Palaeoenvironmental evolution of an ordovician-silurian deep-marine sedimentary succession in the Welsh Basin.

The Llandeilo-Llandovery stratigraphic succession of west Wales comprises fourteen formations and consists of two dominant lithologies, namely sandstone and mudstone, the distribution of which is largely determined by eustatic sea-level changes. The sediments are interpreted as representing distal shelf, sandstone lobes, lobe fringes and slope environments. The turbidite systems developed within the Welsh depositional basin as a Type 1 (unchannelled sandstone lobes) system. The tectonic setting of the basin for the entire period of deposition was that of an active margin. Petrographic and geochemical data, however, indicate a passive margin tectonic setting for the basin. Critical examination of the models suggests that the sedimentary provenance indicators are incorrect and that the signature is a relict one, presumably derived from the Precambrian basement. Laminated hemipelagites are recorded from a number of formations. These formed under anaerobic conditions within the basin. They all contain layered pyrite framboids which are interpreted as having formed diagenetically within the sediment. It is also possible that some of the framboids may have formed syngenetically in the water column. The strata contain a diverse and relatively abundant ichnofaunal assemblage consisting of sixteen ichnogenera: Chondrites, Circulichnis, Cochlichnus, Cosmorhaphe, Desmograpton, Gordia, Helminthoida, Helminthopsi s, Nereites, Neonereites, Palaeophycus, Paleodictyon (Glenodictyon), Paleodictyon (Squamodictyon) , Planolites , Protopaleodictyon and Spirophycus. The ichnogenera are unevenly distributed throughout the succession, the main controlling factors being toponomy, anoxia and a global extinction event.