The Cambrian-Ordovician Potsdam and Beekmantown groups of the Quebec Reentrant (southwestern Quebec and southeastern Ontario) were deposited during rifting and drifting phases of the Laurentian margin and development of the Iapetus Ocean. Previous studies have generally considered a gradual depositional continuity between the two groups. In contrast, our research on the sedimentology and stratigraphy indicates a regional subaerial unconformity exists between the Potsdam and Beekmantown groups. This unconformable boundary is sharp and associated with an abrupt lithological change. The upper formation of the Potsdam Group (Cairnside Formation) is locally missing, possibly due to erosion, and void-filling, spherulitic, chalcedonic cement is developed less than a metre beneath the unconformity. The void-filling, silica cement is interpreted as pedogenic silcrete. Truncation of the Potsdam Group quartz grains at the contact, along with their silica overgrowths, is also observed. Conodonts from the basal part of the Theresa Formation (lowest formation of the Beekmantown Group) indicate an Early Ordovician (Ibexian, Stairsian) age. No fossils were collected from the underlying Cairnside Formation, but conodonts of Late Cambrian age have been recovered from the Strites Pond Formation of the Philipsburg Group, southern Quebec, a unit that is a possible correlative of the Cairnside Formation. These conodont faunas indicate that the Cairnside-Theresa unconformity ranges from latest Cambrian (Cordylodus proavus Zone) to Early Ordovician age, approximately correlative with the early Stairsian "low diversity interval". We correlate this unconformity with another poorly documented unconformity northeast of Montreal, as well as with other coeval hiatuses exposed elsewhere in the Laurentian margin (e.g. southern Quebec and New York State). Coeval unconformities, interpreted as being the result of a latest Cambrian eustatic sea-level fall, have also been recognized in other parts of North America and abroad. The eustatic fall is also interpreted from Cambro-Ordovician slope-deposited, channel-fill quartz arenites and thick debris flow conglomerates in the Appalachian Humber Zone.