Northern Marginal Zone (NMZ), Traverse west, towards Priomh-lochs, Rum - an excursion

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Geological map of Excursion 2: the Northern Marginal Zone. Dykes and cone-sheets omitted. Modified after Emeleus and Bell (2005). (© NERC)
Cone-sheet intruding Torridonian sandstone, Allt Slugan a’Choilich, beside the Coire Dubh path. Scale: hammer shaft 30 cm.

Excursion 2 (optional traverse) from: Emeleus, C. H. and Troll, V. R. A geological excursion guide to Rum: the Paleocene igneous rocks of the Isle of Rum, Inner Hebrides. Edinburgh : Edinburgh Geological Society in association with NMS Enterprises Limited, 2008.

Highlights Excursion 2 and optional traverse. This excursion will examine the rocks of Stage 1 exposed in Coire Dubh and on the low hills south of Kinloch (Cnapan Breaca, Meall Breac and Am Màm). The traverse, from Kinloch into Coire Dubh to the south, crosses in-situ Torridon Group rocks to the Main Ring Fault, then caldera breccias, rhyodacite ash flows and small intrusions of rhyodacite (with evidence of mixing of basaltic and rhyodacitic magmas). On Am Màm, a distinctive intrusive breccia is seen to predate intrusive rhyodacite. Further west, near the Priomh-lochs, Archaean gneisses are overlain by coarse sandstone belonging to the Fiachanis Gritty Sandstone Member (Torridon Group).

The distance for this excursion is about 10 to 15 km, depending on options chosen, and the highest point reached is nearly 350 m (See image). Leave Kinloch by the path from the castle along the west bank of Allt Slugan a’Choilich. Once clear of the trees around Kinloch, sandstone belonging to the Scresort Sandstone Member is encountered, intruded by numerous south-west-dipping sheets of aphyric to sparsely feldspar-phyric basalt that form part of a cone-sheet swarm (See image). Representatives of the swarm are well exposed in the bed of Allt Slugan a’Choilich, particularly above the altitude of 150 m where the resistant basalt sheets form a succession of small waterfalls. Follow the path or stream uphill towards Coire Dubh, noting the increased north to north-north-west dips of the sandstones.

Traverse west, towards Priomh-lochs

From the north-west side of Am Màm (Locality 2.13) traverse along the edge of Loch Gainmhich to gabbro crags at the north end of the loch.

Locality 2.16 Loch Gainmich – small intrusive plug of gabbro inside Main Ring Fault [NM 3805 9895]

A small plug of gabbro is situated just inside the Main Ring Fault. The gabbro is relatively unaltered and much finer grained than the gabbro involved with the Am Màm Breccia. Skirt the southern end of Loch Bealach Mhic Nèill (See image) and follow the western shore for 200 m to a small gabbro mass surrounded by gritty sandstone, then walk north-west for 150 m to a prominent north–south ridge of feldspathic peridotite with excellent examples of steeply dipping matrix banding (Dunham, 1965).

Locality 2.17 Ridge east of Priomh-lochs – ‘matrix banding’ in peridotite tongue [NM 3735 9900]

The ridge is formed by a tongue of peridotite that extends north from the Layered Centre to cut the Main Ring Fault (See image). Follow the ridge to the south for about 250 m, observing variation in the matrix-banding structures (Dunham, 1965), then walk west over sandstone outcrops for about 250 m to a point 170 m east-north-east of the peaty ground between the Priomh-lochs (See image).

Locality 2.18 Priomh-lochs – faulted and unconformable relationships of gritty sandstone to gneiss [NM 3715 9876]

The coarse sandstone hereabouts belongs to the Fiachanis Gritty Sandstone Member (TCDF) and is in contact with gneisses, which form extensive exposures either side of the Priomh-lochs. The boundary is probably an unconformity, complicated by minor faulting. The gneisses nearest the peridotite tongue have been thermally metamorphosed with indications of some melting in the leucocratic variants. However, the deformation and contortion of gneissose banding seen in many exposures pre-dates the Paleocene (Holness and Isherwood, 2003; cf. Emeleus 1997; Figure 25).

Locality 2.19 North of Priomh-lochs – explosion breccia injected along line of Main Ring Fault [NM 367 994]

Pass to the north of the Priomh-lochs and continue in a north-west direction for about 700 m until a prominent west-north-west-trending ridge is reached. The traverse crosses extensive gneiss outcrops, together with gabbro and ill-exposed microgranite; the gabbros are of late date since they cut the Main Ring Fault. The ridge marks the position of a strip of explosion breccias parallel to, and just inside, the Main Ring Fault. The breccia differs from the Am Màm Breccia since it contains fragments of porphyritic rhyodacite in addition to gabbro. It is cut by small rhyodacite intrusions. From here, either return to the north side of Loch Gainmhich and resume the itinerary (see above), or continue a short distance downhill in a north-westerly direction to the track that leads back to Kinloch (which is about 4 km distant).

References

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