Lithostratigraphic unit: Coalbrookdale Formation
Locality: Section in banks of the stream in Whitwell Coppice, 500 m North of the hamlet of Homer, Shropshire (grdi reference SO 6194 0204).
Lithology and fossil constituents
Bassett et al. (1975) described the lithology in the section as thin-bedded olive to grey-green mudstones, typical for the Coalbrookdale Formation. Swire (1991) recorded an 80 mm thick bentonite with its top 0.8 m below the GSSP level, which can be used as a marker horizon, and nodules from 0.22 m to 0.3 m above the GSSP level at which level Swire’s log terminates.
The GSSP level is within the Apedale Member of the Coalbrookdale Fm at the first occurrence of the graptolite Cyrtograptus lundgreni in the section (Holland, in Holland & Bassett 1989).
Bassett et al. (1975) indicated that the macrofossils from Whitwell Coppice comprised low diversity graptolite assemblages and orthoconic nautiloids, although rare smooth brachiopods have been reported. From 1.5 m below the GSSP level the following graptolites were recorded: Cyrtograptus ellesae, Monograptus flemingii and Pristiograptus dubius. From at and above the GSSP level, the graptolites C. lundgreni, M. flemingii and P. dubius, and the dendroid graptolite Dendrograptus sp.
Palynomorphs described by Swire (1991) are exceptionally well preserved and abundance is high with the palynological residues dominated by acritarchs with acanthomorphs dominating the diverse assemblages. Chitinozoans were much less common and with rare scolecodonts.
Graptolite Biostratigraphy
Graptolites in the section include the stratigraphically long-ranging taxa M. flemingii and P. dubius, both of which have ranges that extend well below and above the base of the Homerian. Graptolites occurring 1.5 m below the GSSP level are indicative of the C. ellesae Zone
Zalasiewicz & Williams (1999) critically evaluated the biostratigraphical significance of the Whitwell Coppice C. ellesae and C. lundgreni faunas, since in the Builth Wells district of Wales, they found C. ellesae to occur within the C. lundgreni Biozone. This stratigraphical relationship is also shown in North Wales (Warren, 1971; see Zalasiewicz & Williams, 1999 ), although in Shropshire and in some other regions the C. ellesae and C. lundgreni faunas are stratigraphically distinct (Melchin et al., 2012; Hughes & Ray, 2016). Zalasiewicz & Williams (1999) suggested the rather poor fossil record of Cyrtograptus specimens in the type Wenlock area ‘is insufficient to establish taxon ranges with any precision.’ Hence, their conclusion that the GSSP level may not represent the base of the C. lundgreni Biozone, but some point within it. Part of the uncertainty surrounding this issue may stem from the taxonomic distinction of C. ellesae and C. lundgreni (Loydell, 2011).
Palynomorph Biostratigraphy
Swire (1991) provided a range chart for the palynomorphs through the Whitwell Coppice section. Seven chitinozoan taxa identifiable to species level are recorded: Ancyrochitina ancyrea, A. gutnica, Cingulochitina cingulata, Conochitina argillophila, C. proboscifera, C. tuba and Margachitina margaritana. Of these, C. cingulata is the most common. With the exception of C. tuba, which was encountered in only one sample, all species occur in samples from both above and below the
GSSP level. All of the species recorded are moderately long-ranging and their co-occurrences indicate a level somewhere between the mid Sheinwoodian to mid Homerian interval (see Verniers et al., 1995) which does not help in a better definition of the Homerian GSSP.
In the Lower Hill Farm borehole in Shropshire the correlative position to the GSSP level at Whitwell Coppice (containing ?C. lundgreni above C. ellesae) is some 10 m below a low in d13Ccarb (Hughes & Ray, 2016). This low in d13Ccarb is indicative of a level close to the inferred Sheinwoodian-Homerian boundary within the Vidukle core or perhaps a level within the Cyrtograptus lundgreni biozone (Kaljo & Martma 2006; Melchin et al., 2012).
In conclusion the relationship of the GSSP level to both graptolite and chitinozoan biozonations has some uncertainty, but that data from both fossils and correlative isotope data are a little more consistent with it lying within (rather than at the base) of the Cyrtograptus lundgreni Biozone.
Visiting the site: Strata at the GSSP are variably to poorly exposed. The base of the Homerian Stage can only be studied after site clearance, which must be conducted after consultation with Natural England who are responsible for the locality. See the Hughley Brook description.
Source: Mostly derived directly from Loydell (2011)
(MWH)
Bibliography
Bassett, M.G., Cocks, L.R.M., Holland, C.H., et al. 1975. The type Wenlock Series. Report of the Institute of Geological Sciences 75/13, 1–19.
Holland, C.H. and Bassett, M.G. 1989. A Global Standard for the Silurian System. National Museum of Wales Geological Series 10, 325 pp.
Hughes, H. E. & Ray, D. C. 2016. The carbon isotope and sequence stratigraphic record of the Sheinwoodian and lower Homerian stages (Silurian) of the Midland Platform, UK. Palaeogeography, Palaeoclimatology, Palaeoecology, 445, 97-114.
Loydell, D. K. 1998. Early Silurian sea-level changes. Geological Magazine, 135, 447–471.
Loydell, D. 2011. The GSSP for the base of the Homerian Stage, Whitwell Coppice. In: Siluria Revisited: A Field Guide. International Subcommission on Silurian Stratigraphy, Field Meeting 2011 (ed. D.C. Ray), 86 – 91.
Melchin, M. J., Sadler, P. M. Cramer, B. D. et al. 2012. The Silurian Period, In: The Geologic Time Scale 2012. Gradstein F. et al. (eds), 525-558.
Kaljo, D. & Martma, T. O. N. 2006. Application of carbon isotope stratigraphy to dating the Baltic Silurian rocks. GFF, 128, 123-129.
Swire, P.H. 1991. Palynology of a lower Wenlock (Silurian) shelf–basin transect, Wales and the Welsh borderland. Unpublished Ph.D. thesis,etheses.nottingham.ac.uk/1267/1/334865.pdf.
Warren, P. 1971. The sequence and correlation of graptolite faunas from the Wenlock–Ludlow rocks of North Wales. Bureau de Rechérches Géologiques et Minières, Mémoire 73, 451–460.
Verniers, J., Nestor, V., Paris, F., et al. 1995. A global Chitinozoa biozonation for the Silurian. Geological Magazine 132, 651–666.
Zalasiewicz, J.A. & Williams, M. 1999. Graptolite biozonation of the Wenlock Series (Silurian) of the Builth Wells district, central Wales. Geological Magazine 136, 263–283.