Moine geology of the Ross of Mull. Itinerary C. Structure and lithologies within the Ardalanish Striped and Banded Formation, western linb of the Assapol Synform - 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 Tony Harris

Fig. 1.1 Simplified geology map of the Ross of Mull, showing the localities described in the text.

Excursion 1 Ross of Mull is composed of the following articles:[edit]

Itinerary C Structures and lithologies within the Ardalanish Striped and Banded Formation on the western limb of the Assapol Synform; access from Ardalanish.[edit]

Distance from Ardalanish and return is c.3 km, taking 4-5 hours.

In Bunessan, leave the A849 and turn up the steep hill that passes behind the Argyll Arms Hotel. After about 600m, turn left (south) following for about 2km the narrow metalled road signposted to Uisken. A narrow metalled road joins the Uisken road from the right (southwest). Follow this past the Ardachy Hotel towards Ardalanish Farm. About 0.5km along this road it is joined from the left by a track running alongside some ruined outbuildings. Parking for minibuses or cars is provided beside the track near its junction with the metalled road [NM 3731 1930], shown with ‘P’ on the 1:25,000 Ordnance Survey topographic map.

Access to several of the localities, notably 1.15 to 1.18, is easy and involves no more than walking along a fairly rough track and thereafter along the extensive sands interspersed with rocky knolls and reefs that is Ardalanish beach. Beyond the eastern limit of the beach, however, access to Localities 1.19 and 1.20 requires quite strenuous scrambling over rocky and bouldery terrain which becomes more difficult eastwards, especially under wet conditions produced either by rain or an outgoing tide. The access to exposures on the main beach (Localities 1.15-1.18) is, to a small degree, tide-dependent. Ideally they would be visited at or below mid-water on an ebbing tide; this would also reduce problems of access further to the east (Localities 1.19 and 1.20). Access to Locality 1.19 is quite easy at low water, but Locality 1.20, a tidal island, and a very rewarding, highly photogenic locality, should be attempted only by fit people capable of strenuous scrambling and minor rock climbing, and then only on an ebbing tide.

Locality 1.15 - Regional and contact metamorphism of the Assapol Group Moine. [NM 3753 1884][edit]

From the car park, pass through the gate and walk south-southeast along the track, noting the extensive flat very gently sloping fields, especially to the northeast of the track. These surfaces comprise higher (older) and lower (younger) raised beaches. The higher beach at 15-20m O.D. has coarse cobble/pebble deposits, and the lower at ~10m O.D. has much finer silty/ sandy deposits. Both have excellent back (fossil cliff) features and the lower beach has raised stacks and sea caves. The lower beach is extensively covered by Blown Sand.

Locality 1.15 is Dun Fuinn which would have been an offshore rocky island at the time the lower raised beach was cut. The exposure, originally described by MacKenzie (1949), comprises largely Assapol Group pelitic and semipelitic schists, remarkable for their metamorphic mineral assemblage. In addition to garnet and micas, some 3-4cm-wide bands contain blades of kyanite 1-2cm long. The high-pressure Al2SiO5 polymorph kyanite formed during the early period of Barrovian regional metamorphism that also produced the garnets. Because the exposure lies within the aureole of the Caledonian Ross of Mull Granite (Zone 2 of Wheeler et al., 2004, figure 1.1), rims of pink andalusite, the low-pressure polymorph, are common around the kyanite blades (Wheeler et al., 2004, figures 1, 4).

Abundant tourmaline also occurs at the head of a shallow gully within the exposure at its western end. These may also be linked to the effects of the granite, the exposed margin of which lies some 500m to the west.

Locality 1.16 - Migmatitic fabrics and polyphase folds. [NM 3772 1876][edit]

Locality 1.16 is a low elongate crag that emerges from the beach sand and extends south-southwest from HWM for some 100m down the beach. The crag consists of interbanded psammite, semipelite and pelite; the last is commonly garnetiferous and carrying quartzofeldspathic lits appears somewhat migmatized. The migmatitic segregations have been shredded by the apparently penetrative fabric that is interpreted as S2, axial planar to sub-isoclinal folds, and well displayed at the northern end of the exposure. Calc-silicate stripes and lenticles up to a few centimetres thick are an important feature of this exposure. These are characteristically pale-weathering and comprise amphibole, garnet, plagioclase feldspar, quartz and epidote. In several parts of the exposure they have clearly been folded into sub-isoclinal (F2?) folds and subsequently refolded by more open (F3?) structures.

Locality 1.17 Garnetiferous amphibolites. [NM 3778 1870][edit]

Locality 1.17 lies some 60m east of Locality 1.16 (Fig. 1.1). It is best examined on its eastern side where it exposes large pods of metabasic garnetiferous amphibolite. The red almandine garnets, commonly a centimetre in diameter, contrast vividly with the dark green groundmass consisting of hornblende, plagioclase quartz, epidote and titanite. Contacts between the metasediments and amphibolite at this locality appear to be concordant with the metasedimentary layering. The protolith of the meta-amphibolite is basaltic with MORB (‘mid-ocean ridge basalt’) chemistry. Its concordant pod-like form is not unique to this locality, but it is unusual and elsewhere the forms are sheet-like and commonly cut bedding.

Locality 1.18 Relationships between amphibolites and host metasediments. [NM 3805 1855][edit]

Locality 1.18 lies ~250m east of Locality 1.17 and is situated about half way down the beach at low tide (Fig. 1.1). It is important for establishing the original intrusive relationships of the metabasic amphibolites and the metasediments. The margin of the 3.5m-wide (not thick) amphibolite at its eastern side strikes N37°E and dips 70°NW. It unambiguously cuts the bedding in the psammitic country rocks which strike N44°E and dip 50° NW. The marginal zone of the amphibolite carries abundant large (~1cm) garnets that rapidly decrease in size while increasing in abundance away from the margin.

Localities 1.19-1.21 demonstrate the nature of the tectonic structures which have been imposed on Assapol Group rocks during polyphase deformation.

Locality 1.19 East-verging F3 folds. [NM 3816 1837][edit]

Locality 1.19 can be reached at all states of the tide, but is best approached at low water (Fig. 1.1). It comprises a ~2m-wide gully with subvertical sides, running approximately east-west for about 15m and ending at its eastern end with a shingly more open space surrounded by low crags to north and south, but a much higher cliff to the east. Access to this gully is possible at low water directly from the eastern end of Ardalanish beach, following a sandy inlet that leads off to the east. Otherwise the gully should be approached by scrambling eastwards from higher up the beach, across and around crags surrounded in most part by shingle, turf and boulder-strewn terrain.

Several structural features are worth noting:# at the southern side of the western entrance to the gully, on a low, sand-abraded rock face are well displayed ptygmatic folds of thin (~1cm) pegmatitic veins as well as stripes of siliceous psammite, set in a more micaceous matrix. The exposure can be used to demonstrate the minimum shortening accomplished by the D3 deformation and the variable wavelengths of buckle folds where the competent layers are of different thickness;

  1. along both sides of the gully F3 folds and related crenulations and crenulation cleavage (S3) in pelitic bands are well displayed, while a lineation (L2) on the folded banding lies obliquely to the F3 fold hinges. The F3 folds verge eastwards to an F3 antiform;
  2. at the eastern end of the gully [NM 3816 1837] a fold pair, antiform/ synform, at least an order of magnitude larger than those displayed on the gully walls, similarly verge towards an antiform to the east;
  3. in the southern wall at the end of the gully, one limb of the antiform is brecciated and, at first sight, appears faulted. However, there is little sign of displacement and psammite flags can be traced, essentially continuously through the breccia;
  4. the psammitic flags carry regularly spaced, quartz-filled tension gashes.


Locality 1.20 - F3 folds. [NM 3824 1832][edit]

Locality 1.20 can be reached from Locality 1.19 by climbing out of the east end of the gully by climbing up the lowest part of the vertical face and walking/scrambling south-southwest down the fold hinge zone as far as the next east-northeast-trending gully some 20m to the south (Fig. 1.1). Alternatively, return to Ardalanish beach and, if the tides permit, walk down the beach to a gain access to the gully via a sandy inlet. This gully marks the line of a strike-slip fault. Walk eastwards along the gully noting the many subvertical garnetiferous amphibolite bodies, many locally crosscutting, exposed in the gully walls. Because of the lateral-slip displacement of the fault these cannot be matched in detail across it.

Passing along the gully eastwards, into the next bay, a very rocky boulder strewn inlet, note in its north wall in particular, good examples of ‘necking’ and boudinage extension of the psammite layers. These are the result of the extension of the competent psammitic layers and are accompanied by tension gashes, similar to those at Locality 1.19.

In the bay itself, scramble across the boulders, which are exceptionally slippery when wet, to the tidal island situated opposite a cliff some 30m high.

The cliff, the face of which is probably the fault along which you walked to gain access to the locality, displays spectacular intermediate-scale folds that verge towards a major antiform the hinge zone of which lies some 100m to the east. The lithologies involved in these structures are striped and banded pelites and psammites with abundant calc-silicates. The crest of the major antiform which forms the roof of a cave marked on the 1:25,000 Ordnance Survey topographic map is not normally accessible from the west, but can be reached, with difficulty, from Uisken to the east (see Itinerary D, below). It forms the lowest structural level of a complex antiformal zone that extends westwards for some 300m from the inlet known as Slochd nam Ba [NM 385 184].

The island itself displays Assapol Group lithologies very well, on clean surfaces. Some psammitic bands, several centimetres thick, have very sharp contacts with extremely micaceous material on one side but pass by rapid transition into micaceous material on the other as the pelitic component of the metasediment increases. These occurrences can be readily, but perhaps not safely, interpreted as graded bedding, indicating the original way-up of the sediments. If this interpretation is to be believed, however, it would imply that the F3 folds at this locality at least, are downward-facing, and if widely applied would be at variance with more convincing younging evidence elsewhere on the Ross.

Leaving the island to return to Ardalanish, glance up the scree slope to the north, to where the structural features displayed on a very large fallen and tilted block are worth a detour. These include excellent examples of ptygmatically folded pegmatitic veins and of the ‘cusp and lobe’ mode of buckle folding of alternating psammite (competent) and pelite (incompetent) layers.

References[edit]

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