Identification and characterisation of the Oligocene-Miocene boundary (base Neogene) in the eastern North Sea Basin - based on dinocyst stratigraphy, micropalaeontology and δ¹³C-isotope data

For the first time a combined palynological and δ¹³C-isotope study has identified the Oligocene-Miocene boundary - and thus the base of the Neogene - within the North Sea Basin. The type section, the Lemme-Carrosio section in northern Italy, is correlated with the Frida-1 well in the eastern (Danish) part of the North Sea Basin using a combination of data from a previous dinocyst stratigraphic study and new δ¹³C-isotope data. The results show that the Oligocene-Miocene boundary is located at a depth of 1440m in the Frida-1 well. The Frida-1 δ¹³C-isotope curve further reflects the Mi-1 glaciation event also recorded in the Lemme-Carrosio section. The dinocyst events bracketing the boundary in Frida-1 are; the last occurrence (LO) of Distatodinium biffii at 1630. m, below the boundary, the LO of Chiropteridium spp. at 1370. m and first occurrence (FO) of Ectosphaeropsis burdigalensis at 1330. m, both above the boundary. An influx of Deflandrea phosphoritica is found in an interval immediately below the boundary (1532-1490. m), while the genus Homotryblium occurs abundantly in a broader interval (1650-1330. m) encompassing the boundary. Hitherto unpublished data combined with new data provide a series of stratigraphically important nanno- and micropalaeontological events that frame/characterise the Oligocene-Miocene boundary within the North Sea Basin; an almost monospecific assemblage of Reticulofenestra bisecta at 1630. m, the LO of Elphidium subnodosum at 1625. m, the LO of Aulacodiscus insignis quadrata (Diatom sp. 3 of King, 1983) at 1610. m, the LO of Karreriella seigliei at 1580. m and the LO of Pararotalia canui at 1570. m, all below the boundary, and the FO of Aulacodiscus aemulans (Diatom sp. 5 of King, 1983) at 1410. m and the LO of Aulacodiscus aemulans at 1250. m, both above the boundary. The dinocyst, nanno- and micropalaeontological studies thus provide a series of bioevents and abundance variations which can be used to locate and to correlate the Oligocene-Miocene boundary within the eastern North Sea Basin more precisely than was previously possible.