Moine geology from Invergarry to Kinloch Hourn, Loch Quoich shore section - an excursion

From Earthwise
Jump to navigation Jump to search
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. 4.4 Geological map of the central part of the Loch Quoich shore section (Locality 4.5) (from Holdsworth & Roberts, 1984).

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

Locality 4.5 Loch Quoich shore section [NH 042 019] to [NH 046 016][edit]

Loch Quoich shore section (Fig. 4.1), (Fig. 4.4). D2 sheath folds, fold interference patterns, boudinage, members of the microdiorite and appinite suites.

From Locality 4.4, drive west for about 2km and park in the large lay-by at [NH 045 018], below the radio mast. This parking space lies above a remarkable stretch of exposures along the north shore of Loch Quoich between [NH 042 019] and [NH 046 016]. The lower the level of the loch the more rock is exposed, but even if the water level is high there is much of interest to be seen.

The rocks of the shore section lie within the Quoich Banded Formation of the Glenfinnan Group (Roberts & Harris, 1983), but unlike the Quoich Banded Formation at Quoich Quarry (Locality 4.4), these rocks lie not on a major fold limb affected by high D3 strains, but rather within the hinge zone of the D3 Spidean Mialach Antiform (Fig. 4.1). The low D3 strain within the hinge zone provides a ‘window’ back to the pre-D3 history of the area. A similar low-strain hinge zone was examined in Coir’ an t-Seasgaich (Locality 4.2).

The structural history of the shore section is extremely complex, and for a full structural analysis accompanied by detailed maps the reader is referred to Holdsworth & Roberts (1984) and Roberts (1984). In this account, only the salient features of the shore section, of interest to the general reader, are described.

Folds produced by all the major deformation events recognized in the area, D1-D4 sensu Roberts & Harris (1983) and Holdsworth & Roberts (1984), can be seen in the shore section. D1 folds are generally small-scale (1m) and most easily recognized where they are folded around D2 folds. Quartzofeldspathic migmatitic segregations lying within S1 occur in some pelitic units, but later deformation has resulted in this fabric being almost entirely transposed. Evidence for the early age of this migmatization is clear only in D2 fold hinges, where S1 passes around the fold. D2 structures are ubiquitous in the southern two-thirds of the section. The folds are tight to isoclinal, with an axial planar crenulation fabric that is commonly so strongly developed that it appears penetrative to the naked eye. Open to close D3 folds have nearly upright NE-SW-trending axial surfaces and an axial planar crenulation cleavage (S3). These folds commonly reorientate the earlier, much tighter D2 structures. A solitary, NW-SE-trending D4 antiform, with no related axial planar fabric, occurs at [NH 043 018].

The shore section is best traversed from SE to NW, and in this guide will be discussed in three sections: (1) [NH 046 016] to [NH 044 017]; (2) [NH 044 017] to [NH 043 018] (Fig. 4.5); (3) [NH 043 018] to [NH 042 019].

(1) [NH 046 016] to [NH 044 017][edit]

Begin this traverse at the most southerly set of exposures on the small headland. At this end of the section, tight to isoclinal D2 folds are exposed, unaffected by later refolding. The folds, both here and throughout the section, are best defined by c.1m-thick bands of quartzite that occur within a striped unit of pelitic gneiss, semipelite, psammite and quartzite. The competence of the quartzite bands in relation to the surrounding metasediments has commonly resulted in tight D2 fold hinges being cut through by small thrust faults as the folds developed. This phenomenon is well developed in the southeastern part of the section. Examples of D1 folds and an S1 fabric can also be seen folded around the D2 folds in the first 50m of the traverse.

About 60m into the traverse, the first effects of upright, NE-SW-trending D3 folding can be seen, in the form of an antiform-synform pair with an associated axial planar crenulation cleavage. These D3 folds are superimposed on earlier, tight D2 folds. The D2 hinges are in places cut through by small thrusts and spectacular boudinage occurs within quartzite bands on the long limbs of D2 folds.

Continuing northwestwards, the slabs are dominated by interference between reclined D2 folds and upright D3 folds. Where the quartzite bands were boudinaged during D2 the boudins have been refolded during D3, adding to the complexity of the deformation. D3 refolding of D2 folds and boudins continues to the end of exposure within section 1 at [NH 045 017], some 140m NW of the start of the traverse.

Several examples of easterly-inclined felsic porphyrites, cutting across all structures within the host rocks, are exposed in this section, at 80m and 100m into the traverse, and at the last exposures 140m into the traverse. This final example is unusual in that the felsic porphyrite has been intruded into an already-present microdiorite sheet. Both sheets cut across minor D3 folds and fabrics but the microdiorite has been extensively foliated whereas the porphyrite is unfoliated, indicating that the microdiorite is late-tectonic but the porphyrite is probably post-tectonic. Nevertheless, both microdiorites and felsic porphyrites have recrystallized to amphibolite facies mineral assemblages in this area.

From this multiple intrusion walk across c.115m of unexposed ground to the start of section 2 at [NH 044 017].

(2) [NH 044 017] to [NH 043 018][edit]

A published map (reproduced here, (Fig. 4.4)) of this part of the section can be found in Holdsworth & Roberts (1984, figure 4). An unpublished map of the whole shore section can be found in Roberts (1984).

Section 2 is again dominated by D3 refolding of D2 structures, although D2 boudinage and fold hinges cut by thrust faults are less common than in section 1. Occasional examples of D1 folds and fabrics can be seen in D2 fold hinge zones, and a solitary, upright, NW-SE-trending D4 antiform has been recognized.

The distinguishing feature of section 2 is the curved nature of the hinges of the D2 folds. As at Garry Quarry (Locality 4.1), D2 folds are curved about a N-S extension lineation; however, the curvature of individual folds is here much greater. In some cases the hinges of individual folds curve through nearly 180° in about 1m, giving them a conical or sheath-fold geometry. When such folds are viewed along their extension direction, closed outcrop patterns are seen (similar to those produced by dome-and- basin interference) as well as double vergence (S and Z) within a single fold pair (e.g. Holdsworth & Roberts, 1984, figure 6). The best examples of such closed outcrop or ‘eye’ structures are exposed in quartzite and psammite bands at stop 5A (Fig. 4.4), (Fig. 4.5). Note the strong extension lineation parallel to the hinges of the folds. Eye structures can also be seen elsewhere in section 2, and less spectacular examples occur in section 1.

Further NW, a number of quartzite bands trace out several upright, NE-SW-trending D3 folds. However, at 5B (Fig. 4.4), two quartzite bands trace out a very obvious, reclined D2 pair (openly folded during D3), showing S vergence. The vergence of this fold pair should be compared with that of a train of D2 folds exposed in a quartzite band 40m further northwest, 5C (Fig. 4.4), where the D2 folds consistently show Z vergence. Examination of the completely exposed section between the S and Z folds shows no evidence for a major D2 fold core. The absence of a major D2 fold is also indicated by sparsely preserved cross lamination within the quartzites, that shows overall younging to the NE. The change in fold vergence from S to Z is attributed to the curved nature of the D2 folds producing ‘double vergence’, the hinges of the D2 folds having swung through c.180° between the S and Z folds. This phenomenon represents a larger scale version of the eye structures seen earlier in the traverse.

From 5C to the end of section 2, approximately 70m, the main structural features seen are a number of upright, S-profile D3 fold pairs.

A number of late Caledonian igneous rocks are also exposed in section 2.

At 5D (Fig. 4.4), a c.5m-wide plug of typical hornblendic appinite is well exposed, cutting across the stratigraphically highest quartzite band. Two easterly-inclined microdiorite sheets are exposed in section 2. At 5C, a c.1m-thick microdiorite has been intensely sheared by late, localized movements within the sheet. The margins of the sheet, however, truncate D2, D3 and D4 structures in the surrounding rocks. A larger, non-foliated microdiorite is exposed 60m further NW (Fig. 4.4). A single, 3m-wide felsic porphyrite is exposed at stop 5F immediately west of the quartzite band at

5C. The porphyrite sheet is inclined 30° to the east and is foliated internally. It clearly truncates the D2 folds, although its relationship to later structures cannot be demonstrated. At 5G a number of structurally early pegmatites are exposed. These pegmatites are folded by D2 structures and carry the strong north-plunging D2 lineation present throughout section 2. The age of these pegmatites is unknown.

(3) [NH 043 018] to [NH 042 019][edit]

There is no exposure gap between section 2 and 3. Section 3 starts at a 20m-wide, south-facing cliff and is approximately 100m long.

Only D3 folds are exposed in section 3, accompanied by an axial planar S3 crenulation fabric. The overall structure of section 3 is that of an open, NNE-plunging D3 antiform, around which several quartzite bands can be traced. Sparse but consistent cross-lamination within the quartzites shows the antiform to be upward facing. There is therefore no major structural inversion between sections 2 and 3.

Three foliated microdiorite sheets, all inclined at c.60° to the east, crop out in section 3, at 25m, 30m and 60m into the traverse. These sheets cut across minor D3 structures, and across the major antiform. The largest and most easterly microdiorite contains xenoliths of country rock quartzite, and also interbanded psammite and pelite containing a pre-existing S3 fabric. This sheet shows a classic example of the sigmoidal internal fabric described by Smith (1979). The margins of the sheet are intensely foliated at a low angle to the contact, but as the centre of the sheet is approached the fabric weakens and its angle to the contact increases. The centre of the sheet is non-foliated.

Having completed section 1 to 3 of the shore section, most people will now wish to return to their vehicles. However, if time permits, those with more than a passing interest in the Moine will find the complete shore section between here and Quoich Bridge, c.3km west, extremely interesting. It affords a well-exposed traverse across the Loch Eil Group outlier, and its Glenfinnan Group envelope, cropping out in the core of the Gleouraich Synform (Roberts & Harris, 1983), the northward continuation of the Glen Dessarry Synform (Roberts et al., 1984).

References[edit]

At all times follow: The Scottish Access Codeand Code of conduct for geological field work