Determining the origin of soft‐sediment deformation structures: a case study from Upper Carboniferous delta deposits in south‐west Wales, UK

Determining the cause of sediment mobilization is a major problem; possible triggers include earthquakes, sediment loading and wave action. A detailed sedimentological and palaeoenvironmental analysis of soft‐sediment deformation in Upper Carboniferous deltaic deposits in SW Wales, UK, shows that two styles of deformation occur. Type A (syndepositional convolute stratification) affects most sandstone beds and was generated by rapid sedimentation. Type B (localized sand‐in‐sand pseudonodules) incorporates beds that already contained Type A deformation, and developed when the substrate was liquefied by disturbance due to movement on a near‐surface gravity slide. Neither type of deformation was triggered by seismic events.