Discussion on ‘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, London , 176, 830–846

Abstract

Ken Amor's recognition that the Stac Fada Member is an impact deposit (Amor et al. 2008) proved a game-changer. I planned only to collect a piece on a holiday visit in 2011 but have since walked the entire outcrop and examined every exposure on multiple occasions. My observations extend far beyond what was outlined in my original paper (Simms 2015) and I recognize errors in the latest contribution by Amor et al. (2019). Our opinions differ on several issues: cited thicknesses for the Stac Fada Member, the unshocked nature of spallation blocks and the erosive–non-erosive contact beneath the impact deposit. I have not observed any north–south decrease in melt clast size along the outcrop. Amor et al . dismiss the Lairg Gravity Low as a possible impact crater based on palaeoenvironments, yet the cited paper specifically states ‘the Stoer Group has nothing in common with any of the Moine sequences discussed here’ (Friend et al. 2003, p. 254). I maintain that the Lairg Gravity Low is a convincing (although unproven, hence the question mark in the title of my 2015 paper) candidate for a buried impact crater. Stac Fada Member intrusions into the underlying sandstones at Stoer are critical to locating the impact. A conspicuous example (Fig. 1a) ‘appears to have travelled from a southerly direction’ (Amor et al. 2019, fig. 2b and c), yet viewed from the south (Fig. 1b) is clearly …