New Red Sandstone, Northern Highlands of Scotland

Johnstone, G S and Mykura, W. 1989. British regional geology: Northern Highlands of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.

Wester Ross

Mesozoic basins in the seas around Northern Scotland. P915496.

New Red Sandtsone basins of north-west Scotland. P915497.

The largest New Red Sandstone outcrop in this area (P915496) extends from Gruinard Bay to the Isle of Ewe. At Udrigle and Laide on Gruinard Bay the succession is over 250 m thick. It consists of a thin basal breccia overlain by c.100 m of thinly-interbedded, purple, pebbly sandstone and conglomerate; these deposits were derived from local uplands in the south-east and south-west, and laid down in alluvial fans. They are in turn succeeded by 135 m of fluvial sediments laid down on the floodplains of meandering rivers, whose source lay to the south-south-west; they show well defined fining-upward cycles, each containing the following units.

3 Fine sandstone with immature cornstones (overbank deposit with incipient fossil soil)

2 Bright orange sandstone (wind deposit)

1 Pebbly sandstone with conglomerate lenses (channel deposits)

Of the small outliers along the coast of Wester Ross which were thought to be of New Red Sandstone age (Phemister, 1960, fig. 25), those at Achiltibuie, Little Loch Broom and near the Rubha Reidh lighthouse (10 km NW of Gairloch) have been shown to be Torridonian (Lowe, 1965). On the Rubha Reidh peninsula the only outcrop of New Red Sandstone age is at Camas Mor on the north coast, where coarse basal conglomerate is overlain by 15 m of pebbly sandstone with conglomerate lenses. Here many of the pebbles and cobbles are rounded and composed of Cambrian quartzite and limestone, and the sedimentary structure of the beds suggests that they were laid down in the channel of a NW-flowing braided river. Brick-red sandstone with conglomerate lenses crops out at Big Sand, Gairloch, and further south at Red Point there is a very small outlier of conglomerate and grit, with a cornstone at the base, which penetrates into the underlying Torridonian. At Applecross conglomerates and pebbly grits with well defined cornstone horizons containing chalcedony underlie marly beds with vertically aligned calcareous concretions.

Inverness-shire

Further south, in Ardnamurchan and Morvern, the New Red Sandstone consists of a thin basal breccio-conglomerate full of carbonate concretions (carbonate also penetrates as veins into the underlying Moine) overlain by a thin sequence of fluvial cycles of red sandstone and mudstone with well developed mature cornstone (caliche) profiles at the tops of some units (Steel, 1974a).

Lewis

A thick sequence of chocolate-red conglomerates with subordinate sandstones and rare siltstones forms extensive coastal exposures north and south of Stornoway, and on the Eye peninsula east of the town. The sediments are bounded by normal faults in the west and north, but they rest unconformably on the basement of Lewisian gneiss in the east. Though the apparent thickness of the formation is nearly 4 km, Steel and Wilson (1975) have shown that most of the succession was laid down in a number of overlapping alluvial fans formed along the foot of a retreating series of fault scarps located to the west and north, and that its true thickness at any one point may not exceed 1 km.

The conglomerates consist entirely of locally derived clasts of gneiss and ‘flinty crush’; many are very coarse with boulders reaching 1 m and, exceptionally, 3 m in size. Steel and Wilson have recognised six distinct cyclic sequences, each representing a period of fan formation. The lower fans are thought to have formed at the foot of a NE-trending fault which lay just east of Stornoway, while the later fans are related to fault scarps which lay progressively further west and north. Four of the cyclic sequences are coarsening upwards, suggesting that the fault scarps were actively moving during deposition.

Thin purple sandstones and siltstones (with incipient cornstones) of fluvial origin are intercalated with the lower cyclic units. In contrast to the suggested palaeoslopes of the fans (from west and north), the fluvial sediments were laid down by streams flowing to the north-east, suggesting that the alluvial fans formed along the western margin of a major alluvial plain which drained to the north-east (P915497).

North Sutherland

Of the small outliers around Tongue (P915496), four consist entirely of conglomerate and basal breccia, with clasts of local Moine rocks and syenite from the eastern outcrops of the Bed Loyal intrusion. The Fifth outcrop, at Coldbackie, has a basal breccia overlain by up to 120 m of bright red fluvial sandstone with fining- upward cycles and some incipient cornstones in the overbank deposits; these are overlain by up to 300 m of massive conglomerate, again with syenite clasts, suggesting a southerly derivation. By contrast, the Ron Islands north of Tongue are made almost entirely of up to 500 m of coarse, grey-brown conglomerate with clasts which include abundant Cambrian quartzites and Durness limestones. Clast composition and pebble imbrication in the Ron conglomerates suggest deposition by braided rivers from a west-north-westerly source.

The Kirtomy outliers occupy three fault-bounded N–S trenches and consist of red-brown pebbly arkose with conglomerate and breccia. They also appear to fill valleys with a northerly palaeoslope.

Though both Blackbourn (1981a;b) and O’Reilly (1983) still favour an Old Red Sandstone age for the North Sutherland sediments, a New Red Sandstone age is here preferred for the following reasons.

1. The fine sediments of the Watch Hill (Tongue) outcrops are very similar to sediments from offshore cores obtained along the southern margin of the Stormy Bank Basin and considered to be of Permo-Triassic or younger age by Evans and others (1982).
2. The bright red colours of the fine sediments match more closely with the red colours of the New Red Sandstone of Wester Ross, and contrast with the dull purple and drab colours of the Old Red Sandstone of Caithness.
3. The fluvial sediments contain incipient cornstones, similar in type to those found in Lewis and along Gruinard Bay. This contrasts with the absence of cornstones and incipient fossil soils in the Caithness Old Red Sandstone.

Palaeogeography of West Scotland

Steel (1978) has shown that the sediments of Hebridean outliers were laid down along the margins of major basins of deposition which accumulated in NNE- and NE-trending half-grabens, bounded by major normal faults on their northwestern margins. P915497 shows the extent, thickness and major paleoslopes of these basins, and the probable direction of their drainage. The alluvial fan deposits had their palaeoslopes roughly normal to the axes of these basins, whereas the fluvial deposits were laid down by streams running subparallel to the basin axes.

East Sutherland

At Dunrobin (P915496) the New Red Sandstone (?Trias) consists of pale calcareous sandstone overlain by red and green marls with bands of cherty limestone. The cherty bands were correlated by Judd (1873) with the Stotfield Cherty Rock of Lossiemouth on the south shore of the Moray Firth. These are, in turn, overlain by intermittently exposed pebbly grits and conglomerates termed the Dunrobin Pier Conglomerate and commonly ascribed to the Rhaetic (though there is no direct evidence to support this). The conglomerate contains pebbles of chert and limestone derived from the underlying Trias.