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

Abstract

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 logand 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 b 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.

Regional stratigraphy and subsidence of Orphan Basin near the time of breakup and implications for rifting processes

“…However, there is still much variability in elastic plate thicknesses employed in rifted margin studies, as the methodologies vary and the constraining data sets are generally incomplete (e.g. Watts & Torne, ; Bell et al ., ).…”

Strain migration during multiphase extension: Observations from the northern North Sea

“…Although flexural backstripping studies have been able to adequately reconstruct the stratigraphy in this area with a low‐uniform value of elastic thickness (Te = 1.5 km) across the Viking Graben [e.g., Roberts et al ., ], the elastic thickness would be expected to be greater in regions that have experienced less overall stretching, like the Horda Platform, Uer Terrace, and Måløy Slope areas [cf. Bell et al ., ].…”

Restored topography of the Po Plain‐Northern Adriatic region during the Messinian base‐level drop—Implications for the physiography and compartmentalization of the palaeo‐Mediterranean basin

“…(d) Area where the dataset has been integrated with observations from Fantoni et al (2001) workflow involves: (i) removal of the shallowest stratigraphic unit and calculation of the flexural-isostatic response, (ii) decompaction of the underlying sedimentary units, (iii) removal of the water layer and calculation of the corresponding flexural-isostatic response. This is a standard methodology in subsidence basin analysis (Allen & Allen, 1990;Bell et al, 2014). Decompaction is conducted by calculating near-surface porosity, decay constants, and bulk densities for lithologies observed in wells and also using the relationships provided by Sclater and Cristie (1980) (see Table 1).…”