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The auriferous late Archaean sedimentation systems of South Africa: unique palaeo-environmental conditions?

โœ Scribed by B.G. Els


Book ID
104167286
Publisher
Elsevier Science
Year
1998
Tongue
English
Weight
894 KB
Volume
120
Category
Article
ISSN
0037-0738

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โœฆ Synopsis


Studies of the auriferous Witwatersrand placers and associated rocks have revealed that certain palaeo-environments, especially braided fluvial, are particularly well represented in this part of the rock record. However, there is a paucity of lithofacies indicating certain other palaeo-environments. Possible reasons for their scarceness or absence are suggested in this paper. It is generally assumed that pre-vegetational fluvial systems would have been characteristically braided, because of the absence of land plants necessary to stabilise river banks. A question arising is what kind of downstream changes such pre-vegetational braided streams underwent. A recent study of a braided stream placer revealed that the depositing system retained its braided character right down to the palaeo-shoreline. However, gravel did not reach the palaeo-beach. As with many modern examples, beach conglomerates are rare in the Witwatersrand rocks. The paucity of conglomeratic beach placers is ascribed to the low probability of gravel being transported across coastal plains, because of the relatively low slopes or depths of rivers here. The Witwatersrand fluvial channels are generally considered to have had high width-to-depth ratios, because of the absence of land vegetation to stabilise channel banks. However, two examples of deep, relatively narrow scour features, with predominantly fine-grained fills, occur near the base of the Central Rand Group. The low width-to-depth ratios of these scour features, which probably represent palaeo-valleys, are ascribed to severe incision during a rapid sea-level fall. The auriferous fluvial systems of the Witwatersrand are generally considered to have been entirely braided, due to the lack of bank stability. However, the fluvial B placer of the Welkom goldfield is confined to discrete channels. Their banks are thought to have been stable, due to the cohesive nature of the lutite into which the gravelly streams incised. In addition, braiding sensu stricto was probably inhibited by initial incision and a low sediment supply. No deposits of specifically deltaic sub-environments have been found in the Central Rand Group. Their absence is attributed to the following factors: (a) the paucity (absence?) of well-defined palaeo-river mouths; (b) low concentrations of suspended sediment; (c) intermittent sediment supply to the palaeo-coastline; and (d) reworking by tidal and longshore currents. Alluvial-fan deposits are also apparently absent in the Witwatersrand rocks. The absence of fan deposits is attributed to the poor potential for development and preservation of fan deposits in the compressive tectonic setting proposed for the Witwatersrand Basin. Although ventifacts have been found in the Witwatersrand rocks, no aeolianites have been reported. Their apparent absence is probably due to (a) reworking in a predominantly humid climate, and (b) transport of sand by dominant winds to areas unfavourable for the preservation of aeolianites.


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