A Nereites ichnofacies from the Ordovician-Silurian Welsh Basin

The sediments of theOrdovician-Silurian succession of west Wales are interpreted as turbiditic sands with associated pelagic and hemipelagic mudstones. The strata contain a trace fossil assemblage consisting of fifteen ichnogenera, namely: Chondrites, Circulichnis, Cochlichnus, Cosmorhaphe, Desmograpton, Gordia, Helminthoida, Helminthopsis, Neonereites, Nereites, Paleodictyon, Palaeophycus, Planolites, Protopaleodictyon and Spirophycus. The ichnofaunal assemblage is typical of the Nereites ichnofacies. Twenty three ichno-species are described, two of which (Helminthoida granulata, Palaeophycus serratus) are new. Turbiditic sands contain the most abundant and diverse ichnoassemblages, a reflection of favorable environmen-tal conditions for habitation by benthic organisms. Both the turbiditic mudstones and anoxic hemipelagites contain just two ichnospecies, namely Chondrites ichnosp. and Planolites montanus both of which are common, albeit less so in the latter facies. The effects of preservation potential, medium and preservation mechanisms are also important controls on distribution.