Grampian Highlands Field Guide: Day 1 - Dunkeld area and Benachally
|This page is part of a category of pages within the Grampian Highlands Field Guide.
Author: J R Mendum, BGS
Day 1 – Dunkeld area and Benachally
Aims: to examine exposures within and close to the hinge zone of the Tay Nappe that throw light on its overall 3D geometry; to observe the folds and cleavages resulting from the three phases of deformation present in this area; to examine the metamorphosed sedimentary rocks of the Dalradian Southern Highland Group; to elucidate views on the overall structure, particularly of the Benachally area.
The Tay Nappe is one of the largest known ‘fold nappe’ structures in the world. Including its south-western extension into Northern Ireland, it is some 430 km in strike length and up to 40 km wide. The structure dominates the south-eastern part of the Grampian Highlands along its entire length. Unsurprisingly, its geometry, age, mode of formation, and its role during Grampian orogenesis, have been paramount in the discussion of the structural development of the Highlands for many decades. We plan to visit 3 localities that together provide some insight into the geometry and structural development of the Tay Nappe, namely Little Glen Shee, Birnam Hill Quarry, and Benachally.
The overall 3D regional structure of the Grampian Highlands is shown in Figure P2, a modification of block diagram originally drawn by P R Thomas that tries to synthesise the mapping and geological work done in the late 19th and 20th centuries. Note that the colours merely define somewhat arbitrary areas rather than structurally distinct regions. In contrast, Figures 1.1 and 1.2 show a map and simplified 2D cross-section that illustrate the overall structure in the Dunkeld-Pitlochry area. These illustrations show how the overall structure can be divided into three distinct parts: the ‘Flat Belt’, consisting of the sub-horizontal inverted lower limb of the nappe, which dominates the outcrop; the Highland Border Steep Belt (the ‘Steep Belt’), which incorporates parts of the original hinge zone of the ‘nappe’ and tracks along the length of the Grampian Highlands near-parallel to the Highland Boundary Fault; and the Highland Border Downbend (the ‘Downbend’). This last relatively late-stage monoformal structure has effectively folded down the hinge zone of the nappe, thereby generating the ‘Steep Belt’. Note that the present level of erosion exposes either the inverted lower limb of the nappe or its ‘downbent’ hinge-zone.
The existence of a ‘Flat Belt’ and a ‘Steep Belt’ has been known since the early days of mapping in the Highlands, e.g. in the SW Highlands Clough (in Gunn et al. 1897) identified the equivalent Cowal Antiform. However, except for the Loch Tay Limestone on the north-western parts of the Tay Nappe, a stratigraphical succession could not be recognised and hence the overall structural geometry remained problematical. E B Bailey had identified an Iltay Nappe farther northwest in 1922 whilst working for the Geological Survey, but it was his Glasgow students who made a significant breakthrough in the Southern Highlands. In 1938, S M K Henderson identified the presence of a major anticline, which he termed the Aberfoyle Anticline, based on stratigraphical considerations within the strata of the Steep Belt. Subsequently, related structures that explained the distribution of slate and ‘grit’ units were identified all along the Highland Border by J G C Anderson in 1947. However, as the hinge zones of these regional-scale folds could not be readily identified and the direction of younging was not mapped across the structures, it was unclear whether they were upward- or downward-facing (in modern terms). Thus, the Aberfoyle Anticline and related structures were assumed to be upward-facing folds.
The structural geometry remained problematic until the 1950s, when R M Shackleton (Liverpool University), assisted by his research students M Stone and P J Stringer, developed the concept of facing, based on their work in the Highland Border region. Shackleton (1958) showed how the relationships between bedding, cleavage and way-up criteria could be used to determine both the locations of antiformal and synformal hinges, and define the facing direction of the major structures. As a result, the Aberfoyle Anticline and related structures were recognised as downward-facing, leading to a wider model that explained the regionally inverted rocks and overall structural geometry of the Tay Nappe, particularly in the central parts of the Southern Highlands.
Although a basic structural model had been elucidated, subsequent work has shown that the regional structure of the Tay Nappe varies considerably along its strike length. Thus, in Northeast Scotland around Collieston the ‘nappe’ is a simple recumbent fold structure of considerably lower amplitude. In the Glen Shee district Crane et al. (2002) showed that the ‘Flat Belt’ encompassed several kilometre-scale D1 and D2 folds. In the Loch Lomond — Roseneath Peninsula area major dislocations were recognised, along which parts of the fold nappe have apparently been excised. Mechanisms for the generation of the Tay Nappe have been the subject of much discussion. For diverse interpretations see Harris et al. (1976), Bradbury et al. (1979), Krabbendam et al. (1997), Mendum & Thomas (1997), Rose & Harris (2000). Recent summaries can be found in the Dalradian GCR volume (Stephenson et al., 2013; Tanner et al., 2013); a further revised tectonic model has been presented recently by Tanner (2013).
The rocks of the southeast part of the Grampian Highlands belong mainly to the Southern Highland Group of the Dalradian Supergroup (Figure P1). They comprise interbedded gritty metasandstones, quartzites, metasiltstones, metamudstones and metavolcaniclastic rocks. The age of deposition of this mainly turbiditic succession is bracketed by a U-Pb zircon age of 601 ± 4 Ma (Dempster et al., 2002) from felsic tuffs in the underlying Tayvallich Volcanic Formation, and the Early Cambrian trilobite fauna of the Leny Limestone (c.515 Ma), which lies within the uppermost part of the group. Originally, three units were recognised in the Dunkeld area: the Birnam Grit (oldest; base not seen), the Birnam Slate, and the Dunkeld Grit (youngest; top not defined). However, Crane et al. (2002) amalgamated the two lower units into the Birnam Slate and Grit Formation, reflecting the more complex sedimentary facies distribution in the Glen Shee district. The coarse-grained sediments commonly show grading and were deposited from turbidity currents in channels on deep-water submarine fans, whereas the finer grained facies were generally laid down as overbank deposits, or in more distal parts of the fan. Although some broad facies associations can be recognized, lateral continuity of a single facies is unlikely and attempts at stratigraphical correlation are problematical, even over relatively short distances.
The structural history of this area can be considered in terms of three of the four principal phases of deformation that can be recognised regionally across the Grampian Highlands. These have been termed D1, D2 and D4 (D3 is present elsewhere in the Tay Nappe, but effectively absent here). The first two phases, D1 and D2, were associated with prograde metamorphism and were responsible for the generation and present geometry of the Tay Nappe. D4 was associated with retrograde metamorphism during or following uplift of the Tay Nappe and formation of the Highland Border Downbend. D1 resulted in the formation of both small and large, even kilometre-scale folds of bedding (F1) associated with a spaced pressure solution cleavage in the metasandstones and a slaty cleavage in the finer-grained more pelitic beds. At high levels of the nappe exposed in the ‘Steep Belt’, these F1 folds are preserved as downward-facing structures with steep NW-dipping axial planes. Note that in the Dunkeld area and further northeast there is no downward-facing major synform comparable with the Aberfoyle Anticline of Shackleton (1958). The entire succession effectively youngs to the north-west from the Highland Boundary Fault to the ‘Downbend’, with any originally SE-younging ‘upper’ limb presumed to have been cut out by this major fault. The second deformation, D2 was only developed at intermediate and lower structural levels of the nappe. It was coincident with the peak of metamorphism (greenschist to amphibolites facies) and has been interpreted by most authors as having a significant component of simple shear. It resulted in pervasive small-scale folding (F2) in much of the Tay Nappe, notably of the S1 pressure-solution striping, examples of which predominate in the ‘Flat Belt’. These small-scale F2 folds characteristically show thickened hinges but attenuated limbs. However, D2 was also responsible for the generation of large-scale F2 folds of the bedding and earlier cleavage. The D4 deformation occurred considerably later; its relationship to the uplift that resulted in formation of the ‘Downbend’ is unclear. The characteristic minor open to close F4 folds and associated steeply NW-dipping S4 crenulation cleavage extend across much of the ‘Flat Belt’ and patently resulted from a compressional event. Note that the Ben Lawers Synform is a large-scale F4 fold.
Little Glen Shee [NN 9792 3453]
From Kindrogan and Pitlochry, take the A9 south to near Birnam and turn right onto the B867. At Bankfoot turn right in the centre of the village and then after 500 m left to take the minor road to Tullybelton and Little Glenshee. Park by the ford across the Shochie Burn at [NN 988 340]. V Cross back over the burn and take the good track northwest for c.1 km past Little Glenshee farm, as far as a large boulder by the track, painted (very faint) with ‘No dogs’ [NN 9792 3447]. Here ascend the mainly heather-covered slope on the right-hand side of the valley for a short distance (about 50 m) to reach the craggy exposures at [NN 9792 3453]. In total the walk should be c.2.2 km with a c.50 m height difference.
These exposures of the Birnam Slate and Grit Formation lie within the ‘Steep Belt’, but were originally located at a relatively high structural level in the Tay Nappe on its inverted limb close to its hinge zone. The beds and S1 cleavage have been have rotated some 100° into their present steep northwesterly dips during the formation of the Highland Border Downbend. Note that metamorphic grade is low (chlorite zone–greenschist facies) and the structures largely reflect only a single phase of deformation, D1. The rocks consist of well-defined graded beds up to 0.5 m thick. Poorly sorted, matrix-supported, coarse- to medium- grained metamorphosed wacke sandstones (or arenites) grade into fine-grained metasandstones and mica-rich metasiltstones and metamudstones (Figure 1.3). Abundant subangular clasts of quartz and feldspar up to 2 mm across occur in the coarser fraction. Bedding lamination and more rarely small-scale ripple cross-lamination are commonly well displayed in the finer grained metasandstones. Where seen, the younging is indicated by both cross-lamination and graded bedding.
The exposed metre-scale F1 folds verge towards the north-west and have an associated S1 spaced pressure-solution cleavage in the arenites that passes into a slaty cleavage in the finer grained pelitic lithologies,. The dominant folds have rounded hinges, straight limbs, interlimb angles of c.70° and a wavelength of c.7 m (Figure 1.4). Smaller parasitic folds with wavelengths of c.1 m, and minor folds with 3–4 cm wavelengths are developed in the main fold-hinge zones. Sedimentary structures, notably grading, show that the beds young downwards towards the cores of the antiforms and away from the cores of synforms. Hence, the folds are downward-facing.
The S1 pressure-solution cleavage is developed in the arenite layers that originally formed the lower parts of the graded beds. It is defined by mica- and chlorite-rich laminae (less than 1 mm wide), here commonly lying at a high angle to bedding. These laminae, spaced at intervals of c.1 cm, anastomose in detail, both laterally and normal to bedding, such that the more quartz-rich zones define 3-dimensional lenticular zones, commonly referred to as ‘lithons’. Differential weathering has left these lithons upstanding with the mica-rich zones forming narrow linear indentations (Figure 1.3). Note that this S1 spaced cleavage fans through as much as 100° and is roughly symmetrical about the F1 fold axial planes (Figures 1.4, 1.5).
In the finer grained upper parts of the graded beds, the spaced pressure-solution cleavage passes into a penetrative, axial-planar, slaty (continuous) cleavage, which has a more uniformly orientation, near parallel to the fold axial planes. In parts the pressure-solution cleavage refracts into the slaty cleavage with its curved trace clearly reflecting the decrease in original grading from sand to silt (Figure 1.4). Do not confuse with true cross-bedding. In contrast, the contact between the coarse-grained base of each bed and the fine-grained top of the bed immediately below it is emphasised by the sharp angular break between the slaty cleavage in the older bed and the pressure-solution cleavage in the younger bed.
Birnam Hill quarry [NO 0404 4040]
From Little Glen Shee return to Bankfoot and turn left onto the B867 north towards Dunkeld. Some 300 m before the junction with the A9, park on the left hand side of the road where a public footpath goes under the railway [NO 0410 4045]. Take the track under the railway and take the footpath to the left signposted Birnam Hill. After some 50 m at [NO 0403 4039], turn right along an overgrown track, partly followed by a stream that leads to the small disused quarry in about 100m, now somewhat overgrown and bosky (Figure 1.6). In total the locality involves some 400 m walking with a c.20 m height difference.
From near the quarry we may get a view northeastwards through the trees along the Highland edge, which emphasises that there are numerous slate quarries hereabouts, both nearby scattered across Birnam Hill, and also on the northeast side of the Strath Tay, notably on Newtyle Hill. The ’Birnam Slates’ are exposed in a larger quarry just to the north at [NO 0378 4052]. These quarries worked the various slaty pelitic bands in the Birnam Slate and Grit Formation, particularly where these units have been ‘thickened’ in the hinge zones of medium-scale F1 folds.
In the quarry, an upright antiformal closure of arenites and interbanded pelitic beds is exposed. The lithologies consist of fawn to pale and mid-grey, thick to medium-bedded, coarse-grained, gritty arenites and wackes with thinner darker greenish- and bluish-grey semipelite and pelite beds with scattered large pyrite cubes. Younging, as indicated by grading, shows that the beds are inverted and hence the fold is downward facing. Note that the S1 cleavage is steeply dipping in the fold hinge area and despite the fold geometry we are still within the ‘Steep Belt’.
The arenites and wackes are feldspar-rich with abundant chlorite and blue quartz; lithic clasts include granitoids, deformed quartzites, and altered basic volcanic rocks. The assemblage suggests derivation from a volcanic source but with exposed high-grade metamorphic basement gneisses, in accord with the detrital zircon age spectra that showed some Archaean ages (Cawood et al., 2003). The fold is associated with fanning, spaced pressure-solution and slaty S1 cleavages. In the arenites Harris et al. (1976) described the occurrence of an additional more penetrative continuous cleavage here designated S1p. This secondary S1p fabric is developed in the cleavage lithons, and is defined by the preferred orientation of chlorite, sericite and micas and the curved overgrowths of quartz on original detrital grains. It lies in the acute angle between bedding and the pressure-solution cleavage on both fold limbs, such that its orientation lies closer to the fold axial plane. Note that the angle between the pressure-solution S1 seams and the penetrative cleavage (S1p) is greater on the northwestern limb of the antiform than on the southeastern limb. As their geometry was congruent with the F1 folding, Harris et al. (1976) interpreted both cleavages as a product of D1 deformation but with differences in their timing of formation.
Benachally [NO 0659 4915]
From Birnam Hill take the A9 north to the right-hand turn for Dunkeld. Proceed through the town centre (pay toilets on northern side) and on the outskirts turn right along the A923 towards Blairgowrie. The large craggy ridge to the northwest of Dunkeld here is Craig a' Barns, which is coincident with the hinge of the ‘Downbend’. Note that near Dunkeld the ‘Steep Belt’ is only some 7 km wide. The D2 deformation, which strongly affects the intermediate and deeper levels of the Tay Nappe, is progressively developed to the northwest across the ‘Steep Belt’ such that F2 folding and the S2 cleavage are the dominant structural features in its northwestern third.
The A923 takes a somewhat sinuous course broadly along the strike of the ‘Steep Belt’. At Butterstone (c.5.5 km) turn left (sharp turn) onto a minor single-track tarmac road that leads up the valley of the Buckny Burn. The road leads to Riechip Lodge from where it is gated. Pass through the gate and over the grassy pasture to park with permission near Craigend cottage (around [NO 0704 4010]) near where the power line crosses. From here we traverse up, around and along the SE ridge of Benachally (Figure 1.7), the round trip is c.3.5 km, generally over irregular rough mainly heathery ground, involving a height difference of c.170m.
Benachally provides an across-strike section through a lithologically mixed sequence mapped as undifferentiated Southern Highland Group on Sheet 56W (Glen Shee). The outcrop of the Birnam Slate and Grit Formation lies in the southeast part of the ‘Steep Belt’ adjacent to the Middleton Muir Fault (≡ Highland Boundary Fault). The trace of the ‘Downbend’ has been mapped trending ENE across the unexposed ground just southeast of Benachally, where it is offset laterally by several NW-trending faults. It is shown as lying some 620 m to the SSE of our starting point, but in this area the ‘Downbend’ hinge is portrayed as a 1–2 km wide zone, marked by the occurrence of medium-scale D4 folds, rather than a simple monoform as seen near Dunkeld. The sequence on Benachally consists of mixed arenites, wackes, and semipelites, with thick units of semipelite and pelite and locally gritty, coarse-grained, quartz + feldspar-rich arenites. These latter units, which common form ridges, are lenticular and have been interpreted as turbiditic fan channel infills (Crane et al., 2002). The stratigraphical relationships of these rocks in relation to the successions defined in the eastern part of the Glen Shee district and in the Southern Highland Group outcrop farther southwest is unclear. Crane et al. (2002) portray the rocks as lying above the Birnam Slate and Grit Formation, which they place near the base of the group. However the presence of volcaniclastic ‘Green Beds’, some 4 km to the northwest, and the gritty arenite-semipelite association would suggest that these rocks may represent relatively low stratigraphical levels in the Southern Highland Group.
Although the section lies within the ‘Flat Belt’ the overall dip of bedding averages about 40° to the NNW. In fact, near the summit bedding is locally sub-vertical. The section shows evidence of both F1 and F2 folding, but F4 folding and the related S4 crenulation cleavage are only weakly developed. These later structures are best seen in the more pelitic units at the southeast end of the traverse (near the mapped ‘Downbend’ trace). Way-up can be determined in places, both from local evidence of grading and the presence of pebbly bases to some of the gritty arenite units. Generally, the beds are inverted. What is unclear is the overall structural geometry of the succession here. The D2 deformation shows all variety of structures; minor and medium-scale folds of bedding and the S1 spaced cleavage are abundantly developed, ranging from centimeter- to decimeter-scale (Figure 1.8). Evidence can be found for the implied shear sense on small-scale F2 folds, the role of pressure solution during D2 deformation, and the degree of attenuation of F1 structures. It is clear that the intensity of D2 deformation increases northwestwards, such that just beyond the summit of Benachally S2 cleavage and related folding dominate. Any D1 structures are ‘pale imitations of their former selves’, having been strongly deformed and overprinted by D2.
Figure 1.9 shows the geology as portrayed on the clean copy NO 04 NE, and Figure 1.10 shows the cross-section through Benachally as drawn on the published 1:50 000 map (Sheet 56W). Note that the clean copy shows no bedding orientations, only cleavages as the dominant planar structures. Figure 1.11 shows the line of the proposed traverse on a 1:10 000 scale topographical map with the vertical transect plotted below. The idea is that the general lithological units, bedding orientations, bedding-cleavage relationships, etc. should be sketched in, either during and/or after your trip. Note that accurate measurements are unnecessary. My rough interpretation is shown here. Hopefully this will lead to some discussion of the larger scale structural geometry in this area and its implications for the structure and origins of the Tay Nappe. Questions to be discussed include: Is there repetition of the lithologies and if so what is the scale and nature of the fold geometry? Are there significant D1 structures present, albeit modified by D2? Alternatively, is the structural pattern dominated mainly by D2 structures? Hopefully, you will see that mapping and interpreting the geology provides some rather interesting challenges. As you should be carrying little or no Dalradian ‘structural baggage’, your objective views will be welcome.
Return to Kindrogan via the A923 to Blairgowrie, the A93 to Bridge of Cally, and then the A924 to Kirkmichael and Enochdhu.