ABSTRACT: Foraminifera from the CÃ¡rdenas Formation (San Luis PotosÃ) represent the upper part of the Gansserina gansseri Zone and are thus of early Maastrichtian age, revising the late Campanian or broadly Maastrichtian dates for these strata in the literature. The stratigraphical relationship of eight microfacies, based on lithology and faunal composition, suggests open marine sedimentation on a shallow platform, temporarily interrupted by a transient increase in terrigenous clastics indicating a regional tectonic event. The foraminiferal fauna was changed as a result, with planktic foraminifera becoming more abundant, and with significant replacements in the large benthic forams indicative of deeper water or stronger currents and eventually with the disappearance of the larger forams altogether. The sequence ends with major influx of poorly fossiliferous terrigenous clastics in the upper member of the formation. The assemblage of larger benthic foraminifera that inhabited the platform environment consisted of cosmopolitan species and others endemic to the Caribbean Province, whose range does not extend past the K/T boundary. In the region of this study, we show that the demise of this end-Cretaceous assemblage, together with associated rudists, came in the mid-Maastrichtian as the result of increased terrigenous clastic input and local subsidence associated with the development of the Sierra Madre Oriental fold belt to the west.