Abstract

Zonal stagnation is an important element in the regional style of deglaciation in areas of moderate bedrock relief (50–150 m), such as the shield terrane of southern Ontario. Bedrock topography played a major role in the stagnation process, as ice blocks were trapped within bedrock basins of all sizes. Stagnation occurred subglacially as the ice sheet thinned and trapped basal ice became increasingly isolated from its source. Sediment–landform relationships support this model. Bedrock lakes are floored by highly faulted laminated sediment, often displaying large ice-block casts. The dominant depositional landforms are ice-contact terraces, which flank valley walls. Sediment forming the terraces is variable, depending upon ice-marginal depositional environment.Models for two depositional systems are developed: (1) localized ice-marginal and subglacial sedimentation in upland terrain, and (2) thick proglacial basin-fill sequences developed in major structural valleys. In upland areas, sediment cover is thin and discontinuous and is composed primarily of complex sediment-flow assemblages. Basal till is rare but occurs as two distinct lithofacies representing (1) meltout from beneath wholly stagnant ice, and (2) deposition by basal melting from thin slabs of stagnant, debris-rich ice lodged beneath an active shear zone. Thick proglacial basin-fill sequences exhibit complex facies relationships. Ice-contact lacustrine terraces comprise fining-upward subaqueous outwash sequences, developed as density underflows were funnelled between rock knobs and blocks of stagnant ice. Kettle lakes developed within large basins where ice blocks persisted for the longest period of time.

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