Discussion on a reassessment of the early Ordovician 'Celtic' brachiopod province

Abstract

R. B. Neuman, D. A. T. Harper & C. R. van Staal write: The results of several recent geological studies relevant to early Ordovician palaeogeography are not mentioned in this recent paper by Cocks & McKerrow (1993). Contributions of particular significance include the recognition that the Dunnage Zone of Newfoundland consists of two distinct parts, the Notre Dame and Exploits subzones of contrasting Ordovician stratigraphy, lithology, structure, metamorphism, plutonism, lead isotope signatures, mineralization, geophysics, and faunas ( Williams et al. 1988 ). The boundary between these subzones, named the Red Indian Line in Newfoundland, is probably a suture. Fossiliferous sedimentary rocks are rare in the Notre Dame Subzone, but Arenig-Llanvirn graptolites occur in shales at three places and at one place conodonts have been extracted from carbonate clasts in volcanic breccias. All these fossils are considered to have Laurentian affinities (Nowlan & Thurlow 1984; Williams 1992 ; Williams et al. 1992 ). Fossiliferous Lower Ordovician rocks are more abundant in the Exploits subzone. In addition to the several occurrences of Celtic assemblage brachiopods (Cl-C6 of Neuman & Harper 1992 ; Harper 1992 ; Cocks & McKerrow 1993), Gondwanan trilobites and graptolites (Cyclopyge and Undulograptus) are known in the Exploits subzone of central Newfoundland ( Williams et al. 1992 ) from shale that overlies a Lower Ordovician ophiolite fragment near an Occurrence of Celtic brachiopods (C5 of Neuman & Harper 1992 ). Palaeomagnetic studies of Newfoundland rocks near the Red Indian Line showed that these two subzones were separated by about 30° of latitude (van der Pluijm et al. 1990;