Abstract:

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 δ13Ccarbonate profile in which values decline from a range of 0 to +2‰ to a nadir of −5‰ and then rise towards and oscillate around 0‰. The Girlsta Limestone is a 700–900 m thick marble unit with values of −9‰ to −11‰ 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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