Palaeozoic, 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.|
The lower part of the Cambro-Ordovician succession, which is comparable with that seen elsewhere in the North West Highlands, crops out in a highly complex structure around Ord on the Sleat peninsula (P914122; Table 2). At the base of the sequence, quartzites of the Eriboll Sandstone Formation (including the False Bedded Quartzite Member and the bioturbated Pipe Rock Member), and the An t-Sron Formation are all of early Cambrian (Comley) age. The An t-Sron Formation consists of the Fucoid Beds and Salterella Grit members. The Fucoid Beds consist mainly of dolomitic siltstone characterised by trace-fossil remains originally thought to resemble seaweed markings. The Salterella Grit Member comprises quartzite with remnants of the small gastropod Salterella. The overlying Ghrudaidh, Eilean Dubh and Sailmhor formations (Durness Group) range from mid Cambrian (St David’s) to earliest Ordovician (Tremadoc) in age and are best exposed in the coastal sections; they consist of dolomitic limestones and dolostones that are commonly cherty. The outcrops of Cambro-Ordovician rocks, together with associated strata of the Torridonian Kinloch and Applecross formations, occur within a structure that has been termed the Ord Window (P914122a). This structure is a stratigraphical outlier in that it is completely surrounded by older rocks, but it is a structural inlier in the sense that it has been interpreted as an area of the foreland that has been exposed by erosion through overlying thrust sheets of the Moine Thrust Belt. The inlier is characterised by folded, steeply dipping, fault-bounded slices, and the detailed structure is difficult to determine. A number of different models have been proposed (P914122b). Clough (in Peach et al. 1907) envisaged the Ord structure as a tectonic window in the core of a relatively open antiform, in which essentially right-way-up sequences are separated by two thrusts within the lower part of the Moine Thrust Belt. Bailey (1939, 1955) re-interpreted Clough’s work, and suggested that the inlier consists of a pair of large recumbent folds, with inversion of strata. The two thrusts were interpreted as branches of the Kishorn Thrust on the lower limb of the recumbent Lochalsh Syncline and the lowest structures in this part of the Moine Thrust Belt. The core of the inlier could therefore be regarded as part of the foreland. Potts (1982) envisaged the rocks of the inlier as part of a recumbent fold, analogous to, but at a lower structural level than, the Lochalsh Syncline, and re-interpreted some of the thrusts as normal stratigraphical junctions. He proposed that the folded rocks were emplaced at their current level through a combination of thrusting on the western, leading edge and extensional normal faulting at the trailing, eastern edge. In this model, the inlier does not constitute a classic ‘window’ through a thrust sheet. As yet there is no consensus on the preferred model and there is considerable scope for further interpretation.
A thick chert-bearing dolostone sequence crops out extensively between Broadford and Loch Slapin. It is separated from the Cambro-Ordovician outcrops of Sleat by several kilometres, with only small fault-bound outcrops of the Eriboll Sandstone Formation in the intervening ground. Gastropods, bivalves and sponges have been obtained, for example near the outlet of Loch Kilchrist, and the dolostones are considered to be of early Ordovician (Arenig) age.
However, the succession cannot be equated directly with the upper part of the Durness Group elsewhere and hence is here termed the Strath Suardal Formation. The dolostones underlie overthrust Torridonian strata and are intruded by igneous rocks of Paleocene age including the Beinn an Dubhaich Granite, the Broadford Gabbro and numerous dykes (P914139/P914140 BR). Dolostones in contact with the granite and gabbro have been altered to a variety of calcsilicate hornfels, including the well-known yellow and green decorative ‘Skye Marble’, which contains diopside, serpentine and brucite. The Ordovician strata are folded into an anticlinal structure, which has the Beinn an Dubhaich Granite at its core.
A thickness of about 300 m of black mudstone and chloritised spilitic pillow lavas, together with rare beds of chert or tuff, is exposed in the North Glen Sannox River. Poorly preserved brachiopod remains obtained from the mudstone indicate an Arenig age for these deposits. The sequence rests on Dalradian metasandstones and is cut by a few sheets of altered dolerite and gabbro. It forms part of the Highland Border Complex, which crops out along the Highland Boundary Fault between Arran and Stonehaven (Stephenson and Gould, 1995).
Caledonian igneous rocks
The Ross of Mull Pluton comprises a number of granitic bodies with a Rb-Sr age of about 414 Ma (Halliday et al., 1979); it intrudes and thermally metamorphoses the Moine metasedimentary rocks of south-west Mull. Cordierite-sillimanite hornfelses have developed from kyanite-bearing pelitic rocks adjacent to the granite and occur as xenoliths. Within the intrusion, the intricate relationships found between quartz-diorite, granite and basic enclaves suggest the co-existence and interaction of basic and silicic magmas.
Minor intrusions of Caledonian age include various felsic types and lamprophyres. The felsic dykes are most common in the vicinity of the Ross of Mull Pluton, and near the Strontian Pluton east of the Morvern lava pile (Johnstone and Mykura, 1989). The Ross of Mull granites have been extensively quarried (p. 173).
Old Red Sandstone
The Old Red Sandstone is mainly Devonian in age but parts may be late Silurian or Early Carboniferous.
Sandstones and conglomerates of the Lower Old Red Sandstone form a thick arcuate outcrop east and south of the North Arran Granite Pluton, and part of the succession is repeated on the southeast side of the Central Arran Ring-complex. These strata, which total about 1235 m in thickness, are faulted against Dalradian metasedimentary rocks, younger sedimentary rocks and the North Arran Granite Pluton in eastern Arran, but on the west side of Glen Rosa they lie unconformably on the Dalradian (Friend et al., 1963; McKerrow and Atkins, 1985). Clasts of schistose gritty metasandstone occur in conglomerates low in the succession, and were clearly derived from local Dalradian sources. However, the very abundant pinkish quartzite boulders and cobbles seen, for example, in conglomerates near the Glen Sannox baryte mine have no local source and match most closely Argyll Group (Dalradian) quartzites on Islay and Jura. Sparse plant remains (Psilophyton princeps, var. ornatus) have been found high in the succession in Glen Shurig (Tyrrell, 1928).
Upper Old Red Sandstone strata occur above all the sequences of Lower Old Red Sandstone but, in contrast to the mainland occurrences, no intervening unconformity has been found. In the east, the sequence is about 870 m thick and comprises sandstone with conglomerate beds containing clasts not only of local origin (vein quartz and Dalradian lithologies) but also of possible Islay and Jura quartzite (Friend et al., 1963). Similar rocks crop out south-east of the ring-complex. On the west coast, conglomerates and sandstones of the North Machrie Breccias total about 375 m in thickness, with the proportion of sandstone increasing eastwards.
The Old Red Sandstone sedimentary rocks probably represent alluvial fan deposits similar to those present on the northern edge of the Midland Valley. They were laid down by braided river systems, with the sediment load derived from a northerly source. Strongly dessicating conditions are indicated by the presence of caliche deposits (cornstones), which are especially well developed at the base of the succession at ‘Hutton’s Unconformity’ north-east of Loch Ranza, where the carbonate pedogenesis extends for up to 50 cm into Dalradian rocks below the unconformity. There, the cornstone-bearing Upper Old Red Sandstone beds above the unconformity are assigned to the Kinnesswood Formation at the base of the Carboniferous succession.
Volcanic rocks within the Old Red Sandstone succession of Arran are limited to an (?)olivine andesite lava recorded from the Lower Old Red Sandstone south of the North Arran Granite Pluton and (altered) olivine basalt lavas interbedded with the Upper Old Red Sandstone strata north of North Glen Sannox. The olivine andesite flow has been compared with lavas found in the Lower Old Red Sandstone of the Midland Valley, whereas the olivine basalt lavas are similar to lavas found low in the Carboniferous succession of the Midland Valley (Tyrrell, 1928).
Basaltic and andesitic lavas belonging to the Lower Old Red Sandstone Lorn Plateau Volcanic Formation crop out in the core of the Loch Don Anticline in south-east Mull. Conglomerates and marly sandstones, possibly of similar age, occur on Frank Lockwood’s Island, south-east of Loch Buie in southern Mull.
Intensively faulted Carboniferous strata crop out on the north-east coast, and between Corrie and Glen Cloy. They range from sandstone and mudstone of the Inverclyde Group (Tournaisian) to siltstone, seatearth and sandstone of the Coal Measures (Westphalian). The sequence includes the richly fossiliferous Corrie Limestone, which is crowded with the brachiopod Productus latissimus and is equated with the Hurlet Limestone at the base of the Lower Limestone Formation (latest Visean) on the mainland. Olivine basalt lavas and volcaniclastic rocks are present in the lower part of the succession exposed on the Corrie foreshore and on the north-east coast. They overlie sandstones of the Clyde Sandstone Formation (Tournaisian) and are equated with lavas belonging to the Clyde Plateau Volcanic Formation of the Strathclyde Group. Altered basalt lavas in the Merkland Burn near Brodick Castle overlie sandstones and mudstones of the Limestone Coal Formation (Namurian).
A small faulted inlier consisting of over 100 m of sandstones and mudstones with thin fireclays and (uneconomic) coal seams underlies Permo-Triassic beds at Inninmore (c, P914121). Fossil plants, including Asterophyllites charaeformis, A. equisetiformis, Calamites cisti, C. schutzeiformis, Mariopteris muricata, Neuropteris gigantea and Samaropsis sp., indicate an early Coal Measures (Westphalian) age for these deposits (Lee and Bailey, 1925; Johnstone and Mykura, 1989). Despite its limited extent, the occurrence is of some importance since it is one of very few outcrops of Carboniferous rocks in western Scotland: the presence of offshore occurrences has been suggested, which could have implications for hydrocarbon generation, but to date none has been proved (Fyfe et al., 1993).
Permo-Triassic rocks crop out widely in the Hebrides, where they are also important constituents of the offshore basins, but it is only on Arran that the Permian and Triassic are Arran separated in published accounts (e.g. Lovell, 1991). The approximately 500 m-thick sequence of sandstones and sedimentary breccias comprising the Corrie Sandstone and Brodick Breccia are Permian. The sandstones are of aeolian origin, with obvious dune bedding and wind polished sand grains (Frederiksen et al., 1998) (Plate 3). Fossil lightning strikes (fulgurites) have been recognised in these beds at Corrie (Harland and Hacker, 1966). The Brodick Breccia consists of coarse sedimentary breccias and conglomerates with fragments of Dalradian vein quartz and quartzose schist, and agate and basalt clasts derived from Devonian and Carboniferous lavas. They are interpreted as flash-flood deposits, formed when debris was periodically flushed out from wadis. The associated finer grained rocks are similar to the Corrie Sandstone. The overlying, diachronous and possibly interdigitating Machrie, Glen Dubh and Lamlash sandstone formations are tentatively equated with the Upper Permian to Middle Triassic Sherwood Sandstone Group (Lovell, 1991; Table 3). Flows of olivine basalt in the upper reaches of the Sliddery Water are of Early Permian age.
Intrusions of Carboniferous and Permian age
Minor intrusions of Carboniferous and Permian age are widespread through the southern Highlands and extend into the western Highlands and Islands. There are two main suites: Stephanian tholeiitic rocks and Visean to Early Permian alkali basalts and lamprophyres. On Arran, the former includes a thick quartz-dolerite dyke at Imachar. East of the Paleocene lavas on Morvern, isolated quartz-dolerite plugs and west- to west-north-west-trending quartz dolerite dykes, which crop out between the lavas and the Strontian Pluton, are regarded as part of the Stephanian tholeiitic suite. At Gribun in western Mull, Moine rocks are intruded by a dyke of olivine nephelinite exposed at low water which contains a suite of xenoliths and megacrysts of lower crustal and possibly upper mantle origin (Upton et al., 1998). This dyke is similar to numerous other xenolith-bearing intrusions of Carboniferous and Permian age in the western Highlands and Islands and the Midland Valley. The xenoliths and xenocrysts found in these intrusions have furnished valuable information about the nature of the Lower Crust and Upper Mantle at the start of the Mesozoic (e.g. Upton et al., 1998).