1.5.2. Salt
Section written in collaboration with Mark Rowan.
In geology, the term “salt” refers to an evaporitic mineral, a rock (halite, also known as rocksalt) and an interbedded sedimentary sequence composed mostly or largely of evaporitic material. Evaporites designate rocks formed by the precipitation of minerals in arid, isolated basins where the loss of water by evaporation exceeds the inflow. There are more than 80 evaporite minerals (Warren 2016), with the most common being halite and anhydrite/gypsum. They are usually interbedded with carbonate and siliciclastic rocks in layered evaporite sequences (LES) that can be up to 5+ km thick.
Halite, anhydrite and other evaporite minerals are distinguished based on their composition and crystal structure: for example, halite is a sodium-chloride (NaCl) isometric crystal, anhydrite is a sulfate (CaSo4) with orthorhombic crystals, and gypsum is a hydrous variation of anhydrite. The minerals precipitate at different stages during water evaporation, in the reverse order of their solubilities (gypsum/anhydrite precipitates before halite, and bittern salts form in extremely evaporative conditions – see Warren (2016)). Halite is typically the most abundant constituent of LES.
In rift/rifted margin settings, similarly to sediments, salt is often classified as pre-rift, syn-rift or post-rift (e.g. Jackson and Vendeville 1994) according to the time of its deposition relative to the tectonic activity. Syn-rift salt can further be subdivided by adhering to the multistage evolution proposed for rifted margins (see Chapter 2) because the relative timing of rift evolution and evaporite deposition controls the spatial and thickness distribution of the salt (Rowan 2014). If salt is deposited early in margin development (syn-stretching or early syn-rift salt) or in rift basins that never evolve into rifted margins, it forms near sea level in elongate basins that are variably isolated or connected with significant thickness variations (Figure 1.32a). If salt is deposited later in the history (syn-thinning or mid syn-rift salt), the salt will be concentrated in the distal domain, with the base salt offset by large-displacement, low-angle faults. Finally, if salt is shortly deposited before the onset of spreading (syn-exhumation or late syn-rift salt), it will be widespread with little/no relief on the base salt except in the outer domain, where it is typically thickest in an outer trough bounded by oceanic crust (Rowan 2014) (see Figure 1.32b).
Figure 1.32. Evaporite deposition at different stages of rifting, shown just prior to the onset of sea-floor spreading: a) syn-stretching (early syn-rift) salt; b) syn-exhumation (late syn-rift) salt. Rift template modified from Ribes et al. (2019); vertical exaggeration 3:1
Salt has physical characteristics and mechanical properties (Jackson and Hudec 2017) that make it special in rift evolution and thus a critical consideration when interpreting subsurface geometries. First, it has a constant low density (2.160 g/cm3 for pure halite) and is thus incompressible. Other sediments become denser with burial and compaction/cementation, so there will be a density inversion for the deeply buried salt. Much more important, however, is that halite has a low viscosity that allows it to easily flow, compared to surrounding rocks. Where the encasing rocks behave as relatively strong, brittle materials, salt is best viewed as a pressurized fluid that flows in a viscous manner (Vendeville and Jackson 1992). The drivers for salt movement are extension, contraction and differential loading, and the consequent way in which salt deforms is known as salt tectonics, halokinesis or halotectonics.
In a rift/rifted margin context, the style of halokinesis largely depends on whether the salt is pre-rift, early syn-rift or late syn-rift (Rowan 2014). In the first two cases, the deformation is thick-skinned in that the basement, salt and its cover are all extended. The salt tends to decouple the supra- and subsalt deformation to some degree, with a greater degree of coupling for thin salt and/or large-displacement faults (Withjack and Callaway 2000). In cases of extreme extension and mantle exhumation, salt can completely decouple low-angle detachment faults in the presalt from thin-skinned detachment faults in the suprasalt (Jammes et al. 2010). In less extended settings, typical structural features (Figure 1.33a) include listric or conjugate faults that are detached on the salt, drape folds and faulted drape folds over basement faults, and diapirs located over or slightly in the footwall of subsalt faults. A salt diapir is a body of salt with a truncating relationship with its overburden; in rifted settings, they are usually triggered by extension, going through reactive, active and passive stages (Vendeville and Jackson 1992). Passive diapirs continue to grow at or just beneath the surface as sediment is deposited around them.
In the case of late syn-rift salt, salt tectonics is dominated by post-rift gravitational failure of the salt and its overburden (see Rowan 2020). The primary drivers are the basinward tilt of the margin, caused by the thermal subsidence of oceanic crust, and proximal sediment loading. The deformation is thin-skinned, comprising of linked systems of proximal extension, translation and distal contraction (Figure 1.33b). Extension is accommodated by both basinward- and landward-dipping normal faults, contraction is manifested by both salt-cored folds and thrust faults as well as the squeezing of diapirs and the translational province has both symmetric and asymmetric salt-evacuation structures. Relief on the base salt can generate ramp-syncline basins. Diapirs are triggered by various processes, and the salt may break out and laterally flow to form salt sheets and canopies.
Salt has commonly been associated with geophysical imaging problems. Firstly, salt bodies may be non-reflective or have complex internal structures depending on the original layering and mode of deformation. Secondly, salt structures such as diapirs, typical of many rifted margins worldwide, usually have steep sub-vertical geometries that are often poorly imaged and that often also generate geophysical noise, making their exact shape and edge-diapir location difficult to define. Thirdly, the contrast between shallow high-velocity salt and low-velocity sediment causes migration problems and artifacts. Finally, there are many cases where little orno energy penetrates salt sheets/canopies into subsalt domains and then back to the surface receivers. However, new acquisition and processing techniques now allow much better imaging (Figure 1.34).
Further reading.– The above descriptions are abbreviated and often simplified. If interested in reading and learning further, the reader is referred to the following list of publications and references:
– General: (Vendeville and Jackson 1992; Jackson and Vendeville 1994; Withjack and Callaway 2000; Jammes et al. 2010; Rowan 2014, 2020; Warren 2016; Jackson et al. 2020).
Figure 1.33. Styles and features of rift-related salt tectonics: a) thick-skinned extension; b) gravity-driven, linked thin-skinned extension, translation and contraction. Not to scale, and a) and b)