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Book Chapter

North Wales

By
A. W. A. Rushton
A. W. A. Rushton
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R. A. Fortey
R. A. Fortey
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Published:
January 01, 2000

Abstract

In North Wales (fig. 9) the Ordovician, mainly represented by the Gwynedd Supergroup of Woodcock (1990), occupies a broad tract surrounding the Cambrian of the Harlech Dome and extends eastwards to the Bala area and westwards over much of the Llŷn Peninsula (Roberts 1979). The lower parts of the Ordovician are preserved as outliers on Anglesey and the upper parts are well developed in the broadly anticlinal structure of the Berwyn Hills. In outline, the Ordovician of the North Welsh Basin consists of mudstones, siltstones and sandstones of great aggregate thickness deposited in oxygenated waters commonly of no great depth (above wave-base). Basin subsidence roughly balanced deposition except during part of the Caradoc, when deposition was at a maximum (Prigmore et al. 1997). The stratigraphy is less complete than in South Wales, the Whitlandian commonly being absent, and there is little evidence for the Llandeilian and Pusgillian stages.

The regional stratigraphy is complicated by a series of volcanic episodes, the deposits of which interfinger with the marine sedimentary units. Rushton & Howells (1998) synthesized a stratigraphical framework for the Tremadoc to Caradoc of Snowdonia, and in Snowdonia and Llŷn alone catalogued nearly 100 named divisions of the Gwynedd Supergroup, with over 20 more that are synonyms or of doubtful value. They distinguished as volcanic groups the products of each volcanic centre. They proposed the term Ogwen Group for the marine sedimentary sequence that envelops the volcanic groups and which commences with the transgressive basal Arenig rocks of North Wales, extending up to the top of the Nod Glas (Fig. 10).

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Contents

Geological Society, London, Special Publications

A revised correlation of Ordovician Rocks in the British Isles

R. A. Fortey
R. A. Fortey
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D. A. T. Harpe
D. A. T. Harpe
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J. K. Ingham
J. K. Ingham
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A. W Owen
A. W Owen
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M. A. Parkes
M. A. Parkes
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A. W. A. Rushton
A. W. A. Rushton
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N. H. Woodcock
N. H. Woodcock
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Geological Society of London
Volume
24
ISBN electronic:
9781862396920
Publication date:
January 01, 2000

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