Bernd Leiss, Yvonne Küster, Klaus Ullemeyer, Torben Seidel, Michael Schramm, in preparation
5.1.1 Introduction
Investigations concerning the development of crystallographic preferred orientations (textures) in halite yield knowledge about deformation mechanisms of rock salt which is important to explain natural deformation processes as well as to understand and predict the long-term rheological behaviour of rock salt formations. From a substantial number of studies that investigated the development of textures in experimentally deformed rock salt (e.g., Kern
& Braun 1973; Skrotzki & Haasen 1981; Skrotzki & Welch 1983; Trimby et al. 2000a, b), a variety of well-defined texture types is known. Studies about textures in naturally deformed rock salts are relatively rare (e.g., Clabaugh 1962; Schwerdtner 1966; Goemann & Schumann 1976, 1977; Kämpf et al. 1986; Ertel et al. 1987) and there are hardly any recent studies (e.g.
Schléder & Urai 2007, Desbois et al. 2010). This is because the measurement of textures in naturally deformed rock salts is especially challenging. Texture analyses with the polarization microscope are not possible due to the optical isotropy of halite, and the applicability of electron- and X-ray diffraction methods is limited, since grain sizes are in the range of mm to cm, even in shear zones. For statistically representative texture analyses, neutron diffraction is particularly suitable, because the low absorption of neutrons in matter allows measurements
of large sample volumes - which provides for a good statistical basis even in coarser-grained samples.
Only a few previous texture studies of naturally deformed rock salt show systematic structure/field-related correlations with, e.g., shear zones and fold structures. The aim of this study is therefore a detailed textural characterization of naturally deformed rock salt by correlating textures, measured via neutron texture goniometry, with micro-, macro- as well as large scale salt structures.
Fig. 5.1: Map showing the salt structures in the former Southern Permian Basin (after Lokhorst 1998).
Inserted rectangle shows the location of the salt structures investigated in this study.
5.1.1 Samples and methods
Sample material for the first four neutron texture measurements (Seidel et al. 2006) was obtained from drilling cores of various salt structures in northern Germany, namely Gorleben, Morsleben, and Teutschenthal, all situated in the former Southern Permian Basin (Fig. 5.1).
The deformation history and intensity of these salt structures is different, varying from a salt pillow structure in Teutschenthal to a diapiric structure in Gorleben. The samples were preferably taken from core segments that show significant grain shape preferred orientations and relatively small grain sizes (long axis several mm). These segments were interpreted to represent mylonitic shear zones and were well comparable to observations directly made in
the drifts of the Morsleben salt mine at a larger scale (Fig. 5.2). The samples belong to the Stassfurt Formation (Z2) of the Zechstein Group.
In this study, measurements were carried out at the Time-Of-Flight neutron texture diffractometer SKAT at the research reactor IBR-2 in Dubna, Russia, that allows sample diameters up to 50 mm (e.g., Ullemeyer et al. 1998). For the measurements, cube-shaped, cylindrical and spherical samples with dimensions up to five centimetres were prepared.
5.1.2 Results and discussion
Samples from Morsleben and Gorleben show clear grain shape anisotropies (Fig. 5.3; Seidel, 2006). Grain sizes range between 1 and 10 mm. In general, the grains‟ aspect ratio is between 2 and 2.5. Samples from Teutschenthal show only weak grain shape anisotropy (Fig. 5.3C).
Fig. 5.2: A) Synclinal fold of rock salt in the Morsleben salt mine. The layers in dark grey are shear zones. The white arrow indicates the approximate position where the sample shown in Fig. 5.2B was taken from. B) Sample of the fold shown in Fig. 5.2A. The dashed lines mark the shear zone. C) Detailed view (clockwise rotated about 45°) of a segment cut from the upper left of the sample in Fig.
5.2B. The sample surface was ground and polished. Note the large difference in grain size between the shear zone and the adjacent rock salt as well as the strong shape preferred orientation of the grains in the shear zone.
The pole figures of the first four measured samples show no crystallographic preferred orientation (Seidel et al. 2006). Further neutron texture measurements have been carried out
on 14 samples from Morsleben, Gorleben, Teutschenthal, and Remlingen (Asse), and, in addition to the Stassfurt Formation rock salt, sample material of the Leine (Z3) and the Aller (Z4) Formation as well as the Muschelkalk (Middle Triassic) group was studied. The results of these measurements support the findings of Seidel et al. (2006). All data sets are currently analyzed and prepared for publication.
Fig. 5.3: Reflected light photographs of rock salt samples from Gorleben (A), Morsleben (B) and Teutschenthal (C) measured via neutron texture goniometry (modified from Seidel et al. 2006).
The lack of a crystallographic preferred orientation in these samples demonstrates that deformation of the rock salt was not or only subordinately controlled by intracrystalline slip.
Therefore, other deformation mechanisms have to be considered to explain the random texture. Natural rock salt samples always contain certain amounts of brine in the form of fluid inclusions or as film at the grain boundaries (Roedder 1984). Due to the high solubility of halite, the presence of only small amounts of brine (>10 ppm) can change the deformation mechanism from dislocation to solution-precipitation creep (e.g., Urai et al. 1986b, Spiers et al. 1990; Peach et al. 2001; Pennock et al. 2006; Urai & Spiers 2007). I.e. fluid assisted mechanisms such as fluid assisted grain boundary migration or solution-precipitation creep may prevent the development of a crystallographic preferred orientation. This argumentation agrees with the findings of studies made on experimentally deformed rock salt (e.g., Trimby et al. 2000a, b) that demonstrate a lack of texture in pole figures of wet samples (water content ~60 ppm).
The results of this study clearly demonstrate that, in nature, the deformation processes of halite need much more attention for the setup of rheological models related to the application of using salt structures as hydrocarbon storage caverns or as host rocks for the disposal of radioactive waste.