Abstract The Dyfed Supergroup of SW Wales ranges from Terreneuvian to Furongian in age and represents a prolonged and nearly continuous phase of siliciclastic sedimentation on the southern margin of the Cambrian Welsh Basin. The lower 564 m of the Supergroup are included in an extended Caerfai Group (six formations), embracing strata previously assigned to the Solva Group. Predominantly arenaceous, the Caerfai Group has important units of conglomerate and pebbly sandstone at its base, middle and top. The Caerfai Group is overlain by 687 m of mainly argillaceous sedimentary deposits of the Porth-y-rhaw Group (five formations), a newly erected unit that includes strata previously assigned to the ‘Menevian Group’ and ‘Lingula Flags’. This two-fold division of the Dyfed Supergroup is comparable with the distinction of the laterally equivalent Harlech Grits and Mawddach groups exposed around the Harlech Dome in North Wales. High resolution sequence-stratigraphical techniques, constrained by biostratigraphical data wherever possible, are applied to the Dyfed Supergroup across southern Britain: the revised lithostratigraphy is thereby integrated with a slightly modified version of the Avalonian chronostratigraphy initially developed for the western Avalonian successions of maritime Canada. Sequences 1 and 2 are not recognized in the Dyfed supergroup of SW Wales, reflecting the inner platform setting of the area. Those sequences are represented in North Wales however, which was situated on the outer platform. The relative ease with which the Cambrian successions of southern Britain can be incorporated into the general Avalonian framework reflects the shared epeirogenic history, sediment sources and accumulation history of a microcontinent unified by early Cambrian times. The gross lithological similarities that exist between Cambrian successions across Avalonia, and between SW Wales and North Wales in particular, are better understood when basin chronostratigraphy is placed within a sequence stratigraphic framework defined by systems tracts. This is particularly evident during Unnamed Cambrian Series 3 (C3) and Furongian times, when clear systems links are demonstrated between paralic depositional environments in SW Wales and deeper basin turbidites in North Wales. Sequences 3–7 (Terreneuvian–C3) in the Welsh Basin and its hinterland were dominated by siliciclastic deposition on an epeirogenically active platform. An extensional rift-like tectonic regime is proposed, where elongate basins dominated by mudstone deposition have time equivalents formed on areas of apparent uplift that were probably tilt-block highs. Local accumulation history, the development of regional unconformities, the extent of marine onlap, and the secular succession of lithofacies were controlled principally by phases of fault-accommodated subsidence along the NE-trending lineaments that bounded these basins and their intervening horsts or platforms. In southern Britain, movements first along the Menai Straits Fault System then the Welsh Borderlands Fault System, both of which are terrane boundaries, were responsible for the dominantly coarse-grained arenaceous sedimentation of pre-Drumian times. In the Welsh Basin, this is highlighted by a change in sediment source from the Monian Superterrane to the Wrekin Terrane. A rapid switch from marginal to inner platform source areas accompanied a major phase of tectonic reorganization of the Avalonian Superterrane during the development of the boundary between sequences 3 and 4. Although the role of secular variations in eustatic sea level as a control on sequence development and architecture has been dismissed previously, prominent lowstands such as those recorded during the basal Ptychagnostus gibbus Biozone, ‘ Solenopleura ’ brachymetopa Biozone (mid- Paradoxides forchhammeri Biosuperzone), and the upper part of the forchhammeri Biosuperzone also influenced the depositional sequence stratigraphy. The broad subdivision of Cambrian Avalonian stratigraphy into tectonically active and passive phases of sedimentation allows two megasequences to be distinguished. Megasequence 1 (sequences 1–7) represents the transform termination of Avalonian subduction following oblique convergence, and the accretion of island arcs onto the northern periphery of Gondwana. Under transpressional regimes, late Precambrian arc-related basins were inverted and recycled into pull-apart successor basins. Initially, the margins of these were dominated by alluvial fans and coarse-grained fan-deltas built by flood-generated sediment dispersal processes. In the later transform stage of Cadomian–Avalonian orogenesis, there was a switch to sediment supplied from highlands to the east. Sequence 8, represented in Pembrokeshire by the Aber Llong and Ogof Velvet formations, lies at the base of Megasequence 2 (late C3–early Ordovician). Excellent sections occur through these formations and their equivalents in North Wales, and their interpretation significantly improves understanding of younger Cambrian sedimentary basins in Avalonia. These successions reflect passive margin sedimentation and the culmination of the Avalonian–Cadomian orogenic cycle. Deposition occurred in part of a subaqueous delta platform at the mouth of a huge river system that drained a substantial part of West Gondwana, with Avalonia acting as a sink for vast quantities of fine-grained sediment. Secular variations in eustatic sea level and/or sediment supply, rather than active tectonism, were the main mechanisms controlling sequence architecture and depositional environments at this time.

Abstract The classic Cambrian succession of SW Wales comprises a succession of clastic rocks some 1250 m thick that make up part of the Dyfed Supergroup. The existing lithostratigraphical framework owes much to the Nineteenth Century researchers, with relatively little detailed work having been published since. We here present a detailed, rationalized and revised lithostratigraphy for the Cambrian part of the Dyfed Supergroup of the area. Where possible and appropriate, we have retained original and well-known names for formations and other units in harmony with current usage. However, the recognition of four lithostratigraphical groups is overly complex and sometimes unworkable, and a two-fold subdivision is proposed instead. The Caerfai and much of the Lower and Middle Solva groups of earlier usage comprise conglomerates, pebbly sandstones and brightly coloured fine- to coarse-grained sandstones: all are here combined into a revised Caerfai Group. By excluding the silt-dominated Upper Solva division, the Caerfai Group is readily divided into lower and upper parts comprising conglomerates and/or pebbly sandstones, with sandstones and siltstones dominant in between. These rocks comprise the Ogof Golchfa (new), St Non's, Caerfai Bay, Caer Bwdy Bay, Newgale (new) and Porth Clais (new) formations. The following members (all new) are recognized within the Newgale Formation: the Trwyncynddeiriog, Pen-y-Cyfrwy and Cwm Mawr members. Ichnofossils from the upper part of the Ogof Golchfa Formation suggest that Cambrian deposition in the area began in Unnamed Cambrian Series 2 (C2) times. Body fossils and radiometric dating show that the Caerfai Bay to Porth Clais formations span the interval from Cambrian Series 2, Unnamed Cambrian Stage 3 (such stages are denoted by CS3, CS4 etc. below), probably to the Ptychagnostus gibbus Biozone (CS5). The former Upper Solva Group, Menevian Group and Lingula Flags comprise the newly erected Porth-y-rhaw Group, a 687 m-thick unit of fine sandstones, siltstones and mudstones, within which a further five formations are recognized. These are the Whitesands Bay, Menevia, Aber Llong (all new), Ogof Velvet (revised) and Treffgarne Bridge formations. The Porth-y-rhaw Group spans the Tomagnostus fissus Biozone (C3, Drumian) to the Olenus cataractes Biosubzone (Furongian, Paibian). The overall two-fold subdivision of the Cambrian part of the Dyfed Supergroup in the area accords broadly with the first lithostratigraphical scheme proposed for the area, and is comparable with the separation of the Harlech Grits and Mawddach groups in northern and central Wales. The oldest part of the Caerfai Group formed in a tectonically active context and records a transition from alluvial fan deposits, through braided stream environments, into transgressive nearshore marine sandstones. These sediments were derived from the west, and as well as locally-sourced material, include lithic clasts and grains derived from a presumed southwestwards extension of the Monian Composite Terrane. Above lie sedimentary deposits formed under a wide range of conditions, ranging from tidally-influenced to turbidites. Further evidence of active tectonism is found in the easterly-derived fan-delta sedimentary deposits of the higher part of the Caerfai Group and lower part of the Porth-y-rhaw Group. Deposition of the latter began with fine-grained turbidites deposited in a mid–outer shelf setting. These pass up first into hemipelagites, then into sedimentary rocks deposited on a storm-dominated shelf, and finally a very extensive shallow subaqueous delta platform formed in a passive margin setting.

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).