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.
Appendices: additional maps and sections (I); field photographs (II); solid-collision equations (III); calculation results (IV); and Brazilian Test data (V).<br>
Basement-hosted fissure fill networks in sub-unconformity settings are increasingly recognised globally and have the potential to act as important sub-surface reservoirs and/or migration pathways for hydrocarbons, geothermal fluids and groundwater. In the present paper we examine well exposed fissures from exhumed crystalline upper Carboniferous basement rocks in southern Italy (Calabria) and describe their nature, origin and evolution. The basement rocks record the emplacement and exhumation of their plutonic protoliths, and an evolution which includes initial intrusion in the upper Carboniferous followed by veining, folding, and rifting events to eventual exhumation at the surface when fissuring occurred in the mid-Miocene. The fissure network hosts fossiliferous marine sediments, wall rock collapse breccias and limited mineralization with vuggy cavities. In the basement below the main erosional unconformity, fissure fills form up to 50% by volume of the exposed rock. The fills are notably porous (up to 15-25% matrix porosity) compared to the ultra-low porosity (< 1%) of the crystalline host rocks. We present field observations, palaeostress analyses of fault slickenlines, and fracture topology analyses which demonstrate that these exceptionally well-connected fissure networks are related to rifting and penetrated to depths of
at least
150 m below the main Miocene erosion surface.
Appendices: additional maps and sections (I); field photographs (II); solid-collision equations (III); calculation results (IV); and Brazilian Test data (V).<br>
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