Welsh Basin

Abstract

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). He recognized equivalents of some of these sequences in selected Welsh and English successions (Landing 1996, p. 51, fig. 7), for example the base of his Sequence 6 (which corresponds to Woodcock’s Ic) and Sequence 9 (part of Woodcock’s Id). Although some correlations are doubtful, the presence in Britain of a hiatus at the level of his Sequence 5, as suggested by Landing’s analysis (1996, fig. 7), is a possibility that merits further investigation. On the other hand, the downward extension of the Arvon ‘Slate Belt’ succession to the base of the Terreneuvian appears speculative