Skye Lava Group, 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 principal lava sequences are summarised in P914128 and Table 9. Details are given in Tables 11–15. They are:
No lavas occur onshore in the St Kilda archipelago and none are seen on Arran apart from the foundered masses within the Central Arran Ring-complex (p. 137).
Skye Lava Group
The Skye Lava Group crops out in north and west-central Skye, borders much of the Skye GROUP Central Complex, and extends south to the islands of Canna, Sanday and north-west Rum (P914127). The position of the Skye Lava Group in the igneous stratigraphy of the Hebridean Igneous Province is considered in Chapter 5. The various parts of the group are not easily correlated, but detailed successions for the lavas of west-central and northern Skye, Strathaird (Skye), and for north-west Rum and Canna are given in P914128 and Tables 11, 12, 13 and 14.
The lavas of west-central Skye are divided into a number of formations and members on the basis of distinct lithological associations, originally termed groups and formations by Williamson and Bell (1994). Interlava sedimentary units have been used to separate the stratigraphical divisions (P914128; Table 11). Numerous faults dissect the predominantly flat-lying flows. The faults, many with only small displacements, are typically orientated parallel, normal and slightly oblique to the north-west-trending Skye Dyke Swarm (see Chapter 7).
The stratigraphically lowest sequence in the group forms the Rubh’ an Dùnain Formation south-east of Loch Brittle. On the north shore of Soay Sound its basal deposits, the An Leac Member, rest upon an irregular pre-lava land surface cut across Upper Cretaceous, Lower Jurassic, Triassic and Torridonian sedimentary rocks. The member is about 10 m thick, and comprises laterally impersistent units of volcanic breccia, lapilli-tuff and hyaloclastite. Large clasts and rafts of pre-lava lithologies occur within the volcaniclastic deposits. This initial hydro-magmatic phase of activity also occurred in north Skye (see below), suggesting that the early Paleocene land surface was covered with shallow, water-filled depressions into which the magmas were erupted. The first lavas are intercalated with lacustrine mudstones and siltstones in the overlying Meacnaish Member. The succeeding Bualintur Formation, which crops out on the west side of Loch Brittle, may possibly interdigitate with the upper part of the Rubh’ an Dùnain Formation. Interbedded with the lavas of this formation are deposits of heterogeneous, poorly sorted stratiform volcaniclastic breccia, conglomerate and sandstone, for example in the inaccessible sea cliffs around Sgurr an Duine.
Overlying the Rubh’ an Dùnain and Bualintur formations, the Minginish Conglomerate Formationcomprises three distinctive, geographically separated members:Culnamean, Allt Geodh’ a’ Ghamhna and Allt Mòr (P914129). All comprise volcaniclastic conglomerate– sandstone–siltstone sequences together with subsidiary mudstones and coals; the conglomerates contain ‘exotic’ clasts. Plant remains are widespread and, in places, build up into leaf-rich beds. These sedimentary sequences are typical of fluvial environments, and in particular those recorded from braided river systems. Sandy, matrix-supported conglomerates and associated pebbly sandstones at the base of the formation were most likely the products of mass-flow or debris-flow processes, initially triggered by volcanotectonic activity, and modified by high water-flow regimes, possibly during flash-flooding. Clasts of granite and porphyritic felsite were derived from unroofing of the uplifted massif overlying the Rum Central Complex that lay to the south. Quartzite, green gritty sandstone and feldspathic sandstone were derived from the east, sourced from an uplifted block to the east of the Camasunary–Skerryvore Fault composed of various Torridonian and Cambrian lithologies associated with the Moine Thrust Belt.
Higher in the sequence, the Eynort Mudstone Formation was deposited during another hiatus in the volcanic activity. Following a period of intense weathering of the lavas, mud and volcaniclastic material transported by the combined effects of rain-wash and wind action were deposited in shallow pools and lakes. Four separate outcrops, all from the same stratigraphical level within the lava pile, have been afforded member status.
In the upper part of the sequence, the Gleann Oraid Formation is dominated by evolved flows, including trachytes and a benmoreite. The initial flows of the lowermost Arnaval Member vary in their nature and thickness and may have filled in topographical depressions produced prior to their eruption. The Sleadale Member contains at least one thick trachyte flow of limited lateral extent, possibly indicating an original dome-like form. The trachytic tuff at the base of the member contains exceptionally fresh anorthoclase and biotite crystals from which an Ar-Ar age of 58. 91 Ma has been obtained (Bell and Williamson, 2002; Table 8). The thick trachyte flow forming the Cnoc Scardall Member occupies a broad palaeovalley excavated out of the subjacent flows of the Skridan and Arnaval members. Close to the presumed base of the Gleann Oraid Formation at Dùn Ard an t-Sabhail is a composite hawaiite flow with a nearly aphyric base and an upper plagioclase-macrophyric unit. Similar composite flows have been identified from the Beinn Totaig Formation of north Skye (see below; Anderson and Dunham, 1966) and it is possible that flows of the two formations (Gleann Oraid and Beinn Totaig) may be interleaved and may have been erupted more or less simultaneously. In the area south-west of Fiskavaig, flows belonging to the lower part of the Gleann Oraid Formation are intercalated or interdigitated with hawaiites and mugearites belonging to the laterally restricted Loch Dubh Formation, which crops out on the northern and western slopes of Beinn nan Dubh Lochan.
The Preshal Beg Conglomerate Formation, which crops out around the base of Preshal Beg and on the north-east slopes of Preshal More, overlies the Arnaval and Sleadale members of the Gleann Oraid Formation (P580466). It comprises a thick, laterally restricted sequence of heterogeneous volcaniclastic material with clasts of local derivation; it is interpreted as talus and alluvial fan deposits that accumulated rapidly within a restricted but broad valley system.
The upper part of the formation is invaded by irregular apophyses (lava pillows, globe breccias and ‘neptunian’ dykes) of the overlying tholeiitic olivine basalt flows of the Talisker Formation, suggesting that the sediments were not lithified prior to eruption of the lavas.
The Talisker Formation comprises two flows of tholeiitic olivine basalt (Tables 9; 11) that were originally impounded in steep-sided canyons and now form the twin hills of Preshal More and Preshal Beg, south-east of Talisker Bay.
In northern Skye, volcanic activity commenced with the deposition of the Portree Hyaloclastite Formation onto flat-lying Middle Jurasssic strata. The formation comprises palagonitised hyaloclastite breccias (P580460), locally developed pillow lavas, and a variety of volcaniclastic sedimentary rocks (sandstone, siltstone and mudstone). These hydromagmatic rocks are exposed around Portree and at other localities on the east side of the Trotternish peninsula (Anderson and Dunham, 1966). Elsewhere in north Skye, for example in the Staffin, Flodigarry, Uig and Loch Bay areas and in the Lealt River of the Trotternish peninsula, there are scattered and poorly exposed occurrences of tuffaceous sandstone and associated strata.
The remainder and vast majority of the Skye Lava Group in north Skye is composed of lavas erupted into a subaerial environment. Pauses in the volcanic activity are marked by the development of interlava sedimentary sequences, ranging from conglomerate and sandstone to plant-bearing mudstone and siltstone. None of the recognised sequences appears to be laterally persistent, leading to problems with correlation and stratigraphical subdivision (England, 1994). However, the scheme set out by Anderson and Dunham (1966) offers a useful and approximate device for the recognition of the overall structure (Table 12). The lavas belong to the alkali olivine basalt–hawaiite–mugearite–benmoreite–trachyte series and are referred to by Thompson et al. (1972) as belonging to the Skye Main Lava Series. No simple correlations between composition and stratigraphical position exist. The evolved flows, comprising about 20 per cent of the total thickness, are intercalated with the more common alkali olivine basalt lavas, possibly becoming more abundant at higher levels in the lava sequence. The entire Formation preserved cumulative thickness of about 1200 m of lavas gives rise to impressive east-facing escarpments on the Trotternish peninsula, together with inland plateaux and coastal cliffs. The most spectacular sequences comprise the cliffs tracable from The Storr, north to Quiraing, inland from Staffin Bay (P580491).
The lowest of the lava units, the Beinn Edra Formation crops out on the Trotternish peninsula in north-east Skye. The most complete section forms The Storr, where at least 24 flows can be identified. Here, and elsewhere on the east side of the outcrop, extensive rotational landslips and rock failures obscure the base of the formation (p. 168; 176), although the lower part is seen at Uig on the west side of the peninsula. To the south of Loch Portree, lavas belonging to this formation can be traced south to Tianavaig Bay and Ollach. The top of the formation is defined by the conglomerate–sandstone–siltstone–mudstone sequence in Glen Tungadal, about 7 km east of Bracadale.
The Ramasaig Formation crops out on the Duirinish and Waternish peninsulas of northwest Skye. It overlies Middle Jurassic strata west of Waterstein Head. The formation consists of porphyritic and non-porphyritic olivine basalts. The top of the formation is separated from the Osdale Formation by trachytic tuff, and by sandstones and plant-bearing fissile sandstones exposed in the Hamra River, Glendale.
The Beinn Totaig Formation has a type locality north-east of Loch Harport, which includes Mugeary, the type locality for mugearite (Harker, 1904). Within this formation are composite flows of hawaiite and mugearite; an aphyric portion at the base is separated by a sharp interface from the main plagioclase-macrophyric portion (Harker, 1904). They may correlate with a similar flow close to the presumed base of the Gleann Oraid Formation in west-central Skye (see above). The top of the Beinn Totaig Formation is taken at the base of the Glen Osdale sedimentary sequence, deposited during a pause in the volcanic activity. However, as pointed out by England (1994), the Glen Osdale deposits are about 250 m below the top of the formation as described by Anderson and Dunham (1966) close to the summit of Healabhal Mhòr. The Glen Osdale deposits comprise about 8 m of conglomerate containing cobbles of red-weathering Torridonian gritty sandstone, rare cobbles of gneiss, and lesser amounts of granite and porphyritic felsite, which do not, however, match clasts found elsewhere on Skye that were derived from the Rum Central Complex. Clasts of locally derived lava lithologies are also present. Interbedded siltstones were deposited in a lacustrine or marginal floodplain environment and contain well-preserved leaf impressions of oak, hazel and plane (Anderson and Dunham, 1966). The plant-bearing, inter-lava beds at Hamra River, Glen Osdale, Forse River, and Red Burn east of Edinbane were considered by Anderson and Dunham (1966, fig. 13) to be lateral equivalents.
The relatively evolved lavas of the Bracadale Formation crop out north-east of Bracadale. With increasing compositional evolution the lateral extent of individual flows diminishes, with the trachytes being particularly restricted. The flows appear to occupy a broad north-westtrending palaeovalley, with a preserved width of about 15 km and a depth of about 300 m, eroded into the underlying lavas. The valley trends parallel to the Skye Dyke Swarm (England, 1994).
The youngest lavas preserved in north Skye (Osdale Formation) crop out west of Bracadale and form many of the islands in Loch Bracadale. They consist mainly of basalt and mugearite.
Other lavas on Skye
There are large areas of basaltic lava in Glen Drynoch, north of Loch Harport and on either side of Loch Sligachan. They form part of the Skye Lava Group, but their equivalence to particular formations described above has not been established. Likewise, isolated areas of basaltic lava occur around the periphery of the Western and Eastern Red Hills granite centres and on the south-east margin of the Cuillin Centre. Of the latter, the most extensive is the lava succession that overlies Jurassic and thin, impersistent Upper Cretaceous strata east of the Camasunary–Skerryvore Fault on Strathaird. Up to 290 m of predominantly olivine basalt lavas, including picritic flows, are preserved on An Stac, where a lava stratigraphy was established by Almond (1964) (Table 13). The lavas show pervasive hydrothermal alteration and a progressive increase in thermal metamorphism over about 1500 m adjacent to the Cuillin Centre, culminating in a sanidinite-facies assemblage being developed immediately adjacent to its margin, as evidenced by the presence of cordierite, sanidine and possibly mullite in altered boles (Almond, 1964). The lavas and underlying strata occur in open folds concentric about, and related to the emplacement of, the Cuillin Centre. The small area of olivine basalt that forms the summit of Dùn Caan on Raasay is an outlier of the north Skye lava pile.
Olivine basalt (alkali and transitional) and tholeiite basalt, together with their evolved differentiates, occur within the lavas of the Canna Lava Formation and are interleaved with fluviatile conglomerate–sandstone sequences, thus permitting local subdivision into a number of recognisable members (Emeleus, 1985, 1997; P914128; Table 9). Eruption of the lavas and deposition of the sediments occurred on a hill-and-valley topography, graphically illustrated in north-west Rum. The main drainage system flowed north from a highland area initially created during uplift associated with the emplacement and growth of the Rum Central Complex (Williamson and Bell, 1994; Emeleus, 1997). Flows on Canna and Rum have been dated at 60.0 Ma (Chambers et al, 2005).
Canna and Sanday
On Canna and Sanday, the lava pile is subhorizontal, with dips of 3° or less. The lowest flows crop out in south-east Sanday, and the uppermost on the high ground of east and west Canna (P914128). The formation comprises at least 200 m of lavas, predominantly of the alkali olivine basalt–hawaiite–mugearite lineage. Individual lavas and sedimentary units show considerable variation in thickness along strike. In general, the stratigraphically higher flows are more evolved and strongly feldspar-phyric basaltic hawaiites; hawaiites form thick, continuous flows on the higher parts of Canna. Lava accumulation was predominantly in a subaerial environment. Many of the flows have brecciated a’a flow tops, a few of which are reddened as, for example on the wave-cut platform between Tarbert and Cùil a’ Bhainne in eastern Canna. At Cùil a’ Bhainne, lavas were erupted into shallow water with the local formation of pillowed flows and hyaloclastite deposits.
Inter-lava fluviatile breccia–conglomerate–sandstone sequences are a prominent feature of the formation. On Canna, they are thickest on Compass Hill, totalling about 50 m, but thin out westwards along the north coast. Sedimentary breccias at Compass Hill contain basalt blocks up to 2 m across and smaller blocks of red feldspathic sandstone. The more widespread conglomerates contain well-rounded cobbles and pebbles of locally derived lava lithologies, red feldspathic sandstone resembling the Torridon Group on Rum, rare schist and gneiss, and porphyritic rhyodacite and granophyric microgranite identical with rock-types within the Rum Central Complex (Emeleus, 1973). No clasts of Mesozoic rocks are known, although Lower Jurassic limestones occur offshore of north-east Canna (Fyfe et al., 1993). Poorly preserved carbonised plants are found in the sedimentary intercalcations at a number of localities.
The lavas and conglomerates of the Canna Lava Formation in north-west Rum (P914128; Table 14) were impounded within a system of steep-sided palaeovalleys. The remains of valleys eroded in Torridon Group sandstones crop out north-east of Fionchra and also on Bloodstone Hill. In the east face of Bloodstone Hill, the cross-section of one valley is infilled with two flows of tholeiitic andesite and underlying conglomerates (P580467). One side of a valley eroded in the Western Granite is recognised on the north side of Orval. The presence of up to 60 m of hyaloclastite breccia and pillowed lava at the base of the Upper Fionchra Member on the north side of Fionchra indicates the former presence of a shallow lake impounded in a valley. The underlying sedimentary rocks include siltstones with delicate leaf impressions, from which material for palynological dating has been obtained (Jolley, 1997; see Chapter 5). Carbonised logs up to 1 m in length occur in conglomerate at Maternity Hollow, east of Fionchra. Secondary minerals in the lavas include the well-known bloodstone and banded agate found in the basaltic andesite flows on Bloodstone Hill (p. 153; 172).
The conglomerates of north-west Rum provide an excellent example of the palaeogeographical information that may be derived from deposits of this type (e.g. Emeleus, 1973). Four groups of clasts are recognised:
- Lewisian gneisses and Torridon Group sandstones derived from the country rocks
- rocks not found on Rum, including quartz-dolerite and vesicular tholeiitic basalt (?lava), and rare psammites (possibly Moine Supergroup)
- rocks from the Rum Central Complex (granophyre and microgranite, porphyritic rhyodacite, bytownite troctolite, gabbro, igneous breccias); the absence of peridotite clasts is attributed to the physical instability of the olivine-rich rocks
- in the younger beds, lava clasts derived from earlier lava flows in the formation
The abundance of clasts derived from the Rum Central Complex provides unequivocal evidence of rapid uplift and unroofing of the central complex during the Paleocene, and its continued vigorous erosion during accumulation of the lavas; the configuration of the deposits shows that they were laid down mainly from fast-flowing streams and rivers in rapidly evolving, steep-sided valleys and, less commonly, in small lakes.