Structural and microstructural analysis of K–Mg salt layers in the Zechstein 3 of the Veendam Pillow, NE Netherlands: development of a tectonic mélange during salt flow

Abstract

In fully developed evaporite cycles, effective viscosity contrasts of up to five orders of magnitude are possible between different layers, but the structures and mechanics in evaporites with such extreme mechanical stratification are not well understood. The Zechstein 3 unit in the Veendam salt pillow in the Netherlands contains anhydrite, halite, carnallite and bischofite, showing this extreme mechanical stratification. The Veendam Pillow has a complex multiphase salt tectonic history as shown by seismic reflection data: salt withdrawal followed by convergent flow into the salt pillow produced ruptures and folds in the underlying Z3-anhydrite–carbonate stringer and deformed the soft Z3-1b layerWe analysed a unique carnallite- and bischofite-rich drill core from the soft Z3-1b layer by macroscale photography, bulk chemical methods, X-ray diffraction and optical microscopy. Results show high strain in the weaker bischofite- and carnallite-rich layers, with associated dynamic recrystallisation at very low differential stress, completely overprinting the original texture. Stronger layers formed by alternating beds of halite and carnallite show complex recumbent folding on different scales commonly interrupted by sub-horizontal shear zones with brittle deformation, veins and boudinage. We attribute this tectonic fragmentation to be associated with a softening of the complete Z3-1b subunit during its deformation. The result is a tectonic mélange with cm- to 10 m-size blocks with frequent folds and boudinage. We infer that these structures and processes are common in deformed, rheologically strongly stratified evaporites.

Stratigraphy, Paleogeography and Depositional Setting of the K–Mg Salts in the Zechstein Group of Netherlands—Implications for the Development of Salt Caverns

“…The Z3K1 is few meters to ~50 m thick and dominantly developed in the northwestern offshore and in the northeastern onshore, around the Lauwerszee Trough and Groningen High area (Figure 6B). In accordance with previous publications [7,31], well log data highlight the occurrence of several meters-thick bischofite-rich layers in the main depocenters of this interval (Figure 3).…”

“…The Zechstein group was up to 2 km thick and drove intensive halokinetic deformations since the Late Triassic [4,5]. The Zechstein evaporites include four main types of lithologies which are marking 5 to 7 basin-scale evaporite sequences: shale, carbonate, sulfate, halite and K-Mg salts (dominantly including polyhalite, kieserite, carnallite and bischofite) (Figure 1B) [6][7][8]. Carbonate and sulphate deposits formed prograding platforms in the marginal part of the basins, whereas halite accumulated in basin centers during drawdown periods.…”

Rheological stratification in impure rock salt during long-term creep: morphology, microstructure, and numerical models of multilayer folds in the Ocnele Mari salt mine, Romania

“…In the Ocnele Mari salt mine, polyharmonic fold structures occur on up to four scales. As shown by Ramberg (1962), polyharmonic folds can develop in a multilayer sequence, where significant variation of the more competent layer thicknesses is observed. However, in the Ocnele Mari salt mine, the salt is finely layered and the variation of the layer thicknesses ranges centimetres to tens of centimetres with a maximum of ca.…”

“…Understanding folding in multilayers is rooted in the analysis of deformation of an isolated more viscous layer embedded in the less viscous matrix. Biot (1961) and Ramberg (1962) described the wavelength selection process, which is responsible for the faster growth of selected wavelengths and leads to development of a semi-regular pattern of the fold train. The wavelength that initially experiences the largest growth rate is referred to as the dominant wavelength.…”

“…The authors illustrated how these structures can be used to infer the viscosity ratio between the layers and also conditions of deformation. The role of spacing between the layers was discussed by Ramberg (1962), who distinguished disharmonic and harmonic folds. He showed that if the separation between the layers is larger than the sum of their dominant wavelength, the layers fold independently, forming disharmonic structures; otherwise they start to interact and fold harmonically.…”