Nuevos datos magnetoestratigráficos del tránsito Oligoceno-Mioceno en el sector SE de la Cuenca del Ebro (provincia de Lleida, Zaragoza y Huesca, NE de España)

Abstract

The study of two late Oligocene-early Miocene stratigraphic sections of alluvial and shallow lacustrine facies in the SE Ebro Basin has enabled the establishment of a high-resolution magnetostratigraphy for the Oligocene-Miocene boundary. The studied samples include alluvial sandstones and mudstones as well as shallow lacustrine limestones. A total of 121 stratigraphic levels, with an average sample spacing of -2.5 m, were sampled in two sections, using a portable gasoline powered dril1 machine. Core samples were cut in the laboratory into two or three standard specimens of 2.54 cm diameter X 2.1 cm in length. A minimum of two specimens per site were thermally or AF demagnetized in 8-9 steps at intervals of 50°C or 30°C, and intervals of 2.5 mT and 5 mT, respectively. The magnetic polarity zonation of the sections have been correlated to the Geomagnetic Polarity Time Scale (GPTS) allowing the recognition of Chrons 9r, 9n, 8r and 8n in the Mina Pilar section, and Chrons 6Cr, 6Cn, 6Br, 6Bn and 6AAr in the Valcuerna section. On the basis of magnetostratigraphic correlation it is deduced an interval of sedimentation time of -1.6 Ma for the Mina Pilar section, and 1.7-1.9 Ma for the Valcuerna section.