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

Carbonate sediments of the upper Middle Ordovician Trenton Group between Montreal and Quebec City in the St. Lawrence Lowland, eastern Canada, contain a diverse and abundant trace fossil assemblage consisting of Arenicolites sp., ?Calycraterion sp., Chondrites spp., Circulichnis montanus, Clematischnia sp., ?Conostichnus sp., Cruziana problematica, Cruziana sp., cf. Diplichnites sp., Furculosus carpathicus, Helminthopsis hieroglyphica, Helminthopsis sp., Oichnus paraboloides, Palaeophycus tubularis, Palaeophycus sp., ?Plagiogmus sp., Planolites beverleyensis, P. montanus, Planolites sp., ?Rhizocorallium cf. R. irregulare, ?Rosselia sp., Scalarituba misouriensis, Scolicia sp., Skolithos linearis, Skolithos sp., Teichichnus rectus, Teichichnus sp., Trichichnus sp., Trypanites weisei, Vermiforichnus clarkei 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 Cruziana 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.