SEDIMENTARY PROCESSES/Post-Depositional Sedimentary Structures 603 mass movement, leading to a phase of resedimentation However, this article deals only with situations in which the loss of strength was short-lived and the sediment regained its strength before it moved downslope, thus preserving structures that record more or less in situ deformation Deformation occurs within sediment when intergranular forces are unable to resist applied stresses, which are usually, but not exclusively, gravitational The shear strength of sediment is normally expressed by the equation: t ẳ C ỵ s pị tan where t is the shear strength, C is the grain cohesion, s is the pressure normal to shear, p is the excess pore fluid pressure, and ’ is the angle of internal friction This means that sediment will fail when the applied stress exceeds t; such a condition will be favoured by a lower cohesive strength C, by changes in grain packing to reduce tan ’, or by an increase in pore fluid pressure p beyond a certain critical value Such conditions can occur for several reasons, discussed below Strain, when it occurs, may be isotropic, dispersed throughout the body of sediment, or concentrated on discrete slip surfaces Loss of Strength In the shear strength equation set out above, the presence of fine-grained, especially muddy sediment tends to increase the role of the cohesive strength, which is largely a function of grain size, whilst in better sorted, mud-free sediment, frictional forces dominate In both cohesive and non-cohesive sediments, high pore water pressure commonly leads to a loss of shear strength, but this condition can have different causes and durations in different sediments In fine-grained, muddy sediments, excess pore fluid pressure is often caused by relatively rapid deposition Low permeability inhibits the escape of pore water and hence retards sediment compaction, giving overpressured conditions In addition, the decay of organic matter within the sediment can generate gases, which, if unable to escape, also contribute to pore pressure Such overpressured conditions occur close to the sediment surface where highly mobile mud may be susceptible to deformation or flowage They also develop at a larger scale, and in a longer time frame, within thick piles of sediment, such as those deposited by prograding deltas or deltaic margins (e.g., Mississippi, Niger) In such cases, increasing pore water pressure with burial may eventually exceed lithostatic pressure, leading to failure This results in plastic flowage of a thick, overpressured layer These movements set up extensional stresses in the overburden, which can lead to the development of discrete slip surfaces that cut up-section as listric faults when the tensile strength of the sediments is exceeded Such surfaces may also become the bounding surfaces of sediment gravity slides These conditions, and the resultant movements, develop and persist over long intervals of time; in the case of the largest deltaic margins, over millions of years Rapidly deposited sands, lacking significant finegrained sediment, commonly have rather loose grain packing They are particularly susceptible to shock, such as by an earthquake, heavy wave action, further rapid deposition, or a sudden rise in water level Shock breaks grain contacts and induces tighter packing and, as a result, excess pore water is present, raising the pore water pressure and causing temporary liquefaction, which will last until the extra pore water escapes In the case of sands, the high permeability usually means that liquefaction is shortlived Whilst liquefied, the sediment–water mixture deforms readily as a Newtonian fluid in response to gravitational and other applied stresses As pore fluid escapes, usually upwards to the sediment surface, grain contacts are re-established and a rising front of reconsolidation ‘freezes’ the deformed sediment Deformation is usually identified through the distortion of depositional lamination but, with extreme or sustained liquefaction and deformation, depositional lamination is totally obliterated and structureless sand or slurried textures are produced The upwards movement of escaping pore water may also create deformation, both by distorting depositional laminae and by creating new vertical structures, such as dishes, pipes, and sheets, within which sediment–water mixtures move as fluidized flows Deforming Forces Virtually all the forces that act upon sediment weakened by the above processes are gravitational These can be divided into two major classes: a downslope component of gravity and the action of gravity on inverted density gradients In addition, shear at the sediment surface due to flowing water or ice, or to sediment gravity flows, may also deform the sediment The down-slope gravitational component may be sufficient to overcome the cohesive strength of muddy sediment on a slope This can lead to the detachment of a slab of sediment on a basal shear surface and to internal deformation within the moving layer The style of deformation varies depending upon the position within the moving sheet Stretching, particularly at the up-slope end, may lead to tensile failure and to the development of brittle extensional structures, whilst compression, commonly in a downslope setting, causes plastic folds These styles of deformation are associated with a family of mass