Graptolite biozonation of the Wenlock Series (Silurian) of the Builth Wells district, central Wales

Abstract

The graptolite biozonation of the Wenlock Series in the Builth Wells district is reassessed. The graptolite fauna is dominated by relatively few, but variable, species of Monograptus s.s., Pristiograptus and Monoclimacis. These species are mostly long-ranging and are of limited biostratigraphical use. Biozonation is based largely on the relatively rare cyrtograptids. The lower Wenlock Series of the district comprises the widely recognized centrifugus, murchisoni and riccartonensis biozones. The middle to upper Wenlock biostratigraphy differs from that previously recognized in the UK. Above the riccartonensis Biozone, dubius, rigidus and lundgreni biozones are distinguished. Separate flexilis and ellesae biozones are not recognized: Monograptus flexilis appears earlier than, then largely co-exists with, Cyrtograptus rigidus s.l.; while Cyrtograptus ellesae appears later than Cyrtograptus lundgreni. The uppermost Wenlock nassa–ludensis Biozone is sparsely fossiliferous; its subdivision has not proved possible. This revised biostratigraphical scheme for the Wenlock Series of the Builth Wells district suggests that the Sheinwoodian–Homerian boundary, as defined in the Welsh Borderland, might occur within, rather than at the base of, the lundgreni Biozone. However, difficulties in correlation with other Wenlock sequences, particularly overseas, suggest significant biofacies control, with the Wenlock seas of the Builth Wells district appearing to have been periodically inhospitable to graptolites.