The Cretaceous post-rift basin configuration of the northern North Sea

Abstract

The present analysis suggests that three stages can be identified in the post-rift Cretaceous development of the northern North Sea, namely the incipient (Ryazanian–latest Albian), the middle (Cenomanian–late Turonian) and the mature (early Coniacian–early Palaeocene). The transition from syn-to post-rift configuration was strongly diachronous, suggesting that the thermal state of the system was not homogeneous at the onset of the post-rift stage. This is supported by observed differences between the early post-rift subsidence histories of the southern Viking Graben, the Stord Basin and the Sogn Graben. The incipient post-rift stage was characterized by diverse subsidence. The major structural features inherited from the syn-rift basin (e.g. crests of rotated fault blocks, relay ramps and sub-platforms) had a strong influence on the basin configuration and, therefore, the sediment distribution. In the middle stage the internal basin relief became gradually drowned by sediments. This is typical for basins where sediment supply outpaces or balances subsidence, as was the case in the northern North Sea. Thus, the influence of the syn-rift basin topography become subordinate to the subsidence pattern which was determined by the crustal thinning profile which, in turn, relies on thermal contraction and isostatic/elastic response to sediment loading. The mature post-rift stage was characterized by the evolution into a wide, saucer-shaped basin where the syn-rift features finally became erased. Since thermal equilibrium was reached at this stage, subsidence ceased, and the pattern of basin filling became, to a larger degree, dependent on extra-basinal processes. This simple pattern was influenced by the structural inhomogeneity of the basin. This inhomogeneity may have included the graben units, in turn related to contrasting geometries of the lithospheric structure. The incipient stage of post-rift development was halted by relative uplift/deceleration of subsidence, locally corresponding to 200 m. This is ascribed to a hitherto undescribed thermo-tectonic event. The mechanism of this event is not yet known.