. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. 220 ANNALS OF THE SOUTH AFRICAN MUSEUM. '?'V' --'?. Fig. 4. Top of an upward-coursening sequence in the Weltevrede Formation, Howison's Poort. The major erosion surface marks the end of the constructional phase of delta out-building and the remaining sediments of the Weltevrede Formation were deposited during a destructional phase. This involved transgressive marine reworking of the delta top (Fisher et al. 1969). The upper 70 m of the Weltevrede Formation comprise quartz arenites, siltstones and sha
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. 220 ANNALS OF THE SOUTH AFRICAN MUSEUM. '?'V' --'?. Fig. 4. Top of an upward-coursening sequence in the Weltevrede Formation, Howison's Poort. The major erosion surface marks the end of the constructional phase of delta out-building and the remaining sediments of the Weltevrede Formation were deposited during a destructional phase. This involved transgressive marine reworking of the delta top (Fisher et al. 1969). The upper 70 m of the Weltevrede Formation comprise quartz arenites, siltstones and shales arranged in a number of upward-fining cycles (Fig. 5). The sandstone units thin upwards and are overlain by vertical alternations of siltstone and shale in which flaser and lenticular bedding are common. Trace fossils are common throughout the sequence and marine invertebrates were recovered from the topmost cycle. The upward-fining cycles are interpreted as being the products of a tidal-flat environment crossed by meandering tidal channels (Tankard & Barwis 1982). The change-over from the Weltevrede Formation to the Witpoort Formation is thought to mark a major change in shoreline configuration from a lobate or arcuate deltaic shoreline to a linear barrier-beach shoreline (Johnson 1976). This change must reflect a reduction in the sediment supply to the shore zone and a decrease in the rate of subsidence so that marginal marine processes rework and redistribute the sediment along the shore. Wave reworking of delta front sands under transgressive conditions produced barrier islands that migrated landwards over the tidal-flat facies. The Witpoort Formation consists mostly of mature sandstones. In the basal 10-12 m these are brownish in colour, fine to medium grained, and display horizontal lamination and ripple cross-lamination. Above this level the sandstones are cleaner, slightly coarser grained, greyish quartz arenites displaying planar and trough cross-bedding and ho
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectnaturalhistory, booky
