OR/18/139 Introduction

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Melchin, M J1, Davies, J R2, De Weirdt, J3, Russell, C4, Vandenbroucke, T R A3, Zalasiewicz, J A. 2018. Integrated stratigraphic study of the Rhuddanian-Aeronian (Llandovery, Silurian) boundary succession at Rheidol Gorge, Wales: a preliminary report. British Geological Survey Internal Report, OR/18/139.

1  Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS, Canada
2  Department of Geography & Earth Sciences, Aberystwyth University, Aberystwyth SY23 3DB, UK (also British Geological Survey, Cardiff Office, Cardiff University, Main Building, Park Place,Cardiff, CF10 3AT, Wales, UK)
3  Department of Geology, Ghent University, Krijgslaan 281 S8, BE-9000, Ghent, Belgium
4  School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK
5  Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK

The first comprehensive geological and palaeontological study of the Rheidol Gorge area was published by Jones (1909)[1]. That paper summarized the previous geological studies in the region and thoroughly documented the stratigraphy, structure and graptolite biostratigraphy of the Plynlimon and Ponterwyd areas, including a detailed geological map and description of the Rheidol Gorge section, just south of Ponterwyd (Figures 1 and 2). Jones showed that the Rheidol Gorge section exposes an almost continuous succession of strata through much of the interval that is now known as the Rhuddanian and Aeronian stages. He also documented the succession of graptolite faunas and biozones through the section. Since that time, a number of papers have included material from Rheidol Gorge in paleontological studies (e.g., Elles and Wood, 1901–18[2]; Challinor, 1928[3], 1945[4]; Packham, 1962[5]; Jones and Rickards, 1967[6]; Cullum and Loydell, 1996[7]; Zalasiewicz et al., 2009[8]). However, the importance of the section was most clearly established, both from a graptolite systematic and biostratigraphic perspective, by the monograph of Sudbury (1958)[9], which provided a detailed analysis of the early Aeronian triangulate monograptids that has since formed the basis for the classification of these taxa, and has also provided a key framework for the regional and global graptolite zonation of the lower Aeronian (e.g., Rickards, 1976[10]; Zalasiewicz et al., 2009[8]). Most recently, Cullum and Loydell (2011)[11] provided a new lithological log of the upper Rhuddanian and lower Aeronian portions of the section and Russell et al. (2013)[12] undertook a very detailed study of the sedimentology of the strata at and just below the Rhuddanian-Aeronian boundary interval. One of the most important points of the Cullum and Loydell (2011)[11] paper was that they showed that graptolites occur at many more stratigraphic levels in the section than had been documented previously, and that graptolites occur throughout the immediate interval that spans the base of the Demirastrites triangulatus (= Monograptus triangulatus) Biozone.

File:OR18139fig1.jpg
Figure 1    Location map and geology of the Ponterwyd area and study section. Modified and updated from Jones (1909)[1] and BGS (1984)[13] Numbers along the margins of (a) refer to the UK National Grid.

In the definition of the Global Stratotype Section and Point for the base of the Aeronian Stage, it is stated “… the marker point for the base is within a continuous lithological section through part of the Trefawr Formation at the base of locality 72 in transect h as described by Cocks and others (1984[14], p. 165, figures 60, 61, 62, 62). This point correlated with the base of the base of the Monograptus triangulatus graptolite Biozone in the section” (Bassett, 1985[15]). It has since been pointed out by a number of authors (e.g. Melchin et al., 2004[16], 2012[17]; Davies et al., 2011[18], 2013[19]), however, that at the GSSP, the D. triangulatus Biozone was only recognized in one sample by the occurrence of one taxon, Monograptus austerus sequens, and that species is elsewhere known only to occur in the middle of the D. triangulatus Biozone.

Therefore, the GSSP does not appear to correlate with the base of the D. triangulatus Biozone as recognized regionally or globally. For this reason, and also the lack of other available data from this section that may be useful for global correlation, a working group was created within the International Subcommission on Silurian Stratigraphy in 2011, to restudy this boundary with the aim of finding a GSSP that permits precise global correlation of the base of the Aeronian. Following from the work of Russell et al. (2013)[12], our research group has undertaken a thorough restudy of the lithological succession, graptolite and chitinozoan biostratigraphy, as well carbon isotope chemostratigraphy (currently under study) of the upper Rhuddanian-lower Aeronian portion of the Rheidol Gorge succession to assess its potential as a new candidate section for this GSSP. The purpose of this informal field guide is to report on our current progress in this work.

References

  1. ↑ Jump up to: 1.0 1.1 Jones, O T. 1909. The Hartfell-Valentian succession in the district around Plynlimon and Pont Erwyd (north Cardiganshire). Quarterly Journal of the Geological Society, 65, 463–537, pls 24, 25.
  2. ↑ Elles, G L, and Wood, E M R. 1901–18. A monograph of British graptolites. Palaeontographical Society, London.
  3. ↑ Challinor, J. 1928. A shelly band in graptolitic shales. Geological Magazine, 65, 364–368.
  4. ↑ Challinor, J. 1945. A graptolite lineage from north Cardiganshire. Geological Magazine, 82, 97–106.
  5. ↑ Packham, G H. 1962. Some diplograptids from the British Lower Silurian. Palaeontology, 5, 498–526, pls 71, 72.
  6. ↑ Jones, W D V, and Rickards, R B. 1967. Diplograptus penna Hopkinson, 1869, and its bearing on vesicular structures. Paläontologische Zeitschrift, 41, 173–185.
  7. ↑ Cullum, A A, and Loydell, D K. 1996. A new species of Neodiplograptus from the Middle Llandovery of the Rheidol Gorge, Wales. Journal of the Czech Geological Society, 41, 217–218.
  8. ↑ Jump up to: 8.0 8.1 Zalasiewicz, J A, Taylor, L, Rushton, A W A, Loydell, D K, Rickards, R B, and Williams, M. 2009. Graptolites in British stratigraphy. Geological Magazine, 146, 785–850.
  9. ↑ Sudbury, M. 1958. Triangulate monograptids from the Monograptus gregarius Zone (Lower Llandovery) of the Rheidol Gorge (Cardiganshire). Philosophical Transactions of the Royal Society of London, 241, 485–555.
  10. ↑ Rickards, R B. 1976. The sequence of Silurian graptolite zones in the British Isles. Geological Journal, 11, 153–188.
  11. ↑ Jump up to: 11.0 11.1 Cullum, A A, and Loydell, D K. 2011. The Rhuddanian-Aeronian transition in the Rheidol Gorge, mid Wales. Proceedings of the Yorkshire Geological Society, 58, 261–266. Cite error: Invalid <ref> tag; name "Cullum 2011" defined multiple times with different content
  12. ↑ Jump up to: 12.0 12.1 Russell, C E, Vandenbroucke, T R A, Zalasiewicz, J A, Williams, M, Davies, J R, Waters, R A, and Molyneux, S G. 2013. The Rheidol Gorge (central Wales) as potential stratotype for the Rhuddanian/Aeronian stage boundary: A progress report. In A. Lindskog & K. Mehlqvist (eds.): Proceedings of the 3rd IGCP 591 Annual Meeting — Lund, Sweden, 9–19 June 2013, 19–20. Lund University Cite error: Invalid <ref> tag; name "Russell 2013" defined multiple times with different content
  13. ↑ British Geological Survey. 1984. Aberytwyth. England and Wales Sheet 163. Solid. 1:50 000. (Keyworth, Nottingham: British Geological Survey).
  14. ↑ Cocks, L R M, Woodcock, N H, Rickards, R B, Temple, J T, and Lane, P D. 1984. The Llandovery Series of the type area. Bulletin of the British Museum of Natural History (Geology), 38, 131–182.
  15. ↑ Bassett, M G. 1985. Towards a “common language” in stratigraphy. Episodes, 8, 87–92.
  16. ↑ Melchin, M J, Cooper, R A, and Sadler, P M. 2004. The Silurian Period. In Gradstein, F M, Ogg, J G, and Smith, A G (eds.), A Geologic Time Scale 2004, Cambridge University Press, pp. 188–201.
  17. ↑ Melchin, M J, Sadler, P M, and Cramer, B D. 2012. The Silurian Period. In Gradstein, F M, Ogg, J G, and Smith, A G (eds.), A Geologic Time Scale 2012, Elsevier Press.
  18. ↑ Davies, J R, Molyneux, S G, Vandenbroucke, T R A, Verniers, J, Waters, R A, Williams, M, and Zalasiewicz, J A, 2011. Pre-conference field trip to the Type Llandovery area. In: Ray, D C. (Ed.), Siluria Revisited: A Field Guide, pp. 29–7 (International Subcommission on Silurian Stratigraphy, Field Meeting, 2011).
  19. ↑ Davies, J R, Waters, R A, Molyneux, S G, Williams, M, Zalasiewicz, J A, Vandenbroucke, T R A, and Verniers, J. 2013. A revised sedimentary and biostratigraphical architecture for the Type Llandovery area, central Wales, UK. Geological Magazine, 150, 300–332.
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