Advances in Ordovician Geology

Citations Over Time

Abstract

The series of the Ordovician System in the Anglo-Welsh area have been a standard reference in Ordovician chronostratigraphy for many years, and should remain so in the interest of a common language for international use. Their original definition was not in accord with modern stratigraphic precision, and to this end it is necessary to redefine bases for the series within continuous sequences. Such sequences have been identified for the bases of ail the series. Even though there are breaks in the succession locally or in the original type areas, these shortcomings are minimized by considering these other sections in the Anglo-Welsh area as potential stratotypes. The Anglo-Welsh faunas include widespread taxa virtually throughout the Ordovician, which permit international correlation; the monographic coverage of these faunas compares with that from anywhere in the world. lt is thought to be possible to modify the horizon at which series boundaries were drawn originally in the interests of international correlation, as long as nomenclature is conserved. The possible stratotypes for bases of the series of the Ordovician System in the type area are reviewed briefly, and new faunal logs are given where necessary. The base of the Tremadoc is taken at an horizon coincident with the appearance of nematophorous graptolites, the only practicable horizon in the type area. The base of the Arenig poses particular problems because of the poorly fossiliferous nature of available sections. However, newly discovered sections in the Lake District indicate that there may have been ,w major break in sedimentation at the top of the Tremadoc. Afossiliferous mixed graptolite-trilobite section is available for the base of the Llanvirn. Although there are candidates for the base of the Llandeilo (current usage), the comparative brevity of this series suggests that it may be better united with the Llanvirn to form a new, combined series commensurate in extent with the other series of the Ordovician. The name Llandeilo could be retained for this new concept. The Llanvirn could also be retained as the earlier stage within it, and a new stage name, for which the Dyneforian is suggested, proposed for the later part, equivalent to some or al! of the traditional Llandeilo; Just how much will de pend on the definition of the base of the Caradoc, for which three possibilities are proposed as a basis for discussion. The base of the Ashgill has been located precisely in a section in the Murthwaite Inlier, Cautley, which is proposed as a suitable international type section.

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