Large grain-size-dependent rheology contrasts of halite at low differential stress: evidence from microstructural study of naturally deformed gneissic Zechstein 2 rock salt (Kristallbrockensalz) from the northern Netherlands
Authors: Barabasch Jessica , Schmatz Joyce , Klaver Jop , Schwedt Alexander , Urai Janos L. 
Published: March 2023

Abstract. Constitutive laws to predict long-term deformation of
solution-mined caverns and radioactive-waste repositories in rock salt play
an important role in the energy transition. Much of this deformation is at
differential stresses of a few megapascals, while the vast majority of laboratory
measurements are at much higher differential stress and require
extrapolation. This can be much improved by including microstructural data
of samples deformed in natural laboratories. Deformation of rock salt can
occur by dislocation creep and grain-size-dependent dissolution–precipitation
creep processes (pressure solution); this mechanism is not commonly included
in current engineering predictions. Here we show evidence for large grain-size-dependent differences in rock
salt rheology based on microstructural observations from Zechstein rock salt
cores of the northern Netherlands that experienced different degrees of
tectonic deformation. We studied the relatively undeformed
horizontally layered Zechstein 2 (Z2) salt (Stassfurt Formation) from Barradeel and
compared it with a much more strongly deformed equivalent in diapiric salt from
Winschoten, Zuidwending, and Pieterburen. We used optical microscopy of
thin gamma-irradiated sections for microtectonic analysis, recrystallized
grain-size measurements and subgrain-size piezometry, electron microscopy
with energy-dispersive X-ray spectroscopy, and X-ray diffraction analysis for
second-phase mineralogy. Subgrain-size piezometry shows that this
deformation took place at differential stresses between 0.5 and 2 MPa. In the undeformed, layered salt from Barradeel we find centimetre-thick layers of
single crystalline halite (Kristalllagen or megacrystals) alternating with
fine-grained halite and thin anhydrite layers. The domal salt samples are
typical of the well-known “Kristallbrockensalz” and consist of centimetre-size
tectonically disrupted megacrystals surrounded by fine-grained halite with a
grain size of a few millimetres. We infer high strains in the fine-grained halite as
shown by folding and boudinage of thin anhydrite layers, as compared to the
megacrystals, which are internally much less deformed and develop subgrains
during dislocation creep. Subgrain size shows comparable differential
stresses in Kristallbrocken as in matrix salt. The fine-grained matrix salt
is dynamically recrystallized to some extent and has few subgrains and
microstructures, indicating deformation by solution–precipitation processes.
We infer that the finer-grained halite deformed dominantly via pressure
solution and the megacrystals dominantly by dislocation creep. The samples show that the fine-grained matrix salt is much weaker than
Kristallbrocken because of different dominant deformation mechanisms. This
is in agreement with microphysical models of pressure solution creep in which grain size has a
significant effect on strain rate at low differential stress. Our results
point to the importance of pressure solution creep in rock salt at low
differential stresses around engineered structures but also in most salt
tectonic settings. We suggest that including results of microstructural
analysis can strongly improve engineering models of rock salt deformation. We recommend that this mechanism of grain-size-dependent rheology is
included more consistently in the constitutive laws describing the
deformation of rock salt.