Palaeogene volcanic districts of Scotland – an introduction
|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 varied geology of western Scotland is spectacularly displayed in the precipitous coastline of the mainland and in the islands of the Inner Hebrides. The area is noted for the widespread volcanism that occurred during the Palaeogene Period, and which has played an important role in the development of the rugged scenery (P521672). The volcanic rocks buried and preserved thick sequences of Mesozoic sedimentary strata that now provide, where exposed by erosion, welcome tracts of fertile land in the otherwise rugged surroundings. In this book, the igneous rocks produced by the Palaeogene volcanism are described in detail, together with the Mesozoic sedimentary rocks, the Quaternary deposits and aspects of the earlier geology. A more detailed account of the Archaean Lewisian Gneiss Complex, the Proterozoic and Palaeozoic rocks and their associated structures can be found in another volume in this series, The Northern Highlands of Scotland (Johnstone and Mykura, 1989) and also in The Geology of Scotland (Trewin, 2002).
The area described in this book, and referred to here as the district, lies largely within the part of the Inner Hebrides north-west of the Great Glen Fault (P914119), where igneous rocks of Palaeogene age occur on the islands of Skye, Raasay, the Small Isles (Canna, Rum, Eigg and Muck) and Mull, and on Ardnamurchan and Morvern on the mainland. Also described are other islands that are dominated by Palaeogene igneous rocks, including Arran, Ailsa Craig, the Shiant Isles, Rockall and St Kilda. Since the Third edition of this book (Richey, 1961), the offshore search for oil and gas has resulted in a vast increase in our knowledge of the geology beneath the seas around Scotland, and the continental shelf in the Hebridean area has been described in two BGS Offshore Geology Reports (Fyfe et al., 1993; Stoker et al., 1993). These provide detailed summaries of the geology of the Mesozoic basins, and much information on the Palaeogene igneous rocks found offshore.
The Palaeogene igneous rocks of Great Britain, Ireland and the surrounding sea form part of the North Atlantic Igneous Superprovince that extends to East and West Greenland, Baffin Island and the Labrador Sea (Upton, 1988; P914120). This volcanism was a precurser to and accompanied the opening of the North Atlantic and continues to the present day on the Mid-Atlantic Ridge and in Iceland. In north-west Scotland, the igneous activity occurred during the Paleocene Epoch, from about 62 Ma to about 55 Ma, with a peak between about 61 Ma and 57 Ma (P914126). The remains of the central volcanoes, basaltic lava fields and innumerable minor intrusions formed at that time comprise the Hebridean Igneous Province. The intrusions extend to the Outer Hebrides and may be traced through southern Scotland into north-east England. There are also scattered occurrences in Wales, in central England, and forming Lundy Island in the Bristol Channel. Intrusions and lavas of similar age constitute an important part of the geology of north-east Ireland and include the well-known Antrim Lava Field and the central complexes of the Mourne Mountains, Slieve Gullion and Carlingford (Preston, 1982, 2001; Mitchell, 2004; P914119). In many publications, including the earlier editions of this book, the Palaeogene igneous rocks of the British Isles are referred to as the British Tertiary Volcanic (or Igneous) Province, but as the term ‘Tertiary’ is no longer used in chronostratigraphy we have chosen to replace it in the title of this book.
Summary of geology
The Hebridean Igneous Province extends from Arran to Skye and St Kilda and thus crosses several tectonic crustal terranes (P914119). Prior to the Caledonian Orogeny (see below), these were all situated on the margin of the supercontinent of Laurentia, which also included the basement rocks of present-day North America and Greenland. The Archaean gneisses of the Lewisian Gneiss Complex are the oldest rocks and comprise ancient crust that has been repeatedly involved in metamorphism and deformation events (Park et al., 1994, 2002). The gneisses are widespread in the Hebridean Terrane, which constitutes the foreland to the Caledonian Orogeny, and most likely underlie the younger rocks of the Northern Highlands Terrane. In the Hebridean Terrane, they are overlain unconformably by the Mesoproterozoic to Neoproterozoic Torridonian sedimentary rocks, which extend south from Cape Wrath to Skye and Rum, and eastwards to the limit of Caledonian deformation at the Moine Thrust Belt (Park et al., 1994, 2002; Stewart, 2002; P914121).
The Torridonian sediments were transported by rivers up to 500 km in length, which drained an area far to the north-west of the Outer Hebrides, and were deposited within a braided river system (Nicholson, 1993). Sedimentological studies and zircon age determina tions show that the sediment source was not the immediately subjacent Lewisian rocks, but Precambrian gneisses of the Laurentian Shield, which range in age from 2500 to about 1100 Ma.
In Skye, and to the north, the Lewisian and Torridonian rocks are typically overlain unconformably by Cambro-Ordovician shelf deposits, which crop out on the foreland and within the Moine Thrust Belt. However, in south-eastern Skye the Torridonian strata have been transported tectonically over these younger rocks as part of the Kishorn Thrust Sheet.
In the Northern Highlands Terrane, there are extensive outcrops of rocks of the Mesoproterozoic Moine Supergroup. These metasedimentary rocks may be laterally equivalent to the Torridonian, from which they are separated by westerly transporting thrusts of the Moine Thrust Belt (Holdsworth et al., 1994; Strachan et al., 2002). They were deformed and metamorphosed during the Knoydartian Orogeny at around 800 Ma and were later affected by Caledonian events. On the Sleat peninsula of south-east Skye, the Tarskavaig Group, a sequence of low-grade metamorphosed psammitic rocks, may provide a link between the Torridonian and the Moine. Rocks of the Moine Supergroup also occur within the Ardnamurchan and Mull central complexes. To the south-east of the Great Glen, the Grampian Terrane is dominated by rocks belonging to the Dalradian Supergroup. However, within the area described in this book they crop out only in eastern Mull and northern Arran. They include a variety of metasedimen tary and meta-igneous rocks, mainly of Neoproterozoic age, although the youngest may be earliest Cambrian. In eastern Arran, black mudstones, cherts and pillow lavas of Ordovician age belong to the Highland Border Complex (Holdsworth et al., 1994).
The Torridonian, Moine and Dalradian sedimentary sequences were all deposited in extensional basins on what was to become a passive margin of Laurentia. The Cambro-Ordovician rocks of the North West Highlands were deposited subsequently on this passive margin, while the rock of Highland Border Complex were formed within the adjacent expanding ocean. In late Neoproterozoic to Early Palaeozoic time, this Iapetus Ocean separated Laurentia from the supercontinent of Gondwana (which included the basement of present-day England, Wales, southern Ireland and western Europe) and the continent of Baltica (the basement of present-day Scandinavia and Russia). It was the complex sequence of plate-tectonic events that led to the closure of this ocean, culminating in the collision and fusion of the bounding continents to form the new supercontinent of Laurussia, that resulted in the Caledonian Orogeny. Closure probably commenced in the early Ordovician and was completed by the end of Silurian time, but post-collision uplift, with associated magmatism, continued throughout most of the Early Devonian.
The effects of the orogeny are seen mostly in the Northern Highlands and Grampian terranes (Strachan et al., 2002). There, the peak of deformation and metamorphism, known as the Grampian Event, occurred during the early to mid-Ordovician and was accompanied by intrusion of large volumes of basic magma and by crustal melting that produced a suite of granite plutons. It is possible that the Highland Border Complex was obducted onto the continental margin at this time. Later, in mid-Silurian times, large-scale thrust movements resulted in considerable crustal shortening and the whole of the Caledonian ‘mobile belt’ was transposed over the foreland of the Hebridean Terrane along the Moine Thrust Belt. Major movements on the Outer Hebrides Fault-zone also occurred at this time. This mid-Silurian deformation or Scandian Event was associated with the emplacement of highly alkaline magmas within and around the thrust belt. Calc-alkaline granitic plutons elsewhere in the Northern Highlands and Grampian terranes continued to be emplaced widely throughout the ensuing continental uplift phase, overlapping with calc-alkaline volcanism in late Silurian to Early Devonian times. Although by this time active subduction beneath the Laurentian margin had ceased following continent–continent collision, all of the Silurian and Devonian magmatism has subduction-related characteristics. The widespread Caledonian intrusions are represented in this district by the Ross of Mull Pluton in south-west Mull and by a wide variety of minor intrusions. The crustal terranes were juxtaposed into their present positions by large sinistral (left-lateral) movements on bounding faults, such as the Great Glen Fault, during the later stages of the orogeny. Major lateral movements on other north-east-trending faults probably occurred at the same time.
Small outcrops of late Silurian to Early Devonian sedimentary and volcanic rocks are found in eastern Mull. On Arran, the country rocks around the Palaeogene intrusions include both Lower and Upper Old Red Sandstone sandstones and conglomerates of late Silurian to Early Carboniferous age, deposited by rivers draining from the north and north-east. The Old Red Sandstone deposits, together with the Lower Carboniferous sandstones, siltstones, limestones and basaltic lavas on Arran, all form part of the Midland Valley Terrane (Cameron and Stephenson, 1985; P914119). Elsewhere, Carboniferous rocks are not common. An inlier of Upper Carboniferous rocks is present on Morvern, and it has been suggested that Carboniferous rocks occur in the offshore basins (Fyfe et al., 1993). A few dykes of Late Carboniferous to Early Permian age cross the region, and quartz-dolerite plugs, for example in Morvern, may also date from this time.
Major sedimentary basins were initiated towards the end of the Palaeozoic, and continued to develop during the Mesozoic and into the Cainozoic. The basins or troughs trend approximately north–south, and include the Sea of the Hebrides–Little Minch Trough, the Inner Hebrides Trough and Blackstones Basin and the Colonsay Basin (P914123). During the Permian and Triassic periods, thick sequences accumulated in these troughs, consisting of both desert and fluviatile sandstones. The Jurassic Period was characterised by shallow water sedimentation, and up sequence shows a progressive change from brackish water to marine conditions. The basin margin deposits are found onshore at several places, where there is evidence that they were faulted, tilted and gently folded after Kimmeridgian times but prior to the deposition of Upper Cretaceous (Cenomanian to Middle Turonian) marine sandstones and limestones (chalk). Cretaceous rocks occur widely in the Inner Hebrides, but are typically very thin and laterally variable.
Regional uplift and ensuing erosion between the end of the Turonian and the early Paleocene (Danian) gave rise to a land surface on which the first Paleocene volcaniclastic rocks and basaltic lavas were deposited. Coarse clastic sedimentary rocks intercalated in the lower parts of the lava sequences provide indications that this land surface may have had significant relief locally. Shallow lakes, ponds and rivers were present and the basal beds of the Paleocene sequences are generally water-laid tuffaceous sandstones and siltstones, commonly with plant remains, and rare hyaloclastite deposits. These are overlain by thick, predominantly basaltic, subaerial lava successions of which the earliest appear to be those of Muck, Eigg and the basal members of the Mull succession. The eruption sites of the lavas are not known, but it is likely that they were fed from isolated vents, which developed along fissure systems now represented by the north-westtrending basaltic dyke swarms. Although evidence of dyke-related feeders is good in the Antrim Lava Field of Northern Ireland (Preston, 1982), it is less clear in western Scotland.
Several well-defined north-west-trending regional dyke swarms cross the region, intensifying in the vicinity of the central igneous complexes. The swarms reflect a regional northeast–south-west extensional stress system (England, 1988) and many of the dykes may have fed lava flows. In addition, sills of dolerite and basalt occur widely, and extensive sill-complexes intrude the Mesozoic basin deposits.
The central complexes are the deeply eroded roots of major volcanoes. With a few exceptions, they postdate the preserved lava fields and overlapped in time with the intrusion of the related dyke swarms. The central complexes represent fundamental changes in the character of the igneous activity, from the widespread feeders of the lava fields to much more localised and intense magmatism of shield volcanoes. There was a marked change too in the composition of the magmas. This is evident when the predominantly basaltic lava fields are compared and contrasted with the compositionally diverse intrusions of the central complexes, which include peridotite, olivine-gabbro, granitic rocks and minor amounts of rock of intermediate composition. Within the central complexes, granites generally form a high proportion of the surface outcrop. However, surveys of the gravity and magnetic fields over the complexes show that each centre has a substantial core of dense basic and/or ultrabasic rocks and that, despite their considerable areal extent, the granitic rocks are relatively thin and superficial bodies.
The combined evidence of radiometric ages and palaeomagnetic determinations indicates that the individual lava fields probably accumulated in little over one million years and that the active life of individual central complexes may also have been as short as one million years (P914126).
The siting of the central complexes has long intrigued geologists. J E Richey of the Geological Survey was responsible for much of the earlier work on this subject in Scotland, and also examined several of the central complexes in Ireland (Richey, 1932; Preston, 2001). He highlighted the possible significance of the proximity of the central complexes to pre-Paleocene faults. Additional contributory factors have been suggested, including the intersection of major faults with intra-basin ridges of Precambrian rocks, the diapiric rise of bodies of granitic magma focussing subsequent basaltic magmatism, and the intersection of dyke swarms with intra-basin ridges, possibly facilitating the initiation of granite magmatism. When a view is taken of the whole of the Palaeogene igneous activity in the British Isles, the most striking feature is the similarity of the magma composition and emplacement style across a wide range of structural settings and terranes. This suggests a region-wide stress field and a widely available heat source. This heat source is generally attributed to proto-Iceland mantle plumes (e.g. White, 1988; White and McKenzie, 1989). When a plume impinged on the area, large amounts of relatively buoyant basaltic magma were formed. As this magma rose up to and through the crust, it became ponded at traps caused by relatively abrupt density changes across major structural discontinuities. Upward movement became progressively more influenced by the local geology and structure the closer the magma approached to the Earth’s surface. These factors facilitated differentiation and contamination of the magmas, and focussed their paths during ascent.
Throughout the Oligocene and Miocene epochs, significant weathering and erosion of the lava fields and central complexes occurred, with much of the detritus shed into sedimentary basins off north-west Scotland. Although Neogene sedimentary rocks are preserved offshore (Fyfe et al., 1993), the next major events recorded in the onshore geology are the Quaternary glaciations. Much of the area, including the sea bed, was covered by the extremities of the thick, Late Devensian ice cap that formed over the central and northern Highlands 29 000 to 14 700 years ago. Following the decay of this ice sheet, the higher mountains of Skye, Rum, Mull and Arran supported corrie and valley glaciers during the Loch Lomond Stadial, 12 500 to 11 500 years ago. The glaciations have left deposits of till, hummocky moraine and various glaciofluvial gravels. The accompanying changes in sea level are recorded in the raised marine beach deposits and marine notches that are common around the coastline (Plate 42). The preglacial topography was drastically modified during deep erosion caused by the local ice caps and glaciers. This contributed to the excellent exposure now found in many of the corries and cliffs of the islands, and is most dramatically expressed in the deep rock basins and jagged skyline of the Skye Cuillin (P521672). Following the melting and retreat of the glaciers, over-steepened slopes collapsed, forming screes and some of the most extensive landslips in the British Isles (P580491), several of which are active to the present day.