Implications for the influence of salt migration-related processes

Former studies implied that the bedded Stassfurt rock salt yields a very regular bromide profile that resembles almost a theoretical profile (Schulze 1958, 1960a). A comparison of these profiles with that of domal salt (Herrmann 2000; Simon & Haltenhof 1970; Bornemann et al. 2008) suggests that diapirism-related processes result in irregular bromide profiles, either due to bromide-rich fluids involved in such processes (Baar 1963; Simon & Haltenhof 1970; Simon 1972), or by brecciation and mixing of rock salt layers (Bornemann et al. 2000, 2008). For that reason, one aim of the present study was to illuminate the relationship between the bromide content and the intensity of deformation of the studied salt by means of a well-standardized and coherent set of bromide profiles. The comparison of the bromide distribution between bedded (Teutschenthal) and domal salt (Morsleben, Gorleben) shows that the classic trend of the bromide profile of a stratigraphically equivalent rock salt sequence was principally preserved in all three studied locations (Fig. 3.9). Therefore, a contribution from bromide-rich fluids that potentially circulated through the Hauptsalz during salt migration and significantly altered the bromide contents for either Morsleben or Gorleben is highly unlikely. In addition, although the fabric of the Hauptsalz in these two locations is most likely a result of brecciation, folding, and homogenization of different layers and halite types (Bornemann et al. 2000, 2008), these processes must have been restricted to formation-parallel units, since the general stratigraphic sequence has been preserved. On the one hand, this is demonstrated by the bromide profiles and, on the other hand, by geological sections that are based on excellent subsurface outcrops and cores from wells.

Typical original depositional features, such as primary fluid inclusions or growth zones like in chevron crystals (Hardie et al. 1983; Lowenstein & Hardie 1985), are only present in very rare cases in the Hauptsalz of the Stassfurt Formation (Pape et al. 2002). Nevertheless, the characteristic lamination within the Kristallbrocken indicates that they are relics of former continuous sedimentary layers. Furthermore, results of a preliminary study showed that,

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Fig. 3.9: Compilation of bromide analyses and interpretation of the observed findings.

Bromide contents are plotted versus the thickness of the Hauptsalz section normalized to 100 m. A) Plot showing for all studied locations the curve through the data of the Kristallbrocken and matrix plotted together as well as the bromide values of the Kristallbrocken. B) Plot showing for each location the running average of five points through the data of the Kristallbrocken and matrix.

within the Kristallbrocken, the bromide content of these laminae increases upward reflecting progressive evaporation and precipitation (Schlechter 2004). Consequently, the Kristallbrocken can be regarded as an unaltered, quasi primary component of the Hauptsalz.

In Teutschenthal, they are arranged in layers and occur throughout the whole Hauptsalz, whereas in Gorleben and Morsleben they are restricted to the upper half of the Hauptsalz and occur more separated from each other surrounded by matrix halite.

In contrast to the Kristallbrocken, the matrix halite can be regarded as secondary, presumably a dynamically recrystallised component of the Hauptsalz. In all locations, matrix halite is mostly composed of clear, fluid inclusion-poor halite, with fluid inclusions and anhydrites arranged at its boundaries. The presence of fluid inclusions at the grain boundaries suggests that the matrix halite of all three locations has been largely recrystallised (Roedder 1984). Reflected light examination of etched sample surfaces revealed a quite similar microfabric of matrix halite for the three locations. In all studied samples, both sub-structure free and sub-structure rich grains could be observed in different parts of the Hauptsalz. The presence of less sub-structured grains replacing highly sub-structured grains as well as lobate grain boundaries and pinning structures points to grain boundary migration recrystallisation (Urai et al. 1987; Drury & Urai 1990; Passchier & Trouw 1998; Schléder & Urai 2005).

Grains with well developed, equidimensional sub-grains indicate intracrystalline deformation (Passchier & Trouw 1998). In all samples from each location, evidence for intracrystalline deformation and recrystallisation can be observed. The microstructural investigations revealed no significant differences within the Hauptsalz and between the three studied locations, although they belong to different tectonic settings. The clearly different regional deformation intensities at the various locations are not reflected in the microfabrics. Only the macrofabric of the Hauptsalz in Morsleben and Gorleben differs considerably from the one in Teutschenthal, as deformation-related processes have destroyed the original sedimentary fabric during salt migration. As a consequence, the Hauptsalz contains less Kristallbrocken and higher amounts of matrix halite, the grain size distribution is more homogeneous, and anhydrite mainly occurs as a fine dispersion in the rock salt or as layer fragments and nodules.

As the bromide characteristics of the Hauptsalz in Morsleben and Gorleben before salt migration are assumed to be similar to that of Teutschenthal at present, the absence of the Kristallbrocken in the lower part of the profile and the low dispersion of bromide contents along the running average curve is interpreted to be the result of deformation related processes. The assumption of comparable bromide characteristics seems plausible because the

investigated profiles belong to equivalent stratigraphic intervals precipitated in analogous time intervals.

The following, presumably deformation-related features in the domal salt of Morsleben and Gorleben can be determined: (1) The absence of the Kristallbrocken in the lower half of the Hauptsalz (Fig. 3.9). (2) A lower dispersion of bromide contents along the running average curve, especially in the lower half of the Hauptsalz (Fig. 3.9). The absence of the laminated Kristallbrocken in the lower half of the domal salt indicates that this halite type was deformed and recrystallised during salt migration, forming clear, medium to coarse grained matrix halite. The comparatively lower dispersion of bromide contents along the running average curve is expected to be associated with a redistribution of bromide, which eventually results in homogenization of the varying bromide contents and smoothing of the bromide profile (Figs 3.7B and C, 3.8B and C). However, the exact process of bromide redistribution and homogenization cannot be satisfactorily explained. This is because it is not yet well understood, what effect deformation-related processes such as dislocation creep, diffusion creep, dynamic recrystallisation, or combinations of these processes (Guillopé & Poirier 1979; Carter & Hansen 1983; Skrotzki & Welch, 1983; Spiers et al. 1986; Urai et al. 1986a, 1986b, 1987; Spiers et al. 1990; Carter et al. 1993; Spiers & Carter 1998) may have on the behavior of trace elements, especially bromide. In any case, the presence of brine is supposed to be crucial, as deformation or recrystallisation would influence the bromide content only if brines are present that have bromide contents different from that of the original brine (Raup &

Hite 1996).

In the upper part of the bromide profiles of Morsleben and Gorleben, the Kristallbrocken are still present and the dispersion of bromide contents is considerably higher than that in the lower part of the profile. Two explanations for this finding are conceivable:

first, the original dispersion of bromide contents may likewise have been higher in the upper half than in the lower half of the Hauptsalz (similar to the profile in Teutschenthal). In this case, due to such initially different dispersions, homogenization of the varying bromide contents and smoothing of the bromide profile would require more time in the upper half.

Second, halite beds of the upper half may have behaved more passively during salt migration, and thus may have been subjected to less intense deformation than the lower half. This explanation would also be in line with the observation that the sedimentary fabric of younger Zechstein sequences of Gorleben could be largely preserved (Bäuerle 2000; Bornemann et al.

2000). Another possibility could be that the kind of deformation mechanism was affected by varying amounts of fluids or impurities in the Hauptsalz. However, this explanation would

only apply for the Hauptsalz of Gorleben, because in Morsleben, there is no significant difference between the anhydrite content of the upper part (ca. 4.1%) and that of the lower part (ca. 4.3%) of the Hauptsalz.