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PALAEOZOIC/Ordovician 177 however, and to date a type section has yet to be chosen A standard time division close to the Middle Ordovician, the Darriwilian, has been recognized on the basis of graptolites The base of the Upper Ordovician has been agreed to be the base of the widespread Nemagraptus gracilis biozone This horizon coincides with the base of the Caradoc ‘Series’ as now defined A type section in the Fa˚gelsang section, Sweden, has been selected for its definition Because this threefold series subdivision has not yet achieved wide currency, it is expedient to continue to use the fivefold subdivision in this chapter Strictly speaking, these should no longer be referred to formally as ‘Series’, as they are effectively regional stages The suffix ‘-ian’ is therefore employed below Because of the high degree of regional differences in Ordovician strata, several widely separated areas have become important in defining Ordovician temporal subdivisions on a regional to global scale Biozonal standards have been based on graptolites, conodonts, and trilobites, but these are found together in only a minority of rock sections and they, too, show provinciality Much effort has accordingly been expended in tying together graptolitic and conodont-based zonal schemes of an upper part of the Tremadocian (Hunnebergian) that is not well-developed in Britain Arenigian Arenigian rocks overlie the Tremadocian rocks in North Wales, and were named after a mountain, Arennig Fawr, where they are well exposed However, in North Wales there is an unconformity at the base of the Arenigian, which is marked by a transgressive sandstone, and so it is inadequate as a type area (Figure 1) The Arenigian succession is better developed in South Wales and Shropshire, where thick sequences of mudstones, shales, sandstones, and turbidites have yielded diverse trilobite faunas of shallow-to-deep shelf aspect with, more locally, graptolites and brachiopods Because of the disparate biogeography of the Early Ordovician, most of the species are of relatively local occurrence, and correlation of these strata internationally can be difficult For some years a de facto base of the Arenigian has been recognized at the base of the widespread graptolite Tetragraptus (Etagraptus) approximatus biozone, which can be identified in North and South America, Tremadocian The small town of Tremadoc lies in North Wales at the northern end of Cardigan Bay A series of dark shales and mudstones of the Dol cyn Afon Formation – often cleaved and weakly metamorphosed – overlie rocks of the Upper Cambrian There is often a stratigraphical break between the two, but in at least one section a complete and conformable transition between the two systems has been demonstrated The Tremadocian is recognized by the incoming of net-like rhabdosomes of the widespread graptolite Rhabdinopora flabelliformis subspecies, the oldest planktonic graptoloid species These rocks were deposited in comparatively open shelf environmental conditions, and the graptolites are accompanied by a variety of trilobites, of which members of the Family Olenidae are a prominent component, some species of which are also very widespread The genus Jujuyaspis, for example, is known from North and South America, Scandinavia, and China in earliest Ordovician strata In North America, the stage name Ibexian (approximately the same as ‘Canadian’ used by earlier authors) is employed for Early Ordovician strata, based on well-exposed sections in Utah in the Great Basin The Ibexian embraces all of the Tremadocian and part of the Arenigian Both the Tremadocian and Ibexian have been further subdivided into chronostratigraphical subdivisions of regional application The graptolite and conodont succession is particularly well-known in the Baltoscandian area and indicates the presence there Figure Ordovician (Arenigian) strata lying unconformably over Cambrian rocks on the Llyn Peninsula, North Wales
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