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Cambrian and Ordovician

Chronostratigraphy The Cambrian and Ordovician systems originated from study of rock successions in Wales, and the resulting classification set a stratigraphical standard that was adopted worldwide. For example, the development of graptolite biostratigraphy owed much to Lapworth’s pioneering 19th century studies of Scottish and Welsh successions, which ultimately contributed to the existing Lower Palaeozoic timescale.

British Cambrian and Ordovician rocks are mostly of marine origin, rangeing from sandstones and limestones from the shore and shallow shelves to mudrock facies that accumulated in deep basins or on the continental slopes. Passive margin transects from shelf to deep basin are seen in both Cambrian and Ordovician rocks, and are recognized in both Anglo-Welsh and Scottish basins. British Cambrian and Ordovician rocks demonstrate great changes, in geography, climate and the evolution of life. Most striking of all are the differences in geology of Anglo-Welsh sites (on Avalonia) as compared with Scottish (on Laurentia), caused by their former large geographic separation across the Iapetus Ocean, leading to dramatic and clear differences between Scottish and English Cambrian-Ordovician successions. During the Cambrian and early Ordovician, most of Wales and England lay at a passive plate margin on the flanks of Gondwana (at least until Floian-Dapingian [Arenig] times). From the Cambrian though the Ordovician the Avalonian plate moved from high southerly latitudes into sub-equatorial southern latitudes. As Avalonia moved it experienced changing climates effecting sedimentary and biotic environments, duly reflected in the rock and fossil record.

In the Cambrian the Welsh Basin has relatively thick sequences of basinal mudcocks (Harlech Grits, Mawddach in North Wales; Caerfai, Solva and Menevian Groups in south wales), as opposed to the adjoining Midland Platform with thinner shelf sequences (e.g. Wrekin Quartzite, Upper and Lower Comley) with a scattering of relatively small but stratigraphically important inliers. The passive ocean-facing margin in northern England accumulated thick clastic deposits in the earlier Ordovician (mainly Floian-Dapingian [i.e. Arenig]; Skiddaw Group), overlain by thick arc volcanics of about mid Ordovician (Borrowdale Volcanic Group). A diachronous marine transgression followed, introducing relatively thin neritic deposits. These include a full succession of the higher Caradoc and Ashgill series (Katian to Hirnantian), which form the base to a thick foreland basin succession deposited in the Silurian. In the Southern Uplands of Scotland there are thick developments of Ordovician rocks (Crawford and Moffat Shale groups) in a basin produced during the closure of the Iapetus Ocean, with Ordovician successions sometimes showing striking changes of thickness and facies. The Hebridean Terrane (formed on the margin of Laurentia) in NW Scotland has a foreland margin succession that consists of relatively thin shelf deposits of mid Cambrian to mid Ordovician in age (Ardvreck and Durness groups).

The early Cambrian was the time of the ‘Cambrian evolutionary explosion’ when a great variety of life-forms first appeared, including first representatives of several groups whose descendants exist today. In the early Ordovician, the Cambrian Fauna was largely replaced by a more diverse ‘Palaeozoic Fauna’, which suffered mass extinction at the end of the Ordovician. The groups most widely used in British Cambrian and Ordovician stratigraphy are brachiopods, trilobites and graptolites. Amongst the other fossil groups, conodonts occur sporadically and where they occur, are often of great value, whilst cephalopod molluscs, ostracods and echinoderms are locally useful and in particular environments. Among the microfossils, organic-walled microfossils (acritarchs and chitinozoa) are locally important, where their vertical distribution is known. Trilobites are valuable for correlation in the Cambrian and remain so in the Ordovician, especially in deeper- water settings. Planktonic graptolites appeared in the early Tremadocian and usefully diversified through the Floian and Darriwilian (i.e Arenig and Llanvirn). Good excursions in Scotland are to the Ballentrae ophiolite and graptolites.

Further Information

Brenchley, P.J., Rushton, A.W.A., Howells, M. & Cave, R. (2006). Cambrian and Ordovician: the early Palaeozoic tectonostratigraphic evolution of the Welsh basin, Midland and Monian Terranes of Eastern Avalonia. In: Brenchley & Rawson, P.F. (eds), The Geology of England and Wales, 2nd Edn, p 25-74, The Geological Society, London.

Rushton, A.W.A., Owen, A.W., Owens, R.M. & Prigmore, J.K., (2000). British Cambrian to Ordovician Stratigraphy , Geological Conservation Review Series, No. 18, Joint Nature Conservation Committee, Peterborough, 436pp.

Robin, L. Cocks, M., Fortey, R.A. & Rushton, A.W.A. (2010).Correlation for the Lower Palaeozoic. Geological Magazine, 147,171–180.