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.