Clastic wedges and patterned ground in the Late Ordovician—Early Silurian tillites of South Africa

Authors

  • B. DAILY,

    1. Department of Geology and Mineralogy, University of Adelaide, South Australia, 5001, and South African Museum, Box 61, Cape Town, South Africa*
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  • M. R. COOPER

    1. Department of Geology and Mineralogy, University of Adelaide, South Australia, 5001, and South African Museum, Box 61, Cape Town, South Africa*
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  • *

    Department of Geology and Mineralogy, Parks Road, Oxford, U.K.

ABSTRACT

The widespread Late Ordovician to Early Silurian glaciations are represented in South Africa by glaciogenic sediments of the Pakhuis Formation. Previously unrecorded clastic dykes within the basal tillite of this formation on Table Mountain, Cape Town, are interpreted as casts of frost-fissures filled directly with clastics (sand-wedges or ground-wedges) rather than filled subsequent to the melting of ice-wedges (ice-wedge casts). This suggests sub-aerial deposition for the tillite of the Sneeukop Member and the presence of a cold, dry climate with steep temperature gradients following deposition of that tillite in the Table Mountain area. A peculiar three-dimensional joint-pattern shown by the Pakhuis Formation tillites is interpreted as a relict of a reticulate ice-vein network that formed in sandy till in permafrost. Such networks have been documented from the modern environment, but so far their relicts have not been recorded in ancient sediments.

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