This study examines the distribution of larger foraminifera in an upper Campanian–lower Maastrichtian siliciclastic–carbonate depositional environment within the Arén Sandstone Formation. Three assemblages have been identified, which refer to prodelta, reef, and lagoon depositional facies. Prodelta deposits are characterized by Lepidorbitoides socialis, Lepidorbitoides aff. minor, Clypeorbis mammillata, and Siderolites cataluniensis. The reef deposit assemblage contains Siderolites calcitrapoides, Siderolites cataluniensis, Orbitoides gruenbachensis, “Orbitoides” aff. concavatus, Lepidorbitoides aff. minor, and Fallotia jacquoti. They are associated with the encrusting foraminifera Solenomeris and Haddonia. The lagoonal assemblage contains Fascispira colomi and Omphalocyclus macroporus. Among the siderolitids, the keeled saucer-shaped Siderolites cataluniensis colonized the deepest environments at the lower photic zone, whereas the canaliculate spine-bearing Siderolites calcitrapoides characterized shallower waters. The former lived on soft media of calm environments whereas the latter crawled on firmground of high-energy environments. Lepidorbitoides socialis shared with Siderolites cataluniensis and Clypeorbis mammillata the deepest, calm environments, while Lepidoritoides aff. minor adapted to a wide range of depths in low to moderate water-energy environments. Orbitoides gruenbachensis and “Orbitoides” aff. concavatus colonized the firm media in the upper photic zone, whereas Omphalocyclus macroporus was an epiphyte in soft media in areas with a moderate-energy water. The meandropsinids occupied the upper photic zone; the odd pair Fallotia jacquoti and Eofallotia simplex colonized high-energy environments, whereas their relative Fascispira colomi adapted to shallow, calm environments. The structure of the assemblages of larger foraminifera described in this paper provides a useful tool for the reconstruction of the Late Cretaceous shallow-water fauna in mixed siliciclastic-carbonate environments.