Acritarchs of Llandeilo and Caradoo age from classic localities in Britain
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The last two decades have seen the publication of a large number of papers dealing with Palaeozoic acritarchs including many devoted to the description of Ordovician assemblages. It has become clear that, these microfossils have considerable potential for correlation and that they are frequently recovered From sequences otherwise devoid of fossil material. In Britain, acritarchs have been described in detail from the Cambrian (Potter 1974 M. S.); Tremadoc (Rasul 1971 M. S., 1974, 1977a, l977b) and from the Silurian (Lister 1966 M. S., 1970; Hill 1974 M. S.) Until the recent work of Booth (l979 M. S.) dealing with Arenig-Llanvirn assemblages, no detailed systematic investigation of British Ordovician acritarchs had been attempted. The primary objective of the present study is to provide a comprehensive description of acritarchs from the Middle and Upper Ordovician of Britain, with particular emphasis on the type-areas of the Llandeilo and Caradoc. In conjunction with the work of Booth it is hoped that this account will go some way towards providing a biostratigraphical framework which will facilitate the use of these microfossils for the purposes of correlation in the Ordovician.Keywords:
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The most radical global change, involving the biology, chemistry and physics of life, occurred near and at the stratigraphical level of the boundary between the Neoproterozoic System (Precambrian) and the Cambrian System. This stratigraphical level is held by consensus to be the Proterozoic Eonothem-Phanerozoic Eonothem Boundary, and is regarded as being of fundamental importance. Research has involved not only archaeocyathans, shelly small problematica, earliest trilobites, stromatolites and algae, trace fossils, soft-bodied metazoans, microphytolites and acritarchs, but also physical and chemical aspects: radio-isotope dating, carbon, strontium and oxygen isotope stratigraphy, polar wandering relating to palaeogeography and magnetostratigraphy. A selected bibliography (1985-1990) is appended to enable further reading of the extensive research contributions.
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The benchmark volume edited by Christopher Scotese & Stuart McKerrow (1990), the so-called ‘Green Book’, marked a turning point in Palaeozoic biogeography and geography. The 40 papers in that volume included substantial new data and important syntheses on palaeoclimatology, palaeomagnetism and the distribution of climatically sensitive sediments, but the majority of contributions were on Palaeozoic biogeography. Since 1990 there have been major advances in the taxonomy of Early Palaeozoic organisms, the correlation of Lower Palaeozoic rocks together with numerical methods for the analysis of fossils and their distributions. Moreover and most significantly there has been a quantum leap in the accuracy and precision of palaeogeographic reconstructions, reconciling in many cases palaeomagnetic data and the distribution of fossil organisms.
The Early or (Lower) Palaeozoic was an interval characterized by a major radiation of marine life, including not only the ‘Cambrian Explosion’, but also the ‘Great Ordovician Biodiversification’. An International Geoscience Programme (formerly International Geological Correlation Programme, IGCP) was dedicated to the Great Ordovician Biodiversification Event (IGCP 410, 1997–2002). This programme focused on the Ordovician, but it also analysed the roots of the Ordovician biodiversification in the Cambrian, the first big mass extinction at the end of the Ordovician, and the Silurian recovery. A subsequent programme, the IGCP 503, ‘Ordovician Palaeogeography and Palaeoclimate’, followed from 2004 to 2009. Several special issues and review papers resulted from this programme, including special issues on Lower Palaeozoic palaeoenvironments (Servais & Owen 2010) and Lower Palaeozoic sea-level and climate (Munnecke et al. 2010).
The papers presented here originated from the ‘Absolutely Final Meeting of IGCP 503: Ordovician Palaeogeography and Palaeoclimate’ in Copenhagen, September, 2009 (see Harper et al. 2011). That symposium focused especially on the palaeogeographical dimension of the Cambrian–Ordovician marine radiation; the current volume critically uses the available world pre-drift maps for the …
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Abstract The major steps in the development of Early Palaeozoic stratigraphy are examined, with special emphasis on early Journal papers by Murchison and Sedgwick, and on their conception of systems and series, which permitted long-distance correlation. Unlike other periods, the Ordovician and Silurian were originally split into series; most stages have only been defined in the past 60 years. From 1880, Lapworth’s graptolite zones have allowed much greater chronological precision. More recently, other methods have been developed for recognizing small time divisions, including studies in gradational evolution. A significant new advance is the correlation (by biostratigraphy) of short-lived physical events such as magnetic reversals and sea-level and climatic changes.
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HARRIES, P. J. (ed.) 2003. High-Resolution Approaches in Stratigraphic Paleontology. Topics in Geobiology Series Volume 21. xv+474 pp. + CD-ROM. Dordrecht, Boston, London: Kluwer. Price Euros 159.00, US $175.00, £110.00 (hard covers). ISBN 1 4020 1443 0 - Volume 142 Issue 3
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The current state of and the latest developments in the conodont-based biostratigraphical studies of Upper Ordovician and lower Silurian strata in Estonia are discussed.An updated conodont zonation is proposed, based on successive first and/or last occurrences of taxa as interpreted from the stratigraphical record of Estonia as well as of other parts of the world.The zonation includes mainly units proposed by different authors at different times, with some revision by the author.Currently, 32 zones (with 14 subzones) and 3 informal units can be recognized in the Upper Ordovician to upper Wenlock (Homerian) of Estonia.
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The proposal for a symposium on the Triassic rocks of Britain sprang from two sources. The first was the inauguration by the Council of the Geological Society of a series of Committees to review the stratigraphical nomenclature and correlation of the Phanerozoic rock succession. It was apparent in the case of the Trias that existing published data provided an inadequate basis for such a review, and that it was desirable to organize up-to-date regional appraisals of modern information. The second incentive was provided by the very vigorous discussion which followed the reading of Dr Audley-Charles' paper on the Stratigraphical Correlation of the Triassic rocks in Britain, a discussion which showed that much unpublished information in fact existed and that particularly critical aspects of the correlation could be investigated on the basis of existing data. In addition results of a study of the correlation of the German succession with the North Sea Trias and that of eastern England have now become available. The major problem which has remained unsolved in British Triassic stratigraphy has been the establishment of reliable time planes, in a series which is essentially continental and lacking macrofossils, and in which classification has hitherto been entirely on a lithostratigraphical basis. New data of correlative value are now being supplied by palynological investigation, by evidence at some horizons from microplankton, by study of cyclothemic units and by electric, radioactive and mechanical logging of boreholes. The original basis of correlation is therefore being replaced by new lines of investigation many
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