M. Williams, 2011. "Cambrian chronology", A Revised Correlation of the Cambrian Rocks in the British Isles, A. W. A. Rushton, P. M. Brück, S. G. Molyneux, M. Williams, N. H. Woodcock
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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).
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A Revised Correlation of the Cambrian Rocks in the British Isles
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