Conodonts from the Davidsville Group, northeastern Newfoundland: Reply

Abstract

McKerrow and Cocks (1976) assume that different plates carry different faunal provinces and that the width of the Iapetus Ocean formed the principal barrier. They suggest that the closing of Iapetus modified the areal extent and distinctiveness of the provincial boundaries. This assumption culminated in the suggestion that the Reach Fault separated different faunal provinces in central Newfoundland and thus was the suture line of the Iapetus Ocean (McKerrow and Cocks, 1977). Based on the distribution of North Atlantic province conodonts on each side of the Reach Fault this author (Stouge 1980a) queried the importance of the Reach Fault as a provincial boundary and thus as a plate boundary. In their counter arguments McKerrow and Cocks correctly state that the North Atlantic province spans the Iapetus Ocean (Bergstrom 1971; and others). Such a distribution has been found to favour a pelagic mode of life for the conodonts (e.g., Bergstrom and Cooper 1973). Therefore, following McKerrow and Cocks, conodonts cannot be used as an argument either for or against (Stouge 1980a) the presence of a suture line-in this case the proposed Reach Fault (McKerrow and Cocks 1977hwhilst only benthic taxa (trilobites and brachiopods) can be used. Barnes and F5hraeus (1975), however, showed that conodonts were benthic, and this is now generally accepted by most conodont workers (see Barnes 1976). Therefore, the conodonts as well as brachiopods and trilobites are here considered to be valid for this discussion. In my note (Stouge 1980a) I indicated that consideration of the Reach Fault as a suture cannot be argued for solely on the basis of benthic (microlmacro) fossils, using conodonts as an example. It is true that the North Atlantic conodont province is oceanic and therefore widespread (Barnes and F5hrzus 1975); however, faunal elements of the Midcontinent conodont province are also present in central Newfoundland in Lower and Middle Ordovician strata (Stouge 1980b, c). These are recorded from limestones associated with volcanic rocks representing volcanic islands and island arc complexes. The Midcontinent province conodonts inhabited marine shallow lagoons, whereas the North Atlantic province conodonts lived along the slopes (Stouge 1980~). The presence of Midcontinent province conodonts within the Iapetus was not noted by Bergstrom (1971), Bergstrom and Cooper (1973), or Barnes and F5hrzus (1975). Therefore, conodont distribution does not indicate distinct plates. This distribution is paralleled by the brachiopods (Neuman 1976). It is also known that several benthic provinces exist on the same plate (e.g., Barnes and F5hraeus 1975; Fortey 1975; Fortey and Barnes 1977). Thus, on the same plate at least one province is present on the craton and another province (oceanic) is present along the continental slope (Fortey 1975; Fortey and Barnes 1977; Ludvigsen 1978), a situation that had prevailed since Cambrian time (Taylor 1976). Such a distribution of faunal provinces cannot be used as evidence for either a wide ocean, plate boundaries, or major plate movements. The explanation for the mixing of faunal province boundaries by a narrow ocean (McKerrow and Cocks 1976) is one interpretation, but it is not the only one possible. For example, simple eustatic changes with or without plate movements may equally well apply (Ludvigsen 1978; Fortey 1980; Stouge 19806, c). Thus, it appears that the idea that the Reach Fault is the Iapetus suture line (McKerrow and Cocks 1977), mainly based on the sporadic occurrence of the benthic fauna elements (i.e., trilobites and brachiopods) of different provincial affinities, should be reassessed, since faunal provinces are not restricted to different plates and hence cannot successfully delineate plate boundaries, as assumed by McKerrow and Cocks (1976,1977).