The Moine Thrust Belt at Loch Eriboll. Transect 4: Creagan Road - 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.
Fig. 11.11 Geology of transect 4 through the mylonite belt along the Creagan Road.
Fig. 11.12  Quartz mylonites and associated quartz veins deformed by F3 folds at Locality 11.4D.

Excursion 11 The Moine Thrust Belt at Loch Eriboll is composed of the following articles:[edit]

Excursion 11 The Moine Thrust Belt at Loch Eriboll. Transect 4: Creagan Road[edit]

Transect 4 – Creagan Road[edit]

(contributed by Rob Strachan, Bob Holdsworth and Ian Alsop)

This is a short traverse to examine the internal tectonostratigraphy of the mylonite belt (Fig. 11.11) that occupies a high structural level in the Moine Thrust Belt (Barber & Soper, 1973; Soper & Wilkinson, 1975; Evans & White, 1984; Law et al., 1986; Holdsworth et al., 2006). The description here makes an interesting counterpoint to that for the Kempie transect in that the naming of thrusts and correlative approaches are different. This debate has continued since the work of Peach et al. (1907; see Barber & Soper, 1973). The traverse involves 2-3 hours of moderate walking on tracks and hillsides.

Park (with permission) adjacent to the entrance to Eriboll Estate at [NC 4323 5630], taking care not to block any gateways or entrances. There is sufficient space for a coach or four to five cars. Take the track (the ‘Creagan Road’) that leads southwards from the telephone box through a gate. Pause further on at a second gate to look northeastwards to the crags in the trees of steeply dipping Durness Limestone on the overturned limb of the Kempie Bay Syncline. After going through the gate, follow the fence southwards and then walk across the hillside to the low-lying crags that form Locality 11.4A [NC 4301 5577]. Here is exposed the tectonic lower boundary of the mylonite belt which is termed the Lochan Riabhach Thrust by Holdsworth et al. (2006). They propose that it is a late, out-of-sequence brittle structure that everywhere underlies the mylonite belt and is entirely distinct from the Moine Thrust that is exposed at a higher structural level. It is a sharp, gently-dipping fault that emplaces intensely deformed quartzo-feldspathic mylonites that are here interpreted to be of Lewisian origin onto largely undeformed and probably inverted Cambrian Salterella Grit. Small pips of carbonate lie along the thrust plane and are interpreted as detached slices of Durness Limestone. The thrust cuts obliquely across the inverted limb of the Kempie Bay Syncline, to rest discordantly on Durness Limestone in the stream section a few hundred metres to the southwest. Walk from here northeastwards, cutting uphill to the first hairpin bend in the Creagan Road. Continue along the track, passing outcrops of Cambrian Pipe Rock, the unexposed Lochan Riabach Thrust and, above that, further outcrops of Lewisian-derived mylonite between the second and third hairpin bends. Pass through a gate and continue along the track.

Locality 11.4B [NC 4356 5534] is by the track, where ‘Oystershell Rock’ mylonites are well exposed. These are platy, white mica-chlorite phyllonites with numerous lunate quartz segregations – the superficial resemblance of the latter to fossil shells gave rise to the informal term that has continued to be used for this lithology (Peach et al., 1907; Soper & Wilkinson, 1975; Holdsworth et al., 2001a, 2006). The protolith for the Oystershell Rock has long been considered as Lewisian (e.g. Barber & Soper, 1973; but see also Soper & Wilkinson, 1975). Holdsworth et al. (2001a) have confirmed a metamorphic protolith and a Lewisian one seems most likely. The phyllonites carry well-developed shear band fabrics (McClay & Coward, 1981) that indicate a top-to-the-west sense of displacement parallel to a locally developed mineral and extension lineation. Continue uphill, passing further outcrops of the Oystershell Rock, some containing early syn-mylonitization isoclinal folds (‘F2’) that are refolded by asymmetric folds (‘F3’). Just beyond the telegraph poles, the track starts to flatten by low outcrops of Oystershell Rock [NC 4367 5525]. At this point, head across the hillside due east towards Am Feur Loch, walking over more low outcrops of Oystershell Rock. Pause at [NC 4388 5530] to view the loch and surrounding outcrops. The steep crags on the east side of the loch comprise in their lower part quartz mylonites derived most probably from deformation of the Eriboll Sandstone Formation. These are separated by what Holdsworth et al. (2006) and others before have interpreted to be the Moine Thrust from overlying mylonites derived from Lewisianoid basement. Exposures of the latter have a characteristic blotchy pale colouration due to greater amounts of lichen cover compared to the more homogeneous grey quartz mylonites.

Whilst there is agreement on the protolith of these quartz mylonites, the problem of their structural setting is central to current debate on the structural evolution of this part of the Moine Thrust Belt. According to Holdsworth et al. (2006), the quartz mylonites overlie an original (albeit highly tectonized) unconformity with the Lewisian protoliths of the Oystershell Rock, both of which are assigned to the Caledonian foreland (British Geological Survey 2002; Holdsworth et al., 2006). An alternative view, consistent with Butler’s (1982) interpretation on Creag Shomhairle (see transect 1), is that the quartzites have been imbricated into the Oystershell Rock. This could mean that the Moine Thrust and the Lochan Riabhach Thrust described here are essentially the same structure repeated by displacement on a breach thrust. If this is the case, the Oystershell Rock need not be derived from the foreland, but could form part of the far-travelled Moine Thrust sheet.

Alternatively some or all of these contacts could be minor structures associated with distributed shearing, as found at the top of the Kempie area (transect 3).

Walk to Locality 11.4C [NC 4395 5525] at the low crags to the north of the loch. The lowest exposures are spectacular quartz mylonites; an intense mylonite fabric dips gently to the ESE and carries a mineral and extension lineation that plunges down-dip. Asymmetrically sheared quartz veins indicate a top-to-the-west sense of shear parallel to the lineation. West-verging, open F3 folds deform the mylonite fabric and associated lineation.

Quartz veins vary from intensely mylonitic to apparently undeformed. Some discrete thrusts are associated with late folds. Higher parts of the crags expose the Moine Thrust as a sharp contact between the white quartz mylonites and grey to cream coloured Lewisianoid-derived mylonites. The latter contain quartz, feldspar and mica, and lack the continuous banding that is a prominent feature of the quartz mylonites. The contrast between the evidently ductile nature of the Moine Thrust here, located within a broad belt of mylonites, and the brittle Lochan Riabhach Thrust, is the rationale for Holdsworth et al. (2006) regarding the structures as entirely separate and not the same thrust repeated by breaching.

Head northwestwards upslope to small outcrops 50m away of quartz mylonite at Locality 11.4D [NC 4388 5545]. These expose excellent examples of F3, tight-to-open, asymmetric S-folds of the mylonite fabric and associated lineation (Fig.11.12). Fold hinges are variably oriented: some are almost parallel to the lineation, others are normal to the lineation (Evans & White, 1984). Also visible are isolated F2 isoclinal folds that formed during mylonitization; note that the mylonite fabric is appreciably more intense on the fold limbs than in the hinges. Numerous quartz veins are present, some elongated parallel to lineation. The F2 and F3 folds are interpreted as resulting from continuous, progressive deformation within the evolving mylonite belt (Holdsworth et al., 2006).

Splendid views may be had of Ben Hope and Ben Loyal to the east, and Cranstackie and the dip-slope of the Cambrian quartzites to the west. Return to the lochside and walk back down the Creagan Road to the vehicles, leaving all gates as you find them.


References[edit]

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