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Notes on some deformation structures of sedimentary origin in the Little Haven—Amroth coalfield, Pembrokeshire

Published online by Cambridge University Press:  01 May 2009

Peter F. Williams
Peter F. Williams
Affiliation:
Chevron Standard Ltd., 400 Fifth Ave. S.W., Calgary, Canada.
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Summary

The morphology, origin, and orientation patterns of distinctive types of sedimentary deformation structure are considered. The structures described are laminar folds, load marks and pouches, and convolute and corrugated lamination. A set of criteria obtained from detailed field analysis of the structures is put forward to indicate minor lateral translations of mobilised sediment.

Laminar folds occur in a laminated mudstone/siltstone lithology and are the result of predominantly lateral movements in response to an unstable palaeoslope. Laminar folds are metadepositional or postdepositional in origin. Orientation patterns are characteristically unimodal. Load marks and pouches are preserved in a banded siltstone/sandstone-mudstone lithology, and result from sediment movements which are either vertical, or vertical with some lateral translation. They are syndepositional or metadepositional in origin. Orientation patterns are predominantly unimodal but bimodal and random patterns are also present. Convolute and corrugated laminations are found in thin siltstone/sandstone beds and have developed through sediment movements which were almost exclusively vertical. The convolutions and corrugations are metadepositional in origin. The majority of orientation patterns are unimodal but there is an increase in the number of random patterns.

The positioning of deformation units which show internal lateral translation in a broader stratigraphical and geographical context, suggests there may be a regional palaeoslope control on the lateral movement of mobilised sediment.

Type
Articles
Information
Geological Magazine , Volume 106 , Issue 5 , October 1969 , pp. 395 - 411
Copyright
Copyright © Cambridge University Press 1969

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