Palaeohydrological Parameters of a Proterozoic Braided Fluvial System (Wilgerivier Formation, Waterberg Group, South Africa) Compared with a Phanerozoic Example

  1. N. D. Smith3 and
  2. J. Rogers4
  1. M. Van Der Neut1 and
  2. P. G. Eriksson pat@scientia.up.ac.za2

Published Online: 17 MAR 2009

DOI: 10.1002/9781444304213.ch27

Fluvial Sedimentology VI

Fluvial Sedimentology VI

How to Cite

Van Der Neut, M. and Eriksson, P. G. (1999) Palaeohydrological Parameters of a Proterozoic Braided Fluvial System (Wilgerivier Formation, Waterberg Group, South Africa) Compared with a Phanerozoic Example, in Fluvial Sedimentology VI (eds N. D. Smith and J. Rogers), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304213.ch27

Editor Information

  1. 3

    Department of Geosciences, 214 Bessey Hall, University of Nebraska, Lincoln, NE 68588-0340, USA

  2. 4

    Cape Town, South Africa

Author Information

  1. 1

    Council for Geoscience, Private Bag X112, Pretoria 0001, South Africa

  2. 2

    Department of Geology, University of Pretoria, Pretoria 0002, South Africa

Publication History

  1. Published Online: 17 MAR 2009
  2. Published Print: 7 OCT 1999

ISBN Information

Print ISBN: 9780632053544

Online ISBN: 9781444304213

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Keywords:

  • Palaeohydrological parameters of Proterozoic braided fluvial system;
  • Wilgerivier Formation - part of Waterberg Group;
  • palaeohydrology - methods;
  • hydrological data;
  • binary plot of mean annual bankfull discharge;
  • Molteno and Wilgerivier Formations - reflecting predominantly braided-river deposition

Summary

Palaeohydrological data for mean and bankfull channel depth and width, mean annual discharge and mean annual bankfull discharge, palaeoslope, drainage area and principal stream length are estimated for braided, bedload rivers of the c. 1800 Ma Wilgerivier Formation red beds in South Africa. The age of these deposits implies deposition within a pre-vegetative continental setting, where soil formation was probably limited, where weathering was aggressive and erosion rapid, and there is no evidence for significant suspended-load influence on the Wilgerivier fluvial style. These data are compared with the same hydrological variables calculated previously for the Middle Triassic Molteno Formation (predominantly braided bedload channels, with subordinate suspended load and floodplain deposits). The drainage area and principal stream length of the Molteno braided channels were significantly higher, and it thus follows that Molteno channel dimensions (and their bankfull equivalents) as well as discharge were higher than for individual braid-channels of the Wilgerivier Formation. Bankfull discharge values estimated for the latter exhibit a greater range than those derived from Molteno Formation cross-bed set thicknesses. Palaeoslope values estimated for the Precambrian Wilgerivier braid-channels lie between those generally found for alluvial fans (> 0.026) and for rivers (< 0.007), including the Molteno channels (mean = 0.0058). These higher palaeoslopes, combined with shorter principal stream lengths, indicate that the Wilgerivier source areas were much closer than those inferred for the Molteno Formation. The margins of the Wilgerivier graben basin would have provided smaller, fault-bounded drainage areas with steep slopes. In the Precambrian, at about 1800 Ma, weathering was still relatively aggressive, providing much sandy detritus close to source terrains, and the generally semi-arid conditions combined with lack of vegetation meant that rapid aeolian and aqueous erosion would have removed fine material. Gravity flow deposits on the high Wilgerivier palaeoslopes were thus uncommon and sandy material was transported by braided channel systems on high-gradient floodplains, most likely as a result of intermittent torrential rainstorms. The different palaeoclimate inferred for the Wilgerivier Formation, combined with unique weathering–erosional regimes of the Precambrian, most probably were responsible for the significant differences in estimated palaeohydraulic parameters when compared with the Phanerozoic Molteno braid-channels.