Lithostratigraphic units: Wormwood and Builth Mudstones formations
Locality: Cefn Cerig Quarry and road section (also known as Fron road section), Llandovery area; Ordnance Survey Grid Reference: SN 774 334 – 775 323. Quarry and road cuttings exposing Telychian GSSP and intra- Wenlock synsedimentary slide complex. Stage name from Pen-lan-Telych Farm.
Lithology and fossil constituents
The Telychian GSSP section is a quarry in part of the Cefn-Cerig road section. It exposes, southeasterly dipping, well-bedded, bioturbated, muddy siltstones and sandstones and sandy mudstones of the Wormwood Formation. The base of the Telychian is taken within a 29 cm thick bed of bioturbated siltstone, below which is a fossiliferous rottenstone horizon that represents the LAD for two key Aeronian brachiopod taxa (i.e. Eocoelia intermedia and Stricklandia lens progressa) at locality 162 of Cocks et al. (1984). Shelly fossils, including corals and trilobites, are present at several other levels in the quarry both below and above the GSSP level, with the brachiopds Clorinda globosa, Coolinia applanata, Dicoelosia alticavata, Eoplectodonta penkillensis, Leangella segmentum, Resserella sp. and Skenidioides lewisii amongst the most common.
The adjacent road cutting sections comprise the original type section for the Cerig Formation (Cocks et al. 1984). These authors obtained a rich shelly fauna which included brachiopods used as diagnostic for the Telychian Stage, specifically Eocoelia curtisi and Stricklandia laevis from a level (their locality 163) assumed to be within the Cerig Formation and 45 m stratigraphically above the GSSP level. Hill and Dorning (Cocks et al. 1984, Appendix 1) obtained a monospinosa acritiarch Biozone assemblage from the same locality. However, a detailed re-examination of the section has led to the stratigraphic significance of these Telychian faunas being challenged (Davies et al. 2010).
Overlying stratigraphic complexity
The discontinuous road section begins 22.5 m above the basal Telychian GSSP level. Here, the Wormwood Formation is terminated by a sharp cross-cutting contact above which is a c. 40 m-thick sequence that includes three separate levels of Builth Mudstones Formation interleaved with variably disturbed units of varied lithology. The sharp basal contact is interpreted as a major slide plane above which is a stack of slump sheets, possibly incorporating pre- and post-Cerig Formation strata. The highly fossiliferous muddy sandstones and sandy mudstones on the west side of the road, which comprise locality 163 of Cocks et al. (1984)- the inferred Telychian brachiopods- are now known to be part of this complex slump/slide disturbed succession (Cefn Cerig Slide Complex).
The sandy mudstones with abundant yellow-weathering shell debris and discontinuous shell beds (coquinas) are very similar to parts of the underlying Wormwood Formation, but also present are units of slump-disturbed and recrystallized laminated mudstone (?Builth Mudstones Formation), thinly interbedded laminated and burrow mottled mudstone (?Dolfawr Formation), and unfossiliferous pale green mudstone with phosphate nodules (?true Cerig Formation). A new type locality for the Cerig Formation (above the GSSP level) and a new well-dated and undisturbed type section has been selected elsewhere(Davies et al. 2010).
The younger part of the Cefn Cerig section provides clear evidence of a major slide complex that displaced much of the late Llandovery succession at a level ca. 31 m above the GSSP level and emplaced an assemblage of slide slices and associated mélange containing material of differing age and composition. Comparable features noted in other sections suggest that this was the product of a slope failure over 10 km in width.
The dating of units within the disturbed succession show down slope movement post-dated deposition of Builth Mudstones of the riccartonesis Biozone (early Sheinwoodian). Largely undisturbed Builth Mudstones of dubius Biozone age overlie the slide complex, as at Cefn Cerig, and, where strata have been lost, drape the basal slide plane (Davies et al. 2010).
Biostratigraphy
Cocks et al. (1984) defined the Telychian GSSP level above a fossiliferous level which yielded the highest known specimens of the brachiopods Eocoelia intermedia (and Stricklandia lens progressa) and below the first appearance of Eocoelia curtisi (Cocks et al., 1984). This level was then regarded as corresponding with the base of the Spirograptus turriculatus graptolite Biozone. However revision of the Spirograptus genus has shown an earlier has shown an earlier cosmopolitan species S. guerchi (Melchin et al., 2012). In the type area, the earliest guerichi Biozone assemblage comes from the base of the Cerig Formation at its new undisturbed type section (Davies et al. 2010).
In the GSSP section, only the graptolite Paradiversograptus runcinatus has been found at a level above the GSSP, a species which ranges between the upper Aeronian and the Telychian with an acme in the S. guerchi biozone. Graptolites recovered from the underlying Wormwood Formation consistently indicate a sedgwickii Biozone age (Davies et al. 2010) and the balance of evidence now suggests that the Telychian GSSP level lies within the upper part of this biozone. This is supported by the last occurrence of the brachiopod E. intermedia which elsewhere in Britain occurs in the upper part of the sedgwickii Biozone. The Cerig Formation at its new undisturbed type section may provide an alternative GSSP in the S. guerchi Biozone in the type Llandovery area (Davies et al. 2010)
Microfossils recovered from below the GSSP level in the Cefn-cerig section belong to the dolioliformis chitinozoa and estillis acritarch biozones and assemblages indicative of younger microfossil biozones are not recorded below the top of the Wormwood Formation, and its correlatives, in the type Llandovery area (Davies et al. 2010). The lowest unit of Builth Mudstones exposed in the Cefn Cerig road cuttings yields riccartonensis Biozone graptolites(i.e. early Sheinwoodian) and margaritana Biozone chitinozoans and the upper unit contains possible dubius Biozone graptolite faunas (i.e. mid Homerian).
Chitinozoans recovered from Cocks et al.’s Locality 163, including Margachitina margaritana and Eisenackitina ithoniensis, are consistent with a Wenlock age (Davies et al. 2010), and it is probable these record the age of the ‘matrix’ or of a ‘clast’ within the mélange that includes rafts of different age and lithostratigraphical provenance. The suspicion is that the monospinosa Biozone acritarch assemblage reported by Hill & Dorning (1984) may also have come from similar material.
Sources: most text and figures taken directly from Davies et al. (2011) and Melchin et al. (2012).
(MWH)
Bibliography
Cocks, L.R.M., Woodcock, N.H., Rickards, R.B., Temple, J.T. & 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. & 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.
Hill, P.J. & Dorning, K.J. 1984. Acritarchs. In: The Llandovery Series of the type area (eds Cocks, L.R.M., Woodcock, N.H., Rickards, R.B., Temple, J.T., and Lane, P.D.). Bulletin of the British Museum (Natural History) Geology 38, 174-176.
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.