A.W. A. Rushton, S. G. Molyneux, 2011. "Welsh Basin", A Revised Correlation of the Cambrian Rocks in the British Isles, A. W. A. Rushton, P. M. Brück, S. G. Molyneux, M. Williams, N. H. Woodcock
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The Cambrian rocks of Wales mostly lie within the Avalon composite terrane, apart from a small area of Cambrian rocks of the Monian composite terrane that is discussed in Chapter 9. The Cambrian rocks of the Welsh Basin form the greater part of the Dyfed Supergroup of Woodcock (1990), the base of which overlies a widespread early Cambrian unconformity and the top of which extends to the late Tremadocian (Ordovician). The Dyfed Supergroup extends onto the Midland microcraton in attenuated form, with substantial gaps in the successions locally. The correlation of the basal parts of the Dyfed Supergroup is uncertain because of the lack of suitable evidence, but Woodcock was able to interpret the supergroup as a megasequence composed of five sequences, labelled in ascending order Ia to Ie, each separated by eustatic, tectonic or volcanic events, or a combination of these (Woodcock 1990, fig. 6). Sequence Ib includes strata assigned to the later Terreneuvian and all of Series 2; the strata of Sequence Ic are those of Stage 5 and the Drumian. The bases of sequences Id and Ie are particularly strongly marked, mainly by eustatic regressions, the former in the Guzhangian and the latter near the top of the Furongian, so that Ie is essentially composed of Tremadocian strata.
Based principally on his extensive work in the eastern North American sector of Avalonia, Landing (1996) divided the Cambrian to Tremadocian successions in Avalonia into ten epeirogenic sequences (Landing 1996, figs 2 & 5).
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A Revised Correlation of the Cambrian Rocks in the British Isles
This work reviews the correlation of the British and Irish Cambrian with the current (though incomplete) international standard for the Cambrian. Since the earlier edition of 1972, the basal and upper limits of the Cambrian system has been internationally agreed; so this account excludes Tremadocian rocks but includes some that were formerly considered Neoproerozoic. Half of the series and stage subdivisions are internationally agreed, but for the undefined divisions of the Cambrian the standard used here makes use of data from Avalonian successions.
Since the first edition was published, almost every aspect of the Cambrian I the British Isles has been subjected to new study. Here, the plate tectonic make-up of the British Isles is reviewed, new radiometric ages and isotopic studies are summarized and the biostratigraphy is enhanced by the study of acritarchs, especially in the Irish successions