Stratigraphical and palaeoecological importance of Caradoc (Upper Ordovician) graptolites from the Cardigan area, southwest Wales

Abstract

Graptolites from more than 60 horizons in the basinal Caradoc succession of southwest Wales, between Fishguard and Cardigan, allow recognition of the multidens, clingani and linearis biozones. The biostratigraphy permits recognition of major differences in the sedimentary rock-sequence north and south of structures associated with the Fishguard–Cardigan Fault Belt. The Penyraber Mudstone Formation, disconformably overlying the Fishguard Volcanic Group (Llanvirn), is partly of multidens Biozone age. It is succeeded south of the Newport Sands Fault by the Cwm yr Eglwys Mudstone Formation of clingani to linearis biozones age. North of the fault the Cwm yr Eglwys Mudstone Formation is replaced laterally by the northwards-thickening, sandstone turbidite-dominated Dinas Island Formation (clingani and linearis biozones). Graptolite stratigraphical distribution indicates that Dicranograptus clingani occurs only rarely within the caudatus Subzone of the clingani Biozone and that Climacograptus antiquuss.l. also does not range above the lower part of the clingani Biozone. The first occurrence of Dicellograptus morrisi, within the upper clingani Biozone, confirms its value as a marker for the morrisi Subzone, and is associated with the first occurrences of Diplacanthograptus dorotheus and Normalograptus minimus. Dicellograptus flexuosus, used to indicate the morrisi Subzone elsewhere, occurs throughout the clingani Biozone in the Cardigan area. The linearis Biozone is recognized by Climacograptus tubuliferus. Oxic bottom conditions in early and early mid-Caradoc times largely precluded the influx of, or preservation of, graptolite faunas in the Penyraber Mudstone Formation. Anoxic mudstones of the Cwm yr Eglwys Mudstone and Dinas Island formations preserve graptolite assemblages of 21 and 26 species, signalling strong open marine influences which persisted in this area until late Caradoc times. This contrasts with the shelfal faunas in the Whitland area (south Pembrokeshire), where the late Caradoc is dominated by low-diversity Normalograptus-dominated assemblages.