Palaeogene lava fields and associated sedimentary rocks, 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.|
Two main lava fields are preserved in western Scotland: the Mull Lava Field (Bailey et al., 1924; Kerr, 1995b) and the Skye Lava Field (Anderson and Dunham, 1966; Williamson and Bell, 1994). The lavas of Canna, Sanday and north-west Rum, comprising the Canna Lava Formation, are considered to be outliers of the Skye Lava Field (Emeleus, 1997). A sequence of somewhat older flows, termed the Eigg Lava Formation, is preserved on the islands of Eigg and Muck and on the south-east part of Rum (Emeleus, 1997). These older lavas may be outliers of the Mull Lava Field (see p. 69); in the same area, a much younger glassy porphyritic rhyodacite flow comprises the Sgurr of Eigg Pitchstone Formation (Emeleus, 1997). The distribution of the lavas is shown in P914127 and the various successions are summarised in Table 9.
Eruption of the lavas and various pyroclastic materials was predominantly subaerial, although during the initial stages of the volcanic activity, the contemporaneous land surface had local lakes and river systems, leading to rare eruptions of magma into water to produce pillow lavas and hyaloclastite breccias. The land surface was essentially a peneplain, but with some relief. Locally, higher ground was associated with the Moine Supergroup in the southwest of Mull and eastern Morvern, and with the Torridonian strata in central Skye and around the Rum Central Complex (Emeleus, 1985;1997; Williamson and Bell, 1994). On Rum, the relief probably increased as the central complex developed. Sedimentary beds within the early, Eigg Lava Formation are virtually free of clasts above granule grade, whereas cobbles and boulders derived from the Rum Central Complex and its surroundings are abundant in the conglomerates interbedded with lavas of the younger Canna Lava Formation. It is likely that a similar situation pertained in Skye as the Skye Central Complex developed.
The climate during the Palaeogene, as indicated by the palynoflora, was generally warm temperate, and marked a period of significant cooling following the tropical conditions of the Late Cretaceous (e.g. Jolley, 1997). Where Upper Cretaceous (Maastrichtian) chalk is still preserved below the lavas, for example in west and south-west Mull and on the Morvern peninsula, residual deposits of ‘clay with flints’ are found together with a karstic surface with a relief of up to several metres. Elsewhere, further evidence of pre-lava relief is provided where Palaeogene, coal-bearing strata and channel-fill conglomerates typical of fluviatile environments are preserved locally within small pockets between the lavas.
The initial phases of volcanic activity in both the Skye and Mull lava fields were marked by the eruption of basaltic scoria from fissure-controlled cones, and by the extrusion of lavas into shallow water with the formation of hyaloclastite deposits (P580460). Magma was also intruded as shallow sills into unconsolidated, water-saturated sediments (mainly silt and mud) of Palaeogene age. On Mull, the presence of basaltic pillow lavas has been used to infer the presence of a water-filled caldera during the development of a later central volcano (Bailey et al., 1924).
Pauses in the predominantly subaerial volcanic activity are marked in the lavas by the development of weathered tops that pass up into relatively thick and laterally continuous red (oxidised) palaeosols (boles). During these periods, lacustrine sediments (clay, silt and peaty mire) accumulated in shallow depressions on the land surface, together with fluviatile channel-fills that formed condensed sequences of conglomerate, sandstone, siltstone and coal. These sedimentary sequences have been used to subdivide the lavas of Skye and Rum into a number of coherent formations (Anderson and Dunham, 1966; Emeleus, 1985, 1997; Williamson and Bell, 1994; Tables 11; 12; 14). On Mull and Morvern, the scarcity of sedimentary beds or other distinctive stratigraphical markers has so far limited subdivision of the lavas (Table 15). The conglomerates within the lava sequences also provide provenance information for the periods during which active erosion occurred. Locally, thick accumulations of debris formed during construction of the lava fields, for example at Maclean’s Nose, south of Ben Hiant on the Ardnamurchan peninsula (Richey and Thomas, 1930), and at Compass Hill on the east side of Canna (Harker, 1908; Emeleus, 1997).
The original limits of the lava fields are not easily estimated. However, Preston (1982) pointed out that the coastal cliffs, up to 300 m high, which mark the onshore truncation of the lava piles on, for example Skye and Mull, taken together with the horizontal attitude and thickness of the lava piles, suggests that they extended at least 50 km beyond their present-day limits. Extensive areas of basaltic lavas are known to exist beneath the sea south-west of Skye and west and north-west of Mull (P914127).