The investigation of foraminiferal assemblages from a series of Oligocene borehole sections allowed paleoenvironment and paleoclimate reconstructions for the Rupelian and Chattian (Lower and Upper Oligocene) Stages in their type region, the southern North Sea Basin. A striking feature coinciding with the Rupelian-Chattian (R-C) unconformity is the major change in paleotemperature and paleobathymetry. The shallow marine to restricted marine subtropical fauna at the base of the Chattian is in strong contrast with the deeper marine and cooler upper Rupelian assemblages. This study suggests that the early Chattian transgression is genetically related to a widespread major warming pulse, known as the Late Oligocene Warming Event.

The classical problem of the nature and age of the Rupelian-Chattian (Early-Late Oligocene) unconformity in its type region is here approached using organic walled dinoflagellate cyst (dinocyst) correlations between the North Sea Basin and well-calibrated central Italian (Tethyan Ocean) sections. Useful Oligocene dinocyst events are the last occurrence of Enneadocysta pectiniformis (~29.3 Ma), and the first occurrences of Saturnodinium pansum (~29.4 Ma), Distatodinium biffii (~27.9 Ma) and Artemisiocysta cladodichotoma (~26.7 Ma).The latter event marks the earliest Chattian. The improved correlations indicate that the Rupelian-Chattian (R-C) boundary is associated with the so-called ‘Oligocene Glacial Maximum’. This phase of important global cooling and glacio-eustatic sea level fall is genetically related to the unconformity between the classic Oligocene stages. Subsequent global warming (so-called ‘Late Oligocene Warming Event’), induced a major sea level rise, leading e.g. to the time-transgressive deposition of the typical basal Chattian glauconitic sands. The oldest of the Chattian units have a GPTS age of-26.7 Ma. It further appears that a hiatus of ~500 kyrs spans the classic Rupelian-Chattian unconformity.