Integrated Upper Ordovician graptolite–chitinozoan biostratigraphy of the Cardigan and Whitland areas, southwest Wales

Abstract

To help calibrate the emerging Upper Ordovician chitinozoan biozonation with the graptolite biozonation in the Anglo-Welsh, historical type basin, the graptolite-bearing Caradoc-Ashgill successions between Fishguard and Cardigan, and at Whitland, SW Wales, have been collected for chitinozoans. In the Cardigan district, finds of Armoricochitina reticulifera within strata referred to the clingani graptolite Biozone (morrisi Subzone), together with accessory species, indicate the Fungochitina spinifera chitinozoan Biozone, known from several Ordovician sections in northern England that span the base of the Ashgill Series. Tanuchitina ?bergstroemi, eponymous of the succeeding chitinozoan biozone, has tentatively been recovered from strata of Pleurograptus linearis graptolite Biozone age in the Cardigan area. The T. ?bergstroemi Biozone can also be correlated with the type Ashgill Series of northern England. Chitinozoans suggest that the widespread Welsh Basin anoxic-oxic transition at the base of the Nantmel Mudstones Formation in Wales, traditionally equated with the Caradoc-Ashgill boundary, is of Cautleyan (or younger Ashgill) age in the Cardigan area. In the broadly time-equivalent, graptolite-rich Whitland section, also in SW Wales, two Baltoscandian chitinozoan biozones and a subzone have been recognized (again using accessory species), namely the Spinachitina cervicornis Biozone?, the Fungochitina spinifera Biozone and the Armoricochitina reticulifera Subzone. The new chitinozoan data provide a more precise means of correlation between the Whitland and Cardigan successions and suggest that the Normalograptus proliferation interval of the Whitland section is at least partly attributable to the Dicellograptus morrisi Subzone of the Dicranograptus clingani Biozone, rather than equating with the overlying Pleurograptus linearis Biozone.

An Upper Ordovician chitinozoan biozonation in British Avalonia (England and Wales)

“…The Sholeshook Limestone contains a late Pusgillian–early Cautleyan shelly fauna and graptolites indicative of the Dicellograptus anceps Biozone (Zalasiewicz et al . 1995; Vandenbroucke et al . 2008; and references therein).…”

“…in press a). Nevertheless, the Eisenackitina rhenana Biozone has tentatively been recognized in the Rorrington Shale Formation, straddling the base of the Nemagraptus gracilis Biozone (Vandenbroucke et al . in press a).…”

“…The Newport Sands section ( Dicranograptus clingani graptolite Biozone, Dicellograptus morrisi Subzone) yielded a chitinozoan assemblage of the Armoricochitina reticulifera Subzone of the Fungochitina spinifera Biozone (Vandenbroucke et al . 2008).…”

“…2005, in press a) as well as data from additional key sections with good graptolite and/or conodont control (Van Nieuwenhove et al . 2006; Vandenbroucke et al . 2008); the latter groups do not range continuously through the type sections.…”

A modern assessment of Ordovician chitinozoans from the Shelve and Caradoc areas, Shropshire, and their significance for correlation

“…Specimens of A. communis from both sections, illustrated in plates 12 and 22.2 of Vandenbroucke (2008b), show their identical appearance. Acanthochitina latebrosa has also been reported from the Fungochitina spinifera Zone in Whitland in south central Wales (Vandenbroucke et al 2008); likewise, Lagenochitina prussica and Lagenochitina baltica, normally typical of the F. spinifera Zone and younger, have been reported from the Onny Valley. These records represent very low yields of specimens (especially when compared to the remainder of the assemblages), which obscures their potential to correlate these two biozones.…”