The geology of the Llandello district, Carmarthenshire

Abstract

Summary The Ordovician and Silurian rocks of the Llandeilo area form the flanks and core of the Towy anticline. The oldest beds exposed are the D. bifidus Shales which pass upwards into a variable set of sediments, ashes and rhyolite lavas. These beds, with the Ffairfach Grit at the base, have hitherto been classified as Lower Llandeilo, but it now seems probable that they represent a facies variation of the D. murchisoni graptolitic shales and should be included in the Upper Llanvirn. The Llandeilo Flags are locally divisible into faunal and lithological units and are followed, with profound lithological and faunal change, by the N. gracilis Shales. The Dicranograptus Shales are poorly exposed but include towards the top the Crûg Limestone, apparently equivalent to the Robeston-Wathen Limestone of South-West Wales. Important folding and faulting movements occured before the deposition of the Ashgillian rocks and resulted in the elevation of an important geographical barrier, the Towy anticline. Ashgillian deposition was limited to a basin west of the anticline, where at least 5000 feet of mudstones accumulated. The barrier persisted throughout most of the Silurian, but was breached by transverse troughs during Llandovery times and was subjected to sporadic sea invasions from the north-east, so that the Llandovery and Wenlock mudstones are involved in uncomformable overlaps and oversteps. The Towy anticline continued to exercise this geographical control until Upper Wenlock times, when it was completely inundated and buried in mudstones which were laid down far beyond the limits of the older Silurian rocks.