Salt thickness and composition influence rift structural style, northern North Sea, offshore Norway

Abstract

“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.

A reconstruction of Iberia accounting for W-Tethys/N-Atlantic kinematics since the late Permian-Triassic

“…4A). This is recorded by several late Permian rift domains located in the southern North Atlantic (Rasmussen et al, 1998;Leleu et al, 2016), in the Adriatic (Scisciani and Esestime, 2017) in the North Sea (Hassaan et al, 2020), in the Germanic rift basins, including the Zechstein basin (Evans, 1990;Van Wees et al, 2000;Jackson et al, 2019) (Channell and Kozur, 1997;Stampfli et al, 2001). As proposed by Schmid et al (2008), the Pindos ocean was probably a western branch of the Neotethys rather than a unique ocean.…”

“…CC BY 4.0 License. Van Wees et al, 2000;Evans et al, 2003;Jackson et al, 2019 Tornquist Zone , 1983;Brunet, 1984;Biteau et al, 2006;Serrano et al, 2006 Salas and Casas, 1993 Hyper-extended rifting 150 to 120 Salas and Casas, 1993;Arche and Gomez, 1996;Salas et al, 2001;Omodeo-Sale et al, 2017;Rat et al, 2019 Southern North Atlantic Initiation of oceanic spreading 133-100 Olivet, 1996;Strivastava et al, 2000;Schettino & Turco, 2009;Whitmarsh and Manatschal, 2012 Bay of Biscay…”

A reconstruction of Iberia accounting for Western Tethys–North Atlantic kinematics since the late-Permian–Triassic

“…We quantify 99 and 19 km for the 140-120 and 120-100 Ma time intervals, respectively, leading to a total of 128 km of strike-slip movement during the Lower Cretaceous, in the range of amounts deduced from offshore and onshore geological observations (Olivet, 1996;Canérot, 2016). By 118 Ma, most of the strike-slip faulting is terminated as extension became orthogonal and Ebro is close to its present-day position (Jammes et al, 2009;Mouthereau et al, 2014). The maximum strain rate of 5 km Myr −1 is obtained for the 140-120 Ma time interval, revealing progressive strain localization in the Pyrenean basins before mantle exhumation (Jammes et al, 2009;Lagabrielle et al, 2010;Mouthereau et al, 2014;Tugend et al, 2014).…”

“…Because of the lack of geological constraints, such a large strike-slip displacement has been supposedly exported along the North Pyrenean Fault (Fig. 1a) (e.g., Choukroune and Mattauer, 1978;Debroas, 1987Debroas, , 1990Lagabrielle et al, 2010;Jammes et al, 2009). Here again, previous studies were not conclusive so there are currently no firm geological constraints nor geophysical evidence across the Pyrenees to argue for significant transcurrent deformation during the Jurassic or the Cretaceous (Olivet, 1996;Masini et al, 2014;Canérot, 2016;Chevrot et al, 2018).…”

“…The Ebro block is delimited to the north by the Pyrenean system. Extension then migrated and localized to the north (Rat et al, 2019) leading to oceanic spreading in the Bay of Biscay and hyper-extension in the Pyrenean rift basins (Jammes et al, 2009;Lagabrielle et al, 2010;Mouthereau et al, 2014;Tugend et al, 2014Tugend et al, , 2015.…”