New ammonites collected from two sections of the Cardium Formation (Upper Cretaceous; upper Turonian/lower Coniacian) exposed along the Bow River near Seebe, Alberta, indicate there is no biostratigraphic basis to support the proposal that the Cardinal/Leyland unconformity (＇＇E5/T5 surface＇＇) represents a significant time gap. Immediately below this unconformity, the sandstones of the Cardinal Member contain ammonites of the Prionocyclus wyomingensis Zone while the lower shales of the overlying Leyland Member yield ammonites of the Prionocyclus quadratus Zone. Thus, the Cardinal/Leyland unconformity represents, at most, a hiatus corresponding to the missing zone of Prionocyclus novimexicanus and perhaps parts of the underlying and overlying P. wyomingensis and P. quadratus zones, respectively. Consequently, this hiatus is much shorter than has been proposed by other workers in developing sequence stratigraphic models for the Cardium Formation. In contrast, sandstones of the upper Pembina River Member, in the subsurface at Pembina, Alberta, extend into the P. quadratus Zone, so that the unconformity between this member and the overlying shales of the Cardium zone Member occurs in the upper parts of the P. quadratus Zone, or even higher. If this unconformity is a basin-wide, chronostratigraphic horizon resulting from a single erosional event, then this event must have occurred within a time interval corresponding to the middle P. quadratus Zone. If the unconformity separating sandstones from conglomerates in the Seebe and Pembina areas occurs at different stratigraphic levels then it does not represent a chronostratigraphic horizon and its use for basin-wide correlation will need to be re-evaluated. Ammonites retrieved from three cores in the Cardium Formation of the Pembina Field permit, for the first time, biostratigraphic confirmation for proposed correlations between surface and subsurface members of the Cardium Formation. Our findings confirm that sandstones of the upper Pembina River Member in the subsurface of Pembina Field are generally correlative with the Cardinal Member of outcrop; however, the biostratigraphic evidence indicates that they are also, in part, correlative with the basal Leyland Member of outcrop. This observation is significant, because it is not apparent from simple allostratigraphic correlations, or model-driven sequence stratigraphic correlations.