Diachronism of the Deflandrea oebisfeldensis acme towards the southern margin of the Belgian Basin

The biostratigraphic significance of the Deflandrea oebisfeldensis acme is lost near the southern margin of the North Sea Basin where it is recorded in higher dinoflagellate zones within the lowermost Ypresian deposits. A tentative explanation of the diachronism of the D. oebisfeldensis acme is based on certain changes of palaeogeography and palaeohydrography of the North Sea Basin during the Early Ypresian.