Intra-Iapetus brachiopods from the Ordovician of eastern Ireland: implications for Caledonide correlation: Discussion

Abstract

In an important paper dealing with the Hilltown brachiopod fauna of eastern Ireland and its implication for Caledonide correlation, the authors reiterate the view that the Iapetus suture of Ireland correlates with the junction between the Dunnage and Gander terranes of Newfoundland, implying thereby that the Dunnage Terrane forms part of the Laurentian oceanic margin. As a consequence they are led to propose that the rocks containing the Indian Bay fauna of the Gander Terrane (Wonderley and Neuman 1984) lie either within a klippe thrust across the suture zone or within a zone gradational with the rocks of the Gander Group. Furthermore, in allowing that the Bellewstown Terrane has affinities with part of the Dunnage, it would seem necessary that in Ireland the Early Ordovician suture be placed between the Bellewstown and Leinster terranes. It is the purpose of this discussion to suggest that the junction of the Dunnage and Gander terranes (the Reach Fault boundary of McKerrow and Cocks (1976, 1977, 1986), McKerrow (1988), and Stillman (1988)), does not represent the Iapetus suture in Newfoundland. It is argued that the faunal evidence allows location of the Iapetus cryptic suture to the north of the Exploits volcanic zone, along a Lukes Arm Red Indian line (cf. Williams et al. 1988). It is further suggested that the Exploits and Grangegeeth terranes of Newfoundland and Ireland, respectively, were located in a more oceanward position within the Avalonian margin than was the Bellewstown Terrane, from which they were separated perhaps by a backarc basin. Consequently, the Indian Bay rocks, even if thrust, need not be exotic, nor is it necessary to locate the Iapetus suture to the southeast of the Bellewstown Terrane. The assumption that the Dunnage rocks formed to the north of the Iapetus suture (e.g., Arnott et al. 1985; Stillman 1988) is largely based on the conclusions of McKerrow and Cocks (1977) concerning the paleogeographic significance of the shelly faunas of central Newfoundland. Counter to earlier interpretations placing Iapetus along a Solway Lukes Arm line (Church and Gayer 1973), McKerrow and Cocks (1977) located the Iapetus suture along the Reach Fault on the basis of their identification of a Caradocian fauna in the Cobbs Arm Limestone of New World Island. The Early Ordovician faunas of New World Island that were obviously not Scoto-Appalachian were explained as having developed endemically (McKerrow and Cocks 1977, p. 493). However, the Early Ordovician faunas of New World Island and Anglesey were subsequently shown by Neuman and Bates (1978) to belong to a common Celtic biogeographic province. Further-