A bigger splat: The catastrophic geology of a 1.2 billion year old terrestrial megaclast, NW Scotland

Rock falls are relatively little described from the ancient geological record, likely due to their poor preservation potential. At Clachtoll, NW Scotland, a megaclast (~90,000m³) of Neoarchaean Lewisian gneiss is associated with basal breccias of the fluvial-lacustrine sequences of the Mesoproterozic Stoer Group. Foliation in the megablock is misoriented by ~90° relative to that in the underlying basement gneisses, and is cut by fracture networks filled with Stoer Group red sandstone. Bedded clastic fracture-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 15m onto unconsolidated wet sediment. Following impact, over-pressure and liquefaction of the water-laden sands below the basement block would be sufficient to cause hydrofracturing and upwards sediment slurry injection. Asymmetrically-distributed structures record extension and shortening at the inferred back and front of the megablock, respectively, as it slid downslope and rotated by at least 90° about a vertical axis. With an age of ca 1.2 billion years, the megablock represents one of the oldest known terrestrial rock fall features on Earth.