Upper Ordovician global stratotype sections and points and the British historical type area: a Chitinozoan point of view
Vandenbroucke, T. R. A. (2005). Upper Ordovician global stratotype sections and points and the British historical type area: a Chitinozoan point of view. PhD Thesis. Universiteit Gent. Faculteit Wetenschappen: Gent. 295 pp.
Recent developments in chronostratigraphic procedure and new biostratigraphical insights necessitated the ongoing drastic revision of the Ordovician system's chronostratigraphy. A new global subdivision of the Ordovician is being established at the expense of the British chronostratigraphical framework, which has long been used as an informal 'global' standard. Carefully evaluated graptolite and conodont index species have already been selected to define the intra-systemic boundaries in their new Global Stratotype Sections and Points, or GSSP's for short. Although historically less well studies, the chitinozoans are a fossil group with a similar biostratigraphical potential as the two aforementioned groups. However, up to the present, they remained virtually unstudied in several of the newly proposed GSSP's for the Ordovician System. This project's main objective consists of the study of the chitinozoan assemblages in those newly proposed, or already ratified Global Stratotype Sections and Points. Because the Ordovician is a long period, the present study has necessarily been restricted to its three uppermost stages, grouped into the Upper Ordovician Series. Chitinozoan abundances and preservation permitting, a biozonation is established and a proxy is selected for each of the investigated boundary levels. This approach was particularly successful in the Swedis Fagelsang section, the new GSSP for the base of the Upper Ordovician Series/ Attempts to recover biostratigraphically equally significant assemblages from the Hartfell Score (Scotland) and Wangjiawan (China) Sections, respectively proposed as GSSP's for the bases of the second and third stage of the Upper Ordovician Series, proved somewhat less rewardingIn the second part of this volume; these new Upper Ordovician GSSP's are compard to the historical type areas of the British equivalent Caradoc and Ashgill Series and their subdivisions in the UK. Concomitantly, the first Upper Ordovican chitinozoan biozonation for British Avalonia is established. The historical Caradoc and Ashgill type sections in the Anglo-Welsh basin yield an important chitinozoan fauna; these data are supplemented with information from other British key sections which are famous for their accurate graptolite control. The latter include several Shelve Inlier sections, the Whitland road cutting, the Cardigan area and the Wye Valley around Rhayader. As a result, the established chitinozoan biozonation for British Avalonia is nicely tied to both the British chronostratigraphical framework and the graptolite biostratigraphy. It consists of thirteen chitinozoan biozones and subzones and is of importante as Avalonia lacks a formal biozonal scheme for the Ordovician, in contrast with the well-established biozonations in the other prominent palaeocontinents of that period in time. Interestingly, the newly drawn British biozonation scheme has a predominantly Baltoscandid signature, supplemented with endemic Avalonian and northern Gondwanan influences which fits Avalonia's migratory pattern during the Ordovician, away from Gondwana and approaching Baltica.Conclusions drawn in this study are based on the observation of 40860 chitinozoans specimens in total, handpicked from 295 samples which have been collected in c. twenty sections, inliers of areas in Great-Britain, China and Sweden.
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