Triassic Lacustrine Sediments from South Wales: Shore-Zone, Evaporites and Carbonates

  1. Albert Matter and
  2. Maurice E. Tucker
  1. Maurice E. Tucker

Published Online: 29 JUN 2009

DOI: 10.1002/9781444303698.ch11

Modern and Ancient Lake Sediments

Modern and Ancient Lake Sediments

How to Cite

Tucker, M. E. (1978) Triassic Lacustrine Sediments from South Wales: Shore-Zone, Evaporites and Carbonates, in Modern and Ancient Lake Sediments (eds A. Matter and M. E. Tucker), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303698.ch11

Author Information

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

Publication History

  1. Published Online: 29 JUN 2009
  2. Published Print: 24 NOV 1978

ISBN Information

Print ISBN: 9780632002344

Online ISBN: 9781444303698

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Keywords:

  • hematitic replacement of limestone;
  • hematite horizon representing an iron-rich soil or ferricrete;
  • Shore-zone sediments and geomorphic features;
  • Shore-flat sediments with non-lacustrine interbeds;
  • ubiquitously hypersaline

Summary

Lacustrine shore-zone clastics, evaporites and carbonates pass laterally and vertically into the offshore lacustrine Keuper Marl (Norian, Triassic) in Glamorgan, South Wales. The shore-zone clastics are well-sorted breccias of beach origin which show a rapid lateral passage, into sandstones and siltstones containing wave-formed ripples, graded beds (of storm origin) and slump folds. Where the Triassic lake margin was in contact with Carboniferous Limestone, shore platforms and wave notches were cut into the bedrock during stillstands of lake level. Screes accumulated upon the terraces and were locally reworked by wave activity. Calcretes developed within the colluvium during the extended periods of lake shoreline retreat. Where the lake margin was distant from contemporaneous Palaeozoic exposures, fine-grained lake shore-flat sediments are interbedded with stream and sheet flood deposits.

The evaporites are continental sabkha-type gypsum-anhydrite deposits (commonly replaced), exhibiting nodular and enterolithic textures. Replaced sulphate crusts with polygonal cracks are also present. The lacustrine carbonates consist chiefly of calcarenites with wave ripples and fenestrae, and cryptalgal limestones. A minor facies is rhythmically-laminated limestone comprised of calcisiltite laminae (suspension current deposits) in calcilutite. Desiccation and tepee horizons and simple calcretes are intercalated. The carbonates are regarded as shallow sublittoral to littoral in origin, with deposition taking place upon a near-horizontal shore-zone mud-flat.

Interpretations of the lacustrine sediments suggest that there were significant fluctuations in lake level; terraces and coarse shore-zone clastics formed at high lake-level stands and calcretes, evaporites and tepee horizons developed during retreats of the lake shoreline. The Keuper Marl was the offshore sediment of this Triassic lake, deposited subaqueously, but periodically exposed during times of regression when the sulphates developed within the sediments. A consideration of the factors controlling sedimentation suggests that both climate and tectonics were important. The sequence of lacustrine facies suggests that there were changes in chemistry of the lake waters.