A revision of the age of the hen‐dy‐capel ooidal ironstone (ordovician), llanengan, N. Wales

Abstract

AbstractPendent didymograptid graptolites are described from mudstones immediately overlying the Hen‐dy‐capel ironstone (St. Tudwal's Peninsula, Lly̌n, North Wales). The graptolites indicate a Llanvirn age for these beds, implying a Llanvirn or Arenig age for the ironstone, in contrast with the previous interpretation of a gracilis Biozone age. The redating of the ironstone horizon suggests that the Pen‐y‐gaer mudstones may be at the base of the local Nant Ffrancon Formation, rather than being equivalent to the lower Caradoc Tyddyn Dicwm beds above the Tremadog ironstone as widely supposed. The ironstones at Trefor, Betws Garmon, and Llandegai, which have been correlated with the Hen‐dy‐capel ironstone on the basis of acritarch assemblages, may also have a similar age. A reinterpretation of the age of these localities as Llanvirn would remove the stratigraphic problems introduced by their previously suggested early gracilis Biozone age. The Hen‐dy‐capel ironstone forms one of a large number of ironstones recorded from close to the Arenig‐Llanvirn boundary in N. Wales, and elsewhere at high southern palaeolatitudes.