606 SEDIMENTARY PROCESSES/Post-Depositional Sedimentary Structures Figure Convolute bedding developed in coarse cross bedded fluvial sandstones as a result of the sediment experiencing a short lived period of liquefaction shortly after deposition Namurian, Yorkshire, England associated with the ripple laminated (Bouma C) interval (see Sedimentary Processes: Depositional Sedimentary Structures; Deep Water Processes and Deposits) All examples reflect liquefaction of the sand, which can happen for a variety of reasons, including seismic shock, rapid sedimentation, breaking waves, or a shift in water table Where seismic shock caused liquefaction, deformed layers may be very widespread and have stratigraphical significance Upward escape of excess pore water, internal density inversions, and down-slope components of gravity may all contribute to deformation Dewatering leads to reconsolidation of the sediment from the bottom up, ‘freezing’ the deforming laminae Protracted liquefaction may lead to total homogenization Overturned Cross-Bedding This is a special case of convolute lamination in which liquefaction occurred in an actively migrating bedform (Figure 4) Current shear on the sediment– water interface dragged the liquefied sand in a downcurrent direction in an essentially laminar style An upward-migrating front of reconsolidation allowed higher parts of the sediment to be sheared longer, giving down-current-facing recumbent folds Dish and Pillar Structures These are most common in thick, otherwise massive sands and result from dewatering following rapid Figure Overturned cross bedding in two small sets within a shallow marine sandstone This was caused by ongoing bed shear by a current during a short lived loss of strength The deformed sand was progressively ‘frozen’ as a front of reconsoli dation moved upwards through the sediment Late Precambrian, north east Greenland