Triassic, Palaeogene volcanic districts of Scotland
|Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
Rocks ranging in age from the Triassic to the Late Cretaceous occur throughout the Inner Hebrides and on Arran (P914123). They are part of the extensive basinal deposits found mainly offshore, and in many instances represent the feather edges of these successions. The basins are important structural elements in the Hebridean Igneous Province. In addition to their dominant control on Mesozoic sedimentation, the basins and associated structures appear to have influenced the emplacement of the Paleocene central complexes and possibly also the accumulation of the lava successions.
Many of the New Red Sandstone occurrences mentioned below are assigned to the Triassic on grounds of lithology, unconformable relationships with older rocks, or a conformable one with overlying, fossiliferous Jurassic strata. Fossils are rare in all but the highest of these beds. In the Hebridean area, the New Red Sandstone sequences are thought to belong to the Upper Triassic and accumulated during the earliest stages of basin formation (Steel 1974a, b; Steel et al., 1975).
The Lamlash Sandstone and Glen Dubh Sandstone formations are well exposed on the east side of the island. These sandstones are massive, reddish to purple, and are stratigraphically continuous with the Permian sandstones. The Permian–Triassic boundary is difficult to identify here, as it is elsewhere in Great Britain, and is arbitrarily located within the sandstones of the Sherwood Sandstone Group (Warrington et al., 1980; Table 3).
The Permian and Triassic rocks of Arran were considered virtually devoid of stratigraphically diagnostic fossils until the discovery of Triassic miospores (Warrington, 1973) in what is now termed the Lag a’Bheith Formation (Lovell, 1991). This formation comprises mudstones with fine-grained sandstones and contains carbonate concretions (‘cornstones’) probably resulting from evaporation under subaerial conditions.
The overlying Auchenhew Mudstone Formation covers much of southern Arran and corresponds with the areas shown as Triassic on the published maps. It extends from Largybeg Point in the east to Drumadoon in the west and there are abundant, excellent exposures on the foreshore, in cliffs and in many stream and river sections. The formation comprises a succession of fine-grained sandstones, mudstones and calcareous mudstones (marl) that has been equated with the Mercia Mudstone Group (Lovell, 1991). The presence of carbonate concretions, pseudomorphs after halite, ripple marks and dessication cracks indicate that the beds formed mainly under shallow-water conditions, but with periods of drying out and deeper flooding.
Mudstones, siltstones and fine micaceous sandstones of the Levencorroch Mudstone Formation are exposed in stream sections near Levencorroch Hill. These beds also contain carbonate concretions. The highest parts of the Triassic succession occur only as downfaulted blocks in the Central Arran Ring-complex (Warrington et al., 1980). There, they are exposed in the Allt nan Dris, where mudstones of the Levencorroch Mudstone Formation are associated with grey-green siltstones of the Derenenach Formation. The latter closely resemble the ‘tea green marls’ of the Collin Glen Formation at the top of the Triassic succession in Northern Ireland. Overlying black mudstones with thin limestones have yielded marine bivalves (e.g. Rhaetavicula contorta, Chlamys valonensis, Protocardia philipiana?, Modiolus minimus?) and these beds have been assigned to the Westbury Formation of the Rhaetian Penarth Group.
Up to about 60 m of conglomerate, sandstone and cornstone are found at Gribun and on Inch Kenneth in western Mull, where the striking unconformity with the underlying Moine rocks is exposed on the foreshore and in cliff sections (P532641). Clasts of Moine rocks, cherty fossiliferous limestone (Cambro-Ordovician Durness Group), red feldspathic sandstone (Torridonian), vein-quartz, quartzite, granite and rare, red andesitic porphyry (?Siluro-Devonian) indicate both local and quite distant provenance for these beds. Upper Triassic miosporeshave been recovered from beds near the top of the Gribun succession that also contain indistinct bivalve remains. The uppermost beds are sandy limestones of Rhaetian age, which contain fish scales and bivalves (e.g. Cardinia sp., Chlamys valoniensis, Protocardia rhaetica, Rhaetavicula contorta). In eastern Mull, Triassic conglomerates and sandstones are exposed in the cores of the anticlines that surround the central complex, from Craignure to Loch Don and Loch Spelve. In addition, there are extensive sandstone outcrops on the west shore of Loch Spelve, which extend up Glen Lussa, where the Triassic rocks occur in screens between inclined basic sheets. Clasts in the conglomerates are mainly quartzite and vein-quartz, but pebbles of Moine rocks are abundant locally where these lithologies are in situ nearby. Small outcrops of Triassic strata are also found within the central complex, where their steep dips indicate considerable disturbance by the Paleocene intrusions.
Up to 100 m of conglomerate, sandstone, calcareous mudstone, mudstone and cornstone are present beneath younger Mesozoic rocks and Paleocene lavas, and in outliers to the north of the lavas. Clasts in the conglomerates come from local Moine rocks and Caledonian felsic minor intrusions. Thickness is very variable. The Triassic beds appear to be absent to the west of Loch Teacuis, and even in the vicinity of Loch Aline and Inninmore Bay, where the thickest and most complete sections have been measured (Lee and Bailey, 1925), the beds may thin out completely. Poorly preserved bivalve remains have been found in rocks of possible Rhaetian age at Inninmore.
Sandstones, cornstones and sedimentary breccias crowded with local schist and vein-quartz fragments underlie lavas on the east side of Ben Hiant. Similar rocks, with abundant cornstone, crop out on the south coast at Mingary and underlie Lias beds on the north coast at Ockle Point and at Swordle.
The Monadh Dubh Sandstone Formation of north-west Rum is 80 m thick, and consists of a fining-upwards sequence of sedimentary breccias, conglomerates, cornstones and sandstones with thin silty sandstones containing plant remains. These north-west-dipping beds unconformably overlie Torridonian sandstones, which have been permeated and replaced by corn-stones. Clasts in the breccias and conglomerates were generally derived from the nearby Torridonian sandstones, although quartzite pebbles may have come from the basal Cambrian rocks of Skye. Near the middle of the succession, conglomerate-sandstone-cornstone cyclothems suggest a change from initial debris flows to fluvial deposition and soil formation (R J Steel in Emeleus, 1997). A general change from continental conditions to nonmarine shallow-water and estuarine deposition is indicated by the upward progression from fine-grained siltstones and calcareous sandstones with abundant plant and wood fragments, to beds containing fish scales and teeth, and conchostracan arthropods (Euestheria minuta). The uppermost beds may correlate with the Mercia Mudstone (Triassic) or Penarth (Rhaetic) groups of England and Wales.
There is a thin, impersistent development of conglomerates, sandstones and cornstones beneath the Lower Jurassic rocks near Broadford and on either side of the shallow syncline of Jurassic rocks between Broadford and Loch Eishort. Similar rocks, predominantly conglomerate and breccia, also crop out near Tarskavaig on the Sleat peninsula and on the north side of Soay Sound. Faulted strips of Mesozoic rocks, including Triassic, are found on the northern slopes of Glamaig where they have been considerably disturbed by the granite intrusions. Pebbles and other clasts in the Triassic conglomerates are of local derivation and are predominantly of Torridonian sandstone, Cambro-Ordovician limestone, chert and, less commonly, quartzite. These New Red Sandstone strata have been assigned to the Stornoway Formation, which ranges in age from Triassic to possibly Hettangian (Morton and Hudson, 1995). However, the presence of Rhaetian strata within this formation is now disputed, and it is possible that most of the New Red Sandstone north of Mull is Jurassic in age.
There are several occurrences of the Stornoway Formation in the south of the island, near Eyre Point. The outcrops are much faulted, and are obscured towards the north-east beneath the immense slipped mass of Jurassic rocks at Beinn na’ Leac, reappearing beyond the landslip at Rubha na’ Leac. Marly beds and calcareous sandstones, including cornstones, are accompanied by conglomerates containing pebbles of Torridonian feldspathic sandstone, vein-quartz, and limestone (in places with chert) and quartzite of presumed Cambro-Ordovician age. The sequence is about 40 m thick at Rubha na’ Leac, but less than 20 m elsewhere.