CYCLIC SEDIMENTATION IN THE LOWER WESTPHALIAN OF NO DEVON, ENGLAND

Authors

  • J. F. M. DE RAAF,

    1. Koninklijke/Shell Exploratie en Produktie Laboratorium, Rijswijk (The Netherlands); Department of Geology and Mineralogy, University Museum, Oxford (Great Britain)
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  • H. G. READING,

    1. Koninklijke/Shell Exploratie en Produktie Laboratorium, Rijswijk (The Netherlands); Department of Geology and Mineralogy, University Museum, Oxford (Great Britain)
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  • R. G. WALKER

    1. Koninklijke/Shell Exploratie en Produktie Laboratorium, Rijswijk (The Netherlands); Department of Geology and Mineralogy, University Museum, Oxford (Great Britain)
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    • 1

      Present address: Department of Geology, Johns Hopkins University, Baltimore, Md. (U.S.A.).


SUMMARY

Eleven facies of the (Lower Westphalian) Abbotsham Formation and upper portion of the Northam Formation of north Devon are described: black mudstone, silty mudstone, “turbidite”, silty streak, sandy streak, three kinds of oscillatory beds, cross-stratified sandstones and mudstones, fining-upwards units and major sandstones. Their relationships to each other are determined and they are shown to recur in a cyclic pattern.

Six cycles are distinguished in the 1,200 ft. of the Abbotsham Formation. Each passes upwards gradually from black mudstone into coarser beds, the top of which is sharply separated from the black mudstone of the succeeding cycle.

We suggest that the environment of deposition of these cycles ranges from a moderately deep basin, in which mudstones and occasional turbidites were deposited, through the sediments of an advancing coastline, fronted by deltas, to coastal plains in which fluviatile sediments were laid down.

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