87Sr/86Sr analysis on calcitic shells of pectinids (Chlamys actinodes and Chesapecten crassus) and oysters (Ostrea sp.) shows that the ?Entrerriense? sequence belonging to the Puerto Madryn Formation, Valdés Península (Chubut), was deposited at about 10.0±0.3 Ma. Thus, a middle Tortonian age is assigned to the sequence. Within the sequence, the pectinids better retain their original 87Sr/86Sr ratio, whilst the oysters are altered and yield older-than-real ages. The latter is the result of greater susceptibility to interaction with diagenetic fluids bearing Sr derived from the alteration of tuffaceous materials and volcanic rocks of intermediate and basic composition.