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Late Ordovician brachiopod biofacies of the Girvan district, SW Scotland

Published online by Cambridge University Press:  03 November 2011

David A. T. Harper
David A. T. Harper
Affiliation:
David A. T. Harper, Geological Museum, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen K,Denmark; e-mail: [email protected]
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Abstract

ABSTRACT

Diverse and abundant brachiopod faunas, associated with unstable outer shelf and slope environments, occur through the Upper Ardmillan Group (upper Caradoc–upper Ashgill) in the Girvan district of SW Scotland. Representatives of the deep-water Foliomena fauna occur intermittently throughout the group, appearing in both the Whitehouse and Drummuck subgroups. This distinctive assemblage of small, thin-shelled brachiopods, including Dedzetina, Christiania, Cyclospira and Foliomena itself, first appeared in South China during the early Caradoc but had colonised the Laurentian margins by the late Caradoc. Within the upper Caradoc–lower Ashgill Whitehouse Subgroup, the Foliomena fauna is interbedded with a variety of other less cosmopolitan deep-water assemblages including the Onniella–Skenidioides and Lingulella–Trimurellina associations. Shallower-water environments in the middle Ashgill Lower Drummuck Subgroup hosted the Fardenia–Eopholidostrophia association in sands, and the Christiania-Leptaena association in muds and silts. The remarkable Lady Burn Starfish Beds in the upper part of the group contain a variety of brachiopod-dominated assemblages including the Eochonetes and Plaesiomys-Schizophorella associations, transported from various shelf locations, within a very diverse mid-Ashgill biota. Nevertheless, elements of the Foliomena fauna persisted to near the top of the Drummuck Subgroup, occurring as rare assemblages in more muddy and silty facies. The upper Ashgill High Mains Formation contains abundant elements of the terminal Ordovician Hirnantia fauna including Eostropheodonta, Hindella and Hirnantia itself, but also some taxa more typical of the Laurentian Edgewood Province. As a whole, the changing brachiopod biofacies monitor environmental fluctuations, on part of the Laurentian margin, driven by mainly eustatic and tectonic events.

Keywords

AshgillBrachiopodaCaradocpalaeobiogeographypalaeoecologypalaeoenvironments
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 91 , Issue 3-4: The Southern Uplands Terrane: Tectonics and Biostratigraphy within the Caledonian Orogen , 2000 , pp. 471 - 477
Copyright
Copyright © Royal Society of Edinburgh 2000

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