Discussion on ‘A reassessment of the proposed ‘Lairg impact structure’ and its potential implications for the deep structure of northern Scotland’ Journal of the Geological Society, London, 176, 817-829

Abstract

The interpretation of the Stac Fada Member of the Stoer Group of NW Scotland as a proximal ejecta blanket of late Mesoproterozoic age (Amor et al. 2008) has led to the search for the impact crater from which it originated. One prominent suggestion has been that this crater lies buried beneath east-central Sutherland, broadly centred on the village of Lairg (Fig. 1; Simms 2015). Simms & Ernstson (2019) develop this suggestion by interpreting the well-known negative anomaly that dominates the Bouguer gravity map of northern Scotland to represent the roots of this crater.However, they qualify their interpretation, noting that the geology at the Stac Fada outcrops implies a more distant crater location, and go on to propose hitherto unreported "crustal shortening structures" to explain this discrepancy. In doing so they challenge current understanding of structural evolution in the Moine Thrust Belt and for crustal structure in NW Scotland. The purpose of this discussion is to demonstrate not only that the Simms & Ernstson (2019) proposals are incompatible with the known structural geology of northern Scotland, but that the geology of the Moine Thrust Belt renders the Lairg site to be one of the very few places that can be regarded as a highly unlikely source for the Stac Fada impact rocks. Internal Earth processes, rather than extra-terrestrial interventions, should be sought as explanations for the "Lairg gravity low". A more general aim is to show, through this discussion, that making small changes to tectonic models commonly carry unintended consequences for geological interpretations that can go unrecognised if only illustrated on unscaled sketches. The Moine Thrust Belt as a crustal dredging machine textClick here to access/download;text file;Lairg discussion_2.docx

The Mesoproterozoic Stac Fada Member, NW Scotland: an impact origin confirmed but refined

“…1), northwest of the outcrop belt (Amor et al 2019), and the other is the Lairg Gravity Low (Fig. 1) (Simms 2015;Simms & Ernstson 2019), which, if correct, has significant implications for the deep crustal structure of northern Scotland (Butler & Alsop 2019;Simms 2019;Simms & Ernstson 2019).…”