• Three lithofacies associations were initially described along the eastern slope of the La Colonia Formation in the proximity of the Cerro Buitre area (Fig. 1). The lowest association is composed of coarse-grained sandy to conglomeratic cross-bedded deposits up to 16 m thick and was initially interpreted as a terrestrial setting drained by low- to moderately sinuous fluvial channels. The middle and thickest association is represented by fine-grained deposits and includes three facies (laminated mudstones, pelite-fine sandstones, and banded siltstones and claystones) thought to have been deposited on coastal plains. The upper association is a single facies consisting of laminated claystones deposited in an intertidal zone.Two of the three facies (pelite-fine sandstones and banded siltstones and claystones) have also been recognized within the middle facies association at a locality north of Cerro Buitre (Fig. 1), where the La Colonia Formation is almost 130 m in thickness. Near Telsen (Fig. 1), the La Colonia Formation is much thinner (around 15 m on average) and starts with fine conglomerates that represent an erosive surface (or ravinement surface) resulting from the Maastrichtian transgression.