Analyses of seismic reflection profiles supported by lithological and palynological studies of core samples from submarine outcrops indicate that the lower Paleozoic succession of the Avalon Terrane, southeast Newfoundland, is continuous offshore. The succession crops out over an area greater than 30 000 km2 and is approximately 8 km thick. The sequence is dominantly siltstone and is of Late Cambrian to ?Devonian or younger age. It is relatively unmetamorphosed, underlain by Hadrynian acoustic basement, and overlain along its eastern and southern margins by a Mesozoic–Cenozoic succession that is economically important from an oil and gas perspective.Lithofacies studies indicate that in Early Ordovician time restricted shallow-marine conditions probably prevailed over a vast area of the Avalon Terrane. Upper Ordovician and Silurian siltstones show evidence of deposition under more-dynamic and well-oxygenated conditions and probably represent a normal shallow-marine environment. Redbeds of possible Devonian or younger age are interpreted to be of continental origin. Black shales of Ordovician age are potential source rocks for the generation of hydrocarbons.