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Author: J R Mendum, BGS
Day 2 – Glen Ogle, Killin and Lochan na Lairige
Aims: to examine the upper Argyll Group stratigraphy in some of the ‘classic’ parts of the inverted limb of the Tay Nappe in Breadalbane; to look at the lithology of Loch Tay Limestone Formation and its transition into the structurally overlying Ben Lui Schist Formation; to look at the Ben Lawers Schist Formation, its structure, and its transition into the structurally underlying Ben Lui Schist Formation; to compare the structural sequence with that seen near the Highland Border.
The area around Loch Tay included is underlain mainly by Dalradian Supergroup rocks that belong to the upper part of the Argyll Group (Easdale, Crinan and Tayvallich subgroups) and the Southern Highland Group. The area contains several lithologically distinctive formations that can be traced laterally over wide areas, e.g. the Loch Tay Limestone Formation, and the Ben Lawers and Ben Lui Schist formations. These units have been intruded in parts by numerous mafic igneous sheets, mainly metadolerite sills, which are now manifest as amphibolite. This succession has then been deformed, folded and faulted during the Caledonian Orogeny, mainly during the Mid-Ordovician Grampian Event, giving rise to a large-scale complex structural pattern whereby the rocks largely lie on the inverted limb of the ‘Tay Nappe’, albeit modified by significant subsequent refolding. The complementary ‘primary’ fold to the Tay Nappe, here termed the Ben Lui Fold Complex, which lies at a deeper structural level and effectively ‘turns’ the sequence right way up, is exposed on the southern flanks of the upper part of Glen Lyon (see Figure P2).
Current structural interpretations of this central part of the Grampian Highlands around Ben Lawers are based upon those proposed E B Bailey, who drew together the results of the primary mapping for the Geological Survey, mainly by J S Grant Wilson. In a series of papers, from 1910 to 1938, Bailey demonstrated that the rocks of the south-west and central Grampian Highlands are disposed in large, Alpine-type recumbent folds, the largest of which he termed the Iltay Nappe (see Bailey, 1925; Bailey and McCallien, 1937). He proposed that the long limbs of many of these folds had been partly replaced by low-angled ductile dislocations, termed ‘slides’, with postulated movements of several kilometres. Most of the major fold structures originally identified and named by Bailey are still recognized, but some of his ‘slides’ have lost their significance as major tectonic and/or stratigraphical boundaries. Subsequent modifications and refinements to Bailey’s model have involved elucidation of the relative ages of the various structures, their recognition as belonging to several phases of deformation, and their relationships with each other. Most of this later work has improved our knowledge of the overall structural history of this notably complex area (see Stephenson and Gould, 1995 for a summary). In the Ben Lawers area Elles (1926) reinterpreted various relationships between the formations, notably with respect to the margins of the Ben Lawers Schist Formation. She recognised that there was a difference in metamorphic grade across its boundaries and proposed that several internal lithological units were absent along its northern margin. As a result she postulated the existence of the Ben Lawers Slide across which beds had been excised. However, the metamorphic grade reflects the differences in lithology. As the Ben Lawers Schist has been strongly deformed during D4 deformation, the related fabrics are dominated by associated greenschist-facies assemblages. In addition there are patently facies changes within the succession, notwithstanding the presence of large-scale F1 and F2 tight folds. Hence, although the boundaries of the Ben Lawers Schist Formation have acted as zones of distinct strain variation and even localized movement, the purported excision of lithological units at the northern margin of the formation is no longer accepted and the presence of a ‘slide’ unnecessary. However, Elles (1926) did also recognise the presence of Ben Eagach Graphitic Schist and Carn Mairg Quartzite infolded within a large-scale tight synform, now called the Meall Corranaich Anticline. Later work in the Ben Lawers area has been carried out by Treagus (1964), who clarified the relationships between the D1, D2 and D4 deformations and related structures near to Lochan na Lairige. The most recent description of the traverse effectively along the Lochan na Lairige road can be found in Treagus, 2013 (GCR site 21). The area lies in 1:50,000 sheet 46E (Killin) that has been recently published (2013) following partial remapping by the Geological Survey.
Glenoglehead [NN 5592 2852]
From Kindrogan we pass through Pitlochry and take the A9 southwards to Ballinluig where we turn left (complex junction) onto the A 827 to Grandtully and Aberfeldy. We continue on past Kenmore, Loch Tay, and Killin to reach the junction with the A85 at Lix Toll. Here turn left (south) and after c.2.2 km stop in the large parking at Glenoglehead [NN 5590 2850].
Take the obvious forestry track leading east from the car park but immediately traverse left along the edge of the forest to reach the small burn. A rough path makes its way up the right-hand side of the burn to a water supply. The intention is to traverse up part of the burn section (low water levels are desirable), a round trip of about 1 km with a c.150 m height difference. No path and some steep slopes — grass, bracken, rocks, burn, etc.
This section lies in the northwestern part of the type area of the ‘Flat Belt’, here in Tayvallich Subgroup strata, namely the Loch Tay Limestone Formation. Note that as we progress structurally (and topographically) upwards, we are moving stratigraphically downwards, into the Ben Lui Schist Formation (Crinan Subgroup, upper Argyll Group). Note that sedimentary way-up criteria are sparse or absent in outcrop and it is the distinctive stratigraphy that enables us to determine the regional structure here. The underlying Southern Highland Group arenites, wackes and semipelites, here with garnetiferous pelite interbeds, lie structurally below, and are exposed at the Falls of Dochart in Killin.
The burn exposes a good cross-section up through the Loch Tay Limestone Formation, which here is about 200 m thick. Note that the main points of geological interest are marked by waterfalls. The formation consists of thinly bedded metalimestone, calc-silicate rock, with abundant pelite and semipelite beds. The inverted beds dip gently east to southeast as they lie on the southern limb of the Ben More Antiform (F4). This broad, open antiformal structure is responsible for the structural inlier that exposes Southern Highland group rocks around Loch Tay. It is complementary to the Ben Lawers Synform whose axial trace lies 4–6 km farther NNW. In the burn section bedding is clearly defined but minor and medium-scale folds (?F2) are present in parts with a prominent axial planar cleavage developed in pelitic interbeds, particularly in the fold hinges. Fold axes generally plunge very gently northeast thereby exposing a true cross-section profile. A good example is seen at the waterfall at [NN 5592 2853] where a pink-fawn weathered felsitic sheet also intrudes the metalimestone (Figure 2.1). Although locally folded, the thickness of the unit is largely stratigraphical. In contrast, when traced southwards the formation becomes markedly thinner and indeed, lenses out just south of Strathyre. Up-section, the metalimestone unit contains increasing amounts of semipelitic and pelitic material and is intruded by a thick, massive amphibolitic metadolerite sheet. Above this amphibolite sheet additional thinner amphibolite sheets are present but metalimestone and calc-silicate beds become sparse and finally at a higher waterfall we pass upwards into garnetiferous muscovite-bearing semipelite and psammite, lithologies typical of the overlying Ben Lui Formation.
Return back to the car park by descending the bracken and grass covered banks adjacent to the burn (Care on descent). From Glenoglehead retrace your route to Lix Toll and turn eastwards onto the A827 to pass through Killin (toilets) and along the north side of Loch Tay to Edramucky (house), where a single-track minor road climbs northwards up past Loch na Lairige and over to Bridge of Balgie. To this point along Loch Tayside the main road has tracked along close to the Loch Tay Limestone Formation-Southern Highland Group boundary. In contrast the Loch na Lairige road crosses almost at right angles to the stratigraphy (and structure) thus providing a good across-strike section. The localities along and adjacent to the road pass from the Loch Tay Limestone up into the Ben Lui Schist Formation, and thence into the Ben Lawers Schist Formation (Easdale Subgroup), which occupies the wide hinge zone of the large-scale F4 Ben Lawers Synform.
Old Limestone Quarry, 500 m N of Edramucky [NN 6212 3678]
After c.500 m park adjacent to a disused quarry on the west (left) side of the road, now much more accessible owing to recent forestry clearance. The best structural features are to be seen in some of the large boulders recently unearthed. (Care clambering about, some boulders are not yet fully stable). The quarry lies within the central part of the Loch Tay Limestone Formation on the southern limb of the Ben Lawers Synform. The overall bedding dips some 30° to the NNW, and the formation structurally overlies Southern Highland Group rocks exposed by Loch Tay (Figure 2.3). The quarry exposes grey, layered metalimestone with dark pelite and semipelite interbeds, in part graphitic. Both calcite and quartz veins and pods are present, some of which show minor folding. Although the main penetrative fabric (? S2 cleavage) is generally orientated near parallel to the lithological layering, F2 minor folds can be found with S2 clearly axial planar (Figure 2.2). Treagus (2013) recorded a gently NE–plunging S0–S2 intersection lineation (e.g. 10° to 025°), which should lie parallel to such F2 fold axes. However, both the layering and S2 cleavage are clearly refolded in places by later F4 structures. A related steeply NNW-dipping crenulation cleavage (S4) is locally developed, notably in the F4 hinges.
From the disused quarry the road climbs up in zig-zags, having crossed into the overlying Ben Lui Schist Formation. Grass gives way to heather reflecting the change in lithology, but exposure is generally poor except in the incised section in the Burn of Edramucky north from the bridge (parking difficult). The Ben Lui Schist Formation dips 25° to 45° NNW and consists here of thinly interbedded semipelite, psammite, and subsidiary pelite with abundant quartz veins and pods. It is notable for the development of garnet and muscovite and has been termed garnet-muscovite-quartz schist. We pass the revised Ben Lawers parking area and information boards and wend our way up to the Lawers dam at Lochan na Lairige. The dam, a concrete buttress-type, some 42 m high, was built between 1951 and 1956 as part of the Breadalbane Hydro-electric Scheme and raises the top water level of the loch by 27 m. The road cuttings on the bends just SSE of the dam expose chevron folded (F4) and crenulated Ben Lawers Schists (calcareous semipelites and subsidiary psammites), which here lie within a pyrite-rich zone giving rise to rusty staining locally. It was from this section that Treagus (1964) described 3 phases of deformation/folding. He recognised that F1 isoclinal folds are refolded by tight F2 folds, which in turn are refolded by the ubiquitous upright F4 folds.
Unfortunately the outcrops have degraded somewhat over time and it is better (and easier) to examine similarly complex folded Ben Lawers Schist rocks on the shore of the lochan, just north of the dam.
Lochan na Lairige [NN 6007 3936]
We will park some 200 m north of the dam [NN 6012 3942] where a good track leads down to the loch. A short distance below are good outcrops of the Ben Lawers Schist Formation, forming part of the rocky and cobbly foreshore (unless water levels are particularly high).
The rocks consist of calcareous semipelites and thick, variably foliated amphibolite sheets, pods and layers, with thin calcareous psammite and more quartzose bands in parts. They have a pale to dark grey-green colour reflecting their greenschist facies retrogression (much chlorite and/or amphibole). F4 open to close and rarely tight, upright folds are abundant together with a pervasive crenulation/spaced S4 cleavage. Although overall they verge towards the SSE, here, in detail their vergence is locally variable, dependent on their scale of development. Tight to isoclinal F2 folds can also be found and some of these structures fold an earlier quartz-chlorite lineation (L1). F4 refolds of both F2 folds and the S2 cleavage occur in several places (Figure 2.4). Hence, with some searching, evidence of three phases of deformation can be found. The ubiquitous F4 folds and general small scale structural complexity conceal the fact that the overall dip of bedding is some 30° to the NNW and that we are still on the southern limb of the Ben Lawers Synform (Figure 2.3a, b).
From this locality proceed northwards along the narrow road that tracks just above the lochan. At around midway, you cross the broad hinge-zone of the Ben Lawers Synform (F4), best viewed in a prominent buttress on the opposite (western) side of the lochan. Faint traces of the overall shallow to sub-horizontal bedding have been accentuated by the weathering, but the steep weathered crags are dominated by sub-vertical features, reflecting the dominant penetrative S4 crenulation cleavage. Binoculars may reveal decametre-scale upright chevron folds (‘crenulations’) in the hinge zone.
Northern end of Lochan na Lairige [NN 5925 4136] to [NN 5917 4087]
At the northern end of the lochan, park, and take the good track leading down to the inlet portal. The track (rough) offers adequate parking either near the top or at the bottom [NN 5950 4128]. The aim here is to carry out a traverse, starting in the Ben Lui Schist Formation and then passing structurally up (but stratigraphically down) through a varied transitional unit to reach the Ben Lawers Schist Formation. Note that the sequence is still inverted, but as we are now on the northern limb of the Ben Lawers Synform the bedding dips at some 20° to 40° to the SSE. This is an approximately 3 km round trip with a height difference of c.110 m. No paths, some steep terrain, and rough, locally hummocky (though soft) ground. Note that the area is renowned for its flora.
The transition between the Ben Lawers Formation (Easdale Subgroup) and Ben Lui Formation (Crinan Subgroup) differs considerably across the district. On the south side of the Ben Lawers Synform the two units have a narrow transitional zone that does not merit the definition of a separate unit. In other areas the distinctive metavolcanic lithologies of the Farragon Volcanic Formation (formerly Farragon Beds) are well-developed between the two units, e.g. in the middle parts of Glen Lyon (See Day 6). In this area the recent 1:50 000 map (Sheet 46E) shows a lenticular development of the Farragon Volcanic Formation along the boundary, but Treagus (2013) has extrapolated this unit as being of constant thickness (Figure 2.3). The transitional unit was originally mapped as the Sron Bheag Schist (Elles, 1926); a laterally equivalent unit a little farther west has recently been termed the Auchlyne Formation. Its importance and origin are enhanced by the presence of local sulphide mineralisation, mainly pyrite but some chalcopyrite in places. In this area there is a varied assemblage of thinly bedded psammite, minor quartzite, schistose semipelite (garnet + mica), graphitic pelite and calcareous quartzite. The succession contains some thin amphibole-bearing units and amphibolites, the latter both massive and finely interbedded with psammites.
From the base of the track we traverse up by a prominent wall to reach the watershed area (or traverse across to obvious stile). The first prominent outcrops are interbedded psammites and semipelites with small garnets and muscovite well-seen on some surfaces. These are typical of the Ben Lui Schist Formation. However a short distance to the southwest on a prominent knoll [NN 5915 4132] they give way to psammites, quartzites and semipelites and some thin amphibolite units and calcareous quartzites.
Calcareous psammitic and semipleitic lithologies, more typical of the Ben Lawers Schist Formation, occur on the south side of the knoll, composed mainly of chlorite, sericite, carbonate and quartz, but locally with dark green hornblende porphyroblasts and some pyrite. Some of these units are interpreted as metavolcanic in origin.
Traverse south at a similar height towards the main cliff face, passing from the calcareous assemblage through quartzite (largely obscured by rock-fall debris) and into the dark grey schistose graphitic pelite at the base of the cliff. The latter clearly illustrates the angular relationship between the late-phase crenulation cleavage and the earlier continuous cleavages. In a gully a few metres above the base of the cliff at [NN 5910 4094] (where a small stream emerges) the contact of the graphitic pelite with the calcareous semipelites and psammites of the overlying Ben Lawers Schist is marked by sheared calc-quartzite.
Return to the vehicles by descending down towards the lochan (Care). Some upstanding gritty quartzite crags are seen (possible grading suggesting younging towards the northwest?). Attain the flat area the northwest end of the loch, and cross the inlet burn that emerges from the tunnel with the brick arch at [NN 59290 4095], all part of the Breadalbane Hydroelectric Scheme.
Return to the vehicles and Kindrogan via Loch Tay.