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Aeronian GSSP: Trefawr track cutting, Crychan Forest, Llandovery

Lithostratigraphic units: Trefawr and Cefngarreg formations

Locality: Trefawr track cutting, Crychan Forest, northern Llandovery area; Ordnance Survey Grid Reference: SN 8380 3953 (location 3 on the map).

Lithology and fossil constituents

The section shows a southwards dipping succession in which the lower part of the section is extensively bioturbated sandy mudstones of the Trefawr Formation, passing upwards into muddy sandstones with scattered coarse sand grains and granules. These form part of a 25 m-thick sandstone unit that includes the GSSP. Cocks et al. (1984) included this unit in their Trefawr Formation, but Davies et al. (2010) interpret these sand-prone strata as an early tongue (unit Ceg0) of the Cefngarreg Sandstone Formation (Schofield et al., 2009). Finer grained and more mud-rich Trefawr Formation facies succeed this sandy unit and gradually coarsen-upwards into an overlying much thicker and widely mapped unit of the Cefngarreg Sandstone Formation (Ceg1).

Cocks et al. (1984) and Temple (1987) document faunal distributions in the section including the key species of graptolite that allow the recognition of revolutus (Coronograptus cyphus), Demirastrites triangulatus and post-triangulatus biozone assemblages. Shelly fossils are scattered throughout and include the brachiopods Clorinda undata, Eoplectodonta duplicata, Leangella scissa, Meifodia prima prima, Plectatrypa tripartita subsp. nov. (Temple, 1987) and Stricklandia lens intermedia. These taxa are present both above and below the GSSP, as are eoplanktonica Biozone acritarch and maennili Biozone chitinozoan assemblages. A prominent slickensided bedding plane 0.5 m below the stratotype level records some slip during regional mid Devonian deformation.


The base of the Aeronian Stage was defined by Cocks et al. (1984) at the entry of the D. triangulatus biozone graptolites within the sandy mudstones of the Rhuddanian to Aeronian Trefawr Formation, in strata now included by Davies et al. (2010) in the Cefngarreg Sandstone Formation (Ceg0). The base of the Triangulatus Biozone was defined in the Trefawr track section on a single occurrence of Monograptus austerus sequens at Locality 72 of Cocks et al. (1984). The LAD of the precursor C. cyphus Biozone subspecies, M. austerus vulgaris, is some 18 m below the boundary stratotype level.

Temple (1988) has argued that there is insufficient precision in the faunal distributions to justify erection of the stage stratotype at this locality. Also, Zalasiewicz et al. (2009) now restrict the range of M. a. sequens to the middle part of the Triangulatus Biozone suggesting that the Aeronian boundary stratotype level lies within, not at the base, of the Triangulatus Biozone.

Graptolites from upper levels of the early Cefngarreg Sandstone tongue (Ceg0) were included by Cocks et al. (1984) in the Neodiplograptus magnus biozone (elsewhere included in the upper part of the triangulatuspectinatus biozone). It is immediately above this sandstone that assemblages containing the biozone’s eponymous and diagnostic N. magnus first appear. Long-ranging graptolite assemblages are present in the overlying upward passage from the Trefawr Formation into a younger unit of Cefngarreg Sandsone (Ceg1).

This younger unit, shown by the presence of Pristiograptus regularis (Locality 39 of Cocks et al., 1984), ranges into the Convolutus Biozone. An assessment of the other faunas from the section fails to show a significant turnover across the stratotype boundary level. Strata above and below the GSSP level contain eoplanktonica Biozone (acritarch) and maennili Biozone (chitinozoan) assemblages, as well as identical subspecies of the brachiopods Meifodia, Plectatrypa and Stricklandia.

Sequence Stratigraphy

In assessing the Rhuddanian to Aeronian succession of the Type Llandovery area, Davies et al. (2010) recognise a series of progradational sequences bounded by major flooding surfaces. They interpret each coarsening upwards unit as the product of glacioeustatic regression and an increase in siliciclastic supply from rejuvenated source areas. The defining flooding events record glacioeustatic transgressions that introduced deeper and more distal conditions and promoted deposition of mud-prone facies.

The earliest major Llandovery progradation recognised in the type succession, represented by the Bronydd, Crychan and Goleugoed formations (Schofield et al., 2009), attained its maximum extent in the mid Rhuddanian (Acinaces graptolite Biozone). An initial flooding event at the base of the Revolutus graptolite Biozone saw the marked contraction of this sand-prone system and deposition of the more muddy Trefawr Formation facies, with the maximum extent (mid Revolutus Biozone) of this unit marking the period of maximum Rhuddanian flooding.

The Aeronian GSSP section reveals evidence of renewed progradation and the advance of an early, Triangulatus Biozone tongue (Ceg0) of the sand-prone Cefngarreg Sandstone system. The flooding event that caps this unit, and which reintroduced Trefawr Formation facies, occurred during the UK Magnus graptolite Biozone in the Type Llandovery area.

Sources: text and figures taken directly from Davies et al. (2011) and  Melchin et al. (2012).



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 (Natural History), Geology Series 38, 131-182.

Davies, J.R., Waters, R.A., Zalasiewicz, J.A., Molyneux, S.G., Vandenbroucke, T.R.A. and Williams, M. 2010. A revised sedimentary and biostratigraphical architecture for the type Llandovery and Garth areas, Central Wales: a field guide. British Geological Survey Open Report, OR/10/037.

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: Siluria Revisited:  A Field Guide (ed. D.C. Ray). International Subcommission on Silurian Stratigraphy, Field Meeting, pp29 – 72.

Melchin, M. J., Sadler, P. M.  Cramer, B. D.  Cooper, R. A.  Gradstein, F. M.  & Hammer, O. 2012. The Silurian Period, In: The Geologic Time Scale 2012. Gradstein F. et al. (eds), 525-558.

Schofield, D.I., Davies, J.R., Jones, N.S., Leslie, A.B., Waters, R. A., Williams, M., Wilson, D., Venus, J. and Hillier, R.D. 2009. Geology of the Llandovery district - a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 Sheet 212, Llandovery (England and Wales).

Temple, J.T. 1987. Early Llandovery brachiopods of Wales. Monograph of the Palaeontological Society, London, Publication No. 572, issued as part of volume 139 for 1985.

Temple, J.T. 1988. Biostratigraphical correlation and stages of the Llandovery. Journal of the Geological Society of London 145, 875-879.

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.