Devonian-Carboniferous Sedimentary Basins

The Cornubian passive margin succession accumulated in six E-W trending basins (Figure 1) which were actively extending from the Lochkovian-Pragian to the Viséan (Leveridge and Hartley, 2006; Leveridge et al., 2002; Merriman et al., 2000). Variscan shortening of the Rhenohercynian passive margin has been estimated to be approximately 50% or not less than 150 km (Franke, 2000; Shackleton et al., 1982), therefore, it can be inferred the original width of the passive margin was some 300 km. The Devono-Carboniferous sedimentary rocks were deposited in the Gramscatho, Looe, South Devon, Tavy, Culm and North Devon basins. The oldest of these is the Looe Basin with periodic basin formation towards the north. The North Devon Basin represents the most continental sedimentary succession and displays cyclic sedimentation relating to the formation of the South Devon, Tavy and Culm basins that lie between it and the Looe Basin. The Gramscatho Basin forms the southern edge of the continental passive margin and represents the transition into an oceanic setting (Leveridge and Hartley, 2006; Leveridge et al., 2002). Between the passive margin basins, structurally controlled topographic highs are observed, although much of the exposures are occluded by later basin inversion.

Figure modified from Shail and Leveridge (2009) and originally based on Leveridge and Hartley (2006) showing the arrangement of Devonian-Carboniferous basins in SW England, the Sticklepath Lustleight Fault Zone (SLFZ), Lizard Complex (LC), Start Complex (SC), Start-Perranporth Zone (SPZ), St Mellion Outlier (SM) and Permian Granites: LEG - Land’s End Granite; CG - Carnmenellis Granite; SAG - St Austell Granite; BMG - Bodmin Granite; DG - Dartmoor Granite.

The Looe Basin hosts the earliest (?Lochkovian) rocks of the Devonian passive margin succession (Hollick et al., 2014; Leveridge and Hartley, 2006; Leveridge et al., 2002). These include the sedimentary successions of the Dartmouth, Meadfoot and Tamar groups. The Looe Basin is separated from the South Devon Basin to the north by the Plymouth High (Leveridge et al., 2002) which is partially occluded and is likely to correlate with the Brixham High described by Leveridge et al. (2003).

The South Devon Basin was first defined in its modern form by Leveridge et al. (2002) and was discussed in a regional context by Leveridge and Hartley (2006). The definition extended the existing South Devon Basin westwards into north Cornwall, and the former “Trevone Basin” became obsolete. The basin comprises two sub-basins (north and south) divided by the mid-basin Torquay High (Leveridge and Hartley, 2006; Leveridge et al., 2003). Despite recent revisions, a complicated lexicon still exists. Formation divisions within the Tamar Group are more dependent on geographic location rather than lithostratigraphic character. Many of the formations in north Cornwall are assimilated from the previous “Trevone Basin” nomenclature or unrevised geological maps simply divide units into Middle and Upper Devonian (e.g. Ivybridge, Sheet 349). A review by Leveridge (2011) attempts to reconcile the inconsistencies between formation names within the basin.

The Landulph High forms a horst separating the South Devon and Tavy basins. The Tavy Basin includes the northwest “Trevone Basin” stratigraphy and is therefore similarly complicated in its nomenclature as the South Devon Basin Leveridge et al. (2002; Leveridge, 2011; Leveridge and Hartley, 2006). The basin formed in the Givetian and has high concentrations of chlorite and pyrite, indicating reducing conditions, which contrast with the oxidising environment (red-purple rocks) of the South Devon Basin (Leveridge and Hartley, 2006). It is inferred by Leveridge and Hartley (2006) that the change in sedimentation is likely due to the presence of the Landulph High which isolated the Tavy Basin from the South Devon Basin.

The northern margin of the Tavy Basin is marked by the occluded Laneast High. The horst block formed in the Famennian following the inception of the extensional Culm Basin; a graben basin that existed into the Viséan (Leveridge and Hartley, 2006). The Culm Basin was extensional prior to Variscan orogenesis which caused inversion and transformation into a foreland basin (Hartley and Warr, 1990). The extensional Culm Basin succession has three sub-basins; the Central Devon Sub-basin forms the main graben with two half-grabens, the Bideford Sub-basin and Launceston Sub-basin to the north and south, respectively (Leveridge and Hartley, 2006; Waters and Davies, 2006; Waters et al., 2009). Furthermore, Mississippian rocks crop out in the St Mellion Outlier, an expelled segment of the Culm Basin (Leveridge et al., 2002). The Mississippian succession has been reviewed by Waters et al. (2009) and has been correlated across the sub-basins as part of the Teign Valley Group.

The North Devon Basin forms the most northerly basin of the Devonian-Carboniferous passive margin and includes rocks of Emsian-Serpukhovian age (Leveridge and Hartley, 2006; Waters et al., 2009; Whittaker and Leveridge, 2011). The Devonian to Tournaisian succession is defined as the Exmoor Group and the succeeding Tournaisian-Serpukhovian succession of the Teign Valley Group (Waters et al., 2009). The Exmoor Group represents a cyclic terrestrial-lacustrine-nearshore-shallow marine environment associated with the largely marine passive margin succession to the south (Leveridge and Hartley, 2006; Waters et al., 2009; Whittaker and Leveridge, 2011). Cycles represent an initial shallow marine transgression immediately after basin inception with a subsequent coarsening-up of the sediment representing shallowing of the basin to terrestrial-lacustrine environments (Leveridge and Hartley, 2006; Whittaker and Leveridge, 2011).

References

Franke, W., 2000. The mid-European segment of the Variscides: tectonostratigraphic units, terrane boundaries and plate tectonic evolution. Geological Society, London, Special Publications 179, 35–61. https://doi.org/10.1144/GSL.SP.2000.179.01.05

Hartley, A.J., Warr, L.N., 1990. Upper Carboniferous foreland basin evolution in SW Britain. Proceedings of the Ussher Society 7, 212–216.

Hollick, L.M., Scrivener, R.C., Burt, C.E., 2014. Geology of the Newquay district - a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 346 Newquay (England; Wales).

Leveridge, B.E., 2011. The Looe, South Devon and Tavy basins: the Devonian rifted passive margin successions. Proceedings of the Geologists’ Association 122, 616–717. https://doi.org/10.1016/j.pgeola.2011.03.005

Leveridge, B.E., Hartley, A.J., 2006. The Varisan Orogeny: the development and deformation of Devonian/Carboniferous basins in SW England and South Wales, in: Brenchley, P.J., Rawson, P.F. (Eds.), The Geology of England and Wales. The Geological Society, London, pp. 225–256.

Leveridge, B.E., Holder, M.T., Goode, A.J.J., Scrivener, R.C., Jones, N.S., Merriman, R.J., 2002. Geology of the Plymouth and south-east Cornwall area. Memoir of the British Geological Survey, Sheet 348. England; Wales.

Leveridge, B.E., Scrivener, R.C., Goode, A.J.J., Merriman, R.J., 2003. The geology of the Torquay district. Memoir of the British Geological Survey, Sheet 350. England; Wales.

Merriman, R.J., Evans, J.A., Leveridge, B.E., 2000. Devonian and Carboniferous volcanic rocks associated with the passive margin sequences of SW England; some geochemical perspectives. Geoscience in South-West England 10, 77–85.

Shackleton, R.M., Ries, A.C., Coward, M.P., 1982. An interpretation of the Variscan structures in SW England. Journal of the Geological Society, London 139, 533–541. https://doi.org/10.1144/gsjgs.139.4.0533

Shail, R.K., Leveridge, B.E., 2009. The Rhenohercynian passive margin of SW England: Development, inversion and extensional reactivation. Comptes Rendus Geoscience 341, 140–155.

Waters, C.N., Davies, S.J., 2006. Carboniferous: extensional basins, advancing deltas and coal swamps, in: Brenchley, P.J., Rawson, P.F. (Eds.), The Geology of England and Wales. The Geological Society, London, pp. 173–223.

Waters, C.N., Waters, R.A., Barclay, W.J., Davies, J.R., 2009. A lithostratigraphical framework for the Carboniferous successions of southern Great Britain (onshore). British Geological Survey Research Report, RR/09/01.

Whittaker, A., Leveridge, B.E., 2011. The North Devon Basin: a Devonian passive margin shelf succession. Proceedings of the Geologists’ Association 122, 718–744. https://doi.org/10.1016/j.pgeola.2011.03.006