CONICET | Buscador de Institutos y Recursos Humanos

The extra-andean Jurassic region of Chubut, between Cerro Cóndor and Sierra de Pichiñanes, is characterized by a magmatic-sedimentary association, the Cañadón Asfalto (CA) Formation, whose sedimentation took place in a pull-apart basin. It is composed by Las Chacritas and Puesto Almada Members. The first is characterized by lacustrine carbonatic and siliciclastic facies associations, interfingering with volcanic deposits grading from lavic in the base to predominantly pyroclastic towards the top. The carbonatic facies/microfacies are represented by mudstones, wackstones, packstones, grainstones and microbialitic limestones indicating littoral/marginal and palustrine environments. The tuffs, tuffites, shales and sandstones of the Puesto Almada Mb. represent the silting facies of the basin. The age of CA Fm. is limited by the ages of the volcano-sedimentary sequences in contact at the base and the top of the Formation. A K/Ar age of 173.1±9.4Ma (Aalenian) (Silva Nieto, 2005) was obtained for the Lonco Trapial Fm., at the base of the CA Basin, representing the beginning of the first depositional cycle of the basin. The CA Fm. is overlain by the Cañadón Calcáreo Fm., assigned by palynologic evidence to the Early Cretaceous (Volkheimer et al., 2009). Furthermore, earlier dating, by K/Ar, of the basal olivinic basalts of the Las Chacritas Mb., indicates an age of 170.9±4.4Ma (Aalenian-Bajocian) for the basal part of the CA Fm. (Salani, 2007).The objective of this contribution is the presentation of new U-Pb radiometric ages from CA Fm, obtained from a middle part of the Las Chacritas Mb. and from the base of the Puesto Almada Mb. One sample from a gray pyroclastic level about 20 cm in thicknesses with rests of plants and pomez up to 2 mm of diameter, for Las Chacritas Mb., and one sample from a fall deposit of about 40 cm from Puesto Almada Mb. were sampled for U-Pb analyses by LA-MC ICP-MS on zircon grains. The zircons are clear, prismatic, up to 100 µm. A U-Pb concordia age of 167±4Ma (Bathonian) for Las Chacritas Mb. and a U-Pb concordia age of 161±3Ma (Callovian-Oxfordian limit) for Puesto Almada Mb. were obtained. The new ages obtained for the Las Chacritas and Puesto Almada Mb., permit to delineate the sedimentary cycles of the basin. According with the new data the sedimentation of Las Chacritas Mb. would extend from 171±5Ma (Bajocian) to, at least, 167±4Ma (Bathonian). The Puesto Almada Mb. would extend from 161±3Ma (Callovian-Oxfordian) to 147.1±3.3Ma (Tithonian) (Cabaleri et al., in press,), embracing most of the Late Jurassic.