The geology of the Wenlock shales around Builth Wells
Detailed field mapping and palaeontological collecting have revealed a complete sequence of Wenlock graptolite biozones around Builth. Bio-stratigraphical work has greatly improved the correlation with other British and European successions. The flexilis Zone, of wider international value, replaces the linnarssoni Zone, and eight subzones are suggested. The earliest Wenlock records the diachronous transgression across the Ordovician inlier. The centrifugus Zone, well developed in the W, thins to the E to a condensed horizon - the Acidaspis Bed- on the inlier. Calcareous mudstone slide deposits occur at four levels: in the rigidus; flexilis; ellesae; and nassa-ludensis zones. The Builth Mudstone Formation (formerly the Wenlock shales) comprises dark, laminated, silty mudstones deposited by hemipelagic processes under largely anoxic bottom conditions. The slide deposits reflected slope failure, possibly generated by fault-related seismic events. The Wenlock strata were greatly folded and then cut by a series of NE-SW trending faults forming the eastern boundary to the Pontesford Lineament. Some faults probably had a strike-slip displacement, others are possibly reactivated basement structures. Deformation and metamorphism are weak, and cleavage is localised to fault zones. Graptolite reflectivity indicates the zeolite- diagenetic grade. The intensity of cleavage and metamorphism increases northwestwards into the Pontesford Lineament. A review of early Palaeozoic and Wenlock palaeo- geography, - climatology, -oceanography, and -biogeography was undertaken. The causes of anoxia in the Wenlock Welsh basin were examined in the light of new Caledonian plate tectonic models. A correlation between graptolite occurrence and inferred sites of upwelling was shown. Three poorly defined graptolite subprovinces were established, which were probably caused by oceanographic and climatic effects. Systematic palaeontological studies have clarified the Monograptus flemingii and Monoclimacis vomerina subspecies, the Monograptus antennularius-retroflexus species, the genus Cyrtograptus, and the retoilitid group. The genus Plectograptus was divided, with a new genus Maculagraptus proposed.