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Journal of the Geological Society; 1998; v. 155; issue.2; p. 401-411;
DOI: 10.1144/gsjgs.155.2.0401
© 1998 Geological Society of London

Article

The Proterozoic–Cambrian transition within the ‘Charnian Supergroup’ of central England and the antiquity of the Ediacara fauna

D. McILROY1,2, M. D. BRASIER1 & J. B. MOSELEY3

1 Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK (e-mail: martin.brasier{at}earth.ox.ac.uk)
2 Currently at: Department of Earth Sciences, University of Liverpool, Liverpool L69 3BX, UK
3 Dingle Nook, Shirley Lane, Longton, Preston PR4 5NJ, UK

A revised interpretation for the ‘Charnian Supergroup’ is suggested from ichnological and lithological comparisons with the Cambrian of Nuneaton and other parts of the Avalonian superterrane. Late Neoproterozoic volcaniclastic sediments contain an Ediacara fauna in the Maplewell Group. These are succeeded by one or more inferred erosional breaks and overlain by passive margin sediments (quartzarenites, glauconitic greywackes, pelites) of the Brand Group which yield Cambrian trace fossils, including abundant large Teichichnus. This transformation is compared with that between the Neoproterozoic Caldecote Volcanic Formation and the Cambrian Hartshill Formation at Nuneaton. The Hanging Rocks Formation of the Brand Group lies above a major erosional break but shares a volcanogenic island arc petrology with the underlying Maplewell Group. Conglomeratic sandstones at the base of the Brand Hills Formation contain purple pelite clasts comparable with underlying lithologies, plus clasts of weathered diorite showing granophyric textures identical to those seen in the enigmatic South Charnwood Diorites. Nd–Sm isotope studies confirm the geochemical similarity between these diorites and those that intruded the Caldecote Volcanics at Nuneaton, previously U–Pb zircon dated at 603±2 Ma. These strong similarities with the Nuneaton succession suggest that deposition of the Brand Hills Formation in the Cambrian was preceded by the local unroofing of diorite and erosion of underlying mudrocks, much as seen in other parts of Avalonia. A date of 603±2 Ma for the South Charnwood Diorites provides a minimum age for the local Ediacara fauna but this is in need of substantiation.


Keywords: Proterozoic, Precambrian, Cambrian, Ediacara, ichnofossils.




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