Tag Archives: Scotland

A major Precambrian impact in Scotland

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The northwest of Scotland has been a magnet to geologists for more than a century. It is easily accessed, has magnificent scenery and some of the world’s most complex geology. The oldest and structurally most tortuous rocks in Europe – the Lewisian Gneiss Complex – which span crustal depths from its top to bottom, dominate much of the coast. These are unconformably overlain by a sequence of mainly terrestrial sediments of Meso- to Neoproterozoic age – the Torridonian Supergroup – laid down by river systems at the edge of the former continent of  Laurentia. They form a series of relic hills resting on a rugged landscape carved into the much older Lewisian. In turn they are capped by a sequence of Cambrian to Lower Ordovician shallow-marine sediments. A more continuous range of hills no more than 20 km eastward of the coast hosts the famous Moine Thrust Belt in which the entire stratigraphy of the region was mangled between 450 and 430 million years ago when the elongated microcontinent of Avalonia collided with and accreted to Laurentia.  Exposures are the best in Britain and, because of the superb geology, probably every geologist who graduated in that country visited the area, along with many international geotourists. The more complex parts of this relatively small area have been mapped and repeatedly examined at scales larger than 1:10,000; its geology is probably the best described on Earth. Yet, it continues to throw up dramatic conclusions. However, the structurally and sedimentologically simple Torridonian was thought to have been done and dusted decades ago, with a few oddities that remained unresolved until recently.

NW Scotland geol

Simplified geological map of NW Scotland (credit: British Geological Survey)

One such mystery lies close to the base of the vast pile of reddish Torridonian sandstones, the Stac Fada Member of the Stoer Group. Beneath it is a common-or-garden basal breccia full of debris from the underlying Lewisian Complex, then red sandstones and siltstones deposited by a braided river system. The Stac Fada Member is a mere 10 m thick, but stretches more than 50 km along the regional NNE-SSW  strike. It comprises greenish to pink sandstones with abundant green, glassy shards and clasts, previously thought to be volcanic in origin, together with what were initially regarded as volcanic spherules – the results of explosive reaction of magma when entering groundwater or shallow ponds. Until 2002, that was how ideas stood. More detailed sedimentological and geochemical examination found quartz grains with multiple lamellae evidencing intense shock, anomalously high platinum-group metal concentrations and chromium isotopes that were not of this world. Indeed, the clasts and the ensemble as a whole look very similar to the ‘suevites’ around the 15 Ma old Ries Impact crater in Germany. The bed is the product of mass ejection from an impact, a designation that has attracted great attention. In 2015 geophysicists suggested that the impact crater itself may coincide with an isolated gravity low about 50 km to the east. A team of 8 geoscientists from the Universities of Oxford and Exeter, UK, have recently reported their findings and ideas from work over the last decade. (Amor, K, et al. 2019. The Mesoproterozoic Stac Fada proximal ejecta blanket, NW Scotland: constraints on crater location from field observations, anisotropy of magnetic susceptibility, petrography and geochemistry. Journal of the Geological Society, online; DOI: 10.1144/jgs2018-093).

The age of the Stac Fada member is around 1200 Ma, determined by Ar-Ar dating of K-feldspar formed by sedimentary processes. Geochemistry of Lewisian gneiss clasts compared with in situ basement rocks, magnetic data from the matrix of the deposit, and evidence of compressional forces restricted to it suggest that the debris emanated from a site to the WNW of the midpoint of the member’s outcrop. Rather than being a deposit from a distant source, carried in an ejecta curtain, the Stac Fada material is more akin to that transported by a volcanic pyroclastic flow. That is, a dense, incandescent debris cloud moving near to the surface under gravity from the crater as ejected material collapsed back to the surface. On less definite grounds, the authors suggest that a crater some 13 to 14 km across penetrating about 3 km into the crust may have been involved.

Together with evidence that I described in Impact debris in Britain (Magmatism February 2018) and Britain’s own impact  (Planetary Science November 2002) it seems that Britain has directly witnessed three impact events. But none of them left a tangible crater.

Impact debris in Britain

These days reports of geological evidence for asteroid impacts are not regarded with a mixture of disbelief, wonder and foreboding: well, not by geologists anyway. But for such a small area as Britain now to have three of widely different ages and in easily accessible places is pretty good for its brand as the place to visit for practically every aspect of Earth history. The first to be discovered lies at the base of Triassic mudstones near Bristol (see Britain’s own impact) and would need some serious grubbing around at a former construction site. The next to emerge was located in one of the best geological districts in the country at several easily accessed coastal exposures in Northwest Scotland. A glass-rich ejecta layer occurs in the basal Torridonian Stoer Group on Stac Fada, Stoer, Sutherland (UK National Grid Reference 203300, 928400). The most recently found (Drake, S.N. and 8 others 2018. Discovery of a meteoritic ejecta layer containing unmelted impactor fragments at the base of Paleocene lavas, Isle of Skye, Scotland. Geology, v. 46, p. 171-174; doi:10.1130/G39452.1) is on the Inner Hebridean island of Skye at the base of its famous Palaeocene flood basalt sequence (UK National Grid Reference 155371,821112).

View to the northwest across Loch Slapin to the Cuillin Hills of Skye (Central Igneous Complex). The flood basalts beneath which the ejecta layer occurs are just above the trees. (Credit: Wikipedia)

The last is perhaps the most spectacular of the three, as it contains the full gamut of provenance, matched only by material from the drill core into the 66 million year-old Chicxulub crater. The 0.9 m thick debris layer rests directly on mid-Jurassic sandstones beneath Palaeocene basalts of the North Atlantic Igneous Province (NAIP). The layer contains a basalt clast dated at 61.54 Ma, but is dominantly reminiscent of a pyroclastic ignimbrite flow as it contains glass shards. But there the resemblance ends for the bulk of small clasts are of quartz and K-feldspar, sandstone and gneiss. Zircons extracted from the debris show shock lamellae and give Archaean and Proterozoic ages commensurate with the local basement, but also with the bulk of the Scandinavian and Canadian Shields. So the impact could have been anywhere in such widespread terrains, although the enclosed basalt narrows this down to areas where basement is overlain by lavas of the NAIP. The Skye impactite contains unmelted meteorite fragments in the form of titanium nitrides alloyed with vanadium and niobium, metallic iron-silicon alloy containing exsolved carbon, and manganese sulfide.

Although it may be coincidental, the situation of the ejecta layer immediately beneath the Skye lavas, its containing a clast of basalt whose age corresponds to the oldest flows anywhere in the NAIP is fascinating. But the actual impact site is, as yet, unknown. Even so, the layer provokes thoughts about whether an impact may have been more than spatially related to the large NAIP flood basalt pile, preserved on either side of the North Atlantic. If the event was large, then surely the ejecta should be preserved near the base of the flood basalts elsewhere in NW Britain and further afield