Moine geology of North Sutherland. South and east side of Tongue Bay - 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. 13.3 (left) ‘Eye’ structure developed within Moine psammites at Locality 13.1B. This structure is interpreted as a cross section through the nose of a D2 sheath fold.
Fig 13.4 Locality 13.2. (a) the Ben Hope Thrust and basement-cover relations in the Ben Hope to Kinloch area of the Moine Nappe; (b) detailed map of the Kinloch area; (c) interpretative sketch of D3 structures in the Kinloch area (from Moorhouse et al., 1988).
Fig. 13.5 Localities 13.4 and 13.5: basement-cover relationships in the Coldbackie Bay and Sletell area of the Moine Nappe (from Moorhouse et al., 1988).

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

Excursion 13 North Sutherland. South and east side of Tongue Bay

Locality 13.2 Kinloch Broch [NC 5500 5282] to [NC 553 531]

Kinloch Broch (Fig. 13.4). The D2 Ben Hope Thrust, highly strained Lewisianoid basement and Moine psammite, complex D2 and D3 folds.

Two to three hours should be allocated to this locality, at the south end of the Kyle of Tongue. Parking is available for a coach or several minibuses in the small gravel pit [NC 5516 5337], 150 m east of the Allt Ach’ an t-Strathain.

Follow the burn for 550m SSW, until it turns sharply west [NC 549 629]. Traverse ESE uphill for 100m, noting the flaggy to platy, highly strained, sparsely gritty psammites. At 2A (Fig 13.4b, [NC 5500 5282]), thin (1m) units of what has been interpreted as an intensely deformed, intraformational conglomerate are interbanded with psammites a few metres below a laterally extensive pelite (cf. stop 1A). The pebble ‘clasts’ are mainly quartzose, resembling closely those in the Strathan Conglomerate (stop 1F), and hence similar questions arise as to their true origin. Psammite adjacent to the overlying pelite displays a marked increase in white mica content. Traverse ESE for 30m over extremely platy psammite, containing the ESE-trending L2 mineral lineation and isolated small, strongly flattened feldspar clasts, elongated parallel to this fabric. At the top of the slope at 2B [NC 5503 5282], these psammites are overlain by hornblende-biotite schists of the Kinloch basement sheet; this boundary is the D2 Ben Hope Thrust (Fig. 13.4)a, (Fig. 13.4)b). This thrust, and the overlying basement sheet, has been traced for at least 25km (Fig. 13.1) from south of Ben Hope to Strathan Bay where it is associated with the zone of intense D2 strain affecting the Strathan Conglomerate (Locality 13.1F). The marked asymmetry across this D2 thrust contact is demonstrated by the main (pre-D1M) Ben Hope Sill intrusion occurring above the accompanying basement body, but never below the lower contact with the platy psammites ((Fig. 13.3)a). The outcrop pattern of the Ben Hope Thrust suggests a minimum WNW displacement of 7.55 km ((Fig. 13.4)a).

Walk NNE for some 150m towards the summit of the hill to 2C. Here a number of open to isoclinal, south- to ESE-plunging folds form a ‘Z’ geometry pair, distorting the lower Moine-basement contact (i.e. the Ben Hope Thrust) and clearly refolding the D2 platy fabric and associated lineation. Thus these folds belong to the local third phase, D3. Note the apparent repetition of the upper pelite and lower basement contacts ((Fig. 13.4)b) by a thrust which must lie within the pelite to the north (Fig. 13.4)b, (Fig. 13.4)c) as the lower boundary of the pelite is unaffected by thrusting or the D3 folding. The common limb of the fold pair is a zone of relatively low strain where D1M-D2 refolded folds occur in basement rocks [NC 5507 5288] along with migmatitic fabrics and ultramafics pods; 50m further south at 2D [NC 5507 5282], D3 folds display up to 100° of curvature of their hinges and clearly deform the L2 lineation.

Traverse SSW for 200m within the basement gneisses. The D3 folds tighten, becoming progressively smeared out into the foliation and indistinguishable from D2 structures. The exposed D3 fold pair appears to form the southern part of a large fold pair of the same age that initially overturned WNW before being modified into highly curved sheath fold geometry by continued shearing (Holdsworth, 1990). The northern part of this structure has been largely removed by erosion, but a remnant of an intensely curved synformal infold of psammite within pelite is still preserved to the northeast at 2E (Fig. 13.4), (Fig. 13.4)c) [NC 553 531]. Within the Moine Nappe, many D3 folds are similarly associated with zones of high D2 strain and so may be genetically related to ductile thrusting processes (Holdsworth, 1990; Alsop & Holdsworth, 2007). This implies that a direct correlation of all D3 structures may not be valid, with the structures forming at various times during a protracted ductile displacement event.

Locality 13.3 Ribigill [NC 5618 5258] to [NC 5860 5455]

Ribigill (Fig. 13.1). Lewisianoid basement gneisses of the Ribigill inlier, the metabasic Ben Hope Sill.

This locality comprises three short stops (each 15-30 minutes), all close to the road between Kinloch Broch and Tongue. Driving from south to north, first of all stop at 3A [NC 5618 5258] where parking is available for three to four cars at intervals along the road. Examine the crag exposures immediately east of the road. The lowest outcrops are of banded Moine psammite with locally well-preserved, inverted cross-bedding. The beds are deformed by mesoscopic, tight to isoclinal D2 folds with a strong axialplanar S2 fabric, as well as asymmetric, open D3 folds. Both sets of folds plunge broadly parallel to L2. The crags upslope expose planar banded gritty psammites; ductile strain is apparently higher as no sedimentary structures are preserved. Their contact with overlying platy, banded hornblendic gneisses of the Ribigill East basement inlier is concordant and interpreted as a tectonically modified unconformity on the inverted limb of a major D2 fold (Holdsworth, 1989a). A 40Ar/39Ar age of c.416 Ma obtained from muscovite within the psammites is interpreted to date cooling through a closure temperature of c.350°C either during or after D2 (Dallmeyer et al., 2001).

Return to the vehicles and drive northwards to [NC 5667 5334] where roadside parking is available for a minibus or four to five cars. Head westwards over the hillside for 150m or so to find west-facing slabs of foliated garnet amphibolite of the Ben Hope Sill at 3B [NC 5660 5338]. The amphibolites carry an intense S2/L2 fabric; thin, concordant quartzo-feldspathic layers and quartz veins are often boudinaged. The garnets are wrapped by the dominant schistosity and show well-developed pressure shadows that are elongate parallel to the lineation, indicating that they formed pre-D2. Inclusion trails of S1M that are highly oblique to S2 are present occasionally. The evidence therefore indicates that garnet growth occurred post-D1M and pre-D2.

Return to the vehicles and drive northwards to park in the entrance to Ribigill Quarry [NC 5860 5455]. This quarry is in intermediate, banded hornblende-biotite gneisses of the Ribigill East basement inlier. At the back of the quarry at 3C, a unit of homogeneous, finely-banded gneiss is exposed. U-Pb SHRIMP dating of zircons from this unit sampled in the main face of the quarry indicates a late Archaean age of c.2850 Ma for the igneous protolith (Friend et al., 2008). Mm-cm scale concordant granitic veins may have formed during migmatization of the basement prior to intense reworking during the Knoydartian and Caledonian events. The dominant planar and linear fabrics within the gneisses are essentially parallel to the S2/L2 fabric in nearby Moine rocks. A 40Ar/39Ar age of c.433 Ma obtained from hornblende within the gneisses is interpreted to date cooling through a closure temperature of c.500°C either after or during D2 (Dallmeyer et al., 2001). High in the quarry face, the gneisses are cut discordantly by pink granitic to syenitic veins that may have been intruded at the same time as the late-Caledonian Loch Loyal Syenite Complex some 5km to the south (Holdsworth et al., 1999).

Locality 13.4 Coldbackie Bay [NC 6124 6053]

Coldbackie Bay (Fig. 13.5). D2 deformation in the Moine Nappe; brittle deformation, faulting, possible New Red Sandstone (Permian) conglomerates and unconformity.

Extensive roadside parking space is available by the A836 at Coldbackie [NC 612 601]. Allow 1 hour for this locality. In the roadside cutting on the south side of the road, 4A (Fig. 13.5), are well-developed mullion structures that are parallel to the hinges of prominent ‘Z’ geometry D2 folds. These deform a well-defined, bedding-parallel D1M planar fabric. Cross-bedding is well preserved in a fold hinge close to the road surface [NC 6102 6003] and indicates that these folds face southwest and lie in the normal limb of a major D2 synform lying just to the southeast. Some 30m to the south and east of these exposures, psammites with gritty bands lie in the inverted limb of this fold as shown by cross-bedding [NC 6105 5997] and ‘S’ geometry minor D2 folds.

Descend through the gate to the beach, 4B, to examine Moine psammites with tight-to-isoclinal D2 folds and a strongly developed S2/L2 fabric. On the east side of the beach, numerous folded, lineated and boudinaged pegmatites and quartz veins were clearly deformed during D2 (e.g. [NC 6124 6053]). A 40Ar/39Ar age of c.419 Ma obtained from muscovite within the psammites is interpreted to date cooling through a closure temperature of c.350°C either during or after D2 (Dallmeyer et al., 2001). Undeformed red conglomerates and occasional sandstone layers unconformably overlie the Moine rocks on the beach. The sediments are thought to have been deposited in a series of alluvial fans and braided channels. Rounded to subrounded clasts within the conglomerates are mostly of Moine psammites, basement gneisses and late Caledonian syenite plutons and are therefore essentially locally derived (Blackbourn, 1981). The age of the conglomerates and sandstones is uncertain, with both Devonian (Old Red Sandstone) and Permian ages proposed (see Holdsworth et al., 2001a and references therein). However, recent studies suggest that strata of both ages may be present, with the NNW-SSE bounding fault, lower conglomerate and sandstone units (only exposed inland) being of Devonian age, and the upper, syenite-bearing conglomerates (seen at this locality) being of Permian age (Wilson et al., 2010).

Locality 13.5 Sleteil & Skullomie Harbour [NC 6281 6283] to [NC 6180 6150]

Sleteil & Skullomie Harbour (Fig. 13.5). D2 infolds of Lewisianoid basement gneisses in unconformable contact with Moine psammites preserving sedimentary structures; late folds and associated brittle detachments.

At Strathtongue on the A836, turn north on the minor road to Skullomie; parking is available for three to four cars in the lay-by at the end of the road [NC 6191 6132]. Allow 2-3 hours for this locality. Walk north through a gate, pass a house on the right and over a footbridge. Follow the footpath through a gate, turn sharp right parallel to a fence and head NE past a ruined croft and up a steep hillside to Carn an Fheidh (Fig. 13.5) [NC 6275 6285] where excellent examples of D2 folds and sedimentary structures occur in the psammites. A further 700m north at 5A [NC 6281 6283] is a well exposed boundary with locally gritty, feldspathic Moine psammites underlying sheared hornblende, biotite and migmatitic feldspathic basement gneisses. This locality is 250m NE of a prominent coastal inlet and north of three large syenite erratics.

This boundary can be followed around the hinge of an east-plunging D2 fold; less than 100m to the north [NC 6282 6291], Moine psammites contain poorly preserved cross-bedding younging away from hornblendic basement schists, thus the D2 fold faces north. Follow the upper boundary of the basement towards 5B [NC 6282 6268] where the Moine psammites young away from the underlying basement schists. In this area, the psammites contain soft-sediment structures and pebbly horizons; crude finingupwards cycles and cross-bedding indicate the direction of younging and enables the disposition and facing of the D2 folds to be determined even though up to 180 degrees of fold hinge curvature is developed about the ESE-plunging L2 mineral lineation. Traverse some 200m SE to 5C that lies just within the inverted limb of a syncline (Fig. 13.5). Around here, examples of D2 ‘eye’ structures and along-strike changes in fold plunge, sense of vergence and facing are well displayed, with hinge curvature occurring about a weakly developed, ESE-plunging L2 mineral lineation. This inverted fold limb underlies basement schists exposed further to the east, with the fold closing to the north and facing SW (Fig. 13.5).

The Sleteil basement body lies in the core of complex, en-echelon, anticlinal D2 folds with curved hinges that appear to close and face both to the north and to the south, and to thus form ‘tongue-shaped’ sheath structures that originally faced and closed upwards to the WNW; D2 folds with sheath-like geometry are present on all scales within this area (Fig. 13.5) (Holdsworth, 1988; Alsop & Holdsworth, 1999). The boundary of the Sleteil inlier is believed to be a slightly modified Moine-basement unconformity (Holdsworth, 1989a).

Return to the vehicles, drive south for 500m and take a sharp turn right at [NC 6155 6086] to follow the track down to Skullomie Harbour. 5D [NC 6180 6150] is located along the low cliffs east of the harbour and best approached at low tide. Here the Moine psammites are deformed by a series of minor brittle-ductile folds that are related kinematically to a series of detachment faults that have both top-to-the-SSE and ESE senses of displacement. These structures are developed preferentially in belts of pre-existing D2 strain in which the foliation is markedly flaggy with few folds developed. The generally eastward-dipping detachment faults lie either parallel to the foliation (‘flats’) or cross-cut at angles of up to 40° (‘ramps’). Examples of ramp-flat geometries are well exposed in the cliffs behind the harbour. These structures are thought to have formed during extensional collapse of the thickened nappe pile at a late stage in the Caledonian orogeny (Holdsworth, 1989b; Holdsworth et al., 1999, 2007).

References

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