Conodont biofacies of lower to lower middle ordovician megaconglomerates, Cow Head group, western Newfoundland

Abstract

Clasts from three conglomerate and megaconglomerate horizons in the Cow Head Group of western Newfoundland were sampled for conodonts to determine age and faunal composition. The Cow Head Group represents a fragment of the paleoslope of the Cambro-Ordovician Iapetus Ocean that was thrust upon age-equivalent shelf strata of the Port-au-Port and St. George groups. Although shelf and slope facies are preserved in sequence, the record of shelf break and upper slope facies is only retained in clasts of the conglomerate units of the Cow Head Group. The Arenig interval with the thickest conglomerate units was chosen for the study. Conodonts from the shelf edge and upper slope environment were compared and correlated with slope and shelf faunas. The 300 samples collected from limestone clasts yielded 12 492 well preserved conodonts with a low colour alteration index of 1.5 to 2.0. Altogether 132 form- and multielement species were determined, and one new genus (Texania) and several new species (Texania heligma, T. teras, Prioniodus marginalis and P. serratus) were established. Conodont faunas from the shelf edge and upper slope derived clasts contain elements of North Atlantic and Midcontinent aspect as well as endemic and cosmopolitan species. They range in age from late Canadian (Fauna D) to basal Whiterock (Fauna 1) of the North American Series and through the British Arenig Series (Paroistodus proteus Zone to earliest ?Eoplacognathus suecicus Zone of Baltoscandia). Five different biofacies were found, beginning with the oldest Paroistodus- Drepanoistodus Biofacies, followed by the slightly younger Prioniodus-Texania Biofacies; both were found in clasts from Bed 10. Bed 12 yielded clasts with the older Oepikodus-Periodon Biofacies and the younger Periodon-Texania Biofacies. Bed 14 clasts contained the Periodon-Parapanderodus Biofacies. The shelf break and upper slope environment was characterized by a distinct faunal realm whose composition was controlled by changes in the adjacent habitats of shelf and lower slope.