Soft-sediment deformation in a pre-vegetation river system: the Neoproterozoic Torridonian of NW Scotland

The Torridonian of NW Scotland is a thick, sandstone-dominated succession that unconformably overlies Archaean to Palaeoproterozoic metamorphosed Lewisian basement of the Hebridean terrane and is unconformably overlain by the Cambrian of the Caledonian foreland. The Applecross Formation comprises over 3000 m of homogeneous, cross-bedded, coarse to pebbly red sandstone with little mudstone, and few reliable stratigraphical markers. Its accumulation pre-dates the advent of land vegetation that took place from Ordovician times. Depositional architecture is characterised by laterally extensive sandstone bodies representing a braided fluvial system with high width-to-depth ratio channels. Soft-sediment deformation structures are abundant, complex in geometry and large in scale. Using criteria relating to the driving forces, deformation mechanism and timing of deformation, distinct styles of soft-sediment deformation are recognised. They relate to liquefaction or fluidization triggered by: (1) flood-induced turbulence or rapid sedimentation in active channels; (2) seismically induced liquefaction of near-surface waterlogged sediment in active or recently active terminal splays; and (3) fluidization by upwelling groundwater in inactive parts of the system, possibly near the toes of inter-channel areas of distributive fluvial systems and triggered by earthquakes or groundwater movements. The soft-sediment deformation demonstrates that groundwater levels remained high in inactive parts of the fluvial system and this is likely a contributing factor to the unusual abundance of soft-sediment deformation in the Applecross Formation. (C) 2014 The Geologists' Association. Published by Elsevier Ltd. All rights reserved.