In the Jurassic–Cretaceous North Sea basin, the synrift sequence is separated from the post-rift sequence by the ‘base Cretaceous’ or ‘late Cimmerian’ unconformity. The unconformity covers almost the entire basin, has a distinct character in seismic reflection data and wireline logs, and hence, is easily identified and correlated, making it the most important marker horizon in the area. The unconformity displays great local complexity (in many localities) and great variability on a regional scale (from one locality to another). Thus the unconformity is classified as a nonconformity, a disconformity and an angular unconformity. We suggest that these variations basically reflect different structural position within the basin, so that the short-wavelength variation is dominated by local structural development (e.g. the rotational history of a fault block), whereas the long-wavelength variation reflects basin-scale tectonic, thermal and isostatic processes. The merging of this unconformity with younger erosional surfaces, its complex configuration and polychronous character makes the general term ‘base Cretaceous unconformity’ inadequate. Thus, the term ‘northern North Sea Unconformity Complex’ is used here.