Geology of salt

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Salt is part of a group of chemically deposited minerals which crystallize by evaporation of salt-rich lake or seawater. Their chemical compositions reflect the original composition of the brine. The largest known, and also economically most important salt occurrences on earth are of marine origin. The sequence of evaporites results from variations in the chemistry of the brine from which they were deposited. The word salt is used as a general name for evaporitic salts, like halite (NaCl), sylvite (KCl), carnallite (KClMgCl₂.6H₂O), kainite (KClMgSO₄.2.75H₂O), kieserite (MgSO₄.H₂O) and bischofite (MgCl₂.6H₂O). It occurs in the subsurface of NW Europe in deposits of Permian, Triassic and Jurassic age.

Salt is in many aspects unlike other sedimentary rocks. Not only is it able to form at geologically impressive sedimentation rates of up to 10 cm/a, i.e. up to 1000 times faster than clastic sediments (Schreiber & Hsü, 1980); furthermore it does not or hardly compact with depth since its density is controlled by its crystal structure. Most important, however, is its ability to deform plastically. This is shown in many different settings around the world, from passive margins (e.g. Gulf of Mexico, West Africa), foldbelts (Zagros, Pyrenees), but also from rift and sag basins. Salt can form important detachment horizons, and it has excellent sealing capacities.

On a geological time scale, and when buried below 500 m, salt behaves in a visco-plastic way, whereas other sedimentary rocks show brittle deformation. Rock salt compacts already during early stages of burial to a tight mass with a constant density of 2168 kg/m³. Other sediments show an increase in density with depth owing to cementation and the reduction of pore volume as a function of overburden and pore pressures. Consequently, in near-surface positions, where sand and clay typically show densities of 1200 to 1400 kg/m³, halite is relatively heavy, whilst below 500 m it is lighter than surrounding rocks. This results in an unstable situation. Under the right conditions, the salt rises while solid rocks sink (Trusheim, 1963; Kockel, 1990; Remmelts, 1995). This process of flowing and rising salt is called halokinesis. It will only start when the salt successions have thicknesses of at least several hundreds of metres, and when a fault with a significant throw affects the base of the salt. Without such a trigger no halokinesis will occur; this is for instance the case for the Zechstein in part of Northern Netherlands where a hardly disturbed 600-m-thick salt layer is present.

Model for the development of salt structures (after RGD, 1993).

Salt will initially flow mainly in a lateral sense, forming what is known as salt pillows. Above a pillow, the sedimentary cover is not yet pierced. In the next stage a pillow can evolve into a salt diapir. The growth of the pillow causes extension and subsequent faulting and weakening of the sedimentary cover above the evolving diapir (Jenyon, 1986; Remmelts, 1996). In cases of strong uplift, erosion may further weaken this cover. These processes take millions of years.

The Zechstein salt in the Netherlands has formed many diapirs. The boundary conditions for diapirism have been met on a wide scale in the geological past. There are several periods of structuration during which halokinesis has been triggered, such as a long period of extension from the Mid Triassic to the Early Cretaceous, and a phase of compression during the Late Cretaceous to Early Tertiary. Many Zechstein salt structures are related to large faults in the substrate; elongated salt walls, like those along the margins of the Dutch Central Graben, are related to long fault zones, whereas isolated, circular domes developed over intersecting faults (Remmelts, 1995, 1996; De Jager, 2007).

The shapes and the internal structures of individual salt bodies are complex; reference is made to descriptions by De Boer (1971), Richter-Bernburg (1972, 1980), Kupfer (1976), Bornemann (1991) and Geluk (1995). Salt diapirs outcropping in the Iranian Dasht-e-Kavir (Jackson et al., 1990) and Zagros Mountains (Kent, 1979), and in the Spanish Pyrenees (Wagner et al., 1971) present analogues for buried salt structures in the Netherlands (Geluk, 1998, 2000).

Example of a salt structure which underwent Late Cretaceous compression. NE of Assen, The Netherlands (NITG, 2000)

A good understanding of the mechanical behaviour of the various components within a salt sequence is a prerequisite to understand their deformation. Some rocks, such as interbedded claystones, thick anhydrites (> 1 m) and carbonates are brittle in the subsurface. Potassium-magnesium salts on the contrary are mostly ductile during deformation. Mixed rock types behave differently than the separate components. For instance, a thin-bedded alternation of anhydrite and salt can be deformed in a ductile manner, while one thick layer of anhydrite embedded in salt deforms in a brittle way (Lotze, 1957). Usually the stiffer layers will be folded within the softer layers, a structure that can often be observed in conventional salt mines and also in salt cores from boreholes.

Different internal tectonic styles can be recognized in salt structures. In literature a separation is commonly made between layers, pillows and domes or diapirs, which represent increasing states of salt deformation (Trusheim, 1963).

In salt layers the top and bottom of the layer are more or less parallel to the interbedded elastic carbonate and anhydrite beds, reflecting the original deposition. Salt layers may be faulted if the throw of sub-salt faults exceeds the thickness of the salt, e.g. in areas with relatively thin Zechstein salt. In basinal areas the thickness of the Z2 (Stassfurt) Salt (500-800 m) generally exceeds the throw of the fault, which then only affects the base. The internal structures of salt layers are mainly flow folds with horizontal axial planes (Lotze, 1957). They occur on the decimetre to decametre-scale. Potassium-magnesium salts, with interbedded layers of halite, usually show strong folding.

Salt pillows are formed by a local thickening of the salt due to horizontal salt flow. In central areas of the Zechstein basin the cores of the pillows are composed of the Z2 (Stassfurt) Salt; in marginal areas the Z1 (Werra) Salt has been mobilized into pillows. Extensional faulting commonly affects the beds over the crest of the pillow. The thickening of salt in a pillow is usually accompanied by a series of stacked, large-scale flow folds in the salt, with horizontal axial planes as has been reconstructed for the Anloo salt pillow. Interbedded carbonate and anhydrite layers within the pillow are folded and their axial planes are generally parallel to its roof. Superimposed upon such large-scale folds one commonly finds a series of smaller parasitic folds. In the late pillow stage, the direction of salt flow changes from horizontal to vertical (Lotze, 1957).

In salt domes or diapirs the movement of the salt changed from mainly horizontal to mainly vertical, and the salt pierced the cover beds. The vertical flow results in folds with vertical axial planes and horizontal fold axes as well as in curtain folds with vertical fold axes. As a result of the diapirism these latter folds usually have deformed the older generation of folds with horizontal fold axes. However, due to the extremely high strains of more than 300% in the diapir, the older deformation phase may be indistinguishable. Along the margins of diapirs, friction between the cover beds and the uprising salt mass results in high vorticities within this mass, with an additional complex fault pattern (Jackson et al., 1990). Sometimes the salt may even envelop blocks of the overlying succession. The salt in a diapir rarely moves at a uniform rate (Kupfer, 1976). Shear zones separate sectors moving at higher speed, known as spines, from those moving more slowly. The development of salt diapirs normally results in a complicated internal structure, with folds of many different scales and orientations (Richter-Bernburg, 1980). Compressional tectonics during the Late Cretaceous accelerated the rate of diapirism, or strongly deformed some of the diapirs. A generalized simplified model of Zechstein salt diapirs is that the central part of the dome contains mostly Z2 Salt and that the marginal parts are made up of younger salt units. This is shown to be true for structures such as the Winschoten and Zuidwending diapirs (Harsveldt, 1980, 1986). In the Pieterburen diapir, however, the central part is made up of younger salts and the Z2 Salt occurs at the margin (Harsveldt, 1980; RGD, 1995); in Germany this occurs at Gorleben (Bornemann, 1991) and Hänigsen-Wathlingen (Schachl, 1987).

Several salt domes in the Netherlands pierced to close to the surface. The shallowest is the Zuidwending dome, where the top of the caprock is between 100 and 110 m below mean sea level. Other shallow domes include Schoonlo (120 m), Pieterburen (190 m), Hooghalen (250 m), Onstwedde (250 m) and Gasselte-Drouwen (350 m). In the western offshore block K9, a salt dome reaches almost to the sea bottom (Giesen & Mesdag, 1995).

Last modified 10/03/2009