The Moine Thrust Belt at Loch Eriboll. Transect 3: Kempie - an excursion
|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 Rob Butler
Excursion 11 The Moine Thrust Belt at Loch Eriboll is composed of the following articles:
- Excursion 11 The Moine Thrust Belt at Loch Eriboll - introduction
- Transect 1: South Eriboll.
- Transect 2: Arnaboll.
- Transect 3: Kempie.
- Transect 4: Creagan Road.
Excursion 11 The Moine Thrust Belt at Loch Eriboll. Transect 3: Kempie
Transect 3: Kempie
Parking is available in the large lay-by on the A838 [NC 4441 5800] where there is room for about six cars. The transect running up the hillside from Kempie follows in the footsteps of Lapworth (1883). It covers the transition from folded Cambrian quartzites and their Lewisian basement through a dramatic deformation gradient that increases upwards into mylonites associated with the Moine Thrust. The lower parts of the transect are concerned with less deformed units and include excellent introductory outcrops for parts of the Cambrian stratigraphy. Indeed the coastal outcrops include the type section for the An t-Sron Formation units. This transect lies exclusively within the hanging-wall to the Arnaboll Thrust sheet. Therefore it provides a continuation from transect 2 into progressively higher structural levels within the Moine Thrust Belt. The whole section contains a number of informative outcrops that can be used to build up a structural interpretation from first principles. Consequently the area has been much used for student training exercises. It has been described most recently by Butler et al. (2006). This transect provides an introduction to some of the critical elements and is illustrated by a sketch geological map (Fig.11.9) and cross-section (Fig.11.10). It is described so that visitors can appreciate the geological interpretations based on individual outcrops that can build up into a coherent structural model through this part of the thrust belt. As described, this transect takes about 4 hours, involving ~2km of walking and ~200m of ascent with some steep terrain.
Start on the shore below the parking area, on the headland of An t-Sron (Fig. 11.9). Walk down to the headland to reach clean outcrops of Cambrian Pipe Rock (Locality 11.3A, [NC 4431 5817]). These quartzites lie in the core of a large anticline. Moving west along the coast (passable at all but the highest part of the tide) provides an excellent introduction to the stratigraphy of Cambrian strata. The stratigraphic top of the Pipe Rock, passing abruptly up into the Fucoid Beds, is found at the base of the low sea-cliff (Locality 11.3B, [NC 4417 5814]). In detail, this boundary is deformed. Bedding planes show evidence for slip and some of the finer-grained layers in the Fucoid Beds are cleaved. Within the Eriboll area, the base of the Fucoid Beds can be inferred to have acted as a detachment horizon from which imbricate thrusts have splayed to repeat Fucoid Beds and the overlying Salterella Grit. Here on the An t-Sron shore line there are no such imbricates, but examples are encountered on the Arnaboll transect described below. There are, however, minor thrust structures, chiefly directed towards the east (‘back-thrusts’) that cut up from the Pipe Rock into the Fucoid Beds (see Coward, 1988).
Continuing along the wave-cut platform (beware slippery rocks) is a journey up stratigraphic section. There are excellent outcrops of the sedimentary structures within the Fucoid Beds that are commonly difficult to study in the sparse outcrops of this formation inland. Towards the top of the Fucoid Beds at Locality 11.3C [NC 4405 5809], there are some spectacular yet enigmatic folds and thrusts with trends that are strongly oblique to the regional orientations.
The Salterella Grit Member on An t-Sron has a significantly lower thickness compared to other parts of the NW Highlands (<5m as opposed to the usual 8-10m). These clean quartzites are well exposed in the low sea cliff section and capped by the Ghrudaidh Formation – the gritty dolostones that form the basal unit of the Durness Group carbonates. Bedding within the dolostones is markedly disturbed – well exposed examples lying on the low rocky promontory, Locality 11.3D [NC 4393 5807]. These structures have been variously interpreted as formed by extensional faults or by backthrusts. The latter of these is preferred here, as there are examples of such structures along the foreshore section and many other contractional structures can be proven within the dolostones in the ground above the shore. The development of thrust structures within the dolostones of the Ghrudaidh Formation, but not in the Salterella Grit, requires the presence of a detachment horizon near their geological boundary. Evidence for this in outcrop include cleavage development (foreland-vergent) and local slip surfaces.
From the shore, return to the roadside of the A838. A useful exercise is to trace the geological boundary between the dolostones and quartzites of the Salterella Grit. This is picked out by vegetation and confirmed by scattered outcrops. The final climb to the road (at a crash barrier) breaks out opposite a small spring and water trough that lie between cuttings with dolostones (west) and Salterella Grit (east: [NC 4437 5799]). For those wishing to find more evidence for deformation within the dolostones, the road cuttings to the west along the A838 provide sections through complex thrust stacks. However, unless experienced with carbonates, it may be difficult to distinguish between depositional bedding surfaces and tectonic surfaces in these sections.
The remaining objectives for this transect lie to the east and up to the Moine Thrust. This route is described in detail so as to identify the necessary evidence for the structural relationships depicted on (Fig.11.10). The road section opposite the parking lay-by (Locality 11.3E, [NC 4439 5798]) is made of Fucoid Beds that are variably thickened up by thrusts. These structures lie in the crest of the An t-Sron Anticline, the western limb of which has formed the focus of attention so far. The excursion now moves into its eastern limb. Regrettably this is rather poorly exposed at the level of the road, but can be proven down on the coast (below the cottages of Kempie). The first outcrop to the east along the road, next to the small stream next to the woods (Locality 11.3F, [NC 4453 5787]) are quartzites partly covered in moss that are interpreted to be Salterella Grit. Bedding is sub-vertical. The adjacent road outcrops to the east are Fucoid Beds [NC 4455 5787]. Consequently the interpretation here is that the Salterella Grit and Fucoid Beds young to the west and therefore lie on the eastern limb of a synform whose axis runs through the dead-ground between these outcrops are the lay-by. This structure is the Kempie Bay Syncline. Cleavage within the Fucoid Beds in the road section here verges east, consistent with the syncline interpretation.
It is worth working carefully eastwards along the low road cutting below the trees, watching carefully for traffic. For the most part the section consists of steeply-dipping Fucoid Beds with inclined, east-vergent cleavage. After 40m [NC 4460 5786] there is a screen of vertically-bedded quartzites with prominent Skolithos burrows, deduced to be the Pipe Rock that stratigraphically underlies the Fucoid Beds. The burrows are sheared with a sense consistent with the vergence of the cleavage in the Fucoid Beds. Similar vergence can be determined for cleavage in the shaley units in the Pipe Rock here. For enthusiasts it is possible to trace the boundary between Pipe Rock and Fucoid Beds, and the Fucoid-Salterella boundary encountered to the west up the wooded hillside to the south of the road.
Given the stratigraphic thickness (>150m) of the Cambrian quartzites, the expectation moving east along the road from the previous outcrops at the stratigraphic top of the Pipe Rock should be to remain in quartzites. However, within a few metres the road section returns to Fucoid Beds. This unit can be found in a small damp gully (Locality 11.1G, [NC 4470 5784]). Just to the east, at the sharp bend in the A838 [NC 4472 5785], is a return to Pipe Rock. As with the Pipe Rock found to the west on the road, the Skolithos burrows here are also sheared with an eastward vergence. Consequently the panel of Fucoid Beds caught between the Pipe Rock outcrops is interpreted as having been incorporated by thrusting rather than in a syncline hinge (Fig.11.10). This is called the Kempie Thrust. It is inferred to lie on the western side of the Fucoid Beds at Locality 11.1G, while their eastern side is interpreted as the original stratigraphic contact at the top of the Pipe Rock, now tilted to be vertical. Note that the inferred Kempie Thrust has also been tilted to vertical, an important corollary that will be explored at the end of this excursion. Thrusting within the Cambrian stratigraphy deduced from consideration of these road sections means that the Kempie Syncline is difficult to prove as there are no simple geological boundaries that can be traced around this fold. Further structural com-plexity can be investigated on the rocky shore at Kempie [NC 4464 5795].
The plan now is to follow the structure eastwards, working back across strike. In doing this, the route is predicted to run down stratigraphic section, through steeply-dipping Cambrian quartzites on the steep, eastern limb of the Kempie Bay Syncline. The thin slice of Fucoid Beds described above (Locality 11.3G) cannot be followed far away from the road and consequently will not add to the structural complexity! To access this ground, return back along the road a few metres to the most western slice of Pipe Rock and follow an indistinct path up into the trees. A number of different routes can be taken up the steep heathery hillside, but the target is the narrow, north-south-trending hanging valley that ends at an elevation of about 130m. It is worth checking the outcrops of quartzites, confirming the general orientation of bedding (subvertical) and the presence of Skolithos burrows (confirming the status of these rocks as being Pipe Rock). The burrows display elliptical bedding sections, with the long axes sub-parallel to strike, implying significant bed-parallel shortening strains. With care, upon entering the hanging valley, it is possible to pick the stratigraphic boundary with the underlying Lower Quartzite. The key distinction lies in identifying mm-scale depositional lamination and therefore the absence of the intense burrowing that characterises the Pipe Rock. Good examples can be found at Locality 11.3H [NC 4478 5771]. It is worth studying these rocks in some detail. Cross-bedding can be identified here and used to determine the westward younging of these strata. Individual sedimentary grains can be readily identified either with the naked eye or through a hand-lens. They are undeformed and show a good granular texture. It might be instructive to collect a small sample of these quartzites from surface detritus for comparison with equivalent rocks further up the section.
Lapworth (1883) interpreted the structure further up the hanging valley in terms of fold structures, the western of which are the An t-Sron Anticline and Kempie Bay Syncline described above. The next fold hinge is found in the cliffs on the east side of the hanging-valley at Locality 11.3I (best seen from [NC 4472 5757]). This anticline shares its steep western limb with the Kempie syncline. Quartzites of its eastern limb dip gently eastwards, indicating that the fold axial surface is inclined to the east (Fig.11.10). The interlimb angle (the measure of fold tightness) is about 100 degrees.
Continue up the hanging-valley consulting outcrops, especially on its northern side. Towards the top of the valley (e.g. [NC 4478 5743]) bedding in the Lower Quartzite dips moderately towards the WNW, with younging determined by cross-lamination towards the WNW. Consequently a synclinal fold axis may be inferred to have been crossed ((Fig.11.10); it can be mapped through the adjacent ground, see (Fig.11.9)). Good clean outcrops of quartzites are to be found at Locality 11.3J [NC 4488 5743]. Here the beds dip at about 50° to the ESE but young westwards, indicating that they are upside-down. Careful inspection reveals that the sedimentary grains are flattened, creating a weak to locally intense protomylonite deformation fabric that is approximately axial planar with respect to the main folds that were encountered on the traverse. A few metres to the east are out-crops of pegmatite-rich Lewisian basement (e.g. [NC 4492 5741]) which also show a weak schistosity defined by chlorite and epidote that is sub-parallel to the foliation in the quartzites. It may be deduced that the contact between the Lewisian and the quartzites is an overturned unconformity. A slight diversion along the plateau reveals this contact in outcrop [NC 4501 5756].
Returning to Locality 11.3J, the next objective is to work carefully up section, best achieved walking south. The small knoll (219m OD) about 200m SSW of Locality 11.3J forms a useful landmark, with outcrops lying along a small escarpment facing the plateau area (Locality 11.3K; [NC 4487 5726]). The upper part of the escarpment consists of distinctive folded mylonites, and again the interpretation (Barber & Soper, 1973; Butler et al., 2006) put forward here is that these were chiefly derived from Moine psammites, although others favour a foreland Lewisian protolith (British Geological Survey 2002; Holdsworth et al., 2006). They contain a strong linear fabric defined by elongate quartz aggregates that plunges towards the ESE. The base of these mylonites is considered by Butler et al. (2006) to be the Moine Thrust. Below lie more mylonites of distinct compositions, arranged in bands of about a few metres thickness. One type of mylonitic layer is highly quartzitic. Others are essentially chloritic phylonites with thin feldspathic seams. Where evident, these units also show stretching lineations that plunge ESE.
The derivation of the mylonites beneath the Moine Thrust can be established by briefly tracing out a deformation gradient. Return to Locality 11.3J. The plan is to walk out these quartzites and the neighbouring Lewisian for a few hundred metres to the WSW, along the strike (Fig.11. 9). As seen previously, at Locality 11.3J, the quartzites retain visible bedding but also display moderate protomylonitic deformation fabrics. Further WSW the deformation increases (e.g. [NC 4470 5731]) to become fully mylonitic with the same ESE-plunging stretching lineation as seen at Locality 11.3K. Thus these mylonites are products of progressive deformation that, at lower strain states, involves folding of the Cambrian quartzites and their Lewisian basement (Fig.11.10). Elsewhere (e.g. [NC 4417 5713]) the shearing focuses onto a discrete thrust that carries mylonites derived from the Cambrian quartzites and their Lewisian basement onto more outlying parts of the fold belt crossed on this transect. Further description of these forms of structural relationship is reserved for the next transect. Return to vehicles by carefully descending the slopes to the A838.
Many interpretations of structural evolution in crustal-scale shear zones, such as the Moine Thrust Belt, assume that there is a simple progression from ductile deformation, manifest by mylonites development into brittle deformation and cataclasis as rocks become progressively exhumed. However, the transition from ductile to brittle deformation need not be controlled simply by depth (or temperature), but also by strain rate. The structural evolution on this transect illustrates this complexity. The main folds (An t-Sron Anticline, Kempie Bay Syncline and un-named folds seen higher on the transect) face WNW. Their axial surfaces become increasingly inclined up-section as the deformation state increases, culminating in mylonite formation directly beneath the Moine Thrust. Therefore these folds formed before or during the latest ductile movements on the Moine Thrust (Butler et al., 2006). Yet the folds deform earlier thrust structures such as those that are now found facing downwards on the steep eastern limb of the Kempie Bay Syncline. These earlier structures represent periods when deformation was strongly localized. It is not clear what the timing of the folding is relative to slip on the Arnaboll Thrust, although it is plausible that this structure is folded by the Kempie Bay Syncline (e.g. Coward, 1984). However, there were periods in the structural evolution of this part of the thrust belt when ductile shear was partitioned strongly onto the Moine Thrust, then distributed across the fold belt, then onto the Moine Thrust again. Thus the deformation has alternated between ductile and brittle styles.
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