Moine geology of North Sutherland. Swordly Bay, Kirtomy Bay and Cnoc Mor - 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 Strachan, Bob Holdsworth, Clark Friend, Ian Burns and Ian Alsop

Fig. 13.10 Geological map of Localities 13.12, 13.13 and 13.14 (from Moorhouse et al., 1988 and Burns, 1994).

Fig. 13.11 Migmatitic gneisses at Locality 13.14, showing extensive partial melting.

Fig. 13.12 Simplified coastal geology of the Strathy Complex (from Moorhouse et al., 1988 and Burns, 1994) showing Localities 13.15, 13.16A and 13.17.

Excursion 13 Moine geology of North Sutherland is composed of the following articles:

• Excursion 13 North Sutherland - introduction
• The Melness area. Locality 13.1
• South and east side of Tongue Bay. Localities 13.2 to 13.5
• Loch Cormaic and Borgie peat cuts. Localities 13.6 to 13.7
• Torrisdale Bay. Locality 13.8
• Craig Ruadh to Glaisgeo. Localities 13.9 to 13.11
• Swordly Bay, Kirtomy Bay and Cnoc Mor. Localities 13.12 to 13.14
• Port Mor to Portskerra. Localities 13.15 to 13.18

Excursion 13 North Sutherland. Swordly Bay, Kirtomy Bay and Cnoc Mor

Locality 13.12 Swordly Bay [NC 7354 6355]

Swordly Bay (Fig. 13.10). Swordly Lewisianoid basement inlier; Swordly Thrust and overlying migmatitic pelites reworked within the Torrisdale Steep Belt.

Turn off the A836 onto a minor road c.5km east of Bettyhill; after 300m take the left-hand fork and follow the road for 1.25km. Park with permission by the farm buildings at [NC 7356 6307] where there is sufficient space for a minibus or three to four cars. Allocate 1 hour for this locality. Walk northwards and turn to the right of a ruined building, passing through a gap in the stone wall and then head across the grass towards Swordly Bay. Cross the fence at a small stone stile and walk down to the beach [NC 7354 6355]. Exposures on the west side are of banded, mafic hornblende gneisses of the Swordly basement inlier that is thought to occupy the core of an early isoclinal fold within the Naver Nappe (Moorhouse, 1979; Moorhouse et al., 1988). It has not at the time of writing been dated isotopically, but its basement affinities have been confirmed by detailed chemical studies (Burns, 1994). The gneissic fabric is deformed by upright, open (D₄?) folds. If the tide and sand levels are low, the eastern contact of the basement with psammitic gneisses is exposed in the centre of the bay. Walk to the east side of the bay, across the unexposed Swordly Thrust, to see the Swordly Pelite, comprising northeast-dipping, pelitic gneisses and schists with numerous sheets and pods of granitic material. This lithology is interpreted as a strongly deformed and mylonitized migmatitic gneiss; the granitic rocks are thought to represent melt layers, consistent with the local presence of garnet and sillimanite, and the abundant muscovite is most likely of retrogressive origin. A strong L₄ lineation plunges gently to the SSE. Pervasive shear bands and asymmetrically sheared boudins of granitic material indicate a dextral sense of shear parallel to the lineation. On the west side of a rocky knoll at the top of the beach, a low strain zone preserves a NW-trending mineral lineation (L₂ ?) that is variably sheared into parallelism with L₄. This early lineation is interpreted to define the direction of tectonic transport along the D₂ Swordly Thrust prior to steepening within the Torrisdale Steep Belt. A 40Ar/39Ar muscovite age of c.423 Ma has been obtained from these mylonitic pelitic gneisses (Dallmeyer et al., 2001).

Locality 13.13 Kirtomy Bay [NC 7413 6408]

Kirtomy Bay (Fig. 13.10). Moine gneisses within the Torrisdale Steep Belt and unconformably overlying Old Red Sandstone (Devonian) sedimentary rocks.

From the previous locality, drive back 1.25km to the fork in the road, turn sharp left and follow the road towards the small hamlet of Kirtomy. After 1km the road turns abruptly right; instead, carry on straight ahead on the grassy track and parking for a minibus or three to four cars is available at the end of the track [NC 7417 6401]. Allow ½-1 hour for this locality. Do not take the path to the right down to the small jetty; instead walk to the west side of the headland and descend down the grassy path on a narrow ridge westwards onto a stony beach. On the west side at [NC 7411 6399] are low polished beach outcrops of steeply-dipping banded semi-pelitic gneisses within the Torrisdale Steep Belt showing numerous shear bands and asymmetrically deformed leucosomes and melt pods that demonstrate a dextral sense of shear parallel to a gently-plunging L₄ lineation. Walk back 100m or so to the east and climb over the bottom of the grassy ridge that you descended into the next small bay [NC 7413 6408] to view the cliffs at the back of the beach that comprise Old Red Sandstone boulder conglomerates with thin sandstone layers. Look across the small bay to the north to see the irregular Moine-Old Red Sandstone faulted landscape unconformity completely exposed where sheets of conglomerate drape steeply-dipping gneisses. Boulders within the conglomerate are mostly of Moine migmatitic gneisses of the Kirtomy migmatites (Burns, 1994) and pink granites typical of the late Caledonian ‘Newer Granite’ Suite. Their rounded appearance most likely indicates fluvial erosion and transportation prior to deposition.

Faults bounding the basin trend NNW-SSE and N-S (the former reactivating basement fabrics) with a system of minor ENE-WSW faults appearing to accommodate along-strike variation in throw. The sedimentary rocks are also cut by various late, brittle faults which are attributed to dextral reactivation of the basin-bounding faults, probably during the Permian (Wilson et al., 2010). Before leaving this locality, take a moment to look east across the bay to see the classic half-graben structure, with conglomerate units adjacent to the bounding fault grading into more blocky, moderately-dipping sandstone units outcropping across much of the bay, which then onlap basement exposures on the far hillside.

Locality 13.14 Cnoc Mor [NC 7567 6344]

Cnoc Mor (Fig. 13.10). Moine migmatites.

Turn off the A836 c.400m east of the minor road to Kirtomy, onto a small tarred track that leads up to the radio and mobile telephone masts on Cnoc Mor. Park in a large space below the mast at [NC 7567 6344]. There is sufficient space for minibuses and cars. Allocate ½ hour for this locality. Walk up the road and examine the first outcrops on the east side. These are of banded migmatitic gneisses with substantial layers of anatectic melt (Fig. 13.11). Some melt layers are concordant, whereas others are clearly discordant and have probably migrated locally from their source. Similar migmatitic gneisses along strike to the NNW at Kirtomy Point have yielded a U-Pb zircon (SHRIMP) age of 461 ± 13 Ma that is interpreted to date melting during the Grampian phase of the Caledonian orogeny (Kinny et al., 1999). Two sets of folds are present at the present locality: (a) early syn-migmatite folds (D₁N?) that are commonly disharmonic and cut by discordant melt layers; and (b) later asymmetric folds (D₄?) associated with the steep foliation and development of the major upright synform located a few hundred metres to the east. Note that the gently-plunging L₄ lineation and its associated dextral shear indicators are now absent. A U-Pb monazite age of 431 ± 10 Ma has been obtained from the migmatites near here, indicating substantial reheating during the Scandian phase of the Caledonian orogeny (Kinny et al., 1999). If weather permits, excellent views can be had from the top of the hill of the entire north coast of Sutherland, the south coast of the Orkney Islands, and various mountains inland.

Locality 13.15 Port Mor [NC 7735 6547]

Port Mor (Fig. 13.12). The boundary between Moine migmatites and the rocks of the Strathy Complex.

If driving from the previous locality, continue east on the A836 to the right-hand bend around Crasbackie Hill [NC 7780 6388]. Stop here briefly to look south and view the escarpment of the Moine gneisses above the lower ground of the Strathy Complex to the east. The Strathy Complex is dominated by siliceous gneisses and amphibolites that may be volcanic in origin (Moorhouse & Moorhouse, 1983; Burns et al., 2004). The age of the complex is uncertain: limited isotopic evidence suggests a late Mesoproterozoic to early Neoproterozoic age, and thus it may represent local basement to the adjacent Moine migmatites (Burns et al., 2004). Turn off the A836 onto the minor road signposted to Armadale. Follow the minor road and continue until the road splits into three at the bus shelter [NC 7861 6466]; coaches should go no further. Smaller vehicles take the left-hand road for 250m and parking is available for a minibus or three to four cars by the house at [NC 7845 6483]. Allow 2 hours for this locality. Take the track signposted to Poulouriscaig westwards across the moor and then follow a narrow valley northwards to reach Port Mor, 15A [NC 7735 6547]. It is best to stay in the valley beside the stream and not to attempt to climb the cliffs on the east side. On the east side of the bay are steeply dipping siliceous gneisses and amphibolites of the Strathy Complex, cut by numerous sheets of discordant granite and pegmatite. At least two sets of folds are present: a tight D₄ fold pair verges east just above high water mark, and later gentle folds with flat-lying to gently inclined axial planes are visible higher in the cliff. The westernmost outcrops before the stream are of a calcite-scapolite-diopside-orange spinel-bearing marble (Harrison & Moorhouse, 1976; Moorhouse & Moorhouse, 1983) that is unlike any Moine lithology in the area. The yellowish-orange marble is interlayered with green calc-silicate bands characterized by abundant diopside with scapolite and plagioclase. A sub-vertical brittle fault located along the western margin of the marble is inferred to separate the Strathy Complex from steeply-dipping and strongly deformed Moine pelitic and psammitic gneisses to the west. These gneisses do not carry any penetrative lineations and the high strains appear to be simply related to flattening: there is no evidence that the two different rock units are separated by, for example, a ductile thrust. A 3-4m-wide vertical sheet of pink, unfoliated granite intrudes the Moine gneisses at the back of the bay. Look up to the steep cliffs on the west side of the bay to see pods of amphibolite within the Moine gneisses, engulfed by ramifying late granite sheets. Walk back up the gorge until it is safe to climb out on the east side. Walk east to Locality 13.15B which corresponds to exposures of bright green ultramafic amphibolite, forming grassy knolls at [NC 7784 6532] and [NC 7777 6545]. The rock comprises anthophyllite with relic clinopyroxene and the Cr-rich chemistry has been interpreted to indicate that they represent retrogressed pyroxenites (Moorhouse & Moorhouse, 1983).

References

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