Stac Fada Formation - possible cooling joints? 1

Stac Fada at the northern end of the Bay of Stoer, north west Scotland, is the type locality for the Stac Fada Formation, recently interpreted as a meteorite impact debris horizon, comparable with suevite from the much younger (Miocene, 15 Ma) Ries meteorite impact event in south Germany. The Stac Fada Formation lies within the Bay of Stoer Formation, mostly fluviatile sandstones and siltstones (Mesoproterozoic, c. 1200 Ma).

The Stac Fada Formation is about 12 metres thick at this location. The photo shows a part of the middle section and consists of a poorly bedded arkosic sand matrix within which there are dark green chloritised clasts and fragments which represent formerly molten debris blasted into the air from the original impact. There is a network of polygonal cracks visible on this 'bedding' surface and I theorise that these may be cooling joints formed as the presumably hot ejecta deposit cooled and contracted. (I have seen no reference to this particular feature in the literature).

The site of the meteorite impact is now correlated with a well-defined, roughly circular, low-gravity anomaly some 50 km to the east, near the village of Lairg, although any impact crater is now buried beneath younger rocks. SEE UPDATE BELOW.

For more details see:
Simms, M. J., 2015. The Stac Fada impact ejecta deposit and the Lairg Gravity Low: evidence for a buried Precambrian impact crater in Scotland?, Proceedings of the Geologists Association, 126 (2015), 742 - 761.

UPDATE June 2019:
Latest research based on detailed field observations, geochemical and geomagnetic work now seems to indicate that the impact site is located in the present day Minch, about 15 - 20 km WNW of Enard Bay. See:
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 176/4 (2019).
dx.doi.org/10.1144/jgs2018-093