Moine geology of North Sutherland. The Melness area - 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. 13.2 Geological map of Locality 13.1 (modified from British Geological Survey, 1997).
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.

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

Excursion 13 North Sutherland. The Melness area[edit]

Locality 13.1 The Melness area [NC 580 643] to [NC 5850 6501][edit]

The Melness area (Fig. 13.2). A comparison of low and high strain Moine metasedimentary rocks of the Moine Nappe; Lewisianoid basement rocks; amphibolite of the Ben Hope Sill Suite; the Strathan basal conglomerate.

From Tongue, turn north off the A838 on the west side of the Kyle of Tongue, continue through Talmine for 1.3km and park where the road forks [NC 580 643] just south of Loch Vasgo. Parking is available for a coach or four to five cars. Allow 4 hours for this locality.

Examine the low cliffs (1A, (Fig. 13.2)) to the east of the fork. Here, relatively low-strain, cross-bedded gritty psammites are folded into a mesoscopic recumbent, north-facing D2 fold pair that plunges sub-parallel to the ESE-trending L2 lineation. The foresets dip north, sedimentary transport is from the south, and the lower limb is right-way up. Variations in the state of strain can be estimated by measuring the angles of cross-beds around the fold. The apparent strain increases dramatically on sections parallel to L2. These psammites are interpreted to rest with modified unconformity upon the basement rocks of the Achinahaugh inlier that are exposed on low-lying outcrops immediately west of Loch Vasgo (Alsop & Holdsworth, 2004a, figure 6). On top of the cliffs, strain and white mica content increase towards a minor ductile thrust that cross-cuts the fold pair. A prominent pelite horizon above the thrust contains garnets up to 5mm in diameter. Small inclusion trails within these porphyroblasts define S1M; the garnets are wrapped by the dominant S2 mica fabric and are associated with pressure shadows that are elongate parallel to L2. The evidence thus suggests that garnet growth occurred post-D1M and pre-D2. Formation of the S1M fabric and garnet growth are tentatively assigned to the Knoydartian orogenic event. Quartz veins are lineated and elongate parallel to L2. Shear bands within the pelites indicate a top-to-the-west sense of displacement parallel to the lineation. S2 and the quartz veins are folded by cm-scale, open to close, asymmetric D3 folds (e.g. [NC 5811 6431]). From here, head northeastwards across the intermittently well exposed, undulating ground. Spectacular rodded quartz veins are present at [NC 5820 6440] on the west side of a large hummocky outcrop. Traverse across psammites and occasional pelite bands, displaying good examples of D2 and D3 folds. 1B ((Fig. 13.2), [NC 5847 6455], 25m on a bearing 195° from the main summit cairn) is marked by a small cairn above a west-facing, 3m high surface displaying D2 and D3 folds, including numerous spectacular ‘eye’ structures (Fig. 13.3) that represent cross-sections through the noses of D2 sheath folds (i.e. not interference patterns). Please do not hammer these localities. Detailed analyses of both D2 and D3 folds in this area are presented by Alsop & Holdsworth (1999, 2002, 2004a & b). A little further to the east (25m on a bearing of 140° from the main summit cairn) another west-facing surface marked by a small cairn [NC 5851 6454] displays various D2 eye structures that are refolded by tight, asymmetric D3 folds. Then walk northeastwards downslope towards the coast.

1C [NC 5892 6487] is by the beach on the west side of the gully (Fig. 13.2), where pelites contain prominent 1-2cm diameter garnets. These show obvious zonation with orange cores (some with aligned inclusion trails) and darker rims wrapped by S2. Prominent shear bands indicate a top-to-the-west sense of displacement parallel to the locally developed L2 lineation. To the east of the gully, above a thin garnet amphibolite of the Ben Hope Sill Suite, east-dipping surfaces of highly deformed psammites display intersections of S2 with S0 /S1M curving about the E-W-trending L2 mineral and extension lineation. This reflects the progressive rotation of fold hinges during west-directed D2 thrusting. Quartzofeldspathic clasts within pebbly horizons show a marked elongation parallel to the intersection lineation. Still further to the east, the L2 mineral lineation disappears, leaving a uniformly N-S-trending stretching direction thought to represent a relict L1M lineation (Holdsworth, 1989a).

Head northwestwards, over inclined surfaces of garnet pelite, with isoclinally folded and rodded quartz veins elongate parallel to L2 . Further excellent examples of garnets wrapped by S2, showing cores and rims and inclusion trails, are present at [NC 5885 6507]. At 1D [NC 5884 6504], at the south end of the gully (Fig. 13.2), an outcrop-scale brittle detachment fault cuts across upright, brittle kink folds of fabric in its footwall. On the west side of the gully are steeply-inclined slabs of garnet amphibolite of the Ben Hope Sill Suite. Garnets are wrapped by the S2 fabric and some have pressure shadows that are elongate E-W parallel to L2. Walk westwards across undulating ground towards Port Vasgo. At [NC 5867 6499], pause to look ENE to view steep rock slabs that show mesoscopic tight to isoclinal D2 folds.

Follow the path down into Port Vasgo (Fig. 13.2) to a stony beach at 1E [NC 5850 6501], where NE-dipping, mylonitic psammites define a zone of high D2 strain. If tides are low, cross the rocks on the east side of the bay to access a pale-weathering lenticular pod of psammite associated with a microdiorite sheet exposed on a SW-dipping cliff at [NC 5862 6508; see Holdsworth et al. 2001a, plate 8). The pale eye-shaped area of psammite is a low-strain augen preserving cross-laminations folded by D2 folds that appear to be cross-cut by a little-deformed, fine-grained sheet of microdiorite, with well-defined chilled margins. On careful inspection, however, it is apparent that the sheet pinches out and passes laterally into bedding-parallel shears that locally preserve irregular isolated pods of microdiorite, often in equivalent small-scale low-strain augen. Other sheets of microdiorite can be traced laterally into the high-strain, platy psammites where they become schistose and carry fabrics parallel to S2 and L2 in the adjacent psammites. Although clearly still discordant in both anticlockwise and clockwise senses, they appear to represent intrusions that were emplaced as curviplanar units that have subsequently been deformed during D2. Similar microdiorite intrusions are common around the Kyle of Tongue and have been grouped as the Port Vasgo Microdiorite Suite by Holdsworth et al. (2001a) who interpreted them as being syn-tectonic with respect to the D2 deformation. It is equally possible, however, that the intrusions pre-date D2, with some or all of the folds occurring close to the discordant sheets representing flanking folds (Passchier, 2001). If the intrusions are pre-D2, they may be the same age as the Loch a’ Mhoid Metagabbro Suite (see Locality 13.6).

Retrace your steps back to the beach, and follow the road up out of Port Vasgo back to the vehicles. Take the left fork and drive to Strathan Bay (Fig. 13.2), parking at the east end of the crash barrier at [NC 5743 6480]. Walk down the slope to the shore on the south side of the bay to 1F [NC 5734 6489] to view exposures of the Strathan Conglomerate (Mendum, 1976), interpreted as a basal Moine conglomerate overlying Lewisianoid basement (Holdsworth, 1987, 1989a; Holdsworth et al., 2001a). The conglomerate lies within a high strain zone and contains numerous highly flattened ‘clasts’ that are up to 30cm in length and elongate parallel with L2 . ‘Clast’ types are, in order of decreasing frequency and degree of strain: pale grey quartzite, fine to medium-grained quartzo-feldspathic gneiss, white quartz, dark grey quartz-magnetite rock, and granite/pegmatite. It is doubtful, however, that all these types were genuinely incorporated as clasts within the Moine sediments prior to D1M. In low strain zones, the grey ‘quartzite’ and white quartz ‘clasts’ appear to represent tectonically disrupted, isoclinally folded metamorphic segregations (compare with the segregated pelites and possible conglomerate layers seen at Kinloch Broch, Locality 13.2 below). According to Holdsworth (1987), the only lithologies that can be identified confidently as pebbles in areas of low strain are those of quartz-magnetite rock and pink granite/pegmatite which typically display lower amounts of strain. At low tide and low sand levels, highly strained hornblendic material within the conglomerate could represent early infolds of the Lewisianoid basement. Upstream, the conglomerate is underlain by flaggy, pink acidic basement gneisses that are interleaved with mafic sheets at [NC 5725 6483].

References[edit]

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