Moine geology of Loch a’ Bhraoin, Braemore and Loch Broom. The Sgurr Beag Thrust section

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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 Simon Kelley

Fig. 9.1 Geology of the Moine rocks between the Fannich mountains and Ullapool, showing the localities to be visited.
Fig. 9.2 Sketch map for Locality 9.2. See Fig. 9.1 for key.

Excursion 9 Loch a’ Bhraoin, Braemore and Loch Broom is composed of the following articles:[edit]

The Sgurr Beag Thrust section [NH 177 768] to [NH 116 723][edit]

The first locality (Fig. 9.1) of the section is on the A832 Gairloch road 4.5km from Braemore junction [NH 209 777] with the A835 Ullapool road (not to be confused with the junction close to Garve) and about 16km from Dundonnell. Roadside exposures between Braemore junction and the parking place are psammites of the Inverbroom Psammite, containing occasional interbedded pelites (Fig. 9.1). Parking for three cars is available by the bridge over Allt Leacach (Fig. 9.1), (Fig. 9.2). This section of the A832 road (‘Destitution Road’) to Dundonnell was first built in 1846 to assist years of poverty and famine in the area after several consecutive years of bad weather causing crop failures.

Locality 9.1 Allt Leacach [NH 177 768][edit]

Allt Leacach (Fig. 9.1). Start of the Sgurr Beag Thrust traverse (900-800m structurally below the Sgurr Beag Thrust).

The Inverbroom Psammite exposed in the waterfalls above and below the bridge, consists of banded psammites and semi-pelites with a flat-lying D2 planar fabric crenulated by D3. Occasional minor D3 folds are exposed in the stream below the bridge. Note particularly the undulating style of the bedding planes with rare cross-bedding, the angular discordance between early quartz veins and the foliation/bedding plane, and the intensity of the stretching lineation in the psammites.

About 1.5km further down the road towards Dundonnell, an untarred track leads from the road to the SW [NH 162 761]. Parking for five cars is available in a lay-by, 100m further along the road.

Locality 9.2 Allt Breabaig [NH 156 752 to NH 166 723][edit]

Allt Breabaig (Fig. 9.2). A section from the Morar Group rocks across the Sgurr Beag Thrust into Glenfinnan Group migmatites and the Fannich Lewisianoid basement inlier (Fig. 9.1).

From the parking place, follow the track for about 1km to the bridge [NH 1585 7500]. Note the exposure of the Sgurr Mor Pelite in the stream by the lochside, 2A [NH 1565 7490] (350m below the Sgurr Beag Thrust, (Fig. 9.2)) which exhibits deformed quartz veining and a stretching lineation (trending at 135°) that is stronger than that at Locality 9.1. Rare sedimentary structures are flattened in the plane of the foliation and pelite-rich horizons have been crenulated by the post-thrusting D3 deformation.

Follow the path leading from the bridge and up the glen between Druim Reidh and Meall a’ Chrasgaidh. As the path drops down to follow the burn, exposures of the Meall a’ Chrasgaidh Psammite are visible in its banks, 2B (250m below the Sgurr Beag Thrust, (Fig. 9.2)). Compare the near-parallelism of sedimentary structures, foliation and early quartz veins with the equivalent relationships in Locality 9.1 and stop 2A that were further beneath the thrust. Note the increased intensity of the stretching lineation in these psammites. A later minor curved D3 fold hinge in one of these exposures folds the foliation and lineation related to the development of the D2 Sgurr Beag Thrust. All localities in this excursion lie on the shallow-dipping limb of a major N-S-trending D3 syncline (Fig. 9.1) overturned towards the west. Many minor folds of the D3 generation exhibit curved hinges.

Large exposures of the planar-bedded psammites, 2C (75m-25m below the thrust, (Fig. 9.2)) have been created by waterfalls further up the stream. These exposures have a ‘tramline’ style foliation and increasingly intense stretching lineations. It is difficult, by this stage of the deformation, to distinguish any angular discordance between the foliation and early quartz veins.

Exposures of the planar-bedded psammites above the waterfalls exhibit occasional dark bands, less than 1cm thick, lying nearly parallel to the foliation. These bands are haematized cataclastic zones, almost certainly associated with late brittle movements on the Moine Thrust Zone that lies only 6km to the west or 1.5km vertically below these exposures.

The lower boundary of the Meall an t-Sithe Pelite is heralded by waterworn exposures of pelitic migmatites in the banks of the burn, 2D (Fig. 9.2). The actual contact is exposed on the Meall an t-Sithe to Creag Rainich ridge (Option A) where it is found in its original shallow lying position, in contrast to the steeply-dipping outcrops in the southwest Moine (see Excursions 4 and 5). Folds of thrust-generation age can be seen at [NH 155 741] by making a diversion at the end of the traverse on the way back to the farmhouse (or close to the summit of Meall an t-Sithe, Option A). These are a feature of the Sgurr Beag Thrust elsewhere (Rathbone & Harris, 1979), but are rare in this area.

In the first exposures above the Sgurr Beag Thrust, the lit-par-lit texture that is characteristic of these rocks has been destroyed by deformation associated with displacement along the thrust. Note that the shallow-lying foliation and stretching lineation (still trending 135°) are stronger than those further from the thrust.

Pelitic migmatites have retained their lit-par-lit texture further up the stream, 2E (50 m above the thrust, (Fig. 9.2)). The migmatization is thought to have occurred during the Neoproterozoic Knoydartian event and the migmatitic fabric was subsequently deformed during Caledonian thrusting.

The composite fabric exhibits later crenulations, due to the post-thrust D3 deformation.

Pale coloured acidic Lewisianoid basement gneisses are exposed in the stream bed and by the stream a few metres above the ruined croft, 2F (150m above the thrust, (Fig. 9.2)). These are characterized by stronger foliations and stretching lineations than those in the Moine rocks further downstream. The orientations of the foliation and lineation correspond closely with those seen in the Moine psammites. Refolded isoclines in some exposures testify to the complex polydeformational history of the rocks.

The Fannich outcrops of Lewisianoid basement gneiss have been the subject of controversy throughout the history of their study. The original Geological Survey workers disagreed over their outcrop pattern, some believing that it represented a Lewisian basement core to the Moine rocks in a ‘fountain of nappes’, and others that the outcrop represented a block of Lewisian translated to its present position along a thrust plane. No field evidence was produced for the latter view, but they believed that it lay at the boundary between the Meall an t-Sithe Pelite and the Meall a’ Chrasgaidh Psammite (now generally accepted to be the position of the Sgurr Beag Thrust). Some years later, Sutton & Watson (1954) discounted the idea that the gneisses of Fannich were basement at all, and maintained that they were an integral part of the Moine succession. Subsequent work on the geochemistry of the gneisses (Winchester, 1971) re-established their basement affinities. Although the Lewisianoid gneisses and Moine migmatites (above and below the gneisses) are certainly interfolded, the precise structural relationship still remains uncertain.

Return to the bridge [NH 1585 7500]. Exposures of sheared isoclines in the Meall a’ Chrasgaidh Psammite lie close to the 450m contour on the NE-trending spur at the head of the glen [NH 155 742]. From these exposures, simply walk down the spur and rejoin the path to the bridge.

References[edit]

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