgregor (1912). Thomas and 61 62 63 64 65 1 2 3 4 Treagus (1968) studied two areas adjacent to Glen Orchy on Beinn 5 Udlaidh in more detail and published a map of the closure of the 6 Beinn Udlaidh Syncline. The wider area around Beinn Udlaidh, 7 including the crucial section that constitutes the River Orchy GCR 8 site, has recently been the subject of an extremely detailed field 9 and petrographic study of the stratigraphy, structure, metamorphism 10 and minor intrusions by Tanner and Thomas (2010). 11 12 The pelitic rocks at this site contain abundant millimetre-sized, 13 partly or wholly chloritized porphyroblasts of garnet, and randomly 14 orientated crystals of biotite, and the entire sequence has been 15 affected by amphibolite-facies regional metamorphism. 16 17 13.2 Description 18 19 20 13.2.1 Stratigraphy 21 22 In the River Orchy section at NN 242 318, rocks belonging to the 23 Grampian Group are mainly of finely banded psammite and semipelite, 24 with some pelitic beds a few centimetres thick that contain 25 chloritized garnets. Thin grey quartzite beds, and thin calcareous 26 seams are also present. The latter occur in a distinctive 27 sedimentary association in which the dark brown-weathering 28 calcareous bands, a few centimetres thick, are separated from the 29 30 pelitic background lithology by a narrow zone of siliceous 31 psammite. These zoned calcareous units generally have an extremely 32 elongated pod-like geometry overall, and die out laterally within a 33 metre or two. 34 The Beinn Udlaidh Quartzite is commonly coarse grained and 35 feldspathic where it is least deformed, as in the hinge-zone of the 36 major fold at NN 248 332, and locally contains gritty and pebbly 37 layers. It varies from pale grey to white or even pink in colour. 38 Excellent examples of festoon cross-bedding (at NN 248 331), as 39 well as at other localities in the quartzite on the hillside to the 40 north-east of this GCR site, show clearly that the unit is younger 41 than the Grampian Group. Where the boundary between the two units 42 is least affected by later deformation, it is generally 43 transitional over several tens of metres, with interbedding of 44 psammite, semipelite, and quartzite ribs (Thomas and Treagus, 1968, 45 p. 127). The northern contact is not exposed in the River Orchy 46 section, the first exposures north of the quartzite, seen 47 48 immediately above the waterfall at NN 248 332, being of psammite 49 with thin quartzite beds. At the south end of the section, the 50 entire quartzite unit is thinned tectonically, and the contact can 51 be located to within a metre or so below the dam at NN 242 319, 52 although Thomas and Treagus (1968) considered that the topmost 33 m 53 of the Grampian Group at this locality constitute a ‗passage 54 group‘. A dyke and a -like apophyse of appinitic rock are 55 intruded close to the stratigraphical base of the quartzite and 56 somewhat obscure its relationship to the psammitic rocks farther 57 downstream. 58 The overlying Leven Schists have a very uniform lithology and 59 consist of finely banded, dark-grey, schistose biotite-muscovite- 60 garnet-graphite pelites with thin layers of psammite and 61 62 63 64 65 1 2 3 4 semipelite. The pelites are characterized by a strong bedding- 5 parallel schistosity. They contain porphyroblasts of garnet, 6 reaching several millimetres across in places, commonly accompanied 7 by millimetre-sized randomly orientated flakes of biotite. Most of 8 the garnets in the pelitic rocks have been altered to chlorite, 9 fresh garnets being most common in the thin siliceous bands. 10 Significantly, in the exceptionally clean exposures in the area of 11 12 the gorge above the Iron Bridge, and where the river runs close to 13 the road farther north at NN 243 323, small-scale zoned calcareous 14 units are found, which are identical to those seen in the Grampian 15 Group and are also accompanied by thin beds of steel-grey 16 quartzite. 17 The boundary between the Leven Schists and the underlying Beinn 18 Udlaidh Quartzite is transitional, as is shown by the presence of 19 thin quartzite beds within the pelite for a distance of a few 20 metres above the main quartzite. This relationship is clearly seen 21 at several places near to the confluence of the River Orchy and the 22 Allt Broighleachan (Thomas and Treagus, 1968, p. 127) (Figure 29). 23 24 13.2.2 Structure 25 26 The Beinn Udlaidh Syncline is a sideways-closing and upward-facing 27 syncline whose gently plunging axis changes trend from 28 approximately east–west to north–south as it is traced southwards 29 30 along the river section. This major change can be monitored by the 31 progressive change in orientation of the hinges of the congruous 32 minor folds (Figure 29, stereoplot b). It consists of an upper 33 limb (with inverted Grampian Group rocks lying above the Appin 34 Group in the south of the area), and a lower limb to the north in 35 which the Leven Schists lie above the Beinn Udlaidh Quartzite 36 (Figure 29). The axial trace of the major hinge-zone passes 37 through afforested ground to the west of the river. The marked 38 curvature of the axial trace, as seen in Figure 29, is due to the 39 intersection of the gently dipping axial surface of the fold with 40 the irregular topography, and is unrelated to the fold axis 41 curvature described above. 42 When viewed down-plunge to between south and west, the minor folds 43 on the upper limb are seen to have a Z-shaped vergence (Figure 30), 44 which changes first to a neutral vergence in the vicinity of a 45 poorly defined major hinge-zone at about the Iron Bridge, and then 46 to a consistent S-shaped vergence on the lower limb. These minor 47 48 folds are best seen in the quartzite and the banded Leven Schists, 49 and have wavelengths that vary from tens of centimetres to over a 50 metre (Figure 31). The axial planes of the minor folds, together 51 with the related penetrative cleavage in the pelitic rocks, dip 52 consistently at less than 20o (Figure 29, stereoplot a). Throughout 53 the section, a stretching lineation, seen sporadically on the 54 bedding or foliation planes, maintains a constant trend of 190–180° 55 and plunges at a gentle angle to either north or south. Evidence 56 that this lineation is a stretching lineation and not a bedding- 57 cleavage intersection lineation is seen in the gritty and pebbly 58 quartzite beds in the hinge-zone of the major fold around NN 248 59 332, where clastic grains are clearly elongated and define a 60 stretching lineation, which lies at right angles to the local 61 62 63 64 65 1 2 3 4 intersection lineation and to minor fold hinges (Tanner and Thomas, 5 2010). 6 Evidence of later ductile deformation superimposed upon the major 7 synclinal structure is restricted to the development of a 8 crenulation cleavage, which is associated with minor folds of S- 9 vergence in the Grampian Group rocks on the upper limb of the fold, 10 and cross-cuts the earlier Z-folds and penetrative fabric. A weak 11 12 development of a similar crenulation cleavage and lineation is also 13 seen in the pelitic rocks on the lower limb of the Beinn Udlaidh 14 Syncline. 15 The structural pattern in these rocks is beguilingly simple, and 16 only rarely, even on the cleanest rock surfaces, are isoclinal 17 minor folds of F1 age seen to be refolded around minor folds 18 congruous to the major syncline (Tanner and Thomas, 2010). Care 19 has to be taken, as some suspected refolded folds have been proved 20 on closer examination, followed by slabbing and sectioning in the 21 laboratory, to be of sedimentary origin In addition, examination 22 of the garnets with a hand-lens reveals that they contain helicitic 23 inclusion trails, which are strongly oblique to an external 24 cleavage, which is axial planar to the F2 minor folds. Thin 25 sections of these rocks show that (i) there is an earlier 26 penetrative cleavage (S1), which pre-dates the formation of the S2 27 fabric associated with the Beinn Udlaidh Syncline, and (ii) the 28 garnets also grew in the interval between these two deformation 29 30 events (Tanner and Thomas, 2010). 31 32 13.3 Interpretation 33 34 The field relationships seen at this GCR site, supported by 35 younging evidence from cross-bedding, show that there is a coherent 36 stratigraphical sequence from the upper part of the Grampian Group 37 into the lower part of the Appin Group, with no evidence of a major 38 stratigraphical or structural discordance between the two groups 39 (Tanner and Thomas, 2010). Of particular importance is the 40 recognition of minor sedimentary rhythms, of unusual character, in 41 rocks belonging to both groups. At the contact between the two 42 groups there is evidence of sedimentary interfingering of beds, 43 rather than tectonic interleaving. This observation is in 44 agreement with the relationships seen at the River Leven and Strath 45 Fionan GCR sites. This is an important conclusion as the boundary 46 marks a major lithological change in the sedimentary record, and 47 48 indeed was formerly taken to be the Moine-Dalradian boundary. 49 The sedimentary sequence is folded into a major syncline, the 50 Beinn Udlaidh Syncline, which faces up to the east, and has a 51 strongly curved axis. Previous authors have regarded this syncline 52 and the complementary Glen Lochy Anticline as F1 structures 53 (Cummins and Shackleton, 1955; Thomas and Treagus, 1968; Roberts 54 and Treagus, 1975) but Tanner and Thomas (2010) have shown that it 55 post-dates an earlier fabric (see below) and hence can be 56 confidently assigned to the D2 regional deformation. However, only 57 a small amount of deformation, and a gentle warping of the axial 58 surface followed this main deformation event. This suggests that 59 the curvature was a primary feature of the D2 deformation, and not 60 a later effect due to refolding. Analysis of the geometrical 61 62 63 64 65 1 2 3 4 results from this GCR site, together with those from the adjoining 5 Beinn Udlaidh massif in which the fold is extensively dissected, 6 indicates that this curvilinearity has resulted from the rotation 7 of the original east–west fold axis, as seen in the least deformed 8 rocks, towards the orientation of the north–south stretching 9 lineation (the X-direction of the strain ellipsoid), with 10 increasing deformation (Figure 29, stereoplot b) (Tanner and 11 12 Thomas, 2010). The upper limb of the fold has been most affected 13 by this increase in strain, and the Beinn Udlaidh Quartzite found 14 there has a platy foliation, lacks sedimentary structures, and is 15 considerably thinner than it is on the lower, less deformed, limb. 16 It also carries a strong stretching lineation. 17 A petrographical study of the garnet-bearing assemblages shows 18 that the major F2 fold and its associated family of minor folds, 19 cleavage, and lineations, formed after a deformation event which 20 had given rise to a penetrative cleavage and was accompanied by 21 amphibolite-facies metamorphism. Evidence of this early, S1 22 cleavage has been all but destroyed by later recrystallization and 23 mineral growth in the overwhelming majority of rocks, and it is 24 best preserved as a helicitic fabric in the garnet porphyroblasts 25 (Tanner and Thomas, 2010). There is no evidence from this GCR site 26 or from the adjoining area to suggest that either minor or major 27 folding accompanied this early tectonothermal event, and its 28 significance is still being assessed. Tanner and Thomas (2010) 29 30 concluded that the rocks belonging to the Grampian Group have been 31 affected by the same number and sequence of events as those of the 32 Appin Group, and that there is no evidence for additional 33 deformation phases in the older rocks. These findings support the 34 conclusion that the Grampian–Appin group boundary is not marked by 35 a significant structural break. 36 37 13.4 Conclusions 38 39 The River Orchy GCR site provides an invaluable section through the 40 upper part of the Grampian Group and its continuation upwards into 41 the Appin Group (Lochaber Subgroup). Transitional contacts between 42 the major rock units, coupled with sedimentary repetition of 43 distinctive lithologies, precludes the presence of a major, 44 orogenic, unconformity at this stratigraphical level. This finding 45 is supported by a microscope study of rocks from this GCR site, 46 which has confirmed the field-based conclusion that both groups 47 48 have been affected by the same number of structural events, having 49 the same intensity of development, and geometry. Thus a boundary 50 that was formerly taken to be the contact between the Moine and 51 Dalradian supergroups can now be confidently recognized as a normal 52 stratigraphical contact between the two lowest groups of the 53 Dalradian succession. This conclusion is supported by observations 54 at the River Leven and Strath Fionan GCR sites. 55 The rocks at this site are folded over into a large downward- 56 closing F2 fold, the Beinn Udlaidh Syncline, which lies on its side 57 and has been deeply incised by the River Orchy, to reveal its 58 internal geometry. The gorges and rocky bed of the river expose a 59 superlative section which is invaluable as a natural laboratory in 60 which to study the intricate three-dimensional shape of this fold 61 62 63 64 65 1 2 3 4 and its associated minor structures, and to enable its mode of 5 development and complex history to be further unravelled. One 6 aspect of the work of special interest, is that it is the first 7 locality in the western Grampian Highlands where it can be 8 demonstrated that the so-called ‗early‘ nappe-like folds formed 9 after an even earlier major deformational and metamorphic event. 10 11 12 14 A9 ROAD CUTTINGS AND RIVER GARRY GORGE 13 (NN 686 717–NN 804 656) 14 15 P.R. Thomas 16 17 18 14.1 Introduction 19 20 The rock exposures in the 20 km-long section of Glen Garry between 21 the Drumochter Pass and Blair Atholl are of national, if not 22 international, importance since they form an almost continuous 23 section through the Grampian Group rocks in the Central Grampian 24 Highlands. Not only does the bed of the River Garry itself have 25 75% exposure, most of it above water level for much of the year, 26 but the parallel A9 road also provides a series of long clean rock 27 cuts. Hence, the exposures are frequently visited by student 28 parties and professional geologists. The GCR site is noteworthy 29 30 for its wealth of minor folds and sedimentary structures, which can 31 be used to demonstrate the position and geometry of some of the 32 major folds that make up the central part of the Grampian fold 33 belt. 34 The site lies on the inverted limb of the recumbent, SE-facing Tay 35 Nappe and provides a unique section through D1 and D2 minor and 36 major folds, related to that regionally important structure. It 37 also provides an essential link between two later major upright 38 folds, the Drumochter Dome to the north-west (see Figure 32) and 39 the Ben Lawers Synform to the south-east (see the Ben Lawers GCR 40 site report); those are the folds that control the outcrop pattern 41 of the Dalradian in much of the Central Grampian Highlands, as 42 illustrated and discussed by Treagus (1987, figure 1b and p.12). 43 The rocks of Glen Garry were originally described by Barrow (1904) 44 as Moine ‘granulites’ (an obsolete term for high-grade psammitic 45 metasedimentary rocks). The paper mentions ‘water-pipe’ structures 46 at Clunes and some overturned folds, but provides little other 47 48 specific data. Apart from a brief description of a fold in a 49 quarry on the A9 road by McIntyre (1950), the main work, which 50 forms the basis for this account, is that of Thomas (1965, 1979, 51 1980, 1988). Further useful comments were made on the 52 sedimentology by Glover and Winchester (1989) and Banks (2007), and 53 on the structure by Lindsay et al (1989). 54 55 14.2 Description 56 57 The following numbered descriptions of key localities start in the 58 north-west, near the summit of Drumochter Pass, and follow the road 59 and river down towards Blair Atholl (Figure 32). All of the 60 exposures are within psammitic rocks of the Grampian Group. The 61 62 63 64 65 1 2 3 4 dominant fold-set is named F2 and its axial plane schistosity S2. 5 Rarely observed folds which clearly pre-date F2 or S2 are 6 designated F1, although S1 is generally parallel to bedding. 7 8 14.2.1 Stalcair Cut (NN 686 717) 9 10 At the third lay-by on the southbound section of the A9 dual 11 12 carriageway descending from the Drumochter Pass, gently dipping 13 inverted beds face down to the south-east on the S2 axial-plane 14 schistosity and lenses of calcsilicate rock in the schistose 15 psammites are well seen. Roadside exposures 200 m to the east, by 16 a pylon buttress, display schistose laminated psammites with superb 17 trough ripple-laminations and larger dune beds as well as sediment 18 slump structures (Banks, 2007; Figure 33). Overturned, NW-vergent, 19 F2 folds in the cut face plunge gently towards 070o and are clearly 20 downward facing. A strong S2 axial-plane cleavage cuts S1, which 0 0 21 is parallel to bedding, and dips at 30 towards 160 at this 22 locality. The exposures are on the north-west, inverted, limb of 23 the F2 Garry Synform, the hinge-zone of which is seen at locality 24 2. 25 26 14.2.2 Wade Stone Cut. (NN 694 716–NN 699 71) and 27 River Garry (NN 697 714–NN 714 706) 28 29 30 At the fourth lay-by on the southbound dual carriageway, a high 31 rock face, on the curve of the road, cuts obliquely across the 32 hinge-zone of a major F2 fold, the Garry Synform. The section east 33 of the lay-by is in the gently dipping inverted limb of the 34 synform, with NW-verging minor folds, but east from the retaining 35 wall F2 minor folds become more neutral in vergence as the hinge is 36 reached. The folds, plunging at 20o to 070o with an axial-plane dip 37 of 40o to the south-east, are downward facing. Evidence is based on 38 the many fine examples of ripple-laminated and dune-bedded 39 schistose psammites. Lenses of calcsilicate rock are deformed by 40 S2, but post-D2 microcline porphyroblasts in the schistose psammites 41 could be evidence of later metasomatism. 42 There are both early quartz veins, which are folded, and later 43 cross-cutting veins. NE-trending microdiorite dykes and faults can 44 also be traced in the rock-face. The major Allt an Stalcair fault- 45 zone, seen in the Allt Stalcair at NN 693 717 to the west, is 46 mineralized with calcite and haematite. 47 48 The northbound lay-by, nearer to the River Garry, enables both 49 road and river sections to be seen. Good way-up evidence is found 50 in the river beneath the railway bridge (NN 6995 7131), and 51 immediately upstream is the hinge of the Garry Synform. One 52 kilometre downstream from the railway bridge, the hinge of the 53 major F2 Creag a’ Mhadaidh Antiform can be traced (NN 7090 7075), 54 plunging at 10–14o to 070–080o with fine examples of overturned 55 folds containing trough cross-laminations on steep limbs and an S2 56 crenulation schistosity dipping at 20–30o to the south. Exposures 57 around the footbridge at NN 7120 7065 have considerably tightened 58 and overturned minor folds involving some refolding of F1 folds. 59 These continue downstream to a point where very regular low dips 60 mark the commencement of the ‘Dalnacardoch Banded Zone’ (see 61 62 63 64 65 1 2 3 4 locality 3b), about 200 m upstream from the confluence with the 5 Edendon Water. 6 7 14.2.3a Edendon Cut (NN 7108 7084–NN 7140 7075) 8 9 The next lay-by on the A9 southbound is east of the major F2 10 closure of the Creag a’ Mhadaidh Antiform. At the west end of the 11 12 rock cut (NN 7114 7083), very tight F1 folds verge north-west and 13 appear to face downwards to the south-east as indicated by deformed 14 current-ripple laminations. At the burn (NN 712 708) a strike-slip o 15 fault trending 024 brings in a more-psammitic lithology and 16 overturned, NW-vergent, F2 folds plunging at 16o to 075o. At the 17 east end of the lay-by, above a low-dipping ductile thrust, o o 18 slightly more-open F2 folds plunging at 20 to 082 with an axial- 19 plane schistosity dipping at 35o to the south, fold thin quartz 20 veins and are cut by a microdiorite dyke. 21 22 14.2.3b Dalnacardoch Cut (NN 719 705) 23 24 Half a kilometre south-east of the last locality, in the deep 25 southbound road cut, an apparently regular sequence of flaggy to 26 schistose psammites dipping at between 25o and 30o is far more 27 complex when closely inspected. This is the ‘Dalnacardoch Banded 28 Zone’, which envelopes very tight F1 and F2 folds as well as a later 29 30 generation of reclined folds on both major and minor scales. At 31 the north-west end of the highest section (NN 7183 7055), very 32 tightly refolded isoclines occur, whereas farther into the cutting 33 isoclinal folds lie within an apparently simple banding (NN 7198 34 7046). The cutting also contains bands of mixed gneissose 35 lithologies, in which post-D2 boudins are quite common. Late 36 rotational shears in the form of large kink bands also occur at the 37 eastern end of the cut. 38 In the River Garry, the Dalnacardoch Banded Zone is at least 4 km 39 wide from NN 716 706 to NN 748 694 and contains both F1 and F2 40 folds, as well as folds of a later deformation. The latter are 41 associated with the north-west limb of the major post-D2 Errochty 42 Synform, which is thought to pass through the River Garry near 43 Dailnafraoich (NN 7375 6983); here, exposures of F1 and F2 44 isoclines are refolded about reclined folds of the later 45 generation, which plunge at 10–30o to 140–170o. The river also 46 exposes a number of interesting crush-zones parallel to banding in 47 48 which angular schistose clasts are preserved in narrow breccia 49 bands less than a metre in thickness. 50 51 14.2.4 Allt Crom Cut (NN 769 690) 52 53 Just to the west of the Allt Crom bridge on the A9 road, a cutting 54 at NN 7688 6896 exposes the hinge of a major antiformal fold in 55 three dimensions (Figure 34). Plunges of both major and minor 56 folds here are in the range 16–25o towards 150–170o, but strong axial 57 cleavage planes appear to be restricted to the hinge-zone, where 58 they dip at 40o to 080o. The western limb dips at 35–45o towards 59 080o and the less-steep eastern limb dips at 20–30o towards 135o; the 60 interlimb angle from here northwards remains at less than 40o. This 61 62 63 64 65 1 2 3 4 undoubted major hinge can be traced for 1 km north in surface 5 exposures and also in the Allt a’ Chireachain, 3 km to the north, at 6 NN 7752 7194, where the plunge is 20o to 140o. In the Garry Gorge 7 immediately to the south, it is difficult to follow the precise 8 trace of the antiform; it is seen towards Glen Errochty, though in 9 a much more-open reclined style at this higher structural level. 10 This fold is equated with the Bohespic Antiform, the major post-D2 11 12 fold complementary to the Errochty Synform (see below). 13 14 14.2.5 Black Tank Cut (NN 773 680) 15 16 This wide cutting is dominated by a major, tight, overturned F2 17 antiform, the hinge of which can be detected opposite the 18 northbound lay-by in schistose psammites dipping at 38o to 116o. 19 The plunge is unusual for the area at 8o to 218o with a cleavage dip 20 of 30o to 136o. The high rock face is controlled by regularly 21 dipping planar structures 1–3 m apart with a dip of 52–55o towards 22 270–290o. Some joint faces are mineralized with quartz and pyrite 23 whilst other fault planes display slickensides, some with dip-slip 24 and others involving strike-slip final movements. Several of these 25 discontinuities caused wedge failures of the face during 26 construction. At the south-east end of the cut a large red 27 microdioritic sill, dipping to the east at 26o in very regular 28 layers of schistose psammite, is downfaulted to the north-west. 29 30 31 14.2.6 Clunes Gorge (NN 782 671–NN 789 667) and 32 Clunes Cut (NN 785 670–NN 789 668) 33 34 This superb river locality has now been rivalled by the new road 35 cut only 150 m to the north (Figure 35). Both localities 36 demonstrate the presence of the Clunes Antiform, a major F2 37 structure, folding cross-bedded and ripple-laminated psammites, 38 quartzites and semipelites, stratigraphically located near the 39 transition between the Bruar Psammite Formation and the Tummel 40 Quartzite and Psammite formations. Linear features seen in the 41 river section, such as the famous `water-pipe’ mullions (NN 784 667) 42 (Barrow et al., 1905, p. 68), plunge consistently parallel to the 43 axes of the F2 minor folds. Minor folds, abundantly displayed in 44 the road cut (Figure 35), plunge consistently at about 10o to 045o, 45 with a strong S2 axial-planar cleavage dipping at 20–30o to 120o. At 46 both localities the SE-vergent, overturned, folds can be seen 47 48 climbing towards the hinge of the major antiform, which lies near 49 the south-east end of the cut and some distance down the Clunes 50 gorge (NN 787 667). The sedimentary structures consistently show 51 that the section is inverted overall and that the F2 folds face 52 downwards to the south-east. 53 Several vertical and inclined microdiorite dykes cut both the 54 river and the road sections. The widest of these is not only 55 porphyritic but also contains xenoliths made up from a variety of 56 metamorphic rocks, some of which are quite distinct from normal 57 Grampian Group lithologies. Immediately west of one of the dykes, 58 at the north-west end of the road cut, early, presumed F1, 59 isoclinal folds can be seen to be refolded by the F2 minor folds. 60 61 62 63 64 65 1 2 3 4 Downstream around NN 790 664, major faults cut the river section 5 and start to bring in lithologies that can be assigned more-readily 6 to the Bruar Psammite Formation. 7 8 14.2.7 Struan Exposures (NN 802 657–NN 808 655) 9 10 Inverted, regularly SE-dipping, flaggy psammites and quartzites 11 12 form the bed of the River Garry for 1 km below the junction with 13 the Allt a’ Chrombaidh (NN 790 664), but from 200 m above the twin 14 bridges at Calvine (NN 802 657) and downstream almost as far as 15 Struan church (NN 808 655), numerous overturned F2 folds have been 16 entrenched by the river. These NW-vergent folds have consistent 17 plunges of 20–30o to 060o, and amplitudes of 5–20 m. 18 19 At the ‗Salmon Leap‘ (NN 8037 6565), sedimentary structures are 20 best preserved on the steep limbs of the folds, in laminated 21 schistose psammites with interbedded semipelitic layers. The proof 22 that the layering is true bedding lies in the presence of a number 23 of well-preserved sedimentary dykes, washouts and slump structures 24 (Figure 36), all of which consistently young to the south-east on 25 steep limbs of folds, which thus face down to the south-east as at 26 Clunes (locality 6). The rocks here are assigned to the topmost 27 part of the Bruar Psammite Formation. 28 29 30 14.3 Interpretation 31 32 In essence, the sections at localities 1 and 2 and 4 to 7 are 33 dominated by minor and major folds of the second generation, which 34 fold all earlier planar structures (S0 and S1). In the more- 35 pelitic lithologies, a strong crenulation cleavage is developed, 36 axial-planar to the folds. Both the folds and the cleavage can be 37 correlated, in style and geometry, with the regional D2 structures. 38 The three-dimensional structure of the area is made more complex by 39 the presence of a third deformation phase, described by Thomas 40 (1980) as D3 but which is thought by others to pre-date the ENE- 41 trending upright folds described as F3 by Treagus (1999, 2000) in 42 the Appin and Argyll group rocks south of the Rannoch–Tummel area. 43 Treagus (2000) gave these late structures a local designation of 44 De, being unable to state categorically whether they pre- or post- 45 date the regional D3 phase. 46 This late phase is associated with the major N-S-trending Errochty 47 48 Synform and the Bohespic Antiform, described by Rast (1958) from 49 the boundary of the Grampian Group with the Appin and Argyll groups 50 to the south (see the Meall Dail Chealaich GCR site report). Minor 51 structures associated with this phase become remarkably intense in 52 the mid-part of the Glen Garry section, where the Errochty Synform 53 produces the tight reclined structures at locality 3. This gives 54 rise to the Dalnacardoch Banded Zone, which destroys most of the 55 stratigraphical continuity and causes the re-orientation of all 56 earlier folds around its hinge. Since no sedimentary structures 57 are preserved in this high-strain zone, it is possible that bedding 58 has been transposed and the schistosity might be composite. The 59 presence of well-preserved sedimentary structures outside the 60 banded zone introduces the possibility of some stratigraphical 61 62 63 64 65 1 2 3 4 subdivision but the psammitic nature of most of the lithologies 5 reduces the confidence of correlations north and west of the 6 Dalnacardoch area. 7 In spite of the refolding by this later fold-pair, the major F2 8 folds described above, and the majority of the minor folds on their 9 long limbs, are consistently overturned and vergent to the north- 10 west, whilst being downward facing to the south-east on the second 11 12 cleavage (Figure 32). The Garry Synform and the complementary 13 Craig a’ Mhadaidh Antiform demonstrate this relationship 14 particularly well on the north-west limb of the Errochty Synform. 15 In the core of the Errochty Synform and on its common limb with the 16 Bohespic Antiform, between localities 2 and 4, the facing of the D2 17 structures is not clear. However, on the south-east limb of the 18 Bohespic Antiform the north-west vergence and south-east facing of 19 the D2 structures is very clear from the exposures of the Clunes 20 Antiform, seen in conjunction with the Meall Reamhar Synform (see 21 the Creag nan Caisean–Meall Reamhar GCR site report). It is likely 22 that the Clunes Antiform is the lateral equivalent of the Creag a’ 23 Mhadhaidh Antiform. 24 The geometry, north-west vergence and south-east facing of the D2 25 structures is consistent with that observed in the Appin and Argyll 26 groups to the south (see, for example, the Strath Fionan, Slatich 27 and Craig an Chanaich to Frenich Burn GCR site reports). This is 28 consistent with their origin on the lower limb of the Tay Nappe, in 29 30 its development during top-to-the-SE D2 simple shear of originally 31 upright F1 folds (Treagus, 1987). This concept is supported by the 32 observation, in the Clunes locality particularly, that the 33 sedimentary structures are best preserved in the steeply dipping, 34 short limbs of the F2 folds, where they have suffered the least 35 rotation and deformation. The rarely observed minor first 36 generation folds also appear to be facing down to the south-east, 37 on the long limbs of second folds. This suggests that, on the 38 removal of the second deformation, these folds would have been part 39 of an upward-facing fold train (see 1.1 Introduction), originally 40 on the south-east limb of a major F1 anticline. 41 42 14.4 Conclusions 43 44 Glen Garry, from Drumochter to Calvine, is traversed by two 45 parallel, excellently exposed ribbons of rock, one natural (the 46 River Garry) and one man made (the A9 road). Both have been 47 48 instrumental in providing vital evidence to help our understanding 49 of the complex geological structure of the Grampian Group and 50 without them very little three-dimensional interpretation of the 51 structure would have been possible. Both minor and major folds, as 52 well as sedimentary structures, are clearly and spectacularly 53 displayed in exceptional clean and continuous exposures of rocks 54 that are poorly exposed elsewhere. Most of the folds belong to the 55 regional D2 phase of deformation, are north-west verging and face 56 downwards to the south-east on the inverted lower limb of the F1 57 Tay Nappe. However, this site also exposes the hinges of an 58 important pair of later folds, the Bohespic Antiform and the 59 Errochty Synform, which exert a major influence on the overall 60 outcrop pattern in the Schiehallion area to the south. 61 62 63 64 65 1 2 3 4 15 CREAG NAN CAISEAN–MEALL REAMHAR 5 (NN 771 6017–NN 783 622) 6 7 P.R. Thomas 8 9 10 11 15.1 Introduction 12 13 The two peaks of Meall Reamhar (493 m O.D.; NN 784 618) and Creag 14 nan Caisean (477 m O.D.; NN 778 607) form part of a NNE-trending 15 gentle rocky ridge surrounded by plantations, north of Tummel 16 Bridge (Figure 37). Exposures within the GCR site provide evidence 17 for the downward-facing Meall Reamhar Synform, formerly interpreted 18 as the primary F1 closure of the Atholl Nappe, structurally beneath 19 the Tay Nappe (Thomas, 1979, 1980), but now regarded as one of the 20 major F2 folds that affect the inverted lower limb of the F1 Tay 21 Nappe (Treagus, 2000). 22 The site also has stratigraphical significance and contains 23 outcrops of two major formations of the Grampian Group, the Bruar 24 Formation and the Tummel Quartzite Formation. On the particularly 25 well-exposed south-east flank of Creag nan Caisean, consistent 26 younging evidence has played a vital role in establishing the 27 stratigraphy on the upper limb of the major fold. This is the type 28 area for the Tummel Quartzite Formation, which here contains a 29 30 number of distinctive quartzite units separated by laminated 31 schistose psammites. . 32 33 15.2 Description 34 35 The southern boundary of the GCR site coincides with a forestry 36 road, which is accessible either from Easter Bohespic (NN 756 603) 37 in the west or Grenich (NN 804 603) in the east. 38 The Bruar Formation is exposed on the forestry road to the west of 39 the surge shaft of the Dalcroy Power station (notably at NN 767 40 603), and on Meall Reamhar. Laminated schistose psammites with 41 way-up evidence are dominant, but there are fewer quartzites and 42 more schistose pelites and semipelites than in the overlying 43 formation. On the summit of Meall Reamhar (NN 784 618) the 44 formation contains more interbedded layers of schistose pelite and 45 is intruded by late-Caledonian NE-trending dykes in the hinge-zone 46 of the Meall Reamhar Synform. 47 48 Some 200 m to the east of the surge shaft, the lowest of the 49 quartzite units at the base of the Tummel Quartzite Formation is 50 well exposed. This is the first of numerous individual 5–30 m- 51 thick beds of feldspathic quartzite, which are characteristic of 52 the Tummel Quartzite in this area and extend north-eastwards for 9 53 km towards Glen Garry. In general they dip steeply to the south- 54 east. Interbedded with the quartzites are schistose psammites, 55 which display laminations with abundant examples of ripple drift, 56 dunes, convolutions and, more rarely, sedimentary dykes, all 57 younging consistently to the south-east, towards the upper boundary 58 with the Tummel Psammite Formation. 59 Small-scale minor folds are common, especially within the 60 quartzites. The polyphase nature of the deformation means that 61 62 63 64 65 1 2 3 4 styles and plunges vary greatly throughout the formation. Two 5 penetrative schistosites are related to two sets of early minor 6 folds, which are commonly difficult to distinguish one from the 7 other. However the dominant vergence of the later of the two sets 8 is to the south-east, and they are associated with an axial planar 9 cleavage dipping steeper than the bedding. Two further sets of 10 crenulation cleavages cross-cut the early sets. 11 12 Immediately to the north-west of the summit of Meall Reamhar, the 13 dip of the bedding and the subparallel schistosity become less o 14 steep (25–40 ) and the second, dominant, penetrative schistosity is o 15 steeper (40–60 ), indicating that the axial trace of the major Meall 16 Reamhar Synform has been crossed. This is confirmed by the 17 occurrence of inverted cross-bedding in the laminated schistose 18 psammites on the southern slopes of Glen Errochty, to the north of 19 the GCR site. The axial trace of the synform trends north-east 20 below the summit ridge, where there is an antiformal plunge 21 culmination; from here the hinge plunges at low angles to the 22 south-west and north-east. 23 24 15.3 Interpretation 25 26 The excellent exposures in this GCR site provide critical evidence 27 for the key, upward-younging, stratigraphical succession of the 28 Grampian Group from the Bruar Formation through the Tummel 29 30 Quartzite into the Tummel Psammite above. 31 The main structural debate concerns the age of the major Meall 32 Reamhar Synform. The presence of small-scale interference 33 structures involving both of the primary deformation phases (D1 and 34 D2), as well as the two later cross-fold phases, indicates the 35 structural complexity of the area, but it is considered that the 36 primary deformation has dominated the large-scale structure. The 37 major closure was initially interpreted by Thomas (1965, 1980) as 38 an F1 closure. Two hundred and fifty metres to the west of the 39 surge shaft at NN 771 601, near the forestry road, there is 40 evidence suggesting that some folding may be earlier than that 41 associated with the Meall Reamhar Synform. Here, very steeply 42 dipping schistose psammites with cross-bedded laminae are cut at 43 right angles by a single (or composite) penetrative schistosity, 44 with no sign of any bedding-parallel schistosity. (Figure 38) 45 This was interpreted by Thomas as being representative of the 46 downward-facing (F1) closure of the major Meall Reamhar Synform. 47 48 However, it is now thought that this might be an early minor F1 49 fold hinge facing south-east, which is not related to the Meall 50 Reamhar closure and certainly does not affect the general upward 51 younging on the steep limb of that fold. 52 The Meall Reamhar Synform has been recently re-interpreted to be 53 of D2 age, complementary to the Balliemore Antiform and other NW- 54 verging major folds within the Appin and Argyll groups to the south 55 (Treagus, 2000). The evidence for this, as well as that in this 56 GCR site, comes from the consistent south-east vergence of F2 minor 57 folds in Grampian Group formations to the south-east (e.g. in the 58 Kynachan Quartzite Formation on Creag Kynachan at NN760 576) and 59 the north-west vergence of the F2 folds to the north-west (as is 60 also described at Clunes–locality 6 of the A9 and River Garry GCR 61 62 63 64 65 1 2 3 4 site report). These folds are associated with a penetrative S2 5 schistosity in semipelites, on which the folds face down to the 6 south-east. 7 8 15.4 Conclusions 9 10 The Creag nan Caisean–Meall Reamhar GCR site provides important 11 12 evidence that helps to establish a succession in the Grampian 13 Group, through its excellent younging evidence and distinctive 14 quartzite lithologies. The Bruar Formation and Tummel Quartzite 15 Formation are both well represented. Creag nan Caisean is one of 16 the best areas in the Central Grampian Highlands to see 17 interference structures in minor fold outcrops, which reflect the 18 regional polyphase fold pattern. The site also provides dramatic 19 evidence, from minor tectonic structures, for the presence of the 20 two limbs of the F2 Meall Reamhar Synform, a downward- and SE- 21 facing anticline that is an important component of the major folds 22 that affect the lower, inverted, limb of the Tay Nappe. This fold 23 is complementary to the Balliemore Antiform described in the Strath 24 Fionan GCR site report. 25 26 16 MEALL DAIL CHEALACH 27 (NN 703 676–NN 716 675) 28 29 30 P.R. Thomas 31 32 33 16.1 Introduction 34 35 The NW-elongated ridge of Meall Dail-Chealach (510 m O.D.) rises 36 above open moorland, 2 km north of the Errochty Dam. Exposed at 37 this GCR site is one of the best examples to be seen in the Central 38 Grampian Highlands of a late, post-metamorphic, major kink fold, 39 the Monoform. This fold refolds the sharp closure of the 40 Errochty Synform and is responsible for its final orientation. 41 The steep middle limb of the monoform is marked by a narrow line 42 of steep dips that was first recorded by the Geological Survey on 43 Sheet 55 (1902) and was described as ‗a line of disturbance‘ in the 44 subsequent memoir (Barrow et al., 1905). It was not however noted 45 by subsequent workers in the area, Anderson (1923), Bailey and 46 McCallien (1937) or Rast (1958). Its significance was realized by 47 48 Thomas (1965, 1980), who interpreted it as the steep limb of one of 49 a number of major post-metamorphic angular kink folds in this area 50 and named it the Trinafour Monoform (Ft on Figure 39a, following 51 the nomenclature of Treagus, 2000). 52 53 16.2 Description 54 55 Exposures on the ridge of Meall Dail-Chealach from NN 708 671 as 56 far north-west as the mid-slopes of Meall Breac (NN 686 697) 57 (Figure 39a) are composed of Grampian Group metasedimentary rocks. 58 The rocks are thinly layered schistose psammites and quartzites 59 containing isoclinal folds of earlier generation structures 60 (D1/D2). These rocks are affected by their proximity to the major 61 62 63 64 65 1 2 3 4 ductile thrust, the Boundary Slide, which separates them from rocks 5 of the Appin and Argyll groups. All of these rocks are affected by 6 the post-D2 Errochty Synform, which is locally associated with 7 minor folds and a schistosity (Fe and Se on Figure 39a, following 8 the nomenclature of Treagus, 2000). The ridge follows the outcrop 9 of the steep limb of the Ft Trinafour Monoform. 10 The bedding and subparallel S1 and S2 schistosities on the steep 11 limb of the Trinafour Monoform range in dip from 65o north-east up 12 o 13 to vertical, with a strike of 155 , which is oblique to the main 14 north-west trend of the steep belt. Some of the exposures display o 15 minor open folds (Ft on Figure 39a), plunging at 20–30 towards 140– 16 150o with axial planes dipping gently to the south-west (Figure 17 39b). Near the base of the crag on the north-east side of the 18 hill, the layering is seen to curve away towards Dubh Lochan (NN 19 711 674), beyond which the laminated gneissose psammites dip at 30– 20 40o towards 210o on the north-eastern gentle limb that marks the 21 north-east margin of the monoformal structure. The south-west 22 closure of the monoform in the Grampian Group rocks is seen 2.5 km 23 to the north-west (at NN 687 695) and also more clearly in the 24 Appin and Argyll group rocks downstream from the Errochty Dam (at 25 NN 718 653) (Figure 39a). The plunge of the major fold varies from 26 10o to 25o to the south-east and both the amplitude and the 27 wavelength of the fold decrease downwards, so that the fold dies 28 out up plunge on the slopes of Meall Breac (NN 685 698). Down 29 30 plunge, the monoform broadens to an open structure south-west of 31 Trinafour, where it is accompanied by the open upright Allt Culaibh 32 Antiform and the asymmetrical Croftnagowan Synform to the north- 33 east and south-west respectively (Figure 39). 34 35 16.3 Interpretation 36 37 It is clear that this major line of disturbance refolds all earlier 38 structures, including the Boundary Slide and associated D1 and D2 39 minor folds and schistosities, and especially the axial trace of 40 the post-D2 Errochty Synform and its associated minor folds and 41 schistosity (the De phase of Treagus, 2000). The Trinafour 42 Monoform is related to distinctive brittle-style minor kink folds 43 with a low plunge to the south-east (Figure 39b). The occurrence 44 of major and minor open and kink folds with gentle south-east 45 plunges is rare in the Central Grampian Highlands, but they appear 46 to be particularly well developed in the flaggy rocks on the gently 47 48 dipping north-west limb of the Errochty Synform, where they are 49 referred to as the Dt phase by Treagus (2000). The north-east 50 vergence of the Trinafour Monoform is reflected by the minor folds, 51 which also occur in conjugate and oblique sets, exemplified by the 52 Sron Chon folds to the south-west. Late folds of presumably the 53 same generation as the monoform that occur to the west and east 54 (the Allt Sleibh Antiform, Meall na Leitreach Synform and Allt 55 Culaibh Antiform) have progressively steeper axial surfaces with 56 depth (Figure 39b). 57 The age relationship of the Trinafour Monoform, and the related Ft 58 folds mentioned above, to the other deformation events late in the 59 history of the Grampian orogenic belt is not clear. Their brittle 60 style would suggest that they post-date the ENE-trending folds of 61 62 63 64 65 1 2 3 4 the D4 phase, such as the Ben Lawers Synform, described in the Ben 5 Lawers GCR site report. The Dt folds must be the consequence of a 6 NE-directed shortening along the length of the orogenic belt. This 7 is in contrast to the supposed transtensional deformation in that 8 direction that is responsible for the NE-trending, late-Silurian, 9 fault set (Treagus et al., 1999) discussed in the Ben Oss GCR site; 10 the age relationship of the Dt folds to these faults is not known. 11 12 13 16.4 Conclusions 14 15 The Meal Dail Chealach GCR site is of national importance since it 16 is representative of a number of spectacular NW-trending folds, the 17 most impressive of which is the Trinafour Monoform, that formed 18 late in the Caledonian deformation process. In this part of the 19 Central Grampian Highlands these semibrittle-style, monoformal kink 20 folds, which are the latest set of folds that can be identified, 21 appear to be restricted in development to the area between 22 Craiganour Forest and Glen Garry, and have formed in the gently 23 dipping, flaggy, Grampian Group rocks. Around the Errochty Dam 24 area they have also deformed the younger Appin Group rocks above 25 the Boundary Slide. The site lies only a few kilometres west of 26 the Trinafour to A9 road and is frequently visited by student 27 parties. 28 29 30 17 STRATH FIONAN 31 (NN 720 580–NN 745 560) 32 33 J.E. Treagus 34 35 36 17.1 Introduction 37 38 This GCR site is located on the slopes of Strath Fionan, 39 immediately adjacent to the minor road which runs around the north 40 side of the isolated mountain, Schiehallion (Figure 41). It 41 provides the only continuous section in the Central Grampian 42 Highlands, from the top of the Grampian Group through the Lochaber 43 and Ballachulish subgroups of the Appin Group (Table 1). Each of 44 the formations has observable sedimentary boundaries with its 45 neighbours and sedimentary structures can be used to establish the 46 continuity and the way up of the succession. Additional 47 48 attractions include minor folds, a great variety of metamorphic 49 minerals and microdiorite dykes and the area is popular for student 50 field parties and mapping projects. 51 The uniformly south-dipping sequence was originally interpreted by 52 Bailey and McCallien (1937) as the folded and faulted repetition of 53 the Blair Atholl ‗Series‘ (now the Blair Atholl Subgroup) and of 54 the Schiehallion Quartzite and Killiecrankie Schist, which occur to 55 the south of the GCR site. The junction at the base of the Blair 56 Atholl ‗Series‘ with the ‗Moines‘ (now the Grampian Group) was 57 interpreted as a major synmetamorphic dislocation, since the 58 interpretation required considerable excision of the 59 stratigraphical succession. This tectonic junction, named the 60 Boundary Slide, the equivalent of the Iltay Boundary Slide in the 61 62 63 64 65 1 2 3 4 south-west Grampian Highlands (MacGregor, 1948; Rast, 1963), plays 5 a major role in Dalradian tectonics. This interpretation was 6 supported subsequently by Rast (1958), who interpreted a persistent 7 outcrop of schist immediately above the slide, the Beoil Schist, 8 not as a stratigraphical formation but as a product of localized 9 ductile deformation. 10 The present interpretation is that of Treagus and King (1978) and 11 12 Treagus (1987) who maintained that the sequence may be directly 13 correlated with the type Appin Group sequence below the Blair 14 Atholl Subgroup and that it is a continuous south-younging 15 sequence, uninterrupted by major folds or dislocations. The zone 16 of Appin Group formations, including the Beoil Schist, immediately 17 above the Grampian Group, although the locus of high strain, was 18 not considered to contain a major dislocation. 19 The geometry of the dominant set of minor folds, seen in most of 20 the lithologies, shows that the succession lies on the short, 21 overturned limb of a fold (the Balliemore Antiform of Bailey and 22 McCallien, 1937), which is one of the major F2 folds in the region 23 according to Treagus (1987). The stratigraphy and structure of the 24 Strath Fionan area has been described more fully in the Geological 25 Survey memoir for Sheet 55W (Treagus, 2000). 26 27 17.2 Description 28 29 30 The principal lithologies of the GCR site may be conveniently 31 examined in transects A–A’, B–B’ and C–C’ (Figure 41). 32 33 17.2.1 Lochaber Subgroup 34 35 Good exposures of the uppermost Grampian Group and of the lowest 36 four formations of the Appin Group can be seen on or close to the 37 line of transect A–A’ (NN 723 577–721 576). The transect provides 38 the type section for the lowest two formations of the Lochaber 39 Subgroup, the Dunalastair Quartzite and Dunalastair Semipelite. 40 The Grampian Group psammites (the Kynachan Psammite Formation of 41 the Glen Spean Subgroup) are in sharp contact with the Dunalastair 42 Quartzite and exhibit clear south-younging cross-beds a few metres 43 from the boundary; there is no evidence of unusually high strain or 44 of structural discordance at the contact. The typically rather 45 pink-weathering quartzites and quartzose psammites of the 46 Dunalastair Quartzite, in well-bedded 100–300 mm-thick units, 47 48 exhibit slight variations in feldspar content, but are rarely as 49 feldspathic as the adjacent Grampian Group psammites. Cross- 50 bedding is evident from feldspar and heavy mineral concentrations, 51 and truncated foresets may be observed within a few metres of the 52 Grampian Group, confirming the age relationships. 53 Good exposures of the Dunalastair Semipelite within the area of 54 Figure 41 can be seen along the ridge crossed by the transect A–A‘, 55 on Speirean Ruadh (NN 735 570) and south-west of Lochan Beoil. 56 Although the dominant lithology in this formation is a biotite- 57 muscovite-garnet semipelite, its characteristic feature is a ribbed 58 appearance on a variety of scales, owing to the presence of 59 psammitic laminae, which are dominant close to the lower boundary. 60 Cross-bedding is apparent in the thicker psammites, which are 61 62 63 64 65 1 2 3 4 similar in character to those of the Grampian Group; no way up has 5 been determined near the line of transect but evidence of the 6 southward younging has been found near Lochan Beoil. 7 The Beoil Quartzite and Beoil Schist are well exposed to the 8 south-west of the above formations in transect A–A‘, but the type 9 section of these formations occurs to the north of transect B–B‘, 10 on the steep south-facing scarp west of Lochan Beoil. Generally 11 12 within the area of Figure 41, the Beoil Quartzite can be traced as 13 discontinuous 100–300 mm-thick ribs of pure white quartzite 14 separated by layers of semipelite 10–500 mm thick. Thin beds (100– 15 300 mm) of pelite of Beoil Schist aspect and of quartz-feldspar 16 psammites similar to those in the Dunalastair Semipelite also occur 17 within the formation. The ribs of quartzite are commonly seen to 18 be affected by tight folds with wavelengths of several metres (F1 19 and F2 of Figure 42, see Interpretation below) and it is clear that 20 this results in considerable duplication. No certain sedimentary 21 structures have been detected in these strongly deformed rocks. 22 The Beoil Schist is well exposed within the area of Figure 41, 23 forming a prominent topographical feature. The schist is 24 characterized by an unusual concentration of muscovite and the 25 presence of pods of vein quartz. The stratigraphically lower half, 26 apart from these characteristics, is very similar to the 27 Dunalastair Semipelite described above. It is a biotite-muscovite- 28 garnet pelite or semipelite with thin ribs of quartzofeldspathic 29 30 psammite and some quartzite ribs towards the lower, well-exposed, 31 gradational boundary. The upper half has striking muscovite-rich 32 schistosity surfaces studded with 1–8 mm garnets and streaked with 33 quartz veins and pods; it is noticeably less biotitic than the 34 lower part and bedding is not as obvious. No certain sedimentary 35 structures have been detected in these strongly deformed rocks. 36 Boudins of garnetiferous amphibolite are present. 37 A pronounced topographical hollow separates the Beoil Schist from 38 exposures of the Meall Dubh Striped Pelite and the junction is 39 nowhere well exposed. It is best seen north of the transect B–B‘ 40 in an area where the stratigraphy is repeated by several F2 41 isoclinal folds. The type section of the Meall Dubh Striped Pelite 42 Formation is at the northern end of transect B–B‘, in a 20 m-long 43 ridge at NN 7386 5661. Here, the delicately striped alternations 44 of fine-grained psammite and schistose muscovite-biotite pelite are 45 well displayed in a 5 m-wide section. Garnet and feldspar 46 porphyroblasts seen here are characteristic of this lithology, as 47 48 is grading in the 10–30 mm stripes of fine-grained psammite. 49 Although the junction with the Meall Dubh Limestone is not well 50 displayed, the youngest beds in this section are calcareous; 51 carbonate-rich rocks are seen a few metres to the south, suggesting 52 a transitional boundary. This junction marks the Boundary Slide of 53 Bailey and McCallien (1937). 54 55 17.2.2 Ballachulish Subgroup 56 57 The exposures along the line of traverse B–B’ (NN 7386 5661–7432 58 5630) give the opportunity to examine the constituent formations of 59 the Ballachulish Subgroup in reasonable proximity, although some of 60 the type sections detailed below occur elsewhere in Strath Fionan. 61 62 63 64 65 1 2 3 4 Yellow-weathering, ‗grey-hearted‘ carbonate rock, calcareous 5 schist and tremolitic amphibole schist are all represented in the 6 thin, poorly exposed, Meall Dubh Limestone Formation. Biotite or 7 phlogopite porphyroblasts are usually conspicuous in the latter two 8 lithologies; colourless tremolite and green actinolite occur 9 locally as rosettes. No sedimentary structures have been observed. 10 The Meall Dubh Graphitic Schist Formation is particularly well 11 12 displayed on the ridge to the east and west of the line of transect 13 B–B‘. The type section is a prominent crag at NN 7409 5645, where 14 the graphitic schist displays 30–100 mm crystals of kyanite (black, 15 as a result of graphite inclusions) as well as common biotite and 16 garnet and rarer staurolite and feldspar porphyroblasts. Bedding 17 is seen as non-graphitic semipelite ribs at 100–200 mm intervals. 18 Concordant garnetiferous amphibolites are a feature of this 19 formation. The boundary with the Meall Dubh Limestone is not well 20 exposed, although isolated occurrences with admixtures of graphitic 21 schist and calcareous schist show that it is gradational. The 22 upper boundary is seen along strike to the east at NN 7422 5640, on 23 the north side of the ridge of Meall Dubh Quartzite. Here 24 graphitic kyanite-bearing semipelite and pelite merge into a metre 25 of transitional muscovitic, rusty quartzite containing graphitic 26 seams. At the eastern edge of the quartzite outcrop on Meall Dubh 27 (NN 727 567), 2–3 m of a very characteristic transitional facies of 28 the graphitic schist is developed. This consists of finely bedded, 29 30 fine-grained quartzite with black graphitic laminae, which exhibits 31 cross-lamination in some exposures. Some of the 20 mm-thick 32 semipelitic beds exhibit grading of their graphite content. The 33 younging is consistently towards the overlying quartzite, allowing 34 for the presence of major and minor folds that affect this 35 boundary. 36 On transect B–B‘ the upper 20 m of the Meall Dubh Quartzite 37 Formation may be examined on the south bank of the Allt Strath 38 Fionan at NN 7424 5634, where 200–500 mm-thick beds display a 39 characteristic strong pebble lineation and feldspar-rich and heavy 40 mineral laminae. Cross-bedding in 100 mm beds youngs south towards 41 the Strath Fionan Banded Semipelite, some 20 metres distant. The 42 junction between the two formations is not exposed here, but may be 43 identified farther east on the south bank at NN 7432 5631; exposure 44 is poor but there appears to be no transition. The exposures on 45 Meall Dubh (NN 729 567), where the outcrop is greatly thickened by 46 folding, provide many clean sections where the detailed mineralogy, 47 48 deformation and sedimentary structures of the formation can be 49 seen. 50 The Meall Dubh Quartzite typically comprises 70–80% coarse- 51 textured quartz; the remainder consists of pink and milky-white 52 feldspar occurring as 2–10 mm-long rod-shaped clastic grains, 53 commonly concentrated in thin (50 mm) beds. Plate-like aggregates 54 of quartz grains (up to 30 mm in maximum dimension and 5 mm thick) 55 appear to represent highly deformed original clasts and indicate 56 that the original rock was partly conglomeratic. Cross-bedding on 57 the ridge above the roadside at NN 7275 5654 and to the east, 58 youngs south towards exposures of the Strath Fionan Banded 59 Semipelite. 60 61 62 63 64 65 1 2 3 4 Exposures of the Strath Fionan Banded Semipelite Formation in the 5 Allt Strath Fionan (transect B–B‘, NN 7424 5634–7432 5631) are 6 typical of the usual, rather friable, slightly rusty-weathering, 7 muscovitic, interbedded pelite and semipelite in this formation. 8 Four metres of schistose metacarbonate rock mark the transitional 9 junction of the pelite with the succeeding metalimestone in the 10 steep south bank of the burn. These exposures comprise the type 11 12 section. Typical lithologies can also be seen along the roadside 13 to the west, as far as the junction with quartzite described above 14 at NN 7275 5656. Bedding is usually a prominent feature on a 10– 15 100 mm scale and broken or cut specimens reveal a wealth of 16 sedimentary structures, in particular channelled cross-laminations 17 and small-scale grading. Sedimentary structures consistently 18 indicate younging to the south. Biotite, but not garnet, is 19 usually evident. 20 The southern end of transect B–B‘ provides an almost continuous 21 section from the Strath Fionan Banded Semipelite Formation, through 22 the Strath Fionan Pale Limestone Formation into the Tullochroisk 23 Semipelite Formation. However, the quality of the exposure is poor 24 and the latter two formations are better seen at the northern end 25 of transect C–C‘. This transect (NN 7137 5675–7186 5658) also 26 permits the examination of all the remaining formations of the 27 Appin Group. 28 The type section for the Strath Fionan Pale Limestone Formation is 29 30 at prominent crags south of the road at NN 7275 5638. Although the 31 base of the formation is not seen here, the crags expose 20 m of 32 white, almost pure, dolomite-tremolite rock. Bedding can usually 33 be detected as 2–3 mm-spaced slightly quartzose laminae; other beds 34 are conspicuously muscovite rich and phlogopite is a minor 35 constituent. The lower beds (to the east) are noticeably more 36 tremolite rich, some exhibiting rosettes 100 mm across. At the 37 west end of this locality the sharp junction with the overlying 38 Tullochroisk Semipelite is seen. Similar rocks occur at the start 39 of transect C–C‘ on the north bank of the burn (NN 7237 5675), 40 although here the rocks are more pelitic. 41 The Tullochroisk Semipelite Formation is well exposed in the 42 burn draining Lochan an Daim, and its sharp lower boundary is well 43 seen in exposures above the type locality of the Pale Limestone at 44 NN 7275 5638. However, the formation is best seen on or near the 45 line of transect C–C‘, (Figure 41). A burn section to the south of 46 the transect (NN 7235 5662–7223 5663), where the outcrop is 47 48 considerably thickened by F2 folding, is the type section of the 49 lower half of the formation. Here the dominant lithology is a 50 rusty-weathering semipelite with psammitic beds, not unlike the 51 Strath Fionan Banded Semipelite, although the Tullochroisk 52 Semipelite contains a greater proportion of fine-grained psammite 53 than the older formation. It is generally a non-graphitic, but 54 pyritic, muscovite-biotite schist, with fine-grained psammite 55 laminae 1–5 mm thick and quartzose psammite ribs 100–200 mm thick. 56 Small-scale cross-laminations and graded bedding have been observed 57 younging south near the boundary with the underlying Strath Fionan 58 Pale Limestone. 59 The upper half of the formation has its type section on the line 60 of transect C–C‘, where the upper junction is well seen. 61 62 63 64 65 1 2 3 4 Lithologies in the upper half of the formation are distinctly more- 5 graphitic than those typical of the lower half, and locally they 6 are as graphitic as the pelites of the overlying Blair Atholl Dark 7 Limestone and Dark Schist Formation. They commonly contain small 8 garnet, staurolite and kyanite crystals, but are characterized by 9 large (c. 5 mm) biotite porphyroblasts. Fine-grained psammitic 10 laminations (2–3 mm) and ribs of coarser grained psammite (100–200 11 12 mm), with rare small-scale grading, are usually evident. This 13 change of character defining the upper part of the formation is 14 marked by a gully between NN 7226 5656 and NN 7220 5663, and by a 15 grey metalimestone (maximum thickness 8 m) to the east, between NN 16 7235 5646 and NN 7278 5636. The junction with the first grey 17 metalimestone of the Blair Atholl Dark Limestone and Dark Schist 18 Formation, marking the start of the Blair Atholl Subgroup, is seen 19 on, and to the north-west of, the line of transect at NN 7186 5660, 20 followed by excellent exposures of the complete Blair Atholl 21 Subgroup (Figure 41). It is also especially well seen to the west 22 and south of Lochan an Daimh (Figure 43). 23 Immediately to the south of transect C–C‘, there is a very clear 24 repetition of the formations of the Blair Atholl Subgroup about a 25 major antiformal core, enclosing the Schiehallion Boulder Bed of 26 the Islay Subgroup. This is the Balliemore Antiform of Bailey and 27 McCallien (1937), a major fold of F2 age according to Treagus 28 (1987). Minor folds, sympathetic to this major closure, verging to 29 30 the south, plunging to the east and associated with an intensely 31 developed crenulation cleavage, can be observed in most of the 32 lithologies described above. 33 34 17.3 Interpretation 35 36 The sequence exposed in Strath Fionan is a continuous 37 stratigraphical succession from the uppermost formation of the 38 Grampian Group through the Lochaber and Ballachulish subgroups of 39 the Appin Group. The boundaries between formations are usually 40 transitional and sedimentary structures show consistent upward 41 younging; there is no evidence for major repetition by folding nor 42 of dislocation. However, the formations of the lower part of the 43 Appin Group, from the Dunalastair Semipelite through to the Meall 44 Dubh Striped Pelite do show, from the intensity of folding and of 45 schistosity and the absence of sedimentary structures, that these 46 rocks have suffered high strain. This interpretation (Treagus and 47 48 King, 1978; Treagus, 1987, 2000) contradicts previous 49 interpretations (Bailey and McCallien, 1937; Rast, 1958) that this 50 is a folded sequence of formations now assigned to the Blair Atholl 51 Subgroup and the lower Argyll Group, and that the contact with the 52 Grampian Group is a ‘slide‘. 53 The present interpretation is supported by the clear correlation 54 that may be made between this succession, albeit in a very 55 condensed sequence, and the Lochaber and Ballachulish subgroups of 56 the Appin Group in the type area (Figure 2). The latter 57 succession, which contains thicker and more-fully developed 58 correlatives of all of the formations seen in Strath Fionan, is 59 illustrated by the GCR sites in the Loch Leven area (River Leven, 60 Nathrach, Rubha Cladaich, St John’s Church, Onich) and Appin area 61 62 63 64 65 1 2 3 4 (Ardsheal Peninsula and Lismore Island) discussed elsewhere in this 5 paper. Of particular significance in making this correlation is 6 the clear similarity of the Meall Dubh Quartzite with the Appin 7 Quartzite of the type area and, in its pebbly nature, its total 8 dissimilarity with the Schiehallion Quartzite, with which it had 9 been previously correlated. 10 Two other aspects of this GCR site are important in the context of 11 12 the interpretation of the Grampian Fold-belt. Firstly, in general 13 the site has an unusual wealth of porphyroblastic metamorphic 14 minerals, including biotite, garnet, staurolite, amphibole, epidote 15 and feldspar. In particular, the spectacular kyanite of the Meall 16 Dubh Graphitic Schist was used in a study by Wells and Richardson 17 (1979) to determine that the Dalradian of the Central Grampian 18 Highlands has been buried to a depth of some 30 km. 19 Secondly, the wealth of minor structures (folds, cleavages and 20 lineations) in the site has allowed it to be shown that the rocks 21 have undergone four distinct episodes of deformation (Treagus, 22 1987). The dominant set of minor structures, which can be 23 confidently correlated with the regional D2, show from their 24 consistent southerly vergence that the succession described lies on 25 the northern limb of a major antiform. This is the regional 26 Balliemore Antiform, the axial trace of which lies immediately to 27 the south of the GCR site and is the complementary fold to the 28 Meall Reamhar Synform described in the Creag nan Caisean–Meall 29 30 Reamhar GCR site report (Treagus, 1987, 2000). 31 32 17.4 Conclusions 33 34 The Strath Fionan GCR site is of national importance in 35 demonstrating the continuity of sedimentation between two of the 36 major groups, the Grampian and the Appin, of the Dalradian 37 Supergroup. Of particular importance is the unusual preservation 38 of sedimentary structures in such strongly deformed rocks. The 39 continuity of both the stratigraphical succession and of the 40 structural history across the Grampian–Appin group boundary 41 precludes this junction being interpreted as a major tectonic 42 unconformity. A comparison can be made in this respect with the 43 River Leven and River Orchy GCR sites, where the same conclusion 44 has been reached. Neither is there convincing evidence for the 45 presence of a major low-angled ductile dislocation, the Boundary 46 Slide, which had been suggested by earlier researchers because of 47 48 the high strain exhibited by units in the lower part of the Appin 49 Group succession. 50 The site is critical to the reconstruction of the Dalradian 51 sedimentary basin, particularly in the comparison of the very thin 52 succession here with the much thicker equivalent sequences seen in 53 other GCR sites in the Central Grampian Highlands (in the Loch 54 Leven and Appin areas), as well as with those on Islay and on the 55 Garvellach Islands in the South-west Grampian Highlands and with 56 GCR sites in the North-east Grampian Highlands. The site reveals a 57 quite exceptional development of minor-scale structural features 58 which, in conjunction with those of other nearby GCR sites, help to 59 demonstrate the presence of the major structures that make up the 60 Grampian Fold-belt. Minerals that grew during metamorphism are 61 62 63 64 65 1 2 3 4 also unusually visible to the naked eye in many of the rocks in the 5 GCR site. The geochemical analysis of the mineral kyanite from 6 this site has been of key importance in establishing the great 7 depth of burial that this part of the Grampian Terrane has 8 undergone. 9 10 11 18 TEMPAR BURN 12 (NN 691 575–NN 696 562) 13 14 J.E. Treagus 15 16 17 18.1 Introduction 18 19 This GCR site, off the road on the south side of 20 Strath Tummel, includes part of the Tempar Burn and exposures along 21 an adjacent track that together comprise one of the classic 22 sections through the Schiehallion Boulder Bed Formation (the local 23 equivalent of the Port Askaig Tillite Formation), at the base of 24 the Argyll Group (Table 1). The particular interest of this 25 formation is that it is interpreted as having been deposited from a 26 grounded ice-sheet, which carried material on to a shallow 27 continental shelf. The tillite generally consists of a gritty 28 matrix, in which are scattered clasts of various underlying rock 29 types a few centimetres in length. Most spectacularly, in the 30 upper half, are boulders of pink granite up to 30 cm across. The 31 site also includes a section through the Schiehallion Quartzite 32 (the equivalent of the Jura Quartzite), which originated as shelf 33 above the tillite, and through a metaconglomerate of reworked 34 tillite. The Schiehallion Boulder Bed Formation is possibly the 35 most important ‘marker’ horizon that has enabled stratigraphical 36 correlation within the Dalradian outcrop from the west of 37 to Banffshire. It has also been a key horizon in the correlation 38 of the Dalradian with Neoproterozoic successions in Greenland and 39 Scandanavia. 40 Although this section through the formation was first described by 41 Bailey (1917) and was correlated by him with the Port Askaig 42 ‗Conglomerate‘ of Islay, it was Anderson (1923) who first discussed 43 the arguments for its glacial origin. The formation has also been 44 described by Treagus (2000). Together with the classic localities 45 of the Port Askaig Tillite (see the Isla and Garvellach Isles 46 GCR site reports), the site has attracted wide international 47 interest in the ongoing investigations and debate on the evidence 48 for and implications of Neoproterozoic glaciations. 49 50 51 18.2 Description 52 53 The Schiehallion Boulder Bed Formation, where most fully developed 54 and exposed as in this section, may be divided into a sequence of 55 lower calcareous metadiamictites with dominantly calcareous clasts 56 and an upper sequence of quartz-rich metadiamictites with a mixture 57 of clast-types, including distinctive pink granitic rocks. The two 58 metadiamictite sequences are generally separated by discontinuous 59 units of quartzite (Figure 44). Exposures and loose material 60 immediately south of East Tempar Farm (at NN 691 575) and along the 61 62 63 64 65 1 2 3 4 track to NN 696 564, are especially good and accessible, 5 illustrating both calcareous and granitic metadiamictites, as well 6 as the intervening quartzites (Figure 44). Further exposures are 7 seen in the tributary of the Tempar Burn downstream of where it 8 crosses the track at NN 6958 5628. 9 The sequence of the lower calcareous metadiamictites, up to 50 m 10 thick, is best seen 100 m east of the sheep dip (NN 6938 5737) and 11 12 100 m east of the track at NN 6954 5705. Exposures of the 13 tremolitic and dolomitic metalimestone that underlies the tillite 14 (the Drumchastle Pale Limestone, equivalent to the Islay Limestone 15 of Islay) can be seen in the grassy hollow some 20 m to the north- 16 east of these exposures, but the contact is not seen. Individual 17 metadiamictites are separated by beds of schistose calcareous rock 18 free of clasts; bedding is not otherwise observed, except for rare 19 thin (0.5–12 mm) beds of cream-coloured schistose dolomitic 20 metalimestone, and tremolitic psammite. 21 The matrix of these lower metadiamictites is a schistose 22 calcareous (dolomitic) rock, locally rich in amphibole (radiating 23 blades of white tremolite or pale green actinolite and large 24 hornblende porphyroblasts) and generally rich in a phlogopitic 25 biotite. The matrix, commonly weathering to a rusty-brown, 26 characteristically shows a regular network of depressions, owing to 27 small patches of carbonate minerals, which are possibly small 28 clasts. According to Bailey and McCallien (1937) ‗lime-rich‘ 29 30 scapolite and diopside occur locally in the matrix. The dominant 31 clasts in the lower metadiamictites are 1–20 cm diameter, disc- 32 shaped fragments of schistose dolomitic limestone or limestone, 33 elongate parallel to the schistosity. Although commonly completely 34 weathered out, vestiges of the clasts can be seen at the rims of 35 the holes. Biotite-muscovite semipelite clasts, which are less 36 common, are very elongate and up to 20 cm in length, and the rarer 37 quartzite and granitic clasts, are up to 25 cm long but less 38 elongate (ratios of 2:1 or 3:1) and more angular; these clasts 39 become more common upwards. 40 Two discontinuous units of quartzite, each up to 50 m thick and 41 occurring at slightly different levels, are seen east of the track 42 around NN 6945 5709 and NN 6963 5678. These quartzites are not as 43 clean as the Schiehallion Quartzite above, being sugary and 44 feldspathic; they are generally massively bedded and locally they 45 are strongly schistose. Although bedding-plane laminations have 46 been observed, cross-stratification has not been widely detected. 47 48 The younger part of the Schiehallion Boulder Bed Formation, up to 49 70 m thick, is best seen on both sides of the track above the small 50 burn that crosses at NN 6956 5672); exposure is particularly good 51 around NN 696 565 (Figure 45). Clasts of grey or pink granite and 52 microgranite are the most common, followed by pink and white 53 quartzite, biotite-muscovite semipelite, and dolomitic limestone 54 (least common). The pink granite is the ‗nordmarkite ‗of Bailey 55 and McCallien (1937). Clasts are rarely in contact and spacing 56 appears to be random. Within the clast-rich metadiamictites, the 57 proportion of the rock composed of clasts (greater than 5 mm in 58 length) varies from 70% to zero. Some beds, up to several metres 59 thick, are free of obvious clasts (greater than 5 mm in length) but 60 still have the gritty, feldspathic texture of the metadiamictite 61 62 63 64 65 1 2 3 4 matrix. The maximum length of clasts, which might be somewhat 5 flattened in the schistosity, is 0.5–30 cm. The more-rare biotite- 6 muscovite semipelite and dolomitic limestone clasts are the 7 smallest, the most strongly deformed (flattened) and consequently 8 appear to be more rounded. The more-common quartzite and 9 microgranite clasts seldom exceed 10 cm, but might display very 10 irregular, angular and non-flattened shapes; the clasts of the 11 12 granite are typically the largest and display similar variations. 13 Clasts might show pressure-shadow fringes of quartz, carbonate 14 minerals and mica. As with the lower metadiamictites, there is no 15 evidence of sorting by size or composition, although some beds that 16 are several metres thick are clearly free of clasts. 17 The contact with the Schiehallion Quartzite is not exposed, but 18 evidence elsewhere in the district shows that it is sharp (Treagus, 19 2000). A thin bed of metadiamictite with reworked granitic clasts 20 is present near the base of the quartzite in the East Tempar track 21 exposures at NN 6953 5631. Clean exposures of the lower part of 22 the Schiehallion Quartzite are exposed in the Tempar Burn 23 downstream of the tributary at NN 6942 5643; scattered clasts of 24 granite and quartzite are present and of particular interest is a 25 30 m-thick metaconglomerate that is well exposed in the burn at NN 26 6935 5654. The quartzitic matrix of the metaconglomerate is very 27 similar to the Schiehallion Quartzite itself, but with a somewhat 28 more-gritty appearance. The rock types and sizes of the clasts are 29 30 much as in the metadiamictites, described above, but they are more- 31 obviously rounded and locally they are in contact with one another. 32 Clasts of grey or pink granite and microgranite are the most 33 common, together with pink and white quartzite, biotite-muscovite 34 semipelite and carbonate rocks. 35 The metaconglomerates have great potential for both 36 sedimentological and strain studies. Where measured, in the Tempar 37 Burn section, the proportion of clasts to matrix is very variable 38 over a few square metres, from 1% with isolated single clasts, 39 usually granite, to approximately 70%, with crowded clasts of a 40 mixture of rock types. The clasts are deformed into disc shapes 41 within the dominant cleavage, which is very close to the bedding; 42 observation of shape is difficult, but most clasts display 43 approximately circular sections. Deformation appears to be more 44 pronounced than in the metadiamictites; measured on surfaces 45 perpendicular to the bedding, granite clast length:width ratios, 46 range between 10.4:1 and 2.1:1, but are commonly about 5:1 and 47 48 longest dimensions ranged from 2 to 47 cm. Semipelite clast ratios 49 are from 5 to 13:1, and sizes range from 3 to 18 cm. 50 Downstream, after a further few metres of quartzite, the burn and 51 banks provide excellent exposure of the Tempar Dolomitic Member, 52 the probable equivalent of the Bonahaven Dolomite of Islay. 53 54 18.3 Interpretation 55 56 Subsequent to Bailey’s (1917) correlation of the Schiehallion 57 Boulder Bed with the Port Askaig ‘Conglomerate’, Anderson (1923) 58 discussed the arguments for its glacial origin. He pointed out 59 that the uniform, unbedded nature of the matrix and the haphazard 60 arrangement of the clasts strongly suggest an origin as a till. He 61 62 63 64 65 1 2 3 4 noted the similarity of the carbonate-rock and the quartzite clasts 5 to the underlying ‘Pale Limestone’ and ‘Banded Group’ formations 6 respectively (now the Drumchastle Pale Limestone and Cnoc an 7 Fhithich Banded Semipelite formations of Treagus, 2000), which are 8 exposed to the east of the GCR site. He used this correlation to 9 demonstrate the order of superposition. He further pointed out 10 that the granitic (‗nordmarkite‘) clasts have no likely local 11 12 source in Britain and must be far travelled. He noted that the 13 base of the Schiehallion Quartite closely approaches the ‘Blair 14 Atholl grey limestone’ (now the upper calcitic metalimestone of the 15 Blair Atholl Dark Schist and Dark Limestone Formation) in the north 16 of the district, but accepted the possibility that this might be 17 due to either erosion of the sub-quartzite surface, or to later 18 tectonic movements. He noted that the presence of amphibolite 19 clasts suggests a period of igneous activity that pre-dated the 20 diamictites. The present investigation, however, has failed to 21 reveal clasts of true amphibolite, as against green-amphibole 22 (actinolite)-bearing mica-schist. 23 More-recent discussion concerning the depositional environment of 24 the diamictites in general has concentrated on evidence from the 25 superior exposures of the Garvellach Islands and Islay, e.g. 26 Kilburn et al. (1965), Spencer (1971, 1981, 1985), Eyles and Eyles 27 (1983), Eyles (1988). These authors considered that the 28 diamictites represent a major glacial event of global extent and 29 30 that they may be correlated with the Varanger tillites of 31 Scandanavia and East Greenland (but see Stephenson et al., 2013a 32 for more-recently suggested correlations with other global glacial 33 events). It is generally accepted that the geographically 34 extensive, though discontinuous, distribution of the deposits in 35 the favours a glacial origin, as opposed to 36 sedimentary slide deposits of a more-local character. The general 37 evidence, throughout their outcrop in the Dalradian, of the mixture 38 of locally derived and exotic clasts, the random distribution and 39 size of the clasts, the lack of bedding within the ill-sorted 40 matrix, the record of dropstone structure and the particular 41 evidence of other glacially-related structures in the South-west 42 Grampian Highlands, makes the glacial origin indisputable. 43 In the Schiehallion district, the massive nature of the 44 diamictites, their inclusion of stratified, sorted sands, and the 45 subrounded shape of some clasts, supports the interpretation of a 46 marginal glaciomarine environment (Edwards, in Reading, 1986). The 47 48 local thinning and probable absence of the formation from some 49 areas and the inclusion of clasts of at least two of the preceding 50 formations in the lower diamictites, strongly suggests an 51 unconformable base. The distinctive sedimentary structures 52 attributed to tillites and present in some exposures of the Port 53 Askaig Tillite (see the Caol Isla and Garvellach Isles GCR site 54 reports) have not been observed at this GCR site. Possible 55 dropstone structures have been recorded, but it is difficult to 56 exclude the effect of the strong tectonic deformation. The clasts 57 show no evidence of having been sorted or of having had their 58 distribution controlled by water movement. 59 There are two beds of metaconglomerate, within the Schiehallion 60 Quartzite, that have clasts of identical lithologies to those in 61 62 63 64 65 1 2 3 4 the metadiamictites (Anderson, 1923; Treagus, 2000). The 5 significant difference between the metaconglomerates and the 6 metadiamictites is that there appears to be a more strongly bimodal 7 size distribution in the metaconglomerates, such that the 8 intermediate 1–5 cm sizes are missing; in addition the clasts are 9 less dispersed, although rarely in contact, and they are 10 significantly more rounded. This supports the interpretation of 11 12 Anderson (1923) that these rocks represent reworked diamictite 13 material derived by fluviatile or marine erosion. 14 15 18.4 Conclusions 16 17 The Tempar Burn GCR site is of international importance for its 18 clear exposures of a Neoproterozoic glacial deposit. The ‘boulder 19 beds’ (metadiamictites) that make up the Schiehallion Boulder Bed 20 Formation are composed of blocks and pebbles up to almost a metre 21 across, sitting isolated in a matrix that was originally and 22 silt. The blocks and pebbles can only have been dropped from 23 floating ice. They include a wide range of rock types, including 24 some derived locally from underlying formations, but particularly 25 significant, are the commonly occurring ‘stones’ of a type of granite 26 that does not crop out in Britain and therefore might have been 27 carried a great distance across an ocean. The GCR site is thus of 28 great importance for the light it sheds upon climate and 29 30 sedimentary processes in the late Precambrian, but it is also 31 celebrated for its significance in the correlation of Dalradian 32 formations across the British Isles, as well as with possibly 33 equivalent deposits in Greenland and Scandinavia. Studies of the 34 deformed shapes of the ‘stones’ are important for estimating the 35 strains that the Dalradian rocks were subjected to during the 36 Caledonian Orogeny. 37 38 19 ALLT DRUIDHE 39 (NN 6422 5723–NN 6423 5662) 40 41 J.E. Treagus 42 43 44 19.1 Introduction 45 46 The lower part of the Allt Druidhe, which drains into 47 48 3 km south-west of Kinloch Rannoch, has been selected as offering 49 the best section across the Boundary Slide, the most important 50 synmetamorphic high-strain zone and dislocation to affect the 51 Dalradian rocks of Scotland. The Boundary Slide is known to extend 52 for at least 90 km from the Dalmally district (see the Ben Oss GCR 53 site report) to the Schiehallion district, although demonstrable 54 dislocation and excision of strata decrease towards the north-east 55 (see the Strath Fionan GCR site report). If proposed correlations 56 with similar structures are correct (e.g. the Benderloch and 57 Ballachulish slides; see the Camas Nathais and St John’s Church GCR 58 site reports; Roberts and Treagus, 1977c) the dislocation can 59 probably be traced for most of the length of the Scottish Dalradian 60 outcrop from the Appin district in the south-west to the North-east 61 62 63 64 65 1 2 3 4 Grampian Highlands (see the Bridge of Brown GCR site report in 5 Stephenson et al., 2013b). 6 In its outcrop between Dalmally and Loch Errochty (9 km north-east 7 of this GCR site), the slide marks a hiatus in the Dalradian 8 stratigraphical succession (Table 1), juxtaposing Grampian Group 9 psammites below, against quartzites, semipelites and pelites of the 10 Argyll Group above. It is only in the area south and east of Loch 11 12 Errochty that members of the missing Appin Group gradually appear 13 between the other two groups, until the complete succession is 14 restored locally in Strath Fionan (Roberts and Treagus, 1977c; 15 Treagus, 1987, 2000). 16 The slide was first recognized in the Schiehallion area, to the 17 north-east of this GCR site, by Anderson (1923) and its outcrop was 18 mapped throughout the district by Bailey and McCallien (1937). Its 19 origin was discussed further by Rast (1958) and Treagus (1987, 20 2000). The present account is based on the mapping and 21 descriptions of P.A.R. Nell (BGS 1:10 000 Sheet NN65NW, and in 22 Treagus, 2000). 23 24 19.2 Description 25 26 The description below is taken principally from the almost 27 continuous exposures in the Allt Druidhe (NN 6419 5731 to 6420 28 5604), complemented by information from the hillside on its two 29 30 sides, especially that towards Meall Druidhe to the west. 31 From NN 6419 5731 to NN 6421 5713, the course of the burn is 32 controlled by a steep NNW-trending fault with SE-dipping Grampian 33 Group psammites exposed on its two sides (Figure 46). Bedding on a 34 1 to 30 cm scale is evident; cross-sets are also seen, but way-up 35 has not been determined. At NN 6421 5713, south of a prominent 36 microdiorite dyke, the psammites become more schistose and flaggy 37 and are seen for a further 100 m as far as NN 6421 5704, where the 38 fault crosses the bed of the burn. The psammites are succeeded, as 39 far as NN 6423 5690, by a sequence of pink and white quartzites, 40 interbedded with platy psammites and semipelites containing a 41 highly muscovitic pelite; several concordant sheets of microdiorite 42 cut these beds. In the lower part of this sequence, the lowest 43 pink, feldspathic quartzite can be correlated clearly with the 44 Dunalastair Quartzite, the lowest formation of the Lochaber 45 Subgroup, and the highly muscovitic pelite is correlated with the 46 Beoil Schist. Striped biotite-muscovite psammites and semipelites 47 48 below and above the Beoil Schist must represent the Dunalastair 49 Semipelite and Meall Dubh Striped Pelite formations respectively, 50 both of the Lochaber Subgroup. Above these units, a few 51 centimetres of graphitic kyanite schist, an upper 15 m-thick unit 52 of pebbly quartzite and a 50 m-thick unit of banded semipelite and 53 pelite can be equated with the Meall Dubh Graphitic Schist, the 54 Meall Dubh Quartzite and the Strath Fionan Banded Semipelite 55 formations, respectively, of the Ballachulish Subgroup as seen in 56 the Strath Fionan GCR site. The Meall Dubh Limestone Formation is 57 not represented. 58 At NN 6421 5704, the Strath Fionan Banded Semipelite Formation 59 (Ballachulish Subgroup) is in contact with strongly deformed and 60 boudinaged strata that can be assigned to the Easdale Subgroup of 61 62 63 64 65 1 2 3 4 the Argyll Group, thereby marking a major dislocation and excision 5 of strata. Some 40 m of strongly deformed, schistose, rusty- 6 coloured, feldspathic quartzite with thin interbeds of amphibolite 7 (equivalent to the Carn Mairg Quartzite), followed by a few metres 8 of graphitic schist (equivalent to the Ben Eagach Schist) are 9 followed by a major sequence of calcareous schist, calcareous 10 quartzites and interbedded amphibolites. The latter sequence can 11 12 be traced laterally, to the north and south into outcrops of 13 certain Ben Lawers Schist Formation (see, for example, the Slatich 14 GCR site report). Above the Ben Lawers Schist outcrop, at NN 6421 15 5638, are 10 m of graphitic schist (better seen on the hillside to 16 the north-east) followed by a major unit of schistose pebbly 17 quartzite, a repetition of the Ben Eagach Schist and Carn Mairg 18 Quartzite formations. 19 From the uppermost psammites of the Grampian Group southwards, 20 throughout the section, a 30–40o SE-dipping, penetrative 21 schistosity, is parallel to bedding where the latter is visible. 22 The schistosity planes in the amphibolites and the pebbly 23 quartzites exhibit a strong lineation of hornblende and of the 24 pebbles, generally pitching at a high angle (over 50o) to the south- 25 west. Amphibolite boudins, intersection lineations and rare 26 isoclinal fold hinges pitch with the same general reclined 27 attitude. 28 The Lochaber Subgroup formations in particular are cut by fault- 29 30 planes, usually associated with brecciation, dipping at lower 31 angles to the south-east than the schistosity and pre-dating the 32 microdiorite sheets. 33 34 19.3 Interpretation 35 36 The Boundary Slide occurs here on the short common limb of the 37 regional Meall Reamhar Synform–Balliemore Antiform F2 fold-pair, the 38 latter being represented at the GCR site by the repetition of the 39 Ben Eagach Schist and Carn Mairg Quartzite about the Ben Lawers 40 Schist in the fold core. The penetrative schistosity at the site 41 (the regional S2) is essentially parallel to bedding but can be 42 demonstrated to be axial planar to the fold-pair elsewhere (e.g. in 43 the Strath Fionan GCR site). 44 No measurement of the scale of displacement on the slide is 45 possible, nor have directional movement indicators been identified. 46 The apparent parallelism of the intersection lineations, the fold- 47 48 hinges and the extension direction is attributed to the high 49 strains in the slide-zone. The locally strong pebble and 50 hornblende stretching lineations, pitching steeply to the south- 51 west on the S2 schistosity planes, give an indication of the 52 intensity of the strain, as do the common boudinage axes pitching 53 in the same direction. These features together suggest strong 54 flattening strains in the slide-zone with a finite extension 55 direction pitching steeply to the south-west on the schistosity. 56 Some of the telescoping of the stratigraphical succession of the 57 Appin Group at the GCR site is considered to be the result of small 58 movements (metre-scale) on the low-angle SE-dipping faults, which 59 cut down section. Evidence in similar sections to that seen at the 60 GCR site, between Loch Rannoch and Loch Errochty, suggest that such 61 62 63 64 65 1 2 3 4 thrusts are numerous in the slide-zone and are commonly occupied by 5 the later, slightly discordant, microdiorite sheets. 6 There can be little doubt concerning the stratigraphical 7 assignations given above for the formations of the Appin Group, as 8 the formations can be traced north-eastwards into the type 9 formations of the Loch Errochty and Strath Fionan areas (Treagus, 10 2000). Similarly, the formations belonging to the Argyll Group can 11 12 be traced southwards into type areas in Glen Lyon. Thus it is 13 clear that a major hiatus exists between the Strath Fionan Banded 14 Semipelite and the Carn Mairg Quartzite formations, in this highly 15 strained sequence. The missing formations include the 16 metalimestones and graphitic semipelites of the upper Appin Group, 17 together with the Schiehallion Boulder Bed, the Schiehallion 18 Quartzite and Killiecrankie Schist of the Argyll Group, certainly 19 representing several hundred metres thickness of the type 20 stratigraphical succession seen in the Strath Fionan GCR site and 21 to the south of Schiehallion. In addition to this major hiatus, it 22 is considered that the whole section from the top of the Grampian 23 Group into at least the lower part of the Ben Lawers Schist 24 represents a zone of high strain, in which there are probably many 25 planes of discrete movement. 26 From consideration of evidence along the Boundary Slide-zone 27 elsewhere in the Schiehallion area (Treagus, 1987, 2000), it is 28 clear that the stratigraphical hiatus can be explained, at least in 29 30 part as a result of top-to-the-NW thrust-like movements during the 31 regional D2 deformation. However, this does not exclude the 32 possibility of movements contemporary with the D1 deformation, 33 which have been identified elsewhere in the Schiehallion area. 34 It is not clear to what extent any, or indeed all, of the hiatus 35 can be attributed to sedimentological causes. Treagus (2000) 36 provided evidence, from elsewhere in the Schiehallion district, 37 that some substantial thinning of parts of the succession is an 38 original sedimentary variation and suggested that such parts were 39 preferentially thinned tectonically. Soper and Anderton (1984) 40 have suggested that movements on some Dalradian slides might have 41 originated as low-angle normal faults during sedimentation, and 42 Anderson (1923) most-perceptively considered the possibility that 43 the hiatus could represent an original normal fault, later 44 subjected to intense shear. Both Anderson (1923) and Bailey and 45 McCallien (1937) considered, but rejected, the idea that the slide 46 might have originated as an unconformity and essentially favoured 47 48 the explanation of a synmetamorphic dislocation, as presented here. 49 Further research at this GCR site could well help to resolve these 50 matters. 51 52 19.4 Conclusions 53 54 The Allt Druidhe GCR site provides a rare opportunity to observe 55 the zone of rocks that contains the Boundary Slide, a high-strain 56 zone critically important in the development of the structure of 57 the Dalradian and contemporary with folding and metamorphism. It 58 can be demonstrated within this GCR site that there is a major gap 59 here in the normal stratigraphical succession, with several 60 hundreds of metres of rocks missing. It is likely that there are 61 62 63 64 65 1 2 3 4 many discrete dislocations within a zone of strongly deformed 5 rocks. The evidence for this high strain is in the unusual nature 6 of the small-scale structures seen in the section. It is 7 considered possible that some of the movements on the slide might 8 be associated with early faulting and changes of thickness during 9 the deposition of the sediments, a hypothesis that could be tested 10 by further work in the area of this GCR site. 11 12 13 20 SLATICH 14 (NN 632 477–NN 641 486) 15 16 J.E. Treagus 17 18 19 20.1 Introduction 20 21 The Slatich GCR site, on the low slopes of middle Glen Lyon, 22 provides exceptional exposures across the hinge-zone of a major F2 23 antiform. It also exhibits minor structures of both the preceding 24 D1 phase and the succeeding D3 phase. These exposures also give a 25 rare, accessible section across three formations of the Argyll 26 Group, from the calcareous schists of the Ben Lawers Schist 27 Formation up through the Farragon Volcanic Formation into the 28 kyanite-garnet schists of the Ben Lui Schist Formation (Table 1). 29 30 The major F2 fold, the Ruskich Antiform, is the correlative of the 31 Ben Lui Fold of the South-west Grampian Highlands, one of the major 32 structures of the Grampian fold-belt. This correlation was first 33 made by Bailey and McCallien (1937) and observations of the minor 34 structures and development of the major structural context have 35 been made by Nell (1984) and by Treagus (1987, 1999). 36 37 20.2 Description 38 39 The Ruskich Antiform in the area of the GCR site is represented by 40 the zig-zag outcrop pattern of three formations of the Argyll 41 Group, which define its hinge-zone (Figure 47). 42 The Ben Lui Schist can be characterized generally as a semipelitic 43 garnet-quartz-mica schist, alternating with pelites and psammites 44 on scales from centimetres to several metres. The formation is 45 typically garnetiferous but, unusually for the area, also contains 46 local kyanite, best seen in crags around NN 635 480, north of 47 48 Slatich. The Farragon Volcanic Formation is well exposed in the 49 crags above and east of Slatich around NN 640 477, where it 50 comprises a delicately striped, centimetre-scale, alternation of 51 amphibolite and quartzite. A good section of the Ben Lawers Schist 52 is seen above Roromore, where virtually the whole formation may be 53 examined in a small burn from NN 6385 4686 to NN 6404 4642; it is 54 typically a chloritic calcareous schist with a hundred metres or so 55 of dolomitic quartzite at its base, and with concordant 56 amphibolites above. This section allows the junctions with the 57 Farragon Formation below and the Ben Eagach Schist above to be 58 seen. 59 The principal interest of the site is in the closure of the 60 Farragon Volcanic Formation around the Ben Lui Schist in the core 61 62 63 64 65 1 2 3 4 of the Ruskich Antiform (NN 640 487). In the south of Figure 47, 5 around NN 638 468, the regional dip of this boundary is 40o to the 6 south-east; farther north, the same boundary on the opposite limb, 7 around NN 634 484, has a regional dip of 60o to the south; the major 8 hinge plunges almost due east at 30o. Minor structures (minor folds 9 and bedding/schistosity relationships) with opposed vergence on the 10 two limbs, sympathetic to the major antiform, can be seen at these 11 12 two localities. 13 The limb south-east of the hinge is considerably corrugated by co- 14 axial intermediate-scale folds (tens of metres limb length), which 15 are well exposed on the hillside north-east of Slatich, around NN 16 640 477. Some of these folds are sympathetic to the major antiform 17 but other intermediate-scale folds, with upright axial planes and a 18 crenulation cleavage, are superimposed. This locality therefore 19 offers the opportunity to examine three generations of minor folds– 20 F1, F2 and F4 of the regional phases. Not only are minor, metre- 21 scale, structures of the latest generation (F4) superimposed on the 22 dominant phase of minor folds (F2) (all plunging east), but also 23 rare refolded minor isoclinal folds of the earliest generation (F1) 24 are developed in the finely-bedded quartzites and amphibolites of 25 the Farragon Volcanic Formation (Figure 48). 26 27 20.3 Interpretation 28 29 30 The Ruskich Fold was first recognized by Bailey and McCallien 31 (1937) in their perceptive paper on the Schiehallion–Glen Lyon 32 district. They interpreted the fold as a major primary nappe 33 closure below that of the ‘Iltay Nappe’ (now the Tay Nappe) and thus 34 equivalent to the Ben Lui Fold of the Dalmally district in the 35 South-west Grampian Highlands (Bailey, 1922). However, the fold 36 has been re-interpreted from a subsequent investigation of the 37 associated minor structures by Nell (1984) and Treagus (1987) as a 38 major F2 fold. The dominant penetrative schistosity is axial 39 planar to the abundant minor folds which are sympathetic in 40 vergence to the major fold. This schistosity is associated with 41 the growth of garnet and can be correlated with the S2 fabric, which 42 dominates the pelitic rocks of the South-west and Central Grampian 43 Highlands. Only rare examples of F1 minor folds are seen, such as 44 the isoclinal folds in the finely banded amphibolites of the 45 Farragon Formation, which are clearly refolded by F2 minor folds 46 (Figure 48). 47 48 It has been confirmed that the Ruskich Fold has a similar geometry 49 and position in the structural pile to the Ben Lui Fold, which has 50 also now been established as a D2 structure by Roberts and Treagus 51 (1975). Although separated by several major, cross-strike faults 52 (Treagus, 1991) the axial traces of the Ruskich and Ben Lui folds 53 have been correlated on the ground (Roberts and Treagus, 1979). 54 The Ruskich Fold can be shown to have been superimposed on major F1 55 folds related to the early, F1 Tay Nappe, which can be identified 56 in Glen Lyon to the north of this GCR site (Nell, 1984; Treagus, 57 1987). The fold is seen as a major component of the zone of D2 58 complication in the Schiehallion area, which marks the northern 59 limit of the dominantly inverted rocks of the Flat Belt to the 60 south. 61 62 63 64 65 1 2 3 4 A further feature of this GCR site, not seen in the Ben Lui area, 5 is the superimposition on the Ruskich Fold of a later fold-set, 6 both intermediate and small scale, with an associated crenulation 7 cleavage. These structures belong to the regional F4 set, related 8 to the major Ben Lawers Fold to the south (Treagus, 1964b) (see the 9 Ben Lawers GCR site report). 10 11 12 20.4 Conclusions 13 14 The Slatich GCR site provides a rare example of almost continuous 15 exposure across the hinge-zone of a major fold, the Ruskich 16 Antiform. The fold hinge can be traced from the detailed mapping 17 of three of the principal formations of the Argyll Group in easily 18 accessible exposures on either side of Glen Lyon. Observations of 19 small-scale folds and cleavages have revealed that the major fold 20 belongs to the second (D2) phase of the four important episodes of 21 deformation that affect the Dalradian rocks. Unusually, small- 22 scale folds of the first generation (F1) can also be identified, 23 clearly pre-dating those of the second generation. The second- 24 generation folds themselves have small-scale structures of a still 25 later generation (the regional F4 phase) superimposed upon them. 26 Thus the small-scale structures can be used not only to establish 27 the sequence of deformation events but also to identify the 28 relative age of a major fold. The Ruskich Antiform is equated with 29 30 the Ben Lui Fold of the Dalmally area (see the Ben Oss GCR site 31 report), and can be followed for several tens of kilometres across 32 the Central Grampian Highlands. It is one of the most significant 33 structures involved in the building of the Grampian Fold Belt. 34 35 21 BEN LAWERS 36 (NN 621 368–NN 595 415) 37 38 J.E. Treagus 39 40 41 21.1 Introduction 42 43 This GCR site, on the western and southern flanks of the Ben Lawers 44 mountain range, illustrates typical structures within the inverted 45 limb of the Tay Nappe, the major fold that dominates the structure 46 of the south-eastern Grampian Terrane. It also contains the type 47 48 locality of one of the major post-nappe folds, the F4 Ben Lawers 49 Synform. In addition to the structure, the exposures described 50 within this very large GCR site, have been selected to illustrate 51 the stratigraphy of the four highest formations of the Argyll Group 52 in the Central Grampian Highlands. 53 The regional inversion of the succession on the lower limb of a 54 major fold has long been established (the Iltay Nappe of Bailey, 55 1922 or the Tay Nappe of Shackleton, 1958). The details of the 56 local structure, particularly that of the Ben Lawers Synform, have 57 been studied by Elles (1926), Elles and Tilley (1930), Treagus 58 (1964b) and Nell (1984). The site, which belongs to the National 59 Trust for Scotland and is famous for its plants and wildlife, has a 60 61 62 63 64 65 1 2 3 4 visitor centre that is much visited by the general public who would 5 benefit from the availability of more geological information. 6 7 21.2 Description 8 9 The localities are described from south to north, up the southern 10 slopes of Ben Lawers which, as the succession is inverted, means 11 12 that the descriptions progress approximately from the youngest to 13 the oldest formations (Figure 49). Three phases of deformation can 14 be discerned in these rocks, designated D1, D2 and D4 in the 15 descriptions below; the attribution to these regional phases is 16 justified in the subsequent ‘Interpretation‘. 17 The youngest formation represented, the Loch Tay Limestone of the 18 Tayvallich Subgroup, is seen in the small quarry at NN 6212 3678 19 (Figure 49, locality 1). The dip of the beds (some 30o to the 20 north-west) in the semipelite laminae in the limestone is typical 21 of the regional dip on the south limb of the F4 Ben Lawers Synform. 22 A gently NE-plunging lineation (e.g. 10o to 025o) results from the 23 intersection of bedding and a nearly bedding-parallel schistosity 24 (S2), dipping less steeply to the north-west, and from the hinges 25 of rare NW-verging isoclinal F2 folds. Pelites within the 26 limestone show a steep NW-dipping S4 crenulation cleavage. 27 A well-exposed section from the next oldest formation, the Ben Lui 28 Schist of the Crinan Subgroup, through the Farragon Volcanic 29 30 Formation and into the Ben Lawers Schist of the Easdale Subgroup, 31 is seen in the Burn of Edramucky from NN 6184 3725 to NN 6125 3910 32 (locality 2). In the Ben Lui Schist, below and above the road at 33 NN 6145 3756, the quartz-rich, garnetiferous, locally graphitic, 34 interbedded semipelites and pelites show structural features 35 similiar to those described above for the Loch Tay Limestone. NW- 36 verging folds are well developed, with a penetrative axial-planar 37 S2 schistosity and with plunges at low angles varying in trend from 38 050–020o. Going up the burn section, the north-north-west dip of 39 the bedding gradually increases to about 50o at the Farragon 40 Volcanic Formation junction (NN 6125 3910), and the F2 folds are 41 more-constantly ESE-trending, with a northerly vergence. The 42 latter formation, comprising finely interbedded semipelites and 43 amphibolites, is exposed in the river gorge and is transitional 44 northwards into the quartzites and calcareous schists of the Ben 45 Lawers Schist. The Farragon Formation here contains small, NW- 46 verging, tight F2 folds which refold earlier F1 isoclines. 47 48 The Ben Lawers Schist is not well exposed in the burn and the 49 rocks at this structural level are better seen in the road section 50 at NN 601 390 (locality 3), described in detail by Treagus (1964b). 51 Here, upright metre-scale folds, associated with a well-developed 52 ENE-trending S4 crenulation cleavage, are seen to refold north- 53 verging, tight F2 folds, related to a subpenetrative schistosity 54 (Figure 50). The earlier folds, which here trend between west- 55 north-west and north-west, themselves fold an earlier S1 56 schistosity and a mineral (quartz-chlorite) lineation with a more 57 north-north-westerly trend. F1 isoclines with a north-west to west 58 trend and northerly vergence are seen rarely and appear to be 59 associated with the earliest schistosity and mineral lineation. 60 The upright F4 folds have an enveloping surface that defines a 61 62 63 64 65 1 2 3 4 gentle northerly dip of the major fold limb, near to the axial zone 5 of the Ben Lawers Synform. Farther north, near the road on the 6 east side of Lochan nan Lairige at about NN 600 406, and on the 7 hillside above, the same generation of F4 folds are seen to have a 8 neutral vergence in the hinge-zone of the Ben Lawers Synform. The 9 hinge-zone is very well seen looking across the lochan to the cliff 10 section on its west side in which the bedding and the upright S4 11 12 cleavage are apparent on a grand scale. 13 The Ben Lawers Schist is not well seen on the north limb of the 14 Ben Lawers Synform but at the tunnel portal at the north end of the 15 lochan (NN 5953 4115; locality 4), the Farragon Volcanic Formation 16 is exposed, with bedding here dipping at a low angle to the south. 17 Many loose specimens provide examples of tight F2 folds that rarely 18 show refolded F1 isoclines, subparallel to a strong hornblende 19 lineation. 20 Within the Ben Lawers Schist on the north limb of the Ben Lawers 21 Synform, there is an isoclinal infold of schistose graphitic 22 pelite, assigned to the Ben Eagach Schist Formation; this is 23 exposed on the north flank of . In the core of the 24 fold, the graphitic pelites are transitional into schistose pebbly 25 quartzites of the Carn Mairg Quartzite Formation. The two limbs of 26 this fold, both dipping at moderate angles to the south-east, are 27 exposed in a burn between NN 624 433 and NN 633 428 and in two 28 burns to the south-west (NN 624 426, north limb and NN 625 416, 29 south limb) (locality 5). In these exposures, the plunge of the 30 o 31 tight F2 fold-set is at moderate angles (10–40 ) to the east-south- 32 east, with vergence both of the folds and of the axial-planar 33 schistosity/bedding intersection consistently to the north; these 34 folds are overprinted by the upright S4 crenulation cleavage. The 35 infold must of D1 age, pre-dating the D2 and D4 deformation phases. 36 37 21.3 Interpretation 38 39 The exposures described in this GCR site have long been 40 acknowledged to lie on the inverted limb of the Tay Nappe, where 41 they are affected by the later Ben Lawers Synform (e.g. Elles and 42 Tilley, 1930; Treagus, 1964b). The dominant north- to NW-verging 43 folds with the penetrative to subpenetrative axial-planar 44 schistosity are of the regional F2 generation, which is considered 45 to be the main phase associated with the generation of the Tay 46 Nappe (Harris et al., 1976). The axial trend of the F2 minor folds 47 48 and of the S0/S2 intersections, which is N–S in the Flat Belt south 49 of the site, curves progressively through the area of the site from 50 north-north-east in the south through north-east to east and then 51 east-south-east in the north. Further research in the Ben Lawers 52 area might contribute to the understanding of this phenomenon and 53 of the evolution of the Tay Nappe. 54 The regional D1 phase is represented by the earlier fabric that 55 can be seen to be crenulated by the S2 cleavage planes, and by the 56 small-scale isoclines and mineral lineation in the Ben Lawers 57 Schist and the Farragon Formation. Data are too sparse to state a 58 trend or vergence for these structures, though the mineral 59 lineation appears to have a north-north-west trend, both in the 60 Farragon Formation and in the Ben Lawers Schist, similar to that 61 62 63 64 65 1 2 3 4 widely reported elsewhere in the Flat Belt. The major infold of 5 the Ben Eagach Schist and Carn Mairg Quartzite in the Meall 6 Corranaich exposures is overprinted by the D2 structures and thus 7 must also be of D1 age; from its outcrop pattern it has a north- 8 east trend and thus must have been (at the time of D2) an anticline 9 facing south-east. According to Treagus (1987, 2000) these major 10 F1 folds originally faced steeply upwards. 11 12 The major fold that affects bedding and S2 within the area of the 13 GCR site is the regionally important F4 Ben Lawers Synform, a 14 gentle deflection of the originally subhorizontal limb of the Tay 15 Nappe. The fold is associated, in all lithologies but particularly 16 within the Ben Lawers Schist, with upright intermediate- and small- 17 scale folds trending east-south-east and their axial-planar 18 crenulation cleavage. The vergence relations of the small-scale 19 folds and of the cleavage across the major fold are particularly 20 clear. 21 22 21.4 Conclusions 23 24 The Ben Lawers GCR site provides exceptional sections through four 25 of the main formations of the Argyll Group and is of national 26 importance in providing remarkably clear evidence of the total 27 inversion of the sequence in this part of the Central Grampian 28 Highlands, on the lower limb of a major early fold, the Tay Nappe. 29 30 These upside-down rocks constitute the Flat Belt, which extends 31 south-east across strike for 25 km to the Highland Border and for 32 some 250 km laterally along strike, and dominates the structure of 33 the south-eastern Grampian Terrane. Two generations of minor 34 structures, folds and cleavage planes, related to the D1 and D2 35 movements that produced the nappe are well displayed. 36 Within the area of the GCR site another major fold that later 37 folded the inverted rocks, is totally displayed, together with 38 associated minor structures. This F4 fold, the Ben Lawers Synform, 39 is one of the most important late folds in the overall Dalradian 40 structure. 41 42 22 CRAIG AN CHANAICH TO FRENICH BURN 43 (NN 811 546–NN 818 545) 44 45 J.E. Treagus 46 47 48 49 22.1 Introduction 50 51 The Craig an Chanaich to Frenich Burn GCR site incorporates the 52 principal exposures of a body of stratiform baryte that is unique 53 in the British Isles and in Europe. As well as the unusually pure, 54 bedded baryte, the body also contains rare barium silicate minerals 55 and stratiform sulphides. It has a maximum thickness of 80 m and 56 extends for 7 km to the east of the GCR site along the Meall 57 Tairneachan–Farragon Hill ridge between Strath Tummel and Strath 58 Tay. The baryte occurs within the Ben Eagach Schist Formation of 59 the Easdale Subgroup; it is involved in all three of the regional 60 phases of deformation and the regional metamorphism. 61 62 63 64 65 1 2 3 4 The mineralization was first discovered by the Geological Survey 5 in 1975 during stream-sediment sampling and was described by Coats 6 et al. (1978, 1981). It was the subject of a PhD. thesis by Moles 7 (1985a) and was summarized by Hall (1993) and by Pattrick and 8 Treagus (1996). The consensus of this research was that the body 9 is a sedimentary deposit, resulting from the exhalation of metal- 10 laden fluids onto the floor of a deep-sea basin. Other barium 11 12 enrichments in the Ben Eagach Schist are now known near at 13 Loch Kander (NO 192 803) and Allt an Loch (NO 196 796) (Gallagher 14 et al., 1989) and near Loch Lyon (NN 414 383) (Coats et al., 1984). 15 The body has been the subject of extensive underground and surface 16 mining (Foss Mine) from 1984 to the time of writing (1999); the GCR 17 site is largely comprised of the artificial exposures created by 18 this mining activity. 19 20 22.2 Description 21 22 Natural exposures of the mineral body are generally poor, the most 23 significant occurring on Creag an Chanaich (NN 8122 5457), in the 24 Frenich Burn East (NN 8196 5492) and on Creag an Loch (NN 8267 25 5504). However, in the area of the GCR site, around Foss Mine, the 26 body has been exposed extensively in opencast workings, along the 27 access road on Creag an Chanaich and around the adits to the 28 underground workings, especially on the north-eastern face of Creag 29 30 an Chanaich (Figure 51). 31 The total thickness of the mineral body reaches 80 m, and the 32 maximum thickness of baryte is 15 m. It is hosted by the graphitic 33 muscovite schist of the Ben Eagach Schist Formation and is close 34 to, or at the contact with, the calcareous schist of the overlying 35 Ben Lawers Schist. There are two main mineral horizons, the Upper 36 and Lower horizons. The high-grade Upper Mineral Horizon is 37 characterized by bedded pyritiferous baryte, generally bounded 38 above and below by quartz-celsian (barium feldspar) rock, barium- 39 enriched muscovite schist and sulphide-bearing graphitic schist. 40 Layers of massive sulphide (pyrite, chalcopyrite, sphalerite and 41 galena) are rare but significant components. The thicknesses and 42 relative importance of the mineral lithologies vary considerably 43 along strike and down dip, but the bedded baryte and the quartz- 44 celsian rock are major components and this is one of the largest 45 occurrences of celsian in the world. The description here is 46 confined to the exposures on or near to the mine track at Creag an 47 48 Chanaich and those in the opencast pits to the east (Figure 53). 49 The most westerly exposure of the body is seen on the western 50 slope of Creag an Chanaich, where massive baryte of the Upper 51 Mineral Horizon, dipping steeply to the south, is overlain by 52 quartz-celsian rock at NN 8109 5461. On the ridge of Creag an 53 Chanaich, between NN 8114 5459 and NN 8122 5457, the 3.5 m-thick 54 baryte body, containing pyritic layers, is overlain by 1 m of 55 siliceous schist and then by quartz-celsian rock. In this area, 8 56 m of graphitic schist separate the Upper Mineral Horizon from the 57 Ben Lawers Schist. On the steep, east slope of Creag an Chanaich, 58 the surface trace of the mineral body is controlled by two WSW- 59 plunging regional fold sets; F2 folds, which were originally 60 upright, are here locally recumbent, having been affected by a 61 62 63 64 65 1 2 3 4 major F3 fold-pair (Figure 52) that is well seen in the outcrop 5 sub-parallel to the hairpin bends of the mine track, especially 6 between NN 8128 5457 and NN 8131 5453 (Figure 51). One metre of 7 baryte is here enclosed in an envelope of quartz-celsian rock, 2 m 8 thick below and over 3 m thick above, which is affected by a F2 9 fold-pair (Figure 51). At NN 8135 5453 the baryte and quartz- 10 celsian rock form the core of an F2 antiform. The more-penetrative 11 12 S2 cleavage, as well as a superimposed well-developed upright S3 13 crenulation cleavage, is well seen in the schists adjacent to these 14 folds. Within the outcrop of the Ben Eagach Schist in this area is 15 a 3 m-thick, broadly concordant sheet of calcareous hornblende 16 schist (a basic meta-), which crosses the track in 17 several places on the hairpin bends. 18 The mineral body is also well exposed in the western wall of the 19 track, in a disused adit (NN 8135 5455). Here the baryte layer is 20 4 m thick, massive, white or grey depending on the concentration of 21 pyrite layers and occupies the synformal core of an F2 fold. A 10 22 cm-thick layer of massive pyrite is present at the stratigraphical 23 base of the baryte, in the north wall of the adit. To the east a 24 further (lower) adit has been driven into the baryte, which is here 25 3.5 m thick in an envelope of quartz-celsian rock in an F3 26 antiformal structure. Farther down the track, the Ben Eagach 27 Schist–Ben Lawers Schist contact is exposed (NN 8137 5450). A 28 structural synthesis of these exposures in the mine track is shown 29 30 in Figure 52. 31 The Lower Mineral Horizon is also present on Creag an Chanaich, 32 although it thins progressively eastwards (Figure 51). A small 33 outcrop is present on the top of the ridge at NN 8122 5464 and 34 comprises quartz-celsian rock with a sheet of heavily weathered 35 calcareous hornblende schist (basic meta-igneous rock) at the base; 36 the associated scree contains massive baryte. A trench by the 37 track at NN 8124 5463 has exposed 2–3 m of pyritic quartz-celsian 38 rock with 1 m of basic meta-igneous rock at the basal contact; the 39 lithologies are repeated in an F3 fold core and the Ben Eagach 40 Schist is exposed in the southerly end of the trench, in which a 41 flat-lying crenulation cleavage (local S4) can be seen as well as 42 D3 and D2 folds and cleavages. In the most-easterly exposure of 43 the Lower Mineral Horizon, at NN 8133 5457, 30 cm of pyritic 44 quartz-celsian rock and sulphide-rich schist are found; here too 45 the lower contact of the horizon is marked by a 50 cm-thick sheet 46 of basic meta-igneous rock. 47 48 The surface trace of the Upper Mineral Horizon is offset by a pair 49 of NNE-trending, left-lateral faults that separate the underground 50 workings from the opencast pits; the area between the faults is 51 occupied by mine buildings and plant (Figure 51). The most 52 easterly fault-plane is exposed in the western end of the opencast 53 workings and contains an altered microdiorite dyke (NN 8144 5446). 54 To the east of the fault, the mineral horizons have been mined in 55 several pits, which are exploiting the thickest, near-surface 56 occurrences of the baryte (Figures 51 and 52). In the south- 57 western part of the opencast workings, exploitation has focussed on 58 baryte rock in the core of a southwards plunging F2 synform. On 59 the southern limb of this synform the Ben Lawers Schist, the 60 muscovitic, graphitic, Ben Eagach Schist and quartz-celsian rock 61 62 63 64 65 1 2 3 4 are well exposed in the pit wall; the Ben Eagach Schist is also 5 exposed in the core of the synform at NN 8147 5448 (Figure 53). An 6 F3 antiformal structure causes the baryte and quartz-celsian rock 7 to crop out again to the north at NN 8152 5450. To the east, the 8 outcrop of the Upper Mineral Horizon is complicated by small NE- 9 trending faults and F2/F3 folding and thins rapidly towards the 10 Frenich Burn Fault. 11 12 13 22.3 Interpretation 14 15 The mineral body in the area of the GCR site is considered to be a 16 SedEx-type (sedimentary exhalative) deposit (Coats et al., 1980; 17 Willan and Coleman, 1983; Russell et al., 1984; Moles, 1985a; Hall, 18 1993). It is thought to have resulted from the exhalation of 19 metal-laden mineralizing fluids onto the floor of a marine basin at 20 the time of Easdale Subgroup sedimentation. The baryte formed by 21 the mixing of barium, carried in reduced hydrothermal fluids, with 22 sulphate in the relatively oxidized seawater. Barium also reacted 23 with the aluminosilicate-rich seafloor muds to produce barian clays 24 and cymrite (BaAl2Si2O8.H2O), the precursors to the barian and 25 barian feldspar (celsian), the transition taking place during 26 metamorphism, with estimates of a pressure of 6.5 kbar and a 27 temperature of 530oC (Moles, 1985b). The hydrothermal fluid was 28 also the source of the metals for the sulphides and iron oxides 29 30 but, in contrast to many SedEx deposits, these rarely formed thick 31 accumulations. Exhalative silica precipitated as cherts in the 32 aluminosilicate muds and recrystallized during metamorphism. 33 The mineral horizons are interpreted as successive exhalative 34 hydrothermal pulses emanating onto the seafloor, the variation in 35 the horizons being a function of both intensity of hydrothermal 36 activity and the distance from exhalative centres. Although two of 37 the horizons are laterally very extensive, occurring along the 2 km 38 strike length in the GCR site area and extending for a further 7 km 39 eastwards, the recognition of exhalative centres within these 40 horizons has not proved possible. To explain the sharp contacts to 41 the massive baryte layers, Russell et al. (1984) suggested that the 42 sulphate for the baryte was derived from evaporitic shallow coastal 43 areas which periodically produced flows of dense sulphate-rich 44 brine into the sulphate-poor basin deeps at the time of 45 hydrothermal activity, thus providing an episodic and finite 46 sulphate supply. 47 48 The fundamental cause of the mineral influx is considered to be 49 the crustal instability during Argyll Group times that resulted in 50 the break-up of the Dalradian basin into several second-order 51 basins (Coats et al., 1984; Russell et al., 1984; Pattrick and 52 Treagus, 1996, figure 7b). The basic meta-igneous rocks of oceanic 53 affinity in the Farragon Volcanic Formation overlying the Ben 54 Lawers Schist are a product of the continuation of this process as 55 the basins extended and deepened. The thick basic meta-igneous 56 sheets in the mine area that post-date the mineralization are 57 interpreted as sills and are also associated with this process. 58 The crustal extension and thinning that led to the break-up of the 59 basin resulted in a high geothermal gradient and large-scale 60 convection of seawater into the underlying lithologies (to a depth 61 62 63 64 65 1 2 3 4 of at least 10 km) where it was heated by interaction with the 5 buried older Dalradian lithologies. This fluid leached metals from 6 the rocks (especially barium from the feldspathic Grampian Group), 7 before rising to the seafloor to form the SedEx deposit. These 8 same conditions prevailed over all of the Dalradian basins at this 9 time and resulted in similar mineralization being developed in the 10 Easdale Subgroup elsewhere. 11 12 In terms of the regional structure, the deposit lies on the steep, 13 south-dipping limb of the major upward-facing F1 Creag an h-Iolaire 14 Anticline (Treagus, 2000, figure 14); there is no significant 15 mineral body on the north limb of this fold. F1 minor folds have 16 not been recorded, but the structural relationships between the 17 well-developed minor folds and cleavages of D2 and D3 age (as well 18 as of the local D4) in the area of the GCR site are exceptionally 19 clear. They also clearly reveal the geometry and superimposition 20 of major folds of both D2 and D3 ages (Figure 52). 21 22 22.4 Conclusions 23 24 The Creag an Chanaich to Frenich Burn GCR site is of international 25 importance in providing excellent exposure of a metamorphosed 26 synsedimentary mineral body, which is unique in British and 27 European geology and has few equals worldwide. The barium sulphate 28 mineral baryte (BaSO ), which forms a large proportion of the body 29 4 30 in an unusually pure concentration, has been an important source of 31 drilling mud for oil exploration in the North Sea. In addition it 32 contains concentrations of sulphide minerals, as well as two 33 unusual barium silicate minerals, celsian (barium feldspar) and 34 cymrite (a hydrous aluminosilicate). The latter two minerals have 35 been used to calculate the temperature and pressure (and hence the 36 depth of burial) to which the Dalradian rocks were subjected during 37 the creation of the mountain-belt; and this is one of the largest 38 known ocurrences of celsian in the world. 39 The unique feature of this body is that it is of sedimentary 40 origin, believed to have been precipitated directly from brines in 41 a small marine basin, fed by metal-rich hot fluids percolating 42 through the underlying sedimentary pile. The site also provides 43 unusually clear minor structures (folds and cleavages) of two 44 generations, which affect the mineral body and which have been used 45 in the understanding of the geometry of some major folds that make 46 up the Grampian fold-belt. 47 48 49 23 AUCHTERTYRE 50 (NN 354 291–NN 368 312) 51 52 R.A. Scott 53 54 55 23.1 Introduction 56 57 The Auchtertyre GCR site, between and Tyndrum, contains 58 the most extensive and most accessible exposures of stratabound 59 sulphide mineralization in the Ben Challum Quartzite Formation, a 60 unit of restricted distribution at the base of the Crinan Subgroup. 61 62 63 64 65 1 2 3 4 Characteristic features of the quartzites are distinctive minor 5 folds with strongly curving axes, which are restricted in this area 6 to this particular formation. 7 The Tyndrum area occupies an important position for structural and 8 stratigraphical correlations with adjacent regions. However, until 9 the 1980s, the area had remained essentially unmapped since the 10 original survey of Sheet 46 (, 1900) by B.N. Peach and 11 12 others. It was the discovery of stratabound units of sulphide 13 mineralization as part of the Mineral Reconnaissance Programme of 14 the Geological Survey that provided the main incentive for renewed 15 stratigraphical and structural study (Smith et al., 1984, 1988; 16 Fortey and Smith, 1986; Hall, 1993). 17 The new mapping in the Auchtertyre area revealed a thick sequence 18 dominated by feldspathic quartzites, between the Ben Lawers Schist 19 Formation and the Ben Lui Schist Formation (Figure 54a). This was 20 named the Ben Challum Quartzite Formation and was considered to be 21 equivalent to the Farragon Volcanic Formation at the top of the 22 Easdale Subgroup, a unit of amphibolites and quartzites, which in 23 upper Glen Lyon has a maximum thickness of 25 m. Subsequently it 24 has been re-assigned to the Crinan Subgroup on sedimentological 25 grounds, although it might merely reflect a diachronous facies 26 variation and hence be broadly equivalent in time to the Farragon 27 Formation. Scott (1987) showed that the Ben Challum Quartzite 28 Formation forms a continuous mappable unit, which can be traced 29 30 from the Tyndrum Fault to the Garabal Hill Fault, a total distance 31 along strike of approximately 20 km. The formation is restricted 32 to the area bounded by these faults. A maximum thickness of 500 m 33 is likely but the formation thins towards the east and possibly 34 also in the west, close to the Tyndrum Fault. With the exception 35 of the Ben Challum Quartzite Formation, the Argyll Group 36 stratigraphy of the Tyndrum area is identical to that in areas to 37 the south-west and north-east (Roberts and Treagus, 1979; Nell, 38 1984). 39 The principal exposures of the formation occur at Auchtertyre, 40 where river sections drain south-westward into Strath Fillan from 41 the watershed with Glen Lyon and Glen Lochay to the north-east 42 (Figure 54a). Other than the river sections, most of the valley 43 floors are filled with thick glacial till and moraine. Exposures 44 also occur on the main peaks that form the watershed (e.g. Ben 45 Challum, 1022 m, NN 388 322) but in general these are not as easily 46 accessible or as informative as the river sections. 47 48 49 23.2 Description 50 51 The GCR site encompasses three main river sections through the Ben 52 Challum Quartzite Formation (Figure 54a): the Allt Gleann a’ 53 Chlachain, the Allt a’ Chaol Ghlinne and, below the confluence of 54 these two rivers, the Allt Auchtertyre. These rivers provide near 55 continuous exposure through the entire formation at its thickest 56 part. 57 The Ben Challum Quartzite Formation contains a variety of rock 58 types but is dominated by well-bedded quartzites rich in 59 plagioclase feldspar in the range albite-oligoclase. In some beds 60 feldspar is more abundant than quartz (Scott et al., 1988). The 61 62 63 64 65 1 2 3 4 rock is generally fine to medium grained, but locally contains 5 coarser pebbly beds. Beds range in thickness up to 2 m but are 6 usually less than 0.5 m; bedding is normally continuous with no 7 internal cut-offs and lacks recognizable way-up criteria. 8 Individual beds are separated by micaeous layers (mainly muscovite) 9 generally less than 10 mm thick. Rarely the rock as a whole 10 becomes more pelitic and garnetiferous, and as such is then 11 12 indistinguishable from the overlying Ben Lui Schist Formation. 13 Conformable amphibolite units up to 2 m thick occur sporadically 14 throughout the Ben Challum Quartzite Formation, and this contrasts 15 with the Farragon Volcanic Formation, which is dominated by 16 amphibolite. The fine-scale layering in some amphibolite units was 17 clearly disrupted before the deformation described below. Several 18 layers of calcareous semipelite up to 1 m thick form a minor 19 component of the sequence (e.g. at NN 3601 3057), and minor 20 occurrences of graphitic pelite occur in the lower part of the 21 formation. 22 The Ben Challum Quartzite Formation contains two units of 23 stratabound sulphide mineralization: the Auchtertyre Horizon and 24 the overlying Ben Challum Horizon (Fortey and Smith, 1986; Smith et 25 al., 1988; Scott et al., 1988, 1991). The weakly mineralized 26 Auchtertyre Horizon is about 80 m thick in total; the best 27 exposures are around the intersection of Allt Gleann a‘ Chlachain 28 and the Allt a‘ Chaol Ghlinne (NN 3543 3023), and a further 100 m 29 30 upstream in the Allt Gleann a‘ Chlachain (Figure 54a). 31 Mineralization occurs as bedding-parallel sulphide laminae, up to 32 10 mm thick, in which pyrite predominates over minor chalcopyrite 33 and sphalerite, and leads to a characteristic rusty weathering of 34 the host quartzites. There are substantial thicknesses of non- 35 mineralized quartzite in the total thickness. 36 The Ben Challum Horizon is approximately 20 - 30 m thick at the 37 type locality on Ben Challum (NN 387 322) to the north-east of the 38 GCR site, and on Creag Bhocan (NN 315 280), to the west of Strath 39 Fillan (Scott et al., 1988). It consists of layers of pyrite, 40 chalcopyrite and sphalerite, up to 0.3 m thick, in chloritic schist 41 and albitic quartzite. In the GCR site, the horizon is seen only 42 as about 10 m of pyritized quartzite immediately below the top of 43 the formation in the Allt Auchtertyre at NN 3541 3013. 44 An enigmatic unit of ultramafic rock characterizes the boundary 45 between the Ben Challum Quartzite Formation and the overlying Ben 46 Lui Schist Formation. It can be traced intermittently across the 47 48 Tyndrum area, but exposure of this part of the succession is poor 49 in the Allt Auchtertyre; the unit is best exposed on the flanks of 50 Ben Challum and on Creag Bhocan (Scott et al., 1988). The variable 51 mineralogy of the unit is compatible with an igneous origin and a 52 subsequent high degree of alteration. It generally contains a 53 combination of chlorite, ankerite, talc, fuchsite (chromian 54 muscovite), quartz and amphibole, with accessory chromium- and 55 zinc-bearing spinel, thiospinel, pyrite, pyrrhotite, pentlandite 56 and millerite (Scott, 1987). The exposures in the Allt Auchtertyre 57 occur close to the railway bridge and include chloritic schist 58 containing carbonate porphyroblasts over 50 mm in diameter. 59 Two principal phases of ductile deformation are recorded in the 60 Ben Challum Quartzite Formation. The most striking feature of the 61 62 63 64 65 1 2 3 4 quartzites is the presence of non-cylindrical minor folds with 5 strongly curving axes, of which numerous examples can be observed 6 at all stratigraphical levels (Scott, 1987). The best examples 7 occur within well-bedded feldspathic quartzites in which individual 8 beds have constant thickness and are separated by thin partings of 9 mica schist (Figure 55). Non-cylindrical minor folds do not occur 10 in any of the adjacent formations. 11 12 The non-cylindrical folds are generally tight and fold hinges 13 become more curved with a decrease in interlimb angle; a number of 14 morphological types are present (Scott, 1987). The overall range 15 of orientation of the hinges approaches 180º (Figures 54b and 55) 16 so that many hinges pitch down the dip of the schistosity. The 17 effect of this is that subparallel hinges within a few metres of 18 each other face in opposite directions. Exposed folds range in 19 amplitude from tens of centimetres to 3 metres, but reversals of 20 vergence indicate that the largest fold amplitudes must be of the 21 order of 10 - 20 m. The vergence of such reclined fold-pairs can 22 be difficult to determine but, when ‗restored‘ to a horizontal 23 position, it can be ascertained that most fold-pairs, minor and 24 major, verge to the south. Disharmonic folding affects sequences 25 where there are marked differences in bed thickness and competence. 26 In suitable pelitic lithologies a well-developed axial-planar 27 crenulation schistosity is present. These folds affect the earlier 28 bedding-parallel schistosity as well as, locally, a strong 29 30 lineation of the quartz and feldspar fabric that is developed on 31 that schistosity. Rarely, isoclinal folds of bedding are seen to 32 pre-date the main fold set (Figure 56). No minor ductile 33 structures post-dating the non-cylindrical folds were identified in 34 the field, although several minor deformation phases have been 35 identified elsewhere in the area (Scott, 1987). 36 A subvertical NNE-trending fault-system, named the Auchtertyre 37 Fault, causes apparent sinistral displacement of the boundary 38 between the Ben Challum Quartzite Formation and Ben Lui Schist 39 Formation by as much as 800 m (Figure 54a). The fault can be 40 observed in the Allt Auchtertyre. 41 42 23.3 Interpretation 43 44 The presence of layers of stratabound sulphide has promoted 45 investigation of the Ben Challum Quartzite, both for scientific 46 reasons, and in terms of its resource potential (e.g. Smith et al., 47 48 1984, 1988; Fortey and Smith, 1986; Fisk, 1986; Scott, 1987; Scott 49 et al., 1988, 1991). Although no economic mineral occurrences have 50 been discovered, the presence of this mineralization is important 51 scientifically for two main reasons: (1) the nature of the 52 mineralization provides information on the tectonic setting of 53 sediment deposition and the geometry of depocentres; (2) 54 stratabound mineralization is formed by relatively short-lived 55 hydrothermal exhalation events on the sea floor. Geographically 56 widespread mineralized units of limited thickness can therefore be 57 regarded as time-lines in a succession otherwise devoid of 58 chronostratigraphical control. In the Tyndrum area, the 59 mineralization indicates a broad consistency between 60 chronostratigraphy and lithostratigraphy. 61 62 63 64 65 1 2 3 4 The sediments that formed the Argyll Group are generally 5 considered to have been deposited in an actively extending basin 6 (e.g. Anderton, 1982), and there are clear indications of 7 synsedimentary faulting at a number of levels within the Argyll 8 Group succession in the area surrounding Auchtertyre. The evidence 9 comes from the nature and geometry of sediment packages, the 10 distribution of stratabound mineralization, and the metal zonation 11 12 within individual mineralized layers (Scott, 1987). The exact 13 position and original orientation of the bounding faults is 14 difficult to constrain because of the subsequent multiphase 15 deformation history. However, all the evidence is consistent with 16 a fault-bounded depression in the Tyndrum area that most probably 17 had a half-graben geometry with the controlling fault on the north- 18 west side. The presence of a fault-bounded basin is the principal 19 reason why the Ben Challum Quartzite Formation accumulated only in 20 this area, and why exhalative submarine brines were also ponded in 21 the same depocentre. 22 Although the evidence for active extension during Argyll Group 23 sedimentation is convincing, the tectonic setting in which it took 24 place has been disputed, and has ranged from a marginal basin 25 setting (e.g. Wright, 1976; Henderson and Robertson, 1982; Nell, 26 1984) to an intracontinental basin setting (e.g. Phillips et al. 27 1976; Harris et al., 1978). There are two principal strands of 28 evidence from the Ben Challum Quartzite Formation that can be used 29 30 to imply an arc-related marginal basin setting. 31 (1) A major change of sandstone provenance in the Dalradian 32 succession appears to occur between the deposition of the Carn 33 Mairg Quartzite and earlier formations, in which K-feldspar is the 34 dominant feldspar, and the Ben Challum Quartzite and later 35 formations, in which plagioclase is the dominant feldspar. It is 36 no coincidence that this change occurs across an interval of the 37 succession characterized by the onset of extension, stratabound 38 mineralization and significant magmatism. 39 The albite-rich composition of the Ben Challum Quartzite Formation 40 has led to speculation that the sequence may include keratophyre 41 tuffs (Fortey and Smith, 1986; Fisk, 1986). No textural evidence 42 now remains for a direct igneous origin but, considering the high 43 plagioclase content, it does not seem unreasonable to conclude that 44 the Ben Challum Quartzite Formation was derived by erosion of 45 igneous rocks of silicic to intermediate composition, possibly with 46 a direct volcanic input. A preliminary whole-rock geochemical 47 48 study of psammites from the Ben Challum Quartzite Formation is 49 consistent with a back-arc or continental margin arc setting 50 (Scott, 1987). 51 The mineralogy of the apparently conformable ultramafic unit at 52 the top of the Ben Challum Quartzite Formation is compatible with 53 an igneous origin, and represents a more-intense state of 54 alteration than serpentinization (Scott, 1987). The chemistry is 55 also consistent with an origin as an igneous body, or as a 56 sedimentary derivative, which was hydrothermally altered on, or 57 close to, the seabed. More-definite conclusions about the unit‘s 58 origin are precluded by the degree of alteration, but the presence 59 of ultramafic rocks is compatible with an extensional setting in a 60 marginal, possibly back-arc basin. 61 62 63 64 65 1 2 3 4 (2) The base-metal content of the mineralized layers in the Ben 5 Challum Quartzite Formation is indicative of volcanic-associated 6 massive sulphide (VMS) deposits, an unsurprising interpretation 7 considering the abundant evidence for magmatic activity in the host 8 succession. The extreme length/thickness ratio of the mineralized 9 units in the Ben Challum Quartzite Formation, and the absence of an 10 Fe-Mn-oxide or baryte facies, suggests similarities with Besshi 11 12 deposits, a type of arc-related VMS deposit described from Japan 13 (Scott et al., 1988, 1991). 14 The closely packed, non-cylindrical minor folds, with strongly 15 curving hinges (Figure 55), occur in this area only in the Ben 16 Challum Quartzite Formation. These folds refold an earlier set of 17 fold hinges as well as the earliest (S1) schistosity and are 18 associated with a tight crenulation cleavage and garnet growth. 19 They can be clearly correlated with the main regional D2 20 deformation phase. The entire outcrop of the Ben Challum Quartzite 21 Formation lies on the right-way-up limb shared by the F2 Dalmally 22 Antiform to the south (Figure 54a) and the F2 Ra Creag Synform to 23 the north (Roberts and Treagus, 1975). This is supported by the 24 dominant southerly vergence sense of minor fold-pairs observable at 25 the site. The axial-plane traces of the major folds trend ENE, and 26 rocks on the shared limb generally dip at moderate angles to the 27 south. The geometrical relationships between the isoclinal folds 28 and mineral lineations that pre-date the dominant D2 set (Figure 29 30 56) are not clear, but both are interpreted to be of the regional 31 D1 phase, and related to the regional major F1 fold, the Ardrishaig 32 Anticline (Roberts and Treagus, 1975). 33 The origin of the non-cylindrical folds with curving axes is not 34 entirely clear. Many other examples of such folds have been 35 ascribed to their occurrence in zones of extreme strain; however, 36 this does not appear to be the case at Auchtertyre. Here, two 37 factors are considered to be important: (1) the mechanical 38 properties of the quartzite; and (2) the position of the outcrop 39 relative to major D2 structures (Scott, 1987). 40 The presence of competent quartzite beds separated by thin pelitic 41 partings has led to pronounced strain partitioning as beds moved 42 relative to each other during folding (Scott, 1987). Internally, 43 the quartzite beds were not subjected to intense stretching; pebbly 44 beds, for example, do not show any significant elongation of 45 clasts, which commonly show only a weak flattening in the 46 approximate plane of bedding. It is not uncommon to find S1 fabric 47 48 elements preserved within quartzite beds. In contrast, pronounced 49 mineral lineations formed by microboudinaged garnet crystals within 50 the pelitic seams indicate high D2 strain. This interbed slip was 51 promoted both by the continuous nature of the bedding and by the 52 overall structural position on a major F2 fold limb. In this 53 location, it can be assumed that the principal D2 elongation 54 direction would lie close to the average plane of layering, thus 55 enhancing the tendency for layer-parallel shear. 56 As folding progressed, the initiation of minor folds in individual 57 quartzite beds was impeded by adjacent beds, from which they were 58 separated by a minimal thickness of pelitic material. In a 59 succession where interbed slip was likely, the presence of F2 60 buckles or existing F1 hinges would cause variations in cohesion 61 62 63 64 65 1 2 3 4 and heterogeneous strain, thus providing a mechanism by which non- 5 cylindrical hinges could develop. There is no evidence to suggest 6 significant transection of axial planes by the schistosity 7 associated with the folds, which elsewhere has been reported to be 8 a feature of this style of folding (Treagus and Treagus, 1981). 9 10 11 23.4 Conclusions 12 13 The Auchtertyre GCR site illustrates a style of stratabound 14 sulphide mineralization that is unknown elsewhere in the Dalradian 15 of Scotland. The mineralization occurs within the Ben Challum 16 Quartzite Formation, a unit restricted to the Tyndrum area, which 17 is important scientifically because of the insight it provides into 18 the depositional settings of Argyll Group sediments. There is 19 clear evidence that faults were active during deposition, that they 20 acted as conduits for mineralizing fluids, and that they created 21 the topography necessary to pond the fluids once they had been 22 exhaled onto the sea bed. Contemporaneous magmatic activity 23 provided the source of heat required to circulate the fluids 24 through crustal rocks beneath the sea floor, where they were able 25 to gather base metals such as copper and zinc. The chemistry and 26 geometry of the mineralized units, combined with the mineralogy of 27 their host sediments, suggest an island-arc-related tectonic 28 setting. 29 30 An additional feature of the Ben Challum Quartzite Formation is 31 the presence of well-exposed minor folds with exceptionally curved 32 hinges, a feature found in this area only in this formation. The 33 distinctive folds are thought to reflect the mechanical properties 34 of the unusually well-bedded quartzites, which were able to slip 35 relative to each other along intervening micaceous seams. The 36 minor folds in the section also provide important information 37 concerning the geometry of the major regional folds of the area. 38 This GCR site provides the only section through the entire Ben 39 Challum Quartzite Formation and offers an excellent opportunity for 40 further research into both stratabound mineralization and fold 41 mechanisms. 42 43 24 BEN OSS 44 (NN 291 265–NN 296 271) 45 46 47 J.E. Treagus 48 49 50 24.1 Introduction 51 52 The Ben Oss GCR site has been selected to exemplify the fracture 53 history of the Tyndrum Fault, one of the major system of NE- 54 trending, dominantly left-lateral faults that dissect the Grampian 55 Terrane between the Great Glen and the Highland Boundary faults 56 (Figure 1 and Stephenson et al., 2013a, fig. 1). The principal 57 period of fault movement was in the late Silurian and is closely 58 associated with Caledonian intrusions, such as the adjacent Garabal 59 Hill–Glen Fyne granitic pluton and microgranite and appinitic 60 microdiorite dykes in the Tyndrum area. Late- quartz- 61 62 63 64 65 1 2 3 4 dolerite dykes, with a regional east to east-north-east trend, cut 5 across the fault-zone. 6 Previous studies have established the strike-slip component of 7 movement on this system of faults, although the importance of dip- 8 slip components has also been recognized (Anderson, 1951; Johnstone 9 and Wright, 1957; Pitcher, 1967). Treagus (1991) constructed 10 geological profiles on either side of some of the principal fault 11 12 planes within the Grampian Terrane to demonstrate that the 13 components of movement on the individual faults were both left- 14 lateral strike-slip (up to 8 km) as well as dip-slip (up to 2 km). 15 The Tyndrum Fault, in particular, was shown to have a significant 16 dip-slip component of 2 km, down to the east, as well as a left- 17 lateral strike-slip component, which is greatest (4 km) in the 18 central portion of the fault, the area of this GCR site. Treagus 19 et al. (1999) have made a detailed study of the fault and its 20 associated mineralization. 21 22 24.2 Description 23 24 This GCR site, which lies some 4 km south-west of the village of 25 Tyndrum, occupies a gully of the Allt Coire Chruinn, which drains 26 the northern slopes of Ben Oss (Figure 57). On these slopes, one 27 major fault and many minor fractures associated with the Tyndrum 28 Fault are particularly well exposed. The fault, termed here the 29 30 Ben Oss Fault, can be traced from NN 295 269 into good exposures in 31 the 300 m-long gully in the Allt Coire Chruinn at NN 291 265 to the 32 south-west. It is a major splay of and is very close to the line 33 of the Tyndrum Fault, which in the area of the site is occupied by 34 a microdiorite dyke. 35 At the north-east end of the gully, a quartz reef is exposed for 36 100 m in the hanging wall of the fault, which here trends 040° and 37 dips at 72° to the south-east. To the south-west the reef becomes 38 a breccia, composed of clasts of vein-quartz, 1–15 cm in length, in 39 a cataclastic matrix of indurated fine-grained quartz and micas, 40 evidence of extensional opening of the fault. In the wall formed 41 by the margin of the breccia, many fault-parallel fractures are 42 seen. These are marked by horizontal grooves, confirming strike- 43 slip movement, and by a series of steeply N-pitching pinnate 44 fractures, whose geometry indicates left-lateral movement. Many 45 steep SE-dipping fractures are seen in the footwall of the fault 46 (the north-west side of the gully), comprising both a fault- 47 ° 48 parallel set and another set orientated some 10–25 anticlockwise 49 to the fault. The former are interpreted as Y shears, the latter 50 as R shears, both with movement senses sympathetic to that of the 51 main fault (see Sibson, 1977). In the schists in the footwall, 2–3 52 m to the west of the fault plane, small-scale, steeply plunging, 53 drag-folding of bedding also indicates left-lateral movement. 54 There is also evidence of left-lateral movement in the area 55 immediately to the north-west of the fault gully. Here, major 56 fractures can be seen on the aerial photographs, branching at some 57 20° anticlockwise from the fault at regular 50 m intervals for 58 several hundred metres to the north of the gully (Figure 58). The 59 fractures contain both illite-quartz and pyritic gouges with 60 textures indicating left-lateral movement. One major fracture at 61 62 63 64 65 1 2 3 4 NN 2925 2677, occupied by the stream, contains a 3 m-thick quartz- 5 vein breccia, kink folds and bedding offsets in the schists that 6 show clear left-lateral displacements (Figure 57, locality marked X 7 and to its south-west). Slickensides and grooves (localities 8 marked S on Figure 57) plunge at an average of 2° towards 221° on 9 the fault-parallel fractures, and at 7° towards 197° on the 10 anticlockwise set, confirming the dominant strike-slip movement 11 12 sense. The anticlockwise set of fractures is not seen to the 13 south-east of the fault. 14 However, there is also evidence of right-lateral movement on the 15 fault. Measurements of bedding to the west of and adjacent to the 16 fault, at about NN 292 267, show a 100 m-wide fold plunging at 43° 17 towards 188°, the sense of deflection indicating right-lateral 18 shear. Moreover, the swing in strike of the regional bedding, 19 where followed for 1 km to the west of the fault, shows a similar 20 pattern that also indicates a right-lateral deflection. Small 21 right-lateral displacements and kinks have also been observed on 22 some anticlockwise fractures (e.g. the two localities indicated on 23 Figure 57), suggesting re-activation, and a 30 cm-wide Caledonian 24 dyke at NN 291 265 shows both left-lateral and right-lateral metre- 25 scale displacements. The north-western margin of the dyke that 26 occupies the Tyndrum Fault shows effects of further brecciation, 27 shearing and hydrothermal alteration (e.g. at NN 295 270), which 28 could represent this right-lateral movement. A thick late- 29 30 Carboniferous dyke shows a slight dextral deflection where it 31 crosses the fault at NN 296 271. 32 33 24.3 Interpretation 34 35 Regional considerations suggest that the earliest movement on the 36 Tyndrum Fault may have been a tensional opening as a normal fault 37 (see Treagus et al., 1999); this is supported by the 70o dip of the 38 Ben Oss Fault and associated fractures, as well as by the 39 hydrothermal breccias. Two fracture sets are evident in the GCR 40 site, a fault-parallel set and another some 10–25° anticlockwise to 41 the fault (Figure 58); these are interpreted as Y shears and R 42 shears, respectively, with sympathetic left-lateral shear in 43 response to movement on the major fracture, in the traditional 44 manner of fault analysis (Sibson, 1977). Many fault-related 45 features on minor fractures (pinnate structures, kink-folds, 46 bedding displacements and gouge textures) support the left-lateral 47 48 displacements. Thus the Ben Oss Fault can be confidently 49 interpreted as a major fault with left-lateral displacement. 50 The principal evidence for the magnitude of the left-lateral 51 displacement comes from the offset of the Ben Lawers Schist/Ben Lui 52 Schist junction. On the west side of the Ben Oss Fault, this 53 junction trends 040° and dips steeply to the south-east; it is 54 apparently displaced by some 3 km on the east side of the fault to 55 near Cononish (Figure 57, inset). However, Treagus (1991, figure 56 3) showed that the latter occurrence of the junction represents the 57 limb of a major F2 fold (a component of the regional Ben Lui Fold) 58 that is structurally lower than the junction on the western side of 59 the fault; the two fold limbs have been brought together by 60 61 62 63 64 65 1 2 3 4 movements which had components of displacement of 1.8 km dip-slip 5 (down to the east) and about 4 km left-lateral strike-slip. 6 Evidence for significant right-lateral movement in the fault-zone 7 can also be established from the displacement of the Ben Lawers 8 Schist/Ben Lui Schist junction in the Ben Oss area. There is a 9 narrow sliver of Ben Lawers Schist directly east of the Ben Oss 10 Fault, in which the junction is shifted some 300 m to the south- 11 12 west (Figure 57). This sliver is separated from the Ben Lui Schist 13 farther east by a 14 m-wide, steeply SE-dipping, Caledonian dyke of 14 porphyritic microdiorite. This dyke must be intruded along the 15 major displacement plane of the Tyndrum Fault-zone, immediately 16 east of the Ben Oss Fault. The interpretation of the movement 17 history of the zone is shown in the inset to Figure 57. The early 18 major movement displaced the Ben Lawers Schist/Ben Lui Schist 19 junction as described above (with components of displacement up to 20 8 km in a left-lateral sense and 1.8 km of dip-slip) from its 21 position at Cononish to that in the fault-bounded sliver. The main 22 fault plane was then intruded and sealed by the microdiorite dyke, 23 which is evidence of further extensional opening. A later right- 24 lateral movement occurred on the Ben Oss Fault, which brought the 25 Ben Lawers Schist/Ben Lui Schist junction back to its present 26 position west of the faults. The Ben Oss Fault is therefore 27 interpreted as a major splay of the Tyndrum Fault, having an 28 unknown amount of left-lateral shear, but with a subsequent right- 29 30 lateral movement of at least 300 m 31 There is no evidence of the early precious-metal-type 32 mineralization, seen elsewhere in the Tyndrum area, apart from 33 minor pyrite mineralization on the faults and minor fractures. 34 However, the later right-lateral movements are equated with those 35 at other localities in the Tyndrum area, where they are associated 36 with the base-metal mineralization (Treagus et al., 1999). 37 38 24.4 Conclusions 39 40 The Ben Oss GCR site provides exceptional exposures of a major 41 fault, the Ben Oss Fault, together with associated minor and major 42 fractures. Many of the features associated with major faults are 43 well displayed, such as fault breccias, fault-gouge clays, 44 slickensides and quartz veins. The fault is a component of the 45 Tyndrum Fault, also exposed at this site, which is one of the major 46 dislocations that traverse the Grampian Terrane from north-east to 47 48 south-west. 49 Evidence from the exposed fault planes, together with that seen on 50 aerial photographs, allows the movement sense on the major 51 structure to be predicted. Both faults have a complex history, 52 which principally involved a left-lateral sense of movement, such 53 that the north-west side of the faults moved sideways to the south- 54 west; the Ben Oss Fault probably only moved a few hundred metres 55 but the Tyndrum Fault moved a minimum of 4 km and possibly as much 56 as 8 km. The evidence also suggests that the history of the 57 faults, involved sideways movement in the opposite (right-lateral) 58 sense, as well as vertical movements. Regionally these movements 59 can be linked to the history of mineralization in the area. 60 61 62 63 64 65 1 2 3 4 5 ACKNOWLEDGEMENTS 6 7 This regional paper is the combined work of 5 authors, many of 8 whom, in addition to their own site descriptions, have made 9 valuable comments on other aspects of the work. Sadly, Peter 10 Thomas died whilst this special issue was being prepared for final 11 12 submission. He made major contributions to his own areas of the 13 Dalradian outcrop and to our overall understanding of Grampian 14 Highland geology. 15 The paper has been compiled by J.E. Treagus and has been edited by 16 D. Stephenson. S.H. Treagus is thanked for additional comments. 17 The GCR editor was P.H. Banham and the referee was M.R.W. Johnson, 18 who also provided valuable editorial suggestions. The project was 19 cofunded by the Joint Nature Conservation Committee (JNCC) and the 20 British Geological Survey (BGS) and has been managed by N.V. Ellis 21 for JNCC and D.J. Fettes and M. Smith for BGS. 22 The initial site selection and site documentation for the 23 Dalradian block of the Geological Conservation Review was by S.J. 24 Moorhouse. Since then, much new mapping and refined interpretation 25 has taken place, and the site list has been revised, firstly by 26 J.E. Treagus and subsequently through a panel consisting of R. 27 Anderton, A.L. Harris, J.R. Mendum, J.L. Roberts, P.W.G. Tanner, R. 28 Threadgould and J.E. Treagus. The necessary amendments to the GCR 29 30 documentation were greatly facilitated by R. Wignall (for Scottish 31 Natural Heritage). 32 K.M. Goodenough and S.W. Horsburgh assisted with preliminary 33 drafts of key maps and all diagrams were drafted for publication by 34 S.C. White (JS Publications, Newmarket) and K.J. Henderson (BGS, 35 Edinburgh). Photographs were scanned and prepared by B.M. McIntyre 36 (BGS, Edinburgh). Photographs from the BGS collection are 37 reproduced by kind permission of the Director, BGS © Natural 38 Environment Research Council; all rights reserved (PR/23–27). 39 The first complete draft of the Dalradian GCR was submitted to the 40 JNCC in June 2009. In 2010, the JNCC terminated its involvement in 41 Earth Science conservation and abandoned its contractual agreements 42 to publish the remaining GCR volumes. So, the authors are greatly 43 indebted to Diarmad Campbell, Chief Geologist Scotland for the BGS, 44 for funding the drafting of remaining figures and to the 45 Geologists‘ Association and Elsevier, for ensuring that this volume 46 is published as a Special Issue of their Proceedings. We are 47 48 particularly grateful to Neil Ellis of the JNCC for his efforts to 49 secure a new publisher and to Professor James Rose, Editor in Chief 50 of the PGA, for making it all happen. 51 Finally, on behalf of all of the site authors, we would like to 52 record our thanks to the owners and managers of land and quarries 53 who have allowed access to the sites, either during previous work 54 or specifically for the GCR exercise. 55 56 REFERENCES 57 58 Agrell, S. O. (1942) 1. A petrological investigation of the 59 adinoles at Dinas Head, Cornwall. 2. A petrofabric study of the 60 61 62 63 64 65 1 2 3 4 Ben Vuroch granite and the adjacent schists in . 5 Unpublished PhD thesis, University of Cambridge. 6 Ahmed-Said Y. and Tanner P. W. G. (2000) P-T conditions during 7 emplacement, and D2 regional metamorphism, of the Ben Vuirich 8 Granite, Perthshire, Scotland. Mineralogical Magazine, 64, 737–53. 9 Ague, J.J. & Baxter, E.F., 2007. 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(1989) An introduction to metamorphic petrology. 20 Longman, Harlow. 21 Yardley, B.W.D. and Valley, J.W. 1997. The petrologic case for a 22 dry lower crust. Journal of Geophysical Research, 106 B6, 12173– 23 12185. 24 25 Zeh, A. and Millar, I. L. 2001. Metamorphic evolution of garnet- 26 epidote-biotite gneiss from the Moine Supergroup, Scotland, and 27 geotectonic implications. Journal of Petrology 42, 529–554. 28 Zenk, M. and Schulz, B. 2004. Zoned Ca-amphiboles and related P-T 29 evolution in metabasites from the classical Barrovian metamorphic 30 zones in Scotland. Mineralogical Magazine, 68, 769-786. 31 32 33 Figure 1 Map of the Central Grampian Highlands, showing 34 Dalradian subgroups, major structures including the Boundary Slide 35 and locations of GCR sites. Only areas described in this regional 36 paper are ornamented. * On the limbs of the late Errochty Synform, 37 to the north of Schiehallion, highly attenuated condensed sequences 38 of the Lochaber and Ballachulish subgroups, too thin to be shown at 39 this scale, are present in the Boundary Slide-zone. 40 GCR sites: 1, River Leven Section, 2 Nathrach, 3 Rubha 41 Cladaich, 4 Tom Meadhoin and Doire Ban, 5 Stob Ban, 6 St John’s 42 Church, Loch Leven, 7 Onich Dry River Gorge and Onich Shore 43 Section, 8 Ardsheal Peninsula, 9 South Coast, Lismore Island, 10 44 Camas Nathais, 11 Port Selma, Ardmucknish, 12 River Orchy, 13 A9 45 Road Cuttings and River Garry Gorge, 14 Creag nan Caisean–Meall 46 Reamhar, 15 Meall Dail Chealach, 16 Strath Fionan, 17 Tempar 47 Burn, 18 Allt Druidhe, 19 Slatich, 20 Ben Lawers, 21 Craig an 48 Chanaich to Frenich Burn, 22 Auchtertyre, 23 Ben Oss. 49 Faults: BBF Bridge of Balgie Fault, ELF Ericht–Laidon Fault, GGF 50 Great Glen Fault, LTF Loch Tay Fault, TF Tyndrum Fault. 51 F1 and F2 folds: AS Appin/Cuil Bay Syncline, AHA Airds Hill 52 Anticline, BCS Beinn Chuirn Synform, BDS Beinn Donn Syncline, BLA 53 Ben Lui Antiform, BSA Beinn Sgluich Anticline, BUS Beinn Udlaidh 54 Syncline, BWA Blackwater Antiform/Treig Syncline, BWS Blackwater 55 Synform, CA Clunes Antiform, IA Inverlair Antiform, KA 56 Kinlochleven Antiform, LDS Loch Dochard Syncline, MRS Meall 57 Reamhar Synform, RA Ruskich Antiform, SBS Stob Ban Synform. 58 59 60 61 62 63 64 65 1 2 3 4 F3, F4 and later folds: BA Bohespic Antiform, BLS Ben Lawers 5 Synform, DD Drumochter Dome, ES Errochty Synform, TM Trinafour 6 Monoform. 7 8 Figure 2 Comparison of Dalradian successions in the Loch Leven 9 and Schiehallion (Strath Fionan) areas. 10 11 Figure 3 12 (a) Map of the Loch Leven area, showing outcrops of the main 13 stratigraphical units, major structures and locations of GCR sites. 14 BS Ballachulish Syncline, BaSl Ballachulish Slide, FWSl Fort 15 William Slide 16 (b) Diagrammatic profile of the area shown in (a), looking up- 17 plunge of F1 folds and showing position of GCR sites. Key, 18 abbreviations and horizontal scale as in (a). 19 20 Figure 4 21 (a) Map of the Schiehallion–Loch Tay area, showing outcrops of 22 the main stratigraphical units, major structures and locations of 23 GCR sites. 24 (b) Cross-section of the Schiehallion–Loch Tay area, showing 25 positions of GCR sites. Key, abbreviations and horizontal scale as 26 in (a). 27 28 Figure 5 Map of the River Leven section east of Kinlochleven. 29 30 Figure 6 Near-vertical bedding in the Binnein Quartzite, on a 31 horizontal surface in the River Leven at NN 1910 6186; the cross- 32 sets young to the west (top of photograph). Coin is 20 mm 33 diameter. (Photo: J.E. Treagus.) 34 35 Figure 7 Map of the area around the Nathrach GCR site, showing 36 the boundary between the Binnein Quartzite and the Binnein Schist 37 on the two limbs of the F1 downward-facing Mamore Syncline. At 38 localities A, B and C, relations between bedding and the three 39 cleavages are shown, as appropriate. The relation of bedding to 40 the direction of younging is shown at localities along the boundary 41 as well as at localities A and C, within the Binnein Schists. 42 43 Figure 8 Thinly-bedded quartzites and semipelites in the Binnein 44 Schists, tightly folded by F2 folds that plunge steeply to the 45 north-east at locality A, Figure 7 in the Nathrach GCR site. Coin 46 is 25 mm diameter. (Photo: J.E. Treagus.) 47 48 Figure 9 Semipelites interbedded in the Binnein Quartzite, as 49 seen on a surface sloping gently to the south-east (bottom right) 50 at locality B, Figure 7 in the Nathrach GCR site. Cross-bedding 51 near the top left corner youngs to the south-east and faces down on 52 the axial-planar fabric of the minor folds; these folds, which are 53 of D1 age, plunge at 40o to the north-east and verge to the north- 54 east. Not clear in the photograph is the S2 crenulation cleavage, 55 which cross-cuts the F1 folds parallel to the length of the pencil. 56 Pencil is 15 cm long. (Photo: J.E. Treagus.) 57 58 59 60 61 62 63 64 65 1 2 3 4 Figure 10 Map of the coastal section at the Rubha Cladaich GCR 5 site, showing the outcrops of the Glen Coe Quartzite, Binnein 6 Schists and Binnein Quartzite. Principal exposures are outlined. 7 8 Figure 11 D2 folds and cleavage in the Binnein Schists at the 9 Rubha Cladaich GCR site. F1 folds can be seen, for instance, at 10 the tip of the pencil. Pencil is 10 cm long. (Photo: J.E. 11 Treagus.) 12 13 Figure 12 Regional geological context of the Tom Meadhoin and 14 Doire Ban GCR site. After Roberts and Treagus (1977b, figure 5). 15 16 Figure 13 Map of the Stob Ban GCR site. After Roberts and 17 Treagus (1977b, figure 5). 18 19 Figure 14 Stob Ban (left) viewed from the east, showing the 20 Binnein Quartzite dipping to the north-west (away from the camera) 21 on the south-east limb of the Stob Ban Synform. In the centre, the 22 area of the Stob Ban GCR site, beds of Leven Schists and 23 Ballachulish Limestone dip to the south-east on the north-west limb 24 of the synform. (Photo: D. Stephenson, BGS No. P 726592.) 25 26 Figure 15 Map of the Loch Leven shore section at St John’s church 27 after Bailey (1960). 28 29 Figure 16 View looking north-east along the outcrop of the 30 Ballachulish Slide on the shore of Loch Leven, near St John’s 31 church. The slide occurs beneath the hammer shaft and can be 32 traced along the black dotted line. To its right is the 33 Ballachulish Limestone; immediately left of the hammer, and for 34 about one metre beyond, are a few centimetres of quartzitic Leven 35 Schists; the remainder of the exposure left of the slide is a NE- 36 trending dyke. Hammer shaft is 30 cm long. (Photo: J.E. Treagus.) 37 38 Figure 17 Map of the Onich Shore Section and the south-west end 39 of the Onich Dry River Gorge GCR sites. Areas outlined by dotted 40 lines are exposures discussed in the text, showing typical 41 measurements of bedding (with direction of younging where 42 appropriate) and plunge (and vergence where appropriate) of F1 43 minor folds. 44 45 Figure 18 Generalized structural profile of the F1 Appin 46 Syncline for the area containing the Onich Dry River Gorge, Onich 47 Shore Section and Ardsheal Peninsula GCR sites, looking up-plunge 48 to the north-east at about 25o. Use has been made of data from the 49 Onich shore section as well as the profile drawn of the Ardsheal 50 area by Treagus and Treagus (1971, figure 2). Data from localities 51 1–5 of the Ardsheal Peninsula GCR site are shown in the top left 52 corner of the profile. 53 54 Figure 19 55 (a) Cross-sets, younging to the north-west (top left) on a 56 horizontal surface of near vertical-dipping Appin Quartzite at 57 Caigean Mor (NN 0437 6105) on the Onich shore section. Lens cap 58 (top left) is 4 cm diameter. 59 (b) View to the north-east of SE-verging F1 folds of a 10 cm- 60 thick limestone bed, and of thinner adjacent beds, in the Appin 61 62 63 64 65 1 2 3 4 Phyllite on the Onich shore section at NN 036 612, 300m SW of the 5 school. Coin (centre) is 30 mm in diameter. 6 (Photos: J.E. Treagus.) 7 8 Figure 20 Map of the Appin Syncline on the Ardsheal Peninsula. 9 Numbers relate to localities discussed in the text. After Treagus 10 and Treagus (1971). 11 12 Figure 21 13 (a) Bedding in the Cuil Bay Slates shows upward grading, crossed 14 by upright S1 cleavage. View to the south-west, down the plunge of 15 a F1 fold hinge from NM 972 554, Rubha Beag, Ardsheal Peninsula. 16 Lens cap is 5 cm in diameter. 17 (b) F1 folds, verging north-west and plunging gently to the 18 south-west in phyllitic semipelites and psammites near the top of 19 the Appin Quartzite on Rubha Mor, Ardsheal Peninsula (NM 9640 20 5565). The fanning S1 cleavage can be seen in the semipelite bed 21 in the centre. Figures for scale. 22 (Photos: J.E. Treagus.) 23 24 Figure 22 Map of southern Lismore Island, showing the ‘units’ of 25 the Lismore Limestone Formation and sedimentary cycles within the 26 Middle unit as mapped by Hickman (1975, 1978). The traces of the 27 major folds are also shown: A and B form a major F2 fold-pair, C is 28 a major F1 syncline. The cross-section (after Hickman, 1978) 29 illustrates the major folds and the relation of the (schematic) 30 vergence of the F2 minor folds. 31 32 Figure 23 Open F2 minor folds, verging north-west in the 33 Lismore Limestone Formation south-west of Miller’s Port (NM 812 34 372); view to the south-west. Figure is 1.5 m tall. (Photo: J.E. 35 Treagus.) 36 37 Figure 24 Map of the Lochnell Peninsula and adjoining area, 38 north of Oban, modified from BGS 1:50 000 Sheet 45E (). It 39 shows the regional setting of both the Camas Nathais and Port 40 Selma, Ardmucknish GCR sites (labelled inset boxes), and the line 41 of the Benderloch Slide. 42 43 Figure 25 Detailed map of an area at the head of Camas Nathais 44 (see inset on Figure 24), based upon a survey by P.W.G. Tanner 45 using GPS for location. The irregularly shaped Palaeogene dykes 46 and sheets provide a guide to location on this featureless rock 47 platform. The map shows the relationship of the rock sequence and 48 structure in the hanging wall of the Benderloch Slide (S), to those 49 in the footwall. 50 51 Figure 26 The Benderloch Slide at Camas Nathais, as seen 52 looking north from locality 1, Figure 25. The slide-plane dips 53 steeply to the north-west and separates graphitic pelite (g) above, 54 from a strongly boudinaged and disrupted unit of orange-weathering 55 dolostone (d) below. To the right of and beneath the dolostone is 56 the intensely fractured Baravullin Quartzite (q). 57 Inset: Lineations in the graphitic pelite just above the 58 slide, as seen looking in the direction of the arrow in the main 59 photo. Two sets of steeply plunging lineations lie in the plane of 60 the phyllitic cleavage The lineation plunging steeply to the left 61 62 63 64 65 1 2 3 4 (north) is the stretching lineation associated with the slide 5 movement, and it is crossed by later crenulation lineations and 6 kink bands plunging to the right (south). 7 (Photos: P.W.G. Tanner.) 8 9 Figure 27 Map of the Port Selma, Ardmucknish area showing the 10 distribution of breccia (B1-5) and interbreccia (IB3, IB4) beds 11 belonging to the Selma Breccia, and their relationship to the Selma 12 Black Slates (modified from Litherland, 1982). Structural 13 interpretation by P.W.G. Tanner. 14 15 Figure 28 The Selma Breccia as seen on the coast at locality 1, 16 Figure 27, showing the variations in clast type and shape within 17 part of breccia units B1-B3 (m, mudstone raft). The direction of 18 view is to the south-east, with the longest dimensions of the 19 clasts being generally aligned parallel to the NE-trending slaty 20 cleavage. (Photo: P.W.G. Tanner.) 21 22 Figure 29 Map of the closure of the F2 Beinn Udlaidh Syncline 23 in Glen Orchy. The curved axial trace of this fold is due to the 24 intersection of the gently dipping axial surface with the irregular 25 topography, and does not reflect the curvilinear hinge as described 26 in the text or later deformation. Equal-area stereographic 27 projections for some of the structural data are shown. Stereoplot 28 (a) shows the poles to the axial planes of minor folds related to 29 the syncline, together with their computed mean orientation as a 30 great circle (solid line). Stereoplot (b) shows the orientations 31 of stretching lineations (solid triangles; N=41) and minor fold 32 hinges (open circles; N=107), related to the major syncline. The 33 solid line represents the computed best-fit plane containing the 34 fold hinges. 35 36 Figure 30 Z-shaped vergence shown by main-phase minor folds, 37 which plunge to the south on the upper limb of the F2 Beinn Udlaidh 38 Syncline in the River Orchy. The structures are seen looking due 39 south from the dam at locality 1, Figure 29, during low-water 40 conditions. The hammer shaft is 78 cm long. (Photo: P.W.G. 41 Tanner.) 42 43 Figure 31 Stacked fold hinges of minor folds (‘fold mullions’) 44 with neutral vergence in the hinge-zone of the F2 Beinn Udlaidh 45 Syncline in the River Orchy at locality 2, Figure 29. The 46 structures are viewed from the south-east, and the hammer shaft is 47 78 cm long. (Photo: P.W.G. Tanner.) 48 49 Figure 32 The major structures of Glen Garry depicted in three 50 dimensions and showing the localities 1–7 that are described in the 51 text. Below is a map of the Glen Garry section showing the A9 52 road, the River Garry and the positions of the numbered localities. 53 Adapted from Thomas (1988, figure 1.2). 54 55 Figure 33 Cross-bedded laminated schistose psammites at the 56 Stalcair Cut on the A9 road (locality 1). The beds young to the 57 right (south-east), and thus the F2 fold is downward facing. The 58 masonry buttress is c. 4 m high. See Thomas (1988, figure 1.5) for 59 an annotated sketch of this view. (Photo: P.R. Thomas.) 60 61 62 63 64 65 1 2 3 4 Figure 34 The exposed hinge of the Bohespic Antiform in the A9 5 road cut near the Allt Chrom, looking north (locality 4). Car 6 gives scale. See Thomas (1988, figure 1.4) for an annotated sketch 7 of this view. (Photo: P.R. Thomas.) 8 9 Figure 35 Downwards- and SE-facing F2 folds on the rock faces 10 of the Clunes Cut on the A9 road, looking north-east (locality 6). 11 Height of section about 8 metres. (Photo: J.E. Treagus.) 12 13 Figure 36 Sedimentary dykes of schistose semipelite in 14 schistose psammite (top, centre), cross-lamination and slump folds 15 in the River Garry, Struan (locality 7). Beds young to the right 16 (south-east) on the steep overturned limb of an F2 fold. The 17 compass is c. 5 cm in diameter. (Photo: P.R. Thomas.) 18 19 Figure 37 Map of the Creag nan Caisean–Meall Reamhar GCR site 20 after Thomas (1965). Minor intrusions are not shown. 21 22 Figure 38 Exposure near the forestry road on the south flank of 23 Creag nan Caisean at NN 771 601, where a strong penetrative 24 schistosity (S1) cuts steeply dipping, cross-laminated (SS) 25 schistose psammite. The hand lens is c. 3 cm in diameter. (Photo: 26 P.R. Thomas.) 27 28 Figure 39 29 (a) Structural map of the Trinafour Monoform and the adjacent 30 area around Meal Dail Chealach and Loch Errochty. AC Allt Culaibh 31 Antiform, AS Allt Sleibh Antiform, CS Croftnagowan Synform, ML Meal 32 na Leitreach Synform, SC Sron Con fold-pair, TM Trinafour Monoform. 33 (b) Schematic profile view of the map in (a), looking east-south- 34 east. Adapted from Thomas (1980). 35 36 Figure 40 Minor Ft monoformal fold related to the Trinafour 37 Monoform on the west flank of Sron Chon (NN 680 668), north of Loch 38 Errochty. The fold is plunging at 20o to 165o (away from the camera) 39 and is cut by a small thrust plane. The hammer shaft is 30 cm 40 long. (Photo: P.R. Thomas.) 41 42 Figure 41 Geology of the Strath Fionan area based on mapping by 43 J.E. Treagus and P.A.R. Nell. After BGS 1:10 000 sheet NN75NW 44 (1997) and Treagus (2000, figure 4). 45 46 Figure 42 Thinly bedded Beoil Quartzite in Strath Fionan exhibits 47 an F2 fold-pair, verging south in the centre of the picture. To 48 the right of the F2 synform is a tight F1 closure folded by another 49 F2 antiform. View to the east from NN 721 576, c. 300 m north-east 50 of Lochan an Daimh. Hammer head is 15 cm long. (Photo: J.E. 51 Treagus.) 52 53 Figure 43 The lower limestone of the Blair Atholl Dark Limestone 54 and Dark Schist Formation in the hinge-zone of an F2 fold in Strath 55 Fionan. View to the south-east from NN 717 570, c. 200 m south- 56 west of Lochan an Daimh. Outcrop is about 10 m wide. (Photo: J.E. 57 Treagus.) 58 59 Figure 44 Map of the Tempar Burn area based on mapping of 60 P.A.R. Nell (BGS 1:10 000 sheet NN65NE). 61 62 63 64 65 1 2 3 4 5 Figure 45 Exposure of a boulder bed close to the Tempar Burn 6 track at NN 696 565, showing elongation of stones parallel to a 7 vertical cleavage; the holes are weathered-out dolomitic clasts, 8 whilst the stones that are proud are of granite, quartzite and 9 biotite-muscovite semipelite (e.g. large clast to the right of the 10 hammer shaft). Hammer shaft is 30 cm long. (Photo: J.E. Treagus.) 11 12 Figure 46 Map of the lower part of the Allt Druidh, after 13 P.A.R. Nell (BGS 1:10 000 sheet NN65NW). 14 15 Figure 47 Map of the hinge-zone of the Ruskich Antiform in the 16 Slatich area based on mapping of P.A.R. Nell (BGS 1:10 000 sheet 17 NN64NW). Adapted from Treagus, 2000, figure 7. 18 19 Figure 48 Cut surface of finely bedded quartzite and 20 amphibolite of the Farragon Formation, c. 800 m east-north-east of 21 Slatich, Glen Lyon (NN 6407 4778). Isoclinal F1 folds (arrowed) 22 are refolded by more-open F2 folds. Scale bar in centimeters. 23 (Photo: J.E. Treagus.) 24 25 Figure 49 Map and cross-section (A–B) of the area south-west of 26 the Ben Lawers mountain range, showing localities 1–5, described in 27 the text. Boundaries are based on the original Geological Survey 28 1:63 360 Sheet 46 (1900), except those of the Farragon Volcanic 29 Formation. The latter boundaries are extrapolated from 30 observations at localities 2 and 4, assuming a constant thickness. 31 Structural data is from Nell (1984) and Treagus (1964b and 32 unpublished). The geometry of folds shown at the key localities on 33 the cross-section are representative of the vergence of the F2 34 folds. 35 36 Figure 50 Cut surface of a typical calcareous schist of the Ben 37 Lawers Schist Formation from the south-east shore of Lochan na 38 Lairige (NN 601 393). The S4 crenulation cleavage is well 39 developed in the lower part of the specimen, while the calcareous 40 quartzite bed near its top exhibits open F4 folding, which at the 41 left-hand edge refolds a tight F2 fold-pair. Scale in centimetres. 42 (Photo: J.E. Treagus.) 43 44 Figure 51 Map of Creag an Chanaich and the area of opencast 45 pits. Mapping west of easting 8145 is based on Moles (1985a) and 46 Treagus (2000); to the east it is based on unpublished mapping by 47 J.M. Maclachlan and N.J. Butcher. Compilation and structural 48 observations by J.E Treagus. 49 50 Figure 52 Schematic down-plunge profile view of major and minor 51 F2 and F3 folds in the area of the Craig an Chanaich mine and 52 opencast pits. A plunge of 25o to 255o is assumed; fault movement 53 restored. After Pattrick and Treagus (1996, figure 6). 54 55 Figure 53 The opencast workings at Creag an Chanaich, in which 56 Ben Eagach Schist (dark area, centre) is exposed in the core of an 57 F2 synform within the stratiform baryte. Cliff exposure is 10 m 58 high. (Photo: J.E. Treagus.) 59 60 Figure 54 61 62 63 64 65 1 2 3 4 (a) Map of the Auchtertyre area. 5 (b) Equal-area stereographic projection of poles to S2 planes and 6 hinges of F2 folds (the range of plunge of folds with strongly 7 curvilinear hinges is shown by the dashed lines). See text for 8 explanation. 9 10 Figure 55 Curvilinear F2 fold hinge in the Allt Gleann a’ 11 Chlachain, Auchtertyre GCR site. Hammer head is 15 cm long. 12 (Photo: J.E. Treagus.) 13 14 Figure 56 Refolded ?F1 folds in the Allt Gleann a’ Chlachain, 15 Auchtertyre GCR site. Pen is 13 cm long. (Photo: J.E. Treagus.) 16 17 Figure 57 Map of the Ben Oss Fault-zone with the Tyndrum Fault 18 immediately to its south-east, occupied by a microdiorite dyke. 19 The outcrop of the Ben Lawers Schist (ornamented) is shown between 20 the two faults and its margin with the Ben Lui Schist is shown to 21 the north-west of the Ben Oss Fault. The inset shows, (a) the 22 left-lateral movement on the Tyndrum Fault and (b) the subsequent 23 locking by the dyke and right-lateral movement transferred to the 24 Ben Oss Fault. 25 26 Figure 58 Vertical aerial photograph of the northern flank of 27 Ben Oss, showing the Ben Oss Fault (stream gully in centre with 28 large arrows at each end) and fractures orientated at 20o 29 anticlockwise to the fault on its north-west side (indicated by 30 small arrows). See text for explanation. (Aerial Photograph © 31 Getmapping.) 32 33 34 Table 1 Summary of the Dalradian sequence in the 35 Schiehallion district (adapted from Treagus, 2000). 36 * the Atholl and Strathtummel subgroups of Treagus (2000) are no 37 38 longer recognized; the strata are now regarded as part of the Glen 39 Spean Subgroup. 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 3.1

Subgr- Group Formation Metasedimentary rocks Igneous rocks

oup

Pitlochry Schist Fm with Gritty semipelite/psammite

Green Beds gritty hornblende schist Basic volcanism — Southern Highland sills + volcaniclastic rocks

Loch Tay Limestone Fm limestone and calc-schist Tayvallich

Ben Lui Schist Fm garnet semipelite/psammite Crinan

Basic volcanism — Farragon Volcanic Fm amphibolite and semipelite sills + volcaniclastic

rocks

Ben Lawers Schist Fm calc-semipelite/amphibolite

Argyll Ben Eagach Schist Fm graphitic pelite/semipelite,

Easdale Ba–Zn deposit near top Carn Mairg Quartzite Fm pebbly quartzite Killiecrankie Schist Fm semipelite/psammite volcaniclastic rocks

Schiehallion Quartzite Fm fine-grained quartzite Tempar Dolomitic Member calc-pelite/semipelite and

dolomitic limestone Schiehallion Boulder Bed diamictite with stones of Islay granite, quartzite and limestone in calc-semipelite matrix

l Drumchastle Pale Limestone Fm tremolitic/dolomitic limestone

Cnoc an Fhithich Banded Semipelite Fm semipelite/psammite Blair Athol Blair Atholl Dark Limestone & Schist Fm limestone/graphitic pelite

Tullochroisk Semipelite Fm banded semipelite/psammite Strath Fionan Pale Limestone Fm tremolitic/dolomitic limestone

Strath Fionan Banded Semipelite Fm pelite/semipelite/psammite Meall Dubh Quartzite Fm pebbly feldspathic quartzite

Appin Meall Dubh Graphitic Schist Fm graphitic pelite

Ballachulish Meall Dubh Limestone Fm tremolite schist/dol. limestone Meall Dubh Striped Pelite Fm calc-schist and semipelite

Beoil Schist Fm muscovite-garnet pelite Beoil Quartzite Fm thin quartzite Dunalastair Semipelite Fm ribbed semipelite/psammite

Lochaber Dunalastair Quartzite Fm quartzite/quartz psammite

Kynachan Psammite Fm thick-bedded psammite -

Kynachan Quartzite Fm feldspathic quartzite

* * Tummel Psammite Fm thick-bedded psammite ‘Strath

tummel’ Tummel Quartzite Fm quartzite/schistose psammite

Grampian

* Bruar Psammite Fm flaggy psammite/semipelite

tholl’ ‘A

Figure 3.1

IA GGF SBS Fort William DD Ben BA Nevis A9 ES CA MRS

Loch Ericht BS BWA 13 TM 15 AS 5 KA

2 4 1 7 3 14 BWS * 6 Loch Leven 8 17 16 18 * BLS 21 Pitlochry Schiehallion 7 50 BBF BS Aberfeldy Loch Linnhe BA 19 Lismore BLA BLS LTF ELF

LDS Loch Tay

9 TF AHA 11 20

10 BCS BUS Loch Etive 12 22 N Tyndrum BLS BLA BLA Dalmally 23 0 kilometres 10 200 250

Old Red Sandstone Supergroup Southern Highland Group Appin Group axial plane trace of F1 and F2 syncline/synform Lorn Plateau and Glencoe volcanic Blair Atholl Subgroup formations and associated axial plane trace of sedimentary rocks Argyll Group F1 and F2 anticline/antiform Late-Caledonian intrusive Ballachulish Subgroup Dalradian Supergoup axial plane trace of igneous rocks Tayvallich Subgroup F3/F4 and later synform Dalradian Lochaber Subgroup fault Supergoup axial plane trace of Crinan Subgroup Grampian Group F3/F4 and later antiform BS mostly Glen Spean boundary slide Easdale Subgroup Subgroup

1 GCR site in Chapter 3 Islay Subgroup Figure 3.2

Strath Fionan

Schiehallion Quartzite Islay Subgroup formations formations Dolomitic ‘beds’ Schiehallion Appin – west Boulder Bed Loch Leven see Fig. 3.41 for formation names

Lismore Limestone Blair Atholl Subgroup Ballachulish – east Loch Leven Struan Flags Cuil Bay Slate

Appin Phyllite and Limestone

Appin Quartzite Ballachulish Subgroup Ballachulish Slate

Ballachulish 500 Limestone metres Leven Schist 0 Glen Coe Quartzite

Binnein Schist Lochaber Subgroup Binnein Quartzite tillite

Eilde pebbly quartzite Schist quartzite Eilde Quartzite carbonate rocks

Grampian carbonaceous pelite Group Eilde Flags pelite/semipelite Figure 3.3a

(a) Stob Ban granite Synform Stob Ban Glencoe Caldera- FWSl volcano Complex Tom Meadhoin Mullach nan and Doire Ban Coirean Pluton slide Loch Linnhe 65 fault rm ifo Kinlochleven Anticline e nt lin A nc en axial plane trace of BSl y ev e S hl or c F1 anticline m lo a in axial plane trace of M K F1 syncline FWSl Nathrach axial plane trace of F2 antiform

axial plane trace of Onich F2 synform Gorge Onich River Rubha Leven L o c h L e v e n shore Cladaich 60 Kinlochleven Anticline

Appin Syncline St John’s BS BSl Church

BS

BSl Glencoe Fault Intrusion Ardsheal Ballachulish Pluton Glencoe Caldera-volcano N Complex 55

0 kilometres 2

00 05 10 15 20

Appin Group Grampian Group Ballachulish Subgroup Lochaber Subgroup Eilde Flags Cuil Bay Slates Appin Quartzite Leven Schists Binnein Quartzite Loch Treig Glen Coe Schist and Appin Phyllites Ballachulish Slates Loch Treig Quartzite Schist and Eilde Schists Quartzite Ballachulish Quartzite Formation Appin Limestone Limestone Binnein Schists Formation Eilde Quartzite Figure 3.3b

(b) South Coast, Lismore Island Tom Meadhoin Stob Ban W and Doire Ban Synform Kinlochleven Mamore E BS Anticline Syncline Stob Ban St John’s BSl Church Kinlochleven Antiform Ardsheal Blackwater Synform

K Rubha Onich in Cladaich lo shore c h River Leven le v e n A Nathrach n t ic l in e

0 kilometres 2 Appin F1 folds with facing Syncline approximate scale Fort William Slide F2 folds Figure 3.4a 65 70Errochty 75 80 Synform Pitlochry Schist Meall Dail-chealach Loch Tay Limestone

65 Ben Lui Schist Glen Errochty

Ben Lawers Schist Ben Eagach Schist and Meall Carn Mairg Quartzite Reamhar Bohespic Killiecrankie Schist Antiform

Schiehallion Quartzite A 60 Blair Atholl Subgroup Strath Tummel B Lochaber and I L o c h II Ballachulish subgroups R annoch Strath III Fionan IV Struan Flags V C Allt D Tempar (a) 60 Druidhe Burn EF 55 D C

III IV V

t F l lt u E u a a F F

n y i a l l T 50 i h K c G o L

H Glen Lyon G Glen Lyon G I Slatich II G G fault 45 axial plane trace of major H F1 folds (A–H) axial plane trace of major Ben Lawers F2 folds (I–V) axial plane trace of major F3, F4 and later folds G

H 40 line of section N Fig. 3.4b Ben Lawers Synform 0 kilometres 5

H Loch Tay Figure 3.4b

(b) N S III Ben Lawers II V A IV Synform

B I flat belt

G H

C D

OD Strath Fionan Tempar Burn Ben Lawers

Slatich 2

Allt Druidhe III

kilometres 0 II 0 2 I facing of F1 folds GH Figure 3.5

19 20 Kinlochleven

B863

R 62 iv e r L footpath e ven

80 88

pipeline 58

40 78 12

Binnein Quartzite

Eilde Schists old river course 68 Eilde Quartzite

Eilde Flags N 88 76 inclined bedding, dip in degrees 61

0 metres 200 direction of younging

overflow Figure 3.7 63 N 0 metres 200

81 85 h

c

a r

h t a

N

t l 76 l A B 73 A 60 80 88 85 B863 82 Kinlochleven C 88

62

Loch Leven 16 17

Binnein Schists

Binnein Quartzite inclined bedding, inclined S3 cleavage, dip in degrees dip in degrees vertical S1 cleavage vertical S3 cleavage axial plane trace of inclined S2 cleavage, F1 Mamore Syncline dip in degrees (downward facing) vertical S2 cleavage direction of younging Figure 3.10 N 0 metres 100 48

B863

HWM F2 folds 610 85 72 70 F1/F2 folds; major plagioclase local F3 folds Rubha and garnet fold Cladaich Loch Leven 125

Loch Treig Schist and Quartzite Formation Glen Coe Quartzite Member

Binnein Schist Member

Binnein Quartzite Member

principal exposures

inclined bedding, dip in degrees

direction of younging Figure 3.12 granite slide

Appin Group axial plane Ballachulish Subgroup trace of F1 fold Appin Quartzite axial plane trace of Ballachulish Slates F2 fold Ballachulish 65 Limestone Lochaber Subgroup

Leven Schists Doire Ban Lairig Mhor Glen Coe Quartzite Ballachulish Binnein Schists Syncline 64 Binnein Quartzite

63

Mam na Gualainn Tom Meadhoin

62

e d li S h s li u Kinlochleven h c a l l a B 61 B863 e d i l S

h s i l Callert u h House c a l l a Kinlochleven Anticline B Loch Leven

North Ballachulish 60 Stob Ban Synform

N B a

l l a c h u l i s h S 0 metres 500 l id e 08 09 10 11 12 Figure 3.13 14

ide h Sl ulis lach Bal

Stob Ban Synform 66

N Stob Ban

0 metres 500

granite slide Appin Group axial plane trace of F2 synform Ballachulish Subgroup Ballachulish Slates

Ballachulish Limestone Lochaber Subgroup Leven Schists Glen Coe Quartzite

Binnein Quartzite Figure 3.15 065 HWM quartz-rich 0 metres 100 N psammite 80 70 B quartzite and graphitic pelite 20 80 85 principal outcrop of graphitic pelite A 80 85 80 82 Loch Leven

Ballachulish Slide

585 Sgorr a’ Choise Slideaxial trace of Ballachulish Syncline St John’s Church

A82

Appin Group Ballachulish Subgroup Appin Quartzite

Appin Transition Formation

slide Ballachulish Slates inclined bedding, Ballachulish dip in degrees Limestone inclined S1 cleavage, Lochaber Subgroup dip in degrees Leven Schists axis of F1 fold, (lower part) plunge in degrees

dykes A localities mentioned in the text Figure 3.17

49 Appin Phyllite with fault dolomitic limestone inclined bedding, dip in degrees Dubh ghlac Appin Limestone axis of minor fold, plunge in 57 degrees; > shows direction of Appin Quartzite vergence track direction of younging A82 Ballachulish Slates axial plane trace of major F1 antiform Onich Hotel axial plane trace of major F1 synform school line of section 28 15 39 86 82 21 26 81 44 43 dolomitic 21 65 limestone dyke 36 quarry 58 85 87 61 Appin N Syncline Caigean Mor

0 metres 500 03 04 Figure 3.18

5 4 3 Ardsheal section 2 1

Appin SynclineAppin

?

Onich section

N

0 metres 200

Appin Group fault Ballachulish Subgroup cleavage/bedding and Cuil Bay Slate Appin Quartzite minor fold convergence direction of younging Appin Phyllite Ballachulish Slate and Appin Transition formations – Appin Limestone 1 5 Ardsheal localities Figure 3.20

98 99 00 late Caledonian fault intrusions Appin Group inclined bedding, 58 dip in degrees Ballachulish Subgroup inclined S1 cleavage, Cuil Bay Slates dip in degrees direction of Ardsheal Appin Phyllites younging 97 Ballachulish Appin Limestone 57 78 5 Appin Quartzite 70 Appin Transition Formation 39 Loch Linnhe 95 96 82 4 56

3 82 2 Rubha 70 65 Mòr Rubha Rubha 1 Meadhonach 85 Beag 55 46 59

Cuil Bay N

54 0 metres 500 Oban A828(T) Figure 3.22

41 C XYA B C Loch Linnhe A B

Bernera Island 40

Kilchevan N Loch

A 0 kilometres 1 39 B X C 83 84 85 A B C Lismore Limestone Formation units

38 upper Bagh Clach Loch an Dobhrain Fiart cycle 3

cycle 2 middle Fiart Miller’s Farm Port 37 cycle 1 Y fault axial plane trace of Lynn of Lorn F1 syncline (C) 36 78 79 80 81 82 axial plane trace of F2 fold Figure 3.24 41 N 0 metres 500 46

No exposure

60 ? 48 40 ? Baravullin ? 86 ?

e 85 id 39 Sl ? h Lynn of Lorn c rlo Kiel 39 de Tralee en Crofts B

86 81 82 75 Port Selma Benderloch

38 Fionn Ard 65 Ardmucknish 78 area of area of Fig. 3.25 B a y Fig. 3.27 67

Garbh Ard 53 37 A828

87 88 89 90 91

Appin Group Old Red Sandstone slide Blair Atholl Subgroup Argyll Group fault Easdale Subgroup Baravullin Dolomite Benderloch Slide inclined bedding, Culcharan Black Slate Ballachulish Subgroup dip in degrees inclined slaty cleavage, Selma Breccia Ballachulish Limestone dip in degrees

Selma Black Slate Lochaber Subgroup

Islay Subgroup Leven Schists Tralee Bay Quartzite

Castle Dolomitic Flags

Baravullin Quartzite

Baravullin Boulder Bed Figure 3.25

HWM Structural sequence: 62 Ballachulish Limestone sand 382 grey-green semipelite and psammite LWM tectonic break N metadolostone with bands of semipelite and psammite

banded metadolostone transitional contact grey-green semipelite 0 metres 50 tectonic break 71 ?Ballachulish Limestone 41 381 L black phyllitic pelite with dark metalimestone (L) Benderloch Slide Islay Subgroup orange metadolostone Baravullin Quartzite quartzite with bands of grey-green phyllitic semipelite (not shown on map)

67 basic dykes and sheets, Palaeogene 51 quartz vein/boudins 380 slide Benderloch Slide ? fault L 1 inclined bedding, dip in degrees

Camas inclined main cleavage, dip in degrees Nathais L limit of exposure

379

L

LWM

HWM

876 877 878 Figure90 3.27 IB4

HWM

Port B4 Selma A828

59 IB3 75 Dun 54 Mac Sniachan

75 Benderloch 88 B1-3 84

73 62 54 Dun-na-Mara N

65 1 0 metres 100 38 B4 B5

Easdale Subgroup fault Selma Breccia inclined bedding, dip in degrees B5 breccia unit 5 inclined slaty cleavage, dip in degrees IB4 interbreccia unit 4 direction of younging B4 breccia unit 4 axial plane trace of anticline IB3 interbreccia unit 3 breccia units 1–3, limit of exposure B1-3 including interbreccia units 1 and 2 Selma Black Slates pebbly mudstone

black slaty mudstone Figure 3.29

N 0 metres 200 Appin Group 3 Leven Schists: banded garnetiferous pelite 21 29 B8074 Beinn Udlaidh Quartzite 30 6 Grampian Group psammite, semipelite and minor pelite 15 and quartzite bands 15 fault roighleacha 18 B n llt A axial plane trace of the 2 Beinn Udlaidh Syncline (F2) 28 33 15 axial plane trace of other F2 folds 36 22 43 24 16 28 2 axis of minor fold, plunge in degrees 22 17 track 20 horizontal axis of minor fold 27 30 22 21 stretching lineation, plunge in degrees 16 15 24 inclined bedding, dip in degrees 9 12 12 5 direction of younging

27 16 1 2 location of photographs 9 22 2 11 18 (a) N 14 pole to Falls of Orchy axial plane 5 20 20 (N= 37) Iron Bridge 8

10 32 10 22 5 dam 21 1 18 13

8 ? 17 21 N (b) minor fold ? hinge (N= 107) 22 stretching 17 23 lineation 18 (N= 41)

Tom na 11 16 Gualainn

25 Figure 3.37 79 77 26 78 N 0 metres 200 30

25

50 25

20 62

40 493m 50 Meall Reamhar 60 70 5 50

46 10 55 35 70 30

Allt Charmaig

70 40 61 48

Allt na Coire Bhuidhe 50

Meall Reamhar Synform axial trace 30 20 26 55 477m Creag nan 45 Caisean 20

60 43 45 30 68 76

surge shaft 60 forestry road 50 60

fault axis of minor fold, Tummel Psammite Formation plunge in degrees Tummel Quartzite Formation inclined bedding, direction of younging with beds of white quartzite dip in degrees

Bruar Formation vertical bedding inclined S2 schistosity, pelite in Bruar Formation dip in degrees Figure 3.39a

68 69 70 20 7210 73 (a) 65 20 15 Appin and Argyll group rocks 70 Meal Grampian group with S0 trend Breac G l e n G a r r y lines in flaggy zone 28 TM 28 fault 69 30 20 Boundary Slide 10 40 10 30 inclined bedding (S ) and S1 20 schistosity, dip in degrees 68 Meal Dail AC Chealach vertical bedding (S ) and 14 18 S1schistosity 20 45 ML 15 22 inclined Se schistosity, 67 SC dip in degrees axis of minor Fe folds, 14 7 20 20 SC 30 plunge in degrees 20 35 axis of minor Ft folds, 66 25 AS plunge in degrees 10 21 5 14 axial plane trace of chty 28 Fe Errochty Synform Erro 20 Loch 65 axial plane trace of CS major Ft synform N Trinafour axial plane trace of major Ft antiform 64 30 0 kilometres 1 28 Figure 3.39b

(b) ENE WSW

G E le nr r o c h ty

CS

SC TM

AS

AC Glen Garry ML 0 kilometres 2 Figure 3.41

70 A 1 66 0 metres 500 N A 58

67 70 Lochan 68 an Daim 55 Speiran Ruadh Lochan Beoil 88 85 Chathaiche 57 75

64 56 Meall 55 Dubh 1 80 C 52 56 30 B1 C 62 71 81 Strath Fionan 62 68

67 B 72 7361 62 74 58 Intrusive igneous rocks Meall Dubh Striped Pelite Formation: striped semipelite microdiorite dykes Beoil Schist Formation: micaceous Metamorphosed intrusive igneous rocks pelite Lochaber late amphibolite Beoil Quartzite Formation: quartzite Subgroup Argyll Group Dunalastair Semipelite Formation: semipelite Schiehallion Boulder Bed Formation: Islay diamictite with limestone clasts Subgroup Dunalastair Quartzite Formation: quartzite Appin Group Grampian Group Drumchastle Pale Limestone Formation: dolomitic metalimestone undivided psammite Cnoc an Fhithich Banded Semipelite Formation: banded semipelite Kynachan Psammite Formation: psammite Glen Spean graphitic Blair Atholl Subgroup metalimestone Subgroup Kynachan Quartzite Formation: Blair Atholl Dark quartzite graphitic pelite Limestone and Dark Schist Tummel Psammite Formation: psammite graphitic Formation metalimestone fault Tullochroisk Semipelite Formation: banded, graphitic semipelite inclined bedding, dip in degrees Strath Fionan Limestone Formation: dolomitic metalimestone AA 1 Strath Fionan Pale Banded Semipelite B B1 transects described in text Formation: banded semipelite CC 1 quartzite Ballachulish Meall Dubh Subgroup Quartzite transitional facies Formation Meall Dubh Graphitic Schist Formation: graphitic pelite Meall Dubh Limestone Formation: metalimestone Figure 3.44 East Tempar 0 metres 200 N Farm 575

75 track

58 72

570

74 54

57 66

565

690 695

Islay Subgroup quartzite

Tempar Dolomitic Member Schiehallion Quartzite Formation conglomerate

upper tillite with quartzite Schiehallion Boulder Bed Formation lower tillite with quartzites

Blair Atholl Subgroup

Drumchastle Pale Limestone Formation

Cnoc an Fhithich Banded Semipelite Formation

inclined bedding, dip in degrees Figure 3.46

0 metres 200 25 642 N 30 Allt Druidhe 38 38 32

644

562 564 566 568 570 572 574 Loch Rannoch

high-strain zone microdiorite (mainly near-concordant Appin Group Boundary Slide sheets with some dykes) Ballachulish Subgroup Argyll Group (quartzites and semipelites) fault Lochaber Subgroup Ben Lawers Schist (quartzites and semipelites) inclined bedding, Easdale dip in degrees Ben Eagach Schist and Subgroup Grampian Group Carn Mairg Quartzite psammites Figure 3.47a 4 30 67 2 36 52 4 4 28 66 52 62 0 metres 200 N 37 55 2 4 67

2 45 34 2 80 2 45 62 70 51 53 2 2 4 43 Ruskich Antiform 4 43 26 60 88 69 4 54 4 45 52 68 2 75

4 38 81 68 4 2 23 244 48 47 2 59 89 74 4 48

2 87 52 50 4 4 56 84 2 35

73 54 Ruskich 4 2 88 30 4 58 2 0 4 47 53 51 82 4 39 2 Slatich 2 30 4 26

4 8 16 4 71 56 2

80 River Lyon

32 60 4 4 81 33 78 2 4 25 83 47

4 Roromore 53 29 38 33 4 32 2 68 78 4 25 2 19 88 2 73 69 2 2 32 4 30 4 28 72 2 72 2 18 86 46 62 87 80 28 82 63 2 20 4 4 64 85 65 Figure 3.47b amphibolite

Crinan Subgroup Ben Lui Schist Easdale Subgroup Farragon Volcanic Formation Ben Lawers Schist with dolomitic quartzite at top Ben Eagach Schist

Carn Mairg Quartzite

fault

inclined bedding, dip in degrees inclined bedding known to be overturned, dip in degrees vertical bedding

2 axis of F2 minor fold or bedding/S2 cleavage intersection, plunge in degrees (flag shows direction of vergence) 4 axis of F4 minor fold, plunge in degrees (flag shows direction of vergence) inclined axial plane of F4 minor fold, dip in degrees axial plane trace of major antiform (a)Figure 3.49aA 70 20 38 43 5 ine 48 ticl 42 An 48 ich ana orr 32 42 66 ll C Meall ea Corranaich M Ben 4 Lawers 34 22 48 50 41 30 18 Lochan 16 na Lairige 18 32 12 36 40

Ben Lawers Synform n r

55 u B

50 39 3 31 y k

c u 22 m a r 2 d E 28 38

32 31 1 30 37 quarry A827

B 36

N 35

Loch Tay 0 kilometres 2 34 58 59 60 61 62 63

Pitlochry Schist

Loch Tay Limestone

Ben Lui Schist

Farragon Volcanic Formation

Ben Lawers Schist Ben Eagach Schist and Carn Mairg Quartzite

fault

inclined bedding, dip in degrees

axial plane trace of major F1 fold

axial plane trace of major F4 fold Figure 3.49b

(b) A B 5

700

(metres) 4 2 3 1 100 ?

0 kilometres 1 Height above OD Figure 3.51

812 813 814 815 816 817 818

0 metres 50 N 20 4 2 25 2 50 85 546 22 2 86 2 2 64 4 25 2 52 4 26 2 mine 15 86 2 28 4 25 2 42 35 2 32 76 2 86 28 28 48 2 4 12 2 2 56 25 27 2 69 Creag an 22 2 Chanaich 68 13 30 60 2 4 4 545 4 26 25 4 89 4 82 66 2 2 26 55 82 4 80 4 52 42 4 19 4 76 55 80 28 4 2 54 4 68 Frenich Burn Fault 62 2 opencast pits 80 19

Ben Lawers Schist Formation: fault axial plane trace of F2 fold calcareous schist inclined bedding, Ben Eagach Schist Formation: graphite-muscovite schist dip in degrees axial plane trace of F4 fold baryte (upper mineral horizon) with quartz-celsian inclined S2 cleavage, 2 axis of F2 fold, rock (not distinguished) 2 dip in degrees plunge in degrees Ben Eagach Schist Formation: graphitic inclined S4 cleavage, 4 4 axis of F4 fold, schist with local basic meta-igneous rock dip in degrees plunge in degrees baryte-celsian rock (lower mineral horizon) L2 intersection lineation, plunge in degrees Figure 3.52

S [8118 5460] N

[8136 5448]

Ben Lawers Schist Formation with local baryte Ben Eagach Schist Formation baryte with upper and lower quartz-celsian rock (dashed lines)

Ben Eagach Schist Formation with local basic meta-igneous rock and baryte

axial plane trace of major F2 fold

axial plane trace of major F4 fold SØ N schematic bedding–cleavage S2 relationships S4 notional projection of track 0 metres 50 open pits Figure 3.54 N 0 kilometres 1 32

Allt a’ Chaol Ghlinne

t l Allt Gleann a’ Chlachain u a F

e r y 31 t r e t h c u

A

30

A85 Dalmally Antiform Auchtertyre

Allt Auchtertyre 29

Strath Fillan (a) 34 35 36 37

Easdale Subgroup Ben Lui Schist Formation

ultramafic rocks Ben Challum Horizon Ben Challum Quartzite Auchtertyre Horizon Formation

Ben Lawers Schist Formation

fault

axial plane trace of antiform

(b) N Gleann a’ Chlachain

pole to S2 F2 fold hinge curvilinear hinge Figure 3.57

(a) (b) dextral 295 0 km 20 kinks S 36 80 Cononish Cononish 20 cm-wide breccia at dyke margin S

85 2 m-wide quartz reef at 270 S dyke margin 72 Ben Lawers Schist Ben Ben fractured, kinked and Oss Oss pyritized at dyke margin 290 2 m-wide breccia in graphitic Tyndrum Fault Ben Lawers Schist; some fractures N-S kinks and fractures show vertical slickensides show right-lateral displacement Ben Oss Fault 75

80 S 3 m-wide breccia on major fracture; left-lateral kinks S 40 m-wide left-lateral displacement of 60 quartzite bed on gouge-filled major fracture

metre-scale displacements on 30 cm-thick microdiorite dyke: N S two left-lateral and one right-lateral 85 80 3 m-thick quartz reef and quartz-cemented 85 breccia on major fault wall; horizontal grooves; 0 metres 100 major fault left-lateral pinnate fractures major fault wall exposed wall exposed 38 late-Silurian microdiorite dyke major fracture observed 265 85 late-Carboniferous major fracture inferred from S quartz-dolerite dyke topography or air photography Southern Highland inclined bedding, dip in Group degrees Loch Tay Limestone inclination of fracture or dyke, Formation dip in degrees

Ben Lui Schist S Formation subhorizontal slickensides Ben Lawers Schist locality mentioned in text Formation Figure 3.32

F3

Bohespic Antiform Drumochter Dome F2

m r fo ti n A h id da ha ’M Boundary Slide g a ea Clunes Antiform Cr

1 4 5 2 6 3a 3b Blair A9(T) Atholl F2 7 A9(T) Garry SynformN Errochty Synform Dalnacardoch Struan folds Banded Zone F2 F3 Meall Reamhar Synform plunge direction of fold axis, with phase direction of younging

70 75 70 80 85 4 A9(T) N 65 2 3b 3a River Garry 5 1 6 7 0 kilometres 2 70 2 70 75 6 80 65 Figure 3.6 colour Click here to download high resolution image Figure 3.8 colour Click here to download high resolution image Figure 3.9 colour Click here to download high resolution image Figure 3.11 colour Click here to download high resolution image Figure 3.14 colour Click here to download high resolution image Figure 3.16 colour Click here to download high resolution image Figure 3.19a colour Click here to download high resolution image Figure 3.19b colour Click here to download high resolution image Figure 3.21a colour Click here to download high resolution image Figure 3.21b colour Click here to download high resolution image Figure 3.23 Click here to download high resolution image Figure 3.26 Click here to download high resolution image Figure 3.28 colour Click here to download high resolution image Figure 3.30 colour Click here to download high resolution image Figure 3.31 colour Click here to download high resolution image Figure 3.33 colour Click here to download high resolution image Figure 3.34 Click here to download high resolution image Figure 3.35 colour Click here to download high resolution image Figure 3.36 Click here to download high resolution image Figure 3.38 Click here to download high resolution image Figure 3.40 Click here to download high resolution image Figure 3.42 colour Click here to download high resolution image Figure 3.43 colour Click here to download high resolution image Figure 3.45 colour Click here to download high resolution image Figure 3.48 Click here to download high resolution image Figure 3.50 colour Click here to download high resolution image Figure 3.53 colour Click here to download high resolution image Figure 3.55 colour Click here to download high resolution image Figure 3.56 colour Click here to download high resolution image Figure 3.58 colour Click here to download high resolution image Figure 3.6 B&W Click here to download high resolution image Figure 3.8 B&W Click here to download high resolution image Figure 3.9 B&W Click here to download high resolution image Figure 3.11 B&W Click here to download high resolution image Figure 3.14 B&W Click here to download high resolution image Figure 3.16 B&W Click here to download high resolution image Figure 3.19a B&W Click here to download high resolution image Figure 3.19b B&W Click here to download high resolution image Figure 3.21a B&W Click here to download high resolution image Figure 3.21b B&W Click here to download high resolution image Figure 3.28 B&W Click here to download high resolution image Figure 3.30 B&W Click here to download high resolution image Figure 3.31 B&W Click here to download high resolution image Figure 3.33 B&W Click here to download high resolution image Figure 3.35 B&W Click here to download high resolution image Figure 3.42 B&W Click here to download high resolution image Figure 3.43 B&W Click here to download high resolution image Figure 3.45 B&W Click here to download high resolution image Figure 3.50 B&W Click here to download high resolution image Figure 3.53 B&W Click here to download high resolution image Figure 3.55 B&W Click here to download high resolution image Figure 3.56 B&W Click here to download high resolution image Figure 3.58 B&W Click here to download high resolution image