Welsh Basin

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

Scotland : Hebridean Terrane

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 & 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

A palynological investigation of samples from the Devonian of SW England

The results of a palynological investigation aimed at determining the ages of 17 samples from the Devonian of SW England are reported. A key element in the extraction of determinable spores from these samples was successful oxidation of the material using a strong oxidising agent (fuming Schultze solution – a mixture of potassium chlorate, KClO3, and fuming nitric acid, HNO3). This rendered originally opaque and therefore largely undeterminable spores translucent so that key morphological features on the surface of the spores could be seen. Stratigraphically important species are figured

The stratigraphy, correlation, provenance and palaeogeography of the Skiddaw Group (Ordovician) in the English Lake District

A new lithostratigraphy is presented for the Skiddaw Group (lower Ordovician) of the English Lake District. Two stratigraphical belts are described. Five formations are defined in the Northern Fells Belt, ranging in age from Tremadoc to early Llanvirn. They are all mudstone or sandstone dominated, of turbidite origin; in ascending order they are named the Bitter Beck, Watch Hill, Hope Beck, Loweswater and Kirk Stile formations. Two formations are defined in the Central Fells Belt, ranging in age from late Arenig to Llanvirn. These are the Buttermere Formation - a major olistostrome deposit - overlain by the Tarn Moor Formation, consisting of turbidite mudstones with volcaniclastic turbidite sandstone beds. A revised graptolite and new acritarch biostratigraphy for the Skiddaw Group is presented with eight graptolite biozones and thirteen acritarch assemblages and sub-assemblages. The provenance of the group is assessed from detailed petrographical and geochemical work. This suggests derivation, in the early Ordovician, largely from an old inactive continental arc terrane lying to the south-east, with the appearance of juvenile volcanic material in the Llanvirn. Comparisons and correlations of the Skiddaw Group are made with the Isle of Man and eastern Ireland

Cambrian of Ireland

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 confined to some areas of the southeast (Fig. 16) 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 is 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 1994a) although there is no direct biostratigraphical evidence

Biostratigraphical divisions

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), though 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, whilst those in the Furongian are local range-zones of selected species

Biostratigraphical dating of the Ingleton Group, northern England

Palynological slides from rocks assigned to the Ingleton Group in the Beckermonds Scar Borehole and from the Ingleton Group at outcrop have been examined in an attempt to replicate the Arenig date assigned to the borehole section. No microfossils have been seen in slides from borehole samples, apart from those originally used to date the section in 1982, from a sample at 396.20 m. However, acritarchs have been observed in slides from three outcrop samples, one from the vicinity of Thornton Force and the other two from the River Doe section above Ingleton. The acritarchs are similar in preservation to those from the borehole. Most specimens recorded are simple acanthomorph acritarchs that are not especially diagnostic of age, but two specimens of Veryhachium trispinosum were recorded from one of the samples from the River Doe. The species has its first appearance in the latest Tremadoc (Early Ordovician) and is also present in the Beckermonds Scar assemblage. The acritarchs provide the first biostratigraphical evidence for the age of the Ingleton Group at outcrop, and strengthen the case for the Beckermonds Scar specimens being in situ. Together, the outcrop and borehole material provide consistent evidence that the Ingleton Group is no older than Early Ordovician

Recycled aritarchs as provenance indicators : implications for Caledonian Terrane Reconstruction

Silurian successions deposited on the margins of the Iapetus Ocean in the English Lake District, Southern Uplands of Scotland and Ireland contain recycled Ordovician acritarchs. These include taxa known only from Avalonia and Gondwana, with other recycled acritarchs possibly derived from Baltica. There is no palynological evidence to indicate that Laurentiaconstituted a source area. The data presented herein suggest that provinciality in derived microfossils may be profitably exploited in provenance studies