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 rocksequence 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 antiquus s.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.

Chapter 26 Graptolite palaeobiogeography

“…Elles & Wood 1908;Hadding 1913;Hede 1951;Jaanusson & Skoglund 1963;Nilsson 1977;Williams et al 2003). Taxa not found in low-palaeolatitude localities include Oepikograptus bekkeri, Hustedograptus uplandicus, Archiclimacograptus rugosus (¼Diplograptus molestus Thorslund), Archiclimacograptus compactus, A. vasae, A. skagensis, Normalograptus rotundatus and N. kukersianus (e.g.…”

“…Graptolites from low-palaeolatitude regions such as Victoria (Australia), Scotland, South China and western Laurentia (including Trail Creek, Idaho; west Texas; and the Canadian Cordillera) consist primarily of species of Orthograptus, Rectograptus, Dicranograptus, Dicellograptus, Amplexograptus and Climacograptus, with a small radiation of archiretiolitids (Berry 1960;Carter & Churkin 1977;Lenz & Chen 1985;Vandenberg & Cooper 1992;Zalasiewicz et al 1995). Zalasiewicz et al 1995;Williams et al 2003). This interval, however, does contain small sub-clades of Diplograptus and derived Archiclimacograptus, the diversity of which is not well understood.…”

“…In Wales, Williams et al (2003) described an interesting Middle Katian differentiation of faunas along an onshore -offshore transect. At Cardigan (south Pembrokeshire), shale and mudstones representing an open marine environment contain a moderately diverse graptolite fauna in the Dicranograptus clingani and Pleurograptus linearis biozones.…”

Chapter 24 Late Ordovician zooplankton maps and the climate of the Early Palaeozoic Icehouse

“…In this model, the first group represents a deep-water biotope and the third one an epipelagic biotope ( Fig. Finney 1984Finney , 1986Finney & Berry 1997;Williams et al 2003;Mitchell et al 2008), and suggests that groups (a) -(c) inhabited water masses at largely the same depth, but at different areas on a shelf to basin transect ( Fig. Recent evidence in favour of such a scenario includes Cooper & Sadler's (2010) constrained optimization experiments that showed differences in mean species duration between species of their epipelagic and those of their deep-water groups: species of the deep-water group generally have much shorter biostratigraphical ranges than the ones from the epipelagic groups.…”

“…Two hypotheses to explain graptolite facies associations: (a) onshore/offshore variation in species distribution (e.g Finney & Berry 1997;Williams et al 2003;Mitchell et al 2008;Vandenbroucke et al 2009Cooper et al 1991)…”