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Reply to comment by Trevor Faulkner on “The Stac Fada impact ejecta deposit and the Lairg Gravity Low: evidence for a buried Precambrian impact crater in Scotland? [Proc. Geol. Assoc. 126, 742–761 (2015)] and the consequence for the formation of the caves within the Durness Limestone outcrops at Assynt, Sutherland”
Proceedings of the Geologists’ Association 127 (2016) 109
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Proceedings of the Geologists’ Association journal homepage: www.elsevier.com/locate/pgeola
Correspondence Reply to comment by Trevor Faulkner on ‘‘The Stac Fada impact ejecta deposit and the Lairg Gravity Low: evidence for a buried Precambrian impact crater in Scotland? [Proc. Geol. Assoc. 126, 742–761 (2015)] and the consequence for the formation of the caves within the Durness Limestone outcrops at Assynt, Sutherland’’ Trevor Faulkner has made an interesting suggestion that the existence of extensive cave systems in the Durness Limestone of Assynt owes much to the presence of a giant impact crater, formed more than 700 million years earlier, that manifests itself today as the Lairg Gravity Low (Simms, 2015). It calls to mind the link between the end-Cretaceous Chicxulub impact crater and the distribution of cenotes in the overlying Cenozoic limestones of the Yucatan (Hildebrand, 1995). Faulkner suggests that the seemingly anomalous embayment of the Moine Thrust at Assynt might be ascribed to the effects of a deep depression – an impact crater – deflecting the westward advance of Moinian metasediments across the Lewisian basement during the Caledonian Orogeny. Although the modelled depth to the base of the Lairg Gravity Low lies several km below the surrounding Lewisian surface (Leslie et al., 2010) this geophysical structure would, if it is an impact crater, likely be occupied by an impact melt sheet overlain by a thick succession of post-impact Stoer Group sediments. Hence even by late Mesoproterozoic times it might have resembled more closely the subdued relief of most terrestrial impact craters than the spectacular, near pristine, bowl of Meteor Crater. In the ensuing 700 million years, between the impact and the advent of the Caledonian Orogeny, the region would have experienced considerable erosion and/or burial. Even assuming that it was exposed at the surface by that time it is
debatable whether the differential relief between the eroded crater rim and the sediment-filled crater bowl would have been substantially greater than the relief across the pre-Torridonian surface, where some Lewisian inliers rise several hundred metres above the base of the Stoer Group. Could such a subdued structure have deflected the Moine Thrust in the manner proposed by Faulkner? However, extensive fracturing of the crust caused by the impact may have been of greater significance. Radial and/or concentric fractures in the basement could act as foci for subsequent fault displacements and thereby influence the advance of the Moine Thrust in this region. Although this differs from the mechanism proposed by Faulkner, the ultimate result – creating the Assynt window – is essentially the same and hence it does not discount a possible link between the Assynt caves and the putative Lairg impact crater. References Hildebrand, A.R., 1995. Size and structure of the Chicxulub Crater revealed by horizontal gravity gradients and cenotes. Nature 376, 415–417. Leslie, A.G., Krabbendam, M., Kimbell, G.S., Strachan, R.A., 2010. Regional-scale lateral variation and linkage in ductile thrust architecture: the Oykel Transverse Zone, and mullions in the Moine Nappe, NW Scotland. In: Law, R.D., Butler, R.W.H., Holdsworth, R.E., Krabbendam, M., Strachan, R.A. (Eds.), Continental tectonics and mountain building: the legacy of Peach and Horne. Geological Society, London, Special Publication, 335, pp. 359–381. 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 (6), 742–761.
http://dx.doi.org/10.1016/j.pgeola.2016.02.007 0016-7878/ß 2016 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.
Michael J. Simms E-mail address: [email protected] (M.J. Simms). Received 3 February 2016 Available online 21 March 2016
Contents lists available at ScienceDirect
Proceedings of the Geologists’ Association journal homepage: www.elsevier.com/locate/pgeola
Correspondence Reply to comment by Trevor Faulkner on ‘‘The Stac Fada impact ejecta deposit and the Lairg Gravity Low: evidence for a buried Precambrian impact crater in Scotland? [Proc. Geol. Assoc. 126, 742–761 (2015)] and the consequence for the formation of the caves within the Durness Limestone outcrops at Assynt, Sutherland’’ Trevor Faulkner has made an interesting suggestion that the existence of extensive cave systems in the Durness Limestone of Assynt owes much to the presence of a giant impact crater, formed more than 700 million years earlier, that manifests itself today as the Lairg Gravity Low (Simms, 2015). It calls to mind the link between the end-Cretaceous Chicxulub impact crater and the distribution of cenotes in the overlying Cenozoic limestones of the Yucatan (Hildebrand, 1995). Faulkner suggests that the seemingly anomalous embayment of the Moine Thrust at Assynt might be ascribed to the effects of a deep depression – an impact crater – deflecting the westward advance of Moinian metasediments across the Lewisian basement during the Caledonian Orogeny. Although the modelled depth to the base of the Lairg Gravity Low lies several km below the surrounding Lewisian surface (Leslie et al., 2010) this geophysical structure would, if it is an impact crater, likely be occupied by an impact melt sheet overlain by a thick succession of post-impact Stoer Group sediments. Hence even by late Mesoproterozoic times it might have resembled more closely the subdued relief of most terrestrial impact craters than the spectacular, near pristine, bowl of Meteor Crater. In the ensuing 700 million years, between the impact and the advent of the Caledonian Orogeny, the region would have experienced considerable erosion and/or burial. Even assuming that it was exposed at the surface by that time it is
debatable whether the differential relief between the eroded crater rim and the sediment-filled crater bowl would have been substantially greater than the relief across the pre-Torridonian surface, where some Lewisian inliers rise several hundred metres above the base of the Stoer Group. Could such a subdued structure have deflected the Moine Thrust in the manner proposed by Faulkner? However, extensive fracturing of the crust caused by the impact may have been of greater significance. Radial and/or concentric fractures in the basement could act as foci for subsequent fault displacements and thereby influence the advance of the Moine Thrust in this region. Although this differs from the mechanism proposed by Faulkner, the ultimate result – creating the Assynt window – is essentially the same and hence it does not discount a possible link between the Assynt caves and the putative Lairg impact crater. References Hildebrand, A.R., 1995. Size and structure of the Chicxulub Crater revealed by horizontal gravity gradients and cenotes. Nature 376, 415–417. Leslie, A.G., Krabbendam, M., Kimbell, G.S., Strachan, R.A., 2010. Regional-scale lateral variation and linkage in ductile thrust architecture: the Oykel Transverse Zone, and mullions in the Moine Nappe, NW Scotland. In: Law, R.D., Butler, R.W.H., Holdsworth, R.E., Krabbendam, M., Strachan, R.A. (Eds.), Continental tectonics and mountain building: the legacy of Peach and Horne. Geological Society, London, Special Publication, 335, pp. 359–381. 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 (6), 742–761.
http://dx.doi.org/10.1016/j.pgeola.2016.02.007 0016-7878/ß 2016 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.
Michael J. Simms E-mail address: [email protected] (M.J. Simms). Received 3 February 2016 Available online 21 March 2016