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).

Abstract Following recognition of the Vendian to mid-Ordovician rotation of Baltica, with more than 55° of that rotation occurring in the Upper Cambrian and Lower Ordovician, the Tornquist Margin of Baltica must have faced northwards towards Laurentia and the Panthalassic Ocean, rather than, as now, southwestwards towards Gondwana (including Avalonia). Unequivocally Baltic endemic trilobite, brachiopod and other faunas are known from both the Cambrian and the Ordovician of the Holy Cross Mountains, Poland, and from both parts of them, i.e. the Małopolska Block and the Łysogóry Block. Whether or not these two blocks were united into a single terrane or were separate as two terranes is equivocal from the faunal evidence, and there is no faunal evidence of substantial strikeslip faulting of the blocks in relation to the main Baltic craton: they are perceived as having made up part of the margin of Baltica itself. However, both Holy Cross Mountain blocks were different and palaeogeographically separate from the Bruno-Silesian Block, whose continental origins are yet to be finally determined. The Ordovician clastic sediments at both Rügen, north Germany, and Pomerania, NW Poland, have yielded no macrofossils other than graptolites, but microfossils (acritarchs and chitinozoa) are interpreted as having been deposited at relatively high palaeolatitudes, i.e. at a higher palaeolatitude than Baltica, and may have been deposited in an ocean basin within the Tornquist Ocean between Baltica and Avalonia.

Abstract Excellent coastal exposures of strongly folded and faulted Cambrian rocks occur around the St David's Peninsula of SW Wales. The best sections occur in the St David's Anticline, in the vicinity of St David's itself, and these have been the subject of scientific study since the mid-Nineteenth Century. Based on this early work, a quadripartite lithostratigraphical scheme emerged, comprising the Caerfai, Solva and Menevian groups and the Lingula Flags Formation. However, this scheme is unnecessarily complex, and sometimes unworkable in practice. The Caerfai Group is here redefined to embrace much of the Solva Group of earlier authors, while the upper part of the Solva Group, the Menevian Group and Lingula Flags are combined in the new Porth-y-rhaw Group. Traditionally, all these Cambrian rocks have been thought of as being deposited on a stable cratonic platform, but there is substantial evidence of tectonic influences on sedimentation, particularly in the older part of the succession. The Cambrian was originally recognized and defined in North Wales. There, as in many other areas, it became common practice to divide the period into three parts, but more recent work favours a four-fold chronostratigraphical subdivision. The oldest part of the Cambrian is missing in South Wales, but rocks of Terreneuvian, C2, C3 and Furongian age are all represented. Age-diagnostic fossils are few in the older part of the local succession. Trace fossils low in the Caerfai Group indicate the avalonensis Ichnozone or Teichichnus Interval of the Terreneuvian, while a radiometric date of early C2 (CS3) age has been obtained higher up. Trilobite faunas occur more commonly in the higher parts of the Caerfai Group and in the Porth-y-rhaw Group, and these facilitate international correlation of the C3 (CS5) to Furongian (Paibian) parts of the succession. Trilobites characteristic of the oelandicus Biosuperzone and the gibbus , fissus , parvifrons , punctuosus , pisiformis and Olenus biozones all occur.