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

Abstract

19 20'Salt' giants are typically halite-dominated, although they invariably contain other evaporite (e.g. anhydrite, 21 bittern salts) and non-evaporite (e.g. carbonate, clastic) rocks. Rheological differences between these rocks 22 mean they impact or respond to rift-related, upper crustal deformation in different ways. Our understanding 23 of basin-scale lithology variations in ancient salt giants, what controls this, and how this impacts later rift-24 related deformation, is poor, principally due to a lack of subsurface datasets of sufficiently regional extent. 25Here we use 2D seismic reflection and borehole data from offshore Norway to map compositional variations 26 within the Zechstein Supergroup (Lopingian), relating this to the structural styles developed during Middle 27Jurassic-to-Early Cretaceous rifting. Based on the proportion of halite, we identify and map four intrasalt 28 depositional zones (sensu Clark et al., 1998) offshore Norway. We show that, at the basin margins, the 29 Zechstein Supergroup is carbonate-dominated, whereas towards the basin centre, it become increasingly 30 halite-dominated, a trend observed in the UK sector of the North Sea Basin and in other ancient salt giants. 31However, we also document abrupt, large magnitude compositional and thickness variations adjacent to 32 large, intra-basin normal faults; for example, thin, carbonate-dominated successions occur on fault-bounded 33 2 footwall highs, whereas thick, halite-dominated successions occur only a few kilometres away in adjacent 34 depocentres. It is presently unclear if this variability reflects variations in syn-depositional relief related to 35 flooding of an underfilled presalt (Early Permian) rift or syn-depositional (Lopingian) rift-related faulting. 36

Tectono‐stratigraphic development of a salt‐influenced rift margin: Halten Terrace, offshore Mid‐Norway

“…In such a setting it is common for the less mobile evaporite and evaporite‐related sediments, such as anhydrites and carbonates, to be deposited (cf. Permian Zechstein Supergroup of the North Sea, Clark et al., 1998; Jackson et al., 2019). The presence of largely immobile rocks on the flanks of the Frøya High would have inhibited supra‐salt footwall collapse, thereby enhancing uplift and associated erosion of the footwall (Figures 5 and 10).…”

The influence of multiple salt layers on rift‐basin development; The Slyne and Erris basins, offshore NW Ireland

“…The exact age of faulting remains ambiguous, with the Late Permian bathymetry either being inherited from Early Permian rifting, or representing syn‐depositional extension in the Late Permian (e.g. Jackson et al., 2019; Jackson & Lewis, 2016; McKie & Shannon, 2011). Applying a similar model to the Slyne and Erris basins, the thicker salt‐prone Zechstein Group in the Slyne Basin and the Southern Erris Sub‐basin would be indicative of active Permian faulting, whereas the presence of thin marginal deposits in the Northern Erris Sub‐basin suggests this area was a relative high with little active faulting.…”

“…This leads to significant differences in the structural styles and evolution of salt‐influenced basins relative to those unaffected by salt. Thick layers of salt have been encountered in several basins across the European Atlantic margin, including offshore Iberia (Alves et al., 2006; Pena dos Reis et al., 2017; Ramos et al., 2017; Wilson et al., 1989; Zamora et al., 2017), offshore France (Chapman, 1989; Ferrer et al., 2012), on the United Kingdom Continental Shelf (UKCS) (Jackson & Stewart, 2017; Stewart et al., 1996) and offshore Norway (Jackson et al., 2019; Rojo et al., 2019). Salt is also present in basins on the conjugate margin of Atlantic Canada (Deptuck & Kendell, 2017; Jansa et al., 1980).…”

Reply to the comment of Schollnberger on ‘Subsidence analysis of salt tectonics‐driven carbonate minibasins’ (Northern Calcareous Alps, Austria), published on Basin Research 2020; 00: 1– 23 doi:10.1111/bre.12500

“…There is one point we would like to stress up front in our reply: what do we refer as to salt? Rock salt is a crystalline aggregate of halite; however, since pure halite sequences are rare, in salt tectonics terms ‘salt’ most commonly refers to any rock composed of halite plus other associated evaporitic and nonevaporitic rocks; there are numerous works that use this term as such (Jackson et al., 2018; Rowan, 2014; Rowan & Vendeville, 2006; Teixell et al., 2017, just for mentioning some). In fact, it may be more appropriate to use the term ‘layered evaporitic sequence’, which commonly includes halite, other evaporites, plus different amounts of carbonates and siliciclastics.…”