The origin of soft-sediment deformation structures in Permo-Carboniferous glacial and proglacial beds, South Africa

ABSTRACT Soft-sediment deformational structures in subsurface glacial and proglacial beds of the Permo-Carboniferous Dwyka Formation include faults, folds, boudins, sigmoidally shaped laminae and sandstone/siltstone injections within specific zones ranging in thickness from 1 to 12 m. The zones consist of rhythmite, shale with ice-rafted debris, siltstone and minor sandstone and diamictite, but often the original character of the sediments is completely destroyed by the deformation. The deformation structures predominantly formed during extensional strain of the sediments and form part of complex shear zones, diapiric structures, load coasts and convolute lamination. The deformation zones are lithologically controlled and intimately associated with debris-flow deposits, indicating that gravitational forces giving rise to slumping were the predominant cause of the deformation. Excess pore-water pressures and subsequent liquefaction of the fine-grained sediments on oversteepened depositional slopes apparently generated the sheetlike slumps, but the possibility of grounded ice initiating slumps in the lowermost deformation zone cannot be excluded. A new type of coarse-grained, matrix-supported deposit, designed melange diamictite, which formed during slumping under a soft-sediment slab, is described for the first time.