Insights into the development of major rift‐related unconformities from geologically constrained subsidence modelling: Halten Terrace, offshore mid Norway

Due to the effects of sediment compaction, thermal subsidence and 'post-rift' fault reactivation, the present-day geometry of buried, ancient rift basins may not accurately reflect the geometry of the basin at any stage of its syn-rift evolution. An understanding of the geometry of a rift basin through time is crucial for resolving the dynamics of continental rifting and in assessing the hydrocarbon prospectivity of such basins. In this study, we have restored the Late Jurassic-Early Cretaceous geometry of the southern Halten Terrace, offshore mid Norway, using a combination of well log-and core-derived, sedimentological and stratigraphic data, seismic-stratigraphic observations and reverse subsidence modelling. This integrated geological and geophysical approach has allowed the large number of input parameters involved in flexural backstripping and post-rift thermal subsidence modelling to be constrained. We have thus been able to determine the regional structure of the basin at the end of the Late Jurassic-Early Cretaceous rift phase and the associated amount of crustal stretching. Our basin geometry reconstructions reveal that, during the latest syn-rift period in the Late Jurassic-Early Cretaceous, the Halten Terrace was characterized by a series of isolated depocentres, located between footwall islands, which were not connected into a single depocentre until the Late Cretaceous (Coniacian). We show that two major unconformities, which are now vertically offset by ca. 2 km and located ca. 60 km apart, formed at similar subaerial elevations in the Late Jurassic-Early Cretaceous and were subsequently vertically offset by thermally induced tilting of the basin margin. Cretaceous sediments were deposited in a single, relatively unconfined basin in water depths of 1-1.5 km. The beta profile that best restores palaeobathymetry to match our geological constraints is the same as that derived from summing visible post-Late Triassic heave on faults plus 25-60% additional extension to account for sub-seismic deformation. This indicates that, at least in the southern part of the Halten Terrace, the amount of upper-crustal stretching during the Late Jurassic-Early Cretaceous rift phase is comparable to the total amount of lithospheric stretching, supporting a uniform pure-shear stretching model.