A reassessment of the proposed ‘Lairg Impact Structure’ and its potential implications for the deep structure of northern Scotland by Michael J. Simms, and Kord Ernstson Journal of the Geological Society Volume ():jgs2017-161 June 9, 2019 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

Regional geology of northern Scotland showing the outcrop of the Stoer Group and its relationship to the residual gravity field (contoured at 2 mGal intervals; from Rollin 2009) for the Lairg Gravity Low. Regional geology of northern Scotland showing the outcrop of the Stoer Group and its relationship to the residual gravity field (contoured at 2 mGal intervals; from Rollin 2009) for the Lairg Gravity Low. Arrows indicate directional azimuths within the Stac Fada Member (from Simms 2015). Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

Gravity stations of the British Geological Survey in northern Scotland. Gravity stations of the British Geological Survey in northern Scotland. The rectangle centred with respect to the Lairg Gravity Low (cross) frames the stations used for the re-evaluation of the Lairg gravity field Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

Bouguer anomaly map for the framed gravity stations in Figure 1 displaying the roughly circular Lairg negative anomaly. Bouguer anomaly map for the framed gravity stations in Figure 1 displaying the roughly circular Lairg negative anomaly. The most northwesterly and southeasterly parts of the Bouguer map lack any gravity stations because of their locations in the sea. Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

A regional trend field computed from the Bouguer anomalies in Figure 3 by radical moving average low-pass filtering. A regional trend field computed from the Bouguer anomalies in Figure 3 by radical moving average low-pass filtering. Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

Bouguer residual anomalies for the Lairg gravity field, with contours at 1 mGal intervals. Bouguer residual anomalies for the Lairg gravity field, with contours at 1 mGal intervals. The residual field results from subtracting the regional field (Fig. 4) from the measured Bouguer field (Fig. 3). It should be noted that in the residual field the central negative anomaly is enclosed by a roughly circular ring of relatively positive anomalies. The dashed red line is a circle of radius 25 km centred on the gravity low. Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

Three dashed lines selected for diametric gravity profiles across the Lairg gravity minimum (see Fig. 5). Three dashed lines selected for diametric gravity profiles across the Lairg gravity minimum (see Fig. 5). From the circle a diameter of nearly 50 km for the ring of positive anomalies can be deduced. Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

The Bouguer gravity profiles taken from the Bouguer map in Figure 6 revealing roughly similar shape with regard to the central negative anomaly and the surrounding ring of relatively positive anomalies that are only faintly developed on the NNW–SSE profile. The Bouguer gravity profiles taken from the Bouguer map in Figure 6 revealing roughly similar shape with regard to the central negative anomaly and the surrounding ring of relatively positive anomalies that are only faintly developed on the NNW–SSE profile. Also noteworthy is the ring of gravity lows (arrowed) beyond the suggested peak ring strongly supporting the peak or inner ring character of an originally much larger impact structure. Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

Results of 2.5D model calculation of a very simple two-layer density model for the NW–SE and the SW–NE gravity profiles. Results of 2.5D model calculation of a very simple two-layer density model for the NW–SE and the SW–NE gravity profiles. For reasons of simplicity, and because effectively the gravity evaluation lacks any ‘true’ zero level, a constant regional shift of −10 mGal has been applied to the curves in Figure 7. Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

Rough approximation of the mass deficit related to the central anomaly by a spherical segment of −0.15 g cm−3 density deficit. Rough approximation of the mass deficit related to the central anomaly by a spherical segment of −0.15 g cm−3 density deficit. Model without vertical exaggeration. Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

Gravity residual anomalies of impact structures of various sizes compared with the Lairg residual anomaly. Gravity residual anomalies of impact structures of various sizes compared with the Lairg residual anomaly. Sources: Chicxulub, modified from Hildebrand et al. (1998); Manicouagan, modified from Sweeny (1978); Popigai, modified from Pilkington et al. (2002); Lairg, this paper; Rochechouart, modified from Schmidt (1984); Ries, digitized from gravity map of Kahle (1969). Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

Some peak-ring impact structures with an approximate double ratio of crater/peak-ring diameter. Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

Gravity-derived mass deficits of terrestrial impact structures as a function of diameter. Gravity-derived mass deficits of terrestrial impact structures as a function of diameter. Modified and supplemented from Ernstson & Fiebag (1992); with data from Pohl et al. (1978) and references therein; Ernstson (1984). Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved

Schematic sequence of the main events necessary to accommodate a 100 km diameter impact crater centred upon the Lairg Gravity Low. 1170 Ma: a recently formed c. 100 km diameter peak-ring impact crater, surrounded by a thick and extensive ejecta deposit blanketing the sediments of the Stoer Group, is filled with lake sediments above a primary fill of impact breccia and impact melt. 1000 Ma: prolonged erosion, prior to deposition of the Diabaig Formation (Neoproterozoic, Torridon Group), removes the outer rim of the crater and much of the ejecta deposit and Stoer Group. Schematic sequence of the main events necessary to accommodate a 100 km diameter impact crater centred upon the Lairg Gravity Low. 1170 Ma: a recently formed c. 100 km diameter peak-ring impact crater, surrounded by a thick and extensive ejecta deposit blanketing the sediments of the Stoer Group, is filled with lake sediments above a primary fill of impact breccia and impact melt. 1000 Ma: prolonged erosion, prior to deposition of the Diabaig Formation (Neoproterozoic, Torridon Group), removes the outer rim of the crater and much of the ejecta deposit and Stoer Group. Just a remnant survives in a downfaulted block far to the west. 400 Ma: during the Caledonian Orogeny thin-skinned thrusting emplaces westwards a thick cover of Moinian metasediments across the region. Thick-skinned thrusts extend deep into the Lewisian basement, perhaps nucleated on fractures associated with the original impact, and translocate the impact crater tens of kilometres westwards to its present position relative to the Stoer Group outcrop. Today: Post-Caledonian erosion has stripped away the Moinian metasediments in the west to expose the Stoer Group. The impact crater remains deeply buried beneath Lairg. Michael J. Simms, and Kord Ernstson Journal of the Geological Society 2019;jgs2017-161 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved