Moine geology of North Sutherland. Port Mor to Portskerra - 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 Rob Strachan, Bob Holdsworth, Clark Friend, Ian Burns and Ian Alsop

Fig. 13.1 Simplified geological map of north Sutherland together with the localities for the excursion.
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:[edit]

Excursion 13 North Sutherland . Port Mor to Portskerra[edit]

Locality 13.15 Port Mor [NC 7735 6547][edit]

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).

Locality 13.16 Strathy road sections [NC 7992 6395] to [NC 8127 6487][edit]

Strathy road sections (Fig. 13.1), (Fig. 13.12). Strathy Complex siliceous gneisses and amphibolites, fold structures and cross-cutting granitic intrusions.

This locality comprises three separate roadside exposures; about 20 minutes could be allocated to each. If driving from Armadale, take the A836 east and cross the Armadale Burn and, as the hill starts to rise, turn onto the south side of the road and park by a small turning at [NC 7992 6395]. Cross the road to the cutting at 16A that exposes amphibolites and siliceous gneisses of the Strathy Complex, here dipping moderately to the west and cut by late sheets of granitic material and pegmatite. Upright open D4 folds verge east towards an antiform. Continue driving eastwards for c.800 km and park on the verge on the north side of the road opposite 16B at [NC 8078 6470]. Care should be taken here – this straight stretch of road is dangerous for unwary pedestrians. The road cutting shows extensive exposures of layered, siliceous gneisses of the Strathy Complex, mostly dipping east and folded by asymmetric D4 folds that verge west. The gneisses and the folds are cross-cut by sheets of undeformed white pegmatitic granitoids and later, finer-grained pink granites. Drive a few hundred metres further on to park either on the verge opposite the next large roadside exposure that is 16C at [NC 8127 6487] or a few hundred metres further east in the designated parking area. The road cutting exposes gently undulating interbanded siliceous gneisses and amphibolites. These are intruded by younger metabasic sheets, seen as a series of en-echelon, folded boudins, and ramifying granite sheets that are essentially undeformed. At the east end of the cutting, U-Pb dating of zircons from one of these granite sheets has yielded a Silurian crystallization age (Kinny & Friend, unpublished data).

Locality 13.17 Strathy Point [NC 8282 6964] to [NC 8340 6680][edit]

Strathy Point (Fig. 13.12). Rock types and structures in the Strathy Complex.

Continue east along the A836 to the western parts of Strathy and turn north on the road to Strathy Point [NC 8296 6553]. Follow this road to Totegan, where the private road to the lighthouse begins, and park in the extensive car park by the sheep pens [NC 8270 6859]. Allocate two hours for this locality. Walk down the road to the lighthouse, noting various outcrops of siliceous gneiss and fold structures en route. Head for the east side of the lighthouse to 17A at [NC 8282 6964]. In an exposure below the wall, crossed by a drainpipe, a prominent boudin carries an interesting mineral assemblage (garnet-staurolite-sillimanite) and a steep gneissic fabric that is oriented approximately normal to the sub-horizontal enveloping S1N fabric within the host siliceous gneisses. A second boudin immediately to the north is less accessible. The margins of the boudins are sheared and retrogressed. The mineral assemblage comprises quartz + plagioclase (An47-63) + garnet + staurolite + sillimanite + anthophyllite + brown and green biotite + spinel. The garnet porphyroblasts may show three growth phases, with sillimanite, staurolite and rutile inclusions found in the second phase (Burns, 1994). The idioblastic outlines of phase 2 garnet are marked by fibrolite, and a second phase of sillimanite may be found outside the third zone, intergrown with biotite. Staurolite is replaced by green spinel. It is clear that the boudins, and presumably also their host siliceous gneisses, have undergone a complex metamorphic history. Metamorphic conditions during the formation of the early mineral assemblage have been estimated by Burns (1994) at c.700°C and 6kb – substantially lower pressures than those deduced for regional Grampian migmatization in the Moine rocks west and east of the complex. Walk NE from the boudin down the slope onto the rocky headland where there are excellent exposures of interbanded siliceous gneisses and amphibolites cut by discordant granitoids and pegmatites. The dominant S1N fabric dips mostly to the west and is associated with a strong, north-south trending L1N lineation defined by aligned minerals as well as rods and mullions in places. Tight to open D4 folds plunge gently to the north, more or less parallel to L1N, but can occasionally be seen to fold the lineation.

Return to the vehicles and drive south along the road to the track running east at [NC 8294 6672]. Walk east, through the gate past the bungalow on the south side, then 150m along keep straight on past ‘Caberfeidh’, through two more gates, and follow the faint track southeast down the cliff to an old fishing slipway and 17B at [NC 8340 6680]. In the exposures behind the winch are flat-lying isoclinal D1N folds refolded by upright, non-cylindrical, tight to open folds, of probable D4 age. A L1N lineation trends north-south, approximately parallel to the axes of the D1N folds. Ramifying networks of coarse white pegmatite are cut by sheets of finer-grained, pink granite that occupy distinctive tension gashes in the cliffs to the south of the slipway.

Locality 13.18 Portskerra [NC 8740 6644][edit]

Portskerra (Fig. 13.1). Moine migmatitic gneisses; late Caledonian igneous intrusions; Devonian sedimentary rocks.

Drive east along the A836, turning left onto the minor road by the Melvich Hotel [NC 8767 6507]. Follow the road for 600m and take the left-hand fork. Continue for 850m around a bend and then park at [NC 8749 6620] or further east at the next bend in the road. Parking here is restricted to two minibuses or cars; coaches would have to park further south in Portskerra. Allocate 1½-2 hours for this locality. Low tide is useful but not essential. From the parking place walk 100m west, turn right at the end of the fence and follow a faint path beside the stream to a grassy flat above the beach. The two stones mark the graves of shipwrecked mariners. Climb down the grassy cliff into a stony cove. Devonian sandstones are exposed on both sides of the cove, resting unconformably on Moine gneisses. The unconformity surface is highly irregular and may be examined at 18A [NC 8755 6633]. The underlying Moine rocks are highly migmatized semi-pelitic gneisses showing several generations of melt layers deformed by complex, often disharmonic folds. These are interpreted to represent the same gneisses seen at Cnoc Mor and Port Mor, on the east side of the broad antiform cored by the Strathy Complex. The gneisses are cut by sheets of undeformed pink granite. Walk along the cliffs on the east side of the cove to emerge on the headland where there are extensive exposures. The semipelitic gneisses appear to pass transitionally eastwards into psammitic gneisses. At 18B [NC 8740 6644], migmatized psammitic gneisses with numerous concordant melt layers are highly veined and sheeted by late, pink granites that are thought to correlate with the c.425 Ma Strath Halladale Granite that intrudes Moine rocks 10km to the SE (Kocks et al., 2006). Some of the late granite sheets contain magmatic fabrics defined by aligned feldpars and micas. Walk east along the cliff line; keep above the next major cove, but look down into it to observe Devonian sandstones again resting unconformably upon Moine gneisses on the west side. Walk around the inlet and head northwards to the next headland. At 18C [NC 8768 6660], psammitic gneisses are intruded by a large, essentially undeformed diorite body which is itself cut by late granite sheets. The diorite is probably the same age as the Reay Diorite 8km to the east. Walk east a few tens of metres into the next bay to examine in detail exposures of the Moine/Devonian unconformity that preserves some 3-4m of relief. The sedimentary rocks are mostly fine to medium-grained sandstones with siltstones; pebble beds are thin and localized. This facies may be contrasted with those seen at Coldbackie and Kirtomy. If tide allows, continue around to the slipway at 18D [NC 8786 6629] (alternatively, return to the previous locality, scramble up the grassy cliff and walk around to the slipway). At the slipway, look west to view the Moine/Devonian unconformity and observe large-scale gentle variations in dip of the lowermost Devonian strata. Whether these variations represent original depositional angles or the effects of compaction and draping over an irregular Moine land surface is uncertain. The rocks at the end of the slipway are psammitic gneisses and late granites as seen before.


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