E. C. Freshney, B. E. Leveridge, R. A. Waters, C. N. Waters, 2011. "Culm Basin", A Revised Correlation of Carboniferous Rocks in the British Isles, C. N. Waters, I. D. Somerville, N. S. Jones, C. J. Cleal, J. D. Collinson, R. A. Waters, B. M. Besly, M. T. Dean, M. H. Stephenson, J. R. Davies, E. C. Freshney, D. I. Jackson, W. I. Mitchell, J. H. Powell, W. J. Barclay, M. A. E. Browne, B. E. Leveridge, S. L. Long, D. McLean
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To the south of the Mississippian platform carbonate successions of South Wales (Chapter 5) and Bristol, Mendips and Somerset (Chapter 6), Carboniferous rocks predominantly occur within the strongly deformed Culm crop of SW England. The Culm Basin has a broad graben architecture, with an inner graben (Central Devon Sub-basin) fianked by half-grabens (Bideford and Launceston sub-basins) (Fig. 14; Leveridge & Hartley 2006; Waters et al. 2009). The Bideford Sub-basin is bounded to the north by the Brushford Fault, the Central Devon Sub-basin by the Greencliff Fault and the Launceston Sub-basin by the Rusey Fault. To the north of the Brush-ford Fault is the northern margin of the Culm Basin. The Tavy Basin has limited development of Famennian-Tournaisian strata. The Laneast High separating the Tavy Basin and Launceston Sub-basin includes a Tournaisian to ?Visean succession (Yeolmbridge and Laneast Quartzite formations). Remnants of Carboniferous strata also occur in the South Devon Basin.
In many areas there is no clear lithological break between the Tournaisian and the underlying Upper Devonian rocks, both of which are dominated by shallow-marine and deeper-water mudstones. The succession, commonly referred to as the Transition Series or Group (Dearman & Butcher 1959; Freshney et al. 1972), are assigned to the Exmoor Group in north Devon, the Hyner Mudstone and Trusham Mudstone formations of the eastern part of the Central Devon Sub-basin and the Tamar Group in south Devon.