A. W. A. Rushton, R. A. Fortey, 2000. "North Wales", A revised correlation of Ordovician Rocks in the British Isles, R. A. Fortey, D. A. T. Harpe, J. K. Ingham, A. W Owen, M. A. Parkes, A. W. A. Rushton, N. H. Woodcock
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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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This Report is revised and expanded from the 1972 publication providing an up-to-the-minute account of the British Ordovician formations and their correlation nationally and internationally. It also includes the most comprehensive treatment of Ireland ever attempted. The reference list of a comprehensive bibliography of papers on the subject published since 1970.
The British sections are the type for the Ordovician System and classical in stratigraphical, tectonic and volcanic studies. The Charts bring together 30 years of research over the period in which plate tectonics has revolutionized our understanding of the Lower Palaeozoic rocks of the British Isles.
This Special Report will be a valuable reference for research and applied geoscientists working with rocks of Ordovician age. It will be of particular interest to those working in, or visiting, the Welsh mountains and the English Lake District.