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A bigger splat: The catastrophic geology of a 1.2-b.y.-old terrestrial megaclast, northwest Scotland

Killingback, Z.; Holdsworth, R.E.; Walker, R.J.; Nielsen, S.; Dempsey, E.; Hardman, K.

A bigger splat: The catastrophic geology of a 1.2-b.y.-old terrestrial megaclast, northwest Scotland Thumbnail


Authors

Z. Killingback

R.J. Walker

E. Dempsey

K. Hardman



Abstract

Rockfalls are relatively little described from the ancient geological record, likely due to their poor preservation potential. At Clachtoll, northwest Scotland, a megaclast (100 m × 60 m × 15 m) of Neoarchean Lewisian gneiss with an estimated mass of 243 kt is associated with basal breccias of the Mesoproterozoic Stoer Group. Foliation in the megablock is misoriented by ∼90° about a subvertical axis relative to that in the underlying basement gneisses, and it is cut by fracture networks filled with Stoer Group red sandstone. Bedded clastic fissure fills on top of the megablock preserve way-up criteria consistent with passive deposition during burial. Sediment-filled fractures on the lateral flanks and base show characteristics consistent with forceful injection. Using numerical calculations, we propose that rift-related seismic shaking caused the megablock to fall no more than 15 m onto unconsolidated wet sediment. On impact, overpressure and liquefaction of the water-laden sands below the basement block were sufficient to cause hydrofracturing and upward sediment slurry injection. In addition, asymmetrically distributed structures record internal deformation of the megablock as it slowed and came to rest. The megablock is unrelated to the younger Stac Fada impact event, and represents one of the oldest known terrestrial rockfall features on Earth.

Citation

Killingback, Z., Holdsworth, R., Walker, R., Nielsen, S., Dempsey, E., & Hardman, K. (2020). A bigger splat: The catastrophic geology of a 1.2-b.y.-old terrestrial megaclast, northwest Scotland. Geology, 49(2), 180-184. https://doi.org/10.1130/g48079.1

Journal Article Type Article
Acceptance Date Aug 22, 2020
Online Publication Date Sep 18, 2020
Publication Date 2020
Deposit Date May 26, 2021
Publicly Available Date Mar 29, 2024
Journal Geology
Print ISSN 0091-7613
Electronic ISSN 1943-2682
Publisher Geological Society of America
Peer Reviewed Peer Reviewed
Volume 49
Issue 2
Pages 180-184
DOI https://doi.org/10.1130/g48079.1

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