The stratigraphic boundary separating the Mannville and the Colorado groups in Alberta, occupying incised valleys cut within the Upper Mannville strata, has previously been interpreted as an unconformity recording post-Mannville erosion followed by a late Albian marine transgression initiated by the deposition of the Joli Fou Shale and locally by the Basal Colorado Sandstone (both of the Colorado Group). Sedimentology, paleontology, organic and inorganic geochemistry, mineralogy, and petrography of strata above and below the unconformity or the sequence boundary have been studied in 106 samples from 37 wells within the Cessford Field covering an area of 3600 km2. Cores through the boundary show a distinct physical break represented by a scoured surface overlain by basal conglomerates. Paleontological data, based on dinoflagellates and foraminifers, show establishment of restricted marine conditions in the Basal Colorado times (initial transgression) and onset of open marine condition (maximum flooding) during the Joli Fou times. Although paleosol horizons have not been found near the boundary, influence of meteoric water in the Upper Mannville sandstones is inferred from development of spherulitic siderite and extensive early kaolinization of the feldspar, mica, and lithic grains. The absence of paleosol or fluvial strata within the incised valley fills suggests that the subaerial unconformity was modified by tidal erosion during the Joli Fou transgression.