Structural and Sedimentological Controls of Coal Deposition in the Nongoma Graben, Northern Zululand, South Africa

  1. J. D. Collinson and
  2. J. Lewin
  1. Brian R. Turner1 and
  2. Michael K. G. Whateley2

Published Online: 29 APR 2009

DOI: 10.1002/9781444303773.ch37

Modern and Ancient Fluvial Systems

Modern and Ancient Fluvial Systems

How to Cite

Turner, B. R. and Whateley, M. K. G. (1983) Structural and Sedimentological Controls of Coal Deposition in the Nongoma Graben, Northern Zululand, South Africa, in Modern and Ancient Fluvial Systems (eds J. D. Collinson and J. Lewin), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303773.ch37

Author Information

  1. 1

    Department of Geology, The University, Newcastle upon Tyne NE1 7RU, UK

  2. 2

    Southern Sphere Mining and Development Company (Pty) Ltd, Box 50065, Randburg 2125, South Africa

Publication History

  1. Published Online: 29 APR 2009
  2. Published Print: 7 FEB 1983

ISBN Information

Print ISBN: 9780632009978

Online ISBN: 9781444303773

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Keywords:

  • structural and sedimentological controls of coal deposition – Nongoma graben, northern Zululand, South Africa;
  • sedimentation patterns;
  • sedimentary basins;
  • interdistributary bay deposits;
  • tectonic influence

Summary

The Nongoma graben in northern Zululand developed in response to crustal thinning and the first phase of extensional tectonics (rifting) prior to continental break-up and the separation of east and west Gondwanaland. Sedimentation contemporaneous with graben formation led to the deposition of a thick sequence of coal-bearing fluvio-deltaic Ecca Group (Permian) sediments, controlled mainly by episodes of tensional stress build-up and release in the crust, and not to factors inherent in the depositional system, as on the flanking craton to the north.

A lower progradational deltaic phase of deposition is succeeded by a fluvial depositional phase characterized by fining-upward sequences which themselves show a gross fining-upward trend throughout the succession. Early fluvial deposition was dominated by high-gradient, low-sinuosity (non-braided) bedload channels. During later fluvial deposition, tectonic events controlling deposition were spaced increasingly further apart. Although low-sinuosity channels developed at first, stabilization of source and depositional site promoted lower gradients, increased production of fines and produced high-sinuosity channels due to denudation and possible drifting of the source area. Thus, late fluvial deposition is dominated by fining-upward depositional couplets, comprising low-sinuosity channel deposits overlain by high-sinuosity channel deposits. Fluvial deposition was terminated by a transgressive deltaic depositional phase.

Economically important coals occur at or near the top of fining-upward sequences associated with the high-sinuosity channel facies. Within this facies thick, laterally persistent coal seams formed in extensive floodbasin peat swamps. Overbank flooding was infrequent, with the result that the coals are low ash, low volatile, bright coals containing few shale partings. Depositional modelling indicates that: coal trends are seldom influenced by the thick, laterally extensive sand sheets at the base of the low–high-sinuosity channel depositional couplets; the better-quality coals are of distal floodbasin origin; roof conditions are generally good and the effects of differential compaction minimal. Because of the strong structural imprint on sedimentation, the coals differ in depositional setting and physico-chemical properties from coals in the nearby northern Natal Coalfield where more stable conditions prevailed. This has wider implications for the structural evolution of this part of Africa and the location of coal elsewhere in the southern hemisphere.