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Journal of the Geological Society; 2009; v. 166; issue.1; p. 129-135;
DOI: 10.1144/0016-76492007-126
© 2009 Geological Society of London

Research Article

Global C cycle perturbations recorded in marbles: a record of Neoproterozoic Earth history within the Dalradian succession of the Shetland Islands, Scotland

A.R. Prave1, R.A. Strachan2 & A.E. Fallick3

1 Geological Research Unit, University of St. Andrews, St. Andrews KY16 9AL, UK
2 School of Earth & Environmental Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK
3 Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, UK

*Corresponding author (e-mail: ap13{at}st-andrews.ac.uk)

The Shetland Islands in Scotland contain a c. 12 km thick, mostly siliciclastic Neoproterozoic succession that experienced deformation and greenschist- to amphibolite-facies metamorphism during the Ordovician–Silurian Caledonian Orogeny. Correlating these rocks with Neoproterozoic successions elsewhere has remained problematic. However, four marble units punctuate the succession; these are, in stratigraphic order, the Weisdale, Whiteness, Girlsta and Laxfirth Limestones, two of which record hallmark C-isotopic excursions diagnostic of Neoproterozoic time. The Whiteness Limestone is lithologically similar to the 635 Ma Marinoan post-glacial cap carbonate sequence and displays a {delta}13Ccarbonate profile in which values decline from a range of 0 to +2{per thousand} to a nadir of –5{per thousand} and then rise towards and oscillate around 0{per thousand}. The Girlsta Limestone is a 700–900 m thick marble unit with values of –9{per thousand} to –11{per thousand} and is interpreted as recording the late Neoproterozoic Shuram–Wonoka event. These correlations permit linking the Shetland succession to Neoproterozoic sections elsewhere and also show that rift-related rocks in Shetland post-date the Shuram–Wonoka event (post c. 600 Ma–pre c. 550 Ma) thereby supporting the concept of a diachronous phase of extensional tectonism, from c. 700 Ma in east–central Laurentia to near the close of the Proterozoic in northeastern Laurentia, prior to the opening of the Iapetus Ocean.





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A.R. Prave, A.E. Fallick, C.W. Thomas, and C.M. Graham
A composite C-isotope profile for the Neoproterozoic Dalradian Supergroup of Scotland and Ireland
Journal of the Geological Society, 2009; 166: 845 - 857.
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