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.

No abstract

Between 62° and 68°N on the Norwegian continental margin, the hypothesis of a Cretaceous or Permian ocean extending northwards from the Rockall Trough can be tested with data from commercial reflection seismic profiles. Two major Mesozoic basin systems occur. The Helgeland Basin is a late Triassic and early Jurassic depocentre with a thin Upper Jurassic and Cretaceous cover except in two late Cretaceous sub-basins. The Møre/lnner Vøring Basin system is a middle to late Cretaceous depocentre in which the pre-Cretaceous section is deeply buried and rarely seen on seismic profiles. The two basin systems are separated by a complex zone of faulted 'marginal highs' active in Jurassic times and reactivated during Cretaceous times. Pre-Cenomanian lavas or sills are interpreted within the Møre Basin and a pre-Cretaceous block-faulted sequence can be identified across the inner Vøring Basin except in a narrow axial zone. These features suggest the presence of an extremely narrow zone of oceanic crust of Cretaceous age. Fault patterns confirm a major tectonic episode of mid-Cretaceous age. The geology off mid-Norway supports a mid-Cretaceous age for sea-floor spreading in the Rockall Trough but suggests that the generated oceanic crust narrowed northwards and that plate motion was partially accommodated by dextral transcurrent faulting. Reconstruction of Jurassic tectonic trends in NW Europe must allow for subsequent Cretaceous distortion.

No abstract

Depositional sequence mapping has been used to analyse the late Devonian to Recent geologic evolution and hydrocarbon habitat of north Alaska and northwest Canada. Eight depositional megasequences have been identified, each of which records a discrete, major phase of basin evolution. The three oldest megasequences are named the Ellesmerian and reflect deposition on a subsiding fold belt terrane. We name thc subsequent two megasequences of early Jurassic to Aptian age, the Beaufortian. They record a 100 Ma period of extension during which a Jurassic failed rift episode was followed by onset of the successful rift episode in the Hauterivian. This extension led to the opening of the oceanic Canada Basin. The final three megasequences record geographically distinct pulses of Brookian orogenesis.The major proven hydrocarbon habitat occurs on the Barrow Arch of north Alaska. This is a volumetrically large, but greatly restricted, hydrocarbon province, which developed as a result of constructive interference between Beaufortian rift and Brookian orogenic tectonics. Two other, relatively minor, hydrocarbon provinces have also been discovered. They are the Mackenzie Delta and Kugmallit Trough provinces of northwest Canada, which developed in passively subsided basins, located just beyond the influencc of Brookian orogenic uplift.