High-resolution magnetic stratigraphy across the Oligocene-Miocene boundary in an alluvial-lacustrine succession (Ebro Basin, northeast Spain)

Abstract

The study of a late Oligocene-early Miocene, 300 m thick section of alluvial and shallow lacustrine sediments in the Ebro Basin, has allowed a high-resolution magnetic polarity stratigraphy for the Oligocene-Miocene boundary to be established. The samples studied include alluvial sandstones and mudstones as well as shallow lacustrine and paludine, micritic and microsparitic limestones. Two-hundred-and-fifty specimens were stepwise thermally or magnetically demagnetized following standard procedures. Magnetic imprints of the distinct lithologies present sharp contrasts, especially for the stability of the secondary components. The VGP data show 18 reversal events, defining a sequence of 16 polarity intervals. Lateral and vertical continuity of the sequences studied has enabled a correspondence between the local paleomastological biozones and the magnetostratigraphic succession to be established. The high-resolution chronology of this magnetostratigraphic succession provides valuable information that allows the non-marine European biochronological scales to be correlated to the global polarity time scale (GPTS). The magnetic polarity zonation of the section has been correlated to the GPTS using the distinct sequence and pattern polarity reversals as well as by considering the fossil mammal assemblages. As a preliminary result, chrons 8, 7A, 7 and 6C have been identified.