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.
Studies of normal fault systems in modern extensional regimes (e.g. Basin and Range), and in exhumed, ancient rift basins (e.g. Gulf of Suez Rift) have shown a link between the evolution of fault-related footwall topography and associated erosional drainage systems. In this study, we use 3D seismic reflection data to image the footwall crest of a gravity-driven fault system developed during late Middle Jurassic to Early Cretaceous rifting on the Halten Terrace, offshore Mid-Norway. This 22-km-long fault system lacks significant footwall uplift, with hangingwall subsidence accommodating throw accumulation on the fault system. Significant erosion has occurred along the length of the footwall crest and is defined by 96 catchments characterized by erosional channels. These erosional channels consist of small, linear systems up to 750 m long located along the front of the fault footwall. Larger, dendritic channel systems extend further back (up to 3 km normal to fault strike) into the footwall. These channels are up to 7 km long, up to 50 m deep and up to 1 km wide. Fault throw varies along strike, with greatest throw in the centre of the fault decreasing towards the fault tips; localized throw minima are interpreted to represent segment linkage points, which were breached as the fault grew. Comparison of the catchment location to the throw distribution shows that the largest catchments are in the centre of the fault and decrease in size to the fault tips. There is no link between the location of the breached segment linkage points and the location and size of the footwall catchments, suggesting that the first-order control on footwall erosion patterns is the overall faultthrow distribution.
“Salt” giants are typically halite‐dominated, although they invariably contain other evaporite (e.g. anhydrite, bittern salts) and non‐evaporite (e.g. carbonate, clastic) rocks. Rheological differences between these rocks mean they impact or respond to rift‐related, upper crustal deformation in different ways. Our understanding of basin‐scale lithology variations in ancient salt giants, what controls this and how this impacts later rift‐related deformation, is poor, principally due to a lack of subsurface datasets of sufficiently regional extent. Here we use 2D seismic reflection and borehole data from offshore Norway to map compositional variations within the Zechstein Supergroup (ZSG) (Lopingian), relating this to the structural styles developed during Middle Jurassic‐to‐Early Cretaceous rifting. Based on the proportion of halite, we identify and map four intrasalt depositional zones (sensu Clark et al., Journal of the Geological Society, 1998, 155, 663) offshore Norway. We show that, at the basin margins, the ZSG is carbonate‐dominated, whereas towards the basin centre, it becomes increasingly halite‐dominated, a trend observed in the UK sector of the North Sea Basin and in other ancient salt giants. However, we also document abrupt, large magnitude compositional and thickness variations adjacent to large, intra‐basin normal faults; for example, thin, carbonate‐dominated successions occur on fault‐bounded footwall highs, whereas thick, halite‐dominated successions occur only a few kilometres away in adjacent depocentres. It is presently unclear if this variability reflects variations in syn‐depositional relief related to flooding of an underfilled presalt (Early Permian) rift or syn‐depositional (Lopingian) rift‐related faulting. Irrespective of the underlying controls, variations in salt composition and thickness influenced the Middle Jurassic‐to‐Early Cretaceous rift structural style, with diapirism characterising hangingwall basins where autochthonous salt was thick and halite‐rich and salt‐detached normal faulting occurring on the basin margins and on intra‐basin structural highs where the salt was too thin and/or halite‐poor to undergo diapirism. This variability is currently not captured by existing tectono‐stratigraphic models largely based on observations from salt‐free rifts and, we argue, mapping of suprasalt structural styles may provide insights into salt composition and thickness in areas where boreholes are lacking or seismic imaging is poor.
Rift basin tectono-stratigraphic models indicate that normal fault growth controls the sedimentology and stratigraphic architecture of syn-rift deposits. However, such models have rarely been tested by observations from natural examples and thus remain largely conceptual. In this study we integrate 3D seismic reflection, and biostratigraphically constrained core and wireline log data from the Vingleia Fault Complex, Halten Terrace, offshore Mid-Norway to test rift basin tectono-stratigraphic models. The geometry of the basin-bounding fault and its hangingwall, and the syn-rift stratal architecture, vary along strike. The fault is planar along a much of its length, bounding a half-graben containing a faultward-thickening syn-rift wedge. Locally, however, the fault has a ramp-flat-ramp geometry, with the hangingwall defined by a fault-parallel anticline-syncline pair. Here, an unusual bipartite syn-rift architecture is observed, comprising a lower faultward-expanding and an upper faultward-thinning wedge. Fine-grained basinfloor deposits dominate the syn-rift succession, although isolated coarse clastics occur. The spatial and temporal distribution of these coarse clastics is complex due to syn-depositional movement on the Vingleia Fault Complex. High rates of accommodation generation in the fault hangingwall led to aggradational stacking of fan deltas that rapidly (<5 km) pinch out basinward into offshore mudstone. In the south of the basin, rapid strain localization meant that relay ramps were short-lived and did not represent major, long-lived sediment entry points. In contrast, in the north, strain localization occurred later in the rift event, thus progradational shorefaces developed and persisted for a relatively long time in relay ramps developed between unlinked fault segments. The footwall of the Vingleia Fault Complex was characterized by relatively low rates of accommodation generation, with relatively thin, progradational hangingwall shorelines developed downdip of the fault block apex, sometime after the onset of sediment supply to the hangingwall. We show that rift basin tectono-stratigraphic models need modifying to take into account along-strike variability in fault structure and basin physiography, and the timing and style of syn-rift sediment dispersal and facies, in both hangingwall and footwall locations.
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