Eigg Lava Formation, 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).
Eigg Lava Formation
The Eigg Lava Formation comprises the sequence of lavas that crops out on Eigg and Muck and, as down-faulted slivers, in south-east Rum (Allwright, 1980; Emeleus, 1997; Table 9). The lavas are intruded by a north-west-trending dyke swarm (Chapter 7), which in turn is cut by the Rum layered intrusions (Chapter 9). Consequently, it may be deduced that the eruption of the Eigg Lava Formation predates at least part of the Rum Central Complex. Precise radiometric ages of 60.6 Ma both for trachytic tuffs interbedded with the basal lavas on Muck (Chambers, 2000; Chambers et al., 2005) and for the Rum Layered Suite (Hamilton et al., 1998) indicate that all of this happened in a very short time (P914126).
The lavas on Eigg lie unconformably on the Upper Cretaceous Strathaird Limestone Formation and the Middle Jurassic Great Estuarine Group. The volcanic succession is about 400 m thick and dips gently towards the south-west. The lowest units, exposed in north and north-east Eigg, are tuffs, typically only a few centimetres thick. The majority of the flows are of alkali olivine basalt and hawaiite. Mugearites occur especially about 50 m above the base of the formation (P914128), and near the top there is a distinctive group of feldspar-phyric basaltic hawaiite flows. Interspersed with the lavas are thin, massive, red to orange mudstones and siltstones, some of which are of volcaniclastic origin. Fluviatile deposits are rare, but a thin, laterally impersistent pebbly conglomerate occurs on the foreshore west of the old pier in the south-east of Eigg.
The volcanic succession on Muck comprises about 140 m of near-horizontal or gently north-westdipping lavas. They are intruded, as on Eigg, by many members of the north-north-west-trending Muck Dyke Swarm (Chapter 7). The lowest units comprise an assemblage of laterally impersistent olivine basalt, feldspar-phyric basaltic hawaiite, hawaiite and mugearite lavas and sedimentary deposits, which overlie the Middle Jurassic Kilmaluag Formation at Camas Mór. The higher flows are mainly of olivine basalt but with a flow of feldsparphyric basalt near the summit of Beinn Airein (P914128). Bright red to orange mudstones are commonly interleaved with the lavas, especially near the base of the succession. Some represent transported material, whereas others grade downwards into reddened flow tops and most likely developed by in-situ lateritic weathering. An example of the former crops out south-east of Port Mór (P580462), where 20 to 30 cm of red interflow siltstone overlies the reddened, brecciated pahoehoe top of an olivine basalt lava. The crystal component of the siltstone (sanidine, clinopyroxene, sphene) was derived from an explosive eruption of peralkaline trachytic magma, during the early stages of development of the lava sequence (Emeleus et al., 1996a). Radiometric age determinations (see also Pearson et al., 1996; Chambers, 2000; Chambers et al., 2005; P914126), obtained from the sanidine crystals are considered to accurately date the start of igneous activity in the Hebridean Igneous Province at 60.6 Ma.
A coarse deposit, termed the Camas Mór Breccia, crops out beneath shoreface boulders at the east end of Camas Mór. It is dominated by clasts, generally up to 30 cm across, of grey and brown limestone (Duntulm and Kilmaluag formations, respectively), in a matrix consisting of much smaller clasts of black carbonaceous siltstone and grey and brown impure limestone. Locally, the breccia contains prominent, thin interbeds rich in quartz granules. The bedding is deformed beneath particularly large limestone clasts, 0.5 to more than 2 m in diameter. The breccia is generally lacking in igneous clasts and predates the lavas (Emeleus, 1997).
Metamorphosed and fractured basalt, intruded by north-west-trending basaltic dykes, occurs as a fault-bound mass, about 1 km in length, within the Main Ring Fault on the south-east slopes of Beinn nan Stac, and on the north-east side of Dibidil (Emeleus, 1997). The basalt unconformably overlies Lower Jurassic Broadford Beds (Smith, 1985) and is considered to be a remnant of the Eigg Lava Formation.