Systematic ichnology of the Middle Ordovician Trenton Group, St Lawrence Lowland, eastern Canada

Carbonate sediments of the upper Middle Ordovician Trenton Croup between Montreal and Quebec City in the St. Lawrence Lowland, eastern Canada, contain a diverse and abundant trace fossil assemblage consisting of Arenlcolltes sp. , ? Calycraterion sp. , Chondrites spp. , Circullchnis montanus, Clematischnia sp. , ? Conoatichniu sp. , Cruziana problematical Cruziana sp. , cf. Diplichnites sp. , Furculosus car-pathicus, Helminthopsis hieroglyphica, Helminthopsis sp. , Oichnus parabololdes, Palaeophycus tubularis, Palaeophycus sp. , ? P! ag1ogmus sp. , Planolites beverleyensls, P. montanus, Planolites sp. , ? Rhizocorallium cf. R. Irregulare, ? Rossella sp. , Scalar 1 tuba misouriensis, Scolicia sp. , Skolithos linearis, Skolithos sp„ Teichichnus rectus, Teichichnus sp. , Trichichnus sp. , Trypanites weisei, Vermiforichnus clarkel and Zoophycos sp. as well as informally diagnosed loop, oblique and pronged burrows and bryozoan borings. Of these forms, only Chondrites spp. , Palaeophycus tubularis, Palaeophycus sp. , Planolites spp. , Teichichnus spp. and Trypanites weisei are abundant; the remainder are rare to only moderately common. Nevertheless, in this paper we describe all the trace fossils in detail and in doing so attempt to resolve several current and controversial problems of nomenclature regarding certain ichnogenera. Sediments of the Trenton Group were deposited Initially In lagoons followed in turn by offshore "bar", shallow and, finally, deeper offshore shelf environments. The trace fossils do not exhibit significant variation with respect to these broad depositional regimes and, Instead, each environment Is characterized by assemblages typical of the Cruzlana ichnofacies as recognized in clastic sequences. The major factor primarily limiting the spatial and temporal trace fossil distribution was substrate and its primary characteristics, in turn a reflectance of food availability and environmental energy levels. RESUME Dans les Basses Terres du St-Laurent de L’Est canadien, les calcaires du Groupe de Trenton (Ordovicien moyen superieur) renferment, entre Montreal et Quebec, un assemblage d'ichnofossiles abondants et varies comprenant: Arenlcolltes sp. , ? Calycraterion sp. , Chondrites spp. , Circullchnis montanus, Clematischnia sp. , ? Conoatichniu sp. , Cruziana problematical Cruziana sp. , cf. Diplichnites sp. , Furculosus car-pathicus, Helminthopsis hieroglyphica, Helminthopsis sp. , Oichnus parabololdes, Palaeophycus tubularis, Palaeophycus sp. , ? P! ag1ogmus sp. , Planolites beverleyensls, P. montanus, Planolites sp. , ? Rhizocorallium cf. R. Irregulare, ? Rossella sp. , Scalar 1 tuba misouriensis, Scolicia sp. , Skolithos linearis, Skolithos sp„ Teichichnus rectus, Teichichnus sp. , Trichichnus sp. , Trypanites weisei, Vermiforichnus clarkel et Zoophycos sp. On y trouve egalement des galeries en boucle et fourchues, des terriers obliques ainsi que des perforations de bryozoaires, tous identifies de facon informelte. De toutes ces formes, seules les suivantes sont presentes en abondance: Chondrites spp. , Palaeophycus tubularis, Palaeophycus sp. , Planolites spp. , Teichichnus spp. et Trypanites weisei; les autres sont rares ou peu communes. Nean-moins, nous decrivons en detail toutes les traces fossiles et tentons par le fait meme de resoudre plusfeurs problemes de nomenclature chez certains Ichnogenres qui font a l'heure actuelle t'objet de debats. Le depot des sediments du Croupe de Trenton s'est inltie dans un milieu de lagune puis de barre, de plate-forme de haut-fond et, flnalement, de plate-forme d'eau profonde. Les traces fossiles ne varient pas de facon significative d'un environnement de deposition a l'autre. On remarque plutot que chaque environnement se caracterlse par des assemblages typiques de L’lchnofacies a Cruziana tel qu'on L’observe dans les sequences detritiques. Le substrat et ses caracterlstiques de base, qui refletent la disponibllite de la nourrlture et les conditions energetiques du milieu, constituent l'element majeur qui a, avant tout, limite la distribution spatiale et temporelte des traces fossiles. [Traduit par le journal]