The majority of the stringer fragments in areas below extremely subsided synclines are flat (Fig. 4.10e, Fig. 4.13, and Fig 4.22a,b,c). In this case, the boudins are classified based on salt flow direction, where initial and large-scale gaps form below the hinge zone after the stringer was stretched by a diverging salt flow and resulted in almost flat boudin shapes (Fig 4.22c). However, gently folded individual fragments were also found (Fig. 4.6 and Fig.4.7). The mechanical properties of the stringer
153 within the salt and the presence of extreme lithological heterogeneities (e.g., weak potash layers below Z3 Stringer, see section 3.5.1.2) are considered factors contributing to folding within high lateral stretching areas. Moreover, the ductile behaviour of the anhydrite stringer itself causes the layer to fold even within zones of regional extension (van Gent el. 2011) and to fragment during extreme stretching.
Highly brittle and competent stringers tend to be less folded and more boudinaged, such as the carbonate stringer in the South Oman Salt Basin (e.g., Li et al. 2012a).
Chapter 4: Kinematic evolution of the Z3 Stringer in areas of salt subsidence
154
Fig. 4.22: Boudinage of the Z3 Stringer. (a) Profile across S1. (b) Simplified sketch of the common Z3 structural styles. (c) Boudinage classified based on salt flow direction where the initial and large-scale gap created below the hinge zone by stretching the stringer via opposite flow directions resulted in almost flat boudins. (d) Further away from the hinge, the stringer is stretched in one dominant direction, and the salt flows faster than the stringer, thus gaps occur. Folding may occur within this zone due to progressive deformation events, because of the transition from extension into contraction, or a combination of these two. (e) Folding of the stringer by salt accumulation from all directions, typical for a spherical pillow (e.g., A3). (f–g) Two boudinage processes (Bons et al. 2004) (f) Inflation and collapse upon emplacement, no lateral extension required. (g) Boudinage by layer-parallel stretching.
155 4.6.6 Interpretation of steep discontinuities below anticline structures
The seismic reflections of steep or thin stringers are strongly reduced or absent (Sleep and Fujita 1997; van Gent et al. 2011; Strozyk et al. 2012; Strozyk et al.
2014). Most steep stringer geometries are not imaged on seismic data (Fig. 4.11, Fig. 4.15, and Fig. 4.16). The most common stringer structure within areas of thick salt sections (i.e., below A1, A2, A3, A4 and A5) is a fold structure; as a result, most of the steep stringer parts are located within fold limbs.
Four possible end-member scenarios for such discontinuities are proposed. First, the stringer is continuous, with steeply inclined fold limbs that are seismically not resolved (Fig. 4.23a). Second, the stringer is too thin, below seismic resolution, and therefore could not be imaged (Fig. 4.23b). Third, the stringer is physically fractured and displaced vertically by salt flow as a boudin fragment (Fig. 4.23c). Fourth, the stringer is fractured parallel to the folded layer into smaller boudins that are either too steep or too small and thin to be imaged (Fig. 4.23d).
The Z3 Stringer was observed to experience brittle deformation only within areas of significant subsidence by Top Salt (e.g., S1, S5). The stringer is still continuous and folded below S2, with no significant brittle deformation (Fig. 4.8). Therefore, it can be inferred that the Z3 Stringer did not deform easily in a brittle manner during the early stages of subsidence, and a high rate of Top Salt subsidence was required for the anhydrite to reach the brittle phase and fail. However, the formation of a regional anticlinal structure is the opposite. Buckling of the basin with salt movement from areas where top salt has subsided into the generated anticline structure creates a contractional system inside the larger salt cored anticline (Hudec and Jackson 2007;
van Gent et al. 2011). This contraction within the anticline structure is likely to fold
Chapter 4: Kinematic evolution of the Z3 Stringer in areas of salt subsidence
156 the ductile anhydrite stringer into high-amplitude folds (Fig. 4.11d). The anhydrite layer is sufficiently ductile to fold when shortening forces are applied parallel to the layering with no significant brittle deformation. Similar examples have been documented in salt mines (e.g., Wagner and Jackson 2011), salt outcrops (Fig.
4.11d) (Seidl 1921; Bornemann 1991), on seismic data in NW Europe (van Gent et al. 2011; Strozyk et al. 2012; Strozyk et al. 2014), and in model experiments (Zulauf et al. 2003; Zulauf et al. 2009). Therefore, the stringer is interpreted to rupture only if it has been highly stretched parallel to layering (e.g., below S1 and S5). A steeply inclined limb of folded stringer within compressional salt zones (e.g., regional anticlines, pillows) is unlikely to be the cause for generating brittle deformation, since stretching would be required to deform the stringer in a brittle fashion within the pillow structure.
The absence of any stringer fragments scattered between the upper and lower hinges, either in the limb area or displaced nearby, supports the continuity model of the anhydrite stringer within steeply folded structures. Therefore, vertically fragmented and displaced stringer limbs (Fig. 4.23c,d) might rarely be the case for non-imaged steeper parts within the core of regional anticlinal structures, except where the stringer has been reworked and dragged from the extensional regions (e.g., S1, S5) into areas of shortening (A1–A5).
The interpretation of the stringer in the Silverpit area cannot be applied as a case study for the late stage of halokinesis that requires vertical ascent of the stringer during the formation of salt domes or salt diapirs (e.g., Bornemann 1991; Koyi 2001;
Peters et al. 2003; Reuning et al. 2009). Based on a cross-section of the Gorleben salt dome in Germany (Bornemann 1991), which is considered to be a late
157 halokinesis stage, the Z3 Stringer is still continuous up until the top of the dome.
However, further examples are needed to confirm the continuity and folding of an anhydrite layer within salt accumulation zones at late stages of halokinesis (e.g., diapir build-up). Therefore, the continuity of the stringer within zones of salt contraction is at least applicable for areas of low to moderate halokinesis grade, similar to the salt pillows in the Cavendish area.
Fig. 4.23: Four possible scenarios to explain the vertical gaps between visible stringer fragments: (a) The stringer is continuous but too steep to be imaged. (b) The stringer is continuous but too thin to be imaged. (c) The stringer is fractured and disrupted vertically. (d) The steeper stringer part is fragmented into smaller boudins parallel to the folded layer; the fold hinges are thickened while the limbs are extended and boudinaged.
158 4.6.7 Kinematic evolution of the Z3 Stringer in areas of salt subsidence