Graptolite assemblages of multidens Biozone age have been recovered from mudstones immediately above and below the Llyn Conwy Formation, the local representative of the regionally extensive acid tuffs that mark the end of vulcanicity in the south and east of the Harlech Dome. They include several taxa new to Wales and are markedly different to contemporaneous faunas from the Welsh Borders. One new species, Pseudoclimacogruptus isknos, is described.
The Lower Ordovician sequence around the eastern and southern sides of the Harlech Dome in Wales is represented by volcanic and sedimentary rocks: the Aran Volcanic Group (Ridgway 1975). The lower and middle parts of this group are laterally variable, fossils are rare and internal correlation is difficult. However, the upper part consists of a thick, regionally extensive sequence of acid tuffs (the Llyn Conwy Formation at Arenig) succeeded by mudstones of the Nant Hir Formation (Figure I).
At Arenig Fawr, the tuffs were originally termed the Upper Ashes of Arenig by Fearnsides (1905), who recovered graptolites from the underlying strata that he assigned to the murchisoni Biozone (upper Llanvirn Series) (Fearnsides 1905; p. 625). A shelly fauna in a thin calcareous (decalcified) bed, the Derfel Limestone, in the basal part of the overlying mudstones includes the probable late Harnagian (lower Caradoc) trilobite Broeggerolifhus ulrichi (Whittington and Williams 1955; Williams rt a1 1972). Whittington and Williams (1955) also noted that Elles (1922) had reported graptolites of supposed gracilis Biozone age from mudstones associated with the Derfel Limestone. From these observations the Llyn Conwy Formation was thought to encompass Llandeilo and Lower Caradoc time.
Recent surveys at Arenig Fawr have yielded well preserved graptolites both below and above the Llyn Conwy Formation. These graptolites place more precise time constraints on the vulcanism at Arenig than was previously possible.
All fossil material described here is lodged with the Sedgwick Museum, Cambridge.
STRATIGRAPHIC FRAMEWORK
2a. Lithostratigraphy
The volcanic and sedimentary sequence of the Serw and Llyn Conwy Formations in the Arenig area (Figure 2 ) is generally comparable with that described by Lynas (1973) in the Migneint area to the north. However, to the south of Arenig, the mudstones and quartz latites of the Serw Formation pass laterally into crystal-rich tuffs with minor basaltic pillow lavas.