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Late Caradoc graptolitic faunal gradients across the Iapetus Ocean

Published online by Cambridge University Press:  01 May 2009

J. A. Zalasiewicz ,
A. W. A. Rushton  and
A. W. Owen
J. A. Zalasiewicz
Affiliation:
British Geological Survey, Key worth, Nottingham NG12 5GG, UK
A. W. A. Rushton
Affiliation:
British Geological Survey, Key worth, Nottingham NG12 5GG, UK
A. W. Owen
Affiliation:
Department of Geology and Applied Geology, Lilybank Gardens, University of Glasgow, Glasgow G12 8QQ, UK
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Abstract

Late Caradoc graptolite assemblages across the Iapetus Ocean in Wales and Scotland became progressively more disparate despite the narrowing of the ocean. We compare faunal distributions in continuous sections from opposite sides of Iapetus, at Whitland in South Wales and Hartfell Score in the Southern Uplands of Scotland. A comprehensive graptolite range-chart is given for each. The graptolite assemblages from the clingani Biozone are subdivided into a lower Ensigraptus caudatus Subzone and upper Dicellograptus morrisi Subzone at both localities, though the faunas differ in detail. Higher in the sequence, the distinctive Scottish linearis Biozone fauna is not recognizable at Whitland, its presumed equivalent being a fauna dominated by variable morphotypes of the genus Normalograptus. This suggests that environmental gradients (depth and/or temperature) were changing more rapidly than the geography. Significantly, the subsequent inception of limestone deposition at Whitland was approximately contemporaneous with widespread replacement of black mudstones by oxic, bioturbated sediments elsewhere in the Welsh Basin and in Scotland. This was possibly a response to an early phase of cooling prior to the end-Ordovician glaciation. In Wales the claimed hiatus at the Caradoc-Ashgill boundary may rather reflect biofacies variation.

Type
Articles
Information
Geological Magazine , Volume 132 , Issue 5 , September 1995 , pp. 611 - 617
Copyright
Copyright © Cambridge University Press 1995

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