Abstract Since publication of the first edition of the Cambrian correlation report (Cowie et al . 1972), there have been important advances in the study of the Cambrian System. New research has resulted in changes that affect the extent, nomenclature and correlation of the Cambrian rocks in every part of the British Isles. The boundaries of the Cambrian, under debate in the 1970s, have now been decided by international agreement and the Cambrian Subcommission is working towards establishing a complete international standard for chronostratigraphical subdivisions of the Cambrian. Work since 1972 has led to a much more detailed understanding of the plate-tectonic and palaeogeographical history of Britain and Ireland; a number of reliable radiometric ages have been determined from Cambrian rocks, both in the British Isles and abroad, and there have been new stratigraphical, sedimentological and biostratigraphical studies of nearly all of the British and Irish outcrops. Syntheses of the palaeogeography of the Cambrian of the British Isles were given by Brasier et al . (1992) and Brenchley et al . (2006). In many cases older work has been confirmed and modern revision has caused little change, but in other cases there have been important new results leading, for example, to the inclusion of parts of the Charnwood massif in central England and the Mona Complex in Anglesey within the Cambrian, and the transfer of the Lickey Quartzite from the Cambrian to the Ordovician (Fortey et al . 2000, p. 29).

Abstract Many chronostratigraphical divisions of the Cambrian have been named world-wide. They were mostly intended to be used regionally in the several palaeogeographical provinces that have been recognized around the various continental plates. An indication of the extent and variety of these schemes can be obtained from international correlation charts that show independent columns for Laurentia, Siberia, Kazakhstan, China, Australia, etc. (e.g. Shergold in Kaesler 1997, pp. 301-311; Geyer & Shergold 2000). In order to stabilize stratigraphical usage in the Britsh Isles, Cowie et al . (1972, p. 8) proposed three regional series, the Comley, St David’s and Merioneth series, which corresponded to the Lower, Middle and the lower half of the Upper Cambrian as then used in Britain, and those authors accepted the long-recognized Tremadoc Series as the uppermost part of the Cambrian. The new series divisions were described but not formally defined. Over the last decade, working groups of the Cambrian Subcommission have sought to identify the most satisfactory and widely acceptable horizons for international correlation, and aimed to base new internationally recognized series and stages on their findings. The results at the time of writing are shown in Figure 1, and indicate the intention to define four series, collectively divided into ten stages, as discussed by Babcock et al . (2005). The application of these divisions to the British regional divisions of the Cambrian are summarized in Cocks et al . (2010).

Abstract Since the 19th century geologists working on Cambrian rocks world-wide have relied largely on trilobites as biostratigraphical guides, and these remain important, especially in Series 3 and the Furongian Series, where they enable refined correlations. In Series 2, especially in its lower part where trilobite biostratigraphy becomes increasingly difficult, other criteria are employed, for example the distribution of small shelly fossils, bradoriid arthropods and, where available, archaeocyaths. In Avalonia there are no archaeocyaths, but the bradoriids have been revised and exploited stratigraphically (Siveter & Williams 1997; Williams & Siveter 1998). The recent development of acritarch biostratigraphy, which has made a vital contribution, is discussed below. Towards the base of the Cambrian, body fossils may be very scarce and trace fossils have been used biostratigraphically (Narbonne & Myrow 1988; Bland & Goldring 1995; McIlroy & Horák 2006), although they may be difficult to work with. All the zones referred to are biozones (Rawson et al . 2002), most of those in the Terreneuvian and Series 2 and 3 being assemblage zones, whereas those in the Furongian are local range-zones of selected species.

Abstract Since the first edition of the Cambrian correlation report was published (Cowie et al . 1972), there have been great improvements in the techniques of radiometric dating. Cowie et al . (1972, p. 12) recognized no reliable radiometric ages from British Cambrian rocks, but provisionally accepted ‘with questionable accuracy’ that the age of the base of the Lower Cambrian was 570 Ma and the top of the Upper Cambrian (including Tremadoc) was about 500 Ma. The subsequent development of the U-Pb dating methods has given much more consistent and reliable ages with small errors, showing that the Cambrian Period was of shorter duration and not as old as Cowie et al . had supposed (Tucker & McKerrow 1995). Thanks to the initiative of Landing and his co-workers (Landing et al . 1998, 2000; Davidek et al . 1998), several radiometric ages have been obtained using zircon crystals extracted from volcanic and volcaniclastic detrital rocks in Avalonia, and from Morocco on the margin of Gondwana, which is depicted as adjacent to Avalonia in Chapter 6. Examples of these radiometric dates that can be correlated to the stratigraphical standard adopted here are shown in Figure 1, whilst interpolated dates, drawn from the syntheses of Ogg et al . (2008), Gradstein et al . (2008) and Peng & Babcock (2008) are given in brackets. Development of a precise chronology for the four series and ten stages of the Cambrian is a work in progress (Shergold & Cooper 2004; Zhu et al . 2006).

Abstract Carbon isotope excursions (CIEs) from the latest Proterozoic and Cambrian have been linked with major global biotic/oceanographic events (e.g. Salzmann et al . 2004) and are viewed as important for Cambrian correlation. Zhu et al . (2006) provide a synthesis of the chronology, chronostratigraphy and carbon isotope stratigraphy for the Cambrian, identifying and naming ten CIEs.

Abstract A complicating factor in correlating Cambrian sections in Britain and Ireland is that host terranes now geographically close to one another may have been widely separated in Cambrian time. This section reviews a terrane map of Britain and Ireland (Fig. 5) and its implications for Cambrian correlation. There are intrinsic limitations in palaeomagnetic restoration of terranes to their geographic positions in Cambrian times. Additional evidence is available from faunal distributions, tectonic reconstructions, and isotopic characterization of sediment source areas. Using these methods, some constraints can be put on the Cambrian positions of terranes relative to each other and to the major continents from which they were detached in post-Cambrian time. Some Neoproterozoic relationships are also discussed, because of doubts in some terranes as to whether late Neoproterozoic successions range up into the Cambrian.

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 Outcrops of Cambrian rocks occur on the Midland Microcraton only in scattered inliers. In the present charts, the inliers are grouped according to their distribution on or around two major basement blocks in the English Midlands, the Wrekin Block to the west and the Charnwood Block to the east (Fig. 5), corresponding to two sub-terranes recognizable in the Neoproterozoic basement (Pharoah et al . 1987). The Cambrian rocks are mostly exposed along the lineaments that form the margins of these blocks (Fig. 7), and some of the successions show evidence of movements during the Cambrian that caused local subsidence or uplift (Smith & Rushton 1993; Prigmore et al . 1997). Numerous borehole records indicate that there is a substantial area of Tremadocian subcrop with Cambrian locally (Molyneux 1991).

Abstract At the time of the report on Cambrian correlation by Cowie et al . (1972), the pre-Ordovician rocks of Anglesey, the ‘Mona Complex’, were generally accepted as being of Precambrian age. Radical re-interpretation of the Mona Complex followed from palaeontological evidence reported by Muir et al . (1979) and from a plate-tectonic evaluation by Barber & Max (1979), and since then strong evidence supporting the Cambrian age of a substantial part of the Mona Complex, the Monian Supergroup, has been published. Summary accounts of the Mona complexes (or terranes) are to be found in works dealing primarily with Precambrian or Neoproterozoic rocks: Gibbons et al . in Gibbons & Harris (1994), Horák & Gibbons in Carney et al . (2000), and McIlroy & Horák in Brenchley & Rawson (2006). In these reviews the former Mona Complex is treated as three fault-bound complexes, of which the Coedana Complex, of granite and gneiss, and the Eastern Schist Belt (Aethwy Terrane), are accepted as of Neoproterozoic age, whereas the Monian Supergroup, a thick metasedimentary succession, is considered to be of Cambrian age (Fig. 13). The lowest division of the Monian Supergroup (column 20), the South Stack Group, has yielded detrital zircons that gave an age of 522 + 6 Ma, that is, well within the Cambrian, and the strata cannot be older than that.

Abstract The Cambrian succession in the Hebridean Terrane extends for about 200 km from near Durness on the northern coast of the Scottish mainland SSW to Skye (Fig. 14). The classic account of the whole region is the memoir by Peach et al . (1907), which work had a great influence on British geological studies in the Lower Palaeozoic during the earlier part of the 20th century. The geology of the region was described by Park et al . (2002) and some critical localities were reviewed by Prigmore & Rushton (1999). Sedimentological studies by Swett and his collaborators (Swett 1969; Swett & Smit 1972) and by McKie (1990a-c, 1993) and new litho-and biostratigraphical work, including the study of microfossil faunas and floras, have led to a re-appraisal of the stratigraphical succession and its correlation. The Cambrian succession (Fig. 15, Column 22) consists of a generally upward-fining sequence of clastic deposits, the Ardvreck Group of early Cambrian age, followed by a succession of carbonate formations, the Durness Group, of Cambrian to early Ordovician age. The strata were deposited on a stable, gently sloping shelf, and because the facies represented are generally similar throughout the length of the outcrop, it is inferred that the trend of the shelf lay approximately parallel to the line of the present outcrop (Bluck 2007).

Abstract Strata of the Dalradian Supergroup occupy a great part of the Central Highlands Terrane (Fig. 5) in Scotland, underlie extensive areas of northwestern and western Ireland, and also form a small part of northern Co. Antrim (Figs 14 & 16). They constitute very thick successions, in places .10 km thick, of variegated metasedimentary rocks, known by radiometric data to be largely of Neoproterozoic age. A few records of fossil algae from the Argyll Group of the Scottish Dalradian (Pantin 1955; Litherland 1975) are compatible with such an age. All the Dalradian rocks are described in detail in the Revised Correlation of the Precambrian Rocks in the British Isles (Gibbons & Harris 1994; Harris et al . 1994; Sanders 1994). They are also brie?y discussed here because the age of the uppermost division in Scotland, the Southern Highland Group, is considered to include Cambrian strata, although this has been controversial. Harris et al . (1994, p. 38) summarized evidence that had been used to infer an early Cambrian or, locally, an early Ordovician age for the upper part of the Dalradian succession. However, in the same work Harris et al . (1994) accepted the conclusions of Rogers et al . (1989) that there was strong isotopic evidence that the whole of the Dalradian succession was of late Precambrian age; and they proposed that neither the Leny Limestone nor any other Cambrian-Ordovician strata occurring adjacent to or within the Dalradian tract in Scotland could be part of the Southern Highland Group.

Abstract As discussed in Chapter 11 of this report, some authors have considered that the uppermost part of the Southern Highland Group of the Dalradian in Scotland might be Lower Palaeozoic, possibly Early Cambrian to Early Ordovician. The age of the Irish Dalradian is also not fully clear (Daly 2001). Thus, in Ireland, strata that are clearly Cambrian in age are restricted in outcrop, being con?ned to some areas of the SE (Fig. 17) in the Leinster Terrane (Murphy et al . 1991; Woodcock 2000; Holland 2001, 2009). There they comprise the Bray and Cahore groups and part of the Lower Palaeozoic Ribband Group (Figs 18 & 19). In addition, in southernmost Leinster (Fig. 20), palynological studies have now shown that the Cullenstown Formation, which is not assigned to any of these groups, is also Cambrian. In the same area, the Ballycogly Group mylonites, which occur along the boundary between the Leinster Terrane and the Precambrian basement of the Rosslare Terrane, are considered to be Cambrian (Tietzsch-Tyler & Sleeman 1994 a ) although there is no direct biostratigraphical evidence. The Leinster Terrane is made up of several sub-terranes: particulars of these and the relevant references are given in Chapter 6 of this report. Within these terranes, Brack et al . (1979) grouped the Lower Palaeozoic rocks into three belts. More recent work has shown that, as far as the Ribband Group is concerned, a fourth belt can be established (McConnell et al . 1999).

Abstract 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