Despite lateral thickness variations and lithostratigraphic complexities, a coherent depositional model can be recognized for carbonates of the upper Middle Ordovician Trenton Group between Montreal and Quebec City in the St. Lawrence lowland of eastern Canada. Between Montreal and Quebec City, the Group was deposited initially on a confined and irregular shallow shelf and latterly on a broader and essentially flatter shelf, whereas northeast the onshore-to-offshore profile was steeper and rapid submergence promoted the early development of deeper shelf and slope-and-basin sediments.

Within the area between Montreal and Quebec City, the Trenton Group contains an abundant but generally poorly preserved assemblage of biogenic structures, the majority of which can only be identified (sometimes only questionably, at a general level). These genera represent a variety of behavioral groups and include Arenicolites?, Calycraterion?, Chondrites, Circulichnis, Cruziana?, Diplichnites?, cf. Furculosus, Helminthopsis, Isopodichnus, Oichnus, Palaeophycus, Plagiogmus?, Planolites, Rosselia, Scalarituba?, Teichichnus, Trichichnus, Trypanites, and cf. Zoophycos, as well as unclassified pronged, looped, oblique, and vertical burrows.

The spatial and temporal distribution and abundance of these traces are examined in context of the environmental model and related depositional patterns. Because limestones show a great susceptibility to early and late diagenesis, trace fossils in skeletal

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shoal sediments (lower Trenton Group) were preserved through toponomic processes, whereas those in lagoonal (basal Trenton Group) and offshore (middle and upper Trenton Group) sediments were preserved through diagenetic processes (diagenesis also obliterated morphological details resulting in a bias towards simple forms). These processes resulted in ichnocoenoses restricted respectively to surface crawling traces for the former and infaunal dwelling and feeding traces for the latter. It is suggested that substrate consistency and rates of deposition were the most important controlling parameters.

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