The Clachtoll megaclast : the forensic reconstruction of a 1.2 billion year old catastrophe with Bob Holdsworth of Durham University
At Clachtoll, NW Scotland, a large area of Lewisian gneiss is associated with basal breccias of the Mesoproterozic (ca 1200 Myr) Stoer Group. The banding in the gneisses is misoriented by ~90° relative to that in the Lewisian rocks to the east, and it is notably cut by huge numbers of fractures filled with red sandstone. What on Earth was going on here?
In this talk, I want to show how we used geological forensics to reconstruct how this feature formed. We discovered that bedded fracture-fills on top of the megablock preserve way-up criteria consistent with passive sediment filling from above during burial. By contrast, sediment-filled fractures on the lateral flanks and base show characteristics consistent with forceful injection. Furthermore, a recently exposed gully shows that the region of misoriented Lewisian actually sits on top of Clachtoll sedimentary breccias. This suggests it represents an enormous ‘megaclast’ of basement rock, with a volume of around 90,000m3, weighing close to 250 kt. This fell into the basal Stoer Group sediments as they were being deposited.
The buried megaclast lies some 300m stratigraphically below the famous Stac Fada impactite deposits and therefore cannot be related to this event. We suggest instead that tectonic seismic shaking caused the megaclast to fall no more than 15m from a cliff onto unconsolidated wet sediment below. Immediately following impact, liquefaction of the water-laden sands below the block generated overpressured slurries of sediment that were injected upwards into it. We can also show that the block then slid downslope and rotated by at least 90° about a vertical axis.
The megaclast represents perhaps the oldest known terrestrial rock fall feature on Earth. The Stoer Group is therefore truly remarkable in its preservation of evidence for two geological catastrophes, albeit of very different magnitude!
Bob Holdsworth is Professor of Structural Geology at Durham University. His research – a substantial proportion of which has utilised world-class NW Scottish geology – is concerned with understanding the geological architecture, deformation processes and fluid transport properties of fault zones by integrating the results of fieldwork, microstructural analysis and rock deformation experiments. Bob is also a member of the Office of Nuclear Regulation (ONR) Expert Panel in Seismic Hazard and is Chair and a Trustee of the Scottish Geology Trust.
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