Sequence stratigraphy of a failed rift system: the Middle Jurassic to Early Cretaceous basin evolution of the Central and Northern North Sea

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Abstract
The structural and stratigraphic evolution of the North Sea rift system is described, from the onset of pre-rift thermal uplift in the Aalenian to the failure of the rift system caused by the transfer of extension out onto the Atlantic margin during the Neocomian. Our sequences are interpreted from a combination of lithostratigraphic, biostratigraphic and seismic stratigraphic correlation. They represent discrete phases of basin infill caused by unique combinations of subsidence and sedimentation rates. Our sequence boundaries are regional unconformities or hiatuses which develop in response to changes in relative sea-level. We recognize three processes which control relative sea-level and, therefore, the nature of the sequence boundaries; they are changes in tectonic subsidence rates, sedimentation rates and eustatic sea-level. Six sequences are described and labelled J20 to J70. Each marks a regionally correctable step in the development of the rift system. We believe the sequences are mainly controlled by changes in the rates of tectonic subsidence and sedimentation. The effects of eustatic sea-level variations are recognized, but are not considered the prime control on sediment distribution within the rift at the scale of the sequences described. Sequences J20 to J70 highlight the struggle between sedimentation rates and tectonic subsidence during the development of the rift system. The overall pattern is one of drowning of the sediment supply and starvation of the basin as faulting propagates. This trend is often interrupted by periods in which sedimentation was able to keep pace with tectonic subsidence and in local examples substantially exceed it. The first rift-related sequence (J20) records deposition during pre-rift thermal uplift over much of NW Europe. During J20 a low-angle, weakly erosive, pre-rift unconformity (the ‘Mid-Cimmerian’ unconformity) developed over much of the North Sea Basin. The domal uplift was centred on the Central Graben volcanic province in the Central North Sea. The second main phase in the development of the rift was the syn-rift period where faulting actively controlled sedimentation. J30 through J60 sequences record the diachronous onset of faulting and breakup of the pre-rift thermal dome, starting at the extremities of a three-armed rift system and propagating in towards the core of the pre-rift thermal dome in the Central Graben. Diachronous faulting is noted by dating the unconformities in fault footwalls and in the onset of fault-controlled deposition. For example, major faulting was initiated during J30 in the Brent Province in the Northern North Sea, but not until J60 in the Central Graben of the Central North Sea. Fault-controlled unconformities can be correlated across sub-provinces but no single basin-wide extension-related unconformity (i.e. no ‘Late Cimmerian’ unconformity) is identified. The third phase of the rift development records the failure of the North Sea rift and the transfer of extension out onto the Atlantic margin. This commences with the J70 sequence which in the North Sea records the onset of thermal subsidence without associated faulting. J70 and younger fault-controlled unconformities and associated depositional systems are recorded only in the Magnus Province bordering the active Atlantic margin. In the North Sea the strong seismic boundary, often known as the ‘Base Cretaceous Unconformity’, is shown to be neither an unconformity nor to coincide with the Jurassic/Cretaceous boundary.