CONICET | Buscador de Institutos y Recursos Humanos

The Jurassic-Cretaceous boundary is one of the last stage boundaries that lack precise geochronological data. The Geological Time Table of 2012 recommends the ID-TIMS U-Pb zircon dating of interstratified tuffs as the most reliable method to be used in order to calibrate the boundary. The present J-K boundary was set in the Shatsky rise, in the west Pacific Ocean, using pillow lavas from an ODP site. The evaluation of the biostratigraphic control of that site shows that it did not meet neither the biostratigraphic standards nor the geochronology recommended for the isotopic dating. The data are mainly based on a linear interpolation of oceanic magnetic anomalies assuming constant spreading rates. The comparison with the first data obtained in the Neuquén Basin by LA-MC-ICPMS, producing 237 concordant ages, shows a statistically robust youngest age peak at ca. 144 Ma that is 5 Ma younger than indicated in the present Geological Time Table. The new ages obtained in interstratified tuff layers near the boundary by U-Pb SHRIMP, confirm the 5 Ma difference. Following the recommendation of the International Commission on Stratigraphy for the International Chronostratigraphic Chart of 2013 new ID-TIMS U-Pb zircon dating was produced that confirm again the 5 Ma difference. Therefore, a new boundary is proposed at 140 Ma based on the geochronological ages, high precision calcareous nanofossils markers, precise ammonite biozonation, and sedimentation rates obtained by cyclostratigraphy. The new data base obtained in the Neuquén Basin led us to suggest a change in the present assumed age of the stage boundary. Moreover, as a byproduct, we propose a nonlinear polarity magnetic chron-scale for the late Jurassic and early Cretaceous. New geochronological studies under way will improve the resolution of the chronometric scale at this stage boundary and eventually will lead to propose the first GSSP in the Southern Hemisphere.