87Sr/86Sr late miocene age of fossil molluscs in the 'entrerriense' of the Valdés Peninsula (Chubut, Argentina)

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.Fil: Scasso, Roberto Adrian. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: McArthur, J. M.. Colegio Universitario de Londres; Reino UnidoFil: del Río, Claudia Julia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"; ArgentinaFil: Martinez, Sergio. Universidad de la República; UruguayFil: Thirwall, M. F.. University College London; Estados Unido