Cara Wells discusses the origins of the Stac Fada impact ejecta.

The Importance of the Stac Fada Member in Understanding Asteroid Impacts

Giant asteroid impacts have occurred throughout Earth’s history, but, perhaps fortunately, have never been witnessed by humans. Therefore, understanding is limited to analogue modelling and geological investigations of the craters and ejecta deposits left behind.  Suevites are impact-melt-bearing ejecta deposits found within and around impact craters. How suevites form is poorly understood, and this limits our understanding of what happens when an asteroid (or comet) collides with a terrestrial planet. Several processes have been proposed to emplace suevite, ranging from atmospheric fall out from some sort of hot atmospheric plume caused by the impact (ref 1) to explosions caused by violent interaction between hot impact-melted rock with surface water (e.g. a lake) (ref 2).

Recently it has been suggested (ref 3,4) that suevites are deposits of ground-hugging ‘impactoclastic density currents’, similar to pyroclastic density currents that deposit ignimbrites around volcanoes. But how similar are they? Ongoing research on the Stac Fada suevite in the NW Highlands of Scotland, suggests very!, The research is being carried out in tandem with research on suevite at the world’s best preserved medium-sized impact crater, Ries, in Germany. The Stac Fada suevite was deposited following a large asteroid impact over a billion years ago and yet is one of the best-exposed examples of a subaerial outflow suevite ejecta blanket anywhere on Earth. Quantitative analysis of the particle shapes, grain size, sorting and lithofacies of the Stac Fada suevite are compared with suevites from Germany and Canada, and with well-studied ignimbrites at volcanoes, to explore similarities and differences, and what this means for our understanding of impact-ejecta emplacement and the formation of impact craters. The study emphasises the importance of geological investigations, and the continued value of Scottish geology, in understanding significant planetary events.

1. Engelhardt, W. von. and Graup, G., 1984, Suevite of the Ries crater, Germany: Source rocks and implications for cratering mechanics. Geologische Rundschau, v. 73, p.447-481.
https://doi.org/10.1007/BF01824968
2. Artemieva, N.A., Wünnemann, K., Krien, F., Reimold, W.U., and Stöffler, D., 2013, Ries crater and suevite revisited- Observations and modelling Part II: Modelling: Meteoritics & Planetary Science, v. 48, p. 590-627.
https://doi.org/10.1111/maps.12086
3. Siegert, S., Branney, M.J., and Hecht, L., 2017, Density current origin of a melt-bearing impact ejecta blanket (Ries suevite, Germany): Geology, v. 45, p. 855-858. https://doi.org/10.1130/G39198.1
4. Branney, M.J., and Brown, R.J., 2011, Impactoclastic density current emplacement of terrestrial meteorite-impact ejecta and the formation of dust pellets and accretionary lapilli: evidence from Stac Fada, Scotland: The Journal of Geology, v. 119, p. 275-292.
https://doi.org/10.1086/659147

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