Solution-precipitation creep and fluid flow in halite: a case study of Zechstein (Z1) rocksalt from Neuhof salt mine (Germany)

Abstract

Zechstein (Z1) rocksalt from the Fulda basin, from the immediate vicinity of the Hessen potash bed is folded into tight to isoclinal folds which are cut by an undeformed, 1 cm thick, coarse-grained halite vein. Microstructures were investigated in etched, gamma-irradiated thin sections from both the wall rock and the vein. The lack of synsedimentary dissolution structures and the widespread occurrence of plate-shaped and hopper grains in the wall-rock suggests that the sedimentary environment was perennial lake. Deformation microstructures are in good agreement with solution-precipitation creep process, and salt flow under very low differential stress. Strength contrast between anhydrite-rich and anhydrite-poor layers caused the small scale folding in the halite beds. The vein is completely sealed and composed mainly of euhedral to subhedral halite grains, which often overgrow the wall-rock grains. Those microstructures, together with the presence of occasional fluid inclusion bands, suggest that the crystals grew into a solution-filled open space. Based on considerations on the maximum value of in-situ differential stress, the dilatancy criteria, the amount of released fluids from the potash bed during metamorphism and the volume change, it is proposed that the crack was generated by hydrofracturing of the rocksalt due to the presence of the salt-metamorphic fluid at near-lithostatic pressure.Keywords Halite Á Potash salt Á Deformation mechanism Á Hydrofracture Fluid transport in rocksaltStudies of Casas and Lowenstein (1989) and Lowenstein and Spencer (1990) on Quaternary halite deposition showed that the effective porosity and permeability decrease drastically in the shallow subsurface. After about 50 m of burial the halite is nearly completely cemented, and is without any visible porosity. At depth of 100 m, the halite is tight, entirely cemented and has no measurable permeability. The very low permeability of halite is demonstrated by laboratory measurements and its ability to seal large hydrocarbon columns and fluid pressure cells. This very low permeability is maintained during halokinesis, although there are some processes which can lead to the increase in permeability.For example, Fokker (1995) considered the long term evolution of salt permeability after abandonment of solution mining caverns. He showed that when fluid pressure in the cavern approaches lithostatic due to creep convergence, diffuse dilatancy of the roof occurs and the permeability of the salt increases to allow the slow escape of the fluid. Similar high-pressure fluid pockets (porous salt) occur also in nature and these are presumably slowly moving upwards by similar processes. This process might operate in other rock types, as suggested by the mobile hydrofracture model in Bons (2001). The detailed nature of the relation between the effective stress, differential stress and permeability was

Rheology of rock salt for salt tectonics modeling

“…However, as an important extrapolation of laboratory data, it is necessary to study the rock salt 0.00E+00 3.00E+01 6.00E+01 Urai and Spiers 2007) deformation in nature in order to observe detailed information for deformation mechanisms (Schléder and Urai 2005;Schléder et al 2008). For creep of salt under high differential stresses, for example, 2.0 MPa, which is in agreement with many studies which used subgrain size piezometry on salt deformed by a combination of dislocation creep and pressure solution creep.…”

Inclusions in anhydrite crystals from blue halite veins in the Kłodawa Salt Dome (Zechstein, Poland)

“…How ever, even if tem per a ture is con sid ered as a main rea son of ha lite colouration, un solved re main the prob lems of col our vari a tions and of the forms of colouration ob served on the mi cro-scale within the sin gle crys tals (Heflik et al, 2008) and on the scale of par tic u lar ex po sures in mine work ings (Natkaniec-Nowak and Tobo³a, 2003;Janiów et al, 2008). Vari ability of blue colouration was ob served also in ar ti fi cially ir ra di ated nat u ral ha lite crys tals Urai, 2005, 2007;Schléder et al, , 2008 and in ter preted gen er ally as a re sult of stress. It must be em pha sized that dur ing the gamma-ir ra di ation, the blue colouration of ha lite was ob tained at the tem per ature of 100°C, whereas at lower tem per a ture (35°C) the brown col our ap peared.…”

Evolution of rheologically heterogeneous salt structures: a case study from the NE Netherlands

“…The interaction of layers with such contrasts in viscosity and rheology during deformation must lead to the development of complex fold and boudin structures. At the meso-scale, these complex internal geometries in layered evaporites are documented in mine galleries and in drill cores (Martini, 1953;Siemeister, 1969;Bornemann and Fischbeck, 1991;Burliga, 1996;Zirngast, 1996;Behlau and Mingerzahn, 2001;Bornemann et al, 2008;Schléder et al, 2008;Hammer et al, 2014). K-Mg salt-rich layers typically show complex, internal folding on the meter to millimeter scale, even if the entire package of layered evaporites is less deformed.…”