ABSTRACT

An arenaceous diamictite rests on fissile silty mudstone and argillaceous diamictite, and is overlain by a laminated muddy siltstone east of Helpmekaar in the Dundee district of northern Natal. The argillaceous deposits originated either in the subglacial or proglacial zone of a partly grounded marine ice sheet, where sediment was supplied by debris-laden floating ice, meltwater plumes, and turbidity currents. The arenaceous diamictite is the product of melt-out debris released close to the grounding line of the ice sheet, where it was subjected to overriding by the ice and slumping and redeposition by sediment gravity flow. The laminated siltstone represents sediment that was deposited in a low-energy setting by turbidity currents and suspension settling downslope from a retreating g ounding line. Dispersed ice-rafted clasts occur in these fine-grained deposits.

The upper surface of the arenaceous diamictite displays regularly spaced linear undulations that are parallel to the former iceflow direction, as are corrugations in discrete subhorizontal slip planes in its upper part. Dike-like bodies of this diamictite are found in the underlying mudstone and argillaceous diamictite. These dikes, or wedge-shaped sheets, of sandy diamictite are nearly vertical in their upper parts but at depth curl to the southeast to form sill-like bodies. These intrusive diamictites record a novel, and previously undescribed, form of subglacial bed shear by repeated hydraulic injections which shuffled the upper layers of the substrate to the south-southeast.

Bed compaction and shear beneath the grounded ice is recorded by folds in steepened portions of the clastic intrusive sheets and by pinch-and-swell (and folded pinch-and-swell) of the clastic sills. Displacement of steep, folded sheet elements along subhorizontal, slicken-sided slip planes indicate that bed shear continued along spaced planes of slip during the process of compaction.

The corrugated and planar slip surfaces and the formation of the listric or spoon-shaped intrusions are all shown to be different expressions of bed shear induced by traction from the overriding ice flowing to the southeast when it was temporarily grounded. The base of an ice sheet flowing from land into the sea undergoes flow acceleration and forms a ramp where it decouples from the high-friction substrate. The arenaceous diamictite mound most likely to have developed beneath this ramp is inferred to have a structural core of sediments that have been squeezed and sheared and have slipped from beneath the ice in the tidal-grounding zone.