Moine geology from Invergarry to Kinloch Hourn, Quoich Bridge, westwards - an excursion

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From: Strachan, Rob, Friend, Clark, Alsop, Ian, Miller, Suzanne (Editors). A Geological excursion guide to the Moine geology of the Northern Highlands of Scotland.: Edinburgh Geological Society, Glasgow Geological Society in association with NMS Enterprises, 2010.

By Alan M. Roberts and David Barr

Fig. 4.1 Location map and general geology of Excursion 4.
Fig. 4.5 D2  sheath fold viewed looking NW. Note the intense L2 mineral lineation above coin (15 mm diameter) (Locality 4.5 [NH 0438 0174]).
Fig. 4.6 West-vergent D3 folds directly below Quoich Bridge (viewed north at Locality 4.6 [NH 0148 0406]).
Fig. 4.7 Geological map of Locality 4.8, Allt Coire Shubh Beag.
Fig. 4.8 Geological map of Locality 4.9, Kinloch Hourn to Skiary (lithostratigraphic symbols as Fig. 4.1).

Excursion 4 Invergarry to Kinloch Hourn is composed of the following articles:[edit]

Locality 4.6 Quoich Bridge [NH 015 041][edit]

Quoich Bridge (Fig. 4.1). Short stop to examine spectacular D2 eye structures refolded by upright D3 folds.

From Locality 4.5 drive west for c.3.5km and park (space available for about five cars) on the west side of the bridge (weight limit 10 tons) over the north arm of Loch Quoich. Look over the SW corner of the bridge, and if the rocks are exposed approximately 6m or more above the level of the loch, descend to them. If not, continue to Locality 4.7 as the level of the loch is too high.

This locality exposes the Garry Banded Formation (Roberts & Harris, 1983) on the west limb of the Gleouraich Synform. Numerous upright, NNE-plunging D3 folds, verging west away from the Gleouraich Synform to the east, can be seen (Fig. 4.6). These tight folds refold earlier, isoclinal D2 folds. The D2 folds have intensely curved hinges, and numerous three dimensional eye structures, with an accompanying axial extension lineation, are exposed. Some of the eye structures are folded by D3 folds. When well exposed this locality is extremely photogenic. Return to the vehicles and proceed to Locality 4.7. Coaches, however, should proceed no further than the sheepfolds 1.5km to the SW [NH 004 033], where there is space to turn around.

Locality 4.7 Coire Shubh road section [NG 9668 0407][edit]

Coire Shubh road section (Fig. 4.1). Short stop to examine the envelope of the Coire Shubh Pegmatite Complex, D3 folds in Glenfinnan Group semi-pelites, both cut by late-D3 pegmatites.

From Locality 4.6, drive on past the western end of Loch Quoich and across the watershed (5.5km). Drive down the first steep hill (with a stone embankment on the left) and park on the firm ground to the right of the road at [NG 9668 0407], 70m past a stone bridge at the foot of the hill. A roche moutonnée beside the passing place consists of migmatitic Glenfinnan Group semipelite and contains several upright, NNE-SSW-trending, reclined refolds of D3 (steep belt) age. Early leucosomes are folded and/or transposed into the strong axial-planar crenulation cleavage. Several NW-SE-trending white pegmatite veins are ptygmatically folded about D3 axial planes. These belong to the suite of late-D3, Caledonian pegmatites that will be visited at the next locality. Several large, white pegmatites are visible in surrounding exposures. The semi-pelites and pelites form the envelope to the Coire Shubh Pegmatite Complex, which is developed in a psammite-cored D3 antiform. Proceed to Locality 4.8.

Locality 4.8 Coire Shubh Beag [NG 960 045][edit]

Coire Shubh Beag (Fig. 4.1), (Fig. 4.7). Late Caledonian Pegmatite Complex in core of D3 steep-belt fold.

From Locality 4.7, drive on c.900m until the road crosses a major stream, the Allt Coire Shubh Beag, at [NG 9610 0468], 200 m after cutting through a rocky spur. One car can be parked 100m east of the stream without blocking the passing place, and there are also two large passing places c.800m to the west, on either side of the abandoned building at Coire Shubh [NG 958 053]. The pegmatite complex is developed in the core of a major, SW-plunging D3 antiform (Fig. 4.7). The psammites and semi-pelites that occupy the core of the fold are probably equivalent to the Reidh Psammite or the Quoich Banded Formation (Tanner, 1971; Roberts et al., 1987). They define a major fold interference pattern, occupying the core of a (sheath-like?) D2 recumbent fold that has been refolded by D3. Climb the hillside to the top of the rocky spur, keeping c.100m SE of the Allt Coire Shubh Beag, observing the white pegmatite dykes visible in slabs across the stream. About 120m from the road, at 8A [NG 9612 0449], bedding strikes almost east-west on the SE limb of the D3 antiform close to its core. Open to close, Z-profile D3 folds have a weak NNE-SSW-trending axial planar fabric. Compare these with the tight D3 folds at Locality 4.7 on the fold limb. Intrafolial, isoclinal D2 folds of bedding and of early migmatitic leucosomes are locally preserved. These folds are present throughout the centre of the psammite, i.e. in the core of the major D3 fold. Move WSW towards the stream and observe late pegmatites of the complex forming ESE-WSW-trending dykes. They define open D3 folds and carry a weak axial-planar fabric.

Proceed upstream to the confluence, noting the small dams that divert water via a conduit to Loch Quoich. Between the two streams, 8B, bedding strikes WNW-ESE in the core of the D3 fold and is disrupted by numerous pegmatites carrying a weak, NNE-SSW-trending D3 fabric. At 8C, a number of sloping exposures consist almost entirely of pegmatite. Several generations of pegmatite are present, along with patches and streaks of micaceous restite and some little-modified psammite blocks. Some of the larger psammite rafts in the centre of the exposure contain tight to isoclinal, E-W-trending D2 folds. The core of the D3 antiform runs up the Allt Coire Shubh Beag to the ridge, and to the north it runs through the tree-covered crags below the pylon line. Walk downhill on a bearing of 025° to 8D, a large sloping exposure with abundant white pegmatites. Bedding trends NW-SE and the psammites are broken up by concordant, weakly foliated pegmatites to form a migmatite. This is cut by NE-SW-trending, muscovite-rich pegmatite dykes up to 4m thick, that in turn are cut by weakly foliated, muscovite-poor dykes striking at 120°. These dykes belong to two major swarms, one approximately axial planar to the major D3 fold of bedding, and one that defines an open fold having the same axial plane as the major fold of bedding but a smaller interlimb angle. Apophyses of the axial planar set define open D3 folds. Sheets, patches and swarms of micaceous restite are common.

Cross the gully and proceed to the large rocky spur at 8E. This locality contains the strongest evidence for the origin of the micaceous restite. The psammites strike at 100° and are cut by 025°-trending pegmatites that have micaceous reaction rims against the host psammite. These zones probably represent restite after extraction of quartzofeldspathic components from the psammite, possibly by partial melting (Barr, 1985). The pegmatites contain streaked-out relics of psammite and restite and, locally, zones of restite are developed with little or no pegmatite. The varying ratios of pegmatite to psammite and restite indicate that quartzofeldspathic material has moved some distance, perhaps as a melt, so that all stages are seen from restite with virtually no pegmatite to pegmatite with virtually no restite.

Walk NNW across the exposure, noting the presence of S-profile D3 folds indicating that we are now on the NW limb of the major antiform. The D3 fabric also becomes stronger to the NW. Both on the exposure scale and on the scale of the NW-SE dyke swarm (Fig. 4.5), the pegmatites show evidence for emplacement after some D3 shortening, i.e. they cut tight D3 folds but themselves carry a weak D3 fabric or have been gently folded. Walk down the hillside to the road and return to the vehicles.

Locality 4.9 Kinloch Hourn to Skiary [NG 953 064] to [NG 933 072][edit]

Kinloch Hourn to Skiary (Fig. 4.1), (Fig. 4.8). Knoydartian pegmatite, steep-belt folds, Sgurr Beag Thrust, Lewisianoid inlier, Caledonian pegmatites, Knoydart Thrust.

From Coire Shubh, drive NW along the road for c.2km to Kinloch Hourn Farm where parking is available for a nominal sum. Overnight parking is also available, as well as a toilet, tea room and accommodation. Details can be obtained by contacting Martin Riley (Tel: 01809 511253). Return on foot to the road cutting 70m east of the junction with the private road to Kinloch Hourn Lodge, 9A, [NG 954 064]. The Kinloch Hourn fault forms a marked topographic feature north of here and is followed closely by the pylon line. A transitional contact is exposed between the Sgurr Beag (= Quoich or Lochailort) pelite to the east and the Reidh Psammite to the west (Tanner, 1971). This locality lies on the eastern limb of the D3 Kinloch Hourn Antiform and so a D3 synform is inferred to lie within the Sgurr Beag Pelite, between here and the Coire Shubh Antiform (Fig. 4.1). The rocks are steeply ESE-dipping and moderately platy, with early migmatitic leucosomes and garnets that form augen (‘eyes’) within the dominant fabric. Feldspar porphyroclasts are wrapped by a quartz ribbon fabric that is present in the pegmatites and in the more psammitic metasedimentary rocks. A number of deformed pegmatite and quartz-muscovite veins and pods are also present. If these are interpreted as syn-metamorphic segregations, the Rb-Sr muscovite age of 755 ± 19 Ma obtained from one of these veins by Piasecki & van Breemen (1983) suggests that the early high-grade metamorphism of these rocks occurred during the Neoproterozoic Knoydartian event.

The exposure of Reidh Psammite behind the fence on the south side of the road contains several reclined, tight-to-isoclinal S-profile fold pairs that occupy low strain augen. These have a strong axial planar quartz fabric but fold a fine lamination and, in more micaceous bands, an earlier fabric. Follow the exposure southwards for c.50m, to a point on a bearing of 152° (from grid north) from the south end of the bridge. A tight, upright fold pair deforms a planar fabric in which the early leucosomes are streaked out and form augen. The fold pair has an associated axial planar and an open-folded pegmatite, similar to those of the Coire Shubh Complex, supporting assignation of at least some of the upright folds to D3 (steep belt), but much of the platiness to an earlier event.

Walk 70m west to the foot of the next spur. This platy semipelite is relatively non-migmatitic and fine-grained. It lies structurally below the Reidh Psammite in the core of the Kinloch Hourn Antiform, and may represent an upfold of the Morar Group lying below the Sgurr Beag Thrust (Barr, 1983, 1985). The semipelite is thoroughly recrystallized, but quartz and feldspar grains define a shape fabric, and quartzofeldspathic segregations, garnets and mica form augen in a manner typical of rocks from the vicinity of the Sgurr Beag Thrust. The platy fabric is crenulated by upright, steeply-plunging folds with variable vergence. Associated quartz rodding plunges at 75° to 148°. The Kinloch Hourn Antiform is inferred to be sheath-like, with variable plunges in an east-west zone near its centre and steep plunges at its northern and southern closures. The northern closure of the semipelite/ psammite contact is not exposed, and is inferred from fold and foliation trends within the semipelite to be a steeply south-plunging synform. This is because fold axes have rotated from north-plunging, through vertical, to south-plunging in approaching the steeply south-plunging extension direction. Return to the parking place and walk along the crags south of the tidal pool towards 9B [NG 9475 0675]. These crags expose the transitional contact between Reidh Psammite (east) and Sgurr Beag Pelite (west). Quartzose psammites and highly migmatitic micaceous psammites and semi-pelites all carry an intense planar fabric whose strike varies from north-south to east-west on open, south-plunging D3 fold pairs verging towards the Kinloch Hourn Antiform to the east. Early leucosomes, quartz veins and large pegmatite pods are intensely deformed (Barr, 1985 fig. 7.4), feldspar augen being wrapped by quartz ribbons. In non-migmatitic psammites, quartz ribbons parallel this fabric and intrafolial, S-profile isoclines are preserved.

The platy fabric is present throughout the Reidh Psammite, and is probably related to the presence of the Sgurr Beag Thrust at its base.

Pelites exposed by the roadside strike north-south, and swing anticlockwise to east-west in the core of a steeply south-plunging D3 synform. A down-dip mineral lineation is observed in the synform core, but the platy fabric clearly passes around this fold. Immediately before the jetty, at the top of the exposure south of the road, intrafolial D2 folds can be seen in low strain augen within the platy fabric. Descend west of the jetty to the point at 9C [NG 9470 0683], where a series of south-plunging D3 folds carry a north-south trending axial-planar crenulation cleavage. The earlier, D2 platy fabric contains augen of leucosome and garnet, and in thin section it too is observed to be a crenulation cleavage. Walk west to the next bay, where a body of feldspathic augen gneiss occupies the core of a complex south-plunging antiform. It lies close to the boundary between the Reidh Psammite and the Sgurr Beag Pelite and contains isoclinally folded pegmatites to which the platy fabric is axial-planar. Also present are some late, cross-cutting pegmatites, and towards the western end of the exposure, several concordant pods and sheets of somewhat biotitized hornblendite. These hornblendites comprise one of the Lewisianoid basement bodies recognized by Tanner (1965, 1971) and interpreted as a tectonically-emplaced inlier along the course of the Sgurr Beag Thrust. The augen gneiss could also be part of the basement sheet, but it has a metasedimentary bulk chemistry (Barr, 1983) and could represent a basal Moine arkose.

Proceed to the west of the jetties, crossing a synform cored by Sgurr Beag Pelite that contains several tight, north-south trending D3 folds. The exposure to the west of the boathouses comprises platy, migmatitic Reidh Psammite, intruded by late Caledonian pegmatites. The final 10m is more quartzitic, and the Sgurr Beag Thrust is inferred to lie in the rubble-choked gully at the end of this exposure (Tanner, 1965, 1971; Barr, 1983). The platy fabric within the Reidh Psammite and the lower part of the Sgurr Beag Pelite is considered to result from intense ductile strain (simple shear) in the hanging-wall of the Sgurr Beag Thrust, and the fact that this fabric is folded by the upright north-south D3 structures indicates that thrusting occurred prior to formation of the steep belt.

The sequence of events observed within the Sgurr Beag Nappe at Kinloch Hourn is: D1 – early fabric, high-grade metamorphism, migmatization, garnet growth; D2 – ductile thrusting, development of platy fabrics; D3 – upright refolding on NNE-SSW axial planes with curved hinges, pegmatite emplacement. It is not possible at this low level within the nappe to identify unambiguously the north-south curvilinear folds seen at Localities 4.1 and 4.5.

The Morar Group west of the Sgurr Beag Thrust carries an intense planar fabric with a down-dip mineral lineation, contains ribboned quartz veins and ?syn-thrusting pegmatites, and becomes less micaceous westwards. All planar discordances (e.g. cross-bedding, cross-cutting quartz veins and cleavages) have been eliminated (see also Rathbone & Harris, 1979; Rathbone et al., 1983). Towards the end of the road, micaceous psammite bands become common. The concordant quartz veins are joined by long-limbed isoclinal folds of quartz veins, pegmatites and calc-silicate ribs, indicating a westward reduction in ductile strain.

The calc-silicate rocks, seen as pale coloured pods and ribs a few centimetres thick, were described in detail by Tanner (1976). They contain garnet, hornblende, calcic plagioclase and occasional pyroxene, and indicate middle to upper amphibolite facies conditions (Tanner, 1976). The enclosing psammites were assigned to the Coire Mhicrail ‘Group’ by Tanner (1971), who suggested that they represent a local psammitic variant of the Morar Schist, rather than Lower Morar (= Barrisdale) Psammite.

Beyond the jetty at 9D, semi-pelites carry a semi-penetrative mica fabric with a strong down-dip lineation, presumably related to the Caledonian Sgurr Beag Thrust, and an earlier, slightly oblique schistosity lying anticlockwise of the dominant foliation. Together with garnet augen within the platy fabric, this observation demonstrates that the Morar Group had been deformed and metamorphosed prior to Caledonian ductile thrusting.Continue westwards around the promontory to 9E where the psammites trend east-west and lack a strong fabric but are folded on north-south axial planes. In the next bay, open-folded zones with ESE-WNW-trending bedding and discordant pegmatites and quartz veins alternate with NNE-SSW-trending zones where a strong fabric dips at 70° towards 110° and is axial planar to tight folds of pegmatites. Pegmatites in the east-west domains both cut and are folded by north-south structures or are axial planar with folded east-west apophyses; they are probably syn- to late-tectonic. Also present are earlier, intrafolial folds and somewhat-deformed sedimentary structures. By 9F the east-west domains preserve sedimentary structures, younging southwards. Some cross-beds are oversteepened, either as a result of east-west shortening or as an original sedimentary feature. Round the promontory at the western end of the bay, isoclinal folds are seen in a north-south zone and boudins of Caledonian pegmatites become common. The age of these north-south folds is unclear. They could be related to the D2 Sgurr Beag Thrust, the north-south trending platy zones representing minor ductile shear zones within the Morar Group, or they could be of D3 ‘steep belt’ age. The latter interpretation is favoured by the presence of syn-tectonic pegmatites, but it is also possible that these pink-weathering pegmatites are not the same as the white-weathering ones at Kinloch Hourn and Coire Shubh.

Continue to walk westwards along the straight section of coast. Note the general steep ESE dip of the psammites, that are flaggy but contain discordant quartz veins and occasional steeply-plunging upright folds. At 9G a Z-profile fold pair is succeeded westwards by a 50m-wide platy zone with abundant concordant quartz and pegmatite veins as well as some that are slightly cross-cutting. About 20m east of the small promontory in the centre of the bay, the micaceous and calc-silicate-bearing psammites of the Coire Mhicrail ‘Group’ give way abruptly to the grey and featureless Barrisdale Psammite. Both units are finely laminated and carry a strong platy fabric.

Walk westwards along the track and descend to the foreshore by the cairn, 9H, observing the variably platy psammites that include a pegmatite-rich zone of late, east-west folding. Towards the western end of this exposure, intrafolial folds and possible deformed cross-beds begin to appear. The next exposure, 50m further along the shore, contains quite large (0.5m wavelength) isoclinal folds. Return along the fence to the track, to a zone of tight to isoclinal folds that deform bedding and ribbon-like quartz veins, 9J. Some of these appear to define sheath folds with steeply-plunging axes. The strain gradient is similar to that observed at the Sgurr Beag Thrust, and, if anything, the high-strain zone in the footwall is wider (200m). As at the Sgurr Beag Thrust (Rathbone et al., 1983), the footwall platy zone is much wider than the hanging-wall platy zone. The sharp boundary between the two psammitic units is considered by Barr et al. (1986) to represent the Knoydart Thrust, emplacing the Coire Mhicrail ‘Group’ onto the Barrisdale Psammite. The thrust cuts up-section to the west so that in Glen Barrisdale it lies at the base of the Knoydart Pelite (line ‘C’ of Tanner, 1971, figure 2) and in western Knoydart it lies at a low level within the Knoydart Pelite. Return along the track to Kinloch Hourn, but if the weather is clear it is worth diverting to the high point on the track, 800 m SW of Skiary, from where excellent views may be had of the surrounding hills. To the WSW lies Meall nan Eun, and in the background, Ladhar Bheinn and Beinn na Caillich. To the north lies Carn nan Caorach, with the Saddle ridge in the background, to the NE lie Sgurr na Sgine and Sgurr a’ Bhac Chaolias, and to the ENE, Buidhe Bheinn.

References[edit]

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