SEDIMENTARY PROCESSES/Post-Depositional Sedimentary Structures 609 Figure Small, synsedimentary growth fault within deltaic sediment Note the thickening of sands into the hanging wall of the fault and the roll over within these sands The synsedimentary nature of the fault is shown by the fact that the uppermost beds pass over the fault without disturbance Namurian, County Clare, Ireland and are aided by progressive sediment loading Their hanging wall areas act as local depocentres in which thickened delta-front sediments are deposited and preserved Continued progradation of individual deltas or of the continental slope causes the active faulting to shift progressively basinwards Large growth faults are important traps for petroleum in deltas such as the Mississippi and the Niger Climatically Induced Structures In addition to the soft-sediment structures described above, in which a temporary loss of strength causes the disturbance, there are some rather diverse structures which result from climatic effects Desiccation and Other Cracks When surface layers of mud or silt dry out, they contract and create isotropic horizontal tension If this exceeds the tensile strength of the mud, polygonal (commonly hexagonal) cracks develop which may be filled with wind-blown or water-lain sand (Figure 9) The dimensions of the polygons are a function of the mud layer thickness, and several scales may coexist Subaqueous shrinkage (synaeresis) of surface mud layers also leads to cracks, although these often form less regular patterns They are thought to result from volume changes of clays within the surface layer Synaeresis cracks may commonly be differentiated from subaerial desiccation cracks because they are Figure Desiccation cracks on an upper bedding surface in ephemeral lacustrine sediments Carboniferous, New Brunswick, Canada infilled with mud, not sand Large-scale polygonal cracks also develop through protracted freeze–thaw processes in periglacial settings