Late Castlemainian (Ca 4, Arenig) graptolites from the Ballantrae Complex, SW Scotland

Sirs — Graptolites of Arenig age have been discussed repeatedly from the Ballantrae Complex (Stone & Strachan 1981; Stone & Rushton 1983, 2003; Rushton et al. 1986; Rushton & Stone 1988). The inferred biostratigraphic ranges were estimated to be from the basal Arenig Tetragraptus approximatus Biozone to the early Yapeenian. As the exact dating of the graptolite faunas in this sequence has some important implications for the interpretation of the tectonic development of the region, precision is relevant to the question. The discovery of Arenig isograptid faunas in the North Ballaird borehole (Stone & Strachan 1981; Stone & Rushton 2003) thus was of crucial importance for the determination of the timing of the collisional event initiating the Grampian Orogeny. Stone & Rushton (2003) described the fauna in detail and discussed a possible early Yapeenian age, but were not able to exclude a late Castlemainian (Ca 4) age. The fauna represent the best preserved and richest isograptid fauna found so far in Britain. British isograptids in general are poorly known from few scattered records (Jenkins 1982) and their use for a detailed biostratigraphy was limited due to tectonic distortion and ambiguous identifications. Isograptid faunal successions are well established from Australasia (Cooper 1973; Cooper & Ni 1986) and eastern North America (Williams & Stevens 1988). Their taxonomy and evolution is known in great detail (Maletz & Mitchell 1996; Maletz & Zhang 2003). Therefore, they are regarded as of high potential for biostratigraphic purposes and inter-continental correlation (Maletz et al. 2003). The illustrations by Stone & Rushton (2003) to support their interpretation of a Yapeenian age of the fauna are excellent and show the material to be well preserved. Therefore a detailed assessment of the fauna is possible.