Reconstruction of the ordovician pakhuis ice sheet, South Africa

Blignault H.J. ; Theron J.N. (2010)


This reconstruction of the Late Ordovician, glacigenic Pakhuis Formation of the Table Mountain Group is based on a variety of diverse datasets. Field observations were made over the full 300 km extent of exposure in a north-south direction. The thin glacigenic package constitutes a minor interlude in the development of a subcontinental, Palaeozoic, siliciclastic basin. The Pakhuis Formation is of special interest because it includes features like the extensive penecontemporaneous folding, an erosional unconformity and the uniform lithological character of the sandy diamictites. A model is presented by which an unusual concurrence of circumstances produced the special features of the Pakhuis glacigenics. These are the ice invasion of a developing basin, an onset zone within basin and a deep trough harboring an ice-stream. Subglacial, grounding zone and iceberg facies associations with their lateral variations are developed. The temperate, glaciation of relative short duration, comprised one major and a second minor advance. During the first advance the grounded ice sheet with slow-moving sheet-flow reached ice-stream characteristics along at least part of a trough. The occurrence of an ice-stream within a slow-moving, low-profile ice sheet provides the glaciological environment for ice-stream draw-down, flanking infill of ice, resulting in compressional ice-flow from the sides. The fundamental requirement for buckling (i.e. the development of the Fold Zone) is layer parallel shortening achieved by the compressional ice-flow. Ice-bed interface features like deforming-bed structures are developed, as well as outsize flutes for which an alternative bulk strain model is proposed. The glacial retreat was rapid, interrupted only by the second, minor advance. A marine environment is attested by the postglacial fauna of a black shale. © 2010 September Geological Society of South Africa.

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