ABSTRACT

The Applecross Formation (upper Proterozoic Torridon Group, northwest Scotland) was deposited as planar-laminated and cross-bedded coarse sands in braided streams. Soft-sediment deformation is described from a 97 m thick study section, most of which comprises contorted stratification. The main structures are cusps and complex recumbent folded cross-bedding, formed by water escape during fluidization.

Four assemblages of structures are recognized. Deformation as isolated structures in an otherwise undeformed unit may have been triggered by fluid turbulence. Discrete and chaotic units record widespread liquefaction and fluidization, respectively, of the most recently deposited sediment. Deformation may have been triggered by upwelling groundwater. Multilayer complexes formed when water escape from a buried layer generated fluidization in the overlying sediment. This may have been triggered by seismically induced liquefaction. Some parts of the succession were disturbed on separate occasions by different processes. Sediment-laden currents are unlikely to have been a major trigger for deformation.

It is concluded that specific soft-sediment deformation structures result from the combination of driving forces, deformation mechanism, and initial geometry. Assemblages of structures relate to the timing of deformation and are of more value in identifying the trigger for deformation.