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. RÉSUMÉ Dans les Basses Terres du St-Laurent de L’Est canadien, les calcaires du Groupe de Trenton (Ordovicien moyen supérieur) 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 également des galeries en boucle et fourchues, des terriers obliques ainsi que des perforations de bryozoaires, tous identifiés de façon informelte. De toutes ces formes, seules les suivantes sont présentes 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 décrivons en détail toutes les traces fossiles et tentons par le fait même de résoudre plusfeurs problèmes de nomenclature chez certains Ichnogenres qui font à l'heure actuelle t'objet de débats. Le dépôt des sédiments 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 façon significative d'un environnement de déposition à l'autre. On remarque plutôt que chaque environnement se caractérlse par des assemblages typiques de L’lchnofaciès à Cruziana tel qu'on L’observe dans les séquences détritiques. Le substrat et ses caractérlstiques de base, qui reflètent la disponibllité de la nourrlture et les conditions énergétiques du milieu, constituent l'élément majeur qui a, avant tout, limite la distribution spatiale et temporelte des traces fossiles. [Traduit par le journal]
The transgressive sequence of the Ordovician intracratonic basin of the present-day Lac-St-Jean and Chicoutimi areas is characterized by a variety of sediments that were deposited in a number of environments. Marginal and nearshore sediments of the basin were a complicated system of clastics derived from the adjacent rugged coastal hinterland and intertidal and shallow subtidal carbonates with variably admixed clastics. Farther from shore, in sheltered bays or lagoons partly enclosed by offshore shoals and bars of skeletal lime sands, fine-grained carbonates with coral-algal-bryozoan thickets accumulated. These sediments exhibit numerous submarine hardgrounds or firm grounds. Variations in sedimentation rate and the development of periodically exposed banks of sediment also resulted in the formation of subaerial microkarstic surfaces. The offshore skeletal shoals and bars, which were composed mainly of pelmatozoan debris but included patches of incipient reef growth, provided an incomplete barrier to circulation. Deeper water offshore subtidal sediments consisted of mixed carbonate and argillaceous muds.Sudden increased rates of transgression, probably related to isostatic movements, resulted in the drowning of these coastal environments and the deposition of dark-coloured argillaceous shales in deeper water. These shales are partly of latest Utica and mainly Lorraine in age and it is likely that the initial submergence of this part of the Canadian Shield was much later than that in the St. Lawrence Lowland, probably occurring in Cobourg time.
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