Abstract Lower Ordovician to lower Middle Ordovician (upper Ibexian to lower Whiterockian; upper Sauk III supersequence) subtidal to peritidal carbonates of the Romaine Formation in the western Anticosti Basin record the evolution during the early Paleozoic of the low-latitude passive margin of eastern North America. A regional paleokarst unconformity, the super-Romaine unconformity corresponding to the North American Sauk-Tippecanoe megase-quence boundary developed on top of the Romaine carbonates during the early Middle Ordovician. The regional distribution of the passive-margin carbonates below the unconformity, however, suggests that significant foreland basin tectonic activity influenced the facies patterns in the uppermost Romaine Formation before the final demise of the Lower Ordovician great American carbonate bank, leading to its eventual subaerial exposure and erosion. The Romaine Formation is mostly composed of peritidal and open-shelf carbonate rocks similar to those in age-equivalent El Paso, Ellenburger, Arbuckle, Knox, Beekmantown, and St. George Groups found along thepresent southern and eastern flanks of the North American craton. Flooding of the Precambrian basement for the first time in the area allowed deposition of a deepening to shallowing carbonate succession in the late Ibexian. A narrow coastal belt of peritidal carbonates onlapped onto the basement with time, but the Romaine platform was mostly covered byopen-marine subtidal carbonate deposits. The latter, assea level receded and offlap began, gave way to peritidal deposition in the latest Ibexian. However, a succession of lower Whiterockian subtidal limestone found locally in the offlapping carbonates indicates that open subtidal conditions resumed briefly before the super-Romaine unconformity formed. This Romaine stratigraphy suggests that two large-scale, third-order, transgressive-regressive sequences are present and can be correlated basinward into the subsurface beneath the northern part of Anticosti Island. Petrographic and geochemical interpretations combined with other geologic and geophysical data provide evidence that the Lower Ordovician carbonates were hydrothermally altered at a regional scale to form porous, structurally controlled dolostone reservoirs. These structurally controlled hydrothermal dolomite reservoirs in the Romaine Formation provide a local but significant trapping mechanism for migrating hydrocarbons along the relatively unde-formed, southwesterly dipping homoclinal succession. Their signature has been recognized along several seismic lines and has served as an exploration guide in the recent round of exploration on Anticosti Island.