Enhancing mutual stratigraphic resolution in Belgium and the Netherlands by cross-border study of mid Eocene successions in the southernmost North Sea Basin

Abstract

The lack of age-indicative calcareous nannoplankton has rendered the mid Eocene stratigraphic framework of the southern North Sea Basin uncertain. Palynological analyses by this study succeed in bridging this gap. A series of wells in the border region between Belgium and the Netherlands was biostratigraphically analysed, based on dinocyst species. The Belgian middle to late Eocene lithostratigraphic framework, comprising the Maldegem and Zelzate formations and their respective members, was applied to interpret the wells, which strongly increased the existing stratigraphic detail for the Dutch wells. A well-log correlation panel shows consistent biostratigraphic ages for the different middle to late Eocene units. It shows that the Zomergem and Buisputten members of the Maldegem Formation are still of Lutetian age, whereas the Onderdijke Member of the same formation is completely of Bartonian age. Palynological assemblages indicate an intra-Bartonian sea-level fall at the boundary between the Maldegem and Zelzate formations. The lowermost few metres of the Zelzate Formation consistently comprise Bartonian dinocyst species, which had been previously interpreted as reworked, but are now considered in situ, placing the Bartonian/Priabonian transition in the basal Zelzate Formation. The newly established Bartonian/ Priabonian boundary corresponds to the start of a new sea-level rise, indicated by an increase in gamma-ray values, in a nummulite level attributed to calcareous nannoplankton NP18 biozone. Hence, the previously established hiatus between the Belgian Maldegem and Zelzate formations now falls within the (upper part of the) Bartonian.