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