The Oligocene/Miocene boundary in the ON-Mol-1 and Weelde boreholes along the southern margin of the North Sea Basin, Belgium

Abstract

Based on palynologic interpretations, the Oligocene/Miocene boundary was identified in the cored samples of the ON-Mol-1 and Weelde boreholes in the southern North Sea Basin. In both boreholes, the presumed location of the Oligocene/Miocene boundary is marked by a thin gravel bed, which testifies of a hiatus, within a continuation of shelly, dark green to black fine glauconitic sands. In the Weelde borehole, the hiatus is relatively short as the dinocysts under- and overlying the gravel bed suggest a late(st) Chattian and early Aquitanian age. This study thereby for the first time indicates the presence of Aquitanian strata in the area. In the ON-Mol-1 borehole, located south of the Weelde borehole, the hiatus is longer as dinocysts under- and overlying the gravel bed suggest an early Chattian and late Aquitanian age. The wider ranges of the hiatus might be the result of the combination of erosion/non-deposition of late Chattian strata and the gradual transgression of the southern North Sea Basin from north to south after the Oligocene/Miocene sea-level fall. The sampled interval around the Oligocene/Miocene boundary contains a lot of reworked Paleogene (including irregular upper Oligocene species occurrences) and Mesozoic taxa, which explains why the Miocene strata in the ON-Mol-1 and Weelde boreholes were dated as Chattian by previous studies.